Shaft position detection

Abstract

An electric motor 3 comprises a shaft 13 and a position sensor 18. A high frequency signal is injected into the electric motor and a first response signal, generated by the electric motor in response

Description

This invention relates to methods of controlling an electric motor and a controller configured to control an electric motor according to those methods. It is becoming increasingly common for electric motors to be used as power sources in vehicles. With this use is a requirement that the control of those electric motors be as reliable and safe as possible so that the vehicles that use them as a power source are, in turn, also as reliable and safe as possible. The electric motors generally have a shaft which is caused to rotate by electrical excitation of drive components of the electric motor. The electrical excitation of these drive components can cause the shaft to rotate relative to a body of the electric motor. The vehicles use various electric motor control methods which use as an input the rotational position of the shaft at a given time. This assists with the control of the electrical inputs to the electric motor to correctly electrically excite the drive components. It is important that the correct inputs, such as the rotational position, are provided to the motor control methods so that the electric motors can operate as reliably and safely as possible. Therefore, it is desired that there be improved methods of controlling an electric motor. According to a first aspect of the present invention there is provided a method for controlling an electric motor, the electric motor comprising a shaft and a position sensor attached to the shaft, the shaft being caused to rotate by electrical excitation of the electric motor and the position sensor being configured to detect a rotational position of the shaft, the method comprising: injecting a high frequency signal into the electric motor; processing a first response signal, generated by the electric motor in response to the high frequency signal, to determine a first rotational position of the shaft at a first time; receiving a second rotational position of the shaft from the position sensor, the second rotational position indicating the rotational position of the shaft at the first time as measured by the position sensor; determining a position offset between the first rotational position and the second rotational position measured by the position sensor; and controlling the electrical excitation of the electric motor to rotate the shaft in dependence on subsequent rotational positions detected by the position sensor and the position offset. The high frequency signal may be a high frequency low voltage signal. Injecting the high frequency signal into the electric motor may comprise injecting the high frequency signal at an injection angle and varying the injection angle; and processing a first response signal comprises measuring impedance of the electric motor to determine the first rotational position. Controlling the electrical excitation of the electric motor to rotate the shaft in dependence on subsequent rotational positions detected by the position sensor and the position offset may comprise correcting the rotational positions detected by the position sensor based on the position offset. Correcting the rotation positions detected by the position sensor based on the position offset may comprise adjusting the rotational positions by the position offset. The position offset may be the rotational difference between the first rotational position and the second rotational position. The method may comprise: processing a second response signal, generated by the electric motor in response to electrical input into the electric motor, to determine a third rotational position of the shaft at a first time; receiving a fourth rotational position of the shaft from the position sensor, the fourth rotational position indicating the rotational position of the shaft at the first time as measured by the position sensor; determining a twist angle between the third rotational position and the fourth rotational position; and determining a torque generated by the electric motor based on the twist angle. According to a second aspect of the present invention there is provided a method for controlling an electric motor, the electric motor comprising a shaft and a position sensor attached to the shaft, the shaft being caused to rotate by electrical excitation of the electric motor and the position sensor being configured to detect a rotational position of the shaft, the method comprising: processing a second response signal, generated by the electric motor in response to electrical input into the electric motor, to determine a third rotational position of the shaft at a first time; receiving a fourth rotational position of the shaft from the position sensor, the fourth rotational position indicating the rotational position of the shaft at the first time as measured by the position sensor; determining a twist angle between the third rotational position and the fourth rotational position; and determining a torque generated by the electric motor based on the twist angle. The electrical input may be electrical excitation of the electric motor to cause the shaft to rotate. The second response signal may be measured back-EMF generated by the electric motor in response to the electrical excitation of the electric motor. The electrical input may be a high frequency signal; and the method may comprise injecting the high frequency signal into the electric motor. The high frequency signal may be a high frequency low voltage signal. Injecting the high frequency signal into the electric motor may comprise injecting the high frequency signal at an injection angle and varying the injection angle; and processing a second response signal comprises measuring impedance of the electric motor to determine the third rotational position. The electric motor may comprise a plurality of drive components to cause the shaft to rotate that are located at a first location on the shaft, the third rotational position may be measured at the first location, and the position sensor may measure the fourth rotational position at a second location on the shaft that is remote from the first location along the shaft. The shaft may have a length between the first location and the second location and the method may comprise determining a torque generated by the electric motor based on the twist angle and the length. The method may comprise controlling the electrical excitation of the electric motor to rotate the shaft in dependence on the torque generated by the electric motor. According to a third aspect of the present invention there is provided a controller for an electric motor, the electric motor comprising a shaft and a position sensor attached to the shaft, the shaft being caused to rotate by electrical excitation of the electric motor and the position sensor being configured to detect a rotational position of the shaft, the controller being configured to: inject a high frequency signal into the electric motor; process a first response signal, generated by the electric motor in response to the high frequency signal, to determine a first rotational position of the shaft at a first time; receive a second rotational position of the electric motor from the position sensor, the second rotational position indicating the rotational position of the shaft at the first time as measured by the position sensor; determine a position offset between the first rotational position and the second rotational position measured by the position sensor; and control the electrical excitation of the electric motor to rotate the shaft in dependence on subsequent rotational positions detected by the position sensor and the position offset. According to a fourth aspect of the present invention there is provided a controller for an electric motor, the electric motor comprising a shaft and a position sensor attached to the shaft, the shaft being caused to rotate by electrical excitation of the electric motor and the position sensor being configured to detect a rotational position of the shaft, the controller being configured to: process a second response signal, generated by the electric motor in response to electrical input into the electric motor, to determine a third rotational position of the shaft at a first time; receive a fourth rotational position of the electric motor from the position sensor, the fourth rotational position indicating the rotational position of the shaft at the first time as measured by the position sensor; determine a twist angle between the first rotational position and the fourth rotational position; and determine a torque generated by the electric motor based on the twist angle. The present invention will now be described by way of example with reference to the accompanying drawings. In the drawings: Figure 1 shows a schematic diagram of a motor control system. Figure 2 shows a flow diagram of the method of calibrating a position sensor. Figure 3 shows a flow diagram of the method of calculating the torque produced by the electric motor. The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art. The general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. The present invention relates to a method for controlling an electric motor, the electric motor comprising a shaft and a position sensor attached to the shaft, the shaft being caused to rotate by electrical excitation of the electric motor and the position sensor being configured to detect a rotational position of the shaft. The method comprises injecting a high frequency signal into the electric motor; and processing a first response signal, generated by the electric motor in response to the high frequency signal, to determine a first rotational position of the shaft at a first time. The method further comprises receiving a second rotational position of the shaft from the position sensor, the second rotational position indicating the rotational position of the shaft at the first time as measured by the position sensor and determining a position offset between the first rotational position and the second rotational position measured by the position sensor. The method further comprises controlling the electrical excitation of the electric motor to rotate the shaft in dependence on subsequent rotational positions detected by the position sensor and the position offset. The present invention also relates to a method for controlling an electric motor, the electric motor comprising a shaft and a position sensor attached to the shaft, the shaft being caused to rotate by electrical excitation of the electric motor and the position sensor being configured to detect a rotational position of the shaft, the method comprising: processing a second response signal, generated by the electric motor in response to electrical input into the electric motor, to determine a third rotational position of the shaft at a first time; and receiving a fourth rotational position of the shaft from the position sensor, the fourth rotational position indicating the rotational position of the shaft at the first time as measured by the position sensor. The method further comprises determining a twist angle between the third rotational position and the fourth rotational position; and determining a torque generated by the electric motor based on the twist angle. Figure 1 shows a schematic diagram of a motor control system 1. The system 1 comprises a motor control unit 2 and an electric motor 3. The motor control unit 2 is connected to a battery 4. The battery 4 provides a DC-link voltage 5 to the motor control unit 2. The motor control unit 2 converts the DC-link voltage 5 into three-phase AC voltages 6 that drive the electric motor 3. The motor control unit 2 receives a drive demand input 7 and uses that to determine how to drive the electric motor 3 to achieve the required drive demand. The motor control unit 2 is connected to the electric motor 3 by drive connection 8. The motor control unit 2 is connected to the battery 4 by supply connection 9. The motor control unit 2 may also be referred to as a motor controller 2 or simply a controller 2. The motor control unit 2 comprise a processor 10 and a memory 11. The memory 11 may be non-volatile memory. The motor control unit 2 may comprise more than one processor and more than one memory. The memory 11 stores a set of program instructions that are executable by the processor 10, and reference data such as lookup tables that can be referenced by the processor 10 in response to those instructions. The processor 10 may be configured to operate in accordance with a computer program stored in non-transitory form on a machine-readable storage medium. The computer program may store instructions for undertaking the motor control methods described herein. The processor 10 may be in the form of a general purpose processor. Alternatively, the processor 10 may be in the form of application specific integrated circuits which are configured to perform the methods as herein described. The motor control unit 2 may comprise additional circuitry and / or processors, shown generally at 12, to perform the functions of the motor control unit 2. For instance, the motor control unit 2 may comprise power electronics to convert the DC-link voltage to the AC voltages that are required to drive the electric motor 3. The circuitry and / or processor 12 may also be used in the methods herein described to generate and process the signals sent and received from the electric motor 3. The electric motor 3 comprises a shaft 13. The shaft 13 is rotatable relative to a housing 14 of the electric motor 3. The shaft 13 is rotatable along its longitudinal direction. The shaft 13 provides the drive output 15 of the electric motor 3. The drive output 15 may be coupled to one or more drive wheels of a vehicle to drive the vehicle. The shaft 13 is caused to rotate by electrical excitation of the electric motor 3. The electrical excitation is provided by power being provided over the drive connection 8. The electric motor 3 comprises a plurality of drive components 16. The drive components 16 cause the shaft 13 to rotate. Some of the drive components may be attached to the shaft 13 and others may be attached to the housing 14. The shaft 13 being rotationally mounted to the housing 14 to allow it to rotate. The electrical excitation of at least some of the drive components 16 causes the shaft 13 to rotate. Some of the drive components 16 may rotate with the shaft 13. The drive components 16 may comprise a rotor attached to the shaft 13. The drive components 16 may comprise a stator attached to the housing 14. The interaction between the rotor and the stator causes the rotor, and thus the shaft 13, to rotate during the electrical excitation of the electric motor 3. The electrical excitation of the electric motor 3 is caused by the motor control unit 2 driving the electric motor 3 over drive link 8. The drive components 16 for the electric motor 16 are generally located at a first location 17 along the shaft 13. It will be appreciated that as the drive components 16 have an amount of extent along the shaft 13, the first location can be viewed as a region along the shaft 13. The rotational position of the shaft 13 at the first location can be calculated using characteristics of the drive components 16 interaction with an electrical input provided to the electric motor 3. The electrical input may be provided over drive link 8 by the motor control unit 2. The electrical input may be the electrical excitation used to drive the electric motor to cause the shaft to rotate. Alternatively, or as well as, the electrical input may be a separate electrical signal that is provided either separately or as well as the electrical excitation used to drive the electric motor. The electrical input may involve be high frequency signal injection. When the electric motor 3 is moving at moderate or high speed, the interaction between the drive components 16 of the electric motor generates a back-EMF (back electromotive force) that is sufficient to be able to be sensed by the motor control unit 2. A moderate or high speed may be above approximately 5% of the base speed of the electric motor 3. This back-EMF is a voltage that is generated when a first set of the drive components 16 move relative to a second set of drive components 16 which causes one set to move in a magnetic field generated by the other set. The properties of these drive components 16 determine the shape of the back-EMF waveform. The back-EMF waveform varies in accordance with the relative position of the various drive components 16. Therefore, the back-EMF can be used to determine the rotational position of the shaft 13 at a given time. There are several approaches that may be used to determine the back-EMF. For example, back-EMF Model Reference Adaptive System (MRAS), Sliding Mode Observer or Extended Kalman Filter may be used to determine the back-EMF. By way of example, back-EMF MRAS works by subtracting two back-EMF values. One back-EMF value is calculated from motor phase voltages and measured phase currents (the may be known as the reference back-EMF) and the other back-EMF value is estimated from stator phase current and estimated speed (estimated back-EMF). The error between the reference and estimated back-EMF values are then fed into a proportional integral controller and that in return estimates the speed and position of the electric motor 3. The proportional integral controller may be implemented by the control unit 2. As the shaft can rotate about its rotational axis, at any given time, the shaft 13 can be at any rotational position around that axis. The rotational position is an angle from a defined zero point. The zero point may be defined with reference to the drive components 16. The back-EMF may be known as a response signal that can be processed by the motor control unit 2 to determine a first rotational position of the shaft 13. The first rotational position being at the first location 17 along the shaft 13. When the electric motor 3 is at rest, and so the shaft 13 is not rotating, or the electric motor 3 is moving at low speed, the drive components 16 either generate no back-EMF or the back-EMF is insufficient to be usefully sensed by the motor control unit 2. Low speed may be below approximately 5% of the base speed of the electric motor 3. In this case, high frequency signal injection (HFSI) may be used to determine the first rotational position of the shaft 13 instead. HFSI may also be used at moderate or high speeds in addition to or instead of back-EMF to estimate the first rotational position of the shaft 13. However, at higher speeds HFSI can be detrimental to the quality of the drive of the electric motor 3 and so back-EMF may be preferred at these motor speeds. HFSI involves injecting a high frequency signal into the electric motor 3. The high frequency signal is generally a low voltage signal. The signal is injected via the drive connection 8. The signal is injected at an injection angle. This may be on an arbitrary axis. The injection angle is then varied. Generally, the injection angle is swept around a complete revolution of all the injection angles. A response signal is generated by the electric motor 3 whilst the high frequency signal is being injected into the electric motor 3. The control unit 2 receives the response signal and processes it to determine the current orientation of the shaft 13. This is done by measuring the impedance of the drive components 16 during the high frequency signal injection. The impedance of the electric motor 3 is at a minimum when the injection angle is aligned with the magnetic pole axis. This thus enables the rotational position of the drive components, and so the rotational position of the shaft, to be determined. Again, the rotational position is an angle from a defined zero point. The zero point may be defined with reference to the drive components 16. The response signal from the HFSI can be processed by the motor control unit 2 to determine a first rotational position of the shaft 13. The first rotational position being at the first location 17 along the shaft 13. HFSI works based on anisotropic effects of the electric motor 3. Generally speaking, HFSI may be implemented using any standard method. By way of example, in a common type of HFSI, a small high-frequency voltage signal is injected into electric motor 3. The small high-frequency voltage signal is injected in a way to excite the drive components 16 of the electric motor in a way that the small high-frequency voltage signal is injected the rotor d-axis. A tracking observer for the HF current processes an anisotropy model of the electric motor 3 and provides rotor speed and position estimation. The tracking observer may be implemented by motor control unit 2. The tracking observer comprises a bandpass filter, proportional integral controller, and low pass filter. The bandpass filter separates high frequency phase current signals from fundamental-components and this high frequency component which contains position information. The positional information may then be processed further to obtain the speed and angle of the electric motor 3 and in particular the speed and angle of the rotor. A position sensor 18 is attached to the shaft 13. The position sensor 18 may be an encoder or a resolver. The position sensor 18 is configured to detect a rotational position of the shaft 13. The position sensor 18 may be known as a resolver. As the shaft 13 can rotate about its rotational axis, at any given time, the shaft 13 can be at any rotational position around that axis. The rotational position is an angle from a defined zero point. The zero point may be defined with reference to the drive components 16. Thus, the position sensor 18 is configured to output a rotation position signal which indicates the rotational position of the shaft. The position signal may be a varying voltage which indicates the rotational position of the shaft 13. The position signal may be a data signal which indicates the rotational position of the shaft 13. The rotational position of the shaft 13 is as measured by the position sensor 18. This rotational position is at a second location 19 along the shaft. This rotational position is a second rotational position of the shaft 13. The second location 19 may be remote from the first location 17. The second location 19 may be located away from the drive components 16 of the electric motor 3. The second location 19 may be located towards the drive output 15 of the electric motor 3. The second location 19 may be located along the shaft 3 between the drive components 16 and the drive output 15. Having two ways of measuring the rotational position of the shaft 13 is useful as it permits calibration of the position sensor 18 and the ability to measure the torque generated by the electric motor 3. The methods by which the motor control unit 2 undertakes these functions will now be described with reference to figures 2 and 3. Figure 2 shows a flow diagram of the method of calibrating the position sensor 18. The method may be undertaken by the control unit 2. As shown in 21, the method comprises injecting a high frequency signal into the electric motor. This injection uses HFSI as described herein. The high frequency signal may be a high frequency low voltage signal. Injecting the high frequency signal into the electric motor may comprise injecting the high frequency signal at an injection angle and varying the injection angle of the high frequency signal as described herein. As shown in 22, the method further comprises processing a response signal generated by the electric motor in response to the high frequency signal. The method may comprise receiving the response signal and then processing the response signal. The response signal is processed to determine a first rotational position of the shaft at a first time. The processing may involve measuring the impedance of the electric motor to determine the first rotational position as described herein. The measuring of the impedance of the electric motor may involve measuring the impedance of the drive components 16 of the electric motor 3. As shown in 23, the method further comprises receiving a second rotational position of the shaft. The second rotational position is received from the position sensor 18. As described herein, the position sensor 18 outputs a position signal and so receiving the second rotational position of the shaft may comprise receiving the position signal. The second rotational position is also as measured at the first time. It will be appreciated that there may be some slight offset between the time at which the first rotational position is measured and when the second rotational position is measured. However, it is advantageous for this difference to be as small as possible so that the method is as accurate as possible. As shown in 24, the method further comprises determining a position offset between the first rotational position and the second rotational position. The position offset gives the error in the position determined by the position sensor 18. As the position sensor 18 may not have been installed precisely aligned with the orientation of the drive components 16 of the electric motor 3, there may be an offset between the position measured by the position sensor and actual position of the drive components 16. This leads to an offset between the rotational position of shaft 3 as measured by the position sensor 18 and the actual rotational position of the shaft 3 which can be determined using HFSI. The position offset may be a value of the angle between the first rotational position and the second rotational position. The position offset may be the rotational difference between the first rotational position and the second rotational position. As shown in 25, the method further comprises controlling the electrical excitation of the electric motor 3 in dependence on subsequent rotational positions detected by the position sensor 18 and the position offset. Subsequent rotational positions may be received from the position sensor 18 and used to control the electric motor 3. The electric motor 3 may be controlled in dependence on the position sensor 18 so that the drive components 16 of the electric motor 3 are driven in the correct manner to give the desired operation of the electric motor. The rotational positions received from the position sensor may be corrected by the position offset before being used as the actual rotational position of the shaft 13. The rotational positions received from the position sensor may be adjusted by the position offset. Figure 3 shows a flow diagram of the method of calculating the torque produced by the electric motor. The method may be undertaken by the control unit 2. As shown in 31, the method comprises processing a response signal that is generated by the electric motor 3. The response signal is processed to determine a third rotational position of the shaft at a first time. The response signal is generated by the electric motor in response to electrical input into the electric motor 3. The electrical input may be into the drive components 16 of the electric motor 3. The electrical input can take different forms. The electric input may be the electrical excitation of the electric motor which causes the shaft to rotate. In this case, the response signal may be back-EMF generated by the electric motor as herein described. The back-EMF may be processed to derive the third rotational position of the electric motor 3 as herein described. The method may therefore comprise electrically exciting the electric motor 3 to cause rotation of the shaft 3. The electric input may be a high frequency signal. The method may therefore comprise injecting the high frequency signal into the electric motor as herein described. This injection uses HFSI as described herein. The high frequency signal may be a high frequency low voltage signal. Injecting the high frequency signal into the electric motor may comprise injecting the high frequency signal at an injection angle and varying the injection angle of the high frequency signal as described herein. The processing may involve measuring the impedance of the electric motor to determine the third rotational position as described herein. The measuring of the impedance of the electric motor may involve measuring the impedance of the drive components 16 of the electric motor 3. The electrical input may be a combination of the electrical excitation and the high frequency signal. As shown in 32, the method further comprises receiving a receiving a fourth rotational position of the shaft. The fourth rotational position is received from the position sensor 18. As described herein, the position sensor 18 outputs a position signal and so receiving the fourth rotational position of the shaft may comprise receiving the position signal. The fourth rotational position is also as measured at the first time. It will be appreciated that there may be some slight offset between the time at which the third rotational position is measured and when the fourth rotational position is measured. However, it is advantageous for this difference to be as small as possible so that the method is as accurate as possible. As shown in 33, the method further comprises determining a position offset between the third rotational position and the fourth rotational position. When the electric motor is being driven, and so the shaft is rotating, the electric motor generates a torque on the drive output 15. As the shaft has an amount of torsional flexibility, the shaft will twist along its length. This twist is due to the torque being generated by the electric motor. The twist means that there can be an offset between the third rotational position and the fourth rotational position. This offset can be a twist angle. The twist angle is a value for the angular difference between the third rotational position and the fourth rotational position. As shown in 34, the method further comprises determining a torque generated by the electric motor based on the twist angle. The twist angle is proportional to the applied torque on the shaft 13. Therefore, the twist angle can be used to calculate the torque being generated by the electric motor 3 at a given time point. The twist angle will also be dependent on the distance, also known as the length, between the first location and the second location. Therefore, the torque may also be calculated in dependence on that distance. The twist angle may be calculated using the following equation: where 0 is the twist angle, T is the torque generated by the electric motor, L is the length between the first location and the second location along the shaft, G is the modulus of rigidity of the shaft, and J is the polar moment of inertia of the shaft. The modulus of rigidity and the polar moment of inertia are properties of the shaft and so are constants associated with a particular shaft. Therefore, if the twist angle and the length between the first and second locations is known then the torque may be calculated based on dividing the twist angle by the distance between the first location and the second location. Thus, determining a torque generated by the electric motor based on the torque angle may comprise determining the torque based dividing the torque angle by the distance between the first location and the second location. The determined torque of the electric motor 3 can be used by the control unit 2 to control the electrical excitation of the electric motor 3. The control unit 2 may be commanded to cause the electric motor 3 to generate a particular torque at a given time. The control unit 2 can therefore use the calculated torque in determining the electrical excitation that should be input into the electric motor 3 at a given time. The two methods herein described may be used together. The calibration method may be used to set the position offset then the torque of the electric motor during operation may be calculated. The two methods are advantageous because they improve the control of an electric motor by providing higher accuracy data to the control unit 2. The applicant hereby discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the present specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the present invention may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims

1. A method for controlling an electric motor, the electric motor comprising a shaft and a position sensor attached to the shaft, the shaft being caused to rotate by electrical excitation of the electric motor and the position sensor being configured to detect a rotational position of the shaft, the method comprising:injecting a high frequency signal into the electric motor;processing a first response signal, generated by the electric motor in response to the high frequency signal, to determine a first rotational position of the shaft at a first time;receiving a second rotational position of the shaft from the position sensor, the second rotational position indicating the rotational position of the shaft at the first time as measured by the position sensor;determining a position offset between the first rotational position and the second rotational position measured by the position sensor; andcontrolling the electrical excitation of the electric motor to rotate the shaft in dependence on subsequent rotational positions detected by the position sensor and the position offset.

2. A method according to claim 1, wherein the high frequency signal is a high frequency low voltage signal.

3. A method according to claim 1 or 2, wherein injecting the high frequency signal into the electric motor comprises injecting the high frequency signal at an injection angle and varying the injection angle; and processing a first response signal comprises measuring impedance of the electric motor to determine the first rotational position.

4. A method according to any preceding claim, wherein controlling the electrical excitation of the electric motor to rotate the shaft in dependence on subsequent rotational positions detected by the position sensor and the position offset comprises correcting the rotational positions detected by the position sensor based on the position offset.

5. A method according to claim 4, wherein correcting the rotation positions detected by the position sensor based on the position offset comprises adjusting the rotational positions by the position offset.

6. A method according to any preceding claim, wherein the position offset is the rotational difference between the first rotational position and the second rotational position.

7. A method according to any preceding claim, the method comprising: processing a second response signal, generated by the electric motor in response to electrical input into the electric motor, to determine a third rotational position of the shaft at a first time;receiving a fourth rotational position of the shaft from the position sensor, the fourth rotational position indicating the rotational position of the shaft at the first time as measured by the position sensor;determining a twist angle between the third rotational position and the fourth rotational position; anddetermining a torque generated by the electric motor based on the twist angle.

8. A method for controlling an electric motor, the electric motor comprising a shaft and a position sensor attached to the shaft, the shaft being caused to rotate by electrical excitation of the electric motor and the position sensor being configured to detect a rotational position of the shaft, the method comprising:processing a second response signal, generated by the electric motor in response to electrical input into the electric motor, to determine a third rotational position of the shaft at a first time;receiving a fourth rotational position of the shaft from the position sensor, the fourth rotational position indicating the rotational position of the shaft at the first time as measured by the position sensor;determining a twist angle between the third rotational position and the fourth rotational position; anddetermining a torque generated by the electric motor based on the twist angle.

9. A method according to claim 7 or 8, wherein the electrical input is electrical excitation of the electric motor to cause the shaft to rotate.

10. A method according to any of claims 7 to 9, wherein the second response signal is measured back-EMF generated by the electric motor in response to the electrical excitation of the electric motor.

11. A method according to any of claims 7 to 10, wherein the electrical input is a high frequency signal; and the method comprises injecting the high frequency signal into the electric motor.

12. A method according to claim 11, wherein the high frequency signal is a high frequency low voltage signal.

13. A method according to claim 11 or 12, wherein injecting the high frequency signal into the electric motor comprises injecting the high frequency signal at an injection angle and varying the injection angle; and processing a second response signal comprises measuring impedance of the electric motor to determine the third rotational position.

14. A method according to any of claims 7 to 13, wherein the electric motor comprises a plurality of drive components to cause the shaft to rotate that are located at a first location on the shaft, the third rotational position is measured at the first location, and the position sensor measures the fourth rotational position at a second location on the shaft that is remote from the first location along the shaft.

15. A method according to claim 14, wherein the shaft has a length between the first location and the second location and the method comprises determining a torque generated by the electric motor based on the twist angle and the length.

16. A method according to any of claims 7 to 15, the method comprising controlling the electrical excitation of the electric motor to rotate the shaft in dependence on the torque generated by the electric motor.

17. A controller for an electric motor, the electric motor comprising a shaft and a position sensor attached to the shaft, the shaft being caused to rotate by electrical excitation of the electric motor and the position sensor being configured to detect a rotational position of the shaft, the controller being configured to:inject a high frequency signal into the electric motor;process a first response signal, generated by the electric motor in response to the high frequency signal, to determine a first rotational position of the shaft at a first time;receive a second rotational position of the electric motor from the position sensor, the second rotational position indicating the rotational position of the shaft at the first time as measured by the position sensor;determine a position offset between the first rotational position and the second rotational position measured by the position sensor; andcontrol the electrical excitation of the electric motor to rotate the shaft in dependence on subsequent rotational positions detected by the position sensor and the position offset.

18. A controller for an electric motor, the electric motor comprising a shaft and a position sensor attached to the shaft, the shaft being caused to rotate by electrical excitation of the electric motor and the position sensor being configured to detect a rotational position of the shaft, the controller being configured to:process a second response signal, generated by the electric motor in response to electrical input into the electric motor, to determine a third rotational position of the shaft at a first time;receive a fourth rotational position of the electric motor from the position sensor, the fourth rotational position indicating the rotational position of the shaft at the first time as measured by the position sensor;determine a twist angle between the first rotational position and the fourth rotational position; anddetermine a torque generated by the electric motor based on the twist angle.

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