Abstract:
A torque sensor has a magnetostrictive member fixed to a shaft. The magnetostrictive member is strained by the torque applied to the shaft. Exciting coils generate flux running through the magnetostrictive member. The flux varies in accordance with the strain of the magnetostrictive member. Detecting coils detect the flux variation. The exciting coils and the detecting coils are wound about bobbins. A stator supports the bobbins such that the coils surround and face the magnetostrictive member. The stator is rotatably supported relative to the shaft and is formed with stator pieces. The stator has annular grooves formed in its inner surface to accommodate the bobbins. One end of each bobbin is fixed to one wall of the corresponding annular groove. A clearance is formed between the outer end of the bobbin and the other wall of the annular groove, and another clearance is formed between the outer circumferential surface of the bobbin and the inner circumferential wall of the annular groove. The clearances accommodate thermal expansion of the bobbins, which improves the performance of the torque sensor.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a magnetostrictive torque sensor that detects torque acting on a shaft by sensing magnetostrictive changes. 
     For example, Japanese Unexamined Patent Publication No. 5-118938 and Japanese Unexamined Patent Publication No. 59-77326 disclose such magnetostrictive torque sensors. A magnetostrictive torque sensor includes a shaft, a magnetostrictive member fitted about the shaft and detecting coil. Torque applied to the shaft deforms the magnetostrictive member, which changes the magnetic permeability of the magnetostrictive member. This varies the flux, or inductance, of the magnetostrictive member. Changes in the inductance are detected based on electromotive force induced in the detecting coil. The torque applied to the shaft is detected accordingly. 
     As illustrated in FIG. 4, a prior art magnetostrictive torque sensor includes a housing  30 , a shaft  32  rotatably supported by the housing  30 , a sleeve  33  fitted about the shaft  32  and a magnetostrictive member  34  fitted about the sleeve  33 . A cylindrical stator  35  is located about the magnetostrictive member  34 . The stator  35  is supported on the shaft  32  by bearings  38 . The bearings  38  allow the stator  35  rotate relative to the shaft  32 . The stator  35  includes a cylindrical yoke  36  and a pair of retainers  37  for supporting the yoke  36 . The yoke  36  has a pair of resin bobbins B. An exciting coil  39  and a detecting coil  40  are wound about each bobbin B. The exciting coils  39  and the detecting coils  40  surround and face the surface of the magnetostrictive member  34 . When an alternating current is supplied to the exciting coils  39 , the exciting coils  39  generate flux. The flux forms a magnetic circuit through the yoke  36  and the magnetostrictive member  34 . 
     The surface of the magnetostrictive member  34  is divided into two detection regions X and Y. Grooves  34   a  are formed in each region X, Y. The grooves  34   a  in the region X are inclined by forty-five degrees relative to the axis of the shaft  32 , and the grooves  34   a  in the region Y are inclined by minus forty-five degrees relative to the axis. When torque is applied to the shaft  32 , a compressive force acts on one of the detection regions X and Y and a tensile force acts on the other region depending on the rotational direction of the shaft  32 . A tensile force increases the magnetic permeability of the detection regions X, Y and a compression force decreases the magnetic permeability of the regions X, Y. Variations in the magnetic permeability of the magnetostrictive member  34  change the voltages induced in the detecting coils  40 . That is, the detecting coils  40  send varying voltage values to a processor (not shown). The processor computes the torque applied to the shaft  32  based on the voltage values from the detecting coils  40 . 
     The yoke  36  is located close to the magnetostrictive member  34  to improve the sensitivity of the torque sensor. To facilitate the installation of the bobbins B (the coils  39 ,  40 ) the yoke  36  includes two semi-cylindrical pieces  41 ,  42 , as illustrated in FIG.  5 . When installing the coils  39 ,  40 , the bobbins B, about which the coils  39 ,  40  are wound, are attached to the inner wall of the yoke  36 . Thereafter, the pieces  41 ,  42  are secured to each other along two joints  43  with non-conductive adhesive. Since the yoke  36  is divided into two pieces along a plane that includes the axis and since the non-conductive adhesive is located between the pieces  41 ,  42 , eddy currents in the circumferential direction of the yoke  36  are blocked by the joints  43 . The eddy currents, which would otherwise adversely affect the sensitivity of the torque sensor, are reduced. This improves the sensitivity of the torque sensor. However, since the magnetic reluctance of the joints  43  is much greater than that of the other parts, the magnetic symmetry about the axis of the yoke  36  is impaired. The lowered magnetic symmetry of the yoke  36  causes the detection voltage of the torque sensor to change in accordance with the rotational position of the shaft  32 . Therefore, even if there is no torque acting on the shaft  32 , the torque sensor may erroneously detect that a torque is acting on the shaft  32 . 
     Torque deforms the magnetostrictive member  34 , and deformation of the member  34  changes the inductance of the coils. The torque sensor detects torque based on the changes of the inductance. However, the inductance also changes when the distance between the magnetostrictive member  34  and the coils  39 ,  40  changes. When the distance between the magnetostrictive member  34  and the coils  39 ,  40  changes, the torque sensor falsely detects torque even if there is no torque acting on the shaft  32 . Therefore, it is imperative that the distance between the member  34  and the coils  39 ,  40  be constant to guarantee the precision of the torque sensor. 
     As described above, the stator  35  includes the yoke  36  and the two retainers  37 , shown in FIG. 4, and the yoke  36  includes two semi-cylindrical pieces  41 ,  42 , shown in FIG.  5 . Due to dimensional errors created during assembly, the cross-section of the stator  35  is not completely round. That is, when assembling the pieces  41  and  42  to form the yoke  36 , their relative positions may vary. Also, the positions of the retainers  37  may vary relative to the yoke  36 . The variation of the relative positions of the parts prevents the inner surface of the retainer  35  from forming a perfect circle about the axis of the shaft  32 . Further, the edges of the pieces  41 ,  42  are exposed on the inner surface of the yoke  36 , which disturbs the distribution of the flux. 
     The resin bobbins B are press fitted in the metal yoke  36 . The bobbins B have a greater coefficient of thermal expansion than that of the yoke  36 . Therefore, when the temperature of the sensor is relatively high, a stress F is generated between each bobbin B and the yoke  36 . The stress F displaces the positions of the exciting coils  39  and the detecting coils  40 . Therefore, even if there is no torque acting on the shaft  32 , the electromotive force induced in the detecting coils  40  is changed. In other words, the magnetic characteristics of the coils  39 ,  40  are affected by temperature changes. This degrades the temperature characteristics of the torque sensor thereby lowering the detection accuracy of the torque sensor. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an objective of the present invention to provide a torque sensor that accurately detects torque despite temperature changes. 
     To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, a torque sensor for sensing torque applied to a shaft is provided. The sensor includes a magnetostrictive member, an exciting coil, a detecting coil, a bobbin and stator. The magnetostrictive member is fixed to the shaft is strained by the torque applied to the shaft. The exciting coil generates flux running through the magnetostrictive member. The flux varies in accordance with the strain of the magnetostrictive member. The detecting coil detects the flux variation. The exciting coil and the detecting coil are wound about the bobbin. The stator supports the bobbin such that the coils surround the magnetostrictive member. The stator is rotatably supported relative to the shaft and is formed with a plurality of stator pieces. The stator has an annular groove formed in its inner surface to accommodate w the bobbin. The annular groove has a first end wall, a second end wall and a circumferential wall. The bobbin further has a first end surface, a second end surface and a circumferential surface. The first end surface of the bobbin is fixed to the first end wall of the annular groove. A first annular clearance is formed between the circumferential surface of the bobbin and the circumferential wall of the annular groove. A second annular clearance is formed between the second end surface of the bobbin and the second end wall of the annular groove. 
     Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: 
     FIG. 1 is a cross-sectional view illustrating a torque sensor according to a first embodiment of the present invention; 
     FIG. 2 is an enlarged partial cross-sectional view illustrating the torque sensor of FIG. 1; 
     FIG. 3 is an enlarged cross-sectional view illustrating a torque sensor according to a second embodiment of the present invention; 
     FIG. 4 is a cross-sectional view illustrating a prior art torque sensor; 
     FIG. 5 is a perspective view illustrating the yoke of the torque sensor shown in FIG. 4; 
     FIG. 6 is a perspective view illustrating a stator according to the present invention; 
     FIG. 7 is an enlarged cross-sectional view illustrating a torque sensor according to a third embodiment of the present invention; and 
     FIG. 7A is an enlargement of the portion of FIG. 7 encircled by a broken line. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A torque  1  sensor according to a first embodiment of the present invention will now be described with reference to FIGS. 1,  2  and  6 . 
     As shown in FIG. 1, the torque sensor  1  includes a detection member  3  and a detector  4 . The detection member  3  is fitted about a shaft  2  and the detector  4  detects magnetic changes due to strain of the member  3 . A pair of radial bearings  6  allow the detection member  3  and the detector  4  to rotate relative to each other. 
     The detection member  3  is welded to the shaft  2  and includes a cylindrical magnetostrictive member  5 , sleeves  5   b  adjacent to the member  5  and threaded portions  5   c  formed at the ends. The magnetostrictive member  5  has a magnetostrictive property. The magnetostrictive member  5  can be made of a soft magnetic material having a high magnetic permeability such as permalloy and iron-nickel-chromium alloy. The magnetostrictive member  5  also can be made of magnetostrictive material including iron and aluminum or an amorphous magnetostrictive material. The magnetostrictive member  5  includes two regions X and Y, in which grooves  5   a  are formed. The grooves  5   a  are equally spaced apart in the circumferential direction. The grooves  5   a  in one of the regions X are inclined by forty-five degrees relative to the axis, and the grooves  5   a  in the other region Y are inclined by minus forty-five degrees relative to the axis. The bearings  6  are press fitted between the sleeves  5   b  and the detector  4 . 
     The detector  4  includes a stator  7  and bobbins B 1 , B 2 . The stator  7  is supported on the detection member  3  by a pair of bearings  6 . An exciting coil  8  and a detecting coil  10  are wound about the bobbin B 1 , and an exciting coil  9  and a detecting coil  11  are wound about the bobbin B 2 . The exciting coils  8 ,  9  are located inside the detecting coils  10 ,  11 . The stator  7  includes an annular inner yoke  14 , which is press fitted to and couples a pair of outer annular yokes  12  and  13 . That is, the stator  7  is formed by the first outer annular yoke  12 , the inner annular yoke  14  and the second outer annular yoke  13 , which are connected end-to-end in series in the axial direction of the shaft  2 . The outer yokes  12 ,  13  and the inner yoke  14  are made of metal such as permalloy and form two circumferential grooves  24 ,  25 . The bobbins B 1 , B 2  are located in the grooves  24 ,  25  and face the detection regions X, Y, respectively. The bobbins B 1 , B 2  are made of a resin such as polybutylene terephthalate (PBT). 
     The supporting structure of the bobbins B 1 , B 2  will now be described with reference to FIG.  2 . Since the yokes  12 ,  13  and the bobbins B 1 , B 2  of the detector  4  are identical, only the left yoke  12  and the bobbin B 1  will be described. The first outer yoke  12  includes an end portion  12   a  and a stopper portion  12   b . The end portion  12   a  is located at the outer end of the first outer yoke  12 . The inner diameter of the stopper portion  12   b  is smaller than that of the end portion  12   a . Therefore, a step is defined between the end portion  12   a  and the stopper portion  12   b . The bearing  6  is fitted in the end portion  12   a  and contacts the step. The first outer yoke  12  also has an intermediate portion  12   c  located axially inward of the stopper portion  12   b . The inner diameter of the intermediate portion  12   c  is smaller than that of the stopper portion  12   b . The inner diameter of the end portion  12   a  is substantially the same as the outer diameter of the bearing  6 , such that the outer ring of the bearing  6  is lightly press fitted in the end portion  12   a.    
     The distance between the inner surface of the intermediate portion  12   c  and the detection region X of the magnetostrictive member  5  is determined such that the sensitivity of the torque sensor is maximized. 
     The first outer yoke  12  also includes a holder portion  12   d  and an inner end portion  12   e . The holder portion  12   d  holds the bobbin B 1  and the inner end portion  12   e  contacts the yoke ring  14 . The inner diameter of the holder portion  12   d  is slightly larger than the outer diameter of the bobbin B 1 . The axial length L 1  of the holder portion  12   d  is longer than the axial length L 2  of the bobbin B 1 . Therefore, a clearance Cl is defined between the inner surface of the holder portion  12   d  and the outer surface of the bobbin B 1 . Also, a clearance C 2  is defined between the right end surface of the bobbin B 1  and the left end surface of the inner yoke  14 . Specifically, the radial and axial dimensions of the clearances C 1  and C 2 , respectively, between 0.1 mm and 0.2 mm. The coefficient of thermal expansion of the bobbins B 1 , B 2  is greater than that of the yokes  12 ,  13  and the yoke ring  14 . The clearances C 1 , C 2  accommodate thermal expansion of the bobbins B 1 , B 2 . 
     An annular projection  15  is formed on the side wall of the holder portion  12   d , and an annular recess  16  is formed in the outer side wall of the bobbin B 1 . Specifically, the recess  16  is located at the radially inner part of the bobbin B 1 , as shown in FIG.  2 . The projection  15  engages with the recess  16 , which facilitates accurate positioning of the bobbin B 1  and maintains the clearance C 1  between the inner surface of the holder portion  12   d  and the outer surface of the bobbin B 1 . 
     The inner diameter of the inner end portion  12   e  of the first outer yoke  12  is larger than the outer diameter of the bobbin B 1 , which allows the bobbin B 1  to be inserted into the holder portion  12   d  from the inner end portion  12   e . The outer diameter of the yoke ring  14  is larger than that of the bobbin B 1  and is substantially the same as the inner diameter of the inner end portion  12   e . The inner diameter of the inner yoke  14  is substantially the same as that of the bobbin B 1 . 
     The manufacture of the stator  7  will now be described. First, adhesive is applied on the surfaces  100 ,  101  of the projection  15  of the first outer yoke  12 . Then, the bobbin B 1  is inserted in the holder portion  12   d  of the yoke  12  until the recess  16  engages with the projection  15 . The bobbin B 1  is thus fixed to the surfaces  100 ,  101 . The adhesive on the surface  100  prevents the bobbin B 1  from moving radially, and the adhesive on the surface  101  prevents the bobbin B 1  from moving axially. Thereafter, the yoke ring  14  is lightly press fitted into the inner end portion  12   e.    
     As illustrated, the holder portion  12   d  of the first outer yoke  12  and the side wall of the inner yoke  14  define the circumferential groove  24  to accommodate the bobbin B 1 . In the same manner, the bobbin B 2  is accommodated in the circumferential groove  25  defined by the holder portion  13 d of the second outer yoke  13  and the other side wall of the inner yoke  14 . 
     As shown in FIG. 1, fasteners  17 ,  18  are engaged with the threaded portions  5   c  to determine the axial position of the torque sensor  1 . The fasteners  17 ,  18  press the stator  7  through the bearings  6  thereby preventing the stator  7  from moving axially. 
     The exciting coils  8 ,  9  are connected to an alternating-current power supply (not shown). The detecting coils  10 ,  11  are connected to a conventional processor (not shown). When an alternating-current is supplied to the exciting coils  8 ,  9 , the exciting coils  8 ,  9  produce flux. The flux forms two magnetic circuits through the stator  7  and the magnetostrictive member  5 . The flux produced by the exciting coil  8  extends along the grooves  5   a  in the region X, and the flux produced by the exciting coil  9  extends along the grooves  5   a  in the region Y. Changes of the flux induce electromotive force in the detecting coils  10 ,  11 . The induced electromotive force is sent to the processor. 
     When torque acts on the shaft  2 , one of the detection regions X, Y receives a compression force and the other region receives a tensile force depending on the direction of the torque. A tensile force increases the magnetic permeability of the magnetostrictive member  5  and a compression force decreases the magnetic permeability of the magnetostrictive member  5 . Each detecting coil  10 ,  11  produces an induced electromotive force representing either a compression force or by tensile force. Specifically, when a region X or Y receives a tensile force, the coil  10  or  11  corresponding to the region generates relatively great electromotive force. When a region X or Y receives a compression force, the coil  10  or  11  corresponding to the region generates a relatively small electromotive force. In short, the detecting coils  10 ,  11  send different voltages to the processor. 
     The processor includes a differential circuit, a rectifier circuit and a torque computing circuit (none of which is shown). The processor receives voltages from the detecting coils  10 ,  11 . The differential circuit computes the difference between the voltages. The rectifier circuit rectifies the computed difference and outputs the rectified signal to the torque computing circuit. The torque detector circuit computes the magnitude and the direction of the torque based on the value and the sign of the rectified signal. In this manner, the processor detects an applied torque based on voltages from the detecting coils  10 ,  11 . The computation of the difference performed by the differential circuit compensates for external noise such as temperature changes. Accordingly, the accuracy of torque detection is improved. The torque computing circuit detects the direction of the torque based on whether the rectified signal has positive value or a negative value relative to a reference value (zero) and also detects the magnitude of the torque based on the magnitude of the rectified signal. The reference value is set such that it becomes zero when no torque acts on the shaft  2 . 
     This torque sensor  1  has the following characteristic features. 
     When the torque sensor  1  is operating, an alternating-current power supply supplies alternating-current to the exciting coils  8 ,  9 . The supplied current has a constant amplitude and frequency. The exciting coils  8 ,  9  produce two magnetic circuits through the stator  7  and the magnetostrictive member  5 . Since the bobbins B 1 , B 2  are made of resin, the coefficient of thermal expansion of the resins B 1 , B 2  is greater than that of the yoke  13  and the inner yoke  14 . When the bobbins B 1 , B 2  are expanded by heat, the clearances C 1 , C 2  accommodate the expansion of the bobbins B 1 , B 2 . Thus, the bobbins B 1 , B 2  are not pressed against the outer yokes  12 ,  13  and the inner yoke  14 , which eliminates undesired stress acting on the bobbins B 1 , B 2 . As a result, the magnetic characteristics of the exciting coils  8 ,  9  and the detecting coils  10 ,  11  are maintained. The thermal characteristics of the torque sensor  1  are maintained accordingly, which improves the accuracy of the torque detection. 
     Further, the annular projection  15  engages with the annular recess  16 , which facilitates positioning of the bobbins B 1 , B 2  in the radial direction. Therefore, the clearances C 1 , C 2  are accurately formed about the bobbins B 1 , B 2 . 
     The bobbins B 1 , B 2  are firmly fixed to the surfaces  100 ,  101 . Thus, when the torque sensor  1  vibrates, the bobbins B 1 , B 2 , or the coils  8  to  11 , are not displaced. Thus, the magnetic characteristics of the coils  8  to  11  are maintained. As a result, the detecting coils  10 ,  11  generate induced electromotive force that accurately corresponds to the strain of the regions X, Y on the magnetostrictive member  5 , or to torque acting on the shaft  2 . Thus, the torque is accurately detected. 
     The torque sensor  1  of FIGS. 1,  2  and  6  does not have the retainers  37  of the prior art torque sensor shown in FIG. 4, and the stator  7  is formed with three parts, namely, the outer yokes  12 ,  13  and the inner yoke  14 . This construction reduces errors produced during assembly. Since the outer yokes  12 ,  13  and the inner yoke  14  are cylindrical, the cross section of the stator  7  is substantially perfectly round, and the stator  7  is therefore substantially coaxially assembled with the magnetostrictive member  5 . Thus, the inductance of each detecting coil  10 ,  11  is scarcely affected by the rotational position of the shaft  2 . Accordingly, the reliability of the torque sensor  1  is improved. 
     Since the outer yokes  12 ,  13  have the same shapes, they are interchangeable. Compared to the case where two different shaped outer yokes are used, the outer yokes  12 ,  13  reduce the manufacturing cost. 
     A torque sensor according to a second embodiment of the present invention will now be described with reference to FIG.  3 . 
     The torque sensor of the second embodiment has a stator  7 A shown in FIG.  3 . An annular groove  26  is formed in the inner wall of the stator  7 A. The shape of the groove  26  is different from the shape of the groove  24  shown in FIGS. 1 C 2  and  2 . A pair of bobbins BA is fixed to an inner yoke  21  by adhesive. Thus, an annular clearance C 3  is defined between the bobbin BA and the end wall of a holder portion  20   a  of the first outer yoke  20 . The axial dimension of the clearance C 3  is between 0.1 mm and 0.2 mm. 
     An annular projection  22  is formed on the end wall of the inner yoke  21 , and an annular recess  23  is formed in the inner end wall of the bobbin BA. The projection  22  engages with the recess  23 . 
     Adhesive is applied to the surfaces  10 A,  101 A of the projection  22  of the inner yoke  21 . The recess  23  of the bobbin BA is engaged with the projection  22 . Then, the connector  21  is lightly press fitted into the first outer yoke  20  to form the stator  7 A. The adhesive on the surfaces  10 A,  101 A prevents the bobbin B 1  from moving radially and axially. 
     In the embodiment of FIG. 3, a pair of bobbins BA are secured to the sides of the inner yoke  21 . This structure allows electric connectors for drawing out the wires of the exciting coils  8 ,  9  and the detecting coils  10 ,  11  to be located in the inner yoke  21 . Further, the structure allows a common connector to be used to draw out wires of the coils  8  to  11 . 
     FIG. 7 illustrates a torque sensor according to a third embodiment. In the embodiments of FIGS. 1 to  3  and  6 , the stators  7  and  7 A are formed with annular parts, which are separated in the axial direction of the shaft  2 . The stator of FIG. 7 is formed with semi-cylindrical parts, which are separated by a plane that includes the axis of the shaft  2 . 
     It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the invention may be embodied in the following forms. 
     In the embodiments of FIGS. 1,  2 ,  3 ,  6  and  7 , the numbers of the bobbins B 1 , B 2  and BA may be changed. 
     In the embodiments of FIGS. 1,  2 ,  3 ,  6  and  7 , the shape and the number of the parts forming the stators  7 ,  7 A may be changed as long as the stators  7 ,  7 A have a space for accommodating the bobbins B 1 , B 2  and BA. Also, the stators  7 ,  7 A may be directly coupled to the shaft  2  by bearings. 
     In the embodiments of FIGS. 1,  2 ,  3 ,  6  and  7 , the inner yokes  14 ,  21  are smaller in diameter than the outer yokes  12 ,  13  and  20 . However, the inner yoke may be larger in diameter than the two outer yokes. Also, the grooves for holding the bobbins may be entirely formed within the inner yoke, rather than being formed between the inner and outer yokes. 
     The present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.