Linear Variable Differential Transducer

LVDT stands for “Linear Variable Differential Transducer“. LVDT is a passive differential inductive transducer. This is a variable inductance displacement transducer.

Construction of LVDT

LVDT, Internal structure of LVDT
Construction of LVDT
  1. It consists of a primary winding and two identical secondary windings. These windings are axially spaced and bound on a cylindrical coil former.
  2. A rod-shaped magnetic core is positioned centrally inside the coil assembly. This rod provides a low reluctance path for the magnetic flux linking the coils (windings).
  3. The moving object displacement of which is measured is coupled to this movable rod.
  • The two secondary winding are connected in series opposition. Hence the voltages induced into these windings are of opposite polarities.
  • The output voltage is given by E0 = E01 – E02
  • Where E01 and E02 are the emf induced in two secondary windings.
  • The transfer characteristic of LVDT is shown in the figure.
  • It is the graph of output voltage against the core position.
Connection of LVDT windings
LVDT Winding

Transfer Characteristics of LVDT

Operation of LVDT

  • The primary winding is connected to the ac source.
  • Assume that the core is exactly at the center of the coil assembly. Then the flux linked to both the secondary windings will be equal.
  • Due to equal flux linkage, the secondary induced voltage is equal, but they have opposite polarities.
  • The output voltage of LVDT i.e. E0 is, therefore, zero correspondings to the central portion of the core. This position of the core is called the “null position”.
  • Now, if the core is displaced from its null position towards secondary-1, then the flux linked to secondary-2 increases and flux linked to the secondary-2 to decreases.
  • Therefore the induced voltage eo1 is now greater than E02 and the output voltage of LVDT i.e. E0 will be positive.
  • Similarly, if the core is displaced downwards i.e. towards the secondary-2 then E02 will be greater than E01 and the output voltage e0 will be negative.
  • Thus the magnitude of the output signal is made a very “linearly” with the mechanical displacement. Hence the word linear is used for LVDT.
  • The output is obtained “differentially” between the two secondary windings. Hence, the word “differential” is used in LVDT.
Operation of LVDT

Advantages

  1. Very fine resolution
  2. High accuracy
  3. Very good stability
  4. The linearity of transfer characteristics
  5. Ease of fabrication and installation
  6. Ability to operate at high temperature
  7. High sensitivity (2mV/Volt/10 microns at 4 kHz excitation)

Disadvantages

  1. It is sensitive to the external magnetic fields. To minimize this effect magnetic shielding is necessary.
  2. Complicated circuitry is needed.
  3. Due to the mass of the core, it is not suitable for dynamic measurement.
  4. Large displacements are needed to get appreciable differential output.

Applications

In addition to displacement measurement, it is used in the measurement of pressure, load, acceleration, force, weight, etc.

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