Abstract:
An accelerator opening degree detection device is equipped with: a rotor that is attached to the shaft of a handlebar and rotates in conjunction with the operation of an accelerator grip; magnets attached to the rotor; magnetic sensors that detect the magnetic force of the magnets; a case that houses the rotor and the magnetic sensors, and is attached to the handlebar; and a sensor holder to which the magnetic sensors are attached, and which is housed in the case on the opposite side of the rotor with respect to the accelerator grip. The sensor holder has an inner diameter protruding part that protrudes at a location opposing the magnets, this location being on the inside of the rotor in the circumferential direction, and the magnet sensors are arranged on the inner diameter protruding part.

Description:
TECHNICAL FIELD 
     The present invention relates to an accelerator opening degree detection device including magnetic sensors for detecting the rotational position of an accelerator grip (also referred to as an “opening degree of an accelerator grip”), which is angularly movably mounted on a handlebar of a saddle-type vehicle such as a two-wheeled vehicle or the like. 
     BACKGROUND ART 
     Saddle-type vehicles such as two-wheeled motor vehicles (including motorized bicycles) or the like, which are powered by an internal combustion engine, include an accelerator grip that is inserted into an end of a handlebar for engagement therewith. The accelerator grip comprises an inner tube having a flange, and a grip member in the form of a rubber member that covers the outside of the inner tube and is combined integrally therewith. 
     Japanese Laid-Open Patent Publication No. 2006-112284 discloses an accelerator opening degree detection device including a magnet circumferentially embedded in a flange of the inner tube of an accelerator grip. The flange of the inner tube is housed in a casing, which is made up of a pair of upper and lower casing members. The casing, which is fixedly mounted on the handlebar, also accommodates a magnetic sensor therein such as a Hall IC or the like. 
     When the accelerator grip is turned, the angular position of the magnet is changed, thereby causing a change in the magnetic force that is detected by the magnetic sensor. 
     An output signal of the magnetic sensor, which represents an accelerator opening degree, is supplied through wires to a controller, which controls the opening of a throttle valve, for example. 
     SUMMARY OF INVENTION 
     With the accelerator opening degree detection device disclosed in Japanese Laid-Open Patent Publication No. 2006-112284, since the magnetic sensor is disposed outside of a rotor that rotates in unison with the accelerator grip, the magnetic sensor is susceptible to external magnetic fields. Further, the accelerator grip tends to be large in size. 
     The present invention has been made in view of the aforementioned problems. It is an object of the present invention to provide an accelerator opening degree detection device, which is less susceptible to external magnetic fields, and does not make the accelerator grip large in size. 
     According to the invention recited in claim  1 , an accelerator opening degree detection device comprises a rotor mounted on a handlebar around an axis of the handlebar, the rotor being capable of being turned in unison with an accelerator grip as the accelerator grip is turned, a magnet mounted on the rotor, a magnetic sensor for detecting a magnetic force of the magnet, a case housing the rotor and the magnetic sensor, the case being mounted on the handlebar, and a sensor holder housed in the case at a position remote from the accelerator grip with the rotor interposed therebetween, the magnetic sensor being installed in the sensor holder, wherein the sensor holder includes an inner projected portion positioned radially inward of the rotor and projecting toward a position confronting the magnet, and wherein the magnetic sensor is disposed in the inner projected portion. 
     According to the invention recited in claim  2 , in the accelerator opening degree detection device of claim  1 , the magnetic sensor is housed in an inner space defined in the inner projected portion. 
     According to the invention recited in claim  3 , the accelerator opening degree detection device of claim  1  further comprises a back yoke mounted on the rotor radially outward of the magnet. 
     According to the invention recited in claim  4 , in the accelerator opening degree detection device of claim  3 , the back yoke has bent ends, which are bent radially inward from outside of opposite circumferential side edges of the magnet. 
     According to the invention recited in claim  5 , in the accelerator opening degree detection device of claim  1 , the magnet includes a first magnet and a second magnet, the magnetic sensor includes a first magnetic sensor and a second magnetic sensor, the first magnet and the second magnet are disposed in respective positions diametrically opposite to each other across the axis of the handlebar, the first magnetic sensor detects a magnetic force of the first magnet, and the second magnetic sensor detects a magnetic force of the second magnet. 
     According to the invention recited in claim  6 , the accelerator opening degree detection device of claim  1  further comprises a return spring having one end thereof supported in the rotor and another end thereof supported in the sensor holder, thereby normally biasing the rotor to move to a turning start point. 
     According to the invention recited in claim  1 , since the sensor holder with the magnetic sensor assembled therein and the case are separate from each other, the magnetic sensor can be installed on the handlebar without concern over how the case is placed in alignment with the handlebar. Therefore, the magnetic sensor can be installed with increased efficiency. The sensor holder has the inner projected portion, which is positioned radially inward of the rotor with the magnet assembled therein and projecting toward a position confronting the magnet, and the magnetic sensor is disposed inside the inner projected portion. Consequently, the accelerator opening degree detection device reduces the effect that an external magnetic field has on the magnetic sensor. Further, the component around the accelerator grip is not increased in size. 
     According to the invention recited in claim  2 , since the magnetic sensor is housed in an inner space defined in the inner projected portion, the rotor is prevented from being brought into contact with the magnetic sensor when the rotor is installed. As a result, the magnetic sensor and the magnet can easily be installed on the handlebar. Unlike the background art, the accuracy with which the magnetic sensor is installed with respect to the magnet does not depend on the dimensional accuracy of the case. In addition, the case is not susceptible to dimensional errors and backlash of the handlebar along the axis of the handlebar. 
     According to the invention recited in claim  3 , inasmuch as the back yoke is disposed inside the rotor at a position radially outward of the magnet, the external magnetic field, which adversely affects the magnetic force of the magnet, is blocked by the back yoke. Therefore, the effect that the external magnetic field has on the magnetic force is eliminated, whereby accuracy with which the magnetic sensor detects the magnetic force of the magnet is increased. 
     According to the invention recited in claim  4 , the back yoke has the bent ends, which are bent radially inward from outside of opposite circumferential side edges of the magnet. The back yoke with the bent ends is capable of blocking an external magnetic field over a wider area, and of eliminating external magnetic fields that adversely affect the magnetic force of the magnet. Consequently, accuracy with which the magnetic force of the magnet is detected by the magnetic sensor can be further increased. 
     According to the invention recited in claim  5 , the magnetic sensors are disposed in respective symmetric positions on an axis perpendicular to the axis of the handlebar. For example, a failure of one of the magnetic sensors can be judged by the other magnetic sensor. Therefore, the accuracy with which failure of the magnetic sensor can be judged to have occurred or not is increased. 
     According to the invention recited in claim  6 , since the return spring is not mounted on the case, the case can be removed without being affected by the return spring. Consequently, when the case is removed to perform maintenance, the case can easily be removed, and therefore maintainability is increased. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a left-hand side elevational view of a saddle-type vehicle; 
         FIG. 2  is a view of the saddle-type vehicle taken along the arrow Y of  FIG. 1 , with a front carrier thereof omitted from illustration; 
         FIG. 3  is a view of an accelerator opening degree detection device according to an embodiment of the present invention, which is incorporated in the saddle-type vehicle; 
         FIG. 4  is a fragmentary cross-sectional view taken along line IV-IV of  FIG. 3 ; 
         FIG. 5  is a perspective view, partially in cross section, of the accelerator opening degree detection device shown in  FIG. 4 ; 
         FIG. 6  is a cross-sectional view taken along line VI-VI of  FIG. 4 ; 
         FIG. 7  is a cross-sectional view similar to  FIG. 6  except that a flange is omitted from illustration; and 
         FIG. 8  is a cross-sectional view taken along line VIII-VIII of  FIG. 4 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An accelerator opening degree detection device according to a preferred embodiment of the present invention will be described in detail below with reference to the accompanying drawings. 
       FIG. 1  is a left-hand side elevational view of a saddle-type vehicle  100  such as a two-wheeled motor vehicle. The saddle-type vehicle  100  is in the form of a scooter-type electric two-wheeled vehicle having a low floor  102 . The saddle-type vehicle  100  has a rear wheel WR, which is rotatable by rotational power generated by an electric motor  108  that is housed in a swing arm  106 . The rear wheel WR has an axle  104  that is rotatably supported on a rear end portion of the swing arm  106 . 
     The saddle-type vehicle  100  has a vehicle body frame, not shown, which is covered by a vehicle body cover  110  made of synthetic resin that provides a vehicle body S. The vehicle body cover  110  includes a front cover  112  covering a front side of a head pipe, not shown, of the saddle-type vehicle  100 , a leg shield  116  joined to the front cover  112  for covering a front side of the legs of a rider seated on a rider seat  114 , a low floor  102  joined to a lower portion of the leg shield  116  on which the feet of the rider seated on the rider seat  114  can be placed, a pair of left and right floor side covers  118  hanging down from opposite sides of the low floor  102 , an under cover  120  interconnecting lower edges of the floor side covers  118 , a lower seat front cover  122  covering a front side of a lower portion of the rider seat  114  and rising up from the rear end of the low floor  102 , a pair of left and right side covers  124  covering opposite sides of the lower portion of the rider seat  114  and joined to opposite sides of the lower seat front cover  122 , and a rear cover  126  covering an upper side of the rear wheel WR and joined to the side covers  124 . 
     A headlight  128  is mounted on the front end of the front cover  112 , and a taillight  130  is mounted on the rear end of the rear cover  126 . A front fender  132 , which is disposed below the front cover  112 , is mounted on a front fork  134  that rotatably supports a front wheel WF. The front fender  132  is disposed in covering relation to an upper side of the front wheel WF. A rear fender  136 , which covers a rear upper side of the rear wheel WR, is joined to the rear cover  126 . A fender  138  also is mounted on a front portion of the swing arm  106  for covering a front upper side portion of the rear wheel WR. 
     A handle  140  is joined to an upper portion of the front fork  134 . The front wheel WF can be steered by the handle  140 . The handle  140  has a front portion covered with a handle cover  142 . A front carrier  144  is disposed in front of the front cover  112 . A luggage base  146  is disposed behind the rider seat  114  and above the rear cover  126 . A carrier  148  is disposed above the luggage base  146 . 
     The vehicle body frame of the saddle-type vehicle  100  includes a pivot plate  150 . A side stand  152 , which holds the vehicle body S in an upright position while being tilted to the left, is angularly and movably mounted on the pivot plate  150 . The swing arm  106  has a front portion, which is swingably supported on the pivot plate  150 . A main stand  154  is angularly and movably mounted on the front portion of the swing arm  106 . 
     As shown in  FIG. 2 , winkers  156  are disposed as a pair respectively on left and right sides of the headlight  128 . The winkers  156  are formed integrally with the headlight  128  in a unitary manner to constitute a lamp unit  158 . The front cover  112  includes first recesses  160  and second recesses  162 , which are inwardly concave. The front carrier  144  has holes defined therein, which correspond respectively to the first recesses  160  and the second recesses  162 . After the holes in the front carrier  144  have been positioned in alignment respectively with the first recesses  160  and the second recesses  162 , bolts are inserted therethrough in order to fix the front carrier  144  and the front cover  122  to the vehicle body frame. The headlight  128  has a housing  164 , which is teardrop-shaped in side elevation, and a lens  166 , which is circular in front elevation and is mounted on the front end of the housing  164 . An accelerator grip  16  is mounted on a right-hand side of the handle  140 . The electric motor  108  is energized so as to rotate at a speed that depends on the angle through which the accelerator grip  16  is turned by the rider. 
       FIG. 3  is a view of an accelerator opening degree detection device  10  according to an embodiment of the present invention, which is incorporated in the saddle-type vehicle  100 . The accelerator opening degree detection device  10  includes a handlebar  12 , a case (casing)  14  mounted on the handlebar  12 , and the accelerator grip  16 , which is mounted on one end portion of the handlebar  12 .  FIG. 3  also shows a brake lever  18  that is mounted on the case  14  for facilitating understanding of the present invention, although the brake lever  18  does not have a direct bearing on the present invention. In  FIG. 4  and the following figures, the brake lever  18  and other components, which are not required for description of the present invention, are omitted from illustration. 
       FIG. 4  is a fragmentary cross-sectional view taken along line IV-IV of  FIG. 3 .  FIG. 5  is a perspective view, partially in cross section, of the accelerator opening degree detection device  10  shown in  FIG. 4 . Reference numeral  20  represents the axis (central axis) of the handlebar  12 . 
     As shown in  FIGS. 4 and 5 , the accelerator grip comprises a hollow throttle member  24  having a flange  22 , which is fitted over the hollow handlebar  12 , and a grip sleeve  26  made of resin or the like, and which is fixedly mounted on the outer circumferential surface of the throttle member  24 . The accelerator grip  16  is angularly movably (rotatably) mounted on the handlebar  12 . The case  14  houses therein a rotor  28 , which is disposed around the axis  20  of the handlebar  12  for angular movement in unison with the accelerator grip  16  as the rotor  28  turns in the direction indicated by the arrow a in  FIG. 3 , and a sensor holder  32 . The sensor holder  32  is of a hollow, substantially two-stepped cylindrical shape, and holds respective Hall ICs (magnetic sensors)  30   a ,  30   b  thereon. In other words, the case  14  serves as a casing that covers the rotor  28  and the sensor holder  32  in a protected manner. The sensor holder  32  is housed in the case  14  at a position remote from the accelerator grip  16  with the rotor  28  interposed therebetween. 
     The rotor  28  has a protrusion  34  that projects toward the sensor holder  32 . The protrusion  34  supports magnets  36   a ,  36   b  and back yokes  38   a ,  38   b  thereon. The back yokes  38   a ,  38   b  are disposed on an outer circumferential side relative to the magnets  36   a ,  36   b . The protrusion  34 , the magnets  36   a ,  36   b , and the back yokes  38   a ,  38   b  are disposed radially outward of a hollow cylindrical member, which has a shorter radius (hereinafter referred to as an “inner projected portion  32   a ”), of the sensor holder  32 , which is of a substantially two-stepped cylindrical shape. The Hall ICs  30   a ,  30   b  are disposed inside of an outer circumferential surface of the inner projected portion  32   a . The inner projected portion  32   a  projects toward a position that confronts the magnets  36   a ,  36   b , at a location located inward of the circumferential direction of the rotor  28  (the protrusion  34  of the rotor  28 ). The Hall ICs  30   a ,  30   b  are accommodated in an inner space  32   b , which is defined in the inner projected portion  32   a.    
     In this manner, the magnets  36   a ,  36   b  and the back yokes  38   a ,  38   b  are disposed outside of the Hall ICs  30   a ,  30   b  in the sensor holder  32 . The Hall IC (first magnetic sensor)  30   a  detects a magnetic force of the magnet (first magnet)  36   a  in order to detect a turning angle of the rotor  28  (the opening degree of the accelerator grip  16 ), whereas the Hall IC (second magnetic sensor)  30   b  detects a magnetic force of the magnet (second magnet)  36   b  in order to detect a turning angle of the rotor  28  (the opening degree of the accelerator grip  16 ). The back yokes  38   a ,  38   b  block external magnetic fields that could adversely affect the magnetic forces of the magnets  36   a ,  36   b , thus resulting in an increase in detection accuracy of the magnetic forces of the magnets  36   a ,  36   b  by the Hall ICs  30   a ,  30   b.    
     The sensor holder  32  covers the Hall ICs  30   a ,  30   b , so as to prevent the Hall ICs  30   a ,  30   b  from coming into contact with the outer components. A portion of the sensor holder  32  is interposed between the magnets  36   a ,  36   b  and the Hall ICs  30   a ,  30   b , thereby preventing the magnets  36   a ,  36   b  and the Hall ICs  30   a ,  30   b  from coming into contact with each other. The sensor holder  32  also prevents the rotor  28  and the Hall ICs  30   a ,  30   b  from contacting each other in directions along the axis  20 . 
     The rotor  28  has a groove  40  defined therein, which houses a return spring  42  between the groove  40  and the sensor holder  32 , for normally biasing the rotor  28  to move in the direction indicated by the arrow b in  FIG. 3 . The return spring  42  has one end that engages and is supported by a hole  44  defined in the rotor  28 , and another end that engages and is supported by a hole  46  defined in the sensor holder  32 . The sensor holder  32  and the case  14  are fixed to the handlebar  12  by a bolt B. The bolt B also fastens the sensor holder  32  and the case  14  to each other. The case  14  comprises an upper case  14   a  and a lower case  14   b . Detection signals, which are generated by the Hall ICs  30   a ,  30   b , are transmitted over wires  47  to a controller such as an ECU (Engine Control Unit) or the like, which is mounted on the saddle-type vehicle. The upper case  14   a  has a mounting boss  15  on which a non-illustrated side mirror is mounted. 
       FIG. 6  is a cross-sectional view taken along line VI-VI of  FIG. 4 , and  FIG. 7  is a cross-sectional view similar to  FIG. 6 , except that the flange  22  is omitted from illustration. 
     As shown in  FIG. 6 , the flange  22  of the throttle member  24  has recesses  50 ,  52 , with a small flange sector  54  and a large flange sector  56  defined between the recesses  50 ,  52 . The rotor  28  has a tooth  58  that projects toward the flange  22  and is positioned in the recess  50 . The upper case  14   a  has a stopper  60  that projects radially inward (toward the flange  22 ) and is positioned in the recess  52 . The stopper  60  limits rotational movement of the accelerator grip  16  in the direction indicated by the arrow a, and also limits rotational movement of the accelerator grip  16  in the direction indicated by the arrow b. 
     As shown in  FIG. 7 , the rotor  28  also has a recess  62  defined therein. The lower case  14   b  has a stopper  64  for limiting rotational movement of the rotor  28  in the direction indicated by the arrow b under the bias of the return spring  42 . When an edge of the recess  62  abuts against the stopper  64 , the rotor  28  is prevented from rotating further in the direction indicated by the arrow b. When the rotor  28  abuts against the stopper  64 , the angular position of the rotor  28  is referred to as a turning start point. In other words, the rotor  28  is biased to return to the turning start point. 
     When the rider turns the accelerator grip  16  in the direction indicated by the arrow a, a first abutment surface  54   a  of the small flange sector  54  moves in the direction of the arrow a, whereupon the first abutment surface  54   a  comes into abutment against the tooth  58 . When the rider further turns the accelerator grip  16  in the direction of the arrow a, the small flange sector  54  and the tooth  58  move together in the direction of the arrow a. At this time, the rotor  28  turns and the Hall ICs  30   a ,  30   b  detect the turning angle of the rotor  28 . When the rider turns the accelerator grip  16  to a certain angular position, a fully open abutment surface  56   a  of the large flange sector  56  comes into abutment against a fully open stopper surface  60   a  of the stopper  60 , thereby preventing the accelerator grip  16  from turning further in the direction of the arrow a. 
     Thereafter, when the rider subsequently releases the accelerator grip  16  (reverses the direction in which the accelerator grip  16  is turned), the small flange sector  54  and the tooth  58  are turned by the return spring  42  in the direction indicated by the arrow b. The rotor  28  and the accelerator grip  16  are turned in the direction of the arrow b until the edge of the recess  62  of the rotor  28  abuts against the stopper  64 , whereupon the rotor  28  is stopped at the turning start point. Even if the rotor  28  returns to the turning start point, the accelerator grip  16  continues to turn in the direction of the arrow b until a second abutment surface  54   b  of the small flange sector  54  comes into abutment against an excessive return prevention stopper surface  60   b  of the stopper  60 , at which time the rotor  28  stops turning. An interval between the position at which the second abutment surface  54   b  of the small flange sector  54  abuts against the excessive return prevention stopper surface  60   b  of the stopper  60  and the position at which the first abutment surface  54   a  abuts against the tooth  58  is referred to as a play interval of the accelerator grip  16 . Accordingly, the tooth  58  serves to transmit turning movement of the accelerator grip  16  to the rotor  28 . The stopper  60  serves as a limiting member for limiting turning movement of the accelerator grip  16  (turning movement of the accelerator grip  16  in the direction of the arrow a, as well as turning movement of the accelerator grip  16  in the direction of the arrow b). 
       FIG. 8  is a cross-sectional view taken along line VIII-VIII of  FIG. 4 . The magnets  36   a ,  36   b  are disposed in respective positions, which are diametrically opposite to each other across the axis  20  of the handlebar  12 , and the magnets  36   a ,  36   b  are curved along the circumferential direction of the handlebar  12 . The back yokes  38   a ,  38   b  are disposed respectively on outer circumferential surfaces of the magnets  36   a ,  36   b . The back yokes  38   a ,  38   b  open centrally along the axis  20 . The back yokes  38   a ,  38   b  are curved along the circumferential direction of the handlebar  12 . The back yoke  38   a  has bent ends  66   a ,  66   b , which are bent radially inward from outside of the opposite outer circumferential side edges of the magnet  36   a , and the back yoke  38   b  has bent ends  68   a ,  68   b , which are bent radially inward from outside of the opposite outer circumferential side edges of the magnet  36   b . The back yokes  38   a ,  38   b  including the bent ends  66   a ,  66   b ,  68   a ,  68   b  thereof are capable of blocking an external magnetic field over a wider area, and of further eliminating an external magnetic field that adversely affects the magnetic forces of the magnets  36   a ,  36   b , thereby resulting in a further increase in the accuracy with which the Hall ICs  30   a ,  30   b  are capable of detecting the magnetic forces of the magnets  36   a ,  36   b . The back yokes  38   a ,  38   b  need not necessarily open centrally along the axis  20 . 
     The opening degree of the accelerator grip  16  may be detected by either one of the Hall ICs  30   a ,  30   b , whereas the other one of the Hall ICs  30   a ,  30   b  may be used to judge whether or not the Hall IC used to detect the opening degree of the accelerator grip  16  has failed. For example, only the Hall IC  30   a  may be used to detect the opening degree of the accelerator grip  16 , whereas the Hall IC  30   b  may be used to judge whether or not the Hall IC  30   a  has failed. If the opening degree of the accelerator grip  16 , which is detected by the Hall IC  30   a , and the opening degree of the accelerator grip  16 , which is detected by the Hall IC  30   b , differ from each other, then the controller can determine that the Hall IC  30   a  has failed. 
     A process of assembling the accelerator opening degree detection device  10  will be described below. After the sensor holder  32  with the Hall ICs  30   a ,  30   b  assembled therein has been fitted over the handlebar  12  from one end thereof and positioned on the handlebar  12 , the rotor  28  with the return spring  42  placed therein also is fitted over the handlebar  12  from the one end thereof and is mounted on the handlebar  12 . At this time, one end of the return spring  42  engages and is supported in the hole  44  defined in the rotor  28 , whereas the other end of the return spring  42  engages and is supported in the hole  46  defined in the sensor holder  32 . 
     Next, the accelerator grip  16  is fitted over the handlebar  12  from the one end thereof. When the accelerator grip  16  is fitted over the handlebar  12 , as shown in  FIG. 6 , the accelerator grip  16  is positioned such that the small flange sector  54  and the large flange sector  56  of the flange  22  sandwich the tooth  58  therebetween. Further, at this time, the accelerator grip  16  is positioned such that the tooth  58  is positioned in the recess  50  defined in the flange  22 . 
     Thereafter, the lower case  14   b  is fitted over the sensor holder  32 , the rotor  28 , and the flange  22  of the accelerator grip  16 , such that the stopper  64  fits in the recess  62  defined in the rotor  28  (see  FIG. 7 ). In addition, the upper case  14   a  is fitted over the sensor holder  32 , the rotor  28 , and the flange  22  of the accelerator grip  16 , such that the stopper  60  fits in the recess  52  defined in the flange  22  (see  FIG. 6 ). The upper case  14   a , the lower case  14   b , and the sensor holder  32  are fastened to the handlebar  12  by the bolt B, thereby completing assembly of the accelerator opening degree detection device  10 . 
     Since the sensor holder  32  with the Hall ICs  30   a ,  30   b  assembled therein and the case  14  are separate from each other, the Hall ICs  30   a ,  30   b  can be installed on the handlebar  12  without concern over how the case  14  is placed in alignment with the handlebar  12 . Therefore, the Hall ICs  30   a ,  30   b  can be installed with increased efficiency. The sensor holder  32  includes the inner projected portion  32   a , which is positioned radially inward of the rotor  28  with the magnets  36   a ,  36   b  assembled therein, and which projects toward the position confronting the magnets  36   a ,  36   b , while in addition, the Hall ICs  30   a ,  30   b  are disposed on the inner projected portion  32   a . Consequently, the accelerator opening degree detection device  10  reduces the effect that the external magnetic field has on the Hall ICs  30   a ,  30   b , and does not make the component (i.e., the case  14 ) around the accelerator grip  16  large in size. 
     Since the Hall ICs  30   a ,  30   b  are housed in the inner space  32   b  defined in the inner projected portion  32   a , the rotor  28  is prevented from coming into contact with the Hall ICs  30   a ,  30   b  when the rotor  28  is installed. As a result, the Hall ICs  30   a ,  30   b  and the magnets  36   a ,  36   b  can easily be installed on the handlebar  12 . Unlike the background art, installation accuracy of the Hall ICs  30   a ,  30   b  with respect to the magnets  36   a ,  36   b  does not depend on the dimensional accuracy of the case  14 , and the case  14  is not susceptible to dimensional errors and backlash of the handlebar  12  along the axis  20  of the handlebar  12 . 
     Inasmuch as the back yokes  38   a ,  38   b  are disposed in the rotor  28  at a radially outward position from the magnets  36   a ,  36   b , external magnetic fields, which could adversely affect the magnetic forces of the magnets  36   a ,  36   b , are blocked by the back yokes  38   a ,  38   b . Thus, any effect that the external magnetic field may produce on the magnetic forces of the magnets  36   a ,  36   b  is eliminated, thereby enhancing accuracy with which the magnetic forces of the magnets  36   a ,  36   b  are detected by the Hall ICs  30   a ,  30   b.    
     The back yokes  38   a ,  38   b  include the bent ends  66   a ,  66   b ,  68   a ,  68   b , which are bent radially inward from outside of the opposite circumferential side edges of the magnets  36   a ,  36   b . The back yokes  38   a ,  38   b , which include the bent ends  66   a ,  66   b ,  68   a ,  68   b , are capable of blocking external magnetic fields over a wider area, and of eliminating external magnetic fields that adversely affect the magnetic forces of the magnets  36   a ,  36   b , thereby resulting in a further increase in accuracy with which the Hall ICs  30   a ,  30   b  detect the magnetic forces of the magnets  36   a ,  36   b.    
     The magnets  36   a ,  36   b  are disposed in respective positions, which are diametrically opposite to each other across the axis  20  of the handlebar  12 . The Hall IC  30   a  detects the magnetic force of the magnet  36   a , whereas the Hall IC  30   b  detects the magnetic force of the magnet  36   b . The Hall IC  30   a  may be used to detect the angle of rotation of the rotor  28 , whereas the Hall IC  30   b  may be used to judge whether the Hall IC  30   a  has failed or not. Therefore, whether or not the Hall IC  30   a  has failed can be judged with increased accuracy. 
     Insofar as the return spring  42  is not mounted on the case  14 , the return spring  42  is not dislodged even at times that the case  14  is removed for maintenance. Consequently, the case  14  can easily be removed for improving ease of maintenance. 
     The present invention has been described above with respect to a preferred embodiment thereof. However, the technical scope of the present invention is not limited to the embodiment illustrated above. It will be obvious to those skilled in the art that various improvements or modifications can be made with respect to the aforementioned embodiment. It is apparent from the scope of the claims that configurations, which are based on such improvements or modifications, fall within the technical scope of the present invention. The parenthetical reference characters, which are referred to in the patent claims, correspond with the reference characters shown in the accompanying drawings for thereby facilitating understanding of the present invention. However, the present invention should not be construed as being limited to the elements denoted by such reference characters.