Steering actuator and method for operating a rear axle steering system

Abstract

The invention relates to a steering actuator (1) comprising: an electric motor (2); a push rod (6) that is linearly adjustable by means of the electric motor (2) via a gear assembly (3); and a sensor system (12) which is designed to detect the position of the push rod (6) and to which two different sensors (7, 8) can be assigned. One of the two sensors (8) covers the entire measurement range (M2) in which the position of the push rod (6) can be detected, whereas the other sensor (7) is designed to detect the position of the push rod (6) in smaller, adjacent measurement ranges (M1) located within the entire measurement range (M2). A control unit (9) interacts with both sensors (7, 8) and, in cooperation with the electric motor (2), is designed to move the push rod (6), as required in each of the smaller measurement ranges (M1), into a neutral position (NR) located in the centre of said measurement range (M1) without moving towards a reference position outside said measurement range (M1).

Description

[0001] Steering actuator and method for operating a rear axle steering system

[0002] The invention relates to an electromechanical steering actuator according to the preamble of claim 1. Furthermore, the invention relates to a method for operating a rear-axle steering system of a motor vehicle.

[0003] A steering actuator of this type is known, for example, from DE 10 2008 032 046 A1. The known steering actuator, intended for use in a rear-axle steering system, comprises a position-determining system that operates with two different sensors, namely a linear sensor and a rotary sensor. Within the scope of a calibration method described in DE 10 2008 032 046 A1, calibration information is generated that includes zero-point information from the linear sensor and sector information relating to the rotary sensor. For this purpose, the measuring range of the rotary sensor is divided into at least two sectors. The sector information is intended to identify the angle at which the angular position of a rotor of the steering actuator, as detected by the rotary sensor, lies when a displaceable actuator element of the steering actuator is arranged in its geometric center position.According to an embodiment described as advantageous in DE 10 2008 032 046 A1, the angular range covered by the sectors is larger than the angular resolution of the rotation sensor.

[0004] Another steering actuator, which has the features of the preamble of claim 1 and is also intended for use in a rear-axle steering system, is known from DE 10 2018 129 119 A1. This steering actuator comprises a two-stage gear arrangement and a sensor arrangement designed to detect the setting of the gear arrangement and has two spatially separated, independently functional sensors. Both sensors of the steering actuator according to DE 10 2018 129 119 A1 are located on one and the same output-side, linearly displaceable element of the gear arrangement of the steering actuator. A chassis actuator disclosed in DE 10 2015 206 678 A1 comprises an electric motor, a spindle drive having a threaded spindle and a spindle nut, and a gear with a fixed gear ratio connected between the electric motor and the spindle nut.The electric motor has a first angle sensor. Furthermore, a second angle sensor is arranged on an output gear of the gearbox, which is connected to the spindle nut.

[0005] The invention is based on the object of further developing electromechanical actuators which are suitable for adjusting the steering angle of the rear wheels of a motor vehicle, compared to the cited prior art, whereby a particularly favorable ratio between equipment expenditure and operational reliability is to be achieved and at the same time a good level of comfort is sought, in particular during the starting phase of the vehicle.

[0006] This object is achieved according to the invention by a steering actuator having 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 8. 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, the entire rear-axle steering system, and the motor vehicle equipped therewith, and vice versa.

[0007] In a basic concept known per se, the steering actuator comprises an electric motor, a push rod that can be adjusted linearly by means of the electric motor via a gear arrangement, and a sensor system for detecting the position of the push rod, which includes two different sensors.

[0008] According to claim 1, one of the two sensors covers the entire measuring range in which the position of the push rod can be detected, whereas the other sensor is designed to detect the position of the push rod in smaller, adjacent measuring ranges located within the entire measuring range. Furthermore, the steering actuator according to the application comprises a control unit that interacts with both sensors and, in cooperation with the electric motor and the gear arrangement, is designed to move the push rod, as needed, in each of the smaller measuring ranges, into a neutral position located centrally within said narrow measuring range without approaching a reference position located outside the exceeding, narrow measuring range.

[0009] Thus, there is a number of neutral positions corresponding to the number of smaller, i.e. narrow, measuring ranges, which can be used for calibration positions.

[0010] These neutral positions can take on the function of reference positions, which in older, unused positioning systems are provided, for example, by stop positions.

[0011] When using the steering actuator in a motor vehicle's rear-axle steering system, this means that after starting the vehicle, no calibration procedure is required, which would require the rear wheels to be turned to one of their extreme positions. Likewise, pivoting the rear wheels—if they are already turned—to their center position is not required. Rather, it is possible to drive off directly with the rear wheels turned, for example, when exiting a parking space. Eliminating the need to return the rear wheels to the center position, i.e., the position for straight-ahead travel, which is unnecessary for driving dynamics, significantly increases comfort.

[0012] According to one possible embodiment, the steering actuator sensor intended for detecting measured values ​​in the smaller measuring range is designed as a rotary sensor. This can, in particular, be a rotary sensor designed for absolute value measurement. The rotary sensor can, for example, detect the angular position of the shaft of the steering actuator's electric motor. Variants are also feasible in which the rotary sensor detects the angular position of an element of the steering actuator's gear arrangement that rotates at a lower speed than the motor shaft of the electric motor. In both cases, the rotary sensor is, in particular, a multi-turn sensor, i.e., a sensor that detects a rotation angle over several revolutions.The multi-turn functionality can be implemented in a conventional manner, either electronically or with the aid of a reduction gear that serves purely metrological purposes and converts the rotation of the shaft on which the measurement is to be carried out into a slower rotary movement.

[0013] The sensor that covers the entire measuring range is typically designed as a linear sensor. This sensor detects, for example, displacements of the push rod or movements of an output-side element of the gear assembly that is rigidly connected to the push rod. In any case, the linear sensor can be designed as an incremental sensor.

[0014] Regardless of the design of the two sensors, the wider measuring range corresponds, for example, to at least four times and at most twenty times the smaller measuring range, which means that the wider measuring range is divided into four or twenty smaller measuring ranges, respectively. The width of the smaller measuring range can be determined, in particular, by a full revolution of an input-side element of the gear arrangement, whereby the input-side element of the gear arrangement can be connected in a rotationally fixed manner to the motor shaft of the electric motor or can be identical. Alternatively, the smaller measuring range can correspond to a full revolution of an element of the gear arrangement rotating at a reduced speed compared to the motor shaft of the electric motor.

[0015] The gear arrangement of the steering actuator comprises, for example, a belt-driven transmission, in particular in the form of a belt or chain drive, as well as a rotary-linear transmission downstream of the belt-driven transmission, in particular in the form of a spindle drive. The spindle drive is designed, for example, as a ball screw drive or a planetary roller screw drive. In principle, a simple motion screw, i.e., a spindle drive without rolling elements, can also be used. The steering actuator is generally suitable for use in an electromechanically actuated steering system of a motor vehicle. In particular, such a steering system can also be a steer-by-wire steering system for steering the front wheels of a motor vehicle. Thus, embodiments that refer to rear-axle steering are also transferable to front-wheel steering systems.

[0016] In addition to steering variants in which the measuring range, realized in particular with the help of the linear sensor, covers the entire adjustment range of the push rod, variants are also possible in which the physically possible adjustment range, i.e. the entire range from stop to stop, extends beyond the entire measuring range, which is divided into numerous smaller measuring ranges. In such a case, at least one signal must be present that indicates the direction in which the push rod has been moved beyond the entire measuring range. This signal is sufficient, as a minimum, to adjust the push rod back to the range in which the position of the push rod can be determined using both sensors.

[0017] The method for operating a rear-axle steering system according to the application generally assumes that a push rod, by means of which the steering angle of the rear wheels can be varied, is electromechanically adjusted via a gear arrangement. The position of the push rod is determined using a combination of two sensors, one of which covers the entire measuring range in which the position of the push rod can be detected, while the other, particularly more precisely resolving sensor, is used to detect the position of the push rod in smaller, non-overlapping measuring ranges located within the entire measuring range.With the aid of a control unit interacting with both sensors, in conjunction with the electric motor, the push rod is moved, as needed, in each of the smaller measuring ranges into a neutral position centrally located within said small measuring range without approaching a reference position outside the small measuring range. Within the scope of the operating method according to the application, the position of the push rod is directly sensed, in particular, by one of the two sensors, while a change in the angular position of a rotatable element of the gear arrangement is detected by the second sensor. The shaft of the electric motor can also represent an element of the gear arrangement.

[0018] The operating procedure is characterized in particular by the possibility that immediately after starting a vehicle equipped with rear-axle steering, the position of the push rod is calibrated by moving to one of the neutral positions which does not correspond to the central position of the push rod, i.e. the setting for straight-ahead travel.

[0019] As far as the functional principle of the sensors is concerned, all principles known in length and angle measurement technology, such as optical or magnetic measuring methods, can be used.

[0020] An exemplary embodiment of the invention is explained in more detail below with reference to a drawing. The drawings show, partly schematically:

[0021] Fig. 1 a rear axle steering system including an electromechanical steering actuator,

[0022] Fig. 2 a control circuit of the steering actuator,

[0023] Fig. 3 shows a flow chart of a method for operating the rear axle steering according to Fig. 1.

[0024] A steering actuator, designated overall by reference number 1, is used for

[0025] Intended for use in a rear-axle steering system 10 of a motor vehicle not shown in detail. Regarding the basic function of the rear-axle steering system 10, including the steering actuator 1, reference is made to the prior art cited at the beginning.

[0026] The steering actuator 1 comprises an electromechanical actuator, an electric motor 2, and, in this case, a two-stage gear arrangement 3. The gear arrangement 3 includes a belt drive 4, i.e., a belt-and-tie transmission, as a rotary-rotary transmission, and a spindle drive 5 as a rotary-linear transmission. A push rod 6 is provided as the output element of the spindle drive 5. This push rod is connected to forks 16, to which further, not shown, chassis elements are articulated, enabling adjustment of the steering angle of the rear wheels of the motor vehicle. The housing of the steering actuator 1 is designated overall by 15.

[0027] The sensor system 12 comprises a narrow measuring range M1 and a comparatively wide measuring range M2. The measuring range M2 covers the entire adjustment range of the push rod 6. The position of the push rod 6 can only be determined in the sense of an absolute value measurement within the narrow measuring range M1. In the measuring range M2, however, only incremental measurements of the change in the position of the push rod 6 are possible. In the exemplary embodiment, it is assumed that the linear sensor 8 covers the entire measuring range M2. The absolute value measurement in the measuring range M1, which lies within the measuring range M2, is possible using the sensor 7, which detects the angular position of the rotor of the electric motor 2 and thus the setting of a rotatable input-side element of the gear arrangement 3. Deviating from this constellation, it is also conceivable for both sensors 7, 8 to detect the angular position or rotation of a rotatable element.Likewise, embodiments are feasible in which both sensors 7, 8 detect the position or displacement of a linearly movable element of the steering actuator 1. The number of narrow measuring ranges M1, into which the wide measuring range M2 is divided, is greater than shown in simplified form in Fig. 1. The steering actuator 1 is designed to implement a control method, which is illustrated in Figure 2. A setpoint value SW of the position of the push rod 6 is processed with the aid of a processing module 17, which in the present case is designed as a ramp generator, and fed to a comparator 13.

[0028] The comparator 13 has the task of comparing the value supplied by the processing module 17 with a measured value MW obtained with the aid of the sensor system 12, whereby in the cases considered, measured values ​​MW from both sensors 7, 8 are always available. The result of the comparison between the setpoint SW and the measured value MW is fed to a controller 14, which, like the comparator 13, is assigned to the control unit 9. The controlled system of the control loop comprises the mechanical components of the steering actuator 1. In the present case, the push rod 6 is adjustable in a range of at least -25 mm to +25 mm, whereby the linear sensor 8 covers the entire adjustment range, i.e. measuring range M2.

[0029] In the case outlined, the entire measuring range M2 is completely divided into a multitude of smaller, equally sized measuring ranges M1. It is also possible to subdivide only a portion of the measuring range M2 into smaller measuring ranges M1. In any case, the center position of the push rod 6 is in a range covered by both the wide measuring range M2 and the narrow measuring ranges M1. One full revolution of the shaft of the electric motor 2 corresponds to the width of each of the small measuring ranges M1.

[0030] As outlined in Figure 1, which shows only a portion of the small measuring ranges M1, a signal SR is generated within each measuring range M1, which is visualized in Figure 1 as a linearly rising curve. A neutral position within each measuring range M1, which is scanned using the sensor 7, is designated NR. In total, there is therefore a number of neutral positions NR of the sensor system 12, which corresponds to the number of narrow measuring ranges M1. Each of these neutral positions NR can be used as a reference position for calibrating the sensor system 12. With regard to the calibration of the sensor system 12, reference is made to the flow chart in Figure 3. The start of the calibration process is designated as step S1. In step S2, a query is made as to whether the push rod 6 is within the range that can be detected using the linear sensor 8.If this is not the case, in step S7, the system switches to a method described in patent application DE 10 2023 109 655.9. This method allows the push rod 6 to be moved back into the measuring ranges M1, M2, in particular into its central position, even though it has left the detection range of the linear sensor 8, i.e., the wide measuring range M2, which is not shown in the simplified flow diagram of Figure 3.

[0031] If the test carried out in step S2 shows that the push rod 6 is within the measuring range M2, a check is carried out in step S3 to determine whether and, if so, in what way there is a deviation from the neutral position NR of the narrow measuring range M1, i.e. the rotary measuring range. In most cases, such a deviation will be detectable. If the deviation is positive, the push rod 6 is adjusted in the negative direction in step S4. If, on the other hand, a negative deviation is detected in step S3, the push rod 6 is moved in the positive direction in step S5. In any case, the query carried out in step S3 and the subsequent adjustment carried out in step S4 or step S5 culminate in a calibration of the steering actuator 1 including the sensor system 12, referred to as step S6. Step S8 marks the end of the calibration process.

[0032] List of reference symbols

[0033] 1 steering actuator

[0034] 2 electric motor

[0035] 3 Gear arrangement

[0036] 4 Belt drive

[0037] 5 spindle drive

[0038] 6 push rod

[0039] 7 Sensor on the electric motor

[0040] 8 linear sensors

[0041] 9 Control unit

[0042] 10 Rear-axle steering

[0043] 11 electrical cable

[0044] 12 Sensor technology

[0045] 13 comparators

[0046] 14 controllers

[0047] 15 housings

[0048] 16 Fork piece

[0049] 17 Processing module

[0050] M1 narrow measuring range, rotary measuring range

[0051] M2 wide measuring range, linear measuring range

[0052] NR Neutral position within the rotary measuring range

[0053] S1 ... S8 process step

[0054] SR signal within the rotary measuring range

[0055] SW setpoint

Claims

Patent claims 1. Steering actuator (1) comprising an electric motor (2), a push rod (6) which is linearly adjustable by means of the electric motor (2) via a gear arrangement (3), and a sensor system (12) provided for detecting the position of the push rod (6), which sensor system comprises two different sensors (7, 8), characterized in that one of the two sensors (8) covers the entire measuring range (M2) in which the position of the push rod (6) can be detected, whereas the other sensor (7) is designed to detect the position of the push rod (6) in smaller, adjacent measuring ranges (M1) located within the entire measuring range (M2), and wherein there is a control unit (9) which interacts with both sensors (7, 8) and which, in interaction with the electric motor (2), is designed toif necessary, in each of the smaller measuring ranges (M1), the push rod (6) is to be moved into a neutral position (NR) located centrally in the said measuring range (M1) without approaching a reference position located outside the respective measuring range (M1).

2. Steering actuator (1) according to claim 1, characterized in that the sensor (7) provided for detecting measured values ​​in the smaller measuring range (M1) is designed as a rotation sensor.

3. Steering actuator (1) according to claim 2, characterized in that the rotation sensor (7) is designed for absolute value measurement.

4. Steering actuator (1) according to claim 3, characterized in that the sensor (8) provided for detecting measured values ​​in the further measuring range (M2) is designed as a linear sensor.

5. Steering actuator (1) according to claim 4, characterized in that the linear sensor (8) is designed as an incremental sensor.

6. Steering actuator (1) according to one of claims 1 to 5, characterized in that the wider measuring range (M2) corresponds to at least four times and at most twenty times the narrower measuring range (M1), the width of the narrower measuring range (M1) being given by a full revolution of an input-side element of the gear arrangement (3).

7. Steering actuator (1) according to one of claims 1 to 6, characterized in that the gear arrangement (3) comprises a belt transmission (4) and a spindle drive (5) connected downstream of the latter, in particular in the form of a ball screw drive.

8. A method for operating a rear-axle steering system (10), wherein a push rod (6) is electromechanically adjusted via a gear arrangement (3) and the position of the push rod (6) is determined by means of a combination of two sensors (7, 8), wherein one of the two sensors (8) covers the entire measuring range (M2) in which the position of the push rod (6) is detectable, whereas the other sensor (7) is used to detect the position of the push rod (6) in smaller, adjacent measuring ranges (M1) located within the entire measuring range (M2), and wherein, with the aid of a control unit (9) interacting with both sensors (7, 8) in cooperation with the electric motor (2), the push rod (6) is moved, as required, in each of the smaller measuring ranges (M1) without moving to a reference position located outside the respective measuring range (M1), into a neutral position (NR) located centrally in said measuring range (M1). becomes.

9. Method according to claim 8, characterized in that the position of the push rod (6) is directly sensed by means of one of the two sensors (8), whereas a change in an angular position of a rotatable element of the gear arrangement (3) is detected by means of the second sensor (7).

10. Method according to claim 8 or 9, characterized in that immediately after starting a vehicle equipped with the rear axle steering (10), the position of the push rod (6) is calibrated by moving to one of the neutral positions which does not correspond to the central position of the push rod (6).

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