Patent GB-1474449-A (GB)
Title: Adaptive braking system for a vehicle
IPC: B60T8/1764
Filed: 1974-09-03
Abstract: 1474449 Adaptive braking systems BENDIX CORP 3 Sept 1974 [26 Sept 1973] 38557/74 Heading F2F An adaptive braking system 10 has a solenoid valve 48 which is actuated to effect brake pressure decay under a number of modes of operation in response to the rotational behaviour of wheels 16, 18 on a common axle; in the primary mode the valve 48 is actuated when wheel deceleration drops below a reference level 38 and the actuation is terminated when deceleration increases above a reference level 42; in the unbalanced mode the valve 48 is actuated when one of the wheels 16, 18 is rotating at a much lower speed than the other as may be caused when the wheels are travelling on surfaces having very different coefficients of friction; in the secondary mode the valve 48 is actuated when both wheels 16, 18 are rotating at such a low speed that locking is imminent; the system 10 also includes a velocity tracking circuit 30 which generates a signal decaying at a rate substantially representing the rate of decrease of vehicle velocity and if this signal becomes less than a reference signal 128 a signal is generated to stop actuation of valve 48 and to terminate brake pressure decay. As shown the system 10 has respective wheel speed sensors 12, 14 and a selecting circuit 26 selects the signal representing the faster rotating wheel and feeds it to a differentiator 32 which generates a signal corresponding to wheel acceleration and deceleration. This signal is supplied to comparators 36, 40 having the reference levels 38 4 , 42 L  corresponding to different deceleration levels, 38 L  being higher than 42 L ; an AND gate 44 generates a primary mode signal which signal is fed to an OR gate 46 the output of which actuates valve 48 via an AND gate 78. The wheel speed signals are also fed to respective comparators 50, 52 having identical reference levels 54, 56 representing relatively low wheel speeds, and the output terminals of the comparators 50, 52 are connected to an OR gate 55 and an AND gate 57. One input of another AND gate 58 is connected to the output of the OR gate 55, another input of the AND gate 58 is connected to the inverted output of AND gate 57, and a third input of the AND gate 58 is connected to the inverted output of a timer 60, whose input is connected to the output of the OR gate 55. AND gate 58 generates an unbalanced mode signal to OR gate 46 when only one but not both wheels 16, 18 are rotating below its corresponding reference levels. Such signal is terminated when both wheels are rotating above or below the reference levels, or when the timer 60 which is responsive to the leading edge of the output signal from OR gate 55 generates an output signal after a predetermined time. Should both wheels be rotating below the reference level speed the AND gate 57 produces an output signal which fires a one-shot 61 connected to an AND gate 62 whose other input is connected to a comparator 66 which generates a signal when the wheel deceleration signal from differentiator 32 is less than the reference level 70. When AND gate 62 generates a signal in response to the two inputs a flip-flop 72 is set, and an output comprising the secondary mode signal is transmitted to OR gate 46. The reset terminal of flip-flop 72 is connected to an OR gate 74 having inputs from a timing means 76 and a secondary mode reset circuit 80. The timing means 76 generates a signal when a predetermined time period has elapsed after the leading edge of the signal generated by OR gate 46 is received by timing means 76, the timing means 76 then supplies a signal which is inverted to close AND gate 78 and also to reset flip-flop 72 via OR gate 74. The reset circuit 80 acts to determine when the faster rotating wheel is rotating at a speed substantially synchronous with vehicle speed so that brake pressure decay will be terminated when this occurs. The circuit 80 includes a comparator 82 connected to differentiator 32 and having a high acceleration reference level 84, so that the flip-flop 72 is reset via an OR gate 86 and a one shot 88 when the acceleration exceeds the reference level 84. Further the circuit 80 includes a comparator 94 having an acceleration reference level 96 which represents a substantially zero acceleration level, and also a peak fall off detector 89 connected to differentiator 32; thereby, an AND gate 92 connected to OR gate 86 produces a signal when the value of the acceleration signal has decreased a predetermined amount from its peak value, and the value of the acceleration signal after this decrease from peak value is still greater than zero. Still further, the circuit 80 includes another peak fall-off detector 98 having a smaller fall-off detector 89 and the detector 98 supplies a signal to an AND gate 102 whose other input is connected to a timing means 104 started when AND gate 78 generates an output signal to valve 48; thereby AND gate 102 generates a signal to OR gate 86 only when the fall-off allowed by detector 98 occurs after the solenoid valve 48 has been on for a time period equal to that of timing means 104. The velocity tracking circuit 30 (as seen in Fig. 3) has a capacitor 113 whose charge increases as the rotational velocity of the faster wheel increases: when the wheel speed decreases the capacitor discharges through current sources 116, 120 at a limited rate. However during operation of the system in the secondary mode the output signal of flip-flop 72 switches off a transistor 124 to change the decay rate of transistor 113. The output signal of velocity tracking circuit 30 is supplied to a comparator 126 having a reference velocity 128 corresponding to a relatively low value of vehicle speed and the comparator 126 generates an-output signal to a timer 130 when the output signal of circuit 30 decreases to a value less than the reference velocity 128. The timer 130 produces an output signal a predetermined time period after it is actuated-and the inverted output of timer 130 is supplied to AND gate 78 to halt brake pressure decay.