Fail safe vehicle wheel braking anti-lock system

Claim 1

In a vehicle wheel anti-lock system in which fluid pressure applied to a vehicle wheel brake is relieved, having a solenoid (24) to operate a bleed valve, and means (10, 11) providing a signal representative of wheel speed; differentiating means (12) delivering a differentiated derivative signal representative of rate of change of wheel speed; threshold switch means (13) having the differentiated speed signal applied thereto and providing a pressure relief control signal to the solenoid when the differentiated signal exceeds a predetermined limit; valve switching means (23) responsive to the pressure relief control signal to control energization of the bleed valve solenoid; a first timing circuit means (14, 21) interposed between the threshold switch means (13) and the bleed valve switching means (23) and energized upon beginning of the pressure relief signal to start measuring a predetermined time interval and to conduct said pressure relief signal to the bleed valve switching means (23) during said time interval only to permit response of the solenoid coil to relieve brake pressure, said timing circuit means (13), after said predetermined time interval, interrupting application of said pressure relief signal to the valve switching means (23) regardless of persistence or nonpersistence of the pressure relief signal, whereby control of relief of braking pressure by said pressure relief signal is limited to said time interval, and the time of relief of fluid pressure applied to the brake is always limited to said predetermined time interval; and an additional turn-off circuit (26) including a turn-off relay (25, 251) connected between the power supply (35, 36) and said bleed valve solenoid (24) to interrupt current to the solenoid upon response of said turn-off circuit when energized, said turn-off circuit comprising a solenoid condition sensing circuit (27) connected to provide a signal when current flows through the solenoid to operate said solenoid; a turn-off timing circuit (28) having a second timing period longer thaN the timing period of the first timing circuit (14); a logic gate (29) connected to the sensing circuit (27) and to the turn-off timing circuit (28) and providing an output upon logical conjunction of: a. current flow to the solenoid, and b. elapse of said second timing period, the output of said logic gate being connected to said turn-off control relay (25, 251) to interrupt power to the solenoid upon response of said logic circuit after elapse of the second timing period of said turn-off timing circuit (28).