Method for controlling a steer-by-wire steering system of a road vehicle during power-concentrated maneuvers

Abstract

The present invention relates to a method of controlling a steer-by-wire steering system (1) for a road vehicle, the steer-by-wire steering system (1) comprising steering means (3), road wheel actuators (5) for actuating road wheels (4) via racks, and a control unit (10) including a position controller for calculating a target rack position (21) for the road wheel actuators (5) based on an actual rack position and a rack position request (17), the method comprising: (a) limiting the requested rack speed to the maximum rack speed value (15) used by the position controller for calculating the target rack position (21) of the road wheel actuators (5) if the requested rack speed exceeds a predetermined maximum value.

Description

[Technical Field]

[0001] The present invention relates to a method for controlling a steer-by-wire steering system for a road vehicle according to the preamble of claim 1 and to a steer-by-wire steering system for a road vehicle according to the preamble of claim 10. [Background technology]

[0002] In a steer-by-wire steering system, there is no mechanical connection between the steering wheel, steering rack, and steerable wheels. Steering movement is achieved by a position-controlled road wheel actuator, whose nominal position is based on the steering wheel angle.

[0003] The position controller always operates on the motor torque so that the road wheel actuator operates after the reference position signal. Due to the physical limitations of the road wheel actuator motor (e.g., maximum available motor torque at a certain motor speed) and the limitations (bandwidth) of the position controller, the road wheel angle exhibits a transient response.

[0004] In extreme cases, for example when the road wheel actuators are in a degraded state, the controller settling time can become even longer, which adversely affects vehicle handling.

[0005] If the road wheel actuator does not have the required mechanical power for satisfactory settling behavior, the so-called post-steering effect occurs. For example, if a very fast step-steering maneuver is performed on the steering wheel, the road wheels continue to rotate during the transient response, even though the steering wheel maneuver has already ended. This can be felt and heard by the driver, and is very unpleasant. In extreme cases (e.g., when changing lanes quickly), the rack and steering wheel may briefly move in opposite directions, which can lead to undesirable situations. Summary of the Invention

[0006] It is therefore an object of the present invention to provide a method for controlling a steer-by-wire steering system for a road vehicle, and a steer-by-wire steering system itself, which do not have the above-mentioned drawbacks.

[0007] This object is achieved by a motor steer-by-wire system having the features of claim 1 and a method for controlling a motor steer-by-wire steering system having the features of claim 10.

[0008] There is therefore provided a method for controlling a steer-by-wire steering system for a road vehicle, the steer-by-wire steering system comprising steering means, wheel actuators for actuating the wheels via racks, and a control unit including a position controller, the position controller calculating a target rack position for the road wheel actuators based on an actual rack position and a rack position request, the method comprising the following steps: If the requested rack speed exceeds a predetermined maximum, limit the requested rack speed to the maximum rack speed value used by the position controller to calculate the target rack position of the road wheel actuator.

[0009] Modifying the target rack position increases maneuverability, reduces post-steering, and improves noise, vibration, and harshness (NVH) of steer-by-wire steering systems.

[0010] Preferably, the maximum rack speed value is calculated based on vehicle signals and steering system signals, for example based on vehicle speed and position offset values. For normal steering inputs, the maximum rack speed value is typically higher than the actual rack speed, and the rack speed is not limited.

[0011] It is advantageous if the following steps are provided (in the order mentioned): b. calculating, by the position controller, a position tracking error as the deviation between the actual rack position and the target rack position; c. detecting a road wheel actuator power limiting condition; d. setting the position tracking error to a predetermined value, and calculating a target rack position of the road wheel actuator based on the predetermined value.

[0012] These steps will maintain vehicle maneuverability.

[0013] Preferably, in step c) to detect a power limited condition, the position tracking behavior of the system is monitored such that a power limited condition is detected if the road wheel actuator is unable to follow the target rack position with a position tracking error less than a predetermined value.

[0014] In step d, the position tracking error is reset (the forced control is terminated), preferably as soon as the driver-demanded rack speed falls below the actual rack speed, thereby allowing the tracking capability of the position controller to be restored.

[0015] Preferably in step d) the steering force that the driver must apply to the steering means to steer is increased to inform the driver of the situation.

[0016] As soon as the driver-requested rack speed falls below the actual rack speed, the position offset between the rack position request and the target rack position is advantageously gradually reduced by synchronizing the target rack position with the driver's rack position request, and preferably the steering force is gradually reduced to a normal level.

[0017] The synchronization can be based on (but is not necessarily limited to) the following signals: steering wheel angle, steering wheel angular velocity, steering wheel torque, actual misalignment, vehicle speed, yaw rate, lateral acceleration, and longitudinal acceleration.

[0018] For good steering feel, it is advantageous for the position controller to react to driver input during synchronization.

[0019] There is further provided a steer-by-wire steering system for a road vehicle, the system comprising steering means, road wheel actuators for actuating the road wheels via racks, and a control unit for calculating a target rack position for the road wheel actuators based on an actual rack position and a rack position request based on the position of the steering means, the control unit being designed to perform the method described above.

[0020] Preferably, the control unit includes a position controller having a switch, said switch comprising: -Normal state, in which the rack speed limit is used to calculate a rack offset value, which is then used to calculate the target rack position; and A power-limited state is taken in which the position tracking error is set to a predetermined value, and a rack offset value is calculated based on this to calculate a target rack position.

[0021] Hereinafter, embodiments of the present invention will be described with reference to the drawings. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 1 is a schematic diagram of a steer-by-wire steering system for an automobile. [Figure 2] FIG. 2 shows a diagram of rack position over time with rack speed limiting and power limiting operation of the road wheel actuator. [Figure 3] FIG. 3 shows a block diagram of a method for controlling a steer-by-wire system in a motor vehicle. DETAILED DESCRIPTION OF THE INVENTION

[0023] Figure 1 is a schematic diagram of a steer-by-wire system 1 in which a steering shaft 2 is connected to steering means 3. There is no mechanical connection between the steering means 3 and the wheels 4. A wheel actuator 5 operates a gear rack 6 via a rack-and-pinion gear 7 that is part of a front axle 8. The front axle 8 has two tie rods 9 for the road wheels 4, of which only one road wheel 4 is shown.

[0024] When the driver operates the steering means 3, the steering shaft 2 rotates, and this is detected by a shaft sensor (not shown). When the vehicle is switched on, the control unit 10 calculates an operation signal for the wheel actuator 5 from the signal detected by the shaft sensor. By operating the gear rack 6 with the operation signal, the front axle 8 moves laterally and the road wheel 4 rotates. At the same time, the force introduced from the road wheel 4 to the axle 8 is detected by another sensor (not shown), and a feedback signal is calculated to be applied to the steering shaft 2 by the feedback actuator 11, allowing the operator to recognize the feedback on the steering means 3.

[0025] The control unit 10 includes a position controller that calculates the motor torque based on the actual or estimated position of the rack 6 and the desired rack position. Thus, the road wheel actuators 5 follow the target rack position. The desired rack position depends on the steering angle.

[0026] Figure 2 shows the rack position x as a function of time t. The solid line represents the requested rack position. The dashed line shows the target rack position, and the dotted line shows the actual rack position. Under normal conditions, the road wheel actuators are able to follow the requested rack position, and the requested and actual rack positions are synchronized. This is shown in the first section of the plot. If the requested rack speed exceeds a predefined maximum, the requested rack speed is limited to the maximum rack speed value. This condition is displayed in the second section of the figure.

[0027] The required rack speed is optionally limited to reduce operational noise and unexpected and erratic vehicle motion. The maximum rack speed value is calculated based on vehicle level input signals (such as, but not limited to, vehicle speed, yaw rate, lateral acceleration, longitudinal acceleration, etc.) and steering system signals (such as, but not limited to, steering wheel angle, steering wheel angular velocity, steering wheel angular acceleration, steering wheel torque, etc.). In a specific example, the maximum rack speed is determined based on vehicle speed and misalignment.

[0028] Normally, the road wheel actuator should be able to track the rack-speed-limited target rack position in a satisfactory manner. However, if the position tracking error increases beyond a predetermined threshold (which may be dependent on the vehicle and / or steering system signals), this indicates that the road wheel actuator is unable to follow the rack-speed-limited target rack position. Therefore, at this point, a correction of the target rack position is initiated.

[0029] In the second section of the diagram, the target rack position is the same as the initial actual rack position. Over time, the deviation between the actual rack position and the target rack position (position tracking error) increases up to a threshold value. This can occur, for example, due to a high steering wheel angle input (e.g., a rapid lane change). Such driver input must mean that the mechanical performance of the road wheel actuators must not be intentionally limited (because such highly dynamic maneuvers occur in emergency situations). In this situation, the position tracking error is forced to a predetermined value. This condition is shown in the third section of the diagram. Forcing the position tracking error forces the actuator to operate at its power limit, ensuring that the actuator follows the target position as quickly as possible.

[0030] At the same time, the steering force the driver must apply to the steering wheel to steer (the counter-steering torque of the feedback actuator) is also increased to signal the "catch-up effect" to the driver. The logic exits the forced position tracking error state as soon as the rack speed requested by the driver falls below the actual rack speed. This is shown in the fourth section. At this point, the system's ability to track the rack position is restored. At the same time, the increased steering force is gradually reduced to a normal level. The actual rack position is less than the target rack position. The target and requested rack positions are no longer synchronized.

[0031] The resulting position offset is handled by logic that gradually synchronizes the target rack position with the operator's rack position request.

[0032] The term "logic," as used herein, may include software and / or firmware running on one or more programmable processors, application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), digital signal processors (DSPs), hardwired logic, or combinations thereof.

[0033] Synchronization (reducing position offset) is done using, but not limited to, the following signals: steering wheel angle, steering wheel angular velocity, steering wheel torque, actual position offset, vehicle speed, yaw rate, lateral acceleration, and longitudinal acceleration. This is done so that the position controller reacts to driver input during synchronization (i.e., if the driver steers left, the rack also turns the wheel left). If the steering wheel is not turned by the driver, synchronization still occurs over time.

[0034] 3 shows a block diagram of the above-described method. A rack speed limiter 12 calculates a rack speed limit 15 based on steering system signals 13 and vehicle signals 14. The limit value is passed as an input to limit logic 16. Additionally, a driver's rack position request (requested rack position) 17 is sent to limit logic 16. Limit logic 16 calculates a rack offset value 18.

[0035] The rack position tracking evaluator 19 also calculates a forced control offset 20 based on the target rack position 21 and the actual rack position 22. A switch 23 is used upstream of the synchronization logic 24. The switch 23 is in a normal mode (true). In this normal mode, the rack offset value 18 is passed from the limit logic 16 to the synchronization logic 24 as an offset value 25. The offset value 25, the steering system signal 13, and the vehicle signal 14 are used to calculate a rack offset value 26. The rack offset value 26 is used in the calculation in addition to the requested rack position 17 and the target rack position 21. If the rack position tracking evaluator 19 detects a power limit condition, the switch 23 is switched to the power limit mode (false), and the forced control offset 20 is passed to the synchronization logic 24 as the offset value 25, and the synchronization logic 24 calculates the rack offset value 26 based on the offset value 25.

[0036] As soon as the rack position tracking and evaluation unit 19 detects the end of the power limiting condition, the switch 23 is switched to normal mode.

Claims

1. A method for controlling a steer-by-wire steering system (1) for a road vehicle, comprising: a steer-by-wire steering system (1) with steering means (3); road wheel actuators (5) for actuating road wheels (4) via racks; and a control unit (10) having a position controller for calculating a target rack position (21) for the road wheel actuators (5) based on an actual rack position and a rack position request (17), If the requested rack speed exceeds a predetermined maximum, limiting the requested rack speed to a maximum rack speed value (15) used by a position controller to calculate a target rack position (21) of a road wheel actuator (5).

2. 2. The method of claim 1, wherein the maximum rack speed value (15) is calculated based on a vehicle signal (14) and a signal of the steering system (13).

3. b. Calculating the position tracking error as the deviation between the actual rack position and the target rack position (21) by the position controller; c. detecting a road wheel actuator power limiting condition; d. setting the position tracking error to a predetermined value (20), and calculating a target rack position (21) of the road wheel actuator (5) based on the predetermined value.

4. 4. The method of claim 3, wherein for the detection of a road wheel actuator power limited situation in step c) the position tracking behavior of the system is monitored and a power limited situation is detected if the road wheel actuator (5) is unable to follow the target rack position (21) with a position tracking error less than a predetermined value.

5. 5. The method of claim 3, wherein in step d, the position tracking error is reset as soon as the rack speed requested by the driver falls below the actual rack speed.

6. A method according to any one of claims 3 to 5, wherein in step d, the steering force that the driver must apply to the steering means (3) to steer is increased.

7. 6. The method of claim 5, wherein the position offset between the rack position request (17) and the target rack position (21) is gradually reduced by synchronizing the target rack position (21) with the rack position request (17) of the driver as soon as the rack speed requested by the driver falls below the actual rack speed.

8. 8. The method of claim 7, wherein the synchronization is based on steering wheel angle, steering wheel angular velocity, steering wheel torque, actual displacement, vehicle speed, yaw rate, lateral acceleration, and longitudinal acceleration signals.

9. 9. The method of claim 7 or 8, wherein during synchronization, the position controller reacts to driver input.

10. A steer-by-wire steering system (1) for a road vehicle comprising steering means (3), road wheel actuators (5) for actuating road wheels (4) via racks, and a control unit (10) for calculating a target rack position (21) for the road wheel actuators (5) based on an actual rack position and a rack position request (17) based on the position of the steering means (3), wherein the control unit (10) is designed to perform the method according to any one of claims 1 to 9.

11. The control unit (10) comprises a position controller (23) having a switch (23), The switch (23) A normal state in which the rack speed limit is used to calculate a rack offset value (18), which is then used to calculate the target rack position (17); and and a power limited state in which the position tracking error is set to a predetermined value (20) based on which a rack offset value is calculated to calculate a target rack position (21).

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