Patent Publication Number: US-2023150481-A1

Title: Apparatus and method for controlling an electric machine of a vehicle

Description:
TECHNICAL FIELD 
     The present disclosure relates to controlling an electric machine and particularly, but not exclusively, to controlling coupling of an electric machine. Aspects of the invention relate to a control system, to a powertrain, to a vehicle, to a method and to computer software. 
     BACKGROUND 
     It is increasingly known for vehicles to be powered by more than one motive or traction power source, such as an internal combustion engine and one or more electric machines or motors. However, management of multiple traction power sources may be problematic. 
     It is an object of embodiments of the invention to at least mitigate one or more of the problems of the prior art. 
     SUMMARY OF THE INVENTION 
     Aspects and embodiments of the invention provide a control system, a powertrain, a vehicle, a method and computer software as claimed in the appended claims. 
     According to an aspect of the invention, there is provided an electric machine control system for a vehicle, the electric machine control system comprising one or more controllers, wherein the vehicle comprises an electric machine arranged to be selectively coupleable to provide torque to at least one wheel of an axle of the vehicle processing means arranged to determine a coupling state of the electric machine to the at least one wheel of the axle. Advantageously the processing means is arranged to determine the coupling of the electric machine to the at least one wheel of the axle. 
     According to an aspect of the invention, there is provided an electric machine control system for a vehicle, the electric machine control system comprising one or more controllers, wherein the vehicle comprises an electric machine arranged to be selectively coupleable to provide torque to at least one wheel of an axle of the vehicle, the control system comprising: input means arranged to receive a status signal indicative of a status of a coupling of the electric machine to the at least one wheel of an axle of the vehicle; output means arranged to output a coupling signal to control coupling of the electric machine to the at least one wheel of the axle; and processing means arranged to determine a coupling state of the electric machine to the at least one wheel of the axle and to control the output means to output a coupling signal indicative of the determined coupling state, wherein the processing means is arranged to control the output means to output the coupling signal indicative of a retry of the change in the coupling state in dependence on the speed signal. Advantageously the change in coupling state is retried. 
     According to an aspect of the invention, there is provided an electric machine control system for a vehicle, the electric machine control system comprising one or more controllers, wherein the vehicle comprises an electric machine arranged to be selectively coupleable to provide torque to at least one wheel of an axle of the vehicle, the control system comprising: input means arranged to receive a speed signal indicative of a speed of the vehicle and a status signal indicative of a status of a coupling of the electric machine to the at least one wheel of an axle of the vehicle; output means arranged to output a coupling signal to control coupling of the electric machine to the at least one wheel of the axle; and processing means arranged to determine a coupling state of the electric machine to the at least one wheel of the axle and to control the output means to output a coupling signal indicative of the determined coupling state, wherein the processing means is arranged, in dependence on the status signal being indicative of a failure to change the coupling state of the electric machine to the at least one wheel of the axle in dependence on a change in the determined coupling state, to control the output means to output the coupling signal indicative of a retry of the change in the coupling state in dependence on the speed signal.
         Advantageously the change in coupling state is retried in dependence on the status signal.       

     The processing means may be arranged to defer controlling the output means to output the coupling signal indicative of the retry of the change in the coupling state in dependence on the speed signal being indicative of the speed of the vehicle being at least a predetermined minimum speed. Advantageously the change in coupling state is retried in dependence on the speed of the vehicle. 
     The predetermined speed is optionally a speed greater than substantially 0 kmh −1 . Advantageously the change in coupling state is retried when the vehicle is moving. In some embodiments, the change in coupling state is only retried when the vehicle is moving. 
     Optionally the processing means is arranged to defer controlling the output means to output the coupling signal indicative of the retry of the change in the coupling state in dependence on the speed signal being indicative of the speed of the vehicle being less than or equal to a predetermined maximum speed. Advantageously the change in coupling state is retried up to the predetermined maximum speed. The maximum speed may be less than 50 kmh −1 . The maximum speed may be less than 20 kmh −1 . 
     The processing means is optionally arranged to control the output means to output the coupling signal indicative of the retry of the change in the coupling state in dependence on the speed signal up to a predetermined maximum number of times. Advantageously the change in coupling state is retried up to the predetermined maximum number of times. The predetermined maximum number of times may be 5. 
     According to an aspect of the invention, there is provided a powertrain comprising the system as described above. 
     According to an aspect of the invention, there is provided a vehicle comprising the control system as described above or the powertrain as described above. 
     The electric machine may be arranged to be selectively coupleable to provide torque to at least one wheel of a first axle of the vehicle, and the vehicle comprises a second motive power source arranged to provide torque to at least one wheel of a second axle of the vehicle. 
     The second motive power source may comprise a second electric machine. 
     According to an aspect of the invention, there is provided a method of controlling coupling of an electric machine to provide torque to at least one wheel of an axle of a vehicle, the method comprising: receiving a speed signal indicative of a speed of the vehicle and a status signal indicative of a status of a coupling of the electric machine to the at least one wheel of an axle of the vehicle; determining a coupling state of the electric machine to the at least one wheel of the axle; outputting a coupling signal indicative of the determined coupling state to control the coupling of the electric machine to the at least one wheel of the axle; determining, in dependence on the status signal, a failure to change the coupling state of the electric machine to the at least one wheel of the axle in dependence on a change in the determined coupling state; and outputting, in dependence on the determined failure, the coupling signal indicative of a retry of the change in the coupling state in dependence on the speed signal. 
     The method may comprise deferring the outputting of the coupling signal indicative of the retry of the change in the coupling state in dependence on the speed signal being indicative of the speed of the vehicle being at least a predetermined minimum speed. 
     The predetermined speed may be a speed greater than substantially 0 kmh −1 . 
     The method may comprise deferring outputting the coupling signal indicative of the retry of the change in the coupling state in dependence on the speed signal being indicative of the speed of the vehicle being less than or equal to a predetermined maximum speed. 
     The maximum speed may be less than 50 kmh −1 . The maximum speed may be less than 20 kmh −1 . 
     The method may comprise outputting the coupling signal indicative of the retry of the change in the coupling state in dependence on the speed signal up to a predetermined maximum number of times. The predetermined maximum number of times may be 5. 
     According to another aspect of the invention, there is provided computer software which, when executed by a computer, is arranged to perform a method as described above. The computer software may be stored on a computer-readable medium. The computer software may be tangibly stored on the computer readable medium. 
     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    shows a vehicle according to an embodiment of the invention; 
         FIG.  2    shows a system according to an embodiment of the invention; 
         FIG.  3    shows a control system according to an embodiment of the invention; 
         FIG.  4    shows an illustration of modules of the control system according to embodiments of the invention; 
         FIG.  5    shows a method according to an embodiment of the invention; 
         FIG.  6    illustrates operation of a module according to an embodiment of the invention; 
         FIG.  7    further illustrates operation of a module according to an embodiment of the invention; 
         FIG.  8    illustrates operation of another module according to an embodiment of the invention; 
         FIG.  9    shows a method according to another embodiment of the invention; 
         FIG.  10    shows a method according to still another embodiment of the invention; 
         FIG.  11    shows a method according to yet another embodiment of the invention; 
         FIG.  12    shows a method according to further embodiment of the invention; 
         FIG.  13    shows a method according to a yet further embodiment of the invention; 
         FIG.  14    shows a method according to still further embodiment of the invention; 
         FIG.  15    illustrates operation of a module according to an embodiment of the invention; and 
         FIG.  16    illustrates operation of the system according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    illustrates a vehicle  100  according to an embodiment of the invention. The vehicle  100  provides space within a cabin of the vehicle  100  for one or more occupants. In some embodiments, the vehicle  100  may be manually driven by one of the occupants representing a driver of the vehicle  100 , although the vehicle  100  may have an at least partly autonomous driving capability in some embodiments. The vehicle  100  is an at least partly electric-powered vehicle  100 , as will be explained, with an internal combustion engine and one or more electric machines or traction electric motors for providing motive torque, thereby the vehicle being a hybrid electric vehicle (HEV). In some embodiments the vehicle  100  may be entirely electric powered i.e. a battery electric vehicle (BEV) without an internal combustion engine. 
       FIG.  2    illustrates a system  20  for a parallel HEV  10 . The system  20  defines, at least in part, a powertrain of the HEV. The system  20  comprises a control system  208 . The control system  208  comprises one or more controllers. The control system  208  may comprise one or more of: a hybrid powertrain control module; an engine control unit; a transmission control unit; a traction battery management system; and/or the like. 
     The system  20  comprises an engine  202 . The engine  202  is a combustion engine. The illustrated engine  202  is an internal combustion engine. The illustrated engine  202  comprises three combustion chambers, however a different number of combustion chambers may be provided in other examples. 
     The engine  202  is operably coupled to the control system  208  to enable the control system  208  to control output torque of the engine  202 . The output torque of the engine  202  may be controlled by controlling one or more of: air-fuel ratio; spark timing; poppet valve lift; poppet valve timing; throttle opening position; fuel pressure; turbocharger boost pressure; and/or the like, depending on the type of engine  202 . 
     The system  20  comprises a vehicle transmission arrangement  204  for receiving output torque from the engine  202 . The vehicle transmission arrangement  204  may comprise an automatic vehicle transmission or a semi-automatic vehicle transmission. The vehicle transmission arrangement  204  comprises a fluid-coupling torque converter  217  between the engine  202  and a gear train. 
     The system  20  may comprise a differential (not shown) for receiving output torque from the gear train. The differential may be integrated into the vehicle transmission arrangement  204  as a transaxle, or provided separately. 
     The engine  202  is mechanically connected or connectable to a first set of vehicle wheels (FL, FR) via a first torque path  220 . The first torque path  220  extends from an output of the engine  202  to the vehicle transmission arrangement  204 , then to axles/driveshafts, and then to the first set of vehicle wheels (FL, FR). In a vehicle overrun and/or friction braking situation, torque may flow from the first set of vehicle wheels (FL, FR) to the engine  202 . Torque flow towards the first set of vehicle wheels (FL, FR) is positive torque, and torque flow from the first set of vehicle wheels (FL, FR) is negative torque. 
     The illustrated first set of vehicle wheels (FL, FR) comprises front wheels, and the axles are front transverse axles. Therefore, the system  20  is configured for front wheel drive by the engine  202 . In another example, the first set of vehicle wheels (FL, FR) comprises rear wheels (RL, RR). The illustrated first set of vehicle wheels (FL, FR) is a pair of vehicle wheels, however a different number of vehicle wheels could be provided in other examples. 
     In the illustrated system  20 , no longitudinal (centre) driveshaft is provided, to make room for hybrid vehicle components. Therefore, the engine  202  is not connectable to a second set of rear wheels (rear wheels RL, RR in the illustration). The engine  202  may be transverse mounted to save space. 
     A torque path connector  218  such as a clutch is provided inside and/or outside a bell housing of the vehicle transmission arrangement  204 . The clutch  218  is configured to connect and configured to disconnect the torque path  220  between the engine  202  and the first set of vehicle wheels (FL, FR). The system  20  may be configured to automatically actuate the clutch  218  without user intervention. 
     The system  20  comprises a first electric traction motor  216 . The first electric traction motor  216  may be an alternating current induction motor or a permanent magnet motor, or another type of motor. The first electric traction motor  216  is located to the engine side of the clutch  218 . 
     The first electric traction motor  216  may be mechanically coupled to the engine  202  via a belt or chain. For example, the first electric traction motor  216  may be a belt integrated starter generator (BiSG). In the illustration, the first electric traction motor  216  is located at an accessory drive end of the engine  202 , opposite a vehicle transmission end of the engine  202 . In an alternative example, the first electric traction motor  216  is a crankshaft integrated motor generator, located at a vehicle transmission end of the engine  202 . 
     The first electric traction motor  216  is configured to apply positive torque and configured to apply negative torque to a crankshaft of the engine  202 , for example to provide functions such as: boosting output torque of the engine  202 ; deactivating (shutting off) the engine  202  while at a stop or coasting; activating (starting) the engine  202 ; and regenerative braking in a regeneration mode. In a hybrid electric vehicle mode, the engine  202  and first electric traction motor  216  are both operable to supply positive torque simultaneously to boost output torque. The first electric traction motor  216  may be incapable of sustained electric-only driving, although in other embodiments the first electric traction motor  216  may be capable of electric only driving particularly an embodiment without the engine  202 . One or both of the engine  202  and the first electric traction motor  216  are able to provide torque to a first axle  221  of the vehicle. 
     However, when the torque path  220  between the engine  202  and the first set of vehicle wheels (FL, FR) is disconnected, a torque path  220  between the first electric traction motor  216  and the first set of vehicle wheels (FL, FR) is also disconnected. 
       FIG.  2    illustrates a second electric traction motor  212  configured to enable at least an electric vehicle mode comprising electric-only driving. In some, but not necessarily all examples, a nominal maximum torque of the second electric traction motor  212  is greater than a nominal maximum torque of the first electric traction motor  216 . 
     Even if the torque path  220  between the engine  202  and the first set of vehicle wheels (FL, FR) is disconnected by the clutch  218 , the vehicle  10  can be driven in electric vehicle mode because the second electric traction motor  212  is connected to at least one vehicle wheel. The at least one vehicle wheel may be one, or both, of the rear wheels (RL, RR) of the vehicle  100  associated with a second axle  222  of the vehicle  100 . 
     The illustrated second electric traction motor  212  is configured to provide torque to the illustrated second set of vehicle wheels (RL, RR) of the second axle  222  of the vehicle. The second set of vehicle wheels (RL, RR) comprises vehicle wheels not from the first set of vehicle wheels (FL, FR). The illustrated second set of vehicle wheels (RL, RR) comprises rear wheels, and the second electric traction motor  212  is operable to provide torque to the rear wheels (RL, RR) via rear transverse axles forming the second axle  222 . Therefore, the vehicle  10  may be rear wheel driven in electric vehicle mode. 
     The control system  208  may be configured to disconnect the torque path  220  between the engine  202  and the first set of vehicle wheels (FL, FR) in electric vehicle mode, to reduce parasitic pumping energy losses. For example, the clutch  218  may be opened. In the example of  FIG.  2   , this means that the first electric traction motor  216  will also be disconnected from the first set of vehicle wheels (FL, FR). 
     Another benefit of the second electric traction motor  212  is that the second electric traction motor  212  may also be configured to operable in a hybrid electric vehicle mode, to enable four-wheel drive operation despite the absence of a centre driveshaft. 
     The second electric traction motor  212  may be selectively coupled to one or both wheels RL, RR of the second axle  222 . Coupling of a torque path between the second electric traction motor  212  and the one or both wheels RL, RR of the second axle  222  may be achieved via a second clutch  219 . The second clutch  219  may be controlled to open, such as via an actuator under control of a received signal, to disconnect the torque path between the second electric traction motor  212  and the one or both wheels (RL, RR) of the second axle  222 . In some embodiments the second clutch  219  may be a dog cutch. 
     Thus it will be appreciated that the second electric traction motor  212  is arranged to be selectively coupleable to provide torque to at least one wheel (RL, RR) of an axle of the vehicle  100 . In some embodiments, the vehicle  100  comprises another motive power source arranged to provide torque to at least one wheel (FL, FR) of another axle of the vehicle  100 . In the illustrated embodiment the another motive power source power source comprises another electric machine  216  in the form of the first electric traction motor  216 . The another motive power source may, in some embodiments, comprise an internal combustion engine  202  which may provide positive torque alone or in combination with the first electric traction motor  216 . 
     In order to store electrical power for the electric traction motors  212 ,  216 , the system  20  comprises a traction battery  200 . The traction battery  200  provides a nominal voltage required by electrical power users such as the electric traction motors. If the electric traction motors  212 ,  216  run at different voltages, DC-DC converters (not shown) or the like may be provided to convert voltages. 
     The traction battery  200  may be a high voltage (HV) battery. High voltage traction batteries provide nominal voltages in the hundreds of volts, as opposed to traction batteries for mild HEVs which provide nominal voltages in the tens of volts. The traction battery  200  may have a voltage and capacity to support electric only driving for sustained distances. The traction battery  200  may have a capacity of several kilowatt-hours, to maximise range. The capacity may be in the tens of kilowatt-hours, or in the hundreds of kilowatt-hours. 
     Although the traction battery  200  is illustrated as one entity, the function of the traction battery  200  could be implemented using a plurality of small traction batteries in different locations on the vehicle  10 . 
     In some examples, the first electric traction motor  216  and second electric traction motor  212  may be configured to receive electrical energy from the same traction battery  200 . By pairing the first (mild) electric traction motor  216  to a high-capacity battery (tens to hundreds of kilowatt-hours), the first electric traction motor  216  may be able to provide the functionality of the methods described herein for sustained periods of time, rather than for short bursts. In another example, the electric traction motors  212 ,  216  may be paired to different traction batteries. 
     Finally, the illustrated system  20  comprises one or more inverters. Two inverters  210 ,  214  are shown, one for each electric traction motor  212 ,  216 . In other examples, one inverter or more than two inverters could be provided. 
     It can be appreciated from the foregoing that the vehicle  100  may be provided with motive torque from a combination of sources. Embodiments of the present invention relate to determining which of the sources of motive torque to utilise. 
       FIG.  3    illustrates a control system  300  according to an embodiment of the invention. The control system  300  may be formed by one or more controllers  305 . The control system  300  illustrated in  FIG.  2    comprises one electronic controller  305  although it will be appreciated that this is merely illustrative. The, or each, controller  305 , comprises a processing means  310  and a memory means  320 . The processing means  310  may be one or more electronic processors  310  or processing devices  310 , such as CPUs, for executing computer readable instructions. The memory means  320  may be one or more memory devices  320 . The one or more memory devices  320  may store computer-readable instructions for execution by the at least one processing device  310 . 
     The controller  305  comprises an input means  330  and an output means  340 . The input means  330  is arranged to receive one or more signals  335 . The input means  330  may be an electrical input to the controller  305  for receiving one or more electrical signals  335 . The output means  340  is arranged to output at least one signal  345 , which is provided in  FIG.  3    to one or both of the second clutch  219  and second electric traction motor  212  to control coupling to the second torque path to provide torque to one or both wheels of the second axle  222 . The output means  340  is an electrical output of the controller  305 . The output means  340  is operable by the processing device  310  to output the signal  345  under control thereof. The signal  345  may cause the second electric traction motor  212  to ‘spin-up’ or accelerate to a rotation speed suitable to couple with the second axle  222  i.e. bearing in mind that the vehicle  100  may be in motion through torque provided by the first electric traction motor  216  and/or engine  202 . The signal  345  may cause closing of the second clutch  219  to couple the second electric traction motor  212  to the second torque path. 
     The electrical input  330  and output  340  of the controller  305  may be provided to/from a communication bus or network of the vehicle, such as a CANBus or other communication network which may, for example, be implemented by an Internet Protocol (IP) based network such as Ethernet, or FlexRay or a Single Edge Nibble Transmission (SENT) protocol, although other protocols may be used. 
       FIG.  4    schematically illustrates a portion of the controller  305  comprising the input means  330  and output means  340  of the system  300 .  FIG.  4    illustrates inputs  410 ,  420 ,  430 ,  440 ,  450 ,  460 ,  470  to the input means  330  of the controller  305  which form the signal  335  illustrated in  FIG.  3   .  FIG.  4    further illustrates modules  510 ,  520 ,  530 ,  540 ,  550 ,  560 ,  570 , or functional units, which may operatively execute on the processing device  310  of the controller  305 . Each of the inputs  410 ,  420 ,  430 ,  440 ,  450 ,  460 ,  470  provides information relating to one or more aspects or attributes of the vehicle  100  or the powertrain  20  thereof. 
     The inputs  410 ,  420 ,  430 ,  440 ,  450 ,  460 ,  470  may comprise one more of one or more speed signals  410 , a temperature signal  420 , a fault-derived coupling state request (FDCSR) signal  430 , a driving mode (DM) signal  440 , a state of charge (SoC) signal  450  and an inhibit signal  460  which provide information or data on which a desired coupling state is determined by one or more of the modules  510 ,  520 ,  530 ,  540 ,  550 ,  560 ,  570  as will be explained. The desired coupling state is a desired coupling of the torque path between the second electric traction motor  212  and the one or both wheels RL, RR of the second axle  222  of the vehicle  100  which is determined by one or more of the modules  510 ,  520 ,  530 ,  540 ,  550 ,  560 ,  570 . 
     The one or more speed signals  410  is indicative of one or more of a speed of the vehicle  100  i.e. a speed of the vehicle  100  over ground, a wheel speed signal indicative of a speed of rotation of one or more wheels of the vehicle and a motor speed signal indicative of a speed of one or both of the speed of the first and second electric traction motors  216 ,  212 . 
     The temperature signal  420  is indicative of one or more of an ambient temperature and a temperature of one or more units, or a temperature of fluids associated with one or more units, particularly fluids used for cooling said units i.e. coolant fluid, of the vehicle  100 . For example the coolant fluid may be a coolant fluid of one or both traction electric motors  212 ,  216 . In some embodiments, the temperature signal  420  comprises a temperature associated with one more units of the powertrain. In some embodiments, the temperature associated with one more units of the powertrain comprises a temperature of one or more of one or both of the inverters  210 ,  214 , one or both of the electric traction motors  212 ,  216 , a coolant temperature, and an indication of a temperature of the traction battery  200 . The indication of the temperature of the traction battery  200  may be indicative of a power capability of the traction battery  200 , which is a function of temperature and a State of Charge (SoC) of the traction battery  200 . Thus in some embodiments the temperature signal  420  may comprise a signal indicative of the power capability of the traction battery  200 , this being indicative of temperature. 
     The fault-derived coupling state request signal (FDCSR)  430  is indicative of a request for a coupling state derived in determination of a fault associated with the vehicle  100 , such as a fault associated with the powertrain. For example, where a fault associated with the second clutch  219  is detected by a fault management module (not shown), the fault management module may request that a coupling state of coupled or decoupled in order to control a state of the clutch  219  i.e. open or closed, in order to manage or resolve the fault. Other faults may be appreciated to cause a desired coupling state to manage or ameliorate the fault. In some embodiments, a fault management module  530  may be executed upon the processing device  310  and thus the FDSCR signal  430  may be generated internal to the controller  305 . 
     The driving mode signal  440  may be indicative of a driving mode of the vehicle  100  which may be automatically determined, such as by an intelligent driving mode or terrain response (TR) determination unit, an autonomous driving controller, such as an ADAS system, or selected by an occupant of the vehicle  100 . The driving mode signal  440  may be indicative of selection of an efficiency-based driving mode i.e. to provide minimal fuel and/or energy usage, a four wheel-drive driving mode, such as where a number of driven wheels may be automatically selected, and a selected driving gear i.e. neutral, drive (D), reverse (R) etc. 
     The state of charge (SoC) signal  450  is indicative of the SoC of the traction battery  200 . 
     The inhibit signal  460  is indicative of one or more inhibited coupling states. For example, the inhibit signal  460  may indicate that a state of coupled is inhibited to prevent coupling of the second electric traction motor  212  to the one or both wheels (RL, RR) of the second axle  222 , or that a state of decoupled is inhibited to prevent decoupling of the second electric traction motor  212  from the one or both wheels (RL, RR) of the second axle  222 . 
     The inputs  410 ,  420 ,  430 ,  440 ,  450 ,  460 ,  470  may, in some embodiments, comprise a coupling status signal  470  which is indicative of an actual coupling status of the second electric traction motor  212  to the one or both wheels of the second axle  222 . In some embodiments, the coupling status signal  470  has states of coupled and decoupled indicative the respective coupling. The coupling status signal  470  reports the physical status of the coupling of the second electric traction motor  212  to the second torque path via the second axle  222  and is thus indicative of successful coupling or decoupling of the second electric traction motor  212 . 
     In some embodiments, the modules  510 ,  520 ,  530 ,  540 ,  550 ,  560 ,  570  comprise a high-speed module  510 , a low-speed module  520 , a fault management module (FMM)  530 , an anti-fussiness module  540 , an inhibit module  550 , a driving mode module (DMM)  560  and an arbitrator  570 . It will be appreciated that not all modules are present in all embodiments, thus embodiments of the present invention may comprise one or more of the aforementioned modules. Each of the modules will be explained below. Each of the high-speed module  510 , the low-speed module  520 , the fault management module  530 , the anti-fussiness module  540 , the inhibit module  550 , the efficiency module  560 , as present in the relevant embodiment, may determine a respective desired coupling state. An indication of the desired coupling state is provided to the arbitrator  570  to determine the coupling state of the electric machine  212  to the axle  222  i.e. as an arbitrated coupling state. 
     An embodiment of the high-speed module (HSM)  510  will now be explained with reference to  FIGS.  5  &amp;  6   . The HSM  510  is operatively executable by the processing device  310  to determine a coupling state of the electric machine  212  to the at least one wheel of the axle  222  in dependence on the speed signal  410  indicative of the speed of the vehicle  100 . In some embodiments, the HSM  510  and the arbitrator  570  are arranged to cause the controller  305  to output a coupling signal  345  to control coupling of the second electric traction motor  212  to the at least one wheel of the axle  222  dependent on the speed signal  410  as will be explained. The HSM  510  is arranged to cause decoupling of the second electric traction motor  212  from the at least one wheel of the axle  222  a high-speeds of the vehicle  100  which, advantageously, prevents rotation of the second electric traction motor  212  at excessive speeds which may damage the second electric traction motor  212 . 
       FIG.  5    illustrates a method  600  according to an embodiment of the invention which may be performed by the HSM  510  executed by the processing device  310  of the controller  305 . The method  600  will be explained with reference to  FIG.  6    which illustrates a speed of the vehicle  100 , as indicated by the speed signal  410 , over a period of time. Also illustrated in a lower portion of  FIG.  6    is a desired coupling signal  515  output by the HSM  510  which represents a request  730 ,  740  for the desired coupling state from the HSM  510  determined in dependence on the speed signal  410 . 
     The method  600  comprises a step  610  of receiving one or more signals, such as data representing the one or more signals, at the HSM  510 . In the illustrated embodiment the HSM  510  is arranged to receive the speed signal  410 , which as discussed above may be indicative of the speed of the vehicle  100 . In some embodiments, the HSM  510  is arranged to receive the temperature signal  420  as discussed above. In some embodiments, the HSM  510  is arranged to receive the SoC signal  460  indicative of the state of charge of one or more traction batteries  200  for providing electrical power to the traction electric machines  212 ,  216 . In some embodiments, the HSM  510  may receive a signal indicative of a power limit or capability of the traction battery  200  which, as discussed above, is indicative of the temperature of the traction battery  200 . 
     Step  620  comprises determining a desired coupling state of the second electric traction motor  212  to the at least one wheel (RL, RR) of the second axle  222  in dependence on the speed signal  410 . Step  620  comprises determining whether the speed of the vehicle  100  is equal to or greater than a first high-speed threshold  710  shown in  FIG.  6   . Thus step  620  comprises comparing the speed of the vehicle  100  against one or more thresholds  710 ,  720 , where the one or more thresholds  710 ,  720  comprise the first high-speed threshold  710 . In some embodiments, the one or more thresholds  710 ,  720  comprise a second high-speed threshold  720 . The second high-speed threshold  720  represents a vehicle speed lower than the first high-speed threshold  710 . The first  710  and second  720  high-speed thresholds are illustrated in  FIG.  6   . 
     If the speed of the vehicle  100  is equal to or greater than a first high-speed threshold  710  then the method  600  moves to step  630 . If, however, the speed of the vehicle  100  is less than the first high-speed threshold  710  then the method  600  moves to step  640 . 
     In the example of  FIG.  6   , the method  600  progresses to step  640  prior to time t 1 . Prior to time t 1  as will be appreciated the vehicle  100  is generally accelerating which may be caused by positive torque applied by the first electric traction motor  216  and/or engine  202 , and the second electric traction motor  212  which is coupled to the second torque path via the second axle  222 . 
     In step  630  the desired coupling state is determined as decoupled in dependence on the speed signal  410  being indicative of a vehicle speed equal to or greater than the first high-speed threshold  710 . In step  630  the HSM  510  may output an indication  515  of the desired coupling state of decoupled to the arbitrator  570  indicative of a request to decouple  740  the second electric traction motor  212  from the second axle  222 . The indication  515  of the desired coupling state of decoupled  740  may be referred to as the high-speed coupling state request  515 ,  740 . The arbitrator  570  may in some embodiments arbitrate between multiple requests for desired coupling states as will be explained. In the absence of any other competing requests from other modules, the arbitrator  570  is arranged to output, via the output means  340 , the high-speed coupling state request  515  for the decoupled state  740  as output signal  345 . In some embodiments, the high-speed coupling state request  515  may be provided from the HSM  510  directly to the output means  340  of the controller  305 . 
     After time t 1 , i.e. once the speed of the vehicle  100  exceeds the first high-speed threshold  710 , it has been determined that it is desirable to decouple the second electric traction motor  212 . Continued coupling of the second electric traction motor  212  to the wheel(s) of the vehicle  100  causes the second electric traction motor  212  to exceed a predetermined rotation speed. The predetermined rotation speed may be a motor speed of 12,000 rpm, although it will be appreciated that other predetermined rotation speeds may be selected. The predetermined rotation speed may correspond to a vehicle speed of 140 kmh −1  although it will be appreciated that this depends on a gearing between the second electric traction motor  212  and the wheels of the vehicle  100  and a diameter of the wheels. Furthermore, in some embodiments, the vehicle speed corresponding to the first high speed threshold  710 , and thus the rotation speed of the second electric traction motor  212 , may be determined in dependence on temperature as will be explained with reference to  FIG.  7   . 
     The output means  340  of the controller  305  is arranged to output the coupling signal  345 ,  730 ,  740  indicative of a request to decouple  740  the second electric traction motor  212  from the at least one wheel of the second axle  222  in dependence on the desired coupling state being decoupled. 
     If, in step  620 , the speed of the vehicle  100  is less than the first high speed threshold  710 , the method moves to step  640 . In step  640  it is determined whether the speed of the vehicle  100  is less than or equal to the second high speed threshold  720 . If the speed of the vehicle  100  is less than or equal to the second high speed threshold  720  the method moves to step  660 . 
     In step  660  the HSM  510  is arranged not to request a desired coupling state of the second electric machine  212 . The HSM  510  outputs a request for a coupling state to the arbitrator  570  or may, as illustrated in  FIG.  5   , output a ‘no-request’ signal  730  to the arbitrator  570 , where the no-request signal  730  is indicative of the HSM  510  not requesting a specific coupling state of the second electric traction motor  212  to the one or more wheels of the second axle  222 . Thus, prior to time t 1  in  FIG.  6   , the HSM  510  outputs the no-request signal  730  to the arbitrator  570 , or may output no signal to the arbitrator  570  in other embodiments. The arbitrator  570  may have a default coupling state. The default coupling state may be coupled i.e. for the second electric traction motor  212  to be coupled to the torque path of the second axle  222 . Thus when either a ‘no-request’ signal  730 , or no request signal is received by the arbitrator  570 , the arbitrator  570  may output a determined coupling request via the output means  340 . 
     In some embodiments, the HSM  510  is arranged to output the coupling signal  345 , indicative of a request to couple the second electric traction motor  212  machine to the at least one wheel of the second axle  222 . It will be appreciated that the HSM  510  may, in some embodiments, request the default state of coupled when the speed signal  410  is indicative of a low vehicle speed. 
     In some embodiments, the HSM  510  may apply hysteresis to the speed signal  410  to determine the coupling state. That is, the coupling state of decoupled may be determined for a vehicle speed greater than that at which the second electric traction motor  212  is recoupled to the torque path via the second axle  222  i.e. above the second high-speed threshold  720 . Advantageously this assists in preventing ‘hunting’ or ‘flickering’ between the decoupled and coupled states as the speed of the vehicle varies around (above and below) the first high speed threshold  710 . Use of the second high speed threshold  720  provides the hysteresis in some embodiments. As can be appreciated from  FIG.  6   , between t 1  and prior to time t 2  the vehicle deaccelerates from a peak speed, such that the speed signal  410  drops below the first high speed threshold  710 . As can be appreciated from the lower portion of  FIG.  6   , the ‘no-request’ signal  730  is not output immediately upon the speed of the vehicle  100  falling below the first high-speed threshold  710 . 
     Instead, in a region between the first and second high speed thresholds  710 ,  720  the coupling state of decoupled  740  is maintained until the vehicle speed falls below the second high-speed threshold  720 . In step  650 , which is reached when the vehicle speed is between the first and second high speed thresholds  710 ,  720  the desired coupling state is determined in dependence on the speed signal  410  in dependence on a last intersected of the first and second high-speed thresholds  710 ,  720 . Thus, prior to time t 2  when the speed signal  410  is below the first high-speed threshold  710  the coupling state is determined in step  650  as decoupled based on last-intersecting the first high speed threshold  710 . Thus the method moves to step  630 . Similarly, prior to time t 1 , when the speed signal  410  is above the second high-speed threshold  720 , the method moves to step  660  wherein the ‘no request’ output signal  730  is maintained such that the arbitrator  570  in the example embodiment determines the coupling state as coupled. 
     Thus it can be appreciated that embodiments of the invention select coupling of the second electric traction motor  212  in dependence on the speed of the vehicle  100 . 
       FIG.  7    illustrates motor speed, i.e. speed (RPM) of the second electric traction motor  121 , against temperature according to an embodiment of the invention. Illustrated in  FIG.  7    is the first high speed threshold  710  which, according to some embodiments of the invention varies dependent upon temperature. As described above, in some embodiments of the invention the controller  305  receives the temperature signal  420 . In some embodiments, the first high speed threshold  710  adopts a first value  710  between first and second temperatures  740 ,  750 . The first temperature  740 , below which one or both of the first and second high-speed thresholds  710 ,  720  reduces may correspond to a cold-temperature, such as a temperature below 0° C., such as −5° C., although it will be realised that other temperatures may be selected. It will be appreciated that, although not illustrated, the second high speed threshold  720  may follow the first high speed threshold  710 . 
     Below the first temperature  740 , in some embodiments the first high speed threshold  710  reduces i.e. to value  810 , such that the coupling state of the second electric traction motor  212  is determined as decoupled at a lower speed, as illustrated. In some embodiments, one or both of the first and second high-speed thresholds  710 ,  720  may reduce proportional to temperature during one or more temperature regions. Advantageously, the reduction in the first high speed threshold  710 ,  810  allows for changes in, for example, coolant of the second electric traction motor  212  or a reduced viscosity of fluids associated with the second torque path via the second axle  222 , such that rotation of the motor  212  may consume more energy and therefore lower-speed decoupling is more efficient. In the embodiment shown in  FIG.  7   , the first high speed threshold  710 ,  810  is arranged to decrease in dependence on temperature over a first temperature range  740 ,  730 . The temperature range may be between −10° C. and −20° C., although other temperature ranges may be selected. In other embodiments, the first high speed threshold  710  may reduce instantaneously, however advantageously having a gradual change may be less noticeable to occupants of the vehicle  100 . Below a third temperature  730  the first thigh speed threshold  810  corresponds to a minimum threshold speed  810 . 
     Similarly, in some embodiments, above the second temperature  750  the first thigh speed threshold  710  is arranged to decrease in dependence on temperature over a second temperature range  750 ,  760  to a fourth temperature  760 . Above the fourth temperature  760  the first thigh speed threshold  710  adopts a constant value  820  in some embodiments, which may be different to the minimum threshold speed  810  as shown in  FIG.  7   , although in other embodiments the two speeds  810 ,  820  may be equal. Advantageously the reduction in the first thigh speed threshold  710 ,  820  at higher speeds may reduce cooling issues associated with the second electric traction motor  212 . The temperature  750  may be at least 25° C. or at least 35° C., such as in some embodiments a temperature of between 50° C. and 60° C. 
     As described above, in some embodiments, the controller  305  is arranged to receive the SoC signal  450 . In some embodiments, one or both the first high speed threshold  710  and second high speed threshold  720  is determined in dependence on the SoC of the traction battery  200 . As described above, in some embodiments, the arbitrator  570  may be arranged to achieve the default coupling state of coupled in absence of a request from the HSM  510  for the decoupled state. In this way, the HSM  510  and arbitrator  570  operate to decouple the second electric traction motor  212  when the vehicle  100  speed is above first high speed threshold  710  and coupled when the vehicle  100  speed is below the second high speed threshold  720 . To couple the second electric traction motor  212  to the second axle in some embodiments the second electric traction motor  212  is required to ‘spin-up’ or accelerate from a low, such as zero, rotation speed to generally a speed of rotation of the rear axle  222  before the second clutch  219  can be closed to couple the second electric traction motor  212  to the axle  222 . As can be appreciated, accelerating the second electric traction motor  212  consumes energy from the traction battery  200 . When the vehicle  100  is operative with a traction battery  200  having a low SoC, one or both the first high speed threshold  710  and second high speed threshold  720  may be reduced in dependence on the SoC. Advantageously, by reducing the speed corresponding to one or both the first high speed threshold  710  and second high speed threshold  720 , the second electric traction motor  212  is only required to ‘spin-up’ to a lower rotation speed to recouple to the second axle  222 , thereby requiring less energy consumption when the traction battery  200  has a lower SoC. 
     An embodiment of the low-speed module (LSM)  520  will now be explained with reference to  FIGS.  8  and  9   . The LSM  520  is operatively executable by the processing device  310  to determine a coupling state of the electric machine  212  to the at least one wheel of the second axle  222  in dependence on the speed signal  410  indicative of the speed of the vehicle  100 . In some embodiments, the LSM  510  and arbitrator  570  are arranged to cause the controller  305  to output a coupling signal  345  to control coupling of the electric machine  212  to the at least one wheel of the second axle  222  dependent on the speed signal  410 , as will be explained. As will be explained the LSM  520  is arranged to cause coupling of the electric machine  212  to the at least one wheel of the axle  222  a low-speeds which, advantageously, enables the electric machine  212  to provide motive torque for the vehicle at low speeds, especially from stationary. Furthermore, the LSM  520  is arranged to control the coupling of the electric machine to avoid, or reduce, undesirable characteristics which may be noticeable to an occupant of the vehicle  100  as will be explained. 
       FIG.  9    illustrates a method  1000  according to an embodiment of the invention which may be performed by the LSM  520  executed by the processing device  310  of the controller  305 . The method  1000  will be explained with reference to  FIG.  8    which illustrates a speed of the vehicle  100 , as indicated by the speed signal  410 , over a period of time. Also illustrated in a lower portion of  FIG.  8    is a desired coupling signal  525  output by the LSM  520  which represents a request  730 ,  750  for the desired coupling state from the LSM  520  determined in dependence on the speed signal  410 . 
     The method  1000  comprises a step  1010  of receiving one or more signals, such as data representing the one or more signals, at the LSM  520 . In the illustrated embodiment the LSM  520  is arranged to receive the speed signal  410  indicative of the speed of the vehicle  100 . 
     Step  1020  comprises determining a desired coupling state of the second electric traction motor  212  to the at least one wheel (RL, RR) of the second axle  222  in dependence on the speed signal  410 . Step  1020  comprises determining whether the speed of the vehicle  100  is equal to or less than a first low-speed threshold (LST)  910 . Thus step  1020  comprises comparing the speed of the vehicle  100  against one or more thresholds  910 ,  920 , where the one or more thresholds  910 ,  920  comprise the first LST  910 . In some embodiments, the one or more low-speed thresholds  910 ,  920  comprise a second LST  920 , as shown in  FIG.  8   . The second LST  920  represents a vehicle speed greater than the first LST  910 . The first  910  and second  920  LSTs are illustrated in  FIG.  8   . 
     In step  1030  the desired coupling state is determined as coupled in dependence on the speed signal  410  being indicative of a vehicle speed equal to or below than the first LST  910 . In step  1030  the LSM  520  may output an indication  525  of the desired coupling state of coupled to the arbitrator  570  indicative of a request to couple  750  the second electric traction motor  212  to the second axle  222 . The indication  525  of the desired coupling state of coupled  750  may be referred to as the low-speed coupling state request  525 ,  750 . The arbitrator  570  may in some embodiments arbitrate between multiple requests for desired coupling states. In the absence of any other competing requests from other modules, the arbitrator  570  is arranged to output, via the output means  340 , the low-speed coupling state request  525  for the coupled state  750  as output signal  345 . In some embodiments, the low-speed coupling state request  525 ,  750  may be provided from the LSM  520  directly to the output means  340  of the controller  305 . 
     Referring to  FIG.  8   , after time t 3 , i.e. once the speed of the vehicle  100  is equal to or below the first LST  910 , it has been determined that it is desirable to couple the second electric traction motor  212 . For example, it can be envisaged that the vehicle  100  is about to stop and that torque from the second electric traction motor  212  will be useful e.g. for a standing start. A predetermined vehicle speed corresponding to the first LST  910  may be a vehicle speed of 10 kmh −1  although it will be appreciated that other vehicle speeds may be selected. In some embodiments, the vehicle speed corresponding to the first LST  910  may be selected or determined based on a deacceleration rate of the vehicle  100 , which may be determined based on a rate of change of the speed signal  410 . In the presence of a large deceleration i.e. above a deacceleration threshold the vehicle speed corresponding to the first LST  910  may be increased to advantageously allow for coupling of the second electric traction motor  212  prior to the vehicle  100  stopping. 
     The output means  340  of the controller  305  is arranged to output the coupling signal  345 ,  750  indicative of a request to couple  750  the second electric traction motor  212  to the at least one wheel of the second axle  222  in dependence on the desired coupling state being coupled, as in step  1030 . 
     In some instances, due to a default state being coupled as shown in Table 1 below, the request to couple  750  shown in  FIG.  8    output as a result of the speed of the vehicle dropping through the LST  910  will have no practical effect (change in state) as the second electric traction motor  212  will already be coupled to the second axle  222  as a result of the default state being coupled. However, in some instances, the second electric traction motor  212  will be decoupled from the second axle  222  when the speed of the vehicle drops through the LST  910 . In such situations, the arbitrator  570  may determine an arbitrated coupling state with respect to the LTS  910  in dependence on a reason why the second electric traction motor  212  is disconnected. If the arbitrated coupling state is decoupled whilst the vehicle speed is above the LST  910  for a high priority reason, such as a fault, then the arbitrator  570  will not change the arbitrated coupling state to coupled responsive to the request to couple  750  from the LSM  520 . However, if the reason for the decoupled state is lower priority, such as a preferential reason, the arbitrator  570  may change the arbitrated coupling state to coupled responsive to the request to couple  750  from the LSM  520 . 
     If, in step  1020 , if the speed of the vehicle  100  is greater than the first LST  910 , the method moves to step  1040 . In step  1040  it is determined whether the speed of the vehicle  100  is greater than or equal to the second LST  920 . If the speed of the vehicle is greater than or equal to the second LST  920  the method moves to step  1060 . 
     In step  1060  the LSM  520  is arranged not to request a desired coupling state of the second electric machine  212 . The LSM  520  outputs a request for a coupling state to the arbitrator  570  and may, as illustrated in  FIG.  8   , output a ‘no-request’ signal  730  to the arbitrator  570 , where the no-request signal  730  is indicative of the LSM  520  not requesting a specific coupling state of the second electric traction motor  212  to the one or more wheels of the second axle  222 . Thus, prior to time t 3  in  FIG.  8   , the LSM  520  outputs the no-request signal  730  to the arbitrator  570 , or may output no signal to the arbitrator  570  in other embodiments. The arbitrator  570  may have a default coupling state. The default coupling state may be coupled i.e. for the second electric traction motor  212  to be coupled to the torque path of the second axle  222 . Thus when either a ‘no-request’ signal  730 , or no request signal is received by the arbitrator  570 , the arbitrator  570  may output a determined coupling request via the output means  340 . 
     In some embodiments, the LSM  520  is arranged to output the coupling signal  345 , indicative of a request to couple the second electric traction motor  212  to the at least one wheel of the second axle  222 . It will be appreciated that the LSM  520  may, in some embodiments, request the default state of coupled when the speed signal  410  is indicative of a low vehicle speed i.e. below the first LST  910 . 
     In some embodiments, the LSM  520  may apply hysteresis to the speed signal  410  to determine the coupling state. That is, the coupling state of coupled may be determined for a vehicle speed greater than that at which the second electric traction motor  212  is determined to be coupled to the torque path via the second axle  222  i.e. above the first LST  910 . Advantageously this assists in preventing ‘hunting’ or ‘flickering’ between decoupled and coupled states as the speed of the vehicle varies around (above and below) the first LST  910 . Use of the second LST  920  provides the hysteresis in some embodiments. As can be appreciated from  FIG.  8   , between t 3  and prior to time t 4  the vehicle accelerates from a minimum speed, such that the speed signal  410  exceed the first LST  910  for a period of time prior to time t 4 . As can be appreciated from the lower portion of  FIG.  8   , the ‘no-request’ signal  730  is not output immediately upon the speed of the vehicle  100  exceeding the first LST  910  i.e. coupled  750  is maintained. 
     Instead, in a region between the first and second LSTs  910 ,  920  the coupling state of coupled  750  is maintained until the vehicle speed falls exceeds the second LST  920  at time t 4 . In step  1050 , which is reached when the vehicle speed is between the first and second LSTs  910 ,  920  the desired coupling state is determined in dependence on the speed signal  410  in dependence on a last intersected of the first and second LSTs  910 ,  920 . Thus, prior to time t 4  when the speed signal  410  is below the second LST  920  the coupling state is determined in step  1050  as coupled based on last-intersecting the first LST  910 . Similarly, immediately prior to time t 3 , when the speed signal  410  is above the first LST  920 , the ‘no request’ output signal  730  is maintained as the last intersected threshold is the second LST  920 . 
     As can be appreciated from  FIG.  8   , some embodiments of the LSM  520  comprise a third LST  930 . The coupling of the motor  212  is inhibited if not successfully coupled to the second torque path via the second axle  222  when the vehicle speed  410  is equal to or less than the third LST  930 . The third LST  930  may correspond to a speed of, for example, 5 kmh −1  although it will be appreciated that other speeds may be selected. 
     In some embodiments, the LSM  520  is arranged to receive a signal indicative of a coupling status  470  of the second electric traction motor  212  to the at least one wheel of the axle  222 . The signal  470  reports whether the second electric traction motor  212  is successfully coupled to the at least one wheel of the axle  222 . In some situations, the coupling state may be determined as coupled and a corresponding request output by the controller  305 . However for electrical and/or mechanical reasons it may not be possible, at least immediately, to couple the motor  212  to the second torque path. For example, the second clutch  219  may not have yet successfully engaged a drive output of the motor  212  to the axle  222 . In particular, it may be difficult to successfully couple the motor  212  when the vehicle is moving slowly or has become stationary. Furthermore, attempted coupling of the motor  212  to the axle may be increasingly noticeable, such as in the form of noise and/or vibration, to occupants of the vehicle  100  at slow speeds and may possibly cause damage if attempted whilst stationary. Use of the third LST  930  reduces such risks. 
     The LSM  520  in some embodiments determines a coupling inhibited state. The LSM  520  in some embodiments outputs a coupling inhibit signal  526  in the coupling inhibited state when the speed signal  410  is indicative of a vehicle speed equal to or below the third LST  930 . The LSM  520  may output the coupling inhibit signal  526  when the vehicle speed is below the third LST  930  and the coupling status signal  470  is indicative of the second electric traction motor  212  being decoupled from the second axle  222  i.e. successful coupling caused by the vehicle speed being below the first LST  910  has not yet occurred. 
     In some embodiments, the LSM  520  may apply hysteresis to the speed signal  410  to determine the coupling inhibited state. That is, the coupling inhibited state may be determined for a vehicle speed greater than the third LST  930 . Advantageously this assists in preventing ‘hunting’ or ‘flickering’ between the decoupled and coupled states as the speed of the vehicle varies around (above and below) the third LST  930 . Use of a fourth LST  950 , as shown in  FIG.  9    provides the hysteresis in some embodiments. The fourth LST  950  defines a maximum speed of a coupling inhibition region  940  defining the coupling inhibited state. The third and fourth LSTs  930 ,  950  act as described above with respect to the first and second LSTs  910 ,  92  and the speed signal  410 . 
     Some embodiments of the invention comprise a fault management module (FMM)  530 . The FMM  530  is arranged to determine a desired coupling state of the second electric traction motor  212  to the at least one wheel (RL, RR) of the second axle  222  in dependence on detection or determination of one or more faults associated with the vehicle  100 . The coupling state determined by the FMM  530  is selected to manage or mitigate faults associated with the vehicle  100 . For example, the FMM  530  may receive the temperature signal  420 , wherein the temperature signal  420  is indicative of an invertor temperature associated with the second electric traction motor  212 . In the event of the temperature signal  420  indicating that the invertor has a high temperature (above a predetermined threshold), the FMM  530  is arranged to determine the coupling state as decoupled in order to allow the second electric traction motor  212  to be inactive thereby allowing the invertor to cool for a period of time. In another example, the FMM  530  is arranged to receive the coupling status signal  470  discussed above. The coupling status signal  470  may be indicative of a failure to decouple the second electric traction motor  212  to the axle. Therefore the FMM  530  may determine the coupling state as coupled in dependence thereon to reduce problems associated with the problematic decoupled state. The FMM  530  is arranged to output a fault-derived coupling state request (FDCSR) signal  535  in dependence on one more received signals indicative of fault state associated with the vehicle  100 . The FDCSR signal  535  is indicative of a coupling state request determined by the FMM  530  in response to one or more faults or undesirable conditions or parameters associated with the vehicle. The FDCSR signal  535  is received by the arbitrator  570  in some embodiments as shown in  FIG.  4   . 
     In some embodiments, the FMM  530  is arranged to manage retries, i.e. further attempts, of changes in the coupling state of the second electric traction motor  212  in the presence of a failure to successfully change the coupling state. In particular, in some embodiments, the FMM  530  is arranged to control the output means  340  of the controller  305  to output a signal  345  indicative of a retry, i.e. to request a further attempt, of a change in the coupling state of the second electric traction motor  212  as will be explained. 
       FIG.  11    illustrates a method  1200  according to an embodiment of the invention. The method  1200  is a method of managing retries of a change in coupling state of the second electric traction motor  212 . 
     In step  1210  the coupling state of the second electric traction motor  212  is determined. The coupling state may be determined by one of the modules  510 - 560  and a consequent coupling state request signal received at the arbitrator  570 , or by the arbitrator  570  such as in the case of the default coupling state in the absence of any requests from the modules  510 - 560 . 
     In step  1220 , a coupling state request signal  345  is output from the controller  305  via the output means  340  to request the determined coupling state. For example, the coupling state request may be a request for one of a coupled or decoupled state of the second electric traction motor  212  to the second axle  222 . 
     In step  1230  the FMM  530  is arranged to determine whether a failure to change the coupling state of the second electric traction motor  212  to the second axle  222  has occurred. As discussed above, the coupling status signal  470  is indicative of the actual coupling status of the second electric traction motor  212  to the one or both wheels of the second axle  222 . Therefore, the FMM  530  is able to determine, in dependence on the coupling status signal  470 , whether the failure has occurred i.e. whether the actual coupling state reflects the requested coupling state. Step  1230  may be performed after a delay to allow a change in coupling state to be implemented, such as the second clutch  219  being opened or closed. If the change in coupling state is successful the method returns to step  1210 . If, however, the change was not successful i.e. a failure to change the coupling state of the second electric traction motor  212  has occurred as indicated by the coupling status signal  470 , the method moves to step  1240 . 
     In step  1240  a speed of the vehicle  100  is determined. Step  1240  comprises receiving the speed signal  410  indicative of the speed of the vehicle  100 . Controlling the output means  340  of the controller  305  to output the coupling signal  345  indicative of a retry of the change in the coupling state is performed in dependence on the speed signal  410  as will be explained. 
     In some embodiments, the FMM  530  is arranged to defer controlling the output means  340  to output the coupling signal  345  indicative of the retry of the change in the coupling state in dependence on the speed signal  410  being indicative of the speed of the vehicle  100  being at least a predetermined minimum speed. The predetermined minimum speed may be, for example, a speed greater than substantially 0 kmh −1 . Other predetermined minimum speeds may be, for example, 5 kmh −1  although it will be appreciated that other minimum speeds may be selected. Advantageously, preventing a retry of the change in coupling state, particularly from changes from decoupled to coupled, at to too low a vehicle speed may prevent the retry of the engagement of the second electric traction motor  212  with the axle being noticeable to occupants of the vehicle  100 . For example, such as (although not exclusively) where the second clutch  219  is a dog clutch, attempting the retry may cause noise and/or vibration at low vehicle speeds. 
     In some embodiments, the FMM  530  is arranged to defer controlling the output means  345  to output the coupling signal  345  indicative of the retry of the change in the coupling state in dependence on the speed signal being indicative of the speed of the vehicle  100  being less than a maximum speed. The maximum speed may be, for example up to 50 kmh −1  or up to 30 kmh −1  or up to 20 kmh −1  although other maximum speeds may be chosen. As noted above, in order to couple the second electric traction motor  212  to the second axle  222  it may be necessary to ‘spin-up’ or accelerate the motor  212  to approximately the rotation speed of the axle  222 . Advantageously the maximum speed prevents or reduces energy used in coupling the motor  212  to the axle  222 . Furthermore, changing from the decoupled to the coupled state at vehicle speed below the maximum speed may avoid attempting to couple the second electric traction motor  212  to the axle during periods of large deacceleration i.e. during heavy braking or other slowing of the vehicle  100  when it may be difficult to match the rotation speed of the second electric traction motor  212  to the axle  222 . Thus the FMM  530  defers controlling the output means  340  to output the coupling signal  345  indicative of the retry of the change in the coupling state in dependence on the speed signal  410  being indicative of the speed of the vehicle being less than or equal to the predetermined maximum speed. 
     In step  1250  the FMM  530 , when it is determined that the speed of the vehicle  100  is either above the minimum speed or above the minimum speed and below the maximum speed considered in step  1240 , the FMM  530  is arranged to output a signal  535  indicative of a request to retry the change of coupling state. The signal  535  may be a further request for the change in coupling state such as a request for one or the coupled or decoupled state. The request may be received by the arbitrator  570  which outputs a corresponding request or signal  345  via the output means  340  to cause the retry of the change in coupling state. Once the retry of the change has been requested the method returns to step  1230 , where it is considered whether the retry has been successful. 
     In some embodiments, for every iteration of step  1250  a counter is maintained to track a number of retries of the change in coupling state. The FMM  350  in some embodiments is arranged to attempt the retry up to a predetermined maximum number of times. That is, to perform step  1250  up to the maximum number of times. The maximum number of times may be 5, 3 or 2 in some embodiments. Advantageously the maximum number of retries may prevent excessive numbers of retries to avoid damaging the system  300 , and/or reduces energy wasted ‘spinning-up’ the second electric traction motor  212  to attempt further retries. 
     Some embodiments of the invention comprise an anti-fussiness module (AFM)  540 . The AFM  540  is arranged to control changes in coupling state of the second electric traction motor  212 . In particular, the AFM  540  is arranged to control a timing of changes in the coupling state of the second electric traction motor  212 . The AFM  540  may ensure that changes in the coupling state of the second electric traction motor  212  do not occur too frequently i.e. that at least a predetermined period of time is provided between changes in coupling state of the second electric traction motor  212 . The AFM  540  is illustrated in  FIG.  4    as forming part of the arbitrator  570 . It will, however, be realised that the AFM  540  may be located elsewhere i.e. that other structures may be envisaged. 
       FIG.  12    illustrates a method  1300  according to an embodiment of the invention. The method  1300  is a method of controlling changes in coupling state of the second electric traction motor  212  according to an embodiment of the invention. The method  1300  may be performed by the AF module  540 . 
     In step  1310  of the method a coupling state of the second electric traction motor  212  to the second torque path via the second axle  222  is determined. In other words, step  1310  comprises determining whether the second electric traction motor  212  is coupled to one or more wheels (RR, RL) of the second axle  222  of the vehicle  100 . The determination is performed in dependence on at least one attribute signal, such as the speed signal  410  indicative of the speed of the vehicle  100 , or the driving mode signal  440 . As described above, the coupling state of the second electric traction motor  212  may be determined by one of modules  510 ,  520 ,  530 ,  550 ,  560  and a corresponding signal or request provided to the arbitrator  570 . For example, the HSM  510  may provide a request to decouple the second electric traction motor  212  from the rear axle  222 , whilst the FMM  530  may provide a request to couple the second electric traction motor  212  to the rear axle  222 . Thus requests for various coupling states may originate from different modules. Advantageously the AF module  540  is arranged to prevent frequent changes in coupling state of the second electric traction motor  212  in order to avoid such changes being noticeable to occupants of the vehicle  100 . Step  1310  may comprise one or more requests for a coupling state being received at the arbitrator  570  and, in particular, the AFM  540 . 
     Step  1320  comprises determining whether a predetermined period of time has elapsed since a last, or most recent previous, change in coupling state of the second electric traction motor  212 . The predetermined period of time may be a period of time since a last request for a change in coupling state was output by the controller  305 , or since a successful change in coupling state reported by the coupling status signal  470 . The predetermined period of time may be, for example, at least 5 second, at least 10 seconds, at least 20 seconds or at least 30 seconds. It will be appreciated that other periods of time may be envisaged. If the predetermined period of time has elapsed the method  1300  moves to step  1340 . 
     If the predetermined period of time has not elapsed, the method moves to step  1330  where the AFM  540  is arranged to wait i.e. to defer controlling the output means  340  of the controller  305  to output the coupling signal  345  indicative of the requested change in the coupling state until expiry of the predetermined period of time since the last change in the coupling state. The AFM  540  may buffer incoming or received coupling state requests from the modules  510 ,  520 ,  530 ,  550 ,  560  until expiry of the predetermined period of time, as it will be appreciated that the desired coupling state may be continuously re-evaluated during the predetermined period of time. Thus upon expiry of the predetermined period of time the coupling state may be determined based upon most recently-received coupling state requests rather than implementing a first-buffered request. Advantageously this ensures that the requested coupling state upon expiry of the predetermined period of time reflects most recent attributes of the vehicle  100 . Upon expiry of the predetermined period of time the method moves to step  1340 . 
     In step  1340  the AF module  540  is arranged to control the output means  340  of the controller  305  to output the coupling request signal  345  to control coupling of the second electric traction motor  212  to the rear axle  222 . In some embodiments, the inhibit module is provided with a signal  575  indicative of an arbitrated coupling request, as will be explained. 
     Some embodiments of the invention comprise an inhibit module  550 . The inhibit module  550  is arranged to control changes in coupling state of the second electric traction motor  212 . In particular, the inhibit module  550  is arranged to allow for inhibition of one or more coupling states of the second electric traction motor  212  to the rear axle  222 . The inhibition of a coupling state prevents the inhibited coupling state being requested by the controller  305 . The inhibit module  550  is illustrated in  FIG.  4    as forming part of the arbitrator  570 . It will, however, be realised that the inhibit module  550  may be located elsewhere i.e. that other structures may be envisaged. 
     The inhibit module  550  is arranged to receive the inhibit signal  460 . The inhibit signal is indicative of one or more coupling states of the second electric traction motor  212  to the rear axle  222  which are prohibited or inhibited. The inhibit signal  460  may be indicative or one of the coupled and decoupled states of the second electric traction motor  212  to the rear axle  222 . Whilst the inhibit signal  460  is shown as one signal it will be appreciated that in other embodiments a respective signal may be provided for each of the coupled and decoupled coupling states to indicate whether each state is inhibited. The inhibit module is arranged to output a coupling state inhibit signal  555  to the arbitrator which is indicative of a request for a coupling state as described below. In particular, the coupling state inhibit signal  555  is indicative of a request for a coupling state when that coupling state is not inhibited, thereby further indicating which coupling states are not inhibited. 
       FIG.  13    illustrates a method  1400  according to an embodiment of the invention. The method  1400  is a method of controlling changes in coupling state of the second electric traction motor  212  according to an embodiment of the invention. The method  1400  may be performed by the inhibit module  550 . 
     In step  1410  of the method a coupling state of the second electric traction motor  212  to the second torque path via the second axle  222  is determined. In other words, step  1410  comprises determining whether the second electric traction motor  212  is coupled to one or more wheels (RR, RL) of the second axle  222  of the vehicle  100 . The determination may be performed in dependence on a determination of an expected amount of power required to spin-up the second electric traction motor  212  to the speed of the rear axle as compared to an amount of power available from the traction battery  200 . As described above, the coupling state of the second electric traction motor  212  may be determined by one of modules  510 ,  520 ,  530 ,  560  and a corresponding signal or request provided to the arbitrator  570 . For example, the HSM  510  may provide a request to decouple the second electric traction motor  212  from the rear axle  222 , whilst the FMM  530  may provide a request to couple the second electric traction motor  212  to the rear axle  222 . Thus requests for various coupling states may originate from different modules. Advantageously the inhibit module  550  is arranged to prevent a coupling state of the second electric traction motor  212  being selected, such as in order to avoid a state associated with a fault. For example, when it is determined that a fault exists which prevents the second electric traction motor  212  from coupling to the rear axle  222 , the inhibit module  550  may inhibit the coupled state to avoid the coupled state being selected. Similarly, in some embodiments, one or more coupling states may be inhibited dependent upon one or more of a power limit or capability of the traction battery  200 . For example, if it is determined that the capability of the traction battery  200  to provide sufficient power to spin up the second electric traction motor  212  for coupling to the rear axle  222 , the coupled state may be inhibited in step  1410 . 
     Step  1410  may comprise one or more requests for a coupling state being received at the arbitrator  570  and, in particular, the inhibit module  550 . As explained below, the arbitrator  570  may determine an arbitrated coupling state in dependence on the received requests. 
     In step  1420  it is determined whether the determined coupling state is inhibited. The determined coupling state may be the arbitrated coupling state determined by the arbitrator  570 . Step  1420  comprises comparing the determined coupling state against the one or more inhibited coupling states, such as where the coupled state is indicated as inhibited by the inhibit signal  460 . Where the determined coupling state and the coupling state indicated by the inhibit signal differ, or no coupling state is indicated as inhibited, the method moves to step  1430 . If, however, the determined coupling state is indicated as inhibited by the inhibit signal  460  the method returns to step  1410 . In other words, the method  1400  prevents a request for an inhibited coupling state being output in step  1430 . 
     In step  1430  the inhibit module  550  is arranged to control the output means  340  of the controller  305  to output the coupling request signal  345  to control coupling of the second electric traction motor  212  to the rear axle  222 . That is, when the determined coupling state is not indicated as inhibited by the inhibit signal  460  a request for the determined coupling state is output by the controller  305 . 
     Some embodiments of the invention comprise a driving mode module (DMM)  560 . The DMM  560  is arranged to determine a coupling state of the second electric traction motor  212  in dependence on a driving mode of the vehicle  100 . The driving mode of the vehicle  100  is indicated by the driving mode signal  440 . The driving mode of the vehicle  100  may be selected by a driver or occupant of the vehicle  100 , or may at least in part be determined by a module or system of the vehicle  100 , such as a terrain-response (TR) module which adaptively selects a driving mode including one or more settings of the vehicle and, in particular, a powertrain thereof such as a traction control mode thereof, for example. The driving mode may include driving selected settings, such as of the powertrain, including a driving mode of the vehicle including one of forward, reverse or neutral in the case of an automatic gearbox or a gear selection of a manual gearbox. The driving mode may include a selection of one of sport, normal or economy driving modes where settings of one or more of the engine, first and/or second electric motors, suspension etc of the vehicle  100  may be adapted accordingly. Data indicative of the selected driving mode(s) is provided by the driving mode signal. 
       FIG.  14    illustrates a method  1500  according to an embodiment of the invention. The method  1300  is a method is a method of controlling changes in coupling state of the second electric traction motor  212  according to an embodiment of the invention. Some of the steps of the method  1500  may be performed by the DMM  560 . 
     In step  1510  an attribute-based coupling state of the second electric traction motor  212  to the second axle  222  is determined. The determination in step  1510  is performed in dependence on at least one attribute signal, such as the speed signal  410  indicative of the speed of the vehicle. As described above, the coupling state of the second electric traction motor  212  may be determined by one of modules  510 ,  520 ,  530 ,  560  and a corresponding signal or request provided to the arbitrator  570 . For example, the HSM  510  may provide a request to decouple the second electric traction motor  212  from the rear axle  222 , whilst the FMM  530  may provide a request to couple the second electric traction motor  212  to the rear axle  222 . Thus requests for various coupling states may originate from different modules. Step  1510  may be performed by one of more the HSM  510 , the LSM  520 , and FMM  530 . Step  1510  may be performed in dependence on signals  515 ,  525 ,  535  except for the driving mode signal  440 . One of more signals indicative of the determined coupling states is provided to the arbitrator  570 . The one or more coupling states determined in step  1510  may be together referred to as first coupling states of the second electric traction motor  212 . 
     In step  1520  a driving-mode-based coupling state of the second electric traction motor  212  to the second axle  222  is determined. Step  1520  is determined in dependence on the driving mode signal  440 . 
     In one example, the driving mode signal  440  may indicate a selected driving mode of the vehicle including selection of an efficiency-based driving mode. The efficiency-based driving mode is selected in order to provide improved efficiency of the vehicle  100 , i.e. reduced energy consumption, such as the expense of performance of the vehicle  100 . The efficiency may be to improve consumption of fuel provided to the engine  202  or to conserve electrical power consumed the motors  212 ,  216 . The driving mode signal  440  is indicative of the selection of the efficiency based driving mode which may be manually or automatically selected. Similarly, in another example, the driving mode signal may be indicative of a neutral gear of the vehicle  100  being selected. 
     In dependence on the driving mode signal  440  the DMM  560  is arranged to determine the coupling state of the second electric traction motor  212  to the rear axle  222 , such as one of coupled and decoupled. A signal  565  indicative of the driving-mode-based coupling state is provided to the arbitrator  570 . The driving-mode-based coupling state may be referred to as a second coupling state of the second electric traction motor  212 . Thus the coupling state determined in step  1520  may be decoupled. 
     In another example, the driving mode signal  440  may indicate either a driver-selected or automatically-selected, such as by the terrain response module, driving mode such as requesting four-wheel drive of the vehicle  100  which requires coupling of the second electric traction motor  212  to provide power to the rear axle  222 . Thus the coupling state may be determined as coupled in step  1520 . 
     In step  1530  it is determined whether the first and second coupling states are the same i.e. equal. That is, whether the first coupling state as one of coupled or decoupled is equal to the second coupling state as one of coupled or decoupled. If the first and second coupling states are equal then, method moves to step  1540 . If, however, the first and second coupling states differ then the method moves to step  1550 . 
     In step  1540 , the output means  340  is controlled to output the coupling signal  345  indicative of the first and second coupling states i.e. one of coupled or decoupled. 
     In step  1550 , the output means  340  is controlled to output the coupling signal  345  indicative of the first coupling state i.e. the attribute-based coupling state when the determined first and second coupling states differ. That is, the arbitrator  570  is arranged to allocate a higher priority to the first coupling state than the second coupling state. This is reflected in Table 1 below, as will be explained, by the efficiency column being right-most such that coupling states determined e.g. by the HSM  510  etc take precedence. Only when no requests are received from the other modules does arbitrated coupling state independently follow the coupling state determined by the DMM  560 . 
       FIG.  15    illustrates a coupling state determined by the DMM  560  according to some embodiments of the invention. In some embodiments, the DMM  560  is arranged to determine the coupling state of the second electric traction motor  212  in dependence on the driving mode signal  440  being indicative of a mode, or gear selection, of the powertrain in particular a gearbox thereof, such as one of drive (D), neutral (N) and Reverse (R) i.e. a shifter position. As can be appreciated, the DMM  560  is arranged to not to request  1630  a coupling state  1630  when the powertrain is not in neutral i.e. one of D or R is selected, or a gear of the gearbox is selected. In such a state the DMM  560  may output the no-request NR signal. However, when N is selected  1620 , as indicated by the driving mode signal  440 , the DMM  560  is arranged to output the coupling signal  565  to request the decoupled state  1640 . Thus the second electric traction motor  212  is requested to be decoupled when N is selected. 
     As described above, some embodiments of the present invention comprise the arbitrator  570 . The arbitrator  570  is arranged to receive one or more requests for coupling states of the second electric traction motor  212  and to determine an overall or arbitrated coupling state of the second electric traction motor  212  to the second axle  222 . The arbitrator  570  is arranged to control the output means  340  of the controller  305  to output the coupling signal  345  indicative thereof. The arbitrator  570  is arranged to allocate a predetermined precedence or priority to at least some of the requests for coupling states from different modules. Table 1 below identifies requests for coupling state requests received from the various modules of the controller  305 , a default coupling state i.e. in the absence of any other requests, and a determined coupling state of the arbitrator  570 . 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 FMM 
                 HSM 
                 LSM 
                 Neutral 
                 AWD 
                 AF 
                 Efficiency 
                 De- 
                 Arbitrated 
               
               
                 530 
                 510 
                 520 
                 560 
                 560 
                 540 
                 560 
                 fault 
                 570 
               
               
                   
               
             
            
               
                 D 
                 X 
                 X 
                 X 
                 X 
                 X 
                 X 
                 X 
                 D 
               
               
                 NR 
                 D 
                 X 
                 X 
                 X 
                 X 
                 X 
                 X 
                 D 
               
               
                 C 
                 D 
                 X 
                 X 
                 X 
                 X 
                 X 
                 X 
                 D 
               
               
                 C 
                 NR 
                 X 
                 X 
                 X 
                 X 
                 X 
                 X 
                 C 
               
               
                 NR 
                 NR 
                 C 
                 X 
                 X 
                 X 
                 X 
                 X 
                 C 
               
               
                 NR 
                 NR 
                 NR 
                 D 
                 X 
                 X 
                 X 
                 X 
                 D 
               
               
                 NR 
                 NR 
                 NR 
                 NR 
                 C 
                 X 
                 X 
                 X 
                 C 
               
               
                 NR 
                 NR 
                 NR 
                 NR 
                 NR 
                 NR 
                 C 
                 X 
                 C 
               
               
                 NR 
                 NR 
                 NR 
                 NR 
                 NR 
                 NR 
                 D 
                 X 
                 D 
               
               
                 NR 
                 NR 
                 NR 
                 NR 
                 NR 
                 NR 
                 NR 
                 C 
                 C 
               
               
                   
               
               
                 C = Coupled, 
               
               
                 D = Decoupled, 
               
               
                 NR = No Request, 
               
               
                 X = Don&#39;t Care. 
               
            
           
         
       
     
     The arbitrator  570  is arranged to receive the FDCSR signal  535  from the FMM  530  at an input means thereof. It can be appreciated that the arbitrator  570  also receives a plurality of further coupling state request signals  515 ,  525 ,  565 , i.e. from each of modules  510 ,  520 ,  560 . Each coupling state request signal is indicative of a request for a coupling state of the second electric traction motor  212  to the at least one wheel of the second axle  222 . 
     Referring to  FIG.  10   , which illustrates a method of determining the coupling state in the presence of an FDCSR  535  from the FMM  530 . The arbitrator  570  is arranged to determine an arbitrated coupling state of the second electric traction motor  212  to the at least one wheel of the second axle  222  in dependence on the FDCSR signal  535  and the at least one further coupling state request signal  515 ,  525 ,  565 . The arbitrator  570  is arranged to determine the arbitrated coupling state of the second electric traction motor  212  in precedence on the FDCSR signal  535  over the at least one further coupling state request signal  515 ,  525 ,  565 . 
     In  FIG.  10   , in step  1110  the arbitrator  570  is arranged to receive the FDCSR signal  535  from the FMM  530 . The FDCSR signal  535  is indicative of a coupling state request as explained above. For example, the FDCSR signal  535  is indicative of a request for one of a coupled or decoupled state as indicated in Table 1. 
     In step  1120  the arbitrator  570  is arranged to receive any other coupling state request signals i.e. from modules  510 ,  520 ,  525 ,  525 ,  565 ,  560 . It will be appreciated, as contemplated by Table 1, that at some points in time no other coupling state request are received at the same time as the FDCSR  535 . 
     In step  1130 , a coupling state of the second electric traction motor  212  is determined in dependence on the FDCSR  535  and any other received coupling state requests. As can be appreciated from Table 1 above, when the FDCSR signal  535  is indicative of the decoupled state (D), the arbitrator  570  is arranged to determine the arbitrated coupling state as decoupled irrespective of a state of the further coupling state request signals  515 ,  525 ,  565 . Thus the arbitrator  570  is arranged to determine the coupling state of the electric machine  212  in precedence on the FDCSR signal  535  over the any further coupling state request signals. In particular, the arbitrator  570  is arranged to determine the decoupled state of the second electric traction motor  212  in precedence on the FDCSR signal  535  being indicative of a request to decouple the second electric traction motor  212  over any further coupling state requests. 
     When the arbitrator  570  receives the high-speed coupling state request  515 , HSCSR, signal from the HSM  510  which is indicative of a request to disconnect (D) the second electric traction motor  212  from the second axle  222 , as can be appreciated from Table 1, when no FDCSR  535  is received (NR) or when the FDCSR signal  535  is indicative of a coupled (C) request, the arbitrator  570  determines the arbitrated coupling state of the second electric traction motor  212  as decoupled in dependence on the request from the HSM  510  to, advantageously, protect the second electric traction motor  212  from excessive rotation speed. Thus, the decoupled request from the HSM  510  takes precedence over the FDCSR  535  when in contradiction. 
     In step  1140 , the arbitrator  570  is arranged to output an arbitrated coupling request signal  575  indicative of the arbitrated coupling state to control coupling of the second electric traction motor  212  to the at least one wheel (RL, RR) of the second axle  222 . The arbitrated coupling request signal  575  is output via the output means  340  of the controller  305  to control the coupling of the second electric traction motor  212 . 
       FIG.  16    illustrates an overall operation of the system  300 . Trace  1701  represents an arbitrated coupling state request output by the controller  305  as signal  345 . Trace  1702  represents an actual coupling state of the second electric traction motor  212  to the at least one wheel (RL, RR) of the second axle  222 . Trace  1703  is a connection inhibit signal and trace  1704  is disconnection or decoupled state inhibit signal. 
     As can be appreciated, during period  1710  the DMM  560  determines the coupling state is decoupled such as based on the driving mode signal  440  being indicative of the efficiency-based driving mode. The coupling state of coupled being inhibited, as indicated by  1703 , does not have an effect since the arbitrated coupling state is decoupled. During period  1720  the DMM  560  determines the coupling state as coupled based on the IDD driving mode indicated by the driving mode signal  440 . However, the connection inhibit signal  1703  indicates that the coupled state is inhibited, thereby the actual state of the coupling is decoupled i.e. the coupling inhibited state takes precedence over the coupled state requested by the DMM  560 . However, during period  1730  once the coupled state inhibit signal  1703  indicates that the coupled state is not inhibited, the coupled state is achieved. During period  1740  the coupled state is requested as a default coupling state of the arbitrator  570 , although partially during period  1740  the decoupled state is inhibited as shown by trace  1704  although this does not affect the coupling state during period  1740  as coupled is still requested by the arbitrator  570 . However, during period  1750  when the decoupled state requested by the HSM  510 , due to decoupled still being inhibited, the coupled state is maintained. Once the inhibition is cancelled during period  1760  the coupling state of decoupled is requested by the arbitrator  570  corresponding to the requested state of the HSM  510 . During period  1760  the LSM  520  requests the coupled state. 
     It will be appreciated that embodiments of the present invention can be realised in the form of hardware, software or a combination of hardware and software. Any such software may be stored in the form of volatile or non-volatile storage such as, for example, a storage device like a ROM, whether erasable or rewritable or not, or in the form of memory such as, for example, RAM, memory chips, device or integrated circuits or on an optically or magnetically readable medium such as, for example, a CD, DVD, magnetic disk or magnetic tape. It will be appreciated that the storage devices and storage media are embodiments of machine-readable storage that are suitable for storing a program or programs that, when executed, implement embodiments of the present invention. Accordingly, embodiments provide a program comprising code for implementing a system or method as claimed in any preceding claim and a machine readable storage storing such a program. Still further, embodiments of the present invention may be conveyed electronically via any medium such as a communication signal carried over a wired or wireless connection and embodiments suitably encompass the same. 
     All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. 
     Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. 
     The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. The claims should not be construed to cover merely the foregoing embodiments, but also any embodiments which fall within the scope of the claims.