The present invention relates to a method to control a steer-by-wire steering system of a motor vehicle according to the preamble of claim 1 and a steer- by-wire steering system designed to carry out the method.

In a steer-by-wire steering system, the vehicle's steering wheel is disengaged from the steering mechanism. In such a steering system, there is no mechanical coupling between the steering wheel and the steering gear. Steering movement is achieved by a steering actuator 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 etc. The detected values are communicated electronically to the steering actuator from sensors, whereby the electric motor drives the rack and orients the steerable wheels in the desired direction.

One of the main requirements in steer-by-wire steering systems is that the steering system has to be fault tolerant, which means that if one failure occurs the steering system has to operate further, providing the main functionality. If a malfunctioning in the system occurs, degradation might take place. Degradation of the system leads to limited function of the steering system. A limited function might include limited feedback actuator output torque. It is known to linearly limit the feedback actuator output torque of the feedback actuator during degradation, which hardly influences 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 of scaling down the overall steering feel to fit the remaining torque range after degradation has its disadvantages.

Therefore, it is an object of the present invention to provide a method to control a steering system of a road vehicle that allows the feedback actuator torque output to be efficiently and drivably influenced during degradation.

This object is achieved by a method to control 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.

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 feedback actuator for applying a feedback torque to a steering wheel and a road wheel actuator for turning of steerable road wheels. The method includes the following steps: a. Calculating a feedback actuator output torque, and b. When degradation of the steer-by-wire-steering system occurs or the end of the life of the steer-by-wire-steering system is detected, the feedback actuator output torque is reduced by a limiter with a non-linear scaling function, preferably only if the feedback actuator output torque is above a threshold.

The limitation creates an adequate steering feel and allows to improve the operability of the vehicle steering.

Preferably, the non-linear scaling function depends on the feedback actuator output torque value. A limitation of the limiter can increase non-linearly with increasing feedback actuator output torque value. Preferably, the limited feedback actuator output torque increases monotonously with increasing feedback actuator output torque value.

It is preferred, that below a threshold value, no limitation takes effect.

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

Further it is preferred, that the road vehicle speed is reduced under method step b.

It is preferred, if the method includes under method step b the following step: Adjusting the nonlinear scaling function with time in case of one side failure of feedback actuator or adjusting the nonlinear scaling function according to the degree of degradation to further reduce the feedback actuator output torque.

Furthermore, a steer-by-wire steering system for a road vehicle designed to carry out the method described above is provided.

The method can be implemented in steering systems with a feedback actuator with only one side (redundant and non-redundant) or with two sides, where the method is implemented on one or both sides, or in steering systems with a special operation mode (for redundancy e.g. specific/not usual steering feedback).

A preferred embodiment of the present invention will now be described with reference to the drawings, in which:

Figure 1 : is a schematic view of a steer-by-wire steering system, and

Figure 2: is a schematic diagram of a feedback actuator output torque plotted against steering wheel angle with a normal output curve and a limited output curve.

Figure 1 is a schematic drawing of a steer-by-wire steering system 1 with a steering shaft 2 connected to a steering wheel 3. There is no mechanical connection between the steering wheel 3 and the road wheels 4. A road wheel actuator 5 operates a gear rack 6 via a rack-and-pinion gear 7. When a driver operates the steering wheel 3, steering shaft 2 is rotated, which is detected by a shaft sensor, which is not shown in the drawings. A control unit 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 road wheels 4 are turned. At the same time, forces introduced in the gear rack 6 from the road wheels 4 are recognized by another sensor not shown in the drawings, and a feedback signal is calculated, which is applied to the steering shaft 2 by a steering wheel actuator 8, also called 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 the control of the feedback torque. The ECU determines a feedback actuator output torque on a 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.

The relationship between the feedback actuator output torque and the steering wheel angle is shown in 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 coordinate origin and then flattens out, becoming nearly horizontal before rising sharply again in a kind of hyperbolical progression. The shape is determined by a number of factors (e.g. the vehicle speed) and can differ a lot.

In case of degradation due to malfunctioning in or of the system of the steer- by-wire steering system, especially degradation of the feedback actuator and/or the road wheel actuator the feedback actuator output torque is limited by a limiter which includes a non-linear scaling function.

The feedback actuator output torque is multiplied by the non-linear 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 value, limitation takes place in the form of a reduction of the feedback actuator output torque value. At medium values, the influence of the non-linear scaling function is moderate. At high values, the reduction is high and the feedback actuator output torque is limited by a predefined maximum value. In other words, the limitation increases non-linearly with increasing feedback actuator output torque value. The limited feedback actuator output torque can still be described by a monotonic function.

The dashed line represents the limited output curve 10. From the coordinate origin, the limited output curve rises sharply, but flatter than the normal output curve. After that, the limited output curve is also almost horizontal, below the normal output curve. In the area of the hyperbolic course of the normal output curve, the limited output curve rises slightly until a maximum value is reached, which corresponds to about half the value of 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 higher the feedback actuator output torque rises, the higher the deviation (= the intervention of the non-linear scaling function, the applied scaling) will be.

The method is advantageous for both single side feedback actuators and redundant feedback actuator architectures.

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

The method can also be applied towards the end of the life of the steer-by- wire steering system. Degradation of the actuator can happen at any time due to for instance drop of battery voltage or overheating of electrical components. The above described method provides a satisfactory steering feel, even if just a fraction of the nominal torque is available. The solution is independent of the root cause of the degradation. It is possible that the limitation will be increased over time. The limited feedback actuator output torque should be dependent on the vehicle speed and steering speed (e.g. damping and dynamic road wheel actuator degradation feedback function can be implemented).

During degradation, the method described results in lower power consumption, which helps maintain the steering system's ability to generate torque. However, over time, complete torque loss cannot be prevented. The nonlinear scaling function can be changed with time in case of one side failure of feedback actuator. In the event of degradation due to a malfunction, the nonlinear scaling function can be changed according to the degree of degradation of the feedback actuator. This opens up the possibility of 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.