Abstract:
A clutch control device is used to engage and disengage a clutch of a straddle-type vehicle. A method of using the device comprises detecting a predetermined operation, such as positioning a key switch in an ON position, for starting use of the straddle-type vehicle, detecting application of a brake of the straddle-type vehicle, and actuating clutch control means that disconnects the clutch upon detection of application of the brake if the predetermined operation for starting use is detected.

Description:
RELATED APPLICATIONS 
   The present application claims the priority benefit of Japanese Patent Application No. 2004-359223, filed on Dec. 10, 2004, which is hereby incorporated by reference in its entirety. 
   The systems and methods described herein also can be used with other types of transmissions, clutch mechanisms and associated control systems and routines, such as those described in one or more of the following copending U.S. patent applications: (1) application Ser. No. 11/301,288, filed on even date herewith, which is entitled STRADDLE-TYPE VEHICLE HAVING CLUTCH ENGAGEMENT CONTROL DEVICE AND METHOD OF USING CLUTCH ENGAGEMENT CONTROL DEVICE; (2) application Ser. No. 11/301,646, filed on even date herewith, which is entitled CLUTCH ACTUATOR FOR STRADDLE-TYPE VEHICLE; (3) application Ser. No. 11/299,720, filed on even date herewith, which is entitled APPARATUS AND METHOD FOR CONTROLLING TRANSMISSION OF STRADDLE-TYPE VEHICLE; and (4) application Ser. No. 11/299,858, filed on even date herewith, which is entitled GEAR CHANGE CONTROL DEVICE AND METHOD. The contents of all of the above-noted copending U.S. patent applications are hereby incorporated by reference in their entireties. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to a clutch control device, a method of using the clutch control device and a straddle-type vehicle that employs the clutch control device. More specifically, the present invention relates to a clutch control device that controls operation of the clutch when starting the straddle-type vehicle. 
   2. Description of the Related Art 
   Conventionally, there are known automatic transmissions of automobiles for electronic control of gear shift and engagement/disengagement of a clutch based upon a pattern preset according to an operating condition of a vehicle. Such automatic transmissions include ones in which, when the key switch is turned OFF, the engine speed is zero and the vehicle speed are zero, the clutch is engaged and then control is terminated (for example, see JP-UM-B-6-8903). With such a construction, the clutch is engaged when the engine and the vehicle are stopped. With the clutch engaged, it is possible to reduce the likelihood of undesired vehicle movement. Accordingly, when the vehicle is stopped on a slope, the vehicle will be more likely to remain stationary while the clutch is engaged. 
   In the four-wheel vehicle arts, the vehicles usually comprise a parking brake. When a parking brake is used, the vehicle is less likely to move even when a clutch is disengaged upon starting of the engine of the vehicle following the vehicle being stopped with the clutch engaged. However, most straddle-type vehicles, examples of which include motorcycles, are not equipped with parking brakes. Accordingly, measures are desired that can reduce the likelihood of inadvertent movement of the straddle-type vehicle when the clutch is disengaged for starting. 
   SUMMARY OF THE INVENTION 
   Thus, one aspect of the present invention involves a clutch control device of a straddle-type vehicle that is used to engage and disengage a clutch with a clutch actuator. The clutch control device comprises an operation detection means that detects a predetermined operation indicative of the vehicle being started. A brake application detection means is provided that detects application of a brake of the vehicle. Clutch control means is provided that disengages the clutch upon detection of brake application if the predetermined operation indicative of the vehicle being started is detected. 
   Another aspect of the present invention involves a clutch control method for engagement and disengagement of a clutch of a straddle-type vehicle by a clutch actuator. The clutch control method comprises detecting a predetermined operation for starting use of the straddle-type vehicle, detecting application of a brake of the straddle-type vehicle, and putting the clutch in a disengaged state upon detection of application of the brake if the predetermined operation for starting use is detected. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects and advantages of the present invention will now be described with reference to the drawings of a preferred embodiment, which embodiment is intended to illustrate and not to limit the invention, and in which figures: 
       FIG. 1  is a side view of a straddle-type vehicle that is arranged and configured in accordance with certain features, aspects and advantages of the present invention. 
       FIG. 2  is a schematic view showing a hardware configuration of a clutch and a transmission control system that can be used on the straddle-type vehicle of  FIG. 1 . 
       FIG. 3  is a schematic view showing a preferred construction of a sensor/switch group. 
       FIG. 4  is a flow diagram illustrating how the control state changes when a main key is operated. 
       FIG. 5  is a flowchart illustrating a main-key off control routine. 
       FIG. 6  is a flowchart illustrating a main-key on control routine. 
       FIG. 7  is an illustration of a clutch control system that is arranged and configured in accordance with certain features, aspects and advantages of the present invention. 
       FIG. 8  is a graph illustrating the relationship among torque imparted to a rotating member from a clutch side, torque imparted to the rotating member by an auxiliary spring, and a composite torque thereof. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   With reference now to  FIG. 1 , a straddle-type vehicle is illustrated that has been arranged and configured in accordance with certain features, aspects and advantages of the present invention. In the illustrated configuration, the straddle-type vehicle is a motorcycle  100 . In other configurations, the straddle-type vehicles can comprise, for example but without limitation, motorcycles, motorized bicycles, scooters, three-wheeled and four-wheeled buggies such as all terrain vehicles, snowmobiles and the like. 
   The illustrated motorcycle  100  comprises a front wheel  110  and a rear wheel  112 . A front fork is connected to the front wheel  112 . A handle  116  extends laterally across the motorcycle  100  and is connected to a top of the front fork  114 . The handle  116  comprises a grip  102  and a clutch lever  104  that are mounted at a first end and an accelerator grip and a brake lever (not shown) that are mounted at a second end. 
   The motorcycle  100  comprises an engine  106  and a fuel tank  108  that is positioned generally vertically above the engine  106 . The motorcycle further comprises a seat  118  that is provided on an upper portion of the motorcycle  100  at a location rearward of the fuel tank  108  in the illustrated configuration. A rider can ride the motorcycle  100  while straddling the seat  118 . 
   The motorcycle  100  comprises a clutch actuator  11 . A control device  120  (see  FIG. 2 ) controls the clutch actuator  11 . The engagement and disengagement of the clutch are performed by a drive force, such as that supplied by electric power rather than that provided by human power. The clutch actuator  11  preferably uses a motor to operate a clutch that is provided in a crankcase of the engine  106 . In the preferred configuration, the motorcycle  100  comprises a single clutch and torque of the engine  106  is input into the transmission by the action of the single clutch. The clutch actuator  11  can be mounted above the engine  106 . In a preferred configuration, the clutch actuator  11  is mounted in a region above the engine  106  and below the fuel tank  108 . The clutch lever  104  can be connected to the clutch actuator  11  by a length of wire so that engagement and disengagement of the clutch also can be performed by operator of the vehicle by manipulating the clutch lever  104 . 
   The motorcycle  100  also comprises a shift actuator  13 . In one configuration, the shift actuator  13  comprises a motor that operates a transmission provided in a transmission casing of the engine  106 . The shift actuator  13  can be controlled by the control device  120  and the shift actuator  13  causes shifting of the transmission. 
   With reference now to  FIG. 2 , a sensor/switch group  130 , the clutch actuator  11 , and the shift actuator  13  are connected to the control device  120 . A battery  126  also can be connected to the control device  120  such that electric power from the battery  126  is supplied to the control device  120 . The electric power from the battery  126  also can be supplied to the clutch actuator  11  and the shift actuator  13  via the control device  120  or in any other suitable manner. Thus, the electric power of the battery  126  can be used to operate the control device  120  and can be used to operate the clutch actuator  11  and the shift actuator  13 . 
   In one preferred configuration, the clutch actuator  11  comprises a direct current (dc) motor  11   a  (see  FIG. 7 ). Other suitable types of motors or linear actuators also can be used. In the illustrated configuration, forward rotation of the dc motor  11   a  can disengage the clutch while reverse rotation of the dc motor can engage the clutch or put the clutch in a desired position between the disengaged state and the engaged state. 
   A clutch potentiometer  11   b  preferably is mounted to the clutch actuator  11 . The clutch potentiometer  11   b  can comprise a resistor and other suitable electrical components such that voltage indicative of an operational state of the clutch actuator  11  (e.g., a voltage indicative of a clutch position) can be supplied to the control device  120 . Any suitable configuration can be used that supplies information regarding the clutch position to the control device. In the preferred embodiment, the voltage value is used to supply information regarding the clutch position to the control device  120 . 
   The shift actuator  13  also comprises a dc motor (not shown). Other suitable types of motors or linear actuators also can be used. In the illustrated configuration, normal rotation of the motor can cause up-shifting of the transmission and reverse rotation of the motor can cause down-shifting of the transmission. 
   Preferably, the shift actuator  13  is mounted to a shift arm of the transmission. The dc motor is rotated in a first direction to move the shift arm in one direction and the dc motor is rotated in a second direction to move the shift arm in the other direction. A shift potentiometer  13   b  preferably is mounted to the shift actuator  13 . The shift potentiometer  13   b  can comprise a resistor and other suitable electrical components such that voltage indicative of an operational state of the shift actuator  13  (e.g., a voltage indicative of a shift actuator/shift lever position) can be supplied to the control device  120 . Any suitable configuration can be used that supplies information regarding either or both of the shift actuator position and shift lever position. In the preferred embodiment, the voltage value is indicative of a turning angle of the shift arm. The voltage value can be supplied to the control device  120  and the voltage value can be used by the control device  120  as information relating to a turning angle of the shift actuator. In one configuration, the transmission is shiftable among neutral and multiple forward speeds. Preferably, the transmission is shiftable among neutral and five forward speeds. The transmission can feature a shift dog or any other suitable shifting configuration. 
   With reference now to  FIG. 3 , the sensor/switch group  130  preferably comprises an accelerator sensor  130   a , a foot brake sensor  130   b , a hand brake sensor  130   c , a gear position sensor  130   d , a key switch sensor  130   e , and a vehicle speed sensor  130   f . Other sensors also can be used. In addition, while the illustrated sensors preferably directly detect the states of the associated components, the sensors also can comprise so-called pseudo sensors that indirectly detect the states of the components desired to be sensed. 
   The accelerator sensor  130   a  preferably detects the accelerator opening degree and inputs a representative value into the control device  120 . The foot brake sensor  130   b  preferably inputs into the control device  120  the tread angle of a rear wheel brake mounted to the rear wheel  112  or data indicative of whether the rear wheel brake is operated. The gear position sensor  130   d  preferably is mounted to the transmission and is used to input into the control device  120  data indicative of a present transmission position, such as a present gear ratio (e.g., degree of rotation of the shift drum). The key switch sensor  130   e  detects the state (e.g., ON, OFF or the like) of a key switch and inputs a representative value into the control device  120 . The vehicle speed sensor  130   f  detects data indicative of vehicle speed (or information equivalent thereto) and inputs data or corresponding data into the control device  120 . In one configuration, the vehicle speed sensor  130   f  may detect the number of rotations of the drive shaft of the transmission (e.g., a sensor associated with a counter shaft), or may detect movement of a chain which can be connected to the drive shaft without significant play, or may detect movement of a shaft drive, or a tire hub. The detected data can be used to compute the vehicle speed in any suitable manner. In some configurations, the vehicle speed may be obtained by detecting the rotating speed of a main shaft and a middle shaft (an idler shaft) of the transmission, and multiplying the value thereof by a reduction ratio of the present gear position (number) of the transmission. 
   With reference again to  FIG. 2 , the control device  120  chiefly comprises a main microcomputer  144  that controls operation of the clutch actuator  11  and that controls operation of the shift actuator  13 . Preferably, the control device  120  controls the clutch actuator based upon various information indicating the state of the vehicle. The information can be input from the sensor/switch group  130 , the clutch potentiometer  11   b , and the shift potentiometer  13   b . Other configurations also can be used. In general, the control device  120  comprises the main microcomputer  144 , a power circuit  146 , a motor drive circuit  138  that drives the clutch actuator  11 , and a motor drive circuit  140  that drives the shift actuator  13 . 
   The power circuit  146  preferably comprises a switch (not shown) that is switched to an ON position when the key switch is moved to the ON position. The power circuit  146  also preferably comprises a holding circuit  146   a . When the switch is moved to the ON position, the power circuit  146  preferably applies the voltage of the battery  126  to the main microcomputer  144 . When the key switch is moved to the OFF position, the power circuit switch preferably is held in the ON position by the holding circuit  146   a . While the power circuit switch is held in the ON position, the power circuit  146  continues to apply the drive voltage to the main microcomputer  144  until the shutdown operations of the main microcomputer  144  have been completed. When the shutdown operations have been completed, the main microcomputer  144  instructs the holding circuit  146   a  to stop supplying electric power so that the supply of electric power to the main microcomputer  144  from the power circuit  146  ceases. 
   In one configuration, the motor drive circuit  138  comprises a known H bridge circuit. The motor drive circuit  138  feeds electric current from the battery  126  to the dc motor of the preferred clutch actuator  11 . The current is delivered to rotate the dc motor in the direction and at the speed corresponding to the clutch actuator drive signal supplied from the main microcomputer  144 . Similarly, in one configuration, the motor drive circuit  140  comprises a known H bridge circuit. The motor drive circuit  140  feeds electric current from the battery  126  to the dc motor of the preferred shift actuator  13 . The current is delivered to rotate the dc motor in the direction and at the speed corresponding to the shift actuator drive signal supplied from the main microcomputer  144 . 
   With reference now to  FIG. 4 , control by the microcomputer  144  in the illustrated embodiment changes depending upon the position of the key switch (i.e., the main key). The main microcomputer  144  can execute a number of different routines or programs and does so depending upon various criteria. As shown in the figure, the main microcomputer  144  executes a termination time control program  200 , a clutch release control program  202 , a clutch engagement control program  204 , a shutdown program  206 , a starting program  208 , and a starting time control program  210 . Preferably, these programs are stored in a ROM, or any other suitable information storage medium. In one configuration, the memory location can be in the main microcomputer  144 . 
   The termination time control program  200  preferably is executed when data is transmitted that is indicative of the key switch being positioned in the OFF position is input from the key switch sensor  130   e . The terminate time control program  200  starts the clutch disengagement control program  202  if the gears of the transmission are in any torque-transferring position and if the clutch is engaged. After the clutch is disengaged by the clutch disengagement control program  202 , control by the termination time control program  200  is resumed. In addition, the transmission is considered to be engaged if any of the torque-transferring gears are engaged and such information is conveyed when data input from the gear position sensor  130   d  indicates that the transmission of the motorcycle  100  is put in any one of the multiple gear combinations (e.g., the preferred transmission is in any of the 5 speed combinations available such that it is not in neutral). Also, clutch engagement corresponds to situations in which the clutch position data obtained by the clutch potentiometer  11   b  indicate that the clutch is engaged state (i.e., the clutch is ON). 
   With continued reference to  FIG. 4 , the termination time control program  200  executes the clutch engagement control program  204  if the transmission is in-gear, the clutch is disengaged, and the vehicle is stopped (e.g., the engine is turned off or stalled), or if the transmission is in neutral and the clutch is disengaged. The clutch engagement control program  204  engages the clutch. 
   After the clutch is engaged by the clutch engagement control program  204 , the shutdown program  206  is executed. Advantageously, the shutdown program  206  initiates the shutdown operation of the main microcomputer  144  and feeds to the holding circuit  146   a  a signal to stop supply of electric power to the main microcomputer  144 . Thus, the signal advantageously causes the stopping of the supply of electric power to the main microcomputer  144  from the power circuit  146 . Preferably, clutch disengagement is confirmed if the clutch position data obtained by the clutch potentiometer  11   b  indicates that the clutch is disengaged state (the clutch is OFF). Vehicle stopping preferably is confirmed if the vehicle state data input from the vehicle speed sensor  130   f  indicates a predetermined speed (for example but without limitation, a speed per hour of 3 km) or less, for a predetermined time (for example but without limitation, 3 seconds). The predetermined speed can be one at which the vehicle can be estimated to be substantially stationary and/or one at which, even when the clutch is engaged, the motorcycle  100  generally does not move in any significant manner. A neutral gear position preferably is confirmed if the data indicating that the transmission of the motorcycle  100  is in a neutral state is input from the gear position sensor  130   d.    
   The termination time control program  200  can start the shutdown program  206  if the transmission is in neutral and the clutch is engagement. If data indicative of the key switch being positioned in the ON position is input while the termination time control is on-going, the starting program  208  is executed. In other words, if the key switch is moved to an ON position while the termination time control is proceeding, the termination time control  200  is stopped and the starting time control  210  is begun. 
   The clutch release control program  202  is a control program that feeds a clutch actuator drive signal of a predetermined pattern to the motor drive circuit  138  to rotate the dc motor  11   a  in a first direction and disengage the clutch. On the other hand, the clutch engagement control program  204  is a control program that feeds a clutch actuator drive signal of a predetermined pattern to the motor drive circuit  138  to rotate the dc motor  11   a  in the other direction and engage the clutch. 
   The starting program  208  can be executed when the key of the motorcycle  100  is inserted into the key switch prior to being turned toward an ON side (main key on) while the stopping time or termination time control program  200  is being executed. More preferably, the starting program  208  is executed if, based on output of the key switch sensor  130   e , the key is inserted into the key switch and has yet to be turned toward the ON position. In one configuration, movement from the insertion position and the ON position triggers execution of the starting program  208 . 
   The starting program  208  initializes some of the settings of the control device  120  and determines whether the transmission of the motorcycle  100  is in a neutral position. If the transmission is in neutral, the starting program shifts to normal control (e.g., stopping state control, traveling state control, up-shift control, down-shift control, clutch release and up-shift control, clutch release and down-shift control, etc.). 
   If upon starting the transmission is in any other position other than neutral (i.e., in a torque-transferring mode), the starting program  208  initiates the starting time control program  210 . The starting time control program  210  monitors whether the brake is actuated and starts the clutch release control program  202  when the brake is actuated (i.e., the brake is engaged such that it is detected as being in an ON position). Thus, when the brake is not actuated (e.g., disengaged), control by the starting time control program  210  is resumed and, if the transmission is in a gear engagement position and the clutch is disengaged, the starting program shifts to normal control. Whether the brake is engaged is determined based upon input from the foot brake sensor  130   b  and the hand brake sensor  130   c . Specifically, in one preferred configuration, where data indicating that the rear wheel brake is applied is input from the foot brake sensor  130   b  or data indicating that the front wheel brake is applied is input from the hand brake sensor  130   c , it is determined that the brake is applied. Thus, it preferably is judged that the brake of the motorcycle  100  is engaged provided that either the front wheel brake or the rear wheel brake is applied. In some configurations, however, it may be judged that the brake of the motorcycle  100  is not applied unless both brakes are applied. 
   With reference now to  FIG. 5 , the main-key off control will be described in greater detail. As illustrated in  FIG. 5 , whether the vehicle is in a state of gear engagement and clutch engagement is checked first at the time of main-key off by the termination time control program  200  (S 101 ). If the gear is engaged and the clutch is engaged, the clutch release control program  202  disengages the clutch (S 102 ). On the other hand, if either the gears are disengaged (e.g., the transmission is in neutral) or the clutch is disengaged, S 102  can be skipped. Subsequently, the termination time control program  200  judges whether the key switch has been turned ON (S 103 ). If the key switch is turned ON, the control shifts to the main key-on-time control (see  FIG. 6 ). On the other hand, when the key switch remains in the OFF position, it is judged whether the transmission gears of the motorcycle  100  are engaged (S 104 ). If the gears are engaged, the control stands by until the vehicle slows to a speed equal to or less than the predetermined speed (for example, 3 km/h) for at least a predetermined period of time (for example, 3 seconds) (S 105 ). When this occurs, the clutch is engaged by the clutch engagement control program  204  (S 107 ). When it is judged in S 104  that the transmission gears of the motorcycle  100  are not engaged (e.g., the transmission is in neutral), it is then judged whether the clutch is disengaged (S 106 ). If the clutch is disengaged, the clutch is engaged by the clutch engagement control program  204  (S 107 ). When the clutch is engaged in S 107 , the shutdown program  206  subsequently begins shutdown operations (S 108 ) and then the main-key off time control is terminated. If it is judged in S 106  that the clutch is engaged, the shutdown operations is begun by the shutdown program  206  with the clutch as it is (S 108 ) and then the main-key off time control is terminated. 
   According to the main-key off time control, it is possible to stop the motorcycle  100  and the clutch will be surely engaged such that it is possible to reduce the likelihood of the vehicle moving even if the motorcycle  100  is stopped on a slope. 
   With reference now to  FIG. 6 , a main-key on time control used in one embodiment of the vehicle  100  will be discussed. The processing illustrated in the figure begins when key switch is positioned in the ON position and the starting program  208  performs various initializing operations (S 201 ). Subsequently, it is judged whether the transmission is in a neutral state (S 202 ). If the transmission is in neutral, the routine shifts to a normal control without manipulating the transmission or the clutch (S 205 ). On the other hand, if the transmission is not in neutral (e.g., one of the gears is engaged), the starting time control program  210  stands by until the brake is engaged (S 203 ). When the brake is engaged, the clutch release control program  202  disengages the clutch (S 204 ). Thereafter, the control shifts to normal control (S 205 ). 
   Thus, if the transmission is in gear, normal control does not begin until the brakes are actuated by the rider, which causes the clutch release control program  202  to disengage the clutch. The action of the clutch when the main-key is in the ON position can be varied depending on whether the transmission is in a torque-transmitting position or the transmission is in a neutral position at the time of main-key off. If the main key is turned OFF when the transmission is in-gear, parking on a slope can be properly accommodated and, when the main key is turned ON, the clutch will be disengaged as soon as the brakes are applied. Thus, by having the clutch engaged or the brakes engaged, the transmission reduces the likelihood of vehicle movement until the brakes are applied so that it is possible to greatly reduce the likelihood of the vehicle moving inadvertently when the vehicle is intended to be stopped. 
   With reference now to  FIG. 7 , the clutch actuator  11  will be described in greater detail. In one preferred configuration, the clutch is a wet type multiple-disc clutch  10 . The clutch  10  preferably is positioned within a crankcase of the engine  106 . The illustrated clutch  10  can be engaged or disengaged by manipulation of a push rod  12 . The illustrated clutch  10  also comprises a clutch spring whose biasing force pushes a clutch disc, which is connected either directly or indirectly to a crankshaft (on a drive side) of the engine, against a friction disc, which is connected either directly or indirectly to a transmission main shaft (on a driven side). Therefore, the clutch  10  can be engaged as desired. The clutch  10  can be semi-engaged by pushing the push rod  12  toward the clutch  10  against the push force of the clutch spring and can be disengaged by further pushing the push rod. The clutch can have any suitable configuration and, therefore, the details of the clutch itself are not important. 
   With the clutch control system, the push rod  12  of the clutch  10  is manipulated by a hydraulic mechanism and the spring of the clutch. In other words, the hydraulic mechanism pushes the push rod  12  and the push rod  12  is returned by the spring of the clutch. The hydraulic mechanism preferably comprises a fluid hose  18  through which fluid passes. Hydraulic fluid, oil or any other suitable fluid can be used. A clutch disengagement cylinder  14  can be connected to one end of the hose  18  and a master cylinder  40  can be connected to the other end of the hose  18  through a suitable connector  43 . A piston  15  is mounted in the clutch disengagement cylinder  14 . The piston  15  is pushed by fluid flowing from the hose  18  into the clutch disengagement cylinder  14 . A clutch disengagement spring  16  preferably biases the piston  15  toward the clutch  10  such that the travel of the piston  15  away from the clutch  10  can be somewhat limited. When the fluid flows in from the hose  18 , the piston  15  is moved toward the clutch  10  which pushes the push rod  12  toward the clutch  10 . As a result, the clutch  10  is either semi-engaged or disengaged. Conversely, when the flow of fluid from the master cylinder  40  ceases, the push rod  12  can be returned toward the piston and the piston can be returned toward the spring  16  by the clutch spring. As such, the clutch  10  is engaged. 
   A reservoir tank  42  preferably is connected to the master cylinder  40  through a connector  41 . The reservoir tank  42  can correct a hydraulic pressure in the system. 
   In the illustrated configuration, the clutch actuator  11  comprises the master cylinder  40  and a mechanism that exerts and releases pushing pressure on a piston  36  of the master cylinder  40 . Preferably, the piston  36  of the master cylinder  40  is moved by a first piston push rod  35  (e.g., a clutch operating member, a first piston pushing pressure mechanism). A manipulation arm  58  and a second piston push rod  34  preferably are provided in a manner such that they abut independently against the first piston push rod  35 . 
   The manipulation arm  58  can be connected to the clutch lever  104  through a length of wire or cable  63  such that, when a rider manipulates the clutch lever  104 , the manipulation arm  58  is moved by the wire  63 . Accordingly, the manipulation arm  58  moves the first piston push rod  35  and the piston  36  is pushed toward the interior of the master cylinder  40 . Also, when the rider returns the clutch lever  104  to its original position, the manipulation arm  58  is returned to its initial position whereby the piston  36  is also returned to its original position. 
   Preferably, the second piston push rod  34  is driven by the dc motor  11   a , which is included in the clutch actuator  11 , to move the first piston push rod  35  independently of the manipulation arm  58  such that the piston  36  is pushed toward the interior of the master cylinder  40 . When the dc motor  11   a  rotates in the second direction, the second piston push rod  34  returns to its initial position (a clutch connection position) and the clutch  10  correspondingly returns to an engaged state. 
   As described above, a force that moves the clutch  10  toward engagement is applied to the push rod  12  by the clutch spring provided in the clutch  10 . It is noted that varied types of springs can be used in clutches for this purpose. In order to cause the second piston push rod  34  to push the piston  36 , the illustrated dc motor  11   a  rotates a rotating member  32  against this restoring force. Thus, in the illustrated clutch actuator  11 , an auxiliary spring  22  is mounted to the rotating member  32  to lower the required output power for the dc motor  11   a.    
   The rotating member  32  preferably comprises a rotating shaft  28  to which the auxiliary spring  22  can be connected. The rotating member  32  also comprises a rotating shaft  30  to which the second piston push rod  34  can be connected. In one configuration, the rotating shaft  28  extends in a parallel direction relative to a rotating shaft  48  on the other side of the rotating member  32 . 
   The auxiliary spring assembly  22  comprises a first auxiliary spring shaft  26 . In one configuration, the first auxiliary spring shaft  26  is rotatably connected to the rotating shaft  28 . In such a configuration, the other end of the auxiliary spring assembly  22  can abut against an inner wall of an actuator casing  70 . Preferably, the actuator casing  70  contains elements of the clutch actuator  11 . The auxiliary spring  22  can comprise a coil spring  20 , which is compressed to a length that is shorter than its natural length. Thus, the coil spring  20  exerts an expansion pressure. In the illustrated configuration, one end of the auxiliary spring assembly  22  is supported by the inner wall of the actuator casing  70 , which is fixed on a vehicle side, and the other end of the auxiliary spring assembly  22  pushes the first auxiliary spring shaft  26  toward the rotating member  32 . Accordingly, an elastic force of the coil spring  20  causes the first auxiliary spring shaft  26  to push the rotating shaft  28  in a direction of expansion, which imparts an auxiliary torque to the rotating shaft  28 . 
   With continued reference to  FIG. 7 , the clutch potentiometer  11   b  is mounted on a worm wheel  50 . The clutch potentiometer  11   b  comprises a resistor and other suitable electrical components such that a voltage value corresponding to an angle of rotation of the rotating member  32  can be output. The voltage value can be input into the control device  120  and, therefore, can be used to control of movement of the dc motor  11   a  or the like. The control device  120  preferably determines the rotation angle of the rotating member  32  from the output of the clutch potentiometer  11   b  and so is able to set the rotating member  32  to a desired rotation angle between an angle (angle of engagement) at which there is clutch engagement and an angle (angle of disengagement) at which there is disengagement of the clutch  10 . In some configurations, the rotation of a rotating shaft  46  of the motor  10  may be detected for use in control. 
     FIG. 8  is a graph illustrating the relationship among the torque (curve A) imparted to the rotating member  32  by the second piston push rod  34  (under the force of the clutch spring), the torque (curve B) imparted to the rotating member  32  by the auxiliary spring  22 , and a composite torque thereof (curve C). The Y-axis on the graph indicates a torque value. A negative value indicates a torque acting to rotate the rotating member  32  clockwise in  FIG. 7  to push the second piston push rod  34  toward the master cylinder  40 , thereby disengaging the clutch  10 . A positive value indicates a torque acting to rotate the rotating member  32  counterclockwise in  FIG. 7  to return the second piston push rod  34  towards its original position, thereby engaging the clutch  10 . The X-axis in the graph indicates a rotational angle of the rotating member  32 . In the illustrated embodiment, the clutch is engaged when the rotational angle is about 40 degrees, and the clutch is disengaged when the rotational angle is about 180 degrees. 
   With reference to curve A in the figure, the torque imparted to the rotating member  32  by the second piston push rod  34  at about 40 degrees (corresponding to a clutch engaged position) is zero (or thereabout) as a result of the clutch spring relaxing to a substantially unloaded state. When the rotating member  32  is rotated in a direction of clutch disengagement (clockwise), the torque represented by curve A gradually increases to resist the clockwise rotation of the rotating member  32 . That is, rotation of the rotating member  32  causes the piston  36  to move leftward in  FIG. 7 , thereby loading the clutch spring. The clutch spring force produces a positive torque on the rotating member  32  (e.g., a torque in the counterclockwise direction). Thereafter, as the rotating member  32  continues to rotate, a distance (or moment arm) between a rotational axis of the rotating member  32  and a vector of the clutch spring force first increases and then begins to decrease. The vector of the clutch spring force extends along the longitudinal axis of the second piston push rod  34 . As a result of this decreasing moment arm, the torque on the rotating member  32  produced by the clutch spring begins to decrease as the rotating member  34  continues to rotate clockwise. In a disengaged state of the clutch  10 , the vector of the clutch spring force extends through the rotational axis of the rotating member  32  so as to produce about zero torque on the rotating member  32 . That is, while the clutch spring is fully loaded, its produces no or substantially no torque on the rotating member  32  because the clutch spring force vector intersects with the rotational axis of the rotating member. There is no moment arm (or a very small one) at the state of clutch disengagement. 
   Curve B shows the torque acting on the rotating member  32  that is caused by the auxiliary spring  22 . This torque will herein be referred to as an “assist torque.” At the state of clutch engagement (e.g., the about 40 degree position of the rotating member), the auxiliary spring is loaded and produces an auxiliary spring force vector that extends to the left side of the rotational axis of the rotating member  32  in  FIG. 7 . As such, the auxiliary spring  22  produces a positive assist torque, thereby promoting clutch engagement (e.g., biasing counterclockwise rotation of the rotating member  32 ). When the rotating member  32  is rotated in a direction of clutch disengagement (e.g., clockwise), the positive assist torque decreases to about zero as the moment arm between the auxiliary spring force and the rotational axis shrinks to about zero. Further rotation of the rotating member  32  produces a moment arm between the auxiliary spring force and the rotational axis on the other side of the rotational axis (e.g., on the right side in  FIG. 4 ). Consequently, the auxiliary spring  22  generates a negative assist torque on the rotating member  32 , thereby assisting to disengage the clutch  10 . The negative assist torque continues to increase with rotation of the rotating member  32  despite the additional clockwise rotational movement unloading the auxiliary spring  22 . The increase in the negative assist torque continues through a range of rotation that corresponds to a semi-engaged state as the moment arm continues to increase. Further rotation of the rotating member  32  in the clockwise direction beyond the semi-engaged state causes the negative assist torque to decrease as a consequence of further unloading of the auxiliary spring  22  and/or a decrease of the corresponding moment arm. Once the rotating member  32  has rotated to a position corresponding to the disengaged state (e.g., 180 degrees in the illustrated embodiment), the auxiliary spring  22  remains loaded, but only slightly. The moment arm also has decreased so as to produce only a small amount of negative assist torque on the rotating member  32 . 
   As represented by curve C in the  FIG. 8 , the torque imparted by the second piston push rod  34  and the torque imparted by the auxiliary spring  22  act in opposite directions to offset each other over a substantial portion of the rotating member&#39;s motion. Therefore, an absolute value of torque imparted to the rotating member  32  becomes a relatively small value, so that the motor  44  can readily drive the rotating member  32 . Also, at the rotational angle of engagement (i.e., a clutched state), the composite torque acts in a direction of clutch engagement (e.g., counterclockwise in the illustrated embodiment). Therefore, the members (e.g., plates) of the clutch  10  remain biased towards one another during the engaged state (i.e., clutched state) to stabilize this operational state of the clutch  10 . Also, at the rotational angle of disengagement (i.e., unclutched state), the composite torque acts in a direction of clutch disengagement (e.g., clockwise in the illustrated embodiment). Therefore, the members (e.g., plates) of the clutch  10  remain biased apart from one another during the disengaged state (i.e., unclutched state) to stabilize this operational state of the clutch  10 . Further, at rotational angles corresponding to the semi-engaged state of the clutch  10 , torque in a direction of clutch engagement is generated as the rotating member  32  rotates in a direction of clutch disengagement, and torque in a direction of clutch disengagement is generated as the rotating member  32  rotates in a direction of clutch engagement. As a consequence of the composite torque counteracting rotational movement of the rotating member  32  through the rotational range corresponding to the semi-engaged state, the clutch  10  has increased stability in the semi-engaged state as compared to prior clutch actuator mechanisms. 
   Although the present invention has been described in terms of a certain embodiment, other embodiments apparent to those of ordinary skill in the art also are within the scope of this invention. Thus, various changes and modifications may be made without departing from the spirit and scope of the invention. For instance, various components may be repositioned as desired. Moreover, not all of the features, aspects and advantages are necessarily required to practice the present invention. Accordingly, the scope of the present invention is intended to be defined only by the claims that follow.