Electric motor feedback drive for steering system

Abstract

The invention relates to an electric motor-type feedback drive (2) for a steering system, comprising a brushless electric motor (4) having a wound stator (8) and having a rotor (10), the rotor (10) having a permanent magnet rotor magnet (26), the rotor magnet (26) having a number of magnetic poles magnetized in a Halbach arrangement, and the rotor magnet (26) having a plurality of magnetic poles magnetized in a Halbach arrangement. The rotor magnet (26) is arranged in a rotor housing (24) which engages in a rotationally fixed manner with the bearing cover plate (6).

Description

[0001] This patent application is a divisional of the following patent applications:

[0002] Invention Title: Electric motor type feedback drive for steering system;

[0003] Application date: August 4, 2022;

[0004] Application number: 202210931833.4. Technical Field

[0005] The invention relates to an electric motor-type feedback drive for a steering system, comprising a brushless electric motor with a wound stator and a permanently excited rotor. The invention also relates to a steering system having such a feedback drive. Background Art

[0006] In modern motor vehicles, electric motors are used in many ways as drives for various adjustment elements. For example, electric motors are used as window regulator adjustment drives, sunroof adjustment drives or seat adjustment drives, as steering drives (EPS, Electrical Power Steering), as cooling fan drives or as transmission actuators. Such electric motors can also be used in so-called feedback drives (English: force feedback drive) for steer-by-wire systems.

[0007] Here and hereinafter, steer-by-wire is understood to mean, in particular, a steering concept in which a manual steering command from a vehicle user is transmitted from a sensor, in particular a steering wheel, via a control unit only electrically to an electromechanical actuator, which executes the steering command. In other words, in such a steering system, there is no mechanical connection between the steering wheel and the steering gear or the steered wheel.

[0008] Therefore, in a steer-by-wire system, there is no direct mechanical feedback to the vehicle user regarding the steering command. This lack of tactile feedback reduces driving safety and steerability because the vehicle user cannot reliably grasp the current driving situation.

[0009] Therefore, steer-by-wire systems usually have a separate control actuator as a feedback drive in order to physically transmit the load torque as a reaction of the steering wheel to the vehicle user as a haptic feedback signal and, if necessary, also to build up a braking torque that simulates the steering dynamics and the steering stop. The feedback signal thus gives the vehicle user a feedback torque that corresponds to the actual reaction torque, so that the vehicle user gets the experience of conventional steering.

[0010] The feedback drive can be arranged as a direct drive (ie without additional transmission) on the steering column of the steering system. In such direct drives, high functional requirements are imposed on the feedback drive. Thus, for example, the feedback drive should have a high adjustment torque in a small installation space and a sufficiently high efficiency in terms of thermal stability and low torque fluctuations in terms of good acoustics. In addition, low cogging torques and good adjustability with high adjustment accuracy are required.

[0011] A motor type that satisfies all criteria adequately is a multi-pole brushless DC electric motor (BLDC motor), which is designed, for example, as an inner or outer rotor. A particular challenge here is to achieve a very sinusoidal air gap induction between the rotor and the stator in order to be able to control the electric motor unobtrusively in terms of acoustics and haptics and to meet the high market requirements in this regard.

[0012] In principle, conventional BLDC motor designs have an approximately trapezoidal air gap induction and can only conditionally meet the required standards. The multipolarity means more effort when assembling the rotor magnets, and due to the limited rotor diameter, magnetic short circuits can occur in the edge region of the rotor magnets, which prevents optimal utilization of the magnet material used. Summary of the invention

[0013] The object of the present invention is to specify a particularly suitable feedback drive for a steering system. In particular, an air gap induction which is as sinusoidal as possible should be achieved between the rotor and the stator. The object of the present invention is also to specify a particularly suitable steering system.

[0014] According to the invention, this object is achieved with respect to the feedback drive by the features of claim 1 and with respect to the steering system by the features of claim 4. Advantageous embodiments and improvements are the subject matter of the dependent claims. The advantages and embodiments listed with respect to the feedback drive can also be transferred to the steering system and vice versa.

[0015] The electric motor-type feedback drive according to the invention is provided for a steering system of a motor vehicle, in particular for a steer-by-wire system, and is designed and suitable for this purpose. The feedback drive has a multi-pole, brushless electric motor with a wound stator and a rotor.

[0016] The rotor is designed as a permanently excited rotor and has a permanent rotor magnet as a magnetic flux source for generating torque. The rotor magnet has a plurality of magnetic poles which are magnetized in a Halbach arrangement (Halbach array, Halbach magnetization). In such an arrangement, the magnetic field is increased on the side of the arrangement facing the stator, while the magnetic field is reduced on the opposite side (facing away from the stator). According to the invention, the magnetic poles are oriented in such a way that a sinusoidal field strength curve is obtained on the side facing the stator, thereby reducing the cogging torque in particular.

[0017] In summary, the rotor magnets magnetized in the Halbach arrangement have a sinusoidal magnetic field strength curve with respect to the radial direction, i.e. perpendicular to the motor axis, on their side facing the stator and correspondingly in the air gap formed between the rotor and the stator. This results in a sinusoidal electromagnetic force (EMK) as an air gap induction in the circumferential direction of the rotor. In particular, due to the magnetization, the sinusoidal EMK is achieved here without or at least with relatively few and / or weakly formed harmonics. As a result, relatively small torque fluctuations occur, so the motor efficiency is advantageously improved.

[0018] Here, the rotor magnets are arranged in a rotor housing which is joined to the bearing cover in a rotationally fixed manner. The bearing cover preferably forms the mechanical interface of the feedback drive to the steering system. For example, it is conceivable that the feedback drive can be coupled to the steering column by means of the bearing cover in order to directly transmit the generated torque. This creates a particularly suitable feedback drive.

[0019] Due to the Halbach magnetization of the rotor magnets, low torque ripple is achieved during motor operation, which has a favorable effect on the acoustics of the feedback drive due to the sinusoidal air gap induction in combination with the high pole number.

[0020] The electric motor is preferably designed as a radial flux motor. Alternatively, the electric motor can also be designed as an axial flux motor, thereby enabling different mounting and assembly concepts and advantages in terms of installation space requirements. Axial flux motors also have a higher torque density than radial flux motors.

[0021] The motor torque acting on the rotor by means of the rotating magnetic field is proportional to the square of the rotor diameter. In other words, the motor torque therefore increases with increasing rotor diameter. According to the invention, the electric motor is therefore designed as an outer rotor. In other words, the rotor is an outer rotor, so that a torque increase can be achieved by a larger rotor diameter. Therefore, compared with an electric motor designed as an inner rotor, the motor torque is greater in this way with the same electric motor size.

[0022] No individual magnets are used in the rotor magnet. According to the invention, the rotor magnet is formed by means of a closed ring magnet (magnetic ring). The rotor magnet is designed as a one-piece, i.e., integral or monolithic ring magnet. This results in a significant reduction in assembly effort compared to the assembly of individual magnets. In addition, a durable pole symmetry with reduced tolerances can be achieved compared to individually assembled and sintered magnets.

[0023] According to the invention, the rotor magnet has between twenty (20) and fifty (50) poles. For example, the stator has twenty-four (24) stator teeth, to which the rotating field winding is applied. The rotating field winding is, for example, two stator windings, each having twelve coils, which are each interconnected to form a star circuit. In this case, the rotor or rotor magnet has in particular twenty-eight (28) poles, which are arranged in a Halbach arrangement.

[0024] According to the invention, the rotor magnet is made of a magnetic ring as a composite material that is bonded to plastic and pressed. It has magnetic powder embedded in a plastic matrix. This allows a lower plastic content to be achieved compared to magnets that are injected with plastic bonding, thereby achieving higher magnetic properties when using the same magnetic powder.

[0025] In a suitable embodiment, the rotor magnets are made of neodymium iron boron (NdFeB) material. Preferably, the rotor magnets have a remanence between 0.2 T (Tesla) and 0.75 T and a coercive field strength of magnetic polarization between 150 kA / m (kiloamperes per meter) and 1200 kA / m at room temperature (20°C).

[0026] The flux source of the rotor is, for example, an isotropic NdFeB Halbach array having a number of poles in the range of 6 to 50. The isotropic NdFeB Halbach array has a remanence between 0.65 T and 0.75 T at 20°C and a magnet coercive field strength of, for example, between 450 kA / m and 550 kA / m. In the case of a pressed magnetic ring, the magnet coercive field strength is preferably between 700 kA / m and 770 kA / m. With such magnetic properties, a flux density in the air gap of the motor in the range of 0.8 T to 1.0 T is achieved. Compared to the magnet, the concentration factor of the air gap flux density (KFC) is in the range of 1.2 to 1.5.

[0027] Instead of NdFeB magnet material, ferrite magnets can also be used. Injection molded ferrite Halbach arrays have a remanence between 0.2T and 0.32T and a magnet coercive field strength between 150kA / m and 300 kA / m at 20°C. With such magnetic properties, a magnetic flux density in the air gap of the motor in the range of 0.3T to 0.5T is achieved. Compared with magnets, the concentration factor of the air gap magnetic flux density is in the range of 1.2 to 1.5.

[0028] Due to the Halbach arrangement, the rotor implemented as an outer rotor has a reduced field strength as leakage flux on the (outer) side opposite to the stator. According to the present invention, an additional (rotor) yoke is provided to reduce the leakage flux. According to the present invention, the yoke ring on the outer peripheral side is placed on the rotor magnet. The yoke ring is made of ferromagnetic material here. Such a yoke ring has a significantly smaller thickness than a rotor with a conventional magnet without the Halbach effect. According to the present invention, the yoke ring has a radial thickness of less than 2 mm (millimeter). For example, the yoke ring has a thickness of about 1 mm (millimeter). In the following, the term "approximately" in the thickness given particularly refers to a certain thickness range around a given thickness range value, such as ±0.5 mm. For example, a thickness of about 1 mm should be understood as (1.0 ± 0.5) mm, i.e. a thickness range between 0.5 mm and 1.5 mm. Therefore, the rotor is characterized by having a particularly low rotor weight and reduced rotor inertia.

[0029] The steering wheel system according to the invention is in particular designed as a steer-by-wire system and has a steering wheel which is coupled to a feedback drive as described above. This results in a particularly suitable steering wheel system with low noise generation. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The embodiments of the present invention are described in more detail below with reference to the accompanying drawings, wherein:

[0031] Figure 1 A feedback actuator for a steer-by-wire system is shown in a perspective exploded view; and

[0032] Figure 2 The electric motor of the feedback drive is shown in a sectional view.

[0033] In all figures, parts and dimensions which correspond to one another are always provided with the same reference symbols. DETAILED DESCRIPTION

[0034] Figure 1 A feedback drive 2 is shown for a steer-by-wire system (not shown in detail) of a motor vehicle. In this case, the feedback drive 2 is arranged in the region of the steering wheel and generates a haptic feedback signal or feedback torque for a vehicle user during operation.

[0035] The feedback drive 2 has a multi-pole brushless electric motor 4 which can be coupled directly to the steering wheel by means of a bearing cover 6 designed as a motor bracket.

[0036] The electric motor 4 is designed as an external rotor and has a radially inner stator 8 and a radially outer rotor 10. In this exemplary embodiment, the stator 8 has twenty-four radially outwardly pointing stator teeth 12, on which a rotating field winding 14 is wound. The stator teeth 12 are provided with reference numerals in the figure only by way of example. The rotating field winding 14 has, for example, two parallel winding systems or stator windings with twelve individual coils each, which are each connected to form a three-phase star circuit. The stator 8 is placed on a mandrel 16.

[0037] The permanently excited rotor 10 surrounds the stator 8 and is mounted rotatably about a motor axis formed by a mandrel 16. To support the rotor 10, the feedback drive 2 has two rolling bearings 18, 20, which act on the rotor 10 from axially opposite sides. The axial play of the rotor 10 between the two rolling bearings 18, 20 is spring-loaded here, for example, by a spring washer 22.

[0038] The rotor 10 has a ring-shaped rotor housing 24 and an annular rotor magnet 26. The rotor housing 24 can be connected to the bearing cover 6 in a rotationally fixed manner by means of six fastening screws 28. The approximately annular bearing cover 6 has a central bearing seat for the rolling bearing 20, by means of which the bearing cover 6 and thus the rotor 10 are rotatably supported on the spindle 16. In this case, the bearing cover 6 is designed as a mechanical interface for directly coupling the feedback drive 2 to the steering wheel.

[0039] The hollow cylindrical or tubular rotor magnet 24 has, for example, 6 to 50 poles which are magnetized in a Halbach arrangement. The ring magnet or rotor magnet 26 is made in one piece, i.e. in one piece or monolithic, of a pressed composite material bonded to plastic, wherein the magnetic powder is embedded in the plastic matrix.

[0040] The rotor magnet 24 is made of, for example, neodymium iron boron (NdFeB) material. Preferably, the rotor magnet 24 has a remanence between 0.2 T (Tesla) and 0.75 T and a coercive field strength of the magnetic polarization between 150 kA / m (kiloamperes per meter) and 1200 kA / m at room temperature (20° C.).

[0041] The rotor magnet 24 is implemented, for example, as an isotropic NdFeB Halbach array having a number of magnetic poles in the range of 6 to 50. The isotropic NdFeB Halbach array has a remanence between 0.65 T and 0.75 T and a magnet coercive field strength between 450 kA / m and 550 kA / m at 20° C. With such magnetic properties, a magnetic flux density in the air gap of the motor in the range of 0.8 T to 1.0 T is achieved.

[0042] exist Figure 2 , a second embodiment for a feedback drive 2 is shown. In this embodiment, a ferromagnetic yoke ring 30 is placed on the outer circumference of the rotor magnet 24. The rotor magnet 26 has a radial thickness 32 of 4 mm, for example, wherein the yoke ring 30 has a thickness 34 of approximately 1 mm. The central recess of the stator 8 for accommodating the mandrel 16 has an inner radius 36 of approximately 51 mm, for example, wherein the outer radius 38 of the electric motor 4 or the yoke ring 30 is approximately 99 mm.

[0043] The present invention is not limited to the above-described embodiments. Instead, those skilled in the art may also derive other variations of the present invention without departing from the subject matter of the present invention. In particular, all individual features described in conjunction with the embodiments may also be combined with each other in other ways without departing from the subject matter of the present invention.

[0044] Reference numerals list

[0045] 2 Feedback Driver

[0046] 4 Electric motors

[0047] 6 Bearing cover

[0048] 8 Stator

[0049] 10 rotors

[0050] 12 stator teeth

[0051] 14 Rotating field winding

[0052] 16 spindles

[0053] 18 Rolling bearings

[0054] 20 Rolling bearings

[0055] 22 Spring coil

[0056] 24 rotor housing

[0057] 26 rotor magnets

[0058] 28 Fastening screws

[0059] 30 yoke ring

[0060] 32 thickness

[0061] 34 thickness

[0062] 36 inner radius

[0063] 38 outer radius

Claims

1. An electric motor feedback drive (2) for a steering system, comprising a brushless electric motor (4) with a wound stator (8) and a rotor (10), - the electric motor (4) is designed as an external rotor, - in, The rotor (10) has permanent rotor magnets (26). - wherein the rotor magnet (26) is produced as a one-piece ring magnet consisting of a pressed composite material bonded to plastic, wherein the composite material has magnetic powder embedded in a plastic matrix, - wherein the rotor magnet (26) has between 20 and 50 poles which are magnetized in a Halbach arrangement, so that a sinusoidal field strength curve is achieved on the side facing the stator (8), - wherein a yoke ring (30) on the outer peripheral side made of ferromagnetic material is placed on the rotor magnet (26), - wherein the yoke collar (30) has a radial thickness (34) of less than 2 mm, and - wherein the rotor magnets (26) are arranged in a rotor housing (24) which is joined to the bearing cover plate (6) in a rotationally fixed manner.

2. The feedback driver (2) according to claim 1, It is characterized in that The rotor magnet (26) is made of NdFeB material.

3. The feedback driver (2) according to claim 1 or 2, It is characterized in that The rotor magnet (26) has a remanence of between 0.2 T and 0.75 T at 20° C. and a coercive field strength of the magnetic polarization of between 150 kA / m and 1200 kA / m.

4. A steering system for a motor vehicle, comprising a steering wheel and a feedback drive (2) according to claim 1 coupled to the steering wheel.

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