Rotary Variable Differential Transformer - Operation of RVDT's

Operation of RVDT's

The two induced voltages of the secondary windings, and, vary linearly to the mechanical angle of the rotor, θ:

where is the gain or sensitivity. The second voltage can be reverse determined by:

The difference gives a proportional voltage:

and the sum of the voltages is a constant:

This constant gives the RVDT great stability of the angular information, independence of the input voltage or frequency, or temperature, and enables it to also detect a malfunction.

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 degrees 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 zero position or E.Z. When the rotor shaft is displaced from E.Z., the resulting output signals have a magnitude and phase relationship proportional to the direction of rotation. Because RVDT’s perform essentially like a transformer, excitation voltages changes will cause directly proportional changes to the output (transformation ratio). However, the voltage out to excitation voltage ratio will remain constant. Since most RVDT signal conditioning systems measure signal as a function of the transformation ratio (TR), excitation voltage drift beyond 7.5% typically has no effect on sensor accuracy and strict voltage regulation is not typically necessary. Excitation frequency should be controlled within +/- 1% to maintain accuracy

Although the RVDT can theoretically operate between ±45°, accuracy decreases quickly after ±35°. Thus, its operational limits lie mostly within ±30°, but some up to ±40°. Certain types can operate up to ±60°.

The advantages of the RVDT are :

• low sensitivity to temperature, primary voltage & frequency variations
• sturdiness
• low cost
• simple control electronics
• small size