METHOD FOR DETERMINING THE STEERING FEEL OF A STEER-BY-WIRE STEERING SYSTEM

Description

[0001] The present invention relates to a method for controlling a steer-by-wire steering system with the features of the preamble of claim 1, and to a steer-by-wire steering system with the features of the preamble of claim 9.

[0002] In steer-by-wire systems, the position of the steered wheels is not directly linked to the steering input device, such as a steering wheel. Instead, the steering wheel and the steered wheels are connected via electrical signals. The driver's steering input is detected by a steering angle sensor, and a steering actuator adjusts the position of the steered wheels accordingly. There is no mechanical connection to the wheels, so no immediate feedback is provided to the driver after turning the steering wheel. However, appropriately adjusted feedback is provided, for example, when parking or driving straight ahead, where a steering torque, adapted to the vehicle's response and varying depending on the vehicle manufacturer, is desired.When cornering, reaction forces act as lateral forces on the steering gear, which the feedback actuator replicates as a torque opposite to the steering direction. This provides the driver with a predictable steering feel. To simulate the road's effects on the steering wheel in steer-by-wire systems, a feedback actuator (FBA) is necessary on the steering wheel or steering column. This actuator imparts a steering feel depending on the desired feedback from the steering input.

[0003] Traditionally, steering feedback is determined by the rack force, which is exerted on the rack by the tie rods connected to the wheels via the suspension. This rack force is significantly influenced by the current lateral forces. Thus, a substantial portion of the current rack force corresponds to lateral acceleration. However, the rack force is not solely determined by the lateral forces occurring while cornering; numerous other factors influencing the current driving situation also affect it. One example is the road surface condition (unevenness, ruts, coefficient of friction).

[0004] It is known in electric power steering (EPS) systems to determine the currently applied rack force using a torque sensor arranged on the rack or by estimation using a so-called observer based on a model of the steering system. German patent application DE 10 2015 216 040 A1 discloses an electromechanical power steering system in which a rack force is determined, its temporal profile is analyzed and compared with a reference value, in order to subsequently provide the driver with haptic feedback, e.g., in the form of a steering torque applied to a steering wheel. It is also known from German patent application DE 10 2017 105 370 A1 to estimate a rack force and generate a feedback signal depending on the estimated rack force. EP 2 647 548 discloses the preamble of claim 1 (therefore also of claim 9). .

[0005] The object of the present invention is to provide a method for controlling a steer-by-wire steering system for motor vehicles that enables improved steering feel and simultaneously exhibits low-noise behavior in the center range. Furthermore, a steer-by-wire steering system that enables improved steering behavior is to be provided.

[0006] This problem is solved by a method for controlling a steer-by-wire steering system for motor vehicles with the features of claim 1 and a steer-by-wire steering system for motor vehicles with the features of claim 9. Advantageous embodiments of the invention are described in the dependent claims.

[0007] Accordingly, a method for controlling a steer-by-wire steering system for a motor vehicle includes: an electronically controlled steering actuator acting on the steered wheels, a control unit, a feedback actuator which can be acted upon by a driver via a steering input device with a driver request for a steering angle and which outputs a feedback signal to the steering input device in response to the driver request and a driving state of the motor vehicle, a signal transmission which transmits the driver request to the control unit, wherein the control unit controls the steering actuator in order to transform the driver request into a deflection of the steered wheels, wherein the method comprises the following process steps: determining the wheel slip of at least one front wheel, frequency analysis of the time course of the wheel slip signal of the at least one front wheel, determining the feedback signal as a function of the result of the frequency analysis.

[0008] Preferably, the feedback signal can be determined via the following steps: Generating a transmission model from a vehicle speed vector and from a front wheel speed vector for each of the left and right front wheels. Outputting a steering torque to the feedback actuator, which transmits the feedback from the road to the driver via the steering wheel.

[0009] By determining the feedback signal from the velocity vectors, a stable signal about the road condition can be transmitted to the driver, which is independent of forces resulting from the mechanical components of the steering actuator and transmitted to the driver in the form of torque, which in turn lead to an unwanted deflection via the control unit on the steering rod of the steering actuator.

[0010] Preferably, dominant frequencies are determined during frequency analysis and classified using predefined frequency values. The dominant frequencies are preferably determined in a frequency range of 2 to 50 Hz, and more preferably in a frequency range of 8 to 20 Hz.

[0011] Depending on the classification, a predefined feedback signal is preferably assigned to the dominant frequencies.

[0012] The feedback signal preferably includes a steering torque and a center position.

[0013] Furthermore, it is preferred that the wheel slip of at least one front wheel S,wF be determined according to the following equation: S , wF = V , wF − V , veh V , veh , where v ,wF is the front wheel speed of at least one front wheel and v ,veh is the vehicle speed.

[0014] Preferably, the wheel slippage of both front wheels is analyzed.

[0015] Furthermore, a steer-by-wire steering system for a motor vehicle is provided, comprising: an electronically controlled steering actuator acting on the steered wheels, a control unit, a feedback actuator which can be acted upon by a driver via a steering input device with a driver request for a steering angle and which outputs a feedback signal to the steering input device in response to the driver request and a driving state of the motor vehicle, a signal transmission device which transmits the driver request to the control unit, wherein the control unit controls the steering actuator in order to transform the driver request into a deflection of the steered wheels, wherein the steer-by-wire steering system is configured to perform a previously described

[0016] A preferred embodiment of the invention is explained in more detail below with reference to the drawings. Similar or functionally equivalent components are designated with the same reference numerals in the figures. The figures show: Fig. 1: a schematic representation of a steer-by-wire steering system, Fig. 2: a block diagram of a steer-by-wire steering system control unit with a module for determining a steering torque, Fig. 3: a block diagram of the module for determining the steering torque, and Fig. 4: a block diagram of another module for determining the steering torque.

[0017] In the Figure 1A steer-by-wire steering system 1 is shown. A rotary angle sensor (not shown) is attached to a steering shaft 2, which detects the driver's steering angle applied by turning a steering input device 3, which in this example is designed as a steering wheel. A steering torque can also be detected. A joystick can serve as the steering input device. Furthermore, a feedback actuator 4 is attached to the steering shaft 2, which serves to simulate the feedback from the road surface 70 to the steering wheel 3 and thus provide the driver with feedback on the steering and driving behavior of the vehicle. The driver's steering request is transmitted via signal lines to a feedback actuator monitor unit 10 via the rotary angle α of the steering shaft 2 measured by the rotary angle sensor, as shown in the Figure 2This is illustrated. The feedback actuator monitor unit 10 transmits the driver's steering request to the control unit 60. The feedback actuator monitor unit 10 preferably also controls the feedback actuator 4. The feedback actuator monitor unit 10 can also be integrated with the control unit 60. Depending on the signal from the steering angle sensor and other input variables, the control unit 60 controls an electric steering actuator 6, which controls the position of the steered wheels 7. The steering actuator 6 acts indirectly on the steered wheels 7 via a steering linkage 8, such as a rack and pinion steering gear, as well as via tie rods 9 and other components.

[0018] Figure 2Figure 1 shows the control of the feedback actuator 4. The feedback actuator 4 receives signals via signal line 50, including from the rotary angle sensor, which measures and stores the rotary angle α, the steering angle acceleration, and the steering angle velocity. The feedback actuator 4 communicates with the feedback actuator monitor unit 10, which controls the feedback actuator 4. The feedback actuator monitor unit 10 receives the actual wheel steering angle β of the steered wheels 7 from a control unit 60 of the steering actuator 6, as well as other parameters determined by the control unit 60. The rack position 120 measured on a rack 12 and other road information 13 are transmitted to the control unit 60. The control unit 60 also receives driver steering commands 51, such as the steering angle status. The feedback actuator monitor unit 10 includes a module for determining steering feel 11.The module generates a signal that controls the feedback actuator 4, resulting in a steering torque Tfb, which creates a steering feel. Additionally, a steering torque for the straight-ahead position of the steering wheel, Tnpos, is transmitted to the feedback actuator 4.

[0019] In Figure 3 A module for determining steering feel 11 is shown. In a first step 111, the vectors of the road wheel speeds of the two front wheels are used to calculate v ,wFL , v ,wFR , as well as the vehicle speed vThe steering torque for the straight-ahead position of the steering wheel, Tnpos, is determined by means of a frequency analysis or a transfer model, for example a bandpass filter, and is applied to the resulting steering torque, Tfb, and transmitted to the feedback actuator 4. By using the road wheel speeds and the vehicle speed, a stable steering torque can be returned to the driver, which is independent of forces acting on the steering gear. These forces result from the mechanical components of the steering actuator and are transmitted to the driver as torque, which in turn can lead to an undesired deflection via the control unit on the steering actuator's steering rod.

[0020] In Figure 4 Figure 11 shows another module for determining steering feel. In a first step, Figure 110 calculates the road wheel speed vectors of the two front wheels. v ,wFL , v,wFR , as well as the vehicle speed v The wheel slip of the two front wheels s, wFL ,s, wFR is determined using the following relationships: S , wFL = V , wFL − V , veh V , veh , S , wFR = V , wFR − V , veh V , veh .

[0021] The wheel slip of the two front wheels s, wFL, s, wFR can be used to determine whether the vehicle is driving on a level / dry road or, for example, on a wet road surface or a gravel path. If the vehicle is driving on a gravel path, for example, the wheel slip of the two front wheels s, wFL, s, wFR oscillates, so these irregularities can be identified by analyzing the values. Therefore, in a subsequent step 112, a frequency analysis of the wheel slip of the two front wheels s, wFL, s, wFR is performed. Preferably, the frequency analysis analyzes an oscillation of the time-dependent wheel slip profile in the range of approximately 2 to 50 Hz, particularly in the range of approximately 8 to 20 Hz, and determines the frequencies or dominant frequencies from this. The determined dominant frequencies f,dom are then compared with predefined frequency values ​​in a subsequent step 113.The determined dominant frequencies f,dom are assigned to specific frequency ranges based on their cause. Frequency ranges deemed desirable are forwarded, while those perceived as disruptive are suppressed or at least significantly attenuated in a feedback path. Based on the forwarded dominant frequencies, a value for the steering torque T,fb is generated in a subsequent step (114) to provide the driver with the most realistic steering feel possible in relation to the road information. Furthermore, a steering torque for the straight-ahead position of the steering wheel, T,npos, is transmitted to the feedback actuator (4). The steering torques determined in this way are free of interference, as they depend only on the measured speeds and are therefore completely independent of the rack force.

Claims

1. A method for controlling a steer-by-wire steering system for a motor vehicle comprising: - an electronically controllable steering actuator (6) which acts on the steered wheels (7), - a control unit (60), - a feedback actuator (4) to which a driver's request for a steering angle can be applied by a driver via a steering input means (3) and which outputs a feedback signal to the steering input means as a reaction to the driver's request and a driving condition of the motor vehicle, - a signal transmission which transmits the driver's request to the control unit (60), - wherein the control unit (60) controls the steering actuator (6) in order to convert the driver's request into a deflection of the steered wheels (7), - characterized in that the method comprises the following method steps: - determination of the wheel slip of at least one front wheel, - frequency analysis of the profile over time of the wheel slip signal of the at least one front wheel, - determination of the feedback signal in dependence on the result of the frequency analysis.

2. The method as claimed in claim 1, characterized in that in the frequency analysis there are determined dominant frequencies which are classified by means of predefined frequency values.

3. The method as claimed in claim 2, characterized in that a predefined feedback signal is allocated to the dominant frequencies in dependence on the classification.

4. The method as claimed in one of the preceding claims, characterized in that the feedback signal comprises a steering torque and a center position.

5. The method as claimed in one of the preceding claims, characterized in that the wheel slip of at least one front wheel (s,wF) is determined in accordance with the following equation: S , wF = V , wF − V , veh V , veh , wherein v,WF is the front wheel velocity of at least one front wheel and v,veh is the vehicle velocity.

6. The method as claimed in one of the preceding claims, characterized in that the dominant frequencies are determined in a frequency range of from 2 to 50 Hz.

7. The method as claimed in claim 6, characterized in that the dominant frequencies are determined in a frequency range of from 8 to 20 Hz.

8. The method as claimed in one of the preceding claims, characterized in that the wheel slip of both front wheels is analyzed.

9. A steer-by-wire steering system (1) for a motor vehicle comprising: - an electronically controllable steering actuator (6) which acts on the steered wheels (7), - a control unit (60), - a feedback actuator (4) to which a driver's request for a steering angle can be applied by a driver via a steering input means (3) and which outputs a feedback signal to the steering input means as a reaction to the driver's request and a driving condition of the motor vehicle, - a device for signal transmission which transmits the driver's request to the control unit (60), - wherein the control unit (60) controls the steering actuator (6) in order to convert the driver's request into a deflection of the steered wheels (7), characterized in that the steer-by-wire steering system (1) is adapted to carry out a method as claimed in one of claims 1 to 8.