Patent GB-1401043-A (GB)
Title: Variable speed sewing machine drive control system
IPC: D05B69/22
Filed: 1972-06-14
Abstract: 1401043 Sewing machine KOLLMORGEN CORP 14 June 1972 [16 June 1971] 27712/72 Heading DIG [Also in Division H2] A control system for a sewing machine comprises a low inertia DC motor coupled to the machine drive means, transformer means controllable by the operator to develop a variable AC output which is converted into DC of low source impedance to supply the motor, switch means controllable by the operator to provide a stop command and disconnect the motor from its supply and needle positioning means responsive to a stop command and to the position of the drive means relative to a reference position to develop a limited duration torquing current to drive the motor subsequent to the stop command until the needle reaches a predetermined position. In the embodiment shown in Figure 6, the sewing machine treadle 14 is mechanically coupled to the wiper 82 of a rotary autotransformer 80 and to switches 51, 52, 53, of which the first and third control the running supply to a low inertia DC motor 26 and the braking thereof and the second controls a positioning circuit 96 for driving the motor to a desired needle position after the end of a running period. Movement of the treadle 14 from its rest position changes the switch 51 to its NC position to connect the rectifier 32 to the segment 88 of switch 53, which is designed so that its wiper 90 does not engage segment 88 until the wiper 82 has moved a small distance from its zero voltage position, whereby voltage is applied abruptly to the motor to overcome the friction of the system. This allows the use of a motor which avoids the need for speed regulation to compensate for load variations during sewing the motor torque being selected to produce a mechanical time constant for the system which is less than 500 milliseconds and preferably about 50 milliseconds. The motor speed is varied by variation of the position of wiper 82 and the motor is stopped by releasing the treadle 14 to allow wiper 82 to return to its zero voltage position, the return movement of switch 53 connecting its members 90, 92 to short circuit and so brake the motor, assisted if desired by an electromechanical brake, after which switches S1, S3 are opened (S1 in N.O. position and S3 with wiper 90 in the gap between segments 88, 92). Simultaneously the switch S2 changes from its N.C. to its N.O. position to supply the motor with half-wave rectified AC from transformer 80 through one or other of controlled switches SCR1, SCR2 according to whether the motor shaft is on one or other side of a desired position corresponding to the required needle position. To this end the motor shaft 24 carries a magnet 102 controlling sets of normally-open read switches Fl, F2, F3, B1, B2, B3 spaced about the desired position, Fig. 8 (not shown) successive closure of the switches in either set by the magnet causing one of the switches SCR1, SCR2 to be disabled (opened) and the other to be fired (closed) to supply the motor with pulses of current until it reaches its desired position. Variable resistors 110, 112 allow these current pulses to be regulated to prevent overshoot of the position. The position-sensing switches may be cam-operated ones or light-sensitive diodes and the desired position may be deduced from a particular condition of the switches or transition in switching. Normally-closed seriesconnected switches may be used. In another modification Fig. 9 (not shown) the switches are omitted and the shaft magnet (113) rotates between the poles (115, 116) of an electromagnet (114) having a winding (118) which is energised with DC by operation of the switch S2, the motor then being turned by the magnet (113) until it is aligned with the poles (115, 116). As shown in Figure 1, the treadle 14 is coupled by a linkage 36 to a pulley 38 secured to the rotor 40 of the transformer 28 electrically connected by a cable 30 to the rectifier bridge 32, a return spring 40 automatically restoring the rotor 40 to its zero-output position when the treadle is released. The motor 26 is located in a housing 27 which also contains the bobbin winding mechanism shown in Figures 2 and 5. A coupling 47 connects the motor shaft 24 and machine drive shaft 20 and also engages the rubber ring 44 of a friction drive wheel 45 on whose shaft 48 the bobbin 56 is mounted. With an empty or partially-wound bobbin, the wheel 45 is urged into contact with the coupling 47 by the combined action of a spring 66 acting on an extension 68 of a pivoted arm 51 carrying the wheel 45 and of a spring 63 acting on a lug 60 which has a shoulder 70 engaging the arm 51 and a feeler arm 58 engaging the bobbin thread and moving anti-clockwise as the thread accumulates. When the bobbin is full, the arm 58 moves members 60, 63 past their operational dead centre and spring 63 overcomes spring 66 to cause lug 60 to rotate the arm 51 in an arcuate slot 54, thereby disengaging the wheel 45 from the coupling 47.