LVDT - Linear Variable Differential Transformer

Q: What are the advantages of LVDT?

The key advantage of a LVDT is that they are frictionless, so no parts can rub together or wear out . Infinite resolution is present in Linear Variable Differential Transformer High output

LVDT Full Form – Linear Variable Differential Transformer

Definition of LVDT:

LVDT Linear Variable Differential Transformer is an electromechanical sensor used to convert mechanical motion or vibrations, specifically rectilinear motion, into a variable electrical current, voltage or electric signals, and the reverse. It is the most widely used inductive transducer that converts the linear motion into the electrical signal.

Function of LVDT:

Linear Variable Differential Transformer is an absolute position/displacement transducer that converts a distance from a mechanical reference (zero, or null position) into a proportional electrical signal containing phase (for direction) and also amplitude (for distance) information.
The Linear Variable Differential Transformer operation does not require an electrical contact between the moving part (probe or core assembly) and also the coil assembly, but instead relies on electromagnetic coupling.

LVDT Construction:

The LVDT consists of a primary coil (of magnet wire) wound over the whole length of a non-ferromagnetic boreliner (or spool tube) or a cylindrical, non-conductive material (usually a plastic or ceramic) form. Two secondary coils are wound symmetrically on top of the primary coil for “long stroke” LVDTs (i.e. for actuator rod position) or each side of the primary coil for “Short stroke” LVDTs (i.e. for electro-hydraulic servo-valve or EHSV).
The two secondary windings are typically connected in “series opposing” (Differential). A ferromagnetic core, which length is a fraction of the coil assembly length, magnetically couples the primary to the secondary winding turns that are located over the length of the core.
Even though the secondary windings of the long stroke Linear Variable Differential Transformer are shown on top of each other in the above illustration, nowadays TE Connectivity winds them both at the same time using custom designed, dual carriage computerized machines.
This method reduces manufacturing time and also creates secondary windings with the same exact resistance and symmetrical capacitance distribution, therefore allows better performance (linearity, phase symmetry, lower null voltage, etc.).

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.

LVDT Diagram

Working Of LVDT:

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

Working Of Linear Variable Differential Transformer

Now three cases arise according to the locations of core which explains the working of LVDT are discussed below as,

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. So for no displacement the value of output eout is zero as e1 and e2 both are equal. 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 II :

When the core is moved to upward of null position (For displacement to the upward of reference point). In the this case the flux linking with secondary winding S1 is more as compared to flux linking with 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 III:

When the core is moved to downward of Null position (for displacement to the downwardof the reference point). In this case magnitude of e2 will be more as that of e1 Hence E2>E1 and Net differential output voltage E0 = E1 – E2 will be negative.This means the output voltage E0 will be in phase opposition (180 degrees out of phase) with the primary voltage.

Types Of LVDT:

Captive Armature LVDT
Unguided Armatures
Force Extended Armatures

Advantages And Disadvantages of LVDT:

LVDT Advantages:

Infinite resolution is present in Linear Variable Differential Transformer
High output
LVDT gives High sensitivity
Very good linearity
Ruggedness
LVDT Provides Less friction
Low hysteresis
LVDT gives Low power consumption.

LVDT Disadvantages:

Very high displacement is required for generating high voltages.
Shielding is required since it is sensitive to magnetic field.
The performance of the transducer gets affected by vibrations
It is greatly affected by temperature changes.

Applications of LVDT:

Linear Variable Differential Transformer is used to measure displacement ranging from fraction millimeter to centimeter.
Acting as a secondary transducer, LVDT can be used as a device to measure force, weight and also pressure
LVDTs can operate without any built-in electronic circuitry are the primary reasons why they have been widely used in applications where long life and high reliability under very severe environments are a required, such as
Military/Aerospace
Process Controls
Automation
Robotics
Nuclear
Chemical Plants
Hydraulics
Power Turbines, etc.

LVDT FAQ

What is a LVDT used for?

What is LVDT and its working principle?

Linear Variable Differential Transformer is an absolute position/displacement transducer. 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.

What are the three main parts of LVDT?

The main components of an LVDT are a transformer and a core. The transformer consists of three coils a primary and two secondary’s (S1 and S2) wound on a hollow form, which is typically made of glass-reinforced polymer.

What are the advantages of LVDT?

The key advantage of a LVDT is that they are frictionless, so no parts can rub together or wear out.
Infinite resolution is present in Linear Variable Differential Transformer
High output

What are the disadvantages of LVDT?

Very high displacement is required for generating high voltages.
Shielding is required since it is sensitive to magnetic field.
The performance of the transducer gets affected by vibrations
It is greatly affected by temperature changes.

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LVDT- Construction, Working Principle , Applications, Advantages, and Disadvantages