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] The present invention relates to a steering system provided in a vehicle. [Background technology]

[0002] Technologies such as those described in the following patent documents are being considered, namely, technologies that detect wheel lift while a vehicle is moving and prevent the vehicle from rolling over by warning the driver or braking the vehicle. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2005-239078 A Summary of the Invention [Problem to be solved by the invention]

[0004] In the technology described in the above patent document, for example, when the front wheels pass over an uneven road surface, the rear wheels cannot be prevented from passing over the uneven road surface, and it is difficult to say that the stability of the vehicle can be maintained well. If a steering system that can ensure the stability of the vehicle's running well is realized, the steering system will be highly practical. The present invention has been made in view of such a situation, and an object of the present invention is to provide a steering system that is highly practical. [Means for solving the problem]

[0005] In order to solve the above problems, the steering system of the present invention comprises: A steering system provided in a vehicle, a front wheel steering device for steering front wheels, a rear wheel steering device for steering rear wheels, and a controller for controlling the front wheel steering device and the rear wheel steering device so that the front wheels and the rear wheels can be steered independently of each other; The controller: When the front wheels pass over an uneven surface, rear wheel avoidance control is executed to prevent the rear wheels from passing over the uneven surface, and in the rear wheel avoidance control, a rear wheel avoidance side slip angle, which is the vehicle center of gravity point side slip angle to be realized, is determined, and a target steering angle, which is the steering angle at which each of the front wheels and rear wheels should be steered, is determined based on the rear wheel avoidance side slip angle. Effect of the Invention

[0006] "Vehicle center of gravity side slip angle" can also be called vehicle body slip angle, and is the angle between an axis that passes through the center of gravity of the vehicle and extends in the longitudinal direction of the vehicle (hereinafter sometimes referred to as "vehicle longitudinal axis") and a line that extends along the direction of travel of the vehicle (hereinafter sometimes referred to as "driving direction line"). In "rear wheel avoidance control", the vehicle center of gravity side slip angle is set to an appropriate angle so that the rear wheels do not pass over uneven surfaces. This angle is the above-mentioned "rear wheel avoidance side slip angle". In rear wheel avoidance control, the steering angle of the front wheels and the steering angle of the rear wheels are controlled to achieve this rear wheel avoidance side slip angle, and this control makes it possible to prevent the rear wheels from passing over uneven surfaces that the front wheels have passed over. As a result, the running stability of the vehicle is improved. Aspects of the Invention

[0007] When the steering system of the present invention (hereinafter sometimes referred to as "the present steering system") is applied to a front-rear, left-right, four-wheel vehicle having two front wheels (left and right) and two rear wheels (left and right), the "front wheel steering device" may be one that steers the two front wheels integrally, 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 integrally, 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 device, or may be one that includes, 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 together. Note that the present steering system is a system in which both the front wheels and the rear wheels can be steered by the control of the controller, and is suitable for an automatically driven vehicle.

[0008] The "unevenness" to be avoided by the rear wheel avoidance control is a general term for depressions, depressions, convexities, rises, protrusions, steps, etc. on the road surface that disrupt the running of the vehicle when the wheels pass over them. The unevenness to be avoided by the rear wheel avoidance control can be limited to unevenness that significantly disrupts the running of the vehicle. Specifically, it is possible to target unevenness that causes the front and rear wheels to not share the load of the vehicle body when the front wheels pass over the unevenness, or in simple terms, causes both the front and rear wheels to be in a floating state. In the above-mentioned front-rear, left-right, and right-left four-wheel vehicle, it is sufficient to target unevenness that causes both the front and rear wheels to be in a floating state on either the left or right side. In other words, the steering system may be configured to execute the rear wheel avoidance control on the condition that both the front and rear wheels are detected to be floating. A method based on ZMP (zero moment point) can be adopted as a method for detecting the floating of the front and rear wheels. This method will be described later.

[0009] In the rear wheel avoidance control, in order to prevent the rear wheels from passing over the unevenness, the rear wheel avoidance sideslip angle may be determined based on, for example, a distance by which the rear wheels should be shifted laterally from the unevenness (hereinafter, may be referred to as "rear wheel shift distance") and the wheelbase of the vehicle. A specific determination method will be described later. Incidentally, in the rear wheel avoidance control, the rear wheels may be shifted to the outside of the turn relative to the front wheels, or to the inside of the turn, or may be shifted toward the center of the vehicle, or may be shifted toward the outside of the vehicle.

[0010] In order to reliably prevent the rear wheels from passing over uneven surfaces, the rear wheel avoidance control may be executed until the vehicle travels a distance equivalent to the wheelbase of the vehicle after the front wheels have passed over the uneven surfaces. In other words, the rear wheel avoidance control may be terminated when the vehicle travels a distance equivalent to the wheelbase of the vehicle. The travel distance of the vehicle may be obtained by estimating the vehicle's own position based on odometry information obtained by a sensor that measures the amount of wheel rotation, a GPS, a camera, etc.

[0011] In this steering system, a "target line running control" may be executed to steer the front and rear wheels so that the vehicle runs along a target line, which is the line along which the vehicle should run. The target line running control enables the vehicle to run along an appropriate running line. When the front wheels pass over an uneven surface while the target line running control is being executed, the target line running control may be continued while the rear wheel avoidance control is executed. By executing these two controls in parallel, the vehicle can be run along an appropriate running line while preventing the rear wheels from passing over an uneven surface.

[0012] Specifically, the target line running control may be performed by, for example, 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 "standard side slip angle" which is a preset vehicle center of gravity point side slip angle. The standard side slip angle characterizes the turning characteristics of the vehicle, and for example, if the standard side slip angle is set to 0°, the front and rear wheels are steered in opposite phases to each other, enabling so-called zero beta turning. By the way, if this zero beta turning is realized, the delay in response can be reduced and the steering performance can be improved. The standard side slip angle may be set to a fixed value, or may be set to a variable value according to the vehicle running speed (hereinafter sometimes referred to as "vehicle speed"), turning radius, etc.

[0013] In order to execute rear wheel avoidance control while performing target line driving control as described above, for example, a rear wheel avoidance side slip angle may be adopted instead of the standard side slip angle, and the target steering angles of the front and rear wheels may be determined based on the rear wheel avoidance side slip angle. [Brief description of the drawings]

[0014] [Figure 1] FIG. 2 is a schematic diagram showing a hardware configuration of a steering system according to an embodiment of the present invention. [Diagram 2] This is a conceptual diagram showing a front and rear two-wheel model regarding wheel steering and how the vehicle travels along a target line. [Diagram 3] 1A and 1B are conceptual diagrams for explaining how rear wheels avoid bumps by rear wheel avoidance control, and conceptual diagrams showing vehicle behavior when rear wheel avoidance control is performed while target line driving control is being performed; [Figure 4] FIG. 1 is a conceptual diagram for explaining a method for detecting wheel lift using ZMP. [Diagram 5] 4 is a flowchart of a target steering angle determination processing program executed in the steering system of the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Hereinafter, as a mode for carrying out the present invention, a steering system according to an embodiment of the present invention will be described in detail with reference to the drawings. In addition to the following embodiment, the present invention can be carried out in various forms including the form described in the above section [Modes of the Invention], and various modifications and improvements based on the knowledge of those skilled in the art. EXAMPLES

[0016] [A] Steering system hardware configuration The steering system of the embodiment (hereinafter sometimes referred to as "this system") is mounted on a front-rear four-wheel vehicle having a left front wheel 10FL, a right front wheel 10FR, a left rear wheel 10RL, and a right rear wheel 10RR, as shown in FIG. 1. This system is provided 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 about the kingpin axis. This system is provided 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 that generalizes and controls this system. The controller of this system is configured to include these four steering ECUs 14 and the general ECU 16.

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

[0018] Each of the steering devices 12 has a general structure, and therefore a description of the structure will be omitted. The wheel steering device 12 has a steering motor 18, which is a DC brushless motor, as a 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 having a CPU, ROM, RAM, etc. as a main component, and has an inverter as a drive circuit (driver) for the steering motor 18. The management ECU 16 is configured to include a computer having a CPU, ROM, RAM, etc. as a main component. Each steering ECU 14 and the management ECU 16 are connected to a CAN (car area network or controllable area network) 20, and are capable of communicating with each other via this CAN 20.

[0020] The vehicle is provided with various devices, equipment, and systems, and those related to the system will be briefly described below. The vehicle in which the system is provided is provided with an automatic driving electronic control unit (hereinafter, sometimes referred to as "automatic driving ECU") 22 that controls the automatic driving (hereinafter, sometimes referred to as "automatic driving"). The vehicle is provided with a driving information acquisition device 24 that includes a camera for monitoring the surroundings, a LiDAR, a communication device for communicating information with the outside, a GPS device, etc., and the driving information acquisition device 24 acquires driving information that is information necessary for automatic driving. The automatic driving ECU 22 and the driving information acquisition device 24 are also connected to the CAN 20, and the automatic driving ECU 22 determines a target line, which is a driving line on which the vehicle should travel, a vehicle speed, etc., based on the driving information acquired by the driving information acquisition device 24. The vehicle is also provided with a sensor for detecting the attitude, behavior, etc. of the vehicle. Specifically, a stroke sensor 26 that detects the stroke, which is the vertical distance between the wheel 10 and the vehicle body, a rotational speed sensor 28 that detects the rotational speed of the wheel 10, and an IMU (inertial measurement unit) 30 that detects the acceleration in the front-rear, left-right, and up-down axial directions of the vehicle and the rotational speed around the front-rear, left-right, and up-down axes are provided for each wheel 10. These sensors 26, 28 and IMU 30 are also connected to the CAN 20.

[0021] [B] Control in steering system In simple terms, the present system is configured to detect a target steering angle δ of each wheel 10. * The steering angle δ of each wheel 10 is determined based on the determined target steering angle δ * In detail, as a basic control that is always performed, the vehicle is steered along a target line L *A target line running control is executed to make the vehicle run along the uneven surface. Then, when any of the front wheels 10F passes over an uneven surface, a rear wheel avoidance control is executed to prevent the rear wheel 10R located behind the front wheel 10F from passing over the uneven surface. The target line running control and the rear wheel avoidance control are described in detail below, and then the flow of the target steering angle determination process performed in these controls is described.

[0022] (a) Target line driving control In the target line running control, the general ECU 16 sets the target steering angle δ of each wheel 10. * The steering ECU 14 executes a target steering angle determination process to determine the steering angle δ of each wheel 10. * The wheel steering process is executed to steer each of the wheels 10 so that the wheel steering angle is set to the predetermined angle. The wheel steering process will be described in detail below.

[0023] i) Processing for determining a target steering angle for traveling along a target line The central ECU 16 calculates the front wheel steering angle δ, which is the steering angle δ of the front wheel 10F, according to the front and rear two-wheel model shown in FIG. F The target front wheel steering angle δ F * , the steering angle δ of the rear wheel 10R is R The target rear wheel steering angle δ R * Therefore, in this system, for convenience, the left and right front wheels 10F and the rear wheels 10R are both steered to the same steering angle δ.

[0024] In the above front and rear two-wheel model, the following two equations of motion hold in a steady state:

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[0025] Here, the turning curvature κ is defined as follows:

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

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[0027] On the other hand, in this system, as shown in FIG. 2(b), the target line L* In other words, the vehicle is controlled to travel so that the center of gravity C moves along the target travel line. * A target point G is set on the vehicle. In other words, the target point G is set so as to have a specific positional relationship with the vehicle. The target distance, which is the distance between the target point G and the vehicle center of gravity C in the vehicle front-rear direction, is defined as S G The target point angle, which is the angle between the line connecting the target point G and the vehicle center of gravity C and the vehicle longitudinal axis X, is defined as δ G Then, the turning curvature κ can be expressed as follows:

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[0028] In this system, the target distance S G is set according to the vehicle speed V, specifically, set to a distance that reaches the target point G in several seconds (for example, 2 seconds). Also, the vehicle center of gravity point side slip angle β in the above formula is set to the target side slip angle β * This target side slip angle β * determines the turning characteristics of the vehicle. Therefore, the target side slip angle β * can be set arbitrarily according to the steering characteristics to be realized, but in this system, during turning, the target side slip angle β * But the standard side slip angle β REF Specifically, the standard side slip angle β REF may be set to, for example, 0° to achieve a so-called zero beta turn. REF can be set to change according to the vehicle speed V, etc. During turning, the control ECU 16 sets the target sideslip angle β * Standard sideslip angle β REF By substituting the above, the turning curvature κ is calculated from the above formula, and the turning curvature κ and the target side slip 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 * Furthermore, in this system, the target line driving control is executed regardless of whether the vehicle is traveling straight or turning. Therefore, when traveling straight, that is, when the target point angle δ G When the target side slip angle β is 0°, * is set to 0°.

[0029] In addition, the front wheel target steering angle δ is set so that the steering angle δ of the wheels 10 changes according to the vehicle speed V. F * , Rear wheel target steering angle δ R * Specifically, for example, the front wheel target steering angle δ F * , Rear wheel target steering angle δ R * may be determined so that the higher the vehicle speed V, the smaller the target steering angle δ is, and the lower the vehicle speed V, the larger the target steering angle δ is. * For example, the target steering angles δ of the left and right wheels 10 may be determined so that the steering ratio of the left and right wheels 10 approaches the Ackermann geometry when the vehicle speed V is low and approaches the parallel geometry when the vehicle speed V is high. * It is possible to determine the following.

[0030] ii) Wheel steering processing The wheel steering process is a process performed by each of the four steering ECUs 14, and the steering angle δ of the corresponding wheel 10 is calculated based on the target steering angle δ sent via the CAN 20. * Specifically, the steering ECU 14 steers the corresponding wheels 10 so that the target steering angle δ * The deviation of the actual steering angle δ from the *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 the motor rotation angle.

[0031] Then, according to the following formula, that is, according to the PID control law, the target steering torque T, which is the steering torque T required to steer the wheels 10 based on the steering angle deviation Δδ, is calculated. * By the way, G in the following equation P ,G I ,G D are the proportional gain, integral gain, and differential gain, respectively.

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

[0033] (b) Rear wheel avoidance control When either the left or right front wheel 10F passes over an uneven surface that disrupts the running of the vehicle, if the rear wheel 10R located on the same left or right side also passes over the uneven surface, the running of the vehicle becomes considerably unstable. Therefore, in this system, rear wheel avoidance control is executed so that the rear wheel 10R does not pass over the uneven surface that the front wheel 10F passed over. The processing in the rear wheel avoidance control will be described below.

[0034] i) Changing the target sideslip angle In the rear wheel avoidance control, the integrated ECU 16 sets the target side slip angle β * 0° when going straight or standard sideslip angle β when turning REFFrom the above, the rear wheel avoidance slip angle β A With reference to FIG. 3(a), the rear wheel avoidance side slip angle β A is determined according to the following formula. Note that FIG. 3(a) shows a situation in which the left front wheel 10FL passes over an uneven surface R while the vehicle is turning left. A is the distance by which the rear wheel 10R is to be avoided, that is, the rear wheel shift distance. Incidentally, the rear wheel 10R may be shifted to the left or right, but in this system, the rear wheel 10R is shifted toward 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 slip angle β A The change to the wheel speed v of each wheel 10 is continued until the rear wheel 10R passes over the bump. Specifically, the change is performed from the time when the front wheel 10F passes over the bump R until the vehicle travels a distance equivalent to the wheelbase l. The supervisory ECU 16 acquires the travel distance D of the vehicle based on the odometry information. Specifically, for example, the wheel speed v of each wheel 10 is W The control ECU 16 acquires the target side slip angle β when the vehicle travels a distance equivalent to the wheelbase l. * The rear wheel avoidance slip angle β A From this, when going straight, it is set to 0°, and when turning, it is set to the standard sideslip angle β REF Return to.

[0036] By the above-described rear wheel avoidance control, the vehicle moves along the target line L * In other words, the center of gravity C is moving along the target line L * It is possible for the rear wheel 10R to avoid passing over the unevenness R while the vehicle travels as if moving over the unevenness R.

[0037] ii) Starting conditions The rear wheel avoidance control is started when one of the front wheels 10F passes over an uneven surface R. The uneven surface R that serves as the trigger is an uneven surface that is so large that it is estimated that the running of the vehicle would become considerably unstable if the rear wheel 10R also passes over the uneven surface R. The load of the vehicle body is shared by the four wheels 10, but when one front wheel 10F passes over a certain uneven surface, depending on the degree of the uneven surface, a state may occur in which not only the one front wheel 10F but also one rear wheel 10 located on the same side of the left or right wheel does not share the weight of the vehicle body. Simply put, both the front wheel 10F and the rear wheel 10R of one of the left or right wheels are in a floating state (hereinafter, sometimes referred to as a "two-wheel floating state on one side"). In this system, the execution of the rear wheel avoidance control is started on the condition that the two-wheel floating state on one side is reached.

[0038] The detection of the two wheels off the ground is performed according to a method based on the ZMP (zero moment point). This method is a common method, so it will be briefly explained below with reference to Figure 4. Note that "ZMP" refers to the point where the resultant force of gravity and inertia force acting on the vehicle intersects with the road surface.

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

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[0040] The figure shows the center of gravity of the spring C S , the unsprung center of gravity C corresponding to each wheel 10 U is shown, and the plane defined by connecting the points where each of the four wheels 10 contacts the road surface is defined as the support plane PL. The position of the projection point p of the ZMP onto the support plane PL (p X ,p Y ) moves on the support plane PL. When one of the two front wheels 10F passes over an unevenness R, if the height or depth of the unevenness R is relatively large, the ZMP moves significantly and the projection point p moves to a position on either the left or right side of the support plane PL. At this time, a two-wheel lift state occurs on either the left or right side of the vehicle. In this system, when the projection point p of the ZMP moves to a position on either the left or right side of the support plane PL, it is determined that the front wheel 10F has passed over the unevenness R that needs to be addressed, and rear wheel avoidance control is started.

[0041] (c) Flow of target steering angle determination process The target steering angle determination process in the target line running control including the rear wheel avoidance control described above is performed by the integrated ECU 16 repeatedly executing the target steering angle determination process program shown in the flowchart in Fig. 5 at short intervals (for example, several to several tens of msec). The flow of the target steering angle determination process will be described below with reference to the flowchart.

[0042] Target line L of the vehicle * The data on the target point angle δ is constantly transmitted from the automatic travel ECU 22. In the process 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 the data. GIn the next step S2, it is determined whether or not 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-mentioned rear wheel avoidance control is being performed, and is set to 0 when the rear wheel avoidance control is not being performed.

[0043] If rear wheel avoidance control is not being performed, in S3, it is detected based on the above-mentioned ZMP method whether either the left or right front wheel 10F has passed over the unevenness R that should 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. Next, in S5, it is determined whether the vehicle is going straight or turning. Specifically, the target point angle δ G When it 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. When the vehicle is going straight, in S6, the above-mentioned target side slip angle β * is set to 0, and in the case of turning, in S7, the target side slip angle β * is the standard side slip angle β REF is set to.

[0044] If it is determined in S3 that any of the front wheels 10F has passed over the unevenness R, the rear wheel avoidance flag FL is set to 1 in S8, and rear wheel avoidance control is started. Then, in S9, the rear wheel avoidance side slip angle β A is determined as described above, and in S10, the target side slip angle β * is the rear wheel avoidance side slip angle β A Then, in S11, the traveled distance D is reset to 0. The traveled distance D is the distance traveled by the vehicle after the front wheels 10F pass 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, the travel distance D is updated by an update distance ΔD in S12. This update distance ΔD is determined by the execution time pitch of the program and the wheel rotation speed v of each wheel 10. WWhen the rear wheel avoidance control is being performed, it is determined in S13 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, the rear wheel avoidance flag FL is reset to 0 in S4, and the execution of the rear wheel avoidance control is terminated.

[0046] The target side slip angle β set as above * and the target point angle δ G and the target distance S G Based on the above, in S14, the front wheel target steering angle δ is calculated according to the above-mentioned method. F * , Rear wheel target steering angle δ R * are determined, and in S15, the determined front wheel target steering angle δ F * , Rear wheel target steering angle δ R * A signal regarding the target steering angle δ is transmitted to each steering ECU 14 via the CAN 20. * Each steering ECU 14 receives the signal and converts the steering angle δ of the corresponding wheel 10 into the target steering angle δ * The corresponding wheel is steered as described above so that [Explanation of symbols]

[0047] 10F: Front wheels 10R: Rear wheels 12F: Front wheel steering device 12R: Rear wheel steering device 14: Steering electronic control unit (steering ECU) [controller] 16: General electronic control unit (general ECU) [controller] β A : Rear wheel slip angle S A : Rear wheel shift distance L * : Target line X: Vehicle longitudinal axis l: Wheelbase δ F :Front wheel steering angle δ R : Rear wheel steering angle R: Unevenness C: Center of gravity of vehicle

Claims

1. A steering system provided in a vehicle, a front wheel steering device for steering front wheels, a rear wheel steering device for steering rear wheels, and a controller for controlling the front wheel steering device and the rear wheel steering device so that the front wheels and the rear wheels can be steered independently of each other; The controller: A steering system configured to execute rear wheel avoidance control for preventing the rear wheels from passing over an uneven surface when the front wheels pass over the uneven surface, determine a rear wheel avoidance side slip angle, which is the vehicle center of gravity point side slip angle to be realized, in the rear wheel avoidance control, and determine a target steering angle, which is the steering angle at which each of the front and rear wheels should be steered, based on the rear wheel avoidance side slip angle.

2. The controller:

2. The steering system according to claim 1, configured to determine the rear wheel avoidance sideslip angle in the rear wheel avoidance control based on a distance by which the rear wheels should be shifted laterally from the unevenness and a wheelbase of the vehicle.

3. The controller: The steering system of claim 1, further comprising: a target line driving control for steering the front and rear wheels so that the vehicle drives along a target line, which is the driving line on which the vehicle should drive; and a steering system configured to execute the rear wheel avoidance control while continuing to execute the target line driving control when the front wheels pass over an uneven surface.

4. The controller: In the target line traveling control, a target point is set on the target line in front of the vehicle, and target steering angles of each of the front wheels and the rear wheels are determined based on a positional relationship between the target point and the vehicle and a standard sideslip angle which is a preset vehicle center-of-gravity point sideslip angle, 4. The steering system according to claim 3, wherein the target steering angles of the front and rear wheels are determined based on the rear wheel avoidance sideslip angle instead of the standard sideslip angle, thereby executing the rear wheel avoidance control while executing the target line traveling control.

5. The controller:

2. The steering system according to claim 1, wherein the rear wheel avoidance control is executed on condition that lifting of both the front and rear wheels is detected.

6. The controller:

2. The steering system according to claim 1, configured to execute the rear wheel avoidance control until the vehicle travels a distance equivalent to a wheelbase of the vehicle after the front wheels have passed over an uneven surface.

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