SENSORLESS DETERMINATION OF THE ROTOR POSITION IN A PERMANENT MAGNET SYNCHRONOUS MOTOR

Description

[0001] The invention relates to a method for determining the rotor position of a three-phase permanent magnet synchronous motor, wherein the permanent magnet synchronous motor has, as a rotor-fixed reference system with respect to a d / q coordinate system having a d-axis and a q-axis, a d-axis inductance (Ld) and a q-axis inductance (Lq) that differs from the d-axis inductance (Ld), wherein each phase of the permanent magnet synchronous motor is assigned an inductance which changes depending on the rotor position, and wherein the rotor position is determined by means of the changing inductances.The invention further relates to a method for field-oriented control of a permanent magnet synchronous motor, in particular a permanent magnet synchronous motor used in a motor vehicle for an assistance system, wherein a rotor position of the permanent magnet synchronous motor necessary for field-oriented control is determined by means of a aforementioned method for determining a rotor position.

[0002] The invention further relates to a control unit for the operation of a permanent magnet synchronous motor, as well as a steering system with such a control unit.

[0003] A permanent magnet synchronous motor is essentially a combination of an induction motor and a brushless DC motor. Like a brushless DC motor, the permanent magnet synchronous motor comprises a permanent magnet rotor and coils on the stator. However, the stator's coil configuration results in a sinusoidal waveform of the back electromotive force (BEMF), giving the permanent magnet synchronous motor a greater resemblance to an induction motor in this respect. The various winding strands of the coils are designated, as is common in electrical machines, by U, V, and W.

[0004] Permanent magnet synchronous motors are frequently used in motor vehicles, particularly as servo motors. Their use is especially common in electromechanical power steering systems, but also in other vehicle assistance systems that incorporate an electric motor. For example, such permanent magnet synchronous motors can also be used to drive an air conditioning compressor. Since a vehicle's electrical system provides a direct current (DC) voltage, methods for controlling permanent magnet synchronous motors are typically based on pulse-width modulation (PWM), which allows for the electronic simulation of a multiphase AC system.

[0005] For controlling a permanent magnet synchronous motor, especially for field-oriented control (FOC), it is crucial to consider not only the rotational speed but also the rotor position. The rotor position must be determined for this purpose. It is known to use rotor position sensor units, which can include Hall effect sensors and angle sensors. For safety-critical applications, such as those found in the automotive sector, for example, in electromechanical power steering systems as disclosed in DE 10 2019 202 142 A1, reliable control of the synchronous motor must be guaranteed in all cases. For this reason, a third rotor position sensor is regularly used in such applications, which increases complexity and costs.

[0006] Another known method for determining rotor position is back EMF-based, also called BEMF-based. In this method, a high-frequency signal is applied to the base voltage, the resulting current is measured, and the rotor position is derived from this. A disadvantage of this method is that the high-frequency signal interferes with the base voltage signal. Another disadvantage is the requirement to measure the current waveform caused by the applied signal. Furthermore, the method is essentially only applicable once a predetermined rotational speed has been reached.

[0007] It is also known to derive the rotor position of a synchronous motor by determining the neutral point voltage. This method exploits the fact that the neutral point voltage depends on the inductance of the respective phase, which in turn depends on the rotor position. The PWM signal used serves as the excitation voltage. A prerequisite for this method is that the inductances in the d- and q-axes are different in the d / q coordinate system. Furthermore, the motor's neutral point must be connected to the associated control unit. To determine the neutral point voltage for different space vectors, a special DC bus-compatible pulse-width modulation must also be employed.

[0008] In HUA Y ET AL: "Improved sensorless control of a permanent magnet machine using fundamental pulse width modulation excitation", IET ELECTRIC POWER APPLICATIONS, Vol. 5, No. 4, April 1, 2011, a method for estimating the rotor position of permanent magnet synchronous motors is described. This method measures the derivative of the motor current in response to the standard pulse width modulation (PWM) sequence to estimate the rotor position. No additional sensors are required.

[0009] RAVIKUMAR SETTY A ET AL: "Comparison of high frequency signal injection techniques for rotor position estimation at low speed to standstill of PMSM", POWER ELECTRONICS (IICPE), 2012 IEEE 5TH INDIA INTERNATIONAL CONFERENCE ON, IEEE, December 6, 2012, further provides an overview of various approaches for determining rotor position using HFSI (high frequency signal injection) techniques. The common idea of ​​the described approaches is to apply a test voltage signal to the permanent magnet synchronous motor to excite the winding inductances. The current response is then used to determine position information.

[0010] A system for controlling an electric motor is also disclosed in WO 2020 / 161496 A1, wherein the motor has a rotor and stator windings. The system comprises a sensor arrangement with a first sensor that measures the current in one phase of the motor and with a second sensor that measures the rate of change of the current (dl / dt) in the phase of the motor. The system also includes a controller designed to drive the electric motor based on the feedback received from the sensor arrangement.

[0011] Against this background, it is an object of the present invention to provide a cost-effective solution for determining the rotor speed of a permanent magnet synchronous motor, which is preferably applicable over the entire operating range of the permanent magnet synchronous motor.

[0012] To solve this problem, a method for determining the rotor position of a three-phase permanent magnet synchronous motor, a method for field-oriented control of a permanent magnet synchronous motor, a control unit for operating a permanent magnet synchronous motor, and a steering system according to the independent claims are proposed. Further advantageous embodiments of the invention are described in the dependent claims and the description and are illustrated in the figures.

[0013] The proposed solution provides a method for determining the rotor position of a three-phase permanent magnet synchronous motor, wherein the permanent magnet synchronous motor, with respect to a d / q coordinate system having a d-axis and a q-axis as a rotor-fixed reference system, has a d-axis inductance (Ld) and a q-axis inductance (Lq) that differs from the d-axis inductance (Ld), wherein each phase of the permanent magnet synchronous motor is assigned an inductance that changes depending on the rotor position, and wherein the rotor position is determined by means of the changing inductances.According to the invention, a voltage is applied to the phases, the applied voltage inducing a ripple current for each phase. The ripple current is determined for at least two of the three phases, and the inductance of each phase is determined based on the ripple current determined for that phase. Advantageously, the inductance of each phase is determined according to the known relationship u(t) = L di(t) / dt, which describes the relationship between electrical voltage u(t) and the change in electrical current i(t) over time t for an inductance L. Since the proposed solution does not require sensors for detecting the rotor position, such as Hall sensors, it is advantageously less expensive to implement than a solution with a rotor position detection sensor unit.Furthermore, this solution advantageously increases system availability, as the rotor position can be detected over the entire working range of a permanent magnet synchronous motor using the proposed method.

[0014] Preferably, the permanent magnet synchronous motor is a motor used in a motor vehicle, in particular a motor used in a safety-related system of a motor vehicle, and further, in particular, a motor used in a steering system of a motor vehicle. In particular, the permanent magnet synchronous motor is a motor with a salient pole rotor.

[0015] Furthermore, it is specifically provided that only the voltage applied to the phases is used for the operation of the permanent magnet synchronous motor in the method according to the invention, and that no additional voltage superimposed on the applied voltage is applied. This advantageously prevents the applied voltage from being affected by a further superimposed voltage. In particular, it is provided that the voltage is applied as a pulse-width modulated signal, also called a PWM signal.

[0016] Advantageously, the permanent magnet synchronous motor is controlled by means of a PWM signal, wherein the PWM signal, particularly without superposition with another signal, is advantageously used to generate the ripple current. The PWM signal has a frequency of at least 10 kHz (kHz: kilohertz). In particular, it is provided that the permanent magnet synchronous motor is controlled by means of space vector modulation using voltage space vectors, wherein the applied voltage space vector is advantageously used to generate the ripple current.

[0017] For one phase of the permanent magnet synchronous motor, a ripple current is advantageously determined as a current caused by a ripple voltage, wherein the ripple voltage is advantageously derived from the voltage difference between an instantaneous voltage and an average phase voltage. The instantaneous voltage is determined by the corresponding voltage space vector.

[0018] The determination of the average phase voltage is provided in particular based on the phase duty cycles and / or an alpha-beta filter and / or the dq voltage vectors. Advantageously, the average phase voltage is determined to be the phase voltage corresponding to the respective harmonic current.

[0019] The ripple current is thus advantageously generated for each phase by the excitation voltage, which results from the voltage difference between the applied voltage space vector and the average phase voltage. This excitation voltage is derived from the pulse-width modulation-based space vector modulation used to control the permanent magnet synchronous motor and therefore advantageously does not need to be generated separately. This means that the generation and application of an additional voltage signal to an existing voltage is advantageously eliminated. Due to the high frequency at the inverter output for the space vector modulation, particularly above 10 kHz, and especially in the range of 12 kHz to 24 kHz, the excitation voltage is also advantageously high-frequency. In particular, an inverter output frequency of 16 kHz or 20 kHz is provided.The voltage space vectors exhibit, in particular, discrete voltage levels that interact with the phase voltage.

[0020] The instantaneous voltages for the respective phases can advantageously be determined, taking into account the inductance Lu, Lv, Lw assigned to each winding strand U, V, W and thus to each phase, as shown in the following table, where Ubat denotes the voltage of the main voltage source, and in the case of use of the method in a motor vehicle, in particular the voltage of the vehicle battery: Spannungsraumzeiger Phase U (SV_ph_U) Spannungsraumzeiger Phase V (SV_ph_V) Spannungsraumzeiger Phase W (SV_ph_W) Up Ubat* (Lu / (Lu + Lv x Lw)) SV_ph_U - Ubat SV_ph_U - Ubat Vp Ubat* (Lv / (Lv + Lu x Lw)) SV_ph_V - Ubat SV_ph_V - Ubat Wp Ubat* (Lw / (Lw + Lu x Lv)) SV_ph_W - Ubat SV_ph_W - Ubat Um -Ubat* (Lu / (Lu + Lv x Lw)) SV_ph_U +Ubat SV_ph_U + Ubat Vm -Ubat* (Lv / (Lv + Lu x Lw)) SV_ph_V + Ubat SV_ph_V + Ubat Wm -Ubat* (Lw / (Lw + Lu x Lv)) SV_ph_W + Ubat SV_ph_W + Ubat Zp oder Zm 0V 0 V 0 V Anmerkung: p = plus, m = minus, Z = Nullspannungsraumzeiger

[0021] According to a further advantageous embodiment of the method, a ripple current gradient is determined for at least two of the three phases, and the inductance of each phase is determined from the gradient determined for one phase and the resulting ripple voltage. In particular, a ripple current gradient is determined for all three phases, and the inductance of each phase is determined from the gradient determined for one phase and the resulting ripple voltage. Determining the ripple current gradient for all three phases is particularly useful when different voltage space vectors are applied. In this way, the inductance for each phase can be determined for each determined gradient, taking the ripple voltage into account.

[0022] An advantageous embodiment of the method provides that the gradient of the ripple current is determined for only two of the three phases, and the inductances of the two phases are determined from the gradient determined for one phase and the resulting ripple voltage for that phase, whereby the determination of the inductances for the two phases is carried out under the same voltage space vector. In this case, where the voltage space vector is the same, the inductance of the third phase is advantageously determined for this voltage space vector from the inductances determined for the other two phases.

[0023] According to a further advantageous embodiment of the method, the gradient of the ripple current is determined by measuring the phase current. Another advantageous embodiment provides that the gradient of the ripple current is determined by measuring the DC link current. Advantageously, a microcontroller unit with an analog-to-digital converter is used to control the permanent magnet synchronous motor, and it is particularly provided that the inputs of the analog-to-digital converter of the microcontroller used to control the permanent magnet synchronous motor are used to measure the ripple current. For the same voltage space vector, at least two gradients of the ripple current of the phases can be determined by measuring the phase current, and one gradient of the ripple current can be determined by measuring the DC link current.If the ripple current gradient is determined via a DC link current measurement or a low-side current measurement, it is particularly advantageous that a measurement is only taken a predetermined time after a switching operation, the predetermined time preferably being dimensioned such that the signal at the operational amplifier output has settled. Furthermore, it is advantageous to increase the duty cycle, i.e., the ratio of pulse duration to period, with respect to the voltage space vector, in order to determine the ripple current gradient with sufficient accuracy. Since, with a DC link current measurement, only the ripple current for one phase can initially be determined for a given applied voltage space vector, at least two different additional voltage space vectors are advantageously used to determine the ripple current for all three phases.Advantageously, the inductance for the respective phase is determined from the specified gradient of the ripple current and the associated voltage.

[0024] According to the invention, a first rotor angle is determined from the specified inductance for a given phase using a Clarke transform. The Clarke transform is well-known and is also referred to as the α,β transform, as it is used to convert multiphase quantities, such as those in a three-phase machine with axes U, V, W, into a two-axis coordinate system with an α-axis and a β-axis. The determined first rotor angle does not correspond to the actual rotor angle of the permanent magnet synchronous motor rotor. In particular, the first rotor angle rotates at twice the rotor speed in the opposite direction.

[0025] According to the invention, a correction calculation is applied to the first rotor angle to determine a second rotor angle. Preferably, the actual rotor angle is determined as the second rotor angle using this correction calculation. In this case, the second rotor angle rotates at half the speed and with the opposite sign to the first rotor angle.

[0026] According to the invention, the correction calculation takes into account a period profile with an alternation between a first period and a second period that is offset by 180° relative to the first period. Advantageously, the periods are counted by means of a counter, wherein the counter is advantageously initialized when the permanent magnet synchronous motor starts up, particularly taking into account the settling time of the current. According to an advantageous embodiment of the method, the rotor position is determined at a predetermined speed of the permanent magnet synchronous motor using a BEMF-based method.In particular, it is therefore intended that the rotor position at a rotational speed below the predetermined rotational speed is determined by determining the inductances of the phases based on the determination of the respective gradients of the ripple current, and that when the predetermined rotational speed is reached, the method is switched to the BEMF-based method.

[0027] Furthermore, it is specifically provided that different PWM signals are applied to the permanent magnet synchronous motor in a steady-state, unloaded condition and under load. This advantageously prevents the generation of ripple current at an instantaneous voltage of 0 V (V: volts), thus preventing the determination of inductance. Specifically, a modified space vector modulation is used to generate ripple current in the steady state. This advantageously takes into account that the fundamental harmonic of the phase voltage is zero in the steady state, which can lead to errors in current measurement. Therefore, an error correction is advantageously implemented that considers the known magnitude of the ripple current for each voltage space vector and further takes into account that the ripple current does not contribute to the fundamental harmonic of the phase current.With this error correction, the sampling for the control of the permanent magnet synchronous motor is advantageously adjusted in such a way that the ripple current is not measured when it is superimposed on the fundamental harmonic.

[0028] According to a particularly advantageous embodiment of the invention, the proposed method for determining the rotor position of a three-phase permanent magnet synchronous motor with the aforementioned features is to be used individually or in combination in a method for field-oriented control of a permanent magnet synchronous motor, in particular a permanent magnet synchronous motor used in a motor vehicle for an assistance system. In particular, a method for field-oriented control of a permanent magnet synchronous motor, in particular a permanent magnet synchronous motor used in a motor vehicle for an assistance system, is proposed, wherein a rotor position of the permanent magnet synchronous motor necessary for field-oriented control is determined, and the determination of the rotor position is carried out according to a method proposed according to the invention for determining a rotor position.According to a preferred embodiment, the rotor position is determined primarily using at least one rotor position sensor and – to increase reliability – secondarily by applying the rotor position determination method proposed according to the invention. In particular, it is intended to use the rotor position determination method proposed according to the invention instead of a third sensor, especially as a backup function in the event of a rotor position sensor failure.

[0029] The control unit also proposed for solving the aforementioned problem is advantageously designed for field-oriented control of a permanent magnet synchronous motor according to a method proposed according to the invention for field-oriented control of a permanent magnet synchronous motor. The further proposed steering system, in particular an electromechanical steering system, especially an electromechanical power steering system, advantageously comprises such a control unit. The steering system is designed to detect steering commands given via a steering handle and to transmit them via a steering actuator to a rack on which wheels steerable via tie rods are arranged, the steering actuator comprising a permanent magnet synchronous motor.

[0030] Further advantageous details, features and embodiments of the invention are explained in more detail in connection with the exemplary embodiments shown in the figures (Fig.: Figure). These show: Fig. 1 shows a simplified schematic block diagram of an embodiment of a control unit designed according to the invention for the operation of a permanent magnet synchronous motor, also shown schematically; Fig. 2 shows an exemplary difference between the d-axis inductance and the q-axis inductance of a permanent magnet synchronous motor, as for example in a d / q coordinate system. Fig. 1 Fig. 3 shows an exemplary representation of the phase inductance profiles Lu, Lv, Lw of a rotating rotor of a permanent magnet synchronous motor, as for example in Fig. 1 Fig. 4a shows an embodiment of an embodiment for a device connected to the phase of a permanent magnet synchronous motor according to the invention, such as in Fig. 1 shown, applied voltage signal; Fig. 4 shows the result of the voltage signal according to Fig. 4a resulting current; Fig. 5 shows an embodiment for a PWM signal to be used in an operating state of the permanent magnet synchronous motor when the average phase voltage and the instantaneous voltage are approximately 0 V to prevent ripple current; Fig. 6 shows an embodiment for a first rotor angle determined according to the invention and a second rotor angle derived therefrom; Fig. 7 shows a simplified schematic representation of an embodiment for a motor vehicle with various applications according to the invention for a permanent magnet synchronous motor with a control unit designed according to the invention, such as in Fig. 1 Fig. 8ain a simplified perspective view shows an embodiment of a steering system designed according to the invention; and Fig. 8bin a simplified perspective view shows a further embodiment of a steering system designed according to the invention.

[0031] With reference to Fig. 1 An advantageous embodiment of the proposed invention is explained. Fig. 1 A three-phase permanent magnet synchronous motor 1 is schematically represented, comprising one winding strand U 101, one winding strand V 102, and one winding strand W 103. Each winding strand U 101, V 102, W 103, and thus each phase, is assigned an inductance Lu, Lv, Lw. The phase inductances of the permanent magnet synchronous motor 1 change as a function of the rotor angle. With respect to a d / q coordinate system, which has a d-axis and a q-axis as a rotor-fixed reference system, the permanent magnet synchronous motor 1 exhibits a d-axis inductance (Ld) and a q-axis inductance (Lq) that differs from the d-axis inductance (Ld), as shown in the d / q coordinate system according to [reference missing]. Fig. 2 Illustrated by example. Fig. 2 The inductance in µH (µH: microhenry) is plotted on axes Ax1 and Ay2 as the unit. The phases U 101, V 102, W 103 are shown in Fig. 2 entered as well as the Ld-Lq difference 120, which is modeled as an ellipse that rotates with the rotor of the permanent magnet synchronous motor 1, leading to the change in the phase inductances Lu, Lv, Lw.

[0032] The inductances Lu, Lv, Lw can be represented as sine functions of the rotor position of the permanent magnet synchronous motor 1, as exemplified in Fig.3 shown. In Fig. 3 The angle in radians (rad) is plotted on the Ax2 axis and the inductance in µH (µH: microhenry) is plotted on the Ay2 axis.

[0033] The permanent magnet synchronous motor 1 according to the in Fig. 1 In the illustrated embodiment, the motor is controlled by means of the control unit 2. The control unit 2 comprises a control block 200, which includes functions known per se for controlling a permanent magnet synchronous motor, without describing them in detail, such as, in particular, an analog-to-digital converter, an SVM modulator (SVM: space vector modulation), and a PWM unit. This means that the permanent magnet synchronous motor 1 is controlled by means of a PWM signal, specifically with space vector modulation using voltage space vectors. The PWM signal at the converter output can, in particular, have a frequency of 16 kHz or 20 kHz.

[0034] The control unit 2 is designed for field-oriented control of the permanent magnet synchronous motor 1, with a block 21 for the field-oriented control function, which is also known per se, being shown within the control block 200. For the field-oriented control of the permanent magnet synchronous motor 1, the rotor position of the permanent magnet synchronous motor 1 must be provided to the block 21 as an input variable. In the control unit 2, the rotor position RP2 is determined sensorlessly in the block 20 using a method for determining the rotor position of a three-phase permanent magnet synchronous motor, whereby the rotor position is determined by means of the inductances that change as the rotor rotates.The method is designed for use with three-phase permanent magnet synchronous motors, wherein, as in this embodiment, the permanent magnet synchronous motor, with respect to a d / q coordinate system having a d-axis and a q-axis as a rotor-fixed reference system, has a d-axis inductance (Ld) and a q-axis inductance (Lq) that differs from the d-axis inductance (Ld), and each phase of the permanent magnet synchronous motor is assigned an inductance Lu, Lv, Lw, which changes depending on the rotor position. The method for determining the rotor position is also executed by the control unit 2 outside of block 21. The assignment to block 21 essentially serves for a more intuitive representation in [reference missing]. Fig. 1 , whereby in this embodiment, the rotor position is ultimately determined in block 21. In particular, the necessary parameters for determining the inductances Lu, Lv, Lw are also supplied to block 21 of the control unit 2.

[0035] The method for determining the rotor position involves applying a voltage u(t) to the phases via the connecting lines 111, 112, 113 of the permanent magnet synchronous motor, whereby the applied voltage induces a ripple current i(t) for each phase. Fig. 4a Figure 1 shows an embodiment of such a given voltage u(t) from time t=0 to time t=Ts, where time t is plotted on axis Ax3. The axis Ay3 represents the voltage. The difference between the instantaneous voltage u(t) and the average phase voltage ua(Ts) results in a ripple voltage ur(t), which causes a ripple current i(t). The ripple current i(t) caused by the ripple voltage ur(t) is shown in Fig. 4b The graph is displayed and is determined for at least two of the three phases. Time is plotted on axis Ax4, corresponding to axis Ax3. Axis Ay4 represents the current. The gradient of the ripple current i(t) can be determined, in particular, by measuring the phase current and / or the DC link current. Specifically, the inputs of an analog-to-digital converter of a microcontroller unit in control unit 2 can be used to determine the gradient of the ripple current. The respective inductance is then determined using the relationship u(t) = L di(t) / dt with the determined gradient of the ripple current.

[0036] In this embodiment, the method for determining the rotor position involves applying different PWM signals when the permanent magnet synchronous motor 1 is in a load-free steady state and when it is under load. Fig. 5 A special PWM signal for application to the respective connecting lines 111, 112, 113 is shown, which advantageously ensures that a ripple current i(t) is generated in the load-free steady state, which would otherwise not occur if the PWM signal were not adapted, thus preventing a determination of the phase inductances Lu, Lv, Lw.

[0037] Once the inductances Lu, Lv, Lw for the respective phases have been determined from the determined gradients of the ripple current for the respective phases in block 20 of the control unit 2, a Clarke transformation is applied to the determined inductances in block 20 to convert them into a two-dimensional coordinate representation, from which a first rotor angle w1 is determined. An example of such a determined first rotor angle w1 is shown in Fig. 6 The diagram shows the samples on axis Ax5 and the angle in radians on axis Ay5. A correction calculation is used to determine a second rotor angle w2 from the first rotor angle w1, which corresponds to the actual rotor angle. Block 20 also includes a counter that counts the periods for inclusion in the correction calculation. When the permanent magnet synchronous motor 1 starts up, the counter is initialized, taking into account the current's settling time. The rotor position RP2 determined in this way is then provided to block 21 as an input for the field-oriented control of the permanent magnet synchronous motor 1.

[0038] According to an optional advantageous embodiment, the rotor position RP2 is a rotor position determined redundantly to a rotor position RP1 determined by an optional rotor position sensor unit 3. Both the sensor-determined rotor position RP1 and the sensorless rotor position RP2 are transmitted to the block 21, so that in the event of a failure of the rotor position sensor unit 3, the permanent magnet synchronous motor 1 can advantageously continue to be operated.

[0039] In particular, a further optional embodiment provides that the control unit 2 has a block 22 for BEMF-based rotor position determination, whereby the BEMF-based rotor position determination of block 22 is intended to replace the ripple current-based rotor position determination of block 20 at a predetermined rotational speed of the rotor of the permanent magnet synchronous motor 1. The BEMF-based determined rotor position RP3 is also supplied to block 21 for the control of the permanent magnet synchronous motor 1.

[0040] A permanent magnet synchronous motor 1 with a control unit 2, as shown in Fig. 1 As shown, it is intended in particular for use in a motor vehicle 4, as exemplified in Fig. 7 The following is outlined. In particular, it is planned to integrate the permanent magnet synchronous motor 1 with the control unit 2 into a steering actuator 53 of a steering system 5. Another application is as a motor 61 in an air conditioning compressor used in a motor vehicle 4. A further application is as an actuator 73 in a braking system 7 of a motor vehicle 4, wherein the actuator regulates the braking force acting on the respective brake 71.

[0041] In Fig. 8a und Fig. 8b Exemplary embodiments of a steering system 8 for a motor vehicle are shown. Fig. 8a This includes an electromechanical steering system 8 and in Fig. 8b A steer-by-wire steering system 8 is shown. The steering systems 8 each comprise a steering column 81 with a steering spindle 82 and a steering gear 83. The steering gear 83 comprises a pinion 835 and a rack 836, which can also be referred to as a toothed connecting rod. The steering gear 83 serves to translate a rotational movement of the pinion 835 into a translational movement of the rack 836 along its longitudinal axis. A steering wheel 87 is attached to the end of the steering column 81, and thus the steering spindle 82, facing the driver. This steering wheel allows the driver to input a steering input or command, and the driver can rotate the steering wheel 87 in a known manner to input their steering input. The rack 836, which moves linearly along its longitudinal axis, is mechanically coupled to a tie rod 838 on both sides of the vehicle. The tie rods 838 are in turn mechanically coupled to the vehicle wheels 84.In the case of the electromechanical steering system 8 according to . Fig. 8a The steering column 81 is mechanically coupled to the steered wheels 84 of the motor vehicle via the steering gear 83. In the steer-by-wire steering system 8 according to Fig. 8b The detected steering command is electronically transmitted from the steering column 81 to the steering actuator 831 of the steering gear. Both the steering actuator 831 according to Fig. 8a as well as the steering actuator 831 according to Fig. 8b Each comprises a permanent magnet synchronous motor 1 with a control unit 2, as described in reference to Fig. 1 explained.

[0042] The embodiments shown in the figures and explained in connection with them serve to illustrate the invention and are not limiting to it. Bezugszeichenliste

[0043] 1 Permanent magnet synchronous motor 101 Winding strand U 102 Winding strand V 103 Winding strand W 111 Connection cable for winding strand U (101) 112 Connection cable for winding strand V (102) 113 Connection cable for winding strand W (103) 120 Ld-Lq difference 2 Control unit 20 Block of control unit (2) for ripple current-based determination of rotor position (RP2) 21 Block of control unit (2) for field-oriented control of the permanent magnet synchronous motor (1) 22 Optional block of control unit (2) for BEMF-based determination of rotor position (RP3) 200 Block of control unit (2) with known functionalities 3 Sensor unit for determining rotor position (RP1) using rotor position determination sensor 4 Motor vehicle 5 Steering system 51 Steering handle 52 Steering shaft 53 Steering actuator 54 Steerable wheel 6 Air conditioning compressor 61 Air conditioning compressor motor (6) 7 Braking system 71 Brake 72 Central brake booster 73 Brake system actuator (7) 8 Steering system 81 Steering column 82 Steering spindle 83 Steering gear831 Steering actuator with permanent magnet synchronous motor (1) and control unit (2) 835 Pinion of steering gear (83) 836 Rack and pinion 838 Tie rod 84 Steerable wheel 87 Steering handle RP1 Rotor position, determined by rotor position sensor RP2 Rotor position, determined by ripple current-based method RP3 Rotor position, determined by BEMF-based method w1 First rotor angle w2 Second rotor angle i(t) Ripple current u(t) Instantaneous voltage ua(Ts) Average phase voltage ur(t) Ripple voltage Lu Phase inductance Lv Phase inductance Lw Phase inductance Ldd Axial inductance Lqq Axial inductance t Time Ts Time Ax1 x-axis Fig. 2 Ay1y-axis Fig. 2 Ax2x axis Fig. 3 Ay2y axis Fig. 3 Ax3x axis Fig. 4a Ay3y-axis Fig. 4a Ax4x axis Fig. 4b Ay4y axis Fig. 4b Ax5x axis Fig. 6 Ay5y axis Fig. 6

Claims

1. Method for determining a rotor position (RP2) of a three-phase permanent magnet synchronous motor (1), the permanent magnet synchronous motor (1) having, with respect to a d / q coordinate system as a rotor-fixed reference system having a d-axis and a q-axis, a d-axis inductance (Ld) and a q-axis inductance (Lq) differing from the d-axis inductance (Ld), wherein each phase of the permanent magnet synchronous motor (1) is assigned an inductance (Lu, Lv, Lw) which varies as a function of the rotor position, and wherein the rotor position is determined by means of the varying inductances, a voltage (u(t)) is applied to the phases, wherein the applied voltage (u(t)) causes a ripple current (i(t)) for each phase, wherein the ripple current (i(t)) is determined for at least two of the three phases and the inductance of the respective phase is determined based on the ripple current (i(t)) determined for the respective phase, characterized in that a first rotor angle (w1) is determined from the determined inductance for a respective phase using a Clarke transformation, a correction calculation for determining a second rotor angle (w2) is applied to the first rotor angle (w1), and the correction calculation takes into account a period variation with an alternation between a first period and a second period which is offset by 180° with respect to the first period.

2. Method according to claim 1, characterized in that the periods are counted by means of a counter.

3. Method according to claim 2, characterized in that initialization of said counter takes place when the permanent magnet synchronous motor (1) starts up.

4. Method according to one of the preceding claims, characterized in that the permanent magnet synchronous motor (1) is controlled by means of a PWM signal, the PWM signal being used to generate the ripple current (i(t)).

5. Method according to one of the preceding claims, characterized in that the permanent magnet synchronous motor (1) is controlled by means of a space vector modulation using voltage space vectors.

6. Method according to one of the preceding claims, characterized in that, for a phase, a ripple current (i(t)) is determined as a current which is caused by a ripple voltage (ur(t)) resulting from the voltage difference between an instantaneous voltage (u(t)) and an average phase voltage (ua(t)), the instantaneous voltage (u(t)) being determined by the associated voltage space vector.

7. Method according to claim 6, characterized in that a gradient of the ripple current (i(t)) is determined for at least two of the three phases, the inductance of the respective phase being determined from the gradient determined for a phase and the ripple voltage (ur(t)) resulting for this phase.

8. Method according to claim 7, wherein the gradient of the ripple current (i(t)) is determined for only two of the three phases and the inductances of the two phases are determined from the gradient determined for one phase and the resulting ripple voltage (ur(t)) for this phase, characterized in that the inductances for the two phases are determined under the same voltage space vector, wherein the inductance of the third phase for this voltage space vector is determined from the inductances determined for the two other phases.

9. Method according to claim 7 or claim 8, characterized in that the gradient of the ripple current (i(t)) is determined by means of phase current measurement; and / or in that the gradient of the ripple current (i(t)) is determined by means of intermediate circuit current measurement.

10. Method according to one of claims 7 to 9, characterized in that a microcontroller unit is used for controlling the permanent magnet synchronous motor (1), wherein inputs of an analog-to-digital converter of the microcontroller unit are used for determining the gradient of the ripple current (i(t)).

11. Method according to one of the preceding claims, characterized in that at a predetermined rotational speed of the permanent magnet synchronous motor (1), the rotor position (RP3) is determined using a BEMF-based method.

12. Method according to one of the preceding claims, characterized in that PWM signals that differ from one another are used in no-load steady state of the permanent magnet synchronous motor (1) and in a loaded state of the permanent magnet synchronous motor (1).

13. Method for the field-oriented control of a permanent magnet synchronous motor (1), in particular a permanent magnet synchronous motor (1) used in a motor vehicle (4) for an assistance system, wherein a rotor position (RP1, RP2, RP3) of the permanent magnet synchronous motor (1) necessary for the field-oriented control is determined, characterized in that the rotor position is determined by a method according to one of claims 1 to 12.

14. Control unit (2) for the operation of a permanent magnet synchronous motor (1), characterized in that the control unit (2) is designed for field-oriented control of the permanent magnet synchronous motor (1) according to the method according to claim 13.

15. Steering system (5, 8) which is designed to detect steering commands issued via a steering handle (51, 87) and to transmit them via a steering actuator (53, 831) to a rack (836) on which steerable wheels (54, 84) are arranged via tie rods (838), the steering actuator (53, 831) comprising a permanent-magnet synchronous motor (1), characterized in that the permanent-magnet synchronous motor (1) comprises a control unit (2) according to claim 14.