Patent GB-763252-A (GB)
Title: Improvements in or relating to servo-devices
IPC: G05B19/39
Filed: 1953-11-13
Abstract: 763,252. Lathes; machine details. SOC D'ELECTRONIQUE ET D'AUTOMATISME. Nov. 13, 1953, [Nov. 22, 1952], No. 31544/53. Class 83 (3). [Also in Groups XXXVII and XL (b)] The carriage 1 of a lathe &c. is fed through a screw 5, Figs. 1 and 3, by a reversible servomotor 6 and a differential gear 12 driven at one side by motor 6 and at the other side by an auxiliary motor 13. The output shaft of the differential gear controls a slider 15 of a potentiometer 16 which is supplied with a direct reference-voltage. A scale 19 having graduations in the form of slots 20 co-operates with an impulse-generating device 21 on the toolcarriage, the device 21 operating in response to a change of reluctance of its magnetic circuit when passing over a slot 20. Coils 24, 25, produce opposing fluxes so that the current derived from coil 26 takes the form shown at 27, Fig. 2, the signal from the coil 26 being amplified at 29 and then passed to a demodulator 28. By counting the number of pulses from the device 21 when it passes over the scale 19, a measure of the distance travelled by the carriage is obtained. Triggering pulses for the actuation of an electrical counter are produced by a circuit 34, the pulses travelling through leads 48 or 45 to counters 36 or 46 according to the direction of travel of the carriage. The number of pulses that the counter 36 receives before maximum capacity of the counter is reached and the counter is automatically brought back to its zero condition, is pre-set by selector-switches 55, the counter 46 being similarly pre-set by selector switches 77. Voltage decoders 40, 39, are associated with the counters 36, 46, each counter having an output lead 38, 37. The output voltage derived from each counter has a definite value when the counter is inoperative and decreases step-bystep each time the counter advances one digit. The decoders have equal outputs for the same condition of the counters and, as one counter remains at zero during a cycle of operations corresponding to one direction of travel of the carriage, a reference for the varying voltage of the other counter is provided. The outputs of the decoders are applied to a summing amplifier 41 leading to a power amplifier 44 which controls the motor 6, the arrangement being such that the motor rotates until the outputs of the decoders become equal, whereupon the signal from amplifier 41 reaches zero. After the motor has stopped, the last signal from the demodulator 28 is applied directly to the amplifier 44 and the motor is re-started until the signal becomes zero, so ensuring accurate positioning of the carriage. Further control means may be provided for adjusting the position of the carriage with an accuracy equal to a fraction of the distance between graduations on the scale. For this purpose, after the motor 6 has been stopped in the manner described above, the auxiliary motor 13 brings the slider of the potentiometer 16 back to the zero position. The motor 6 is then driven to a position corresponding to the setting of a potentiometer 59 which provides a voltage for moving the carriage a fraction of the distance between graduations on the scale 19, the motor being finally halted when voltages derived from potentiometers 16, 59 are balanced and the input to amplifier 67 reaches zero. The potentiometer 16 may be replaced by other forms of control devices and the pulsegenerating device 21 may be of the photoelectric type.