Patent GB-1517318-A (GB)
Title: Wheel slip control system for automotive vehicles and the like
IPC: B60T8/70
Filed: 1975-11-04
Abstract: 1517318 Antiskid braking systems WAGNER ELECTRIC CORP 4 Nov 1975 [3 Feb 1975] 45840/75 Heading F2F In a method of controlling the degree of slip of at least one brakeable vehicle wheel, the wheel speed and acceleration are monitored and a reference increment of wheel speed is generated. The rate of change of braking force is modified at least during each period of time in which the wheel acceleration exceeds a predetermined deceleration threshold, the braking force is reduced at least during each period of time in which the reduction in wheel speed, measured from the value at the time the acceleration exceeds the said threshold, exceeds the variable reference increment of wheel speed, the said increment being variable in dependence on both wheel acceleration and reduction in wheel speed during wheel deceleration and in dependence on wheel acceleration alone during acceleration. The increment may be generated in linear or in non-linear relationship to the rate of change of wheel speed during wheel acceleration. Sensors 10, 12 generate pulses of a frequency in proportion to wheel speed and feed the pulse train to frequency/D.C. converters 14, 16, an input bias network 15 being connected to the converters 14, 16. The analogue signals are fed into wheel speed selector circuit 18 which passes a signal representative of the lowest wheel speed to a differentiating circuit 20, which provides a wheel acceleration signal, to a velocity threshold circuit 30 and to the variable reference signal generator 34 which in turn receives a deceleration signal from circuit 20 and a signal from a deceleration threshold circuit 22 via an inverter 24. If the wheel speed falls below its initial value by more than the variable reference increment, the velocity threshold circuit 30 generates a variable-width, positive pulse of constant amplitude which is applied to the input of inverter 36 which in turn applies a negative pulse to an input of NOR gate 38 and to an input of NOR gate 26. Gate 38 generates a positive output pulse which is amplified by amplifier 40 to energize solenoid valve S2 of the modulator valve assembly and effect sharp relief of brake line fluid pressure. The positive pulse generated by NOR gate 26 in response to the inverted deceleration pulse from threshold circuit 22 maintains solenoid valve S1 energized. The positive pulse from circuit 30 is also applied to NAND gate 42 which normally receives a positive signal from NAND gate 44 at its other input so that a negative pulse is produced at the output of gate 42 and a timer 46 is actuated to provide a positive input to an inverter 48 and hence a negative input to a gate 32 for a predetermined time interval so that the speed signal can pass to threshold circuit 30 even after the deceleration pulse from circuit 22 is terminated. In the modulator valve assembly valve S 1 has a service inlet and service exhaust while valve S2 has a service exhaust only giving the following modes of operation: (1) S1, S2 both de-energized, rapid application of fluid pressure to control chamber of modulator valve assembly. (2) S1 energized, S2 de-energized: slow exhaust rate. (3) S1, S2 both energized: rapid exhaust rate. (4) S1 de-energized, S2 energized: reduced application rate. During acceleration a timer 54 ensures that the acceleration pulse has a minimum duration so that spurious outputs from the sensors 10, 12 do not undesirably affect the skid control cycle. A fail safe circuit includes an OR gate 68, a timer 70 and a circuit 72 containing a siliconcontrolled rectifier which when caused to fire by malfunction in the circuitry shorts to ground and blows a fuse. Malfunction of a sensor operates a warning circuit 64 to energize a device such as a buzzer.