WO1992020556A1

WO1992020556A1 - Vehicle braking system

Info

Publication number: WO1992020556A1
Application number: PCT/GB1992/000880
Authority: WO
Inventors: John Patrick Bayliss
Original assignee: Lucas Industries Public Limited Company
Priority date: 1991-05-15
Filing date: 1992-05-15
Publication date: 1992-11-26
Also published as: GB9110641D0; JPH06507586A; DE4291463T1; GB9322512D0; GB2271822A

Abstract

A vehicle braking system capable of acting in a traction control mode includes a master cylinder (7) operable to supply fluid under pressure to a brake associated with a wheel to be braked, and a servo booster (1) containing a pair of chambers (3, 4) separated by a movable partition (5) which is connected to force output means (11) and provides power assistance in operation of the master cylinder. The system includes a pilot valve (12) which, in response to a skid signal, acts under the influence of pressure in one of the booster chambers to create a pressure differential between the chambers such as to effect a traction control braking operation without intervention by the vehicle driver.

Description

VEHICLE BRAKING SYSTEM

This invention relates to a vehicle hydraulic braking system capable of operating in a traction control mode for correcting a sensed slip condition of a vehicle wheel, the system including a servo booster primarily useful for providing power assistance in the actuation of a master cylinder of the system, but which may operate, in response to a sensed wheel slip condition, to operate the master cylinder automatically for brake actuation in the traction control mode without intervention by the driver, in order to correct the slip condition.

In one conventional arrangement, illustrated in our British patent specification 0303470, the supply of air to the booster in the traction control mode is controlled by a solenoid valve carried on the forward wall of the booster casing. Following operation of the booster in the traction control mode, a large air flow is required to enable the booster to return rapidly to its normal mode of operation, which means that the solenoid valve is required to control a large port and therefore needs a correspondingly large armature. This leads to penalties in terms of manufacturing costs and power consumption and also to noisy operation. Because the installation space around the booster is cramped, there can sometimes be a requirement for the solenoid valve to be separated and mounted at a remote location of the engine compartment, such as on the fire wall separating this compartment from the passenger cabin. This can itself lead to potential problems, including increased response times due to the extra length of pipe run between the booster and valve, and the possibility of an unwanted and uncontrolled brake application, should the pipe become ruptured or detached. There is also a possibility of excessive operational noise being transmitted to the cabin.

An object of the invention is to provide a braking system in which a brake booster effects operation of the system in a traction control mode and in which the aforesaid potential problems are minimised or avoided.

According to the present invention, the braking system includes a pilot valve which acts, under the influence of pressure in one of the booster chambers, to create a pressure differential between the booster chambers, such as to effect a traction control braking application.

Preferably, the pilot valve incorporates a pressure differential responsive member which assumes a first position when no pressure differential is present across said member and a second position when it is subjected to a pressure differential, said member being arranged to actuate a pair of valves, such that, in the first position it permits the supply of pressure to one of the booster chambers in order to establish a pressure therein substantially equal to the pressure of the other chamber, and in the second position it causes another pressure to be supplied to one of the chambers in order to establish said pressure differential for traction control braking application.

Preferably, the pressure differential responsive member carries a pair of valve elements for engagement with respective valve seats, each seat being engaged by its associated valve element when the other is not so engaged.

The pressure across the responsive member is controlled by a switching valve, preferably an electrical solenoid valve, which conveniently acts to switch the pressure at one side of the member from the same pressure as that on the other side of the member to a different pressure, the latter pressure preferably being that in said other chamber of the booster.

The invention will now be described, by way of example, with reference to the accompanying drawings in which:-

Figure 1 is a side elevation showing part of one form of the braking system of the invention;

Figures 2 and 3 are views, to an enlarged scale, of part of the system of Figure 1, illustrating respectively different operational conditions, and

Figure 4 is a diagrammatic illustration of part of an alternative form of the system of the invention.

Referring to Figure 1, the system of the invention includes a servo booster, indicated generally at 1, having a housing 2 which is divided into internal chambers 3 and 4 by a movable partition in the form of a diaphragm assembly 5 carried by a movable hub 6. The booster is attached to a master cylinder 7 supplied with hydraulic fluid from a reservoir 8. The internal components of the booster and master cylinder are conventional and are not illustrated in detail. It will be understood that an input force applied, for example, via a drivers foot pedal (not shown), to an input rod 9 actuates a valve mechanism 10 to enable atmospheric air to enter the chamber 4 which applies power assistance via an output rod 11 to the master cylinder 7, in conventional manner. The illustrated booster is adapted to function in a traction control mode, without the intervention of the driver, so as to apply the brakes in order to correct a sensed wheel spin and this mode is controlled by a pilot valve assembly 12 mounted, in this case, on the front wall of the booster and acting to supply alternatively vacuum or atmospheric air to the rear chamber 4 of the booster, in the manner to be described.

The pilot valve assembly 12 is illustrated in more detail in Figures 2 and 3. The assembly includes a solenoid valve 14 having a coil assembly 15 which is energised, when required, in response to electrical signals received from spin sensing means, in conventional manner. An armature 16 of the solenoid valve carries a valve element 17 which controls air flow through a passage 18 which leads to the booster chamber 3 via a passage 18A, and also through a passage 15A communicating with the exterior. A valve element 13 controls air flow through an opening 13A in the booster housing leading to the booster chamber 3 and is connected by a rod 19 to a diaphragm 20 which is supported so as to define a pair of mutually isolated chambers 21, 22. The chamber 21 communicates by way of an orifice 23 and passage 24 with a chamber 25 of the solenoid valve 12 which, with the valve element in its position shown in Figure 2, communicates with the atmosphere.

The rod 19 carries a valve element 26, which, in the embodiment described is conveniently formed as part of the diaphragm 20. A spring 27, urges the valve element 26 into engagement with a seat 28 formed around a port 29 extending between the chamber 22, which communicates permanently with the atmosphere, and a passage 30 leading via the interior of the booster to the rear chamber 4 thereof, in known manner.

For normal operation of the booster under driver control via the rod 9, the solenoid valve is in its non- energised condition, as shown in Figure 2, such that the chamber 25 is connected to the atmosphere inlet 15A and the valve element 17 shuts off the passage 18 connected to the vacuum chamber 3 of the booster. Under these conditions, both sides of the pilot valve diaphragm 20 are subjected to atmospheric pressure and the diaphragm is urged by the spring 27 to its illustrated position in which the valve element 26 engages the seat 28 around the port 29 to seal the latter. With the diaphragm in this position, the rod 19 assumes the illustrated position in which the valve element 13, also carried by the rod 19, is displaced from the opening 13A in the booster housing, allowing vacuum to be supplied from the chamber 3 into the passage 30 and thence to the chamber 4, so that the booster is inoperative. Vacuum is also applied to the underside of the valve element 26 and acts to hold this valve in engagement with the seat 28.

The traction control mode is initiated upon the sensing of a wheel spin condition, when an electrical signal is supplied from suitable conventional control means to energise the solenoid valve, the armature 16 is moved to the position illustrated in Figure 3 in which the valve element 17 closes off the atmosphere passage 15A and simultaneously places the vacuum chamber 3 of the booster in communication with the chamber 25 via the passages 18A and 18. It will thus be seen that vacuum is communicated to the orifice 23 via passage 24, admitting vacuum to the upper side of the diaphragm 20, thereby creating a differential pressure which causes atmospheric pressure in chamber 22 beneath the diaphragm to lift the latter. The effect of this is to bring the valve element 13 into closing engagement over the opening 13A and also simultaneously lift the valve element 26 from the seat 28, which has the effect of placing the passage 30 in communication with atmospheric pressure beneath the diaphragm, such pressure being transmitted to the rear chamber 4 of the booster to cause a traction control application of the brake without the intervention of the driver. When the skid control signal ceases, the solenoid valve returns to its de-energised condition shown in Figure 2, causing the valve element to unblock the passage 15A once more and the valves 13 and 26 to return to their original positions, so that the traction control mode ceases.

Figure 4 illustrates diagrammatically an alternative valve arrangement which employs a pair of solenoid valves 40, 41 operating in conjunction to control a diaphragm 20 and its associated valve elements 13 and 26 which are arranged similarly to those illustrated in Figures 2 and 3. The chamber 21 above the diaphragm 20 is normally connected to atmosphere via a passage 42 of the valve 41, an orifice 43 and passage 44. The chamber 30 beneath the diaphragm 20 communicates with the atmosphere via a passage 45. The vacuum chamber 3 of the booster is connected by a passage 46 to an inlet port 47 of the other solenoid valve 40 and it will be seen that the passage 46 contains a restrictor 48. With both valves 40, 41 de-energised and in their illustrated positions, it will be seen that atmospheric pressure exists at both sides of the diaphragm 20, enabling the spring 27 to hold the valve element 26 in closing engagement with the port 28 and also holding the valve element 13 in its open position so that the vacuum of the booster chamber 3 is communicated via passage 31 to the rear booster chamber. Under these conditions, it is not possible for the booster to operate without intervention from the driver as explained previously. The solenoid valve 40, in its illustrated de-energised position, interrupts communication between the booster chamber 3 and the chamber 21 above the diaphragm.

When traction control is required, both solenoid valves 40, 41 are energised causing their respective armatures 40A, 41A to be lifted, which has the effect of applying vacuum to the upper chamber 21 via passage 46, port 47 and passage 47A, and cutting off atmospheric pressure from the same chamber by closing orifice 43. The valves associated with the diaphragm 20 operate to cause actuation of the booster in the traction control mode, as described above. Should the valve 41 fail to open after de-energisation upon ceasing of the traction control mode, a small additional orifice 50 connecting the chamber 21 to atmosphere permits a slow bleed of atmospheric pressure back into chamber 21, causing the pressures in chamber 21 and 30 eventually to equalise and enable the diaphragm to reassume its illustrated position to prevent unwanted operation of the brakes. Should the valve 40 fail to close the port 47 upon de-energisation, the combined air flow through orifice 50 and the orifice 43 in the valve 41 is sufficient to choke the restrictor 48 and cause a back pressure to be established in the chamber 21 which can rise to at least a level permitting the diaphragm to reassume its condition shown in Figure 4 so that vacuum is reapplied to the rear booster chamber from the chamber 3 and the unwanted braking ceases. All of the various orifices illustrated diagrammatically in Figure 4 may be produced in any convenient manner and may, for example, be incorporated into the valve seats.

It will be understood that the solenoid valve or valves may be disposed remotely from the booster and each connected to the diaphragm valve assembly by a flexible pipe. Any rupture or detachment of such a pipe would not cause an unwanted actuation of the brakes because the pilot valve is closed under the action of atmospheric pressure and this condition can only be changed by application of vacuum to the upper chamber of the pilot valve. The use of a solenoid-actuated pilot valve to switch the booster between normal and traction control operation permits a relatively small solenoid valve to be employed, thereby minimising the power required and noise ensuing from operation of the solenoid valve.

Claims

1. A vehicle hydraulic braking system capable of operating in a traction control mode for correcting a sensed slip condition of a vehicle wheel, the system comprising a master cylinder operable to supply fluid under pressure to a brake associated with the wheel for brake actuation, a servo booster containing a pair of chambers separated by a movable partition which is connected to force output means and, under the influence of pressure differential between the booster chambers provides power assistance via said output means in the operation of the master cylinder, and a pilot valve which, in response to a signal resulting from the sensing of an incipient wheel slip condition, acts under the influence of pressure in one of the booster chambers, to create a pressure differential between the booster chambers such as to effect a traction control braking operation without intervention by the vehicle driver.

2. A braking system according to Claim 1, wherein the pilot valve incorporates a pressure differential responsive member which assumes a first position when no pressure differential is present across said member and a second position when it is subjected to a pressure differential, said member being arranged to actuate a pair of valves, such that, in the first position it permits the supply of pressure to one of the booster chambers in order to establish a pressure therein substantially equal to the pressure of the other chamber, and in the second position it causes another pressure to be supplied to one of the chambers in order to establish said pressure differential for traction control braking application.

3. A braking system according to Claim 2, wherein the pressure differential responsive member carries a pair of valve elements for engagement with respective valve seats, each seat being engaged by its associated valve element when the other is not so engaged, said valve elements and their respective seats constituting said pair of valves.

4. A braking system according to Claim 2 or Claim 3, wherein said responsive member is spring-urged towards its first position.

5. A braking system according to any one of Claims 2 to 4, wherein said responsive member is a diaphragm.

6. A braking system according to any one of Claims 2 to 5, wherein the pressure across the responsive member is controlled by a switching valve which acts to switch the pressure at one side of the member from the same pressure as that on the other side of the member to a different pressure, such pressure switching causing the responsive member to move from its first position to its second position and thereby change the state of said pair of valves as between open and closed.

7. A braking system according to Claim 6, wherein the switching valve is actuated by an electrical solenoid in response to said signal.

8. A braking system according to any one of Claims 2 to 5, wherein an armature of the solenoid carries a valve element arranged to cooperate alternatively with a pair of valve ports disposed at either side thereof and communicating respectively with atmosphere and vacuum, so that movement of the armature from one operative position to another changes the pressure acting on one side of the responsive member from atmosphere to vacuum and vice versa.

9. A braking system according to any one of Claims 2 to 5, wherein the pressure across the responsive member is controlled by a pair of switching valves, a first of which is arranged in an atmosphere path normally connecting one side of the responsive member to atmosphere and the other of which is in a vacuum path between the vacuum chamber of the booster and said one side of the responsive member, the other side of the responsive member being permanently connected to atmosphere, whereby, upon simultaneous actuation of the valves, the first valve closes a port to interrupt the atmosphere path and the other valve opens the vacuum path to admit vacuum to said one side of the responsive member, such that said responsive member moves from its first position to its second position and thereby change the state of said one pair of valves as between open and closed.

10. A servo booster for use in a braking system according to any one of the preceding claims and comprising a pair of chambers separated by a movable partition which is connected to force output means and, under the influence of pressure differential between the booster chambers provides power assistance via said output means in the operation of the master cylinder, and a pilot valve which, in response to a signal resulting from the sensing of an incipient wheel slip condition, acts, in use, under the influence of pressure in one of the booster chambers, to create a pressure differential between the booster chambers such as to effect a traction control braking operation without intervention by the vehicle driver.

11. A servo booster according to Claim 10, wherein the pilot valve incorporates a pressure differential responsive member which assumes a first position when no pressure differential is present across said member and a second position when it is subjected to a pressure differential, said member being arranged to actuate a pair of valves, such that, in the first position it permits the supply of pressure to one of the booster chambers in order to establish a pressure therein substantially equal to the pressure of the other chamber, and in the second position it causes another pressure to be supplied to one of the chambers in order to establish said pressure differential for traction control braking application.