SPACE VECTOR PULSE WIDTH MODULATION FOR MULTI-PHASE MACHINES

Abstract

Control system (24) for controlling the operation of a multiphase electrical machine (19), comprising: a signal generator (210) configured to generate a number n of duty cycle signals of a pulse width modulation (PWM) scheme as a combination of a continuous PWM scheme and a non-continuous PWM scheme, the combination being based on a value of a mixing coefficient Σ; wherein the signal generator (210) is configured to use each of the duty cycle signals to control the switching of a DC power to a corresponding phase of the multiphase electrical machine (19); wherein the signal generator (210) is configured to use a mixing function to determine a relative weighting of both the continuous PWM scheme and the non-continuous PWM scheme to generate the number n of duty cycle signals;wherein the mixing function is calculated on the basis of a difference between the value of the mixing coefficient γ and two thresholds a1, a2; and wherein the duty cycle signals are generated via the combination of the continuous PWM scheme and the discontinuous PWM scheme, based on the fact that the value of the mixing coefficient γ is greater than or equal to a first threshold a1 and less than a second threshold a2, where the second threshold a2 is greater than the first threshold a1; characterized in that n is an integer greater than 3 and that the mixing function determines the mixing coefficient γ as a function of the modulation index mi and a phase current Is using the following formula: γ(mi,Is)={0.50≤γ; <a10.5×(1−&lgr;−a1a2−a1)a1≤&lgr;<a20a2≤&lgr;<1wobei &lgr; eine Zwischenvariable ist, die definiert ist durch:&lgr;(mi,Is)=b⋅IsIsmax+(1−b)⋅mi,0≤b≤1 where b is a tunable constant and Ismax is the maximum current of the multiphase electric machine (19).;

Description

BACKGROUND The present application relates generally to methods and systems for controlling electrical machines and in particular to methods and systems for generating duty cycle signals used to control an n-phase alternating current (AC) machine, where n is an integer greater than 3. A control unit typically controls an AC machine motor by generating duty cycle signals for each motor phase, for example, using pulse width modulation (PWM) techniques to supply phase voltage signals to the motor. Electric motors are generally controlled by a feedback system that includes a current regulator and a modulator using a sinusoidal PWM scheme to generate gate driver signals and send them to a three-phase inverter (DC-AC converter). The inverter provides voltage signals for each phase of the electric motor. The voltages generated by the inverter depend heavily on the inverter control scheme used, and suboptimal techniques can lead to undesirable torque ripple and audible noise in high-power applications such as electric power steering (EPS). Redundancy is required in safety-critical applications, such as motors used in EPS systems. Typically, two or more three-phase machines are used in conventional safety-critical applications to ensure the necessary redundancy. Multi-phase AC machines, i.e., machines with more than three phases, have significant potential to replace the currently ubiquitous electric motor drive configurations that provide redundancy in safety-critical applications. A system for controlling the operation of a multiphase electrical machine according to the preamble of claim 1 is disclosed in DE 10 2017 127 780 A1. Further prior art is known from O. Ojo (“The Generalized Discontinuous PWM Scheme for Three-Phase Voltage Source Inverters”, in IEEE Transactions on Industrial Electronics, Vol. 51, No. 6, December 2004, pp. 1280-1289), DE 10 2011 004 817 A1, J. Prieto et al. (“Comparative Analysis of Discontinuous and Continuous PWM Techniques in VSI-Fed Five-Phase Induction Motor”, in IEEE Transactions on Industrial Electronics, Vol. 58, No. 12, 2011, pp. 5324-5335) and US 2015 / 0280619 A1. SUMMARY One objective of the invention is to create an improved control system and control method for a multi-phase electrical machine. To solve the problem, a control system with the features of claim 1 and a control method with the features of claim 7 are provided. Further developments of the invention are the subject of the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS Advantages and further features of the invention will become apparent from the following detailed description in conjunction with the accompanying drawings, in which: Fig. 1 shows a block diagram of an exemplary embodiment of an electric power steering system according to aspects of the present disclosure; Fig. 2 shows a block diagram of a control unit for a five-phase permanent magnet synchronous motor according to aspects of the present disclosure; Fig. 3 shows a block diagram of a DC link inverter for a five-phase permanent magnet synchronous motor according to aspects of the present disclosure; Fig. 4 shows a diagram of five-phase duty cycle waveforms of a discontinuous PWM minimum (DPWMMIN) scheme according to aspects of the present disclosure; Fig. 5 shows a diagram of five-phase duty cycle waveforms of a continuous PWM (CPWM) scheme according to aspects of the present disclosure; Fig.Figure 6 shows a diagram of five-phase duty cycle waveforms of a combination of continuous and discontinuous PWM schemes according to aspects of the present disclosure; Figure 7 is a diagram illustrating an example of a mixing function for combining a continuous and discontinuous PWM scheme according to aspects of the present disclosure; and Figure 8 shows a flowchart of a method for controlling a five-phase motor in a steering system according to aspects of the present disclosure. DETAILED DESCRIPTION The terms module and submodule used here refer to one or more processing circuits such as an application-specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or grouped) with memory that executes one or more software or firmware programs, a combinational logic circuit, and / or other suitable components that provide the described functionality. As can be seen, the submodules described below can be combined and / or further subdivided. Now, with reference to the figures, in which the technical solutions are described with reference to specific embodiments without limiting them, Fig. 1 is an exemplary embodiment of an electric power steering (EPS) system 40 suitable for implementing the disclosed embodiments. The steering mechanism 36 is a rack-and-pinion system and comprises a rack (not shown) within a housing 50 and a pinion (also not shown) located beneath a gearbox housing 52. When the operator input, hereinafter referred to as the steering wheel 26 (e.g., handwheel and the like), is turned, the upper steering shaft 29 rotates, and the lower steering shaft 51, which is connected to the upper steering shaft 29 via a universal joint 34, rotates the pinion.The rotation of the pinion moves the rack, which moves the tie rods 38 (only one is shown), which in turn move the steering knuckles 39 (only one is shown), which turn or steer one or more steerable wheels 44 (only one is shown). Electric power steering assistance is provided by the control device, generally designated by reference numeral 24, which comprises the control unit 16 and an electric machine 19, which could be a permanent magnet synchronous motor (PMSM) and is hereinafter referred to as the electric machine 19. The control unit 16 is powered by the vehicle power supply 10 via a line 12. The control unit 16 receives a vehicle speed signal 14, representative of the vehicle speed, from a vehicle speed sensor 17. A steering angle is measured by a position sensor 32, which may be an optical coding sensor, a variable resistance sensor, or any other suitable type of position sensor, and provides a position signal 20 to the control unit 16. The engine speed may be measured by a speedometer or other device and transmitted to the control unit 16 as an engine speed signal 21.A motor speed, denoted ωm, can be measured, calculated, or determined by a combination of these methods. For example, the motor speed ω can be calculated as the change in motor position θ, as measured by a position sensor 32 over a prescribed time interval. Alternatively, the motor speed ω can be determined as the derivative of the motor position θ from the equation ωm = Δθ / Δt, where Δt is the sampling time and Δθ is the change in position during the sampling interval. The motor speed can also be derived from the motor position as the rate of change of position over time. It should be noted that numerous known methods exist for performing a derivative function. When the steering wheel 26 is turned, a torque sensor 28 detects the torque applied to the steering wheel 26 by the driver. The torque sensor 28 can include a torsion bar (not shown) and a variable resistance sensor (also not shown), which outputs a variable torque signal 18 to the control unit 16 depending on the degree of twisting of the torsion bar. Although this is one type of torque sensor, any other suitable torque sensor device using known signal processing techniques is sufficient. In response to the various input signals, the control unit sends a command 22 to the electric motor 19, which provides torque assistance to the steering system via a worm gear 47 and a worm wheel 48, thus providing torque assistance for the vehicle steering. It should be noted that although the disclosed embodiments are described by reference to a motor control system for electric steering applications, these references are for illustrative purposes only, and the disclosed embodiments can be applied to any motor control application with an electric motor, such as steering, valve control, and the like. Furthermore, the references and descriptions contained herein can apply to many types of parameter sensors, including, but not limited to, torque, position, speed, and the like. It should also be noted that, for the sake of brevity and simplicity, references to electrical machines, including but not limited to motors, will henceforth refer exclusively to motors. In the depicted control system 24, the control unit 16 uses the torque, position, speed, and similar parameters to calculate one or more commands to deliver the required output power. The control unit 16 communicates with the various systems and sensors of the motor control system. The control unit 16 receives signals from each of the system sensors, quantifies the received information, and in response delivers one or more output command signals, in this case, for example, to the motor 19. The control unit 16 is configured to develop the appropriate voltage(s) from the inverter (not shown), which can optionally be integrated into the control unit 16 and is referred to here as the control unit 16, so that when applied to the motor 19, the desired torque or position is generated.In one or more examples, the control unit 16 operates in a feedback control mode as a current controller to generate command 22. Alternatively, in one or more examples, the control unit 16 operates in a feedforward control mode to generate command 22. Since these voltages are related to the position and speed of the motor 19 and the desired torque, the position and / or speed of the rotor and the torque applied by an operator are determined. A position sensor is connected to the lower steering shaft 51 to detect the angular position θ. The sensor can detect the rotational position based on optical sensing, changes in the magnetic field, or other methods. Typical position sensors include potentiometers, resolvers, synchros, encoders, and the like, as well as combinations containing at least one of the aforementioned elements.The position sensor outputs a position signal 20, which indicates the angular position of the lower steering shaft 51 and thus of the motor 19. The desired torque can be determined by one or more torque sensors 28 that transmit torque signals 18 indicating an applied torque. One or more exemplary embodiments comprise such a torque sensor 28 and the torque signal(s) 18 thereof, which can respond to a compliant torsion bar, T-bar, spring, or similar device (not shown) configured to provide a response indicating the applied torque. In one or more examples, one or more temperature sensors 23 are located on the electric machine 19. Preferably, the temperature sensor 23 is configured to directly measure the temperature of the monitored section of the motor 19. The temperature sensor 23 transmits a temperature signal 25 to the control unit 16 to enable the processing and compensation described herein. Typical temperature sensors are thermocouples, thermistors, thermostats, and the like, as well as combinations of at least one of the aforementioned sensors, which, when appropriately arranged, provide a calibratable signal proportional to the respective temperature. The control unit 16 receives, among other things, the position signal 20, the speed signal 21, and one or more torque signals 18. The control unit 16 processes all input signals to generate values ​​corresponding to each signal, so that a rotor position value, a motor speed value, and a torque value are available for processing in the algorithms described here. Measurement signals, such as those mentioned above, are also typically linearized, compensated, and filtered as desired to improve the characteristics or eliminate undesired characteristics of the acquired signal. For example, the signals can be linearized to improve processing speed or to address a large dynamic range of the signal. In addition, frequency- or time-based compensation and filtering can be used to eliminate noise or avoid undesirable spectral characteristics. To perform the prescribed functions and the desired processing, as well as the corresponding calculations (e.g., identification of engine parameters, control algorithm(s), and the like), the control unit 16 may include, among other things, one or more processors, computers, DSPs, main memory, mass storage, registers, timers, interrupts, communication interfaces, and input / output signal interfaces, as well as combinations of at least one of the aforementioned. For example, the control unit 16 may include input signal processing and filtering to enable accurate sampling and conversion or acquisition of such signals from communication interfaces. Further features of the control unit 16 and certain processes it contains will be discussed in detail later. The need for fault tolerance in safety-critical applications such as EPS systems is constantly increasing. Double-wound, three-phase permanent magnet synchronous machines (PMSMs) are currently used to provide fault tolerance in the event of failures in the electrical actuator used in EPS systems. These machines are typically combined with two inverters and / or microcontrollers to ensure full electrical redundancy. However, if half of the system fails, the total support that can be provided is reduced by half. This can only be increased slightly if the remaining half of the system is overloaded. A potential alternative to double-winding machines are multiphase synchronous machines (PMSMs falling into the broader category of synchronous machines). These machines consist of a number n phases, where n is an integer greater than three, and each phase can be controlled independently. Therefore, the overall system capacity in the event of single-point inverter or machine failures is much higher than in electrical drive systems based on double-winding machines. However, very little work has been done on the control of power converters used with multiphase machines, so such machines cannot be readily used for industrial applications. The basic sinusoidal PMW (SPWM) commutation technique, which has a relatively low voltage utilization, is currently typically used for multiphase drive systems. Fig. 2 shows a block diagram of a five-phase PMSM control unit 16 according to aspects of this disclosure. However, the PMSM control unit 16 and the motor 19 can have any number n of phases, where n is an integer greater than 3. In other words, the system and method of this disclosure can be used with a control unit and / or an electrical machine, such as a motor, having a number n of 4, 5, 6, 7, 8, or 9 phases. A current command module (e.g., the current reference computer 202) converts the torque command Tc into current commands, which can take the form of d / q current references Idqc. These current commands are then sent to a current controller module (e.g., the current controller 206). The current controller 206 can be a feedback controller that uses current measurements or a feedforward compensator.The temperature T can be fed into a parameter compensator 204 to adjust the electrical parameters of the PMSM as needed. The current controller 206 then generates the d / q voltage command corresponding to a commanded modulation index mi and phase advance angle δ. In other words, the current controller module generates the modulation index mi and phase advance angle δ based on the current commands. The modulation index mi and phase advance angle δ are converted by the duty cycle generator 210 of the converter commutation module 208 into an equivalent duty cycle dph for each phase. Once the equivalent duty cycle dph for each phase is generated, the pulse width modulator 212 then generates the on-times tph for the various switches (e.g., FETs) in the phase arms of the converter (e.g., the inverter 216). The inverter 216, which is powered by a source 214, then supplies the desired voltages to the motor (e.g., the PMSM 19), which generates currents Iph and an electromagnetic torque Te. The position θ and the currents Iph are then measured and fed back to the position measuring device 220 and the current measuring device 222 of the control system, respectively, to close the control loop. The measured position θm is used with the phase currents to calculate the measured d / q currents Idqm, which are used for current control. A voltage-source inverter (VSI) is typically used as a power converter in PMSM-based electric drives. While the design and structure of the actual machine differ for three-phase and multi-phase (more than three phases) PMSMs, the same principle of vector control in the synchronous or d / q reference frame is possible for all these machines by using the appropriate transformation matrices to convert phase currents and voltages into the equivalent DC quantities in the synchronous frame. This disclosure focuses on various techniques for commutation of the converter (e.g., inverter 216) for a multi-phase electric machine 19. In some exemplary embodiments, the multi-phase electric machine 19 is a permanent magnet synchronous motor (PMSM). However, the multi-phase electric machine 19 can be any type of motor or motor / generator with four or more phases. The multi-phase electric machine 19 can, for example, be aIt could be an induction motor, a reluctance motor, or a permanent magnet motor. Fig. 3 shows an example of the inverter 216 according to aspects of the present disclosure. Each of the five phases of the multiphase electrical machine 19 (e.g., the PMSM 19) is connected to a corresponding phase leg consisting of two switches. For example, the inverter 216 includes five phase legs 310, 320, 330, 340, 350, each with two switches (e.g., phase leg 310 includes switches 312, 314; phase leg 320 includes switches 322, 324; phase leg 330 includes switches 332, 334; phase leg 340 includes switches 342, 344; and phase leg 350 includes switches 352, 354). The duty cycles (equivalent to the on-times) of the upper and lower switches (e.g., switches 312 and 314 represent the upper and lower switches of phase leg 310, respectively) are denoted as dxb and dx', respectively, where x = a, b, c, d, e for the five phase legs. The phase-to-ground voltages are denoted as Vxg (e.g., Vag for phase leg 310, Vbg for phase leg 320, etc.). These voltages Vxg are controlled by the duty cycles calculated by the duty cycle generator 210 in Fig. 2. One of the simplest continuous PWM schemes for multiphase AC machines is the sinusoidal PWM technique (SPWM technique). The duty cycles for the SPWM scheme are mathematically expressed as follows: where mid is the modulation index, θr is the electrical position, n is the number of phases, and i = 0, 1, ... n-1. The SPWM scheme can generate sinusoidal voltages with relatively low harmonics. However, a disadvantage of the SPWM scheme is that the maximum AC output voltage is not optimal. For example, there are other PWM schemes that utilize the DC link more effectively to generate higher phase-to-phase output voltages than is possible with SPWM. Other PWM schemes, such as space vector PWM (SVPWM), which can operate continuously or discontinuously, can be used to supply alternating current to multiphase AC machines. SVPWM schemes have been shown to utilize the DC link more effectively than SPWM schemes. In other words, SVPWM schemes can deliver a higher phase-to-phase output voltage than SPWM schemes. This higher phase-to-phase output voltage can increase the torque-speed capacity of the multiphase machine at the same DC voltage. The SPWM scheme can be used to obtain the generalized expression of the duty cycle for the discontinuous PWM minimum technique (DPWMMIN technique), which is expressed in the following formula: The amplitude of the fundamental voltage with DPWMMIN is greater than 1. Fig. 4 shows a diagram 400 of five duty cycle waveforms A, B, C, D, E of a DPWMMIN scheme according to aspects of the present disclosure. Specifically, Fig. 4 shows duty cycle waveforms for each of the five phases A, B, C, D, E of the multiphase electrical machine 19, generated using the DPWMminimum (DPWMMIN) technique. The duty cycle waveforms for each of the DPWMMIN duty cycle waveforms A, B, C, D, E remain at zero for 1 / n of an electrical revolution, where n is the number of phases of the multiphase electrical machine 19. The generalized SVPWM can be obtained from DPWMMIN using the following equation: Fig. 5 shows a diagram 500 of five duty cycle waveforms A, B, C, D, E of a continuous PWM scheme (CPWM scheme) according to aspects of the present disclosure. In particular, Fig. 5 shows duty cycle waveforms for each of the five phases A, B, C, D, E of the multiphase electrical machine 19, which were generated using the continuous SVPWM scheme. The equation above represents a generic way to calculate CPWM from DPWMMIN, which can simplify the calculation compared to conventional methods. Additionally, by changing the coefficient in the equation above, a combined PWM scheme can be obtained that generates mixed PWM waveforms from CPWM and DPWMMIN. The combined adaptive PWM (APWM) can be written with the following equation: where α is the mixing coefficient, which has a value from 0.0 to 0.5. In some embodiments, a sinusoidal PWM scheme can be used to form the continuous PWM scheme within the combined PWM scheme. In some embodiments, a discontinuous minimum PWM scheme (DPWMMIN) can be used to form the non-discontinuous PWM scheme used in the combined PWM scheme. Alternatively or additionally, a discontinuous minimum PWM scheme with offset (DPWMMINO) can be used to form the non-discontinuous PWM scheme used in the combined PWM scheme. In some embodiments, two or more different schemes can be used for the continuous PWM scheme and / or the non-discontinuous PWM scheme within the combined PWM scheme. Fig. 6 shows a diagram 600 with five-phase SVPWM duty cycle waveforms for each of the five phases A, B, C, D, E of the multiphase electric machine 19, which were produced using the combined APWM scheme, which was mixed from CPWM and DPWMMIN. In particular, Fig. 6 shows the duty cycle waveforms for each of the five phases A, B, C, D, E of the multiphase electric machine 19, which were generated using the combined PWM scheme with a mixing coefficient α = 0.2. Continuous PWM schemes, such as SVPWM, and discontinuous PWM schemes, such as DPWMMIN, each have their own advantages under specific, non-overlapping operating conditions. By varying the value of the mixing coefficient α according to the different operating conditions, the combined PWM scheme can leverage the advantages of both PWMs under their favorable conditions and switch continuously and gradually between them. The mixing coefficient α can be constructed as a function to maximize the advantages of the two PWMs. One example is using the modulation index mi to determine the value of the mixing coefficient α. In some embodiments, the value of the mixing coefficient α is a function of the magnitude of a phase current supplied to or from the multiphase electric machine 19. In some embodiments, the value of the mixing coefficient α is a function of the rotational speed of the multiphase electric machine 19. The rotational speed of the multiphase electric machine 19 can be a mechanical speed, such as the speed of an output shaft. Alternatively, the rotational speed of the multiphase electric machine 19 can be an electrical speed, such as the rotational speed of a rotating magnetic field in a stator of the multiphase electric machine 19. In some embodiments, the value of the mixing coefficient α is a function of a torque of the multiphase electric machine 19.For example, the mixing coefficient can be a function of a target torque or of an actual torque generated by the multiphase electric machine 19. In some embodiments, the mixing coefficient α can be determined using a piecewise function. An example of such a piecewise function is shown below. In the exemplary piecewise function, the mixing coefficient α is a constant when the modulation index mi is outside the range defined by a1 and a2, and inversely proportional to the modulation index mi when the modulation index mi is within the range. Using such a piecewise linear function to determine the value of the mixing coefficient α allows for a smooth transition from SVPWM to DPWMMIN with only a few additional calculations. An example of this function is as follows: In some embodiments, a mixing function is used to determine relative weights of the continuous PWM scheme and the non-continuous PWM scheme, which are used to generate the number n of duty cycle signals. A signal generator (e.g., the duty cycle generator 210) can execute the mixing function to generate a mixed PWM scheme to produce the number n of duty cycle signals. In some embodiments, the mixing function is a piecewise function. Alternatively or additionally, the mixing function can be a continuous function of one or more different variables, such as modulation index mi, phase current Is, torque of the multiphase electric machine 19, and / or speed of the multiphase electric machine 19. Figure 7 is a diagram illustrating an exemplary mixing function for the combination of a continuous and a non-continuous PWM scheme according to aspects of this disclosure. In particular, Figure 7 illustrates a piecewise mixing function using the mixing coefficient α. Curve 700 represents the percentage or weighting of a continuous PWM scheme as a function of the mixing coefficient α, and curve 702 represents the percentage or weighting of a non-continuous PWM scheme as a function of the mixing coefficient α. As shown, the continuous PWM scheme (e.g., the SVPWM scheme) dominates the mixing coefficient α in a relatively low range of values. This low range is shown to lie between zero and a first threshold value a1.If the mixing coefficient α lies in an intermediate range between the first threshold a1 and a larger second threshold a2, the continuous PWM scheme is mixed with the non-continuous PWM scheme (e.g. the DPWMMIN scheme), so that with increasing mixing coefficient α, the relative weight of the non-continuous PWM scheme increases until the non-continuous PWM scheme dominates at values ​​of the mixing coefficient α that are greater than or equal to the second threshold a2. In other words, the exemplary mixing function is calculated based on the difference between the mixing coefficient α and the two thresholds a1 and a2, where the second threshold a2 is greater than the first threshold a1. In some embodiments, each of the duty cycle signals is generated via the continuous PWM scheme, based on the value of the mixing coefficient α being less than the first threshold a1. In some embodiments, each of the duty cycle signals is generated via the discontinuous PWM scheme, based on the value of the mixing coefficient α being greater than or equal to a second threshold a2 that is greater than the first threshold a1.In some embodiments, the duty cycle signals are generated via the combination of the continuous PWM scheme and the non-continuous PWM scheme, based on the fact that the value of the mixing coefficient α is greater than or equal to the first threshold a1 and less than the second threshold a2. In some embodiments, the mixing function is a function of two or more variables. In one example, the mixing function determines the mixing coefficient α as a function of the modulation index mi and a phase current Is using the following formula: where λ is an intermediate variable defined by: where b is a tunable constant and Ismax is the maximum current of motor 19. Fig. 8 illustrates a flowchart of a method 800 for controlling a multiphase electrical machine, such as an AC motor, according to aspects of the present disclosure. The method can be implemented, for example, in an EPS system 40, as shown in Fig. 1, or in any other suitable machine or system. Fig. 4 will now be described with reference to elements from Fig. 1 and / or 2. In block 802, a processing device, such as the duty cycle generator 210, generates a number n of duty cycle signals using a pulse width modulation (PWM) scheme, where n is an integer greater than 3. The PWM scheme used, which can be called a combined PWM scheme, is a combination of a continuous PWM scheme and a non-continuous PWM scheme, the combination being based on the value of a mixing coefficient α. More precisely, the value of the mixing coefficient α determines the weighting, or amount, that both the continuous PWM scheme and the non-continuous PWM scheme have within the combined PWM scheme and that are used to generate the duty cycle signals. In some embodiments, the continuous PWM scheme used in block 802 is a sinusoidal PWM scheme. In some embodiments, the non-continuous PWM scheme used in block 802 is a discontinuous PWM minimum scheme (DPWMMIN) or a discontinuous PWM minimum scheme with offset (DPWMMINO). In block 804, a power converter, such as the inverter 216, switches the power to each of the n phases of the multiphase electrical machine 19 using an associated duty cycle signal. In some embodiments, the method 800 for controlling the multiphase electric machine 19 also includes receiving a torque command and outputting a current command by a current command module, such as the current controller 206. The method may further include generating a modulation index value based on the current command. In some embodiments, the method includes determining the value of the mixing coefficient α as a function of the modulation index value mi. In some embodiments, the value of the mixing coefficient α is equal to the value of the modulation index mi multiplied by a scaling constant. In some embodiments, the scaling constant is between 0.5 and 1.0. More precisely, in some embodiments, the scaling constant may be equal to 0.5, so that the mixing coefficient α corresponds to 0.5 multiplied by the modulation index m. In some embodiments, the method 800 further comprises determining the relative weights of the continuous PWM scheme and the non-continuous PWM scheme using a mixing function based on a difference between the value of the mixing coefficient and two threshold values. In some embodiments, and as shown in Fig. 7, the mixing function is calculated based on the difference between the mixing coefficient α and the two thresholds a1 and a2, where the second threshold a2 is greater than the first threshold a1. In some embodiments, each of the duty cycle signals is generated via the continuous PWM scheme, based on the value of the mixing coefficient α being less than the first threshold a1. In some embodiments, each of the duty cycle signals is generated via the discontinuous PWM scheme, based on the value of the mixing coefficient α being greater than or equal to a second threshold a2 that is greater than the first threshold a1.In some embodiments, the duty cycle signals are generated via the combination of the continuous PWM scheme and the non-continuous PWM scheme, based on the fact that the value of the mixing coefficient α is greater than or equal to the first threshold a1 and less than the second threshold a2. In some embodiments, the value of the mixing coefficient α is calculated based on the difference between the value of the modulation index mi and the two thresholds a1 and a2. In some embodiments, each of the duty cycle signals is generated using the continuous PWM scheme, based on the fact that the value of the modulation index mi is less than the first threshold a1. In some embodiments, each of the duty cycle signals is generated using the discontinuous PWM scheme, based on the fact that the value of the modulation index mi is greater than or equal to the second threshold a2, which is greater than the first threshold a1. In some embodiments, the duty cycle signals are generated using a combination of the continuous and discontinuous PWM schemes, based on the fact that the value of the modulation index mi is greater than or equal to the first threshold a1 and less than the second threshold a2. In some embodiments, the combination is calculated based on the following relationship: where dAPWM is one of the duty cycle signals, dDPWMMIN is a duty cycle calculated based on the non-continuous PWM scheme, and max(dDPWMMIN) is a maximum value of the duty cycle, and α is the mixing coefficient. In some embodiments, the multiphase electric machine 19 is configured to exert a supporting torque on a vehicle's steering system and / or to control the vehicle's steering system.

Claims

Control system (24) for controlling the operation of a multiphase electrical machine (19), comprising: a signal generator (210) configured to generate a number n of duty cycle signals of a pulse width modulation (PWM) scheme as a combination of a continuous PWM scheme and a non-continuous PWM scheme, the combination being based on a value of a mixing coefficient α; wherein the signal generator (210) is configured to use each of the duty cycle signals to control the switching of a DC power to a corresponding phase of the multiphase electrical machine (19); wherein the signal generator (210) is configured to use a mixing function to determine a relative weighting of both the continuous PWM scheme and the non-continuous PWM scheme to generate the number n of duty cycle signals;where the mixing function is calculated on the basis of a difference between the value of the mixing coefficient α and two threshold values ​​a1, a2; and wherein the duty cycle signals are generated via the combination of the continuous PWM scheme and the non-continuous PWM scheme, based on the fact that the value of the mixing coefficient α is greater than or equal to a first threshold a1 and less than a second threshold a2, where the second threshold a2 is greater than the first threshold a1; characterized in that n is an integer greater than 3 and that the mixing function determines the mixing coefficient α as a function of the modulation index mi and a phase current Is using the following formula: α ( mi , Is ) = { 0.5 0 ≤ λ < a 1 0.5 × ( 1 − λ − a 1 a 2 − a 1 ) a 1 ≤ λ < a 2 0 a 2 ≤ λ < 1; where λ is an intermediate variable defined by: λ ( mi , I s ) = b ⋅ I s I smax + ( 1 − b ) ⋅ mi , 0 ≤ b ≤ 1 where b is a tunable constant and I smax the maximum current of the multiphase electric machine (19) is. Control system (24) according to claim 1, wherein the continuous PWM scheme is a sinusoidal PWM scheme. Control system (24) according to claim 1, wherein the non-continuous PWM scheme is a discontinuous PWM minimum scheme (DPWMMIN) or a discontinuous PWM minimum scheme with offset (DPWMMINO). Control system (24) of claim 1, further comprising: a current command module (202) configured to receive a torque command Tce and output a current command; a current control module (206) configured to generate the modulation index value mi on the basis of the current command. Control system (24) according to claim 1, wherein the duty cycle signals are generated via the continuous PWM scheme based on the fact that the value of the mixing coefficient α is less than a first threshold a1; and wherein the duty cycle signals are generated via the non-continuous PWM scheme based on the fact that the value of the mixing coefficient α is greater than or equal to a second threshold a2, which is greater than the first threshold a1. Control system (24) according to claim 1, wherein the multiphase electric machine (19) is configured to perform at least one of the following functions: applying an assist torque to a steering system of a vehicle and controlling the steering system. Method for controlling a multiphase electrical machine (19), comprising: generating a number n of duty cycle signals by a signal generator (210) using a pulse width modulation (PWM) scheme which is a combination of a continuous PWM scheme and a non-continuous PWM scheme, the combination being based on a value of a mixing coefficient α; switching power to each of the n phases of the multiphase electrical machine (19) using an associated duty cycle signal;and Determining relative weights of the continuous PWM scheme and the non-continuous PWM scheme using a mixing function based on a difference between the value of the mixing coefficient α and two thresholds a1, a2, wherein the duty cycle signals are generated via the combination of the continuous PWM scheme and the non-continuous PWM scheme, based on the fact that the value of the mixing coefficient α is greater than or equal to a first threshold a1 and less than a second threshold a2, where the second threshold a2 is greater than the first threshold a1; characterized in that n is an integer greater than 3 and that the mixing function determines the mixing coefficient α as a function of the modulation index mi and a phase current Is using the following formula: α ( mi , Is ) = { 0.5 0 ≤ λ < a 1 0.5 × ( 1 − λ − a 1 a 2 − a 1 ) a ≤ λ < a 2 0 a 2 ≤ λ < 1; where λ is an intermediate variable defined by: λ ( mi , I s ) = b ⋅ I s I smax + ( 1 − b ) ⋅ mi , 0 ≤ b ≤ 1 where b is a tunable constant and I smax the maximum current of the multiphase electric machine (19) is. Method according to claim 7, wherein the continuous PWM scheme is a sinusoidal PWM scheme. Method according to claim 7, wherein the non-continuous PWM scheme is a discontinuous PWM minimum scheme (DPWMMIN) or a discontinuous PWM minimum scheme with offset (DPWMMINO). Method according to claim 7, further comprising: Receiving a torque command Tc and outputting a current command by a current command module (202); Generating the modulation index value mi on the basis of the current command. The method of claim 7, wherein each of the duty cycle signals is generated via the continuous PWM scheme based on the fact that the value of the mixing coefficient α is less than a first threshold a1; and wherein each of the duty cycle signals is generated via the non-continuous PWM scheme based on the fact that the value of the mixing coefficient α is greater than or equal to a second threshold a2, which is greater than the first threshold a1. Method according to claim 7, wherein the multiphase electric machine (19) is configured to perform at least one of the following methods: applying an assist torque to a steering system of a vehicle and controlling the steering system.

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