GB2568097A - Apparatus and method for controlling an electric machine - Google Patents

Apparatus and method for controlling an electric machine Download PDF

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Publication number
GB2568097A
GB2568097A GB1718334.4A GB201718334A GB2568097A GB 2568097 A GB2568097 A GB 2568097A GB 201718334 A GB201718334 A GB 201718334A GB 2568097 A GB2568097 A GB 2568097A
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GB
United Kingdom
Prior art keywords
torque
electric machine
vehicle
controller
input
Prior art date
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Granted
Application number
GB1718334.4A
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GB201718334D0 (en
GB2568097B (en
Inventor
Loftus Daniel
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Jaguar Land Rover Ltd
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Jaguar Land Rover Ltd
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Priority to GB1718334.4A priority Critical patent/GB2568097B/en
Publication of GB201718334D0 publication Critical patent/GB201718334D0/en
Publication of GB2568097A publication Critical patent/GB2568097A/en
Application granted granted Critical
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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B60—VEHICLES IN GENERAL
    • B60L—PROPULSION OF ELECTRICALLY-PROPELLED VEHICLES; SUPPLYING ELECTRIC POWER FOR AUXILIARY EQUIPMENT OF ELECTRICALLY-PROPELLED VEHICLES; ELECTRODYNAMIC BRAKE SYSTEMS FOR VEHICLES IN GENERAL; MAGNETIC SUSPENSION OR LEVITATION FOR VEHICLES; MONITORING OPERATING VARIABLES OF ELECTRICALLY-PROPELLED VEHICLES; ELECTRIC SAFETY DEVICES FOR ELECTRICALLY-PROPELLED VEHICLES
    • B60L15/00—Methods, circuits, or devices for controlling the traction-motor speed of electrically-propelled vehicles
    • B60L15/20—Methods, circuits, or devices for controlling the traction-motor speed of electrically-propelled vehicles for control of the vehicle or its driving motor to achieve a desired performance, e.g. speed, torque, programmed variation of speed
    • B60L15/2072—Methods, circuits, or devices for controlling the traction-motor speed of electrically-propelled vehicles for control of the vehicle or its driving motor to achieve a desired performance, e.g. speed, torque, programmed variation of speed for drive off
    • B60L15/2081—Methods, circuits, or devices for controlling the traction-motor speed of electrically-propelled vehicles for control of the vehicle or its driving motor to achieve a desired performance, e.g. speed, torque, programmed variation of speed for drive off for drive off on a slope
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
    • H02P1/00—Arrangements for starting electric motors or dynamo-electric converters
    • H02P1/16—Arrangements for starting electric motors or dynamo-electric converters for starting dynamo-electric motors or dynamo-electric converters
    • H02P1/26—Arrangements for starting electric motors or dynamo-electric converters for starting dynamo-electric motors or dynamo-electric converters for starting an individual polyphase induction motor
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B60—VEHICLES IN GENERAL
    • B60L—PROPULSION OF ELECTRICALLY-PROPELLED VEHICLES; SUPPLYING ELECTRIC POWER FOR AUXILIARY EQUIPMENT OF ELECTRICALLY-PROPELLED VEHICLES; ELECTRODYNAMIC BRAKE SYSTEMS FOR VEHICLES IN GENERAL; MAGNETIC SUSPENSION OR LEVITATION FOR VEHICLES; MONITORING OPERATING VARIABLES OF ELECTRICALLY-PROPELLED VEHICLES; ELECTRIC SAFETY DEVICES FOR ELECTRICALLY-PROPELLED VEHICLES
    • B60L3/00—Electric devices on electrically-propelled vehicles for safety purposes; Monitoring operating variables, e.g. speed, deceleration or energy consumption
    • B60L3/0023—Detecting, eliminating, remedying or compensating for drive train abnormalities, e.g. failures within the drive train
    • B60L3/003—Detecting, eliminating, remedying or compensating for drive train abnormalities, e.g. failures within the drive train relating to inverters
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
    • H02P27/00—Arrangements or methods for the control of AC motors characterised by the kind of supply voltage
    • H02P27/04—Arrangements or methods for the control of AC motors characterised by the kind of supply voltage using variable-frequency supply voltage, e.g. inverter or converter supply voltage
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
    • H02P29/00—Arrangements for regulating or controlling electric motors, appropriate for both AC and DC motors
    • H02P29/60—Controlling or determining the temperature of the motor or of the drive
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B60—VEHICLES IN GENERAL
    • B60L—PROPULSION OF ELECTRICALLY-PROPELLED VEHICLES; SUPPLYING ELECTRIC POWER FOR AUXILIARY EQUIPMENT OF ELECTRICALLY-PROPELLED VEHICLES; ELECTRODYNAMIC BRAKE SYSTEMS FOR VEHICLES IN GENERAL; MAGNETIC SUSPENSION OR LEVITATION FOR VEHICLES; MONITORING OPERATING VARIABLES OF ELECTRICALLY-PROPELLED VEHICLES; ELECTRIC SAFETY DEVICES FOR ELECTRICALLY-PROPELLED VEHICLES
    • B60L2220/00—Electrical machine types; Structures or applications thereof
    • B60L2220/40—Electrical machine applications
    • B60L2220/46—Wheel motors, i.e. motor connected to only one wheel
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B60—VEHICLES IN GENERAL
    • B60L—PROPULSION OF ELECTRICALLY-PROPELLED VEHICLES; SUPPLYING ELECTRIC POWER FOR AUXILIARY EQUIPMENT OF ELECTRICALLY-PROPELLED VEHICLES; ELECTRODYNAMIC BRAKE SYSTEMS FOR VEHICLES IN GENERAL; MAGNETIC SUSPENSION OR LEVITATION FOR VEHICLES; MONITORING OPERATING VARIABLES OF ELECTRICALLY-PROPELLED VEHICLES; ELECTRIC SAFETY DEVICES FOR ELECTRICALLY-PROPELLED VEHICLES
    • B60L2240/00—Control parameters of input or output; Target parameters
    • B60L2240/40—Drive Train control parameters
    • B60L2240/42—Drive Train control parameters related to electric machines
    • B60L2240/421—Speed
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B60—VEHICLES IN GENERAL
    • B60L—PROPULSION OF ELECTRICALLY-PROPELLED VEHICLES; SUPPLYING ELECTRIC POWER FOR AUXILIARY EQUIPMENT OF ELECTRICALLY-PROPELLED VEHICLES; ELECTRODYNAMIC BRAKE SYSTEMS FOR VEHICLES IN GENERAL; MAGNETIC SUSPENSION OR LEVITATION FOR VEHICLES; MONITORING OPERATING VARIABLES OF ELECTRICALLY-PROPELLED VEHICLES; ELECTRIC SAFETY DEVICES FOR ELECTRICALLY-PROPELLED VEHICLES
    • B60L2240/00—Control parameters of input or output; Target parameters
    • B60L2240/40—Drive Train control parameters
    • B60L2240/42—Drive Train control parameters related to electric machines
    • B60L2240/423—Torque
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B60—VEHICLES IN GENERAL
    • B60L—PROPULSION OF ELECTRICALLY-PROPELLED VEHICLES; SUPPLYING ELECTRIC POWER FOR AUXILIARY EQUIPMENT OF ELECTRICALLY-PROPELLED VEHICLES; ELECTRODYNAMIC BRAKE SYSTEMS FOR VEHICLES IN GENERAL; MAGNETIC SUSPENSION OR LEVITATION FOR VEHICLES; MONITORING OPERATING VARIABLES OF ELECTRICALLY-PROPELLED VEHICLES; ELECTRIC SAFETY DEVICES FOR ELECTRICALLY-PROPELLED VEHICLES
    • B60L2240/00—Control parameters of input or output; Target parameters
    • B60L2240/40—Drive Train control parameters
    • B60L2240/52—Drive Train control parameters related to converters
    • B60L2240/525—Temperature of converter or components thereof
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B60—VEHICLES IN GENERAL
    • B60L—PROPULSION OF ELECTRICALLY-PROPELLED VEHICLES; SUPPLYING ELECTRIC POWER FOR AUXILIARY EQUIPMENT OF ELECTRICALLY-PROPELLED VEHICLES; ELECTRODYNAMIC BRAKE SYSTEMS FOR VEHICLES IN GENERAL; MAGNETIC SUSPENSION OR LEVITATION FOR VEHICLES; MONITORING OPERATING VARIABLES OF ELECTRICALLY-PROPELLED VEHICLES; ELECTRIC SAFETY DEVICES FOR ELECTRICALLY-PROPELLED VEHICLES
    • B60L2270/00—Problem solutions or means not otherwise provided for
    • B60L2270/10—Emission reduction
    • B60L2270/14—Emission reduction of noise
    • B60L2270/145—Structure borne vibrations
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02T—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION
    • Y02T10/00—Road transport of goods or passengers
    • Y02T10/60—Other road transportation technologies with climate change mitigation effect
    • Y02T10/64—Electric machine technologies in electromobility
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02T—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION
    • Y02T10/00—Road transport of goods or passengers
    • Y02T10/60—Other road transportation technologies with climate change mitigation effect
    • Y02T10/72—Electric energy management in electromobility

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Transportation (AREA)
  • Mechanical Engineering (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Sustainable Development (AREA)
  • Sustainable Energy (AREA)
  • Electric Propulsion And Braking For Vehicles (AREA)

Abstract

A controller for electrical machine having compliant connection between output and load, particularly a connection between the output of an electric machine and a wheel of an electric vehicle, includes a control system 300 for ensuring that the speed of a rotor of the electric machine 200 does not remain below a threshold value for long enough for overheating of the inductor output stages to occur during acceleration from rest. The control system comprises a controller 304 having a processor 340p, an input 304i and an output 304o. In a first aspect the torque output of the electrical machine is fluctuated depending on whether a torque requirement is above a first threshold and a speed of the rotor is below a second threshold. In a second aspect the torque output of the electrical machine is increased according to a first or second function in dependence on determined values of driver torque demand and a required balance torque. The invention is suitable for allowing electric vehicles to move away from rest on steep gradients or in other situations in which resistance to forward motion is high.

Description

APPARATUS AND METHOD FOR CONTROLLING AN ELECTRIC MACHINE
TECHNICAL FIELD
The present disclosure relates to an apparatus and method for controlling an electric machine, particularly, but not exclusively, to a controller for controlling an electric machine, optionally an electric machine of an electric vehicle. Aspects of the invention relate to a controller, to an electric machine, to a vehicle and to a method.
BACKGROUND
Electric machines are known to be operable to produce high torque starting from low or zero speed. Accordingly, electric vehicles are capable of high acceleration from a stationary condition. However, for typical electric drive systems for use in vehicles high torque is only available at zero speed for a short period of time if the output of the electric machine is stalled. This is because when the output of an electric machine is stationary and a high mechanical load is applied substantially all of the current flows through a single phase winding of the machine and the associated power electronics, as opposed to switching relatively rapidly between different phases as would be the case when the rotational velocity of the rotor is high. Accordingly, overheating of the inverter output stages can occur in a short space of time if the output of an electric machine is stalled.
It is an object of embodiments of the invention to at least mitigate one or more of the problems of the prior art.
SUMMARY OF THE INVENTION
Aspects of the invention provide a controller, a method, an electric machine and a vehicle as claimed in the appended claims.
According to an aspect of the invention for which protection is sought there is provided a controller for an electric machine having a compliant connection between an output thereof and a load, the controller comprising:
input means configured to receive a first input indicative of a torque requirement of the electric machine and a second input indicative of a rotational velocity of a rotor of the electric machine;
output means operable to control the electric machine; and processing means configured to communicate with the input means and the output means, wherein the processing means is configured:
to determine whether the torque requirement is above a first threshold value in dependence on the first input;
to determine whether the rotational velocity of the rotor of the electric machine is below a second threshold value in dependence on the second input; and if the torque requirement is above the first threshold and the rotational velocity is below the second threshold, to control the output means to cause the electric machine to cause the torque produced by the electric machine to fluctuate. The torque fluctuation gives rise to fluctuation in torsional deflection levels of the compliant connection and hence angular displacement of the electric machine. This may ensure that the speed of the rotor of the electric machine does not remain below a threshold value for a significant amount of time, thereby preventing overheating. The input means may be an input portion of the controller, for example an electrical input or an input board and the output means may be an output portion of the controller, for example an electrical output or an output board. In some embodiments a combined input and output means may be provided, for example in the form of an electrical input/output (I/O) interface or an I/O board.
In an embodiment the controller is a controller for an electric vehicle, the electric machine is a traction motor of the vehicle, and the compliant connection is a connection between the output of an electric machine and a wheel of the vehicle. The controller may be particularly useful for allowing the vehicle to move away from rest without overheating the electric machine when a very high resistance to forward movement of the vehicle is present.
Optionally, the controller may be a controller for an electric vehicle having a plurality of electric machines, each of said electric machines having a compliant connection between the output of the electric machine and a respective group of one or more wheels of the vehicle. The electric machines may each be arranged to drive a respective wheel, or they may each be arranged to drive an axle having two or more wheels attached thereto.
Further optionally, the controller may be arranged to control the torque fluctuations produced by at least one of the electric machines to be at least partially out of phase with the torque fluctuations produced by at least one other electric machine. Advantageously, the frequency, magnitude and phase controls for the torque fluctuation may be arranged so as the effect on overall vehicle acceleration and vibration partially or wholly cancel each other out, reducing vibration and overall powertrain torque disturbance perceptible to the occupants. Alternatively, the controller may be arranged to control the torque fluctuations produced by at least one of the electric machines to be substantially in phase with the torque fluctuations produced by at least one other electric machine. Although controlling the fluctuations to be in phase may lead to increased vibration and powertrain torque disturbance, this may also increase the average acceleration that can be provided without overheating the electric machines. This may be because the peak difference between the forces acting to initiate motion of the vehicle in the intended direction of travel and forces acting to prevent motion of the vehicle in that direction will be higher when the fluctuations are in phase than if they were out of phase.
In some embodiments the torque fluctuations produced by at least one of the electric machines may have either magnitude or phase determined by a defined relationship to the magnitude or phase of the torque fluctuations of at least one other electric machine. In an embodiment the defined relationship to the magnitude or phase of the torque fluctuations of at least one other electric machine may be determined in dependence on an amount of torque required to initiate forward motion of the vehicle. For example, the fluctuations may be out of phase unless the average amount of torque required is so high that it can only be produced by making the fluctuations in phase.
In an embodiment, wherein causing torque produced by the electric machine to fluctuate comprises at least partially inhibiting an anti-resonance routine. This may provide a simple means for causing the torque to fluctuate.
In an embodiment the first input comprises an input indicative of a driver torque request.
Optionally, the first threshold value corresponds to at least 80% of the rated maximum torque of the electric machine.
In an embodiment the second threshold value corresponds to less than 50 radians per second, preferably less than 30 radians per second. It will be understood that the second threshold value may be equal to the speed below which, for a given system, overheating becomes likely and the system would otherwise need to apply a current de-rating strategy.
According to an aspect of the invention for which protection is sought there is provided a controller for an electric vehicle having a compliant connection between an output of an electric machine and a wheel of the vehicle, the controller comprising: input means configured to receive a first input indicative of a required balance torque, the input means being further configured to receive a second input indicative of the vehicle being substantially stationary and a third input indicative of a driver torque demand;
output means operable to control the electric machine; and processing means configured to communicate with the input means and the output means, wherein upon receipt of the second input the processing means is configured to: determine if the required balance torque is above a threshold value;
determine if the driver torque demand is greater than the balance torque; and, if the required balance torque is above the threshold value and the driver torque demand is greater than the required balance torque, control the output means to cause the torque produced by the electric machine to increase according to a first function when the torque produced by the electric machine is less than the required balance torque and to increase according to a second function when the torque produced by the electric machine is greater than the required balance torque, wherein the first function provides a higher average rate of increase than the second function. The input means may be an input portion of the controller, for example an electrical input or an input board and the output means may be an output portion of the controller, for example an electrical output or an output board. In some embodiments a combined input and output means may be provided, for example in the form of an electrical input/output (I/O) interface or an I/O board.
Advantageously, such a controller may be particularly useful for allowing the vehicle to move away from rest without overheating the electric machine when a very high resistance to forward movement of the vehicle is present. Further advantageously, the controller may not introduce additional vibrations into the driveline.
It will be understood that the term “balance torque” is used herein to refer to the torque that will be required to be produced by the electric machine to overcome the forces resisting forward motion of the vehicle when all of the braking means (e.g. the foundation brakes, friction brakes, parking brake and any automated vehicle hold systems) have been fully released.
Optionally, the controller may be arranged to transition between the first function for controlling the torque produced by the electric machine to the second function for controlling the torque produced by the electric machine when the torque produced by the electric machine is substantially equal to the balance torque.
Optionally, the threshold value corresponds to at least 80% of the rated maximum torque of the electric machine.
Optionally, the first input comprises an input indicative of a gradient of the surface on which the vehicle is located and the processing means is configured to calculate the required balance torque in dependence on the first input.
In an embodiment the first input comprises an input indicative of a total mass of the vehicle and the processing means is configured to calculate the required balance torque in dependence on the first input.
In an embodiment the first input comprises an input indicative of a rolling resistance and/or a morphological feature of a surface on which the vehicle is located and the processing means is configured to calculate the required balance torque in dependence on the first input.
It will be understood that the balance torque may be calculated by conventional means given known forces acting on the vehicle.
According to an aspect of the invention for which protection is sought there is provided a method of controlling an electric machine having a compliant connection between an output of the electric machine and a load, the method comprising:
determining whether a torque requirement of the electric machine is above a first threshold value and whether a rotational velocity of the electric machine is below a second threshold value; and if the torque requirement is above the first threshold value and the rotational velocity is below the second threshold value, controlling the electric machine cause the torque produced by the electric machine to fluctuate.
Optionally, the electric machine is a traction motor of the vehicle, and the compliant connection is a connection between the output of an electric machine and a wheel of the vehicle.
Optionally the method comprises controlling a plurality of electric machines according to the method described above, wherein each of said electric machines has a compliant connection between the output of the electric machine and a respective group of one or more wheels of the vehicle. Further optionally, the method may comprise controlling the torque fluctuations produced by at least one of the electric machines to be at least partially out of phase with the torque fluctuations produced by another of the electric machines.
In an embodiment the method comprises receiving a third input indicative of a rotational velocity of a road-engaging wheel of the vehicle.
In an embodiment causing torque produced by the electric machine to fluctuate comprises at least partially inhibiting an anti-resonance routine.
In an embodiment the first input comprises an input indicative of a driver torque request.
Optionally, the first threshold value corresponds to at least 80% of the rated maximum torque of the electric machine.
In an embodiment the second threshold value corresponds to less than 50 radians per second, preferably less than 30 radians per second.
According to an aspect of the invention for which protection is sought there is provided a method of controlling an electric machine of an electric vehicle having a compliant connection between an output of the electric machine and a wheel of the vehicle, the method comprising:
receiving an input indicative of the vehicle being substantially stationary and an input indicative of a driver torque demand;
determining if a required balance torque is above a threshold value;
determining if the driver torque demand is above the balance torque; and if the required balance torque is above the threshold value and the driver torque demand is above the balance torque, controlling the output means to cause the torque produced by the electric machine to increase according to a first function when the torque produced by the electric machine is less than the required balance torque and to increase according to a second function when the torque produced by the electric machine is greater than the required balance torque, wherein the first function provides a higher average rate of increase than the second function.
In an embodiment the threshold value corresponds to at least 80% of the rated maximum torque of the electric machine.
Optionally, the method may comprise receiving a first input indicative of a gradient of the surface on which the vehicle is located and calculating the required balance torque in dependence on the first input.
The method may comprise receiving a first input indicative of a total mass of the vehicle and calculating the required balance torque in dependence on the first input.
Optionally, the method may comprise receiving a first input indicative of a rolling resistance and/or a morphological feature of a surface on which the vehicle is located and calculating the required balance torque in dependence on the first input.
According to an aspect of the invention for which protection is sought there is provided a controller arranged to implement a method as described above.
According to an aspect of the invention for which protection is sought there is provided an electric machine comprising a controller as described above.
According to an aspect of the invention for which protection is sought there is provided a vehicle comprising a controller as described above or an electric machine as described above.
In an embodiment the compliant connection may comprise a shaft between a group of one or more wheels of the vehicle and an electric machine of the vehicle. The shaft may comprise a compliant element arranged to increase the overall compliance of the shaft. The compliant element may comprise a torsional spring or a torsional spring and damper.
Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.
BRIEF DESCRIPTION OF THE DRAWINGS
One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:
Figure 1 shows an electric vehicle in an embodiment of the present invention;
Figure 2 shows an electric machine that may be used as a traction motor for an electric vehicle;
Figure 3 shows a drive unit for an electric vehicle (PRIOR ART);
Figure 4 shows a schematic diagram of controller for an electric machine in an embodiment of the present invention;
Figure 5 shows a graph illustrating the variation of vehicle road speed and electric machine rotor speed during starting of an electric vehicle in an embodiment of the present invention;
Figure 6 shows a schematic diagram of a controller for an electric vehicle in an embodiment of the present invention; and
Figure 7 shows another graph illustrating the variation of vehicle road speed and electric machine rotor speed during starting of an electric vehicle in an embodiment of the present invention.
DETAILED DESCRIPTION
Figure 1 shows an electric vehicle 100 powered by an electric machine 200 for use as a traction motor of a vehicle. The electric machine 200 has a stator 202 having a plurality of windings 206 disposed around teeth 208, and a rotor 204, as shown in figure 2. In the present embodiment the electric machine 200 is a three-phase machine, although it will be understood that the present invention is also applicable to machines having a different number of phases. The windings 206 of the motor 200 are arranged to be supplied with current by an inverter (not shown) or another power electronics controller in the conventional way. As the rotor 204 rotates the power electronics controller changes the amount of current that is sent to the windings associated with each phase, either by switching the current between phases or adjusting the current sent to the windings of each phase as a function of the rotor position.
As can be seen from figure 1, the vehicle is located on a steep slope 102. Accordingly, the weight W of the vehicle acts to cause the vehicle to roll down the slope 102. When the vehicle is stationary a braking force B is provided by a foot brake or a handbrake of the vehicle, the force B being sufficient to prevent the vehicle from rolling down the slope 102 as a result of the component of the weight W that acts to cause the vehicle to roll down the slope 102.
When a driver wishes to accelerate the vehicle 100 it is necessary to release the handbrake and the food brake, and when the brakes have been released and so the force B is no longer provided sufficient torque must be provided by the electric machine to provide a force F that overcomes the component of the weight W that acts to cause the vehicle to roll down the slope 102 and also a rolling resistance R which may act to prevent the vehicle from moving away. Provided the force F is sufficient to overcome the rolling resistance R and the component of the weight W acting to cause the vehicle to roll down the slope 102 the vehicle will accelerate away.
It will be well understood by the skilled person that the component of the weight W that acts to cause the vehicle to roll down the slope 102 is dependent on the total mass of the vehicle (i.e. the mass of the vehicle including all of the passengers and items currently in the vehicle) and the steepness of the slope 102. It will also be understood that the rolling resistance R will be dependent on various factors, including but not limited to the type of surface that the vehicle is on, the morphology of the surface 102, and the type and pressure of the tyres 104.
Although figure 1 illustrates a situation in which a particularly high torque is required to initiate motion of the vehicle 100 because the vehicle 100 is located on a steep slope, the skilled person will understand that the present invention is equally applicable in other situations in which a particularly high torque is required to initiate motion of a vehicle. For example, the present invention may be applicable in the event that a vehicle is required to overcome an obstacle such as a kerb, a step or a rock before forward motion can begin.
The present inventors have recognised that if the maximum torque that can be produced by an electric machine of an electric vehicle corresponds to a force that is only slightly greater than the forces that act to prevent forward motion of the vehicle then the vehicle may only accelerate at a very slow rate. This can lead to the rotor 204 of the traction machine only turning at a very slow rate despite the high current flowing in the energised windings and inverter output stages. Furthermore, while the rotor 204 is only turning at a very slow rate the current may only switch between different sets of windings and corresponding inverter output stages relatively infrequently, which may lead to overheating of the inverter output stages or another component relatively quickly. In the event of a component of the electric machine or the associated power electronics overheating the electric machine 200 may no longer be able to supply positive torque to the driveline of the vehicle 100. Accordingly, under certain circumstances the vehicle 100 may be unable to commence and maintain forward motion on a slope 102, despite the electric machine 200 being capable of producing sufficient torque to initiate motion, as the maximum torque that the electric machine 200 is capable of producing may only be sufficient to accelerate the vehicle at a rate that would be too slow to be maintained without overheating.
A popular configuration of a driveline of an electric machine is to provide a drive unit 210 incorporating the electric machine 200 at each driven axle. As can be seen in figure 3, the drive unit 210 comprises a differential 214, a cage 212 of which is driven by an output shaft 216 of the electric machine 200. The right and left output gears 218R, 218L are connected to respective wheels 220R, 220L via respective shafts 222R, 222L. It will be understood that a gearbox may be provided in line with the electric machine 200, and that the ratio of such a gearbox may determine a direct linear relationship between the angular velocity of the rotor 204 and the shafts 222R, 222L. There will also be a direct linear relationship between the angular velocity of the shafts 222R, 222L and the velocity of the vehicle 100. Accordingly, the angular velocity of the rotor 204 is directly proportional to the speed of the vehicle, assuming that the shafts 222R, 222L behave rigidly.
The present inventors have recognised that when a vehicle 100 incorporating a drive unit 210 moves away from rest on a steep slope or in another circumstance where the resistance to forward movement of the vehicle is high, compliance in the shafts
222R, 222L may cause the instantaneous rotor angular velocity to be different to the angular velocity that would be expected based on the vehicle speed. This effect is transient and under normal circumstances it will cease to occur when the shafts 222R, 222L reach an equilibrium state for the amount of torque being transmitted from the electric machine 200 to the wheels 220R, 220L. However, it has been recognised that by employing appropriate control on the torque provided by the electric machine 200, the point at which an equilibrium state is reached can be delayed or prevented. This can allow for the vehicle speed to increase to a value above which overheating of the output stages of the power controller or the electric machine 200 can occur, without allowing the rotor 204 to be kept in a “stalled” state during the acceleration to the speed above which overheating of the inductor output stages can no longer occur at the rated current. Although the speed above which overheating of the inverter output stages will not occur will vary between different electric machines, dependent upon various factors including gearing, the number of phases, the number of pole pairs in the machine and cooling arrangements, it will be understood that this threshold will generally be in the range of 10 to 50 radians per second, which may correspond to a vehicle speed of 1 to 3km/hr. For example, threshold above which overheating of the inverter output stages will not occur may be 30 radians per second.
Figure 4 shows a control system 300 for ensuring that the speed of a rotor of an electric machine 200 does not remain below a threshold value for long enough for overheating of the inductor output stages of an electric machine 200 to occur during acceleration from rest of a vehicle 100.
Control system 300 comprises a controller 304 having a processing means in the form of a processor 340p arranged to communicate with an electronic memory 304m, an input means 304i and an output means 304o. The input means 304i may be an input portion of the controller, for example an electrical input or an input board and the output means 304o may be an output portion of the controller, for example an electrical output or an output board. In some embodiments a combined input and output means may be provided, for example in the form of an electrical input/output (I/O) interface or an I/O board.
The output means is arranged to send control signals to a power controller 250 of electric machine 200 and the input means is arranged to receive a signal indicative of the current rotor speed and a driver input for use in calculating the torque request from driver input means 308 (which may be an accelerator pedal of a vehicle in which the electric machine 200 is installed or a demand from an automated control system). In the illustrated embodiment the power controller 250 is an inverter, although it will be realised that other power controllers could also be employed.
The processing means 302p is arranged to receive the signals indicative of the driver input and the rotor speed and to calculate the driver-requested torque. The processing means then determines whether or not the driver-requested torque is above a first threshold value and whether or not the rotational velocity of the rotor 204 is below a second threshold value. As discussed above, in circumstances where a high amount of torque is required and the rotational velocity of the rotor is low there is a risk of the inverter output stages or another component of the electric machine 200 overheating because of the high current requirement and infrequent switching between phases of the electric machine. The first threshold value may be approximately 80% of the maximum rated torque of the electric machine and the second threshold value may be a value that corresponds to a vehicle speed of approximately 2kph, which may be a rotor rotational velocity of approximately 30 radians per second.
In the event that the processing means 304p determines that the driver-requested torque is above a first threshold value and the rotational velocity of the rotor 204 is below a second threshold value it is arranged to initiate a control routine in which the torque provided by the electric machine is fluctuated about a mean value, thereby to ensure that the rotational velocity of the rotor 204 does not remain below the threshold value for long enough to cause overheating of the inverter output stages or any other component. The effects of this control routine are illustrated in figure 5, in which line 400 shows the variation of rotor speed ω against time t, line 402 shows vehicle road speed v against time t, line 404 shows the torque τ produced by the electric machine against time t and line 406 shows the deflection δ of the half-shafts 222R, 222L against time t. It will be understood that figure 5 is a simplified diagram, and that not all of the physical effects that would be expected to occur during the operation of the method of the embodiment illustrated in figure 5 are accounted for. In particular, inertial effects of the components are not taken into account in figure 5.
Figure 5 illustrates a situation in which an electric vehicle 100 is initially at rest at t0 and is located on a steep slope or in another situation in which a significant proportion of the torque that the electric machine of the vehicle is capable of producing is required to initiate forward movement of the vehicle. At t0 the driver wishes to accelerate the vehicle away from rest and therefore provides a high torque request via an accelerator pedal of the vehicle substantially at the same time as any applicable manual or automatic system for holding the vehicle stationary, such as a park brake or hill hold system, is released. The torque provided by the electric machine therefore initially increases rapidly to η, which is a torque value greater than the balance torque. Because the resistance to forward motion of the vehicle is high this initial increase in torque only causes a slow increase in the vehicle speed v, but it causes the deflection of the half-shafts 222R, 222L to rapidly increase to an equilibrium value 6eq.
At this point the speed of the rotor 204 would become directly proportional to the speed of the vehicle 100 if the torque was to remain constant. As the initial rate of torque increase and the development of consequent deflections in the system are rapid processes, while the acceleration of the vehicle is relatively long duration, the vehicle speed and road wheel rotational velocity would still be too low at this time to avoid overheating of the power controller output stages 250 or the electric machine 200 because the current required to produce the torque η is relatively high and switching between phases of the electric machine 202 would occur only infrequently.
To prevent overheating in the electric machine 200 or the power controller 250 the processing means 304p is configured to control the torque produced by the electric machine 200 to fluctuate between η and t2. This causes the deflection in the halfshafts 222R, 222L to oscillate rather than remaining at 5eq. As the instantaneous rotational velocity of the rotor 204 is related to both the instantaneous velocity of the vehicle 200 and the instantaneous rate of change of deflection in the half-shafts 222R, 222L the continuously changing deflection in the half shafts whilst the torque is fluctuating can prevent the rotational velocity of the rotor from remaining low enough to cause overheating for too long, thereby preventing overheating in the electric machine 200.
The fluctuation in the torque τ is continued until the vehicle speed v reaches speed v1, at which point the vehicle speed is high enough that overheating of the inverter output stages of the electric machine 200 will not occur if the rotational velocity ω of the rotor 204 is related to vehicle speed by the normal linear relationship that exists when the half-shafts 222R, 222L behave rigidly. Accordingly, the torque produced by the electric machine may be controlled in the conventional way after time q. As the fluctuation in torque is ended at time ti, the deflection in the half-shafts 222R, 222L also ceases to fluctuate after time ti, and the deflection of the half-shafts then becomes quasi constant.
The skilled person will understand that the fluctuation in torque demand may be applied with any characteristic frequency desired. In some embodiments a torque demand fluctuation applied in-phase with an existing mass-elastic resonance (or a harmonic thereof) may allow a total torque fluctuation with a greater peak magnitude, greater deflection magnitude and therefore higher average speed fluctuation at the electric machine than would otherwise be achievable.
Alternatively, if such magnitude reinforcement is not required, a fluctuation frequency avoiding resonance frequencies of the system or selected to coincide with nodal frequencies where the sensitivity of the vibration transfer function from road wheel to occupants is minimised may be preferable in order to minimise the perception of vibrations by the vehicle occupants.
It will be understood that controller 304 is arranged to provide a non-fluctuating torque request to power controller 250 in the event that the torque request is not above the first threshold value or the rotor velocity is above second the threshold value. Calculation of the torque request signal may be done in the conventional way, and will not be described in detail here.
Although figure 5 illustrates torque fluctuations of constant amplitude, it will be understood that in some embodiments the amplitude of the fluctuations may not be constant. For example, as the vehicle speed increases towards v> the frequency and shaping of the fluctuating torque demand signal may be altered as the amount of additional movement of the rotor 204 required to be caused by the fluctuations in half-shaft displacement in order to maintain sufficiently frequent switching between phases of the electric machine is lower when the vehicle speed is close to v>.
In some embodiments controller 304 may be arranged under normal operation to control the torque produced by the electric machine to counteract vibrations that may occur as a result of deflections in the half-shafts 222R, 222L when a change in the torque applied by the electric machine 200 occurs. Such control routines will be well known to the skilled person and may be referred to as “anti-shuffle” or “antiresonance” routines. In some embodiments the controller 304 may be operable to reduce or completely prevent the effects of such an anti-resonance control routine when it is determined that the torque request is above the first threshold value and the rotor velocity is below second the threshold value, thereby allowing torsional vibrations in the shafts 222R, 222L to at least some extent. Such vibrations may be sufficient to ensure that switches between windings of different phases occurs frequently enough that overheating of the inverter output stages does not occur. Once the speed of the vehicle is above a threshold speed so that overheating will not occur even if the displacement of the shafts 222R, 222L is constant the anti-shuffle routine may be allowed to function as normal again.
It will be understood that the method of controlling torque produced by an electric machine to optimise the starting capability in high resistance conditions shown in figure 5 may be considered to be a reactive method, as the control of the fluctuations may be performed once the processing means 304p has determined that the vehicle speed is below the threshold value and the torque being produced by the electric machine is already above a threshold value. Figure 6 shows a control system 500 that may be used to implement a predictive method for allowing a vehicle to start on a steep incline or another situation in which resistance to forward motion is high.
Control system 500 comprises a controller 504 having a processing means in the form of a processor 504p arranged to communicate with an electronic memory 504m, an input means 504i and an output means 504o. The input means 504i may be an input portion of the controller, for example an electrical input or an input board and the output means 504o may be an output portion of the controller, for example an electrical output or an output board. In some embodiments a combined input and output means may be provided, for example in the form of an electrical input/output (I/O) interface or an I/O board.
The output means is arranged to send control signals to a power controller 250 of electric machine 200 and the input means is arranged to receive a signal indicative of the current rotor speed and a signal indicative of a driver torque request from the input means 308 (which may be an accelerator pedal of a vehicle in which the electric machine 200 is installed). Input means 504i is also arranged to receive signals from one or more sensors 506 (of which only one is illustrated), which signals are indicative of a required balance torque and may be used to calculate an estimate of the required balance torque. It will be understood that the term “balance torque” is used herein to refer to the torque that will be required to be produced by the electric machine to overcome the forces resisting forward motion of the vehicle when all of the braking means (e.g. the foundation brakes, friction brakes, parking brake and any automated vehicle hold systems) have been fully released.
Processor 504p is arranged to determine that the torque that will be required to accelerate a vehicle in which is installed away from rest will be greater than a threshold value in dependence on the inputs from sensors 506. The inputs from the sensors may be indicative of an incline of a surface 102 on which the vehicle is located, a total mass of the vehicle, information on external forces acting on the vehicle such as towing or recovery attachment points loads and one or more parameters of the surface 102 on which the vehicle is located such as a rolling resistance or a surface morphology. It will be understood that the force acting to keep the vehicle stationary may be computed from such inputs, and that the torque that is required from the electric machine 200 may be readily calculated from the required force.
In the event that the processor 504p detects that the balance torque is above a threshold value and that therefore the electric machine will likely be unable to cause the vehicle in which the controller 504 is installed to move away from rest without overheating the inverter output stages of the electric machine 200, the processor 504p is arranged to implement a control routine that causes the torque produced by the electric machine 200 when the driver desires the vehicle to move away from rest to initially increase according to a first function that causes the torque to increase at a first linear rate until the balance torque is reached and then to increase according to a second function that causes the torque to increase at a second linear rate that is slower than the first linear rate once the balance torque is exceeded. It will be understood that in other embodiments the rates of increase may be non-linear, provided the first function results in a more rapid average increase in torque than the second function.
Figure 7 shows the effects of the control routine implemented by processor 504p, in which line 600 shows the variation of rotor speed ω against time t, line 602 shows vehicle road speed v against time t, line 604 shows the torque τ produced by the electric machine against time t and line 606 shows the deflection δ of the half-shafts 222R, 222L against time t. It will be understood that figure 7 is a simplified diagram, and that not all of the physical effects that would be expected to occur during the operation of the method of the embodiment illustrated in figure 7 are accounted for. In particular, inertial effects of the components are not taken into account in figure 7
Figure 7 illustrates a situation in which an electric vehicle 100 is initially at rest at t3 and is located on a steep slope or in another situation in which a significant proportion of the torque that the electric machine of the vehicle is capable of producing is required to initiate forward movement of the vehicle. At t3 the driver wishes to accelerate the vehicle away from rest and therefore provides a high torque request via an accelerator pedal of the vehicle substantially at the same time as any braking means, such as a hand brake or a foot brake which was previously holding the vehicle stationary is released. The torque provided by the electric machine therefore initially increases rapidly to t3, which is equal to or greater than the balance torque required to initiate forward movement of the vehicle and is achieved at t4. After t4 the rate of increase of torque is reduced. This causes the deflection δ of the shafts 222R, 222L to continue to increase, but at a reduced rate between t4 and t5. The continuously increasing deflection between t4 and t5 causes the rotor velocity to remain above the threshold rotational velocity ωτ, below which overheating of the inverter output stages can occur until t5. At time t5 the maximum torque value (which may limited either by a driver demand or the capability of the electric machine 200) is reached. Although the vehicle road speed v does not correspond to a rotor velocity ω that is above the threshold value ωτ, provided the vehicle can be expected to accelerate to a speed corresponding to the threshold value before overheating of the inverter output stages actually occurs (that is, the time between t5 and t6 is short), no further control intervention will be required, as the time during which the rotor speed is below the threshold value has been reduced to an acceptable level. Accordingly, it will be understood that no further special control is required after t5 provided the road speed at t5 is greater than the critical road speed above which overheating will not occur or, if the road speed is still less than the critical road speed, the road speed can be expected to increase above the critical road speed before overheating is actually likely to occur.
In some embodiments, the controller 504 may be arranged to cause the torque produced to fluctuate between t5 and t6, thereby further reducing the amount of time during which the rotor velocity is below the threshold value ωτ.
In some embodiments the shafts 222R, 222L may each include a compliant element arranged to increase the overall compliance of the shaft. This may increase the amount of motion of the rotor that can be obtained without corresponding motion of the wheel or group of wheels that the electric machine is arranged to drive. The compliant element may comprise a torsional spring or a torsional spring and damper.
Unless otherwise stated, references to rates of change (or to rates of increase or decrease) of a parameter in the present specification refer to the rate of change (or increase or decrease) with respect to time.
It will be understood that as used herein the term “electric vehicle” is used to describe vehicles in which motive power is at least partially provided by an electric machine. Accordingly, the term does not exclude hybrid electric vehicles in which motive power is provided by an electric machine and another actuator, such as a combustion engine.
It will be appreciated that embodiments of the present invention can be realised in the form of hardware, software or a combination of hardware and software. Any such software may be stored in the form of volatile or non-volatile storage such as, for example, a storage device like a ROM, whether erasable or rewritable or not, or in the form of memory such as, for example, RAM, memory chips, device or integrated circuits or on an optically or magnetically readable medium such as, for example, a CD, DVD, magnetic disk or magnetic tape. It will be appreciated that the storage devices and storage media are embodiments of machine-readable storage that are suitable for storing a program or programs that, when executed, implement embodiments of the present invention. Accordingly, embodiments provide a program comprising code for implementing a system or method as claimed in any preceding claim and a machine readable storage storing such a program. Still further, embodiments of the present invention may be conveyed electronically via any medium such as a communication signal carried over a wired or wireless connection and embodiments suitably encompass the same.
All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.
Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.
The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed. The claims should not be construed to cover merely the 10 foregoing embodiments, but also any embodiments which fall within the scope of the claims.

Claims (32)

1. A controller for an electric machine having a compliant connection between an output thereof and a load, the controller comprising:
input means configured to receive a first input indicative of a torque requirement of the electric machine and a second input indicative of a rotational velocity of a rotor of the electric machine;
output means operable to control the electric machine; and processing means configured to communicate with the input means and the output means, wherein the processing means is configured:
to determine whether the torque requirement is above a first threshold value in dependence on the first input;
to determine whether the rotational velocity of the rotor of the electric machine is below a second threshold value in dependence on the second input; and if the torque requirement is above the first threshold and the rotational velocity is below the second threshold, to control the output means to cause the electric machine to cause the torque produced by the electric machine to fluctuate.
2. A controller as claimed in claim 1, wherein the controller is a controller for an electric vehicle, the electric machine is a traction motor of the vehicle, and the compliant connection is a connection between the output of an electric machine and a wheel of the vehicle.
3. A controller as claimed in claim 2, wherein the controller is a controller for an electric vehicle having a plurality of electric machines, each of said electric machines having a compliant connection between the output of the electric machine and a respective group of one or more wheels of the vehicle.
4. A controller as claimed in claim 3, wherein the controller is arranged to control the torque fluctuations produced by at least one of the electric machines to be at least partially out of phase with the torque fluctuations produced by another of the electric machines.
5. A controller as claimed in any one of claims 2 to 4, wherein the controller is further configured to receive a third input indicative of a rotational velocity of a roadengaging wheel of the vehicle.
6. A controller as claimed in any preceding claim, wherein causing torque produced by the electric machine to fluctuate comprises at least partially inhibiting an anti-resonance routine.
7. A controller as claimed in any preceding claim, wherein the first input comprises an input indicative of a driver torque request.
8. A controller as claimed in any preceding claim, wherein the first threshold value corresponds to at least 80% of the rated maximum torque of the electric machine.
9. A controller as claimed in any preceding claim, wherein the second threshold value corresponds to less than 50 radians per second, preferably less than 30 radians per second.
10. A controller for an electric vehicle having a compliant connection between an output of an electric machine and a wheel of the vehicle, the controller comprising:
input means configured to receive a first input indicative of a required balance torque, the input means being further configured to receive a second input indicative of the vehicle being substantially stationary and a third input indicative of a driver torque demand;
output means operable to control the electric machine; and processing means configured to communicate with the input means and the output means, wherein upon receipt of the second input the processing means is configured to:
determine if the required balance torque is above a threshold value;
determine if the driver torque demand is greater than the balance torque; and, if the required balance torque is above the threshold value and the driver torque demand is greater than the required balance torque, control the output means to cause the torque produced by the electric machine to increase according to a first function when the torque produced by the electric machine is less than the required balance torque and to increase according to a second function when the torque produced by the electric machine is greater than the required balance torque, wherein the first function provides a higher average rate of increase than the second function.
11. A controller as claimed in claim 10, wherein the controller may be arranged to transition between the first function for controlling the torque produced by the electric machine to the second function for controlling the torque produced by the electric machine when the torque produced by the electric machine is substantially equal to the balance torque.
12. A controller as claimed in claim 10 or claim 11, wherein the threshold value corresponds to at least 80% of the rated maximum torque of the electric machine.
13. A controller as claimed in any one of claims 10 to 12, wherein the first input comprises an input indicative of a gradient of the surface on which the vehicle is located and the processing means is configured to calculate the required balance torque in dependence on the first input.
14. A controller as claimed in any one of claims 10 to 13, wherein the first input comprises an input indicative of a total mass of the vehicle and the processing means is configured to calculate the required balance torque in dependence on the first input.
15. A controller as claimed in any one of claims 10 to 14, wherein the first input comprises an input indicative of a rolling resistance and/or a morphological feature of a surface on which the vehicle is located and the processing means is configured to calculate the required balance torque in dependence on the first input.
16. A method of controlling an electric machine having a compliant connection between an output of the electric machine and a load, the method comprising:
determining whether a torque requirement of the electric machine is above a first threshold value and whether a rotational velocity of the electric machine is below a second threshold value; and if the torque requirement is above the first threshold value and the rotational velocity is below the second threshold value, controlling the electric machine cause the torque produced by the electric machine to fluctuate.
17. A method as claimed in claim 16, wherein the electric machine is a traction motor of a vehicle, and the compliant connection is a connection between the output of an electric machine and a wheel of the vehicle.
18. A method comprising controlling a plurality of electric machines according to the method of claim 17, wherein each of said electric machines has a compliant connection between the output of the electric machine and a respective group of one or more wheels of the vehicle.
19. A method as claimed in claim 18, comprising controlling the torque fluctuations produced by at least one of the electric machines to be at least partially out of phase with the torque fluctuations produced by another of the electric machines.
20. A method as claimed in any one of claims 17 to 19, comprising receiving a third input indicative of a rotational velocity of a road-engaging wheel of the vehicle.
21. A method as claimed in claim any one of claims 16 to 20, wherein causing torque produced by the electric machine to fluctuate comprises at least partially inhibiting an anti-resonance routine.
22. A method as claimed in any one of claims 16 to 21, wherein the first input comprises an input indicative of a driver torque request.
23. A method as claimed in any one of claims 16 to 22, wherein the first threshold value corresponds to at least 80% of the rated maximum torque of the electric machine.
24. A method as claimed in any one of claims 16 to 25, wherein the second threshold value corresponds to less than 50 radians per second, preferably less than 30 radians per second.
25. A method of controlling an electric machine of an electric vehicle having a compliant connection between an output of the electric machine and a wheel of the vehicle, the method comprising:
receiving an input indicative of the vehicle being substantially stationary and an input indicative of a driver torque demand;
determining if a required balance torque is above a threshold value; determining if the driver torque demand is above the balance torque; and if the required balance torque is above the threshold value and the driver torque demand is above the balance torque, controlling the output means to cause the torque produced by the electric machine to increase according to a first function when the torque produced by the electric machine is less than the required balance torque and to increase according to a second function when the torque produced by the electric machine is greater than the required balance torque, wherein the first function provides a higher average rate of increase than the second function.
26. A method as claimed in claim 25, wherein the threshold value corresponds to at least 80% of the rated maximum torque of the electric machine.
27. A method as claimed in claim 25 or claim 26, comprising receiving a first input indicative of a gradient of the surface on which the vehicle is located and calculating the required balance torque in dependence on the first input.
28. A method as claimed in any one of claims 25 to 27, comprising receiving a first input indicative of a total mass of the vehicle and calculating the required balance torque in dependence on the first input.
29. A method as claimed in any one of claims 25 to 28, comprising receiving a first input indicative of a rolling resistance and/or a morphological feature of a surface on which the vehicle is located and calculating the required balance torque in dependence on the first input.
30. A controller arranged to implement a method as claimed in any one of claims 16 to 25.
31. An electric machine comprising a controller as claimed in any one of claims 1 to 15.
32. A vehicle comprising a controller as claimed in any one of claims 1 to 15 or 30, or an electric machine as claimed in claim 31.
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