GB2126673A

GB2126673A - Anti-lock braking system

Info

Publication number: GB2126673A
Application number: GB08225737A
Authority: GB
Inventors: Glyn Philip Reginald Farr
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1982-09-09
Filing date: 1982-09-09
Publication date: 1984-03-28

Abstract

An anti-lock braking system (1) comprises a master cylinder (2), a booster 8 supplying fluid from outlet (12) at the same pressure as master cylinder outlet 6, a plurality of modulator assemblies (4) and a detector (5) for detecting an impending wheel lock condition. Normally first inlet (13) of the modulator (4) communicates with the first outlet (16) to a brake through valve (19) and bore (23) with reservoir (11) through port (14), valve (15) and vent (26). When an impending wheel lock is detected an electrical signal is sent via lines (30, 31) to open a normally closed solenoid valve (33) and to shift a valve head (34) of valve (15) to a position to isolate inlet (14) from vent outlet (26) and connect it to booster outlet (12) to move piston 22 to the left. This allows valve (19) to close and permits fluid to flow from brake actuators to reservoir via ports (16) and (17), thereby releasing the brakes. On cessation of the wheel lock signal the valve (33) closes, raising the pressure in the bore (23) and supplying the brake actuators with fluid from booster outlet (12) at the service braking pressure through inlet 14 and passages 37 and 20 to outlet port 16. <IMAGE>

Description

SPECIFICATION Anti-lock braking system This invention relates to an anti-lock braking system, that is to say a braking system which includes at least one detector for detecting when a wheel is decelerating in a manner which indicates that it is about to lock (this condition being referred to herein as "an impending wheel lock condition"), and a modulator assembly which is operative when an impending wheel lock condition is detected to control the hydraulic brake pressure supplied to the wheel which is tending to lock in order to prevent locking thereof.

An anti-lock braking system is described in our British patent application 81 30883 in which fluid from a hydraulic accumulator is used to power a modulator assembly which includes a de-boost piston. The system described is particularly suitable for applications where the fluid in the accumulator is incompatible with the fluid of the brake circuit, e.g. where the accumulator is charged with a mineral based fluid and the brake circuit operates with a vegetable or polyglycol based fluid.

The present invention relates to an anti-lock system in which a source of pressurised working fluid of the same type as that used in the brake circuit is available. The preferred embodiments of the present invention provide anti-lock braking systems which are considerably cheaper and easier to manufacture than the systems described in the above mentioned patent application, and which offer significant other advantages.

According to the present invention there is provided an anti-lock braking system comprising: a driver-controlled hydraulic fluid supply device having at least one service outlet for delivering brake applying fluid; a source of said brake applying fluid which is at an instantaneous pressure substantially equal to the pressure prevailing at said service outlet; a detector for detecting an impending wheel lock condition; and a modulator assembly having a first inlet connected to said service outlet, second inlet connectable with said source, a first outlet connected to a brake actuator, a second outlet connected to a reservoir, a normally open valve between the first inlet and first outlet, and control valve means responsive to said detector detecting an impending wheel lock condition to close the normally open valve and to establish communication between the first and second outlets and responsive to subsequent recovery of the wheel to isolate the first outlet from the second outlet and to connect the second inlet to the first outlet to increase the pressure at the first outlet.

Preferably, the driver controlled hydraulic fluid supply device is a tandem hydraulic master cylinder in which the pressure generating seals are each permanently exposed on one side thereof to atmospheric pressure.

Preferably, the hydraulic master cylinder is hydraulically boosted with a hydraulic boost pressure which is at any instant substantially equal to the output pressure of the master cylinder. In this case, the said source of brake applying fluid is conveniently the booster chamber of the master cylinder.

Preferably, each wheel is provided with a detector, and each front wheel has a respective modulator assembly. The rear wheels may be controlled by a single modulator assembly.

Preferably, a valve is interposed between the said source of brake applying fluid and the second inlet of the or each modulator assembly, said valve normally connecting the second inlet to reservoir and being operative upon detection of an impending wheel-lock condition by any detector to connect the second inlet of the or each modulator assembly to the said source of brake applying fluid, and to maintain this connection until no detector detects an impending wheel-lock condition.

The above and other features of the invention, and advantages thereof, will become clear from the following description of three embodiments thereof, given by way of example only, reference being had to the accompanying drawings wherein: Figures 1, 2 and 3 are respectively schematic views of three embodiments of the invention, like reference numerals having been used, where appropriate, throughout the Figures.

Referring firstly to Figure 1 there is shown an anti-lock braking system 1 comprising a master cylinder 2 having an input rod 3, a plurality of modulator assemblies 4 (only one of which is shown) and a detector 5 for detecting an impending wheel lock condition.

The master cylinder 2 has two pressure generating chambers supplying respective service outlets 6 and 7. As is conventional in tandem master cylinders the pressure generating seals associated with the two pressure generating chambers are each exposed on one side thereof to atmospheric pressure so that each time the brakes are applied a pressure differential is generated across the seals. This ensures that any failure of the pressure generating seals is immediately detected. The master cylinder assembly includes a hydraulically powered booster mechanism 8 which is supplied with pressurised hydraulic fluid from an accumulator 9.The accumulator 9 is charged by a pump 10 which draws hydraulic fluid from a reservoir 1 The reservoir 11 also supplies fluid to the pressure generating chambers of the master cylinder and accordingly the fluid delivered from the accumulator is identical to that delivered from the service outlets 6, 7 of the master cylinder. The booster 8 is designed so that, at any particular moment, the pressure existing within the booster chamber is substantially equal to the pressure subsisting at the service outlets 6, 7. Accordingly, an outlet 12 from the booster chamber constitutes a source of pressurised fluid which is of the same type of fluid as that delivered from the service outlets 6, 7 and which is instantaneously at the same pressure as that subsisting at the service outlets.

The service outlet 6 is connected to a first inlet 13 of the modulator assembly 4, and the outlet 12 is connected to a second inlet 14 of the modulator 4 via a valve 1 5, the function of which will be described in more detail hereinafter. The modulator 4 includes a first outlet 1 6 connected to one or more brake actuators, e.g. disc brake assemblies, and a second outlet 1 7 which is connected to the reservoir 11.

The first inlet 13 and first outlet 1 6 communicate with each other via a passage 18, normally open valve 19, and passage 20. The valve 19 is held open by an extension 21 of a piston 22 which is slidably mounted in a bore 23 of the modulator assembly body. The piston 22 is biased into the position illustrated, holding the valve 19, open by a spring 24 which, in the absence of hydraulic pressure in the bore 23, is sufficiently strong to maintain the valve 19 open under the influence of all working pressures which are liable to be encountered in the brake circuit. The bore 23 is normally connected to the reservoir 11 via second inlet 14, connecting pipe 25, and valve 1 5 which includes a vent outlet 26 connected to the reservoir 11.Thus, under normal operating conditions the modulator assembly components adopt the position shown in Figure 1 and the master cylinder 2 operates the brakes in conventional manner.

Preferably, all wheels of the vehicle in question are provided with modulators 4, the service outlets 6, 7, being used to provide dual-circuit braking in a conventional manner.

The detector 5 comprises a sensor 27 associated with each wheel, each sensor being adapted to provide an output on a respective line 28 indicative of speed of rotation of its associated wheel. Only one wheel sensor is shown in the interests of clarity. The lines 28 are connected to a central controller 29 which monitors the signals from the sensors 27 and provides an output on a line 30 when an impending wheel lock condition is detected. The number of lines 30 will equal the number of modulator assemblies 4, a signal being provided on a particular line 30 when the wheel or wheels associated with the modulator assembly to which that line 30 is connected is detected as being in an impending wheel lock condition. In addition, a signal is provided on a line 31 whenever any sensor 27 detects an impending wheel lock condition.

Thus, when the sensor 27 associated with the wheel supplied by the illustrated modulator assembly 4 detects an impending wheel lock condition an electrical signal appears on the illustrated lines 30 and 31. The signal on the line 30 energises a normally closed solenoid valve 32 to move the valve head 33 of the solenoid valve out of engagement with its seat and to establish communication between the left-hand end of the bore 23 and reservoir via the second outlet 17.

Simultaneously, the valve head 34 of the valve 15 is shifted to its opposite extreme position to isolate the inlet 14 from the outlet 26, and to connect the inlet 14 to the outlet 12. This causes hydraulic fluid at a pressure substantially equal to that subsisting at the service outlet 6 to enter the inlet 14 and flow via a throttle 35 into the lefthand end of bore 23 and then on to reservoir. The throttle 35 produces a pressure in the right-hand end of bore 23 larger than the pressure in the lefthand end, and this pressure differential together with the service brake pressure operating over the piston extension 21 is sufficient to overcome the effects of spring 24 and move the piston 22 to the left.This allows the valve 1 9 to close, thereby isolating the service outlet 6 from the brake actuator and further movement of the piston causes the extension 21 to move pasta seal 36 thereby connecting the outlet 16 to the outlet 1 7 via passage 37. This allows pressurised hydraulic fluid from the brake actuator to return to reservoir, and thereby releases the wheel in question to allow recovery thereof. When recovery is detected the signal on line 30 ceases and the solenoid valve 32 closes. Fluid continues to flow through throttle 35 causing the pressure in the left-hand end of bore 23 to increase, and this increased pressure is supplied to the brake outlet 1 6 via passages 37 and 20. This causes the brakes to be re-applied.If an impending wheel lock condition is again detected the solenoid 32 is again opened and the pressure within the brake actuator is relieved. The cycle is again repeated and pressure within the brake actuator is alternately increased and relieved until an impending wheel lock condition is no longer detected.

It will be noted that during the entire cycle described above the driver will feel no change in pedal characteristics since, as soon as an impending wheel lock condition is detected, the valve 1 9 is closed and no further fluid flows from the service outlet 6. The control valve within the booster 8 operates to maintain pressure in the booster chamber substantially equal to the pressure at the service outlet 6 despite the fact that fluid is removed from the booster chamber via the outlet 12.

It will be noted that, during a recovery part of the cycle, when the solenoid valve 32 is closed the pressure rises in the left-hand end of the bore 23. When the piston 22 is substantially pressure balanced it will start to move to the right under the influence of spring 24. This will occur when the pressure in the left-hand end of the bore 23 is substantially equal to the pressure at the inlet 1 4, i.e. substantially equal to the pressure in the boost chamber of the master cylinder. This pressure in turn is substantially equal to the pressure at the service outlet 6. Thus, when the piston 22 starts to move to the right the pressure at the outlet 1 6 (which at this time is connected to the passage 37) will be substantially equal to the pressure at the inlet 13, and when the valve 19 is opened there will be no dramatic pressure change within the brake actuator and no discharge of brake fluid from the service outlet 6.

It will also be noted that the piston 22 acts as a constant flow regulator. This is because, as the piston 22 moves to the left under the influence of a pressure differential, a control edge 43 on the piston forms a variable area throttle with the passage 38 which extends from the inlet 14 to the bore 23. As the piston 22 moves progressively to the left the area of the variable throttle decreases, thereby limiting the pressure available in the right-hand end of bore 23. The piston will find an equilibrium position in which flow through the variable throttle is just sufficient to maintain a pressure differential across the piston 22 to hold it in an equilibrium position against the effect of spring 24.Any increase in pressure at inlet 14 will cause an increase in pressure in the right-hand end of bore 23 which will move the piston further to the left, thereby reducing the variable throttle area. The variable throttle accordingly ensures that a substantially constant pressure differential exists across the piston 22 and this constant pressure differential, in combination with the fixed area of the throttle 35 ensures a constant flow rate through the throttle 35 regardless of pressure at the inlet 14.

This is an important advantage since it ensures that the rate at which brakes are re-applied after an impending wheel lock condition is sense is constant and independent of the pressure at the inlet 14.

It will be noted that in the event of a complete failure of booster power the modulator assembly will be completely inoperative since the spring 24 will maintain the valve 19 open. Thus, in the event of hydraulic power failure the master cylinder can operate in its usual emergency mode. The valve 1 5 provides an additional safety feature in that in its normal non-actuated position the valve head 34 prevents flow of fluid from the outlet 12. This, in the event that the solenoid valve 32 fails there will be no movement of the piston 22 until an impending wheel lock condition is detected. Accordingly, if failure of the solenoid valve 32 is detected the vehicle may be driven with all brakes operative to a garage for repair.

Even if an impending wheel lock condition is detected under these conditions, the only effect will be to render the brake associated with the failed modulator inoperative until the valve 1 5 next closes.

It will be noted that the modulator 4 includes only a single dynamic seal 36 associated with the piston 22. This considerably reduces the cost and increases the inherent reliability of the modulator 4 as compared with modulators making use of multiple dynamic seals.

The master cylinder 2 may advantageously be of the type described.

Referring now to Figure 2, the anti-lock braking system shown includes a modulator assembly 4 and a detector substantially as described above with reference to Figure 1. However, the hydraulically powered tandem master cylinder 2 of Figure 1 has been replaced by a tandem master cylinder 2A powered by a conventional pneumatic servo booster. Under these circumstances, a source of hydraulic fluid at a pressure instantaneously equal to the pressures subsisting at the master cylinder outlet 6A is provided by a pump 39 and a balance valve 40. The effect of the balance valve 40 is to ensure that the pressure supplied to the inlet 14 is always equal to the instantaneous pressure at the inlet 1 3.

Referring now to Figure 3 the servo powered master cylinder 2A, the modulator assembly 4 and the detector 5 are as described above with reference to Figure 2. However, the pump 39 has been replaced by a pump 41 and accumulator 42.

A modified balance valve 40A is again provided for ensuring that the pressure supplied to the inlet 14 is equal to the instantaneous pressure subsisting at the inlet 1 3.

It will be noted that the balance valve 40 and 40A both operate to connect the inlet 14 to reservoir when no pressure subsists at the master cylinder outlet 6A.

Claims

1. An anti-lock braking system comprising: a driver-controlled hydraulic fluid supply device having at least one service outlet for delivering brake applying fluid; a source of said brake applying fluid which is at an instantaneous pressure substantially equal to the pressure prevailing at said service outlet; a detector for detecting an impending wheel lock condition; and a modulator assembly having a first inlet connnected to said service outlet, a second inlet connectable with said source, a first outlet connected to a brake actuator, a second outlet connected to a reservoir, a normally open valve between the first inlet and first outlet, and control valve means responsive to said detector detecting an impending wheel lock condition to close the normally open valve and to establish communication between the first and second outlets and responsive to subsequent recovery of the wheel to isolate the first outlet from the second outlet and to connect the second inlet to the first outlet to increase the pressure at the first outlet.

2. An anti-70ck braking system according to claim 1 wherein the driver controlled hydraulic fluid supply device is a tandem hydraulic master cylinder in which the pressure generating seals are each permanently exposed on one side thereof to atmospheric pressure.

3. An anti-lock braking system according to claim 2 where the master cylinder is hydraulically boosted by a hydraulic boost pressure which is substantially equal to the instantaneous pressure subsisting at the service outlet, and wherein said source of brake applying fluid is the boost chamber of the master cylinder.

4. An anti-lock braking system according to any preceding claim wherein a normally closed valve is positioned between the source of brake applying fluid and the second inlet, and wherein the second inlet is normally connected to reservoir.

5. An anti-lock braking system according to any preceding claim wherein the control valve means comprises a constant flow valve controlled by a single solenoid valve.