Linear Variable

Differential Transformer

Prin. Dr. Ashok D Gaikwad

Linear Variable Differential Transformer (LVDT)
►
The LVDT is one of the most
widely used electromechanical
transducers for measuring linear
displacement. It is based on the
principle of mutual induction and
converts small linear
displacements of a core into a
corresponding electrical signal.
►
It is widely used in
instrumentation due to its high
accuracy, infinite resolution,
ruggedness, and frictionless
operation.
Linear Variable Differential Transformer (LVDT)
►
Principle of Working:
►
According to Faraday’s law of
electromagnetic induction, which
states that “the net induced emf in
the circuit is directly proportional to
the rate of change of magnetic flux
across the circuit, and the magnetic
flux of the coil wounded with wires
can be changed by moving a bar
magnet through the coil,” the
operation of an LVDT is based on
this principle.
Linear Variable Differential Transformer (LVDT)
►
Principle of Working:
►
According to Faraday’s law of
electromagnetic induction, which
states that “the net induced emf in
the circuit is directly proportional to
the rate of change of magnetic flux
across the circuit, and the magnetic
flux of the coil wounded with wires
can be changed by moving a bar
magnet through the coil,” the
operation of an LVDT is based on
this principle.
The primary coil is supplied with a sinusoidal AC Linear Variable Differential Transformer
voltage.
The magnetic field induces voltages in both secondary (LVDT)
coils, depending on the position of the movable core.
Cases:
1.Core at Null Position (center):
Equal flux linkage in S1 and S2.
Induced voltages in S1 and S2 are equal in magnitude
but opposite in phase (since connected in series
opposition).
Output = 0 (null).
2. Core Displaced Toward S1:
Coupling with S1 > S2.
Net output voltage ∝ displacement, in phase with input.
3. Core Displaced Toward S2:
Coupling with S2 > S1.
Net output voltage ∝ displacement, 180° out of phase
with input.
Thus, both magnitude and phase of output indicate
displacement and direction.
 Linear Variable Differential Transformer (LVDT)

The important considerations

are range and sensitivity of
the transducer. The range
depends on the construction.
Commercial transducers
have a range varying from
±0.002 cm to several cms. In
certain special types,
detection of even ±20 mcm
displacement can be made.
 Linear Variable Differential Transformer (LVDT)

Sensitivity of LVDT | Transference ratio of the LVDT

The relationship between the LVDT’s output voltage and core displacement is described by
the sensitivity of the device. It is also referred to as the LVDT transference ratio.

When the primary AC source is maintained at a specific voltage (3 Vrms) and the core is
moved by the full-scale displacement from the null position, the sensitivity of the LVDT is
determined.

Next, the voltage across the windings S1 and S2 is measured to determine the LVDT’s net
output voltage. The collected numbers are then used to modify the following equation to
determine the LVDT’s sensitivity.

Sensitivity = V output / (Vprimary × Core Displacement)

V output / (Vprimary Core Displacement) is the formula for sensitivity.

It is written as mV/V/mm or mV/V/in, which stands for millivolt output per volt of excitation
per core displacement in millimetres or inches.
 Linear Variable Differential Transformer (LVDT)

Advantages of LVDT


LVDTs are constructed using high-quality components and methods that are
easily resistant to rust, pressure, and extreme temperatures. Even at
temperatures higher than its operating temperature, the LVDT’s null point often
remains constant.


Since there is no friction during LVDT operation, the core’s location can be
adjusted quickly, leading to dynamic reactions from the LVDT. The bulk of the core
is the single factor thought to restrict the LVDT’s ability to respond dynamically.


The absolute value is provided by the LVDT. It implies that in the event of an
unexpected power outage, the LVDT does not lose its location data. If the
measurement is redone, the output value stays the same as it was before to the
power outage.
 Linear Variable Differential Transformer (LVDT)

Advantages of LVDT

As there is no direct contact between the moving core and the fixed coil structure,
LVDT is a frictionless device (former). As a result of friction-related wear and tear
being reduced, the device is less likely to sustain damage. As a result, the
mechanical life of an LVDT is significantly longer than that of other devices that
experience friction during operation.

It can be used to calculate an object’s displacement that spans from a few
centimetres to a few millimetres. In laboratories and for industrial uses, modern
LVDTs that can measure displacement over wide ranges (±100μm to ±25 cm) are
frequently utilised.

They typically use less than 1 W of electricity and exhibit less hysteresis loss,
which improves their reliability.
 Linear Variable Differential Transformer (LVDT)

Advantages of LVDT

In spite of their tiny size and lightweight nature, LVDT are capable of withstanding
mechanical shocks and vibrations. Because of their small size and lightweight
nature, LVDT may be readily managed and aligned to meet requirements.

Since there is no direct connection between the coil and core of the LVDT and
instead they are magnetically connected, they can be removed from one another.
This can be accomplished by placing a non-magnetic tube between the core and
the former; at this point, pressurised fluid is supplied to the tube that has been
placed there. Typically, this assembly is used in hydraulics for a variety of
measures.

Due to its strong output signal and sensitivity to even tiny displacements, LVDT
does not require the use of an amplifier to amplify the signals.
 Linear Variable Differential Transformer (LVDT)

Disadvantages of LVDT

The primary drawback of the LVDt is the need for an auxiliary circuit to handle the
stray magnetic field it generates across the electric circuit. The LVDT’s inductive
transducer mechanism produces the stray magnetic field.

Due to unwanted vibrations or temperature fluctuations in the device, the LVDT’s
performance may lag.

Since the LVDT produces AC output, a demodulator is needed to produce DC
output.

Due to the mass of the movable core or the frequency of the applied primary
voltage, the LVDT’s quick dynamic responses may be restricted.
 Linear Variable Differential Transformer (LVDT)

Applications of LVDT

When an object is submerged in non-corrosive, non-conductive fluids, hydraulics
uses it to check for leaks or other damage. Robotic manipulators also use LVDT
sensors.

It is employed in the aerospace sector to keep an eye on numerous mechanisms,
including pilot and flight control. The moving core is attached to the moving parts,
such as landing gears, and a variety of mini-positon transducers are mounted at
the fixed points. Depending on the sensitivity of the LVDT and the mounting
system, different electrical output signals supply the angles, lengths, motion, and
rate of the moving when the landing gears are moved.
 Linear Variable Differential Transformer (LVDT)

Applications of LVDT

If utilised as a secondary transducer, the LVDT can be used to monitor physical
properties such as force, pressure, and weight in addition to displacement. For
instance, a Bourdon tube can be used as the primary transducer to measure
pressure by converting it to linear displacement. The linear displacement is then
converted by an LVDT into voltage or electrical signals, allowing us to obtain the
pressure reading.

Additionally, it is employed in the creation of pills in the medical industry. This is
accomplished via a computer-controlled hybrid mechanism that properly
measures the weight and thickness of the tablets while minimising human error. It
consists of primary and secondary winding transducers.
 Linear Variable Differential Transformer (LVDT)

Applications of LVDT

In civil engineering, LVDT is used to evaluate things like spring tensions,
weight, and displacement as well as to test the durability of different soil
samples and rocks that utilised to build buildings or bridges.

By observing the waveform at the LVDT output terminal, they can also be
used to test the quality of flat display panels.
Thanks...