Rear axle steering system, and method for operating a rear axle steering system

Abstract

A rear axle steering system (1) of a motor vehicle comprises a linear actuator (2) and a push rod (3) which can be moved by means of the linear actuator (2), wherein: a push rod section (19) to be coupled to the left-hand rear wheel has a length (LS1) which differs from the length (LS2) of a push rod section (20) to be coupled to the right-hand rear wheel; each push rod section (19, 20) has significant elasticity in the longitudinal direction of the push rod (3); stops (16, 17, 18) for limiting the adjustment range of the push rod (3) are provided; there is a geometric measuring system (10) for detecting the position of the push rod (3); and said measuring system (10) in cooperation with a measuring and control unit (15) is designed to determine specific elasticity properties of each push rod section (19, 20) on the basis of a measured resilience of the respective push rod section (19, 20) when approaching the stop (17, 18) and to control the linear actuator (2) taking into account the determined elasticity properties.

Description

[0001] Rear-axle steering and method for operating a rear-axle steering

[0002] The invention relates to a rear-axle steering system of a motor vehicle. Furthermore, the invention relates to a method for operating a rear-axle steering system.

[0003] DE 10 2018 129 119 A1 describes an electromechanical chassis actuator designed for rear-axle steering. It has a gear arrangement comprising a rotary-rotary gear and a downstream gear designed to convert rotation into linear motion, i.e., a rotary-linear gear. Two independently functioning sensors are arranged on a linearly displaceable element of the gear arrangement of the known chassis actuator. In addition, an electric motor associated with the chassis actuator is equipped with a rotor position sensor.

[0004] A position measuring device for a rear-axle steering system of a motor vehicle disclosed in DE 10 2022 102 104 A1 comprises a permanent magnet and a Hall sensor. The Hall sensor is rigidly connected to a housing in which a push rod is movably guided, with the permanent magnet located on the push rod.

[0005] The invention is based on the object of providing possibilities which are more advanced than the cited prior art for adapting the operation of a rear axle steering system designed for use in a motor vehicle to its mechanical properties, in particular elastic properties.

[0006] This object is achieved according to the invention by a rear-axle steering system comprising a linear actuator with the features of claim 1. The object is also achieved by a method for operating a rear-axle steering system of a motor vehicle, as defined in claim 5. The embodiments and advantages of the invention explained below in connection with the operating method also apply mutatis mutandis to the devices, i.e., the steering actuator and the entire rear-axle steering system, and vice versa.

[0007] The rear-axle steering system according to the application comprises a linear actuator and a push rod that can be moved by means of the linear actuator and is to be articulatedly coupled to the rear wheels of a motor vehicle to be steered. A push rod section to be coupled to the left rear wheel has a length that differs from the length of a push rod section to be coupled to the right rear wheel. The length of each push rod section is to be measured from the center of a force introduction area of ​​the linear actuator, i.e., the area of ​​the linear actuator in which it introduces a force into the push rod, to an end of the respective push rod section coupled to a joint element.

[0008] Each push rod section has a significant elasticity in the longitudinal direction of the push rod, wherein stops are provided to limit the adjustment range of the push rod and a geometric measuring system exists to detect the position of the push rod, and wherein said measuring system is designed in cooperation with a measuring and control system to determine specific elastic properties of each push rod section on the basis of a measured compliance of the respective push rod section when approaching the stop and to control the linear actuator taking into account the determined elastic properties.

[0009] This design not only takes into account the fact that the push rod or other elements of the rear-axle steering system, especially elements of the linear actuator, exhibit a technically relevant elastic compliance, but also considers the directional dependence of this compliance. This means that the rear-axle steering system reacts elastically differently when approaching the left stop than when approaching the right stop, and these differences in elasticity are incorporated into the control of the linear actuator.

[0010] With regard to differences in elasticity depending on the adjustment direction of the push rod, a wide variety of constellations are conceivable. For example, the longer of the two push rod sections may have a lower elastic compliance relative to its length than the shorter push rod section. It is even possible for the longer of the two push rod sections to have a lower absolute elastic compliance than the shorter push rod section. This means that a certain longitudinal force acting on the longer push rod section leads to a smaller change in the length of this section than a force of the same magnitude would compress or stretch the shorter push rod section. Analogous relationships with regard to forces that lead to stretching or compression can also apply to the housing sections of the linear actuator in which the push rod sections are guided.

[0011] According to various possible embodiments, the geometric measurement system of the rear-axle steering can comprise at least two measuring devices based on different measuring principles. The term "measuring principle" can refer to physical principles of measurement and / or geometric conditions. For example, the measurement can operate using optical or magnetic means. Irrespective of this, it can be a linear displacement measurement or the measurement of an angular position. In all cases, the measurement can be implemented as an incremental measurement or as an absolute value measurement.

[0012] The method according to the application for operating a rear axle steering system of a motor vehicle having a linear actuator assumes that the linear actuator comprises the following components:

[0013] - a push rod which is movable between two stops and which is at least minimally elastic in its longitudinal direction, said push rod being connected at both ends to joint elements by means of which the steering angle of the rear wheels of the motor vehicle can be varied,

[0014] - an electric motor,

[0015] - a gear arrangement actuated by the electric motor, the output of which is placed asymmetrically between the ends of the push rod.

[0016] As already mentioned in connection with conceivable expansions and compressions, an elastic housing structure or the elasticity of elements rigidly connected to the housing of the linear actuator has the same effect as an elastic push rod. To simplify the explanation of technical relationships, this text primarily considers the elastic compliance of the push rod, without loss of generality.

[0017] As further stated in the independent method claim, the claimed method provides

[0018] - to move the push rod to one of the two stop positions and to determine elastic properties at the stop position,

[0019] - to adjust the push rod to the second stop position and also to determine elastic properties,

[0020] - to compare the elastic properties depending on the stop position and to control the electric motor based on this during further operation of the linear actuator.

[0021] According to one possible method variant, the time elapsed until the push rod assumes a stable position upon reaching each stop position is measured. In particular, the difference between the time elapsed upon reaching the first stop position until the push rod has assumed a stable, vibration-free position and the time elapsed at the second stop position during the corresponding process, i.e. the stabilization of the push rod, can be calculated. A first sub-variant provides that the push rod is moved back in the opposite direction immediately after stabilization. In contrast, according to an alternative sub-variant, the push rod remains in this position for a finite time after reaching the stabilized position. Within the time period that elapses until the push rod has stabilized at the stop, vibrations in the longitudinal direction of the push rod can be detected.As soon as these vibrations have subsided below the detection limit or a defined threshold, the position of the push rod is considered to be stabilized. In cases where the push rod subsequently remains in this position for a defined period of time, this period can be more than twice as long as the period during which vibrations are measured.

[0022] A further variant of the method provides for a comparison between the behavior of the push rod at the two stop positions by comparing the time required to adjust the push rod from the first stop position to the second stop position with the time which elapses for the adjustment of the push rod between the stops in the opposite direction.

[0023] In each process variant, the prerequisites for particularly precise control of the linear actuator are created by determining its elastic properties bidirectionally. The linear actuator is suitable not only for use in a rear-axle steering system, but also, for example, for use in a steer-by-wire system for steering the front wheels of a vehicle.

[0024] Several embodiments of the invention are explained in more detail below with reference to a drawing. In the drawings:

[0025] Fig. 1 shows a rear axle steering system of a motor vehicle, Fig. 2 shows a diagram showing the adjustment of a push rod of the rear axle steering system according to Figure 1,

[0026] Fig. 3 and 4 in further diagrams evaluation options for adjustments of the push rod of the rear axle steering according to Figure 1,

[0027] Fig. 5 a modified way of determining the properties of a rear axle steering,

[0028] Fig. 6 in a flow chart steps for determining elastic properties of the rear axle steering according to Figure 1 ,

[0029] Fig. 7 and 8 further features of the data processing process according to Figure 6.

[0030] Unless otherwise stated, the following explanations refer to all exemplary embodiments. Corresponding or essentially equivalent parts are identified by the same reference numerals in all figures.

[0031] A rear-axle steering system of a motor vehicle (not shown in detail), designated overall by reference numeral 1, comprises a linear actuator in the form of an electromechanical steering actuator 2. The steering actuator 2 serves to displace a push rod 3, which is guided in a housing and connected at each of its two ends to a fork-shaped connecting element 4, 5. The connecting elements 4, 5, designed as articulated elements, are provided for articulated coupling with other chassis elements (not shown) and thus for changing the steering angle of the rear wheels of the vehicle. Regarding the basic structure and function of the rear-axle steering system 1, reference is made to the prior art cited at the beginning.The linear actuator 2 includes a gear assembly 7, which comprises a rotary-to-rotary gear 8, in this case in the form of a belt drive, and a rotary-to-linear gear 14 connected downstream of the latter, which in the example shown is designed as a planetary roller screw drive. The planetary roller screw drive 14 includes a cage 23 in which pitchless profiled planets 22 are guided. In this case, the cage 23 represents the driving element of the planetary roller screw drive 14. A nut, designated 24, of the planetary roller screw drive 14 has no drive or output function.

[0032] An electric motor, designated 6, of the steering actuator 2 has a motor shaft aligned parallel to the push rod 3 and rigidly connected to a pulley 9 of the transmission 8. The belt running over the pulley 9, which acts as a traction mechanism and drives an input-side element of the planetary roller gear 14, namely a sleeve 25 concentric with the push rod 3 and connected in a rotationally fixed manner to the cage 23, is designated 13.

[0033] The total length of the push rod 3, measured between its ends, each of which is connected to one of the connecting elements 4, 5, is designated LS.

[0034] The length LS is composed of a length LS1 of a first push rod section 19 and a length LS2 of a second, comparatively long push rod section 20. The use of the term "push rod sections" does not imply any statement about the single-part or multi-part design of the push rod 3. The boundary between the two push rod sections 19, 20 lies in the middle of a force introduction area, designated 21, of the planetary roller screw drive 14.

[0035] The rear-axle steering system 1 is operated using a sensor arrangement 10, referred to as the sensor system for short, which is designed as a geometric measuring system and includes a rotary sensor 11 and a linear sensor 12. In general, the rotary sensor 11 represents a first measuring system and the linear sensor 12 a second measuring system. The rotary sensor 11 detects the rotation of the motor shaft of the electric motor 6 and thus also of the pulley 9, which represents an input-side machine element of the rotary-rotary gear 8. In the design according to Figure 1, the rotary sensor 11 is located on the end face of the electric motor 6 facing away from the pulley 9. The rotary sensor 11 is designed as a multi-turn sensor. Since the gear arrangement 7 has a fixed transmission ratio, the rotary sensor 11 allows a clear conclusion to be drawn about the position of the push rod 3.In addition, the position of the push rod 3 is directly detected by the linear sensor 12, which in this case is located between the electric motor 6 and the push rod 3. Overall, this provides redundant detection of the position of the push rod 3.

[0036] A measuring and control unit 15 is linked to the measuring systems 11, 12 and to the electric motor 6. This unit can be arranged at any location within the steering actuator 2 and, in deviation from the symbolic representation in Figure 1, can also be decentralized. A stop contour 16 is located on the push rod 3, which is intended to interact with two housing-fixed stops 17, 18. The shape and positioning of the stop contour 16 and the housing-fixed stops 17, 18 are shown in Figure 1 without reference to the actual geometric design, merely to illustrate the basic function.

[0037] The diagram in Figure 2 illustrates the adjustment of the push rod 3 between a first stop A1 and a second stop A2, where the first stop A1 means that the stop contour 16 rests against the first stop 17 fixed to the housing. The stop A2 means that the push rod 3 with its stop contour 16 contacts the second stop 18 fixed to the housing. PS generally denotes the position of the push rod 3. A push rod position SP is assumed at each of the stops A1, A2. Over the course of time t, the push rod 3 is initially moved in the direction of the stop A2, as can be seen from Figure 2. As soon as the stop contour 16 reaches the stop 17 fixed to the housing, noticeable oscillations occur which depend significantly on the elastic properties of the push rod 3. A resulting oscillation phase OP1 extends over the duration Ö1 and is stored.After the oscillation phase OP1, no further significant oscillations occur; the push rod 3 remains at rest. This applies until the next adjustment process, in which the push rod 3 is moved to the stop A1. At this point, the behavior of the push rod 3 is similar to the behavior described at the stop A2. However, an oscillation phase OP2 now occurs, which extends over a duration Ö2 that differs from the duration Ö1. The oscillation phases OP1, OP2, with their specific durations Ö1, Ö2, provide important information about the direction-dependent elastic properties of the linear actuator 2, in particular of the push rod 3.

[0038] Figures 3 and 4 illustrate a further possibility of determining direction-dependent, i.e. bidirectional, elastic properties of the push rod 3. In the idealized case sketched in Figure 3, among other things, the adjustment of the push rod 3 from stop A1 to stop A2 can be seen. Oscillations of the push rod 3 are not apparent from this illustration. A time period At1 is marked, in which the push rod 3 is adjusted, starting from the first stop A1, until it reaches a stable position at the second stop A2. In this case, the time period At1 does not end when the stop 18 fixed to the housing is first touched, but only after the push rod position has stabilized. The same applies to the adjustment process according to Figure 4, in which the push rod 3 is moved from the second stop A2 to the first stop A1.The total time required for the travel process, which is required until the push rod 3 reaches a stable position, is designated Δt2 in this case and differs from the time required Δt1 shown in Figure 3, which is required for adjustment in the opposite direction. The difference between the time required Δt1 and Δt2 provides information about the directional dependence of the elasticity of the push rod 3.

[0039] Figure 5 illustrates, likewise in an idealized representation, another method for determining elastic properties of the rear-axle steering system according to Figure 1. This method, like the methods according to Figures 2 to 4, is also applicable to rear-axle steering systems 1 that have a structure that differs from the exemplary embodiment according to Figure 1 and, for example, operate with a ball screw drive or a simple motion thread to convert rotation into linear movement. According to Figure 5, as in the case of Figure 2, it is assumed that the push rod 3 is initially in its central position ML. Starting from this, the push rod 3 is moved successively to the various stops A2, A1, whereby the push rod 3 only remains at the respective stop A2, A1 until its position has stabilized, i.e., until no significant oscillations occur.As soon as this state is reached, i.e., after the durations Ö1, Ö2 have elapsed, a new adjustment process is initiated. In this case, information about the direction-dependent elasticity of push rod 3 is also obtained from the durations Ö1, Ö2 and their difference.

[0040] The flow chart in Figure 6 relates to the adjustment process in Figure 5. The method for determining elastic properties of the linear actuator 2 including the push rod 3 begins with step S1. In step S2, the first contact of the push rod 3 with the stop A2 is detected. This corresponds to the start of the first oscillation phase OP1. A stopwatch is started in step S2. Regarding the triggering of step S2, reference is made to Figure 7. After that, a position PS of the push rod 3 is repeatedly compared with a position determined in a previous query (Z-1, where Z stands for the numbering of the queries). The difference between the current position PS and the position determined in the previous query represents an absolute value AW. If the absolute value AW is at most as large as a limit value SW, the stop A2 is considered to have been reached.

[0041] Step S3 means that a renewed movement, this time in the opposite direction, of push rod 3 is detected. At this moment, the stopwatch is stopped and the time period Ö1 is saved. The adjustment process, now in the direction of the first stop A1, ends with step S4. Analogous to step S2, in step S4, the first contact with stop A1 is used as a trigger signal to start the stopwatch. The stopwatch is stopped in step S5, whereby the duration Ö2 is determined. Step S6 marks the end of the process according to Figure 6.

[0042] Regarding the input of the determined durations Ö1, Ö2 into the position control of the push rod 3, reference is made to Figure 8. Overall, this is a PID control. The angular position of the rotor of the electric motor 6, which is detected by the sensor 11, is recorded and fed to a comparator taking into account the transmission ratio GR. When evaluating the signals from the linear sensor 12, no such transmission ratio GR needs to be taken into account. The sensor data or derived values ​​are compared with a target position DP and used for control with the inclusion of various control and activation parameters K, IG, DG. A processing module, which can also be decentralized, is designated VGR. In each case, there is a function F that is dependent on the durations Ö1, Ö2 and is used within the framework of the control according to Figure 8.Overall, a position control of the push rod 3 is realized, which takes into account not only its elasticity, but also the directional dependence of the elasticity.

[0043] List of reference symbols

[0044] 1 rear-axle steering

[0045] 2 linear actuator, steering actuator

[0046] 3 push rod

[0047] 4 connecting element

[0048] 5 connecting element

[0049] 6 Electric motor

[0050] 7 Gear arrangement

[0051] 8 Belt drives, rotary-rotary drives

[0052] 9 Pulley, machine element

[0053] 10 Sensor arrangement, sensor technology

[0054] 11 Rotary sensor, first measuring system

[0055] 12 Linear sensor, second measuring system

[0056] 13 belts

[0057] 14 Planetary screw drive, rotary-linear gear

[0058] 15 Measuring and control unit

[0059] 16 Stop contour of the push rod

[0060] 17 first housing-fixed stop

[0061] 18 second housing-fixed stop

[0062] 19 short push rod section

[0063] 20 long push rod section

[0064] 21 Force introduction area

[0065] 22 Planet

[0066] 23 Cage

[0067] 24 Mother

[0068] 25 sleeve

[0069] A1 first stop

[0070] A2 second stop

[0071] AW absolute value

[0072] DP target position

[0073] Ö1 , 52 Duration of the oscillation or holding phase At1 , At2 Time required for changing the stop position

[0074] F(ö1 , ö2) function

[0075] GR gear ratio

[0076] K, IG, DG control and control parameters

[0077] ML middle layer

[0078] LS Length of the push rod

[0079] LS1 , LS2 Length of the push rod sections

[0080] OP1 , OP2 oscillation phases

[0081] P, I, D control structure

[0082] PS Position of the push rod

[0083] S 1 ... S6 process step

[0084] SP push rod position at the stop

[0085] SW limit t time

[0086] VGR processing module

[0087] Z number (integer)

Claims

Patent claims 1. Rear axle steering (1) of a motor vehicle, comprising a linear actuator (2) and a push rod (3) which is displaceable by means of the linear actuator (2) and which is to be coupled in an articulated manner to the rear wheels of the motor vehicle to be steered, wherein a push rod section (19) to be coupled to the left rear wheel has a length (LS1) which differs from the length (LS2) of a push rod section (20) to be coupled to the right rear wheel, and wherein each push rod section (19, 20), the length (LS1, LS2) of which is to be measured from the center of a force introduction region (21) of the linear actuator (2) to an end of the respective push rod section (19, 20) coupled to a connecting element (4, 5), has a significant elasticity in the longitudinal direction of the push rod (3), wherein further stops (16, 17,18) are provided to limit the adjustment range of the push rod (3) and a geometric measuring system (10) exists to detect the position of the push rod (3), and wherein said measuring system (10) is designed in cooperation with a measuring and control unit (15) to determine specific elastic properties of each push rod section (19, 20) on the basis of a measured compliance of the respective push rod section (19, 20) when approaching the stop (17, 18) and to control the linear actuator (2) taking into account the determined elastic properties.

2. Rear axle steering (1) according to claim 1, characterized in that the longer of the two push rod sections (19, 20) has a lower elastic compliance related to the length than the shorter push rod section (19, 20).

3. Rear axle steering (1) according to claim 2, characterized in that the longer of the two push rod sections (19, 20) has absolutely a lower elastic compliance than the shorter push rod section (19, 20).

4. Rear axle steering (1) according to one of claims 1 to 3, characterized in that the geometric measuring system (10), that is to say the entire sensor arrangement, comprises at least two measuring devices (11, 12) based on different measuring principles.

5. Method for operating a rear axle steering system (1) of a motor vehicle, which comprises a linear actuator (2) which - a push rod (3) which is displaceable between two stops (17, 18; A1, A2) and which is at least minimally elastic in its longitudinal direction and which is connected at both ends to joint elements (4, 5) via which the steering angle of the rear wheels of the motor vehicle can be varied, - an electric motor (6) and a gear arrangement (7) operable by the latter, the output of which is placed asymmetrically between the ends of the push rod (3), wherein - the push rod (3) is moved to one of the two stop positions (A1, A2) and elastic properties are determined at the stop position (A1, A2), - the push rod (3) is moved to the second stop position (A1, A2) and elastic properties are also determined, - the elastic properties dependent on the stop position (A1, A2) are compared with each other and, based on this, the electric motor (6) is controlled during further operation of the linear actuator (2).

6. Method according to 5, characterized in that when each stop position (A1, A2) is reached, the time (Ö1, Ö2) elapsed until the push rod (3) assumes a stable position is measured.

7. Method according to 6, characterized in that the difference between the time (Ö1, Ö2) which elapses upon reaching the first stop position (A1, A2) until the push rod (3) has assumed a stable, vibration-free position and the time (Ö1, Ö2) which elapses at the second stop position (A1, A2) during the corresponding process, that is to say the stabilization of the push rod (3), is calculated.

8. Method according to 7, characterized in that the push rod (3) is moved in the opposite direction immediately after stabilization.

9. Method according to 7, characterized in that the push rod (3) is held in this position for a finite time after it has been stabilized in the stop position (A1, A2).

10. Method according to 5, characterized in that a comparison is made between the behavior of the push rod (3) at the two stop positions (A1, A2) by comparing the time (At1, At2) required to adjust the push rod from the first stop position (A1, A2) to the second stop position (A1, A2) with the time (At1, At2) required for the Adjustment of the push rod (3) between the stops (A1, A2) in the opposite direction.

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