Patent GB-1303519-A (GB)
Inventor: HALFHILL MARTIN O
IPC: H02P7/29
Filed: 1970-02-05
Abstract: 1303519 Control of DC motors INFORMATION STORAGE SYSTEMS Inc 5 Feb 1970 [21 March 1969] 5645/70 Heading H2J In a servo system (e.g. for data processing plant) a D.C. servomotor is controlled in dependence upon any imbalance between a feedback signal derived from the motor armature current and having a polarity significant of its direction of rotation and a position error signal P (as embodiment, Fig. 4, not shown) or between the said feedback signal and the differential output of a summing amplifier 12 (Fig. 1) on lines 34, 35 the inputs to the amplifier being a position error signal P and a velocity signal V. In Fig. 1 when P is larger than V, the line 34 is more positive than line 35 and vice-versa. The centre point of a pot 36 provides a reference potential. A comparator 13 compares a motor current feedback signal If (corresponding to motor current when it is rotating in a forward direction) with the summing amplifier output on line 34 when line 34 is more positive than the reference point. Comparator 14 compares a feedback signal Ir (corresponding to motor current when it is rotating in the reverse direction) with the amplifier 12 output when line 35 is more positive than the reference point. The comparators 13, 14 control switches 21, 19 (Fig. 2) respectively. A pulse generator 15 emits a 5 Ásec. pulse every 50 Ásecs. which opens switches 16, 17 in the current feedback lines, interrupting the current feedback and causing imbalance of the comparator which is controlling the motor, e.g. comparator 13 when the motor is rotating forward. The comparator 13 consequently closes the switch 21 whereby full drive current is applied to the motor 11. Thus full drive current is applied for at least 1 Ásec. every 50 Ásecs. The motor armature current is monitored by the circuit, Fig. 2, the voltage drop across R18 being a measure thereof and either a transistor 27 or 28 is turned ON depending upon the polarity across R18. The feedback signal If is produced by Tr 27 and Ir by Tr 28. A two-way and off switch 22 (e.g. a transistor switch) is closed in dependence upon the direction the motor 11 is to rotate. After a drive pulse is terminated by the reclosing of switches 16, 17 and the rebalance of the comparator inputs and consequent opening of switch 21 (or 19), the motor inductance causes the voltage level at the end of R18 adjacent switch 21 to fall below -48 V. so that a diode 25 conducts and the motor current decays very little before the next pulse from generator 15 closes switch 16 again and causes imbalance. In effect the motor inductance drives the terminal voltage to that necessary to maintain constant motor current. The output of each comparator is a pulse train the width of each pulse being proportional to the magnitude of the difference between the position P and velocity V signals applied to amplifier 12, the pulse widths varying while their fre quency remains constant as established by the pulse generator 15. In the alternative embodiment Fig. 4 (not shown) the pulse gen. 15 and switches 16, 17 are omitted and a dual polarity ramp generator waveform is applied to a differential summing amplifier together with the position error signal and an integrated current feedback signal from the motor. The result is an h.f. pulsed drive signal to the motor, the pulse widths of which vary with the magnitude of the difference between the position error signal and motor current.