Controlling an oil circuit of an electric drive system of a vehicle

Abstract

A system (400, Fig. 4) controls an oil circuit (310, Fig. 3) of an electric drive system 208 of a vehicle 1. A first signal is received (906, Fig. 9) which indicates a requested speed (904, Fig. 9) of

Description

TECHNICAL FIELD The present disclosure relates to controlling an oil circuit of an electric drive system of a vehicle. Aspects of the invention relate to a control system, to a system, to a vehicle, to a method, and to computer readable instructions. BACKGROUND Oil sumps for oil circuits of vehicle electric drive systems may be shallower in height than typical oil sumps for vehicle internal combustion engines. This is because electric drive systems tend to be in areas of the vehicle with limited vertical packaging space, such as under tailgate floors and / or near battery packs. An oil pickup tube is located within the shallow oil sump. The oil pickup tube comprises an inlet which is elevated above a base of the shallow oil sump. While the inlet of the oil pickup tube is fully immersed in oil, then rotation of an impeller of an oil pump will generate a pressure differential that sucks oil into the oil pickup tube. The oil is then circulated through the oil circuit to provide lubrication and cooling of the electric drive system. If the inlet of the oil pickup tube is only partially immersed in oil or not immersed at all, then air will also be sucked into the oil pickup tube. Due to the reduced thermal conductivity or air compared to oil, and the lack of lubricating properties of air, this will result in less cooling and lubrication of the electric drive system. The electric drive system may lack a sensor for directly measuring the presence of air in the oil circuit. It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a control system, a 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 controlling an oil circuit of an electric drive system of a vehicle, the control system comprising one or more processors collectively configured to: a) receive a first signal indicative of a requested speed of an oil pump of the electric drive system; b) determine a minimum pressure threshold in dependence on at least the requested speed of the oil pump; c) receive a second signal indicative of a sensed oil pressure of the electric drive system; d) determine whether the sensed oil pressure is below the minimum pressure threshold; and e) output a control signal to reduce an operating speed of the oil pump in dependence on the second signal indicating that the sensed oil pressure is below the minimum pressure threshold. Reducing the oil pump speed in response to lubrication issues may seem counter-intuitive, but in this instance it has been found that reducing the pump speed will not reduce the oil pressure, or will only negligibly reduce the oil pressure. Reducing the oil pump speed gives rise to the advantage of improved oil circuit management, comprising several specific advantages which outweigh the penalties (if any) associated with reducing oil pump speed. A first advantage is helping to prevent underprediction of future temperatures by thermal models that base their predictions on an assumed relationship between measured oil pump speed and actual oil flow rate. This is because the now-reduced pump speed may be fed back to the thermal model, so the thermal model has an improved indication of the actual lower flow rate in the oil circuit. The thermal model then predicts (more accurately) the electric drive system temperature, which will be higher if there is air in the oil circuit. The thermal model can therefore better anticipate excessive electric drive system temperatures in the specific situation where air is being sucked into the oil circuit. The control system can then apply pre-emptive cooling functions. The above method does not require air in the oil circuit to somehow be sensed directly, and may not require the thermal model itself to be modified or recalibrated. A second advantage of reducing the pump speed is based on fluid dynamics: the reduced pump speed may reduce turbulence and splashing of the oil in the oil sump, to reduce the amount of air being aspirated. If not for the present method, the pump speed may continue to rise as the electric drive system temperature unexpectedly rises, making the problem even worse due to increasing turbulence and splashing of the oil in the oil sump, and greater air aspiration. A third advantage related to the second advantage is that less turbulence and splashing of the oil results in lower noise, vibration, or harshness from the electric drive system. A fourth advantage of reducing the pump speed is that the oil pump is protected from wear that may otherwise occur from cavitation and scavenging. Optionally, the minimum pressure threshold is an expected minimum oil pressure dependent on at least the requested speed of the oil pump. Optionally, the control system is configured to determine an expected current oil pressure in dependence on the requested speed of the oil pump and on a sensed current oil temperature, and wherein the determination of the minimum pressure threshold is dependent on the expected current oil pressure and is less than the expected current oil pressure. An advantage is accurate detection of oil pickup issues, by establishing that the oil pump is producing less oil pressure than expected for a given oil pump speed and oil temperature. Optionally, determining the minimum pressure threshold comprises applying a correction factor to the expected current oil pressure. Optionally, the correction factor configures the minimum pressure threshold to track the expected current lubricant pressure while being less than the expected current lubricant pressure. An advantage is responding to oil pickup issues more appropriately, due to the correction factor which provides a variable acceptable delta between minimum and expected oil pressure across a wide range of oil pressures. Optionally, the correction factor is a variable dependent on sensed parameter. Optionally, the sensed parameter comprises temperature associated with the electric drive system. Optionally, the temperature comprises oil temperature. Optionally, the parameter is other than temperature. An advantage is responding to oil pickup issues more appropriately, by providing the option to tune / calibrate how far below the expected oil pressure the minimum can be. Optionally, the control system is configured to determine a first target speed of the oil pump, and output a first control signal requesting the first target speed of the oil pump in dependence on the sensed oil pressure being greater than the minimum pressure threshold, and wherein the control signal to reduce the operating speed of the oil pump is a second control signal requesting a modified target speed less than the first target speed. Optionally, the first target speed of the oil pump requested by the first control signal is dependent on at least one of: a current rotor speed of the electric drive system; or a currently demanded torque of the electric drive system. Optionally, the first target speed of the oil pump is further dependent on a current oil flow rate. Optionally, the control system is configured to determine the modified target speed in dependence on the current oil flow rate, wherein determining the modified target speed is independent of the current motor speed and the currently demanded torque. An advantage is responding to oil pickup issues more appropriately, because the oil pump may stop depending on the current motor speed or load of the electric drive system, and may only depend on the current sensed flow rate (temperature, pressure). Therefore, when oil pickup issues are detected (pressure below threshold), the oil pump speed will no longer increase with other parameters such as the current motor speed of the electric drive system, or the current requested torque of the electric drive system. Optionally, the control system is configured to: determine whether a cumulative time for which the modified target speed has been requested exceeds an allowable time; and in dependence on the cumulative time exceeding the allowable time, output the first control signal requesting the first target speed and repeat at least operations a) to d). In dependence on the cumulative time exceeding the allowable time, the control system may output the first control signal requesting the first target speed even if the sensed oil pressure is above the minimum pressure threshold. An advantage is that the control system will periodically reinitialise to check real system behaviour. This involves allowing the oil pump speed to return to its normal ‘first target speed’, and then checking again whether sufficient oil pressure can be generated to exceed the minimum pressure threshold. Optionally, the control system is configured to determine a current or predicted thermal state of the electric drive system in dependence on the reduced operating speed requested by the control signal, and output one or more thermal protection control signals in dependence on the determined current or predicted thermal state to control one or more of: a torque derate of the electric drive system; or a position of a heat exchanger bypass valve of the electric drive system. Optionally, determining the current thermal state may comprise receiving a signal from a temperature sensor such as an oil temperature sensor. Optionally, determining the current or predicted thermal state may depend on one or more of: an oil pump measured speed (an indication of the reduced operating speed); sensed oil temperature by a temperature sensor; or sensed oil pressure by a pressure sensor. An advantage is that the control system comprises a thermal model which can better predict future temperatures, as described in the ‘first advantage’ earlier. Optionally, determining whether the sensed oil pressure is below the minimum pressure threshold comprises determining whether the sensed oil pressure is below the minimum pressure threshold for at least a predetermined time period. An advantage is that the control system will respond to oil pickup issues appropriately, while ignoring transient ‘slosh’ events. Transient events shorter than the predetermined time period do not require modifications to the oil pump speed. According to another aspect of the present invention there is provided a system comprising the control system of any one of the preceding claims, and an electric drive system comprising: a rotor-stator pair for converting between electrical energy and mechanical torque; the oil pump; and an oil pressure sensor, wherein the sensed oil pressure is from the oil pressure sensor. According to a further aspect of the present invention there is provided a vehicle comprising the control system or the system. According to a further aspect of the present invention there is provided a method of controlling an oil circuit of an electric drive system of a vehicle, the method comprising: a) receiving a first signal indicative of a requested speed of an oil pump of the electric drive system; b) determining a minimum pressure threshold in dependence on at least the requested speed of the oil pump; c) receiving a second signal indicative of a sensed oil pressure of the electric drive system; d) determining whether the sensed oil pressure is below the minimum pressure threshold; and e) outputting a control signal to reduce an operating speed of the oil pump in dependence on the second signal indicating that the sensed oil pressure is below the minimum pressure threshold. According to a further aspect of the present invention there is provided a control system for controlling oil pressure of a torque source of a vehicle, the control system comprising one or more processors collectively configured to execute the below steps, and / or there is provided a method comprising the below steps: receive a signal indicative of a sensed oil pressure of the torque source; determine whether the sensed oil pressure is below a minimum pressure threshold; and output a control signal to perform at least one of the following actions in dependence on the second signal indicating that the sensed oil pressure is below the minimum pressure threshold: reduce the operating speed of the oil pump; or apply a torque derate to the torque source. According to a further aspect of the present invention there is provided a control system for controlling oil pressure of a torque source of a vehicle, the control system comprising one or more processors collectively configured to execute the below steps, and / or there is provided a method comprising the below steps: receive a signal indicative of a sensed oil pressure of the torque source; determine whether the sensed oil pressure is below a pressure threshold; and output a control signal to perform at least one of the following actions in dependence on the second signal indicating that the sensed oil pressure is below the pressure threshold: reduce the current operating speed of the oil pump; or apply a torque derate to the torque source. According to a further aspect of the present invention there is provided a control system for controlling oil pressure of a torque source of a vehicle, the control system comprising one or more processors collectively configured to execute the below steps, and / or there is provided a method comprising the below steps: receive an inertial signal indicative of an angle (pitch angle and / or roll angle) of the vehicle; determine whether the angle is greater than a threshold; and output a control signal to perform at least one of the following actions in dependence on the second signal indicating that the angle is greater than the threshold: reduce the current operating speed of the oil pump; or apply a torque derate to the torque source. The control system as described in any one or more of the preceding statements comprises one or more controllers collectively comprising at least one electronic processor having an electrical input for receiving an input signal; and at least one memory device electrically coupled to the at least one electronic processor and having instructions stored therein; and wherein the at least one electronic processor is configured to access the at least one memory device and execute the instructions thereon so as to execute the operations as defined above. According to a further aspect of the present invention there is provided computer readable instructions which, when executed by a computer, are arranged to perform any one or more of the above-described methods. 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 vehicle; FIG. 2 illustrates a schematic view of the vehicle where the vehicle is at least partially electrically driven; FIG. 3 schematically illustrates components of an oil circuit and of a coolant circuit; FIG. 4 schematically illustrates a control system; FIG. 5 schematically illustrates a non-transitory computer-readable medium; FIG. 6 illustrates a cross-section of an example oil sump of an electric drive system; FIG. 7 illustrates a graph of oil pressure relative to oil pump speed; FIG. 8 illustrates a graph of stator temperature relative to time; FIG. 9 illustrates a flowchart depicting an example method; and FIG. 10 illustrates a graph of oil pressure relative to oil pump speed. 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 illustrates a schematic view of the vehicle 1 where the vehicle is a battery electric vehicle (BEV) or hybrid electric vehicle (HEV). FIG. 2 illustrates the vehicle 1 comprising a traction battery 202 or an equivalent electrical energy storage means. The traction battery 202 is electrically connected to a DC-DC power converter 204. The DC-DC power converter 204 is electrically connected to an inverter 206 (DC-AC power converter). DC means direct current and AC means alternating current. The inverter 206 is part of an electric drive system, referred to herein as an electric drive unit (EDU) 208 for illustrative purposes. The EDU 208 comprises an electric machine 210 operable as a traction electric machine. The traction electric machine 210 is operable as an electric motor and as an electric generator. The EDU 208 further comprises a transmission 212 associated with the traction electric machine 210. In use, the traction electric machine 210 provides torque to one or more wheels of the vehicle 1 via the transmission 212. The traction electric machine 210 is associated with the inverter 206, for providing an electrical supply from the traction battery 202 to the traction electric machine 210. The transmission 212 provides at least one gear ratio between an output of the traction electric machine 210 and the one or more wheels of the vehicle 1. In another example, the traction electric machine 210 comprises a direct drive motor so lacks a transmission 212. In some embodiments, the traction electric machine 210 and the transmission 212 may be integrated into a single unit or housing to provide the EDU 208 for the vehicle 1. In some embodiments, the traction electric machine 210 and the transmission 212 are associated with an axle of the vehicle 1 arranged to provide torque to first and second wheels associated with the axle, which may be each disposed at respective ends of the axle. However it will be appreciated that the EDU 208 may be associated with only one wheel of the vehicle 1. FIG. 3 illustrates an oil circuit 310 and a coolant circuit 350, thermally coupled together by a coolant-to-oil (coolant-to-lubricant) heat exchanger (‘heat exchanger’ herein) 324. In an example, the oil circuit 310 is a lubricant circuit forming part of the EDU 208. Oil within the oil circuit 310 both lubricates and cools the EDU 208. The coolant circuit 350 is part of a thermal management system of the vehicle 1 which may be a vehicle-level coolant system, referred to concisely as a coolant system 340. In other examples, the fluids are other than oil and coolant. The coolant system 340 is for exchanging thermal energy between the coolant and various modules thermally coupled or couplable to the coolant system 340. FIG. 3 depicts three such modules. These modules comprise: a traction battery cooler 360; a climate control module 362 (e.g., coolant-to-coolant heat exchanger) for a heating ventilation and cooling system (HVAC); and a heat transfer module 345 for the EDU 208. The order of the modules, and the number of modules, are outside the scope of this disclosure. 5 Components of the heat transfer module 345 for the EDU 208 are illustrated in FIG. 3. The heat transfer module 345 comprises an inlet 352 and an outlet 358 for receiving and outputting coolant fluid, respectively, flowing around a coolant circuit 350 of the coolant system 340. The heat transfer module 345 comprises a coolant temperature sensor 354 between the inlet 352 and outlet 358. The coolant temperature sensor 354 can therefore sense real-time coolant temperature locally within the heat transfer module 345. Coolant temperature sensors could alternatively, or additionally, be provided elsewhere in the coolant system 340. The heat transfer module 345 further comprises an inverter portion 356 for thermally coupling to the inverter 206. The inverter 206 may therefore be coolant-cooled. The coolant circuit 350 within the heat transfer module 345 flows through the heat exchanger 324. The oil circuit 310 also flows through the heat exchanger 324. The heat exchanger 324 provides a thermal bridge between the coolant circuit 350 of the coolant system 340 and the oil circuit 310 of the EDU 208. As will be described, the heat exchanger 324 can be selectively bypassed. In this implementation, the EDU 208 is fully oil-cooled. The oil temperature can be decreased by controlling coolant heat sources or sinks to make the coolant temperature cooler than the oil temperature. The coolant indirectly controls the temperature of the EDU 208 via the heat exchanger 324. The illustrated heat transfer module 345 does not further comprise a coolant-filled jacket circulating coolant around a portion of the EDU 208 such as the stator. The stator jacket may instead be oil-filled. This means that the temperature of the EDU 208 can be managed independently of the coolant temperature, by controlling whether the heat exchanger 324 is bypassed. This hardware configuration means that the thermal condition of the EDU 208 is strongly correlated with the oil temperature therein, with coolant temperature not being an interfering variable if the heat exchanger 324 is bypassed. It will be noted that the order of the inverter portion 356, the heat exchanger 324, and the coolant temperature sensor 354 between the inlet 352 and outlet 358 may be different from that illustrated in FIG. 3 and is not restricted in this way. It will also be noted that the EDU 208 does not have to be fully oil cooled. In other examples, coolant may directly flowthrough a coolant jacket of the EDU 208. The illustrated oil circuit 310 of the EDU 208 is now described. It circulates oil through both the traction electric machine 210 and the transmission 212. An oil sump 312 is illustrated, from which oil flows via a pickup through an oil pump 314. The oil sump 312 is also shown in FIG. 6, and described later. The oil pump 314 is controllable by a pump control signal to vary an oil pressure and / or flow rate within the oil circuit 310. The oil pump 314 may be a positive displacement pump, so that as long as there is no air being picked up, there is a direct or close relationship between pump speed and oil flow rate in the oil circuit 310. As described later, this direct or close relationship may be used by a control system to estimate oil flow rates based on requested pump speed, in order to predict a temperature of the EDU 208. The oil then flows through an oil filter 316 to an active bypass valve 318. The active bypass valve 318 is an electronic valve that is controllable by a valve control signal to control whether the oil flows through a passage 322 comprising the heat exchanger 324 or through a bypass passage 320 that bypasses the heat exchanger 324. When the oil is hotter than the coolant and the active bypass valve 318 is closed, that is, the bypass passage 320 is closed and the heat exchanger passage 322 is open, the hotter oil will flow through the heat exchanger 324 and lose its heat to the coolant. The coolant in turn will heat up. If the active bypass valve 318 is open, that is, the bypass passage 320 is open and the heat exchanger passage 322 is closed, the oil will flowthrough the bypass passage 320 and will not lose its heat to the coolant. It would be appreciated that the terms ‘closed’ and ‘open’ are non-limiting and merely refer to first and second states of the active bypass valve 318. By extension, if the oil is colder than the coolant and the active bypass valve 318 is closed, the coolant will warm up the oil via the heat exchanger 324. But if the active bypass valve 318 is open, the coolant will not warm up the oil. The temperature of the oil can be detected in real-time via the illustrated oil temperature sensor 326 (lubricant temperature sensor) in the oil circuit 310. The pressure of the oil can be detected in real-time via the illustrated oil pressure sensor 327. The same block comprises the oil temperature sensor 326 and oil pressure sensor 327, and may be regarded as an OTP (oil temperature-and-pressure) sensor device. By measuring and comparing the sensed real-time temperatures of the coolant and the oil via the sensors 354, 326, the active bypass valve 318 can be controlled with a full sensory awareness of the effect that a particular position of the active bypass valve 318 will have on oil temperatures and coolant temperatures. In the present example, the active bypass valve 318 is a binary valve such that all the oil flows through one or the other of those passages 320, 322. In other examples, the active bypass valve 318 could be controllable to provide a variable ratio of oil flow through each of the passages 320, 322. Although in the present example the active bypass valve 318 and bypass passage 320 belong to the oil circuit 310, in other examples they may belong to the coolant circuit 350 to provide the same function by directing coolant either through or around the heat exchanger 324. FIG. 3 further illustrates an oil stator jacket 328 in the oil circuit 310, to circulate oil around the stator of the electric traction machine. A transmission oil sprayer 330 is also illustrated, to spray oil at a mechanism of the transmission 212. The oil then passes back to the oil sump 312. It will be noted that the order of the oil pump 314, oil filter 316, active bypass valve 318, heat exchanger 324, oil temperature sensor 326, oil stator jacket 328, and transmission oil sprayer 330, may be different from that illustrated in FIG. 3 and is not restricted in this way. With reference to FIG. 4, there is illustrated a control system 400 for a vehicle 1. The control system 400 comprises one or more controllers 401. The control system 400 is configured to receive oil pressure and oil temperature data from a plurality of sources including an oil pressure sensor 327 and an oil temperature sensor 326. The control system 400 may be further configured to receive pump speed data such as requested pump speed data requested by the control system 400. The control system 400 may then output a control signal to control the speed of the oil pump 314, in dependence on the data. Additionally, other control signals of the control system 400 may depend on the data, such as a control signal for controlling a state of the active bypass valve 318. The control system 400 as illustrated in FIG. 4 comprises one controller 401, although it will be appreciated that this is merely illustrative. The controller 401 comprises processing means 404 and memory means 406. The processing means 404 may be one or more electronic processing device 404 which operably execute computer-readable instructions. The memory means 406 may be one or more memory device 406. The memory means 406 is electrically coupled to the processing means 404. The memory means 406 is configured to store instructions, and the processing means 404 is configured to access the memory means 406 and execute the instructions stored thereon. The controller 401 comprises an input means 410 and an output means 412. The input means 410 may comprise an electrical input 410 of the controller 401. The output means 412 may comprise an electrical output 412 of the controller 401. The controller 401 may have an interface 402 comprising an electrical input / output I / O 410, 412, or an electrical input 410, or an electrical output 412, for receiving information and interacting with 7 external components. The input 410 is arranged to receive temperature signals from the oil and coolant temperature sensors 326, 354 and the oil pressure sensor 327. The temperature signals are electrical signals which are indicative of absolute temperatures of the oil and coolant, respectively. The output 412 is arranged to output a valve control signal to the active bypass valve 318, and / or a pump control signal to the oil pump 314, indicative of a request for controlling a state of the active bypass valve 318 and / or a speed of the oil pump 314. FIGS. 3 and 4 together illustrate a system 300 comprising a control system 400 and the EDU 208, the active bypass valve 318 for the oil circuit 310 of the EDU 208 being controllable by a valve control signal from the control system 400 to control a fluid flow rate through the heat exchanger 324 relative to a fluid flow rate through the bypass passage 320. FIG. 5 illustrates a non-transitory computer-readable storage medium 500 comprising the instructions (computer software). FIG. 6 schematically illustrates a cross-section view of the oil sump 312 of the EDU 208. As shown, the oil sump 312 for the EDU 208 is significantly shallower in height than a typical oil sump of an internal combustion engine. This is because EDUs tend to be in areas of the vehicle 1 with limited vertical packaging space, such as under tailgate floors and / or near battery packs 202. FIG. 6 illustrates an oil pickup tube 313 located within the oil sump 312. The oil pickup tube 313 optionally has an inverted conical shape in crosssection. The oil pickup tube 313 comprises an inlet which is elevated above a base of the oil sump 312. If the inlet of the oil pickup tube 313 is fully immersed in oil, then rotation of an impeller of the oil pump 314 will generate a pressure differential that sucks oil into the oil pickup tube 313. The height of the inlet of the oil pickup tube 313 may ensure full immersion for a range of vehicle pitch and / or roll angles. The exact angles are not specified herein but are greater than 10 degrees and may be at least 15 degrees. However, due to the shallow height of the oil sump 312, there is a greater chance that the oil level will fall below the level of the inlet of the oil pickup tube 313 in some situations such as: - when the vehicle 1 is at an unusually high angle (e.g., pitch angle); - when a lateral g-force is particularly high for a prolonged period such as when driving fast in a circle; - oil sloshing at a high level for a prolonged duration, such as when driving over moguls; - when there is a slow oil leak (if undetected by other means); - malfunctioning of the oil pump 314 (if undetected by other means). If the inlet of the oil pickup tube 313 is only partially immersed in oil or not immersed at all, then air will also be sucked into the oil pickup tube 313. Due to the reduced thermal conductivity or air compared to oil, and the lack of lubricating properties of air, this will result in less cooling and lubrication of the EDU 208. For example, FIG. 7 shows oil gauge pressure P (y-axis) relative to pump speed S (x-axis). The dashed line represents an example pressure-speed relationship when no air pickup issues occur. FIG. 7 shows that when there is no air mixed with the oil, the oil pressure increases from 0 to P2 as pump speed increases from 0 to S2. If 100% air is aspirated, then the pressure in the oil circuit 310 will be very low, close to zero, even at high pump speeds. The solid line in FIG. 7 represents the pressure-speed relationship if a mixture of air and oil are being aspirated. The oil pressure for a given pump speed reaches P1 which is substantially less than P2, and less than the dashed line across the range S1 to S2. Furthermore, the oil pressure may hardly rise at all between pump speed S1 (e.g., 40% of S2) and speed S2, or may remain static or even fall. The lines diverge especially at higher pump speeds between S1 and S2. When there is air mixed with the oil, a thermal model of the EDU 208 which is unable to detect this situation will overpredict the actual oil flow rate in the oil circuit 310. Therefore, it will underpredict the future temperature of the EDU 208. The control system 400 may predict the dashed line when 8 the actual pressure is as shown by the solid line. This overprediction will impede functions of the control system 400 for pre-emptively controlling EDU temperature. FIG. 8 is a graph schematically illustrating stator temperature (y-axis) over time (x-axis), while air is being aspirated and circulated with the oil. The dashed line represents the actual stator metal temperature at a stator winding of the EDU 208. The solid line represents predicted stator temperature predicted by a thermal model of the control system 400. In other examples, the thermal model predicts oil temperature or both stator temperature and oil temperature. The vertical bars represent the prediction time window W of the thermal model of the control system 400. As shown, the predicted stator temperature is lower than the actual stator temperature because the presence of air may be unknown to the control system 400. The lines diverge meaning that the delta between actual and predicted temperature increases over time, with a continuous underprediction. Eventually, the predicted stator temperature indicated by the thermal model of the control system 400 may reach a protection threshold, prompting the control system 400 to output a thermal protection control signal(s) to reduce EDU temperature, for example by derating the maximum requestable torque of the EDU 208. In addition, the sensed oil temperature indicated to the control system 400 by the oil temperature sensor 326, and the predicted rotor temperature may reach a protection threshold, prompting the control system 400 to output a thermal protection control signal(s) to reduce EDU temperature, for example by derating the maximum requestable torque of the EDU 208. It would be advantageous to avoid these scenarios and to ensure that the thermal model’s predicted temperatures are accurate, so that thermal management of the EDU 208 can occur pre-emptively without altering noticeable attributes of the vehicle 1 such as maximum requestable torque. Embodiments of the present invention modify the functionality of the control system 400 without having to deepen the oil sump 312, and without needing additional dedicated sensors. The use of existing sensors was considered as a novel approach for detecting a likelihood of air aspiration. For example, the vehicle 1 may already comprise inertial sensors capable of detecting vehicle pitch and / or roll angles, such as an accelerometer(s) and / or gyroscope(s). The control system 400 may receive one or more signals indicative of a pitch angle and / or roll angle of the vehicle 1, from one or more of the inertial sensors. The control system’s output of a control signal (e.g., pump speed request) for managing EDU temperature could depend on said signal(s), for example when the angle is above a threshold or outside a range. However, this approach does not account for oil sloshing or slow oil leaks. Nonetheless, this novel aspect is useful and may be claimed separately. Embodiments of the present invention, described below, account for all the above-listed scenarios in which air aspiration may occur. FIG. 9 illustrates a method 900 according to an embodiment of the invention. The method 900 is a method of controlling an oil circuit 310 of an electric drive system 208 of a vehicle 1, such as the vehicle 1 illustrated in FIG. 1. In particular, the method 900 is a method of controlling a pump speed of an oil pump 314 of the oil circuit 310. The method 900 may be performed by the control system 400 illustrated in FIG. 4. In particular, the memory 406 may comprise computer-readable instructions 408 which, when executed by the processor 404, perform the method 900. The illustrated flowchart defines an ‘under-pressure’ function, to modify an oil pump speed requested by an oil pump control strategy (OPC) of the control system 400. The term “OPC” is used herein to refer to the oil pump control strategy of the control system 400. The OPC comprises a thermal model of the EDU 208. Assuming that all the optional features of the flowchart are included, then the under-pressure function (method 900) can be summarised as follows below. The method 900 determines an acceptable minimum pressure threshold (blocks 910-916) for oil pressure based on the pump speed requested by the OPC (blocks 904, 906) and oil temperature (block 908). If the sensed oil pressure (block 918) falls below the acceptable minimum pressure threshold (decision block 920), this indicates an under-pressure state in which air is being sucked into the oil pump 314. Therefore, the OPC-requested 9 pump speed (block 904) is reduced (blocks 922-930,940) to a level commensurate with the oil flow rate that is actually being achieved, and to prevent the oil pump 314 from sucking in more air. This reduction of the OPC-requested pump speed can have several advantageous effects. A first advantage arises if the thermal model of the control system 400 works in a certain way. If the thermal model comprises a thermal model of the EDU 208 which is based on the oil pump speed as an indicator of oil flow rate, then the accuracy of the thermal model is improved. The reduced pump speed may be fed back to the thermal model, so the thermal model has a corrected indication of the actual flow rate in the oil circuit 310. The thermal model predicts (more accurately) the EDU temperature, then predicts whether the EDU temperature will reach a protection threshold within a prediction time window. The advantage of the more accurate temperature prediction is that the thermal model can anticipate excessive EDU temperatures in a situation where air is being sucked into the oil circuit 310. Based on the thermal model, the control system 400 may also provide pre-emptive cooling functions such as connecting or disconnecting the heat exchanger 324. A second advantage of reducing the pump speed is based on fluid dynamics rather than software and is that the reduced pump speed may reduce turbulence and splashing of the oil in the oil sump 312, to reduce the amount of air being aspirated. If not for the present method 900, the pump speed may continue to rise as the EDU temperature unexpectedly rises, making the problem even worse due to increasing turbulence and splashing of the oil in the oil sump 312, and greater air aspiration. A third advantage related to the second advantage is that less turbulence and splashing of the oil results in lower noise, vibration, or harshness associated with the oil pump 314. A fourth advantage of reducing the pump speed is that the oil pump 314 is protected from wear that may otherwise occur from cavitation and scavenging. The individual blocks of the method 900 are now described in more detail. Block 902 is a start block. Block 904 comprises determining a first target speed of the oil pump 314, optionally using information from an optional thermal model shown as block 946. Block 906 comprises sending a first signal indicative of a requested speed of the oil pump 314 of the EDU 208, wherein the requested speed is the first target speed, i.e., the first signal requests the first target speed. The first target speed is a pre-arbitrated signal. The first target speed is sent to the under-pressure function of blocks 910 to 940, which comprises one or more arbitration blocks. If an under-pressure state is detected, the first target speed will be modified or replaced with another signal via arbitration, so the final arbitrated oil pump speed request will differ from the first target speed. If an under-pressure state is not detected, then the arbitrated requested operating speed may substantially match the first target speed. Optionally, determining the first target speed of the oil pump 314 is executed by the thermal model of block 946 and the OPC of block 904 as defined above. Therefore, the first target speed can be referred to as an “OPC oil pump speed”. The determination of the first target speed is dependent on at least one of: a current rotor speed of the EDU 208; a currently demanded torque of the EDU 208; or a current temperature of the oil of the oil circuit 310 of the EDU 208. The current rotor speed of the EDU 208 for determining the first target speed may be referred to as current EDU speed. The currently demanded torque of the EDU 208 may be an arbitrated torque request, in other words the torque demanded from the EDU 208. 10 The rotor speed and torque of the EDU 208 are indicative of the current load applied to the EDU 208. Higher load generates more heat. Therefore, the determined first target speed may increase as one or both current rotor speed and current demanded torque increase. Blocks 946 and 904 of the method 900 may further comprise determining the current oil flow rate, based on feedback. The determination of the current oil flow rate is a function of up to three sensed feedback parameters: 1) An oil pump measured speed (block 944). This is the speed currently requested by the control system 400. 2) Sensed oil temperature sensed by the oil temperature sensor 326; 3) Sensed oil pressure sensed by the oil pressure sensor 327; Since the oil pump 314 may be a direct displacement pump, there is a direct or close relationship between oil pump measured speed and flow rate and may be corrected by the sensed oil pressure and temperature. In summary, the determined first target speed may depend on up to five parameters: the current rotor speed of the EDU 208; the currently demanded torque of the EDU 208; the oil pump measured speed; the sensed oil temperature; and the sensed oil pressure. The determined first target speed depends on a combination of feedback parameters (the oil pump measured speed, sensed oil temperature, and sensed oil pressure) and feedforward parameters (current rotor speed of the EDU 208, currently demanded torque of the EDU 208, sensed oil temperature). The use of feedforward parameters ensures that when the oil pump 314 is controlled to deliver the first target speed, overheating will be pre-emptively prevented. Sensed oil temperature is also a feedforward parameter because the 3-D oil flow prediction map may be a function of rotor speed, torque and oil temperature. The predicted oil flow is then converted into the first target speed of the oil pump 314. The precise number of parameters, and their nature, may vary with implementation. If air is present in the oil circuit 310, then the current oil flow rate is inaccurate and the oil pump 314 may be incapable of delivering the flow rate actually needed. Therefore, the first target speed is a first input received by the under-pressure function of blocks 910-940. Block 908 represents a second input received by the under-pressure function of blocks 910-940. Specifically, block 908 comprises the sensed oil temperature sensed by the oil temperature sensor 326. The under-pressure function starts with block 910, which comprises determining an expected current oil pressure for the requested speed and determining a minimum pressure threshold dependent on the expected current oil pressure and providing the minimum pressure threshold at block 916 for use by the next block 920. The expected current oil pressure is the target pressure which would be expected for the requested speed and oil temperature. As the pump speed rises, a higher oil pressure would be expected for similar temperature as shown by the dashed line in FIG. 7. As the oil temperature rises, the viscosity of the oil decreases so a lower oil pressure would be expected for similar oil flow rate. The expected current oil pressure is determined by sub-block 912, in dependence on the requested speed of the oil pump from block 906 (e.g., first target speed), and further in dependence on the sensed current oil temperature from block 908. Specifically, sub-block 912 comprises determining a lookup value (“Pdesired”) indicative of the expected current oil pressure from a first lookup table, using the requested speed (block 906) and sensed oil temperature (block 908) as inputs. Both inputs are variables, so the illustrated lookup table is two-dimensional. The expected current oil pressure may be based substantially or wholly on these two parameters. As shown, the relationship between these parameters may be predetermined in the control system 400 as a lookup table (map) or a function stored in the memory 406. The predetermined relationship may be based on an oil circuit 310 that does not contain air, or at least does not contain any more air than what is normal or acceptable. The relationship may be substantially fixed. Values in the first lookup table may be substantially fixed calibration values. The minimum pressure threshold is the acceptable minimum pressure threshold described earlier. The minimum pressure threshold is less than the expected current oil pressure, for example being within the range 0% to 100% of the expected current oil pressure. The reason for being less than the expected current oil pressure is to account for calculation uncertainty. This provides confidence that when actual oil pressure falls below the minimum pressure threshold, the oil circuit 310 is in an under-pressure state. Since the expected current oil pressure is a variable, the minimum pressure threshold is also a variable. The minimum pressure threshold tracks the expected current oil pressure. Therefore, as the pump speed rises, the minimum pressure threshold rises with expected pressure. As the oil temperature rises, the minimum pressure threshold falls with expected pressure. To determine the minimum pressure threshold, the expected current oil pressure is modified by another parameter, for example by multiplying it by a correction factor of less than one. In the illustrated example, determining the minimum pressure threshold comprises multiplying the expected current oil pressure by a correction factor. The correction factor may be either a linear correction factor such as a fixed predetermined multiplier (e.g., 0.95), or may be a nonlinear correction factor, the calculation of which is shown in sub-block 914. Sub-block 914 comprises determining a nonlinear correction factor (“F_corr”) which is a nonlinear correction factor based on one or more sensed variables. In the illustrated example, but not necessarily all examples, the correction factor is an oil temperature correction factor, based on the sensed oil temperature (block 908) as an input parameter. This is because the expected oil pressure is nonlinear across the oil temperature range. The result is a nonlinear delta between the minimum pressure threshold and the expected current oil pressure, rather than being a fixed percentage of the expected current oil pressure. Decision block 920 receives two signals. The first signal is the minimum pressure threshold from block 916 which was calculated at block 910. The second signal is indicative of a sensed oil pressure of the EDU 208, sensed by the oil pressure sensor 327. The second signal is received from block 918. These two signals allow a comparison between actual pressure and the minimum acceptable pressure. Specifically, decision block 920 comprises determining whether the current sensed oil pressure (block 918) is below the minimum pressure threshold (block 916). In dependence on being below the threshold, the method 900 may progress to blocks 922-930 which define an under-pressure treatment process. In dependence on the current sensed oil pressure being above the threshold, the method 900 may instead progress to blocks 932-938 which define a normal-pressure process. In some, but not necessarily all examples, block 920 determines whether the sensed oil pressure has been below the minimum pressure threshold for longer than a predetermined time period. In dependence on being below the threshold for longer than said period, the method 900 may progress to blocks 922-930. In dependence on being above the threshold or below the threshold for less than said period, then the method 900 may instead progress to blocks 932-938. The predetermined time period may comprise a debounce time. In other words, the control system 400 waits for a debounce time to make sure that there is a real under-pressure state. This ensures that transient events such as temporary oil slosh are ignored. In an example, the predetermined time period may be selected from the range milliseconds to several seconds (less than 10 seconds or less than 1 minute). The predetermined time period may be calibratable and may be re-calibratable. The under-pressure treatment process is now described, with reference to block 922. The graph of FIG. 10 illustrates the effect of the process. FIG. 10 is the same type of graph as FIG. 7. The oil pressure ‘P’ is in the y-axis, and the pump speed ‘S’ is in the x-axis. FIG. 10 shows the failure to build oil pressure when air starts to be aspirated with the oil. The aim is to reduce the requested pump speed from the right vertical bar representing the first target speed ‘T1 ’, to the left vertical bar representing a modified target speed ‘T2’. At block 922, the method 900 comprises determining the modified target speed of the oil pump 314, which is less than the first target speed (OPC oil pump speed) of block 906. Block 924 comprises outputting the modified target speed of the oil pump 314 for the next operations 926-930 of the flowchart. The modified target speed is determined based on the current oil flow rate (actual flow rate) as indicated by the oil temperature and pressure sensors 326, 327. The determination calculates a pump speed which is expected to correspond to the current oil flow rate which is actually being achieved. Therefore, if the pump speed is reduced to this modified speed T2, the oil pressure and oil flow rate should not fall even if the pump speed is significantly less than the first target speed T1. For example, dropping the pump speed from 5000rpm to 2000rpm may substantially not affect oil flow rate at all, since mostly air is being picked up. The exact numerical values of T1 and T2 and the delta therebetween are outside the scope of this disclosure. The modified pump speed T2 may however be a positive nonzero value. The delta therebetween may be predetermined through calibration. In an implementation, block 922 determines the modified target speed as a lookup value (“OPC_speed_actual”) indicative of the pump speed which is expected to deliver the actual flow rate which is currently sensed by the oil temperature and pressure sensors 326, 327. The lookup value is determined from a lookup table, using the sensed oil pressure and sensed oil temperature as inputs. Both inputs are variables, so the illustrated lookup table is two-dimensional. It should be noted that the use of lookup tables in general is optional, as other techniques exist. In other examples, sensed oil temperature is not taken into account. Furthermore, other variables (not specified) may be taken into account. Unlike the first target speed, the modified target speed is determined based on a different number of parameters than the first target speed. For example, the illustrated modified target speed is determined based on fewer parameters than the first target speed. In an example, the first target speed was determined based on five parameters: the current rotor speed of the EDU 208, the currently demanded torque of the EDU 208, the oil pump measured speed, the sensed oil temperature, and the sensed oil pressure. However, the modified target speed is based on two of the parameters: the sensed oil temperature and sensed oil pressure. Therefore, the modified target speed is independent of each of the following parameters: the current rotor speed (motor speed) of the EDU 208; the currently demanded torque of the EDU 208; and the oil pump measured speed. The modified target speed may be substantially invariate to these parameters. In other examples, the modified target speed is independent of one or more of the above parameters. Furthermore, although the first target speed was based on both feedback parameters (e.g., sensed oil temperature and pressure and motor speed) and feedforward parameters (e.g., EDU speed and load), the modified target speed is based only, or to a greater extent, on feedback parameters (e.g., sensed oil temperature and pressure). In other examples, the modified target speed may be determined in dependence on the same parameters as the first target speed, but the calculation may differ to achieve a different, lower target. For example, different calibration parameters or different parameter weights may be applied. Block 926 comprises setting a flag to a state indicating an under-pressure mode. The setting of flags is related to an optional decision at block 936, which is described later. The flag may be stored in the memory 406 of the control system 400. The flag may comprise a pair of states, including a first state indicating a normalpressure mode, and a second state indicating the under-pressure mode. The flag may be in its second state. The term ‘flag’ is to be interpreted broadly as any persistent stored value that represents a state or mode of a system and acts as a controlling parameter for one or more control operations. Going back a few steps, if the decision block 920 determines that the normal-pressure mode is to be used, then the method 900 proceeds to block 932 instead of block 922. The normal-pressure process is now described, with reference to blocks 932 and 934. Block 932 comprises setting the flag to the first state indicating the normal-pressure mode. The first target speed is used, without being modified or replaced by a lower speed, because block 932 is in a branch of the flowchart that does not include blocks 922-926. Block 928 is a speed arbitration block connected to the flag-setting blocks 926 and 932. At the speed arbitration block 928, the method 900 comprises setting an oil pump speed request to the first target speed T1 of the oil pump 314 in dependence on the flag being in the first state (block 932), and setting the oil pump speed request to the modified target speed T2 of the oil pump 314 in dependence on the flag being in the second state (block 926). Block 934 looks up the state of the flag from blocks 926 and 932, and passes the flag to a state arbitration decision block 936. Decision block 936 comprises determining whether a cumulative (e.g., continuous) time for which the flag has been in the second state (underpressure mode) is longer than an allowable time. This allows the control system 400 to periodically re-apply the first target speed so that the real system behaviour can be checked. The allowable time may comprise a debounce timer, the duration of which depends on calibration. If the cumulative time of the second state exceeds the allowable time, the method 900 progresses to a request-setting block 938 which sets an arbitrated oil pump speed request as the first target speed of block 906, even though earlier decision block 920 in the present iteration of the method 900 indicates that the under-pressure treatment process / modified target speed should be used. In other words, the arbitrated oil pump speed request is not allowed to be set as the modified target speed. Consequently, the oil pump 314 speeds up back to its first target speed, and the next iteration of the method 900 will check whether the air pickup problem has resolved. Furthermore, the control system 400 will reset the cumulative time to zero for the next iteration of the method 900. If the cumulative time of the second state does not exceed the allowable time, the method 900 instead progresses to request-setting block 930 which sets the arbitrated oil pump speed request as the oil pump speed request determined at the speed arbitration block 928. In other words, the arbitrated oil pump speed request is allowed to be set as the modified target speed or the first target speed depending on the flag. If the flag is in the second state, the modified target speed will be set and the cumulative time will continue running. If the flag is in the first state, the first target speed will be set. Block 940 is connected to blocks 930 and 938, and comprises the control system 400 outputting a control signal as requested by the request-setting block 930, 938. The control signal is either a first control signal to maintain the operating speed of the oil pump 314 at the first target speed, or a second control signal to reduce the operating speed of the oil pump 314 from the first target speed to the modified target speed, or a third control signal to increase the operating speed of the oil pump 314 from the modified target speed to the first target speed. The first, second, and third control signals may refer to different values of the same control signal. The control signal may be sent from the control system 400 to a lowest-level controller which is a speed controller of the oil pump 314, such as a controller supplied with or built into the oil pump 314. If there is additional signal processing or arbitration (not shown), then the request may be sent to a lower-level controller before a final control signal is sent to the lowest-level controller. Block 942 represents a controller of the oil pump 314 receiving the control signal sent / transmitted at block 940, and consequently controlling the oil pump speed to match the speed requested by the control signal. Block 944 comprises determining an oil pump measured speed, which can be used if there is another iteration of the method 900 (blocks 904-906). The determination may be determined by either internal lookup if it is known to the control system 400, or by communication with an external controller which knows the speed of the oil pump 314. Block 946 comprises the thermal model described earlier, which is connected to the block 904 which determines the first target speed of the oil pump 314 for the next iteration of the method 900. As described earlier, the thermal model may predict the thermal state (e.g., stator temperature) of the EDU 208 based on the estimated flow rate, which itself may be based on the oil pump measured speed from block 944, and optionally on sensed oil temperature and pressure. The prediction will now be accurate in situations where air pickup is occurring, because the reduced oil pump measured speed is an accurate indicator of the oil flow rate being actually achieved. Therefore, there is no longer a situation where the pump speed is very high while the oil flow rate is very low, so the thermal model can continue to accurately infer the oil flow rate from the pump speed. As described earlier, this benefit is achieved because the oil pump speed has been reduced to a speed which corresponds to the flow rate that is actually occurring. Based on the predictions, the control system 400 may be configured to output a thermal protection control signal in dependence on the determined current or predicted thermal state to control a torque derate of the EDU 208. For example, the torque derate may be executed in dependence on the predicted temperature at the end of the prediction time window exceeding a protection threshold. In some examples, based on the predictions the control system 400 may be configured to output a thermal protection control signal to control position of the active bypass valve 318 of the EDU 208. For example, the active bypass valve 318 may be switched to connect the heat exchanger 324 to the oil circuit 310 in dependence on the predicted temperature at the end of the prediction time window exceeding a protection threshold, and further in dependence on comparing the coolant and oil temperature sensors 354, 326 to determine that the coolant temperature is lower than the oil temperature. In other examples, the thermal model may only determine the current thermal state of the EDU 208, rather than predicting it. In summary, the Figures showa control system 400 and method 900 for controlling an oil circuit 310 of an electric drive system 208 of a vehicle 1, the control system 400 comprising one or more processors 404 collectively configured to: a) receive a first signal 906 indicative of a requested speed 904 of an oil pump 314 of the electric drive system 208; b) determine a minimum pressure threshold 916 in dependence on at least the requested speed 904 of the oil pump 314; c) receive a second signal indicative of a sensed oil pressure 918 of the oil circuit 310 of the electric drive system 208; d) determine 920 whether the sensed oil pressure 918 is below the minimum pressure threshold 916; and e) output a control signal 930, 940 to reduce an operating speed / requested speed of the oil pump 314 in dependence on the second signal indicating that the sensed oil pressure 918 is below the minimum pressure threshold 916. The other blocks are implemented in some, but not necessarily all, implementations. The Figures further show that optionally the control system 400 is configured to determine an expected current oil pressure 912 in dependence on the requested speed 904 of the oil pump 314 and on a sensed current oil temperature 908, and wherein the determination of the minimum pressure 15 threshold 916 is dependent on the expected current oil pressure 912 and is less than the expected current oil pressure 912. The Figures further show that optionally, determining the minimum pressure threshold 916 comprises applying a correction factor 914 to the expected current oil pressure 912. The Figures further show that optionally, the correction factor 914 is a variable dependent on a sensed parameter 908. The Figures further show that optionally, the control system 400 is configured to determine a first target speed 946, 904 of the oil pump 314, and output a first control signal 938, 940 requesting the first target speed 946, 904 of the oil pump 314 in dependence on the sensed oil pressure 918 being greater than the minimum pressure threshold 916, and wherein the control signal 930, 940 to reduce the operating speed of the oil pump is a second control signal requesting a modified target speed 922 less than the first target speed 946, 904. The Figures further show that optionally, the first target speed 946, 904 of the oil pump 314 requested by the first control signal 938, 940 is dependent on at least one of: a current rotor speed of the electric drive system 208; a current torque of the electric drive system 208; or sensed oil temperature. The Figures further show that optionally, the first target speed 946, 904 of the oil pump 314 is further dependent on a current oil flow rate. The Figures further show that optionally the control system 400 is configured to determine the modified target speed 922 in dependence on the current oil flow rate, wherein determining the modified target speed 922 is independent of the current motor speed and the currently demanded torque. The Figures further show that optionally the control system 400 is configured to: determine whether a cumulative time 936 for which the modified target speed 922 has been requested exceeds an allowable time; and in dependence on the cumulative time 936 exceeding the allowable time, output the first control signal 938, 940 requesting the first target speed 946, 904 and repeat at least operations a( to d). The Figures further show that optionally the control system 400 is configured to determine a current or predicted thermal state 946 of the electric drive system 208 in dependence on the reduced operating speed requested by the control signal 930, 940, and output one or more thermal protection control signals in dependence on the determined current or predicted thermal state to control one or more of: a torque derate of the electric drive system 208; or a position of a heat exchanger bypass valve 318 of the electric drive system 208. The Figures further show that optionally, determining 920 whether the sensed oil pressure 918 is below the minimum pressure threshold 916 comprises determining whether the sensed oil pressure 918 is below the minimum pressure threshold 916 for at least a predetermined time period. It is to be understood that the or each controller 401 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 401 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 401 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 408 could be embedded in said one or more electronic processors 404 of the controller 401; or alternatively, the set of instructions 408 could be provided as software to be executed in the controller 401. 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 404 may comprise any suitable electronic processor (e.g., a microprocessor, a microcontroller, an ASIC, etc.) that is configured to execute electronic instructions 408. The, or each, electronic memory device 406 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 406 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 404 may access the memory 16 device 406 and execute and / or use that or those instructions and information to carry out or perform some or all of the functionality and methodology described herein. The at least one memory device 406 may comprise a computer-readable storage medium (e.g. a non-transitory or non-transient storage medium) that may comprise any mechanism for storing information in a form readable by a machine or electronic processors / computational devices. Examples of the form include, without limitation: a magnetic storage medium (e.g. floppy diskette); optical storage medium (e.g. CD-ROM); magneto optical storage medium; 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. 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. 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. One such modified embodiment would comprise a control system 400 for controlling oil pressure of a torque source (e.g., electric drive system 208 or an internal combustion engine) of a vehicle 1, the control system 400 comprising one or more processors 406 collectively configured to: receive a signal indicative of a sensed oil pressure of the torque source; determine whether the sensed oil pressure is below a pressure threshold; and output a control signal to perform at least one of the following actions in dependence on the second signal indicating that the sensed oil pressure is below the pressure threshold: reduce the current operating speed of the oil pump 314; or apply a torque derate to the torque source. Another such modified embodiment would comprise a control system 400 for controlling oil pressure of a torque source of a vehicle 1, the control system 400 comprising one or more processors 406 collectively configured to: receive an inertial signal indicative of an angle (pitch angle and / or roll angle) of the vehicle 1; determine whether the angle is greater than a threshold; and output a control signal to perform at least one of the following actions in dependence on the second signal indicating that the angle is greater than the threshold: reduce the current operating speed of the oil pump 314; or apply a torque derate to the torque source. The blocks illustrated in FIG. 9 may represent steps in a method and / or sections of code in the computer program 408. 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 described or not.

Claims

1. A control system for controlling an oil circuit of an electric drive system of a vehicle, the control system comprising one or more processors collectively configured to:a) receive a first signal indicative of a requested speed of an oil pump of the electric drive system;b) determine a minimum pressure threshold in dependence on at least the requested speed of the oil pump;c) receive a second signal indicative of a sensed oil pressure of the electric drive system;d) determine whether the sensed oil pressure is below the minimum pressure threshold; ande) output a control signal to reduce an operating speed of the oil pump in dependence on the second signal indicating that the sensed oil pressure is below the minimum pressure threshold.

2. The control system of claim 1, configured to determine an expected current oil pressure in dependence on the requested speed of the oil pump and on a sensed current oil temperature, and wherein the determination of the minimum pressure threshold is dependent on the expected current oil pressure and is less than the expected current oil pressure.

3. The control system of claim 2, wherein determining the minimum pressure threshold comprises applying a correction factor to the expected current oil pressure.

4. The control system of claim 3, wherein the correction factor is a variable dependent on a sensed parameter.

5. The control system of any preceding claim, configured to determine a first target speed of the oil pump, and output a first control signal requesting the first target speed of the oil pump in dependence on the sensed oil pressure being greater than the minimum pressure threshold, and wherein the control signal to reduce the operating speed of the oil pump is a second control signal requesting a modified target speed less than the first target speed.

6. The control system of claim 5, wherein the first target speed of the oil pump requested by the first control signal is dependent on at least one of: a current rotor speed of the electric drive system; a currently demanded torque of the electric drive system; or sensed oil temperature.

7. The control system of claim 6, wherein the first target speed of the oil pump is further dependent on a current oil flow rate.

8. The control system of claim 7, configured to determine the modified target speed in dependence on the current oil flow rate, whereindetermining the modified target speed is independent of the current motor speed and the currently demanded torque.

9. The control system of any one of claims 5 to 8, configured to: determine whether a cumulative time for which the modified target speed has been requested exceeds an allowable time; and in dependence on the cumulative time exceeding the allowable time, output the first control signal requesting the first target speed and repeat at least operations a) to d).

10. The control system of any preceding claim, configured to determine a current or predicted thermal state of the electric drive system in dependence on the reduced operating speed requested by the control signal, and output one or more thermal protection control signals in dependence on the determined current or predicted thermal state to control one or more of: a torque derate of the electric drive system; or a position of a heat exchanger bypass valve of the electric drive system.

11. The control system of any preceding claim, wherein determining whether the sensed oil pressure is below the minimum pressure threshold comprises determining whether the sensed oil pressure is below the minimum pressure threshold for at least a predetermined time period.

12. A system comprising the control system of any one of the preceding claims, and an electric drive system comprising: a rotor-stator pair for converting between electrical energy and mechanical torque; the oil pump; and an oil pressure sensor, wherein the sensed oil pressure is from theoil pressure sensor.5 13. A vehicle comprising the control system of any one of claims 1 to 11, or the system of claim 12.

14. A method of controlling an oil circuit of an electric drive system of a vehicle, the method comprising:a) receiving a first signal indicative of a requested speed of an oil pump of the electric drive system;b) determining a minimum pressure threshold in dependence on at least the requested speed of the oil pump;10 c) receiving a second signal indicative of a sensed oil pressure of the electric drive system;d) determining whether the sensed oil pressure is below the minimum pressure threshold; ande) outputting a control signal to reduce an operating speed of the oil pump in dependence on the second signal indicating that the sensed oil pressure is below the minimum pressure threshold.15 15. Computer readable instructions which, when executed by a computer, are arranged to perform a method according to claim 14.

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