GB2540193A

GB2540193A - Controller, system and method

Info

Publication number: GB2540193A
Application number: GB1512002.5A
Authority: GB
Inventors: Clegg David; Reynolds Timothy
Original assignee: Jaguar Land Rover Ltd
Current assignee: Jaguar Land Rover Ltd
Priority date: 2015-07-09
Filing date: 2015-07-09
Publication date: 2017-01-11
Anticipated expiration: 2035-07-09
Also published as: GB201512002D0; GB2540193B; WO2017005775A1

Abstract

A controller (140C, fig 1) for a motor vehicle hydraulic disc braking system detects the release of an accelerator input device (e.g. pedal 110P, fig 1), and in response outputs a signal for the amount of brake prefill to be generated, and implementing closed loop feedback so that the prefill magnitude is adjusted to maintain a substantially consistent detected dead travel of a brake input device (e.g. pedal 140P, fig 1). A target prefill pressure is reduced iteratively if the vehicle speed excessively decreases during prefill; or by assessing the pedal position S1 that a gradient of brake pressure S2 increase or a predetermined pressure S2 is achieved. Target dead travel & prefill pressure can vary with speed, lateral acceleration & temperature, and be stored in a lookup table. Thus variations due to aging P1-P3, wear and construction differences are reduced.

Description

CONTROLLER, SYSTEM AND METHOD

TECHNICAL FIELD

The present invention relates to vehicle braking systems. In particular but not exclusively the invention relates to a controller for controlling a vehicle braking system.

BACKGROUND

In known braking systems, a braking system controller is configured to implement a ‘brake prefill’ function in which the braking system is pressurised by a predetermined amount when a driver releases an accelerator pedal. Release of the accelerator pedal may also be referred to as ‘lift off or ‘tip out’. In known vehicles a hydraulic fluid pump associated with the braking system is operated for a predetermined amount of time at tip out in order to prefill the braking system. It is found that a size of a dead travel associated with brake pedal actuation, being the distance the brake pedal must be moved before an increase in brake pressure occurs in response to brake pedal movement, varies considerably from vehicle to vehicle whether or not a brake prefill function is implemented. A driver swapping from one vehicle to another may therefore find that the responsiveness of the braking system to movement of the brake pedal may vary from vehicle to vehicle, even where the vehicles are of the same or similar model.

Brake pedal dead travel for a given effort/input can vary from vehicle to vehicle as a result of system compliance. Typical causes include hydraulic brake fluid bleed differences (small amounts of air in the system left over from the evacuation and fill process), the brake pad not being substantially flat (both the friction surface and the backplate of the pad), the brake pad friction surface and backplate surface not being parallel, variations in calliper stiffness, variations in brake hose hydraulic stiffness, differences in brake hose/brake pipe routing length between left and right-hand drive vehicles, and therefore different hydraulic stiffnesses, and seal cut off travels in the brake master cylinder. Differences in each of these features may exist between in any two vehicles of a given type to at least some extent, stacking up to not insignificant differences in the amount of dead travel.

It is to be understood that both radial and tangential pad taper wear can occur with vehicle use, leading to an increase in dead travel. Brake fluid compressibility can also increase over time, resulting in greater dead travel. With regard to piston seal rollback performance, the amount of rollback or ‘pull back’ decreases with time and temperature, and therefore less prefill is required at higher seal temperatures and with repeated brake actuation. Similarly, the amount of rollback typically decreases with ageing of the piston seal.

It is an aim of embodiments of the present invention to address disadvantages associated with the prior art.

SUMMARY OF THE INVENTION

Embodiments of the invention may be understood with reference to the appended claims.

Aspects of the present invention provide a system, a vehicle and a method.

In one aspect of the invention for which protection is sought there is provided an electronic controller for controlling a motor vehicle hydraulic braking system including a brake pad and a brake disc, the controller being configured to receive: a braking system control input device position signal indicative of a position of a braking system control input device; and an accelerator control input device position signal indicative of a position of an accelerator control input device, the controller being configured to: detect the release of the accelerator control input device, and in response thereto output a predetermined brake prefill demand signal indicative of an amount of brake prefill to be generated by the braking system, wherein the controller implements a closed loop feedback arrangement in which the magnitude of the brake prefill demand is adjusted to maintain a substantially consistent detected braking system control input device dead travel.

The braking system control input device dead travel may be a value of travel of the braking system control input device before contact of the brake pad with the brake disc is detected.

It is to be understood that the size of (or amount of) the braking system control input device dead travel or dead band, i.e. the size or amount of travel of the braking system control device from a released position before contact between the brake pad and brake disc is detected, is dependent at least in part on the brake prefill pressure demand signal. Accordingly, by adjusting this signal, the amount of the dead travel can be adjusted. This feature allows the controller to maintain the braking system in a condition in which a substantially consistent and reproducible dead travel characteristic of the braking system is enjoyed.

Optionally, the electronic controller comprises: an electronic processor having an electrical input for receiving the braking system control input device position signal and the accelerator control input device position signal; and an electronic memory device electrically coupled to the electronic processor and having instructions stored therein, and wherein the processor is configured to access the memory device and execute the instructions stored therein such that it is operable to detect the release of the accelerator control input device, and in response thereto output a predetermined brake prefill demand signal indicative of an amount of brake prefill to be generated by the braking system, the magnitude of the brake prefill demand being adjusted by means of a closed loop feedback arrangement to maintain a substantially consistent detected braking system control input device dead travel, the dead travel being a value of travel of the braking system control input device before contact of the brake pad with the brake disc is detected,

Optionally, the predetermined brake prefill demand signal and predetermined dead travel size are determined at least in part on vehicle speed.

Optionally, the controller is configured wherein the predetermined dead travel size increases with increasing vehicle speed.

Optionally, the controller is configured wherein the dead travel size decreases with increasing vehicle speed.

The controller may be configured to cause a reduction in the amount of braking system hydraulic fluid prefill pressure if vehicle speed decreases prior to actuation of the braking system control input device following initial output of the brake prefill pressure demand signal.

This feature has the advantage that, if a driver does not actuate the braking system control device following release of the accelerator control input device and brake prefill causes a brake pad to come into contact with and apply pressure to a brake disc, the amount of any braking experienced by the vehicle due to the increase in braking system hydraulic fluid pressure in response to the brake prefill pressure demand signal may be reduced as the vehicle slows. This has the advantage that excessive deceleration of the vehicle as the speed decreases may be avoided, enhancing a user’s enjoyment of the vehicle.

In some embodiments the decrease in hydraulic fluid pressure with speed reduction may be achieved by reducing the value of demanded brake prefill pressure to the value that would be applied at the instant value of vehicle speed and any one or more other parameters that determine the demanded brake prefill pressure as speed reduces, causing the braking system to reduce the amount of the hydraulic fluid pressure bias below the value that was generated in response to the initial demand for brake prefill pressure.

Optionally, the controller is configured to store at least one value of predetermined brake prefill demand signal and a corresponding at least one predetermined target value of braking system control input device dead travel, the controller being configured to compare the predetermined target value of braking system control input device dead travel with a measured value of braking system control input device dead travel when the braking system control input device is actuated sufficiently to cause braking, the controller being configured to adjust the at least one predetermined brake prefill demand signal in dependence on the comparison in order to reduce a difference between future measured values of braking system control input device dead travel and the predetermined at least one target value of braking system control input device dead travel, i.e, iterative corrections of the brake prefill are manifested at each subsequent application of brakes.

The predetermined brake prefill demand signal may be determined by reference to predetermined stored brake prefill demand signal data, for example in the form of a look-up table.

The controller may be configured to receive a signal indicative of braking system brake pressure, the controller being configured to determine the measured value of braking system dead travel in dependence at least in part on a relationship between braking system control input device position signal and braking system brake pressure.

The controller may be configured to determine the measured value of braking system dead travel to correspond substantially to the position of the braking system control input device at which the rate of change of braking system brake pressure as a function of braking system control input device position exceeds a predetermined rate.

The controller may be configured to determine the measured value of braking system dead travel in dependence at least in part on a relationship between braking system control input device position signal and vehicle rate of deceleration.

Optionally the controller is configured to determine the measured value of braking system dead travel to correspond substantially to the position of the braking system control input device at which the rate of change of vehicle deceleration as a function of braking system control input device position exceeds a predetermined rate.

The rate of change of deceleration may be referred to as jerk. Thus, when the value of jerk exceeds a predetermine amount the controller may determine that the limit of dead travel has been reached. Other arrangements may be useful in some embodiments.

In some embodiments the controller may monitor an amount of drag the vehicle, taking into account the gradient of a driving surface, and determine the limit of brake control input device as the position at which an amount of drag increases above a predetermined amount or above a predetermined rate of change of drag that is not due to driving surface gradient.

The controller may be configured wherein the amount of demanded brake prefill pressure is dependent at least in part on an amount of lateral acceleration being experienced by the vehicle.

That is, the brake prefill pressure that the controller causes to be applied may be dependent at least in part on the instant amount of lateral acceleration to which the vehicle is subject at a given moment in time. This allows the controller to maintain a consistent dead travel size by adjusting the brake prefill pressure to compensate for changes in the distance between working surfaces of the brake pad and brake disc due to lateral acceleration forces.

Optionally the controller is configured wherein the amount of demanded brake prefill pressure reduces as a function of increasing amount of lateral acceleration experienced by the vehicle.

The controller may be configured wherein the amount of demanded brake prefill pressure is dependent at least in part on a temperature of one or more of a brake pad, brake disc and brake calliper, the brake calliper being arranged to support the brake pad.

Optionally, the controller is configured to reduce the amount of demanded brake prefill pressure as a function of increasing temperature of one or more of the brake pad, brake disc and brake calliper.

Optionally the controller is configured wherein the amount of demanded brake prefill pressure is dependent at least in part on historical data in respect of braking system actuation.

Optionally, the controller is configured wherein the amount of demanded brake prefill pressure is dependent at least in part on data indicative of an amount of elapsed time since the braking system was previously actuated.

The controller may be configured wherein the amount of demanded brake prefill pressure is dependent at least in part on data indicative of an amount of elapsed time since the braking system was previously actuated to cause the amount of brake pressure to exceed a predetermined value.

The predetermined value may be any suitable value, for example a value sufficient to cause displacement of a calliper supporting the brake pad away from the brake disc, optionally around 2 bar. It is to be understood that such displacement can result in a change in the distance between working surfaces of the brake pad and brake disc, thereby affecting the value of braking system control input device dead travel.

In an aspect of the invention for which protection is sought there is provided a motor vehicle braking system comprising a controller according to another aspect.

In one aspect of the invention for which protection is sought there is provided a motor vehicle comprising a body, a plurality of wheels, a powertrain to drive said wheels, a braking system to brake said wheels, and a controller according to another aspect.

In a further aspect of the invention for which protection is sought there is provided a method of controlling a motor vehicle hydraulic braking system including a brake pad and a brake disc, comprising: receiving a braking system control input device position signal indicative of a position of a braking system control input device; and receiving an accelerator control input device position signal indicative of a position of an accelerator control input device, the method comprising: detecting release of the accelerator control input device, and in response thereto outputting a predetermined brake prefill demand signal indicative of an amount of brake prefill to be generated by the braking system, the method comprising implementing a closed loop feedback arrangement in which the magnitude of the brake prefill demand is adjusted to maintain a substantially consistent detected braking system control input device dead travel.

The dead travel may be a value of travel of the braking system control input device before contact of the brake pad with the brake disc is detected.

In an aspect of the invention for which protection is sought there is provided a non-transitory computer readable medium carrying computer readable code for controlling a vehicle to carry out the method of another aspect.

In one aspect of the invention for which protection is sought there is provided a computer program product executable on a processor so as to implement the method of another aspect.

In an aspect of the invention for which protection is sought there is provided a computer readable medium loaded with the computer program product of another aspect.

In an aspect of the invention for which protection is sought there is provided a processor arranged to implement the method of another aspect, or the computer program product of another aspect.

In one aspect of the invention for which protection is sought there is provided a controller for controlling a motor vehicle hydraulic braking system, the controller being configured to receive: a signal indicative of a braking system hydraulic fluid pressure; a signal indicative of a position of a braking system control input device; and a signal indicative of a position of an accelerator control input device, the controller being configured to output a brake prefill pressure demand signal indicative of an amount of brake prefill pressure to be generated by the braking system when release of the accelerator control input device is detected, the amount of brake prefill pressure being dependent at least in part on vehicle speed.

Optionally, the amount of brake prefill pressure is further dependent at least in part on an amount of lateral acceleration being experienced by the vehicle.

The brake prefill pressure may be dependent at least in part on the instant amount of lateral acceleration to which the vehicle is subject at a given moment in time.

It is to be understood that the controller or controllers described herein may comprise a control unit or computational device having one or more electronic processors. The system may comprise a single control unit or electronic controller or alternatively different functions of the controller may be embodied in, or hosted in, different control units or controllers. As used herein the term “control unit” will be understood to include both a single control unit or controller and a plurality of control units or controllers collectively operating to provide the stated control functionality. A set of instructions could be provided which, when executed, cause said computational device to implement the control techniques described herein. The set of instructions could be embedded in said one or more electronic processors. Alternatively, the set of instructions could be provided as software to be executed on said computational device. The speed controller may be implemented in software run on one or more processors. One or more other controllers may be implemented in software run on one or more processors, optionally the same one or more processors as the speed controller. Other arrangements are also useful.

Within the scope of this application it is envisaged that the various aspects, embodiments, examples and alternatives, and in particular the individual features thereof, set out in the preceding paragraphs, in the claims and/or in the following description and drawings, may be taken independently or in any combination. For example features described in connection with one embodiment are applicable to all embodiments, unless such features are incompatible.

For the avoidance of doubt, it is to be understood that features described with respect to one aspect of the invention may be included within any other aspect of the invention, alone or in appropriate combination with one or more other features.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying figures in which: FIGURE 1 is a schematic illustration of (a) a motor vehicle according to an embodiment of the present invention.

Figure 2 is a schematic illustration of a portion of a brake module of the vehicle of FIG. 1 showing the module in (a) a brake released condition with a braking system hydraulic fluid pressure at a baseline value and (b) in a primed condition following application of a brake prefill by the braking system and prior to actuation of a brake pedal by a user; FIGURE 3 is a schematic illustration of a portion of the brake module of the vehicle of FIG. 1 showing the module in (a) a brake released condition with the braking system hydraulic fluid pressure at a baseline value; (b) in a brake-applied condition following depression of a brake pedal of the vehicle, with a calliper of the module deflected from the position shown in (a); (c) in a brake-released condition following at least partial retraction of the piston into a housing of the calliper but with the calliper still in an at least partially deflected position; (d) following return of the calliper to the condition in which it was prior to brake application; and (e) following a cornering manoeuvre in which a brake disc was deflected towards the brake pad (shown in dashed outline) causing further retraction of the cylinder into the calliper housing and deflection of the calliper; FIGURE 4 is a schematic illustration of a brake controller of the vehicle of the embodiment of FIG. 1; FIGURE 5 is a schematic plot of brake pressure signal S2 as a function of brake pedal position signal S1; FIGURE 6 is a plot of S2 as a function of S1 at different stages during the life of a braking system, plot P1 corresponding to a braking system a substantially new condition, plot P3 corresponding to a braking system close to an end of life condition with relatively highly worn brake pads, brake discs and relatively old brake fluid, and P2 corresponds to a braking system in a substantially mid-life condition; and FIGURE 7 is a schematic illustration of a brake controller of a vehicle according to a further embodiment of the present invention.

DETAILED DESCRIPTION FIG. 1 shows a vehicle 100 according to an embodiment of the present invention. The vehicle 100 has an internal combustion engine 110 operable to provide motive torque to a transmission 108. The vehicle 100 has a driveline 109 by means of which the transmission 108 may be coupled to a pair of rear wheels 103 of the vehicle 100 by means of a rear prop shaft 109RP and rear drive unit 109RDU. The transmission 108 is releasably connectable to a pair of front wheels 101, 102 by means of a power transfer unit (PTU) 109PTU having a power transfer clutch (not shown), front prop shaft 109FP and front differential gear box 109FD, which also form part of the driveline 109. The PTU 109PTU allows operation of the vehicle 100 in a two wheel drive mode or a four wheel drive mode. It is to be understood that embodiments of the invention may be suitable for vehicles having more than four wheels or where only two wheels are driven, for example two wheels of a three wheeled vehicle or four wheeled vehicle or a vehicle with more than four wheels. It is to be further understood that embodiments of the present invention are also suitable for use in vehicles with manual transmissions, continuously variable transmissions or any other suitable transmission.

The vehicle has a powertrain controller 110C and a brake controller 140C. The powertrain controller 110C is configured to control operation of the engine 110 in dependence at least in part on a position of an accelerator pedal 11 OP. The powertrain controller 110C receives an accelerator position electrical signal S1 indicative of the position of the accelerator pedal 110P with respect to an allowable range of travel and controls the engine 110 accordingly.

The brake controller 140C is configured to control operation of a hydraulic braking system 140 that includes a hydraulic fluid pump module 140HF and a hydraulic fluid line 140L that supplies pressurised hydraulic fluid from the hydraulic fluid pump module 140HF to brake actuator modules 140B of the system 140. The brake actuator modules 140B in turn cause friction braking of wheels 101 -104 of the vehicle 100 under the control of the brake controller 140C. The hydraulic fluid pump module 140HF includes a hydraulic fluid pump and a hydraulic fluid reservoir, the pump being configured to maintain a predetermined pressure of hydraulic fluid in the reservoir. It will be appreciated by the person versed in the art that Figure 1 is purely schematic and that most modern vehicles would comprise an individual brake fluid lines for supplying pressurised brake fluid to each of the brakes such that braking at each wheel is individually controllable

The brake pedal 140P is coupled to a brake master cylinder 140MC that is in turn coupled to the hydraulic fluid pump module 140HF. In response to actuation of the brake pedal 140P and therefore of the master cylinder 140MC, the hydraulic fluid pump module 140HF causes an increase in the pressure of hydraulic fluid in the hydraulic fluid line 140L above a default or baseline value causing the brake actuator modules to cause a brake force to be applied to the wheels 101-104.

The brake controller 140C receives an electrical brake pedal position signal S1 indicative from the brake pedal 140P indicative of a position of the brake pedal 140P with respect to an allowable range of travel thereof. The controller 140C also receives a brake pressure signal S2 from the hydraulic fluid pump module 140HF indicative of the pressure of fluid in the hydraulic fluid line 140L. In addition, the brake controller 140C receives from the powertrain controller 110C an accelerator position signal S3 indicative of the position of the accelerator pedal 110P, and a wheel speed signal S4 indicative of the speed of rotation of each wheel of the vehicle from wheel speed sensors associated with each wheel. In the present embodiment the wheel speed sensors are comprised by the brake actuator modules 140B but in some alternative embodiments the wheel speed sensors may be separate therefrom.

In the present embodiment the brake controller 140C is configured to calculate a value of vehicle speed referred to as a reference speed value. The reference speed value is employed by the brake controller 140C as well as by other vehicle controllers including the powertrain controller 110C. In the present embodiment the controller 140C sets the reference speed value substantially equal to the speed of the second slowest turning wheel. FIG. 2(a) is a schematic illustration of a portion of a brake module 140B in a released condition with a braking system hydraulic fluid pressure at the baseline value, which in the present embodiment is substantially equal to atmospheric pressure. Each module 140B has a brake pad 142P that is arranged to be urged against a corresponding brake disk 142D that is fixedly coupled to a respective wheel 101-104 by a brake calliper 143. The brake pad 142P is attached to a brake piston 142 that is slidable within a calliper housing 143 in a direction parallel to longitudinal axis A of the piston 142. A brake seal 144 provides a hydraulic seal between the piston 142 and housing 143, allowing hydraulic brake fluid from hydraulic fluid line MOL to be fed into the housing 143 behind the seal 144. As shown in FIG. 2(a), the seal 144 sits partially within a recess 143R formed in a sidewall of the housing 143. The seal 144 is resiliently deformable when a shear force is applied thereto parallel to axis A.

The brake module 140B is configured such that, with the hydraulic fluid pressure in the hydraulic fluid line 140L at the baseline value the seal is substantially undeformed in shear. However, as the pressure of brake fluid increases due for example to actuation of the brake pedal 140P, the piston 142 is caused to move towards the brake disc 142D, causing shear deformation of the seal 144 and sliding of the piston 142 with respect to the seal 144. Brake fluid fills a region 146 of the calliper housing 143H behind the piston 142 as the piston 142 slides axially towards the brake disc 142D. A distance between working surfaces of the brake pad 142P and brake disc 142D at a given moment in time will be referred to herein as distance D. It is to be understood that when the working surfaces are in contact, D is substantially zero. FIG. 2(b) shows the brake module 140B in an actuated condition in which the piston 142 has been translated axially parallel to axis A to cause the brake pad 142P to contact the brake disc 142D. The brake seal 144 can be seen to be experiencing shear deformation, as compared with its undeformed condition illustrated in FIG. 2(a).

Upon release of the brake pedal, the hydraulic fluid pump module 140HF ceases operation and valves between the master cylinder 140MC and hydraulic fluid module 140HF open to permit release of pressurised hydraulic fluid from the master cylinder 140MC to a hydraulic fluid reservoir within the hydraulic fluid module 140HF. This causes a decrease in the pressure of fluid in the hydraulic fluid line 140L back to the baseline value. The decrease in pressure causes the seal 144 to relax, which in turn causes the piston 142 to be displaced axially away from the brake pad 142P, causing the brake pad 142P to break contact with the disc 142D. This phenomenon is referred to as seal ‘fall-back’ or ‘pull-back’, the amount of which decreases with age of the seal due to reduced resilience and hardening of the seal with age.

It is to be understood that the braking system 140 is configured such that, with the pressure of hydraulic fluid in the fluid line MOL at the baseline level, the brake pedal 140P must travel a finite and non-zero distance before the pressure in the hydraulic fluid line MOL reaches a value sufficient to cause the brake pad 142P to contact the brake disc 142D. This distance may be referred to as ‘dead travel’ as noted above since substantially no braking action is effected by the system 140 over this relatively small range of pedal travel.

The size of the amount of dead travel is found to change over time, with both short term and long term variations in the amount of dead travel experienced by a user at a given moment in time. FIG. 3(a) - (e) illustrates the variations in distance between the brake pad 142P and brake disc 142D before, during and immediately following a brake actuation cycle. FIG. 3(a) shows the brake module 140B in a full relaxed condition with a pressure of brake fluid in region 146 behind the piston 142 substantially at the baseline pressure, i.e. substantially at atmospheric pressure in the present embodiment. In the condition shown, the working surface of the brake pad 142P is positioned a distance D=D1 from the corresponding working surface of the brake disc 142D. FIG. 3(b) shows the brake module 140B in a fully actuated condition following depression of brake pedal 140P. The piston 142P has been urged against the brake disc 142D by a pressure of brake fluid in region 146 behind the piston 142. It can be seen that, in this condition, brake seal 144 is experiencing resilient deformation. A reaction force on the calliper 143 due to actuation of the brake module 140B causes the calliper 143 to be displaced axially away from the brake disc 142D. Accordingly the amount by which the brake piston 142 is displaced relative to the calliper housing 143H is greater than distance D1. FIG. 3(c) shows the brake module 140B following release of the brake pedal 140P. As shown, brake seal 144 has relaxed as a result of the decrease in brake pressure, causing the brake pad 142P initially to be displaced axially away from the disc 142D by a distance D=D2. Distance D2 is referred to herein as the ‘fall-back’ or ‘pull-back’ distance.

The calliper 143 returns to its original undisplaced (or undeflected) position (illustrated in FIG. 3(a)) more slowly than the brake seal 144. FIG. 3(d) illustrates the brake module 140B following return of the calliper 143 to its original position. As shown in FIG. 3(d) the calliper 143 relaxes to a position in which the working surfaces of the brake pad 142P and brake disc 142D are a distance D=D3 apart. This distance is known as the ‘running clearance’. The amount R by which the calliper 143 relaxes is given by the difference between D2 and D3, i.e. R=D2-D3.

It is to be understood that the amount of dead travel of the brake pedal 140P is dependent on the distance D between the working surfaces of the brake pad 142P and brake disc 142D. Accordingly, the amount of dead travel varies with time following release of the brake pedal 140P following a braking event.

It is to be understood that, lateral forces on a vehicle 100, for example due to cornering, can also cause variations in distance D. When a vehicle experiences relatively high lateral acceleration, for example during cornering at high speed, the brake disc 142D can be displaced towards and into contact with the piston 142, causing displacement of the piston 142 axially, back into the calliper housing 143H. This phenomenon is referred to as ‘knock-back’. Once the lateral acceleration forces fall substantially to zero, the value of D is found to have increased to a value (D3+D4) where D4 is referred to as the ‘knock-back’ distance. This phenomenon is illustrated schematically in FIG. 3(e), where the position of the brake disc when displaced by relatively high lateral acceleration forces is shown in dashed outline at 142D’, and the substantially undisplaced position shown in solid outline. It is to be understood that, following knock-back due to cornering, the amount of dead travel will increase due to the relatively large distance (D3+D4) between working surfaces of the brake pad 142P and disc 142D. FIG. 4 is a schematic illustration of the brake controller 140C showing the signals S1, S2, S3, S4 and S5 that are input thereto. In response to the signals input thereto, the controller 140C generates a brake pressure prefill demand signal S6 that is output to the hydraulic fluid pump module 140HF. The prefill demand signal S6 is indicative of a desired hydraulic brake fluid pressure bias that is to be established in the hydraulic fluid line 140L in addition to the baseline pressure which is substantially equal to atmospheric pressure in the present embodiment as noted above. Accordingly, in response to receipt of the prefill demand signal S6, the hydraulic fluid pump module 140HF causes an increase in the pressure of hydraulic fluid in the hydraulic fluid line 140L by an amount dependent on the prefill demand signal S6. This increase in pressure may be referred to as a prefill bias pressure since it is a pressure that is in addition to any increase in pressure due to actuation of the master cylinder 140MC by the brake pedal 140P. In the absence of a prefill bias pressure or pressure due to actuation of the brake pedal 140P the braking system is configured to release the hydraulic fluid pressure therein such that it is substantially at atmospheric pressure.

In the present embodiment the brake controller 140C is configured to determine the required value of brake pressure prefill demand signal S6 in dependence on detection of release of the accelerator pedal 110P prior to actuation of the brake pedal 140P when the vehicle is moving (i.e. the reference speed value is greater than zero).

The brake controller 140C is configured to set the value of prefill demand signal S6 to a value of substantially zero when the vehicle is moving with the accelerator pedal 11 OP depressed. However, the controller 140C detects that a user is releasing the accelerator pedal 11 OP at a rate exceeding a predetermined rate, regardless of whether actual release of the brake pedal 140P takes place, the controller 140C sets the value of prefill demand signal S6 to a predetermined value that depends at least in part on vehicle speed. The predetermined rate may be determined empirically, and may for example correspond to a rate equivalent to release of the accelerator pedal 11 OP from a 50% depressed condition to fully released condition within a time period of around 0.5s. Other values and other rates may be useful in some embodiments. The controller 140C may also set the value of prefill demand signal S6 to the predetermined value referred to above if the controller 140C detects that a user has fully released the accelerator pedal 11 OP regardless of the rate at which the user released the accelerator pedal 11 OP.

In the event that the controller 140C detects that the accelerator pedal 11 OP has been depressed (i.e. the amount of travel has increased from its minimum amount when pre-fill was triggered) the prefill demand signal S6 is set to correspond substantially to no prefill pressure. As a consequence, the brake pressure is set substantially to atmospheric pressure.

The controller 140C is provided with a look-up table (LUT) that stores values of prefill demand signal S6 as a function of vehicle reference speed value and brake temperature signal S5. In addition, for each value of vehicle reference speed, the LUT also stores a value of target dead travel. The target value of dead travel corresponds to a target value of the amount of dead travel of the brake pedal 140P, being the desired range of initial travel or stroke of the brake pedal 140P over which substantially no increase in brake force developed by the braking system is experienced by a driver in order to maintain a consistent braking performance in terms of responsiveness of the braking system to actuation of the brake pedal 140P.

When a user releases the accelerator pedal 11 OP at a rate exceeding the predetermined rate whilst the vehicle 100 is moving, or the accelerator pedal 11 OP is otherwise fully released, the controller 140C determines the required value of prefill demand signal S6 from the look-up table and sets the value of prefill demand signal S6 equal to the value specified by the look-up table. It is to be understood that the values of brake prefill pressure demand signal S6 are arranged to be lower for higher values of vehicle speed in order to increase the amount of dead travel at higher vehicle speeds, reducing the risk that a driver unintentionally causes excessive braking when the driver initially depresses the brake pedal 140P. The values of brake prefill pressure demand signal S6 are also arranged to be lower for higher values of brake temperature, brake temperature being determined by reference to signal S5. This is at least in part because, with increasing brake temperature, expansion of components of the brake module 140B causes the distance between working surfaces of the brake pad 142P and brake disc 142D to reduce.

The controller 140C then monitors the value of the brake pressure signal S2 and brake pedal position signal S1 to determine, upon actuation of the brake pedal 140P, the value of brake pedal position signal S1 at which a rate of increase of the value of brake pressure signal S2 as a function of signal S1 exceeds a predetermined value, indicating that the brake pad 142P has contacted the brake disc 142D. Thus, if the brake pedal 140P is actuated sufficiently to cause the rate of change of brake pressure signal S2 to exceed the predetermined value, the value of brake pedal position signal S1 at which the rate of increase of brake pressure signal S2 exceeds the predetermined value is stored temporarily by the controller 140C as a value of actual measured brake pedal dead travel.

By way of example, FIG. 5 is a schematic illustration of a typical plot of brake pressure signal S2 as a function of brake pedal position signal S1 when the braking system of the vehicle 100 of FIG. 1 is actuated. The value of S1 at which the rate of increase of S2 as a function of S1 exceeds a predetermined rate is ST as shown in FIG. 5. The predetermined rate is given by the gradient G of the plot at S=S1 ’. FIG. 6 illustrates schematically a relationship between S1 and S2 as a function of aging of a braking system. Plot P1 illustrates the relationship between S1 and S2 with a braking system in a substantially as-new condition with fresh hydraulic braking fluid and new brake pads 142P and brake discs 142D. The value of S1 at which the end point of dead travel is detected is S1a as shown in the figure.

Plot P3 illustrates the relationship between S1 and S2 with a braking system in a substantially end-of-life condition, the value of S1 at which the end point of dead travel is detected being S1c, corresponding to a greater amount of travel of the brake pedal compared with S1a.

Plot P2 illustrates the relationship between S1 and S2 with a braking system in a substantially mid-life condition, the value of S1 at which the end point of dead travel is detected being S1b, substantially midway between S1a and S1c. In each case, the limit of dead travel is given by the value of S1 at which the gradient of the plot of S2 as a function of S1 is substantially equal to parameter G, the value of which is determined empirically and found to correspond substantially to the onset of contact between brake pad and brake disc.

In some alternative embodiments the value of brake pedal dead travel is determined to be the value of brake pedal position signal S1 at which the brake pressure has increased by a predetermined amount above the prefill demand signal S6. The predetermined amount may be an absolute amount, such as 2 bar, or a relative amount, such as an amount dependent on the value of prefill demand signal S6. For example, the relative amount may be an amount substantially equal to the value of prefill demand signal S6 (such that the total brake pressure signal value S2 corresponding to the limit of dead travel is substantially twice the predetermined prefill brake pressure), or be a proportion or multiple thereof.

In some further alternative embodiments, instead of detecting the end of the range of dead travel by reference to brake pressure value or rate of increase of brake pressure value, the controller 140C may monitor vehicle speed and detect the onset of braking by detecting a change in vehicle speed that occurs in response to braking. It is to be understood that this may be referred to as detecting a ‘bite point’ of the braking system, where contact between brake pad 142P and brake disc 142D occurs and braking begins to be effected. Monitoring of vehicle speed may be performed by monitoring the vehicle reference speed signal, or the speed of one or more wheels directly by reference to wheel speed signal S4. The controller 140C may for example monitor rate of change of vehicle speed as brake pressure increases, and monitor rate of change of vehicle acceleration. When the rate of change of acceleration (also referred to as ‘jerk’) exceeds a predetermined amount as brake pressure increases, the controller HOC may determine that the end of brake pedal dead travel has been reached and store data indicative of the instant value of brake pedal position value according to signal S1.

The amount of actual measured brake pedal dead travel according to signal S1 is then compared with the value of target dead travel stored in the look-up table described above, and retrieved by the controller 140C when determining the value of prefill pressure signal S6. If the magnitude of a difference (error) between actual measured brake pedal dead travel and target dead travel stored in the look-up table exceeds a predetermined amount, the value of brake prefill pressure signal S6 stored in the look-up table for the particular vehicle reference speed value prevailing when the look-up table was accessed is adjusted by a predetermined amount.

In the present embodiment, the value of brake prefill pressure signal S6 stored in the lookup table is adjusted to reduce the value of the signal S6 if the value of actual measured brake pressure dead travel is less than the value of target dead travel, indicating the braking system is more highly responsive to initial brake pedal actuation than is desired and that a lower prefill pressure is required. Conversely, the value of signal S6 stored in the look-up table is adjusted to increase the value of signal S6 stored if the value of actual measured brake pressure dead travel is greater than the value of target dead travel, indicating the braking system is less responsive to initial brake pedal actuation than is desired and that a higher prefill pressure is required.

In an alternative embodiment, instead of modifying the value of brake prefill pressure signal S6 stored in memory, the controller 140C is configured to calculate a value of a multiplier being a value by which to adjust the stored value of prefill pressure signal S6 before outputting the resultant value as signal S6. Byway of example, the default value of multiplier may be unity, and the value increased if it is determined that an increase in signal S6 is required, and reduced below unity if a decrease in signal S6 is required. For example the value of the multiplier may be reduced by 10% each time it is determined that a decrease in the value of S6 is required, and increased by 10% each time it is determined that an increase in the value of S6 is required. Use of a multiplier has the advantage that each stored value of signal S6 may be adjusted, rather than only individual values, by reference to a single multiplier. Alternatively, a plurality of multipliers may be employed in some embodiments, for example one multiplier per stored value of S6.

It is to be understood that in the present embodiment the controller 140C operates in an iterative manner to attempt to maintain the values of actual measured dead travel for a given vehicle reference speed substantially equal to the target value of dead travel. Consequently, factors that would otherwise affect the value of actual measured dead travel such as temperature, wear, aging and variations in component characteristics between vehicles due to manufacturing processes, may be prevented from affecting the actual measured dead travel, or the effects of these variations at least reduced.

In some embodiments, the controller 140C may be configured to set the target value of dead travel to be the actual measured dead travel value when the vehicle is first manufactured. For example, in some embodiments the controller 140C may be configured to calculate the average value of actual measured dead travel over a predetermined distance of travel such as the first 10 to 100km of travel, or over a predetermined number of actuations of the brake pedal 140P, and store this average value as the target dead travel value.

In some alternative embodiments, the controller may be configured to take into account one or more additional parameters in determining the value of brake prefill pressure signal S6. FIG. 7 illustrates a brake controller 240C according to a further embodiment of the invention. The controller 240C is configured to receive input signals brake pressure S2, brake pedal position S1 and accelerator pedal position S3 in a similar manner to the controller 140C of the embodiment of FIG. 1 and FIG. 4. However, in addition, the controller 240C receives a lateral acceleration signal S7 indicative of the instant amount of lateral acceleration to which the vehicle is subject at a given moment in time. The controller 240C is configured to feed the value of lateral acceleration signal S7 into a look-up table that stores values of brake prefill pressure S6 as a function of signal S7 and vehicle reference speed. In the event that the controller 240C detects release of the accelerator pedal 11 OP by a driver at a rate exceeding a predetermine rate whilst the vehicle is moving, the controller 240C determines the value of brake pressure prefill signal S6 from the look-up table according to the prevailing values of lateral acceleration signal S7 and vehicle reference speed and sets the value of brake prefill pressure S6 to the value given by the look-up table.

The controller 240C is further configured to detect when the value of lateral acceleration signal S7 swings from a value exceeding a predetermined threshold value in one lateral direction followed by a value exceeding a predetermined threshold value in the opposite lateral direction, within a predetermined time period and/or distance travelled. When such a transition is detected, the value of brake prefill pressure signal S6 is determined in dependence on the two respective values of lateral acceleration signal S7 and on the value of vehicle reference speed according to a further look-up table. It is to be understood that this scenario may be experienced, for example, in the event that a vehicle negotiates a chicane at speed.

It is to be understood that embodiments of the present invention that take lateral acceleration into account may compensate at least in part for the effects of knock-back discussed above with respect to FIG. 3.

In some embodiments the brake controller 240C is configured to determine the value of the brake prefill pressure signal S6 in further dependence at least in part on historical data in respect of brake pedal actuation. This is so as to attempt to compensate for the effects of calliper relaxation whereby the running clearance between brake pad 142P and brake disc 142D is found to reduce over time following release of the brake pedal 140P after a braking event. The running clearance is illustrated as reducing from value D2 to value D3 in FIG. 3(c) and (d).

In some embodiments, when the controller 140C determines that the conditions for establishing a prefill brake pressure are met, the controller 140C determines a required value of prefill pressure S6 from a look-up table according to the prevailing values of signals S7, S5 and vehicle reference speed. The controller 140C then adjusts the value of signal S6 by an amount that is inversely proportional to the amount of time that has elapsed since the brake pedal 140P was released after a brake pressure exceeding a predetermined value had been attained by depression of the brake pedal 140P. In some embodiments the predetermined value of brake pressure is 2 bar although other values are useful in some embodiments.

In one embodiment, the value of brake prefill signal S6 is reduced by 50% if the conditions for establishing a brake prefill pressure are met within 5s of release of the brake pedal 140P following attainment of a brake pressure of 2 bar by depression of the brake pedal 140P, and 30% during the subsequent 5s. The value of signal S6 is not reduced in dependence on historical brake actuation information after this subsequent 5s period has elapsed. It is to be understood that other methodologies for reducing the value of S6 according to historical brake use information may be useful. For example, in some embodiments the value of S6 may be reduced by a predetermined amount such as 50% during a first predetermined period following release of the brake pedal 140P, and the amount of the reduction reduced by 5% of the value of S6 determined according to the look up table each time a further predetermined period elapses such as 1s, 2s, 5s or any other suitable value. Thus the amount by which the signal S6 is reduced may decrease from 50% to 0% in ten steps of 5%, i.e. from 50% to 45%, from 45% to 40%, and so forth.

In some embodiments, the controller may be provided with a signal indicative of brake disc temperature in addition to or instead of brake module temperature. It is to be understood that thermal expansion of both the brake disc, brake calliper and brake pad can cause a change in the distance between the working surfaces of the brake pad and brake disc, in use. In some embodiments, the distance tends to decrease with increasing temperature of either component.

In some embodiments, brake prefill pressure signal S6 corresponds to a value of brake pressure that is to be generated by a hydraulic brake fluid pump. The brake pressure developed may be controlled by a closed loop feedback arrangement in which a real-time measurement of brake fluid pressure is provided to the controller 140, 240. In some alternative embodiments the brake prefill pressure signal S6 may correspond to an amount of time for which a hydraulic brake fluid pump is to be driven in order to raise the pressure of hydraulic brake fluid. The longer the pump is driven, the greater the increase in brake pressure, enabling variable amounts of brake prefill pressure to be established. Thus the pump may operate in an open-loop manner without reference to brake pressure signal S2. It is to be understood that the amount of time for which the pump would be run, indicated by signal S6, would be dependent in part on vehicle reference speed and one or more other parameters in a similar manner to that described above.

In some embodiments, where the controller 140, 240 is caused to issue signal S6 indicating that a non-zero value of brake prefill pressure is required, the controller 140, 240 may be configured to adjust the value of brake prefill pressure if vehicle speed changes substantially before the brake pedal 140P is depressed. This is particularly useful in embodiments in which the amount of prefill brake pressure that is caused to be applied at higher speeds is higher than that at lower speeds. If a vehicle begins to decelerate following accelerator pedal release (lift-off) at a rate sufficient to trigger the establishment of a non-zero prefill brake pressure, the amount of the prefill brake pressure may cause noticeable braking at lower speeds due to the presence of a relatively high prefill brake pressure. Accordingly, the controller 140, 240 may be configured to reduce the amount of prefill brake pressure as speed decreases and attempt to cause the brake prefill pressure to be substantially equal to that which would be established under the prevailing value of vehicle speed and one or more other parameters employed by the controller 140, 240 to determine the value of prefill brake pressure signal S6.

Some embodiments of the present invention have the advantage that a braking system of a vehicle 100 may provide a driver with a more consistent response to actuation of the brake pedal 140P over the course of a service life of the vehicle 100. In addition or instead, some embodiments of the present invention have the advantage that a variation between vehicles of a given model in respect of the response of a braking system to brake pedal actuation may be reduced by causing the value of brake pedal dead travel exhibited by each vehicle to be substantially the same under similar driving conditions. This may be particularly useful to a driver who regularly drives different examples of vehicles of a given model, reducing a workload on a driver in familiarising him or herself with the characteristics of a vehicle each time a transition to a different vehicle is made. In some embodiments, the response of a vehicle braking system to actuation of a brake pedal 140P may be made consistent between different models of vehicle, as well as between individual examples of a given model.

Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “comprises”, means “including but not limited to”, and is not intended to (and does not) exclude other moieties, additives, components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.

Claims

CLAIMS:

1. A controller for controlling a motor vehicle hydraulic braking system including a brake pad and a brake disc, the controller being configured to receive: a braking system control input device position signal indicative of a position of a braking system control input device; and an accelerator control input device position signal indicative of a position of an accelerator control input device, the controller being configured to: detect the release of the accelerator control input device, and in response thereto output a predetermined brake prefill demand signal indicative of an amount of brake prefill to be generated by the braking system, wherein the controller implements a closed loop feedback arrangement in which the magnitude of the brake prefill demand is adjusted to maintain a substantially consistent detected braking system control input device dead travel.

2. A controller according to claim 1 configured wherein the predetermined brake prefill demand signal and predetermined dead travel size are determined at least in part on vehicle speed.

3. A controller according to claim 2 configured wherein the predetermined dead travel size increases with increasing vehicle speed.

4. A controller according to claim 2 configured wherein the dead travel size decreases with increasing vehicle speed.

5. A controller according to any one of claims 2 to 4 configured to cause a reduction in the amount of braking system hydraulic fluid pressure if vehicle speed decreases prior to actuation of the braking system control input device following initial output of the brake prefill pressure demand signal.

6. A controller according to any preceding claim configured to store at least one value of predetermined brake prefill demand signal and a corresponding at least one predetermined target value of braking system control input device dead travel, the controller being configured to compare the predetermined target value of braking system control input device dead travel with a measured value of braking system control input device dead travel when the braking system control input device is actuated sufficiently to cause braking, the controller being configured to adjust the at least one predetermined brake prefill demand signal in dependence on the comparison in order to reduce a difference between future measured values of braking system control input device dead travel and the predetermined at least one target value of braking system control input device dead travel.

7. A controller according to claim 6 configured to receive a signal indicative of braking system hydraulic fluid pressure, the controller being configured to determine the measured value of braking system dead travel in dependence at least in part on a relationship between braking system control input device position signal and braking system hydraulic fluid pressure.

8. A controller according to claim 7 configured to determine the measured value of braking system dead travel to correspond substantially to the position of the braking system control input device at which the rate of change of braking system hydraulic fluid pressure as a function of braking system control input device position exceeds a predetermined rate.

9. A controller according to any one of claims 6 to 8 configured to determine the measured value of braking system dead travel in dependence at least in part on a relationship between braking system control input device position signal and vehicle rate of deceleration.

10. A controller according to claim 9 as dependent on claim 6 configured to determine the measured value of braking system dead travel to correspond substantially to the position of the braking system control input device at which the rate of change of vehicle deceleration as a function of braking system control input device position exceeds a predetermined rate.

11. A controller according to any preceding claim configured wherein the amount of demanded brake prefill pressure is dependent at least in part on an amount of lateral acceleration being experienced by the vehicle.

12. A controller according to claim 11 configured wherein the amount of demanded brake prefill pressure reduces as a function of increasing amount of lateral acceleration experienced by the vehicle.

13. A controller according to any preceding claim configured wherein the amount of demanded brake prefill pressure is dependent at least in part on a temperature of one or more of a brake pad, brake disc and brake calliper, the brake calliper being arranged to support the brake pad.

14. A controller according to claim 13 configured to reduce the amount of demanded brake prefill pressure as a function of increasing temperature of one or more of the brake pad, brake disc and brake calliper.

15. A controller according to any preceding claim wherein the amount of demanded brake prefill pressure is dependent at least in part on historical data in respect of braking system actuation.

16. A controller according to any preceding claim wherein the amount of demanded brake prefill pressure is dependent at least in part on data indicative of an amount of elapsed time since the braking system was previously actuated.

17. A controller according to claim 16 wherein said amount of demanded brake prefill pressure is dependent at least in part on data indicative of an amount of elapsed time since the braking system was previously actuated to cause the amount of braking system hydraulic fluid pressure to exceed a predetermined value.

18. A controller according to any preceding claims comprising: an electronic processor having an electrical input for receiving the braking system control input device position signal and the accelerator control input device position signal; and an electronic memory device electrically coupled to the electronic processor and having instructions stored therein, and wherein the processor is configured to access the memory device and execute the instructions stored therein such that it is operable to detect the release of the accelerator control input device, and in response thereto output a predetermined brake prefill demand signal indicative of an amount of brake prefill to be generated by the braking system, the magnitude of the brake prefill demand being adjusted by means of a closed loop feedback arrangement to maintain a substantially consistent detected braking system control input device dead travel.

19. A controller according to any preceding claim wherein the braking system control input device dead travel is a value of travel of the braking system control input device before contact of the brake pad with the brake disc is detected.

20. A motor vehicle braking system comprising a controller according to any preceding claim.

21. A motor vehicle comprising a body, a plurality of wheels, a powertrain to drive said wheels, a braking system to brake said wheels, and a controller according to any one of claims 1 to 19.

22. A method of controlling a motor vehicle hydraulic braking system including a brake pad and a brake disc, comprising: receiving a braking system control input device position signal indicative of a position of a braking system control input device; and receiving an accelerator control input device position signal indicative of a position of an accelerator control input device, the method comprising: detecting release of the accelerator control input device, and in response thereto outputting a predetermined brake prefill demand signal indicative of an amount of brake prefill to be generated by the braking system, the method comprising implementing a closed loop feedback arrangement in which the magnitude of the brake prefill demand is adjusted to maintain a substantially consistent detected braking system control input device dead travel.

23. A method according to claim 22 whereby the dead travel is a value of travel of the braking system control input device before contact of the brake pad with the brake disc is detected.

24. A non-transitory computer readable medium carrying computer readable code for controlling a vehicle to carry out the method of claim 22 or 23.

25. A computer program product executable on a processor so as to implement the method of claim 22 or 23.

26. A computer readable medium loaded with the computer program product of claim 25.

27. A processor arranged to implement the method of claim 22 or 23, or the computer program product of claim 25.

28. A system, vehicle, method, computer program or carrier medium substantially as hereinbefore described with reference to the accompanying drawings.