Control system for a vehicle powertrain

Abstract

A control system 10 for controlling a powertrain system 120 of a vehicle 100 with first and second electric traction motors 122, 124. The control system is configured to receive inputs including a veh

Description

TECHNICAL FIELD The present disclosure relates to a control system for a powertrain system of a vehicle. Aspects of the invention relate to a control system, to a system, a vehicle, a method, and to computer readable instructions. BACKGROUND It is known for some vehicles with a powertrain having one or more electric traction motors, for example hybrid vehicles, Plug-in Hybrid Electric Vehicles (PHEV), Battery Electric Vehicles (BEV), and Fuel Cell Electric Vehicles (FCEV), to be controlled such that the electric traction motor(s) can be operated to generate both accelerative (“positive”) torque and decelerative (“negative”) torque as desired in dependence on a powertrain torque demand. This enables the vehicle to be slowed and / or brought to a stop using electric traction motor(s) either alone or in combination with the foundation brakes. However, when the torque demand changes direction, it can be difficult to accurately control the torque response due to lash (or “backlash”) crossing in torque transmitting components of the powertrain, such as the motors themselves, when the torque changes direction. This can be exacerbated at low speeds at which the motor response may be deliberately reduced and smoothed for driveability reasons by the motor mapping. If a driver were to apply a brief “tip-in” to the accelerator pedal to move the vehicle a small distance then lift or “tip-out” after the vehicle had started to move, the lash crossing could cause inaccuracy in the distance travelled and require the driver to correct manually. It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a control system for controlling a powertrain system of a vehicle, a system, a vehicle, a method and computer readable instructions as claimed in the appended claims. According to an aspect, there is provided a control system for controlling a powertrain system of a vehicle, the powertrain comprising first and second electric traction motors, the control system comprising one or more processors collectively configured to: receive at least one input comprising: a torque demand signal indicating a powertrain torque demand; determine, in dependence on the at least one input, that an operating condition is met; output, in response to the determination that the operating condition is met, first and second motor control signals to the powertrain system indicating first and second motor torques required from the first and second electric traction motors to provide a combined torque which approximates the powertrain torque demand, wherein the first and second motor control signals are modulated such that the first and second motor torques act in opposite directions, with one of the first and second electric traction motors providing an opposing torque acting in an opposite direction in relation to the powertrain torque demand. According to an aspect, there is provided a control system for controlling a powertrain system of a vehicle, the powertrain comprising first and second electric traction motors, the control system comprising one or more processors collectively configured to: receive a plurality of inputs comprising: a vehicle speed signal indicative of a vehicle speed; and a torque demand signal indicating a powertrain torque demand; determine, in dependence on the inputs, that an operating condition is met, wherein the operating condition requires at least that the vehicle speed is less than a speed threshold; output, in response to the determination that the operating condition is met, first and second motor control signals to the powertrain system indicating first and second motor torques required from the first and second electric traction motors to provide a combined torque which approximates the powertrain torque demand, wherein the first and second motor control signals are modulated such that the first and second motor torques act in opposite directions, with one of the first and second electric traction motors providing an opposing torque acting in an opposite direction in relation to the powertrain torque demand. By modulating the motor control signals such that the first and second motors act in opposition when the vehicle is moving slowly, at least one, if not both, of the motors can avoid lash crossing when the torque demand changes direction. One motor provides torque in the same direction as the torque demand while the other motor is held on the other side of the lash crossing. Consequently, that motor, and the powertrain in general, can react more quickly in response to a change in direction of the torque demand compared to arrangements in which both motors act in the same direction to provide their share of the combined torque. This can be particularly advantageous when a driver of the vehicle briefly tips-in and tips-out of acceleration in order to move the vehicle only a short distance, since it can improve the accuracy and controllability of the powertrain response and reduce the extent to which the vehicle might overshoot the desired distance of movement. It can also be particularly advantageous when the vehicle is travelling on steep gradients in which a faster reaction of the powertrain can help to avoid unintended acceleration downhill or vehicle roll-back against the intended direction of travel. Once the torque demand has changed direction, both motors could be controlled to provide torque in the same direction as the torque demand. In this case, only one of the motors needs to go through lash crossing. Alternatively, the motors could be controlled so that they continue to provide torque in opposing direction. In this manner, it is possible to control the powertrain at low speed without either motor needing to go through lash crossing. The first and second motor control signals are modulated such that the electric traction motor providing torque in the same direction as the powertrain torque demand is varied to compensate for the opposing torque provided by the other electric traction motor. In this manner, the combined torque can still approximate the powertrain torque demand despite the provision of the opposing torque. The control system comprises one or more controllers collectively comprising at least one electronic processor having an electrical input for receiving an input signal; and at least one memory device electrically coupled to the at least one electronic processor and having instructions stored therein; and wherein the at least one electronic processor is configured to access the at least one memory device and execute the instructions thereon so as to: receive a plurality of inputs comprising: a vehicle speed signal indicative of a vehicle speed; and a torque demand signal indicating a powertrain torque demand; determine, in dependence on the inputs, that an operating condition is met, wherein the operating condition requires at least that the vehicle speed is less than a speed threshold; output, in response to the determination that the operating condition is met, first and second motor control signals to the powertrain system indicating first and second motor torques required from the first and second electric traction motors to provide a combined torque which approximates the powertrain torque demand, wherein the first and second motor control signals are modulated such that the first and second motor torques act in opposite directions, with one of the first and second electric traction motors providing an opposing torque acting in an opposite direction in relation to the powertrain torque demand. In an embodiment, the plurality of inputs comprises a gradient signal indicating a gradient of a surface on which the vehicle is travelling. The one or more processors may be collectively configured to determine, in dependence on the gradient signal, that the vehicle is on a slope. In an embodiment, the first electric traction motor is associated with a first axle of the vehicle and the second electric traction motor is associated with a second axle of the vehicle. In such embodiments, the one or more processors may be collectively configured to: responsive to determining that the vehicle is on a slope, modulate the first and second motor control signals such that the first or second motor torque associated with the first or second axle located down slope acts in an uphill direction. With this arrangement, the downslope axle is held on the correct side of the lash crossing (i.e. on the same side of the lash crossing as the new torque demand direction) in the event that the torque demand changes direction. This can further improve the controllability of the powertrain, since the downhill axle is the axle with the greatest tractive capability due to load transfer and this is the first to provide torque in the correct direction when a torque demand reversal occurs. Alternatively, the processors may be collectively configured to modulate the first and second motor control signals such that the motor torque associated with the axle located down slope acts in a downhill direction. As a further alternative, both electric traction motors may be associated with the same axle of the vehicle. Where the plurality of inputs comprises a gradient signal indicating a gradient of a surface on which the vehicle is travelling, the one or more processors may be collectively configured to: determine, in dependence on the gradient signal, that the gradient of the surface on which the vehicle is travelling is higher in magnitude than a gradient threshold. The determination that the operating condition is met may further require that the gradient of the surface on which the vehicle is travelling is higher in magnitude than the gradient threshold. This can simplify control of the system by avoiding the need to provide opposing torque when the vehicle is operating on a flat or shallow sloping surface from which the gravitational effect on the acceleration of the vehicle is generally sufficiently low that distance overshoot is minimal or can be avoided entirely. The gradient threshold may be any suitable value. In an embodiment, the gradient threshold is 5%. In other embodiments, the gradient threshold may be 0%, 0.5%, 1%, 2%, 3%, 4%, 6%, 7%, 8%, 9% or 10%. In an embodiment, the plurality of inputs includes a direction signal indicative of a direction of travel of the vehicle. In such an embodiment, the one or more processors may be collectively configured to: determine, in dependence on the gradient signal and the direction signal, that the vehicle is travelling downhill. The determination that the operating condition is met may further require that the vehicle is travelling downhill. With this arrangement, the control system modulates the motor control signals such that the first and second motor torques act in opposite directions only when the vehicle is travelling downhill and not when travelling uphill. This can simplify control by avoiding modulation in situations in which the road gradient will act to slow the vehicle during lash crossing and, therefore, reduce the risk of overshoot without the requirement for an intervention by the control system. The speed threshold may be any suitable value. In an embodiment, the speed threshold is 20 kph. In other embodiments, the speed threshold may be 2 kph, 5 kph, 10 kph, 15 kph, 25 kph, 30 kph, 35 kph, or 40 kph. In an embodiment, the one or more processors are collectively configured to modulate the first and second motor control signals such that the opposing torque is greater than or equal to a lash crossing torque associated with the first or second electric traction motor. That is, the motor control signals are modulated such that the electric traction motor which provides the opposing torque, provides that torque at a level which is greater than or equal to the las crossing torque associated with that electric traction motor. For example, where the first electric traction motor provides the opposing torque, the opposing torque amount is greater than or equal to the lash crossing torque associated with the first electric traction motor. With this arrangement, the opposing torque can effectively take up all lash in the driveline associated with the traction motor to which it is applied and allow that driveline to be held at or beyond the point of lash crossing. This can further improve the speed of response of the powertrain to a reversal in the powertrain torque demand. The electric traction motor providing the opposing torque can immediately provide additional torque in the same direction on demand, without first needing to account for any lash associated with that electric traction motor. In this manner, where the torque demand direction reverses rapidly, the electric traction motor providing the opposing torque can immediately compensate for lash associated with the other electric traction motor as that motor passes through lash crossing. In some embodiments, the one or more processors are collectively configured to modulate the first and second motor control signals such that the opposing torque is equal to the lash crossing torque associated with the first or second electric traction motor. With this arrangement, the above advantages can be provided while minimising the additional torque required from the other electric traction motor to compensate for the opposing torque. In an embodiment, the one or more processors are collectively configured to modulate the first and second motor control signals such that the opposing torque is constant while the one or more operating conditions are satisfied. With this arrangement, the process of modulation can be simplified by allowing variations in the powertrain torque demand to be applied only to one of the electric traction motors while the other is held at a constant opposing torque on the opposite side of lash crossing. The control system may be configured to modulate the first and second motor control signals as required irrespective of the operating mode of the vehicle. In an embodiment, the one or more processors are collectively configured to ascertain if a current operating mode of the vehicle is a single pedal operating mode in which acceleration and braking functions are requested through a single pedal. In such an embodiment, the determination that the operating condition is met may further require that the vehicle is in the single pedal driving mode. In the “single pedal driving” or “one pedal driving” mode, the electric traction motors can provide deceleration torque, for example during regenerative braking. In such a mode of operation it is known to provide electric powertrain deceleration torque (or “negative torque”) when a driver modulates an accelerator pedal / input, such that the vehicle can be slowed or brought to a stop by the driver in a controlled manner using only the accelerator pedal / input (that is, without making an input to a foundation brake system on the vehicle). For example, in the context of a car, a driver lifting an accelerator pedal is used to instruct the powertrain to provide deceleration torque, thereby reducing the car’s speed. Deceleration torque provided by the electric powertrain in response to a driver modulating the accelerator pedal / input can be known as “overrun”. The control system may be configured to modulate the first and second motor control signals as required irrespective of whether the powertrain torque demand is accelerative or decelerative. For example, where the powertrain torque demand is decelerative, the one or more processors may be collectively configured to modulate the first and second motor control signals such that the opposing torque is accelerative. This enables the powertrain to respond quickly in the event of a torque demand reversal from decelerative to accelerative. In an embodiment, the one or more operating conditions comprise that the powertrain torque demand is an accelerating powertrain torque demand. With this arrangement, the control logic can be simplified by requiring the opposing torque to be provided only in situations in which the powertrain torque demand is accelerative. According to an aspect, there is provided a system comprising the control system of the preceding aspect and a powertrain comprising first and second electric traction motors. The powertrain is controlled by the control system. According to an aspect, a vehicle is provided comprising the control system of any previously defined aspect. According to an aspect, there is provided a method for controlling a powertrain system of a vehicle, the powertrain comprising first and second electric traction motors, the method comprising: receiving a plurality of inputs including: a vehicle speed signal indicative of a vehicle speed; and a torque demand signal indicating a powertrain torque demand; determining, in dependence on the inputs, that an operating condition is met, wherein the operating condition requires at least that the vehicle speed is less than a speed threshold; outputting, in response to the determination that the operating condition is met, first and second motor control signals to the powertrain system indicating first and second motor torques required from the first and second electric traction motors to provide a combined torque which approximates the powertrain torque demand, wherein the first and second motor control signals are modulated such that the first and second motor torques act in opposite directions, with one or the first and second electric traction motors providing an opposing torque acting in an opposite direction in relation to the powertrain torque demand. The method may further comprise any of the additional steps found in any control system of the control system aspect, or system of the system aspect, or vehicle of the vehicle aspect. According to an aspect, there is provided computer readable instructions which, when executed by a computer, are arranged to perform a method according to the previous aspect. Within the scope of this application, it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a vehicle in accordance with an embodiment of the invention; Figure 2 is a schematic representation of a powertrain and control system of the vehicle of Figure 1; Figure 3 is a flow chart of a method in accordance with an embodiment of the invention; and Figure 4 is a graph illustrating various signals relating to the method of Figure 3. DETAILED DESCRIPTION A vehicle 100 in accordance with an embodiment of the present invention is described herein with reference to the accompanying Figures 1 and 2. With reference to Figure 2, a control system 10 and a powertrain system 120 of the vehicle 100 of Figure 1 are illustrated schematically. The powertrain system 120 comprises a first electric traction motor 122 and a second electric traction motor 124. Each electric traction motor is an electric motor arranged to convert electrical energy into kinetic energy in the form of mechanical torque applied to wheels 116 of the vehicle. Each electric traction motor is also arranged to convert kinetic energy, mechanical torque applied by the wheels, into electrical energy (for example during regenerative braking). One or both electric traction motors may be an alternating current induction motor or a permanent magnet motor, or another type of suitable electric machine. The electric traction motors are configured to drive the vehicle 100. Another term for the electric traction motors is electric drive unit (EDU). Each electric traction motor is configured to drive at least one of the wheels 116. In the illustrated embodiment, the first electric traction motor 122 is configured to drive (and be driven by) the front axle 112 of the vehicle 100 and the second electric traction motor 124 is configured to drive (and be driven by) the rear axle 114 of the vehicle 100. However, it will be understood that the first and second electric traction motors could be swapped such that the first electric traction motor 122 drives the rear axle and vice versa and the second electric traction motor 124 drives the front axle and vice versa. In other embodiments, one or both of the first and second electric traction motors 122, 124 may be configured to drive the wheels 116 of the vehicle in a different manner, such as via a transmission shaft and / or via one or more gears, differentials or transaxles. One or both of the first and second electric traction motors may comprise a plurality of electric traction motors, for example a pair of electric traction motors, each arranged to drive (and by driven by) a single one of the wheels. Although the vehicle is illustrated as having two pairs of wheels 116, it will be appreciated that the vehicle may have any suitable number of wheels, for example two wheels, three wheels, or more than four wheels. The powertrain system 120 may further comprise one or more further prime movers, such as an internal combustion engine (ICE). The powertrain system optionally further comprises a powertrain controller 128, or powertrain control module (PCM), configured to control the operation of the prime movers of the powertrain. The vehicle 100 also comprises an electrical energy storage 130, for example a traction battery. The electrical energy storage 130 is configured to deliver electrical energy to the first and second electric traction motors 122, 124 to drive one or more of the wheels 116, and to receive and store electrical energy generated by the first and second electric traction motors 122, 124 (for example during regenerative braking). The first and second electric traction motors 122, 124 are electrically connected to the electrical energy storage 130, optionally by an inverter (not shown). In some embodiments, the electrical energy storage 130 is communicatively coupled to the powertrain controller 128. The electrical energy storage 130 is optionally a high voltage battery. The electrical energy storage 130 may have a voltage and capacity to support electric only driving for sustained distances. The electrical energy storage 130 may have a capacity of several kilowatt-hours, to increase range. The capacity may be in the tens of kilowatt-hours, or over a hundred kilowatt-hours. The control system 10 is configured to control the powertrain system 120 of the vehicle 100. The control system 10 as illustrated in Figure 2 comprises one controller 11, although it will be appreciated that this is merely illustrative. The controller 11 comprises processing means 12 and memory means 13. The processing means 12 may be one or more electronic processing device 12 which operably executes computer-readable instructions. The memory means 13 may be one or more memory device 13. The memory means 13 is electrically coupled to the processing means 12. The memory means 13 is configured to store instructions, and the processing means 12 is configured to access the memory means 13 and execute the instructions stored thereon. The controller 11 comprises an input means 14 and an output means 15. The input means 14 may comprise an electrical input of the controller 11. The output means 15 may comprise an electrical output of the controller. The input 14 is arranged to receive signals from other components in the vehicle 10, for example from powertrain controller 128. In some embodiments, the received signals are electrical signals indicative of, for example, modulation of an accelerator pedal, a powertrain torque demand, a vehicle speed, a gradient of the surface on which the vehicle is travelling, a current operating mode of the vehicle, or a state of charge of the electrical energy storage 130, among other things, as described in more detail below. The output 15 is arranged to output control signals, for example electrical signals, for example motor control signals indicating required motor torques of the first and second electric traction motors, as described in more detail below. Optionally the control system 10 is, or is part of, the powertrain controller 128. The control system 10 is electrically connected to the first and second electric traction motors, optionally by one or more inverters. Figure 3 illustrates a method 300 according to an embodiment of the invention. The method 300 is a method for controlling a powertrain system of a vehicle, such as the vehicle 100 illustrated in Figures 1 and 2. In particular, the method 300 is a method for controlling a powertrain system comprising first and second electric traction motors. The method 300 may be performed by the control system 10 illustrated in Figure 2. In particular, the memory 13 may comprise computer-readable instructions which, when executed by the processor 12, perform the method 300 according to an embodiment of the invention. At step S-301, the control system receives at least one input 301 relating to the operating state or condition of the vehicle. The at least one input 301 includes at least a torque demand signal 301-2 indicating a powertrain torque demand. The at least one input 301 may be a plurality of inputs including at least a vehicle speed signal 301-1 indicative of a vehicle speed and a torque demand signal 301-2 indicating a powertrain torque demand. There are several ways in which the vehicle speed signal 301-1 can be implemented. For example, the vehicle speed signal 301-1 may comprise information from which the control system can derive the vehicle speed based on other known characteristics of the vehicle and / or powertrain, such as wheel rolling radius. For example, the vehicle speed signal 301-1 may comprise a rotational speed signal from one or both electric traction motors, from which the vehicle speed is derivable by the control system. The vehicle speed signal 301-1 may comprise a signal which directly specifies the vehicle speed. Such a signal may be received by the control system from one or more other systems which have derived the vehicle speed to generate the vehicle speed signal, or from direct measurement, such as from a wheel speed sensor. There are several ways in which the torque demand signal 301-2 can be implemented. For example, the torque demand signal 301-2 may be based on driver input, for example driver input provided by a driver via an accelerator pedal / input. Alternatively or in addition, the torque demand signal 301-2 may be received from, or supplemented by, a vehicle system, such as an ADAS controller. The input means of the controller may receive the demand signal via a controller of the vehicle, e.g., the powertrain controller, that is coupled to a driver input sensor. Alternatively or additionally, the control system may by directly coupled to a driver input sensor associated with the driver input, such as an accelerator pedal, to receive the torque demand signal 301-2. The plurality of inputs 301 may further comprise additional signals relating to the operating state or condition of the vehicle, for example, the plurality of inputs 301 may further comprise one or more of: a gradient signal 301-3 indicating a gradient of a surface on which the vehicle is travelling, a direction signal 301-4 indicative of a direction of travel of the vehicle, and an operating mode signal 301-5 indicative of a current operating mode of the vehicle. The gradient signal 301-3 may be a direct measurement of the gradient from one or more sensors on the vehicle, for example from one or more inclinometers. Alternatively, or in addition, the gradient signal may be derived from one or more other signals or measurements, such as the vehicle speed signal and / or from an acceleration signal. In one example, the control system receives a longitudinal acceleration signal from an accelerometer (such as an accelerometer forming part of a restraint control system of the vehicle), and a vehicle speed signal from a speed sensor (such as an ABS sensor or a motor speed sensor), from which the control system calculates a linear acceleration. The control system then subtracts the linear acceleration from the longitudinal acceleration to determine the acceleration in the direction of gravity and thereby estimate the gradient of the surface on which the vehicle is travelling. The direction signal 301-4 may be received from any suitable sensor or device. In one example, the direction signal is derived by the control system from a vehicle speed signal and a gradient signal, such as the estimated gradient discussed above. In that example, the gradient estimation switches sign when the driver changes the intended direction of travel and the control system can use this change in gradient sign to derive the direction signal. The operating mode signal 301-5 may be received from one or more controllers of the vehicle, for example from the powertrain controller, and may directly inform the control system of the current operating mode. Alternatively, the operating mode signal may indicate one or more operating parameters of the vehicle from which the control system can determine the current operating mode. The operating mode signal may be received or ascertained by the control system from any suitable signal, sensor, or device. For example, the operating mode may selectable by the driver from an input device, such as an infotainment system and / or an operating mode selector switch or lever, from which the operating mode signal is transmitted to the control system either directly or via an intermediate controller or system, such as the vehicle network. Typical operating parameter modes that may be employed include gear selection, steering direction, or driving mode (such as sport mode, low friction surface mode, hill climb / descent mode or single pedal acceleration / deceleration control mode, etc). Optionally, at step S-302, the control system determines, in dependence on a gradient signal 301-3 indicating a gradient of a surface on which the vehicle is travelling, whether the vehicle 100 is on a slope. Optionally, at step S-303, the control system determines, in dependence on the gradient signal 301-3 indicating a gradient of a surface on which the vehicle is travelling, whether the gradient of the surface on which the vehicle 100 is travelling is higher in magnitude than a gradient threshold. Optionally, at step S-304, the control system determines, in dependence on a gradient signal 301-3 indicating a gradient of a surface on which the vehicle is travelling and on a direction signal 301-4 indicative of a direction of travel of the vehicle, whether the vehicle 100 is travelling uphill or downhill. Optionally, at step S-305, the control system ascertains, in dependence on an operating mode signal 301-5 indicative of a current operating mode of the vehicle, whether the vehicle is in a single pedal operating mode in which acceleration and braking functions are requested through a single pedal. At step S-306, the control system determines, in dependence on the plurality of inputs 301, whether an operating condition 306 is met. The operating condition 306 may require at least that signal 306-1, the vehicle speed, is less than a speed threshold. With this requirement, the generation of an opposing torque occurs only at speed below the speed threshold. This can avoid potentially unnecessary intervention by the control system at higher speeds, at which the likelihood of unexpected distance overshoot / undershoot is lower. In an embodiment, the speed threshold is 20 kph, although it will be understood that any suitable speed could be selected in particular circumstances. For example, the speed threshold could alternatively be any of 2 kph, 5 kph, 10 kph, 15 kph, 25 kph, 30 kph, 35 kph, or 40 kph, or higher. Optionally, the operating condition 306 may require at least that signal 306-2, the gradient of the surface on which the vehicle is travelling, is greater in magnitude than a gradient threshold. With this requirement, the generation of an opposing torque occurs only when the vehicle is travelling on an incline which is steeper than the gradient threshold. This can avoid potentially unnecessary intervention by the control system when the vehicle is on flat terrain or only on a shallow slope, in which vehicle acceleration due to gravity, and the likelihood of distance overshoot / undershoot, is lower. This requirement may apply to all vehicle operating modes. Alternatively, this requirement may apply only in certain vehicle operating modes, such as one or more single pedal operating modes. In such examples, the operating condition 306 may require at least that the gradient of the surface on which the vehicle is travelling is greater in magnitude than a gradient threshold and that the vehicle operating mode is of a certain type of vehicle operating mode. Optionally, the operating condition 306 may require at least that signal 306-3, the gradient signal 301-3 in combination with the direction signal 301-4, indicate that the vehicle is currently travelling downhill. This can avoid potentially unnecessary intervention by the control system when the vehicle is travelling uphill, or on flat terrain, in which the vehicle would tend to slow in the absence of accelerative torque from the powertrain, rather than be caused to accelerate due to gravity. Optionally, the operating condition 306 may require at least that signal 306- indicate that the vehicle is currently travelling uphill. This requirement may apply to all vehicle operating modes. Alternatively, this requirement may apply only in certain vehicle operating modes, such as one or more single pedal operating modes in which the vehicle is prevented by the control system from rolling backwards when on a positive gradient. In such examples, the operating condition 306 may require at least that the gradient of the surface on which the vehicle is travelling is greater in magnitude than a gradient threshold, that the direction signal 301-4 indicates that the vehicle is currently travelling uphill and that the vehicle operating mode is of a certain type of vehicle operating mode. Optionally, the operating condition 306 may require at least that signal 306-4, the current operating mode, is a single pedal operating mode. As used herein, “single pedal operating mode” refers to an operating mode in which the driver is able to control deceleration of the vehicle 100 via operation of an accelerator input (for example an accelerator pedal) only, that is to say without operation of a brake input (for example a brake pedal). It is noted that although the driver has the ability to control deceleration of the vehicle via modulation of the accelerator pedal / input during single pedal driving, the driver is still able to separately modulate a brake pedal / input to further control deceleration of the vehicle. The present invention is primarily described in relation to driver modulation of an accelerator pedal. However, it will be appreciated that the present invention may also be applied to driver modulation of a different accelerator input, for example a hand-operated throttle as typically found on motorcycles. Optionally, the operating condition 306 may require at least that signal 306-5, the powertrain torque demand, is an accelerating powertrain torque demand. That is, a powertrain torque demand which leads to acceleration of the vehicle. Such a torque demand might also be termed a “positive torque demand”. Alternatively, or in addition, the operating condition may require at least that the powertrain torque demand is below a threshold torque demand, optionally with a hysteresis or delay. This can help to avoid the opposing torque logic from being introduced in situations in which the driver “tips-in” very heavily, which may require access to the full powertrain capability, even if this degrades a subsequent “tip-out” behaviour of the vehicle. The operating condition 306 may require any combination of the above requirements, or sub-conditions, to be satisfied in order for the operating condition 306 to be met. In an embodiment, the operating condition 306 requires that signal 306-1, the vehicle speed, is less than a speed threshold, that signal 306-2, the gradient of the surface on which the vehicle is travelling, is greater in magnitude than a gradient threshold, and that signal 306-3, the gradient signal 301-3 in combination with the direction signal 301-4, indicate that the vehicle is currently travelling uphill. In that embodiment, the operating condition is met only when all sub-conditions are met. This is the same for any other combination of sub-conditions which must be satisfied in order for the operating condition to be met. If the control system determines that the operating condition 306 is not met, the powertrain output is controlled (for example by the control system 10 and / or by the powertrain controller) in a conventional manner to provide a combined torque which approximates the powertrain torque demand, and the method returns to step S-301. If the control system determines that the operating condition 306 is met, the method proceeds to step S-307. At step S-307, the control system modulates first and second motor control signals in dependence on the powertrain torque demand. The first and second motor control signals indicate first and second motor torques required from the first and second electric traction motors to provide a combined torque which approximates the powertrain torque demand. The control system modulates the first and second control signals such that one of the electric traction motors provides an opposing torque which acts in opposition to the powertrain torque demand, while the other electric traction motor provides a motor torque which acts in the same direction (i.e. accelerative or decelerative) as the powertrain torque demand and compensates for the opposing torque, i.e. provides a motor torque which approximately equals the powertrain torque demand plus the magnitude of the opposing torque. The control system may be free to select which of the first and second electric traction motors provides the opposing torque based on characteristics of each electric traction motor or the axle with which it is associated. For example, the control system may modulate the first and second control signals such that the opposing torque is provided by the electric traction motor which is associated with the greatest amount of lash crossing torque. By modulating the motor control signals such that the first and second motors act in opposition when the vehicle is travelling below the speed threshold, at least one, if not both, of the motors can avoid lash crossing when the torque demand changes direction. One motor provides torque in the same direction as the torque demand while the other motor provides the opposing torque and is held on the other side of the lash crossing. Consequently, that motor, and the powertrain in general, can react more quickly in response to a change in direction of the torque demand compared to arrangements in which both motors act in the same direction to provide their share of the combined torque. This can be particularly advantageous when a driver of the vehicle briefly tips-in and tips-out of acceleration in order to move the vehicle only a short distance, since it can improve the accuracy and controllability of the powertrain response and reduce the extent to which the vehicle might overshoot the desired distance of movement. It can also be particularly advantageous when the vehicle is travelling on steep gradients in which a faster reaction of the powertrain can help to avoid unintended acceleration downhill or vehicle roll-back against the intended direction of travel when travelling uphill. The first and second motor control signals are modulated such that the electric traction motor providing torque in the same direction as the powertrain torque demand is varied to compensate for the opposing torque provided by the other electric traction motor. In this manner, the combined torque can still approximate the powertrain torque demand despite the provision of the opposing torque. When travelling uphill, such modulation of the control signals can help to prevent rollback because one of the axles is held positive when the driver demand goes negative during a tip out. This then helps the powertrain to meet its torque demand when the torque becomes positive as the vehicle is coming to a rest, since only one axle needs to cross lash. The opposing torque may be a predetermined constant amount. This may equal or be greater than a lash crossing torque amount associated with that electric traction motor or the driveline or system with which that electric traction motor is associated. Optionally, the lash crossing torque amount accounts for the lash in the entire system for a given electric traction motor, from the inverter to the wheel hubs. This can be estimated via CAE analysis and / or from a calibration process with an actual vehicle, such as a vehicle prototype. This is typically a constant predetermined value regardless of operating conditions. Once the lash crossing torque amount has been estimated or calibrated for a given electric traction motor, the opposing torque implemented for that motor may equal that lash crossing torque amount. Alternatively, the control system may modulate the control signal for that motor such that the opposing torque is no less than the lash crossing torque amount. Optionally, in response to determining at step S-302 that the vehicle 100 is on a slope, the control system may modulate in step S-307 the first and second motor control signals such that the first or second motor torque associated with the first or second axle located down slope acts in an uphill direction. In this manner, the control system ensures that the downhill axle, which has the greatest tractive torque capability, provides torque in a direction which opposes movement of the car under gravity. With this arrangement, when the rear axle is downhill of the front axle (i.e. when the vehicle is pointing uphill), the rear axle would be provided with a torque which acts uphill irrespective of whether the vehicle is reversing downhill or in drive going uphill. The converse is true when the front axle is downhill of the rear axle (i.e. when the vehicle is pointing downhill). Optionally, in response to determining at step S-302 that the vehicle 100 is on a slope, the control system may modulate in step S-307 the first and second motor control signals such that the first or second motor torque associated with the first or second axle located down slope is the opposing torque. In other words, the electric traction motor associated with the downhill axle is controlled to provide the opposing torque. In this manner, the control system ensures that the downhill axle, which has the greatest tractive torque capability, is held on the opposite side of the lash crossing to the powertrain torque demand. This enables the powertrain to react more quickly to a reversal in the powertrain torque demand by ensuring that the axle with the greatest tractive torque capability can immediately provide torque in the “new” direction without first passing through lash crossing. At step S-308, the control system outputs the first and second motor control signals to the powertrain system. The control system may output the first and second motor control signals directly to the first and second electric traction motors, or to one or more electric motor controllers or inverters, or to the powertrain controller for subsequent output to the first and second electric traction motors. The control system continues to modulate at step S-307 and output at step S-308 the first and second motor control signals in dependence on the powertrain torque demand until the operating conditions are no longer met at step S-306. Figure 4 is a graph 400 illustrating powertrain behaviour when the method 300 above is followed. The graph 400 includes several plots illustrating how various signals associated with the method 300 vary in relation to each other. Plot (a) shows a gradient signal 401 indicating a gradient of a surface on which the vehicle is travelling, the plot showing percentage gradient along the y-axis and time along the x-axis. Plot (b) shows a torque demand signal 402 indicating a powertrain torque demand in the form of an accelerator pedal position measurement, the plot showing pedal position along the y-axis (for which y=0 denotes that the pedal is not depressed) and time along the x-axis. Plot (c) shows a vehicle speed signal 403 indicating a vehicle speed, the plot showing speed along the y-axis and time along the x-axis. Plot (d) shows a first motor control signal 404 indicating a first motor torque required from the first electric traction motor, the plot showing torque along the y-axis and time along the x-axis. Plot (e) shows a second motor control signal 405 indicating a second motor torque required from the second electric traction motor, the plot showing torque along the y-axis and time along the x-axis. In the scenario illustrated in graph 400, the vehicle 100 is pointing downhill in reverse and is operating in the single pedal operating mode in which accelerative and decelerative torques can be demanded by the driver by modulating only the accelerator pedal position. At time to, the control system receives the gradient signal 401, the torque demand signal 402, and the vehicle speed signal 403 (see step S-301 above) and determines that the vehicle is on a slope (see step S-302 above), that the gradient of the surface on which the vehicle is travelling is higher in magnitude than a gradient threshold (see step S-303 above) and that the vehicle speed is less than the speed threshold. Driver modulation of the accelerator pedal leads to a determination that the powertrain torque demand is less than a powertrain torque demand threshold and, consequently, that the operation condition is met (see step S-306 above). In response to the determination that the operating condition is met, the control system modulates and outputs first and second motor control signals 404 and 405 such that the second electric traction motor provides an opposing torque Top acting in an opposite direction in relation to the powertrain torque, i.e. in a decelerative direction, while the first electric traction motor provides a torque acting in the same direction in relation to the powertrain torque and compensating for the opposing torque Top. The second electric traction motor is held at a generally constant opposing torque of around 20 Nm from shortly after to until to. The powertrain torque demand indicated by the pedal position continues to increase until ti. At time ti, the driver starts to lift-off from the accelerator pedal and the powertrain torque demand as indicated by torque demand signal 402 reduces. The first motor control signal 404 continues to increase to reflect the increasing powertrain torque demand prior to ti. The first motor control signal 404 lags the torque demand signal 402 due to the torque mapping of the powertrain, which reduces the motor response at low vehicle speed to provide a smoother torque response. At, or shortly after ti, the vehicle starts to accelerate (in reverse), as indicated by vehicle speed signal 403. Between ti and t2, the torque demand signal 401 continues to reduce as the driver continues to ease-off from the accelerator pedal, and the first motor control signal 404 peaks at around 90Nm shortly after ti before also reducing to approximate the reducing powertrain torque demand. At a certain amount of accelerator pedal travel, the control system determines that the overall powertrain demand has reversed from accelerative to decelerative torque, As will be understood, the point at which this is determined to occur may vary in dependence on vehicle speed and / or the gradient of the slope on which the vehicle is travelling. At time t2, the accelerator pedal has returned to the ‘zero’ position at which the driver has lifted off completely. At this point, the powertrain torque demand is negative and this is reflected in the combined torque from the first and second traction motors, which is also negative - i.e. decelerative. This is because the first electric traction motor is providing a positive torque which is now lower in magnitude than the opposing torque provided by the second electric traction motor. This is also illustrated in the vehicle speed signal 402 which shows that the rate of acceleration of the vehicle decreases to a maximum speed at or shortly after ta. At time ts, the first motor control signal reaches zero and, at this point, the front axle has to slow down due to lash crossing, and the rate of decrease of the first motor control signal slows under the influence of lash crossing. The delta between the unaltered torque request and the modified torque request during lash crossing of the front axle is compensated for by the second electric traction motor. This is achieved by modulating the second motor control signal to request a decelerative torque beyond that of the opposing torque amount Top. This ensures that the combined torque continues to reduce at the required rate despite the slowed response of the first electric traction motor due to lash crossing. The reverse of this behaviour happens if the vehicle is driving uphill in order to help prevent the vehicle rolling backwards because of lash crossing. At time U, the first electric traction motor has passed through lash crossing and both electric traction motors are on the same side of the lash crossing. Shortly before this point, the first motor control signal is modulated to increase the magnitude of the negative torque requested from first electric traction motor and, conversely, the second motor control signal is modulated to reduce the magnitude of the negative torque requested from the second electric traction motor so that the combined torque is shared more evenly between the first and second electric traction motors. After t4, the magnitude of the combined torque gradually reduces to bring the vehicle smoothly to a halt. In the example illustrated in Figure 4, the control system modulates and outputs first and second motor control signals 404 and 405 such that the second electric traction motor provides the opposing torque Top. The second electric traction motor is associated with the rear axle which, in this example, is the downhill axle since the driver intends to drive in reverse down a gradient. Since the load transfer under gravity is such that vehicle weight shifts towards the downhill axle, the downhill axle can be considered to have the greatest tractive torque capability. In this manner, the control system modulates the first and second motor control signals such that the motor torque associated with the downhill axle, and thus which has the greatest tractive torque capability, provides the opposing torque. Consequently, that axle remains on the negative side of lash and will be the first to generate torque in the “new” direction when a torque demand reversal occurs. The accelerator pedal input from the driver is a rapid “tip-in” and “tip-out” demand in which the pedal is moved rapidly to a large pedal position before being released even more rapidly. The time over which this process occurs is relatively short. For example, the duration from to to ti might be in the region of 2 seconds and the 5 duration from ti to to might be in the region of 1.5 seconds. By modulating the motor control signals such that the second motor provides an opposing torque, the overall powertrain response can be improved. In this example, when the first electric traction motor passes through lash crossing, the reduced rate of decrease of the first motor torque can be immediately or almost immediately compensated by the second electric traction motor, which is already on the correct side of its own lash crossing. This enables the combined torque output 10 to continue to change at the desired rate despite the lash crossing of the first electric traction motor. With such a brief tip-in and tip-out of acceleration, this method can improve the predictability of the vehicle response and reduce the extent to which the vehicle might overshoot the desired distance of movement. It will be appreciated that various changes and modifications can be made to the present invention without 15 departing from the scope of the present application.

Claims

1. A control system for controlling a powertrain system of a vehicle, the powertrain comprising first and second electric traction motors , the control system comprising one or more processors collectively configured to:receive a plurality of inputs comprising:a vehicle speed signal indicative of a vehicle speed; anda torque demand signal indicating a powertrain torque demand;determine, in dependence on the inputs, that an operating condition is met, wherein the operating condition requires at least that the vehicle speed is less than a speed threshold;output, in response to the determination that the operating condition is met, first and second motor control signals to the powertrain system indicating first and second motor torques required from the first and second electric traction motors to provide a combined torque which approximates the powertrain torque demand, wherein the first and second motor control signals are modulated such that the first and second motor torques act in opposite directions, with one of the first and second electric traction motors providing an opposing torque acting in an opposite direction in relation to the powertrain torque demand.

2. The control system of claim 1, wherein the plurality of inputs comprises a gradient signal indicating a gradient of a surface on which the vehicle is travelling, and wherein the one or more processors are collectively configured to determine, in dependence on the gradient signal, that the vehicle is on a slope.

3. The control system of claim 2, wherein the first electric traction motor is associated with a first axle of the vehicle and the second electric traction motor is associated with a second axle of the vehicle, wherein the one or more processors are collectively configured to:responsive to determining that the vehicle is on a slope, modulate the first and second motor control signals such that the first or second motor torque associated with the first or second axle located down slope acts in an uphill direction.

4. The control system of claim 2 or claim 3, wherein the one or more processors are collectively configured to:determine, in dependence on the gradient signal, that the gradient of the surface on which the vehicle is travelling is higher in magnitude than a gradient threshold, andthe determination that the operating condition is met further requires that the gradient of the surface on which the vehicle is travelling is higher in magnitude than the gradient threshold.

5. The control system of claim 4, wherein the gradient threshold is 5%.

6. The control system of any of claims 2 to 5, wherein the plurality of inputs includes a direction signal indicative of a direction of travel of the vehicle, wherein the one or more processors are collectively configured to:determine, in dependence on the gradient signal and the direction signal, that the vehicle is travelling downhill, and the determination that the operating condition is met further requires that the vehicle is travelling downhill; ordetermine, in dependence on the gradient signal and the direction signal, that the vehicle is travelling uphill, and the determination that the operating condition is met further requires that the vehicle is travelling uphill.

7. The control system of any preceding claim, wherein the speed threshold is 20 kph.

8. The control system of any preceding claim, wherein the one or more processors are collectively configured to modulate the first and second motor control signals such that the opposing torque is greater than or equal to a lash crossing torque associated with the first or second electric traction motor providing the opposing torque.

9. The control system of any preceding claim, wherein the one or more processors are collectively configured to modulate the first and second motor control signals such that the opposing torque is constant while the operating condition is met.

10. The control system of any preceding claim, wherein the one or more processors are collectively configured to:ascertain if a current operating mode of the vehicle is a single pedal operating mode in which acceleration and braking functions are requested through a single pedal, andthe determination that the operating condition is met further requires that the vehicle is in the single pedal driving mode.

11. The control system of claim 10, wherein the operating condition further requires that the powertrain torque demand is an accelerating powertrain torque demand.

12. A system comprising the control system of any preceding claim and a powertrain comprising first and second electric traction motors .

13. A vehicle comprising the system of claim 12 or the control system of any of claims 1 to 11.

14. A method for controlling a powertrain system of a vehicle, the powertrain system comprising first and second electric traction motors, the method comprising:receiving a plurality of inputs including:a vehicle speed signal indicative of a vehicle speed; and5 a torque demand signal indicating a powertrain torque demand;determining, in dependence on the plurality of inputs, that an operating condition is met, wherein the operating condition requires at least that the vehicle speed is less than a speed threshold;outputting, in response to the determination that the operating condition is met, first and second motor control signals to the powertrain system indicating first and second motor torques required from the first10 and second electric traction motors to provide a combined torque which approximates the powertrain torque demand, wherein the first and second motor control signals are modulated such that the first and second motor torques act in opposite directions, with one or the first and second electric traction motors providing an opposing torque acting in an opposite direction in relation to the powertrain torque demand.15 15. Computer readable instructions which, when executed by a computer, are arranged to perform amethod according to claim 14.

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