Motor controller for determining a position of a rotor of an AC motor, AC motor system, and method of determining a position of a rotor of an AC motor

A motor controller for determining a position of a rotor of an AC motor, the motor controller comprises a control input for receiving a control signal, an output for providing a power control signal for controlling power applied to a stator of the AC motor, and an input for receiving a feedback signal representative of the current in the stator. The control input and output are coupled by a reference path, and the input is coupled to the control input by a feedback path. A carrier signal injection element injects a high frequency carrier signal in the reference path at an injection node. The motor controller is arranged to generate the power control signal in dependence on the control signal, the feedback signal, and the high frequency carrier signal. A position determining element generates a position signal representing the position of the rotor in dependence on the feedback signal which includes a carrier signal component comprising rotor position information. The motor controller further comprises a filter block coupled between the control input and the injection node in the reference path for filtering signals in the reference path in a frequency range including the frequency of the high frequency carrier signal.

FIELD OF THE DISCLOSURE

This disclosure relates to a motor controller for determining a position of a rotor of an AC motor an AC motor system and a method of determining a position of a rotor of an AC motor.

BACKGROUND

AC motors are widely used in many applications, including consumer applications such as washing machines, dish washers, electric fans, and automotive applications such as window lift control, electrical power steering systems, electromechanical brake systems and the like.

AC motor systems typically comprise a motor comprising a stator and a rotor and a motor controller to control the power supplied to drive the motor. In order to ensure good control of the motor, for example in order to meet specified motor performance requirements, the motor controller is required to know the position of the motor rotor.

Position sensors, such as position and velocity transducers, and the cabling and connectors required for such position sensors, increase the size, weight and complexity of the AC motor system and have also been a source of failure for AC motor systems. In order to eliminate such position sensors, particularly for small low cost motor controllers, much research has taken place into sensorless techniques for determining rotor position for different classes of motors under a variety of different operating conditions.

A simple technique uses the induced back electromotive force (EMF) generated in the motor. However, at rotor standstill or low speed there is insufficient back electromotive force (EMF) generated in the motor to enable an accurate estimate of rotor position.

More complex techniques are based upon injection of appropriate reference signals and the tracking of the response of the AC motor to the injected reference signal in order to determine the rotor position. The basis for most low and zero speed sensorless control techniques is the presence of a difference in the d-axis and q-axis characteristics of a motor: the d-axis and the q-axis define the dq rotating reference frame. This difference is used to determine the rotor position and is referred to as saliency. The motor characteristics may include for example inductance, or resistance. A salient motor is a motor that exhibits saliency, for example, a difference in inductance in the d-axis and q-axis depending on the position of the rotor. In a Permanent Magnet (PM) motor, there are several sources of saliencies, for example, rotor inherent saliency, saturation based saliency (stator, teeth).

Typically, as described for example, in US patent application no. 2006/0061319 and U.S. Pat. No. 6,894,454, a high frequency carrier signal is injected into the stator by combining the carrier signal with the reference voltage signal that controls the power provided to the stator of the AC motor. The resulting high frequency components, which carry the saliency position information and which are part of the feedback current from the stator, are then processed by a processor in the motor controller to determine the rotor position. The feedback current is also fed back as part of a control loop in the controller to control the power applied to the stator.

High frequency harmonic components are generated in the motor due to the carrier signal and the reference voltage signal and the operation of the control loop, for example during changes in motor load. Known sensorless control methods do not take account of the interference caused between the high frequency components carrying the saliency position information with the high frequency harmonic components generated in the control loop. Such interference impacts the performance of the motor controller in determining the rotor position.

Thus, there is a need for an improved motor controller.

SUMMARY

The present invention provides a motor controller, an AC motor system, a method of determining a position of a rotor of an AC motor, a computer program and a computer-readable medium as described in the accompanying claims.

DETAILED DESCRIPTION OF THE DRAWINGS

The present disclosure will be described with reference to a 3-phase Permanent Magnet Synchronous (PMS) motor. It will be appreciated that the disclosure is not limited to use with a 3-phase PMS motor and may apply to any AC motor with saliency, for example 2-phase AC motors, a PM motor with the permanent magnet buried in the motor or surface mounted, Synchronous Reluctance Motor (SynRM), Switched Reluctance Motor (SRM), AC Induction Motor (ACIM) or AC linear motors.

Referring first toFIG. 1, an AC motor system2comprises an AC motor6having a stator7and rotor9, a power converter8for providing power to the stator7and a motor controller4in accordance with the disclosure for controlling the power converter8and for determining the position of the rotor9. The power converter8is coupled to an AC supply (not shown) and has an output coupled to an analog-to-digital converter (ADC)10. The dq rotating reference frame is shown inFIG. 1for illustrative purposes.

The motor controller4comprises a control input12for receiving a control signal which includes a current control signal component in the d-axis id* and a current control signal component in the q-axis iq*. The control signal at the control input12is generated for example by a master controller, such as the main vehicle controller in an automotive application or washer controller in a consumer washer application, which generates a torque request. The current control signal components id* and iq* are processed in processing block28and the output of processing block28are voltage control signal components in the d-axis ud* and q-axis uq*. Processing block28in accordance with an embodiment comprises combining elements20and22which are arranged to subtract corrected estimations of the current control signal components id—estim_corrected and iq—estim_corrected from the current control signal components id* and iq*, and Proportional Integration (PI) controllers24and26. The PI controllers24and26compare the corrected estimations of the current control signal components id—estim_corrected and iq—estim_corrected with the current control signal components id* and iq* and the difference (or ‘error’ signal) is used by the motor controller4to adjust the control signals applied to the power converter8so that the corrected estimations of the current control signal components id—estim_corrected and iq—estim_corrected are adjusted to the current control signal components id* and iq* in a closed control loop. The processing block28may further comprise in an embodiment limiting elements30,32which are arranged to limit the power control signals at the output of the motor controller4to predetermined limits which limits are determined by the output power achievable by the power converter8, and compensation elements34,36for combining compensation voltages in the d-axis and q-axis to the voltage control signal components at the output of the PI controllers24and26. Compensation voltages may be added to cancel any dependency between the d and q axis which helps to achieve linear and independent control of the d and q axis currents.

The voltage control signal components in the d-axis ud* and q-axis uq* at the output of the processing block28are coupled to a filter block39and the filtered signals at the output of the filter block39are coupled to a transformation element50which is coupled to a carrier signal injection element42. The filtered voltage control signal components ud* and uq* are transformed in transformation element50from the dq rotating reference frame to the αβ stationary reference frame using a corrected rotor angular position θcorrected which is generated by a position determining element or saliency extraction element18. The carrier signal injection element42injects a high frequency carrier signal uhf(t) into the transformed voltage control signal components in the d-axis ud* only and provides voltage control signal components in the αβ stationary reference frame which are coupled to a Space Vector Modulation (SVM)44and which provide power control signals at an output13of the motor controller4. The power control signals at the output13of the motor controller4are coupled to the power converter8so as to control the power applied to the stator7. Thus, a reference path is provided between the control input12and the output13including processing block28, filter block39, the transformation element50and the carrier signal injection element42which injects a high frequency carrier signal uhf(t) into the voltage control signal components provided in the reference path at an injection node in the reference path. More details of carrier signal injection element42and the filter block39are provided below with reference toFIG. 2.

Space Vector Modulation (SVM) are well known in the art and will not be described in any more detail herein.

The motor controller4further includes an input14coupled to the ADC10for receiving measured current signals from the power converter8and a current reconstruction element16for deriving a feedback signal representative of the current signal in the stator from the current signals at the input14. In the embodiment shown and described herein, the AC motor6is a 3-phase motor and so the power converter8is a 3-phase power converter. Thus, a function of the reconstruction element16is to reconstruct the current signal in the stator from the measured current signals which reconstruction is necessary due to the 3-phase nature of the power converter8. The current signal in the stator includes torque and flux producing components and carrier signal current components. As discussed in the introduction, the carrier signal current components contain rotor position information. Another function of the reconstruction element16is to transform the received measured current signals to the αβ stationary reference frame. Thus, the feedback signal at the output of the reconstruction element includes a feedback signal current component iαin the α axis of the stationary reference frame and a feedback signal current component iβin the β axis of the stationary reference frame.

Descriptions of example current reconstruction elements that may be used in the motor controller4in accordance with the disclosure is given in Application Note AN 1930 (section 7.6.1) and in Application Note AN1931 (section 4.3.2) produced by Freescale Semiconductor, Inc. It will be appreciated that the current reconstruction element16may be constructed differently (e.g. iα, iβcan be reconstructed by single shunt measurement of DC bus current) such as to provide the αβ stationary components of the stator current.

The feedback signal current component iαand the feedback signal current component iβare coupled to the position determining element or saliency extraction element18. More details of the position determining element18will be given below with reference toFIG. 2. The position determining element18generates a position signal representing the position of the rotor in dependence on the feedback signal current component iαand the feedback signal current component iβof the feedback signal. The position signal includes an estimate of the rotor angular position θestim and a corrected rotor angular position θcorrected. In addition, the position determining element18generates an estimate of the rotor speed ωestim and corrected estimations of the current control signal component in the d-axis id—estim_corrected and a current control signal component in the q-axis iq—estim_corrected. The corrected estimations of the current control signal components id—estim_corrected and iq—estim_corrected are combined in combining elements20and22with the current control signal components id* and iq* provided at the control input12so that a feedback path is provided coupling the feedback signal representative of the current signal in the stator to the control input12so as to provide a closed loop control of the current components to control the power supplied to the stator7. Thus, the power control signals at the output13depend on the control signal at the control input12, the feedback signal which is part of the closed loop control, and the high frequency carrier signal CS which is injected into the reference path.

Referring now also toFIG. 2which provides a more detailed schematic diagram of the filter block39, the carrier signal injection element42and the position determining element18.

The filter block39comprises filter elements38and40arranged to filter the voltage control signal components ud* and uq* in the reference path in a frequency range including the frequency of the high frequency carrier signal uhf(t). The filter elements38and40thus filter or remove any harmonic components, for example due to the operation of the closed loop control, in the proximity of or around the carrier signal frequency so that the frequency spectra of the voltage control signal components ud* and uq* around the carrier signal frequency is empty. InFIG. 2, the filter elements38and40are represented by Band Stop Filters (BSF)38and40. However, it will be appreciated that the filter elements38and40may be implemented in other ways. For example, a combination of low and high pass filters or a bandpass filter in complementary arrangement (Input-BPF{Input}).

The filtered voltage control signal components ud* and uq* are then transformed in transformation element50, which may be part of the carrier signal injection element42or a separate element as shown inFIGS. 1 and 2, from the dq rotating reference frame to the αβ stationary reference frame using the corrected rotor angular position θcorrected which is generated by the position determining element18. Such a transformation element is well known in the art. The corrected rotor angular position θcorrected is used for the transformation due to the fact that the estimated rotor angular position θestim is displaced from the actual position due to armature reaction whereas the corrected rotor angular position has been corrected to compensate for armature reaction. Armature reaction is described in more detail below. However, the estimated rotor angular position θestimated may be used to perform the transformation when armature reaction is not an issue.

The carrier signal injection element42comprises a carrier signal generator52for generating a high frequency carrier signal (uhf(t)=UM*sin(ωhft)). It is desirable to choose the frequency of the carrier signal to be as high as possible so as to allow for easy spectral separation from the control signals. However, the frequency selection must also account for the fact that the frequency must not be too high compared to the switching frequency of the power converter8. Thus, the frequency of the carrier signal is typically several tenths greater than the frequency of the control signal in the reference path. For example, the carrier signal may have a frequency in the range of 500 Hz to 2000 Hz. The high frequency carrier signal is injected on the d-axis and so the voltage component of the high frequency carrier signal on the d-axis is ud—hfand there is no voltage component on the q-axis. Since in the dq rotating reference frame, the d-axis current component is the electromagnetic field producing current and the q-axis current component is the torque producing current, by keeping the injected voltage component in the q-axis zero (or small), any unwanted torques are also kept small. The high frequency carrier signal components are transformed in transformation element54from the dq rotating reference frame to the αβ stationary reference frame using the estimate of the rotor angular position θestim which is generated by the position determining element18. The estimate of the rotor angular position θestim is used for the transformation because the injection of the high frequency carrier signal as well as the estimation of the control signal current must be performed in the estimated frame since this is where the local minimum of high frequency coupling impedance exists which minimum corresponds to the equilibrium point where the saliency tracking observer (described below) will stabilise.

The output of the transformation element54is coupled to combination elements56and58so that the transformed high frequency carrier signal components in the α-axis and β-axis are combined with the filtered voltage control signal components in the α-axis and β-axis to provide voltage control signal components in the αβ stationary reference frame which define the power control signal for controlling power to the stator7.

The power control signal therefore includes the voltage control signal components based on the control signal at the input12and superimposed or injected high frequency carrier signal components. The high frequency carrier signal induces a current signal in the stator7, which is amplitude modulated depending on the effective rotor saliency (which varies according to rotor position). The voltage control signal components and the high frequency carrier signal components are sufficiently separated in the frequency spectra to not interfere with each other. The rotor position information can be extracted from the feedback signal derived from the measured current signals from the power converter8.

The position determining element18includes a transformation element62for transforming the feedback signal current components iαand iβprovided by the current reconstruction element16from the αβ stationary reference frame to the dq rotating reference frame using the estimate of the rotor angular position θestimate which is generated by the position determining element18. As above, the estimate of the rotor angular position θestim is used for the transformation because the estimation of the control signal current as well as the injection of the high frequency carrier signal must be performed in the estimated frame since this is where the local minimum of high frequency coupling impedance exists which minimum corresponds to the equilibrium point where the saliency tracking observer (described below) will stabilise. The output of the transformation element62is an estimate of the current control signal component in the q-axis iq—estimbased on the feedback signal and is coupled to combining element64. The current control signal component in the q-axis iq* provided at the control input12is subtracted from the estimate of the current control signal component in the q-axis iq—estimby the combining element64to provide a signal which represents the error in the estimate of the current control signal and includes the high frequency carrier signal current components in the feedback signal. The output of the combining element64is fed to an extracting element including combining element68and a filter element66: the output of the combining element64is fed to one input of combining element68and via the filter element66to another input of the combining element68. The filter element66is shown inFIG. 2as being implemented as a Band Stop Filter (BSF). However, it will be appreciated that the filter element66may be implemented in other ways. For example, a high order band pass filter with small phase shift may be used.

The combining element68subtracts the signal at the output of combining element64from the filtered signal to extract the high frequency carrier signal components from the feedback signal which include the saliency information or rotor position information. The extracted high frequency carrier signal components hf carrier=func(position_error) are passed to a saliency tracking observer70or position signal generator which uses the rotor position information in the extracted high frequency carrier signal components to generate the estimate of the rotor angular position θestim, and the estimate of the rotor speed ωestim. An example implementation of the saliency tracking observer70is shown inFIG. 3. The saliency tracking observer70comprises an amplitude detector80and a tracking filter82. The amplitude detector utilises a homodying process, which by definition is the demodulation of a signal by multiplication with a sinusoidal signal that is in phase and frequency synchronism with the incoming carrier frequency. The detected amplitude of the extracted high frequency carrier signal components is directly proportional to the estimated position error. The resulting signal is used as an input to the tracking filter82which provides estimates of rotor angular position θestim, and rotor speed ωestim. The following publications provide more information on implementations of a saliency tracking observer: B. P. Lathi, Modern Digital and Analog Communication Systems, Oxford University Press, USA; 3 edition (Mar. 11, 1998), M. Mienkina, P. Pekarek, F. Dobes: 56F80x Resolver Driver and Hardware Interface, Freescale Semiconductor, Inc. Application Note AN1942, R. D. Lorenz, Observers and State Filters in Drives and Power Electronics, Keynote paper, IEEE IAS OPTIM 2002, Brasov, Romania, May 16-18, 2002, and G. Ch. Hsieh, J.C. Hung, Phase-lock loop techniques—a survey, IEEE Transaction on IE, vol. 43, No. 6, December 1996.

By subtracting the current control signal component in the q-axis from the estimate of the current control signal component in the q-axis iq—estimin the combining means64, the motor controller4according to the disclosure improves the attenuation of the DC component and other harmonic components in the estimate of the current control signal component in the q-axis iq—estim. This makes it easier to filter out the high frequency components which carry the rotor position information.

In addition, by extracting the high frequency carrier signal components using the filter element66and combining element68, unwanted harmonic signals around the frequency of the carrier signal are removed which results in reduced phase distortion of the carrier signal in the feedback signal and so an improvement in the control response of the saliency tracking observer70.

A transformation element60transforms the feedback signal current components iαand iβprovided by the current reconstruction element16from the αβ stationary reference frame to the dq rotating reference frame using the estimate of the rotor angular position θcorrected which is generated by the position determining element18. The corrected rotor angular position θcorrected is used for the transformation due to the fact that the estimated rotor angular position θestim is displaced from the actual position due to armature reaction whereas the corrected rotor angular position has been corrected to compensate for armature reaction. Armature reaction is described in more detail below. However, the estimated rotor angular position θestimated may be used to perform the transformation when armature reaction is not an issue. The corrected estimations of the current control signal component in the d-axis id—estim_corrected and a current control signal component in the q-axis id—estim_corrected are provided at the output of transformation element60.

The current control signal component in the q-axis iq* provided at the control input12is also coupled to a load correction element72. Saliency depends on applied load. In other words, if the load on the AC motor6changes, there is an effect of armature reaction which in turn affects the saliency of the AC motor6. The current control signal component in the q-axis iq* is directly related to the load. The load correction element72provides a correction to the estimated position of the rotor due to the response of the rotor9to variations in load. The load correction element72may be implemented by means of a look-up table having a list of correction values to the estimated rotor position for a number of different loads as indicated by different values of the current control signal component iq*. In another embodiment, load correction element72may comprise a memory for storing parameters of the AC motor so that the value of the correction to the estimated position of the rotor can be calculated in real-time based on the current control signal component iq* at the input of the load correction element72.

In an embodiment, the correction element72performs a phase compensation of the saliency tracking observer70estimate of the rotor angular position θestim. In general, a commissioning process is used to obtain the phase compensation table72. The following publication provides more information on an off-line commissioning process: Joachim Holtz and Lothar Springob: Identification and Compensation of Torque Ripple in High-Precision Permanent Magnet Motor Drives.

In the case of a 3-phase Permanent Magnet Synchronous (PSM) motor, the values of the phase compensation table72are obtained by an analytical calculation expressed as:
θ_compensation=Arc Tan((iq*×Lq(iq))/Ψpm)  Equation 1

Iq* is the torque producing current control signal component in the q-axis,

Lq(iq) is the quadrature axis inductance in dependence of loading and is specified by the motor manufacturer,

Ψpm is the magnetic flux of the rotor permanent magnets.

The use of an analytical calculation such as that of Equation 1 avoids the need for a commissioning process and only the values of the magnitude of the magnetic flux and quadrature inductance are necessary. Similar analytical equations can be derived for other types of motors.

The correction value provided by the load correction element72and corresponding to the current control signal component iq* at the input of the load correction element72is added to the estimate of the rotor angular position θestim provided at the output of the saliency tracking observer70at combining element74so as to provide the corrected rotor angular position θcorrected.

It will be appreciated that in an alternative embodiment, the filter block39and the carrier signal injection element42may be used with a known position determining element which known position determining element does not include the filter element66, combining element68and load correction element72as described above. It will however be appreciated that an embodiment including the filter block39, the carrier signal injection element42and a position determining element including the filter element66, combining element68and load correction element72has enhanced performance over embodiments which include one or more of such elements by way of improved control response and stability of the saliency tracking observer in the case of load current transients.

In summary, by having filter elements in the reference path prior to injecting the high frequency carrier signal, the motor controller in accordance with the disclosure ensures that unwanted frequency harmonic components in the proximity of the frequency of the carrier signal are removed which improves phase distortion and reduces interference with the carrier signal resulting in improved extraction of the high frequency components carrying the saliency information and hence improved performance in determining the position of the rotor by the position determining element.

In an embodiment having a filter element to extract the carrier signal from the feedback signal and by subtracting the current control signal component iq* further enhances the performance of determining rotor position by improving phase distortion and improving the response of the saliency tracking observer.

In an embodiment having a load correction element, corrections can be made to the determined rotor position to compensate for load transients.

The motor controller4shown inFIGS. 1 and 2and described above with reference toFIGS. 1 and 2is represented by a number of blocks or elements for performing different functions. The blocks may be implemented in software for execution in a controller, microprocessor or similar device. In such an implementation, the motor controller4may be the device or may be part of one or more controllers, microprocessors or similar devices with the different functions distributed across the devices.

In the description above, the high frequency carrier signal is injected into the d-axis of the dq rotating reference frame. It will be appreciated that the present disclosure may apply to an arrangement in which the high frequency carrier signal is injected into a different rotating frame (e.g. rotating frame is phase shifted relative to the rotor) or the stationary reference frame. An injection technique into the stationary reference frame is well known and produces different current motor response due to saliency as that when the injection is into the rotating reference frame.

The described technique for determining a position of a rotor of an AC motor in accordance with the disclosure may be used for motor rotation speeds in the range of zero to medium speeds. At high speeds, such a technique can be used but the back EMF techniques described above in the introduction work more efficiently at high speeds compared to injection techniques.