Steering system

Abstract

To upgrade the practicality of a steering system capable of turning both front wheels and rear wheels.SOLUTION: When a front wheel 10F passes over an irregularity R, rear wheel avoidance control is executed to prevent the rear wheel 10R from passing over the same irregularity. In this rear wheel avoidance control, a rear wheel avoidance slip angle βA, which is the vehicle center-of-gravity slip angle to be realized, is determined. Based on this rear wheel avoidance slip angle, a target steering angle, being the steering angle to which both the front and rear wheels should be turned, is determined. While target line tracking control is being executed, which steers the front and rear wheels so that the vehicle travels along a target line L*, which is a travel line along which the vehicle should travel, the rear wheel avoidance control may also be executed. In this way, the rear wheel avoids the irregularity while the vehicle is guided along an appropriate travel path.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] This invention relates to a steering system installed in a vehicle. [Background technology]

[0002] Technologies such as those described in the following patent documents, namely technologies that detect wheel lift during vehicle operation and prevent the vehicle from overturning by warning the driver or braking the vehicle, are being considered. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2005-239078 [Overview of the project] [Problems that the invention aims to solve]

[0004] In the technology described in the above-mentioned patent document, for example, when the front wheels pass over an uneven surface in the road, the rear wheels cannot avoid passing over the same uneven surface, and it cannot necessarily be said that the stability of the vehicle can be well maintained. If a steering system that can ensure good stability while the vehicle is running can be realized, that steering system will be highly practical. The present invention has been made in view of such circumstances, and aims to provide a highly practical steering system. [Means for solving the problem]

[0005] To solve the above problems, the steering system of the present invention is A steering system installed in a vehicle, The system comprises a front wheel steering device for steering the front wheels, a rear wheel steering device for steering the rear wheels, and a controller that controls the front wheel steering device and the rear wheel steering device so that the front wheels and rear wheels can be steered independently of each other. The aforementioned controller When the front wheels pass over an uneven surface, rear wheel avoidance control is performed to prevent the rear wheels from passing over that surface. In this rear wheel avoidance control, the rear wheel avoidance lateral slip angle, which is the lateral slip angle of the vehicle's center of gravity that should be achieved, is determined. Based on this rear wheel avoidance lateral slip angle, the target steering angle, which is the steering angle at which the front and rear wheels should be steered, is determined. [Effects of the Invention]

[0006] The "vehicle center of gravity lateral slip angle," also called the vehicle body slip angle, is the angle between the axis extending in the longitudinal direction of the vehicle through the vehicle's center of gravity (hereinafter sometimes referred to as the "vehicle longitudinal axis") and the line extending along the direction of travel of the vehicle (hereinafter sometimes referred to as the "travel direction line"). In "rear wheel avoidance control," the vehicle center of gravity lateral slip angle is set to an appropriate angle so that the rear wheels do not pass over bumps. This angle is the "rear wheel avoidance lateral slip angle" mentioned above. In rear wheel avoidance control, the steering angles of the front wheels and rear wheels are controlled to achieve this rear wheel avoidance lateral slip angle, and this control makes it possible to prevent the rear wheels from passing over bumps that the front wheels have passed over. As a result, the vehicle's driving stability is improved. Embodiment of the Invention

[0007] When the steering system of the present invention (hereinafter sometimes referred to as "this steering system") is applied to a four-wheeled vehicle with two front wheels and two rear wheels, the "front wheel steering device" may be one that steers the two front wheels as a whole, or one that steers the two front wheels independently of each other, that is, one that includes two single-wheel steering devices. Similarly, the "rear wheel steering device" may be one that steers the two rear wheels as a whole, or one that steers the two rear wheels independently of each other, that is, one that includes two single-wheel steering devices. The "controller" may be a single unit, or it may be composed of, for example, a controller that controls the front wheel steering device, a controller that controls the rear wheel steering device, and a controller that controls them all together. This steering system is a system in which both the front and rear wheels can be steered by the control of the controller, and is suitable for autonomously driven vehicles.

[0008] The "unevenness" targeted by rear-wheel avoidance control is a general term for road surface irregularities such as depressions, dips, bumps, bulges, protrusions, and steps that disrupt the vehicle's movement when the wheels pass over them. It is also possible to limit the target of rear-wheel avoidance control to irregularities that significantly disrupt the vehicle's movement. Specifically, it is possible to target irregularities such that when the front wheels pass over them, neither the front nor the rear wheels bear any of the vehicle's load—in short, both the front and rear wheels are lifted off the ground. In the case of the four-wheeled vehicle described above, it is sufficient to target irregularities such that both the front and rear wheels are lifted off the ground on either the left or right side. In other words, this steering system may be configured to execute rear-wheel avoidance control only when it detects that both the front and rear wheels are lifted off the ground. A method based on ZMP (zero moment point) can be used to detect the lifting of the front and rear wheels. This method will be described later.

[0009] In rear-wheel avoidance control, the rear-wheel avoidance lateral slip angle can be determined, for example, based on the distance the rear wheels should be shifted laterally from the uneven surface (hereinafter sometimes referred to as the "rear-wheel shift distance") and the wheelbase of the vehicle. The specific determination method will be described later. Incidentally, in this rear-wheel avoidance control, the rear wheels may be shifted to the outside of the turn relative to the front wheels, to the inside of the turn, or towards the center of the vehicle, or towards the outside of the vehicle.

[0010] Furthermore, in order to reliably avoid the rear wheels passing over bumps, rear-wheel avoidance control should be performed after the front wheels have passed over the bumps and until the vehicle has traveled a distance equivalent to its wheelbase. In other words, rear-wheel avoidance control should be terminated when the vehicle has traveled a distance equivalent to its wheelbase. The vehicle's travel distance can be obtained by estimating its own position based on odometry information obtained from sensors that measure wheel rotation, GPS, cameras, etc.

[0011] This steering system may also perform "target line driving control," which steers the front and rear wheels so that the vehicle travels along a target line, which is the intended driving path. Target line driving control enables the vehicle to travel along the appropriate driving path. Furthermore, if the front wheels pass over an uneven surface while target line driving control is being performed, rear wheel avoidance control may be performed while continuing the target line driving control. By performing these two controls in parallel, it becomes possible to avoid the rear wheels passing over uneven surfaces while the vehicle travels along the appropriate driving path.

[0012] Specifically, target line driving control can be performed by setting a target point on a target line in front of the vehicle, and determining the target steering angles of the front and rear wheels based on the positional relationship between the target point and the vehicle, and a preset "standard lateral slip angle," which is the lateral slip angle of the vehicle's center of gravity. The standard lateral slip angle characterizes the turning characteristics of the vehicle. For example, if the standard lateral slip angle is set to 0°, the front and rear wheels will be steered in opposite phases, enabling so-called zero-beta turning. Incidentally, achieving this zero-beta turning can reduce responsiveness lag and improve steering performance. The standard lateral slip angle may be fixed, or it may be variable depending on the vehicle speed (hereinafter sometimes referred to as "vehicle speed"), turning radius, etc.

[0013] To perform rear-wheel avoidance control while simultaneously controlling the vehicle to follow the target line as described above, one could, for example, adopt a rear-wheel avoidance lateral slip angle instead of the standard lateral slip angle, and determine the target steering angles of the front and rear wheels based on that rear-wheel avoidance lateral slip angle. [Brief explanation of the drawing]

[0014] [Figure 1] This is a schematic diagram showing the hardware configuration of the steering system in the embodiment. [Figure 2] This is a conceptual diagram showing a two-wheeled model (front and rear) related to wheel steering, and how it travels along a target line. [Figure 3] This is a conceptual diagram illustrating the rear wheel's ability to avoid uneven surfaces using rear wheel avoidance control, and also a conceptual diagram showing the vehicle's behavior when rear wheel avoidance control is performed while target line driving control is being performed. [Figure 4] This is a conceptual diagram illustrating a method for detecting wheel lift using ZMP. [Figure 5] This is a flowchart of the target steering angle determination process program executed in the steering system of the embodiment. [Modes for carrying out the invention]

[0015] Hereinafter, a steering system, which is an embodiment of the present invention, will be described in detail with reference to the drawings as an embodiment for carrying out the present invention. In addition to the embodiments described below, the present invention can be carried out in various forms by making various changes and improvements based on the knowledge of those skilled in the art, starting with the forms described in the section [Embodiments of the Invention] above. [Examples]

[0016] [A] Steering system hardware configuration The steering system of this embodiment (hereinafter sometimes referred to as "this system") is mounted on a four-wheeled vehicle with left front wheel 10FL, right front wheel 10FR, left rear wheel 10RL, and right rear wheel 10RR, as shown in Figure 1. This system is equipped with a left front wheel steering device 12FL, a right front wheel steering device 12FR, a left rear wheel steering device 12RL, and a right rear wheel steering device 12RR in order to independently steer these four wheels 10FL, 10FR, 10RL, and 10RR, in other words, to rotate them around the kingpin axis. It is equipped with four steering electronic control units (hereinafter sometimes referred to as "steering ECUs") 14 that control these four wheel steering devices 12FL, 12FR, 12RL, and 12RR, and a general electronic control unit (hereinafter sometimes referred to as "general ECU") 16 for overall control of this system. The controller of this system is composed of these four steering ECUs 14 and the general ECU 16.

[0017] In the following explanation, when it is not necessary to distinguish between the left and right front wheels 10FL and 10FR, they will be collectively referred to as front wheel 10F, and when it is not necessary to distinguish between the left and right rear wheels 10RL and 10RR, they will be collectively referred to as rear wheel 10R. When it is not necessary to distinguish between front wheel 10F and rear wheel 10R, they will be collectively referred to as wheel 10. Similarly, the wheel steering devices will be collectively referred to as front wheel steering device 12F, rear wheel steering device 12R, and wheel steering device 12. Furthermore, when it is not necessary to distinguish between left and right or front and rear, the subscripts F, R, FL, FR, RL, and RR may be omitted for the specifications and various parameters corresponding to each wheel 10. Incidentally, it can also be considered that the left front wheel steering device 12FL and the right front wheel steering device 12FR constitute one front wheel steering device, and the left rear wheel steering device 12RL and the right rear wheel steering device 12RR constitute one rear wheel steering device.

[0018] Since each steering device 12 has a general structure, a description of its structure will be omitted. The wheel steering device 12 has a steering motor 18, which is a DC brushless motor, as its drive source, and steers the wheels 10 by an angle corresponding to the amount of rotation of the steering motor 18.

[0019] Each steering ECU 14 includes a computer with a CPU, ROM, RAM, etc. as its main component and has an inverter as the drive circuit (driver) for the steering motor 18. The central ECU 16 is composed of a computer with a CPU, ROM, RAM, etc. as its main component. Each steering ECU 14 and the central ECU 16 are connected to a CAN (car area network or controllable area network) 20 and are able to communicate with each other via this CAN 20.

[0020] This vehicle is equipped with various devices, equipment, and systems, and those related to this system are briefly described below. Since the vehicle equipped with this system operates autonomously, it is equipped with an autonomous driving electronic control unit (hereinafter sometimes referred to as "autonomous driving ECU") 22 that controls autonomous driving (hereinafter sometimes referred to as "autonomous driving"). This vehicle is equipped with a driving information acquisition device 24 which includes a camera for monitoring the surroundings, LiDAR, a communication device for communicating information with the outside, a GPS device, etc. The driving information acquisition device 24 acquires driving information, which is information necessary for autonomous driving. Both the autonomous driving ECU 22 and the driving information acquisition device 24 are connected to CAN 20, and the autonomous driving ECU 22 determines the target line, vehicle speed, etc., which is the driving line that the vehicle should follow, based on the driving information acquired by the driving information acquisition device 24. In addition, the vehicle is equipped with sensors for detecting the vehicle's attitude, behavior, etc. Specifically, each wheel 10 is equipped with a stroke sensor 26 that detects the vertical distance between the wheel 10 and the vehicle body (stroke), and a rotational speed sensor 28 that detects the rotational speed of the wheel 10. Additionally, an IMU (inertial measurement unit) 30 is provided that detects the acceleration in the longitudinal, lateral, and vertical axes of the vehicle, as well as the rotational speed around the longitudinal, lateral, and vertical axes. These sensors 26, 28 and the IMU 30 are also connected to the CAN bus 20.

[0021] [B] Control in the steering system In simple terms, this system is a target steering angle δ for each of the 10 wheels. * The steering angle δ of each wheel 10 was determined, and the target steering angle δ was determined. * To achieve this, each wheel is steered by 10. More specifically, as a basic control that is always performed, the target line L, which is the driving line L that the vehicle should follow, is controlled. *Execute target line driving control for driving the vehicle along a certain path. When any of the front wheels 10F passes over an unevenness, while executing the target line driving control, execute rear wheel avoidance control to prevent the rear wheel 10R located behind the front wheel 10F from passing over the unevenness. Below, these target line driving control and rear wheel avoidance control will be explained in detail, and then, the flow of the target steering angle determination process performed in these controls will be explained.

[0022] (a) Target line driving control In the target line driving control, the overall ECU 16 executes a target steering angle determination process, which is a process of determining the target steering angle δ of each wheel 10, and each steering ECU 14 executes a wheel steering process, which is a process of steering each wheel 10 so that the steering angle δ of each wheel 10 becomes the target steering angle δ. Below, these processes will be explained in detail.

[0023] i) Target steering angle determination process for driving along the target line The overall ECU 16 determines the target front wheel steering angle δ F as the target of the front wheel steering angle δ F and the target rear wheel steering angle δ R as the target of the rear wheel steering angle δ R according to the front and rear two-wheel model shown in Fig. 2(a). Therefore, in this system, for the sake of convenience, both the front wheel 10F and the rear wheel 10R are steered at the same steering angle δ on the left and right.

[0024] In the above front and rear two-wheel model, in the steady state, the following two equations of motion hold. <000023​​​​​​​​​​​​​​​​​​​​​​​​​​​​β is the distance between the centers of gravity of the front wheels and the distance between the centers of gravity of the rear wheels, respectively; β is the lateral slip angle at the lateral point of the vehicle's center of gravity; M is the vehicle weight; V is the vehicle speed; γ is the yaw rate; K F ,K R These represent the front wheel cornering stiffness and rear wheel cornering stiffness, respectively. In the diagram, X represents the vehicle's longitudinal axis, and X' represents the direction of travel. The front wheel 10F and rear wheel 10R are steered in the same phase. In other words, the steering directions of the front wheel 10F and rear wheel 10R are drawn to be the same. In this system, the front wheel 10F and rear wheel 10R can also be steered in opposite phases, that is, their steering directions are opposite. In that case, the steering angle δ of the front wheel 10F is... F and the steering angle δ of the rear wheel 10R R These values ​​are distinct in terms of their positive and negative signs.

[0025] Here, we define the rotational curvature κ as follows:

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[0026] By solving the two equations of motion above, the front wheel steering angle δ can be determined. F , rear wheel steering angle δ R This can be calculated as follows:

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[0027] On the other hand, in this system, as shown in Figure 2(b), the target line L is the target travel path L.* The system performs target line driving control to make the vehicle travel along the target line L. In other words, it controls the vehicle to travel so that the center of gravity C moves along the target driving line. Therefore, the target line L is located in front of the vehicle. * Above, a target point G is set. In other words, the target point G is set so that it is in a specific positional relationship with the vehicle. The target distance, which is the distance between this target point G and the vehicle's center of gravity C in the longitudinal direction of the vehicle, is S G The target point angle, which is the angle between the line connecting the target point G and the vehicle's center of gravity C and the vehicle's longitudinal axis X, is δ G Therefore, the turning curvature κ can be expressed as follows:

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[0028] In this system, the target distance S G This is set according to the vehicle speed V, and specifically, it is set to the distance that will reach the target point G in a few seconds (for example, 2 seconds). Also, the vehicle center of gravity lateral slip angle β in the above formula is the target lateral slip angle β as the target of the vehicle center of gravity lateral slip angle β. * This is set to the target sideslip angle β. * This determines the turning characteristics of the vehicle. Therefore, the target sideslip angle β * This can be set arbitrarily according to the steering characteristics you want to achieve, but in this system, during turning, the target sideslip angle β * However, the standard side-slip angle β REF It is set to the standard lateral slip angle β. Specifically, the standard lateral slip angle β REF This can be set to, for example, 0° in order to achieve so-called zero-beta turning. Incidentally, the standard sideslip angle β REF This can also be set to change according to the vehicle speed V, etc. The overall ECU16 sets the target sideslip angle β during cornering. * Standard lateral slip angle β REF By substituting the values, we can find the turning curvature κ from the above equation, and then compare that turning curvature κ with the target sideslip angle β. * Based on the above two equations, the front wheel steering angle δ F The target front wheel steering angle δF * , rear wheel steering angle δ R The target rear wheel steering angle δ R * This determines the target line. Furthermore, in this system, target line travel control is performed regardless of whether the vehicle is traveling in a straight line or turning. Therefore, when traveling in a straight line, that is, the target point angle δ G When it is 0°, the target sideslip angle β * It is set to 0°.

[0029] Furthermore, the steering angle δ of the wheel 10 changes according to the vehicle speed V, and the target steering angle δ of the front wheel is set accordingly. F * , Rear wheel target steering angle δ R * You may decide on the target steering angle δ of the front wheels. Specifically, for example, the target steering angle δ of the front wheels. F * , Rear wheel target steering angle δ R * The target steering angle δ of the left and right front wheels 10F and left and right rear wheels 10R may be determined such that it becomes smaller as the vehicle speed V increases and larger as the vehicle speed V decreases. * The target steering angle δ of the left and right wheels 10 may be determined such that the steering ratio of the left and right wheels 10 approaches Ackermann geometry when the vehicle speed V is low, and approaches parallel geometry when the vehicle speed V is high. * It is permissible to make such a decision.

[0030] ii) Wheel steering process Wheel steering is performed by each of the four steering ECUs 14, and the steering angle δ of the corresponding wheel 10 is sent via CAN 20 to the target steering angle δ. * This process is for steering the corresponding wheel 10 to achieve the desired steering angle δ. Specifically, the steering ECU 14 controls the target steering angle δ. * The steering angle deviation Δδ (=δ) is the deviation from the actual steering angle δ relative to the given value. *The steering angle δ is determined. Note that the actual steering angle δ is proportional to the motor rotation angle of the steering motor 18, and the steering ECU 14 detects the actual steering angle δ based on that motor rotation angle.

[0031] Then, according to the following equation, that is, according to the PID control law, the target steering torque T is the steering torque T required to steer the wheel 10 based on the steering angle deviation Δδ. * This will determine G in the following equation. P ,G I ,G D These are the proportional term gain, the integral term gain, and the differential term gain, respectively.

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[0032] The steering torque T can be considered to be roughly proportional to the steering current I, which is the current supplied to the steering motor 18. Accordingly, the steering ECU 14 sets the determined target steering torque T * The coefficient of determination of steering current K I By multiplying by this, the target steering current I is obtained. * Determine the target steering current I * Based on this, current is supplied to the steering motor 18.

[0033] (b) Rear wheel avoidance control If either the left or right front wheel 10F passes over an uneven surface that disrupts the vehicle's movement, and the rear wheel 10R on the same side also passes over that same surface, the vehicle's movement becomes considerably unstable. Therefore, this system performs rear wheel avoidance control to prevent the rear wheel 10R from passing over the same uneven surface that the front wheel 10F has passed over. The processing in rear wheel avoidance control is described below.

[0034] i) Changing the target sideslip angle In rear-wheel avoidance control, the overall ECU16 determines the target lateral slip angle β in the target line driving control described above. * This is the 0° angle when moving straight or the standard lateral slip angle β when turning. REFTherefore, rear wheel avoidance side slip angle β A This will be changed to the rear wheel avoidance side slip angle β. Referring to Figure 3(a), the rear wheel avoidance side slip angle β will be explained. A This is determined according to the following formula. Note that Figure 3(a) shows the situation where the left front wheel 10FL passes over the uneven surface R while the vehicle is turning left, and S A This is the distance at which you want to avoid the rear wheel 10R, i.e., the rear wheel shift distance. Incidentally, the rear wheel 10R can be shifted to either the left or the right, but in this system, the rear wheel 10R is shifted closer to the center of the vehicle relative to the front wheel 10F.

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[0035] Rear wheel avoidance control, i.e., target sideslip angle β * Rear wheel avoidance side slip angle β A The change is performed until the rear wheel 10R passes over the bump. Specifically, it is performed from the moment the front wheel 10F passes over the bump R until the vehicle has traveled a distance equivalent to the wheelbase l. The central ECU 16 obtains the vehicle's mileage D based on odometer information. Specifically, for example, the wheel speed v of each wheel 10 W Based on the above, the target lateral slip angle β is obtained. The overall ECU16 determines the target lateral slip angle β when the vehicle has traveled a distance equivalent to the wheelbase l. * Rear wheel avoidance side slip angle β A Therefore, it is 0° when moving straight, and the standard lateral slip angle β when turning. REF Return to the original state.

[0036] As shown schematically in Figure 3(b), the vehicle moves along the target line L through the rear-wheel avoidance control described above. * While traveling along this line, in other words, the center of gravity C is on the target line L. * This allows the vehicle to move over the surface while avoiding passing over the 10R curve of the rear wheels.

[0037] ii) Starting conditions The rear-wheel avoidance control is started triggered by any one of the front wheels 10F passing over the unevenness R. The unevenness R that triggers is such that if even the rear wheels 10R pass over it, it is presumed that the running of the vehicle will become considerably unstable. The load of the vehicle body is shared by the four wheels 10. However, when one front wheel 10F passes over a certain unevenness, depending on the degree of the unevenness, not only that one front wheel 10F but also one rear wheel 10 located on the same side on the left and right may not share the weight of the vehicle body. Simply put, both one front wheel 10F and one rear wheel 10R on one side are in a floating state (hereinafter, sometimes referred to as the "one-side two-wheel floating state"). In this system, the execution of the rear-wheel avoidance control is started on the condition that the above one-side two-wheel floating state occurs.

[0038] The detection of the above one-side two-wheel floating state is performed according to a method based on ZMP (zero moment point). Since this method is a common method, this method will be briefly described below while referring to FIG. 4. Incidentally, "ZMP" means the point where the resultant force of the gravity and the inertial force acting on the vehicle intersects with the road surface.

[0039] In the (x, y, z) coordinate system, if the center-of-gravity position vector 《C》, the vehicle momentum vector 《P》, the vehicle angular momentum vector 《L》, the gravitational acceleration vector 《g》, and the ZMP position vector 《p》 are defined as follows, 《C》 = [xyz] T 《P》 = [P X P第16行: Y P Z T 《L》 = [L X L Y L Z T 《g》 = [00 - g] T <9000336>《p》 = [p X p Y p Z T Regarding the position of the ZMP, the following two equations hold. Incidentally, M is the vehicle weight and g is the gravitational acceleration.​​​

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[0040] The diagram shows the center of gravity C on the sprung mass. S , unsprung center of gravity C corresponding to each of the 10 wheels U The diagram shows that the plane demarcated by connecting the points where each of the four wheels 10 touches the road surface is defined as the support plane PL. The position of the projection point p of ZMP onto the support plane PL is determined by the road surface input from the wheels 10. X ,p Y The front wheels move along the support plane PL. When one of the two front wheels 10F passes over a bump or irregularity R, if the height or depth of the bump or irregularity R is relatively large, the ZMP moves significantly, and the projection point p moves to the position of either the left or right side of the support plane PL. At this time, two wheels on one side of the vehicle are lifted off the ground. In this system, when the projection point p of the ZMP moves to the position of either the left or right side of the support plane PL, the system assumes that the front wheels 10F have passed over the bump or irregularity R that needs to be addressed, and initiates rear wheel avoidance control.

[0041] (c) Flowchart for determining the target steering angle The target steering angle determination process in the target line driving control, including the rear wheel avoidance control described above, is performed by the integrated ECU16 repeatedly executing the target steering angle determination process program, whose flowchart is shown in Figure 5, at short time intervals (for example, several to tens of milliseconds). The flow of the target steering angle determination process will be explained below in accordance with its flowchart.

[0042] Target line L of the vehicle in question * The data related to this is constantly transmitted from the automatic driving ECU22, and in the processing according to the above program, first, in step 1 (hereinafter abbreviated as "S1"; the same applies to the other steps), the above target point angle δ is calculated based on that data. Gis specified. In the next S2, it is determined whether the rear-wheel avoidance flag FL is 1. This rear-wheel avoidance flag FL is a flag that is set to 1 when the above-described rear-wheel avoidance control is being performed and set to 0 when the rear-wheel avoidance control is not being performed.

[0043] When the rear-wheel avoidance control is not being performed, in S3, it is detected based on the above-described ZMP method whether either of the left and right front wheels 10F has passed over the unevenness R to be dealt with. If neither of the front wheels 10F has passed over the unevenness R, in S4, the rear-wheel avoidance flag FL is reset to 0. Subsequently, in S5, it is determined whether the vehicle is going straight or turning. Specifically, when the target point angle δ G is 0, it is determined that the vehicle is going straight, and when it is not 0, it is determined that the vehicle is turning. In the case of going straight, in S6, the above-described target lateral slip angle β * is set to 0, and in the case of turning, in S7, the target lateral slip angle β * is set to the standard lateral slip angle β REF .

[0044] If it is determined in S3 that either of the front wheels 10F has passed over the unevenness R, in S8, the rear-wheel avoidance flag FL is set to 1 and the rear-wheel avoidance control is started. Then, in S9, the rear-wheel avoidance lateral slip angle β A is determined as described above, and in S10, the target lateral slip angle β * is set to the rear-wheel avoidance lateral slip angle β A . Subsequently, in S11, the travel distance D is reset to 0. The travel distance D is the distance that the vehicle has traveled since the front wheel 10F passed over the unevenness R.

[0045] When it is determined in S2 that the rear-wheel avoidance flag FL is 1, that is, when the rear-wheel avoidance control is being executed, in S12, the travel distance D is updated by the update distance ΔD. This update distance ΔD is the product of the execution time pitch of the program and the wheel rotation speed v of each wheel 10 WThe decision is made based on the above. When rear wheel avoidance control is being performed, in S13 it is determined whether the travel distance D exceeds the distance corresponding to the wheelbase l of the vehicle. If the travel distance D exceeds the distance corresponding to the wheelbase l, in S4 the rear wheel avoidance flag FL is reset to 0 and the execution of rear wheel avoidance control is terminated.

[0046] The target lateral slip angle β set as described above * And, the target point angle δ G And, target distance S G Based on the above, in S14, the target steering angle δ of the front wheels is determined according to the method described above. F * , Rear wheel target steering angle δ R * The target steering angle δ of the front wheels is determined in S15. F * , Rear wheel target steering angle δ R * A signal regarding this is sent to each steering ECU14 via CAN20. Target steering angle δ * Upon receiving a signal about this, each steering ECU 14 sets the steering angle δ of the corresponding wheel 10 to its target steering angle δ * To achieve this, the corresponding wheel is steered as described above. [Explanation of Symbols]

[0047] 10F: Front wheels 10R: Rear wheels 12F: Front wheel steering system 12R: Rear wheel steering system 14: Steering electronic control unit (Steering ECU) [Controller] 16: Integrated electronic control unit (Integrated ECU) [Controller] β A Rear wheel avoidance side slip angle S A Rear wheel shift distance L * :Target line X:Vehicle front / rear axle line l:Wheelbase δ F :Front wheel steering angle δ R :Rear wheel steering angle R:Unevenness C:Vehicle center of gravity

Claims

1. A steering system installed in a vehicle, The system comprises a front wheel steering device for steering the front wheels, a rear wheel steering device for steering the rear wheels, and a controller that controls the front wheel steering device and the rear wheel steering device so that the front wheels and rear wheels can be steered independently of each other. The aforementioned controller A steering system configured to perform rear-wheel avoidance control when the front wheels pass over an uneven surface, to prevent the rear wheels from passing over that uneven surface, to determine the rear-wheel avoidance lateral slip angle, which is the lateral slip angle of the vehicle's center of gravity that should be achieved in the rear-wheel avoidance control, and to determine the target steering angle, which is the steering angle at which the front wheels and rear wheels should each be steered, based on the rear-wheel avoidance lateral slip angle.

2. The aforementioned controller The steering system according to claim 1, configured to determine the rear wheel avoidance lateral slip angle based on the distance by which the rear wheels should be shifted laterally from the uneven surface and the wheelbase of the vehicle, in the rear wheel avoidance control.

3. The aforementioned controller The steering system according to claim 1, which is configured to perform target line driving control, steering the front and rear wheels so that the vehicle drives along a target line which is the driving line to be traveled, and to perform rear wheel avoidance control while continuing to perform the target line driving control when the front wheels pass over an uneven surface.

4. The aforementioned controller In the aforementioned target line driving control, a target point is set on the target line in front of the vehicle, and the target steering angles of the front and rear wheels are determined based on the positional relationship between the target point and the vehicle, and a standard lateral slip angle which is a preset lateral slip angle of the vehicle's center of gravity. The steering system according to claim 3, configured to perform rear wheel avoidance control while performing target line driving control, by determining the target steering angles of the front wheels and rear wheels based on the rear wheel avoidance lateral slip angle instead of the standard lateral slip angle.

5. The aforementioned controller The steering system according to claim 1, configured to perform the rear wheel avoidance control on the condition that both the front wheel and the rear wheel are lifted off the ground.

6. The aforementioned controller The steering system according to claim 1, configured to perform the rear wheel avoidance control after the front wheels have passed over an uneven surface until the vehicle has traveled a distance equivalent to the vehicle's wheelbase.

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