Method for controlling a brushless direct current electric motor

Abstract

A method for controlling a brushless and sensorless direct current electric motor (3) for a motor vehicle device, in which the electric motor (3) comprises a rotor and phases (A, B, C) powered by a pulse width modulation applied to a power inverter (1) of the electric motor (3), and in which, beyond a minimum threshold (S min ) of the rotational speed of the rotor, the position of the rotor is determined by the measurement of the electromotive forces at the phases (A, B, C) of the electric motor (3), the control method being characterized in that, in the case where a stop (104) of the electric motor (3) is commanded, the rotational speed of the rotor is reduced (105) from a nominal speed to a predetermined rotational speed (V1) in a range between the minimum threshold (S min ) and 10% above said minimum threshold (S min ), then the electric motor (3) is stopped (106) at a predetermined position by short-circuiting the branches (A, B, C) of the inverter (1) when the predetermined position is reached.

Description

TECHNICAL FIELD

[0001] The present invention relates to the field of electric motors for motor vehicle equipment, and more particularly to brushless DC electric motors for wiper devices. BACKGROUND

[0002] Controlling a brushless DC electric motor requires determining the position of the rotor at least at certain precise points in order to be able to apply a pulse width modulation command, making it possible to achieve the desired rotational speed.

[0003] To this end, it is known to use position sensors, in particular Hall effect sensors, which make it possible to determine certain positions of the rotor.

[0004] However, in order to reduce the cost of the electric motor, it can be beneficial to dispense with these position sensors, the cost of which is not negligible.

[0005] It is also possible to use a zero-crossing method, by measuring the electromotive force of the phases of the electric motor, to determine the position of the rotor. However, the electric motor must be rotated at a speed greater than a predetermined rotational speed, in order for the electromotive force to be sufficiently high to be able to be detected.

[0006] Thus, by applying a predetermined command at start-up until a predetermined rotational speed is reached, it is possible to drive the electric motor without sensors.

[0007] However, in the case of a wiper device, it is also necessary to stop the motor at a predetermined position corresponding to a stop position (or "parking position"). This stop position is generally provided by a sensor associated with the linkage mechanism of the wiper device. Thus, one way of stopping the electric motor is to short-circuit the branches of the inverter supplying the phases of the electric motor when the stop position is reached, in order to completely stop the electric motor. However, this short-circuiting of the branches of the inverter generates significant current peaks when the electric motor is rotating at its nominal speed, which can damage the transistors used in the inverter, leading to a failure of the electric motor. One way of overcoming this problem is to increase the size of the transistors in order to be able to withstand these current peaks, but this involves considerable costs, which are contrary to the desire to reduce the overall cost of the electric motor.

[0008] It is therefore necessary to find a solution that makes it possible to control a brushless and sensorless electric motor, while still allowing the electric motor to be stopped at a predetermined position, and without damaging the transistors used to supply the electric motor. SUMMARY

[0009] To this end, a method for controlling a brushless and sensorless direct current electric motor for a motor vehicle device is proposed, wherein the electric motor comprises a rotor and phases supplied by pulse width modulation applied to an inverter of the electric motor, and wherein, above a minimum threshold of the rotational speed of the rotor, the position of the rotor is determined from the measured values of the electromotive forces at the phases of the electric motor,

[0010] characterized in that, in the case where the electric motor is commanded to stop, the rotational speed of the electric motor is reduced from a nominal rotational speed to a second predetermined rotational speed, by modifying the pulse width modulation, in an interval between a first predetermined rotational speed and 10% above said first predetermined rotational speed, then, when the predetermined position is reached, the electric motor is stopped at the predetermined position by short-circuiting the branches of the inverter.

[0011] A use of a method for controlling a brushless and sensorless electric motor, wherein, when the electric motor is stopped, the speed is reduced to a rotational speed substantially equal to a minimum speed for which the electromotive forces of the phases of the electric motor can be measured in order to detect the position of the rotor, then the branches of the inverter are short-circuited in order to stop the electric motor when the desired stop position is reached, so that the intensity of the current peak generated when the branches of the inverter are short-circuited can be reduced. This advantageously makes it possible to stop the electric motor at the desired position without damaging the transistors used to supply the electric motor.

[0012] According to one of the embodiments, the reduction of the rotational speed to the predetermined rotational speed comprises a plurality of increments associated with various reductions of the rotational speed of the electric motor.

[0013] According to another embodiment, the electric motor is associated with a gear reduction device, forming a geared motor configured to drive a motor vehicle wiper device comprising at least one wiper arm. The wiper device comprises a stop sensor associated with a stop position of the wiper arm. The predetermined position is given by the stop sensor and the signal from the stop sensor is used to determine the time at which the rotational speed should be reduced.

[0014] According to another embodiment, the stop sensor is modified to provide a deceleration position from which the rotational speed of the electric motor should be reduced to allow the wiper device to stop at the stop position, for example 5° or 10° before the stop position.

[0015] According to another embodiment, when the rotational speed of the electric motor is greater than the minimum threshold, the position of the wiper arm is deduced from the rotor position determined from the electromotive forces and from the gear reduction ratio of the gear reduction.

[0016] The present invention also relates to a geared motor for a motor vehicle wiper assembly, the geared motor comprising a brushless and sensorless DC motor, wherein the motor includes a rotor, a control unit, and phases powered by pulse width modulation applied to an inverter of the motor. The control unit is configured to determine the position of the rotor based on measurements of the electromotive force of each phase of the motor exceeding a minimum threshold of the rotor's rotational speed.

[0017] In the event of a command to stop the electric motor, the control unit is configured to reduce the motor's rotational speed from the nominal speed to a predetermined speed within a range between the minimum threshold and 10% above the minimum threshold by modifying the pulse width modulation, and then stop the electric motor at the predetermined position by short-circuiting a branch of the inverter when the predetermined position is reached.

[0018] According to one embodiment, the predetermined position is given by a stop sensor associated with the stop position of the wiping device, and the control unit is configured to use the output signal from the stop sensor to determine the time when the rotor's rotational speed should decrease.

[0019] According to another embodiment, the stop sensor is modified to provide a deceleration position from which the rotational speed of the electric motor should be reduced to allow the wiping device to stop at the stop position.

[0020] The present invention also relates to a wiping device, particularly a wiping device for motor vehicles, comprising a geared motor as described above. Attached Figure Description

[0021] Other features and advantages of the invention will become more apparent from the following description and accompanying drawings, which are given by way of illustrative and non-limiting example, in which:

[0022] Figure 1 A schematic diagram of an electric motor and its control inverter is shown.

[0023] Figure 2 A schematic perspective view of a portion of the wiping device is shown;

[0024] Figure 3 A schematic diagram of a portion of the stop sensor is shown;

[0025] Figure 4 A schematic diagram of a wheel on a metal track equipped with a stop sensor is shown;

[0026] Figure 5 A flowchart illustrating the steps of a method for controlling an electric motor is shown.

[0027] In these figures, the same elements have the same reference numerals. Detailed Implementation

[0028] The following examples are illustrative. Although described with reference to one or more embodiments, this does not necessarily mean that every aspect referred to will be included in one or more embodiments. Various features of the various embodiments can also be combined or interchanged, so as to create other embodiments.

[0029] The present invention relates to a method for controlling an electric motor of a motor vehicle wiper device.

[0030] Figure 1 A circuit diagram of a power inverter 1 for a three-phase electric motor 3, in particular for an electric motor 3 of a motor vehicle wiper device, is shown. The inverter 1 comprises three branches, labeled B1, B2 and B3, configured to supply power to the three phases A, B and C of the electric motor 3, respectively. Each branch B1, B2, B3 is connected on one hand to a positive terminal of a power source 5, for example a vehicle battery, and on the other hand to ground, corresponding to a negative terminal of the power source 5. Each branch B1, B2, B3 comprises two switches 7, typically formed by transistors, connected in series. A diode 8 is typically arranged in parallel with each transistor 7. A center tap between the two transistors 7 of the respective branch B1, B2, B3 is connected to the respective phase A, B, C of the electric motor 3.

[0031] In order to control the rotation of the electric motor 3, a pulse width modulated command is applied to the phases A, B, C of the electric motor 3. This command is applied by commanding the opening and closing of the transistors 7 via a control unit 18. During normal operation, the two switches 7 of the branches B1, B2, B3 are in opposite states (one open, while the other is closed). Moreover, there is always a phase A, B or C that is not powered (the switch 7 is connected to the positive terminal of the power source in the open position).

[0032] The electric motor 3 comprises a rotor. When the rotor rotates fast enough, that is to say, at a speed greater than a minimum threshold S min of the rotation speed of the rotor, the electromotive force at the phase A, B or C that is not powered can be measured, and the position of the rotor of the electric motor 3 can be deduced by detecting the time at which the measured voltage crosses zero, which is called the "zero-crossing" technique.

[0033] The pulse width modulated command is applied, for example, by a control unit 18 for controlling the electric motor 3. Moreover, when the electric motor 3 is started, a predetermined sequence of commands can be applied so that the rotation speed S min can be reached without determining the position of the rotor, and therefore without using a position sensor. In the same way, another predetermined sequence of commands can be applied to stop the electric motor without a position sensor, but this method does not make it possible to stop the electric motor 3 at a desired stop position.

[0034] Figure 2A schematic view of a wiper device 9 for a motor vehicle is shown. As Figure 1 illustrated, the wiper device 9 comprises an electric motor 3 controlled by an inverter 1. A gear reduction mechanism (not visible) is arranged at the output of the electric motor 3 in order to form a gear motor 10. The gear reduction ratio of the gear reducer is for example 1 / 69. The output of the gear reducer is connected to a linkage mechanism 11 enabling a mechanical connection between the output of the gear reducer and one or more wiper arms (not shown) of the wiper device 9. The linkage mechanism 11 comprises for example a set of links and cranks enabling the conversion of the rotational movement of the electric motor 3 into a reciprocating movement of the one or more wiper arms. In order to ensure that the wiper arms stop at a predetermined stop position, a stop sensor 13, also called "parking finger", is arranged in the wiper device 11, for example in the linkage mechanism 11 or at the output of the gear reducer.

[0035] Figure 3 and 4 An exemplary embodiment of such a stop sensor 13 is shown. The sensor 13 comprises a metal and thus a conductive track 15 having for example a resistance of less than 10 Mega Ohm. The metal track 15 is arranged on a wheel 17 made of a non-conductive material, for example plastic. The metal track 15 comprises a circular portion 15a from which an appendix 15b extends outwardly over a limited angular portion. The sensor 13 further comprises two contacts 19a and 19b, for example formed by two metal blades, configured to be in contact with the metal track 15. The first contact 19a is configured to be in contact with the circular portion 15a of the metal track 15 and is in permanent contact with the metal track 15, while the second contact 19b is configured to be in contact with the appendix 15b so that the second contact is in contact with the metal track 15 only over the limited angular portion corresponding to the appendix 15b and is in contact with the non-conductive wheel 17 the rest of the time.

[0036] Therefore, by measuring the resistance between the two contacts 19a and 19b, it is possible to detect the time when the appendix 15b of the metal track 15 is in contact with the second contact 19b, since the resistance between the two contacts 19a and 19b becomes substantially zero due to the conductive properties of the metal track 15. The wheel 17 is thus configured so that the angular region associated with the appendix 15b corresponds to the stop position. However, the configuration of the wheel, in particular of the metal track 15, can be modified so that the position of the appendix 15b corresponds to a position other than the stop position, for example 5° or 10° before the stop position.

[0037] According to a first embodiment, in order to limit the intensity of these current peaks when the electric motor 3 stops, the stop sensor 13 is modified so that the appendage corresponds to a deceleration position located before the stop position, for example 5° or 10° before the stop position. Moreover, during operation and when the wiper device 11 stops, when the deceleration position is reached and detected by the stop sensor 13, the control unit 18 is configured to reduce the rotational speed of the electric motor 3. Thus, the control unit 18 is configured to receive and use the output signal from the stop sensor 13. The rotational speed of the electric motor 3 is reduced to a predetermined rotational speed V1 corresponding to a minimum threshold S min , for which the position of the rotor can be determined from the electromotive forces measured at the phases A, B, C or from a speed slightly greater than this minimum threshold S min . The predetermined rotational speed V1 is for example within an interval between the minimum threshold S min and a rotational speed greater than the minimum threshold S min by 10%.

[0038] Thus, when the electric motor 3 stops, the speed is first reduced in order to change from the nominal operating speed (there are generally several nominal speeds) to a speed close to the minimum threshold S min . The reduction from the nominal speed to the predetermined rotational speed V1 can be implemented in a substantially linear manner or in an incremental manner, passing through various intermediate speeds at various predetermined positions in order to reach the predetermined speed V1 before reaching the stop position. In particular, the incremental configuration can differ according to the nominal speed at which the command to stop the electric motor 3 is sent.

[0039] The control unit 18 is configured to stop the rotor of the electric motor 3 by short-circuiting the branches B1, B2, B3 of the inverter 1 when the stop position is reached. This stop position is determined from the electromotive forces measured at the phases of the electric motor 3 and from the reduction ratio of the gear reducer.

[0040] Indeed, the electromotive forces make it possible to determine the position of the rotor (this is possible because the predetermined rotational speed is greater than the minimum threshold S min ) and the reduction ratio of the gear reducer makes it possible to deduce the position of the wiper arm(s) from the position of the rotor.

[0041] When the stop position is reached, the branches of the short-circuit inverter 1 thus result in current peaks of low intensity due to the reduced rotational speed (compared to the nominal rotational speed of the electric motor 3).

[0042] According to a second embodiment, the stop sensor 13 is not modified and indicates the stop position. The first predetermined position is then determined according to the rotor position estimated from the electromotive forces and from the reduction ratio of the gear reducer.

[0043] In practice, as mentioned above, when the rotation speed of the electric motor is sufficient, in other words greater than said minimum threshold S min the position of the wiper arm(s) can be estimated as a function of the rotor position determined from the electromotive force.

[0044] However, since the determination of the rotor position is a measurement of the relative position, it is necessary to have a position reference using the stop sensor 13. All types of sensorless control, such as the measurement of the electromotive force, make it possible to estimate the position of the wiper arm between the two stop positions of the wiper arm(s). This makes it possible, in particular, to determine the position of the arm before it reaches the stop position, for example to a first predetermined position located 5° before the stop position. Then, when the estimated position corresponds to the first predetermined position, the speed is reduced to a predetermined rotation speed VI. Next, when the stop position given by the stop sensor 13 is reached (at VI speed), the branches Bl, B2, B3 of the inverter 1 are short-circuited in order to stop the electric motor 3 in this stop position. In the same way as in the preceding embodiment, the peak of current generated in the transistors 7 of the inverter 1 is reduced (compared to the peak generated when a short-circuit is created when the electric motor 13 rotates at nominal speed).

[0045] The various steps of the method for controlling the electric motor 3 of the wiper device, as described above, will now be described. The invention relates more particularly to the control of the electric motor 3 when it is stopped, but steps relating to the start-up and normal operation of the electric motor 3 will also be described. Figure 5

[0046] The first step 101 relates to the activation of the wiper device 11. This activation corresponds, for example, to a start-up of the hand command of the user of the vehicle, which leads to an activation signal being sent to the control unit 18.

[0047] The second step 102 relates to the start-up of the electric motor 3. After receiving the actuation command, the control unit 18 for controlling the electric motor 3 applies a predetermined sequence of pulse width modulation commands to the inverter 1. This step can be performed with or without knowledge of the rotor position of the electric motor 3.

[0048] The third step 103 relates to the stabilisation of the rotation speed of the electric motor 3 at a nominal speed. This nominal speed is greater than the minimum threshold Smin, which means that the pulse width modulation control for achieving this nominal speed is achieved by the rotor position determined from the electromotive force measured at the branches of the inverter 1. The wiper device 1 can comprise a plurality of nominal speeds, generally two, so that the user can change the nominal speed over time as required. In all cases, the electric motor 3 is regulated using the electromotive force measured at the branches Bl, B2, B3 of the inverter 1.

[0049] ​The fourth step 104 involves a command to stop the wiper device 1. This command corresponds to a manual command from the user for example, which causes a stop signal to be sent to the control unit 18.

[0050] The fifth step 105 involves reducing the rotational speed of the electric motor 3 to a predetermined rotational speed. This reduction is performed when the wiper device 1 is in a predetermined position, for example 5° before the stop position.

[0051] According to a first embodiment, this position is given by the stop sensor 13, modified so as to detect this predetermined position, in other words the deceleration position.

[0052] According to a second embodiment, this position is determined from the estimated position of the rotor by means of the electromotive force measured at the branches B1, B2, B3 of the inverter 1, the gear reduction ratio of the gear reducer and the previous signal from the stop sensor 13.

[0053] The reduction in rotational speed can be linear or incremental (a first speed reduction is applied between the positions -15° and -10° (corresponding to 0° reference position before the stop position), then a second speed reduction is applied between the positions -10° and -5°). A non-linear reduction can also be applied.

[0054] The sixth step 106 involves stopping the electric motor 3 in the stop position by short-circuiting the branches B1, B2, B3 of the inverter 1 when the stop position is reached.

[0055] According to a first embodiment, the arrival of the stop position is determined from the estimated position of the rotor by means of the electromotive force measured at the branches B1, B2, B3 of the inverter 1, the gear reduction ratio of the gear reducer and the previous signal from the stop sensor 13 corresponding to the position 5°.

[0056] According to a second embodiment, the stop position is given by the stop sensor 13. The short-circuiting of the branches B1, B2, B3 of the inverter 1 causes a very rapid stop of the electric motor 3, thus corresponding to the stop of the wiper device 11 in the stop position. Furthermore, thanks to the speed reduction before the short-circuiting, the peak current generated in the transistors 7 of the inverter 1 is reduced, thus making it possible to avoid using oversized transistors 7, thus limiting the cost of the inverter 1.

[0057] The management of the stop of the electric motor 3 of the wiper device 11 as described above thus makes it possible to use a brushless and sensorless direct current electric motor 3 in such a device, while using transistors 7 of limited capacity, thus having a limited cost. It is thus possible to reduce the overall cost of the wiper device 11 while still maintaining the same operating quality for the user (stopping the wiper arm in the stop position).

Claims

1. A method for controlling a brushless and sensorless direct current electric motor (3) for a motor vehicle device, wherein the electric motor (3) comprises a rotor and phases (A, B, C) supplied by pulse width modulation, said pulse width modulation being applied to an inverter (1) for supplying the electric motor (3), and wherein, a minimum threshold (S min ) is exceeded, the position of the rotor is determined from the measured values of the electromotive force at the phases (A, B, C) of the electric motor (3), characterized in that, in the case of a command (104) to stop the electric motor (3), by modifying the pulse width modulation, the rotation speed of the rotor is reduced (105) from the nominal speed to a predetermined rotation speed (V1) in an interval between a minimum threshold (S min ) and 10% above said minimum threshold (S min ), and then, when the predetermined position is reached, the electric motor (3) is stopped (106) in the predetermined position by short-circuiting the branches (A, B, C) of the inverter (1), wherein said electric motor (3) is associated with a gear reduction device so as to form a gear motor (10) configured to drive a motor vehicle wiper device (11) comprising at least one wiper arm, said wiper device (11) comprising a stop sensor (13) associated with a stop position of the wiper arm, wherein said stop sensor (13) comprises an electrically conductive track comprising a circular portion and an appendix extending outwardly from a portion of the circular portion; a wheel of non-conductive material surrounding the circular portion; a first contact in contact with said circular portion during rotation of the rotor; and a second contact contacting the wheel or the appendix depending on the rotational position of the rotor, said predetermined position being detected by measuring the electrical resistance between the first contact and the second contact.

2. The control method of claim 1, wherein, The reduction (104) of the rotational speed of the rotor to a predetermined rotational speed (V1) comprises a plurality of increments associated with various reductions of the rotational speed of the electric motor (3).

3. The control method according to claim 1 or 2, wherein said predetermined position is given by said stop sensor (13) and an output signal from the stop sensor (13) is used to determine a time from which the rotational speed of the rotor should be reduced.

4. The control method of claim 3, wherein, Said stop sensor (13) is modified to provide a deceleration position from which the rotational speed of said electric motor (3) should be reduced in order to allow said wiper device (11) to stop at said stop position, for example 5° or 10° before said stop position.

5. The control method according to claim 3 or 4, wherein When the rotational speed of the rotor is greater than a minimum threshold (S min ) the position of the wiper arm is derived from the rotor position determined from the electromotive force and from the gear reduction ratio of the gear reducer.

6. A gear motor (10) for driving a motor vehicle wiper device (11), comprising a gear reduction and a brushless and sensorless direct current electric motor (3), wherein the electric motor (3) comprises a rotor, a control unit (18) and phases (A, B, C) supplied by pulse width modulation applied to an inverter (1) of the electric motor (3), and wherein the control unit (18) is configured so as to determine the position of the rotor from the measured values of the electromotive forces at the phases (A, B, C) of the electric motor (3) which exceed a minimum threshold (S min ) of the rotational speed of the rotor, characterized in that, In the case of a command to stop the electric motor (3), the control unit (18) is configured so as to reduce the rotational speed of the rotor from the nominal speed to a predetermined rotational speed (V1) in an interval between a minimum threshold (S min ) and 10% above said minimum threshold (S min ), by modifying the pulse width modulation, and then to stop the electric motor (3) at the predetermined position by short-circuiting the branches (B1, B2, B3) of the inverter (1) when the predetermined position is reached, wherein said electric motor (3) is associated with a gear reduction device so as to form a gear motor (10) configured to drive a motor vehicle wiper device (11) comprising at least one wiper arm, said wiper device (11) comprising a stop sensor (13) associated with a stop position of the wiper arm, wherein said stop sensor (13) comprises an electrically conductive track comprising a circular portion and an appendix extending outwardly from a portion of the circular portion; a wheel of non-conductive material surrounding the circular portion; a first contact in contact with said circular portion during rotation of the rotor; and a second contact contacting the wheel or the appendix depending on the rotational position of the rotor, said predetermined position being detected by measuring the electrical resistance between the first contact and the second contact.

7. The gear motor (10) of claim 6, wherein, Said predetermined position is given by a stop sensor (13) associated with a stop position of said wiper device (11), said control unit (18) being configured so as to use an output signal from said stop sensor (13) in order to determine a time from which the rotational speed of the rotor should be reduced.

8. The gear motor (10) of claim 7, wherein, Said stop sensor (13) is modified to provide a deceleration position from which the rotational speed of said electric motor (3) should be reduced in order to allow said wiper device (11) to stop at said stop position.

9. The gear motor (10) as claimed in claim 7 or 8, wherein, The control unit (18) is configured so as to derive the position of the wiper arm from the position of the rotor determined from the electromotive force and from the gear reduction ratio of the gear reduction device when the rotational speed of the rotor is greater than the minimum threshold (S min ).

10. A wiper device (11) especially for a motor vehicle comprising a gear motor (10) according to any one of claims 6 to 9.

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