Patent GB-1281488-A (GB)
Title: System for controlling needle positioning drive and thread trimming functions of sewing machines
IPC: H02P7/292
Filed: 1970-05-20
Abstract: 1281488 Control of DC motors SINGER CO 20 May 1970 [24 June 1969] 24323/70 Heading H2J  To stop a sewing machine in a needle-down position, a treadle is relaxed whereby switches 28-31 are arranged as shown and a flip-flop 101 is set so that gate 130 saturates transistor 44 to turn-off the drive SCRs 13 in circuit with the motor 10. Capacitor 133 charges until switch 135 emits a trigger pulse to brake SCR 32, the same pulse turning-on transistor 136. Flip-flop 137 activates a speed sensor circuit comprising photo-transistor 121. Darlington amplifier 139 and programmable unijuction transistor 140. The transistor 121 forms part of a tachometer device responsive to motor speed, the transistor 140 emitting a pulse when the speed has been reduced to a given value. This pulse is transmitted through transistor 151, inverter 152, gate 153, and flip-flop 154 to flip-flop 137 which discharges capacitor 146 to de-activate the speed sensor circuit. The unit 154 is associated with gate 157 and inverters 158, 159, which are coupled through transistor 160 to the gate of the brake-release SCR 53. Capacitor 54 discharges in opposition to the existing brake current and turns-off the SCR 32. The SCR 32 may be replaced by a power transistor, Fig. 9 (not shown). When the multivibrator 157, 158, returns to its stable state, the flip-flop 101 responds to turn-off the transistor 44 whereby the motor is energized to operate at a speed determined by the setting of potentiometer 25. After this slowing-down operation, phototransistor 103 emits a needle-down sensor pulse to set the flip-flop 101 whereby transistor 44 saturates to turn-off the drive circuit. The SCR 32 is again triggered to introduce dynamic braking until the motor is stopped. A switch 200 is closed to move the machine from a needledown to a needle-up position. Flip-flop 201 couples a negative pulse to flip-flop 101 which re-starts the motor, capacitor 43 providing gentle starting operation. Needle-up position pulses from photo-transistor 123 are now coupled to flip-flop 101 which turns-off the drive and applies dynamic braking until the machine stops in the desired position. After the machine has reached a needle-down rest position, a thread-trimming operation is initiated by heeling the treadle to open switch 212. The output of photo-transistor 122 is low so that inverter 213 provides a high signal to gate 214. When the switch 212 is opened, flipflop 208 responds to turn-on transistor 220 (Fig. 3) and energize picker solenoid 209 for the quasi-stable period of multivibrator 215- 216. After this period, the solenoid 209 is de-energized and flip-flop 201 responds to drive the machine to the needle-up position. Photo-transistor 123 then causes transistor 227 to energize a trimmer solenoid 210 for the quasi-stable period of the multivibrator 223- 224. The output of inverter 225 is coupled through a delay circuit 228, 229, to transistor 231 which energizes wiper solenoid 211. The solenoids 210, 211 are de-energized when the multivibrator 223-224 returns to its stable state, the machine being now at rest in the needle-up position with the bobbin and needle thread trimmed and the needle thread wiped. The sensor arrangement for generating the speed and position signals comprises two lamps mounted on a bracket secured to the motor which drives a disc carrying near the periphery a ring of equally-spaced reflecting dots for producing a speed signal. A single reflecting dot and two reflecting segments are also carried by the disc so that light is reflected by the respective dot or segment on to the appropriate phototransistor for generating a time-related waveform signal indicative of the rotational speed or position of the motor shaft, Figs. 4, 5, 6 (not shown).