GB2218766A

GB2218766A - Brake system with anti-skid control for a motor vehicle

Info

Publication number: GB2218766A
Application number: GB8911616A
Authority: GB
Inventors: Karl-Heinz Willmann
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1988-05-20
Filing date: 1989-05-19
Publication date: 1989-11-22
Also published as: GB8911616D0; FR2631596A1; DE3817179A1; JPH0218147A

Description

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DESCRIPTION

BRAKE SYSTEM WITH ANTI-SKID CONTROL FOR A M01OR VEHICLE The invention relates to a pressure-medium-operated brake system, for example a hydraulic or pneumatic brake system, with anti-skid control for a motor vehicle.

Pressure modulations occurring during anti-skid control in a brake force booster, operated as a pressure modulator, react upon a brake pedal through a connection between a booster piston and the brake pedal, and are perceptible to the driver operating the brake pedal. In order to reduce this effect on the brake pedal, an additional modulator, independent of the brake force booster/pressure modulator, is provided for brake pressure modulation. In the case of an average spread of modulation, that is, the value of the pressure changes, pressure modulation is effected exclusively by the additional modulator, while the booster piston is retained in its instantaneous axial position in such a way that it is not displaceable in a direction to build up pressure. On the other hand, in the case of a large modulation spread, the brake force booster also becomes effective as a pressure modulator for the brake pressure.

In a known brake system (German Offenlegungsschrift No. 36 12 793), the booster piston is connected to a master brake piston by way of a thrust rod which in turn abuts axially and only loosely against the booster piston. The additional modulator is integrated into the brake force booster. For this purpose, a modulation piston is rigidly connected to the thrust rod and subdivides a booster cylinder return or counter-chamber., which is located at the end of the booster piston remote from a working or pressure chamber, into two chamber portions of which the chamber portion facing the booster piston forms a modulation chamber of the additional modulator and is connected to a valve arrangement. In order to carry out pressure modulations of average modulation spread, the modulation chamber is relieved of pressure, or pressure is admitted thereto, by corresponding control of the valve arrangement, whereby the modulation piston relieves or loads the master brake piston by way of the thrust rod. The booster piston is essentially decoupled from the movement of the thrust rod and retains the axial displacement position assumed before commencement of the anti-skid control. In another known brake system, (German Offenlegungsschrift No. 34 44 828), the additional modulator is accommodated in the master brake cylinder. In this case, the modulation piston is formed by a second booster piston which serves for brake force %L L boosting during braking operation, and which defines the modulation chamber connected to the valve arrangement. The booster piston forms, together with a pump piston, a piston combination which is mounted on the master brake piston so as to be axially displaceable. An end annular surface of the pump piston, together with the master brake cylinder, defines the brake chamber. During its displacement in a direction to build up pressure, the pump piston can carry along the master brake cylinder by way of a positive drive means. In the case of pressure modulation with average modulation spread, the master brake piston is locked by way of the booster piston of the brake force booster which is hydraulically fixed in its instantaneous axial displacement position, while the piston combination, comprising the second booster piston and pump piston, is axially displaced on the master brake piston due to the activation of the valve arrangement, and thus effects a reduction or increase in brake pressure in the brake chamber.

In accordance with the present invention a brake system with anti-skid control for a motor vehicle is provided, having a master brake cylinder in which at least one master brake piston is axially displaceably guided to introduce a brake pressure into at least one brake circuit, a brake force booster which is operable as a pressure modulator for the anti-skid control and which comprises a booster cylinder'and a booster piston which is displaceable therein and which is controllable by means of a brake valve by actuation of a brake pedal and which acts mechanically upon the master brake cylinder, and an additional modulator, independent of the pressure modulator, for brake pressure modulation, which additional modulator has a modulation piston and a modulation chamber which is defined thereby and is connectible alternately to a pressure source or a relief point by way of a valve arrangement, wherein the additional modulator is integrated in the master brake cylinder in such a manner that the master brake piston directly forms the modulation piston, and a surface of the master brake piston defines a portion of the master brake cylinder which forms the modulation chamber, and wherein the master brake piston and the booster piston are in free transmission relationship with one another in the direction of displacement.

This has the advantage of a technically simpler and less expensive design of the additional modulator. A separate modulation piston is omitted, as well as the two-part construction of the booster piston and thrust rod in the booster cylinder. A relatively expensive construction of pistons which are disposed concentrically of one another and which are displaceab 1 e Ga k.

4.

relative to one another, and the sealing problems thereby occurring, are avoided. Compared with a master brake piston for a brake system operating without an additional modulator, only the master brake piston has to be constructed in such a way that pressure can act upon its rear side remote from the brake chamber.

The invention will be described further hereinafter, way of example only, with reference to and as illustrated in the accompanying drawing, which is a diagrammatic illustration of one embodiment of a hydraulic brake system with anti-skid control accordance with the present invention.

A hydraulic brake system, is supplied from a pressure accumulator 10 which is stressed to a predetermined pressure by means of a hydraulic pump 12 driven by an electric motor 11. Alternatively, the drive moment for the hydraulic pump 12 may be derived from a drive, already provided in the vehicle, such as the vehicle engine, and transmitted by way of a transmission such as a V-belt drive. The pressure medium, brake fluid in the present case, is drawn by the hydraulic pump 12 from a reservoir 13 into which the pressure medium flows back by way of various return lines. The brake system has two brake circuits I and II which are separated from one another and which are fed from separate brake chambers 14, 15 respectively of a master - 6 brake cylinder 16. The brake circuit I supplies brake pressure to the right hand front wheel VR and to the left hand rear wheel HL, while the left hand front wheel VL and the right hand rear wheel HR are fed from the brake circuit II. Brake pressure holding valves 17 to 20 are connected in a known manner into the brake lines. leading to the wheel brake cylinders of the front and rear wheels.

Two axially displaceable master brake pistons 21, 22 with integrated charging valves 23, 24 are disposed one behind the other in the master brake cylinder 16. Each master brake piston 21, 22 defines a respective brake chamber 14 or 15 which can be filled with pressure medium by way of the respective, associated filling valve 23 or 24 from a filling reservoir 25 connected to the reservoir 13. After the master brake pistons 21, 22 have covered a predetermined amount of displacement travel, the charging valves 23, 24 are closed and communication with the reservoir 25 is shut off. Each master brake piston 21, 22 is loaded by a respective one of return springs 26 and 27 which abut against a respective stop 28 or 29 fixed relative to the housing and seek to return the associated master brake piston 21 or 22 into its normal position. While the second master brake piston 22 is applied by the return spring 27 11 1 11 1 1 7 against the fixed stop 28 of the return spring 26 loading the first master brake piston 21, and defines the first brake chamber 14 by its end face 211, the first master brake cylinder 21 is applied by its return spring 26 against an axial push rod 30 extending into the end of the master brake cylinder 16, wherein an end face 211 of the master brake piston 21 and the end wall 161 of the master brake cylinder 16 define a cylinder portion which forms a modulation chamber 31 of an additional modulator 32 for brake pressure modulation during anti-skid control. The modulation chamber 31 is connected to the reservoir 13 by way of a first switching valve 33, and may be connected to the pressure accumulator 10 by way of a second switching valve 34. The two switching valves 33, 34 are in the form of twoport, two-position solenoid valves, the first switching valve 33 being open and the second switching valve 34 being closed when in their non-energised normal positions.

The push rod 30, guided through the end wall 161 of the master brake cylinder 16 in a fluid-tight and axially movable manner, is rigidly connected to a booster piston 35 of a brake force booster 36 which is assembled with the master brake cylinder 16. The booster piston 35 is axially displaceably guided in a booster cylinder 37 which is axially contiguous to the master brake cylinder 16. When in its normal position phown in the drawing, the booster piston 35 is applied against a fixed stop 38 by the master brake piston 21 acting upon the push rod 30. The right hand end 351, viewed in the drawing, of the booster piston 35 defines a pressure or working chamber 39, and its left hand end wall 352, as viewed in the drawing, defines a counteror return chamber 40. A brake valve 41 is integrated in the booster piston 35 and is controlled mechanically by a brake pedal 42. The brake valve 41 connects the working chamber 39 alternately to the reservoir 13 and to the pressure accumulator 10. In a known manner, the brake valve 41 has a control spool 44 with various control ports which is connected to the brake pedal 42 by way of a pedal rod 43, and which is loaded by a weak return spring 45 against a displacement movement of the brake pedal 42. The construction and mode of operation of the brake valve 41 are known and are described in detail in, for example, German Offenlegungsschrift 34 44 828.

The brake force booster 36 serves as a pressure modulator in the case of anti-skid control, for which purpose its return chamber 40 may be connected to either the reservoir 13 or to an outlet 52 of the working chamber 39, or shut off, by way of a valve configuration i 1 S 1 -1 - 9 comprising three switching valves 46, 47 and 48. Each.switching valve 46 to 48 is a two-port, two-position solenoid valve. The two switching valves 46 and 47 are open when in their non-energised normal positions, wherein a throttle is provided in the switching valve 46, while the switching valve 48 is closed when in its normal position. It may also be mentioned that one working port of the switching valve 34 is not directly connected to the pressure accumulator 10, but is connected directly to the outlet 52 of the working chamber 39 of the brake force booster 36, and directly to the pressure accumulator 10 by way of the working chamber 39.

The mode of operation of the brake system described above is as follows:

When the brake pedal 42 is in its normal position, the return spring 45 maintains the control spool 44 in its normal position. The working chamber 39 of the brake force booster 36 is connected to a return line (shown by broken lines in the drawing), opening into the reservoir 13, by way of the control ports in the control spool 44 and by way of an outlet bore 49 in the booster cylinder 37. Actuation of the brake pedal 42 first displaces the control spool 44 by way of the pedal rod 43 in such a way that communication between the working chamber 39 and the outlet bore 49 is shut off, and communication is established between the working chamber 39 and an inlet 50 which is connected to the pressure accumulator 10 by way of an inlet line. Pressure medium then flows out of the pressure accumulator 10 into the working chamber 39. A portion of the pressure medium flows by way of a non-return valve 51 to the outlet 52 of the working chamber 39, to which is connected the switching valve 34 which is in its closed position. The booster piston 35 is displaced to the left, as viewed in the drawing, by the pressure building up in the working chamber 39, wherein the push rod 30 of the booster piston 35 displaces the master brake piston 21 in the same manner. The booster piston 35 is displaced for as long as the working chamber 39 remains connected to the pressure accumulator 10. It is stopped when the control spool 44 interrupts this connection by corresponding displacement relative to the booster piston 35.

The charging valve 23 in the master brake piston 21 is closed after a small amount of displacement travel has been covered. The brake pressure building up in the brake chamber 14 displaces the second master brake piston 22 in the same direction, so that the charging valve 24 also closes and a brake pressure is also built up in the second brake chamber 15. The two brake pressures are introduced into the two brake circuits I 1 D i 1 c 1 - 1 1 - and II, and hence into the wheel brake cylinders of the wheels.

If the brake pedal 42 is actuated too violently after a braking operation has been initiated, so that the wheel brakes are applied too violently with the result of undesired wheel slip, the anti-skid control becomes effective. Switching signals for the valve, arrangement comprising the valves 33, 34 and 46 to 48 are generated as a result of the wheel slip detected by wheel sensors (not illustrated). All the switching valves are switched over by the first switching signal. A pressure is built up in the return chamber 40 and displacesthe booster piston 35 slightly to the right as viewed in the drawing. A pressure is built up in the same way in the modulation chamber 31 by way of the switching valve 34 and allows only slight displacement of the master brake piston 21 in the same direction, that is, towards the right as viewed in the drawing. Thus, the positive coupling between the booster piston 35 and the master brake piston 21 is cancelled. Changeover of the switching valve 48 shuts off the return chamber 40, so that the booster piston 35 is locked in its instantaneous displaced position by the same pressure in the working chamber 39 and in the return chamber 40. The wheel slip is compensated by pressure modulations which are triggered by simultaneous 12 switching of the switching valves 33 and 34 in the modulation chamber 31. The pressure in the modulation chamber 31 may thereby be reduced and increased in a pulsed manner with the booster piston 35 and the brake pedal 42 stationary, so that the brake pressure introduced by the brake pedal 42 finally performs slipless deceleration.

In this anti-skid control, only the wheel brake cylinders which are instantaneously connected to the brake chambers 14, 15 by the brake pressure holding valves 17 to 20 are those whose associated wheels exhibit wheel slip. In the case of anti-skid control, the other wheel brake cylinders are isolated from the brake chambers 14, 15 by the brake pressure holding valves 17 to 20 switched over to their pressure-holding positions.

If the reductions in brake pressure obtainable by the additional modulator 32 are inadequate, the brake force booster 36 is used as a further pressure modulator. In this case, the switching valve 48 switches into its open position again, whereby an increasing restoring pressure is built up in the return chamber 40 of the booster piston 35 and displaces the booster piston 35 to smaller brake forces, that is, to the right as viewed in the drawing. The push rod 30 is 1 1 7 t 13 withdrawn in the same manner and thereby enables larger displacement of the master brake pistons 21, 22 in a direction to reduce brake pressure.

Claims

14 CLAIMS

A brake system with anti-skid control for a [notor vehicle, having a master brake cylinder in which at least one master brake piston is axially displaceably guided to introduce a brake pressure into at least one brake circuit, a brake force booster which is operable as a pressure modulator for the anti-skid control and which comprises a booster cylinder and a booster piston which is displaceable therein and which is controllable by means of a brake valve by actuation of a brake pedal and which acts mechanically upon the master brake cylinder, and an additional modulator, independent of the pressure modulator, for brake pressure modulation, which additional modulator has a modulation piston and a modulation chamber which is defined thereby and is connectible alternately to a pressure source or a relief point by way of a valve arrangement, wherein the additional modulator is integrated in the master brake cylinder in such a manner that the master brake piston directly forms the modulation piston, and a surface of the master brake piston defines a portion of the master brake cylinder which forms the modulation chamber, and wherein the master brake piston and the booster piston are in free transmission relationship with one another in the direction of displacement.

2. A brake system as claimed in claim 1, wherein -W t the valve arrangement comprises two switching valves, the first of which switching valves is closed when in its energised working position, and the second of which switching valves is closed when in its non-energised normal position, and that a port of each of the switching valves is connectible to that portion of the master brake cylinder which forms the modulation chamber, and that another port of the first switching valve is connectible to a relief point, and that another port of the second switching valve is connectible to a pressure source.

3. A brake system as claimed in claim 1 or 2, wherein the booster piston has a push rod extending axially into the master brake cylinder, and wherein the master brake piston abuts against one end of the push rod under the action of a spring.

4. A brake system as claimed in any one of claims 1 to 3, wherein the booster piston sub-divides the booster cylinder into a working chamber controlled by the brake valve and a return chamber which, by means of a valve configuration, is selectively connectible to the working chamber or a relief point, or may be shut off.

5. A brake system as claimed in claim 4, wherein the brake valve is integrated in the booster piston and is connectible to the brake pedal for selectively - 16 connecting the working chamber to the pressure source or to the relief point.

6. A brake system as claimed in any one of claims 2 to 5, wherein the second switching valve is connectible to the pressure source by way of the working chamber of the brake force booster.

7. A brake system with anti-skid control for a motor vehicle substantially as herein described with reference to and as illustrated in the accompanying drawing.

Published 1989 at The Patent Office, State House, e6,'71 High Holborn, London WClR- 4TP Further copies maybe obtained from The Patent Office. Sales Branch, St Maz7 Cray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd, St Mary Cray, Kent, CoiL 1/87