Patent GB-1392389-A (GB)
Title: Dc control circuits for direct drive motors continuously coupled to sewing machines for controlling sewing functions
IPC: D05B69/22
Filed: 1972-06-14
Abstract: 1392389 *Control of D.C. motors; protective systems SINGER CO 14 June 1972 [21 June 1971] 27781/72 Headings H2J and H2K [Also in Division G3]  A D.C. permanent magnet motor 10 continuously coupled to the drive shaft 46 of a sewing machine is driven at a desired speed or braked by a control system including a transistor circuit 22 connected between a D.C. supply 24 and the motor armature 12, a comparator circuit 60 receiving a variable reference voltage 54 determined by the position of a foot pedal 56 and a voltage representative of the motor speed provided by a tachometer 48 and diode bridge 50, the error signal from circuit 60 controlling the circuit 22 via first and second driver circuits 28, 30, a comparator circuit 38 receiving a signal from divider 34, 36 representative of the motor voltage and the voltage of a Zener diode 40, the error signal from comparator 38 controlling the circuit 28 to limit the voltage applied to the motor and so determine its maximum speed, and a dynamic brake consisting of a coil 14 in series with the motor armature 12 and controlling normally-closed contacts 18 in parallel with armature 12 through a resistor 20, the arrangement being such that movement of the pedal 56 to a forward position adjusts the impedance of the circuit 22 to produce the desired motor speed and return of the pedal towards its relaxed position makes the circuit 22 assume a high impedance to de-energize members 12, 14, thus allowing contacts 18 to close and connect resistor 20 in parallel with the armature 12. If the pedal is returned to its relaxed position the motor is braked to a stop, whereas return to an intermediate position brakes the motor to a lower speed set by the pedal, after which the circuit 22 assumes a corresponding impedance state to re-energize the motor and so open contacts 18 to terminate the braking action. The sewing machine may be stopped in the "needle up" or "needle down" position by energization of the coil 70 of an electro-mechanical brake in response to the alignment of a permanent magnet position locator 66 on the shaft 46 with the sensing coil corresponding to the desired needle position, the pedal 56 being returned to its relaxed position to obtain "needle down" and being momentarily urged rearward from its relaxed position to obtain "needle up". Figs. 2 and 3 show the detailed circuits of Fig. 1 and include the following features: Thermal overload protection of the unit 22 (Fig. 2).-This unit comprises parallel-connected transistors 81 ... 86 mounted on a cooling unit 80 and controlled by an emitter follower transistor 93 through a thermal switch 32 mounted in close proximity to the unit 22. If an excessive temperature rise of the transistor mounting surface occurs, the switch 32 opens to remove the driving current from transistors 81 ... 86 which change to their high impedance condition to reduce the current therethrough to substantially zero. Electrical overload protection of the motor (Fig. 2).-Transistors 121, 122 and resistors 126, 127, 128, 130, 132, 134, 136 form a bi-stable multivibrator controlled by the voltage across the coil 14, which voltage causes the transistor 122 to be off when the motor current is below a predetermined upper limit. When this is exceeded, transistor 122 turns on and the multivibrator action changes a transistor 124 from a high to a low impedance to earth the base of a transistor 111 and so remove the drive current from transistors 81 ... 86 which assume their high impedance state. The voltage to the control system must be turned off and then on, in order to reset this overload protection circuit. Automatic de-energization of the motor if the machine is turned on its side.-This condition closes a mercury switch 120 to earth the base of transistor 111 and remove the drive current from transistors 81 ... 86. Provision of a multiple selector switch 74 (Fig. 3) controlled by the pedal 56.-Movement of the latter simultaneously rotates wiper arms 261 ... 264 to any one of ten positions during which resistors 265, 266, 267 are inserted into or shorted out of circuit to vary the reference voltage applied to a transistor 118 controlling transistors 81 ... 86 through transistors 119, 110, 111 and 93, subject to the maximum speed limitation imposed by members 36, 40 and 114. Transistors 224 and 226 respectively control relays 162 and 160 to stop the machine in "needle down" or "needle up" when the switch 74 is returned to its relaxed position or pressed momentarily rearward to the position R. In both cases the transistors 224, 226 are switched off to de-energize the relays 162, 160 when the position locator 66 reaches sensors 76, 78, the resultant energization of the coil 70 being obtained by switching on transistors 97, 98, which action also switches off the transistor 118. In the case of "needle up", movement to position R causes relay 160 to be energized to change the resistor values associated with the switch 74, whereby the motor runs at a slower speed for a short time until members 66, 78 coincide to turn off members 226, 246. Energization of the relay 160 also charges a capacitor 104 whose discharge through a resistor 101 consequent upon the relay de-energization causes transistors 97, 98 to switch on, members 101, 104 having a time constant long enough to permit activation of a thread cutting mechanism when present.