WO1989008572A1

WO1989008572A1 - Vehicle antilock braking systems

Info

Publication number: WO1989008572A1
Application number: PCT/GB1989/000168
Authority: WO
Inventors: David Parsons
Original assignee: Automotive Products Plc
Priority date: 1988-03-19
Filing date: 1989-02-23
Publication date: 1989-09-21
Also published as: GB8806589D0; KR900700325A

Abstract

A vehicle braking system includes: a master cylinder (20) which is connected to a brake circuit including two brake actuators (11, 17) associated with different wheels of the vehicle; control means (23, 31) which is associated with one of said wheels and senses changes in velocity of that wheel, said control means (23, 31) acting to reduce pressure of fluid in the brake circuit when the wheel deceleration increases above a first predetermined value; and a proportioning valve (34) which at a predetermined pressure in the brake circuit will increase the ratio of the rates at which pressure is increased to the actuators (11, 17) associated with said one and the other wheel respectively, actuator means (60) being provided to control the proportioning valve (34) to reduce the pressure applied to the actuator (17) associated with said other wheel, when said other wheel locks or is liable to lock.

Description

VEHICLE ANTILOCK BRAKING SYSTEMS if

The invention relates to vehicle antiloc braking systems.

Hitherto, dual circuit braking systems have been proposed in which each circuit controls one front wheel and the diagonally opposite rear wheel. With such systems, wheel speed sensors may be provided on each of the front wheels to control brake pressure modulation means to reduce the pressure of fluid in the circuit serving that front wheel and the associated rear wheel, in response to deceleration of the front wheel. Locking of the associated rear wheel of such systems is avoided by applying a greater percentage of the braking effort through the front wheels. This may be achieved by means of a proportioning valve which is reactive to pressure in the brake circuit to reduce the rate at which pressure is applied to the rear brakes, once the pressure in the circuit rises above a predetermined value. The predetermined value at which the proportioning valve reduces the rate at which pressure is applied to the rear brakes may be adjustable, for example in response to axle load.

In order to prevent excessive underbraking of the rear wheels in such systems, the predetermined pressure and rate of increase of pressure to the rear brakes may be set such that it will just prevent the rear wheels locking, when the front and rear brakes are operating with their friction linings at the design frictional coefficients. However, should the frictional coefficients of the friction linings vary significantly due, for example, to fade of the front friction linings, the rear wheels will be liable to lock. The risk may be reduced by reducing the pressure at which the proportioning valve begins to reduce the rate of increase in pressure of the rear wheels as well as reducing the rate of increase, but at the cost of excessive underbraking when the front and rear brakes are operating at their design friction lining frictional coe ficients.

According to one aspect of the present invention, a vehicle antilock braking system comprises; a master cylinder which will produce a source of high pressure fluid upon brake actuation, said master cylinder being connected to a brake circuit including two brake actuators, each actuator being associated with a different wheel of the vehicle, control means being associated with one of the wheels for sensing a change in the velocity of that wheel and reducing the pressure of fluid in the brake circuit when wheel deceleration increases above a first predetermined value and a proportioning valve which at a predetermined pressure in the brake circuit will increase the ratio of the rates at which pressure is increased to the actuators associated with said one and the other wheel respectively, characterised in that actuator means is provided to control the proportioning valve to reduce the pressure applied to the actuator associated with said other wheel, when said other wheel locks or is liable to lock.

Various embodiments of the invention are now described, by way of example only, with reference to the accompanying drawings, in which:-

Figure 1 is a schematic diagram of an -antilock braking system formed in accordance with the invention;

Figure 2 is a diagrammatic illustration of a propo tioning valve that may be used in the system illustrated in figure 1; and

Figure 3 is a diagrammatic illustration of a modification to the system illustrated in figure 1.

In the system illustrated in figure 1, the front wheels of the vehicle are provided with disk brakes 10 and 12 controlled by actuating calipers 11 and 13 respectively and rear wheels are provided with drum brakes 14 and 16 being controlled by hydraulic cylinders 15 and 17 respectively. The braking system is a dual system controlled by servo 572 . -

-4 -

assisted dual master cylinder 20 of conventional design. The master cylinder 20 has two outlets 22 and 22* which provide a source of hydraulic pressure for the two circuits of the dual system, one circuit serving the caliper 11 of the off side front wheel and the cylinder 17 of the near side rear wheel and the other circuit serving the caliper 13 of the near side front wheel and the cylinder 15 of the off side rear wheel.

The two circuits of the system are identical in arrangement and operation, and only one circuit is described in detail below. Similar components have been identified with similar reference numerals.

Outlet 22 of master cylinder 20 is connected to the hydraulic fluid inlet port 36 of an antilock servo pressure modulator 23 of conventional design and outlet port 37 from servo pressure modulator 23 is connected via line 25 to the off side front caliper 11 and near side rear cylinder 17.

The servo pressure modulator 23 comprises a plunger controlled valve which is positioned between the inlet port 36 and outlet port 37, the plunger being slidably sealed within a cylinder on the outlet side of the valve. Movement of the plunger is controlled by a vacuum actuator in which a piston divides a chamber into two fluid tight compartments, one compartment being connected to vacuum reservoir 39 and the other being selectively connected to vacuum reservoir 39 or atmosphere by means of a solenoid valve 28. The pressure differential across the piston may thus be controlled by means of the solenoid valve 28 to control movement of the piston and hence the plunger. Movement of the plunger away from the plunger control valve will first interrupt communication between the master cylinder 20 and, caliper 11 and cylinder 17. Further movement of the plunger in the cylinder will then permit fluid to flow back into the cylinder from the caliper 11 and cylinder 17, thus reducing the braking effect.

A toothed wheel 18 is mounted for rotation with the off side front wheel and an electromagnetic pickup 19 is associated with the toothed wheel 18 to provide a signal which alternates at a frequency proportional to the rotational speed of the off side front wheel. The pickup 19 is connected to an electronic control module 31 which processes the signal therefrom, to provide a measure of the deceleration or acceleration of the off side front wheel. An output is provided from the control module 31 to solenoid valve 28, by means of which the solenoid valve 28 may be energised to connect the chamber of the pressure modulator 23 to atmosphere and thus reduce braking effort applied to caliper 11 and cylinder 17, when deceleration of the off side front wheel rises above a predetermined value, locking of the wheel being liable to occur above this value. The solenoid valve 28 remains energised until acceleration of the off side front wheel rises above a predetermined value, when it will be de-energised and the servo pressure modulator 23 will re-apply normal braking.

A proportioning valve 34, illustrated in greater detail in figure 2, is provided in the line 25 between the servo pressure modulator 23 and the brake cylinder 17.

The proportioning valve 34 illustrated in figure 2 has a stepped cylindrical bore 40 which is closed at one end, the closed end portion 41 being of relatively large diameter. A stepped piston 43 is slidingly located in cylindrical bore 40, an enlarged diameter end portion 44 being sealed with respect to end portion 41 of bore 40 and dividing that portion 41 into two chambers 46 and 47, while a reduced diameter portion 45 of piston 43 is sealed with respect to a reduced diameter portion 42 of the bore 40. Fluid connections 48 and 49 are provided to chambers 46 and 47 respectively, by which chamber 46 is connected to the antilock servo pressure modulator 23 via line 25 and chamber 47 is connected to brake cylinder 17.

Chambers 46 and 47 are interconnected, via a ball valve 50, by bore 51 which extends through the enlarged diameter portion 44 of piston 43. The ball valve "50 comprises a ball 52 which is urged towards chamber 47 and a seat 54 by means of spring 55. A rod 56 extending from the closed end of cylinder bore 40 and into bore 51, engages the ball 52 and holds the valve 50 open when the piston 43 is hard over to the right as illustrated in figure 2.

A vacuum actuator 60 is disposed coaxially of the cylindrical bore 40. The vacuum actuator 60 comprises a cylindrical casing 61, a piston 62 being mounted within the casing 61 and being sealed with respect to the cylindrical wall thereof by means of a diaphragm unit 63, thereby dividing the casing 61 into two fluid tight working chambers 64 and 65. The reduced diameter portion 42 of bore 40 opens into chamber 65 of the vacuum actuator 60, so that the reduced portion 45 of piston 43 will extend into chamber 65. A plunger 66 is formed on piston 62 and extends into engagement with piston 43 . The end of the plunger 66 engaging piston 43 has a flange formation 67 and a spring 68 acts between this flange formation 67 and a shoulder portion 69 of the casing 61 to apply an axial load forcing the piston 43 towards the closed end of bore 40. Chambers 64 and 65 are provided with fluid connections 70 and 71 respectively. As illustrated in figure 1, connections 70 and 70' of the proportioning valves 34 and 34' of the two circuits of the brake system are interconnected and are connected to vacuum reservoir 39, while connections 71 and 71* are selectively connected to the vacuum reservoir 39 or to atmosphere via solenoid valve 75. The connection of solenoid valve 75 to atmosphere is via restriction 76, and to vacuum reservoir 39 via restriction 77, the rate of flow of air to or from chambers 65 being controlled by the restrictions 76 and 77 respectively.

Toothed wheels 18 and 18' are mounted for rotation with each of the rear wheels and electro-magnetic pickups 19 and 19* associated with the toothed wheels 18 and 18', provide signals which alternate at a frequency proportional to the rotational speed of the associated wheel, in similar manner to the similar components associated with each of the front wheels. The pickups 19 and 19* associated with the rear wheels are connected to a control unit 80 which processes the signals from the pickups 19 and 19' to provide a measure of decleration or acceleration of each of the rear wheels and provides an output to energise solenoid valve 75 when deceleration of either rear wheel reaches a second predetermined value above which locking of the wheel is liable to occur. The second predetermined value may be the same as or different than the predetermined value of deceleration which triggers control modules 31 and 31'. Alternatively the control unit 80 may compare the velocity of each rear wheel with that of the associated front wheel and provide an output to energise solenoid valve 75 when the rear wheel is locked or the difference in velocities rises above a predetermined value.

In normal operation, solenoid valve 75 is de-energised and connects chambers 65 of the proportioning valves 34 and 34' to vacuum so that there is no pressure differential across the pistons 62. Considering one brake circuit when a braking operation commences, fluid is delivered from the master cylinder 22 via servo pressure modulator 23 and line 25 to chamber 46 via inlet 48. This fluid passes through the bore 51 past ball valve 50, which is held open by rod 56, into chamber 47 and from there via outlet 49 to the brake cylinder 17. Initially the pressure in brake cylinder 17 will therefore increase at the same rate as the pressure in caliper 11.

As the pressure in chamber 47 increases, the area differential between the face 85 and the annular face 86 at the shoulder between portions 44 and 45 of piston 43, will cause a force to be exerted on the piston 43 moving it to the left against the load applied to the piston 43 by spring 68. At a predetermined pressure of fluid in chamber 47, which depends upon the spring rate of spring 68, valve 50 will close preventing the passage of further fluid from chamber 46 to chamber 47.

Further increase in pressure in line 25 while ball valve 50 is closed, will cause the piston 43 to move to the right and increase pressure in chamber 47 and the brake cylinder 17, but only at a reduced rate proportional to the ratio of the areas of faces 86 and 85.

Upon release of the brakes or when an antilock operation controlled by a servo pressure modulator 23 is triggered by the sensor 19 associated with the near side front wheel, the excess pressure in chamber 47 will force open the ball valve 50, permitting fluid to flow out of chamber 47 releasing the brake cylinder 17 and allowing the piston 43 to move back to the right under the influence of spring 68.

If the sensor 19 associated with either rear wheel senses that the wheel has locked or is liable to lock, solenoid valve 75 is energised and connects the chambers 65 of proportioning valves 34 and 34' to atmosphere. This generates a pressure differential across piston 62, the resulting force acting against spring 68 to reduce the load applied thereby to piston 43. Reduction of the load applied to piston 43 will reduce the pressure at which the ball valve 50 will close and will permit further movement of piston 43 to the left, until the pressure acting upon face 85 again balances that acting upon face 86 and the reduced load applied by spring 68. The fluid pressure applied to brake cylinders 15 and 17 and the braking effort applied to the rear wheels is thereby reduced. The restriction 76 which controls the rate at which air is allowed into the chambers 65, will produce a gradual decrease in the braking effort applied to the rear wheels which will continue until both rear wheels have accelerated

_r » sufficiently to overcome the liability of either wheel to lock. The solenoid valve 75 will then be de-energised to reconnect chambers 65 to vacuum, so that the full load of springs 68 are gradually reapplied to pistons 43 and the pistons 43 move back to the right reapplying pressure to the brake cylinders 15 and 17.

In the modified embodiment illustrated in Figure 3, chamber 64 of each of the vacuum actuators 60 are open to atmosphere while chamber 65 is selectively connected to the vacuum reservoir 39 via a first solenoid valve 90 or to atmosphere via a second solenoid valve 91. The spring 68 acts directly on a flange portion 92 formed on the reduced diameter portion 45 of piston 43.

A suspension travel sensor 93 applies a signal to control odule 80 indicative of the load applied to the vehicle and a pressure sensor 94 applies a signal to the control module 80 indicative of the pressure in chambers 65 of vacuum actuators 60.

With this embodiment, under normal braking the pressure at which ball valve 50 closes is set by the axial load applied to piston 43 by spring 68 plus the load applied by piston 62 through plunger 66.

The load applied by piston 62 depends upon the pressure differential across the piston 62 which may be adjusted by connecting chamber 65 to vacuum via solenoid valve 90 or atmosphere via solenoid valve 91. This pressure differential is controlled in accordance with the signal from the suspension travel sensor 93, so that the load applied by piston 62 to piston 43 and thus the pressure at which ball valve 50 closes increases with increase in the load applied to the vehicle. The pressure sensor 94 provides a feed back to the control module 80 so that the solenoid valves 90 and 91 may be de-energised and closed when the appropriate load is applied by piston 62.

Under normal braking, pressure fluid will be delivered to the rear brake cylinders 15 and 17 as disclosed with reference to figures 1 and 2, the pressure at which the rate of increase to the rear wheels is reduced will however depend upon the load applied to the vehicle. _;

If the sensor 19 or 19' associated with either of the rear wheels indicates that the wheel has locked or is liable to lock the control module 80 energises solenoid 90 to connect the chambers 65 of both proportioning valves 34 and 34', to atmosphere. The pressure differential across piston 62 is thereby reduced, this reduces the pressure at which ball valve 50 closes and reduces the pressure of fluid in the brake cylinders 15 and 17 and' the braking 'effort applied to the rear wheels, in similar manner to the embodiment illustrated in figures 1 and 2.

Various modifications may be made without departing from the invention. For example, while a dual braking system with diagonal split has been described above, the invention will cover single or multi-circuit systems, where the or each proportioning valve may control pressure to one or more brake actuators. While in the embodiments described with reference to the accompanying drawings, the proportioning valves are controlled together to reduce the braking effect on both rear wheels should either of the rear wheels lock or be liable to lock, each proportioning valve may be provided with individual solenoid valves, so that they may be controlled independently to reduce the braking effort applied only to the wheel that has locked or is liable to lock. While vacuum actuation means has been used in the embodiments described above other forms of actuator, for example hydraulic piston actuators or electric solenoids, may be used.

Claims

1. A vehicle braking system comprising a master cylinder (20) which will produce a source of high pressure fluid upon brake actuation, said master cylinder (20) being connected to a brake circuit including two brake actuators (11, 17; 12, 15), each actuator (11, 17; 12, 15) being associated with a different wheel of the vehicle, control means (23, 31; 23', 31') being associated with one of the wheels for sensing a change in the velocity of that wheel and reducing the pressure of fluid in the brake circuit when wheel deceleration increases above a first predetermined value and a proportioning valve (34; 34') which at a predetermined pressure in the brake circuit will increase the ratio of the rates at which pressure is increased to the actuators (11, 17; 12, 15) associated with said one and the other wheel respectively, characterised in that actuator means (60) is provided to control the proportioning valve (34; 34') to reduce the pressure applied to the actuator (17; 15) associated with said other wheel, when said other wheel locks or is liable to lock.

2. A vehicle antilock braking system according to Claim 1 characterised in that said proportioning valve (34; 34') has valve means (50) to control flow of fluid from the master cylinder (20) to the brake actuator (17; 15) associated with said other wheel; a piston (43) to control said valve means (50), said piston (43) having a first face (85) which is subjected to pressure of fluid in the brake actuator (17; 15), a second face (86) which is subjected to pressure of fluid in the brake circuit, and means (60) to apply a load to the piston (43) which will oppose the force applied thereto by fluid pressure applied to the first face (85), said first face (85) being of greater area than the second face (86), so that when said valve means (50) is open, the excess force acting upon the first face (85) will cause the piston (43) to move against the load applied thereto, until when the pressure reaches a predetermined value the piston (43) will cause the valve means (50) to close, thus interrupting communication between the master cylinder (20) and brake actuator (17; 15) associated with said other wheel.

3. A vehicle antilock braking system according to Claim 2 characterised in that further increase in pressure of fluid in the brake circuit after the valve means (50) is closed, will cause the piston (43) of the proportioning valve (34; 34') to move increasing the pressure of fluid applied to the brake actuator (17; 15), but at reduced rate.

4. A vehicle antilock braking system according to Claim 2 or 3 characterised in that the load applied to the piston (43) of the proportioning valve (34; 34') and hence the predetermined pressure at which the valve means (50) closes is adjustable with the load applied to the vehicle.

5. A vehicle antilock braking system according to any one of Claims 2 to 4 characterised in that spring means (68) is provided to apply a load to the piston (43) of the proportioning valve (34; 34').

6. A vehicle antilock * braking system according to Claim 5 characterised in that said actuator means (60) acts to reduce the load applied by the spring means (68) to the piston (43) of the proportioning valve (34; 34'), when said other wheel locks or is liable to lock.

7. A vehicle antilock braking system according to Claim 6 characterised in that said actuator means (60) is a vacuum actuator which comprises a piston (62) mounted within a cylindrical casing (61) and sealed with respect thereto to divide the casing into two fluid tight chambers (64, 65), the piston (62) of the vacuum actuator (60) having a plunger (66) which abuts the piston (43) of the proportioning valve (34; 34'), said plunger (66) having a flange formation (67) at the end which abuts the piston (43) of the porpo tioning valve (34; 34*) and said spring means (68) acting against the flange formation (67) to urge it into engagement with the piston (43) of the proportioning valve (34; 34'), the chamber (64) of the vacuum actuator (60) remote from the piston (43) of the proportioning valve (34; 34') being connected to vacuum and the other chamber (65) being selectively connected to vacuum or to atmosphere via a solenoid control valve (75), so that when said other wheel locks or is liable to lock the solenoid valve (75) may be controlled to connect said other chamber (65) to atmosphere thus establishing a pressure differential across' the piston (62) of the vacuum actuator (60), this pressure differential producing a force which opposes the load applied to the piston (43) of the proportioning valve (34; 34') by the spring means (68).

8. A vehicle antilock braking system according to Claim 7 characterised in that means (76, 77) is provided for controlling the rate of flow of air to and/or from said other chamber (65) of the vacuum actuator (60).

9. A vehicle antilock braking system according to Claim 5 characterised in that the actuator means (60) is arranged to apply a load to the piston (43) of the proportioning valve (34; 34'), means (90, 91) being provided for reduction of this load when the other wheel locks or is liable to lock.

10. A vehicle antilock braking system according to Claim 9 characterised in that the load applied by the actuator means (60) is variable with the load applied to the vehicle.

11. A vehicle antilock braking system according to Claim 9 or 10 characterised in that the actuator means (60) is a vacuum actuator (60) which comprises a piston (62) mounted within a cylindrical casing (61) and sealed with respect thereto to divide the" casing into^' two fluid tight chambers (64, 65), the piston (62) of the vacuum actuator (60) having a plunger (66) which abuts the piston (43) of the proportioning valve (34; 34'), the chamber (64) of the vacuum actuator (60) remote from the piston (43) of the proportioning valve (34; 34') being open to atmosphere and the other chamber (65) being connected to vacuum via a first solenoid valve (90) or to atmosphere via a second solenoid valve (91), control means (80, 93, 94) being provided to actuate said first and second solenoid valves. (90, 91) to provide an appropriate pressure differential across the piston (62) of the vacuum actuator (60) and to reduce that pressure differential when said other wheel locks or is liable to lock.

12. A vehicle antilock braking system according to Claim 11 characterised in that said control means (80, 93, 94) acts to vary the pressure differential across the piston (62) of the vacuum actuator (60) in proportion to the load applied to the vehicle.

13. A vehicle antilock braking system according to any one of the preceding claims characterised in that the system has a plurality of circuits, each circuit including two brake actuators (11, 17; 12, 15) and a proportioning valve (34; 34').

14. A vehicle antilock braking system according to Claim 13 characterised in that the actuator means (60) associated with each of the proportioning valves (34; 34") are controlled together, so that if the other wheel controlled by any of the brake circuits locks or is liable to lock, all the proportioning valves (34; 34') will be actuated to reduce braking effort applied to all said other wheels.