GB1567066A

GB1567066A - Anti skid control unit

Info

Publication number: GB1567066A
Application number: GB3312377A
Authority: GB
Original assignee: Nissan Motor Co Ltd
Current assignee: Nissan Motor Co Ltd
Priority date: 1976-08-13
Filing date: 1977-08-08
Publication date: 1980-05-08
Also published as: FR2361256B1; FR2361256A1; DE2736449A1

Description

(54) AN ANTI SKID CONTROL UNIT (71) We, NISSAN MOTOR COM PANY, LIMITED, a corporation organized under the laws of Japan, of No. 2, Takaramachi, Kanagawa-ku, Yokohama City, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a hydraulic braking system for a motor vehicle and more particularly to an anti skid unit incorporatable therein.

As is well known it is desirable in view of the ever increasing performance capabilities of the modern motor vehicle to provide therein an anti skid device or unit which will prevent excessive pressure being fed to the cylinders of the brake units during sudden deceleration and thus prevent dangerous wheel lock up and resulting skidding of the vehicle. This is particularly so in today's motor vehicles wherein efforts are being constantly made to reduce the overall weight and complexity of the vehicle components and thus the vehicle so as to reduce the consumption of dwindling petroleum supplies.

One particular anti skid unit is activatable by a computing circuit which temporarily cuts or severely limits the communication between the master cylinder of the braking system and the brake units and simultaneously expands the pressurized hydraulic pressure entrapped in the brake unit cylinders to reduce the braking effect generated thereby. This operation is repeatable to maintain the rate of deceleration below a predetermined safe level.

However this unit has suffered from overcomplexity and excessive weight in that it uses three spool type valves or valve units slidably disposed in three separately formed stepped bores. The resulting conduiting required to interconnect the three bores presents many locations requiring sealing and is difficult to machine and assemble during manufacture of same.

Furthermore a rather large heavy bulky housing is required to house the three valve units thus incurring the afore mentioned weight penalty.

Thus a control unit of the afore described type has been developed in which the weight, size and complexity has been reduced by using only two valve units slidably disposed in two stepped bores.

However, despite its simplicity the control unit according to the present invention performs identically with the more complex heavy unit of the prior art.

In detail a check valve is integrally formed on one end of an expansion valve slidably disposed in a first stepped bore and arranged to cut communication between the master cylinder and the brake units upon the pressure of the hydraulic fluid biasing the expansion valve to a first normal position being reduced due to either a signal from a computing circuit indicating a dangerously high rate of deceleration or cessation of the supply of pressurized hydraulic fluid. The movement of the expansion valve from its normal position permits the fluid in the brake units to pressure reducingly expand.

A by-pass valve slidably disposed in a second stepped bore responds to increased master cylinder pressure to increase the pressure biasing the expansion valve to the first normal position to balance the reverse effect of the increased master cylinder pressure. A differential valve integrally connected to the by-pass valve maintains a fail safe pressure transmission path closed until cessation of the supply of pressurized fluid whereupon the master cylinder is directly connected with the brake units.

A regulator valve can be slidably dis posed in a third step formed in the by-pass valve to be responsive to the change of both the master cylinder pressure and the supplied pressurized fluid to modulate the pressure biasing the expansion valve to its normal position.

Thus it is an object of the present invention to provide an anti skid control unit which is light and compact.

It is another object of the present invention to provide an anti skid control unit which is simple in construction and therefore easy to manufacture and assemble.

Another object of the present invention is to provide an anti skid control unit which is formed with only two stepped bores within the housing of same thereby reducing the amount of conduiting interconnecting said bores to a minimum.

Yet another object of the present invention is to provide an anti skid control unit which requires the minimum amount of sealing members and therefore reduces the risk of hydraulic fluid leakage to a minimum.

It is yet another object of the present invention to provide an anti skid control unit wheich demonstrates rapid response to changes in master cylinder pressure by providing a third valve unit which is slidably disposed in one of the two valve units which are respectively slidably disposed in the afore mentioned two stepped bores.

It is yet another object of the present invention to provide an anti-skid control unit in which in the case of pump failure or a similar malfunction causing a loss of pressurized hydraulic fluid, used as the source of motive power in the anti-skid control unit, the amount of pressure relaxation, as a fail safe pressure transmission path is opened to replace a normal path, is reduced to a minimum.

According to the present invention, there is provided an anti skid unit for a motor vehicle hydraulic braking system comprising a first valve which is responsive to the hydraulic fluid pressure supplied into said anti skid unit from a source of hydraulic fluid under pressure and which permits fluid communication between first and second ports via a first pressure transmission path when it assumes a first normal position under the biasing influence of the hydraulic fluid pressure fed into said anti skid unit and cut off said fluid communication when it assumes a second position in the absence of said hydraulic fluid pressure and provide an anti skid inducing pressure reduction at said second port by moving from said first normal position to said second position; an electromagnetic valve which isolates said source of hydraulic fluid under pressure from said first valve to cause said absence of hydraulic fluid pressure upon receiving an energizing signal; and a second valve which is responsive to said hydraulic fluid pressure and which, under the biasing influence of said hydraulic fluid pressure, is maintained in a position where it maintains a second pressure transmission path between said first and second ports closed and opens said second pressure transmission path in the absence of said hydraulic fluid pressure, said second valve being sensitive to small variations in the pressure introduced into said anti skid unit through said first port and modulates said hydraulic fluid pressure under the influence of which said first valve is biased to said first normal position so that the fluid connection between said first and second ports is maintained constant irrespective of said small variations in pressure while said hydraulic fluid pressure is fed into said anti skid unit.

In the accompanying drawings: Fig. 1 is a cross sectional view of a prior art anti skid control unit; Fig. 2 is a cross sectional view of a first preferred embodiment of a control unit for an anti skid system according to the present invention; Fig. 2(a) is an enlarged view of a portion of the control valve of the control unit shown in Fig. 2; Figs. 2(b) and 2(c) are enlarged views showing details of the check valve which is integrally connected to the expansion valve shown in the first preferred embodiment in Fig. 2; Fig. 3 is a cross sectional view of a second embodiment of a control unit for an anti skid system according to the present invention; and Fig. 4 is a cross sectional view of a third embodiment of a control unit for an an anti skid system according to the present invention.

Fig. 1 shows an example of a prior art anti skid system incorporated into a braking system of a motor vehicle wherein the letters A, B, C, CC, PS and R represent the master cylinder; the wheel brake units (only one is shown); a source of hydraulic fluid under pressure such as a pump; a computing circuit which is fed various signals representative of various vehicle operating parameters such as rate of deceleration, vehicle speed etc; a power steering unit and a reservoir; respectively.

The above mentioned computing circuit cc is arranged to issue an activation signal upon the rate of deceleration of the vehicle being sensed as exceeding a value which represents the upper limit of safe deceleration with respect to various road conditions such as inclination etc. A solenoid valve 110 is disposed within the con trol unit Sl of the anti skid system which upon receiving the activation signal from the computing circuit induces a situation therein where the pressure fed to the wheel brake units is temporarily reduced or relaxed to reduce the rate of deceleration and thereby reduce the risk of dangerous wheel lock up and subsequent skidding of the vehicle.

Now let us consider the construction and arrangement of the control unit and the components disposed therein in detail.

Formed within the control unit denoted by S, are three stepped bores 191, 192 and 193. Slidably disposed in the first bore is a first valve unit consisting of an expansion valve 117 and a check valve 120. As seen the check valve is a two stage type and consists of two members 121 and 122.

The first member 121 is as shown fixedly connected to the expansion valve for integral movement therewith. The second member 122 is slidably arranged within the first. Two springs are arranged to bias the first and second members to the right as seen in the figure. Although shown in an open state for the purpose of illustration the second member will normally be biased to a closed position under the effect of the spring abutting the end thereof.

With this arrangement a two stage operation of the check valve is possible.

Slidably disposed in the second stepped bore 192 is a second valve unit consisting of a differential valve 123 and a by-pass valve 104. As in the case of the expansion valve and the check valve these two valves are fixedly interconnected for simultaneous integral movement; and slidably disposed in the third stepped bore 193 is a third valve unit consisting of a regulator valve 101. The pump C is arranged to supply pressurized hydraulic fluid into the control unit through a port Pc. As shown the port Pc fluidly communicates with chambers 105, 115 and 125 through conduits 113', 113 and 103. A conduit 109 is shown communicating with the chamber 125.

This conduit is connected to a servo motor or the like which utilizes pressurized hydraulic fluid as a source of motive power. In this case a power steering unit is shown fluidly connected to the pump via the afore mentioned conduit 109.

The afore mentioned solenoid valve 110 is shown interposed between the chamber 115 and the pump C. The valve consists of a solenoid coil 111, a conically shaped coil spring 110a, a disc like armature member lIOb and a suitable sealing member 112 fixed to the armature 110b. The spring 110a is, as seen, arranged to bias the armature member to a position where it sealingly abuts the sealing member, in this case a ball shaped member, against an orifice which communicates a conduit 116 with the chamber 115. The other end of the conduit 116 is arranged to constitute a port PD which in turn is communicated with the afore mentioned reservoir R.

Upon energization of the coil the armature is arranged to be attracted downwardly (as seen in the drawings) to open the afore mentioned orifice and close the orifice communicating the conduit 113 with the chamber 115. A restriction 113a is formed at the upstream end of the conduit 113.

This is provided to limit the rate at which pressure can build in the chamber 115 thus providing smooth control of the expansion valve 117 which is movable via said pressure. As will be appreciated the pressurized fluid from the pump C is permitted to reach and enter the chambers 105, 115 and 125, and be transmitted to the power steering unit PS and or be returned from the chamber 115 to the reservoir R upon energization of the solenoid valve 110.

The master cylinder A is arranged as shown to communicate freely with chambers 118 and 126 via conduits 100 and 102 and be communicable with a chamber 108 via the afore mentioned differential valve 123 and a conduit 106. The latter conduit 106 is as seen branched off from the conduit 100. Two other conduits are shown formed in the control unit namely conduit 119 and conduit 107. The first 119 is as shown arranged to interconnect chamber 108 and chamber 118 via the check valve 120. The second 107, is arranged to communicate the chamber 108 with the wheel brake units B via port BB.

The foregoing construction will be more clearly understood as the description of the operation proceeds.

During normal operation with all components functioning correctly, and no braking of the vehicle being initiated, then the pressurized fluid supplied into the chambers 105, 115 and 125 will urge all three valve units i.e. the by pass valve and integral differential valve, the expansion valve and the integral check valve and the regulator valve leftwardly as seen in the drawings due to the differential piston effect induced by the difference in diameter of the spools constituting the ends of the valves. eg.Since the diameter of the end of the by-pass valve 104 exposed to the chamber 105 is greater than that exposed to the chamber 108 then by suitable selection of the spring (no numeral) biasing the bypass valve to the right as seen in the drawings the normal operating pressure supplied to the chamber 105 will urge integrally connected differential valve into abutment with an orifice which communicates the conduit 106 and the chamber 108 to cut communication therebetween.In the case of the expansion valve 117 the pressure from the pump acting on the surface area of the expansion valve exposed to the chamber 115 will bias the integrally connected expansion valve and check valve leftwardly to a position where the check valve permits free communication between conduit 100 and conduit 119. i.e. the biasing forces of the two springs abutting the first and second members of the check valve compete with the biasing force applied to the expansion valve by the pressurized hydraulic fluid in the chamber 115 and maintains the valve in a position intermediate of the two extremes of its possible travel.On the other hand the regulator valve is arranged to be biased by a spring which is so selected that the normal pressure supplied to the port PC and the chamber 125 biases it completely to its extreme left position and thus permits full communication between port PC and the conduit 103.

Hence under the above described conditions when braking is initiated by the depression of the brake pedal pressure is transmitted from the master cylinder to the cylinders of the wheel brake units via what shall be termed hereafter a first pressure transmission path, namely port PA, conduit 100 chamber 118 past check valve 120, conduit 119, chamber 108 conduit 107 and finally port PB and to the brake units B, as they will be referred to hereinafter. This situation will continue until such time that the computing circuit senses a rate of deceleration greater than that appropriate for the situation whatever it may be.

If the computing circuit fails to issue a signal to the solenoid valve 110 then the as described unmodified connection between the master cylinder and the brake units will continue and normal braking will thus continue. However upon the computing circuit issuing a signal indicative of a dangerously high rate of deceleration then the armature 110b will be attracted from its home position by the energizing of the solenoid coil 111 to open the conduit 116 and close the conduit 113. Thus the pressure prevailing in the chamber 115 is permitted to fall to zero as the drain port PD is now freely communicated with the chamber 115. However the same time as the rate of deceleration has increased i,e, before the energisation of the solenoid coil 111. the pressure prevailing in the chambers 118 and 126 increases due to the depression of the brake pedal.This induces a slight change in the positions of the expansion valve 117 and the regulator valve 101. The increased pressure will of course move the expansion valve so as to slightly reduce the communication between conduit 100 and conduit 119, by moving the integrally connected check valve 120 slightly to the right as seen in the draw ings. The regulator valve will move so as to slightly reduce the degree of communi cation between the port Pps and the con duit 103 thereby slightly increasing the pressure prevailing in the chamber 105.

However, on the chamber 115 being communicated with the drain port PD the expansion valve will be urged completely to the right as seen in the drawings moving the check valve to a position where very little communication between the conduit 100 and the conduit 119 is possible and simultaneously enlarging the volume of the chamber defined within the portion of the stepped bore within which the expansion valve is slidably disposed, i.e. that chamber with which conduit 119 is freely commun icable with and which is defined between the check valve and the expansion valve when the expansion valve has assumed the just described position.Thus due to the reduction of communication between con duits 100 and 119 and the increase in the volume of the afore mentioned chamber (which is brought about by the movement of the expansion valve into the chamber 115) the pressure prevailing in the cylin ders of the brake units will cease to increase and decrease respectively. Hence the braking force exerted by same will be accordingly decreased to prevent wheel lock up. Several repetitions of this opera tion i.e. pressure relaxation, normal opera tion, pressure relaxation may be necessary to ensure safe braking the number of which will be determined by the comput ing circuit cc.

Should the pump fail (malfunction), the engine stop, a fan belt break (assuming the pump is connected to the engine via the fan belt) and/or conduit interconnecting the pump and the control unit S1 break or otherwise cease to function properly so that the supply of pressurized fluid stops or is reduced then the regulator valve will be immediately biased toward and into the chamber 125 according to the change in pressure therein, whereby if some pressure is still supplied then that pressure will be directed to the chambers 105 and 115 and either cut off from, or sparingly supplied to the associated mechanism such as the power steering PS.

For ease of explanation lets assume that all pressure has disappeared and as a result all three valve units have been biased rightwardly by the master cylinder pressure and the forces of the associated springs. At this time a second pressure transmission path for the pressure from the master cylinder will be established namely conduit 100 conduit 106 chamber 108 and conduit 107 due to the closure of the check valve 120 and the opening of the differential valve 123. It will be noted that at this time no anti skid action is possible however the system is fail safe in that the second path is opened to provide restriction free connection between the master cylinder and the brake units.

In Fig. 2 a first preferred embodiment of the present invention is shown wherein S,' indicates a control unit which has two stepped bores 91 and 92 formed therein. Slidably disposed in the first bore 91 is a first valve unit consisting of an expansion valve denoted by the numeral 61. Slidably disposed in the second bore 92 is a second valve unit consisting of a control valve 63. As seen the control valve 63 consists of a by-pass valve portion 63' and a differential valve portion 636B, however for simplicity of explanation of the first embodiment reference will be made to only "a control valve" 63. A check valve 62 is integrally formed on one end of the expansion valve 61 as shown. The pump C is fluidly communicated with a chamber 65 through a port Pc. As shown two conduits 70 and 67 open in the chamber 65.The first conduit 70 is arranged to communicate with a chamber 93 via a conduit 75 and a solenoid valve consisting of a solenoid coil 74, a conically shaped coil spring 73, an armature 69 and a sealing member 72 which is either formed integrally with or attached to the top thereof (as seen in the drawings). As shown the armature is normally biased to a position where it closes a port P5 which interconnects the chamber 93 with the drain port PD. The second conduit 67 is arranged to communicate the pump C with a power steering unit PS or the like via the chamber 65 and a restrictuion 66 formed in the conduit 67. A spring 77 is disposed in the chamber 65 and arranged to bias the control valve 63 to the left as seen in the drawings.The end of the control valve 63 which is abutted by the spring 77 is formed with a chamber (no numeral) which if we refer briefly to fig. 2(a) is, as shown arranged to reduce the degree of communication between the chamber 65 and the conduit 67 by reducing the degree of opening of port P2. The position of the control valve 63 shown on broken lines indicates a fully open position while the position shown in solid lines indicates the situation wherein the communication between the conduit 67 and the chamber 65 is restricted by the movement of the control valve into the chamber 65. Further if the control valve moves into the chamber sufficiently a valve member 63a formed thereon abuts and closes the port P3 which communicates the conduit 70 and the chamber 65.

With the expansion valve 117' differential valve 215, by-pass valve 210 and regulator valve 214 in their effective leftmost positions a first pressure transmission path will be established for the transmission of pressure from the master cylinder to the brake units B, which is namely: conduit 100 chamber 118 chamber 206 conduit 119 chamber 234 and conduit 107'.

Now upon pressing of the brake pedal to initiate deceleration of the vehicle the pressure prevailing in the chambers which are fluidly communicated with the master cylinder A, will rise.

Hence the interconnected check valve 120' and the expansion valve 117 and the regulator valve 214 will be urged slightly rightwardly as seen in the drawings while the interconnected differential valve 215 and by-pass 210 remain stationary to maintain the ball valve 216 securely in a closed position. This induces a feed back phenomenon to occur within the control unit via the regulator valve 214 moving rightwardly as seen in the drawings with respect to the by-pass valve 210 in which it is slidably housed to increase the volume of the chamber 212 and to be thus urged into close proximity of the orifice via which the conduit 109 communicates with the chamber 213. This of course restricts the possible flow of pressurized hydraulic fluid therethrough and accordingly causes an increase in the pressure prevailing in the chamber 115.The expansion valve is thus urged leftwardly via this increased pressure back toward the position it assumed in normal non braking operation. Thus normal fluid communication between the chambers 118 and 206 is re-established following the temporary restriction therebetween caused by the initial effect of the increase in the master cylinder pressure and the resulting slight closing of the check valve. Via this feed back operation the first pressure transmission path is rapidly restored to normal and maintained substantially constant.

Now should the rate of deceleration of the vehicle exceed a level beyond which safe braking of the vehicle is not possible then the computing circuit will issue a signal to energize the solenoid coil 111. Upon energization of the coil 111 the armature 110b will be attracted so as to open the conduit 116 and close the conduit 113. This of course permits free communication between the chamber 115 and the drain port PD. The pressure prevailing in the chamber 115 quickly drops to zero permitting the pressure in the chamber 206 to urge the expansion valve to its rightmost position. The check valve 120' is moved to a position cutting communication between the chambers 118 and 206. The first pressure transmission path is thus closed isolating the brake units B and the master cylinder A.Simultaneously the volume of the chamber 206 is increased markedly whereby the pressurized fluid retained in the brake unit cylinders is permitted to expand thereinto to decrease the pressure prevailing in said cylinders. The braking effect inducing the afore mentioned dangerous rate of deceleration is thus reduced to temporarily reduce the rate of deceleration. Several repetitions of this pressure relaxation may be necessary to induce and maintain the rate of deceleration of the vehicle at the maximum safe level. The frequency and number of said pressure relaxations will of course be decided by the computing circuit and will undoubtedly vary from situation to situation.

Now in the case the supply of pressurized hydraulic fluid ceases or is drastically reduced as a result of any one of a number of possible malfunctions such as pump malfunction, loss of mechanical connection between the motor driving the pump and the pump, rupturing of the conduit interconnecting the pump and the control unit, or the like then the a second fail safe pressure transmission path will be opened, namely; conduit 209 annular chamber 218 conduit 211 ball valve 216 chamber 234 (part thereof) and conduit 107. This is a result of the movement of both valve units to the right which is a direct consequence of the pressure in the chambers 115 and 213 falling to, or approaching, zero whereupon the expansion valve 117' moves into the chamber 115 and the by-pass valve 210 moves into the chamber 213.

It will be noted that as the by-pass valve 210 moves to the right under the influence of both the master cylinder pressure and the biasing force of the spring 232 the chamfer formed on the differential valve will be urged into contact with the step (no numeral) adjacent same. The chamber 234 will thus be effectively divided into two subchambers whereupon communication between the conduit 119 and the conduit 107' will be cut but fluid communication between conduits 211 and 107' established by the opening of the ball valve 216.It will be understood that the by-pass valve 210 is not permitted to move so far as to cut communication between the conduit 209 and the annular chamber 218 due to the afore mentioned engagement of the chamfer and the step and most importantly that the latter mentioned engagement isolates the second pressure transmission path from the chamber 206 so that the increase in the volume of same due to the movement of the expansion valve toward the chamber 115 does not induce an anti skid type pressure relaxation during pump failure or the like. With this arrangement the master cylinder pressure is of course constantly supplied into the chamber 212 and conduit 211 irrespective of the pressure conditions within the control unit.

Hence it will be understood that the embodiment of the present invention just described is fail safe in the event of loss of the supply of pressurized hydraulic fluid the just described second pressure transmission path being opened while the first is closed at two points, at the check valve 120' and by the abutting of the chamfer formed on the differential valve with the afore mentioned step formed within the stepped bore 292. It will be further appreciated that under these conditions there will be no anti-skid operation by the control unit even if a signal from the computing circuit is fed to the solenoid coil thereof.

Fig. 4 shows a third preferred embodiment of the present invention which is similar to the afore described second preferred embodiment but which is equipped with a different differential valve arrangement and has provision made for air entrapped within the hydraulic fluid employed in the master cylinder the brake units and the control unit to be simply and easily bled off.

Since the arrangements of the expansion valve 117', check valve 120' by-pass valve 210' and the regulator valve 214 are basically the same as in the previous embodiment no description of same will be given for brevity.

In this case the differential valve 215' is not fixedly connected to the by-pass valve 210' and thus is a separate unit which is arranged to be integrally movable therewith.

A chamfer is formed on the differential valve and arranged to sealingly abut a step formed in the stepped bore denoted in this case by the numeral 293. On abutment with the step the chamfer divides the chamber 234 effectively into two subchambers and this isolates the conduit 119 and conduit 107'. Thus in operation when a loss of pressurized fluid is experienced and the by-pass valve is urged into the chamber 213 by the force of the master cylinder pressure acting on the end thereof exposed to the chamber 234 the ball valve 216' will open the orifice communicating the conduit 106 and conduit 100 with said chamber 234. Hence a second pressure transmission path will be opened which is namely: conduit 209, conduit 100, conduit 106, chamber 234 and finally conduit 107'.

As before the movement of the expansion valve into the chamber 115 cuts the first pressure transmission path which in this case is: conduit 209, conduit 100, chamber 118, check valve 120', chamber 206, conduit 119, chamber 234 and conduit 107'. It will be noted that this first path is cut at two places as before; at the check valve 120' and at chamber 234 by the abutment of the afore mentioned chamfer and step to advantageously isolate the chamber 206 and the second transmission path as previously described in connection with the second preferred embodiment. Once again a spring 232 is disposed in the chamber 234 and arranged to abut the end of the differential valve 215' to urge it and the by-pass valve 210' in the direction of the chamber 213.It will be also noted that the chamber 212 is not directly communicated with the ball valve and the chamber 234 in this case this communication being replaced by the communication via conduits 106, 100 and 209.

Thus operation is as before; via a first pressure transmission path during normal operation with no braking, temporal cutting of the first pressure transmission path during anti skid operation and use of the second pressure transmission path only during loss of supply of pressurized hydraulic fluid which ensures a fail safe direct connection between the master cylinder and the brake units.

Now a further feature is found in this embodiment and that is the provision for air bleeding. As shown an air bleed 252 is fluidly connected with the chamber 234 via a conduit 208. A lock member 250 is as shown screwed into a threaded bore formed through the body of the control unit and arranged to be extendable into the stepped bore 291 so as to prevent the movement of the expansion valve 117' in a rightward direction (as seen in the drawings). Thus with this arrangement when it is necessary to bleed the hydraulic braking system per se to eliminate any air entrapped therein the motor should be started to activate the pump C and thus urge the expansion valve to the position as illustrated in Fig. 4. The locking member should be then screwed so as to project into the stepped bore 291.The motor should be stopped to permit the expansion valve 117' to move slightly toward the chamber 115 and thus abut the locking member 250. At this time despite the fact that the motor has stopped and all supply of pressurized hydraulic fluid has ceased accordingly the expansion valve is maintained in a position which prevents cutting of the first pressure transmission path by the check valve. It is however cut by the movement of the by-pass valve into the chamber 213 which engages the chamfer on the differential valve with the step of the stepped bore as previously described. Hence if the air bleed 252 is opened at this time then air bleeding can be simply accomplished by pumping of the pedal operatively connected to the master cylinder to displace any air within the braking system.Upon completion of the air bleeding the air bleed should be closed and the locking member rectracted via screwing of same to a position where interference with the expansion valve will not take place.

At this time it is considered appropriate to further clarify the differential piston effect which is vital to provide the desired operation of the control units as described in connection with the second and third embodiments of the present invention. As seen in Fig. 4 the diameters of the end of the by-pass valve 210' the end of the regulator valve 214 exposed to the pressurized hydraulic fluid from the master cylinder and the diameter of the ball valve 216' are respectively Dl, D2 and D3.As will be appreciated by one skilled in the art by carefully selecting the ratio of D1 D2: D3 in accordance with parameters such as the pressure supplied to the control unit by the pump C and the biasing effect of the spring 232 then the effect of the master cylinder pressure on the afore mentioned two valve ends and the ball valve can be optimally selected.

WHAT WE CLAIM IS: 1. An anti skid unit for a motor vehicle hydraulic braking system comprising: a first valve which is responsive to the hydraulic fluid pressure supplied into said anti skid unit from a source of hydraulic fluid under pressure and which permits fluid communication between first and second ports via a first pressure transmission path when it assumes a first normal position under the biasing influence of the hydraulic fluid pressure fed into said anti skid unit and cut off said fluid communication when it assumes a second position in the absence of said hydraulic fluid pressure and provide an anti skid inducing pressure reduction at said second port by moving from said first normal position to said second position;; an electromagnetic valve which isolates said source of hydraulic fluid under pressure from said first valve to cause said absence of hydraulic fluid pressure upon receiving an energizing signal; and a second valve which is responsive to said hydraulic fluid pressure and which, under the biasing influence of said hydraulic fluid pressure, is maintained in a position where it maintains a second pressure transmission path between said first and second ports closed and opens said second pressure transmission path in the absence of said hydraulic fluid pressure, said second valve bemg sensitive to small variations in the pressure introduced into said anti skid unit through said first port and modulates said hydraulic fluid pressure under the influence of which said first valve is biased to said first normal position so that the fluid connection between said first and second ports is maintained constant irrespective of said small variations in pressure while said hydraulic fluid pressure is fed into said anti skid unit.

2. An anti skid unit as claimed in claim 1, wherein said second valve takes the form of a single body which is movable through a predetermined distance without opening said second pressure transmission path and by said movement within said predetermined distance modulates the pressure under the influence of which said first valve is biased to said first normal position.

3. An anti skid unit as claimed in claim 1, wherein said second valve includes a third

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. noted that the chamber 212 is not directly communicated with the ball valve and the chamber 234 in this case this communication being replaced by the communication via conduits 106, 100 and 209. Thus operation is as before; via a first pressure transmission path during normal operation with no braking, temporal cutting of the first pressure transmission path during anti skid operation and use of the second pressure transmission path only during loss of supply of pressurized hydraulic fluid which ensures a fail safe direct connection between the master cylinder and the brake units. Now a further feature is found in this embodiment and that is the provision for air bleeding. As shown an air bleed 252 is fluidly connected with the chamber 234 via a conduit 208. A lock member 250 is as shown screwed into a threaded bore formed through the body of the control unit and arranged to be extendable into the stepped bore 291 so as to prevent the movement of the expansion valve 117' in a rightward direction (as seen in the drawings). Thus with this arrangement when it is necessary to bleed the hydraulic braking system per se to eliminate any air entrapped therein the motor should be started to activate the pump C and thus urge the expansion valve to the position as illustrated in Fig. 4. The locking member should be then screwed so as to project into the stepped bore 291.The motor should be stopped to permit the expansion valve 117' to move slightly toward the chamber 115 and thus abut the locking member 250. At this time despite the fact that the motor has stopped and all supply of pressurized hydraulic fluid has ceased accordingly the expansion valve is maintained in a position which prevents cutting of the first pressure transmission path by the check valve. It is however cut by the movement of the by-pass valve into the chamber 213 which engages the chamfer on the differential valve with the step of the stepped bore as previously described. Hence if the air bleed 252 is opened at this time then air bleeding can be simply accomplished by pumping of the pedal operatively connected to the master cylinder to displace any air within the braking system.Upon completion of the air bleeding the air bleed should be closed and the locking member rectracted via screwing of same to a position where interference with the expansion valve will not take place. At this time it is considered appropriate to further clarify the differential piston effect which is vital to provide the desired operation of the control units as described in connection with the second and third embodiments of the present invention. As seen in Fig. 4 the diameters of the end of the by-pass valve 210' the end of the regulator valve 214 exposed to the pressurized hydraulic fluid from the master cylinder and the diameter of the ball valve 216' are respectively Dl, D2 and D3.As will be appreciated by one skilled in the art by carefully selecting the ratio of D1 D2: D3 in accordance with parameters such as the pressure supplied to the control unit by the pump C and the biasing effect of the spring 232 then the effect of the master cylinder pressure on the afore mentioned two valve ends and the ball valve can be optimally selected. WHAT WE CLAIM IS:

1. An anti skid unit for a motor vehicle hydraulic braking system comprising: a first valve which is responsive to the hydraulic fluid pressure supplied into said anti skid unit from a source of hydraulic fluid under pressure and which permits fluid communication between first and second ports via a first pressure transmission path when it assumes a first normal position under the biasing influence of the hydraulic fluid pressure fed into said anti skid unit and cut off said fluid communication when it assumes a second position in the absence of said hydraulic fluid pressure and provide an anti skid inducing pressure reduction at said second port by moving from said first normal position to said second position;; an electromagnetic valve which isolates said source of hydraulic fluid under pressure from said first valve to cause said absence of hydraulic fluid pressure upon receiving an energizing signal; and a second valve which is responsive to said hydraulic fluid pressure and which, under the biasing influence of said hydraulic fluid pressure, is maintained in a position where it maintains a second pressure transmission path between said first and second ports closed and opens said second pressure transmission path in the absence of said hydraulic fluid pressure, said second valve bemg sensitive to small variations in the pressure introduced into said anti skid unit through said first port and modulates said hydraulic fluid pressure under the influence of which said first valve is biased to said first normal position so that the fluid connection between said first and second ports is maintained constant irrespective of said small variations in pressure while said hydraulic fluid pressure is fed into said anti skid unit.

valve reciprocatively received therein so as to be responsive to said small variations in the pressure introduced into said anti skid unit through said first port to be movable with respect to said second valve to modulate the pressure under the influence of which said first valve is biased to said first normal position.

4. An anti skid unit as claimed in claim 1 wherein said first valve comprises: a first valve unit slidably disposed in a first stepped bore, said first valve unit being arranged to define within said first stepped bore first second and third fluid chambers, said first chamber being arranged to communicate with said first port and further arranged to communicate with said second chamber when said first valve unit assumes said first normal position and be isolated therefrom when said first valve unit assumes said second position, said second chamber being arranged to be communicable with said second port; said second valve comprises:: a second valve unit slidably disposed in a second stepped bore said second valve unit being arranged to define within said second stepped bore fourth fifth and sixth fluid chambers, said fourth chamber being arranged to directly communicate with a third port and communicable with a fourth port and said third chamber, said fifth chamber being arranged to communicate with said second chamber and said second port when said second valve unit assumes a first normal position and which is so constructed and arranged as to be dividable or isolatable to permit fluid communication between said first port and said second port when said second valve unit assumes a second position, said sixth chamber being arranged to directly fluidly communicate with said first port; and said electromagnetic valve is interposed between third and fourth chambers and is so constructed and arranged as to cut normal communication therebetween and communicate said third chamber with a fifth port during the period an energizing signal is fed to said electromagnetic valve.

5. An anti skid unit as claimed in claim 4 wherein said third valve comprises: a third valve unit slidably disposed in a third stepped bore formed in said second valve unit, said third valve unit being arranged to define within said third stepped bore a seventh fluid chamber, said seventh chamber being directly communicated with said sixth chamber, said third valve unit being responsive to the pressure fed into said seventh chamber from said sixth chamber to reduce the communication betweeh said fourth chamber and said fourth port in accordance with the increase of the pressure in said sixth and seventh chambers.

6. An anti skid unit as claimed in claim 4 wherein said first port is arranged to communicate with a master cylinder of said hydraulic braking system for the introduction of the master cylinder pressure from said master cylinder into said first and sixth chambers; said second port is arranged to communicate with the hydraulic fluid cylinders of the brake units; said third port being arranged to communicate with said source of hydraulic fluid under pressure; said fourth port being arranged to communicate with a reservour the arrangement being that the communication between said fourth port and said reservoir restricts the flow of hydraulic fluid therebetween; and said fifth port is arranged to communicate directly with said reservoir.

7. An anti skid unit as claimed in claim 6 wherein: said source of hydraulic fluid under pressure is a pump which is fluidly connected to said reservoir; and a power steering unit is fluidly interposed between said fourth port and said reservoir, said power steering unit functioning to maintain said predetermined pressure in said fourth chamber and which uses the hydraulic fluid discharged from said fourth port as a source of motive energy.

8. An anti skid unit as claimed in claim 5 further comprising: a ball valve disposed in said fifth chamber which is so constructed and arranged as to cut fluid communication between said fifth and sixth chambers when said second valve unit assumes said first normal position and permit fluid communication between said chambers when said second valve unit assumes said second position.

9. An anti skid unit as claimed in claim 4 wherein said first valve unit is formed with a single action check valve on the end thereof, said check valve controlling said communication between said first and second chambers and comprising: a shaft reciprocatively passed through an orifice formed in a partition formed between said first and second chambers; means formed on the end of said shaft, said means being abutable with said partition upon said first valve unit assuming said second position to cut fluid communication between said first and second chambers; said shaft being arranged to permit fluid communication thereover when said first valve unit assumes said first normal position; and first biasing means disposed between said means and the end wall of said first chamber arranged to bias said check valve and thus said first valve unit in a direction which urges said means into abutment with said partition.

10. An anti skid unit as claimed in claim 4 wherein the diameter of the end of said first valve unit exposed to said third chamber is larger than the diameter of the end of same exposed to said second chamber; and the diameter of the end of the second valve unit exposed to said fourth chamber is larger than the diameter of the end of same which is exposed to said fifth chamber the arrangement of the foregoing being such that the pressurized hydraulic fluid normally supplied into said fourth chamber and said third chamber urges said first and second valve units to their respective first normal positions.

11. An anti skid unit as claimed in claim 5 wherein the diameter of the end of the third valve unit which is exposed to said fourth chamber is greater than the diameter of the end of same exposed to said seventh chamber.

12. An anti skid unit as claimed in any of the foregoing claims and as hereinbefore described with reference to and as illustrated in Figs 2 to 4 of the accompanying drawings.