When the primary winding is connected to an AC supply, current starts to flow in the secondary coils. The secondary coils are connected in series opposition so that two voltages have opposite phase so when the core is in the centre position then voltages of equal magnitude but opposite phase will be induced in each secondary coil and the output is zero. As the core is moved in one direction the voltage in the corresponding secondary coil increases while the other coil experiences a complementary voltage reduction. The effect is a transducer voltage output that is proportional to displacement. Knowledge of the magnitude and the phase of the output with respect to the excitation signal allows one to deduce the position and direction of the core motion from the null position.

Low cost solutions to displacement measurement
Friction free operation with reduced guidance accuracy
Stainless steel body and push rod

Linear Variable Differential Transformers (LVDT)

These devices comprise a movable nickel iron core and three coils - one for energisation and two for pick-up. They need to be driven by a sine wave to produce an output amplitude and phase directly proportional to the position of the core with respect to the pick-up coils. A phase sensitive detector is required to demodulate the output signal. The main advantages of LVDTs are;

No wear problems as with potentiometers
Infinite resolution, high accuracy and linearity
No friction introduced into the measured system
Available in ranges from ±1mm to ±50mm