Patent Publication Number: US-2007108968-A1

Title: Rotation angle detection device

Description:
TECHNICAL FIELD  
      The present invention relates to a rotation angle detection device which detects the rotation angle of an object to be detected and, more particularly, to a technique effectively applied to a detection device for detecting the open degree of an engine throttle valve.  
     BACKGROUND ART  
      There is known, as a non-contact type rotation angle detection device, a configuration as disclosed in, e.g., Japanese Patent No. 2842482, in which a change in flux caused by a magnet attached to an object to be detected is caught to detect the rotation angle of the object.  FIG. 13  is an explanatory view showing a general configuration of such a non-contact type rotation detection device.  
      In a rotation angle detection device of  FIG. 13 , a rotor core  52  of magnetic material is attached to a shaft  51  which is subject to rotation angle detection. A dipole ring magnet  53  is fixed to the inner circumference of the rotor core  52 . A stator core  54  is disposed outside the ring magnet  53  in a coaxial manner with the shaft  51 . The stator core  54  is formed of a magnetic material and is constituted by two core pieces  54   a ,  54   b  each having a semicircular cross-section. A Hall IC  56  is disposed in a gap  55  between the core pieces  54   a ,  54   b.    
      As shown in  FIG. 13 , the stator core  54  forms a magnetic path for directing the flux of the ring magnet  53 , and a voltage signal corresponding to the density of the interlinkage flux is output from the Hall IC  56 . When the ring magnet  53  rotates together with the shaft  51 , the flux amount passing through the Hall IC  56  changes to change the output signal from the Hall IC  56 . The change in the output signal occurs in response to a change in the rotation angle of the shaft  51 , and the signal changes almost linearly with the rotation of the shaft  51 . This makes it possible to detect the rotation angle of the shaft  51  based on a signal from the Hall IC  56 .  
      In recent years, so-called an electronically-controlled throttle apparatus which drives an engine throttle valve by a motor has widely been used along with computerization of automobile parts. In the electronically-controlled throttle apparatus, a throttle valve is not mechanically activated by a conventional accelerator wire, but controlled by an electrical signal. The accelerator pedal depression amount is electrically detected by a potentiometer or the like and, in accordance with the detected value, a motor is driven to open/close the throttle valve.  
      The rotation angle detection device as shown in  FIG. 13  is also used for detecting the open degree of the throttle valve in such an electronically-controlled throttle apparatus. In this case, a throttle valve is attached to the shaft  51 , and the open degree of the throttle valve is detected based on the rotation angle of the shaft  51 . Operational control for a motor is performed based on the detected valve open degree. At this time, the valve open degree corresponding to the accelerator pedal depression amount or engine load is set.  
      [Patent Document 1] 
      Japanese Patent No. 2842282  
      [Patent Document 2] 
      Jpn. Pat. Appln. Laid-Open Publication No. 8-35809  
      [Patent Document 3] 
      Jpn. Pat. Appln. Laid-Open Publication No. 2001-4315  
      [Patent Document 4] 
      Jpn. Pat. Appln. Laid-Open Publication No. 2001-59702  
      [Patent Document 5] 
      Jpn. Pat. Appln. Laid-Open Publication No. 2001-188003  
      [Patent Document 6] 
      Jpn. Pat. Appln. Laid-Open Publication No. 2001-208510  
      [Patent Document 7] 
      Jpn. Pat. Appln. Laid-Open Publication No. 2001-289609  
      [Patent Document 8] 
      Jpn. Pat. Appln. Laid-Open Publication No. 2001-289610  
      [Patent Document 9] 
      Jpn. Pat. Appln. Laid-Open Publication No. 2001-303979  
      [Patent Document 10] 
      Jpn. Pat. Appln. Laid-Open Publication No. 2001-317909  
      [Patent Document 11] 
      Jpn. Pat. Appln. Laid-Open Publication No. 2001-211201  
      [Patent Document 12] 
      Jpn. Pat. Appln. Laid-Open Publication No. 2001-257723  
      [Patent Document 13] 
      Japanese Patent Application No. 2002-258901  
      [Patent Document 14] 
      Japanese Patent Application No. 2002-83092  
     DISCLOSURE OF INVENTION  
      However, in the rotation angle detection device having the configuration as shown in  FIG. 13 , output characteristics are structurally vulnerable to center displacement. That is, when the centers of the stator core  54  and magnet  53  are displaced from each other, the air-gap between them changes with the rotation angle due to core displacement. This makes a change in the amount of the flux passing through the Hall IC  56  non-uniform with respect to the rotation angle, resulting in detection error.  
      Therefore, in the configuration of the rotation angle detection device shown in  FIG. 13 , both the rotor core  52  and magnet  53  must be accurately fixed to the shaft  51  in a coaxial manner with respect to the shaft  51 , correspondingly increasing assembly man-hour to increase manufacturing cost. While it is possible to omit the rotor core  52  and use the shaft  51  as a magnetic path in order to increase the coaxial accuracy, a limitation on the size of the shaft diameter limits the size of the magnet diameter, so that the travel distance of the magnet in the circumferential direction becomes small, correspondingly reducing the angle detection accuracy.  
      In the rotation angle detection device for an electronically-controlled throttle apparatus, the rotation angle detection device itself is provided in an engine room. Therefore, in order to efficiently dispose various devices in the narrow space, high layout flexibility is required for the rotation angle detection device itself, and a reduction in the size and thickness thereof is demanded.  
      An object of the present invention is to reduce the size and thickness of a rotation angle detection device without deteriorating its detection accuracy to increase its layout flexibility.  
      A rotation angle detection device according to the present invention is characterized by comprising: a yoke member which is formed of a magnetic material rotating in synchronization with a rotary shaft and fixed to a nonmagnetic member attached to the rotary shaft; a magnet which is disposed with a predetermined space apart from the yoke member in the axial direction and has a magnetic pole facing the yoke member on the axial direction edge surface thereof; and a magnetic detection element disposed in a magnetic path which is formed by the magnet and passes through the yoke member.  
      In the present invention, the yoke member and a magnet pole of the magnet face each other with a predetermined space in the axial direction, so that opposed surfaces between them extend in the radial direction to reduce the axial direction length of the device. Therefore, the thickness of the rotation angle detection device can be reduced to thereby increase the layout flexibility of the device.  
      In the rotation angle detection device, the magnet may be attached to a fixing member formed of a magnetic material, and the fixing member may have a magnet attachment portion to which the magnet is fixed and a magnetic detection element attachment portion which is magnetically connected to the magnet attachment portion and to which the magnetic detection element is fixed with a predetermined space apart from the yoke member in the axial direction. In this case, the fixing member may be disposed adjacent to the magnet in the axial direction.  
      In the rotation angle detection device, the magnet may be axially magnetized. This makes it possible to detect the rotation angle of the rotary shaft without using a magnet with radial magnetization in which high accurate magnetization is difficult, thereby reducing manufacturing cost of the rotation angle detection device.  
      The nonmagnetic member may be a throttle gear for driving a throttle valve which is provided in an electronically-controlled throttle valve of an engine.  
      Further, a rotation angle detection device is characterized by comprising: a magnet which rotates in synchronization with a rotary shaft, which has a magnetic pole on the axial direction edge surface thereof, and which is magnetically connected to the rotary shaft; a yoke member which is disposed with a predetermined space apart from the magnet in the axial direction and is formed of a magnetic material facing the magnetic pole of the magnet; and a magnetic detection element disposed in a magnetic path which is formed by the magnet and passes through the yoke member.  
      In the present invention, the yoke member and a magnet pole of the magnet face each other with a predetermined space in the axial direction, so that opposed surfaces between them extend in the radial direction to reduce the axial direction length of the device. Therefore, the thickness of the rotation angle detection device can be reduced to thereby increase the layout flexibility of the device.  
      In the rotation angle detection device, the magnet may be axially magnetized. This makes it possible to detect the rotation angle of the rotary shaft without using a magnet with radial magnetization in which high accurate magnetization is difficult, thereby reducing manufacturing cost of the rotation angle detection device.  
      In the rotation angle detection device, the magnetic detection element may be attached to a fixing member formed of a magnetic material, and the yoke member may be attached to the axial direction edge surface of the magnetic detection element.  
      In the rotation angle detection device, the yoke member and magnet may be formed into a partially cylindrical shape, and the outer diameter of the yoke member may be larger than the outer diameter of the magnet. This makes it possible to suppress a change in the opposed areas between the yoke member and magnet even when center displacement between them occurs. In addition, coaxial accuracy level between the movable side and stationary side can be lowered.  
      In the rotation angle detection device, opposed surfaces between the yoke member and magnet may extend in the radial direction. Further, opposed areas between the yoke member and magnet may change with a rotation of the rotary shaft, and the change in the opposed areas may change the flux density in the magnetic path.  
      In the rotation angle detection device, the rotary shaft may be a valve shaft to which a throttle valve in an electronically-controlled throttle valve of an engine is fixed. In this case, the yoke member may be fixed to a gear formed of a nonmagnetic member attached to the valve shaft.  
      According to the rotation angle detection device of the present invention, the magnet is disposed with a predetermined space apart from the yoke member formed of a magnetic material rotating in synchronization with the rotary shaft in the axial direction, and the magnetic detection element is disposed in the magnetic path formed by the magnet. Accordingly, opposed surfaces between the yoke member and magnet extend in the radial direction to reduce the axial direction length of the device. Therefore, the thickness of the rotation angle detection device can be reduced to thereby increase the layout flexibility of the device.  
      Further, according to the rotation angle detection device of the present invention, the yoke member formed of a magnetic material is disposed with a predetermined space apart from the magnet rotating in synchronization with the rotary shaft in the axial direction, and the magnetic detection element is disposed in the magnetic path formed by the magnet. Accordingly, opposed surfaces between the yoke member and magnet extend in the radial direction to reduce the axial direction length of the device. Therefore, the thickness of the rotation angle detection device can be reduced to thereby increase the layout flexibility of the device.  
      Further, the outer diameter of the yoke member is made larger than the outer diameter of the magnet. This makes it possible to suppress a change in the opposed areas between the yoke member and magnet even when center displacement between them occurs, so that a change in the output characteristics due to the center displacement between the movable side and stationary side hardly occurs. Thus, it is possible to stably and accurately detect the angle. In addition, coaxial accuracy level between the movable side and stationary side can be lowered, resulting in a reduction in manufacturing cost.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  Explanatory views showing a configuration of a rotation angle detection device according to a first embodiment of the present invention; (a) is a front view, and (b) is a bottom view.  
       FIG. 2  An exploded perspective view of the rotation angle detection device of  FIG. 1 .  
       FIG. 3  Explanatory views showing a configuration of a rotation angle detection device according to a second embodiment of the present invention; (a) is a front view, and (b) is a bottom view.  
       FIG. 4 A  cross-sectional view showing a configuration of an electronically-controlled throttle valve using a rotation angle detection device according to a third embodiment of the present invention.  
       FIG. 5 A  bottom view of the electronically-controlled throttle valve of  FIG. 4 .  
       FIG. 6  Explanatory views schematically showing a configuration of the rotation angle detection device of  FIG. 4 ; (a) is a front view, and (b) is a cross-sectional view taken along A-A line of (a).  
       FIG. 7  is an explanatory view showing configurations of components of the rotation angle detection device shown in  FIG. 6 .  
       FIG. 8  Explanatory views schematically showing a configuration of a rotation angle detection device in the case where the rotation angle detection device using two magnets is used for the electronically-controlled throttle valve of  FIG. 4 ; (a) is a front view of the rotation angle detection device, and (b) is a cross-sectional view taken along B-B line of (a).  
       FIG. 9  An explanatory view showing configurations of components of the rotation angle detection device shown in  FIG. 8 .  
       FIG. 10  An exploded perspective view of a rotation angle detection device according to a fifth embodiment of the present invention.  
       FIG. 11  Explanatory views schematically showing a configuration of a rotation angle detection device for the electronically-controlled throttle valve according to a sixth embodiment of the present invention; (a) is a front view of the rotation angle detection device, and (b) is a cross-sectional view taken along A-A line of (a).  
       FIG. 12  An explanatory view showing configurations of components of the rotation angle detection device shown in  FIG. 11 .  
       FIG. 13  An explanatory view showing a general configuration of a non-contact type rotation angle detection device. 
    
    
     EXPLANATION OF REFERENCE SYMBOLS  
     
         
           1 : Rotation angle detection device  
           2 : Rotary shaft  
           3 : Yoke plate (yoke member)  
           3   a ,  3   b : Yoke plate  
           3   p : Axial direction edge surface  
           4 : Rotor core  
           4   p : Axial direction edge surface  
           5 : Magnet  
           5   a ,  5   b : Magnet  
           5   p ,  5   q : Axial direction edge surface  
           6 : Fixing plate (fixing member)  
           6   a : Arc portion  
           6   b : Chord portion  
           7 : Magnet attachment portion  
           8   a ,  8   b : Pole piece  
           9   a ,  9   b : Hall IC (magnetic detection element)  
           10 : Rotation angle detection device  
           11 : throttle valve  
           12 : Valve shaft  
           13 : Brushless motor  
           14 : Housing  
           15 : Cover  
           16 : Circuit board  
           17 : Rotation angle detection device  
           18 : Connection portion  
           19 : Intermediate gear  
           20 : Gear shaft  
           21 : Throttle gear  
           22 : Gear  
           23 : Gear  
           24 : Gear  
           25 : Speed reduction mechanism  
           26 : Rotor shaft  
           27 : Rotation angle detection device  
           28 : Rotation angle detection device  
           29 : Rotation angle detection device  
           51 : Shaft  
           52 : Rotor core  
           53 : Ring magnet  
           54 : Stator core  
           54   a : Core piece  
           55 : Gap  
           56 : Hall IC  
          G 1 : Air gap  
          G 2 : Air gap  
          G 3 : Air gap  
          M: Magnetic path  
          M 1 : Magnetic path  
          M 2 : Magnetic path  
          R 1 : Outer diameter of yoke plate  
          R 2 : Outer diameter of magnet  
          R 3 : Inner diameter of yoke plate  
          R 4 : Inner diameter of magnet  
          S 1 , S 2 : Opposed area  
       
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
      Embodiments of the present invention will be described below in detail with reference to the accompanying drawings.  
     First Embodiment  
       FIG. 1  is explanatory views showing a configuration of a rotation angle detection device according to a first embodiment of the present invention.  FIG. 1  ( a ) is a front view of the rotation angle detection device, and  FIG. 1  ( b ) is a bottom view thereof.  FIG. 2  is an exploded perspective view of the rotation angle detection device of  FIG. 1 . As shown in  FIG. 1 , yoke plates (yoke members)  3   a ,  3   b  rotating in synchronization with a rotary shaft  2  are provided in a rotation angle detection device  1 . Each of the yoke plates  3   a ,  3   b  is formed of a magnetic material and formed in a partially cylindrical shape having a central angle of about 160°. The yoke plates  3   a ,  3   b  are circumferentially fixed to the rotary shaft  2  through a rotor core  4  of nonmagnetic material in even shares.  
      A magnet  5  having a sector shape is disposed on the axial direction edge surface  3   p  side of the yoke plates  3   a ,  3   b  with a predetermined space apart from the yoke plates  3   a ,  3   b . The magnet  5  is fixed to a fixing plate (fixing member)  6  of magnet material to constitute a stationary side relative to a rotary side constituted by the yoke plates  3   a ,  3   b . The magnet  5  is formed in a partially cylindrical shape having a central angle of 120° and is axially magnetized such that its edge surfaces  5   p ,  5   q  opposed in the axial direction have different magnetic polarities. Magnetization in the axial direction is relatively easy and, accordingly, it is possible to reduce manufacturing cost of the magnet  5  more than the cost involving the manufacturing of a magnet with radial magnetization in which high accurate magnetization is difficult. The edge surface  5   p  of the magnet  5  faces both the edge surfaces  3   p  of the yoke plates  3   a ,  3   b  and, between the surface  5   p  and surfaces  3   p , an air gap G 1  is formed. The air gap G 1  is kept unchanged irrespective of the rotation of the yoke plates  3   a ,  3   b.    
      The outer diameter R 1  of the yoke plates  3   a ,  3   b  is larger than the outer diameter R 2  of the magnet  5 . The inner diameter R 3  of the yoke plates  3   a ,  3   b  is smaller than the inner diameter R 4  of the magnet  5  (R 1 &gt;R 2 , R 3 &lt;R 4 ). That is, the yoke plates  3   a ,  3   b  are so formed as to entirely cover the area of the magnet  5 . Therefore, even when center displacement between the yoke plates  3   a ,  3   b  and magnet  5  occurs, the opposed areas between them do not change. Further, the center displacement does not affect the air-gap G 1  because of structural reasons. Therefore, a change in the output characteristics due to the center displacement between the movable side and stationary side hardly occurs in the rotation angle detection device  1 , enabling accurate and stable angle detection. Further, coaxial accuracy level between the movable side and stationary side can be lowered to lower the level of accuracy required for individual parts as well as to make assembly work easier, resulting in a reduction in manufacturing cost.  
      While the stator core  54  is provided outside the magnet  53  in the rotation angle detection device shown in  FIG. 13 , it is possible to omit the stator core in the configuration of the rotation angle detection device  1 . Therefore, in the case where the devices of  FIGS. 1 and 13  are designed to have the same outer diameter, the outer diameter R 2  of the magnet  5  can be made larger than that of the magnet  53 . Accordingly, the travel distance of the magnet  5  in the circumferential direction associated with the rotation of the rotary shaft  2  becomes larger than that of the magnet  53  of  FIG. 13 , correspondingly increasing the rotation angle detection accuracy. Further, in the case where the outer diameter R 2  of the magnet  5  is made equal to the outer diameter of the magnet  53 , it is possible to reduce the size of the device by the size corresponding to the stator core  54  without deteriorating the accuracy.  
      Further, in the configuration of the rotation angle detection device of  FIG. 13 , the magnet  53  and stator core  54  are formed into substantially a cylindrical shape and their opposed surfaces extend in the axial direction, with the result that the axial direction length of the device tends to increase. On the other hand, in the configuration of the rotation angle detection device  1 , the opposed surfaces of the yoke plates  3   a ,  3   b  and magnet  5  extend in the radial direction, making it possible to reduce the axial direction length of the device. As a result, it is possible to reduce the thickness of the rotation angle detection device, thereby increasing layout flexibility of the device.  
      The edge surface  5   q  of the magnet  5  is fixed to the fixing plate  6 . The fixing plate  6  is formed into substantially a semilunar shape having a central angle of about 200° and is fixed to the chassis of the device. A magnet attachment portion  7  is provided along a circular arc portion  6   a  of the fixing plate  6 . The magnet  5  is fixed to the magnet attachment portion  7 .  
      Pole pieces (magnetic detection element attachment portions)  8   a ,  8   b  are so formed near a chord portion  6   b  of the fixing plate  6  as to protrude in the axial direction. Each of the pole pieces  8   a ,  8   b  is also formed of a magnetic material and magnetically connected to the magnet attachment portion  7 . Hall ICs  9   a ,  9   b  are attached to the leading end of the pole pieces  8   a ,  8   b . Each of the Hall ICs  9   a ,  9   b  is an IC obtained by integrating a Hall element and a signal amplifier circuit. In this case, a linear output Hall IC is used. The Hall ICs  9   a ,  9   b  are space apart from the yoke plates  3   a ,  3   b  in the axial direction, so that an air gap G 2  is formed between the Hall ICs  9   a ,  9   b  and edge surface  3   p  of the yoke plates  3   a ,  3   b . In this example, G 1  is set equal to G 2 .  
      As shown in  FIG. 1  ( b ), in the rotation angle detection device  1 , a magnetic path M is formed circulating from the magnet  5 , through the yoke plates  3   a ,  3   b , pole pieces  8   a ,  8   b , fixing plate  6  to the magnet  5 . The Hall ICs  9   a ,  9   b  are disposed in the magnetic path M, and a signal output from the Hall IC changes in accordance with the flux change. In the magnetic path M, the flux is converged by the pole pieces  8   a ,  8   b  to pass through the Hole ICs  9   a ,  9   b . Therefore, it is possible to efficiently guide the flux of the magnet  5  to the Hole ICs  9   a ,  9   b , improving the accuracy for detecting the flux change.  
      In the rotation angle detection device  1 , the rotation of the rotary shaft  2  changes the opposed areas S 1 , S 2  between the magnet  5  and yoke plates  3   a ,  3   b . For example, when the rotary shaft  2  rotates clockwise in  FIG. 1  ( a ), S 1  increases and S 2  decreases. Then, on the side (S 1  side) where the opposed area increases, the flux induced in the yoke plate  3  increases to increase the flux in the magnetic path M. Contrary, on the side (S 2  side) where the opposed area decreases, the flux in the magnetic path M decreases. That is, when the rotary shaft  2  rotates, the density of magnetic flux passing through the Hall ICs  9   a ,  9   b  in the magnetic path M also linearly changes in proportion to the rotation angle. The Hall ICs  9   a ,  9   b  catch this change and output a linear voltage signal proportional to the rotation angle of the rotary shaft  2 . The rotation angle of the rotary shaft  2  is then calculated based on a relationship between the voltage signal value and rotation angle.  
     Second Embodiment  
       FIG. 3  is explanatory views showing a configuration of a rotation angle detection device according to a second embodiment of the present invention.  FIG. 3  ( a ) is a front view of the rotation angle detection device, and  FIG. 3  ( b ) is a bottom view thereof. In the following embodiments, the same reference numerals as the first embodiment are given to the components which are common to the first embodiment, and the overlapped description is omitted.  
      As shown in  FIG. 3 , two magnets  5  ( 5   a ,  5   b ) are provided in a rotation angle detection device  10  according to the second embodiment. The magnets  5   a ,  5   b  are so magnetized as to have magnetic poles of different polarities alternately arranged in the rotational direction. The fixing plate  6  is formed not into a semilunar shape, but into a circular shape for placement of the two magnets. In this configuration, as shown in  FIG. 3 ( b ), two separate magnetic paths M 1 , M 2  are formed by the magnets  5   a ,  5   b . That is, the magnetic paths M 1 , M 2  are formed each circulating from the magnet  5   a ,  5   b , through the yoke plates  3   a ,  3   b , pole pieces  8   a ,  8   b , fixing plate  6  to the magnet  5   a ,  5   b . The Hall ICs  9   a ,  9   b  are disposed in the magnetic paths M 1 , M 2 . The use of the two magnets  5  allows the flux of the two separate magnetic paths to pass through the Hall ICs  9   a ,  9   b , correspondingly increasing the flux amount. Therefore, it is possible to sensitively grasp the flux change, thereby increasing the detection accuracy of the rotation angle.  
      In the rotation angle detection device  10 , the rotation of the rotary shaft  2  changes the opposed areas S 1 , S 2 , S 3 , S 4  between the magnet  5   a ,  5   b  and yoke plates  3   a ,  3   b . For example, when the rotary shaft  2  rotates clockwise in  FIG. 3  ( a ), S 1 , S 3  increase and S 2 , S 4  decrease. Then, on the sides (S 1 , S 3  sides) where the opposed areas increase, the flux induced in the yoke plate  3  increases. Contrary, on the sides (S 2 , S 4  sides) where the opposed areas decrease, the flux induced in the yoke plate  3  decreases. When this point is seen about Hall ICs  9   a ,  9   b , in the Hall IC  9   a , the flux increases on the magnetic path M 1  side and decreases on the magnetic path M 2  side. Thus, assuming that the flux traveling along the magnetic path M 1  is positive side, the flux passing through the Hall IC  9   a  changes in the positive direction. Contrary, in the Hall IC  9   b , the flux decreases on the magnetic path M 1  side and increases on the magnetic path M 2  side, so that the flux passing through the Hall IC  9   b  changes in the negative direction. The above change in the flux causes the Hall ICs  9   a ,  9   b  to output a linear output signal proportional to the rotation angle of the rotary shaft  2  and, thereby, the rotation angle of the rotary shaft  2  is detected.  
     Third Embodiment  
       FIG. 4  is a cross-sectional view showing a configuration of an electronically-controlled throttle valve using a rotation angle detection device according to a third embodiment.  FIG. 5  is a bottom view of the electronically-controlled throttle valve of  FIG. 4 . The electronically-controlled throttle valve shown in  FIGS. 4 and 5  is disposed in the inlet path of an engine. Intake air flow of the engine is controlled by the open degree of the throttle valve  11 . The throttle valve  11  is fixed to the valve shaft (rotary shaft)  12  and driven by a brushless motor  13  (hereinafter, abbreviated as motor  13 ) through a speed-reduction mechanism  25  constituted by gears  21  to  24 .  
      The valve shaft  12  is pivotably supported by a not shown bearing fixed to a metal housing  14 . A plastic cover  15  is attached to the lower portion of the housing  14 , as shown in  FIG. 4 . A circuit board  16  is fixed inside the cover  15 . A torsion coil spring is attached to the throttle gear  21  fixed to the valve shaft  12 . The throttle valve  11  automatically returns to its totally closed position by a biasing force of the torsion coil spring.  
      A rotation angle detection device  17  which detects the open degree of the throttle valve  11  is provided in one end portion of the valve shaft  12 .  FIG. 6  is explanatory views schematically showing a configuration of the rotation angle detection device  17 .  FIG. 6  ( a ) is a front view of the rotation angle detection device  17 , and  FIG. 6  ( b ) is a cross-sectional view taken along A-A line of  FIG. 6  ( a ).  FIG. 7  is an explanatory view showing configurations of components of the rotation angle detection device  17 .  
      Like the rotation angle detection device  1  of  FIG. 1 , the rotation angle detection device  17  includes the yoke plates  3   a ,  3   b , magnet  5 , and fixing plate  6 . In this example, the yoke plates  3   a ,  3   b  are fixed by being embedded in the throttle gear  21  formed of a nonmagnetic material such as plastic and rotates in synchronization with the valve shaft  12 . The fixing plate  6  is fixed to the cover  15  together with the circuit board  16 . In this case, the yoke plates  3   a ,  3   b  can be insert-molded into the throttle gear  21 , reducing assembly man-hour and increasing space efficiency.  
      An air gap G 1  is formed between the yoke plates  3   a ,  3   b  and magnet  5  also in the rotation angle detection device  17 . The outer diameter R 1  of the yoke plates  3   a ,  3   b  is larger than the outer diameter R 2  of the magnet  5 . The inner diameter R 3  of the yoke plates  3   a ,  3   b  is smaller than the inner diameter R 4  of the magnet  5  (R 1 &gt;R 2 , R 3 &lt;R 4 ). The magnet attachment portion  7 , pole pieces  8   a ,  8   b , and a connection portion  18  connecting the magnet attachment portion  7  and pole pieces  8   a ,  8   b  are provided on the fixing plate  6 . The magnet attachment portion  7  and pole pieces  8   a ,  8   b  are magnetically connected to each other through the connection portion  18 . The hall ICs  9   a ,  9   b  are attached to the leading end of the pole pieces  8   a ,  8   b . An air gap G 2  is formed between the Hall ICs  9   a ,  9   b  and yoke plates  3   a ,  3   b.    
      The Hall ICs  9   a ,  9   b  are disposed in a magnetic path M circulating from the magnet  5 , through yoke plates  3   a ,  3   b , pole pieces  8   a ,  8   b , fixing plate  6 , to the magnet  5 . When the valve shaft  12  rotates, the yoke plates  3   a ,  3   b  rotate together with the throttle gear  21  to change opposed areas S 1 , S 2  between the magnet  5  and yoke plates  3   a ,  3   b . Accordingly, the flux density in the magnetic path M linearly changes in proportion to the rotation angle to cause the Hall ICs  9   a ,  9   b  to output a linear voltage signal proportional to the rotation angle of the valve shaft  12 .  
      The motor  13  is a brushless motor of inner rotor type. A gear  24  is formed on a rotor shaft  26  of the motor  13  and is engaged with a gear  23  of an intermediate gear  19 . The intermediate gear  19  is rotatably supported by a gear shaft  20 . A gear  22  is formed integrally with the gear  23  on the intermediate gear  19  and is engaged with the throttle gear  21 . With the above configuration, the rotation of the rotor shaft  26  in the motor  13  is transmitted to the valve shaft  12  while being decelerated.  
      In such an electronically-controlled throttle valve, the valve shaft  12  rotates at angles from 0° to about 90° in accordance with a totally closed state/totally open state of the throttle valve  11 , and the flux density of the magnetic path M accordingly changes. A voltage signal is output from the Hall ICs  9   a ,  9   b  in accordance with the change in the flux density and is then transmitted to a control unit (CPU). The control unit stores, in the form of, e.g., a table, the output change of the Hall ICs  9   a ,  9   b  in association with the rotation angle of the valve shaft  12 . While referring to the table, the control unit calculates the rotation angle of the valve shaft  12 , i.e., the open degree of the throttle valve  11  based on the output change of the Hall ICs  9   a ,  9   b.    
     Fourth Embodiment  
      The configuration of the rotation angle detection device according to the above mentioned second embodiment, in which two magnets  5  are used, can be applied to the electronically-controlled throttle valve exemplified in  FIGS. 4 and 5 .  FIG. 8  is explanatory views schematically showing a configuration of a rotation angle detection device in the case where the rotation angle detection device using two magnets is used for the electronically-controlled throttle valve according to the third embodiment.  FIG. 8  ( a ) is a front view of the rotation angle detection device  27 , and  FIG. 8  ( b ) is a cross-sectional view taken along A-A line of  FIG. 8  ( a ).  FIG. 9  is an explanatory view showing configurations of components of the rotation angle detection device  27 .  
      As shown in  FIGS. 8 and 9 , two magnets  5  ( 5   a ,  5   b ) are provided in the rotation angle detection device  27  according to the fourth embodiment. The magnets  5   a ,  5   b  are so magnetized as to have magnetic poles of different polarities alternately arranged in the rotational direction. Two magnet attachment portions  7  are symmetrically provided on the fixing plate  6  with respect to the connection portion  18  for placement of the two magnets. As is the case with the second embodiment, two separate magnetic paths M 1 , M 2  are formed by the magnets  5   a ,  5   b . The Hall ICs  9   a ,  9   b  are disposed in the magnetic paths M 1 , M 2 .  
      In the rotation angle detection device  27 , the rotation of the valve shaft  12  changes the opposed areas S 1 , S 2 , S 3 , S 4  between the magnet  5   a ,  5   b  and yoke plates  3   a ,  3   b . Accordingly, as described above, the flux passing through the Hall ICs  9   a ,  9   b  changes, causing the Hall ICs  9   a ,  9   b  to output a linear voltage signal proportional to the rotation angle of the valve shaft  12  and, thereby, the rotation angle of the valve shaft  12  is detected and the open degree of the valve shaft  11  is calculated.  
     Fifth Embodiment  
      Next, as a fifth embodiment of the present invention, a rotation angle detection device having a configuration in which the magnet is disposed on the movable side, contrary to the abovementioned embodiments will be described.  FIG. 10  is an exploded perspective view of a rotation angle detection device  28  according to the fifth embodiment of the present invention. In the rotation angle detection device  28 , the magnet  5  rotates in synchronization with the rotary shaft  2 . The rotor core  4  formed of a magnetic material is attached to the rotary shaft  2 . The magnet  5  is fixed to the axial direction edge surface  4   p  of the rotor core  4  and magnetically connected to the rotary shaft  2 . The magnet  5  is formed into a partially cylindrical shape having a center angle of 120° and is axially magnetized such that its edge surfaces  5   p ,  5   q  opposed in the axial direction have different magnetic polarities.  
      The yoke plates (yoke members)  3   a ,  3   b  are provided on the axial direction edge surface  5   p  side of the magnet  5  with a predetermined space apart from the magnet  5 . Each of the yoke plates  3   a ,  3   b  is formed of a magnetic material and formed in a partially cylindrical shape having a central angle of about 160°. The yoke plates  3   a ,  3   b  are circumferentially arranged in even shares. The edge surface  5   p  of the magnet  5  faces both the edge surfaces  3   p  of the yoke plates  3   a ,  3   b  and, between the surface  5   p  and surfaces  3   p , an air gap is formed. The outer diameter R 1  of the yoke plates  3   a ,  3   b  is larger than the outer diameter R 2  of the magnet  5 . The inner diameter R 3  of the yoke plates  3   a ,  3   b  is smaller than the inner diameter R 4  of the magnet  5  (R 1 &gt;R 2 , R 3 &lt;R 4 ).  
      The yoke plates  3   a ,  3   b  are fixed onto the Hall ICs  9   a ,  9   b . The Hall ICs  9   a ,  9   b  are fixed to the fixing plate  6 . The fixing plate  6  is formed into substantially a semilunar shape having a central angle of about 200° and is fixed to the chassis of the device. A shaft hole  6   a  is formed in the center of the fixing plate  6 . The rotary shaft  2  is inserted through the shaft hole  6   a . An air-gap G 3  is formed between the rotary shaft  2  and peripheral wall of the shaft hole  6   a.    
      In the rotation angle detection device  28 , a magnetic path is formed circulating from the magnet  5 , through the yoke plates  3   a ,  3   b , fixing plate  6 , rotary shaft  2 , rotor core  4  to the magnet  5 . The Hall ICs  9   a ,  9   b  are disposed in the magnetic path. The rotation of the rotary shaft  2  changes the opposed areas between the magnet  5  and yoke plates  3   a ,  3   b . Accordingly, the density of magnetic flux passing through the Hall ICs  9   a ,  9   b  in the magnetic path linearly changes in proportion to the rotation angle. The Hall ICs  9   a ,  9   b  catch this change and output a linear voltage signal proportional to the rotation angle of the rotary shaft  2 . The rotation angle of the rotary shaft  2  is then calculated based on a relationship between the voltage signal value and rotation angle.  
     Sixth Embodiment  
      It is possible to use the rotation angle detection device having the configuration shown in  FIG. 10  in the electronically-controlled throttle valve.  FIG. 11  is explanatory views schematically showing a configuration of a rotation angle detection device  29  for the electronically-controlled throttle valve according to a sixth embodiment of the present invention.  FIG. 11  ( a ) is a front view of the rotation angle detection device  29 , and  FIG. 11  ( b ) is a cross-sectional view taken along A-A line of  FIG. 11  ( a ).  FIG. 12  is an explanatory view showing configurations of components of the rotation angle detection device  29 .  
      Like the rotation angle detection device  28  of  FIG. 10 , the rotation angle detection device  29  includes the yoke plates  3   a ,  3   b , magnet  5 , and fixing plate  6 . In this example, the magnet  5  is fixed by being embedded in the throttle gear  21  formed of a magnetic material such as metal. The throttle gear  21  is fixed to the valve shaft  12 . The magnet  5  is magnetically connected to the valve shaft  12  and rotates in synchronization with the valve shaft  12 . The fixing plate  6  is fixed to the cover  15  together with the circuit board  16 .  
      An air gap G 1  is formed between the yoke plates  3   a ,  3   b  and magnet  5  also in the rotation angle detection device  29 . The outer diameter R 1  of the yoke plates  3   a ,  3   b  is larger than the outer diameter R 2  of the magnet  5 . The inner diameter R 3  of the yoke plates  3   a ,  3   b  is smaller than the inner diameter R 4  of the magnet  5  (R 1 &gt;R 2 , R 3 &lt;R 4 ). The yoke plates  3   a ,  3   b  are fixed onto the Hall ICs  9   a ,  9   b . The Hall ICs  9   a ,  9   b  are fixed to the fixing plate  6 . A shaft hole  6   a  is formed in the center of the fixing plate  6 . The valve shaft  12  is inserted through the shaft hole  6   a . An air-gap G 3  is formed between the valve shaft  12  and peripheral wall of the shaft hole  6   a.    
      The Hall ICs  9   a ,  9   b  are disposed in a magnetic path M circulating from the magnet  5 , through the yoke plates  3   a ,  3   b , fixing plate  6 , valve shaft  12 , throttle gear  21  to the magnet  5 . When the valve shaft  12  rotates, the magnet  5  rotates together with the throttle gear  21  to change opposed areas S 1 , S 2  between the magnet  5  and yoke plates  3   a ,  3   b . Accordingly, the flux density in the magnetic path M linearly changes in proportion to the rotation angle to cause the Hall ICs  9   a ,  9   b  to output a linear voltage signal proportional to the rotation angle of the valve shaft  12 . The rotation angle of the rotary shaft  2  is then calculated based on a relationship between the voltage signal value and rotation angle.  
      The present invention is not limited to the above embodiments and various changes and modifications can be made within the spirit and scope of the present invention.  
      For example, the center angle of the yoke plates  3   a ,  3   b , or magnet  5  is just an example and may be appropriately be changed depending on the angle range to be detected. Further, although the two Hall ICs are used in a redundant design in the above embodiments, it is possible to detect the rotation angle using only one Hall IC.  
      Although the magnet  5  is embedded in the throttle gear  21  of magnetic material in the sixth embodiment, another configuration may be used in which a plate of magnetic material fixed to the valve shaft  12  is fixed to a throttle gear of nonmagnetic material such as plastic, and the magnet  5  is attached to the plate.  
      The rotation angle detection device has been used in detecting the open degree of the electronically-controlled throttle valve in the above embodiments. However, the application range of the present invention is not limited to this, but the present invention can widely be applied to the rotation angle detection for a rotating body such as a motor rotary shaft.