TECHNICAL FIELD OF THE INVENTION

The invention relates to a method for controlling an electrical motor, especially a synchronous motor, for instance a brushless motor.

BACKGROUND OF THE INVENTION

Control of synchronous motors and especially brushless motors is often realized by six-steps control or by sinusoidal control. These methods use sensors adapted to detect the angular position of the rotor with respect to the stator in order to adjust the currents provided to the armatures defining the phases of the stator. The goal is to obtain a substantially constant output torque.

These methods cannot overcome the flaws of the motor. For example, mechanical flaws on the coils or on the magnets and other issues imply undesired output torque variations, which are called "torque ripple". The torque ripples may be due to an eccentricity of the rotor, an attraction between the teeth on which the coils of the stator are fixed and the magnets of the rotor, or to imperfections on the back electromotive force of the motor. These issues may also provoke noises, vibrations, and other phenomenons that may depreciate the operation of the motor or damage its environment.

SUMMARY OF THE INVENTION

This invention aims at proposing a new method for controlling an electrical motor allowing to overcome the issues provoked by mechanical flaws, magnetic interactions and electrical imperfections of the motor, in order to have a substantially constant output torque.

To this end, the invention concerns a method for controlling an electrical motor comprising a magnetic rotor comprising at least a couple of magnetic poles, a stator comprising at least three coils rolled around teeth, said coils being fed in electrical current, the output torque being controlled by acting on the electrical currents provided to the coils. This method is characterized in that corrective harmonic currents are provided to the coils to obtain a constant output torque, and in that said corrective harmonic currents are computed on the basis of some characteristics of the electrical motor.

Thank to the invention, the control currents provided to the coils, which corresponds to the phases of the motor, are corrected on the basis of data concerning the mechanical structure of the motor, the magnetic interactions, the electrical imperfections and all the possible issues that may provoke torque ripples and reduce the performances and the accuracy of the motor.

According to further aspects of the invention which are advantageous but not compulsory, such a method may incorporate one or several of the following features:

- The corrective harmonic currents are computed on the basis of a determination of the excentricity of the rotor.

- The corrective harmonic currents are computed on the basis of the number of magnets of the rotor and the number of teeth of the stator.

- The corrective harmonic currents are computed on the basis of the variations of the distance between the stator and the rotor, along a radial axis of the motor.

- The corrective harmonic currents are computed on the basis of the harmonic profile of the back electromotive force of the motor, whereas the method comprises, prior to feeding the coils with the corrective currents, the steps of a) determining the harmonic profile of the back electromotive force of the motor, and b) determining the corrective harmonic currents on the basis of the harmonic profile of the back electromotive force of the motor, in order to obtain a constant output torque.

- The harmonic profile comprises, prior to step a), a step c) measuring the variations of the output torque of the motor under a normal control method, whereas the harmonic profile of the back electromotive force is determined on the basis of the measurements of step c).

- The harmonic profile of the back electromotive force is determined by running the motor in generator mode and measuring voltage variations between the terminals of the coils.

- The harmonic profile of the back electromotive force is determined by fast Fourier transformation.

- The motor is controlled by a six-step process, and wherein the corrective harmonic currents are injected during a current control step of the six-steps process.

- The motor is controlled by a sinusoidal control process, and wherein the corrective harmonic currents are injected during the mathematical transformations of the sinusoidal control process.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained in correspondence with the annexed figures and as an illustrative example, without restricting the objet of the invention. In the annexed figures:

- figure 1 is a schematic representation of an electrical motor with which the method of the invention can be implemented,

- figures 2 and 3 are block diagrams representing electrical motor control processes with which the method according to the invention can be implemented,

- figure 4 is a schematic representation of a device adapted to determine the harmonic profile of the back electromotive force of the motor of figurel .

DETAILED DESCRIPTIONS OF SOME EMBODIMENTS

As illustrated on figure 1 , an electrical synchronous motor 1 , such as a brushless motor, comprises a stator 1 1 and a rotor 12. The stator 1 1 comprises several teeth or armatures 1 1 1 , on which are mounted coils 1 13. Coils 1 13 are rolled around teeth 1 1 1 and linked with each other and with a non-represented electric power supply known per se. In the present example, stator 1 1 comprises three coils arranged with respective angles of 120 ⁰ from each other with respect to a rotation axis X-X' of motor 1 . Rotor 12 is adapted to rotate with respect to stator 1 1 around axis X-X'. To this end, rotor 12 comprises one or several permanent magnets 121 with a north pole N and a south pole S. In the example represented, rotor 12 comprises three magnets 121 extending along an angular sector of 120° with respect to axis X-X'. As it appears on the drawing, each north pole N is opposed to a south pole S with respect to axis X-X'.

Electrical motor 1 is operated by feeding each coil 1 13 with electrical current. Each coil 1 13 creates a magnetic field which provokes a rotation of rotor 12 around axis X-X'.

To control the output torque of motor 1 , the electrical properties of the current which is provided to coils 1 13 are controlled. Each coil 1 13 is provided with a dedicated electrical current of a predefined intensity.

The goal of the control of motor 1 is to obtain a substantially constant output torque T in order to make sure the mechanical energy provided by motor 1 to, for instance, a non-represented gear wheel, is adapted to said wheel. A constant output torque is the ideal case of operation of an electrical motor. In fact, output torque T of motor 1 is not constant and varies, because of structure flaws of motor 1 and magnetic phenomenons that occur between stator 1 1 and rotor 12. These variations of output torque are called "torque ripples".

Torque ripples can be caused by an eccentricity of rotor 12 provoking periodic variations of output torque T. Other mechanical flaws can also influence variations of output torque T.

Torque ripples can also come from magnetic interactions between teeth 1 1 1 of stator 1 1 and magnets 121 of rotor 12. This phenomenon is called "cogging torque" and its intensity depends on the number of teeth 1 1 1 and on the number of magnets 121 on rotor 12.

Torque ripples can also come from a phenomenon called "reluctancy" which is provoked by variations of the distance between stator 1 1 and rotor 12 along a radial axis Y-Y', perpendicular to axis X-X' during the rotation of rotor 12.

Torque ripples can also be provoked by the fact that the back electromotive force (emf) of motor 1 should have, in an ideal case, a sinusoidal waveform. This is actually not the case: back emf waveforms show some periodic flaws that provoke torque ripple.

In order to obtain a substantially constant output torque T, these issues must be included in the computation of the currents that are provided to the coils. To this end, corrective harmonic currents are injected in the current controls of motor 1 , for each of its coils 1 13, hereafter called "phases". The corrective harmonic currents are computed on the basis of the characteristics of motor 1 that are involved in the aforementioned torque ripples provoking issues. Each issue can be analysed and measured in order to determine an harmonic profile, allowing to determine corrective harmonic currents to be delivered to the current control of motor 1 .

The harmonic corrective currents can be injected at different points of the control of the motor, depending on the control process used. From here, the term "angular position" denotes the electric angle of the rotor, unless precised.

A six-steps control process can be used, which separates rotation of rotor 12 in six angular sectors and uses three position sensors adapted to determine in which angular sector rotor 12 is. This control process is well known by those skilled in the art and is not precisely described here. This control process represented on figure 2 comprises a current control step 20, during which the intensity of the current provided to coils 1 13 is determined on the basis of the angular position of rotor 12 and on the basis of the actual intensities of the currents delivered to the motor. In a step 22, the corrective harmonic currents can be incorporated in the current control step 20. They can be substituted to the normal control currents or be added to these currents.

A sinusoidal control process can also be used to control motor 1 . This system is known from those skilled in the art. The sinusoidal control delivers constant values to the phases of motor 1 , these constant values being amplitudes of the sinusoidal currents that are delivered to the phases depending on the angular position of rotor 12 detected by a non-shown sensor. This control process represented on figure 3 involves mathematical transformations adapted to transform sinusoidal functions into discrete values, and to transform discrete values into sinusoidal functions. These transformations involve Park and Clark transforms, like represented on steps 30, 32, 34 and 36 on figure 3. The corrective harmonic currents are injected directly in one of these calculation steps. In the example shown, the corrective harmonic currents are injected, during a step 40, in a first Park transform 32.

In the following example, a method for determining corrective harmonic currents will be explained, implementing the computation of the corrective harmonic currents on the basis of the correction of a non-sinusoidal back emf. In order to determine the corrective harmonic currents, one determines the harmonic profile of the back emf of each of the phases of motor 1 . In this example, the back emf of the phases are determined by an experimental device later decribed and three components, noted e1 r, e2r and e3r are determined. Values 1 to 3 denote each phase of motor 1 and letter r, which stands for "real", denotes the experimental provenance of the data.

Each of these three back emf are sinusoidal harmonic sums which can be written as follows:

k

eir(6) = Εία ήη(αθ) (1 )

α=0

where i e [1 ; 3], where k is the number of harmonics computed and where Θ is the angular position. The output torque T of motor 1 can be written as follows:

T = il . el + i2 . e2 + i3 . e3 (2)

This equation is the general expression of the value of output torque T of motor 1 , where i1 to i3 are the currents normally delivered to phases 1 to 3 in a classical control method.

The same process is realized for each phase. In the following, the determination of the back emf is explained for phase 1 .

The output torque values obtained with an ideal back emf e1 under a normal control current i can be easily determined in a manner known form the one skilled in the art. This output torque T, which should be constant under a predefined control setting, must be obtained with a real back emf, that-is-to-say non ideal, which can be non sinusoidal, under a corrected control current i1 c, the letter c denoting the corrected nature of this current. The goal is then to determine i1 c. To this end, one must determine e1 r by an experimental method.

To compute i1 c, the following relation is used:

ilc. elr = il . el (3)

Finally, the relation to determine i 1 c can be written as follows:

il. el

ilc = (4)

elr These litteral equations are given in simplified form. Theoretically, each component of these relations can be developed into sums of sinusoidal harmonics. Once the computations are realized, i1 c can be written as follows: ilc = a sm (θ) + βήη (2Θ) + <S sin(3#) + ... (5) where Θ is the electric angular position of each magnet of rotor 12.

The number k of harmonics computed depends on the accuracy required to control motor 1 . For instance, twenty harmonics can be computed.

The aforementioned computations are realized for each phase 1 , 2 and 3 of motor 1 and injected at the steps predetermined in the control process chosen.

To determine the real back emf harmonic profiles, electrical motor 1 can be operated in a generator mode. By measuring the voltage between the terminals of each of the phases of motor 1 , the real back emf can be determined for each phase. This measurement is realized during one mechanical turn of motor 1 . By a fast Fourier transform, the harmonic profile of each of the back emf of the phases can be determined and injected into equation (4) with which correction currents i 1 c to i3c are determined.

According to an alternate embodiment represented on figure 4, motor 1 can be operated in motor mode under a normal control process 50. In this method, output torque T of motor 1 is detected thanks to a torque sensor 52 which is coupled to an output shaft 13 of motor 1 by a mechanical clutch 54. This operation is realized under a predefined load 56.

The variations of output torque T are measured on one mechanical turn of output shaft 13. This permits, knowing the control currents that have been delivered to motor 1 , to determine the harmonic profile of the back emf of motor 1 thanks to an equation similar to equation (2). This correction method permits to obtain a substantially constant output torque without changing the structure of motor 1 , the computation involves use of only a supplementary microprocessor or a microchip in a control unit of the motor. To realize this control, experimental measurements or theoretical calculations have to be realized in order to determine the corrective harmonic currents.

It is possible to operate several harmonic corrective processes at the same time, computed on the basis of different torque ripple origins.