LVDT Linear Variable Differential Transformer

Table of Contents

Full form of LVDT isLinear Variable Differential Transformer. Its 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).

Linear Variable Differential Transformer (LVDT) is an electromechanical type inductive transducer that converts rectilinear displacement into the ac electrical signal.

The 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 the corresponding electrical signal. Hence, LVDT is used as secondary transducer.

Why it is called LVDT?

Linear: It measures Linear displacement
Transformer: It has one primary coil and two secondary coils. Also, it has AC input and AC output.
Variable: It has movable (variable) core and fixed coils.
Differential: The output voltage is the difference between two secondary voltages.

Construction of LVDT

The transformer consists of one primary coil (winding) P and two secondary coils S₁and S₂wound on a cylindrical core (which is hollow in nature).

Both the secondary windings have an equal number of turns, and they are placed on either side of primary winding.
The primary winding is connected to an AC source which produces a flux in the air gap and voltages are induced in secondary windings.
A movable soft iron core is placed inside the former and arm is connected to the core.
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.
The iron core is generally of high permeability which helps in reducing harmonics and high sensitivity.
The secondary windings are connected in such a way that resulted output is the difference between the voltages of two windings.
Usually, this AC output voltage is converted by suitable electronic circuitry to high level DC voltage or current that is more convenient to use.
It is placed inside a stainless-steel housing because it provides electrostatic and electromagnetic shielding.

Working Principle of LVDT

Theworking principle of LVDT is based on the mutual induction principle.

The emf induced in one coil by the change of current in the other coil is called as mutually induced emf
Consider two coils A and B lying close to each other. When switch S is closed, current flows through coil B and flux is developed.

Part of this flux cuts the coil A as shown.
When current in coil B is changed by varying resistance R, flux of coil B changes. Flux which cuts coil A also changes. emf is induced in coil A and coil B.
Emf induced in coil B is self-induced emf. Emf induced in coil A is mutually induced emf.
The changing emf can also be obtained by an AC source because in AC the magnitude of emf changes. This is the principle is used in transformer. LVDT also uses same principle.

Working of LVDT

Case I: When the core is at null position (for no displacement)
When the core is at null position then the flux linking with both the secondary windings is equal so the induced emf is equal in both the windings. Therefore, for null position (i.e. no displacement) the value of output V_Ois zero as E₁and E₂both are equal.

V_O=0

Case II: When the core is moved toward S₁

In this case the flux linking with secondary winding S₁is more as compared to flux linking with S₂. Therefore, for this case E₁will be more as that of E₂. Due to this,

Net differential output voltageV0 = E1 – E2will be positive

Case III:When the core is moved toward S₂

In this case the flux linking with the secondary winding S₂ is more as compared to flux linking with S₁. Therefore, the magnitude of E₂will be more as that of E₁. Due to this

Net differential output voltageV0 = E1 – E2will be negative.

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 V_O is positive then this means an object is moving towards the Left from the Null position.
Similarly, If the output voltage V_O is negative then this means the object is moving towards the Right of the Null position.

Video on Construction and Working of LVDT

Types of LVDT

On the basis of armature type

Unguided Armature
Captive Armature
Spring-extended Armature

Based on electrical output

AC LVDT
DC LVDT

Unguided Armature

Armatures can be free unguided to measure targets that move parallel to the LVDT or need frequent measurements.
In this type of unit, the armature is disconnected from the LVDT body.
There is no wear on the LVDT because no contact is made between armature and core. So, free unguided LVDTs have a practically infinite mechanical life.
LVDT does not restrict the resolution of measured data (infinite resolution).

Captive Armature

This armature type is specialized for longer measurement ranges of ±0.5 inch to ±18.5 inches. Here, the armature is attached to the body as well as the structure it is measuring.
The armature is threaded to allow free movement across the machined bearings.

Advantages compared to unguided armature:

Better for longer working ranges.
Preferred when misalignment may occur.

Spring-extended Armature

Also known as captive guided spring return type LVDT.
It is made for measuring multiple targets or for targets that move transverse to the armature.
In spring return armatures, an internal spring makes contact with the target’s surface to measure surface displacement.
This is particularly suited for applications measuring changes in a structure’s surface.

AC LVDT

Alternating current (AC) types have a better shock and vibration resistance.
It can operate over higher temperature ranges (–200°C to 500°C).
It can also work with remotely located electronics.
AC LVDT’s cost is less than DC.
AC LVDTs require separate signal conditioning equipment.

DC LVDT

AC LVDTs require separate signal conditioning equipment, while DC LVDTs include signal conditioning equipment on the device.
DC LVDT is shown in Figure 2.
Direct current LVDTs can function in temperatures from –40° C to 200°C, and are compatible with internal electronics.
Built-in electronics eliminate the volume, weight and cost of external AC excitation equipment.
These can send digital outputs directly to computer systems.
It more cost-effective and can work just as well in most environments.

Advantages of LVDT

In normal use, there is no mechanical contact between the core and coil assembly, so there is no rubbing, dragging, or other source of friction.
There is normally no contact between the core and coil structure, no parts can rub together or wear out. This means that an it has huge mechanical life.
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. It is shock and vibration proof.
The absence of friction during ordinary operation permits an LVDT to respond very fast to changes in core position.
It has very wide operating temperature range.
It consumes very low power of approximately 1W during its operation.
They convert linear displacement directly to the corresponding electrical voltage signal which are easy to process further.

Disadvantages of LVDT

It is an inductive transducer, so it is sensitive to Stray Magnetic Field therefore extra setup is required to protect it from Stray Magnetic Field.
It also gets affected by the vibrations and temperature variation.

Applications of LVDT

It is used to measure linear displacement.
It 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.