Diagnosis of demagnetization by flux linkage

Abstract

Method for determining the magnetization of a permanent magnet of a permanent magnet excited synchronous machine (4) of a steering system comprising the step: Determining the flux linkage Ψ p of the permanent magnet excited synchronous machine (4), wherein the flux linkage Ψ p from a calculated torque M and the torque-generating current i sq is calculated, where the torque-generating current i sq the actual stator current in the transverse direction to the rotor magnetic field.

Description

AREA OF INVENTION

[0001] The present invention relates to a method for determining the magnetization of a permanent magnet of a permanent magnet excited synchronous machine of a steering system and a steering system for a vehicle. BACKGROUND OF THE INVENTION

[0002] Permanent magnet synchronous machines (PMSM) are known in the prior art. Such synchronous machines have permanent magnets in the rotor. SUMMARY OF THE INVENTION

[0003] For example, in highly automated driving (HAD), it must be ensured that the electric motors of a vehicle's steering system can generate a minimum torque with a torque-generating current. This is not the case if the permanent magnets in the rotor of the relevant synchronous machine are demagnetized. The permanent magnets can be made of NdFeB. Moisture can lead to corrosion of the permanent magnets. This corrosion destroys the surface of the permanent magnet, thereby reducing its magnetization. Demagnetization can also occur due to high motor currents and / or high motor temperatures. Demagnetization due to high motor temperatures does not occur abruptly by exceeding a threshold, but rather gradually.Above a certain temperature, with increasing temperature and corresponding motor currents, an increasingly strong and irreversible demagnetization of the permanent magnets occurs.

[0004] A corresponding method for determining the magnetization of a permanent magnet in a permanent magnet synchronous machine of a steering system is known, for example, from DE 10 2013 201 648 A1. In this case, however, the flux linkage is determined via an induced voltage, which can then be used to calculate a torque and / or to detect demagnetization of the permanent magnets.

[0005] Furthermore, corresponding methods for determining the magnetization of a permanent magnet of a permanent magnet excited synchronous machine are known from DE 10 2014 206 400 A1 and JP S59-47 994 A, although their intended use is not directly directed towards application in a steering system.

[0006] Similar procedures have also been proposed in DE 10 2012 211 315 A1 and DE 10 2011 088 729 A1.

[0007] One task is therefore to provide a method that allows for an assessment of the current state of magnetization, in particular the demagnetization that has occurred to date, of the permanent magnets in a permanent magnet synchronous machine. A simple and cost-effective method for this determination should be provided.

[0008] As a first embodiment of the invention, a method for determining the magnetization of a permanent magnet of a permanent magnet excited synchronous machine of a steering system is provided, comprising the step: determining the flux linkage Ψ p of the permanent magnet excited synchronous machine, wherein the flux linkage Ψ pcalculated from a calculated torque and the torque-generating current, where the torque-generating current is the actual stator current in the transverse direction to the rotor magnetic field.

[0009] Based on the flux linkage of the permanent magnet synchronous machine, it is possible to determine the magnetization state of the permanent magnets. By determining the flux linkage, it is possible to easily infer the state of demagnetization of the permanent magnets in the synchronous machine.

[0010] As a second embodiment of the invention, a steering system for a vehicle is provided, wherein the steering system is configured to perform a method according to one of claims 1 to 11.

[0011] Exemplary embodiments are described in the dependent claims.

[0012] According to an exemplary embodiment of the invention, a method is provided comprising the step: if the flow chain Ψ p below a threshold value: Generate a message indicating that the magnetization of the permanent magnet is too low.

[0013] By determining a threshold value, a simple statement can be obtained about the current magnetization of the permanent magnets.

[0014] In a further embodiment of the invention, a method is provided wherein the flow linkage Ψ p and / or the threshold is adjusted using the motor temperature of the synchronous machine.

[0015] Using the motor temperature allows for a more accurate assessment of the current magnetization of the permanent magnets.

[0016] According to an exemplary embodiment of the invention, a method is provided comprising the step: calculating the flow chain Ψ p , where the river chain Ψ p from a calculated torque M and the torque-generating current, where the torque M = P ab,Motor / (2·π·n) where p z is the number of pole pairs of the synchronous machine.

[0017] According to a further embodiment of the present invention, a method is provided comprising the step: calculating the flux linkage Ψ p with the formula: Ψ p = (2 · M) / (3 · p z · i sq ), where the torque M = P ab,motor / (2·π·n) where p z is the number of pole pairs of the synchronous machine.

[0018] Simple formulas allow for a quick and accurate assessment of magnetization.

[0019] According to an exemplary embodiment of the invention, a method is provided wherein the calculation of the torque M at a rotational speed n above a threshold n schwelle This has been done.

[0020] By performing calculations only at high speeds, the problems encountered when calculating at low speeds can be avoided.

[0021] In a further embodiment of the invention, a method is provided comprising the step: calculating the torque M from the motor output power: P ab,Motor = P Wirk,ab,Umrichter - P V,Umrichter - P V,Mot,R - P V,Mot,Fe .

[0022] According to a further embodiment of the present invention, a method is provided wherein the PWM1, PWM2, PWM3 to be provided of the inverter are not in the voltage limiting and / or wherein PWM min < PWM1 < PWM max and / or PWM min <PWM2 < PWM max and / or PWMmin < PWM3 < PWM max is and / or where PWM min >= 5% and PWM max <= 95%, where in particular PWM min >= 10% and PWM max <= 90%, preferably PWM min >= 20% and PWM max <= 80%.

[0023] If the PWM values ​​to be set are chosen within appropriate limits, voltage limitation can be avoided.

[0024] According to an exemplary embodiment of the invention, a method is provided wherein P V,Umrichter = 0 and / or P V,Mot,R = 0 and / or P V,Mot,Fe is set to = 0.

[0025] If power losses are disregarded, a very simple calculation of the magnetization can be performed.

[0026] In a further embodiment of the invention, a method is provided wherein P Wirk,ab,Umrichter = 3 · U strang · I strang · cosφ and / or P V,Umrichter= i1 2 · R 1.Phase, Umrichter + i2 2 · R 2.Phase, Umrichter + i3 2 · R 3.Phase, Umrichter and / or P V,Mot,R = i1 2 · R S + i2 2 · R S + i3 2 · R S is.

[0027] By taking power losses into account, the calculation of the magnetization of the permanent magnets can be made more accurately.

[0028] According to a further embodiment of the present invention, a method is provided comprising the step of first waiting a cycle time before the method is carried out.

[0029] Periodic calculations can limit the required computing capacity.

[0030] According to an exemplary embodiment of the invention, a method is provided wherein the method is applied to a partial drive of a redundant drive of a steering system of a vehicle.

[0031] A redundant steering system design can ensure a high level of safety for the vehicle in question.

[0032] According to a further embodiment of the present invention, a method is provided wherein the method is used for automatically driving the vehicle.

[0033] One aspect of the invention is the determination of the current torque of the synchronous machine based on a power balance. For this purpose, only the easily determined ohmic stator resistance is used as a motor parameter. The ohmic stator resistance can be obtained by measuring its resistance. Alternatively, the ohmic stator resistance can be assumed, for example, based on comparative measurements. In an alternative embodiment, the temperature-dependent change in the stator resistance is taken into account. The current flux linkage is calculated using the current torque and the current required to produce it. Advantageously, the current flux linkage is only determined once a certain rotational speed is exceeded. To increase accuracy, the temperature-dependent internal resistance of the ECU can be considered.Furthermore, it can be advantageous to take the iron losses of the electric motor into account when determining the demagnetization.

[0034] This can be achieved, for example, by using a speed-dependent characteristic curve for iron losses.

[0035] The individual features can of course also be combined with each other, which can sometimes result in advantageous effects that go beyond the sum of the individual effects. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Further details and advantages of the invention will become clear with reference to the exemplary embodiment shown in the drawing. It shows Fig. 1 A schematic representation of a control system for a permanent magnet synchronous machine. DETAILED DESCRIPTION EXAMPLE FORMS

[0037] Fig.Figure 1 shows a schematic representation of a control system for a permanent magnet excited synchronous machine in the rotor field-oriented d / q coordinate system.

[0038] To determine the magnetization of a permanent magnet in a synchronous machine, the flux linkage Ψ is used. p calculated. Ψ p For a permanent magnet excited synchronous machine with negligible reluctance torque, this results in: Ψ p = (2 · M) / (3 · p z · i sq ), where p z is the number of pole pairs of the permanent magnet excited synchronous machine.

[0039] Is the river chain Ψ p If the magnetization falls below a minimum threshold, it can be assumed that the magnetization is too low and therefore damaged. This detection of damage can be transmitted to the steering system's control unit.

[0040] If necessary, the accuracy of the diagnosis can be improved by taking into account the dependence of the flow chain Ψ p The flux linkage Ψ can be increased by the motor temperature of the synchronous machine in question. This relationship can be determined using a formula or a characteristic curve. The motor temperature itself can be determined, for example, using temperature sensors in the motor winding or by means of temperature models. p The permanent magnet can be checked regularly, cyclically, or periodically, especially at times when specific evaluation conditions exist. The magnetization state can be assessed based on the flux linkage.

[0041] To calculate the flow chain Ψ p The effective torque M is required. The torque M is determined using the motor output power P. ab,Motor This results in the torque M being: M = Pab,Motor / (2·π·n), with n = rotational speed of the rotor of the synchronous machine.

[0042] Low rotational speeds can cause problems when calculating torque. Therefore, it is advisable to perform calculations only for rotational speeds n above a certain threshold: n>=nthreshold.

[0043] For a lossless ideal inverter and a lossless motor, the motor output power P is calculated as follows: ab,Motor : P ab,Motor = P Wirk,ab,Umrichter , where P Wirk,ab,Umrichter corresponds to the active power delivered in the three strands: PWirk,ab,Umrichter=3⋅Ustrang⋅Istrang⋅cosφ.

[0044] U strang and the angle of the phase voltage results from u sd and u sq . I Strang and the angle of the current flow is determined from i sd and i sqThe phase angle φ is calculated from the angle of the phase voltage and the angle of the phase current. Alternatively, the phase voltages, phase currents, and phase angle can be calculated from the stator-fixed quantities.

[0045] Alternatively, the active power output of the ideal inverter can be calculated from the PWM, the DC link voltage and the phase currents.

[0046] It is only useful to perform a calculation if the PWM values ​​to be set for each phase are not within the voltage limit, i.e., if the inverter generates PWM values ​​to be set that are within the threshold values. min and PWM max lay: PWMmin <PWM1<PWMmax PWMmin <PWM2<PWMmax PWMmin <PWM3<PWMmax.

[0047] For example, PWM can min = 5% and PWM max = 95% will be elected.

[0048] However, losses are to be expected. Taking these losses into account will increase the accuracy of the calculation. Losses can arise from: losses of the inverter P V,Umrichter , ohmic losses of the stator winding of the permanent magnet synchronous machine P V,Mot,R and the iron losses of the engine P V,Mot,Fe The iron losses P V,Mot,Fe can be calculated using a formula or easily determined using a characteristic curve based on the engine speed.

[0049] The losses of the inverter P V,Umrichter result as: PV, Inverter = i12⋅R1st Phase, Inverter + i22⋅R2nd Phase, Inverter + i32⋅R3rd Phase, Inverter and the ohmic losses of the stator winding of the permanent magnet synchronous machine P V,Mot,R are calculated as: PV,Mot,R=i12⋅RS+i22⋅RS+i32⋅RS,with RS=RS,25°C⋅(1+α⋅ΔT), where R S The temperature-dependent stator string resistance is R S,25°CΔT is the stator winding cold resistance at the winding cold temperature, here for example 25° Celsius, ΔT is the temperature difference between the winding temperature and the winding cold temperature, and α is the temperature coefficient, for example 0.004 1 / K for a winding made of copper (K=Kelvin).

[0050] In an alternative embodiment, the temperature-dependent change in the stator string resistance R is S not taken into account.

[0051] In an alternative embodiment, the temperature-dependent change in the phase resistances of the ECU (Electronic Control Unit) is taken into account to increase the accuracy of the calculation.

[0052] This results in: Pab,Motor=PWirk,ab,Umrichter−PV,Umrichter−PV,Mot,R−PV,Mot,Fe.

[0053] It should be noted that the term "include" does not exclude further elements or procedural steps, just as the terms "a" and "an" do not exclude multiple elements and steps.

[0054] The reference symbols used serve only to increase clarity and should in no way be considered restrictive, the scope of protection of the invention being defined by the claims. LIST OF REFERENCE MARKS 1 current regulator 2 PWM modules that generate PWM1, PWM2, PWM3 3 Power stage / converter / inverter 4 permanent magnet synchronous machines 5 Transformation from the stator-fixed coordinate system to the rotor field-oriented coordinate system 6. Calculating rotational speed from the rotor position 7 Target values ​​for the flows i sq and i sd 8 measured values: rotational speed n, currents i sq , i sd 9 tensions u sq, u sd 10 control signals PWM1, PWM2, PWM3 for the power stage 11 measured phase currents i1, i2, i3 ECU Electronic Control Unit i1 Phase current in string U i2 Phase current in string V i3 Phase current in string W i sd Actual stator current in the longitudinal direction to the rotor magnetic field i sq Actual stator current in the direction perpendicular to the rotor magnetic field i sd,Soll Target stator current in the longitudinal direction to the rotor magnetic field i sq,Soll Target stator current in the direction perpendicular to the rotor magnetic field I strang String current M torque n rotational speed n schwelle Speed ​​threshold P ab,Motor Engine output power P Wirk,ab,Umrichter Active power delivered in the three strands of the inverter P V,Umrichter Inverter losses P V,Mot,Rohmic losses of the stator winding of the permanent magnet synchronous machine P V,Mot, Fe Iron losses of the engine p z Number of pole pairs PWM1 to be set PWM of the first phase PWM2 to be set PWM of the second phase PWM3 to be set PWM of the third phase R S temperature-dependent stator string resistance R S,25°C Stator winding cold resistance at winding cold temperature U strang String voltage u sd Stator voltage in the longitudinal direction to the rotor magnetic field u sq Stator voltage in the direction perpendicular to the rotor magnetic field U ZK DC link voltage α temperature coefficient ΔT Temperature difference between the winding temperature and the winding cold temperature Ψ p River chain

Claims

[1] Method for determining the magnetization of a permanent magnet of a permanent magnet excited synchronous machine (4) of a steering system comprising the step: Determining the flux linkage Ψ p of the permanent magnet excited synchronous machine (4), wherein the flux linkage Ψ p from a calculated torque M and the torque-generating current i sq is calculated, where the torque-generating current i sq the actual stator current in the transverse direction to the rotor magnetic field. [2] The method of claim 1 comprising the step: if the river chain Ψ p below a threshold value: Generate a message indicating that the magnetization of the permanent magnet is too low. [3] Method according to one of claims 1 or 2, characterized by , that the river chain Ψ p and / or the threshold value is adjusted by means of the motor temperature of the synchronous machine (4). [4] Method according to any one of the preceding claims, comprising the step: Calculating the river chain Ψ p with the formula: Ψ p = (2 · M) / (3 · p z · i sq ), where the torque M = P ab,Motor / (2·π·n) where p z the number of pole pairs of the synchronous machine (4) is. [5] Method according to any one of the preceding claims, characterized by , that the calculation of the torque M at a rotational speed n above a threshold n schwelle This has been done. [6] Method according to any one of the preceding claims, comprising the step: Calculating the torque M using the motor output power: P ab,Motor = P Wirk,ab,Umrichter - P V,Umrichter - P V,Mot,R - P V,Mot,Fe . [7] Method according to any one of the preceding claims, characterized by , that the PWM1, PWM2, PWM3 signals of the inverter are within the specified PWM threshold values min and PWM max lie, so that PWM min< PWM1 < PWM max and PWM min < PWM2 < PWM max and PWM min < PWM3 < PWM max is, and where PWM min >= 5% and PWM max <= 95%, where PWM1 is the PWM to be set of a first phase of the synchronous machine (4), PWM2 is the PWM to be set of a second phase of the synchronous machine (4) and PWM3 is the PWM to be set of a third phase of the synchronous machine (4). [8] Method according to one of claims 6 or 7, characterized by , that P V,Umrichter = 0 and / or P V,Mot,R = 0 and / or P V,Mot,Fe is set to = 0. [9] Method according to any one of claims 6 to 8, characterized by , that P Wirk,ab,Umrichter = 3 · U strang · I strang · cosφ and / or P V,Umrichter = i1 2 · R 1.Phase , Inverter + i2 2 · R 2.Phase, Umrichter + i3 2 · R 3.Phase, Umrichter and / or P V,Mot,R = i1 2 · R S + i2 2 · R S + i3 2 · R S is. [10] Method according to any one of the preceding claims, characterized by that the procedure is applied to a partial drive of a redundant drive of the steering system. [11] Method according to any one of the preceding claims, characterized by that the procedure is used in the automatic driving of a vehicle comprising the steering system. [12] Steering system for a vehicle, characterized by that the steering system is configured to perform a method according to one of the preceding claims.

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