Circuit for regulating the rpm and phase of a motor

Abstract

A timing signal has a value changing during its period and a frequency dependent on actual motor rpm. A reference signal has an average value indicating desired rpm and a superimposed sawtooth component used, after desired rpm is reached, to generate phase-error information. A comparator receives the two signals and produces an output pulse which begins when the timing signal reaches the value of the reference signal, and which ends at the end of the period of the timing signal. In the case of rpm error, the comparator output pulse duration serves to regulate rpm; in the steady state, it serves to regulate phase. A sample-and-hold capacitor converts the aforementioned pulse duration into a lengthier motor-control signal. In the case of rpm error, the pulse duration inaccurately indicates the amount of rpm error, the inaccuracy corresponding to the difference between the average and instantaneous values of the reference signal. During motor start-up, the amplitude of the sawtooth component of the reference signal is attenuated, to thereby reduce the difference between such average and instantaneous values and thus increase the accuracy of indication of the rpm error. The falling sawtooth flank is substantially more than half the period of the sawtooth, so that phase error can be indicated within a correspondingly large range of values.