Method for controlling a steer-by-wire steering system of an automobile having a 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 as described in the preamble of claim 1, and to a steer-by-wire steering system designed to execute this method.

Background Art

[0002] In a steer-by-wire steering system, the steering wheel (handle) of the vehicle is disengaged from the steering mechanism. In such a steering system, there is no mechanical coupling between the steering wheel and the steering gear. The steering operation is achieved by a steering actuator equipped with an electric motor. The steering actuator operates according to the detected values of various steering parameters such as the steering wheel angle and the vehicle speed. The detected values are electronically transmitted from the sensor to the steering actuator, whereby the electric motor drives the rack and turns the steerable wheel in the desired direction.

[0003] One of the main requirements in 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 continue to operate and provide the main functions. If the system malfunctions, a reduction in functionality may occur. The reduction in the functionality of the system leads to the limitation of the functions of the steering system. The limited functions may include a limited feedback actuator output torque.

[0004] It is known that linearly limiting the feedback actuator output torque of a degraded feedback actuator has little effect on the feedback torque output. It is also known that this can limit the gain of the feedback actuator output torque, potentially leading to undesirable steering behavior. A simple solution of reducing the overall steering feel to fit the remaining torque range after degradation has drawbacks. [Overview of the Initiative]

[0005] Therefore, an object of the present invention is to provide a method for controlling a road vehicle steering system that can efficiently and drivably influence the feedback actuator torque output when the system is malfunctioning.

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

[0007] Therefore, a method for controlling a steer-by-wire steering system of a road vehicle is provided. The steer-by-wire steering system comprises a feedback actuator for applying feedback torque to the steering wheel and a wheel actuator for turning the steerable wheels. This method, a) A step of calculating the output torque of the feedback actuator, b) The step of limiting the feedback actuator output torque by a limiter having a nonlinear scaling function, preferably only when a degradation in the steer-by-wire steering system occurs or the end of the lifespan of the steer-by-wire steering system is detected, and the feedback actuator output torque exceeds a threshold.

[0008] This restriction allows for a proper steering feel and improves the operability of the vehicle's steering.

[0009] Preferably, the nonlinear scaling function depends on the feedback actuator output torque value. The limiter limit may increase nonlinearly with increasing feedback actuator output torque value.

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

[0011] It is preferable that the limit does not become effective when the threshold is not met.

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

[0013] Furthermore, it is preferable that the speed of road vehicles is reduced in step b of this method.

[0014] The present method preferably includes, under step b of the present method, a step of adjusting the nonlinear scaling function over time in the case of a one-side failure of the feedback actuator, or a step of adjusting the nonlinear scaling function according to the degree of degradation in order to further reduce the output torque of the feedback actuator.

[0015] Furthermore, a steer-by-wire steering system for road vehicles designed to perform the methods described above is provided.

[0016] This method can be implemented in a steering system having feedback actuators on only one side (redundant and non-redundant), or in a steering system having two sides on which this method is implemented, either on one or both sides, or in a steering system having a special operating mode (e.g., specific / unusual steering feedback). [Modes for carrying out the invention]

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

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

[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 acts on 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 this rotation is detected by a shaft sensor (not shown). The control unit calculates an operating signal for the wheel actuator 5 from the signal detected by the shaft sensor. By operating the gear rack 6 with the operating signal, the wheel 4 rotates. Simultaneously, the force introduced from the wheel 4 to the gear rack 6 is recognized by another sensor (not shown), and a feedback signal is calculated. This feedback signal is applied to the steering shaft 2 by the steering wheel actuator 8 (also called the feedback actuator), allowing the driver to perceive the feedback in the steering wheel 3. The feedback actuator is equipped with an ECU (electronic control unit) for controlling the feedback torque. The ECU determines the feedback actuator output torque based on the requested amount of feedback torque 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] Figure 2 shows the relationship between the output torque of the feedback actuator and the steering wheel angle. The x-axis represents the steering wheel angle. The feedback actuator output torque is represented on the y-axis. The normal 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 kind of hyperbolic progression. The shape can vary considerably, determined by several factors (e.g., vehicle speed).

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

[0023] The feedback actuator output torque is multiplied by a non-linear 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. When the threshold is exceeded, a limitation occurs in a manner such that the feedback actuator output torque value decreases. For medium values, the influence of the non-linear scaling function is medium. For high values, the decrease is large and the feedback actuator output torque is limited by a predetermined maximum value. In other words, the limitation increases non-linearly with an increase in the 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 output curve 10. Starting from the origin of coordinates, the limited output curve rises steeply but becomes flatter than the normal output curve. Thereafter, the limited output curve also becomes almost horizontal and is below the normal output curve. In the region of the hyperbolic course of the normal output curve, the limited output curve rises slightly until it reaches the maximum value, which corresponds to a value approximately half of the feedback actuator output torque of the normal output curve. Generally speaking, the dashed line does not differ significantly from the solid line at low feedback actuator output torques, but the deviation (intervention of the non-linear scaling function, applied scaling) increases as the feedback actuator output torque increases.

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

[0026] When a degradation occurs and the limiter is activated using a non-linear scaling function, the vehicle speed should be reduced so that preferably 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. The degradation of the actuator can occur at any time, for example due to a drop in battery voltage or overheating of electrical components. The method described above provides a satisfactory steering feel even when only a small part of the nominal torque is available. This solution is independent of the root cause of the degradation. The limits may increase over time.

[0028] The limited feedback actuator output torque should depend on the vehicle speed and the steering speed (for example, an attenuation and dynamic wheel actuator degradation feedback function can be implemented).

[0029] During degradation, the method described keeps power consumption lower, which helps maintain the ability of the steering system to generate torque. However, over time, it is not possible to prevent complete torque loss. The non-linear scaling function can be changed over time in case of a single-sided failure of the feedback actuator. In case of degradation due to malfunction, the non-linear scaling function can be changed according to the degree of degradation of the feedback actuator. This opens the possibility of a more gradual loss of feedback actuator torque. That is, it is safer because the driver has time to get used to the reduced torque feedback and ultimately to the complete loss of torque feedback.

Claims

1. A method for controlling a steer-by-wire steering system (1) of a road vehicle, wherein the steer-by-wire steering system (1) comprises a feedback actuator (8) for applying feedback torque to a steering wheel (3) and a wheel actuator (5) for turning a steerable wheel (4), and the method is: a) A step of calculating the output torque of the feedback actuator, b) A method comprising the step of limiting the output torque of a feedback actuator by a limiter having a nonlinear scaling function when a deterioration occurs in the function of the steer-by-wire steering system or when the end of the life of the steer-by-wire steering system is detected.

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

3. The method according to claim 1 or 2, wherein the nonlinear scaling function depends on the output torque of the feedback actuator.

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

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

6. The method according to claim 2, wherein the restriction does not take effect when the value falls below the first threshold.

7. The method according to claim 2, wherein when a second threshold greater than the first threshold is exceeded, the limited feedback actuator output torque is a predetermined maximum value.

8. The method according to claim 1 or 2, wherein the method includes a step of reducing the speed of road vehicles in step b of the method.

9. A steer-by-wire steering system (1) for a road vehicle designed to perform the method according to claim 1 or 2.

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