DC Servomotor circuit having drive current controlled as a function of motor speed

An apparatus for predetermining the maximum current supplied by a drive circuit to a bidirectional DC motor coupled to a tachometer providing a tachometer signal having a magnitude representing the actual motor speed and a polarity representing the actual direction of motor rotation, said drive circuit being controlled by the magnitude and polarity of a bipolar speed error signal generated by a comparator in response to the algebraic difference between the tachometer signal and velocity command signal, said command signal having a magnitude representing the desired motor speed and a polarity representing the desired direction of motor rotation, the apparatus comprising a bipolar limiting means having an input responsive to the tachometer signal and an output connected between the comparator and the drive circuit for limiting the magnitude of the speed error signal to positive and negative limiting levels for both polarities of the tachometer signal, said limiting means causing the magnitude of the limiting level to be

• (1) varied continuously in accordance with a plurality of linear functions, and in response to the tachometer and command signals having polarities representing the same direction of motor rotation, each linear function controlling over a portion of the full speed range of the motor to produce a nearly exponential profile of motor current, and
• (2) directly proportional to the tachometer signal in response to the polarity of the tachometer signal representing a direction of motor rotation opposite the direction of motor rotation being represented by the polarity of the command signal, whereby the maximum current available to the motor is predetermined exclusively by the speed error signal over the full speed range of the motor.