Introduction to Temperature Controllers

Introduction to Temperature Controllers
Introduction to
Temperature Controllers
On/Off
An on-off controller is the simplest form of temperature
control device. The output from the device is either
on or off, with no middle state. An on-off controller will
switch the output only when the temperature crosses
the setpoint. For heating control, the output is on when
the temperature is below the setpoint, and off above
setpoint.
Since the temperature crosses the setpoint to change
the output state, the process temperature will be cycling
continually, going from below setpoint to above, and
back below. In cases where this cycling occurs rapidly,
and to prevent damage to contactors and valves,
an on-off differential, or “hysteresis,“ is added to the
controller operations. This differential requires that the
temperature exceed setpoint by a certain amount before
the output will turn off or on again. On-off differential
prevents the output from “chattering” (that is, engaging
in fast, continual switching if the temperature’s cycling
above and below the setpoint occurs very rapidly).
On-off control is usually used where a precise control
is not necessary, in systems which cannot handle the
energy’s being turned on and off frequently, where
the mass of the system is so great that temperatures
change extremely slowly, or for a temperature alarm.
The Miniature CN77000 is a full featured
microprocessor-based controller in a 1⁄16 DIN
package.
How Can I Control My Process Temperature Accurately
and Reliably?
One special type of on-off control used for alarm is a
limit controller. This controller uses a latching relay,
which must be manually reset, and is used to shut down
a process when a certain temperature is reached.
To accurately control process temperature without
extensive operator involvement, a temperature control
system relies upon a controller, which accepts a
temperature sensor such as a thermocouple or RTD as
input. It compares the actual temperature to the desired
control temperature, or setpoint, and provides an output
to a control element.
ON
ON
ON
ON
Heater
OFF
The controller is one part of the entire control system,
and the whole system should be analyzed in selecting
the proper controller. The following items should be
considered when selecting a controller:
OFF
OFF
OFF
Temperature
1. Type of input sensor (thermocouple, RTD) and temperature range
2. Type of output required (electromechanical relay, SSR, analog output)
3
Control algorithm needed (on/off, proportional, PID)
4Number and type of outputs (heat, cool, alarm, limit)
Setpoint
On-Off
Differential
(Deadband)
What Are the Different Types of Controllers, and How
Do They Work?
There are three basic types of controllers: on-off,
proportional and PID. Depending upon the system to be
controlled, the operator will be able to use one type or
another to control the process.
Z-131
Time
ON/Off Temperature Control Action
Proportional
Proportional controls are designed to eliminate the
cycling associated with on-off control. A proportional
controller decreases the average power being supplied
to the heater as the temperature approaches setpoint.
This has the effect of slowing down the heater, so
that it will not overshoot the setpoint but will approach
the setpoint and maintain a stable temperature. This
proportioning action can be accomplished by turning
the output on and off for short intervals. This “time
proportioning” varies the ratio of ‘on’ time to ‘off’ time
to control the temperature. The proportioning action
occurs within a “proportional band” around the setpoint
temperature. Outside this band, the controller functions
as an on-off unit, with the output either fully on (below
the band) or fully off (above the band). However, within
the band, the output is turned on and off in the ratio
of the measurement difference from the setpoint. At
the setpoint (the midpoint of the proportional band),
the output on:off ratio is 1:1; that is, the on-time and
off-time are equal. If the temperature is further from
the setpoint, the on- and off-times vary in proportion to
15 Sec. On
the temperature difference. If the temperature is below
setpoint, the output will be on longer; if the temperature
is too high, the output will be off longer.
The proportional band is usually expressed as a
percent of full scale, or degrees. It may also be referred
to as gain, which is the reciprocal of the band. Note,
that in time proportioning control, full power is applied
to the heater, but is cycled on and off, so the average
time is varied. In most units, the cycle time and/or
proportional band are adjustable, so that the controller
may better match a particular process.
In addition to electromechanical and solid state relay
outputs, proportional controllers are also available with
proportional analog outputs, such as 4 to 20 mA or 0
to 5 Vdc. With these outputs, the actual output level is
varied, rather than the on and off times, as with a relay
output controller.
One of the advantages of proportional control is
simplicity of operation. It may require an operator
to make a small adjustment (manual reset) to bring
the temperature to setpoint on initial startup, or if the
process conditions change significantly.
5 Off
Time Proportional
4-20 mA
Proportional
Percent On Time Off Time Temp Output Percent
On SecondsSeconds (ºF) Level Output
Repetitive
20 sec. Cycle
Time
Time Proportioning at 75% Output Level
0.0
0.0
20.0 over 540 4 mA
0.0
0.0
0.0
20.0
540.0
4 mA
0.0
12.5
2.5
17.5
530.0
6 mA
12.5
25.0
5.0
15.0
520.0
8 mA
25.0
37.5
7.5
12.5
510.0
10 mA 37.5
50.0
10.0
10.0
500.0
12 mA 50.0
62.512.57.5490.0
14 mA
62.5
75.015.05.0480.0
16 mA
75.0
87.517.52.5470.0
18 mA
87.5
100.020.00.0460.0
20 mA
100.0
100.020.00.0
under 460
20 mA
100.0
Proportional Bandwidth
Example: heating
Setpoint: 500°F
Proportional Band: 80°F
(±40°F)
Systems that are subject to wide temperature cycling
will also need proportional controllers. Depending upon
the process and the precision required, either a simple
proportional control or one with PID may be required.
The CN3251 controller features ramp and soak, the
ability to control temperature over time.
Processes with long time lags and large maximum
rate of rise (e.g., a heat exchanger), require wide
proportional bands to eliminate oscillation. The wide
band can result in large offsets with changes in the
load. To eliminate these offsets, automatic reset
(integral) can be used. Derivative (rate) action can be
used on processes with long time delays, to speed
recovery after a process disturbance.
Z-132
Z
Introduction To Temperature Controllers Cont’d
There are also other features to consider when
selecting a controller. These include auto- or selftuning, where the instrument will automatically calculate
the proper proportional band, rate and reset values for
precise control; serial communications, where the unit
can “talk” to a host computer for data storage, analysis,
and tuning; alarms, that can be latching (manual reset)
or non-latching (automatic reset), set to trigger on high
or low process temperatures or if a deviation from
setpoint is observed; timers/event indicators which can
mark elapsed time or the end/beginning of an event.
In addition, relay or triac output units can be used with
external switches, such as SSR solid state relays or
magnetic contactors, in order to switch large loads up to
75 A.
PID The third controller type provides proportional with
integral and derivative control, or PID. This controller
combines proportional control with two additional
adjustments, which helps the unit automatically
compensate for changes in the system. These
adjustments, integral and derivative, are expressed
Offset
Temp.
PB SP
Time
Process with Temperature Offset
in time-based units; they are also referred to by their
reciprocals, RESET and RATE, respectively.
The proportional, integral and derivative terms must be
individually adjusted or “tuned” to a particular system,
using a “trial and error” method. It provides the most
accurate and stable control of the three controller types,
and is best used in systems which have a relatively
small mass, those which react quickly to changes
in energy added to the process. It is recommended
in systems where the load changes often, and the
controller is expected to compensate automatically due
to frequent changes in setpoint, the amount of energy
available, or the mass to be controlled.
What Do Rate and Reset Do, and How Do They Work?
Rate and reset are methods used by controllers to
compensate for offsets and shifts in temperature. When
using a proportional controller, it is very rare that the
heat input to maintain the setpoint temperature will be
50%; the temperature will either increase or decrease
from the setpoint, until a stable temperature is obtained.
The difference between this stable temperature and the
setpoint is called offset. This offset can be compensated
for manually or automatically. Using manual reset, the
user will shift the proportional band so that the process
will stabilize at the setpoint temperature. Automatic
reset, also known as integral, will integrate the deviation
signal with respect to time, and the integral is summed
with the deviation signal to shift the proportional band.
The output power is thus automatically increased or
decreased to bring the process temperature back to
setpoint,
The rate or derivative function provides the controller
with the ability to shift the proportional band, to
compensate for rapidly changing temperature. The
amount of shift is proportional to the rate of temperature
change.
A PID, or three-mode controller, combines the
proportional, integral (reset) and derivative (rate)
actions, and is usually required to control difficult
processes. These controllers can also be made with
two proportional outputs, one for heating and another
for cooling. This type of controller is required for
processes which may require heat to start up, but then
generate excess heat at some time during operation.
What are the Different Output Types That Are Available
for Controllers?
The output from the controller may take one of several
forms. The most common forms are time proportional
and analog proportional. A time proportional output
applies power to the load for a percentage of a fixed
cycle time. For example, with a 10 second cycle time, if
the controller output were set for 60%, the relay would
be energized (closed, power applied) for 6 seconds,
and de-energized (open, no power applied) for 4
seconds. Time proportional outputs are available in
three different forms: electromechanical relay, triac or
ac solid state relay, or a dc voltage pulse (to drive an
external solid state relay). The electromechanical relay
is generally the most economical type, and is usually
chosen on systems with cycle times greater than 10
seconds, and relatively small loads.
An ac solid state relay or dc voltage pulse are chosen
for reliability, since they contain no moving parts.
Recommended for processes requiring short cycle
times, they need an additional relay, external to the
controller, to handle the typical load required by a
heating element. These external solid state relays are
usually used with an ac control signal for ac solid state
relay output controllers, or with a dc control signal for dc
voltage pulse output controllers.
An analog proportional output is usually an analog
voltage (0 to 5 Vdc) or current (4 to 20 mA). The output
level from this output type is also set by the controller;
if the output were set at 60%, the output level would
be 60% of 5 V, or 3 V. With a 4 to 20 mA output (a 16
mA span), 60% is equal to (0.6 x 16) + 4, or 13.6 mA.
These controllers are usually used with proportioning
valves or power controllers.
What Should I Consider When Selecting a Controller for
My Application?
When you choose a controller, the main considerations
include the precision of control that is necessary, and
how difficult the process is to control. For easiest tuning
and lowest initial cost, the simplest controller which will
produce the desired results should be selected.
Simple processes with a well matched heater (not overor undersized) and without rapid cycling can possibly
use on-off controllers. For those systems subject to
cycling, or with an unmatched heater (either over- or
undersized), a proportional controller is needed.
Z-133
Temperature Controllers
Selection Considerations
CONTROLLABILITY OF
ELECTRIC HEAT
The basic function of a controller is to
compare the actual temperature with its
setpoint and produce an output that will
maintain the setpoint.
The controller is one part of the entire
control system, and the whole system
should be analyzed in selecting the
proper controller. The following items
should be considered when selecting a
controller.
1.Type of input sensor (thermocouple,
RTD, card, and temperature range)
2.Placement of sensor
3.Control algorithm needed (on/off,
proportional, PID, autotune PID)
4.Type of output hardware required
(electromechanical relay, SSR, analog
output signal)
5.Additional outputs or requirements
of system (display required of
temperature and/or setpoint, cooling
outputs, alarms, limit, computer
communication, etc.)
TYPE OF INPUT
The type of input sensor will depend
on the temperature range required,
the resolution and accuracy of the
measurement required, and how and
where the sensor is to be mounted.
PLACEMENT OF THE SENSOR
The correct placement of the sensing
element with respect to the work and
heat source is of the utmost importance
for good control. If all three can be
located in close proximity, a high
degree of accuracy, up to the limit of the
controller, is relatively easy to achieve.
However, if the heat source is located
some distance from the work, widely
different accuracies can be obtained
just by locating the sensing element at
various places between the heater and
the work.
Before selecting the location for the
sensing element, determine whether
the heat demand will be predominantly
steady or variable. If the heat demand
is relatively steady, placement of the
sensing element near the heat source
will hold the temperature change at the
work to a minimum.
On the other hand, placing the sensing
element near the work, when heat
demand is variable, will enable it to
more quickly sense a change in heat
requirements. However, because of
the increase in thermal lag between
the heater and the sensing elements,
more overshoot and undershoot can
occur, causing a greater spread between
maximum and minimum temperature.
This spread can be reduced by selecting
a PID controller.
CONTROL ALGORITHM (MODE)
This refers to the method in which the
controller attempts to restore system
temperature to the desired level. The two
most common methods are two-position
(on-off) and proportioning (throttling)
controls.
ON/OFF CONTROL
On/off control has the simplest of control
modes. It has a deadband (differential)
expressed as a percentage of the input
span. The setpoint is usually in the
center of the deadband. Therefore, if
the input is 0 to 1000°F, the deadband
is 1% and the setpoint is set at 500°F,
the output will be full on when the
temperature is 495°F or below and
will stay full on until the temperature
reaches 505°F, at which time the output
will be full off. It will stay full off until the
temperature drops to 495°F.
If the process has a fast rate of
response, the cycling between 495 and
505°F will be fast. The faster the rate of
response of the process, the greater the
overshoot and undershoot and the faster
the cycling of the contactor when used
as a final control element.
On/off control is usually used where
a precise control is not necessary, for
example, in systems that cannot handle
having the energy turned on and off
frequently, where the mass of the system
is so great that the temperature changes
extremely slowly, or for a temperature
alarm.
One special type of on/off control used
for alarm is a limit controller. This
controller uses a latching relay, which
must be manually reset, and is used
to shut down a process when a certain
temperature is reached.
PROPORTIONAL
Proportional controls are designed to
eliminate the cycling associated with
on/off control. A proportional controller
decreases the average power being
supplied to the heater as the temperature
approaches setpoint. This has the effect
of slowing down the heater so that it
will not overshoot the setpoint, but will
approach the setpoint and maintain a
stable temperature. This proportioning
action can be accomplished by turning
the output on and off for short intervals.
This “time proportioning” varies the ratio
of “on” time to “off” time to control the
temperature.
The time period between two successive
turn-ons is known as the “cycle time” or
“duty cycle.” The proportioning action
occurs within a “proportional band”
around the setpoint temperature. Outside
this band, the controller functions as an
on/off unit, with the output either fully
on (below the band) or fully off (above
the band). However, within the band,
the output is turned on and off in the
Z-134
ratio of the measurement difference
from the setpoint. At the setpoint (the
midpoint of the proportional band), the
output on/off ratio is 1:1, that is, the
on-time and off-time are equal. If the
temperature is further from the setpoint,
the on- and off-times vary in proportion
to the temperature difference. If the
temperature is below setpoint, the output
will be on longer. If the temperature is too
high, the output will be off longer.
The proportional band is usually
Above
Temp.
At
Setpoint
Temp.
On
Time
Off
Time
Below
Temp.
Figure 1: Proportional control
expressed as a percentage of full input
range scale, or in degrees. It may also
be referred to as gain, which is the
reciprocal of the band. In many units, the
cycle time and/or proportional bandwidth
are adjustable, so that the controller
can be better matched to a particular
process.
Proportional controllers have a manual
reset (trim) adjustment, which can be
used to adjust for an offset between
the steady state temperature and the
setpoint.
In addition to electromechanical and
solid state relay outputs, proportional
controllers are also available with
proportional analog signal outputs, such
as 4 to 20 mA or 0 to 5 Vdc. With these
outputs, the actual output level amplitude
is varied, rather than the proportion of on
and off times.
PROPORTIONAL PLUS INTEGRAL
PLUS DERIVATIVE CONTROL MODE
(PID):
This controller operates the same way
a proportional controller does, except
that the function of the trim adjustment
is performed automatically by the
integral function (automatic reset).
Thus, load changes are compensated
for automatically and the temperature
agrees with the setpoint under all
operating conditions. Offset is eliminated.
The derivative function (rate action)
Z
Temperature Controllers
Selection Considerations
Low Rate
Setting
Set
Point
Rate Set
Properly
Figure 2: Rate function compensates for rapid
changes.
compensates for load changes which
take place rapidly. An example is a
traveling belt oven where the product
is fed intermittently. When the product
enters the oven, there is a sharp rise in
the demand for heat, and when it stops,
there is an excess of heat. Derivative
action reduces the undershoot and
overshoot of temperature under these
conditions and prevents bad product due
to over or under curing.
Set
Point
Offset
Figure 3: Reset function eliminates offset.
PID control provides more accurate and
stable control than on/off or proportional
controller types. It is best used in
systems that have a relatively small
mass and which react quickly to changes
in energy added to the process. It is
recommended in systems where the load
changes often. The controller is expected
to automatically compensate the amount
of energy available or the mass to be
controlled, due to frequent changes in
setpoint.
The proportional, integral and derivative
terms must be “tuned,” i.e., adjusted
to a particular process. This is done by
trial and error. Some controllers called
Autotune controllers attempt to adjust the
PID parameters automatically.
TYPE OF CONTROL OUTPUT
HARDWARE
The output hardware in a temperature
controller may take one of several
forms. Deciding on the type of control
hardware to be used depends on the
heater used and power available, the
control algorithm chosen, and the
hardware external to the controller
available to handle the heater load. The
most commonly used controller output
hardware is as follows:
Time Proportional or On/Off
1)Mechanical Relay
2)Triac (ac solid state relay)
3)dc Solid State Relay Driver (pulse)
Analog Proportional
1) 4-20 mA dc
2) 0-5 Vdc or 0-10 Vdc
A time proportional output applies power
to the load for a percentage of a fixed
cycle time. For example, with a 10
second cycle time, if the controller output
were set for 60%, the relay would be
energized (closed, power applied) for 6
seconds, and de-energized (open, no
power applied) for 4 seconds.
The electromechanical relay is generally
the most economical output type, and
is usually chosen on systems with cycle
times greater than 10 seconds and
relatively small loads.
Choose an ac solid state relay or dc
voltage pulse to drive an external
SSR with reliability, since they contain
no moving parts. They are also
recommended for processes requiring
short cycle times. External solid state
relays may require an ac or dc control
signal.
An amplitude proportional output is
usually an analog voltage (0 to 5 Vdc)
or current (4 to 20 mA). The output level
from this output type is also set by the
controller. If the output were set at 60%,
the output level would be 60% of 5 V, or
3 V. With a 4 to 20 mA output (a 16 mA
span), 60% is equal to (0.6 x 16) + 4, or
13.6 mA. These controllers are usually
used with SCR power controllers or
proportioning valves.
The power used by an electrical
resistance heater will usually be given in
watts. The capacity of a relay is given in
amps. A common formula to determine
the safe relay rating requirements is:
W = V(A)(1.5) or A = W/(V)(1.5)
Where A = relay rating in amps
W = heater capacity in watts
V = voltage used
1.5 = safety factor
The types of hardware available, external
Z-135
to the controller, to allow it to handle the
load, are as follows:
1) Mechanical Contactor
2) ac controlled solid state relay
3) dc controlled solid state relay
4) Zero crossover SCR power controller
5) Phase angle fired SCR power controller
Mechanical contactors are external
relays, which can be used when a higher
amperage than can be handled by the
relay in the controller is required, or for
some three-phase systems. They are
not recommended for cycle times shorter
than 15 seconds.
Solid state relays have the advantage
over mechanical contactors, in that they
have no moving parts, and thus can be
used with short cycle times. The shorter
the cycle time, the less dead lag and the
better the control. The “switching” takes
place at the zero voltage crossover point
of the alternating current cycle; thus, no
appreciable electrical noise is generated.
An ac controlled solid state relay is
used with either a mechanical relay or
triac output from the controller, and is
available for currents up to 90 amps at
voltages of up to 480 Vac. DC solid state
relays are used with dc solid state driver
(pulse) outputs. The “turn on” signal can
be from 3 to 32 Vdc and models are
available to control up to 90 amps at up
to 480 Vac.
Zero crossover SCR power controllers
are used to control single or three-phase
power for even larger loads. They can
be used for currents up to 200 amps at
480 volts. A 4-20 mA dc control signal is
usually required from the controller. The
zero crossover SCR power controllers
convert the analog output signal to a
time proportional signal with a cycle time
of about two seconds or less, and also
provide switching at the zero crossover
point to avoid generating electrical noise.
Phase angle SCR power controllers also
are operated by a 4-20 mA dc controller
output. Power to the load is controlled
by governing the point of turn on (firing)
of each half cycle of a full ac sine wave.
This has the effect of varying the voltage
within a single 0.0167 second period. By
comparison, time proportional controllers
vary the average power over the cycle
time, usually more than 1 second, and
often more than 15 seconds. Phase
angle SCR’s are only recommended
for low mass heating elements such as
infrared lamps or hot wire heaters.
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