WO1995011823A2

WO1995011823A2 - Vehicle brake system

Info

Publication number: WO1995011823A2
Application number: PCT/GB1994/002379
Authority: WO
Inventors: William Sidney Broome; Colin Ford Ross; Robert David Prescott
Original assignee: Grau Limited
Priority date: 1993-10-29
Filing date: 1994-10-28
Publication date: 1995-05-04
Also published as: WO1995011823A3

Abstract

A vehicle brake system comprising a fluid pressure converter responsive to a fluid pressure brake command signal to provide a hydraulic brake pressure signal which is supplied to a hydraulic brake actuator to operate a vehicle brake, and an anti-slip brake control means wherein the fluid pressure converter supplies a hydraulic brake operating signal and the anti-slip brake control means is operable to modulate the hydraulic brake operating signal to provide the hydraulic brake pressure signal.

Description

Title: Vehicle Brake System

Description of Invention

This invention relates to a vehicle brake system comprising a fluid pressure converter responsive to a fluid pressure brake command signal to provide a hydraulic brake pressure signal which is supplied to a hydraulic brake actuator to operate a vehicle brake and which has an anti-slip brake control means.

The anti-slip brake control means may be an anti-skid brake control means where the slip is as a result of a wheel decelerating faster than contact with the ground permits or an anti-spin brake control means where the slip is as a result of a driven wheel accelerating faster than contact with the ground permits.

In a multi-channel vehicle brake system, that is where independent anti-slip brake control means are provided for at least two wheels or group of wheels, it would be necessary to provide each wheel or group of wheels with a respective fluid pressure converter and a separate fluid pressure brake operating signal provided by modulating a fluid pressure brake demand signal with an anti¬ skid brake control means or by providing a brake operating signal by means of an anti-spin brake control means.

However, such a system is expensive because of the need to provide a respective fluid pressure converter for each channel and, in addition, problems arise with across-axle imbalance.

An object of the invention is to provide a new and improved system of the kind specified above.

According to a first aspect of the present invention we provide a vehicle brake system comprising a fluid pressure converter responsive to a fluid pressure brake command signal to provide a hydraulic brake pressure signal which is supplied to a hydraulic brake actuator to operate a vehicle brake and an anti- slip brake control means, wherein the fluid pressure converter supplies a hydraulic brake operating signal and the anti-slip brake control means is operable to modulate the hydraulic brake operating signal to provide the hydraulic brake pressure signal.

The anti-slip brake control means may generate a plurality of individual hydraulic brake pressure signals for a plurality of brake actuators.

Each hydraulic brake pressure signal may be generated by independently modulating the hydraulic brake operating signal.

The anti-slip brake control means may comprise a hydraulic fluid control valve for each hydraulic brake pressure signal.

Each hydraulic fluid control valve may be independently operable to transmit or block passage of the hydraulic brake operating signal to provide said individual brake pressure signals.

According to a second aspect of the invention we provide a vehicle brake system comprising a fluid pressure converter responsive to a fluid pressure brake command signal to provide a hydraulic brake operating signal, means to supply said hydraulic brake operating signal to an anti-slip brake control means comprising a plurality of hydraulic fluid control valves which are operable independently of each other to transmit or block passage of said hydraulic brake operating signal and thereby provide an independent brake pressure signal for each of a plurality of brake actuators.

According to a third aspect of the present invention we provide a vehicle brake system comprising a plurality of hydraulic brake actuators each being responsive to an individual hydraulic brake pressure signal fed thereto by a respective hydraulic fluid control valve of an anti-slip brake control means, said control valves being operable independently of each other to transmit or block a hydraulic brake operating signal to provide said individual brake pressure signals and wherein said hydraulic brake operating signal is supplied from a fluid pressure converter responsive to a fluid pressure brake command signal whereby a selected hydraulic brake pressure may be relieved by reducing, such as by exhausting to a low pressure region, fluid pressure on the brake command signal side of said converter.

In a first more specific aspect of the invention the fluid pressure brake demand signal may be provided by a manually operable brake control means.

In a second more specific aspect of the invention the fluid pressure brake demand signal may be provided by an anti-spin valve to control supply of fluid pressure from a source thereof.

In a third more specific aspect of the invention the fluid pressure brake demand signal may be provided selectively by a manually operable brake control means or by an anti-spin valve to control supply of fluid pressure from a source thereof.

Selector valve means may be provided to select the source of the fluid pressure brake demand signal depending upon the higher pressure present at a pair of inlets of the selector valve means.

The fluid pressure convertor may be a pneumatic to hydraulic convenor and the fluid pressure brake command signal is a pneumatic signal.

The pneumatic brake demand signal may be modulated by an electrically operated modulating valve to provide said pneumatic brake command signal.

According to a fourth aspect of the present invention, or in any one of the first to third aspects of the invention, we provide a vehicle brake system comprising a pneumatic to hydraulic pressure converter responsive to a pneumatic brake command signal to provide a hydraulic brake pressure signal which is supplied to a hydraulic brake actuator to operate a vehicle brake and wherein the pneumatic brake command signal is supplied from a source of fluid under pressure by a relay valve responsive to a pneumatic brake control signal wherein the brake control signal is controlled by first and second solenoids and said first solenoid is adapted alternately to supply a brake demand signal from a source under the control of a pneumatic brake demand control and to exhaust said brake control signal and the second of said solenoid valves is adapted to supply either said pneumatic brake demand signal or air from said source to provide an anti- spin brake demand signal.

When the second valve supplies air from said source it interrupts supply of said brake demand signal from the first solenoid.

The system therefore provides an anti-spin facility by virtue of a control means sensing a condition of incipient wheel spin and controlling the second solenoid so as to cause said air to be fed to the relay valve from said source. Normally when a condition of incipient spin occurs no brake demand signal will be being applied by the first solenoid valve and the first solenoid will be exhausting the brake operating signal. When the second solenoid is operated to provide the anti-spin brake operating signal the second solenoid valve connects said source to the relay valve and blocks the line from the first solenoid valve.

Alternatively, the fluid pressure converter may be a hydraulic to hydraulic converter and the fluid pressure brake command signal may be a hydraulic signal.

The hydraulic brake operating signal may be provided by a closed centre, or an open centre, powered hydraulic system.

The power hydraulic system may comprise a hydraulic brake demand signal control valve, actuated by a manual brake control, which provides an hydraulic brake demand signal at a pressure, proportional to the extent of operation of the manual brake control, to the hydraulic-hydraulic converter from a pressurised source and provided with a low pressure region to which the hydraulic brake demand signal may be exhausted.

Two hydraulic-hydraulic converters may be provided supplied by separate brake demand signal control valves actuated by the manual brake control through a balancing means and each converter being adapted to control a respective set of wheels and provided with a desired plurality of hydraulic control valves. In all aspects of the invention, particularly when the converter is a pneumatic to hydraulic converter, the or each hydraulic fluid control valve may be pneumatically operated.

Operating air for the or each pneumatically operated hydraulic fluid control valve may be controlled by an electrically operated valve having an electrically driven valve member, such as a solenoid valve.

The operating air may be supplied to the or each electrically operated valve from a pneumatic brake demand signal.

Alternatively, the or each hydraulic fluid control valve may comprise an electrically operated valve having an electrically driven valve member such as a solenoid valve.

The modulating valve may be controlled by an electrical brake operating signal provided by an electrical controller responsive to wheel slip of a respective wheel.

According to a fifth aspect of the invention in any one of the first to fourth aspects, we provide a brake installation for a vehicle having un-driven wheels at opposite ends of a first axle, such as a front axle, and driven wheels at opposite ends of a second axle, such as a rear axle, and wherein a brake system according to the first aspect of the invention is provided for at least the un-driven wheels or the driven wheels of the vehicle.

The brake installation may comprise a first brake system, according to the first more specific aspect of the invention provided for said un-driven wheels and a second brake system, according to the first or to the second or to the third more specific aspect of the invention for said driven wheels.

Alternatively, the installation may comprise a brake system according to the first or the second, or the third more specific aspect of the invention for the driven wheels and a further fluid pressure converter to provide a hydraulic brake pressure signal which is supplied to both of the un-driven wheels to operate vehicle brakes and the further fluid pressure converter being provided with a fluid pressure brake command signal which may be provided by modulating a fluid pressure brake demand signal with anti-skid brake control means.

Further alternatively, the installation may be for a vehicle having further un-driven wheels at opposite ends of an axle spatially associated with said driven wheels and wherein a brake system according to the third more specific aspect of the invention is provided for said driven wheels and said further un- driven wheels are provided with an independent hydraulic brake pressure signal from a blocking valve means which is supplied with the hydraulic brake pressure signals supplied to each of said driven wheels and there being means to permit said blocking valve means to transmit said brake pressure signal of each driven wheel to an associated further un-driven wheel when the fluid pressure brake demand signal is provided by said manually operable brake control means and to prevent transmission of said brake pressure signal of each driven wheel to the associated further un-driven wheel when said fluid pressure brake demand signal is provided by said anti-spin valve.

Blocking valve means may be placed in condition to prevent transmission of said brake pressure signals by a pneumatic operating signal for the blocking valve supplied thereto when said anti-spin valve provides said pneumatic brake demand signal.

Means may be provided to feed said pneumatic brake demand signal provided by said anti-spin valve to said blocking valve to provide said operating signal therefor.

According to a sixth aspect of the present invention or in any one of the first to fifth aspects of the invention, we provide a vehicle brake system comprising a fluid pressure converter responsive to a fluid pressure brake command signal to provide a hydraulic brake operating signal to operate a vehicle brake, a hydraulic fluid control valve which is operable to transmit or block passage of the hydraulic brake operating signal and thus provide a brake pressure signal for a brake actuator and wherein the hydraulic fluid control valve comprises a first valve member moveable by an actuating means into and out of sealing engagement with a first valve seat of a shuttle valve member, the shuttle valve member being moveable with the first valve member relative to a valve body into and out of sealing engagement with a second valve seat to control flow of fluid between two ports of the valve body and means being provided whereby when the shuttle valve member is in sealing engagement with the second valve seat, fluid may flow between said ports past the first valve seat under the control of the first valve member and when the shuttle valve member is not in sealing engagement with the second valve seat flow of fluid between the ports past the first valve seat is prevented and is permitted past the second valve seat.

The first valve member may comprise a shaft disposed within the shuttle valve member which comprises a sleeve, the first valve seat being provided between an end part of the shaft and a valve seat provided internally of the sleeve, the sleeve being slidably mounted relative to the valve body and being provided with transverse passage means which is obturated by the valve body when the sleeve is out of engagement with the second valve seat and which is exposed to fluid from one of said ports when the sleeve is in engagement with the second valve seat.

The first valve member may be moveable by an actuating means comprising a solenoid which may act directly on the first valve member or which may act indirectly on the first valve member by controlling passage of fluid to act on a piston or like member, movement of which is communicated to the first valve member.

According to a seventh aspect of the invention, or in any one of the first to sixth aspects of the invention, we provide a vehicle brake system comprising a fluid pressure converter responsive to a fluid pressure brake command signal to provide a hydraulic brake operating signal, a hydraulic fluid control valve which is operable to transmit or block passage of the hydraulic brake operating signal and thus provide a brake pressure signal for a brake actuator and wherein the hydraulic fluid control valve comprises a spool valve comprising a sleeve piston slidable longitudinally in a bore in a valve body, the bore having adjacent one end a transverse port which is, at least in a first longitudinal position of the sleeve piston in the bore, in communication with a transverse passage in the sleeve piston which communicates with a longitudinally extending passage in the sleeve piston, the bore having adjacent its opposite end a transverse port which is in commumcation with the longitudinally extending passage of the sleeve piston when the sleeve piston is in said first position and which is closed by the sleeve piston when the sleeve piston is in a second longitudinal position in the bore.

The longitudinally extending passage of the sleeve piston may terminate in a longitudinally facing port at the end of the sleeve piston adjacent said opposite end of the bore.

Spring means may be provided to act on the sleeve piston to bias it towards said one end of the bore.

The spring may be disposed in a counter-bore provided at said opposite end of the sleeve.

Bleed means be provided whereby fluid under pressure may act on opposite ends of the sleeve piston.

The sleeve piston may be adapted to be moved between the first and second positions by an element acting on the one end of the sleeve piston and the element may be moved in the longitudinal direction by electromagnetic means.

The components of the control valve are configured to avoid any regions where air may be trapped so that after the system has been bled, no air is present in the system.

Where appropriate features of any aspect of the invention may be provided in association with any one or more of the other aspects of the invention.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings wherein:

FIGURE 1 is a diagrammatic illustration of a 4S/4M brake operating system embodying the invention, FIGURE 2 is a diagrammatic illustration of a 4S/3M brake operating system embodying the invention,

FIGURE 3 is a diagrammatic illustration of a 6S/3M brake operating system embodying the invention,

FIGURE 4 is a diagrammatic cross-section to an enlarged scale through an electrically operated pneumatic-hydraulic holding/blocking valve used in the systems of Figures 1 - 3,

FIGURE 5 is a diagrammatic cross-sectional view through a pneumatic hydraulic holding/blocking valve used in the system of Figure 3,

FIGURES 6 to 8 are diagrammatic illustrations of valve combinations for use as modulating valves and/or anti-spin valves in the embodiments of Figures 1-3,

FIGURE 9 is a diagrammatic illustration of a modification of the pneumatic air supply means of the previous embodiments,

FIGURE 10 is a fragmentary cross-section, to an enlarged scale, showing a modification of the valve of Figure 4,

FIGURE 11 is a cross-sectional view through another holding/blocking valve,

FIGURE 12 is a diagrammatic illustration of a wholly hydraulic brake system embodying the invention, and

FIGURE 13 is a diagrammatic illustration of a modification of the system of Figure 12.

Referring now to Figure 1, a four-wheel vehicle has un-driven wheels 11, 12 at opposite ends of a front axle and driven wheels 13, 14 at opposite ends of a rear axle of the vehicle.

Each wheel 11 - 14 is provided with a respective wheel speed sensor 15 which provides individual inputs on lines 15a. - d_. to an electronic controller 16 which is responsive to slip signals of the wheels 11 - 14. When the slip detected is a skid or incipient skid condition, i.e. a wheel deceleration which is greater than that permitted by contact with the ground, the controller 16 provides an electrical brake operating signal on line 17 if skid of wheels 13 and/or 14 is detected, and an independent electrical brake operating signal on line 18 if skid of one or both of wheels 11 and 12 is detected. When the slip detected is a spin or incipient spin condition, i.e. a greater wheel acceleration than that permitted by contact with the ground of the driving wheels of the vehicle 13, 14, the controller 16 provides an anti-spin signal on line 19 and an electrical brake operating signal on line 17.

Each wheel 11 - 14 is provided with a wheel brake operated by a hydraulic actuator 20 - 23. The actuators 20, 21 associated with the front wheels 11, 12 are operated to apply the brake by means of a hydraulic brake pressure signal supplied thereto on lines 24, 25 respectively from a first pneumatic- hydraulic holding valve 26, whilst the actuators 22, 23 are similarly supplied with a respective hydraulic brake pressure signal on lines 27, 28 respectively from a second pneumatic-hydraulic holding valve 29.

The first pneumatic-hydraulic holding valve 26 is supplied with a hydraulic brake operating signal on line 30 from a first fluid pressure converter 31. The converter 31 is of a conventional pneumatic/hydraulic kind and typically boosts the pressure of the hydraulic fluid relative to the pneumatic fluid, for example in the ratio 30:1. The construction of such a converter is well-known and does not require further description.

The converter 31 is supplied with a pneumatic brake command signal on line 32 from a modulating valve 33 responsive to the electrical brake operating signal supplied on the line 18.

In order to minimise the current consumption the modulating valve 33 preferably comprises a combination of a solenoid valve which acts as a "pilot valve" for a further valve. Figure 6 shows such a combination where the further valve is a relay valve R and the solenoid valve is indicated at S whilst Figure 7 shows a combination where the solenoid valve S provides a pilot valve for a pneumatically controlled 3/2 in-line valve I. Other valve configurations may be provided if desired and, if appropriate the modulating valve 33 may comprise the solenoid valve S alone. A pneumatic brake demand signal is supplied to the valve 33 on line 34 from a "front" outlet of a conventional brake control 35 of the vehicle, air being supplied to a "front" inlet of the control 35 from a reservoir 36 or other source in conventional manner.

A branch line 34a. also supplies the pneumatic brake demand signal to the first holding valve 26.

The second holding valve 29 is similarly supplied with a hydraulic brake operating signal on line 40 from a second fluid pressure converter 41 similar to the first pneumatic-hydraulic converter 31 described hereinbefore. A pneumatic brake command signal is supplied to the converter 41 on line 42 from a second modulating valve 43 which is responsive to the electrical brake operating signal supplied on line 17. The second modulating valve 43 may be a valve or valve combination as described hereinbefore in connection with modulating valve 33.

The second modulating valve 43 is provided with a pneumatic brake demand signal on line 44 which extends from a selector valve means which comprises in this example a shuttle valve 45, one inlet 46 of which is connected by line 47 to the brake control 35 and a second inlet 48 of which is connected to an anti-spin valve 49. If desired any other suitable kind of selector/shuttle valve means may be provided which may automatically select a source of fluid pressure depending upon the higher pressure present at a pair of inlets of the selector/shuttle valve means. The anti-spin valve 49 is responsive to the anti-spin signal provided on the line 19.

As described in connection with the modulating valve 33, 43 the anti- spin valve 49 may comprise a combination of valves in which a solenoid valve S acts as a pilot valve to a further valve. Figure 8 shows such a further valve in the form of an in-line valve I. If desired other valve combinations of known kind may be provided.

The anti-spin valve 49 is supplied with air by line 50 which extends from a second reservoir or other source 51. A branch 52 of the line 50 supplies air to a "rear" inlet of the brake control 35 to provide the source of the "rear" brake demand signal supplied by the brake control 35. A branch 44a. of the line 44 supplies the brake demand signal to the second holding valve 29. If desired a load sensing valve L of conventional type may be provided on the line 44.

Each pneumatic-hydraulic holding valve 26, 29 is identical and hence only one of these valves will be described in detail with reference to Figure 4.

Each holding valve comprises a body 100 having a first outlet I for hydraulic fluid connectable to one of the wheels 11, 12 or 13, 14 with which the holding valve is associated. For convenience, in the present example the outlet I is shown connected to line 24/27 and for convenience of description the wheel 11, 13 will be referred to as the "first" wheel of the pair of wheels with which the respective holding valve 26, 29 is associated.

The body 10 also has a second outlet II to supply hydraulic fluid to the brake actuator of the other wheel of the pair of wheels with which it is associated and hence this line is shown marked 25/28 and the associated wheel 12, 14 will be referred to herein as the "second" wheel of the respective pair of wheels.

The body 10 has a pair of hydraulic fluid inlets A, B both of which are supplied with the hydraulic brake operating signal on the respective line 30, 40 and which may be provided by a common port.

Passage of hydraulic fluid from the inlet A to the outlet I is controlled by a first hydraulic fluid control valve 101 whilst passage of hydraulic fluid from the inlet B to the second outlet II is controlled by a second hydraulic fluid control valve 102. Each control valve 101, 102 comprises a valve stem 103 having a valve insert of suitable sealing material 104 which is movable into closing engagement with a valve seat 105 of the outlet I/II. A seal 106 is provided for the valve stem 103 and a ring seal 107 is also provided between each valve stem and the body 100.

The valve stems 103 are slidably mounted in bores 108 in the body 100 and at their upper ends have heads 109 which act as pistons slidable in cylinders 110 with an O"-ιϊng seal 111 between each head 109 and the circumferential extending wall of the associated chamber 110. A coil spring 112 is provided to bias each piston head and hence each valve stem upwardly so as normally to maintain the hydraulic fluid control valves 101, 102 open.

Control air to act on the upper surfaces 113 of the pistons 109 is supplied under the control of electrically operated valves 114, 115 which control the air for the valve 101, 102 respectively.

The valves 114, 115 each comprise a solenoid valve having a valve member 116 having an insert 117 of suitable sealing material which is movable into and out of sealing engagement with a valve seat 118. Normally the valve members 116 are biased into their closed position by a coil spring 119 but are movable out of sealing engagement with the valve seat 118 by a respective solenoid coil 120. An electrical control signal is supplied to each solenoid of valve 26 on lines 26a, b. and of valve 29 on lines 29a, b_.

The valve body 100 is provided with an exhaust passage 121, the outer end of which is provided with a valve member 121a_. to prevent ingress of detritus. The passage 121 commumcates, by transversely extending passages 122, with chambers 123 in which the valve members 116 of the electrically operated valves 114, 115 slide and by further transversely extending passages 124 and 125 to exhaust the chambers 110 and passages 108 below the piston heads 109.

The valve seats 118 communicate with a passage 126 which communicates with an inlet 127 for the pneumatic brake demand signal supplied on line 34a, 44a.

The seat 118 of the solenoid valve 115 communicates via passage 128 with the chamber 110 above the piston 109 of the control valve 102 whilst the seat 118 of the valve 114 communicates by passage 129 with the chamber 110 above the piston 109 of the control valve 101.

In use, during normal brake operation, the brake control 35 will send a pneumatic brake demand signal on lines 34, 34a_. and 44, 44a, During normal braking the solenoids of the valves 114, 115 are de-energised and therefore the valve members 116 are in the position illustrated in Figure 4, being biased thereto by coil springs 119. Thus, in this condition the pneumatic brake demand signal supplied by the lines 34a, 44a. is blocked by the valves 114, 115 and the space above the pistons 109 in the chambers 110 is exhausted to atmosphere past the valve members 116 and through passages 122, 121. Accordingly the control valves 101, 102 are in the position shown in Figure 4.

The pneumatic pressure brake demand signal supplied on the lines 34, 44 is transmitted by the modulating valves 33, 43 without modulation, since in this condition the solenoids of these valves are de-energised and hence the valves are maintained in the position shown in Figure 1. Accordingly the respective pneumatic-hydraulic converter 31, 41 supplies a hydraulic brake operating signal to the holding valves 26, 29 in dependence upon the pneumatic brake command signal supplied thereto from the modulating valves 33, 43. In these circumstances the pneumatic brake command signal is the same as the pneumatic brake demand signal or as modified by any further means such as the load sensing valve L.

Since the valves 101, 102 are open, the hydraulic brake operating signal supplied to the valves 26, 29 is simply transmitted thereby without modulation and thus provides the hydraulic brake pressure signal to the hydraulic actuators via lines 24/27 and 25/28.

In these conditions the hydraulic brake pressure signal is the same as the hydraulic brake operating signal although, of course, two hydraulic brake pressure signals are supplied by each holding valve 26, 29 from a single hydraulic brake operating signal supplied thereto.

If a detector 15 detects a wheel skid condition of one of the wheels 11 and 12 the system operates as follows. In this example it will be assumed that the sensor has detected skidding of the wheel 11, which is referred to hereinafter as the first wheel of the pair of wheels 11 and 12, and thus the wheel 12 will be referred to hereinafter as the second wheel.

The driver will, of course, have already demanded the brake application and thus the brake pressure signal will be being supplied through lines 24, 25 to the first and second wheels 11, 12 respectively. On detection of skidding of the first wheel 11 the controller 16 sends an electrical brake operating signal on line 18 to the modulating valve 33. This will cause the solenoid of the valve 33 to be energised and for the valve to move to its second position so as to block supply of further pneumatic brake command signal to the converter 31 and to permit the air in line 32 to be exhausted to atmosphere. Accordingly the hydraulic brake operating signal similarly reduces in line 30 and hence at the inlets A and B.

At the same time the solenoid of valve 115 is energised on line 26b to feed air from the inlet 127 supplied with pneumatic brake demand signal on line 934a, to act on the upper side of the piston 109 of the control valve 102 to close the seat 105 and so block the reduction in pressure in outlet II so that the hydraulic brake pressure signal supplied to the actuator 21 of the second wheel 12 is held. The solenoid 120 of the valve 114 remains de-energised so that the control valve 101 remains open so that the hydraulic brake pressure signal in the line 24 can fall in accordance with the fall in hydraulic brake operating signal in the line 30 described previously.

When the controller 16 detects recovery in the skid condition, i.e. when the first wheel begins to rotate or to increase in speed, the controller 16 maintains the solenoid of valve 102 energised on line 26b_ so that the hydraulic brake pressure signal of the second wheel 12 is maintained held and the controller 16 provides a signal on line 18 to the modulating valve 33 to de-energise the valve so as to again supply brake demand signal to the converter 31 so as to cause a corresponding increase in the hydraulic brake operating signal which, since the valve 101 is open, is supplied from inlet A to outlet 1 and hence to the actuator 20 of the first wheel 11 so as to start brake application.

The controller 16 also causes the solenoid of the valve 114 to successively to energise and de-energise so that the control valve 101 is opened and closed to cause a stepwise increase in the hydraulic brake pressure signal which is fed on line 24 to the actuator 20 in conventional ABS manner. The controller 16 may also energise and de-energise the solenoid of valve 115 so that the control valve 102 is opened and closed to cause stepwise increase in the hydraulic brake pressure which is fed on line 25 to the actuator 21 in conventional anti-skid manner. This is continued unless and until a further skid condition of one of the wheels 11, 12 is detected, whereupon the above mentioned procedure is repeated with the appropriate solenoid valve 114, 115 being energised on line 26a, 26b depending on the skidding wheel.

An exactly similar sequence of operations occurs if a skid condition is detected of one of the rear wheels 13 or 14 and, of course, the brake system can operate so that such anti-skid brake operation in respect of the wheels 11, 12 or the wheels 13, 14 can take place independently.

If a spin condition of one of the driving wheels 13, 14 is detected by the controller 16 the system operates as follows. It will be assumed in this example that the wheel 13 comprises a first wheel which is spinning, and the wheel 14 a second wheel where spinning has not been detected.

In these circumstances, of course, there is no braking being demanded. Accordingly the controller 16 sends an anti-spin signal on line 19 to the anti-spin valve 49 to cause it to move from its at rest position shown in Figure 1 in which supply of air on line 50 from reservoir 51 is blocked, to its energising position where such air is supplied to the second inlet 48 of the shuttle valve 45 so as to supply a pneumatic brake demand signal on lines 44 and 44a. The system then works exactly as described hereinbefore in connection with an anti-skid condition except that the controller 16 monitors the spinning condition of the first wheel 13 and sends appropriate electrical brake operating signals to the modulating valve 43 on line 17 and signals on line 29a. and 29b. to energise solenoid valves 114 & 115 to operate the control valves 101 & 102 analogously to their operation described hereinbefore in connection with anti-skid operation but in accordance with normal anti-spin criteria.

Thus,in outline, initially, on energisation of the anti-spin valve 49,the modulating valve 43 remains de-energised, so that air is fed to the convertor 41 and the solenoid of the control valve 101,102 associated with the non-spinning wheel is energised to close the relevant control valve and block supply of brake pressure to the actuator of the non-spinning wheel, the solenoid of the other control valve is repeatedly energised and de -energised to cause step wise supply of brake pressure to the spinning wheel. When the spinning wheel stops spinning, the valves 101,102 are opened and air is exhausted from the air side of the convertor 41 by energisation of modulating valve 43 and/or de-energisation of anti-spin valve 49.

Since the system of Figure 1 is provided with four sensors and four brake operating signal modulators, it is referred to herein as a 4S/4M system.

The controller 116 may be programmed to provide a modified individual control for the front wheels in which the controller starts operation in a "select low" condition and then lets the pressures diverge gradually to provide individual control. This ensures that the steering of the vehicle is not adversely affected during an anti-lock brake application.

If it is desired to reduce the cost of the brake system in circumstances where a less sophisticated control system is acceptable, a 4S/3M system may be provided, as illustrated in Figure 2, where the same reference numerals have been used as were used in Figure 1 to refer to corresponding parts, but with a preceding 2.

The brake system with respect to the rear wheels 13, 14 is identical to that described with regard to Figure 1 and hence does not require further description.

The brake system with regard to the front wheels 11 and 12 differs in that it is not provided with a pneumatic-hydraulic holding valve. Instead the pneumatic-hydraulic converter 231 supplies a hydraulic brake operating signal to provide, directly, a hydraulic brake pressure signal on lines 224, 225 to the actuators 220, 221 of the front wheels 210, 212. The converter 231 is provided with a pneumatic brake command signal on line 232 from a modulating valve 233 provided with suitable signals, in conventional manner, from the controller 216 on lines 218a, 218b.

As in the case of modulating valves 33,34 the modulating valve 233 may comprise a combination which in this case comprises a pair of solenoid valves a,b. acting as pilot valves to a relay valve Rl. If desired other valve combinations may be provided such as an in-line valve.

In use, when normal braking is demanded solenoids a. and b_ are de- energised and therefore the pneumatic brake demand signal on line 234 is supplied as the pneumatic brake command signal on line 232 to the converter 231.

When a skid condition of one of the wheels is detected then, irrespective of the skidding wheel, the controller 216 sends a signal to the solenoid valve b_so as to exhaust the pneumatic brake command signal on the line 232 to atmosphere and thus reduce the hydraulic brake pressure to release the brakes and, of course, to block the supply of further brake demand signal from line 234.

When the controller 216 detects that neither of the wheels 210, 212 are skidding the solenoid valve b_ is de-energised to increase the brake operating pressure. Thereafter, the solenoid valve a. is energised briefly to hold the brake operating pressure constant. Solenoid a_ is then de-energised to increase the brake pressure temporarily and then again energised so as to hold the brake pressure in steps and so on in conventional ABS manner. If desired other valve configuration and sequence of operation may be provided as desired to achieve ABS control.

Referring now to Figure 3, there is shown a brake system of a 6S/3M type suitable for use with a vehicle having a tag axle.

In Figure 3 the same reference numerals are used as were used in connection with Figure 1 to refer to corresponding parts, but with the addition of an initial figure 3. In addition to front, undriven wheels 311, 312 and driven rear wheels 313, 314 which, in the present example, are "double" wheels mounted to rotate together in conventional manner, there are further un-driven wheels 70, 71 provided at opposite ends of a tag axle.

As will be seen from Figure 3, the system is basically similar to that of Figure 2 except for the part of the brake system associated with the tag axle, and hence only these features of difference will be described.

The wheels 70, 71 are provided with hydraulic brake actuators 72, 73 respectively which are provided with a brake pressure signal on lines 74, 75 respectively from a pneumatically operated shut-off valve 76.

The shut-off valve 76 is provided with hydraulic fluid from lines 77, 78 which branch from line 327, 328 respectively. A pneumatic operating signal is supplied to the valve 76 on line 79 which branches from the line 334.

Referring now to Figure 5, there is shown, in cross-section to an enlarged scale, the construction of the valve 76.

The valve 76 below the line A-A is of identical configuration and construction as the pneumatic-hydraulic holding valves 26, 29 described previously and illustrated in Figure 4 and hence the same reference numerals, but with a preceding numeral 4, are used to refer to corresponding parts as were used in Figure 4. It should be noted that whilst a common hydraulic feed can be provided to the valve assembly shown in Figure 4, separate hydraulic feeds must be provided for the valve 76 of Figure 5.

Valve 76 differs from the valves 26, 29 by virtue of being unprovided with solenoid valves and the pneumatic brake demand signal control air is supplied on line 79 to an inlet port 80 which feeds air via passages 81, 82, 83 to act on the top surface 4113 of the piston heads 4101, 4102.

In use, the brake system of Figure 3 operates, so far as the wheels 311 to 314 are concerned in an exactly similar manner to that of Figure 2. So far as the wheels 70, 71 are concerned, during normal braking and anti-skid braking the hydraulic brake pressure in the lines 327, 328 will be fed via lines 77, 78 to inlets 4 A, 4B of valve 76 and will be transmitted by the valves 4101 and 4102 to outlets 41, 411, and by lines 74, 75 to the actuators 72, 73 and hence the wheel 70 will be braked in exactly the same way as the wheel 313 whilst the wheel 71 will be braked in exactly the same way as the wheel 314.

However, on anti-spin braking, when a brake demand signal is provided by the anti-spin valve 349, then a pneumatic brake demand signal will be supplied on line 79 to the inlet port 80 and thus will cause the valves 4101, 4102 to close, thus preventing transmission of a brake pressure signal to the actuators 72, 73. Accordingly only the driven wheels 313, 314 are braked during anti-spin braking.

If desired, the front wheels of the vehicle may be braked in a similar manner to that illustrated in Figure 1.

The above described brake systems, and in particular where the front wheels are braked by the system shown in Figure 1, provide a brake system which provides four channel ABS braking, and ASR braking, i.e. anti-spin braking, with the use of only two pneumatic-hydraulic converters and a total of seven solenoids. No hydraulic power supply is required since when hydraulic brake pressure is reduced this is simply as a result of the piston of the pneumatic-hydraulic converter moving in the direction to reduce pressure without there being any actual loss of hydraulic fluid. In addition, modified individual control may be provided on the front axle whilst independent control is provided on the rear axle.

If desired, any particular axle of a vehicle may be provided with any one or more of the arrangements described hereinbefore.

An un-driven axle may be braked by an arrangement as described with reference to Figure 1 or Figure 2 or either of the un-driven axle arrangements of Figure 3. A driven axle may be braked by an arrangement as described in Figure 1 (the arrangements of Figs. 2 and 3 being the same) but may be provided with an anti-skid facility alone or an anti-spin facility alone instead of both facilities as described hereinbefore. Although the control valves 101, 102 described hereinbefore are pneumatically operated with the air being controlled by solenoid valves, if desired the control valves 101, 102 may be directly electrically operated by providing suitable solenoid operated hydraulic control valves.

In the above described embodiments an alternate means of providing an anti-spin facility may be provided, as illustrated in Figure 9. In this modification the pneumatic brake command signals supplied on those of lines 32, 42; 232, 242; 332, 342 which relate to driving wheels are supplied from source of fluid under pressure which comprises a reservoir 51 supplied from a prime source such as an engine driven compressor on line 402 via a one-way valve 403. Air from the source 51 is fed on line 400 to a relay valve 404 to provide the brake command signal on line 32 -342. The relay valve 404 is responsive to a brake control signal supplied on line 405.

During brake application the brake control signal is provided by a conventional driver operated brake demand valve 35 which is supplied with air from the reservoir 51 on line 52.

The brake control signal is controlled by first and second solenoid valves A, B. The valve A provides ABS control and is adapted alternately to supply a brake demand signal on line 406 from the demand valve 35 when the solenoid valve A is in its at rest position shown in Figure 9, and to interrupt said supply and exhaust the line 405 to atmosphere when it is in its energised condition. The second solenoid valve B is adapted alternately to transmit the brake demand signal from the first solenoid valve A when it is in its at rest position shown in Figure 9 and, when it is in its energised position to interrupt the above referred to transmission of the brake demand signal or exhaust of line 405, and alternately to supply an anti-spin brake demand signal from reservoir 51 on line 407 to provide ASR control.

This embodiment has the advantage that the anti-spin facility (ASR) is provided by the solenoid B which simply connects the relay valve 404 to the reservoir 51. It enables a separate ASR valve such as that illustrated at 49, 249, 349 in the previously described embodiments to be eliminated as well as the shuttle valve 45, 245, 345. This is of a considerable commercial advantage, particularly if the engine has an electronic engine control. Under these circumstances it is simply necessary to provide one extra solenoid valve, i.e. the valve B and connect it to the existing control means.

Referring now to Figure 10 there is shown a modification of the holding or blocking valve arrangement shown in Figures 4 and 5. The arrangement shown in Figure 10 replaces the valves shown at 4101 and 4102 in Figure 5, although Figure 10 illustrates only the valve 4101.

In the embodiment shown in Figure 10 flow of hydraulic fluid from port 4A/4B to port 41/411 is controlled by a first valve member comprising a shaft 500 connected to a piston such as the piston 109 or 4109 of Figures 4 and 5. The shaft 500 is of relatively small diameter, for example of the order of 3mm diameter.

The shaft 500 has a conical tip 501 adapted to sealingly engage with a frusto-conical valve seat 502 provided on a shuttle valve member comprising a cylindrical sleeve 503 which is slidable in a passage 504 of the body 505 of the valve into and out of sealing engagement with a valve seat 506. The sleeve 503 is provided with a pair, or if desired a different number, of radial through passages 507. In addition, bleed passages 507a are provided to allow the pressure in ports 4A and 4B to act on the end of the sleeve 503 which is disposed in a chamber 508.

In use, in an at rest position, as shown in Figure 10, there is a relatively large flow area between ports 4A/4B and 41/411 and this is the condition when there is no ABS control since there is no restriction between the inlet and outlet ports 4A/4B and 41/411.

When ABS operation is commanded to close the passage between the ports initially the shaft 500 is moved into sealing engagement with the valve seat 502 and the sleeve 503 is then moved forwards by the shaft 500 until it makes contact with a valve seat 506 of the ports 41/411. At this stage the transverse passages 507 are uncovered and fluid can flow from passage 4A/4B through the passages 507 into the centre of the sleeve 503.

With increasing pressure in port 4A/4B there is a pressure drop between the port 4A/4B and the port 41/411 and then, for stepping purposes it is possible to oscillate the shaft 500 in order to obtain steps in the hydraulic system.

The sleeve 503, however, will remain on its seat 506 due to the pressure drop from port 4A/4B to port 41/411 and so the only flow path available is via the small opening.existing between the sleeve 503 and the shaft 500 as the shaft 500 is oscillated into and out of sealing engagement with the valve seat 502.

The above described arrangement is particularly advantageous for a relatively stiff hydraulic system where the displacement per unit increase in pressure is relatively small, since opening and closing a relatively large diameter valve member would result in the pressure steps being too great.

The invention is particularly suitable for drum brakes where a large displacement is required initially whilst the shoes are being moved into engagement with the drum and then there is a relatively stiff system thereafter. It is therefore necessary during normal braking to keep a large port open initially and then during pressure stepping at higher pressures to do so on a relatively smaller diameter avoiding larger pressure steps due to the stiffness of the system at high pressure.

In the illustrated embodiments there is no seal between the sleeve 503 and the body 505 since it is sufficient to make these items a good running fit, since under relatively large volume flow conditions through the port 4A/4B there is a bulk flow of fluid trying to keep the sleeve 503 in its at rest position due to there being instantaneous pressure drop between the volume outside the sleeve 503 and the internal volume existing between the shaft 500 and the sleeve 503. These pressure drops are transient and do not exist once the sleeve has been moved forward by the shaft 500 due to the fact that the transverse passages 507 have now been uncovered. It is important that the sleeve 503 does not move accidentally onto its seat 506 due to the effects of fluid flow since this would cause unwanted restriction in the non-operable condition. The position or location of the inlet port 4A/4B is therefore important since the fluid, by entering largely in the direction shown by arrow D by virtue of its reaction on the sleeve 503 keeps the sleeve at its at rest position shown in Figure 10.

The sleeve 503 also acts as a non-return valve in that with the shaft 500 in its at rest position shown in Figure 10 and with the pressure in the port 4A/4B reducing, the instant pressure in port 4A/4B falls below that in port 41/411 then the sleeve will move into its at rest position shown in Figure 10 to open the large diameter port provided by the space between the valve seat 506 and the conical end of the sleeve 503 once again.

Referring now to Figure 11, there is shown an alternative form of shut- off valve which may be used in the system previously described in place of the shut-off valve illustrated in Figure 4 or which may be used in a wholly hydraulic system, hereinafter to be described with reference to Figure 12.

The valve of Figure 11 comprises a body 600 having first and second outlets I for hydraulic fluid similar to the outlets I and II of the valve of Figure 4. The body 600 has single hydraulic fluid inlet 601. Passage of hydraulic fluid from the inlet 601 to the outlet I is controlled by a first hydraulic fluid control valve 602 whilst that of the hydraulic fluid from the inlet 601 to the second outlet II is controlled by a second hydraulic fluid control valve 603.

Each control valve 602, 603 comprises a valve sleeve 604 slidably received in a bore 605 in the valve body. Each sleeve 604 has a radial passage

606 which provides communication between an interior axially extending passage

607 and exterior of the sleeve. When the sleeve is in its de-energised position, as shown in Figure 11, passages 606 are aligned with a respective gallery 608 which communicates with the inlet passage 601.

At their upper ends, the sleeve 604 have a counter bore in which is received a coil compression spring 609 which engages a circlip 610 held in place by a 0-ring 611 engaged by a screw 612. The internal passage 607 communicates with respective outlet passages 613, 614 which communicate with the outlet ports I and II.

The sleeves 604 provide pistons which are moved between the position shown in Figure 11 and a position in which the sleeves 604 are moved upwardly in Figure 11 in order to obturate the passages 613, 614 respectively. This movement of the sleeves 604 is achieved by means of armature extension shafts 616 operated by solenoids 617. The shafts 616 are guided by a fixed sleeve 619 which is sealed to the valve body. An 0-ring seal 620 is provided between each shaft 616 in its sleeve 619.

The valves and their associates ports and passageways are designed to avoid air traps so that after bleeding of the system no air is retained.

Fluid under pressure from the inlet 601 acts on the underside 618 of the piston sleeve 604 through appropriate bleed spaces so that the fluid pressure acting on opposite ends of each piston sleeve 604 is equalised.

By virtue of such balance of fluid forces, satisfactory operation of the valve is achieved whereas if such balances were not provided, the solenoid 617 would not be strong enough to maintain the valve closed. In other respects the valves shown in Figure 11 operate in a similar manner to the valve described with reference to Figure 4.

Referring now to Figure 12, there is shown an alternative embodiment in which a four wheel vehicle, similar to that of Figure 1, has actuators 20, 21 associated with front wheels and 22, 23 associated with rear wheels.

The actuators 20, 21 for the front wheels are operated to apply the brakes by means of a hydraulic brake pressure signal supplied thereto by a first solenoid operated holding valve 726 whilst the actuators 22, 23 are similar supplied with a respective hydraulic brake pressure signal from a second holding valve 729. The holding valves may, for example, comprise a valve as described hereinbefore with reference to Figure 11. The first holding valve 726 is supplied with a hydraulic brake operating signal on line 730 from a first fluid pressure converter 731. The converter 731 is of a conventional hydraulic-hydraulic converter which may boost the pressure by a desired amount or may provide a one-to-one pressure ratio between the hydrostatic side of the converter which is connected to the holding valve 726 and the power hydraulics side connected to line 732.

The holding valve 729 is similarly connected on line 733 to a second hydraulic-hydraulic converter 734 similar to the converter 731.

The converter 731 is supplied with a hydraulic brake command signal on the line 732 from a modulating valve 735 whilst the converter 734 is supplied with a hydraulic brake command signal on line 736 from a second modulating valve 737.

Each modulating valve 735, 737 is responsive to an electrical brake operating signal supplied on lines 738, 739 respectively. When the valves 735, 737 are in their de-energised position, as shown in Figure 12, they are arranged to feed hydraulic fluid to lines 732 and 736 respectively from a power hydraulic supply line 740, 741 respectively whilst when they are in their energised position, the lines 732 and 736 are connected to low pressure reservoir 742 by lines 743 and 744 respectively.

The power hydraulic system of the present embodiment is of the open centre type in which hydraulic fluid is drawn from the reservoir 742 on line 745 by a motor driven pump 746 which delivers fluid on line 747 to the lines 740, 741 described hereinbefore. A return line 748 is provided which connects the lines 741 and 740 to the reuurn lines 743, 744 and hence the low pressure reservoir 742.

The line 747 feeds the lines 740, 741 through manually operated spool valves 750, 751 connected by a mechanical linkage comprising an equalising link 752 pivotally connected to operating rods 753, 754 of the valves 750, 751 respectively and pivotally connected at its mid-point to an operating member 755 operated by a foot pedal or the like. When the driver operates the pedal to move the link 755 to the left in Figure 12, the spools 756, 757 of the valves 750, 751 respectively are moved to the left progressively to close the return line 748 and open the feed lines 740, 741 so as to provide a hydraulic brake demand signal at a pressure in the lines 740, 741 which is proportional to the extent of depression of the operator control.

During normal brake operation, the modulating valves 735, 737 are in a de-energised position shown in Figure 12 to permit free flow of fluid to provide a hydraulic brake-operating signal of the same pressure as the hydraulic brake demand signal. Similarly the holding valves 726, 729 are in their de-energised position, permitting free flow of fluid to the brake actuators.

When a control system detects skidding of a wheel leading to the requirement of ABS operation, an electronic controller similar to that described in connection with the previous embodiments provides an electrical signal to the solenoids of the valves 735 and/or 737 and to the solenoids of the valves 726 and/or 729. This causes the respective solenoid to be energised so as to block supply of further power hydraulic fluid on line 732 and/or 736 and to permit the lines 732, 736 to be exhausted to the low pressure reservoir 742; i.e. to modulate the hydraulic brake demand signal on line 740/741 to provide a hydraulic brake command signal on line 732/736.

Accordingly, the hydraulic brake operating signal is similarly reduced in the lines 730 and/or 733 by virtue of the response of the hydraulic-hydraulic converters 731, 734 respectively to fall in pressure of the brake command signal in the lines 732 and/or 736. At the same time, the relevant solenoids of the valves 726 and/or 729 are energised so as to lock the reduction in pressure at one of the outlets I/II so that the hydraulic brake pressure signal supply to the desired actuator is held whilst the other solenoid remains de-energised so that its control valve remains ope n so that the hydraulic brake pressure signal to the respective actuator can fall in accordance with the fall in the hydraulic brake operating signal in line 730 and/or 733 as previously described. When the controller detects recovery of the skid condition the controller maintains the respective solenoid of the holding valve or valves 726, 729 is energised so that the hydraulic brake pressure signal is maintained thereby and the controller causes the modulating valve 735 and/or 737 to be de-energised so as again to supply a brake command signal on lines 732 and/or 736 to cause a corresponding increase in the hydraulic brake operating signal, supplied to the brake actuator associated with the open holding valve.

The controller also causes the solenoid of the closed holding valve to be successively energised and de-energised so that this control valve is opened and closed to cause a stepwise increase in the hydraulic brake pressure signal in conventional ABS manner. The control may also energise and de-energise the solenoid of the other valve so that the other valve is opened and closed to cause a stepwise increase in the hydraulic brake pressure which is fed to the other actuator.

Except for the hydraulic-hydraulic nature of the fluid pressure converter, and the manner in which the brake command signal is supplied by the power hydraulic side of the converters, this embodiment operates in a similar manner to that of the other embodiment and features of the other embodiments may be provided in such a wholly hydraulic system where appropriate.

Although in the embodiment illustrated in Figure 10 an open centre system has been illustrated, if desired, a closed centre system may be provided as illustrated in Figure 13 in which the same reference numerals have been used to refer to corresponding parts.

The closed centre system differs in that instead of fluid being supplied from pump 746 directly to line 747, instead pump 746 pressurises an hydraulic accumulator 746a via a non-return valve 746b. and the line 747 extends from the accumulator 746a. In other respects, the system of Figure 13 is as described in connection with Figure 12.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in the terms or means for performing the desired function, or a method or process for attaining the disclosed result, may, separately or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

CLAIMS:

1. A vehicle brake system comprising a fluid pressure converter responsive to a fluid pressure brake command signal to provide a hydraulic brake pressure signal which is supplied to a hydraulic brake actuator to operate a vehicle brake, and an anti-slip brake control means wherein the fluid pressure converter supplies a hydraulic brake operating signal and the anti-slip brake control means is operable to modulate the hydraulic brake operating signal to provide the hydraulic brake pressure signal.

2. A system according to claim 1 wherein the anti-slip brake control means generates a plurality of individual hydraulic brake pressure signals for a plurality of brake actuators.

3. A system according to claim 2 wherein each hydraulic brake pressure signal is generated by independently modulating the hydraulic brake operating signal.

4. A system according to claim 3 wherein the anti-slip brake control means comprises a hydraulic fluid control valve for each hydraulic brake pressure signal.

5. A system according to claim 4 wherein each hydraulic fluid control valve is independently operable to transmit or block passage of the hydraulic brake operating signal to provide said individual brake pressure signals.

6. A vehicle brake system comprising a fluid pressure converter responsive to a fluid pressure brake command signal to provide a hydraulic brake operating signal, means to supply said hydraulic brake operating signal to an anti- slip brake control means comprising a plurality of hydraulic fluid control valves which are operable independently of each other to transmit or block passage of said hydraulic brake operating signal and thereby provide an independent brake pressure signal for each of a plurality of brake actuators.

7. A vehicle brake system comprising a plurality of hydraulic brake actuators each being responsive to an individual hydraulic brake pressure signal fed thereto by a respective hydraulic fluid control valve of an anti-slip brake control means, said control valves being operable independently of each other to transmit or block a hydraulic brake operating signal to provide said individual brake pressure signals and wherein said hydraulic brake operating signal is supplied from a fluid pressure converter responsive to a fluid pressure brake command signal whereby a selected hydraulic brake pressure may be relieved by reducing fluid pressure on the brake command signal side of said converter.

8. A system according to claim 7 wherein said fluid pressure on the brake command signal side of the converter is reduced by exhausting to a low pressure region.

9. A system according to any one of the preceding claims wherein the fluid pressure brake command signal is provided by a manually operable brake control means.

10. A system according to any one of claims 1 to 8 wherein the fluid pressure brake command signal is provided by an anti-spin valve to control supply of fluid pressure irom a source thereof.

11. A system according to any one of claims 1 to 8 wherein the fluid pressure brake command signal is provided selectively by a manually operable brake control means or by an anti-spin valve to control supply of fluid pressure from a source thereof.

12. A system according to claim 11 wherein selecter valve means are provided to select the source of the fluid pressure brake demand signal depending upon the higher pressure present at a pair of inlets of the selecter valve means.

13. A system according to any one of the preceding claims wherein the fluid pressure convertor is a pneumatic to hydraulic convertor and the fluid pressure brake command signal is a pneumatic signal.

14. A system according to claim 13 wherein a pneumatic brake demand signal is modulated by an electrically operated modulating valve to provide said pneumatic brake command signal.

15. A system according to any one of claims 1 to 14 comprising a pneumatic to hydraulic pressure converter responsive to a pneumatic brake command signal to provide a hydraulic brake pressure signal which is supplied to a hydraulic brake actuator to operate a vehicle brake and wherein the pneumatic brake command signal is supplied from a source of fluid under pressure by a relay valve responsive to a pneumatic brake control signal wherein the brake control signal is controlled by first and second solenoids and said first solenoid is adapted alternately to supply a pneumatic brake demand signal from a source under the control of a brake, demand control and to exhaust said brake control signal and the second of said solenoid valves is adapted to supply either said pneumatic brake demand signal or air from said source to provide an anti-spin brake demand signal.

16. A vehicle brake system comprising a pneumatic to hydraulic pressure converter responsive to a pneumatic brake command signal to provide a hydraulic brake pressure signal which is supplied to a hydraulic brake actuator to operate a vehicle brake and wherein the pneumatic brake command signal is supplied from a source of fluid under pressure by a relay valve responsive to a pneumatic brake control signal wherein the brake control signal is controlled by first and second solenoids and said first solenoid is adapted alternately to supply a pneumatic brake demand signal from a source under the control of a brake demand control and to exhaust said brake control signal and the second of said solenoid valves is adapted to supply either said pneumatic brake demand signal or air from said source to provide an anti-spin brake demand signal.

17. A system according to claim 15 or claim 16 wherein when the second valve supplies air from said source it interrupts supply of said pneumatic brake demand signal from the first solenoid.

18. A system according to any one of claims 1 to 12 wherein the fluid pressure converter is a hydraulic to hydraulic converter and the fluid pressure brake command signal is a hydraulic signal.

19. A system according to claim 18 wherein the hydraulic brake operating signal is provided by a closed centre powered hydraulic system.

20. A system according to claim 18 wherein the hydraulic brake operating signal is provided by an open centre powered hydraulic system.

21. A system according to claim 19 or claim 20 wherein the power hydraulic system comprises a hydraulic brake demand signal control valve, actuated by a manual brake control, which provides an hydraulic brake demand signal at a pressure, proportional to the extent of operation of the manual brake control, to the hydraulic-hydraulic converter from a pressurised source and provided with a low pressure region to which the hydraulic brake demand signal may be exhausted.

22. A system according to claim 21 wherein two hydraulic-hydraulic converters are provided supplied by separate brake demand signal control valves actuated by the manual brake control through a balancing means and each converter being adapted to control a respective set of wheels and provided with a desired plurality of hydraulic control valves.

23. A system according to claim 4 or any one of claims 5 to 22 when directly or indirectly dependent on claim 4, wherein the or each hydraulic fluid control valve is pneumatically operated.

24. A system according to claim 23 wherein operating air for the or each pneumatically operated hydraulic fluid control valve is controlled by an electrically operated valve having an electrically driven valve member.

25. A system according to claim 24 wherein the operating air is supplied to the or each electrically operated valve from a pneumatic brake demand signal.

26. A system according to claim 4 or any one of claims 5 to 22 when directly or indirectly dependent on claim 4 wherein the or each hydraulic fluid control valve comprises an electrically operated valve having an electrically driven valve member such as a solenoid valve.

27. A system according to claim 26 wherein the modulating valve is controlled by an electrical brake operating signal provided by an electrical controller responsive to wheel slip of a respective wheel.

28. A system according to any one of the preceding claims wherein the system is for a vehicle having un-driven wheels at opposite ends of a first axle and driven wheels at opposite ends of a second axle and the brake system is provided for at least the un-driven wheels or the driven wheels of the vehicle.

29. A system according to claim 28 wherein the brake system comprises a first brake system, according to claim 9, or any one of claims 10 to 27 when dependent directly or indirectly on claim 9, provided for said un-driven wheels and a second brake system, according to claim 9, claim 10 or claim 11, or any one of claims 12 to 27 when dependent directly or indirectly on claim 9, claim 10, or claim 1 , for said driven wheels.

30. A system according to claim 28 wherein the system comprises a brake system according to claim 9 or claim 10, or claim 11, or any one of claims 12 to 27 when dependent directly or indirectly on claim 9, or claim 10, or claim 11, for the driven wheels and a further fluid pressure converter to provide a hydraulic brake pressure signal which is supplied to both of the un-driven wheels to operate vehicle brakes and the further fluid pressure converter being provided with a fluid pressure brake command signal provided by modulating a fluid pressure brake demand signal wi-:h anti-skid brake control means.

31. A system according to claim 28 wherein the system is for a vehicle having further un-driven wheels at opposite ends of another axle and wherein a brake system according to claim 10 or claim 11, or any one of claims 12 to 27 when dependent directly or indirectly on claim 10, or claim 11, is provided for said driven wheels and said further un-driven wheels are provided with an independent hydraulic brake pressure signal from a blocking valve means which is supplied with the hydraulic brake pressure signals supplied to each of said driven wheels and there being means to permit said blocking valve means to transmit said brake pressure signal of each driven wheel to an associated further un-driven wheel when the fluid pressure brake demand signal is provided by said manually operable brake control means and to prevent transmission of said brake pressure signal of each driven wheel to the associated further un-driven wheel when said fluid pressure brake demand signal is provided by said anti-spin valve.

32. A system according to claim 31 wherein the blocking valve means are placed in condition to prevent transmission of said brake pressure signals by a pneumatic operating signal for the blocking valve supplied thereto when said anti- spin valve provides said pneumatic brake demand signal.

33. A syst m according to claim 32 wherein means are provided to feed said pneumatic brake demand signal provided by said anti-spin valve to said blocking valve to provide said operating signal therefor.

34. A system according to any one of the preceding claims comprising a fluid pressure converter responsive to a fluid pressure brake command signal to provide a hydraulic brake operating signal to operate a vehicle brake, a hydraulic fluid control valve which is operable to transmit or block passage of the hydraulic brake operating signal and thus provide a brake pressure signal for a brake actuator and wherein the hydraulic fluid control valve comprises a first valve member moveable by an actuating means into and out of sealing engagement with a first valve seat of a shuttle valve member, the shuttle valve member being moveable with the first valve member relative to a valve body into and out of sealing engagement with a second valve seat to control flow of fluid between two ports of the valve body and means being provided whereby when the shuttle valve member is in sealing engagement with the second valve seat, fluid may flow between said ports past the first valve seat under the control of the first valve member and when the shuttle valve member is not in sealing engagement with the second valve seat flow of fluid between the ports past the first valve seat is prevented and is permitted past the second valve seat.

35. A vehicle brake system comprising a fluid pressure converter responsive to a fluid pressure brake command signal to provide a hydraulic brake operating signal to operate a vehicle brake, a hydraulic fluid control valve which is operable to transmit or block passage of the hydraulic brake operating signal and thus provide a brake pressure signal for a brake actuator and wherein the hydraulic fluid control valve comprises a first valve member moveable by an actuating means into and out of sealing engagement with a first valve seat of a shuttle valve member, the shuttle valve member being moveable with the first valve member relative to a valve body into and out of sealing engagement with a second valve seat to control flow of fluid between two ports of the valve body and means being provided whereby when the shuttle valve member is in sealing engagement with the second valve seat, fluid may flow between said ports past the first valve seat under the control of the first valve member and when the shuttle valve member is not in sealing engagement with the second valve seat flow of fluid between the ports past the first valve seat is prevented and is permitted past the second valve seat.

36. A vehicle brake system according to claim 34 or claim 35 wherein the first valve member comprises a shaft disposed within the shuttle valve member which comprises a sleeve, the first valve seat being provided between an end part of the shaft and a valve seat provided internally of the sleeve, the sleeve being slidably mounted relative to the valve body and being provided with transverse passage means which is obturated by the valve body when the sleeve is out of engagement with ihe second valve seat and which is exposed to fluid from one of said ports when the sleeve is in engagement with the second valve seat.

37. A vehicle brake system according to claim 36 wherein the first valve member is moveable by an actuating means comprising a solenoid which may act directly on the first valve member or which may act indirectly on the first valve member by controlling passage of fluid to act on a piston or like member, movement of which is communicated to the first valve member.

38. A vehicle brake system according to any one of the preceding claims comprising a fluid pressure converter responsive to a fluid pressure brake command signal to provide a hydraulic brake operating signal, a hydraulic fluid control valve which is operable to transmit or block passage of the hydraulic brake operating signal and thus provide a brake pressure signal for a brake actuator and wherein the hydraulic fluid control valve comprises a spool valve comprising a sleeve piston slidable longitudinally in a bore in a valve body, the bore having adjacent one end a transverse port which is, at least in a first longitudinal position of the sleeve piston in the bore, in communication with a transverse passage in the sleeve piston which communicates with a longitudinally extending passage in the sleeve piston, the bore having adjacent its opposite end a transverse port which is in communication with the longitudinally extending passage of the sleeve piston when the sleeve piston is in said first position and which is closed by the sleeve piston when the sleeve piston is in a second longitudinal position in the bore.

39. A brake system comprising a fluid pressure converter responsive to a fluid pressure brake command signal to provide a hydraulic brake operating signal, a hydraulic fluid control valve which is operable to transmit or block passage of the hydraulic brake operating signal and thus provide a brake pressure signal for a brake actuator and wherein the hydraulic fluid control valve comprises a spool valve comprising a sleeve piston slidable longitudinally in a bore in a valve body, the bore having adjacent one end a transverse port which is, at least in a first longitudinal position of the sleeve piston in the bore, in commumcation with a transverse passage in the sleeve piston which communicates with a longitudinally extending passage in the sleeve piston, the bore having adjacent its opposite end a transverse port which is in commumcation with the longitudinally extending passage of the sleeve piston when the sleeve piston is in said first position and which is closed by the sleeve piston when the sleeve piston is in a second longitudinal position in the bore.

40. A vehicle brake system according to claim 38 or claim 39 wherein the longitudinally extending passage of the sleeve piston may terminate in a longitudinally facing port at the end of the sleeve piston adjacent said opposite end of the bore.

41. A brake system according to claim 40 wherein spring means are provided to act on the sleeve piston to bias it towards said one end of the bore.

42. A brake system according to claim 41 wherein the spring means is disposed in a cou. ter-bore provided at said opposite end of the sleeve.

43. A brake system according to any one of claims 38 to 42 wherein bleed means are provided whereby fluid under pressure may act on opposite ends of the sleeve piston.

44. A brake system according to any one of claims 38 to 43 wherein the sleeve piston is adapted to be moved between the first and second positions by an element acting on the one end of the sleeve piston and the element may be moved in the longitudinal direction by electromagnetic means.

45. A brake system according to any one of claims 38 to 44 wherein the components of the control valve are configured to avoid any regions where air may be trapped so that after the system has been bled, no air is present in the system.

46. A brake system substantially as hereinbefore described with reference to the accompanying drawings.

47. Any novel feature or novel combination of features described herein and/or in the accompanying drawings.