GB2230578A - Hydraulic brake system - Google Patents

Description

J.Burgdorf 165-21-21 1 HYDRAULIC BRAKE SYSTEM The present invention

relates to hydraulic brake systems and, in particular, but not exclusively to hydraulic brake systems of the type incorporating brake slip control and/or traction slip control.

A hydraulic brake system is usually composed of a master brake cylinder hydraulically coupled via brake lines to wheel brake cylinders, the cylinders and the brake lines being filled with brake fluid. In such systems, the brake circuits are arranged, when the master brake cylinder is not being actuated, to be connected to a supply reservoir for the following reasons.

Firstly, provisions must be made to ensure that the brake circuits are always filled completely with pressure fluid since otherwise air bubbles will be generated in the brake circuits on operation of the brake system, which air bubbles due to their compressibility prevent a direct transmission of the pedal force onto the wheel brakes. Furthermore, heat expansion of the brake fluid must be accounted for. If the brake systems were closed-loop systems, the expansion of the brake fluid would result in application of the wheel brakes, whereby the vehicle would be decelerated in an undesirable way.

Formerly, two ways have been chosen to realise the connection of the brake circuits to the supply reservoir when the brake is not operated. The most simple solution resides in filling the brake system through a so-termed 2 breather bore (transverse bore in the master brake cylinder housing) which is in communication with the supply reservoir and terminates directly in the working chamber of the master brake cylinder in front of the working pistons of the master brake cylinder. Upon operation of the brake, the working pistons are displaced and, in doing so, override their respectively assigned breather bore, a closed brake-circuit system resulting on brake actuation. When the driver removes his foot from the brake pedal, the working pistons are reset by resetting springs into their initial positions so that the connection between the working chamber and the supply reservoir is re- established via the breather bore. As long as a brake system of the above type is not used for controlling brake slip, refilling of the brake circuits by way of breather bores entails no problems at all.

In anti-lock hydraulic brake systems, pressure fluid from an auxiliaryenergy source is introduced into the brake circuits for regulating the. wheel braking pressure. As a consequence, a residual pressure prevails in the brake circuits and thus in the master brake cylinder when the working pistons of the master brake cylinder release the breather bores. In view of the pressure in the brake circuits, sealing cups of the working pistons are pressed into the breather bores, causing damage to the sealing cups. Therefore, it has also been proposed to replace the breather bores by sotermed central valves in anti-lock hydraulic brake systems. These valves which are arranged in the working pistons of the master cylinder normally consist of a valve ball which, in the initial position of the working pistons, is kept at a distance from a valve seat by way of a tappet abutting on a pin formed fast with the cylinder housing. When the brake is applied, the working pistons are displaced and the valve balls can move to sit on their valve seat, as a result of which the brake circuits are hydraulically isolated. Although this measure appears quite simple, considerable problems are encountered in practice. Furthermore, the central valves are much more expensive to 1 3 produce than the breather bores, and assembly of the central valve parts is difficult.

Moreovert both arrangements result in a master brake cylinder of considerably longer length, since the working pistons must have at least once more the length of the working chambers.

Another disadvantage resides in that both systems exhibit lost travel due to their construction. Before it is possible to build up a pressure in the master brake cylinder, first the breather bores must be overridden, or the central valves must be closed, respectively. This lost travel implies that, during a braking operation, no braking effect is accomplished in the first phase of the brake pedal depression.

is Furthermore, the working pistons must be adjusted precisely when the master brake cylinder is fitted to a vehicle in order to minimise the lost travel. The situation becomes particularly complicated when the master brake cylinder is operated by a booster. The booster piston, too, must be precisely adjusted so that the initial positions of working piston and booster piston conform accurately to each other.

Therefore, this invention has for its object the provision of a straightforward re-fill system which, in addition thereto, permits the mode of construction of the master brake cylinders to be simplified. Furthermore, the master brake cylinder shall have an overall length which is as short as possible and lends itself to ease of assembly.

Accordingly, there is provided a hydraulic brake system, in particular for brake slip control and/or traction slip control operations, comprising a pedal-operated master brake cylinder and wheel brake cylinders connected thereto, as well as comprising a pressure fluid supply reservoir, a pressure fluid conduit between the working chambers of the master brake cylinder and the pressure fluid supply reservoir existing in an initial position of the pistons of the master brake cylinder, which conduit includes at least 4 one electromagnetically actuated valve, characterised in that a sensor for pedal actuation is provided, and in that in the non-applied position of the pedal the electromagnetically operated valve is switched at least temporarily into a position opening the pressure fluid conduit.

The principles embodied in the present invention can be characterised as follows. While in the known state of the art the hydraulic system is an open system in the absence of any brake actuation and is not closed until actuation of the master brake cylinder, the present invention discloses a system which is closed even when the brake is not applied. Hence the above-mentioned drawbacks no longer exist. For instance, the system no longer exhibits lost travel. There is merely an electromagnetically controllable pressure fluid conduit which is opened for a short time only when precisely defined criteria apply. It may, for example, be provided that the pressure fluid conduit is opened after every braking operation in order to decrease the pedal pressure in the system. Furthermore, the inventive system affords the added advantage that the brake system, which is usually closed, can be monitored more easily for leakage.

The pressure fluid conduit with the solenoid valve can terminate in a favourable manner into the mastercylinder working chamber at a certain distance from the initial or rest position of the working piston. This eliminates adaptation problems with the pedal assembly and with a booster which is possibly inserted in front of the master cylinder. Pedal assembly and booster also comprise stops which define the initial positions of the pedal and of the booster pistons. The latter positions have to be conformed to the basic position of the working pistons so that the breather bore terminates into the working chamber or the central valve is opened, respectively, when the pedal is not depressed. These problems are not manifest in the system proposed by the present invention.

In principle, it is also possible to employ the inventive arrangement in conventional brake systems, that means those without slip control. The additional costs for the solenoid valves have to be contrasted with the cost for 5 a central valve or a breather bore, respectively.

Therefore, the present invention permits being used particularly expediently in slip-controlled brake systems, since these systems already comprise solenoid valves which, by virtue of a changed actuation, can be used for the replenishment of the brake circuits with pressure fluid.

German published patent application 36 35 846 discloses a brake system wherein the working chamber of the master brake cylinder communicates with the supply reservoir via a central valve. one solenoid valve, each of which is open in its de-energised state, is arranged in the connecting line between the working chamber o the master brake cylinder and the supply reservoir, namely between supply reservoir and central valve. To control traction slip, pressure fluid is supplied into the brake lines by way of a pump so that pressure can be built up in the wheel brakes irrespective of brake pedal depression. However, the fluid return flow to the supply reservoir must be prevented. This is achieved in that the mentioned solenoid valves between the supply reservoir and the central valves will be closed during a traction slip control operation.

When employing the inventive idea and when further developing the brake system disclosed in the above-mentioned published patent application, the mentioned solenoid valves can be made use of as replenishment valves. In a favourable manner, these valves are closed as long as the solenoid valves are de- energised. Therefore, the brake circuits are normally shut off hydraulically, thereby allowing pressure to develop on brake application. The valves will then be opened for a short time according to predetermined criteria so that no pressure is allowed to develop in the brake circuits when the pedal is not applied.

in the event of the brake system having this form.

6 the pressure fluid conduit between the master brake cylinder and the supply reservoir can be a housing bore in the master cylinder housing. The pump line for replenishment of pressure fluid during a slip control operation terminates into this housing bore.

it is also possible to use the solenoid valve that is closed in its deenergised state as a replenishment valve in the connection of the wheel brake to the supply reservoir. The expenditure in valves is thus still minimised. The pressure-fluid connection is then established via the brake line and a relief line.

The signal to open the replenishment valve can be generated in various ways. The basic precondition is that the pedal is in its initial position. This can be determined easily by means of a pedal switch which is coupled to the pedal. It can be ensured thereby that the replenishment valve is opened for a short time at least at the end of a braking operation.

In addition, the signals of a pressure sensor or a temperature probe can be taken into account as further criteria. When pressure develops in the master brake cylinder when the pedal is not depressed, the replenishment valve will be opened for a short time. The valve may also be opened in response to a rise in temperature detected by a suitable temperature sensor. It is also possible to detect e.g. grinding noises of brake pads on the brake disc or on the brake drum and to open the replenishment valve whenever it can be judged from the noises detected that the brake is applied in an undesired fashion.

To compensate for pressure fluid losses due to minor leakages, an alarm switch may be arranged which serves to initiate the pump supply when the brake pedal is depressed beyond a threshold.

Another switch on the brake pedal is used for controlling the pressure fluid supply of the pump during a slip control action. The pressure fluid supply is controlled such that the pedal is moved to assume a 7 predefined position during a slip control action. The quantity of pressure fluid which is delivered into the brake lines can be controlled by switching the pump drive on and off or by the use of short-duration pulses opening the 5 solenoid valves in the supply line.

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

Figure 1 shows a schematic representation of one embodiment of a hydraulic brake system in accordance with the present invention; Figure 2 shows a cross sectional view of a brake master cylinder for use in the present invention; and Figure 3 shows a schematic representation of a further embodiment of a hydraulic brake system according to the present invention.

A brake system comprises a master brake cylinder 1, in front of which a vacuum booster 2 is inserted. Two working pistons, a floating piston 5 and a push rod 6 are sealingly guided in a bore 3 of a housing 4 of the master brake cylinder 1. The push rod piston 6 is exposed to the direct action of the boosted pedal force.

By virtue of the pistons 5 and 6, two working chambers 7 and 8 are formed in the master cylinder housing 4 with the floating piston 5 exposed to the hydraulic pressure in the working chamber 8. The working chambers 7 and 8 communicate via brake lines 9, 10 with wheel brakes 11. An inlet valve 12 is inserted into each of the brake lines 9,10 and/or into branch lines to the wheel brakes 11.

The inlet valves 12 are controlled electromagnetically, and are arranged to adopt an open position when the magnet coils are not energised.

Additionally, the wheel brakes 11 are in communication with a supply reservoir 14 via a relief line 13 which includes a branch line for each of the wheel brakes. Connected into each of the branch lines of the relief line 13 to the wheel brakes 11 is an outlet valve 15 8 which is controlled electromagnetically and which assumes its closed position when the coils are not energised.

The brake system is also furnished with two pumps 16,17 which are driven by a joint electric motor M. The pumps supply fluid out of the supply reservoir 14 into the working chambers 7 and 8 of the master brake cylinder 1. According to the embodiment of Figure 3, the pressure lines 18,19 of the pumps terminate into the brake lines 9 and 10, and that is to say directly below the master brake cylinder.

In the embodiment of Figure 1, there is a direct connection between the working chambers 7,8 and the supply reservoir 14 via supply lines 20,21. A replenishment valve 22 is inserted into each of the supply lines 20,21. These replenishment valves 22 are controlled electromagnetically and are arranged to assume a closed position as long as the magnet coils are de-energised. The pressure lines of the pumps 16,17 terminate into the supply lines 20,21, namely between the working chamber 7 and/or 8 and the replenishment valves 22.

Figure 2 shows a cross-sectional view of a possible embodiment of a brake cylinder including part of the hydraulic circuit according to Figure 1. The supply lines 20,21 of Figure 1 are illustrated in Figure 2 as channels or housing bores in the master cylinder housing 4 which extend from the working chambers 7,8 to connecting sockets or ports 23,24 for coupling to the supply reservoir 14.

A side view of the brake unit is shown in the right-hand part of Figure 2. A receiving bore 25 for a replenishment valve 22 can be seen. The pressure port 26 for coupling to the respective pump 16 or 17 is placed on the opposite side. The associated non-return valve is integrated in the port 26.

Important safety functions are illustrated by three switches AB,C which detect the pedal position. These switches are shown symbolically and are actuated directly by the pedal in Figure 3. The switch operation can of course also be performed in an equivalent manner by any other part Lk 9 of the system, as long as the switch operation is coupled with the pedal position in a definite way. For example, switch operation may also be caused by the push rod piston 6 or the movable wall (not shown) of the booster.

It is the function of switch A to detect whether the pedal is in its initial or rest position. Switch B is required to regulate the rate of delivery of the pump and to thereby bring about positioning of the pedal. Fluid losses due to leakage are to be detected by virtue of switch C.

The operation of the various valves is controlled by suitable evaluating electronics (not shown).

The mode of operation of the brake system shall be described hereinbelow. The driver operates the brake by depressing the (symbolically illustrated) pedal. In consequence thereof, the push rod piston 6 and, consequently, also the floating position 5 are displaced. Pressure fluid is forced out of the working chambers 7 and 8 to the wheel brakes 11, whereby the brakes are operated and the vehicle is decelerated. Compared to systems known from the state of the art, there is the advantage that the pressure buildup is effected without lost travel since the brake circuits are closed hydraulically from the very start of a brake operation. Lost travel is incurred in prior art systems since, initially, the breather bore must be overridden or the central valve must be closed.

When the sensors monitoring the rotational behaviour of the wheels now detect an imminent locked condition at any one of the wheels, the system switches into the anti-lock mode. This implies that the pump drive M is switched on, and the pumps 16,17 deliver fluid into the working chambers 7 and 8. Switching of the inlet and outlet valves 12 and 15 allows the pressure in the wheel brakes to be decreased or increased depending on the rotational behaviour of the corresponding wheel so that locking of the wheel is prevented and, simultaneously, an optimal slip value is adjusted. This is a known method so that no further explanations need be given herein.

The regulation of the pump pressure is now performed by means of switch B. Pressure fluid is permanently removed from the working chambers 7,8 of the master brake cylinder 1, by the brake slip control operation. The result thereof is that the push rod piston 6 is increasingly shifted into the master cylinder, the brake pedal following this movement. The pedal depresses through its full travel. When the pedal reaches a position which is characterised by switch B, then pump drive M is switched on. In general, the flow rate of the pumps is in excess of the pumping quantity discharged per time unit out of the working chambers due to the control operation. This causes resetting of the pistons 5 and 6 to their initial rest positions. The brake pedal is thus reset, whereby switch B re-assumes its initial position and the pump drive is switched off. In this manner, pressure proportional to the pedal force prevails in the brake circuits and the pedal is fixed in a specific position which is predefined by switch B. Regulation of the discharge flow of the pump can be attained not only by switching on and off of the pump drive but also by a hydraulic shortcircuiting of the pump.

This is done by virtue of the replenishment valves 22 which cause a short-term coupling of the pressure side and suction side of the pumps 16,17.

The brake system of the present invention can also be used for traction slip control. In a traction slip control operation, braking pressure is applied to the driven wheels so that an excessive drive torque is reduced to such extent that the remaining torque can be transmitted by the friction forces between tyre and road. In this way, spinning of the wheels in the initial phase of driving can be prevented. Pressure fluid must be introduced into the wheel brakes for traction slip control to take effect. This pressure fluid is supplied by the pump and delivered into the brake circuits. In the state of the art, there is a pressure fluid connection between the working chambers of the master cylinder and the supply reservoir when the pedal 1 11 is not applied. Firstly, this connection must be closed. Since, according to the suggestion of th e present invention, the working chambers are shut off hydraulically from the inception of a braking operation, this step is deleted so that an immediate pressurisation of the wheel brakes of the driven wheels can be performed.

Since the inventive suggestion does not provide for a permanent connection between the supply reservoir and the working chambers of the master brake cylinder when the brake is not applied, there is need for a limited number of additional switches. The most important switch is switch A which responds as soon as the pedal is applied.

As soon as a braking operation is completed and switch A is reset, a short-term connection will be established between the working chambers 7,8 of the master brake cylinder and the supply reservoir 14. It is thereby possible to discharge the residual pressure which is possibly prevailing in the wheel brakes. This is effected in that either the valves 22 according to Figure 1 or the valves 15, closed in their de-energised state, according to Figure 3 are energised for a short period of time. There is a connection via the supply lines 20,21 in thefirst- mentioned case, and via the brake lines 9,10 and the relief line 13 in the second-mentioned case.

Heat expansion of the brake fluid when the brake is not applied can be detected either by way of a pressure switch not illustrated or by a temperature probe (also not shown). It would also be possible to detect grinding noises due to application of the brake pads, caused by heat expansion of the brake fluid. As soon as one of the sensors responds, there is a short-term operation of the valves 22 and 15, respectively. In this way it can be ensured that the inventive system remains unpressurised as long as the brake pedal is not applied or as long as a traction slip control action is not to take place.

The permanent pressure fluid connection of the brake system of the invention when the brake is not applied 12 can also be used to compensate for pressure fluid losses caused by minor leakages. Switch C is provided to permit realisation of this goal in the inventive system. When pressure fluid losses occur the brake pedal travel will gradually be extended so that switch C responds when a defined pressure fluid loss is reached. When such a response occurs, the pumps 16,17 start to operate, and pressure fluid is supplied into the working chambers 7,8.

The essential idea of the present invention can be formulated as follows: elimination of the breather bores and/or the central valve permits a considerably shorter design of the working pistons of the master brake cylinder so that the master brake cylinder as a whole becomes shorter. A shortened pedal travel is accomplished at the same time, since the lost travel entailed in known systems when overriding the breather bores or closing the central valves are avoided. The functions performed by the breather bores and by the central valve can be substituted by a pedal-travel monitoring means, a solenoid valve being operated under certain conditions which is arranged in any one of the connecting lines between the master bake cylinder and the supply reservoir.

13

Claims (15)

CLAIMS:

1. A hydraulic brake system, in particular for brake slip control and/or traction slip control operations, comprising a pedal-operated master brake cylinder (1) and wheel brake cylinders (11) connected thereto, as well as comprising a pressure fluid supply reservoir (14), a pressure fluid conduit between the working chambers (7,8) of the master brake cylinder (1) and the pressure fluid supply reservoir (14) existing in an initial position of the pistons (5,6) of the master brake cylinder (1), which conduit includes at least one electromagnetically actuated valve (22,12,15), characterised in that a sensor (A) for pedal actuation is provided, and in that in the non- applied position of the pedal the electromagnetically operated valve (22.12,15) is switched at least temporarily into a position opening the pressure fluid conduit.

2. A brake system as claimed in claim 1r characterised in that the pressure fluid conduit (20,21,9,13) terminates into the working chamber (7,8) at a port arranged at a distance from the initial position of the working piston (5,6) of the master brake cylinder (1) so that the port is always open irrespective of the position of the working piston (5,6).

3. A brake system as claimed in claim 1 or claim 2, characterised in that the electromagnetically operated valve (22,12,15) is a two-way/two-position directional control valve.

4. A brake system as claimed in any one of claims 1 to 3, characterised by a pressure sensor for sensing pressure in the master bake cylinder (1), and in that the electromagnetically operated valve (22,12,15) is open when the pedal is not applied and when pressure is sensed by the pressure sensor.

5. A brake system as claimed in any one of the preceding claims, characterised by a temperature sensor for sensing the temperature of the brake fluid.

6. A brake system as claimed in any one of the 14 preceding claims, characterised by a noise sensor for detecting grinding noise of brake pads on a brake disc.

7. A brake system as claimed in any one of the preceding claims, characterised in that the pressure fluid conduit comprises a brake line (9,10) in communication with the wheel brake cylinders and a supply line (20,21) in communication with the pressure fluid supply reservoir, and in that the electromagnetically operated valve comprises a replenishment valve (22) located in the supply line (20,21).

8. A brake system as claimed in claim 7, characterised in that the replenishment valve (22) is closed during a traction slip control operation.

9. A brake system as claimed in claim 7, characterised in that the supply line (20,21) is a housing bore in the master cylinder housing.

10. A brake system as claimed in claim 9, characterised by a pump pressure line (18,19) terminating via a non-return valve into the housing bore.

11. A brake system as claimed in claim 10, characterised in that the master cylinder housing includes bores (25) for receiving the replenishment valve (22).

12. A brake system as claimed in any one of the preceding claims, comprising a solenoid valve (12) open in its de-energised state in the pressure fluid conduit between the master brake cylinder (1) and the wheel brakes (11) and a valve (15) closed in its de-energised state in a relief line (13) between the wheel brake (11) and the supply reservoir (14), characterised in that the valve (15) closed in its de-energised state is initiated by evaluating electronics to perform as a replenishment valve.

13. A brake system as claimed in any one of the preceding claims, characterised by a further sensor for detecting pedal actuation beyond a nominal value, and in that braking pressure fluid is supplied to the pressure fluid conduit in the event that pedal actuation beyond the nominal value is sensed by the further sensor.

14. A brake system as claimed in any one of the k is preceding claims, characterised in that the supply reservoir (14) is arranged at a location separate from the master brake cylinder.

15. A brake system substantially as hereinbefore described with reference to Figure 1, Figures 1 and 2, or Figure 3 of the accompanying drawings.

Published 1990 at The Patent 0Mce. State Rouse.5671 High Holborn. Undon WCIR 4TP. PurLher copies may be obtainedfrom The PatentWice IWs Branch, St Mary Cray. Orpington. Xent BP.5 $RD. Printed by Multiplex techniques ltd. St MLry Cray. Xent. Con. 1187