GB1361686A - Electrical drive system for a sewing machine - Google Patents

Abstract

1361686 Control of D.C. motors CUTTERS MACHINE CO Inc 19 Nov 1971 [20 Jan 1971] 53738/71 Heading H2J [Also in Division D1] An electric drive system for a sewing machine comprises an electric drive motor, a speed control circuit for operating the motor at a predetermined low speed, operator controlled means for varying the speed of the motor above the low speed, a dynamic braking circuit across the electric motor, a logic circuit connected to the speed control circuit, a connection for feeding a signal corresponding to the back E.M.F. from the dynamic braking circuit to the logic circuit, means in the logic circuit for de-energizing the speed control circuit and for energizing the dynamic braking circuit to reduce the speed of the motor to said low speed, a needle position switch in the logic circuit, brake switch means in the logic circuit connected to the dynamic braking circuit and means responsive to the closing of the needle position switch for actuating the brake switch to stop the motor only at said low speed. When the machine treadle is depressed the logic circuit 141 is de-energized and the dynamic braking circuit 140 is also deenergized, the clamping circuit 168 is opened and the motor speed control circuit 139 comes into operation by means of the foot feed potentiometer 146. When the treadle is subsequently released the logic circuit 141 is re-energized and the back E.M.F. of the decelerating motor 112 is fed to a sensing transistor 186 but an S.C.R. 187 remains off until that E.M.F. reaches a value corresponding to the needle positioning low speed and transistors 166, 167 are energized, thus closing the damping circuit 168 and deenergizing the motor speed control and motor 112. At this time the dynamic braking circuit still operates. When the low speed is reached transistor 186 turns on SCR 187 and the clamping circuit is opened and the speed control reenergized at the low speed until the next time a down reed switch 134 is closed by a magnet 136 rotating on the needle drive-shaft. A Zener diode 160 at this point receives a pulse above its threshold value and the motor 112 is de-energized by a clamping circuit 168 and the dynamic braking circuit is energized. A filter circuit produces a slight delay so that the dynamic braking circuit remains energized after the motor stops and then the dynamic braking circuit itself is de-energized. A knee-operated switch 128 allows the needle to be stopped in its up position by causing the up reed switch 133 to be actuated by the magnet 136.