TO SUCH A METHOD

TECHNICAL FIELD OF THE INVENTION

The invention concerns a method for controlling an electrical motor. The invention also concerns a sensor adapted to such a method.

BACKGROUND OF THE INVENTION

Electrical motors, such as synchronous brushless motors, are usually controlled thanks to a "six steps" control method or a "sinusoidal" control method.

The six step control method uses a sensor which gives one square signal for each phase of the motor. The data delivered by such a sensor are then used directly by the power stage or by a micro-processor to elaborate, in closed loops in current, the currents to be delivered to the phases of the motor.

The sinusoidal control, also named vectorial control, uses heavy calculations and mathematical transforms in order to convert signals to angular values by arctangent calculations, to transform currents in constant values and then amplitude values into sinusoidal functions. These computations need specifical micro-processors that induce a high cost and complex softwares. Moreover, this type of control causes accuracy issues and torque ripple.

SUMMARY OF THE INVENTION

This invention aims at proposing a new method for controlling an electrical motor allowing to control the motor in a simple way and to reduce the torque ripple, by reducing the number of calculations.

To this end, the invention concerns a method for controlling an electrical motor comprising a given number of phases fed with electrical current. This method is characterized in that each phase is fed with a sinusoidal current directly generated on the basis of a sinusoidal output signal delivered by a position sensor and in that each sinusoidal current is phase-shifted with respect to each other sinusoidal current by an angle equal to a multiple of 360°divided by the nu mber of phases of the motor.

Thanks to the invention, the data delivered by the sensor can be directly used in the control of the motor, without involving mathematical computations or transforms. This permits the use of a simply-structured micro-processor and therefore reduces the cost 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 sinusoidal currents are sine currents.

- The sinusoidal currents are cosine currents.

- Corrective harmonic currents are injected in the sinusoidal signals delivered by the sensor.

- The number of phases is 3 and each sinusoidal current is phase shifted, with respect to each other sinusoidal current, by an electrical angle equal to a multiple of 120°.

The invention also concerns a sensor for detecting the angular position of the rotor of an electrical motor comprising a given numbers of phases fed with electrical current. This sensor is characterized in that it is adapted to deliver a number of outputs corresponding to the number of phases of the motor, and in that each output is a sinusoidal electrical signal, each signal having, with respect to each other signal, an angular phase shift equal to a multiple of 360° div ided by the number of phases of the motor.

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

- The sensor comprises hall cells.

- The sensor comprises three hall cells arranged with a 120° mechanical and/or electrical offset with respect to each other.

- The sensor comprises inductive cells.

- The sensor comprises three inductive cells arranged with a 120° mechanical and/or electrical offset with respect to each other.

- The sensor comprises magnetic cells.

- The sensor comprises three magnetic cells arranged with a 120° mechanical and/or electrical offset with respect to each other.

- The sensor comprises optic cells.

- The sensor comprises three optic cells arranged with a 120° mechanical and/or electrical offset with respect to each other.

- The sensor comprises Giant MagnetoResistance or Anisotropic

MagnetoResistance cells.

- The sensor comprises three Giant MagnetoResistance or Anisotropic MagnetoResistance cells arranged with a 120° mechan ical and/or electrical offset with respect to each other.

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 block diagram of a control method according to the invention,

- figure 2 is a schematic view of a sensor according to the invention mounted to detect the angular position of a rotor of a motor,

- figure 3 is a chart representing sinusoidal currents elaborated in the method of the invention and sinusoidal signals delivered by the sensor of the invention, with respect to the angular position of the rotor of figure 2,

- figure 4 is a block diagram of a control method according to a second embodiment of the invention.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

As represented on figures 1 and 2, a method for controlling an electrical motor M involves the detection of the angular position of a rotor 2 of the motor, thanks to a sensor S. Sensor S comprises several detection cells 4 adapted to deliver electrical signals to an amplifier 6. Amplifier 6 elaborates, on the basis of the signals delivered by cells 4, sinusoidal signals to be used in the control method of the motor. This amplifier operates in a manner known from those skilled in the art.

In the following example, the control method is explained for a three phases-motor.

In other words, the number N of phases of motor M equals 3. In that case, three cells 4 are used to detect the angular position Θ of rotor 2. Each cell 4 is mounted around rotor 2 with a mechanical and/or electrical angular offset of 120°with respect to the other cells.

Sensor S delivers, on the basis of the signals of cells 4, three sinusoidal signals A1 , A2 and A3. As represented on figure 3, each of signals A1 to A3 is phase shifted with respect to each other signal, by an angle equal to a multiple of 360° divided by the number N of phases of motor M. In the present example, signal A2 is phase shifted with respect to signal A1 by an angle a equal to 120°, and signal A3 is phase shifted with respect to signal A2 by an angle β equal to 120° and with respect to signal A1 by an angle δ equal to 240°.

Each of signals A1 to A3 is then used in respective current control steps 101 , 102 and 103. During steps 101 to 103, a specific current is elaborated for each of the three phases of motor M on the basis of signals A1 to A3 and on the basis of a current request, which is set by a user or by a computer program in a dedicated device D to obtain a desired output torque or a desired rotation speed. The possibility for steps 101 to 103 to use sinusoidal signals A1 to A3 obtained from amplifier 6 allows to use simple electronical components to operate the control. Indeed, on the contrary to a normal sinusoidal control, there is no need to elaborate sinusoidal signal on the basis of electrical angles calculations. Therefore, the method of the invention allows to use very simple micro-processors.

In a manner similar to classical control methods, each of current control steps 101 to 103 is followed by a Pulse Width Modulation (PWM) step 121 , 122 and 123 during which specific electrical control currents E1 , E2 and E3 with pre-determined duty cycles are elaborated for each phase of motor M in a procedure known from those skilled in the art.

In a further so-called power stage step 130 fed with the control currents E1 , E2 and

E3, sinusoidal electrical drive currents C1 , C2 and C3 are generated , and then delivered to motor M. Sinusoidal signals C1 to C3 used to drive motor M and sinusoidal signals A1 to A3 can be either sine or cosine signals. Using sinusoidal currents to drive motor M reduces the torque ripple of motor M. Drive currents C1 to C3 delivered to the motor are phase shifted with respect to each other in the same way as signals A1 to A3. Drive currents C1 to C3 are superimposed with signals A1 to A3 on figure 3 for the understanding of the drawing. Drive currents delivered to the phases of motor M are elaborated so as to obtain a number of sinusoidal periods on a mechanical turn of rotor 2 corresponding to the number of pairs of magnetic poles of rotor 2.

Sinusoidal control currents E1 , E2 and E3 are injected into current control steps 101 to 103 in order to slave the operation of motor M.

The invention allows using a completely analogic control method, which is very simple and economical.

According to a second embodiment of the invention represented on figure 4, harmonic corrective signals H1 , H2 and H3 are injected, in correction steps 141 , 142 and 143, directly in the output signals A1 to A3 of sensor S, allowing a very simple correction of the mechanical, electrical or magnetic flaws of motor M.

The invention can be used with various kinds of sensors using, for example, three hall cells, magnetic cells, inductive cells, or optic cells arranged in a manner described before. The invention can also be used with three Giant MagnetoResistance (GMR) cells or Anisotropic MagnetoResistance (AMR) cells arranged in a manner described before.

The invention can be used with a number N of phases different from 3, for example

2.

Sensor S can also be equipped with a number of cells 4 different from 3.

Current control steps 101 to 103, harmonic correction steps 141 to 143 and PWM steps 121 to 123 can be implemented by a single microprocessor.