GB2146080A

GB2146080A - Vehicle anti-skid braking systems

Info

Publication number: GB2146080A
Application number: GB08323577A
Authority: GB
Inventors: Glyn Phillip Reginald Farr
Original assignee: Lucas Industries Ltd
Current assignee: ZF International UK Ltd
Priority date: 1983-09-02
Filing date: 1983-09-02
Publication date: 1985-04-11
Also published as: JPH0241462B2; ES8505592A1; GB8700977D0; ES535591A0; GB2146080B; JPS60151157A; FR2551409A1; GB2184184A; FR2551409B1; GB8323577D0; GB2184184B

Abstract

A modulator piston assembly (6) is of generally dumb-bell shape, consisting of a first modulator piston (7) and a second modulator piston (9) having an integral stem (10) carrying an isolation valve member (14) engageable with a seat (16) carried by the first piston (7). First, second and third modulator chambers (23, 24 and 41) respectively are bounded by the modulator piston assembly, such that on movement of the modulator piston assembly from its normal position the volumes of first and third chambers (23, 41) increase, and that of second chamber (24) decreases. A master cylinder (28) is connected permanently to the second chamber (24), and a wheel brake (25) to the first chamber (23), and a normally closed solenoid controlled dump valve (35, 36) is interposed between the master cylinder inlet (31) and the third chamber (41). On a skid signal the dump valve (35, 36) opens, and fluid flows from the second chamber (24) to the third chamber (41), isolation valve (14, 16) closes, and the first chamber (23) is expanded to relieve the wheel brake pressure. A pump piston (51) is arranged in association with one-way seals (42, 43) to pump fluid from the third chamber (41) back to the inlet of the dump valve (35, 36), and on cancellation of the skid signal thereby returns fluid from the third chamber (24). The pump piston on the Fig. 2 construction is remote from the modulator assembly and is connected thereto by only a single hydraulic line (56). <IMAGE>

Description

SPECIFICATION Vehicle anti-skid braking systems This invention relates to vehicle anti-skid braking systems.

There have been many previous proposals for anti-skid braking systems. Most of these fall broadly into the following types. In a first type the wheel brakes are connected to a reservoir to relieve the brake pressure. In a second type a support chamber is connected to a reservoir. In a third type the wheel brakes are connected to an expansion chamber, and in a fourth type an accumulator is connected to a chamber to push back against the master cylinder.

Each of these known types has its associated disadvantages. The use of a separate reservoir or expansion chamber to which fluid is dumped, either directly from the brakes or from a support chamber, adds.considerably to the system piping. It is also generally considered to be undesirable for a large force to be reacted back to the brake pedal in the event of a skid signal. Some of these systems can be difficult to bleed.

In attempting to overcome the disadvantages of the existing types of systems we have now devised a further type of system.

According to one aspect of the invention in a vehicle anti-skid braking system a modulator assembly comprises a modulator housing, a modulator piston assembly axially movable in a bore in the housing, the modulator piston assembly and the housing bore being shaped to define first, second and third modulator chambers bounding portions of the modulator piston assembly such that on movement of the modulator piston assembly in one direction from its normal position the volumes of the first and third chambers are both increased whereas the volume of the second chamber is decreased, the first chamber is connected to a wheel brake, the second chamber is connected to a master cylinder, and the third chamber is connected to the second chamber through a normally closed dump valve responsive to a skid sensing means, a normally open isolation valve is connected between the first and second chambers and is arranged to be closed in response to a skid condition signal, and means for returning fluid from the third to the second chamber on termination of the skid signal.

On receipt of a skid condition signal the dump valve is opened to dump fluid from the second chamber to the third chamber thereby to move the modulator piston assembly from its normal position to expand the volume of the first chamber connected to the wheel brake to relieve the wheel brake pressure.

Since there is no requirement for an exter nal reservoir, the complications associated with an external reservoir may be avoided. As will be explained hereafter this enables a pump working chamber of the fluid returning means to be positioned remote from the modulator housing if desired with only a single pipe connection.

Preferably the isolation valve is closed by relative movement of two parts of the modulator piston assembly in response to initial expansion of the third chamber.

A particular advantage of a system in accordance with the invention is that by making substantially equal the pressure effective areas of the modulator piston assembly that are exposed respectively to the second and third chambers, little fluid reaction is applied to the master cylinder on dumping of fluid or on reapplication of the brakes, since the mean volume change of fluid transferring from the second to the third chamber or vice versa will vary very little on shuttling of the piston assembly.

In order to 'tune' the system the said pressure effective areas may be made slightly unequal if desired. In particular, when it is required to reduce the wheel brake pressure to zero it may be desirable to arrange the pressure effective area of the modulator piston assembly exposed to the third chamber to be larger than that exposed to the second chamber.

The master cylinder may be a hydrostatic master cylinder or a power valve.

In a preferred arrangement the modulator piston assembly is of generally dumb-bell shape having opposite end portions connected by a reduced diameter central portion which extends sealably through a partition wall of the modulator housing, the second and third chambers are defined at opposite ends of the modulator piston assembly, and the first chamber is an annular chamber adjacent to the partition wall and encircling said reduced diameter portion.

The modulator piston is preferably biassed to its normal position by resilient means which urges the modulator piston assembly in the direction to reduce the volume of the third chamber.

In the last-mentioned preferred arrangement, the resilient means is conveniently located in a further annular chamber located on the opposite side of the partition from said annular chamber, the further annular chamber being vented to atmosphere or containing fluid at low pressure.

The means for returning fluid from the third to the second chamber on termination of the skid signal preferably comprises a pump piston actuated by an eccentric on a drive shaft, the pump piston normally being held out of engagement with the eccentric by a piston biassing means, but being urged into engagement with the eccentric on supply of fluid to the third chamber by the increase in fluid pressure in the third chamber.

The in let and outlet valves of the pump are preferably both located in the modulator assembly, but the working chamber of the pump may be located remotely from the pump if desired and connected by a single hydraulic line to a valve chamber of the modulator assembly located between the pump inlet and outlet valves. Since there need only be a single hydraulic connection between the pump working chamber and the pump valve chamber the pump working chamber may be located at any convenient position in the vehicle where there is a suitable drive means for the pump.

The drive means may be a vehicle wheel, engine, electric motor, or transmission etc.

This remote positioning of the pump working chamber from the pump valve chamber has wider applicability than to systems in accordance with the first aspect of the invention.

Thus, in accordance with a second aspect of the invention a vehicle anti-skid braking system comprises a modulator assembly provided with a chamber to which fluid is dumped in response to a skid condition signal from a skid sensing means, a pump valve chamber located within the modulator assembly and connected to said chamber by a pump inlet valve, a pump outlet valve housed within the modulator assembly, and a pump working chamber remote from the modulator assembly and connected to the pump valve chamber by a single hydraulic line.

Two embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a cross-section through a selfcontained modulator, pump and skid detector assembly, the modulator piston assembly being shown in its normal position; and Figure 2 is a similar view but of a modulator assembly, and remote pump driving assembly combined with a skid detector.

With reference to Figure 1, a modulator housing 1 is provided with a stepped throughbore 2 inclined with respect to a second stepped through-bore 3 and closed at its lower end by a threaded plug 4'. The bore 2 is partitioned into an upper bore part 3 and a lower bore part 4 by a partition wall 5. A modulator piston assembly 6 of substantially dumb-bell shape is slidable in bore 2 and comprises a first modulator piston 7 provided with a frusto-conical bore 8, and a second modulator piston 9 having an integral stem 10 which extends axially through partition wall 5 and through the bore 8 of first modulator piston 7. The first and second modulator pistons 7, 9 are sealed to the wall of bore 2 by O-ring seals 11, 12 respectively, and stem 10 is sealed to wall 5 by an O-ring 1 3.

A frusto-conical portion 14 of stem 10 spaced from its headed free end 1 5 acts as an isolation valve member for co-operation with an isolation valve seal 1 6 carried by first modulator piston 7, the seal 1 6 being located in place by a seal retainer 1 7. In the normal condition of the modulator assembly, when there is no skid signal, a protrusion 1 8 on the second piston 9 abuts a fixed insert 1 9 and the piston 9 is held in that position by a coiled compression spring 20 acting between partition wall 5 and piston 9.A skirt 21 on piston on first modulator piston abuts partition wall 5 to define the normal position of first modulator piston 7, the piston 7 being urged to that position by a compression spring 22 acting between head 1 5 and retainer 1 7. It will be seen from Figure 1 that the isolation valve defined by members 14 and 1 6 is thereby arranged to be open in the normal condition of the modulator assembly to provide unrestricted communication between a first modulator chamber 23, defined in bore part 4 between wall 5 and first piston 7. and a second modulator chamber 24 defined in bore part 4 between first modulator piston 7 and plug 4'.

First modulator chamber 23 is permanently connected to a wheel brake 25 by way of a port 26 and brake line 27, and second modu lator chamber 24 is permanently connected to a master cylinder 28, which may be a hydrostatic master cylinder or a power valve, by way of an inclined drilling 29, a drilling 30 parallel to bore 2, an inlet port 31 and a brake line 32.

Thus in the normal condition of the modulator assembly the wheel brake is actuated in the normal way by fluid pressure from master cylinder 28 through the open isolation valve 14, 16.

The insert 1 9 is threadedly secured at its upper end to the body 33 of a solenoid 34, the body 33 acting as a closure plug for the upper end of bore 2. Insert 1 9 is provided with an axial through-bore 34 incorporating a restriction which acts as a dump valve seat 35 for engagement by a dump valve member 36 carried by the solenoid armature 37. In the normal, unenergised condition of solenoid 34 the armature is biassed downwards by a spring 38 to hold the dump valve 35, 36 closed. The upper side of the dump valve 35, 36 is permanently connected to the master cylinder and to second modulator chamber 24 by diametral drillings 39 in insert 1 9 an annular chamber 39' and by a drilling 40 connecting with inlet port 31.

A third modulator chamber 41 is defined in bore 2 between second modulator piston 9 and insert 1 9 and communicates with the lower side of valve seat 35. Since valve seat 35 is normally closed there cannot normally be any flow from inlet 31 to chamber 41.

Insert 1 9 is provided with two external annular recesses in which are located respective annular one-way seals 42, 43 which permit fluid flow along the wall of bore 2 in the direction from third modulator chamber 41 to drilling 40. As will be explained hereafter, the one-way seal 42 acts as a pump inlet valve, and seal 43 acts as a pump outlet valve, of a pump means for recovering fluid dumped to third modulator chamber 41.

The second housing bore 3 mounts a drive shaft 44 for a pump eccentric ring 45, the shaft 44 running in spaced bearings 46, 47 and carrying at its inner end a rotor 48 of an opto-sensor assembly 49 for measuring the shaft speed. Shaft 44 may be driven by a vehicle wheel through any suitable drive connection, such as a toothed belt. Eccentric ring 45 is freely rotatable on a bearing 50 mounted eccentrically of the axis of shaft 44.

A pump piston 51 is housed in a bore 52 extending transversely of bore 3 to define a pump working chamber 53 at the lower end of bore 52 sealed by a piston seal 54. Piston 51 is normally held out of engagement with ring 54 by a compression spring 55.

Pump working chamber 53 is permanently connected with bore 2 by a drilling 56, which connects with bore 2 at a position intermediate one-way seals 42 and 43.

An electronic control unit, not shown, is connected to the opto-sensor assembly 49 and to the solenoid 34 by a plug and socket connection 57. The control unit works in conventional manner in monitoring the output of the opto-sensor assembly to detect exessive deceleration of the shaft 44 indicative of an imminent skid situation. A skid signal is produced when such a situation is detected, and in response to the skid signal the solenoid 34 is energised.

The operation of the assembly of Figure 1 will now be described. Normally, in the absence of a skid signal, the isolation valve 14, 1 6 is open as shown, dump valve 35, 36 is shut and the modulator pistons 7, 9 are in the positions shown, in which the volumes of first and third chambers 23, 41 respectively are substantially minimised, and the volume of chamber 24 is substantially maximised. On application of the master cylinder 28 the wheel brake is applied, as previously explained, by fluid supplied through open isolation valve 14, 16.

An important advantage of the arrangement is that in the event of a sudden braking effort by the driver, the isolation valve 14, 1 6 will not close under the transient fluid pressure forces, since both pistons 7, 9 are engaged with stops.

When the electronic control unit detects an imminent skid condition from the signals of opto-sensor 49 the solenoid 34 is energised to move armature 37 upwards to open dump valve seat 35 and allow fluid to commence flowing from second modulator chamber 24 to third modulator chamber 41 by way of drillings 29, 30, 40 and 39. Initial downward displacement of second modulator piston 9 due to fluid entering third modulator chamber 41 results in closure of the isolation valve 16, the first modulator piston initially remaining in engagement with wall 5 under the action of spring 22.Closure of isolation valve 14, 1 6 isolates the first modulator chamber 23 from the second modulator chamber 24 and thereby isolates the wheel brake 25 from the supply, so that as the pistons 7, 9 move downwards in unison on continued flow of fluid from chamber 24 to chamber 41 the first modulator chamber 23 is expanded to reduce the wheel brake pressure. Chamber 58 in which the spring 20 is located is vented to atmosphere to permit the movement of piston 9. Eventually the modulator piston assembly 6 reaches a new equilibrium position in which the sum of the pressure and spring forces on the assembly 6 are balanced.

In the drawings the diameter of pistons 7 and 9 are equal, but if it is required to reduce the wheel brake pressure to zero it will be desirable to increase the diameter of piston 9 relative to that of piston 7 to increase the pressure-effective area of piston 9 exposed to chamber 41.

An advantage of this arrangement, and in particular in having the pistons 7 and 9 of substantially the same diameter, is that little fluid volume change is felt at the master cylinder 28 by the dumping of fluid to the chamber 41, because the total volume of fluid in chambers 24 and 41 will remain substantially constant as the modulator piston assembly moves. Thus the driver will not feel a violent reaction on the brake pedal.

On initial supply of fluid to third modulator chamber 41, some fluid will flow past oneway seal 42 and by way of drilling 56 to the pump working chamber to move the pump piston 51 upwards into engagement with the ring 45, and thereafter the pump will be operative to pump fluid from third chamber 41 past seals 42, 43 to chamber 39', the fluid returning to chamber 41 by way of open dump valve seat 35. It will be understood that as the piston 51 moves upwards fluid is drawn past seal 42 from chamber 41, and then on downward movement of piston 51 fluid is forced from drilling 56 past seal 43, the seal 43 acting as a pump outlet valve.

This pumping action will cause some vibration which will probably be felt by the driver's foot, but this will be useful in indicating the presence of an imminent skid situation.

When the wheel speed has recovered sufficiently for the electronic control unit to terminate the skid signal, solenoid 34 is de-energised to cause the dump valve 35, 36 to close. The pump means continues to operate in pumping fluid from third modulator chamber 41 to drilling 39, but now the fluid flows to passage 40 and back to second modulator chamber 24 by way of drillings 30, 29. Thus fluid is progressively pumped by pump piston 51 from chamber 41 back to chamber 24, and as chamber 41 decreases so chamber 24 expands so that the net volume change at the master cylinder is zero, or very small. Owing to the pumping action the modulator piston assembly 6 moves upwards to reduce the volume of first modulator chamber 23 to redevelop the wheel brake pressure.In the final stage of pumping of fluid from chamber 41, the first piston 7 will re-engage with wall 5, and isolation valve 14, 1 6 will re-open to reconnect the master cylinder 28 with the wheel brake 25.

Once piston stop 1 8 re-engages with insert 1 9 under the force of fluid pressure in chamber 24 and spring 20, no more fluid can be pumped from chamber 41. When, in due course, the brakes are released by the driver, pump piston 51 will move downwards under the force of spring 55 which is sufficient to cause fluid to be urged past seal 43 back to the master cylinder line 32, to enable spring 55 to retract the pump piston 51 clear of the ring 45.

It will be appreciated that with this arrangement by which the third modulator chamber 41 is expanded in unison with contraction of the second modulator chamber 24, as the modulator piston assembly 6 moves downwards in response to a skid signal, there is no requirement for an external reservoir, and the modulator piston assembly can be accommodated within a single bore 2 of the housing.

Since there is only a single fluid connection, the drilling 56 in Figure 1, between the pump working chamber 53 and the bore in which the pump valve seals 42 and 43 are located, it is possible to arrange the pump working chamber remote from the modulator piston assembly, as in the arrangement of Figure 2 described hereafter.

In Figure 2 parts corresponding to those of Figure 1 have been given corresponding reference numerals. Only the major parts have, however, been referenced. In this case the drive assembly 44, 45, 51 of the pump means, and the opto-sensor 49 are arranged as a unit 1' which is intended to be mounted remote from the housing 1 containing the modulator assembly, but connected thereto by a single hydraulic brake line 56, and by an electrical lead 57 from the opto-sensor 49.

It will be appreciated that the provision of two units connected by only a single hydraulic line and an electrical lead considerably assists in accommodating the assemblies in a vehicle.

For example, the modulator assembly may be closely associated with the master cylinder, and the small sized unit 1' may more easily be accommodated at a wheel or other drive source for shaft 44.

In the arrangement of Figure 2, a flow regulator valve assembly 58 has been arranged between the pump working chamber 53 and the brake line 56 to control the rate at which fluid is pumped by the pump means from chamber 41 to chamber 24 on reapplication of the wheel brake pressure thereby to control the rate of reapplication. Any suitable kind of flow regulator may be employed. The regulator valve shown is described in detail in published Patent Application G.B. No.

2045372A. The regulator is provided to accommodate any changes in pump output with wheel speed.

Another difference between the arrangement of Figure 2 and that of Figure 1 is the provision in the Figure 2 construction of a flow restrictor 59 to prevent sudden changes in the flow rate through passage 34, the restrictor 59 comprising a spring biassed cup with restrictor recesses in the rim of the cup.

It will be appreciated that the regulator valve 58 and restrictor 59 may equally be applied to the arrangement of Figure 1.

Although the dump valve member 26 has been shown as operated by a solenoid 34 in response to the output of an opto-source 49, it would be possible to operate the dump valve by a deceleration responsive flywheel mechanism, such as that shown in Patent No.

G.B. 2029914B.

It will also be appreciated that the shape and disposition of the first, second and third modulator chambers 23, 24, 41 may be substantially altered whilst still achieving a similar result.

Whilst the pump inlet and outlet valves have been provided by one-way seals 42, 43 it would obviously be possible to use other constructions of one-way valve.

In a complete brake system there will usually be several modulator assemblies, with their inlet ports 31 connected to a common master cylinder 28, which may of course be a tandem master cylinder, in which case some ports 31 would be connected to a different output of the master cylinder.

Claims

1. A vehicle anti-skid braking system comprising a modulator assembly connected between a master cylinder and a wheel brake, in which the modulator assembly comprises a modulator housing and a modulator piston assembly axially movable in a bore in the housing, the modulator piston assembly and the housing bore are shaped to define first, second and third modulator chambers bounding portions of the modulator piston assembly such that on movement of the modulator piston assembly in one direction from its normal position the volumes of the first and third chambers are both increased whereas the volume of the second chamber is decreased, the first chamber is connected to the wheel brake, the second chamber is connected to the master cylinder, and the third chamber is connected to the second chamber through a normally closed dump valve responsive to a skid sensing means, a normally open isolation valve is connected between the first and second chambers and is arranged to be closed in response to a skid condition signal, and comprising means for returning fluid from the third to the second chamber on termination of the skid signal.

2. A braking system as claimed in claim 1 in which the isolation valve is arranged to be closed by relative movement of two parts of the modulator piston assembly in response to initial expansion of the third chamber.

3. A braking system as claimed in claim 1 or claim 2 in which the pressure effective areas of the modulator piston assembly that are exposed respectively to the second and third chambers are made substantially equal.

4. A braking system as claimed in any of the preceding claims in which the modulator piston assembly is of generally dumb-bell shape having opposite end portions connected by a reduced diameter central portion which extends sealably through a partition wall of the modulator housing, the second and third chambers are defined at opposite ends of the modulator piston assembly, and the first chamber is an annular chamber adjacent to the partition wall and encircling said reduced diameter portion.

5. A braking system as claimed in any of the preceding claims in which the modulator piston assembly is biassed to its normal position by resilient means which urges the modu lator piston assembly in the direction to reduce the volume of the third chamber.

6. A braking system as claimed in claim 5 in which the resilient means is located in a further annular chamber located on the opposite side of the partition wall from said annular chamber, the further annular chamber being vented to atmosphere or containing fluid at low pressure.

7. A braking system as claimed in any of the preceding claims in which the means for returning fluid from the third to the second chamber on termination of the skid signal comprises a pump piston actuated by an eccentric on a drive shaft, the pump piston normally being held out of engaqement with the eccentric by a piston biassing means, but being urged into engagement with the eccentric on supply of fluid to the third chamber by the increase in fluid pressure in the third chamber.

8. A braking system as claimed in any of the preceding claims in which the inlet and outlet valves of the pump are both located in the modulator assembly, the working chamber of the pump being located remotely from the modulator assembly and connected by a single hydraulic line to a valve chamber of the modulator assembly located between the pump inlet and outlet valves.

9. A modulator assembly for a vehicle antiskid braking system and substantially as described with reference to Figure 1 of the accompanying drawings.

10. A modulator assembly for a vehicle anti-skid braking system and substantially as described with reference to Figure 2 of the accompanying drawings.

11. A vehicle anti-skid braking system incorporating a modulator assembly as claimed in claim 9 or claim 10.