Patent GB-993631-A (GB)
Title: Improvements in and relating to clutches
IPC: H02P29/00
Filed: 1961-08-14
Abstract: 993,631. Clutch control. ROBERT BOSCH G.m.b.H. Aug. 14, 1961 [Aug. 24, 1960], No. 29213/61. Heading F2L. [Also in Divisions G1 and H3] In a control for an electromagnetic powder clutch for motor vehicle I.C. engines, wherein, to enable the clutch to transmit, above a predetermined speed, a torque rising with engine speed, a winding 35 thereof is fed, during starting, in impulses of frequency dependent upon engine speed, said impulses are generated by means coupled, at least indirectly, to the engine crank-shaft and comprising a multi-vibrator having at least an input transistor 70 and an output transistor 71, and to increase the clutch engagement speed at low temperatures to prevent uneven running or stalling of the engine there is provided a transformer 73 having a primary winding 72 connected in series with the output transistor emitter-collector path and a variable resistor 100 responsive to engine cooling water temperature, and a secondary winding 74 connected to the input transistor base, whereby the impulse length is increased with engine temperature. The circuit comprises a contact breaker arm 53 which is raised twice per crank shaft revolution by a four-lobed cam 55 in the usual distributer to interrupt ignition current through an ignition coil primary winding 52 to induce thereon a voltage 11 which is fed to the multivibrator through circuitry comprising resistors 60, 64, 65, capacitors 61, 66, and a rectifier 63 and delivering a narrow control impulse 43 to the base of the input transistor 70 at each ignition interruption. With the engine stationary, the input transistor 70 is conductive. As soon as ignition is interrupted the induced voltage U renders the base of the input transistor 70 positive relative to its emitter so that said transistor is blocked thereby rendering a switching transistor 82 non-conductive and the output transistor 71 conductive. In consequence, a power transistor 94 is rendered conductive via a transistor 93 and delivers a current pulse Jk to the winding 35. A centrifugal switch arm 90, connecting the base of the switching transistor 82 to the negative lead, opens above vehicle speeds about 15 k.p.h. to block the transistor 82 and render the transistors 71, 93 and the power transistor 94 continuously conductive and the winding 35 continuously energized. Due to the inductance of the transformer primary winding 72 the collector current Jc of the output transistor 71 rises with a time constant = L/R , where L is the primary winding inductance and R the total resistance, including that of the variable resistor 100, in the output transistor emitter-collector path. The rising collector current induces in the secondary winding a voltage which drops with rising collector current and gives rise to a current Js through a resistor 75 and a rectifier 76 to hold the base of the input transistor 70 at a positive potential even after the control impulse 43 has disappeared. In this way the input transistor 70 is held blocked until the output transistor collector current Jc approaches its final value and a voltage Us across the resistor 75 has dropped to a bias voltage Uv across a resistor 79, whereupon the output transistor is again blocked. The variable resistor 100 has a negative thermal coefficient so that its resistance in its cold state is high, and vice versa; thus with rising cooling water temperature the time L constant - is increased and the current pulse R Jk becomes longer. A means whereby the pulse length may be altered by using a preceding impulse consists in altering the bias voltage Vv of the resistor 79 by the collector current of a transistor 105 in the intervals between the control impulses. The emitter of the transistor 105 is connected to the positive lead through a potentiometer 106, its collector is connected to the junction of an adjustable resistor 80 and a resistor 81 arranged between the positive and negative leads, in series with a temperature responsive variable resistor 77 and a resistor 78, and its base is connected through a resistor 107 to a timer circuit comprising resistors 108, 145, capacitors 109, 113, and rectifiers 110, 111, one plate of the capacitor 113 being connected to the collector of the transistor 93. At the moment the transistor 93 is blocked the capacitor 113 commences to be charged through the resistor 108, together with the capacitor 109, and the rectifier 110. The transistor 105 is thereby rendered conductive. Charging of the capacitor 113 continues through the resistor 108 and through the resistor 107 in series with the emitter-base path of the transistor 105. With rising voltage at the junction Q between the resistors 107, 108 the output current of the transistor 105 flowing through the resistor 81 drops slowly to slowly increase the bias voltage Uv. With low engine speed there is a greater time interval between preceding and succeeding control impulses and due to increasing discharge of the capacitor 109, the voltage Uv continues to rise after the production of said succeeding control impulse and therefore the intersection of the curves, plotted against time, of the voltages Uv and Us follows more quickly after the start of the said succeeding impulse, to shorten it, than in the case of higher speeds. In this way, vehicle creep is prevented by reducing the torque at idling speed, and full torque is transmitted at not excessively high speeds so that clutch engagement is sufficiently hard. The shortening of the impulse necessary with a cold engine is achieved by an engine temperature responsive negative temperature coefficient variable resistor 116, arranged in series with the resistor 145 in the base circuit of the transistor 105, and by the variable resistor 100. The voltage Us drops more rapidly with low engine temperature and reaches much earlier the value of the voltage drop Uv across the resistor 79 which has been increased by the variable resistor 116. During gear change a switch arm 86 connected to the gear shift lever connects the base of the input transistor 70 to the negative lead to keep the input transistor conductive and the transistors 71, 94 blocked, the switching transistor being conducting; and to cancel residual magnetism in the winding 35, a demagnetizing current Je flows from the junction of a resistor 138 and highly-doped PN germanium semi-conductors 97, 98, in series between the positive and negative leads, reversely through the winding 35, a rectifier 139 and the switch arm 86. A resistor 95 bridged by an accelerator pedal switch 140 is connected in series between the collector of the power transistor 94 and the winding 35, so that following upshifts, depression of the accelerator pedal closes the switch 140 for full battery effect, whilst when the accelerator pedal is released, as for engine braking following downshifts, the resistor 95 limits winding energization. In the electromagnetic powder clutch (Fig. 2) the driving member, secured to the crankshaft and forming the flywheel therefor, comprises half shells 21, 22 enclosing the winding 35, and the driven member comprises a circumferentially grooves rotor 25 secured to the usual gear-box input shaft. Magnetizable powder 30 is centrifuged, during running, into an annular gap, 1 to 2 millimetres wide, between the shells and the rotor periphery. The winding 35 is fed through insulated brushes and slip-rings 38, 39 and 36, 37, respectively, secured to an engine-flanged casing 12 and the shell 22. Specification 993,632 is referred to.