Description

The invention is about a device and a method for monitoring the torque of a brush-less AC motor.

Brushless AC motors are often used as servo motors, for example in power steering systems for steering vehicles. With power steering it is a challenge to ensure that the current of the servo motor is adjusted by a control unit so that the actual torque corre sponds to the required torque. A problem with conventional power steering systems is that the actual torque cannot be measured directly. These problems can be caused, for example, by user interference or road conditions influencing the behaviour of the power steering.

An approach known to the inventor is to get a value of the torque to estimate the ac tual torque based on the phase currents. An observer is designed to calculate the ac tual torque based on the phase currents. This observer may be subject to failure of the electronic used. Even if some diagnostics are preventing most of the critical failures, some residual FIT rate remains.

There is therefore a need to further reduce the FIT rate. This is where this invention comes in.

This invention is based on the problem of proposing a further device and procedure to obtain a value for the current value of torque.

This task is solved according to the invention by the fact that the device has an ob server. The device according the invention may have inputs for voltages across wind ings of the brushless AC motor, and may have an input for an angular velocity of a ro tor of the brushless AC motor and an input for a pole wheel angle of the brushless AC motor. In contrast to the solution previously known to the inventor, which is based on a calcu lation of the torque based on the phase currents by means of an observer, an ob server is used in the device according to the invention, to which the voltages across the windings of the motor could be fed as input variables. 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 by simple means and can thus serve as a basis for a robust observation of the torque by means of an observer of an inventive device.

The voltages applied to the windings of the motor are generated from a AC voltage by means of inverters made up of power electronic components. These power electronic components of the inverters can be controlled by PWM signals. The voltages can con tain harmonics which are caused, for example, by the generation of the power elec tronic components and their control. To minimize the influence of harmonics on the calculation of the torque, it makes sense to minimize the harmonics by means of low- pass filters.

The cut-off frequency of the low-pass filter or filters used is preferably selected so that voltages with the highest frequency of the voltages occurring across the motor wind ings can pass unhindered through the filter or filters. Since the rotor speed of the brushless AC motor depends on the frequency of the voltages applied across the windings, and the frequency of the voltages can be changed to adjust the rotor speed, the cut-off frequency of the filter or the cut-off frequencies of the filters are tuned to the maximum speed that the brushless AC motor is to achieve. For a motor with four pole pairs (p=4), which should achieve a maximum speed of 4000 rpm, an electrical speed of

^J L ^{e L} imax 60 60 = 266 Hz is achieved. Then, for examp ^r le, a filter with a cut-off frequency of 400 Hz is sufficiently designed to filter possibly interfering harmonics with a 1 st order Butterworth filter.

However, filtering can result in a phase and attenuation in a range around f _eimax that is preferably compensated. The measured and filtered voltages dropping across the windings of the motor can now be digitized using one or more analogue-to-digital converters so that they can be digitally processed. Alternatively, the signals could also be processed analogously with the aid of analogue computers.

The filtered voltage signals oscillate around a curve of a continuous and continuously changing alternating voltage, which could be applied to the windings to achieve the same speed and torque. The oscillations of the filtered voltage signals are caused by the PWM signals used to control the power electronic components of the inverters.

The filtered voltage signals hit this continuous and continuously changing voltage al ways in the middle of the pulses or the pulse pauses of the PWM signals. Preferably, therefore, the analogue-to-digital converters are controlled in such a way that the fil tered voltage signals are discretized in the middle of the pulses or the pulse pauses of the PWM signals which are used to control the power electronic components of the in verters.

The compensation of the error caused by the described analogue filtering of the meas ured voltage signals can take place after digitization. For this purpose, a digital filter may be provided that in the frequency range of interest has a transmission behaviour that is inverse to the transmission behaviour of the analogue filter(s), so that in this fre quency range of interest the effect of the analogue filter(s), i.e. the error caused by the analogue filter(s), and the effect of the digital filter neutralize each other. The error or errors of the analogue filter or filters are compensated in the frequency range of inter est.

The advantageously digitized and advantageously compensated voltages can be fed to a module for transforming the voltages into a d/q space. The polar wheel angle can also be used for the d/q transformation by the module.

In the d/q space, a module can then be used to calculate the torque. The calculation module is supplied with the stresses transformed into the d/q space and the angular velocity of the rotor. In the calculation, parameters of the brushless AC motor can be taken into account as constants. These can be the following parameters: Ld direct in ductance in the d/q space, Lq quadratic inductance in the d/q space, R phase re sistance, p number of pole pairs, and <p _f rotor magnet flux.

It is possible that the torque calculated with a device acording to the invention is com pared with a torque determined by other means or with a measured torque. To do this, a difference can be formed to calculate an error.

The module for the transformation into the d/q space and the module for the calcula tion of the torque are preferably parts of the observer of a device according to the in vention. The observer may be implemented by software or part of software, preferably provided on an integrated circuit.

The invention is explained in more detail below based on the attached drawings. The drawings show: fig. 1 a block diagram of the invented device,

fig. 2 an analogue filter of the device according to the invention,

fig. 3 a bode diagram of the device according to invention,

fig. 4 shows a course of a filtered voltage across one of the windings of the mo tor and a PWM signal to generate the voltage across the winding, fig. 5 a block diagram of the software modules of a device according to the in vention,

fig. 6 a block diagram of a phase feedback monitoring module of the device, and

fig. 7 a block diagram of a calculation module for calculating the torque in the d/q space.

The invented device for monitoring the torque of a brushless AC motor has as hard ware components voltage divider 1 , filter 2 and a programmable integrated circuit 3, which is suitable for calculating the torque of the brushless AC motor. The voltage dividers 1 have resistors Rup, Rdown, which are electrically connected in series and arranged electrically parallel to the windings of the motor. Above the resis tors Rup, Rdown the same voltages drop as above the motor windings. The voltage across the resistors Rdown drops a voltage proportional to the voltages across the windings. These voltages V’a, V’b, V’c are applied to the filters 2.

Filters 2 are low-pass filters whose cut-off frequency is selected in such a way that harmonics caused by the generation of alternating voltages falling over the windings are damped. Amplitude errors and phase errors in the output voltages of filter 2 caused by filter 2 cannot be avoided, but they can be compensated, which will be ex plained later. In the bottom diagram of Figure 3, the amplitude response and the fre quency response of filter 2 are marked with the reference signs A_F and F_F respec tively.

The filtered voltages Va, Vb, Vc are fed to the integrated circuit 3, for which the out puts of filter 2 are connected to the inputs of circuit 3.

Circuit 3 also has an input to which a signal is applied indicating the angular speed <¾ of the rotor. At another input there is a signal which indicates the polar wheel angle q ₅ .

Furthermore, circuit 3 is supplied via an input with a signal obtained outside the device according to the invention which indicates the torque c' _em . This is compared within the device according to the invention with the torque c _em . determined in the device. A dif ference error_nm is calculated.

In the programmable integrated circuit 3, the input variables of various software mod ules schematically represented in figures 5 to 7 are processed to calculate the output variable, namely the torque observed by the device and the deviation of the torque ob served by the device from the known torque. The voltages Va, Vb, Vc present at the analog inputs are first converted into digital signals in circuit 3. The discretization is done with a clock, which is given by the PWM signal, which is used to generate the voltages over the windings of the motor. The time points of discretization are selected so that discretization always takes place in the middle of a pulse and/or in the middle of a pulse pause. Ideally, the value obtained by discretization should be on a voltage curve corresponding to the alternating voltage with which the brushless AC motor could be operated to achieve the same speed and torque.

The discretized voltages Va, Vb, Vc are digitally amplified to compensate for the errors caused by the analog filtering by filter 2. The amplifiers used for the amplification are means for compensating for faults. The result is digital voltage signals which result in the unfiltered analog voltage signals cleaned of harmonics. The bodediagram shows the amplitude response of this digital gain with the reference characters A_V and the frequency response with the reference character F_V. A superposition of the transfer functions produces an almost horizontal result A_R, F_R. The errors of the analog fil tering can thus be almost compensated.

The voltage signals amplified in this way are fed to a module 32 in which the torque is determined from the voltages across the windings, also known as phase voltages, 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 without being able to measure the torque directly.

This phase feedback module 32 can in turn be divided into two software modules (Fig. 6), namely a transformation module 321 and a calculation module 322.

A Clarke Park transformation is performed in the transformation module 321 so that further calculations can be performed in d/q space. The image sizes Vsd and Vsq are obtained in the d/q space from the digitized and amplified voltages Va, Vb, Vc and the polar wheel angle. These are included in the further calculation by the calculation module 322 as well as the angular velocity.

The calculation within the calculation module can be understood from fig. 7. The cal culation depends on several parameters given by the motor, namely the parameters Ld direct inductance in the d/q space, Lq quadratic inductance in the d/q space, R phase resistance, p number of pole pairs, and cp_f rotor magnet flux.

From the torque calculated or observed by the calculation module 322 and the previ ously known torque, the difference error_nm, i.e. the deviation of the torques deter mined on the various paths, is then determined in the error calculation module 33 and made available at the output of the integrated circuit 3.

reference sign list

1 voltage divider

2 analog filters

3 integrated circuit

31 amplifier

32 phase feedback module

33 error calculation module

321 transformation module

322 calculation module