Linear variable differential transformer (LVDT):

The most widely used inductive Transducer translate the linear motion into electrical signals is linear variable differential transformer. Here the change in the displacement corresponds to change in inductance, which gives differential voltage output.

(a) A sectional view of a linear variable differential transformer (LVDT).

(b) Circuit diagram for an LVDT.

An LVDT consists of three symmetrically spaced coils wound onto an insulated bobbin. The magnetic core can move within the bobbin and provides a path for the magnetic flux linkage between the primary and the secondary coils. The motion (from a mechanical input) changes the characteristics of the flux path and the changes can be detected in the circuit.

The primary coil is excited by an a.c. signal and voltages are induced in the two secondary coils. The induced voltages depend on the position of the core inside the bobbin. The circuit is shown in part (b) of the figure. The secondary coils are wired in a series-opposing circuit so that when the core is centered between them the voltages induced are opposite but equal. When the core is centered between the two secondary coils, the induced voltages v {1} and v {2} are equal but out of phase by 180 degrees, and they cancel to give a zero output voltage. When the core moves from the center position, an output voltage v {0} =v {1}-v {2} is developed, and as long as the displacement is within the working range of the LVDT, the voltage and displacement will be linearly related.

• Displacement

• extensometers, butterfly valve control, temperature transducers

• Deflection of Beams, Strings, or Rings

• load cells, force transducers, pressure gage, Diaphragm Pressure Gage

• Thickness Variation of Work Pieces

• dimension gages, thickness and profile measurements, product sorting by size 

• Fluid Level

• fluid level and fluid flow measurement, position sensing in hydraulic cylinders 

Fluid Level Gage