Method for controlling steer-by-wire steering system of road vehicle using virtual end-stop feedback function

Abstract

The present disclosure relates to a method of controlling a steer-by-wire steering system (1) for a road vehicle, the method comprising the steps of: providing a basic feedback torque, when steering towards the mechanical end stop and the steering wheel angle is greater than or equal to the activated steering position: i. Activating a first virtual end stop feedback function for determining a reaction torque that counteracts an operation of the steering wheel by the driver, where the first virtual end stop feedback function depends on the steering wheel angle and the steering speed; ii. Adding the reaction torque to the basic feedback torque to generate a steering wheel torque, wherein the procedure of the first virtual end stop feedback function is such that: a predefined maximum steering wheel torque is reached at the virtual steering end stop position; and iii. Sending the resulting steering wheel torque (16) to the feedback actuator (10) and controlling the feedback actuator (10) accordingly. When steering from mechanical rearward, a second virtual end stop feedback function is similarly provided.

Description

Technical Field

[0001] The present disclosure relates to a method of controlling a steer-by-wire steering system of a road vehicle according to the preamble of claim 1 and to a steer-by-wire steering system for a road vehicle. Background Art

[0002] In a steer-by-wire steering system, the vehicle's steering wheel is decoupled from the steering mechanism. In such a steering system, there is no mechanical coupling between the steering wheel and the steering gear. The steering movement is achieved by a steering actuator having an electric motor. The steering actuator operates in response to the sensed values ​​of various steering parameters, such as steering wheel angle and vehicle speed. The sensed values ​​are electronically transmitted from the sensor to the steering actuator, whereby the electric motor drives the rack and guides the steering wheel in the desired direction.

[0003] Although the mechanical linkage between the steering wheel and the road wheels has been eliminated, steer-by-wire steering systems are expected to produce the same function and steering feel as conventional mechanical linkage steering systems.

[0004] In steer-by-wire steering systems, the steering wheel can rotate freely without restriction. However, safe operation requires defined steering end stop positions due to drivability, controllability and other mechanical constraints, such as wires connected to the steering wheel switches. Adjustable mechanical steering wheel range limiters used to solve this problem require high complexity and cost.

[0005] It is known from EP 3 315 383 B1 that a virtual steering limit position can be realized. In the case where the driver has operated the steering wheel to a threshold value close to the desired steering angle value (the desired steering angle value is determined as the virtual steering end stop position), the first correction value calculation circuit calculates a first correction value, thereby increasing the force resisting the driver's operation of the steering wheel, and corrects the basic command value with the first correction value. This makes it difficult for the driver to further operate the steering wheel to a position beyond the virtual steering end stop position when the position of the steering wheel becomes close to the virtual steering end stop position. Therefore, the driver's operation of the steering wheel actually stops near the virtual steering end stop position. Summary of the invention

[0006] An object of the present disclosure is to provide a method for a steer-by-wire steering system for a road vehicle that provides a natural steering feel close to mechanical end stop positions.

[0007] This object is achieved by a method and a steer-by-wire steering system for a road vehicle having the features of claim 1 .

[0008] Therefore, a method of controlling a steer-by-wire steering system for a road vehicle is provided. The steer-by-wire steering system comprises a steering wheel, a road wheel actuator for actuating the road wheel, a feedback actuator having a mechanical end stop for applying a feedback torque to the steering wheel, and a sensor for detecting a steering wheel angle and a steering speed of the steering wheel. The method comprises the following steps: a) Provide basic feedback torque, b) when steering towards the mechanical end stop and the steering wheel angle is greater than or equal to the activation steering position, i. starting a first virtual end stop feedback function, which determines a counter torque that counteracts the driver's operation of the steering wheel, wherein the first virtual end stop feedback function depends on the steering wheel angle and the steering speed, ii. adding the reaction torque to the basic feedback torque to generate the steering wheel torque, wherein the process of the first virtual end stop feedback function is such that at the virtual steering end stop position, a predefined maximum steering wheel torque is reached, and iii. sending the resulting steering wheel torque (16) to the feedback actuator (10) and controlling the feedback actuator (10) accordingly, c) when steering backwards from the mechanical end stops and the steering wheel angle is greater than or equal to the deactivation steering position, i. starting a second virtual end stop feedback function, which determines a counter torque that counteracts the driver's operation of the steering wheel, wherein the second virtual end stop feedback function depends on the steering wheel angle and the steering speed, ii. adding the reaction torque to the basic feedback torque to generate the steering wheel torque, wherein the process of the second virtual end stop feedback function is such that at the virtual steering end stop position, a predefined maximum steering wheel torque is reached, and iii. Sending the resulting steering wheel torque (16) to the feedback actuator (10) and controlling the feedback actuator (10) accordingly.

[0009] It is understood that both mechanical end stops are preferred. These virtual end stop feedback functions provide a more natural end stop feel than using only mechanical end stops. Another purpose of the virtual end stop feedback function is to make the driver aware of reaching the mechanical end stop. In addition, the virtual end stop feedback function can reduce the contact of the mechanical end stop by increasing the counter torque. Preferably, for the same steering speed, the first virtual end stop feedback function is different from the second virtual end stop feedback function and produces a hysteresis.

[0010] Preferably, the first virtual end stop feedback function and the second virtual end stop feedback function are implemented based on a look-up table.

[0011] It is advantageous if the counter-torque provided by the first virtual end stop feedback function increases with increasing steering wheel angle.

[0012] Preferably, these virtual end stop feedback functions are dependent on the steering speed, so that when the corresponding virtual end stop feedback function is activated, the modification of the steering speed is determined by the current steering speed. This ensures that no torque fluctuations due to steering speed changes occur when the virtual end stop feedback function is activated.

[0013] Preferably, at higher steering speeds the virtual steering end stop position is at a smaller steering wheel angle than at lower steering speeds.

[0014] Preferably, in order to prevent the mechanical end stop position from being reached, the virtual steering end stop position is smaller than or equal to the mechanical end stop position.

[0015] Preferably, in the event of a parking maneuver, these virtual end stop feedback functions are modified by gain factors.

[0016] Preferably, for the same mechanical end stop, the virtual steering end stop positions of the first virtual end stop feedback function and the second virtual end stop feedback function are identical.

[0017] It is advantageous if, in the event that the wheel is blocked by a curb, the virtual steering end stop position is close to the actual steering position, ie closer than in normal conditions.

[0018] If the mechanical end stops are different, the first virtual end stop feedback function preferably has the same absolute starting steering position and the same absolute virtual steering end stop position. At the same steering speed, these functions are symmetrical relative to the neutral position (i.e., left and right). This compensates for the deviation of the mechanical end stop between the left and right sides of the steering system. For example, if the left mechanical end stop is at X degrees and the right mechanical end stop is at X+d degrees, the first virtual end stop feedback function establishes the counter-steering torque from the same absolute steering angle (starting steering position). The contact feel of the end stop is provided by the first virtual end stop feedback function and is preferably the same for both sides. If the steering speed when turning toward one or the other mechanical end position is different, the contact feel of the end stop may be different. For example, before reaching the mechanical end stop, the counter-torque may enter a saturated state at the maximum torque of the feedback actuator.

[0019] Furthermore, a steer-by-wire steering system for a road vehicle is provided, which is designed to carry out the above-mentioned method. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Preferred embodiments of the present disclosure will be described with reference to the accompanying drawings.

[0021] Figure 1 is a schematic diagram of a steer-by-wire steering system for a motor vehicle; and Figure 2 A graph of steering wheel torque versus steering wheel angle is shown. DETAILED DESCRIPTION

[0022] Figure 1 is a schematic diagram of a steer-by-wire steering system 1, in which a steering shaft 2 is connected to a steering wheel 3. There is no mechanical connection between the steering wheel 3 and the road wheel 4. A road wheel actuator 5 operates a gear rack 6 via a rack-and-pinion gear 7, wherein the gear rack 6 is part of a front axle 8. The front axle 8 has two tie rods 9 for the road wheels 4, only one of which is shown in the figure.

[0023] When the driver operates the steering wheel 3, the steering shaft 2 rotates, wherein the rotation of the steering shaft 2 can be detected by an axle sensor, wherein the axle sensor is not shown in the figure. The control unit calculates an operation signal for the road wheel actuator 5 based on the signal detected by the axle sensor. By operating the gear rack 6 using the operation signal, the front wheel shaft 8 is moved laterally and the road wheel 4 is turned. At the same time, the force introduced into the front wheel shaft 8 from the road wheel 4 is detected by another sensor not shown in the figure, and a feedback signal is calculated, which is applied to the steering shaft 2 by the feedback actuator 10, thereby generating a steering wheel torque so that the operator can recognize the feedback in the steering wheel 3.

[0024] Figure 2 A total of three hysteresis curves 11 , 12 , 13 are shown for the steering wheel torque as a function of the steering wheel angle.

[0025] When steering from the activated steering position towards the mechanical end stops, the virtual end stop feedback function is activated and added to the basic feedback torque, resulting in a steering wheel torque plotted on the y-axis. The virtual end stop feedback function determines the additional reaction torque. The basic feedback torque is calculated based on the steering wheel angle, steering wheel speed, vehicle dynamics (e.g. longitudinal motion of the vehicle, lateral motion, etc.), rack force and rack motion. When steering backward from the mechanical end stops, the virtual end stop feedback function is active until the deactivated steering position is reached.

[0026] The steering wheel torque has a maximum value, and therefore the steering wheel angle has a maximum value determined by the mechanical end stop positions. Preferably, the course of the reaction torque as a function of the steering wheel angle is adjusted by a lookup table. Steering position breakpoints (predefined steering wheel positions) and reaction torque values ​​are defined as value pairs, up to a maximum steering wheel torque value and a virtual steering end stop position. The virtual steering end stop position can be defined by the mechanical end stop position, or a lower value close to the mechanical end position. The steering position breakpoints are adjustable parameters. Points between two breakpoints are interpolated.

[0027] When steering towards the virtual steering end stop position, the course of the steering wheel torque as a function of the steering wheel angle is different than when steering backwards. Two independent virtual end stop feedback functions are used. One virtual end stop feedback function is used for steering towards the virtual steering end stop position and one virtual end stop feedback function is used for steering backwards. This achieves a unique torque hysteresis and thus a natural feeling of the end stops. The activation steering position and the deactivation steering position of the virtual end stop feedback function are preferably the same. However, due to the different feedback characteristics, this creates the feeling that the function is deactivated at a position different from the position where the function is activated.

[0028] The software algorithm calculates the counter-torque based on the steering wheel angle of the steering wheel and the steering speed.These virtual end stop feedback functions each comprise a basic function which is preferably modified by a steering speed-dependent parameter.

[0029] Figure 2 Steering wheel torque hysteresis curves 11, 12 at two different steering speeds, and a steering wheel torque hysteresis curve 13 at a small steering direction change are shown. The arrow indicates the steering direction. Solid lines 110, 111 represent the basic hysteresis curve 11 at normal steering speeds, which are preferably at most about 50 degrees / second. When turning towards the virtual steering end stop position 112, the upper line 110 is applied. From the starting position, the counter torque calculated based on the first virtual end stop feedback function is added to the basic feedback torque. The steering process starts from the starting steering wheel angle 113. The resulting steering wheel torque first increases slowly and then rises rapidly until the maximum value of 200 is reached at the virtual steering end stop position 112. The counter torque counteracts the driver's operation, and the steering feel is similar to a conventional electromechanical steering system with a mechanical end stop. When turning backward from the virtual steering end stop position 112, the lower line 111 and the second virtual end stop feedback function are applied until the deactivated steering position not shown is reached. The steering wheel torque is first reduced rapidly, then reduced moderately, until the steering comes to a stop at 114 .

[0030] A second hysteresis curve 12 is indicated by a dotted line and represents the course at higher steering wheel steering speeds, preferably steering speeds exceeding approximately 200 degrees / second. The maximum value 200 of the steering wheel torque is identical to the maximum value of the hysteresis curve 11 at normal speed. However, the virtual steering end stop position 120 is at a smaller steering wheel angle than the hysteresis curve 11 at normal speed. The basic course changes depending on the steering speed, since the activation position and deactivation steering position of the virtual end stop feedback function also change. The hysteresis curve shape remains the same. Basically, for higher steering wheel steering speeds, the basic hysteresis curve in the figure drifts to the left. Compared to the hysteresis curve 11 at normal speed (not at Figure 2 The active steering position is at a smaller steering wheel angle than the active steering position (marked in the middle). The same is true for the inactive steering position.

[0031] The modification of the hysteresis curve based on the steering speed is determined at the activation point. This ensures that no torque fluctuations occur due to steering speed changes when the virtual end stop feedback function is activated.

[0032] A third hysteresis curve 13 is also indicated by a dashed line, which lies within the basic course of the first hysteresis curve 11. It represents small changes in the steering direction, such as occur during a parking maneuver, for example.

[0033] The steering direction is changed at 135 and the steering wheel is turned backwards until the steering direction is changed a second time at 130. From this position, the steering wheel is turned toward the virtual steering end stop position until the steering operation is stopped at 134.

[0034] The hysteresis curve 13 is based on the basic hysteresis curve 11 and the basic look-up table, which can be modified by further adjustable parameters (such as gain factors) to produce the hysteresis curve 13. Such gain factors modify the basic steer-out and steer-in curves in order to connect them to each other. The basic shape of the third hysteresis curve 13 is derived from the steer-out function look-up table, but it is modified to produce more hysteresis (torque difference) between steer-out and steer-in.

[0035] The virtual steering end stop positions 112, 120 can be defined at any position within the steering range. The virtual steering end stop positions can be set near or at the actual mechanical end stop, or closer to the actual steering position in case the wheel is caught by the curb (curb block). It is also possible that in case the wheel is caught by the curb, the virtual end stop feedback function is informed of this situation to initiate feedback to the driver.

Claims

1. A method for controlling a steer-by-wire steering system (1) for a road vehicle, the steer-by-wire steering system (1) comprising a steering wheel (3), a road wheel actuator (5), a feedback actuator (10) and a sensor, the road wheel actuator (5) being used to actuate a road wheel (4), the feedback actuator (10) having a mechanical end stopper for applying a feedback torque to the steering wheel, the sensor being used to detect a steering wheel angle and a steering speed of the steering wheel, characterized in that: The method comprises the following steps: a) Provide basic feedback torque, b) when steering towards the mechanical end stop and said steering wheel angle is greater than or equal to the starting steering position, i. starting a first virtual end stop feedback function, which determines a counter torque that counteracts the driver's operation of the steering wheel, wherein the first virtual end stop feedback function depends on the steering wheel angle and the steering speed, ii. adding the reaction torque to the basic feedback torque to generate a steering wheel torque, wherein the process of the first virtual end stop feedback function is such that a predefined maximum steering wheel torque is reached at the virtual steering end stop position, and iii. sending the resulting steering wheel torque (16) to the feedback actuator (10) and controlling the feedback actuator (10) accordingly, c) when steering backwards from the mechanical end stop and said steering wheel angle is greater than or equal to the deactivated steering position, i. starting a second virtual end stop feedback function, which determines a counter torque that counteracts the driver's operation of the steering wheel, wherein the second virtual end stop feedback function depends on the steering wheel angle and the steering speed, ii. adding the reaction torque to the basic feedback torque to generate a steering wheel torque, wherein the process of the second virtual end stop feedback function is such that at the virtual steering end stop position, the predefined maximum steering wheel torque is reached, and iii. Sending the resulting steering wheel torque (16) to the feedback actuator (10) and controlling the feedback actuator (10) accordingly.

2. The method according to claim 1, characterized in that The first virtual end stop feedback function and the second virtual end stop feedback function are implemented based on a lookup table.

3. The method according to claim 1 or 2, characterized in that: The counter-torque provided by the first virtual end stop feedback function increases with increasing steering wheel angle.

4. The method according to claim 3, characterized in that: The virtual end stop feedback functions are dependent on the steering speed, wherein a modification of the steering speed is determined when the respective virtual end stop feedback function is activated.

5. The method according to any one of the preceding claims, characterized in that At higher steering speeds, the virtual steering end stop position is at a smaller steering wheel angle than at lower steering speeds.

6. The method according to one of the preceding claims, characterized in that The virtual steering end stop position is less than or equal to the mechanical end stop position.

7. The method according to claim 4, characterized in that In the event of a parking maneuver, the virtual end stop feedback function is modified by a gain factor.

8. The method according to any one of the preceding claims, characterized in that The virtual steering end stop positions are the same for the first virtual end stop feedback function and the second virtual end stop feedback function.

9. The method according to any one of the preceding claims, characterized in that In the event that the wheel is caught by a curb, the virtual steering end stop position approaches the actual steering position.

10. The method according to any one of the preceding claims, characterized in that In case the mechanical end stops are different, the first virtual end stop feedback function has the same absolute starting steering position and the same absolute virtual steering end stop position for the same steering speed.

11. A steer-by-wire steering system (1) for a road vehicle, designed to perform a method according to any one of the preceding claims.

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