Abstract:
A steering angle sensor comprising an annular drive gear, which rotates in accompaniment with rotation of a steering wheel, and further comprising a first driven gear and a second driven gear, which have magnets disposed therein and rotate in a driven manner in accordance with rotation of the annular drive gear, the steering angle sensor generating angle-of-rotation information of the steering wheel utilizing changes in magnetic fields based on rotation of the first driven gear and the second driven gear, wherein an intermediate gear that meshes with the annular drive gear, the first driven gear, and the second driven gear is interposed between the annular drive gear and the first driven gear and the second driven gear.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a steering angle sensor for generating angle-of-rotation information of a steering wheel of an automobile. 
     Conventionally, steering angle sensors for generating angle-of-rotation information of a steering wheel of an automobile have been known. 
       FIG. 7  shows an exploded view of a conventional steering angle sensor  100 . The steering angle sensor  100  is equipped with an annular drive gear  101 , which rotates in accompaniment with the rotation of a non-illustrated steering wheel, and a first driven gear  103  and a second driven gear  105 , which mesh with the annular drive gear  101  and rotate in a driven manner. 
     Magnets  104  and  106  are integrally disposed in the first driven gear  103  and the second driven gear  105 , respectively. A circuit board  110  is disposed in the steering angle sensor  100 , and elements  111  and  113  that detect changes in magnetic fields based on the rotation of the magnets  104  and  106  are mounted on the circuit board  110  (e.g., see JP-A-2003-294409). 
     SUMMARY OF INVENTION 
     In the conventional steering angle sensor  100 , the annular drive gear  101  is molded using a resin material. Further, there are many cases where the steering angle sensor  100  has a complex shape; for example, the central portion of the steering angle sensor  100  has a large opening for inserting a steering shaft through the steering angle sensor  100 , and engagement portions  101   a  for transmitting the rotational driving of the steering shaft are disposed in the steering angle sensor  100 . In conjunction with the annular drive gear  101  having such a structure, ensuring that the roundness of the annular drive gear  101  is extremely high is difficult in terms of molding characteristics such as sink marks and shrinkage of the resin. 
     The first driven gear  103  and the second driven gear  105  mesh with the annular drive gear  101  at different positions, so errors in the angles of rotation arising in the first driven gear  103  and the second driven gear  105  because of the roundness of the annular drive gear  101  are respectively different. Consequently, there has been the concern that error in the absolute angle of the annular drive gear  101  computed on the basis of the rotation of the first driven gear  103  and the second driven gear  105  will become greater than mechanical errors caused by backlash of the gears. 
     As a result of considering this problem, the inventors of the present invention discovered that this problem can be solved by interposing, between the annular drive gear and the first driven gear and the second driven gear, an intermediate gear that meshes with these gears. 
     That is, it is an object of the present invention to provide a steering angle sensor in which the effect of errors in the angles of rotation arising in the first driven gear and the second driven gear because of the roundness of the annular drive gear is reduced and which can generate angle-of-rotation information of the steering wheel with good precision. 
     According to the present invention, there is provided a steering angle sensor equipped with an annular drive gear, which rotates in accompaniment with the rotation of a steering wheel, and a first driven gear and a second driven gear, which have magnets disposed in them and rotate in a driven manner in accordance with the rotation of the annular drive gear, the steering angle sensor being for generating angle-of-rotation information of the steering wheel utilizing changes in magnetic fields based on the rotation of the first driven gear and the second driven gear, wherein an intermediate gear that meshes with the annular drive gear, the first driven gear, and the second driven gear is interposed between the annular drive gear and the first driven gear and the second driven gear; and thus the above problem can be solved. 
     That is, in the steering angle sensor of the present invention, the intermediate gear that meshes with the annular drive gear, the first driven gear, and the second driven gear is interposed between the annular drive gear and the first driven gear and the second driven gear, so the effect that the roundness of the annular drive gear directly has on the angles of rotation of the first driven gear and the second driven gear can be reduced. Specifically, in contrast to the annular drive gear through which the steering shaft is inserted, it is not necessary to dispose a large opening in the intermediate gear or dispose engagement portions for transmitting drive on the intermediate gear. For that reason, the intermediate gear can be molded with a higher roundness than the annular drive gear. 
     Consequently, even in a case where the roundness of the annular drive gear is low, the lowness of the roundness of the annular drive gear can be absorbed by the intermediate gear. As a result, error in the angle-of-rotation information output from the steering angle sensor can be reduced. 
     Further, in the steering angle sensor of the present invention, it is preferred that the diameter of the intermediate gear be smaller than the diameter of the annular drive gear. By configuring the intermediate gear in this way, the roundness of the intermediate gear can easily be made higher than the roundness of the annular drive gear. 
     Further, in the steering angle sensor of the present invention, it is preferred that the intermediate gear have first tooth portions that mesh with the annular drive gear and second tooth portions that mesh with the first driven gear and the second driven gear and that the number of the first tooth portions and the number of the second tooth portions be equal. By configuring the intermediate gear in this way, the rotational speed of the annular drive gear is no longer accelerated or decelerated, and the angle-of-rotation information of the steering wheel can be generated with good precision without changing the content of angle-of-rotation computation processing between a case where the intermediate gear is interposed and a case where the intermediate gear is not interposed. 
     Further, in the steering angle sensor of the present invention, it is preferred that the annular drive gear, the intermediate gear, the first driven gear, and the second driven gear be placed in such a way that the axial direction of the rotational axes of the intermediate gear, the first driven gear, and the second driven gear intersects the axial direction of the rotational axis of the annular drive gear. By placing the gears in this way, the steering angle sensor can be miniaturized. 
     Further, in the steering angle sensor of the present invention, it is preferred that the first driven gear and the second driven gear be rotatably supported in a housing of the steering angle sensor and that the intermediate gear be rotatably supported on the annular drive gear, the first driven gear, and the second driven gear regardless of the housing. By rotatably supporting the intermediate gear in this way, the structure of the intermediate gear can be simplified, and an intermediate gear having a higher roundness can be molded. Further, by adjusting the positions of the axes of the first driven gear and the second driven gear, all the gears can be accurately caused to mesh. 
     Further, in the steering angle sensor of the present invention, it is preferred that the intermediate gear be made of resin and be formed as a thick member having no hole disposed therein. By configuring the intermediate gear in this way, a drop in roundness caused by sink marks and shrinkage of the resin can be suppressed, and an intermediate gear having a high roundness can be molded. 
     Further, in the steering angle sensor of the present invention, it is preferred that the intermediate gear be made of a metal material. By configuring the intermediate gear in this way, an intermediate gear having a high roundness can be molded. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view for describing the overall configuration of a steering angle sensor pertaining to a first embodiment of the present invention. 
         FIG. 2  illustrates perspective views showing a state in which an annular drive gear, an intermediate gear, a first driven gear, and a second driven gear of the first embodiment are meshed. 
         FIG. 3  illustrates a front view and a side view of the intermediate gear of the first embodiment. 
         FIG. 4  is an exploded perspective view for describing the overall configuration of a steering angle sensor pertaining to a second embodiment of the present invention. 
         FIG. 5  illustrates a front view and a side view of an intermediate gear of the second embodiment. 
         FIG. 6  illustrates drawings for describing alignment between the annular drive gear, the intermediate gear, the first driven gear, and the second driven gear. 
         FIG. 7  is an exploded view for describing the configuration of a conventional steering angle sensor. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments relating to a steering angle sensor pertaining to the present invention will be specifically described below on the basis of the drawings. 
     Constituent elements denoted by the same reference signs in the drawings represent the same constituent elements unless otherwise indicated, and description thereof will be omitted as appropriate. 
       FIG. 1  is an exploded perspective view for describing the overall configuration of a steering angle sensor  10  pertaining to a first embodiment.  FIGS. 2( a ) and 2( b )  illustrate perspective views showing a state in which an annular drive gear  21 , an intermediate gear  23 , a first driven gear  25 , and a second driven gear  27  are meshed.  FIGS. 3 ( a ) and ( b )  illustrate a front view and a side view for describing the configuration of the intermediate gear  23 . 
     The steering angle sensor  10  pertaining to the first embodiment is equipped with the first driven gear  25  and the second driven gear  27 , which have non-illustrated magnets disposed in them and rotate in tandem with the rotation of the annular drive gear  21 , and generates angle-of-rotation information by means of the so-called vernier principle in accordance with changes in magnetic fields produced by rotation. 
     The steering angle sensor  10  is equipped with an upper housing  11 , a lower housing  17 , and a side housing  19 . Through holes  11   b  and  17   b  concentric with a steering shaft are formed in the upper housing  11  and the lower housing  17 , respectively. The annular drive gear  21 , the intermediate gear  23 , the first driven gear  25 , the second driven gear  27 , and a circuit board  31  are housed in a space surrounded by the upper housing  11 , the lower housing  17 , and the side housing  19 . 
     A first driven gear housing portion  12  that houses the first driven gear  25 , a second driven gear housing portion  13  that houses the second driven gear  27 , and an intermediate gear housing portion  14  that houses the intermediate gear  23  are formed in the upper housing  11 . A first bearing portion  12   a  that rotatably supports the first driven gear  25  is formed in the first driven gear housing portion  12 . A second bearing portion  13   a  that rotatably supports the second driven gear  27  is formed in the second driven gear housing portion  13 . A support shaft  14   a  that rotatably supports the intermediate gear  23  is formed in the intermediate gear housing portion  14 . 
     The annular drive gear  21  is housed in an annular drive gear housing portion  11   a  of the upper housing  11  and an annular drive gear housing portion  17   a  of the lower housing  17  in a state in which the annular drive gear  21  is rotatably positioned therein. The annular drive gear  21  has an annular shape in which tooth portions  21   a  are formed on the outer peripheral portion and an open portion  21   b  is formed in the center. Further, engagement portions  21   c  for engaging with the steering shaft and transmitting the rotation of the steering shaft to the annular drive gear  21  are formed on the inner peripheral surface of the annular drive gear  21  surrounding the open portion  21   b , whereby the annular drive gear  21  is driven to rotate in accompaniment with the rotation of the steering shaft. 
     The tooth portions  21   a  of the annular drive gear  21  are formed on the outer peripheral side of the open portion  21   b , extend radially from the center of the rotational axis, and have shapes whose height becomes lower (whose thickness becomes thinner) away from the center of the rotational axis. 
     The first driven gear  25  is housed in the first driven gear housing portion  12  formed in the upper housing  11  in a state in which the first driven gear  25  is engaged with the intermediate gear  23  so as to be capable of transmitting rotation and is rotatably positioned. Tooth portions  25   a  are formed on the outer peripheral portion of the first driven gear  25 , and an open portion  25   c  is formed in the center of the first driven gear  25 . Further, an engagement portion  25   b  is formed adjacent to the open portion  25   c , and the engagement portion  25   b  engages with the first bearing portion  12   a  of the upper housing  11 , whereby the first driven gear  25  is rotatably supported on the first bearing portion  12   a.    
     The second driven gear  27  is housed in the second driven gear housing portion  13  formed in the upper housing  11  in a state in which the second driven gear  27  is engaged with the intermediate gear  23  so as to be capable of transmitting rotation and is rotatably positioned. Tooth portions  27   a  are formed on the outer peripheral portion of the second driven gear  27 , and an open portion  27   c  is formed in the center of the second driven gear  27 . Further, an engagement portion  27   b  is formed adjacent to the open portion  27   c , and the engagement portion  27   b  engages with the second bearing portion  13   a  of the upper housing  11 , whereby the second driven gear  27  is rotatably supported on the second bearing portion  13   a.    
     Non-illustrated magnets are integrally disposed in the first driven gear  25  and the second driven gear  27 . The first driven gear  25  and the second driven gear  27  are placed in such a way that the directions of the magnetic fluxes formed by the magnets have a predetermined phase difference. Further, in the steering angle sensor  10  pertaining to the first embodiment, the number of the tooth portions  25   a  on the first driven gear  25  and the number of the tooth portions  27   a  on the second driven gear  27  are different, and angle-of-rotation information corresponding to the angle of rotation of the steering shaft is generated. 
     The intermediate gear  23  is housed in the intermediate gear housing portion  14  formed in the upper housing  11  in a state in which the intermediate gear  23  is engaged with the first driven gear  25  and the second driven gear  27  so as to be capable of transmitting rotation and is rotatably positioned. A bearing hole  23   c  is formed in the center of the intermediate gear  23 , and the support shaft  14   a  formed in the upper housing  11  is inserted into the bearing hole  23   c , whereby the intermediate gear  23  is rotatably supported on the support shaft  14   a.    
     Further, first tooth portions  23   a  that engage with the annular drive gear  21  and second tooth portions  23   b  that engage with the first driven gear  25  and the second driven gear  27  are formed on the intermediate gear  23 . The first tooth portions  23   a  and the second tooth portions  23   b  are formed in the same number and are placed on straight lines along the axial direction in such a way that, in a case where the intermediate gear  23  is seen from the front (the state in  FIG. 3( a ) ), the teeth of the first tooth portions  23   a  and the second tooth portions  23   b  lie on top of one another. Further, the first tooth portions  23   a  have tapered shapes that become smaller in diameter away from the second tooth portions  23   b  (see  FIG. 3 ( b ) ). 
     The intermediate gear  23  does not have a hole other than the bearing hole  23   c  whose diameter is relatively small, and it is also not necessary to dispose on the intermediate gear  23  engagement portions other than the tooth portions  23   a . Further, the intermediate gear  23  has a small diameter compared to the annular drive gear  21 , and the teeth of the first tooth portions  23   a  and the second tooth portions  23   b  are placed on straight lines along the axial direction and have radially symmetrical shapes as seen from the center of the rotational axis—that is, they are point-symmetrical about the rotational axis. For that reason, even in the case of die-molding the intermediate gear  23  using a resin material, sink marks and shrinkage occur uniformly, strain and so forth at the time of molding does not easily arise compared to the annular drive gear  21 , and a drop in roundness can be suppressed. 
     The method of molding the intermediate gear  23  is not limited to die molding using a resin material. A high roundness can be obtained no matter what kind of molding method is used, because the intermediate gear  23  is point-symmetrical about the rotational axis. Particularly in the case of the intermediate gear  23  whose diameter is relatively small, a relatively high roundness can be obtained regardless of the raw material. Further, with a metal material, sink marks and shrinkage at the time of molding do not easily arise compared to a resin material, so a high roundness can also be obtained by die molding or process molding using a metal material. 
     Further, in the steering angle sensor  10  pertaining to the first embodiment, in order to miniaturize the sensor, the intermediate gear  23 , the first driven gear  25 , and the second driven gear  27  are placed in such a way that an axial direction of the rotational axis of the intermediate gear  23 , the first driven gear  25 , and the second driven gear  27  intersects an axial direction a of the rotational axis of the annular drive gear  21 . In the example in  FIGS. 2( a ) and 2( b ) , an angle  8  formed by the axial direction of the rotational axis of the intermediate gear  23 , the first driven gear  25 , and the second driven gear  27  and the axial direction a of the rotational axis of the annular drive gear  21  is configured to be 90 degrees. Of the first tooth portions  23   a  and the second tooth portions  23   b  of the intermediate gear  23 , the first tooth portions  23   a  positioned on the side of the open portion  21   b  in the center of the annular drive gear  21  mesh with the tooth portions  21   a  of the annular drive gear  21  and receive the rotation of the annular drive gear  21 . On the other hand, the second tooth portions  23   b  of the intermediate gear  23  mesh with the tooth portions  25   a  of the first driven gear  25  and the tooth portions  27   a  of the second driven gear  27  and transmit rotation to the first driven gear  25  and the second driven gear  27 . 
     At this time, because the height of the tooth portions  21   a  of the annular drive gear  21  becomes lower away from the center of the rotational axis and the first tooth portions  23   a  of the intermediate gear  23  gradually become smaller in diameter in the direction toward the center of the rotational axis of the annular drive gear  21 , the intermediate gear  23  is sandwiched between and held by the annular drive gear  21 , the first driven gear  25 , and the second driven gear  27  and does not easily move in the axial direction. 
     Further, because the number of the first tooth portions  23   a  and the number of the second tooth portions  23   b  of the intermediate gear  23  are configured in the same number, the number of teeth with which the intermediate gear  23  receives rotation from the annular drive gear  21  and the number of teeth with which the intermediate gear  23  transmits rotation to the first driven gear  25  and the second driven gear  27  become the same, so that rotational acceleration and deceleration does not occur. For that reason, it is not necessary to change the content of computational processing when processing the signal output from the steering angle sensor  10  and detecting the angle of rotation of the steering wheel between a case where the intermediate gear  23  is interposed and a case where the intermediate gear  23  is not interposed. 
     Non-illustrated elements that convert changes in the magnetic fields produced by the rotation of the first driven gear  25  and the second driven gear  27  into electrical signals and output the electrical signals are mounted on the circuit board  31 . The elements generate the electrical signals as angle-of-rotation information in accordance with the angle of rotation of the steering shaft. Further, connector pins  33  disposed on the circuit board  31  are fitted together with a connector portion  19   a  disposed on the side housing  19 , and the connector portion  19   a  is electrically connected to the circuit board  31 . 
     In this way, in the steering angle sensor  10  pertaining to the first embodiment, the intermediate gear  23  that has a relatively high roundness is interposed between the first driven gear  25  and the second driven gear  27 , so even in a case where the roundness of the annular drive gear  21  disposed with the open portion  21   b  and the engagement portions  21   c  is low, error in the angle of rotation transmitted from the annular drive gear  21  to the first driven gear  25  and the second driven gear  27  can be reduced. 
     In particular, in the steering angle sensor  10  pertaining to the first embodiment, the intermediate gear  23  is formed as a thick member that is smaller in diameter than the annular drive gear  21  and in which the first tooth portions  23   a  and the second tooth portions  23   b  are placed on straight lines along the axial direction, so moderate strength is obtained. Consequently, a drop in roundness is further suppressed, and the angle-of-rotation information of the steering shaft can be generated with good precision. 
     Further, the steering angle sensor  10  pertaining to the first embodiment is configured in such a way that the axial direction a of the rotational axis of the annular drive gear  21  and the axial direction of the rotational axes of the intermediate gear  23 , the first drive gear  25 , and the second drive gear  27  form a 90-degree angle. For that reason, the steering angle sensor  10  can be miniaturized compared to a case where the rotational directions of the rotational axes of the annular drive gear  21 , the intermediate gear  23 , the first driven gear  25 , and the second driven gear  27  are parallel—that is, a case where the annular drive gear  21 , the intermediate gear  23 , the first driven gear  25 , and the second driven gear  27  are all placed along the same plane. 
     Further, the steering angle sensor  10  pertaining to the first embodiment has a structure in which the intermediate gear  23  is sandwiched between and held by the annular drive gear  21 , the first driven gear  25 , and the second driven gear  27  and is therefore fixed, so the position of the intermediate gear  23  does not easily shift in the direction of the rotational axis. Consequently, the concern that the rotation of the annular drive gear  21  will be erroneously transmitted to the first driven gear  25  and the second driven gear  27  is reduced, and error in the angle-of-rotation information that is generated can be further reduced. 
     A steering angle sensor pertaining to a second embodiment of the present invention differs from the steering angle sensor pertaining to the first embodiment in terms of the configurations of the intermediate gear and the upper housing. 
       FIG. 4  is an exploded perspective view for describing the overall configuration of a steering angle sensor  10 A pertaining to the second embodiment.  FIGS. 5( a ) and ( b )  illustrate a front view and a side view for describing the configuration of an intermediate gear  23 A.  FIG. 6  illustrates drawings for describing alignment between the annular drive gear  21 , the intermediate gear  23 A, the first driven gear  25 , and the second driven gear  27 . 
     The steering angle sensor  10 A pertaining to the second embodiment differs from the steering angle sensor  10  pertaining to the first embodiment in that the intermediate gear  23 A is formed as a thick member having no shaft insertion hole disposed therein. Further, in accompaniment with this, an upper housing  11 A has no support shaft in the intermediate gear housing portion  14 . With this intermediate gear  23 A, the structure of the intermediate gear  23 A can be simplified and the roundness can be further raised. 
     Moreover, in the case of a configuration where a shaft insertion hole is formed in the intermediate gear and the intermediate gear is supported on a support shaft in the upper housing, it is necessary to form the gears and the upper housing in such a way that the precision of the positions of the axes in the three locations of an intermediate gear  23 ′, a first driven gear  25 ′, and a second driven gear  27 ′ is ensured as shown in  FIG. 6( a )  in order to align the meshing (pitch) between the gears. In contrast, in a case where the shaft insertion hole in the intermediate gear is omitted, the intermediate gear  23 A can be accurately caused to mesh with all three of the annular drive gear  21 , the first driven gear  25 , and the second driven gear  27  by adjusting the positions of the axes in the two locations of the first driven gear  25  and the second driven gear  27  as shown in  FIG. 6( b ) . 
     Consequently, according to the steering angle sensor  10 A pertaining to the second embodiment, effects that are the same as those of the steering angle sensor  10  pertaining to the first embodiment can be obtained, and error that occurs when transmitting the angle of rotation of the annular drive gear  21  to the first driven gear  25  and the second driven gear  27  can be further reduced. 
     The steering angle sensors  10  pertaining to the first and second embodiments described above represent one aspect of the present invention and are not intended to limit the invention, and each of the embodiments can be arbitrarily changed in the scope of the present invention. The steering angle sensors  10  and  10 A pertaining to the first and second embodiments can be changed as follows, for example. 
     (1) In the steering angle sensors  10  pertaining to the first and second embodiments, the axial direction a of the rotational axis of the annular drive gear  21  and the axial direction of the rotational axes of the intermediate gear  23 , the first driven gear  25 , and the second driven gear  27  are configured to form a 90-degree angle, but the angle formed by these axial directions may be less than 90° or may exceed 90°. Regardless of the angle, error in the angle-of-rotation information caused by the roundness of the annular drive gear  21  can be reduced. 
     (2) In the steering angle sensors  10  pertaining to the first and second embodiments, the diameter of the intermediate gear  23  is smaller than the diameters of the first driven gear  25  and the second driven gear  27 , but the size of the intermediate gear  23  is not particularly limited. For example, as long as the roundness of the intermediate gear  23  is formed so as to be high, the diameter of the intermediate gear  23  may even be larger than the diameter of the annular drive gear  21 . 
     (3) In the steering angle sensors  10  pertaining to the first and second embodiments, the first tooth portions  23   a  of the intermediate gear  23  that mesh with the tooth portions  21   a  of the annular drive gear  21  and the second tooth portions  23   b  of the intermediate gear  23  that mesh with the first driven gear  25  and the second driven gear  27  are independently disposed and placed on straight lines along the axial direction, but the intermediate gear  23  may be configured in such a way that the annular drive gear  21 , the first driven gear  25 , and the second driven gear  27  all mesh with common tooth portions.