LVDT Basics

LVDT Basics
LVDT Basics
Technical Bulletin
What Is An LVDT?
The letters LVDT are an acronym for Linear Variable Differential
Transformer, a common type of electromechanical transducer
that can convert the rectilinear motion of an object to which it is
coupled mechanically into a corresponding electrical signal. LVDT
linear position sensors are readily available that can measure
movements as small as a few millionths of an inch up to several
inches, but are also capable of measuring positions up to ±20
inches (±0.5 m).
Stainless Steel Housing and End Caps
High Permeability
Magnetic Shell
High Density Glass Filled
Polymer Coil Form
Coil Assembly
Core
Primary Winding
Secondary Windings
Threaded Hole
(both ends)
Epoxy
Encapsulation
Core
of thermally stable glass reinforced polymer, encapsulated
against moisture, wrapped in a high permeability magnetic shield,
and then secured in a cylindrical stainless steel housing. This coil
assembly is usually the stationary element of the position sensor.
The moving element of an LVDT is a separate tubular armature
of magnetically permeable material called the core, which is free
to move axially within the coil’s hollow bore, and mechanically
coupled to the object whose position is being measured. This
bore is typically large enough to provide substantial radial
clearance between the core and bore, with no physical contact
between it and the coil.
In operation, the LVDT’s primary winding is energized by
alternating current of appropriate amplitude and frequency, known
as the primary excitation. The LVDT’s electrical output signal is
the differential AC voltage between the two secondary windings,
which varies with the axial position of the core within the LVDT
coil. Usually this AC output voltage is converted by suitable
electronic circuitry to high level DC voltage(0-5V) or current
(4-20mA) or digital(RS-485) signal that is more convenient to use.
High Permeability Nickel-Iron Core
Figure 1
The features that make an LVDT environmentally robust are evident in this cutaway view.
Figure 1 shows the components of a typical LVDT. The
transformer’s internal structure consists of a primary winding
centered between a pair of identically wound secondary windings,
symmetrically spaced about the primary. The coils are wound on
a one-piece hollow form
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How Does An LVDT Work?
Figure 2 illustrates what happens when the LVDT’s core is
in different axial positions. The LVDT’s primary winding, P, is
energized by a constant amplitude AC source. The magnetic flux
thus developed is coupled by the core to the adjacent secondary
windings, S1 and S2. If the core is located midway between S1 and
S2, equal flux is coupled to each secondary so the voltages, E1
and E2, induced in windings S1 and S2 respectively, are equal. At
this reference midway core position, known as the null point, the
differential voltage output, (E1 - E2), is essentially zero.
EOUT = E 1 - E 2
S1
P
S2
CORE
MAX. LEFT
EOUT = E 1 - E 2 = 0 V
S1
P
S2
CORE
NULL
EOUT = E 2 - E 1
S1
P
S2
CORE
MAX. RIGHT
As shown in Figure 2, if
the core is moved closer to
S1 than to S2, more flux is
coupled to S1 and less to
S2, so the induced voltage
E1 is increased while E2 is
decreased, resulting in the
differential voltage (E1 - E2).
Conversely, if the core is
moved closer to S2, more flux
is coupled to S2 and less to
S1, so E2 is increased as E1
is decreased, resulting in the
differential voltage (E2 - E1).
Figure 3A shows how the
magnitude of the differential
output voltage, EOUT, varies
with core position. The
value of EOUT at maximum
core displacement from null
depends upon the amplitude
of the primary excitation
voltage and the sensitivity
factor of the particular LVDT,
but is typically several volts
CORE POSITION
E OUT
MAGNITUDE OF
DIFFERENTIAL
AC OUTPUT
3A
NULL POSITION
% OF FULL RANGE
-100
+100
PHASE ANGLE (DEGREES)
+
PHASE ANGLE OF
OUTPUT RELATIVE
TO PRIMARY
20
10
NULL POSITION
3B
-175
% OF FULL RANGE
-100
+100
+E DC
DC OUTPUT FROM
ELECTRONICS
0 V DC
0 V DC
NULL POSITION
3C
-E DC
-100
% OF FULL RANGE
+100
Figure 3
The output characteristics of an LVDT vary with different positions of the core.
Full range output is a large signal, typically a volt or more, and often requires no
amplification. Note that an LVDT continues to operate beyond 100% of full
range, but with degraded linearity.
RMS. The phase angle of this AC output voltage, EOUT, referenced
to the primary excitation voltage, stays constant until the center
of the core passes the null point, where the phase angle changes
abruptly by 180 degrees, as shown graphically in Figure 3B.
This 180 degree phase shift can be used to determine the
direction of the core from the null point by means of appropriate
circuitry. This is shown in Figure 3C, where the polarity of the
output signal represents the core’s positional relationship to the
null point. The figure shows also that the output of an LVDT is
very linear over its specified range of core motion, but that the
sensor can be used over an extended range with some reduction
in output linearity.
Figure 2
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LVDT Support Electronics
Why Use An LVDT?
Although an LVDT is an electrical transformer, it requires AC
power of an amplitude and frequency quite different from ordinary
power lines to operate properly (typically 3 Vrms at 3 kHz).
Supplying this excitation power for an LVDT is one of several
functions of LVDT support electronics, which is also sometimes
known as LVDT signal conditioning equipment.
LVDTs have certain significant features and benefits, most of
which derive from its fundamental physical principles of operation
or from the materials and techniques used in its construction.
Other functions include converting the LVDT’s low level AC
voltage output into high level DC signals that are more convenient
to use, decoding directional information from the 180 degree
output phase shift as an LVDT’s core moves through the null
point, and providing an electrically adjustable output zero level.
A variety of LVDT signal conditioning electronics is available,
including chip-level and board-level products for OEM
applications as well as modules and complete laboratory
instruments for users.
The support electronics can also be self-contained, as in the DCLVDT shown in Figure 4. These easy-to-use position transducers
offer practically all of the LVDT’s benefits with the simplicity
of DC-in, DC-out operation. Of course, LVDTs with integral
electronics may not be suitable for some applications, or might
not be packaged appropriately for some installation environments.
Figure 5 is an example of an LVDT signal conditioner that accepts
an AC output signal from the sensor and converts it to analog
voltage, current or digital outputs.
Friction-Free Operation
One of the most important features of an LVDT is its friction-free
operation. In normal use, there is no mechanical contact between
the LVDT’s core and coil assembly, so there is no rubbing,
dragging or other source of friction. This feature is particularly
useful in materials testing, vibration displacement measurements,
and high resolution dimensional gaging systems.
Infinite Resolution
Since an LVDT operates on electromagnetic coupling principles
in a friction-free structure, it can measure infinitesimally small
changes in core position. This infinite resolution capability is
limited only by the noise in an LVDT signal conditioner and the
output display’s resolution. These same factors also give an LVDT
its outstanding repeatability.
Unlimited Mechanical Life
Because there is normally no contact between the LVDT’s
core and coil structure, no parts can rub together or wear
out. This means that an LVDT features unlimited mechanical
life. This factor is especially important in high reliability
applications such as aircraft, satellites and space vehicles, and
nuclear installations. It is also highly desirable in many industrial
process control and factory automation systems.
Signal Conditioning
Electronics Module
Figure 4
The cross sectional view of the DC-LVDT at left shows the built-in
signal conditioning electronics module. The module is secured with
a potting compound that is not shown in this drawing.
Figure 5
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Overtravel Damage Resistant
The internal bore of most LVDTs is open at both ends. In the
event of unanticipated overtravel, the core is able to pass
completely through the sensor coil assembly without causing
damage. This invulnerability to position input overload makes an
LVDT the ideal sensor for applications like extensometers that are
attached to tensile test samples in destructive materials testing
apparatus.
Single Axis Sensitivity
An LVDT responds to motion of the core along the coil’s axis,
but is generally insensitive to cross-axis motion of the core or to
its radial position. Thus, an LVDT can usually function without
adverse effect in applications involving misaligned or floating
moving members, and in cases where the core doesn’t travel in a
precisely straight line.
Separable Coil And Core
Because the only interaction between an LVDT’s core and coil
is magnetic coupling, the coil assembly can be isolated from the
core by inserting a non-magnetic tube between the core and the
bore. By doing so, a pressurized fluid can be contained within the
tube, in which the core is free to move, while the coil assembly
is unpressurized. This feature is often utilized in LVDTs used for
spool position feedback in hydraulic proportional and/or servo
valves.
Environmentally Robust
The materials and construction techniques used in assembling an
LVDT result in a rugged, durable sensor that is robust to a variety
of environmental conditions. Bonding of the windings is followed
by epoxy encapsulation into the case, resulting in superior
moisture and humidity resistance, as well as the capability to
take substantial shock loads and high vibration levels in all axes.
And the internal high-permeability magnetic shield minimizes the
effects of external AC fields.
Both the case and core are made of corrosion resistant metals,
with the case also acting as a supplemental magnetic shield. And
for those applications where the sensor must withstand exposure
to flammable or corrosive vapors and liquids, or operate in
pressurized fluid, the case and coil assembly can be hermetically
sealed using a variety of welding processes.
Ordinary LVDTs can operate over a very wide temperature
range, but, if required, they can be produced to operate down
to cryogenic temperatures, or, using special materials, operate
at the elevated temperatures and radiation levels found in many
nuclear reactors.
Null Point Repeatibility
The location of an LVDT’s intrinsic null point is extremely stable
and repeatable, even over its very wide operating temperature
range. This makes an LVDT perform well as a null position sensor
in closed-loop control systems and highperformance servo
balance instruments.
Fast Dynamic Response
The absence of friction during ordinary operation permits an LVDT
to respond very fast to changes in core position. The dynamic
response of an LVDT sensor itself is limited only by the inertial
effects of the core’s slight mass. More often, the response of
an LVDT sensing system is determined by characteristics of the
signal conditioner.
Absolute Output
An LVDT is an absolute output device, as opposed to an
incremental output device. This means that in the event of loss of
power, the position data being sent from the LVDT will not be lost.
When the measuring system is restarted, the LVDT’s output value
will be the same as it was before the power failure occurred.
© 2014 Macro Sensors 04/14/14
MACRO SENSORS • 7300 US ROUTE 130 NORTH, BLDG. 22, PENNSAUKEN, NJ 08110-1541
PHONE (856) 662-8000 • FAX (856) 317-1005 • WWW.MACROSENSORS.COM
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