GB2063402A

GB2063402A - Vehicle Anti-skid Braking Systems

Info

Publication number: GB2063402A
Application number: GB7938469A
Authority: GB
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1979-11-06
Filing date: 1979-11-06
Publication date: 1981-06-03

Abstract

A modulator assembly (1) is interposed between a master cylinder and a wheel brake. In an imminent skid situation brake fluid is dumped, from the brake line (3) connected to the wheel brake, to a self-contained reservoir (28). On termination of the skid signal fluid is pumped by a reciprocating pump piston (8) back into the brake circuit (2) upstream of a cut-off valve provided by a recess (21) in the pump piston and by ports (22) in the pump cylinder. The pump piston (8) is conveniently reciprocated by an electromagnetic actuator (9) of the kind disclosed in Patent G.B. Nos. 1 504 873 or 1 539 514. <IMAGE>

Description

SPECIFICATION Vehicle Anti-skid Braking Systems This invention relates to vehicle anti-skid braking systems of the kind in which fluid is dumped from a brake line connected to a wheel brake actuator on the detection of an imminent skid condition, and the dumped fluid is recovered by the use of a pump.

If anti-skid systems are to become generally adopted in vehicles, and cars in particular, it is essential that the cost of such systems be kept to a minimum. It is therefore highly desirable to produce anti-skid systems employing a minimum number of components but which are capable of dealing efficiency with a potential skid situation.

According to one aspect of the invention in an anti-skid braking system of the kind set forth the pump comprises a pump piston and reciprocatory means for reciprocating the pump piston, and a valve controlling connection of the wheel brake actuator to the outlet of a master cylinder is controlled by the reciprocatory means.

Thus a separate actuator is not required for operating the valve.

Preferably the reciprocatory means comprises an electromagnetic actuator.

Although a solenoid may be used as the electromagnetic actuator, a device of the kind disclosed in British Patent Specification Nos.

1,504,873 or 1,539,514, or in the complete specification of British Patent Application No.

28435/75 is preferably used. Such devices basically comprise a pair of relatively movable magnetisable members and one or more windings through which an electric current can be passed to produce a magnetic field to effect relative movement of the members. The magnetisable members are each formed with a series of slots and the windings are located in the slots. The disposition of the slots and the electrical connections to the windings is such that large local forces of attraction are generated between the projections defined between the slots of the magnetisable members. Such a device can be made capable of producing a relatively large output force for its size as compared with a conventional solenoid. Such a device will hereinafter be referred to as a "slotted electromagnetic actuator".

When a slotted electromagnetic actuator is employed to reciprocate the pump piston it preferably incorporates a resilient means for biassing the magnetisable members relative to each other in one direction to a rest position which the magnetisable members adopt when the winding is in the normal, de-energised condition.

Preferably the pump is arranged to pump dumped fluid back into that portion of the brake line communicating with the outlet of the master cylinder, that is upstream of said valve.

Preferably said valve comprises a spool valve member associated with or combined with the pump piston.

The valve is preferably arranged to be open when the pump is de-energised.

The reservoir to which fluid is dumped on the detection of an imminent skid condition may be in the form of an hydraulic accumulator comprising a resiliently biassed movable wall defining one wall of a variable volume reservoir chamber, and in a preferred arrangement said chamber comprises a chamber of the electromagnetic actuator housing the actuating mechanism.

A dump valve independent of the pump may be provided for controlling the dumping of fluid to the reservoir from the wheel brake actuator, but preferably the dump valve is also actuated by the reciprocating means of the pump.

When a spool valve member is used to cut-off communication between the master cylinder outlet and the wheel brake actuator, the dump valve may also comprise a portion of the spool valve member, the arrangement being such that the cut-off valve is normally open and the dump valve is normally closed, but that on displacement of the reciprocatory means in one direction from its normal position, on detection of an imminent skid condition, the cut-off valve closes followed by opening of the dump valve.

According to another aspect of the invention in a braking system of the kind set forth the pump comprises a pump piston that is reciprocated by the pulsing of an electromagnetic actuator during re-application of the brakes following dumping of fluid, and the brake pressure re-application rate is varied during the re-application period by varying the mark/space ratio of the pulse train.

The invention will now be further described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a cross-sectional view of a modulator assembly in accordance with the invention with the pump piston being shown in its normal position; Figure 2 is three graphs illustrating the operation of the modulator assembly of Figure 1; Figure 3 is a view, on a larger scale than that of Figure 1, of a modified pump arrangement; Figure 4 is a cross-sectional view of a further modulator assembly in accordance with the invention, the pump piston being shown in its normal position; Figure 5 is a view on a larger scale of the spool valve of the modulator assembly of Figure 4; Figure 6 is three graphs illustrating one mode of operation of the modulator assembly of Figure 4; and Figure 7 is a similar set of graphs showing another mode of operation of the assembly of Figure 4.

The moduiator assembly of Figure 1 comprises a housing 1 having an inlet port 2 for connection to the outlet of an hydrostatic master cylinder, not shown, and an outlet port 3 for connection to a wheel brake actuator, not shown. Housing 1 is formed with a stepped bore 4 having bore portions 5 and 6 of smaller and larger diameter respectively. A valve sleeve 7 is secured against movement in smaller bore portion 5 and a cylindrical pump piston 8 is slidable in the valve sleeve.A reciprocating means for the pump piston 8 in the form of a slotted electromagnetic actuator, as previously defined, is housed in larger bore portion 6 and comprises first and second magnetisable members 10 and 11. The first magnetisable member 10 has the form of a hollow cylinder which is closed at its upper end by an end wall 12 abutting piston 8 and is reciprocable in bore 6. The second magnetisable member 11 is also generally cylindrical in shape and is formed with a bore 13 in its upper half to receive a coiled compression spring 14 which biasses the member 10 upwardly, the lower end of member 11 being secured to an annular closure plate 18 8 closing bore 6 and secured to housing 1.The members 10 and 11 are formed with interengageable screw threads 15 and 16, the threads being formed with appreciable axial play, and various electrical windings 1 7 are located between the threads of the member 11.

For a detailed description of the arrangement of the threads and the windings reference should be made to Patent Specification No. 1,504,873. The arrangement is such that on energisation of the windings 17 a strong downward force is exerted on the member 10 to draw the pump piston downwards. Slotted electromagnetic actuators are sold under the trade mark "HELENOID" by Lucas Industries Limited.

When the pump piston 8 is in its normal, upward position as shown in Figure 1, the inlet port 2 communicates freely with outlet port 3 by way of a drilling 19, a first radial port 20 in valve sleeve 7, an annular recess 21 in piston 8, a second radial port 22 in sleeve 7, and a drilling 23. On detection of an imminent skid condition the winding 24 of a solenoid 25 is energised to retract the ball valve member 26 of a normally closed dump valve controlling communication between drilling 23 and a passage 27 leading into a reservoir space 28 defined in a bore 29 by a cup-shaped reservoir piston 30 biassed by a coiled compression spring 31. The reservoir piston 30 is sealed in bore 29 by an annular seal 32 located in a recess in the bore wall and the left-hand end of the bore 29 is vented to atmosphere by a port 33.

A pump chamber 34 is defined in bore portion 5 above piston 8 and is connected through a first one-way valve 35 and passage 36 to the reservoir space 28, and through a second one-way valve 37 and passage 38 to drilling 19 and inlet port 2, the arrangement of the valves being such that on reciprocation of pump piston 8 fluid is pumped from reservoir space 28 to the passage 38.

Simultaneously with energisation of winding 24 winding 1 7 is energised and magnetisable member 10 is drawn downwards followed by pump piston 8 under the pressure of fluid in pump chamber 34. Downward movement of the pump piston 8 results in axial displacement of annular recess 21 from ports 20 and 22 thereby isolating the wheel brake actuator from the master cylinder.

The operation of the modulator assembly of Figure 1 will now be described with reference to the graphs (a), (b) and (c) of Figure 2. At time to the brakes are applied and wheel brake pressure builds up until time t, when an imminent skid condition is detected in the usual way by measurement of the deceleration of the braked wheel. As previously mentioned, the solenoid 24 and winding 17 are energised by an electronic control unit, not shown in response to the skid signal so that pump piston 8 is retracted to cut-off port 2 from port 3 and to dump fluid from the brake line connected to port 3 to reservoir space 28, reservoir piston 30 being retracted against the force of spring 31.As a result the wheel brake pressure falls rapidly as shown in graph (a), until at time t2 the electronic controller decides that the wheel brake pressure has fallen sufficiently, the decision being taken in known manner in response to measurements of the wheel deceleration/acceleration for example, and dump solenoid 24 is de-energised to hold the wheel brake pressure constant until a time t3 when the electronic controller decides that it is now appropriate to increase the wheel brake pressure again.

During the time period t, to t3 the winding 1 7 was kept continuously energised so that pump piston 8 remains in its fully downward position to keep the ports 20 and 22 cut-off. From time t3 onwards the winding 17 is pulsed so as to provide a controlled step-wise increase of the wheelbrake pressure. This is brought about as follows.

On de-energisation of winding 17 the pump piston 8 moves upwards under the force of spring 14 to pump hydraulic fluid from pump pressure space 34 to passage 38. During the initial part of the pumping stroke the ports 20 and 22 are cutoff and fluid passes from passage 38 back to the master cylinder, but during the latter part of the pumping stroke the recess 21 provides communication between ports 20 and 22 so that fluid passes from drilling 19 to drilling 23 to increase the wheel brake pressure.

It will be appreciated that the volume of fluid which passes from drilling 19 to drilling 23 in a given time depends upon the mark/space ratio of the pulses applied to winding 17 and also upon the master cylinder pressure. The markXspace ratio can easily be varied by the electronic controller to obtain any desired rate of brake reapplication. At time t4 all of the fluid originally dumped to reservoir space 28 has been pumped back into the master cylinder line. The piston 8 then remains in its uppermost position, because any downward movement would produce a partial vacuum above it. Thus ports 20 and 22 are put in communication and the wheel brake pressure increases thereafter at a rate determined by the master cylinder pressure. In Figure 2(a) the wheel brake pressure at time t4 is shown as being equal to the pressure at time tl, but this will not always be so, since the rate of wheel brake pressure increase during the period t3 to t4 depends upon the current master cylinder pressure.

Since the rate of wheel brake pressure increase after time t4 is determined only by the master cylinder pressure, it would be possible in some circumstances for the wheel brake pressure to overshoot the optimum value with the Figure 1 construction. The problem of overshoot is minimised in the modified arrangement of Figure 3.

In the Figure 3 modification the pump piston is made in two parts, a lower spool valve part 39 secured to end wall 12 and formed with the annular recess 21 and with a pressure-balancing passage 40, and an upper, solid pumping part 41.

When all of the fluid has become exhausted from reservoir space 28 during pulsing of winding 17 at time t4, then the part 41 will stay in an upward position whilst valve part 39 continues to be reciprocated to allow fluid ta be metered from the master cylinder to wheel brake to build up wheel brake pressure. Although the quantity of fluid passing to the wheel brake after time t4 depends upon master cylinder pressure, the metering action minimises overshoot.

In both the Figure 1 and 3 constructions pulsing of the winding 1 7 is terminated after a predetermined time.

In Figures 4 and 5 corresponding reference numerals have been applied to parts corresponding to those of Figure 1. In this modulator valve assembly the solenoid 25 and ball valve 26 of the Figure 1 embodiment have been dispensed with, the dump valve now being provided by additional formations in the pump piston 8 and sleeve 7, and the reservoir piston 30 now being housed in a lower part 29 of bore 13 in the fixed magnetisable member 11.

As shown in Figure 5, the ports 20 and 22 are axially displaced from each other but normally communicate with each other by way of axially spaced annular recesses 42 and 43 and passages 44, 45 and 46 in piston 8. In order to provide the dump valve function the interior of the sleeve 7 is provided with a series of radial ports 47 axially displaced from ports 22 and communicating freely with bore 6; which provides the reservoir 28, by way of an annular space 48 and holes 52 provided in wall 12. In the normal, upward position of piston 8 shown in Figure 5 recess 43 is axially displaced from ports 47 so that ports 22 are cut-off from the reservoir space 28.

When the winding 1 7 is energised in the assembly of figure 4, initial downward movement of pump piston 8 will lead to the cutting off of ports 20 from recess 42 and to the interconnection of ports 22 and 47 to dump fluid from the wheel brake actuator to reservoir space 28.

In this construction a resilient connection is provided between wall 12 and pump piston 8 by a Belleville washer 49. Also a reservoir switch 50 is provided on the lower end of member 11 to detect the position of reservoir piston 30 and switch off the pulsing of winding 1 7 on return of piston 30 to its normal position. A further oneway valve 53 is arranged between drillings 19 and 23 to provide rapid release of wheel brake pressure during normal brake release, through the bypassing of ports 20 and 22.

Figure 6 shows the operation of the modulator assembly of Figure 4 during one brake release/reapplication cycle. In this case, in order to provide a linear wheel brake pressure re-application rate the mark/space ration is progressively reduced by the electronic control unit during the period t2-t4 (there is no hold-stage in this case, and therefore no t3), in order to compensate for the reducing pressure difference between wheel brake and master cylinder pressures.

Figure 7 shows another mode of operation of the assembly of Figure 4 in which the electronic controller is arranged to provide various periods of controlled wheel brake pressure change. During the period t, to t2 the winding 1 7 is continuously energised to provide a fast dump of brake fluid to reservoir. During the period t2 to t3 the winding 17 is pulsed with a high mark/space ratio pulse train to provide restricted brake pressure release. A hold period t3 to t4 is achieved by pulsing winding 17 with a pulse train of an intermediate mark/space ratio. During the period t4 to t5 a rapid initial re-application of the brakes is brought about by de-energisation of the winding 1 7, and this is followed by a period t5 to to during which the re-application rate is restricted by the use of a pulse-train of low mark/space ratio.

In other modulating valve assemblies in accordance with the invention, not illustrated, the spring 14 is arranged to bias the pumping piston 8 in the opposite direction to that shown, the actuator 9 is arranged such that the pump piston 8 is moved upwards on energisation of winding 17, and the pump piston 8 normally adopts its lowermost position, in which ports 20 and 22 are arranged to be connected.

Claims

Claims 1. An anti-skid braking system of the kind set forth in which the pump comprises a pump piston and reciprocatory means for reciprocating the pump piston, and a valve controlling connection of the wheel brake actuator to the outlet of a master cylinder is controlled by the reciprocatory means. 2. A braking system as claimed in claim 1 in which the reciprocatory means comprises an electromagnetic actuator. 3. A braking system as claimed in claim 2 in which the electromagnetic actuator comprises a slotted electromagnetic actuator, as hereinbefore defined. 4. A braking system as claimed in claim 3 in which the slotted electromagnetic actuator incorporates a resilient means for biassing the magnetisable members relative to each other in one direction to a rest position which the magnetisable members adopt when the winding is in the normal, de-energised condition. 5. A braking system as claimed in any of the preceding claims in which the pump is arranged to pump dumped fluid back into that portion of the brake line communicating with the outlet of the master cylinder. 6. A braking system as claimed in any of the preceding claims in which said valve comprises a spool valve member associated with or combined with the pump piston. 7. A braking system as claimed in any of the preceding claims in which the valve is arranged to be open when the pump is de-energised. 8. A braking system as claimed in any of the preceding claims in which a dump valve controlling the dumping of fluid to a reservoir from the wheel brake actuator is also actuated by the reciprocating means of the pump. 9. A braking system as claimed in claim 8 as appended to claim 6 in which the dump valve comprises a portion of the spool valve member, the arrangement being such that the cut-off valve is normally open and the dump valve is normally closed, but that on displacement of the reciprocatory means in one direction from its normal position, on detection of an imminent skid condition, the cut-off valve closes followed by opening of the dump valve. 10. A braking system of the kind set forth in which the pump comprises a pump piston that is reciprocated by the pulsing of an electromagnetic actuator during re-application of the brakes following dumping of fluid, and the brake pressure re-application rate is varied during the re application period by varying the mark/space ratio of the pulse train. New Claims or Amendments to Claims filed on 5/11/80. Superseded Claims 1-10 New or Amended Claims:- 1-12

1. An anti-skid braking system of the kind set forth in which the pump comprises a pump piston and reciprocatory means for reciprocating the pump piston, a valve controls connection of the wheel brake actuator to the outlet of a master cylinder, the reciprocatory means is arranged to close and open the valve during each cycle of the reciprocatory-means, and the pump is arranged to pump dumped fluid back into that brake line portion which connects the valve to the master cylinder outlet.

2. A braking system as claimed in claim 1 in which the reciprocatory means comprises an electromagnetic actuator.

3. A braking system as claimed in claim 2 in which the electromagnetic actuator comprises a slotted electromagnetic actuator, as hereinbefore defined.

4. A braking system as claimed in claim 3 in which the slotted electromagnectic actuator incorporates a resilient means for biassing the -magnetisable members relative to each other in one direction to a rest position which the magnetisable members adopt when the winding is in the normal, de-energised condition.

5. A braking system as claimed in any of the preceding claims in which said valve comprises a spool valve member associated with or combined with the pump piston.

6. A braking system as claimed in any of the preceding claims in which the valve is arranged to be open when the pump is de-energised.

7. A braking system as claimed in any of the preceding claims in which a dump valve controlling the dumping of fluid to a reservoir from the wheel brake actuator is also actuated by the reciprocating means of the pump.

8. A braking system as claimed in claim 7 as appended to claim 5 in which the dump valve comprises a portion of the spool valve member, the arrangement being such that the cut-off valve is normally open and the dump valve is normally closed, but that on displacement of the reciprocatory means in one direction from its normal position, on detection of an imminent skid condition, the cut-off valve closes followed by opening of the dump valve.

9. A braking system of the kind set forth in which the pump comprises a pump piston that is reciprocated by the pulsing of an electromagnetic actuator during re-application of the brakes following dumping of fluid, and the brake pressure re-application rate is varied during the reapplication period by varying the mark/space ratio of the pulse train.

10. A braking system substantially as described with reference to Figures 1 and 2 of the accompanying drawings.

11. A braking system as claimed in Claim 10 and modified substantially as described with reference to Figure 3 of the accompanying drawings.

12. A braking system substantially as described with reference to Figures 4 to 7 of the accompanying drawings.