Patent Publication Number: US-2022212641-A1

Title: Control system and control method

Description:
BACKGROUND OF THE INVENTION 
     The present disclosure relates to a control system and a control method capable of appropriately executing slip suppression control for a straddle-type vehicle. 
     Conventionally, as control related to behavior of a straddle-type vehicle such as a motorcycle, slip suppression control for suppressing slippage of the straddle-type vehicle has been available. As the slip suppression control, for example, as disclosed in JP-A-2018-024324, control such as anti-lock brake control for suppressing locking of a wheel during braking of the straddle-type vehicle has been available. 
     However, in the related art, there is a case where it is difficult to appropriately execute the slip suppression control due to insufficient optimization of a target amount (for example, a tire friction circle (hereinafter also simply referred to as a friction circle)) used in the slip suppression control. By the way, compared to other vehicles, the behavior of the straddle-type vehicle such as the motorcycle tends to become unstable. Thus, it is particularly important to suppress the slippage of the straddle-type vehicle. Therefore, proposal of a technique of further appropriately executing the slip suppression control for the straddle-type vehicle has been desired. 
     SUMMARY OF THE INVENTION 
     The present invention has been made with the above-described problem as the background and therefore obtains a control system and a control method capable of appropriately executing slip suppression control for a straddle-type vehicle. 
     A control system according to the present invention is a control system for controlling behavior of a straddle-type vehicle that includes a suspension, and includes a control section that controls a damping characteristic of the suspension. The control section estimates a damping force to be generated to the suspension on the basis of the damping characteristic and a stroke speed of the suspension, and executes slip suppression control for suppressing slippage of the straddle-type vehicle by using a target amount corresponding to the estimated damping force. 
     A control method according to the present invention is a control method controlling behavior of a straddle-type vehicle that includes a suspension, and a damping characteristic of the suspension is controlled by a control system. The control method includes: an estimation step of estimating a damping force to be generated to the suspension on the basis of the damping characteristic and a stroke speed of the suspension; and an execution step of executing slip suppression control for suppressing slippage of the straddle-type vehicle by using a target amount corresponding to the estimated damping force. 
     In the control system and the control method according to the present invention, the damping characteristic of the suspension is controlled. In addition, the damping force to be generated to the suspension is estimated on the basis of the damping characteristic and the stroke speed of the suspension, and the slip suppression control for suppressing the slippage of the straddle-type vehicle is executed by using the target amount (for example, a friction circle) corresponding to the estimated damping force. In this way, it is possible to execute the slip suppression control after optimizing the target amount used in the slip suppression control. Therefore, it is possible to appropriately execute the slip suppression control for the straddle-type vehicle. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a configuration of a motorcycle to which a control system according to an embodiment of the present invention is mounted. 
         FIG. 2  is a schematic view of a configuration of a brake system according to the embodiment of the present invention. 
         FIG. 3  is a block diagram of an exemplary functional configuration of the controller system according to the embodiment of the present invention. 
         FIG. 4  is a graph for illustrating a damping characteristic of a suspension. 
         FIG. 5  is a flowchart of an example of a processing procedure that is executed by the control system according to the embodiment of the present invention and is related to estimation of a friction circle. 
         FIG. 6  is a flowchart of an example of a processing procedure that is executed by the control system according to the embodiment of the present invention and is related to execution of slip suppression control. 
     
    
    
     DETAILED DESCRIPTION 
     A description will hereinafter be made on a control system according to the present invention with reference to the drawings. Hereinafter, a description will be made on the control system used for a two-wheeled motorcycle. However, the control system according to the present invention may be used for a straddle-type vehicle other than the two-wheeled motorcycle (for example, a three-wheeled motorcycle, an all-terrain vehicle, a bicycle, or the like). The straddle-type vehicle means a vehicle that a rider straddles, and includes a scooter and the like. 
     A description will hereinafter be made on a case where each of a front-wheel brake mechanism and a rear-wheel brake mechanism is provided in one unit; however, at least one of the front-wheel brake mechanism and the rear-wheel brake mechanism may be provided in multiple units. Alternatively, one of the front-wheel brake mechanism and the rear-wheel brake mechanism may not be provided. 
     In addition, a description will hereinafter be made on a case where an engine is mounted as a drive source capable of outputting power for driving wheels of the motorcycle. However, as the drive source for the motorcycle, a drive source other than the engine (for example, a motor) may be mounted, or a plurality of the drive sources may be mounted. 
     A description will hereinafter be made on a case where a control section in the control system can execute anti-lock brake control and traction control as slip suppression control. However, the control section only needs to be able to execute at least one type of the slip suppression control and, for example, may be able to execute only one of the anti-lock brake control and the traction control. 
     A configuration, operation, and the like, which will be described below, merely constitute one example. The control system and the control method according to the present invention are not limited to a case with such a configuration, such operation, and the like. 
     The same or similar description will appropriately be simplified or will not be made below. In the drawings, the same or similar members or portions will not be denoted by a reference sign or will be denoted by the same reference sign. In addition, a detailed structure will appropriately be illustrated in a simplified manner or will not be illustrated. 
     &lt;Configuration of Motorcycle&gt; 
     A description will be made on a configuration of a motorcycle  100  to which a control system  90  according to an embodiment of the present invention is mounted with reference to  FIG. 1  to  FIG. 4 . 
       FIG. 1  is a schematic view of a configuration of the motorcycle  100  to which the control system  90  is mounted.  FIG. 2  is a schematic view of a configuration of a brake system  10  for the motorcycle  100 .  FIG. 3  is a block diagram of an exemplary functional configuration of the control system  90 .  FIG. 4  is a graph for illustrating a damping characteristic of a suspension. 
     As illustrated in  FIG. 1 , the motorcycle  100  includes: a trunk  1 ; a handlebar  2  that is held by the trunk  1  in a freely turnable manner; a front wheel  3  that is held by the trunk  1  in the freely turnable manner with the handlebar  2 ; a rear wheel  4  that is held by the trunk  1  in a freely rotatable manner; a front suspension  5 ; a rear suspension  6 ; the brake system  10  that includes a hydraulic pressure control unit  50 ; and an engine  60 . The motorcycle  100  further includes: master-cylinder pressure sensors  41  (see  FIG. 2 ), wheel-cylinder pressure sensors  42  (see  FIG. 2 ), a front-wheel rotational frequency sensor  43 , a rear-wheel rotational frequency sensor  44 , a front stroke sensor  45 , and a rear stroke sensor  46 . Moreover, as illustrated in  FIG. 3 , the motorcycle  100  includes the control system  90  that controls behavior of the motorcycle  100 . 
     Each of the front suspension  5  and the rear suspension  6  corresponds to an example of a suspension of the motorcycle  100 , and is interposed between the trunk  1  and the wheel. More specifically, the front suspension  5  is provided to a front fork  7  that connects the handlebar  2  and the front wheel  3 , and can be extended/compressed along an axial direction of the front suspension  5 . The rear suspension  6  connects the trunk  1  and a swing arm  8  that is swingably supported by the trunk  1  and holds the rear wheel  4  in the freely turnable manner, and can be extended/compressed along an axial direction of the rear suspension  6 . 
     More specifically, each of the front suspension  5  and the rear suspension  6  includes a spring and a damper. When the spring and the damper are extended/compressed along the axial direction of the suspension, vibrations from a road surface are absorbed, and transmission of the vibrations to a body of the motorcycle  100  is suppressed. For example, in a hydraulic oil channel that is formed in the damper of the suspension, a control valve is provided to control the damping characteristic of the suspension (more specifically, a characteristic of a damping force with respect to a stroke speed of the suspension). By controlling operation of the control valve, the damping characteristic of the suspension is controlled, which further controls the damping force of the suspension. 
     The damping characteristic of the suspension may be controlled by a method other than the above (for example, a method for controlling a magnetic field produced in the damper by using a magnetic fluid as the hydraulic oil in the damper of the suspension). 
     The front stroke sensor  45  detects a stroke amount of the front suspension  5  and outputs a detection result. The front stroke sensor  45  may detect another physical quantity (for example, stroke acceleration of the front suspension  5 , a force applied to the front suspension  5 , or the like) that can substantially be converted to the stroke amount of the front suspension  5 . The front stroke sensor  45  is provided to the front suspension  5 , for example. 
     The rear stroke sensor  46  detects a stroke amount of the rear suspension  6  and outputs a detection result. The rear stroke sensor  46  may detect another physical quantity (for example, stroke acceleration of the rear suspension  6 , a force applied to the rear suspension  6 , or the like) that can substantially be converted to the stroke amount of the rear suspension  6 . The rear stroke sensor  46  is provided to the rear suspension  6 , for example. 
     As illustrated in  FIG. 1  and  FIG. 2 , the brake system  10  includes: a first brake operation section  11 ; a front-wheel brake mechanism  12  that brakes the front wheel  3  in an interlocking manner with at least the first brake operation section  11 ; a second brake operation section  13 ; and a rear-wheel brake mechanism  14  that brakes the rear wheel  4  in an interlocking manner with at least the second brake operation section  13 . The brake system  10  also includes the hydraulic pressure control unit  50 , and the front-wheel brake mechanism  12  and the rear-wheel brake mechanism  14  are partially included in the hydraulic pressure control unit  50 . The hydraulic pressure control unit  50  is a unit that has a function of controlling a braking force to be generated on the front wheel  3  by the front-wheel brake mechanism  12  and a braking force to be generated on the rear wheel  4  by the rear-wheel brake mechanism  14 . 
     The first brake operation section  11  is provided to the handlebar  2  and is operated by the rider&#39;s hand. The first brake operation section  11  is a brake lever, for example. The second brake operation section  13  is provided in a lower portion of the trunk  1  and is operated by the rider&#39;s foot. The second brake operation section  13  is a brake pedal, for example. 
     Each of the front-wheel brake mechanism  12  and the rear-wheel brake mechanism  14  includes: a master cylinder  21  that includes a piston (not illustrated); a reservoir  22  that is attached to the master cylinder  21 ; a brake caliper  23  that is held by the trunk  1  and has a brake pad (not illustrated); a wheel cylinder  24  that is provided to the brake caliper  23 ; a primary channel  25  through which a brake fluid in the master cylinder  21  flows into the wheel cylinder  24 ; and a secondary channel  26  through which the brake fluid in the wheel cylinder  24  is released. 
     An inlet valve (EV)  31  is provided to the primary channel  25 . The secondary channel  26  bypasses a portion of the primary channel  25  between the wheel cylinder  24  side and the master cylinder  21  side from the inlet valve  31 . The secondary channel  26  is sequentially provided with an outlet valve (AV)  32 , an accumulator  33 , and a pump  34  from an upstream side. 
     The inlet valve  31  is an electromagnetic valve that is opened in an unenergized state and closed in an energized state, for example. The outlet valve  32  is an electromagnetic valve that is closed in an unenergized state and opened in an energized state, for example. 
     The hydraulic pressure control unit  50  includes: components such as the inlet valves  31 , the outlet valves  32 , the accumulators  33 , and the pumps  34  used to control a brake hydraulic pressure; and a base body  51  in which those components are provided and channels constituting the primary channels  25  and the secondary channels  26  are formed. 
     The base body  51  may be formed of one member or may be formed of multiple members. In the case where the base body  51  is formed of the multiple members, the components may separately be provided in the different members. 
     The control system  90  controls operation of each of the components in the hydraulic pressure control unit  50 . In this way, the braking force to be generated to the motorcycle  100  (more specifically, the braking force to be generated on the front wheel  3  by the front-wheel brake mechanism  12  and the braking force to be generated on the rear wheel  4  by the rear-wheel brake mechanism  14 ) is controlled. 
     For example, during a normal time (that is, when the braking force corresponding to a brake operation by the rider is generated to the motorcycle  100 ), the control system  90  opens the inlet valves  31  and closes the outlet valves  32 . When the first brake operation section  11  is operated in such a state, in the front-wheel brake mechanism  12 , the piston (not illustrated) in the master cylinder  21  is pressed to increase a hydraulic pressure of the brake fluid in the wheel cylinder  24 , the brake pad (not illustrated) of the brake caliper  23  is then pressed against a rotor  3   a  of the front wheel  3 , and the braking force is thereby generated on the front wheel  3 . Meanwhile, when the second brake operation section  13  is operated, in the rear-wheel brake mechanism  14 , the piston (not illustrated) in the master cylinder  21  is pressed to increase the hydraulic pressure of the brake fluid in the wheel cylinder  24 , the brake pad (not illustrated) of the brake caliper  23  is then pressed against a rotor  4   a  of the rear wheel  4 , and the braking force is thereby generated on the rear wheel  4 . 
     The master-cylinder pressure sensor  41  detects a hydraulic pressure of the brake fluid in the master cylinder  21  and outputs a detection result. The master-cylinder pressure sensor  41  may detect another physical quantity that can substantially be converted to the hydraulic pressure of the brake fluid in the master cylinder  21 . The master-cylinder pressure sensor  41  is provided in each of the front-wheel brake mechanism  12  and the rear-wheel brake mechanism  14 . 
     The wheel-cylinder pressure sensor  42  detects the hydraulic pressure of the brake fluid in the wheel cylinder  24  and outputs a detection result. The wheel-cylinder pressure sensor  42  may detect another physical quantity that can substantially be converted to the hydraulic pressure of the brake fluid in the wheel cylinder  24 . The wheel-cylinder pressure sensor  42  is provided to each of the front-wheel brake mechanism  12  and the rear-wheel brake mechanism  14 . 
     The engine  60  corresponds to an example of a drive source for the motorcycle  100 , and can output power for driving the wheel (for example, the rear wheel  4 ). For example, the engine  60  is provided with: one or multiple cylinders, each of which is formed with a combustion chamber therein; a fuel injector that injects fuel into the combustion chamber; and an ignition plug. When the fuel is injected from the fuel injector, air-fuel mixture containing air and the fuel is produced in the combustion chamber, and the air-fuel mixture is then ignited by the ignition plug and burned. Consequently, a piston provided in the cylinder reciprocates to cause rotation of a crankshaft. A throttle valve is provided in an intake pipe of the engine  60 , and an intake air amount for the combustion chamber varies in accordance with a throttle opening amount as an opening amount of the throttle valve. 
     The front-wheel rotational frequency sensor  43  is a rotational frequency sensor that detects a rotational frequency of the front wheel  3  (for example, a rotational frequency of the front wheel  3  per unit time [rpm], a travel distance per unit time [km/h], or the like), and outputs a detection result. The front-wheel rotational frequency sensor  43  may detect another physical quantity that can substantially be converted to the rotational frequency of the front wheel  3 . The front-wheel rotational frequency sensor  43  is provided to the front wheel  3 . 
     The rear-wheel rotational frequency sensor  44  is a rotational frequency sensor that detects a rotational frequency of the rear wheel  4  (for example, the rotational frequency of the rear wheel  4  per unit time [rpm], a travel distance per unit time [km/h], or the like), and outputs a detection result. The rear-wheel rotational frequency sensor  44  may detect another physical quantity that can substantially be converted to the rotational frequency of the rear wheel  4 . The rear-wheel rotational frequency sensor  44  is provided to the rear wheel  4 . 
     The control system  90  controls operation of each device in the motorcycle  100 , so as to control the behavior of the motorcycle  100 . 
     The control system  90  may be formed of a single controller or may be formed of multiple controllers. The controller that constitutes the control system  90  may partially or entirely be constructed of a microcomputer, a microprocessor unit, or the like, may be constructed of a member in which firmware or the like can be updated, or may be a program module or the like that is executed by a command from a CPU or the like, for example. 
     As illustrated in  FIG. 3 , the control system  90  includes an acquisition section  91  and a control section  92 , for example. In the case where the control system  90  is formed of the multiple controllers, functional sections that are the acquisition section  91  and the control section  92  may be provided in the single controller or may separately be provided in the multiple controllers. For example, a suspension control section  92   a , a brake control section  92   b , and a brake control section  92   c  may separately be provided in the different controllers. 
     The acquisition section  91  acquires information that is output from each of the devices mounted to the motorcycle  100 , and outputs the acquired information to the control section  92 . For example, the acquisition section  91  acquires the information output from the master-cylinder pressure sensors  41 , the wheel-cylinder pressure sensors  42 , the front-wheel rotational frequency sensor  43 , the rear-wheel rotational frequency sensor  44 , the front stroke sensor  45 , and the rear stroke sensor  46 . 
     The control section  92  controls the damping characteristics of the suspensions in the motorcycle  100 . The control section  92  further controls the braking force and drive power generated to the motorcycle  100 . The control section  92  includes the suspension control section  92   a , the brake control section  92   b , and the drive control section  92   c , for example. 
     The suspension control section  92   a  controls the damping characteristics of the front suspension  5  and the rear suspension  6 , and thereby controls the damping forces of the front suspension  5  and the rear suspension  6 . 
     More specifically, the damping force of each of the suspensions is generated in a direction that corresponds to the positive or negative stroke speed (that is, a stroke direction of each of the suspensions). A magnitude (an absolute value) of the damping force of the suspension is increased as a magnitude (an absolute value) of the stroke speed is increased. For example,  FIG. 4  exemplifies damping characteristics C 1 , C 2 , C 3  as the damping characteristics of the suspension.  FIG. 4  illustrates each of the damping characteristics with a stroke speed V on a horizontal axis and a damping force F on a vertical axis. 
     In the example illustrated in  FIG. 4 , the magnitudes of the damping forces generated at the same stroke speed are sequentially reduced in an order of the damping characteristics C 1 , C 2 , C 3 . Thus, for example, by changing the damping characteristic of the suspension from the damping characteristic C 2  to the damping characteristic C 1 , the damping force of the suspension can be increased (that is, the suspension can be hardened). Meanwhile, by changing the damping characteristic of the suspension from the damping characteristic C 2  to the damping characteristic C 3 , the damping force of the suspension can be reduced (that is, the suspension can be softened). In order to facilitate understanding,  FIG. 4  illustrates the three damping characteristics in a discrete manner. However, the damping characteristic of each of the front suspension  5  and the rear suspension  6  may be continuously changeable. 
     More specifically, the suspension control section  92   a  controls the damping forces of the front suspension  5  and the rear suspension  6  so as to suppress pitching of the motorcycle  100  (that is, a change in a posture of the motorcycle  100  in a direction in which the motorcycle  100  rotates about an axis in a vehicle right-left direction). For example, the suspension control section  92   a  controls the damping forces of the front suspension  5  and the rear suspension  6  by using the stroke amount of the front suspension  5  detected by the front stroke sensor  45  and the stroke amount of the rear suspension  6  detected by the rear stroke sensor  46 . In this way, the suspension control section  92   a  can suppress the pitching of the motorcycle  100 . In detail, the suspension control section  92   a  hardens the front suspension  5  and the rear suspension  6  as the pitching of the motorcycle  100  is increased, for example. In this way, the suspension control section  92   a  can suppress the pitching of the motorcycle  100 . 
     The brake control section  92   b  controls the braking force to be generated to the motorcycle  100  by controlling operation of each of the components of the hydraulic pressure control unit  50  in the brake system  10 . 
     In the normal time, as described above, the brake control section  92   b  controls the operation of each of the components in the hydraulic pressure control unit  50  such that the braking force corresponding to the brake operation by the rider is generated to the motorcycle  100 . Meanwhile, under a particular situation, the brake control section  92   b  executes control for adjusting the braking force to be generated to the motorcycle  100  from the braking force corresponding to the brake operation by the rider (for example, the anti-lock brake control, which will be described later). 
     For example, the brake control section  92   b  brings the motorcycle  100  into a state where the inlet valve  31  is closed and the outlet valve  32  is opened, and drives the pump  34  in such a state. In this way, the brake control section  92   b  reduces the hydraulic pressure of the brake fluid in the wheel cylinder  24 , and can thereby reduce the braking force to be generated on the wheel. In addition, for example, the brake control section  92   b  closes both of the inlet valve  31  and the outlet valve  32  from the above state. In this way, the brake control section  92   b  keeps the hydraulic pressure of the brake fluid in the wheel cylinder  24 , and can thereby keep the braking force to be generated on the wheel. Furthermore, for example, the brake control section  92   b  opens the inlet valve  31  and closes the outlet valve  32  from the above state. In this way, the brake control section  92   b  increases the hydraulic pressure of the brake fluid in the wheel cylinder  24 , and can thereby increase the braking force to be generated on the wheel.[ 0050 ] 
     The drive control section  92   c  controls the drive power to be generated to the motorcycle  100  by controlling operation of each of the devices (the throttle valve, the fuel injector, the ignition plug, and the like) in the engine  60 . 
     In a normal time, the drive control section  92   c  controls the operation of each of the devices in the engine  60  such that the drive power corresponding to an accelerator operation by the rider (more specifically, an operation to twist an accelerator grip of the handlebar  2 ) is generated to the motorcycle  100 . Meanwhile, under a particular situation, the drive control section  92   c  executes control for adjusting the drive power to be generated to the motorcycle  100  from the drive power corresponding to the accelerator operation by the rider (for example, the traction control, which will be described later). 
     For example, the drive control section  92   c  controls the operation of the throttle valve in the engine  60  so as to reduce the throttle opening amount. In this way, the drive control section  92   c  can reduce the intake air amount and thereby reduce the drive power to be transmitted to the wheel. In addition, for example, the drive control section  92   c  controls the operation of the throttle valve in the engine  60  so as to increase the throttle opening amount. In this way, the drive control section  92   c  can increase the intake air amount and thereby increase the drive power to be transmitted to the wheel. 
     Here, the control section  92  executes the slip suppression control for suppressing slippage of the motorcycle  100 . For example, as the slip suppression control, the control section  92  can execute the anti-lock brake control and the traction control. 
     The anti-lock brake control is control for suppressing locking of the wheel during braking of the motorcycle  100 . 
     For example, when a slip rate of the wheel (that is, a value that is obtained by dividing a difference between a vehicle speed and the wheel rotational frequency by the vehicle speed) exceeds a reference slip rate during braking of the motorcycle  100 , the control section  92  executes the anti-lock brake control. For example, the control section  92  executes the anti-lock brake control for the front wheel  3  as a target when the slip rate of the front wheel  3  exceeds the reference slip rate during braking of the motorcycle  100 , and executes the anti-lock brake control for the rear wheel  4  as the target when the slip rate of the rear wheel  4  exceeds the reference slip rate during braking of the motorcycle  100 . Here, the reference slip rate is a value set such that it is possible to appropriately determine whether the slip rate is high enough to cause locking or possible locking of the wheel, and can appropriately be set according to a specification of the vehicle. For example, the control section  92  can estimate the slip rate by using the front-wheel rotational frequency sensor  43  and the rear-wheel rotational frequency sensor  44 , for example. 
     Then, in the anti-lock brake control, the control section  92  estimates a friction circle as a target amount of a friction force that can be generated by a tire, and adjusts the braking force to be generated on the wheel as the control target. In detail, in the anti-lock brake control, the control section  92  reduces the braking force to be generated on the wheel as the control target such that a point indicative of the friction force applied to the tire is within the friction circle on a coordinate plane that has coordinate axes indicating the friction force in a front-rear direction and the friction force in a lateral direction. In this way, it is possible to suppress locking of the wheel during braking of the motorcycle  100 . 
     The traction control is control for suppressing a wheel slip during acceleration of the motorcycle  100 . 
     For example, the control section  92  executes the traction control when a slip rate of a drive wheel exceeds the reference slip rate during the acceleration of the motorcycle  100 . In detail, when the slip rate of the rear wheel  4  as the drive wheel exceeds the reference slip rate during the acceleration of the motorcycle  100 , the control section  92  executes the traction control so as to adjust the drive power to be generated to the rear wheel  4 . 
     Then, in the traction control, the control section  92  estimates the friction circle as the target amount of the friction force that can be generated by the tire, and adjusts the drive power to be generated to the rear wheel  4  as the drive wheel. In detail, in the traction control, the control section  92  reduces the drive power to be generated to the rear wheel  4  such that the point indicative of the friction force applied to the tire is within the friction circle on the coordinate plane that has the coordinate axes indicating the friction force in the front-rear direction and the friction force in the lateral direction. In this way, it is possible to suppress the slippage of the wheel (more specifically, the drive wheel) during the acceleration of the motorcycle  100 . 
     As described above, the control section  92  in the control system  90  executes the slip suppression control for suppressing the slippage of the motorcycle  100 . Here, the control section  92  estimates the damping force to be generated to each of the suspensions on the basis of the damping characteristic and the stroke speed of each of the suspensions in the motorcycle  100 , and executes the slip suppression control by using a target amount corresponding to the estimated damping force. In this way, the slip suppression control for the motorcycle  100  is appropriately executed. Processing for such slip suppression control, which is executed by the control system  90 , will be described later in detail. 
     &lt;Operation of Control System&gt; 
     A description will be made on operation of the control system  90  according to the embodiment of the present invention with reference to  FIG. 5  and  FIG. 6 . 
     A description will hereinafter be made on processing related to the estimation of the friction circle used in the slip suppression control with reference to  FIG. 5 , and then a description will be made on processing related to the execution of the slip suppression control with reference to  FIG. 6 . 
       FIG. 5  is a flowchart of an example of a processing procedure that is executed by the control system  90  and is related to the estimation of the friction circle. More specifically, the control flow illustrated in  FIG. 5  is processing that is executed by the control section  92  in the control system  90  and is, for example, executed during braking, the acceleration, or turning of the motorcycle  100  (that is, under a situation where the relatively large friction force is generated to the tires during travel). In addition, step S 510  and step S 590  in  FIG. 5  respectively correspond to initiation and termination of the control flow illustrated in  FIG. 5 . 
     When the control flow illustrated in  FIG. 5  is initiated, in step S 511 , the control section  92  estimates the stroke speed of each of the suspensions in the motorcycle  100 . 
     For example, the control section  92  can estimate the stroke speed of the front suspension  5  on the basis of a transition of the stroke amount of the front suspension  5  detected by the front stroke sensor  45 . In addition, the control section  92  can estimate the stroke speed of the rear suspension  6  on the basis of a transition of the stroke amount of the rear suspension  6  detected by the rear stroke sensor  46 . 
     Next, in step S 512 , the control section  92  estimates the damping force to be generated to each of the suspensions in the motorcycle  100 . 
     Here, the control section  92  estimates the damping force to be generated to each of the suspensions on the basis of the damping characteristic and the stroke speed of each of the suspensions. For example, the control section  92  can estimate the damping force to be generated to the front suspension  5  on the basis of the damping characteristic of the front suspension  5  and the stroke speed of the front suspension  5  estimated in step S 511 . In addition, the control section  92  can estimate the damping force to be generated to the rear suspension  6  on the basis of the damping characteristic of the rear suspension  6  and the stroke speed of the rear suspension  6  estimated in step S 511 . 
     Next, in step S 513 , the control section  92  determines whether the damping force to be generated to each of the suspensions in the motorcycle  100  estimated in step S 512  is larger than a threshold value. If it is determined that the estimated damping force is larger than the threshold value (step S 513 /YES), the processing proceeds to step S 514 . On the other hand, if it is determined that the estimated damping force is equal to or smaller than the threshold value (step S 513 /NO), the processing proceeds to step S 515 . 
     For example, as the damping force that is a comparison target with the threshold value used in the determination in step S 513 , the control section  92  may use a value to which both of the damping force to be generated to the front suspension  5  and the damping force to be generated to the rear suspension  6  are reflected (for example, an average value of both of the damping forces), or may use one of the damping force to be generated to the front suspension  5  and the damping force to be generated to the rear suspension  6  (for example, the smaller damping force of the damping forces). 
     As will be described below, the damping force that is generated when the suspension is compressed can be used as an index of a repulsive force (that is, a restoring force) of the tire and the spring of the suspension and can also be used as an index of a force generated between the tire and the road surface. The friction circle that is determined as the target amount used in the slip suppression control (in other words, a target tire friction circle) is determined on the basis of the index. More specifically, in the control flow illustrated in  FIG. 5 , the target tire friction circle is switched between a first friction circle and a second friction circle depending on whether the damping force to be generated to each of the suspensions is larger than the threshold value. Thus, more specifically, the threshold value used for the determination in step S 513  is set to a value with which it is possible to appropriately determine whether likeliness of generation of the friction between the tire and the road surface is high enough to use the first friction circle in the slip suppression control or low enough to use the second friction circle in the slip suppression control. 
     If it is determined YES in step S 513 , in step S 514 , the control section  92  determines the first friction circle as the friction circle that is the target amount used in the slip suppression control. On the other hand, if it is determined NO in step S 513 , in step S 515 , the control section  92  determines the second friction circle as the friction circle that is the target amount used in the slip suppression control. 
     Here, the first friction circle is the larger friction circle (for example, the friction circle having a larger radius) than the second friction circle. The friction circles, such as the first friction circle and the second friction circle, used in the slip suppression control may each be a true circle or an oval. 
     After step S 514  or step S 515 , the control flow illustrated in  FIG. 5  is terminated. 
     As described above, the control section  92  estimates the damping force to be generated to each of the suspensions on the basis of the damping characteristic and the stroke speed of each of the suspensions in the motorcycle  100 , and determines the target amount (for example, the friction circle) used in the slip suppression control according to the estimated damping force. 
     Here, the friction force generated between the tire and the road surface (that is, the friction force applied to the tire) is correlated with a ground load of the tire. More specifically, as the ground load of the tire is increased, the friction force generated between the tire and the road surface is increased. In addition, the ground load of the tire is expressed by a sum of the repulsive force of the tire (that is, the restoring force corresponding to a spring constant of the tire), the repulsive force of the spring of the suspension (that is, the restoring force corresponding to a spring constant of the suspension), and the damping force generated to the suspension (the damping force by the damper of the suspension). Accordingly, as the damping force generated to the suspension is increased, the ground load of the tire is increased, and further the friction force generated between the tire and the road surface is increased. Thus, the damping force generated to the suspension can be considered as the index of the likeliness of the generation of the friction between the tire and the road surface. 
     The target amount used in the slip suppression control is determined according to the damping force generated to each of the suspensions. In this way, the target amount can be determined according to the likeliness of the generation of the friction between the tire and the road surface. Therefore, it is possible to optimize the target amount used in the slip suppression control. 
     More specifically, from a perspective of optimizing the friction circle that is the target amount used in the slip suppression control, it is desired that, when the damping force generated to the suspension is larger than the threshold value as in the control flow illustrated in  FIG. 5 , the control section  92  determines, as the target amount used in the slip suppression control, the larger friction circle than the friction circle that is used when the damping force is smaller than the threshold value. Thus, the friction circle can be determined according to the likeliness of the generation of the friction between the tire and the road surface. 
       FIG. 6  is a flowchart of an example of a processing procedure that is executed by the control system  90  and is related to the execution of the slip suppression control. More specifically, the control flow illustrated in  FIG. 6  corresponds to the processing that is executed by the control section  92  in the control system  90 , and is repeatedly executed. In addition, step S 610  and step S 690  in  FIG. 6  respectively correspond to initiation and termination of the control flow illustrated in  FIG. 6 . 
     When the control flow illustrated in  FIG. 6  is initiated, in step S 611 , the control section  92  determines whether an execution condition of the slip suppression control is satisfied. If it is determined that the execution condition of the slip suppression control is satisfied (step S 611 /YES), the processing proceeds to step S 612 . On the other hand, if it is determined that the execution condition of the slip suppression control is not satisfied (step S 611 /NO), the control flow illustrated in  FIG. 6  is terminated. 
     More specifically, the control section  92  determines whether the execution conditions corresponding to the anti-lock brake control and the traction control are each satisfied. 
     For example, in the case where the motorcycle  100  currently brakes and the slip rate of the wheel exceeds the reference slip rate, the control section  92  determines that the execution condition of the anti-lock brake control is satisfied. 
     For example, in the case where the motorcycle  100  currently accelerates and the slip rate of the drive wheel exceeds the reference slip rate, the control section  92  determines that the execution condition of the traction control is satisfied. 
     If it is determined YES in step S 611 , in step S 612 , the control section  92  executes the slip suppression control. More specifically, the control section  92  executes the slip suppression control that corresponds to the satisfied execution condition. 
     As described above with reference to  FIG. 5 , the target amount (for example, the friction circle) used in the slip suppression control is determined according to the damping force that is generated to each of the suspensions and is estimated on the basis of the damping characteristic and the stroke speed of each of the suspensions. Then, in step S 611 , the slip suppression control is executed by using the determined target amount. More specifically, the control section  92  executes the slip suppression control by using the target amount corresponding to the damping force that is generated to each of the suspensions and is estimated on the basis of the damping characteristic and the stroke speed of each of the suspensions. In this way, it is possible to execute the slip suppression control after optimizing the target amount used in the slip suppression control. 
     Next, the control flow illustrated in  FIG. 6  is terminated. 
     The above description has been made on the example in which the damping force generated to each of the suspensions is used as a parameter for determining the target amount used in the slip suppression control. However, as the parameter for determining the target amount used in the slip suppression control, in addition to the damping force generated to each of the suspensions, a parameter (for example, the drive power or the braking force generated to the motorcycle  100  or an inclination angle of the body) other than the damping force may further be used. 
     For example, from a perspective of further effectively optimizing the target amount used in the slip suppression control, the control section  92  preferably estimates the restoring force to be generated to each of the suspensions on the basis of a stroke position of each of the suspensions in the motorcycle  100 , and then preferably executes the slip suppression control by using, in addition to the damping force, the target amount corresponding to the estimated restoring force. 
     As described above, the ground load of the tire is expressed by the sum of the restoring force generated to the tire, the restoring force generated to the suspension, and the damping force generated to the suspension. Accordingly, as the restoring force generated to the suspension is increased, the ground load of the tire is increased. As a result, the friction force generated between the tire and the road surface is increased. Thus, in addition to the damping force generated to the suspension, the restoring force generated to the suspension can also be considered as the index of the likeliness of the generation of the friction between the tire and the road surface. 
     By determining the target amount used in the slip suppression control according to the damping force and the restoring force generated to the suspension, it is possible to further appropriately determine the target amount according to the likeliness of the generation of the friction between the tire and the road surface. Therefore, it is possible to further effectively optimize the target amount used in the slip suppression control. 
     More specifically, from the perspective of further effectively optimizing the friction circle that is the target amount used in the slip suppression control, the control section  92  preferably changes the friction circle, which is the target amount used in the slip suppression control, on the basis of the damping force and the restoring force. For example, the radius of the friction circle is increased as a total value of the damping force and the restoring force generated to the suspension is increased. In this way, the friction circle that is used as the target amount in the slip suppression control can further appropriately be determined. 
     &lt;Effects of Control System&gt; 
     A description will be made on effects of the control system  90  according to the embodiment of the present invention. 
     The control system  90  includes the control section  92  that controls the damping characteristic of the suspension in the motorcycle  100 . In addition, the control section  92  estimates the damping force to be generated to the suspension on the basis of the damping characteristic and the stroke speed of the suspension, and executes the slip suppression control for suppressing the slippage of the motorcycle  100  by using the target amount corresponding to the estimated damping force. In this way, it is possible to execute the slip suppression control after optimizing the target amount used in the slip suppression control. Therefore, it is possible to appropriately execute the slip suppression control for the motorcycle  100 . 
     Preferably, as the slip suppression control, the control section  92  in the control system  90  executes the anti-lock brake control for suppressing locking of the wheel during braking of the motorcycle  100 . In this way, it is possible to execute the anti-lock brake control after optimizing the target amount used in the anti-lock brake control. Therefore, it is possible to appropriately suppress locking of the wheel during braking of the motorcycle  100 . 
     Preferably, as the slip suppression control, the control section  92  in the control system  90  executes the traction control for suppressing the wheel slip during the acceleration of the motorcycle  100 . In this way, it is possible to execute the traction control after optimizing the target amount used in the traction control. Therefore, it is possible to appropriately suppress the wheel (more specifically, the drive wheel) slip during the acceleration of the motorcycle  100 . 
     Preferably, when the damping force generated to the motorcycle  100  is larger than the threshold value, the control section  92  in the control system  90  executes the slip suppression control by using, as the target amount, the larger friction circle than the friction circle that is used when the damping force is smaller than the threshold value. In this way, it is possible to execute the slip suppression control by using such a friction circle that optimizes the target amount used in the slip suppression control. Therefore, it is possible to appropriately execute the slip suppression control in which the friction circle is used as the target amount. 
     Preferably, the control section  92  in the control system  90  estimates the restoring force to be generated to the suspension on the basis of the stroke position of the suspension, and executes the slip suppression control by using the target amount corresponding to the estimated restoring force. In this way, it is possible to execute the slip suppression control after further effectively optimizing the target amount used in the slip suppression control. Therefore, it is possible to further effectively and appropriately execute the slip suppression control for the motorcycle  100 . 
     Preferably, the control section  92  in the control system  90  changes the friction circle, which is the target amount used in the slip suppression control, on the basis of the damping force and the restoring force generated to the motorcycle  100 . As a result, for example, the radius of the friction circle is increased as the total value of the damping force and the restoring force generated to the suspension is increased. In this way, the friction circle that is used as the target amount in the slip suppression control can further appropriately be determined. Therefore, it is possible to further effectively and appropriately execute the slip suppression control in which the friction circle is used as the target amount. 
     The present invention is not limited to each of the embodiments. For example, all or parts of the embodiments may be combined, or only a part of each of the embodiments may be implemented. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1 : Trunk 
               2 : Handlebar 
               3 : Front wheel 
               3   a : Rotor 
               4 : Rear wheel 
               4   a : Rotor 
               5 : Front suspension 
               6 : Rear suspension 
               7 : Front fork 
               8 : Swing arm 
               10 : Brake system 
               11 : First brake operation section 
               12 : Front-wheel brake mechanism 
               13 : Second brake operation section 
               14 : Rear-wheel brake mechanism 
               21 : Master cylinder 
               22 : Reservoir 
               23 : Brake caliper 
               24 : Wheel cylinder 
               25 : Primary channel 
               26 : Secondary channel 
               31 : Inlet valve 
               32 : Outlet valve 
               33 : Accumulator 
               34 : Pump 
               41 : Master-cylinder pressure sensor 
               42 : Wheel-cylinder pressure sensor 
               43 : Front-wheel rotational frequency sensor 
               44 : Rear-wheel rotational frequency sensor 
               45 : Front stroke sensor 
               46 : Rear stroke sensor 
               50 : Hydraulic pressure control unit 
               51 : Base body 
               60 : Engine 
               90 : Control system 
               91 : Acquisition section 
               92 : Control section 
               92   a : Suspension control section 
               92   b : Brake control section 
               92   c : Drive control section 
               100 : Motorcycle