An LVDT (linear variable differential transformer) inductive displacement sensor is constructed using a static transformer primary winding and two secondary windings.The windings are formed on a hollow bobbin through which a magnetic core can travel. The core provides a path for linking the magnetic flux generated in the primary coil, when this is energized with an AC signal, to the secondary coils. The position of the core in the bobbin controls how much flux is coupled to each of the secondary coils.
The secondary coils A and B are connected in series opposition so that the two voltages VA and VB have opposite phase and the transducer output is VA-VB. If the core is in the central position then voltages of equal magnitude but opposite phase are induced in each secondary and the resulting output is zero. When the core is moved in one direction, the voltage in one secondary increases and that in the other decreases. The net effect is an output that is proportional to the position of the core. Knowledge of the magnitude and phase of the output with respect to the excitation signal allows one to deduce the position of the core with respect to the zero position.
The output of an LVDT is a linear function of the displacement over its calibrated range. Beyond this range the output becomes increasingly non-linear. Measurement range is defined as ± distance from the transducer zero or null position.