LVDT is Electromechanical type Inductive Transducer that converts rectilinear displacement into the Electrical Signal. LVDT is a secondary transducer.
LVDT full form is Linear
Variable Differential Transformer. As the name suggests, many people get
confused that it is a Transformer. But actually, it is a Transducer not a
Transformer. Because it’s working principle is same as Transformer (i.e. Mutual
Induction Principle) and also the output across its secondary coil is in the
form of differential voltage, that’s why it is named as Linear Variable
Differential Transformer (LVDT).
It is categorized as an Inductive Transducer used
to measure the speed or position of an object. Generally most of Inductive
Sensors work on the principle of Transformer. Now let’s discuss the complete
concepts of LVDT in detail.
What is LVDT?
Introduction of LVDT:
Linear Variable Differential
Transformer (LVDT) is an Electromechanical type Inductive Transducer that converts
rectilinear displacement into the Electrical Signal. Since LVDT is a secondary
transducer, hence physical quantities such as Force, Weight, Tension, Pressure,
etc are first converted into displacement by a primary transducer and then LVDT
is used to measure it in terms of corresponding Electrical signal. As LVDT is an AC
controlled device, so there is no any electronics component inside it. It is the most widely used Inductive Sensor due to its high accuracy level. Its
electrical output is obtained because of the difference of secondary voltages,
hence it is called Differential Transformer.
LVDT Construction:
LVDT consists of one primary
winding P and two secondary windings S1 & S2 mounted on a cylindrical
former. Both the secondary windings (S1
& S2) has an equal number of turns and placed identically on either side of
the primary winding in such a way that the net output will be the difference of the voltage of both secondary windings.
There is a movable soft iron core placed inside the former.
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LVDT Construction |
Hydrogen annealing is done on Iron core to reduce harmonics, residual voltage
of core and thus provides high sensitivity. The movable core also is laminated
in order to reduce the eddy current losses. The displacement to be measured is
attached to this movable soft iron core. LVDT is placed inside the stainless
steel housing because it will provide electrostatic and electromagnetic
shielding.
LVDT working principle:
The working principle of LVDT is based on the mutual induction
principle. When AC excitation of 5-15 V at a frequency of 50-400Hz is applied
to the primary winding, then a magnetic field is produced. This magnetic field
induces a mutual current in secondary windings. Due to this, the induced
voltages in secondary windings (S1 & S2) are E1 & E2 respectively.
Since
both the secondary windings are connected in series opposition, So the net output voltage will be the difference of both induced voltages (E1 & E2) in
secondary windings.
Hence Differential Output of LVDT will be E0 = E1 – E2
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LVDT Working |
Now according to the position of the core, there are three cases that arise.
So Let’s discuss these three cases one by one in detail.
Case 1: When the core moves towards S1 (Max Left).

When the core of LVDT moves toward Secondary winding S1. Then, in this case, the flux linkage with S1 will be more as compared to S2. This
means the emf induced in S1 will be more than induced emf in S2. Hence E1>E2
and Net differential output voltage E0 = E1 – E2 will be positive. This means
the output voltage E0 will be in phase with the primary voltage.
Case 2: When the core is at Null position.
When the core is at the null position then the flux linkage with
both the secondary windings will be the same. So the induced emf (E1 & E2) in both the windings
will be the same. Hence the Net differential output voltage E0 = E1 – E2 will be
zero (E0 = E1 – E2 = 0). It shows that no displacement of the core.
Case 3: When the core moves towards S2 (Max Right).

From all three cases, we can have the following conclusions:
- The direction of the movement of an object can be identified with the help of the differential output voltage of LVDT. If the output voltage E0 is positive then this means an object is moving towards Left from the Null position.
- Similarly, If the output voltage E0 is negative then this means the object is moving towards the Right of the Null position.
- The amount or magnitude of displacement is proportional to the differential output of LVDT. The more the output voltage, the more will be the displacement of the object.
- If we take the core out of the former then the net differential the output of LVDT will be zero.
- In fact corresponding to both the cases, whether the core is moving either Left or Right to the Null position. Then the output voltage will be increased linearly up to 5mm from the Null position and after 5 mm output E0 will be non-linear.
Advantages and Disadvantages of LVDT:
Following are the main advantages and disadvantages of using
LVDT as a secondary Transducer.
Advantages of LVDT:
- Smooth and Wide Range of Operation
:- LVDT has a very wide range of
measurement of displacement. It can measure displacement ranging from 1.25mm to
250 mm.
- High Sensitivity:- LVDT gives high output value so that there is no need for any Amplifier circuit for the amplification process. Typically the sensitivity of LVDT is recorded as 40V/mm.
- Low Hysteresis Losses:- LVDT gives low hysteresis losses
hence repeatability is excellent under all the conditions.
- Low Friction Losses:- As the core moves in a hollow Former, So there is no concept of friction
losses. Hence it gives accurate output value.
- Rugged Operation:- It can tolerate a high degree of
shock and variation, especially when the core is loaded with spring.
- Low Power consumption:- LVDT consume very low power of
approx 1W during its operation.
- Direct conversion to Electrical
Signal:- They convert
linear displacement directly to the corresponding electrical voltage signal
which are easy to process further.
- Fast dynamic Response:- Due to the absence of Friction, Its dynamic response becomes very fast to change in a core position.
Disadvantages of LVDT:
- Since LVDT is Inductive Transducer, so it is sensitive to Stray Magnetic Field. Hence an extra setup is required to protect it from Stray Magnetic Field.
- Since it is an electromagnetic device, so it also gets affected by the vibrations and temperature variation.
Application of LVDT:
- LVDT is used to measure
the physical quantities such as Force, Tension, Pressure, Weight, etc. These
quantities are first converted into displacement by the use of primary transducers
and then it is used to convert the displacement to the corresponding Electrical
voltage signal.
- It is mostly used in industries as well as a servomechanism.
- It is also used in Industrial Automation, Aircraft. Turbine, Satellite, hydraulics, etc.
LVDT Specifications:
- Range of Measurement: ± 0.25 mm to ± 750 mm
- Operating Temperature: (-265 to 600) degree Celsius.
- Frequency Range: 50 Hz to 20 kHz.
Easy to understand
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