Controlling a vehicle braking system

Abstract

A control system (110) for controlling a braking system (101) of a vehicle (10) when an electric machine brake (122) of the braking system (101) is in a lash region. A vehicle braking request is received and an allowable torque is determined which is less than or equal to the available torque of the electric machine brake (122), such that the electric machine brake (122) is limited during lash, and the determined allowable torque is output. In this way, the movement of a vehicle (10) may be smoother when a rapid increase in braking torque is requested.

Description

TECHNICAL FIELD The present disclosure relates to controlling a vehicle braking system. Aspects of the invention relate to a control system for controlling a braking system of a vehicle, to a system comprising the control system, to a braking system, to a vehicle comprising the braking system, and to a method for controlling a braking system of a vehicle. BACKGROUND It is known to provide vehicles with an electric machine that, upon deceleration of the vehicle, reverses polarity so as to function as a generator that converts the kinetic energy of the vehicle into electrical power. This is known as regenerative braking. During this change of torque direction, each electric machine may pass through lash. Lash, sometimes called backlash, is where a mechanical system may move without transferring a force to a second part of the system, due to clearance in gears, elasticity in mountings, or similar. A rapid increase in torque demand from the electric machine when it is in the lash region can induce shunts and shuffles. Similar driveline disturbances may occur outside the lash region when a rapid increase of torque occurs. It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a control system, a system, a vehicle, a method for controlling a braking system of a vehicle, and computer readable instructions as claimed in the appended claims. According to an aspect of the present invention there is provided a control system for controlling a braking system of a vehicle having an electric machine arranged to act as an electric machine brake of the vehicle, the control system comprising one or more processors collectively configured when the electric machine is in a lash region to: determine an allowable torque for the electric machine brake, wherein the allowable torque is less than or equal to an available torque for the electric machine brake; and output the determined allowable torque to the electric machine brake. According to another aspect of the invention there is provided a control system for controlling a braking system of a vehicle having an electric machine arranged to act as an electric machine brake of the vehicle, the control system comprising one or more processors collectively configured when the electric machine is in a lash region to: receive a signal indicative of a vehicle braking request; receive a signal indicative of an available torque of the electric machine brake; determine an allowable torque for the electric machine brake, less than or equal to the available torque; and output a determined allowable torque signal representative of the determined allowable torque to be applied by the electric machine brake. Terminology referring to an electric machine or a torque command entering lash, being in a lash region or leaving lash may be interpreted as an electric machine generating a torque having a magnitude reducing below a lash threshold, a magnitude being maintained below a lash threshold, and a magnitude increasing above a lash threshold respectively. While lash thresholds are commonly symmetrical about the zero-torque point, it will be understood that an asymmetrical lash threshold may also exist and that a lash threshold for a positive torque may be different from a lash threshold for a negative torque. Further, it will be recognised that lash may be found throughout a powertrain, such as in the transmission, the differential, the electric machine mountings and the driveshaft(s). However, for ease of reference, it may be stated that the electric machine passes through lash, as opposed to referencing each and every one of the various components of the associated powertrain. The ability of the powertrain system to perform category B regenerative braking depends on the capabilities of the electric machine brakes. A rapid increase in torque demand requested from the electric machine brakes can induce shuffles and shunts in the powertrain. This can be avoided by determining an available torque that the electric machine brake is capable of delivering without inducing shunts and shuffles. An allowable torque may then be determined to be a torque value less than or equal to the available torque, and the allowable torque may then be applied by the electric machine brake. In this context, ‘less than’ refers to the magnitude of the signal, often a braking signal may be considered to be a negative torque value. The one or more processors may be further configured to: repeat determination of the allowable torque to progressively increase the allowable torque. The allowable torque can be determined so as to steadily increase the allowable torque as the capability of the electric machine brake increases, while still remaining less than or equal to the available torque. The one or more processors may be further configured to: receive the allowable torque signal; in dependence on the difference between the available torque and the allowable torque signal, repeat determination of the available torque which maintains or reduces said difference; and output the determined available torque signal. This can prevent any subsequent sudden step change in the outputted allowable torque that might otherwise result in shunts. The calculation of the available torque may occur within an electric machine brake controller, and the allowable torque may be calculated within a braking controller. In dependence on the available torque for the electric machine brake, the one or more processors may repeatedly determine the allowable torque for the electric machine brake until the braking system exits the lash region; and output the allowable torque to the electric machine brake. The allowable torque can be redetermined repeatedly until it is no longer required. The increments in the allowable torque may be monotonic (same sized) or steadily increase. The one or more processors may be collectively configured to: determine a required vehicle braking torque to satisfy the vehicle braking request; determine a difference between the required vehicle braking torque and the determined allowable torque; and output a signal indicative of a required friction braking torque to a friction brake in dependence on the difference. The residual of the required braking torque can be taken up by the friction brakes to ensure the vehicle braking request is met whilst the capabilities of the electric machine brakes increase. The lash region may be defined as a torque value of the electric machine brake within a non-zero tolerance band around zero torque. The electric machine brake may be limited in its ability to withstand sudden increases in torque demand over a range of drivetrain torque values, including negative and positive torques. This ability may relate to inverter or power supply constraints, or may relate to undesirable noises, vibrations, or harshness within the driveline or mounting systems. The allowable torque may be determined based on an instantaneous torque capability of the electric machine brake within the above constraints. The allowable rate of torque increase is expected to vary according to several factors including the current vehicle speed and drivetrain torque. Providing an instantaneous torque capability helps to ensure the capabilities of the electric machine brakes in regenerative braking are optimised. The one or more processors may be collectively configured to: determine a required vehicle braking torque to satisfy the vehicle braking request; determine a distribution of the required vehicle braking torque between a first set of one or more electric machine brakes associated with a first axle of the vehicle and a second set of one or more electric machine brakes associated with a second axle of the vehicle, the distribution of the required vehicle braking torque to each electric machine brake being determined in dependence on a respective allowable torque rate for each electric machine brake; and output a respective determined allowable torque to each electric machine brake. The distribution of braking torque requested of the electric machine brakes can be affected by the capabilities of other electric machine brakes. For instance, once one of the electric machine brakes exits the lash region, the requested braking torque from all electric machine brakes may be increased. Other factors may also affect the braking torque distribution, such as maintaining stability of the vehicle. The control system comprises one or more controllers collectively comprising at least one electronic processor having an electrical input for receiving an input signal; and at least one memory device electrically coupled to the at least one electronic processor and having instructions stored therein; and wherein the at least one electronic processor is configured to access the at least one memory device and execute the instructions thereon so as to carry out the operation of the control system as defined with reference to the above-described aspect of the invention. According to yet another aspect of the invention there is provided a system comprising the control system and a powertrain control module, the powertrain control module configured to: determine an instantaneous torque capability for the electric machine brake or brakes in dependence on a configuration of the electric machine brake across the lash region; output the instantaneous torque capability to the control system as the available torque signal; and wherein the control system is configured to determine the allowable torque for the electric machine brake less than or equal to the available torque signal for the electric machine brake. The powertrain control module may be configured to calibrate the allowable rate of increase of the instantaneous torque capability of the electric machine brake so that an up-to-date allowance can be determined based on the current capabilities of the electric machine brake according to current vehicle parameters. The allowable torque of the electric machine brake may be in dependence on a speed of the vehicle and the powertrain torque. The parameters determining the allowable torque of the electric machine brake may be determined based on various factors including vehicle speed and powertrain torque. According to a further aspect of the invention there is provided a braking system comprising the control system or system; at least one friction brake; and at least one electric machine brake. According to yet further aspect of the invention there is provided a vehicle comprising the braking system. The vehicle may comprise a first powertrain and a second powertrain. Each powertrain may drive a wheel of the vehicle or may drive a pair of wheels. The first powertrain may drive a first axle, which may drive the front wheels of a vehicle and the second powertrain may drive a second axle, which may drive the rear wheels of the vehicle. Alternatively, the first and second powertrains may drive separate wheels, such as a first wheel on a left side of a vehicle and a second wheel on a right side of a vehicle. Each powertrain may comprise an electric machine brake associated with each respective wheel. Each wheel may be associated with a respective friction brake, the friction brake configured to apply a friction braking torque to the wheel. According to a still further aspect of the invention there is provided a method for controlling a braking system of a vehicle when an electric machine brake of the braking system is in a lash region, the method comprising: receiving a signal indicative of a vehicle braking request; receiving a signal indicative of an available torque for the electric machine brake; determining an allowable torque for the electric machine brake, wherein the allowable torque is less than or equal to the available torque; and outputting the determined allowable torque to the electric machine brake. The ability of the powertrain system to perform category B regenerative braking depends on the capabilities of the electric machine brakes. A rapid increase in torque demand requested from the electric machine brakes can induce shuffles and shunts in the powertrain. This can be avoided by determining an allowable torque that will be a positive torque value less than the available torque that the electric machine brake is capable of delivering without inducing shunts and shuffles. The method may further comprise repeatedly determining an increasing allowable torque that is less than or equal to the available torque; and outputting the determined allowable torque to the electric machine brake. The allowable torque can be determined so as to steadily increase as the capability of the electric machine brake increases. The method may further comprise; in dependence on the difference between the available torque and the allowable torque, repeatedly determining an available which maintains or reduces said difference; and outputting the determined available torque. This can prevent the available torque increasing when the allowable torque does not, and hence prevent any subsequent sudden step change in the outputted allowable torque that might otherwise result in shunts. According to yet further still aspect of the invention there is provided a computer readable instructions which, when executed by a computer, are arranged to perform the method. Within the scope of this application, it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. 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 drawings, in which: Figure 1 shows a vehicle in accordance with an embodiment of the invention; Figure 2 shows a block diagram illustrating components of a vehicle in accordance with an embodiment of the invention; Figure 3 shows a flow chart illustrating a method in accordance with an embodiment of the invention. Figure 4 shows a graph indicating the increase in allowable torque to the electric machine brake according to an embodiment of the invention; Figure 5 shows a graph indicating the restriction of the allowable torque to the electric machine brake according to a further embodiment of the invention; Figure 6 shows a block diagram illustrating components of a vehicle in accordance with a further embodiment of the invention; and Figure 7 shows a flow chart illustrating a method in accordance with a further embodiment of the invention. DETAILED DESCRIPTION A vehicle 10 in accordance with an embodiment of the present invention is described herein with reference to the Figure 1. The vehicle 10 includes a control system 110, a powertrain 120 and a friction braking system 130. The control system 110 is arranged to control the powertrain 120 and the friction braking system 130. The powertrain 120 and friction braking system 130 form part of a vehicle braking system 101 of the vehicle 10, as will be explained further below. The vehicle 10 may be a battery electric vehicle (BEV) powered by one or more electric machines only, or may be a hybrid electric vehicle (HEV) having an electric machine and an internal combustion engine both arranged to drive the wheels of the vehicle. Figure 2 shows a schematic diagram of components of the vehicle 10. The control system 110 of the vehicle 10 comprises a plurality of processors. The processors are collectively configured to receive data from different sensors, systems, and input devices of the vehicle 10, to process the data and to output commands for controlling the vehicle 10. The processors may each or communally comprise processing means and memory means. The processing means may be one or more electronic processing devices which operably execute computer readable instructions. The memory means may be one or more memory devices. The memory means is electrically coupled to the processing means. The memory means is configured to store instructions, and the processing means may be configured to access the memory means and to execute the instructions stored thereon. The control system 110 is arranged to control the powertrain 120. The powertrain 120 comprises one or more electric machines 122. In an acceleration drive mode of the vehicle 10, the electric machine 122 may act as a power source 122 configured to power one or more axles 140 of the vehicle 10, with the axle 140 arranged to drive a wheel of the vehicle 10. The control system 110 is arranged to output an electric machine torque requirement signal 121 to the powertrain 120 to cause the electric machine 122 to generate the required amount of torque to meet the requested acceleration. The powertrain 120 comprises a drivetrain 124 that is arranged to receive the torque 125a from the electric machine 122, to convert the torque, and to provide a converted torque 127a to the axles 140. The control system 110 is also arranged to receive data 123 from the powertrain 120. The received data 123 may include performance data related to the powertrain 120. In a deceleration drive mode of the vehicle 10, the negative torque 127b from the axles 140 is provided to the drivetrain 124, and the torque 125b from the drivetrain 124 is transferred to the electric machine 122. In this mode, the electric machine 122 functions as a generator that converts the torque to electrical power, and in doing so decelerates the vehicle 10. This may be referred to as regenerative braking, and the electric machine 122 in this driving mode referred to as an electric machine brake 122. The vehicle 10 includes a friction braking system 130 that may supplement the deceleration provided by the electric machine brake 122, The friction braking system 130 is arranged to receive commands 131 from the control system 110 and to transfer data 133 to the control system 110. The friction braking system 130 comprises a brake actuator 132 and a friction brake 134. The term “friction brake” is generally used herein to mean any friction brake that may decelerate the vehicle by converting kinetic energy into heat by friction. The friction braking system 130 may alternatively be referred to as a foundation braking system. The brake actuator 132 may be a hydraulic pump or an electromechanical actuator for moving components of the friction brake 134 that are arranged to engage frictionally in order to provide a braking torque. The brake actuator 132 may therefore apply a force 135 to the friction brake 134. The friction brakes 134 may be any friction brake, such as brake discs with moveable brake callipers or drum brakes with moveable brake shoes. By moving components of the friction brakes 134 into engagement, the friction braking system 130 may provide a friction braking torque 137 to the axles 140 to decelerate the vehicle. It will be understood that the control system 110 may be formed of a plurality of separate controllers or processors, and that signals which are generally described as being received by the control system or output by the control system may be published on a central CAN bus or merely output by one portion of a computer program and received by a further portion of the computer program. Signals described as being received may also be retrieved from an internal memory or determined by a program within the control system 110. Other vehicle networks are available and herein equivalent to the CAN bus. The control system 110 is arranged to receive an acceleration or deceleration request 103 from a driver input device 102, such as an accelerator pedal or brake pedal. In transitioning between the acceleration mode, in which the electric machine 122 drives the drivetrain 124, and the deceleration mode, in which the electric machine 122 is driven by the drivetrain 124, the torque direction of the electric machine 122 changes. In other words, the electric machine 122 transitions from producing a positive torque to receiving a negative torque. A request for braking may be from the brake pedal, or may be provided when the accelerator pedal is depressed less than a threshold position in a similar manner to overrun braking in a vehicle with a combustion engine. Braking requests may also arise from driver assistance systems such as cruise control. During at least a portion of this transition, the electric machine 122 will pass through a transition region referred to as a lash region. Lash can occur when the torque applied through the components of the powertrain 120 is removed and subsequently reapplied during this transition due to lost motion caused by slack or clearance between components of the powertrain 120 when the applied torque reverses direction. Lash may produce a shuffle, shunt, bump or other disturbance of the vehicle 10 perceivable by the occupants of the vehicle 10 that is generally undesirable. It is generally desirable to minimise frictional losses incurred through use of the friction braking system 130 such that deceleration is entirely or primarily provided by the powertrain 120 (via the electric machine brake 122), and thereby maximise the utilization of regenerative braking. However, as overrun levels decrease in new generation electric vehicles, regenerative braking in the coasting state of the vehicle 10 becomes more challenging. In particular, the electric machine brake 122 is more likely to be in a lash region during coasting. In this lash region, the electric machine brake 122 is unable to deliver a rapid change in torque request without inducing shuffles, shunts and other disturbances. To mitigate against the occurrence of these disturbances, a limit can be set on the allowable torque for the electric machine brake 122. Often the limit may comprise a ramp, with the rate of change of the allowable torque being calibrated. Figure 3 illustrates a method 200 according to an embodiment of the invention. The method 200 is a method of controlling a vehicle braking system 101 of a vehicle 10, such as the vehicle 10 illustrated in Figure 1, when an electric machine brake 122 of the vehicle braking system 101 is in a lash region. Herein, the lash region includes a zero-torque value of the electric machine brake 122, although the lash region will typically include a non-zero tolerance band around zero torque such that the lash region is defined between positive (upper) and negative (lower) torque thresholds. In addition, the ability of the electric machine brake 122 to withstand sudden changes in torque demand (both positive and negative) may extend over a range of drivetrain torque values. While lash thresholds are commonly symmetrical about the zero-torque point, it will be understood that an asymmetrical lash threshold may also exist and that a lash threshold for a positive torque may be different from a lash threshold for a negative torque. The method 200 may be performed by the control system 110 illustrated in Figures 1 and 2. In particular, the control system may comprise a memory, which may comprise computer-readable instructions which, when executed by a processor, perform the method 200 according to an embodiment of the invention. At step 201, the control system 110 is arranged to receive a signal 103 indictive of a vehicle braking request (i.e. a deceleration request signal). The vehicle braking request may be received from a driver input device 102, such as a brake pedal. Alternatively, the vehicle braking request may be received from an autonomous driving system, or an advanced driver assistance system (ADAS) such as emergency braking or adaptive cruise control. In some driving modes, it may also be inferred from a removal of acceleration requests, that is when the driver releases pressure on an accelerator pedal. At step 202, the control system 110 is arranged to determine a required vehicle braking torque to satisfy the vehicle braking request. The required vehicle braking torque is the combined braking torque required from the powertrain 120 and friction braking system 130 to meet the requested deceleration of the vehicle 10 received at step 201. At step 203, the control system 110 is arranged to receive a signal indicative of an available torque for the electric machine brake 122. The available torque may be considered as the maximum capability of the electric machine brake 122 in normal operation. In the event that the powertrain 120, and thereby the electric machine brake 122, is required to produce a torque in a lash region, full utilization of the available torque may in some instances induce disturbances, such as shuffles and shunts, into the drive performance. The control system 110 is therefore arranged to control the allowable torque for the electric machine brake 122. At step 204, the control system 110 is arranged to determine an allowable torque for the electric machine brake 122 less than or equal to the available torque for the electric machine brake 122. The allowable torque is determined as a value that will avoid or substantially reduce disturbances that might otherwise be produced at full utilization of the available torque. When the electric machine brake 122 is in a lash region, the allowable torque will be a non-zero and non-negative value less than the available torque. In this instance, a non-negative value refers to a braking torque. Returning to Figure 2, the powertrain 120 includes a powertrain control module 126 that can be arranged to determine the instantaneous torque capability for the electric machine brake 122 and continuously feed this instantaneous torque capability 129 to the control system 110. In this manner, an up-to-date allowance can be determined based on the current capabilities of the electric machine brake 122 according to current vehicle parameters. For instance, a powertrain torque value 128 may be sent to the powertrain control module 126. The instantaneous torque capability may be dependent on several parameters, including vehicle speed and battery level, and include a tolerance based on the condition of the components of the powertrain 120. For example, a battery which is approaching its maximum capacity may require a reduced rate of charging which may limit the braking torque available from the electric machine brake. At step 205, the control system 110 outputs the determined allowable torque of the electric machine brake 122. An upper limit on the torque request from the vehicle braking system 101 to the powertrain 120 is thereby provided. To meet the required vehicle braking torque, the utilization of the allowable torque is compared to the required vehicle braking torque, with the difference indicative of a required friction braking torque to be sent to the friction braking system 130. At a step 304, to utilise the electric machine brake 122 most effectively, the control system 110 is arranged to determine a further allowable torque. It is expected that the powertrain 120 moves towards a threshold of the lash region, so as to exit the lash region, and therefore the further allowable torque will generally be larger than the allowable torque determined at step 204. At step 305, the control system 110 outputs the determined further allowable torque to the electric machine brake 122. It will be appreciated that the control system 110 will repeatedly determine a further allowable torque for the electric machine brake 122 until the vehicle braking system 101, and in particular the powertrain 120, exits the lash region. This process is illustrated in Figure 4, which shows a graph illustrating an example of the variation in required vehicle braking torque 401, allowable torque 402, and friction braking torque 403 against time. In particular, it can be seen that the required vehicle braking torque 401 increases in accordance with the deceleration request 103 from a driver input device 102. The allowable torque for the electric machine brake 122 lags behind the required vehicle braking torque 401 in accordance with the instantaneous torque capability for the electric machine brake 122. The residue of the required vehicle braking torque 401 is accounted for by friction braking torque 403 of the friction brakes 134. It will be appreciated that whilst Figure 4 shows linearly increasing portions of the allowable torque for the electric machine brake 122, the change in allowable torque over time may change non-linearly, e.g. parabolically, and may be observed as step changes in the allowable torque. The step changes may be monotonic, or may increase with time, so as to most efficiently utilise regenerative braking of the vehicle 10. In some instances, it may be necessary to limit or cap the further allowable torque. The control system 110 is arranged to receive a signal indicative of a utilization of the allowable torque for the electric machine brake 122. If the utilization falls below a predetermined threshold, the increase in allowable torque (i.e. the further allowable torque) may be capped. In this manner, the allowable torque may remain substantially fixed until the utilization reaches the predetermined threshold. An example is shown in Figure 5, in which the utilised torque 504 of the electric machine brake 122 is less than the allowable torque 502. In other words, the allowable torque 502 is not fully utilised. Upon meeting the required vehicle braking torque 501, the allowable torque 502 may continue to increase relative to the utilised torque 504. This large discrepancy between the allowable torque 502 and the utilised torque 504 may cause large steps in the requested torque from the electric machine brake 122, for instance in the event the required vehicle braking torque 501 is suddenly increased, which may result in shunts and shuffles. In the event of an under-utilisation of the allowable torque 502, the control system 110 is arranged to limit the allowable torque 502, as indicated by the plateau region 502a. In this manner, the further allowable torque is equal to the preceding allowable torque. Consequently, the under-utilisation of the allowable torque is held at or below a maximum utilisation value. The minimum utilisation may be set below which the allowable torque 502 is prevented from increasing, for instance the minimum utilisation may be set to 90% or more, or 80% or more. Whilst the abovementioned examples refer to a single powertrain 120 and friction braking system 130 of the vehicle braking system 101, it will be appreciated that the vehicle 10 may have multiple powertrains 120a, 120b and multiple friction brake systems 130a, 130b, each controlled by the control system 110, such as illustrated in Figure 6. Each of the separate powertrains 120a, 120b and friction braking systems 130a, 130b may control separate wheels and axles 140a, 140b. For example, a first pair of wheels associated with a first, front axle 140a of the vehicle 10 may be controlled by a first powertrain 120a and a first friction braking system 130a, and a second pair of wheels associated with a second, rear axle 140b ofthe vehicle 10 may be controlled by a second powertrain 120b and a second friction braking system 130b. In this example, the control system 110 determines a distribution of the required vehicle braking torque between the respective powertrains 120a, 120b and friction braking systems 130a, 130b associated with each axle 140. Each powertrain 120a, 120b including a respective electric machine brake 122a, 122b and drivetrain 124a, 124b, and each friction brake system 130a, 130b including a respective brake actuator 132a, 132b and friction brake 134a, 134b. The method is substantially the same as that illustrated in Figure 3, except that step 202 is replaced by step 208, as illustrated in Figure 7. At step 208, the control system 110 is arranged to determine a distribution of the required vehicle braking torque between a first set of electric machine brakes 122a associated with a first axle 140a of the vehicle 10 and a second set of electric machine brakes 122b associated with a second axle 140b of the vehicle 10. It will be appreciated that each set of electric machine brakes 122a, 122b may include one or more electric machine brakes 122a, 122b. Similarly, the control system 110 is arranged to determine a distribution of the required vehicle braking torque between a first set of friction brakes 134a associated with the first axle 140a of the vehicle 10 and a second set of friction brakes 134b associated with the second axle 140b of the vehicle 10. At steps 203a and 203b, the control system 110 is arranged to receive a signals indicative of an available torque for each respective set of electric machine brakes 122a, 122b. At steps 204a and 204b, the control system 110 is arranged to determine an allowable torque for the respective electric machine brake 122a, 122b. At step 205a and 205b, the control system 110 outputs the determined allowable torque to the respective electric machine brake 122a, 122b. At a steps 304a and 304b, the control system 110 is arranged to determine a further allowable torque for each respective electric machine brake 122a, 122b. At steps 305a and 305b, the control system 110 outputs the determined further allowable torque to the respective electric machine brake 122. The distribution of the required vehicle braking torque between the respective powertrains 120a, 120b and friction brake systems 130a, 130b may be determined based on the instantaneous torque capabilities of each electric machine brake 122a, 122b. For instance, one electric machine brake 122a, 122b may compensate for the performance of another electric machine brake 122a, 122b. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application.

Claims

1. A control system for controlling a braking system of a vehicle having an electric machine arranged to act as an electric machine brake of the vehicle, the control system (110) comprising one or more processors collectively configured when the electric machine brake is in a lash region to:receive a signal indicative of a vehicle braking request;receive a signal indicative of an available torque of the electric machine brake;determine an allowable torque for the electric machine brake less than or equal to the available torque; andoutput a determined allowable torque signal representative of the determined allowable torque to be applied by the electric machine brake.

2. The control system of claim 1, wherein the one or more processors are further configured to repeat determination of the allowable torque to progressively increase the allowable torque.

3. The control system of claim 1 or 2, wherein the one or more processors are further configured to: receive the allowable torque signal;in dependence on the difference between the available torque and the allowable torque signal, repeat determination of the available torque which maintains or reduces said difference; andoutput the determined available torque signal.

4. The control system of claim 2 or 3, wherein the one or more processors are collectively configured to repeatedly determine the allowable torque for the electric machine brake until the braking system exits the lash region; and output the available torque to the electric machine brake.

5. The control system of any preceding claim, wherein the one or more processors are collectively configured to:determine a required vehicle braking torque to satisfy the vehicle braking request;determine a difference between the required vehicle braking torque and the determined allowable torque; andoutput a signal indicative of a required friction braking torque to a friction brake controller in dependence on the difference.

6. The control system of any preceding claim, wherein the lash region is defined as a torque value of the electric machine brake within a non-zero tolerance band around zero torque.

1. The control system of any preceding claim, wherein the one or more processors are collectively configured to:determine a required vehicle braking torque to satisfy the vehicle braking request;determine a distribution of the required vehicle braking torque between a first set of one or more electric machine brakes associated with a first axle of the vehicle and a second set of one or more electric machine brakes associated with a second axle of the vehicle, the distribution of the required vehicle braking torque to each electric machine brake being determined in dependence on a respective allowable torque for each electric machine brake; andoutput a respective determined allowable torque to the or each electric machine brake.

8. A braking system comprising the control system of claims 1 to 7; at least one friction brake; and at least one electric machine brake.

9. A vehicle comprising the braking system of claim 8.

10. A method for controlling a braking system of a vehicle when an electric machine brake of the braking system is in a lash region, the method comprising:receiving a signal indicative of a vehicle braking request;receiving a signal indicative of an available braking torque for the electric machine brake;determining an allowable torque for the electric machine brake, wherein the allowable torque is less than or equal to the available torque; andoutputting the determined allowable torque to the electric machine brake.

11. The method for controlling a braking system of a vehicle of claim 10, further comprising:repeatedly determining an increasing allowable torque which is less than or equal to the available torque; andoutputting the determined allowable torque to the electric machine brake.

12. The method for controlling a braking system of a vehicle of claim 10 or 11, further comprising: in dependence on the difference between the available torque and the allowable torque,repeatedly determining an available torque which maintains or reduces said difference; and outputting the determined available torque signal.

13. Computer readable instructions which, when executed by a computer, are arranged to perform a method according to any of claims 10 to 12.14

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