SENSOR MAGNET AND MOTOR

Abstract

A sensor magnet (30) which is an annular sensor magnet (30) fixed to a motor shaft (20) by pressing, comprising a ring portion (32) on the outer peripheral portion of which magnetic poles are alternately formed, a fixing portion (34) provided facing from the inner peripheral portion of the ring portion (32) toward the center and supporting and fixing the pressed motor shaft (20), and a stress-reducing portion (38) which reduces stresses arising at a contact portion of the fixing portion (34) with the motor shaft (20) when the motor shaft (20) is pressed into the fixing portion (34), wherein the sensor magnet (30) is formed by a bonded magnet in which a magnetic material is dispersed in a plastic material, wherein the fixing portion (34) has a plurality of arms (40) extending from the inner peripheral portion of the ring portion (32) toward the center,the stress-reducing section (38) is a circular arc region between the fixing ends of the arms (40), characterized in that the thickness (W1) of the circular arc region in the radial direction of the central section is greater than the thickness (W2) in the radial direction at both ends.

Description

Technical area

[0001] The present invention relates to a sensor magnet used in a motor. General state of the art

[0002] Typically, a motor is provided with a rotation detection device that detects the speed and rotational position of the motor. In the rotation detection device, a ring-shaped magnet, called a sensor magnet, whose magnetic poles are alternately arranged in the circumferential direction, is fixed to the motor shaft, and a magnetic induction element is provided nearby. The sensor magnet is magnetized such that its magnetic flux changes with the rotation of the rotor. The change in the magnetic pole position of the sensor magnet with the rotation of the motor shaft, which is detected by the magnetic induction element, corresponds to the change in the rotational position of the rotor.

[0003] Therefore, the sensor magnet must be fixed to the motor shaft. A direct pressing method has been proposed as a method for fixing a sensor magnet to a motor shaft, in which the motor shaft is pressed into the center of the sensor magnet, but this method carries the risk of breaking the sensor magnet. Therefore, another method for fixing the sensor magnet to the motor shaft is a method in which the sensor magnet is fixed to the motor shaft via a sensor magnet holder (see JP 2010-35411 A).

[0004] In the sensor magnet holder of this method, a tubular section with a compression section into which the motor shaft is pressed is integrally formed, and a plurality of plate-shaped snap joints projecting from the tubular section in the axial direction are formed integrally. The sensor magnet is fixed to the magnet holder by the elastically deformable snap joints. Therefore, stresses generated when the motor shaft is pressed into the sensor magnet holder are not directly transmitted from the motor shaft to the sensor magnet, thus preventing breakage of the sensor magnet. DE 603 09 084 T2 deals with the production of an annular sensor magnet in which a magnetic material is dispersed in a plastic material. WO 97 / 46 850 A1 discloses a sensor magnet according to the preamble of claim 1, and WO 2016 / 132 827 A1 discloses a motor according to the preamble of independent claim 4. Summary of the inventionObject of the present invention

[0005] However, the method of fixing the sensor magnet to the motor shaft via the sensor magnet mount requires the sensor magnet mount as a separate component, which increases the number of parts and the number of assembly steps. Compared to directly pressing the sensor magnet onto the shaft, this method is also more prone to assembly errors.

[0006] The present invention has been made in view of the circumstances described, and its object is to provide a novel technique for fixing a sensor magnet to a shaft. Means of solving the task

[0007] To achieve this object, a sensor magnet according to one aspect of the present invention is an annular sensor magnet fixed to a motor shaft by pressing, comprising a ring portion on whose outer peripheral portion magnetic poles are alternately formed, a fixing portion provided facing from the inner peripheral portion of the ring portion toward the center and supporting and fixing the pressed motor shaft, and a stress relief portion that reduces stress generated at a contact portion of the fixing portion with the motor shaft when the motor shaft is pressed into the fixing portion. The sensor magnet is formed by a bonded magnet in which a magnetic material is dispersed in a plastic material.

[0008] According to this aspect, stresses generated near the contact portion of the fixing portion when the motor shaft is pressed into the central portion of the sensor magnet can be alleviated, so that the sensor magnet can be directly fixed to the motor shaft. Furthermore, the sensor magnet can be more easily manufactured into various shapes. It is sufficient for the stress-relieving portion to be configured to alleviate stresses concentrated at a specific location when the motor shaft is pressed into the sensor magnet, and it does not necessarily need to be located near the contact portion. The stress-relieving portion may also occupy at least a part of the fixing portion or the ring portion.

[0009] The fixing section has several arms extending from the inner peripheral portion of the ring portion toward the center. This allows the sensor magnet to be fixed to the motor shaft without significantly reducing the degree of coaxiality when pressing the motor shaft into the sensor magnet.

[0010] The contact section that contacts the motor shaft can be provided at the front end of the arms. This allows the fixing section to support the motor shaft with a simple structure.

[0011] The stress-reducing section is also a circular arc between the fixing ends of the arms. This reduces the stress concentration at the fixing section in an area remote from the fixing section.

[0012] The radial thickness of the circular arc region of the middle section is greater than the radial thickness at both ends. This allows some of the stress acting on the middle section of the circular arc region to be distributed to the two end sections.

[0013] The arms may have a radial portion extending radially inward from the inner peripheral portion of the ring portion and an axial portion extending axially from an end portion on the center side of the radial portion. The contact portion in contact with the motor shaft may also be provided on the inner peripheral surface of the axial portion. This makes the arms easier to bend.

[0014] The stress-relieving section can also be an arm. This prevents excessive stress concentration in an area other than the fixation section.

[0015] If the number of arms is N1 and the number of magnetic poles formed on the ring section is N2, then N2 can be an even number, and N2=N1×n (where n is a natural number) can be satisfied. In this way, the magnetic characteristics of the magnetic poles of the ring section can be determined with high precision.

[0016] Another aspect of the present invention is a motor. The motor includes a motor section having a brush, a sensor magnet fixed to a motor shaft of the motor section, a magnetic sensor arranged to oppose a ring portion of the sensor magnet, a worm fixed to the motor shaft, and a worm gear meshed with the worm.

[0017] According to this aspect, by detecting the rotation of the sensor magnet directly fixed to the motor shaft of the motor portion having the brush by the magnetic sensor, the position of a driven object such as a power window or seat driven by a reduction gear composed of the worm and the worm wheel can be controlled.

[0018] Any combination of the structural elements described above and expression of the present invention as a method, apparatus or system are also valid aspects of the present invention. Effect of the invention

[0019] According to the present invention, a sensor magnet can be fixed to a shaft. Short description of the characters

[0020] They show: Fig. 1 is a front view of a worm gear reduction motor with a sensor magnet according to an embodiment; Fig. 2 is a sectional view showing essential parts of the worm gear reduction motor with the sensor magnet of the present embodiment; Fig. 3 a perspective view of the sensor magnet according to a first embodiment; Fig. 4A is a front view of the sensor magnet of the first embodiment and Fig. 4B is a sectional view along line AA of the sensor magnet Fig. 4A; Fig. 5A is a perspective view of a sensor magnet according to a second embodiment and Fig. 5B is a perspective view of the sensor magnet of the second embodiment from a different direction than Fig. 5A considered; Fig. 6A is a front view of the sensor magnet of the second embodiment and Fig. 6B is a sectional view along line BB of the sensor magnet Fig. 6A; Fig. 7A is a front view of the sensor magnet according to a first modification example and Fig. 7B is a front view of a sensor magnet holder according to a second modification example; Fig. 8 is a front view of a sensor magnet holder according to a third modification example; and Fig. 9 is a front view of a sensor magnet holder according to a fourth modification example. Embodiments of the invention

[0021] Embodiments of the present invention will be described below with reference to the figures. In the description of the figures, like elements are designated by like reference numerals, and duplicate descriptions are omitted. The configuration discussed below is exemplary and is not intended to limit the scope of the present invention.

[0022] The technique of the present invention can be applied to various motors, such as brushed or brushless DC motors, or worm geared motors used in these motors, where speed and rotational position detection are required. The following description will be given using the application to a worm geared motor as an example. First embodiment: Reduction gear motor

[0023] Fig. 1 shows a front view of a worm gear reduction motor with a sensor magnet according to an embodiment. Fig. 2 is a sectional view showing essential parts of the worm gear reduction motor with the sensor magnet of the present embodiment. Fig. 1, part of the housing of a reduction gear section was omitted to make the sensor magnet visible.

[0024] A reduction gear motor 10 is mainly composed of a motor section 12 and a reduction gear section 14. The reduction gear motor 10 is a conventional brushed DC motor. The motor section 12 is formed of a metal material in the shape of a tube with a bottom and includes a motor housing 16 with a magnet attached to its inner peripheral surface, and an end cap 18 fixed to close the opening portion of the motor housing 16. A front end of a motor shaft 20 extending outward from the end cap 18 of the motor section 12 is supported by a shaft bearing provided inside a reduction gear housing 22.

[0025] The reduction gear section 14 is formed, among other things, of a worm 24 coupled to the extending motor shaft 20, a worm gear 26 meshed with the worm 24, and an output shaft 27 extending from the center of the worm gear 26. The output shaft 27 is connected, for example, to an electrically operated window device for a vehicle.

[0026] A rotation detection device 28 is formed by a combination of a magnetic induction element 29 (e.g., a Hall element, a GMR (Giant Magneto Resistive) element) on the fixed side and a sensor magnet 30 on the rotating side. The Hall element shown as an example is mounted on a substrate 31 fixed to the reduction gear housing 22, opposite the rotating sensor magnet 30. The rotation detection device 28 has leads for supplying power to the Hall element and outputting signals. Furthermore, the rotation detection device 28, including the Hall element, detects a magnetic flux that changes due to the relative movement of the sensor magnet 30 and the Hall element due to the motor rotation, and outputs a pulse signal. In this way, it is possible to detect and control the speed and rotational position of the motor. Sensor magnet

[0027] Fig. 3 shows a perspective view of the sensor magnet 30 according to a first embodiment. Fig. 4A shows a front view of the sensor magnet 30 of the first embodiment and Fig. 4B is a sectional view along the line AA of the sensor magnet 30 of Fig. 4A.

[0028] The sensor magnet 30 is a ring-shaped magnet that is directly fixed to the motor shaft 20 by compression. The sensor magnet 30 according to the present embodiment is formed by a bonded magnet in which a magnetic material is dispersed in a plastic material. For example, isotropic or anisotropic barium ferrite, strontium ferrite, SmCo, NdFeB, SmFeN, AlNiCo, or the like can be used as the magnetic material.

[0029] As the plastic material serving as a binder, in the case where the sensor magnet is formed by injection molding, a thermoformable plastic is advantageous. For example, polyamide plastic (polyamide 6, polyamide 12) can be used as the thermoformable plastic. The mixing ratio of the magnet material and the plastic material (weight ratio) can be selected depending on the properties of the magnet material and is, for example, in a range of approximately 95:5-70:30. A residual magnetic flux density Br of the sensor magnet 30 of the present embodiment is approximately 10-500 mT, and a holding force bHc is approximately 40-250 kA / m.

[0030] By melting a mixture in which the magnetic material is dispersed in the plastic material and injecting it into a mold, sensor magnets of various shapes can be manufactured with high precision and efficiency.

[0031] The sensor magnet 30 has a ring section 32, on the outer peripheral section of which magnetic poles are formed alternately, a fixing section 34, which is provided from the inner peripheral section of the ring section 32 towards the center and supports and fixes the pressed motor shaft 20, and as a stress reducing section, a circular arc region 38, which reduces stresses arising when the motor shaft 20 is pressed into the fixing section 34 at a contact section 36 of the fixing section 34 with the motor shaft 20.

[0032] With the sensor magnet 30 of the present embodiment, stress generated when the motor shaft 20 is pressed into the central portion of the sensor magnet near the contact portion 36 of the fixing portion 34 can be alleviated, making it less likely that excessive stress concentration will occur at a portion of the sensor magnet. Therefore, the sensor magnet 30 can be directly fixed to the motor shaft 20 without any other components between the motor shaft and the sensor magnet.

[0033] It is sufficient if the stress relief portion is configured to alleviate stresses concentrated at a specific location when the motor shaft 20 is pressed into the sensor magnet 30, and it need not necessarily be located near the contact portion 36. Therefore, the stress relief portion of the present embodiment is a circular arc region 38 that forms a part of the ring portion 32 remote from the fixing portion 34. By elastically deforming the circular arc region 38, stress concentrations near the contact portion 36 are alleviated when the motor shaft 20 is pressed into the sensor magnet 30, thereby reducing the overall rigidity of the sensor magnet 30.

[0034] According to CAE (Computer Aided Engineering) analysis, the presence of the stress relief portion on a part of the sensor magnet 30 can reduce the peak value of stress generated when the motor shaft 20 is pressed into the sensor magnet 30, compared to a rigid body without the stress relief portion on the sensor magnet 30. Therefore, a condition in which a part of the sensor magnet 30 is broken or excessive force beyond the specifications is required during press-fitting can be reduced.

[0035] In addition, the fixing section 34, as shown in Fig. 4A, the sensor magnet 30 has a plurality of (four) arms 40 extending from the inner peripheral portion of the ring portion 32 toward the center. The four arms 40 are arranged in a radial pattern at 90-degree intervals with high symmetry around the position where the motor shaft 20 is press-fitted as the center. Thus, the sensor magnet 30 can be fixed to the motor shaft 20 without excessively reducing the degree of coaxiality when the motor shaft 20 is pressed into the sensor magnet 30.

[0036] The contact portion 36, which is in contact with the motor shaft 20, is provided at the front end of the arms 40. Thus, the fixing portion 34 can support the motor shaft 20 with a simple structure. The front and back sides of the sensor magnet 30 are formed symmetrically, so that confusion between the front and back sides cannot occur when installing the motor shaft 20.

[0037] The circular arc region 38 is provided between the fixing ends 42 of the arms 40. In this way, the stress concentration at the fixing section 34 can be reduced in a region remote from the fixing section 34, which increases the degree of freedom in the design of the sensor magnet.

[0038] A thickness W1 of the circular arc region 38 in the radial direction of the central portion is greater than a thickness W2 in the radial direction at both ends. In this way, part of the stress acting on the central portion of the circular arc region 38 can be distributed to the two end portions. This means that when the motor shaft 20 is pressed into the sensor magnet 30, the peak value of the stress generated in the circular arc region 38 can be reduced.

[0039] For the sensor magnet 30 according to the present embodiment, as shown in Fig. As shown in FIG. 4A, when a distance between the contact portions 36 of a pair of opposing arms 40 is L and a diameter of the motor shaft 20 is D, a range of 1.0>L / D≥0.94 is set. More preferably, a range of 0.99≥L / D≥0.97 is set.

[0040] Furthermore, if the number of arms of the sensor magnet is N1 and the number of magnetic poles formed at the ring portion is N2, then N2 may be an even number, and N2=N1×n (where n is a natural number). Specifically, the number of arms 40 of the sensor magnet 30 is four, while the number of magnetic poles formed at the ring portion 32 should be four, eight, twelve, or the like. In this way, the magnetic poles can be positioned with high symmetry with respect to the arms. For example, if there are four arms and four magnetic poles, the position of each magnetic pole corresponds to the adjacent arm. In this way, the magnetic characteristics of the magnetic poles of the ring portion 32 can be determined with high precision. On the other hand, if the number of arms is three and the number of magnetic poles is four, the position of each magnetic pole cannot correspond to an adjacent arm.

[0041] In this way, the reduction gear motor 10 of the present embodiment includes the motor portion 12 having the brush, the sensor magnet 30 fixed to the motor shaft 20 of the motor portion 12, the magnetic induction element 29 arranged to oppose the ring portion 32 of the sensor magnet 30, the worm 24 fixed to the motor shaft 20, and the worm wheel 26 meshed with the worm 24.

[0042] Thus, by detecting the rotation of the sensor magnet 30 fixed directly to the motor shaft 20 of the motor section 12 with the brush by the magnetic induction element 29, the position of a driven object, such as an electrically operated window or seat, driven by the reduction gear section 14 constructed of the worm 24 and the worm wheel 26 can be controlled. Second embodiment

[0043] Fig. 5A is a perspective view of a sensor magnet according to a second embodiment and Fig. 5B is a perspective view of the sensor magnet of the second embodiment from a direction other than Fig. 5A is considered. Fig. 6A is a front view of the sensor magnet of the second embodiment and Fig. 6B is a sectional view along line BB of the sensor magnet of Fig. 6A. Structural elements identical to those of the sensor magnet 30 of the first embodiment will not be described again.

[0044] The sensor magnet 50 includes a ring portion 52 with alternating magnetic poles formed on its outer peripheral portion, a fixing portion 54 extending from the inner peripheral portion of the ring portion 52 toward the center and supporting and fixing the pressed motor shaft 20, and a stress-relieving portion that reduces stresses generated at a contact portion 56 of the fixing portion 54 with the motor shaft 20 when the motor shaft 20 is pressed into the fixing portion 54. The stress-relieving portion according to the present embodiment occupies at least a portion of the fixing portion 54.

[0045] In addition, the fixing section 54, as shown in Fig. 5A, Fig. 5B and Fig. 6A, a plurality of (six) arms 58 extending from the inner peripheral portion of the ring portion 52 toward the center. The arms 58 have, as shown in Fig. 6B, the arms 58 have a radial portion 58a extending radially inward from the inner peripheral portion of the ring portion 52, and an axial portion 58b extending axially from an end portion on the center side of the radial portion 58a. A contact portion 56 is provided on the inner peripheral surface of the axial portion 58b, which is in contact with the motor shaft 20. This facilitates bending of the arms 58.

[0046] The six arms 58 are arranged radially at 60-degree intervals with high symmetry around the point where the motor shaft 20 is pressed in, as the center. This allows the sensor magnet 50 to be fixed to the motor shaft 20 without excessively reducing the degree of coaxiality when the motor shaft 20 is pressed into the sensor magnet 50.

[0047] In this way, in the sensor magnet 50 of the present embodiment, stresses generated when the motor shaft 20 is pressed into the central portion of the sensor magnet near the contact portion 56 of the fixing portion 54 can be alleviated, making it less likely that excessive stress concentration will occur at a portion of the sensor magnet. Therefore, the sensor magnet 50 can be directly fixed to the motor shaft 20 without any other components between the motor shaft and the sensor magnet.

[0048] The stress relief portion of the present embodiment occupies a portion of the arms 58. This prevents excessive stress concentration from occurring in a region other than the fixing portion 54. By bending (elastically deforming) the arms 58, stress concentrations near the contact portion 56 when the motor shaft 20 is pressed into the sensor magnet 50 are alleviated by reducing the overall rigidity of the sensor magnet 50.

[0049] According to CAE analysis, by providing the stress relief portion on a part of the sensor magnet 50, the peak value of the stress generated when the motor shaft 20 is pressed into the sensor magnet 50 can be reduced compared to a rigid body without the stress relief portion on the sensor magnet 50. Therefore, a situation where a part of the sensor magnet 50 breaks or excessive force beyond the specifications is required during press-fitting can be reduced.

[0050] The sensor magnet 50 according to the present embodiment is constructed such that, as shown in Fig. 6B, when a diameter of an opening portion at an end surface on the insertion side of the motor shaft 20 (a distance between the radial portions 58a of a pair of opposing arms 58) is L1 and a diameter of an opening portion at an end surface opposite the insertion side of the motor shaft 20 (a distance between the contact portions 56 of a pair of arms 58) is L2, L2 <D<L1 erfüllt wird.

[0051] Therefore, when the motor shaft 20 is first inserted into the sensor magnet 50, the front end of the motor shaft 20 slides along a rounded section leading from the radial section 58a to the axial section 58b of the arms 58 without much resistance, making it easier to position the motor shaft 20 in the center of the sensor magnet 50. Subsequently, the arms 58 of the motor shaft 20 are gradually bent until the motor shaft 20 is fully pressed into the sensor magnet 50, reaching the contact section 56.

[0052] Also, the number of arms 58 of the sensor magnet 50 is six, while the number of magnetic poles formed on the ring portion 52 should be six, twelve, or the like. In this way, the magnetic poles can be positioned with high symmetry with respect to the arms. For example, if there are six arms and six magnetic poles, the position of each magnetic pole corresponds to the adjacent arm. In this way, the magnetic characteristics of the magnetic poles of the ring portion 52 can be determined with high precision. On the other hand, if the number of arms is six and the number of magnetic poles is four, the position of each magnetic pole cannot correspond to an adjacent arm. Variation examples

[0053] In the following, modification examples of the sensor magnet will be described, but mainly the characteristic parts will be described, while the description of structural elements identical to those of the above embodiments will be omitted. Fig. 7A is a front view of the sensor magnet according to a first modification example and Fig. 7B is a front view of a sensor magnet holder according to a second modification example.

[0054] The sensor magnet 60 from Fig. 7A includes an outer ring portion 62, a ring portion 64 provided further inside than the ring portion 62 and having a smaller diameter than the ring portion 62, and three arms 66 extending from the inner peripheral portion of the ring portion 64 toward the center. The ring portion 62 and the ring portion 64 are coupled by three coupling portions 68 formed at equal intervals.

[0055] The three arms 66 are arranged radially at intervals of 120 degrees with high symmetry around the point where the motor shaft 20 is pressed in, as the center. The arms 66 are each formed between two coupling sections 68. That is, the three arms 66 and the three coupling sections 68 are arranged offset from one another in the circumferential direction.

[0056] When the motor shaft 20 is pressed into the center of the sensor magnet 60 constructed in this way, the ring portion 64 at the fixing end of the arms 66 bends. That is, the ring portion 64 serves as a stress reducing portion.

[0057] One in Fig. The sensor magnet 70 shown in FIG. 7B has two fixing portions 74 provided opposite each other on the inner peripheral side of a ring portion 72. The fixing portion 74 includes a circular arc portion 76, a coupling portion 78 that couples the ring portion 72 and the circular arc portion 76, and a pair of arms 80 extending from both ends of the circular arc portion 76 toward the center.

[0058] When the motor shaft 20 is pressed into the center of the sensor magnet 70 constructed in this way, the circular arc portion 76 at the fixing end of the arms 80 bends. That is, the circular arc portion 76 serves as a stress reducing portion.

[0059] Fig. 8 is a front view of a sensor magnet holder according to a third modification example.

[0060] The Fig. The sensor magnet 82 shown in Figure 8 has four fixing portions 86 formed on the inner peripheral side of a ring portion 84. The fixing portions 86 include arms 90, at the front end of which a contact portion 88 is formed that comes into contact with the press-fitted motor shaft 20, a circular arc portion 92 supporting the arm 90 at one end, and a coupling portion 94 that couples the circular arc portion 92 and the ring portion 84. This allows the circular arc portion 92 to be extended between the coupling portion 94 and the arms 90, thereby allowing some of the fixing portions 86 to bend more easily.

[0061] Fig. 9 is a front view of a sensor magnet holder according to a fourth modification example.

[0062] In the Fig.A fixing portion 110 is coupled to the inside of the ring portion 100, which is connected to the sensor magnet 108 shown in Figure 9. Two linear portions 104 are formed on the fixing portion 110 such that they are opposite each other with the center between them. A recessed portion 106 corresponding to the outer diameter of the motor shaft 20 is formed at the middle portion of the linear portions 104.

[0063] When the motor shaft 20 is pressed into the center of the sensor magnet 108 constructed in this way, the linear sections 104 of the fixing section 110 bend. That is, the linear sections 104 serve as a stress reducing section.

Claims

[1] A sensor magnet (30) which is an annular sensor magnet (30) fixed to a motor shaft (20) by pressing, comprising a ring portion (32) on the outer peripheral portion of which magnetic poles are alternately formed, a fixing portion (34) provided facing from the inner peripheral portion of the ring portion (32) toward the center and supporting and fixing the pressed motor shaft (20), and a stress-reducing portion (38) which reduces stresses arising at a contact portion of the fixing portion (34) with the motor shaft (20) when the motor shaft (20) is pressed into the fixing portion (34), wherein the sensor magnet (30) is formed by a bonded magnet in which a magnetic material is dispersed in a plastic material, wherein the fixing portion (34) has a plurality of arms (40) extending from the inner peripheral portion of the ring portion (32) toward the center,the stress reduction section (38) is a circular arc region between the fixing ends of the arms (40), characterized by that the thickness (W1) of the circular arc region in the radial direction of the middle section is greater than the thickness (W2) in the radial direction at both ends. [2] Sensor magnet (30) according to claim 1, characterized by that the contact section which is in contact with the motor shaft (20) is provided at the front end of the arms (40). [3] The sensor magnet (30) according to claim 1 or 2, wherein when the number of arms (40) is N1 and the number of magnetic poles formed on the ring portion (32) is N2, N2 is an even number and N2=N1 × n (where n is a natural number). [4] A motor (10) comprising a motor section (12) with a brush, a sensor magnet (30) according to any one of claims 1 to 3 fixed to a motor shaft (20) of the motor section (12), a magnetic sensor arranged to oppose a ring section (32) of the sensor magnet (30), a worm (24) fixed to the motor shaft (20), and a worm wheel (26) toothed with the worm (24).

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