STEER-BY-WIRE STEERING SYSTEM WITH ARTIFICIAL POSITION HYSTERESIS FOR STEERING WHEEL ANGLE CHANGE

Description

[0001] The present invention relates to a method to control 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 according to the preamble of claim 15.

[0002] In an electromechanical power steering mechanism a steering shaft is connected to a steering wheel for operation by the driver. The steering shaft is coupled to a steering rack via a gear pinion. Steering rack rods are connected to the steering rack and to steered wheels of the motor vehicle. A rotation of the steering shaft causes an axial displacement of the steering rack by means of the gear pinion which is connected to the steering shaft in a torque-proof manner. Assist force is applied to a steering mechanism by driving an electric motor. Sensors disposed within the vehicle are used to detect the motion and torque of the steering wheel. A computer module calculates an amount of assistive torque to apply by the electric motor.

[0003] The mechanical connection between the steering wheel and the pinion has compliance, which results in angular difference between the steering means and the pinion, if torque is applied on the steering column. The compliance is noticed as a hysteresis component in rotational movement of the steering wheel with respect to the pinion, which is dependent upon torque applied on the steering column.

[0004] In steer-by-wire steering systems there is no mechanical connection between the steering wheel and the steering rack and the steerable wheels and steering movement is achieved by a position controlled road wheel actuator, which reference position is based on the steering wheel angle. If the connection between the road wheel actuator and the steering wheel is simulated and handled to be "infinite stiff", the driver experiences a notable difference in driving feel compared to electric power steering systems, especially during high dynamic steering maneuvers and when the steering wheel is moved quickly sinusoidal around a non-center steering wheel position.

[0005] US 2019 / 0367074 A1 discloses a method to deal with steering wheel stop angles. Once the steering wheel stop angle associated with a maximum steering angle is reached the steering wheel is allowed to rotate beyond the steering wheel stop angle, but the steerable wheels remain at their maximum steering angle. When the driver starts to turn the steering wheel back in the reverse direction, the controller immediately activates the steering actuator to begin reducing the steering angle of the steerable wheels in accordance with a new transmission ration. The transmission ratio may be a constant or variable.

[0006] It is an object of the present invention to provide a method to control a steer-by-wire steering system of a road vehicle and a steer-by-wire steering system itself, which creates a steering feel like electric assisted power steering systems.

[0007] This object is achieved by a method to control a steer-by-wire system of a motor vehicle having the features of claim 1 and a steer-by-wire steering system for a motor vehicle having the features of claim 15.

[0008] Accordingly, a method to control a steer-by-wire steering system of a road vehicle is provided, wherein the steer-by-wire steering system comprises a steering wheel coupled to a steering shaft, a road wheel actuator for turning the steerable road wheels, a shaft sensor, which detects a rotation of the steering shaft and a control unit, which calculates an operation signal for the road wheel actuator from the signal detected by the shaft sensor, and wherein the method includes the following steps: Determining the real steering wheel angle by the shaft sensor; Modifying the real steering wheel angle by applying an artificial hysteresis, which creates an offset between the real steering wheel angle and the modified steering wheel angle to mimic the hysteresis component of electromechanical power steering mechanisms in rotational movement of the steering wheel in respect to the pinion, which depends on the torque applied to the steering column; and Calculating a motor torque request for the road wheel actuator based on the modified steering wheel angle.

[0009] The resulting steering feel is similar to electromechanical power steering systems and thus comfortable for the driver.

[0010] Preferably, the artificial hysteresis is only able to result in equal or higher modified steering wheel angle compared to the real steering wheel angle. It is advantageous, if the modified steering wheel angle has always the same direction (sign) as the real steering wheel angle. In other words, the sign between the change in value of the modified steering wheel angle and the real steering wheel angle are not opposite.

[0011] In a preferred embodiment the offset is limited by a maximum offset, which is defined by an artificial torsion rate parameter, which describes linear connection between the real steering wheel angle and the offset. The maximum offset can be dependent on a vehicle speed dependent parameter.

[0012] It is preferred, that the offset is zero for real steering wheel angles from 0° to a predefined end value v1 in absolute terms.

[0013] In particular, the end value v1 lies in a range between 0° and 15° absolute real steering wheel angle.

[0014] Preferably, the maximum offset is increasing linearly from the end value up to the maximum steering wheel angle.

[0015] It is advantageous, if the offset is decreasing from the actual offset to zero offset during steering to an endlock position.

[0016] The decrease can be tunable.

[0017] It is further preferred, that after reaching the zero offset, it is used as the actual offset during turning of the steering wheel to a larger absolute real steering wheel angle.

[0018] Preferably, during steering back to center, the offset increases in absolute terms up to the actual maximum offset.

[0019] The characteristic of the increase can be tunable.

[0020] It is preferred, that once the maximum offset is reached, when turning back, the actual maximum offset is used as the actual offset for the rest of the steering operation back to the end value.

[0021] Further, a steer-by-wire steering system for a road vehicle comprising a road wheel actuator and a control unit is provided, which calculates an operation signal for the road wheel actuator, wherein the control unit is designed to carry out the above-described method. Preferably, the road wheel actuator operates a gear rack via a rack-and-pinion gear, which is part of a front wheel axle. It is preferred, that the control unit includes a position controller, which calculates a motor torque request based on the actual or an estimated position of the rack and a requested rack position, wherein the requested rack position is dependent on the modified steering wheel angle.

[0022] A preferred embodiment of the present invention will be described with reference to the drawings. Figure 1:is a schematic illustration of a steer-by-wire steering system of a motor vehicle; and Figure 2:shows a diagram with the influence of an artificial position hysteresis on the steering wheel angle.

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

[0024] When a driver operates the steering means 3, steering shaft 2 is rotated, which is detected by a shaft sensor, which is not shown in the drawings. When the vehicle is switched on, a control unit 10 calculates an operation signal for the road wheel actuator 5 from the signal detected by the shaft sensor. By operating gear rack 6 with the operation signal, the front wheel axle 8 is moved sideways and the road wheels 4 are turned. A feedback torque on the steering means 3 is calculated mainly based on the position of the steering means 3 (steering shaft) and modified according to the operating conditions of the vehicle and the road wheel actuator 5. The feedback torque is applied to the steering shaft 2 by a feedback actuator 11, so that the operator can recognize the feedback in the steering means 3.

[0025] The control unit 10 includes a position controller, which calculates a motor torque request based on the actual (measured) position of the rack 6 and a requested rack position. The road wheel actuator 5 thus follows a reference position signal. The requested rack position is dependent on the steering wheel angle α. To mimic electromechanical power steering mechanisms the steering wheel angle α is modified by applying an artificial hysteresis, which creates an offset in the steering wheel angle and the requested rack position, respectively. This way the road wheel actuator is falling behind in movement compared to the steering wheel movement, which is similar to the behavior in electromechanical power steering mechanisms. The actual value of the offset, a build-up and a fade-away of the offset can be adapted and are tunable.

[0026] The offset is created between the real steering wheel angle α and a modified steering wheel angle α, which is used instead of the real steering wheel angle α in the position controller for calculating the requested rack position.

[0027] Figure 2 shows the absolute modified steering wheel angle α mod , including the offset, plotted against the absolute real steering wheel angle α. The steering wheel is steered from 0 to 360 degree, than a repeated maneuver is coming, every iteration with a different build-up and fade-away rates: steering back from 360 degree to 100 degree and then from 100 degree to 360 degree (just to show the effect of the different shape values during steering in and steering out). After some repetition the steering wheel is steered back from 360 degree to 0 degree. The dashed lines are used for reference only. The arrows represent the steering direction. They mark the relevant dependencies.

[0028] The offset is a hysteresis, which is only able to result equal or higher modified steering wheel angle a than the real steering wheel angle α in absolute terms and the sign of the modified steering wheel angle is equal with the sign of the real steering wheel angle. The offset is not able to result in opposite sign between the change in value of the real steering wheel angle α and the modified steering wheel angle α mod .

[0029] During steering from the straight-ahead, center position of the steering wheel (absolute real steering wheel angle is zero) to the endlock (360°) the offset is zero, which means the steering wheel angle is not modified.

[0030] During steering back from the endlock position to the center position the offset increases in absolute terms up to the actual maximum offset.

[0031] The characteristic of the offset build-up is tunable. The build-up is shown for four different examples. In these cases the maximum offset is reached at real steering wheel angles α between 200° and 260°. Once the maximum offset is reached, the actual maximum offset is used as the actual offset for the rest of the steering operation back to v1.

[0032] A maximum offset is defined by an artificial torsion rate parameter, which describes linear connection between the real steering wheel angle α and the offset starting from a predefined end value v1. The range between the straight-ahead, center position of the steering wheel (absolute real steering wheel angle is zero) and the end value v1 is called "dead zone". Within the dead zone the maximum offset is zero and thus the modified steering wheel angle α mod is equal to the real steering wheel angle α. From the starting value v1 on the maximum offset is increasing linearly based on the artificial torsion rate parameter. The maximum offset can be influenced by a vehicle speed dependent parameter.

[0033] If during steering back from the endlock position to the center position the steering direction is changed (to the endlock position instead of to the center position) the offset decreases from the maximum offset to zero offset, which is reached in the example of figure 2 at around 300° real steering wheel angle α. The characteristic of the offset fade-away is tunable, independent of the offset build-up. The fade-away is shown for four different examples. Once the zero offset is reached, it is used as the actual offset during turning of the steering wheel to a larger absolute angle. In other words, the modified steering wheel angle α mod equals the real steering wheel angle α.

Claims

1. Method to control a steer-by-wire steering system (1) of a road vehicle, wherein the steer-by-wire steering system (1) comprises a steering wheel (3) coupled to a steering shaft (2), a road wheel actuator (5) for turning steerable road wheels (4), a shaft sensor, which detects a rotation of the steering shaft (2) and a control unit (10), which calculates an operation signal for the road wheel actuator (5) from the signal detected by the shaft sensor, characterized in that the method includes the following steps: - Determining the real steering wheel angle (α) by the shaft sensor; - Modifying the real steering wheel angle (α) by applying an artificial hysteresis, creating a modified steering wheel angle (αmod), offset from said real steering wheel angle in order to mimic the hysteresis component of electromechanical power steering mechanisms in rotational movement of the steering wheel with respect to the pinion, said hysteresis component depending on the torque applied to the steering column; and - Calculating a motor torque request for the road wheel actuator (5) based on the modified steering wheel angle (αmod).

2. Method according to claim 1, characterized in that the artificial hysteresis is only able to result in equal or higher modified steering wheel angle (αmod ) than the real steering wheel angle (α) in absolute terms.

3. Method according to claim 1 or claim 2, characterized in that the sign between the change in value of the modified steering wheel angle (αmod) and the real steering wheel angle (α) are not opposite.

4. Method according to one of the preceding claims, characterized in that the offset is limited by a maximum offset, which is defined by an artificial torsion rate parameter, which describes linear connection between the real steering wheel angle (α) and the offset.

5. Method according to claim 4, characterized in that the maximum offset is dependent on a vehicle speed dependent parameter.

6. Method according to one of the preceding claims, characterized in that the offset is zero for real steering wheel angles (α) from 0° to a predefined end value (v1) in absolute terms.

7. Method according to claim 6, characterized in that the starting value (v1) lies in a range between 0° and 15° absolute real steering wheel angle (α).

8. Method according to claim 6 or claim 7, characterized in that the maximum offset is increasing linearly from the end value (v1) up to the maximum steering wheel angle.

9. Method according to claim 8, characterized in that the offset is decreasing from the actual offset to zero offset during steering to an endlock position.

10. Method according to claim 9, characterized in that the decrease is tunable.

11. Method according to claim 8 or claim 9, characterized in that after reaching the zero offset, it is used as the actual offset during turning of the steering wheel to a larger absolute real steering wheel angle (α).

12. Method according to one of the preceding claims 4 to 11, characterized in that during steering back to center, the offset increases in absolute terms up to the actual maximum offset.

13. Method according to claim 12, characterized in that the characteristic of the increase is tunable.

14. Method according to claim 12 or claim 13, characterized in that once the maximum offset is reached, when turning back, the actual maximum offset is used as the actual offset for the rest of the steering operation back to the end value (v1).

15. Steer-by-wire steering system (1) for a road vehicle comprising a road wheel actuator (5) and a control unit (10), which calculates an operation signal for the road wheel actuator (5), wherein the control unit (10) is designed to carry out the method according to claims 1 to 14.

16. Steer-by-wire steering system according to claim 15, characterized in that the road wheel actuator (5) operates a gear rack (6) via a rack-and-pinion gear (7), which is part of a front wheel axle (8).

17. Steer-by-wire steering system according to claim 15 or claim 16, characterized in that the control unit (10) includes a position controller, which calculates a motor torque request based on the actual position of the rack (6) and a requested rack position, wherein the requested rack position is dependent on the modified steering wheel angle (αmod).