Abstract:
An object of the present invention is to provide a compact torque detector. The present invention solving the object has a conversion member  23  provided on the outer circumferences of an input shaft  20  and an output shaft  21  for convert the amount of relative rotation of the input and output shafts to a displacement in the axis direction; an annular-shaped ring member  26  secured on the outer circumference of the conversion member  23  and made up of a magnet; and a plurality of magnetically sensitive devices  27  arranged at intervals around the ring member  26  and facing the ring member. The magnetically sensitive device  27  detects the amount of travel of the ring member  26  in the axis direction and supplying the detected amount as a voltage signal to a controller C.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to a power steering for applying an assist force using an electric motor, and particularly, to a torque detector for detecting steering torque supplied from a steering wheel.  
           [0003]    2. Description of Related Art  
           [0004]    Torque detectors for a power steering include, for example, the torque detector disclosed in Japanese Patent Laid-open No. 3-74258 by the present applicant.  
           [0005]    In the torque detector, as shown in FIG. 12 and FIG. 13, an output shaft  2  is rotatably mounted in a gear case  1  and has an end at which a pinion  3  is provided. The pinion  3  is engaged with a rack shaft  4  having both ends linking with wheels (not shown).  
           [0006]    The rack shaft  4  is linked with an electric motor (not shown) for applying an assist force.  
           [0007]    In the gear case  1 , an input shaft  5  linking with the steering wheel (not shown) is rotatably mounted coaxially with the output shaft  2 . A torsion bar  6  is provided in a hollow portion  5   a  of the input shaft  5 , and has one end secured to the input shaft  5  and the other end secured to the output shaft  2 .  
           [0008]    In the detector constructed as above, upon turning the steering wheel (not shown), the torsion bar  6  is twisted in accordance with the input torque of the turned steering wheel, to cause a relative rotation between the input shaft  5  and the output shaft  2 .  
           [0009]    A sensor  18  detailed later detects the amount of relative rotation and the direction of the input and output shafts, and a controller (not shown) controls output of the electric motor based on the detected amount and direction.  
           [0010]    The electric motor thus controlled by the controller exerts a driving force in accordance with the input torque on the rack shaft  4  to drive the rack shaft  4  for application of an optimum assist force.  
           [0011]    Next, the configuration for detecting the input torque will be explained.  
           [0012]    A flange  7  is provided on the input shaft  5 . In the flange  7 , three planetary gears  8  are arranged at regular intervals along the circumferential direction with respective pins  9  as shown in FIG. 13. The planetary gears  8  engage with a sun gear  10  rotatably provided on the outer circumferential face of the input shaft  5  and a rotatable ring gear  11  rotatably mounted in the gear case  1 .  
           [0013]    Another flange  12  is provided on the output shaft  2 , and three planetary gears (not shown) are also arranged at regular intervals along the circumferential direction with respective pins. The planetary gears engage with the sun gear  10  and a fixed ring gear  13  secured in the gear case  1 .  
           [0014]    In the flange  7  provided on the input shaft  5 , two notches  14  are provided as shown in FIG. 13. Protrusions  15  are provided in the flange  12  on the output shaft  2 , and respectively inserted into the notches  14 .  
           [0015]    A clearance  16  is provided between each notch  14  and the corresponding protrusion  15  to allow the input shaft  5  to rotate relative to the output shaft  2  in the circumferential direction. When the notch  14  and the protrusion  15  make contact with each other, the input shaft  5  and the output shaft  2  integrally rotate. This prevents the torsion bar  6  from becoming extremely twisted and thereby damaged.  
           [0016]    A magnetic piece  17  is incorporated in a portion of the outer circumference of the aforementioned rotatable ring gear  11 . The sensor  18  made up of a magnetic resistor element is provided on the inner surface of the gear case  1  facing the magnetic piece  17 .  
           [0017]    The magnetic resistor element has a facility in that it varies an electric resistance therein upon reception of external action of magnetic field and detects the resistance variation as a variation of voltage.  
           [0018]    The sensor  18  made up of such a magnetic resistor element detects a voltage in accordance with the magnetic flux variation when the magnetic piece  17  is moved by the rotation of the rotatable ring gear  11 . The sensor  18  inputs the detected voltage to the controller (not shown) and the controller controls output of the electric motor. The electric motor exerts the assist force in response to signals from the controller to reduce a steering force to an optimal force.  
           [0019]    Next, the operation of the conventional torque detector will be explained.  
           [0020]    In the state that a load is transmitted from the wheels (not shown) to the rack shaft  4 , when the steering wheel (not shown) is turned, the turning force is transmitted to the input shaft  5 . However, since the load from the wheels interferes with the rotation of the output shaft  2 , the torsion bar  6  is twisted by the rotation of the input shaft  5 . Thus, the input shaft  5  and the output shaft  2  are relatively rotated within the range of the clearance  16 .  
           [0021]    When the input shaft  5  and the output shaft  2  relatively rotate in this way, the rotatable ring gear  11  rotates as follows.  
           [0022]    The limitation of the rotation of the output shaft  2  does not allow the planetary gears (not shown) linked with the output shaft  2  to revolve around the sun gear  10 . Additionally, the planetary gears (not shown) cannot rotate due to the engagement with the fixed ring gear  13 . For those reasons, the sun gear  10  engaging with the planetary gears (not shown) is under the condition that its rotation is limited.  
           [0023]    Under such condition, when the input shaft  5  rotates in relation to the output shaft  2 , the planetary gears  8  provided in the flange  7  on the input shaft  5  rotate and revolve around the sun gear  10 . Therefore, the rotatable ring gear  11  engaging with the planetary gears  8  rotates slightly.  
           [0024]    Upon the rotatable ring gear  11  slightly rotating as explained above, the magnetic piece  17  secured in the ring gear  11  also travels. The sensor  18  detects the amount of travel of the magnetic piece  17 , and the controller (not shown) controls the electric motor in response to the detected signal. In this way, the electric motor applies an optimum assisting force.  
           [0025]    Since the sensor  18  and the magnetic piece  17  are out of contact with each other as explained above, there is no significant influence of wear and the like in use of a contact type sensor. This allows the detection of high accurate values.  
           [0026]    On the other hand, when the input shaft  5  and the output shaft  2  integrally rotate while the torsion bar  6  is twisted, the rotatable ring gear  11  is adapted to stop rotating. This situation will be detailed below.  
           [0027]    When the input shaft  5  and the output shaft  2  integrally rotate, the planetary gears (not shown) provided on the output shaft  2  side rotate and revolve around the sun gear  10 . Therefore, the sun gear  10  engaging with the planetary gears rotates in the same direction as that of the input shaft  5  and output shaft  2 .  
           [0028]    The planetary gears  8  provided on the input shaft  5  side revolve in the same direction as that of the input shaft  5  and output shaft  2 , while being rotated by the rotation of the sun gear  10 . That is to say, the planetary gears  8  rotate while revolving around the sun gear  10 .  
           [0029]    Such planetary gears  8  travel on the inner face of the rotatable ring gear  11  while engaging therewith due to their rotation. The speed of travel of the planetary gear  8  on the rotatable ring gear  11  is set to be equal to the speed of voluntary revolution of the planetary gear  8  around the sun gear  10 . In other words, the planetary gears  8  travel on the inner face of the rotatable ring gear  11  with simply engaging therewith such that the rotating force of the planetary gears  8  does not act on the rotatable ring gear  11 . Naturally, the rotatable ring gear  11  does not rotate as long as the rotating force of the planetary gears  8  does not act thereon.  
           [0030]    The reason for limiting the rotation of the rotatable ring gear  11  as described above is in order to prevent the magnetic piece  17 , provided in the rotatable ring gear  11 , from departing from the position facing the sensor  18  even in the integral rotation of the input shaft  5  and the output shaft  2 .  
           [0031]    Specifically, the magnetic piece  17  is provided only in a portion of the outer circumference of the rotatable ring gear  11 . Therefore, if the rotatable ring gear  11  rotates integrally with the input shaft  5  and output shaft  2 , the magnetic piece  17  naturally departs from the position facing the sensor  18 . In this event, there is the disadvantage that a displacement of the rotatable ring gear  11  is impossible to detect.  
           [0032]    For this reason, the conventional detector establishes limitation for the rotation of the rotatable ring gear  11  in order that the magnetic piece  17  faces the sensor  18  at all times.  
           [0033]    In the above conventional detector, in order to prevent the magnetic piece  17  provided in the rotatable ring gear  11  from departing from the position facing the sensor  18  even when the input shaft  5  and the output shaft  2  integrally rotate, it is needed to provide the planetary gear unit composed of the planetary gears  8 , the rotatable ring gear  11  and so on.  
           [0034]    However, providing such a planetary gear unit naturally produces a disadvantage of increasing in size of the gear case  1  housing the planetary gear unit.  
           [0035]    Further, the power steering device including such torque detector must be placed typically in a very small space between the foot of the driver seat and the engine room. Therefore, if the gear case  1  is increased in size as explained above, this produces a disadvantage in which the detector cannot be mounted depending on the car models or vehicle types.  
         SUMMARY OF THE INVENTION  
         [0036]    It therefore is an object of the present invention to provide a compact torque detector.  
           [0037]    A first invention is a torque detector for a power steering, which detects input torque from the amount of relative rotation of an input shaft, linking with a steering wheel, and an output shaft linking with wheels, and supplies the detected signal to a controller, characterized by including: a conversion member provided on the outer circumferences of the input shaft and the output shaft for convert the amount of relative rotation of the input and output shafts to a displacement in the axis direction; an annular-shaped ring member formed of a magnet and secured on the outer circumference of the conversion member; and a plurality of magnetically sensitive devices arranged at intervals around the ring member to oppose each other, in which the magnetically sensitive device detects the amount of travel of the ring member in the axis direction and supplying the detected amount as a voltage signal to the controller.  
           [0038]    It should be noted that the magnet in the present invention refers to a magnet generating a magnet flux.  
           [0039]    According to the first invention, the conversion member traveling in the axis direction in accordance with the amount of relative rotation of the input shaft and the output shaft is provided with the annular-shaped ring member made up of a magnet. The amount of travel of the ring member in the axis direction is detected by the magnetically sensitive device. The reason for such a configuration is that even when the input shaft and the output shaft integrally rotate, the ring member faces the magnetically sensitive device at all times. Hence, torque can be detected without using the planetary gear unit conventionally, resulting in the reduction in size of the torque detector equal to the space conventionally used for the planetary gear unit.  
           [0040]    A second invention is characterized in that the magnetically sensitive devices are connected to a fail detecting mechanism, and the fail detecting mechanism detects a difference between the voltage signals supplied from the magnetically sensitive devices and supplies a fault signal to the controller when the difference exceeds a set value.  
           [0041]    According to the second embodiment, when a false signal is input to the controller C because of a failure of the magnetically sensitive device or the like, the fail detecting mechanism supplies a fault signal to the controller, and then the controller stops the electric motor.  
           [0042]    Hence, the electric motor does not exert an unexpected assist force.  
           [0043]    A third invention is characterized in that a plurality of the magnetically sensitive devices are arranged around the ring member, and the controller averages the detected signals supplied from the magnetically sensitive devices.  
           [0044]    According to the third invention, since an average of the detected values supplied from a plurality of the magnetically sensitive devices is found, it is possible to accomplish exact detection of the input torque.  
           [0045]    A fourth invention is characterized in that a plurality of the magnetically sensitive devices are arranged at regular intervals in the circumferential direction.  
           [0046]    According to the fourth invention, a plurality of the magnetically sensitive devices provided around the ring member are arranged at regular intervals in the circumferential direction. This allows the detection of balanced information, resulting in detection of the input torque with further improved precision.  
           [0047]    A fifth invention is characterized in that the conversion member is a tubular member.  
           [0048]    According to the fifth invention, since the conversion member is the tubular member that requires a small mounting space, further reduction in size of the torque detector can be accomplished.  
           [0049]    A sixth invention is characterized in that the ring member is secured on the tubular member made of metal and a non-magnetic substance is interposed between the tubular member and the ring member.  
           [0050]    According to the sixth invention, by interposing the non-magnetic substance between the ring member and the tubular member, it becomes possible to make the tubular member of robust metal.  
           [0051]    A seventh invention is a torque detector for a power steering, which detects input torque from the amount of relative rotation of an input shaft linking with a steering wheel and an output shaft linking with wheels and supplies the detected signal to a controller, characterized by including: a conversion member made of up a magnet and provided on the outer circumferences of the input shaft and the output shaft for convert the amount of relative rotation of the input and output shafts to a displacement in the axis direction; a plurality of magnetically sensitive devices arranged at intervals around the conversion member to opposite each other; and a fail detecting mechanism connected to the magnetically sensitive devices, in which the magnetically sensitive device detects the amount of travel of the conversion member in the axis direction and supplying the detected amount as a voltage signal to the controller, while the fail detecting mechanism detect detects a difference between the voltage signals supplied from the magnetically sensitive devices and supplies a fault signal to the controller when the detected difference exceeds a set value.  
           [0052]    According to the seventh invention, since the conversion member itself is made up of a magnet, it is unnecessary to additionally provide a ring member to the conversion member. Omitting the ring member enables the reduction of costs.  
           [0053]    It should be mentioned that in the sixth invention, the fail detecting mechanism is provided. Therefore, the electric motor does not exert an unexpected assist force.  
           [0054]    Since the conversion member is different in shape from those of the ring member, the magnetic flux is also different. However, the controller for detecting the magnetic flux can correct the difference. Accordingly, even in use of the conversion member itself made up of a magnet, precise torque can be detected.  
           [0055]    An eighth invention is characterized by further including an adder connected between the magnetically sensitive device and the controller to correct a deviation of an output signal from the magnetically sensitive device in a neutral state.  
           [0056]    According to the eighth invention, even when a signal supplied from the magnetically sensitive device in the neutral state is deviated, the deviation can be readily corrected by the adder. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0057]    [0057]FIG. 1 is a sectional view illustrating a torque detector of a first embodiment according to the present invention.  
         [0058]    [0058]FIG. 2 is a perspective view illustrating a tubular member  23  of the torque detector according to the first embodiment.  
         [0059]    [0059]FIG. 3 is a sectional view of the tubular member  23 .  
         [0060]    [0060]FIG. 4 is a schematic diagram illustrating a relationship between a ring member  26  and a magnetically sensitive device  27 .  
         [0061]    [0061]FIG. 5 is a diagram illustrating a relationship between a horizontal component B 1  and a vertical component B 2  of a magnetic flux B.  
         [0062]    [0062]FIG. 6 is a circuit diagram of the torque detector of the first embodiment.  
         [0063]    [0063]FIG. 7 is a graph illustrating a fail detection area.  
         [0064]    [0064]FIG. 8 is a sectional view illustrating a tubular member  28  according to a second embodiment.  
         [0065]    [0065]FIG. 9 is a sectional view illustrating a tubular member  31  according to a third embodiment.  
         [0066]    [0066]FIG. 10 is a sectional view illustrating a torque detector according to a fourth embodiment.  
         [0067]    [0067]FIG. 11 is a circuit diagram of the torque detector according to the fourth embodiment.  
         [0068]    [0068]FIG. 12 is a sectional view illustrating a conventional torque detector.  
         [0069]    [0069]FIG. 13 is a sectional view illustrating the conventional torque detector which is taken along the  100 - 100  line of FIG. 11. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0070]    [0070]FIG. 1 to FIG. 7 illustrate a first embodiment of a torque detector according to the present invention.  
         [0071]    In a gear case  19  as illustrated in FIG. 1, an input shaft  20  linking with a steering wheel (not shown) is rotatably mounted, while an output shaft  21  is also rotatably mounted coaxially with the input shaft  20 . The input shaft  20  and the output shaft  21  are connected with each other through a torsion bar  22 .  
         [0072]    The output shaft  21  is provided with a rack (not shown) as in the conventional torque detector. The rack is engaged with a rack shaft so as to transmit a driving force of an electric motor thereto.  
         [0073]    On the outer circumferences of the input shaft  20  and the output shaft  21 , a tubular member  23  made up of a non-magnetic material is slidably provided.  
         [0074]    The tubular member  23  has a cylindrical shape as illustrated in FIG. 2 and is provided with a screw portion  23   a  on its inner circumference as illustrated in FIG. 3. The screw portion  23   a  is screw-coupled with a screw portion  20   a  provided on the outer circumference of the input shaft  20 .  
         [0075]    The tubular member  23  is provided with a pair of axial grooves  24  extending in the axis direction at opposite positions shifting in a phase of 180 degrees from each other. In the respective axial grooves  24 , pins  25  secured to the output shaft  21  are slidably fitted. This construction allows the pins  25  to limit the rotation of the tubular member  23  in relation to the output shaft  21 .  
         [0076]    Thus, upon relative rotation of the input shaft  20  and the output shaft  21 , the tubular member  23  rotates in relation to the input shaft  20 , while travelling in the axis direction in accordance with the direction of the rotation because the tubular member  23  and the input shaft  20  are screw-coupled.  
         [0077]    The two pins  25  allow the tubular member  23  to stably rotate in relation to the input shaft  20  as explained above such that the tubular member  23  does not produce twisting.  
         [0078]    On the outer circumference of the tubular member  23 , an annular-shaped ring member  26  made up of a magnet is secured. In the gear case  19  facing the ring member  26 , magnetically sensitive devices  27  are implanted as a pair at opposite positions shifting in a phase of 180 degrees. The pair of magnetically sensitive devices  27  has a function of detecting a magnetic flux from the ring member  26  and outputting the detected value as variations in voltage.  
         [0079]    The magnet used in the first embodiment may be a magnet which generates a magnetic flux, for example, a permanent magnet, an electromagnet using a coil made of a magnetic material or non-magnetic material, and so on.  
         [0080]    For the magnetically sensitive device  27 , a hall device, magnetic resistor element or the like can be used.  
         [0081]    [0081]FIG. 4 is a schematic diagram illustrating the relationship between the ring member  26  and the magnetically sensitive device  27 . As illustrated in FIG. 4, magnetic flux are released from the ring member  26 , and have different curvatures depending on their individual positions. Accordingly, a magnitude and an angle of the magnetic flux in the tangential direction are varied depending on the detecting positions. Magnitude B of the magnetic flux in the tangential direction can be divided into a horizontal component B 1  and a vertical component B 2  as illustrated in FIG. 5. The magnetically sensitive device  27  detects only the horizontal component B 1  of the magnetic flux and outputs the detected value as a voltage.  
         [0082]    Since the horizontal component B 1  of the magnetic flux becomes zero at the midpoint of the ring member  26  in the axis direction, the midpoint is set as a neutral position.  
         [0083]    The tubular member  23  is installed to align the neutral position of the ring member  26  with the detecting position of the magnetically sensitive device  27 .  
         [0084]    With the above configuration, as the tubular member  23  travels from the neutral position toward the axis direction, the horizontal component B 1  of a magnetic flux at the detecting position increases. The magnetically sensitive device  27  detects the horizontal component B 1  of the magnetic flux and outputs the resulting signal to a controller C and a fail detecting mechanism F as illustrated in FIG. 6.  
         [0085]    Next, the operation in the first embodiment will be described.  
         [0086]    During the state that load is transmitted from wheels (not shown) to the output shaft  21 , when a steering wheel (not shown) is turned to rotate the input shaft  20 , the input shaft  20  and the output shaft  21  relatively rotate while the torsion bar  22  twists.  
         [0087]    Upon the relative rotation of the input shaft  20  and output shaft  21  as explained above, the tubular member  23  slides in the axis direction, thereby to move the ring member  26 . When the ring member  26  thus moves in the axis direction, the horizontal component B 1  of the magnetic flux facing each magnetically sensitive device  27  varies. The magnetically sensitive devices  27  respectively detect the horizontal components B 1  of the magnetic flux and output the resultants as voltage signals Va, Vb to the controller C.  
         [0088]    The voltage signals Va, Vb detected by the magnetically sensitive devices  27  are input to the controller C as illustrated in FIG. 6. The controller C averages both the voltage signals Va, Vb and controls the output of the electric motor M based on the averaged signal. The electric motor M controlled by the controller C exerts an assist force to optimize a steering force.  
         [0089]    The reason for averaging two signals from the magnetically sensitive devices  27  as explained above is to achieve high safety.  
         [0090]    Specifically, in installing or sliding, the tubular member  23  is occasionally inclined to the axis of the gear case  19 . If the tubular member  23  is inclined, as a natural consequence, the ring member  26  secured on the tubular member  23  is inclined and the magnetic flux is also inclined. In this event, if the magnetic flux is detected at one position, this increases an error. The error may cause an unexpected assist force.  
         [0091]    Therefore, for the detector according to the first embodiment, the two magnetically sensitive devices  27  are located at opposite positions shifting in a phase of 180 degrees, and the detected values are averaged. In this way, even when the tubular member  23  is inclined, the error produced by the inclination can be substantially reduced. Hence, an unexpected assist force can be avoided and high safety is ensured.  
         [0092]    In the first embodiment, the average value between the detected values of the two magnetically sensitive devices  27  is found. However, if the number of magnetically sensitive devices  27  is further increased and an average value among the detected values of the increased number of magnetically sensitive devices  27  is found, the detection accuracy for input torque is further improved.  
         [0093]    Especially, when the ring member  26  has a magnetic error, i.e. when the magnetic flux vary, it is advantageous to find an average value of many detected values as explained above.  
         [0094]    Further, in the use of a plurality of magnetically sensitive devices  27  in this way, the arrangement of the magnetically sensitive devices  27  at regular intervals in the circumferential direction enables the balanced detection, resulting in the improved detection of the input torque with high accuracy.  
         [0095]    However, if the precision of installing the tubular member  23  is increased and this can prevent the tubular member  23  from inclining in installing and also in sliding, only one magnetically sensitive device  27  will suffice.  
         [0096]    In this case, the number of magnetically sensitive devices  27  is reduced by one, and moreover, it is not needed to provide a calculating unit for averaging the detected values from the magnetically sensitive device  27  in the controller C. This results in a reduced cost.  
         [0097]    The controller C averages the two output values Va, Vb of the respective magnetically sensitive devices  27  as discussed above. Therefore, when one of the magnetically sensitive devices  27  fails, or when the tubular member  23  is inclined to the axis of the gear case  19 , the controller C cannot determine such a failure or inclination. For example, in an abnormal case where one of the output values Va is ten and the other output value Vb is zero, and in a normal case where both the output values Va, Vb are five, the average values in both cases is five. It is impossible to determine an abnormal state by a failure or the like from such an average value.  
         [0098]    Further, it is very dangerous to continue using the one of the magnetically sensitive devices  27  without finding the failure thereof.  
         [0099]    Hence, the signals from the magnetically sensitive devices  27  are additionally input to the fail detecting mechanism F.  
         [0100]    The fail detecting mechanism F calculates to subtract one of the output values from the other output value, |Va−Vb|. When the calculated result exceeds a predetermined value, the fail detecting mechanism F determines the abnormal state and outputs a fault signal to the controller C.  
         [0101]    [0101]FIG. 7 illustrates a graph of the relation |Va−Vb|, in which the non diagonally-shaded area f represents a safe area and the diagonally-shaded areas dl, d 2  represent fault areas.  
         [0102]    When both the output values of the respective magnetically sensitive devices  27  are normal and equal, the calculated result of |Va−Vb| is within the safe area f. In this case, the signal averaged by the controller C is supplied to the electric motor M as it is.  
         [0103]    However, when the calculated result of |Va−Vb|is within the diagonally-shaded fault area dl or d 2 , the fail detecting mechanism F supplies the fault signal to the controller C. Upon reception of the fault signal, the controller C determines the fault state, stops the electric motor M, and switches to the manual steering mode. Thus, safety is maintained.  
         [0104]    It should be noted that the safe area f is instituted in the fail detecting mechanism F for treating a small difference resulting from the above calculation as a tolerance.  
         [0105]    In the first embodiment, the difference between the only two magnetically sensitive devices  27  is found. However, the more than two magnetically sensitive devices  27  can be provided and the differences among the output values of the more than two magnetically sensitive devices  27  may be found. In this case, the fail detection can be achieved with an improved degree of reliability, resulting in further safety.  
         [0106]    Next, when the input shaft  20  and the output shaft  21  integrally rotate while the torsion bar  22  is twisted, the tubular member  23  rotates in relation to the gear case  19 . In this case, the provision of the ring member  26  of an annular shape on the outer circumference of the tubular member  23  allows the magnetically sensitive devices  27  to detect the magnetic flux at all times.  
         [0107]    In other words, with the detector of the first embodiment, the amount of relative rotation of the input shaft  20  and output shaft  21  corresponding to the input torque is converted into the amount of travel of the tubular member  23  in the axis direction. Furthermore, even when the input shaft  20  and the output shaft  21  integrally rotate while maintaining an amount of relative rotation, the amount of travel of the tubular member  23  can be detected by the annular shaped ring member  26 . In consequence, the planetary gear unit which is absolutely needed in the conventional torque detectors discussed in DESCRIPTION OF RELATED ART is not required, resulting in reduction in size of the gear case  19 .  
         [0108]    Further, since the ring member  26  is out of contact with the magnetically sensitive devices  27 , there is no significant influence of wear and the like produced in use of a contact type sensor. This allows the detection of high accurate values.  
         [0109]    In second and third embodiment illustrated in FIG. 8 and FIG. 9, the structure of the tubular member  23  of the first embodiment is changed.  
         [0110]    A tubular member  28  according to the second embodiment illustrated in FIG. 8 has a tubular main body  28   a  made up of metal and an annular-shaped step portion  28   b  to which a holding tube  29  made up of a non-magnetic material is secured. An annular-shaped ring member  30  is secured on the outer circumference of the holding tube  29 .  
         [0111]    The reason why the ring member  30  is secured to the tube main body  28   a  through the holding tube  29  made up of the non-magnetic material is as follows: if the ring member  30  is secured directly to the tube main body  28   a,  the tube main body  28   a  bears a magnetic force. This effects a variation in a magnetic flux of the ring member  30 . As a result, there is a problem in which the amount of travel cannot be detected precisely.  
         [0112]    According to the second embodiment, since the holding tube  29  made up of the non-magnetic material is interposed between the tube main body  28   a  and the ring member  30 , the above problem does not arise. Moreover, the robust metal made-tube main body  28   a  has long life.  
         [0113]    On the other hand, in the third embodiment illustrated in FIG. 9, a tubular member  31  itself is made up of a magnet.  
         [0114]    With the above configuration of the third embodiment, the simple structure of the tubular member  31  allows the reduction in cost thereof.  
         [0115]    In this case, since the tubular member  31  is different in shape from those of the corresponding ring members  26  and  30  of the first and second embodiment, the magnetic flux is also different. However, the controller C for detecting the magnetic flux can correct the difference. Accordingly, even in use of the tubular member  31  itself made up of a magnet, precise torque can be detected.  
         [0116]    In a fourth embodiment illustrated in FIG. 10 and FIG. 11, adders  32  are secured on the gear case  19 , and each is connected between the magnetically sensitive device  27  and the controller C. The remaining configuration is the same as that of the first embodiment.  
         [0117]    In the neutral state, when the voltage signals Va, Vb supplied from the magnetically sensitive devices  27  are not zero representing the neutral state, the adders  32  alter the signals to zero for correction and output them to the controller C.  
         [0118]    Specifically, each adder  32  is composed of an adding section  33  and an offset voltage setting section  34 . The adder  32  inputs a voltage signal Va, Vb, being set in the offset voltage setting section  34 , to a voltage signal input from the magnetically sensitive device  27  to the adding section  33 .  
         [0119]    For example, in the neutral state, when the voltage signal Va, Vb supplied from the magnetically sensitive device  27  to the adding section  33  is smaller than zero, the offset voltage setting section  34  adds the shortage to increase an output voltage to zero. When the voltage signal Va, Vb input to the adding section  33  is larger than zero, a negative voltage signal Va, Vb is output to reduce an output voltage to zero.  
         [0120]    The reason for correcting the output signal in the adder  32  as explained above is as follows: in the case that the voltage signal Va, Vb supplied from the magnetically sensitive device  27  is set as, e.g. zero in the neutral state, when the input torque is zero, the voltage signal Va, Vb must become also zero.  
         [0121]    However, parts for the input shaft  20 , output shaft  21 , tubular member  23  and so on have variations in sizes, and mounting errors are produced in mounting the parts. Therefore, it is extremely difficult to set the neutral state with precision. If the ring member  26  is inclined as explained above, the voltage signals Va, Vb supplied from the magnetically sensitive devices  27  may not be zero despite the neutral state.  
         [0122]    In the fourth embodiment, therefore, when the output voltage from the magnetically sensitive device  27  is not zero notwithstanding that the neural state has been set in the assembling process of the detector, the adder  32  corrects the deviation.  
         [0123]    According to the torque detector of the fourth embodiment, if the output signal supplied from the magnetically sensitive device  27  in the neutral state is deviated, the deviation can be effortlessly corrected by adjusting the adder  32 . Accordingly after the input shaft  20 , output shaft  21 , tubular member  23  and so on are mounted in the gear case  19 , it is not necessary to disassemble the gear case  19  for the adjustment.  
         [0124]    It should be mentioned that it might be possible to automatically correct the deviation of the neutral position using the controller C instead of the adders  32 . In this case, however, when the gear case  19  is required to replace due to a failure or the like, the controller C must be also replaced.  
         [0125]    However, in the torque detector of the fourth embodiment, since the adder  32  independent of the controller C corrects the deviation of the neutral position, the replacement of controller C is not needed. Accordingly, costs for replacing can be reduced.  
       EXPLANATION OF CODES  
       [0126]    [0126]                                                       20   INPUT SHAFT           21   OUTPUT SHAFT           23, 28, 31   TUBULAR MEMBER           26   RING MEMBER           27   MAGNETICALLY SENSITIVE DEVICE           29   HOLDING TUBE           32   ADDER           C   CONTROLLER           F   FAIL DETECTING MECHANISM