Analog DC motor velocity control loop

Claim 1

A velocity control system for a DC motor comprising: a source of DC voltage drive pulses applicable to drive said motor; means for controlling said source for maintaining the velocity of said DC motor substantially constant under variable loading conditions including, a feedback loop connected to modulate the duty cycle of said drive pulses provided by said source to said motor, said feedback loop including means for producing digital feedback control pulses at a frequency proportional to the acutal velocity of said motor, said digital control pulses having a reference interval of fixed time duration independent of the frequency of said control pulses; and analog circuit means operable by said feedback control pulses for controlling the duty cycle of said drive pulse in accordance with the relative proportion of said reference interval and the period of said digital control pulses at the actual velocity including a first current source providing a constant DC reference current, a second current source switchable by said digital control pulses for providing a DC feedback current of fixed time duration correspondong to the reference interval of said binary control pulse; said reference and feedback currents having opposite polarities and different amplitudes whereby a binary current signal is provided, integrating means for converting said binary current signal to a sawtooth voltage signal which oscillates relative to a fixed DC reference voltage, said sawtooth signal having one ramp of fixed slope and duration as a result of the integration of said binary current signal having said fixed time duration, and means for modulating the width of said DC voltage drive pulses applied to said motor in accordance with the interval between crossovers of said sawtooth voltage signal and said reference DC voltage.