Patent Publication Number: US-2018029585-A1

Title: Vehicle controller, vehicle, and control system

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Application No. 2016-151346 filed on Aug. 1, 2016 including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The disclosure relates to a vehicle and a vehicle controller and a control system that perform traveling control of a vehicle. Particularly, the disclosure relates to a vehicle, a vehicle controller, and a control system that execute lane departure prevention control. 
     2. Description of Related Art 
     “Lane departure prevention control” for preventing lane departure of a vehicle is known. Specifically, a controller for a vehicle detects a state in which a driver does not intend to change a traveling lane but the vehicle is likely to depart from the traveling lane. When such a state is detected, the controller automatically turns the vehicle in a direction in which lane departure is avoided. This control is called lane departure alert (LDA) or lane keeping assist (LKA). 
     Japanese Patent Application Publication No. 2006-282168 (JP 2006-282168 A) discloses lane departure prevention control based on brake control. According to this control, a controller generates a difference in braking force between right and left wheels in order to turn the vehicle in a direction in which lane departure is avoided. 
     Japanese Patent Application Publication No. 2010-100120 (JP 2010-100120 A) discloses lane departure prevention control using an electric power steering (EPS) device. According to this control, a controller applies a steering torque using the electric power steering device in order to turn the vehicle in a direction in which lane departure is avoided. 
     SUMMARY 
     There is room for further improvement in lane departure prevention control. The disclosure provides a technique capable of controlling behavior of a vehicle more finely than in existing lane departure prevention control. 
     Since the lane departure prevention control operates regardless of a driver&#39;s intention, the lane departure prevention control causes the driver to feel discomfort depending on situations. For example, in a case of lane departure prevention control based on brake control, a vehicle is decelerated even when the driver does not depress a brake pedal. This deceleration causes discomfort to the driver. In a case of lane departure prevention control based on application of a steering torque, the steering torque is transmitted to a driver&#39;s hand gripping a steering wheel. The driver feels a torque different from a road-surface reaction force, which serves as discomfort. When a degree of turning of the vehicle increases due to the lane departure prevention control, a lateral acceleration and a roll angle increase. The driver feels roll behavior which does not correspond to steering, which serves as discomfort. 
     The disclosure also provides a technique capable of reducing discomfort due to lane departure prevention control. 
     A first aspect of the disclosure relates to a vehicle controller. The vehicle controller includes at least one electronic control unit configured to execute: lane departure prevention control of controlling a first actuator such that a vehicle is turned in a direction in which lane departure of the vehicle is avoided; and roll stiffness control of controlling a second actuator such that a roll stiffness of the vehicle is changed, wherein the at least one electronic control unit executes the roll stiffness control by coupling with execution of the lane departure prevention control. 
     A second aspect of the disclosure relates to a vehicle. The vehicle includes: a lane departure prevention device that executes lane departure prevention control of turning the vehicle in a direction in which lane departure of the vehicle is avoided; and a roll stiffness control device that executes roll stiffness control of changing a roll stiffness of the vehicle, wherein the roll stiffness control device executes the roll stiffness control by coupling with execution of the lane departure prevention control by the lane departure prevention device. 
     A third aspect of the disclosure relates to a control system that executes traveling control of a vehicle. The control system includes: a first actuator configured to turn the vehicle; a second actuator configured to change a roll stiffness of the vehicle; and at least one electronic control unit configured to control the first actuator such that the vehicle is turned in a direction in which departure of the vehicle from a traveling lane is avoided, and control the second actuator by coupling with control of the first actuator. 
     According to the above aspects of the disclosure, the roll stiffness control is executed by coupling with the lane departure prevention control. The lane departure prevention control is for turning the vehicle and the roll stiffness control affects steering characteristics of the vehicle. Accordingly, by combining the roll stiffness control with the lane departure prevention control, it is possible to control behavior of a vehicle more finely than in existing lane departure prevention control. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the disclosure will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
         FIG. 1  is a diagram schematically illustrating an example of a configuration of a vehicle according to an embodiment of the disclosure; 
         FIG. 2  is a conceptual diagram illustrating lane departure prevention control; 
         FIG. 3  is a flowchart illustrating an example of the lane departure prevention control; 
         FIG. 4A  is a conceptual diagram illustrating a relationship between a roll stiffness distribution and steering characteristics; 
         FIG. 4B  is a conceptual diagram illustrating a relationship between a roll stiffness distribution and steering characteristics; 
         FIG. 4C  is a conceptual diagram illustrating a relationship between a roll stiffness distribution and steering characteristics; 
         FIG. 5  is a graph illustrating a relationship between a cornering power and a load; 
         FIG. 6  is a block diagram illustrating functions of a controller according to the embodiment; 
         FIG. 7  is a flowchart illustrating a process flow in the controller according to the embodiment in brief; 
         FIG. 8  is a timing chart illustrating roll stiffness control in a first example of traveling control according to the embodiment; 
         FIG. 9  is a conceptual diagram illustrating advantages achieved by the first example of the traveling control according to the embodiment; 
         FIG. 10  is a conceptual diagram illustrating lane departure prevention control in a second example of the traveling control according to the embodiment; 
         FIG. 11  is a timing chart illustrating lane departure prevention control in a third example of the traveling control according to the embodiment; 
         FIG. 12  is a timing chart illustrating roll stiffness control in a fourth example of the traveling control according to the embodiment; 
         FIG. 13  is a flowchart illustrating the roll stiffness control in the fourth example of the traveling control according to the embodiment; 
         FIG. 14  is a flowchart illustrating roll stiffness control in a fifth example of the traveling control according to the embodiment; and 
         FIG. 15  is a timing chart illustrating the roll stiffness control in the fifth example of the traveling control according to the embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     An embodiment of the disclosure will be described below with reference to the accompanying drawings. 
     1. Example of Configuration 
       FIG. 1  is a diagram schematically illustrating an example of a configuration of a vehicle  1  according to an embodiment of the disclosure. The vehicle  1  includes wheels  10 , a steering mechanism  20 , a brake mechanism  30 , a roll stiffness varying mechanism  40 , various sensors, a camera unit  60 , and a controller  100 . 
     &lt;Wheels  10 &gt; 
     The wheels  10  include a front-left wheel  10 FL, a front-right wheel  10 FR, a rear-left wheel  10 RL, and a rear-right wheel  10 RR. In the following description, the front-left wheel  10 FL and the front-right wheel  10 FR may be referred to as “front wheels  10 F,” and the rear-left wheel  10 RL and the rear-right wheel  10 RR may be referred to as “rear wheels  10 R.” 
     &lt;Steering Mechanism  20 &gt; 
     The steering mechanism  20  is an electric power steering (EPS) mechanism. More specifically, the steering mechanism  20  includes a steering wheel  22 , a steering shaft  24 , a steering gear  26 , and an electric actuator  28 . 
     The steering wheel  22  is an operation member which is used for a driver to perform a steering operation. The steering shaft  24  connects the steering wheel  22  and the steering gear  26  to each other and transmits a rotational motion of the steering wheel  22  to the steering gear  26 . The steering gear  26  is connected to the front wheels  10 F via a link mechanism and converts the rotational motion input from the steering shaft  24  into a motion of the link mechanism. A direction of the front wheels  10 F, that is, a traveling direction of the vehicle  1 , can be changed by the motion of the link mechanism. 
     The electric actuator  28  includes an electric motor and generates a steering torque by rotation of the electric motor. The electric actuator  28  applies the generated steering torque to the steering shaft  24  or the steering gear  26 . The generation of the steering torque from the electric actuator  28  may be coupled with the driver&#39;s steering operation using the steering wheel  22  or may be independent therefrom. By generating the steering torque by coupling with the driver&#39;s steering operation, it is possible to assist the steering operation. On the other hand, by generating the steering torque independently from the driver&#39;s steering operation, for example, it is possible to automatically control a posture of the vehicle  1 . 
     &lt;Brake Mechanism  30 &gt; 
     The brake mechanism  30  includes a brake pedal  32 , a master cylinder  34 , a brake actuator  36 , and wheel cylinders  38 FL,  38 FR,  38 RL, and  38 RR. 
     The brake pedal  32  is an operation member which is used for a driver to perform a braking operation. The master cylinder  34  is connected to the wheel cylinders  38 FL,  38 FR,  38 RL, and  38 RR via the brake actuator  36 . The wheel cylinders  38 FL,  38 FR,  38 RL, and  38 RR are disposed in the front-left wheel  10 FL, the front-right wheel  10 FR, the rear-left wheel  10 RL, and the rear-right wheel  10 RR, respectively. 
     The master cylinder  34  supplies a brake fluid with a pressure based on an amount by which the brake pedal  32  is operated by a driver to the brake actuator  36 . The brake actuator  36  distributes the brake fluid from the master cylinder  34  to the wheel cylinders  38 FL,  38 FR,  38 RL, and  38 RR. Braking forces generated in the front-left wheel  10 FL, the front-right wheel  10 FR, the rear-left wheel  10 RL, and the rear-right wheel  10 RR are determined depending on the pressures of the brake fluid supplied to the wheel cylinders  38 FL,  38 FR,  38 RL, and  38 RR. 
     The brake actuator  36  includes a valve or a pump and can independently adjust the pressures of the brake fluid supplied to the wheel cylinders  38 FL,  38 FR,  38 RL, and  38 RR. That is, the brake actuator  36  can independently control the braking forces of the front-left wheel  10 FL, the front-right wheel  10 FR, the rear-left wheel  10 RL, and the rear-right wheel  10 RR. The control of the braking forces by the brake actuator  36  may be coupled with the driver&#39;s braking operation using the brake pedal  32  or may be independent therefrom. By controlling the braking forces of the wheels  10  independently from the driver&#39;s braking operation, for example, it is possible to automatically control the posture of the vehicle  1 . 
     &lt;Roll Stiffness Varying Mechanism  40 &gt; 
     In this embodiment, a roll stiffness of the vehicle  1  is variable. The variable control of the roll stiffness is performed by the roll stiffness varying mechanism  40 . More specifically, the roll stiffness varying mechanism  40  independently controls the roll stiffness of the front wheels  10 F side and the roll stiffness of the rear wheels  10 R side. Examples of the roll stiffness varying mechanism  40  include an active stabilizer and an active suspension. 
     In the example illustrated in  FIG. 1 , the roll stiffness varying mechanism  40  is an active stabilizer. Specifically, the roll stiffness varying mechanism  40  includes a front active stabilizer  40 F of the front wheels  10 F side and a rear active stabilizer  40 R of the rear wheels  10 R side 
     The front active stabilizer  40 F adjusts the roll stiffness of the front wheels  10 F side. More specifically, the front active stabilizer  40 F includes an actuator  42 F and stabilizer bars  44 FL and  44 FR. The stabilizer bar  44 FL connects the actuator  42 F to a support member of the front-left wheel  10 FL. The stabilizer bar  44 FR connects the actuator  42 F to a support member of the front-right wheel  10 FR. The actuator  42 F includes a motor or a reduction gear. The actuator  42 F can increase or decrease the roll stiffness of the front wheels  10 F side by adjusting amounts of torsion of the stabilizer bars  44 FL and  44 FR. 
     The rear active stabilizer  40 R adjusts the roll stiffness of the rear wheels  10 R side. More specifically, the rear active stabilizer  40 R includes an actuator  42 R and stabilizer bars  44 RL and  44 RR. The stabilizer bar  44 RL connects the actuator  42 R to a support member of the rear-left wheel  10 RL. The stabilizer bar  44 RR connects the actuator  42 R to a support member of the rear-right wheel  10 RR. The actuator  42 R includes a motor or a reduction gear. The actuator  42 R can increase or decrease the roll stiffness of the rear wheels  10 R side by adjusting amounts of torsion of the stabilizer bars  44 RL and  44 RR. 
     &lt;Various Sensors&gt; 
     Various sensors include a wheel speed sensor  50 , a steering angle sensor  52 , a vehicle speed sensor  54 , a lateral acceleration sensor  56 , and a yaw rate sensor  58 . 
     The wheel speed sensor  50  includes wheel speed sensors  50 FL,  50 FR,  50 RL, and  50 RR which are disposed in the front-left wheel  10 FL, the front-right wheel  10 FR, the rear-left wheel  10 RL, and the rear-right wheel  10 RR, respectively. The wheel speed sensors  50 FL,  50 FR,  50 RL, and  50 RR detect rotation speeds of the front-left wheel  10 FL, the front-right wheel  10 FR, the rear-left wheel  10 RL, and the rear-right wheel  10 RR, respectively. 
     The steering angle sensor  52  detects a steering angle due to rotation of the steering wheel  22 . The vehicle speed sensor  54  detects a speed of the vehicle  1 . The lateral acceleration sensor  56  detects a lateral acceleration (lateral G) acting on the vehicle  1 . The yaw rate sensor  58  detects an actual yaw rate which is generated in the vehicle  1 . 
     &lt;Camera Unit  60 &gt; 
     The camera unit  60  is used for lane departure prevention control to be described later. The camera unit  60  includes a camera that captures an image of a front side of the vehicle  1  and an image processing unit that processes imaging data. The image processing unit performs processing of the imaging data and extracts boundary lines such as a white line, a yellow line, and a center line on a road surface on the front side of the vehicle  1 . The image processing unit recognizes a traveling lane based on the extracted boundary lines and generates traveling lane information indicating a relationship between the traveling lane and the position of the vehicle  1 . 
     &lt;Controller  100 &gt; 
     The controller  100  is a vehicle controller that performs traveling control of the vehicle  1 . More specifically, the controller  100  is connected to various actuators (the electric actuator  28 , the brake actuator  36 , and the actuators  42 F and  42 R), various sensors, and the camera unit  60 . The controller  100  receives detection information from various sensors and receives traveling lane information from the camera unit  60 . The controller  100  activates necessary actuators to perform traveling control based on the received information. 
     For example, the controller  100  can perform “steering control” of applying a necessary steering torque by activating the electric actuator  28  of the steering mechanism  20 . The controller  100  and the steering mechanism  20  can be said to constitute a “steering controller” that performs the steering control. 
     The controller  100  can perform “brake control” of applying a necessary braking force by activating the brake actuator  36  of the brake mechanism  30 . The controller  100  and the brake mechanism  30  can be said to constitute a “brake controller” that performs the brake control. 
     The controller  100  can execute “roll stiffness control” of changing a roll stiffness of the vehicle  1  by activating the actuators  42 F and  42 R of the roll stiffness varying mechanism  40 . The controller  100  and the roll stiffness varying mechanism  40  can be said to constitute a “roll stiffness controller” that executes the roll stiffness control. 
     Typically, the controller  100  is a microcomputer including a processor, a memory, and input and output interfaces. The controller  100  is also referred to as an electronic control unit (ECU). The controller  100  receives detection information from various sensors and transmits commands to various actuators via the input and output interfaces. A control program is stored in the memory, and the function of the controller  100  is realized by causing the processor to execute the control program. 
     2. Lane Departure Prevention Control 
       FIG. 2  is a conceptual diagram illustrating lane departure prevention control. A state in which a driver does not intend to change a traveling lane but the vehicle  1  is likely to depart from the traveling lane will be considered. In the lane departure prevention control, when such a state is detected, the vehicle  1  is automatically turned in a direction in which lane departure is avoided. This lane departure prevention control is also referred to as lane departure alert (LDA) or lane keeping assist (LKA). 
     The controller  100  according to this embodiment executes the lane departure prevention control. A specific method of the lane departure prevention control is not particularly limited. For example, a method disclosed in JP 2006-282168 A or JP 2010-100120 A may be used. 
     An example of the lane departure prevention control according to this embodiment will be described below.  FIG. 3  is a flowchart illustrating an example of the lane departure prevention control. The flow illustrated in  FIG. 3  is repeatedly performed while the vehicle  1  is traveling. 
     Step S 1 : The controller  100  receives traveling information from various sensors mounted in the vehicle  1 . The traveling information includes rotation speeds of wheels detected by the wheel speed sensor  50 , a steering angle detected by the steering angle sensor  52 , a vehicle speed detected by the vehicle speed sensor  54 , a lateral acceleration detected by the lateral acceleration sensor  56 , and an actual yaw rate detected by the yaw rate sensor  58 . 
     The camera unit  60  of the vehicle  1  captures an image of the front side of the vehicle  1 . The camera unit  60  performs processing of the imaging data and extracts boundary lines such as a white line, a yellow line, and a center line on a road surface on the front side of the vehicle  1 . The camera unit  60  recognizes a traveling lane based on the extracted boundary lines and generates traveling lane information indicating a relationship between the traveling lane and the position of the vehicle  1 . The controller  100  receives the traveling lane information from the camera unit  60 . 
     Step S 2 : The controller  100  determines whether to execute the lane departure prevention control based on the traveling information and the traveling lane information. For this purpose, the controller  100  determines (a) whether the vehicle  1  is likely to departure from the traveling lane and (b) whether a driver intends to actively change the traveling lane. 
     In Determination (a), the controller  100  estimates a course of the vehicle  1  in the traveling lane using the traveling information and the traveling lane information. Then, the controller  100  calculates a predicted time until the vehicle  1  departs from the traveling lane based on the traveling lane information, the estimated course, the vehicle speed, and the like. When the predicted time is less than an allowable value, the controller  100  determines that the vehicle  1  is likely to depart from the traveling lane. 
     In Determination (b), the controller  100  determines whether a driver performs a steering operation, for example, based on a change in the steering angle. When the driver does not perform the steering operation, the controller  100  can determine that the driver does not intend to actively change the traveling lane. A signal from a direction indicator which is not illustrated can also be used. When the driver does not operate the direction indicator, the controller  100  can determine that the driver does not intend to actively change the traveling lane. 
     When the driver does not intend to change the traveling lane but the vehicle  1  is likely to depart from the traveling lane, the controller  100  determines that the lane departure prevention control is executed (YES in Step S 2 ). In this case, the flow transitions to Step S 3 . Otherwise (NO in Step S 2 ), this cycle ends and the flow returns to Step S 1 . 
     Step S 3 : The controller  100  executes the lane departure prevention control. Specifically, the controller  100  automatically turns the vehicle  1  in a direction in which lane departure is avoided. 
     For example, the controller  100  can generate a steering torque and change the direction of the vehicle  1  by activating the electric actuator  28  of the steering mechanism  20 . In the example illustrated in  FIG. 2 , in order to avoid lane departure, the controller  100  generates a steering torque to cause turning to right. The steering mechanism  20 , the camera unit  60 , and the controller  100  can be said to constitute a “lane departure prevention device” that executes the lane departure prevention control. 
     The controller  100  can also change the direction of the vehicle  1  by generating a difference in a braking force between the left wheels  10 FL and  10 RL and the right wheels  10 FR and  10 RR. For example, in the example illustrated in  FIG. 2 , in order to avoid lane departure, it is necessary to cause turning to right. For this purpose, the controller  100  can apply a braking force to the front-right wheel  10 FR or both the front-right wheel  10 FR and the rear-right wheel  10 RR. Accordingly, a difference in braking force is generated between the right and left sides of the vehicle  1 , and thus the vehicle  1  is turned to right. In order to control the braking forces of the wheels, the controller  100  can appropriately activate the brake actuator  36  of the brake mechanism  30 . The brake mechanism  30 , the camera unit  60 , and the controller  100  can be said to constitute a “lane departure prevention device” that executes the lane departure prevention control. 
     3. Roll Stiffness Distribution and Steering Characteristics 
     A relationship between a roll stiffness distribution and steering characteristics will be described below with reference to  FIGS. 4A to 4C . The roll stiffness distribution refers to distribution of the roll stiffness to the front wheels  10 F side and the rear wheels  10 R side of the vehicle  1 . In  FIGS. 4A to 4C , reference sign RSf denotes the roll stiffness on the front wheels  10 F side and reference sign RSr denotes the stiffness on the rear wheels  10 R side. 
     In  FIGS. 4A to 4C , numerical values surrounded with circles are described beside the front-left wheel  10 FL, the front-right wheel  10 FR, the rear-left wheel  10 RL, and the rear-right wheel  10 RR. These numerical values denote magnitudes of loads applied to the wheels. The numerical values are relative values for the purpose of simple description and do not mean actual loads. The size of the circle surrounding each numerical value is drawn to increase as the numerical value (load) increases. That is, the circle surrounding each numerical value corresponds to a friction circle. 
     As illustrated in  FIG. 4A , when the vehicle  1  travels straight, loads “100” are applied to the front-left wheel  10 FL, the front-right wheel  10 FR, the rear-left wheel  10 RL, and the rear-right wheel  10 RR, respectively. Thereafter, it is assumed that the vehicle  1  is turned to right. It is assumed that a load with a magnitude of “40” moves from the right side (the inner wheel side) of the vehicle  1  to the left side (the outer wheel side) by this turning to right. That is, the loads applied to the front-left wheel  10 FL and the rear-left wheel  10 RL on the outer wheel side increase by “40” in total. On the other hand, the loads applied to the front-right wheel  10 FR and the rear-right wheel  10 RR on the inner wheel side decrease by “40” in total. 
     (1) Case of RSf=RSr 
     First, a case in which the roll stiffness RSf on the front wheels  10 F side and the roll stiffness RSr on the rear wheels  10 R side are the same is considered with reference to  FIG. 4B . In this case, the increase “40” in the load on the outer wheel side is evenly distributed to the front-left wheel  10 FL and the rear-left wheel  10 RL. Accordingly, the loads applied to the front-left wheel  10 FL and the rear-left wheel  10 RL increase by “20” and become “120.” On the other hand, the loads applied to the front-right wheel  10 FR and the rear-right wheel  10 RR on the inner wheel side decrease by “20” and become “80.” The total load applied to the front wheels  10 F and the total load applied to the rear wheels  10 R are “200,” respectively, which is the same as at the time of traveling straight. 
     (2) Case of RSf&lt;RSr 
     Then, a case in which the roll stiffness RSr on the rear wheels  10 R side is higher than the roll stiffness RSf on the front wheels  10 F side is considered with reference to  FIG. 4C . In this case, the increase “40” in the load on the outer wheel side is distributed to the front-left wheel  10 FL and the rear-left wheel  10 RL in accordance with the ratio of RSf and RSr. For example, a case of RSr/RSf=3 is assumed. In this case, the load applied to the rear-left wheel  10 RL increases by “30” and becomes “130,” and the load applied to the front-left wheel  10 FL increase by “10” and becomes “110.” On the other hand, the load applied to the rear-right wheel  10 RF on the inner wheel side decreases by “30” and becomes “70,” and the load applied to the front-right wheel  10 FR decreases by “10” and becomes “90.” The total load applied to the front wheels  10 F and the total load applied to the rear wheels  10 R are “200,” respectively, which is the same as at the time of traveling straight. 
     A difference in steering characteristics is present between the case of (1) and the case of (2). This will be described below with reference to  FIG. 5 .  FIG. 5  is a graph illustrating a relationship between a cornering power and a load. The horizontal axis represents a load [kN] applied to a certain wheel, and the vertical axis represents a cornering power [kN/deg] of the wheel. For example, when a load W0 is applied to a certain wheel, the cornering power of the wheel is CP0. 
     As known well, the cornering power varies depending on the load and increases as the load increases. A ratio of an increase in the cornering power to an increase in the load decreases as the load increases. That is, as illustrated in  FIG. 5 , the curve illustrating a relationship between the cornering power and the load is convex upward. 
     Here, an average cornering power of the rear wheels  10 R (the rear-left wheel  10 RL and the rear-right wheel  10 RR) will be considered. It is assumed that the vehicle  1  is turned to right, a load W0+ΔW is applied to the rear-left wheel  10 RL on the outer wheel side, and a load W0−ΔW is applied to the rear-right wheel  10 RR on the inner wheel side. The average cornering power of the rear wheels  10 R in this case is CP1 which is lower than CP0 as can be seen from  FIG. 5 . 
     Then, it is assumed that a load W0+2ΔW is applied to the rear-left wheel  10 RL on the outer wheel side and a load W0−2ΔW is applied to the rear-right wheel  10 RR on the inner wheel side. The average cornering power of the rear wheels  10 R in this case is CP2 which is lower than CP1 as can be seen from  FIG. 5 . That is, when the vehicle is turned, the average cornering power of the inner wheel and the outer wheel decreases as the difference in load between the inner wheel and the outer wheel increases. On the other hand, the average cornering power of the inner wheel and the outer wheel increases as the difference in load between the inner wheel and the outer wheel decreases. 
       FIGS. 4B and 4C  are compared. That is, the case of (1) and the case of (2) are compared. Paying attention to the rear wheels  10 R, the difference in load between the rear-left wheel  10 RL and the rear-right wheel  10 RR is “40 (=120−80)” in the case of (1) and is “60 (=130−70)” in the case of (2). Accordingly, the average cornering power of the rear wheels  10 R is lower in the case of (2) than in the case of (1). 
     On the other hand, paying attention to the front wheels  10 F, the difference in load between the front-left wheel  10 FL and the front-right wheel  10 FR is “40 (=120−80)” in the case of (1) and is “20 (=110−90)” in the case of (2). Accordingly, the average cornering power of the front wheels  10 F is higher in the case of (2) than in the case of (1). 
     In this way, in comparison with the case of (1), the average cornering power in the case of (2) decreases in the rear wheels  10 R and increases in the front wheel  10 F. This means that an over-steering tendency is stronger in the case of (2). That is, the over-steering tendency become stronger as the roll stiffness distribution to the rear wheels  10 R becomes larger. 
     A “rear distribution ratio RSr/RSf” is considered as an example of a parameter indicating the roll stiffness distribution to the rear wheels  10 R. The rear distribution ratio RSr/RSf is a ratio of the roll stiffness RSr on the rear wheels  10 R side to the roll stiffness RSf on the front wheels  10 F side. As the rear distribution ratio RSr/RSf increases, the over-steering tendency increases. On the other hand, as the rear distribution ratio RSr/RSf decreases, an under-steering tendency increases. 
     In this way, by changing the roll stiffness RSf and RSr of the vehicle  1 , it is possible to change the steering characteristics of the vehicle  1 . The controller  100  according to this embodiment executes the “roll stiffness control” of changing the roll stiffness RSf and RSr. When the roll stiffness RSf and RSr change, the controller  100  can activate the actuators  42 F and  42 R of the roll stiffness varying mechanism  40 . Accordingly, the controller  100  and the roll stiffness varying mechanism  40  can be said to constitute a “roll stiffness control device” that executes the roll stiffness control. 
     4. Interlinking of Lane Departure Prevention Control and Roll Stiffness Control 
       FIG. 6  is a block diagram illustrating the functions of the controller  100  according to this embodiment. The controller  100  includes an information acquiring unit  110 , a lane departure prevention control unit  120 , and a roll stiffness control unit  130  as functional blocks. These functional blocks are realized by causing the processor of the controller  100  to execute the control program stored in the memory. 
     The information acquiring unit  110  receives traveling information from various sensors mounted in the vehicle  1 . The traveling information includes rotation speeds of the wheels detected by the wheel speed sensor  50 , a steering angle detected by the steering angle sensor  52 , a vehicle speed detected by the vehicle speed sensor  54 , a lateral acceleration detected by the lateral acceleration sensor  56 , and an actual yaw rate detected by the yaw rate sensor  58 . The information acquiring unit  110  receives traveling lane information from the camera unit  60 . The process of the information acquiring unit  110  corresponds to Step S 1  in  FIG. 3 . 
     The lane departure prevention control unit  120  receives the traveling information and the traveling lane information from the information acquiring unit  110 . The lane departure prevention control unit  120  determines whether the lane departure prevention control should be executed based on the traveling information and the traveling lane information (Step S 2  in  FIG. 3 ). When it is determined that the lane departure prevention control should be executed, the lane departure prevention control unit  120  executes the lane departure prevention control (Step S 3  in  FIG. 3 ). 
     The roll stiffness control unit  130  executes the roll stiffness control and changes the roll stiffness RSf and RSr. One feature of this embodiment is that the roll stiffness control unit  130  executes the roll stiffness control coupling with execution of the lane departure prevention control. That is, when the lane departure prevention control unit  120  executes the lane departure prevention control, the roll stiffness control unit  130  executes the roll stiffness control in response to execution of the lane departure prevention control. Here, “in response to the execution of the lane departure prevention control” includes both concepts of “in response to the determination (YES in Step S 2  in  FIG. 3 ) of execution of the lane departure prevention control” and “in response to “start” (Step S 3  in  FIG. 3 ) of execution of the lane departure prevention control.” 
     The roll stiffness control according to this embodiment can be said to be triggered directly by execution of the lane departure prevention control. For example, as illustrated in  FIG. 6 , the lane departure prevention control unit  120  outputs a trigger signal TRG to the roll stiffness control unit  130 . The output timing of the trigger signal TRG may be a time at which execution of the lane departure prevention control is determined (YES in Step S 2 ) or a time at which execution of the lane departure prevention control is started (Step S 3 ). In any case, the trigger signal TRG is output directly due to execution of the lane departure prevention control. The roll stiffness control unit  130  receives the trigger signal TRG from the lane departure prevention control unit  120  and executes the roll stiffness control in response to the trigger signal TRG 
     So long as the roll stiffness control is executed by coupling with the lane departure prevention control, the start time of the roll stiffness control may be earlier than, later than, or the same as the start time of the lane departure prevention control. For example, in a case in which the trigger signal TRG is output when execution of the lane departure prevention control is determined, there is a likelihood that the roll stiffness control will be started before the lane departure prevention control is started. 
       FIG. 7  is a flowchart illustrating a process flow in the controller  100  according to this embodiment in brief. In  FIG. 7 , an aspect of “interlocking of the roll stiffness control with the lane departure prevention control” is incorporated into the flowchart illustrated in  FIG. 3 . More specifically, Step S 3  in  FIG. 3  is replaced with Step S 3 ′. Steps S 1  and S 2  are the same as in  FIG. 3  and detailed description thereof will not be repeated. 
     Step S 3 ′: The controller  100  executes the lane departure prevention control. Specifically, the controller  100  automatically turns the vehicle  1  in a direction in which lane departure is avoided. The controller  100  executes the roll stiffness control of changing the roll stiffness RSf and RSr by coupling with execution of the lane departure prevention control. When lane departure is avoided and the lane departure prevention control is completed, the controller  100  ends the roll stiffness control and returns the roll stiffness RSf and RSr to the states before the roll stiffness control is started. 
     As described above, according to this embodiment, the roll stiffness control is executed by coupling with the lane departure prevention control. The lane departure prevention control turns the vehicle  1  and the roll stiffness control affects the steering characteristics of the vehicle  1 . Accordingly, by combining the roll stiffness control with the lane departure prevention control, it is possible to control behavior of the vehicle  1  more finely than in existing lane departure prevention control. Hereinafter, various examples of traveling control based on the combination of the lane departure prevention control and the roll stiffness control will be described. 
     5. Various Examples of Traveling Control 
     5-1. First Example 
     The lane departure prevention control is executed regardless of a driver&#39;s intention and thus causes discomfort to the driver depending on situations. For example, in a case of the lane departure prevention control based on the brake control, even when a driver does not depress the brake pedal  32 , the vehicle  1  is decelerated. The deceleration gives discomfort to the driver. In a case of the lane departure prevention control based on application of a steering torque, the steering torque is transmitted to a driver&#39;s hand gripping the steering wheel  22 . The driver feels a torque different from a road-surface reaction force, which causes discomfort. The first example relates to traveling control that can reduce such discomfort. 
       FIG. 8  is a timing chart illustrating the roll stiffness control in the first example. In  FIG. 8 , the horizontal axis represents time and the vertical axis represents the rear distribution ratio RSr/RSf. According to this example, the controller  100  increases the rear distribution ratio RSr/RSf by coupling with execution of the lane departure prevention control. For example, in  FIG. 8 , the rear distribution ratio RSr/RSf before the roll stiffness control is executed is ra. The controller  100  sets the rear distribution ratio RSr/RSf to rb which is higher than ra in the roll stiffness control. 
     As described above with reference to  FIGS. 4A to 4C , the over-steering tendency increases as the rear distribution ratio RSr/RSf increases. The increase of the over-steering tendency means that the vehicle  1  is more easily turned by the lane departure prevention control. That is, by increasing the rear distribution ratio RSr/RSf by coupling with the lane departure prevention control, the vehicle  1  can be more easily turned. 
       FIG. 9  is a conceptual diagram illustrating advantages which are achieved in the example. In  FIG. 9 , the horizontal axis represents LDA control quantity and the vertical axis represents yaw moment which is generated by the lane departure prevention control. Here, the LDA control quantity is a parameter which reflects a quantity of control which is performed by the lane departure prevention control. In a case of the lane departure prevention control based on the brake control, the LDA control quantity is a parameter based on the braking force applied to the wheels. In a case of the lane departure prevention control based on application of a steering torque, the LDA control quantity is a parameter based on the applied steering torque. 
     In a case of RSr/RSf=ra, the LDA control quantity required for generating the yaw moment y is xa. On the other hand, in a case of RSr/RSf=rb (&gt;ra), the over-steering tendency increases and the vehicle  1  is more easily turned. In this case, the LDA control quantity required for generating the same yaw moment y is xb which is smaller than xa. That is, by increasing the rear distribution ratio RSr/RSf, it is possible to suppress the LDA control quantity. When the LDA control quantity (a braking force, a steering torque) decreases, discomfort due to the lane departure prevention control is reduced by as much. 
     In this example, the roll stiffness control is executed to assist turning of the vehicle  1  by the lane departure prevention control. Accordingly, a burden of the lane departure prevention control is reduced and the LDA control quantity decreases. As a result, it is possible to reduce discomfort due to the lane departure prevention control. 
     5-2. Second Example 
     A second example is a modification of the first example and is applied particularly to the lane departure prevention control based on the brake control. 
       FIG. 10  is a conceptual diagram illustrating the lane departure prevention control according to the second example. It is assumed that the vehicle  1  is turned to right by the lane departure prevention control based on the brake control. In this case, for example, the controller  100  applies a front-wheel braking force Bf and a rear-wheel braking force Br to the front-right wheel  10 FR and the rear-right wheel  10 RR, respectively, on the turning side. 
     On the other hand, the rear distribution ratio RSr/RSf is increased by the roll stiffness control described in the first example. As can be seen from comparison of  FIG. 4A  (RSf=RSr) and  FIG. 4B  (RSf&lt;RSr), when the rear distribution ratio RSr/RSf increases, the friction circle of the front-right wheel  10 FR is enlarged and the friction circle of the rear-right wheel  10 RR is reduced. Accordingly, when the rear-wheel braking force Br is applied, the rear-right wheel  10 RR slips more easily. When an amount of slip of the rear-right wheel  10 RR increases, a lateral force (a turning force) of the rear wheels  10 R as a whole decreases and there is a likelihood that the direction of the vehicle  1  will be changed more than supposed. In other words, there is a likelihood that the lane departure prevention control will not be executed as supposed. 
     Therefore, the controller  100  adjusts distribution of the front-wheel braking force Bf and the rear-wheel braking force Br to suppress slip of the rear-right wheel  10 RR. Specifically, as illustrated in  FIG. 10 , the controller  100  sets the rear-wheel braking force Br to be smaller than the front-wheel braking force Bf (Bf&gt;Br). That is, regarding the rear-right wheel  10 RR having a relatively small friction circle, the rear-wheel braking force Br is set to be relatively small in order to suppress slip. On the other hand, in order to maintain the total braking force, the front-wheel braking force Bf of the front-right wheel  10 FR having a relatively large friction circle is set to be relatively large. By this braking force distribution adjustment, it is possible to maintain the total braking force and to suppress slip. As a result, it is possible to stably execute the lane departure prevention control. 
     The braking force distribution adjustment according to this example has only to be performed in at least a partial period during the lane departure prevention control (the brake control). Even when the braking force distribution adjustment is performed in the partial period, the effect of stabilization of the lane departure prevention control is achieved. 
     5-3. Third Example 
     A third example is a modification of the second example. As described in the second example, when the amount of slip of the rear-right wheel  10 RR increases, the lateral force (the turning force) of the rear wheels  10 R as a whole decreases and there is a likelihood that the direction of the vehicle  1  will be changed more than supposed. In the third example, the vehicle  1  is more rapidly turned reversely using the increase in the turning force due to slip of the rear-right wheel  10 RR. 
       FIG. 11  is a timing chart illustrating lane departure prevention control (brake control) according to the third example. Particularly,  FIG. 11  illustrates switching of distribution of the front-wheel braking force Bf and the rear-wheel braking force Br in the brake control. In  FIG. 11 , a brake control period includes an initial distribution adjustment period PP and a main distribution adjustment period PM. The initial distribution adjustment period PP is an initial stage of the brake control. The main distribution adjustment period PM is a period subsequent to the initial distribution adjustment period PP. 
     In the initial distribution adjustment period PP, the controller  100  sets the rear-wheel braking force Br to be larger than the front-wheel braking force Bf (Bf&lt;Br). Since the friction circle of the rear-right wheel  10 RR is small, the rear-right wheel  10 RR slips likely when the large rear-wheel braking force Br is applied thereto. When the amount of slip of the rear-right wheel  10 RR increases, the lateral force (the turning force) of the rear wheels  10 R as a whole decreases and the yaw moment increases. Accordingly, the vehicle  1  is early turned. The braking force distribution adjustment (Bf&lt;Br) in the initial distribution adjustment period PP can be said to enhance initial responsiveness of the lane departure prevention control. 
     In the main distribution adjustment period PM subsequent to the initial distribution adjustment period PP, the controller  100  sets the rear-wheel braking force Br to be smaller than the front-wheel braking force Bf (Bf&gt;Br). This braking force distribution adjustment is the same as in the second example and stabilizes the lane departure prevention control. 
     According to this example, it is possible to stably execute the lane departure prevention control and to enhance initial responsiveness. When the initial distribution adjustment period PP is excessively long, there is concern that the vehicle  1  spins due to the increase in the amount of slip of the rear-right wheel  10 RR. Accordingly, the initial distribution adjustment period PP is set to such an extent that the vehicle does not spin. 
     For example, a transition time from the initial distribution adjustment period PP and the main distribution adjustment period PM is determined based on whether the amount of slip of the rear-right wheel  10 RR is greater than a threshold value. Specifically, the controller  100  calculates the amounts of slip (slip ratios) of the wheels based on the rotation speeds of the wheels and the speed of the vehicle  1 . The rotation speeds of the wheels are detected by the wheel speed sensors  50 FL,  50 FR,  50 RL, and  50 RR. The vehicle speed of the vehicle  1  is detected by the vehicle speed sensor  54 . Alternatively, the speed of the vehicle  1  may be calculated from the rotation speeds of the front-left wheel  10 FL, the front-right wheel  10 FR, the rear-left wheel  10 RL, and the rear-right wheel  10 RR. The controller  100  monitors an amount of slip of the rear-right wheel  10 RR in the initial distribution adjustment period PP, and compares the amount of slip with a threshold value. The threshold value is set to such an amount of slip that the vehicle  1  does not spin. When the amount of slip of the rear-right wheel  10 RR is greater than the threshold value, the controller  100  switches the braking force distribution adjustment from “Bf&lt;Br” to “Bf&gt;Br.” Accordingly, it is possible to enhance initial responsiveness of the lane departure prevention control within a range in which the vehicle  1  does not spin. 
     5-4. Fourth Example 
     When a degree of turning of the vehicle  1  increases by the lane departure prevention control, the lateral acceleration and the roll angle increase. A driver feels roll behavior which does not correspond to steering, which causes discomfort. A fourth example relates to traveling control that can reduce such discomfort. 
       FIG. 12  is a timing chart illustrating roll stiffness control in the fourth example. According to this example, the controller  100  increases both the roll stiffness RSf of the front wheels  10 F side and the roll stiffness RSr of the rear wheels  10 R side by interlinking with execution of the lane departure prevention control. Accordingly, an increase in roll angle at the time of the lane departure prevention control is suppressed. As a result, it is possible to reduce discomfort due to the increase in roll angle. 
       FIG. 13  is a flowchart illustrating the roll stiffness control in this example. After the LDA control quantity (a braking force, a steering torque) by the lane departure prevention control is determined, the controller  100  calculates an expected yaw rate which is expected to be generated by the LDA control quantity (Step S 11 ). Subsequently, the controller  100  converts the expected yaw rate into a lateral acceleration (Step S 12 ). Then, the controller  100  determines a roll control quantity with a magnitude corresponding to the calculated lateral acceleration (Step S 13 ). Here, the roll control quantity is an increase in roll stiffness RSf and RSr in the roll stiffness control according to this example. The roll control quantity is determined to increase as the lateral acceleration (the expected yaw rate) increases. Thereafter, the controller  100  executes the roll stiffness control using the determined roll control quantity (Step S 14 ). 
     As another example of the method of determining the roll control quantity, a method using an actual lateral acceleration which is detected by the lateral acceleration sensor  56  can be used. That is, when the lane departure prevention control is executed, the vehicle  1  is turned and the lateral acceleration increases. Thereafter, the controller  100  determines the roll control quantity based on the actual lateral acceleration detected by the lateral acceleration sensor  56 . However, in this method, the time at which the roll control quantity is determined is after the lateral acceleration and the roll angle increase actually. That is, the roll angle first increases once and then decreases by the roll stiffness control. 
     On the other hand, in the method illustrated in  FIG. 13 , the roll control quantity is determined from the LDA control quantity in a feedforward manner. Accordingly, in comparison with the method using the lateral acceleration sensor  56 , it is possible to determine the roll control quantity earlier and to start the roll stiffness control earlier. Accordingly, it is possible to prevent an increase in roll angle in the initial stage and to realize more smooth behavior. 
     5-5. Fifth Example 
     The vehicle  1  is turned by the lane departure prevention control. However, when the degree of turning at this time is great and the vehicle  1  is in an over-steering state, a driver also feels discomfort. The fifth example relates to traveling control that can reduce such discomfort. 
       FIG. 14  is a flowchart illustrating roll stiffness control according to the fifth example. After the lane departure prevention control is started, the controller  100  determines whether the vehicle  1  is in an over-steering state. More specifically, the followings are performed. 
     Step S 21 : In the lane departure prevention control based on application of a steering torque, the controller  100  calculates a target yaw rate using a known method based on the steering angle and the vehicle speed. The steering angle is detected by the steering angle sensor  52 . The vehicle speed is detected by the vehicle speed sensor  54 . Alternatively, the vehicle speed may be calculated by the rotation speeds of the front-left wheel  10 FL, the front-right wheel  10 FR, the rear-left wheel  10 RL, and the rear-right wheel  10 RR which are detected by the wheel speed sensors  50 FL,  50 FR,  50 RL, and  50 RR. 
     In the lane departure prevention control based on brake control, since the steering angle does not vary, the steering angle cannot be used to calculate the target yaw rate. Therefore, the LDA control quantity (a braking force) is used instead of the steering angle. The controller  100  calculates the target yaw rate based on the LDA control quantity or the vehicle speed. For example, a map indicating a relationship between the LDA control quantity and the target yaw rate is prepared in advance. After the LDA control quantity in the brake control is determined, the controller  100  acquires the target yaw rate based on the map. 
     Step S 22 : Then, the controller  100  calculates a yaw rate deviation by subtracting the target yaw rate from an actual yaw rate. The actual yaw rate is detected by the yaw rate sensor  58 . Then, the controller  100  compares the yaw rate deviation with a threshold value Dth. When the yaw rate deviation is equal to or greater than the threshold value Dth (YES in Step S 22 ), the controller  100  determines that the vehicle  1  is in the over-steering state. In this case, the process flow transitions to Step S 23 . On the other hand, when the yaw rate deviation is less than the threshold value Dth (NO in Step S 22 ), this cycle ends. 
     Step S 23 : The controller  100  decreases the rear distribution ratio RSr/RSf.  FIG. 15  is a timing chart illustrating the process of Step S 23 . In  FIG. 15 , the horizontal axis represents time, and the vertical axis represents rear distribution ratio RSr/RSf. Here, rc denotes the rear distribution ratio RSr/RSf before the roll stiffness control is executed. In Step S 23 , the controller  100  sets the rear distribution ratio RSr/RSf to rd which is lower than rc. 
     As described above with reference to  FIGS. 4A to 4C , as the rear distribution ratio RSr/RSf decreases, the under-steering tendency increases. Accordingly, over-steering behavior due to the lane departure prevention control is relaxed. As a result, it is possible to reduce discomfort given to a driver. 
     As long as they are consistent with each other, the first to fifth examples can be appropriately combined.