Engineering Reference and Application Solutions

Engineering Reference and Application Solutions
Engineering
Reference and
Application
Solutions
Drill
Head
Table
Ballscrew
Rotating Nut
Motor
Transfer
Machine
Drive/Controller
Drive
Motor
Drive
Motor
Joystick
Indexer
Circuit Board
Rotary Indexer
PLC
Programmable
Logic Controller
Controller
Drive
A1
Motor
Overview
Technologies
Introduction
Motion control, in its widest sense, could relate to
anything from a welding robot to the hydraulic
system in a mobile crane. In the field of Electronic
Motion Control, we are primarily concerned with
systems falling within a limited power range,
typically up to about 10HP (7KW), and requiring
precision in one or more aspects. This may involve
accurate control of distance or speed, very often
both, and sometimes other parameters such as
torque or acceleration rate. In the case of the two
examples given, the welding robot requires precise
control of both speed and distance; the crane
hydraulic system uses the driver as the feedback
system so its accuracy varies with the skill of the
operator. This wouldn’t be considered a motion
control system in the strict sense of the term.
Our standard motion control system consists of
three basic elements:
Fig. 1 Elements of motion control system
High-Level
Commands
Host
Computer
or PLC
Command
Signals
Indexer/
Controller
Drive
Motor
Hybrid Stepper
DC Servo
Brushless
Servo Linear
Stepper
The motor. This may be a stepper motor (either
rotary or linear), a DC brush motor or a brushless
servo motor. The motor needs to be fitted with
some kind of feedback device unless it is a stepper
motor.
Fig. 2 shows a system complete with feedback to
control motor speed. Such a system is known as a
closed-loop velocity servo system.
Fig. 2 Typical closed loop (velocity) servo system
Tachometer
Controller
Drive
Motor
Velocity Feedback
The drive. This is an electronic power amplifier that
delivers the power to operate the motor in response
to low-level control signals. In general, the drive will
be specifically designed to operate with a particular
motor type – you can’t use a stepper drive to
operate a DC brush motor, for instance.
A2
The control system. The actual task performed by
the motor is determined by the indexer/controller; it
sets things like speed, distance, direction and
acceleration rate. The control function may be
distributed between a host controller, such as a
desktop computer, and a slave unit that accepts
high-level commands. One controller may operate
in conjunction with several drives and motors in a
multi-axis system.
We’ll be looking at each of these system elements
as well as their relationships to each other.
Table of Contents
Motor Applications
Step Motor Technology
Linear Step Motor Technology
Common Questions Regarding Step Motors
DC Brush Motor Technology
Brushless Motor Technology
Hybrid Servo Technology
Direct Drive Motor Technology
Step Motor Drive Technology
Microstepping Drive Technology
Analog and Digital Servo Drives
Brushless Servo Drive Technology
Servo Tuning
Feedback Devices
Machine Control
Control System Overview
Understanding I/O Modules
Serial & Parallel Communications
Electrical Noise Symptoms & Solutions
Emergency Stop
System Selection Considerations
Motor Sizing and Selection Software
System Calculations – Move Profiles
System Calculations – Leadscrew Drives
System Calculations – Direct Drives
System Calculations – Gear Drives
System Calculations – Tangential Drives
System Calculations – Linear Motors
Glossary of Terms
Technical Data
Application Examples
A3
A4
A9
A12
A13
A17
A20
A21
A23
A29
A31
A34
A36
A39
A45
A46
A48
A51
A52
A54
A55
A57
A58
A60
A63
A64
A65
A66
A68
A71
A72
Motor Technologies
The following section gives some idea of the
applications that are particularly appropriate for
each motor type, together with certain
applications that are best avoided. It should be
stressed that there is a wide range of
applications that can be equally well met by
more than one motor type, and the choice will
tend to be dictated by customer preference,
previous experience or compatibility with
existing equipment.
Cost-conscious applications will always be
worth attempting with a stepper, as it will
generally be hard to beat the stepper’s price.
This is particularly true when the dynamic
requirements are not severe, such as “setting”
type applications like positioning a guillotine
back-stop or a print roller.
High-torque, low-speed, continuous-duty
applications are also appropriate for step
motors. At low speeds, it is very efficient in
terms of torque output relative to both size and
input power. Microstepping can improve lowspeed applications such as a metering pump
drive for very accurate flow control.
Start Here
Will a stepper meet
the torque/speed
requirements?
Do you need to run
continuously at
speeds above
2000 rpm?
Must the motor
EITHER
1) Be maintenancefree
2) Operate in any
environment
No
Do you need to
control torque?
Are there any
other brush
servos in the
system?
Does the load
change rapidly
during operation?
Is low-speed
smoothness
important?
Use a brushless
servo.
No
If there are other
brushless motors, it may
be better to be consistent
with this one.
Otherwise use a brush servo.
Is there a hybrid
servo which meets
the torque/speed
requirements?
Is rapid settling
important?
Yes
Yes
Try a hybrid
servo with
encoder feedback
if necessary.
Use a microstepping
with encoder
feedback?
Yes
Is quiet operation
important?
No
Yes
Yes
Really quiet?
No
No
No
Use a stepper
No
No
No
Is quiet operation
important?
Higher
torque/speed
technology.
Yes
Use a brush servo.
Yes
No
Yes
Yes
No
Short, rapid, repetitive moves are the natural
domain of steppers or hybrid servos due to their
high torque at low speeds, good torque-toinertia ratio and lack of commutation problems.
The brushes of the DC motor can limit its
potential for frequent starts, stops and direction
changes.
Will a brushless
servo meet
the torque/speed
requirements?
No
No
No
Do you need to
detect position
loss OR measure
actual load
position to correct
for backlash?
Will a brush
servo meet
the torque/speed
requirements?
Yes
Yes
High-torque, high-speed, continuous-duty
applications suit the servo motor, and in fact, a
step motor should be avoided in such
applications because the high-speed losses can
cause excessive motor heating. A DC motor
can deliver greater continuous shaft power at
high speeds than a stepper of the same frame
size.
Low-friction, mainly inertial loads can be
efficiently handled by the DC servo provided the
start/stop duty requirements are not excessive.
This type of load requires a high ratio of peak to
continuous torque and in this respect the servo
motor excels.
No
Use a microstepping,
hybrid servo, or
direct drive servo
Very arduous applications with a high
dynamic duty cycle or requiring very high
speeds may require a brushless motor. This
solution may also be dictated when
maintenance-free operation is necessary.
Low-speed, high-smoothness applications
are appropriate for microstepping or direct drive
servos.
Applications in hazardous environments or in
a vacuum may not be able to use a brush
motor. Either a stepper or a brushless motor is
called for, depending on the demands of the
load. Bear in mind that heat dissipation may be
a problem in a vacuum when the loads are
excessive.
A3
Yes
A Engineering Reference
Application Areas of Motor Types
Motor Technologies
Stepper Motor Disadvantages
Stepper motors have the following disadvantages:
Stepper Motors
Stepper Motor Benefits
Stepper motors have the following benefits:
•
•
•
•
•
•
•
•
•
•
Low cost
Ruggedness
Simplicity in construction
High reliability
No maintenance
Wide acceptance
No tweaking to stabilize
No feedback components are needed
They work in just about any environment
Inherently more failsafe than servo motors.
There is virtually no conceivable failure within the
stepper drive module that could cause the motor to
run away. Stepper motors are simple to drive and
control in an open-loop configuration. They only
require four leads. They provide excellent torque at
low speeds, up to 5 times the continuous torque of
a brush motor of the same frame size or double the
torque of the equivalent brushless motor. This often
eliminates the need for a gearbox. A stepper-driven
system is inherently stiff, with known limits to the
dynamic position error.
Permanent Magnet (P.M.) Motors. The tin-can or
“canstack” motor shown in Fig. 1.1 is perhaps the
most widely-used type in non-industrial
applications. It is essentially a low-cost, low-torque,
low-speed device ideally suited to applications in
fields such as computer peripherals. The motor
construction results in relatively large step angles,
but their overall simplicity lends itself to economic
high-volume production at very low cost. The axialair gap or disc motor is a variant of the permanent
magnet design which achieves higher performance,
largely because of its very low rotor inertia.
However this does restrict the applications of the
motor to those involving little inertia. (e.g.,
positioning the print wheel in a daisy-wheel printer).
• Resonance effects and relatively long settling
times
• Rough performance at low speed unless a
microstep drive is used
• Liability to undetected position loss as a result of
operating open-loop
• They consume current regardless of load
conditions and therefore tend to run hot
• Losses at speed are relatively high and can cause
excessive heating, and they are frequently noisy
(especially at high speeds).
• They can exhibit lag-lead oscillation, which is
difficult to damp. There is a limit to their available
size, and positioning accuracy relies on the
mechanics (e.g., ballscrew accuracy). Many of
these drawbacks can be overcome by the use of
a closed-loop control scheme.
Note: The Compumotor Zeta Series minimizes or
reduces many of these different stepper motor
disadvantages.
There are three main stepper motor types:
• Permanent Magnet (P.M.) Motors
• Variable Reluctance (V.R.) Motors
• Hybrid Motors
Variable Reluctance (V.R.) Motors. There is no
permanent magnet in a V.R. motor, so the rotor
spins freely without “detent” torque. Torque output
for a given frame size is restricted, although the
torque-to-inertia ratio is good, and this type of motor
is frequently used in small sizes for applications such
as micro-positioning tables. V.R. motors are seldom
used in industrial applications (having no permanent
magnet). They are not sensitive to current polarity
and require a different driving arrangement than the
other motor types.
Fig. 1.2 Variable reluctance motor
Fig. 1.1 “Canstack” or permanent magnet motor
N
N
S
S
N
N S N S N
S
S
N
N
S
S
S N
S N
S
N
Rotor
Stator cup A
Coil A
Coil B
Stator cup B
Output shaft
Courtesy Airpax Corp., USA
A4
Hybrid Motors. The hybrid motor shown in Fig. 1.3
is by far the most widely-used stepper motor in
industrial applications. The name is derived from the
fact that it combines the operating principles of the
other two motor types (P.M. & V.R.). Most hybrid
motors are 2-phase, although 5-phase versions are
available. A recent development is the “enhanced
hybrid” motor, which uses flux-focusing magnets to
give a significant improvement in performance,
albeit at extra cost.
Fig. 1.3 Hybrid stepper motor
Non-magnetic
Stainless
Steel Shaft
Prelubricated
Bearing
The rotor of this machine consists of two pole
pieces with three teeth on each. In between the
pole pieces is a permanent magnet that is
magnetized along the axis of the rotor, making one
end a north pole and the other a south pole. The
teeth are offset at the north and south ends as
shown in the diagram.
The stator consists of a shell having four teeth that
run the full length of the rotor. Coils are wound on
the stator teeth and are connected together in
pairs.
With no current flowing in any of the motor
windings, the rotor will take one of the positions
shown in the diagrams. This is because the
permanent magnet in the rotor is trying to minimize
the reluctance (or “magnetic resistance”) of the flux
path from one end to the other. This will occur
when a pair of north and south pole rotor teeth are
aligned with two of the stator poles. The torque
tending to hold the rotor in one of these positions is
usually small and is called the “detent torque”. The
motor shown will have 12 possible detent positions.
The operation of the hybrid motor is most easily
understood by looking at a very simple model that
will produce 12 steps per rev. (Fig. 1.4).
If current is now passed through one pair of stator
windings, as shown in Fig. 1.5(a), the resulting north
and south stator poles will attract teeth of the
opposite polarity on each end of the rotor. There
are now only three stable positions for the rotor, the
same as the number of rotor teeth. The torque
required to deflect the rotor from its stable position
is now much greater, and is referred to as the
“holding torque”.
Fig. 1.4 Simple 12 step/rev hybrid motor
Fig. 1.5 Full stepping, one phase on
Housing
Rotor
Stator
(a)
1A
(b)
N
N
S
2A
N
N
N
S
N
N
N
S
S
2B
S
N
S
S
S
N
S
S
N
S
N
N
S
S
N
S
N
S
(c)
(d)
S
1B
N
S
S
S
N
N
S
N
N
By changing the current flow from the first to the
second set of stator windings (b), the stator field
rotates through 90° and attracts a new pair of rotor
poles. This results in the rotor turning through 30°,
corresponding to one full step. Reverting to the first
set of stator windings but energizing them in the
opposite direction, we rotate the stator field
through another 90° and the rotor takes another
30° step (c). Finally, the second set of windings are
energized in the opposite direction (d) to give a
third step position. We can now go back to the
first condition (a), and after these four steps the
rotor will have moved through one tooth pitch. This
simple motor therefore performs 12 steps per rev.
Obviously, if the coils are energized in the reverse
sequence, the motor will go round the other way.
A5
A Engineering Reference
Motor Technologies
Motor Technologies
If two coils are energized simultaneously (Fig. 1.6),
the rotor takes up an intermediate position since it
is equally attracted to two stator poles. Greater
torque is produced under these conditions because
all the stator poles are influencing the rotor. The
motor can be made to take a full step simply by
reversing the current in one set of windings; this
causes a 90° rotation of the stator field as before. In
fact, this would be the normal way of driving the
motor in the full-step mode, always keeping two
windings energized and reversing the current in
each winding alternately.
motor and drive characteristics). In the half-step
mode, we are alternately energizing two phases
and then only one as shown in Fig. 1.9. Assuming
the drive delivers the same winding current in each
case, this will cause greater torque to be produced
when there are two windings energized. In other
words, alternate steps will be strong and weak.
This does not represent a major deterrent to motor
performance—the available torque is obviously
limited by the weaker step, but there will be a
significant improvement in low-speed smoothness
over the full-step mode.
Fig. 1.6 Full stepping, two phase on
Clearly, we would like to produce approximately
equal torque on every step, and this torque should
be at the level of the stronger step. We can achieve
this by using a higher current level when there is
only one winding energized. This does not overdissipate the motor because the manufacturer’s
current rating assumes two phases to be energized
(the current rating is based on the allowable case
temperature). With only one phase energized, the
same total power will be dissipated if the current is
increased by 40%. Using this higher current in the
one-phase-on state produces approximately equal
torque on alternate steps (see Fig. 1.10).
N
S
S
N
N
N
N
S
S
N
S
N
N
S
S
S
N
S
S
N
S
S
S
N
N
S
N
S
S
N
S
N
N
S
S
N
Fig. 1.8 Full step current, 2-phase on
N
N
N
2
1
4
3
S
By alternately energizing one winding and then two
(Fig. 1.7), the rotor moves through only 15° at each
stage and the number of steps per rev will be
doubled. This is called half stepping, and most
industrial applications make use of this stepping
mode. Although there is sometimes a slight loss of
torque, this mode results in much better
smoothness at low speeds and less overshoot and
ringing at the end of each step.
Phase 1
Phase 2
Fig. 1.9 Half step current
Fig. 1.7 Half stepping
1
S
S
S
N
S
N
S
N
N
S
S
N
4
5
6
8
7
Phase 1
N
N
S
3
S
N
S
2
N
S
S
N
N
N
S
N
Phase 2
Current Patterns in the Motor Windings
When the motor is driven in its full-step mode,
energizing two windings or “phases” at a time (see
Fig. 1.8), the torque available on each step will be
the same (subject to very small variations in the
Fig. 1.10 Half step current, profiled
1
Phase 1
Phase 2
A6
2
3
4
5
6
7
8
We have seen that energizing both phases with
equal currents produces an intermediate step
position half-way between the one-phase-on
positions. If the two phase currents are unequal, the
rotor position will be shifted towards the stronger
pole. This effect is utilized in the microstepping
drive, which subdivides the basic motor step by
proportioning the current in the two windings. In this
way, the step size is reduced and the low-speed
smoothness is dramatically improved. Highresolution microstep drives divide the full motor step
into as many as 500 microsteps, giving 100,000
steps per revolution. In this situation, the current
pattern in the windings closely resembles two sine
waves with a 90° phase shift between them (see
Fig. 1.11). The motor is now being driven very much
as though it is a conventional AC synchronous
motor. In fact, the stepper motor can be driven in
this way from a 60 Hz-US (50Hz-Europe) sine wave
source by including a capacitor in series with one
phase. It will rotate at 72 rpm.
If we imagine that a tooth is placed in each of the
gaps between the stator poles, there would be a
total of 48 teeth, two less than the number of rotor
teeth. So if rotor and stator teeth are aligned at 12
o’clock, they will also be aligned at 6 o’clock. At 3
o’clock and 9 o’clock the teeth will be misaligned.
However, due to the displacement between the
sets of rotor teeth, alignment will occur at 3 o’clock
and 9 o’clock at the other end of the rotor.
Fig. 1.11 Phase currents in microstep mode
Note that there are as many detent positions as
there are full steps per rev, normally 200. The
detent positions correspond with rotor teeth being
fully aligned with stator teeth. When power is
applied to a stepper drive, it is usual for it to
energize in the “zero phase” state in which there is
current in both sets of windings. The resulting rotor
position does not correspond with a natural detent
position, so an unloaded motor will always move by
at least one half step at power-on. Of course, if the
system was turned off other than in the zero phase
state, or the motor is moved in the meantime, a
greater movement may be seen at power-up.
Phase 1 Current: Zero
+
-
Phase 2 Current: Zero
+
-
Standard 200-Step Hybrid Motor
The standard stepper motor operates in the same
way as our simple model, but has a greater number
of teeth on the rotor and stator, giving a smaller
basic step size. The rotor is in two sections as
before, but has 50 teeth on each section. The halftooth displacement between the two sections is
retained. The stator has 8 poles each with 5 teeth,
making a total of 40 teeth (see Fig. 1.12).
Fig. 1.12 200-step hybrid motor
Stator
The windings are arranged in sets of four, and
wound such that diametrically-opposite poles are
the same. So referring to Fig. 1.12, the north poles
at 12 and 6 o’clock attract the south-pole teeth at
the front of the rotor; the south poles at 3 and 9
o’clock attract the north-pole teeth at the back. By
switching current to the second set of coils, the
stator field pattern rotates through 45°. However, to
align with this new field, the rotor only has to turn
through 1.8°. This is equivalent to one quarter of a
tooth pitch on the rotor, giving 200 full steps per
revolution.
Another point to remember is that for a given
current pattern in the windings, there are as many
stable positions as there are rotor teeth (50 for a
200-step motor). If a motor is de-synchronized, the
resulting positional error will always be a whole
number of rotor teeth or a multiple of 7.2°. A motor
cannot “miss” individual steps – position errors of
one or two steps must be due to noise, spurious
step pulses or a controller fault.
Rotor
A7
A Engineering Reference
Motor Technologies
Motor Technologies
Bifilar Windings
Most motors are described as being “bifilar wound”,
which means there are two identical sets of
windings on each pole. Two lengths of wire are
wound together as though they were a single coil.
This produces two windings that are electrically and
magnetically almost identical – if one coil were to be
wound on top of the other, even with the same
number of turns, the magnetic characteristics
would be different. In simple terms, whereas almost
all the flux from the inner coil would flow through
the iron core, some of the flux from the outer coil
would flow through the windings of the coil
underneath.
The origins of the bifilar winding go back to the
unipolar drive (see Drive Technologies section,
page A23). Rather than have to reverse the current
in one winding, the field may be reversed by
transferring current to a second coil wound in the
opposite direction. (Although the two coils are
wound the same way, interchanging the ends has
the same effect.) So with a bifilar-wound motor, the
drive can be kept simple. However, this
requirement has now largely disappeared with the
widespread availability of the more-efficient bipolar
drive. Nevertheless, the two sets of windings do
give us additional flexibility, and we shall see that
different connection methods can be used to give
alternative torque-speed characteristics.
If all the coils in a bifilar-wound motor are brought
out separately, there will be a total of 8 leads (see
Fig. 1.13). This is becoming the most common
configuration since it gives the greatest flexibility.
However, there are still a number of motors
produced with only 6 leads, one lead serving as a
common connection to each winding in a bifilar
pair. This arrangement limits the motor’s range of
application since the windings cannot be connected
in parallel. Some motors are made with only 4
leads, these are not bifilar-wound and cannot be
used with a unipolar drive. There is obviously no
alternative connection method with a 4-lead motor,
but in many applications this is not a drawback and
the problem of insulating unused leads is avoided.
Fig. 1.13 Motor lead configurations
4-lead
5-lead
6-lead
Fig. 1.14 Longitudinal section through single
stack motor
Fig. 1.14 also shows that the rotor flux only has to
cross a small air gap (typically 0.1mm or 0.004")
when the rotor is in position. By magnetizing the
rotor after assembly, a high flux density is obtained
that can be largely destroyed if the rotor is
removed. Stepper motors should therefore not be
dismantled purely to satisfy curiosity, since the
useful life of the motor will be terminated.
Because the shaft of the motor passes through the
center of the permanent magnet, a non-magnetic
material must be used to avoid a magnetic shortcircuit. Stepper shafts are therefore made of
stainless steel, and should be handled with care.
Small-diameter motors are particularly vulnerable if
they are dropped on the shaft end, as this will
invariably bend the shaft.
8-lead
Occasionally a 5-lead motor may be encountered.
These are not recommended since they cannot be
used with conventional bipolar drives requiring
electrical isolation between the phases.
Looking at the motor longitudinal section (Fig. 1.14),
we can see the permanent magnet in the rotor and
the path of the flux through the pole pieces and the
stator. The alternating flux produced by the stator
windings flows in a plane at right angles to the
page. Therefore, the two flux paths are at right
A8
angles to each other and only interact in the rotor
pole pieces. This is an important feature of the
hybrid motor – it means that the permanent magnet
in the rotor does not “see” the alternating field from
the windings, hence it does not produce a
demagnetizing effect. Unlike the DC servo motor, it
is generally impossible to de-magnetize a stepper
motor by applying excess current. However, too
much current will damage the motor in other ways.
Excessive heating may melt the insulation or the
winding formers, and may soften the bonding
material holding the rotor laminations. If this
happens and the laminations are displaced, the
effects can be the same as if the rotor had been
de-magnetized
To produce a motor with a higher torque output,
we need to increase the strength of both the
permanent magnet in the rotor and the field
produced by the stator. A stronger rotor magnet
can be obtained by increasing the diameter, giving
us a larger cross-sectional area. However,
increasing the diameter will degrade the
acceleration performance of the motor because
the torque-to-inertia ratio worsens (to a first
approximation, torque increases with diameter
squared but inertia goes up by the fourth power).
Nevertheless, we can increase torque output
without degrading acceleration performance by
adding further magnet sections or “stacks” to the
same shaft (Fig. 1.15). A second stack will enable
twice the torque to be produced and will double the
inertia, so the torque-to-inertia ratio remains the
same. Hence, stepper motors are produced in
single-, two- and three-stack versions in each
frame size.
Fig. 1.15 Three-stack hybrid stepping motor
As a guideline, the torque-to-inertia ratio reduces by
a factor of two with each increase in frame size
(diameter). So an unloaded 34-size motor can
accelerate twice as rapidly as a 42-size, regardless
of the number of stacks.
Linear Stepping Motors
Fig. 1.16 Linear stepping motor
Forcer
Permanent
Magnet
Phase B
Electromagnet
Phase A
Electromagnet
Field Windings
Platen Teeth
A1
S
A2
B1
Pole Faces
{
{
{
N
{
Air
Gap
Platen
B2
The linear stepper is essentially a conventional
rotary stepper that has been “unwrapped” so that it
operates in a straight line. The moving component
is referred to as the forcer and it travels along a
fixed element or platen. For operational purposes,
the platen is equivalent to the rotor in a normal
stepper, although it is an entirely passive device
and has no permanent magnet. The magnet is
incorporated in the moving forcer together with the
coils (see Fig. 1.16).
The forcer is equipped with 4 pole pieces each
having 3 teeth. The teeth are staggered in pitch
with respect to those on the platen, so that
switching the current in the coils will bring the next
set of teeth into alignment. A complete switching
cycle (4 full steps) is equivalent to one tooth pitch
on the platen. Like the rotary stepper, the linear
motor can be driven from a microstep drive. In this
case, a typical linear resolution will be 12,500 steps
per inch.
The linear motor is best suited for applications that
require a low mass to be moved at high speed. In a
leadscrew-driven system, the predominant inertia is
usually the leadscrew rather than the load to be
moved. Hence, most of the motor torque goes to
accelerate the leadscrew, and this problem
becomes more severe the longer the travel
required. Using a linear motor, all the developed
force is applied directly to the load and the
performance achieved is independent of the length
of the move. A screw-driven system can develop
greater linear force and better stiffness; however,
the maximum speed may be as much as ten times
higher with the equivalent linear motor. For
example, a typical maximum speed for a linear
motor is 100 in/sec. To achieve this with a 10-pitch
ballscrew would require a rotary speed of 6,000
rpm. In addition, the linear motor can travel up to
12 feet using a standard platen.
How the Linear Motor Works
The forcer consists of two electromagnets (A and B)
and a strong rare earth permanent magnet. The
two pole faces of each electromagnet are toothed
to concentrate the magnetic flux. Four sets of teeth
on the forcer are spaced in quadrature so that only
one set at a time can be aligned with the platen
teeth.
The magnetic flux passing between the forcer and
the platen gives rise to a very strong force of
attraction between the two pieces. The attractive
force can be up to 10 times the peak holding force
of the motor, requiring a bearing arrangement to
maintain precise clearance between the pole faces
and platen teeth. Either mechanical roller bearings
or air bearings are used to maintain the required
clearance.
When current is established in a field winding, the
resulting magnetic field tends to reinforce
permanent magnetic flux at one pole face and
cancel it at the other. By reversing the current, the
reinforcement and cancellation are exchanged.
Removing current divides the permanent magnetic
flux equally between the pole faces. By selectively
applying current to phase A and B, it is possible to
concentrate flux at any of the forcer’s four pole
faces. The face receiving the highest flux
concentration will attempt to align its teeth with the
platen. Fig. 1.17 shows the four primary states or
full steps of the forcer. The four steps result in
motion of one tooth interval to the right. Reversing
the sequence moves the forcer to the left.
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Repeating the sequence in the example will cause
the forcer to continue its movement. When the
sequence is stopped, the forcer stops with the
appropriate tooth set aligned. At rest, the forcer
develops a holding force that opposes any attempt
to displace it. As the resting motor is displaced from
equilibrium, the restoring force increases until the
displacement reaches one-quarter of a tooth
interval. (See Fig. 1.18.) Beyond this point, the
restoring force drops. If the motor is pushed over
the crest of its holding force, it slips or jumps rather
sharply and comes to rest at an integral number of
tooth intervals away from its original location. If this
occurs while the forcer is travelling along the platen,
it is referred to as a stall condition.
Fig. 1.17 The four cardinal states or full steps of
the forcer
Phase B
Phase A
Fig. 1.18 illustrates the static torque curve of the
hybrid step motor. This relates to a motor that is
energized but stationary. It shows us how the
restoring torque varies with rotor position as it is
deflected from its stable point. We’re assuming that
there are no frictional or other static loads on the
motor. As the rotor moves away from the stable
position, the torque steadily increases until it
reaches a maximum after one full step (1.8°). This
maximum value is called the holding torque and it
represents the largest static load that can be
applied to the shaft without causing continuous
rotation. However, it doesn’t tell us the maximum
running torque of the motor – this is always less
than the holding torque (typically about 70%).
Fig. 1.18 Static torque-displacement
characteristic
Direction of MMF due
to electromagnet
Flux Lines
N
S
B 2 Aligned
N
S
N
S
Max
Torque
Stable
Unstable
Stable
As the shaft is deflected beyond one full step, the
torque will fall until it is again at zero after two full
steps. However, this zero point is unstable and the
torque reverses immediately beyond it. The next
stable point is found four full steps away from the
first, equivalent to one tooth pitch on the rotor or
1/50 of a revolution.
Although this static torque characteristic isn’t a
great deal of use on its own, it does help explain
some of the effects we observe. For example, it
indicates the static stiffness of the system, (i.e.,
how the shaft position changes when a torque load
is applied to a stationary motor). Clearly the shaft
must deflect until the generated torque matches the
applied load. If the load varies, so too will the static
position. Non-cumulative position errors will
therefore result from effects such as friction or outof-balance torque loads. It is important to
remember that the static stiffness is not improved
by using a microstepping drive—a given load on the
shaft will produce the same angular deflection. So
while microstepping increases resolution and
smoothness, it may not necessarily improve
positioning accuracy.
A 2 Aligned
B 1Aligned
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4 Motor Steps
Angle
Clockwise
Torque
A 1 Aligned
S
There are numerous step motor performance
characteristics that warrant discussion. However,
we’ll confine ourselves to those traits with the
greatest practical significance.
Counter Clockwise
N
Step Motor Characteristics
When the stepper performs a single step, the
nature of the response is oscillatory as shown in
Fig. 1.19. The system can be likened to a mass that
is located by a “magnetic spring”, so the behavior
resembles the classic mass-spring characteristic.
Looking at it in simple terms, the static torque curve
indicates that during the step, the torque is positive
during the full forward movement and so is
accelerating the rotor until the new stable point is
reached. By this time, the momentum carries the
rotor past the stable position and the torque now
reverses, slowing the rotor down and bringing it
back in the opposite direction. The amplitude,
frequency and decay rate of this oscillation will
depend on the friction and inertia in the system as
well as the electrical characteristics of the motor
and drive. The initial overshoot also depends on
step amplitude, so half-stepping produces less
overshoot than full stepping and microstepping will
be better still.
Angle
Fig. 1.19 Single step response
Time
Attempting to step the motor at its natural
oscillation frequency can cause an exaggerated
response known as resonance. In severe cases,
this can lead to the motor desynchronizing or
“stalling.” It is seldom a problem with half-step
drives and even less so with a microstepper. The
natural resonant speed is typically 100-200 full
steps/sec. (0.5-1 rev/sec).
Under full dynamic conditions, the performance of
the motor is described by the torque-speed curve as
shown in Fig. 1.20. There are two operating ranges,
the start/stop (or pull in) range and the slew (or pull
out) range. Within the start/stop range, the motor can
be started or stopped by applying index pulses at
constant frequency to the drive. At speeds within this
range, the motor has sufficient torque to accelerate
its own inertia up to synchronous speed without the
position lag exceeding 3.6°. Clearly, if an inertial load
is added, this speed range is reduced. So the start/
stop speed range depends on the load inertia. The
upper limit to the start/stop range is typically between
200 and 500 full steps/sec (1-2.5 revs/sec).
Fig. 1.20 Start/stop and slew curves
Holding
Torque
Start/Stop Curve
Slew Curve
Torque
Under dynamic conditions with the motor running,
the rotor must be lagging behind the stator field if it
is producing torque. Similarly, there will be a lead
situation when the torque reverses during
deceleration. Note that the lag and lead relate only
to position and not to speed. From the static
torque curve (Fig. 1.18), clearly this lag or lead
cannot exceed two full steps (3.6°) if the motor is to
retain synchronism. This limit to the position error
can make the stepper an attractive option in
systems where dynamic position accuracy is
important.
Start/
Stop
Range
Slew
Range
Steps per second
To operate the motor at faster speeds, it is
necessary to start at a speed within the start/stop
range and then accelerate the motor into the slew
region. Similarly, when stopping the motor, it must
be decelerated back into the start/stop range
before the clock pulses are terminated. Using
acceleration and deceleration “ramping” allows
much higher speeds to be achieved, and in
industrial applications the useful speed range
extends to about 3000 rpm (10,000 full steps/sec).
Note that continuous operation at high speeds is
not normally possible with a stepper due to rotor
heating, but high speeds can be used successfully
in positioning applications.
The torque available in the slew range does not
depend on load inertia. The torque-speed curve is
normally measured by accelerating the motor up to
speed and then increasing the load until the motor
stalls. With a higher load inertia, a lower
acceleration rate must be used but the available
torque at the final speed is unaffected.
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Common Questions and Answers
About Step Motors
1.
2.
Why do step motors run hot?
Two reasons: 1. Full current flows through the
motor windings at standstill. 2. PWM drive
designs tend to make the motor run hotter.
Motor construction, such as lamination
material and riveted rotors, will also affect
heating.
What are safe operating temperatures?
The motors have class B insulation, which is
rated at 130°C. Motor case temperatures of
90°C will not cause thermal breakdowns.
Motors should be mounted where operators
cannot come into contact with the motor case.
3.
What can be done to reduce motor heating?
Many drives feature a “reduce current at
standstill” command or jumper. This reduces
current when the motor is at rest without
positional loss.
4.
What does the absolute accuracy specification
mean?
This refers to inaccuracies, non-cumulative,
encountered in machining the motor.
5.
How can the repeatability specification be
better than that of accuracy?
Repeatability indicates how precisely a
previous position can be re-obtained. There
are no inaccuracies in the system that affect a
given position, returning to that position, the
same inaccuracy is encountered.
6.
Will motor accuracy increase proportionately
with the resolution?
No. The basic absolute accuracy and
hysteresis of the motor remain unchanged.
7.
Can I use a small motor on a large load if the
torque requirement is low?
Yes, however, if the load inertia is more than
ten times the rotor inertia, cogging and
extended ringing at the end of the move will be
experienced.
8.
9.
How can end of move “ringing” be reduced?
Friction in the system will help damp this
oscillation. Acceleration/deceleration rates
could be increased. If start/stop velocities are
used, lowering or eliminating them will help.
Why does the motor stall during no load
testing?
The motor needs inertia roughly equal to its
own inertia to accelerate properly. Any
resonances developed in the motor are at their
worst in a no-load condition.
10. Why is motor sizing important, why not just go
with a larger motor?
If the motor’s rotor inertia is the majority of the
load, any resonances may become more
pronounced. Also, productivity would suffer as
excessive time would be required to accelerate
the larger rotor inertia. Smaller may be better.
11. What are the options for eliminating
resonance?
This would most likely happen with full step
systems. Adding inertia would lower the
resonant frequency. Friction would tend to
A12
dampen the modulation. Start/stop velocities
higher than the resonant point could be used.
Changing to half step operation would greatly
help. Ministepping and microstepping also
greatly minimize any resonant vibrations.
Viscous inertial dampers may also help.
12. Why does the motor jump at times when it's
turned on?
This is due to the rotor having 200 natural
detent positions. Movement can then be ±3.6°
in either direction.
13. Do the rotor and stator teeth actually mesh?
No. While some designs used this type of
harmonic drive, in this case, an air gap is very
carefully maintained between the rotor and the
stator.
14. Does the motor itself change if a microstepping
drive is used?
The motor is still the standard 1.8° stepper.
Microstepping is accomplished by
proportioning currents in the drive (higher
resolutions result). Ensure the motor’s
inductance is compatible.
15. A move is made in one direction, and then the
motor is commanded to move the same
distance but in the opposite direction. The
move ends up short, why?
Two factors could be influencing the results.
First, the motor does have magnetic hysteresis
that is seen on direction changes. This is in the
area of 0.03°. Second, any mechanical
backlash in the system to which the motor is
coupled could also cause loss of motion.
16. Why are some motors constructed as eightlead motors?
This allows greater flexibility. The motor can be
run as a six-lead motor with unipolar drives.
With bipolar drives, the windings can then be
connected in either series or parallel.
17. What advantage do series or parallel
connection windings give?
With the windings connected in series, lowspeed torques are maximized. But this also
gives the most inductance so performance at
higher speeds is lower than if the windings
were connected in parallel.
18. Can a flat be machined on the motor shaft?
Yes, but care must be taken to not damage
the bearings. The motor must not be
disassembled. Compumotor does not warranty
the user’s work.
19. How long can the motor leads be?
For bipolar drives, 100 feet. For unipolar
designs, 50 feet. Shielded, twisted pair cables
are required.
20. Can specialty motors, explosion-proof,
radiation-proof, high-temperature, lowtemperature, vacuum-rated, or waterproof, be
provided?
Compumotor is willing to quote on most
requirements with the exception of explosion
proof.
21. What are the options if an explosion-proof
motor is needed?
Installing the motor in a purged box should be
investigated.
DC Brush Motors
The history of the DC motor can be traced back to
the 1830s, when Michael Faraday did extensive
work with disc type machines (Fig. 1.21).
Fig. 1.21 Simple disc motor
Conductive Disc
Practical Considerations
The problem now is that of using this force to
produce the continuous torque required in a
practical motor.
To achieve maximum performance from the motor,
the maximum number of conductors must be
placed in the magnetic field, to obtain the greatest
possible force. In practice, this produces a cylinder
of wire, with the windings running parallel to the axis
of the cylinder. A shaft is placed down this axis to
act as a pivot, and this arrangement is called the
motor armature (Fig. 1.23).
Fig. 1.23 DC motor armature
N
S
Resultant
Field Due to
Armature
Current
Magnet
Shaft
Brush
This design was quickly improved, and by the end
of the 19th century the design principles of DC
motors had become well established.
About that time; however, AC power supply
systems came into general use and the popularity
of the DC motor declined in favor of the less
expensive AC induction motor. More recently, the
particular characteristics of DC motors, notably high
starting torque and controllability, have led to their
application in a wide range of systems requiring
accurate control of speed and position. This
process has been helped by the development of
sophisticated modern drive and computer control
systems.
Principles
It is well known that when a current-carrying
conductor is placed in a magnetic field, it
experiences a force (Fig. 1.22).
Fig. 1.22 Force on a conductor in a
magnetic field
Magnetic Field (B)
Armature
Direction
of Current
Into Page
Stator Field
As the armature rotates, so does the resultant
magnetic field. The armature will come to rest with
its resultant field aligned with that of the stator field,
unless some provision is made to constantly
change the direction of the current in the individual
armature coils.
Commutation
The force that rotates the motor armature is the
result of the interaction between two magnetic
fields (the stator field and the armature field). To
produce a constant torque from the motor, these
two fields must remain constant in magnitude and
in relative orientation.
Fig. 1.24 Electrical arrangement of the armature
2
Conductor
Carrying
Current (I)
(Into Page)
Force (F)
Force on Conductor F = I x B
The force acting on the conductor is given by:
F=IxB
where B = magnetic flux density and I = current
If this single conductor is replaced by a large
number of conductors (i.e., a length of wire is
wound into a coil), the force per unit length is
increased by the number of turns in the coil. This is
the basis of a DC motor.
1
3
Current
In
Out
6
4
5
This is achieved by constructing the armature as a
series of small sections connected in sequence to
the segments of a commutator (Fig 1.24). Electrical
connection is made to the commutator by means of
two brushes. It can be seen that if the armature
rotates through 1/6 of a revolution clockwise, the
current in coils 3 and 6 will have changed direction.
As successive commutator segments pass the
brushes, the current in the coils connected to those
segments changes direction. This commutation or
switching effect results in a current flow in the
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armature that occupies a fixed position in space,
independent of the armature rotation, and allows
the armature to be regarded as a wound core with
an axis of magnetization fixed in space. This gives
rise to the production of a constant torque output
from the motor shaft.
The axis of magnetization is determined by the
position of the brushes. If the motor is to have similar
characteristics in both directions of rotation, the
brush axis must be positioned to produce an axis of
magnetization that is at 90° to the stator field.
DC Motor Types
Brushless. The major limiting factor in the
performance of iron-cored motors is internal
heating. This heat escapes through the shaft and
bearings to the outer casing, or through the airgap
between the armature and field magnets and from
there to the casing. Both of these routes are
thermally inefficient, so cooling of the motor
armature is very poor.
Fig. 1.28 Brushless motor
Backiron
Return
Path
Several different types of DC motor are currently
in use.
S
Iron cored. (Fig. 1.25). This is the most common
type of motor used in DC servo systems. It is made
up of two main parts; a housing containing the field
magnets and a rotor made up of coils of wire
wound in slots in an iron core and connected to a
commutator. Brushes, in contact with the
commutator, carry current to the coils.
Fig. 1.25 Iron-cored motor
Magnets
N
S
Stator
Lam Teeth
Stator Magnets
Windings
In the brushless motor, the construction of the iron
cored motor is turned inside out, so that the rotor
becomes a permanent magnet and the stator
becomes a wound iron core.
Rotor Winding
Commutator
N
Brushes
Moving coil. There are two principle forms of this
type of motor. 1. The “printed” motor (Fig. 1.26),
using a disc armature. 2. The “shell” type armature
(Fig. 1.27).
Since these types of motors have no moving iron in
their magnetic field, they do not suffer from iron
losses. Consequently, higher rotational speeds can
be obtained with low power inputs.
Fig. 1.26 Disc-armature “printed” motor
Motion
The current-carrying coils are now located in the
housing, providing a short, efficient thermal path to
the outside air. Cooling can further be improved by
finning the outer casing and blowing air over it if
necessary (to effectively cool an iron-cored motor, it
is necessary to blow air through it.) The ease of
cooling the brushless motor allows it to produce a
much higher power in relation to its size.
The other major advantage of brushless motors is
their lack of a conventional commutator and brush
gear. These items are a source of wear and
potential trouble and may require frequent
maintenance. By not having these components, the
brushless motor is inherently more reliable and can
be used in adverse environmental conditions.
To achieve high torque and low inertia, brushless
motors do require rare earth magnets that are
much more expensive than conventional ceramic
magnets. The electronics necessary to drive a
brushless motor are also more complex than for a
brush motor. A more thorough explanation of
brushless motors is provided on page A17.
Permanent magnet
(8 pole)
Fig. 1.27 Shell-armature motor
Magnet pole
Air gap
S
Core
S
Motion
Armature
(Hollow cup, shaped
conductor array)
Diagrams courtesy of Electro-Craft Ltd.
A14
Flux path
Losses in DC Motors
DC motors are designed to convert electrical power
into mechanical power and as a consequence of
this, during periods of deceleration or if externally
driven, will generate electrical power. However, all
the input power is not converted into mechanical
power due to the electrical resistance of the
armature and other rotational losses. These losses
give rise to heat generation within the motor.
Motor losses can be divided into two areas: Those
that depend on the load and those that depend on
speed (Fig. 1.29).
Fig. 1.29 Losses in a DC motor
Motor losses
Load
Short circuit currents. As the brushes slide over
the commutator, the brush is in contact with two
commutator segments for a brief period. During this
period, the brush will short out the coil connected
to those segments (Fig. 1.30). This condition
generates a torque that opposes the main driving
torque and increases with motor speed.
Fig. 1.30 Generation of short-circuit currents
Speed
Commutator
Brush
Winding
losses
Iron
losses
Friction
losses
Brush
losses
Short-cut
circuit losses
Winding losses. These are caused by the electrical
resistance of the motor windings and are equal to
I2R (where I = armature current and R = armature
resistance).
As the torque output of the motor increases, I
increases, which gives rise to additional losses.
Consideration of winding losses is very important
since heating of the armature winding causes an
increase in R, which results in further losses and
heating. This process can destroy the motor if the
maximum current is not limited. Furthermore, at
higher temperatures, the field magnets begin to
lose their strength. Hence, for a required torque
output the current requirement becomes greater.
Brush contact losses. These are fairly complex to
analyze since they depend upon several factors that
will vary with motor operation. In general, brush
contact resistance may represent a high proportion
of the terminal resistance of the motor. The result of
this resistance will be increased heating due to I2R
losses in the brushes and contact area.
Iron losses. Iron losses are the major factor in
determining the maximum speed that may be
attained by an iron-cored motor. These fall into two
categories:
• Eddy current losses are common in all
conductive cored components experiencing a
changing magnetic field. Eddy currents are
induced into the motor armature as it undergoes
changes in magnetization. These currents are
speed-dependent and have a significant heating
effect at high speeds. In practice, eddy currents
are reduced by producing the armature core as a
series of thin, insulated sections or laminations,
stacked to produce the required core length.
• Hysteresis losses are caused by the resistance
of the core material to constant changes of
magnetic orientation, giving rise to additional heat
generation, which increases with speed.
Friction losses. These are associated with the
mechanical characteristics of the motor and arise
from brush friction, bearing friction, and air
resistance. These variables will generate heat and
will require additional armature current to offset this
condition.
Winding
All these losses will contribute heat to the motor
and it is this heating that will ultimately limit the
motor application.
Other Limiting Considerations
Torque ripple. The requirement for constant torque
output from a DC motor is that the magnetic fields
due to the stator and the armature are constant in
magnitude and relative orientation, but this ideal is
not achieved in practice. As the armature rotates,
the relative orientation of the fields will change
slightly and this will result in small changes in torque
output called “torque ripple” (Fig. 1.31).
Fig. 1.31 Torque ripple components
Torque
Torque Ripple
Steady Torque
O/P
Time
This will not usually cause problems at high speeds
since the inertia of the motor and the load will tend
to smooth out the effects, but problems may arise
at low speeds.
Motors can be designed to minimize the effects of
torque ripple by increasing the number of windings,
or the number of motor poles, or by skewing the
armature windings.
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Demagnetization. The permanent magnets of a
DC motor field will tend to become demagnetized
whenever a current flows in the motor armature.
This effect is known as “armature reaction” and will
have a negligible effect in normal use. Under high
load conditions, however, when motor current may
be high, the effect will cause a reduction in the
torque constant of the motor and a consequent
reduction in torque output.
Above a certain level of armature current, the field
magnets will become permanently demagnetized.
Therefore, it is important not to exceed the
maximum pulse current rating for the motor.
Mechanical resonances and backlash. It might
normally be assumed that a motor and its load,
including a tachometer or position encoder, are all
rigidly connected together. This may, however, not
be the case.
Motor Equations
Unlike a step motor, the DC brush motor exhibits
simple relationships between current, voltage,
torque and speed. It is therefore worth examining
these relationships as an aid to the application of
brush motors.
The application of a constant voltage to the
terminals of a motor will result in its accelerating to
attain a steady final speed (n). Under these
conditions, the voltage (V) applied to the motor is
opposed by the back emf (nKE) and the resultant
voltage drives the motor current (I) through the
motor armature and brush resistance (Rs).
The equivalent circuit of a DC motor is shown in
Fig. 1.34.
Fig. 1.34 DC motor equivalent circuit
L
It is important for a bi-directional drive or positioning
system that the mechanics are free from backlash,
otherwise, true positioning will present problems.
In high-performance systems, with high
accelerations, interconnecting shafts and couplings
may deflect under the applied torque, such that the
various parts of the system may have different
instantaneous velocities that may be in opposite
directions. Under certain conditions, a shaft may go
into torsional resonance (Fig. 1.32).
Fig. 1.32 Torsional oscillation
Shaft
Load
Motor
Tach
Back emf
As described previously, a permanent magnet DC
motor will operate as a generator. As the shaft is
rotated, a voltage will appear across the brush
terminals. This voltage is called the back
electromotive force (emf) and is generated even
when the motor is driven by an applied voltage. The
output voltage is essentially linear with motor speed
and has a slope that is defined as the motor voltage
constant, KE (Fig. 1.33). K E is typically quoted in
volts per 1000 rpm.
Fig. 1.33 Back-emf characteristic
Output
volts
Shaft speed
A16
I
v
Rs
RL
Vg
Rs = motor resistance
L = winding inductance
Vg = back emf and
RL represents magnetic losses.
The value of RL is usually large and so can be
ignored, as can the inductance L, which is generally
small.
If we apply a voltage (V) to the motor and a current
(I) flows, then:
V = IRs + Vg
but
Vg = nKE
so
V = IRs + nK E
(1)
This is the electrical equation of the motor.
If KT is the torque constant of the motor (typically in
oz/in per Amp), then the torque generated by the
motor is given by:
T = IKT
(2)
The opposing torque due to friction (TF) and viscous
damping (KD) is given by:
TM = TF + nKD
If the motor is coupled to a load TL ,then at
constant speed:
T = TL + TF + nKD (3)
Equations (1), (2) and (3) allow us to calculate the
required current and drive voltage to meet given
torque and speed requirements. The values of KT,
KE, etc. are given in the motor manufacturer’s data.
Brushless Motors
Before we talk about brushless motors in detail,
let’s clear up a few points about terminology. The
term “brushless” has become accepted as referring
to a particular variety of servo motor. Clearly a step
motor is a brushless device, as is an AC induction
motor (in fact, the step motor can form the basis of
a brushless servo motor, often called a hybrid
servo, which is discussed later). However, the socalled “brushless” motor has been designed to have
a similar performance to the DC brush servo
without the limitations imposed by a mechanical
commutator.
Within the brushless category are two basic motor
types: trapezoidal and sine wave motors. The
trapezoidal motor is really a brushless DC servo,
whereas the sine wave motor bears a close
resemblance to the AC synchronous motor. To fully
explain the difference between these motors, we
must review the evolution of the brushless motor.
Fig. 1.35 Conventional DC brush motor
Brushless Motor Operation
To turn this motor into a brushless design, we must
start by eliminating the windings on the rotor. This
can be achieved by turning the motor inside out. In
other words, we make the permanent magnet the
rotating part and put the windings on the stator
poles. We still need some means of reversing the
current automatically – a cam-operated reversing
switch could be made to do this job (Fig. 1.36).
Obviously such an arrangement with a mechanical
switch is not very satisfactory, but the switching
capability of non-contacting devices tends to be
very limited. However, in a servo application, we
will use an electronic amplifier or drive which can
also be used to do the commutation in response to
low-level signals from an optical or hall-effect
sensor (see Fig. 1.37). This component is referred
to as the commutation encoder. So unlike the DC
brush motor, the brushless version cannot be
driven by simply connecting it to a source of direct
current. The current in the external circuit must be
reversed at defined rotor positions. Hence, the
motor is actually being driven by an alternating
current.
Fig. 1.37 Brushless motor
S
N
+
Commutator
-
N
S
A simple conventional DC brush motor (Fig. 1.35)
consists of a wound rotor that can turn within a
magnetic field provided by the stator. If the coil
connections were made through slip rings, this
motor would behave like a step motor (reversing the
current in the rotor would cause it to flip through
180°). By including the commutator and brushes,
the reversal of current is made automatically and
the rotor continues to turn in the same direction.
Commutation
Encoder
Drive
Fig. 1.36 “Inside out” DC motor
Going back to the conventional brush motor, a
rotor consisting of only one coil will exhibit a large
torque variation as it rotates. In fact, the
characteristic will be sinusoidal, with maximum
torque produced when the rotor field is at right
angles to the stator field and zero torque at the
commutation point (see Fig. 1.38). A practical DC
motor has a large number of coils on the rotor,
each one connected not only to its own pair of
commutator segments but to the other coils as
well. In this way, the chief contribution to torque is
made by a coil operating close to its peak-torque
position. There is also an averaging effect produced
by current flowing in all the other coils, so the
resulting torque ripple is very small.
N
S
Reversing Switch
+
-
A17
A Engineering Reference
Motor Technologies
Motor Technologies
Fig. 1.38 3-phase brushless motor
Fig. 1.41 Position of rotor at commutation point
A1
A1
Stator
Field
B2
C2
A1
C2
C1
A2
B2
C2
N
N
B1
B2
A2
C1
B1
We would like to reproduce a similar situation in the
brushless motor; however, this would require a
large number of coils distributed around the stator.
This may be feasible, but each coil would require its
own individual drive circuit. This is clearly
prohibitive, so a compromise is made. A typical
brushless motor has either two or three sets of coils
or “phases” (see Fig. 1.38). The motor shown in Fig.
1.38 is a two-pole, three-phase design. The rotor
usually has four or six rotor poles, with a
corresponding increase in the number of stator
poles. This doesn’t increase the number of
phases—each phase has its turns distributed
between several stator poles.
B1
A2
A1
I
C2 A2
B2
B1
C1
Stator
Field
A1
B2
C2
N
N
S
Fig. 1.39 Position-torque characteristic
S
S
C1
S
C1
B1
A2
A1
+
Torque
-
0°
90°
Direction of Rotor
Field Relative
to Stator Field
180°
I
C2
A2
B2
B1
C1
Fig. 1.40 Stator field positions for different
phase currents
A1
C2
B2
S
N
S
N
Stator
Field
B1
C1
A2
S
Stator
Field
A1
Rotation
N
60°
C2 A2
B2
120°
C1
Stator
Field
Rotor
Field
Average Lag = 90°
The torque characteristic in Fig. 1.39 indicates that
maximum torque is produced when the rotor and
stator fields are at 90° to each other. Therefore, to
generate constant torque we would need to keep
the stator field a constant 90° ahead of the rotor.
Limiting the number of phases to three means that
we can only advance the stator field in increments
of 60° (Fig. 1.40). This means we must keep the
stator field in the same place during 60° of shaft
rotation. So we can’t maintain a constant 90°
torque angle, but we can maintain an average of
90° by working between 60° and 120°. Fig. 1.41
shows the rotor position at a commutation point.
When the torque angle has fallen to 60°, the stator
field is advanced from 1 to 2 so that the angle now
increases to 120°, and it stays here during the next
60° of rotation.
A18
I
B1
Motor Technologies
With a fixed current level in the windings, the use of
this extended portion of the sinusoidal torque
characteristic gives rise to a large degree of torque
ripple. We can minimize the effect by manipulating
the motor design to “flatten out” the characteristic –
to make it trapezoidal, (Fig. 1.42). In practice, this is
not very easy to do, so some degree of non-linearity
will remain. The effect of this tends to be a slight
“kick” at the commutation points, which can be
noticeable when the motor is running very slowly.
Fig. 1.42 Trapezoidal motor characteristic
60°
60°
Torque ripple resulting from non-linearity in the
torque characteristic tends to produce a velocity
modulation in the load. However, in a system using
velocity feedback the velocity loop will generally
have a high gain. This means that a very small
increase in velocity will generate a large error signal,
reducing the torque demand to correct the velocity
change. So in practice, the output current from the
amplifier tends to mirror the torque characteristic
(Fig. 1.43) so that the resulting velocity modulation
is extremely small.
Fig. 1.43 Current profile in velocity-controlled
servo
(Torque)
angle very accurately, resulting in very smooth lowspeed rotation and negligible torque ripple. A
simplified explanation of why the sine wave motor
produces constant torque is given in the next
section.
The drive for a sine wave motor is more complex
than for the trapezoidal version. We need a
reference table from which to generate the
sinusoidal currents, and these must be multiplied by
the torque demand signal to determine their
absolute amplitude. With a star-connected threephase motor, it is sufficient to determine the
currents in two of the windings—this will
automatically determine what happens in the third.
As previously mentioned, the sine wave motor
needs a high-resolution feedback device. However,
this device can also provide position and velocity
information for the controller.
Why constant torque from a sine wave
motor?
To understand this, it’s easier to think in terms of a
two-phase motor. This has just two sets of
windings that are fed with sinusoidal currents at 90°
to each other. If we represent shaft position by an
angle θ, then the currents in the two windings are of
the form Isinθ and Icosθ.
Going back to our original motor model, you’ll
remember that the fundamental torque
characteristic of the motor is also sinusoidal. So for
a given current I, the instantaneous torque value
looks like:
T = I KT sinθ
Where KT is the motor torque constant
By making the motor current sinusoidal as well, and
in phase with the motor torque characteristic, the
torque generated by one phase becomes:
T1 = (I sinθ) KT sinθ
= I KT sin2θ
Similarly, the torque produced by the other phase
is:
T2 = I KT cos2θ
The total torque is:
Current
The Sine Wave Motor
In the sine wave motor (sometimes called an AC
brushless servo), no attempt is made to modify the
basic sinusoidal torque characteristic. Such a motor
can be driven like an AC synchronous motor by
applying sinusoidal currents to the motor windings.
These currents must have the appropriate phase
displacement, 120° in the case of the three-phase
motor. We now need a much higher resolution
device to control the commutation if we want
smooth rotation at low speeds. The drive needs to
generate 3 currents that are in the correct
relationship to each other at every rotor position. So
rather than the simple commutation encoder
generating a handful of switching points, we now
need a resolver or high-resolution optical encoder.
In this way, it’s possible to maintain a 90° torque
T1 + T2 = I KT (sin2θ + cos2θ)
but:
sin2θ + cos2θ = 1 for any value of θ
therefore: T1 + T2 = IKT
So for sinusoidal phase currents with a constant
amplitude, the resultant torque is also constant and
independent of shaft position.
For this condition to remain true, the drive currents
must accurately follow a sine-cosine relationship.
This can only occur with a sufficiently high
resolution in the encoder or resolver used for
commutation.
A19
A Engineering Reference
The Trapezoidal Motor
Motor Technologies
The Hybrid Servo
In terms of their basic operation, the step motor
and the brushless servo motor are identical. They
each have a rotating magnet system and a wound
stator. The only difference is that one has more
poles than the other, typically two or three polepairs in the brushless servo and 50 in the stepper.
You could use a brushless servo as a stepper – not
a very good one, since the step angle would be
large. But by the same token, you can also use a
stepper as a brushless servo by fitting a feedback
device to perform the commutation. Hence the
“hybrid servo”, so called because it is based on a
hybrid step motor (Fig. 1.44). These have also been
dubbed ‘stepping servos’ and ‘closed-loop
steppers’. We prefer not to use the term ‘stepper’
at all since such a servo exhibits none of the
operating characteristics of a step motor.
The hybrid servo is driven in precisely the same
fashion as the brushless motor. A two-phase drive
provides sine and cosine current waveforms in
response to signals from the feedback device. This
device may be an optical encoder or a resolver. Since
the motor has 50 pole pairs, there will be 50 electrical
cycles per revolution. This conveniently permits a 50cycle resolver to be constructed from the same rotor
and stator laminations as the motor itself.
A hybrid servo generates approximately the same
torque output as the equivalent step motor,
assuming the same drive current and supply
voltage. However, the full torque capability of the
motor can be utilized since the system is operating
in a closed loop (with an open-loop step motor, it is
always necessary to allow an adequate torque
margin). The hybrid servo system will be more
expensive than the equivalent step motor systems,
but less costly than a brushless servo. As with the
step motor, continuous operation at high speed is
not recommended since the high pole count results
in greater iron losses at high speeds. A hybrid servo
also tends to run quieter and cooler than its step
motor counterpart; since it is a true servo, power is
only consumed when torque is required and
normally no current will flow at standstill. Lowspeed smoothness is vastly improved over the
open-loop full step motor.
It is worth noting that the hybrid servo is entirely
different from the open-loop step motor operated in
‘closed loop’ or ‘position tracking’ mode. In position
tracking mode, an encoder measures the load
movement and final positioning is determined by
encoder feedback. While this technique can provide
high positioning accuracy and eliminates
undetected position loss, it does not allow full
torque utilization, improve smoothness or reduce
motor heating.
Fig. 1.44 Hybrid servo motor with resolver feedback
Stator
Rotor
Two MS Style
Connectors
Position Feedback
Device Rotor
Bearing
Housing
Position Feedback
Device Stator
A20
Motor Technologies
Motor Construction and Operation
Direct drive systems couple the system’s load
directly to the motor without the use of belts or
gears. In some situations, brushed or brushlesss
servo motors may lack adequate torque or
resolution to satisfy some applications’ needs.
Therefore, mechanical means, such as gear
reduction systems to increase torque and
resolution, are used to meet system requirements.
The Dynaserv Direct Drive can provide very high
torque in a modest package size and solves many
of the performance issues of the gear reducer. All in
a system that is as easy to use as a stepping
motor.
Fig. 1.45 below shows the construction of the
Dynaserv DM Series direct drive motor compared
to a conventional motor with a gear reducer. The
gear reducer relies on large amounts of frictional
contact to reduce the speed of the load. This
gearing effectively increases torque and resolution
but sacrifices speed and accuracy. The direct drive
motor is brushless and gearless so it eliminates
friction from its power transmission Since the
feedback element is coupled directly to the load,
system accuracy and repeatability are greatly
increased and backlash is eliminated.
The motor contains precision bearings, magnetic
components and integral feedback in a compact
motor package (see Fig. 1.46). The motor is an
outer rotor type, providing direct motion of the
outside housing of the motor and thus the load.
The cross roller bearings that support the rotor
have high stiffness, to allow the motor to be
connected directly to the load. In most cases, it is
not necessary to use additional bearings or
connecting shafts.
Fig. 1.46 Expanded motor view—
Dynaserv Model DM
Encoder Plate
LED Kit
Clamp Ring
Housing Kit
Retaining
Ring
Stator Core
PDA Kit
Encoder
Rotor Core
Housing
Hub
Core
Fig. 1.45 Construction comparison
Conventional Motor
Gear
Reducer
Direct Drive Motor
DC/AC
Motor
Bearing
Rotating
Element
Encoder PCB
Slit Plate
Stator
Element
Encoder
Rotor Core
Stator Core
The torque is proportional to the square of the sum
of the magnetic flux (Øm), of the permanent magnet
rotor and the magnetic flux (Øc), of the stator
windings. See Fig. 1.47. High torque is generated
due to the following factors. First, the motor
diameter is large. The tangential forces between
rotor and stator act as a large radius, resulting in
higher torque. Secondly, a large number of small
rotor and stator teeth create many magnetic cycles
per motor revolution. More working cycles means
increased torque.
Fig. 1.47 Dynaserv magnetic circuit
Φm
Rotor
Φm
Φc
Excitation
Coil
Stator A
T
Permanent
Magnet
Stator B
A21
A Engineering Reference
Direct Drive Motors
Motor Technologies
Direct Drive Motor Advantages
High Precision
Dynaserv motors eliminate the backlash or
hysteresis inevitable in using any speed reducer.
Absolute positioning of 30 arc-sec is typical with a
repeatability of ±2 arc-sec.
Faster Settling Time
The Dynaserv system reduces machine cycle times
by decreasing settling times. This result is realized
because of the “gearless” design and sophisticated
“I-PD” control algorithm.
High Torque at High Speed
The torque/speed curve of the various Dynaserv
models is very flat. This results in high acceleration
at high speeds (4.0 rps) with good controllability.
Optimum Tuning
Dynaserv systems offer the user a tuning mode that
simplifies the setting of optimum parameters for the
actual load. Turning on the “test” switch on the
front panel of the drive produces a test signal.
Using an oscilloscope, the gain settings are quickly
optimized by adjusting the digital switches and
potentiometers on the front panel.
Clean Operation
The Dynaserv system is brushless and gearless,
which results in a maintenance-free operation. With
preparation, the Dynaserv can operate in class 10
environments.
Smooth Rotation
The very low velocity and torque ripple of the
Dynaserv contribute to its excellent speed
controllability with a more than 1:1,000 speed ratio.
Fig. 1.48 Dynaserv velocity/torque ripple
Speed Ripple
(DM1150A)
Torque Ripple
(DM1015A)
20
Conditions
• Load 30 x Rotor Inertia
• Rotation: CW
• Speed Mode
15.3
15
14.7
5%
10
Torque (N • m)
Ripple (%)
15
5
3
5
0
0.2
0.4
0.6
0.8
1.0
Revolution (rps)
A22
1.2
0°
90°
180°
270°
Rotational Angle (degrees)
360°
Drive Technologies
The stepper drive delivers electrical power to the
motor in response to low-level signals from the
control system.
The motor is a torque-producing device, and this
torque is generated by the interaction of magnetic
fields. The driving force behind the stator field is the
magneto-motive force (MMF), which is proportional
to current and to the number of turns in the
winding. This is often referred to as the amp-turns
product. Essentially, the drive must act as a source
of current. The applied voltage is only significant as
a means of controlling the current.
Input signals to the stepper drive consist of step
pulses and a direction signal. One step pulse is
required for every step the motor is to take. This is
true regardless of the stepping mode. So the drive
may require 200 to 101,600 pulses to produce one
revolution of the shaft. The most commonly-used
stepping mode in industrial applications is the halfstep mode in which the motor performs 400 steps
per revolution. At a shaft speed of 1800 rpm, this
corresponds to a step pulse frequency of 20kHz.
The same shaft speed at 25,000 steps per rev
requires a step frequency of 750 kHz, so motion
controllers controlling microstep drives must be
able to output a much higher step frequency.
Fig. 2.1 Stepper drive elements
The simplest type of switch set is the unipolar
arrangement shown in Fig. 2.2. It is referred to as a
unipolar drive because current can only flow in one
direction through any particular motor terminal. A
bifilar-wound motor must be used since reversal of
the stator field is achieved by transferring current to
the second coil. In the case of this very simple
drive, the current is determined only by the motor
winding resistance and the applied voltage.
Fig. 2.2 Basic unipolar drive
V+
1A
1B
TR1
2A
TR2
2B
TR3
TR4
0V
Such a drive will function perfectly well at low
stepping rates, but as speed is increased, the
torque will fall off rapidly due to the inductance of
the windings.
Stepper Drive
Elements
Step
Phase 1
Translator
Switch
Set
Motor
Direction
Phase 2
Step
Direction
The logic section of the stepper drive is often
referred to as the translator. Its function is to
translate the step and direction signals into control
waveforms for the switch set (see Fig. 2.1). The
basic translator functions are common to most
drive types, although the translator is necessarily
more complex in the case of a microstepping drive.
However, the design of the switch set is the prime
factor in determining drive performance, so we will
look at this in more detail.
A23
A Engineering Reference
Stepping Motor Drives
Drive Technologies
Inductance/Water Analogy
For those not familiar with the property of
inductance, the following water analogy may be
useful (Fig. 2.3). An inductor behaves in the same
way as a turbine connected to a flywheel. When the
tap is turned on and pressure is applied to the inlet
pipe, the turbine will take time to accelerate due to
the inertia of the flywheel. The only way to increase
the acceleration rate is to increase the applied
pressure. If there is no friction or leakage loss in the
system, acceleration will continue indefinitely for as
long as the pressure is applied. In a practical case,
the final speed will be determined by the applied
pressure and by friction and the leakage past the
turbine blades.
Fig. 2.3 Inductance water analogy
Pressure Equivalent
to Applied Voltage
Kinetic Energy
of Flywheel
Equivalent to
Energy Stored
in Magnetic Field
Tap
I
1-Way
Valve
Turbine
Water Flow
Equivalent
to Current
Higher Pressure
Causes Flywheel
to Accelerate
More Rapidly
Voltage
(Pressure)
Reverse Pressure
When Flow
Interrupted
Current
(Flow)
Applying a voltage to the terminals of an inductor
produces a similar effect. With a pure inductance
(i.e., no resistance), the current will rise in a linear
fashion for as long as the voltage is applied. The
rate of rise of current depends on the inductance
and the applied voltage, so a higher voltage must
be applied to get the current to rise more quickly. In
a practical inductor possessing resistance, the final
current is determined by the resistance and the
applied voltage.
Once the turbine has been accelerated up to
speed, stopping it again is not a simple matter. The
kinetic energy of the flywheel has to be dissipated,
and as soon as the tap is turned off, the flywheel
drives the turbine like a pump and tries to keep the
water flowing. This will set up a high pressure
across the inlet and outlet pipes in the reverse
direction. The equivalent energy store in the
inductor is the magnetic field. As this field
collapses, it tries to maintain the current flow by
generating a high reverse voltage.
A24
By including a one-way valve across the turbine
connections, the water is allowed to continue
circulating when the tap is turned off. The energy
stored in the flywheel is now put to good use in
maintaining the flow. We use the same idea in the
recirculating chopper drive, in which a diode allows
the current to recirculate after it has built up.
Going back to our simple unipolar drive, if we look
at the way the current builds up (Fig. 2.4) we can
see that it follows an exponential shape with its final
value set by the voltage and the winding resistance.
To get it to build up more rapidly, we could increase
the applied voltage, but this would also increase the
final current level. A simple way to alleviate this
problem is to add a resistor in series with the motor
to keep the current the same as before.
R-L Drive
Bipolar Drive
The principle described in the Inductance/Water
Analogy (p. A24) is applied in the resistance-limited
(R-L) drive see Fig. 2.4. Using an applied voltage of
10 times the rated motor voltage, the current will
reach its final value in one tenth of the time. If you
like to think in terms of the electrical time constant,
this has been reduced from L/R to L/10R, so we’ll
get a useful increase in speed. However we’re
paying a price for this extra performance. Under
steady-state conditions, there is 9 times as much
power dissipated in the series resistor as in the
motor itself, producing a significant amount of heat.
Furthermore, the extra power must all come from
the DC power supply, so this must be much larger.
R-L drives are therefore only suited to low-power
applications, but they do offer the benefits of
simplicity, robustness and low radiated interference.
An obvious possibility is the simple circuit shown in
Fig. 2.6, in which two power supplies are used
together with a pair of switching transistors.
Current can be made to flow in either direction
through the motor coil by turning on one transistor
or the other. However, there are distinct drawbacks
to this scheme. First, we need two power supplies,
both of which must be capable of delivering the
total current for both motor phases. When all the
current is coming from one supply the other is
doing nothing at all, so the power supply utilization
is poor. Second, the transistors must be rated at
double the voltage that can be applied across the
motor, requiring the use of costly components.
Fig. 2.4 Principle of the R-L drive
L
R
TR1
I
I
V
0V
R
R
R
TR2
2V
I
V-
I
2V
2V
2R
Unipolar Drive
Fig. 2.5 Basic unipolar drive
V+
1A
V+
V
V
L
Fig. 2.6 Simple bipolar drive
1B
2A
2B
The standard arrangement used in bipolar motor
drives is the bridge system shown in Fig. 2.7.
Although this uses an extra pair of switching
transistors, the problems associated with the
previous configuration are overcome. Only one
power supply is needed and this is fully utilized;
transistor voltage ratings are the same as that
available for driving the motor. In low-power
systems, this arrangement can still be used with
resistance limiting as shown in Fig. 2.8.
Fig. 2.7 Bipolar bridge
V+
TR1
TR2
TR3
TR4
TR1
TR3
TR2
TR4
0V
A drawback of the unipolar drive is its inability to
utilize all the coils on the motor. At any one time,
there will only be current flowing in one half of each
winding. If we could utilize both sections at the
same time, we could get a 40% increase in ampturns for the same power dissipation in the motor.
To achieve high performance and high efficiency,
we need a bipolar drive (one that can drive current
in either direction through each motor coil) and a
better method of current control. Let’s look first at
how we can make a bipolar drive.
0V
Fig. 2.8 Bipolar R-L drive
V+
TR1
TR3
TR2
TR4
0V
A25
A Engineering Reference
Drive Technologies
Drive Technologies
Recirculating Chopper Drive
The method of current control used in most stepper
drives is the recirculating chopper (Fig. 2.9). This
approach incorporates the four-transistor bridge,
recirculation diodes, and a sense resistor. The
resistor is of low value (typically 0.1 ohm) and
provides a feedback voltage proportional to the
current in the motor.
Fig. 2.9 Recirculating chopper drive
+
TR1
TR2
+
TR3
D1
D2
TR1
TR4
TR2
TR3
D1
D2
TR4
Vs
Vs
Rs
Rs
–
–
Injection
Recirculation
Motor current
Current is injected into the winding by turning on
one top switch and one bottom switch, and this
applies the full supply voltage across the motor.
Current will rise in an almost linear fashion and we
can monitor this current by looking across the
sense resistor. When the required current level has
been reached, the top switch is turned off and the
stored energy in the coil keeps the current
circulating via the bottom switch and the diode.
Losses in the system cause this current to slowly
decay, and when a pre-set lower threshold is
reached, the top switch is turned back on and the
cycle repeats. The current is therefore maintained at
the correct average value by switching or
“chopping” the supply to the motor.
A26
This method of current control is very efficient
because very little power is dissipated in the
switching transistors other than during the transient
switching state. Power drawn from the power
supply is closely related to the mechanical power
delivered by the shaft (unlike the R-L drive, which
draws maximum power from the supply at
standstill).
A variant of this circuit is the regenerative chopper.
In this drive, the supply voltage is applied across
the motor winding in alternating directions, causing
the current to ramp up and down at approximately
equal rates. This technique tends to require fewer
components and is consequently lower in cost,
however, the associated ripple current in the motor
is usually greater and increases motor heating.
Regeneration and Power Dumping
Like other rotating machines with permanent
magnets, the step motor will act as a generator
when the shaft is driven mechanically. This means
that the energy imparted to the load inertia during
acceleration is returned to the drive during
deceleration. This will increase the motor current
and can damage the power switches if the extra
current is excessive. A threshold detector in the
drive senses this increase in current and
momentarily turns off all the bridge transistors
(Fig. 2.10). There is now a path for the regenerated
current back to the supply capacitor, where it
increases the supply voltage. During this phase, the
current is no longer flowing through the sense
resistors, so the power switches must be turned on
again after a short period (typically 30µS) for
conditions to be reassessed. If the current is still too
high, the drive returns to the regenerative state.
The circuit of a simple power dump is shown in
Fig. 2.11. A rectifier and capacitor fed with AC from
the supply transformer provide a reference voltage
equal to the peak value of the incoming AC. Under
normal conditions this will be the same as the drive
supply voltage. During excess regeneration, the
drive supply voltage will rise above this reference,
and this will turn on the dump transistor connecting
the 33-ohm resistor across the power supply.
When the supply voltage has decreased sufficiently,
the transistor is turned back off. Although the
instantaneous current flowing through the dump
resistor may be relatively high, the average power
dissipated is usually small since the dump period is
very short. In applications where the regenerated
power is high, perhaps caused by frequent and
rapid deceleration of a high inertia, a supplementary
high-power dump resistor may be necessary.
Fig. 2.10 Current flow during regeneration
Fig. 2.11 Power dump circuit
+V
HV
D2
Power
dump
circuit
Power
supply
capacitor
D1
AC
in
R6
33 Ω
10W
R2
TR1
1K
R5
100K
R3
C1
A small increase in supply voltage during
regeneration is acceptable, but if the rise is too
great the switches may be damaged by overvoltage rather than excessive current. To resolve
this problem, we use a power dump circuit that
dissipates the regenerated power.
R1
TR2
R4
0V
A27
A Engineering Reference
Drive Technologies
Drive Technologies
Stepper Drive Technology Overview
Full-Step and Half-Step Systems
Full-step and half-step systems do not have the
resolution capability of the ministepping or
microstepping systems. However, the drive
technology is not as complex and the drives are
relatively inexpensive. Full-step and half-step systems
will not have the same low-speed smoothness as
higher resolution systems.
An inherent property of a stepper motor is its lowspeed resonance, which may de-synchronize a
motor and cause position loss. Full-step and halfstep drives are more prone to resonance effects and
this may limit their application in low-speed systems.
Full-step and half-step systems can be operated at
speeds above the motor’s resonant speed without
loss of synchronization. For this reason, full-step and
half-step systems are normally applied in high-speed,
point-to-point positioning applications. In these types
of applications, the machine designer is primarily
concerned with selecting a motor/drive system
capable of producing the necessary power output.
A28
Step motor systems typically require the motor to
accelerate to reach high speed. If a motor was
requested to run instantaneously at 3000 rpm, the
motor would stall immediately. At slow speeds, it is
possible to start the motor without position loss by
applying unramped step pulses. The maximum speed
at which synchronization will occur without ramping is
called the start/stop velocity. The start/stop velocity is
inversely proportional to the square-root of the total
inertia. The start/stop capability provides a benefit for
applications that require high-speed point-to-point
positioning—since the acceleration to the start/stop
velocity is almost instantaneous, the move-time will
be reduced. No additional time is required to
accelerate the motor from zero to the start/stop
velocity. While the move-time can be reduced, it is
generally more complicated for the controller or
indexer to calculate the motion profile and implement
a start/stop velocity. In most applications, using start/
stop velocities will eliminate the need to run the motor
at its resonant frequency and prevent desynchronization.
Velocity
Stepper drive technology has evolved—being driven
by machine builders that require more shaft power in
smaller packages, higher speed capability, better
efficiency, and improved accuracy. One trend of the
technology is towards microstepping, a technique
that divides each full step of the motor into smaller
steps. This is achieved electronically in the drive by
proportioning the current between the motor
windings. The higher the resolution, the more
precision is required in the current control circuits. In
its simplest form, a half-step system increases the
resolution of a standard 1.8° full-step motor to 400
steps/rev. Ministepping drives have more precise
current control and can increase the resolution to
4,000 steps/rev. Microstep drives typically have
resolutions of 50,000 steps/rev, and in addition to
improved current control, they often have
adjustments to balance offsets between each phase
of the motor and to optimize the current profile for
the particular motor being used.
Since power is the product of torque and speed, a
high-torque system with low-speed capability may
not produce as much power as a low-torque, highspeed system. Sizing the system for torque only may
not provide the most cost-effective solution, selecting
a system based on power output will make the most
efficient use of the motor and drive.
Velocity
Within the various drive technologies, there is a
spectrum of performance. The uni-polar resistancelimited (R-L) drive is a relatively simple design, but it
lacks shaft power performance and is very inefficient.
A uni-polar system only uses half of the motor
winding at any instant. A bi-polar design allows
torque producing current to flow in all motor
windings, using the motor more efficiently, but
increasing the complexity of the drive. A bi-polar R-L
drive improves shaft performance, but is still very
inefficient—generating a lot of wasted heat. An
alternative to resistance-limiting is to control current
by means of chopper regulation. ␣A chopper
regulator is very efficient since it does not waste
power by dropping voltage through a resistor.
However, good current control in the motor is
essential to deliver optimum shaft power. Pulse width
modulation (PWM) and threshold modulation are two
types of chopper regulation techniques. PWM
controls the average of the motor current and is very
good for precise current control, while threshold
modulation controls current to a peak level.
Threshold modulation can be applied to a wider
range of motors, but it does suffer greater loss of
performance than PWM when the motor has a large
resistance or long motor cables are used. Both
chopper regulation techniques can use recirculating
current control, which improves the power
dissipation in the motor and drive and overall system
efficiency. As system performance increases, the
complexity and cost of the drive increases.
Time
Time
Ministep Systems
Applications that require better low-speed
smoothness than a half-step system should
consider using a microstepping or ministepping
solution. Microstepping systems, with resolutions
of 50,000 steps/rev, can offer exceptional
smoothness, without requiring a gear-reducer.
Ministepping systems typically do not have wavetrimming capability or offset adjustment to achieve
the optimum smoothness, but offer a great
improvement over full-step and half-step systems.
Ministepping systems have resolutions between
1,000 and 4,000 steps/rev.
The motor is an important element in providing
good smoothness. Some motor designs are
optimized for high-torque output rather than
smooth rotation. Others are optimized for
smoothness rather than high torque. Ministepping
systems are typically offered with a motor as a
“packaged” total solution, using a motor that has
been selected for its premium smoothness
properties.
Ministep systems are sometimes selected to
improve positional accuracy. However, with an
open-loop system, friction may prevent the
theoretical unloaded accuracy from being achieved
in practice.
Drive Technologies
Accurate microstepping places increased demands
on the accuracy of current control in the drive,
particularly at low current levels. A small phase
imbalance that may be barely detectable in a halfstep drive can produce unacceptable positioning
errors in a microstep system. Pulse-width
modulation is frequently used to achieve higher
accuracy than can be achieved using a simple
threshold system.
The phase currents necessary to produce the
intermediate steps follow an approximately
sinusoidal profile as shown in Fig. 2.12. However
the same profile will not give the optimum response
with all motors. Some will work well with a
sinusoidal shape, whereas others need a more
filled-out or trimmed-down shape (Fig. 2.12). So a
microstep drive intended to operate with a variety
of motors needs to have provision for adjusting the
current profile. The intermediate current levels are
usually stored as data in an EPROM, with some
means of selecting alternative data sets to give
different profiles. The change in profile may be
thought of in terms of adding or subtracting a
third-harmonic component to or from the basic sine
wave.
Fig. 2.12 Microstep current profile
Sinewave
Filled out
Due to this dependence on motor type for
performance, it is usual for high-resolution
microstep systems to be supplied as a matched
motor-drive package.
The Stepper Torque/Speed Curve
We have seen that motor inductance is the factor
that opposes rapid changes of current and
therefore makes it more difficult to drive a stepper
at high speeds. Looking at the torque-speed curve
in Fig. 2.13, we can see what is going on. At low
speeds, the current has plenty of time to reach the
required level and so the average current in the
motor is very close to the regulated value from the
drive. Changing the regulated current setting or
changing to a drive with a different current rating
will affect the available torque accordingly.
Fig 2.13 Regulated and voltage-limited regions
of the torque-speed curve
Regulated Region
Torque
As we mentioned earlier, subdivision of the basic
motor step is possible by proportioning the current
in the two motor windings. This produces a series
of intermediate step positions between the onephase-on points. It is clearly desirable that these
intermediate positions are equally spaced and
produce approximately equal torque when the
motor is running.
Voltage-Limited
Region
Drive with
Higher Supply
Voltage
Speed
Average
Current
During
Pulse
Trimmed
In the case of high-resolution microstep drives
producing 10,000 steps per rev or more, the best
performance will only be obtained with a particular
type of motor. This is one in which the stator teeth
are on a 7.5° pitch, giving 48 equal pitches in 360°.
In most hybrid steppers, the stator teeth have the
same pitch as the rotor teeth, giving equal
increments of 7.2°. This latter arrangement tends to
give superior torque output, but is less satisfactory
as a microstepper since the magnetic poles are
“harder” – there is no progressive transfer of tooth
alignment from one pole to the next. In fact, with
this type of motor, it can be quite difficult to find a
current profile that gives good static positioning
combined with smooth low-speed rotation. An
alternative to producing a 7.5°-pitch stator is to
incorporate a slight skew in the rotor teeth. This
produces a similar effect and has the benefit of
using standard 7.2° laminations throughout.
Skewing is also used in DC brush motors as a
means of improving smoothness.
As speed increases, the time taken for the current
to rise becomes a significant proportion of the
interval between step pulses. This reduces the
average current level, so the torque starts to fall off.
As speed increases further, the interval between
step pulses does not allow the current time to reach
a level where the chopping action can begin. Under
these conditions, the final value of current depends
only on the supply voltage. If the voltage is
increased, the current will increase more rapidly and
hence will achieve a higher value in the available
time. So this region of the curve is described as
“voltage limited”, as a change in the drive current
setting would have no effect. We can conclude that
at low speeds the torque depends on the drive
current setting, whereas at high speeds it depends
on the drive supply voltage. It is clear that highspeed performance is not affected by the drive
current setting. Reducing the current simply
“flattens out” the torque curve without restricting
the ability to run at high speeds. When performance
is limited by the available high-speed torque, there
is much to be said for running at the lowest current
that gives an adequate torque margin. In general,
dissipation in motor and drive is reduced and lowspeed performance in particular will be smoother
with less audible noise.
A29
A Engineering Reference
Microstepping Drives
Drive Technologies
With a bipolar drive, alternative possibilities exist for
the motor connections as shown in Fig. 2.14. An
8-lead motor can be connected with the two halves
of each winding either in series or in parallel. With
a 6-lead motor, either one half-winding or both
half-windings may be connected in series. The
alternative connection schemes produce different
torque-speed characteristics and also affect the
motor’s current rating.
However, having doubled the effective number of
turns in the winding means that we have also
increased the inductance by a factor of 4. This
causes the torque to drop off much more rapidly as
speed is increased, and as a result, the series
mode is most useful at low speeds. The maximum
shaft power obtainable in series is typically half that
available in parallel (using the same current setting
on the drive).
Fig. 2.14 Series & parallel connections
Connecting the two half-windings of an 8-lead
motor in parallel allows the current to divide itself
between the two coils. It does not change the
effective number of turns and the inductance
therefore remains the same. So at a given drive
current, the torque characteristic will be the same
for two half-windings in parallel as for one of the
windings on its own. For this reason, “parallel” in
the context of a 6-lead motor refers to the use of
one half-winding only.
1A
1B 2A
2B 1A
Series
1B 2A
2B
Parallel
Torque
Fig. 2.15 Series & parallel torque/speed curves
Series
Parallel
Speed
Compared with using one half-winding only,
connecting both halves in series requires the drive
current to flow through twice as many turns. For the
same current, this doubles the “amp-turns” and
produces a corresponding increase in torque. In
practice, the torque increase is seldom as high as
100% due to the non-linearity of the magnetic
material. Equally, the same torque will be produced at
half the drive current when the windings are in series.
A30
As has already been mentioned, the current rating
of a step motor is determined by the allowable
temperature rise. Unless the motor manufacturer’s
data states otherwise, the rating is a “unipolar”
value and assumes both phases of the motor are
energized simultaneously. So a current rating of 5A
means that the motor will accept 5A flowing in each
half-winding.
When the windings of an 8-lead motor are
connected in parallel, half of the total resistance is
produced. For the same power dissipation in the
motor, the current may now be increased by 40%.
Therefore, the 5A motor will accept 7A with the
windings in parallel, giving a significant increase in
available torque. Conversely, connecting the
windings in series will double the total resistance
and the current rating is reduced by a factor of 1.4,
giving a safe current of 3.5A for our 5A-motor in
series.
As a general rule, parallel is the preferred
connection method as it produces a flatter torque
curve and greater shaft power (Fig. 2.15). Series is
useful when high torque is required at low speeds,
and it allows the motor to produce full torque from
a lower-current drive. Care should be taken to avoid
overheating the motor in series since its current
rating is lower in this mode. Series configurations
also carry a greater likelihood of resonance due to
the high torque produced in the low-speed region.
DC Brush Motor Drives
Linear and Switching Amplifiers
Linear amplifiers – this type of amplifier operates in
such a way that, depending on the direction of
motor rotation, either TR1 or TR2 will be in series
with the motor and will always have a voltage (V)
developed across it (Fig. 2.16).
This characteristic is the primary limitation on the
use of linear amplifiers (since there will always be
power dissipated in the output stages of the
amplifier). To dissipate this power, large transistors
and heat sinks will be required, making this type of
amplifier unsuitable for use in high power systems.
However, the linear amplifier does offer the benefit
of low radiated electrical noise.
Fig. 2.16 Linear servo amplifier
+ Ve
Overview – The Analog Drive
In the traditional analog drive, the desired motor
velocity is represented by an analog input voltage
usually in the range ±10 volts. Full forward velocity
is represented by +10v, and full reverse by -10v.
Zero(D) volts represents the stationary condition and
intermediate voltages represent speeds in
proportion to the voltage.
The various adjustments needed to tune an analog
drive are usually made with potentiometers. With a
little experience, this can usually be performed
quite quickly, but in some difficult applications it
may take longer. Repeating the adjustments on
subsequent units may take the same time unless
there is an easy way of duplicating the potentiometer settings. For this reason, some proprietary
drives use a plug-in “personality card” that may be
fitted with preset components. However, this not
only increases the cost but may preclude the
possibility of fine tuning later.
TR1
V
M
TR2
- Ve
Switching amplifiers – this amplifier is the most
commonly used type for all but very low-power
requirements and the most commonly used
method of output control is by pulse width
modulation (PWM).
Power dissipation is greatly reduced since the
output transistors are either in an “on” or an “off”
state. In the “off” state, no current is conducted and
so no power is dissipated. In the “on” state the
voltage across the transistors is very low (1-2 volts),
so that the amount of power dissipated is small.
Such amplifiers are suitable for a wide range of
applications (including high power applications).
The operation of a switching or chopper amplifier is
very similar to that of the chopping stepper drive
already described. Only one switch set is required
to drive a DC brush motor, making the drive
simpler. However, the function of a DC drive is to
provide a variable current into the motor to control
the torque. This may be achieved using either
analog or digital techniques.
Analog and Digital Servo Drives
Unlike stepper drives, amplifiers for both brush and
brushless servo motors are either analog or digital.
The analog drive has been around for many years,
whereas the digital drive is a relatively recent
innovation. Both types have their merits.
Overview – The Digital Drive
An alternative to the analog system is the digitallycontrolled drive in which tuning is performed by
sending data from a terminal or computer. This leads
to easy repetition between units and, since such
drives are invariably processor-based, facilitates
fully-automatic self tuning. The input signal to such a
drive may also be an analog voltage but can equally
take the form of step and direction signals, like a
stepper drive.
Digital drives are used more in conjunction with
brushless servo motors than with DC brush motors.
Such drives are almost wholly digital with the
exception of the power stage that actually delivers
current to the motor. Velocity feedback is derived
either from an encoder or resolver and again is
processed as digital information. It becomes logical
to incorporate a position controller within such a
drive, so that incoming step and direction signals
can be derived from a conventional stepper-type
indexer. Equally, the positioner may be controlled
by simple commands using a high-level motion
control language – see the X-code products in this
catalog.
A Comparison of Analog and Digital Drives
The analog drive offers the benefit of lower cost
and, in the case of a drive using tach feedback,
very high performance. The wide bandwidth of the
brush tach allows high gains to be used without
inducing jitter at standstill, resulting in a very “stiff”
system.
The digital drive, while more costly, is comparatively
easy to set up and adjustments can be quickly
repeated across several units. Automatic self-tuning
can be a distinct advantage where the load
parameters are unknown or difficult to measure.
The digital drive also offers the possibility of
dynamic tuning – sometimes vital where the load
inertia changes dramatically during machine
operation. Such an option is clearly out of the
question with a potentiometer-adjusted drive.
A31
A Engineering Reference
Drive Technologies
Drive Technologies
Analog DC Drive Operation
The elements of an analog velocity amplifier are
shown in Fig. 2.17. The function of the system is to
control motor velocity in response to an analog
input voltage.
Fig. 2.17 Elements of an analog servo system
Velocity
Control
Signal
Torque
Control
Signal
A
Drive
Amplifier
B
M
T
Torque Feedback Loop
R
Velocity Feedback Loop
Motor velocity is measured by a tach generator
attached to the motor shaft. This produces a
voltage proportional to speed that is compared with
the incoming velocity demand signal, and the result
of this comparison is a torque demand. If the speed
is too low, the drive delivers more current, which in
turn creates torque to accelerate the load. Similarly,
if the speed is too high or the velocity demand is
reduced, current flow in the motor will be reversed
to produce a braking torque.
This type of amplifier is often referred to as a fourquadrant drive. This means that it can produce
both acceleration and braking torque in either
direction of rotation. If we draw a diagram
representing direction of rotation in one axis and
direction of torque in the other (see Fig. 2.18), you
will see that the motor can operate in all four
“quadrants”. By contrast, a simple variable-speed
drive capable of running only in one direction and
with uncontrolled deceleration would be described
as single-quadrant.
Fig. 2.18 Four-quadrant operation
Torque
CW
Braking
CCW
Accelerating
CW
CCW
CW
Accelerating
CCW
Braking
CW
CCW
A32
Torque
The velocity amplifier in Fig. 2.17 has a high gain so
that a small velocity difference will produce a large
error signal. In this way, the accuracy of speed
control can be made very high even when there are
large load changes.
A torque demand from the velocity amplifier
amounts to a request for more current in the motor.
The control of current is again achieved by a
feedback loop that compares the torque demand
with the current in the motor. This current is
measured by a sense resistor R, which produces a
voltage proportional to motor current. This inner
feedback loop is frequently referred to as a torque
amplifier since its purpose is to create torque in
response to a demand from the velocity amplifier.
The torque amplifier alone may be used as the
basis of a servo drive. Some types of position
controller generate a torque output signal rather
than a velocity demand, and there are also
applications in which torque rather than speed is of
primary interest (winding material onto a drum, for
instance). Most analog drives can be easily
configured either as velocity or torque amplifiers by
means of a switch or jumper links. In practice, the
input signal is still taken to the same point, but the
velocity amplifier is bypassed.
Drive Technologies
Tuning
RS-232C
Step
Microprocessor
Direction
D to A
Converter
PWM
Control
Amplifier
Encoder
M
Digital Servo Drive Operation
Fig. 2.19 shows the components of a digital drive
for a servo motor. All the main control functions are
carried out by the microprocessor, which drives a
D-to-A convertor to produce an analog torque
demand signal. From this point on, the drive is very
much like an analog servo amplifier.
Feedback information is derived from an encoder
attached to the motor shaft. The encoder generates
a pulse stream from which the processor can
determine the distance travelled, and by calculating
the pulse frequency it is possible to measure
velocity.
The digital drive performs the same operations as
its analog counterpart, but does so by solving a
series of equations. The microprocessor is
programmed with a mathematical model (or
“algorithm”) of the equivalent analog system. This
model predicts the behavior of the system. In
response to a given input demand and output
position. It also takes into account additional
information like the output velocity, the rate of
change of the input and the various tuning settings.
E
The tuning of a digital servo is performed either by
pushbuttons or by sending numerical data from a
computer or terminal. No potentiometer
adjustments are involved. The tuning data is used
to set various coefficients in the servo algorithm and
hence determines the behavior of the system. Even
if the tuning is carried out using pushbuttons, the
final values can be uploaded to a terminal to allow
easy repetition.
In some applications, the load inertia varies
between wide limits – think of an arm robot that
starts off unloaded and later carries a heavy load at
full extension. The change in inertia may well be a
factor of 20 or more, and such a change requires
that the drive is re-tuned to maintain stable
performance. This is simply achieved by sending
the new tuning values at the appropriate point in the
operating cycle.
To solve all the equations takes a finite amount
of time, even with a fast processor – this time is
typically between 100µs and 2ms. During this
time, the torque demand must remain constant
at its previously-calculated value and there will
be no response to a change at the input or
output. This “update time” therefore becomes a
critical factor in the performance of a digital
servo and in a high-performance system it must
be kept to a minimum.
A33
A Engineering Reference
Fig. 2.19 Digital servo drive
Drive Technologies
Brushless Motor Drives
The trapezoidal drive
Fig. 2.20 shows a simplified layout of the drive for a
three-phase trapezoidal motor. The switch set is
based on the familiar H-bridge, but uses three bridge
legs instead of two.The motor windings are
connected between the three bridge legs as shown,
with no connection to the star point at the junction of
the windings. By turning on the appropriate two
transistors in the bridge, current can be made to flow
in either direction through any two motor windings.
At any particular time, the required current path
depends on rotor position and direction of rotation,
so the bridge transistors are selected by logic driven
from the commutation encoder.
A PWM recirculating chopper system controls the
current in the same way as in the DC brush drive
described previously. The required current
feedback information is provided by sense
resistors connected in series with two of the motor
leads. The voltage signals derived from these
resistors must be decoded and combined to
provide a useful current reference, and the circuit
that does this also uses the commutation encoder
to determine how to interpret the information. In
fact, this is not a simple process because the
relatively small feedback voltage (about 1V) must
be separated from the large voltage excursions
generated by the chopping system (240V in the
case of a typical high-power drive).
The input stages of the brushless drive follow the
same pattern as a conventional analog brush drive
(using a high-gain velocity amplifier that generates
the torque demand signal). Velocity feedback can
be derived in a number of ways, but it is clearly
desirable to use a brushless method in conjunction
with a brushless motor. Some motors incorporate
a brushless tach generator that produces
multi-phase AC outputs. These signals have to
be processed in a similar way to the current
feedback information using additional data from a
tach encoder. Again, this is not a particularly
straightforward process and it is difficult to obtain
a smooth, glitch-free feedback signal. A more
satisfactory alternative is to use a high-resolution
optical encoder and convert the encoder pulse
frequency to an analog voltage. The encoder
can also be used as the feedback device for a
position controller.
Fig. 2.20 Simplified trapezoidal brushless servo drive
Velocity Amp
Velocity
Input
+
-
Torque
Amp
+
Logic
PWM &
Circuit
Velocity
Feedback
-
Current
Sense
Selector
Motor
Communication
Encoder
A34
Drive Technologies
Sine wave brushless motors can be two- or threephase, and the drive we'll look at is for the twophase version (see Fig. 2.21). This uses two Hbridges to control current in the two motor
windings, and the power section of this drive
closely resembles a pair of DC brush drives. By
contrast with the previous example, this drive uses
a digital processor-based control section that takes
its input in the form of step and direction signals.
We need to generate currents in the two motor
windings that follow a sine and cosine pattern as
the shaft rotates. The drive shown in Fig. 2.21 uses
a brushless resolver and a resolver-to-digital
converter (RDC) to detect the shaft position. From
this, we will get a number that can be fed to a
reference table to determine the instantaneous
current values for that particular shaft position. Bear
in mind that the reference table will only indicate
relative currents in the two windings—the absolute
values will depend on the torque demand at the
time. So the processor must multiply the sine and
cosine values by the torque demand to get the final
value of current in each phase. The resulting
numbers are fed to D-to-A converters that produce
an analog voltage proportional to demanded
current. This is fed to the two PWM chopper
amplifiers.
Commutation information for a sine wave drive may
also be derived from an absolute or incremental
optical encoder. An incremental encoder will be less
expensive for the same resolution, but requires
some form of initialization at power-up to establish
the required 90° torque angle.
A “pseudo-sine wave” drive using feedback from a
low-resolution absolute encoder can offer a costeffective alternative. The pseudo sine wave drive
gives superior performance to the trapezoidal drive
at lower cost than the standard high-resolution sine
wave system.
Fig. 2.21 Two-phase sine wave brushless drive
Step
D-to-A
Converter
PWM
Control
H - Bridge
D-to-A
Converter
PWM
Control
H - Bridge
Microprocessor
Direction
Resolver
to Digital
Converter
Motor
Resolver
A35
A Engineering Reference
The Sine Wave Drive
Servo Tuning
Tuning a Servo System
Any closed-loop servo system, whether analog or
digital, will require some tuning. This is the process
of adjusting the characteristics of the servo so that
it follows the input signal as closely as possible.
Why is tuning necessary?
A servo system is error-driven, in other words, there
must be a difference between the input and the
output before the servo will begin moving to reduce
the error. The “gain” of the system determines how
hard the servo tries to reduce the error. A high-gain
system can produce large correcting torques when
the error is very small. A high gain is required if the
output is to follow the input faithfully with minimal
error.
Now a servo motor and its load both have inertia,
which the servo amplifier must accelerate and
decelerate while attempting to follow a change at
the input. The presence of the inertia will tend to
result in over-correction, with the system oscillating
or “ringing” beyond either side of its target (Fig. 3.1).
This ringing must be damped, but too much
damping will cause the response to be sluggish.
When we tune a servo, we are trying to achieve the
fastest response with little or no overshoot.
Fig. 3.1 System response characteristics
Output
Underdamped
Response
Critical Damping
So why doesn’t the feedback stay negative all the
time?
To answer this, we need to clarify what we mean by
“negative”. In this context, it means that the input
and feedback signals are in antiphase. If the input is
driven with a low frequency sinewave, the feedback
signal (which will also be a sinewave) is displaced in
phase by 180°. The 180°-phase displacement is
achieved by an inversion at the input of the
amplifier. (In practice it’s achieved simply by
connecting the tach the right way round – connect
it the wrong way and the motor runs away.)
The very nature of a servo system is such that its
characteristics vary with frequency, and this
includes phase characteristics. So feedback that
starts out negative at low frequencies can turn
positive at high frequencies. The result can be
overshoot, ringing or ultimately continuous
oscillation.
We’ve said that the purpose of servo tuning is to
get the best possible performance from the system
without running into instability. We need to
compensate for the characteristics of the servo
components and maintain an adequate stability
margin.
What determines whether the system will be stable
or not?
Closed-loop systems can be difficult to analyze
because everything is interactive. The output gets
fed back to the input in antiphase and virtually
cancels it out, so there seems to be nothing left to
measure. The best way to determine what’s going
on is to open the loop and then see what happens.
Fig. 3.2 Closed-loop velocity servo
Overdamped Response
Time
In practice, tuning a servo means adjusting
potentiometers in an analog drive or changing gain
values numerically in a digital system. To carry out
this process effectively, it helps to understand
what’s going on in the drive. Unfortunately, the
theory behind servo system behavior, though well
understood, does not reveal itself to most of us
without a struggle. So we’ll use a simplified
approach to explain the tuning process in a typical
analog velocity servo. Bear in mind that this
simplified approach does not necessarily account
for all aspects of servo behavior.
Servo
Amplifier
Velocity
Input
+
-
Tach
Feedback Signal
Fig. 3.3A Measuring open-loop characteristics
Motor
Oscillator
+
A Brief Look at Servo Theory
A servo is a closed-loop system with negative
feedback. If you make the feedback positive, you
will have an oscillator. So for the servo to work
properly, the feedback must always remain
negative, otherwise the servo becomes unstable. In
practice, it’s not as clear-cut as this. The servo can
almost become an oscillator, in which case it
overshoots and rings following a rapid change at
the input.
A36
Motor
-
Tach
Scope
Measuring the open-loop characteristic allows us to
find out what the output (and therefore the
feedback) signal will be in response to a particular
input. We need to measure the gain and phase
shift at different frequencies, and we can plot the
results graphically. For a typical servo system, the
results might look like this:
Fig. 3.3B Open-loop gain and phase characteristics
The open-loop gain must be less than unity
when the phase shift is 180°.
When this condition is only just met (i.e., the phase
shift is near to180° at unity gain) the system will ring
after a fast change on the input.
+30
+20
We’ve said that a problem can occur when there is
a phase shift of 180° round the loop. When this
happens, the open-loop gain must be less than one
(1) so that the signal fed back is smaller than the
input. So here is a basic requirement for a stable
system:
Gain
Fig. 3.4 Underdamped response
Ringing at
unity-gain
frequency
+10
db
0
-10
Frequency
Input
Output
Characteristics of a Practical
Servo System
0
-90
Phase
Shift
-180
Typical open-loop gain and phase characteristics of
an unloaded drive-motor-tach system will look
something like Fig. 3.5.
Fig. 3.5 Characteristics of a practical system
Crossover frequency
40–300 Hz typical
+dB
The gain scale is in decibels (dB), which is a
logarithmic scale (a 6dB decrease corresponds to a
reduction in amplitude of about 50%). The 0dB line
represents an open-loop gain of one (unity), so at
this frequency the input and output signals will have
the same amplitude. The falling response in the gain
characteristics is mainly due to the inertia of the
motor itself.
Gain
The phase scale is in degrees and shows the phase
lag between input and output. Remember that the
feedback loop is arranged to give negative
feedback at low frequencies, (i.e., 180° phase
difference). If the additional phase lag introduced by
the system components reaches 180°, the
feedback signal is now shifted by 360° and
therefore back in phase with the input. We need to
make sure that at no point do we get a feedback
signal larger than the original input and in phase
with it. This would amount to positive feedback,
producing an ever-increasing output leading to
oscillation.
Phase
Fortunately, it is possible to predict quite accurately
the gain and phase characteristics of most servo
systems, provided that you have the necessary
mathematical expertise and sufficient data about
the system. So in practice, it is seldom necessary to
measure these characteristics unless you have a
particular stability problem that persists.
Shaft resonance
2 kHz typical
0
-dB
0
-180°
B
-360°
The first thing we notice is the pronounced spike in
the gain plot at a frequency of around 2kHz. This is
caused by shaft resonance, torsional oscillation in
the shaft between the motor and the tach. Observe
that the phase plot drive dramatically through the
critical 180° line at this point. This means that the
loop gain at this frequency must be less than unity
(0dB), otherwise the system will oscillate.
The TIME CONSTANT control determines the
frequency at which the gain of the amplifier starts to
roll off. You can think of it rather like the treble
control on an audio amplifier. When we adjust the
time constant control, we are changing the highfrequency gain to keep the gain spike at 2kHz just
below 0dB. With too high a gain (time constant too
low) the motor will whistle at about 2kHz.
A37
A Engineering Reference
Servo Tuning
Servo Tuning
The second point of interest is the CROSSOVER
FREQUENCY, which is the frequency at which the
gain curve passes through 0dB (unity gain). This
frequency is typically between 40 and 300Hz. On
the phase plot, ß (beta) is the phase margin at the
crossover frequency. If ß is very small, the system
will overshoot and ring at the crossover frequency.
So ß represents the degree of damping – the
system will be heavily damped if ß is large.
The DAMPING control increases the phase margin
at the crossover frequency. It operates by applying
lead compensation, sometimes called acceleration
feedback. The compensation network creates a
phase lead in the region of the crossover frequency,
which increases the phase margin and therefore
improves the stability.
Increasing the damping also tends to reduce the
gain at the 2kHz peak, allowing a higher gain to be
used before instability occurs. Therefore, the time
constant should be re-adjusted after the damping
has been set up.
What’s the effect of adding load inertia?
An external load will alter both the gain and phase
characteristics. Not only will the overall gain be
reduced owing to the larger inertia, but an
additional gain spike will be introduced due to
torsional oscillation between the motor and the
load. This gain spike may well be larger than the
original 2kHz spike, in which case the motor will
start to buzz at a lower frequency when the time
constant is adjusted.
A38
Fig. 3.6 Characteristics of a system with inertial load
+dB
Gain
Motor alone (no load)
0
-dB
Motor
plus load
0
Phase
-180°
-360°
The amplitude of this second spike will depend on
the compliance or stiffness of the coupling
between motor and load. A springy coupling will
produce a large gain spike; this means having to
reduce the gain to prevent oscillation, resulting in
poorer system stiffness and slower response. So,
if you’re after a snappy performance, it’s important
to use a torsionally-stiff coupling between the
motor and the load.
Feedback
ServoDevices
Tuning
A permanent magnet DC motor may be used as a
tachometer. When driven mechanically, this motor
generates an output voltage that is proportional to
shaft speed (see Fig. 4.1). The other main
requirements for a tachometer are that the output
voltage should be smooth over the operating range
and that the output should be stabilized against
temperature variations.
Small permanent magnet DC “motors” are
frequently used in servo systems as speed sensing
devices. These systems usually incorporate
thermistor temperature compensation and make
use of a silver commutator and silver loaded
brushes to improve commutation reliability at low
speeds and at the low currents, which are typical of
this application.
To combine high performance and low cost, DCservo motor designs often incorporate a
tachometer mounted on the motor shaft and
enclosed within the motor housing (Fig. 4.1).
Fig. 4.1 Tachometer output characteristics
Output
Volts
Shaft Speed
Fig. 4.2 Motor with integral tachometer
Tachometer
Collimated
Light Source
Grating
Mask
Detector
An incremental encoder generates a pulse for a
given increment of shaft rotation (rotary encoder), or
a pulse for a given linear distance travelled (linear
encoder). Total distance travelled or shaft angular
rotation is determined by counting the encoder
output pulses.
An absolute encoder has a number of output
channels, such that every shaft position may be
described by its own unique code. The higher the
resolution the more output channels are required.
The Basics of Incremental Encoders
Since cost is an important factor in most industrial
applications, and resetting to a known zero point
following power failure is seldom a problem, the
rotary incremental encoder is the type most favored
by system designers. Its main element is a shaft
mounted disc carrying a grating, which rotates with
the grating between a light source and a masked
detector. The light source may be a light emitting
diode or an incandescent lamp, and the detector is
usually a phototransistor or more commonly a
photo-voltaic diode. Such a simple system,
providing a single low-level output, is unlikely to be
frequently encountered, since quite apart from its
low output signal, it has a DC offset that is
temperature dependent, making the signal difficult
to use (Fig. 4.4).
Fig. 4.4 Encoder output voltage
V
Output
Voltage
Motor
Optical Encoders
In servo control systems, where mechanical
position is required to be controlled, some form of
position sensing device is needed. There are a
number of types in use: magnetic, contact,
resistive, and optical. However, for accurate
position control, the most commonly used device is
the optical encoder. There are two forms of this
encoder – absolute and incremental.
Optical encoders operate by means of a grating,
that moves between a light source and a detector.
When light passes through the transparent areas of
the grating, an output is seen from the detector.
For increased resolution, the light source is
collimated and a mask is placed between the
grating and the detector. The grating and the mask
produce a shuttering effect, so that only when their
transparent sections are in alignment is light
allowed to pass to the detector (Fig. 4.3).
DC
Offset
Shaft
Rotation
In practice, two photodiodes are used with two
masks, arranged to produce signals with 180°
phase difference for each channel, the two diode
outputs being subtracted so as to cancel the DC
offset (Fig. 4.5). This quasi-sinusoidal output may
be used unprocessed, but more often it is either
amplified or used to produce a square wave output.
Hence, incremental rotary encoders may have sine
wave or square wave outputs, and usually have up
to three output channels.
A39
A Engineering Reference
Fig. 4.3 Principle of optical encoder
Tachometers
Feedback
Devices
Servo Tuning
Fig. 4.5 Output from dual photodiode system
V1
CMOS (Complimentary Metal-Oxide
Semiconductor) – Available for compatibility with
the higher logic levels, it normally used with CMOS
devices.
Output 1
V1
Output 2
DC
Offset
Shaft
Rotation
0
Combined
Output (1-2)
A two-channel encoder, as well as giving position
of the encoder shaft, can also provide information
on the direction of rotation by examination of the
signals to identify the leading channel. This is
possible since the channels are normally arranged
to be in quadrature (i.e., 90° phase shifted:
Fig. 4.6).
For most machine tool or positioning applications, a
third channel known as the index channel or Z
channel is also included. This gives a single output
pulse per revolution and is used when establishing
the zero position.
Line driver – These are low-output impedance
devices designed for driving signals over a long
distance, and are usually used with a matched
receiver.
Complementary outputs – Outputs derived from
each channel give a pair of signals, 180° out of
phase. These are useful where maximum immunity
to interference is required.
Noise problems
The control system for a machine is normally
screened and protected within a metal cabinet. An
encoder may be similarly housed. However, unless
suitable precautions are taken, the cable
connecting the two can be a source of trouble due
to its picking up electrical noise. This noise may
result in the loss or gain of signal counts, giving rise
to incorrect data input and loss of position.
Fig. 4.7 Corruption of encoder signal by noise
+ Ve Noise Pulse
Channel A
Fig. 4.6 Quadrature output signals
- Ve Noise Pulse
Channel A
Fig. 4.7, shows how the introduction of two noise
pulses has converted a four-pulse train into one of
six pulses.
A number of techniques are available to overcome
problems due to noise. The most obvious resolution
is to use shielded interconnecting cables.
Channel B
90 Phase Shift (± Tolerance)
Fig. 4.6 shows that for each complete square wave
from channel A, if channel B output is also
considered during the same period, four pulse
edges will occur. This allows the resolution of the
encoder to be quadrupled by processing the A and
B outputs to produce a separate pulse for each
square wave edge. For this process to be effective,
however, it is important that quadrature is
maintained within the necessary tolerance so that
the pulses do not run into one another.
Square wave output encoders are generally
available in a wide range of resolutions (up to about
5000 lines/rev), and with a variety of different output
configurations, some of which are listed below.
TTL (Transistor-Transistor Logic) – This is a
commonly available output, compatible with TTL
logic levels, and normally requiring a 5 volt supply.
TTL outputs are also available in an open-collector
configuration which allows the system designer to
choose from a variety of pull-up resistor value.
A40
However, since the signals may be at a low level
(5 volts) and may be generated by a highimpedance source, it may be necessary to go to
further lengths to eliminate the problem.
The most effective way to resolve the problem is
to use an encoder with complementary outputs
(Fig. 4.8) and connect this to the control system by
means of shielded, twisted-pair cable.
The two outputs are processed by the control
circuitry so that the required signal can be
reconstituted without the noise.
Fig. 4.8 Complementary output signals
Channel A
Noise Spike
A
If the A signal is inverted and is fed with the A signal
into an OR gate (whose output depends on one
signal or the other being present), the resultant
output will be a square wave (Fig. 4.9).
Fig. 4.9 Reduction of noise in a complementary
system
Channel A
OR
Inverted A
The simple interconnection of encoder and
controller with channel outputs at low level may be
satisfactory in electrically “clean” environments or
where interconnections are very short. In cases
where long interconnections are necessary or
where the environment is “noisy”, complementary
line driver outputs will be needed, and
interconnections should be made with shielded,
twisted-pair cable.
Factors Affecting Accuracy
Slew rate (speed) – An incremental rotary encoder
will have a maximum frequency at which it will
operate (typically 100KHz), and the maximum
rotational speed, or slew rate, will be determined by
this frequency. Beyond this, the output will become
unreliable and accuracy will be affected.
Eccentricity
This may be caused by bearing play, shaft run out,
incorrect assembly of the disc on its hub or the hub
on the shaft. Eccentricity may cause a number of
different error conditions.
a) Amplitude Modulation – In a sine wave encoder,
eccentricity will be apparent as amplitude
modulation (Fig. 4.11).
Fig. 4.11 Amplitude modulation caused by
eccentricity
Nominal Signal
Level
Signal Amplitude
b) Frequency modulation – As the encoder is
rotated at constant speed, the frequency of the
output will change at a regular rate (Fig. 4.12). If
viewed on an oscilloscope, this effect will appear as
“jitter” on the trace.
Fig. 4.12 Encoder frequency modulation
Increased Frequency (f 2 )
Consider a 600-line encoder rotated at 1rpm (gives
an output of 10Hz). If the maximum operating
frequency of the encoder is 50KHz, its speed will be
limited to 5000 times this (i.e., 50KHz • 10Hz =
5000 rpm).
If an encoder is rotated at speeds higher than its
design maximum, there may be conditions set up
that will be detrimental to the mechanical
components of the assembly. This could damage
the system and affect encoder accuracy.
Quantization error – All digital systems have
difficulty, interpolating between output pulses.
Therefore, knowledge of position will be accurate
only to the grating width (Fig. 4.10).
Fig. 4.10 Encoder quantization error
Nominal Frequency (f 1 )
c ) Inter-channel jitter – If the optical detectors for
the two encoder output channels are separated by
an angular distance on the same radius, then any
“jitter” will appear at different times on the two
channels, resulting in “inter-channel jitter”.
Environmental Considerations
Like electrical noise, other environmental factors
should be considered before installing an optical
encoder.
In particular, temperature and humidity should be
considered (consult manufacturers’ specifications).
Quantization Error
In environments contaminated with dust, oil vapor
or other potentially damaging substances, it may be
necessary to ensure that the encoder is enclosed
within a sealed casing.
Quantization Error = F (1, 2N) (N = no. of lines/disk rotation)
Quantization error = F(1,2N) (N = # of lines/disk
rotation)
A41
A Engineering Reference
Feedback
ServoDevices
Tuning
Feedback
Devices
Servo Tuning
Mechanical Construction
Shaft encoder (Fig. 4.13). In this type of encoder,
which may be either incremental or absolute, the
electronics are normally supported on a substantial
mounting plate that houses the bearings and shaft.
The shaft protrudes from the bearings on the
“outside” of the encoder, for connection to the
rotating system, and on the “inside”, so that the
encoder disc may be mounted in the appropriate
position relative to the light source and detector.
The internal parts are covered by an outer casing,
through which the interconnecting leads pass.
Fig. 4.13 Shaft encoder
Interconnecting
Leads
Cover
Mounting Plate
Shaft
For use in extreme environmental or industrial
conditions, the whole enclosure may be specified to
a more substantial standard (heavy duty) with
sealed bearings and sealing between the mounting
plate and cover. Also the external electrical
connections may be brought out through a high
quality connector.
Modular or kit encoder (Fig. 4.14). These are
available in a number of forms, their principal
advantage being that of reduced cost.
Fig. 4.14 Modular encoder
Cover
Hub and Disc
Electronics
Interconnecting
Leads
Body
Since many servo motors have a double-ended
shaft, it is a simple matter to fit a kit encoder onto a
motor.
A42
The kit encoder will usually be less robust than the
shaft encoder, but this need not be a problem if the
motor is mounted so that the encoder is protected.
If this cannot be done, it will normally be possible to
fit a suitable cover over the encoder.
A typical kit encoder will include a body, on which
will be mounted the electronic components and a
hub and disc assembly for fitting to the shaft.
Some form of cover will also be provided, mainly to
exclude external light and provide some mechanical
protection.
Linear encoder. These encoders are used where it
is required to make direct measurement of linear
movement. They comprise a linear scale (which
may be from a few millimeters to a meter or more in
length), and a read head. They may be incremental
or absolute and their resolution is expressed in lines
per unit length (normally lines/inch or lines/cm).
Feedback
ServoDevices
Tuning
An absolute encoder is a position verification device
that provides unique position information for each
shaft location. The location is independent of all
other locations, unlike the incremental encoder,
where a count from a reference is required to
determine position.
Fig. 4.15 Absolute disk
Fig. 4.17 Absolute encoder output
Bit
Current
Position
1
1
0
1
1011 = Decimal 11
The disk pattern of an absolute encoder is in
machine readable code, usually binary, grey code
or a variety of grey. The figure above represents a
simple binary output with four bits of information.
The current location is equivalent to the decimal
number 11. Moving to the right from the current
position, the next decimal number is 10 (1-0-1-0
binary). Moving to the left from the current position,
the next position would be 12 (1-1-0-0).
Fig. 4.18 Multi-turn absolute encoders
High Resolution
Main Disk
Fig. 4.16 Incremental disk
Bearing
Seals
In an absolute encoder, there are several concentric
tracks, unlike the incremental encoder, with its
single track. Each track has an independent light
source. As the light passes through a slot, a high
state (true “1”) is created. If light does not pass
through the disk, a low state (false “0”) is created.
The position of the shaft can be identified through
the pattern of 1’s and 0’s.
The tracks of an absolute encoder vary in slot size,
moving from smaller at the outside edge to larger
toward the center. The pattern of slots is also
staggered with respect to preceding and
succeeding tracks. The number of tracks
determines the amount of position information that
can be derived from the encoder disk – resolution.
For example, if the disk has ten tracks, the
resolution of the encoder would usually be 1,024
positions per revolution or 210.
For reliability, it is desirable to have the disks
constructed of metal rather than glass. A metal disk
is not as fragile, and has lower inertia.
Additional Turns
Stages
Gearing an additional absolute disk to the primary
high-resolution disk provides for turns counting, so
that unique position information is available over
multiple revolutions.
Here is an example of how an encoder with 1,024
counts per revolution becomes an absolute device
for 524,288 discrete positions.
The primary high-resolution disk has 1,024 discrete
positions per revolution. A second disk with 3
tracks of information will be attached to the highresolution disk geared 8:1. The absolute encoder
now has 8 complete turns of the shaft or 8,192
discrete positions. Adding a third disk geared 8:1
will provide for 64 turns of absolute positions. In
theory, additional disks could continue to be
incorporated. But in practice, most encoders stop
at or below 512 turns. Encoders using this
technique are called multi-turn absolute encoders.
This same technique can be incorporated in a rack
and pinion style linear encoder resulting in long
lengths of discrete absolute locations.
Advantages of Absolute Encoders
Rotary and linear absolute encoders offer a number
of significant advantages in industrial motion control
and process control applications.
No Position Loss During Power Down or
Loss of Power
An absolute encoder is not a counting device like an
incremental encoder, because an absolute system
reads actual shaft position. The lack of power does
not cause the encoder lose position information.
A43
A Engineering Reference
Basics of Absolute Encoders
Feedback
Devices
Servo Tuning
Whenever power is supplied to an absolute system,
it can read the current position immediately. In a
facility where frequent power failures are common,
an absolute encoder is a necessity.
Operation in Electrically Noisy Environments
Equipment such as welders and motor starters
often generate electrical noise that can often look
like encoder pulses to an incremental counter.
Electrical noise does not alter the discrete position
that an absolute system reads.
High-speed Long-distance Data Transfer
Use of a serial interface such as RS-422 gives the
user the option of transmitting absolute position
information over as much as 4,000 feet.
Eliminate Go Home or Referenced Starting
Point
The need to find a home position or a reference
point is not required with an absolute encoding
system since an absolute system always knows its
location. In many motion control applications, it is
difficult or impossible to find a home reference
point. This situation occurs in multi-axis machines
and on machines that can't reverse direction. This
feature will be particularly important in a “lights-out”
manufacturing facility. Significant cost savings is
realized in reduced scrap and set-up time resulting
from a power loss.
Provide Reliable Position Information in
High-speed Applications
The counting device is often the factor limiting the
use of incremental encoders in high-speed
applications. The counter is often limited to a
maximum pulse input of 100 KHz. An absolute
encoder does not require a counting device or
continuous observation of the shaft or load location.
This attribute allows the absolute encoder to be
applied in high-speed and high-resolution
applications.
Resolvers
A resolver is, in principle, a rotating transformer.
If we consider two windings, A and B (Fig. 4.19),
and if we feed winding B with a sinusoidal voltage,
then a voltage will be induced into winding A. If we
rotate winding B, the induced voltage will be at
maximum when the planes of A and B are parallel
and will be at minimum when they are at right
angles. Also, the voltage induced into A will vary
sinusoidally at the frequency of rotation of B so that
EOA = Ei Sinø. If we introduce a third winding (C),
positioned at right angles to winding A, then as we
rotate B, a voltage will be induced into this winding
and this voltage will vary as the cosine of the angle
ø, so that EOC = Ei Cosø
Winding B
Fig. 4.19 Resolver principle
Winding A
A44
Fig. 4.20 Resolver output
E iCos
360°
E i Sin
1 Electrical Cycle
The information output from the two phases will
usually be converted from analog to digital form, for
use in a digital positioning system, by means of a
resolver-to-digital converter (Fig. 4.21). Resolutions
up to 65,536 counts per revolution are typical of
this type of system.
Fig. 4.21 Resolver-to-digital converter
Sine
Multiplier
Integrator
Phase
Comparitor
Cosine
Multiplier
Up/Down
Counter
Digital Output
(Shaft Angle)
Voltage
Control
Oscillator
Integrator
DC Signal
(Velocity)
In addition to position information, speed and
direction information may also be derived. The
resolver is an absolute position feedback device.
Within each electrical cycle, Phase A and Phase B
maintain a constant (fixed) relationship.
The excitation voltage Ei may be coupled to the
rotating winding by slip rings and brushes, though
this arrangement is a disadvantage when used with a
brushless motor. In such applications, a brushless
resolver may be used so that the excitation voltage is
inductively coupled to the rotor winding (Fig. 4.22).
Fig. 4.22 Brushless resolver
Stator
Phase 1
q
Ei
EOA
Referring to Fig 4.20, we can see that if we are able
to measure the relative amplitudes of the two
winding (A & C) outputs at a particular point in the
cycle, these two outputs will be unique to that
position.
Stator
Phase 2
Rotor
Control Systems
Many industrial designers are concerned with
controlling an entire process. Motion control is one
important and influential aspect of complete
machine control. The primary elements of machine
control include:
Fig. 5.1 Primary machine control elements
Motion Control
Servos
Steppers
Hydraulics
Mainframes
MIS
SPC
Switches
Indicators
Readout
Actuators
Ne
two
ital
rks
I/O
Dig
Machine
Control
f
ter
r In
ato
er
Op
ace
Displays
Keyboards
Touchscreens
An
alo
gI
/O
Sensors
Gauges
Meters
Data Acquisition
Proportional Valves
Motion Control: For precise programmable load
movement using a servo motor, stepper motor,
or hydraulic actuators. Feedback elements are
often employed.
Analog and Digital I/O: For actuation of an
external process, devices such as solenoids,
cutters, heaters, valves, etc.
Operator Interface: For flexible interaction with
the machine process for both setup and on-line
variations. Touchscreens, data pads, and
thumbwheels are examples.
Communications Support: For process
monitoring, diagnostics and data transfer with
peripheral systems.
There are many different machine control
architectures that integrate these elements. Each
results in varying levels of complexity and
integration of both motion and non-motion
elements. PLC-based, bus-based and integrated
solutions are all commercially available. Your
selection of a machine control strategy will often be
based on performance, total application cost, and
technology experience.
PLC-based Control
The PLC-based architecture is utilized for I/O
intensive control applications. Based upon banks of
relays that are scanned, or polled, by a central
processor, the PLC provides a low-cost option for
those familiar with its ladder logic programming
language. Integration of the motion, I/O, operator
interface, and communication are usually supported
through additional cards that are plugged in its
backplane.
have not historically concentrated on motion
control, plug-in indexers or those that communicate
over BCD are preferable. Because these indexer
boards often include their own microprocessor,
they prevent slow polling rates, but incorporate a
separate programming language. In general, this
compromise is acceptable for all but the most
complicated motion/machine applications.
Bus-based Systems
Bus-based machine control systems are common
in today's industrial environment. STD, VME and
PC-AT are only a few of the numerous options.
Most of these options can operate through a
standard operating system (DOS, OS/2, OS/9) that
can be used to program add-on cards for I/O,
motion and communication interfaces. Flexible
graphical operator interfaces remain one of the
computer's major advantages.
Some successful examples of bus-based machine
control applications include gear grinding and
dressing. PCB placement machines, hard disk
manufacturing, and automotive glass bending.
Wherever intensive communications or data
processing are required, the benefits of the bus
structure can be realized.
There are some disadvantages to the bus-based
machine control system that relate to the amount of
integration between the motion and I/O structure.
Separate cards are required for each, resulting in a
need for software integration of different
programming languages. Motion control operations,
such as servo loops, should be polled and updated
on a more immediate basis than auxiliary I/O or the
operator interface. The programmer must develop
this polling hierarchy to thread the system together.
Integrated controllers
A more integrated approach to machine control
uses a stand-alone architecture that builds in the
same essential elements of I/O, motion, operator
interface, and communication. This approach uses
a single software and hardware platform to control
an entire machine application. The polling of servo
loops, I/O points, and the operator interface are
handled internally, invisible to the user. A common
software language is provided to integrate the
motion and I/O actuation. This pre-tested approach
allows a typical machine control application to be
developed with a minimum of effort and cost.
The total application cost is the major consideration
when selecting an integrated machine controller.
While the initial hardware cost is typically higher
than other solutions, the software investment and
maintenance of a single language is an overriding
and positive factor. Software development and
maintenance costs for any machine control
application can dwarf the initial hardware expense.
The integrated approach offers a more economical
solution.
The addition of scanning points decreases the
polling rate of any individual point, and can thus
lead to lower machine response. Because PLCs
A45
A Engineering Reference
Machine Control
Control Systems
Control System Overview
The controller is an essential part of any motion
control system. It determines speed, direction,
distance and acceleration rate – in fact all the
parameters associated with the operation that the
motor performs. The output from the controller is
connected to the drive’s input, either in the form of
an analog voltage or as step and direction signals.
In addition to controlling one or more motors, many
controllers have additional inputs and outputs that
allow them to monitor other functions on a machine
(see Machine Control, p. A45).
Controllers can take a wide variety of forms. Some
examples are listed below.
Standalone – This type of controller operates
without data or other control signals from external
sources. A standalone unit usually incorporates a
keypad for data entry as well as a display, and
frequently includes a main power supply. It will also
include some form of nonvolatile memory to allow it
to store a sequence of operations. Many controllers
that need to be programmed from a terminal or
computer can, once programmed, also operate in
standalone mode.
Bus-based – A bus-based controller is designed to
accept data from a host computer using a standard
communications bus. Typical bus systems include
STD, VME and IBM-PC bus. The controller will
usually be a plug-in card that conforms to the
standards for the corresponding bus system. For
example, a controller operating on the IBM-PC bus
resides within the PC, plugging into an expansion
slot and functioning as an intelligent peripheral.
PLC-based – A PLC-based indexer is designed to
accept data from a PLC in the form of I/O
communication. Typically, the I/O information is in
BCD format. The BCD information may select a
program to execute, a distance to move, a time
delay, or any other parameter requiring a number.
The PLC is well suited to I/O actuation, but poorly
suited to perform complex operations such as math
and complicated decision making. The motion
control functions are separated from the PLC's
processor and thus do not burden its scan time.
X Code-based – X Code is a command language
specifically developed for motion control and
intended for transmission along an RS-232C link.
Controllers using this language either accept realtime commands from a host computer or execute
stored sequences that have been previously
programmed. The simplicity of RS-232C
communication allows the controller to be
incorporated into the drive itself, resulting in an
integrated indexer/drive package.
A46
X Code Programming
X Code has been designed to allow motion control
equipment to be programmed by users with little or
no computer experience. Although the language
includes more than 150 commands, depending on
the product, it is only necessary to learn a small
percentage of these to write simple programs.
Most command codes use the initial letter of the
function name, which makes them easy to
remember. Here are some examples of frequently
used commands.
V – velocity in revs/sec
D – distance in steps
A – acceleration rate in revs/sec2
G – go; start the move
T – time delay in seconds
A typical command string might look like this:
V10 A50 D4000 G T2 G
This would set the velocity to 10 revs/sec,
acceleration to 50 revs/sec2 and distance to 4000
steps. The 4000-step move would be performed
twice with a 2-second wait between moves.
Please refer to specifications of X Code products
for a list of all the available X Code commands.
Single-axis and Multi-axis Controllers
A single-axis controller can, as the name implies,
only control one motor. The controller in an
integrated indexer/drive comes into this category.
However, such units are frequently used in systems
using more than one motor where the operations
do not involve precise synchronization between
axes.
A multi-axis controller is designed to control more
than one motor and can very often perform
complex operations such as linear or circular
interpolation. These operations require accurate
synchronization between axes, which is generally
easier to achieve with a central controller.
A variant of the multi-axis controller is the
multiplexed unit, which can control several motors
on a time-shared basis. A printing machine having
the machine settings controlled by stepper motors
could conveniently use this type of controller when
the motors do not need to be moved
simultaneously.
Hardware-based Controllers
Control systems designed without the use of a
microprocessor have been around for many years
and can be very cost-effective in simpler
applications. They tend to lack flexibility and are
therefore inappropriate where the move parameters
are continually changing. For this reason, the
hardware-based controller has now given way
almost exclusively to systems based on a
microprocessor.
Control Systems
XCode
Commands
RS-232C
Communications
Interface
Nonvolatile
RAM
Step
Microprocessor
Programmable
Pulse
Generator
Direction
Inputs
I/O
Interface
Program
Memory ROM
Output
to Drive
Outputs
Processor-based Controllers
The flexibility offered by a microprocessor system
makes it a natural choice for motion control.
Fig. 5.2 shows the elements of a typical step and
direction controller that can operate either in
conjunction with a host computer or as a standalone unit.
All the control functions are handled by the
microprocessor whose operating program is
stored in ROM. This program will include an
interpreter for the command language, which may
be X Code for example.
X Code commands are received from the host
computer or terminal via the RS-232C
communications interface. These commands are
simple statements that contain the required speed,
distance and acceleration rate, etc. The processor
interprets these commands and uses the
information to control the programmable pulse
generator. This in turn produces the step and
direction signals that will control a stepper or servo
drive.
The processor can also switch outputs and
interrogate inputs via the I/O interface. Outputs
can initiate other machine functions such as
punching or cutting, or simply activate drive panel
indicators to show the program status. Inputs may
come from sources such as operator pushbuttons
or directional limit switches.
When the controller is used in a standalone mode,
the required motion sequences are programmed
from the host and stored in nonvolatile memory
(normally battery-backed RAM). These sequences
may then be selected and executed from switches
via the I/O interface or from a separate machine
controller such as a PLC.
A47
A Engineering Reference
Fig. 5.2 Processor-based controller
Control Systems
Understanding Input and Output Modules
The primary reason for using I/O modules is to
interface 5VDC logic signals from an indexer to
switches and relays on the factory floor, which
typically run on voltage levels ranging from 24VDC
to 220VAC. Solid-state I/O modules are essentially
a relay, utilizing light emitting diode (LED) and a
transistor along with a signal conditioning circuit to
activate a switch. These I/O modules isolate (no
direct connection) the internal microprocessor
circuitry of an indexer from oversized DC and AC
voltages. The lack of a physical connection
between the indexer and external devices, protects
the indexer from hazardous voltage spikes and
current surges.
Most motion controllers/indexers offer
programmable inputs and outputs to control and
interact with other external devices and machine
elements.
Programmable Output Example
After indexing a table to a preset position, energize
a programmable output to activate a knife that will
cut material on the table.
Programmable Input Example
After indexing a table in a pick and place
application, the indexer waits for an input signal
from a robot arm, signaling the indexer that a part
has been located on the table.
DC Input and Output Modules
As with all DC devices, this is a polarized, + and –
input module. Since current will flow in only one
direction, care must be taken to observe these
polarities during installation.
DC input modules typically feature an input signal
conditioning circuit. This circuit requires the input to
remain on/off for a minimum of 5 milliseconds
before recognizing the switch. This eliminates a
short voltage spike or “de-bounce” contact closure
less than 5 milliseconds in duration. However, a 0.1
microfarad, ceramic disc capacitor across the
actual switching contacts is still recommended to
prevent switch bounce that can be as long as 1080 milliseconds.
Fig. 5.3 Typical DC input connection diagram
4KV
Isolation Barrier
Optional
Circuit Board
Indicating
LED
Switch #1
+5VDC
+
+
10 to 24VDC
Floating
Source
-
10 to 24 mA
Logic Signal
Signal
Conditioning
1K
Coupling
LED
Photo
Transistor
Screw
Terminals
Ground
DC Input Operational Sequence
As switch #1 closes, current flows through the
limiting resistor (1K ohm), and then into the LED.
The light issued by the LED due to this forward
current flow in turn simulates the photo transistor.
Hence the term “opto” or optically isolated. The
phototransistor then drives the base of the second
transistor to a high level, bringing its output, or
collector, to a low level.
The operation of the DC output model is similar to
the DC input module. A 5VDC signal from an
indexer is used to activate an LED. The output of
the module is defined as open collector.
Fig. 5.4 represents a typical DC output schematic.
Note the diode across the relay coil. These should
always be installed to eliminate the leading inductive
kick caused by the relay. A typical part number for
such a diode is 1N4004. Failure to provide this
protection can cause noise problems or the
destruction of the output device.
Fig. 5.4 Typical DC output connection diagram
Freewheeling
Diode
Indicator
+5VDC
(+)
Amplifier
Logic
(equivalent
circuit)
A48
LED
Photo
Transistor
(-)
Output
Transistor
Load 10
(solenoid)
+
24VDC
Screw
Terminals
2.4 Amps
Control Systems
voltage to DC levels. AC input modules also include
transient protection to filter out spikes from the AC
line (caused by lightning strikes, arc welders, etc.).
AC modules are not polarized devices. This makes
it virtually impossible to install a unit backwards.
AC input modules operate like DC input modules
with the addition of a bridge rectifier to change AC
Fig. 5.5 Typical AC input connection diagram
Pushbutton
Indicator
Rectifying
Bridge
+5VDC
LED
8mA
Logic Signal
Signal
Conditioning
Screw
Terminals
Photo
Transistor
14K
120 VAC
60Hz
Power Line
Ground
AC output modules feature a Triac power device as
its output. A Triac output offers three distinct
advantages.
1. Zero voltage turn on eliminates in-rush currents
to the load.
2. Zero voltage turn off eliminates inductive kick
problems.
3. A snubber across the output.
Fig. 5.6 Zero voltage turn on and off
A
B
C
The module will only turn on or off at points A, B, or
C; where the voltage is zero.
AC output modules do have leakage current, which
may “turn-on” small current loads. To solve
potential problems, add a parallel resistor across
the load, 5K, 5W for 120VAC and a 10K, 10W for
240VAC.
Fig. 5.7 Typical AC output connection diagram
Indicator
+5VDC
C
Zero
Voltage
Circuit
Logic
(equivalent
circuit)
LED
Photo
Transistor
4KV Isolation Barrier
Load
(motor)
R
Triac
Snubber
Screw
Terminals
120VAC
60Hz
Power Line
A49
A Engineering Reference
AC Input and Output Modules
Control Systems
Serial and Parallel Communications
Serial and parallel communications are methods of
transferring data from a host computer to a
peripheral device such as a Compumotor indexer.
In the case of a Compumotor indexer, the data
consist of parameters such as acceleration,
Fig. 5.8 Serial Communications
Data bits
velocity, move distance, and move direction
configured in ASCII characters. Both
communication techniques are generally bidirectional allowing the host to both transmit and
receive information from a peripheral device.
Fig. 5.9 Parallel Communications
0
1
Data bus
0
Parity bit
Start bit
0
Stop bits
IEEE-488
IBM PC
VME Bus
STD Bus
Multi Bus
0
0
0
Time
(baud rate)
Serial
Serial communication transmits data one bit at a
time on a single data line. Single data bits are
grouped together into a byte and transmitted at a
predetermined interval (baud rate). Serial
communication links can be as simple as a 3-line
connection; transmit (Tx), receive (Rx) and ground
(G). This is an advantage from a cost standpoint,
but usually results in slower communications than
parallel communications. Common serial interfaces
include RS-232C, RS-422, RS-485, RS-423.
Troubleshooting
Procedure for troubleshooting 3-wire RS-232C
communication.
1. Verify that the transmit of the host is wired to the
receive of the peripheral, and receive of the host
is wired to the transmit of the peripheral. Note:
Try switching the receive and transmit wires on
either the host or peripheral if you fail to get any
communication.
2. Some serial ports require handshaking. You can
establish 3-wire communication by jumpering
RTS to CTS (usually pins 4 and 5) and DSR to
DTR (usually pins 6 and 20).
3. Configure the host and peripheral to the same
baud rate, number of data bits, number of stop
bits, and parity.
4. If you receive double characters (e.g., typing “A”
and receiving “AA”), your computer is set to half
duplex mode. Change to full duplex mode.
5. Use DC common or signal ground as your
reference, NOT earth ground.
6. Cable lengths should not exceed 50 ft. unless
you are using some form of line driver, optical
coupler, or shield. As with any control signal, be
sure to shield the cable to earth ground at one
end only.
7. To test terminal or terminal emulation software
for proper 3-wire communication, unhook the
peripheral device and transmit a character. An
echoed character should not be received. If a
character is received, you are in half duplex
mode. Jumper the host’s transmit and receive
lines and send another character. You should
receive the echoed character. If not, consult the
manufacturer of the host’s serial interface for
proper pin outs.
A50
1
Signals
A = 0100
1 = 0011
0001
0001
Parallel
Parallel communication requires handshaking and
transmits data one byte (8 bits) at a time. When
data are transferred from the host processor to a
peripheral device, the following steps take place.
1. The host sets a bit on the bus signalling to the
peripheral that a byte of data has been sent.
2. The peripheral receives data and sets a bit on
the bus, signalling to the host that data have
been received.
The advantage of communicating in parallel vs.
serial is faster communications. However, since
parallel communications require more
communication lines, the cost can be higher than
serial communications.
Parallel bus structures include:
IEEE-488, IBM PC, VME, MULTIBUS, Q and STD.
Troubleshooting
Procedure for troubleshooting parallel
communication.
1. Make certain the address setting of the
peripheral device is configured properly.
2. Confirm that multiple boards are not set to the
same address (and each board is sealed
properly into a slot).
3. Verify that peripheral subroutines to reset the
board, write data, and read data work properly.
Follow the handshaking procedure outlined in
the device’s user manual.
Note: Compumotor bus-based indexers come
complete with a diskette that includes pretested
programs to verify system functions and
routines for simple user program development.
Control Systems
Serial and Parallel Communications
NULL MODEM: A simple device or set of
connectors that switches the receive and transmit
lines a 3-wire RS-232C connector.
PARITY: An RS-232C error detection scheme that
can detect an odd number of transmission errors.
SERIAL POLLING: Method of checking the status
of the IEEE-488 device. By reading the status byte,
the host can determine if the device is ready to
receive or send characters.
START BITS: When using RS-232C, one or two
bits are added to every character to signal the end
of a character.
TEXT/ECHO (ON/OFF): This setup allows received
characters to be re-transmitted back to the original
sending device. Echoing characters can be used to
verify or “close the loop” on a transmission.
XON/XOFF: Two ASCII characters supported in
some serial communication programs. If supported,
the receiving device transmits an XOFF character to
the host when its character buffer is full. The XOFF
character directs the host to stop transmitting
characters to the device. Once the buffer empties,
the device will transmit an XON character to signal
the host to resume transmission.
A Engineering Reference
ADDRESS: Multiple devices are controlled on the
same bus, each with a separate address or unit
number. This address allows the host to
communicate individually to each device.
ASCII: American Standard Code for Information
Interchange. This code assigns a number to each
numeral and letter of the alphabet. In this manner,
information can be transmitted between machines
as a series of binary numbers.
BAUD RATE: Number of bits transmitted per
second. Typical rates include 300; 600; 1,200;
2,400; 4,800; 9,600, 19,200. This means at 9,600
baud, 1 character can be sent nearly every
millisecond.
DATA BITS: Since the ASCII set consists of 128
characters, computers may transmit only 7 bits of
data. Most computers do, however, support an 8bit extended ASCII character set.
DCE: Data Communications Equipment transmits
on pin 3 and receives on pin 2.
DTE: Data Terminal Equipment. Transmits on pin 2
and receives on pin 3.
FULL DUPLEX: The terminal will display only
received or echoed characters.
HALF DUPLEX: In half duplex mode, a terminal will
display every character transmitted. It may also
display the received character.
HANDSHAKING SIGNALS:
RTS: Request To Send DTR: Data Terminal Ready
CTS: Clear To Send
IDB: Input Data Buffer
DSR: Data Set Ready
ODB: Output Data Buffer
ASCII Table
DEC
HEX
GRAPHIC
DEC
HEX
GRAPHIC DEC
000
001
002
003
004
005
006
007
008
009
010
011
012
013
014
015
016
017
018
019
020
021
022
023
024
025
026
027
028
029
00
01
02
03
04
05
06
07
08
09
0A
0B
0C
0D
0E
0F
10
11
12
13
14
15
16
17
18
19
1A
1B
1C
1D
NUL
SOH
STX
ETX
EOT
ENQ
ACK
BEL
BS
HT
LF
VT
FF
CR
SO
S1
DLE
DC1
DC2
DC3
DC4
NAK
SYN
ETB
CAN
EM
SUB
ESC
FS
GS
030
031
032
033
034
035
036
037
038
039
040
041
042
043
044
045
046
047
048
049
050
051
052
053
054
055
056
057
058
IE
1F
20
21
22
23
24
25
26
27
28
29
2A
2B
2C
2D
2E
2F
30
31
32
33
34
35
36
37
38
39
3A
RS
US
SPACE
!
"
#
$
%
&
'
(
)
*
+
,
.
/
0
1
2
3
4
5
6
7
8
9
:
059
060
061
062
063
064
065
066
067
068
069
070
071
072
073
074
074
075
076
077
078
080
081
082
083
084
085
086
087
HEX
3B
3C
3D
3E
3F
40
41
42
43
44
45
46
47
48
49
4A
4B
4C
4D
4E
4F
50
51
52
53
54
55
56
57
GRAPHIC
;
<
=
>
?
@
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
DEC
HEX
GRAPHIC DEC
HEX
088
089
090
091
092
093
094
095
096
097
098
099
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
58
59
5A
5B
5C
5D
5E
5F
60
61
62
63
64
65
66
67
68
69
6A
6B
6C
6D
6E
6F
70
71
72
73
74
X
Y
Z
[
/
]
V
75
76
77
78
79
7A
7B
7C
7D
7E
7F
117
118
119
120
121
122
123
124
125
126
127
’
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
p
q
r
s
t
A51
GRAPHIC
u
v
w
x
y
z
{
I
}
~
DEL
Control Systems
Electrical Noise . . .
Sources, Symptoms and Solutions
Noise related difficulties can range in severity from
minor positioning errors to damaged equipment
from runaway motors crashing blindly through limit
switches. In microprocessor controlled equipment,
the processor is constantly retrieving instructions
from memory in a controlled sequence. If an
electrical disturbance occurs, it could cause the
processor to misinterpret an instruction, or access
the wrong data. This is likely to be catastrophic to
the program, requiring a processor reset. Most
Compumotor indexers are designed with a
watchdog timer that shuts down the system if the
program is interrupted. This prevents the more
catastrophic failures.
Sources of Noise
Being invisible, electrical noise can be very
mysterious, but it invariably comes from the
following sources:
•
•
•
•
Power line disturbances
Externally conducted noise
Transmitted noise
Ground loops
Externally Conducted Noise
Externally conducted noise is similar to power line
noise, but the disturbances are created on signal
and ground wires connected to the indexer. This
kind of noise can get onto logic circuit ground or
into the processor power supply and scramble the
program. The problem here is that control
equipment often shares a common DC ground that
may run to several devices, such as a DC power
supply, programmable controller, remote switches
and the like. When some noisy device, particularly a
relay or solenoid, is on the DC ground, it may cause
disturbances within the indexer.
The solution for DC mechanical relays and
solenoids involves connecting a diode backwards
across the coil to clamp the induced voltage “kick”
that the coil will produce. The diode should be rated
at 4 times the coil voltage and 10 times the coil
current. Using solid-state relays eliminates this
effect altogether.
Fig. 5.11 Coil Suppression Methods
Diode
Some common electrical devices generate
electrical noise.
DC
• Coil driven devices: conducted and power line
noise
• SCR-fired heaters: transmitted and power line
noise
• Motors and motor drives: transmitted and power
line noise
• Welders (electric): transmitted and power line
noise
Power line disturbances are usually easy to solve
due to the wide availability of line filtering equipment
for the industry. Only the most severe situations call
for an isolation transformer. Line filtering equipment
is required when other devices connected to the
local power line are switching large amounts of
current, especially if the switching takes place at
high frequency. Corcom and Teal are two
manufacturers of suitable power line filters.
Also, any device having coils is likely to upset the
line when it is switched off. Surge suppressors such
as MOVs (General Electric) can limit this kind of
noise. A series RC network across the coil is also
effective, (resistance; 500 to 1,000 Ω, capacitance;
0.1 to 0.2µF). Coil-driven devices (inductive loads)
include relays, solenoids, contactors, clutches,
brakes and motor starters.
Fig. 5.10 Typical RC Network
Inductive
Load
C
A52
AC or DC
Multiple devices on the same circuit should be
grounded together at a single point.
Furthermore, power supplies and programmable
controllers often have DC common tied to Earth
(AC power ground). As a rule, it is preferable to
have indexer signal ground or DC common floating
with respect to Earth. This prevents noisy
equipment that is grounded to Earth from sending
noise into the indexer. The Earth ground
connection should be made at one point only as
discussed in “Ground Loops” on p. A53.
In many cases, optical isolation may be required to
completely eliminate electrical contact between the
indexer and a noisy environment. Solid-state relays
provide this isolation.
R
AC or DC
Varistor (MOV)
Transmitted Noise
Transmitted noise is picked up by external
connections to the indexer, and in severe cases,
can attack an indexer with no external connections.
The indexer enclosure will typically shield the
electronics from this, but openings in the enclosure
for connection and front panel controls may “leak”.
As with all electrical equipment, the indexer chassis
should be scrupulously connected to Earth to
minimize this effect.
When high current contacts open, they draw an
arc, producing a burst of broad spectrum radio
frequency noise that can be picked up on an
indexer limit switch or other wiring. High current and
high voltage wires have an electrical field around
them, and may induce noise on signal wiring
(especially when they are tied in the same wiring
bundle or conduit).
When this kind of problem occurs, consider
shielding signal cables or isolating the signals. A
proper shield surrounds the signal wires to intercept
electrical fields, but this shield must be tied to Earth
to drain the induced voltages. At the very least,
wires should be run in twisted pairs to limit straight
line antenna effects.
Most Compumotor cables have shields tied to
Earth, but in some cases the shields must be
grounded at installation time. Installing the indexer
in a NEMA electrical enclosure ensures protection
from this kind of noise, unless noise-producing
equipment is also mounted inside the enclosure.
Connections external to the enclosure must be
shielded.
Even the worst noise problems, in environments
near 600 amp welders and 25kW transmitters, have
been solved using enclosures, conduit, optical
isolation, and single-point ground techniques.
Ground Loops
Ground Loops create the most mysterious noise
problems. They seem to occur most often in
systems where a control computer is using
RS-232C communication. Garbled transmission
and intermittent operation symptoms are typical.
The problem occurs in systems where multiple
Earth ground connections exist, particularly when
these connections are far apart.
Defeating Noise
The best time to handle electrical noise problems is
before they occur. When a motion system is in the
design process, the designer should consider the
following system wiring guidelines (listed by order of
importance).
1. Put surge suppression components on all
electrical coils: resistor/capacitor filters, MOVs,
Zener and clamping diodes.
2. Shield all remote connections and use twisted
pairs. Shields should be tied to Earth at one
end.
3. Put all microelectronic components in an
enclosure. Keep noisy devices outside. Monitor
internal temperature.
4. Ground signal common wiring at one point.
Float this ground from Earth if possible.
5. Tie all mechanical grounds to Earth at one point.
Run chassis and motor grounds to the frame,
frame to Earth.
6. Isolate remote signals. Solid-state relays or opto
isolators are recommended.
7. Filter the power line. Use common RF filters
(isolation transformer for worst-case situations).
A noise problem must be identified before it can be
solved. The obvious way to approach a problem
situation is to eliminate potential noise sources until
the symptoms disappear, as in the case of ground
loops. When this is not practical, use the above
guidelines to troubleshoot the installation.
References
Information about the equipment referred to may be
obtained by calling the numbers listed below.
• Corcom line filters (312) 680-7400
• Opto-22 optically isolated relays (714) 891-5861
• Crydom optically isolated relays (213) 322-4987
• Potter Brumfield optically isolated relays
(812) 386-1000
• General Electric MOVs (315) 456-3266
• Teal power line isolation filters (800) 888-8325
Example
Suppose a Model 500 is controlling an axis, and the
limit switches use an external power supply. The
Model 500 is controlled by a computer in another
room. If the power supply Common is connected to
Earth, ground loop problems may occur (most
computers have their RS-232C signal common tied
to Earth). The loop starts at the Model 500’s limit
switch ground, goes to Earth through the power
supply to Earth at the computer. From there, the
loop returns to the Model 500 through RS-232C
signal ground. If a voltage potential exists between
power supply Earth and remote computer Earth,
ground, current will flow through the RS-232C
ground creating unpredictable results.
The way to test for and ultimately eliminate a ground
loop is to lift or “cheat” Earth ground connections in
the system until the symptoms disappear.
A53
A Engineering Reference
Control Systems
Control Systems
Stopping in an Emergency
For safety reasons, it is often necessary to
incorporate some form of emergency stop system
into machinery fitted with stepper or servo motors.
There are several reasons for needing to stop
quickly.
• To prevent injury to the operator if he makes a
mistake or operates the machinery improperly.
• To prevent damage to the machine or to the
product as a result of a jam.
• To guard against machine faults. You should
consider all the possible reasons for stopping to
make sure that they are adequately covered.
How should you stop the system?
There are several ways to bring a motor to a rapid
stop. The choice depends partly on whether it is
more important to stop in the shortest possible time
or to guarantee a stop under all circumstances. For
instance, to stop as quickly as possible means
using the decelerating power of the servo system.
However, if the servo has failed or control has been
lost, this may not be an option open to you. In this
case, removing the power will guarantee that the
motor stops; but if the load has a high inertia, this
may take some time. If the load is moving vertically
and can back-drive the motor, this introduces
additional complications. In extreme cases where
personal safety is at risk, it may be necessary to
mechanically lock the system even at the expense
of possible damage to the machine.
Emergency Stop Methods
1. Full-torque controlled stop.
Applying zero velocity command to a servo amplifier
will cause it to decelerate hard to zero speed in
current limit, in other words, using the maximum
available torque. This will create the fastest possible
deceleration to rest. In the case of a digital servo
with step and direction inputs, cutting off the step
pulses will produce the same effect.
The situation is different for a stepper drive. The
step pulse train should be decelerated to zero
speed to utilize the available torque. Simply cutting
off the step pulses at speeds above the start-stop
rate will de-synchronize the motor and the full
decelerating torque will no longer be available. The
controller needs to be able to generate a rapid
deceleration rate independent of the normal
programmed rate, to be used only for overtravel
limit and emergency stop functions.
2. Disconnect the motor.
Although this method is undoubtedly safe, it is not
highly recommended as a quick-stop measure. The
time taken to stop is indeterminate, since it
depends on load inertia and friction, and in highperformance systems the friction is usually kept to a
minimum. Certain types of drives may be damaged
by disconnecting the motor under power. This
method is particularly unsatisfactory in the case of a
vertical axis, since the load may fall under gravity.
A54
3. Remove the AC input power from the drive.
On drives that incorporate a power dump circuit, a
degree of dynamic braking is usually provided when
the power is removed. This is a better solution than
disconnecting the motor, although the power
supply capacitors may take some time to decay
and this will extend the stopping distance.
4. Use dynamic braking.
A motor with permanent magnets will act as a
generator when driven mechanically. By applying a
resistive load to the motor, a braking effect is
produced that is speed-dependent. Deceleration is
therefore rapid at high speeds, but falls off as the
motor slows down.
A changeover contactor can be arranged to switch
the motor connections from the drive to the
resistive load. This can be made failsafe by ensuring
that braking occurs if the power supply fails. The
optimum resistor value depends on the motor, but
will typically lie in the 1-3 ohms range. It must be
chosen to avoid the risk of demagnetization at
maximum speed as well as possible mechanical
damage through excessive torque.
5. Use a mechanical brake.
It is very often possible to fit a mechanical brake
either directly on the motor or on some other part of
the mechanism. However, such brakes are usually
intended to prevent movement at power-down and
are seldom adequate to bring the system to a rapid
halt, particularly if the drive is delivering full current
at the time. Brakes can introduce friction even when
released, and also add inertia to the system – both
effects will increase the drive power requirements.
What is the best stopping method?
It is clear that each of the methods outlined above
has certain advantages and drawbacks. This leads
to the conclusion that the best solution is to use a
combination of techniques, ideally incorporating a
short time delay.
We can make use of the fact that a contactor used
for dynamic braking will take a finite time to drop
out, so it is possible to de-energize the contactor
coil while commanding zero speed to the drive. This
allows for a controlled stop to occur under full
torque, with the backup of dynamic braking in the
event that the amplifier or controller has failed.
WARNING! – there is a risk that decelerating a
servo to rest in full current limit could result in
mechanical damage, especially if a high-ratio
gearbox is used. This does not necessarily ensure a
safe stop, be sure that the mechanism can
withstand this treatment.
A mechanical brake should also be applied to a
vertical axis to prevent subsequent movement. An
alternative to the electrically-operated brake is the
differential drag brake, which will prevent the load
from falling but creates negligible torque in the
opposite direction.
System Selection
System
Considerations
Calculations
Accuracy
An accuracy specification defines the maximum
error in achieving a desired position. Some types of
accuracy are affected by the application. For
example, repeatability will change with the friction
and inertia of the system the motor is driving.
Accuracy in a rotary motor is usually defined in
terms of arcminutes or arcseconds (the terms
arcsecond and arcminute are equivalent to second
and minute, respectively). There are 1,296,000
seconds of arc in a circle. For example, an
arcsecond represents 0.00291 inches of
movement on a circle with a 50-foot radius. This is
equivalent to about the width of a human hair.
Stepper Accuracy
Servo & Closed-Loop Stepper Accuracy
There are several types of performance listed under
Compumotor’s motor specifications: repeatability,
accuracy, relative accuracy, and hysteresis.
Repeatability, accuracy and relative accuracy in
servos and closed-loop stepper systems relate as
much to their feedback mechanisms as they do to
the inherent characteristics of the motor and drive.
Repeatability
The motor’s ability to return to the same position
from the same direction. Usually tested by moving
the motor one revolution, it also applies to linear
step motors moving to the same place from the
same direction. This measurement is made with the
motor unloaded, so that bearing friction is the
prominent load factor. It is also necessary to ensure
the motor is moving to the repeat position from a
distance of at least one motor pole. This
compensates for the motor’s hysteresis. A motor
pole in a Compumotor is 1/50 of a revolution.
Accuracy
Also referred to as absolute accuracy, this
specification defines the quality of the motor’s
mechanical construction. The error cancels itself
over 360° of rotation, and is typically distributed in a
sinusoidal fashion. This means the error will
gradually increase, decrease to zero, increase in the
opposite direction and finally decrease again upon
reaching 360° of rotation. Absolute accuracy
causes the size of microsteps to vary somewhat
because the full motor steps that must be traversed
by a fixed number of microsteps varies. The steps
can be over or undersized by about 4.5% as a
result of absolute accuracy errors.
Relative Accuracy
Also referred to as step-to-step accuracy, this
specification tells how microsteps can change in
size. In a perfect system, microsteps would all be
exactly the same size, but drive characteristics and
the absolute accuracy of the motor cause the steps
to expand and contract by an amount up to the
relative accuracy figure. The error is not cumulative.
Servos
Compumotor servos use resolver feedback to
determine their resolution and position. It is
essentially the resolution of the device reading the
resolver position that determines the highest
possible accuracy in the system. Digiplan servos
use encoder feedback to determine their resolution
and position. In this case, it is the encoder’s
resolution that determines the system’s accuracy.
The positional accuracy is determined by the drive’s
ability to move the motor to the position indicated
by the resolver or encoder. Changes in friction or
inertial loading will adversely affect the accuracy
until the system is properly tuned.
Closed-Loop Steppers
Compumotor closed-loop stepper systems use an
encoder to provide feedback for the control loop.
The encoder resolution determines the system’s
accuracy. When enabled, the controlling indexer
attempts to position the motor within the specified
deadband from the encoder. Typically, this means
the motor will be positioned to within one encoder
step. To do this satisfactorily, the motor must have
more resolution than the encoder. If the step size of
the motor is equal to or larger than the step size of
the encoder, the motor will be unable to maintain a
position and may become unstable. In a system
with adequate motor-to-encoder resolution, the
motor is able to maintain one encoder step of
accuracy with great dependability. This is a
continuous process that will respond to outside
events that disturb the motor’s position.
Hysteresis
The motor’s tendency to resist a change in
direction. This is a magnetic characteristic of the
motor, it is not due to friction or other external
factors. The motor must develop torque to
overcome hysteresis when it reverses direction. In
reversing direction, a one revolution move will show
hysteresis by moving the full distance less the
hysteresis figure.
A55
A Engineering Reference
Application Considerations
Selection
Considerations
System Calculations
Application Considerations
Load characteristics, performance requirements,
and coupling techniques need to be understood
before the designer can select the best motor/drive
for the job. While not a difficult process, several
factors need to be considered for an optimum
solution. A good designer will adjust the
characteristics of the elements under his control –
including the motor/drive and the mechanical
transmission type (gears, lead screws, etc.) – to
meet the performance requirements. Some
important parameters are listed below.
Torque
Rotational force (ounce-inches) defined as a linear
force (ounces) multiplied by a radius (inches). When
selecting a motor/drive, the torque capacity of the
motor must exceed the load. The torque any motor
can provide varies with its speed. Individual speed/
torque curves should be consulted by the designer
for each application.
Inertia
An object’s inertia is a measure of its resistance to
change in velocity. The larger the inertial load, the
longer it takes a motor to accelerate or decelerate
that load. However, the speed at which a motor
rotates is independent of inertia. For rotary motion,
inertia is proportional to the mass of the object
being moved times the square of its distance from
the axis of rotation.
Friction
All mechanical systems exhibit some frictional force,
and this should be taken into account when sizing
the motor, as the motor must provide torque to
overcome any system friction. A small amount of
friction is desirable since it can reduce settling time
and improve performance.
Torque-to-Inertia Ratio
This number is defined as a motor’s rated torque
divided by its rotor inertia. This ratio is a measure of
how quickly a motor can accelerate and decelerate
its own mass. Motors with similar ratings can have
different torque-to-inertia ratios as a result of
varying construction.
Load Inertia-to-Rotor Inertia Ratio
For a high performance, relatively fast system, load
inertia reflected to the motor should generally not
exceed the motor inertia by more than 10 times.
Load inertias in excess of 10 times the rotor inertia
can cause unstable system behavior.
Torque Margin
Whenever possible, a motor/drive that can provide
more motor torque than the application requires
should be specified. This torque margin
accommodates mechanical wear, lubricant
hardening, and other unexpected friction.
Resonance effects, while dramatically reduced with
the Compumotor microstepping system, can cause
the motor’s torque to be slightly reduced at some
speeds. Selecting a motor/drive that provides at
least 50% margin above the minimum needed
torque is good practice.
A56
Velocity
Because available torque varies with velocity,
motor/drives must be selected with the required
torque at the velocities needed by the application.
In some cases, a change in the type of mechanical
transmission used is needed to achieve the
required performance.
Resolution
The positioning resolution required by the
application will have an effect on the type of
transmission used and the motor resolution. For
instance, a leadscrew with 4 revolutions per inch
and a 25,000-step-per-revolution motor/drive
would give 100,000 steps per inch. Each step
would then be 0.00001 inches.
Duty Cycle
Some motor/drives can produce peak torque for
short time intervals as long as the RMS or average
torque is within the motor’s continuous duty rating.
To take advantage of this feature, the application
torque requirements over various time intervals
need to be examined so RMS torque can be
calculated.
Solving Duty Cycle Limitation Problems
Operating a motor beyond its recommended duty
cycle results in excessive heat in the motor and
drive. The drive cycle may be increased by
providing active cooling to the drive and the motor.
A fan directed across the motor and another
directed across the drive’s heatsink will result in
increased duty cycle capability.
In most cases, it is possible to tell if the duty cycle is
being exceeded by measuring the temperature of
the motor and drive. Refer to the specifications for
individual components for their maximum allowable
temperatures.
Note: Motors will run at case temperatures up to
100°C (212°F)—temperatures hot enough to burn
individuals who touch the motors.
To Improve Duty Cycle:
• Mount the drive with heatsink fins running
vertically
• Fan cool the motor
• Fan cool the drive
• Put the drive into REMOTE POWER SHUTDOWN
when it isn’t moving, or reduce current
• Reduce the peak current to the motor
(if possible)
• Use a motor large enough for the application
A wide range of applications can be solved
effectively by more than one motor type. However,
some applications are particularly appropriate for
each motor type. Compumotor’s Motor Sizing and
Selection Software package is designed to help
you easily identify the proper motor size and type
for your specific motion control application.
torque curves that are based on user-provided
information. This advanced graphics package is
VGA/EGA compatible and allows data entry with
either a mouse or keypad.
Contact your local Automotive Technology Center
to obtain a copy.
This software helps calculate load inertias and
required torques—information that is reflected
through a variety of machine transmissions and
reductions, including leadscrews, gears, belts, and
pulleys. This software then produces graphs of the
results, allowing you to select the proper motor
from a comprehensive, detailed database of more
than 200 motor models.
IBM ® PC-compatible, Motor Sizing & Selection
software also generates a number of applicationspecific reports, including profiles and speed/
A57
A Engineering Reference
Motor Sizing andSystem
Selection
Calculations
Software
System Calculations
Move Profile
Before calculating torque requirements of an
application, you need to know the velocities and
accelerations needed. For those positioning
applications where only a distance (X) and a time
(S) to move that distance are known, the
trapezoidal motion profile and formulas given below
are a good starting point for determining your
requirements. If velocity and acceleration
parameters are already known, you can proceed to
one of the specific application examples on the
following pages.
Move distance X in time S.
Assume that:
1. Distance X/4 is moved in time S/3 (Acceleration)
2. Distance X/2 is moved in time S/3 (Run)
3. Distance X/4 is moved in time S/3 (Deceleration)
The acceleration (a), velocity (v) and deceleration (d)
may be calculated in terms of the knowns, X and S.
( X4 )
( S3 )
2
a = -d = 2X
=
t2
2
v = at = 4.5X x S
S2
3
X
= 2 2x 9 = 4.5X
S
S2
= 1.5X
S
Example
You need to move 6" in 2 seconds
a = -d =
inches
4.5 (6 inches)
= 6.75
second2
(2 seconds)2
The graph would appear as follows:
v = 1.5 (6 inches) = 4.5 inches
(2 seconds)
second
Velocity
V
a
0
d
S/3
S/3
2S/3
S/3
S
time
S/3
Common Move Profile Considerations
Distance: ______________ Inches of Travel ________________________
revolutions of motor
Move Time: ____________________________________________________
seconds
Accuracy: ______________________________________________________
arcminutes, degrees or inches
Repeatability: ___________________________________________________
arcseconds, degrees or inches
Duty Cycle
on tme: ____________________________________________________
off time: ___________________________________________________
seconds
seconds
Cycle Rate: ____________________________________________________
sec. min. hour
Motor/Drive Selection
Based on Continuous Torque Requirements
Having calculated the torque requirements for an
application, you can select the motor/drive suited
to your needs. Microstepping motor systems
(S Series, Zeta Series OEM650 Series, LN Series)
have speed/torque curves based on continuous
duty operation. To choose a motor, simply plot total
torque vs. velocity on the speed/torque curve. This
point should fall under the curve and allow
approximately a 50% margin for safety. An S106178 and an S83-135 curve are shown here.
Note: When selecting a ZETA Series product, a
50% torque margin is not required.
Example
Assume the following results from load calculations:
TF = 25 oz-in
Friction torque
T
A = 175 oz-in Acceleration torque
TT = 200 oz-in Total torque
V = 15 rev/sec Maximum velocity
You can see that the total torque at the required
velocity falls within the motor/drive operating range
for both motors by plotting TT .
A58
oz-in
1500
1200
900
106-178
600
83-135
300
TT = 200
0
0
10
20
30
40
50
RPS (Vmax)
The S83-135 has approximately 250 oz-in available
at V max (25% more than required). The S106-178
has 375 oz-in available, an 88% margin.
In this case, we would select the S106-178
motor/drive to assure a sufficient torque margin
to allow for changing load conditions.
System Calculations
Based on peak torque requirements
Servo-based motor/drives have two speed/torque
curves: one for continuous duty operation and
another for intermittent duty. A servo system can
be selected according to the total torque and
maximum velocity indicated by the continuous duty
curve. However, by calculating the root mean
square (RMS) torque based on your duty cycle, you
may be able to take advantage of the higher peak
torque available in the intermittent duty range.
Duty Cycle
Index 4 revs in 0.3 seconds, dwell 0.3 seconds
then repeat.
If you look at the S106-178 speed/torque curve,
you’ll see that the requirements fall outside the
curve.
T1 =
Torque reqired to accelerate the load from
zero speed to maximum speed (TF + TA)
Torque required to keep the motor moving
once it reaches max speed (TF)
Torque required to decelerate from max
speed to a stop (TA - TF)
Torque required while motor is sitting still at
zero speed (Ø)
Time spent accelerating the load
Time spent while motor is turning at
constant speed
Time spent decelerating the load
Time spent while motor is at rest
T2 =
T3 =
MM
Ti2 ti
ti
TRMS =
T4 =
Where:
•
M
• Ti is the torque required over the time interval ti
means “the sum of”
TF
= 25 oz-in
Friction Torque
TA
= 775 oz-in
Acceleration Torque
TT
= 800 oz-in
Total Torque
Vmax = 20 rps
t1 =
t2 =
t3 =
t4 =
Example
Assume the following results from your load
calculations.
A Engineering Reference
Motor/Drive Selection
T12t1 + T22 t 2 + T32 t3 + T42 t 4
TRMS =
t1 + t2 + t3 + t4
(800)2(.1) + (25)2(.1) + (750)2(.1) + (0)2(.3)
=
(.1) + (.1) + (.1) + (.3)
Maximum Velocity
TRMS = 447 oz. in.
Now plot TRMS and TT vs. Tmax on the speed/torque
curve.
The drawing below resembles the
speed/torque curve for the Z606 motor.
Motion Profile
20 rps
1800
0
0.1
t1
0.2
t2
0.3
t3
0.4
0.5
0.6 t
T T (800)
t4
600
T RMS (506)
10
20
30
40
50
60
The Z606 motor will meet the requirements.
RMS torque falls within the continuous duty
cycle and total torque vs. velocity falls within
the intermittent range.
How to Use a Step Motor
Horsepower Curve
Applications requiring the most power the motor can
generate, not the most torque, should use a motor
speed that is just below the peak of the HP curve.
oz-in (N-m)
175 (1.22)
(HP)
.175
Torque
140 (1.98)
.140
Torque
105 (.73)
.105
70 (.49)
.070
Horsepower
35 (.24)
Power
Horsepower (HP) gives an indication of the motor’s
top usable speed. The peak or “hump” in a
horsepower curve indicates a speed that gives
maximum power. Choosing a speed beyond the peak
of the HP curve results in no more power: the power
attained at higher speeds is also attainable at a lower
speed. Unless the speed is required for the
application, there is little benefit to going beyond the
peak as motor wear is faster at higher speeds.
.035
0
0
0
6
12
18
24
30
Speed-rps
A59
System Calculations
Leadscrew Drives
Leadscrews convert rotary motion to linear motion
and come in a wide variety of configurations.
Screws are available with different lengths,
diameters, and thread pitches. Nuts range from the
simple plastic variety to precision ground versions
with recirculating ball bearings that can achieve very
high accuracy.
The combination of microstepping and a quality
leadscrew provides exceptional positioning
resolution for many applications. A typical 10-pitch
(10 threads per inch) screw attached to a 25,000
step/rev. motor provides a linear resolution of
0.000004" (4 millionths, or approximately 0.1
micron) per step.
A flexible coupling should be used between the
leadscrew and the motor to provide some damping.
The coupling will also prevent excessive motor
bearing loading due to any misalignment.
Microscope Positioning
Application Type: X/Y Point to Point
Motion: Linear
Description: A medical research lab needs to
automate their visual inspection process. Each
specimen has an origin imprinted on the slide with
all other positions referenced from that point. The
system uses a PC-AT Bus computer to reduce data
input from the operator, and determines the next
data point based on previous readings. Each data
point must be accurate to within 0.1 microns.
Machine Objectives
• Sub-micron positioning
• Specimen to remain still during inspection
• Low-speed smoothness (delicate equipment)
• Use PC-AT Bus computer
Motion Control Requirements
• High resolution, linear encoders
• Stepper (zero speed stability)
• Microstepping
• PC-AT Bus controller
Compumotor Solution: Microstepping motors
and drives, in conjunction with a precision ground
40 pitch leadscrew table, provide a means of submicron positioning with zero speed stability.
Conventional mechanics cannot provide 0.1 micron
accuracies without high grade linear encoders. It is
necessary for the Compumotor Model AT6400
indexer, which resides directly on the computer
bus, to provide full X, Y, Z microscope control and
accept incremental encoder feedback.
Microstepping
motors
Encoders
A60
System Calculations
Other Leadscrew Drive Applications
• XY Plotters
• Facsimile transmission
• Tool bit positioning
• Cut-to-length machinery
• Back gauging
• Microscope drives
• Coil winders
• Slides
• Pick-and-Place machines
• Articulated arms
A Engineering Reference
Precision Grinder
A bearing manufacturer is replacing some
equipment that finishes bearing races. The old
equipment had a two-stage grinding arrangement
where one motor and gearbox provided a rough cut
and a second motor with a higher ratio gearbox
performed the finishing cut. The designer would like
to simplify the mechanics and eliminate one motor.
He wants to use a single leadscrew and exploit the
wide speed range available with microstepping to
perform both cuts. This will be accomplished by
moving a cutting tool mounted on the end of the
leadscrew into the workpiece at two velocities; an
initial velocity for the rough cut and a much reduced
final velocity for the finish cut.
The torque required to accelerate the load and
overcome the inertia of the load and the rotational
inertia of the leadscrew is determined to be 120 ozin. The torque necessary to overcome friction is
measured with a torque wrench and found to be 40
oz-in. A microstepping motor with 290 oz-in of
torque is selected and provides adequate torque
margin.
This grinder is controlled by a programmable
controller (PC) and the environment requires that
the electronics withstand a 60°C environment. An
indexer will provide the necessary velocities and
accelerations. The speed change in the middle of
the grinding operation will be signaled to the PC
with a limit switch, and the PC will in turn program
the new velocity into the indexer. Additionally, the
indexer Stall Detect feature will be used in
conjunction with an optical encoder mounted on
the back of the motor to alert the PC if the
mechanics become “stuck.”
Leadscrew Application Data
Inertia of Leadscrews per Inch
Diameter
In.
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
2.25
2.50
Steel
0.0017
0.0275
0.1392
0.4398
1.0738
2.2266
4.1251
7.0372
11.2723
17.1807
Brass
0.0018
0.0295
0.1491
0.4712
1.1505
2.3857
4.4197
7.5399
12.0774
18.4079
Diameter
Alum.
0.0006
0.0094
0.0478
0.1512
0.3691
0.7654
1.4180
2.4190
3.8748
5.9059
In.
2
oz-in
oz-in2
oz-in2
oz-in2
oz-in2
oz-in2
oz-in2
oz-in2
oz-in2
oz-in2
Coefficients of Static Friction Materials
(Dry Contact Unless Noted)
Steel on Steel
Steel on Steel (lubricated)
Aluminum on Steel
Copper on Steel
Brass on Steel
Teflon on Steel
2.75
3.00
3.25
3.50
3.75
4.00
4.25
4.50
4.75
5.00
Steel
25.1543
35.6259
49.0699
66.0015
86.9774
112.5956
143.4951
180.3564
223.9009
274.8916
Brass
Alum.
26.9510
38.1707
52.5749
70.7159
93.1901
120.6381
153.7448
193.2390
239.8939
294.5267
8.6468
12.2464
16.8678
22.6880
29.8985
38.7047
49.3264
61.9975
76.9659
94.4940
oz-in2
oz-in2
oz-in2
oz-in2
oz-in2
oz-in2
oz-in2
oz-in2
oz-in2
oz-in2
Leadscrew Efficiencies
µS
0.58
0.15
0.45
0.22
0.19
0.04
Type
Efficiency (%)
High
Median Low
Ball-nut
Acme with metal nut*
Acme with plastic nut
95
55
85
90
40
65
85
35
50
* Since metallic nuts usually require a viscous
lubricant, the coefficient of friction is both speed
and temperature dependent.
A61
System Calculations
Leadscrew Drives
Vertical or Horizontal Application:
ST – Screw type, ball or acme
e – Efficiency of screw
µS – Friction coefficient
L – Length ofscrew
D – Diameter of screw
p—Pitch
W – Weight of load
F—Breakaway force
Directly coupled to the motor?
If yes, CT – Coupling type
If no, belt & pulley or gears
Radius of pulley or gear
Gear: Number of teeth – Gear 1
Number of teeth – Gear 2
Weight of pulley or gear
Weight of belt
ST =
e=
µS =
L=
D=
p=
W=
F=
yes/no
%
inches
inches
threads/inch
lbs.
ounces
inches
ounces
ounces
Leadscrew Formulas
The torque required to drive load W using a
leadscrew with pitch (p) and efficiency (e) has the
following components:
TTotal = TFriction + TAcceleration
TFriction =
F
2πpe
Where:
F = frictional force in ounces
p = pitch in revs/in
e = leadscrew efficiency
F = µs W for horizontal surfaces where µs =
coefficient of static friction and W is the weight of
the load. This friction component is often called
“breakaway”.
Dynamic Friction: F = µD W is the coefficient to use
for friction during a move profile. However, torque
calculations for acceleration should use the worst
case friction coefficient, µs.
TAccel = 1 (J Load + JLeadscrew + JMotor) ω
g
t
ω = 2πpv
4
JLoad = w 2 ; JLeadscrew = πLρR
(2πp)
2
Where:
T = torque, oz-in
ω = angular velocity, radians/sec
t = time, seconds
v = linear velocity, in/sec
L = length, inches
R = radius, inches
ρ = density, ounces/in3
g = gravity constant, 386 in/sec2
The formula for load inertia converts linear inertia
into the rotational equivalent as reflected to the
motor shaft by the leadscrew.
Problem
Find the torque required to accelerate a 200-lb steel
load sliding on a steel table to 2 inches per second
in 100 milliseconds using a 5 thread/inch steel
leadscrew 36 inches long and 1.5 inches in
A62
diameter. Assume that the leadscrew has an Acme
thread and uses a plastic nut. Motor inertia is given
as 6.56 oz-in2. In this example, we assume a
horizontally oriented leadscrew where the force of
gravity is perpendicular to the direction of motion.
In non-horizontal orientations, leadscrews will
transmit varying degrees of influence from gravity to
the motor, depending on the angle of inclination.
Compumotor Sizing Software automatically
calculates these torques using vector analysis.
1. Calculate the torque required to overcome
friction. The coefficient of static friction for steel-tosteel lubricant contact is 0.15. The median value of
efficiency for an Acme thread and plastic nut is
0.65. Therefore:
(16 oz)
F = µsW = 0.15 (200 lb)
= 480 oz
lb
480 oz
F
TFriction = 2πpe = 2π x 5 rev x 0.65 = 23.51 oz-in
rev
in
2. Compute the rotational inertia of the load and the
rotational inertia of the leadscrew:
JLoad = W 2 = 200 lb x 16 oz = 3.24 oz-in2
(2πp)
lb
(2π5)2
in2
JLeadscrew = πLρR4 = π (36 in) (4.48 oz) (0.75 in)4
2
2
in3
= 80.16 oz-in2
3. The torque required to accelerate the load may
now be computed since the motor inertia was
given:
1
ω
TAccel = g (JLoad + JLeadscrew + JMotor) t
e
ω = 2π
( )( 2secin ) =
5
in
20π
sec
20π.
1
2
= 386 in/sec2 (4.99 + 80.16 + 6.56(oz-in )) sec
0.1sec
= 149 oz-in
TTotal = TFriction + TAccel
TTotal = 23.51 oz-in + 149 oz-in = 172.51 oz-in
System Calculations
There are many applications where the motion
being controlled is rotary and the low-speed
smoothness and high resolution of a Compumotor
system can be used to eliminate gear trains or other
mechanical linkages. In direct drive applications, a
motor is typically connected to the load through a
flexible or compliant coupling. This coupling
provides a small amount of damping and helps
correct for any mechanical misalignment.
Direct drive is attractive when mechanical simplicity
is desirable and the load being driven is of
moderate inertia.
Direct Drive Formulas
R2
R
L
R1
5.96
L
R – Radius
R(1) – Inner radius
R(2) – Outer radius
L – Length
W – Weight of disc
ρ – Density/Material
g – Gravity constant
R=
R(1) =
R(2) =
L=
W=
ρ=
g=
Solid Cylinder (oz-in2)
Inertia: JLoad = WR 2
2
Where weight and radius are known
4
Inertia (oz-in2) JLoad = πLρR
2
Where ρ, the material density is known
Weight W = πLρR2
Inertia may be calculated knowing either the weight
and radius of the solid cylinder (W and R) or its
density, radius and length (ρ, R and L.)
The torque required to accelerate any load is:
T (oz-in) = Ja
a = ω2 - ω1 = 2π (accel.) for Accel. in rps2
t
Hollow Cylinder
w
JLoad = 2 (R21 + R22)
Where W, the weight, is known
or
JLoad =
πLρ
2
(R42 – R41)
Where ρ, the density, is known
W=1
πLρ (R22 – R21)
ω
g
T=
(JLoad + JMotor) t
inches
inches
inches
inches
ounces
ounces/inch3
386 in/sec2
Where:
a = angular acceleration, radians/sec2
ω2 = final velocity, radians/sec
ω1 = initial velocity, radians/sec
t = time for velocity change, seconds
J = inertia in units of oz-in2
The angular acceleration equals the time rate of
change of the angular velocity. For loads
accelerated from zero, ω1 = 0 and a = ω
t
TTotal = 1 (JLoad + JMotor) ω
g
t
TTotal represents the torque the motor must deliver.
The gravity constant (g)
in the denominator
represents acceleration
due to gravity (386 in/
R
sec2) and converts
L
inertia from units of ozin2 to oz-in-sec2.
Problem
Calculate the motor torque required to accelerate a
solid cylinder of aluminum 5" in radius and 0.25"
thick from rest to 2.1 radians/sec (0.33 revs/sec) in
0.25 seconds. First, calculate JLoad using the
density for aluminum of 1.54 oz/in3.
4
4
JLoad = πLρR = π x 0.25 x 1.54 x 5 = 378 oz-in2
2
2
Assume the rotor inertia of the motor you will use is
37.8 oz-in2.
TTotal = 1
g
=
R2
R1
(J Load + JMotor) x ω
t
1 x (378 + 37.8) x 2.1
386
0.25
= 9.05 oz-in
L
5.96
A63
A Engineering Reference
Directly Driven Loads
System Calculations
Gear Drives
reflected back to the motor through the gearing is
divided by the square of the gear ratio.
Traditional gear drives are more commonly used
with step motors. The fine resolution of a
microstepping motor can make gearing
unnecessary in many applications. Gears generally
have undesirable efficiency, wear characteristics,
backlash, and can be noisy.
In this manner, large inertial loads can be moved
while maintaining a good load-inertia to rotor-inertia
ratio (less than 10:1).
Gears are useful, however, when very large inertias
must be moved because the inertia of the load
Gear Driven Loads
R – Radius
R(1) – Radius gear #1
R(2) – Radius gear #2
N(1) – Number of teeth G#1
N(2) – Number of teeth G#2
G – Gear ratio N(1)
N(2)
W – Weight of load
W(1) – Weight G#1
W(2) – Weight G#2
L – Length
F – Friction
BT – Breakaway torque
R=
R(1) =
R(2) =
N(1) =
N(2) =
G=
inches
inches
inches
W=
W(1) =
W(2) =
L=
F=
BT =
ounces
ounces
ounces
inches
ounce/inches
N1
G1
R
Gears
R1
W
R2
G2
N2
Gear Drive Formulas
W Load
NGear 2 2
2
JLoad =
R
Load
2
NGear 1
or
πLLoadρLoad 4
NGear 2
R Load
JLoad =
2
NGear 1
(
)
(
W
JGear1 = Gear1 R2Gear1
2
JGear2 =
TTotal =
A64
(
N1
NGear 2
NGear 1
G1
R
2
)
W
R2
G2
2
)
WGear2
R2Gear2
2
1
g
Gears
R1
(JLoad + JGear1 + JGear2 + JMotor) ω
t
N2
Where:
J
T
W
R
N
L
ρ
ω
t
g
=
=
=
=
=
=
=
=
=
=
inertia, oz-in (gm-cm 2) “as seen by the motor”
torque, oz-in (gm-cm)
weight, oz (gm)
radius, in. (cm)
number of gear teeth (constant)
length, in (cm)
density, oz/in 3 (gm/cm3)
angular velocity, radians/sec @ motor shaft
time, seconds
gravity constant, 386 in/sec2
System Calculations
W
A Engineering Reference
Tangential Drives
R
R– Radius
R=
inches
W – Weight (include weight of belt or chain)
W=
ounces
W(P) – Weight of pulley or material
W(P)
ounces
F – Breakaway force
F=
ounces
V – Linear velocity
V=
inches/sec
CT – Coupling type
CT =
SL – Side load
SL =
Tangential Drive Formulas
TTotal = TLoad + TPulley + TBelt + TMotor + TFriction
TTotal =
1 (J
+ JPulley + JBelt + JMotor) ω
t
g Load
+TFriction
JLoad = WLR2
JLoad = WLR2 = 5 lb x 16 oz x (1 in)2 = 80 oz-in2
lb
2
JPulley =
WpR
2
Problem
What torque is required to accelerate a 5-lb load to
a velocity of 20 inches per second in 10
milliseconds using a flat timing belt? The motor
drives a 2-inch diameter steel pulley 1/2-inch wide.
The timing belt weighs 12 oz. Load static friction is
30 ozs. Motor rotor inertia is 10.24 oz-in.2
(Remember to multiply by 2
if there are 2 pulleys.)
JBelt = WBR2
JPulley =
2(πLρR4)
= π x 0.5 in x (4.48 oz/in3) (1 in)4
2
= 7.04 oz-in2
TFriction = FR
JBelt = WBR2 = 12 oz (1 in)2 = 12 oz-in2
V
ω= R
TFriction = F x R = 30 oz x 1 in = 30 oz-in
Where:
T = torque, oz-in (gm-cm)
ω = angular velocity, radians/sec
t = time, seconds
WL = weight of the load, oz
WP = pulley weight, oz
W B = belt or rack weight, oz
F = frictional force, oz (gm)
R = radius, in (cm)
V = linear velocity
g = gravity constant, 386 in/sec2
ρ = density, oz/in3
V
in
1 rad
rad
ω = R = 20 sec x 1 in = 20 sec
1
20
TTotal = 386 (80 + 7.04 + 12 + 10.24) .01 + 30
TTotal = 596.2 oz-in
A65
System Calculations
Linear Step Motors
There are many characteristics to consider when
designing, selecting and installing a complete
motion control system. The applications data
worksheet and the application considerations
detailed below will help determine if a linear motor
system is recommended for a given application. A
linear motor, when properly specified, will provide
the optimum performance and the greatest
reliability.
Application Data Worksheet #1
Application: Single Axis √
Multi-Axis
X-Y Gantry
Description of system operation: A part is moved in and out of a
machine very quickly. The part comes to rest at the same point in the
machine each time. An operator sets this distance with a thumbwheel
switch.
Step 1: Total mass to be accelerated
Mtotal = Mload (10.0) + Mforcer (2.0) = 12.0 lbs.
Step 2: Acceleration rate
A. Average velocity = move distance
move time
= (40 inches)
(1.0 sec)
= 40.0 in/sec
B. Maximum velocity
(Based on trapezoidal move profile)
Vmax = 1.33 x Vavg (40.0 in/sec)
= 53.2 in/sec
Velocity
(in/sec)
Vmax
53.2
Sketch the proposed mechanical configuration:
Vave
40
4 lbs.
1/4
0
The Solution
Actual and assumed factors that contribute to the
solution are:
1. Force (F) = mass (M) x acceleration (A)
Note: mass units are in pounds
2. Acceleration due to gravity
(1g=386 inches/sec2)
3. L20 forcer weighs 2.0 lb.
4. Attractive force between L20 forcer and platen
= 200 lbs.
5. Trapezoidal velocity profile:
Accel time = 1.0 sec/4
= 0.250 sec.
Vmax
= 1.33 x Vavg
A66
1/4
Time (sec)
1/4
1.0
4
20"
1.0
C. Minimum acceleration rate
A = Vmax (53.2 in/sec) = 212.8 in/sec2
Accel. time (.250 sec)
A = Minimum acceleration (212.8 in/sec2)
386 in/sec 2 per 1 G
= 0.551 g's
Step 3: Calculate maximum acceleration rate of
L20 (using constant acceleration indexer).
Based on the speed/force curve below, the L20
has 14.0 lbs of force at 53.2 in/sec (Vmax).
Force vs. Speed L-L20
20
(9.08)
16
(7.26)
Force lbs (kg)
1. Motor Sizing
AXIS 1
A. Weight of payload (lbs)
10.0
B. Fixed forces, if any (lbs)
0
C. Known move distance (in)
40
time (sec)
1.0
D. Angle from horizontal (degrees)
0
2. Total length of travel (inches)
40
3. Desired repeatability (in)
001
4. Desired resolution (in)
.0005
5. Necessary settling time after move
100 ms to within
.001 inches
6. Life expectancy:
Percent duty cycle
20%
Estimated number of moves/year
200,000
7. Is the center of gravity significantly changed?
no
8. What is the environment? clean [√] dirty [ ]
Specifics
9. Operating temperature range
65° to 85°F
10. Can air be available?
yes
1/4
14.0 lbs
12
(6.45)
8
(3.83)
53.2 ips
4
(1.82)
0
20
(50.8)
40
60
(101.6) (152.4)
Speed in (cm)
80
(203.2)
100
(254.0)
Step 4: Non-damped safety margin
If all available force could be used, the maximum
calculated acceleration rate:
Amax =
Force (14.0 lb) = 1.16 g's
Mtotal (12.0 lbs.)
The calculated acceleration rate should be reduced
by 50% (100% non-damped safety margin) netting
an acceleration rate for the L20 of 0.58 g's. The
application requires a 0.55 g's acceleration rate.
The L20 meets the requirements of this application.
Velocity Ripple
Velocity ripple is most noticeable when operating
near the motor’s resonant frequency. Rotary
stepping motor’s have this tendency as well, but it
is usually less noticeable due to mechanical losses
in the rotary-to-linear transmission system, which
dampens the effects. Velocity ripple due to
resonance can be reduced with the electronic
accelerometer damping option (-AC).
Platen Mounting
The air gap between the forcer and the platen
surface can be as small as 0.0005 inches. Properly
mounting the platen is extremely important. When
held down on a magnetic chuck, the platen is flat
and parallel within its specifications, however, in its
free state, slight bows and twists may cause the
forcer (L20) to touch the platen at several places.
Compumotor recommends mounting the platen
using all its mounting holes on a ground flat piece of
steel, such as an I-beam, U-channel or tube.
Environment
Due to the small air gap between the forcer and
platen, care should be taken to keep the platen
clean. A small amount of dirt or adhesive material
(such as paint) can cause a reduction in motor
performance. When appropriate, mounting the
motor upside down or on its side will help keep
foreign particles off the platen. Protective boots that
fold like an accordion as the motor travels can also
be used to assist in keeping the platen clean.
Linear Step Motors
Life Expectancy
The life of a mechanical bearing motor is limited by
wearing of the platen surface over which the
bearings roll. Factors that affect wear and life of a
mechanical bearing system include:
A. High velocities – Life is inversely proportional to
velocity cubed. Increasing velocity raises the
temperature of the platen due to eddy current
losses in the solid platen material. (In normal
high-speed, high duty cycle operation over a
small piece of platen, the platen can become
almost too hot to touch.)
B. Load on the forcer – Load has some effect on
the life expectancy of the linear motor. Users are
urged to adhere to the load specifications for
each motor.
Yaw, Pitch and Roll
In applications such as end effector devices or
where the load is located far from the motor’s
center of gravity, the stiffness characteristics of the
forcer must be considered. Moment producing
forces tend to deflect the forcer, and if strong
enough, will cause the motor to stall or be removed
from the platen. Yaw, pitch and roll specifications
are used to determine the maximum torque you can
apply to the forcer.
Accuracy
In linear positioning systems, some applications
require high absolute accuracy, while many
applications require a high degree of repeatability.
These two variables should be reviewed to
accurately evaluate proper system performance.
In the “teach mode”, a linear motor can be
positioned and subsequently learn the coordinates
of any given point. After learning a number of points
in a sequence of moves, the user will be concerned
with the ability of the forcer to return to the same
position from the same direction. This scenario
describes repeatability.
In a different application, a linear motor is used to
position a measuring device. The size of an object
can be measured by positioning the forcer to a
point on the object. Determining the measured
value is based on the number of steps required to
reach the point on the object. System accuracy
must be smaller than the tolerance on the desired
measurement.
Open-loop absolute accuracy of a linear step motor
is typically less than a precision grade leadscrew
system. If a linear encoder is used in conjunction
with a linear motor, the accuracy will be equivalent
to any other transmission system.
The worst-case accuracy of the system is the
sum of these errors:
Accuracy = A + B + C + D + E + F
A = Cyclic Error – The error due to motor
magnetics that recurs once every pole pitch
as measured on the body of the motor.
B = Unidirectional Repeatability – The error
measured by repeated moves to the same
point from different distances in the same
direction.
C = Hysteresis – The backlash of the motor
when changing direction due to magnetic
non-linearity and mechanical friction.
D = Cumulative Platen Error – Linear error of
the platen as measured on the body of the
motor.
E = Random Platen Error – The non-linear
errors remaining in the platen after the linear
is disregarded.
F = Thermal Expansion Error – The error
caused by a change in temperature
expanding or contracting the platen.
A67
A Engineering Reference
System Calculations
System Calculations
Glossary
of Terms
Absolute Positioning
Refers to a motion control system
employing position feedback devices
(absolute encoders) to maintain a given
mechanical location.
Absolute Programming
A positioning coordinate referenced
wherein all positions are specified relative
to some reference, or “zero” position. This
is different from incremental programming,
where distances are specified relative to
the current position.
AC Servo
A general term referring to a motor drive
that generates sinusoidal shaped motor
currents in a brushless motor wound as to
generate sinusoidal back EMF.
Acceleration
The change in velocity as a function of
time. Acceleration usually refers to
increasing velocity and deceleration
describes decreasing velocity.
Accuracy
A measure of the difference between
expected position and actual position of a
motor or mechanical system. Motor
accuracy is usually specified as an angle
representing the maximum deviation from
expected position.
Ambient Temperature
The temperature of the cooling medium,
usually air, immediately surrounding the
motor or another device.
ASCII
American Standard Code for Information
Interchange. This code assigns a number
series of electrical signals to each numeral
and letter of the alphabet. In this manner,
information can be transmitted between
machines as a series of binary numbers.
Bandwidth
A measure of system response. It is the
frequency range that a control system can
follow.
BCD
Binary Coded Decimal is an encoding
technique used to describe the numbers 0
through 9 with four digital (on or off) signal
lines. Popular in machine tool equipment,
BCD interfaces are now giving way to
interfaces requiring fewer wires – such as
RS-232C.
Bit
Abbreviation of Binary Digit, the smallest
unit of memory equal to 1 or 0.
Back EMF
The voltage produced across a winding of
a motor due to the winding turns being cut
by a magnetic field while the motor is
operating. This voltage is directly
proportional to rotor velocity and is
opposite in polarity to the applied voltage.
Sometimes referred to as counter EMF.
A68
Block Diagram
A simplified schematic representing
components and signal flow through a
system.
Damping
An indication of the rate of decay of a
signal to its steady state value. Related to
settling time.
Bode Plot
A graph of system gain and phase versus
input frequency which graphically
illustrates the steady state characteristics
of the system.
Damping Ratio
Ratio of actual damping to critical
damping. Less than one is an
underdamped system and greater than
one is an overdamped system.
Break Frequency
Frequency(ies) at which the gain changes
slope on a Bode plot (break frequencies
correspond to the poles and zeroes of the
system).
Dead Band
A range of input signals for which there is
no system response.
Brushless DC Servo
A general term referring to a motor drive
that generates trapezoidal shaped motor
currents in a motor wound as to generate
trapezoidal Back EMF.
Byte
A group of 8 bits treated as a whole, with
256 possible combinations of one’s and
zero’s, each combination representing a
unique piece of information.
Commutation
The switching sequence of drive voltage
into motor phase windings necessary to
assure continuous motor rotation. A
brushed motor relies upon brush/bar
contact to mechanically switch the
windings. A brushless motor requires a
device that senses rotor rotational position,
feeds that information to a drive that
determines the next switching sequence.
Closed Loop
A broadly applied term relating to any
system where the output is measured and
compared to the input. The output is then
adjusted to reach the desired condition. In
motion control, the term describes a
system wherein a velocity or position (or
both) transducer is used to generate
correction signals by comparison to
desired parameters.
Critical Damping
A system is critically damped when the
response to a step change in desired
velocity or position is achieved in the
minimum possible time with little or no
overshoot.
Crossover Frequency
The frequency at which the gain intercepts
the 0 dB point on a Bode plot (used in
reference to the open-loop gain plot).
Daisy-Chain
A term used to describe the linking of
several RS-232C devices in sequence
such that a single data stream flows
through one device and on to the next.
Daisy-chained devices usually are
distinguished by device addresses, which
serve to indicate the desired destination for
data in the stream.
Decibel
A logarithmic measurement of gain. If G is
a system’s gain (ratio of output to input),
then 20 log G = gain in decibels (dB).
Detent Torque
The minimal torque present in an
unenergized motor. The detent torque of a
step motor is typically about 1% of its
static energized torque.
Direct Drive Servo
A high-torque, low-speed servo motor with
a high resolution encoder or resolver
intended for direct connection to the load
without going through a gearbox.
Duty Cycle
For a repetitive cycle, the ratio of on time
to total cycle time.
On Time
Duty cycle =
(On Time + Off Time)
Efficiency
The ratio of power output to power input.
Electrical Time Constant
The ratio of armature inductance to
armature resistance.
Encoder
A device that translates mechanical motion
into electronic signals used for monitoring
position or velocity.
Form Factor
The ratio of the RMS value of a harmonic
signal to its average value in one halfwave.
Friction
A resistance to motion. Friction can be
constant with varying speed (Coulomb
friction) or proportional to speed (viscous
friction).
Gain
The ratio of system output signal to system
input signal.
Holding Torque
Sometimes called static torque, it specifies
the maximum external force or torque that
can be applied to a stopped, energized
motor without causing the rotor to rotate
continuously.
Home
A reference position in a motion control
system derived from a mechanical datum
or switch. Often designated as the “zero”
position.
Hybrid Servo
A brushless servo motor based on a
conventional hybrid stepper. It may use
either a resolver or encoder for
commutation feedback.
Hysteresis
The difference in response of a system to
an increasing or a decreasing input signal.
IEEE-488
A digital data communications standard
popular in instrumentation electronics. This
parallel interface is also known as GPIB, or
General Purpose Interface Bus.
Incremental Motion
A motion control term that describes a
device that produces one step of motion
for each step command (usually a pulse)
received.
Incremental Programming
A coordinate system where positions or
distances are specified relative to the
current position.
Inertia
A measure of an object’s resistance to a
change in velocity. The larger an object’s
inertia, the larger the torque that is
required to accelerate or decelerate it.
Inertia is a function of an object’s mass
and its shape.
Inertial Match
For most efficient operation, the system
coupling ratio should be selected so that
the reflected inertia of the load is equal to
the rotor inertia of the motor.
Indexer
See PMC.
I/O
Abbreviation of input/output. Refers to
input signals from switches or sensors and
output signals to relays, solenoids etc.
Lead Compensation Algorithm
A mathematical equation implemented by
a computer to decrease the delay
between the input and output of a system.
Limits
Properly designed motion control systems
have sensors called limits that alert the
control electronics that the physical end of
travel is being approached and that
motion should stop.
Logic Ground
An electrical potential to which all control
signals in a particular system are
referenced.
Mechanical Time Constant
The time for an energized DC motor to
reach 2/3rds of its set velocity. Based on a
fixed voltage applied to the windings.
Mid-range Instability
Designates the condition resulting from
energizing a motor at a multiple of its
natural frequency (usually the third orders
condition). Torque loss and oscillation can
occur in underdamped open-loop
systems.
Microstepping
An electronic control technique that
proportions the current in a step motor’s
windings to provide additional intermediate
positions between poles. Produces
smooth rotation over a wide speed range
and high positional resolution.
Open Collector
A term used to describe a signal output
that is performed with a transistor. An
open collector output acts like a switch
closure with one end of the switch at
ground potential and the other end of the
switch accessible.
Open Loop
Refers to a motion control system where
no external sensors are used to provide
position or velocity correction signals.
Opto-isolated
A method of sending a signal from one
piece of equipment to another without the
usual requirement of common ground
potentials. The signal is transmitted
optically with a light source (usually a Light
Emitting Diode) and a light sensor (usually
a photosensitive transistor). These optical
components provide electrical isolation.
Parallel
Refers to a data communication format
wherein many signal lines are used to
communicate more than one piece of data
at the same time.
Phase Angle
The angle at which the steady state input
signal to a system leads the output signal.
Input
Output
Phase Angle
Phase Margin
The difference between 180° and the
phase angle of a system at its crossover
frequency.
PLC
Programmable logic controller; a machine
controller that activates relays and other I/
O units from a stored program. Additional
modules support motion control and other
functions.
PMC
Programmable motion controller,
primarily designed for single- or multiaxis motion control with I/O as an
auxiliary function.
Pole
A frequency at which the transfer
function of a system goes to infinity.
Pulse Rate
The frequency of the step pulses applied
to a motor driver. The pulse rate
multiplied by the resolution of the motor/
drive combination (in steps per
revolution) yields the rotational speed in
revolutions per second.
PWM
Pulse Width Modulation. A method of
controlling the average current in a
motors phase windings by varying the
on-time (duty cycle) of transistor
switches.
Ramping
The acceleration and deceleration of a
motor. May also refer to the change in
frequency of the applied step pulse train.
Rated Torque
The torque producing capacity of a
motor at a given sped. This is the
maximum torque the motor can deliver
to a load and is usually specified with a
torque/speed curve.
Regeneration
Usually refers to a circuit in a drive
amplifier that accepts and drains energy
produced by a rotating motor either
during deceleration or free-wheel
shutdown.
Registration Move
Changing the predefined move profile
that is being executed, to a different
predefined move profile following receipt
of an input or interrupt.
Repeatability
The degree to which the positioning
accuracy for a given move performanced
repetitively can be duplicated.
Resolution
The smallest positioning increment that
can be achieved. Frequently defined as
the number of steps required for a
motor’s shaft to rotate one complete
revolution.
Resolver
A feedback device with a construction
similar to a motor’s construction (stator
and rotor). Provides velocity and position
information to a drive’s microprocessor
or DSP to electronically commutate the
motor.
A69
A Engineering Reference
System
Glossary
Calculations
of Terms
Glossary of Terms
Resonance
Designates the condition resulting from energizing a
motor at a frequency at or close to the motor’s
natural frequency. Lower resolution, open-loop
systems will exhibit large oscillations from minimal
input.
Ringing
Oscillation of a system following a sudden change
in state.
RMS Torque
For an intermittent duty cycle application, the RMS
Torque is equal to the steady- state torque that
would produce the same amount of motor heating
over long periods of time.
(Ti2 ti)
MM
TRMS =
ti
Where: Ti = Torque during interval i
ti = Time of interval i
RS-232C
A data communications standard that encodes a
string of information on a single line in a time
sequential format. The standard specifies the
proper voltage and time requirements so that
different manufacturers devices are compatible.
Servo
A system consisting of several devices which
continuously monitor actual information (position,
velocity), compares those values to desired
outcome and makes necessary corrections to
minimize that difference.
Slew
In motion control, the portion of a move made at a
constant non-zero velocity.
Static Torque
The maximum torque available at zero speed.
Step Angle
The angle the shaft rotates upon receipt of a single
step command.
Stiffness
The ability to resist movement induced by an
applied torque. Is often specified as a torque
displacement curve, indicating the amount a motor
shaft will rotate upon application of a known
external force when stopped.
Synchronism
A motor rotating at a speed correctly corresponding
to the applied step pulse frequency is said to be in
synchronism. Load torques in excess of the
motor’s capacity (rated torque) will cause a loss of
synchronism. The condition is not damaging to a
step motor.
Torque
Force tending to produce rotation.
A70
Torque Constant
KT = The torque generated in a DC motor per unit
Ampere applied to its windings.
KT = T oz-in
A amp
Simplified for a brushless motor at 90° commutation
angle.
Torque Ripple
The cyclical variation of generated torque at a
frequency given by the product of motor angular
velocity and number of commutator segments or
magnetic poles.
Torque-to-Inertia Ratio
Defined as a motor’s holding torque divided by the
inertia of its rotor. The higher the ratio, the higher a
motor’s maximum acceleration capability will be.
Transfer Function
A mathematical means of expressing the output to
input relationship of a system. Expressed as a
function of frequency.
Triggers
Inputs on a controller that initiate or “trigger” the
next step in a program.
TTL
Transistor-Transistor Logic. Describes a common
digital logic device family that is used in most
modern digital electronics. TTL signals have two
distinct states that are described with a voltage – a
logical “zero” or “low” is represented by a voltage of
less than 0.8 volts and a logical “one” or “high” is
represented by a voltage from 2.5 to 5 volts.
Voltage Constant
KE = The back EMF generated by a DC motor at a
defined speed. Usually quoted in volts per 1000
rpm.
Zero
A frequency at which the transfer function of a
system goes to zero.
Technical Data
Don’t confuse mass-inertia with weight-inertia: mass inertia = wt. inertia
g
To convert from A to B, multiply by entry in Table.
lb-ft-s2
B
kg-m2
A
kg-cm2
g-cm2
kg-m-sec2
kg-cm-sec 2 g-cm-sec 2
oz-in2
oz-in-s2
lb-in-2
lb-in-s2
lb-ft2
(slug-ft-2)
kg-m2
1
104
107
0.10192
10.1972
1.01972-104
5.46745-104
1.41612-102
3.41716-103
8.850732
23.73025
0.73756
kg-cm2
10-4
1
103
1.01972-10-5
1.01972-10-3
1.01972
5.46745
1.41612-10-2
0.341716
8.85073-10-4
2.37303-10-3
7.37561-10-5
7.37561-10-8
g-cm2
10-7
10-3
1
1.01972-10-8
1.01972-10-6
1.01972-10-3
5.46745-10-3
1.41612-10-5
3.41716-10-4
8.85073-10-7
2.37303-10-6
9.80665
9.80665-104
9.80665-107
1
102
105
5.36174-105
1.388674-103
3.35109-104
86.79606
2.32714-102
7.23300
kg-cm-s2 9.80665-10-2
9.80665-102
9.80665-105
10-2
1
103
5.36174-103
13.88741
3.35109-102
0.86796
2.327143
7.23300-10-2
g-cm-s2 9.80665-10-5
0.980665
9.80665-102
10-5
10-3
1
5.36174
1.38874-10-2
0.335109
8.67961-10-4
2.32714-10-3
7.23300-10-5
oz-in2
0.182901
1.82901-102
1.86506-10-6
1.86506-10-4
0.186506
1
2.59008-10-3
6.250-10-2
1.61880-10-4
4.34028-10-4
1.34900-10-5
oz-in-s2 7.06154-10-3
70.6154
7.06154-104
7.20077-10-4
7.20077-10-2
72.00766
3.86089-102
1
24.13045
6.250-10-2
0.167573
5.20833-10-3
lb-in2
2.92641-10-4
2.92641
2.92641-103
2.98411-10-5
2.98411-10-3
2.98411
16
4.14414-10-2
1
2.59008-10-3
6.94444-10-3
2.15840-10-4
lb-in-s2
0.112985
1.12985-103
1.12985-106
1.15213-10-2
1.152126
1.15213-103
6.17740-103
16
3.86088-102
1
2.681175
8.3333-10-2
lb-ft2
lb-ft-s2
4.21403-10-2
4.21403-102
4.21403-105
4.29711-10-3
0.429711
4.297114-102
2.304-103
5.96755
144
0.372971
1
3.10809-10-2
1.35583
1.35582-104
1.35582-107
0.138255
13.82551
1.38255-104
7.41289-104
192
4.63306-103
12
32.1740
1
kg-m-s2
1.82901-10-5
(slug ft2)
Torque Conversion Table
To convert from A to B, multiply by entry in Table.
B
A
N-m
N-m
1
N-cm
10 -2
dyn-cm
10
-7
N-cm
dyn-cm
kg-m
kg-cm
102
107
0.1019716
1
105
1.019716-10-3
10
-5
1
2
1.019716-10
7
g-cm
10.19716
-8
0.1019716-3
1.01972-10
-6
2
oz-in
1.019716-104
141.6119
1.019712-102
1.41612
1.01972-10
-3
5
1.41612-10
ft-lbs
in-lbs
0.737562
-5
8.85074
7.37562-10 -3
8.85074-10 -2
-8
8.85074-10 -7
7.37562-10
3
kg-m
9.80665
9.80665-10
9.80665-10
1
10
10
1.38874-10
7.23301
kg-cm
9.80665-10 -2
9.80665
9.80665-105
10 -2
1
103
13.8874
7.23301-10 -2
0.86792
g-cm
9.80665-10 -5
9.80665-10 -3
9.80665-102
10 -5
10 -3
1
1.38874-10 -2
7.23301-10 -5
8.679624-10-4
oz-in
7.06155-10 -3
0.706155
7.06155-104
7.20077-10 -4
7.20077-10 -2
72,0077
1
5.20833-10 -3
6.250-10-2
192
1
12
16
8.33333-10 -2
1
2
7
4
ft-lbs
1.35582
1.35582-10
1.35582-10
0.1382548
13.82548
1.382548-10
in-lbs
0.112085
11.2985
1.12985-106
1.15212-10 -2
1.15212
1.15212-103
Densities of Common Materials
Material
86.79624
Calculate Horsepower
3
3
oz/in
gm/cm
Aluminum (cast or hard-drawn)
1.54
2.66
Brass (cast or rolled 60% CU; 40% Zn)
4.80
8.30
Bronze (cast, 90% CU; 10% Sn)
4.72
8.17
Copper (cast or hand-drawn)
5.15
8.91
Plastic
0.64
1.11
Steel (hot or cold rolled, 0.2 or 0.8% carbon)
4.48
7.75
Hard Wood
0.46
0.80
Soft Wood
0.28
0.48
Horsepower = Torque x Speed
16,800
Torque = oz-in
Speed = revolutions per second
* The horsepower calculation uses the torque
available at the specified speed
1 Horsepower = 746 watts
Most tables give densities in lb/ft3. To convert to oz/in3
divide this value by 108. To convert lb/ft3 to gm/cm3
divide by 62.5. The conversion from oz/in 3 to gm/cm3
is performed by multiplying oz/in3 by 1.73.
Reference: Elements of Strength of Materials,
S. Timoshinko and D.H. Young, pp. 342-343.
A71
A Engineering Reference
Rotary Inertia Conversion Table
Application Examples
Summary of Application Examples
Feed-to-length
Tool Feed
Applications in which a continuous web, strip, or
strand of material is being indexed to length, most
often with pinch rolls or some sort of gripping
arrangement. The index stops and some process
occurs (cutting, stamping, punching, labeling,
etc.).
Applications where motion control is used to feed a
cutting or grinding tool to the proper depth.
Application No.
Page
1: BBQ Grill-Making Machine .................... A73
16: Flute Grinder ........................................... A88
Application No.
Page
14: Surface Grinding Machine ...................... A86
15: Transfer Machine .................................... A87
17: Disc Burnisher ........................................ A89
2: Film Advance .......................................... A74
3: On-the-Fly Welder .................................. A75
X/Y Point-to-point
Applications that deal with parts handling
mechanisms that sort, route, or divert the flow of
parts.
Application No.
Page
4: Optical Scanner ...................................... A76
5: Circuit Board Scanning ........................... A77
Metering/Dispensing
Applications where controlling displacement and/
or velocity are required to meter or dispense a
precise amount of material.
Application No.
Page
6: Telescope Drive ...................................... A78
Winding
Controlling the process of winding material around
a spindle or some other object.
Application No.
Page
18: Monofilament Winder .............................. A90
19: Capacitor Winder ................................... A91
Following
Applications that require the coordination of motion
to be in conjunction with an external speed or
position sensor.
Application No.
Page
20: Labelling Machine ................................... A92
21: Window Blind Gluing .............................. A93
22: Moving Positioning Systems ................... A94
7: Engine Test Stand .................................. A79
8: Capsule Filling Machine .......................... A80
Indexing/Conveyor
Applications where a conveyor is being driven in a
repetitive fashion to index parts into or out of an
auxiliary process.
Application No.
Page
9: Indexing Table ........................................ A81
10: Rotary Indexer ........................................ A82
11: Conveyor ................................................ A83
Contouring
Applications where multiple axes of motion are
used to create a controlled path, (e.g., linear or
circular interpolation).
Application No.
Page
12: Engraving Machine ................................. A84
13: Fluted-Bit Cutting Machine ..................... A85
A72
Injection Molding
Applications where raw material is fed by gravity
from a hopper into a pressure chamber (die or
mold). The mold is filled rapidly and considerable
pressure is applied to produce a molded product.
Application No.
Page
23: Plastic Injection Molding ......................... A95
Flying Cutoff
Applications where a web of material is cut while
the material is moving. Typically, the cutting device
travels at an angle to the web and with a speed
proportional to the web.
Application No.
Page
24: Rotating Tube Cutting ............................ A96
Application Examples
Application Type: Feed-to-Length
Motion: Linear
Application Description: A manufactuer was
using a servo motor to feed material into a
machine to create barbeque grills, shopping carts,
etc. The process involves cutting steel rods and
welding the rods in various configurations.
However, feed-length was inconsistent because
slippage between the drive roller and the material
was too frequent. Knurled nip-rolls could not be
used because they would damage the material.
The machine builder needed a more accurate
method of cutting the material at uniform lengths.
The customer used a load-mounted encoder to
provide feedback of the actual amount of material
fed into the cutting head.
Machine Objectives:
• Compnesate for material slippage
• Interface with customer’s operator panel
• Smooth repeatable operation
• Variable length indexes
• High reliability
Motor and
Drive Roll
Spool
Motion Control Requirements:
• Accurate position control
• Load-mounted encoder feedback
• High-speed indexing
• XCode language
Application Solution: By using the global
position feedback capability of the BLHX drive, the
machine builder was able to close the position
loop with the load-mounted encoder, while the
velocity feedback was provided by the motormounted encoder and signal processing. The twoencoder system provides improved stability and
higher performance than a single load-mounted
encoder providing both position and velocity
feedback. The load-mounted encoder was
coupled to friction drive nip-rollers close to the cut
head.
Product Solutions:
Controller/Drive
Motor
BLHX75BN
ML3450B-10
Nip-Roll and
Load Mounted
Encoder
BLHX150BN
Servo Drive
Cutting Head
A73
A Engineering Reference
1. BBQ Grill-Making Machine
Application Examples
2. Film Advance
Application Type: Feed-to-Length
Motion: Linear
Tangential drives consist of a pulley or pinion
which, when rotated, exerts a force on a belt or
racks to move a linear load. Common tangential
drives include pulleys and cables, gears and
toothed belts, and racks and pinions.
Tangential drives permit a lot of flexibility in the
design of drive mechanics, and can be very
accurate with little backlash. Metal chains should
be avoided since they provide little or no motor
damping.
Application Description: A movie camera is
being modified to expose each frame under
computer control for the purpose of generating
special effects. A motor will be installed in the
camera connected to a 1/2-inch diameter, 2-inch
long steel film drive sprocket and must index one
frame in 200 milliseconds. The frame spacing is 38
mm (1.5").
Machine Requirements:
• Index one frame within 200 milliseconds
• Indexer must be compatible with BCD interface
• Fast rewind and frame indexing
Motion Control Requirements:
• Little to no vibration at rest—∴ Stepper
• Minimum settling time
• Preset and slew moves
Application Solution:
In this application, the move distance and time are
known, but the required acceleration is not known.
The acceleration may be derived by observing
that, for a trapezoidal move profile with equal
acceleration, slew and deceleration times, 1/3 of
the move time is spent accelerating and 1/3 of the
total distance is travelled in that time (a trapezoidal
move).
It is determined that the acceleration required is
107.4 rps2 at a velocity of 7.166 rps. Assume that
the film weighs 1 oz. and total film friction is 10 ozin. The rotor, sprocket, and film inertia is calculated
to be 0.545 oz-in/sec2. Solving the torque formula
indicates that the motor for this application must
provide 11.9 oz-in to drive the film and pulley (refer
to Direct Drive Formulas on p. A63).
An indexer is selected to be connected to a BCD
interface in the camera electronics. Preset and
Slew modes on the indexer are then controlled by
the camera electronics to provide fast rewind and
frame indexing.
Product Solutions:
Drive/Indexer
Motor
SX
S57-51-MO
Drive/Indexer
Motor
A74
Application Examples
Application Type: Feed-to-Length
Motion: Linear
Description: In a sheet metal fabrication process,
an unfastened part rides on a conveyor belt
moving continuously at an unpredictable velocity.
Two spot-welds are to be performed on each part,
4 inches apart, with the first weld 2 inches from the
leading edge of the part. A weld takes one second.
Machine Objectives
• Standalone operation
• Position welder according to position and
velocity of each individual part
• Welding and positioning performed without
stopping the conveyor
• Welding process must take 1 second to
complete
Motion Control Requirements
• Programmable I/O; sequence storage
• Following
• Motion profiling; complex following
• High linear acceleration and speed
Application Solution:
This application requires a controller that can
perform following or motion profiling based on a
primary encoder position. In this application, the
controller will receive velocity and position data
from an incremental encoder mounted to a roller
on the conveyor belt carrying the unfastened parts.
The conveyor is considered the primary drive
system. The secondary motor/drive system
receives instructions from the controller, based on
a ratio of the velocity and position information
supplied by the primary system encoder. The linear
motor forcer carries the weld head and is mounted
on an overhead platform in line with the conveyor.
Linear motor technology was chosen to carry the
weld head because of the length of travel. The
linear step motor is not subject to the same linear
velocity and acceleration limitations inherent in
systems converting rotary to linear motion. For
example, in a leadscrew system, the inertia of the
leadscrew frequently exceeds the inertia of the
load and as the length of the screw increases, so
does the inertia. With linear motors, all the force
generated by the motor is efficiently applied
directly to the load; thus, length has no effect on
system inertia. This application requires a 54-inch
platen to enable following of conveyor speeds over
20 in/sec.
Application Process
1. A sensor mounted on the weld head detects
the leading edge of a moving part and sends a
trigger pulse to the controller.
2. The controller receives the trigger signal and
commands the linear motor/drive to ramp up to
twice the speed of the conveyor. This provides
an acceleration such that 2 inches of the part
passes by the weld head by the time the weld
head reaches 100% of the conveyor velocity.
3. The controller changes the speed ratio to 1:1,
so the weld head maintains the speed of the
conveyor for the first weld. The weld takes 1
second.
4. The following ratio is set to zero, and the
welder decelerates to zero velocity over 2
inches.
5. The controller commands the linear forcer to
repeat the same acceleration ramp as in step␣1
above. This causes the weld head to position
itself, at an equal velocity with the conveyor, 4
inches behind the first weld.
6. Step 3 is repeated to make the second weld.
7. Once the second weld is finished, the controller
commands the linear forcer to return the weld
head to the starting position to wait for the next
part to arrive.
Product Solutions:
Indexer
Model 500
Drive
Motor
L Drive PO-L20-P54
Encoder
-E
Indexer
Linear Motor
Spot Welds
Microstepping
Drive
Encoder
(Mounted
to Conveyor)
Weld Head
A75
A Engineering Reference
3. On-the-Fly Welder
Application Examples
4. Optical Scanner
Application Type: X-Y Point-to-Point
Motion: Rotary
Application Description: A dye laser designer
needs to precisely rotate a diffraction grating under
computer control to tune the frequency of the
laser. The grating must be positioned to an angular
accuracy of 0.05°. The high resolution of the
microstepping motor and its freedom from
“hunting” or other unwanted motion when stopped
make it ideal.
Machine Requirements:
• System must precisely rotate a diffraction
grating to tune the frequency of the laser
• PC-compatible system control
• Angular accuracy of 0.05º
• IEEE-488 interface is required
Motion Control Requirements:
• High resolution—∴ Microstepper
• Little to no vibration at rest—∴ Stepper
• No “hunting” at the end of move—∴ Stepper
• Limited space is available for motor—∴ small
motor is required
Application Solution:
The inertia of the grating is equal to 2% of the
proposed motor’s rotor inertia and is therefore
ignored. Space is at a premium in the cavity and a
small motor is a must. A microstepping motor,
which provides ample torque for this application, is
selected.
The laser’s instrumentation is controlled by a
computer with an IEEE-488 interface. An indexer
with an IEEE-488 interface is selected. It is
mounted in the rack with the computer and is
controlled with a simple program written in BASIC
that instructs the indexer to interrupt the computer
at the completion of each index.
Product Solutions:
Indexer
Drive
Motor
Model 4000
LN Drive
LN57-51
Drive
Model 4000
Motor
A76
Application Examples
Application Type: X-Y Point-to-Point
Motion: Linear
Application Description: An Original Equipment
Manufacturer (OEM) manufactures X-Ray Scanning
equipment used in the quality control of printed
circuit boards and wafer chips.
the AT6400 to control both axes. A microstepping
drive is used because its linear amplifier
technology produces little EMI. The PC monitor is
the operator interface.
The OEM wants to replace the DC motors,
mechanics and analog controls with an automated
PC-based system to increase throughput and
eliminate operator error. The host computer will
interact with the motion control card using a “C”
language program. The operator will have the
option to manually override the system using a
joystick.
A “C” language program controls the machine.
This machine operates in an environment where
PWM (pulse width modulation) related EMI
emission is an issue.
Machine Requirements:
• 2-Axis analog joystick
• Joystick button
• Travel limits
• Encoder feedback on both axes
Display Requirements:
• X and Y position coordinates
Operator Adjustable Parameters:
• Dimensions of sample under test
• (0,0) position—starting point
Motion Control Requirements:
• AT-based motion controller card
• Replace velocity control system (DC motors) and
mechanics with more accurate and automated
positioning scheme
• Manual Joystick control
• Continuous display of X & Y axis position
• User-friendly teach mode operations
• Low EMI amplifiers (drives)
Application Solution:
The solution of this application uses the existing
PC by providing a PC-based motion controller and
Motor
Machine operation begins with a display to the operator
of a main menu. This main menu lets the operator
select between three modes: Automated Test, Joystick
Position and Teach New Automated Test.
In Automated Test mode, the PC displays a menu
of preprogrammed test routines. Each of these
programs has stored positions for the different test
locations. This data is downloaded to the controller
when a test program is selected. The controller
controls the axes to a home position, moves to
each scan position, and waits for scan completion
before moving to the next position.
In Joystick Position mode, the controller enables
the joystick allowing the operator to move in both
X and Y directions using the joystick. The AT6400
waits for a signal from the PC to indicate that the
joystick session is over.
When Teach mode is selected, the PC downloads a
teach program to the controller (written by the user).
After the axes are homed, the controller enables the
joystick and a “position select” joystick button. The
operator then jogs axes to a position and presses
the “position select” button. Each time the operator
presses this “position select” button, the motion
controller reads this position into a variable and
sends this data to the PC for memory storage.
These new position coordinates can now be stored
and recalled in Automated Test mode.
Product Solutions:
Controller
Drive
Motor
AT6400-AUX1 LN Drive LN57-83-MO
Accessories
-E
Daedal X-Y Table
Joystick
Drive
Drive
Motor
Joystick
Indexer
A77
A Engineering Reference
5. Circuit Board Scanning
Application Examples
6. Telescope Drive
Application Type: Metering/Dispensing
Motion: Rotary
Traditional gear drives are more commonly used
with step motors. The fine resolution of a
microstepping motor can make gearing
unnecessary in many applications. Gears generally
have undesirable efficiency, wear characteristics,
backlash, and can be noisy.
Gears are useful, however, when very large inertias
must be moved because the inertia of the load
reflected back to the motor through the gearing is
divided by the square of the gear ratio.
In this manner large inertial loads can be moved
while maintaining a good load inertia-to-rotor
inertia ratio (less than 10:1).
Application Description: An astronomer
building a telescope needs to track celestial events
at a slow speed (15°/hour) and also slew quickly
(15° in 1 second).
Machine Requirements:
• Smooth, slow speed is required–∴ microstepper
• High data-intensive application–∴ bus-based
indexer
• Future capabilities to control at least 2 axes of
motion
• Visual C++ interface
Motion Control Requirements:
• High resolution
• Very slow speed (1.25 revolutions per hour)—
microstepping
• AT bus-based motion controller card
• Dynamic Link Library (DDL) device driver must
be provided with indexer. This helps Windows™
programmers create Windows-based
applications (i.e., Visual C++) to interface with
the indexer
Application Solution:
A 30:1 gearbox is selected so that 30 revolutions
of the motor result in 1 revolution (360°) of the
telescope. A tracking velocity of 15°/hour
corresponds to a motor speed of 1.25 revs/hour or
about 9 steps/sec. on a 25,000 steps/rev. Moving
15° (1.25 revolutions) in 1 second requires a
velocity of 1.25 rps.
The inverse square law causes the motor to see 1/
900 of the telescope’s rotary inertia. The equations
are solved and the torque required to accelerate
the telescope is 455 oz-in. The step pulses
required to drive the motor are obtained from a
laboratory oscillator under the operator’s control.
Product Solutions:
Indexer
Drive
Motor
AT6200-AUX1*
S Drive
S106-178
* To control up to four axes, refer to the AT6400.
Drive
Motor
A78
Computer
(Indexer installed
in a PC)
Application Examples
Application Type: Metering/Dispensing
Motion: Rotary
Application Description: A jet engine
manufacturer is building a test facility for making
operational measurements on a jet engine. The
throttle and three other fuel flow controls need to
be set remotely. While the application only calls for
a rotary resolution of 1 degree (1/360 rev.), the
smoothness and stiffness of a microstepping
system is required.
Application Solution:
Each valve is measured with a torque wrench.
Two valves measure at 60 oz-in and the other two
measure at 200 oz-in. Two high-power and two
low-power microstepping motor/drives systems
are selected. These choices provide
approximately 100% torque margin and result in a
conservative design.
Motor speeds are to be low and the inertias of the
valves connected to the motors are insignificant.
The main torque requirement is to overcome valve
friction.
The operator would like to specify each valve
position as an angle between 0° and 350°.
Machine Requirements:
• Low wear
• Remote operation
• High reliability
Motion Control Requirements:
• Motor velocity is low
• High stiffness at standstill
• Slow-speed smoothness
• Four axes of control
• Homing function
Home position switches are mounted on the test
rig and connected to each indexer to allow for
power-on home reference using the indexer’s
homing feature.
Product Solutions:
Indexer
Drive
Motor
AT6400*
S Drive
S57-102
* A standalone indexer could also be used
(instead of a bus-based indexer), refer to the
Model 4000.
Motor
Motor
Drive
Motor
Drive
Computer
(Indexer installed in a PC)
Motor
Drive
Drive
A79
A Engineering Reference
7. Engine Test Stand
Application Examples
8. Capsule Filling Machine
Application Type: Metering/Dispensing
Motion: Linear
Application Description: The design requires a
machine to dispense radioactive fluid into capsules.
After the fluid is dispensed, it is inspected and the
data is stored on a PC. There is a requirement to
increase throughput without introducing spillage.
Machine Requirements:
• Increase throughput
• No spilling of radioactive fluid
• Automate two axes
• PC compatible system control
• Low-cost solution
• Smooth, repeatable motion
Motion Control Requirements:
• Quick, accurate moves
• Multi-axis controller
• PC bus-based motion control card
• Open-loop stepper if possible
• High-resolution motor/drive (microstepping)
Application Solution:
The multi-axis indexer is selected to control and
synchronize both axes of motion on one card
residing in the IBM PC computer. An additional
feature is the integral I/O capability that’s
necessary to activate the filling process. The
horizontal axis carrying the tray of capsules is
driven by a linear motor. The simple mechanical
construction of the motor makes it easy to apply,
and guarantees a long maintenance-free life. The
vertical axis raises and lowers the filling head and
is driven by a microstepping motor and a
leadscrew assembly. A linear motor was also
considered for this axis, but the fill head would
have dropped onto the tray with a loss of power
to the motor. Leadscrew friction and the residual
torque of the step motor prevents this occurrence.
Product Solutions:
Indexer
AT6200
Drive
Axis 1: ZETA Drive
S57-51
Axis 2: ZETA Drive PO-L20-P18
Filling Heads
Top View
Full
Capsules
Tray of
Empty Capsules
Hose
Side View
Platen
Linear Motor
Motor with Leadscrew
Computer
(Indexer
installed
in a PC)
Drive
A80
Drive
Motor
Application Examples
Application Type: Indexing/Conveyor
Motion: Linear
Application Description: A system is required
to plot the response of a sensitive detector that
must receive equally from all directions. It is
mounted on a rotary table that needs to be
indexed in 3.6° steps, completing each index
within one second. For set-up purposes, the table
can be positioned manually at 5 rpm. The table
incorporates a 90:1 worm drive.
Machine Requirements:
• Low-EMI system
• Repeatable indexing
• Remote operation
• Table speed of 5 rpm
Motion Control Requirements:
• Jogging capability
• Sequence select functionality
• Capable of remote drive shutdown
Application Solution:
The maximum required shaft speed (450 rpm) is
well within the capability of a stepper, which is an
ideal choice in simple indexing applications.
Operating at a motor resolution of 400 steps/rev,
the resolution at the table is a convenient 36,000
step/rev. In this application, it is important that
electrical noise is minimized to avoid interference
with the detector. Two possible solutions are to
use a low-EMI linear drive or to shut down the
drive after each index (with a stepper driving a
90:1 worm gear there is no risk of position loss
during shutdown periods).
Product Solutions:
Indexer
Drive
Motor
Model 500
LN Drive
LN57-102
* The SX drive/indexer and PK2 drive are other
products that have been used in these types of
applications.
Radiation
Source
Detector
Drive
Indexer
Motor
Rotary
Stage
A81
A Engineering Reference
9. Indexing Table
Application Examples
10. Rotary Indexer
Application Type: Indexing Conveyor
Motion: Rotary
Application Description: An engineer for a
pharmaceutical company is designing a machine
to fill vials and wants to replace an old style
Geneva mechanism. A microstepping motor will
provide smooth motion and will prevent spillage.
The indexing wheel is aluminum and is 0.250-inch
thick and 7.5" in diameter. Solving the equation for
the inertia of a solid cylinder indicates that the
wheel has 119.3 oz-in2. The holes in the indexing
wheel reduce the inertia to 94 oz-in2. The vials
have negligible mass and may be ignored for the
purposes of motor sizing. The table holds 12 vials
(30° apart) that must index in 0.5 seconds and
dwell for one second. Acceleration torque is
calculated to be 8.2 oz-in at 1.33 rps2. A triangular
move profile will result in a maximum velocity of
0.33 rps. The actual torque requirement is less
than 100 oz-in. However, a low load-to-rotor
inertia ratio was necessary to gently move the vials
and fill them.
Machine Requirements:
• Smooth motion
• PLC control
• Variable index lengths
Motion Control Requirements:
• Smooth motion
• Sequence select capability
• I/O for sequence select
• Programmable acceleration and deceleration
Application Solution:
The index distance may be changed by the
engineer who is controlling the machine with a
programmable controller. Move parameters will be
changing and can therefore be set via BCD inputs.
The indexer can be “buried” in the machine and
activated with a remote START input.
Product Solutions:
Drive Indexer
Motor
SX Drive Indexer*
S83-135
* The 6200, AT6200, and Model 500 are other
indexer products that have been used in these
types of applications.
PLC
Programmable
Logic Controller
A82
Controller
Drive
Application Examples
Application Type: Indexing/Conveyor
Motion: Linear
Tangential drives consist of a pulley or pinion
which, when rotated, exerts a force on a belt or
racks to move a linear load. Common tangential
drives include pulleys and cables, gears and
toothed belts, and racks and pinions.
Tangential drives permit a lot of flexibility in the
design of drive mechanics, and can be very
accurate with little backlash. Metal chains should
be avoided since they provide little or no motor
damping.
Application Description: A machine vision
system is being developed to automatically inspect
small parts for defects. The parts are located on a
small conveyor and pass through the camera’s
field of view. The conveyor is started and stopped
under computer control and the engineer wants to
use a system to drive the conveyor because it is
necessary for the part to pass by the camera at a
constant velocity.
It is desired to accelerate the conveyor to a speed
of 20 inches/sec. in 100 milliseconds. A flat timing
belt weighing 20 ozs. is driven by a 2-inch diameter
aluminum pulley 4 inches wide (this requires a
motor velocity of 3.2 rps). The maximum weight of
the parts on the pulley at any given time is 1 lb.
and the load is estimated to have an inertia of 2 ozin2. Static friction of all mechanical components is
30 oz-in. The required motor toque was
determined to be 50.9 oz-ins (refer to Direct Drive
Formulas on p. A63).
Machine Requirements:
• Computer-controlled system
• High accuracy
• Low backlash
Motion Control Requirements:
• Accurate velocity control
• Linear motion
• High resolution
• AT bus-based motion control card
Application Solution:
A computer controls the entire inspection
machine. A bus-based compatible indexer card
was selected. A microstepping motor/drive system
that supplied 100 oz-in of static torque was also
chosen to complete the application.
Product Solutions:
Indexer
Drive
Motor
PC21*
S Drive
S57-83
* The AT6200 and AT6400 are other PC-based
indexer products that are often used in these
types of applications.
Motor
Drive
Computer
(Indexer installed
in a PC)
A83
A Engineering Reference
11. Conveyor
Application Examples
12. Engraving Machine
Application Type: Contouring
Motion: Linear
Application Description: An existing engraving
machine requires an upgrade for accuracy beyond
0.008 inches, capability and operating
environment. Using a personal computer as the
host processor is desirable.
Machine Requirements:
• Positional accuracy to 0.001 inches
• Easy-to-use, open-loop control
• CNC machining capability
• Interface-to-digitizer pad
• Compatibility with CAD systems
Motion Control Requirements:
• High resolution
• Microstepping
• G-Code compatibility
• IBM PC compatible controller
Application Solution:
A four-axis motion controller resides on the bus of
an IBM compatible computer, allowing full
integrated control of four axes of motion. Axes 3
and 4 are synchronized to prevent table skew.
CompuCAM’s G-Code package allows the user to
program in industry-standard machine tool
language (RS274 G-Code) or to import CAD files
with CompuCAM-DXF. Open-loop microstepping
drives with precision leadscrews give positional
accuracies better than the desired ±0.001 inch.
This simple retrofit to the existing hardware greatly
improved system performance.
Product Solutions:
Indexer
Drives
Motor
AT6400*
S Drives
S83-135
* The Model 4000 (standalone) and AT6450 are
servo controller products that have also been
used in these types of applications.
IBM PC with Indexer
Drives
Digitizer Pad
Motor
Axis 2
Axis 3
Axis 1
Motor
Motor
Axis 4
A84
Motor
Application Examples
Application Type: Contouring
Motion: Linear
Application Description: The customer
manufactures a machine that cuts a metal cylinder
into fluted cutting bits for milling machines. The
machine operation employed a mechanical cam
follower to tie the bit’s rotation speed to the
traverse motion of the bit relative to the cutting
tool. The cut depth was manually adjusted using a
hand crank.
This arrangement was acceptable when the
company had a bit for the cam they wanted to
grind. Unfortunately, custom prototype bits made
of titanium or other high-tech metals required that
they make a cam before they could machine the
bit, or do those parts on a $10,000 CNC screw
machine. Both of these alternatives were too
expensive for this customer.
Machine Requirements:
• Machine must be capable of making lowvolume custom bits as well as high-volume
standard bits—an be economical for both
processes.
• Quick set-up routine
• Operator interface for part entry
Motion Control Requirements:
• Smooth motion
• Four axes of coordinated motion
• 2 axes of linear interpolation
• Math capabilities
Application Solution:
Controlled by a multi-axis step and direction
controller, microstepping motors and drives are
attached to four axes for smooth, programmable
motion at all speeds.
• Axis 1: Alignment
• Axis 2: Chamfer (cutting depth)
• Axis 3: Traverse
• Axis 4: Rotation
To allow for the flexibility required to cut a bit at a
desired pitch, the traverse and rotation axes (axes
3 and 4) are synchronized along a straight line.
The controller’s linear interpolation allows this
functionality. Both the alignment and chamfer axes
(axes 1 and 2) remain stationary during the cutting
process.
The controller’s operator input panel and math
capabilities allow the operator to enter the bit
diameter, desired pitch, depth, and angle. Using
these part specifications, the controller generates
all motion profiles and stores them in nonvolatile
battery-backed RAM. Programming is
accomplished with the controller’s menu-driven
language. The typical process is as follows:
1. Axis 1 aligns the center line of the bit to the
cutting tool.
2. Axis 2 lowers the cutting tool to the desired
cutting depth (chamfer).
3.
Axis 3 traverses the bit along the cutting tool.
4. While axis 3 traverses, axis 4 rotates the bit to
create the desired pitch.
Product Solutions:
Indexer
Drives
Motor
Model 4000*
S Drives
S83-135
* The Model AT6400 and AT6450 are other
controllers that have been used in these types
of applications.
Indexer
Drive
Drive
Drive
Motor
(Axis 2 - Chamfer)
Motor
(Axis 4 Rotation)
Drive
Cutting
Tool
Bit
Motor
(Axis 3 Traverse)
Motor
(Axis 1 - Alignment)
A85
A Engineering Reference
13. Fluted-Bit Cutting Machine
Application Examples
14. Surface Grinding Machine
Application Type: Tool Feed
Motion: Linear
Application Description: A specialty machine
shop is improving the efficiency of its surface
grinding process. The existing machine is sound
mechanically, but manually operated. Automating
the machine will free the operator for other tasks,
which will increase overall throughput of the
machine shop.
Machine Requirements:
• Allow flexibility to machine various parts
• Easy set up for new parts
• Automate all three axes
• Keep operator informed as to progress
• Low-cost solution
• High-resolution grinding
Motion Control Requirements:
• Nonvolatile memory for program storage
• Teach mode
• Multi-axis controller
• Interactive user configurable display
• Open-loop stepper if possible
• High resolution motor/drive (microstepping)
Application Solution:
A four-axis motion controller with a userconfigurable front panel is required for this
application. An indexer with a sealed, backlit
display would be ideal for the application’s
industrial environment (machine shop). The
controller’s Teach mode and sizable nonvolatile
memory allows for easy entry and storage of new
part programs. Microstepping drives, which plenty
of power, resolution, and accuracy are selected
instead of more expensive closed-loop servo
systems. The operator utilizes the controller’s jog
function to position the grinding head at the proper
“spark off” height. From this point, the controller
takes over and finishes the part while the operator
works on other critical tasks. Increasing the parts
repeatability and throughput of the process
justified the cost of automating the machine.
Product Solutions:
Indexer
Drive
Motor
Model 4000*
S Drive
S83-93
* The AT6400 PC-based indexer has also been
used to solve similar applications.
Motor
Safety
Guard
Grinding
Wheel
Control Panel
Motors
Indexer
A86
Application Examples
Application Type: Tool Feed
Motion: Linear
Application Description: A stage of a transfer
machine is required to drill several holes in a
casting using a multi-head drill. The motor has to
drive the drill head at high speed to within 0.1" of
the workpiece and then proceed at cutting speed
to the required depth. The drill is then withdrawn at
an intermediate speed until clear of the work, then
fast-retracted and set for the next cycle. The
complete drilling cycle takes 2.2 seconds with a
0.6-second delay before the next cycle.
Due to the proximity of other equipment, the length
in the direction of travel is very restricted. An
additional requirement is to monitor the machine
for drill wear and breakage.
Machine Requirements:
• Limited length of travel
• Limited maintenance
• Monitor and minimize drill damage
• High-speed drilling
Motion Control Requirements:
• Packaged drive controller
• Complex motion profile
• High speed
• High duty cycle
Application Solution:
The combined requirements of high speed, high
duty cycle and monitoring the drill wear all point to
the use of a servo motor. By checking the torque
load on the motor (achieved by monitoring drive
current), the drilling phase can be monitored (an
increased load during this phase indicates that the
drill is broken).
This type of application will require a ballscrew
drive to achieve high stiffness together with high
speed. One way of minimizing the length of the
mechanism is to attach the ballscrew to the
moving stage and then rotate the nut, allowing the
motor to be buried underneath the table. Since
access for maintenance will then be difficult, a
brushless motor should be selected.
Product Solutions:
Drive/Controller
Motor
APEX6152
606 Motor
Drill
Head
Table
Ballscrew
Rotating Nut
Motor
Drive/Controller
A87
A Engineering Reference
15. Transfer Machine
Application Examples
16. Flute Grinder
Application Type: Tool Feed
Motion: Linear
Application Description: A low-cost machine
for grinding the flutes in twist drills requires two
axes of movement—one moves the drill forwards
underneath the grinding wheel, the other rotates
the drill to produce the helical flute. At the end of
the cut, the rotary axis has to index the drill round
by 180° to be ready to grind the second flute. The
linear speed of the workpiece does not exceed 0.5
inches/sec.
Machine Requirements:
• Two-axis control
• Low cost
• Easy set-up and change over of part programs
• Smooth, accurate cutting motion
Motion Control Requirements:
• Two-axis indexer
• Linear interpolation between axes
• Nonvolatile program storage
• Flexible data pad input
• Moderate speeds
• Programmable I/O
Grinding Wheel
Application Solution:
This is a natural application for stepper motors,
since the speeds are moderate and the solution
must be minimum-cost. The grinding process
requires that the two axes move at accurately
related speed, so the controller must be capable
of performing linear interpolation. The small
dynamic position error of the stepper system
ensures that the two axes will track accurately at
all speeds.
Product Solutions:
Controller
Drive
Motor
Operator
Interface
6200*
S Drive
S83-135
RP240
* The Model 4000-FP has also been used to
solve similar applications.
Axis
Motor
Rotary
Head
Twist
Drill
Drive
X-Axis
Motor
Axis Drive
X-Axis
Drive
Operator Interface
A88
Controller
Drive
Application Examples
Application Type: Tool Feed
Motion: Rotary
Application Description: Rigid computer discs
need to be burnished so that they are flat to within
tight tolerances. A sensor and a burnishing head
move together radially across the disc. When a
high spot is sensed, both heads stop while the
burnishing head removes the raised material. The
surface speed of the disc relative to the heads
must remain constant, and at the smallest
diameter, the required disc speed is 2400 rpm.
The machine operates in a clean environment, and
takes approximately one minute to scan an
unblemished disk.
Machine Requirements:
• High-speed burnishing
• Surface speed of disc relative to the heads must
remain constant
• Clean environment—∴ no brushed servo
motors
Motion Control Requirements:
• Variable storage, conditional branching and
math capabilities
• Linear interpolation between the head axes
(axes #1 and #2)
• Change velocity on-the-fly
• Programmable inputs
Axis 1
Sensing Head
Application Solution:
The drive for the disc requires continuous
operation at high speed, and a brushless solution
is desirable to help maintain clean conditions. The
natural choice is a brushless servo system. The
speed of this axis depends on head position and
will need to increase as the heads scan from the
outside to the center. To successfully solve this
application, the multi-axis indexer requires variable
storage, the ability to perform math functions, and
the flexibility to change velocity on-the-fly.
The sense and burnishing heads traverse at low
speed and can be driven by stepper motors.
Stepper motors—since the sense and burnishing
heads need to start and step at the same time,
linear interpolation is required.
Product Solutions:
Controller Drive #1
Model 4000* S Drive
Drive #2
Drive #3
S Drive
Z Drive
Motor #1
Motor #2
Motor #3
S83-93
S83-93
Z60
* The AT6400 PC-based indexer has also been
used in these types of applications.
Axis 2
Burnishing Head
Motor
Motor
Disc
Axis 3
Disc Drive Motor
Drive
Multi Axis
Controller (4000)
Drive
Drive
A89
A Engineering Reference
17. Disc Burnisher
Application Examples
18. Monofilament Winder
Application Type: Winding
Motion: Rotary
Application Description: Monofilament nylon is
produced by an extrusion process that results in
an output of filament at a constant rate. The
product is wound onto a bobbin that rotates at a
maximum speed of 2000 rpm. The tension in the
filament must be held between 0.2 lbs. and 0.6 lbs
to ensure that it is not stretched. The winding
diameter varies between 2" and 4".
The filament is laid onto the bobbin by a ballscrewdriven arm, which oscillates back and forth at
constant speed. The arm must reverse rapidly at
the end of the move. The required ballscrew speed
is 60 rpm.
Machine Requirements:
• Controlled tension on monofilament
• Simple operator interface
• High throughput
Motion Control Requirements:
• 2 axes of coordinated motion
• Linear interpolation
• Constant torque from motor
Application Solution:
The prime requirement of the bobbin drive is to
provide a controlled tension, which means
operating in Torque mode rather than Velocity
mode. If the motor produces a constant torque,
the tension in the filament will be inversely
proportional to the winding diameter. Since the
winding diameter varies by 2:1, the tension will fall
by 50% from start to finish. A 3:1 variation in
tension is adequate, so constant-torque operation
is acceptable. (To maintain constant tension,
torque must be increased in proportion to winding
diameter.)
This requirement leads to the use of a servo
operating in torque mode (the need for constantspeed operation at 2000 rpm also makes a
stepper unsuitable). In practice, a servo in Velocity
mode might be recommended, but with an
overriding torque limit, the programmed velocity
would be a little more than 2000 rpm. In this way,
the servo will normally operate as a constanttorque drive. However, if the filament breaks, the
velocity would be limited to the programmed
value.
The traversing arm can be adequately driven by a
smaller servo.
Product Solutions:
Indexer
Drive
Motor
6250*
BL30
ML2340
* The AT6450 PC-based servo controller and the
APEX20/APEX40 servo controllers have also
been used in this type of application.
Bobbin
Drive
Torque
Motor
Servo
Drive
A90
Controller
Application Examples
Application Type: Winding
Motion: Linear
Application Description: The customer winds
aluminum electrolytic capacitors. Six reels, two
with foil (anode and cathode) and four with paper,
are all wound together to form the capacitor. After
winding the material a designated number of turns,
the process is stopped and anode and cathode
tabs are placed on the paper and foil. The tabs
must be placed so that when the capacitor is
wound, the tabs end up 90° (±0.1°) from each
other. This process is repeated until the required
number of tabs are placed and the capacitor
reaches its appropriate diameter.
The previous system used a PLC, conventional DC
drives, and counters to initiate all machine
functions. DIP switches were used to change and
select capacitor lengths. Lengthy set-up and
calibration procedures were required for proper
operation. In addition, material breakage was
common, resulting in extensive downtime. An
operator had to monitor the machine at all times to
constantly adjust the distances for accurate tab
placement.
Machine Requirements:
• Constantly monitor the linear feed length of the
paper and foil and calculate the constantly
changing capacitor circumference as a function
of that length
• A complete motion control package is required
to eliminate the need for a PLC and separate
motion cards
• Reduce time and complexity of set-up (too much
wiring in previous system)
• Reduce machine downtime caused by material
breakage
Motion Control Requirements:
• Following
• Two axes of coordinated motion
• Math capability
• AT-based control card
Drive
Application Solution:
Precise motion control of the material feed axes
demands closed-loop servo commands. Actuation
of external cylinders and solenoids requires both
analog and digital I/O. A flexible operator interface
is needed for diagnostics and other alterations of
machine function. Motion, I/O, and an operator
interface should be provided with a machine
controller.
The first motorized axis (mandril) pulls all six
materials together and feeds an appropriate
distance. An encoder is placed on this motor as
well as on the materials as they are fed into the
mandril. The controller constantly compares the
two encoders to get an exact measurement of
linear distance, and compensates for material
stretching.
When the linear distance is achieved, the first
motor comes to an abrupt stop while a second
axis places a tab. The controller then initiates a
cold weld (pressure weld) of the tab onto the
paper and foil.
To avoid material breakage, constant tension is
applied to each of the six reels via air cylinders.
Sensors are installed on all axes so that if a break
occurs, the controller can stop the process.
A computer makes this process easy to use and
set up. PC/AT-based support software allows the
user to build his controller command program.
The operator sets the diameter of the appropriate
capacitor, the operating speed and the number of
capacitors (all via the keyboard). After this
process, the machine runs until a malfunction
occurs or it has completed the job.
Product Solutions:
Controller
Drive
AT6250*
BL30
ML2340 -E Encoder
* The 6250 standalone 2-axis servo controller and
APEX20/APEX40 servo drives have also been
used in these types of applications.
Drive
Encoder
Input
Motor Accessories
Anode
Tab
Reel
Motor
Output
Cathode
Tab
Reel
Anode
Foil Reel
Tab Feeder
Axis Motor
Spindel
Axis Motor
and Encoder
Paper
Reel
Paper
Reel
Capacitor
Wound Onto
Spindle
Opto I/O Rack
Paper
Reel
Encoder
I/O to Limits,
Cylinders and
Solenoids
Computer
(Indexer installed in a PC)
Cathode
Foil Reel
Paper
Reel
A91
A Engineering Reference
19. Capacitor Winder
Application Examples
20. Labelling Machine
Application Type: Following
Motion: Linear
Application Description: Bottles on a conveyor
run through a labelling mechanism that applies a
label to the bottle. The spacing of the bottles on
the conveyor is not regulated and the conveyor
can slow down, speed up, or stop at any time.
Velocity
Machine Requirements:
• Accurately apply labels to bottles in motion
• Allow for variable conveyor speed
• Allow for inconsistent distance between bottles
• Pull label web through dispenser
• Smooth, consistent labelling at all speeds
Motion Control Requirements:
• Synchronization to conveyor axis
• Electronic gearbox function
• Registration control
• High torque to overcome high friction
• High resolution
• Open-loop stepper if possible
Application Solution:
A motion controller that can accept input from an
encoder mounted to the conveyor and reference
all of the speeds and distances of the label roll to
the encoder is required for this application. A
servo system is also required to provide the
torque and speed to overcome the friction of the
dispensing head and the inertia of the large roll of
labels. A photosensor connected to a
programmable input on the controller monitors the
bottles’ positions on the conveyor. The controller
commands the label motor to accelerate to line
speed by the time the first edge of the label
contacts the bottle. The label motor moves at line
speed until the complete label is applied, and then
decelerates to a stop and waits for the next bottle.
Product Solutions:
Controller
Motor
APEX6152*
APEX604
* The ZXF single-axis servo controller has also
been used in these types of applications.
Primary Axis
Secondary Axis
Time
Registration Input
Start Photocell
Servo
Encoder
Drive/Controller
A92
Application Examples
Application Type: Following
Motion: Linear
Application Description: A window blind
manufacturer uses an adhesive to form a seam
along the edge of the material. It is critical that the
glue be applied evenly to avoid flaws; however, the
speed that the material passes beneath the
dispensing head is not constant. The glue needs
to be dispensed at a rate proportional to the
varying speed of the material.
Program
Machine Requirements:
• Allow for varying material speed
• Dispense glue evenly
• Allow for multiple blind lengths
Motion Control Requirements:
• Synchronization to material speed
• Velocity following capabilities
• Sequence storage
Application Solution:
A step and direction indexer/follower and a
microstepping motor/drive are used to power a
displacement pump. The indexer/follower is
programmed to run the motor/drive at a velocity
proportional to the primary velocity of the material,
based on input from a rotary incremental encoder.
This assures a constant amount of glue along the
length of the material.
One input will be configured to start motion, a
second input will be used to stop motion. The
motor has 10000 steps/revolution. The encoder
that is placed on the motor pulling the material
has 4000 pulses/revolution. It is desired to have
the motor dispensing the glue turning twice as
fast as the encoder sensing the material.
When the start button is depressed, the glue will
begin dispensing and can be discontinued with the
stop button. If a new speed ratio is desired, FOR
can be changed with either the front panel
pushbutton, thumbwheels, or with the RS-232C
serial link.
Two following commands are used.
FOR
Sets the ratio between the secondary
motor resolution and the primary
encoder resolution
FOL
Sets the ratio of the speed between
the primary and secondary motor
FOR2.5
Set the motor to encoder ratio
FOL2ØØ
The following speed ratio is 200% or
twice as fast
A1Ø
Set acceleration to 10 rps2
AD1Ø
Set deceleration to 10 rps2
MC
The controller is placed in Continuous
mode
Product Solutions:
Drive/Controller
Motor
SXF Drive/Controller*
S57-102
* The Model 500 single-axis controller and the
S Drive have also been used in these types of
applications.
Motor
Encoder
Drive/Controller
A93
A Engineering Reference
21. Window Blind Gluing
Application Examples
22. Moving Positioning System
Application Type: Following
Motion: Linear
Application Description: In a packaging
application, a single conveyor of boxes rides
between 2 conveyors of product. The product
must be accurately placed in the boxes from
alternate product conveyors without stopping the
center conveyor of boxes. The line speed of the
boxes may vary. When the product is ready, the
controller must decide which box the product can
be placed into and then move the product into
alignment with the moving box. The product must
be moving along side of the box in time for the
product to be pushed into the box.
Machine Requirements:
• Reliable product packaging on the fly
• Standalone operation
• Multiple product infeeds
• Continuous operation without stopping the box
conveyor
Motion Control Requirements:
• Programmable I/O
• Sequence storage
• Complex following capabilities
• Moving positioning system functionality
• Multitasking
Product
Application Solution:
A standalone multiple-axis controller provides the
control for this application. The controller can
perform motion profiling based on an external
encoder that is mounted on the center conveyor of
boxes. The two product conveyors are driven by
servo motors for high speeds and accelerations.
The controller looks for a product ready signal
from a sensor mounted on the product infeed
conveyor and then makes a move based on the
status of the boxes on the box conveyor and the
status of the product on the other product
conveyor. The controller is multitasking the control
of the two product conveyors and the external
encoder input, as well as a sensor input to monitor
the status of the boxes. Thus the controller can
instantaneously decide into which box the product
should be placed and where that box is located.
The controller then accelerates the product into
alignment with the appropriate box in time for the
product to be completely placed in the box, and
continues to monitor the other rest of the product
and box positions.
Product Solutions:
Controller
Drive
Motor
Encoder
Model 500
L Drive
L20
-E Encoder
Product
Infeed
Box
Conveyor
Product
Product
Infeed
Product
Synchronization
Product
Synchronization
Controller
A94
Drive
Drive
Drive
Drive
Application Examples
Application Type: Injection Molding
Motion: Linear
Application Description: A manufacturer of
injection molding machines wants a system that
will close a molding chamber, apply pressure to
the molding chamber for 5 seconds and then open
the mold. This action needs to be synchronized
with other machine events. When the molding
chamber is open the motor must be ‘parked’ at a
designated position to allow clearance to remove
the molded part. The manufacturer would like an
electronic solution (this is the only hydraulic axis on
the current machine).
Machine Requirements:
• Electronic solution
• Computer-controlled solution
• 4000N (900lbs.) force
Motion Control Requirements:
• Position and torque control
• Serial link to computer and other drives
• Ability to change pressure and dwell
Application Solution: A BLHX75BP brushless
servo drive with an ML345OB-25 motor and an
ETS8O-BO4LA Electro-Thrust Electric Cylinder
were used. The motor drives the rod inside the
cylinder and extends/retracts the top molding
chamber. During this portion of the machine cycle,
the servo drive must control the position of the
motor. When the top molding chamber closes on
the bottom molding chamber, a pressure must be
applied. While pressure is being applied to the
mold the position of the motor is not important.
However, the motor must control the pressure on
the molding chamber by applying a torque from
the motor. A regular positioning servo can only
apply torque by generating a position error—trying
to control torque through position is not very
accurate and can create instabilities. The BLHX
servo was chosen because it can switch between
position control and torque control on-the-fly
without instability or saturation and then, while in
torque control mode, directly controls motor
torque.
Product Solutions:
Controller/Drive
Motor
Actuator
BLHX75BN
ML3450B-10
-ET580-BO4LA
ML3450B-25
Motor
Electric
Cylinder
Drive
"HOME" Position
Top Mold
Chamber
"PARK" Position
Bottom Mold
Chamber
"CLOSE" Position
A95
A Engineering Reference
23. Plastic Injection Molding
Application Examples
24. Rotating Tube Cutter
Application Type: Flying Cutoff
Motion: Linear
Application Description: Metal tubing feeds off
of a spool and needs to be cut into predetermined
lengths. A rotating blade mechanism is used to cut
the tube, and the blade mechanism must spin
around the tube many times in order to complete
the cut. The throughput of this machine must be
maximized, so the tubing cannot be stopped while
this cut is being made. Therefore, to make a clean
cut on the tube, the blade must move along with
the tube while the cut is being performed.
Machine Requirements:
• Standalone operation
• Move cutting mechanism with the tubing to
make the cut without stopping
• Simple user interface to set different tube
lengths
• High accuracy on cut
Motion Control Requirements:
• Programmable I/O
• Program storage
• Position following
• High acceleration and speed
Application Solution:
A single-axis servo controller/drive was chosen to
solve this application. An external encoder
monitors the tube output and sends this
information back to the servo system. The servo
system tracks the length of the tube that is being
fed past the cutting blade. Once the appropriate
amount of material has been fed past the blade,
the servo accelerates the cutting device up to the
speed of the tube, sends an output to start the
cutter, and then follows the tube speed exactly.
Product Solutions:
Drive/Controller
Motor
APEX6152
APEX610
Coil of Tube
Encoder
Cutting
Mechanism
Motor
RP240
Leadscrew
Table
A96
Controller/Drive
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