Patent GB-1231512-A (GB)
IPC: H02P29/00
Filed: 1969-02-13
Abstract: 1,231,512. Clutch control. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 13 Feb., 1969 [16 Feb., 1968], No. 7914/69. Heading F2L. [Also in Division H3] To facilitate push-starts of a motor vehicle engine, the coil 56 of an electromagnetic clutch, e.g. of the magnetic powder type, is controlled by an electronic circuit which supplies it with maximum current before the engine is started; minimum current while the engine is idling; and then with a current increasing to a maximum in accordance with engine speed as sensed by a magnetic transducer E. The minimum current is insufficient to engage the clutch. Circuit.-The transducer signal is shaped by clipping by an amplifier transistor 3, and is applied through a capacitor 7 to the base of a trigger transistor 11 which forms a monostable circuit with the time constant of a resistor 9 and the capacitor 7. A capacitor 19 integrates the signals and supplies them to a comparator transistor 32 having its emitter at a reference voltage determined by a potentiometer 24. The transistor 32 controls a monostable power circuit including transistors 37, 47, such that: (a) in its saturation condition, the current corresponds to that required during engine idling; (b) during its variable conduction condition the current correpsonds to that required during increase with engine speed; and (c) during its cut-off condition, the current corresponds to the maximum. Operation.-(a) During saturation, the transistor 37 is driven into saturation, and transistor 47 into cut-off, determining the first stable state of the power circuit. A weak current flows through coil 56 determined by a resistance 53, and the clutch remains disengaged while the engine idles. (b) During conduction, the transistor 37 comes out of saturation and transistor 47 begins to conduct. The duration of the conduction of these transistors is determined by a feedback circuit comprising a capacitor 42 and a resistor 48, and by the collector current of transistor 32. This current decreases with increasing engine speed, decreasing the duration of the conductive condition of transistor 37 and increasing that of transistor 47 to determine the gradual rise in the mean coil current in accordance with engine speed. (c) During cut-off, the transistor 47 is driven into saturation and the power circuit is in its second stable state and a continuous current is delivered to the coil 56. Push starts are provided by causing the transistor 3 to conduct at standstill. A resistor 4 is connected to the positive terminal of source VB and diode 6 is cut-off; transistors 11, 32 and 37 are consequently cut-off and transistor 47 continuously delivers a current to the coil 56 until the engine starts running, rendering the diode 6 conductive and charging the capacitor 8 which prevents transistor 11 from being permanently cut-off again. The coil current is reduced to disengage the clutch when the transistor 32 is again driven into its saturation condition. Accelerator switch 39 smooths clutch reengagement after a transmission down shift. The switch short circuits a resistor 45 when the accelerator is depressed, cutting off diodes 30, 31 and allowing normal operation. However, with the switch open, an additional control voltage is applied to the collector of transistor 47 by an integrating circuit comprising resistor 50 and capacitor 41, modifying the bias of transistor 32 to prevent the power circuit from reaching its second stable state, and limiting the current to a predetermined value. Gear lever switch 57 is operative during a gear shift to ensure complete clutch disengagement by connecting the coil 56 to a contact b so that a weak reverse current, determined by resistors 52, 55, and diode 54, flows through the coil to cancel out the remnant magnetism. Choke switch 36 is normally open but is closed during choke operation so that the increase in idling speed due to choke operation does not cause the clutch to be engaged too early. The operation of the monostable power circuit is rendered largely independent of voltage variations of the source VB by the double integrating circuit comprising resistors 43, 49 and capacitors 38, 46 forming a feed back circuit from transistor 47 to transistor 32. Temperature stabilization of the circuit characteristics is effected by diodes 22, 29 and NTC- resistors 13, 15.