LVDT stands for “Linear Variable Differential Transducer“. LVDT is a passive differential inductive transducer. This is a variable inductance displacement transducer.
Construction of LVDT
- It consists of a primary winding and two identical secondary windings. These windings are axially spaced and bound on a cylindrical coil former.
- 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).
- 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.
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.
- Very fine resolution
- High accuracy
- Very good stability
- Linearity of transfer characteristics
- Ease of fabrication and installation
- Ability to operate at high temperature
- High sensitivity (2mV/Volt/10 microns at 4 kHz excitation)
- It is sensitive to the external magnetic fields. To minimize this effect magnetic shielding is necessary.
- Complicated circuitry is needed.
- Due to the mass of the core, it is not suitable for dynamic measurement.
- Large displacements are needed to get appreciable differential output.
In addition to displacement measurement, it is used in the measurement of pressure, load, acceleration, force, weight, etc.