GB2098292A - Modulators in anti-skid hydraulic braking systems for vehicles - Google Patents

Abstract

An anti-skid hydraulic braking system for vehicles includes a modulator (3) which, in response to an incipient skid signal relieves the brake- applying pressure at a braked wheel by expanding the volume available to fluid from the brake (2). The modulator (3) provides a permanent connection between a master cylinder (1) and a brake-line to the brake (2). An expansion piston (40) thereof is movable in a first direction, drawing fluid from the brake-line and into an expansion chamber (42) through a first one-way valve (71), and introduces restrictive means (6) between the master cylinder (1) and the brake-line; the piston (40) moves in a second direction expelling fluid from the expansion chamber (42) through a second one-way valve (72). A change in pressure in a support chamber (43) in response to the skid signal permits the piston (40) to move in the first direction and, at the termination of the skid signal, the pressure in the support chamber (43) changes again to urge the piston (40) in the second direction, and the restrictive means (6) is removed preferably when the brake pressure is equal to the master cylinder pressure. In other embodiments (Figs. 3 to 7, not shown) the expansion piston is moved pneumatically, and differences in the layout of the modulator are described. <IMAGE>

Description

SPECIFICATION Improvements in anti-skid hydraulic braking systems for vehicles This invention relates to anti-skid hydraulic braking systems for vehicles of the kind which include a modulator which, in response to a skid signal at a skid point from means to sense the presence of skid conditions of the wheel during braking, is operative to relieve the brake-applying pressure at a braked wheel by releasing fluid from the brake.

In one known anti-skid system of the kind set forth which incorporates an hydraulic pump and an isolating valve located downstream of the master cylinder, the pump comprises a scavenger pump which is operative at least at the termination of the skid to withdraw fluid from a dump chamber to which the fluid was dumped to relieve the brake in response to the skid signal and to return such fluid to the master cylinder.

This known type of system has the advantage that the isolating valve may be replaced by a restricting means which is adapted to allow permanent communication between the master cylinder and the brake, but in such known antiskid braking systems the hydraulic pump is provided exclusively for the braking system, in addition to any other power source already present in a vehicle, for example a separate power steering pump, an air compressor, or vacuum generated by the engine of the vehicle.

According to our invention in an anti-skid hydraulic braking system of the kind set forth the modulator provides a permanent connection between a master cylinder for applying the brake and a brake-line to the brake itself, and a displacement piston working in a bore in a housing is movable in first and second opposite directions, movement of the displacement piston in the first direction drawing fluid from the brakeline and into a displacement chamber through a first one-way valve, and acting to introduce restrictive means between the master cylinder and the brake-line, and movement of the displacement piston in the second direction expelling fluid from the displacement chamber through a second oneway valve leading to the master cylinder, a power source being provided for pressurising a support chamber whereby normally to hold the displacement piston in an advanced position defining the extent to which the piston can be moved in the said second direction and in which the effective volume of the displacement chamber is at a minimum, a change in pressure in the support chamber in response to the skid signal permitting the piston to move in the first direction, and at the termination of the skid signal the pressure in the support chamber changes again to urge the piston in the second direction.

Preferably the restrictive means is removed when the brake pressure is substantially equal to the pressure from the master cylinder.

The displacement piston separates the fluid used in such a power source from that in the braking system, which fluids are normally of different type. The power source may comprise an hydraulic pump and/or hydraulic accumulator, for example for the power steering or suspension of the vehicle. Alternatively we can use as the power source a source of positive or negative pneumatic pressure, for example a source of vacuum such as suction from the manifold of an engine of a vehicle, or of compressed air.

Preferably the fluid from the master cylinder is applied to the brake line through a first flowcontrol regulator valve incorporating first restrictor means and the fluid from the power source may be supplied to the support chamber through a second flow-control regulator valve incorporating second restrictor means.

The modulator may comprise a wall movable in a housing to divide the housing into first and second chambers and co-operates with the displacement piston to oppose pressure applied to the brake through a flow-control regulator valve, and the power source normally subjecting one of the chambers to a corresponding fluid pressure to hold the wall in an inoperative position in which the displacement piston is held in the corresponding, advanced, position, and both chambers being interconnected in response to a skid signal whereby the wall can move into an operative position accompanied by retraction of the displacement piston whereby to increase the effective volume of the displacement chamber, whereafter, at the termination of the skid signal, only the said one chamber is connected to the power source.

Where used herein the term "positive" pressure applies to pressure in excess of atmospheric pressure, and the term "negative" pressure refers to a pressure less than atmosphere, suitably vacuum.

When the source comprises compressed air, the air acts in the first chamber and the second chamber is normally connected to atmosphere.

When a skid occurs the chambers are interconnected, and at the termination of the skid signal the compressed air in the second chamber is exhausted to atmosphere.

When the source comprises vacuum the first chamber is connected to atmosphere during normal brake application. The second chamber is connected to vacuum before and after a skid signal but is connected to the first chamber during a skid signal.

In each case communication between the two chambers is controlled by a solenoid-operated valve of which the solenoid is energised in response to the skid signal to place the two chambers in communication.

When the source comprises hydraulic fluid under pressure, the second chamber would normally be connected to a tank for fluid during normal brake application, and after a skid signal, the first chamber always being connected to the pressure source.

Alternatively, the fluid pressure from the power source may be applied to that end face of the displacement piston which is remote from the displacement chamber, and which defines at least in part the support chamber, with the solenoidoperated valve controlling a normally closed connection between the support chamber and a low pressure reservoir.

This arrangement is particularly suitable when the power source comprises a supply of hydraulic fluid under pressure, in which case the displacement piston may comprise a differential piston in order to generate in the braking system higher pressures than are available from the power source.

Conveniently we provide means for monitoring regularly the deceleration of the braked wheel and, after a skid signal has first become operative, the skid signal is cancelied when, over two successive readings, the deceleration of the wheel has fallen.

The skid signal is operative to cause energisation of the solenoid of a solenoid operated dump valve for dumping the fluid from the support chamber to the displacement piston, and the solenoid is released when the braked wheel is accelerated. If the braked wheel is slow to recover the solenoid-operated valve is cycled until the wheel accelerates.

Three embodiments of our invention are illustrated in the accompanying drawings in which: Figure 1 is a layout of an anti-skid hydraulic braking system of a vehicle; Figure 2 is a graph of modulated brake pressures plotted against time; Figure 3 is a longitudinal section through a modulator assembly; Figure 4 is a section on the line 4-4 of Figure 3; Figure 5 is a section similar to Figure 3 of a modulator assembly but showing a modification; Figure 6 is a section similar to Figure 3 but showing a further modification; and Figure 7 is another section similar to Figure 3 but showing yet another modification.

The anti-skid hydraulic braking system illustrated in Figure 1 of the drawings comprises a pedal-operated hydraulic master cylinder 1 for operating a wheel brake 2, and fluid under pressure from the master cylinder 1 is supplied to the brake 2 through a modulator 3.

The modulator 3 comprises a housing 4 in which a solenoid-operated pressure dump valve 5, a first flow-control regulator valve 6, an expander piston 7 working in a bore portion 8 which defines a dump chamber 9, and a pressure intensifier 10 are located.

The dump valve 5 comprises a valve head 11 which is normally urged by a spring into engagement with a seating in order to cut-off communication between a port 12 connected to the intensifier 10 and the dump chamber 9.

The valve 6 is housed in one end of a stepped longitudinally extending bore 13 in the housing 4 of which the opposite end, which is of smallest diameter, defines the bore portion 8 which forms the dump chamber 9. The valve 6 comprises a sleeve 14 of differential outline of which the two portions are sealingly housed in the portions of bore 1 3 which are of greatest and of intermediate diameter, a hollow spool 1 5 working in the bore 1 6 of the sleeve 14, and a valve head 1 7 of coneshaped outline slidably guided in the bore 1 8 of the spool 1 5 for engagement with a seating 1 9 defined by shoulder in the spool 1 5. The head 17 is provided with a longitudinal notch 20 and the seating 1 9 is provided with a diametral scratch or slight notch which defines between the head 1 7 and the seating 19 a first restriction of fixed area when the head 1 7 is in engagement with the seating 1 9. The head 1 7 is carried at one end of a stem 21 of reduced diameter of which the opposite end carries an enlarged head 22.

A force-transmitting member 23 is guided to slide through a seal 24 in a partition 25 which is received in the portion 8 of the bore 1 3 which is of smallest diameter. The expander piston 7 carries one end of the member 23 and the opposite end disposed in the opposite side of the partition 25 is provided with a longitudinally extending bore 26 in which a spring 27 acting on the head 22 is received. The head 22 is retained in position by means of a retainer 28 of top-hat outline, and the spring 27 urges the head 1 7 towards the seating 1 9. A further regulating compression spring 32 acts between the spool 15, and a radial flange 33 on the force-transmitting member 23.The spring 32 acts to urge the spool 15 against the stop 31 at the end of the bore 11 and also urges the flange 33 into engagement with the partition 25, in turn to define a retracted position for the expander piston 7 in which the volume of the dump chamber 9 is at a minimum. The relative lengths of the stem 23 and the head 1 7 are chosen such that, in this inoperative retracted position shown, the head 1 7 is spaced from the seating 1 9.

A second restriction 34 of variable area is defined in the valve 6 by a diametral port 35 in the sleeve 14 which is in permanent communication with the master cylinder 1 through a passage 36 in the housing 4 and an annular recess 37 in the spool 1 5 which meters flow through the port 35 and into a radial port in the spool 1 5 in accordance with the position of the spool 13 within the bore of the sleeve 14.

In the retracted position shown the restriction 34 is in a maximum open position to permit an unrestricted flow of fluid from the master cylinder 1 to the brake 2, through the unrestricted space between the head 1 7 and the seating 19, the notch 20 and through the intermediate portion of the bore 13 and a communicating passage 38 in the housing 4. The stop 31 is provided with a diametrical or other slot 39 to enable fluid pressure within the bore of the spool 1 5 act on the adjacent end of the spool 1 5.

The pressure intensifier 10 comprises a displacement piston 40 of differential outline working in a second, stepped, bore 41 in the housing. A displacement chamber 42 is defined in the bore 41 adjacent to the end of the piston 40 which is of smallest area, and a support chamber 43 is defined in the bore 41 between the opposite end of the piston 40 which is of greatest area, and a second flow-control regulator valve 44.

The modulator 3 is connected to a conventional power braking and steering system 45 of a vehicle. As illustrated the system 45 comprises a pump 46, which may be driven from the engine of the vehicle or from an electric motor, and which is adapted to draw fluid from a tank 47 and pump it to a flow divider 48 which is arranged to supply fluid to an accumulator 49 and to a steering valve 50, from whence it is returned to the tank 47. The accumulator 49 supplies fluid under pressure to the regulator valve 44, and to a brake servo (not shown) and a low pressure warning switch 51 is provided to monitor pressure in the accumulator 49.The tank 47 is connected through a pipe-line 52 to a port 53 in the housing 4, the port 53 in turn being in communication with the dump chamber 9 through an orifice 54 and a spring loaded one-way valve 55, and through a separate passage 56 incorporating a restriction 57.

The secondary flow-control regulator valve 44 comprises a sleeve 60 of differential outline of which the two portions are sealingly housed in the portions of the bore 41 which are of greatest and of intermediate diameters, a hollow spool 61 working in the bore in the sleeve 60 and having an axial through-bore 62 of which the innermost end is provided with a fixed orifice 63 defining a first restriction. A second restriction 64 of variable area is defined in the valve 44 by a radial port 65 in the sleeve 60 which is in permanent communicaton with the accumulator 49 through a spring-loaded one-way valve 66 and a annular recess 67 in the spool 61 which meters flow through the port 65 and into a radial port in the spool 61 in accordance with the position of the spool 61 within the bore of the sleeve 60.In the retracted position shown the restriction 64 is in a maximum open position assisted by a light compression spring 68 to give a fluid pressure connection from the accumulator 49 to the support chamber 43 through the orifice 63.

A stop 69 for the outer end of the sleeve 60 is provided with a diametrical or other slot 70 to enable fluid pressure within the bore of the sleeve 60 act on the adjacent end of the spool 61.

In the inoperative position shown in the drawings, the solenoid is de-energised so that the valve head 11 is spring loaded onto the seating 1 2 to trap accumulator pressure in the support chamber 43. This holds the displacement piston 40 in an inoperative position in which the effective volume of the displacement chamber 42 is at a minimum.

The portion of the bore 1 3 between the valve 6 and the partition 25 is in communication with the displacement chamber 42 through a springloaded one-way valve 71 and the displacement chamber 42 is in communication with the radial port 35 in the sleeve 14 through a spring-loaded one-way valve 72.

Finally the flow control valve 6 is in the fully open position as described above to provide open communication between the master cylinder 1 and the brake 2.

When the brake is to be applied, operation of the master cylinder 1 forces fluid to the brake 2 through the bore 1 3 and pressurises the displacement chamber 42 through the one-way valve 71.

If the braked wheel skids, a skid signal from means sensing the deceleration of the wheel produces an electric current which energises the solenoid of the valve 5. The solenoid is energised so that the pressure in the support chamber 43 is displaced through the one-way valve 55 into the dump chamber and, since the restrictions 54 and 57 will not permit free flow to the tank 47, sufficient back pressure is generated to urge the expander piston 7 relatively towards the valve 6, in turn compressing the spring 32 to cause the head 1 7 to engage with the seating 19, thereafter compressing the spring 27 also.Simultaneously the pressure in the displacement chamber 42 urges the displacement piston 40 relatively towards the valve 44 so that fluid from the line to the brake 2 is drawn into the displacement chamber 42 through the valve 71 to reduce the pressure applied to the brake 2. Fluid will also start to flow from the accumulator 49 to the support chamber 43 through the secondary flow control valve 44, and from whence it is returned to the tank 47 through the restrictors 54 and 57.

Typically the sizes of the restrictors 54 and 57 are chosen such that the flow through the control valve 44, plus the flow due to the initial rate of movement of the displacement piston 40, less that taken by the expander piston 7, will cause a back pressure of substantially 1.3 bar.

The brake-line pressure applied to the brake 2 falls as the effective volume of the displacement chamber 42 is increased to accommodate fluid from the brake-line plus residual flow through the flow control valve 6 at a rate determined by the load in the springs 32 and 27.

When the displacement piston 40 has moved through a distance sufficient to reduce the pressure applied to the brake 2 to, say, 3.5 bar it slows down to a rate of travel corresponding to the rate of flow through the flow control valve 6.

Thus the brake pressure remains constant. Since the body pressure in the support chamber 43 will now have dropped, the expander piston 7 starts to expel fluid from the dump chamber 9 through the restrictor 57 under the influence of the springs 32 and 27.

If the wheel is slow to accelerate the solenoid valve 5 will close, arresting the movement of the displacement piston 40 towards the flow control valve 44. The displacement piston 40 then starts to move relatively away from the valve 44 due to action of fluid from the accumulator which continues to flow into the support chamber 43 through the flow control valve 44. By this movement fluid is expelled by the displacement piston 40 from the displacement chamber 42 and through the one-way valve 72 to the master cylinder side of the flow control valve 6. The one way valve 71 isolates the brake-line to the brake 2 from the fluid in the displacement chamber 42 which during this movement of the displacement piston away from the valve 44, will overcome the master cylinder pressure.However, during this period, the brake pressure will increase slightly, typically to substantially 11 or 1 2 bar, since the displacement chamber 42 cannot, during this phase, accommodate the flow from the flow control valve 6 which will thus start to increase the brake pressure at some low rate, the precise value of which will depend upon the load in the springs 27 and 32 at that time.

After the solenoid has been de-energised for a fixed period, typically 1 6 ms, it will be reenergised for a further variable period provided that the skid has not yet been adequately corrected. Fluid from the support chamber 43 will again be displaced through the solenoid-operated valve 5 as the displacement piston 40 starts to move relatively towards the valve 44 under the influence of the pressure in the brake-line applied to the brake 2 and the flow through the flow control valve 6. However, such movement will now be much reduced in terms of both speed and travel since the brake-line pressure driving the piston is much lower than that during the initial brake-pressure release described above.

Therefore, such return movement of the expander piston 7 will not be greatly affected, merely slowed temporarily or perhaps halted for a brief period.

The steps described above are then repeated in a closed loop until the skid has been corrected.

The solenoid then remains de-energised so that brake re-application continues under the control of the flow control valve 6, and the displacement piston 40 returns to its original inoperative position.

In the construction described above the master cylinder 1 is always in communication with the brake 2. However, should the accumulator 49 fail, the pressure switch 51 cuts off the solenoidoperated valve 5 so that this valve will remain closed which together with the one-way valve 66 will trap a volume of fluid in the support chamber 43, thereby preventing movement, or further movement, of the displacement piston 40 towards the valve 44. The brake 2 can therefore be freely applied and released.

As described, the brake pressure acting on the displacement piston 40 causes a pressure rise in the support chamber 43 since this fluid is trapped by the one-way valve 66 and the solenoid operated valve 5. If either of these valves 66 and 5 leak then pedal travel will increase as the displacement piston 40 moves. However, as the master cylinder 1 recuperates each time the pedal is released, the moment the displacement piston 40 "bottoms", for example engages with a stop member 73, no further travel is lost. When the system is repaired, pressure from the accumulator 49 forces the displacement piston 40 to its inoperative position and returns the "lost" fluid back to the reservoir of the master cylinder 1.

A typical comparison, on a surface having a low coefficient of friction, between wheel speed, brake pressure, and solenoid current from a common time base is shown in Figure 2 of the drawings. It will be noted that, in this example, the adaptive and fixed periods during which the solenoid is energised are shown as being equal, and as being equal to the periods when the solenoid is deenergised. As described above, this will not normally be the case and the adaptive periods will depend upon the stored figures of deceleration of the braked wheel.

In a modified construction the power source for brake re-application may be pneumatic in character and comprise a source of vacuum, such as a connection to the manifold of the engine of the vehicle, or a source of compressed air. In either use the intensifier 10 takes the form of a movable wall subjected to the said pneumatic power source and acts directly on the expander piston 7.

The solenoid-operated valve 5 controls the application of the power source of the movable wall to regulate the position of the displacement piston 40 according to whether or not the skid signal is operative.

The modulator assembly illustrated in Figures 3 and 4 of the drawings comprises a housing 81 containing a piston 82 which divides the housing 81 into a first chamber 83 and a second chamber 84. The piston 82 acts as a support for a fiexible diaphragm 85 which is connected at its peripheral edge to the housing 81.

A block 86 is carried from one end of the housing 81 and has a longitudinal bore 87 in which is guided to slide a displacement piston 88 carried by the piston 82. A displacement chamber 89 is defined in the bore 87 between the piston 88 and a fixed partition 90, and a flow-control valve regulator 91 is housed in the bore 87 on the opposite side of the partition 90.

The valve block 86 houses a solenoid-operated valve 92 for controlling pressurisation of the chambers 83 and 84. The solenoid 93 of the valve 92 is energised in response to a skid signal.

Normally the solenoid 93 is de-energised and the chamber 83 is subjected to vacuum from the manifold of the engine of a vehicle through a pipe 94. The other chamber 84 is at all times in communication with atmosphere through spaced outlet ports 95, the flow through which takes place through a filter 96. In the position illustrated a valve member 97 is in engagement with a first seating 98 to isolate the supply of vacuum from an external pipe 99 which leads to the chamber 84, and is spaced from a second seating 100 to provide communication between the supply of vacuum and the chamber 83 through a passage 101 disposed between the seatings 98 and 100.

The valve 91 is housed in an enlarged stepped portion at the outer end of the bore 87. The valve 91 comprises a sleeve 102 of differential outline of which the two portions are sealingly housed in the portions of bore 87 which are of greatest and of intermediate diameter, a hollow spool 103 working in the bore 104 of the sleeve 102, and a valve head 105 of cone-shaped outline for engagement with a seating 107 defined by a shoulder at the outer end of the bore of the spool 1 03. The seating 107 is provided with a diametrical scratch or slight notch 109 which defines between the head 105 and the seating 1 07 a first restriction of fixed area when the head 105 is in engagement with the seating 1 07. The head 105 is carried at one end of a stem 110 of reduced diameter which extends through the bore of the spool 83 and works through a bore in the partition 90, engaging at its free end with the expander piston 88. A light spring 111 urges the head 105 towards the seating 107, and a further compression spring 11 2 acts between the block 86, and a radial flange on the spool 103.The spring 112 acts to hold the flange of the spool 93 against the adjacent end of the sleeve 92. The relative lengths of the stem 110 and the piston 88 are chosen such that, in this inoperative retracted position shown, in which the volume of the expansion chamber 89 is at a minimum, the head 105 is spaced from the seating 1 07.

A second restriction 11 4 of variable area is defined in the valve 91 by a radial port 115 in the sleeve 102 which is in permanent communication with a master cylinder through a passage 11 6 in the block 86 and an annular recess 11 7 in the spool 103 which meters flow through the port 11 5 and into a radial port in the spool 103 in accordance with the position of the spool 1 03 within the bore of the sleeve 102. In the retracted position shown the restriction 11 4 is in a maximum open position to permit an unrestricted flow of fluid from the master cylinder to an outlet port 11 8, through the unrestricted space between the head 105 and the seating 107 and through a radial port 11 9 in the spool 103.

The displacement chamber 89 communicates with the passage 106 through a first spring loaded one-way valve 120, and the passage 108 communicates with the displacement chamber 89 through a second one-way valve 121.

In the inoperative position shown in the drawings the displacement piston 88 is in its inoperative position due to the negative pressure in the chamber 83, and the effective volume of the displacement chamber 89 is at a minimum, and the valve 91 is fully open.

Normally, when the brake is to be applied hydraulic fluid from the master cylinder is freely supplied to the outlet port through the passage 11 8 by way of the open valve 11 5.

When a skid signal is produced by means sensing the deceleration of the braked wheel, the solenoid 93 of the valve 912 is energised and the valve head 97 is urged away from the seating 98 and into engagement with the seating 99. This cuts off the supply of vacuum to the chamber 83 and interconnects the two chambers 83 and 84 so that they are both at the same pressure.The pressure applied to the brake and acting on the displacement piston 88 through the one-way valve 121, causes the displacement piston 88 to retract, thereby permitting the head 105 to engage with the seating 109, this restriction causing a back pressure to act on the inner end of the spool 103 which moves against the spring 111, 112, to regulate the flow through the port 11 5. The rate at which fluid is supplied to the brake is thereby reduced by the valve 91, and the pressure applied to the brake is relieved by the displacement chamber 89.

At the termination of the skid signal the solenoid is de-energised and a spring 122 acts to urge the head 97 into engagement with the seating 98. This restores the supply of vacuum to the chamber 83, and the piston 82 is returned to its original retracted position shown with the effective volume of the displacement chamber 89 being returned towards its original minimum value, at a rate determined by the withdrawal of air to the vacuum source. This can be regulated by the inclusion of a restrictor in the line 94 so that the flow back to the master cylinder equates to the flow into the brake. During this movement of the displacement piston 88, fluid displaced from the displacement chamber 89 is returned to the master cylinder and the passage 116 through the one-way valve 120, and the brake are re-applied by fluid flowing at a predetermined rate through the flow control valve 91.When the displacement piston 88 reaches its rest position the valve head 105 is disengaged from the seat 1 09.

In the embodiment described above means are provided for switching off the solenoid valve should the source of vacuum fail.

In a modification, the pipe 94 can be connected to atmosphere or vacuum, and the outlet 95 can be connected to a source of compressed air. Thus, normally, the piston 82 will be held in its retracted position by a supply of positive pressure.

Similarly the outlet 95 can be connected to a source of hydraulic pressure, suitably the pump or accumulator of an hydraulic power steering system, with the pipe 14 connected to tank for hydraulic fluid.

In the modification of Figure 5 the stem 110 carries at its free end a head 131 which is guided to slide in a bore 132 in the expander piston 88, and a spring 133 acts between the head 131 and an abutment 134 on the expander piston 88. The spring 111 is reacted against an extended portion of the spool 103.

In this construction the flow rate setting of the valve 91 depends upon the combined load of the springs 112 and 133. Since the load in the spring 1 33 varies with the displacement of the piston 88, the flow rate through the valve 91 during skid control will vary in accordance with the volume of fluid dumped from the brake.

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

In the modified construction shown in Figure 6 the expander piston 88 does not co-operate with the stem 11 0. In this construction the valve 91 and the stem 110 are arranged in a bore which is normal to the bore 89, and the free end of the stem 110 remote from the valve 91 is carried by a piston 140 of substantial diameter. Normally the piston 140 is subjected on both sides of the pressure, negative or atmospheric, in the chamber 3. When the solenoid 93 is energised in response to a skid signal and the head 97 is urged into engagement with the seating 99, opposite sides of the piston 140 are isolated from each other.Since the pressures in both chambers 83 and 84 are equalised and one side of the piston 140 is exposed to such pressures, and since the opposite side is subjected to its original pressure, then the piston 140 is subjected to a differential pressure which moves it relatively away from the valve 91 against the loading in a spring 141.The valve 91 and the remainder of the modulator operate as described above with reference to Figures 3 and 4 and corresponding reference numerals have been applied to corresponding parts.

This ensures that if the support of the movable wall fails, for any reason, then a failure switch cuts out the solenoid and free-flow can pass to and from the brakes, even if the expander piston moves away from its normal position.

In the modulator assembly illustrated in Figure 7 of the drawings the external pipe 99 is replaced by an internal passage 145 of large diameter, and a second internal passage 66 of similar large diameter provides communication between the seating 98 and the second chamber 84. At an intermediate point in its length the passage 146 is connected to the filter 96 in the air inlet through an air inlet valve 147. The air inlet valve 147 comprises a head 148 which is carried by a stem 149 in freely separable engagement at its free end with the valve member 97, and a spring 1 50 for urging the head 148 towards a seating 151 at an intermediate point in the length of a passage 1 52 between the passage 146 and the filter 96.When the valve member 97 is in engagement with the seating 98 in the inoperative position shown, the stem 149 holds the head 148 away from the seating 1 51 so that the chamber 84 is in free communication with the inlet port 95 through the passage 1 52.

When the solenoid 93 is energised and the valve head 97 is urged out of engagement with the seating 98, before the valve head 97 can engage with the seating 99 to isolate the chamber 83 from vacuum and to interconnect the two chambers 83 and 84, the head 148 first engages with the seating 1 51 to cut-off communication between atmosphere and the chamber 84.

Thereafter the valve head engages with the seating 99 and equal pressure applied to opposite sides of the piston 82 which are of different areas due to the presence of the rod 88. The piston 82 withdraws the rod 8 to relieve the brakes as described above, with the effective volume of the chamber 83 increasing with decrease in volume of the chamber 84.

During this movement of the piston 82 the valve 147 has no effect, since during the displacement of air at higher pressure from the chamber 84 into the chamber 83 which is of lower pressure, no air is withdrawn from the passages 145 and 146 by the source of vacuum.

At the termination of the skid signal when the solenoid 93 is de-energised the valve 147 opens before valve member 97 is returned to the position shown in the drawing. Air is withdrawn from the chamber 83, and additional air enters the chamber 84 through the valve 147 to make up the volume of that chamber 84.

If, whilst a skid signal is operative, the piston 82 has moved to the extreme end of the bore before the signal ends, and, at the termination of the skid signal, the piston 82 has returned substantially half-way towards its initial position, then should a further skid occur, the valve 147 again closes before the valve member 97 engages with the seating 99 and only the volume of air trapped in the chamber 84 can be used to fill-up the chamber 83, previously subjected to vacuum.

The provision of the valve 147 therefore conserves the air supply when a skid signal is operative which, in turn, limits the substantial quantity of air which must be withdrawn by vacuum to re-apply the brakes following a skid, and the valve 147 is particularly effective when the volume of the chamber 83 is substantial.

Claims (22)

1. An anti-skid hydraulic braking system of the kind set forth in which the modulator provides a permanent connection between a master cylinder for applying the brake and a brake-line to the brake itself, and a displacement piston working in a bore in a housing is movable in first and second opposite directions, movement of the displacement piston in the first direction drawing fluid from the brake-line and into a displacement chamber through a first one-way valve, and acting to introduce restrictive means between the master cylinder and the brake-line, and movement of the displacement piston in the second direction expelling fluid from the displacement chamber through a second one-way valve leading to the master cylinder, a power source being provided for pressurising a support chamber whereby normally to hold the displacement piston in an advanced position defining the extent to which the piston can be moved in the said second direction and in which the effective volume of the displacement chamber is at a minimum, a change in pressure in the support chamber in response to the skid signal permitting the piston to move in the first direction, and at the termination of the skid signal the pressure in the support chamber changes again to urge the pistion in the second direction.

2. A system as claimed in Claim 1, in which the restrictive means is removed when the brake pressure is substantially equal to the pressure from the master cylinder.

3. A system as claimed in Claim 1 or Claim 2, in which the power source is hydraulic.

4. A system as claimed in any preceding claim, in which the displacement piston separates the fluid used in the power source from that in the braking system.

5. A system as claimed in any preceding claim, in which the power source comprises an hydraulic pump and/or hydraulic accumulator for the power steering or suspension of the vehicle.

6. A system as claimed in any preceding claim in which the displacement piston comprises a differential piston which is adapted to generate in the braking system higher pressures than are present in the support chamber, thereby acting as a pressure intensifier.

7. A system as claimed in Claim 1 or Claim 2, in which the power source is pneumatic.

8. A system as claimed in Claim 7, in which the power source comprises suction from the manifold of an engine of a vehicle.

9. A system as claimed in Claim 7, in which the power source comprises compressed air.

10. A system as claimed in any preceding claim, in which the fluid from the master cylinder is applied to the brake line through a first flowcontrol regulator valve incorporating first restrictor means and the fluid from the power source is supplied to the support chamber through a second flow-control regulator valve incorporating second restrictor means.

11. A system as claimed in any of Claims 1 to 9, in which the modulator comprises a wall movable in a housing to divide the housing into first and second chambers and to co-operate with the displacement piston to oppose pressure applied to the brake through a flow-control regulator valve, and the power source normally subjecting one of the chambers to a corresponding fluid pressure to hold the wall in an inoperative position in which the displacement piston is held in the corresponding, advanced, position, and both chambers being interconnected in response to a skid signal whereby the wall can move into an operative position accompanied by retraction of the displacement piston whereby to increase the effective volume of the displacement chamber, whereafter, at the termination of the skid signal, only the said one chamber is connected to the power source.

12. A system as claimed in Claim 11, in which the source comprises compressed air which acts in the first chamber and the second chamber is normally connected to atmosphere, the chambers being interconnected when a skid occurs, and at the termination of the skid signal the compressed air in the second chamber being exhausted to atmosphere.

13. A system as claimed in Claim 11, in which the source comprises vacuum and the first chamber is normally connected to atmosphere during normal brake application, the second chamber being connected to vacuum before and after a skid signal but being connected to the first chamber during a skid signal.

14. A system as claimed in any of Claims 10 to 13, in which communication between the two chambers is controlled by a solenoid-operated valve of which the solenoid is energised in response to the skid signal to place the two chambers in communication.

1 5. A system as claimed in Claim 11, in which the source comprises hydraulic fluid under pressure, the second chamber being normally connected to a tank for fluid during normal brake application, and after a skid signal, the first chamber always being connected to the pressure source.

16. A system as claimed in any of Claims 1 to 10, in which the fluid pressure from the power source is applied to that end face of the displacement piston which is remote from the displacement chamber and which defines at least in part the support chamber, with the solenoidoperated valve controlling a normally closed connection between the support chamber and a low pressure reservoir.

17. A system as claimed in any preceding claim in which means are provided for monitoring regularly the deceleration of the braked wheel and, after a skid signal has first become operative, the skid signal is cancelled when, over two successive readings, the deceleration of the wheel has fallen.

18. An anti-skid hydraulic braking system for a vehicle substantially as described herein with reference to Figures 1 and 2 of the accompanying drawings.

19. An anti-skid hydraulic braking system for a vehicle substantially as described herein with reference to Figures 3 and 4 of the accompanying drawings.

20. An anti-skid hydraulic braking system for a vehicle substantially as described herein with reference to Figure 5 of the accompanying drawings.

21. An anti-skid hydraulic braking system for a vehicle substantially as described herein with reference to Figure 6 of the accompanying drawings.

22. An anti-skid hydraulic braking system for a vehicle substantially as described herein with reference to Figure 7 of the accompanying drawings.