Patent Publication Number: US-2022236045-A1

Title: Rotation sensing device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Japanese Patent Application No. 2021-010607, filed Jan. 26, 2021, the contents of which are incorporated herein by reference in its entirety for all purposes. 
     BACKGROUND 
     Technical Field 
     The present invention relates to a rotation sensing device for sensing the rotation of an object through the use of magnetism. 
     Related Art 
     Patent Document 1 below has described a rotation sensing device that employs a large Barkhausen effect as a rotation sensing device for sensing the rotation of an object through the use of magnetism. To illustrate the basic structure of a rotation sensing device that employs a large Barkhausen effect, let us consider, for example, a case in which the rotation of a rotary shaft in a motor is sensed with the help of a rotation sensing device that employs a large Barkhausen effect. 
     The rotation sensing device that employs a large Barkhausen effect comprises a magnet and multiple magnetic sensors. The magnet is secured, for example, to a rotary shaft and rotates along with the rotary shaft. Since the N poles and S poles of the magnet rotate along with the rotary shaft, this forms a magnetic field (a rotating magnetic field) that rotates about the rotary shaft. The multiple magnetic sensors are secured, for example, to the housing of the motor, etc. The multiple magnetic sensors are arranged in a fixed, spaced-apart relationship in the circumferential direction on the outer periphery of the rotational trajectory of the magnet and sense the rotating magnetic field formed by the magnet. Each magnetic sensor comprises a magnetic wire that produces a large Barkhausen effect and a coil arranged on the outer periphery of the magnetic wire. As the magnet rotates along with the rotary shaft and a rotating magnetic field is formed around the rotary shaft, each magnetic sensor is placed in the rotating magnetic field. Consequently, as the rotary shaft rotates, the direction of the magnetic field around each magnetic sensor changes and the direction of magnetization of the magnetic wire of each magnetic sensor is reversed in response to changes in the direction of the magnetic field. When the direction of magnetization of the magnetic wire is reversed, a pulse is output from the coil. The rotation of the rotary shaft can be sensed based on this pulse. 
     In addition, a rotation sensing device that employs a large Barkhausen effect, in which yokes are provided on the outer periphery of the rotational trajectory of the magnet, has been described in Patent Document 2 hereinbelow. The yokes, which are secured to the housing of the motor, etc., along with the multiple magnetic sensors, are arranged to partially cover the section facing the rotational trajectory of the magnet in each magnetic sensor. The yokes have the ability to control the direction of the magnetic flux of the rotating magnetic field such that the magnetic flux passes through the magnetic wire of each magnetic sensor in the direction of extension thereof. Providing the yokes makes it possible to focus the magnetic flux of the rotating magnetic field on the magnetic wire of each magnetic sensor and increase the accuracy with which the rotation of the rotary shaft is sensed. 
     PATENT DOCUMENTS 
     [Patent Document 1] 
     International Publication No. 2016/002437 [Patent Reference 2] 
     International Publication No. 2016/021074 
     SUMMARY 
     Problems to be Solved 
     As devices such as motors and so forth, to which rotation sensing devices that employ a large Barkhausen effect are applied, become more compact, reducing the dimensions of rotation sensing devices that employ a large Barkhausen effect becomes more desirable. Rotation sensing devices that employ a large Barkhausen effect can be made more compact by shortening the spacing between the magnetic sensors as well as the spacing between the magnet and each magnetic sensor. However, if a rotation sensing device that employs a large Barkhausen effect comprises yokes, such as in the case of the rotation sensing device described in Patent Document 2, the yokes are arranged between the magnetic sensors as well as between the magnet and each magnetic sensor, thereby making it difficult to shorten the spacing between the magnetic sensors as well as the spacing between the magnet and each magnetic sensor. 
     In addition, rotation sensing devices that employ a large Barkhausen effect can be made more compact by making the magnet smaller or by shortening the length of the magnetic wire of each magnetic sensor. However, if the magnet is made smaller or the length of the magnetic wire of each magnetic sensor is shortened, the voltage of the pulse output from the coil will be reduced, thereby making it difficult to improve the accuracy of rotation sensing. Accordingly, in order to minimize the reduction in the voltage of the pulse output from the coil, it would be desirable to increase the degree to which the magnetic flux of the rotating magnetic field is focused on the magnetic wire with the help of a yoke. In order to meet such demands, it is necessary to explore yoke shapes capable of increasing the degree to which magnetic flux is focused on the magnetic wire. 
     The present invention was made by considering problems such as those described above and it is an object of the invention to provide a rotation sensing device which, despite being equipped with yokes, can be made more compact and the accuracy of rotation sensing can be improved. 
     Technical Solution 
     It is an object of the present disclosure to make a rotation sensing device equipped with yokes more compact while improving the accuracy of rotation sensing. 
     In order to eliminate the above problems, the inventive rotation sensing device is a rotation sensing device that is provided within a structure having a supporting portion and a rotating portion which rotates, in relative terms, with respect to the supporting portion, and that senses the rotation of the rotating portion with respect to the supporting portion, wherein the device comprises: a base that is secured to the supporting portion and has one face parallel to a plane orthogonal to the rotational axis of the rotating portion; a magnetic field forming portion that is secured to the rotating portion, rotates along with the rotating portion about the rotational axis, and forms a magnetic field around the periphery of the rotational axis; a plurality of magnetic field sensing portions that are secured to said one face of the base, are arranged with equal angular spacing to one another in the circumferential direction on the outer periphery of the rotational trajectory of the magnetic field forming portion, and sense the magnetic field formed by the magnetic field forming portion; and a plurality of yokes that are secured to said one face of the base, are arranged on the outer periphery of the rotational trajectory in one-to-one correspondence with the plurality of magnetic field sensing portions, and control the magnetic flux of the magnetic field formed by the magnetic field forming portion; the magnetic field sensing portions comprise a magnetic wire extending in a direction parallel to the tangent of the rotational trajectory, a coil provided on the outer periphery of the magnetic wire, and a bobbin holding the magnetic wire and the coil; and, for each magnetic field sensing portion, the direction of extension of the rotational axis is the up-down direction, the direction of extension of the magnetic wire is the left-right direction, the direction of extension of a straight line orthogonal to both the rotational axis and the magnetic wire is the forward-backward direction, the direction in which said one face of the base is facing is “up”, the direction toward the rotational axis in the forward-backward direction is “front”, the left and right of each magnetic field sensing portion are defined by viewing each magnetic field sensing portion from the front thereof as a point of reference, the bobbin is formed in a columnar shape extending in the left-right direction; a wire receiving portion, in which the magnetic wire is received, is formed inside the bobbin; a conductor winding portion, which is the central portion of the bobbin in the left-right direction and around which the conductor wire of the coil is wound, is formed on the outer periphery of the wire receiving portion; each yoke comprises a left yoke piece that partially covers the left portion of the magnetic field sensing portion, and a right yoke piece that partially covers the right portion of the magnetic field sensing portion; the left yoke piece has a left front plate portion, which extends linearly from the front of the left portion of the bobbin, which is left of the conductor winding portion, to the front of the coil; the left front plate portion is inclined with respect to the direction of extension of the magnetic wire such that the right end of said left front plate portion is located rearwardly of the left end of said left front plate portion; the right yoke piece has a right front plate portion, which extends linearly from the front of the right portion of the bobbin, which is right of the conductor winding portion, to the front of the coil; the right front plate portion is inclined with respect to the direction of extension of the magnetic wire such that the left end of said right front plate portion is located rearwardly of the right end of said right front plate portion; and the right end of the left front plate portion and the left end of the right front plate portion are mutually opposed across a gap in front of the central portion of the coil in the left-right direction. 
     In addition, in the inventive rotation sensing device, the left yoke piece may be adapted to have a left plate portion covering the bobbin from the left side of said bobbin, with the left plate portion being inclined with respect to a straight line orthogonal to both the rotational axis and the magnetic wire such that the rear end of said left plate portion is located to the left of the front end of said left plate portion, and the right yoke piece may be adapted to have a right plate portion covering the bobbin from the right side of said bobbin, with the right plate portion being inclined with respect to a straight line orthogonal to both the rotational axis and the magnetic wire such that the rear end of said right plate portion is located to the right of the front end of said right plate portion. In this case, the left plate portion may be adapted to be inclined with respect to a straight line orthogonal to both the rotational axis and the magnetic wire by an angle equal to one-half the angular spacing such that the rear end of said left plate portion is located to the left of the front end of said left plate portion, and the right plate portion may be adapted to be inclined with respect to a straight line orthogonal to both the rotational axis and the magnetic wire by an angle equal to one-half the angular spacing such that the rear end of said right plate portion is located to the right of the front end of said right plate portion. Furthermore, the left plate portion may be adapted to cover only the section of the bobbin forward of the center in the forward-backward direction from the left side of said bobbin, and the right plate portion may be adapted to cover only the section of the bobbin forward of the center in the forward-backward direction from the right side of said bobbin. 
     In addition, the inventive rotation sensing device may be adapted to have a structure in which the plurality of magnetic field sensing portions and the plurality of yokes are arranged respectively in one-to-one correspondence on said one face of the base on an arc centered on the rotational axis, and, when said one face is viewed from above the plurality of magnetic field sensing portions, a first dummy yoke piece having the same configuration as the left yoke piece is arranged on the clockwise side of the magnetic field sensing portion arranged on the most clockwise side among the plurality of magnetic field sensing portions, a second dummy yoke piece having the same configuration as the right yoke piece is arranged on the counterclockwise side of the magnetic field sensing portion arranged on the most counterclockwise side among the plurality of magnetic field sensing portions, and, on said one face, there are neither magnetic field sensing portions corresponding to the first dummy yoke piece nor magnetic field sensing portions corresponding to the second dummy yoke piece. 
     Technical Effect 
     According to the present invention, despite being equipped with yokes, the rotation sensing device can be made more compact and the accuracy of rotation sensing can be improved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective diagram showing a rotation sensing device, etc., according to an embodiment of the present invention. 
         FIG. 2  is an illustrative diagram showing the rotation sensing device according to the embodiment of the present invention, as viewed from above in  FIG. 1 . 
         FIG. 3  is a cross-sectional diagram showing a cross-section of the rotation sensing device according to the embodiment of the present invention taken along section line in  FIG. 2 , as viewed from below in  FIG. 2 . 
         FIG. 4  is an illustrative diagram showing a magnetic sensor used in the rotation sensing device according to the embodiment of the present invention. 
         FIG. 5  is an illustrative diagram showing the magnetic sensor of  FIG. 4  with its coil removed. 
         FIG. 6  is a perspective diagram showing a yoke used in the rotation sensing device according to the embodiment of the present invention. 
         FIG. 7  is an illustrative diagram showing the magnetic sensor and yoke used in the rotation sensing device according to the embodiment of the present invention, as viewed from above. 
         FIG. 8  is an illustrative diagram showing the magnetic sensor and yoke used in the rotation sensing device according to the embodiment of the present invention, as viewed from the front. 
         FIG. 9  is an illustrative diagram showing the left yoke piece and the left portion of the magnetic sensor used in the rotation sensing device according to the embodiment of the present invention, as viewed from above. 
         FIG. 10  is an illustrative diagram showing the magnet, magnetic sensors, and yokes, as viewed from above, provided to illustrate, in greater detail, the shape of the yokes used in the rotation sensing device according to the embodiment of the present invention. 
         FIG. 11  is an illustrative diagram showing the left yoke piece and the left portion of the magnetic sensor used in the rotation sensing device according to the embodiment of the present invention, as viewed from the front. 
         FIG. 12  is an illustrative diagram showing the left yoke piece and the left portion of the magnetic sensor used in the rotation sensing device according to the embodiment of the present invention, as viewed from above. 
         FIG. 13  is an illustrative diagram showing the flow of magnetic flux controlled by the yokes in the rotation sensing device according to the embodiment of the present invention. 
         FIG. 14  is an illustrative diagram showing the flow of magnetic flux controlled by the left upper plate portion and left plate portion of the left yoke piece as well as the right upper plate portion and right plate portion of the right yoke piece in the rotation sensing device according to the embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     (Rotation Sensing Device) 
       FIG. 1  shows a rotation sensing device  1  along with a rotary shaft  83  according to an embodiment of the present invention.  FIG. 2  shows the rotation sensing device  1  as viewed from above in  FIG. 1 .  FIG. 3  shows a cross-section of the rotation sensing device  1  taken along section line in  FIG. 2 , as viewed from below in  FIG. 2 . 
     The rotation sensing device  1  is a device for sensing the rotation of an object that employs a large Barkhausen effect. The present embodiment illustrates a case in which the rotation sensing device  1  is applied to a motor  81 . As shown in  FIG. 3 , the motor  81  is provided with a motor main body  82  and a rotary shaft  83 . The rotary shaft  83  rotates relative to the motor main body  82 . The motor main body  82 , which has a mechanism for rotating the rotary shaft  83  as well as a housing that holds this mechanism, etc., rotatably supports the rotary shaft  83 . The rotation sensing device  1  is provided in the motor  81  and senses the rotation of the rotary shaft  83 . It should be noted that the motor  81  is a specific example of the structure, the motor main body  82  is a specific example of the supporting portion, and the rotary shaft  83  is specific example of the rotating portion. 
     As shown in  FIG. 1 , the rotation sensing device  1  comprises a base  2 , a magnet  3  serving as a magnetic field forming portion, three magnetic sensors  11  serving as the plurality of magnetic field sensing portions, three yokes  30 , and two dummy yoke pieces  51 ,  52 . 
     The base  2  is formed in a disk-like configuration and, as shown in  FIG. 3 , is secured to the housing of the motor main body  82  by means of a securing bracket  84 . An insertion hole  2 A is formed in the center of the base  2  and the rotary shaft  83  is inserted into the insertion hole  2 A. The diameter of the insertion hole  2 A is larger than the diameter of the rotary shaft  83  and the base  2  is spaced apart from the rotary shaft  83 . In addition, the base  2  has a mounting face  2 B, to which each magnetic sensor  11 , each yoke  30 , and each dummy yoke piece  51 ,  52  is mounted. The base  2  is arranged such that the mounting face  2 B is parallel to a plane orthogonal to the rotational axis A of the rotary shaft  83 . It should be noted that the mounting face  2 B is a specific example of one face. 
     The magnet  3  is formed in an annular configuration from, for example, ferrite or another magnetic material. The magnet  3  has the rotary shaft  83  inserted internally of its periphery and is thus arranged on the outer periphery of the rotary shaft  83 . The magnet  3  is secured to the rotary shaft  83  and rotates along with the rotary shaft  83 . The center of the magnet  3 , which is formed in an annular configuration, is positioned on the rotational axis A. In addition, the magnet  3  is positioned above the mounting face  2 B of the base  2 . In other words, the magnet  3  is secured to the section of the rotary shaft  83  that passes through the insertion hole  2 A in the base  2  and protrudes above the mounting face  2 B of the base  2 . 
     In addition, as shown in  FIG. 2 , the magnet  3  is magnetized such that four magnetic poles, i.e., an N pole, an S pole, an N pole, and an S pole, are arranged in the outer peripheral section of the magnet  3  with equal angular spacing in the circumferential direction, more specifically, with 90-degree spacing. In addition, in the magnet  3 , these four magnetic poles are arranged such that magnetic poles mutually adjacent in the circumferential direction are different from each other. When the magnet  3  rotates along with the rotary shaft  83 , the four magnetic poles of the magnet  3  rotate and thereby form a rotating magnetic field around the rotational axis A. 
     The three magnetic sensors  11 , all of which have an identical configuration and shape, are arranged on the outer periphery of the rotational trajectory K of the magnet  3  with equal angular spacing in the circumferential direction, for example, with 60-degree spacing. Each magnetic sensor  11  has a magnetic wire  12 , as described hereinafter, and each magnetic sensor  11  is arranged such that the direction of extension of the magnetic wire  12  is a direction orthogonal to the rotational axis A (direction parallel to the mounting face  2 B). In addition, as shown in  FIG. 2 , each magnetic sensor  11  is arranged such that the central portion of the magnetic wire  12  in the direction of extension is tangent to a circular arc C 1  that is centered on the rotational axis A and has a radius larger than the radius of the rotational trajectory K of the magnet  3 . In addition, the three magnetic sensors  11  are arranged on circular arc C 1  within an angular range of 180 degrees, as a result of which the three magnetic sensors  11  are arranged within half the area of the mounting face  2 B of the base  2 . Each magnetic sensor  11  is secured on top of the mounting face  2 B of the base  2 . The three magnetic sensors  11  sense the rotating magnetic field formed by the magnet  3 . Each magnetic sensor  11  will be further discussed later herein. 
     The three yokes  30 , all of which have an identical configuration and shape, are arranged on the outer periphery of the rotational trajectory K of the magnet  3  in one-to-one correspondence with the three magnetic sensors  11 . Each yoke  30  is secured on top of the mounting face  2 B of the base  2 . In addition, each yoke  30  comprises a left yoke piece  31  and a right yoke piece  41 . Each yoke  30  controls the magnetic flux of the rotating magnetic field formed by the magnet  3 . Each yoke  30  will be further discussed later herein. In addition, each dummy yoke piece  51  will be discussed later herein. 
     Further, an outer wall  55  is provided on the outer periphery of the base  2 . The magnet  3 , each magnetic sensor  11 , each yoke  30 , and each dummy yoke piece  51 ,  52  are arranged inwardly of the outer wall  55 . The outer wall  55  is formed, for example, from a metallic material. The outer wall  55  is effective for preventing external objects from making contact with the magnet  3 , each magnetic sensor  11 , each yoke  30  or each dummy yoke piece  51 ,  52  and is effective for minimizing leakage of the magnetic field formed by the magnet  3  to outside. 
     (Magnetic Sensors) 
       FIG. 4  shows a magnetic sensor  11 , as viewed from above.  FIG. 5  shows the magnetic sensor  11  of  FIG. 4  with the coil  13  removed. Here, when each magnetic sensor  11  and each yoke  30  is described, for ease of discussion, directions are defined as follows. The direction of extension of the rotational axis A is the “up-down direction”, and the direction in which the mounting face  2 B of the base  2  is facing is “up”. Further, the direction of extension of the magnetic wire  12  is the “left-right direction”. In addition, the direction of extension of a straight line orthogonal to both the rotational axis A and the magnetic wire  12  is the “forward-backward direction”, and the direction toward the rotational axis A in the forward-backward direction is “front”. In addition, the left and right of the magnetic sensor  11  are defined based on a front view of the magnetic sensor  11  as a point of reference. In addition, the left and right of the yoke  30  are defined based on a front view of the yoke  30  as a point of reference. The arrows at the bottom right of  FIGS. 4 through 14  illustrate the above definitions of “up” (Ud), “down” (Dd), “front” (Fd), “back” (Bd), “left” (Ld), and “right” (Rd). 
     As shown in  FIG. 4 , each magnetic sensor  11  comprises a magnetic wire  12 , a coil  13  and a bobbin  14 . 
     The magnetic wire  12 , which is a magnetic wire that generates a large Barkhausen effect, is called a composite magnetic wire. The magnetic wire  12  is a wire that is formed, for example, from a semi-rigid magnetic material including iron and cobalt, and has a diameter of, e.g., approximately 0.1 mm to 1 mm and a length of, e.g., approximately 10 mm to 30 mm. The magnetic wire  12  is formed, for example, by drawing the semi-rigid magnetic material and twisting it multiple times while changing the direction. The magnetic wire  12  exhibits uniaxial anisotropy, in which the easy direction of magnetization is the direction of the central axis B of said magnetic wire  12 . In addition, in the magnetic wire  12 , its coercivity is larger in the central section than in the outer peripheral sections. The magnetic wire  12  has a property that the direction of magnetization of the magnetic wire  12  (its outer peripheral section) is suddenly reversed in response to changes in the direction of the external magnetic field. The magnetic wire  12  is arranged such that its direction of extension, i.e., the direction of its central axis B, is a direction parallel to the tangent to the rotational trajectory K of the magnet  3 . Specifically, the magnetic wire  12  is arranged such that its central portion in the direction of extension is tangent to the circular arc C 1 . 
     The coil  13  is provided on the outer periphery of the magnetic wire  12 . The coil  13  is formed by winding a conducting wire such as, for example, enameled wire on the conductor winding portion  18  of the bobbin  14 . 
     The bobbin  14 , which is a member that holds the magnetic wire  12  and coil  13  in place, is formed from a nonmagnetic material such as resin or the like. As shown in  FIG. 5 , when viewed as a whole, the bobbin  14  is formed in a columnar shape extending in the direction of extension of the magnetic wire  12 , i.e., in the left-right direction. In addition, a wire receiving portion  15  that receives the magnetic wire  12  is formed inside the bobbin  14 . In the present embodiment the wire receiving portion  15  is an upward-opening groove extending in the left-right direction from the left end to the right end of the bobbin  14  through the section of the bobbin  14  at the center in the forward-backward direction and at the center in the up-down direction. The magnetic wire  12  is arranged within this wire receiving portion  15 . It should be noted that the wire receiving portion may be in the form of a hole passing through the inside of the bobbin  14  in the left-right direction. 
     In addition, connecting member retaining portions  16 ,  17 , which hold connecting members  25  in place, are formed in the left end section and right end sections of the bobbin  14 . Further, a conductor winding portion  18 , on which the conductor wire of the coil  13  is wound, is formed on the outer periphery of the wire receiving portion  15 , which is the central portion of the bobbin  14  in the left-right direction, i.e., the section between the two connecting member retaining portions  16 ,  17 . The conductor winding portion  18  is made smaller in diameter than each connecting member retaining portion  16 ,  17  in order to make it possible to wind the conductor wire of the coil  13  in numerous layers. 
     In addition, a protruding portion  19  that protrudes to the left is formed in the central portion in the forward-backward direction on the left edge face of the bobbin  14 , i.e., the left edge face of the left-side connecting member retaining portion  16 . Similarly, a protruding portion  20  that protrudes to the right is formed in the central portion in the forward-backward direction on the right edge face of the bobbin  14 , i.e., the right edge face of the right-side connecting member retaining portion  17 . Further, the left end side of the wire receiving portion  15  extends to the left end of the left-side protruding portion  19  and the right end side of the wire receiving portion  15  extends to the right end of the right-side protruding portion  20 . In addition, the left end side of the magnetic wire  12  extends to a location in very close proximity to the left end of the left-side protruding portion  19 , and the right end side of the magnetic wire  12  extends to a location in very close proximity to the right end of the right-side protruding portion  20 . 
     Further, inclined faces  21 ,  22  are formed respectively in the front and rear portions on the left edge face of the left-side connecting member retaining portion  16 . The inclined face  21  formed in the front portion on the left edge face of the left-side connecting member retaining portion  16  is inclined such that the rear end of said inclined face  21  is located to the left of the front end of said inclined face  21 . In addition, the inclined face  22  formed in the rear portion on the left edge face of the left-side connecting member retaining portion  16  is inclined such that the front end of said inclined face  22  is located to the left of the rear end of said inclined face  22 . Further, inclined faces  23 ,  24  are formed symmetrically with respect to the inclined faces  21 ,  22  on the right edge face of the right-side connecting member retaining portion  17 . 
     In addition, connecting members  25  are secured to each connecting member retaining portion  16 ,  17 . The connecting members  25  are members formed in an L-shaped configuration from, for example, metal or another electrically conductive material, with one end thereof extending downward, and the other end extending forward. The connecting members  25  are effective for securing the magnetic sensor  11  to the base  2 , connecting the coil  13  to circuits formed on the base  2 , and attaching the end portion of the conductor wire of the coil  13 . 
     When the magnet  3  rotates along with the rotary shaft  83  and a rotating magnetic field is formed by the magnet  3 , the direction of magnetization of the magnetic wire  12  that each magnetic sensor  11  has is suddenly reversed by this rotating magnetic field and a pulse is output from the coil  13  by electromagnetic induction. In specific terms, in one magnetic sensor  11 , when an N pole of the magnet  3  approaches the left end portion of said magnetic sensor  11  while an S pole of the magnet  3  approaches the right end portion of said magnetic sensor  11  in a state in which the direction of magnetization of the magnetic wire  12  is to the left, the direction of magnetization of the magnetic wire  12  is suddenly reversed from left to right and, for example, a positive pulse is output from the coil  13 . Thereafter, when an S pole of the magnet  3  approaches the left end portion of said magnetic sensor  11  while an N pole of the magnet  3  approaches the right end portion of said magnetic sensor  11 , the direction of magnetization of the magnetic wire  12  is suddenly reversed from right to left and, for example, a negative pulse is output from the coil  13 . The rotation of the rotary shaft  83  can be sensed based on these pulses. The method described in Patent Document 1 above can be used as a method for sensing rotation with the help of such pulses. 
     (Yoke) 
       FIG. 6  shows a yoke  30  as viewed from the top right front.  FIG. 7  shows a magnetic sensor  11  and a yoke  30  as viewed from above.  FIG. 8  shows a magnetic sensor  11  and a yoke  30  as viewed from the front. 
     As shown in  FIG. 6 , each yoke  30  comprises a left yoke piece  31  and a right yoke piece  41 . The left yoke piece  31  and right yoke piece  41  are each formed from a soft magnetic material such as iron or the like. The left yoke piece  31  and right yoke piece  41  are shaped to be symmetrical to each other. As shown in  FIG. 7  and  FIG. 8 , the left yoke piece  31  partially covers the left portion of the magnetic sensor  11 . The left yoke piece  31  has a left front plate portion  32 , a left plate portion  33 , a left upper plate portion  34 , and coupling portions  35 ,  36 . In addition, the right yoke piece  41  partially covers the right portion of the magnetic sensor  11 . The right yoke piece  41  has a right front plate portion  42 , a right plate portion  43 , a right upper plate portion  44 , and coupling portions  45 ,  46 . 
     (Left Front Plate Portion &amp; Right Front Plate Portion of Yokes) 
       FIG. 9  shows the left yoke piece  31  and the left portion of the magnetic sensor  11  as viewed from above. To describe the details of the shape of the left yoke piece  31  and right yoke piece  41 ,  FIG. 10  shows the magnet  3 , the magnetic sensor  11  and the yoke  30  as viewed from above. It should be noted that  FIG. 9  and  FIG. 10  show cross-sections of the left yoke piece  31  taken along section line IX-IX in  FIG. 8 , as viewed from above in  FIG. 8 . In addition,  FIG. 10  also provides a partial cross-section of the right yoke piece  41 , similarly to the left yoke piece  31 . 
     As shown in  FIG. 9 , the left front plate portion  32  of the left yoke piece  31  covers the left-side connecting member retaining portion  16  of the bobbin  14  and the left portion of the coil  13  from the front. The left front plate portion  32  is formed in a planar configuration extending in a direction that is parallel to the mounting face  2 B of the base  2 , and is inclined with respect to the central axis B of the magnetic wire  12  (direction of extension of the magnetic wire  12 ) as well as in a direction perpendicular to the mounting face  2 B of the base  2 . In addition, the left front plate portion  32  extends linearly from the front of the left-side connecting member retaining portion  16  to the front of the left portion of the coil  13 . In addition, the right end of the left front plate portion  32  reaches a location in very close proximity to the front of the center of the coil  13  in the left-right direction. 
     In addition, the left front plate portion  32  is inclined with respect to the central axis B of the magnetic wire  12  such that the right end of said left front plate portion  32  is located rearwardly of the left end of said left front plate portion  32 . Due to the fact that the left front plate portion  32  is inclined in this manner, the distance D 1  between the right end of the left front plate portion  32  and the central axis B of the magnetic wire  12  is smaller than the distance D 2  between the left end of the left front plate portion  32  and the central axis B of the magnetic wire  12 . In other words, the right end of the left front plate portion  32  is closer to the central axis B of the magnetic wire  12  than the left end of the left front plate portion  32 . As a result, the left front plate portion  32  approaches the front outer surface of the coil  13  as one moves to the right. 
     In addition, the right front plate portion  42  of the right yoke piece  41  covers the right-side connecting member retaining portion  17  of the bobbin  14  and the right portion of the coil  13  from the front. The right front plate portion  42  is symmetrical in shape to the left front plate portion  32  of the left yoke piece  31 . In other words, as can be appreciated by examining  FIG. 10 , the right front plate portion  42  is formed in a planar configuration extending in a direction that is parallel to the mounting face  2 B of the base  2 , and is inclined with respect to the central axis B of the magnetic wire  12  as well as in a direction perpendicular to the mounting face  2 B of the base  2 . In addition, the right front plate portion  42  extends linearly from the front of the right-side connecting member retaining portion  17  to the front of the right portion of the coil  13 . In addition, the left end of the right front plate portion  42  reaches a location in very close proximity to the front of the center of the coil  13  in the left-right direction. 
     In addition, the right front plate portion  42  is inclined with respect to the central axis B of the magnetic wire  12  such that the left end of said right front plate portion  42  is located rearwardly of the right end of said right front plate portion  42 . Due to the fact that the right front plate portion  42  is inclined in this manner, the left end of the right front plate portion  42  is closer to the central axis B of the magnetic wire  12  than the right end of the right front plate portion  42 . As a result, the right front plate portion  42  approaches the front outer surface of the coil  13  as one moves to the left. 
     In addition, as shown in  FIG. 10 , the angle of inclination of the left front plate portion  32  of the left yoke piece  31  with respect to the central axis B of the magnetic wire  12  and the angle of inclination of the right front plate portion  42  of the right yoke piece  41  with respect to central axis B of the magnetic wire  12  are set such that the left front plate portion  32  and the right front plate portion  42  are generally in line with the circular arc C 2  centered on the rotational axis A. As a result, the distance between the left end of the left front plate portion  32  and the outer peripheral surface of the magnet  3 , the distance between the right end of the left front plate portion  32  and the outer peripheral surface of the magnet  3 , the distance between the right end of the right front plate portion  42  and the outer peripheral surface of the magnet  3 , and the distance between the left end of the right front plate portion  42  and the outer peripheral surface of the magnet  3  are equal to one another. 
     Further, as shown in  FIG. 7 , the right end of the left front plate portion  32  and the left end of the right front plate portion  42  are opposed across a gap G in front of the central portion of the coil  13  in the left-right direction. The right end of the left front plate portion  32  and the left end of the right front plate portion  42  are positioned in proximity to one another, but do not come into contact with each other. 
     (Left Plate Portion &amp; Right Plate Portion of Yokes) 
     As shown in  FIG. 9 , the left plate portion  33  of the left yoke piece  31  covers the inclined face  21  of the front portion on the left edge face of the left-side connecting member retaining portion  16  of the bobbin  14  from the left. As shown in  FIG. 10 , the left plate portion  33  is formed in a planar configuration extending in a direction inclined with respect to straight line L 1  orthogonal to both the rotational axis A and the central axis B of the magnetic wire  12 , as well as in a direction perpendicular to the mounting face  2 B of the base  2 . In addition, the left plate portion  33  is inclined with respect to straight line L 1  orthogonal to both the rotational axis A and the central axis B of the magnetic wire  12  such that the rear end of said left plate portion  33  is located to the left of the front end of said left plate portion  33 . In addition, the angle of inclination Q of the left plate portion  33  with respect to straight line L 1  is set to one-half the angular spacing P used in the arrangement of the three magnetic sensors  11 . Since in the present embodiment the angular spacing P used in the arrangement of the three magnetic sensors  11  is 60 degrees, the angle of inclination Q of the left plate portion  33  with respect to straight line L 1  is set to 30 degrees. In addition, the angle of inclination, with respect to the straight line L 1 , of the inclined face  21  of the front portion on the left edge face of the left-side connecting member retaining portion  16  of the bobbin  14  is also set to one-half the angular spacing P used in the arrangement of the three magnetic sensors  11  (30 degrees). As a result, the left plate portion  33  and said inclined face  21  are parallel to each other. It should be noted that straight line L 2  in  FIG. 10  is a straight line inclined 30 degrees to straight line L 1  in the counterclockwise direction. 
     In addition, although the left plate portion  33 , as shown in  FIG. 9 , covers the inclined face  21  of the front portion on the left edge face of the left-side connecting member retaining portion  16  of the bobbin  14 , the protruding portion  19  and the inclined face  22  of the rear portion on the left edge face of the left-side connecting member retaining portion  16  are not covered. In other words, the rear end of the left plate portion  33  is located in front of the protruding portion  19 . 
     Further, the right plate portion  43  of the right yoke piece  41  covers the inclined face  23  of the front portion on the right edge face of the right-side connecting member retaining portion  17  of the bobbin  14  from the right. The right plate portion  43  is symmetrical in shape to the left plate portion  33  of the left yoke piece  31 . As shown in  FIG. 10 , the right plate portion  43  is formed in a planar configuration extending in a direction that is inclined with respect to straight line L 1  as well as in a direction perpendicular to the mounting face  2 B of the base  2 . In addition, the right plate portion  43  is inclined with respect to straight line L 1  such that the rear end of said right plate portion  43  is located to the right of the front end of said right plate portion  43 . In addition, the angle of inclination R of the right plate portion  43  with respect to straight line L 1  is set to one-half the angular spacing P used in the arrangement of the three magnetic sensors  11 . In the present embodiment, the angle of inclination R of the right plate portion  43  with respect to straight line L 1  is set to 30 degrees. In addition, the angle of inclination, with respect to the straight line L 1 , of the inclined face  23  of the front portion on the right edge face of the right-side connecting member retaining portion  17  of the bobbin  14  is also set to one-half the angular spacing P used in the arrangement of the three magnetic sensors  11  (30 degrees). As a result, the right plate portion  43  and said inclined face  23  are parallel to each other. It should be noted that straight line L 3  in  FIG. 10  is a straight line inclined 30 degrees to straight line L 1  in the clockwise direction. 
     In addition, although the right plate portion  43  covers the inclined face  23  of the front portion on the right edge face of the right-side connecting member retaining portion  17  of the bobbin  14 , the protruding portion  20  and the inclined face  24  of the rear portion on the right edge face of the right-side connecting member retaining portion  17  are not covered. In other words, the rear end of the right plate portion  43  is located in front of the protruding portion  20 . 
     (Left Upper Plate Portion &amp; Right Upper Plate Portion of Yokes) 
       FIG. 11  shows the left yoke piece  31  and the left portion of the magnetic sensor  11 , as viewed from the front. It should be noted that  FIG. 11  shows a cross-section of the left yoke piece  31  taken along section line XI-XI in  FIG. 7 , as viewed from below in  FIG. 11 .  FIG. 12  shows the left yoke piece  31  and the left portion of the magnetic sensor  11  as viewed from above. 
     As shown in  FIG. 11  and  FIG. 12 , the left upper plate portion  34  of the left yoke piece  31  covers the left-side connecting member retaining portion  16  of the bobbin  14  from the above. The left upper plate portion  34  is formed in a planar configuration extending in a direction parallel to the mounting face  2 B of the base  2 . In addition, the left upper plate portion  34  largely covers the section of the magnetic wire  12  protruding to the left from the coil  13 . Specifically, the left upper plate portion  34  spans across said section of the magnetic wire  12  in the forward-backward direction. 
     In this manner, while the left upper plate portion  34  largely covers the left-side connecting member retaining portion  16  of the bobbin  14  from above, the range covered by the left upper plate portion  34  remains above the left-side connecting member holding portion  16  and does not extend to above the coil  13 . In other words, the left upper plate portion  34  does not enter the region above the coil  13 . The coil  13  is not covered by the yoke  30  from above. 
     In addition, as shown in  FIG. 11 , the left upper plate portion  34  is disposed very close to the top face of the left-side connecting member retaining portion  16  of the bobbin  14  such that the distance D 3  between said left upper plate portion  34  and the magnetic wire  12  is reduced. Specifically, the position of the bottom face of the left upper plate portion  34  in the up-down direction is at or below the position of the uppermost portion of the outer surface of the coil  13  in the up-down direction. 
     In addition, as shown in  FIG. 12 , the right face  34 A of the left upper plate portion  34  is perpendicular to the central axis B of the magnetic wire  12  and extends above the magnetic wire  12  in the forward-backward direction. 
     In addition, the right upper plate portion  44  of the right yoke piece  41  covers the right-side connecting member retaining portion  17  of the bobbin  14  from above. The right upper plate portion  44  is symmetrical in shape to the left upper plate portion  34  of the left yoke piece  31 . The right upper plate portion  44  is formed in a planar configuration extending in a direction parallel to the mounting face  2 B of the base  2  and largely covers the section of the magnetic wire  12  protruding to the right from the coil  13 . However, the range covered by the right upper plate portion  44  remains above the right-side connecting member retaining portion  17  and does not extend to above the coil  13 . In addition, the position of the right upper plate portion  44  in the up-down direction is set such that the distance between the right upper plate portion  44  and the magnetic wire  12  is reduced. Specifically, the position of the bottom face of the right upper plate portion  44  in the up-down direction is at or below the position of the uppermost portion of the outer surface of the coil  13  in the up-down direction. Furthermore, the left face  44 A of the right upper plate portion  44  is perpendicular to the central axis B of the magnetic wire  12  and extends above the magnetic wire  12  in the forward-backward direction. 
     (Other Yoke Configurations) 
     As shown in  FIG. 6 , in the left yoke piece  31 , the coupling portion  35  couples the left front plate portion  32  and the left plate portion  33 . Further, the coupling portion  36  couples the left plate portion  33  and the left upper plate portion  34 . In addition, in the right yoke piece  41 , the coupling portion  45  couples the right front plate portion  42  and the right plate portion  43 . In addition, the coupling portion  46  couples the right plate portion  43  and the right upper plate portion  44 . Each coupling portion  35 ,  36 ,  45 ,  46  is formed in a gently curved rounded shape. 
     In addition, as shown in  FIG. 8 , in the left yoke piece  31 , a notch  37  used for adjusting the strength of the magnetic field acting on the magnetic wire  12  is formed at the bottom of the right end side of the left front plate portion  32 . Further, a notch  38  used for passing a connecting member  25  therethrough is formed at the bottom of the section extending from the left end of the left front plate portion  32  to the coupling portion  35 . In a similar manner, notches  47 ,  48  are also formed in the right yoke piece  41 . 
     In addition, as shown in  FIG. 6 , in the left yoke piece  31 , support pieces  39  used for securing the left yoke piece  31  to the base  2  are provided at the bottom of the left front plate portion  32  and at the bottom of the left plate portion  33 . In a similar manner, support pieces  49  are also provided in the right yoke piece  41 . The left yoke piece  31  and right yoke piece  41  are secured to the base  2  by inserting each support piece  39 ,  49  into apertures formed in the base  2  and, for example, soldering them in place. 
     (Magnetic Action of Yokes) 
       FIG. 13  diagrammatically illustrates the flow of the magnetic flux controlled by the yokes  30 .  FIG. 14  diagrammatically illustrates the flow of the magnetic flux controlled by the left plate portion  33  and left upper plate portion  34  of the left yoke piece  31  as well as the right plate portion  43  and right upper plate portion  44  of the right yoke piece  41 . 
     When the magnet  3  rotates along with the rotary shaft  83 , an N pole of the magnet  3  approaches the left end portion of one magnetic sensor  11  while an S pole of the magnet  3  approaches the right end portion of said magnetic sensor  11 , and said magnetic sensor  11  is thereby placed in the magnetic field formed by the N poles and S poles of the magnet  3 . At such time, as shown in  FIG. 13  and  FIG. 14 , the path of the magnetic flux from the N poles to the S poles of the magnet  3  is controlled by the yoke  30  partially covering said magnetic sensor  11 . This makes it possible to focus the magnetic flux on the magnetic wire  12  of said magnetic sensor  11 . 
     To consider this in more detail, the left front plate portion  32  extends from the front of the left-side connecting member retaining portion  16  of the bobbin  14  to a location in very close proximity to the front of the center of the coil  13  in the left-right direction and, in addition, the right front plate portion  42  extends from the front of the right-side connecting member retaining portion  17  of the bobbin  14  to a location in very close proximity to the front of the center of the coil  13  in the left-right direction, and the right end of the left front plate portion  32  and the left end of the right front plate portion  42  are mutually opposed across a small gap G. This configuration makes it possible to focus the magnetic flux on the central portion of the magnetic wire  12  in the left-right direction. 
     In addition, the left front plate portion  32  is inclined with respect to the central axis B of the magnetic wire  12  such that the right end of said left front plate portion  32  is located rearwardly of the left end of said left front plate portion  32 , as a result of which the left front plate portion  32  approaches the central portion of the magnetic wire  12  in the left-right direction as one moves to the right. In a similar manner, the right front plate portion  42  is inclined with respect to the central axis B of the magnetic wire  12  such that the left end of said right front plate portion  42  is located rearwardly of the right end of said right front plate portion  42 , as a result of which the right front plate portion  42  approaches the central portion of the magnetic wire  12  in the left-right direction as one moves to the left. This configuration makes it possible to increase the degree to which magnetic flux is focused on the central portion of the magnetic wire  12  in the left-right direction. 
     In addition, the right face  34 A of the left upper plate portion  34  and the left face  44 A of the right upper plate portion  44  are perpendicular to the central axis B of the magnetic wire  12  and extend above the magnetic wire  12  in the forward-backward direction. The magnetic flux has the property of exiting the faces of the yoke in a direction perpendicular to said faces as well as the property of entering the faces of the yoke in a direction perpendicular to said faces. Therefore, some of the magnetic flux in the magnetic field formed by the magnet  3  exits the right face  34 A of the left upper plate portion  34  in a direction perpendicular to said right face  34 A and enters the left face  44 A of the right upper plate portion  44  in a direction perpendicular to said left face  44 A. This makes it possible to increase the magnetic flux flowing along the direction of the central axis B of the magnetic wire  12 . Furthermore, since the right face  34 A of the left upper plate portion  34  and the left face  44 A of the right upper plate portion  44  extend above the magnetic wire  12  in the forward-backward direction, the magnetic flux flowing along the direction of the central axis B of the magnetic wire  12  can be further increased. 
     In addition, since the left plate portion  33  is in the vicinity of the left end portion of the magnetic wire  12  and the right plate portion  43  is in the vicinity of the right end side of the magnetic wire  12 , the magnetic flux is collected by the left plate portion  33 , and the magnetic flux toward the right plate portion  43  can be focused on the magnetic wire  12  and the magnetic flux flowing along the direction of the central axis B of the magnetic wire  12  can be increased. 
     It should be noted that when the magnet  3  rotates along with the rotary shaft  83 , an S pole of the magnet  3  approaches the left end portion of said magnetic sensor  11  while an N pole of the magnet  3  approaches the right end portion of said magnetic sensor  11 , and said magnetic sensor  11  is thereby placed in a magnetic field reversed in direction from when an N pole of the magnet  3  approaches the left end portion of said magnetic sensor  11  and an S pole of the magnet  3  approaches the right end portion of the magnetic sensor  11 . In such a case, the direction of the flow of the magnetic flux along the path illustrated in  FIG. 13  and  FIG. 14  is reversed. 
     In this manner, the magnetic flux of the magnetic field formed by the magnet  3  is focused on the magnetic wire  12  by the yoke  30 , and the magnetic flux is caused to flow along the direction of the central axis B of the magnetic wire  12 , thereby making it possible to increase the strength of the magnetic field acting on the magnetic wire  12 . As a result, due to the reversal of the direction of magnetization of the magnetic wire  12 , the height (voltage) of the pulse output from the coil  13  can be increased and, in addition, the pulse can be made sharper. Therefore, the accuracy in sensing the rotation of the rotary shaft  83  can be improved. 
     In addition, focusing the magnetic flux of the magnetic field formed by the magnet  3  on the central portion of the magnetic wire  12  in the left-right direction with the help of the left front plate portion  32  and right front plate portion  42  of the yoke  30  makes it possible to increase the strength of the magnetic field acting on the central portion of the magnetic wire  12  in the left-right direction. This makes it possible to equalize the strength of the magnetic field acting on both end sections of the magnetic wire  12  and the strength of the magnetic field acting on the central section of the magnetic wire  12  in the left-right direction. As a result, due to the reversal of the direction of magnetization of the magnetic wire  12 , the height of the pulse output from the coil  13  can be further increased, and, in addition, the pulse can be made even sharper. Therefore, the accuracy in sensing the rotation of the rotary shaft  83  can be further improved. 
     (Dummy Yoke Pieces) 
     As shown in  FIG. 2 , when the mounting face  2 B of the base  2  is viewed from above, one dummy yoke piece  51  of the two dummy yoke pieces  51 ,  52  has the same configuration and shape as the left yoke piece  31  of the yoke  30 . Dummy yoke piece  51  is arranged on the clockwise side of the magnetic sensor  11  arranged on the most clockwise side among the three magnetic sensors  11 . Further, another dummy yoke piece  52  has the same configuration and shape as the right yoke piece  41  of the yoke  30 . The dummy yoke piece  52  is arranged on the counterclockwise side of the magnetic sensor  11  arranged on the most counterclockwise side among the three magnetic sensors  11 . In addition, there are neither magnetic sensors corresponding to the dummy yoke piece  51  nor magnetic sensors corresponding to the dummy yoke piece  52  on the mounting face  2 B of the base  2 . 
     To illustrate this in greater detail, the three magnetic sensors  11  are arranged on the mounting face  2 B of the base  2  at 60-degree spacing on the circular arc C 1  centered on the rotational axis A. Further, as can be appreciated by examining  FIG. 2 , the three yokes  30  corresponding to the three magnetic sensors  11  on a one-to-one basis are arranged at 60-degree spacing on the circular arc C 1  on the mounting face  2 B of the base  2 . In addition, since the spacing at which the left yoke piece  31  and the right yoke piece  41  are arranged in each yoke  30  is the same in all three yokes  30 , the three left yoke pieces  41  disposed on the mounting face  2 B of the base  2  are arranged on the circular arc C 1  at 60-degree spacing and, in addition, the three right yoke pieces  41  disposed on the mounting face  2 B of the base  2  are arranged on the circular arc C 1  at 60-degree spacing. 
     The dummy yoke piece  51 , which has the same configuration and shape as the left yoke piece  31 , is arranged on the circular arc C 1  on the mounting face  2 B of the base  2  and is arranged on the clockwise side of the left yoke piece  31  arranged on the most clockwise side among the three left yoke pieces  31  while being spaced 60 degrees from said left yoke piece  31 . In other words, the three left yoke pieces  31  and the dummy yoke piece  51  are arranged on the same circular arc C 1  with equal angular spacing. 
     In addition, the dummy yoke piece  52 , which has the same configuration and shape as the right yoke piece  41 , is arranged on the circular arc C 1  on the mounting face  2 B of the base  2  and is arranged on the counterclockwise side of the right yoke piece  41  arranged on the most counterclockwise side among the three right yoke pieces  41  while being spaced 60 degrees from said right yoke piece  41 . In other words, the three right yoke pieces  41  and the dummy yoke piece  52  are arranged on the same circular arc C 1  with equal angular spacing. 
     In this manner, as a result of arranging the dummy yoke pieces  51 ,  52  on the mounting face  2 B, the left yoke piece  31  of the intermediate yoke  30  and the right yoke piece  41  of the yoke  30  on the most counterclockwise side are disposed adjacent each other on the counterclockwise side of the intermediate magnetic sensor  11  among the three magnetic sensors  11 , and the right yoke piece  41  of the intermediate yoke  30  and the left yoke piece  31  of the yoke  30  on the most clockwise side are disposed adjacent each other on the clockwise side of this intermediate magnetic sensor  11 . In addition, the left yoke piece  31  of the yoke  30  on the most clockwise side and the right yoke piece  41  of the intermediate yoke  30  are disposed adjacent each other on the counterclockwise side of the magnetic sensor  11  on the most clockwise side among the three magnetic sensors  11 , and the right yoke piece  41  of the yoke  30  on the most clockwise side and the dummy yoke piece  51  are disposed adjacent each other on the clockwise side of the magnetic sensor  11  on the most counterclockwise side. Further, the left yoke piece  31  of the yoke  30  on the most counterclockwise side and the dummy yoke piece  52  are disposed adjacent each other on the counterclockwise side of the magnetic sensor  11  on the most counterclockwise side among the three magnetic sensors  11 , and the right yoke piece  41  of the yoke  30  on the most counterclockwise side and the left yoke piece  31  of the intermediate yoke  30  are disposed adjacent each other on the clockwise side of the magnetic sensor  11  on the most counterclockwise side. As a result of the above, the configuration of the yoke pieces arranged around each magnetic sensor  11  is identical for all three magnetic sensors  11 . Therefore, the magnetic circuits formed by the yoke pieces around each magnetic sensor  11  are identical for all three magnetic sensors  11 . This makes it possible to equalize the strength of the rotating magnetic field acting on the magnetic wire  12  of each magnetic sensor  11  among the three magnetic sensors  11  when the three magnetic sensors  11  are placed in a rotating magnetic field. Therefore, the accuracy in sensing the rotation of the rotary shaft  83  can be improved. 
     As described above, in the rotation sensing device  1  according to the embodiment of the present invention, the left yoke piece  31  of each yoke  30  has a left front plate portion  32 , and the left front plate portion  32  extends linearly from the front of the left-side connecting member retaining portion  16  of the bobbin  14  to the front of the left portion of the coil  13  while also being inclined with respect to the central axis B of the magnetic wire  12  such that the right end of said left front plate portion  32  is located rearwardly of the left end of said left front plate portion  32 . In addition, the right yoke piece  41  of each yoke  30  has a right front plate portion  42 , and the right front plate portion  42  extends linearly from the front of the right-side connecting member retaining portion  17  of the bobbin  14  to the front of the right portion of the coil  13  while also being inclined with respect to the central axis B of the magnetic wire  12  such that the left end of said right front plate portion  42  is located rearwardly of the right end of said right front plate portion  42 . With this configuration, as described above, when the N poles and S poles of the magnet  3  approach the magnetic sensor  11 , for example, as shown in  FIG. 13 , the magnetic flux of the magnetic field formed by the magnet  3  can be focused on the central portion of the magnetic wire  12  of each magnetic sensor  11  in the left-right direction, and the accuracy in sensing the rotation of the rotary shaft  83  can be improved. 
     In addition, as shown in  FIG. 10 , with this configuration, the left front plate portion  32  and right front plate portion  42  can be arranged so as to be generally in line with circular arc C 2  centered on rotational axis A. For this reason, the distance between the left end of the left front plate portion  32  and the outer peripheral surface of the magnet  3 , the distance between the right end of the left front plate portion  32  and the outer peripheral surface of the magnet  3 , the distance between the right end of the right front plate portion  42  and the outer peripheral surface of the magnet  3 , and the distance between the left end of the right front plate portion  42  and the outer peripheral surface of the magnet  3  can be made equal to one another. As a result, by shortening the spacing between the magnet  3  and each magnetic sensor  11 , the rotation sensing device  1  can be made more compact and false detection of rotation of the rotary shaft  83  can be minimized. In specific terms, if the left front plate portion  32  were to extend in parallel to the central axis B of the magnetic wire  12 , the distance between the right end of the left front plate portion  32  and the outer peripheral surface of the magnet  3  would be smaller than the distance between the left end of the left front plate portion  32  and the outer peripheral surface of the magnet  3 . In this case, for example, not only when an N pole of the magnet  3  approaches the left end of the left front plate portion  32  (i.e., when an N pole of the magnet  3  approaches the left end portion of the magnetic sensor  11 ), but also when an N pole of the magnet  3  approaches the right end of the left front plate portion  32  (i.e., when an N pole of the magnet  3  approaches the vicinity of the center of the magnetic sensor  11  in the left-right direction), some of the magnetic flux in the magnetic field formed by the magnet  3  is controlled by the left front plate portion  32  of the left yoke piece  31  and ends up being focused on the magnetic wire  12  of the magnetic sensor  11 . In such a case, the direction of magnetization of the magnetic wire  12  of said magnetic sensor  11  is reversed not when the N pole of the magnet  3  approaches the left end portion of the magnetic sensor  11 , but when the N pole of the magnet  3  approaches the vicinity of the center of the magnetic sensor  11  in the left-right direction, and consequently a pulse is output from the coil  13  in a mistimed manner. Phenomena wherein pulses are output this way in a mistimed manner are likely to occur if the spacing between the magnet  3  and the magnetic sensor  11  is small. In the present embodiment, due to the fact that the left front plate portion  32  is inclined with respect to the central axis B of the magnetic wire  12  such that the right end of said left front plate portion  32  is located rearwardly of the left end of said left front plate portion  32 , phenomena wherein pulses are output in a mistimed manner can be minimized even if the spacing between the magnet  3  and the magnetic sensor  11  is shortened. The same applies to the right front plate portion  42 . 
     In addition, in the rotation sensing device  1  according to the embodiment of the present invention, the left yoke piece  31  of each yoke  30  has a left plate portion  33 , and the left plate portion  33  is inclined with respect to straight line L 1  orthogonal to both the rotational axis A and the central axis B of the magnetic wire  12  such that the rear end of said left plate portion  33  is located to the left of the front end of said left plate portion  33 . Further, the right yoke piece  41  of each yoke  30  has a right plate portion  43 , and the right plate portion  43  is inclined with respect to straight line L 1  such that the rear end of said right plate portion  43  is located to the right of the front end of said right plate portion  43 . This configuration makes it possible to extend the length of the magnetic wire  12  of each magnetic sensor  11 . In specific terms, inclining the left plate portion  33  with respect to straight line L 1  such that the rear end of said left plate portion  33  is located to the left of the front end of said left plate portion  33  makes it possible to separate the rear end of the left plate portion  33  from the left end of the bobbin  14 . Since a space appears between the rear end side of the left plate portion  33  and the left end of the bobbin  14  as a result, a protruding portion  19  can be provided at the left end of the bobbin  14  and the left end side of the wire receiving portion  15  can be extended to the protruding portion  19 . In a similar manner, since inclining the right plate portion  43  with respect to straight line L 1  such that the rear end of said right plate portion  43  is located to the right of the front end of said right plate portion  43  makes it possible to separate the rear end side of the right plate portion  43  from the right end of the bobbin  14 , a protruding portion  20  can be provided at the right end of the bobbin  14  and the right end side of the wire receiving portion  15  can be extended to the protruding portion  20 . Extending both ends of the wire receiving portion  15  in this manner makes it possible to extend the length of the wire receiving portion  15  and makes it possible to extend the maximum length of the magnetic wire  12  that can be received within the wire receiving portion  15 . 
     Further, in the rotation sensing device  1  according to the embodiment of the present invention, the angle of inclination Q of the left plate portion  33  with respect to straight line L 1  is set to one-half the angular spacing P used in the arrangement of the three magnetic sensors  11  and, in addition, the angle of inclination R of the right plate portion  43  with respect to straight line L 1  is set to one-half the angular spacing P used in the arrangement of the three magnetic sensors  11 . With this configuration, even if the length of the magnetic wire  12  of each magnetic sensor  11  is extended, the spacing between two magnetic sensors  11  adjacent to each other can be shortened and the rotation sensing device  1  can be made more compact. In specific terms, as shown in  FIG. 10 , as a result of setting angle of inclination Q and angle of inclination R respectively to one-half the angular spacing P, in two yokes  30  adjacent to each other in the circumferential direction, the right plate portion  43  of the right yoke piece  41  of the yoke  30  on the counterclockwise side and the left plate portion  33  of the left yoke piece  31  of the yoke  30  on the clockwise side can be made parallel to each other. By making the mutually adjacent right plate portion  43  and left plate portion  33  parallel to each other in this manner, the spacing between the right plate portion  43  and the left plate portion  33  can be shortened, and the spacing between two magnetic sensors  11  adjacent to each other can also be shortened. In other words, even though the maximum length of the magnetic wire  12  that can be received within the wire receiving portion  15  is extended by inclining the right plate portion  43  and the left plate portion  33 , respectively, with respect to straight line L 1 , placing the mutually adjacent right plate portion  43  and left plate portion  33  parallel to each other makes it possible to shorten the spacing between two magnetic sensors  11  adjacent to each other. 
     In addition, in the rotation sensing device  1  according to the embodiment of the present invention, an inclined face  21  is formed in the front portion on the left edge face of the left-side connecting member retaining portion  16  of the bobbin  14 , and the angle of inclination of this inclined face  21  with respect to the straight line L 1  is set to one-half the angular spacing P used in the arrangement of the three magnetic sensors  11 . This makes it possible to make the left plate portion  33  of the left yoke piece  31  parallel to the front portion on the left edge face of the left-side connecting member retaining portion  16  of the bobbin  14  and reduce the distance between the left plate portion  33  and the front portion on the left edge face of the left-side connecting member retaining portion  16  of the bobbin  14 . In addition, an inclined face  23  is formed in the front portion on the right edge face of the right-side connecting member retaining portion  17  of the bobbin  14 , and the angle of inclination of the inclined face  23  with respect to straight line L 1  is set to one-half the angular spacing P used in the arrangement of the three magnetic sensors  11 . This makes it possible to make the right plate portion  43  of the right yoke piece  41  parallel to the front portion on the right edge face of the right-side connecting member retaining portion  17  of the bobbin  14  and reduce the distance between the right plate portion  43  and the front portion on the right edge face of the right-side connecting member retaining portion  17  of the bobbin  14 . 
     Further, in the rotation sensing device  1  according to the embodiment of the present invention, the left plate portion  33  covers only the inclined face  21  of the front portion on the left edge face of the left-side connecting member retaining portion  16  of the bobbin  14  from the left, and the right plate portion  43  covers only the inclined face  23  of the front portion on the right edge face of the right-side connecting member retaining portion  17  of the bobbin  14  from the right. With this configuration, even if the spacing between two magnetic sensors  11  adjacent to each other is shortened, the length of the magnetic wire  12  of each magnetic sensor  11  can be further extended. In specific terms, as shown in  FIG. 9 , the rear end of the left plate portion  33  is located in front of the protruding portion  19 . As a result, the protruding portion  19  can be made to protrude to the left to a significant extent and the length of the protruding portion  19  in left-right direction can be extended. In addition, the longer the protruding portion  19  is made in the left-right direction, the farther the left end side of the wire receiving portion  15  can be extended. In a similar manner, the rear end of the right plate portion  43  is located in front of the protruding portion  20 . As a result, the protruding portion  20  can be made to protrude to the right to a significant extent and the length of the protruding portion  20  in the left-right direction can be extended, and the longer the protruding portion  20  is made in the left-right direction, the farther the right end side of the wire receiving portion  15  can be extended. Extending both ends of the wire receiving portion  15  in this manner makes it possible to extend the maximum length of the magnetic wire  12  that can be received within the wire receiving portion  15 . In addition, due to the fact that the rear end of the left plate portion  33  is located in front of the protruding portion  19  and the rear end of the right plate portion  43  is located in front of the protruding portion  20 , in two magnetic sensors  11  adjacent to each other in the circumferential direction, neither the right plate portion  43  nor the left plate portion  33  is interposed between the right end of the protruding portion  20  of the bobbin  14  of the magnetic sensor  11  on the counterclockwise side and the left end of the protruding portion  19  of the bobbin  14  of the magnetic sensor  11  on the clockwise side. Consequently, the right end of the protruding portion  20  and the left end of the protruding portion  19  of these two magnetic sensors  11  can be brought closer to each other, thereby making it possible to shorten the spacing between the two magnetic sensors  11  adjacent to each other and the rotation sensing device  1  can be made more compact. 
     Further, in the rotation sensing device  1  according to the embodiment of the present invention, the left yoke piece  31  of each yoke  30  has a left upper plate portion  34 , and the range covered by the left upper plate portion  34  remains above the left-side connecting member holding portion  16  and does not extend to above the coil  13 . In addition, the right yoke piece  41  of each yoke  30  has a right upper plate portion  44  and the range covered by the right upper plate portion  44  remains above the right-side connecting member retaining portion  17  and does not extend to above the coil  13 . In other words, neither the left upper plate portion  34  nor the right upper plate portion  44  enters the region above the coil  13 , and the coil  13  is not covered by the yoke  30  from above. Along with making it possible to increase the amount of windings of the coil  13 , this configuration makes it possible to bring the left upper plate portion  34  and the right upper plate portion  44  respectively closer to the left end portion and the right end portion of the magnetic wire  12 . In specific terms, when the amount of the conductor wire of the coil  13  wound upon the conductor winding portion  18  of the bobbin  14  is increased, the diameter of the coil  13  is expanded. Since increasing the amount of windings of the conductor wire of the coil  13  could create the risk that the outer surface of the coil  13  might make contact with the left upper plate portion  34  or the right upper plate portion  44 , if the left upper plate portion  34  or the right upper plate portion  44  were to cover the region above the coil  13 , the amount of windings of the conductor wire of the coil  13  is restricted in order to avoid this. In the present embodiment, neither the left upper plate portion  34  nor the right upper plate portion  44  reach above the coil  13 , and, as a result, the coil  13  is not covered by the yoke  30  from above. Consequently, even if the amount of windings of the conductor wire of the coil  13  is increased, the outer surface of the coil  13  will not make contact with the left upper plate portion  34  or the right upper plate portion  44 . Therefore, the amount of windings of the coil  13  can be increased as long as the outer surface of the coil  13  does not make contact with the right end of the left front plate portion  32  or the left end of the right front plate portion  42 . Increasing the amount of windings of the conductor wire of the coil  13  makes it possible to increase the height (voltage) of the pulse output from the coil  13  as a result of reversal of the direction of magnetization of the magnetic wire  12 . Therefore, the accuracy in sensing the rotation of the rotary shaft  83  can be improved. In addition, since neither the left upper plate portion  34  nor the right upper plate portion  44  enters the region above the coil  13 , even if the amount of windings of the coil  13  is increased, as shown in  FIG. 11 , the position of the bottom faces of the left upper plate portion  34  and the right upper plate portion  44  in the up-down direction can be at or below the position of the uppermost portion of the outer surface of the coil  13 . Therefore, the left upper plate portion  34  and the right upper plate portion  44  can be brought closer respectively to the left end portion and the right end portion of the magnetic wire  12 . This makes it possible to increase the extent to which the magnetic flux of the magnetic field formed by the magnet  3  is focused on the magnetic wire  12  when the N poles and S poles of the magnet  3  approach the magnetic sensor  11 , for example, as shown in  FIG. 13 . 
     In addition, in the rotation sensing device  1  according to the embodiment of the present invention, the right face  34 A of the left upper plate portion  34  of the left yoke piece  31  and the left face  44 A of the right upper plate portion  44  of the right yoke piece  41  are perpendicular to the central axis B of the magnetic wire  12  and extend above the magnetic wire  12  in the forward-backward direction. When the N poles and S poles of the magnet  3  approach the magnetic sensor  11 , for example, as shown in  FIG. 13 , this configuration, as described above, makes it possible to increase the magnetic flux flowing along the direction of the central axis B of the magnetic wire  12 . Therefore, the accuracy in sensing the rotation of the rotary shaft  83  can be increased. 
     In addition, in the rotation sensing device  1  according to the embodiment of the present invention, the dummy yoke pieces  51 ,  52  can equalize the strength of the rotating magnetic field acting on the magnetic wire  12  of each magnetic sensor  11  in all three magnetic sensors  11 . Therefore, the accuracy in sensing the rotation of the rotary shaft  83  can be improved. 
     It should be noted that, while the magnet  3  used in the above embodiment has two pairs of magnetic poles consisting of an N pole, an S pole, an N pole and an S pole, in the present invention, the magnet may have 1 pair or 3 pairs or more of magnetic poles. In addition, the magnet  3  may be cut into quarters and a rotating magnetic field may be formed using four magnets  3 . Further, while the rotation sensing device  1  of the above embodiment comprises three magnetic sensors  11  and three yokes  30 , in the present invention, the number of the magnetic sensors  11  and yokes  30  may be 2, 4 or more. In addition, the present invention can be used to sense the rotation of objects other than rotary shafts of motors. In addition, while the embodiment described above used an example in which the magnet  3  rotates relative to the magnetic sensors  11  and the yokes  30 , it is possible to use a configuration in which the magnetic sensors  11  and the yokes  30  rotate relative to the magnet  3 . 
     In addition, in the present invention, appropriate modifications can be made without departing from the gist or idea of the invention that can be read from the claims and the specification as a whole, and rotation sensing devices resulting from such modifications are also included in the technical concept of the present invention. 
     DESCRIPTION OF THE REFERENCE NUMERALS 
     
         
           1  Rotation sensing device 
           2  Base 
           2 B Mounting face (one face) 
           3  Magnet (magnetic field forming portion) 
           11  Magnetic sensor (magnetic field sensing portion) 
           12  Magnetic wire 
           13  Coil 
           14  Bobbin 
           15  Wire receiving portion 
           18  Conductor winding portion 
           30  Yoke 
           31  Left yoke piece 
           32  Left front plate portion 
           33  Left plate portion 
           34  Left upper plate portion 
           34 A Right face 
           41  Right yoke piece 
           42  Right front plate portion 
           43  Right plate portion 
           44  Right upper plate portion 
           44 A Left face 
           51  Dummy yoke piece (first dummy yoke piece) 
           52  Dummy yoke piece (second dummy yoke piece) 
           81  Motor (structure) 
           82  Motor main body (supporting portion) 
           83  Rotary shaft (rotating portion)