GB2403521A

GB2403521A - Vacuum brake booster having two assistance levels

Info

Publication number: GB2403521A
Application number: GB0315527A
Authority: GB
Inventors: Robert Alan Anderson
Original assignee: TRW Ltd
Current assignee: TRW Ltd
Priority date: 2003-07-03
Filing date: 2003-07-03
Publication date: 2005-01-05
Anticipated expiration: 2023-07-03
Also published as: GB2403521B; GB0315527D0

Abstract

A vacuum brake booster comprises a vacuum chamber (14) and a working chamber (15) separated from each other by a movable wall (16), a control valve (17) operatively coupled to said movable wall (16) and which is capable of controlling supply of atmospheric pressure or above-atmospheric to the working chamber (15) in dependence upon the displacement of an input element of the brake booster to achieve a pressure difference at the movable wall (16), and an output member operatively coupled to the moveable wall, in which the booster is so arranged as to provide a first, normal, mode of operation in which force is transmitted from the input element to the output element with a first level of assistance from the booster and a second, emergency braking, mode in which force is transmitted from the input element to the output element with a second, higher level of assistance from the booster and in which increased forces on the input element are transmitted to the output element during emergency braking mode.

Description

240352 1

VACUUM BRAKE BOOSTERS

The invention relates to a vacuum brake booster.

Vacuum brake boosters are provided in many braking systems to reduce the force that needs to be applied by a driver of a vehicle to achieve a given brake pressure. The booster typically comprises a vacuum chamber and a working chamber separated from each other by a movable wall and a control valve, which comprises a valve body operatively coupled to said movable wall and to an input element. Operation of the input element by pressing the brake pedal opens the valve which in turn controls the supply of atmospheric pressure or above-atmospheric to the working chamber in dependence upon the displacement of the input element. This pressure achieves a pressure difference across the movable wall causing it to move in the direction of movement of the input element. The movement of the wall therefore applies an assistance force to the input element making it easier to move.

The amount of assistance is dependent upon the force applied to the input member since this dictates how far the valve opens to allow air into the chamber. The force applied to the input member is controlled to an extent by the compliance between input member and the output member. The compliance can be controlled by altering the properties of a resilient reaction disk that is squashed between the input member and the output member.

Also known are so-called emergency braking aids, which are frequently referred to as "Brake Assistants". These devices provide a driver with increased braking power in an emergency braking situation for substantially the same actuating force. Emergency braking aids may be divided into electro-magnetically actuated and mechanically actuated systems. For reasons of cost, the use of a mechanical system is preferred for applications in vehicles at the lower end of the price range.

During tests with average drivers it was found out that most of the drivers apply the brakes at the beginning of an emergency braking situation quick I enough but cannot hold the high pedal press-down forces over a longer period of time, needed to achieve the shortest braking distance.

With the use of a brake assistant device the boost ratio of the booster can be changed in case of an emergency braking condition allowing to the device to provide an increased servo assistance. This means higher brake pressure with less brake force input at the brake pedal in comparison to normal braking conditions. By boost ratio we mean the amount of assistance provided by the movable wall for a given force applied to the; input element.

We are aware of German patent application DE 198 3l 961 Al (Continental Teves AG), which shows a brake assistant device for use under emergency braking conditions. The reaction system comprises an input piston, a reaction piston supported by a spring, and an elastically deformable coupling-ring. Forces are applied to the input piston by an input element, typically operatively coupled to a brake pedal and through the coupling ring to the reaction piston. The reaction piston acts on a rubber reaction disc operatively coupled to an output member. The coupling member can be urged by the input piston to a retract into the reaction piston to release the reaction piston to allow relative movement of the reaction and input pistons when a relative movement between the ring and the input-piston exceeds a predetermined value characteristic of emergency braking. This uncouples the input and reaction pistons and allows greater travel of the input element, causing a valve of the booster of which the brake assistant device is a part to open more fully and so I more servo assistance to be developed.

While releasing the reaction piston, the input piston moves backwards and the reaction forces from the master cylinder will be taken by the ring, the ring being biased onto an abutment of the control valve housing. Due to this, the boost ratio will be increased giving the greatest possible level of servo assistance. I We are also aware of the US patent application published as US; 2003/0041726 Al (Lucas Automotive), which describes a brake assistant device for use in a vacuum brake booster. A coupling arrangement (66) connects the reaction piston (72) to the control valve housing (22) under emergency braking conditions and initiates a ratio change giving increased servo assistance.

Accordingly, the present invention provides a vacuum brake booster comprising a first chamber and a second chamber separated from each other by a movable wall, a control valve operatively coupled to said movable wall and which is capable of controlling supply of atmospheric pressure or above-atmospheric to the second chamber in dependence upon the displacement of an input element of the brake booster to achieve a pressure difference at the movable wall, and an output member operatively coupled to the moveable wall, in which the booster is so arranged as to provide a first, normal, mode of operation in which force is transmitted from the input element to the output element with a first I level of assistance from the booster and a second, emergency braking, mode in which force is transmitted from the input element to the output element with a second, higher, level of assistance from the booster and in which increased forces on the input element are transmitted to the output element during emergency braking mode.

The invention has the advantage that if the brake assistant device is in the emergency mode of operation and a further pressure has been applied to the input member, the driver of a vehicle provided with this system should have the same control of the brake pressure as during normal braking albeit with an additional amount of boost at all times.

The input element may be connected to the output member through a I variable length connection, which may act on a reaction disk or other resilient element. The moveable wall may also act upon the reaction disk to apply force to the output member. In the first mode the connection may have a first length and in the second mode it may have a second length and in which the amount of boost is dependent upon the length of the connection. This may be achieved in at least one embodiment by making the amount of opening of the valve dependent upon the length of the connection, thereby making the amount of force provided by the movable wall dependent upon the length of the connection.

In a most preferred arrangement, the second length may be shorter than the first length. This allows for greater opening of the valve in the second mode and hence greater servo assistance.

One convenient way to make the amount of valve opening - and hence boost greater with a change in length of the variable connection is to connect a first portion of the valve to one end of the connection member and a second portion of the valve to the other such that a change in the length of the variable connection changes the spacing between the two parts of the valve. In a most convenient arrangement one part of the valve assembly may be connected to the movable wall and the other to the input member. By connect we do not necessarily mean directly connected, but simply that they may be operably connected to one another, perhaps I through one or more intermediate parts.

In this way, on normal braking force applied to the input member moves the connection to compress the reaction disk, thereby opening the valve by an amount dependent upon the force applied. In the emergency mode, an offset is present in the amount of valve opening due to the changed length of the connection, thereby applying an increased amount of boost I overlaid with an amount of boost determined by the driver applying depressing the input member. The amount of additional boost can be controlled by careful design of the valve and the amount of difference between the first and second lengths of the variable connection.

The variable length connection may comprise two telescopic parts through which the force is transmitted from input to output elements. In one mode the two telescopic parts may be fixed relative to one another and may have a first fixed overall length. This may correspond to the first mode.

When a predetermined emergency force or rapid increase in force has been applied to the connection member by the driver pressing on the input member, the two parts may collapse relative to one another such that their overall length is reduced. Their length may reduce to a second fixed overall length. They may be collapsed in the emergency mode of operation. The collapse may be initiated by a rapid application of force, which causes a large relative movement between the connection and the movable wall.

The connection may be arranged such that it can transmits force from the input to the output element when it is at its first and its second length, i.e. at all times.

With the above-described arrangement, and with a preferred arrangement of components, upon a normal application of force the connection member acts to compress the reaction disk, opening the valve to create a pressure difference across the movable wall, which thereafter applies a force to the reaction disk causing the valve to close. In the emergency mode, the reduced length of the connection means that the valve is further open so that the movable wall applies relatively more force to the reaction disk for a given force applied to the input member.

One or more coupling members may be provided which in the normal mode fix the connection at its first length and in the emergency mode permit the connection to adopt its second length. The coupling members may permit the connection to move from its first to its second length in the event that relative movement between the variable connection and the movable wall exceeds a predefined limit. This will occur on a rapid application of high force to the input member as a driver panics, which compresses the reaction disk a great amount relative to the amount that it is compressed by the movable wall.

The two telescopic parts may be of the form of first and second pistons.

The engagement between the two parts may be of the form of captive coupling members such as balls held in a groove or slot in one or both of the two telescopic parts.

In a convenient arrangement a sleeve may be provided in which the first and second pistons may be able to slide. The sleeve may be adapted to slide within the valve housing. In the first mode, the sleeve may hold the coupling members captive in the groove or slot thereby keeping the first and second pistons fixed relative to one another. In the second mode the sleeve may allow the coupling members to at least partially exit at least one of the first and second pistons thereby permitting relative movement between the first and second pistons.

In the first mode the first and second pistons may slide within the sleeve and the sleeve may be fixed in position relative to the valve housing by a return spring which forces the sleeve against an abutment on the valve housing.

In the second mode the first and second pistons may be fixed relative to the sleeve by the connection members to prevent them sliding relative to one another. The sleeve may move relative to the valve housing in this mode as the force applied to the connection overcomes the force applied by the return spring. The collapse in the second mode may be caused by deformation of the reaction disk following release of the coupling members.

The coupling members may engage the pistons at a position at which the first piston is situated within a bore of the second piston. The coupling members may be retained within a groove in the first piston and may also be retained in a slot in the second piston. The portion of the sleeve that engages the slot in the first mode may be substantially flush with the slot so the coupling members remain within the slot. Force may then be transmitted through first piston, the coupling members and the second piston.

The portion of the sleeve that engages the slot in the second mode may be provided with a groove into which the coupling members can at least partially pass. This allows relative movement of the first piston relative to the second piston. During normal mode this groove may be spaced from the coupling members.

The bore in the second piston may be blind such that, on activation of the emergency mode the first piston moves within the second piston from the position in which it is held by the coupling members in the first position to a position in which the first piston is in contact with the end of the blind bore of the second piston. This allows force to be transmitted directly from the first piston to the second piston.

The return member, which acts between the input member and the sleeve, may additionally provide the function of retracting the telescopic parts on t return to the first mode. This may comprise a spring acting between the first piston and the sleeve. The spring may be biased against the input element and act on the first piston through a retainer plate. This may urge the first piston in the direction out of the bore of the second piston into the position where the coupling members hold the first and second pistons fixed relative to one another.

In a most preferred embodiment, the first chamber is a vacuum chamber, and may be provided with connections means for connection to an evacuation means whereby the chamber is kept at less than atmospheric pressure. The first chamber may be a vacuum chamber of the brake booster, and the second chamber may be a working chamber of the brake booster.

There now follows, by way of example only, an embodiment of the present invention described with reference to the accompanying drawings in which: Figure 1 shows a prior-art vacuum booster; Figure 2 shows the control valve arrangement for a vacuum booster according to the present invention in a normal, released, position; Figure 3 shows the control valve arrangement of Figure 2 in the beginning of an emergency braking situation; Figure 4 shows the control valve arrangement of Figure 2 at the end of an emergency braking situation; and Figure 5 shows a curve of deceleration against pedal force (in arbitrary units), following the dotted line when the brake assistant device is active and the solid line in normal braking.

A vacuum booster according to the prior art can be seen in Figure 1 of the accompanying drawings. It comprises a vacuum chamber 14 and a working chamber 15 separated from each other by a movable wall 16. A control valve 17, which comprises a housing 2 coupled to the movable wall l 6, controls the supply of air at atmospheric pressure or above atmospheric pressure to the working chamber 14. The control valve 17 admits air dependent upon the displacement of an input rod 1 of the brake booster to achieve a pressure difference at the movable wall 16, the input rod being coupled in use to the brake pedal of a vehicle.

A vacuum booster according to the present invention can be seen in Figure 2 to 5 of the accompanying drawings. Features common to the prior art booster described with reference to Figure 1 of the accompanying drawings have been assigned the same reference numerals.

The control valve of the vacuum booster according to the present invention is shown in detail in Figures 2 to 4 of the accompanying lo drawings. The valve 17 comprises an input piston 5, which is disposed in an axially displaceable manner inside the control valve housing 2. Travel of this piston allows increasing amounts of air to enter working chamber past valve seat 5a. A coupling device generally indicated as 20, has first and second telescopically arranged force transfer pistons 11, 6. The first force transfer piston 11 contacts on a first side the input piston 5 and on the other, second side a first side of the second force transfer piston 6.

The other, second, side of the second force transfer piston 6 contacts a reaction disc 4. This reaction disc 4 transfers the input force and the additional servo force to the output rod 3, which is in interaction with a F master cylinder (not shown) to activate the vehicle brakes.

The telescopically arranged force transfer pistons 6,11 are contained and may slide within a sleeve 7. The telescopic arrangement of force transfer pistons 6 and 11 is such that in normal use they are held fixed relative to; - one another. First piston 11 is provided with a groove 11a of generally semi-circular cross-section in which a plurality of coupling members being balls 8 are held captive. The second piston 6 has a circumferential slot in which the balls are retained, in normal use, by the sleeve 7. The portion of the wall of the sleeve 7 adjacent to the balls 8 retains the balls 8 from draining radially out and ensures that, in normal use, force transmitted by input piston 5 is transmitted through the force transfer pistons 6,11 through the balls 8 to the reaction disk 4 and hence to the master cylinder.

During normal braking the length of the telescopic elements, reaction piston 6 and piston 11, stays unchanged transferring the input force from the input piston 5 into the reaction disc 4.

A return spring 9 located between the right hand face of the sleeve 7 and the retainer plate 10 is fixed to the piston 11 for example by caulking the extending material of piston]1 onto a retainer plate 10 coupled to the input piston 5. This acts to retract the force transfer pistons 6,11 on release of the braking force.

In case of an emergency braking condition (Figure 3 of the accompanying drawings) the relative movement between the telescopically arranged parts 6,11 and the housing 2 exceeds a predetermined value and the sleeve 7 abuts against the control valve housing 2. During further movement, relative movement between the force transfer pistons 6,11 and the sleeve 7 now takes place such that a the slot in the second piston 6 is adjacent to a groove 12 in the wall of the sleeve allowing the balls 8 to drain radially out of the groove 11a into the now opposite groove 12. The connection of the telescopically arranged parts 6,11 will than be disconnected allowing the parts 6,11 to slide over their length telescopically until the first piston 11 abuts against the end face of the bore inside the second piston 6.

Accordingly, this length reduction causes the force required to open the valve 17 to reduce, and therefore an increase in servo assistance is obtained. This can be seen as a decrease in the pedal effort to deceleration ratio of the brake booster as represented by the vertical part of the dotted trace in Figure 5 of the accompanying drawings - a socalled "vertical jump-in".

If the driver releases the brake pedal (Figure 4 of the accompanying drawings) the return spring 9 urges the pistons 6,11 back to their initial overall length and hence the balls 8 return into the groove 1 la. The sleeve 7 snaps over the balls 8 to re-lock the coupling system, and the standard ratio of pedal effort to deceleration (the solid line of Figure 5 of the accompanying drawings) is obtained.

Figure 5 of the accompanying drawings shows the inventive effect on a conventional vacuum booster curve. The dotted line in the diagram, which is the characteristic curve when the brake assistant device is active shows at the beginning a vertical jump-in effect (sudden deceleration increase) due to the ratio change initiated by the brake assistant device. At the end of this jump-in section the gradient will fall down to a gradient similar to the one for normal braking (full line shown below) but with a constant offset to this line. The higher servo support compared to normal braking ends when the servo force has reached the maximum possible level defined by the maximum possible pressure difference between atmosphere and vacuum or vacuum and overpressure when available, applied onto the vacuum booster diaphragm.

Claims

1. A vacuum brake booster comprising a vacuum chamber and a working chamber separated from each other by a movable wall, a control valve operatively coupled to said movable wall and which is capable of controlling supply of atmospheric pressure or above-atmospheric to the working chamber in dependence upon the displacement of an input element of the brake booster to achieve a pressure difference at the movable wall, and an output member operatively coupled to the moveable wall, in which the booster is so arranged as to provide a first, normal, mode of operation in which force is transmitted from the input element to the output element with a first level of assistance from the booster and a second, emergency braking, mode in which force is transmitted from the input element to the output element with a second, higher, level of assistance from the booster and in which increased forces on the input element are transmitted to the output element during emergency braking mode.

2. A vacuum booster according to any preceding claim in which one part of the valve assembly is connected to the movable wall and the other to the input member.

3. A vacuum brake booster according to claim I or claim 2 in which the input element is connected to the output member through a variable length connection which acts on a reaction disk or other resilient element, the moveable wall also acting upon the reaction disk through the control valve to apply force to the output member, the connection having a first length in the first mode and a second length in the second mode and in which the amount of boost is dependent upon the length of the connection.

4. A vacuum booster according to claim 3 in which the second length is shorter than the first length.

5. A vacuum booster according to claim 3 or claim 4 in which a first portion of the valve is connected to one end of the connection member and a second portion of the valve to the other such that a change in the length of the variable connection changes the spacing between the twit portions of the valve.

6. A vacuum booster according to any one of claims 3 to 5 in which the variable length connection comprises two telescopic parts through which the force is transmitted from input to output elements.

7. A vacuum booster according to any one of claims 3 to 6 in which a predetermined emergency force or rapid increase in force applied to the connection member by the driver pressing on the input member is arranged to cause the two parts to collapse relative to one another such that their overall length is reduced.

8. A vacuum booster according to any one of claims 3 to 7 in which the connection is arranged such that it can transmits force from the input to the output element when it is at its first and its second length.

9. A vacuum booster according to any one of claims 3 to 8 in which one or more coupling members are provided which in the normal mode fix the connection at its first length and in the emergency mode permit the connection to adopt its second length.

10. A vacuum booster according to claim 9 in which the coupling members permit the connection to move from its first to its second length in the event that relative movement between the variable connection and the movable wall exceeds a predefined limit.

11. A vacuum booster according to claim 10 in which the two portions of the connection comprise first and second pistons and in which the engagement between the two parts is of the form of captive coupling members such as balls held in a groove or slot in one or both of the two telescopic parts.

12. A vacuum booster according to claim 11 in which a sleeve is provided in which the first and second pistons are able to slide, the sleeve being arranged to slide within the valve housing and in which in the first mode the sleeve holds the coupling members captive in the groove or slot thereby keeping the first and second pistons fixed relative to one another and in the second mode the sleeve allows the coupling members to at least partially exit at least one of the first and second pistons thereby permitting relative movement between the first and second pistons.