Abstract:
A rotation detecting device detects rotation of a rotating object by means of a biasing permanent magnet and plural magnetoresistance sensors. The sensors are disposed along the rotation direction of the rotating object to convert magnetic vectors of a magnetic field to a plurality of sensor signals. A selection circuit is connected to the magnetoresistance sensors and selects one of the sensor signals whose magnitude is the largest.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     The present application is based on and claims priority from Japanese Patent Application 2005-124061, filed Apr. 21, 2005, the contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a device for detecting rotation of a rotating object by magneto-resistance elements that are located in a biasing magnetic field.  
         [0004]     2. Description of the Related Art  
         [0005]     U.S. Pat. No. 6,452,381 B1 or JP-A-11-237256, which is a counterpart application of the U.S. patent, discloses such a rotation detecting device. This device includes a hollow biasing permanent magnet and a sensor chip on which a pair of magneto-resistance elements is formed. The sensor chip is inserted into the inside hollow portion of the permanent magnet so as to confront gear teeth formed at the periphery of a rotating object.  
         [0006]     The magneto-resistance sensor is constituted of a pair of magneto-resistance elements formed on a sensor chip in such a shape of reversed V that the distance between the pair of sensor elements becomes shorter as they approach the gear teeth. Therefore, the resistance of one of the sensor elements changes differently from the other when the rotating object rotates. Because the sensor elements form into a half bridge circuit having a middle point, an electric output signal that changes as the rotation angle of the rotating object changes can be obtained from the middle point of the half bridge circuit. In other words, the potential of the middle point cyclically changes in response to the concave-convex portions of the gear teeth.  
         [0007]     The resistance of the magneto-resistance sensor element changes when the magnetic vector of the biasing magnetic field provided by the permanent magnets changes. However, if the sensor chip is not correctly positioned relative to the gear teeth, the magnitude of change in the magnetic vector may become insufficient for the magneto-resistance sensor to provide sensor signals.  
       SUMMARY OF THE INVENTION  
       [0008]     Therefore, an object of the invention is to provide an improved rotation detecting device that can provide sufficient magnitude of sensor signals without work for precisely positioning the sensor chip relative to the rotating object.  
         [0009]     According to a feature of the invention, a rotation detecting device includes a biasing permanent magnet, plural magnetoresistance sensors and output circuit connected to the magnetoresistance sensors. The magneto resistance sensors are disposed along the rotation direction of the rotating object in parallel to each other to convert magnetic vectors of the magnetic field to a plurality of sensor signals, and, the output circuit provides a rotation signal based on a selected signal that are selected from the sensor signals.  
         [0010]     Preferably, the selected signal has a largest magnitude of the sensor signals. Each of the magnetoresistance sensors may be constituted of a pair of magnetoresistance elements each of which is disposed to provide one of two component signals having phase reversed to the other. Therefore, each sensor signal corresponds to a difference between the two component signals. The output circuit preferably includes a selection circuit connected to the magnetoresistance sensors to select from the sensor signals one of the sensor signals whose magnitude is the largest. As a preferred embodiment, an elongated chip mount having surfaces that are perpendicular to the rotation axis of the rotating object is provided to hold the magnetoresistance sensors on the surfaces. The chip mount may be replaced with a lead frame for transmitting the rotation signal. The magnetoresistance sensors may be formed in two or more sensor chips. The biasing permanent magnet may have a hollow cylindrical shape so that the magnetoresistance sensors can be disposed inside the biasing permanent magnet. Further, a resinous mold member may be filled inside the biasing permanent magnet to cover and protect the magnetoresistance sensors.  
         [0011]     According to another feature of the invention, the rotation detecting device may include a hollow cylindrical biasing permanent magnet magnetized to provide a biasing magnetic field toward the magnetic teeth of a rotation object, a pair of magnetoresistance sensors disposed inside the biasing permanent magnet in parallel with each other along the rotation direction of the rotating object and a selecting circuit for selecting from signals of the magnetoresistance sensors one of signals whose magnitude is larger than the other.  
         [0012]     According to another feature of the invention, the rotation detecting device may include a hollow cylindrical biasing permanent magnet magnetized to provide a biasing magnetic field toward the magnetic teeth of the rotation object, a pair of sensor chips disposed inside said biasing permanent magnet in parallel with each other along the rotation direction of the rotating object, a selection circuit for selecting from the sensor signals of the sensors formed at corresponding positions on the pair of sensor chips a selected pair of the sensor signals whose magnitude is larger than the other; and a calculation circuit for calculating a difference between the selected pair of sensor signals.  
         [0013]     According to a further feature of the invention, the rotation detecting device includes a hollow cylindrical biasing permanent magnet magnetized to provide a biasing magnetic field extending toward the magnetic teeth of the rotation object, a plurality of magnetoresistance sensors disposed inside the biasing permanent magnet in parallel with each other along the rotation direction of the rotating object and a selection circuit for selecting from signals of the magnetoresistance sensor one signal whose magnitude is the largest. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     Other objects, features and characteristics of the present invention as well as the functions of related parts of the present invention will become clear from a study of the following detailed description, the appended claims and the drawings. In the drawings:  
         [0015]      FIG. 1  is a longitudinal cross-sectional view of a rotation detecting device according to the first embodiment of the invention;  
         [0016]      FIG. 2A  is a plan view of the rotation detecting device according to the first embodiment,  FIG. 2B  is a bottom view thereof, and  FIG. 2C  is a circuit diagram of the rotation detecting device according to the first embodiment;  
         [0017]      FIG. 3  is a cross-sectional view of the rotation detecting device viewed from a rotating object;  
         [0018]      FIG. 4  is a longitudinal cross-sectional view of a rotation detecting device according to the second embodiment of the invention;  
         [0019]      FIG. 5A  is a schematic plan view of the rotation detecting device according to the second embodiment,  FIG. 5B  is a schematic bottom view of the same, and  FIG. 5C  is a circuit diagram of the rotation detecting device according to the second embodiment;  
         [0020]      FIG. 6  is a cross-sectional view of the rotation detecting device according to the second embodiment viewed from a rotating object;  
         [0021]      FIG. 7  is a cross-sectional view of the rotation detecting device according to the third embodiment viewed from a rotating object;  
         [0022]      FIG. 8  is a fragmentary cross-sectional view of a modification of the above rotation detecting devices;  
         [0023]      FIG. 9A  is a fragmentary cross-sectional view of a modification of the above rotation detecting devices, and  FIG. 9B  is a fragmentary cross-sectional view of a further modification of this rotation detecting device. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]     Some preferred embodiments of the present invention will be described with reference to the appended drawings.  
         [0025]     A rotation detecting device  100  according to the first embodiment of the invention will be described with reference to  FIGS. 1-3 .  
         [0026]     As shown in  FIG. 1 , the rotation detecting device  100  includes a biasing permanent magnet  2 , a pair of magnetoresistance sensors  3   a  and  3   b  respectively formed on sensor chips  4   a  and  4   b , a chip mount  5 , a circuit chip  10 , a cylindrical cover  13 , wires  14 , a terminal  15 , a lead frame  16  and a cap member  18 . The rotation detecting device  100  is disposed to confront a rotor or rotating object  6 , which is a gear-like magnetic member having teeth along its periphery and a rotation axis  11  at its center.  
         [0027]     The biasing permanent magnet  2  provides a biasing magnetic field that extends to the rotor  6 . The biasing permanent magnet  2  is made of a bonded composite of magnet powder and organic binder. The magnetic powder preferably includes rare earth powder. The biasing permanent magnet  2  may be a ferrite permanent magnet or other sintered permanent magnet. The biasing permanent magnet  2  is formed by means of compression molding or injection molding to have a hollow cylindrical shape that has a hollow portion  7 . The biasing permanent magnet  2  is magnetized so that an N-pole is formed at one of the axial ends thereof near the rotor  6  and an S-pole at the other end. However, the biasing permanent magnet  2  may be magnetized in the other direction.  
         [0028]     The magnetoresistance sensors  3   a ,  3   b  have magnetic anisotropy. The sensor chips  4   a ,  4   b  provide electric signals related to the magnetic resistances of the magnetoresistance sensors  3   a ,  3   b . The sensor chips  4   a ,  4   b  are covered with or packaged in a resinous material.  
         [0029]     The chip mount  5  is an elongated member that has front and back surfaces extending along the rotation direction of the rotating object  6  or in the direction perpendicular to the rotating axis  11  of the rotor  6 . The sensor chips  4   a ,  4   b  are respectively fixed to the front and back surfaces of the chip mount  5  by adhesive bond  12 .  
         [0030]     The circuit chip  10  includes a selection circuit  20   a , which is shown in  FIG. 2C , and receives electric signals from the sensor chips  4   a ,  4   b  via the wires  14  and the terminal  15 . The selection circuit  20   a  selects one of the electric signals that has a larger power and sends the selected signal outside via the lead frame  16 . The cylindrical cover  13  is a magnetic member that has an opening at one end and a bottom at the other end. The opening is closed by the cap member  18 , which is made of an insulating material, after the biasing permanent magnet  2  is held in the inside of the cover  13 .  
         [0031]     As shown in  FIGS. 1, 2A  and  2 B, the sensor chips  4   a ,  4   b  are disposed on the surfaces of the chip mount  5  so that the respective center lines  9   a ,  9   b  of the sensor chips  4   a ,  4   b  can be positioned on the magnetic center line  17  of the biasing permanent magnet  2 .  
         [0032]     The magnetoresistance sensors  3   a ,  3   b  are made of nickel-cobalt alloy or nickel-iron alloy, which are vapor-deposited on the sensor chips  4   a ,  4   b  to form thin films extending along the rotation direction of the rotating object.  
         [0033]     As shown in  FIGS. 2A, 2B  and  2 C, the magnetoresistance sensor  3   a  is constituted of a pair of series-connected magnetoresistance elements  3   a   1 ,  3   a   2 , and the magnetoresistance sensors  3   b  is constituted of a pair of series-connected magnetoresistance elements  3   b   1 ,  3   b   2 . Each of the magnetoresistance elements  3   a   1 ,  3   a   2 ,  3   b   1 ,  3   b   2  has the same length. The magnetoresistance elements  3   a   1 ,  3   a   2  are formed on the sensor chip  4   a  in a shape of reversed V so that the extensions thereof can cross the magnetic center line  17  at an angle of 45° and cross each other at an angle of 90°. The magnetoresistance elements  3   b   1 ,  3   b   2  are also formed on the sensor chip  4   b  in a shape of reversed V so that the extensions thereof can cross the magnetic center line  17  at an angle of 45° and cross each other at an angle of 90°.  
         [0034]     As shown in  FIG. 3 , the biasing permanent magnet  2  has a hollow cylindrical shape, and the sensor chips  4   a ,  4   b  are respectively fixed to the front and back surfaces of the chip mount  5  disposed inside the permanent magnet  2  so as to face opposite directions that are in parallel with the rotating axis  11  of the rotor  6 . In other words, the sensor chips  4   a ,  4   b  are disposed in parallel with each other inside the permanent magnet along the rotation direction of the rotor  6 .  
         [0035]     Accordingly, the rotation of the rotor  6  can be detected without very accurate position of the sensor chips  4   a ,  4   b  relative to the rotor  6 . Even if the position of the rotor  6  relative to the sensor chips  4   a ,  4   b  shifts from the center line between the sensor chips  4   a ,  4   b  to the side of the sensor chip  4   a  as shown by a two-dot line  6   a  in  FIG. 3 , the distance between the sensor chip  4   a  and the rotor  6  does not change very much, so that the sensor chip  4   a  can still detect sufficient magnitude of magnetic vector when the rotor  6  rotates although the sensor chip  4   b  cannot.  
         [0036]     For the above purpose, the circuit chip  10  includes a selection circuit  20   a , which is connected to the magnetoresistance sensors  3   a ,  3   b , as shown in  FIG. 2C , to select a larger magnitude of the output signals of the elements  3   a ,  3   b . The magnitude may be detected based on a range defined by the maximum and the minimum of the output signals.  
         [0037]     Each of the magnetoresistance sensors  3   a ,  3   b  may be constituted of a full bridge of four magnetoresistance elements to increase sensing accuracy of the rotation detecting device  100 .  
         [0038]     A rotation detecting device  101  according to the second embodiment of the invention will be described below with reference to  FIGS. 4-6 . Incidentally, the same reference numeral indicates the same or substantially the same portion, part or component as the rotation detecting device according to the first embodiment.  
         [0039]     As shown in  FIGS. 5A, 5B , and  5 C, each of a pair of sensor chips  34   a ,  34   b  is disposed along the rotation direction of the rotor  6  and has a pair of magnetoresistance sensors  32   a ,  33   a  (or  32   b ,  33   b ), each of which has a pair of series-connected magnetoresistance elements  33   a   1  and  33   a   2  (or  33   a   3  and  33   a   4 ,  33   b   1  and  33   b   2 , or  33   b   3  and  33   b   4 ). Each of the eight magnetoresistance elements ( 33   a   1 ,  33   a   2 ,  33   a   3 ,  33   a   4 ,  33   b   1 ,  33   b   2 ,  33   b   3  and  33   b   4 ) has the same length.  
         [0040]     The sensor chip  34   a  is positioned relative to the rotor  6  so that its center line  9   d  can be positioned on the magnetic center line  17  of the biasing magnetic field, and the magnetoresistance sensors  32   a  and  33   a  are positioned in symmetric with each other with respect to the center line  9   d . The sensor chip  34   b  is also positioned relative to the rotor  6  so that its center line  9   e  can be positioned on the magnetic center line  17 , and the magnetoresistance sensors  32   b  and  33   b  are positioned in symmetric with each other with respect to the center line  9   e.    
         [0041]     The magnetoresistance elements  33   a   1 ,  33   a   2  are formed on the sensor chip  34   a  in a shape of reversed V so that the extensions thereof can cross the magnetic center line  17  at an angle of 45° and cross each other at an angle of 90°. Other pairs of magnetoresistance elements are also formed on the respective sensor chips  34   a ,  34   b  in the same shape of reversed V. Therefore, the output signal of the sensor  32   a  and the output signals of the sensor  33   a  are reversed in phase to each other, and the output signal of the sensor  32   b  and the output signals of the sensor  33   b  become reversed in phase to each other.  
         [0042]     As shown in  FIG. 6 , the sensor chips  34   a ,  34   b  are fixed to the front and back surfaces of the chip mount  5  to extend perpendicular to the rotating axis  11  of or along the rotation direction of the rotor  6 . In other words, the magnetoresistance sensors  32   a ,  33   a  of the sensor chip  34   a  are disposed in parallel with the magnetoresistance sensors  32   b ,  33   b  of the sensor chip  34   b  along the rotation direction of the rotor  6 .  
         [0043]     As shown in  FIG. 5C , the circuit chip  10  includes a pair of selection circuits  20   b ,  20   c  and a differential circuit  20   d . The selection circuits  20   b  receives electric signals from the magnetoresistance sensor  32   a  (front-right in view of  FIG. 5A ) and the magnetoresistance sensor  33   b  (rear-right). The selection circuit  20   c  receives electric signals from the magnetoresistance sensor  33   a  (front left) and the magnetoresistance sensor  32   b  (rear left). Each of the selection circuits  20   b ,  20   c  respectively selects such one of the two electric signals of the magnetoresistance sensors, disposed on the same side of the chip mount along the rotation direction of the rotor  6 , that has a larger magnitude. The selection circuits  20   b ,  20   c  send the selected signals to an outside circuit via a lead frame.  
         [0044]     If the plane  6   c  of the rotation of the rotor  6  leans, as shown by a two-dot-chain line in  FIG. 6 , the selection circuit  20   b  selects the electric signal from the front left magnetoresistance sensor  32   a , and the selection circuit  20   c  selects the electric signal from the rear right magnetoresistance sensor  33   b . Therefore, the sensor chips  34   a ,  34   b  can still detect sufficient magnitude of magnetic vector even if the rotor  6  leans, so that the difference between the electric signals from the sensor chips  34   a  and  34   b  is provided by the differential circuit  20   d.    
         [0045]     A rotation detecting device  102  according to the third embodiment of the invention will be described below with reference to  FIG. 7 .  
         [0046]     A sensor chip  24  is held on the circuit chip  10  via a bump  22 . The sensor chip  24  is constituted of a pair of chip plates  24   a ,  24   b  on which magnetoresistance elements  23   a ,  23   b  are respectively formed in the same manner as the magnetoresistance elements  3   a ,  3   b  described above relating to the first embodiment. The sensor chip  24  may be replaced with a pair of sensor chips  34   a ,  34   b  of the second embodiment.  
         [0047]     As a modification of the above-described embodiments, the sensor chips  4   a ,  4   b  and the circuit chip  10  may be covered with a resinous material  27  such as polyphenilene sulfide (PPS) or epoxy resin, as shown in  FIG. 8 , in order to prevent the rotation detecting device from being affected by water, dust and/or vibration.  
         [0048]     Further, the chip mount  5  can be replaced with a lead frame  36 , as shown in  FIG. 9A . The sensor chips  4   a ,  4   b  and the lead frame  36  may be covered with a resinous material  27  such as polyphenilene sulfide or epoxy resin, as shown in  FIG. 9B .  
         [0049]     In the foregoing description of the present invention, the invention has been disclosed with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made to the specific embodiments of the present invention without departing from the scope of the invention as set forth in the appended claims. Accordingly, the description of the present invention is to be regarded in an illustrative, rather than a restrictive, sense.