GB2049080A

GB2049080A - Vehicle Braking Systems

Info

Publication number: GB2049080A
Application number: GB7916280A
Authority: GB
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1979-05-10
Filing date: 1979-05-10
Publication date: 1980-12-17
Also published as: GB2049080B

Abstract

A vehicle braking system incorporates a modulating valve 6 for reducing outlet pressure applied to brakes on the rear wheels 7 as compared with inlet pressure supplied to the brakes on the front wheels 4. The valve 6 is controlled in dependence on the pressure in the front and rear brake lines 3, 9 and on the vehicle deceleration as detected by measuring the rear wheel deceleration. Excessive deceleration indicative of a skid, results in pressure being reduced by the valve. <IMAGE>

Description

SPECIFICATION Vehicle Braking Systems This invention relates to vehicle braking systems of the kind which allow proportions of braking efforts to be varied between front and rear wheels.

Some known vehicle braking systems of the kind set forth incorporate proportioning valves which reduce the outlet pressure applied to rear wheel brakes, as compared to the full inlet pressure supplied to the front wheel brakes, after a predetermined inlet pressure is attained.

In addition it is known to alter the operating characteristics of such proportioning valves by the incorporation of inertia-responsive means, for example a ball or pendulum, to take into account the deceleration of the vehicle and the shift of load from the rear to the front axle during braking.

Such known proportioning valves have the disadvantage that during sudden braking, for example in an emergency stop, the full inlet pressure is built up in the rear wheel brakes before the inertiaresponsive means has had time to respond.

According to our invention in a vehicle braking system of the kind set forth incorporating a proportioning valve for reducing the outlet pressure applied to the rear wheel brakes as compared with the inlet pressure supplied to the front wheel brakes, the proportioning valve is responsive to control means responsive to the deceleration of the rear wheels or the transmission of the rear wheels.

Producing control means responsive to deceleration of the rear wheels or the transmission to the rear wheels, rather than to the deceleration of the vehicle itself, has the advantage that the attitude of the vehicle does not activate deceleration control means. In addition if the deceleration of the rear wheels or the transmission to the rear wheels is excessive then the pressure applied to the rear wheel brakes can be reduced or fully released. Furthermore the same control means can be utilised for any vehicle ranging from a small car to a heavy lorry.

Some embodiments of our invention are illustrated in the accompanying drawings in which Figure 1 is a layout of an hydraulic braking system for a vehicle; Figure 2 is a graph showing the ratios between the pressure applied to front and rear wheel brakes; Figure 3 is a layout of an air-operated anti-skid braking system for a vehicle; Figure 4 is a section through the proportioning valve of Figure 3; Figure 5 is a layout of an hydraulic power-braking system including a section through a proportioning valve; Figure 6 is a system similar to the layout of Figure 1 but as applied to an hydrostatic braking system; Figure 7 is a layout of a further hydrostatic braking system; Figure 8 is a longitudinal section through the proportioning valve of Figure 7; Figure 9 is an end view of the valve of Figure 8;; Figure 10 is a layout of an hydrostatic braking system similar to Figure 7; Figure 11 is a longitudinal section through the proportioning valve and mechanical speed sensor of Figure 10; Figure 12 is an end view of the proportioning valve of Figure 1 1; Figure 13 is the layout of a further braking system; Figure 1 4 is a longitudinal section through the proportioning valve and mechanical speed sensor of Figure 13; Figure 1 5 is an end view of the proportioning valve of Figure 14; Figure 1 6 is a graph showing the ratios between front and rear brake pressures for the braking system of Figures 13-1 5; Figure 1 7 is a layout of a braking system similar to Figure 1 3 but showing a modification;; Figure 1 8 is a longitudinal section through a proportioning valve incorporated in the layout of Figure 17; Figure 19 is an end view of the valve at Figure 1 8; Figure 20 is a graph showing the ratio between the pressure applied to the brakes on the front wheels, and on the rear wheels, for different braking conditions; Figure 21 is the layout of a further braking system; Figure 22 is a longitudinal section through an auxiliary master cylinder and proportioning valve assembly for the braking system of Figure 21; Figure 23 is an end elevation of the assembly illustrated in Figure 22; and Figure 24 is a graph showing the ratio between the pressure applied to the brakes on the front wheels, and on the rear wheels, for different braking conditions.

In the braking system illustrated in Figures 1 and 2 of the drawings, 1 is an hydraulic tandem power valve having a first pressure space 2 connected directly through a pipe-line 3 to brakes on front wheels 4 of the vehicle, and a second pressure space 5 connected through a proportioning valve 6 to brakes on rear wheels 7 of the vehicle.

The proportioning valve 6 incorporates a solenoid-operated inlet valve 8 controlling communication between the power valve 1 and the brakes on the rear wheels 7 through a pipe-line 9, and a solenoid-operated exhaust valve 10 controlling communication between the inlet valve 8 and an exhaust port 11 which leads to a reservoir for hydraulic fluid (not shown).

Operation of the solenoid-operated valves 8 and 10 is controlled by an electronic control module 12. The control module 1 2 receives a voltage signal VF from a pressure sensor 1 3 sensing pressure applied to the brakes on the front wheels 4 and which is proportioned to that pressure, a voltage signal VR from a pressure sensor 14 sensing pressure applied to the brakes on the rear wheels 7 and which is proportional to that pressure, and a voltage signal VG which is proportional to the deceleration of the rear wheels 7 of the vehicle, or to the transmission to the rear wheels 7, from a speed sensor 1 5.

Normally the inlet 8 is open, and the exhaust valve 10 is closed. When the power valve 1 is operated, fluid is supplied directly to the brakes on the front wheels 4 and fluid under pressure is supplied to the brakes on the rear wheels 7 in an unrestricted manner through the open inlet valve 8.

The system sets up an equation: Constant x Front brake pressure = rear brake pressure + rear wheel deceleration x constant It follows therefore that for any given front wheel brake pressure, a deceleration requirement is established for the rear wheels 7 by the control module 12. That is to say even if no brakes were fitted to the rear wheels 7, the rear wheels would decelerate at vehicle deceleration. Specifically the control module 12 is operative to actuate the solenoid operated valves 8 and 10 to modulate the pressure applied to the brakes on the rear wheel of the vehicle 7. If the expected deceleration is not seen by the control module 12, the equation becomes unbalanced and the valve 8 is held open so that the pressure applied to the brakes on the rear wheels 7 can be increased further.If the deceleration of the rear wheels exceeds the expected value the equation becomes unbalanced in the opposite sense and the valve 8 is closed and the valve 11 is opened so that the pressure applied to the brakes on the rear wheels 7 is reduced until balance is restored.

Figure 2 is a graph showing the relationship between the pressure applied to the rear wheel brakes and the pressure applied to the front wheel brakes for an unladen vehicle of the lower part of the graph and for a fully laden vehicle at the upper part of the graph. The respective pressures are shown in broken lines and it will be seen that they are very near to the ideal ratios for that particular vehicle, which are shown in full lines.

In the air-operated braking system illustrated in Figures 3 and 4 of the accompanying drawings air from a main supply 16 is supplied to the brakes 17 on rear wheels of the vehicle through a proportioning valve 1 8 under the control of a brake-applying valve 1 9. The speed of the rear wheels, or of the transmission to the rear wheels, is measured by a sensor 20 which supplies a voltage signal to an electronic control module 21. This differentiates the signal from the sensor 20 in preparation for use in the control logic.

The proportioning valve 1 8 comprises a housing 22 having a stepped bore 23 in which works a differential piston 24. The inner end of the piston 24, which is of smaller area, acts on a load cell 25 through a rod 26. The rod 26 carries a first valve member 27 which is normally spaced from a second hollow valve member 28, through which the rod 26 extends with a substantial clearance therebetween, the second valve member 28 works through a bushing 29 in a second bore 30 in the housing 22 and is urged by a spring 31 into engagement with a seating 32 in the housing 22 between a passage 33 for connection to the main supply on the upstream side of the valve 19 through a branch pipe-line 34 and an outlet passage 35 for connection to the brakes on the rear wheels and to which the inner end of the differential piston 24 is exposed.

An electrical signal from the load cell 25 is fed to the control module 21, and the control module 21 emits an output voltage for energising a solenoid 36 of a valve 37 which is normally closed to isolate an input passage 38 connected to the brake-applying valve 1 9 from a chamber 40a. In addition the input passage 38 communicates with the end of the piston 24 which is of greater area.

The exhaust port 39 is disposed between the valve member 28 and a piston 40 which works in a bore 41 in the housing 22 and against which the load cell 25 abuts. A restricted passage 42 provides communication between opposite sides of the piston 40.

In operation of the system of Figures 3 and 4, actuation of the brake-applying valve 1 9 admits high pressure fluid to the housing 22 which acts on the end of the piston 24 which is of greater area.

This moves the piston 24 inwardly, initially to cause the valve member 27 to engage with the valve member 28, thereby isolating the brakes 1 7 from the exhaust port 39, through the clearance between the valve member 28 and the rod 26. Subsequent movement of the piston 24 in the same direction urges the valve member 28 away from the seating 32 in the housing to place the passage 33 in communication with the brakes 1 7.

Opposite ends of the piston 24 are therefore subjected to input and output pressures. As the pressure in the brake actuators builds up, the resulting wheel deceleration causes the control module 21 to energise the solenoid 36 to hold the valve 37 open so that air under pressure from the input passage 38 can act on the adjacent face of the piston 40. This produces a force which is transmitted by the load cell 25 and the rod 26 to the piston 24, thus augmenting the effect of the pressure applied to the brakes 1 7. The load cell 25 measures the force produced by the piston 40 and de-energises the solenoid 36 when it reaches the correct value. That is to say the load cell 25 gives an electrical signal of equivalent strength to the wheel deceleration signal to the control module 21 and the solenoid valve closes.Thereafter the pressure acting on the piston 40 can pass to the exhaust port 39, through the restricted passage 42.

The force proportional to deceleration on the piston 40 and the input pressure acting on the.

piston 24 determine what proportion of the input pressure is supplied to the rear brakes.

Should the road surface be slippery and the deceleration of the rear wheels exceeds the deceleration of the vehicle then the solenoid valve 37 opens to act to increase the pressure differential across the piston 40. This causes the valve member 28 to operate and relieve the pressure applied to the brakes on the rear wheels 7.

The ratio between input pressure and the pressure applied to the brakes 1 7 for a laden and unladen vehicle is the same as that shown in the graph of Figure 2.

The braking system illustrated in Figure 5 is similar to that of Figures 3 and 4 except it comprises a full hydraulic power system.

As illustrated a pedal-operated control valve 45 directs hydraulic fluid from an hydraulic accumulator 46 directly to front wheel brakes 47 and to the input passage 38 of the proportioning valve 18.

In the proportioning valve 1 8 the restricted passage 42 comprises a restriction by-passing the piston 40, the valve members 27 and 28 comprise an extension 49 acting as a spool to control communication between the passage 33 and the outlet passage 35, and an armature 50 movable by the solenoid 36 carries a needle valve member 51 for engagement with a seating 52 between the inlet passage 38 and the end of the bore 41 which is remote from the load cell 25. Finally the exhaust port 39 is connected to a reservoir 53 for hydraulic fluid.

The construction and operation of the braking system of Figure 5 is otherwise the same as that of Figures 3 and 4 and corresponding reference numerals have been applied to corresponding parts.

The ratios between input pressure applied to the brakes 1 7 for a laden and an unladen vehicle is the same as that shown in the graph of Figure 2.

The braking system illustrated in the layout of Figure 6 is similar to the system of Figure 1 but modified for hydrostatic booster operation.

As illustrated the exhaust connection 11 is replaced by an air outlet 54 and an inlet 55 to the inlet valve 8 instead of being connected to the master cylinder 1 is connected to a source of vacuum.

A booster 56 for operating an hydraulic master cylinder 57 for applying the brakes on the rear wheels 7 incorporates a movable wall 58 which acts on the piston 59 of the master cylinder 57. The movable wall 58 is exposed on its face adjacent to the master cylinder 57 to the vacuum through a connection from the inlet 55 and, normally, when the valve 8 is open, the opposite face of the movable wall 58 is also exposed to vacuum through a passage 60.

When the master cylinder 1 is operated to apply the brakes on the front wheels 4, the control module is operative to energise the solenoids of the valves 8 and 10. Initially the valve 8 closes to isolate both sides of the movable wall 58. Subsequently the valve 10 is opened so that the face of the movable wall 58 which is remote from the master cylinder 57 is exposed to atmosphere to energise the booster 56. This actuates the master cylinder 57 to apply the brakes on the rear wheels 7 of the vehicle.

The construction and operation of the braking system of Figure 6 is otherwise the same as that of Figure 1 and corresponding reference numerals have been applied to corresponding parts.

In the braking system shown in the layout of Figure 7 a booster-assisted hydraulic tandem master cylinder 60 operates brakes on front wheels 61 of a vehicle directly through a pipe-line 62, and brakes on rear wheels 63 through a pipe-line 64 leading to a proportioning valve and auxiliary master cylinder assembly 65, which is shown in detail in Figure 8, and a pipe-line 66 leading from the assembly 65 to the brakes on the rear wheels.

The assembly 65 comprises auxiliary master cylinder 67 which is operated by a booster 68 of the vacuum-suspended type. The master cylinder 67 comprises a piston 69 which is operative to pressurise hydraulic fluid in a pressure space 70 for application to the brakes on the rear wheels through the pipelines 66. The piston 69 is advanced into the pressure space 70 by the application of differential pneumatic pressures to opposite sides of the movable wall 71 of the booster 68; and energisation of the booster 68 is controlled by a booster valve 72 operable in response to pressure from the master cylinder 60 through the pipe-line 64.

The movable wall 71 divides a housing 73 into a forward constant pressure chamber 74, which is connected permanently to a source of vacuum, and a rear energising chamber 75.

The booster valve 72 is located in a stepped bore 76 which is transverse to the axis of the pressure space 70. The valve 72 comprises a resilient valve member 77 of generally cup-shaped outline of which a portion terminating at the end of the skirt seals against the wall of the bore 76 and lies against a filter 78 for atmospheric air. The crown of the valve member 77 has a central aperture 79 surrounded by a seating into engagement with which a second valve member 80 is urged by means of a spring 81. This isolates an air inlet 82 to the filter 78 from a chamber 83 which communicates with the energising chamber 75 through an external connection 84.

A control member 85 comprising a diaphragm divides a compartment into a lower vacuum chamber 86 which is in permanent communication with the constant pressure chamber 74 through an internal passage 87, and an upper chamber 88 which is normally sealed from atmosphere by means of a normally closed solenoid-operated valve 89. A permanent leakage between the chambers 86 and 88 takes place through a bleed orifice 90 in the diaphragm 85. In addition the lower chamber 86 communicates with the chamber 83 through an axial passage 91 which is spaced radially outwards from an annular seating 92 with respect to which the crown of the valve member 77 is normally spaced.

A piston 93 working in a portion 94 of the bore 76 which is of smallest diameter acts on the diaphragm 85 through a piston rod 95, a thrust member 96 with which the piston rod 95 engages, and a load cell 97 which is disposed between the thrust member 96 and the diaphragm 85.

A speed sensor 98 for sensing the deceleration of the rear wheels of the vehicle, or the transmission to the rear wheels, emits a voltage signal which is supplied to a control module 99. The control module 99 also receives a signal from the load cell 97 and emits an energising voltage to energise the solenoid of the valve 89 until the load cell voltage balances the voltage proportional to wheel deceleration. Then the valve 89 closes.

The pressure space 70 is connected through a passage 100 to the inner annular area of the piston 93. The opposite outer face of the piston 93 is exposed to pressure from the master cylinder 60 via the passage 64, and is also connected via an internal passage 103 to that side of the piston 69 which is remote from the pressure space 70. In the inoperative position shown, the piston 69 is spaced from the shoulder 101 at a step in the diameter of the master cylinder 67 and is surrounded by a spring-loaded shroud 102 in which it is slidably movable. The shroud 102 and the shoulder 101 constitute a recuperation valve for the auxiliary master cylinder 67 so that when the shroud 102 is spaced from the shoulder the passage 103 communicates with the pressure space 70 through angularly spaced axial passages 104 between splines in the outer face of the shroud 102.Thus, in an inoperative position, the pressure space 70 is exhausted td the reservoir of the master cylinder 60.

When the booster operated master cylinder 60 is operated hydraulic pressure is fed directly to the brakes on the front wheels 61 through the pipe-line 62 and through the pipe-line 64, through the passage 103 and the open recuperation valve to the brakes on the rear wheels 63 through the pressure space 70. The pressure in the pipe-line 64 acts on the piston 93 to advance it in the bore 94. Initially this urges the valve member 77 into engagement with the seating 92 to isolate the chambers 74 and 75 from each other and subsequently urges the valve member 80 away from the valve member 77 so that air is admitted to the chamber 75 to energise the booster. Initially this advances the shroud 102 and the piston 69 together until the recuperation valve closes.Further movement in the same direction causes the piston 69 to move relative to the shroud 102 and augment the pressure applied to the rear wheel brakes by pressurising the fluid trapped in the pressure space 70. Alternatively the seal on the piston 69 could pass over a small recuperating hole, in a similar manner to that of a seal carried by the piston of a master cylinder.

At the same time as the rear wheels or the transmission of the rear wheels decelerates, a signal is given to the control module 99 to activate the solenoid valve 89. This allows air to enter the chamber 88 and act upon the control diaphragm 85 so that the boost ratio of the booster 68 is modified.

When the force exerted by the control diaphragm 85 on the load cell 97 is sufficient for an electrical signal to be generated which is equal to that of the electrical signal generated by the deceleration of the rear wheels, the solenoid valve closes. Since air pressure in the chamber 88 is allowed to leak slowly to the chamber 86 through the bleed orifice 90 in the diaphragm 85 the solenoid of the valve 89 continues to operate to maintain the pressure differential across the control diaphragm 85.

This balance of forces on the control diaphragm 85 determines the boost ratio of the booster 68 and hence the magnitude of pressure supplied to the brakes on the rear wheels 63 in proportion to the pressure applied to the brakes on the front wheels 61.

Should the road surface be slippery and the deceleration of the rear wheels exceeds the deceleration of the vehicle then the solenoid valve 89 opens to act to increase the pressure differential across the control diaphragm 85. The boost ratio of the booster 68 is changed so that the valve 72 operates to relieve the pressure applied to the brakes on the rear wheels 63.

The ideal ratio between front and rear brake pressures and, in dotted lines, the actual ratio achieved is also shown in the graph of Figure 2.

In a modification the bleed orifice 90 can be replaced by small inlet and outlet valves operated by the solenoid.

In the embodiment described above with reference to Figures 7-9, in the event of failure of the supply of vacuum the brakes on the rear wheels are applied by fluid from the master cylinder 60 which passes to the brakes through the passages between the splines 1 04.

In addition it may be necessary to indlude a restrictor, for example in the passage 103, to ensure that the valve 72 can be operated to energise the booster 68 before any significant pressure rise in the rear brake line can occur by the supply of fluid from the master cylinder 60 and to the brakes on the rear wheels through the open recuperation valve defined by the shroud 102 and the shoulder 1 01.

The braking system shown in Figures 10-12 is similar to that of the embodiment of Figures 7-9 except that the electronic control module 99 and speed sensor 98 have been replaced by a mechanical assembly 105 for sensing the deceleration of the rear wheels 63, or of the transmission to the rear wheels 63. In addition the solenoid-operated valve 89 and the load cell 97 have been omitted.

The skid sensing assembly 105 controls the application of a control pressure to the control diaphragm 85.

As illustrated the assembly 105 comprises a shaft 106 which is journalled for rotation in a housing 107 and is driven by the rear wheels 63 or by transmission to the wheels 63. An axially fixed flywheel 108 is journalled for rotation on the inner end of the shaft 106, and the flywheel 108 is driven from the shaft 106 through an expander mechanism 109. The expander mechanism 109 comprises a sleeve 110 which is slidably keyed to the shaft 1 06 and which is formed at its outer end with a radial flange 111, a collar 112 concentric with the sleeve 110 and driven from it through a clutch 11 3 defined by the engagement of the collar 112 with the flange 111, and a series of angularly spaced balls 114 which are received in complementary recesses 115,116 in adjacent faces of the flywheel 108 and the collar 112.

A normally-closed air inlet 117, comprising a head 118 for engagement with a seating 119 in the housing 107, and a stem 120 carrying the head 118, is controlled by a sensor diaphragm 121. The sensor diaphragm 1 21 is engaged by the free end of the stem 1 20 and on its opposite side, carries a stem 122 which engages at an intermediate point in the length of a lever 123 of which the outer end is pivotally connected to a fixed fulcrum 1 24 on the housing 107, and the opposite end engages with a thrust washer 125 acting on the sleeve 110.

The flywheel 108 is enclosed within a sealed end closure 1 26 for that end of the housing 107, and a chamber formed between the closure 126 and the diaphragm 121 is connected to a source of vacuum which also provides the energisation for the booster 68. Since the flywheel 108 is housed within a chamber which is subjected to vacuum, this has the advantage of eliminating resistance to rotation due to windage. If the orifice 90 is omitted from the control diaphragm 85, the sensor diaphragm 121 is provided with a small bleed orifice so that, normally, the diaphragm 121 is held in a balanced position with opposite faces subjected to equal pressures.The control diaphragm 85 is also subjected on opposite sides to vacuum from the booster 68 and through a pipe-line 127 which interconnects the chamber 88 above the diaphragm 85 with a passage 1 29 between the seating 11 7 and the diaphragm 121.

When the shaft 106 decelerates the flywheel 108 overruns and expands the ball and ramp mechanism 114, 11 5, 116 which, in turn, opens the inlet valve 117 by pivotal movement of the lever 123. This admits atmosphere to the passage 129 to act on the diaphragm 121, and to the chamber 88 through the pipe-line 127, to act on the diaphragm 85.

The pressure differential to which the diaphragm 1 21 is exposed provides a force which, when it is equal to the force generated by the ball and ramp 114, 11 5, 11 6, causes the valve 117 to close.

Since the torque on the flywheel 108 is proportional to deceleration the axial force applied to the sleeve 110 through the ball and ramp 114,115,116 and which acts on the valve 117 is also proportional to deceleration. It follows therefore that the pressure differential acting across the diaphragm 121 and the diaphragm 85 is proportional to deceleration. Due to the orifice in the diaphragm 121 (or the diaphragm 85) pressure can leak away to atmosphere so that the flywheel 108 gently oscillates to maintain the pressure differential across the diaphragm 121.

In a modification the bleed orifice is omitted, and the inner end of the stem 120 carries a head for engagement with one end of a passage through the diaphragm 121. This forms an exhaust valve which is open when the inlet valve 11 7 is closed, but which closes before the inlet valve 11 7 opens.

Should the wheel deceleration increase above that determined by the brake pressure, then the pressure differential on the sensor diaphragm 121 will increase. As this pressure is fed to the control diaphragm 85 the brake pressure will be reduced. The reduction in brake pressure will cause the wheel to accelerate and cancel the flywheel signal. This allows the signal pressure to leak away and gently re apply the brakes.

The clutch 113 allows relative movement between the collar 112 and the sleeve 111 to take place due to overrun of the flywheel 108, after the pressure differential has reached a maximum value.

The sensor diaphragm 121 is biassed away from the inlet valve 117 by means of a light compression spring 1 30. This spring force has first to be overcome by the force applied to the diaphragm 121 through the lever 123. After the spring force has been overcome the pressure difference across the diaphragm 1 21 is proportional to deceleration, which prevents the sensing assembly 105 from operating at extremely small decelerations.

The construction and operation of the braking system illustrated in Figures 10-22 is otherwise the same as that of Figures 7-9 and corresponding reference numerals have been applied to corresponding parts.

The ratio between front and rear wheel pressures for ideal and actual ratios is again shown in the graph of Figure 2.

In the braking system illustrated in Figures 13-1 S of the accompanying drawings the brakes on the front wheels, instead of being applied directly from the master cylinder 60, which is "boosteroperated", -are applied by a booster-operated auxiliary master cylinder assembly 140. The booster of the assembly 140 is operated in respect to a supply of hydraulic pressure from the master cylinder 60 through a pipe-line 141.

The brakes on the rear wheels 63 are again applied by the proportioning valve and auxiliary master cylinder assembly 65 which is operated by the booster 68, and operation of the booster 68 is controlled by the skid sensing assembly 1 05.

The auxiliary master cylinder 65 incorporates a recuperation valve 1 42 which is normally open to place the pressure space 70 in communication with a reservoir 143 for the master cylinder 60 through a pipeline 144.

In this construction the pressure from the master cylinder 60 acts only on the piston 93 and cannot be transferred to the pressure space 70. Thus the brakes on the rear wheels 61 can be applied only in response to energisation of the booster 68. This has the advantage that during brake release during the "deboost mode" the pressure applied to the brakes on the rear wheels 61 can be reduced to the pressure in the reservoir. However there is no provision for applying the brakes on the rear wheels 61 upon failure of the booster 68.

The construction and operation of the assembly 65 and of the skid sensing assembly 105 is otherwise the same as that of Figure 11, and corresponding reference numerals have been applied to corresponding parts.

The graph of Figure 1 6 shows the ratio between the pressure applied to the rear wheels and to the front wheels under laden, and unladen conditions, with the deceleration signal, when inoperative, being shown in chain-dotted outline.

In the braking system shown in Figures 1 7-20, the mechanical sensor 105 is replaced by an electrical speed sensor 145, the assembly 65 incorporates a load cell 146 and a solenoid-operated valve 147, and an electronic control module 148 operates the valve 147 in accordance with signals received from the control module 1 48 in response to signals from the load cell 1 46 and the sensor 145.

As in the braking system of Figures 7-9 described above as the rear wheels or the transmission to the rear wheels decelerates when the master cylinder 60 is operated, a signal is given to the control module 148 to actuate the solenoid valve 147. This allows air to enter the chamber 88 and act upon the control diaphragm 85 so that the boost ratio is modified.

Simiiarly when the force exerted by the control diaphragm 85 on the load cell 97 is sufficient for an electrical signal to be generated which is equal to that of the electrical signal generated by the deceleration of the rear wheels the solenoid valve 147 closes. Since air pressure in the chamber 88 is allowed to leak slowly to the chamber 86 through the bleed orifice 90 in the diaphragm 85, the solenoid of the valve 147 continues to operate to maintain the pressure differential across the control diaphragm 85.

The construction and operation of the braking system of Figures 1 7-20 is otherwise the same as that of Figures 13-1 5 and corresponding reference numerals have been applied to corresponding parts.

In the anti-skid braking system of Figures 21-24 a passage 1 50 interconnects the inlet passage 64 to the remote end of the recuperation valve 142 and the connector 144 is omitted.

This enables the brakes on the rear wheels 63 to be applied in the event of failure of the booster 73, by means of fluid supplied directly to the pressure space 70, from the master cylinder 60, and through the passage 1 50 and the open recuperation valve 1 42.

The construction and operation of the system of Figures 21-24 is otherwise the same as that of Figures 1 7-20, and corresponding reference numerals have been applied to corresponding parts.

In the braking systems described above with reference to Figures 3-5, and 7-24 the formula for each proportioning valve is as follows: RP1A1-P2A2=K3g-(1) where R = ratio of input pressure/brake pressure P, = front wheel brake pressure P2= rear wheels brake pressure A, = area of the input piston 24, 93 A2 = annular area of the output piston 49, 69 K3 = deceleration force amplification for conversion from wheel deceleration in g to the force output of the control member 85 The formula for the vehicle is as follows: K1P1+K2P2=Mg+K4-(2) where K, = front brake constant K2 = rear brake constant M = weight of vehicle K4 = threshold pressure constant From (1) and (2):: K3(K,P,+K2P2K4)=M(RP,A,P2A2) P,(RMA1K,K3)+K4K3 P2= MA2+K2K3 This formula gives the relationship between the pressure applied to the front wheels and the pressure applied to the rear wheels. For a given vehicle the only variabie is M, the vehicle weight.

In the graph of Figure 2 we illustrate the relationship between P1 and P2 for an unladen vehicle, and for a laden vehicle. These lines are very near the ideal curves for that particular vehicle.

The advantage of providing active proportioning valves of the kind described in the foregoing embodiments is that they are abie to increase or reduce the pressure applied to the rear wheels until an appropriate deceleration is achieved. Previous passive designs could be beaten by rapid actuation which trap an excessive pressure in the brakes for the rear wheels before the deceleration sensing device had time to operate.

However, valves operated by an inertia-responsive means, for example a ball or pendulums, are not the complete answer because on low T surfaces the rear wheels could still lock. This would reduce the vehicle deceleration (y/slip curve) and the inertia-responsive means would act in the wrong sense by calling for a higher brake pressure.

Making the proportioning valve responsive to wheel deceleration corrects this fault and ensures that the rear wheels should not lock. This type of control is especially advantageous because the wheel does not have to skid before corrective action is taken.

With conventional anti-skid braking systems the wheel deceleration has to exceed at least 1 g before a skid is signalled. Usually the threshold is closer to 2 g to prevent spurious operation. This results in jerky control.

With the systems described above however, control action occurs whenever the wheel deceleration exceeds the capability of the surface, (as predicted by prevailing front brake pressure) be it 0.1 g or 1 g. Hence much smoother control results.

Correct functioning is however dependent upon the driver to adjust the master cylinder pressure so as to avoid locking the front wheels. If the driver fails to do this, for example, in an emergency,the valve will be fooled into assuming that the surface adhesion co-efficient is higher than it really is, and will allow a higher wheel deceleration before taking action. A further consequence is that the control member would need to be large enough to ensure that it could always overcome the force from the control piston 24, 93.

Both these factors can be alleviated to some degree by fitting an ordinary pressure limiting valve in the line between the master cylinder or control valve and the control piston, and by delaying the signal to reduce the force from the control member at least during brake pressure cycling. The limiting valve would be set to cut off at the pressure normally needed in the rear brakes during a 0.9 g stop.

The addition of the limiting valve ensures that the deceleration requirement remains within realistic limits and enables the use of a control member of smaller area.

The adhesion utilisation will be reduced under overbraking conditions but should be no worse than conventional systems employing a 1 g threshold. If pedal pressure appropriate to 0.8 g stop is applied on a 0.3 g surface the brakes will not be released until the wheel exceeds a 0.8 g deceleration and would normally be reapplied as it drops below 0.8 g. The provision of a delay would enable the wheel to return to 0.3 g before the next cycle.

If the front brake circuit is equipped with anti-skid pressure modulators, the pressure limiting valve referred to above can be arranged to operate at a pressure dependent upon the instantaneous front brake pressure.

Because the system detects a skid very early it could be especially useful in conjunction with brake actuators having a high hysteresis such as the 'S' cam brake. Early detection may avoid the need to drop the brake pressure almost to zero before the wheel responds.

Claims

1. A vehicle braking system of the kind set forth incorporating a proportioning valve for reducing the outlet pressure applied to the rear wheel brakes as compared with the inlet pressure supplied to the front wheel brakes, in which the proportioning valve is responsive to control means responsive to the deceleration of the rear wheels, or to the transmission to the rear wheels.

2. A vehicle braking system as claimed in claim 1, in which the proportioning valve incorporates a solenoid-operated inlet valve controlling communication between a source of pressure fluid and the rear wheel brakes, and a solenoid-operated exhaust valve controlling communication between the inlet valve and exhaust port, and operation of the solenoid-operated valves is controlled by an electronic control module, the control module receiving a voltage signal proportional to the pressure applied to the brakes on the front wheels, a voltage signal proportional to the pressure applied to the brakes on the rear wheels, and a voltage signal proportional to the deceleration of the rear wheels, or to the transmission to the rear wheels.

3. A vehicle braking system as claimed in claim 1, in which operation of the proportioning valve is controlled by a solenoid-operated valve which is operative to change the characteristics of the proportioning valve, the solenoid-operated valve is, in turn, controlled by a control module, and the proportioning valve incorporates a load cell which emits an electrical signal which is transmitted to the control module, the control module receiving an electrical signal proportional to the deceleration of the rear wheels, or to the transmission to the rear wheels, and being adapted to emit a signal to energise and open the solenoid-operated valve in response to such a signal, the solenoid-valve closing when a force on the load cell causes the electrical signal which it emits to be of equivalent strength to that proportional to the deceleration of the rear wheels, or to the transmission to the rear wheels.

4. A vehicle braking system as claimed in claim 3, in which the proportioning valve incorporates a control member subjected to differential pressures by means of which the boost ratio of a booster for applying the rear wheel brakes; the control member and the load cell co-operating with each other, and the solenoid-operated valve being operative to control the magnitude of the differential pressure acting on opposite sides of the control member.