Patent GB-1390310-A (GB)
Title: Monitoring and protecting circuit for an electronic braking system
IPC: B60T8/1761
Filed: 1973-10-29
Abstract: 1390310 Electric protective systems SOC ANON DBA 29 Oct 1973 [3 Nov 1972] 50239/73 Heading H2K [Also in Division F2] An electronic anti-skid braking system is connected to a switch K which is operable to inhibit the operation of the system in response to signals appearing at a point D and generated by a monitoring and protecting circuit. The circuit has input terminals E1-E4 which receive signals representative of the rotational speeds of respective wheels of the vehicle from speed sensor units. The terminals E1-E4 are respectively connected to diode d 1 -d 4  of a "SELECT HIGH" diode array and also to diodes d 5 -d 8  of a "SELECT LOW" diode array. The cathodes of diodes d 1 -d 4  are connected to point A and the anodes of diodes d 5 -d 8  are connected to point B, and point A is connected to the base of an NPN transistor T1 whose emitter is connected through a resistor R1 to the point B. The collector of transistor T1 is connected to the base of PNP transistor T2 whose collector designated by the point D is earthed by way of a parallel resistance/capacitor network R4/C1. The collector of transistor T1 is also connected through a diode d 9  to a brake light switch I. The point A is connected to the base of an NPN transistor T3 whose emitter is connected to the base of an NPN transistor T4 by way of a capacitor C2, a resistor R6 being provided between the base of transistor T4 and earth. The collector of transistor T4 is earthed by way of a capacitor C3 and is connected to the base of a PNP transistor T5, whose collector is connected to the base of transistor T2.  When the vehicle is travelling on a straight road with brakes released the speed signals appearing at E1-E4 will normally be equal and the transistor T1 is non-conducting, and the switching device K occupies the condition in which it allows the anti-skid system to operate. When the vehicle turns with brakes released, a slight speed difference occurs between the various wheels and produces between points A and B a low voltage too small to switch transistor T1. Should a fault develop in at least one of the speed sensor units during unbraked travel the voltage difference between A and B may cause transistor T1 to conduct, and a voltage drop to appear at its collector. This causes switching of transistor T2 which results in the application of a positive voltage delayed by time-delay circuit R4/C1 to the point D to cause the switching device K to shift into its condition to prevent the anti-skid system from operating.  When the vehicle is braked, the light switch I causes the collector of transistor T1 to be connected to a positive voltage + as soon as braking begins, so switching off transistor T2 and preventing switching device K from operating. To prevent the switching device K from operating when the vehicle is accelerating and the driven wheels are skidding the circuit comprising transistors T3, T4, T5 and associated components is effective: in this circumstance the signal at point A is transmitted via transistor T3 to be differentiated by network C2/R6 and if the signal appearing at the base of transistor T4 denotes acceleration of the driven wheels above a given threshold transistor T4 switches over and the capacitor C3 discharges almost instantaneously to the transistor T4. Assuming that the transistor T4 switches over again rapidly because the acceleration of the driven wheels falls below the threshold, the capacitor C3 recharges slowly by way of a resistor R7, the transistor T5 remains conductive during the charging time and applies the positive voltage + to the base of transistor T2 to prevent the switching device K from inhibiting the antiskid braking system.