Method of controlling steer-by-wire steering system of motor vehicle with limited feedback actuator output torque

Abstract

The present disclosure relates to a method of 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 a feedback torque to a steering wheel (3) and a road wheel actuator (5) for turning steerable road wheels (4), the method comprises 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 a performance degradation occurs in the steer-by-wire steering system or an end of life of the steer-by-wire steering system is detected.

Description

Technical Field

[0001] The present disclosure relates to a method of 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 the method. Background Art

[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 coupling between the steering wheel and the steering gear. The steering movement is achieved by a steering actuator having an electric motor. The steering actuator operates in response to the sensed values ​​of various steering parameters, such as steering wheel angle and vehicle speed. The sensed values ​​are electronically transmitted from the sensor to the steering actuator, whereby the electric motor drives the rack and guides the steering wheel in the desired direction.

[0003] One of the main requirements of a steer-by-wire steering system is that the steering system must be fault tolerant, meaning that if a single fault occurs, the steering system must continue to operate and provide the primary functionality. When a system fault occurs, degradation may occur. Degradation of the system can result in limited functionality of the steering system. Limited functionality can include limited feedback actuator output torque.

[0004] It is known to linearly limit the feedback actuator output torque of the feedback actuator during performance reduction, which has little effect on the output of the feedback torque. It is also known that the gain of the feedback actuator output torque is limited, which can potentially lead to undesirable steering behavior. A simple solution is to scale down the overall steering feel to accommodate the remaining torque range after performance reduction, but this solution has its disadvantages. Summary of the invention

[0005] It is therefore an object of the present disclosure to provide a method of controlling a steering system of a road vehicle which allows for efficient and drivable influencing of the feedback actuator torque output during degraded performance.

[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 for a road vehicle having the features of claim 10 .

[0007] Therefore, a method of controlling a steer-by-wire steering system of a road vehicle is provided, wherein the steer-by-wire steering system comprises a feedback actuator for applying a feedback torque to a steering wheel and a road wheel actuator for turning a steerable road wheel. The method comprises the following steps: a. Calculate the feedback actuator output torque, and b. When the steer-by-wire steering system performance degradation occurs or the end of life of the steer-by-wire steering system is detected, the feedback actuator output torque is preferably reduced by a limiter with a non-linear scaling function only when the feedback actuator output torque is above a threshold.

[0008] This restriction creates adequate steering feel and allows for improved vehicle steering maneuverability.

[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 increasing feedback actuator output torque value.

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

[0011] Preferably, below the threshold, no restrictions are in effect.

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

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

[0014] Preferably, the method comprises the following steps under method step b: in case of a single-sided fault of the feedback actuator, adjusting the nonlinear scaling function over time, or adjusting the nonlinear scaling function according to the degree of performance degradation, to further reduce the feedback actuator output torque.

[0015] Furthermore, a steer-by-wire steering system for a road vehicle is provided, which is designed to carry out the above-mentioned method.

[0016] The method can be implemented in a steering system with a feedback actuator having only one side (redundant and non-redundant) or having both sides, wherein the method is implemented on one side or both sides; or in a steering system with special operating modes (for redundancy, such as specific / non-usual steering feedback). BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Preferred embodiments of the present disclosure will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a steer-by-wire steering system, and Figure 2 Schematic diagram of the relationship between the output torque of the feedback actuator and the steering wheel angle, showing a normal output curve and a limited output curve. DETAILED DESCRIPTION

[0018] Figure 1 is a schematic diagram of a steer-by-wire steering system 1, in which a steering shaft 2 is connected to a steering wheel 3. There is no mechanical connection between the steering wheel 3 and the road wheel 4. The road wheel actuator 5 operates a gear rack 6 via a rack-and-pinion gear 7.

[0019] When the driver operates the steering wheel 3, the steering shaft 2 rotates, wherein the rotation of the steering shaft 2 can be detected by a shaft sensor, wherein the shaft sensor is not shown in the figure. The control unit calculates an operation signal for the road wheel actuator 5 according to the signal detected by the shaft sensor. By operating the gear rack 6 using the operation signal, the road wheel 4 is turned. At the same time, the force introduced into the gear rack 6 from the road wheel 4 is recognized by another sensor not shown in the figure, and a feedback signal is calculated, which is applied to the steering shaft 2 by the steering wheel actuator 8 (also referred to as the feedback actuator) so that the operator can recognize 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 according to the request amount of the 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 signals related to the vehicle and the steering system.

[0020] The relationship between the feedback actuator output torque and the steering wheel angle is as follows: Figure 2 . 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 origin of the coordinate system, then flattens out to an almost horizontal state, and then rises sharply again in a hyperbolical progression. This shape is determined by many factors (such as vehicle speed) and may vary greatly.

[0021] When the steer-by-wire steering system degrades due to a fault in the system or a malfunction of the system, in particular the feedback actuator and / or the road wheel actuator, the feedback actuator output torque is limited by a limiter comprising a non-linear scaling function.

[0022] The feedback actuator output torque is multiplied by a nonlinear scaling function which 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 is applied in the form of a reduction of the feedback actuator output torque value. At medium values, the effect of the nonlinear scaling function is moderate. At high values, the reduction is high and the feedback actuator output torque is limited to a predefined 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.

[0023] The dashed line represents the limited output curve 10. Starting from the origin of the coordinate system, the limited output curve rises sharply, but is flatter than the normal output curve. Thereafter, the limited output curve is also almost horizontal and is located below the normal output curve. In the hyperbolic course region of the normal output curve, the limited output curve rises slightly until it reaches a maximum value, which is approximately half the value of the feedback actuator output torque of the normal output curve. In general, at low feedback actuator output torques, the dashed line is not significantly different from the solid line, but as the feedback actuator output torque increases, the deviation between the two (= the degree of intervention of the nonlinear scaling function, the scaling applied) becomes larger and larger.

[0024] This approach is beneficial for both single-sided feedback actuator and redundant feedback actuator architectures.

[0025] When performance degradation occurs and the limiter with a non-linear scaling function is activated, the vehicle speed should preferably also be reduced to enable the vehicle to steer as intended by the driver.

[0026] The 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 due to, for example, battery voltage drop or overheating of electronic components. The method described above still provides a satisfactory steering feel even if only a portion of the nominal torque is available. The solution is independent of the root cause of the degradation. The limitations may increase over time.

[0027] The limited feedback actuator output torque should depend on the vehicle speed and steering speed (eg damping and performance reduction feedback functions of dynamic road wheel actuators may be implemented).

[0028] During periods of reduced performance, the described approach results in lower power consumption, which helps maintain the torque generating capability of the steering system. However, over time, a complete loss of torque cannot be avoided. In the event of a unilateral failure of the feedback actuator, the nonlinear scaling function can be varied over time. In the event of reduced performance due to a failure, the nonlinear scaling function can be varied depending on the degree of degradation of the performance of the feedback actuator. This opens up the possibility for a more gradual loss of feedback actuator torque, which is safer because the driver has time to get used to the reduced torque feedback and eventually to the complete loss of torque feedback.

Claims

1. A method of controlling a steer-by-wire steering system (1) of a road vehicle, in, The steer-by-wire steering system (1) comprises a feedback actuator (8) and a road wheel actuator (5), wherein the feedback actuator (8) is used to apply feedback torque to a steering wheel (3), and the road wheel actuator (5) is used to rotate a steerable road wheel (4), and is characterized in that the method comprises the following steps: a. Calculate the feedback actuator output torque, and b. In the event of degradation of the steer-by-wire steering system or detection of the end of life of the steer-by-wire steering system, limiting the feedback actuator output torque by a limiter having a nonlinear scaling function.

2. The method according to claim 1, It is characterized in that In step b, the feedback actuator output torque is limited only when the feedback actuator output torque is higher than a first threshold.

3. The method according to claim 1 or 2, It is characterized in that The non-linear scaling function is dependent on the feedback actuator output torque value.

4. The method according to claim 3, It is characterized in that The limit of the limiter increases non-linearly with increasing output torque of the feedback actuator.

5. The method according to claim 3 or 4, It is characterized in that The limited feedback actuator output torque increases monotonically with increasing steering wheel angle.

6. The method according to any one of the preceding claims, It is characterized in that Below the first threshold, no restriction is in effect.

7. The method according to any one of the preceding claims, It is characterized in that Above a second threshold, the limited feedback actuator output torque is a predefined maximum value.

8. The method according to any one of the preceding claims, It is characterized in that The method comprises the following steps under method step b: - Reduce the speed of vehicles on said road.

9. The method according to any one of the preceding claims, It is characterized in that The method comprises the following steps under method step b: - In the event of a single-sided fault of the feedback actuator, the nonlinear scaling function is adjusted over time, or according to the degree of performance degradation, to further reduce the feedback actuator output torque.

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

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