Compensated shift scheduling

Abstract

Control of an automatic transmission system (12 fig. 2) of a vehicle (1. fig. 1) comprising: receiving (410 fig. 6) an input signal indicative of a current temperature within an electric machine (26);

Description

TECHNICAL FIELD The present disclosure relates to compensated shift scheduling. Aspects of the invention relate to a control system, to an automatic transmission system, to a vehicle, to a method, and to computer readable instructions. BACKGROUND It is known to provide automatic transmissions in vehicles. Automatic transmissions do not require input from a driver to shift gears. Gear management is instead provided to determine when it is appropriate to shift gear and what gear is appropriate to adopt in a given set of circumstances. The performance of a vehicle’s powertrain can vary substantially in different circumstances, and this has an impact on the quality of gear management. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a control system, an automatic transmission system, a vehicle, a method, and computer readable instructions as claimed in the appended claims. According to an aspect of the present invention there is provided a control system for an automatic transmission of a vehicle comprising any one or more of the features described herein. According to an aspect of the present invention there is provided a control system for an automatic transmission of a vehicle. The control system comprises one or more controllers. The control system configured to: receive an input signal indicative of at least one current temperature within an electric machine determine a first estimate of a torque limit imposed on the electric machine under a thermal derate strategy for the electric machine based on the at least one current temperature within the electric machine; obtain a first shift map which is used when the electric machine is operating according to its rated torque capability, wherein the first shift map defines gear shift thresholds for a speed parameter as functions of accelerator input; generate a compensated shift map by modifying at least some of the gear shift thresholds of the first shift map to increase minimum values of the speed parameter at which respective gear shifts are triggered, wherein the increased minimum values are based on the first estimate of the torque limit; and control the automatic transmission according to the compensated shift map. An advantage of increasing the minimum values of the speed parameter at which gear shifts are triggered has the effect of maintaining lower gear ratios for longer to get maximum mechanical advantage from being in a lower gear ratio in order to compensate for the reduced responsiveness to accelerator input, which would 1 otherwise result from shift scheduling according to the first shift map while the electric machine is operating at a derated torque capability. The ability to reliably estimate the torque limit which derates the torque capability contributes to improving the compensation. Optionally the first shift map defines at least gear shift thresholds which are associated with gear shifts out of a current gear ratio. Optionally the at least some of the gear shift thresholds of the first shift map which are modified to generate the compensated shift map comprise gear shift thresholds which are associated with gearshifts out of a current gear ratio. Optionally at least some of the gear shift thresholds of the first shift map are modified to increase values of the speed parameter at which respective gear shifts are triggered for a range of accelerator inputs. Optionally the control system is configured to: receive an input signal indicative of a current speed of the electric machine; receive an input signal indicative of a current power limit imposed on the electric machine; receive an input signal indicative of a current torque capability of the electric machine; and determine a second estimate of the torque limit imposed on the electric machine by comparing the current torque capability of the electric machine with an expected torque capability of the electric machine for the current speed of the electric machine and for the current power limit imposed on the electric machine. The increased minimum values of the speed parameter are based on the most limiting of the first and second estimates of the torque limit. An advantage of determining a second estimate is that this can account for cases in which a more limiting torque limit may be imposed on the electric machine than that proposed under the thermal derate strategy. Optionally the control system is configured to: store the second estimate of the torque limit when the current speed of the electric machine is within a tuneable offset of a speed at which an electric machine becomes power-limited; and use the stored second estimate of the torque limit when the current speed of the electric machine exceeds the speed at which the electric machine becomes power-limited. An advantage of this is that, since the second estimate may not be reliably calculated when the electric machine is power-limited, reliance on such calculations is avoided. Optionally the control system is configured to obtain a second shift map which is used when the electric machine is operating according to a predefined derated torque capability. The second shift map defines gear shift thresholds for the speed parameter as functions of accelerator input. At least some of the gear shift thresholds of the second shift map have higher minimum values of the speed parameter than corresponding gearshift thresholds of the first shift map. Modifying at least some of the gearshift thresholds of the first shift map to increase minimum values of the speed parameter at which respective gear shifts are triggered comprises interpolating between corresponding gear shift thresholds of the first and second shift maps using an interpolation factor based on the first estimate of the torque limit. An advantage of modifying gear shift thresholds using interpolation between gear shift thresholds in two different maps is that aspects of calibration can be maintained into the compensated shift map. Optionally at least some of the gear shift thresholds of the second shift map have higher values of the speed parameter for a range of accelerator inputs than corresponding gear shift thresholds of the first shift map. Optionally the interpolation factor is also based on a power limit imposed on the electric machine. An advantage of this is that the compensation may be based on a more complete picture of potential sources of derate. Optionally the control system is configured to receive an input signal indicative of a status of a propulsion system of the vehicle. Optionally the interpolation is dependent on the propulsion system being in a parallel hybrid mode. Optionally modifying at least the gear shift thresholds of the first shift map which are associated with gearshifts out of the current gear to increase minimum values of the speed parameter at which respective gearshifts out of the current gear are triggered comprises: determining minimum speed limits to impose on respective gear shifts based on the first estimate of the torque limit; and clipping the gear shift thresholds of the first shift map to an accelerator input of value zero at the determined minimum speed limits to provide corresponding gear shift thresholds of the compensated shift map. An advantage of this is that the number of shift maps that need to be stored is reduced as compared to the interpolative approach. Optionally generating the compensated shift map further comprises: modifying at least one gear shift threshold of the first shift map which is associated with a downshift out of the current gearto decrease a maximum value of the speed parameter at which the downshift out of the current gear is triggered. This comprises: determining a maximum speed limit to impose on the downshift based on a power limit imposed on the electric machine; and clipping the gear shift threshold of the first shift map to an accelerator input of maximum value at the determined maximum speed limit to provide a corresponding gear shift threshold of the compensated shift map. Optionally the control system is configured to receive an input signal indicative of a status of a propulsion system of the vehicle. Optionally the clipping is dependent on the propulsion system being in an electric mode. Optionally the control system is configured to receive one or more input signals indicative of current road load; and obtain the first shift map based on the current road load. Optionally the first shift map is obtained from a first set of shift maps, wherein different shift maps in the first set are associated with different road loads. Optionally obtaining the first shift map from the first set of shift maps comprises: selecting a pair of shift maps associated with a road load interval in which the current road load resides from the first set of shift maps; and interpolating between the pair of shift maps based on the current road load to generate the first shift map. Optionally the control system is configured to: receive one or more input signals indicative of a current air pressure; and obtain the first shift map based on the current air pressure. Optionally obtaining the first shift map from the first set of shift maps comprises: receiving one or more input signals indicative of a current air pressure; selecting a first pair of shift maps associated with a road load interval in which the current road load resides from a first subset of the first set of shift maps, each shift map in the first subset being associated with a first air pressure; interpolating between the first pair of shift maps based on the current road load to generate a first interim shift map; selecting a second pair of shift maps associated with a road load interval in which the current road load resides from a second subset of the first set of shift maps, each shift map in the second subset associated with a second air pressure, wherein the first and second air pressures represent end points of a range of air pressures in which the vehicle is expected to operate; interpolating between the second pair of shift maps based on the current road load to generate a second interim shift map; interpolating between the first and second interim shift maps based on the current air pressure to generate the first shift map. Optionally the speed parameter is a speed parameter of the vehicle or of a powertrain component. Optionally the speed parameter is one of the following: a vehicle speed, a wheel speed, a transmission output shaft speed, a transmission input shaft speed, or an engine and / or electric machine speed. According to a further aspect of the invention, there is provided an automatic transmission system comprising the control system. According to a further aspect of the invention, there is provided a vehicle comprising the automatic transmission system. According to a further aspect of the invention, there is provided a method of controlling an automatic transmission of a vehicle, the method comprising: receiving an input signal indicative of at least one current temperature within an electric machine determining a first estimate of a torque limit imposed on the electric machine under a thermal derate strategy for the electric machine based on the at least one current temperature within the electric machine; obtaining a first shift map which is used when the electric machine is operating according to its rated torque capability, wherein the first shift map defines gear shift thresholds for a speed parameter as functions of accelerator input; generating a compensated shift map by modifying at least some of the gear shift thresholds of the first shift map to increase minimum values of the speed parameter at which respective gear shifts are triggered, wherein the increased minimum values are based on the first estimate of the torque limit; and controlling the automatic transmission according to the compensated shift map. According to a further aspect of the invention there is provided computer readable instructions which, when executed by a computer, are arranged to perform any one or more of the methods described herein. According to a further aspect of the invention there is provided a non-transitory computer readable medium comprising computer readable instructions that, when executed by one or more electronic processors, causes the one or more electronic processors to carry out any one or more of the methods described herein. 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 falls within the scope of the appended claims. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination that falls within the scope of the appended claims, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: FIG. 1 illustrates an example of a vehicle; FIG. 2 illustrates an example of at least part of a powertrain of a vehicle; FIG. 3 illustrates an example of a control system; FIG. 4 illustrates an example of a non-transitory computer-readable storage medium; FIG. 5A illustrates an example of the effect of power limits on an electric machine; FIG. 5B illustrates an example of the effect of torque limits on an electric machine; FIG. 6 illustrates an example of a method; FIG. 7 illustrates a further example of the method; FIG. 8 illustrates a further example of the method; FIG. 9 illustrate examples of upshift thresholds; FIG. 10 illustrate examples of downshift thresholds; FIGS 11A and 11B illustrate further examples of the method; FIG. 12 illustrates a further example of the method; and FIG. 13 illustrate further examples of downshift thresholds. DETAILED DESCRIPTION A vehicle 1 in accordance with an embodiment of the present invention is described herein with reference to the accompanying FIG. 1. In some, but not necessarily all examples, the vehicle 1 is a passenger vehicle, also referred to as a passenger car or as an automobile. In other examples, embodiments of the invention can be implemented for other applications, such as commercial vehicles. FIG. 2 schematically illustrates an example of at least part of a powertrain of the vehicle 1. In this example, the vehicle 1 comprises a propulsion system 22 comprising a plurality of torque sources 24, 26 which are selectively operable for the purpose of providing drive torque for accelerating the vehicle 1. A torque source refers to a prime mover, such as an internal combustion engine, an electric machine such as a traction motor, or the like. In the illustrated example, the propulsion system 22 comprises two torque sources 24,26. A first torque source 24 is an internal combustion engine (‘engine’). A second torque source 26 is an electric machine. The electric machine 26 is an electric motor arranged to convert electrical energy into kinetic energy in the form of mechanical torque and vice versa. The electric motor 26 may be an alternating current induction motor ora permanent magnet motor, or another type of motor. The electric machine 26 is a traction motor configured to enable at least an electric mode comprising electric-only driving. That is, the electric machine 26 can drive the vehicle by itself (without an engine). This propulsion system 22 Is configured to operate in a plurality of predefined operating modes. These include at least an electric mode and a parallel hybrid mode. In the electric mode (also known as electric vehicle (EV) mode or electric-only mode) the vehicle 1 is propelled only by torque generated by the electric machine 26. The engine 24 may be off (in a non-running state) such that fuel is not combusted, though it may still be rotating if not disconnected from the wheels 34. The engine 24 may alternatively be on but only for the purpose of generating electrical energy and not connected to a torque path to the wheels 34. In the parallel hybrid mode the vehicle 1 is propelled by torque generated by both the engine 24 and by the electric machine 26. The predefined operating modes may also include an engine-only mode in which the vehicle 1 is propelled only by torque generated by the engine 24 and there is no electric propulsion. Transitioning between predefined operating modes of the propulsion system 22 comprises turning on or off one of the torque sources 24, 26 so that, respectively, it either does or does not output torque. In some examples, transitioning between predefined operating modes further comprises mechanically connecting (coupling) or disconnecting (uncoupling) one of the torque sources 24, 26 to the drivetrain. A coupling clutch 25 is provided to mechanically connect and disconnect the engine 24 from the drivetrain. It will be appreciated that the transition between the predefined operating modes is not instantaneous. While the propulsion system 22 is transitioning between predefined operating modes, its mode status is ‘in transition’. The vehicle 1 comprises an automatic transmission system 10 comprising an automatic transmission 12 and a control system 100 such as a transmission control unit / module for controlling the automatic transmission 12. The automatic transmission 12 comprises a launch device 14 which transfers torque output by the operating torque sources 24, 26 of the propulsion system 22 to the transmission input shaft 18. The launch device 14 may be a fluid coupling such as a torque converter or one or more automatically-actuated friction clutches as found in, for example, a dual-clutch transmission. The launch device may alternatively comprise one of a plurality of clutches within the transmission. The automatic transmission 12 also comprises a gear set and accompanying shifting mechanism, referenced in combination as 16. The gear set 16 comprises a plurality of gears which are selectively couplable into different gear trains to enable multiple gear ratios between the transmission input shaft 18 and the transmission output shaft 20. Each gear may have a corresponding clutch configured to couple them (directly or indirectly) to the transmission output shaft 20. Alternatively a plurality of clutches may engage selected gear combinations to provide the overall transmission ratio, multiple clutches may be required to provide each ratio. The clutches and their actuators form the shifting mechanism. The actuators may be solenoid valves controlling hydraulic pressure to clutches. The shifting mechanism is controlled to establish a selected gear ratio in accordance with a control signal output by the control system 100. The control system 100 may determine which clutches are involved in shifting to the selected gear ratio and determine how the pressure at the clutches should be controlled to establish the selected gear ratio. The control system 100 may then directly control the actuators associated with these clutches to increase or decrease pressure at these clutches as required. The control system 100 may also be capable of controlling actuation of the launch device 14. The transmission output shaft 20 is connected to a final set of gears 32, such as a pinion gear meshed with a ring gear, to transfer torque to the wheel axles and thus the vehicle wheels 34. In order to store electrical energy for the electric machine 26, the vehicle 1 comprises an electrical energy storage means 28. The electrical energy storage means 28 can be a traction battery. The traction battery 28 provides a nominal voltage required by electrical power users such as the electric machine 26. The traction battery 28 may be a high voltage battery. The traction battery 28 may have a voltage and capacity to support electric only driving for sustained distances. The traction battery 28 may have a capacity of several kilowatt-hours, to maximise range. The capacity may be in the tens of kilowatt-hours, or even over a hundred kilowatt-hours. Although the traction battery 28 is illustrated as one entity, the function of the traction battery 28 could be implemented using a plurality of small traction batteries in different locations on the vehicle 1. An inverter 30 converts between the DC output of the traction battery 28 and the AC input required for the electric machine 26. In view of the above description of the vehicle 1, it will be understood that the vehicle 1 is a full hybrid electric vehicle (HEV) or plug in hybrid electric vehicle (PHEV). However, in some examples the vehicle 1 may be other than as shown in FIG. 2. The vehicle 1 may be a mild HEV, an internal combustion engine vehicle (ICEV) or otherwise. Mild HEVs do not have an electric-only mode of propulsion, but the electric machine 26 may be configured to provide assistance such as boosting output torque of the engine 24. The electric machine 26 is not sufficiently powerful to drive the vehicle 1 under electric power alone. ICEV are propelled solely by an engine 24. On board electric machines comprise starter and generator or a combined starter-generator but not an electric traction motor. At least some of the disclosures made herein can find application in any of these vehicles. FIG. 3 illustrates an example of the control system 100 for the automatic transmission 12 of the vehicle 1. The control system 100 comprises one or more controllers 102. The control system 100 is configured to receive data from multiple sensors 116 and / or other control systems. The control system 100 is configured to receive data about at least one current temperature within the electric machine 26, such as a current rotor temperature and / or a current stator temperature. The control system 100 is configured to determine an estimate of the torque limit imposed on the electric machine 26 and to generate a compensated shift map by modifying at least some gear shift thresholds of a first shift map, which is used when the electric machine 26 is operating according to its rated torque capability. The control system 100 may then control the automatic transmission 12 according to the compensated shift map, for example by outputting a control signal comprising instructions to cause initiation of a shift (by for example controlling the automatic transmission 12, and specifically the shifting mechanism which functions to establish a gear ratio) in respect of gear ratios proposed by the compensated shift map. The control system may also be configured to receive accelerator input data from, for example, an accelerator pedal sensor or a system processing the output of the accelerator pedal sensor to determine a torque to request of the propulsion system 22 or from an automated driving system (ADS) or an advanced driverassistance system (ADAS). The control system may also be configured to receive speed parameter data from, for example, a crank position sensor, a drivetrain speed sensor, a wheel speed sensor, or the like. The control system 100 may be configured to determine a destination gear ratio based on the accelerator input data and 8 the speed parameterdata using the compensated shift map. The control system 100 may then output a control signal comprising instructions to cause initiation of a shift to the destination gear ratio. The control system 100 as illustrated in FIG. 3 comprises one controller 102, although it will be appreciated that this is merely illustrative. The controller 102 comprises processing means 106 and memory means 108. The processing means 106 may be one or more electronic processing devices 106 which operably execute computer-readable instructions. The memory means 108 may be one or more memory devices 108. The memory means 108 is electrically coupled to the processing means 106. The memory means 108 is configured to store instructions, and the processing means 106 is configured to access the memory means 108 and execute the instructions stored thereon. The controller 102 comprises an input means 112 and an output means 114. The input means 112 may comprise an electrical input 112 of the controller 102. The output means 114 may comprise an electrical output 114 of the controller 102. The controller 102 may have an interface 104 comprising an electrical input / output I / O 112, 114, or an electrical input 112, or an electrical output 114, for receiving information and interacting with external components. The input 112 is arranged to receive a plurality of signal from a plurality of sensors 116 or other control systems. At least one signal is an electrical signal which is indicative of at least one current temperature within the electric machine 26, such as a current rotor temperature and / or a current stator temperature. Other received signals may include an electrical signal which is indicative of an accelerator input and an electrical signal which is indicative of a speed parameter. The output 114 is arranged to output control signals comprising instructions to cause initiation of a gearshift. FIG. 4 illustrates a non-transitory computer-readable storage medium 200 comprising the instructions (computer software). It is to be understood that the, or each, controller 102 can comprise a control unit or computational device having one or more electronic processors (e.g., a microprocessor, a microcontroller, an application specific integrated circuit (ASIC), etc.), and may comprise a single control unit or computational device, or alternatively different functions of the or each controller 102 may be embodied in, or hosted in, different control units or computational devices. As used herein, the term “controller,” “control unit,” or “computational device” will be understood to include a single controller, control unit, or computational device, and a plurality of controllers, control units, or computational devices collectively operating to provide the required control functionality. A set of instructions could be provided which, when executed, cause the controller 102 to implement the control techniques described herein (including some or all of the functionality required for the method(s) described herein). The set of instructions 110 could be embedded in said one or more electronic processors 106 of the controller 102; or alternatively, the set of instructions 110 could be provided as software to be executed in the controller 102. A first controller or control unit may be implemented in software run on one or more processors. One or more other controllers or control units may be implemented in software run on one or more processors, optionally the same one or more processors as the first controller or control unit. Other arrangements are also useful. The, or each, electronic processor 106 may comprise any suitable electronic processor (e.g., a microprocessor, a microcontroller, an ASIC, etc.) that is configured to execute electronic instructions 110. The, or each, electronic memory device 108 may comprise any suitable memory device and may store a variety of data, information, threshold value(s), lookup tables or other data structures, and / or instructions therein or thereon. In an embodiment, the memory device 108 has information and instructions for software, firmware, programs, algorithms, scripts, applications, etc. stored therein or thereon that may govern all or part of the methodology described herein. The processor, or each, electronic processor 106 may access the memory device 108 and execute and / or use that orthose instructions and information to carry out or perform some or all of the functionality and methodology described herein. The at least one memory device 108 may comprise a computer-readable storage medium (e.g. a non-transitory or non-transient storage medium) that may comprise any mechanism forstoring information in a form readable by a machine or electronic processors / computational devices. Examples of the form include, without limitation: a read only memory (ROM); random access memory (RAM); erasable programmable memory (e.g. EPROM ad EEPROM); flash memory; or electrical or other types of medium for storing such information / instructions. FIGS 6 to 13 illustrate examples of a method 400 (including submethods) of controlling the automatic transmission 12 of the vehicle 1. The method 400 may be performed by the control system 100 illustrated in FIG. 3. In particular, the memory 108 may comprise computer-readable instructions 110 which, when executed by the processor 106, perform the method 400. The method 400 facilitates shift scheduling that compensates for reduced performance of the propulsion system 22, particularly, though not necessarily exclusively, reduced performance of the electric machine 26. The torque capability of an electric machine 26 is a function of the speed at which the electric machine 26 is running. The torque capability of an electric machine is characterised by a torque-speed curve, examples of which are illustrated in FIGS 5A and 5B. The electric machine 26 has a rated torque capability which corresponds to the maximum torque that it is designed to be able to continuously produce at a given running speed under normal operating conditions. This should not be confused with peak torque, which is the maximum torque which may be produced at a given running speed for only a short period of time and cannot be sustained. Under adverse operating conditions, which are harsher than normal operating conditions, the maximum torque that the electric machine 26 may be able to continuously produce at a given running speed may be reduced. An example of adverse operating conditions includes cold temperatures which can cause, for example, increased internal resistance of the traction battery 28. To prevent damage to the battery 28 during operation in cold temperatures, the amount of power available to the electric machine 26 may be reduced by imposing a power limit. A battery power limit may be converted, accounting for efficiencies and losses, into a tractive power limit to be imposed on the electric machine 26. Another example of adverse operating conditions 10 includes hot temperatures which can, for example, affect cooling. Components such as the inverter 30 and the traction battery 28 may therefore be operated at lower currents (therefore reducing power) to avoid thermal runaway which would damage them. This can be achieved by imposing a torque limit on the electric machine 26. Adverse operating conditions are not only related to temperature. For example, severe vibrations, which might result from the vehicle traversing certain surfaces, can cause, for example, mechanical stresses to the bearing, rotor, and stator of the electric machine 26. These stresses may be amplified by the production of high torque. To prevent damage, therefore, torque and / or power limits may be imposed. The imposition of torque and / or power limits on the electric machine 26 leads to derating of the torque capability of the electric machine 26. Examples of various derated torque capabilities (torque-speed curves 312-318; 322-324) are illustrated in comparison to a rated torque capability (torque-speed curve 302) in FIGS 5A and 5B, which show electric machine torque Tem on the y-axis with respect to electric machine speed coem on the x-axis. FIG. 5A shows, in its different torque-speed curves 302; 312-318, an example of the effect of imposing different power limits on the electric machine 26. The torque-speed curves 312-318 result from limiting to the electric machine 26 to successively lower power. FIG. 5B shows, in its different torque-speed curves 302; 322-324, an example of the effect of imposing different torque limits on the electric machine 26. The torque-speed curves 322-324 result from limiting to the electric machine 26 to successively lower torque. In FIG. 5A it can be seen that the electric machine torque is constant (dictated by a torque limit on the electric machine 26) with increasing electric machine speed until a certain speed at which the power limit is reached. Beyond that speed, the electric machine 26 is no longer torque-limited, but is instead power-limited. The electric machine 26 is torque-limited when the torque limit is the limiting factor on the torque capability, regardless of whether other limits on the electric machine 26 are active. The electric machine 26 is powerlimited when the power limit is the limiting factor on the torque capability, regardless of whether other limits on the electric machine 26 are active. As the speed increases, the electric machine torque falls further below the level set by the torque limit. The more that the electric machine 26 is limited to lower power, the lower the speed at which the transition from torque-limited to power-limited occurs. In contrast, in FIG. 5B it can be seen that when the electric machine 26 is limited to lowertorque, the transition from torque-limited to power-limited occurs at a higher electric machine speed. When the electric machine 26 is power-limited, the electric machine torque is not affected by differences in the imposed torque limit. This can be seen by the convergence of the torque-speed curves 302; 322-324 once power-limited. It will be appreciated from the different ways in which the shape of the torque-speed curves is altered by the imposition of a power limit compared to the imposition of a torque limit that these limits affect the torque capability of the electric machine 26 differently. The torque limit derates the torque capability at speeds where 11 the electric machine 26 is torque-limited. The power limit derates the torque capability at speeds where the electric machine 26 is power-limited. Therefore, a torque limit represents one component (or “factor”) of the derate of the torque capability of the electric machine 26 and a power limit represents another component (or “factor”) of derate of the torque capability of the electric machine 26. The derate of the torque capability of the electric machine 26 comprises contribution from one or both components. Shift scheduling according to a single shift map will not be sufficient to cover both the scenario where the electric machine 26 is operating with its rated torque capability and the scenario where it is operating with a derated torque capability without a resulting detrimental effect to operating qualities of the vehicle powertrain, in particular to the power delivery and the responsiveness to accelerator input. For example, reaching maximum torque at a given operating point during driving with the electric machine 26 operating according to its rated torque capability, may require accelerator input at 60% of its maximum whilst, at a derated torque capability, maximum torque may be reached with accelerator input at 45% of its maximum. In this example, the point at which to request a downshift during operating according to the rated torque capability could be when the accelerator input is at 60%. But during operation according to this derated torque capability, this point and accelerator input could move to 45%. Therefore, the use of a single shift map to cover both rated and derated torque capabilities could involve a compromise on performance at either rated or derated conditions or both. If the single shift map was calibrated for operating according to the rated torque capability, then at this derated torque capability, for the aforementioned theoretical operating point, varying accelerator input between 45% and 60% may induce limited response from the powertrain. FIG. 6 illustrates the method 400 of controlling the automatic transmission 12 of the vehicle 1. The method 400 compensates for a derate of the torque capability of the electric machine 26 by the modifying gear shift thresholds in accordance with which the automatic transmission 12 is controlled. The compensation may aim to achieve operating qualities of the vehicle powertrain when the torque capability is derated which are substantially consistent with those achieved during operation of the electric machine 26 according to its rated torque capability. The method 400 comprises, at block 410, receiving an input signal indicative of at least one current temperature within the electric machine 26. For example, an input signal indicative of a current rotor temperature within the electric machine 26 may be received. Additionally or alternatively, an input signal indicative of a current stator temperature within the electric machine 26 may be received. These input signals may be received from a control module for the invertor 30. Block 420 comprises determining an estimate of a torque limit imposed on the electric machine 26 under a thermal derate strategy for the electric machine 26. This estimate of the torque limit is based on the at least one current temperature within the electric machine 26 received at block 410. The thermal derate strategy for the electric machine 26 is a control strategy for derating the torque capability of the electric machine 26 as a function of temperature(s) within the electric machine 26 such as, for example, the temperature of the rotor and / or stator. Its purpose is to prevent overheating and protect the electric machine 26 from damage. It involves imposing different torque limits on the electric machine 26 depending on the temperature(s). By way of example, the thermal derate strategy may begin to impose a torque limit which decreases (which is to say becomes more limiting by way of limiting torque to lower and lower values) linearly with increase in temperature once a first threshold temperature has been exceeded. The torque limit may decrease until the electric machine 26 may provide no torque at a second, higher threshold temperature. The second temperature threshold may be indicative of a fault. At temperatures below the first threshold temperature, the electric machine 26 operates according to its rated torque capability. The thermal derate strategy is not necessarily implemented by control system 100, however the control system 100 may store a model of the thermal derate strategy in the memory 108 and so, based on at least one current temperature within the electric machine 26 (received at block 410), can inferred what response would be implemented under the thermal derate strategy and thus estimate what torque limit will be currently being imposed on the electric machine 26. The thermal derate strategy may be implemented by the inverter 30 or an associated control module. Block 430 comprises obtaining a first shift map, which will be subsequently modified to generate a shift map which compensates for the effect that the torque limit imposed on the electric machine 26 would have on the operating qualities of the vehicle powertrain. Shift maps are used to determine a destination gear ratio for the automatic transmission 12 based on a speed parameter and an accelerator input. The speed parameter may be any speed parameter suitable for enabling a determination of a gear ratio in which to place the automatic transmission 12. For example, the speed parameter may be a speed parameter of the vehicle 1, such as its longitudinal speed (a longitudinal direction being defined by an axis between the front and rear of the vehicle 1). Alternatively, the speed parameter may be a speed parameter of a powertrain component, such as a rotational speed of one or more of the wheels 34, or of a transmission output shaft 20, or of a transmission input shaft 18 (which may be measured from a rotational speed of a turbine of the torque converter 14), or of the engine 24 and / or electric machine 26. In some examples, the speed parameter may be a ratio between foregoing parameters, particularly between those pertaining to speeds on either side of the gear set 16, such as a ratio between the vehicle’s longitudinal speed and the engine / electric machine speed ora ratio between the wheel speed and the transmission input shaft speed. In some examples the speed of the transmission output shaft 20 may be used because it may be measured within the automatic transmission system 10 and accordingly network latency and communication issues do not affect the signal which is indicative of it. The accelerator input may be dependent on for example, accelerator pedal depression (APD) or autonomous driving torque demand from an automated driving system (ADS) or an advanced driver-assistance system (ADAS). The accelerator input may be indicative of a torque requested or to be requested of the propulsion system 22. Shift maps define a plurality of regions of a parameter space which is spanned by the speed parameter and the accelerator input. Each of the regions of the parameter space defined by a shift map is associated with a different gear ratio. The lowest gear ratio provides the fastest transmission output speed for a given transmission input speed. The highest gear ratio provides the slowest transmission output speed for a given transmission input speed. Upshifting increases the gear ratio whilst downshifting reduces the gear ratio. A region associated with the highest gear ratio comprises a lower speed parameter boundary (downshift threshold) as a function of the accelerator input. A region associated with the lowest gear ratio comprises an upper speed parameter boundary (upshift threshold) as a function of the accelerator input. Each region associated with intervening gear ratios comprises a lower speed parameter boundary (downshift threshold) as a function of the accelerator input and an upper speed parameter boundary (upshift threshold) as a function of the accelerator input. The functions which define the gear shift (ratio change) thresholds can be specified in various forms. For example, each of the functions could be specified by a mathematical formula or mathematical formulas (for example in the case of a piecewise function). Alternatively, each of the functions could be specified by listing values of the function, for example in a table or array. The table or array can give the values of the function at specific values of the accelerator input. If a value of the function associated with an intermediate value of the accelerator input is needed, interpolation can be used to estimate the value of the function. The values of the function at specific values of the accelerator input are known as “shift points”. Shift points associated with a downshift are known as “downshift points”. Shift points associated with an upshift are known as “upshift points”. A gear ratio which is different to the current gear ratio may be proposed by a shift map when a current value of the accelerator input and a current value of the speed parameter describe a point within the defined parameter space which is not within the region associated with the current gear ratio. The proposed gear ratio is the one associated with the region in which the point lies. For each gear ratio, the region associated with this gear ratio may at least partially overlaps with the regions for its nearest neighbouring gear ratios (by size), though, in general, none of the lower speed parameter boundaries (downshift thresholds) overlap or intersect with one another and, in general, none of the upper speed parameter boundaries (upshift thresholds) overlap or intersect with one another. If the point described by a current value of the accelerator input and a current value of the speed parameter lies within an overlap between two or more regions, then the gear ratio selected depends on the direction of entry into the overlap region. It will be appreciated however that a gear ratio proposed by a shift map may be further subjected to restrictions on gear availability before the destination gear ratio is determined in final arbitration. For example, the number of gear ratios which may be skipped over in shifting from the current gear ratio to a destination gear ratio may be limited. A gear ratio proposed by a shift map may also be further subjected to gear shift inhibit conditions before the destination gear ratio is determined in final arbitration. Examples of shift inhibit conditions include, without limitation: being within a defined period of time from completion of a previous shift; advanced driver-assistance systems (ADAS) features such as adaptive cruise control being active; being engaged in downhill driving, or at least downhill driving above a threshold gradient; a rate of decrease in the accelerator input exceeding a threshold rate; being engaged in cornering, or at least cornering exceeding a threshold based on a lateral acceleration; being engaged in reversing; a propulsion system of the vehicle being in transition between predefined operating modes; a different target gear ratio being determined based on a change to at least one of: the speed parameter, the accelerator input, or the active shift map; and a rate of decrease in a powertrain torque exceeding a threshold rate. A plurality of shift mapsfordifferent particular scenarios, including different operating conditions, may be stored in the memory 108 of the control system 100. The plurality of shift maps stored in the memory 108 cover a finite, discrete set of specific operating conditions. If current operating conditions do not match these, then interpolation between stored shift maps associated with operating conditions which are similar but different to the current operating conditions may be performed to generate a shift map tailored to the current operating conditions. Other, non-interpolative modifications to the gear shift thresholds may be additionally or alternatively applied to account for the current operating conditions. Though, as stored, each shift map comprises gear shift thresholds for all gear ratios, as retrieved from the memory 108 and as used for determining the destination gear ratio, the shift maps may define only those gear shift thresholds associated with gear shifts out of a current gear ratio or at least those gear shift thresholds. 15 For example, the first shift map, which may be retrieved directly from the memory 108 or which may be determined from one or more shift maps retrieved from the memory 108, can define at least gear shift thresholds which are associated with gear shifts out of a current gear ratio. In some examples, the first shift map may be limited to defining gear shift thresholds which are associated with gear shifts out of a current gear ratio. In other examples, the first shift map may define more gear shift thresholds such as gear shift thresholds associated with any selectable gear ratio within the automatic transmission 12. The first shift map is one which is used when the electric machine 26 is operating according to its rated torque capability. That is, the first shift map is one in accordance with which the automatic transmission 12 is controlled while the electric machine is operating according to its rated torque capability. It may be specifically calibrated for operation of the electric machine 26 at its rated torque capability. For example, it may be tuned in relation to a torque demand map which relates accelerator input to a torque demand at different electric machine speeds. This tuning may be such that the first shift map proposes shifts to gear ratios which result in electric machine speeds which are optimal for meeting this demand. The first shift map may be one among a plurality which can be used when the electric machine 26 is operating according to its rated torque capability. It may be the one of these which is associated with one or more of the following operating conditions: current road load (a reflection of driving resistance calculated from weight, slope, or the like), current air pressure (accounting for both altitude and temperature), or current operating mode of the propulsion system 22. The method 400 may accordingly also comprise receiving: one or more input signals indicative of the current road load, one or more input signals indicative of the current air pressure, and / or one or more input signals indicative of a current status of a propulsion system 22 such as which of the predefined operating modes the propulsion system 22 is currently in or between which predefined operating modes it is currently transitioning. The first shift map may therefore be, for example, one in accordance with which the automatic transmission 12 is controlled while the vehicle 1 is operating: in a parallel hybrid mode; at an air density equal to the current air density; against a road load equal to the current road load; and with the electric machine 26 operating according to its rated torque capability. The first shift map may alternatively be, for example, one in accordance with which the automatic transmission 12 is controlled while the vehicle 1 is operating: in an electric-only mode; against a road load equal to the current road load; and with the electric machine 26 operating according to its rated torque capability. The first shift map may be specifically calibrated for either such use. Block 440 comprises generating a compensated shift map. The compensated shift map is generated by modifying at least some of the gear shift thresholds of the first shift map. These gear shift thresholds are modified to increase minimum values of the speed parameter at which respective gear shifts are triggered. The increased minimum values are based on the estimate of the torque limit (determined at block 420). The compensated shift map is, in some examples, generated by modifying at least the gear shift thresholds of the first shift map which are associated with gear shifts out of the current gear ratio to increase the minimum 16 values of the speed parameter at which respective gear shifts out of the current gear ratio are triggered. The increased minimum values are based on the estimate of the torque limit (determined at block 420). If the first shift map defines gear shift thresholds associated with other gear shifts, then in some examples, at least some of these may also be modified to increase minimum values of the speed parameter at which they are triggered, again with the increase being based on the estimated torque limit. In some examples, gear shift thresholds associated with certain gear shifts may not be modified to increase to increase minimum values of the speed parameter at which they are triggered. Increasing the minimum values of the speed parameter at which gear shifts are triggered has the effect of maintaining lower gear ratios for longer to get maximum mechanical advantage from being in a lower gear ratio in order to compensate for the reduced responsiveness to accelerator input, which would otherwise result from shift scheduling according to the first shift map while the electric machine 26 is operating at a derated torque capability. The ability to reliably estimate the torque limit (as at block 420) which derates the torque capability contributes to improved compensation than would otherwise have been achieved. In some examples, the gear shift thresholds of the first shift map which are modified to generate the compensated shift map may be raised to higher values of the speed parameter for all accelerator inputs or for at least a range of accelerator inputs. For example, these gear shift thresholds may be raised to higher values of the speed parameter for all accelerator inputs below a certain value, which may vary between gear shift thresholds associated with different gear shifts. For example, a gear shift threshold associated with one gear shift may be raised to higher values of the speed parameter for all accelerator inputs below a first value whereas a gear shift threshold associated with another gear shift may be raised to higher values of the speed parameter for all accelerator inputs below a second, different value. It will be appreciated that, in some examples, the compensated shift map may not be generated in its entirety at any given time. Instead, the compensated shift map may be generated in the form of one or more compensated shift points which are relevant at a given time. For example, at a given time, relevant shift points may be those associated with a gear shift out of the current gear that could be triggered at or close to the current value of the speed parameter or the current value of the accelerator input. Block 450 comprises controlling the automatic transmission 12 according to the compensated shift map. Controlling the automatic transmission 12 according to the compensated shift map can comprise shifting from a current gear ratio to a destination gear ratio when a current value of the accelerator input and a current value of the speed parameter describe a point within the parameter space defined by the compensated shift map which is not within the region associated with the current gear ratio. The destination gear ratio may be determined as the gear ratio associated with the region of the compensated shift map in which the described point lies. As described in the foregoing, if the described point lies within an overlap between two or more regions of the compensated shift map, then the destination gear ratio will depend upon the direction of entry into the overlap region. Though in the majority of examples, the torque limit imposed on the electric machine 26 is due to the thermal derate strategy, there are occasions where this is not the case. FIG. 7 illustrates an example of a method 500 of estimating the torque limit imposed on the electric machine 26 which accounts for occasions when the electric machine 26 is further limited. The method 500 can be used to determine an estimate of the torque limit imposed on the electric machine 26 for use in the method 400. The method 500 may therefore be implemented as a submethod of the method 400 which may be performed at block 420. The method 500 comprises receiving at least some of the following as input parameters: a current rotor temperature within the electric machine 26, received at block 502; a current stator temperature within the electric machine 26, received at block 504; a current torque capability of the electric machine 26, received at block 506; a current power limit imposed on the electric machine 26, received at block 508; and a current speed of the electric machine 26, received at block 510. These input parameters may be indicated by signals received by the control system 100 or may be output parameters from another method performed by the control system 100. They may therefore be received from an external source (with respect to the to the control system 100) or have an internal source. A first estimate of the torque limit imposed on the electric machine 26 is determined at block 512 based on at least one current temperature within the electric machine 26 and a model of a thermal derate strategy for the electric machine 26, as described in relation to block 420 of method 400 in the foregoing. In some examples, the model of the thermal derate strategy for the electric machine 26 is implemented by lookup table stored in the memory 108 of the control system 100 which defines torque limits expected to be imposed on the electric machine 26 at different rotor and stator temperatures. In such examples, block 512 comprises using the current rotor temperature (received at block 502) and the current stator temperature (received at block 504) as an index to the lookup table to extract the corresponding torque limit. The first estimate of the torque limit can be specified as a derate factor. The derate factor represents the estimated torque limit as a fraction of the maximum torque that the electric machine can continuously produce when operated according to its rated torque capability. The method 500 determines a second estimate of the torque limit imposed on the electric machine 26 in case the torque limit imposed on the electric machine 26 is not due to the thermal derate strategy. It will be appreciated that a torque limit may still be proposed under the thermal derate strategy on such occasions, but a more limiting torque limit may be anyway imposed due to other adverse operating conditions. The second estimate of the torque limit imposed on the electric machine 26 is determined by comparing the current torque capability of the electric machine 26 with an expected torque capability of the electric machine 26 for the current speed of the electric machine 26 and for the current power limit imposed on the electric machine 26. In some examples, the expected torque capability of the electric machine 26 at different speeds while under a given power limit can be derived from experimental data, theoretical modelling, or a combination thereof and recorded in a lookup table associated with that power limit. A plurality of such lookup tables, associated with different power limits, can be stored in the memory 108 of the control system 100. In such examples, block 514 comprises using the current speed of the electric machine 26 (received at block 510) as an index to the lookup tables to extract the corresponding expected torque capabilities of the electric machine 26 under different power limits. In the illustrated example, there are five different lookup tables associated with five different power limits, though it will be appreciated that there may be more or less. Block 516 then comprises interpolating between the expected torque capabilities of the electric machine 26 under different power limits based on the current power limit (received at block 508) in order to return an expected torque capability of the electric machine 26 for the current speed and for the current power limit. Block 518 comprises dividing the current torque capability of the electric machine 26 (received at block 506) by the expected torque capability of the electric machine 26 for the current speed and for the current power limit (determined at block 516). In some examples the order of performance of blocks 516 and 518 may effectively be reversed such that the current torque capability of the electric machine 26 is divided respectively by each of the extracted expected torque capabilities of the electric machine 26 under different power limits and then interpolating between the resultant values to obtain a single interpolated value. The result of block 516 and 518, regardless of the order of their performance, is a second estimate of the torque limit imposed on the electric machine 26. Block 534 comprises comparing the first and second estimates of the torque limit imposed on the electric machine 26 to determine which is the most limiting and comprises selecting the most limiting of these to provide as an output parameter (from method 500) at block 536. It will be understood that this output parameter forms an input parameter for another block in the method 400 (specifically block 440) and is not an output from the control system 100. The increased minimum values of the speed parameter at which respective gearshifts out of the current gear are triggered according to the compensated shift map (generated in block 440) are based on the estimate of the torque limit provide as an output parameter at block 536. By selecting the most limiting estimated torque limit on which to base the generation of the compensated shift map, the amount of compensation achieved through use ofthis shift map will not be based on an underestimate of what is required. Blocks 514 to 518 are sufficient to achieve a reliable second estimate of the torque limit imposed on the electric machine 26 when the electric machine 26 is torque-limited. Once the electric machine 26 transitions to powerlimited, the convergence of the torque-speed curves characterising the torque capability of the electric machine 26 under different torque limits (as seen in FIG. 5B) means that the division at block 518 will not produce a reliable second estimate of the torque limit. An example of additional steps which may be included in the method 500 to improve the reliability of the second estimate of the torque limit will now be described. Block 520 comprises determining the speed at which the electric machine 26 becomes power-limited. The speed at which the electric machine 26 becomes power-limited is the speed at which the torque capability of the electric machine 26, given the current power limit imposed on the electric machine 26, becomes less than the torque limit imposed on the electric machine 26. Assuming that the current electric machine speed is less than this speed (and thus that the electric machine 26 is not currently power-limited), the current torque capability should align with the imposed torque limit. Thus, through relation of power being the product of torque and speed, the speed at which the electric machine 26 becomes power-limited can be determined based on: the current torque capability of the electric machine 26 (received at block 506) and the current power limit imposed on the electric machine 26 (received at block 508). A tuneable (speed) offset is obtained at block 522. This may be obtained by accessing data from the memory 108 of the control system 100 and may also involve operations such as calculating or computing or selecting or choosing. The tuneable offset is subtracted from the speed at which the electric machine 26 becomes power-limited (the result of block 520) at block 524 to define an electric machine speed at which the electric machine 26 is on the verge of becoming power-limited. The result of block 524 is compared to the current speed of the electric machine 26 (received at block 510) at block 526 to determine if the current speed of the electric machine 26 is on the verge of becoming powerlimited. Once the electric machine 26 becomes power-limited, the reliability of the following steps may be compromised, so reaching this speed (result of block 524) is a trigger for performing the following steps. If the result of block 526 indicates that the current speed of the electric machine 26 is within the tuneable offset of the speed at which the electric machine 26 becomes power-limited, then at block 528 the second estimate of the torque limit (the result of block 518) is stored. Block 530 comprise comparing the speed at which the electric machine 26 becomes power-limited (the result of block 520) with the current speed of the electric machine 26 (received at block 510) to determine which is greater. When the current speed of the electric machine 26 exceeds the speed at which the electric machine 26 becomes power-limited, the stored result is used as the second estimate of the torque limit. Otherwise, the current result from block 518 is used. The selection between the stored and current results can be made at block 532 in dependence on the result of block 530. Alternatively, blocks 530 and 532 may be omitted and the result of block 518 may be latched at block 528 and only unlatched once the current speed of the electric machine 26 drops back below the result of block 524. The result of block 524 may be latched to avoid recalculation and reset if the power limit changes. In either case, a stored (for example, latched) second estimate of the torque limit is used when the current speed of the electric machine 26 exceeds the speed at which the electric machine 26 becomes power-limited. FIG. 8 illustrates a method 600 which provides an example of how to generate the compensated shift map for use in the method 400. The method 600 may therefore be implemented as a submethod of the method 400 which may be performed at block 440. In some examples, the method 600 may be used to generate the compensated shift map in dependence on the propulsion system 22 being in (or in transition to) a parallel hybrid mode. If the propulsion system 22 is in an electric-only mode, then the method 800, which will be described in relation to FIG. 12, may instead be used to generate the compensated shift map. However, it will also be understood that either method 600, 800 could be used for parallel hybrid mode or for electric-only mode. The method 600 comprises interpolating between a first shift map 602 and a second shift map 604. While the first shift map 602 is, as described in the foregoing, one which is used when the electric machine 26 is operating according to its rated torque capability, the second shift map 604 is, by contrast, one which is used when the electric machine 26 is operating according to a predefined derated torque capability. The predefined derated torque capability may correspond to the maximum derate of the torque capability of the electric machine 26 at which the vehicle 1 is intended to still be able to operate. It can therefore depend on the operating mode of the propulsion system 22. For example, in a parallel hybrid mode, the torque capability of the electric machine 26 could be derated to the point at which no torque is produced by the electric machine 26 because the vehicle 1 may still be driven by engine torque. In such instances, the second shift map 604 may be one which is or could be used in engine-only mode. In an electric-only mode, the predefined derated torque capability would be non-zero. The predefined derated torque capability may correspond to a minimum torque capability of the electric machine 26 which is capable of sustaining electric-only driving. The second shift map 604 is one in accordance with which the automatic transmission 12 is controlled while the electric machine 26 is operating according to the predefined derated torque capability. It may be specifically calibrated for operation of the electric machine 26 at the predefined derated torque capability. For example, it may be tuned in relation to a torque demand map which relates accelerator input to a torque demand at different electric machine speeds. This tuning may be such that the second shift map proposes shifts to gear ratios which can meet this demand in spite of the derate of the torque capability of the electric machine 26. Since more torque can be produced in a lower gear ratio, it may be preferable to delay upshifts and to downshift earlier. Accordingly, at least the minimum values of the speed parameter at which upshifts are triggered are raised (to cause upshifts to be delayed) and at least the minimum values of the speed parameter at which downshift are triggered are raised (to cause downshifts to be made earlier) in the second shift map 604 as compared to the first shift map 602. Like the first shift map 602, the second shift map may define at least gear shift thresholds which are associated with gear shifts out of a current gear ratio. In some examples, the second shift map 604 may be limited to defining gear shift thresholds which are associated with gear shifts out of a current gear ratio. In other examples, the second shift map 604 may each define more gear shift thresholds such as gear shift thresholds associated with any selectable gear ratio within the automatic transmission 12. To aid understanding of the differences between the first and second shift maps 602,604, gear shift thresholds of an example first shift map for an eight-speed transmission and an example second shift map for the same eight-speed transmission are shown in FIGS 9 and 10. Upshift thresholds for both shift maps are shown in FIG. 9. Downshift thresholds for both shift maps are shown in FIG. 10. In FIGS 9 and 10, the speed parameter is shown on the x-axis with respect to accelerator input on the y-axis. FIG. 9 illustrates seven upshift thresholds of the example first shift map, which each comprise the left-most branch (solid line) at the lower region of each line. FIG. 9 also illustrates seven upshift thresholds of the example second shift map, which each comprise the right-most branch (dashed line) at the lower region of each line. Both sets of upshift thresholds comprise the portion of each line above where the branches converge. The left-most of the lines represents the upshift threshold for triggering an upshift out of the first gear (lowest gear ratio) and the right-most of these lines represents the upshift threshold for triggering an upshift into the eighth gear (highest gear ratio). Intervening lines successively represent upshift thresholds for triggering upshifts from the second gear through to the seventh gear. FIG. 10 illustrates seven downshift thresholds of the example first shift map, which each comprise the left-most branch (solid line) at the lower region of each line. FIG. 10 also illustrates seven downshift thresholds of the example second shift map, which each comprise the right-most branch (dashed line) at the lower region of each line. Both sets of downshift thresholds comprise the portion of each line above where the branches converge. The right-most of these lines represents the downshift threshold for triggering a downshift out of the eighth gear and the left-most of the lines represents the downshift threshold for triggering a downshift into the first gear. Intervening lines successively represent downshift thresholds for triggering downshifts from the seventh gear through to the second gear. It will be observed from FIGS 9 and 10, which are exemplary on at least this point, that at least some of the gear shift thresholds of the second shift map 604 have higher minimum values of the speed parameter than corresponding gear shift thresholds of the first shift map 602. This enables the second shift map 604 to propose lower gear ratios at higher values of the speed parameter than the first shift map 602. Accordingly control of the automatic transmission 12 according to the second shift map 604 can maintain lower gears (lower gear ratios) for longer (as compared to control according to the first shift map 602). There is more maximum mechanical advantage from being in a lower gear ratio, and this can be used in order to compensate forthe reduced responsiveness to accelerator input, which would otherwise result from the derate of the torque capability of the electric machine 26. In some examples, at least some of the gear shift thresholds of the second shift map 604 have higher values of the speed parameter for all accelerator inputs or for at least a range of accelerator inputs. For example, these gear shift thresholds of the second shift map 604 may have higher values of the speed parameter than corresponding ones in the first shift map 602 for all accelerator inputs below a certain value, which may vary between gear shift thresholds associated with different gear shifts. In some examples (not illustrated), one or more, but not all, corresponding gear shift thresholds of the first and second shift maps 602, 604 are the same, including in respect of minimum values of the speed parameter at which an associated gear shift is triggered. Typically, this will be in respect of shifts between the highest gear ratios. In other examples, all of the corresponding gear shift thresholds of the first and second shift maps 602, 604 differ, at least in respect of the minimum values of the speed parameter at which associated gear shifts are triggered. The magnitude of the difference in minimum values of the speed parameter between corresponding gear shift thresholds of the first and second shift maps 602, 604 may vary between gear shift thresholds associated with different gear shifts. For gear shift thresholds associated with shifts between low gear ratios, the magnitude of this difference may be greater than for gear shift thresholds associated with shifts between high gear ratios. For gear shift thresholds associated with gear shifts in and out of the highest gear ratio, for example, there may be little or no difference between those of the first shift map 602 and those of the second shift map 604. It will be understood that the exact shape of gear shift thresholds of the first and second map and the values of the shift points which may specify these thresholds will depend on, for example: the calibration strategy employed; the rated and derated capability, respectively, of the electric machine 26; the capabilities of the engine 24; the transmission type; etc. Though the gear shift thresholds have been illustrated with straight line segments, it will be appreciated that the lines may be curved in whole or in part. Returning to the method 600, block 612 specifically comprises an interpolation between at least the gearshift thresholds of the first shift map 602 which are associated with gear shifts out of the current gear and those corresponding gear shift thresholds of the second shift map 604 is performed. The result of the interpolation is a set of gear shift thresholds which define the compensated shift map 614. 23 In some examples, the interpolation is effected by interpolating between a value of the speed parameter according to the first shift map 602 and a value of the speed parameter according to the second shift map 604 at corresponding values of accelerator input. This may be performed at specific values of the accelerator input. If a gear shift threshold value of the speed parameter associated with an intermediate value of the accelerator input is needed, another interpolation can be used to estimate this. The interpolation may be a linear interpolation and the interpolation factor may indicate a magnitude of the displacement of the gear shift threshold values of the speed parameter in the compensated shift map 614 to higher speeds, where said displacement is along interpolants between speed parameter values corresponding to the same accelerator input value in corresponding gear shift thresholds of the first and second shift maps 602,604. The interpolation factor may lie in a range of 0 to 1. Where the interpolation factor is 0, the interpolation is weighted entirely towards the first shift map 602. Accordingly, an interpolation factor of 0 indicates operation of the electric machine 26 according to its rated torque capability. Where the interpolation factor is 1, the interpolation is weighted entirely towards the second shift map 604. Accordingly, an interpolation factor of 1 indicates operation of the electric machine 26 according to the predefined derated torque capability. Where, for example, the interpolation factor is 0.5, the gear shift thresholds of the compensated shift map 614 lie hallway between the corresponding gear shift thresholds of the first shift map 602 and the second shift map 604. Because at least some gear shift thresholds of the second shift map 604 have higher minimum values of the speed parameter than corresponding gear shift thresholds of the first shift map 602 interpolation between corresponding gear shift thresholds of the first and second shift maps 602,604 to produce gear shift thresholds of the compensated shift map 614 results in an increase in the minimum values of the speed parameter at which respective shifts out of a gear are triggered according to the compensated shift map 614 in comparison to the first shift map 602. Since, in some examples, at least some of the gear shift thresholds of the second shift map 604 have higher values of the speed parameter than corresponding gear shift thresholds of the first shift map 602 for all accelerator inputs or for at least a range of accelerator inputs, as a result of the interpolation, at least some of the gear shift thresholds of the compensated shift map 614 will also have higher values of the speed parameter than corresponding gear shift thresholds of the first shift map 602 for all accelerator inputs or for at least a range of accelerator inputs. In some examples, the interpolation may be effected using an interpolation factor based on the torque limit imposed on the electric machine 26, as estimated in any manner described in the foregoing. To this end, the method 600 comprises, at block 606, receiving an estimate of the current torque limit imposed on the electric machine 26, as estimated in any manner described in the foregoing, as an input parameter. 24 The aforementioned advantages of raising the minimum value of the speed parameter at which upshifts and downshifts are triggered are therefore achieved to an extent commensurate with the torque limit imposed on the electric machine 26. Overcompensation, for example by controlling the automatic transmission 12 in accordance with the second shift map when the torque capability of the electric machine 26 is not so derated is avoided. This allows operating qualities of the vehicle powertrain to feel substantially consistent. It may also avoid the disadvantages of an unnecessarily aggressive shift scheduling strategy, which might include increased wear on components and reduced fuel efficiency in addition to reduced driver satisfaction. In some examples, the interpolation factor can also be based on a power limit imposed on the electric machine 26. The interpolation factor may therefore be based on the current derate of the torque capability of the electric machine 26 as a result of the torque and power limits estimated (or determined) to be imposed upon the electric machine 26. To this end, the current power limit imposed on the electric machine 26 may also be received as an input parameter at block 608. The current power limit may be indicated by an input signal received by the control system 100. The input signal may be from, for example, a powertrain control module (PCM) of the vehicle 1 or from a vehicle supervisory controller (VSC) of the vehicle 1, which may be employed to coordinate control between the conventional powertrain and the electrified powertrain. The VSC may be hosted within the PCM. In some examples, the input signal may indicate the current power limit in the form of a powertrain performance ratio, which is a dimensionless value which characterises the relative performance of electrified powertrain components, in respect to their optimum performance, due to the effect of electrical power limits. Another method performed by the control system 100 may determine the current power limit imposed on the electric machine 26 based on the powertrain performance ratio. For example, a lookup table stored in the memory 108 of the control system 100 may define power limits imposed on the electric machine 26 for different powertrain performance ratios. Accordingly, the received powertrain performance ratio may be used as an index to the lookup table to extract the corresponding power limit. Block 610 comprises determining an interpolation factor based on the estimate of the current torque limit (received at block 606) and the current power limit (received at block 608). The interpolation factor may be determined from a lookup table of calibrated values for the interpolation factor, for example. The lookup table comprises specific values for the torque limit that index a first dimension of the lookup table and specific values for the power limit that index a second dimension of the lookup table. Accordingly, the estimate of the current torque limit and the current power limit may be used to extract a corresponding value of the interpolation factor. The calibrated values forthe interpolation factorare larger when the torque limit is more limiting. The calibrated values forthe interpolation factorare larger when the power limit is more limiting. FIG. 11A illustrates a method 700A which provides an example of how to obtain the first shift map for use in the method 400. The method 700A may therefore be implemented as a submethod of the method 400 which may be performed at block 430. The first shift map (referenced again as 602) is obtained from a first set of shift maps 702. Each of these shift maps are associated with operation of the electric machine 26 at its rated torque capability. Different shift maps in the first set 702 are associated with different road loads. Different shift maps in the first set are therefore used for different road loads when the electric machine 26 is operating according to its rated torque capability. Each of the shift maps in the first set of shift maps may be specifically calibrated for operation of the electric machine 26 at its rated torque capability. They may each be further specifically calibrated to compensate for a different road load, typically adopting a more aggressive profile where gear shifts are triggered at higher values of the speed parameter for higher road loads. In this example, the first set of shift maps 702 comprises two subsets 702A, 702B. Shift maps in the first subset 702A are associated with a first (ambient) air pressure. That is, these shift maps are calibrated to provide shift scheduling having desirable characteristics during operation of the vehicle 1 in conditions involving the first air pressure. Shift maps in the second subset 702B are associated with a second (ambient) air pressure. That is, these shift maps are calibrated to provide shift scheduling having desirable characteristics during operation of the vehicle 1 in conditions involving the second air pressure. The first and second air pressures represent end points of a range of air pressures in which the vehicle 1 is expected to operate. For example, the first air pressure may be an air pressure expected at sea level and the second air pressure may be an air pressure expected at a maximum altitude at which the vehicle 1 is intended to operate. A current road load is received as an input parameter for the method 700A at block 706. The current road load may be indicated by a signal received by the control system 100 or may be an output parameter from another method performed by the control system 100, for example a method that determines road load from one or more of estimated vehicle mass, estimated slope (road gradient), estimated aerodynamic resistance, and estimated rolling resistance. The road load may comprise a variance, due to one or more factors,from the road load of the unladen vehicle on a level road. Block 708 comprises selecting a first pair of shift maps associated with a road load interval in which the current road load resides from the first subset 702A of the first set of shift maps 702. In some examples, the two shift maps associated with the smallest road load interval in which the current road load resides are selected. The current road load is used at block 710 to interpolate between the first pair of shift maps to generate a first interim shift map 712. The relative position of the current road load in the road load interval provides the interpolation factor upon which the interpolation is based. The closer the current road load is to one end of the road load interval, the more the gear shift thresholds defining the first interim shift map 712 will be weighted towards those of the shift map associated with that end of the road load interval. Blocks 708' and 710' repeat blocks 708 and 710 in respect of the second subset 702B of the first set of shift maps 702 to generate a second interim shift map 714. That is, a second pair of shift maps associated with a road load interval in which the current road load resides (for example, the smallest road load interval in which the current road load resides) are selected from the second subset 702B and the second interim shift map 714 is generated by interpolating between this second pair of shift maps based on the current road load. In some examples, if there is a shift map in the first subset 702A which is associated with the current road load, then this shift map may be selected as the first interim shift map 712 and the steps described in relation to blocks 708 and 710 in the foregoing may not be performed. Likewise, if there is a shift map in the second subset 702B which is associated with the current road load, then this shift map may be selected as the second interim shift map 714 and blocks 708' and 710' in the foregoing may not be performed. A current air pressure is received as an input parameter at block 720. The current air pressure may be indicated by a signal received by the control system 100 or may be an output parameter from another method performed by the control system 100. The current air pressure is used at block 722 to interpolate between the first and second interim shift maps 712, 714 to generate the first shift map 602. The relative position of the current air pressure In the Interval between the first and second air pressures provides the interpolation factor upon which the interpolation is based. The first shift map 602, obtained according to the method 700A, is therefore compensated for road load and air pressure. Air pressure varies with both altitude and temperature so the first shift map 602 is compensated for both of these. FIG. 11B illustrates a method 700B which provides an example of how to obtain the second shift map 604 for use in the method 600. In examples in which the method 700B is implemented, it is implemented in conjunction with the method 700A. Contrastingly, the method 700A may be implemented independently of the method 700B. The second shift map 604 is obtained from a second set of shift maps 704. Each of these shift maps are associated with operation of the electric machine 26 according to the predefined derated torque capability. Different shift maps in the first set 704 are associated with different road loads. Different shift maps in the second set are therefore used for different road loads when the electric machine 26 is operating according to the predefined derate torque capability. Each of the shift maps in the second set of shift maps may be specifically calibrated for operation of the electric machine 26 at the predefined derated torque capability. They may each be further specifically calibrated to compensate for a different road load, typically adopting a more aggressive profile where gear shifts are triggered at higher values of the speed parameter for higher road loads. In this example, the second set of shift maps 704 comprises two subsets 704A, 704B. Shift maps in the first subset 704A are associated with the aforementioned first air pressure. That is, these shift maps are calibrated to provide shift scheduling having desirable characteristics during operation of the vehicle 1 in conditions involving the first air pressure. Shift maps in the second subset 704B are associated with the aforementioned second air pressure. That is, these shift maps are calibrated to provide shift scheduling having desirable characteristics during operation of the vehicle 1 in conditions involving the second air pressure. The current road load, received as an input parameter for the method 700A at block 706, is also received at this block as an input parameter for the method 700B. Blocks 708" and 710" repeat blocks 708 and 710 in respect of the first subset 704A of the second set of shift maps 704 to generate a third interim shift map 716. Block 708" comprises selecting a third pair of shift maps associated with a road load interval in which the current road load resides from the first subset 704A of the second set of shift maps 704. In some examples, the two shift maps associated with the smallest road load interval in which the current road load resides are selected. Block 710" comprises interpolating between the third pair of shift maps to generate the third interim shift map 716 based on the current road load. The relative position of the current road load in the road load interval provides the interpolation factor upon which the interpolation is based. The closer the current road load is to one end of the road load interval, the more the gearshift thresholds defining the third interim shift map 716 will be weighted towards those of the shift map associated with that end of the road load interval. Blocks 708"' and 710"' repeat blocks 708" and 710" in respect of the second subset 704B of the second set of shift maps 704 to generate a fourth interim shift map 718. That is, a fourth pair of shift maps associated with a road load interval in which the current road load resides (for example, the smallest road load interval in which the current road load resides) are selected from the second subset 704B and the fourth interim shift map 718 is generated by interpolating between this fourth pair of shift maps based on the current road load. In some examples, if there is a shift map in the first subset 704A which is associated with the current road load, then this shift map may be selected as the third interim shift map 716 and the steps described in relation to blocks 708" and 710" in the foregoing may not be performed. Likewise, if there is a shift map in the second 28 subset 704B which is associated with the current road load, then this shift map may be selected as the fourth interim shift map 718 and blocks 708"' and 710"' in the foregoing may not be performed. The current air pressure, received as an input parameter for the method 700A at block 720, is also received at this block as an input parameter for the method 700B. Block 722' repeats block 722 in respect of the third and fourth interim shift maps 716, 718 to generate the second shift map 604. Block 722' comprises interpolating between third and fourth interim shift maps 716, 718 to generate the second shift map 604 based on the current air pressure. The relative position of the current air pressure in the interval between the first and second air pressures provides the interpolation factor upon which the interpolation is based. The second shift map 604, obtained according to the method 700B, is therefore compensated for road load and air pressure. Air pressure varies with both altitude and temperature so the second shift map 604 is compensated for both of these. When the first shift map 602, obtained according to the method 700A, and the second shift map 604, obtained according to the method 700B, are used in the method 800 to generate the compensated shift map 614, eight stored shift maps are therefore blended together via multiple layers of interpolation. Each layer of Interpolation provides increased refinement of the gear shift thresholds in respect of one parameter ofthe current operating conditions: first road load, then air pressure, then derate of torque capability. The process of generating the compensated shift map 614 is therefore one of fine-tuning the gear shift thresholds to the current operating conditions. Although both methods 700A and 700B, as described in the foregoing, compensate for road load and air pressure, either or both methods 700A and 700B may omit the steps compensating for air pressure. The first shift map 602 may be obtained by: selecting a pair of shift maps associated with a road load interval in which the current road load resides from the first set of shift maps 702; and interpolating between the pair of shift maps based on the current road load. The second shift map 604 may be obtained by: selecting a pair of shift maps associated with a road load interval in which the current road load resides from the second set of shift maps 704; and interpolating between the pair of shift maps based on the current road load. Alternatively, either or both methods 700A and 700B may omit the steps compensating for road load. The first shift map 602 may be obtained by: selecting a shift map associated with the first air pressure from the first set of shift maps 702; selecting a shift map associated with the second air pressure from the first set of shift maps 702; and interpolating between these two shift maps based on the current air pressure. The second shift map 604 may be obtained by: selecting a shift map associated with the first air pressure from the second set of shift maps 704; selecting a shift map associated with the second air pressure from the second set of shift maps 704; and interpolating between these two shift maps based on the current air pressure. FIG. 12 illustrates a method 800 which provides another example of how to generate the compensated shift map for use in the method 400. The method 800 may therefore be implemented as a submethod of the method 400 which may be performed at block 440. The first shift map, or at least relevant gear shift thresholds thereof, are received as an input parameter to the method 800 at block 802. The relevant gear shift thresholds can comprise those associated with gear shifts out of a current gear. In some examples, the first shift map may be obtained by the method 700A. The torque limit imposed on the electric machine 26, as estimated in any manner described in the foregoing, is received as an input parameter to the method 800 at block 804. In some examples, the power limit imposed on the electric machine 26 may also be received as an input parameter and may be received as such at block 806. Block 808 comprises determining minimum speed limits to impose on respective gear shifts. The minimum speed limits are based on the estimate of the torque limit (received at block 804). In some examples they may be further based on the power limit (received at block 806). In some examples a lookup table stored in the memory 108 of the control system 100 may define one or more minimum speed limits associated with different torque limits and, in some examples, different power limits. The lookup table may be derived from experimental data, theoretical modelling, or a combination thereof. Accordingly, the estimate of the torque limit (received at block 804) and, in some examples, the power limit (received at block 806) may be used as an index to the lookup table to extract the corresponding one or more minimum speed limits. The lookup table may define a plurality of minimum speed limits corresponding to minimum values of the speed parameter at which different gear shifts are triggered. Alternatively, the lookup table may define a single minimum speed limit. This may be summed with predefined speed offsets associated with different gear shifts to provide minimum speed limits corresponding to minimum values of the speed parameter at which different gearshifts should be triggered. Block 810 comprises clipping at least some of the gear shift thresholds of the first shift map received at block 802 to an accelerator input of value zero (0%) at the determined minimum speed limits. To aid understanding, FIG. 13 shows an example of the clipping applied to downshifts defined in an example first shift map. In FIG. 13 the speed parameter is shown on the x-axis with respect to accelerator input on the y-axis. The downshift thresholds of the first shift map are shown as solid lines. The clipped downshift thresholds are shown by the dashed lines which branch off from the downshift thresholds of the first shift map in the lower region of the plot at values of the speed parameter corresponding to the minimum speed limits. Clipping a gear shift threshold to an accelerator input of value zero at the minimum speed limit for the associated gear shift comprises removing the section of the gear shift threshold that enables gear shifts to be triggered at speed parameter values which are less that the minimum speed limit. The speed parameter value at which the associated gear shift is triggered is set equal to the minimum speed limit for accelerator input values associated with the removed section. As a result, the minimum value of the speed parameter at which the associated gear shift is triggered is raised to the minimum speed limit. The section of the gear shift threshold that enables gear shifts to be triggered at speed parameter values above the minimum speed limit is retained. In some examples, the clipped gearshift thresholds resulting from block 810 provide the gearshift thresholds of the compensated shift map. The compensated shift map is provided as an output parameter of the method 100 at block 816. It will be understood that this output parameter forms an input parameter for another block in the method 400 (specifically block 450) and is not an output from the control system 100. In other examples, further modification of the gearshift thresholds of the first shift map may be effected in order to provide corresponding gear shift thresholds of the compensated shift map. For example, at block 812, maximum speed limits to impose on the gear shifts are determined. In some examples, the maximum speed limits may only be determined and imposed on gearshifts thresholds received at block 802 which represents downshifts. The maximum speed limits are based on the power limit (received at block 806). In some examples they may be further based on the estimate of the torque limit (received at block 804). In some examples a lookup table stored in the memory 108 of the control system 100 may define one or more maximum speed limits associated with different power limits and, in some examples, different torque limits. The lookup table may be derived from experimental data, theoretical modelling, or a combination thereof. Accordingly, the power limit (received at block 806) and, in some examples, the estimate of the torque limit (received at block 804) may be used as an index to the lookup table to extract the corresponding one or more maximum speed limits. The lookup table may define a plurality of maximum speed limits corresponding to maximum values of the speed parameter at which different gearshifts are triggered. Alternatively, the lookup table may define a single maximum speed limit. This may be summed with predefined speed offsets associated with different gear shifts to provide maximum speed limits corresponding to maximum values of the speed parameter at which different gearshifts should be triggered. Where maximum speed limits are determined, these are used in block 814 to further modify the gear shift thresholds of the first shift map, for example by modifying the clipped gear shift thresholds resulting from block 810. Block 814 comprises clipping at least the downshift thresholds to an accelerator input of maximum value at the determined maximum speed limits. The maximum value may be the maximum value achievable without kickdown (not shown). Clipping a downshift threshold to an accelerator input of maximum value at the maximum speed limit for the associated downshift comprises removing the section of the downshift threshold that enables gearshifts to be triggered at speed parameter values which are above the maximum speed limit. The speed parameter value at which the associated downshift is triggered is set equal to the maximum speed limit for accelerator input values associated with the removed section. As a result, the maximum value of the speed parameter at which the associated downshift is triggered is lowered to the maximum speed limit. The section of the downshift threshold that enables gear shifts to be triggered at speed parameter values below the maximum speed limit is retained. In some examples, block. 814 additionally comprises clipping the upshift thresholds to an accelerator input of maximum value at the determined maximum speed limits. As used herein, the term ‘obtain / obtaining’ (and grammatical variants thereof) can include, not least: calculating, computing, processing, deriving, measuring, investigating, identifying, looking up (for example, looking up in a table, a database or another data structure), ascertaining and the like. Also, ‘obtain / obtaining’ can include receiving (for example, receiving information), accessing (for example, accessing data in a memory), determining and the like. Also, ‘obtain / obtaining’ can include resolving, selecting, choosing, establishing, and the like. It will be appreciated that embodiments of the present invention can be realised in any suitable form of hardware, software or a combination of hardware and software. For example, it is contemplated that the present invention is not limited to being implemented by way of programmable processing devices, and that at least some of, and in some embodiments all of, the functionality and or method steps of the present invention may equally be implemented by way of non-programmable hardware, such as by way of non-programmable ASIC, Boolean logic circuitry, etc. For purposes of this disclosure, it is to be understood that reference to ‘the control system being configured to’ is to be understood to mean ‘the one or more controllers of the control system are collectively configured to’. The controllers) described herein can each comprise a control unit or computational device having one or more electronic processors, the one or more processors collectively configured to perform the control system functionality set out in the control system claims. 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 blocks illustrated in the FIGS 6, 7, 8, 11A, 11B, and 12 may represent steps in a method and / or sections of code in the computer program 110. The illustration of a particular order to the blocks does not necessarily imply that there is a required or preferred order for the blocks and the order and arrangement of the block may be varied. Furthermore, it may be possible for some steps to be omitted. Features described in the preceding description may be used in combinations other than the combinations explicitly described. Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not. Although features have been described with reference to certain embodiments, those features may also be present in other embodiments whether 5 described or not.

Claims

1. A control system for an automatic transmission of a vehicle , the control system comprising one or more controllers , the control system configured to:receive an input signal indicative of at least one current temperature within an electric machine ;determine a first estimate of a torque limit imposed on the electric machine under a thermal derate strategy for the electric machine based on the at least one current temperature within the electric machine ;obtain a first shift map which is used when the electric machine is operating according to its rated torque capability, wherein the first shift map defines gear shift thresholds for a speed parameter as functions of accelerator input;generate a compensated shift map by modifying at least some of the gear shift thresholds of the first shift map to increase minimum values of the speed parameter at which respective gear shifts are triggered, wherein the increased minimum values are based on the first estimate of the torque limit; andcontrol the automatic transmission according to the compensated shift map.

2. The control system of claim 1, wherein the first shift map defines at least gear shift thresholds which are associated with gear shifts out of a current gear ratio3. The control system of claim 1 or claim 2, wherein the at least some ofthe gear shift thresholds of the first shift map which are modified to generate the compensated shift map comprise gear shift thresholds which are associated with gear shifts out of a current gear ratio.

4. The control system of any preceding claim, wherein at least some ofthe gear shift thresholds ofthe first shift map are modified to increase values ofthe speed parameter at which respective gear shifts are triggered for a range of accelerator inputs.

5. The control system of any preceding claim, wherein the control system is configured to:receive an input signal indicative of a current speed ofthe electric machine ;receive an input signal indicative of a current power limit imposed on the electric machine ;receive an input signal indicative of a current torque capability ofthe electric machine ;determine a second estimate ofthe torque limit imposed on the electric machine by comparing the current torque capability ofthe electric machine with an expected torque capability ofthe electric machine for the current speed ofthe electric machine and for the current power limit imposed on the electric machine ,wherein the increased minimum values ofthe speed parameter are based on the most limiting ofthe first and second estimates ofthe torque limit.

6. The control system of claim 5, wherein the control system is configured to:when the current speed of the electric machine is within a tuneable offset of a speed at which an electric machine becomes power-limited, store the second estimate ofthe torque limit; andwhen the current speed of the electric machine exceeds the speed at which the electric machine becomes power-limited, use the stored second estimate ofthe torque limit.

7. The control system of any preceding claim, wherein the control system is configured to:obtain a second shift map which is used when the electric machine is operating according to a predefined derated torque capability, wherein the second shift map defines gearshift thresholds for the speed parameter as functions of accelerator input, and wherein at least some of the gear shift thresholds of the second shift map have higher minimum values of the speed parameter than corresponding gear shift thresholds of the first shift map,wherein modifying at least some of the gearshift thresholds of the first shift map to increase minimum values of the speed parameter at which respective gear shifts are triggered comprises:interpolating between corresponding gear shift thresholds of the first and second shift maps using an interpolation factor based on the first estimate of the torque limit.

8. The control system of claim 7, wherein at least some of the gear shift thresholds of the second shift map have higher values of the speed parameter for a range of accelerator inputs than corresponding gearshift thresholds of the first shift map.

9. The control system of claim 7 or claim 8, wherein the interpolation factor is also based on a power limit imposed on the electric machine .

10. The control system of any preceding claim, wherein modifying at least the gear shift thresholds of the first shift map which are associated with gear shifts out of the current gear to Increase minimum values of the speed parameter at which respective gear shifts out of the current gear are triggered comprises:determining minimum speed limits to impose on respective gearshifts based on the first estimate of the torque limit; andclipping the gear shift thresholds of the first shift map to an accelerator input of value zero at the determined minimum speed limits to provide corresponding gear shift thresholds of the compensated shift map.

11. The control system of any preceding claim, wherein generating the compensated shift map further comprises:modifying at least one gear shift threshold of the first shift map which is associated with a downshift out of the current gear to decrease a maximum value of the speed parameter at which the downshift out of the current gear is triggered, comprising:determining a maximum speed limit to impose on the downshift based on a power limit imposed on the electric machine; andclipping the gearshift threshold of the first shift map to an accelerator input of maximum value at the determined maximum speed limit to provide a corresponding gear shift threshold of the compensated shift map.

12. An automatic transmission system comprising the control system of any preceding claim and an automatic transmission .13, A vehicle comprising the automatic transmission system of claim 12,14. A method of controlling an automatic transmission of a vehicle , the method comprising:receiving an input signal indicative of at least one current temperature within an electric machine ;determining a first estimate of a torque limit imposed on the electric machine under a thermal derate strategy for the electric machine based on the at least one current temperature within the electric machine ;obtaining a first shift map which is used when the electric machine is operating according to its rated torque capability, wherein the first shift map defines gear shift thresholds for a speed parameter as functions of accelerator input;generating a compensated shift map by modifying at least some of the gear shift thresholds of the first shift map to increase minimum values of the speed parameter at which respective gearshifts are triggered, wherein the increased minimum values are based on the first estimate of the torque limit; andcontrolling the automatic transmission according to the compensated shift map.

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

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