A rotary variable differential transformer (RVDT) is a type of electrical transformer used for measuring angular displacement. The transformer has a rotor which can be turned by an external force. The transformer acts as an electromechanical transducer that outputs an alternating current (AC) voltage proportional to the angular displacement of its rotor shaft. In operation, an alternating current (AC) voltage is applied to the transformer primary to energize the RVDT. When energized with a constant AC voltage, the transfer function (output voltage vs. shaft angular displacement) of any particular RVDT is linear (to within a specified tolerance) over a specified range of angular displacement. RVDTs employ contactless, electromagnetic coupling, which provides long life and reliable, repeatable position sensing with high resolution, even under extreme operating conditions. Most RVDTs consist of a wound, laminated stator and a salient two-pole rotor. The stator, containing four slots, contains both the primary winding and the two secondary windings, which may be connected together in some cases. RVDTs offer advantages such as sturdiness, relatively low cost, small size, and low sensitivity to temperature, primary voltage and frequency variations. Operation The two induced voltages of the secondary windings, and , vary linearly with the mechanical angle of the rotor, θ: where is the gain or sensitivity. The second voltage can be determined by: The difference gives a proportional voltage: and the sum of the voltages is a constant: Consequently, the angular information output by a RVDT is independent of the input voltage, frequency and temperature. Putting the above mathematical equations in some theoretical form, the working of RVDT can be explained as below. Basic RVDT construction and operation is provided by rotating an iron-core bearing supported within a housed stator assembly. The housing is passivated stainless steel. The stator consists of a primary excitation coil and a pair of secondary output coils. A fixed alternating current excitation is applied to the primary stator coil that is electromagnetically coupled to the secondary coils. This coupling is proportional to the angle of the input shaft. The output pair is structured so that one coil is in-phase with the excitation coil, and the second is 180° out-of-phase with the excitation coil. When the rotor is in a position that directs the available flux equally in both the in-phase and out-of-phase coils, the output voltages cancel and result in a zero value signal. This is referred to as the electrical