Steering unit for a steer-by-wire steering system and method for producing a feedback actuator

Abstract

A steering unit (1) for a steer-by-wire steering system comprises a feedback actuator (2) having at least one coil (5) for generating a magnetic field. At least one inherently rigid actuator element (8, 9, 12, 13) with location-dependent permeability is provided to influence the magnetic field.

Description

The invention relates to a steering unit provided for use in a steer-by-wire steering system of a motor vehicle and designed according to the preamble of claim 1. The invention further relates to a method for producing a feedback actuator for such a steering unit.A steering unit of the generic type is known, for example, from WO 2020 / 038522 A1. The known steering unit comprises an electric motor and a brake which is separate from the electric motor and which has both electromagnetic and mechanical elements. Another component of the known steering unit operates as a purely passive device with spring force or pneumatically. This already gives a basic characteristic of the steering unit, wherein a counter torque, which is applied to a steering shaft in the sense of a braking torque, is dependent on the angular position of the steering wheel. This basic characteristic can be refined by torques generated by the electric motor. Torque peaks can be generated by the separate brake if necessary and in the extreme case be perceived as a steering stop. Overall, the steering unit described in WO 2020 / 038522 A1 thus provides various means for generating feedback which is generally to be artificially generated in steer-by-wire steering.A steering unit for steer-by-wire steering, which is described in EP 1 409 326 B1 and likewise has the features according to the preamble of claim 1, operates to generate steering feedback with an electromagnetic brake, which comprises a stator and a rotatable disc made of a highly permeable material, which disc is connected to a steering wheel. The stator is assigned a number of energizable coils. The device according to EP 1 409 326 B1 is intended to be suitable in particular for use in a forklift truck.A further steering resistor device is disclosed in DE 101 45 982 A1. As possible devices for generating a braking torque, there are mentioned in this case, among others, a fluid brake, a magnetic particle brake, an electric motor, a hysteresis brake and a friction brake.A feedback system for a steering system which operates with a magnetorheological fluid is disclosed, for example, in EP 1 211 159 A1. This is a steering force simulator which is intended to be suitable both for use in motor vehicles and for use in devices which serve for entertainment purposes.Possible designs of further magnetorheological brake devices are evident from the documents DE 10 2021 111 973 A1 and DE 10 2011 111 965 A1. These braking devices are designed for use in haptic operators, for example thumb rollers in steering wheels.The invention is based on the object of specifying possibilities for feedback in steer-by-wire steering which are developed further compared to the prior art, wherein a particularly favorable ratio between production-related outlay and energy requirement during operation in a motor vehicle is sought.This object is achieved according to the invention by a steering unit having the features of claim 1. Likewise, the object is achieved by a method designed according to claim 7 for producing a feedback actuator of a steering unit for steer-by-wire steering. Embodiments and advantages of the invention explained below in connection with the production method also apply analogously to the devices, that is to say the steering unit including the feedback actuator, and vice versa.The steering unit provided for use in steer-by-wire steering comprises a feedback actuator which has at least one coil for generating a magnetic field. In this case, there is at least one inherently rigid actuator element with location-dependent magnetic properties in order to influence the magnetic field generated by the coil. In particular, the permeability within the relevant actuator element can be location-dependent. Other magnetic properties that may be non-uniform from volume region to volume region within the actuator element are, for example, the saturation flux density, the coercive field strength and the residual magnetic field.The invention is based on the consideration that in electric machines, whether electric motors or electrically actuated linear actuators, the magnetic flux is significantly influenced by the permeability of the materials used. In order to conduct the magnetic flux in a targeted manner through specific parts of the motor or actuator and at the same time to prevent magnetic flux through other parts as far as possible, it is possible, for example, to install parts which are mechanically connected to one another and differ greatly from one another with regard to their magnetic permeability.The solution according to the application is deliberately applied to this approach by using, instead of a plurality of parts whose permeability is different, at least one single-piece element, namely actuator element, which has volume regions whose permeability is significantly different. If permeability is mentioned in the present text, the corresponding explanations, if applicable, also apply to other magnetic properties. For example, the permeability in a first volume region of the actuator element is at least twice, five times or more than ten times as high as in a second volume region of the same actuator element. With regard to the basic shape of the actuator element, which has regions of different magnetic permeability and / or other material properties, there are no principal restrictions. For example, the actuator element has a planar basic shape, optionally with a three-dimensional form.In principle, the actuator element mentioned can be a rotating element, i.e. a rotor element, or a non-rotatable element, i.e. a stator element. In particular, embodiments of the steering actuator can be realized which comprise at least one rotor element and at least one stator element, each with regions of different permeability, saturation flux density and / or other location-dependent properties relevant during operation of the feedback actuator.This applies, for example, to embodiments of the steering actuator which comprise at least two stator-fixed pot-shaped actuator elements and at least two rotor-fixed, likewise pot-shaped actuator elements, wherein the two rotor-fixed actuator elements are mechanically and magnetically connected to one another and are arranged in a cavity formed by the stator-fixed actuator elements. For example, the steering actuator comprises exactly two stator-fixed pot-shaped actuator elements and exactly two rotor-fixed, likewise pot-shaped actuator elements which are connected to one another at their bases.In addition to the connection of actuator elements at their bases, i.e. at disk-shaped or ring-disk-shaped surfaces, connections can also be provided on other, in particular conical, spherically curved or cylindrical surfaces, in order to improve the magnetic flux. This applies, inter alia, to cases in which the feedback actuator is based on the principle of a powder brake.In particular in the case of a plurality of rotor-side actuator elements which are mechanically and magnetically connected to one another, shear gaps, for example four or more shear gaps, are formed at different points of the feedback actuator. In general, shear gap regions are regions in which the lines of flux around the stator coil lead into the rotor and out of the rotor again. In the present case, different shear gap diameters can be present at different locations of the feedback actuator.In the case of a mirror-symmetrical configuration of two rotor-side actuator elements, at least one pair of identical shear gap diameters can exist. Alternatively, a shear gap diameter of a first rotor-side actuator element deviates from a shear gap diameter of a second rotor-side actuator element, which can be expressed in pot shapes of different widths, which are described by the two actuator elements each belonging to the rotor. The same applies analogously to pot-shaped stator-side actuator elements.Both on the stator side and on the rotor side, at least one actuator element having a plurality of regions of different permeability can describe a stepped pot shape, wherein individual, in particular cylindrical, regions of the actuator element in question each delimit a shear gap and can be separated by an intermediate region, in particular a planar or conical shape. In this case, the magnetic properties of the intermediate region can differ from the magnetic properties of the regions adjoining the shear gaps. This applies, just like the explanations for simpler, non-stepped stator- and rotor-side geometric designs, both for embodiments in which the rotor is designed as an inner rotor and for embodiments with a rotor designed as an outer rotor.In a wide variety of embodiments, the at least one actuator element, the permeability of which is location-dependent, can be designed as a sheet metal part. A primary forming production of the actuator element, for example as a cast part or 3D pressure part, is also possible. Regardless of the type of production of a blank from which the actuator element is to be produced, machining or post-machining is considered.Overall, the feedback actuator is designed, for example, as a powder brake. With regard to a possible design of a powder brake, reference is made by way of example to EP 0 458 465 B1, which is the subject matter of voltage control in connection with a magnetic brake system. Alternatively or additionally to a powder brake, an electric motor, for example, can be used as feedback actuator of the steering unit according to the application. In such a case, the electric motor can generate braking torques in a generator mode. An active drive by the electric motor can also be provided, for example, in order to guide the steering wheel into its central position. In all cases, the location dependence of the magnetic properties of the actuator element reduces magnetic shunt values, i.e. field lines which are generated by coil current but bypass the rotor without any useful effect.The method according to the application for producing a feedback actuator of a steering unit for steer-by-wire steering comprises the following steps:providing a plurality of actuator elements manufactured from metal, in particular formed or rolled steel sheet, namely at least one stator element and at least one rotor element, each of these actuator elements being designed as a passive element which cannot be supplied with current,providing at least one current-carrying coil,heat treatment of at least one of the actuator elements in such a way that regions of different magnetic permeability are established within the actuator element in question,assembling said actuator elements and the at least one coil, wherein a powder which can be influenced by the magnetic field of the coil is filled into an interior of the actuator formed between the stator- and rotor-side actuator elements, said powder developing a braking action when the coil is energized during subsequent operation of the feedback actuator.The permeability of the at least one actuator element, which differs depending on location, can be generated, for example, by inductive heating, in particular within the scope of a curing method. Targeted local heating by induction can be achieved by shaping and placing the induction coil. In this case, the permeability in defined regions of the actuator element can be reduced in a targeted manner by the heat treatment carried out in the form of inductive curing. The remaining regions of the actuator element thus become regions of relatively high magnetic permeability.It is likewise possible to influence the permeability of the workpiece being machined, that is to say of the metallic actuator element, by annealing. The thermal annealing again increases the permeability reduced after the heat treatment. Depending on the dimensions and shape of the workpiece, a permeability inside the workpiece that differs depending on location can be at least partially maintained after the annealing. Otherwise, the annealing represents a possibility of optimizing the permeability of the entire workpiece.The heat treatment can be combined with a gas treatment, wherein carbon- and / or nitrogen-containing gases can be used in particular. Local masking can also be used to influence the magnetic properties close to the surface by location-dependent indiffusing. The simultaneous influencing of the mechanical properties is of secondary importance. The processes to which the actuator element is subjected to influence its magnetic properties--and thereby also other properties, inter alia its hardness--can comprise rapid cooling processes.The selection of the machining parameters during the production of the actuator element serves in each case to optimize the magnetic flux within the finished feedback actuator, in particular to reduce unwanted secondary fluxes. This contributes substantially to a high achievable braking torque with at the same time moderate energy requirement of the actuator.In comparison with powder brakes which are not claimed, as can be used in feedback actuators, minimizing the number of rotatable and non-rotatable parts, in particular metal parts, represents an assembly-related advantage, wherein gaps between rotating and non-rotating elements of exactly defined geometry can be produced not least on account of the given geometric precision of the one-piece elements which are fixed to the rotor or stator and which have a location-dependent magnetic permeability, even under the conditions of series production.An exemplary embodiment of the invention is explained in more detail below with reference to a drawing. Shown herein are: FIG. 1 shows a section of a feedback actuator of a steering unit of a steer-by-wire steering system in a sectional illustration, FIG. 2 shows a stator element of the arrangement according to FIG. 1, having regions of different magnetic permeability and manufactured as a sheet metal part, FIG. 3 shows a further stator element of the arrangement according to FIG. 1, FIGS. 4 and 5 each show a rotor element of the arrangement according to FIG. 1.A steering unit, which is identified overall by the reference symbol 1, is provided for use in a steer-by-wire steering system of a motor vehicle, which steering system is not shown in any more detail. The steering unit 1 comprises a feedback actuator 2 which simulates the driving feel of a conventional steering system with a mechanical connection between the steering wheel and the steered wheels. A basic characteristic of steer-by-wire steering is the lack of a mechanical connection between the steering wheel and the wheels. For the rest, as regards steer-by-wire steering systems for passenger cars or other vehicles, for example construction machines, reference is made to the prior art cited at the beginning.In the present case, the feedback actuator 2 comprises a powder brake 3, wherein a powder that can be influenced by a magnetic field and is contained in an interior space of the powder brake 3 denoted by 4 is not shown. A coil 5 is provided for generating the magnetic field. The coil 5 is held in a stator 7. The stator 7 comprises two stator elements 8, 9, which will be discussed in more detail below. A stator ring denoted by 6 is situated at the same height as the coil 6, as viewed in the radial direction, and connects to an end face of the coil 5 in the axial direction.A shaft 11 is mounted in the stator 7 by means of a rolling bearing 10, in the exemplary embodiment a ball bearing. The shaft 11 is fixedly coupled to the steering wheel of the steer-by-wire steering system. Mounted on the shaft 11 are two rotor elements 12, 13 which are attributable to a rotor designated overall as 14. The rotor elements 12, 13 are located completely within the interior space 4. two sealing elements 15, 16 are provided for the dynamic sealing of the interior space 4, wherein the sealing element 15 contacts the stator element 9 and the sealing element 16 contacts a housing ring 17. The housing ring 17 is fastened to the inside of the stator element 8 with the aid of fastening elements 18, namely rivets. Further fastening elements 43, which in the exemplary embodiment are likewise rivets, connect the rotor elements 12, 13 to one another.The two stator elements 8, 9 and the two rotor elements 12, 13 are collectively also referred to as actuator elements 8, 9, 12, 13. The coil 5 has the task of generating a magnetic field which penetrates gaps Sp formed between the stator 7 and the rotor 14, in which gaps a powder, for example in the form of iron filings, is located. The gaps Sp are formed in the present case between portions of the inner circumferential surface of the stator element 9 and cylindrical outer circumferential surfaces of the rotor elements 12, 13. Each of the actuator elements 8, 9, 12, 13, which are sheet metal parts, is substantially cup-shaped. In this case, the stator element 8 has approximately the same wall thickness as each of the two rotor elements 12, 13. The stator element 9, on the other hand, is of substantially thinner-walled configuration. The four actuator elements 8, 9, 12, 13 can be produced from sheet metal by forming methods.As for the shape of the stator element 8, reference is made to FIGS. 1 and 3. The stator element 8 is also referred to as an outer stator element. An outer cylindrical section of the stator element 8, designated 19, at the same time forms the outer circumferential surface of the section of the steering unit 1 shown in FIG. 1. The outer cylindrical section 19 extends in the axial direction over the majority of the extent of the feedback actuator 2 to be measured in the same direction. The width of the outer annular disk-shaped section 20 to be measured in the radial direction is to be measured on one of its two end faces, together with the wall thickness of the stator element 9 denoted by d 9 determining for the height of the at least approximately rotationally symmetrical interior space 4 denoted by H 37 to be measured in the radial direction, any roundings at the edge of the outer annular disk-shaped section 20 being ignored.The inner edge of the outer annular disc-shaped section 20 is adjoined by a central cylindrical section 21 of the stator element 8, which is substantially shorter compared to the outer cylindrical section 19. The middle cylindrical section 21 is in turn adjoined by a transition section 22 with a conical, slightly convex basic shape. The inner edge of the transition section 22 merges seamlessly into an inner annular disk-shaped section 23. The inner edge of the inner annular disc-shaped section 23 merges into an inner cylindrical section 24, the width, i.e. axial extension, of which is greater than the axial extension of the middle cylindrical section 21, but less than the width of the outer cylindrical section 19 of the stator element 8. The rolling bearing 10 is inserted between the inner circumferential surface of the inner cylindrical portion 24 and the shaft 11, with its outer ring denoted by 25 being supported on the housing ring 17 in the axial direction.Overall, the stator element 8, which can be connected to housing components of the steering unit 1 that are not shown or can itself be designed as a housing, has a high mechanical stability. The wall thickness of the stator element 8, to be measured at the outer cylindrical section 19, is denoted by d 8 and in the exemplary embodiment is more than twice, namely somewhat three times, the wall thickness d 9 of the stator element 9.The interior 4, the shape of which is already partially predefined by the stator element 8 and the housing ring 17, is closed by the stator element 9 and the shaft 11 with the two sealing elements 15, 16 held on the shaft 11. The sealing elements 15, 16 bear against a shaft shoulder 26 which, in the present case, is an integral component of the shaft 11, but could also be present as a separate part which is pressed onto the shaft 11, for example.The thin-walled stator element 9 describes, in principle comparable to the stator element 8, a complex-shaped part with a pot-like basic shape. An outer cylindrical section of the stator element 9, designated 27, contacts the inner circumferential surface of the outer cylindrical section 19 of the stator element 8 and simultaneously abuts the outer annular disk-shaped section 20 of the relatively thick-walled stator element 8. In particular, the thin-walled stator element 9 can be pressed into the thick-walled stator element 8.The outer cylindrical section 27 of the stator element 9 is more expanded in the axial direction than the central cylindrical section 21 of the stator element 8, and an outer annular disc-shaped section 28 and a central cylindrical section 29 of the stator element 9 adjoin the outer cylindrical section 27. The sections 29, 28 form an annularly encircling step in which the coil 5 is arranged. The complete outer cylindrical section 27 and that region of the middle cylindrical section 29 which is not surrounded by the coil 5 have an increased magnetic permeability compared to the rest of the stator element 9. Together with the configuration of the rotor elements 12, 13, the stator element 8 and the stator ring 6, which configuration is explained in greater detail below, the location dependence of the permeability of the stator element 9 contributes substantially to avoiding undesired secondary fluxes within the powder brake 3.On the end face of the powder brake 3 opposite the outer annular-disc-shaped section 20, in the arrangement according to FIG. 1 on the left-hand side, the middle cylindrical section 29 is adjoined by a middle annular-disc-shaped section 30, an inner cylindrical section 31, a middle annular-disc-shaped section 32, a transition section 33 with a conical basic shape, and an inner annular-disc-shaped section 34. The sealing lip of the sealing element 15 contacts the inner side of the inner annular disc-shaped section 34.The interior 4 can be roughly divided into a disk space 35 functioning as a powder space, in which both rotor elements 12, 13 are located in contact with one another, a first annular space 36, in which the rotor element 12 engages, and a second annular space 37, which is partially filled by the second rotor element 13. The annular chamber 37 is arranged radially further outward than the annular chamber 36 on average, with respect to the central axis of the shaft 11.The subspaces 35, 36, 37 of the interior 4 merging into one another describe overall a modified Y-shape. 38, 39 designate annular disk-shaped sections of the rotor elements 12, 13 within the disk space 35. The annular disk-shaped sections 38, 39 are also referred to as bases of the rotor elements 12, 13. The fastening elements 43 are likewise located in the disc space 35, and the annular disc-shaped section 38 of the rotor element 12 merges at its outer edge into a cylindrical section 40 which is arranged in the annular space 36. The annular space 36 is bounded radially outwards by the central cylindrical section 29 of the stator element 9 and radially inwards by the inner cylindrical section 31 of the same stator element 9.Adjoining the outer edge of the annular disc-shaped section 39 of the rotor element 13 is a transition section 41, which is curved just like the transition section 22. The transition section 41 again merges into a cylindrical section 42 of the rotor element 13 which extends in both the radial and axial directions in comparison with the rotor element 12, the cylindrical section 42 being located within the annular space 37 and being arranged without any overlap in the radial direction outside the cylindrical section 40. The annular space 37 is bounded radially outwards by the outer cylindrical section 27 of the stator element 9 and radially inwards by the central cylindrical section 21 of the stator element 8.Like the stator element 9, the rotor elements 12, 13 have regions of increased permeability. In the case of the rotor element 12, these are the cylindrical section 40 and, in some cases, the annular disk-shaped section 38; in the case of the second rotor element 13, the regions of increased permeability are the cylindrical section 42, the transition section 41 and, in some cases, the annular disk-shaped section 39.The location dependence of the magnetic permeability is obtained without making any of the components 8, 9, 12, 13 multi-part. Rather, by inductively curing those regions of the elements 8, 9, 12, 13 which are intended to conduct a magnetic flux to the smallest possible extent, a relatively high magnetic flux is achieved in the remaining regions of the corresponding elements 8, 9, 12, 13. In FIG. 1, the magnetic flux generated by the coil 5 is shown in an idealized manner, with regions of relatively high permeability also being marked. In fact, in FIG. 1, non-visualized secondary flows that do not contribute to the action of the powder brake 3 occur at most to a small extent.List of reference characters1 Steering unit 2 feedback actuator 3 powder brake 4 interior space of powder brake 5 coil 6 stator ring 7 stator 8 stator element 9 stator element 10 rolling bearing 11 shaft 12 rotor element 13 rotor element 14 rotor 15 sealing element 16 sealing element 17 housing ring 18 fastening element between stator element 8 and stator ring 17 19 outer cylindrical section of stator element 8 20 outer annular disk-shaped section of stator element 8 21 middle cylindrical section of stator element 8 22 transition section 23 inner annular disk-shaped section 24 inner cylindrical section 25 outer ring 26 shaft shoulder 27 outer cylindrical section of stator element 9 28 outer annular disk-shaped section of stator element 9 29 middle cylindrical section of stator element 9 30 middle annular disk-shaped section of stator element 9 31 inner cylindrical section of stator element 9 32 middle annular disk-shaped section of stator element Stator element 9 33 transition section 34 inner annular-disk-shaped section of the stator element 9 35 disk space, powder space 36 first annular space 37 second annular space 38 annular-disk-shaped section of the rotor element 12 39 annular-disk-shaped section of the rotor element 13 40 cylindrical section of the rotor element 12 41 transition section 42 cylindrical section of the rotor element 13 43 fastening element between the rotor elements 12, 13 d 8 wall thickness of the stator element 8 d 9 wall thickness of the stator element 9 H 36 height of the annular space 36 H 37 height of the annular space 37 Sp gapReferences included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedWO 2020 / 038522 A1

[0002] EP 1 409 326 B1

[0003] DE 101 45 982 A1

[0004] EP 1 211 159 A1

[0005] DE 10 2021 111 973 A1

[0006] DE 10 2011 111 965 A1

[0006] EP 0 458 465 B1

[0019]

Claims

Steering unit (1) for steer-by-wire steering, comprising a feedback actuator (2) which has at least one coil (5) for generating a magnetic field, characterized in that at least one inherently rigid actuator element (8, 9, 12, 13) having location-dependent magnetic properties is provided for influencing the magnetic field.Steering unit (1) according to Claim 1, characterized in that at least one actuator element (12, 13) is designed as a rotor element.Steering unit (1) according to Claim 1 or 2, characterized in that at least one actuator element (8, 9) is designed as a stator element.Steering unit (1) according to Claims 2 and 3, characterized byat least two stator-fixed pot-shaped actuator elements (8, 9) and at least two rotor-fixed, likewise pot-shaped actuator elements (12, 13), wherein the two rotor-fixed actuator elements (12, 13) are connected mechanically and magnetically to one another and are arranged in an interior space (4) formed by the stator-fixed actuator elements (8, 9).Steering unit (1) according to one of Claims 1 to 4, characterized in that the actuator element (8, 9, 12, 13) is designed as a sheet metal part.Steering unit (1) according to one of Claims 1 to 5, characterized in that the feedback actuator (2) is designed as a powder brake.Method for producing a feedback actuator (2) of a steering unit (1) for steer-by-wire steering, having the following steps: - provision of a plurality of actuator elements (8, 9, 12, 13) each produced from metal, namely at least one stator element (8, 9) and at least one rotor element (12, 13), - provision of at least one energizable coil (5), - heat treatment of at least one of the actuator elements (8, 9, 12, 13) in such a way that regions of different magnetic permeability are established within this actuator element (8, 9, 12, 13), - assembly of all actuator elements (8, 9, 12, 13) and the coil (5), wherein a powder which can be influenced by the magnetic field of the coil (5) is filled into a space formed between the stator-side and rotor-side actuator elements (8, 9, 12, 13).Method according to claim 7, characterised in that the location-dependently different permeability of the actuator elements (8, 9, 12, 13) is generated by local inductive hardening.Method according to claim 7 or 8, characterized in that the permeability is influenced by annealing.Method according to one of Claims 7 to 7, characterized in that, during the assembly of the coil (5) and actuator elements (8, 9, 12, 13), a gap-shaped space to be filled with the powder is formed, which space is bounded on both sides, that is to say on the rotor side and on the stator side, by surface sections (40, 42, 27, 29) of the rotor- or stator-side actuator elements (8, 9, 12, 13) which have an increased magnetic permeability compared with other surface sections of the respective elements (8, 9, 12, 13).

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