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

Abstract

A method to control a steer-by-wire steering system of a road vehicle including a steering wheel coupled to a steering shaft, a road wheel actuator to turn steerable road wheels, a shaft sensor to detect a rotation of the steering shaft and a controller configured or programmed to generate an operation signal for the road wheel actuator from a signal detected by the shaft sensor, includes determining a real steering wheel angle by the shaft sensor, modifying the real steering wheel angle by applying an artificial hysteresis to create an offset between the real steering wheel angle and a modified steering wheel angle to mimic a behavior of an electromechanical power steering mechanism, and generating the motor torque request for the road wheel actuator based on the modified steering wheel angle.

Claims

1. A method to control a steer-by-wire steering system of a road vehicle including a steering wheel coupled to a steering shaft, a road wheel actuator to turn steerable road wheels, a shaft sensor to detect a rotation of the steering shaft and a controller configured or programmed to generate an operation signal for the road wheel actuator from a signal detected by the shaft sensor, the method comprising: determining a real steering wheel angle (α) by the shaft sensor; modifying the real steering wheel angle (α) by applying an artificial hysteresis to create an offset between the real steering wheel angle (α) and a modified steering wheel angle (α.sub.mod) to mimic a behavior of an electromechanical power steering mechanism; and generating a motor torque request for the road wheel actuator based on the modified steering wheel angle (α.sub.mod).

2. The method according to claim 1, wherein the artificial hysteresis is only able to result in an equal or higher modified steering wheel angle (α.sub.mod) than the real steering wheel angle (α) in absolute terms.

3. The method according to claim 1, wherein a sign between a change in value of the modified steering wheel angle (α.sub.mod) and the real steering wheel angle (α) are not opposite.

4. The method according to claim 1, wherein the offset is limited by a maximum offset, which is defined by an artificial torsion rate parameter, which describes a linear connection between the real steering wheel angle (α) and the offset.

5. The method according to claim 4, wherein the maximum offset is dependent on a vehicle speed dependent parameter.

6. The method according to claim 1, wherein the offset is zero when the real steering wheel angle (α) is from 0° to a predefined end value (v1) in absolute terms.

7. The method according to claim 6, wherein the predefined end value (v1) is in a range between 0° and about 15° absolute real steering wheel angle.

8. The method according to claim 6, wherein a maximum offset increases linearly from the predefined end value (v1) up to the maximum steering wheel angle.

9. The method according to claim 8, wherein the offset decreases from the actual offset to zero offset during steering to an endlock position.

10. The method according to claim 9, wherein an amount that the offset decreases is tunable.

11. The method according to claim 8, wherein 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. The method according to claim 4, wherein during steering back to center, the offset increases in absolute terms up to an actual maximum offset.

13. The method according to claim 12, wherein an amount that the offset increases is tunable.

14. The method according to claim 12, wherein once the maximum offset is reached, when turning back, the actual maximum offset is used as the actual offset for a remainder of the steering operation back to the predefined end value (v1).

15. A Steer-by-wire steering system for a road vehicle comprising a road wheel actuator and a controller configured or programmed to generate an operation signal for the road wheel actuator, wherein the controller is configured or programmed to carry out the method according to claim 1.

16. The steer-by-wire steering system according to claim 15, wherein the road wheel actuator operates a gear rack via a rack-and-pinion gear, which is part of a front wheel axle.

17. The steer-by-wire steering system according to claim 15, wherein the controller includes a position controller configured or programmed to generate a motor torque request based on an actual position of the rack and a requested rack position, wherein the requested rack position is dependent on the modified steering wheel angle (α.sub.mod).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 is a schematic illustration of a steer-by-wire steering system of a motor vehicle.

[0023] FIG. 2 shows a diagram with the influence of an artificial position hysteresis on the steering wheel angle.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] FIG. 1 is a schematic drawing of a steer-by-wire system 1 with a steering shaft 2 connected to steering device 3. There is no mechanical connection between the steering device 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 includes two tie rods 9 for road wheels 4, of which only one road wheel 4 is shown.

[0025] When a driver operates the steering device 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 controller 10 generates 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 device 3 is calculated mainly based on the position of the steering device 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 device 3.

[0026] The controller 10 includes a position controller configured or programmed to generate 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. For the actual value of the offset, a build-up and a fade-away of the offset can be adapted and are tunable.

[0027] 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 to calculate the requested rack position.

[0028] FIG. 2 shows the absolute modified steering wheel angle α.sub.mod, including the offset, plotted against the absolute real steering wheel angle α. The steering wheel is steered from 0 to 360 degrees, than a repeated maneuver is coming, every iteration with a different build-up and fade-away rates: steering back from about 360 degrees to about 100 degrees and then from about 100 degrees to about 360 degrees (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 about 360 degrees to 0 degree. The dashed lines are used for reference only. The arrows represent the steering direction. They mark the relevant dependencies.

[0029] The offset is a hysteresis, which is only able to result equal or higher modified steering wheel angle α 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.sub.mod.

[0030] 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.

[0031] During steering back from the endlock position to the center position the offset increases in absolute terms up to the actual maximum offset.

[0032] 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 about 200° and about 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.

[0033] 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.sub.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.

[0034] 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 FIG. 2 at about 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.sub.mod equals the real steering wheel angle α.

[0035] While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.