Method for controlling a steer-by-wire steering system for a road vehicle with virtual end-stop feedback

Abstract

The present invention relates to a method for controlling a steer-by-wire steering system (1) for a road vehicle, comprising the steps of: providing a base feedback torque when steering toward a mechanical end stop and the steering wheel angle is equal to or greater than an activation steering position. The method includes the steps of: activating a first virtual end stop feedback function that determines a counter torque that reacts to the driver's steering input, the first virtual end stop feedback function being dependent on the steering wheel angle and the steering wheel speed; adding the counter torque to the base feedback torque to produce a steering wheel torque, the first virtual end stop feedback function reaching a predetermined maximum steering wheel torque at the virtual steering end stop; and sending the resulting steering wheel torque (16) to a feedback actuator (10) and controlling the feedback actuator (10) accordingly. The feedback works similarly when steering backward from a machine that provides a second virtual end stop.

Description

[Technical Field]

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

[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 connection between the steering wheel and the steering gear. Steering movement is achieved by a steering actuator equipped with an electric motor. The steering actuator operates in response to detected values ​​of various steering parameters, such as steering wheel angle and vehicle speed. The detected values ​​are electronically transmitted from sensors to the steering actuator, which then drives a rack to point the steerable wheels in the desired direction.

[0003] Even though there is no mechanical link between the steering wheel and the road wheels, steer-by-wire steering systems are expected to provide the same functionality and steering feel as traditional mechanically linked steering systems.

[0004] In steer-by-wire steering systems, the steering wheel can rotate freely without restriction. However, safe operation requires a defined steering end-stop position due to drivability, controllability, and other mechanical constraints, such as wires connected to steering wheel switches. An adjustable mechanical steering wheel range limiter to solve this problem would be highly complex and expensive.

[0005] European Patent Publication No. 3,315,383 B1 discloses a method for implementing a virtual steering limit position. A first correction value calculation circuit calculates a first correction value so that the force of the steering wheel resisting the driver's operation increases when the driver operates the steering wheel to a threshold value close to a desired steering angle value determined as a virtual steering end stop position, and corrects the base command value with the first correction value. This makes it difficult for the driver to further operate the steering wheel beyond the virtual steering end stop position when the steering wheel position approaches the virtual steering end stop position. This allows the driver's steering wheel operation to be virtually stopped near the virtual steering end stop position. Summary of the Invention

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

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

[0008] Accordingly, there is provided a method of controlling a steer-by-wire steering system for a road vehicle, the steer-by-wire steering system comprising a steering wheel, wheel actuators for actuating the wheels, a feedback actuator having a mechanical end stop for applying a feedback torque to the steering wheel, and sensors for detecting a steering wheel angle and a steering speed of the steering wheel.

[0009] The method comprises: a) providing a basic feedback torque; b) When steering towards a mechanical end stop and the steering wheel angle is equal to or greater than the actuation steering position, i) activating a first virtual end-stop feedback function that determines a counter torque that reacts to a driver's input of the steering wheel, the first virtual end-stop feedback function being dependent on the steering wheel angle and the steering wheel velocity; ii) adding a counter torque to the basic feedback torque to generate a steering wheel torque, the process of the first virtual end-stop feedback function reaching a predetermined maximum steering wheel torque at a virtual steering end-stop position; iii) sending the resulting steering wheel torque (16) to the feedback actuator (10) and controlling the feedback actuator (10) accordingly; c) When steering backward from the mechanical end stop and the steering wheel angle is equal to or greater than the deactivated steering position, i) activating a second virtual end-stop feedback function that determines a counter torque that reacts to a driver's input of the steering wheel, the second virtual end-stop feedback function being dependent on the steering wheel angle and the steering wheel velocity; ii) adding a counter torque to the basic feedback torque to generate a steering wheel torque, the process of the second virtual end-stop feedback function reaching a predetermined maximum steering wheel torque at the virtual steering end-stop position; iii) sending the resulting steering wheel torque (16) to the feedback actuator (10) and controlling the feedback actuator (10) accordingly.

[0010] It should be understood that this is preferable for both mechanical end stops. The virtual end stop feedback function provides a more natural end stop feel than the mechanical end stops alone. Another purpose of this function is to make the driver aware that they are reaching the mechanical end stop. It also reduces mechanical end stop contact by increasing the counter torque. The first and second virtual end stop feedback functions are preferably different for the same steering speed, creating hysteresis.

[0011] Preferably, the first and second virtual endstop feedback functions are based on look-up tables.

[0012] It is advantageous if the counter torque provided by the first virtual endstop feedback function increases with increasing steering wheel angle.

[0013] Preferably, the virtual end-stop feedback function is dependent on the steering speed, so that when each virtual end-stop feedback function is activated, the steering speed correction is determined by the current steering speed, thereby ensuring that no torque fluctuations occur due to changes in steering speed while the virtual end-stop feedback function is active.

[0014] Preferably, the virtual steering end stop position is at a smaller steering angle for higher steering speeds compared to lower steering speeds.

[0015] To prevent reaching the mechanical end stop position, the virtual steering end stop position is preferably equal to or less than the mechanical end stop position.

[0016] Preferably, the virtual endstop feedback function is modified by a gain factor for parking maneuvers.

[0017] Preferably, the virtual steering endstop position is the same for the first and second virtual endstop feedback functions for the same mechanical endstop.

[0018] In the case of a wheel block on a curb, it is advantageous if the virtual steering end stop position is close to the actual steering position, i.e. closer than the virtual steering end stop position under normal conditions.

[0019] When 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, the function is symmetrical about the neutral position (i.e., left and right). This compensates for deviations in the mechanical end stops 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 is at (X + d) degrees, the first virtual end stop feedback function generates a counter steering torque from the same absolute steering angle (starting steering position). The end stop contact feel provided by the first virtual end stop feedback function is preferably the same on both sides. When steering to one or the other mechanical end position, different steering speeds may result in different end stop contact feel. For example, the counter torque may saturate at the maximum torque of the feedback actuator before reaching the mechanical end stop.

[0020] There is also provided a steer-by-wire steering system for a road vehicle designed to carry out the above-described method. DETAILED DESCRIPTION OF THE INVENTION

[0021] Hereinafter, embodiments of the present invention will be described with reference to the drawings. [Brief explanation of the drawings]

[0022] [Figure 1]1 is a schematic diagram of a steer-by-wire steering system for a vehicle; [Figure 2] 1 shows a plot of steering wheel torque versus steering wheel angle.

[0023] Figure 1 is a schematic diagram of a steer-by-wire system 1 having a steering shaft 2 connected to a steering wheel 3. There is no mechanical connection between the steering wheel 3 and the wheels 4. A wheel actuator 5 operates a gear rack 6 via a rack-and-pinion gear 7 that is part of a front axle 8. The front axle 8 has two tie rods 9 for the wheels 4, of which only one wheel 4 is shown.

[0024] When the driver operates the steering wheel 3, the steering shaft 2 rotates, and this is detected by a shaft sensor (not shown). The control unit calculates an operation signal for the wheel actuator 5 from the signal detected by the shaft sensor. By operating the gear rack 6 with the operation signal, the front wheel axle 8 moves laterally and the wheels 4 rotate. At the same time, the force introduced from the wheels 4 to the axle 8 is detected by another sensor (not shown), and a feedback signal is calculated to be applied to the steering shaft 2 by the feedback actuator 10. As a result, a steering wheel torque is generated, and the driver can perceive the feedback at the steering wheel 3.

[0025] FIG. 2 shows a total of three hysteresis curves 11, 12, 13 for the steering wheel torque as a function of the steering wheel angle.

[0026] When steering from the activation steering position toward the mechanical end stop, the virtual end stop feedback function is added to the base feedback torque, resulting in the steering wheel torque plotted on the y-axis. The virtual end stop feedback function determines the additional counter torque. The base feedback torque is calculated based on the steering wheel angle, steering wheel velocity, vehicle dynamics (e.g., vehicle longitudinal and lateral movement), rack force, and rack movement. When steering backward from the mechanical end stop, the virtual end stop feedback function remains active until the activation steering position is reached.

[0027] The steering wheel torque has a maximum value, and as a result, the steering wheel angle has a maximum value determined by the mechanical end stop position. The sequence of counter torque as a function of steering wheel angle is preferably adjusted by a lookup table. The steering position breakpoint (for a given steering wheel position) and counter torque value are defined as a value pair from the maximum steering wheel torque value to a virtual steering end stop position. The virtual steering end stop position can be defined by the mechanical end stop position or by a lower value closer to the mechanical end stop position. The steering position breakpoint is an adjustable parameter. Points between the two breakpoints are interpolated.

[0028] The course of steering wheel torque as a function of steering wheel angle is different when steering toward a virtual steering end stop position than when steering backward. Two independent virtual end stop feedback functions are used: one for steering toward the virtual steering end stop position and one for steering backward. This provides a unique torque hysteresis that achieves a natural feel for the end stop. The activation and deactivation steering positions for the virtual end stop feedback function are preferably identical. However, because the feedback characteristics are different, this creates the feeling that the function is deactivating at a different position than where the function is activated.

[0029] The software algorithm calculates the counter torque based on the steering angle of the steering wheel and the steering speed of the steering wheel. The virtual end-stop feedback functions each preferably include a basic function modified by a steering speed dependent parameter.

[0030] FIG. 2 shows two steering wheel torque hysteresis curves 11, 12 for two different steering speeds and a steering wheel torque hysteresis curve 13 for small steering direction changes. The arrows indicate the steering direction. The solid lines 110, 111 represent the basic hysteresis curve 11 for normal steering speeds, preferably up to approximately 50 degrees per second. The upper line 110 applies when steering toward a virtual steering end stop position 112. From the start position, a counter torque calculated based on the first virtual end stop feedback function is added to the basic feedback torque. The steering process begins from a starting steering wheel angle 113. The resulting steering wheel torque initially increases slowly and then rapidly increases to a maximum value 200, which is reached at the virtual steering end stop position 112. The counter torque counteracts the driver's input, resulting in a steering feel comparable to that of a conventional electromechanical steering system with a mechanical end stop. When steering backward from the virtual steering end stop position 112, the lower line 111 and second virtual end stop feedback function are applied up to a non-operating steering position (not shown). The steering wheel torque is initially reduced rapidly and then moderately until steering is stopped (114).

[0031] The second hysteresis 12, shown by the dashed line, represents the course for higher steering wheel steering speeds, preferably above approximately 200 degrees per second. The maximum value of the steering wheel torque 200 is the same as the hysteresis 11 for normal speeds. However, the virtual steering end stop position 120 is at a smaller steering angle compared to the hysteresis 11 for normal speeds. The basic course is modified depending on the steering speed in that the activation and deactivation steering positions of the virtual end stop feedback function also change. The hysteresis shape remains the same. Essentially, the basic hysteresis curve in the figure is shifted to the left for higher steering wheel steering speeds. The activation steering position is at a smaller steering wheel angle compared to the hysteresis 11 for normal speeds (not shown in FIG. 2). The same is true for the deactivation steering position.

[0032] The steering speed based hysteresis curve modification is determined at the activation point, which ensures that no torque fluctuations occur with steering speed changes while the virtual end-stop feedback function is active.

[0033] A third hysteresis 13, also shown in dashed lines, is within the basic course of the first hysteresis 11. It represents small changes in steering direction, such as occur during a parking maneuver.

[0034] The steering direction is changed at 135 and the steering wheel is rotated backward until a second change in steering direction at 130. The steering wheel is then rotated towards a virtual steering end stop position until the steering effort stops at 134.

[0035] The hysteresis curve 13 is based on the basic hysteresis curve 11 and an underlying lookup table. The lookup table can be modified by further adjustable parameters, such as a gain factor, to obtain the hysteresis curve 13. Such a gain factor modifies the basic steer-out and steer-in curves to interconnect them. The basic shape of the third hysteresis 13 is derived from the steer-out function lookup table, but is modified to create more hysteresis (torque difference) between steer-out and steer-in.

[0036] The virtual steering end stop position 112, 120 can be defined anywhere on the steering range. It can be set near or on the actual mechanical end stop, or in the case of a wheel blocking on a curb (curb block), much closer to the actual steering position. It is also possible for a virtual end stop feedback function to be notified when a wheel is blocked on a curb, in order to trigger feedback to the driver.

Claims

1. A method of controlling a steer-by-wire steering system (1) for a road vehicle, the steer-by-wire steering system (1) comprising a steering wheel (3), wheel actuators (5) for actuating road wheels (4), a feedback actuator (10) with mechanical end stops for applying a feedback torque to the steering wheel, and sensors for detecting the steering angle and steering speed of the steering wheel; The method comprises: a) provides a basic feedback torque; b) steering towards a mechanical end stop and the steering wheel angle is equal to or greater than the actuation steering position; i) activating a first virtual end-stop feedback function that determines a counter torque that reacts to a driver's input of the steering wheel, the first virtual end-stop feedback function being dependent on the steering wheel angle and the steering wheel velocity; ii) adding a counter torque to the basic feedback torque to generate a steering wheel torque, the process of the first virtual end-stop feedback function reaching a predetermined maximum steering wheel torque at a virtual steering end-stop position; iii) sending the resulting steering wheel torque (16) to the feedback actuator (10) and controlling the feedback actuator (10) accordingly; c) When steering backward from the mechanical end stop and the steering wheel angle is equal to or greater than the deactivated steering position, i) activating a second virtual end-stop feedback function that determines a counter torque that reacts to a driver's input of the steering wheel, the second virtual end-stop feedback function being dependent on the steering wheel angle and the steering wheel velocity; ii) adding a counter torque to the basic feedback torque to generate a steering wheel torque, the process of the second virtual end-stop feedback function reaching a predetermined maximum steering wheel torque at the virtual steering end-stop position; iii) sending the resulting steering wheel torque (16) to the feedback actuator (10) and controlling the feedback actuator (10) accordingly; A method comprising:

2. The method of claim 1 , wherein the first and second virtual endstop feedback functions are based on lookup tables.

3. 3. The method of claim 1 or claim 2, wherein the counter torque provided by the first virtual endstop feedback function increases as steering wheel angle increases.

4. The method of claim 3 , wherein the virtual end-stop feedback function is dependent on steering velocity, and the steering velocity correction is determined upon activation of the virtual end-stop feedback function.

5. The method of any one of claims 1 to 4, wherein the virtual steering end stop positions are at smaller steering angles for higher steering speeds compared to lower steering speeds.

6. The method of any one of claims 1 to 5, wherein the virtual steering end stop position is less than or equal to the mechanical end stop position.

7. The method of claim 4 , wherein the virtual end-stop feedback function is modified by a gain factor for parking maneuvers.

8. The method of any one of claims 1 to 7, wherein the virtual steering endstop position is the same for the first and second virtual endstop feedback functions.

9. The method according to any one of claims 1 to 8, wherein in case of a wheel block on a curb, the virtual steering end stop position is close to the actual steering position.

10. 10. The method of claim 1, wherein the first virtual endstop feedback function has the same absolute start steering position and the same absolute virtual steering endstop position for the same steering speed when the mechanical endstops are different.

11. A steer-by-wire steering system (1) for a road vehicle designed to carry out the method according to any one of claims 1 to 10.

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