Apparatus and method for generating digital position signals for a rotatable shaft

An apparatus and method for generating digital electrical signals which are representative of the angular position and rotational velocity of a rotatable shaft component in a servomechanism includes a resolver which is connected to the rotatable component. The resolver generates analog electrical output signals which are representative of an excitation signal modulated by both the sine and cosine of the electrical angular or position of the motor. The output signals from the resolver are fed to a digital signal processing circuit which employs software algorithm techniques to define a plurality of tasks which are implemented sequentially to obtain the desired angular position and velocity signals. First, the sine and cosine signals are read from the resolver through respective analog-to-digital converters. The sine and cosine signals are demodulated by multiplication with the excitation signal, and the undesired higher frequency components are deleted by filtering. Then, the demodulated sine and cosine signals are conditioned to insure quadrature and to correct for amplitude and offset variations. Next, an electrical angular position signal for the rotatable member is determined by combining the demodulated and conditioned sine and cosine signals, and a correction is made for repeatable harmonic errors. A calculation is then made to determine the rotational velocity of the rotatable member. The electrical angular position signal is next corrected for velocity related errors to generate an actual electrical position signal. Lastly, an actual mechanical position signal for the rotatable member is calculated.