Abstract:
A rotation sensor functioning as both a rotation-angle sensor and a torque sensor is provided which, when applied to a steering sensor for an automobile, for example, permits reduction in the number of components and thus in the weight of the automobile and contributes to conservation of global environment. The rotation sensor has slip rings covered with conductive synthetic resin having small coefficient of friction, allowing the contact pressure of brushes disposed in sliding contact with the slip rings to be kept low and the life duration of the slip rings to be prolonged. Further, no metal powder is produced when the brushes slide on the respective slip rings, and it is therefore possible to prevent the formation of unwanted insulating film.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
         [0001]    The present application claims priority based on Japanese Patent Application No. 2002-14185, which is incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a rotation sensor, such as a steering sensor for an automobile, which is capable of measuring the angle of rotation of a shaft and torque acting thereon.  
           [0004]    2. Description of the Related Art  
           [0005]    With the recent movement toward the conservation of global environment, researches have been devoted to reducing the weight of a vehicle body, as a measure to reduce the exhaust gas from the automobile. One of such measures for lightening the vehicle body is to replace a hydraulic power steering assembly, which is one of the causes of increased weight of the vehicle body, with an electric power steering assembly. However, in the case of using an electric power steering assembly, an assisting force for a steering shaft needs to be computed using a computer. Accordingly, the electric power steering assembly requires two sensors, i.e. a rotation angle sensor (steering angle sensor) for measuring the rotational angle (steering angle) of the steering shaft and a torque sensor for measuring the torque applied to rotate the steering shaft. Thus, the number of components increases, making it difficult to reduce the weight of the vehicle body as intended.  
           [0006]    Such a rotation angle sensor or torque sensor conventionally includes a type having mechanism where a brush is disposed in sliding contact with a conductive ring arranged concentrically with the shaft, for detecting the rotation angle or torque of the shaft. The mechanism, however, has a disadvantage that metal powder scatters due to friction when the brush and the slip ring, both made of metal, are in siding contact with each other. The metal powder undergoes a mechanochemical reaction with organic gas in the air, causing the formation of an insulating film and consequent unexpected increase of electric resistance.  
           [0007]    It is possible to avert such increase of electric resistance by increasing the contact pressure of the brush and thereby scraping away the insulating film. However, this shortens the life duration of the slip ring.  
         SUMMARY OF THE INVENTION  
         [0008]    It is an object of the present invention to provide a rotation sensor which functions as both a rotation angle sensor and a torque sensor and is capable of overcoming the aforementioned shortcomings.  
           [0009]    It is another object of the present invention to provide a rotation sensor which ensures a long life duration of a slip ring.  
           [0010]    According to the present invention, to accomplish the aforementioned objects there is provided a rotation sensor comprising an active rotor, a follower rotor arranged adjacent to the active rotor, the follower rotor sharing a rotation axis with the active rotor and capable of rotation following the active rotor, and a casing containing the active and follower rotors and allowing the rotors to rotate freely, wherein a disk-like slip-ring retainer plate is secured to an outer periphery of the active rotor for rotation together therewith, feeder slip rings, rotation-angle measuring slip rings, and torque measuring slip rings are all disposed on the slip-ring retainer plate and concentrically with the active rotor, the rotation-angle measuring slip rings and the torque measuring slip rings are covered with conductive resin, the torque measuring slip rings are arranged on one side of the slip-ring retainer plate facing the follower rotor, the feeder slip rings and the rotation-angle measuring slip rings are arranged on the other side of the slip-ring retainer plate opposite to the torque measuring slip rings, feeder brushes and rotation-angle measuring brushes are fixed to the casing for sliding contact with the feeder slip rings and the rotation-angle measuring slip rings, respectively, a torque measuring brush retainer plate is connected to an outer periphery of the follower rotor for rotation together therewith, torque measuring brushes are fixed to the torque measuring brush retainer plate for sliding contact with the respective torque measuring slip rings, the feeder bushes are electrically connected to each other and are connected to a power supply device, the rotation-angle measuring brushes are electrically connected to the feeder brushes and a rotation-angle computing device, and the torque measuring brushes are electrically connected to the feeder brushes and a torque computing device.  
           [0011]    Other objects, features and advantages of the present invention will be apparent from the following detailed description. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a block diagram illustrating a steering assembly for an automobile employing a rotation sensor in accordance with one embodiment of the present invention;  
         [0013]    [0013]FIG. 2 is a perspective view of the rotation sensor shown in FIG. 1;  
         [0014]    [0014]FIG. 3 is a cross sectional view taken along the line A-A in FIG. 2;  
         [0015]    [0015]FIG. 4 is a view of a slip-ring retainer plate shown in FIG. 3;  
         [0016]    [0016]FIG. 5 is an exploded perspective view of a coupler shown in FIG. 3;  
         [0017]    [0017]FIG. 6 is a cross sectional view taken along the line B-B in FIG. 3;  
         [0018]    [0018]FIG. 7 is a perspective view of a coupler in accordance with another embodiment of the present invention;  
         [0019]    [0019]FIG. 8 is a plan view of the coupler of FIG. 7 coupled to a lower brush retainer plate shown in FIG. 3;  
         [0020]    [0020]FIG. 9 is a perspective view of a lower surface of the slip-ring retainer plate shown in FIG. 3;  
         [0021]    [0021]FIG. 10 is a graph depicting change of voltages relating to torque measurement;  
         [0022]    [0022]FIG. 11 is a graph depicting change of voltages relating to torque measurement in case of abnormality;  
         [0023]    [0023]FIG. 12 is a perspective view of torque measuring slip rings laid out differently from those in accordance with the embodiment of the present invention;  
         [0024]    [0024]FIG. 13 is a graph depicting change of voltages relating to torque measurement observed in cases where vibration occurs in the torque measuring brushes disposed in sliding contact with the torque measuring slip rings shown in FIG. 12; and  
         [0025]    [0025]FIG. 14 is a graph depicting change of voltages relating to torque measurement observed in cases where the vibration occurs in the torque measuring brushes disposed in sliding contact with the torque measuring slip rings shown in FIG. 9. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]    Embodiments of the present invention will be described hereinafter in detail with reference to the accompanying drawings.  
         [0027]    [0027]FIG. 1 is a block diagram illustrating a power steering system  1  for an automobile to which is applied a rotation sensor according to an embodiment of the present invention. In the power steering system  1 , as a steering wheel  2  is turned, a steering shaft  3  turns together with the steering wheel. The steering shaft  3  has a torsion bar, not shown, arranged therein and extending coaxially therewith, and the torsion bar further extends through a column shaft  4  coaxially therewith. Thus, when the steering shaft  3  is rotated, this rotation is transmitted via torsion of the torsion bar to the column shaft  4  so that the column shaft also rotates.  
         [0028]    A rotation sensor  5  is disposed at a joint between the steering shaft  3  and the column shaft  4 , so as to cover the adjoining ends of the respective shafts. This rotation sensor  5  detects not only the angle of rotation of the steering shaft  3  (i.e. steering angle of the steering wheel) but also torque applied to rotate the steering shaft. The steering angle and torque detected by the rotation sensor  5  are transmitted to a controller  6 .  
         [0029]    The column shaft  4  is connected to a steering gear  7 , to which an assisting force is provided by a servo motor M. Based on the detected steering angle and torque, the controller  6  instructs servo motor M to output a required assisting force.  
         [0030]    The rotation of the steering gear  7  is transmitted via a rack-and-pinion assembly  8  to tires  9 , the direction of the tires changes.  
         [0031]    [0031]FIG. 2 is a perspective view of the rotation sensor  5  in accordance with the embodiment shown in FIG. 1. The rotation sensor  5  has an active rotor  21  and a follower rotor (not shown) having a common rotation axis Art and contained in a casing  11  which includes a vessel  11   a  and a lid  11   b . The rotation sensor  5  is supplied with electric power through a feeding cable  13  from a feeder unit (not shown) inside a controller  6 . Additionally, the rotation sensor  5  sends detected voltages indicative of steering angle and torque to be measured, respectively, through a steering-angle measuring cable  14  and a torque measuring cable  15  to a steering-angle measuring computing unit and a torque computing unit arranged inside the controller  6 , respectively.  
         [0032]    [0032]FIG. 3 is a cross sectional view of the rotation sensor  5  taken along the line A-A in FIG. 2. The active rotor  21 , which is located at an upper part of the rotation sensor  5 , is secured to the steering shaft  3 . On the other hand, the follower rotor  31 , which is located at a lower part of the rotation sensor  5 , is secured to the column shaft  4 . The steering shaft  3  and the column shaft  4  share the rotation axis Art. A torsion bar  32  extends through the steering shaft  3  and column shaft  4  coaxially therewith and is connected to both of the shafts.  
         [0033]    An upper brush retainer plate  33  is secured to the inner surface of the lid  11   a  of the casing  11 , in a manner interposed between the lid  11   a  and the vessel  11   b . This upper brush retainer plate  33  retains feeding brushes  34   a ,  34   b  and steering-angle measuring brushes  35   a ,  35   b . The feeding brushes  34   a ,  34   b  are electrically connected to the feeding cable  13  shown in FIG. 2. On the other hand, the steering-angle measuring brushes  35   a ,  35   b  are electrically connected to the steering-angle measuring cable  14  shown in FIG. 2. Only the steering-angle measuring cable  14  connected to the steering-angle measuring brush  35   a  is shown in FIG. 3.  
         [0034]    Under these brushes, a slip-ring retainer plate  40  is positioned and secured to the periphery of the end of the active rotor  21 . The slip-ring retainer plate  40  rotates together with the active rotor  21 . The slip-ring retainer plate  40  has feeder slip rings  41  and steering-angle measuring slip rings  42  arranged on an upper surface thereof concentrically with the rotation axis A rt . These feeder slip rings  41  and steering-angle measuring slip rings  42  are disposed in sliding contact with the corresponding feeding brushes  34   a ,  34   b  and steering-angle measuring brushes  35   a ,  35   b.    
         [0035]    The rotation sensor  5  is fed with electric power through the feeding brushes  34   a ,  34   b  which are in sliding contact with the feeder slip rings  41 . Also, the feeder slip rings  41  are electrically connected to the steering-angle measuring slip rings  42 .  
         [0036]    Further, the slip-ring retainer plate  40  has two torque measuring slip rings  43   a ,  43   b  (each with the shape of an arc) disposed on a lower surface thereof concentrically with the rotation axis Art. These torque measuring slip rings  43   a ,  43   b  are electrically connected to the feeder slip rings  43   a ,  43   b , respectively.  
         [0037]    A lower brush retainer plate  45  is located under the torque measuring slip rings  43   a ,  43   b . The lower brush retainer plate  45  is connected via an elastic coupler  44  to a flange  31   a  of the follower rotor  31  and retains torque measuring brushes  46   a ,  46   b  in position such that the brushes  46   a ,  46   b  come into sliding contact with the respective torque measuring slip rings  43   a ,  43   b . The torque measuring brushes  46   a ,  46   b  are connected to the torque measuring cable  15 .  
         [0038]    When the steering wheel is turned, the steering shaft  3  rotates inside the casing  11  of the rotation sensor  5 . Thereupon, the torsion bar  32  twists elastically, so that the column shaft  4  rotates following the twisting of the torsion bar. Thus, when the steering shaft  3  rotates, the active rotor  21  fixedly connected thereto rotates, and also the follower rotor  31  secured to the column shaft  4  rotates.  
         [0039]    When the steering shaft  3  rotates, the radial position of the feeder slip rings  41  does not move since the feeder slip rings  41  are retained concentrically with the slip-ring retainer plate  40  rotating together with the active rotor  21 . Therefore, the feeding brushes  34   a ,  34   b , which are secured to the casing  11  via the slip-ring retainer plate  33 , remain in sliding contact with the feeder slip rings  41 , so that the supply of electric power is never interrupted.  
         [0040]    The steering-angle measuring slip rings  42  are fed with electric power from the feeder slip rings  41  electrically connected thereto. Like the feeding brushes  34   a ,  34   b , the steering-angle measuring brushes  35   a ,  35   b  are kept in sliding contact with the respective steering-angle measuring slip rings  42 . As the steering-angle measuring slip rings  42  rotate and thus their position relative to the steering-angle measuring brushes  35   a ,  35   b  changes in the circumferential direction, the voltage ratio along the steering-angle measuring slip rings  42  changes. Accordingly, the controller  6  can computes a steering angle of the steering wheel, based on this voltage ratio.  
         [0041]    When the follower rotor  31  rotates, the torque measuring brushes  46   a ,  46   b  retained on the lower brush retainer plate  45  rotate together with the follower rotor  31  via the coupler  45 . While the torque reaction transmitted via the tires from the road surface is small, there is no difference of rotation angle occurs between the steering shaft  3  (and the active rotor  21 ) and the column shaft  4  (and the follower rotor  31 ). Thus, the contact position of the torque measuring slip rings  43   a ,  43   b  rotating together with the active rotor  21  relative to the torque measuring brushes  46   a ,  46   b  rotating together with the follower rotor  31  remains unchanged.  
         [0042]    However, as the torque reaction increases, the torsion bar  32  begins to twist, causing a difference of rotation angle between the column shaft  4  (and the follower rotor  31 ) and the steering shaft  3  (and the active rotor  21 ). Consequently, the contact position of the torque measuring brushes  46   a ,  46   b  relative to the torque measuring slip rings  43   a ,  43   b  shifts in the circumferential direction. In this case, since the electric resistance changes by an amount corresponding to the distance over which the torque measuring brushes  46   a ,  46   b  move along the torque measuring slip rings  43   a ,  43 , the voltage changes correspondingly. Therefore, the controller  6  can compute a torque based on this voltage change.  
         [0043]    Accordingly the rotation sensor  5  serves not only as a rotation sensor but as a torque sensor. Thus, where the rotation sensor is incorporated in an automobile, for example, it is possible to reduce the number of components, thereby reducing the weight of the vehicle body.  
         [0044]    [0044]FIG. 4 is an enlarged view of the slip-ring retainer plate  45  shown in FIG. 3. As is apparent from the figure, the steering-angle measuring slip rings  42  and torque measuring slip rings  43   a ,  43   b  are each covered with a resin film  50  with low frictional resistance. This synthetic resin film  50  is made of epoxy resin containing carbon and thus has electric conductivity.  
         [0045]    According to this embodiment, the steering-angle measuring brushes  35   a ,  35   b  and torque measuring brushes  46   a ,  46   b  do not directly contact the metal slip rings, and thus no metal powder is produced due to the friction. Thus, no insulating film is formed through the mechanochemical reaction of metal powder. Furthermore, the contact pressure of each brush can be made small by virtue of the low-frictional synthetic resin film  50 .  
         [0046]    Accordingly, the steering-angle measuring brushes  35   a ,  35   b  and torque measuring brushes  46   a ,  46   b  do not directly contact the corresponding metal slip rings  41 ,  42 ,  43   a ,  43   b , and further, since the contact pressure is small, the life duration of each slip ring can be prolonged.  
         [0047]    The feeder slip rings  41  are not covered with synthetic resin film in order to minimize the potential drop.  
         [0048]    To cut down the production cost, no synthetic resin film may be formed on those portions of the slip rings which are significantly remote from the initial position (i.e. neutral position of the steering wheel) and thus which scarcely come into sliding contact with the respective brushes.  
         [0049]    [0049]FIG. 5 is a perspective view of the elastic metal coupler  44  shown in FIG. 3. Two pairs of bolt holes  51  are formed on opposite sides of a circular base  44   a  as viewed in the direction of x-axis. Inner mounting protrusions  44   b  protrude from the base  44   a  and are situated under the bolt holes  51 , respectively. Two bolt holes  52  are formed in each of the inner mounting protrusions  44   b  in alignment with the respective bolt holes  51  in the direction of z-axis. Outer mounting protrusions  44   c  are formed on opposite sides of the circular base  44   a  as viewed in the direction of y-axis via curved portions  44   b . Two bolt holes  53  are formed in each of the outer mounting protrusions  44   c.    
         [0050]    The coupler  44  is secured to the flange  31   a  of the follower rotor  31  by means of nuts  54   b  and bolts  54   a  inserted through the bolt holes  51 ,  52 . Also, the coupler  44  is secured to the lower brush retainer plate  45  by means of nuts  55   b  and bolts  55   a  inserted through the bolt holes  53 .  
         [0051]    The cross sectional view of FIG. 3 illustrates the coupler  44  as viewed from the direction of C in FIG. 5 so that the mounting of the coupler  44  may be understood more easily.  
         [0052]    [0052]FIG. 6 is a cross sectional view taken along the line B-B in FIG. 3. The circular base  44   a  of the coupler  44  is fitted in the follower rotor  31  and is coupled to the flange  31   a  (not shown in FIG. 6) by means of bolts  54   a , as mentioned above. The coupler  44  is further coupled, by means of bolts  55   a , to the lower brush retainer plate  45  contained in the vessel  11   b . The two torque measuring brushes  46   a ,  46   b  connected to the torque measuring cable  15  are disposed on the same side of the lower brush retainer plate  45  in alignment with a line L crossing the rotation axis Art. As the lower brush retainer plate  45  rotates, the torque measuring brushes  46   a ,  46   b  slide on the respective torque measuring slip rings  43   a ,  43   b  in the circumferential direction while keeping their alignment with the line L.  
         [0053]    The function of the coupler  44  is as follows: During rotation of the follower rotor  31 , the rotation axis thereof can occasionally become misaligned with the rotation axis A rt  in the x-y plane. In such a case, the curved portions  44   c  absorb the misalignment in the direction of y-axis by their elastic deformation as seen from FIGS. 5 and 6. Moreover, since the curved portions  44   c  each have a small width in the direction of x-axis, the base  44   a  secured to the flange  31   a  of the follower rotor  31  is allowed to move elastically in the direction of x-axis by the distance of misalignment with respect to the outer mounting protrusions  44   d  secured to the lower brush retainer plate  45  by means of the bolts  55   a.    
         [0054]    Thus, the lower brush retainer plate  45  can keep the position thereof on the x-y plane irrespective of axial misalignment of the follower rotor, thereby preventing the torque measuring brushes  46   a ,  46   b  from being misaligned in the x-y plane and thus from becoming out of contact with the torque measuring slip rings  43   a ,  43   b.    
         [0055]    Even though the follower rotor  31  tilts, the coupler  44  can keep the lower brush retainer plate  45  in position since the coupler  44  has elasticity also in the direction of z-axis by virtue of the curved portions  44   c.    
         [0056]    The coupler  44  has sufficiently high rigidity in the circumferential direction, as is apparent from the shape thereof shown in FIG. 5. Thus, the lower brush retainer plate  45  is capable of rotating together with the follower rotor  31  via the coupler  44 .  
         [0057]    [0057]FIG. 7 is a perspective view of a coupler  71  according to another embodiment, which can be formed more easily than the aforementioned coupler. The coupler  71  is formed by punching a square metal plate and has an outer part  72 , an inner part  73  and an intermediate part  74 . The intermediate part  74  has two pairs of bolt holes  74   a  on opposite sides thereof as viewed in the direction of x-axis. The outer part  72  has two pairs of bolt holes  72   a  on opposite sides thereof as viewed in the direction of y-axis. The inner part  73  has a bolt hole  73   a  formed in each of its four corners. The outer part  72  has folded portions  72   b  bent in the direction of z-axis at both sides thereof as viewed in the direction of x-axis. The intermediate part  74  also has folded portions  74   b  bent in the direction of z-axis at both sides thereof as viewed in the direction of y-axis. The inner part  73  and the intermediate part  74  are connected to each other by y-axis direction bridges  75 , and the outer part  72  and the intermediate part  74  are connected to each other by x-axis direction bridges  76 .  
         [0058]    [0058]FIG. 8 is a plan view illustrating the coupler  71  of FIG. 7 attached to both the follower rotor  31  and the lower brush retainer plate  45 . The inner part  73  of the coupler  71  is fitted in the follower rotor  31  and is secured to the flange  31   a  by means of bolts  77 . The outer part  72  and the intermediate part  74  are secured to the lower brush retainer plate  45  by means of bolts  78  and  79 , respectively.  
         [0059]    When the axis of the follower rotor  31  becomes misaligned in the x-y plane, the misalignment in the direction of x-axis is absorbed by elastic deformation of the y-axis direction bridges  75  in the direction of x-axis. On the other hand, the misalignment in the direction of y-axis is absorbed with the thin x-axis direction bridge  76  elastically deforming in the direction of y-axis. Therefore, even when the misalignment of the follower rotor  31  occurs, the lower brush retainer plate  45  can remain in its proper position.  
         [0060]    [0060]FIG. 9 is a perspective view of the slip-ring retainer plate  40 , showing a lower surface thereof on which are provided the torque measuring slip rings  43   a ,  43   b . As mentioned above, the torque measuring slip rings  43   a ,  43   b  each have the shape of an arc having a center angle θ about the rotation axis At, since the difference of rotation angle between the active rotor and the follower rotor cannot become very great. The inner torque measuring slip ring  43   a  is connected to the feeder slip ring  41  so that its back end may be at a ground voltage P 0  (=0V). On the other hand, the outer torque measuring slip ring  43   b  is connected to the feeder slip ring  41  so that its front end may be at a ground voltage P 0 .  
         [0061]    As mentioned above, the torque measuring brushes  46   a ,  46   b  slide on the respective torque measuring slip rings  43   a ,  43   b  about the rotation axis A rt , while being located on the line L passing the rotation axis A rt . As shown in the figure, the initial position (neutral position) of the torque measuring brushes  46   a ,  46   b  is set at an intermediate point (i.e. the position of θ/2) of the torque measuring slip rings  43   a ,  43   b . The sign indicative of the difference of rotation angle between the active and rotors is defined as follows: The sign is plus when the torque measuring brushes  46   a ,  46   b  relatively move clockwise; and the sign is minus when they relatively move counterclockwise.  
         [0062]    [0062]FIG. 10 depicts voltages detected as the torque measuring brushes  46   a ,  46   b  slide on the respective slip rings. As the torque measuring brushes  46   a ,  46   b  slide in the positive (+) direction of the angle difference, the voltage detected by the torque measuring brush  46   a  shows a straight line T 1 . Because the detected voltage decreases down to the ground voltage. On the other hand, the voltage detected by the torque measuring brush  46   b  shows a different straight line T 2 , because the detected voltage increases more and more from the ground voltage. As far as no abnormality occurs, both of the torque measuring brushes  46   a ,  46   b  keep their positions aligned with the line L. Thus, the absolute value of the gradient of the voltage T 1  is equal to that of the voltage T 2 .  
         [0063]    To detect the occurrence of abnormal voltage, a voltage T 3  is used which is the sum of the voltages T 1  and T 2 . As far as no abnormality occurs, the voltage T 3  takes a constant value. Based on the voltages T 1  and T 2  the controller  6  computes the sum voltage T 3  and detects the occurrence of abnormality.  
         [0064]    As shown in FIG. 11, if the voltage T 2  shows an abnormal value T ir2  for some reason, then the voltage T 3  takes an abnormal value T ir3 , deviating from the constant value. In such a case, the controller recognizes the occurrence of abnormality and carries out a predetermined process. The controller, however, is set in such a manner that a deviance falling within a predetermined range may not to be regarded as an abnormality.  
         [0065]    [0065]FIG. 12 illustrates the layout of torque measuring slip rings different from that of the embodiment of the present invention. The torque measuring slip rings  81   a ,  81   b  are disposed on diametrically opposite side with respect to the rotation angle A rt  and occupy regions corresponding to a center angle θ. Torque measuring brushes  82   a ,  82   b  are also positioned diametrically opposite to each other with respect to the rotation angle A rt . However, this layout is similar to that shown in FIG. 10 in that the initial position of the torque measuring brushes  82   a ,  82   b  lies on the intermediate point of the torque measuring slip rings  81   a ,  81   b  and that the sign indicative of the difference of rotation angle is plus when the torque measuring brushes  82   a ,  82   b  move clockwise and is minus when the brushes  82   a ,  82   b  move counterclockwise. The back ends of the torque measuring slip rings  81   a ,  81   b  as viewed in FIG. 12 may be at a ground voltage.  
         [0066]    [0066]FIG. 13 depicts voltages detected by the torque measuring slip rings  81   a ,  81   b  and the torque measuring brushes  82   a ,  82   b . While no abnormality is observed, voltages T 1 ′ and T 2 ′ detected by the torque measuring brushes  82   a ,  82   b , respectively, indicate the same values as the voltage T 1  and T 2 .  
         [0067]    However, it is very often that the automobile undergoes a sudden vibration while the torque measuring brushes  82   a ,  82   b  slide in the circumferential direction, with the result that both of the torque measuring brushes  82   a ,  82   b  translate or move in the same direction on the x-y plane as indicated by arrows Q. In this case, the voltage T 1 ′ detected by the torque measuring brush  82   a  deviates in the direction away from the ground voltage P 0 , showing an increased voltage Td 1 ′. On the other hand, the voltage T 2 ′ detected by the torque measuring brush  82   b  also deviates in the direction away from the ground voltage P 0 , similarly showing an increased voltage Td 1 ′.  
         [0068]    Consequently, the sum voltage T 3 ′ of the voltages T 1 ′ and T 2 ′ increases up to Td 3 ′ in a region where the voltages Td 1 ′ and Td 2 ′ are summed up. Accordingly, the voltage T 3 ′ exceeds a threshold voltage V t  for discriminating the occurrence of abnormality. It is, however, undesirable that such vibration to be frequently observed and to fall within a normal range be regarded as abnormality.  
         [0069]    According to the embodiment of the present invention shown in FIG. 9, the voltage T 1  detected by the torque measuring brush  46   a  deviates in the direction away from the ground voltage P 0 , showing a voltage Td 1 , as shown in FIG. 14, in case of the aforementioned translation or movement in the direction Q. On the other hand, the voltage T 2  detected by the torque measuring brush  46   b  deviates in the direction toward the ground voltage P 0 , showing a voltage Td 2 . Therefore, the voltages Td 1  and Td 2  offset each other, thereby keeping the sum voltage Td 3  constant and preventing a normal vibration from being regarded as abnormality.  
         [0070]    It should be noted that the present invention is not limited to the embodiments described hereinabove. For example, the coupler  44  may be provided on the active rotor side.  
         [0071]    It is also possible to apply the rotation sensor of the present invention to the links of a plurality of arms of a robot, etc.  
         [0072]    It is to be noted that all suitable modifications and equivalents may be regarded as falling within the scope of the invention in the appended claims and their equivalents.