Abstract:
In a reactive force pedal device, between a motor-side output shaft and a pedal member, a one-way clutch is provided which allows transmittance of rotational force when performing a depressing operation of the pedal member and disables transmittance of the rotational force when performing a revert operation of the pedal member.

Description:
TECHNICAL FIELD 
     The present invention relates to a reaction force pedal device having a pedal to be depressed by the driver of a vehicle and a motor for applying a reaction force to the pedal. 
     BACKGROUND ART 
     There is known an arrangement for applying a reaction force from an actuator to an accelerator pedal depending on the amount through which the accelerator pedal is operated {for example, Japanese Laid-Open Patent Publication No. 2007-026218 (hereinafter referred to as “JP2007-026218A”)}. According to JP2007-026218A, an accelerator pedal 3 and a servomotor 19 are operatively coupled to each other by a pedal lever 5 and an arm 15. The servomotor 19 has a drive shaft 19b supporting on a distal end thereof a gear 19a that is held in mesh with a gear segment 15b on the pedal lever 5. The servomotor 19 applies a reaction force through the above structure to the accelerator pedal 3 (see Abstract and FIG. 1). 
     According to Japanese Laid-Open Patent Publication No. 2005-132225 (hereinafter referred to as “JP2005-132225A”), a depressing force changing means which comprises a variable friction plate 7, a fixed shaft 8, and an actuator 9 (e.g., an electromagnetic solenoid) is used to indicate a switchover of driving characteristics to the driver of a vehicle (see Abstract and [0011]). According to JP2005-132225A, furthermore, a one-way clutch 12 is disposed between a rotational shaft 3 to which an accelerator pedal 2 is fixed and the variable friction plate 7 (FIG. 10). When the accelerator pedal 2 is returned, the one-way clutch 12 prevents a frictional force of the variable friction plate 7 from being transmitted to the rotational shaft 3 (see [0038]). 
     SUMMARY OF INVENTION 
     As described above, according to JP2007-026218A, a reaction force is transmitted by a speed reducer mechanism which includes the gear 19a and the gear segment 15b. However, no details (such as a gear ratio, etc.) about the speed reducer mechanism are found in the description of the document. 
     JP2007-026218A shows the gear 19a and the gear segment 15b in FIGS. 1, 2, and 4. A review of these figures reveals that even when the gear 19a of the servomotor 19 is turned maximally, the gear 19a has only a portion thereof held in mesh with the gear segment 15b, and has another portion that remains out of mesh with the gear segment 15b. Consequently, a large force continues to be applied only to the portion of the gear 19a, which is thus partly worn badly and partly not worn at all. As a result, when the gear 19a of the servomotor 19 is turned through a maximum angular interval, the gear 19a may possibly become lower in overall durability than when the gear 19a is fully circumferentially brought into mesh with the gear segment 15b. 
     The fact that the gear 19a has only a portion thereof held in mesh with the gear segment 15b even when the gear 19a of the servomotor 19 is turned through a maximum angular interval means that the drive shaft 19b of the servomotor 19 rotates through a rotational angle less than 360°. Therefore, not only the gear 19a, but also various components of the servomotor 19 have localized portions to which forces are applied. The overall durability of the servomotor 19 thus becomes lower than when averaged forces are applied thereto. If the servomotor 19 is a brush motor, then since the brush motor has a commutator and brushes held in contact with each other in a limited range, only certain portions tend to be worn. As a consequence, there are developed steps between those portions which are worn badly and those portions which are not worn, resulting in a reduction in the overall durability of the servomotor 19. Differently worn surfaces are liable to cause noise and load steps (different resistances against the rotation of the servomotor 19), which are likely to make the user feel strange. 
     According to JP2005-132225A, the variable friction plate 7 and the one-way clutch 12 are mounted on the rotational shaft 3 to which the accelerator pedal 2 is fixed (FIG. 10). If the variable friction plate 7 and the one-way clutch 12 disclosed in JP2005-132225A are applied to the structure shown in JP2007-026218A, then the variable friction plate 7 and the one-way clutch 12 should be mounted on a lever shaft 7 (FIG. 1) to which the accelerator pedal 3 and the pedal lever 5 are fixed. Therefore, even if the variable friction plate 7 and the one-way clutch 12 disclosed in JP2005-132225A are applied to the structure shown in JP2007-026218A, the positional relationship between the gear 19a of the servomotor 19 and the gear segment 15b of the pedal lever does not change, and hence the problem of unevenly worn surfaces (uneven wear) remains unsolved. 
     The present invention has been made in view of the above problems. It is an object of the present invention to provide a reaction force pedal device which will increase the durability of a transmission system for transmitting a reaction force. 
     Another object of the present invention is to provide a reaction force pedal device which will not make the user feel strange. 
     According to the present invention, there is provide a reaction force pedal device comprising a pedal member to be depressed by the driver of a vehicle, a motor for applying a reaction force in a direction to return the pedal member when the driver depresses the pedal member, and a motor-side output shaft disposed on the motor for transmitting rotation of the motor to the pedal member, wherein a one-way clutch is disposed between the motor-side output shaft and the petal member, for allowing a rotational force to be transmitted when the pedal member is depressed and preventing a rotational force from being transmitted when the pedal member is returned. 
     According to the present invention, when the pedal member is depressed, a power force from the motor is transmitted through the one-way clutch to the pedal member to apply a reaction force to the pedal member that is depressed by the driver. When the pedal member is returned, the one-way clutch prevents a rotational force applied by the returning pedal member from being transmitted to the motor-side output shaft. Accordingly, the position (the operation amount) of the pedal member and the rotational angle of the motor-side output shaft are different before the pedal member starts being depressed and after the pedal member ends its returning movement (the pedal member is in its original position in each case). Stated otherwise, the corresponding relationship between the position (the operation amount) of the pedal member and the rotational angle of the motor-side output shaft changes when the pedal member in the original position starts to be depressed for the first time and when the pedal member in the original position starts to be depressed for the second time. 
     If the motor comprises a brush motor, for example, then it is possible to prevent the commutator and the brushes of the brush motor from contacting each other in a limited range and hence to prevent only certain portions from being unevenly worn. 
     The reaction force pedal device may further comprise a speed reducer disposed between the pedal member and the motor, for transmitting a drive force from the motor to the pedal member, wherein the speed reducer may include at least one pair of speed reducer gears and a speed-reducer-side output shaft for transmitting rotation of the motor-side output shaft to the pedal member, and the one-way clutch may be disposed between the motor-side output shaft and the speed-reducer-side output shaft. 
     If the one-way clutch is disposed between at least one speed reducer gear and the speed-reducer-side output shaft, then a rotational force applied by the returning pedal member is prevented from being transmitted to speed reducer gears that are closer to the motor than the one-way clutch. 
     Accordingly, the position (the operation amount) of the pedal member and the rotational angles of the speed reducer gears that are closer to the motor than the one-way clutch are different before the pedal member starts being depressed and after the pedal member ends its returning movement (the pedal member is in its original position in each case). Stated otherwise, the corresponding relationship between the position (the operation amount) of the pedal member and the rotational angles of the speed reducer gears that are closer to the motor than the one-way clutch changes when the pedal member in the original position starts to be depressed for the first time and when the pedal member in the original position starts to be depressed for the second time. 
     Therefore, the speed reducer gears that are closer to the motor than the one-way clutch have gear teeth meshing in different positions each time the pedal member is depressed, and hence have worn portions distributed rather than having only certain portions unevenly worn. 
     The pedal member may comprise a pad to be operated by the driver, a pedal-side arm having an end coupled to the pad and another end angularly movably supported on a vehicle body of the vehicle, a motor-side arm coupled to the speed-reducer-side output shaft and held displaceably against the pedal-side arm, for transmitting the drive force from the motor to the pedal-side arm, and an urging unit for urging the motor-side arm into contact with the pedal-side arm, wherein the speed reducer may have a plurality of pairs of speed reducer gears, and the one-way clutch may be disposed between one of the speed reducer gears which is closest to the speed-reducer-side output shaft and the speed-reducer-side output shaft. 
     Accordingly, the urging force of the urging unit can be reduced. In addition, it is possible to improve the driver&#39;s feeling at the time the driver depresses the pedal member. 
     Specifically, if at least one speed reducer gear (hereinafter referred to as “speed-reducer-output-shaft-side speed reducer gear”) is disposed closer to the speed-reducer-side output shaft than the one-way clutch in an arrangement having a plurality of pairs of speed reducer gears, then the motor-side arm and the speed-reducer-output-shaft-side speed reducer gear are coupled to each other. If the urging unit should bring the motor-side arm into contact with the pedal-side arm while overcoming the inertia and frictional force of the speed-reducer-output-shaft-side speed reducer gear when the pedal member is returned, then it is necessary to relatively increase the urging force of the urging unit. 
     If the urging force of the urging unit is relatively increased, then the urging force that is transmitted to the driver when the pedal member is depressed is also relatively increased. Therefore, the load to be applied to the pedal member to depress the pedal member may possibly be unnecessarily large. 
     According to the present invention, however, the one-way clutch is disposed between the speed reducer gear closest to the speed-reducer-side output shaft and the speed-reducer-side output shaft. The one-way clutch prevents the inertia and frictional force of the speed reducer gear from acting on the motor-side arm when the pedal member is returned. Consequently, the urging force of the urging unit can be relatively reduced. In addition, the load to be applied to the pedal member to depress the pedal member does not need to be unnecessarily large, making it possible to improve the driver&#39;s feeling at the time the driver depresses the pedal member. 
     The reaction force pedal device may further comprise a torque limiter disposed between one of the speed reducer gears that is closest to the speed-reducer-side output shaft and the speed-reducer-side output shaft, for preventing a torque in excess of a predetermined value from being transmitted. Even in the event that the motor or either one of the speed reducer gears fails to move on account of some fault at the time the pedal member is depressed, the torque limiter allows the driver to depress the pedal member. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram of a vehicle incorporating an accelerator pedal device as a reaction force pedal device according to an embodiment of the present invention; 
         FIG. 2  is a perspective view schematically showing the appearance of components of the accelerator pedal device; 
         FIG. 3  is a diagram showing the relationship between the operation amount through which an accelerator pedal is depressed and a first urging force generated by an urging force generator; 
         FIG. 4  is a view schematically showing the internal structure of a reaction force generator of the accelerator pedal device; 
         FIG. 5  is a view showing the internal structure of a motor in the embodiment; and 
         FIG. 6  is a diagram showing the movements of various components of the accelerator pedal device at the time the accelerator pedal is operated. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A. Embodiment 
     1. Arrangement of Vehicle  10   
     (1) Overall Arrangement 
       FIG. 1  is a block diagram of a vehicle  10  incorporating an accelerator pedal device  12  as a reaction force pedal device according to an embodiment of the present invention.  FIG. 2  is a perspective view schematically showing the appearance of components of the accelerator pedal device  12 . For example, the vehicle  10  comprises a gasoline-powered vehicle. Alternatively, the vehicle  10  may comprise an electric vehicle including a hybrid vehicle and a fuel battery vehicle. 
     The vehicle  10  includes, in addition to the accelerator pedal device  12 , a powertrain system  14  and an electronic control unit  16  (hereinafter referred to as “ECU  16 ”). 
     (2) Accelerator Pedal Device  12   
     The accelerator pedal device  12  has a pad  20  to be depressed by the driver, a pedal-side arm  22 , an urging force generator  24 , a reaction force generator  26 , and a motor-side arm  28 . The pad  20  and the pedal-side arm  22  will collectively be referred to as an accelerator pedal  30 . 
     (a) Pedal-Side Arm  22   
     The pedal-side arm  22  has an end fixed to the pad  20  and another end swingably supported by the urging force generator  24  (see  FIG. 2 ). 
     (b) Urging Force Generator  24   
     The urging force generator  24  mechanically generates an urging force (hereinafter referred to as “first urging force Fs 1 ” [N]) for returning the accelerator pedal  30  that has been depressed to its original position. The urging force generator  24  applies the generated urging force through the pedal-side arm  22  to the pad  20 . As shown in  FIG. 1 , the urging force generator  24  includes a return spring  40 , a hysteresis property generator  42 , and an operation amount sensor  44 . 
     The hysteresis property generator  42  generates a hysteresis property to be added to the first urging force Fs 1  that is generated by the return spring  40 . Specifically, as shown in  FIG. 3 , the hysteresis property generator  42  increases the first urging force Fs 1  when the accelerator pedal  30  is depressed and reduces the first urging force Fs 1  when the accelerator pedal  30  is returned. 
     The return spring  40  and the hysteresis property generator  42  may be of structures disclosed in International Publication No. WO 01/019638, for example. 
     The operation amount sensor  44  detects the angle θ [degrees] through which the accelerator pedal  30  is operated from its original position, depending on the displacement of the pedal-side arm  22 , and supplies the detected angle θ to the ECU  16 . The operation amount sensor  44  may be positioned outside of the urging force generator  24 . 
     (c) Reaction Force Generator  26   
       FIG. 4  is a view schematically showing the internal structure of the reaction force generator  26  of the accelerator pedal device  12 . As shown in  FIG. 4 , the reaction force generator  26  generates a power force (hereinafter referred to as “motor power force Fm” [N]) to be applied to the accelerator pedal  30 . The motor power force Fm is used as a reaction force that acts in a direction to return the accelerator pedal  30  when the driver depresses the accelerator pedal  30 . As shown in  FIGS. 1 and 4 , the reaction force generator  26  has a motor  50  as an actuator, a driver  52  for controlling the motor  50 , and a speed reducer  54 . 
       FIG. 5  is a view showing the internal structure of the motor  50 . The motor  50  generates a motor power force Fm based on a control signal from the driver  52 . According to the present embodiment, the motor  50  comprises a DC brush motor. Alternatively, the motor  50  may comprise a DC brushless motor or an AC three-phase motor. Further alternatively, the motor  50  may be replaced with another drive force generating means such as a pneumatic actuator, for example. 
     As shown in  FIG. 5 , the motor  50  has a permanent magnet  60  fixedly mounted in a case  62 , an armature  64  rotatable around the axis of an output shaft  66  of the motor  50  (hereinafter also referred to as “motor output shaft  66 ”), a commutator  68  for controlling the direction of an electric current, and brushes  70  for supplying an electric current to the armature  64  through the commutator  68 . The motor  50  generates motor power force Fm that is transmitted through the motor output shaft  66  (motor-side output shaft) to the speed reducer  54 . 
     The driver  52  controls the motor  50  depending on a control signal Sm from the ECU  16 . 
     As shown in  FIG. 4 , the speed reducer  54  has first through sixth gears  80 ,  82 ,  84 ,  86 ,  88 ,  90  as speed reduction gears, the sixth gear  90  including a spur gear, a torque limiter  92 , a one-way clutch  94 , an output shaft  96  (hereinafter also referred to as “speed reducer output shaft  96 ”), and an additional spring  98  (urging unit). 
     The first gear  80  is mounted on the motor output shaft  66 . The second gear  82  is mounted on a first intermediate shaft  100  rotatably supported on an inner wall surface, not shown, of a housing and held in mesh with the first gear  80 . The first gear  80  and the second gear  82  jointly make up a first speed reducer unit  102 . The third gear  84  is mounted on the first intermediate shaft  100 , as with the second gear  82 . The fourth gear  86  is mounted on a second intermediate shaft  104  rotatably supported on the inner wall surface of the housing and held in mesh with the third gear  84 . The third gear  84  and the fourth gear  86  jointly make up a second speed reducer unit  106 . The fifth gear  88  is mounted on the second intermediate shaft  104 , as with the fourth gear  86 . The sixth gear  90  is mounted on the output shaft  96  (speed-reducer-side output shaft) that is rotatably supported by bearings  108 ,  110  and fixed to the motor-side arm  28 . The sixth gear  90  is held in mesh with the fifth gear  88 . The fifth gear  88  and the sixth gear  90  jointly make up a third speed reducer unit  112 . The speed reducer  54 , which has three pairs of speed reducer gears as described above, includes three speed reducer units (first through third speed reducer units  102 ,  106 ,  112 ) for reducing speeds through three stages. 
     The torque limiter  92  has an inner limiter element, not shown in  FIG. 4 , fixed to the one-way clutch  94  on the side of the output shaft  96  and an outer limit element, not shown in  FIG. 4 , fixed to the sixth gear  90  on the side of the motor  50 . When a predetermined torque is applied to the torque limiter  92 , one of the inner limiter element and the outer limiter element slips against the other. Therefore, even in the event that the motor  50  or any one of the first through third speed reducer units  102 ,  106 ,  112  fails to move, the output shaft  96  can be turned by the accelerator pedal  30  when it is depressed. 
     The one-way clutch  94  has an inner clutch element, not shown in  FIG. 4 , fixed to the output shaft  96  and an outer clutch element, not shown in  FIG. 4 , fixed to the inner limiter element of the torque limiter  92  on the side of the motor  50 . When the accelerator pedal  30  is depressed, the inner clutch element and the outer clutch element are turned in unison with each other. When the accelerator pedal  30  is returned, only the inner clutch element is turned, and the outer clutch element is not turned. 
     The additional spring  98  comprises a helical spring having an end fixed to the output shaft  96  and another end fixed to a bracket  114 . The additional spring  98  generates an urging force (hereinafter referred to as “second urging force Fs 2 ” [N]) for urging the output shaft  96  to return the motor-side arm  28  coupled to the output shaft  96  to its original position. The motor-side arm  28  has a portion that is held in contact with a portion of the pedal-side arm  22  at all times (see  FIG. 2 ). The speed reducer  54  applies the second urging force Fs 2  and the motor power force Fm through the motor-side arm  28  to the pedal-side arm  22 . 
     (d) Motor-Side Arm  28   
     The motor-side arm  28  has an end coupled to an end of the speed reducer output shaft  96  (see  FIG. 4 ). Therefore, the motor-side arm  28  and the speed reducer output shaft  96  are coordinated with each other in operation. 
     (3) Powertrain System  14   
     The powertrain system  14  applies a drive force to the vehicle  10 , and includes an engine, a transmission, road wheels, etc., not shown. 
     (4) ECU  16   
     The ECU  16  controls the powertrain system  14  and the reaction force generator  26  based on the operation amount θ of the accelerator pedal  30  detected by the operation amount sensor  44  and the vehicle speed detected by a vehicle speed sensor, not shown, and the like. The ECU  16  may control the motor power force Fm according to the arrangement disclosed in International Publication No. WO 2009/136512, for example. 
     2. Overall Movement made when the Accelerator Pedal  30  is Operated 
     The accelerator pedal device  12  according to the present embodiment is constructed as described above. When the accelerator pedal  30  is depressed and returned, the accelerator pedal device  12  makes overall movement as described below. When necessary, a direction in which various components are moved or turned when the accelerator pedal  30  is depressed will be referred to as “forward direction”, whereas a direction in which various components are moved or turned when the accelerator pedal  30  is returned will be referred to as “reverse direction”. 
     (1) When the Accelerator Pedal  30  is Depressed 
     When the driver depresses the accelerator pedal  30 , the accelerator pedal  30  is turned in the forward direction about the urging force generator  24 , and has its distal end moved downwardly (see  FIG. 2 ). The pedal-side arm  22  has its end turned downwardly while changing a relative angle formed between itself and the accelerator pedal  30 . At this time, the pedal-side arm  22  receives the first urging force Fs 1  from the urging force generator  24  (return spring  40 ). 
     When the pedal-side arm  22  is turned downwardly, the portion of the pedal-side arm  22  presses the portion of the motor-side arm  28 . As a result, the portion of the pedal-side arm  22  moves downwardly in unison with the portion of the motor-side arm  28 . Since the additional spring  98  is torsionally tensioned as the motor-side arm  28  is turned, the motor-side arm  28  is subject to the second urging force Fs 2  as an origin returning force. 
     Based on the operation amount θ detected by the operation amount sensor  44 , the ECU  16  sets an output power force of the motor  50 , i.e., controls the motor  50  to generate the motor power force Fm. The motor power force Fm is transmitted through the speed reducer  54  to the motor-side arm  28  (movements in the speed reducer  54  will be described later). 
     Therefore, the motor-side arm  28  is subject to the depressing force that the driver has applied to the accelerator pedal  30 , the first urging force Fs 1  from the return spring  40 , the motor power force Fm from the motor  50 , and the second urging force Fs 2  from the additional spring  98  (see  FIG. 1 ). 
     (2) When the Accelerator Pedal  30  is Returned 
     When the driver returns the accelerator pedal  30 , the accelerator pedal  30  is turned in the reverse direction about the urging force generator  24  under the first urging force Fs 1  from the return spring  40 . At this time, the second urging force Fs 2  from the additional spring  98  acts on the speed reducer output shaft  96 . Therefore, the motor-side arm  28  coupled to the speed reducer output shaft  96  is turned in the reverse direction, keeping itself in contact with the pedal-side arm  22 . 
     When the driver returns the accelerator pedal  30 , the components of the reaction force generator  26  which are positioned closer to the motor  50  than the one-way clutch  94 , i.e., the motor output shaft  66 , the first through sixth gears  80 ,  82 ,  84 ,  86 ,  88 ,  90 , and the torque limiter  92 , are disconnected from the speed reducer output shaft  96 , by the operation of the one-way clutch  94 , as described in detail later. 
     3. Movements in the Speed Reducer  54  when the Accelerator Pedal  30  is Operated 
       FIG. 6  is a diagram showing the movements of various components of the accelerator pedal device  12  at the time the accelerator pedal  30  is operated. In  FIG. 6 , clockwise arrows indicate movements in the forward direction, i.e., the direction in which various components are moved or turned when the accelerator pedal  30  is depressed, and counterclockwise arrows indicate movements in the reverse direction, i.e., the direction in which various components are moved or turned when the accelerator pedal  30  is returned. It should be noted that the illustrated directions do not necessarily agree with directions in which the various components are actually moved or turned. Cross marks in  FIG. 6  indicate that the corresponding components are not moved when the accelerator pedal  30  is depressed or returned. 
     As described above, the speed reducer  54  according to the present embodiment includes the one-way clutch  94 . Therefore, as shown in  FIG. 6 , the components are moved differently when the accelerator pedal  30  is depressed and returned. 
     Specifically, when the accelerator pedal  30  is depressed, i.e., when it is operated normally, the speed reducer output shaft  96 , the one-way clutch  94  (the inner clutch element and the outer clutch element), the torque limiter  92  (the inner limiter element and the outer limiter element), the third speed reducer unit  112  (the fifth gear  88  and the sixth gear  90 ), the second speed reducer unit  106  (the third gear  84  and the fourth gear  86 ), the first speed reducer unit  102  (the first gear  80  and the second gear  82 ), and the motor  50  (the output shaft  96 ) are turned in the same direction, i.e., the forward direction (see the arrows in the first line of  FIG. 6 ). It should be noted that the motor power force Fm generated by the motor  50  at this time is in the reverse direction. 
     When the accelerator pedal  30  is returned, the one-way clutch  94  is operated to allow the speed reducer output shaft  96  and the inner clutch element of the one-way clutch  94  to turn in the reverse direction, and to keep the other components, i.e., the outer clutch element of the one-way clutch  94 , the torque limiter  92  (the inner limiter element and the outer limiter element), the first through third speed reducer units  102 ,  106 ,  112 , and the motor  50 , unturned and still (see the arrows in the second line of  FIG. 6 ). 
     Consequently, the correlation between the operation amount θ of the accelerator pedal  30  and rotational angles of the motor output shaft  66  and the first through third speed reducer units  102 ,  106 ,  112  (the first through sixth gears  80 ,  82 ,  84 ,  86 ,  88 ,  90 ) becomes different. 
     The speed reducer  54  according to the present embodiment includes the torque limiter  92 . Therefore, if the motor  50  and the first through third speed reducer units  102 ,  106 ,  112  fail to move in the event that the motor  50  or any one of the first through third speed reducer units  102 ,  106 ,  112  (the first through sixth gears  80 ,  82 ,  84 ,  86 ,  88 ,  90 ) is unable to move, then the various components are moved as indicated by the cross marks in the third line of  FIG. 6 . 
     Specifically, when the driver depresses the accelerator pedal  30  while some of the components fail to move as described above, the speed reducer output shaft  96  and the one-way clutch  94  (the inner clutch element and the outer clutch element) are turned, applying a torque in excess of a predetermined value to the torque limiter  92 . When such a torque is applied to the torque limiter  92 , the inner limiter element thereof slips against the outer limiter element thereof. Therefore, only the speed reducer output shaft  96 , the one-way clutch  94 , the inner limiter element are turned, and the other components, i.e., the outer limiter element of the torque limiter  92 , the first through third speed reducer units  102 ,  106 ,  112  (the first through sixth gears  80 ,  82 ,  84 ,  86 ,  88 ,  90 ), and the motor output shaft  66  keep unturned and still (see the arrows in the third line of  FIG. 6 ). 
     Consequently, even if the motor  50  and the first through third speed reducer units  102 ,  106 ,  112  fail to move in the event that the motor  50  or any one of the first through third speed reducer units  102 ,  106 ,  112  (the first through sixth gears  80 ,  82 ,  84 ,  86 ,  88 ,  90 ) is unable to move, it is possible to turn the speed reducer output shaft  96 , i.e., to operate the accelerator pedal  30 . 
     4. Advantages of the Present Embodiment 
     According to the present embodiment, as described above, when the accelerator pedal  30  (pedal member) is depressed, the motor power force Fm is transmitted through the speed reducer  54  to the accelerator pedal  30 , applying a reaction force to the accelerator pedal  30  against the depressing force applied by the driver. When the accelerator pedal  30  is returned, the one-way clutch  94  disposed between the sixth gear  90  (speed reducer gear) and the speed reducer output shaft  96  prevents a rotational force applied by the returning accelerator pedal  30  from being transmitted to the motor output shaft  66  (motor-side output shaft) and the first through sixth gears  80 ,  82 ,  84 ,  86 ,  88 ,  90  (speed reducer gears). 
     Accordingly, the position (the operation amount θ) of the accelerator pedal  30  and the rotational angles of the motor output shaft  66  and the first through sixth gears  80 ,  82 ,  84 ,  86 ,  88 ,  90  are different before the accelerator pedal  30  starts being depressed and after the accelerator pedal  30  ends its returning movement (the accelerator pedal  30  is in its original position in each case). Stated otherwise, the corresponding relationship between the position (the operation amount θ) of the accelerator pedal  30  and the rotational angles of the motor output shaft  66  and the first through sixth gears  80 ,  82 ,  84 ,  86 ,  88 ,  90  changes when the accelerator pedal  30  in the original position starts to be depressed for the first time and when the accelerator pedal  30  in the original position starts to be depressed for the second time. 
     Therefore, it is possible to prevent the commutator  68  and the brushes  70  of the motor  50  from contacting each other in a limited range and hence to prevent only certain portions from being unevenly worn. In addition, the first through sixth gears  80 ,  82 ,  84 ,  86 ,  88 ,  90  have gear teeth meshing in different positions each time the accelerator pedal  30  is depressed, and hence have worn portions distributed rather than having only certain portions unevenly worn. 
     According to the present embodiment, the speed reducer  54  has the first through third speed reducer units  102 ,  106 ,  112  (the first through sixth gears  80 ,  82 ,  84 ,  86 ,  88 ,  90 ), or stated otherwise, has a plurality of pairs of speed reducer gears, and the one-way clutch  94  is disposed between the sixth gear  90  closest to the speed reducer output shaft  96  and the speed reducer output shaft  96 . The one-way clutch  94  thus positioned makes it possible to reduce the second urging force Fs 2  generated by the additional spring  98 , and also to improve the driver&#39;s feeling at the time the driver depresses the accelerator pedal  30 . 
     Specifically, if at least one speed reducer gear (hereinafter referred to as “speed-reducer-output-shaft-side speed reducer gear”) is disposed closer to the speed reducer output shaft  96  than the one-way clutch  94  in an arrangement having the first through third speed reducer units  102 ,  106 ,  112 , then the motor-side arm  28  and the speed-reducer-output-shaft-side speed reducer gear are coupled to each other. For example, if the one-way clutch  94  is disposed between the fifth gear  88  and the second intermediate shaft  104 , then the motor-side arm  28  and the sixth gear  90  (the speed-reducer-output-shaft-side speed reducer gear) are coupled to each other. If the additional spring  98  (urging unit) should bring the motor-side arm  28  into contact with the pedal-side arm  22  while overcoming the inertia and frictional force of the speed-reducer-output-shaft-side speed reducer gear when the accelerator pedal  30  is returned, then it is necessary to relatively increase the urging force of the additional spring  98 . 
     If the second urging force Fs 2  of the additional spring  98  is relatively increased, then the urging force (the sum of the first urging force Fs 1 , the second urging force Fs 2 , and the motor power force Fm) that is transmitted to the driver when the accelerator pedal  30  is depressed is also relatively increased. Therefore, the load to be applied to the accelerator pedal  30  to depress the accelerator pedal  30  may possibly be unnecessarily large. 
     According to the present embodiment, however, the one-way clutch  94  is disposed between the sixth gear  90  closest to the speed reducer output shaft  96  and the speed reducer output shaft  96 . The one-way clutch  94  prevents the inertia and frictional force of the speed reducer gear from acting on the motor-side arm  28  when the accelerator pedal  30  is returned. Consequently, the second urging force Fs 2  generated by the additional spring  98  can be relatively reduced. In addition, the load to be applied to the accelerator pedal  30  to depress the accelerator pedal  30  does not need to be unnecessarily large, making it possible to improve the driver&#39;s feeling at the time the driver depresses the accelerator pedal  30 . 
     According to the present embodiment, the accelerator pedal device  12  includes the torque limiter  92  that is disposed between the sixth gear  90  closest to the speed reducer output shaft  96  and the speed reducer output shaft  96 , for limiting the transmission of a torque in excess of a predetermined value. Even in the event that the motor  50  or any one of the first through sixth gears  80 ,  82 ,  84 ,  86 ,  88 ,  90  fails to move on account of some fault at the time the accelerator pedal  30  is depressed, the torque limiter  92  allows the driver to depress the accelerator pedal  30 . 
     B. Modifications 
     The present invention is not limited to the above embodiment, but may adopt various arrangements based on the disclosure of the above description. For example, the present invention may adopt the following arrangements: 
     1. Vehicle  10   
     In the above embodiment, the vehicle  10  is a gasoline-powered vehicle. However, the vehicle  10  is not limited to a gasoline-powered vehicle, but may be an electric vehicle including a hybrid vehicle and a fuel battery vehicle. 
     2. Accelerator Pedal  30   
     In the above embodiment, the pedal that applies the motor power force Fm is the accelerator pedal  30 . However, the same arrangement may be applied to a brake pedal. Specifically, at least one of the torque limiter  92  and the one-way clutch  94  may be applied to an arrangement which is capable of applying the motor power force Fm to a brake pedal. 
     3. Urging Force Generator  24   
     In the above embodiment, the urging force generator  24  is only of a mechanical structure. However, the urging force generator  24  may have an electric or electromagnetic mechanism. 
     4. Reaction Force Generator  26   
     In the above embodiment, the motor  50  is used to generate a reaction force (urging force) to be applied to the accelerator pedal  30 . However, the present invention is not limited to the motor  50 , but may use any drive force generating means that is capable of adjusting a reaction force depending on a command from the ECU  16 . For example, a pneumatic actuator may be used instead of the motor  50 . 
     In the above embodiment, the motor power force Fm is transmitted through the motor-side arm  28  to the accelerator pedal  30 . However, the present invention is not limited to such a transmission system, but the motor power force Fm may be transmitted directly from the reaction force generator  26  to the accelerator pedal  30 . Alternatively, the motor-side arm  28  may be inseparably, but relatively displaceably, coupled to the pedal-side arm  22 , e.g., may be rotatably supported on a portion of the pedal-side arm  22 . 
     In the above embodiment, the speed reducer  54  includes the three pairs of speed reducer gears, i.e., the first through sixth gears  80 ,  82 ,  84 ,  86 ,  88 ,  90  (the first through third speed reducer units  102 ,  106 ,  112 ). However, the number of speed reducer gears and the number of speed reducer units are not limited to the illustrated numbers. The number of speed reducer gears may be 2 or 4 (one pair or two pairs) (the number of speed reducer units is 1 or 2), or the number of speed reducer gears may be 8 (four pairs) or greater (the number of speed reducer units is 4 or greater). While each of the first through sixth gears  80 ,  82 ,  84 ,  86 ,  88 ,  90  comprises a spur gear in the illustrated embodiment, it may be any of various other gear types, e.g., a helical gear, a double helical gear, a rack and pinion, or the like. 
     In the above embodiment, the speed reducer  54  is disposed between the motor-side arm  28  and the motor  50 . However, the speed reducer  54  may be dispensed with, and the torque limiter  92  and the one-way clutch  94  may be mounted on the motor output shaft  66 , for example. 
     In the above embodiment, the speed reducer  54  includes both the torque limiter  92  and the one-way clutch  94 . However, the speed reducer may include either one of the torque limiter  92  and the one-way clutch  94 . 
     In the above embodiment, the torque limiter  92  is disposed between the sixth gear  90  and the one-way clutch  94 . However, if the one-way clutch  94  is included, then the torque limiter  92  may be disposed anywhere between the accelerator pedal  30  and the motor output shaft  66 . For example, the torque limiter  92  may be disposed between the one-way clutch  94  and the speed reducer output shaft  96 . Alternatively, the torque limiter  92  may be disposed between the fifth gear  88  and the second intermediate shaft  104 . 
     In the above embodiment, the one-way clutch  94  is disposed between the torque limiter  92  and the speed reducer output shaft  96 . However, the one-way clutch  94  may be disposed anywhere between the accelerator pedal  30  and the motor output shaft  66 . For example, the one-way clutch  94  may be disposed between the sixth gear  90  and the torque limiter  92 . Alternatively, the one-way clutch  94  may be disposed between the fifth gear  88  and the second intermediate shaft  104 . 
     In the above embodiment, the additional spring  98  comprises a helical spring (see  FIG. 4 ). However, the additional spring  98  may comprise another urging means. For example, the additional spring  98  may comprise a spring other than a helical spring, for example, a leaf spring. 
     In the above embodiment, the additional spring  98  has an end fixed to the speed reducer output shaft  96  (see  FIG. 4 ) for directly urging the speed reducer output shaft  96 . However, the additional spring  98  may be positioned otherwise insofar as it can urge the motor-side arm  28  toward the pedal-side arm  22 . For example, the additional spring  98  may have an end directly fixed to the pedal-side arm  22 .