Peak torque control system and method

Abstract

Figure 4 shows peak torque capability CPTQ 110 of a torque generating machine (e.g. electric motor of a vehicle). A vehicle controller progressively limits the peak torque output CPTQ 110 of the torqu

Description

10 06 25 TECHNICAL FIELD The present disclosure relates to a control system and method. The control system and 5 method are suitable for controlling an output torque of a torque generating machine. More particularly, but not exclusively, the present disclosure relates to a control system for determining a peak output torque limit to restrict the peak torque output of the torque generating machine. Aspects of the invention relate to a control system, a system, a vehicle, a non-transitory computer-readable medium and a computer software. 10 BACKGROUND The operation of a torque generating machine to generate a peak torque may result in a cumulative fatigue in driveline components in a road vehicle. This problem has been exacerbated by electric drive units which are capable of generating high peak torques. The 15 repeated operation of the electric drive unit to generate a high peak torque may result in the cumulative fatigue of components, for example resulting in the bending or wear of transmission gears. This cumulative fatigue may mean that the vehicle is not able to satisfy expected duty cycles. One approach to addressing this problem would be to increase the size of the transmission gears, for example, to increase diameter and / or width. However, this increases 20 the weight and cost of the transmission gears and may cause packaging issues. Another approach would be to implement a torque cap (limit) which restricts the maximum torque output of the torque generating machine. The torque cap is operative to limit the peak torque of the torque generating machine, either across all operating ranges or in specific gears. The torque cap may reduce functional capability of the vehicle, for example resulting in a reduced 25 towing capacity and / or off-road capability. Alternatively, a torque cap may be activated in certain vehicle operating modes to restrict the available maximum peak torque output. The torque cap may be deactivated in other vehicle operating modes. The torque cap may reduce fatigue damage of the driveline components by reducing the number of cycles at peak torque. However, the torque cap may be perceptible to a user of the vehicle as reduced vehicle 30 performance. It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. 35 10 06 25 SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a control system, a system, a vehicle, a non-transitory computer-readable medium and computer software as claimed in the appended claims. 5 According to an aspect of the present invention there is provided a control system for controlling a peak torque output of a torque generating machine disposed in a vehicle, the control system comprising one or more controllers, the control system configured to: receive a motor speed signal indicating an operating speed of the torque generating 10 machine; determine a peak torque output limit to restrict the peak torque output of the torque generating machine, the peak torque output limit being determined in dependence on the operating speed of the torque generating machine; and output a peak torque control signal representing the determined peak torque output 15 limit; wherein the control system is configured progressively to modify the peak torque output limit in dependence on changes in the operating speed of the torque generating machine. 20 The control system is configured to control the peak torque output deliverable by the torque generating machine. The peak torque output may be reduced to less than a design capability of the torque generating machine. At least in certain embodiments, the peak torque output limit may restrict the peak torque output to help protect a driveline in the vehicle from damage. In particular, the peak torque output may be restricted to reduce or prevent cumulative fatigue 25 damage. The peak torque output may, for example, be restricted to help reduce or avoid fatigue damage to transmission gears. The peak torque output limit may reduce the cumulative cycles at a peak torque, thereby reducing a cumulative damage. The control system modifies the peak torque output limit in dependence on changes in the operating speed of the torque generating machine. Other factors may also be taken into account. The control system may 30 progressively modify the peak torque output limit such that changes occur in parallel with (or in unison with) changes in the operating speed of the torque generating machine. The control system may implement incremental changes or substantially continuous changes in the peak torque output limit in dependence on changes in the operating speed. The control system is configured progressively to change the peak torque output limit. At least in certain 35 embodiments, the changes in the available peak torque output from the torque generating machine are gradual and may be imperceptible to a user of the vehicle. 10 06 25 At least in certain embodiments, the peak torque output limit may be a dependent variable which changes in dependence on changes in the operating speed of the torque generating machine. A change in the operating speed results in a corresponding change in the peak torque output limit. The control system may be configured to determine the peak torque output 5 limit as a continuous variation (i.e. a change without abrupt changes or discontinuities). The continuous variation may be defined with reference to the operating speed of the torque generating machine. At least in certain embodiments, the control system may implement a continuous function to define the peak torque output limit as a continuous variation. 10 The peak torque output limit may be defined with respect to the operating speed of the torque generating machine by a curve having a positive gradient or a negative gradient (i.e., a nonzero gradient). The peak torque output limit may be defined by a curve having a negative gradient as the operating speed of the torque generating machine increases. The peak torque output limit may be defined by a curve having a positive gradient as the operating speed of 15 the torque generating machine decreases. The peak torque control signal may be configured to limit the peak torque output of the torque generating machine to below a peak torque capability of the torque generating machine. The limit may be applied over part or all of an operating speed range of the torque generating 20 machine. The peak torque control signal may be applied over the whole of the operating speed range of the torque generating machine. Alternatively, the peak torque control signal may be applied over only part of the operating speed range of the torque generating machine. The torque generating machine may be an internal combustion engine. Alternatively, the 25 torque generating machine may be an electric machine. The torque generating machine may be an electric traction machine for propelling the vehicle. The electric traction machine may be provided in an electric drive unit for a vehicle. The electric traction machine may be configured to drive one or more wheels of the vehicle. 30 The one or more controllers may collectively comprise: at least one electronic processor having an electrical input for receiving receive the motor speed signal indicating the operating speed of the torque generating machine; and at least one memory device electrically coupled to the at least one electronic processor and having instructions stored therein. The at least one electronic processor may be configured to access the at least one memory device and 35 execute the instructions thereon so as progressively to modify the peak torque output limit in dependence on changes in the operating speed of the torque generating machine. 10 06 25 The peak torque output limit may be inversely proportional to the operating speed of the torque generating machine. The control system may be configured progressively to decrease the peak torque output limit in dependence on an increase in the operating speed of the torque generating machine. Alternatively, or in addition, the control system may be configured 5 progressively to increase the peak torque output limit in dependence on a decrease in the operating speed of the torque generating machine. The operating speed of the torque generating machine may be indicated by a motor speed signal. The peak torque output limit may be inversely proportional to the operating speed of 10 the torque generating machine indicated by the motor speed signal. The torque generating machine has a peak torque capability. The peak torque capability may represent a design torque capability. The peak torque output limit is defined as less than the peak torque capability of the torque generating machine. The peak torque control signal may 15 be configured to limit the peak torque output of the torque generating machine to below the peak torque capability of the torque generating machine. The peak torque output may be controlled in dependence on a current (i.e., an instantaneous) operating speed of the torque generating machine. The peak torque capability of the torque generating machine may vary with respect to an operating speed of the torque generating machine. 20 The peak torque output limit may be defined as a proportion of the peak torque capability of the torque generating machine. The proportion of the peak torque capability may be determined in dependence on the operating speed of the torque generating machine. The proportion may, for example, be defined as a percentage of the peak torque capability for a 25 given operating speed of the torque generating machine. The control system may be configured progressively to decrease the peak torque output limit as a proportion of the peak torque capability of the torque generating machine as the operating speed of the torque generating machine increases. Alternatively, or in addition, the control 30 system may be configured to progressively increase the peak torque output limit as a proportion of the peak torque capability of the torque generating machine as the operating speed of the torque generating machine decreases. The control system may be configured to determine the peak torque capability dynamically, 35 for example by implementing a peak torque algorithm. The peak torque algorithm may comprise a continuous function which defines the peak torque output limit as a continuous 10 06 25 variation. The continuous function may define the continuous variation of the peak torque output limit in dependence on the operating speed of the torque generating machine. Alternatively, the continuous variation of the peak torque output limit may be predefined. The 5 control system may be configured to determine the peak torque output limit by accessing a look-up table. The look-up table may, for example, define the peak torque output limit with reference to an operating speed of the torque generating machine. The peak torque output limit may restrict the peak torque output to below the peak torque 10 capability of the torque generating machine when the operating speed of the torque generating machine is greater than a first speed threshold. The peak torque output limit may be substantially equal to the peak torque capability of the torque generating machine when the operating speed of the torque generating machine is less 15 than the first speed threshold. The peak torque output limit may restrict the peak torque output to below the peak torque capability of the torque generating machine when the operating speed of the torque generating machine is less than a second speed threshold. The peak torque output limit may restrict the 20 peak torque output limit to less than the peak torque capability when the operating speed is within the operating range defined by the first and second speed thresholds. The peak torque output limit may be equal to the peak torque capability of the torque generating machine when the operating speed of the torque generating machine is greater 25 than the second speed threshold. Thus, the peak torque output limit may restrict the peak torque output to below the peak torque capability of the torque generating machine when the operating speed of the torque generating machine is greater than a first speed threshold and less than a second speed threshold, and 30 the first speed threshold may be greater than zero and the second speed threshold may be less than a maximum speed of the torque generating machine. The control system is configured to modify the peak torque output limit at least substantially continuously in dependence on changes in the operating speed of the torque generating 35 machine. The substantially continuous changes in the peak torque output limit may be represented by a curved line (i.e., not a straight line). 10 06 25 The control system is configured to modify the peak torque output limit in a plurality of steps (or intervals) in dependence on changes in the operating speed of the torque generating machine. The plurality of steps (or intervals) may represent incremental changes in the peak torque output limit. 5 The torque generating machine may an electric machine. For example, the torque generating machine may be an electric traction machine. The electric machine may be disposed in an electric drive unit (EDU) of the vehicle. 10 According to a further aspect of the present invention there is provided a system comprising: the control system of any preceding claim, including at least a first controller, wherein the at least a first controller is arranged to output a signal for limiting a peak torque output of a torque generating machine; and means configured to receive the signal and to control operation of the torque generating machine in dependence on the signal. 15 According to a further aspect of the present invention there is provided a vehicle comprising a control system as described herein. According to a further aspect of the present invention there is provided a method of controlling 20 a peak torque output of a torque generating machine, the method comprising: determining an operating speed of the torque generating machine; and setting a peak torque output limit of the torque generating machine over at least a portion of an operating speed range of the torque generating machine; wherein the peak torque output limit of the torque generating machine is set in 25 dependence on the determined operating speed of the torque generating machine; and the method comprises progressively modifying the peak torque output limit in dependence on changes in the operating speed of the torque generating machine. The method may comprise modifying the peak torque output limit at least substantially 30 continuously in dependence on changes in the operating speed of the torque generating machine. The method may comprise modifying the peak torque output limit in a plurality of steps in dependence on changes in the operating speed of the torque generating machine. 35 10 06 25 According to a further aspect of the present invention there is provided a non-transitory computer-readable medium having a set of instructions stored therein which, when executed, cause a processor to perform the method as described herein. 5 According to a further aspect of the present invention there is provided a computer software that, when executed, is arranged to perform a method as described herein. Any control unit or controller described herein may suitably comprise a computational device having one or more electronic processors. The system may comprise a single control unit or 10 electronic controller or alternatively different functions of the controller may be embodied in, or hosted in, different control units or controllers. As used herein the term “controller” or “control unit” will be understood to include both a single control unit or controller and a plurality of control units or controllers collectively operating to provide any stated control functionality. To configure a controller or control unit, a suitable set of instructions may be provided which, 15 when executed, cause said control unit or computational device to implement the control techniques specified herein. The set of instructions may suitably be embedded in said one or more electronic processors. Alternatively, the set of instructions may be provided as software saved on one or more memory associated with said controller to be executed on said computational device. The control unit or controller may be implemented in software run on 20 one or more processors. One or more other control unit or controller may be implemented in software run on one or more processors, optionally the same one or more processors as the first controller. Other suitable arrangements may also be used. Within the scope of this application it is expressly intended that the various aspects, 25 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 30 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 35 One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 10 06 25 Figure 1 shows a schematic representation of a vehicle having a control system for controlling operation of a torque generating machine in accordance with an embodiment of the present invention; 5 Figure 2 show a schematic representation of a first electronic control unit for determining a reference speed of the vehicle shown in Figure 1; Figure 3 show a schematic representation of a peak torque control unit for determining a peak torque output limit of the torque generating machine shown in Figure 1; 10 Figure 4 shows a first torque plot representing a torque curve of the torque generating machine shown in Figure 1 (Torque, in Nm, being plotted against Engine Speed, in revolutions per minute (rpm), with Peak torque shown in heavy line and Continuous torque in fine line); and 15 Figure 5 shows a first power plot representing a power curve of the torque generating machine shown in Figure 1 (Power, in KW, being plotted against Engine Speed, in revolutions per minute (rpm), with Peak power shown in heavy line and Continuous power in fine line). DETAILED DESCRIPTION 20 A control system 3 for controlling operation of at least one torque generating machine 7 of a vehicle 5 in accordance with an embodiment of the present invention is described herein with reference to the accompanying Figures. The control system 3 may form part of a larger system. 25 As shown in Figure 1, the vehicle 5 is a road vehicle having a plurality of front and rear wheels WF, WR. The vehicle 5 in the present embodiment is an automobile. The control system 3 may be implemented in other types of road vehicle, such as a utility vehicle or a sports utility vehicle. The at least one torque generating machine 7 is operative to generate a tractive force to propel the vehicle 5. The or each torque generating machine 7 is in the form of a first electric 30 drive unit 7 in the present embodiment. The first electric drive unit 7 is an electric traction motor for driving the front wheels WF and / or the rear wheels WR of the vehicle 5. The vehicle 5 comprises a traction battery unit 9 and an inverter 11 for supplying electric current to the first electric drive unit 7. The battery unit 9 is a high voltage (HV) battery and is 35 configured to supply electrical current to the inverter 11. A motor control unit 13 is provided for controlling operation of the first electric drive unit 7. The motor control unit 13 outputs a control signal SCON to control operation of the inverter 11. The inverter 11 controls an operating 10 06 25 speed of the first electric drive unit 7 in dependence on the control signal SCON. In the present embodiment, the first electric drive unit 7 is operative to drive the front wheels WF. In use, the or each electric drive unit 7 is powered by the traction battery unit 9. The first electric drive unit 7 comprises or consists of an electric traction motor configured to drive the front wheels WF. 5 It will be understood that the first electric drive unit 7 may be configured to drive the rear wheels WR of the vehicle 5. The present embodiment is described with reference a vehicle 5 having a single electric drive unit. The vehicle 5 may comprise one or more electric drive units. The vehicle 5 may, for example, comprise a second electric drive unit (not shown) for driving the rear wheels WR. Alternatively, first and second electric drive units may be associated with 10 wheels on the left and right sides of the vehicle 5. For example, the first and second electric drive units may drive the front wheels WF or the rear wheels WR of the vehicle 5. The control strategy described herein in respect of the first electric drive unit 7 may be applied to each of the one or more electric drive units. 15 The first electric drive unit 7 has a peak torque capability CPTQ. The peak torque capability CPTQ is a design parameter that represents the maximum (theoretical or design) intermittent torque that the first electric drive unit 7 is capable of generating. The peak torque capability CPTQ varies in dependence on the operating speed (rpm) of the first electric drive unit 7. 20 As shown in Figure 2, the motor control unit 13 comprises at least one first electronic processor 17 and a first memory device 19. The at least one first electronic processor 17 has at least one electrical input 21A and at least one electrical output 21B. The at least one electrical input 21A may be configured to receive a torque demand signal TDS (see Figure 1) indicating a torque request. The torque demand signal TDS may, for example, be a driver torque demand 25 generated in dependence on the operation of an accelerator pedal 37. The at least one electrical output 21B is configured to output a motor speed signal SMS indicating an instantaneous speed (rpm) of the first electric drive unit 7. Alternatively, or in addition, the speed (rpm) of the first electric drive unit 7 may be measured by a rotational sensor (not shown) which measures an operating speed of the first electric drive unit 7 and outputs the 30 motor speed signal SMS. Alternatively, the speed (rpm) of the first electric drive unit 7 may be determined with reference to an instantaneous reference velocity of the vehicle 5. For example, the speed (rpm) of the first electric drive unit 7 may be determined with reference to known driveline parameters, such as an effective driveline gear ratio and a rotational speed of a driven wheel of the vehicle 5. 35 The vehicle 5 comprises a peak torque control unit 23 which is configured to determine a peak torque output limit OPTQ of the first electric drive unit 7. The determined peak torque output 10 06 25 limit OPTQ defines a maximum peak torque output of the first electric drive unit 7. As described herein, the peak torque output limit OPTQ is determined in dependence on the instantaneous speed (rpm) of the first electric drive unit 7. The peak torque output limit OPTQ is less than the peak torque capability OPTQ of the first electric drive unit 7 over at least a portion of the 5 operating speed range of the first electric drive unit 7. The peak torque control unit 23 is configured progressively to modify the peak torque output limit OPTQ in dependence on changes in the speed (rpm) of the first electric drive unit 7. The peak torque control unit 23 is configured to output the peak torque output limit OPTQ to the motor control unit 13. The motor control unit 13 is configured to control the first electric drive unit 7 in dependence on the peak 10 torque output limit OPTQ. In particular, the motor control unit 13 is configured to control the first electric drive unit 7 to limit or restrict the peak torque output to less than or equal to the peak torque output limit OPTQ. The motor control unit 13 is configured to control operation of the first electric drive unit 7 to generate a torque in dependence on the torque demand signal TDS. If the torque demand signal TDS requests an instantaneous torque greater than the 15 determined peak torque output limit OPTQ, the motor control unit 13 restricts the maximum peak torque generated by the first electric drive unit 7 to the peak torque output limit OPTQ. As shown in Figure 3, the peak torque control unit 23 comprises at least one second electronic processor 27 and a second memory device 29. A set of computational instructions are stored 20 on the second memory device 29. When executed, the computational instructions cause the at least one second electronic processor 27 to perform the method(s) described herein. The at least one second electronic processor 27 is configured to determine the peak torque output limit OPTQ of the first electric drive unit 7 in dependence on the speed (rpm) of the first electric drive unit 7. The peak torque output limit OPTQ may be predefined for a given operating speed 25 (rpm) of the first electric drive unit 7. For example, the at least one second electronic processor 27 may access a look-up table to determine the peak torque output limit OPTQ. For example, the look-up table may define the peak torque output limit OPTQ in relation to the operating speed (rpm) of the first electric drive unit 7. Alternatively, the peak torque output limit OPTQ may be calculated by the peak torque control unit 23. For example, a peak torque algorithm 30 may define a relationship between the peak torque output limit OPTQ and the operating speed (rpm) of the first electric drive unit 7. The peak torque control unit 23 may execute the peak torque algorithm to determine the peak torque output limit OPTQ. The at least one second electronic processor 27 has at least one electrical input 31A and at 35 least one electrical output 31B. The at least one electrical input 31A is configured to receive the motor speed signal SMS indicating the speed (rpm) of the first electric drive unit 7. The at least one second electronic processor 27 is configured to determine the peak torque output 10 06 25 limit OPTQ in dependence on the speed (rpm) of the first electric drive unit 7. The at least one second electronic processor 27 is configured progressively to modify the peak torque output limit OPTQ in dependence on changes in the speed (rpm) of the first electric drive unit 7. For example, changes in the peak torque output limit OPTQ may follow a substantially continuous 5 (i.e., uninterrupted) curve in dependence on changes in the speed (rpm) of the first electric drive unit 7. Alternatively, changes in the peak torque output limit OPTQ may follow a stepped profile in dependence on changes in the speed (rpm) of the first electric drive unit 7. The stepped profile may, for example, comprise a plurality of steps to approximate a continuous curve. 10 In the present embodiment, the peak torque control signal SPTQ is output to the motor control unit 13 to control operation of the first electric drive unit 7. In particular, the motor control unit 13 is configured to receive the peak torque control signal SPTQ and to restrict the peak torque output of the first electric drive unit 7 to less than or equal to the peak torque output limit OPTQ. 15 The peak torque control signal SPTQ may, for example, be published to a communication bus and read by the at least one first electronic processor 17 of the motor control unit 13. In a variant, the peak torque control unit 23 may be incorporated into the motor control unit 13. The peak torque control signal SPTQ is operative to restrict the peak torque output of the first electric drive unit 7 to below the peak torque capability CPTQ of the first electric drive unit 7. 20 The peak torque control signal SPTQ is effective over at least a part of the operating range of the first electric drive unit 7. The peak torque output limit OPTQ effectively derates the first electric drive unit 7 to reduce the peak torque generated for a given speed (rpm) (or range of speeds) of the first electric drive unit 7. At least in certain embodiments, this may reduce the cumulative damage at higher torque components (defined by Wohler’s rule). The peak torque 25 capability CPTQ represents the maximum theoretical or design peak torque that the first electric drive unit 7 is capable of generating. The peak torque capability CPTQ varies with the speed of the first electric drive unit 7. As such, the peak torque capability CPTQ is defined with reference to a speed of the first electric drive unit 7. 30 In the present embodiment, the peak torque output limit OPTQ is inversely proportional to the speed (rpm) of the first electric drive unit 7. The at least one second electronic processor 27 is configured to reduce the peak torque output limit OPTQ as the speed (rpm) increases. The determined peak torque output limit OPTQ changes progressively as the speed of the vehicle 5 increases or decreases. The progressive changes in the peak torque output limit OPTQ may 35 help to reduce or avoid step changes in the peak torque generated by the first electric drive unit 7 as the speed of the vehicle 5 changes. In a variant, the peak torque output limit OPTQ may be defined as a plurality of step changes or intervals. The step changes or intervals may 10 06 25 be relatively small in size in order to reduce or avoid changes in the torque output of the first electric drive unit 7. The step changes may approximate the desired shape or profile of the torque output limit OPTQ. 5 The peak torque output limit OPTQ is limited to less than the peak torque capability CPTQ of the first electric drive unit 7 over at least a portion of an operating speed range of the vehicle 5. The peak torque output limit OPTQ of the first electric drive unit 7 may be implemented above and / or below a predetermined speed (rpm) of the first electric drive unit 7. 10 The at least one second electronic processor 27 in the present embodiment is configured to define the peak torque output limit OPTQ as being less than the peak torque capability CPTQ of the first electric drive unit 7 (i.e., OPTQ<CPTQ) when the speed (rpm) of the first electric drive unit 7 is greater than or equal to a first speed threshold STH1. The peak torque output limit OPTQ may be at least substantially equal to the peak torque capability CPTQ of the 15 torque generating machine 7 (i.e., OPTQ=CPTQ) when the speed (rpm) of the first electric drive unit 7 is less than the first speed threshold STH1. The first speed threshold STH1 is greater than zero (0). At least in certain embodiments, the peak torque output limit OPTQ is substantially equal to the peak torque capability CPTQ during acceleration of the vehicle 5 from rest (i.e., from a standstill). At least in certain embodiments, the peak torque capability 20 CPTQ of the first electric drive unit 7 is available to the driver at low speeds or when starting from rest. The at least one second electronic processor 27 is configured to define the peak torque output limit OPTQ as being less than the peak torque capability CPTQ of the first electric drive unit 7 25 (i.e., OPTQ<CPTQ) when the speed (rpm) of the first electric drive unit 7 is less than or equal to a second speed threshold STH2. The peak torque output limit OPTQ may be at least substantially equal to the peak torque capability CPTQ of the torque generating machine 7 (i.e., OPTQ=CPTQ) when the speed (rpm) of the first electric drive unit 7 is greater than the second speed threshold STH2. It will be understood that the peak torque output limit OPTQ is 30 less than the peak torque capability CPTQ when the speed (rpm) of the first electric drive unit 7 is within an operating speed range delimited by the first and second speed thresholds STH1, STH2. The peak torque output limit OPTQ may comprise a lower transition region and / or an upper 35 transition region progressively to implement the peak torque output limit OPTQ at the first speed threshold and / or the second speed threshold. The lower transition region may comprise or consist of a continuous transition from the peak torque capability CPTQ of the torque 10 06 25 generating machine 7 to the peak torque output limit OPTQ as the speed (rpm) of the first electric drive unit 7 increases above the first speed threshold TH 1. The upper transition region may comprise or consist of a continuous transition from the peak torque output limit OPTQ to the peak torque capability CPTQ of the torque generating machine 7 as the speed (rpm) of 5 the first electric drive unit 7 increases above the second speed threshold TH2. The progressive changes in the peak torque output limit OPTQ may help to reduce or avoid step changes in the peak torque generated by the first electric drive unit 7 as the speed of the vehicle 5 changes. It will be understood that the lower transition region and / or the upper transition region are also applicable to reduce changes in the peak torque output limit OPTQ as the speed 10 (rpm) of the first electric drive unit 7 decreases. The implementation of the peak torque output limit OPTQ is illustrated with reference to a first torque plot 100 shown in Figure 4. The first torque plot 100 shows torque (Nm) on the Y-axis and speed (rpm) of the first electric drive unit 7 on the X-axis. A first curve 105 represents a 15 continuous torque output OCTQ of the first electric drive unit 7. A second curve 110 represents a peak (intermittent) torque capability CPTQ of the first electric drive unit 7. The peak torque control unit 23 is configured to define the peak torque output limit OPTQ (represented by a broken line 115) of the first electric drive unit 7 within an operating speed range of the first electric drive unit 7. The peak torque output limit OPTQ is less than the peak torque capability 20 CPTQ of the first electric drive unit 7. As outlined above, the peak torque output limit OPTQ is determined in dependence on speed (rpm) of the first electric drive unit 7. The peak torque output limit OPTQ decreases as the speed (rpm) of the first electric drive unit 7 increases. The speed (rpm) of the first electric drive unit 7 is directly related to the reference speed of the vehicle 5. It will be understood that the peak torque output limit OPTQ may be determined in 25 dependence on the reference speed of the vehicle 5. The peak torque output limit OPTQ in the present embodiment is implemented over only a portion of the operating speed range of the first electric drive unit 7. The operating speed range is represented by the first and second speed thresholds STH1, STH2 of the first electric drive unit 7, as shown in Figure 4. 30 The implementation of the peak torque output limit OPTQ is illustrated with reference to a first power plot 200 shown in Figure 5. The first power plot 200 shows power (kW) on the Y-axis and a speed (rpm) of the first electric drive unit 7 on the X-axis. A first curve 205 represents a continuous power output of the first electric drive unit 7. A second curve 210 represents a peak power capability of the first electric drive unit 7. The peak torque output limit OPTQ defined by 35 the peak torque control unit 23 reduces the peak power output of the first electric drive unit 7 over a portion of the operating speed range of the vehicle 5. The reduced peak power output is represented by a broken line 215 in Figure 5. The peak power output is less than the peak 10 06 25 power capability of the first electric drive unit 7 for a given speed (rpm) of the first electric drive unit 7 (i.e. less than the theoretical or design peak power output of the first electric drive unit 7 at that speed). The peak power output is directly related to available torque at the rotational speed (rpm) of the first electric drive unit 7. The peak power output decreases as the speed 5 (rpm) of the first electric drive unit 7 increases. The peak torque output limit OPTQ in the present embodiment is implemented over only a portion of the operating speed range of the first electric drive unit 7. The operating speed range of the first electric drive unit 7 is defined by the first and second thresholds STH1, STH2 of the first electric drive unit 7, as shown in Figure 5. 10 The peak torque output limit OPTQ of the first electric drive unit 7 may be defined as a proportion of the peak torque capability CPTQ of the first electric drive unit 7. The peak torque output limit OPTQ is less than the peak torque capability CPTQ over at least a portion of the operating range of the first electric drive unit 7. The peak torque control signal SPTQ is 15 configured to decrease the peak torque output limit OPTQ of the torque generating machine 7 as a proportion of the peak torque capability CPTQ as the speed (rpm) of the first electric drive unit 7 increases. The difference between the peak torque output OPTQ and the peak torque capability CPTQ may increase as the speed (rpm) of the first electric drive unit 7 increases. 20 The peak torque output limit OPTQ is defined to limit the torque generated by the first electric drive unit 7 to less than the peak torque capability CPTQ. The peak torque control signal SPTQ is output to the motor control unit 13 to control operation of the first electric drive unit 7. The motor control unit 13 is configured to receive a torque demand signal TDS. The torque demand 25 signal TDS may, for example, represent a driver torque demand generated in dependence on a position of the accelerator pedal 37. Alternatively, or in addition, the torque demand signal TDS may be generated by a vehicle controller, for example an autonomous vehicle controller or an adaptive speed controller. A position sensor 39 is provided to monitor a position of the accelerator pedal 37 and output an accelerator pedal position signal SPP to the motor control 30 unit 13. The torque demand signal TDS is generated in dependence on the accelerator pedal position signal SPP. The motor control unit 13 controls operation of the first electric drive unit 7 in dependence on the torque demand signal TDS and the peak torque control signal SPTQ. The peak torque control signal SPTQ defines the peak torque output limit OPTQ with respect to the current speed of the vehicle 5. If the torque demand signal TDS represents a torque 35 request which is less than the determined peak torque output limit OPTQ, the motor control unit 13 controls the first electric drive unit 7 to generate torque which is at least substantially equal to the torque request. If the torque demand signal TDS represents a torque request which is greater than the determined peak torque output limit OPTQ, the motor control unit 13 controls the first electric drive unit 7 to generate torque which is substantially equal to the peak torque output limit OPTQ. The motor control unit 13 thereby controls the first electric drive unit 7 to restrict the torque to less than or equal to the determined peak torque output limit OPTQ. 5 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. The peak torque control signal SPTQ may be configured to restrict the peak torque output limit 10 OPTQ of two or more electric drive units 7. The peak torque output limit OPTQ of the two or more electric drive units 7 may be controlled independently of each other. This may provide protection for each of the electric drive units 7 and other components in the driveline. Alternatively, the peak torque output limit OPTQ may be controlled collectively, for example to limit the combined peak torque output of the two or more electric drive units 7. This may 15 provide protection for other components in the driveline. Alternatively, or in addition, the peak torque control signal SPTQ may limit the peak torque output of an internal combustion engine (not shown). 10 06 25

Claims

CLAIMS 1. A control system for controlling a peak torque output of a torque generating machine disposed in a vehicle, the control system comprising one or more controllers, the control system configured to: receive a motor speed signal indicating an operating speed of the torque generating machine; determine a peak torque output limit to restrict the peak torque output of the torque generating machine, the peak torque output limit being determined in dependence on the operating speed of the torque generating machine; and output a peak torque control signal representing the determined peak torque output limit; wherein the control system is configured progressively to modify the peak torque output limit in dependence on changes in the operating speed of the torque generating machine.

2. The control system of claim 1, wherein the one or more controllers collectively comprise: at least one electronic processor having an electrical input for receiving the motor speed signal indicating the operating speed of the torque generating machine; 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 progressively to modify the peak torque output limit in dependence on changes in the operating speed of the torque generating machine.

3. A control system as claimed in claim 1 or claim 2, wherein the control system is configured progressively to decrease the peak torque output limit in dependence on an increase in the operating speed of the torque generating machine; and / or the control system is configured progressively to increase the peak torque output limit in dependence on a decrease in the operating speed of the torque generating machine.

4. A control system as claimed in any one of claims 1, 2 or 3, wherein the torque generating machine has a peak torque capability and wherein the peak torque output limit is less than the peak torque capability of the torque generating machine. 5, A control system as claimed in claim 4, wherein the control system is configured to: progressively decrease the peak torque output limit as a proportion of the peak torque capability of the torque generating machine as the operating speed of the torque generating machine increases: and / or progressively increase the peak torque output limit as a proportion of the peak torque capability of the torque generating machine as the operating speed of the torque generating machine decreases.

6. A control system as claimed in claim 4 or claim 5, wherein the peak torque output limit restricts the peak torque output to below the peak torque capability of the torque generating machine when the operating speed of the torque generating machine is greater than a first speed threshold and less than a second speed threshold.

7. A control system as claimed in claim 6, wherein the first speed threshold is greater than zero and the second speed threshold is less than a maximum speed of the torque generating machine.

8. A control system as claimed in claim 6 or 7, wherein the peak torque output limit is equal to the peak torque capability of the torque generating machine when the operating speed of the torque generating machine is greater than the second speed threshold or less than the first speed threshold.

9. A control system as claimed in any one of the preceding claims, wherein, when the control system modifies the peak torque output limit, the control system is configured to modify the peak torque output limit at least substantially continuously in dependence on changes in the operating speed of the torque generating machine.

10. A control system as claimed in any one of claims 1 to 9, wherein, when the control system modifies the peak torque output limit, the control system is configured to modify the peak torque output limit in a plurality of steps in dependence on changes in the operating speed of the torque generating machine.

11. A control system as claimed in any one of the preceding claims, wherein the torque generating machine is an electric machine disposed in an electric drive unit of the vehicle.

12. A vehicle comprising a control system as claimed in any one of claims 1 to 10.

13. A method of controlling a peak torque output of a torque generating machine, the method comprising: determining an operating speed of the torque generating machine; and setting a peak torque output limit of the torque generating machine over at least a portion of an operating speed range of the torque generating machine; wherein the peak torque output limit of the torque generating machine is set in dependence on the determined operating speed of the torque generating machine; and the method comprises progressively modifying the peak torque output limit in dependence on changes in the operating speed of the torque generating machine.

14. A non-transitory computer-readable medium having a set of instructions stored therein which, when executed, cause a processor to perform the method according claim 13.

15. Computer software that, when executed, is arranged to perform a method according to claim 13.

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