Method for controlling a steer-by-wire steering system for an automobile having limited feedback actuator output torque

Abstract

The present invention relates to a method for controlling a steer-by-wire steering system (1) of a road vehicle, the steer-by-wire steering system (1) comprising a feedback actuator (8) for applying a feedback torque to a steering wheel (3) and wheel actuators (5) for turning steerable wheels (4), the method comprising the steps of: a) calculating a feedback actuator output torque, and b) limiting the feedback actuator output torque by a limiter having a non-linear scaling function when degradation of the steer-by-wire steering system occurs or when the end of life of the steer-by-wire steering system is detected.

Description

[Technical Field]

[0001] The present invention relates to a method for controlling a steer-by-wire steering system of a motor vehicle according to the preamble of claim 1, and to a steer-by-wire steering system designed to carry out this method. [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. The steering action 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] One of the main requirements for a steer-by-wire steering system is that the steering system must be fault-tolerant. This means that if a single failure occurs, the steering system must still operate and provide its primary function. If the system malfunctions, degradation of functionality may occur. Degraded system functionality leads to limited functionality of the steering system. The limited functionality may include limited feedback actuator output torque.

[0004] It is known to linearly limit the feedback actuator output torque of the feedback actuator during degradation, which has little effect on the feedback torque output. It is also known that the gain of the feedback actuator output torque is limited, which can result in undesirable steering behavior. The simple solution of scaling down the overall steering feel to fit the remaining torque range after degradation has drawbacks. Summary of the Invention

[0005] It is therefore an object of the present invention to provide a method for controlling a steering system of a road vehicle that can efficiently and operably influence feedback actuator torque output during impairments.

[0006] This object is achieved by a method for controlling a steering system of a road vehicle having the features of claim 1 and by a steering system of a road vehicle having the features of claim 10.

[0007] Accordingly, there is provided a method for controlling a steer-by-wire steering system of a road vehicle, the steer-by-wire steering system comprising a feedback actuator for applying a feedback torque to a steering wheel and wheel actuators for turning steerable wheels, the method comprising: a) calculating a feedback actuator output torque; b) limiting the feedback actuator output torque by a limiter having a non-linear scaling function when degradation of the steer-by-wire steering system occurs or when the end of life of the steer-by-wire steering system is detected, preferably only if the feedback actuator output torque exceeds a threshold value.

[0008] This restriction allows for an appropriate steering feel and improves the operability of vehicle steering.

[0009] Preferably, the non-linear scaling function depends on the feedback actuator output torque value. The limit of the limiter may increase non-linearly with an increase in the feedback actuator output torque value.

[0010] Preferably, the limited feedback actuator output torque increases monotonically as the feedback actuator output torque value increases.

[0011] Preferably, below the threshold, the limit is not effective.

[0012] Preferably, above the second threshold, the limited feedback actuator output torque is a predetermined maximum value.

[0013] Furthermore, under step b of the method, the speed of the road vehicle is preferably reduced.

[0014] Preferably, the method includes, under step b of the method, a step of adjusting the non-linear scaling function over time in the event of a one-side failure of the feedback actuator or adjusting the non-linear scaling function according to the degree of degradation in order to further reduce the feedback actuator output torque.

[0015] There is also provided a steer-by-wire steering system for a road vehicle designed to carry out the above-described method.

[0016] The method can be implemented in steering systems with feedback actuators on only one side (redundant and non-redundant), or in steering systems with two sides where the method is implemented on one or both sides, or in steering systems with special modes of operation (e.g., specific / unusual steering feedback). DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a schematic diagram of a steer-by-wire steering system. [Figure 2] FIG. 2 is a schematic diagram of feedback actuator output torque plotted against steering wheel angle with a normal output curve and a limited output curve.

[0019] Figure 1 is a schematic diagram of a steer-by-wire steering 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.

[0020] When the driver operates the steering wheel 3, the steering shaft 2 rotates and 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 wheels 4 rotate. At the same time, the force applied from the wheels 4 to the gear rack 6 is detected by another sensor (not shown), and a feedback signal is calculated. The feedback signal is applied to the steering shaft 2 by a steering wheel actuator 8 (also called a feedback actuator), allowing the driver to perceive the feedback at the steering wheel 3. The feedback actuator has an ECU (electronic control unit) for controlling the feedback torque. The ECU determines the feedback actuator output torque based on the requested feedback torque amount and controls the feedback torque output of the feedback actuator based on the feedback actuator output torque. The feedback actuator output torque is calculated using vehicle- and steering system-related signals.

[0021] The relationship between feedback actuator output torque and steering wheel angle is shown in Figure 2. The x-axis represents steering wheel angle. Feedback actuator output torque is represented on the y-axis. The normalized output curve 9, represented by the solid line, rises sharply from the coordinate origin, then flattens out and becomes nearly horizontal before rising sharply again in a sort of hyperbolic progression. The shape is determined by several factors (e.g., vehicle speed) and can vary widely.

[0022] In the event of malfunction of the steer-by-wire steering system causing degradation, particularly degradation of the feedback actuator and / or wheel actuator, the feedback actuator output torque is limited by a limiter including a nonlinear scaling function.

[0023] The feedback actuator output torque is multiplied by a nonlinear scaling function that depends on the feedback actuator output torque value. For small feedback actuator output torque values, the feedback actuator output torque is not limited. Above a threshold, limitation occurs in such a way that the feedback actuator output torque value is reduced. For medium values, the effect of the nonlinear scaling function is medium. For high values, the reduction is large and the feedback actuator output torque is limited by a predetermined maximum value. In other words, the limitation increases nonlinearly with increasing feedback actuator output torque value. The limited feedback actuator output torque can still be described by a monotonic function.

[0024] The dashed line represents the limited power curve 10. From the coordinate origin, the limited power curve rises sharply but is flatter than the normal power curve. Thereafter, the limited power curve also becomes approximately horizontal and is below the normal power curve. In the region of the hyperbolic course of the normal power curve, the limited power curve rises slightly until it reaches a maximum value, which corresponds to approximately half the value of the feedback actuator output torque of the normal power curve. Generally speaking, the dashed line does not differ significantly from the solid line at low feedback actuator output torques, but the higher the feedback actuator output torque, the greater the deviation (intervention of the nonlinear scaling function, applied scaling).

[0025] This method is advantageous for both single-sided feedback actuator and redundant feedback actuator architectures.

[0026] When degradation occurs and the limiter is activated using a non-linear scaling function, the vehicle speed should preferably be reduced so that the vehicle can be steered as intended by the driver.

[0027] This method can also be applied at the end of the life of a steer-by-wire steering system. Actuator degradation can occur at any time, for example, due to low battery voltage or overheating of electrical components. The method described above provides a satisfactory steering feel even when only a small fraction of the nominal torque is available. This solution is independent of the underlying cause of the degradation. The limitation may increase over time.

[0028] The limited feedback actuator output torque should be dependent on vehicle speed and steering speed (e.g., damping and dynamic wheel actuator derating feedback functions can be implemented).

[0029] During degradation, the described method keeps power consumption lower, which helps maintain the steering system's ability to generate torque. However, it cannot prevent complete torque loss over time. The nonlinear scaling function can be changed over time in the case of a single-sided failure of the feedback actuator. In the case of degradation due to a malfunction, the nonlinear scaling function can be changed depending on the degree of feedback actuator degradation. This opens up the possibility of a more gradual loss of feedback actuator torque, i.e., it is safer because the driver has time to get used to the reduced torque feedback and eventually the complete loss of torque feedback.

Claims

1. A method of controlling a steer-by-wire steering system (1) of a road vehicle, the steer-by-wire steering system (1) comprising a feedback actuator (8) for applying a feedback torque to a steering wheel (3) and wheel actuators (5) for turning steerable wheels (4), said method comprising: a) calculating a feedback actuator output torque; b) limiting the feedback actuator output torque with a limiter having a non-linear scaling function when degradation of the steer-by-wire steering system occurs or when end of life of the steer-by-wire steering system is detected.

2. The method of claim 1 , wherein in step b, the feedback actuator output torque is limited only if the feedback actuator output torque exceeds a first threshold value.

3. The method of claim 1 or 2, wherein the non-linear scaling function depends on the feedback actuator output torque value.

4. The method of claim 3 , wherein the limit of the limiter increases non-linearly with increasing feedback actuator output torque.

5. 5. The method of claim 3 or 4, wherein the limited feedback actuator output torque increases monotonically with increasing steering wheel angle.

6. The method of any one of claims 1 to 5, wherein below a first threshold the restriction has no effect.

7. A method according to any one of claims 1 to 6, wherein above the second threshold the limited feedback actuator output torque is a predetermined maximum value.

8. The method according to any one of claims 1 to 7, wherein the method comprises under method step b) a step of reducing the road vehicle speed:

9. The method according to any one of claims 1 to 7, wherein the method comprises under method step b: adjusting the nonlinear scaling function over time upon failure of one side of the feedback actuator or adjusting the nonlinear scaling function according to the degree of degradation in order to further reduce the feedback actuator output torque;

10. 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 9.

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