The present invention relates to a method to control a steer-by-wire steering system of a road vehicle according to the preamble of claim 1 and to a steer- by-wire steering system for a road vehicle.

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.

Even though the mechanical linkage between the steering wheel and the road wheels has been eliminated, a steer-by-wire steering system is expected to produce the same functions and steering feel as a conventional mechanically linked steering system. The forces generated in moving the road wheels have to be fed back to the steering wheel to provide information for directional control to the driver. The feedback also contributes to a feeling of steering referred to as steering feel. In steer-by-wire steering systems the feedback and steering feel respectively is generated with a feedback actuator connected to the steering wheel.

When the vehicle is turned off, the steering shaft and the steering means are per se free to rotate as they are not connected to any mechanical resistance. However, many operators of vehicles are used to use the steering means as a handle during ingress and egress. From conventional steering columns they are used that the steering means are hard-steering although the vehicle is turned off, as they are mechanically connected to the wheels.

It is an object of the present invention to provide a method by which a steering means of a steer-by-wire steering system of a road vehicle can be used as a handle during ingress and egress.

This object is achieved by a method having the features of claim 1 and by a steer-by-wire steering system for a road vehicle.

Accordingly, a method to control a steer-by-wire steering system for a road vehicle is provided, the steer-by-wire steering system comprising steering means, a road wheel actuator to actuate road wheels, and a feedback actuator for applying a feedback torque to the steering means, wherein the method includes the following steps:

• If an ingress situation is detected, switching on the feedback actuator and providing a resistance torque against rotation of the steering means in an ingress mode;

• If the normal steering system shut down procedure is finished, activating an egress mode of the feedback actuator and providing a resistance torque against rotation of the steering means.

Even though a mechanical resistance is missing in steer-by-wire systems, with this method the steering means, preferably a steering wheel, can be used as a handle when getting in or out of the vehicle.

Preferably, the resistance torque is provided by a position control mode of the feedback actuator, which maintains the initial position of the steering wheel and makes it difficult to move the steering wheel, when the ingress or egress mode is activated.

It is advantageous, if the ingress and/or egress mode is terminated due to at least one signal and/or after a certain time period has elapsed. The time period is preferably an adjustable parameter. The at least one signal can be the detection of a drop in grid voltage below a certain value and/or a vehicle request signal.

Preferably, the feedback actuator is shut down, when the egress mode is terminated.

It is preferred, that the normal operation mode of the feedback actuator is started or the feedback torque actuation for providing the resistance torque is switched off, when the ingress mode is ended.

Preferably, the ingress situation is detected based on

- signals from the vehicle communication,

- the steering angle signal, and/or

- the steering torque signal, and/or

- other signals which represent a hands-on situation on the steering means.

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

A preferred embodiment of the present invention will be described with reference to the drawings.

Figure 1 : is a schematic illustration of a steer-by-wire steering system of a motor vehicle; and

Figure 2: shows a block diagram of an ingress and egress mode.

Figure 1 is a schematic drawing of a steer-by-wire system 1 with a steering shaft 2 connected to steering means 3. There is no mechanical connection between the steering means 3 and 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 has two tie rods 9 for road wheels 4, of which only one road wheel 4 is sketched.

When a driver operates the steering means 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 control unit 10 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 front wheel axle 8 is moved sideways and the road wheels 4 are turned. At the same time, forces introduced in the wheel axle 8 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 feedback actuator 11, so that the operator can recognize the feedback in the steering means 3.

Figure 2 shows schematically a block diagram of a method to control the steering system with an ingress/egress mode. In a first step 12 the system detects an ingress or egress situation.

An ingress situation can be detected based on signals from the vehicle communication 13, e.g. doors state, doors lock state, enable signal from the vehicle, wake-up signal based on the steering angle signal 14, e.g. if there is a change in steering angle of the steering wheel higher than a defined threshold value, and/or based on other signals which represent a hands-on situation on the steering means 15, e.g. generated by a capacitive sensor implemented in the steering wheel. If an ingress situation is detected the feedback actuator is switched on in an ingress mode and simulates a mechanical resistance, making it difficult to move the steering wheel, such that it can be used as a handle to get into the car. This can be achieved by a position control mode of the feedback actuator 16, which maintains the initial position of the steering wheel when the feedback actuator is activated. To imitate high mechanical resistance, the feedback actuator applies a resistance against rotation to the steering shaft. The ingress mode can be terminated due to signals, e.g. when the power supply voltage drops below a certain value and/or due to a vehicle request and/or after a certain time period has elapsed, the time period being an adjustable parameter. When the ingress mode is terminated, the normal operation mode of the feedback actuator is started or the feedback torque actuation for providing a resistance torque is ended.

The feedback actuator activates an egress mode 12, after the normal steering system shut down procedure is finished. In egress mode the feedback actuator simulates a high mechanical resistance, making it difficult to move the steering wheel, such that it can be used as a handle to get out of the car. This can be achieved by a position control mode of the feedback actuator 16, which maintains the initial position of the steering wheel when the feedback actuator is activated. The egress mode can be terminated due to signals, e.g. when the power supply voltage drops below a certain value and/or due to a vehicle request and/or after a certain time period has elapsed, the time period being an adjustable parameter. When the egress mode is terminated, the feedback actuator is shut down.