This invention relates to vehicle hydraulic brakes and more particularly to improved control means for so-called "hill holders" which hold hydraulic brakes applied when the associated vehicle is parked on an incline.

Hill holders are well known and generally include an inclination sensitive valve member which traps the brake pressure when the vehicle is parked on an incline thus allowing the vehicle driver to take his foot off the brake pedal and move it onto the accelerator to make a hill start without any danger of vehicle rolling back down the hill.

Such hill holders have, however, suffered from problems in timing the release of the brake pressure so that residual brake pressure due to too slow a brake pressure release does not inhibit the hill start nor, at the other extreme, does too fast a release of brake pressure result in roll-back of the vehicle.

It is an object of the present invention to provide a hill holder control system which at least mitigates the above brake pressure release timing

problems .

Thus according to the present invention there is provided a hill holder control system for a vehicle hydraulic braking system the control system comprising a valve having an inlet connectable to a pedal-operated source of braking pressure and an outlet connectable to one or more brake actuators, an inclination sensitive valve member which is capable of closing off a valve seat to trap brake pressure in any brake actuator connected to the outlet, a command means operated from an associated clutch pedal which renders the valve member operative to close off the valve seat when the clutch pedal is operated, the control system being characterised by the provision of valve member disabling means operable to unseat the valve member from the valve seat to release any trapped brake pressure, and an electrical control circuit including a "brakes operating" condition sensor, a "brakes released" condition sensor, and a clutch bite point condition sensor, the electrical control circuit being arranged to operate the disabling means when the three sensors indicate the simultaneous presence of all three conditions to ensure a timed release of brake pressure on re-engagement of the clutch.

In a preferred arrangement the three sensors comprise a series arrangement of switches

comprising a pressure sensitive switch which closes when the brake actuator pressure exceeds a predetermined level, a brake pedal proximity switch which is closed when the brake pedal is released, and a clutch proximity switch which closes at the bite point of the clutch, and the control means operates the disabling means when all the switches are closed simultaneously. The disabling means ^" preferably comprises a solenoid-operated plunger which knocks the valve member off the valve seat.

The command means may be fluid pressure operated or cable operated.

In a preferred construction the valve member is located in a first chamber to which the inlet and outlet are connected, the valve member being housed in a cage which carries an annular sealing means for sealing around the inlet and the valve member being engageable with the sealing means under the action of gravity, the cage being biased away from the inlet and being movable towards the inlet against the bias by the command means to engage the sealing means around the inlet.

The command means may be operated by clutch operating pressure acting in a second chamber of the valve to move the cage towards the inlet.

A piston in the second chamber may be moved by the

clutch operating pressure to act on a cage mounted spigot which projects from the first chamber.

The present invention also provides an improved form of hill holder valve with a solenoid-operated valve member disabling means.

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

Fig 1 is a hill holding valve with solenoid disabling means shown in longitudinal cross section;

Fig 2 is a schematic diagram of a hill holder control system including the valve of Fig 1;

Fig 3 is a large scale view of part of the valve of

Fig 1, and

Fig 4 is a cross section through a dual circuit hill holding valve with solenoid disabling means.

With reference to Fig 1 there is illustrated a hill holder valve 1 for use on a vehicle with hydraulic brakes and a hydraulic clutch actuation system. The hill holder valve 1 has a valve body 2 with a cylindrical blind bore 3 having an end wall 4. The open end 5 of the bore is closed by an end cap 6 which screws into the bore to form a first fluid chamber 7. The fluid chamber 7 is connected to a pedal-operated source of brake applying pressure

such as a hydraulic brake master cylinder (not shown) through an inlet port 9 and to at least one hydraulic brake actuator (not shown) through an outlet port 10.

The hill holder valve includes an inclination sensitive ball valve member 14 housed in a cage 11 which is axially movable in the bore 3 and is biased away from the inlet port 9 by a compression spring 12. The axial end of the cage 11 adjacent to the end cap 6 carries an annular sealing means in the form of back to back annular seals 13a and 13b which encircle a passage 8 in cage 11. Seal 13a is engageable around inlet port 9 to close off the inlet when seal 13b is also engaged by ball valve 14 as will be described in more detail below. Seal 13a is secured to cage 11 by plate lib and screws lie to withstand the high pressure entering chamber 7 via inlet port 9.

The cage 11 has a cylindrical guide spigot 15 which is sealingly slidable in an annular spacer 16. The spacer 16 is located and secured within bore 3 by a locking ring 17 and includes a second bore 20 in which a piston 19 is sealingly slidable. Piston 19 divides the second bore 20 into a second fluid chamber 18 at the open end of the second bore and an air chamber 21 adjacent to the spacer 16. the air chamber 21 is connected to atmosphere via passageways 22 and 23.

The guide spigot 15 abuts the piston 19 and a rod 24 extending from piston is sealingly slidable in a passageway 4a through the end wall 4 and terminates in a threaded portion 25. This threaded portion 25 and associated nut 25a compresses and locks a spring 26 between the valve body 2 and an annular retainer 27. A boot 28 seals the passageway 4a.

The second fluid chamber 18 is connected to a pedal operated hydraulic clutch master cylinder (not shown) through an inlet port 29 and to the clutch hydraulic actuator (not shown) through an outlet port 30.

A plunger 31 extends into the first fluid chamber 7 via a passageway 31a through the valve body 2, perpendicular to the longitudinal axis of cylindrical bore 3. Plunger 31 forms part of an electrically-operated solenoid 32 attached to the valve. Solenoid 32 is connected to an electrical logic circuit 40 (see figure 2). The end of the plunger 31 entering the fluid chamber 7 has a chamfered nose 31b and extends through a slot 11a in the cage 11 so that when solenoid 32 is activated chamfered nose 31b contacts ball valve 14 to ensure that it no longer engages seal 13b. Plunger 31 thus acts a disabling means for ball valve 14.

Referring to Figure 2, one end of the coil of solenoid 32 is connected by line 33a to the first of three electrical switches, connected in series. The first switch in the series is a brake pedal proximity switch 34, positioned so as to be closed when an associated brake pedal 37 returns to the raised rest position, as the brake pedal 37 is released by the driver. The next switch in the series is a brake pressure switch 35, which is located at some point in the brake applying line (not shown) connected to outlet port 10, and which is closed when brake hydraulic pressure rises above a predetermined low level of say 1 bar indicating that a brake application is taking place. The third switch in the series is the clutch pedal proximity switch 36, positioned so as to be closed when an associated clutch pedal 38 displacement eguates to the "bite point" of the clutch system (i.e. that point at which the clutch just begins to transmit torque) .

Switches 34,35 and 36 form part of an electrical control or logic circuit 40 is completed by connection to the positive side of a battery 39 and by line 33b to the other end of the coil of solenoid 32.

Operation of the above hill holder arrangement will now be described. If the vehicle is facing upwards on an incline (arrow F in figures 1 and 3 denotes

the forward direction of the vehicle with respect to valve 1) with the hydraulic brakes applied, the brake pressure switch 35 will be closed and the ball valve will be seated against the seal 13b but seal 13a will not be surrounding the inlet port 9. In this condition the master cylinder is still in direct communication with the brake actuator connected to outlet port 10. If the driver then depresses the clutch pedal 38 to disengage the clutch, hydraulic pressure is generated by the clutch master cylinder (not shown) which increases the pressure acting in the second chamber 18 thus causing the piston 19 to move leftwards (as viewed in figures 2 and 3) forcing the cage 11 to the left via spigot 15 to bring the seal 13a into contact around the brake master cylinder inlet port 9.

This closes off chamber 7 from the associated brake master cylinder so that pressurised hydraulic fluid is trapped in the brake actuators connected to outlet port 10. When the brake master cylinder is released, as long as the clutch master cylinder pressure is maintained, the brakes will remain applied.

Referring to Figure 3, the level of hydraulic fluid pressure trapped in the brake actuator system connected to the outlet port 10 can be determined by consideration of the various forces acting on cage 11, piston 19 and rod 24. If the level of

brake pressure in the first fluid chamber 7 is Pb, this pressure acts on the effective area Ab of the

_ 2. cylindrical bore 3, given by ιTDl/4 and acts to the right as viewed in figure 3. If the applied clutch pressure in the second fluid chamber 18 with the clutch disengaged is Pc this pressure acts on the effective area Ac of the second fluid chamber 18 which is given by TT(D2 - D3)/4 and acts to the left as viewed in figure 3. Springs 12 and 26 apply forces Kl and K2 respectively to the cage 11 and rod 24. Both these forces act to the right as viewed in figure 3.

The complete force balance relationship can be expressed as:-

Pc.Ac = Pb.Ab + Kl + K2

When the driver releases the brake pedal 37 the hydraulic fluid pressure maintained in the braking system will drop until a state of equilibrium is reached in the valve 1 governed by the above relationship. The areas Ac and Ab and the spring forces Kl and K2 are set to trap a brake pressure level of say 25 bar in the brake actuators to ensure that the braking capability is retained when the brake pedal is fully released thus holding the vehicle stationary on the incline. The brake pedal proximity switch 34 will be closed as the driver fully releases the brake pedal 37.

The driver may then engage the vehicle change speed gearing (not shown) to select a low gear in preparation for pulling away and then begin to release the clutch pedal 38 to engage the clutch. As the clutch pedal 38 is released the clutch pedal proximity switch 36 is closed as the clutch reaches the "bite point" and the electrical logic circuit 40 is completed so that current flows to the solenoid 32. Activation of solenoid 32 causes plunger 31 to be displaced further into the first fluid chamber 7 so that the chamfered end 31b of the plunger 31 comes into contact with the ball 14 thus knocking the ball 14 off valve seal 13b. The opening of valve seal 13b dumps (via inlet port 9) the brake pressure in chamber 7 and the brake actuators connected to outlet port 10 thus allowing the vehicle to make a smooth pull-away.

The separate springs 12 and 26 may be combined into a single spring replacing spring 12. The separate spring 26 is included in the described construction to facilitate adjustment of the forces acting on the cage 11 during development and testing of prototypes. This spring is not present in the productionised construction shown in Figure 4.

Figure 4 shows a hill holder valve 100 in accordance with the present invention for use with a dual circuit cross-coupled vehicle braking system in which components of a similar function to

components of valve 1 of figures 1 and 3 are numbered with the same reference numerals increased by 100.

The valve includes two ball valves 114a and 114b which trap the brake pressure in the two separate brake circuits. Inlet 109a and outlet 110a are connected with the brake circuit which operates the front left and rear right brakes and inlet 109b and outlet 110b are connected with the other brake circuit which operates the front right and rear left brakes. Both ball valves 114a and 114b are housed within cages Ilia and 111b and operate to close annular seals 113b(i) and 113b(ii) respectively.

A solenoid plunger 131 is again provided to unseat ball valve 114a and clutch operating pressure is connected to an inlet 129. The clutch pressure acts on piston 119a which in turn acts on the end of spigot 115a to move the cage Ilia to bring valve seal 113a(i) into contact around inlet 109a.

Ball valve 114a thus acts in exactly the same manner as ball valve 14 described above to contact valve seal 113b(i) to trap brake pressure in chamber 107a when the associated vehicle is on an incline and the clutch is released.

The ball valve 114b which traps the brake pressure

in the second brake circuit is controlled from ball valve 114a by a drilling 150 which connects chamber 107a with a piston 119b which in turn acts on the end of a spigot 115b associated with the cage 111b to move the cage and thus bring valve seal 113a(ii) into contact around inlet 109b.

Thus when a high pressure is trapped in chamber 107a by ball valve 114a with the vehicle on an incline ball valve 114b contacts valve seal 113b(ii) to also trap the braking pressure in the second brake circuit connected to outlet 110b.

As will be appreciated, the solenoid which operates plunger 131 is connected into the same logic circuit shown in figure 2 so that ball valve 114a is knocked off valve seal 113b(i) as the clutch reaches its "bite point". The unseating of ball valve 114a causes the brake pressure trapped in chamber 107a to be released which means that piston 119b is no longer subjected to a high brake pressure so that spring 112b associated with cage 111b moves cage 111b to the right thus unseating seal 113a(ii) from around inlet 109b. This releases the brake pressure in chamber 107b so that both brake circuits are released.