GB2498794A - Vehicle brake system having a friction braking system and a regenerative braking system and control means for corrosion removal from the friction brake - Google Patents

Abstract

Motor vehicle brake control means operable to control a braking system of the vehicle to apply a brake torque to one or more wheels by a friction braking system and a regenerative braking system. The control means is further operable to determine an amount of corrosion product present on one or more brake surfaces of a friction braking system. The control means is operable to control the braking system to deploy a friction braking system to provide brake torque responsive to an amount of corrosion product thereby to reduce an amount of corrosion product on one or more brake surfaces. When corrosion is detected the friction means may be used instead of the regeneration means, and this may be specific to a certain set of wheels.

Description

MOTOR VEHICLE BRAKE SYSTEM CONTROLLER AND METHOD

FIELD OF THE INVENTION

The present invention relates to a brake system controller and to a method of controlling a brake system of a motor vehicle. In particular, but not exclusively, the invention relates to a brake system controller and method for a vehicle having first and second foundation braking systems in which the first braking system is a friction braking system.

BACKGROUND

It is known to provide a motor vehicle having a first foundation braking system provided by a friction braking system (typically a disc braking system) and a second foundation braking system provided by a regenerative braking system. The regenerative braking system may employ one or more traction motors operated as generators or one or more dedicated generators in order to apply a braking torque to one or more wheels of the vehicle to slow the vehicle.

The present inventors have recognised that the problem exists that a vehicle may be driven in such a manner that the friction braking system may be employed relatively rarely during normal vehicle operations. Thus if a driver applies only relatively light braking pressure during the course of a drivecycle, the friction braking system may be either not employed during a drivecycle or employed for only a relatively short period of time. In friction braking systems employing disc brakes, corrosion products can build up relatively quickly on a surface of a disc of the disc brake. In vehicles not having regenerative braking systems, the corrosion products are typically removed relatively quickly during the first few seconds of the application of the friction brakes to slow the vehicle.

However if the friction brakes are not used (for example if the vehicle is parked for several days) or in the case of a vehicle having regenerative braking functionality the regenerative braking system is employed in preference to the disc braking system for an extended period, corrosion products can build to levels at which an effectiveness of the friction brakes is reduced. Thus an amount of braking torque applied by the friction brakes for a given driver demanded brake torque may be less than that applied in the absence of corrosion products on a surface of the brakes.

In addition or instead a driver may experience noticeable noise, vibration or harshness (NVH) when the brakes are applied due to the relatively uneven surface of a corroded disc brake. Vibration may for example be experienced through a brake pedal when the brakes are applied. In addition or instead excessive brake noise may be heard when the brakes are applied.

It is against this background that the present invention has been conceived.

Embodiments of the invention may at least partially mitigate the disadvantages of known foundation braking systems having friction and regenerative braking systems.

STATEMENT OF THE INVENTION

Aspects of the invention provide motor vehicle brake control means, a motor vehicle and a method as claimed in the appended claims According to a further aspect of the invention for which protection is sought there is provided a motor vehicle brake control means operable to control a foundation braking system of a motor vehicle to apply a brake torque to one or more wheels of a vehicle responsive to a value of a driver brake torque demand by means of a friction braking system and a regenerative braking system, the control means being further operable to determine an amount of corrosion product present on one or more brake surfaces of a friction braking system, wherein the control means is operable to control a foundation braking system to deploy a friction braking system to provide brake torque responsive to an amount of corrosion product thereby to reduce an amount of corrosion product on one or more brake surfaces.

By foundation" braking system herein is meant a braking system that is primarily used to reduce vehicle speed during driving and may typically be actuated by a driver-operable foot pedal. A foundation braking system may include one or niore friction brakes and, in the case of a hybrid electric vehicle or the like, a regenerative braking system. No limitation is meant by the term as used herein except where expressly indicated otherwise.

The control means may be operable to determine an amount of corrosion product in various ways including, by way of example and without limitation, by means of physical detection using appropriate sensors or the like, or by estimating the likelihood of formation of corrosion product in dependence on the work cycle of the braking system for example, by monitoring of a length of time since the braking system was last operated. No limitation is nieant by the term as used herein except where expressly indicated otherwise.

Embodiments of the invention have the advantage that an amount of corrosion product that may build up on a brake surface may be prevented from becoming excessively high.

This is because the control means is operable to monitor build-up of corrosion product and to deploy a friction braking system to reduce the amount of corrosion product responsive to the amount of corrosion product present. This has the advantage that a friction braking system niay remain operational within prescribed brake torque performance requirements. Thus a deterioration of brake performance due to corrosion build up may be prevented or reduced.

Advantageously this increases a consistency of a response of a vehicle to a driver demand for brake torque.

Furthermore, undesirable NVH (noise, vibration and harshness) associated with corrosion build-up on a brake surface may be reduced.

Advantageously the control means may be operable to control a foundation braking system to employ a friction brake system to apply at least a portion of a driver demanded brake torque instead of a regenerative braking system responsive to an amount of corrosion product present.

Thus the control means may control the foundation braking system so as to adjust a relative amount of torque provided by friction and regenerative braking systems responsive to an amount of corrosion product present on a brake surface. This process may be described as a blending of the relative amounts of torque.

Thus in the case where it is required to reduce an amount of corrosion product and the control means would normally deploy a regenerative braking system to provide brake torque, the control means may be arranged to control a friction braking system to provide a proportion of a driver demanded brake torque thereby to reduce an amount of corrosion present.

In some embodiments the control means may be configured not to employ a friction braking system in order to reduce the amount of corrosion product unless a driver deniands brake torque. In some alternative embodiments the control means may be operable to employ a friction braking system to reduce the amount of corrosion product even if a driver does not demand brake torque. For example the control means may control the friction braking system to apply a relatively small amount of brake torque by means of the friction braking system in order to reduce the amount of corrosion product even when a driver has not applied the brakes. In some embodiments the control means may apply friction brake torque during acceleration of the vehicle, so as to reduce a risk that a driver detects application of the friction braking system due to noise, vibration of harshness (NVH) associated therewith.

Further advantageously the control means may be operable to employ a friction braking system instead of a regenerative braking system responsive to an amount of corrosion product present on one or more brake surfaces of a friction braking system.

Thus in some arrangements the control means may employ the friction braking system instead of the regenerative braking system Advantageously the control means may be operable to control a friction braking system to apply brake torque to a first group of one or more wheels instead of or in addition to a second group of one or more wheels responsive to an amount of corrosion product present on one or more brake surfaces associated with a first group of one or more wheels.

Thus in the case that the control means is configured to apply a friction braking system to apply brake torque to a second set of one or more wheels when relatively light braking is required (such as a pair of rear wheels of a motor vehicle) in preference to only a first set of one or more wheels (such as a pair of front wheels of a motor vehicle) the control means may be configured to control a friction braking system to apply brake torque to a first set of one or more wheels when an amount of corrosion product present on one or more brake surfaces associated with a first set of one or more wheels exceeds a prescribed amount.

It is to be understood that some vehicles are configured to deploy a friction braking system when a vehicle speed is below a prescribed value where regenerative braking is not available. Some vehicles apply friction braking only to rear wheels in such circumstances when the driver demanded brake torque is relatively low. Accordingly it is possible that friction braking of the front wheels is not employed for an extended period of time, allowing corrosion of one or more friction brake surfaces associated with the front wheels to build up to an excessive level. Some embodiments of the invention have the advantage that the control means can elect to deploy a friction braking system to apply friction braking to front wheels in addition to or instead of to rear wheels under such circumstances thereby to reduce an amount of corrosion product present on one or more brake surfaces associated with the front wheels.

It is to be understood that in some embodiments the reverse arrangement may be employed, with a friction braking system being employed to apply brake torque to front wheels of the vehicle and not to rear wheels when speed is below a prescribed value and an amount of driver demanded brake torque is relatively low. The control means may therefore be arranged to employ friction braking of rear wheels in addition to or instead of front wheels if an amount of corrosion of brake surfaces associated with friction braking of rear wheels becomes excessive.

Optionally the control means is operable to determine an amount of corrosion product present on one or more brake surfaces of a friction braking system responsive to one or more input signals, the one or more input signals corresponding to one or more selected from amongst an ambient humidity level, an ambient temperature, a rain sensor signal, a windscreen wiper activity, a geographical location of a vehicle and a friction braking system activity.

It is to be understood that a rate of build-up of corrosion product may increase with increasing ambient humidity and temperature. In addition a rate of build-up of corrosion product may increase with an amount of time and a frequency with which a vehicle is exposed to rain. An output of a rain sensor and/or a windscreen wiper activity indicator may therefore provide a useful signal for estimating an amount of corrosion product. In addition or instead the control means may employ data in respect of a geographical location of a vehicle to estimate an amount of corrosion product. Thus if a vehicle is operating in a region where corrosion is known to build up relatively quickly, such as a region known to experience relatively high levels of humidity, the control means may be arranged to estimate corrosion build-up at a higher rate than if a vehicle were operating at a location known to experience relatively low levels of humidity.

Advantageously the control means may be operable to determine an amount of corrosion product present on one or more brake surfaces of a friction braking system responsive to friction braking system activity, the control means being arranged to calculate an amount of work done by a friction braking system using one or more brake surfaces thereby to determine friction braking system activity.

Further advantageously the control means may be operable to determine an amount of corrosion product present on one or more brake surfaces of a friction braking system by reference to a corrosion counter index, the control means being operable to change a value of the counter index towards a value indicating an amount of corrosion product is relatively low responsive to friction braking system activity.

Advantageously the control means may be operable to change a value of the counter index towards either a first value indicating an amount of corrosion product is relatively low or a second value indicating an amount of corrosion product is relatively high responsive to an amount of work done by a friction braking system using one or more brake surfaces as a function of time.

It is to be understood that the value of the counter index may be changed towards the first value when work is done by the friction braking system. The value of the counter index may be changed towards the second value over time when work is not done by the friction braking system. A rate at which the counter index is changed towards the second value may be responsive to one or more of the input signals referred to above, such as temperature, humidity, rain sensor output and wiper activity.

According to another aspect of the invention for which protection is sought there is provided a motor vehicle having a foundation braking system comprising a friction braking system and a regenerative braking system, the vehicle further comprising control means as claimed in any preceding claim.

According to a further aspect of the invention for which protection is sought there is provided a method of controlling a foundation braking system of a motor vehicle comprising: applying a brake torque to one or more wheels of the vehicle responsive to a value of a driver brake torque demand by means of a friction braking system and a regenerative braking system; determining an amount of corrosion product present on one or more brake surfaces of the friction braking system; and controlling a foundation braking system to deploy a friction braking system to provide brake torque responsive to an amount of corrosion product determined.

Within the scope of this application it is envisaged that the various aspects, embodiments, examples and alternatives, and in particular the 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 thereof. For example, features described in connection with one embodiment are applicable to all embodiments, unless such features are incompatible.

BRIEF DESCRIPTION OF THE DRAWINGS

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 hybrid electric vehicle according to an embodiment of the present invention; FIGURE 2 is a plot of total brake torque as a function of pedal position showing a torque split between regenerative braking and friction braking in an embodiment of the invention; and FIGURE 3 is a plot of a rear brakes corrosion counter value as a function of time during a drivecycle.

DETAILED DESCRIPTION

FIG. 1 shows a hybrid electric vehicle (HEV) 100 according to an embodiment of the present invention. The vehicle 100 has an internal combustion engine 121 releasably coupled to a crankshaft integrated motor/generator (CIMG) 123 by means of a clutch 122. The CIMG 123 is in turn coupled to an automatic transmission 124. The vehicle 100 is operable to provide drive torque to the transmission 124 by means of the engine 121 alone, the CIMG 123 alone or the engine 121 and CIMG 123 in parallel.

It is to be understood that in some embodiments the transmission 124 may be a manual transmission instead of an automatic transmission. The transmission may comprise a manual gearbox, a continually variable transmission or any other suitable transmission.

It is to be understood that embodiments of the present invention are suitable for use with vehicles in which the transmission 124 is arranged to drive only a pair of front wheels 111, 112 or only a pair of rear wheels 114, 115, i.e. front wheel drive or rear wheel drive vehicles in addition to all wheel drive or selectable two wheel drive/four wheel drive vehicles. Embodiments of the invention are also suitable for vehicles having less than four wheels or more than four wheels.

The vehicle 100 has a battery 150 connected to an inverter 151 that generates a three-phase electrical supply that is supplied to the CIMG 123 when the CIMO 123 is operated as a motor. The battery 150 is arranged to receive charge from the 01MG 123 when the CIMG 123 is operated as a generator.

The vehicle 100 is configured to operate in one of a hybrid electric vehicle (HEV) mode, a HEV inhibit mode (in which the engine 121 alone provides motive torque) and a driver-selectable electric vehicle only (EV-only) mode according to the state of a HEV mode selector 169.

In the HEV mode of operation the vehicle 100 is arranged to operate in one of a parallel boost mode, a parallel recharge mode, a parallel idle mode and a vehicle-selected EV mode.

In the parallel boost mode the engine 121 and 01MG 123 both apply positive torque to the transmission 124 (i.e. clutch 122 is closed) to drive the vehicle 100. In the parallel recharge mode the engine 121 applies a positive torque whilst the 01MG 123 applies a negative torque whereby charge is generated by the CIMG 123 to charge the battery 150. In the parallel idle mode the engine 121 applies a positive torque whilst the CIMO 123 applies substantially no torque. In the vehicle-selected EV mode (and in the driver selected EV-only mode) the clutch 122 is opened and the engine 121 is switched off.

The vehicle has a powerirain controller 140 configured to control the vehicle 100 to operate in the parallel boost mode, parallel charge mode or EV mode according to an energy management strategy implemented by the controller 140. The energy management strategy may also be referred to as a HEV control methodology.

The vehicle 100 has a foundation braking system operable by means of a brake pedal 168 under the control of a foundation braking system controller or brake controller 180.

The brake controller 180 is operable to apply friction brakes 111B, 112B, 114B, 115B in the form of disc brakes associated with each wheel 111, 112, 114, 115 and regenerative braking. The brake controller 180 is operable to apply friction brakes 111 B, 11 2B associated with front wheels 111, 112 of the vehicle 100 independently of friction brakes 114B, 115B associated with rear wheels 114, 115. Friction brakes 111B, 112B associated with front wheels 111, 112 of the vehicle 100 may be referred to as front axle friction brakes 111 B, 11 2B whilst friction brakes 1 14B, 1 15B associated with rear wheels 114, 115 may be referred to as rear axle friction brakes 114B, 1153. As noted elsewhere the front axle friction brakes 111 B, 11 2B may be controlled independently of the rear axle friction brakes 11 4B, 11 SB.

Regenerative braking is provided by the CIMG 123. When a driver demands brake torque by depressing brake pedal 168 and the brake controller 180 determines that regenerative braking is required, the brake controller 180 provides a signal to the powertrain controller 140 which controls the 01MG 123 to apply a negative torque to the wheels 111, 112, 114, 115.

The brake controller 180 is arranged to control the 01MG 123 to apply regenerative braking when required and to apply friction brakes 111 B, 11 2B, 11 4B, 11 SB when required, according to a brake control strategy implemented by the brake controller 180.

In some embodiments regenerative braking is not available below a certain critical speed Vmin(regen). Accordingly, friction braking alone is provided below Vmin(regen).

The controller 180 is configured to control the foundation braking system to apply regenerative braking exclusively if the speed of the vehicle 100 is above Vmin(regen) and the amount of driver demanded brake torque does not exceed a critical value Tregen_only(max). The critical brake torque value Tregen_only(max) corresponds to the maximum brake torque that regenerative braking may provide for the vehicle 100. If the value of driver demanded brake torque exceeds Tregen_only(max), the brake controller 180 is configured to fulfil the driver demanded brake torque Tbrake by means of the friction brakes in addition to providing regenerative braking.

In the present embodiment, the controller 180 is arranged to determine the amount of driver demanded brake torque Ibrake responsive to a position of the brake pedal 168.

In some alternative embodiments the controller 180 may determine Tbrake responsive to a pressure of brake fluid in a brake cylinder such as a master brake cylinder. Other arrangements are also useful.

FIG. 2 is a plot of total brake torque applied by the brake controller 180 as a function of pedal travel P. It can be seen that over the first few millimetres of pedal travel (around 2mm in the example shown) no braking is applied by the controller 180 and this range (or band) of pedal travel may be referred to as a dead band'. Thus between position P0 (corresponding to zero pedal travel) and position Al (corresponding to the end of the dead band) no brake torque is applied.

Between position Al and position P2 of the pedal 168 only regenerative braking is employed. It is to be understood that position P2 corresponds to a value of driver demanded brake torque of Tregen_only(max).

Beyond position P2 both regenerative braking and friction braking are employed. FIG. 2 shows the amount of brake torque applied by regenerative braking, Tregen as a function of pedal travel P and in addition the total amount of brake torque Tbrake being the sum of Tregen and the amount of brake torque applied by friction braking, Tfriction. It can be seen that the relative proportion of the driver demanded brake torque Tbrake provided by friction braking, Tfrictionlmrake increases with increased driver demand as the amount of regenerative brake torque available begins to saturate.

The brake controller 180 is configured to employ rear friction brakes 114B, 115B when the amount of driver demanded brake torque is above Tregen only(max) or the vehicle speed is below Vmin(regen). If the driver demanded brake torque is above Tregenonly(max) the controller 180 may employ the front friction brakes 111 B, 11 2B in addition to the rear friction brakes 11 4B, 11 5B if the value of driver demanded brake torque is sufficiently high.

The brake controller 180 is provided with a clock signal by means of which the controller monitors elapsed time between friction braking events, being events in which the controller 180 employs friction braking to slow the vehicle 100. The controller 180 also monitors, for each friction braking event: (a) a pressure of brake fluid applied to front friction brakes 1113, 1123 (if the front friction brakes are employed) and an amount of time for which the front friction brakes are applied; (b) a pressure of brake fluid applied to rear friction brakes 11 4B, 1153 (if the rear friction brakes are employed) and an amount of time for which the rear friction brakes are applied; and (c) vehicle speed.

The controller 180 is configured to estimate an amount of corrosion product present on brake surfaces associated with the front friction brakes 111 B, 11 2B and on brake surfaces associated with the rear friction brakes 1143, 1153. To accomplish this the controller 180 employs two corrosion counters, a front friction brake corrosion counter and a rear friction brake corrosion counter.

The front friction brake corrosion counter has a value FCCV corresponding to an amount of corrosion determined to be present on the brake surfaces associated with the front friction brakes 111 B, 11 2B. The rear friction brake corrosion counter has a value corresponding to an amount of corrosion determined to be present on the brake surfaces associated with the rear friction brakes 1143, 11 SB.

The value of each respective corrosion counter is incremented at a prescribed rate as a function of time between braking events in which the front and/or rear friction brakes are employed.

The rate of increase of each counter value increases responsive to a determination that the vehicle may be operating in wet conditions. Thus if a rain sensor detects rain or if a driver employs a windscreen wiper function the rate of increase of each counter value is increased.

When the controller 180 deploys the front friction brakes 111 B, 11 2B, the controller 180 calculates the amount of work done by the front friction brakes 111 B, 11 2B, being the amount of brake torque applied multiplied by a speed of the front wheels 111, 112. The value of the front friction brake corrosion counter is then decreased by an amount corresponding to the amount of work done.

Similarly, when the controller 180 deploys the rear friction brakes 11 4B, 11 5B, the controller 180 calculates the amount of brake work done by the rear friction brakes 114B, 115B, being the amount of brake torque applied multiplied by a speed of the rear wheels 114, 115. The value of the rear friction brake corrosion counter is then decreased by an amount corresponding to the amount of work done by the rear friction brakesll4B,115B.

FIG. 3 is a schematic illustration of an example of a plot of corrosion counter value for rear friction brakes 11 4B, 11 SB of the vehicle 100 of FIG. 1 as a function of time.

At time tO, the value of the rear friction brakes corrosion counter value (RCCV) is zero.

Between time tO and ti the rear friction brakes 11 4B, 11 5B of the vehicle 100 are applied and the value of the RCCV decreases. The gradient gl of the plot of RCCV as a function of time has a gradient that is responsive to the rate at which work is done by the rear friction brakes 11 4B, 11 SB.

Between time tl and t2 the value of RCCV is incremented at a substantially constant rate corresponding to a rate at which the controller 180 determines that corrosion products are likely to be building up. The gradient g2 of this portion of the plot corresponds to the rate of build up of corrosion product.

Between time t2 and t3 rear friction brakes 114B, 115B of the vehicle 100 are again applied and the value of RCCV decreases. The gradient g3 of the plot of RCCV as a function of time is greater than the gradient gl of the portion of the plot corresponding to the previous occasion on which the rear friction brakes 114B, 115B were applied. This indicates that the rate at which work is done by the rear friction brakes 114B, 11SB over the period between t2 and t3 is greater than that between time ti and tl.

From time t3 to t5 the rear friction brakes 11 4B, 11 SB were not applied and the value of RCCV increases until it becomes positive, at time t4. At time t4 the brake controller 180 sets a corroded rear disc' flag to a value of 1. Setting the corroded rear disc flag to a value of 1 disables regenerative brake functionality by setting a regenerative braking flag to a value of 0.

It is to be understood that the front friction brake corrosion counter value FCCV is controlled in an analogous manner to the rear friction brakes corrosion counter value. If the value of FCCV becomes positive, the controller 180 sets a corroded front disc' flag to a value of 1, also disabling regenerative brake functionality. That is, the value of the regenerative braking flag is set to 0.

Table 1 provides values of the corroded front disc flag, corroded rear disc flag, regenerative braking flag and a front axle (FA) active pressure flag (see below) for different combinations of values of corroded front disc flag and corroded rear disc flag.

The FA active pressure flag is set when it is determined that the front axle brakes 111 B, 11 2B should be used to provide friction braking when friction braking is next required.

It is to be understood that regenerative braking functionality is disabled if either the corroded rear disc flag or the corroded front disc flag is set to a value of 1. If both flags are set to a value of 0 then the regenerative brake functionality may be enabled, i.e. the value of the regenerative braking flag may be set to 1.

As noted above, the brake controller 180 is configured to employ the rear friction brakes 11 4B, 11 5B (a) when the amount of driver demanded brake torque cannot be fulfilled by regenerative braking alone, unless the value of driver demanded brake torque is sufficiently high to require the front friction brakes 11B, 112B in addition to the rear friction brakes 11 4B, 11 SB; or (b) the vehicle 100 speed is below Vmin(rcgen). It is to be understood that if a driver drives the vehicle 100 in such a manner that regenerative braking and the rear friction brakes 114B, 115B alone are used above Vmin(regen) and the rear friction brakes 114B, 115B alone are used below Vmin(regen), the brake surfaces of the front friction brakes 111 B, 11 2B may experience excessive corrosion product build up due to lack of use.

Accordingly, when the value of the front brake corrosion counter value FCCV exceeds a value of zero the controller 180 is configured to employ the front axle friction brakes 111B, 112B to slow the vehicle 100 when the driver demands brake torque. The controller sets a front axle (FA) active pressure flag to a value of 1 in order to force their deployment when brake torque is next applied. As noted above, in some embodiments rear axle friction brakes 1143, 115B may be employed in addition to the front axle friction brakes 111 B, 11 2B in these circumstances.

As noted above, regenerative braking functionality is also suspended when either of the corrosion counter values FCCV, RCCV is positive. Thus friction braking is employed instead of regenerative braking, reducing the amount of corrosion product build up on the front friction brake surfaces.

In some alternative embodiments in which regenerative braking is not suspended entirely, the controller 180 may be configured to employ the front friction brakes 111 B, 1123 to provide at least a portion of the driver demanded brake torque when the brake pedal 168 is between positions Al and P2, i.e. the driver demanded brake torque is in the range where regenerative braking only is normally employed. Thus in some embodiments the controller 180 may be arranged to employ the front friction brakes 111B, 112B to provide (say) 5%, 10% or some other suitable proportion of a driver demanded brake torque, the amount of brake torque provided by regenerative braking being reduced by a corresponding amount.

It is to be understood that a lower limit and an upper limit may be set to the value of FCCV and RCCV. Thus once the upper or lower limit is reached the value of FCCV or RCCV cannot change any further in a direction away from zero.

Other arrangements are also useful for controlling the friction brakes 1113, 11 2B, 1143, 11 SB to reduce an amount of corrosion product present on one or more brake surfaces thereof.

In some embodiments, the brake controller 180 may be operable to apply a relatively small brake torque by means of the front and/or rear friction brakes 11 2B, 11 4B, 1153 to reduce an amount of corrosion product present on one or more brake surfaces thereof when it is determined that excessive corrosion product is present even if the driver has not depressed the brake pedal 168. The amount of brake torque applied may be sufficiently small that a driver does not notice that brake torque has been applied.

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 (1)

1. <claim-text>CLAIMS: 1. A braking system for a motor vehicle comprising a friction braking system and a regenerative braking system for applying a braking torque to one or more wheels of a vehicle responsive to a driver braking torque demand, the braking system comprising control means configured to determine whether an amount of corrosion product is present on one or more brake surfaces of the friction braking system and to control the friction braking system to provide braking torque in dependence on the presence of corrosion product thereby to reduce an amount of corrosion product on said one or more brake surfaces.</claim-text> <claim-text>2. A braking system as claimed in claim 1 wherein the control means is operable to employ the friction braking system to apply at least a portion of the driver demanded braking torque instead of a regenerative braking system in dependence on an amount of corrosion product present.</claim-text> <claim-text>3. A braking system as claimed in claim 1 or claim 2 wherein the control means is operable to employ the friction braking system instead of the regenerative braking system responsive to an amount of corrosion product present on one or more brake surfaces of a friction braking system.</claim-text> <claim-text>4. A braking system as claimed in any preceding claim wherein the control means is operable to control the friction braking system to apply brake torque to a first group of one or more wheels instead of or in addition to a second group of one or more wheels responsive to an amount of corrosion product present on one or more brake surfaces associated with a first group of one or more wheels.</claim-text> <claim-text>5. A braking system as claimed in any preceding claim wherein the control means is operable to determine an amount of corrosion product present on one or more brake surfaces of the friction braking system responsive to one or more input signals, the one or more input signals corresponding to one or more selected from amongst an ambient humidity level, an ambient temperature, a rain sensor signal, a windscreen wiper activity, a geographical location of a vehicle and a friction braking system activity.</claim-text> <claim-text>6. A braking system as claimed in claim 5 wherein the control means is operable to determine an amount of corrosion product present on one or more brake surfaces of the friction braking system responsive to friction braking system activity, the control means being arranged to calculate an amount of work done by the friction braking system using one or more brake surfaces thereby to determine friction braking system activity.</claim-text> <claim-text>7. A braking system as claimed in any preceding claim wherein the control means is operable to determine an amount of corrosion product present on one or more brake surfaces of the friction braking system by reference to a corrosion counter index, the control means being operable to change a value of the counter index towards a value indicating an amount of corrosion product is relatively low responsive to friction braking system activity.</claim-text> <claim-text>8. A braking system as claimed in claim 7 as dependent on claim 6 wherein the control means is operable to change a value of the counter index towards either a first value indicating an amount of corrosion product is relatively low or a second value indicating an amount of corrosion product is relatively high responsive to an amount of work done by the friction braking system using one or more brake surfaces as a function of time.</claim-text> <claim-text>9. The controller of the braking system according to any preceding claim.</claim-text> <claim-text>10. A motor vehicle having a braking system or a controller as claimed in any preceding claim.</claim-text> <claim-text>11. A method of controlling a braking system of a motor vehicle comprising: applying a brake torque to one or more wheels of the vehicle responsive to a value of a driver brake torque demand by means of a friction braking system and a regenerative braking system; determining an amount of corrosion product present on one or more brake surfaces of the friction braking system; and controlling a foundation braking system to deploy a friction braking system to provide brake torque responsive to an amount of corrosion product determined.</claim-text> <claim-text>12. A braking system, a controller, a vehicle or a method constructed and/or arranged substantially as described herein with reference to one or more of the accompanying drawings.</claim-text>