Steering actuator and method for operating a rear-axle steering system

Abstract

Steering actuator (1) for a rear-axle steering system (10), with an electric motor (8) and a gear arrangement (9) actuated by the electric motor (8), which comprises a rotary-rotary gear (11) and a rotary-linear gear (12) connected downstream thereof, characterized in that the electric motor (8) has two rotors (27, 28), namely a main rotor (27) and an auxiliary rotor (28), which can be coupled to the gear arrangement (9) in different ways in different operating modes.

Description

The invention relates to a steering actuator according to the preamble of claim 1 suitable for use in a rear axle steering system of a motor vehicle.A steering actuator of the generic type is known, for example, from DE 10 2021 104 649 A1. The known steering actuator comprises a two-stage transmission arrangement, wherein a belt transmission is provided as the first transmission stage and a rotational linear transmission in the form of a planetary roller transmission or a ball screw is provided as the second transmission stage. The axis of an electric motor, which feeds mechanical power into the belt transmission, is arranged parallel to the central axis of the steering actuator.A further steering actuator, which has a planetary roller gearing or a ball screw drive as a gearing for converting a rotation into a linear movement, is described in DE 10 2021 104 646 A1. In this case, too, an output-side element of the steering actuator can be displaced in a housing in a manner secured against rotation.A chassis actuator described in DE 10 2018 129 119 A1 and provided for use in a rear axle steering system comprises two spatially separated sensors which are operable independently of one another and are arranged on one and the same output-side, linearly displaceable element of a transmission arrangement of the chassis actuator. The transmission arrangement also comprises in this case two transmission stages connected in series.DE 10 2020 106 785 A1 relates to steering angle measurement at the rear axle of a motor vehicle. For this purpose, two rotor position encoders are provided, namely a first rotor position encoder on an electric motor and a second rotor position encoder for state detection on a spindle drive. The actuator described in DE 10 2020 106 785 A1 is also suitable for steering the front wheels of a motor vehicle.Various rear wheel steering devices comprising planetary gearing are described, for example, in documents EP 3 127 780 B1 and US 2014 / 0353067 A1.EP 3 132 995 B1 discloses a rear wheel steering device with two electric motors. In this case, each electric motor is assigned a separate speed reducer.The invention is based on the object of further developing rear axle steering mechanisms compared to the aforementioned prior art with regard to a particularly favorable relationship between apparatus complexity and operating reliability.This object is achieved according to the invention by a steering actuator having the features of claim 1. Likewise, the object is achieved by a method for operating a rear axle steering system according to claim 10.The steering actuator comprises, in a basic design known in principle, an electric motor and a two-stage transmission arrangement actuated by the electric motor, wherein the first transmission stage is designed as a rotational-rotational transmission and the second transmission stage is designed as a rotational-linear transmission. According to claim 1, the electric motor comprises two rotors, namely a main rotor and an auxiliary rotor, which can be coupled to the transmission arrangement in different operating modes in different ways.The electric motor is designed, for example, as a permanent magnet-excited synchronous motor. The main rotor can be permanently coupled to the two-stage transmission arrangement, whereas the auxiliary rotor can be switched on and off.According to various possible configurations, both rotors are arranged concentrically with respect to the transmission arrangement of the steering actuator. That is, the central axis of the linearly displaceable output element of the rotary-linear transmission coincides with the central axis of the electric motor, just like the central axis of the rotary-rotary transmission. In particular, the auxiliary rotor of the electric motor concentrically surrounds the main rotor. Conversely, embodiments can also be realized in which the auxiliary rotor is arranged centrally in the main rotor.In the first case, i.e. with the main rotor on the inside, the auxiliary rotor can have an external toothing and the main rotor an internal toothing, wherein the named toothings are each provided as a spline toothing for coupling with an input-side component of the two-stage transmission arrangement. In a space-saving configuration, the external toothing of the auxiliary rotor can overlap with the internal toothing of the main rotor in the axial direction of the rotors and thus of the entire steering actuator. This is equivalent to at least one plane normal to the longitudinal axis of the steering actuator existing, intersecting both the external toothing and the internal toothing.A possible further development, which is also advantageous with regard to space utilization, provides that a control unit provided for actuating the electric motor is attached to the end face. The control unit is located on that end face of the electric motor which faces away from the transmission arrangement of the steering actuator and occupies an annular space which is concentric with the rotors and with a common stator of the electric motor. Optionally, the control unit comprises a linear sensor, whereby it is designed as a control and sensor unit. The linear sensor can cooperate directly with the threaded structure of the output element of the steering actuator, which is designed as a threaded spindle, as a material measure. Further sensors for detecting the setting and / or adjustment of the steering actuator, in particular in the form of stop sensors or rotary position sensors, can be present at another point of the steering actuator.In order to make it possible to connect the auxiliary rotor as required, a clutch arrangement can be connected between the auxiliary rotor and the input-side element of the transmission arrangement. In this case, regardless of the operating state of the auxiliary rotor, there can be a permanent, invariable coupling between the said element of the transmission arrangement and the main rotor of the electric motor. The connection of the auxiliary rotor and actuation of the clutch arrangement is triggered in particular by the already mentioned control unit which is attached to the electric motor or integrated into the electric motor. For switching on the auxiliary rotor, a solenoid can be used in particular. For technical background, reference is made by way of example to DE 10 2012 108 583 A1, which relates to an electromagnet. As in the case of the electromagnet according to DE 10 2012 108 583 A1, position detection, i.e. detection of the switching state of the clutch, can also be provided in the electromagnetically actuated clutch arrangement of the steering actuator according to the application.The first gear stage of the gear arrangement can be designed in a space-saving design as a reduction gear, the input-side gear element of which, like the output-side gear element of which, has an axis of rotation coinciding with the central axis of the electric motor and of the entire steering actuator. For example, the first transmission stage is a planetary transmission with a ring gear mounted in a rotationally fixed manner in the housing of the steering actuator. The input-side transmission element is designed as a sun wheel in this case, while the planet carrier functions as an output-side transmission element.As a clutch arrangement, which, if necessary, transmits a torque between the auxiliary rotor and the input-side transmission element of the multi-stage transmission arrangement, an electromagnetic clutch can be provided in particular. In the case of using a planetary gear as a rotary-rotary transmission, the electromagnetic clutch establishes a direct, i.e. gearless, connection between the auxiliary rotor and the sun gear of the first transmission stage, i.e. the rotary-rotary transmission designed as a reduction gear.As a rotary linear gear connected downstream of the rotary rotary rotary gear, for example a rolling screw drive, in particular in the form of a ball screw drive, is provided. Alternatively, for example, a planetary rolling screw drive can be used as a transmission with a linearly movable output element.The operating method according to the application assumes that an electric motor designed as a two-rotor motor and a multistage transmission arrangement arranged concentrically to the electric motor downstream of the electric motor are provided for steering actuation. As long as the electric motor is freely operable, only its first rotor, i.e. main rotor, feeds a torque into the transmission arrangement. In the event of a failure of the first rotor, the second rotor, i.e. auxiliary rotor of the electric motor, is automatically activated and a clutch between the auxiliary rotor and the transmission arrangement is closed, while the main rotor is dragged along in this operating state, which is to be understood as emergency or standby operation. In normal operation of the electric motor, the auxiliary rotor can be stationary or can be in an undefined movement state. Theoretically, it is also possible to specifically block movements of the auxiliary rotor. A locking mechanism provided for this purpose can cooperate mechanically with the mentioned clutch arrangement in such a way that a rotation of the auxiliary rotor is enabled during the closing of the clutch.The steering actuator according to the application is suitable not only for installation in a rear axle steering system, but also for use as an electromechanical actuator of a front axle steering system.In all embodiments, an important advantage of the steering actuator is that it is constructed in a particularly space-saving manner in the radial direction despite its functionality and high reliability. Thanks to its auxiliary rotor, the steering actuator is able to be at least still reset to a normal position in the event of failure of the regular drive using the main rotor. Depending on the design of the auxiliary drive realized with the auxiliary rotor, it is even possible to maintain all the steering functions in the event of failure of the regular electric drive. All control functions required for this are optionally taken over by the control unit, which is integrated into the housing of the steering actuator, which is overall slender and at the same time also compact in the axial direction. In all cases, a high positioning accuracy is achievable, to which not least the geometrically simple structure of the housing, in which the various electrical and mechanical components are installed in a tightly packed manner, contributes.An exemplary embodiment of the invention is explained in more detail below with reference to a drawing. Shown herein are: FIG. 1 shows an exploded view of a steering actuator of a rear axle steering system, FIG. 2 shows the steering actuator in a sectional perspective view, FIG. 3 shows drive and clutch components of the steering actuator in an exploded view, FIG. 4 shows a linear sensor of the steering actuator, FIG. 5 shows the completely assembled steering actuator in a perspective view, FIG. 6 shows an electric motor of the steering actuator, which is designed as a two-rotor motor, in a sectional perspective view, FIG. 7 shows stator components of the electric motor without windings, FIG. 8 shows the stator together with windings, FIG. 9 shows an inner rotor of the electric motor provided as a main rotor, FIG. 10 shows the main rotor in an exploded view, FIG. 11 shows an internally toothed component of the main rotor, FIG. 12 shows an outer rotor of the electric motor provided as an auxiliary rotor, FIG. 13 shows the auxiliary rotor in a representation analogous to FIG. 10, FIG. 14 shows the electric motor including a plurality of components of a clutch device in a perspective view, FIG. 15 shows the arrangement according to FIG. 14 in exploded view, FIGS. 16, 17 show a housing ring of the electric motor provided for receiving bearings, FIGS. 18, 19 show a housing cover of the electric motor, FIG. 20 shows an annular disk-shaped winding arrangement of the coupling device of the steering actuator, FIG. 21 shows a cover ring of the electric motor, FIGS. 22, 23 show a plurality of mutually cooperating components of the steering actuator, namely the electric motor, the clutch device and a multi-stage transmission arrangement, in two perspective views, FIG. 24 shows the transmission arrangement in exploded view, FIG. 25 shows the transmission arrangement in a completely assembled state, FIG. 26 shows the transmission arrangement in a cut-away perspective view, FIG. 27 is a view for receiving the gear arrangement, FIG. 28 shows further components of the clutch device of the steering actuator, FIG. 29 shows the arrangement according to FIG. 28 in exploded illustration, FIG. 30 shows the arrangement according to FIG. 29 and a planetary gear provided as a first gear stage of the gear arrangement in exploded illustration, FIG. 31 shows the arrangement according to FIG. 30 in a perspective view, FIG. 32 shows the planetary gear in a sectional perspective view, FIG. 33 shows an exploded view of a ball screw provided as a second gear stage of the gear arrangement, FIG. 34 shows the ball screw in a perspective view, FIG. 35 shows an arrangement which has not yet been finally assembled and which comprises the electric motor, the transmission arrangement and a threaded spindle as output element of the steering actuator, FIG. 36 shows the steering actuator with deactivated auxiliary drive in a sectional illustration with the force flow drawn in, FIG. 37 shows a detail from FIG. 36, FIG. 38 shows the steering actuator with activated auxiliary drive in a sectional illustration with the force flow drawn in, FIG. 39 shows a detail from FIG. 38.A steering actuator 1 is part of a rear axle steering system of a motor vehicle, which steering system is designated overall as 10. A push rod 2 of the steering actuator 1 is guided in a housing 3 which is composed of two housing parts 4, 5. A section of the push rod 2 is designed as a threaded spindle 6. Fork pieces 7 connected to the push rod 2 protrude from the housing 3, which are movably coupled to further chassis elements, not shown. With regard to the basic function of the rear axle steering system 10, reference is made to the prior art cited at the beginning.The steering actuator 1 is designed as an electromechanical actuator with an electric motor 8 and a two-stage transmission arrangement 9. In the exemplary embodiment, the steering actuator 1 is designed for actuating forces of up to 10 kN. The mass of the steering actuator 1 is less than 9 kg.The gear arrangement 9 comprises a planetary gear 11 as a rotational-rotational gear and a ball screw 12 as a rotational-linear gear. The electric motor 8, like the planetary gear 11, is arranged as a first gear stage and the ball screw 12 as a second gear stage inside the housing 3, the housing parts 4, 5 of which are connected to one another by screws 13. The electric and mechanical components 8, 11, 12 of the steering actuator 1 are arranged concentrically to one another and to the central axis of the push rod 2. Electrical cables are designated 14 and, if necessary, are passed through openings in the housing 3.The electric motor 8 has its own housing 15 which is located in the housing 3 of the entire steering actuator 1. A rotation-prevention contour 16 on the outer circumferential surface of the housing 15 of the electric motor interacts with a corresponding inner contour of the housing 3. Suitable materials for housing and / or stator components of the electric motor 8 are, in particular, composite materials which are distinguished by minimizing eddy current losses. In particular, such materials can comprise Al 2 O 3.The stator of the electric motor 8, which is denoted overall by 17, comprises an inner stator region 18 and an outer stator region 19, wherein the stator regions 18, 19 are delimited from one another by a stator ring 20. Inner stator teeth 21 are located in the inner stator region; outer stator teeth 22 are located in an analogous manner in the outer stator region 19; the stator teeth 21, 22 carry inner stator windings 24 and outer stator windings 25, respectively.The various stator windings 24, 25 cooperate with an inner rotor 27 and an outer rotor 28, respectively. The inner rotor 27 functions as the main rotor and the outer rotor 28 as the auxiliary rotor of the electric motor 8, and the rotors 27, 28 are mounted with ball bearings 26.A control and sensor unit 29 is flanged onto the housing 15 of the electric motor 8, wherein the outer diameter of the control and sensor unit 29 corresponds to the outer diameter of the housing 15, apart from the anti-rotation contour 16. The control and sensor unit 29 comprises a circular disk-shaped plate 30, which is concentric to the central axis of the push rod 2 and thus of the entire steering actuator 1 and on which, among other things, a linear sensor 31 is located. The linear sensor 31 uses the thread of the threaded spindle 6 as a measurement standard. The entire control and sensor unit 29 is fastened to the housing 15 by means of screws 32, wherein the pot-shaped housing of the control and sensor unit 29 is also referred to as the housing base 33. A housing cover arranged on the opposite end face of the electric motor 8 is denoted by 34.The inner rotor 27 directly surrounding the lead screw 6 has a metallic sleeve member 35 having a stepped cylindrical shape. Here, a main section extending over the major part of the length of the sleeve element 35 is denoted by 36 and an extended section adjoining the latter is denoted by 37. On the inner circumferential surface of the expanded portion 37 there is an internal toothing 38; strip-shaped permanent magnets 39 are fastened, in the present case adhesively bonded, to the cylindrical main portion 36. A support ring 40 which supports the permanent magnets 39 in the axial direction and is made of a material which is advantageous with regard to the electromagnetic properties, in particular composite material, is arranged in an annular space between the arrangement of all the permanent magnets 39 and the section 37 of enlarged diameter.As for the structure of the outer rotor 28, i.e., auxiliary rotor, there are design commonities with the inner rotor 27, and in the case of the outer rotor 28, permanent magnets 39 are disposed on the inner circumferential surface of a main portion 42 of a sleeve member 41. Instead of a portion having an enlarged diameter, the sleeve element 41 has a tapered portion 43 which adjoins the main portion 42 and is provided with an external toothing 44. For receiving the ball bearings 26 which support the rotors 27, 28, a housing ring 45 is provided, among other things. An outer bearing receptacle of the housing ring 45 is designated 46, and an inner bearing receptacle of the same housing ring 45 is designated 47. Securing rings 48 prevent, in particular, axial displacements of components of the steering actuator 1.The steering actuator 1 is further associated with a clutch device generally designated 49, which can be operated as an electromagnetic clutch in the manner of a solenoid and serves for the required coupling of the auxiliary rotor 28 to the transmission arrangement 9. The coupling device 49 comprises an annular multi-layer winding arrangement 50, which is also referred to simply as copper ring 50. The winding arrangement 50 can be configured as a printed circuit. In this context, reference is made by way of example to DE 10 2011 015 287 A1.A cover ring, which is arranged between the electric motor 8 and the gear arrangement 9 and is attributable to the clutch device 49, is denoted 51. A clutch ring 52 has an internal toothing 53 with which it is plugged onto the external toothing 44. The cover ring 51 is arranged axially between the copper ring 50 and the coupling ring 52, wherein a receptacle 56 for the copper ring 50 is located in the cover ring 51. In a comparable manner, a receptacle 54 for the copper ring 50 is formed by the housing cover 34. In addition, the housing cover 34 has a bearing receptacle 55 for one of the ball bearings 26.A pressure plate 57 of the clutch device 49 is attracted to the non-displaceable clutch ring 52 when the winding arrangement 50 is energized, so that a force-fit, torque-transmitting connection is produced between the pressure plate 57 and the clutch ring 52 and thus also the auxiliary rotor 28. The various shift states of the clutch device 49 are shown in Figs. 36 to 39.The pressure plate 57 is coupled to a hub component 60 in a rotationally fixed manner in an axially at least slightly displaceable manner with the aid of a plurality of metal strips 58 and rivets 59. The hub component 60 has an electric motor-side external toothing 61 and a transmission-side external toothing 62. The external toothing 61 on the electric motor side is permanently coupled to the internal toothing 38 and thus rotationally fixedly coupled to the main rotor 27. The external toothing 61 on the electric motor side is thus a spline toothing. The same applies to the external toothing 62 on the transmission side, which is provided for introducing a torque into the transmission arrangement 9. A disc portion of the hub component 60 axially located between the toothings 61, 62 is denoted 63.The external toothing 62 of the hub component 60 on the transmission side is inserted into an internal toothing 69 of a sun gear 67 of the planetary transmission 11. the sun gear 67 furthermore has an external toothing 68, which, in contrast to the internal toothing 69, is a running toothing. Planet gears 70 roll on the external toothing 68, which are guided in a planet carrier 71. The planet gears 70 moreover mesh with an internal toothing 65 of a ring gear 64 of the planetary gear 11, and on its outer circumferential surface, the ring gear 64 has a rotation-preventing contour 66, which interacts with a rotation-preventing contour 77 in the housing 3. On its end side facing the ball screw 12, the planetary gear 11 has a cover 72. In modified embodiments, not shown, in which the electric motor 8 provides a sufficiently high torque for actuating the ball screw 12, the planetary gear 11 can be omitted without fundamentally changing the overall function of the steering actuator 1.The ball screw 12 is housed in a housing 73 inserted into the housing 5 of the steering actuator 1. A nut assembly of the ball screw 73 designated as a whole as 74 is connected in a rotationally fixed manner to the planet carrier 71 and has an internal toothing 75 for this purpose. Balls 76 roll off between the threaded spindle 6 and the nut assembly 74 in a manner known per se as rolling bodies, wherein a rolling body return is provided in the exemplary embodiment.List of reference characters1 Steering actuator 2 Push rod 3 Housing 4 Housing part 5 Housing part 6 Threaded spindle 7 Fork piece 8 Electric motor 9 Transmission arrangement 10 Rear-axle steering 11 Planetary transmission 12 Ball screw 13 Screw 14 Cable 15 Housing of the electric motor 16 Anti-rotation contour 17 Stator 18 Inner stator region 19 Outer stator region 20 Stator ring 21 Inner stator tooth 22 Outer stator tooth 23 Electrical connection 24 Inner stator winding 25 Outer stator winding 26 Ball bearing 27 Inner rotor, main rotor 28 Outer rotor, Auxiliary rotor 29 Control and sensor unit 30 Printed circuit board 31 Linear sensor 32 Screw 33 Housing base 34 Housing cover 35 Sleeve element of the inner rotor 36 Main section of the sleeve element 35 37 Expanded section of the sleeve element 35 38 Internal toothing of the expanded section 37 39 Permanent magnet of the inner rotor 40 Support ring 41 Sleeve element of the outer rotor 42 Main section of the sleeve element 41 43 Tapered section of the sleeve element 41 44 External toothing of the tapered section 43 45 Housing ring 46 Outer bearing receptacle 47 Inner bearing receptacle 48 Securing ring 49 Coupling device 50 Copper ring, multilayer winding arrangement 51 Cover ring 52 Coupling ring, The inner teeth 53 of the inner teeth of the clutch ring 54 are the receptacles for the copper ring in the housing cover 55 are the receptacles for the housing cover 56 are the receptacles for the copper ring in the cover ring 57 are the pressure plate 58 are the metal strips 59 are the rivets 60 are the hub component 61 are the external teeth of the hub component 62 are the external teeth of the hub component 63 are the disk section of the hub component 64 are the ring gear 65 are the internal teeth of the ring gear 66 are the anti-rotation contour of the ring gear 67 are the sun gear 68 are the external teeth of the sun gear 69 are the internal teeth of the sun gear 70 are the planetary gear 71 are the planetary gear 72 are the cover of the planetary gear 73 are the housing of the ball screw 74 is the nut assembly 75 is the internal teeth of the nut assembly 76 are the ball, the rolling body 77 are the anti-rotation contour in the housing 3

Claims

Steering actuator (1) for a rear axle steering system (10), having an electric motor (8) and a transmission arrangement (9) which is actuated by the electric motor (8) and comprises a rotational-rotational transmission (11) and a rotational-linear transmission (12) connected downstream of the rotational-rotational transmission, characterized in that the electric motor (8) has two rotors (27, 28), namely a main rotor (27) and an auxiliary rotor (28), which can be coupled to the transmission arrangement (9) in different operating modes in different ways.Steering actuator (1) according to claim 1, characterized in that both rotors (27, 28) are placed concentrically to the gear arrangement (9).Steering actuator (1) according to claim 2, characterized in that the auxiliary rotor (28) concentrically surrounds the main rotor (27).Steering actuator (1) according to Claim 3, characterized in that the auxiliary rotor (28) has an external toothing (44) and the main rotor (27) has an internal toothing (38), in each case as a spline toothing for coupling to a component of the transmission arrangement (9).Steering actuator (1) according to one of Claims 1 to 4, characterized bya control unit (29) which is attached at the end face to the electric motor (8) and is located in an annular space which is concentric with the rotors (27, 28) and with a common stator (17) of the electric motor (8).Steering actuator (1) according to claim 5, characterized in that the control unit (29) comprises a linear sensor (31).Steering actuator (1) according to one of Claims 1 to 6, characterized bya clutch device (49) which is connected between the electric motor (8) and the transmission arrangement (9) and is designed for coupling the auxiliary rotor (28) to an input-side element of the transmission arrangement (9) as required.Steering actuator (1) according to Claim 6, characterized in that the clutch device (49) provided is an electromagnetic clutch which is designed to produce a torque-transmitting transmission-less connection between the auxiliary rotor (28) and a sun wheel (67) of the first transmission stage, that is to say the rotary-rotary transmission (11).Steering actuator (1) according to one of Claims 1 to 8, characterized in that a rolling screw drive, in particular a ball screw drive, is provided as the rotational linear gear (12).Method for operating a rear axle steering system (10), wherein an electric motor (8) designed as a two-rotor motor and a multistage transmission arrangement (9) arranged concentrically with respect to the electric motor (8) downstream of the electric motor are provided for the steering actuation, and wherein, in the case of unlimited operability of the electric motor (8), the first rotor (27) thereof, namely the main rotor, feeds a torque into the transmission arrangement (9), whereas, in the event of failure of the first rotor (27), the second rotor (28), namely the auxiliary rotor, of the electric motor (8) is automatically activated and a clutch (49) between the auxiliary rotor (28) and the transmission arrangement (9) is closed, while the main rotor (27) is dragged along.

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