Patent GB-1334782-A (GB)
Title: Anti-skid control systems for vehicles
IPC: B60T8/1761
Filed: 1971-09-20
Abstract: 1334782 Brakes NIPPONDENSO CO Ltd 20 Sept 1971 [16 Nov 1970] 43756/71 Heading F2F An anti-skid control system for a vehicle wheel to prevent locking of the wheel under braking pressure comprises means 3 for comparing a variable reference-wheel-speed voltage with voltage corresponding to the actual wheel speed and which generates a braking pressure relieving signal when a predetermined relationship exists between the speeds, braking pressure modulating means for receiving the braking pressure relieving signal to relieve the braking pressure, a timer circuit for generating a plurality of timing signals after the braking pressure relieving signal has been generated and a reference-wheel-speed modification circuit 19 for receiving the timing signals from the timer circuit to vary the rate at which the reference-wheelspeed voltage varies, wherein the timer circuit comprises a counter for counting clock pulses having a fixed frequency and a plurality of flipflops controlled by the counter for producing the timing signals. As shown the comparing means comprises a reference wheel speed voltage generating circuit 3, a comparator 5 and a circuit for producing a voltage corresponding to a predetermined speed difference between the actual wheel speed voltage and the reference-wheelspeed voltage such voltage being subtracted from the voltage produced by the circuit 3 and the difference being supplied to the comparator 5; the comparator 5 also receives a voltage produced by a digital-to-analogue converter 2 which is fed with digital pulses from a wheel speed sensor 1 and which produces a voltage output corresponding to the actual wheel speed. The output of the comparator 5 is a digital 0 signal when the actual wheel speed voltage is less than the difference voltage thereby setting a flip-flop 6 which produces the braking pressure relieving signal at its Q output. The circuit 3 includes a comparator 3b one input of which is fed from the converter 2 and the other input of which is fed by an integrator 3a; the circuit 3 also includes a deceleration setting circuit 3c a capacitor 3d and a diode 3e and the arrangement is such that when the voltage provided by the circuit 2 is increasing or in a steady state the comparator 3b causes the capacitor 3d to charge through the diode 3e so as to make the output voltage of the integrator 3a become equal to the wheel-speed voltage provided by the converter 2 whereas when the wheel-speed voltage is decreasing the comparator 3b interrupts the charging of the capacitor and allows the discharge of the capacitor by the deceleration setting circuit 3c. When the wheel-speedvoltage becomes greater than the referencewheel-speed voltage provided by the integrator 3a the comparator 3b provides a digital 0 signal to reset the flip-flop 6, so terminating the braking pressure relieving signal and allowing the brake to act on the wheel again. The circuit 3c consists of a variable resistor 3f which sets an initial rate of decrease of the reference voltage and a transistor 3h biased by resistors 3i, 3j and with a variable resistor 3g in its collector circuit which when the transistor is turned on increases the rate at which the capacitor 3d is discharged and so varying the rate at which the referencewheel-speed voltage reduces. As explained below, the transistor is pulsed at a varying rate by the reference-wheel-speed voltage modification circuit 19 to achieve a desired variation rate of the reference-wheel-speed voltage. The voltage which is subtracted from the referencewheel-speed voltage is determined by the current through a resistor 4a which is maintained at a predetermined value by a transistor 4 the value being controlled by a variable resistor 4b in the emitter circuit of the transistor 4. A set/reset flip-flop 9 is connected to the Q output of the flip-flop 6 and another set/reset flip-flop 10 is connected to the Q output of the flip-flop 6 and by means of 3 NAND gates and a delay circuit 12 the flip-flops 9 and 10 aer caused to produce short pulses when their set inputs receive a logic 1 signal from the flip-flop 6; the pulse from the flip-flop 9 which occurs when the Q output of the flip-flop 6 goes to logic 1 resets a counter 8 consisting of 12 JK flip-flops to the zero state and also sets four set/reset flip-flops 13 to 16 so that the Q output of each goes to logic 1; the pulse from the flip-flop 10 which occurs when the Q of the flip-flop 6 goes to logic 1 resets the counter circuit 8 to the zero and sets two set/reset flip-flops 17, 18 so that their Q outputs go to logic 1. The counter circuit 8 is connected to a clock-pulse oscillator 7 which operates at the frequency of 2 KHz and the counter circuit counts from zero immediately it is reset, appropriate binary outputs of the counter circuit are connected to the flip-flops 13 to 18 to reset them at specific counts of the counter thus providing the various time delay signal to the modification circuit 19; thus if a time delay of 80 milliseconds is required before the flip-flop 13 is reset it is connected to the binary output of the 6th and 8th flip-flop in the counter. The Q output of the flip-flop 6 is also connected to the base of a transistor 22 and the transistor 22 turns on a solenoid 20 to cause the braking pressure to be relieved; braking pressure is restored when the flip-flop 6 changes state. The modification circuit 19 is connected to each of the two outputs of the flip-flops 13 to 18 and produces pulse signals the width of which is fixed and the frequency of which is varied under the control of the flip-flops 13 to 18 to cause the reference signal to reduce at different reference rates of deceleration; thus if the transistor 3h in the deceleration setting circuit 3c is turned on and off at a frequency of two thousand cycles per second, the deceleration is set at 3À6 g., if it is pulsed at one thousand cycles per second the deceleration is set at 1À8 g. and if it is varied at five hundred cycles per second the deceleration is set at 0À9 g. In any braking cycle the reference deceleration is initially 1 g. until the pressure relieving signal is produced and the reference deceleration in the first braking cycle then changes to a 3À6 g. for a time set by the flip-flop 13 after which it changes to a 1 g. until the pressure relieving signal is terminated and the brakes are reapplied, the reference rate then increases in step with the increasing wheel speed and in the second cycle the reference rate initially decreases at 1 g. until the pressure relieving signal is produced and then changes either at 3À6 g. or 1À8 g. depending upon the braking circumstances in the previous cycle; thus if braking pressure was relieved in the previous cycle for a time shorter than the time set by the flip-flop 14 and the subsequent braking action in that cycle took place for a period in excess of the period set by the flip-flop 17 then the reference rate is reduced at the 3À2 g. rate; however, if braking pressure in the previous cycle was relieved for a period shorter than that set by the flip-flop 14 and the subsequent braking in that cycle was less than the time set by the flip-flop 17 then the flip-flop 17 remains set and controls the modification circuit so that the reference rate drops at 1À8 g. In any cycle if the braking pressure relieving time exceeds that set by the flip-flop 14 then the reference rate reduces at 1 g. and when the time exceeds that set by the flip-flop 15 is exceeded, the reference rate is 3À2 g. on the assumption that the vehicle has come to a standstill; hence when the capacitor 3 is completely discharged the braking pressure is restored by the resetting of the flip-flop 6. If the braking pressure is on for a time exceeding that set by the flip-flop 18 the reference rate of deceleration is changed to 0.9 g. If the pressure relieving signal remains on for a time in excess of that set by the flip-flop 16 it is assumed that there is an abnormal condition in a part of the circuitry and when the flip-flop 16 is reset it produces a signal which operates a relay 23 which cuts off current to the coil 21 so that braking pressure is restored. If the braking pressure is restored before the flip-flop 13 has been reset the signal from the Q terminal of the flip-flop 6 resets the flip-flop 13 in addition to setting the flip-flops 17 and 18.