Sensor arrangement for an electric motor

Abstract

The present invention relates to a sensor arrangement for an electric motor comprising a rotor (30) of the electric motor and a magnet (32) fixedly attached to the rotor (30) to prevent rotation. The magnet (32) has a plastic matrix and magnetic powder embedded in the plastic matrix. The invention also relates to a motor arrangement and a method for manufacturing a sensor arrangement.

Description

Technical FieldThe present invention relates to a sensor arrangement for an electric motor. The invention also relates to an electric motor and to a method for producing a sensor arrangement.Prior ArtFor controlling electric motors, it may be desirable to know an angular position of a rotor relative to a stator. This makes it possible, for example, to efficiently commutation the electric motor. In addition, by detecting the angular position, it is also possible to infer further operating variables, such as a rotational speed. This can also be helpful or even necessary in the operation of a vehicle with such an electric motor.In order to be able to detect the angular position, a magnet is usually fastened to the rotor of an electric motor. A magnetic field of the magnet can have a specific orientation and alternatively or additionally strength in a measurement range depending on the angular position of the rotor. The magnetic field can be detected by a sensor. The angular position can then be determined by a corresponding evaluation.However, magnets are usually difficult to assemble. A magnet can be very brittle. In addition, a change in the magnetic field can occur in the event of excessive deformation. A magnet can thereby be partially or completely demagnetized, for example. Therefore, the magnet for angular position sensing in electric motors is usually fixed to the rotor via a carrier. However, the support constitutes an additional part, as a result of which the fastening of the magnet is complicated and expensive. In addition, the support can require additional installation space.It is an object of the present invention to enable simple and cost-effective sensing of an angular position of a rotor relative to a stator in an electric motor.SUMMARY OF THE INVENTIONThis object is achieved by the subject matters of the subordinate patent claims.A first aspect relates to a sensor arrangement for an electric motor. The electric motor may be part of a powertrain of a vehicle. The electric motor can be, for example, a drive motor of a pedelec. The electric motor has a rotor. The rotor forms part of the sensor arrangement. The rotor may be operatively connectable to an output of the powertrain. The rotor may include a motor shaft of the electric motor. The rotor may be the part of the electric motor which rotates during operation. The electric motor may have a stator. The stator can have windings of the electric motor, for example. The stator can be energized during operation of the electric motor. The sensor arrangement can have the stator or even the entire electric motor. The electric motor can be designed, for example, as a synchronous machine or an asynchronous machine. The electric motor can be designed, for example, as a permanent magnet-excited synchronous motor.The sensor arrangement has a magnet fixed to the rotor in a rotationally fixed manner. Fixed in a rotationally fixed manner can mean that the magnet rotates with the rotor. Any rotational movement of the rotor can thus cause a corresponding rotational movement of the magnet. The magnet can be connected to the motor shaft, for example. The magnet may have a magnetic field. For example, the magnet can be designed as a dipole permanent magnet or lateral permanent magnet. During a rotation of the rotor, respective field lines of the magnet can correspondingly move together. The magnet can be designed to enable sensing of an angular position of the rotor relative to a stator of the electric motor with its magnetic field. The magnet can form a signal transmitter for a sensor for detecting an angular position of the rotor. The sensor arrangement can have a sensor. However, the sensor can also not be part of the sensor arrangement. The sensor can be designed to detect the magnetic field of the magnet. The sensor can be designed alternatively or additionally for determining the angular position of the rotor relative to the stator. The sensor can be designed, for example, as a chip which can detect magnetic field lines. The sensor can be fastened, for example, to the stator or a housing of the electric motor. The sensor therefore does not move with the rotor, for example.The magnet includes a plastic matrix and a magnetic powder embedded in the plastic matrix. The plastic matrix can have an elasticity which is greater than a magnet consisting exclusively of magnetic material. As a result, the mounting on the rotor can be simplified. In addition, the magnet can thus be adapted more easily in its shape to the rotor and its attachment to the rotor. The plastic matrix, for example in contrast to a metallic carrier, cannot influence the magnetic field of the magnet. Due to the plastic matrix, the magnet can be particularly compact. The magnetic powder may have been magnetized only after a shaping of the magnet, whereby the magnetic field can be specified relatively independently of a shape of the magnet. The magnet can be designed, for example, as a plastic-bonded magnet. The magnetic powder may include, for example, ferrite powder or neodymium powder. The plastic matrix can comprise polyamide, such as PA12, for example. The magnet may have a body formed by the plastic matrix with the embedded magnetic powder.The magnet may be directly attached to the rotor. The sensor arrangement may be free of a carrier by means of which the magnet is fastened to the rotor. The rotor may include one or more parts. The parts of the rotor may have a function in addition to holding the magnet. The sensor arrangement can thus be compact and cost-effective. The magnet can be rotationally symmetrical or can have at least one rotationally symmetrical basic shape, for example. An axis of symmetry of the magnet may be coaxial with an axis of rotation of the rotor.In a further embodiment of the sensor arrangement, it can be provided that the magnet is connected to the rotor by means of a press fit. In the case of a press fit, two diameters of components pushed onto one another can have an overlap. The press fit can form a rotationally fixed connection. The press fit may also axially fix the magnet to the rotor. For press fit connection, the rotor and alternatively or additionally the magnet may form a stop defining a relative axial position of the magnet and the rotor to each other. The attachment of the magnet to the rotor may be provided solely or at least partially by the press fit. For example, the magnet can be pressed into a recess of the rotor, such as in a central blind hole. The magnet can thus be pressed radially on the outside onto an inner periphery of the rotor. For example, the magnet can also be pressed on the outside around the rotor. For example, the rotor can have a sleeve-shaped section which is pushed onto the rotor. The rotor can have a recess, such as a central blind hole, in which the rotor is pressed. The magnet can thus be pressed radially on the inside onto an outer periphery of the rotor. The press fit can enable a compact and cost-effective fastening, in which no further parts, such as a carrier, are necessary for fixing.The magnet can have at least one axial or radial recess, such as a central bore or an outside groove, which vents a space between the magnet and the rotor during pressing on. This venting makes it possible to produce the press fit more easily and, in addition, to be more reliable. This recess can also reduce stress in the magnet after pressing. For example, a central through-opening can allow material to escape radially inward at the magnet when it is pressed into a recess of the rotor, as a result of which internal stresses in the magnet can be low. Alternatively or additionally, the rotor can also have at least one axial or radial recess, such as a central bore or an outer groove, which vents a space between the magnet and the rotor during pressing on.In a further embodiment of the sensor arrangement, it can be provided that the magnet is connected to the rotor in a form-fitting manner. For example, the magnet can have a profile on a circumferential surface facing the rotor, such as a toothing or individual projections. The rotor can have a profile corresponding thereto on a circumferential surface facing the magnet, such as a toothing or individual recesses. These profiles can interlock and lock the magnet and rotor in their relative angular position to each other. The form fit can provide a particularly robust connection for the fastening, for example as an alternative or in addition to the press fit. For example, the press fit can mainly axially secure the magnet and the form fit can additionally reliably prevent rotation due to vibrations. Alternatively or additionally, the magnet is connected to the rotor, for example, by an adhesive bond.In a further embodiment of the sensor arrangement, it can be provided that the magnet has a fastening region. The fastening region can be generally sleeve-shaped, for example. The sleeve-shaped fastening region can be formed, for example, by a recess in a journal region, in which the rotor is inserted. The sleeve-shaped fastening region can also be a pin with a central ventilation bore, which is inserted into the rotor. The fastening region can also be designed as a different type of pin, for example without recesses. The magnet may be fixed to the rotor with the fixing portion. This region can thereby be particularly well adapted in its shape for the fastening. The fastening region is arranged, for example, axially in the same region as the rotor. Other regions of the magnet, on the other hand, can thus be optimized for the magnetic field generation, for example, and protrude from the rotor, whereby the magnetic field of this region of the magnet is less shielded or not shielded at all by the rotor. The rotor can be designed, for example, as a metallic component.In a further embodiment of the sensor arrangement, it can be provided that the fastening region has a cross-sectional shape by which a radial elasticity of the fastening region is increased. For example, the fastening region can have an increased radial elasticity compared to other regions. For example, the fastening region can have a higher elasticity due to its cross-sectional shape than a disk-shaped region of the magnet protruding from the rotor. For example, the fastening region can have increased elasticity due to a groove profile and, alternatively or additionally, wave profile on the inner circumference and, alternatively or additionally, outer circumference. For example, a circumferential surface of the magnet may have such a profile that is not in contact with the rotor. As a result, a press fit can further have a large contact surface and enable a secure fastening to the rotor. Further shapes for the increased elasticity are, for example, a pleated skirt or an concertina-like shape. The cross section of the magnet in the attachment region may have a geometric shape that reduces stress in the magnet after it is pressed onto the rotor. As a result, voltage in the magnet can be subcritical, as a result of which a magnetic field can be maintained and the magnet can remain undamaged even in the case of strong press fits.In a further embodiment of the sensor arrangement, it can be provided that the magnet has a magnet region. The magnet portion may protrude from the rotor. The magnetic region may provide the magnetic field sensed by the sensor mainly or even alone. By the protrusion, the magnetic field of the magnetic region can be shielded from the rotor little or not at all with respect to the sensor. Due to the protrusion, the magnet region is also deformed only slightly or not at all when the magnet is fastened to the rotor. The magnet region can be designed, for example, in the form of a disk. The magnet region can be arranged, for example, axially in front of the rotor. The magnet region can thus be arranged axially next to the rotor without overlapping with the rotor. The magnet region may be the only magnetic region of the magnet. Non-magnetic regions can be free of the magnetic powder and consist, for example, only of the plastic matrix. However, the magnet can also have other magnetic regions, such as the fastening region. The magnet can be, for example, a one-piece component which is formed everywhere from the same material combination or has at least one continuous plastic matrix.In a further embodiment of the sensor arrangement, it can be provided that the magnet region has a higher concentration of magnetic powder than another region of the magnet. For example, the concentration of magnetic powder in the attachment portion may be lower than that in the magnet portion. The magnet area may be the area having the highest concentration of magnetic powder in the magnet. As a result, a required amount of magnetic powder can be small. In the attachment region, a concentration of magnetic powder may also be zero or close to zero. For example, the magnetic powder concentration can be adjusted differently in different areas by 2K injection molding or material accumulation of magnetic powder can be influenced otherwise.A second aspect relates to an engine assembly, for example for a vehicle such as a bicycle. The motor arrangement comprises an electric motor. The electric motor has a rotor and a stator. The stator is, for example, designed fixed to the housing and has respective coils. The rotor has, for example, an axis of rotation. A driving force can be provided on the rotor by the electric motor. The motor assembly may include a housing. The motor assembly may include a power supply for the electric motor, such as a battery. The motor arrangement may comprise power electronics for controlling the electric motor, such as an inverter. The motor arrangement comprises a sensor arrangement according to the first aspect. The magnet can thus be fixed to the rotor in a rotationally fixed manner. The electric motor can be designed to be controlled as a function of an angular position of the rotor relative to the stator, which angular position is detected by means of the sensor arrangement. Respective further features, embodiments and advantages can be gathered from the descriptions of the first aspect. Conversely, features, embodiments and advantages of the second aspect also represent features, embodiments and advantages of the first aspect.A third aspect relates to a method for producing a sensor arrangement. By means of the method, for example, a sensor arrangement according to the first aspect can be produced. An engine arrangement according to the second aspect can also be produced by the method according to the first aspect. Respective further features, embodiments and advantages can be gathered from the descriptions of the first and of the second aspect. Conversely, features, embodiments and advantages of the third aspect also represent features, embodiments and advantages of the first and second aspects.The method comprises a step of providing a rotor of an electric motor. The entire electric motor can also be provided. The method comprises a step of forming a magnet blank from a plastic matrix and magnetizable powder embedded therein. The magnetizable powder may not be magnetized at this time. The forming may include, for example, injection molding and, alternatively or additionally, pressing. For example, a plastic and the magnetizable powder can be mixed and liquefied in an extruder and injected into a mold for producing the magnet blank. The method comprises a step of magnetizing the magnetizable powder to produce from the magnet blank a magnet having a plastic matrix and magnetic powder embedded therein. The magnetizing can take place before or after the production of the magnet blank. The magnetizable powder in the magnet blank can be magnetized, for example, by a strong external magnetic field or by application of a voltage. The magnet blank can be held in a predetermined orientation. The method includes a step of attaching the magnet to the rotor. The fastening can comprise, for example, pressing the magnet onto the rotor or pressing the magnet into the rotor. As a result, a connection can be provided by a press fit. The magnet can be attached to the rotor with a predetermined relative orientation. Alternatively or additionally, a sensor or else a motor controller can be adapted, after the fastening, to an orientation of the magnet on the rotor and thus to an orientation of the magnetic field by means of calibration.In a further embodiment of the sensor arrangement, it can be provided that the fastening of the magnet to the rotor comprises a connection by means of an interference fit. For example, the sleeve-shaped fastening region of the magnet can be hydraulically pressed onto the rotor.Brief Description of the FiguresFIG. 1 shows a schematic sectional view of a sensor arrangement for an electric motor according to the prior art. FIG. 2 shows a schematic sectional view of a first embodiment of a sensor arrangement for an electric motor with a rotor and a magnet fastened thereto. FIG. 3 shows a schematic perspective view of the magnet of the sensor arrangement according to FIG. 2. FIG. 4 shows a schematic sectional view of a second embodiment of a sensor arrangement for an electric motor with a rotor and a magnet fastened thereto. FIG. 5 shows a schematic perspective view of the magnet of the sensor arrangement according to FIG. 4. FIG. 6 shows a schematic sectional view of a third embodiment of a sensor arrangement for an electric motor with a rotor and a magnet fastened thereto. FIG. 7 shows a schematic sectional view of a fourth embodiment of a sensor arrangement for an electric motor with a rotor and a magnet fastened thereto. FIG. 8 shows a schematic perspective view of the magnet of the sensor arrangement according to FIG. 7. FIG. 9 shows a further schematic sectional view of the sensor arrangement according to FIG. 7.Detailed Description of EmbodimentsFIG. 1 shows a schematic sectional view of a sensor arrangement for an electric motor. The sensor arrangement has a rotor 10 of the electric motor, which is formed by a metallic motor shaft 12 and a sheet metal pack 16 with permanent magnets 14 arranged therein. The rotor 10 is rotatably mounted by two radial bearings 18 on a housing, not shown, of the electric motor. In operation, the rotor 10 is rotated by energizing a not-shown stator of the electric motor to provide a driving force to the motor shaft 12.In addition, the sensor arrangement has a magnet 20. The magnet 20 is formed as a single-piece disk and consists of a magnetic material, in this case magnetized ferrite. The sensor arrangement has a carrier 22. The magnet 20 is held in a recess of the carrier 22. The carrier 22 is connected to the motor shaft 12 with an interference fit at one axial end. For the pressing-on, the magnet 20 and the carrier 22 each have a central through bore for venting. Via the carrier 22, the magnet 20 is fixed in a rotationally fixed manner to the motor shaft 12 and thus to the rotor 10. The magnet 20 protrudes from the motor shaft 12 and has a magnetic field. This magnetic field thus rotates with the rotor 10. This magnetic field is detected by a sensor that is spaced apart from the magnet 20 and fixed to the housing, whereby an angular position of the rotor 10 can be deduced. As a result, a rotational position of the rotor 10 relative to the stator is known.FIG. 2 shows a schematic sectional view of a first embodiment of a sensor arrangement. The sensor arrangement also has a rotor 30 and a magnet 32. The rotor 30 is formed here only by a motor shaft. In other embodiments, the rotor 30 also includes a sheet metal pack 16 and permanent magnets 14. For simplification, however, these are not shown here, as are the radial bearings 18.The first embodiment of the sensor arrangement does not have a carrier 22. Instead, the magnet 32 has a plastic matrix and magnetic powder embedded therein. The magnet 32 is formed as a one-piece component with a continuous plastic matrix and magnetic powder uniformly distributed therein. The magnet 32 is directly connected to the rotor 30 without the aid of a carrier. In the example shown, the magnet 32 was produced in an injection molding process, wherein a magnetizable powder was first used, which was only magnetized after shaping of the magnet 32.In FIG. 3, the magnet 32 is shown in a schematic perspective view. A pin-shaped fastening region 34 is inserted into a corresponding blind hole 36 of the rotor 30 for fastening the magnet 32. In this case, a press fit is produced with the rotor 30 and the magnet is thus connected to the rotor 30 in a rotationally fixed manner. The magnet 32 has a central through-opening 38. The through-opening 38 vents the blind hole 36 when the magnet 32 is inserted, and an end of the pin-shaped fastening region 34 facing the rotor 30 when inserted is chamfered to facilitate insertion. A circumferential surface of the magnet 32 is formed cylindrically, as a result of which friction and pressing and thus also a hold of the magnet 32 on the rotor 30 can be high. The magnet 32 also has a disk-shaped magnet portion 40 which axially protrudes from the rotor 30. When the magnet 32 is pressed in, the magnet 32 is inserted into the rotor 30 until it abuts.FIG. 4 shows a second embodiment of the sensor arrangement in a schematic sectional view. Differences from the first embodiment will be explained below.In the second embodiment, an outer periphery of the mounting portion 34 of the magnet 32 has a groove profile. This can also be easily seen in the schematic perspective view of the magnet 32 according to the second embodiment in FIG. 5. Here, axially extending panels are provided which are uniformly spaced apart in the circumferential direction. The fastening region 34 thus has a cross-sectional shape by which a radial elasticity of the fastening region 34 is increased. When the fastening region 34 is pressed into the blind hole 36 of the rotor 30, material of the magnet 32 can thus be pressed into the free space formed by the grooves. As a result, an internal stress due to the press fit in the fastening region 34 of the magnet 32 is lower. In addition, the grooves can also serve as a vent for the blind hole 36 during the pressing-in of the magnet 32. Accordingly, the central through hole 38 in the magnet 32 according to the second embodiment has been omitted. In another embodiment, however, such a through-opening 38 can also be additionally provided.FIG. 6 shows a third embodiment of the sensor arrangement in a schematic sectional view. Differences from the first embodiment will be explained below.The magnet 32 of the third embodiment has a sleeve-shaped fastening region 44. This sleeve-shaped fastening region 44 is pushed or fitted onto an axial end of the rotor 30, instead of being introduced into a blind hole 36. Accordingly, the rotor 30 according to the third embodiment does not have a blind hole bore. Instead, the rotor is beveled radially on the outside at this end in order to facilitate pushing on the magnet 32. As in the first embodiment, the magnet region 40 has a disk shape which, however, does not have a radial shoulder to the fastening region 44 but merges smoothly radially on the outside into the latter. The through-opening 38 in the magnet 32 for venting an intermediate space between the rotor 30 and the magnet 32 during pressing-on extends only through the magnet region 40. The magnet 32 however engages around the rotor 30 with its fastening region 44 instead of being inserted into the latter. The fastening region 44 is designed as a cylindrical sleeve with a smooth radially inner and radially outer lateral surface.FIG. 7 shows a fourth embodiment of the sensor arrangement in a schematic sectional view. Differences from the third embodiment will be explained below.In the fourth embodiment, the magnet 32 is likewise pressed onto the rotor 30 radially on the outside. However, the fastening region 44 does not have a cylindrically smooth radially inner and radially outer lateral surface and thus a completely annular cross section. Instead, these two lateral surfaces have a wave profile, which can be seen well in the schematic perspective view in FIG. 8 and the further sectional view in FIG. 9. This wave-profile cross-sectional shape increases a radial elasticity of the fixing portion 44, similarly to the groove profile in the fixing portion 34 of the magnet 32 according to the second embodiment. The shafts can flatten out when the magnet 32 is pressed onto the rotor 30, whereby the fastening region 44 widens radially and in the circumferential direction. As a result, a stress in the fastening region 44 is lower than in the third embodiment. In addition, the radially inner shaft profile allows venting of the intermediate space between the magnet 32 and the rotor 30 when the fastening region 44 of the magnet 32 is pressed onto the rotor 30.Reference numerals denote reference numerals10 Rotor 12 Motor shaft 14 Permanent magnet 16 Sheet metal pack 18 Radial bearing 20 Magnet 22 Carrier 30 Rotor 32 Magnet 34 Fastening region 36 Blind hole 38 Through-opening 40 Magnet region 44 Fastening region

Claims

Sensor arrangement for an electric motor, having a rotor (30) of the electric motor and having a magnet (32) which is fixed to the rotor (30) in a rotationally fixed manner, wherein the magnet (32) has a plastic matrix and magnetic powder which is embedded in the plastic matrix.Sensor arrangement according to Claim 1, characterized in that the magnet (32) is connected to the rotor (30) by means of an interference fit.Sensor arrangement according to Claim 1 or 2, characterized in that the magnet (32) is connected to the rotor (30) in a positively locking manner.Sensor arrangement according to one of the preceding claims, characterized in that the magnet (32) has a fastening region (34; 44), by means of which the magnet (32) is fastened to the rotor (30).Sensor arrangement according to Claim 4, characterized in that the fastening region (34; 44) has a cross-sectional shape by means of which a radial elasticity of the fastening region (32; 44) is increased.Sensor arrangement according to one of the preceding claims, characterized in that the magnet (32) has a magnet region (40) which projects from the rotor (30).Sensor arrangement according to Claim 6, characterized in that the magnet region (40) has a higher concentration of magnetic powder than another region (34; 44) of the magnet (32).Motor arrangement having an electric motor which has a rotor (30) and a stator, and a sensor arrangement according to one of the preceding claims.Method for producing a sensor arrangement, wherein the method comprises at least the following steps: - providing a rotor (30) of an electric motor; - forming a magnet blank from a plastic matrix and magnetizable powder embedded therein; - magnetizing the magnetizable powder in order to produce from the magnet blank a magnet (32) having a plastic matrix and magnetic powder embedded therein; - fastening the magnet (32) to the rotor (30).The method of claim 9, characterized in that the securing of the magnet (32) to the rotor (30) comprises a press-fit connection.

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