Abstract:
First and second GMR devices formed in a first block and fifth and sixth GMR devices formed in a third block are disposed in a symmetrical manner as to a point of origin. The first and fifth GMR devices, and the second and sixth GMR devices are disposed in a symmetrical manner as to the point of origin, and accordingly the intensity and direction of an external magnetic field applied from a rotating magnet is the equal between the first and fifth GMR devices, and also equal between the second and sixth GMR devices. Thus, combining the first, fifth, second, and sixth GMR devices to form a Wheatstone bridge circuit enables detection precision of rotation angles to be improved.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a rotation angle detecting device using a GMR device which indicates great change in resistance according to change in external magnetic fields.  
           [0003]    2. Description of the Related Art  
           [0004]    A conventional rotation angle detecting device is illustrated in FIGS. 4 and 5, with FIG. 4 being a plan view illustrating the principal portions of a conventional rotation angle detecting device, and FIG. 5 a circuit configuration diagram of a magnetism detecting unit. The rotation angle detecting device shown in FIG. 4 is a rotation angle detecting device capable of two outputs for the rotation angle output thereof.  
           [0005]    As shown in FIG. 4, with a conventional rotation angle detecting device  1 , a board (unshown) is divided into four block (regions) A, B, C, and D, by perpendicularly intersecting imaginary axial lines for the X axis and Y axis. A pair of Giant Magneto-Resistance devices (hereafter referred to as “GMR devices”) G 1  and G 2  are disposed in the block A, a pair of GMR devices G 3  and G 4  are disposed in the block B, a pair of GMR devices G 5  and G 6  are disposed in the block C, and a pair of GMR devices G 7  and G 8  are disposed in the block D.  
           [0006]    A disc-shaped or ring-shaped rotating member  2  is provided facing the four blocks A, B, C, and D, so as to be capable of rotating in the a1 and a2 directions on the point of origin O of the X axis and Y axis.  
           [0007]    Magnets M 1  and M 2  are provided on the perimeter of the rotating member  2  at positions symmetrical with regard to the point of origin O. The one magnet M 1  has the inner circumferential side thereof magnetized to the N pole, and the other magnet M 2  has the inner circumferential side thereof magnetized to the S pole. Accordingly, a magnetic field heading from the N pole of the magnet M 1  to the S pole of the magnet M 2  is generated by the rotating member  2 , with the GMR devices G 1  through G 8  of the blocks A through D situated within this magnetic field.  
           [0008]    [0008]FIGS. 4 and 5 shows the direction of the fixed magnetism of the fixed magnetic layers of the GMR devices with arrows e. As shown in FIG. 4, the direction e of the fixed magnetism is the same within each pair of GMR devices. Also, the direction e of the fixed magnetism is the same for the A and B blocks, and the same for the C and D blocks, but opposite between the A and B blocks and the C and D blocks.  
           [0009]    The GMR devices G 1  through G 8  form two Wheatstone bridge circuits by being connected as shown in FIG. 5. The first Wheatstone bridge circuit is configured of the GMR devices G 1  and G 2  of the block A and the GMR devices G 5  and G 6  of the block C, and the second Wheatstone bridge circuit is configured of the GMR devices G 3  and G 4  of the block B and the GMR devices G 7  and G 8  of the block D.  
           [0010]    With the first Wheatstone bridge circuit, a predetermined electric power source voltage V is applied to the connection between the GMR devices G 1  and G 5 , and the connection between the GMR devices G 2  and G 6  is grounded. The connection between the GMR devices G 1  and G 6 , and the connection between the GMR devices G 2  and G 5 , are output terminals T 1  and T 2 .  
           [0011]    In the same way, with the second Wheatstone bridge circuit, a predetermined electric power source voltage V is applied to the connection between the GMR devices G 3  and G 7 , and the connection between the GMR devices G 8  and G 4  is grounded. The connection between the GMR devices G 3  and G 8 , and the connection between the GMR devices G 7  and G 4 , are output terminals T 3  and T 4 .  
           [0012]    Upon the rotating member  2  being rotated in the a1 and a2 directions, the direction of the magnetic field changes according to the rotation angle. In the event that the magnetic field rotates within a plane parallel to the board on which the GMR devices G 1  through G 8  are formed, the direction of magnetization of the free magnetic layer (not shown) provided within the GMR devices G 1  through G 8  changes according to the direction of the rotating magnets M 1  and M 2 . Consequently, the resistances R1 through R8 of the GMR devices G 1  through G 8  cyclically change with regard to the rotation angle of the rotating member  2 .  
           [0013]    The direction of the fixed magnetism is opposite for the GMR devices G 1  and G 6 , the GMR devices G 2  and G 5 , the GMR devices G 3  and G 8 , and the GMR devices G 4  and G 7 , so the change due to rotation of the magnets is opposite between the resistance values R1 and R6, the resistance values R2 and R5, the resistance values R3 and R8, and the resistance values R4 and R7.  
           [0014]    Accordingly, the differential output between the output terminals T 1  and T 2  of the first block circuit, and the differential output between the output terminals T 3  and T 4  of the second block circuit, can both be obtained as sin θ functions, wherein θ is a variable representing the rotation angle of the rotating member  2 .  
           [0015]    However, with the above-described conventional rotation angle detecting device  1 , the GMR devices are arrayed in sets of pairs of GMR devices arrayed in parallel, so in the event of configuring Wheatstone bridge circuits by combining the GMR devices as shown in FIG. 5, with the first block circuit, the center point P 1  of the GMR device G 1  and the GMR device G 6 , and the center point P 2  of the GMR device G 2  and the GMR device G 5 , do not agree with the point of origin O. In the same way, with the second block circuit, the center point of the GMR device G 3  and the GMR device G 8 , and the center point of the GMR device G 4  and the GMR device G 7 , do not agree with the point of origin O.  
           [0016]    This means that magnetic field of the same intensity and direction cannot be applied from the magnets M 1  and M 2  to the GMR device G 1  and the GMR device G 6 . In the same way, a magnetic field is applied to the GMR device G 2  and the GMR device G 5 , the GMR device G 3  and the GMR device G 8 , and the GMR device G 4  and the GMR device G 7 , with different intensity and direction.  
           [0017]    Accordingly, the amount of change as to the rotation angle θ is not the same for the resistances R1 through R8, leading to deterioration in the sine waveform which is the differential output detecting from the first Wheatstone bridge circuit, and in the sine waveform which is the differential output detecting from the second Wheatstone bridge circuit, which has been a problem since the detection accuracy of the rotation detecting device  1  accordingly deteriorates.  
         SUMMARY OF THE INVENTION  
         [0018]    The present invention has been made to solve the above-described problems of the conventional art, and accordingly, it is an object thereof to provide a rotation detecting device having high detection precision regarding the rotation angle detected with the first Wheatstone bridge circuit and the second Wheatstone bridge circuit, by an optimal arrangement of GMR devices.  
           [0019]    The rotation angle detecting device according to the present invention comprises: a rotating shaft; a fixed portion where a plurality of GMR devices are provided; a rotating portion for forming a magnetic field and rotating facing the fixed portion; and a Wheatstone bridge circuit formed by connecting the GMR devices; wherein, with the center of the rotating shaft as the point of origin, and imaginary axial lines perpendicularly intersecting at this point of origin as the X axis and Y axis, each of four blocks sectioned by the X axis and Y axis have provided therein a pair of GMR devices disposed parallel to the X axis or Y axis, and such that the GMR devices provided in one block are symmetrical across the point of origin with the GMR devices provided in another block facing the block across the point of origin; and wherein the Wheatstone bridge circuit has a resistance formed by serially connecting a GMR device provided at a position closer to the point of origin within the one block and a GMR device provided at a position closer to the point of origin within the other block, and a resistance formed by serially connecting a GMR device provided at a position farther from the point of origin within the one block and a GMR device provided at a position farther from the point of origin within the other block, with the resistances connected in parallel.  
           [0020]    The direction of fixed magnetism of the GMR devices may be the same direction for each GMR device in a pair within a block, and formed opposite for blocks symmetrical across the point of origin.  
           [0021]    With the rotation angle detecting device according to the present invention, the center of the magnets forming the magnetic field and the center of GMR devices on opposite sides of the point of origin can be made to agree, by an optimal arrangement of the GMR devices disposed in the blocks. Thus, the intensity and direction of the magnetic field applied from the magnets to the GMR devices can be made the same, such that the resistance values of the GMR devices making up the Wheatstone bridge circuits change by the same amount with regard to the same change in rotation angle, i.e., so that the resistance values of the GMR devices making up the Wheatstone bridge circuits accurately change according to the rotation angle. Thus, the precision of detecting the rotation angle can be improved. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1 is a cross-sectional diagram illustrating a rotation detecting device according to the present invention;  
         [0023]    [0023]FIG. 2 is a plan view of the board in the device shown in FIG. 1;  
         [0024]    [0024]FIGS. 3A and 3B illustrate the circuit configuration of magnetism detecting units, with  3 A illustrating a first Wheatstone bridge circuit, and  3 B illustrating a second Wheatstone bridge circuit;  
         [0025]    [0025]FIG. 4 is a plan view illustrating the principal portions of a conventional rotation angle detecting device; and  
         [0026]    [0026]FIG. 5 is a circuit configuration diagram of a magnetism detecting unit. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    [0027]FIG. 1 is a cross-sectional diagram illustrating a rotation detecting device according to the present invention, FIG. 2 is a plan view of the board in the device shown in FIG. 1, and FIGS. 3A and 3B illustrate the circuit configuration of magnetism detecting units, with  3 A illustrating a first Wheatstone bridge circuit, and  3 B illustrating a second Wheatstone bridge circuit.  
         [0028]    As shown in FIG. 1, the rotation angle detecting device according to the present invention comprises a case  20 , a board (fixed portion) fixed within the case  20 , and a rotating portion having a disc-shaped magnet  22  formed of ferrite, facing the board  21  in a parallel manner. The magnet  22  provided to the rotating portion has been magnetized so as to be polarized into the N pole and S pole on the face facing the board  21 , the direction connecting the N pole and S pole being the radial direction thereof. GMR devices G 1  through G 8  are formed on the face of the board  21  facing the magnet  22 .  
         [0029]    A rotating shaft  23  formed of a non-magnetic material is fixed to the rotating portion, and protrudes outward from the case  20 , being rotatably supported thereby.  
         [0030]    As shown in FIG. 2, the GMR devices G 1  through G 8  are provided in pairs in four blocks A, B, C, and D, formed by sectioning the board  21  by imaginary axial lines serving as the X axis and Y axis. Note that while FIG. 2 shows all of the pairs of GMR devices being arrayed in parallel to the X axis, but these may be arrayed in parallel to the Y axis instead.  
         [0031]    The point of origin O where the X axis and Y axis intersect matches the center of the rotating shaft  23 , as well as the rotating center of the magnet  22 .  
         [0032]    The GMR devices disposed at positions close to the point of origin O, i.e., the GMR device G 1 , G 3 , G 5 , and G 7 , are all at the same distance from the point of origin O, and the GMR devices disposed at positions farther from the point of origin O, i.e., the GMR device G 2 , G 4 , G 6 , and G 8 , are all at the same distance from the point of origin O.  
         [0033]    As shown in FIG. 2, the direction of fixed magnetism (represented by “e” in FIGS. 2 through 3B) of the GMR devices G 1  and G 2  provided in the block A and the GMR devices G 3  and G 4  provided in the block B is the same at −Y, and also, the direction of fixed magnetism of the GMR devices G 5  and G 6  provided in the block C and the GMR devices G 7  and G 8  provided in the block D is the same at +Y.  
         [0034]    The GMR devices G 1 , G 2 , G 5 , and G 6 , formed in the blocks A and C, make up a first Wheatstone bridge circuit such as shown in FIG. 3A. As shown in FIG. 3A, with the first Wheatstone bridge circuit, the pair of GMR devices G 1  and G 5  which are formed at positions close to the point of origin O and have opposite directions of fixed magnetism are serially connected at a node  8 , the pair of GMR devices G 6  and G 2  which are formed at positions farther from the point of origin O and have opposite directions of fixed magnetism are serially connected at a node  9 , and further, the serially-connected GMR devices G 1  and G 5  and the serially-connected GMR devices G 6  and G 2  are connected in parallel. Electric power source voltage V is connected to the connection of the GMR devices G 1  and G 6 , and the connection of the GMR devices G 5  and G 2  is grounded. Output terminals T 1  and T 2  are connected to the nodes  8  and  9 , respectively, so that differential output can be obtained from the output terminal T 1  and the output terminal T 2 .  
         [0035]    On the other hand, the direction of fixed magnetism is opposite between the two GMR devices G 3  and G 4  formed in the block B, and the two GMR devices G 7  and G 8  formed in the block D. The fixed magnetism of the GMR devices G 3  and G 4  is in the −Y direction, and the fixed magnetism of the GMR devices G 7  and G 8  is in the +Y direction.  
         [0036]    As shown in FIG. 3B, a second Wheatstone bridge is formed of the four GMR devices G 3 , G 4 , G 7 , and G 8 , of the blocks B and D, in the same way as the first Wheatstone bridge formed of the four GMR devices G 1 , G 2 , G 5 , and G 6 , of the blocks A and C.  
         [0037]    The magnet  22  of the rotating portion has the face thereof facing the board  21  serving as a supporting member magnetized, forming an external magnetic field indicated by the symbol F in FIG. 1, but the GMR devices detect the components of the magnetic field generated by the magnet  22  that are parallel to the surface of the blocks A, B, C, and D.  
         [0038]    As shown in FIG. 2, with the first Wheatstone bridge circuit, the center point of an imaginary line connecting the GMR devices G 1  and G 5  matches the point of origin O, and the center point of an imaginary line connecting the GMR devices G 2  and G 6  also matches the point of origin O. That is to say, in the first Wheatstone bridge circuit, the GMR devices G 1  and G 2  formed in the block A and the GMR devices G 5  and G 6  formed in the block C are disposed symmetrically with regard to the point of origin O. Accordingly, the intensity and direction of the external magnetic field F perpetually applied from the magnet  22  is the same for the GMR devices G 1  and G 5 , and the intensity and direction of the external magnetic field F perpetually applied from the magnet  22  is also the same for the GMR devices G 2  and G 6 .  
         [0039]    In the same way, in the second Wheatstone bridge circuit, the GMR devices G 3  and G 4  formed in the block B and the GMR devices G 7  and G 8  formed in the block D are disposed symmetrically with regard to the point of origin O, so the intensity and direction of the external magnetic field F perpetually applied from the magnet  22  is the same for the GMR devices G 3  and G 7  and for the GMR devices G 4  and G 8 .  
         [0040]    Upon the rotating shaft  23  being rotated and the magnet  22  rotating while facing the GMR devices, the external magnetic field F of the magnet  22  rotates within a plane parallel to the face of the board  21 , and the direction of magnetization of the free magnetic layers of the GMR devices change so as to match the direction of rotation of the magnetic field of the magnet  22 .  
         [0041]    Due to the Giant Magneto-Resistive effects of the GMR devices G 1  through G 8 , the resistance value drops when the direction of magnetization of the free magnetic layer in a device matches the fixed magnetism direction e, and rises when the direction of magnetization of the free magnetic layer is opposite to the fixed magnetism direction e.  
         [0042]    With the rotation angle θ of the magnet  22  representing the angle between the direction of the external magnetic field F of the magnet  22  and the direction of the fixed magnetism of the GMR devices G 1  and G 2  formed in the block A (the −Y direction), the resistance values R1 and R2 of the GMR devices G 1  and G 2  formed on the board  21  are maximal at a rotation angle θ of 0°, decrease as the rotation angle θ heads clockwise from 0° toward 180°, are minimal at a rotation angle θ of 180°, increase as the rotation angle θ heads from 180° toward 360°, and are maximal again.  
         [0043]    On the other hand, the direction and intensity of the external magnetic field F applied to the GMR devices G 5  and G 6  formed in the block C is the same as that of the external magnetic field F applied to the GMR devices G 1  and G 2  formed in the block A, but the direction of the fixed magnetism of the GMR devices G 5  and G 6  formed in the block C is opposite to the direction of the fixed magnetism of the GMR devices G 1  and G 2  formed in the block A. Accordingly, the change in the resistance values R5 and R6 of the GMR devices G 5  and G 6  formed in the block C is opposite to that of the fixed magnetism of the GMR devices G 1  and G 2  formed in the block A.  
         [0044]    That is to say, the resistance values R5 and R6 of the GMR devices G 5  and G 6  formed in the block C are minimal at a rotation angle θ of 00, increase as the rotation angle θ heads clockwise from 0° toward 180°, are maximal at a rotation angle θ of 180°, decrease as the rotation angle θ heads from 180° toward 360°, and are minimal again.  
         [0045]    With the first Wheatstone bridge circuit, the resistance values R1 and R2 of the GMR devices G 1  and G 2  formed in the block A and the resistance values R5 and R6 of the GMR devices G 5  and G 6  formed in the block C change with mutually opposite properties.  
         [0046]    Moreover, the intensity and direction of the external magnetic field F of the magnet  22  is the same, so the resistance values R1 and R5 of the GMR devices G 1  and G 5  change with an equal ratio, and the resistance values R2 and R6 of the GMR devices G 2  and G 6  also change with an equal ratio. For example, in the event that the resistance values R1 and R2 of the GMR devices G 1  and G 2  each increase by 20%, the resistance values R5 and R6 of the GMR devices G 5  and G 6  each decrease by 20%.  
         [0047]    Accordingly, under rotations of the magnet  22 , the aggregate resistance value R1 of the GMR device G 1  and the resistance value R5 of the GMR device G 5  is always maintained constant, and the aggregate resistance value R2 of the GMR device G 2  and the resistance value R6 of the GMR device G 6  is always maintained constant, so the current flowing through the GMR devices G 1 , G 2 , G 5 , and G 6  is also constant.  
         [0048]    Accordingly, by rotating the magnet  22  with a rotation angle θ, the output waveforms output from the output terminal T 1  on the side of the GMR devices G 1  and G 5 , and from the output terminal T 2  on the side of the GMR devices G 2  and G 6  are sin θ functions mutually off-phase by 180°.  
         [0049]    The differential output of the voltage output from the output terminal T 1  and the voltage output from the output terminal T 2  (sin θ function) is the output of the first Wheatstone bridge circuit. The differential output from the first Wheatstone bridge circuit is a sin θ function with noise signals cancelled, and also is a high-precision output signal corresponding to the rotational angle.  
         [0050]    The GMR devices G 3  and G 4  formed in the block B and the GMR devices G 7  and G 8  formed in the block D to make up the second Wheatstone bridge circuit have the same fixed magnetism direction e as the GMR devices G 1  and G 2  formed in the block A and the GMR devices G 5  and G 6  formed in the block C to make up the first Wheatstone bridge circuit.  
         [0051]    Further, with the second Wheatstone bridge circuit, the center point of an imaginary line connecting the GMR devices G 3  and G 7  matches the point of origin O, and the center point of an imaginary line connecting the GMR devices G 4  and G 8  also matches the point of origin O, with both being disposed symmetrically with regard to the point of origin O, so the intensity and direction of the external magnetic field F applied from the magnet  22  is the same, and accordingly the resistance values R3 and R7 of the GMR devices G 3  and G 7  change inversely with an equal ratio, and the resistance values R4 and R8 of the GMR devices G 4  and G 8  also change inversely with an equal ratio.  
         [0052]    Accordingly, with the second Wheatstone bridge circuit as well, by rotating the magnet  22  with a rotation angle θ, the output waveforms output from the output terminal T 3  on the side of the GMR devices G 3  and G 7 , and from the output terminal T 4  on the side of the GMR devices G 4  and G 8  are sin θ functions mutually off-phase by 180°. Thus, the differential output from the second Wheatstone bridge circuit is a sin θ function with noise signals cancelled, and also is a high-precision output signal corresponding to the rotational angle.  
         [0053]    As described above, with the angle detecting device  1 , rotation angle with high precision can be obtained from both the first Wheatstone bridge circuit and second Wheatstone bridge circuit configured of GMR devices.  
         [0054]    Note that while the above-described embodiment comprises first and second Wheatstone bridge circuits, an arrangement may be made with only one Wheatstone bridge circuit.  
         [0055]    Also, while the magnet  22  has been described as being a disc in the above embodiment, magnets M 1  and M 2  may be disposed so as to be symmetrical as to the point of origin O of the ring-shaped rotating member.  
         [0056]    Moreover, while the GMR devices have been described as being provided on the board  21 , the GMR devices may be formed within an IC chip, whereby the rotation angle detecting device can be miniaturized.