Patent GB-1439185-A (GB)
Title: Anti-locking control systems for pressure-medium operated vehicle brakes
IPC: B60T8/173
Filed: 1973-09-06
Abstract: 1439185 Anti-skid control ROBERT BOSCH GmbH 6 Sept 1973 [7 Sept 1972] 41959/73 Heading F2F A vehicle wheel brake, having an inlet valve and an outlet valve, is controlled by acceleration threshold switches controlled by signals from a wheel speed transducer 30 via a low pass filter 32 and a differentiating circuit 33; a first switch 35 being connected to a second low pass filter 34 connected to the differentiating circuit, and a second switch 36 being connected to the differentiating circuit, the outlet valve being controlled by the switches through an AND gate 42. In Fig. 2, wheel speed frequency signals from the transducer 30 pass through a frequency/ D.C. converter 31 to the low pass filter 32, damping spurious signals, then to the differentiating circuit 33. Outputs from the second low pass filter 34 pass to the first switch 35 and a third threshold switch 37, all these switches being triggered at different predetermined acceleration values to produce signals which cease upon attainment of further acceleration values, the switches being connected to a solenoid 25 of the outlet valve via the AND gate 42 and an OR gate 44, the valve when actuated allowing fluid to drain from a brake cylinder to a reservoir, and to a solenoid 28 of the inlet valve via an OR gate 45, the valve when actuated shutting off communication between the cylinder and a master cylinder for constant pressure. The OR gate 45 prevents both valves from being open simultaneously to drain the master cylinder. This circuit produces, upon high wheel deceleration (impending skid) a pressure decrease (both valve actuated) then constant pressure phases to allow wheel acceleration back to the vehicle speed (only the inlet valve actuated) and then a pressure increase for full braking for a possible second pressure cycle. A differentiating circuit at the output of the third threshold switch 37 causes a threshold shift (different acceleration value) at the first switch 35, producing a similar pressure cycle as before. A vehicle speed simulating circuit 40 (in detail in Fig. 4, not shown) accepts D.C. signals via diodes from low pass filters 32 of other wheel circuits, the fastest speed value being accepted to provide the simulated vehicle speed, the circuit comprising a controlled discharge circuit feeding an amplifier of a comparator 41 to provide an output, if wheel speed falls below the simulated vehicle speed by at least a predetermined amount, to open the outlet valve and close the inlet valve except when the third switch 37 is actuated (during wheel reacceleration). Fig. 3 shows a circuit providing a more sensitive pressure control during the cycles, by automatic shifting of the threshold acceleration values for the switches 35, 36, 37 and the position of a fourth switch 38; the circuit input is from the low pass filter 32. The threshold shift for the switch 36 occurs for a specific duration upon a signal issued by the switch 35 via a monostable timing element 48 for accommodating differing road conditions; similarly a shift occurs for the switch 35 (V2) from the third switch 37 (at the end of the signal) via a monostable element 49 for damping out vehicle frame vibrations. Other threshold shifting inputs at the switches 35, 37 are from the switch 35, at signal commencement, via a bi-stable memory circuit 46 which also controls an astable mutlivibrator 47 providing pulsed signals to the inlet valve via the OR gate 45 for intermittent valve actuation during subsequent pressure cycles. The reset input for the circuit 46 (Fig. 7, not shown) is connected via diodes to threshold switches 37 of circuits for different wheels and via resistors to switches 35 of these circuits, via a resetting circuit and via an inverter; a terminal for the vehicle stop lights is coupled to the diodes. Circuits 46 for other wheels or axles are also connected to this resetting input, e.g. for independent front wheel circuits and a common rear axle circuit of a two-axled vehicle. For more sensitive pressure control, the first switch may have coupled thereto two timing elements 53, 55, Fig. 5, the former having a PNP transistor and connected to the latter by a differentiating circuit, the latter being a monostable circuit, so that it is triggered after a delay by the output of the switch 35 and provides a pulse for a predetermined duration which shifts the threshold of the switch.