WO1988001954A1

WO1988001954A1 - Vehicle antilock braking systems

Info

Publication number: WO1988001954A1
Application number: PCT/GB1987/000620
Authority: WO
Inventors: David Anthony Harries
Original assignee: Automotive Products Plc
Priority date: 1986-09-11
Filing date: 1987-09-04
Publication date: 1988-03-24
Also published as: GB8621863D0; GB2213893A; GB8904088D0

Abstract

A vehicle antilock braking system has a master cylinder (20) which upon brake application will provide a source of high pressure fluid. The master cylinder (20) serves at least one braking circuit, said braking circuit including a first brake actuator (11; 13) associated with one wheel of the vehicle and a second brake actuator (17; 15) associated with another wheel of the vehicle. Control means (24, 26, 28; 25, 27, 29) includes means (26, 28; 27, 29) for sensing a change of velocity of said one wheel and valve means (24, 25) for reducing the pressure of fluid delivered to the brake actuators (11, 17; 13, 15) when wheel deceleration increases above a predetermined limit. The valve means (24; 25) is arranged to reduce the pressure to said first brake actuator (11; 13) at a slower rate than the pressure to said second brake actuator (17; 15).

Description

VEHICLE A TILOCK BRAKING SYSTEMS

The invention relates to vehicle antilock braking systems.

Hitherto dual circuit braking systems have been proposed in which the brakes of each front wheel and the diagonally opposite rear wheel are associated in separate braking circuits. With such systems, wheel speed sensors may be provided on each of the front wheels to control an antilock mechanism to reduce the pressure of fluid in the circuit associated with that wheel and the associated rear wheel in response to deceleration of the front wheel. Locking of the associated rear wheel is avoided in such systems by applying a greater percentage of the braking effort through the front wheels. This may be achieved by using smaller size brakes on the rear wheels or by means of proportioning valves which are reactive to some condition, for example pressure in the brake line or vehicle deceleration, to reduce the pressure of fluid applied to the rear brakes under some predetermined condition.

The braking ratio is selected to be as near as possible to the ideal but allowance has to be made for production tolerances and variations in brake performance with wear and use, for example fade, while still maintaining vehicle stability during braking particularly near the maximum deceleration achievable. This maximum level can only be achieved with non-antilock braking systems , in an almost straight line stop, that is without steering.

With antilock braking systems however, it is possible to achieve near maximum levels of decleration for a given road surface and steer at the same time. In these situations it is sometimes desirable to reduce the level of braking on the rear wheels still further, to maintain vehicle stability during dynamic manoeuvres by ensuring that the rear wheels run at reduced slip.

This may be achieved by reducing -the braking ratio on the rear wheels still further but this has the disadvantage in normal use of putting too much work on the front brakes, generating excessive heat and possibly accelerating wear of the friction linings.

The problem may be overcome by providing wheel sensors on both the front and rear wheels and either controlling the brakes on the rear wheels independently of those on the front wheels or controlling pressure in the brake circuit in response to deceleration of either the front or rear wheels. Use of sensors -on the front and rear wheels will of course considerably increase the cost and complexity of the system and with the latter arrangement would probably result in underbraking of the front wheels in order to prevent locking of the rear wheels.

The object of the present invention is to provide a vehicle antilock braking system having only one wheel speed sensor per circuit, in which maximum braking can be maintained on the front and rear wheels under normal braking conditions, the percentage braking effort applied to the rear wheels only being reduced during potential unstable conditions, that is during an antilock cycle.

According to one aspect of the present invention a vehicle antilock braking system has a master cylinder which upon brake application will provide .a source of high pressure fluid, said master cylinder being connected to a brake circuit including a first brake actuator associated with one wheel of the vehicle and a second brake actuator associated with another wheel of the vehicle, control means including means for sensing a change in velocity of said one wheel and valve means for reducing the pressure of fluid delivered to the brake actuators when wheel deceleration increases above a predetermined limit, characterised in that said valve means reduces the pressure to said first brake actuator at a slower rate than the pressure to said second brake actuator.

In this manner, during the antilock cycle, the brake effort applied to the rear wheel will be reduced at a faster rate than the brake effort applied to the front wheel thus avoiding the risk of locking of the rear wheels. Preferably, the control means will be arranged to reapply full braking effort, once acceleration of the front wheel reaches a predetermined limit.

Several 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 present invention;

Figure 2 is a diagrammatic illustration of one form of servo control means that may be used in the system illustrated in Figure 1;

Figure 3 is a diagrammatic illustration of an alternative form of servo control means that may be used in the system illustrated in Figure 1;

Figure 4 is a diagrammatic illustration of a further alternative form of servo control means that may be used in the system illustrated in Figure 1; and Figure 5 is a diagrammatic illustration of a direct power control means that may be used in the system illustrated in Figure 1.

In the system illustrated in Figure 1, the front wheels of a vehicle are provided with disc brakes 10 and 12, controlled by actuating calipers 11 and 13 respectively and the 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 a dual master cylinder 20 of conventional design. The master cylinder 20 has two outlets 22 and 23 which provide a source of hydraulic pressure for the two legs of the dual system, one leg serving caliper 11 of the off side front wheel and cylinder 17 of the near side rear wheel and the other leg serving caliper 13 of the near side front wheel and cylinder 15 of the off side rear wheel.

Toothed wheels 26 and 27 are mounted for rotation with the off side and near side front wheels respectively and electromagnetic pickups 28 and 29 are associated with the toothed wheels 26 and 27 respectively, to provide a signal which alternates at a frequency proportional to the rotational speed of the wheel with which they are associated.

The pickup 28 associated with the off side front wheel is connected to electronic control means 24. The control means 24 processes the signal from the pickup 28 to provide a measure of deceleration or acceleration of the off side front wheel and modulates the pressure of fluid in the leg of the system serving the off side front and near side rear wheel in response to the acceleration or deceleration measured. Similarly pickup 29 is connected to electronic control means 25 to modulate pressure of fluid in the leg of the system serving the near side front and off side rear wheels in response to the acceleration or deceleration of the near side front wheel. The control means 24 and 25 of the two legs of the system are identical in arrangement and operation and the control means in only one leg of the system is described in detail below.

According to one embodiment of the invention, the electronic control means 24 may include an antilock servo mechanism 30 as illustrated in Figure 2. The servo mechanism 30 comprises a pair of cylinders 31 and 32 having a common inlet port 33. Valve seats 34 and 35 are provided between the inlet port 33 and the bores of cylinders 31 and 32 and balls 36 and 37 are biassed towards the seats 34 and 35 by springs 38 and 39. Control plungers 40 and 41 are slidable in the bores of cylinders 31 and 32 respectively, plunger 40 being of smaller cross-sectional area than plunger 41. Cylinders 31 and 32 are provided with outlet ports 42 and 43.

Movement of the plungers 40 and 41 in cylinders 31 and 32 is controlled by a piston 44 which is located within a cylindrical casing 45. A flexible diaphragm 46 is secured at its outer periphery to the casing 45 and at its inner periphery to the piston 44, to divide the casing 45 into two chambers 47 and 48. A spring 49 urges the piston 44 towards the cylinders 31 and 32. Chamber 47 is connected to a source of vacuum and chamber 48 is selectively connected via a solenoid valve 50 to a source of vacuum or to atmosphere.

The inlet port 33 of the servo mechanism 30 is connected to the outlet port 22 of master cylinder 20, outlet port 42 of cylinder 31 is connected to caliper 11 and outlet port 43 of cylinder 32 is connected to cylinder 17.

Under normal braking conditions the solenoid 50 connects chamber 48 of the servo mechanism 30 to vacuum, so that pressure on either side of the diaphragm 46 is balanced and spring 49 will force the piston 44 towards the cylinders 31 and 32. The plungers 40 and 41 are thus forced towards the closed ends of cylinders 31 and 32 and extensions of the plungers 40 and 41 engage the balls 36 and 37 to unseat them, so that the outlet ports 42 and 43 are open to the inlet port 33 and fluid may pass from the master cylinder 20 to the caliper 11 and cylinder 17 of the brakes 10 and 16.

The signal from the pickup 28 is fed to an electronic control module (not shown) which energises solenoid 50, to open chamber 48 to atmosphere, when the deceleration of the off side front wheel rises above a predetermined limit, above which locking of the wheel is likely to occur.

Opening of chamber 48 to atmosphere will produce a pressure differential across diaphragm 46 which will oppose spring 49 and move piston 44 away from cylinders 31 and 32. Plungers 40 and 41 will move with piston 44, so that the balls 36 and 37 will engage seats 34 and 35 to close connection between the master cylinder 20 and the caliper 11 and cylinder 17. Further movement of the plungers 40 and 41 away from the closed ends of the cylinders 31 and 32 will then expand the volume in which the fluid is contained, thus reducing the pressure of the fluid and the effort applied by the brakes 10 and 16. As piston 41 is of greater cross sectional area than piston 40, the volume in cylinder 32 will increase at a faster rate than the volume in cylinder 31 and consequently the pressure of fluid applied to the cylinder 17 will decrease at a faster rate and to a lower level than the pressure of fluid applied to caliper 11. Braking effort applied to the near side rear wheel will consequently be reduced at a greater rate than that applied to the off side front wheel, thereby altering the front/rear braking ratio and reducing the risk of the rear brake locking.

The solenoid valve 50 will remain engaged until the acceleration of the off side front wheel rises above a predetermined limit, when it will be de-energised and the servo mechanism 30 will reapply normal braking.

Figure 3 illustrates a modification to the servo mechanism illustrated in Figure 2. In this modified arrangement, the plungers 60 and 61 are of equal cross section, but movement thereof is controlled through piston 44 via a lever 62. In this manner, plunger 60 which is positioned nearer to the pivot 63 of lever 62 will move at a slower rate than plunger 61, thus providing the required differential rates of pressure reduction to caliper 11 and cylinder 17.

In the servo mechanism 70 illustrated in Figure 4, the cylinder 72/plunger 74 assembly controlling pressure of fluid to the cylinder 17 of brake 16 on the near side rear wheel, is formed concentrically of the cylinder 71/plunger 73 assembly which feeds caliper 11. The cross-sectional areas of plungers 73 and 74 may be selected to provide the appropriate variation in rates of decrease in braking effort, as described above.

In an alternative embodiment of the electronic control means 24, as illustrated in Figure 5, the caliper 11 of brake 10 associated with the off side front wheel is selectively connected to the outlet port 22 of master cylinder 20 and a drain 80 via a solenoid valve 81, and the cylinder 17 of brake 16 on the near side rear, wheel is selectively connected to outlet port 22 of master cylinder 20 and a drain 82 via solenoid valve 83. The drain 80 includes a restriction 84, and the branch line 85 between the- outlet port 22 of master cylinder 20 and solenoid 83 includes a restriction 86. Under normal braking conditions, solenoid valves 81 and 83 will connect the caliper 11 and cylinder 17 respectively, with the outlet port 22 of master cylinder 20. When deceleration of the off side front wheel reaches a predetermined value, below which locking of the wheel is likely to occur, an electronic control module (not shown) will energise solenoid valves 81 and 83 to connect caliper 11 and cylinder 17 to drains 80 and 82 respectively. The pressure of fluid in the caliper 11 and cylinder 17 will thus be permitted to reduce with a corresponding reduction in the braking effort. The restriction 84 in drain 80 will cause pressure in the caliper 11 to reduce at a slower rate than that in cylinder 17, thus providing the required variation in front/rear brake effort, during the antilock cycle. Once the off side front wheel accelerates above a predetermined value, the solenoids 81 and 83 will be de-energised to reconnect caliper 11 and cylinder 17 to the master cylinder 20. As a result of the restriction 86 in line 85, reapplication of pressure to the cylinder 17 will be at a slower rate than that to caliper 11, so that reapplication of braking effort to the off side front wheel will be faster than to the near side rear wheel.

Various modifications may be made without departing from the invention. For example, while a dual braking system is described above, the invention may be applied to single circuit braking systems where a single antilock servo would be controlled by sensors on one or both front wheels.

Also, with dual braking systems the wheels need not be linked diagonally, but may be linked for example two fronts/two rears, two fronts and one rear/two fronts and other rear, or even all four wheels on each separate circuit. The invention is applicable to any circuit in which the number of wheel speed sensors is less than the number of brake actuators served by the circuit, the actuators on wheels without sensors being de-energised at a faster rate than those with sensors.

Claims

CL I MS

1. A vehicle antilock braking system having a master cylinder (20) which upon brake application will provide a source of high pressure fluid, said master cylinder (20) being connected to a brake circuit including a first brake actuator (11;13) associated with one wheel of the vehicle and a second brake actuator (17;15) associated with another wheel of the vehicle, control means (24 ,26,28;25,27,29) including means (26,28;27,29) for sensing a change in velocity of said one wheel and valve means (24;25) for reducing the pressure of fluid delivered to the brake actuators (11,17;13,15) when wheel deceleration increases above a predetermined limit, characterised in that said valve means ^"(24;25) reduces the pressure to said first brake actuator (11;13) at a slower rate than the pressure to said second brake actuator (17;15).

2. A vehicle antilock braking system according to Claim 1 characterised in that the pressure of fluid delivered to the brake actuator (17;15) associated with the other wheel is reduced to a lower level than the pressure of fluid delivered to the brake actuator (11;13) associated with said one wheel.

3. A vehicle antilock braking system according to Claim 1 or 2 characterised in that the control means (24;25) reapplies full braking effort, once acceleration of said one wheel reaches a predetermined limit.

4. A vehicle antilock braking system according to Claim 3 characterised in that the fluid pressure to the brake actuator (17;15) associated with said other wheel is reapplied at a slower rate than the fluid pressure to the brake actuator (11;13) associated with said one wheel.

5. A vehicle antilock braking system according to any one of the preceding claims characterised in that said control means (24;25) includes a servo mechanism (30;70) with a pair of plungers (40,41;60,61;73,74) , the first plunger (40;60;73) controlling the pressure of fluid to the brake actuator (11;13) associated with said one wheel and the second plunger (41;61;74) controlling the pressure of fluid to the brake actuator (17;15) associated with the other wheel.

6. A vehicle antilock braking system according to Claim 5 characterised in that the first plunger (40;73) is of smaller cross-sectional area than the second plunger (41;74).

7. A vehicle antilock braking system according to Clai 6 characte ised in that the second plunger (74) is concentric of the first plunger (73).

8. A vehicle antilock braking system according to Claim 4 characterised in that the first and second plungers (60,61) are of the same cross-sectional area, a lever mechanism (62,63) being used to control movement of the first and second plungers (60,61), the first plunger (60) being positioned nearer to the pivot (63) of the lever (62) than the second plunger (61).

9. A vehicle antilock braking system according to any one of Claims 1 to 4 characterised in that the control means (24;25) includes a first valve means (81) for selectively connecting the brake actuator (11;13) associated with said one wheel to the master cylinder (20) or to a first drain (80) and a second valve means (83) for selectively connecting the brake actuator (17;15) associated with said other wheel to the master cylinder (20) or to a second drain (82), the flow rate from the brake actuator (17;15) associated with said other wheel - through the second drain (82) being greater than the flow rate from the brake ^' actuator (11;13) associated with, said one wheel through the first drain (80).

10. A vehicle antilock braking system according to Claim 9 characterised in that the flow rate from the master cylinder (20) through the second valve means (83) to the brake actuator (17;15) associated with said other wheel is slower than the flow rate from the master cylinder (20) through the first valve means (81) to the brake actuator (11;13) associated with said one wheel.

11. A vehicle antilock braking system according to any one of the preceding claims characterised in that it includes two or more brake circuits.