Patent Publication Number: US-2020290596-A1

Title: Method for controlling a vehicle

Description:
TECHNICAL FIELD 
     Aspects of the present disclosure relate to a method for controlling a vehicle, to a controller configured to implement the method, and also to a vehicle stability control system. 
     BACKGROUND 
     It is known for electric vehicles or hybrid vehicles to be provided with vehicle braking systems comprising both regenerative braking and frictional braking components. In such systems it is often the case that the regenerative braking components provide the majority of the braking function, with the frictional braking components being employed only where a braking request cannot be entirely satisfied by the regenerative braking components, or where it would be less efficient or effective to employ regenerative braking. A Vehicle Supervisory Controller (VSC) may be used to determine the level of regenerative braking available at any time, the VSC communicating the level of regenerative braking to a Brakes Control Module (BCM). 
     In such systems it is possible for a malfunction of a vehicle propulsion system to cause unintended regenerative braking to be applied to driven wheels of the vehicle. For example, a malfunction within the VSC may cause excessive regenerative braking to be requested by the BCM in one scenario. Alternatively, a fault with the invertor may result in excessive levels of regenerative braking being applied to a driven axle of the vehicle, or a mechanical failure in the gearbox may lead to selection of the wrong gear in a transmission system of the vehicle. Such malfunctions lead to the delivery of excessive positive or negative torque at the driven axle of the vehicle, resulting in a potential instability event due to loss of traction, such as understeer, oversteer or skidding of the vehicle. 
     It is known for a vehicle to be provided with a vehicle stability control system (SCS) that is configured to address the problem, mitigating the effect of a vehicle instability event. In particular, since November 2014, a vehicle SCS must be fitted in all vehicles sold within the European Union. At present, SCS functionality compares one or more inputs from the driver of the vehicle to a vehicle response, making a determination as to whether the response from the vehicle is as expected. An SCS can employ various sensors to monitor the vehicle behaviour, including a steering wheel angle sensor, a yaw rate sensor, a lateral acceleration sensor and wheel speed sensors. As an example, data from the steering wheel angle sensor and wheel speed sensor may be used to determine the desired yaw rate of the vehicle, while the yaw rate sensor can be used to determine an actual yaw rate of the vehicle. 
     When the actual state of the vehicle does not correspond to the desired state, the SCS makes a determination that the vehicle is not responding to the driver&#39;s inputs as the driver would expect. For example, the driven wheels of the vehicle may have begun to slip, or the vehicle may be beginning to skid. The SCS thus determines that the vehicle is in an unstable state, and applies corrective action. In dependence on the particular strategy employed by the SCS, the corrective action may involve reducing, or limiting, the level of braking applied to the driven axle of the vehicle, or adjusting the torque applied to the driven wheels of the vehicle. 
     Such functionality helps to mitigate the effect of the application of excessive torque to the driven axle, attempting to regain control of the vehicle when slip or skidding is detected. However, in some instances, the functionality provided by a conventional SCS has been shown not to be able to mitigate the condition in a sufficient time frame to prevent the vehicle from deviating significantly from the path intended by the driver. There remains a need to provide a system with enhanced functionality, that more effectively and more efficiently attends to an instability event. 
     The present invention has been devised to mitigate or overcome at least some of the above-mentioned problems. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, there is provided a method for controlling a vehicle, the method comprising: receiving data relating to a wheel slip event; determining a predicted vehicle yaw rate in dependence on the data relating to the wheel slip event; comparing the predicted vehicle yaw rate to a target yaw rate; and controlling a braking torque applied by a braking mechanism to at least one wheel of the vehicle, in dependence on the predicted vehicle yaw rate. 
     The predicted yaw rate is a future predicted yaw rate based on the current wheel slip data. Calculation of a predicted vehicle yaw rate allows the vehicle to respond to a future yaw rate, rather than a current yaw rate, such that the vehicle is able to react earlier to a determined wheel slip event. In this way, pre-emptive action can be taken so as to mitigate the effect of the wheel slip event, such as applying a braking torque at at least one wheel of the vehicle, for example. The possibility of the vehicle deviating from its intended path and intended orientation is thus minimised, and the driver is assisted in controlling the vehicle. 
     In one embodiment, the method may further comprise determining the predicted vehicle yaw rate in dependence on one or more of a measured vehicle speed, a value for longitudinal slip for one or more driven wheels of the vehicle and a value for vehicle lateral acceleration. 
     Advantageously, the method may comprise determining a yaw torque that is required to adjust the predicted vehicle yaw rate such that the predicted vehicle yaw rate equals or falls below the target yaw rate, and control the application of the determined yaw torque to the vehicle. A yaw rate of the vehicle is thus reduced before the vehicle yaw reaches the predicted vehicle yaw rate, so as to mitigate the effects of a potential instability event due to loss of traction. 
     The method may comprise receiving a steering wheel angle signal associated with a steering wheel angle sensor of the vehicle and receiving a wheel speed signal associated with one or more wheel speed sensors of the vehicle. The target yaw rate may be determined in dependence on the steering wheel angle signal and the wheel speed signal. 
     In an example, the method may comprise controlling the braking mechanism to increase the braking torque on a selected front wheel of the vehicle, such that the required yaw torque is achieved. Alternatively, the method may comprise controlling the braking mechanism to increase the braking torque on a pair of front wheels of the vehicle, such that the required yaw torque is achieved and such that the vehicle is decelerated. 
     The method may comprise calculating a value for longitudinal slip of one or more driven wheels of the vehicle in dependence on a wheel speed signal associated with one or more wheels speed sensors of the vehicle. In this case, the method may further comprise determining whether the calculated value for longitudinal slip of one or more driven wheels of the vehicle exceeds a predetermined longitudinal slip threshold value. 
     In one example, the method may comprise determining whether the calculated value for longitudinal slip is a positive or a negative value. Determining whether the calculated value for longitudinal slip is a positive or a negative value allows for the method of controlling the vehicle to depend on whether driven wheels of the vehicle are spinning or skidding, relative to a surface the vehicle is traversing. 
     The method may comprise determining whether a value for lateral acceleration of the vehicle exceeds a predetermined lateral acceleration threshold value. 
     Advantageously, the method may comprise determining a driver acceleration demand, in the event that the value for longitudinal slip is a positive value and the calculated value for longitudinal slip of one or more driven wheels of the vehicle exceeds the predetermined longitudinal slip threshold value. Additionally, the method may comprise comparing the driver acceleration demand to an acceleration demand threshold level. The method may comprise increasing a braking torque applied to selected wheels of the vehicle in the event that the driver acceleration demand falls below the acceleration demand threshold value. Such a check ensures that a braking torque is not applied to the selected wheels of the vehicle in the event that the vehicle is responding to an acceleration demand from the driver. 
     In one embodiment, the method may comprise controlling the braking mechanism to increase the braking torque on a pair of front wheels of the vehicle simultaneously, in the event that: the predetermined longitudinal slip threshold value of one or more driven wheels of the vehicle exceeds the predetermined longitudinal slip threshold value; a measured vehicle speed exceeds a predetermined vehicle speed threshold; and, the vehicle lateral acceleration equals or falls below the predetermined lateral acceleration threshold value. Advantageously, applying braking to both front wheels of the vehicle decelerates the vehicle in a controlled manner. The method may comprise controlling the braking mechanism to decelerate the vehicle until a vehicle speed falls below a vehicle speed threshold value. Further, the braking mechanism may be controlled such that the deceleration of the vehicle tends to zero as the vehicle speed tends towards the vehicle speed threshold value. The braking torque applied may be different to each of the front wheels so as to apply a net yaw to the vehicle to counter the predicted yaw. 
     The method may comprise taking action to mitigate a vehicle instability event by performing one or more of: increasing a steering gain associated with the vehicle, pre-charging friction brakes of the vehicle and triggering hazard warning lights of the vehicle. The steering gain may be increased in a direction that would counter a predicted yaw direction of the vehicle. Increasing the steering gain therefore increases the ease with which the driver is able to regain full control of the vehicle. 
     Advantageously, the method may comprise taking action to mitigate a vehicle instability event by performing one or both of: requesting one or more battery contactors to open, and requesting a driveline disconnect clutch to open. 
     The predicted yaw rate may be a yaw rate predicted between approximately 0.25 second and 1 second in advance. The predicted yaw rate may be a yaw rate predicted 0.5 seconds in advance. 
     According to another aspect of the invention there is provided a system for controlling a vehicle, the system comprising: an electronic processor having one or more electrical inputs for receiving a signal indicative of data relating to a wheel slip event; and an electronic memory device electrically coupled to the electronic processor and having instructions stored therein, wherein the electronic processor is configured to access the memory device and execute the instructions stored therein such that it is configured to: determine a predicted vehicle yaw rate in dependence on the signal indicative of data relating to the wheel slip event; comparing the predicted vehicle yaw rate to a target yaw rate; and outputting a signal to control a braking torque applied by a braking mechanism to at least one wheel of the vehicle in dependence on the predicted vehicle yaw rate. 
     The one or more electrical inputs may receive a one or more signal indicative of one or more of a measured vehicle speed, a value for longitudinal slip for one or more driven wheels of the vehicle, and a value for vehicle lateral acceleration; and the electronic processor may be configured to access the memory device and execute the instructions stored therein such that it is configured to determine the predicted vehicle yaw rate in dependence on said one or more signal indicative of one or more of a measured vehicle speed, a value for longitudinal slip for one or more driven wheels of the vehicle and a value for vehicle lateral acceleration. 
     The electronic processor may be configured to access the memory device and execute the instructions stored therein such that it is configured to determine a required yaw torque that is required to adjust the predicted vehicle yaw rate such that the predicted vehicle yaw rate equals or falls below the target yaw rate. 
     The one or more electrical inputs may receive one or more signal indicative of a steering wheel angle signal associated with a steering wheel angle sensor of the vehicle and of a wheel speed associated with one or more wheel speed sensors of the vehicle, and the electronic processor may be configured to access the memory device and execute the instructions stored therein such that it is configured to determine the target yaw rate in dependence on the steering wheel angle signal and the wheel speed signal. 
     The electronic processor may be configured to access the memory device and execute the instructions stored therein such that it is configured to control said braking mechanism to increase the braking torque on a selected front wheel of the vehicle, such that the required yaw torque is achieved. 
     The electronic processor may be configured to access the memory device and execute the instructions stored therein such that it is configured to calculate a value for longitudinal slip of one or more driven wheels of the vehicle in dependence on a wheel speed signal associated with one or more wheels speed sensors of the vehicle. 
     In an embodiment the electronic processor may be configured to access the memory device and execute the instructions stored therein such that it is configured to: determine whether the calculated value for longitudinal slip of one or more driven wheels of the vehicle exceeds a predetermined longitudinal slip threshold value; determine whether the calculated value for longitudinal slip is a positive or a negative value; determine a driver acceleration demand in the event that the value for longitudinal slip is a positive value and that the calculated value for longitudinal slip of one or more driven wheels of the vehicle exceeds the predetermined longitudinal slip threshold value; compare the driver acceleration demand to an acceleration demand threshold level; and output a signal to increase a braking torque applied to selected wheels of the vehicle in the event that the driver acceleration demand falls below the acceleration demand threshold value. 
     In an embodiment the electronic processor may be configured to access the memory device and execute the instructions stored therein such that it is configured to: determine whether the calculated value for longitudinal slip of one or more driven wheels of the vehicle exceeds a predetermined longitudinal slip threshold value; determine whether a value for lateral acceleration of the vehicle exceeds a predetermined lateral acceleration threshold value; and, in the event that: the predetermined longitudinal slip threshold value of one or more driven wheels of the vehicle exceeds the predetermined longitudinal slip threshold value; a measured vehicle speed exceeds a predetermined vehicle speed threshold; and the vehicle lateral acceleration equals or falls below the predetermined lateral acceleration threshold value, output a signal to control the braking mechanism to increase the braking torque on a pair of front wheels of the vehicle simultaneously, 
     The electronic processor may be configured to access the memory device and execute the instructions stored therein such that it is configured to mitigate a vehicle instability event by outputting a signal to instruct one or more or more of: increasing a steering gain associated with the vehicle; pre-charging friction brakes of the vehicle; one or more battery contactors to open; and a driveline disconnect clutch to open. 
     According to a further aspect of the invention, there is provided a controller configured to implement a method in accordance with a previous aspect of the invention. 
     According to another aspect of the invention, there is provided a vehicle comprising a controller or system in accordance with a previous aspect of the invention. 
     According to another aspect of the invention, there is provided a computer program product downloadable from a communication network and/or stored on a machine readable medium, comprising program code instructions for implementing a method in accordance with a previous aspect of the invention. 
     According to another aspect of the invention, there is provided a non-transitory machine readable storage medium having instructions stored thereon that when executed by one or more electronic processors causes the one or more electronic processors to carry out the method of a previous aspect of the invention. 
     For the purposes of this disclosure, it is to be understood that the controller and control system described herein can comprise a control unit or computational device having one or more electronic processors. A vehicle and/or a system thereof may comprise a single control unit or electronic controller or alternatively different functions of the controller(s) may be embodied in, or hosted in, different control units or controllers. As used herein, the term “vehicle control system” will be understood to include both a single control unit or controller and a plurality of control units or controllers collectively operating to provide the required control functionality. A set of instructions could be provided which, when executed, cause said controller(s) or control unit(s) to implement the control techniques described herein (including the method(s) described below). The set of instructions may be embedded in one or more electronic processors, or alternatively, the set of instructions could be provided as software to be executed by one or more electronic processor(s). For example, a first controller may be implemented in software run on one or more electronic processors, and one or more other controllers may also be implemented in software run on or more electronic processors, optionally the same one or more processors as the first controller. It will be appreciated, however, that other arrangements are also useful, and therefore, the present invention is not intended to be limited to any particular arrangement. In any event, the set of instructions described above may be embedded in a computer-readable storage medium (e.g., a non-transitory storage medium) that may comprise any mechanism for storing information in a form readable by a machine or electronic processors/computational device, including, without limitation: a magnetic storage medium (e.g., floppy diskette); optical storage medium (e.g., CD-ROM); magneto optical storage medium; read only memory (ROM); random access memory (RAM); erasable programmable memory (e.g., EPROM ad EEPROM); flash memory; or electrical or other types of medium for storing such information/instructions. 
     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: 
         FIG. 1  is a schematic plan view of a vehicle having a vehicle stability control system of one embodiment of the invention; and 
         FIG. 2  is a flowchart illustrating a general method for controlling a vehicle according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a schematic plan view of vehicle  10 , and in particular, a hybrid vehicle. The vehicle  10  comprises control means in the form of a stability control system  12 , the stability control system  12  having the following functional modules: an Engine Control Module (ECM)  14 ; a Vehicle Supervisory Controller (VSC)  16  a Brakes Control Module (BCM)  18 ; a Transmission Control Module (TCM)  20 ; and a high voltage power invertor  22 . 
     A driveline  24  of the vehicle  10  is depicted in  FIG. 1 , the driveline  24  being connected to two prime mover devices in the form of a crankshaft-integrated motor generator (CIMG)  26  and an internal combustion engine  28 , with connection and disconnection between the engine  28  and the CIMG  26  being facilitated by way of a disconnect clutch  30 . The CIMG  26  is further coupled to the input of a transmission  32  of the vehicle  10 , an output of the transmission  32  being coupled to the driveline  24 . 
     The driveline  24  is configured so as to transmit power from one or both of the internal combustion engine  28  and the CIMG  26  to a front axle  34  and/or a rear axle  36  of the vehicle  10 , in order to drive a pair of front wheels  38 ,  40  and a pair of rear wheels  42 ,  44  of the vehicle  10 , respectively. The TCM  20  is responsible for controlling operation of the transmission  32  of the vehicle  10 , controlling gearing between the relevant prime mover device  26 ,  28  and the driveline  24  so as to achieve optimum vehicle performance and fuel economy. 
     The hybrid vehicle  10  is provided with both frictional and regenerative braking mechanisms for generating a brake torque to be applied to the front and/or rear axle  34 ,  36  of the vehicle  10 . Alternatively, or additionally, one or more of the braking mechanisms may be configured to generate a brake torque to be applied to each wheel  38 ,  40 ,  42 ,  44  of the vehicle  10  independently. The BCM  18  is configured to control selection and operation of the braking mechanisms of the vehicle  10 , and may be arranged to control any number of regenerative and frictional braking systems, such as an electro-hydraulic braking system or an electro-mechanical braking system. 
     The braking mechanisms associated with the frictional braking system are in the form of friction brakes  46 , each friction brake  46  corresponding to a separate wheel  38 ,  40 ,  42 ,  44  of the vehicle  10 . As is conventional, each friction brake  46  is made up of a brake calliper having a pair of brake pads. The brake pads are configured so as to be applied at the respective wheel  38 ,  40 ,  42 ,  44  to slow rotation of the wheel  38 ,  40 ,  42 ,  44 , the brake pads being operable so as to be applied at each wheel  38 ,  40 ,  42 ,  44  independently of the remaining three wheels. Since frictional braking mechanisms are known, further explanation will be omitted for clarity. 
     As is known, the regenerative braking function is implemented by way of the CIMG  26  that comprises an electric motor capable of functioning as a generator when operated in a reverse torque direction. Whilst functioning as a generator, electromagnetic forces generated by the CIMG  26  create braking torque that is applied to one or more axles  34 ,  36  of the vehicle  10 . The CIMG  26  can therefore apply regenerative braking to one or both axles  34 ,  36  of the vehicle  10 , simultaneously reducing the speed of the vehicle  10  and producing electrical energy. The high voltage power invertor  22  is arranged so as to link the CIMG  26  and a battery  48  of the vehicle, such that the invertor  22  facilitates charging of the battery  48  whilst the CIMG acts as a generator. 
     The VSC  16  is arranged so as to control a number of functions of the hybrid vehicle  10 , co-ordinating the individual prime mover devices  26 ,  28  and energy storage associated with the vehicle battery  48 . The levels of energy stored within the battery  48  are monitored by the ECM  14 , the ECM  14  continuously transmitting electrical signals to the VSC  16  to inform the VSC  16  as to the state of charge of the battery  48 . In response to signals from the ECM  14 , the VSC  16  is configured to control the interaction between the transmission  32 , the CIMG  26  and the BCM  18 , informing the BCM  18  of an amount of regenerative braking that is available. 
     The vehicle stability control system  12  is configured to receive input signals from a number of input devices that provide the BCM  18  with real-time information relating to the vehicle  10 . The input devices include a brake pedal  50  of the vehicle  10 . As is conventional, the brake pedal  50  may be used by the driver of the vehicle  10  to manually input brake commands to the vehicle  10 . One or more sensors are configured to provide a signal that is indicative of a required level of braking force, or braking demand, associated with the brake pedal  50  of the vehicle. Such a brake pedal sensor may comprise an optical sensor, an electro-magnetic sensor, a potentiometer, or any other suitable sensor. The BCM  18  is configured to determine the blend of braking in dependence on the regenerative braking capacity information received from the VSC  16 , calculating an appropriate level of regenerative braking and frictional braking that will satisfy the braking demand. 
     The input devices of the vehicle  10  extend to four wheel speed sensors  52 , each wheel speed sensor  52  being associated with a separate wheel  38 ,  40 ,  42 ,  44  of the vehicle  10 . As is known, each wheel speed sensor  52  transmits a signal to the BCM  18  of the stability control system  12  that is indicative of the speed of rotation of the respective wheel  38 ,  40 ,  42 ,  44 . In addition, one or more sensors may be configured to provide a signal that is indicative of a required level of acceleration, or acceleration demand, associated with an accelerator pedal of the vehicle. As previously described in relation to the brake pedal sensor, the accelerator pedal sensor may comprise any suitable sensor, for example an optical sensor, an electro-magnetic sensor or a potentiometer. 
     The vehicle  10  is further provided with an accelerometer  54  and a steering wheel angle sensor  56 . The accelerometer  54  and steering wheel angle sensor  56  send signals to the BCM  18  that are indicative of the lateral acceleration of the vehicle  10  and the orientation of a steering wheel of the vehicle  10 , respectively. The orientation of the steering wheel is determined relative to a reference position. In one embodiment, the reference position corresponds to a natural resting position of the steering wheel and is assigned a value of 0°. 
     Operation of the vehicle stability control system  12  in use will now be described, with reference to the vehicle control method or process  100  shown in  FIG. 2 . A known stability control system is configured to respond to a current vehicle state in a reactive manner, comparing the current state of the vehicle to an ideal state, with tolerable limits for deviation. For example, a current rate of change of wheel speed is compared to an acceptable rate of change of wheel speed. In the event that the rate of change of wheel speed is deemed to be unacceptable, torque to the wheel is reduced. Such known systems thus use existing, real-time error signals to assess the current state of the vehicle and to respond accordingly. The system  12  of the invention adds a predictive element, calculating a predicted vehicle yaw rate in dependence on current vehicle parameters. The system  12  of the invention is thus configured to respond to a predicted future state of the vehicle. 
     The vehicle stability control functionality is available automatically upon engine start. When activated, the vehicle stability control system  12  executes a series of steps to monitor a stability state of the vehicle  10 , controlling stopping or slowing of the vehicle  10  when it is determined that the vehicle state is undesirable. 
     Consider, for example, a scenario in which the vehicle  10  is executing a relatively high speed cornering manoeuver. A malfunction within a propulsion system of the vehicle  10  may cause unintended positive or negative torque to be applied to the driven axle of the vehicle  10 , such that the driven wheels begin to slip, for example. The system  12  continuously monitors the state of the vehicle  10 , and applies corrective action, such as the application of a specific braking pattern, in order to counteract slip of the wheels  38 ,  40 ,  42 ,  44  once the predicted yaw rate exceeds a predetermined yaw rate threshold. 
     In this way, the vehicle stability control system  12  makes a prediction as to how the vehicle  10  is about to respond to a detected slip condition, and takes pre-emptive action so as to mitigate the effects of the slip condition. The vehicle stability control system  12  is able to apply corrective action before the vehicle  10  begins to depart from its intended orientation, minimising the possibility of any lateral deviation of the vehicle  10  from the desired path. In order that the vehicle state may be monitored continuously in real time, and for corrective action to be taken promptly, the vehicle stability control system  12  may execute the method  100  at a suitably high frequency, for example between around 20 Hz and 100 Hz. 
     Each wheel speed sensor  52  of the vehicle  10  is configured to communicate continuously with the BCM  18 , such that the BCM  18  is provided with real-time information relating to the speed of rotation of each wheel  38 ,  40 ,  42 ,  44 . In an initial step, the BCM  18  is able to use the information to carry out a vehicle speed check  110 , calculating an instantaneous vehicle speed associated with the vehicle  10 . The vehicle speed is determined in dependence on the wheel speed sensor readings at each one of the four wheels  38 ,  40 ,  42 ,  44 . 
     The BCM  18  is provided with a predetermined threshold value for vehicle speed, against which the calculated vehicle speed is compared  112  in a next step, below which the yaw stability state of the vehicle  10  is considered to be insignificant. For example, below the predetermined threshold value for vehicle speed, the vehicle may not be expected to experience oversteer or understeer in the event that unintended regenerative braking torque is applied to an axle  34 ,  36  of the vehicle  10 . Therefore, if it is determined that the vehicle speed falls below the predetermined vehicle speed threshold, the stability control system  12  does not take any action, and continues to monitor signals transmitted from the sensors  52 ,  54 ,  56 . If the vehicle speed is above the threshold value, the BCM  18  makes an assessment as to whether or not wheels  38 ,  40 ,  42 ,  44  of the vehicle  10  are slipping. 
     In the event that the calculated vehicle speed exceeds the predetermined threshold value, the BCM  18  proceeds to calculate a longitudinal wheel slip  114  associated with each wheel  38 ,  40 ,  42 ,  44  of the vehicle  10 , using data received from the wheel speed sensors  52 . The BCM  18  is able to determine whether the level of longitudinal wheel slip is as expected. An undesirable level of slip may result from one of a number of situations in which excessive positive or negative torque has been applied to one or more axles  34 ,  36  of the vehicle  10 , by way of regenerative braking, for example. 
     The longitudinal wheel slip calculation employs the determined vehicle speed and the wheel speed sensor reading at the wheel  38 ,  40 ,  42 ,  44  of interest. For example, with respect to a rear left wheel  42  of the vehicle  10 , the longitudinal wheel slip calculation includes a comparison of the vehicle speed with the wheel speed sensor reading associated with the rear left wheel  42 . Specifically, longitudinal wheel slip is calculated using the following equation (1): 
     
       
         
           
             
               longitudinal 
                
               
                   
               
                
               wheel 
                
               
                   
               
                
               slip 
             
             = 
             
               
                 
                   
                     ω 
                      
                     r 
                   
                   - 
                   v 
                 
                 v 
               
               × 
               100 
                
               % 
             
           
         
       
     
     In equation (1), ω is angular velocity of the wheel, v is the vehicle speed and r is the wheel rolling radius. 
     As previously described in relation to the vehicle speed, the BCM  18  is provided with a predetermined threshold value for longitudinal slip, against which the instantaneous calculated values for longitudinal slip can be compared  116 . In one embodiment, only the instantaneous longitudinal slip values associated with the driven wheels of the vehicle  10  are compared  116  to the threshold value. For the purposes of this description, the driven wheels will be described as being the rear wheels  42 ,  44  of the vehicle  10 . It will be appreciated that the driven wheels may instead be the front wheels  38 ,  40  of the vehicle  10 , or that all four wheels  38 ,  40 ,  42 ,  44  of the vehicle  10  may be driven. 
     In one embodiment, the threshold longitudinal slip value may be between 10% and 30%, for example 20%. In the event that the longitudinal slip value for each of the driven wheels falls below the threshold value, no further steps are executed and the vehicle stability control system  12  continues to monitor information from the wheel speed sensors  52 . 
     In the event that the calculated vehicle speed is above the vehicle speed threshold and that the calculated longitudinal slip value is greater than 20%, the BCM  18  identifies an undesirable slip condition. At this stage then, a calculated longitudinal slip value for one or both of the driven wheels that exceeds the predetermined threshold value is indicative of the driven wheels of the vehicle  10  losing traction with the surface the vehicle  10  is traversing. The BCM  18  proceeds to implement a series of steps to mitigate the slip condition. 
     Firstly, the BCM  18  is configured to control pre-charging  118  of the friction brakes  46  of the vehicle  10  such that the brake pads are arranged to be rapidly engaged to control and slow the vehicle  10 , should this be necessary. The BCM  18  may be further arranged to send a signal to a lights control module of the vehicle  10 , initiating activation of hazard warning lights  120  of the vehicle  10  so as to alert any other nearby vehicle drivers. At this stage, the vehicle stability control system  12  is not configured to directly influence the speed or orientation of the vehicle  10 , but to increase the ease and speed with which the driver is able to take evasive action to counter the slip condition. 
     In a next step, the BCM  18  is configured to determine whether the calculated longitudinal slip is a negative value or a positive value  122 . A negative value for longitudinal slip is indicative of a situation in which the wheels  38 ,  40 ,  42 ,  44  of the vehicle  10  are rotating more slowly than would be expected for the instantaneous vehicle speed. A negative value therefore indicates that the vehicle  10  is skidding, relative to the surface being traversed. Conversely, a positive value for longitudinal slip indicates that the wheels  38 ,  40 ,  42 ,  44  of the vehicle  10  are rotating more quickly than would be expected for the instantaneous vehicle speed, and that the wheels  38 ,  40 ,  42 ,  44  are spinning relative to the surface. 
     In the event that the BCM  18  makes a determination of a positive longitudinal wheel slip, the BCM  18  first executes a check  124  to ensure that the wheels  38 ,  40 ,  42 ,  44  are not spinning as a result of an acceleration request from the driver of the vehicle  10 . In practice, the BCM  18  may analyse a signal received from the accelerator pedal sensor of the vehicle  10 , in order to determine the acceleration demand from the driver  124 . The acceleration demand from the driver may subsequently be compared to a threshold level  126 . If the acceleration demand is determined to fall above the threshold level, it is considered that the vehicle  10  is responding to an acceleration request from the driver, and no mitigating action is taken. 
     On the other hand, if the acceleration demand is determined to fall below the threshold level, the acceleration demand from the driver is considered to be lower than that required to produce the calculated longitudinal wheel slip. In this case, the BCM  18  is configured to send a signal to the friction brakes  46  associated with all four wheels  38 ,  40 ,  42 ,  44  of the vehicle  10 , increasing a braking torque  128  applied to the wheels  38 ,  40 ,  42 ,  44  such that braking is symmetrical across the wheels  38 ,  40 ,  42 ,  44  and the vehicle  10  is decelerated. Controlling braking so as to be symmetrical across all four wheels  38 ,  40 ,  42 ,  44  of the vehicle  10  advantageously guards against an application of any yaw moment of the vehicle, and subsequent spinning of the vehicle  10 . In one embodiment, the rate of deceleration of the vehicle  10  is controlled to tend to zero as the instantaneous vehicle speed tends to a predetermined vehicle speed. In an example, the predetermined vehicle speed is the predetermined threshold value for vehicle speed. Making a determination of the acceleration demand from the driver ensures that no action is taken to reduce torque applied to the driven wheels  42 ,  44  of the vehicle  10  in the event that the torque has been requested by the driver. 
     In the event that the BCM  18  makes a determination of a negative longitudinal wheel slip, this is an indication that unintended regenerative braking may have been applied to the driven wheels  42 ,  44  of the vehicle  10 . In response, the BCM  18  measures the vehicle lateral acceleration  130 , using the signals transmitted from the accelerometer  54  of the vehicle  10 . The instantaneous vehicle lateral acceleration indicates how the vehicle  10  is cornering at the time at which the unintended braking torque is applied. Measuring the vehicle lateral acceleration  130  therefore allows the BCM  18  to predict how the vehicle  10  may respond to the unintended braking torque, and how motion of the vehicle will be affected. Once measured, the instantaneous vehicle lateral acceleration is subsequently compared to a predetermined lateral acceleration threshold value  132  that is stored within the BCM  18 . 
     In the event that the vehicle lateral acceleration is determined to fall below the predetermined lateral acceleration threshold value, the BCM  18  proceeds to transmit a signal to the friction brakes  46  associated with the front wheels  38 ,  40  of the vehicle  10 , so as to slow both front wheels  38 ,  40  of the vehicle  10  simultaneously  134 . The resultant braking may be symmetrical, decelerating the vehicle  10  in a controlled manner and enabling the intended alignment of the vehicle  10  to be maintained. In this way, the BCM  18  initiates a controlled emergency stop. In one embodiment, the friction brakes  46  are controlled so as to continue applying a braking torque to the front wheels  38 ,  40  of the vehicle  10  until the instantaneous vehicle speed falls below the predetermined threshold value for vehicle speed, or until the calculated longitudinal slip falls below the threshold longitudinal slip value. 
     The scenario described above relates to a situation in which the instantaneous lateral acceleration is determined to fall below the predetermined lateral acceleration threshold. In this case, the vehicle  10  is not expected to spin as a result of the application of an unintended braking torque to the driven wheels  42 ,  44  of the vehicle  10 . In an alternative scenario, the instantaneous lateral acceleration may be determined to exceed the lateral acceleration threshold, in effect, indicating that the vehicle  10  may be about to spin about a vertical axis of the vehicle  10  and indicating a direction in which this is likely to occur. 
     Upon a determination that the instantaneous lateral acceleration exceeds the lateral acceleration threshold value, the BCM  18  proceeds to make a prediction as to the expected response of the vehicle  10 , calculating a predicted imminent yaw rate  136 . In this case, the aim of the vehicle stability control system  12  is to predict how the vehicle  10  will react to the determined slip condition, such that appropriate counter-measures may be implemented. In one embodiment, the BCM  18  employs a 3D map, or look-up table, to correlate a number of the calculated vehicle parameters with predicted vehicle yaw rates. Specifically, the BCM  18  is configured to input data corresponding to the instantaneous vehicle speed, the instantaneous rear wheel slip and the instantaneous lateral acceleration, in order to extract the corresponding predicted yaw rate value. It will be appreciated that the BCM  18  may alternatively use a physics model to determine the predicted yaw rate. 
     The predicted yaw rate represents an imminent angular velocity of the vehicle  10  around a vertical axis of the vehicle  10 , giving an indication of the degree to which the vehicle  10  is expected to deviate from its current orientation in the immediate future. In one embodiment, the predicted yaw rate is the yaw rate predicted for 0.5 seconds in the future. 
     In a next step, the BCM  18  increases steering gain  138 , increasing the steering output relative to the steering wheel input, in a direction that would counter the predicted yaw rate. For example, if the predicted yaw rate indicates that the vehicle  10  will imminently tend to yaw in a clockwise direction, the steering gain would be applied so as to increase the ease with which the driver could instruct the vehicle  10  to turn in an anticlockwise direction. As a result, the orientation of the vehicle  10  in this direction is more responsive to the driver input, such that the vehicle  10  reacts more quickly to adjustments made to the steering wheel, assisting the driver in regaining control of the vehicle  10 . An increased steering gain would not be applied in a direction of rotation that would increase the predicted yaw rate, and the driver would thus experience relatively high resistance if they chose to rotate the steering wheel in this direction. 
     The BCM  18  subsequently calculates a yaw rate that would be expected under normal operating conditions of the vehicle  10 . The BCM  18  may employ a second look-up table to correlate an instantaneous steering wheel angle and instantaneous wheel speed values with values for the expected yaw rate. The expected yaw rate is a target yaw rate for the vehicle  10 , against which the predicted yaw rate may be compared. Upon calculation of the target yaw rate, the BCM  18  is configured to execute a further calculation, determining a yaw torque  140  that would be required to influence the predicted yaw rate such that the predicted yaw rate equals or falls below the target yaw rate value. 
     In order to achieve the target yaw rate, the BCM  18  calculates a brake pressure distribution that would achieve the required yaw torque. The brake pressure distribution is selected so as to counteract the imminent rotation of the vehicle  10 , and to apply or increase the braking torque at a front outside wheel of the vehicle  142 . In this context, the term ‘outside wheel’ should be taken to mean the wheel that is rolling in the largest radius during cornering. The term ‘inside wheel’ should be interpreted accordingly to mean the wheel during cornering that follows the smallest cornering radius. In practice, the BCM  18  controls application of the brake pad associated with the front outside wheel, slowing rotation of the wheel to pre-empt and prevent a change in alignment of the vehicle  10 . For example, if the predicted yaw rate implies that the vehicle  10  will imminently rotate in a clockwise direction when viewed from above, the brake pressure distribution is such that a braking torque is applied to the front left wheel  38  of the vehicle  10 . 
     Alternatively, the brake pressure distribution may be configured to apply or increase a braking torque at the front outside wheel of the vehicle  10 , in addition to the application or increase of a smaller braking torque at the front inside wheel of the vehicle  10 . In this case, the brake pressure distribution is similarly configured to achieve the required yaw torque, but the application of braking torque to the front inside wheel simultaneously decelerates the vehicle  10 . 
     The BCM  18  operates a further feedback loop at this stage, continuing to apply the braking pressure  142  until the calculated predicted yaw rate is determined to meet or to have fallen below the target yaw rate. Therefore, if the orientation of the vehicle  10  deviates to a lesser extent than expected, the braking torque may be reduced to account for the updated yaw rate prediction. 
     Once the predicted yaw rate has fallen below the target yaw rate, the BCM  18  may modify the braking distribution such that an even braking torque is applied  134  to both front wheels  38 ,  40  of the vehicle  10 , slowing the vehicle in a controlled manner. As previously described, the symmetrical braking torque is applied until the calculated vehicle speed falls below the vehicle speed threshold value. Further, the deceleration is controlled such that the deceleration of the vehicle tends to zero as the vehicle speed tends towards the vehicle speed threshold value. 
     In response to a determination that the instantaneous longitudinal slip value exceeds a threshold value  116 , it may be beneficial for the vehicle stability control system  12  to execute further mitigating actions to guard against deviation of the vehicle  10  from its intended path. Therefore, after the application of braking torque  128  at all four wheels  38 ,  40 ,  42 ,  44  of the vehicle  10  or, alternatively, after the application of braking torque  134  at both front wheels  38 ,  40  of the vehicle, the VSC  16  may transmit a signal to the battery  48  to request that high voltage contactors of the battery  48  be opened  114 . Opening the contactors  144  removes the ability of the battery  48  to gain energy, and thus removes the ability of the system  12  to apply regenerative braking torque to the driven wheels  42 ,  44  of the vehicle  10 . 
     In a next step, the vehicle stability control system  12  may be further configured to remove propulsion torque  146  from the driveline  24 , opening the disconnect clutch  30  to eliminate the braking torque from the rear wheels  42 ,  44  and to remove a cause of the slip condition. Additionally, or alternatively, the vehicle stability control system  12  may control a change in an air suspension state in response to a determined longitudinal slip value, lowering the vehicle  10  to improve traction at the driven wheels  42 ,  44  and/or influencing a pitch rate and roll rate associated with the vehicle  10 . If the vehicle  10  is provided with a Continuously Variable Damping (CVD) system, the vehicle stability control system  12  may be further configured to control dampers of the CVD, to mitigate the effect of an excessive longitudinal slip value. 
     In summary, the vehicle stability control system  12  is configured to continuously monitor vehicle states. In the event that conditions indicating a specific undesirable state are detected, the vehicle stability control system  12  controls the vehicle  10  in dependence on the predicted, or future, vehicle yaw response that would be expected as a result of the specific undesirable state. In the event that the rear wheels  42 ,  44  of the vehicle  10  are determined to be slipping, the vehicle stability control system  12  configures the vehicle  10  to respond more quickly to inputs from the driver. For example, the steering gain is increased  138  or the friction brakes  46  are pre-charged  118 . 
     Upon a subsequent determination that the vehicle  10  is not rotating, or spinning, about a vertical axis, the stability control system  12  implements application of a braking torque at both front wheels  38 ,  40  of the vehicle  10  simultaneously  134 , bringing the vehicle speed below the vehicle speed threshold value. Alternatively, should the stability control system  12  determine that the vehicle  10  has begun to spin, a braking torque is applied to the front outside wheel of the vehicle  142 . The strategy taken by the stability control system  12  is thus dependent on the current state of the vehicle  10 , such that the most appropriate and effective action is taken. 
     Selection of an appropriate strategy is dependent upon the vehicle application and the driving scenario. For example, the appropriate response may be determined in part by an estimate of a coefficient of friction for the surface the vehicle  10  is traversing, an estimation function for which may be provided to the BCM  18 . Such estimation functions are known. 
     In the described embodiment, each step is described as being executed in series. However, it will be appreciated that a number of steps may instead be carried out by the BCM  18  in parallel, or in an alternative order. For example, determination of the instantaneous vehicle speed  110 , the instantaneous longitudinal wheel slip  114 , the expected yaw rate and the predicted imminent vehicle yaw rate  136  may be continuous, the look-up table being constantly updated or refreshed. Alternatively, the predicted yaw rate may be calculated  136  only in the event that the threshold values for vehicle speed, longitudinal slip and lateral acceleration exceed the respective predetermined threshold levels. 
     Further, while each calculation is described as being executed by the BCM  18 , it will be understood that these steps may be executed by any one of the modules of the vehicle stability control system  12 , for example, the VSC  16 . In addition, a third party module, such as the VSC  16 , may be employed to check the calculated values so as to give a greater confidence in the results. 
     It will be appreciated that the stability control system  12  described is not exclusively applicable to hybrid or electric vehicles, and that the system may be applied to any vehicle having a powertrain that could cause the vehicle to deviate from its intended path or orientation while cornering, should the powertrain malfunction. 
     In one embodiment, it is feasible that the driver may intend for the driven, or rear, wheels  42 ,  44  of the vehicle  10  to lock as part of a driving manoeuver, for example a handbrake turn. In order to account for such a scenario, the stability control system  12  may be configured to determine whether certain conditions are met and, if the conditions are met, to inhibit application of any corrective action. As an example, upon detection of a slip condition, the BCM  18  may be configured to detect whether a signal from an electronic park brake switch is indicative of a parking brake of the vehicle  10  being in an engaged position. The BCM  18  may also be configured to determine whether the steering wheel is at a position 90° or more from the reference position of the steering wheel, as measured by the steering wheel angle sensor. In the event that the parking brake is in the engaged position and that the steering wheel has been rotated through an angle of 90° or more, the BCM  18  temporarily disables operation of the vehicle stability control system  12 . 
     Many modifications may be made to the above examples without departing from the scope of the present invention as defined in the accompanying claims.