Patent Publication Number: US-2023150571-A1

Title: Stroke sensor

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present patent application claims the priority of Japanese patent application No. 2021-187359 filed on Nov. 17, 2021, and the entire contents thereof are hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present invention relates to a stroke sensor. 
     BACKGROUND ART 
     In recent years, the application of steer-by-wire to vehicles is progressing. In the steer-by-wire, unlike the conventional steering mechanism, steered wheels and a steering mechanism are not mechanically connected, while the steered wheels and the steering mechanism are electrically connected. Therefore, the application of steer-by-wire is characterized in that the degree of freedom in the design of the interior of the vehicle can be increased, the weight of the steering mechanism can be reduced, and the steered wheels would not directly receive the road surface reaction force from the wheels. 
     In addition, there exists Patent Literature 1 as prior art document information relevant to the invention of this application. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: WO2021/210125 
     SUMMARY OF THE INVENTION 
     The above steer-by-wire requires highly accurate control in order to reproduce the fine steering and turning that humans have performed, and it is required to acquire a turning angle (i.e., a steering angle) with high accuracy. In order to obtain the turning angle with high accuracy, it is required to obtain the displacement (i.e., a stroke position) of a rack shaft in an axial direction with high accuracy. That is, there is a demand for a stroke sensor that can detect the stroke position of the rack shaft with high accuracy. 
     Also, the stroke sensor is housed inside a housing that houses the rack shaft. The space inside the housing that can accommodate the stroke sensor is limited, and a compact stroke sensor is required. 
     Accordingly, it is an object of the present invention to provide a compact stroke sensor capable of detecting a stroke position with high accuracy. 
     To solve the aforementioned problems, one aspect of the present invention provides a stroke sensor configured to detect a stroke position of a rod-shaped measuring object that strokes in an axial direction, comprising:
         two disk-shaped rotors configured to rotate along with a stroke of the measuring object;   a rotation detecting unit configured to detect rotations of the two rotors, respectively; and   a stroke position detecting unit configured to detect the stroke position of the measuring object based on the rotations of the two rotors detected by the rotation detecting unit,   wherein at least one of the two rotors is provided in direct contact with the measuring object,   wherein the two rotors are provided side by side in an arrangement direction perpendicular to an axial direction of the measuring object and are provided so as to be adjacent to the measuring object in an arrangement perpendicular direction perpendicular to the axial direction and the arrangement direction,   wherein each of the two rotors is provided in such a manner that a rotation axis direction of each of the two rotors is inclined with respect to the arrangement direction.       

     Effects of the Invention 
     According to the present invention, it is possible to provide a compact stroke sensor capable of detecting a stroke position with high accuracy. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1 A  is a schematic diagram of a steering device equipped with a stroke sensor according to the present embodiment. 
         FIG.  1 B  is a cross-sectional view of the stroke sensor in  FIG.  1 A  taken along a line A-A. 
         FIG.  2    is a plan view of a rotor and a rotor-side board. 
         FIG.  3    is a circuitry diagram showing an example of a detection circuit. 
         FIG.  4    is a diagram showing an example of a support structure for the rotor using a support member. 
         FIGS.  5 A and  5 B  are schematic diagrams showing modified examples of the stroke sensor shown in  FIG.  1 B . 
         FIGS.  6 A and  6 B  are plan views showing modified examples of the rotor and its conductor pattern. 
         FIG.  7    is a schematic diagram showing a modified example of the stroke sensor of  FIG.  1 B . 
         FIG.  8    is a schematic diagram showing a stroke sensor according to another embodiment of the present invention. 
         FIG.  9    is a plan view of a rotor and a rotor-side board. 
         FIGS.  10 A and  10 B  are schematic diagrams showing modified examples of the stroke sensor of  FIG.  8   . 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Embodiments 
     Embodiments of the present invention will be described below with reference to the accompanying drawings. 
     (Steering Device  10 ) 
       FIG.  1 A  is a schematic diagram of a steering device  10  equipped with a stroke sensor  1  according to the present embodiment, and  FIG.  1 B  is a cross-sectional view thereof taken along a line A-A. 
     As shown in  FIG.  1 A , the steering device  10  includes a tie rod  12  connected to a rolling wheel  11  (such as a front wheel of a vehicle), a rack shaft  13  connected to the tie rod  12 , a housing  14  that houses the rack shaft  13 , a motor  15  for driving the rack shaft  13 , and a steering mechanism  16  including steering wheels and the like. In this steering device  10 , the motor  15  is driven in accordance with the steering operation by the steering mechanism  16 , and the rack shaft  13  is stroked in an axial direction (i.e., a horizontal direction in the drawing), thereby rolling the rolling wheel  11  to perform the steering operation. The rack shaft  13  has a toothed portion (rack)  13   a  in which teeth are formed at equal intervals along the axial direction, and a pinion  15   a  driven by the motor  15  meshes with the toothed portion  13   a  so that a rack and pinion mechanism is configured. 
     As shown in  FIG.  1 B , the rack shaft  13  has a partially notched shape in a cross-section perpendicular to the axial direction, and its outer peripheral surface is composed of a linear portion  13   b  and an arc portion  13   c . The toothed portion  13   a  is formed on a flat portion of an outer peripheral surface of the rack shaft  13  corresponding to the linear portion  13   b . The rack shaft  13  is a member whose stroke position is to be detected by the stroke sensor  1  according to the present embodiment, and corresponds to a member to be measured (i.e., measuring object) in the present invention. 
     (Stroke Sensor  1 ) 
     The stroke sensor  1  is a sensor that detects the stroke position (displacement) of the rack shaft  13  as a rod-shaped measuring object which strokes in the axial direction. The stroke sensor  1  includes two rotors (i.e., rotating bodies)  2 , a rotation detector (i.e., rotation detecting unit)  3 , a stroke position detector (i.e., stroke position detecting unit)  4 , and a support member  5 . 
     (Rotor  2 ) 
     The rotor  2  is a member that rotates along with the stroke of the rack shaft  13 , which is the measuring object and is formed in a disc shape. In the present embodiment, two rotors  2  composed of a first rotor  21 , and a second rotor  22  are used. The stroke sensor  1  converts the stroke of the rack shaft  13  into the rotations of the first and second rotors  21  and  22 , and detects the stroke position of the rack shaft  13  based on the rotations of the first and second rotors  21  and  22 . 
     The first and second rotors  21  and  22  are provided in such a manner that their rotation axes are perpendicular to the stroke direction, that is, the axial direction of the rack shaft  13 , so as to rotate along with the stroke of the rack shaft  13 . 
     In the stroke sensor  1 , at least one of the two rotors  2  is provided in direct contact with the rack shaft  13  in order to detect the stroke position of the rack shaft  13  with high accuracy. In the present embodiment, both the first and second rotors  21  and  22  are in direct contact with the rack shaft  13 . 
     More specifically, in the present embodiment, both the first and second rotors  21  and  22  are gears that directly mesh with the toothed portion  13   a  of the rack shaft  13 . As described above, the toothed portion  13   a  of the rack shaft  13  is used to stroke the rack shaft  13  by the motor  15 . In the present embodiment, the toothed portion  13   a  is also used to detect the stroke position. 
     In the stroke sensor  1  according to the present embodiment, the two rotors  21  and  22  are arranged side by side in the arrangement direction perpendicular to the axial direction of the rack shaft  13  and arranged adjacent to the rack shaft  13  in the arrangement perpendicular direction, which is perpendicular to the axial direction and the arrangement direction. When viewed from the axial direction, each of the two rotors  21  and  22  is provided in such a manner that the direction of its rotation axis is inclined with respect to the arrangement direction. 
     When defining an orthogonal coordinate system consisting of the X-, Y-, and Z-axes, the Z-axis direction corresponds to the “axis direction”, the Y-axis direction to the “arrangement direction”, and the X-axis direction to the “arrangement perpendicular direction”. That is, in the stroke sensor  1 , when the axial direction of the rack shaft  13  is the Z-axis direction, both rotors  21  and  22  are arranged side by side in the Y-axis direction, both rotors  21  and  22  and the rack shaft  13  are adjacent to each other in the Y-axis direction, and the rotation axis directions of both rotors  21  and  22  are inclined with respect to the Y-axis direction. 
     As a result, it is possible to suppress the rotors  21  and  22  from projecting radially outward with respect to the rack shaft  13 , and to reduce the size of the stroke sensor  1  as a whole. Although the space in the housing  14  of the steering device  10  is very small, even in such a limited space in the housing  14 , the small-sized steering device  10  that can be compactly accommodated on one side of the rack shaft  13  can be achieved. 
     Since both rotors  21  and  22  are inclined with respect to the toothed portion  13   a  of the rack shaft  13 , it is preferable to use bevel gears each having a conical toothed surface as the rotors  21  and  22 . 
     In the present embodiment, both rotors  21  and  22  are arranged in such a manner that their rotation axes intersect each other. Although the details will be described later, in the present embodiment, the rotations of the rotors  21  and  22  are detected using magnetism. Therefore, so as to avoid the interference of magnetism, both rotors  21  and  22  are arranged in such a manner that their rotation axes are orthogonal to each other (straight lines along the rotation axes are orthogonal to each other). The details of this point will be described later. However, a slight error in the angle formed by the rotation axes of both rotors  21  and  22  is allowed. Specifically, the angle formed by the rotation axes of both rotors  21  and  22  should be 80° or more and 100° or less. 
     Both rotors  21  and  22  are arranged in such a manner that their rotation axes are at the same position in the axial direction (X-axis direction) of the rack shaft  13  (namely, in such a manner that the rotation axes are aligned in the Y-axis direction). Further, the angles at which the rotation axes are inclined with respect to the arrangement direction (Y-axis direction) of both rotors  21  and  22  are the same angle (45°). Furthermore, when viewed from the axial direction of the rack shaft  13 , the rotors  21  and  22  are arranged to be inclined in such a manner that a distance between the rotor  21  and the rotor  22  gradually increases from a radially outer side to a radially inner side of the rack shaft  13 . Both rotors  21  and  22  are rotatably supported by the support member  5 . Details of the support member  5  will be described later. 
     Furthermore, in the present embodiment, the outer diameters (numbers of teeth) of the first rotor  21  and the second rotor  22  are made different from each other. Here, the outer diameter (number of teeth) of the first rotor  21  is larger than the outer diameter (number of teeth) of the second rotor  22 . The reason for this will be described later. 
     (Rotation Detector  3 ) 
     The rotation detector  3  detects rotations of the two rotors  21  and  22 , respectively. The rotation detector  3  has two rotor-side boards (i.e., rotary substrates)  31 , two detection coils  32 , and two detection circuits  33 . 
     The rotor-side boards  31  are integrally provided on end faces of the two rotors  21  and  22  in the rotation axis directions, respectively, and rotate together with the rotations of the corresponding rotors  21  and  22 . In the present embodiment, the disk-shaped rotor-side boards  31  are provided integrally with the rotors  21  and  22 , respectively, so as to be coaxial with the rotors  21  and  22  on the end faces of the rotors  21  and  22  in the rotation axis direction opposite to the rack shaft  13 . The rotor-side boards  31  are formed smaller in outer diameter than the corresponding rotors  21  and  22 , respectively (more specifically, smaller in outer diameter than the bottoms of the teeth). 
     As shown in  FIG.  2   , each of the rotor-side boards  31  has a conductor pattern  31   a  formed in a predetermined pattern along a circumferential direction of each of the rotors  21  and  22 . In the illustrated examples, the conductor pattern  31   a  is formed in such a manner that the thickness along the radial direction of each of the rotors  21  and  22  gradually changes along the circumferential direction of each of the rotors  21  and  22 . The conductor pattern  31   a  is formed in such a manner that the thickest portion and the thinnest portion face each other in the radial direction. In the present embodiment, since the outer diameters (numbers of teeth) of the first rotor  21  and the second rotor  22  are different from each other, the sizes of the rotor-side boards  31  and the conductor patterns  31   a  are also different from each other accordingly. However, the pattern structure itself of the conductor pattern  31   a  has the same pattern structure in the circumferential direction of each of the rotors  21  and  22 . 
     The two detection coils  32  are provided so as to face the corresponding rotor-side boards  31 , respectively. Both detection coils  32  are fixed so as not to rotate along with the rotations of the rotors  21  and  22 . When an AC voltage is applied to the detection coil  32 , an eddy current is generated in the conductor pattern  31   a  facing the detection coil  32  due to the magnetic field generated by the detection coil  32 . The inductance of the detection coil  32  changes due to the magnetic field generated by the eddy current generated in the conductor pattern  31   a . Since the change in the inductance of the detection coil  32  changes depending on the shape of the conductor pattern  31   a  (here, the thickness along the radial direction of each of the rotors  21  and  22 ), each of the rotation angles of the corresponding rotors  21  and  22  can be detected based on the change in the inductance of the detection coil  32 . 
     Each of the detection coils  32  is arranged in such a manner that the direction of the magnetic field generated by the detection coil  32  (magnetic field generation direction) is parallel to each of the rotation axes of the rotors  21  and  22 . Furthermore, in the present embodiment, the two detection coils  32  are arranged in such a manner that their magnetic field generation directions are orthogonal to each other (thus, the rotation axes of the rotors  21  and  22  parallel to the magnetic field generation directions of both detection coils  32  are also orthogonal to each other). As a result, it becomes possible to suppress the influence of the magnetic field generated by the detection coil  32  corresponding to one rotor  2  (for example, the first rotor  21 ) on the detection coil  32  corresponding to the other rotor  2  (for example, the second rotor  22 ), thereby improving the detection accuracy. In the present embodiment, one detection coil  32  is provided for each of the rotors  21  and  22 , but two or more detecting coils  32  may be provided for each of the rotors  21  and  22 . That is, the number of detection coils  32  provided for each of the rotors  21  and  22  is preferably one or more. 
     Each of the detection circuits  33  is a circuit that detects the rotation angle of each of the corresponding rotors  21  and  22  based on changes in the inductance of both detection coils  32  when AC voltage is applied to each of the detection coils  32 . In the present embodiment, the detection circuit  33  is configured to detect changes in the inductance of the detection coil  32  based on changes in the resonance frequency. 
       FIG.  3    is a circuitry diagram showing an example of the detection circuit  33 . As shown in  FIG.  3   , L e  is the inductance of the detection coil  32 , L e  is the inductance generated in the conductor pattern  31   a , R e  is the resistance of the conductor pattern  31   a , and M is the mutual inductance between the detection coil  32  and the conductor pattern  31   a.    
     The detection circuit  33  includes a resistor R s  connected in series with the detection coil  32 , a capacitive element C p  connected in parallel to the detection coil  32  and the resistor R s  connected in series, a resonance frequency detector (i.e., resonance frequency detecting unit)  33   b  for detecting the resonance frequency of a resonance circuit  33   a  composed of the detection coil  32 , the resistor R s , and the capacitive element C p , a rotation angle calculator (i.e., rotation angle calculating unit)  33   c  for obtaining rotation angles of the rotors  21  and  22  based on the resonance frequency detected by the resonance frequency detector  33   b.    
     In the circuit shown in  FIG.  3   , the resonance frequency f 0  of the resonance circuit  33   a  detected by the resonance frequency detector  33   b  of the detection circuit  33  can be expressed by the following formula. 
     
       
         
           
             
               
                 
                   
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     The rotation angle calculator  33   c  detects a change in the inductance Lc of the detection coil  32  according to the influence of the conductor pattern  31   a  facing the detection coil  32  based on the resonance frequency f 0  detected by the resonance frequency detector  33   b , to obtain the rotation angles of the rotors  21  and  22 , respectively. 
     As shown in  FIG.  1 B , the detection circuit  33  and the detection coil  32  are mounted on a fixed-side (i.e., stationery) circuit board  34 . The fixed-side circuit board  34  is provided facing the rotor-side board  31  and fixed to the housing  14  via the support member  5 . 
     (Stroke Position Detector  4 ) 
     The stroke position detector  4  calculates the stroke position of the rack shaft  13  based on the rotations (rotational angles) of the two rotors  21  and  22  detected by the detection circuit  33 . The stroke position detecting unit  4  is realized by appropriately combining an arithmetic element such as a CPU, a memory, software, an interface, and the like. 
     As described above, in the present embodiment, the outer diameters (the numbers of teeth) of the two rotors  21  and  22  are different from each other, and the rotation angles of the two rotors  21  and  22  when the rack shaft  13  is stroked are configured differently from each other. Therefore, it is possible to detect the stroke position with high accuracy based on the rotation angles of these two rotors  21  and  22 . Further, by appropriately differentiating a detection period (i.e., detection cycle) of each of the rotors  21  and  22  (i.e., a rotation cycle of the conductor pattern  31   a  corresponding to the number of teeth of each of the rotors  21  and  22 ), the stroke position can be detected even with a longer stroke length than the length of the outer periphery of each of the rotors  21  and  22 . Therefore, for example, even if relatively small rotors  21  and  22  are used, the stroke position of the rack shaft  13  can be accurately detected. 
     The stroke position detector  4  may be mounted, for example, on an electronic control unit of a vehicle. In this case, the fixed-side circuit board  34  on which the detection circuit  33  is mounted and the electronic control unit are appropriately connected by a cable or the like. Alternatively, the stroke position detector  4  may be configured separately from the electronic control unit of the vehicle and may be configured to output the detected stroke position of the rack shaft  13  to the electronic control unit. For example, the stroke position detector  4  may be mounted on the fixed-side circuit board  34  or may constitute a dedicated unit separate from the electronic control unit. 
     (Support Member  5 ) 
     The support member  5  is for supporting the rotors  21  and  22  and the fixed-side circuit board  34  and is preferably made of a non-magnetic material such as resin. The support member  5  integrally has a linear fixed portion  51  fixed to the housing  14  of the steering device  10  and a pair of arm portions  52  extending from both ends of the fixed portion  51  toward the rack shaft  13 . 
     The fixed portion  51  is fixed to the housing  14  by a fixing member  53  such as a bolt. Each arm portion  52  has a parallel portion  52   a  that extends vertically from the fixed portion  51 , and an inclined portion  52   b  that extends at an angle inward (toward the opposing arm portion  52 ) from a distal end portion (i.e., tip portion) of the parallel portion  52   a . Each of the rotors  21  and  22 , which are bevel gears, is attached to a distal end portion (i.e., tip portion) of the inclined portion  52   b  so as to be rotatable with respect to the inclined portion  52   b . The fixed-side circuit board  34  is fixed to the inclined portion  52   b  closer to the parallel portion  52   a  than each of the rotors  21  and  22 . 
       FIG.  4    is a diagram showing an example of a support structure for the rotor  2  by the support member  5 . As shown in  FIG.  4   , the distal end portion of the arm portion  52  of the support member  5  is inserted through a through-hole  2   a  formed in the center of the rotor  2 , and a pair of flanges  52   c ,  52   c  are formed so as to sandwich the through-hole  2   a . The rotor  2  is rotatably supported by the support member  5  by the pair of flanges  52   c ,  52   c  interfering with the support member  5  on the periphery of the through-hole  2   a . The specific shape of the support member  5  is not limited to the illustrated one, and can be changed as appropriate according to the arrangement of each member, the shape of the housing  14 , and the like. In other words, the support member  5  should at least rotatably support the rotors  21  and  22 , support the fixed-side circuit board  34 , and be fixed to the housing  14 . Furthermore, the support member  5  that supports the rotors  21  and  22  and the support member  5  that supports the fixed-side circuit board  34  may be configured separately. 
     Modified Examples 
     In the present embodiment, both the first and second rotors  21  and  22  are gears that directly mesh with the toothed portion  13   a  of the rack shaft  13 . However, the present invention is not limited thereto, and at least one of the first and second rotors  21  and  22  should be a gear that directly meshes with the toothed portion  13   a  of the rack shaft  13 . For example, as shown in  FIG.  5 A , one rotor  2  (here, the first rotor  21 ) may be composed of a gear that directly meshes with the toothed portion  13   a , and the other rotor  2  (here, the second rotor  22 ) may be composed of a gear that meshes with one rotor  2  (here, the first rotor  21 ). Even though it is possible to interpose one or more gears between the first rotor  21  and the second rotor  22 , for example, such a configuration is not preferable, since it may lead to an increase in cost and increase the likelihood of failures. 
     Furthermore, as shown in  FIG.  5 B , it may be configured that only the detection coil  32  is provided on the fixed-side circuit board  34 , and the detection circuit  33  is collectively mounted on a common board  35  provided separately from the fixed-side circuit board  34 . The fixed-side circuit board  34  and the common board  35  are electrically connected by a wire  36 . In this case, the detection circuit  33  may be partially mounted on the fixed-side circuit board  34 . 
     Further, in the present embodiment, the two rotors  21  and  22  have different outer diameters (numbers of teeth), but the present invention is not limited thereto. As shown in  FIG.  6 A , the outer diameters (numbers of teeth) of the two rotors  21  and  22  may be the same. In other words, the two rotors  21  and  22  may be configured in such a manner that they rotate at the same angle when the rack shaft  13  is stroked. In this case, the conductor patterns  31   a ,  31   a  corresponding to the two rotors  21  and  22  are preferably designed to have different pattern configurations in the circumferential directions of the rotors  21  and  22  in such a manner that the two rotors  21  and  22  have different detection cycles. In the illustrated example, on one rotor  2  (first rotor  21 ), the conductor pattern  31   a  has one thickest position and one thinnest position at intervals of 180°. On the other rotor  2  (second rotor  22 ), the thickest positions and the thinnest positions are alternately formed every 90°. As a result, an effect equivalent to that obtained when the outer diameters (numbers of teeth) of the two rotors  21  and  22  are different can be obtained. 
     Furthermore, in the present embodiment, the thickness of the conductor pattern  31   a  is gradually changed along the circumferential direction of the rotor  2 . However, the configuration of the conductor pattern  31   a  is not limited thereto. As shown in  FIG.  6 B , the conductor pattern  31   a  may be configured to have a constant thickness and be formed at predetermined intervals in the circumferential direction of the rotor  2 . 
     Further, in the present embodiment, the case where a part of the rack shaft  13  is cut (notched) and the toothed portion  13   a  is formed therein is described. However, as shown in  FIG.  7   , a cross-sectional shape perpendicular to the axial direction of the rack shaft  13  may be circular, and a toothed portion  13   a  having spiral teeth may be formed on the outer peripheral surface thereof. In this case, crown gears may be used as the two rotors  21  and  22 . 
     Functions and Effects of the Embodiment 
     As described above, the stroke sensor  1  according to the present embodiment comprises the two disk-shaped rotors  2  that rotate along with the stroke of the rack shaft  13 , which is the measuring object, the rotation detector  3  that detects the rotations of the two rotors  2  respectively, the stroke position detector  4  that detects the stroke position of the rack shaft  13  as the measuring object, based on the rotations of the two rotors  2  detected by the rotation detector  3 , in which at least one of the two rotors  2  is provided in direct contact with the rack shaft  13  as the measuring object, the two rotors  2  are arranged side by side in the arrangement direction perpendicular to the axial direction of the rack shaft  13  as the measuring object and arranged to be adjacent to the rack shaft  13  as the measuring object in the arrangement perpendicular direction which is perpendicular to the axial direction and the arrangement direction, and when viewed from the axial direction, each of the two rotors  2  is provided in such a manner that the rotation axis direction is inclined with respect to the arrangement direction. 
     By detecting the stroke position using the two rotors  2 , detection accuracy can be improved. Further, by providing the two rotors  2  to be inclined with respect to the arrangement direction (i.e., obliquely), the rotors  21 ,  22  are suppressed from protruding radially outward with respect to the rack shaft  13 , and the entire stroke sensor  1  can be miniaturized. That is, according to the present embodiment, it is possible to realize a compact stroke sensor  1  capable of detecting a stroke position with high accuracy. Further, in the stroke sensor  1 , at least one of the rotors  2  is brought into direct contact with the rack shaft  13 , and the stroke (displacement) of the rack shaft  13  is directly obtained so the stroke position can be detected with high precision. 
     Further, in the present embodiment, the rotation of the rotor  2  is detected by a method using magnetism by the detection coil  32  and the conductor pattern  31   a . Therefore, the stroke position can be accurately detected without being affected by grease or the like in the housing  14  of the steering device  10 . 
     Another Embodiment 
     The stroke sensor  1   a  shown in  FIG.  8    has basically the same configuration as the stroke sensor  1  shown in  FIG.  1 B , except the configuration of the rotation detector  3 . In the stroke sensor  1   a , the rotation detector  3  includes two magnets  61  integrally provided with the two rotors  21  and  22 , respectively, and magnetic detection elements  62 , each of which is provided so as not to rotate along with the rotation of the corresponding one of the rotors  21  and  22  and configured to detect the magnetic field from the corresponding one of the two magnets  61 , and detection circuits  63 , each of which detects the rotation angle of the corresponding one of the rotors  21  and  22  based on the detection result of the corresponding one of the magnetic detection elements  62 . 
     The magnets  61  are provided integrally with the axial end faces of the corresponding rotors  21  and  22  and rotate together with the rotors  21  and  22 , respectively. As shown in  FIG.  9   , in the present embodiment, a columnar (disk-shaped) magnet  61  having N and S poles formed along the circumferential direction of each of the rotors  21  and  22  is provided. In addition, the shape of the magnet  61  is not limited to that shown in the drawing, and the magnet  61  may be, for example, a rod-like shape. Alternatively, the magnet  61  may be a ring-shaped magnet in which a plurality of N poles and a plurality of S poles are formed along the circumferential direction of each of the rotors  21  and  22 . 
     The two magnetic detection elements  62  are used corresponding to the two rotors  21  and  22  in the stroke sensor  1   a . Both magnetic detection elements  62  are arranged so as to face the axial end faces of the corresponding rotors  21  and  22  (the axial end faces on the side where the magnets  61  are provided), and detect the magnetic field (magnetic field intensity) from the corresponding magnets  61 . A Hall element, for example, can be used as the magnetic detection element  62 . The magnetic detection element  62  is mounted on a fixed-side board  64 , and the fixed-side board  64  is fixed to the housing  14  via the support member  5 . In  FIGS.  8  and  9   , the detection axis of the magnetic detection element  62  is denoted by D. 
     The two magnetic detection elements  62  are provided so as to detect magnetic fields perpendicular to the rotation axis directions of the corresponding rotors  21  and  22 , respectively. The two rotors  21  and  22  are arranged in such a manner that their rotation axes are orthogonal to each other. That is, the two rotors  21  and  22  are arranged in such a manner that the magnetic field detection directions (detection axes D) are orthogonal to each other. As a result, it becomes possible to suppress the influence of the magnetic field generated by the magnet  61  corresponding to one rotor  2  (for example, the first rotor  21 ) on the magnetic detection element  62  corresponding to the other rotor  2  (for example, the second rotor  22 ), thereby improving the detection accuracy. 
     In the illustrated example, the magnetic detection element  62  is arranged at a position on the extension line of the rotation axis of each of the rotors  21  and  22  (i.e., a center position of each of the rotors  21  and  22  when viewed from the direction of the rotation axis). Arrangement of the magnetic detection element  62  may be difficult depending on the structure of the support member  5  or the like. In such a case, the magnetic detection element  62  may be arranged at a position slightly displaced from the position on the extension line of the rotation shaft of each of the rotors  21  and  22  (the center position of each of the rotors  21  and  22  when viewed from the direction of the rotation axis). 
     The detection circuit  63  detects the rotation angle of the corresponding one of the rotors  21  and  22  based on the magnetic field intensity detected by the magnetic detection element  62 . The detection circuit  63  is mounted on the fixed-side board  64 . 
     Functions and Effects of Another Embodiment 
     In the stroke sensor  1   a  of  FIG.  8   , the rotation of each of the rotors  21  and  22  is detected using the magnet  61  and the magnetic detection element  62  instead of the rotor-side board  31  and the detection coil  32  in the stroke sensor  1  of  FIG.  1 B . With such a configuration, the same effects as those of the stroke sensor  1  of  FIG.  1 B  can be obtained. That is, according to the stroke sensor  1   a , the size of the stroke sensor can be reduced and the stroke position can be detected with high accuracy. Further, the stroke position can be accurately detected without being affected by grease or the like in the housing  14 . 
     Modified Example of Another Embodiment 
       FIG.  8    describes the case where two magnetic detection elements  62  are used, but a single biaxial magnetic detection element  62  may be used instead. For example, as shown in  FIG.  10 A , when the distance between the rotors  21  and  22  gradually increases as they approach the rack shaft  13 , the axial end faces of the rotors  21  and  22  on the rack shaft  13 -side are provided with the magnets  61  respectively, and the magnetic detection element  62  may be arranged at a position where the rotation axes of both rotors  21  and  22  intersect. The magnetic detection element  62  is preferably arranged in such a manner that the two detection axes D are perpendicular to the rotation axes of the rotors  21  and  22 , respectively. 
     Further, as shown in  FIG.  10 B , it is also possible to dispose the rotors  21  and  22  in such a manner that the distance between the rotors  21  and  22  gradually increases as the distance from the rack shaft  13  increases. In this case, the magnets  61  is provided on each of the axial end faces of the rotors  21  and  22  on the side not facing the rack shaft  13 , and the single magnetic detection element  62  is arranged at a position where the rotation axes of both the rotors  21  and  22  intersect. The magnetic detection element  62  is preferably arranged in such a manner that the two detection axes D are perpendicular to the rotation axes of the rotors  21  and  22 , respectively. 
     Summary of Embodiment 
     Next, technical ideas understood from the embodiments described above will be described with reference to the reference numerals and the like in the embodiments. However, each reference numeral and the like in the following description do not limit the constituent elements in the claims to the members and the like specifically shown in the embodiment. 
     According to the feature [1], a stroke sensor configured to detect a stroke position of a rod-shaped measuring object  13  that strokes in an axial direction is composed of two disk-shaped rotors  2  configured to rotate along with a stroke of the measuring object, a rotation detecting unit  3  configured to detect rotations of the two rotors  2 , respectively, and a stroke position detecting unit  4  configured to detect the stroke position of the measuring object  13  based on the rotations of the two rotors  2  detected by the rotation detecting unit  3 , wherein at least one of the two rotors  2  is provided in direct contact with the measuring object  13 , wherein the two rotors  2  are provided side by side in an arrangement direction perpendicular to an axial direction of the measuring object  13  and are provided so as to be adjacent to the measuring object  13  in an arrangement perpendicular direction perpendicular to the axial direction and the arrangement direction, wherein each of the two rotors  2  is provided in such a manner that a rotation axis direction of each of the two rotors  2  is inclined with respect to the arrangement direction. 
     According to the feature [2], in the stroke sensor  1  described in the feature [1], the rotation detecting unit  3  is composed of two rotor-side boards  31  each being provided integrally with an end face in the rotation axis direction of each of the two rotors  2  and having a conductor pattern  31   a  formed in a predetermined pattern along a circumferential direction of each of the rotors  2 , two detection coils  32  provided so as to face the two rotor-side boards  31 , respectively, and provided so as not to rotate along with the rotations of the rotors  2 , and detection circuits  33 , each being configured to detect a rotation angle of corresponding one of the rotors  2  based on a change in inductance of each of the detection coils  32  when an AC voltage is applied to each of the detection coils  32 . 
     According to the feature [3], in the stroke sensor  1  described in feature [2], the two rotors  2  are arranged in such a manner that rotation axes of the two rotors  2  are orthogonal to each other, and the two detection coils  32  are arranged in such a manner that magnetic field generation directions in the two detection coils  32  when the AC voltage is applied are orthogonal to each other. 
     According to the feature [4], in the stroke sensor  1  described in the feature [2] or [3], the two rotors  2  are arranged in such a manner that the rotation angles of the two rotors  2  when the measuring object  13  is stroked are different from each other, and the conductor patterns  31   a  formed on the rotor-side boards  31  provided on the two rotors  2  are formed to have a same pattern configuration in respective circumferential directions of the rotors  2 . 
     According to the feature [5], in the stroke sensor  1  described in the feature [2] or [3], the two rotors  2  are arranged in such a manner that the rotation angles of the two rotors  2  when the measuring object  13  is stroked are same, and the conductor patterns  31   a  formed on the rotor-side boards  31  provided on the two rotors  2  are formed to have pattern configurations different from each other in respective circumferential directions of the rotors  2 . 
     According to the feature [6], in the stroke sensor  1   a  described in the feature [1], the rotation detecting unit  3  is composed of two magnets  61 , each being integrally provided with each of the two rotors  2 , a magnetic detection element  62  being provided so as not to rotate along with the rotors  2  and configured to detect magnetic fields from the two magnets  61 , and detection circuits  63 , each being configured to detect a rotation angle of a corresponding one of the rotors  2  based on a detection result of a corresponding one of the magnetic detection elements  62 . 
     According to the feature [7], in the stroke sensor  1   a  described in the feature [6], the two rotors  2  are arranged in such a manner that rotation axes of the two rotors  2  are orthogonal to each other, and the magnetic detection element  62  comprises two magnetic detection elements  62  being arranged in such a manner that magnetic field detection directions of the two magnetic detection elements  62  being orthogonal to each other, and each of the two magnetic detection elements  62  is configured to detect a magnetic field of a corresponding one of the two magnets  61 . 
     According to the feature [8], in the stroke sensor  1   a  described in the feature [6], the two rotors  2  are arranged in such a manner that rotation axes of the two rotors  2  are orthogonal to each other, and the magnetic detection element  62  is configured to detect magnetic fields in two directions orthogonal to each other and detect the magnetic fields of the two magnets  61  in the two directions. 
     According to the feature [9], in the stroke sensor  1   a  described in any one of features [6] to [8], the two magnets  61  have a same configuration, and the two rotors  2  are configured in such a manner that rotation angles of the two rotors  2  when the measuring object  13  is stroked are different from each other. 
     According to the feature [10], in the stroke sensor  1 , la described in any one of the features [1] to [9], the measuring object  13  includes a toothed portion  13   a  on at least a part of an outer peripheral surface of the measuring object  13 , and the toothed portion  13   a  includes teeth formed at equal intervals in an axial direction of the measuring object  13 , and at least one of the two rotors  2  is composed of a gear that directly meshes with the toothed portion  13   a.    
     According to the feature [11], in the stroke sensor  1 , la described in the feature [10], each of both the two rotors  2  is composed of a gear that directly meshes with the toothed portion  13   a.    
     According to the feature [12], in the stroke sensor  1  described in the feature [10], one of the two rotors  2  is composed of a gear that directly meshes with the toothed portion  13   a , and the other of the two rotors  2  is composed of a gear that directly meshes with the one of the rotors  2 . 
     Although the embodiments of the present invention have been described above, the embodiments described above do not limit the invention according to the scope of claims. Also, it should be noted that not all combinations of features described in the embodiments are essential to the means for solving the problems of the invention. 
     The present invention can be appropriately modified and implemented without departing from the gist thereof. For example, in the above embodiment, the case where the rack shaft  13  is used as the measuring object has been described. However, the present invention is also applicable to the detection of the stroke position of a member which is stroked other than the rack shaft. 
     Further, in the above-described embodiment, the case where the rotor  2  is a gear has been described, but the rotor  2  is not limited thereto. The rotor  2  may be a roller or the like that rotates due to friction with the measuring object.