Abstract:
A suspension controlling apparatus for a vehicle includes a suspension whose damping force is variably settable and a control unit capable of controlling the damping force of the suspension on the basis of a target damping force, to control the damping force of the suspension with a desired control amount and a high responsibility. A control unit includes a acceleration calculation device for calculating an acceleration in a forward and rearward direction of a vehicle on the basis of an operation input amount to the vehicle and a target damping force setting device for determining a target value of damping force on the basis of a damping force map determined in advance in response to the acceleration and a differentiation value of the acceleration.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2015-055851 filed Mar. 19, 2015 the entire contents of which are hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a suspension controlling apparatus for a vehicle which includes a suspension whose damping force is variably settable. A control unit controls the damping force of the suspension on the basis of target damping force. 
         [0004]    2. Description of Background Art 
         [0005]    A suspension controlling apparatus is known that is configured such that acceleration of a vehicle in a forward and rearward direction is detected by an acceleration sensor and the damping force of a suspension is controlled on the basis of a detection value of the acceleration sensor. See, for example, Japanese Patent Laid-Open No. 2010-184512. 
         [0006]    In addition, when it is tried to enhance the behavior of the vehicle body in the technology disclosed in Japanese Patent Laid-Open No. 2010-184512, then the suspension controlling apparatus reacts highly sensitively to a detection value of the acceleration sensor and it is difficult to control the damping force of the suspension with a desired controlling amount. Thus, it is recommendable to carry out a filter process for a detection value of the acceleration sensor so that the suspension controlling apparatus may not react highly sensitively. However, this gives rise to a delay with respect to the behavior of the vehicle body. Therefore, an improvement is demanded. 
       SUMMARY AND OBJECTS OF THE INVENTION 
       [0007]    The present invention has been made in view of such a situation as described above. It is an object of an embodiment of the present invention to provide a suspension controlling apparatus for a vehicle for controlling damping force of a suspension with a desired controlling amount and a high responsibility. 
         [0008]    According to an embodiment of the present invention, a suspension controlling apparatus for a vehicle includes a suspension whose damping force is variably settable and a control unit capable of controlling the damping force of the suspension on the basis of a target damping force. The control unit includes acceleration calculation means for calculating an acceleration in a forward and rearward direction of the vehicle on the basis of an operation input amount to the vehicle and target damping force setting means for determining the target damping force on the basis of a damping force map determined in advance in response to the acceleration and a differentiation value of the acceleration. 
         [0009]    According to an embodiment of the present invention, the control unit receives, as an input thereto, a vehicle speed detected by a vehicle speed sensor and includes traveling resistance calculation means for calculating traveling resistance of the vehicle on the basis of the vehicle speed, and the acceleration calculation means calculates an acceleration in the forward and rearward direction of the vehicle on the basis of the traveling resistance obtained by the traveling resistance calculation means and the operation input amount. 
         [0010]    According to an embodiment of the present invention, the suspension controlling apparatus for a vehicle may further include an engine speed sensor configured to detect an engine speed, gear ratio decision means for deciding a gear ratio of a transmission, and a throttle sensor configured to detect a throttle operation amount for controlling an intake air amount of the engine as the operation input amount. The acceleration calculation means calculates a driving force on the basis of the engine speed, gear ratio and throttle operation amount and calculates the acceleration on the basis of the driving force. 
         [0011]    According to an embodiment of the present invention, the suspension controlling apparatus for a vehicle may further include brake operation amount detection means for detecting a brake operation amount as the operation input amount. The acceleration calculation means calculates the acceleration on the basis of deceleration corresponding to the brake operation amount. 
         [0012]    According to an embodiment of the present invention, a variation amount limit value for limiting a variation amount of the target damping force is set in advance in the control unit. 
         [0013]    According to an embodiment of the present invention, a variation amount limit value for acceleration and a variation amount limit value for deceleration which limit a variation amount of the target damping force are set in advance in the control unit. 
         [0014]    According to an embodiment of the present invention, acceleration is calculated on the basis of an operation input amount to the vehicle and target damping force is determined in accordance with a map on the basis of the acceleration and a differentiation value of the acceleration. Therefore, the damping force of the suspension can be controlled with a desired controlling amount and a high responsibility. 
         [0015]    According to an embodiment of the present invention, the acceleration in the forward and rearward direction of the vehicle is calculated taking not only the operation input amount but also traveling resistance of the vehicle calculated on the basis of the vehicle speed into consideration. Therefore, acceleration of a higher degree of accuracy can be obtained and suspension control of a higher degree of accuracy can be performed. 
         [0016]    According to an embodiment of the present invention, acceleration can be calculated eliminating the necessity for a sensor for directly detecting acceleration in the forward and rearward direction of the vehicle. Thus, the suspension can be controlled with a high responsibility. 
         [0017]    According to an embodiment of the present invention, deceleration can be calculated eliminating the necessity for a sensor for directly detecting acceleration in the forward and rearward direction of the vehicle. Thus, the suspension can be controlled with a high responsibility. 
         [0018]    According to an embodiment of the present invention, an excessively great variation of the damping force of the suspension can be suppressed. 
         [0019]    According to an embodiment of the present invention, an excessively great variation of the damping force can be suppressed while the suspension controlling apparatus suitably copes with acceleration and deceleration with the variation amount limit values to the target damping force suitably coping with acceleration and deceleration. 
         [0020]    Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
           [0022]      FIG. 1  is a block diagram depicting a configuration of a driving system of a motorcycle; 
           [0023]      FIG. 2  is a block diagram depicting a configuration of a suspension controlling apparatus; 
           [0024]      FIG. 3  is a block diagram depicting a portion of a control unit which controls a suspension for a front wheel; 
           [0025]      FIG. 4  is a block diagram depicting a configuration of a selection unit; 
           [0026]      FIG. 5  is a block diagram depicting a configuration of a pitch controlling target damping force calculation unit; 
           [0027]      FIG. 6  is a timing chart illustrating a variation caused by a sudden throttle operation upon execution of pitch suppression control; 
           [0028]      FIG. 7  is a block diagram depicting a configuration of a skyhook controlling target damping force calculation unit; 
           [0029]      FIG. 8  is a view depicting a variation of skyhook controlling target damping force in response to a roll angle; and 
           [0030]      FIG. 9  is a timing chart upon execution of skyhook control. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0031]    In the following, an embodiment of the present invention is described with reference to  FIGS. 1 to 9 . Referring first to  FIG. 1 , rotational power of a crankshaft  11  provided on an engine mounted on a vehicle such as a motorcycle is transmitted to an input member  12   a  of a clutch  12  through a primary reduction gear mechanism  13 . The power from an output member  12   b  of the clutch  12  is inputted to a transmission  14 . The transmission  14  is configured from selectively engageable first to sixth speed gear trains provided, for example, between a main shaft  15  and a countershaft  16 . In a neutral state of the transmission  14 , the rotational power transmitted from the output member  12   b  of the clutch  12  to the main shaft  15  is not transmitted to the countershaft  16 . However, when the transmission  14  is not in the neutral state, the rotational power is transmitted to the countershaft  16  at a gear ratio given by selective engagement of the first to sixth speed gear trains. The rotational power of the countershaft  16  is transmitted to a rear wheel WR serving as a driving wheel through a final reduction gear mechanism  17 . 
         [0032]    As a rotational speed of the engine side with respect to the clutch  12 , the rotational speed of the engine, namely, the rotational speed of the crankshaft  11  is detected by an engine speed sensor  18 . Meanwhile, as a rotational speed of the transmission  14  side with respect to the clutch  12 , the rotational speed of the countershaft  16  is detected by a countershaft rotational speed sensor  19 . The gear ratio of the transmission  14  is decided by gear ratio decision means  20  for detecting the rotational angle of a shift drum provided in the transmission  14 . 
         [0033]    Referring to  FIG. 2 , a front wheel WF is suspended at a front portion of a vehicle body B of the motorcycle through a suspension  22 F for a front wheel having a solenoid  21 F by which damping force can be variably set. The rear wheel WR is suspended at a rear portion of the vehicle body B through a suspension  22 R for a rear wheel having a solenoid  21 R by which the damping force can be variably set. 
         [0034]    A hydraulic brake is provided for the front wheel WF. Braking operation of the hydraulic brake is controlled by a brake system  23  which can carry out also antilock braking control. Output hydraulic pressure of a master cylinder (not depicted), provided in the brake system  23 , is detected by a hydraulic pressure sensor  24  and represents a brake operation amount. A stroke sensor  25 F for detecting a stroke of the suspension  22 F for the front wheel is provided between the vehicle body B and the front wheel WF. A stroke sensor  25 R for detecting a stroke of the suspension  22 R for the rear wheel is provided between the vehicle body B and the rear wheel WR. Further, a vertical acceleration sensor  26 F for detecting a vertical acceleration of a front portion of the vehicle body B by the suspension  22 F for the front wheel and a vertical acceleration sensor  26 R for detecting a vertical acceleration of a rear portion of the vehicle body B by the suspension  22 R for the rear wheel are provided on the vehicle body B. 
         [0035]    The damping force of the suspensions  22 F and  22 R varies by control of the operation of the solenoids  21 F and  21 R by a control unit  28 . The control unit  28  receives signals inputted thereto from the engine speed sensor  18 , countershaft rotational speed sensor  19 , gear ratio decision means  20 , hydraulic pressure sensor  24 , stroke sensors  25 F and  25 R and vertical acceleration sensors  26 F and  26 R. Also signals from a roll angle sensor  29  for detecting a roll angle of the motorcycle, a vehicle speed sensor  30  for detecting the vehicle speed, a throttle sensor  31  and an actual acceleration sensor  32  for detecting an actual acceleration in the forward and rearward direction of the vehicle are inputted to the control unit  28 . The throttle sensor  31  may be any sensor if it detects the opening of a throttle valve of the engine, an operation amount of a throttle grip and an intake air negative pressure of the engine. Further, a signal indicative of whether or not antilock braking control is being executed is inputted from the brake system  23  to the control unit  28 . 
         [0036]    Referring to  FIG. 3 , the control unit  28  includes a skyhook controlling damping force calculation unit  34 , a pitch suppression controlling damping force calculation unit  35 , a base damping force calculation unit  36 , high select means  37 , starting-engine brake damping force setting means  38 , a target current calculation unit  41 , changeover means  39 , and a selection unit  40 . The skyhook controlling damping force calculation unit  34  determines damping force for skyhook control of the suspensions  22 F and  22 R so as to stabilize the vehicle body against a road surface input. The pitch suppression controlling damping force calculation unit  35  determines damping force for pitch suppression control by which the suspensions  22 F and  22 R are controlled so that an appropriate pitch behavior in response to an operation input of a vehicle rider may be obtained. The base damping force calculation unit  36  determines damping force as a reference upon normal traveling in which none of the skyhook control and the pitch suppression control is executed. The high select means  37  selects a maximum one of values of the damping force determined by the skyhook controlling damping force calculation unit  34 , pitch suppression controlling damping force calculation unit  35  and base damping force calculation unit  36 . The starting-engine brake damping force setting means  38  determines damping force for vehicle starting and for engine braking. The target current calculation unit  41  calculates target current in response to the damping force inputted on the basis of the characteristics of the suspensions  22 F and  22 R. The changeover means  39  carries out changeover between a state in which an output of the high select means  37  is inputted to the target current calculation unit  41  and another state in which an output of the starting-engine brake damping force setting means  38  is inputted to the target current calculation unit  41 . The selection unit  40  selects one of the states to be assumed by the changeover means  39 . 
         [0037]    To the starting-engine brake damping force setting means  38 , a detection value of the engine speed sensor  18  which detects the rotational speed of the engine as the rotational speed of the engine side with respect to the clutch  12 , a detection value of the countershaft rotational speed sensor  19  which detects the rotational speed of the countershaft  16  and a gear ratio decided by the gear ratio decision means  20  are inputted. The starting-engine brake damping force setting means  38  calculates the rotational speed of the main shaft  15  which is the rotational speed of the transmission  14  side with respect to the clutch  12  from the gear ratio decided by the gear ratio decision means  20  and the rotational speed of the countershaft  16  detected by the countershaft rotational speed sensor  19 . When the rotational speed of the engine side with respect to the clutch  12  is equal to or higher than the rotational speed of the transmission  14  side with respect to the clutch  12 , the starting-engine brake damping force setting means  38  sets the damping force to a given value suitable for starting of the vehicle. However, when the rotational speed of the engine side with respect to the clutch  12  is lower than the rotational speed of the transmission  14  side with respect to the clutch  12 , the starting-engine brake damping force setting means  38  sets the damping force to a given value suitable for engine braking. 
         [0038]    Referring to  FIG. 4 , the selection unit  40  includes rotational speed difference detection means  42 , comparison means  43 , first threshold value setting means  44  and second threshold value setting means  45 . The rotational speed difference detection means  42  calculates the rotational speed difference across the clutch  12  provided between the engine and the transmission  14  on the basis of detection values of the engine speed sensor  18 , countershaft rotational speed sensor  19  and gear ratio decision means  20 . The comparison means  43  inputs, to the changeover means  39 , a signal for determining to which state the changeover means  39  is to be set on the basis of the rotational speed difference obtained by the rotational speed difference detection means  42 . 
         [0039]    The rotational speed difference detection means  42  detects the rotational speed difference across the clutch  12  from the rotational speed of the engine, namely, of the crankshaft  11 , detected by the engine speed sensor  18  and the rotational speed of the main shaft  15  calculated on the basis of the detection values of the countershaft rotational speed sensor  19  and the gear ratio decision means  20 . 
         [0040]    The comparison means  43  receives, as inputs thereto, a first threshold value determined by the first threshold value setting means  44  and a second threshold value determined by the second threshold value setting means  45 . When the rotational speed difference exceeds the first threshold value, the comparison means  43  inputs, to the changeover means  39 , a signal for inputting the given value determined by the starting-engine brake damping force setting means  38  to the target current calculation unit  41 . However, the comparison means  43  inputs, to the changeover means  39 , a signal for inputting the target damping force outputted from the high select means  37  to the target current calculation unit  41  in response to a drop of the rotational speed difference to a level equal to or lower than the second threshold value. 
         [0041]    To the first threshold value setting means  44 , a detection value of the engine speed sensor  18 , a detection value of the countershaft rotational speed sensor  19  and a gear ratio decided by the gear ratio decision means  20  are inputted. When the detection value of the engine speed sensor  18  is equal to or higher than the rotational speed of the main shaft  15  obtained on the basis of the detection values of the countershaft rotational speed sensor  19  and the gear ratio decision means  20 , the first threshold value setting means  44  sets the first threshold value to a threshold value which is suitable for starting of the vehicle and varies in response to the gear ratio. However, when the detection value of the engine speed sensor  18  is lower than the rotational speed of the main shaft  15 , the first threshold value setting means  44  sets the first threshold value to a threshold value which is suitable for engine braking and varies in response to the gear ratio. The second threshold value setting means  45  determines the second threshold value as a value lower than the first threshold value. 
         [0042]    According to such a configuration of the changeover means  39  and the selection unit  40  as described above, when the rotational speed difference across the clutch  12  detected by the rotational speed difference detection means  42  exceeds the first threshold value, the target damping force determined by one of the skyhook controlling damping force calculation unit  34 , pitch suppression controlling damping force calculation unit  35  and base damping force calculation unit  36  is changed to the given value. Further, in response to a drop of the rotational speed difference to a level equal to or lower than the second threshold value which is lower than the first threshold value, the state in which the target value is changed to the given value is canceled. In addition, the first threshold value changes in response to the gear ratio of the transmission  14 , and when the rotational speed of the engine side with respect to the clutch  12  is equal to or higher than the rotational speed of the transmission  14  side with respect to the clutch  12 , the first threshold value is set to a threshold value suitable for starting of the vehicle. However, when the rotational speed of the engine side with respect to the clutch  12  is lower than the rotational speed of the transmission  14  side with respect to the clutch  12 , the first threshold value is set to a threshold value suitable for engine braking. It is to be noted that the given value described above may vary in response to the rotational speed difference higher than the first threshold value. 
         [0043]    To the base damping force calculation unit  36  which determines damping force to be used as a reference upon ordinary travelling in which none of skyhook control and pitch suppression control is executed, the detection values of the roll angle sensor  29  and the vehicle speed sensor  30  are inputted. The base damping force calculation unit  36  determines damping force to be used as a reference upon ordinary traveling on the basis of a map determined in advance which corresponds to the roll angle and the vehicle speed. 
         [0044]    Referring to  FIG. 5 , the pitch suppression controlling damping force calculation unit  35  includes acceleration calculation means  45 A and target damping force setting means  47 . The acceleration calculation means  45 A calculates a forward and rearward direction acceleration of the vehicle on the basis of an operation input amount to the vehicle. The target damping force setting means  47  determines target damping force for pitch suppression control by a map search on the basis of the forward and rearward direction acceleration of the vehicle and a differentiation value of the acceleration. 
         [0045]    The operation input amount to the vehicle is a throttle operation amount obtained by the throttle sensor  31  and a hydraulic pressure obtained by the hydraulic pressure sensor  24 . Engine torque calculation means  48  calculates engine torque on the basis of the engine speed obtained by the engine speed sensor  18  and the throttle operation amount. Driving force calculation means  49  calculates driving force on the basis of the gear ratio obtained by the gear ratio decision means  20  and the engine torque. The driving force and a traveling resistance obtained by traveling resistance calculation means  50  are inputted to the acceleration calculation means  45 A. The traveling resistance calculation means  50  calculates standard traveling resistance on the basis of the vehicle speed obtained by the vehicle speed sensor  30 . 
         [0046]    Ordinary braking force estimation means  51  estimates ordinary braking force on the basis of the hydraulic pressure obtained by the hydraulic pressure sensor  24 , and upon ordinary braking in which antilock braking control is not executed, the ordinary braking force estimated by the ordinary braking force estimation means  51  is inputted to the acceleration calculation means  45 A. On the other hand, braking force for antilock braking control when the signal from the brake system  23  indicates an antilock braking controlling state is estimated by the ABS braking force estimation means  52  on the basis of fixed decelerating force and a detection value of the actual acceleration sensor  32 . 
         [0047]    The braking forces estimated by the ordinary braking force estimation means  51  and the ABS braking force estimation means  52  are changed over by changeover means  53  in response to a braking state obtained from the brake system  23  and are inputted to the acceleration calculation means  45 A. In particular, upon ordinary braking, the ordinary braking force estimated by the ordinary braking force estimation means  51  is inputted to the acceleration calculation means  45 A whereas, upon antilock braking control, the given braking force for antilock braking control estimated by the ABS braking force estimation means  52  is inputted to the acceleration calculation means  45 A. 
         [0048]    The acceleration calculation means  45 A receives driving force, standard traveling resistance and braking force inputted thereto and calculates acceleration in the forward and rearward direction on the basis of the acceleration force and the deceleration force applied to the motorcycle. 
         [0049]    The acceleration in the forward and rearward direction obtained by the acceleration calculation means  45 A is inputted to selection means  54 . The selection means  54  selects the actual acceleration obtained by the actual acceleration sensor  32  when the acceleration in the forward and rearward direction obtained by the acceleration calculation means  45 A when the braking state obtained from the brake system  23  is an antilock braking controlling state is equal to or lower than a given acceleration. However, in any other case, the selection means  54  selects the acceleration in the forward and rearward direction obtained by the acceleration calculation means  45 A. The acceleration outputted from the selection means  54  is inputted to the target damping force setting means  47  and inputted also to first differentiation means  46 . The acceleration is differentiated by the first differentiation means  46 , and a differentiation value of the acceleration obtained by the differentiation is inputted to the target damping force setting means  47 . 
         [0050]    The target damping force determined by the target damping force setting means  47  is inputted to variation amount limit processing means  55 . To the variation amount limit processing means  55 , also a variation amount limit value for limiting the variation amount of the target damping force for the suspensions  22 F and  22 R is inputted. The variation amount limit processing means  55  carries out a limiting process for limiting the variation amount of the target damping force determined by the target damping force setting means  47  so that the variation amount does not to exceed the variation amount limit value. In particular, the given acceleration variation amount limit value obtained by acceleration variation amount limit value setting means  56  as the variation amount limit value upon acceleration and the given deceleration variation amount limit value obtained by deceleration variation amount limit value setting means  57  as a variation amount limit value upon deceleration are changed over by changeover means  58  on the basis of the signal outputted from the selection means  54  and inputted to the variation amount limit processing means  55 . The target damping force for pitch suppression control obtained by the limiting process by the variation amount limit processing means  55  is outputted from the pitch suppression controlling damping force calculation unit  35 . 
         [0051]    According to such pitch suppression control, for example, if a rider carries out a sudden throttle operation, then the target damping force for pitch suppression control is changed to set target damping force before a pitch behavior appears on the vehicle body as indicated by a timing chart of  FIG. 6 . In particular, driving force is estimated on the basis of the throttle opening and the acceleration in the forward and rearward direction is estimated on the basis of the driving force, and then target damping force is determined on the basis of the acceleration and a differentiation value of the acceleration. Therefore, if the throttle opening increases suddenly at time t 1 , then the target damping force immediately changes in response to a variation of the estimated driving force, acceleration in the forward and rearward direction and differentiation value of the acceleration. Thereupon, the target damping force changes so as to increase a time period ΔT (for example, 120 milliseconds) prior to appearance of a pitch behavior on the vehicle body. Further, if the throttle opening decreases suddenly at time t 2 , then the target damping force immediately changes in response to a variation of the estimated driving force, acceleration in the forward and rearward direction and differentiation value of the acceleration. Thereupon, the change of the target damping force appears more than the time period ΔT prior to the time at which the vehicle body pitch angle changes to the decreasing side. 
         [0052]    Referring to  FIG. 7 , the skyhook controlling damping force calculation unit  34  includes target damping force setting means  61  which determines target damping force for skyhook control on the basis of detection values of the vertical acceleration sensor  26 F, roll angle sensor  29  and vehicle speed sensor  30 . 
         [0053]    The vertical acceleration obtained by the vertical acceleration sensor  26 F is inputted to integration means  62  serving as vertical speed detection means. The integration means  62  integrates the vertical acceleration to obtain a vertical speed, which is inputted to the target damping force setting means  61 . Stroke amounts of the suspensions  22 F and  22 R obtained by the stroke sensors  25 F and  25 R are inputted to second differentiation means  70 , which differentiates the stroke amounts to obtain stroke speeds. The stroke speeds are inputted to the target damping force setting means  61 . Further, detection values of the roll angle sensor  29  and the vehicle speed sensor  30  are inputted to skyhook coefficient calculation means  63 , and a skyhook coefficient which depends upon the roll angle and the vehicle speed is inputted to the target damping force setting means  61 . 
         [0054]    The target damping force setting means  61  arithmetically operates and sets target damping force from a sprung vertical speed obtained by the integration means  62 , an unsprung vertical speed determined from the stroke speed obtained by the second differentiation means  70  and the skyhook coefficient on the basis of the skyhook theory. 
         [0055]    The skyhook controlling damping force calculation unit  34  includes suspension state detection means  71  for deciding whether the suspensions  22 F and  22 R are in a compressed state or in a decompressed state, and target damping force limit value determination means  64  for determining a limit value to the target damping force. 
         [0056]    The suspension state detection means  71  decides, from the sprung vertical speed obtained by the integration means  62  and the unsprung vertical speed determined from the stroke speed obtained by the second differentiation means  70 , which one of the compressed state and the decompressed state is taken by each of the suspensions  22 F and  22 R. However, the decision may be made otherwise from the stroke amounts of the suspensions  22 F and  22 R obtained by the stroke sensors  25 F and  25 R as indicated by a chain line in  FIG. 7 . 
         [0057]    The target damping force limit value determination means  64  determines a limit value to the target damping force on the basis of a detection value of the suspension state detection means  71  and a roll angle obtained by the roll angle sensor  29 . In particular, the target damping force limit value determination means  64  determines a target damping force limit value on the compression side when the suspensions  22 F and  22 R are in a compressed state and a target damping force limit value on the decompression side when the suspensions  22 F and  22 R are in a decompressed state such that a value obtained by subtracting the target damping force limit value on the compression side from the target damping force limit value on the decompression side may increase as the roll angle increases as depicted in  FIG. 8 . In particular, the target damping force limit value on the compression side is set so as to decrease as the roll angle increases, and the target damping force limit value on the decompression side is set so as to increase as the roll angle increases. Thereupon, the target damping force limit value on the decompression side may be higher than the target damping force limit value on the compression side irrespective of the roll angle as indicated by a solid line in  FIG. 8 , and in a region in which the roll angle is small, the target damping force limit value on the compression side may be higher than the target damping force limit value on the decompression side as indicated by a chain line in  FIG. 8 . 
         [0058]    Further, as depicted in  FIG. 7 , a vehicle speed obtained by the vehicle speed sensor  30  is inputted to the target damping force limit value determination means  64 . The target damping force limit value determination means  64  changes the magnitude or the gradient of the target damping force limit value on the compression side and the target damping force limit value on the decompression side in response to the vehicle speed. 
         [0059]    Meanwhile, a skyhook coefficient determined by the skyhook coefficient calculation means  63  may be set such that the target damping force set by the target damping force setting means  61  decreases in a state in which the vehicle speed exceeds a given speed or may be set such that it is outputted as map data determined in advance depending upon the roll angle and the vehicle speed. 
         [0060]    The skyhook controlling damping force calculation unit  34  has low select means  65  which selects a lower value from between the target damping force determined by the target damping force setting means  61  and the target damping force limit value determined by the target damping force limit value determination means  64 . In particular, if the target damping force for skyhook control determined by the target damping force setting means  61  exceeds the target damping force limit value determined by the target damping force limit value determination means  64 , then the target damping force limit value is outputted as skyhook target damping force from the low select means  65 . 
         [0061]    The target damping force for skyhook control outputted from the low select means  65  is outputted from execution decision means  66  only when excitation decision means  73  decides that a vibration state of the vehicle in the vertical direction is a given vibration state. 
         [0062]    The excitation decision means  73  receives detection values of the stroke sensors  25 F and  25 R filtered by a filter  68  and an excitation decision threshold value determined by excitation decision threshold value determination means  69  as inputs thereto. The excitation decision means  73  decides that a state in which the detection values of the stroke sensors  25 F and  25 R exceed the excitation decision threshold value is a given vibration state. 
         [0063]    The excitation decision threshold value determination means  69  receives, as inputs thereto, target damping force calculated by the pitch suppression controlling damping force calculation unit  35  and outputted from the changeover means  39  and acceleration in the forward and rearward direction outputted from the selection means  54  of the pitch suppression controlling damping force calculation unit  35 . The excitation decision threshold value determination means  69  changes the excitation decision threshold value to a value on a side on which skyhook control is less likely to be started when the acceleration in the forward and rearward direction is outside a given range. Further, during execution of pitch suppression control, the excitation decision threshold value determination means  69  changes the excitation decision threshold value to a value on the side on which skyhook control is less likely to be started. More specifically, when the acceleration in the forward and rearward direction is outside the given range and while pitch suppression control is being executed, the excitation decision threshold value determination means  69  changes a decision condition for the given vibration state for deciding whether or not skyhook controlling damping force control is to be started to a condition on the side on which skyhook control is less likely to be started. 
         [0064]    According to such skyhook control, skyhook control is started at time t 3  at which the stroke detected by the stroke sensor  25 F and filtered by the filter  68  exceeds the excitation decision threshold value as depicted in  FIG. 9 . Thus, damping force control of the suspension  22 F is executed on the basis of the determined skyhook controlling target damping force. 
         [0065]    While the damping force of the suspensions  22 F and  22 R is controlled by the control unit  28  on the basis of target damping force, the control unit  28  has the rotational speed difference detection means  42  for detecting a rotational speed difference across the clutch  12  provided between the engine and the transmission  14  and changes the target damping force to a given value when the rotational speed difference exceeds the first threshold value. Therefore, it is perceived in advance that a sudden acceleration variation is caused by the rotational speed difference across the clutch  12  between the engine and the transmission  14 , and the damping force of the suspensions  22 F and  22 R can be controlled appropriately in advance. Consequently, the posture of the vehicle body can be stabilized. 
         [0066]    Further, the control unit  28  cancels the state in which the target damping force is changed to the given value in response to a decrease of the rotational speed difference to a value equal to or lower than the second threshold value which is lower than the first threshold value. Therefore, the ordinary target damping force control can be restored in response to a decrease of the rotational speed difference across the clutch  12 . 
         [0067]    Further, since the control unit  28  changes the first threshold value in response to a decision result of the gear ratio decision means  20  for deciding a gear ratio of the transmission  14 , the damping force of the suspensions  22 F and  22 R can be controlled appropriately in response to the gear ratio of the transmission  14 . 
         [0068]    Further, the control unit  28  changes the first threshold value to a threshold value suitable for starting of the vehicle and changes the given value to a given value suitable for starting of the vehicle when the rotational speed on the engine side with respect to the clutch  12  is equal to or higher than the rotational speed on the transmission  14  side with respect to the clutch  12 . Therefore, upon starting of the vehicle, the damping force of the suspensions  22 F and  22 R can be set to an appropriate level. 
         [0069]    The control unit  28  changes the first threshold value to a threshold value suitable for engine braking and changes the given value to a given value suitable for engine braking when the rotational speed on the engine side with respect to the clutch  12  is lower than the rotational speed on the transmission  14  side with respect to the clutch  12 . Therefore, upon engine braking, the damping force of the suspensions  22 F and  22 R can be set to an appropriate level. 
         [0070]    The control unit  28  includes the pitch suppression controlling damping force calculation unit  35  which determines damping force for pitch suppression control in which the suspensions  22 F and  22 R are controlled so that an appropriate pitch behavior corresponding to an operation input of the vehicle rider is obtained. The pitch suppression controlling damping force calculation unit  35  has the acceleration calculation means  45 A and the target damping force setting means  47 . The acceleration calculation means  45 A calculates acceleration in the forward and rearward direction of the vehicle on the basis of an operation input amount to the vehicle. The target damping force setting means  47  determines target damping force for pitch suppression control on the basis of a damping force map determined in advance in response to the acceleration and a differentiation value of the acceleration. The control unit  28  thus calculates acceleration on the basis of an operation input amount to the vehicle and determines target damping force of the suspensions  22 F and  22 R from the map on the basis of the acceleration and a differentiation value of the acceleration. Therefore, the suspensions  22 F and  22 R can react in a good response reading a will of the rider from the operation amount of the rider, and the damping force of the suspensions  22 F and  22 R can be controlled with a desirable control amount prior to a behavior of the vehicle body. 
         [0071]    The pitch suppression controlling damping force calculation unit  35  includes the traveling resistance calculation means  50  which calculates traveling resistance of the vehicle on the basis of the vehicle speed detected by the vehicle speed sensor  30 . Since the acceleration calculation means  45 A calculates acceleration in the forward and rearward direction of the vehicle on the basis of the traveling resistance obtained by the traveling resistance calculation means  50  and the operation input amount, the acceleration calculation means  45 A calculates acceleration in the forward and rearward direction of the vehicle taking not only the operation input amount but also the traveling resistance of the vehicle calculated on the basis of the vehicle speed into consideration. Consequently, acceleration of a higher degree of accuracy can be obtained and more accurate suspension control can be achieved. 
         [0072]    The driving force calculation means  49  calculates driving force on the basis of the engine speed detected by the engine speed sensor  18  and the throttle operation amount detected by the throttle sensor  31 , and the acceleration calculation means  45 A calculates acceleration on the basis of the driving force and the gear ratio of the transmission  14  decided by the gear ratio decision means  20 . Therefore, the acceleration is calculated eliminating the necessity for a sensor for directly detecting acceleration of the vehicle in the forward and rearward direction, and the suspensions can be controlled with a high response. 
         [0073]    The acceleration calculation means  45 A calculates acceleration on the basis of deceleration corresponding to the detection value of the hydraulic pressure sensor  24  which detects the brake operation amount as the operation input amount. Therefore, the deceleration is calculated eliminating the necessity for a sensor for directly detecting acceleration of the vehicle in the forward and rearward direction, and the suspensions  22 F and  22 R can be controlled with a high response. 
         [0074]    In addition, since the acceleration calculation means  45 A calculates acceleration on the basis of the braking force and the driving force of the engine, the acceleration upon braking can be calculated with a high degree of accuracy. Further, since the acceleration calculation means  45 A calculates the driving force of the engine on the basis of the torque of the engine, the acceleration calculation means  45 A can calculate the driving force during braking with a high degree of accuracy and thus can calculate the acceleration during braking with a higher degree of accuracy. Further, since the driving force of the engine is calculated on the basis of the torque of the engine and the gear ratio, the driving force during braking can be calculated with a high degree of accuracy, and the acceleration during braking can be calculated with a higher degree of accuracy. 
         [0075]    Since the variation amount limit value is set in advance by the pitch suppression controlling damping force calculation unit  35  such that the variation amount of the target damping force is limited, an excessively great variation of the damping force of the suspensions  22 F and  22 R can be suppressed. For example, when a sudden damping force variation occurs upon a change from sudden acceleration to steady acceleration, the damping force can be made closer to the target damping force for every variation amount limit value. Therefore, the damping force can be varied smoothly. In addition, since a variation amount limit value during acceleration and a variation amount limit value during deceleration for limiting the variation amount of the target damping force are set in advance, the pitch suppression controlling damping force calculation unit  35  can suppress an excessively great variation of the damping force while it can be appropriately made suitable for acceleration and for deceleration. 
         [0076]    Although the acceleration calculation means  45 A calculates, upon ordinary braking, acceleration on the basis of ordinary braking force corresponding to the detection value of the hydraulic pressure sensor  24 , upon antilock braking control, the acceleration calculation means  45 A calculates acceleration on the basis of given antilock brake controlling braking force. Therefore, also upon antilock braking control, appropriate suspension control can be carried out. 
         [0077]    The pitch suppression controlling damping force calculation unit  35  determines, when the acceleration calculated by the acceleration calculation means  45 A during antilock braking control is equal to or lower than the given acceleration, the target deceleration force is determined on the basis of the actual acceleration obtained by the actual acceleration sensor  32  which detects the actual acceleration of the vehicle in the forward and rearward direction. Therefore, even if a difference appears between the actual acceleration and the acceleration calculated on the basis of the brake hydraulic pressure depending upon the state of the road surface on which the vehicle travels, the damping force can be controlled appropriately using the actual acceleration. 
         [0078]    The control unit  28  includes the skyhook controlling damping force calculation unit  34  which determines the damping force for skyhook control of the suspensions  22 F and  22 R so as to stabilize the vehicle body against a road surface input. The skyhook controlling damping force calculation unit  34  includes the target damping force setting means  61  which arithmetically operates and sets target damping force from a sprung vertical speed obtained by the integration means  62 , an unsprung vertical speed determined from a stroke speed obtained by the second differentiation means  70  and a skyhook coefficient on the basis of the skyhook theory. Therefore, the stability of the vehicle can be promoted further. 
         [0079]    The skyhook controlling damping force calculation unit  34  includes the suspension state detection means  71  for deciding whether the suspensions  22 F and  22 R are in a compressed state or in a decompressed state, and the target damping force limit value determination means  64  for determining a limit value to the target damping force. More particularly, the target damping force limit value determination means  64  determines the limit value to the target damping force such that a value obtained by subtracting the target damping force limit value on the compression side when the suspensions  22 F and  22 R are in a compressed state from the target damping force limit value on the decompression side when the suspensions  22 F and  22 R are in a decompressed state increases as the roll angle of the vehicle obtained by the roll angle sensor  29  increases whereas the target damping force limit value on the compression side decreases as the roll angle increases. The low select means  65  selects a lower value from between the target damping force determined by the target damping force setting means  61  and the target damping force limit value determined by the target damping force limit value determination means  64 . Therefore, the target damping force for the skyhook control when the suspensions  22 F and  22 R are in a compressed state during turning is weakened. Consequently, while a vibration suppression performance is assured, the rolling stability against an input from the road surface can be increased. In addition, a moderate pitching behavior is provided at rising from a corner. Consequently, a turning characteristic is improved and appropriate damping force can be obtained, and besides a response to steering can be assured. 
         [0080]    The skyhook controlling damping force calculation unit  34  determines, when the target damping force for skyhook control determined by the target damping force setting means  61  exceeds the damping force limit value, the damping force limit value as the target damping force for skyhook control. Therefore, the control amount for the suspensions  22 F and  22 R can be suppressed to a value equal to or lower than the damping force limit value. 
         [0081]    Since the damping force limit value changes in response to the vertical speed and the roll angle, the control amount for the suspensions  22 F and  22 R can be suppressed to a value equal to or lower than the damping force limit value in response to various vehicle states. 
         [0082]    The excitation decision threshold value determination means  69  provided in the skyhook controlling damping force calculation unit  34  changes, when the acceleration in the forward and rearward direction is outside the given range and during execution of pitch suppression control, the excitation decision threshold value so that the decision condition for a given vibration state for deciding whether or not the skyhook controlling damping force control is to be started is set to a condition on the side on which skyhook control is less likely to be started. Further, when the excitation decision means  73  decides that the vibration state of the vehicle in the vertical direction is in the given vibration state on the basis of the excitation decision threshold value, the excitation decision threshold value determination means  69  outputs target damping force for executing the skyhook control. Therefore, it is possible to execute suspension control while a pitching behavior is left appropriately. In addition, not turning on-off of skyhook control is changed over directly, but ease of intervention is changed by changing the threshold value. Therefore, upon occurrence of a disturbance for which vibration suppression is required, vibration suppression can be started appropriately. 
         [0083]    Further, during execution of the pitch suppression control, the decision condition of the given vibration state is changed to a condition on the side on which damping force control for skyhook control of the suspensions  22 F and  22 R is less likely to be started. Therefore, upon acceleration or deceleration or when pitching suppression control is being executed, excessive damping force control by the skyhook control is not carried out. 
         [0084]    The excitation decision means  73  decides the given vibration state which is used as a reference for starting the skyhook control from the strokes of the suspensions  22 F and  22 R detected by the stroke sensors  25 F and  25 R. Therefore, the vibration state of the vehicle can be detected appropriately from the stroke amounts of the suspensions  22 F and  22 R. 
         [0085]    While the embodiment of the present invention has been described, the present invention is not limited to the embodiment described above, but allows various design changes without departing from the spirit of the present invention described in the claims. 
         [0086]    For example, the present invention can be applied not only to a motorcycle but also widely to vehicles including four-wheeled vehicles. 
         [0087]    The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.