Patent GB-1318847-A (GB)
Title: Four-wheel anti-lock brake control systems
IPC: B60T8/1761
Filed: 1972-03-23
Abstract: 1318847 Anti-skid control systems GENERAL MOTORS CORP 23 March 1972 [13 May 1971] 13612/72 Heading F2F  A four-wheel, anti-lock brake control system for a vehicle having a pair of braked front wheels and a pair of braked rear wheels comprises means for sensing front and rear wheel speeds, a control circuit responsive to a function of wheel speeds to detect incipient wheel lock-up of each pair of wheels and in response thereto to cyclically release and apply the front and rear brakes, means responsive to the front and rear wheel speeds for producing a signal when the front wheel speed is higher than the rear wheel speed by a predetermined amount, and timing means responsive to the signal to thereupon activate means for disabling the anti-lock brake control of at least the front wheel brakes when the signal is present for a preset time period, the arrangement being such that anti-lock cycling of the front wheel brakes is prevented when the front wheel speed substantially exceeds the rear wheel speed for a preset time period. As shown, toothed wheel, variable reluctance, electromagnetic transducer type sensors 10, 12 are provided for and driven by the front wheels and a sensor 14 detects the speed of the propeller shaft or some other drive member rotating at a speed proportional to the average rear wheel speed. The sensors 10, 12 are connected directly to squaring amplifiers 16, 18 connected to a maximum frequency selector 22 wherein the higher frequency input signal is channelled to the output signal line 24, the output signal having a frequency proportional to the faster front wheel speed. A frequency-tovoltage converter 28 produces on line 30 an output signal varying according to maximum front wheel speed and this output is passed to a front wheel control circuit 32 arranged to sense incipient front wheel lock-up and produce on line 34 a control signal to effect brake release by means of a selenoid driver circuit 36 connected directly to a solenoid valve 38 for controlling the front brake pressure modulator 40. The rear sensor 14 is similarly connected to a rear brake pressure modulator 40<SP>1</SP>. The circuit to prevent anti-cycling of the front brakes includes a differential frequency-voltage converter 42 which in response to inputs from lines 24, 24<SP>1</SP> produces on line 44 an analogue D.C. output which varies according to the difference between the front and rear wheel speeds, that output being fed to a comparator circuit 46 to produce a signal on line 48 whenever the signal on line 44 crosses a pre-set reference threshold thereby indicating that the front wheel speed exceeds the rear wheel speed, The signal on line 48 activates a timer 50 which completes its timing period if the signal on line 48 persists for a predetermined period. The timer output on line 52 activates disabling circuit 54 which, via line 56 grounds line 34, 34<SP>1</SP> thereby disabling both front and rear wheel anti-lock brake controls so that normal braking is applied to all brakes and thereby preventing anti-lock cycling of the front and rear brakes. Alternatively, the line 56 could be applied only to the line 34 of the front wheel anti-lock control to disable the front wheel anti-lock control only so that if the rear wheels recover from the locked condition the rear anti-lock control will remain operative. The disabling circuit also energizes a warning lamp 58 to indicate that the anti-lock control has been disabled.