Steering system

Abstract

To provide a steer-by-wire system having high practicality.SOLUTION: The steer-by-wire system is configured such that, when a stored neutral position (ω0mi + Δωm), which is a neutral steering position stored for controlling steering actuators, is shifted from an actual neutral position, which is an actual neutral steering position, a neutral position shift elimination process for bringing the stored neutral position closer to the actual neutral position (S27) is executed in a state (S26) in which an operation of an operation member for causing a vehicle to travel straight is maintained by a driver. Since the neutral position shift elimination process is executed in a state where the driver is driving the vehicle straight, the process is relatively simple.SELECTED DRAWING: Figure 5

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 a "steer-by-wire system") mounted on a vehicle, generally, an operating member such as a steering wheel and a steered wheel are not mechanically connected. Therefore, regardless of the force (hereinafter sometimes referred to as "operating force") applied to the operating member by the driver, the wheel is steered by the force of the power source of the steering actuator. In the steer-by-wire system, since the steering of the wheel is mainly performed in response to the operation of the operating member, the relationship between the operation position of the operating member and the steering position of the wheel is important in the control of the steering of the wheel (hereinafter sometimes referred to as "steering control"). Specifically, when the operation position of the operating member where the vehicle should be located when going straight is defined as the neutral operation position, when the operating member is located at the neutral operation position, the attitude of the steered wheel should also be the attitude when going straight. That is, the steering position of the wheel 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 the stored neutral operation position and neutral steering position. However, when the toe angle of the steered wheel is adjusted, or when a relatively large force is externally applied to the steered wheel, etc., the stored neutral position, which is the stored neutral steering position, may deviate from the actual neutral steering position. When this deviation (hereinafter sometimes referred to as "neutral position deviation") occurs, it is expected that the appropriate steering of the steering wheel may not be performed in response to the operation of the operation member by the driver, or that the driver may feel discomfort with the operation. Therefore, in a steer-by-wire system, for example, the following processing described in the following patent documents is being considered. The processing is a technique of setting the neutral steering position based on the difference between the target steering position based on the target travel line and the steering position detected by a sensor in the target travel line following control for steering the wheels so that the vehicle travels along the target travel line.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] The technology described in the above patent document relates to target driving line following control, which is one of the driving assistance controls, and a certain degree of high accuracy is required for the setting of the neutral steering position. On the other hand, there are also driving assistance controls for steering the steered wheels for safety, such as vehicle stabilization control and lane departure prevention control. These controls are desired to be executed even before executing the target driving line following control to obtain a highly accurate neutral steering position, that is, even in a state with a certain deviation of the neutral position. In addition, the setting of the neutral steering position in the target driving line following control is based on complex methods, learning, etc., and instead of such a setting, it is also desired to simply eliminate the neutral position deviation. That is, it is considered that the practicality of the steer-by-wire system is improved if the neutral position deviation can be simply eliminated. The present invention has been made in view of such circumstances, and an object thereof is to provide a highly practical steer-by-wire type steering system for a vehicle.

Means for Solving the Problems

[0006] In order to solve the above problems, the steering system of the present invention an operating member operated by a driver, a steering actuator having a power source and steering the wheels by the force of the power source, a controller for controlling the steering actuator to realize the steering of the wheels according to the operation of the operating member and is a steer-by-wire type steering system mounted on a vehicle, when the stored neutral position, which is the neutral steering position stored by the controller for controlling the steering actuator, deviates from the actual neutral position, in a state where the operation of the operating member for the driver to drive the vehicle straight is maintained, it is configured to execute a neutral position deviation elimination process for bringing the stored neutral position closer to the actual neutral position.

Effects of the Invention

[0007] In the steering system of the present invention, since the neutral position deviation elimination process is executed while the driver is driving the vehicle straight ahead, the process is relatively simple. As a result, the steering system of the present invention is highly practical. Aspect of the invention

[0008] In the present invention, the "operation member" is generally a steering wheel, but it may be various things such as a handle, a joystick, or a control lever. The steering system of the present invention is a steer-by-wire system. Generally, the "steering actuator" is not mechanically connected to the operation member. Therefore, the steering actuator has a "power source (which can also be called a "drive source")" such as an electric motor or a hydraulic cylinder, and steers the wheels by the force of the power source regardless of the force (hereinafter sometimes referred to as "operating force") applied by the driver to the operation member. The structure of the steering actuator is not particularly limited. For example, in a vehicle in which a pair of left and right wheels are steered, it may have a steering rod that connects a pair of steering knuckles that hold those wheels respectively, and has a structure that moves the steering rod left and right. In a steering system equipped with a steering actuator having such a structure, for example, a mechanism for adjusting the toe angle of the wheels is provided between the end of the steering rod and the steering knuckle, and the toe angle is adjusted by that mechanism. If such toe angle adjustment is performed, there is a high possibility that the actual neutral steering position will change. Therefore, it is very meaningful to execute the neutral position deviation elimination process after the toe angle adjustment.

[0009] The "steering position" means, in a narrow sense, the attitude of the steered wheels (e.g., "rotation position", "toe angle", etc.). However, in the steering system of the present invention, it is interpreted in a broad sense and also means the operating position of the steering actuator. Therefore, the "neutral steering position" means the attitude that the wheels should take when the vehicle is going straight, or the operating position of the steering actuator. Specifically, for example, the toe angle of the steered wheels in a state where the vehicle is going straight (when there is a pair of steered wheels, the toe angle of each of those wheels), and the operating position of the steering rod in the steering actuator having the above structure, etc. correspond to the neutral steering position. Similarly, regarding the operating member, the position of the operating member (e.g., the rotation angle of the steering wheel when the operating member is the steering wheel) is called the "operating position", and the operating position when trying to make the vehicle go straight is called the "neutral operating position".

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

[0011] The controller executes the above steering control based on the stored neutral position, which is the neutral steering position stored by itself. Therefore, in a state of "neutral position deviation" where the stored neutral position deviates from the actual neutral position, which is the actual neutral steering position, even if the driver positions the operating member at the neutral operating position, the vehicle will not go straight, that is, it will deviate to either the left or the right.

[0012] The above-mentioned "neutral position deviation elimination process" in the present invention is carried out when a neutral position deviation occurs, while the driver maintains the operation of driving the vehicle straight ahead. In other words, it is carried out in a state where the driver has shifted the operation member from the neutral operation position in order to drive the vehicle straight ahead. That is to say, in the neutral position deviation elimination process, the controller shifts (offsets) the stored neutral position in the direction in which the vehicle travels straight ahead on the condition that the driver is performing an operation to maintain that state. That is to say, it makes the stored neutral position approach the actual neutral position. According to this neutral position deviation elimination process, although an operation for the driver to drive the vehicle straight ahead is required, it is possible to simply adjust the neutral steering position.

[0013] The neutral position deviation elimination process may be a process that instantaneously makes the stored neutral position approach the actual neutral position. However, a sudden change in the neutral steering position may give the driver a sense of discomfort and may also hinder the driving stability of the vehicle. Therefore, in this steering system, it is desirable to perform a neutral position deviation elimination process that gradually changes the stored neutral position toward the actual neutral position. In other words, it is desirable to shift it step by step little by little, that is, to shift it so that the neutral steering position changes continuously over a certain period of time.

[0014] It is desirable to end the neutral position deviation elimination process when the stored neutral position coincides with the actual neutral position, or when it can be regarded as coinciding. The determination as to whether the stored neutral position coincides with the actual neutral position, or whether it can be regarded as coinciding, can be made, for example, by whether the stored neutral position has been shifted by that amount when the amount of neutral position deviation can be grasped in advance. Specifically speaking, when the difference between the stored neutral position and the actual neutral position becomes smaller than the set threshold value, the neutral position deviation elimination process may be ended. It may also be carried out by the method described below.

[0015] During the execution of the neutral position deviation elimination process, the driver attempts to keep the operating position of the operating member at the position where the vehicle goes straight. Therefore, when the neutral position deviation is eliminated, the operating position of the operating member will naturally be at the neutral operating position. Taking advantage of this, the neutral position deviation elimination process may be terminated on the condition that the operating position of the operating member is at the neutral operating position. This condition may be that the operating position is generally at the neutral operating position. Also, in a steer-by-wire system, since the steered wheels and the operating member are not mechanically connected, generally, a reaction force applying device that applies an operating reaction force, which is a reaction force against the operation of the operating member, is provided to the operating member. The controller also controls the reaction force applying device. More specifically, for example, the reaction force applying device is controlled to apply an operating reaction force having a component for returning the operating member to the neutral operating position to the operating member. When such control is being performed, when the above neutral position deviation occurs, the driver needs to position the operating member at a position shifted from the neutral operating position by an operating force against the operating reaction force in order to make the vehicle go straight. However, in a state where such an operating reaction force is applied, as the neutral position deviation elimination process progresses, the operating reaction force gradually decreases. Taking advantage of this, the neutral position deviation elimination process may be terminated when the operating reaction force disappears or when the operating force against the operating reaction force disappears. More specifically, when the operating reaction force has almost disappeared or when the operating force against the operating reaction force has almost disappeared, if the stored neutral position is considered to have coincided with the actual neutral position, the neutral position deviation elimination process may be terminated. According to these methods, it is possible to appropriately terminate the neutral position deviation elimination process without knowing the amount of the neutral position deviation.

[0016] In many vehicles, various driving assistance controls are executed. Specifically, controls for driving the vehicle along a target driving line (target driving line following control), controls for preventing the vehicle from deviating from the driving lane (driving lane departure prevention control), controls for stably turning the vehicle along the driving line during turning (vehicle stabilization control), controls for avoiding collisions with forward obstacles (collision avoidance control), and the like. All of these driving assistance controls involve automatic steering of the wheels, that is, steering that does not depend on the operation of the driver's operating member. Such control is preferably not executed when the above neutral position deviation occurs. However, controls that prioritize safety, such as driving lane departure prevention control, vehicle stabilization control, and collision avoidance control, are preferably executed even if there is a slight neutral position deviation and the accuracy of the control itself is somewhat poor. Therefore, in the steering system of the present invention, it is desirable to allow at least one of a plurality of driving assistance controls to be permitted even during the execution of the neutral position deviation elimination process.

[0017] More specifically, the plurality of driving assistance controls are classified into two types. One of them is safety-priority support control aimed at prioritizing the safety of vehicle driving even if the accuracy of the neutral steering position is low, and includes driving lane departure prevention control, vehicle stabilization control, and collision avoidance control. The other is high-precision requirement support control that requires accurate grasping of the steering neutral position, which is target driving line following control. In view of this, it is desirable to allow at least the execution of safety-priority control even during the execution of the neutral position deviation elimination process.

Brief Description of the Drawings

[0018]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Embodiment for Carrying Out the Invention

[0019] Hereinafter, as an embodiment for carrying out the present invention, a steering system which is an embodiment of the present invention will be described in detail with reference to the drawings. It should be noted that the present invention can be implemented in various forms with various modifications and improvements based on the knowledge of those skilled in the art, in addition to the following embodiments and the forms described in the section of 〔Aspects of the Invention〕.

Embodiment

[0020] [1] Overall Configuration of Steering System 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 regardless of the driver's operating force applied to the operating member. As shown in FIG. 1, it includes a reaction force actuator 12 to which the steering wheel 10, which is the operating member, is connected, and a steering actuator 16 to which the two left and right wheels 14 are connected to steer them together. Incidentally, the steering wheel 10 has a shape obtained by deforming the steering wheel, is rotated by the driver, and the reaction force actuator 12 receives the operation and applies a reaction force to the steering wheel 10, specifically, to the operation of the steering wheel 10 (hereinafter sometimes referred to as "operation reaction force").

[0021] The reaction force actuator 12 includes a steering column 20 supported by the reinforcement of the instrument panel, a steering shaft 22 rotatably held by the steering column 20, and a reaction force motor 24 which is an electric motor for applying rotational torque to the steering shaft 22 via a power transmission mechanism. The steering wheel 10 is attached to the rear end portion of the steering shaft 22. The power transmission mechanism includes, although the detailed structure description is omitted, 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 the power source of the reaction force actuator 12. Due to the torque generated by the reaction force motor 24, a reaction force torque as an operation reaction force is applied to the steering wheel 10 connected to the steering shaft 22. The steering shaft 22 functions as a movable member of the reaction force actuator 12, and the reaction force actuator 12 functions as an operation reaction force applying device.

[0022] The steering actuator 16 includes a generally cylindrical housing 30 supported by the chassis in a left-right extending posture, a steering rod 32 held non-rotatably and movable left and right in the housing 30, and a pair of tie rods 34 respectively connected to both left and right ends of the steering rod 32 via ball joints. The tip of each tie rod 34 is connected to the wheel 14 via a ball joint. Specifically, each tie rod 34 is connected via a ball joint to a knuckle arm of a steering knuckle that rotatably holds the wheel 14 and is turnably held by a suspension arm.

[0023] A screw groove 36 is formed in the steering rod 32. Although not shown, a nut that holds bearing balls and engages with the screw groove 36 is held non-movably left and right and rotatably in the housing 30. That is, a ball screw mechanism is constituted by the steering rod 32 and the nut. An electric motor, the steering motor 38, is attached to the housing 30, and the steering motor 38 rotates the nut via a power transmission mechanism. Incidentally, although not shown, the power transmission mechanism includes a pulley attached to the motor shaft of the steering motor 38 and a timing belt wound around the pulley and the outer periphery of 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 is moved left and right, and the left and right wheels 14 are steered together. Note that the steering rod 32 functions as a movable member of the steering actuator 16.

[0024] 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 which is 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 which is a driver (drive circuit) of the steering motor 38, and is powered by a battery.

[0025] The reaction force ECU 40 and the steering ECU 42 cooperate 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. Incidentally, they are both also connected to a CAN (car area network or controllable area network) 46 provided in the vehicle.

[0026] Regarding control, although the detailed structure description of the reaction force actuator 12 is omitted, it has an operation torque sensor 50 that detects the operation torque To as the operation force applied by the driver to the steering wheel 10 by detecting the torsional amount of the steering shaft 22. It also has 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 for detecting the rotation angle (rotation phase) φc of the reaction force motor 24 for the purpose of switching the energized phase, etc.

[0027] In addition, since there is a specific 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 explained, a rack 58 is formed on the steering rod 32, and a pinion shaft 60 that meshes with the rack 58 is held by the housing 30. The steering angle sensor 56 detects the left and right movement amounts of the steering rod 32 by detecting the rotation angle of the pinion shaft 60, and detects the steering angle ω of the wheel 14. Incidentally, 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. Further, the steering actuator 16 has a steering motor rotation angle sensor 62 for detecting the rotation angle (rotation phase) φs of the steering motor 38 for the purpose of switching the energization phase and the like.

[0028] [2] Control of Steering System In this steering system, the steering ECU 42 executes steering control for steering the wheel 14, and the reaction force ECU 40 executes reaction force control for applying a reaction force torque as an operating reaction force to the steering wheel 10. The steering control and the reaction force control will be described below.

[0029] (a) Steering Control The steering control is a control for steering the wheel 14 to a steering angle ω corresponding to the operation angle δ to the steering actuator 16, and is performed by the computer of the steering ECU 42 repeatedly executing a steering control program whose flowchart is shown in FIG. 2 at a short time pitch (for example, several m to several tens of msec). The steering control will be described below according to that program. For ease of understanding, hereinafter, for convenience, the operation angle δ may sometimes be referred to as the "operation angle δ of the steering wheel 10".

[0030] In the process according to the steering control program, first, in Step 1 (hereinafter abbreviated as "S1". The same applies to other steps), the steering ECU 42 acquires the operation angle δ of the steering wheel 10 from the reaction force ECU 40 via the dedicated high-speed communication line 44. Here, regarding the operation angle δ, the operation angle δ as the operation position of the operation member is grasped as the operation position of the reaction force actuator 12, that is, the rotation angle position of the steering shaft 22 which is a movable member. The operation position of the reaction force actuator 12 is defined based on the neutral operation position, that is, the operation position where the vehicle should be located when the vehicle is in a straight-ahead state. Although it will be described in detail later, the reaction force ECU 40 specifies the displacement angle from the neutral operation angle δ0 which is the neutral operation position of the steering shaft 22 as the operation angle δ based on the motor rotation angle φc of the reaction force motor 24, and transmits a signal about the operation angle δ to the steering ECU 42 via the dedicated high-speed communication line 44. The steering ECU 42 acquires the operation angle δ based on the signal. Note that the neutral operation angle δ0 is stored in the reaction force ECU 40.

[0031] In the subsequent S2, the steering ECU 42 determines the target steering angle ω which is the steering angle ω by which the wheels 14 should be steered, based on the acquired operation angle δ. * Here, regarding the steering angle ω, similar to the operation angle δ, the steering angle ω as the steering position is grasped as the operation position of the steering actuator 16, that is, the moving position of the steering rod 32 which is a movable member. The operation position of the steering actuator 16 is defined based on the neutral steering position, that is, the operation position where the vehicle should be located when the vehicle is in a straight-ahead state. The steering ECU 42 detects the amount of movement of the steering rod 32 from the neutral position, that is, the displacement angle from the neutral angle position of the pinion shaft 60 as the steering angle ω based on the detection of the steering angle sensor 56. Incidentally, the neutral steering position can be called the neutral steering angle ω0, and hereinafter, it may be called the neutral steering angle ω0. Although it will be described in detail later, the neutral steering angle ω0 is stored in the steering ECU 42, and the stored neutral steering position is a conceptual one, and hereinafter, it may be called the "stored neutral position" or the "stored neutral steering angle ω0m".

[0032] 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. ω * = Ns·δ Note that the steering gear ratio Ns may be set fixedly, or may be set to change according to, for example, the vehicle speed or the like.

[0033] Since the steering angle ω and the steering motor rotation angle φs are in a relationship of a specific gear ratio Nms, the steering control is performed based on the steering motor rotation angle φs in view of detection accuracy and the like. Therefore, in S3, the steering ECU 42 determines the target steering motor rotation angle φs that is the control target of the steering motor rotation angle φs according to the following formula. * to determine. φs * = Nms·ω * Note that the steering angle sensor 56 is capable of detecting the absolute angle, and even if the ignition switch is once turned off, when it is turned on, the steering angle ω based on the neutral steering position (memory neutral position) can be detected. In this steering system, the steering ECU 42 performs calibration of 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 each time the ignition switch is turned on. Incidentally, the steering motor rotation angle sensor 62 only detects the rotation phase for 360°, that is, one rotation, but the steering ECU 42 is configured to detect the steering motor rotation angle φs exceeding 360° by integration processing.

[0034] Next, in S4, the steering ECU 42 detects the current steering motor rotation angle φs by the steering motor rotation angle sensor 62, and in S5, the steering motor rotation angle deviation Δφs, which is the deviation of the current steering motor rotation angle φs with respect to the target steering motor rotation angle φs, is specified according to the following formula. * to identify. Δφs = φs * - φs

[0035] In subsequent S6, the steering ECU 42 determines the torque that the steering motor 38 should generate, that is, the steering torque Ts that the steering actuator 16 should generate, based on the steering motor rotation angle deviation Δφs and in accordance with the PID feedback control law, specifically, in accordance with the following equation. Ts = Gsp·Δφs + Gsi·∫(Δφs)dt + Gsd·d(Δφs) / dt Incidentally, Gsp, Gsi, and Gsd are the proportional term gain, integral term gain, and derivative term gain, respectively.

[0036] Since the steering torque Ts and the steering current Is, which is the supply current to the steering motor 38 as a power source, are generally in a proportional relationship, in S7, the steering ECU 42 determines the steering current Is based on the steering torque Ts using the current determination coefficient Ks and in accordance with the following equation. Is = Ks·Ts

[0037] The determined steering current Is is supplied from the inverter to the steering motor 38 in S8. Then, in S9, the steering ECU 42 transmits a signal related to the steering current Is to the reaction force ECU 40 via the dedicated high-speed communication line 44 for use in the reaction force control described later, and one execution of the steering control program ends.

[0038] (b) Reaction force control Reaction force control is control that causes the reaction force actuator 12 to apply a reaction force torque as a reaction force to the operation of the steering wheel 10 to the steering wheel 10 for the purpose of giving the driver an appropriate steering operation feeling and returning the steering wheel 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 as a flowchart in FIG. 3 at a short time pitch (for example, several m to several tens of msec). The reaction force control will be described below according to the program.

[0039] In the process according to the reaction force control program, first, in S11, the reaction force ECU 42 detects the motor rotation angle φc of the reaction force motor 24 by 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 specific gear ratio Nmc, the reaction force control is performed based on the reaction force motor rotation angle φs in view of detection accuracy and the like. Therefore, in S12, the reaction force ECU 40 specifies the operation angle δ based on the reaction force motor rotation angle φc according to the following equation. δ = Nmc·φc Note that the operation angle sensor 52 can detect the absolute angle, and can detect the operation angle δ at the time when the ignition switch is turned ON even if the ignition switch is once turned OFF. In this steering system, 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 each time the ignition switch is turned ON. Incidentally, the reaction force motor rotation angle sensor 54 only detects the rotation phase for 360°, that is, one rotation, but the reaction force ECU 40 is configured to detect the reaction force motor rotation angle φc exceeding 360° by integration processing. In S13, the reaction force ECU 40 transmits a signal for the specified operation angle δ to the steering ECU 42 via the dedicated high-speed communication line 44 for use in the steering control described above.

[0040] The reaction force torque Tc consists of two components. One is the neutral position return component Tcc for returning the operating position of the reaction force actuator 12, that is, the steering wheel 10, to the neutral operating position, and the other is the steering load-dependent component Tcs for simulating the load of the steering actuator 16. In S14, the reaction force ECU 40 determines the neutral position return component Tcc according to the following equation using the neutral position return component determination coefficient Cc. Tcc = Cc·δ According to the above equation, the neutral position return component Tcc is determined such that the reaction force torque Tc for returning the steering wheel 10 to the neutral position increases as the operation angle δ increases.

[0041] The load on the steering actuator 16 can be considered to be approximately proportional to the steering current Is, which is the current supplied to the steering motor 38. In the following S15, the reaction force ECU 40 acquires the steering current Is based on the above-described signal transmitted from the steering ECU 42. Then, in S16, the reaction force ECU 40 uses the steering load component determination coefficient Cs and determines the steering load-dependent component Tcs according to the following equation. Tcs = Cs·Is According to the above equation, as the steering current Is increases, that is, as the load on the steering actuator 16 increases, the steering load-dependent component Tcs is determined so that a large reaction force torque Tc is applied.

[0042] Based on the neutral position return 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 equation. Tc = Tcc + Tcs Since the reaction force torque Tc and the reaction force current Ic, which is the current supplied to the reaction force motor 24 as a power source, are approximately in a proportional relationship, in S18, the reaction force ECU 40 determines the reaction force current Ic based on the reaction force torque Tc using the current determination coefficient Kc according to the following equation. Ic = Kc·Tc The determined reaction force current Ic is supplied from the inverter to the reaction force motor 24 in S19, and one execution of the reaction force control program ends.

[0043] (c) Driving support control The vehicle equipped with this steering system is also equipped with a driving support system that executes a plurality of driving support controls to assist the driver in driving the vehicle in addition to this system. The plurality of driving support controls are target traveling line following control (LTA), lane departure prevention control (LDA), vehicle stabilization control (VSC), and collision avoidance control (PCS). All of them involve automatic steering of the wheels 14, that is, steering of the wheels 14 that does not depend on the operation of the driver's steering wheel 10.

[0044] Briefly explaining each driving support control, the target lane tracking control is a control for driving the vehicle along the target driving line set in the center of the driving lane when the vehicle is driving on, for example, a highway. According to this control, when the vehicle deviates from the target driving line due to the driver's steering of the wheels 14, the steering angle ω of the wheels 14 is adjusted. The lane departure prevention control is a control for preventing the vehicle from leaving the driving lane. According to this control, when the vehicle protrudes into either the left or right of the driving lane in which it is driving, a warning is issued to the driver, and in order to keep the vehicle within the driving lane, the wheels 14 are forcibly steered. This is a control that is executed only when the vehicle is traveling at a speed of a certain level or more (for example, 50 km / h or more). The vehicle stabilization control is a control for stabilizing the running or posture of the vehicle when the vehicle is turning. According to this control, when the vehicle tends to understeer or oversteer beyond the limit, in addition to operations such as braking and accelerating, the wheels 14 are steered to suppress the understeer or oversteer. 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 by braking, the wheels 14 are steered to avoid the collision. The vehicle stabilization control and the collision avoidance control are executed regardless of whether the vehicle is driving on an ordinary road or the driving speed of the vehicle.

[0045] Each of the above driving support controls is executed by a driving support electronic control unit (hereinafter sometimes referred to as the "driving support ECU"). As shown in FIG. 1, the driving support ECU 66 (denoted as "DA-ECU" in the figure) is connected to the CAN 46. In the figure, it is shown as one unit, but in reality, a plurality of electronic control units corresponding to a plurality of driving support controls are mounted. The driving support ECU 66 transmits a steering command to the steering ECU 42 via the CAN 46. More specifically, it intervenes in the above-described steering control to steer the wheels 14. Specifically, the target steering angle ω *A command regarding this is sent from the driving support ECU 66, and the steering ECU 42 determines the target steering angle ω determined as described above based on the operation angle δ. * Instead of that, based on the sent target steering angle ω * it executes steering control.

[0046] Although detailed description is omitted, regarding each driving support control, the driving support ECU 66 stores a unique neutral steering position, that is, a neutral steering angle ω0, for each control. Those neutral steering angles ω0 are acquired through learning. Each driving support control has a different required control accuracy, and the learning accuracy, specifically, the accuracy of the neutral steering angle ω0 acquired through learning, is also different. Specifically, the target traveling line following control requires the highest accuracy, and the required accuracy decreases in the order of the lane departure prevention control, the vehicle stabilization control, and the collision avoidance control. Conversely, in view of the safety of vehicle travel, the necessity of executing control is set to be inversely proportional to the accuracy. The collision avoidance control has the highest necessity, and the necessity decreases in the order of the vehicle stabilization control, the lane departure prevention control, and the target traveling line following control. Note that, generally, the higher the required accuracy of the control, the longer the learning time of the neutral steering angle ω0 is required.

[0047] In view of the above, in the vehicle equipped with this steering system, the above driving support controls are classified into two types. One of them is the safety - oriented support control, which includes the lane departure prevention control, the vehicle stabilization control, and the collision avoidance control. The other one is the high - accuracy - required support control, which is the target traveling line following control. As will be described in detail later, these two types of controls are handled differently regarding the neutral position deviation elimination process.

[0048] [3] Neutral Steering Position and Its Setting The steering actuator 16 has an operating range for the left and right wheel steering operations, and the center of the operating range, that is, the exact middle position, is set to the mechanical neutral position. Fig. 4(a) is a diagram schematically showing a part of each of the right side and the left side of the steering actuator 16. As shown in this figure, locking rings 74R and 74L are attached to the part extending from the housing 30 of the steering rod 32 which is a movable member of the steering actuator 16. When the steering rod 32 is moved to the right to a certain extent, as shown by the dashed-dotted line, the locking ring 74L abuts against the left end of the housing 30, and further movement of the steering rod 32 to the right is prohibited. This state is a state where the operating position of the steering rod 32 is at one end, that is, the right end, in the operating range of the steering actuator 16. On the other hand, when the steering rod 32 is moved to the left to a certain extent, as shown by the dashed-dotted line, the locking ring 74R abuts against the right end of the housing 30, and further movement of the steering rod 32 to the left is prohibited. This state is a state where the operating position of the steering rod 32 is at the other end, that is, the left end, in the operating range of the steering actuator 16. In this way, one end and the other end of the operating range are defined. The state where the locking rings 74R and 74L are located at the positions shown by the solid line in Fig. 4(a) is a state where the operating position of the steering actuator 16 is at the center of the above-mentioned operating range, that is, a state where the operating position of the steering actuator 16 is at the mechanical neutral steering position. This neutral steering position is an operating position that is considered to be maintained by the steering actuator 16 when the vehicle is moving straight ahead. Incidentally, the right end and the left end of the housing 30 against which the locking rings 74R and 74L abut function as a pair of stoppers for locking the locking rings 74R and 74L.

[0049] After the vehicle is manufactured, that is, after the steering actuator 16 is mounted on the vehicle body, before shipment, the toe angle of the wheel 14 is adjusted in the factory. Briefly speaking, as schematically shown in Fig. 4(b), when the vehicle is going straight, it is desirable that the toe angle of the wheel 14 is approximately 0°. The adjustment of the toe angle of the wheel 14 is performed by adjusting the tie rod length L, which is the length of the tie rod 34, while maintaining the operating position of the steering actuator 16 at the above-described mechanical neutral steering position. The specific adjustment method may follow a general method, and the description here is omitted.

[0050] After the above-described toe angle adjustment, a process is performed to cause the steering ECU 42 to store the detected value of the steering angle sensor 52 at that time as the neutral steering angle ω0, which is the neutral steering position. By this process, the neutral steering angle ω0 at that time is set as the initial stored neutral position, the initial stored neutral steering angle ω0mi. As described above, the stored neutral steering angle ω0m, which is the stored neutral position, is a conceptual one and may be changed after shipment. Actually, the neutral steering angle ω0 is the sum of the initial stored neutral steering angle ω0mi and the stored steering offset angle Δωm, which is the stored steering offset angle Δω, and the steering control is executed. Therefore, the steering ECU 42 stores the initial stored neutral steering angle ω0mi and the stored steering offset angle Δωm, and the change of the stored neutral steering angle ω0m, that is, the neutral steering angle ω0 used for steering control, is performed by changing the stored steering offset angle Δωm. Incidentally, the stored steering offset angle Δωm is set to 0° immediately after the toe angle adjustment is performed after the vehicle is manufactured. In the following description, the sum of the initial stored neutral steering angle ω0mi and the stored steering offset angle Δωm may be treated as the stored neutral steering angle ω0m.

[0051] Regarding the neutral operation position of the steering wheel 10, which is an operation member, when the steering wheel 10 is attached to the steering shaft 22, the same setting as the neutral steering position is performed, but the description here is omitted.

[0052] [4] Process for neutral position deviation and its elimination (a) Deviation in the neutral steering position The toe angle of the steered wheel 14 may need to be readjusted, for example, at the dealership (dealer) that sold the vehicle, due to some circumstances. Also, for example, even if it does not reach the extent that readjustment at the dealership is required, it is expected that the toe angle will change slightly due to some cause (e.g., collision of the wheel 14 with a curb, the wheel 14 getting stuck in a groove, aging, etc.). As described above, the steering control is executed with the stored neutral steering angle ω0m as the neutral steering angle ω0. However, when the toe angle of the wheel 14 is readjusted or when the toe angle changes, the actual neutral steering position, which is the neutral steering position where the vehicle should be when going straight (which can also be simply called the "actual neutral position"), that is, the actual neutral steering angle ω0r, which is the actual neutral steering angle ω0, will change. In other words, the stored neutral steering angle ω0m set based on the mechanical neutral operating position of the steering actuator 16 as described above will deviate from the actual neutral steering angle ω0r. That is, a neutral position deviation will occur.

[0053] As described above, the steering control is executed with the stored neutral steering angle ω0m as the neutral steering angle ω0. However, when the above neutral position deviation occurs, even if the driver maintains the steering wheel 10 at the neutral operating position, the vehicle will deviate to either the left or the right. That is, it is expected that the vehicle will fall into a situation where proper steering of the wheel 14 is not performed. Conversely, when the driver operates the steering wheel 10 to make the vehicle go straight, the steering wheel 10 must be maintained at a position deviated from the neutral operating position, and the reaction force actuator 12 generates a reaction torque Tc corresponding to the operating angle δ at that time by the above-described neutral position return component Tcc. In that case, the driver needs to maintain the steering wheel 10 against the reaction torque Tc and will feel a sense of discomfort with the steering operation.

[0054] (b) Outline of neutral position deviation elimination process In order to eliminate the above-mentioned neutral position deviation, in this steering system, when the vehicle is running, specifically, in a state where the above-mentioned steering control and reaction force control are being executed, a neutral position deviation elimination process is executed. This neutral position deviation elimination process may be performed, for example, as the work of an adjuster at a dealership after the toe angle adjustment at the dealership, or may be performed by the user of the vehicle. When based on the operation of the adjuster, the neutral position deviation elimination process is performed by connecting the operation terminal 80 to the vehicle as shown in FIG. 1. The operation terminal 80 includes a display, a keyboard, etc., and is connected to the connector provided on the vehicle, and is connected to the steering ECU 42 via the CAN 46. This operation terminal 80 can access various information related to the steering system, such as the stored steering offset angle Δωm currently stored in the steering ECU 42. Although detailed description is omitted, in the neutral position deviation elimination process using the operation terminal 80, the adjuster can more accurately eliminate the neutral position deviation while observing the change in the stored neutral steering angle ω0m. On the other hand, when based on the operation of the user of the vehicle, the neutral position deviation elimination process is performed using the position deviation elimination process start switch 82 provided on the instrument panel.

[0055] Briefly speaking, the neutral position deviation elimination process is a process of making the stored neutral steering angle ω0m, which is the stored neutral steering position, approach the actual neutral steering angle ω0r. In other words, it is a process of changing the stored steering offset value Δωm so that the stored neutral steering angle ω0m approaches the actual neutral steering angle ω0r. The neutral position deviation elimination process is performed in a state where the above-mentioned steering control and reaction force control are being executed, and in a state where an adjuster or a user (hereinafter may be collectively referred to as a "driver") maintains the operation of the steering wheel 10 so that the vehicle travels straight. When a neutral position deviation occurs, when the vehicle is traveling straight, as described above, the operation angle δ of the steering wheel 10 is deviated from the neutral operation angle δ0, and a reaction force torque Tc corresponding to the deviation is generated. Therefore, the neutral position deviation elimination process is performed in a state where the driver applies an operation torque To against the reaction force torque Tc to the steering wheel 10.

[0056] When the driver operates the operation terminal 80 or the position deviation elimination processing start switch 82, the neutral position deviation elimination processing is started. When the neutral position deviation elimination processing is started, the steering ECU 42 first determines whether the execution conditions are satisfied. The execution conditions are that the vehicle speed v, which is the running speed of the vehicle, is within the set speed range and the vehicle is going straight. The vehicle speed v is acquired from a brake electronic control unit (brake ECU), not shown, via the CAN 46, 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 initial processing. In this initial processing, the steering offset angle Δωt for shifting the neutral steering angle ω0, which is the steering offset angle for shifting, is set to the stored steering offset angle Δωm currently stored, and instead of the stored neutral steering angle ω0m, the value obtained by adding the steering offset angle Δωt for shifting to the initial stored neutral steering angle ω0mi is set as the neutral steering angle ω0, and based on the neutral steering angle ω0, steering control is executed.

[0057] Next, the steering ECU 42 identifies whether the neutral steering angle ω0, that is, the currently stored neutral steering angle ω0m, is deviated from the actual neutral steering angle ω0r in either the left or right direction. That is, the steering ECU 42 shifts the stored steering offset value Δωm from the current value to either the left or right, that is, the shift direction of the neutral steering angle ω0 is identified. This identification of the shift direction may be performed, for example, based on the detected value of the steering angle sensor 56. Specifically, at the current time, 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 detected value of the current steering angle ω by the steering angle sensor 56, the deviation angle dω0 of the neutral steering angle ω0, which is the deviation amount of the neutral steering position, can be grasped, and the shift direction can be identified. Also, the identification of the shift direction may be performed, for example, based on the detected value of the operation angle δ of the steering wheel 10 by the operation angle sensor 52, the detected value of the operation torque To by the operation torque sensor 50, the value of the generated reaction torque Tc, etc.

[0058] Based on the shift direction specified as above, on the premise that the above execution conditions are satisfied, the steering ECU 42 gradually changes the neutral steering angle ω0 toward the actual neutral steering angle ω0r. Specifically, the shift steering offset angle Δωt is changed by the set shift angle dΔω, and the value obtained by adding the changed shift steering offset angle Δωt to the initial stored neutral steering angle ω0mi is used as the neutral steering angle ω0, and the execution of the steering control is continued. The change of the shift steering offset angle Δωt is repeatedly executed until the change end condition (hereinafter sometimes simply referred to as the "end condition") described later is satisfied.

[0059] The end condition is that the value P of the determination parameter is a value that can be regarded as approximately 0. The determination parameter P can adopt the deviation angle dω0 of the neutral steering angle ω0, the detected value δ of the operation angle of the steering wheel 10 by the operation angle sensor 52, the detected value To of the operation torque by the operation torque sensor 50, the value of the generated reaction torque Tc, etc., which were described in relation to the specification of the shift direction. Regarding the steering of the wheels 14 and the operation of the steering wheel 10, various numerical parameters such as the steering angle ω and the operation angle δ are treated such that the values corresponding to left turn are + and the values corresponding to right turn are -, and the determination parameter P is treated in the same way. By treating it in this way, |P| < Pt the fact that it is satisfied can be set as the end condition. Note that the threshold value Pt is set to a value that can be regarded as the neutral position deviation being eliminated.

[0060] Specifically, when the determination parameter P is the deviation angle dω0, as the shift steering offset angle Δωt changes, the absolute value |dω0| of the deviation angle decreases. When it becomes smaller than the threshold value dω0t, it can be considered that the neutral position deviation is eliminated. Also, the detected value of the operation angle δ of the steering wheel 10, the detected value of the operation torque To by the operation torque sensor 50, and the value of the generated reaction torque Tc, if the above-described reaction force control is being executed, when the absolute values |δ|, |To|, and |Tc| of these are all smaller than the threshold values δt, Tot, and Tct respectively, which are set close to 0, it can be considered that the neutral position deviation is eliminated. That is, simply put, when the steering wheel 10 is positioned at the neutral operation position, which is the position where the vehicle should be when going straight, when the operating force applied by the driver to the steering wheel 10 disappears, or when the operating reaction force applied to the steering wheel 10 disappears, it can be considered that the neutral position deviation is eliminated. Incidentally, according to the latter, there is an advantage that the deviation of the neutral steering angle ω0 can be estimated without detecting the actual steering angle ω.

[0061] Simply put, when the determination parameter P satisfies the above termination condition, the steering ECU 42 terminates the change of the shift steering offset angle Δωt. Actually, when the above termination condition is satisfied, the shift steering offset angle Δωt is changed once more in the shift direction. If, due to this change, |P| does not become larger than the absolute value (|Pp|) of the previous value Pp, the change of the shift steering offset angle Δωt is terminated as it is. If |P| becomes larger than |Pp|, the change of the shift steering offset angle Δωt is terminated by returning to the shift steering offset angle Δωt when that single change was not made. According to such a change of the shift steering offset angle Δωt, for example, the change of the shift steering offset angle Δωt up to the opposite side across the actual neutral steering angle ω0r is allowed, and it becomes possible to set the neutral steering angle ω0 to a value closer to the actual neutral steering angle ω0r.

[0062] The neutral position deviation elimination process in this steering system is considered as part of the work of the above adjuster and is performed in relation to the operation of the ignition switch (hereinafter sometimes referred to as "IG") of the vehicle. Specifically, when considering that the adjuster confirms the satisfaction of the above end condition by the operation terminal 80 before turning off the IG, the steering ECU 42 determines that the neutral position deviation elimination process has ended when the IG is turned off. More specifically, the difference between the shift steering offset angle Δωt and the stored steering offset angle Δωm currently stored is recognized as the change angle (|Δωt - Δωm|). When the change angle (|Δωt - Δωm|) is larger than the threshold angle Δωs, that is, when the stored neutral steering angle ω0m is changed relatively greatly, the shift steering offset angle Δωt at that time is stored as the stored steering offset angle Δωm. That is, the stored neutral steering angle ω0m is updated, and thereafter, the steering control with 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 shift steering offset angle Δωt and the stored steering offset angle Δωm at that time, the stored neutral steering angle ω0m is not updated.

[0063] Also, considering that when the driver is the adjuster and the adjuster confirms the elimination of the neutral position deviation, when the driver turns on the IG, the steering ECU 42 maintains the determination that the neutral position deviation elimination process has ended for a set time (hereinafter sometimes referred to as "end determination maintenance time ts"). This end determination maintenance time ts may be set, for example, to about 30 seconds to 1 minute, which is the time required for the adjuster to confirm. Note that the description of the process related to the operation of the IG will be described in detail later. 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 has not been performed, the determination that the neutral position deviation elimination process has ended is not made.

[0064] Regarding the relationship between the above driving support control and the neutral position deviation elimination process, it is as follows. As described above, each driving support control is performed based on the neutral steering angle ω0 that is independently learned and memorized. In the target driving lane tracking control (LTA), which is a high-precision requirement support control, when the neutral position deviation elimination process starts, the neutral steering angle ω0 that is independently memorized in that control is reset, and the execution of that control itself is prohibited. Then, after the end confirmation maintenance time ts for the end confirmation of the neutral position deviation elimination process has elapsed, the learning of the neutral steering angle ω0 starts, and after a highly accurate neutral steering angle ω0 is obtained through learning, the execution of that control is resumed.

[0065] On the other hand, in the driving lane departure prevention control (LDA), vehicle stabilization control (VSC), and collision avoidance control (PCS), each of which is a safety-oriented support control, even when the neutral position deviation elimination process starts, the memorized neutral steering angle ω0 is not reset, and the control itself is permitted. However, as described above, when the change angle (|Δωt - Δωm|) is larger than the threshold angle Δωs, that is, when the memorized neutral steering angle ω0m is changed relatively greatly, even in the safety-oriented support control, when the IG is turned on, the neutral steering angle ω0 that is independently memorized in that control is reset and the execution of that control is prohibited, and after a highly accurate neutral steering angle ω0 is obtained through learning, the execution of that control is resumed. Note that regarding the safety-oriented support control, regardless of the change angle (|Δωt - Δωm|), the execution of that control may not be prohibited. Also, the learning of the neutral steering angle ω0 may start from the time when the neutral position deviation elimination process ends.

[0066] (c) Flow of the neutral position deviation elimination process The above-mentioned neutral position deviation elimination process is executed by the steering ECU 42 executing a neutral position deviation elimination process program whose flowchart is shown in FIG. 5. This program is started when the driver operates the operation terminal 80 or the position deviation elimination process start switch 82 while the vehicle is moving straight ahead. Below, while referring to the flowchart, the flow of the neutral position deviation elimination process will be briefly described.

[0067] In the process according to the above program, first, in S21, it is determined whether the execution conditions are satisfied. The execution conditions are that, as described above, the vehicle is traveling within a certain speed range and the vehicle is moving straight ahead. If these execution conditions are satisfied, then in S22, as initial processing, the shift steering offset angle Δωt is set to the stored steering offset 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 shift steering offset angle Δωt.

[0068] Next, in S23, as described above, for example, it is specified whether the stored neutral steering angle ω0m is deviated from the actual neutral steering angle ω0r in which direction, so that the direction of the neutral position deviation is specified. Then, in S24, the processing flag Fm is set to "ON". Briefly speaking, the processing flag Fm is a flag indicating that the neutral position deviation elimination process is being performed. It is set to "ON" when the process is being executed and set to "OFF" when the process is not being executed. Subsequently, in S25, the execution of the high-precision requirement support control (for example, the target travel line following control) is prohibited, and a command to reset the value of the neutral steering angle ω0 acquired by learning in that control is sent to the driving support ECU 66.

[0069] Next, in S26, it is determined whether the above-described execution conditions are satisfied. If the execution conditions are satisfied, then in S27, the shift steering offset angle Δωt is changed by the set shift angle dΔω in the direction opposite to the direction of the neutral position deviation. 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 based on whether the absolute value of the above-described determination parameter P is smaller 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 steering wheel 10, the operation torque To of the steering wheel 10, the reaction torque Tc, and the like.

[0070] When the above end condition is satisfied, in S29, the execution conditions are further determined. If the execution conditions are satisfied, then in S30, the shift steering offset angle Δωt is further changed by the set shift 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 becomes larger than the absolute value of the previous value Pp, then in S32, the shift steering offset angle Δωt is changed in the reverse direction by the set shift angle dΔω. That is, the shift steering offset angle Δωt is returned to the value at which the last change was not made. On the other hand, when the absolute value of the determination parameter P has not become larger than the absolute value of the previous value Pp, the shift steering offset angle Δωt is not returned.

[0071] The processing according to the neutral position deviation elimination processing program is as described above. However, as described earlier, 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 executes the IG-OFF program and the IG-ON program whose flowcharts are shown in FIG. 6, respectively. The flow of the processing according to these programs will be briefly described below.

[0072] In the process according to the IG-OFF program, first, in S41, it is confirmed how much the steering offset angle Δωt for shifting has finally changed from the stored steering offset angle Δωm, that is, the change angle (|Δωt - Δωm|). When this change angle (|Δωt - Δωm|) is larger than the threshold angle Δωs, in S42, the stored steering offset angle Δωm is set to the steering offset angle Δωt for shifting, and in the subsequent S43, a command for prohibiting safety - oriented support control (for example, lane departure prevention control, vehicle stabilization control, collision avoidance control) and resetting the neutral steering angle ω0 already learned and acquired in that control is sent to the driving support ECU66. Then, in S44, the post - processing flag Fa is set to "ON". The post - processing flag Fa is a flag for determining that the neutral position deviation elimination process, specifically, the gradual change from the stored neutral position to the actual neutral position has substantially ended, and it is a flag that is set to "ON" when it is determined that the process has ended.

[0073] In S45, the neutral steering angle ω0 is set to the value obtained by adding the stored steering offset 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, when it is determined in S41 that the change angle (|Δωt - Δωm|) is less than or equal to the threshold angle Δωs, the stored steering offset angle Δωm is not changed, and the steering control is executed with the original neutral steering angle ω0. Also, in that case, the post - processing flag Fa is not set to "ON".

[0074] In the process according to the IG-ON program, first, in S51, the processing flag Fm is reset to "OFF". In the subsequent S52, a command to start learning the neutral steering angle ω0 in the prohibited driving support control is issued to the driving support ECU66. When the safety-oriented support control is also prohibited and the neutral steering angle ω0 in that control is also reset, the learning of the neutral steering angle ω0 is also started based on that command. The driving support control is set to resume on the condition that an appropriate neutral steering angle ω0 has been obtained by learning. Then, in S53, it is determined whether the post-processing flag Fa is "ON". If it is "ON", in S54, the time counter t is reset to 0, and in S55, the time counter t is incremented by Δt. This increment is performed in S56 until the time counter t exceeds the end determination estimation time ts. After the elapse of the end determination estimation time ts, in S57, the post-processing flag Fa is reset to "OFF".

[0075] (d) Specific example of neutral position deviation elimination processing A specific example of the neutral position deviation elimination processing will be described below with reference to the diagrams in FIGS. 7 and 8.

[0076] The diagram in FIG. 7 shows a case where the neutral steering angle ω0 is changed relatively greatly in the neutral position deviation elimination processing. Before executing the neutral position deviation elimination processing, the deviation angle dω0 of the neutral steering angle ω0 is 1.6°. That is, the neutral steering angle ω0 is deviated by 1.6° in the left turning direction. At time t0, the neutral position deviation elimination processing is started. The stored steering offset angle Δωm at that time is 0.0°, and when the neutral position deviation elimination processing is started, the shift steering offset angle Δωt is set to 0.0°. And at that time, the processing flag Fm is set to "ON", and the high-precision requirement support control is prohibited. The neutral steering angle ω0 (0.3°) learned in that control is set to the reset value at that time.

[0077] At time points t1, t2, and t3, the steering offset angle Δωt for shifting is repeatedly changed by 0.5° each, which is the set shift angle dΔω, in the right turning direction. That is, the steering offset angle Δωt for shifting is sequentially changed from 0.0° to -0.5°, -1.0°, -1.5°. As a result, the deviation angle dω0 of the neutral steering angle ω0 is gradually decreased from 1.6° to 1.1°, 0.6°, 0.1°. Due to the change in the steering offset angle Δωt for shifting at time point t3, the above-described determination parameter P (for example, the deviation angle dω0 of the neutral steering angle ω0) becomes smaller than the threshold value Pt (for example, the deviation angle 0.2°), and the change in the steering offset angle Δωt for shifting is terminated.

[0078] After the change in the steering offset angle Δωt for shifting is terminated, the IG is turned off. In this neutral position deviation elimination process, finally, since the change angle (|Δωt - Δωm|), which is the difference between the steering offset angle Δωt for shifting and the stored steering offset angle Δωm, is larger than the threshold angle Δωs (for example, 0.4°), when the IG is turned off, the stored steering offset angle Δωm is set to -1.5°, which is the steering offset angle Δωt at that time, and the post-processing flag Fa is set to "ON". Also, from that point on, the execution of the safety-oriented support control is prohibited, and at that time, the neutral steering angle ω0 (0.3°) learned in that control is set to the reset value.

[0079] Next, when the IG is turned on, the in-processing flag Fm is reset to "OFF", and the learning of the neutral steering angle ω0 in the prohibited driving support controls (high-precision requirement support control and safety-oriented control) is started. After an appropriate neutral steering angle ω0 (0.1°) is obtained by that learning, the learning is terminated and the execution of the prohibited driving support control is permitted. Also, when the elapsed time ts from when the IG is turned on reaches the end determination estimation time, the post-processing flag Fa is reset to "OFF".

[0080] The diagram of FIG. 8 shows a case where, although the neutral position deviation elimination process was performed, since the above-described change angle (|Δωt - Δωm|) was equal to or less than the threshold angle Δωs (for example, 0.4°), the neutral steering angle ω0 was not substantially changed. Different from the case of FIG. 7, the set shift angle dΔω is set to 0.3°.

[0081] In the case of FIG. 8, before executing the neutral position deviation elimination process, the deviation angle dω0 of the neutral steering angle ω0 is 0.4°. That is, the neutral steering angle ω0 is deviated by 0.4° in the left turning direction. At time t0, the neutral position deviation elimination process is started. The stored steering offset angle Δωm at that time is 0.0°, and when the neutral position deviation elimination process is started, the shift steering offset angle Δωt is set to 0.0°. And at that time, the in-process flag Fm is set to "ON", and the high-precision requirement support control is prohibited. The neutral steering angle ω0 (0.3°) learned in that control is reset at that time.

[0082] At time t1, the shift steering offset angle Δωt is changed in the clockwise direction by the set shift angle dΔω, which is 0.3°. Due to this change, the above-described determination parameter P (for example, the deviation angle dω0 of the neutral steering angle ω0) becomes smaller than the threshold value Pt (for example, the deviation angle 0.2°). According to the above program, at time t2, again, the shift steering offset angle Δωt is changed in the clockwise direction by the set shift angle dΔω, which is 0.3°. As a result, the absolute value of the determination parameter P (the deviation angle dω0, which is -0.2°) becomes larger than the previous value Pp (0.1°), and at time t3, the shift steering offset angle Δωt is returned to the value (-0.3) before the change is made.

[0083] After the change of the steering offset angle Δωt for shifting, IG is turned off. However, in this neutral position deviation elimination process, finally, since the change angle (0.3 which is |Δωt - Δωm|) is less than or equal to the threshold angle Δωs (for example, 0.4°), even if IG is turned off, the stored steering offset angle Δωm is not changed, and the post - processing flag Fa is not set to "ON". Furthermore, the execution of the safety - oriented support control is not prohibited, and the neutral steering angle ω0 (0.3°) learned in that control is maintained.

[0084] Next, when IG is turned on, the in - processing flag Fm is reset to "OFF", and the learning of the neutral steering angle ω0 in the prohibited driving support control (high - precision requirement support control) is started. After the appropriate neutral steering angle ω0 (0.1°) is obtained by that learning, the learning is terminated, and the execution of the prohibited driving support control is permitted.

Explanation of symbols

[0085] 10: Steering wheel [operating member] 12: Reaction actuator [reaction force applying device] 14: Wheel 16: Steering actuator 24: Reaction motor [power source] 32: Steering rod [movable member] 34: Tie rod 38: Steering motor [power source] 40: Reaction electronic control unit (reaction 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 support electronic control unit (driving support ECU) 74R, 74L: Locked ring L: Tie rod length 80: Operating terminal 82: Position deviation 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: Deviation angle of the neutral steering angle ω0 Δω: Steering offset angle Δωm: Stored steering offset angle Δωt: Steering offset angle for shifting dΔω: Set shift angle |Δωt - Δωm|: Change angle Δωs: Threshold angle P: Parameter for determination Pt: Threshold value v: Vehicle speed γ: Yaw rate Fm: In - processing flag Fa: Post - processing flag

Claims

1. An operating member operated by a driver, a steering actuator having a power source and steering the wheels by the force of the power source, and a controller that controls the steering actuator to achieve steering of the wheels according to the operation of the operating member are provided, and it is a steer-by-wire type steering system mounted on a vehicle, wherein when the stored neutral steering position, which is the neutral steering position stored by the controller for controlling the steering actuator, is deviated from the actual neutral position, which is the actual neutral steering position, the neutral position deviation elimination process for approaching the stored neutral position to the actual neutral position is executed in a state where the operation of the operating member for keeping the vehicle going straight by the driver is maintained. A configured steering system.

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 controller is configured to end the neutral position deviation elimination process on the condition that the operation position of the operating member is at the neutral operation position where the vehicle should be located when the vehicle is going straight. The steering system according to claim 1.

4. The steering system includes an operation reaction force applying device that applies an operation reaction force, which is a reaction force to the operation of the operating member, to the operating member, and the controller is executes control for applying an operation reaction force including a component for returning the operation position of the operating member to the neutral operation position where the vehicle should be located when the vehicle is going straight, and is configured to end the neutral position deviation elimination process on the condition that the operation reaction force has disappeared or the operation force of the driver against the operation reaction force has disappeared. The steering system according to claim 1.

5. In the vehicle, a plurality of driving support controls each involving automatic steering of the wheels are executed, and the steering system is configured to allow at least one of the plurality of driving support controls even during the execution of the neutral position deviation elimination process. The steering system according to claim 1.

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

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