Abstract:
An improved magnetostrictive torque sensor for sensing torque applied to a shaft that is rotatably supported in a housing. A magnetostrictive cylinder is fixed to the shaft. A stator is supported on the shaft by bearings to surround the magnetostrictive cylinder and is accommodated in the housing. The stator incorporates exciting coils and detecting coils such that the coils are located about the shaft. The magnetosttrictive cylinder is strained by torque applied to the shaft. The exciting coils generate flux running through the magnetostrictive cylinder. The generated flux is changed in accordance with the strain of the magnetostrictive cylinder. The detecting coils detect the flux changes. The stator is also rotatably fixed to the housing by bearings. Rotation of the stator relative to the housing is prevented by connectors that couple the stator with the housing. This prevents tension in wires that lead from the stator.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a torque sensor having a stator that detects changes in magnetic flux passing through a magnetostrictive material fixed on a shaft that is rotatable relative to the stator. The present invention also relates to a member for restricting the rotation of the stator. 
     Magnetostrictive torque sensors typically include a detecting coil, a shaft and magnetostrictive material provided on the peripheral surface of the shaft. Application of torque on the shaft strains the magnetostrictive material and causes changes in the magnetic permeability of the sensor. The changes in the permeability alter flux and thus induce electromotive force in the detecting coil. The applied torque is detected based on the induced electromotive force. Methods for detecting torque applied on the shaft in such sensors are proposed, for example, in Japanese Unexamined Patent Publication No.  5-118938  and Japanese Unexamined Patent Publication No.  59-77326.    
     In a typical magnetostrictive torque sensor, a shaft is rotatably supported in a housing. A stator having a detecting coil is fixed to the inner wall of the housing. Also, magnetostrictive material is fixed to the shaft. A predetermined gap exists between the stator and the material. However, eccentric rotation of the shaft relative to the housing varies the distance between the stator and the material and thus degrades the performance of the sensor. 
     A sensor has been proposed in which a stator is supported on a shaft to overcome this drawback. As shown in FIG. 13, a shaft  51  is supported by bearings  53  in a housing  52  and thus rotates relative to the housing  52 . A cylinder  54  made of magnetostrictive material is fixed to the shaft  51 . Also, a cylindrical stator  55  is rotatably supported on the shaft  51  by bearings  56 . The bearings  56  create a predetermined distance between the inner wall of the stator  55  and the surface of the cylinder  54 . The stator  55  includes an exciting coil  57  and a detecting coil  58 . An alternating electric current is applied to the exciting coil  57 . The current forms a magnetic circuit including magnetic flux through the cylinder  54 . Application of torque on the shaft  51  strains the cylinder  54  and causes changes in the flux through the cylinder  54 . The flux changes are detected by the detecting coil  58 . Terminal wires of the coils  57 ,  58  are soldered to lead wires  59 . The lead wires  59  extend through holes  55   a  formed in the cylinder  55  and holes  52   a  formed in the housing  52 . This construction maintains the predetermined distance between the stator  55  and the cylinder  54  even if the shaft  51  rotates eccentrically relative to the housing  52 . 
     However, due to friction in the bearings  56 , rotation of the shaft  51  applies rotational force to the stator  55 . Therefore, when the shaft  51  is rotated, the stator  55  is not always fixed relative to the housing  52 . In other words, the stator  55  rotates a little relative to the housing  52  as illustrated in FIG.  14 . The rotation of the stator applies tension to the lead wires  59  and the terminal wires. The tension can crack the solder connecting the lead wires  59  with the terminal wires. Thus, the terminal wires are likely to be damaged or broken. 
     Accordingly, it is an objective of the present invention to provide an improved torque sensor in which a stator is rotatable relative to a rotary shaft. Specifically, it is an objective of the present invention to provide a torque sensor and a stator rotation restrictor that prevent terminal wires of a stator from receiving tension and that are easy to assemble. 
     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 that is rotatably supported in a housing is provided. The sensor includes a magnetostrictive member fixed to the shaft and an exciting coil for generating flux running through the magnetostrictive member. The magnetostrictive member is strained by the torque applied to the shaft. A generated flux varies in accordance with the strain of the magnetostrictive member. The sensor further includes a detecting coil for detecting the flux variation, a stator for incorporating the exciting coil and the detecting coil such that the coils are located about the shaft, a support for supporting the stator in the housing and a rotation restrictor for preventing the stator from rotating relative to the housing. 
     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 features of the present invention that are believed to be novel are set forth with particularity in the appended claims. 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 showing 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 exploded perspective view showing the connector in the sensor of FIG. 1; 
     FIG. 4 is a perspective view showing the connector of FIG. 3; 
     FIG. 5 is an exploded perspective view showing a stator and a housing in the sensor of FIG. 1; 
     FIG. 6 is a cross-sectional side view illustrating the torque sensor of FIG. 1; 
     FIG. 7 is an exploded perspective view showing a stator and a housing according to another embodiment; 
     FIG. 8 is an enlarged partial cross-sectional view illustrating a torque sensor according to another embodiment; 
     FIG. 9 is an exploded perspective view showing a stator and a housing of a torque sensor according to another embodiment; 
     FIG. 10 is a cross-section view illustrating the torque sensor of FIG. 9; 
     FIG. 11 is an exploded perspective view showing a stator and a housing of a torque sensor according to another embodiment; 
     FIG. 12 is a cross-sectional side view illustrating the torque sensor of FIG. 11; 
     FIG. 13 is a cross-sectional side view illustrating a prior art torque sensor; and 
     FIG. 14 is a cross-sectional view illustrating the torque sensor of FIG.  13 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first embodiment according to the present invention will now be described with reference to FIGS. 1 to  6 . 
     FIG. 6 is a cross-sectional view illustrating a torque sensor  1  provided on a shaft  2 . The shaft  2  extends through a substantially cylindrical housing  3  and is supported by bearings  4 , which are fixed to the housing  3 . The bearings  4  allow the shaft  2  to rotate relative to the housing  3 . The torque sensor  1  includes a magnetostrictive detection member  5  and a detector  6 . The detection member  5  is fitted about the shaft  2  and the detector  6  detects magnetic changes due to strain of the member  5 . 
     As shown in FIG. 5, the detection member  5  includes a sleeve  7  and a magnetostrictive cylinder  8 . The sleeve  7  is fitted about and is welded to the shaft  2 . Likewise, the cylinder  8  is fitted about and is welded to the sleeve  7 . The cylinder  8  is therefore rotated integrally with the shaft  2 . The cylinder  8  includes a core and a magnetostrictive film formed on the core. The film is made of a soft magnetic material having a magnetostrictive property and a high magnetic permeability such as permalloy, iron-nickel-chromium alloy or iron-nickel-chromium-titanium alloy. The surface of the cylinder  8  is divided into two detection regions. Grooves  8   a  are formed in each region. The grooves  8   a  are equally spaced apart. The grooves  8   a  in one region are inclined by forty-five degrees relative to the axis of the shaft  2 , whereas the grooves  8   a  in the other region are inclined by minus forty-five degrees relative to the axis. An iron-aluminum based magnetostrictive material or an amorphous magnetostrictive material may also be used for the film on the cylinder  8 . 
     As shown in FIG. 6, the detector  6  includes a stator  10 . The stator  10  is supported on the shaft  2  by two bearings  9  and thus rotates relative to the shaft  2 . Two annular recesses are formed in the inner surface of the stator  10 . Each recess corresponds to one of the detection regions on the cylinder surface. A bobbin B is accommodated in each recess. An exciting coil  11  and a detecting coil  12  are wound about each bobbin B. The exciting coil  11  is located inside the detecting coil  12 . The bearings  9  maintain a predetermined distance between the stator  10  and the cylinder  8 . 
     The exciting coils  11  are connected to an alternating-current power supply and the detecting coils  12  are connected to a conventional signal processor. The signal processor controls alternating current having a predetermined frequency (Hz) supplied to the exciting coils  11 . The current produces two magnetic circuits between the stator  10 , the cylinder  8  and the stator  10 . The flux of the magnetic circuits runs along and between the grooves  8   a . Changes in the flux induce electromotive force in the detecting coil  12 . Voltage derived from the flux changes is present at the output terminals of the coils  12 . 
     The electromotive force induced by each detecting coil  12  is proportional to the strain produced in the corresponding detection region or to the torque applied to the shaft  2 . When torque is applied to the shaft  2 , a compressive force acts on one of the detection regions and a tensile force acts on the other region depending on the rotational direction of the shaft  2 . A tensile force increases the magnetic permeability of the cylinder  8  and a compression force decreases the magnetic permeability of the cylinder  8 . Therefore, the induced electromotive force of each detecting coil  12  increases when the corresponding detection region receives a tensile force and decreases when the region receives a compression force. 
     The signal processor executes a subtracting process on the induced electromotive force from the detecting coils  12  by a differential circuit (not shown). The signal obtained in the subtracting process is commutated by a commutation circuit incorporated in the signal processor. A conventional circuit then computes the value of the torque applied to the shaft  2  based on the commutated signal. By performing the subtracting process in the differential circuit, external noise caused by temperature changes is offset. This improves the accuracy of the value of the detected torque. 
     As shown in FIGS. 5 and 6, the stator  10  has two rectangular holes  10   a  in its circumference. The holes  10   a  are aligned in the axial direction of the stator  10 . Each hole  10   a  corresponds to one of the bobbins B. The housing  3  has two rectangular holes  3   a  each radially aligned with one of the holes  10   a . A connector  13  is inserted in each radially aligned pair of the holes  3   a  , and  10   a . The holes  10   a  are larger than the holes  3   a . The connectors  13  prevents the stator  10  from rotating relative to the housing  3 . 
     FIGS. 3 and 4 illustrates one of the connectors  13 . The connector  13  is made of resin and includes a male connector  14  and a female connector  15 . The male connector  14  includes a base  14   a  and four metal pins  14   b . The size of the base  14   a is determined such that the base  14   a  is press fitted into the hole  10   a . The inner end  14   c  of each pin  14   b  protrudes from the inside of the base  14   a . Each exciting coil  11  has terminal wires ll a  and each detecting coil  12  has terminal wires  12   a  (see FIGS.  2  and  5 ). Each of the wires  11   a  and  12   a  is electrically connected to the inner end  14   c  of each pin  14   b  by soldering. The bases  14   a  are press fitted into the holes  10   a  before inserting the stator  10  into the housing  3 . The protruding amount of the pins  14   b  is determined such that the pins  14   b  do not contact the inner wall of the housing  3  during the insertion. 
     The female connector  15  is generally a rectangular solid and has receptacles  15   a  in its inner surface, or bottom. The number of the receptacles  15   a  is equal to the number of the pins  14   b . The connector  15  also includes lead wires  16  protruding from its outer surface, or top. Each lead wire  16  is electrically connected with one of the receptacles  15   a . The male connector  14  and the female connector  15  are mated by inserting the pins  14   b  in the receptacles  15   a . This electrically connects the terminal wires ll a ,  12   a  with the lead wires  16 . The size of the female connector  15  is determined such that the connector  15  is fitted into the hole  3   a  in a manner that permits slight radial movement of the connector  15 . 
     The torque sensor  1  is assembled with the shaft  2  by the following process. 
     First, the sleeve  7 , about which the cylinder  8  is welded, is fitted about the shaft  2 . The cylinder  8  is fixed to the shaft  2  to integrally rotate with the shaft  2 . The shaft  2  is then inserted in the stator  10  and the bearings  9  are fitted in both ends of the stator  10 . The stator  10  is thus rotatably connected to the shaft  2  such that each pair of the coils  11 ,  12  faces one of the detection regions on the magnetostrictive cylinder  8 . 
     Next, the four terminal wires  11   a ,  12   a  extending from each hole  10   a  of the stator  10  are soldered to the inner ends  14   c  of the male connector  14 . The male connector  14  is then fitted in the hole  10   a . In this state, the shaft  2  is inserted in the housing  3 . As described above, the distal ends of the pins  14   b  do not contact the inner wall of the housing  3 . In other words, the pins  14   b  do not hinder the insertion of the shaft  2  into the housing  3 . One of the bearings  4  is then fitted to each end of the housing  3  thereby rotatably supporting the shaft  2  in the housing  3 . 
     The rotational position of the shaft  2  is adjusted to match the holes  3   a  of the housing  3  with the holes  10   a  of the stator  10 . The female connectors  15  are fitted to the holes  3   a . This inserts the pins  14   b  in the receptacles  15   a  of the female connector  15 . In this manner, the female connectors  15  are joined with the male connectors  14  as shown in FIGS. 1,  2  and  4 . As a result, the terminal wires  11   a ,  12   a  are electrically connected with the lead wires  16 . Further, the circumferential positions of the female connectors  15  are fixed by the holes  3   a . Therefore, the connectors  13 ,  15  prevent the stator  10  from rotating relative to the housing  3 . 
     When the shaft  2  rotates, friction in the bearings  9  applies rotational force to the stator  10 . The force is received by the base  14   a  and the pins  14   b  of the connector  13 . Therefore, rotation of the stator  10  relative to the housing  3  is restricted. The terminal wires  11   a ,  12   a , which are soldered to the base  14   a , do not receive tension. In other words, the solder joints coupling the wires  11   a ,  12   a  to the base  14   a  do not receive tension. The solder joints are thus not cracked or weakened. 
     Further, the stator  10  is supported by the bearings  9  to be rotatable relative to the shaft  2 . Therefore, even if the shaft  2  rotates eccentrically relative to the housing  3 , the distance between the stator  10  and the magnetostrictive cylinder  8  is constant. 
     A conventional torque sensor has a shaft and a stator, and the stator is located about the shaft and is fixed to a housing. This construction varies the distance between the stator and a magnetostrictive material located on the shaft when the shaft rotates eccentrically relative to the housing. This may degrade the detection accuracy of the sensor. Therefore, the shaft axis needs to be centered relative to the housing with a relatively high accuracy. Even if eccentric rotation of the shaft is prevented, the position of the stator relative to the shaft varies depending on the machining accuracy of the housing. This fluctuates the distance between the stator and the magnetostrictive material during operation of the sensor. The machining accuracy of the housing therefore needs to be improved. Also, bearings that couple the shaft to the housing need to be located in the vicinity of the stator for preventing eccentric rotation of the shaft. These measures must be taken by users of the torque sensor. However, in the device of FIGS. 1-6, the stator  10  is rotatable relative to the shaft  2 . This construction maintains a constant distance between the stator  10  and the magnetostrictive cylinder  8  thereby solving the drawbacks of the conventional torque sensor. 
     The embodiment of FIGS. 1 to  6  has the following advantages. 
     (1) The connectors  13  prevent the stator  10  from rotating relative to the housing  3 . This eliminates tension applied on part of the terminal wires  11   a ,  12   a  that are connected to the base  14   a  of the male connector  14 . Therefore, the wires  11   a ,  12   a  are not damaged or broken. 
     (2) The male connectors  14  are fitted in the holes  10   a  of the stator  10 . The stator  10  is then fitted about the shaft  2 . Thereafter, the shaft  2  with the stator  10   a  is inserted in the housing  3 . The length of the pins  14   b  is determined such that the pins  14   b  do not contact the inner wall of the housing  3  during the insertion. Thereafter, the female connectors  15  are fitted in the holes  3   a . In this manner, the connectors  13  are easily assembled. 
     (3) The male connectors  14  and the female connectors  15  are assembled by inserting the pins  14   b  in the receptacles  15   a . Therefore, the female connectors  14  are easily coupled with the male connectors  14  by simply fitting the female connectors  15  in the holes  3   a . The connectors  13  are therefore easily assembled. (4) The stator  10  is supported by the bearings  9  to be rotatable relative to the shaft  2 . This construction maintains constant distance between the magnetostrictive cylinder  8  and the stator  10 . In other words, the distance between the cylinder  8  and the stator  10  does not vary. This improves the detection accuracy of the sensor and eliminates the necessity for high machining accuracy of the housing  3 . The construction therefore allows a user to freely change the location of the bearings  4 . 
     The present invention may be alternatively embodied in the following forms: 35 USC 101. 
     As shown in FIG. 7, an elongated single hole  10   b  may be formed in the stator  10  for the terminal wires  11   a ,  12   a  of the two bobbins B. In this case, a single connector  17  is fitted to the hole  10   b . The connector  17  includes a male connector  18  and a female connector  19 . Eight pins  18   b  protrude from a base  18   a  of the male connector  18 . The pins  18   b  are aligned in the longitudinal direction of the base  18   a . The male connector  18  is press fitted in the hole  10   b . The female connector  19  has receptacles  19   a  in its bottom, or inner surface. Each receptacle  19   a  corresponds to and receives one of the pins  18   b . Eight lead wires  16  extend from the top, or outer, surface of the female connector  19 . The housing  3  has a hole  3   b , which is radially aligned with the hole  10   b . The size of the hole  3   b  is determined such that the female connector  19  is fitted in the hole  3   b  while permitting slight radial movements of the connector  19 . In this construction, the eight terminal wires  11   a ,  12   a  of the bobbins B are connected to the single connector  17 . This construction reduces the number of the parts and the number of assembly steps. Further, since the hole  3   b  is relatively large, the pins  18   b  are easily mated with the receptacles  19   a.    
     In the embodiments of FIGS. 1-6 and  7 , steps  10   c  may be formed at the inner end of the holes  10   a  ( 10   b ) and protrusions  14   d  ( 18   d ) may be formed at the lower portion of the base  14   a  ( 18   a ) as illustrated in FIG.  8 . The protrusions  14   d  ( 18   d ) are engaged with the steps  10   c . This construction securely fixes the male connectors  14  ( 18 ) to the stator  10 . Especially, the connectors  13  ( 17 ) are firmly fixed to the stator  10  and are prevented from falling out of or moving radially in the holes  10   a  ( 10   b ). The construction therefore effectively prevents the wires  11   a ,  12   a  from receiving tension. 
     As shown in FIG. 9, axially extending protrusions  10   e  may be formed on the outer surface of the stator  10  at equal angular intervals (for example, four protrusions  10   e  at every ninety degrees). In this case, axially extending recesses  3   c  are formed in the inner wall of the housing  3 . Each recess  3   c  corresponds to one of the protrusions  10   e . The stator  10  is slidably supported in the housing  3  by engaging the protrusions  10   e  with the recesses  3   c  as shown in FIG.  10 . The engagement prevents the stator  10  from rotating relative to the housing  3 . The terminal wires  11   a ,  12   a  and the lead wires  20  receive no tension and are not damaged or broken. There is a small amount of radial play between the protrusions  10   e  and the recesses  3   c . The play permits eccentric rotation of the shaft  2  due to variations of its dimensional accuracy. Alternatively, the protrusions  10   e  may be formed on the inner wall of the housing  3  and the recess  3   c  may be formed in the stator  10 . 
     As shown in FIG. 11, a ring  3   e  may be fitted in the housing  3  to contact a first end face  1 O f  of the stator  10 . Serrations are formed in the first end face  10   f  and in a side of the ring  3   e  that faces the end face  10   f . The serrations of the end face  10   f  form teeth  10   g  that are equally spaced apart in the circumferential direction. Likewise, the ring  3   e  has teeth  3   f  that are equally spaced apart in the circumferential direction. The teeth  10   g  and the teeth  3   f  mesh with each other. A snap ring  21  is fitted in the housing  3  at the opposite end of the stator  10  from the ring  3   e . The snap ring  21  contacts a second end face of the stator  10  and presses the first end face  10   f  of the stator  10  against the ring  3   e . Therefore, even if rotation of the shaft  2  applies rotational force to the stator  10 , engagement of the teeth  3   f  and  10   g  prevents the stator  10  from rotating relative to the housing  3 . This construction thus prevents the terminal wires of the coils  11 ,  12  and lead wires  20  connected to the terminal wires from receiving tension. The wires are therefore not damaged or broken. 
     In the embodiment of FIGS. 1-6, only one connector  13  may be used. The single connector  13  also prevents the stator  10  from rotating relative to the housing  3  and thus prevents the wires  11   a ,  12   a  from receiving tension. 
     In the embodiment illustrated in FIGS. 9 and 10, the protrusions  10   e  and the grooves  3   c  may be omitted and other constructions for preventing the stator  10  from rotating may be employed. For example, aligned holes may be formed in the housing  3  and the stator  10  at positions different from the holes  3   d  and  10   d . A connector is fitted in the aligned holes for restricting rotation of the stator  10 . 
     Instead of initially fitting the male connectors  14  in the stator  10 , the assembled connectors  13  may be fitted in the holes  10   a ,  3   a  before the shaft  2  having the stator  10  is assembled with the housing  3 . Specifically, the shaft  2  having the stator  10  is assembled with the housing  3 . The assembled connectors  13  are then inserted from the holes  3   a  and are fitted to the holes  10   a  and  3   a.    
     A female connector may be fitted in the hole of the stator  10  and a male connector may be fitted in the hole in the housing  3 . 
     The grooves  8   a  on the cylinder  8  may be omitted. In this case, strain of the cylinder  8  is detected by a cross head type pickup.