Shaft Position Sensing at High Speeds

An alternative method for determining the angular position of a rotating shaft is introduced. The required information is obtained from the envelopes of a modulated carrier at the stator terminals of an ac servomechanism. Essentially, a single-phase constant amplitude carrier signal is applied to the rotor winding; with zero stator current, the amplitude of the transformed carrier at any stator winding depends on the cosine of the angle between the rotor and stator windings, provided the ratio of rotor speed to carrier frequency is small. The stator envelopes may be decoded by analog methods to give a single analog signal calibrated to indicate the shaft position within a 360° range. An example of the application of this method to static drives for ac machines is included.