GB2241026A

GB2241026A - Anti-lock brake system

Info

Publication number: GB2241026A
Application number: GB9022077A
Authority: GB
Inventors: Gunther Buschmann; Frank Jourdan
Original assignee: Alfred Teves GmbH
Current assignee: Continental Teves AG and Co oHG
Priority date: 1990-02-20
Filing date: 1990-10-11
Publication date: 1991-08-21
Anticipated expiration: 2010-10-11
Also published as: FR2658462A1; FR2658462B1; JPH04215554A; GB2241026B; GB9022077D0

Abstract

In an electronically controlled anti-lock brake system, wherein upon the commencement of braking pressure control pressure fluid is pumped from a supply reservoir (13) into pressure chambers (7, 8) of tandem master cylinder (1) while inlet valves (14 to 17) of all wheels are closed at the onset of the control action, the braking pressure on the imminently locking wheels is regulated by opening and closing the inlet valves and outlet valves (14 to 17 and 22 to 25), while on the non- locking wheels the braking pressure is slowly adapted to the pressure prevailing in the tandem master cylinder (1) by way of pulsewise actuation at a predefinable pulse-pause ratio of the inlet valves (14 to 17). <IMAGE>

Description

CIRCUIT CONFIGURATION FOR AN ELECTRONICALLY CONTROLLED ANTI-LOCK BRAKE SYSTEM The present invention relates to a circuit configuration for an electronically controlled anti-lock brake system comprising inlet valves and outlet valves located in brake lines and actuatable for the purpose of anti-lock control, comprising a master cylinder and at least one pump which is adapted to be switched on when control is needed and which supplies pressure fluid from a supply reservoir into working chambers of the master cylinder.

A like brake system is known e.g. from DE 3627000A1.

Furthermore, systems are known and made use of in automotive vehicles, in which in each case the braking pressure of the imminently locking wheels is regulated by virtue of the inlet valves and outlet valves as well as electronic circuitry. The braking pressure on the wheels devoid of a tendency to lock remains unaffected.

In the majority of cases the known control methods for operating a brake system like that described above are very effective and guarantee safe braking and mastering of the automotive vehicle. However, an undesirable situation will be encountered under extreme conditions.

When gentle braking is initiated for instance on a snowy road surface the brake pedal is depressed only a small distance and if any one wheel shows a tendency to lock, the pumps will be driven to deliver pressure fluid from the supply reservoir into the brake circuits. The braking pressure is regulated on the wheel showing the tendency to lock. Since the pumps are designed to return a sufficient amount of pressure fluid into the brake circuits even if control is effected on all wheels, in the event of only one wheel being controlled they will deliver so much pressure fluid into the brake circuits that the pressure rises there.

Due to the pressure being increased in the master cylinder as well, augmented force will react on the driver's foot placed on the brake pedal. However, since the average driver applies the brake pedal not in a force-proportional manner but in a travel-proportional way, the driver will yield the higher pressure only after a few seconds and will not release the pedal until then.

In the meantime, after the pressure increase and before the pedal release, the following happens: As a result of the increased braking pressure, even the wheels which had not been jeopardized by wheel lock so far may reach the limit of wheel lock and will then be governed corresponding to the prevailing high braking pressure level. The driver who had not intended to brake so strongly may get startled. If another vehicle is behind such an unexpectedly hard braked automotive vehicle, an accident could occur.

The present invention has for its object to eliminate the above-described shortcoming. This object is achieved in a surprisingly straightforward manner in that in the event of a locking tendency on at least one wheel the circuit configuration closes at first all inlet valves, and in that, during regulation of the braking pressure in the wheel brake of the imminently locking wheel, it opens and/or closes the inlet valves of the other wheels by pulsewise actuation in a predeterminable pulse-pause ratio. By this means the build-up of braking pressure on the wheels not jeopardized by wheel lock takes place in a slightly delayed manner and thus the braking pressure on these wheels will rise only moderately during the reaction time of the driver which is needed to release the pedal as a result of the increased pressure.

It is particularly preferable that the circuit configuration closes the inlet valves on the wheels unendangered by wheel lock for approximately four times as long as it opens them. In the presence of this pulse-pause ratio, the pressure build-up in the wheel brakes of the wheels unendangered by wheel lock takes place in a delayed fashion, on the one hand, yet, on the other hand, fast enough in order to safeguard safe and quick braking of the vehicle at any time.

Preferably the circuit configuration closes the inlet valves on the wheels unendangered by wheel lock in each case for 28 milliseconds and opens them thereafter for 7 milliseconds. This opening and closing is continued cyclically.

The present invention will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 shows schematically a brake system comprising a circuit configuration according to the present invention, and Figure 2 shows a graph of the variation of the braking pressure against time on a wheel braked with and a wheel braked without anti-lock control, and the pumps' condition is shown for comparison.

Figure 1 shows a tandem master cylinder 1 with a vacuum brake power booster 2 connected upstream thereof. Via a push rod 3, the pedal force F applied to a brake pedal 4 is transmitted to the vacuum brake power booster 2 and from the booster 2, boosted by auxiliary force, to the working pistons 5 and 6 of the tandem master cylinder 1.

In the illustrated released position of the brake, pressure chambers 7, 8 of the master cylinder 1 communicate with a supply reservoir 13 via open central control valves 9, 10 and via hydraulic lines 11, 12. The two pressure chambers 7 and 8 are connected with wheel brakes 18, 19, 20 and 21 via two main brake lines 40, 41 and branch lines 36, 37, 38 and 39 in which electromagnetically actuatable inlets valves 14, 15, 16 and 17 are located which are open in their initial position. The parallel connected wheel brakes 18 and 19 and 20 and 21, respectively, are allocated to the diagonals.

The wheel brakes 18 to 21 are connected to electromagnetically actuatable outlet valves 22, 23, 24 and 25 which are closed in their initial position and which communicate via a hydraulic return line 26 with the supply reservoir 13, on the one hand, and via the suction line 27 with the suction sides of hydraulic pumps 28 and 29, on the other hand. The pumps are electromotively driven by a motor M. Electric connections "m" and earth are also illustrated symbolically. There is also provided an electrically operating function-monitoring device 30 for checking the mode of operation of the motor M.

The vehicle wheels VR, VL, HR and HL are equipped with inductive sensors S1 to S4 which generate electric signals indicative of the wheel rotational behaviour, that is the wheel speed and variations thereof. These signals are fed via inputs E1 to E4 to an electronic circuit configuration 31 which generates braking-pressure control signals serving to temporarily switch over the inlet and outlet valves 14 to 17 and 22 to 25 respectively on detection of an imminent locked condition and to thereby keep the braking pressure constant, to decrease it and to re-increase it at the appropriate time. To this end, the inlet and outlet valves 14 to 17 and 22 to 25 have actuating magnets which are driven via outputs A1 to A4. Electric connecting lines between the ports A1 to A4 and the coils of the valves 14 to 17 and 22 to 25 are not illustrated for the sake of simplicity.

The electronic circuit configuration 31 can comprise hard-wired circuits or programmed electronic units such as microcomputers or microcontrollers.

The switch-on signal for the start-up of the motor M of the pumps 28, 29 which must run during a slip control action is applied to the motor M via the connection m.

The brake system operates as follows: on application of the brakes the pedal force F boosted by the vacuum in the brake power booster 2 is transmitted to the working pistons 5 and 6. The central control valves 9 and 10 close, thus allowing braking pressure to develop in the pressure chambers 7 and 8 and hence in the main brake lines 40 and 41 which, via the opened inlet valves 14 to 17 is transmitted to the wheel brakes 18 to 21.

On detection of an imminent locked condition at one or more of the wheels by means of the sensors S1 to S4 and the electronic circuit configuration 31, slip control will commence. The motor M of the pumps 28, 29 will be switched on, whereby pressure develops in the two supply lines 32 and 33 which is transmitted via non-return valves 34, 35 and branch lines 36, 37, 38 and 39 to the now closed inlet valves 14 to 17, on the one hand, and which acts upon the pressure chambers 7 and 8 of the tandem master cylinder 1, on the other hand.Further displacement of the working pistons 5, 6 in the direction of the pedal force F as well as evacuation of the pressure chambers 7 and 8 is prevented, since pressure fluid from the pumps 28, 29 now flows via the supply lines 32, 33, the opened non-return valves 34, 35 and the main brake lines 40, 41 into the pressure chambers 7 and 8 and tends to urge the pistons 5 and 6 back into their initial position.

On wheels not tending to lock, the actual braking pressure variation in the wheel brakes 18 to 21 is adapted to the braking pressure level now prevailing in the master cylinder 1 by way of regularly opening and closing the inlet valves 14 to 17, while on the imminently locking wheels it is controlled by opening the inlet valves and outlet valves 14 to 17 and 22 to 25 corresponding to the control signals transmitted by the electronic circuit configuration 31.

As shown in Figure 1, the inlet valves 14 to 17 are guarded still by non-return valves 42, 43, 44 and 45 connected in parallel. These non-return valves 42 to 45 enable in special cases, in particular on release of the pedal 4 after reduction of pressure in the brake circuits to terminate the braking pressure control and/or to release the wheel brakes, since a small quantity of pressure fluid can flow from the wheel brakes 18 to 21 back into the pressure chambers 7 and 8 even if the inlet valves and outlet valves 14 to 17 and 22 to 25 are closed, in the event that the working pistons 5 and 6 of the tandem master cylinder are moved back by reduction of the pedal force F and/or when the central control valves 8 and 10 are in their opened position.

It can be seen from Figure 2a that at the onset of braking the pumps are disconnected and are switched on when the control commences at time t2. Figure 2b shows the braking pressure variation on a wheel braked with anti-lock control. Starting from time t1 indicating the start of braking, the braking pressure rises approximately exponentially. As soon as a locking tendency is sensed at time t2, the braking pressure will be kept constant and thereafter decreased and re-increased stepwise. As soon as the locking tendency re-occurs, braking pressure will be decreased further and re-increased. Figure 2c shows the braking pressure variation on a wheel braked without anti-lock control. Herein, too, the pressure rises in parallel to the braking pressure variation shown in Figure 2b roughly exponentially from time t1 to time t2. With the commencement of the control, that is at time t2, it will in each case be kept constant for a short time and increased by a small amount until a maximum value is reached. As soon as the driver releases the pedal, the pressure will be somewhat decreased, this is time t3 in the Figure.

Claims

CLAIMS:

1. A circuit configuration for an electronically controlled anti-lock brake system comprising inlet valves and outlet valves located in brake lines and actuatable for the purpose of anti-lock control, comprising a master cylinder and at least one pump which is adapted to be switched on when control is needed and which supplies pressure fluid from a supply reservoir into the working chambers of the master cylinder, characterised in that the circuit configuration closes at first all inlet valves (14, 15, 16 and 17) in the event of an imminent locked condition on at least one wheel (e.g. VR), and in that during regulation of the braking pressure in the wheel brake (19) of the imminently locking wheel (VR) it opens and/or closes the inlet valves (14, 16 and 17) of the other wheels (VL, HR and HL) by pulsewise actuation at a predeterminable pulse-pause ratio.

2. A circuit configuration as claimed in claim 1, characterised in that the circuit configuration closes the inlet valves (14, 16 and 17) on the wheels (VL, HR and HL) unendangered by wheel lock for a period approximately four times as long as it opens the inlet valves (14,16 and 17).

3. A circuit configuration as claimed in claim 2, characterised in that the circuit configuration closes the inlet valves (14, 16 and 17) on the wheels (VL, HR and HL) unendangered by wheel lock for 28 milliseconds and thereafter opens the inlet valves (14,16 and 17) for 7 milliseconds during one cycle.

4. A circuit configuration for an electronically controlled anti-lock brake system substantially as hereinbefore described with reference to the accompanying drawings.