Steering system

Abstract

Provided is a steer-by-wire system that is highly practical. A steer-by-wire system is configured such that, when a stored neutral position ([omega] 0mi + [delta] [omega] m), which is a neutral steering position stored for controlling a steering actuator, is shifted from an actual neutral position, which is an actual neutral steering position, a state in which an operation of an operation member for moving a vehicle straight is maintained by a driver (S26), and the stored neutral position ([omega] 0mi + [delta] [omega] m) is shifted from the actual neutral position ([omega] 0mi + [delta] [omega] m). A neutral position offset elimination process for bringing the stored neutral position closer to the actual neutral position (S27) is executed. The neutral position offset elimination process is executed in a state in which the driver moves the vehicle straight, so that the process is relatively simple.

Description

Technical Field

[0001] The present invention relates to a steer-by-wire type steering system mounted on a vehicle. Background Art

[0002] In a steer-by-wire type steering system (hereinafter sometimes referred to as "steer-by-wire system") mounted on a vehicle, generally, an operating member such as a steering wheel is not mechanically connected to the steered wheels. Therefore, the wheels can be steered by the force of the power source of the steering actuator without depending on the force (hereinafter sometimes referred to as "operating force") applied by the driver to the operating member. In the steer-by-wire system, since the steering of the wheels is mainly performed according to the operation of the operating member, the relationship between the operation position of the operating member and the steering position of the wheels is important in the control of the steering of the wheels (hereinafter sometimes referred to as "steering control"). Specifically, in the case where the operation position where the operating member should be located when the vehicle is going straight is defined as the neutral operation position, when the operating member is located at this neutral operation position, the posture of the steered wheels should also be the posture when going straight. That is, the steering position of the wheels should also be located at the neutral steering position.

[0003] The controller that executes the steering control stores the above neutral operation position and neutral steering position, and executes the steering control based on these stored neutral operation position and neutral steering position. However, in the case where the camber angle of the steered wheels is adjusted, or a relatively large force is externally applied to the steered wheels, etc., the stored neutral steering position, that is, the stored neutral position may also shift from the actual neutral steering position. If this shift (hereinafter sometimes referred to as "neutral position shift") occurs, it can be expected that the steering of the steering wheels will not be appropriately performed for the operation of the operating member by the driver, or a sense of discomfort, etc. for the driver regarding this operation will be brought. Therefore, in the steer-by-wire system, for example, the following processing as described in the following patent document has been developed. This processing is a technique in which, in the target driving line following control for steering the wheels so that the vehicle travels on the target driving line, the neutral steering position is set based on the difference between the target steering position based on the target driving line and the steering position detected by the sensor.

[0004] Patent Document 1: Japanese Patent Application Laid-Open No. 2018-177128

[0005] The technology described in the above patent literature is about the target driving line following control, which is one of the driving assistance controls. For the setting of the neutral steering position, a certain degree of high precision is required. On the other hand, there are also driving assistance controls such as vehicle stabilization control and lane departure prevention control that steer the steering wheel for safety. It is desired to execute the above-mentioned respective controls even before obtaining a highly accurate neutral steering position by executing the target driving line following control, that is, even in a state where there is a certain degree of neutral position deviation. In addition, the setting of the neutral steering position in the target driving line following control is based on complex methods, learning, etc. It is also desired to simply eliminate the neutral position deviation instead of such a setting. That is, it is considered that if the neutral position deviation can be simply eliminated, the practicality of the steer-by-wire system is improved. The present invention has been completed in view of such actual situations, and an object is to provide a steer-by-wire type vehicle steering system with high practicality. Summary of the Invention

[0006] To solve the above problems, the steering system of the present invention is a steer-by-wire type steering system mounted on a vehicle, wherein the steering system includes: an operation member that is operated by a driver; a steering actuator that has a power source and steers the wheels by the force of the power source; and a controller that controls the steering actuator to achieve the steering of the wheels corresponding to the operation of the operation member. The controller is configured to: when a stored neutral position, which is a neutral steering position stored for controlling the steering actuator, deviates from an actual neutral position, which is an actual neutral steering position, execute a neutral position deviation elimination process that makes the stored neutral position approach the actual neutral position while the driver maintains the operation of the operation member for keeping the vehicle straight.

[0007] In the steering system of the present invention, since the above-mentioned neutral position deviation elimination process is executed while the driver is keeping the vehicle straight, the process is relatively simple. As a result, the steering system of the present invention has high practicality.

[0008] Although the "operation component" in the present invention is generally a steering wheel, it can be various operation components such as a handle, a joystick, a rocker, etc. The steering system of the present invention is a steer-by-wire system. Generally, the "steering actuator" and the operation component are not mechanically connected. Therefore, the steering actuator has a "power source (which can also be called a 'drive source')" such as an electric motor, a hydraulic cylinder, etc., and does not rely on the force applied by the driver to the operation component (hereinafter sometimes referred to as "operation force"), but steers the wheels by the force of the power source. The structure of the steering actuator is not particularly limited. For example, in a vehicle that steers a pair of left and right wheels, it can have a steering rod that connects a pair of steering knuckles that respectively hold these wheels, and moves the steering rod left and right. In a steering system with a steering actuator having such a structure, for example, between the end of the steering rod and the steering knuckle, there is a mechanism for adjusting the camber angle of the wheels, and the camber angle is adjusted by this mechanism. If such a camber angle adjustment is performed, the actual neutral steering position is likely to change. Therefore, it is more meaningful to perform the above-mentioned neutral position offset elimination process after the camber angle adjustment.

[0009] The "steering position" in a narrow sense refers to the posture of the steered wheels (such as "rotation position", "camber angle", etc.), but in the steering system of the present invention, it is interpreted in a broad sense and still refers to the action position of the steering actuator. Therefore, the "neutral steering position" refers to the posture of the wheels that should be taken when the vehicle is going straight, or the action position of the steering actuator. Specifically, for example, the camber angle of the steered wheels in the state where the vehicle is going straight (in the case of a pair of steered wheels, the camber angles of these wheels respectively), the action position of the steering rod in the steering actuator with the above structure, etc. correspond to the neutral steering position. Regarding the operation component, the position of the operation component (for example, the rotation angle of the steering wheel in the case where the operation component is a steering wheel) is also called the "operation position", and the operation position when the vehicle is desired to go straight is called the "neutral operation position".

[0010] In a steer-by-wire system, the wheels are steered by the operation of the driver's operation component. That is, the control of the steering of the wheels (hereinafter sometimes referred to as "steering control") is performed based on the operation of the driver's operation component. In this steering control, for example, when the operation component is in the neutral operation position, the steering position becomes the neutral steering position, and when the operation component is operated by a certain operation amount from the neutral operation position, the steering position is changed from the neutral steering position by a steering amount corresponding to this operation amount. This steering control is executed by the "controller". By the way, the controller is mainly a computer, for example, and includes a driver (drive circuit) of the steering actuator.

[0011] The controller performs the above-mentioned steering control based on the neutral steering position stored in itself, that is, the stored neutral position. Therefore, in a state of "neutral position offset" where the stored neutral position is offset from the actual neutral position which is the actual neutral steering position, even if the driver places the operating member at the neutral operating position, the vehicle will not go straight, that is, it will be in a state of deflecting to one side or the other.

[0012] The above-mentioned "neutral position offset elimination process" in the present invention is performed when a neutral position offset occurs, in a state where the driver maintains an operation to make the vehicle go straight, in other words, in a state where the driver changes the operating member from the neutral operating position in order to make the vehicle go straight. That is, in the neutral position offset elimination process, the controller changes (deviates) the stored neutral position in the direction of the vehicle going straight on the condition that the driver is performing an operation to maintain this state. That is, the stored neutral position is made closer to the actual neutral position. According to this neutral position offset elimination process, although it is necessary for the driver to perform an operation to make the vehicle go straight, the neutral steering position can be adjusted simply.

[0013] The neutral position offset elimination process can also be a process such that the stored neutral position instantaneously approaches the actual neutral position. However, a sharp change in the neutral steering position will give the driver a sense of discomfort, and in addition, it may also hinder the driving stability of the vehicle. Therefore, in this steering system, it is preferable to perform the neutral position offset elimination process such that the stored neutral position gradually approaches the actual neutral position. In other words, it is preferable to perform the change little by little in stages, that is, to change the neutral steering position continuously over a relatively long time.

[0014] It is preferable to end the neutral position offset elimination process at the moment when the stored neutral position coincides with the actual neutral position, or when it can be regarded as coinciding. The determination of whether the stored neutral position coincides with the actual neutral position, or whether it can be regarded as coinciding can be, for example: when the amount of neutral position offset can be grasped in advance, by whether the stored neutral position can be changed by that amount. Specifically, when the difference between the stored neutral position and the actual neutral position is less than a set threshold value, it is sufficient to end the neutral position offset elimination process. In addition, it can also be performed by the method described below.

[0015] During the process of the neutral position offset elimination process, the driver needs to keep the operating position of the operating member at the straight-ahead position of the vehicle. Therefore, when the neutral position offset is eliminated, the operating position of the operating member naturally lies at the neutral operating position. This point can also be utilized to end the neutral position offset elimination process on the condition that the operating position of the operating member is at the neutral operating position. This condition can be that the operating position is approximately at the neutral operating position. Additionally, in a steer-by-wire system, since the steered wheels and the operating member are not mechanically connected, a reaction force applying device that applies a reaction force against the operation of the operating member, i.e., an operating reaction force, is usually provided. The controller also controls this reaction force applying device. Specifically, for example, the reaction force applying device is controlled in such a way that an operating reaction force having a component for returning the operating member to the neutral operating position is applied to the operating member. In the case where such control is being performed, when the above-mentioned neutral position offset occurs, in order to keep the vehicle going straight, the driver needs to move the operating member to a position changed from the neutral operating position by an operating force that counteracts the operating reaction force. However, in the state where such an operating reaction force is applied, as the above-mentioned neutral position offset elimination process progresses, the operating reaction force gradually becomes smaller. This point can also be utilized to end the neutral position offset elimination process when the operating reaction force disappears, or when the operating force that counteracts the operating reaction force disappears. Specifically, it can also be ended when the operating reaction force has almost disappeared, or when the operating force that counteracts the operating reaction force has almost disappeared, by storing that the neutral position and the actual neutral position become or can be regarded as being the same. According to these methods, even without knowing the amount of the neutral position offset, the neutral position offset elimination process can be appropriately ended.

[0016] In many vehicles, various driving assistance controls are executed. Specifically, there are controls for making the vehicle travel along a target travel line (target travel line following control), controls for preventing the vehicle from deviating from the travel lane (lane departure prevention control), controls for making the vehicle turn stably along the travel line when turning (vehicle stabilization control), controls for avoiding collisions with obstacles ahead (collision avoidance control), etc. These driving assistance controls are all controls accompanied by automatic steering of the wheels, that is, controls accompanied by steering that does not depend on the operation of the driver's operating member. When the above-mentioned neutral position offset occurs, it is reasonable not to execute such controls. However, it is considered that even if a slight neutral position offset occurs and the accuracy of the control itself deteriorates slightly, it is still desired to boldly execute safety-oriented controls such as lane departure prevention control, vehicle stabilization control, and collision avoidance control. Therefore, in the steering system of the present invention, it is desired to allow at least one of the multiple driving assistance controls even during the execution of the neutral position offset elimination process.

[0017] More specifically, multiple driving assistance controls are classified into two categories. One category is safety - oriented assistance controls that prioritize the safety of vehicle travel despite the low accuracy of the neutral steering position, including lane departure prevention control, vehicle stabilization control, and collision avoidance control. The other category is high - precision assistance controls that require accurate grasping of the neutral steering position, which is target travel line following control. In view of this, it is desirable to allow at least the execution of safety - oriented controls even during the process of performing neutral position offset elimination processing. Description of the Drawings

[0018] Figure 1 It is a diagram showing the overall structure of the steering system of the embodiment.

[0019] Figure 2 It is a flowchart of the steering control program executed in the steering system of the embodiment.

[0020] Figure 3 It is a flowchart of the reaction force control program executed in the steering system of the embodiment.

[0021] Figure 4 It is a schematic diagram for explaining the limiter for the operating range of a specified steering actuator and the adjustment of the wheel camber angle.

[0022] Figure 5 It is a flowchart of the neutral position offset elimination processing program executed in the steering system of the embodiment.

[0023] Figure 6 It is a flowchart of the IG - off program and the IG - on program executed for the neutral position offset elimination processing in the steering system of the embodiment.

[0024] Figure 7 It is a diagram showing a specific example of the neutral position offset elimination processing.

[0025] Figure 8 It is a diagram showing another specific example of the neutral position offset elimination processing.

[0026] Description of the Reference Numerals:

[0027] 10 … Handle [operating component]; 12 … Reaction force actuator [reaction force applying device]; 14 … Wheel; 16 … Steering actuator; 24 … Reaction force motor [power source]; 32 … Steering lever [movable component]; 34 … Tie rod; 38 … Steering motor [power source]; 40 … Reaction force electronic control unit (reaction force ECU) [controller]; 42 … Steering electronic control unit (steering ECU) [controller]; 44 … Dedicated high-speed communication line; 46 … CAN; 56 … Steering angle sensor; 60 … Pinion shaft; 66 … Driving assistance electronic control unit (driving assistance ECU); 74R, 74L … Locking ring; L … Tie rod length; 80 … Operation terminal; 82 … Position offset elimination process start switch; 84 … Yaw rate sensor; ω … Steering angle; ω0 … Neutral steering angle [neutral steering position]; ω0m … Stored neutral steering angle [stored neutral position]; ω0mi … Initial stored neutral steering angle; ω0r … Actual neutral steering angle [actual neutral position]; dω0 … Offset angle of neutral steering angle ω0; Δω … Steering compensation angle; Δωm … Stored steering compensation angle; Δωt … Steering compensation angle for transformation; dΔω … Set transformation angle; |Δωt - Δωm| … Change angle; Δωs … Threshold angle; P … Determination parameter; Pt … Threshold value; v … Vehicle speed; γ … Yaw rate; Fm … Processing flag; Fa … Post-processing flag. Detailed implementation mode

[0028] Hereinafter, as a mode for implementing the present invention, a steering system as an embodiment of the present invention will be described in detail with reference to the drawings. In addition, for the present invention, in addition to the following embodiments, it can be implemented in various modes in which various changes and improvements have been made based on the knowledge of those skilled in the art starting from the modes described in the above [Summary of the Invention] section.

[0029]

Embodiment

[0030] [1] Overall structure of the steering system

[0031] The steering system of the embodiment (hereinafter sometimes referred to as "this steering system" or "this system") is a steer-by-wire type steering system that can steer the wheels without relying on the operating force of the driver applied to the operating component. As Figure 1 shown, it includes a reaction force actuator 12 to which a handle 10 as an operating component is connected, and a steering actuator 16 that connects two left and right wheels 14 to steer them together. Incidentally, the handle 10 is a handle in a deformed shape of a steering wheel and is rotationally operated by the driver. The reaction force actuator 12 is configured to receive the rotational operation of the driver and apply a reaction force (hereinafter sometimes referred to as "operation reaction force") to the operation of the handle 10, specifically, to the operation of the handle 10.

[0032] The reaction force actuator 12 includes a steering column 20 supported by a reinforcement of the instrument panel, a steering shaft 22 rotatably held by the steering column 20, and a reaction force motor 24 as an electric motor for applying a rotational torque to the steering shaft 22 via a power transmission mechanism. A handle 10 is attached to the rear end of the steering shaft 22. Although the detailed structure of the power transmission mechanism is not described, it includes a worm attached to the motor shaft of the reaction force motor 24 and a worm wheel attached to the steering shaft 22 and meshing with the worm. The reaction force motor 24 is a three-phase brushless DC motor and functions as a power source for the reaction force actuator 12. By the torque generated by the reaction force motor 24, a reaction force torque as an operating reaction force is applied to the handle 10 connected to the steering shaft 22. In addition, the steering shaft 22 functions as a movable part of the reaction force actuator 12, and the reaction force actuator 12 functions as an operating reaction force applying device.

[0033] The steering actuator 16 includes a substantially cylindrical housing 30 supported by the chassis in a posture extending left and right, a steering rod 32 held by the housing 30 so as not to rotate and capable of moving left and right, and a pair of tie rods 34 respectively connected to the left and right ends of the steering rod 32 via ball joints. The front end of each tie rod 34 is connected to the wheel 14 via a ball joint. Specifically, each tie rod 34 is connected to a knuckle arm of a knuckle that rotatably and steerably holds the wheel 14 on a suspension arm via a ball joint.

[0034] A threaded groove 36 is formed in the steering rod 32, and a nut that holds bearing balls and engages with the threaded groove 36 is held in the housing 30 so as to be rotatable and not movable left and right, and illustration thereof is omitted. That is, a ball screw mechanism is constituted by the steering rod 32 and the nut. A steering motor 38 as an electric motor is attached to the housing 30, and the steering motor 38 rotates the nut via a power transmission mechanism. Incidentally, although illustration of the power transmission mechanism is omitted, it includes a pulley attached to the motor shaft of the steering motor 38 and a timing belt wound around the outer circumferences of the pulley and the nut. The steering motor 38 is a three-phase brushless DC motor and functions as a power source for the steering actuator 16. By rotating the steering motor 38, the steering rod 32 moves left and right, driving the left and right wheels 14 to turn together. In addition, the steering rod 32 functions as a movable part of the steering actuator 16.

[0035] The control of the reaction force actuator 12 is performed by a reaction force electronic control unit (hereinafter sometimes referred to as "reaction force ECU") 40 attached to the reaction force motor 24. The reaction force ECU 40 includes a computer composed of a CPU, ROM, RAM, etc., and an inverter as a driver (drive circuit) of the reaction force motor 24, and is powered by a battery. Similarly, the control of the steering actuator 16 is performed by a steering electronic control unit (hereinafter sometimes referred to as "steering ECU") 42 attached to the steering motor 38. The steering ECU 42 includes a computer composed of a CPU, ROM, RAM, etc., and an inverter as a driver (drive circuit) of the steering motor 38, and is powered by a battery.

[0036] The reaction force ECU 40 and the steering ECU 42 coordinate with each other, and they constitute one controller of the steering system. Therefore, the reaction force ECU 40 and the steering ECU 42 are connected by a dedicated high-speed communication line 44. By the way, they are also connected to the CAN (Car Area Network or Controllable Area Network) 46 equipped in the vehicle.

[0037] Associated with the control, the reaction force actuator 12 has an operation torque sensor 50 that detects the operation torque To, which is the operation force applied by the driver to the steering wheel 10, by detecting the amount of torsion of the steering shaft 22. In addition, there are an operation angle sensor 52 that detects the operation angle δ of the steering wheel 10 by detecting the rotation angle of the steering shaft 22, and a reaction force motor rotation angle sensor 54 that detects the rotation angle (rotation phase) φc of the reaction force motor 24 for the purpose of switching the energized phase, etc. A detailed description of the structure is omitted here.

[0038] In addition, since there is a definite relationship between the steering angle ω of the wheel 14 and the lateral movement position of the steering rod 32, the steering actuator 16 has a steering angle sensor 56 that detects the movement position of the steering rod 32 in order to detect the steering angle ω of the wheel 14. Briefly described, a rack 58 is formed on the steering rod 32, and a pinion shaft 60 meshing with the rack 58 is held by the housing 30. The steering angle sensor 56 detects the lateral movement amount of the steering rod 32 by detecting the rotation angle of the pinion shaft 60, and further detects the steering angle ω of the wheel 14. By the way, the movement position of the steering rod 32 is the operating position of the steering actuator 16, that is, the steering position of the wheel 14. In addition, the steering actuator 16 has a steering motor rotation angle sensor 62 that detects the rotation angle (rotation phase) φs of the steering motor 38 for the purpose of switching the energized phase, etc.

[0039] [2] Steering system control

[0040] In this steering system, the steering ECU 42 performs steering control for steering the wheels 14, and the reaction force ECU 40 performs reaction force control for applying reaction force torque as an operation reaction force to the handle 10. The steering control and reaction force control will be described below.

[0041] (a) Steering control

[0042] The steering control is to make the steering actuator 16 turn the wheel 14 to a steering angle ω corresponding to the operating angle δ, and the computer of the steering ECU 42 repeatedly executes the control at short time intervals (e.g., several meters to tens of milliseconds). Figure 2 The steering control program shown in the flowchart is performed. Hereinafter, the steering control is explained along this program. In addition, for easy understanding, the operation angle δ is sometimes simply referred to as "the operation angle δ of the handle 10" below.

[0043] In the process according to the steering control program, first, in step 1 (hereinafter referred to as "S1". The other steps are the same), the steering ECU 42 obtains the operating angle δ of the handle 10 from the reaction force ECU 40 via the dedicated high-speed communication line 44. Here, the operating angle δ is explained. The operating angle δ as the operating position of the operating member can be grasped in the form of the operating position of the reaction force actuator 12, that is, the rotation angle position of the steering shaft 22 of the movable member. The operating position of the reaction force actuator 12 is defined based on the neutral operating position, that is, the operating position that the vehicle should be in when it is in a straight state. The reaction force ECU 40 determines the displacement angle of the steering shaft 22 from the neutral operating position, that is, the neutral operating angle δ0, as the operating angle δ based on the motor rotation angle φc of the reaction force motor 24, and sends a signal for the operating angle δ to the steering ECU 42 via the dedicated high-speed communication line 44. The details will be described later. The steering ECU 42 obtains the operating angle δ based on the signal. In addition, the neutral operating angle δ0 is stored in the reaction force ECU 40.

[0044] In the following S2, the steering ECU 42 determines the steering angle ω, i.e., the target steering angle ω*, by which the wheel 14 should be steered, based on the obtained operation angle δ. Here, if the steering angle ω is described, similar to the operation angle δ, the steering angle ω as the steering position can be grasped in the form of the operation position of the steering actuator 16, i.e., the moving position of the steering rod 32 as the movable part. The operation position of the steering actuator 16 is defined based on the neutral steering position, i.e., the operation position when the vehicle is in a straight-ahead state. The steering ECU 42 detects the movement amount of the steering rod 32 from the neutral position, i.e., the displacement angle of the pinion shaft 60 from the neutral angular position, based on the detection of the steering angle sensor 56, and regards it as the steering angle ω. Incidentally, the neutral steering position can be called the neutral steering angle ω0, and hereinafter, it is sometimes called the neutral steering angle ω0. In addition, the neutral steering angle ω0 is stored in the steering ECU 42, and this stored neutral position is conceptual, and hereinafter, it is sometimes called the "stored neutral position" or "stored neutral steering angle ω0m", and the details will be described later.

[0045] Since the operation angle δ and the steering angle ω are in a relationship that satisfies the set steering gear ratio Ns, in S2, the target steering angle ω* is determined according to the following formula.

[0046] ω* = Ns·δ

[0047] In addition, the steering gear ratio Ns can be set to be fixed or can be set to vary according to, for example, the vehicle speed.

[0048] Since the steering angle ω and the steering motor rotation angle φs are in a relationship of a certain gear ratio Nms, in view of detection accuracy, etc., the steering control is performed based on the steering motor rotation angle φs. Therefore, in S3, the steering ECU 42 determines the target steering motor rotation angle φs* that becomes the control target of the steering motor rotation angle φs according to the following formula.

[0049] φs* = Nms·ω*

[0050] In addition, the steering angle sensor 56 can detect the absolute angle, and even if the ignition switch is temporarily turned off, it can detect the steering angle ω based on the neutral steering position (stored neutral position) at the moment when it is turned on. In this steering system, every time the ignition switch is turned on, the steering ECU 42 calibrates the steering motor rotation angle φs detected by the steering motor rotation angle sensor 62 based on the steering angle ω detected by the steering angle sensor 56. Incidentally, the steering motor rotation angle sensor 62 only detects the rotation phase of 360°, i.e., one turn, and the steering ECU 42 detects the steering motor rotation angle φs exceeding 360° through cumulative processing.

[0051] Next, in S4, the steering ECU 42 detects the steering motor rotation angle φs at the current moment through the steering motor rotation angle sensor 62. In S5, the deviation of the steering motor rotation angle φs at the current moment with respect to the target steering motor rotation angle φs*, that is, the steering motor rotation angle deviation Δφs, is determined according to the following formula.

[0052] Δφs = φs* - φs

[0053] In the subsequent S6, based on the steering motor rotation angle deviation Δφs, the steering ECU 42 determines the torque that the steering motor 38 should generate, that is, the steering torque Ts that should be generated by the steering actuator 16, according to the PID feedback control rule, specifically according to the following formula.

[0054] Ts = Gsp·Δφs + Gsi·∫(Δφs)dt + Gsd·d(Δφs) / dt

[0055] Incidentally, Gsp, Gsi, and Gsd are the proportional term gain, integral term gain, and derivative term gain, respectively.

[0056] Since the steering torque Ts is approximately in a proportional relationship with the supply current supplied to the steering motor 38 as the power source, that is, the steering current Is, in S7, the steering ECU 42 determines the steering current Is based on the steering torque Ts, uses the current determination coefficient Ks, and according to the following formula.

[0057] Is = Ks·Ts

[0058] In S8, the determined steering current Is is supplied from the frequency converter to the steering motor 38. Then, in S9, in order to enable the reaction force control related to the steering current Is to be utilized as described later, the steering ECU 42 sends the signal related to the steering current Is to the reaction force ECU 40 via the dedicated high-speed communication line 44, and one execution of this steering control program ends.

[0059] (b) Reaction force control

[0060] Reaction force control is a control that applies a reaction force torque, which is a reaction force against the operation of the handle 10, to the handle 10 by the reaction force actuator 12 for the purpose of giving the driver an appropriate steering operation feeling and returning the handle 10 to the neutral position. The reaction force control is performed by the computer of the reaction force ECU 40 repeatedly executing the reaction force control program shown in the flowchart at short time intervals (for example, several m to several tens of msec). Figure 3 The following is an explanation of the reaction force control according to this program.

[0061] In the process of controlling according to the reaction force control program, first, in S11, the reaction force ECU 40 detects the motor rotation angle φc of the reaction force motor 24 through the reaction force motor rotation angle sensor 54. Since the operation angle δ and the reaction force motor rotation angle φc are in a relationship of a fixed transmission ratio Nmc, the reaction force control is performed based on the reaction force motor rotation angle φc in view of detection accuracy and the like. Therefore, in S12, the reaction force ECU 40 determines the operation angle δ based on the reaction force motor rotation angle φc according to the following formula.

[0062] δ = Nmc · φc

[0063] In addition, the operation angle sensor 52 can detect the absolute angle, and even if the ignition switch is temporarily turned off, it can detect the operation angle δ at the moment when it becomes on. In this steering system, every time the ignition switch becomes on, the reaction force ECU 40 calibrates the reaction force motor rotation angle φc detected by the reaction force motor rotation angle sensor 54 based on the operation angle δ detected by the operation angle sensor 52. By the way, the reaction force motor rotation angle sensor 54 only detects the rotation phase of 360°, that is, one turn, and the reaction force ECU 40 detects the reaction force motor rotation angle φc exceeding 360° through cumulative processing. In S13, the reaction force ECU 40 sends the signal for the determined operation angle δ to the steering ECU 42 via the dedicated high-speed communication line 44 for use in the steering control described above.

[0064] The reaction force torque Tc is composed of two components. One of them is the neutral position restoration component Tcc for returning the operation position of the reaction force actuator 12, that is, the handle 10, to the neutral operation position, and the other is the steering load basis component Tcs for simulating the load of the steering actuator 16. In S14, the reaction force ECU 40 uses the neutral position restoration component determination coefficient Cc to determine the neutral position restoration component Tcc according to the following formula.

[0065] Tcc = Cc · δ

[0066] According to the above formula, the neutral position restoration component Tcc is determined such that the larger the operation angle δ, the larger the reaction force torque Tc for returning the handle 10 to the neutral position.

[0067] It can be considered that the load of the steering actuator 16 is approximately proportional to the current supplied to the steering motor 38, that is, the above-mentioned steering current Is. In the following S15, the reaction force ECU 40 obtains the steering current Is based on the above signal sent from the steering ECU 42. Then, in S16, the reaction force ECU 40 uses the steering load basis component determination coefficient Cs to determine the steering load basis component Tcs according to the following formula.

[0068] Tcs = Cs·Is

[0069] Based on the above formula, the steering load dependent component Tcs is determined in such a way that the greater the steering current Is, that is, the greater the load on the steering actuator 16, the greater the reaction force torque Tc is applied.

[0070] Based on the neutral position restoring component Tcc and the steering load dependent component Tcs determined as described above, in S17, the reaction force ECU 40 determines the reaction force torque Tc according to the following formula.

[0071] Tc = Tcc + Tcs

[0072] Since the reaction force torque Tc is approximately in a proportional relationship with the supply current supplied to the reaction force motor 24 as a power source, that is, the reaction force current Ic, in S18, the reaction force ECU 40 determines the reaction force current Ic based on the reaction force torque Tc, using the reaction force current determination coefficient Kc, according to the following formula.

[0073] Ic = Kc·Tc

[0074] In S19, the determined reaction force current Ic is supplied from the frequency converter to the reaction force motor 24, and one execution of this reaction force control program ends.

[0075] (c) Driving assistance control

[0076] In addition to this system, the vehicle equipped with this steering system is also equipped with a driving assistance system that executes a plurality of driving assistance controls to assist the driver in driving the vehicle. The plurality of driving assistance controls are target travel line following control (LTA), lane departure prevention control (LDA), vehicle stabilization control (VSC), and collision avoidance control (PCS). They are all controls accompanied by automatic steering of the wheels 14, that is, controls accompanied by steering of the wheels 14 that do not depend on the operation of the driver's steering wheel 10.

[0077] If each of the driving assistance controls is simply described, the target driving line following control is, for example, a control for causing a vehicle to travel along a target driving line set at the center of a driving lane when the vehicle is traveling on a highway. According to this control, when the vehicle deviates from the target driving line during the driver's steering of the wheel 14, the steering angle ω of the wheel 14 is adjusted. The control for preventing departure from the driving lane is a control for preventing the vehicle from deviating from the driving lane. According to this control, when the vehicle deviates from its own driving lane to either the left or the right, a warning is issued to the driver, and in order to prevent the vehicle from exceeding the driving lane, the wheel 14 is forced to turn. This is a control that is executed only when the vehicle is traveling at a speed above a certain level (e.g., 50 km / h or more). The vehicle stabilization control is a control for stabilizing the travel or posture of the vehicle when the vehicle is turning. According to this control, when understeer or oversteer is felt beyond a limit, in order to suppress this understeer or oversteer, in addition to operations of the brakes, accelerator, etc., the wheel 14 is also turned. The collision avoidance control is a control for preventing the vehicle from colliding with an obstacle ahead. According to this control, when there is a high possibility that the vehicle will collide with an obstacle ahead even if it brakes, the collision is avoided by turning the wheel 14. Regardless of whether it is driving on a general road or the driving speed of the vehicle, the vehicle stabilization control and the collision avoidance control can be executed.

[0078] Each of the above-described driving assistance controls is executed by a driving assistance electronic control unit (hereinafter sometimes referred to as "driving assistance ECU"). As Figure 1 shown, the driving assistance ECU 66 (denoted as "DA-ECU" in the figure) is connected to the CAN 46. In the figure, although shown as one unit, in reality, multiple electronic control units corresponding to multiple driving assistance controls are mounted. The driving assistance ECU 66 sends a steering command to the steering ECU 42 via the CAN 46. In more detail, by intervening in the above-described steering control, the wheel 14 is turned. Specifically, a command for the target steering angle ω* is sent from the driving assistance ECU 66, and the steering ECU 42 executes the steering control based on the sent target steering angle ω* instead of the target steering angle ω* determined as described above based on the operation angle δ.

[0079] Although detailed description is omitted, for each driving assistance control, the driving assistance ECU 66 stores an independent neutral steering position, that is, a neutral steering angle ω0, for each of the above controls. These neutral steering angles ω0 are obtained through learning. In each driving assistance control, the required control precision varies, and the learning precision, specifically the precision of the neutral steering angle ω0 obtained through learning, also varies. Specifically, the target travel line following control requires the highest precision, and the required precision decreases in the order of the lane departure prevention control, the vehicle stabilization control, and the collision avoidance control. On the contrary, considering the safety of vehicle travel, the necessity of executing the control is set to be inversely proportional to the precision. The necessity of the collision avoidance control is the highest, and the necessity decreases in the order of the vehicle stabilization control, the lane departure prevention control, and the target travel line following control. In addition, for a control that requires a higher precision, generally a longer learning time for the neutral steering angle ω0 is required.

[0080] In view of the above, in a vehicle equipped with this steering system, the above driving assistance controls are classified into two categories. One category is the safety assistance control that emphasizes safety, including the lane departure prevention control, the vehicle stabilization control, and the collision avoidance control. The other category is the high-precision assistance control that requires high precision, which is the target travel line following control. In these two controls, the processing regarding the neutral position offset elimination process is different, and the details will be described later.

[0081] [3] Neutral Steering Position and Its Setting

[0082] The steering actuator 16 has an operating range for the steering actions of the left and right wheels, and the center of this operating range, that is, the exact middle position, is set as the mechanical neutral position. Figure 4 Figure (a) schematically shows a part of each of the right side and the left side of the steering actuator 16. As shown in this figure, in the steering actuator 16, and on the part of the steering rod 32, which is a movable part, extending from the housing 30, locking rings 74R and 74L are attached. If the steering rod 32 is moved to the right by a certain extent, as shown by the double-dot chain line, the locking ring 74L abuts against the left end of the housing 30, thereby prohibiting the steering rod 32 from continuing to move to the right. This state is a state where the operating position of the steering rod 32 becomes one end of the operating range of the steering actuator 16, that is, the right end. On the other hand, if the steering rod 32 is moved to the left by a certain extent, as shown by the double-dot chain line, the locking ring 74R abuts against the right end of the housing 30, thereby prohibiting the steering rod 32 from continuing to move to the left. This state is a state where the operating position of the steering rod 32 becomes the other end of the operating range of the steering actuator 16, that is, the left end. In this way, one end and the other end of the operating range are defined. The locking rings 74R and 74L are located at Figure 4The state of the position indicated by the solid line in (a) is a state where the operating position of the steering actuator 16 becomes the center of the above-mentioned operating range, that is, a state where the operating position of the steering actuator 16 becomes the mechanically neutral steering position. This neutral steering position is considered to be the operating position that the steering actuator 16 should maintain when the vehicle is going straight. Incidentally, the right end and the left end of the housing 30 abutted by the snap rings 74R and 74L function as a pair of limiters for locking the snap rings 74R and 74L.

[0083] After the vehicle is manufactured, that is, after the steering actuator 16 is mounted on the vehicle body, before leaving the factory and within the factory, the toe angle of the wheel 14 is adjusted. Regarding the toe angle of the wheel 14, briefly speaking, as schematically shown in Figure 4 (b), when the vehicle is going straight, it is preferably approximately 0°. The adjustment of the toe angle of the wheel 14 is performed by adjusting the length of the tie rod 34, that is, the tie rod length L, while maintaining the operating position of the steering actuator 16 in the above-mentioned mechanically neutral steering position. The specific adjustment method can be in accordance with the usual method, and the description is omitted here.

[0084] After adjusting the above-mentioned toe angle, the steering ECU 42 performs a process of storing the detected value of the steering angle sensor 56 at this time as the neutral steering position, that is, the neutral steering angle ω0. Through this process, the neutral steering angle ω0 at this time is set as the initial stored neutral position, that is, the initial stored neutral steering angle ω0mi. As described above, the stored neutral steering angle ω0m as the stored neutral position is conceptual and may be changed after leaving the factory. In fact, the angle obtained by adding the stored steering compensation angle Δω, that is, the stored steering compensation angle Δωm, to the initial stored neutral steering angle ω0mi becomes the neutral steering angle ω0 to perform the steering control. Therefore, the steering ECU 42 stores the initial stored neutral steering angle ω0mi and the stored steering compensation angle Δωm, and the change of the stored neutral steering angle ω0m, that is, the neutral steering angle ω0 for steering control, is performed by changing the stored steering compensation angle Δωm. Incidentally, soon after the toe angle adjustment is performed after the vehicle is manufactured, the stored steering compensation angle Δωm is set to 0°. In addition, in the following description, sometimes the angle obtained by adding the initial stored neutral steering angle ω0mi and the stored steering compensation angle Δωm is processed as the stored neutral steering angle ω0m.

[0085] Regarding the neutral operating position of the steering wheel 10 as an operating member, the same setting as the neutral steering position is also performed when the steering wheel 10 is mounted on the steering shaft 22, but the description is omitted here.

[0086] [4] Neutral position offset and processing for eliminating neutral position offset

[0087] (a) Offset of the neutral steering position

[0088] For example, due to certain circumstances, the caster angle of the steered wheel 14 sometimes needs to be readjusted at the dealership (dealer) where the vehicle is sold. Additionally, for example, even if it does not reach the level that requires readjustment at the dealership, due to certain reasons (such as the collision of the wheel 14 with a curb, the wheel 14 getting stuck in a ditch, aging, etc.), it is expected that the caster angle will change slightly. As described above, the steering control is performed with the stored neutral steering angle ω0m as the neutral steering angle ω0. However, when the caster angle of the wheel 14 is readjusted or when the caster angle changes, it will cause a change in the actual neutral steering position (which can also be simply referred to as the "actual neutral position"), that is, the actual neutral steering angle ω0r of the actual neutral steering angle ω0 when the vehicle is going straight. In other words, it causes the stored neutral steering angle ω0m set based on the mechanical neutral operation position of the steering actuator 16 as described above to deviate from the actual neutral steering angle ω0r. That is, a neutral position deviation occurs.

[0089] As described above, although the steering control is performed with the stored neutral steering angle ω0m as the neutral steering angle ω0, if the above-mentioned neutral position deviation occurs, even if the driver maintains the steering wheel 10 in the neutral operation position, the vehicle will deviate to one side or the other. That is, it is expected that the situation where the proper steering of the wheel 14 cannot be performed will occur. Conversely, when the driver operates the steering wheel 10 to make the vehicle go straight, the steering wheel 10 has to be maintained at a position deviated from the neutral operation position, and the reaction force actuator 12 generates a reaction force torque Tc corresponding to the operation angle δ at this time according to the neutral position restoring component Tcc. In this case, the driver needs to resist this reaction force torque Tc to maintain the steering wheel 10, thus feeling a sense of discomfort in the steering wheel operation.

[0090] (b) Outline of neutral position deviation elimination process

[0091] In order to eliminate the above-mentioned neutral position deviation, in this steering system, in the state where the vehicle is traveling, specifically, in the state where the above-mentioned steering control and reaction force control are being executed, a neutral position deviation elimination process is performed. Regarding this neutral position deviation elimination process, for example, it may be performed as the operation of the adjuster at the dealership after adjusting the caster angle, or it may also be performed by the user of the vehicle. In the case of an operator's operation, the neutral position deviation elimination process is as Figure 1As shown, it is performed by connecting the operation terminal 80 to the vehicle. The operation terminal 80 includes a display, a keyboard, etc., and is connected to the steering ECU 42 via the CAN 46 by connecting to a connector provided in the vehicle. The operation terminal 80 can access various information related to the steering system such as the stored steering compensation angle Δωm stored in the steering ECU 42 at the current moment. In the neutral position offset elimination process using the operation terminal 80, the adjuster can more accurately eliminate the neutral position offset while observing the change in the stored neutral steering angle ω0m. Details thereof are omitted. On the other hand, in the case of an operation by the user of the vehicle, the neutral position offset elimination process is performed using the position offset elimination process start switch 82 provided on the instrument panel.

[0092] In short, the neutral position offset elimination process is a process of making the stored neutral steering position, that is, the stored neutral steering angle ω0m, close to the actual neutral steering angle ω0r. In other words, it is a process of changing the stored steering compensation angle Δωm so that the stored neutral steering angle ω0m is close to the actual neutral steering angle ω0r. The neutral position offset elimination process is performed in a state where the above-described steering control and reaction force control are being executed, and in a state where the adjuster and the user (hereinafter collectively referred to as "driver") maintain the operation of the handle 10 such that the vehicle goes straight. When a neutral position offset occurs, when the vehicle goes straight, as described above, the operation angle δ of the handle 10 becomes a state offset from the neutral operation angle δ0, and a reaction force torque Tc corresponding to the offset is generated. Therefore, the neutral position offset elimination process is performed in a state where the driver applies an operation torque To against the reaction force torque Tc to the handle 10.

[0093] The neutral position offset elimination process is started by the driver's operation of the operation terminal 80 or the position offset elimination process start switch 82. At the start of the neutral position offset elimination process, first, the steering ECU 42 determines whether the execution conditions are satisfied. The execution conditions are that the vehicle speed v as the traveling speed of the vehicle is within the set speed range and the vehicle is going straight. The vehicle speed v is obtained via the CAN 46 from a brake electronic control unit (brake ECU) not shown in the figure, and the straight running of the vehicle is determined based on the yaw rate γ detected by a yaw rate sensor 84 provided in the vehicle. When the execution conditions are satisfied, the steering ECU 42 executes the initial process. In this initial process, the steering compensation angle Δω for changing the neutral steering angle ω0, that is, the conversion steering compensation angle Δωt, is set to the stored steering compensation angle Δωm stored at the current moment. Moreover, instead of the stored neutral steering angle ω0m, the angle obtained by adding the conversion steering compensation angle Δωt to the initial stored neutral steering angle ω0mi is used as the neutral steering angle ω0, and the steering control is executed based on the neutral steering angle ω0.

[0094] Next, the steering ECU 42 determines the neutral steering angle ω0, that is, in which direction the stored neutral steering angle ω0m at the current moment is offset from the actual neutral steering angle ω0r, to the left or right. That is, the steering ECU 42 determines in which direction the stored steering compensation angle Δωm changes from the value at the current moment, to the left or right, that is, determines the change direction of the neutral steering angle ω0. The determination of this change direction can also be performed, for example, based on the detection value of the steering angle sensor 56. Specifically, at the current moment, the steering angle ω is the actual neutral steering angle ω0r or approximately the actual neutral steering angle ω0r. Therefore, by comparing the stored neutral steering angle ω0m with the detection value of the steering angle ω detected by the steering angle sensor 56 at the current moment, the offset amount of the neutral steering position, that is, the offset angle dω0 of the neutral steering angle ω0, can be grasped, and the change direction can be determined. In addition, the determination of the change direction can also be performed, for example, based on the detection value of the operation angle δ of the handle 10 detected by the operation angle sensor 52, the detection value of the operation torque To detected by the operation torque sensor 50, the value of the generated reaction force torque Tc, and the like.

[0095] Based on the change direction determined as described above, the steering ECU 42 gradually changes the neutral steering angle ω0 toward the actual neutral steering angle ω0r on the premise of satisfying the above execution conditions. Specifically, the steering compensation angle Δωt for change is changed by the set change angle dΔω, and the angle obtained by adding the changed steering compensation angle Δωt for change to the initial stored neutral steering angle ω0mi is used as the neutral steering angle ω0, and the steering control is continued. The change of the steering compensation angle Δωt for change is repeatedly executed until the following change end condition (hereinafter sometimes simply referred to as the "end condition") is satisfied.

[0096] The end condition is that the value of the determination parameter P becomes a value that can be regarded as approximately 0. For the determination parameter P, the offset angle dω0 of the neutral steering angle ω0, the detection value of the operation angle δ of the handle 10 detected by the operation angle sensor 52, the detection value of the operation torque To detected by the operation torque sensor 50, the value of the generated reaction force torque Tc, and the like, which were described in connection with the determination of the change direction, can be adopted. Regarding the steering of the wheel 14 and the operation of the handle 10, the values corresponding to left turn among various numerical parameters such as the steering angle ω and the operation angle δ are set as +, and the values corresponding to right turn are set as − for processing, and the determination parameter P is also processed in the same way. If processed in this way, the following formula can be the end condition. That is:

[0097] |P| < Pt

[0098] In addition, the threshold value Pt is set to a value that can be regarded as the degree at which the neutral position offset has been eliminated.

[0099] Specifically, when the determination parameter P is the offset angle dω0, as the transformation steering compensation angle Δωt changes, the absolute value |dω0| of this offset angle decreases, and when it is less than the threshold dω0t, it can be considered that the neutral position offset has been eliminated. In addition, when the detection value of the operation angle δ of the handle 10, the detection value of the operation torque To detected by the operation torque sensor 50, and the absolute values |δ|, |To|, and |Tc| of the value of the generated reaction force torque Tc are all less than the thresholds δt, Tot, and Tct respectively set to be close to 0 during the execution of the above reaction force control, it can be considered that the neutral position offset has been eliminated. That is, in short, when the handle 10 is in the position where it should be when the vehicle is going straight, that is, the neutral operation position, when the operation force applied by the driver to the handle 10 disappears, or when the operation reaction force applied to the handle 10 disappears, it can be considered that the neutral position offset has been eliminated. By the way, according to the latter, there is an advantage that the offset of the neutral steering angle ω0 can be estimated without detecting the actual steering angle ω.

[0100] In short, when the determination parameter P satisfies the above end condition, the steering ECU 42 ends the change of the transformation steering compensation angle Δωt. In fact, when the above end condition is satisfied, the transformation steering compensation angle Δωt is changed again in the transformation direction. When |P| is not greater than the absolute value (|Pp|) of the previous value Pp due to this change, the change of the transformation steering compensation angle Δωt is directly ended. When |P| is greater than |Pp|, it returns to the transformation steering compensation angle Δωt when this change was not made, and ends the change of the transformation steering compensation angle Δωt. According to such a change of the transformation steering compensation angle Δωt, for example, it is possible to allow the transformation steering compensation angle Δωt to change to the opposite side across the actual neutral steering angle ω0r, and set the neutral steering angle ω0 to a value closer to the actual neutral steering angle ω0r.

[0101] Considering that the neutral position offset elimination process in this steering system is carried out as part of the operation of the above adjuster, it is associated with the operation of the ignition switch (hereinafter sometimes referred to as "IG") of the vehicle. Specifically, considering that the adjuster confirms whether the above end condition is satisfied through the operation terminal 80 before disconnecting the IG, when the IG is disconnected, the steering ECU 42 determines that the neutral position offset elimination process has ended. In detail, the difference between the transformation steering compensation angle Δωt and the stored steering compensation angle Δωm stored at the current moment is determined as the change angle (|Δωt - Δωm|). When this change angle (|Δωt - Δωm|) is greater than the threshold angle Δωs, that is, when the stored neutral steering angle ω0m is changed relatively greatly, the transformation steering compensation angle Δωt at that moment is stored as the stored steering compensation angle Δωm. That is, the stored neutral steering angle ω0m is updated, and then the steering control using the updated stored neutral steering angle ω0m as the neutral steering angle ω0 is executed. On the other hand, when the change angle (|Δωt - Δωm|) is less than or equal to the threshold angle Δωs, that is, when there is almost no difference between the current transformation steering compensation angle Δωt and the stored steering compensation angle Δωm, the update of the stored neutral steering angle ω0m is not performed.

[0102] In addition, considering that when the driver is the adjuster, the adjuster confirms the elimination of the neutral position offset. When the driver turns on the IG, the steering ECU 42 maintains the determination that the neutral position offset elimination process has ended for a set time (hereinafter sometimes referred to as "end determination maintenance time ts"). This end determination maintenance time ts can be set to, for example, the time required for the adjuster to confirm, that is, about 30 seconds to 1 minute. In addition, the description of the process related to the operation of the IG will be described in detail later. In addition, as described above, when the change angle (|Δωt - Δωm|) is less than or equal to the threshold angle Δωs, that is, when the update of the stored neutral steering angle ω0 is not performed, the determination that the neutral position offset elimination process has ended is not made.

[0103] Next, the relationship between the above driving assistance control and the neutral position offset elimination process will be described. As described above, each driving assistance control is based on the neutral steering angle ω0 stored by independent learning. In the target driving line following control (LTA) that requires high-precision assistance control, at the moment when the neutral position offset elimination process starts, the neutral steering angle ω0 independently stored in this control is reset, and the execution of this control itself is prohibited. Then, after the determination that the neutral position offset elimination process has ended is maintained for the end determination maintenance time ts, the learning of the neutral steering angle ω0 starts. After obtaining a highly accurate neutral steering angle ω0 through learning, the execution of this control starts again.

[0104] On the other hand, in the lane departure prevention control (LDA), vehicle stabilization control (VSC), and pre-crash system (PCS) that respectively emphasize safety assist control, even if the neutral position offset elimination process has started, the stored neutral steering angle ω0 is not reset, and the control itself is permitted. However, as described above, when the change angle (|Δωt - Δωm|) is greater than the threshold angle Δωs, that is, when the stored neutral steering angle ω0m is significantly changed, even in the case of safety assist control, when the ignition switch (IG) is turned on, the neutral steering angle ω0 independently stored in this control is reset and the execution of this control is prohibited. After obtaining a highly accurate neutral steering angle ω0 through learning, the execution of this control is started again. In addition, for safety assist control, it is also possible to not prohibit the execution of this control regardless of the change angle (|Δωt - Δωm|). Further, the learning of the neutral steering angle ω0 can start from the moment when the neutral position offset elimination process ends.

[0105] (c) Flow of the neutral position offset elimination process

[0106] The above-mentioned neutral position offset elimination process is executed by the steering ECU 42 by Figure 5 executing the neutral position offset elimination process program shown in the flowchart. This program starts when the driver operates the operation terminal 80 or the position offset elimination process start switch 82 while the vehicle is going straight. Hereinafter, the flow of the neutral position offset elimination process will be briefly described with reference to this flowchart.

[0107] In the process according to the above program, first, in S21, it is determined whether the execution conditions are satisfied. As described above, the execution conditions are that the vehicle is traveling within a certain speed range and the vehicle is going straight. When these execution conditions are satisfied, in S22, as an initial process, the steering compensation angle for transformation Δωt is set to the stored steering compensation angle Δωm, and the neutral steering angle ω0 in the steering control is set to the value obtained by adding the initial stored neutral steering angle ω0mi and the steering compensation angle for transformation Δωt.

[0108] Next, in S23, as described above, for example, the direction of the neutral position offset is determined by determining whether the stored neutral steering angle ω0m is offset to the left or right from the actual neutral steering angle ω0r. Then, in S24, the process flag Fm is set to "ON". Briefly, the process flag Fm is a flag indicating that the neutral position offset elimination process is in progress. It is a flag that is "ON" when the process is being executed and "OFF" when the process is not being executed. Next, in S25, an instruction for prohibiting the execution of highly accurate assist control (e.g., target travel line following control) and resetting the value of the neutral steering angle ω0 obtained through learning in this control is sent to the driving assist ECU 66.

[0109] Next, in S26, it is determined whether the above-mentioned execution conditions are satisfied. When the execution conditions are satisfied, in S27, the steering compensation angle Δωt for transformation is changed by a set transformation angle dΔω in the direction opposite to the direction of deviation from the above-mentioned neutral position. This change is repeated until it is determined in S28 that the end condition is satisfied. The determination of the end condition in S28 is made by checking whether the absolute value of the above-mentioned determination parameter P is less than the threshold value Pt. As described above, the determination parameter P is the deviation angle dω0 of the neutral steering angle ω0, the operation angle δ of the handle 10, the operation torque To of the handle 10, the reaction force torque Tc, etc.

[0110] When the above-mentioned end condition is satisfied, in S29, the execution conditions are further determined. When the execution conditions are satisfied, in S30, the steering compensation angle Δωt for transformation is further changed by the set transformation angle dΔω once again. The result of this change is confirmed in S31. Specifically, the absolute value of the determination parameter P is compared with the previous determination parameter P (previous value Pp). When the absolute value of the determination parameter P is greater than the absolute value of the previous value Pp, in S32, the steering compensation angle Δωt for transformation is changed in the opposite direction to the set transformation angle dΔω. That is, the steering compensation angle Δωt for transformation returns to the value before the last change. On the other hand, when the absolute value of the determination parameter P is not greater than the absolute value of the previous value Pp, the steering compensation angle Δωt for transformation does not return.

[0111] Although the processing according to the neutral position deviation elimination processing program is as described above, as explained in the previous text, the neutral position deviation elimination processing is related to the operation of the IG. Therefore, in this steering system, when the IG is turned off and when the IG is turned on, the steering ECU 42 respectively executes the IG - off program and the IG - on program shown in the Figure 6 flowchart. Hereinafter, the flow of the processing according to these programs will be briefly described.

[0112] In the process of following the IG-off program, first, in S41, it is confirmed to what extent the transformation steering compensation angle Δωt has finally changed from the stored steering compensation angle Δωm, that is, the change angle (|Δωt - Δωm|) is confirmed. When this change angle (|Δωt - Δωm|) is greater than the threshold angle Δωs, in S42, the stored steering compensation angle Δωm is set as the transformation steering compensation angle Δωt. In the following S43, an instruction to also prohibit the safety-oriented auxiliary control (such as preventing lane departure control, vehicle stabilization control, collision avoidance control) and reset the neutral steering angle ω0 that has been obtained through learning in this control is sent to the driving assistance ECU66. Then, in S44, the processed flag Fa is "turned on". The processed flag Fa is a flag used to recognize the neutral position offset elimination process, specifically, a flag that indicates that the gradual change from the stored neutral position to the actual neutral position has actually ended, and is a flag that is "turned on" when it is determined that this process has ended.

[0113] In S45, the neutral steering angle ω0 is set to the angle obtained by adding the stored steering compensation angle Δωm to the initial stored neutral steering angle ω0mi. Thereafter, the steering control is executed based on the neutral steering angle ω0 set in this way. Incidentally, in S41, when it is determined that the change angle (|Δωt - Δωm|) is below the threshold angle Δωs, the stored steering compensation angle Δωm is not changed, and the steering control is executed while maintaining the original neutral steering angle ω0. Additionally, in this case, the processed flag Fa is not "turned on".

[0114] In the process of following the IG-on program, first, in S51, the in-process flag Fm is reset to "off". In the following S52, an instruction is sent to the driving assistance ECU66 to start the learning of the neutral steering angle ω0 in the prohibited driving assistance control. When both the safety-oriented auxiliary control is prohibited and the neutral steering angle ω0 in this control is reset, based on this instruction, the learning of this neutral steering angle ω0 also starts. On the condition that an appropriate neutral steering angle ω0 has been obtained through learning, the driving assistance control is started again. Then, in S53, it is determined whether the processed flag Fa is "turned on". When it is "turned on", in S54, the time counter t is reset to 0, and in S55, the time counter t is incremented by Δt. This incrementing is performed until the time counter t exceeds the end recognition maintenance time ts in S56. After passing the end recognition maintenance time ts, in S57, the processed flag Fa is reset to "off".

[0115] (d) Specific example of neutral position offset elimination process

[0116] Hereinafter, with reference to Figure 7 、 Figure 8The figure illustrates a specific example of neutral position offset elimination processing.

[0117] Figure 7 The figure shows a case where the neutral steering angle ω0 is changed relatively greatly during the neutral position offset elimination process. Before performing the neutral position offset elimination process, the offset angle dω0 of the neutral steering angle ω0 is 1.6°. That is, the neutral steering angle ω0 is offset 1.6° in the left turn direction. At time t0, the neutral position offset elimination process starts. The stored steering compensation angle Δωm at this time is 0.0°. At the moment when the neutral position offset elimination process has started, the conversion steering compensation angle Δωt is set to 0.0°. In addition, at this moment, the processing flag Fm is set to "ON", and the request for high-precision auxiliary control is prohibited. The neutral steering angle ω0 (0.3°) learned in this control becomes the reset value at this moment.

[0118] At times t1, t2, and t3, the conversion steering compensation angle Δωt is repeatedly changed towards the right turn direction by 0.5° each time as the set conversion angle dΔω. That is, the conversion steering compensation angle Δωt is sequentially changed from 0.0° to -0.5°, -1.0°, -1.5°. As a result, the offset angle dω0 of the neutral steering angle ω0 gradually decreases from 1.6° to 1.1°, 0.6°, 0.1°. By changing the conversion steering compensation angle Δωt at time t3, the above determination parameter P (for example, the offset angle dω0 of the neutral steering angle ω0) becomes smaller than the threshold Pt (for example, the offset angle 0.2°), thus ending the change of the conversion steering compensation angle Δωt.

[0119] After ending the change of the conversion steering compensation angle Δωt, the IG is turned off. In this neutral position offset elimination process, since the difference between the conversion steering compensation angle Δωt and the stored steering compensation angle Δωm, that is, the change angle (|Δωt - Δωm|) is finally greater than the threshold angle Δωs (for example, 0.4°), at the moment when the IG is turned off, the stored steering compensation angle Δωm is set to the conversion steering compensation angle Δωt at this time, that is, -1.5°, and the post-processing flag Fa is set to "ON". In addition, from this moment on, the safety-oriented auxiliary control is also prohibited from being executed. At this moment, the neutral steering angle ω0 (0.3°) learned in this control becomes the reset value.

[0120] Next, when the IG is turned on, the processing flag Fm is reset to "OFF", and the learning of the neutral steering angle ω0 in the prohibited driving assistance control (high-precision auxiliary control request and safety-oriented control) starts. After obtaining an appropriate neutral steering angle ω0 (0.1°) through this learning, the learning is ended, and the prohibited driving assistance control is allowed to be executed. In addition, at the moment when the end determination estimation time ts has elapsed since the IG was turned on, the post-processing flag Fa is reset to "OFF".

[0121] Figure 8 The graph shows the following situation. That is, although the neutral position offset elimination process is performed, since the above-mentioned change angle (|Δωt - Δωm|) is below the threshold angle Δωs (e.g., 0.4°), the actual neutral steering angle ω0 is not changed. Different from Figure 7 the situation, the set transformation angle dΔω is set to 0.3°.

[0122] In Figure 8 the situation, before performing the neutral position offset elimination process, the offset angle dω0 of the neutral steering angle ω0 is 0.4°. That is, the neutral steering angle ω0 is offset by 0.4° in the left turn direction. At time t0, the neutral position offset elimination process starts. The stored steering compensation angle Δωm at this moment is 0.0°, and at the moment when the neutral position offset elimination process has started, the transformation steering compensation angle Δωt is set to 0.0°. In addition, at this moment, the process flag Fm is set to "ON", and the high-precision auxiliary control request is prohibited. The neutral steering angle ω0 (0.3°) learned in this control becomes the reset value at this moment.

[0123] At time t1, the transformation steering compensation angle Δωt is changed by 0.3° (which is the set transformation angle dΔω) in the right turn direction. By this change, the above-mentioned determination parameter P (e.g., the offset angle dω0 of the neutral steering angle ω0) becomes smaller than the threshold value Pt (e.g., the offset angle 0.2°). According to the above procedure, at time t2, the transformation steering compensation angle Δωt is further changed by 0.3° (which is the set transformation angle dΔω) in the right turn direction again. As a result, the absolute value of the determination parameter P (-0.2° as the offset angle dω0) is greater than the previous value Pp (0.1°), and at time t3, the transformation steering compensation angle Δωt is returned to the value (-0.3) before this change.

[0124] After changing the above-mentioned transformation steering compensation angle Δωt, IG is turned off. However, in this neutral position offset elimination process, since the change angle (0.3 as |Δωt - Δωm|) is finally below the threshold angle Δωs (e.g., 0.4°), even if IG is turned off, the stored steering compensation angle Δωm is not changed, and the post-process flag Fa is not set to "ON". Also, the execution of the safety-oriented auxiliary control is not prohibited, and the neutral steering angle ω0 (0.3°) learned in this control is maintained.

[0125] Next, when IG is turned on, the process flag Fm is reset to "OFF", and the learning of the neutral steering angle ω0 in the prohibited driving assistance control (high-precision auxiliary control request) starts. After obtaining an appropriate neutral steering angle ω0 (0.1°) through this learning, this learning is ended, and the execution of the prohibited driving assistance control is permitted.

Claims

1. A steering system is a steer-by-wire type steering system mounted on a vehicle, wherein, the steering system includes: an operation member that is operated by a driver; a steering actuator that has a power source and steers wheels by the force of the power source; and a controller that controls the steering actuator to achieve steering of the wheels corresponding to the operation of the operation member, wherein, the controller is configured to: when a stored neutral steering position stored for control of the steering actuator deviates from an actual neutral steering position which is the actual neutral steering position, perform a neutral position deviation elimination process of making the stored neutral position approach the actual neutral position while the driver maintains the operation of the operation member for straight-ahead running of the vehicle.

2. The steering system according to claim 1, wherein, the neutral position deviation elimination process is a process of gradually changing the stored neutral position toward the actual neutral position.

3. The steering system according to claim 1, wherein, the controller is configured to end the neutral position deviation elimination process on the condition that the operation position of the operation member is at a position where it should be when the vehicle is running straight, that is, the neutral operation position.

4. The steering system according to claim 1, wherein, the steering system includes an operation reaction force applying device that applies a reaction force to the operation of the operation member, that is, an operation reaction force, to the operation member, the controller is configured to: perform control for applying an operation reaction force, wherein the operation reaction force includes a component for returning the operation position of the operation member to a position where it should be when the vehicle is running straight, that is, the neutral operation position, and end the neutral position deviation elimination process on the condition that the operation reaction force disappears or the driver's operating force against the operation reaction force disappears.

5. The steering system according to claim 1, wherein, in the vehicle, a plurality of driving assistance controls accompanied by automatic steering of the wheels are performed, the steering system is configured to allow at least one of the plurality of driving assistance controls even during the execution of the neutral position deviation elimination process.

6. The steering system according to claim 5, wherein, the plurality of driving assistance controls include a safety - priority auxiliary control aimed at giving priority to the safety of the vehicle running over the accuracy of the neutral steering position, the steering system is configured to allow at least the safety - priority auxiliary control even during the execution of the neutral position deviation elimination process.

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