Control system for a torque-limited vehicle

Abstract

A control system 100 for controlling at least one electric traction motor (312, 314) of a torque limited vehicle (200) in which the at least one electric traction motor has a torque limit. The control

Description

TECHNICAL FIELD The present disclosure relates to a control system for a torque limited vehicle. Aspects of the invention relate to a control system, a system, a vehicle, a method, and computer readable instructions. BACKGROUND It is known to provide vehicles with a torque limit. Such torque limits can restrict the amount of available torque output for certain vehicles or operators. For example, applying a torque limit can provide a smoother driving experience, or improved economy to provide extended range for given battery capacity. It may also be useful to limit the torque available to inexperienced drivers, and / or for rental vehicles. This can be achieved by imposing a torque limit on the electric traction motor or motors of the vehicle which caps the available torque to a level below the maximum torque capability of those motors. However, it has been found that liming the available torque could result in insufficient performance or unexpected vehicle behaviour in high torque demand driving situations, i.e. in additional torque demand conditions, such as when attempting to accelerate on a steep incline, when towing a trailer, or when driving on soft terrain, such as mud, snow, or sand 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 or controlling at least one electric traction motor of a torque-limited vehicle, a system, a vehicle, a method for controlling at least one electric traction motor of a torque-limited vehicle, and computer readable instructions, as claimed in the appended claims. According to an aspect of the invention, there is provided a control system for controlling at least one electric traction motor of a torquelimited vehicle, the control system comprising one or more processors collectively configured to: receive an indication of a torque demand; determine an expected acceleration of the vehicle in dependence on the torque demand; receive an indication of the actual acceleration of the vehicle; compare the expected acceleration to the actual acceleration; determine, in dependence on the comparison, if the expected acceleration exceeds the actual acceleration by at least a threshold amount; and, in dependence on a determination that the expected acceleration exceeds the actual acceleration by at least the threshold amount, output a torque limit override signal. According to an aspect of the invention, there is provided a control system for controlling at least one electric traction motor of a torquelimited vehicle in which the at least one electric traction motor has a torque limit, the control system comprising one or more processors collectively configured to: receive an indication of a torque demand; output a torque request signal for the at least one electric machine in dependence on the torque demand; determine an expected acceleration of the vehicle in dependence on the torque demand; receive an indication of the actual acceleration of the vehicle; compare the expected acceleration to the actual acceleration; determine, in dependence on the comparison, if the expected acceleration exceeds the actual acceleration by at least a threshold amount; in dependence on a determination that the expected acceleration exceeds the actual acceleration by at least the threshold amount, output a torque limit override signal. With the claimed control system, the torque limit can be selectively overridden according to the external condition in which the vehicle is operated. This enables the advantages of torque limiting to be obtained during normal driving while permitting additional torque to be made available in certain conditions in which additional torque output is required. Additionally, by determining whether additional torque should be made available in dependence on a comparison of actual to expected acceleration, the control system can rely on signals which are already secured from the point of view of functional safety and can provide additional torque in a wide range of different operating conditions without the need to receive and process additional signals. As used herein, the term “the expected acceleration exceeds the actual acceleration by a threshold amount’ refers to situations in which the expected acceleration exceeds the actual acceleration by at least a threshold amount, rather than by exactly the threshold amount. 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 an indication of a torque demand; output a torque request signal for the at least one electric machine in dependence on the torque demand; determine an expected acceleration of the vehicle in dependence on the torque demand; receive an indication of the actual acceleration of the vehicle; compare the expected acceleration to the actual acceleration; determine, in dependence on the comparison, if the expected acceleration exceeds the actual acceleration by a threshold amount; in dependence on a determination that the expected acceleration. The torque limit override signal may comprise an instruction to temporarily disregard or cancel the torque limit. Optionally, the torque limit override signal comprises a modified torque limit signal defining a further torque limit which is higher than the torque limit. The further torque limit may be the same as or less than the maximum torque capability of the at least one electric traction motor. In this manner, the at least one electric traction motor would be operable to provide a torque output which is greater than the torque limit but not greater than the further torque limit. This can enable the torque output to increase but only within a defined envelope. The one or more processors may be collectively configured to apply the modified torque limit signal to impose the further torque limit instantaneously, i.e. in a step change from the torque limit to the further torque limit. Optionally, the one or more processors are collectively configured to modulate the modified torque limit signal to define a modified torque limit value which is gradually increased from the torque limit to the further torque limit. The one or more processors may be collectively configured to modulate the modified torque limit signal to define a modified torque limit value which is gradually decreased from the further torque limit to the torque limit. The one or more processors may be collectively configured to modulate the modified torque limit signal to define a modified torque limit value which is gradually increased from the torque limit to the further torque limit and gradually decreased from the further torque limit to the torque limit. This can enable the additional torque to be provided more smoothly. The one or more processors may be collectively configured to modulate the modified torque limit signal to gradually increase the modified torque limit value from the torque limit to the further torque limit at a ramp-up rate which remains substantially constant. Optionally, the one or more processors are collectively configured to modulate the modified torque limit signal to gradually increase the modified torque limit value from the torque limit to the further torque limit at a ramp-up rate which varies in dependence on one or more vehicle operating parameters. This enables the speed of implementation to be varied for different circumstances, for example to allow the torque limit increase to be implemented more quickly where needed, or more slowly where a rapid change in torque output might be detrimental to the driving experience. The one or more processors may be collectively configured to modulate the modified torque limit signal to increase the modified torque limit value at a ramp-up rate which is selected in dependence on a comparison of the torque limit with the torque generated by the powertrain at the time at which the determination that the expected acceleration exceeds the actual acceleration by at least the threshold amount is made. For example, the one or more processors may be collectively configured to receive a torque output signal indicative of a torque generated by the at least one electric traction motor at a time when the determination that the expected acceleration exceeds the actual acceleration by at least the threshold amount is made, compare the torque generated to the torque limit, and modulate the modified torque limit signal in dependence on the comparison of the torque generated to the torque limit. In such examples, the modified torque limit signal could be modulated to define a modified torque limit value which increases at a ramp-up rate which varies as a function of the difference between the torque generated and the torque limit. For example, the ramp-up rate could be set at a slow rate when the torque generated is equal to the torque limit and at increasingly quicker rates with increasing distance of the torque generated from the torque limit. This can reduce the extent to which a driver might feel a sudden increase in the available torque when the torque output is already at or close to the torque limit at the time when the torque limit is overridden. Optionally, the one or more processors are collectively configured to receive a torque output signal indicative of a torque generated by the at least one electric traction motor at a time when the determination that the expected acceleration exceeds the actual acceleration by at least the threshold amount is made; to compare the torque generated to the torque limit; and, in response to the comparison, modulate the modified torque limit signal to increase the modified torque limit value at a first ramp-up rate if the torque generated is less than the torque limit and to increase the modified torque limit value at a second ramp-up rate which is slower than the first increase rate if the torque generated is the same as the torque limit. This can provide straight-forward control logic to reduce the extent to which a driver might feel a sudden increase in the available torque when the torque output is already at the torque limit at the time when the torque limit is overridden. Optionally, the one or more processors are collectively configured to: monitor whether the expected acceleration continues to exceed the actual acceleration by at least the threshold amount; and in response to a determination that the expected acceleration no longer exceeds the actual acceleration by at least the threshold amount, reinstate the torque limit. For example, the torque limit can be instated by stopping the output of the torque limit override signal in response to the determination that the expected acceleration no longer exceeds the actual acceleration by the threshold amount. In such examples, the torque limit can be reinstated instantaneously. Alternatively, or in addition, the one or more processors may be collectively configured to modulate the modified torque limit signal to gradually reduce the modified torque limit value from the further torque limit to the torque limit at a ramp-down rate. The ramp-down rate may vary in dependence on one or more vehicle operating parameters, such as vehicle speed. The modified torque limit signal may be modulated independently of vehicle speed. The one or more processors may be collectively configured to: receive an indication of vehicle speed; compare the vehicle speed to a feature speed threshold; and modulate the modified torque limit signal in dependence on the comparison. Optionally, the one or more processors are collectively configured to: receive an indication of vehicle speed; compare the vehicle speed to a feature speed threshold; and in response to a determination that the vehicle speed exceeds the feature speed threshold, modulate the modified torque limit signal to gradually reduce the modified torque limit value from the further torque limit to the torque limit as a function of vehicle speed. This modulation may be carried out independently of any monitoring of whether the expected acceleration continues to exceed the actual acceleration by the threshold amount. In this manner, overriding of the torque limit can be caused to occur to a greater extent at lower vehicle speeds when the additional torque is most desired and / or blended out at higher vehicle speeds. The feature speed threshold may be any suitable value. For example, the feature speed threshold may be 20kph, 25kph, 30kph, 35kph, 40kph, or higher. The one or more processors may be collectively configured to apply a speed reduction coefficient to the modified torque limit signal, wherein the speed reduction coefficient varies as a function of vehicle speed. In such examples, when the vehicle speed is less than the feature speed threshold, the speed reduction coefficient is 1, such that the modified torque limit value is unchanged by the speed reduction coefficient. When the vehicle speed is above the feature speed threshold, the speed reduction coefficient may reduce from 1 to 0 as a function of the difference between the feature speed threshold and the vehicle speed. The speed reduction coefficient enables the torque limit increase to be blended out at higher speeds at which the additional torque capacity is likely to be no longer needed, or its absence less apparent. The speed reduction coefficient may reduce at any suitable rate in relation to vehicle speed. For example, the speed reduction coefficient may reduce linearly at a rate of 0.1 per kph such that the speed reduction coefficient reduces from 1 to 0 over a speed range of 10kph. The speed reduction coefficient may vary above the feature speed threshold according to any suitable function of vehicle speed. For example, the speed reduction coefficient may reduce linearly with vehicle speed. For example, the speed reduction coefficient may reduce linearly from 1 at the feature speed threshold to 0 at a feature speed limit. The feature speed limit may be any suitable value above the feature speed threshold. For example, the feature speed limit may be 30kph, 35kph, 40kph, 45kph, 50kph, or higher. In one example, the feature speed threshold is 30kph and the feature speed limit is 40kph, such that the modified torque limit value is blended down to the torque limit between 30kph to 40kph. Optionally, the one or more processors are collectively configured to: receive an indication of an external condition in which the vehicle is operating; determine if the external condition is an additional torque demand condition; and, in dependence on the determination that the that the external condition is an additional torque demand condition, output the torque limit override signal. This can enable the control system to selectively override the torque limit in dependence on one or more indications of the external condition, independently of the comparison of actual acceleration to expected acceleration. This can enable the control system to permit additional torque to be made available more quickly, for example by determining that the external condition is an additional torque demand condition and outputting the torque limit override signal before the vehicle has pulled away from a stationary start. As used herein, the term “external condition” refers to one or more factors exterior to the vehicle which influence the performance of the vehicle. These factors may increase the resistance to acceleration of the vehicle and, consequently, the required torque output for a given vehicle acceleration. As used herein, the term “additional torque demand condition” refers to one or more pre-determined conditions in which the resistance to acceleration of the vehicle is greater than for normal driving conditions so that additional torque is required at the wheels to overcome that resistance and maintain performance. For example, the additional torque demand conditions may be a set of conditions in which torque at the wheel must be increased by at least 500 Nm, 1000 Nm, 1500 Nm, or 2000 Nm to provide comparable performance to typical conditions, such as driving the vehicle on a flat surface with single occupancy. Optionally, the indication of an external condition comprises one or more of: a gradient signal indicating a gradient of a surface on which the vehicle is supported; a tow signal indicating a presence or absence of a vehicle trailer connected to the vehicle; and a terrain signal indicating a type of terrain on which the vehicle is supported. In such an embodiment, the additional torque demand condition may require at least that one or more of the following apply: the gradient is greater than a gradient threshold; the tow signal indicates the presence of a vehicle trailer connected to the vehicle; and the terrain signal indicates that the type of terrain is an additional torque terrain. These signals provide the control system with a clearly defined set of external conditions in which the available torque output may need to be increased above the torque limit. The additional torque demand condition may require only a single one of the conditions to apply in order for the control system to determine that the external condition is an additional torque condition. Alternatively, the additional torque demand condition may require multiple of the conditions to apply in order for the control system to determine that the external condition is an additional torque condition. The gradient signal 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 a 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 due to gravity and thereby estimate the gradient of the surface on which the vehicle is travelling. The indication of actual acceleration may be implemented in any suitable manner. For example, the indication may be based on a signal from an accelerometer mounted on or within the vehicle. Optionally, the indication of the actual acceleration of the vehicle is a speed signal from which the one or more processors are collectively configured to derive the actual acceleration of the vehicle. This can enable more accurate determination of acceleration than from an accelerometer, particularly where the vehicle is travelling on an incline, since the speed signal enables determination of linear acceleration, rather than longitudinal acceleration, as is generally the case with an accelerometer. Such a vehicle speed signal may comprise a signal derived from a speed sensor measurement, such as from a wheel speed sensor or a rotational speed sensor associated with the driveline of the vehicle. Optionally, the speed signal is a motor speed signal indicative of a rotational speed of the at least one electric traction motor and wherein the one or more processors are collectively configured to calculate a vehicle speed based on the rotational speed to derive the actual acceleration of the vehicle. In such embodiments, the indication of actual acceleration is provided by the rotational speed signal. When the driver demands increased torque, for example with a large application of the accelerator pedal, there can be a delay between the demand and a change to the vehicle speed. By deriving vehicle acceleration from the motor rotational speed, that delay can be reduced, enabling the control system to react more quickly and permit increased torque beyond the torque limit closer to the point at which it is first required. This approach can also avoid excessive triggering of the logic which could result from any difference between actual acceleration and expected acceleration due to that delay. The indication of the actual acceleration of the vehicle can be received by the control system directly from a sensor, or from one or more other processors of the vehicle which have derived or the vehicle speed and / or acceleration and output that to the control system. Optionally, the one or more processors are collectively configured to: determine whether a limitation condition is met; and in response to determining that a limitation condition is met, impose the torque limit on the at least one electric traction motor irrespective of a determination that the expected acceleration exceeds the actual acceleration by the threshold amount. This prevents the torque output from exceeding the torque limit in certain situations in which the limitation condition is met, for example in situations in which the additional torque might not be desirable or necessary, and / or to avoid excessive triggering of the torque limit override. The limitation condition is met if: a brake demand signal indicates that a braking pressure is requested; a gradient signal indicates that a gradient of a surface on which the vehicle is supported is less than a gradient threshold; the expected acceleration exceeds the actual acceleration by at least the threshold amount for less than a threshold period of time; and / or the vehicle speed exceeds a maximum speed threshold. The limitation condition may require only a single one of the above conditions to be satisfied for the one or more processors to determine that the limitation condition is met. Alternatively, the limitation condition may require a combination of the above conditions to be satisfied together for the one or more processors to determine that the limitation condition is met. According to another aspect of the invention, there is provided a system comprising the control system of the preceding aspect and at least one electric traction motor. The at least one electric traction motor is controlled by the control system. According to another aspect of the invention, there is provided a vehicle comprising the control system or the system of the preceding aspects. According to a further aspect of the invention, there is provided a method for controlling at least one electric traction motor of a torquelimited vehicle in which the at least one electric traction motor has a torque limit, the method comprising: receiving an indication of a torque demand; outputting a torque request signal for the at least one electric machine in dependence on the torque demand; determining an expected acceleration of the vehicle in dependence on the torque demand; receiving an indication of the actual acceleration of the vehicle; comparing the expected acceleration to the actual acceleration; determining, in dependence on the comparison, if the expected acceleration exceeds the actual acceleration by a threshold amount; and, in dependence on a determination that the expected acceleration exceeds the actual acceleration by the threshold amount, outputting a torque limit override signal. 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 a further aspect of the invention, 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 is a schematic representation of a control system in accordance with an embodiment of the invention; Figure 2 shows a vehicle in accordance with an embodiment of the invention; Figure 3 is a schematic illustration of part of the vehicle of Figure 2, including the powertrain and control system; Figure 4 shows a flow chart illustrating a method in accordance with an embodiment of the invention; Figure 5 shows a flow chart illustrating an example method forming part of the method of Figure 4; and Figure 6 shows a graph illustrating example vehicle behaviour in accordance with the method of Figure 4. DETAILED DESCRIPTION A control system in accordance with an embodiment of the present invention is described herein with reference to the accompanying Figures 1-6. As shown in Figure 2, the control system is installed in a vehicle 200. With reference to Figure 1, there is illustrated a control system 100 for controlling at least one electric traction motor of a torque-limited vehicle. The control system 100 comprises one or more controller 110. The control system 100 as illustrated in Figure 1 comprises one controller 110, although it will be appreciated that this is merely illustrative. The controller 110 comprises processing means 120 and memory means 130. The processing means 120 may be one or more electronic processing device 120 which operably executes computer-readable instructions. The memory means 130 may be one or more memory device 130. The memory means 130 is electrically coupled to the processing means 120. The memory means 130 is configured to store instructions, and the processing means 120 is configured to access the memory means 130 and execute the instructions stored thereon. The controller 110 comprises an input means 140 and an output means 150. The input means 140 may comprise an electrical input 140 of the controller 110. The output means 150 may comprise an electrical output 150 of the controller 110. The input 140 is arranged to receive a torque demand signal 145. The torque demand signal 145 is an electrical signal which is indicative of a torque demand from a driver and / or ADAS request. For example, the torque demand signal 145 may be received or derived from an accelerator pedal position sensor. The input 140 is also arranged to receive an acceleration signal 146 indicative of the actual acceleration of the vehicle. Although this is termed an “acceleration” signal, the signal need only provide an indication of the acceleration, rather than the acceleration itself. For example, the acceleration signal could comprise information relating to the speed of the vehicle from which the processing means 120 derives the acceleration of the vehicle. Optionally, the input is arranged to receive one or more further signals 147 from sensors or controllers associated with the vehicle. For example, from an inclinometer, an accelerometer, a terrain mode controller, and / or a tow hitch sensor. Optionally, the output 150 is arranged to output a torque request signal 155 indicative of the torque required from the at least one electric traction motor in dependence on the torque demand. The output 150 is arranged to output a torque limit override signal 156 to permit the at least one electric traction motor to temporarily increase torque output beyond a torque limit of the at least one electric traction motor. Optionally, the output 150 is also arranged to output a modified torque limit signal 157 for the at least one electric traction motor defining an additional torque limit which is greater than the torque limit. This is discussed in more detail below in relation to Figures 4 to 6. Figure 2 illustrates a vehicle 200 according to an embodiment of the present invention. The vehicle 200 comprises a control system 100 as illustrated in Figure 1. Figure 3 is a schematic diagram illustrating a powertrain system 300 of the vehicle 200 of Figure 2. The powertrain system 300 comprises at least one electric traction motor configured to provide motive force to the vehicle. In the illustrated embodiment, the powertrain system 300 comprises a first electric traction motor 312 and a second electric traction motor 314. Each electric traction motor is an electric motor arranged to convert electrical energy into kinetic energy in the form of mechanical torque and may also be arranged to convert kinetic energy into electrical energy (for example during regenerative braking). One or both electric traction motor may be an alternating current induction motor or a permanent magnet motor, or another type of suitable electric machine. 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. In the illustrated embodiment, the first electric traction motor 312 is configured to drive the front axle 212 of the vehicle 200 and the second electric traction motor 314 is configured to drive the rear axle 214 of the vehicle 200. However, it will be understood that the first and second electric traction motors could be swapped such that the first electric traction motor 312 drives the rear axle 214 and the second electric traction motor 314 drives the front axle 212. In other embodiments, one or both of the first and second electric traction motors 312, 314 may be configured to drive the wheels 216 of the vehicle in a different manner, such as via a transmission shaft and / or via one or more gears, differentials or transaxles, or directly. In other embodiments, the powertrain system may comprise more than two electric traction motors, for example three or four electric traction motors, each arranged to drive one of the wheels 216 of the vehicle 200. Although the vehicle is illustrated as having two pairs of wheels 216, 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 300 may further comprise one or more further prime movers, such as an internal combustion engine (not shown). The powertrain system 300 optionally further comprises a powertrain controller 320, or powertrain control module (PCM), configured to control the operation of the prime movers of the powertrain and / or one or more motor controllers electrically connected to the electric traction motors and configured to receive one or more motor control signals from the control system 100 and / or the PCM 320 and to output motor control signals to the electric traction motors 312, 314. In the illustrated embodiment, the powertrain system 300 comprises a front motor controller 322 and a rear motor controller 324, although these could be combined into a single motor controller. The control system 100 is electrically connected to the first and second electric traction motors 312, 314, either directly, or via one or more other components, such as the motor controllers 322, 324, as shown in Figure 3. Optionally, the control system 100 is, or comprises, or is part of, the powertrain controller 320. Optionally, the control system 100 is, or comprises, or is part of, the front and rear motor controllers 322, 324. The vehicle 200 also comprises an electrical energy storage 230, for example a traction battery. The electrical energy storage 230 is configured to deliver electrical energy to the first and second electric traction motors 312, 314, and optionally to receive and store electrical energy generated by the first and second electric traction motors 312, 314 (for example during regenerative braking). The first and second electric traction motors 312, 314 are electrically connected to the electrical energy storage 230, optionally by an inverter (not shown). In some embodiments, the electrical energy storage 230 is communicatively coupled to the powertrain controller 320. The electrical energy storage 230 is optionally a high voltage battery. The electrical energy storage 230 may have a voltage and capacity to support electric only driving for sustained distances. The electrical energy storage 230 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 vehicle 200 also includes one or more sensors configured to output an indication of the acceleration of the vehicle. For example, the vehicle may include one or more accelerometers. Alternatively, or in addition, the one or more sensors may comprise one or more speed sensors configured to output a speed signal from which the acceleration of the vehicle can be derived. For example, one or more rotational speed sensors may be associated with the electric traction motors 312, 314 to output a motor speed signal indicative of the rotational speed of one or both electric traction motors 312, 314. Alternatively, or in addition, the one or more speed sensors may include a wheel speed sensor configured to output a wheel speed signal indicative of the rotational speed of the wheels 216 of the vehicle. Optionally, the vehicle 200 includes one or more further sensors configured to output signals indicative of vehicle performance or one or more external conditions in which the vehicle is operating. For example, the vehicle may include one or more of: an inclinometer, an accelerometer, a tow hitch sensor, and a wheel torque sensor. Figure 4 illustrates a method 400 according to an embodiment of the invention. The method 400 is a method of controlling at least one electric traction motor of a torque-limited vehicle 200, such as the vehicle 200 illustrated in Figure 2. In particular, the method 400 is a method of controlling at least one electric traction motor of a torque-limited vehicle in which the at least one electric traction motor has a torque limit. The method 400 may be performed by the system 100 illustrated in Figure 1. In particular, the memory 130 may comprise computer-readable instructions which, when executed by the processor 120, perform the method 400 according to an embodiment of the invention. At step S-401, the control system 100 receives an indication of a torque demand in the form of a torque demand signal. The torque demand signal is an electrical signal which is indicative of a torque demand from a driver and / or ADAS request. There are several ways in which the torque demand signal can be implemented. For example, the torque demand signal may be based on driver input, for example driver input provided by a driver via an accelerator pedal / input. For example, the torque demand signal may be received or derived from an accelerator pedal position sensor signal. Alternatively, or in addition, the torque demand signal may be received from, or supplemented by, a torque demand from a vehicle control system, such as an ADAS controller. The torque demand signal may be a wheel level torque demand derived from an accelerator pedal position torque demand in combination with one or more further signals, such as a vehicle speed limiting signal from a stability control system, an ADAS torque demand modified signal, and / or a vehicle speed signal. Optionally, at step S-402, the control system 100 outputs a torque request signal for the at least one electric traction motor, the torque request signal indicating the torque output required from the at least one electric traction motor in dependence on the torque demand signal. There are several ways in which the torque request signal can be implemented. Where the powertrain comprises a plurality of electric traction motors, the torque request signal may comprise a plurality of torque request signals. For the powertrain 300 illustrated in Figure 3, the torque request signal may comprise a first torque request signal for the first electric traction motor and a second torque request signal for the second electric traction motor, the first and second torque request signals together providing a combined torque request for the powertrain. The torque request signal may be output directly to the at least one electric traction motor from the control system 100, and / or via a PCM 320, electric motor controller, or another intermediate component. In some embodiments, the torque request signal may be output by a powertrain control module or other controller of the vehicle. At step S-403, the control system receives an indication of the actual acceleration of the vehicle in the form of an acceleration signal. There are several ways in which the indication of the actual acceleration of the vehicle can be implemented. For example, the indication may be based on a signal from an accelerometer mounted on or within the vehicle. The indication may be derived by the control system from a vehicle speed signal. Such a vehicle speed signal may comprise a signal derived from a speed sensor measurement, such as from a wheel speed sensor or a motor rotational speed sensor. The indication of the actual acceleration of the vehicle can be received by the control system directly from a sensor, or from one or more other processors of the vehicle which have derived or the vehicle speed and / or acceleration and output that to the control system. In this example, the acceleration signal is a motor rotational speed signal indicating the rotational speed of one or more motors of the powertrain from which the vehicle speed and, consequently, actual vehicle acceleration can be derived. However, the acceleration signal could be a speed signal from a different source, such as a wheel speed sensor, or an acceleration signal from an accelerometer on or in the vehicle. The control system 100 determines the actual acceleration in dependence on the acceleration signal. In this example, where the acceleration signal is a motor rotational speed signal, the control system multiplies the motor rotational speed signal by a conversion factor to obtain a vehicle speed signal. The conversion factor takes into account the relationship between the rotational speed of the motor and the rotational speed of the wheels and the rolling radius of the wheels to convert the motor rotational speed signal into the vehicle speed signal. The control system then calculates the rate of change of the vehicle speed signal to obtain the acceleration signal indicating the actual acceleration of the vehicle. At step S-404, the control system 100 determines the expected acceleration of the vehicle in dependence on the torque demand. There are several ways in which determination by the control system of the expected acceleration can be implemented. For example, the control system 100 may ascertain a wheel level torque demand and convert that torque demand into an expected acceleration using vehicle-specific parameters, such as tyre rolling radius, estimated road load data, and vehicle mass. In one example, the control system receives the torque demand signal and subtracts from the torque demand signal an estimated road load data (RLD) torque to obtain an acceleration torque value. The RLD torque provides an estimation of the torque losses on a vehicle as a function of vehicle speed due to factors such as rolling resistance, driveline losses, and aerodynamic drag. RLD is typically obtained from testing, as will be understood by the skilled addressee, and typically indicates the torque losses when the vehicle is driven on a flat surface. The acceleration torque value provides an estimate of the amount of torque demand which is used to accelerate the vehicle once the RLD torque losses are taken into account. The acceleration torque value is divided by the rolling radius of the wheels to obtain an acceleration force value. The acceleration force value is divided by an estimation of vehicle mass to obtain an expected acceleration value. In this manner, the control system by wherein the wheel torque request signal defines the torque demand in terms of the torque determines the expected acceleration of the vehicle in dependence on the torque demand. At step S-405, the control system 100 compares the actual acceleration derived during step S-403 to the expected acceleration derived during step S-404. At step S-406, the control system determines, in dependence on the comparison during step S-405, whether the expected acceleration exceeds the actual acceleration by at least a threshold amount. There are several ways in which the comparison of the expected acceleration to the actual acceleration and the determination of whether the expected acceleration exceeds the actual acceleration by at least a threshold amount can be implemented. For example, the amount of actual acceleration can be subtracted from the amount of expected acceleration to obtain an acceleration delta, and subsequently to compare the acceleration delta to the threshold amount to determine whether the acceleration delta is equal to or greater than the threshold amount. Alternatively, or in addition, the threshold amount can be added to the actual acceleration amount and the total compared to the expected acceleration amount to see if the total is equal to or greater than the expected acceleration amount. One or both of the actual acceleration and expected acceleration signals may be half-wave rectified, i.e. clipped at zero, prior to the comparison such that only positive values of acceleration are considered. In this example, the actual acceleration and expected acceleration signals are clipped at zero so that only positive values of acceleration are used. The actual acceleration is then subtracted from the expected acceleration to obtain an acceleration delta. The acceleration delta is compared to the threshold amount to determine whether the acceleration delta is equal to or greater than the threshold amount. The threshold amount can be any suitable value. The threshold amount may be constant. For example, the threshold amount may be set at 1 m / s2,2 m / s2,3 m / s2,4 m / s2.5 m / s2, or higher. The threshold amount may vary according to one or more operating conditions or vehicle parameters. For example, the threshold amount may vary as a function of vehicle speed. The threshold amount may be stored on a memory and accessed by the control system from the memory. Alternatively, or in addition, the threshold amount may be determined by the control system from a look-up table in which the threshold amount is defined in relation to one or more operating conditions or vehicle parameters. If the control system determines that the expected acceleration exceeds the actual acceleration by at least the threshold amount, then the method optionally proceeds to step S-407 or directly to step S-408. If the control system determines that the expected acceleration does not exceed the actual acceleration by at least the threshold amount, then the method ends at step S-420. Optionally, at step S-407, the control system determines whether a limitation condition is met. In response to a determination that the limitation condition is met, the control system torque limit of the at least one electric traction motor is maintained, and the method ends at step S-420. Otherwise, the method proceeds to step S-408. There are several ways in which the determination of whether a limitation condition is met can be carried out. For example, the control system may receive a brake demand signal and the limitation condition may comprise determining from the brake demand signal that a braking pressure is requested. The brake demand signal is an electric signal indicating a braking pressure request, for example from a brake pedal position sensor. Alternatively, or in addition, the control system may receive a gradient signal as defined above and the limitation condition may comprise determining from the gradient signal that the gradient of the surface on which the vehicle is supported is less than the gradient threshold. The gradient threshold may be the same gradient threshold as defined below in the discussion of external conditions. Alternatively, or in addition, the control system may be configured to determine an elapsed time during which the expected acceleration exceeds the actual acceleration by at least the threshold amount. In such embodiments, the limitation condition may comprise determining that the elapsed time is less than a threshold period. This debouncing can reduce potential noise and reduce or prevent additional overrides of the torque limit by the control system where the expected acceleration exceeds the actual acceleration by at least the threshold amount only for a brief period. The threshold period may be any suitable period. For example, the threshold period may be 0.5 seconds, 1 second, 1.5 seconds, 2 seconds, or higher. At step S-408, the control system outputs a torque limit override signal. The torque limit override signal permits the at least one electric traction motor to temporarily increase torque output beyond its torque limit should excess or additional torque be requested. As will be understood, the torque limit is a pre-set maximum torque output which is below the maximum torque output capability of the at least one electric traction motor. The torque limit override signal enables the at least one electric traction motor to provide a torque output in excess of the usual torque limit amount if requested. The torque limit override signal may be output to any suitable component of the powertrain, for example directly to the at least one traction motor, to an electric motor controller connected to the at least one traction motor, or to a powertrain control module connected to the at least one traction motor. There are several ways in which the torque limit override signal can be implemented. For example, the torque limit override signal could be a simple instruction to temporarily disregard or cancel the torque limit. In such examples, the at least one electric traction motor would be operable to provide the full extent of its maximum torque capability should such a torque output be requested. Alternatively, the torque limit override signal could define a further torque limit which is higher than the torque limit. The further torque limit may be equal to or lower than the maximum torque capability of the at least one electric traction motor. Optionally, at step S-409, the control system 100 outputs a modified torque limit signal defining a further torque limit which is higher than the torque limit. In this manner, the at least one electric traction motor would be operable to provide a torque output which is greater than the torque limit. The further torque limit may be less than the maximum torque capability of the at least one electric traction motor. There are several ways in which the modified torque limit signal can be implemented. For example, the modified torque limit signal could impose the further torque limit instantaneously, i.e. in a step change from the torque limit to the further torque limit. In other examples, the modified torque limit signal could be modulated to define a modified torque limit value which is gradually increased from the torque limit to the further torque limit and / or gradually decreased from the further torque limit to the torque limit. The modified torque limit value may be gradually increased from the torque limit to the further torque limit at a ramp-up rate. The ramp-up rate could be constant. Alternatively, the ramp-up rate could be different for different circumstances and / or vehicle operating parameters. Optionally, at step S-410, the control system modulates the modified torque limit signal to increase the modified torque limit value at a ramp-up rate which is selected in dependence on a comparison of the torque limit with the torque generated by the powertrain at the time at which the determination that the expected acceleration exceeds the actual acceleration by the threshold amount is made. For example, the control system receives a torque output signal indicative of a torque generated by the at least one electric traction motor at a time when a determination that the expected acceleration exceeds the actual acceleration by the threshold amount is made, compares the torque generated to the torque limit, and modulates the modified torque limit signal in dependence on the comparison of the torque generated to the torque limit. In such examples, the modified torque limit signal could be modulated to define a modified torque limit value which increases at a ramp-up rate which varies as a function of the difference between the torque generated and the torque limit. For example, the ramp-up rate could be set at a slow rate when the torque generated is equal to the torque limit and at increasingly quicker rates with increasing distance of the torque generated from the torque limit. This can reduce the extent to which a driver might feel a sudden increase in the available torque when the torque output is already at the torque limit at the time when the torque limit is overridden. In one example, the modified torque limit signal is modulated to increase the modified torque limit value at a first ramp-up rate when the torque generated is less than the torque limit and at a second ramp-up rate when the torque generated is equal to the torque limit. Optionally, at step S-411, the control system varies the modified torque limit value as a function of vehicle speed. In this manner, overriding of the torque limit can be caused to occur only at lower speeds when the additional torque is most desired and / or be blended out at higher speeds. In an example step, the control system receives an indication of vehicle speed, compares the vehicle speed to a feature speed threshold, and modulates the modified torque limit signal in dependence on the comparison. For example, the control system may apply a speed reduction coefficient to the modified torque limit signal, wherein the speed reduction coefficient varies as a function of vehicle speed. In such examples, when the vehicle speed is less than the feature speed threshold, the speed reduction coefficient is 1, such that the modified torque limit value is unchanged by the speed reduction coefficient. When the vehicle speed is above the feature speed threshold, the speed reduction coefficient reduces from 1 to 0 as a function of the difference between the feature speed threshold and the vehicle speed. The speed reduction coefficient enables the torque limit increase to be blended out at higher speeds at which the additional torque capacity is likely to be no longer needed, or its absence less apparent. The speed reduction coefficient may reduce at any suitable rate in relation to vehicle speed. For example, the speed reduction coefficient may reduce linearly at a rate of 0.1 per kph such that the speed reduction coefficient reduces from 1 to 0 over a speed range of 10kph. The feature speed threshold may be any suitable value. For example, the feature speed threshold may be 20kph, 25kph, 30kph, 35kph, 40kph, or higher. The speed reduction coefficient may vary above the feature speed threshold according to any suitable function of vehicle speed. For example, the speed reduction coefficient may reduce linearly with vehicle speed. For example, the speed reduction coefficient may reduce linearly from 1 at the feature speed threshold to 0 at a feature speed limit. The feature speed limit may be any suitable value above the feature speed threshold. For example, the feature speed limit may be 30kph, 35kph, 40kph, 45kph, 50kph, or higher. In one example, the feature speed threshold is 30kph and the feature speed limit is 40kph, such that the modified torque limit value is blended down to the torque limit between 30kph to 40kph. In an embodiment, once the torque limit override signal has been outputted, the method ends at step S-420. Optionally, at step S-412, the control system monitors whether the expected acceleration continues to exceed the actual acceleration by the threshold amount, in response to a determination that the expected acceleration no longer exceeds the actual acceleration by the threshold amount, reinstates the torque limit. There are several ways in which the reinstatement of the torque limit can be implemented. For example, the torque limit can be reinstated by stopping the output of the torque limit override signal in response to the determination that the expected acceleration no longer exceeds the actual acceleration by the threshold amount. In such examples, the torque limit can be reinstated instantaneously. Alternatively, or in addition, the control system may reinstate the torque limit by modulating the modified torque limit signal to gradually reduce the modified torque limit value from the further torque limit to the torque limit over a ramp-down period and / or at a ramp-down rate. The ramp-down rate could be constant across multiple scenarios. Alternatively, the ramp-down rate could be different for different circumstances and / or vehicle operating parameters. For example, at different vehicle speeds and / or in different vehicle modes. The ramp-down rate may be the same as or different to the ramp-up rate used to increase the modified torque limit value. Once the torque limit has been reinstated, the method ends at step S-420. With this method, the control system can detect when an additional torque output beyond the torque limit is required, and override the torque limit, based on signals which are already secured from a functional safety point of view. Figure 5 illustrates a method 500 according to an embodiment of the invention. The method 500 is a method of controlling at least one electric traction motor of a torque-limited vehicle 200, such as the vehicle 200 illustrated in Figure 2, and can be carried out in parallel to the method 400 discussed above in relation to Figure 4. The method 500 may be performed by the system 100 illustrated in Figure 1. In particular, the memory 130 may comprise computer-readable instructions which, when executed by the processor 120, perform the method 500 according to an embodiment of the invention. At step S-501, the control system 100 receives an indication of an external condition in which the vehicle is operating, in the form of an external condition signal. The external condition signal is an electrical signal. The term “external condition” refers to aspects of the driving environment in which the vehicle is operating, particularly where these may result in an increased torque required to propel the vehicle. This includes, but is not limited to, the gradient of the surface on which the vehicle is driven, the type of terrain on which the vehicle is driven and whether this is a high rolling resistance terrain, such as soft ground, and whether the vehicle is towing a trailer or other device attached to the tow hitch or otherwise connected to the vehicle. There are several ways in which the external condition signal can be implemented. For example, the external condition signal may comprise a gradient signal indicating a gradient of a surface on which the vehicle is supported, for example as received or derived from an inclinometer. Alternatively, or in addition, the external condition signal may comprise a tow signal indicating a presence or absence of a vehicle trailer connected to the vehicle, for example as received or derived from a tow hitch sensor or from a networked controller which indicates when an electrical device has been plugged into an electrical connector on the tow hitch, and / or indicates when a towing program has been selected by the driver via an input device such as an infotainment system. Alternatively, or in addition, the external condition signal may comprise a terrain signal indicating a type of terrain on which the vehicle is supported, for example as received from a terrain mode selector. In such examples, the terrain mode selector may be configured to enable manual selection by a driver of a terrain mode, such as a soft ground driving mode, and / or automatic detection and selection of a terrain mode based on one or more sensors, such as a suspension travel sensor, and / or cameras associated with the vehicle. The external conditional signal may comprise multiple signals, for example for different sources. At step S-502, the control system determines if the external condition is an additional torque demand condition. An additional torque demand condition is one or more predefined conditions in which resistance against the acceleration of the vehicle is sufficiently high that a torque output beyond the usual torque limit of the at least one electric traction motor may be or is required to maintain appropriate vehicle performance. These may be considered as adverse driving conditions in which an additional torque output may be required. There are several ways in which the determination by the control system of whether the external condition is an additional torque demand condition can be implemented. This may depend on the nature of the external condition signal. For example, where the external condition signal comprises a gradient signal indicating a gradient of a surface on which the vehicle is supported, the additional torque demand condition may require at least that the gradient is greater than a gradient threshold. In other words, the determination may be carried out by determining, in dependence on the gradient signal, if the gradient is greater than a gradient threshold. The gradient threshold may be any suitable value of incline. The gradient threshold may be selected in dependence on one or more of: vehicle characteristics, vehicle operating mode, powertrain characteristics and driver characteristics. For example, the gradient threshold maybe 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25% or higher. Where the external condition signal comprises a tow signal indicating a presence or absence of a vehicle trailer connected to the vehicle, the additional torque demand condition may require at least that the tow signal indicates the presence of a vehicle trailer connected to the vehicle. In other words, the determination may be carried out by determining, in dependence on the tow signal, if a vehicle trailer is connected to the vehicle. Where the external condition signal comprises a terrain signal indicating a type of terrain on which the vehicle is supported, the additional torque demand condition may require at least that the terrain signal indicates that the type of terrain is an additional torque terrain. In other words, the determination may be carried out by determining, in dependence on the terrain signal, if the terrain is an additional torque terrain. Additional torque terrains are surfaces for which rolling resistance is substantially higher than for typical road surfaces and for which, consequently, the required torque outputfor a given vehicle acceleration is also substantially higher. Additional torque terrains may include, but are not limited to, at least one of: loose terrain, soft terrain, and / or rough terrain. Example terrains may include gravel, sand, snow, mud, and / or water. The determination of whether the external condition is an additional torque demand condition may require only one of the above requirements to be met. Alternatively, the determination may require a combination of the above requirements to be met before the external condition is deemed an additional torque demand condition. If the control system determines that the external condition is not an additional torque demand condition, no further action is taken, and the method 500 ends at step S-520. If the control system determines that the external condition is an additional torque demand condition, the method proceeds optionally to step S-503 or directly to step S-504. Optionally, at step S-503, the control system determines whether a limitation condition is met. This may be carried out in the same manner to that described above in relation to optional step S-407 of method 400. In response to a determination that the limitation condition is met, the control system torque limit of the at least one electric traction motor is maintained, and the method 500 ends at step S-520. Otherwise, the method proceeds to step S-504. At step S-504, the control system outputs the torque limit override signal. This may be carried out in the same manner to that described above in relation to one or more of steps S-408 to S-411 of method 400. The torque limit override signal permits the at least one electric traction motor to temporarily increase torque output beyond its torque limit should excess or additional torque be requested. Optionally, at step S-505, the control system monitors the external condition, determines if the external condition is still an enhanced torque demand condition, and reinstates the torque limit in response to a determination that the external condition is no longer an enhanced torque demand condition. There are several ways in which the reinstatement of the torque limit can be implemented. For example, the torque limit can be reinstated by stopping the output of the torque limit override signal in response to the determination that the external condition is no longer an enhanced torque demand condition. In such examples, the torque limit can be reinstated instantaneously. Alternatively, or in addition, the control system may output a modified torque limit signal which defines a modified torque limit value and modulate the modified torque limit signal value to gradually decrease the modified torque limit value down to the torque limit over a ramp-down period and / or at a ramp-down rate. The ramp-down rate could be constant across multiple scenarios. Alternatively, the ramp-down rate could be different for different circumstances and / or vehicle operating parameters. For example, at different vehicle speeds and / or in different vehicle modes. The ramp-down rate may be the same as, or different to, the ramp-up rate used to increase the modified torque limit value. Once the torque limit has been reinstated, the method ends at step S-520. Figure 6 shows a graph 600 illustrating vehicle behaviour when the method 400 is followed. The graph 600 includes several plots illustrating how various signals associated with the method 600 vary in relation to each other during a pull-away from rest on a gradient. In such a scenario, the vehicle is facing forward towards an incline and accelerates from a stationary position. Plot (a) illustrates the relationship between torque output and the torque limit imposed on the powertrain. Plot (a) illustrates torque in Nm along the y-axis against time on the x-axis. Plot (a) includes a torque output signal 601 indicating an actual torque output from the at least one electric traction motor of the powertrain, a torque limit 602 indicating the torque limit value imposed on the at least one electric traction motor, a maximum torque capability 603 indicating the maximum possible torque output from the at least one electric traction motor, and a modified torque limit signal 604 indicating the modified torque limit value imposed on the at least one electric traction motor when the torque limit overridden. Plot (b) illustrates the vehicle speed in km / h along the y-axis against time on the x-axis. Plot (b) includes an indication of vehicle speed in the form of a vehicle speed signal 610. In this example, the vehicle speed signal 610 is a direct measurement of vehicle speed taken from a wheel rotation sensor. However, the vehicle speed signal could be any indication of vehicle speed, such as a motor rotational speed signal, which can be converted by the control system into a vehicle speed measurement. The vehicle speed signal 610 can be used by the control system to derive the actual acceleration of the vehicle. As such, the vehicle speed signal 610 can be regarded as an indication of the actual acceleration of the vehicle. Plot (c) illustrates torque demand as a function of time. Plot (c) includes an indication of torque demand in the form of a torque demand signal 620. In this example, the torque demand signal 620 is an accelerator pedal position signal from an accelerator pedal position sensor and is illustrated as the percentage of total accelerator pedal travel along the y-axis (where 0% indicates zero accelerator pedal depression and 100% indicates a fully depressed position) against time on the x-axis. At time to, the vehicle is stationary, the accelerator pedal is not depressed, the torque output is zero and the torque limit is imposed. Consequently, the torque output signal 601 is zero, the vehicle speed signal 610 is zero, and the torque demand signal 620 is zero. At time ti, the driver begins to depress the accelerator pedal and, as a result, the torque demand signal 620 and the torque output signal 601 begin to rise. At this point, the vehicle speed signal 610 remains at zero, indicating that the vehicle is still stationary. At time t?, the vehicle speed signal 610 begins to fall, indicating that the vehicle is decelerating despite no reduction in torque output, and the driver responds by modulating the accelerator pedal position to increase vehicle speed, as illustrated by the increase to the torque demand signal 620 and the torque output signal 604. This speed reduction indicates that the external conditions are such that the resistance to acceleration of the vehicle are higher than for typical driving situations. For example, the gradient of the incline may have increased and / or the terrain may have changed to a higher resistance type of terrain, such as mud or sand. During this process, the control system monitors the torque demand signal 620 and the vehicle speed signal 610 to determine and compare the expected vehicle acceleration and the actual vehicle acceleration, for example as outlined above in relation to Figure 4. Between band b, the vehicle speed signal 610 continues to drop despite the continued increase in the torque demand signal 620. The control system determines that the expected acceleration exceeds the actual acceleration by more than the threshold amount and the control system determines that the external condition in which the vehicle is operating is now an additional torque demand condition. The control system carries out a debouncing step in which it determines an elapsed period during which the actual acceleration remains below the expected acceleration and compares this to the threshold period for implementation. At time b, once the elapsed time has exceeded the threshold period, the control system outputs a torque limit override signal indicating that the torque limit 602 can be temporarily disregarded. In this example, the control system outputs a modified torque limit signal 604 to define a modified torque limit value which is gradually increased from the torque limit 602 to the maximum torque capability 603 over a ramp-up period defined by (t4 — b) and at a ramp-up rate defined by (maximum torque capability - torque limit) / (t4—b). Since the actual powertrain torque output at time b as indicated by the torque output signal 601 is less than the torque limit 602, the control system increases the modified torque limit value at a fast ramp-up rate. If the torque output was already at the torque limit at time b, a slower ramp-up rate may have been used by the control system to prevent any sudden increase in actual torque output when the torque limit is overridden. At time U, the torque limit is overridden, and the modified torque limit signal has lifted the available torque to the maximum torque capability 603. This enables the torque output to exceed the torque limit 602. At time ts, the vehicle speed signal 610 begins to increase as a result of the additional torque output provided by the at least one electric traction motor, and the driver maintains the torque demand and torque output at generally constant levels. At time b, the driver lifts completely off of the accelerator, causing the torque demand signal 620, and shortly afterwards the torque output signal 601, to drop to zero and the vehicle is brought to a stop. At time t?, the torque output signal 601, vehicle speed signal 610, and torque demand signal 620 are all at zero and the control system determines that the expected acceleration (now zero) no longer exceeds the actual acceleration (also now zero) by the threshold amount and that this has not been the case for an elapsed period which is greater than the threshold period. As a result, the control system determines that the torque limit may be reinstated. The control system reinstates the torque limit by modulating the modified torque limit signal to gradually decrease the modified torque limit value from the maximum torque capability 603 back down to the torque limit 602 over a ramp-up period defined by (b-1?) and at a ramp-down rate defined by (torque limit - maximum torque capability) / (b-1?). As can be seen, the ramp-down rate is slower than the ramp-up rate over which the modified torque limit was increased when the torque limit was lifted. At time b, the torque limit 602 has been reimposed on the powertrain and the method ends. In this manner, the control system can determine when the external condition in which the vehicle operates is such that additional torque output beyond the torque limit is required to ensure appropriate acceleration. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application.

Claims

1. A control system for controlling at least one electric traction motor of a torque-limited vehicle in which the at least one electric traction motor has a torque limit, the control system comprising one or more processors collectively configured to:receive an indication of a torque demand;output a torque request signal for the at least one electric machine in dependence on the torque demand; determine an expected acceleration of the vehicle in dependence on the torque demand;receive an indication of the actual acceleration of the vehicle;compare the expected acceleration to the actual acceleration;determine, in dependence on the comparison, if the expected acceleration exceeds the actual acceleration by a threshold amount; andin dependence on a determination that the expected acceleration exceeds the actual acceleration by the threshold amount, output a torque limit override signal.

2. The control system of claim 1, wherein the torque limit override signal comprises a modified torque limit signal defining a further torque limit which is higher than the torque limit.

3. The control system of claim 2, wherein the one or more processors are collectively configured to modulate the modified torque limit signal to define a modified torque limit value which is gradually increased from the torque limit to the further torque limit and / or gradually decreased from the further torque limit to the torque limit.

4. The control system of claim 3, wherein the one or more processors are collectively configured to modulate the modified torque limit signal to gradually increase the modified torque limit value from the torque limit to the further torque limit at a ramp-up rate which varies in dependence on one or more vehicle operating parameters.

5. The control system of claim 4, wherein the one or more processors are collectively configured to:in dependence on the determination that the expected acceleration exceeds the actual acceleration by the threshold amount, receive a torque output signal indicative of a torque generated by the at least one electric traction motor at a time when the determination that the expected acceleration exceeds the actual acceleration by the threshold amount is made, compare the torque generated to the torque limit; andin response to the comparison, modulate the modified torque limit signal to increase the modified torque limit value at a first ramp-up rate if the torque generated is less than the torque limit and to increase the modified torque limit value at a second ramp-up rate which is slower than the first increase rate if the torque generated is the same as the torque limit.

6. The control system of any of claims 3 to 5, wherein the one or more processors are collectively configured to: monitor whether the expected acceleration continues to exceed the actual acceleration by the threshold amount; and in response to a determination that the expected acceleration no longer exceeds the actual acceleration by the threshold amount, reinstate the torque limit by modulating the modified torque limit signal to gradually reduce the modified torque limit value from the further torque limit to the torque limit at a ramp-down rate that varies in dependence on one or more vehicle operating parameters.

7. The control system of any of claims 3 to 6, wherein the one or more processors are collectively configured to:receive an indication of vehicle speed;compare the vehicle speed to a feature speed threshold; andin response to a determination that the vehicle speed exceeds the feature speed threshold, modulate the modified torque limit signal to gradually reduce the modified torque limit value from the further torque limit to the torque limit as a function of vehicle speed.

8. The control system of any preceding claim, wherein the one or more processors are collectively configured toreceive an indication of an external condition in which the vehicle is operating;determine if the external condition is an additional torque demand condition; andin dependence on the determination that the that the external condition is an additional torque demand condition, output the torque limit override signal.

9. The control system of any preceding claim, wherein the indication of the actual acceleration of the vehicle is a speed signal from which the one or more processors are collectively configured to derive the actual acceleration of the vehicle.

10. The control system of claim 9, wherein the speed signal is a motor speed signal indicative of a rotational speed of the at least one electric traction motor and wherein the one or more processors are collectively configured to calculate a vehicle speed based on the rotational speed to derive the actual acceleration of the vehicle.

11. The control system of any preceding claim, wherein the one or more processors are collectively configured to:determine whether a limitation condition is met; andin response to determining that a limitation condition is met, impose the torque limit on the at least one electric traction motor irrespective of a determination that the expected acceleration exceeds the actual acceleration by the threshold amount, wherein the limitation condition is met if:a brake demand signal indicates that a braking pressure is requested;a gradient signal indicates that a gradient of a surface on which the vehicle is supported is less than a gradient threshold;the expected acceleration exceeds the actual acceleration by the threshold amount for less than a threshold period of time; andthe vehicle speed exceeds a maximum speed threshold.

12. A system comprising the control system of any preceding claim and at least one electric traction motor.

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

14. A method for controlling at least one electric traction motor of a torque-limited vehicle in which the at least one electrictraction motor has a torque limit, the method comprising:receiving an indication of a torque demand;outputting a torque request signal for the at least one electric machine in dependence on the torque demand;determining an expected acceleration of the vehicle in dependence on the torque demand;receiving an indication of the actual acceleration of the vehicle;comparing the expected acceleration to the actual acceleration;determining, in dependence on the comparison, if the expected acceleration exceeds the actual acceleration by a threshold amount; andin dependence on a determination that the expected acceleration exceeds the actual acceleration by the threshold5 amount, outputting a torque limit override signal.

15. Computer readable instructions which, when executed by a computer, are arranged to perform a method according to claim 14.

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