In the first diagram the variable inductor is part of an oscillator circuit.

If the position of the core is moved then the oscillator frequency changes.

The change in frequency can be displayed as a change in millimetres.

The following diagram shows a variable reluctance transducer.

As the air gap changes the reluctance of the circuit changes.

This causes a change of inductance.


This can be used as shown by the next illustration. 

As the inductance changes so the frequency of the oscillator changes.

The output of the oscillator can be converted to DC for display on a digital meter calibrated in inches etc.

The next diagram shows a linear variable displacement transformer (LVDT).

There is one primary and two secondary windings.

If AC is applied to the primary then voltages are induced in the secondaries.

The secondaries are connected so their outputs are opposite.

When the core is central the two voltages are equal in amplitude and cancel out.

If the core is moved then there will be more voltage in one secondary than the other.

The voltages will not cancel out and there will be an AC signal at the output proportional to the distance the core has moved.

Using a phase detector circuit it is also possible to indicate the direction the core has moved.

The graph below shows the output voltage/position characteristics.


Copyright Graham Knott 2004