GB2272031A

GB2272031A - Improvements in hydraulic braking systems for vehicles

Info

Publication number: GB2272031A
Application number: GB9322411A
Authority: GB
Inventors: Glyn Phillip Reginald Farr
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1992-10-29
Filing date: 1993-10-29
Publication date: 1994-05-04
Anticipated expiration: 2013-10-29
Also published as: GB2272031B; GB9222689D0; GB9322411D0

Abstract

An hydraulic braking system for a vehicle comprises a front wheel brake (4), a rear wheel brake (6), inlet and outlet valves (13, 14 and 11, 12) associated with the wheel brakes, and a pump (20) and expander chamber (26). When front wheel (5) tends to skid (ABS), valve (13) closes and valve (11) opens and fluid is dumped to the expander chamber, and when it recovers valve (11) closes and pump (20) is energised to return fluid from the chamber. When the wheel (5) tends to spin (TC), valve (13) closes, valve (12) opens and the pump is energised and draws fluid from the circuit associated with the rear wheel (7). When the vehicle is facing up-hill, a sensor will cause the pump to be energised, inlet valves (13, 14) to be closed and outlet valve (12) to be opened, so that fluid is transferred from the rear brake (6) to the front brake (4). <IMAGE>

Description

IMPROVEMENTS IN HYDRAULIC BRAKING SYSTEMS FOR VEHICLES This invention relates to four-channel hydraulic braking systems for vehicles of the kind in which brakes on wheels on front and rear axles of the vehicle are adapted to be applied by a pedal-operated master cylinder, and an electrically driven pump and solenoid-controlled valve means are incorporated for achieving brake intervention in a traction control mode, brake pressure retention to hold a vehicle on an upwardly facing hill, or brake re-application in an anti-lock mode in response to energising currents from an electronic control module in turn responsive to signals from a speed sensors for respective wheels, each of the four channels comprising a circuit between the master cylinder and a respective brake controlled by the valve means.

Such systems of the kind set forth comprise modern anti-lock braking systems which are expected to be readily adapted to provide extra features which include:- a) the said brake intervention, for traction control on surfaces of differing friction; and b) the said hill hold, to enable the vehicle to be held on an upward facing hill.

To provide these extra features, the systems normally includes extra solenoid valves and require special master cylinder recuperation valves which can open against a positive pressure. This tends to complicate the standard braking system particularly when anti-lock is an option and is not a fitment on every vehicle. Ideally, therefore, the anti-lock system, with additional features, should not require a special master cylinder or extra solenoid valves.

According to our invention, in a four channel braking system of the kind set forth for vehicles each channel includes a solenoid-controlled outlet valve for controlling the supply of fluid from the pump to the respective brake, the outlet valve being movable between a closed position for normal braking and an open position in which brake operation is initiated by pressure fluid from the pump with the master cylinder isolated from the brake, the system being so constructed and arranged that the source of fluid for applying or intensifying the pressure applied to the brake on a wheel on one axle incorporates the channel of a brake on the wheel of the other axle.

When the vehicle is of the front wheel drive type we can utilise the channel of a rear wheel as the source of fluid for applying the brake on a spinning front wheel for traction control. Similarly we can transfer fluid from a rear brake to a front brake to intensify the holding pressure of the front brake, irrespective of whether the vehicle is of the front or rear wheel drive type.

Normally the system is so constructed and arranged that the source of fluid for applying or intensifying the pressure applied to a brake on a front wheel incorporates the channel of a brake on a rear wheel.

When the control module recognises a signal from a wheel speed sensor as indicative of a spinning driven front wheel it is operative to close the channel between the master cylinder and the spinning wheel and open the solenoid-controlled outlet valve of the channel of the undriven rear wheel so that the pump can withdraw fluid from the master cylinder through the open solenoid-controlled outlet valve and supply it to the brake on the spinning wheel, the solenoid-controlled outlet valve of the channel of the driven wheel remaining closed.

Similarly by isolating the master cylinder from front and rear brakes to retain pressure after the brakes have been applied, and then opening the solenoid-controlled outlet valve of the rear wheel channel the pump can withdraw fluid from the rear channel and transfer it to the front channel to intensify the holding pressure applied to the front brake following a braked stop on an upward facing hill. The brakes are released whenever the traction force exceeds the backward force on the vehicle caused by the incline or when the engine clutch is fully engaged.

In one preferred layout of the braking system, each brake is connected to a normally-open inlet valve, a normally-closed outlet valve, and a motor driven pump. In addition, the inlet valve is connected to the master cylinder and the outlet valve is connected to a low pressure expander chamber. The pump is also connected to the expander chamber. Thus fluid may flow between the master cylinder and brake, the brake and expander chamber, the expander chamber and the pump and finally the pump and the brake.

With this layout, during an anti-lock mode a proportion of fluid relieved from the front brake is transferred to the master cylinder so that the front pressure is automatically re-applied to a pressure just below the skid pressure.

The pressurised fluid is preferably returned to the master cylinder via the outlet valves of front and rear channels or via the inlet valves of the channels of the brakes in the driven wheels.

Both outlet valves may be temporarily opened following anti-lock operation by a signal from the brake light switch "off" position. This releases pressure from the respective brake and from the expander chamber.

Some embodiments of our invention are illustrated in the accompanying drawing in which: Figure 1 is a layout of a four-channel hydraulic braking system for a vehicle;

Figure 2 is a graph of brake pressure

time for the system of Figure 1; Figure 3 is a layout of a system similar to that of Figure 1 but showing a modification; and Figure 4 is a layout of another braking system similar to Figure 3 but showing further modifications.

The four-channel hydraulic braking system illustrated in Figure 1 of the drawings comprises a tandem hydraulic master cylinder 1 which operated by a pedal 2 under the control of a pneumatic booster 3.

The tandem master cylinder 1 incorporates a primary pressure space and a secondary pressure space. Each pressure space is incorporated in two of the four channels. As illustrated in Figure 1 the primary pressure space is connected to a brake 4 on the front wheel 5 of the vehicle, and to a brake 6 on the diagonally opposite rear wheel 7 of the vehicle. The secondary pressure space is similarly connected to the brakes on the remaining two wheels, but the components comprising these two further channels have been omitted for clarity.

The behaviour of each wheel 5 and 6 is sensed by a respective sensor 8, 9, and signals from the sensors are fed to an electronic control module 10 which differentiates the signals and emits energising currents for controlling operation of a pair of solenoid-controlled normally-closed outlet valves 11 and 12, and solenoid-controlled normally-open inlet valves 13 and 14. Each inlet valve 13, 14 is disposed in a supply line from the primary pressure space to the respective front brake 4 and the rear brake 7.

An hydraulic pump 20 driven by a- motor 21 in response to an energising current from the control module 10 comprises a differential piston 22 working in a stepped bore 23 to define two axially spaced pump chambers 24 and 25 each for controlling one channel of the braking system. The pump chamber 24 is larger than the pump space 25. The pump chamber 24 communicates with a low pressure expander chamber 26 through an one-way inlet valve 27 and to the brake 4 through an one-way outlet valve 28. Similarly the pump space 25 communicates with the expander chamber 26 through an one-way inlet valve 30, and with the rear-wheel brake 6 through an one-way outlet valve 31.

The two solenoid-controlled valves 11 and 12 control communication between the corresponding brakes 4 and 6 and the expander chamber 26, and between the pump spaces 24 and 25 and the opposite brake 6 or 4 in a manner to be described.

For normal brake operation the pump 20 is inoperative, the solenoid-controlled valves 13 and 14 are open, and the solenoid-controlled valves 11 and 12 are closed.

Operation of the master cylinder causes fluid to be displaced to the brakes 4 and 6 through the open valves 13 and 14 with fluid being prevented from reaching the expander chamber 26 and the pump chambers 24 and 25 due to closure of the valves 11 and 12 and the presence of the one-way valves 28 and 31. When the control module 10 receives a signal from the wheel speed sensor 8 indicating that the brake wheel 5 is about to lock, it closes the valve 13 to prevent any further increase in pressure. If the skid signal is recognised as genuine, the valve 11 is opened to relieve the pressure acting on the brake 4 to the expander chamber 26. Upon recovery of the wheel 5, the valve 11 closes.At the same time the motor driven pump 20 is energised and fluid from the expander chamber 26 is pumped through the one-way valves 28 and 30, not only to the front brake 4, but also to the rear brake 6 and back through the open valve 14 to the master cylinder.

Since the pump space 24 is larger than the pump space 25, the output applied to the front brake 4 is substantial 75% of the total. The front brake 4 is re-applied to just below the pressure which generated the skid, i.e. the optimum level. Subsequently, the valve 13 is cycled to increase the pressure applied to the front brake 4 further in order to take advantage of any improvement in road surface conditions. During this sequence, the cycling reaction at the input is minimal because only 25% of the pump output is forced back to the master cylinder 1.

The sequences described above are illustrated in the graph of Figure 2 of the drawings.

If the brake 6 on the rear wheel 7 is also controlled, the valve 14 closes and the valve 12 opens to relieve pressure to the expander chamber 26.

Closing the valve 14 reduces input reaction even further since both pump chambers 24 and 25 are isolated from the master cylinder 1.

If the rear wheel 7 locks before the front wheel 5 or is the only wheel to lock, then, because the rear brake volume is small, only 25% of the small volume is returned to the master cylinder 1. This has a negligible effect on the pedal 2.

When the front wheels 5 comprise the driven wheels and the vehicle is travelling on a split friction surface with the brakes inoperative, should one of the driven wheels, say the wheel 5, start to spin, the motor driven pump 20 is energised and the valve 13 closes. At the same time the valve 12 opens so that the pump 20 can draw fluid from the reservoir 30 for the master cylinder 1 back through the open valves 14 and 12 and pump it to the brake 4 through the one-way outlet valve 28. Upon recovery of the wheel 5, the valve 11 opens to relieve pressure fluid from the brake 4 back to the master cylinder 1 and the reservoir 30. Subsequent control is achieved by opening and closing the valve 11 until the wheel 5 is fully controlled.At this point the motor 21 stops, the valves 11 and 12 remain open temporarily until the brake 4 is fully released, and the valve 13 opens to reconnect the master cylinder 1 and the brake 4.

Brake intervention for traction control is therefore provided with the existing anti-lock power source, namely the pump 20, and valves 11, 12, 13, 14.

The conventional actuation equipment of the master cylinder 1 and the pneumatic booster 3 can be used.

When the vehicle is braked to a stop on an upwardly facing hill, which condition is detected by an inclination sensor or other suitable means, the valves 13 and 14 are closed to retain the applied pressure. At the same time the valve 12 is opened and the pump 20 is started in order to transfer the volume of the rear brake 6 into the front brake 4 through the one-way inlet valve 20 and the one-way outlet valve 28 to intensify the holding pressure. The driver may now remove his foot from the brake pedal 2.

When the driver wishes the vehicle to move away and the traction force exceeds the force acting on the vehicle in a rearward direction caused by the incline, or the engine clutch is fully engaged, the valves 13 and 14 are opened to relieve pressure.

With a conventional vehicle anti-lock system, all inlet solenoid-controlled valves between a master cylinder and the brakes have a one-way valve in parallel with them to ensure brake pressure cannot exceed the pressure applied by the master cylinder and in order to guarantee that the brakes can always be released whenever the applied force is removed.

In the system described above with reference to Figure 1 it is necessary to permit the pressure applied to the brakes 4 on the front wheels 5 of a front wheel drive car to exceed that of the master cylinder 1. To ensure pressure release from the front brakes 4, the valves 13 and 14 are temporarily opened whenever the brake lights are switched off following an anti-lock sequence. Thus, if the valve 13 fails in its closed position, the front brakes can still be released via the two valves 11 and 12, and the valve 14 leading to the rear brake 6. If both front and rear valves 13 and 14 fail in their closed positions, both brakes can still be released via the valves 11 and 12 and a one-way valve 31 in parallel with the valve 14.

If the system described above with reference to Figure 1 is installed in a vehicle of the rear wheel drive type, the one-way valve 31 would be incorporated in the solenoid-controlled valve 13 rather than in the solenoid-controlled valve 14.

In the system illustrated in the layout of Figure 3 the solenoid-controlled valve 14 has been replaced by a flow valve 40 of known type, and the outlet from the one-way valve 31 is connected into the line between the solenoid-controlled valve 13 and the flow valve 40.

During anti-lock control of the rear wheel 7, opening the valve 12 to relieve pressure in the rear brake 6 creates a pressure drop in the flow valve 40 which is operative to isolate the master cylinder from the rear brake 6.

Re-application of the rear brake 6 following correction of the wheel 7 is achieved at a rate determined by the flow characteristics of the flow valve 40.

The construction and operation of the system illustrated in the layout of Figure 3 are otherwise the same as that of Figure 1 and corresponding reference numerals have been applied to corresponding parts. The electronic circuit has been omitted from the layout in the interest of clarity.

In the system illustrated in the layout of Figure 4 the pump 2 comprises a piston 50 of constant diameter working in a complementary bore 1 and with a one-way outlet valve 52 cooperating with the inlet passage passing through the piston and leading to a pump space 53 from which fluid is pumped through the one-way outlet valve 31 and back to the front brake 4 through a flow valve 55 and to the master cylinder through a one-way valve 56. The flow valve 55 is disposed between the outlet from the solenoid-controlled valve 13, the front brake 4, and the outlet from the solenoid-controlled valve 11.

When the brake front wheel 5 commences to lock, the valve 13 closes to isolate the master cylinder 1 from the brake 4, and the valve 11 opens so that fluid from the brake 4 is dumped to the expander chamber 26 through the flow valve 55. Upon recovery of the wheel 5, the valve 11 closes and the motor driven pump 20 is energised to withdraw fluid from the expander chamber 26 and pump it back to the front brake 4, at a rate determined by the flow characteristics of the flow valve 26 and also to the master cylinder 1 and the rear brake 6 through the flow valve 40 to achieve the result described above with reference to Figure 3.

When the front wheel 5 is a driven wheel which commences to spin with the brakes unapplied, the motor driven pump 20 is energised and the solenoid-controlled valve 13 closes. At the same time the valve 12 opens and fluid which is drawn from the reservoir 30 through the flow valve 40 is pumped into the front brake through the one-way valve 31, and the flow valve 55.

The construction and operation of the system described in the layout of Figure 4 is otherwise the same as that of Figure 3 and corresponding reference numerals have been applied to corresponding parts.

Claims

1. A four-channel hydraulic braking system for vehicles of the kind in which brakes on wheels on front and rear axles of the vehicle are adapted to be applied by a pedal-operated master cylinder, and an electrically driven pump and solenoid-controlled valve means are incorporated for achieving brake intervention in a traction control mode, brake pressure retention to hold a vehicle on an upwardly facing hill, or brake re-application in an anti-lock mode in response to energising currents from an electronic control module in turn responsive to signals from a speed sensor for each respective wheel, each of the four channels comprising a circuit between the master cylinder and a respective brake controlled by the valve means, in which each channel includes a solenoid-controlled outlet valve for controlling the supply of fluid from the pump to the respective brake, the outlet valve being movable between a closed position for normal braking and an open position in which brake operation is initiated by pressure fluid from the pump with the master cylinder isolated from the brake, the system being so constructed and arranged that the source of fluid for applying or intensifying the pressure applied to the brake on a wheel on one axle incorporates the channel of a brake on the wheel of the other axle.

2. An hydraulic braking system according to claim 1, in which the source of fluid for applying or intensifying the pressure applied to a brake on a front wheel incorporates the channel of a brake on a rear wheel.

3. An hydraulic braking system according to claim 2, in which when the control module recognises a signal from a wheel speed sensor as indicative of a spinning driven front wheel it is operative to close the channel between the master cylinder and the spinning wheel and to open the solenoid-controlled outlet valve of the channel of the undriven rear wheel so that the pump can withdraw fluid from the master cylinder through the open solenoid-controlled outlet valve and supply it to the brake on the spinning wheel, the solenoid-controlled outlet valve of the channel of the driven wheel remaining closed.

4. An hydraulic braking system according to claim 2, in which following a braked stop on an upward facing hill, the master cylinder is isolated from the front and the rear brakes to retain pressure after the brakes have been applied, the solenoid-controlled outlet valve of the rear wheel channel opens, the pump withdraws fluid from the rear channel and transfers it to the front channel to intensify the holding pressure applied to the front brake.

5. An hydraulic braking system according to claim 4, in which the brakes are released whenever the traction force on the vehicle exceeds the backward force on the vehicle caused by the incline or when an engine clutch of the vehicle is fully engaged.

6. An hydraulic braking system according to any preceding claim, in which each brake is connected to a normally-open inlet valve, a normally-closed outlet valve, and a motor driven pump, the inlet valve being connected to the master cylinder, the outlet valve being connected to a low pressure expander chamber, and the pump being connected to the expander chamber.

7. An hydraulic braking system according to claim 6, in which during an anti-lock mode a proportion of fluid relieved from the front brake is transferred to the master cylinder so that the front brake pressure is automatically re-applied to a pressure just below a skid pressure leading to the anti-lock mode.

8. An hydraulic braking system according to either claim 6 or claim 7, in which the fluid is returned to the master cylinder via the outlet valves of front and rear channels or via the inlet valves of the channels of the brakes in the driven wheels.

9. An hydraulic braking system according to any of claims 6 to 8, in which both outlet valves are temporarily opened following anti-lock operation by a signal from a brake light switch in an "off" position.

10. An hydraulic braking system according to any previous claim, in which the source of fluid for applying or intensifying the pressure applied to a brake on a rear wheel incorporates the channel of a brake on a front wheel.

11. An hydraulic braking system substantially as described herein with reference to and as illustrated in Figure 1 of the accompanying drawings.

12. An hydraulic braking system substantially as described herein with reference to and as illustrated in Figure 3 of the accompanying drawings.

13. An hydraulic braking system substantially as described herein with reference to and as illustrated in Figure 4 of the accompanying drawings.