GB1470782A - Electronic motor control - Google Patents

Abstract

1470782 Automatic control of speed FERRANTI Ltd 4 Dec 1974 [4 Dec 1973] 56111/73 Heading G3R In a digital motor control system, a reversible counter, which controls drive current to the motor, counts clockpulses in a direction dependent upon whether the measured motor speed is above or below a desired value, and the clockpulse rate is altered in response to the desired motor speed. As shown, counter 14 counts clockpulses during each cycle of pulse train from tachometer 12, whose output is shaped at 16 and has a frequency dependent upon the speed of motor 10. At least the most significant stages of counter 14 are compared with a desired motor speed signal as set in register, 18. A bi-stable circuit 20 is set if the count is greater than the desired value and reset by each pulse from shaper 16. A reversible counter 24 counts up or down according to the state of bi-stable circuit 20, and the contents of the counter are used to control the motor drive current e.g. by varying the firing angle of a thyristor or triac, provided the contents of counter 24 exceeds that of counter 26, which is reset for each half-cycle of the motor A.C. supply. To maintain stability, the maximum rate of change of the firing angle is limited by the frequency of clock 31 or by a submultiple of this frequency. The pulse rate of the clock 31 is lowered using the output of counter 14 and bi-stable circuit 39 if the motor speed is within a given range i.e. for small motor speed errors. Motor 10 may be a multi or single phase A.C. motor, a D.C. motor, a linear or stepping motor. The system can be used in a washing machine or an aerial attitude control.