Patent GB-1436418-A (GB)
Title: Anti-skid braking system for a vehicle
IPC: G01P15/16
Filed: 1973-08-14
Abstract: 1436418 Antiskid braking systems WABCO WESTINGHOUSE GmbH 14 Aug 1973 [6 Sept 1972] 38396/73 Heading F2F An antiskid vehicle braking system wherein brakes are released by a blocking system 5 on detection of a skid, has a wheel-speed sensor 2, a differentiating circuit 3 for forming a signal proportional to the deceleration of the wheel 1 and including a filter for smoothing the deceleration signal, and a threshold value switch 4 which compares the deceleration signal with an initial threshold value and, upon the threshold value being exceeded by the deceleration signal, sends a control signal to actuate the blocking system 5, which control signal terminates when the deceleration signal falls below a second threshold value, wherein the threshold value switch 4 has means for automatically raising the second threshold value above the initial threshold value in dependence upon the extent to which the deceleration signal exceeds the initial threshold value. In the circuit of Fig. 2 the differentiating circuit 3 consists of a differentiating capacitor 6 and an operational amplifier 7 having a filter, consisting of a capacitor 8 and a resistor 9 in parallel, connected to provide negative feedback. The switch 4 includes a voltage divider 10, which may be adjustable, and comprising two resistors 11, 12 and a tap 13. The tap 13 is connected through a diode 14 with a capacitor 15 having a resistor 16 in parallel. A lead 17 runs from circuit 3 to a junction 18 which is connected by way of a lead 19 to one input 20 of an operational amplifier 21. The junction 18 is also connected through a diode 22 to a second input 23 of amplifier 21 by way of a lead 24 which is also connected with diode 14. In operation a first voltage forming the deceleration signal is produced by circuit 3 and is applied to input 20; a second voltage produced by divider 10 and forming the initial threshold value is applied to capacitor 15 and thence by way of lead 24 to input 23. The amplifier 21 compares the first and second voltages and transmits a control signal to the blocking system 5 when the first voltage is greater than the second. From this moment on the capacitor 15 is charged by way of diode 22 to a voltage less than the first voltage by an amount equal to the drop in voltage due to diode 22. When the first voltage drops as deceleration decreases with the brakes released, the capacitor 15 begins to discharge through resistor 16 and the voltage on capacitor 15 is applied to input 23. When the diminishing first voltage falls below the capacitor voltage. The amplifier 21 ceases to produce the control signal, the capacitor 15 thus forming the second threshold value. In the modification of Fig. 3 the first voltage formed by the differentiating circuit 3 is applied to the emitter E of a transistor 33, and the second voltage forming the initial threshold value is applied through the diode 14 to the capacitor 15 and to the base B of transistor 33. The collector C of transistor 33 provides a control signal to the blocking system 5 when the voltage at emitter E is greater than at the base B. At the same time the capacitor 15 is charged to a voltage which is lower than the first voltage by an amount equal to the drop in voltage due to the base-emitter path of the transistor 33. When wheel deceleration decreases after release of the brakes, the first voltage drops and the capacitor 15 then begins to discharge through resistor 16 and when the difference between the first voltage and the voltage across the capacitor 15 becomes appreciably less than the voltage drop due to the base-emitter path of the transistor 33 the transistor becomes non-conductive and the collector C no longer produces a control signal for the blocking system 5. A plurality of diodes may replace the single diodes 14 and 22. The resistor 16 may also be adjustable to vary the time constant of the capacitor 15.