GB2245323A

GB2245323A - Vehicle braking system

Info

Publication number: GB2245323A
Application number: GB9112909A
Authority: GB
Inventors: Eiji Yagi
Original assignee: Nissan Motor Co Ltd
Current assignee: Nissan Motor Co Ltd
Priority date: 1990-06-14
Filing date: 1991-06-14
Publication date: 1992-01-02
Also published as: DE4118461A1; GB9112909D0; JPH0446853A

Abstract

A vehicle braking system adapted to control brake forces at each of the vehicle wheels independently has a solenoid- operated shutoff valve (6) between a master cylinder (2) and a brake line (5) which includes a chamber (7a) of a modulating cylinder (7). A solenoid-operated flow control valve (9) controls the pressure delivered from an auxiliary source (12) to a control chamber (7b) of the cylinder (7). A control unit (13) controls operation of the shutoff valve (6) and flow control valve (9) in accordance with the vehicle running conditions e.g. as detected by pressure, steering, yaw, linear and lateral vehicle acceleration, vehicle speed and wheel speed sensors (15-22). The valve (9) controls the cylinder (7) to increase or decrease the brake pressure as required with respect to the master cylinder pressure and the reaction on the pedal is sudden, smooth or absent depending on whether the shutoff valve (6) is open, cycled or closed, respectively. The system may be used to effect differential front/rear or left/ right brake control. <IMAGE>

Description

1 VEHICLE BRAKING SYSTEM

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to vehicle braking systems.

2. Description of the Prior Art

Prior art vehicle braking systems provided with means for controlling brake forces for respective vehicle wheels independently are disclosed in Japanese Patent Provisional Publications Nos. 62-149542 and 62 149543.

The braking system disclosed in Japanese Patent Provisional Publication No. 62-149542 comprises a shutoff valve between a master cylinder and a wheel cylinder, and a boost pressure transmitter having inlet and outlet fluid chambers divided by a movable piston. The inlet fluid chamber can communicate with a second fluid pressure source by way of a control valve, and the outlet fluid chamber communicates with the wheel cylinder.

The braking system disclosed in Japanese Patent Provisional Publication No. 62-149543 comprises a pressure intensifier for producing a boost pressure in response to an accumulator pressure and a master cylinder pressure, and a pressure modulator for is 2 producing a brake pressure which is the sum of the boost pressure and the master cylinder pressure.

A disadvantage of the former prior art braking system is that the brake pressure cannot be reduced beyond the master cylinder pressure since the wheel cylinder is cut off from the master Cylinder when the master cylinder pressure (caused by depression of the brake pedal) is increased beyond a predetermined value and then boosted by boost pressure transmitter.

A disadvantage of the latter prior art braking system is that operation of the brake pedal feels strange to the driver since the boosting of the brake pressure by means of the pressure modulator. etc. is accompanied by a sudden variation of the master cylinder pressure, i.e. a sudden variation of the brake pedal reaction force.

SUMMARY OF THE INVENTION

In accordance with an embodiment of the present invention which is described hereinafter, there is provided a vehicle braking system which comprises: manual brake operating means; a master cylinder for producing a master cylinder pressure in response to operation of the manual brake operating means; a wheel cylinder for producing a brake force in response to a brake pressure supplied thereto; first pressure passage means communicating with the master cylinder; 3 second pressure passage means communicating with the wheel cylinder, shutoff valve means between the first and second pressure passage means for selectively determining the fluid pressure communication via said first and second passage means between the master cylinder and the wheel cylinder; fluid pressure source means for producing a predetermined fluid pressure irrespective of the operation of the manual brake cperat-,ng means; fluid cylinder means for varlably controlling the fluid pressure in the second pressure passage means in response to the master cylinder pressure and a control pressure supplied thereto, the fluid cylinder means comprising a first fluid chamber constituting part of the second pressure passage means, a second fluid chamber communicable with the fluid pressure source means and receiving the control pressure, and a movable piston between the first and second fluid chambers; flow control valve means between the fluid pressure source means and the fluid cylinder means for variably controlling communication between the fluid pressure source means and the second fluid chamber of the fluid cylinder means and thereby variably controlling the control pressure in the second fluid chamber; and control means for controlling the operation of the shutoff valve means and the flow control valve means in accordance with 4 vehicle running conditions.

The above structure can solve the above noted problems inherent in the prior art systems, as will hereinafter be described. In particular, the system of the invention can attain a good "feel" in the operation of the brake Pedal, and can obtain a brake pressure which is variable over a wide range. The invention can furthermore reduce design restrictions, and is usejul and applicable in various vehicle control svstems.

The above and further features and aspects of the present invention are set forth with particularity in the appended claims and will become clear from consideration of the following detailed description of exemplary embodiments which is given with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a diagrammatic view of a vehicle braking system according to an exemplary embodiment of the present invention; Fig. 2 is an illustration of various modes of operation of the braking system of Fig. 1; Fig. 3 is an enlarged view of various variants of the portion "E" of Fig. 2; and Fig. 4 is a view similar to Fig. 1 but showing another exemplary embodiment of the present invention. DETAILED DESCRIPTICN OF THE EMBODIMENTS Referring first to Fig. 1, the illustrated vehicle braking system is adapted to attain an ideal distribution of brake forces between front and rear wheels or between left and right wheels.

The vehicle braking system consists of a master cylinder 2 for producing a master cylinder pressure in response to depression of a brake peda]'L I, a wheel cylinder 3 provided on each vehicle wheel (not shown) for applying a brake force to the vehicle wheel in response to a brake pressure, a shutoff valve 6 disposed between a master cylinder pressure passage 4 in communication with the master cylinder 2 and a brake pressure passage 5 in communication with the wheel cylinder 3, a fluid cylinder 7 having a first fluid chamber 7a forming part of the bra'- .e pressure passage 5 and a second fluid chamber 7b in communication with a hydraulic pump 11 through a control pressure passage 8 and a flow control valve 9, an accumulator 12 for producing a constant fluid pressure irrespective of braking operation, i.e. irrespective of depression of the brake pedal 1, and a control unit 13 f or controlling the operation of the shutoff valve 6 and of the flow control valve 9. The shutoff valve 6 is a normally open solenoid valve 6 operated by a solenoid 6a, and the f low control valve 9 is a solenoid valve operated by a solenoid 9a.

A respective shutof f valve 6, f luid cylinder 7 and flow control valve 9 are provided for each of the vehicle wheels, though omitted from the drawing for the sake of clarity.

The fluid cylinder 7 includes a first cylindrical fluid chamber 7a of smaller diameter, a second cylindrical fluLd chamber 7b of Larger diameter, a stepped piston 7c movable in the first and second fluid chambers 7a and 7b, and a pair of centering springs 7d and 7e installed in the respective first and second fluid chambers 7a and 7b for urging the piston 7c toward its central position.

The hydraulic pump 11 and the accumulator 12 communicate with each other by way of a check valve 14 which allows fluid flow in one direction only, namely from the hydraulic pump 11 to the accumulator 12.

The control unit 13 receives signals from a steering angle sensor 15, a yaw rate sensor 16, a lateral acceleration sensor 17, a longitudinal acceleration sensor 18, a vehicle speed sensor 19, a vehicle wheel speed sensor 20, a master cylinder pressure sensor 21 and a brake pressure sensor 22.

The control unit 13 is adapted to determine a target brake pressure on the basis of a predetermined target 7 brake pressure characteristic and the vehicle braking or running conditions as known from the signal from the above sensors, and to supply signals to the solenoid 6a of the shutoff valve 6 and the solenoid 9a of the flow control valve 9 so that the difference between the target brake pressure and the actual brake pressure is eliminated.

The operation of the system will be described in detail herei-below.

(A) Normal braking:

During normal braking, the control unit 13 produces a signal to cause the shutoff valve 6 to open through deenergization of the solenoid 6a and a signal to cause the f low control valve 9 to maintain a predetermined reference control pressure in the passage 8 and in the second f luid chamber 7b of the fluid cylinder 7 through control of the energization and deenergization of the solenoid 9a.

Accordingly, when the master cylinder 2 produces a master cylinder pressure in response to depression of the brake pedal 1, the master cylinder pressure is supplied through the pressure passages 4 and 5 to the wheel cylinder 3, thus subjecting the vehicle wheel to a brake force resulting from the master cylinder pressure (the flow I in Fig. 2).

(B) Boost of brake pressure:

a 8 Depending upon the kind of signal supplied from the control unit 13 to the shutoff valve 6, a plurality of boost modes are available in which the brake pressure is boosted beyond the master cylinder pressure, that is, a boost mode of a kind accompanied by a sudden variation of the brake pedal reaction characteristic (flow II-1 in Fig. 2), a boost mode of a kind accompanied by a smooth variation of the brake pedal reaction characteristic (flow 11-2), and a boost mode of a kind accompanied by substantially no variation of the brake pedal reaction characteristic (flow 11-3 in Fig. 2).

In the case of the boost mode which is accompanied by a sudden variation of the brake pedal reaction characteristic (the flow II-1), the control unit 13 produces a signal to cause the shutoff valve 6 to open through deenergization of the solenoid 6a and signal to cause the flow control valve 9 to maintain fluid pressure higher than the reference control pressure in the passage 8 and in the second f luid chamber 7b of the fluid cylinder 7 through control of energization and deenergization of the solenoid 9a.

Accordingly, when the brake pedal 1 is depressed, the master cylinder 2 produces a master cylinder pressure in response to depression of the brake pedal 1. On the other hand, the second fluid chamber 7b of 9 the fluid cylinder 7 is supplied with a fluid pressure higher than the reference control pressure. By virtue of this, the piston 7c is urged in a direction such as to cause the first fluid chamber 7b of the fluid cylinder 7 to reduce in volume, thus increasing or boosting the brake pressure in the passage 5 and the master cylinder 3. A boosted braking force is thus applied to the associated vehicle wheel. on the other hand, since the shutoff valve 6 is held open in the case of the f low II-1 in Fig. 2, the pressure within the master cylinder 2 is also increased or boosted, thus increasing the brake pedal reaction force.

In the case of the boost mode which is accompanied by a smooth variation of the brake pedal reaction characteristic (the flow 11-2 in Fig. 2), the control unit 13 produces a signal to cause the shutof f valve 6 to open and close repeatedly, and a signal to cause the flow control valve 9 to maintain a fluid pressure higher than the reference control pressure in the passage 8 and in the second fluid chamber 7b of the fluid cylinder 7.

Accordingly, as shown in the flow 11-2 in Fig. 2, while the C-point pressure characteristic (namely the pressure characteristic at point C in Fig. 1) or brake pressure characteristic is similar to that in the case of the boost mode which is accompanied by a sudden variation of the brake pedal reaction characteristic. the B-point pressure characteristic or master cylinder pressure characteristic exhibits a gradual and stepwise variation by virtue of the effect of the repeated opening and closing of the shutoff valve 6, and this stepwise pressure variation is damped and mitigated by the fluid within the master cylinder 2, thus causing the Apoint pressure characteristic or brake pedal rreaction chara--ter--s-,--c to exhi-bilt. a smooth variation and therefore making it possible to attain a good feel in the operation of the brake pedal 1.

As shown in Fig. 3 which shows various variants of the portion "E" of Fig. 2, the master cylinder pressure variation pattern can be changed variously by changing the rate of energization and deenergization of the solenoid 6a of the shutoff valve 6.

In the case of the boost mode which is accompanied by substantially no variation of the brake pedal reaction characteristic, the control unit 13 produces a signal to cause the shutof f valve 6 to close and a signal to cause the f low control valve 9 to maintain a pressure higher than the reference control pressure in the passage 8 and in the second fluid chamber 7b of the fluid cylinder 7.

Accordingly, as shown in the flow 11-3 of Fig. 2, the C-point pressure characteristic or brake pressure characteristic is equal to that in the case of the boost mode which is accompanied by a sudden variation of the brake pedal reaction characteristic. However, the B-point pressure characteristic or master cylinder pressure characteristic is not at all affected by the boosted brake pressure due to the closure of the master cylinder pressure control valve 6, and -1 L.

L.,iere.'&-ore there is no variation of the brake pedal reaction characteristic.

(C) Reduction of brake pressure:

There are three kinds of reduction mode in which the brake pressure is reduced beyond the master cylinder pressure, that is, a reduction mode of a kind which is accompanied by a sudden variation of the brake pedal reaction characteristic (flow III-1 in Fig. 2), a reduction mode of a kind which is accompanied by a smooth variation of the brake pedal reaction characteristic (flow 111-3 of Fig. 2), and a reduction mode of a kind which is accompanied by substantially no variation of the brake pedal reaction characteristic.

In the case of the reduction mode which is accompanied by a sudden variation of the the brake pedal reaction characteristic, the control unit 13 produces a signal to cause the shutoff valve 6 to open 12 and a signal to cause the f low control valve 9 to maintain a pressure lower than the reference control pressure in the passage 8 and in the second f luid chamber 7b of the fluid cylinder 7.

Accordingly, since a pressure lower than the reference control pressure will prevail in the second fluid chamber 7b of the fluid cylinder 7 upon braking, the piston 7c is urged in a direction such as to cause the first fluid chamber 7a to increase in volume. As a result, the brake pressure is reduced by the effect of an increase in volume of the first fluid chamber 7a of the fluid cylinder 7, and the thus reduced brake pressure is supplied to the wheel cylinder 3 to apply a reduced brake force to the vehicle wheel. Further, since the shutoff valve 6 is held open, the pressure within the master cylinder 2 is reduced by the effect of the reduction of the brake pressure, thus causing a variation of the brake pedal reaction characteristic.

In the case of the reduction mode which is 20 accompanied by a smooth variation of the brake pedal reaction characteristic. the control unit 13 produces a signal to cause the shutof f valve 6 to open and close repeatedly, and a signal to cause the flow control valve 9 to maintain a pressure lower than the reference control pressure in the passage 8 and in the second fluid chamber 7b of the fluid cylinder 7.

13 Accordingly, as shown in the flow 111-2 of Fig. 2, the C-point pressure characteristic or brake pressure characteristic is similar to that in the case of the reduction mode which is accompanied by a sudden 5 variation of the brake pedal reaction characteristic. However, the B-point pressure characteristic or master cylinder pressure characteristic exhibits a gradual and stepwise variation by virtue of the repeated opening and closing of the shutoff valve 6. This pressure variation is damped and mitigated by the fluid within the master cylinder 2, thus causing the brake pedal reaction characteristic or the A- point characteristic to exhibit a smooth variation and thereby making it possible to attain a good "feel" in operation of the brake pedal 1.

In the case of the reduction mode which is accompanied by substantially no variation of the brake pedal reaction characteristic as shown in the flow II3 of Fig. 2, the control unit 13 produces a signal to cause the shutof f valve 6 to close and a signal to cause the flow control valve 9 to maintain a pressure lower than the reference control pressure in the passage 8 and in the second fluid chamber 7b of the fluid cylinder 7.

Accordingly, while the brake pressure characteristic or the C-point pressure characteristic 14 is similar to that in the case of the reduction mode which is accompanied by a sudden variation of the brake pedal reaction characteristic, the brake pressure characteristic or the B-point pressure characteristic is not at all affected by the brake pressure due to the closure of the shutoff valve 6, thus causing substantially no variation of the brake pedal reaction characteristic.

(D) Application of a brake force when the brake pedal is not in operation:

When it is desired to apply a brake force to each vehicle wheel without operating the brake pedal 1, the control unit 13 is actuated to produce a signal to cause the shutoff valve 6 to close and thereafter produce a signal to cause the flow control valve 9 to maintain a certain boost pressure in the passage 8 and in the second fluid chamber 7b of the fluid cylinder 7, which boost pressure is higher than the pressure in the passage 5 and the master cylinder 2.

Accordingly, by virtue of the closure of the shutoff valve 6, the working fluid is confined in the brake pressure passage 5, and the piston 7c of the f luid cylinder 7 is urged by the boost pressure in a direction such as to cause the first fluid chamber 7a to reduce in volume in proportion to the intensity of the boost pressure.

As a result, each vehicle wheel has applied thereto a brake force in response to the boost pressure or the resulting variation in volume of the first fluid chamber 7a of the fluid cylinder 7. That is, through control of the boost pressure supplied to the fluid cylinder 7 and through feedback control by monitoring the resulting brake pressure P. by means of the brake pressure sensor 22, a desired braking force can be applied to the vehicle wheel without operating the brake pedal 1.

Next, examples of brake force control when the brake pedal is in operation will be described.

Control of ideal brake force distribution between front and rear wheels:

Brake pressures supplied to front and rear wheel cylinders are increased and reduced relative to the master cylinder pressure on the basis of the vehicle longitudinal acceleration condition or front and rear loading condition which is known from the longitudinal acceleration XG, etc. By virtue of this, an ideal distribution of braking forces between front and rear wheels is obtained without causing rear wheel lock at an early stage of braking.

Control of ideal brake force distribution between left and right wheels:

Brake pressures supplied to left and right wheel 16 cylinders are increased and decreased relative to the master cylinder pressure on the basis of the vehicle turning condition which is known from the lateral acceleration YG, yaw rate T, steering angle 0. vehicle speed V (or left and right wheel loading conditions).

etc. and a predetermined ideal brake force distribution characteristic for left and right wheels.

By virtue of this, an ideal brake force distribution between left and right wheels is obtained without causing one of the left and right wheels to lock at an early stage of braking.

Anti-skid control:

A slip rate for each wheel is computed on the basis of the vehicle speed which is obtained from an integrated value of the vehicle longitudinal acceleration XG and the vehicle wheel speed Vw, and the brake pressure is feedback controlled so that the slip rate approaches of an ideal value. By virtue of this, wheel lock in the event of braking on a low-p road surface or in the event of urgent braking can be prevented, thus making it possible to attain reduced braking distance and braking stability.

Next, examples of brake force control when the brake pedal is not in operation will be described.

Steering characteristic control:

The brake pressure is controlled so that an r, 17 actual steering characteristic during turning, which is detected on the basis of lateral acceleration YG, yaw rate i steering angle 0 and vehicle speed V (or left and right wheel loading condition), etc., becomes that of a neutral steer. By virtue of this, it becomes possible to prevent the occurrence of a strong understeer or a strong oversteer during turning.

Straight running stability control; When a variation of the lateral acceleration or yaw rate of a vehicle occurs due to a disturbance such as a concave or convex road surface, lateral wind, etc., it is immediately detected and corrected through control of the brake pressure and force. By virtue of this, straight running stability can be maintained without being affected by the disturbance.

Yaw rate control:

The difference between the yaw rate T known from the yaw rate sensor 16 and a target yaw rate known from a vehicle speed V and steering angle 0 is first obtained, and the brake force is controlled through control of brake pressure so that the above difference becomes zero. By virtue of this, it becomes possible to obtain an optimal yaw rate characteristic during straight running or turning.

From the foregoing, it will be understood that the braking system of this invention includes a 18 shutof f valve 6 which is installed at the junction between the master cylinder pressure passage 4 and the brake pressure passage 5 and is opened and closed in response to external commands. By virtue of this it 5 becomes possible to eliminate any strangeness in the "feel" of ope=ation of the brake pedal 1 at the time of boost or reduction of the brake pressure, while at the same time making it possible to vary the brake pressure over a wider range under control, and therefore it becomes possible to utilize the above described braking system in various brake controls.

It will be further understood that in the braking system of this invention, the brake pedal reaction force cannot be reduced during increase of the brake pressure and cannot be increased during reduction of same.

It will be further understood that the braking system of this invention makes it possible to attain a good "feel" in operation of the brake pedal 1 by controlling the shutoff valve 6 to open and close repeatedly.

It will be further understood that in the event of pressure reduction during application of the brake. it becomes possible to reduce the brake pressure beyond the master cylinder pressure, thus making it possible to increase the controllable range of the 19 brake pressure for thereby reducing the design restrictions of the braking system.

Fig. 2 shows a modified embodiment of this invention. In this embodiment, the hydraulic pump 11 is f or example a power steering pump and is adapted for common usage. Specifically, the braking system is further provided with a flow dividing valve 30, a solenoid-controlled shutoff valve 31, a pressure switch 32 and a reservoir 33.

In operation, when the pressure within the accumulator 12 reduces beyond a predetermined value, the pressure switch is turned on for thereby causing the shutoff valve 6 to close. By means of this, the flow of fluid towards the reservoir 33 is stopped and the fluid from the hydraulic pump 11 is supplied to the accumulator 12 for thereby causing the accumulator 12 to accumulate a predetermined fluid pressure.

Except for the above, this embodiment is substantially similar to the previous embodiment of Fig. 1 and can produce substantially the same effect.

While the invention has been described in the foregoing with particular reference to specific embodiments, it is to be appreciated that the invention is not limited to the embodiments described and that modifications and variations can be made thereto without departure from the scope of the invention as set forth in the appended claims.

2 1

Claims

CLAIMS:

1. A vehicle braking system comprising: manual brake operating means; a master cylinder for producing a master cylinder pressure in response to operation of said manual brake operating means; a wheel cylinder for producing a brake force in response to a brake pressure supplied thereto; first pressure passage means communicating with said master cylinder; second pressure passage means communicating with said wheel cylinder; shutoff valve means between said first and second pressure passage means for selectively determining the fluid pressure communication via said first and second passage means between said master cylinder and said wheel cylinder; fluid pressure source means for producing a predetermined fluid pressure irrespective of the operation of said manual brake operating means; fluid cylinder means for variably controlling the fluid pressure in said second pressure passage means in response to the master cylinder pressure and a q p Sex, uo:s-yd aTqpAoui pTL2s PUP -TaquiL2xl::) pTri-[-; pu006s PTRS Upq-4 Z6.4amRTP UT aeTTRUS ST suleam zePUTTAD pTuT; pTR9;o =equileijo pTnTj 4s-TT; pFPs uTe-TailmE J:0 1 U1TRID W 'BUTPa03DE UIe:ISAS ISUTXpaq el.OTna.& v c SuRaul 6ATI9A Pe'4P-TaCIO-PTOUaTOS e2P SUReUI 6ATRA OE Toa1U00 MOT; PTR5 pup SUPOUI GATPA;;04nns PTR9 UTeaegm T UITIeTO 04 BUTPa000P UIG4SAS 5UTX'eztq 9TOTIXaA 'V z SUOT,4TPU00 BUTUUnz eTDTT.16A n:TM az)uppaoDOR UT sueam eATRA TOX;UOD MOT9 PTUS PUP SUPGur SATRA J;0'4Wqs PTR5 go uo-r:ui:ado 611.4 BU-F-E-roa;uoo 20; SUPeui Toa4U00 PUP laequlpqD PTn1J puoDes PTRS UT exnSS6=d TOZ:UOO PTR5 BUTTTOZ4UOO.-TqlgTZRA Aqa=ail,4 pup suR@ui aapuTTAo pTnT; pTps go aequip,4o PTnTJ Pu00es PTRS PUP SUR6M 6DZnOS eanssead pTnT; p,Fles ueam4ec[ uoT;RoTunuwoD fDuT"[To-T;uoo A-EqP-p-TP.& jo; OT SW9a1U1 ZePU-&.-LA:) P-En1_; PTUS pup su-eaiu a.-).5:nos iaznssGad PTnT; pTps ueam:aq SUP6DI 6ATRA TOZZUOO MOT; saec[ulptlo p-,nl; puooes p-uz 4sjT; pTes ueem4eq UO'4STd alqu.Aow P pup leanssead ToalUOD PTPS BUT.ATeDea PUP supeul 60anos eanssea:d pTnT; p-yles M:-pm a-rqEo-punmmoo aaquileijo pTnT; puooes P Isupeur e5PssRd eanssead puooes pTps go jaRd fSuTin.z-p-4suoo zeqmeTjz) pTn-Cg 4s.TT; P fSuTsT-Tduioo supew Z9PUTTAO PM; PTRS 104e-T6nZ PeTTddns eanss6ad TOZ:UOO E.

91

2.

3 stepped shape and movable in said first and second fluid chambers.

4. A vehicle braking system according to any preceding claim, wherein said control means comprises means for producing a signal to cause said shutcf--" valve means to open and a signal to cause said f low control valve means to maintain a predetermined reference control pressure in said second fluid chamber of said fluid cylinder means.

5. A vehicle braking system according to claim 4, wherein said control means further comprises means for producing a signal to cause said shutoff valve means to open and a signal to cause said flow control valve maintain a pressure higher than said control pressure in said second fluid said fluid cylinder means.

means to reference chamber of

6. A vehicle braking system according to claim 5, wherein said control means further comprises means for producing a signal to cause said shutoff valve means to open and close repeatedly and a signal to cause said flow control valve means to maintain a pressure higher than said reference control pressure in said second fluid chamber of said fluid cylinder means.

24

7. A vehicle braking system according to claim 6, wherein said control means further comprises means for producing a signal to cause said shutoff valve means to open and close repeatedly and a signal to cause said flow control valve means to ma-4-tain a pressure lower than said reference control pressure in said second fluid chamber of said fluid cylinder means.

8. A vehicle braking system according to claim 7, wherein said control means further comprises means for producing a signal to cause said shutoff valve means to open and close repeatedly and a signal to cause said flow control valve means to maintain a pressure lower than said reference control pressure in said second fluid chamber of said fluid cylinder means.

is

9. A vehicle braking system according to claim 8, wherein said control means further comprises means for producing a signal to cause said shutoff valve means to close and a signal to cause said flow control valve means to maintain a pressure higher than said reference control pressure in said second fluid chamber of said fluid cylinder means.

10. A vehicle braking system according to claim 9, wherein said control means further comprises means for producing a signal to cause said shutof f valve means to close and a signal to cause said flow control valve means to maintain a pressure lower than said reference control pressure in said second fluid chamber of said f'Lu.j-d cylinder.

ii. A vehicle braking system according to any preceding claim, wherein said control means comprises a steering angle sensor, yaw rate sensor, lateral acceleration sensor, vehicle speed sensor and a brake pressure sensor for detecting vehicle running condition.

12. A vehicle braking system according to any preceding claim, wherein said pressure source means is comprises a hydraulic pump, an accumulator and a check valve for allowing flow of fluid in one direction only from said pump to said accumulator.

13. A vehicle braking system according to claim 12 r wherein said pressure source means further comprises a flow dividing valve for enabling discharge of said pump to another place in addition to said accumulator, a pressure sens= for detecting the pressure in said accumulator, a fluid pressure passage extending )6 between said flow dividing valve and said check valve, a reservoir, and a shutof f valve for draining said f luid pressure passage to said reservoir when said pressure sensor detects that the pressure in said accumulator is lower than a predetermined value.

14. A vehicle braking system according to any preceding claim, wherein said manual brake operating means comprises a brake pedal.

15. A vehicle braking system comprising:

first means for producing a master fluid pressure in response to operation of a brake operating means; second means f or producing a brake f orce at a vehicle wheel in response to a brake f luid pressure supplied thereto; is first control means interposed in a fluid pressure line interconnecting said first and second means for selectively determining the fluid pressure communication therebetween; a fluid pressure source for producing a constantly available reservoir of fluid pressure; second control means for variably controlling the fluid pressure provided to said second means in dependence upon the master fluid pressure produced by said first means and upon a control pressure supplied t 27 thereto; third control means interposed between said fluid pressure source and said second control means for determining the fluid pressure communication between said fluid pressure source and said second control pressure; means and thereby determining said and fourth control means for determining the operation of said first control means and said third control means in accordance with vehicle running conditions.

16. A vehicle braking system wherein the brake force developed at the vehicle wheels is dependent upon a fluid pressure developed in response to brake operation and upon a control pressure, and wherein a first control means determines the communication of the first-mentioned fluid pressure to a second control means, the second control means determines the development of the brake force in dependence upon the fluid pressure communicated by said first control means and upon a control pressure, and the control pressure is derived in dependence upon vehicle operating conditions.

17. A vehicle braking system substantially as herein -78 described with reference to Fig. 1 or Fig. 4 of the accompanying drawings.

18. A vehicle incorporating a braking system as claimed in any of the preceding claims.

Published 1991 at 7he Patent Office. Concept House. Cardiff Road. Newport. Gwent NP9 I RH. Further copies may be obtained from Sal- Branch. Unit 6. Nine Mile Point. Cwmfehnfach. Cross Keys. Newport. NPI 7HZ. Printed by Multiplex techniques ltd. St Mary Cray. Kent.