The present invention relates to a method for controlling steering locking of a steer-by-wire steering system of a motor vehicle according to the preamble of claim 1.

In a steer- by- wire steering system, the vehicle's steering wheel is disengaged from the steering mechanism. 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 of the vehicle 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.

Steering columns comprise locking devices, the so-called steering lock, which is intended to secure the parked vehicle against unauthorized use and to block the steering wheel, which is often used as a handle to facilitate getting into and out of the vehicle.

The patent application DE 10 2017 104 510 A1 discloses a steering column of a steer-by-wire steering system with an electrically actuated steering lock. A latching star is arranged on an outer part of the steering shaft, in which a latching bolt, connected to the chassis via a sleeve assembly, can engage in a known manner in order to arrest the steering shaft and therefore prohibit a steering procedure to prevent unauthorized use of the motor vehicle.

However, in between the snap-in positions of the latching bolt the steering wheel moves freely which is very uncomfortable for the driver, if he uses the steering wheel as a handle to get in or out of the car.

It is an object of the present invention to provide a method for controlling steering locking of a steer-by-wire steering system of a motor vehicle, which generates a better steering lock.

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

Accordingly, a method for controlling steering locking of a steer-by-wire steering system of a motor vehicle is provided, the steer-by-wire steering system comprising a feedback actuator to simulate a steering feel to a steering device, wherein said feedback actuator has an electric motor with a motor shaft connected to a steering shaft to be able to transmit a torque, and a steering lock unit with a latching element connected to the steering shaft in a torsion-resistant manner and a locking element that is configured to engage with the latching element at discrete locking positions to block a rotation of the steering shaft, wherein the method comprises the following steps: • If the ignition is switched-off and a movement of the steering device is detected, determining the position of the steering shaft and the distance between the locking element and the next locking position in direction of the movement of the steering device;

• If the distance is greater than a predefined value, transmitting a counter torque to the steering shaft by the feedback-actuator with opposite direction to the movement of the steering device until the locking position reaches the locking element and the locking element can engage with the latching element.

This way after the power is switched-off and driver uses the steering device, in particular steering wheel, as a handle the actuator generates a torque, which makes steering difficult until the rotation of the steering shaft is locked. This mimics the same feeling as the driver experiences in electromechanical steering systems. The angular distance between the locking positions is preferably between 210 and 360 degrees for one locking position systems. The angular distance between the locking positions is preferably between 150 and 210 degrees for two locking position systems. The angular distance between the locking positions is preferably between 20 and 40 degrees, more preferably between 30 and 40 degrees for systems with more than two locking positions.

Preferably, position sensors are used to determine the relative position between the latching element and the locking element of the steering lock unit. The feedback actuator can be kept active as long as the driver is still in the vehicle or can be put on active when it is detected that the driver unlocks the vehicle. Even though the ignition is off, the feedback actuator is connected to a power source.

Preferably, the counter-torque applied by the feedback actuator to the steering shaft is depending on the determined distance between the locking element and the next locking position in direction of the movement of the steering device. It is advantageous, if the counter-torque increases with decreasing angular distance between the locking element and the next locking position in direction of the movement of the steering device. Preferably, the increase is described by an exponential function.

The driver's feeling is further improved, if the maximum counter-torque generated by the feedback actuator is reached during engagement of the locking element into the latching element at the next locking position in direction of the steering wheel movement. Preferably, the transition from the counter-torque generated by the feedback actuator and the torque generated by mechanical blocking of the steering lock unit over the angular position of the steering shaft is continuous and can be constantly differentiated.

The method can further comprise the following step:

• If the ignition is switched-off and no movement of the steering device is detected within a predefined time interval, pro-actively rotating the steering shaft by the feedback actuator until the closest locking position, irrespective of the direction of rotating, reaches the locking element and the locking element can engage with the latching element.

This way it can be rest assured that the steering lock unit is locking the rotation of the steering, if, for example, the driver unlocks the car and gets in, because the locking already occurred before the driver locked the car.

In an embodiment, the locking element is pretensioned inwardly, in an radial direction with respect to the rotational axis of the steering shaft by means of a spring.

The latching element can be a latching star with a discrete number of equal recesses and equal protrusions arranged on the outer surface, next to each other and evenly spaced in circumferential direction and the locking element can be a latching bolt, wherein the positions of the recesses define the locking or snap-in positions of the latching bolt.

It is preferred that the steering shaft has an inner shaft and an outer shaft rotatable around an axis of rotation and with compatible outer and inner profiles so that the two steering shaft parts can engage in one another and form a torsion-resistant but telescopic arrangement. In this case it is preferred that the latching star circumferentially surrounds the outer shaft and is fastened to the outer shaft in a torsion-resistant manner. The feedback actuator preferably acts on the outer shaft. Further, a tolerance ring can be used, which is coaxially arranged between the outer shaft and the latching star.

One exemplary embodiment of the present invention is described below with aid of the drawings. In all figures the same reference signs denote the same components or functionally similar components.

Figure 1 shows a steer-by-wire steering system in a schematic illustration,

Figure 2 shows a cross-section of a steering shaft with a steering lock unit in a locked position,

Figure 3 shows a cross-section of a steering shaft with the steering lock unit in an unlocked position,

Figure 4 shows a cross-section of a steering shaft with the steering lock unit in another unlocked position, and

Figure 5 shows a diagram of a torque applied by the feedback actuator to the steering shaft in dependence of an angular steering wheel position. Figure 1 is a schematic representation of a steer-by-wire steering system 1 that comprises an actuation control system 2 to actuate road wheels 3 and a feedback actuator 4 to simulate the steering feel of a conventional mechanically linked steering system. A steering device 5, which is in the example a steering wheel, is connected to a steering shaft 6. Not shown position sensors and torque sensor are operably connected to steering shaft 6. Position sensors electronically detect the angular position of the steering shaft 6, while the torque sensor electronically detects and evaluates the torsional force acting on the steering shaft 6. The angular displacement of the steering wheel 5 is detected, transmitted to the actuation control system 2, processed in the actuation control system 2, and applied to a servo motor 7 to move the steerable road wheels 3 via a rack 81 and pinion 82 system 8.

The feedback actuator 4 includes an electric motor 9 having a motor shaft rotatively driven by the motor 9 and connected to the steering shaft 6 (not shown). Since there is no direct mechanical coupling between the actuation control system and the steerable wheels, the driver does not receive any feedback from the road surface through the steering mechanism. Therefore, the feedback actuator 4 generates a reaction torque to the steering wheel 5, based upon a number of steering parameters such as vehicle speed, steering device angle, the steering device angle speed, the steering device turning acceleration, the yaw rate of the vehicle, road surface condition, and further driving parameters of the vehicle.

Figure 2 shows a steering lock unit 10 with a latching star 11 and a latching bolt 12, the latching star 11 being illustrated in an engaged position. The steering shaft 6 has an inner shaft 61 and an outer shaft 62 rotatable around an axis of rotation 100. The inner shaft 61 comprises a non-circular, in particular an approximately cloverleaf-shaped, outer profile. The outer shaft 62 comprises a correspondingly compatible inner profile so that the two steering shaft parts can engage in one another and form a torsion-resistant but telescopic arrangement. The steering wheel 5 connected to the steering shaft is schematically shown next to the cross-section.

The latching star 11 circumferentially surrounds the outer shaft 62 and is fastened to the outer shaft 62 in a torsion-resistant manner.

A tolerance ring 13 is coaxially arranged between the outer shaft 62 and the latching star 11. The tolerance ring 13 makes it possible for the latching star 11 to be moved relative to the outer shaft 62 if a predetermined torque is exceeded, acting like a overload clutch, thus avoiding unwanted damage.

A discrete number of equal recesses 14 and equal protrusions 15 are arranged on the outer surface of the latching star 11, next to each other and evenly spaced in circumferential direction. The positions of the recesses 14 define the locking or snap-in positions of the latching bolt 12. The angular distance between the locking positions, defined as the middle of the recesses 14 in circumferential direction, can be for example between 30 and 40 degrees. The latching bolt 12 can be moved into the recesses 14 of the latching star 11 so that the steering shaft 6 is blocked with play in its rotational movement, as shown in Fig. 2. The play of the mechanical blocking is defined by the width of the recesses 14 in relation to the width of the latching bold 12 in the area of engagement.

To release the steering shaft 6 the latching bolt 12 is moved outwardly in radial direction with respect to the rotational axis of the steering shaft 100 and out of the recess 14 of the latching star.

Figures 3 and 4 show unlocked positions of the steering lock unit 10.

It is likely, that in the event where the ignition is switched off and the driver wants to use the steering wheel as a handle, either to get into the vehicle or out of the vehicle, the steering shaft 6 is in an angular position, where the steering lock unit 10 cannot lock the rotation of the steering shaft 6, because the latching bolt 12 can not engage with a recess 14 of the latching star 11. In this case the steering wheel 5 will move freely, which is very uncomfortable for the driver. To overcome the free movement of the steering wheel, the not shown feedback actuator is used to introduce a counter-torque TFBA, with opposite direction to the steering wheel movement implied by the driver, until a lock position is reached. The feedback actuator has power to do so even if the ignition is switched off.

Preferably, the counter-torque TFBA depends on the angular distance between the actual angular position of the steering shaft 6 and the next locking position in direction of the steering wheel movement, as shown in Fig. 5. This counter torque TFBA makes angular movement of the steering wheel difficult.

Fig. 5 shows the counter-torque TFBA applied by the feedback actuator to the steering shaft in dependence of an angular steering wheel position. The torque hindering a steering wheel movement Tsw is plotted against the steering wheel angle a. The three dashes on the x-axis represent three consecutive locking positions Li,l_2,l_ ₃ . The angular steering wheel position at switch-off of the ignition defines the starting point SP. The staring point SP lies in between two locking points. The next locking position in direction of the movement of the steering wheel introduced by the driver is locking position Li. It is preferred that the counter torque TFBA increases with decreasing angular distance of the actual steering wheel position to the next locking position. This behavior is known from electromechanically steering systems given by the rotation of the steering wheel against road wheel friction. The increase can be described by an exponential function. The maximum torque generated by the feedback actuator TFBA, max shall be reached during engagement of the latching bold into the latching star at the next locking position in direction of the steering wheel movement, which corresponds to Li in the shown example. The transition from the counter-torque generated by the feedback actuator TFBA and the torque generated by mechanical blocking of the steering lock unit TSLU over the angular position of the steering shaft a is preferably continuous and can be constantly differentiated so as to prevent singularities.

In addition it is possible that the feedback actuator actively moves the steering wheel to the closest locking position irrespective of the direction of rotating to actively lock the steering wheel.

Preferably, the feedback actuator acts on the outer shaft.

In a preferred embodiment an electric motor moves the latching bolt in and out of the latching star for locking and unlocking.

It is also possible to use a latching bolt, which is pretensioned inwardly in an radial direction with respect to the rotational axis of the steering shaft by means of a spring.