Abstract:
The rotational angle of a rotatable object (such as the throttle valve of an automobile engine) is accurately detected by a rotational angle detector having a magnetic sensor placed in a magnetic field, the direction of which varies according to the rotational angle. The magnetic sensor generates an output that varies sinusoidally according to rotational angle. The sinusoidal output is converted into a signal that is proportional to the rotational angle under arc-sine or arc-cosine transformation. The converted signal correctly represents the rotational angle in a wide angle range. The sensor element and the circuit for converting the sensor output into the proportional output signal may be built in a single-chip integrated circuit to simplify detector structure.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims benefit of priority of Japanese Patent Application No. Hei-11-304826 filed on Oct. 27, 1999, the content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a detector for detecting the rotational angle of a rotatable object, the detector using a magnetic sensor element. 
     2. Description of Related Art 
     An example of rotational detectors of this kind is disclosed in JP-A-5-157506. The essence of the disclosed detector is illustrated in FIGS. 7A and 7B of accompanying drawings. A Hall element  12  is placed in a parallel magnetic field generated by a cylindrical permanent magnet  11 , as shown in FIG.  7 A. The cylindrical magnet  11  connected to a rotatable object rotates together with the rotatable object, while the Hall element  12  stays in the magnetic field. An angle θ made between an magnetosensitive surface of the Hall element  12  and the magnetic field direction changes according to rotation of the rotatable object, as shown in FIG.  7 B. The Hall element  12  generates the following output voltage VH: 
       VH=V   0 . sin θ, 
     where V 0  is a maximum value of VH that is generated when the angle θ is 90° The output VH is fed to an outside microcomputer that calculates the rotational angle of the rotatable object based on VH. 
     The output VH, however, is a sinusoidal curve that is not perfectly proportional to the angle θ, though it is substantially proportional in a limited narrow range. In other words, the angle θ is not correctly detected in a range beyond the limited narrow range. It may be possible to process the output VH in the computer to convert it into a correct rotational angle, but such conversion process makes the computer more complex. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved rotational angle detector that correctly detects the rotational angle in a wide range without making the detector complex. 
     The rotational angle detector is composed of a rotor, a magnetic sensor element and circuits for processing the sensor signal. The rotor is connected to a rotatable object such as a throttle valve, the rotational angle of which is to be detected. Magnets are mounted on the rotor to generate a magnetic field therein, and a magnetic sensor element such as a Hall element is disposed in the magnetic field. The magnetic sensor element generates an output in a form of a sine or cosine wave according to the rotational angle of the rotor. The sensor output is not exactly proportional to the rotational angle because it varies in a sinusoidal waveform. A linear converter provided in the circuits converts the sensor output to a signal which is proportional, or linear, to the rotational angle of the rotor. This conversion is preferably performed under arc-sine or arc-cosine transformation. The converted sensor signal represents a correct rotational angle in a wide angle range. 
     The magnetic sensor element may be rotated relative to the magnetic field while making the magnetic field stationary. The linear converter and the magnetic sensor element may be built in a single-chip-integrated circuit to simplify the detector structure, or they may be separately build in respective integrated circuits if such is convenient for structuring the detector. 
     Preferably, the analog sensor output is converted into a digital signal, and then the digital signal is converted into the signal linear to the rotational angle under the arc-sine or arc-cosine transformation. The rotational angle detector may include two output terminals, one for an analog signal and the other for a digital signal, so that the output is adaptable to both of analog and digital outside controllers. 
     The rotational angle detector according to the present invention is able to correctly detect the rotational angle over a wide range without making the detector structure complex. 
     Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiments described below with reference to the following drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing an entire circuit in a Hall IC mounted in a rotational angle detector of the present invention; 
     FIG. 2 is a cross-sectional view showing a rotational angle detector as a first embodiment of the present invention; 
     FIG. 3A is a cross-sectional view showing a structure of permanent magnets used in the rotational angle detector shown in FIG. 2, taken along line  3 A— 3 A in FIG. 2; 
     FIG. 3B is a cross-sectional view showing an alternative structure of the permanent magnets shown in FIG. 3A; 
     FIGS. 4A and 4B are drawings illustrating a rotational angle θ relative to a magnetosensitive surface of a Hall IC; 
     FIG. 5 is a graph showing an output VH of a Hall element and a converted output VS versus the rotational angle θ; 
     FIG. 6 is a cross-sectional view showing a rotational angle detector as a second embodiment of the present invention; and 
     FIGS. 7A and 7B are drawings illustrating operation of a Hall element in a conventional rotational angle detector. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first embodiment of the present invention will be described with reference to FIGS. 1 to  5 . First, referring to FIG. 2, the structure of a rotational angle detector will be described. The rotational angle detector includes a rotor housing  21  in which a rotor is rotatably housed and a cover housing  28  to which Hall ICs  30  are held. The rotor housing  21  is cylinder-shaped, its one side (left side) being closed and the other side being open. The open side is closed with the cover housing  28 . 
     A rotor shaft  22  connected to a rotatable object such as a throttle valve of an automotive engine, the rotational speed of which is to be detected, is rotatably supported by a bearing  23  held in the rotor housing  21 . A cup-shaped rotor core  24  is fixed to the end of the rotor shaft  22 . A pair of permanent magnets  25  are fixed to the inner bore of the cup-shaped rotor core  24  by adhesive, molding or the like. The magnets  25  are magnetized to form a parallel magnetic field in the direction perpendicular to the longitudinal direction of the rotor shaft  22 . Plural through-holes  26  for preventing the magnetic flux from being short-circuited are formed in a flat end portion of the magnetic core  24  to encircle the rotor shaft  22 . The outside of the rotor core  24  is molded with resin  27 . 
     The cover housing  28  made of resin is fixed to the rotor housing  21  to close its open side. A boss  29  of the cover housing  28  extends into the inner space between the pair of magnets  25 . A pair of Hall ICs  30  are held in the boss  29 . The pair of Hall ICs  30  are overlapped on each other, so that outputs from both Hall ICs  30  are compared with each other to check whether they are properly functioning. Terminals  31  of both Hall ICs  30  are connected to a connector pin  32  inserted in the cover housing  28 . The pair of Hall ICs  30  may be placed side by side instead of overlapping, or only one Hall IC  30  may be used instead of two. 
     Referring to FIGS. 3A and 3B, taken along line  3 A— 3 A of FIG. 2, the arrangement of the pair of permanent magnets  25  and the pair of Hall ICs  30  will be described. In FIG. 3A, a pair of half-circular permanent magnets  25  are abutted with each other and magnetized as illustrated to form a parallel magnetic field running vertically in the drawing. The pair of Hall ICs  30  are positioned in the center of the parallel magnetic field. Alternatively, the permanent magnets  30  may be arranged as illustrated in FIG. 3B. A pair of arc-shaped magnets  25  are disposed with a certain space therebetween. The Hall ICs  30  are similarly positioned in the center of the parallel magnetic field. Other arrangements of the magnets are also possible, as long as a parallel magnetic field is formed in the inner space of the rotor where the Hall ICs  30  are positioned. 
     Referring to FIG. 1, the structure and function of the Hall IC  30  will be described. The Hall IC  30  is a one-chip IC which includes: a Hall element  33  as a magnetic sensor element; a temperature sensor element  34 ; an A/D converter  35 ; a digital signal processor  36 ; an EEPROM  37 ; a D/A converter  38 ; and an EMI filter  39 . The Hall element  33  generates its output voltage according to the magnetic field direction. The temperature sensor element  34  outputs a signal according to a temperature of the Hall IC  30 . The A/D converter  35  converts analog signals fed from the Hall element  33  and the temperature sensor element  34  into digital signals. The converted digital signals are fed to the digital signal processor  36 . 
     The digital signal processor (DSP)  36  performs the following functions: a function  40  for converting the output voltage VH of the Hall element  33  into a signal VS proportional to the rotational angle θ (the portion of the DSP performing this function  40  will be referred to as a linear converter); a function  41  for correcting an offset; a function  42  for adjusting a gain; and a function  43  for correcting the output according to the temperature. 
     More particularly, the linear converter converts the sensor output VH (after it is converted to a digital signal through the A/D converter  35 ) into the signal VS that is proportional to the rotational angle θ under arc-sine or arc-cosine transformation. When the rotational angle θ is measured from a base position set on the magnetosensitive surface of the Hall element  33 , as shown in FIG. 4A, the output VH of the Hall element  33  is expressed as follows: 
     VH=V 0 . sin θ, where V 0  is the maximum output obtained at a position, θ=90°. The rotational angle θ is calculated under the arc-sine transformation according to the following formula: 
     
       
         θ=arcsin( VH/V   0 ) 
       
     
     The linear converter outputs the signal VS expressed in the following formula: 
     
       
           VS=G . arcsin( VH/V   0 ) 
       
     
     where G is a gain of the conversion. The signal VH of the Hall element  33  and the converted signal VS are shown in the graph of FIG.  5 . This graph shows the output voltage versus the rotational angle θ, where V 0 =1 volt and G=1. Accordingly, in this graph, VH and VS are expressed as: VH=sin θ; and VS=arcsin VH. As seen in this graph, the output VH of the Hall element  33  is converted into the signal VS which is proportional, or linear, to the rotational angle θ under the arc-sine transformation. 
     Similarly, when the rotational angle θ is measured from a base position set at a line perpendicular to the magnetosensitive surface of the Hall element  33 , as shown in FIG. 4B, the output VH of the Hall element  33  is expressed as follows: 
     
       
           VH=V   0 . sin(π/2−θ)=V 0 . Cos θ 
       
     
     The rotational angle θ is calculated under the arc-cosine transformation according to the following formula: 
     
       
         θ=arccos( VH/V   0 ) 
       
     
     The linear converter outputs the signal VS expressed in the following formula: 
     
       
           VS=G . arccos( VH/V   0 ) 
       
     
     The signal VS that is proportional to the rotational angle θ may be calculated in other manners than the above. For example, a map showing the relation between VH and VS is stored in the processor, and the VS is calculated based on the map. 
     An offset angle of the rotational angle, e.g., a deviation of a (θ=0) position from the magnetosensitive surface of the Hall element  33 , is corrected by the offset correction function  41  in the DPS  36 . The gain G in the process of converting VH to VS is adjusted by the gain adjustment function  42 . An output deviation of the Hall element  33  due to temperature changes is corrected by the temperature correction function  43  based on the temperature signal fed from the temperature sensor  34 . A program performing those functions  40 - 43  in the DSP  36  is stored in the EEPROM  37 , and the program can be modified from outside by electrical trimming. Some other functions than the functions  40 - 43  may be added in the DSP  36 , or one or more functions may be eliminated from among the functions  41 - 43 . If the temperature correction function  43  is eliminated, the temperature sensor  34  is also eliminated. Alternatively, the temperature sensor  34  may be disposed separately from the one-chip Hall IC  30 . 
     As shown in FIG. 1, the output digital signal VS from the DSP  36  is converted into an analog signal by the D/A converter  38  and then fed to the EMI filter  39  that eliminates electromagnetic interference. The analog signal is output from an analog signal terminal  44 . On the other hand, the digital signal VS is directly output from a digital signal terminal  45 . If a control circuit connected to the Hall IC  30  is an analog circuit, the analog signal VS is used. If the control circuit is digital, the digital signal VS is used. In other words, the Hall IC  30  is adaptable to both analog and digital control circuits. It is, of course, possible to provide only one signal VS as either an analog or digital signal. 
     Though all the components  33 - 39  are built in a single-chip Hall IC  30  in the particular embodiment shown in FIG. 1 to simplify the circuit, the Hall element  33  may be separated from the Hall IC  30 . A conventional rotational angle detector having only the Hall element outputting the signal VS may be modified by adding a circuit including the digital signal processor  36  and other necessary components. 
     A second embodiment of the present invention will be described with reference to FIG. 6, in which the components or parts performing the same function as in the first embodiment carry the same reference numbers. A rotatable lever  51  connected to a rotatable object is formed by molding the rotor core  24  and the permanent magnets  25  together with molding resin  50 . A resin housing  61  includes a boss  52  in which one or two Hall ICs  30  are inserted and a connector terminal housing  56  in which a connector pin  58  is housed. The inner bore of the rotatable lever  51 , formed by the molding resin  50 , is rotatably supported by the boss  52 . A stopper plate  53  is fixed to the end portion of the boss  52  to prevent the rotatable lever  51  from sliding off from the boss  52 . A spring washer  54  is interposed between the stopper plate  53  and the rotatable lever  51  to adjust an axial movement of the rotatable lever  51 . The rotatable lever  51  is biased to its initial position by a twisted coil spring  55 . The rotatable lever  51  is rotated by the rotatable object connected thereto against the biasing force of the twisted coil spring  55 . 
     The Hall IC  30  (one or two) inserted in the boss  52  is placed in the parallel magnetic field generated by the permanent magnets  25 . A circuit board  57  is held by the projection  62  formed in the housing  61  and electrically connected to a connector pin  58  housed in the connector housing  56 . The Hall IC terminal  31  is electrically connected to the circuit board  57 . An opening at the right side of the housing  61  is closed with a cover  60  with a seal member  59  interposed therebetween. The second embodiment described above operates in the same manner as the first embodiment. 
     The Hall IC  30  shown in FIG. 1 is inserted in the boss  52  in the embodiment shown in FIG.  6 . However, the temperature sensor  34 , the digital signal processor  36  and other circuit components other than the Hall element  33  may be separated from the Hall IC  30 , and they may be mounted on the circuit board  57 . 
     Other circuits, such as an amplifier for amplifying the output of the Hall element  33 , may also be included in the Hall IC  30  shown in FIG. 1. A magnetoresistance element may be used as a magnetic sensor in place of the Hall element  33 . Though the rotor having permanent magnets are rotated relative to the Hall element  33  in both embodiments described above, it is also possible to rotate the Hall element  33  in the stationary magnetic field. 
     While the present invention has been shown and described with reference to the foregoing preferred embodiments, it will be apparent to those skilled in the art that changes in form and detail may be made therein without departing from the scope of the invention as defined in the appended claims.