GB2502575A

GB2502575A - Braking control system having a first braking region without front axle friction braking and a second region that includes front axle friction braking

Info

Publication number: GB2502575A
Application number: GB1209677.2A
Authority: GB
Inventors: Anna Newberry; Richard Devenport
Original assignee: Jaguar Cars Ltd
Current assignee: Jaguar Land Rover Ltd
Priority date: 2012-05-31
Filing date: 2012-05-31
Publication date: 2013-12-04
Anticipated expiration: 2032-05-31
Also published as: GB2502575B; GB201209677D0

Abstract

A brake control system for a motor vehicle having front and rear axles. A controller defining a brake map is provided for controlling vehicle deceleration in response to a braking request signal. The brake map defines a series limit threshold 230, 240 delimiting a first brake region A & B and a second brake region C & D. Within the first brake region, A & B, vehicle deceleration is performed at least substantially without front axle friction braking. Within the second brake region C & D, vehicle deceleration is performed by braking which includes front axle friction braking. The series limit threshold is profiled such that a transition from said first brake region to said second brake region varies with a dynamic operating parameter of the vehicle, preferably vehicle speed. A method defining a portion of the series limit threshold for rear axle braking 230 to approximate a maximum deceleration available from regenerative braking 240 is also disclosed.

Description

BRAKING CONTROL SYSTEM

TECHNICAL FIELD

The present invention relates to a braking control system for a motor vehicle having front axle and rear axles. Aspects of the invention also relate to a method, a brake map, a brake system and a vehicle.

BACKGROUND OF THE INVENTION

It is known to provide a brake-by-wire system for controlling motor vehicle braking. The brake-by-wire systems are typically operated by a brake pedal which provides a braking control signal. The total braking force applied to the vehicle typically comprises front and rear axle braking combined with overrun (engine) braking and frictional/aerodynamic braking.

The brake-by-wire system can initiate braking by first applying rear axle braking followed by the application of supplementary front axle braking. As outlined in more detail herein with reference to Figures 1A and 1 B, a series limit defines the transition from exclusively rear axle braking to a combination of rear axle braking and supplementary front axle braking. The series limit is configured to maintain a uniform maximum rear axle braking irrespective of the prevailing operating characteristics of the vehicle.

The present invention sets out to provide an improved braking system for a motor vehicle.

SUMMARY OF THE INVENTION

Aspects of the present invention relate to a braking control system, a method, a brake map, a brake system and a vehicle as claimed in the appended claims.

In a further aspect, the invention relates to a brake control system for a motor vehicle having front and rear axles, the brake control system comprising: a controller defining a brake map for controlling vehicle deceleration in response to a braking request signal; the brake map comprising a series limit threshold delimiting a first brake region in which vehicle deceleration is performed by braking comprising rear axle braking at least substantially without front axle friction braking; and a second brake region in which vehicle deceleration is performed by braking comprising front axle friction braking; wherein the series limit threshold is profiled such that the component of vehicle deceleration performed by rear axle braking before a transition from said first brake region to said second brake region varies with one or more dynamic operating parameters of the vehicle.

The controller may provide a non-uniform brake map for modifying the maximum rear axle braking applied before front axle braking is applied. The maximum rear axle braking may be modified in response to changes in said dynamic operating parameter(s). This can help to protect the rear axle from overheating. The brake control system can also provide an improved braking feel for the driver, for example by reducing rear axle braking at higher speeds whilst maintaining a higher level of rear axle braking at lower speeds.

A regenerative braking apparatus can be provided at the front axle and/or the rear axle.

When engaged, the regenerative braking device will apply a braking force at the front axle and/or the rear axle (referred to herein as regenerative braking). Front axle braking can comprise front axle friction braking and/or front axle regenerative braking. Similarly, rear axle braking can comprise rear axle friction braking and/or rear axle regenerative braking. The controller can be configured to control the regenerative braking apparatus to control front axle regenerative braking and/or rear axle regenerative braking.

The brake map can be configured such that vehicle deceleration in said first brake region is performed by braking comprising rear axle friction braking; and optionally also regenerative braking performed at the front axle and/or the rear axle. The brake map can be configured such that vehicle deceleration in said second brake region is performed by braking comprising front axle friction braking and optionally also front axle regenerative braking. The brake map can be configured such that vehicle deceleration in said second brake region is also performed by rear axle friction braking and optionally also rear axle regenerative braking.

The controller can be configured to control front and rear axle braking. The brake control system can comprise a brake-by-wire system for controlling both the front and rear axle braking. Alternatively, the brake control system can comprise a partial brake-by-wire system which provides wired control of rear axle braking while front axle braking is provided by a hydraulic system coupled directly to the brake pedal.

The controller can define a rear axle braking threshold which defines the maximum deceleration to be performed at least substantially without front axle friction braking. Braking is performed without front axle friction braking up to the series limit threshold. Above the series limit threshold, additional braking can be provided by front axle friction braking. The series limit threshold can thereby control the transition from rear axle braking without front axle friction braking to a combination of front and rear axle braking. The offset between the series limit threshold and either a vehicle overrun curve or a vehicle coastdown curve can change based on said one or more dynamic operating parameters of the vehicle.

The series limit threshold defined by the controller can maintain the rear axle braking within a pre-defined stability margin. The stability margin can limit an offset between rear axle braking and front axle braking. For example, the stability margin can limit the offset between rear axle and front axle braking to an equivalent deceleration of less than or equal to 1 mIs2.

The vehicle deceleration performed by rear axle braking in the first brake region can be inversely proportional to the one or more dynamic operating parameters of the vehicle.

Thus, a reduction in said one or more dynamic operating parameters can result in an increase in the component of vehicle deceleration provided by rear axle braking. This inversely proportional relationship can be defined across a portion of the range of said one or more dynamic operating parameters.

The energy recovered by the regenerative braking apparatus could be stored in a flywheel, for example. Alternatively, the regenerative braking apparatus can comprise a generator and a battery for storing the recovered energy. There is usually a limited energy storage capacity, for example dictated by the rotational speed of the flywheel or the charge of the battery, and this determines whether the regenerative braking device can be engaged. If there is no available regenerative capacity, the regenerative braking apparatus cannot provide regenerative braking. The ability of a generator and battery to recover energy also varies depending on the speed of the vehicle and is typically higher at low speeds.

Accordingly, the maximum available regenerative braking varies with the vehicle speed.

Across a portion of the range of said one or more dynamic operating parameters, the series limit threshold can be defined to approximate a maximum deceleration available from regenerative braking performed by the regenerative braking device.

Across a portion of the range of said one or more operating parameters, the series limit threshold can be defined to approximate a maximum deceleration available from regenerative braking performed by a regenerative braking apparatus coupled to the front axle and/or the rear axle. The series limit threshold for rear axle braking can be a series limit for transition from at least substantially exclusively rear axle braking to a combination of front and rear axle braking. When a braking request equals or exceeds the series limit threshold, supplementary front axle friction braking can be applied. The controller could be configured also to control front axle friction braking and/or front axle regenerative braking. The controller can provide independent control of front and roar axle braking. A portion of said series limit threshold can approximate the maximum regeneration capacity over a range, for example for a pre-defined vehicle speed range (greater than or equal to 6Qkph, 7okph, 8Okph or9Okph).

The regenerative braking apparatus can provide some or all of the rear axle braking. The rear axle braking can comprise rear axle friction braking and/or regenerative braking. The controller can operate to adjust the friction braking force and/or the regenerative braking applied at the rear axle of the vehicle depending on the available capacity of the regenerative braking device.

The braking request signal can be generated in response to a driver braking request, typically generated by operating the brake pedal. Alternatively, or in addition, the braking request could be automated, for example generated by a vehicle safety system.

The one or more dynamic operating parameters can comprise one or more of the following: vehicle speed; driving mode, such as Sport, Dynamic, Drive, Winter etc.; selected transmission gear; transmission mode, for example High transmission range or Low transmission range; a terrain driving mode, for example tailored for driving on snow, sand, grass, rocks; vehicle turning angle; or regenerative energy storage capacity. The dynamic operating parameter can consist of vehicle speed since this can provide a predictable brake pedal feel for the driver. The brake control system can be adapted to provide increased rear axle braking (comprising rear axle friction and/or rear axle regenerative braking) as the vehicle speed decreases.

The controller can be configured to vary the rear axle braking based on the speed of the vehicle when a braking request is initiated. Alternatively, the controller can dynamically alter the rear axle braking as the speed of the vehicle changes.

In a still further aspect, the present invention relates to a motor vehicle comprising a brake control system as described herein.

In a yet further aspect, the invention relates to a method of controlling the braking of a motor vehicle having front and rear axles, the method comprising: defining a series limit threshold delimiting a first brake region in which vehicle deceleration is performed by braking comprising rear axle braking at least substantially without front axle friction braking; and a second brake region in which vehicle deceleration is performed by braking comprising front axle friction braking; wherein the component of vehicle deceleration performed by rear axle braking before a transition from said first brake region to said second brake region varies with one or more dynamic operating parameters of the vehicle.

The method can provide non-uniform rear axle braking. Again, said one or more vehicle operating parameters can comprise or consist of vehicle speed.

The rear axle braking can comprise friction braking and/or regenerative braking. Similarly, the front axle braking can comprise friction braking and/or regenerative braking.

The series limit threshold can be defined with reference to the front axle braking, for example to limit an offset between rear axle braking and front axle braking. Across a portion of the range of said one or more dynamic operating parameters, the series limit threshold can be defined to approximate a maximum deceleration available from regenerative braking performed by the regenerative braking device.

The vehicle deceleration performed by rear axle braking in said first brake region can be inversely proportional to said one or more dynamic operating parameters of the vehicle. This relationship can be defined across a portion of the range of said one or more dynamic operating parameters. Across a portion of the range of said one or more dynamic operating parameters, the series limit threshold can be defined to approximate a maximum deceleration available from regenerative braking performed by the regenerative braking device.

In a further aspect, the present invention relates to a method of controlling the braking of front and rear axles of a motor vehicle, the method comprising defining a series limit threshold for rear axle braking; wherein a portion of said series limit threshold is defined to approximate a maximum deceleration available from regenerative braking performed by a regenerative braking device coupled to the rear axle.

In a still further aspect, the present invention relates to a brake map for controlling vehicle braking, the brake map defining front and rear axle braking components to satisfy a deceleration request at a given vehicle speed, the brake map comprising: an overrun line approximating vehicle deceleration when no positive engine torque is applied; and a series limit threshold defining a transition from vehicle deceleration performed by braking comprising rear axle braking at least substantially without front axle friction braking to a combination of rear axle braking with front axle friction braking; wherein an offset between the overrun line and the series limit line changes with one or more dynamic operating parameters of the vehicle. The component of vehicle deceleration performed by rear axle braking before a transition to combined rear axle braking and front axle friction braking can vary with the one or more dynamic operating parameters of the vehicle. The one or more dynamic operating parameters of the vehicle can comprise or consist of vehicle speed.

Braking within the region defined between the overrun line and the series limit line can be performed at least substantially exclusively by the rear axle. A brake controller can be configured to implement the brake map. The invention also relates to a method of controlling the braking of a motor vehicle utilising the brake map.

The method(s) described herein can be machine-implemented. The method described herein can be implemented on a computational device comprising one or more processors, such as an electronic microprocessor. The processor(s) can be configured to perform computational instructions stored in memory or in a storage device. The device described herein can comprise one or more processors configured to perform computational instructions.

In a further aspect the present invention relates to a computer system comprising: programmable circuitry; and software encoded on at least one computer-readable medium to program the programmable circuitry to implement the method described herein.

According to a still further aspect the present invention relates to one or more computer-readable media having computer-readable instructions thereon which, when executed by a computer, cause the computer to perform all the steps of the method(s) described herein.

The references herein to the front and rear axles are each to be given their conventional meaning in relation to their relative positions at the front and rear of the assembled vehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying figures, in which: Figures 1 A and 1 B show a first brake map illustrating a prior art braking control system for a motor vehicle having a regenerative braking system; Figure 2 shows a second brake map illustrating a braking control system for a motor vehicle according to a first embodiment of the present invention; and Figure 3 shows a brake pressure chart representing the braking pressure applied at the front and rear axle friction brakes.

DETAILED DESCRIPTION OF AN EMBODIMENT

The present invention relates to a brake system for a motor vehicle having a front axle, a rear axle, an internal combustion engine and a regenerative braking system. A controller controls front and rear axle braking to satisfy a deceleration request generated by a driver actuating a brake pedal.

Front and rear hydraulic brakes are provided for applying friction braking forces to the front and rear axles respectively. The regenerative braking system comprises electric motors coupled to the front and rear axles of the vehicle which can function as generators to recover energy from the respective axles under braking. The recovered energy is stored in a battery to power the electric motors. When engaged, the regenerative braking system applies a regenerative braking force to the front axle and/or the rear axle. The regenerative braking system is not engaged if there is no capacity available for storing the recovered energy, for example if the battery is fully charged.

The rear axle braking is controlled by electrical braking signals transmitted over a wired connection (so-called brake-by-wire). The front axle braking is controlled by a hydraulic system coupled to the brake pedal in conventional manner. This control arrangement is referred to herein as a partial brake-by-wire system.

The controller enables independent control of the braking force applied to the respective front and rear axles. Front axle braking can be performed by a combination of friction braking (performed by brake callipers contacting a disc brake coupled to the front axle) and regenerative braking (performed by the electric motor coupled to the front axle). Rear axle braking can comprise a combination of friction braking (performed by the electric motor coupled to the rear axle) and regenerative braking (performed by the electric motor coupled to the rear axle). The controller can control the balance between friction braking and regenerative performed at each axle independently. For example, regenerative braking at each axle can be controlled depending on the available capacity of the regenerative energy braking system (referred to herein as the generator capacity). The controller defines a series limit when rear axle braking is supplemented with front axle friction braking. In addition, the vehicle will experience engine braking on the overrun. Braking forces due to friction and aerodynamic loading will also slow the vehicle and these forms of braking are collectively referred to herein as coastdown braking.

By way of background, a prior art controller for a partial brake-by-wire system will now be described with reference to a first brake map 100 shown in Figures 1 A and 1 B. The brake map shows the relationship between the various braking forces acting on the vehicle for a given rate of deceleration (mis2) at a particular vehicle speed (kph). The first brake map 100 shows deceleration curves for a first coastdown curve 110, a first overrun curve 120 and a first series limit curve 130 plotted against vehicle speed (kph) on the X-axis and vehicle deceleration (mis2) on the Y-axis.

The first overrun curve 120 shows the vehicle deceleration when no positive engine torque is applied to the driven wheels and the deceleration request is zero. The first overrun curve represents the amount of deceleration which will occur as a result of coastdown and overrun braking. Vehicle deceleration which is greater than that afforded by overrun and coastdown braking is performed by front and rear axle braking in response to the driver-generated deceleration request.

In response to a deceleration request, the controller balances front and rear axle braking based on the first brake map 100. If the deceleration request is less than the first series limit curve 130, deceleration is performed exclusively by rear axle braking. When the deceleration request exceeds the first series limit curve 130, the rear axle braking is supplemented with front axle braking (comprising front axle friction braking and/or front axle regenerative braking).

A first generator capacity curve 140 is shown on the first brake map 100 to represent the deceleration which can be performed by the regenerative braking system when regenerative capacity is available, as shown in Figure 1A. The ability of the regenerative braking system to recover energy varies with vehicle speed. Accordingly, the deceleration which can be performed by the regenerative braking system (represented by the first generator capacity curve 140) is non-linear. When the battery is fully charged, the regenerative braking system cannot provide regenerative braking and the first generator capacity curve 140 is set to zero, as shown in Figure 1 B.

S

With reference to Figure 1A, when the regenerative braking system can provide regenerative braking, a first region A is bounded by the first overrun curve 120, the first series limit curve and the first generator capacity curve 140. A second region B is bounded by the first generator capacity curve 140 and the first series limit curve 130. For deceleration requests which fall within the first region A or the second region B, requested deceleration is performed exclusively by braking forces applied to the rear axle of the vehicle. In the first region A, rear axle braking is performed exclusively by the regenerative braking system. In the second region B, the deceleration request exceeds the deceleration capacity of the generator at a given vehicle speed and the shortfall in deceleration request is provided by rear axle friction braking. Thus, in the second region B, the deceleration request is met by a combination of rear axle regenerative braking and rear axle friction braking. The amount of deceleration performed by rear axle friction braking is the difference between the first series limit curve 130 and the first generator capacity curve 140.

is A third region C exists below both the first series limit curve 130 and the first generator capacity curve 140. In the third region C, front axle friction braking supplements the rear axle braking. The vehicle braking in the third region C is performed by a combination of front axle friction braking supplemented with rear axle braking (performed by friction braking and regenerative braking).

In a fourth region D below the first series limit curve 130 and above the first generator capacity curve 140, there is capacity for the regenerative braking system to recover energy.

Accordingly, deceleration in the fourth region D can be provided by a combination of front axle friction braking and regenerative braking. Provided generator capacity is available, it is not necessary to provide rear axle friction braking to meet deceleration requests which fall within the fourth region D. The control scheme implemented when there is no available capacity for regenerative braking will now be described with reference to Figure 1 B. The regenerative braking system cannot provide any deceleration capacity and the first generator capacity curve 140 is zero (i.e. coincident with the x-axis). The first region A is thereby replaced with the second region B; and the fourth region D is replaced by the third region C. The deceleration request up to the first series limit curve 130 is provided exclusively by friction braking on the rear axle. The component of a deceleration request which exceeds the series limit 130 is provided by a combination of rear axle friction braking and front axle friction braking.

Considering the rear axle braking in more detail, it will be noted that the offset between the first overrun curve 120 and the first series limit curve 130 is constant irrespective of the vehicle speed. The first series limit curve 130 thereby defines a maximum rear axle braking force (made up of friction braking and regenerative braking, if available) applied before supplementary front axle braking is applied, and this is independent of the vehicle speed. In the illustrated arrangement, the offset is uniform and sets a constant available vehicle deceleration performed by the rear axle of approximately 1 mis2 (the difference between the first overrun curve 120 and the first series limit curve 130). If deceleration greater than the first series limit is requested, the brake pedal is coupled to the front axle brakes, and front axle friction braking is provided to supplement rear axle braking. Front axle regenerative braking can also be applied to satisfy the driver brake request.

A controller in accordance with an embodiment of the present invention will now be described with reference to a second brake map 200 shown in Figure 2.

The second brake map 200 shows the relationship between the various braking forces acting on the vehicle for a given rate of deceleration (mis2) at a particular vehicle speed (kph). The brake map shows a second coastdown curve 210, a second overrun curve 220 and a second series limit curve 230 plotted against vehicle speed (kph) on the X-axis and vehicle deceleration (mis2) on the V-axis. The second brake map 200 also includes a second generator capacity curve 240 which represents the deceleration which can be performed by the regenerative braking system when regenerative capacity is available. When regenerative capacity is not available, the second generator capacity curve 240 is set to zero.

The relationship between the braking components in the present embodiment is generally unchanged from that described above in respect of the prior art arrangement. Like references have been used herein for the four regions A' to D' of the second brake map 200 shown in Figure 2, albeit suffixed with a letter prime modifier to aid clarity.

The second coastdown curve 210, the second overrun curve 220 and the second generator capacity curve 240 are the same as the brake map of the prior art controller. The primary distinction rests in the profile of the second series limit curve 230 in the brake map shown in Figure 2. The first series limit curve 130 in the prior art arrangement is parallel to the first overrun curve 120 over the entire speed range of the vehicle. In contrast, the controller according to the present invention implements the second series limit curve 230 which approximates the second generator capacity curve 240 over a portion of the speed range of the vehicle (the range from approximately Bokph-lBOkph). Consequently, the second region B' in the second brake map 200 is significantly smaller than the corresponding region in the first brake map 100.

As outlined above, the second overrun curve 220 is unchanged and, therefore, the offset between the second overrun curve 220 and the second series limit curve 230 changes depending on the speed of the vehicle. Accordingly, the controller varies the component of the deceleration performed by rear axle braking (comprising rear axle friction braking and regenerative braking, if available) in relation to the speed of the vehicle.

Based on the second series limit curve 230, the controller dynamically alters the transition from solely rear axle braking to a combination of rear axle braking and front axle braking as the speed of the vehicle changes. Thus, the proportion of the total vehicle braking made up of rear axle braking can change for a constant pedal position (i.e. a constant deceleration request). This is most evident in the third region C' of the brake map shown in Figure 2.

Unlike the prior art arrangement shown in the first brake map 100, the proportion of the deceleration provided solely by rear axle braking does not remain constant as the speed of the vehicle decreases. Rather, the rear axle braking changes with the vehicle speed since the controller defines a non-uniform offset between the second overrun curve 220 and the second series limit curve 230.

By way of example, if the vehicle is decelerating from l2Okph at a constant rate of -2m/s2, the deceleration provided by rear axle braking will initially increase as the vehicle slows (since the offset between the second overrun curve 220 and the second series limit curve 230 initially increases) to a maximum, occurring at approximately 7Okph. The component of the deceleration provided by rear axle braking remains substantially constant as the vehicle continues to slow (since the offset between the second overrun curve 220 and the second series limit curve 230 is uniform). Even if a constant braking force is maintained as the vehicle slows (as would happen if the brake pedal position was constant throughout), the balance between the front and rear axle braking would change since the deceleration component provided by rear axle braking is modified as vehicle speed changes.

The controller defines a profile for the second series limit curve 230 which helps to maximise usage of the regenerative braking system whilst protecting the rear axle from overheating by reducing the rear axle braking. This is achieved by defining the second series limit curve 230 to more closely follow the second generator capacity curve 240 such that rear axle braking is reduced when the regenerative braking system cannot recover energy (i.e. the area of the third region C' of the brake map shown in Figure 2 is reduced). By reducing thermal loading of the rear axle, the regenerative braking system can be maintained within its operating parameters for a greater proportion of time when the vehicle is in use. The controller can thereby help to provide improved vehicle efficiency.

The second series limit curve 230 preserves vehicle stability by maintaining rear axle braking within a prescribed threshold of lmIs2. This is achieved by maintaining the offset between the second series limit curve 230 and the second overrun curve 220 within 1 mIs2. Above the second series limit curve 230, increased deceleration is provided by both front and rear axle braking. Although it would be desirable to follow the second generator capacity curve 240 over the entire range to maximise energy recovery, this would provide unacceptable stability limitations and provide excessive rear axle braking as the vehicle speed reduces.

To illustrate the change in rear axle brake force, a brake pressure chart 300 is shown in Figure 3 illustrating an applied braking pressure (bar) for a given brake pedal travel (mm).

For the sake of clarity, the brake pressure chart 300 assumes that the battery regenerative braking system does not have any available storage capacity and so the rear axle braking force is applied entirely by friction braking. This is the worst case scenario for potential overheating of the rear axle friction brakes.

With reference to Figure 3, a first rear axle brake pressure curve 310 shows the braking pressure applied to the rear axle at approximately 1 SOkph; and a second rear axle brake pressure curve 320 shows the braking pressure applied to the rear axle at approximately 7okph. A front axle brake pressure curve 330 is shown for completeness.

There is an initial portion of pedal travel (from 0mm to approximately 10mm) in which the applied rear axle braking increases while the applied front axle braking pressure is substantially zero. This initial portion of pedal travel generates a deceleration request which is less than the second series limit curve 230. Thus, the initial portion of pedal travel corresponds to the first and second regions A', B' bounded by the second overrun curve 220 and the second series limit curve 230 in the second brake map 200. After this initial portion of pedal travel, the generated deceleration request exceeds the second series limit curve 230. Accordingly, both front and rear axle braking occurs, corresponding to the third and fourth regions C', Din the second brake map 200.

In accordance with the present invention, the braking force applied at the rear axle changes with the speed of the vehicle for a fixed brake pedal displacement, as speed decreases to approximately 7okph. Thus, when deceleration of the vehicle occurs, the amount of rear axle braking at a constant pedal position will increase as vehicle speed decreases, until the vehicle has decelerated to approximately 7okph, i.e. the speed at which the series limit is equal to lm/s2 of deceleration. The offset between the first rear axle brake pressure curve 310 and the second rear axle brake pressure curve 320 highlights the change in rear axle braking pressure applied for a constant brake pedal displacement of 10mm as vehicle speed decreases from approximately 1 BOkph to approximately 7Okph.

It will be understood that a component of deceleration provided by rear axle braking increases as the vehicle speed decreases. The rear axle brake pressure for a pedal displacement of 10mm at a speed of lSOkph is highlighted by a first arrow A; and the increased rear axle brake pressure for the same pedal displacement at a speed of 7okph is illustrated by a second arrow A' The change in rear axle brake pressure due to application of the dynamic brake map according to the present invention could, for example, correspond to an increase in the deceleration caused by rear axle braking from 0.5m/s2 at 1 8okph to 1 mIs2 at7Okph.

It will be appreciated that various changes and modifications can be made without departing from the scope of the present invention. For example, rather than dynamically vary the available rear axle braking, the controller could fix the maximum available braking force at the rear axle of the vehicle based on the speed of the vehicle when a braking request is initiated.

Furthermore, the controller according to the present embodiment has been described as varying the available rear axle braking based on the vehicle speed, but other operating parameters could be used instead of or as well as vehicle speed. For example, the rear axle braking could be altered depending on whether a vehicle is operating in a High or Low transmission range. In a Low transmission range, the regenerative braking system and/or brake-by-wire control systems could be inhibited.

Although the present invention has been described with reference to a hybrid vehicle having an internal combustion engine, it could be employed in an electric vehicle (EV). The front and/or rear axles of the vehicle could be driven by one or more electric motors.

Claims

CLAIMS: 1. A brake control system for a motor vehicle having front and rear axles, the brake control system comprising: a controller defining a brake map for controlling vehicle deceleration in response to a braking request signal; the brake map comprising a series limit threshold delimiting a first brake region in which vehicle deceleration is performed by braking comprising rear axle braking at least substantially without front axle friction braking; and a second brake region in which vehicle deceleration is performed by braking comprising front axle friction braking; wherein the series limit threshold is profiled such that the component of vehicle deceleration performed by rear axle braking before a transition from said first brake region to said second brake region varies with one or more dynamic operating parameters of the vehicle.
2. A brake control system as claimed in claim 1, wherein the brake map is configured such that vehicle deceleration in said first brake region and/or said second brake region is also performed by braking comprising rear axle friction braking and/or rear axle regenerative braking.
3. A brake control system as claimed in claim 1 or claim 2, wherein the brake map is configured such that vehicle deceleration in said first brake region and/or said second brake region is also performed by braking comprising front axle regenerative braking.
4. A brake control system as claimed in any one of claims 1, 2 or 3, wherein the brake map is configured such that vehicle deceleration in said first brake region is performed by rear axle braking which is inversely proportional to said one or more dynamic operating parameters of the vehicle across a portion of the range of said one or more dynamic operating parameters.
5. A brake control system as claimed in any one of the preceding claims, wherein the brake map is configured such that, across a portion of the range of said one or more dynamic operating parameters, said series limit threshold is defined to approximate a maximum deceleration available from regenerative braking.
6. A brake control system as claimed in claim 5, wherein said portion of the series limit threshold extends over a range of said one or more dynamic operating parameters.
7. A brake control system as claimed in claim S or claim 6, wherein the controller is operable to adjust the friction braking force applied at the front axle and/or the rear axle depending on the available regenerative braking capacity.
8. A brake control system as claimed in any one of the preceding claims wherein said one or more dynamic operating parameters comprises or consists of the vehicle speed.
9. A brake control system as claimed in claim 8, wherein the controller is configured to vary the rear axle braking based on the speed of the vehicle when a braking request is initiated; or to dynamically alter the rear axle braking as the speed of the vehicle changes.
10. A motor vehicle comprising a brake control system as claimed in any one of the preceding claims.
11. A method of controlling the braking of a motor vehicle having front and rear axles, the method comprising: defining a series limit threshold delimiting a first brake region in which vehicle deceleration is performed by braking comprising rear axle braking at least substantially without front axle friction braking; and a second brake region in which vehicle deceleration is performed by braking comprising front axle friction braking; wherein the component of vehicle deceleration performed by rear axle braking before a transition from said first brake region to said second brake region varies with one or more dynamic operating parameters of the vehicle.
12. A method as claimed in claim 11, wherein vehicle deceleration in said first brake region and/or said second brake region is also performed by braking comprising rear axle friction braking and/or rear axle regenerative braking.
13. A method as claimed in claim 11 or claim 12, wherein vehicle deceleration in said first brake region and/or said second brake region is also performed by braking comprising front axle regenerative braking.
14. A method as claimed in any one of claims 11, 12 or 13, wherein the vehicle deceleration performed by rear axle braking in said first brake region is inversely proportional to said one or more dynamic operating parameters of the vehicle across a portion of the range of said one or more dynamic operating parameters.
15. A method as claimed in any one of claims 11 to 14, wherein, across a portion of the range of said one or more dynamic operating parameters, said series limit threshold is defined to approximate a maximum deceleration available from regenerative braking.
16. A method of controlling the braking of a motor vehicle having front and rear axles, the method comprising defining a series limit threshold for rear axle braking; wherein a portion of said series limit threshold is defined to approximate a maximum deceleration available from regenerative braking.
17. A braking system substantially as herein described with reference to the accompanying figures.
18. A method of controlling a motor vehicle brake control system substantially as herein described with reference to the accompanying figures.