Device and method for monitoring the torque of a brushless AC motor

Abstract

The present invention relates to a torque monitoring device for a brushless AC motor, characterized in that the torque monitoring device includes an observer (32), the observer having inputs for the voltages (Va, Vb, Vc) at both ends of the windings of the brushless AC motor, having an input for the angular velocity (ω S ) of the rotor of the brushless AC motor and an input for the pole wheel angle (θ S ) of the brushless AC motor.

Description

Technical Field

[0001] The present invention relates to a device and a method for monitoring the torque of a brushless AC motor. Background Art

[0002] Brushless AC motors are commonly used as servo motors, for example, in power steering systems for vehicle steering. For power steering, it is a challenge to ensure that the current of the servo motor is regulated by a control unit such that the actual torque corresponds to the required torque. A problem with conventional power steering systems is that the actual torque cannot be directly measured. These problems may be caused, for example, by user disturbances or road conditions that affect the performance of the power steering.

[0003] A method known to the inventors for obtaining a value of torque is to estimate the actual torque based on the phase current. An observer is designed to calculate the actual torque based on the phase current. This observer may be affected by faults in the electronic devices used. Even though some diagnostics can prevent most serious faults, there is still some residual FIT rate.

[0004] Therefore, there is a need to further reduce the FIT rate. This is the origin of the present invention. Summary of the Invention

[0005] The present invention is based on the following problem: to propose further devices and processes for obtaining a value representing the current value of torque.

[0006] According to the present invention, this task is solved by a device having an observer. The device according to the present invention may have an input for the voltage across the windings of a brushless AC motor, and may have an input for the angular velocity of the rotor of the brushless AC motor and an input for the pole wheel angle of the brushless AC motor.

[0007] Compared with the solution known to the inventors for calculating torque based on the phase current by means of an observer, in the device according to the present invention, when using an observer, the voltage across the windings of the motor can be fed as an input variable to the observer. The speed and pole wheel angle of the rotor of the brushless AC motor can also be used for the calculation. These quantities can be measured in a simple manner and can thus serve as a basis for a robust observation of torque by the observer of the device according to the present invention.

[0008] The voltage applied to the windings of the motor is generated from an AC voltage by means of an inverter composed of power electronic components. These power electronic components of the inverter can be controlled by a PWM signal. The voltage may contain harmonics, which are caused, for example, by the generation by the power electronic components and the control of the power electronic components. In order to minimize the influence of the harmonics on the torque calculation, it makes sense to minimize the harmonics by means of a low-pass filter.

[0009] The cut-off frequency of one or more low-pass filters used is preferably selected such that the voltage having the highest frequency of the voltage occurring across the motor winding can pass through the one or more filters unhindered. Since the rotor speed of a brushless AC motor depends on the frequency of the voltage applied across the winding and the frequency of the voltage can be changed to adjust the rotor speed, the cut-off frequency of the filter or the cut-off frequencies of the multiple filters are adjusted to suit the maximum speed to be achieved by the brushless AC motor. For a motor having four pole pairs (p = 4) that should achieve a maximum speed of 4000 rpm, to reach the electrical speed it is sufficient to design a filter with a cut-off frequency of, for example, 400 Hz in order to filter out possible interfering harmonics using a first-order Butterworth filter.

[0010] However, filtering may result in phase and attenuation in the range around and it is preferred to compensate for the phase and attenuation.

[0011] One or more analog-to-digital converters can now be used to digitize the measured and filtered voltage dropping across the motor winding so that the voltage can be processed digitally. Alternatively, the signal can also be processed analogously with the aid of an analog computer.

[0012] The filtered voltage signal oscillates around a continuous and continuously changing AC voltage curve, and the AC voltage can be applied to the winding to achieve the same speed and torque. The oscillation of the filtered voltage signal is caused by the PWM signal of the power electronic components for controlling the inverter. The filtered voltage signal always intersects this continuous and continuously changing voltage in the middle of the pulse or pulse pause of the PWM signal. Therefore, preferably, the analog-to-digital converter is controlled such that the filtered voltage signal is discretized in the middle of the pulse or pulse pause of the PWM signal of the power electronic components for controlling the inverter.

[0013] Compensation for the error caused by the analog filtering of the measured voltage signal described above can be carried out after digitization. For this purpose, a digital filter can be set up, which has a transfer behavior opposite to that of the analog filter in the frequency range of interest, such that in the frequency range of interest the influence of the analog filter (i.e., the error caused by the analog filter) and the influence of the digital filter cancel each other out. The errors of one or more analog filters are compensated in the frequency range of interest.

[0014] The advantageously digitized and advantageously compensated voltage can be fed into a module for transforming the voltage into the d / q space. The pole wheel angle can also be used for the d / q transformation carried out by the module.

[0015] Then, in the d / q space, a module can be used to calculate the torque. The module for calculation is provided with the voltage (stresses) transformed into the d / q space and the angular velocity of the rotor. In the calculation, the parameters of the brushless AC motor can be considered as constants. The parameters can be the following: the direct-axis inductance Ld in the d / q space, the quadrature-axis inductance Lq in the d / q space, the phase resistance R, the number of pole pairs p, and the rotor magnetic flux

[0016] The torque calculated by using the device according to the present invention can be compared with the torque determined by other means or with the measured torque. For this purpose, a difference can be formed to calculate the error.

[0017] The module for transformation into the d / q space and the module for calculating the torque are preferably part of an observer of the device according to the present invention. The observer can be implemented by software or a part of the software, and the observer is preferably arranged on an integrated circuit. Description of the Drawings

[0018] The present invention will be explained in more detail based on the drawings. In the drawings:

[0019] Figure 1 shows a block diagram of the device of the present invention,

[0020] Figure 2 shows an analog filter of the device according to the present invention,

[0021] Figure 3 shows a Bode plot of the device according to the present invention,

[0022] Figure 4 shows the filtered voltage across one of the windings of the motor and the trend of the PWM signal for generating the voltage across the winding,

[0023] Figure 5 shows a block diagram of a software module of the device according to the present invention,

[0024] Figure 6 shows a block diagram of the phase feedback monitoring module of the device, and

[0025] Figure 7 shows a block diagram of the calculation module for calculating the torque in the d / q space. Detailed Description of the Invention

[0026] The device for monitoring the torque of a brushless AC motor according to the present invention has a voltage divider 1, a filter 2, and a programmable integrated circuit 3 as hardware components, which are suitable for calculating the torque of the brushless AC motor.

[0027] The voltage divider 1 has resistors Rup and Rdown, which are electrically connected in series and arranged in parallel with the windings of the electric machine. The voltage drops across the resistors Rup and Rdown are the same as the voltage drop across the windings of the electric machine. The voltage drop across the resistor Rdown is proportional to the voltage across the windings. These voltages V’a, V’b, V’c are applied to the filter 2.

[0028] The filter 2 is a low-pass filter, and its cut-off frequency is selected such that the harmonics caused by the generation of the alternating voltage across the windings are suppressed. The amplitude error and phase error in the output voltage of the filter 2 caused by the filter 2 cannot be avoided, but they can be compensated for, which will be described later. In Figure 3 the lower part of the figure, the amplitude response and frequency response of the filter 2 are marked with the reference signs A_F and F_F, respectively.

[0029] The filtered voltages Va, Vb, Vc are fed to the integrated circuit 3. For this purpose, the output of the filter 2 is connected to the input of the circuit 3.

[0030] The circuit 3 also has an input for the signal indicating the angular velocity ω S of the rotor applied to this input. At another input, there is a signal indicating the pole wheel angle θ S .

[0031] In addition, a signal obtained outside the device according to the invention is provided to the circuit 3 via an input, and this signal indicates the torque c’ em . This torque is compared with the torque c em determined in the device according to the invention. The difference error_nm is calculated.

[0032] In the programmable integrated circuit 3, the input variables of the respective software modules schematically represented in Figures 5 to 7 are processed to calculate the output variables, namely the torque observed by the device and the deviation of the torque observed by the device from the known torque.

[0033] The voltages Va, Vb, Vc present at the analog inputs are first converted into digital signals in the circuit 3. The discretization is carried out using a clock given by a PWM signal, which is used to generate the voltage across the windings of the electric machine. The discretization time points are selected such that the discretization always occurs in the middle of the pulse and / or in the middle of the pulse pause. Ideally, the values obtained by discretization should be on the voltage curve corresponding to the alternating voltage, and the brushless AC motor can be operated using this alternating voltage to achieve the same speed and torque.

[0034] The discretized voltages Va, Vb, and Vc are digitally amplified to compensate for the errors caused by the analog filtering performed by filter 2. The amplifier used for amplification is a device for compensating errors. The result is a digital voltage signal that clears the unfiltered analog voltage signal of harmonics. The Bode plot shows the magnitude response of this digital gain with reference numeral A_V and the frequency response with reference numeral F_V. The superposition of the transfer functions produces an almost horizontal result A_R, F_R. Thus, the errors of the analog filtering can be compensated for almost completely.

[0035] The voltage signal amplified in this way is fed to module 32, where the torque is determined based on the voltage across the windings (also known as the phase voltage), the angular velocity of the rotor of the motor, and the pole wheel angle of the rotor (pole wheel). In Figure 5 this, module 32 is also referred to as the "phase feedback module". The phase feedback module 32 forms an observer for observing the torque that cannot be directly measured.

[0036] This phase feedback module 32 can in turn be divided into two software modules ( Figure 6 ), namely, the transformation module 321 and the calculation module 322.

[0037] The Clarke Park transformation is performed in the transformation module 321, enabling further calculations to be performed in the d / q space. The image dimensions Vsd and Vsq in the d / q space are obtained from the digitized and amplified voltages Va, Vb, Vc and the pole wheel angle. The said image dimensions together with the angular velocity are included in the further calculations performed by the calculation module 322.

[0038] The calculations in the calculation module can be understood from Figure 7 the following. The calculations are performed based on several parameters given by the motor, namely the following parameters: the direct-axis inductance Ld in the d / q space, the quadrature-axis inductance Lq in the d / q space, the phase resistance R, the number of pole pairs p, and the rotor magnetic flux

[0039] Based on the torque calculated or observed by the calculation module 322 and the previously known torque, the difference error_nm (i.e., the deviation of the torque determined on each path) is then determined in the error calculation module 33 and is available at the output of the integrated circuit 3.

[0040] List of reference numerals

[0041] 1 Voltage divider

[0042] 2 Analog filter

[0043] 3 Integrated circuit

[0044] 31 Amplifier

[0045] 32 Phase feedback module

[0046] 33 Error calculation module

[0047] 321 Transformation module

[0048] 322 Calculation module

Claims

1. A torque monitoring device for a brushless AC motor, characterized in that, The torque monitoring device includes an observer (32) having inputs for the voltages (Va, Vb, Vc) at the two ends of the windings of the brushless AC motor. The torque monitoring device includes a low-pass filter (2) for the voltage on the windings of the brushless AC motor, and the cut-off frequency of the low-pass filter is selected such that the voltage having the highest frequency of the voltages appearing at the two ends of the motor windings can pass through the low-pass filter unimpeded. The torque monitoring device includes an analog-to-digital converter connected to the inputs, which is used to convert the analog input quantity into a digital input quantity. The analog-to-digital converter is controlled such that the filtered voltage signal is discretized in the middle of the pulse or pulse pause of the PWM signal for the power electronic components controlling the inverter, and the inverter is used to generate the voltage applied to the windings of the motor.

2. The torque monitoring device according to claim 1, wherein The observer has an input for the angular velocity (ω S ) of the rotor of the brushless AC motor and an input for the pole wheel angle (θ S ) of the brushless AC motor.

3. The torque monitoring device according to claim 1 or 2, characterized in that, The torque monitoring device includes a device (31) for compensating errors, and by means of this device, the errors caused by filtering using the low-pass filter (2) can be compensated.

4. The torque monitoring device according to claim 1, characterized in that, The observer (32) includes a transformation module (321) for converting the voltage at the two ends of the windings of the brushless converter into a voltage in the d / q space.

5. The torque monitoring device according to claim 2, wherein, The observer (32) includes a transformation module (321) for converting the voltage at the two ends of the windings of the brushless converter into a voltage in the d / q space.

6. The torque monitoring device according to claim 5, wherein The observer has a torque calculation module (322) for calculating the torque according to the voltage in the d / q space and the angular velocity.

7. The torque monitoring device according to claim 6, characterized in that, The torque calculation module (322) takes into account the following characteristic values of the brushless AC motor when calculating torque: the direct-axis inductance Ld in the d / q space, the quadrature-axis inductance Lq in the d / q space, the phase resistance R, the number of pole pairs p, and the rotor magnetic flux φ f .

8. The torque monitoring device according to claim 6 or 7, characterized in that, The torque monitoring device has a device (33) for error calculation, and by using this device, the difference between the calculated torque and the measured torque can be calculated.

9. The torque monitoring device according to claim 1 or 2, characterized in that, The observer (32) is a program or a part of a program that can be executed by means of an integrated circuit.

10. A method for monitoring the torque of a brushless AC motor, wherein, Detect the voltage passing through the windings of the brushless AC motor, detect the pole wheel angle and the angular velocity of the rotor of the brushless AC motor, transform the pole wheel angle and the voltage into a voltage in the d / q space, and calculate the torque of the brushless AC motor according to the voltage in the d / q space and the angular velocity. Wherein, the voltage on the windings of the brushless AC motor is filtered by a low-pass filter, and the cut-off frequency of the low-pass filter is selected such that the voltage having the highest frequency of the voltages appearing at the two ends of the motor windings can pass through the low-pass filter unimpeded. And an analog input quantity is converted into a digital input quantity by an analog-to-digital converter, and the analog-to-digital converter is controlled such that the filtered voltage signal is discretized in the middle of the pulse or pulse pause of the PWM signal for the power electronic components controlling the inverter, and the inverter is used to generate the voltage applied to the windings of the motor.

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