Differential Transformer
The linear variable differential transformer (LVDT) is an accurate and
reliable method for measuring linear distance. LVDTs find uses in
modern machine-tools, robotics, avionics, and computerized
manufacturing.
LVDT
The linear variable differential
transformer (LVDT) is a type of
electrical transformer used for
measuring linear displacement.
The transformer has three
solenoidal coils placed end-toend around a tube. The centre
coil is the primary, and the two
outer coils are the secondary. A
cylindrical ferromagnetic core,
attached to the object whose
position is to be measured,
slides along the axis of the tube
Linear Voltage Differential Transformer
(LVDT)

As shown in the last, the LVDT is a position-to-electrical
sensor whose output is proportional to the position of a
movable magnetic core. The core moves linearly inside a
transformer consisting of a center primary coil and two
outer secondary coils wound on a cylindrical form. The
primary winding is excited with an ac voltage source
(typically several kHz), inducing secondary voltages that
vary with the position of the magnetic core within the
assembly. The core is usually threaded in order to
facilitate attachment to a non-ferromagnetic rod which, in
turn, is attached to the object whose movement or
displacement is being measured.
LVDT

The secondary windings are wound out of phase with
each other. When the core is centered, the voltages in
the two secondary windings oppose each other, and
the net output voltage is zero. When the core is moved
off center, the voltage in the secondary toward which
the core is moved increases, while the opposite
voltage decreases. The result is a differential voltage
output that varies linearly with the core’s position.
Linearity is excellent over the design range of
movement, typically 0.5% or better. The LVDT offers
good accuracy, linearity, sensitivity, infinite resolution,
frictionless operation, and mechanical ruggedness
LVDT

A wide variety of measurement ranges are
available in different LVDTs, typically from
±100 μm to ±25 cm. Typical excitation voltages
range from 1 V to 24 V rms, with frequencies
from 50 Hz to 20 kHz. Note that a true null
does not occur when the core is centered
because of mismatches between the two
secondary windings and leakage inductance.
Also, a simple measurement of the output
voltage, VOUT, will not tell the side of the null
position on which the core resides.
LVDT
A signal conditioning circuit that removes these difficulties is shown in Figure ,
where the absolute values of the two output voltages are subtracted. Using this
technique, both positive and negative variations about the center position can be
measured.
IC related to LVDT use
AD598 an IC

The industry-standard AD598 LVDT signal conditioner, shown in
simplified form in Figure 4, performs all required LVDT signal
processing. The on-chip excitation frequency oscillator can be set
from 20 Hz to 20 kHz with a single external capacitor. Two absolute
value circuits followed by two filters are used to detect the
amplitude of the A- and B channel inputs. Analog circuits are then
used to generate the ratiometric function (A–B)/(A+B). Note that this
function is independent of the amplitude of the primary winding
excitation voltage, assuming the sum of the LVDT output voltage
amplitudes remains constant over the operating range. This is the
case for most LVDTs, but the user should always check with the
manufacturer if it is not specified on the LVDT data sheet. Note also
that this approach requires the use of a five-wire LVDT.
AD698 IC with (LVDT)
The AD698 LVDT signal conditioner (Figure 5) has similar specifications as the AD598, but
processes the signals slightly differently using synchronous demodulation. The A and B
signal processors each consist of an absolute value function and a filter. The A output is
then divided by the B output to produce a final output that is ratiometric and independent
of the excitation voltage amplitude. Note that the sum of the LVDT secondary voltages
does not have to remain constant in the AD698
Strain Gauge
L
R= r
A
Under tension the strain
narrows area therefore the
effective resistance
increases
Under compression area
of the strain thickens ,
therefore effective
resistance decreases
Strain Gauge
Strain Gauge
The strain gauge has been in use for many
years and is the fundamental sensing element
for many types of sensors, including pressure
sensors, load cells, torque sensors, position
sensors, etc. The majority of strain gauges are
foil types, available in a wide choice of shapes
and sizes to suit a variety of applications.
They consist of a pattern of resistive foil which
is mounted on a backing material. They operate
on the principle that as the foil is subjected to
stress, the resistance of the foil changes in a
defined way.
Strain Gauge

The strain gauge is connected into a Wheatstone
Bridge circuit with a combination of four active
gauges (full bridge), two gauges (half bridge), or,
less commonly, a single gauge (quarter bridge). In
the half and quarter circuits, the bridge is
completed with precision resistors.
As the signal value is small, (typically a
few millivolts) the signal
conditioning electronics provides
amplification to increase the signal level
to 5 to 10 volts, a suitable level for
application to external data collection
systems such as recorders or PC Data
Acquistion and Analysis Systems.
Temperature compensation
Under certain industrial
conditions the temperature
causes changes in resistance of
the strain gauge and hence
disguises any stress or
compression measurement.
Under such condition a dummy
strain gauge is connected that
remains unstressed but within
same temperature environment
therefore compensating any
change caused by temperature.
½ Bridge arrangement
However, when a downward
force is applied to the free
end of the specimen, it will
bend downward, stretching
gauge #1 and compressing
gauge #2 at the same time.
Such complimentary forces will cause larger differential
of output voltage to be measured and hence provide
bridge with better resolution.
Full Bridge arrangement
In applications where
such complementary
pairs of strain gauges
can be bonded to the
test specimen, it may
be advantageous to
make all four elements
of the bridge "active"
for even greater
sensitivity. This is
called a full-bridge
circuit:
Strain Gauge
Most manufacturers of strain
gauges offer extensive
ranges of differing patterns
to suit a wide variety of
applications in research and
industrial projects.
They also supply all the
necessary accessories
including preparation
materials, bonding
adhesives, connections tags,
cable, etc.
Varieties of Strain Gauges