Patent Publication Number: US-7583080-B2

Title: Rotation angle detection device and rotation angle correction method

Description:
This application is a U.S. National Phase Application of PCT International Application PCT/JP2006/302228. 
     TECHNICAL FIELD 
     The present invention relates to a rotation angle detection device used in a vehicle body control system, and the like. In particular, it relates to a rotation angle detection device for detecting an absolute rotation angle of a multiple rotation steering wheel and a rotation angle correction method. 
     BACKGROUND ART 
       FIG. 26  shows a conventional rotation angle detection device. Gear  38  is attached via engaged spring  39  to a rotation axis (not shown) whose rotation angle is intended to be detected. Gear  38  is engaged with gear  41  having an outer circumferential end face provided with code plate  40  to which a plurality of magnetic poles are magnetized. Magnetic poles provided on code plate  40  move in accordance with rotation of the rotation axis to be detected. The number of the magnetic poles is counted by detection element  42  provided facing the outer circumferential end face so as to detect a rotation angle. Furthermore, as a device for detecting a rotation angle of a multiple rotation rotor such as an absolute encoder, a measurement method for detecting a rotation angle of an axis to be detected from rotation angles of a plurality of rotors having phase difference is known. 
     Prior art information relating to the invention of this application includes Japanese Patent Unexamined Publications No. H11-194007 and No. S63-118614. 
     In the thus configured rotation angle detection device, the rotation angle of the axis is detected by counting the number of a plurality of moved magnetic poles disposed on the outer circumferential end face of the code plate. Therefore, in order to improve the resolution of the detection angle, the dimension of the magnetized magnetic pole should be made to be fine. Furthermore, since rotation of the code plate and rotation of the axis are carried out via a gear, it is somewhat difficult to enhance the detection accuracy, by backlash and the like. Furthermore, since this rotation angle detection device can be employed only for detection of the relative rotation angle, it is not suitable for detecting an absolute rotation angle. 
     Furthermore, in the above-mentioned rotation angle detection device, due to arrangement accuracy and center deflections of gears, detection error in the rotation angle detection unit, or the like, the detection accuracy of the rotation angle of an axis to be detected may be deteriorated. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the problems discussed above and provides a rotation angle detection device capable of detecting an absolute rotation angle of multiple rotation with a high accuracy and a high resolution by using a target connected to a rotation angle and having an outer circumferential surface to which magnetic poles of alternate polarities are magnetized. 
     Furthermore, the preset invention provides a rotation angle correction method of a rotation angle detection device for correcting a mechanical error of a gear and an electrical error of a rotation angle detection unit with a high accuracy. 
     The rotation angle detection device of the present invention has a first rotor holding a target connected to an input axis and having an outer circumferential surface to which magnetic poles of alternate polarities are magnetized at an identical interval, and a having a multi-rotatable gear. Furthermore, the rotation angle detection device has a first detector for detecting a rotation angle of the first rotor; a second rotor coupled to the gear of the first rotor, rotated at a lower speed than the first rotor, and having a magnet on the center portion; and a second detector for detecting a rotation angle of the second rotor. The first detection unit detects fine rotation angle, and the second detection unit detects rough absolute rotation angle. From these absolute rotation angles, multiple rotation angle of the first rotor is detected. With such a configuration, it is possible to detect an absolute rotation angle with a high accuracy and a high resolution by using a simple structure and a simple circuit configuration. 
     Furthermore, in the rotation angle detection device of the present invention, the first and second detectors include magnetic detection elements disposed in positions facing the target and the magnet. Since it is possible to detect an absolute rotation angle of the first and second rotors by a non-contact method, durability and reliability of the rotation angle detection device can be improved. 
     Furthermore, the rotation angle detection device of the present invention includes a nonvolatile memory (hereinafter, referred to as EEPROM) for storing sensitivities of a sine wave signal and a cosine wave signal output from the first and second detectors. After the first and second rotors are incorporated, the sine wave signal and the cosine wave signal are corrected with the respective sensitivities every time an electric power is turned on. The angle detection error caused by variation in sensitivities of the detection element and detection element amplifier is not generated, and thus, the rotation angle of the rotor can be exactly detected. 
     Furthermore, the rotation angle detection device of the present invention includes a sensitivity detection unit for detecting whether the sensitivity is within a specified value when the sensitivities of the respective magnetic detection elements are stored. When a signal whose sensitivity is out of the specified range is input due to variation in the sensitivity of the magnetic detection elements, such an unnecessary signal can be eliminated. 
     Furthermore, the rotation angle detection device of the present invention includes a signal amplitude detection unit for detecting whether a center of an amplitude of an output signal is within a specified value when the sensitivities of the respective magnetic detection elements are stored. Thus, even when an unnecessary signal having a center of the amplitude out of the specified range is input due to variation in characteristics of the magnetic detection elements, such unnecessary signals can be eliminated. 
     Furthermore, the rotation angle detection device of the present invention includes a signal detection unit for detecting a sine wave signal and a cosine wave signal at a plurality of times when the sensitivities of the respective magnetic detection elements are stored. Thus, even if the sine wave signal and the cosine wave signal are affected by noise and the like, detection error can be suppressed. 
     Furthermore, the rotation angle detection device of the present invention includes a position determination unit for determining a certain position of each magnetic detection element. When values of the sine wave signal and the cosine wave signal at the position are stored in, for example, an EEPROM, it is possible to detect an absolute rotation angle from a certain position in a certain rotation range. 
     The rotation angle detection device of the present invention has an advantage that by employing the above-mentioned configuration, a multiple rotation absolute rotation angle can be detected with a high accuracy and a high resolution. 
     Furthermore, another rotation angle detection device of the present invention includes: 
     (a) a multi-rotatable first rotor holding a first target connected to an input axis and having an outer circumferential surface to which magnetic poles of alternate polarities are magnetized at an identical interval; 
     (b) a first detection unit for detecting a rotation angle of the first rotor, which is disposed facing the magnetic pole of the first target; 
     (c) a second rotor coupled to the input axis and having a gear; 
     (d) a third rotor coupled to the gear of the second rotor and having a gear provided with a second target in a center portion; 
     (e) a second detection unit for detecting a rotation angle of the third rotor, which is disposed facing the second target; 
     (f) a fourth rotor coupled to the gear of the third rotor and having a gear provided with a third target in a center portion; and 
     (g) a third detection unit for detecting a rotation angle of the fourth rotor, which is disposed facing the third target. 
     A multiple rotation angle of the first rotor can be detected with a high resolution and a high accuracy by combining a rotation angle of the first rotor detected by the first detection unit and a multiple rotation angle of the second rotor calculated from the rotation angles of the third and fourth rotors detected by the second and third detection units. 
     Furthermore, the rotation angle detection device of the present invention includes a pair of magnetic sensors including a multi-rotatable first target having an outer circumferential surface to which magnetic poles of alternate polarities are magnetized at an identical interval, and a first detection unit disposed facing the magnetic poles of the first target. When the configuration including the first target and the first detection unit is provided on the input axis, it is possible to detect a rotation angle of the first rotor with a high resolution and a high accuracy. Furthermore, by combining with the multiple rotation angle calculated from the difference between the third and fourth rotor having gears with different numbers of teeth, the multiple rotation angle can be detected with a high resolution and a high accuracy. Furthermore, by employing a magnetic detection element for the detection unit, it is possible to detect the rotation angle of the target by non-contacting method. Therefore, the durability and reliability of the rotation angle detection device can be improved. Furthermore, by comparing a rotation angle of the target to which magnetic poles are magnetized and a rotation angle of the gear with each other, abnormality in the rotation angle detecting device can be detected relatively easily. 
     Furthermore, another aspect of the present invention relates to a rotation angle correction method of a rotation angle detection device including a first rotation angle detection unit disposed in a position facing a target connected to an axis to be detected; a mechanism for reducing a rotation speed of the axis to be detected; and a second rotation angle detection unit for detecting a rotation angle whose rotation speed is reduced. Furthermore, in the rotation angle detection device for calculating a rotation angle of an axis to be detected by output signals from the first rotation angle detection unit and second rotation angle detection unit, by using a motor for rotating the axis to be detected, a motor controller for controlling a rotation angle of the motor, and an encoder for detecting the rotation angle of the motor, a difference between a rotation angle of the axis to be detected actually rotated by the motor and a calculated rotation angle of the axis to be detected obtained by the first and the second rotation angle detection units is stored as a corrected angle in an EEPROM, and the calculated rotation angle of the axis to be detected is corrected with this corrected angle. 
     Furthermore, in the rotation angle correction method of the present invention, the corrected angle is stored in the EEPROM every predetermined rotation angle in the entire detection range and the calculated rotation angle of the axis to be detected is corrected. In addition, between the predetermined rotation angles, correction is carried out by using a corrected angle estimated from an approximate line obtained from the corrected angles stored before and after the predetermined rotation angle. 
     Furthermore, in the rotation angle correction method of the present invention, a target is provided as a multipole ring magnet to which magnetic poles of reverse polarities are magnetized at an identical interval in a circumferential direction of the axis to be detected. In a rotation range corresponding to each magnetic pole width, an average value of each magnetic pole error is stored as a corrected angle common to each magnetic pole in the EEPROM. Thus, the calculated rotation angle of the axis to be detected is corrected with the corrected angle. 
     Furthermore, in the rotation angle correction method of the present invention, the target is provided as a gear having convex portions disposed at an identical interval in a circumferential direction of the axis to be detected. In the rotation range corresponding to each tooth width, an average value of errors of teeth is stored as a corrected angle common to the teeth in the EEPROM. Thus, the calculated rotation angle of the axis to be detected is corrected with the corrected angle. 
     Furthermore, in the rotation angle correction method of the present invention, the target has concave portions and non-concave portions disposed at a predetermined interval in a circumferential direction of the axis to be detected. In the rotation range corresponding to each concave portion width, an average value of errors of each concave portion is stored as a corrected angle common to each concave portion in the EEPROM. Thus, the calculated rotation angle of the axis to be detected is corrected with the corrected angle. 
     Furthermore, in the rotation angle correction method of the present invention, in the rotation range corresponding to the target interval, a corrected angle common to each target is stored in the EEPROM every predetermined rotation angle. The calculated rotation angle of the axis to be detected is corrected, and between the predetermined rotation angles, correction is carried out based on a corrected angle estimated from an approximate line property obtained from the corrected angles stored before and after the predetermined rotation angle. 
     Furthermore, in summary, in the rotation angle correction method of the present invention, correction data with less capacity are stored in the EEPROM and the calculated rotation angle of the axis to be detected is corrected by using the corrected data. Such a correction method of the rotation angle can significantly improve the detection accuracy of the calculated rotation angle of the axis to be detected including a mechanical error due to the dimension variation of the constituting components, a magnetic error due to the characteristic variation of a magnet, and an electrical characteristic error of a rotation angle detection unit or a detection circuit unit. 
     Furthermore, the correction method of the rotation angle of the present invention can correct the reduction of the rotation angle detection accuracy caused by a mechanical error, a magnetic error, an electrical characteristic error, and the like, of a multipole ring magnet or a rotation angle detection unit by using the corrected angle stored in an EEPROM with less capacity. Thus, it is possible to provide a correction method of the rotation angle detection device capable of improving the detection accuracy of rotation angle of the axis to be detected. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a basic configuration of a rotation angle detection device in accordance with a first exemplary embodiment of the present invention. 
         FIG. 2A  is a view showing a rotation angle detection signal of a first magnetic detection element in accordance with the first exemplary embodiment of the present invention. 
         FIG. 2B  is a view showing a rotation angle (electrical angle) of a first rotor in accordance with the first exemplary embodiment of the present invention. 
         FIG. 3  is a graph showing a rotation angle detection signal of a second magnetic detection element in accordance with the first exemplary embodiment of the present invention. 
         FIG. 4  is a circuit block diagram showing a rotation angle detection device in accordance with the first exemplary embodiment of the present invention. 
         FIG. 5  is a graph showing an ideal value and an actual value of an absolute rotation angle of the first rotor in accordance with the first exemplary embodiment of the present invention. 
         FIG. 6  is a characteristic graph showing a rotation angle operation output signal in each element in a CPU and an absolute rotation angle of the rotation angle detection device in accordance with the first exemplary embodiment of the present invention. 
         FIG. 7  is a graph showing output signals output from first and second magnetic detection elements in accordance with the first exemplary embodiment of the present invention. 
         FIG. 8A  is a side sectional view showing a basic configuration of a rotation angle detection device in accordance with a second exemplary embodiment of the present invention. 
         FIG. 8B  is a plan view showing a basic configuration of a rotation angle detection device in accordance with the second exemplary embodiment of the present invention. 
         FIG. 8C  is a partial sectional view showing a basic configuration of a rotation angle detection device in accordance with the second exemplary embodiment of the present invention. 
         FIG. 9  is a circuit block diagram showing a rotation angle detection device in accordance with the second exemplary embodiment of the present invention. 
         FIG. 10A  is a view showing an output signal from a first detection unit in accordance with the second exemplary embodiment of the present invention. 
         FIG. 10B  is a view showing a relation between a mechanical angle and an electrical angle of the output signal from the first detection unit in accordance with the second exemplary embodiment of the present invention. 
         FIG. 11A  is a graph showing an output signal from a third detection unit in accordance with the second exemplary embodiment of the present invention. 
         FIG. 11B  is a graph showing a rotation angle (electrical angle) of the output signal from the third detection unit in accordance with the second exemplary embodiment of the present invention. 
         FIG. 12A  is a graph showing an output signal from a fourth detection unit in accordance with the second exemplary embodiment of the present invention. 
         FIG. 12B  is a graph showing a rotation angle (electrical angle) of the output signal from the fourth detection unit in accordance with the second exemplary embodiment of the present invention. 
         FIG. 13A  is a graph showing a rotation angle of a third rotor used for detecting a rotation angle of a first rotor in accordance with the second exemplary embodiment of the present invention. 
         FIG. 13B  is a graph showing a rotation angle of a fourth rotor used for detecting a rotation angle of the first rotor in accordance with the second exemplary embodiment of the present invention. 
         FIG. 13C  is a graph showing a difference in the rotation angle between the first rotor and the fourth rotor used for detecting a rotation angle of the first rotor in accordance with the second exemplary embodiment of the present invention. 
         FIG. 13D  is a graph showing a rotation angle of a first target calculated by the first detection unit in accordance with the second exemplary embodiment of the present invention. 
         FIG. 14  is a graph showing output signals from the first to third detection units in accordance with the second exemplary embodiment of the present invention. 
         FIG. 15  is a diagram showing a configuration of a rotation angle detection device in accordance with a third exemplary embodiment of the present invention. 
         FIG. 16  is a diagram showing a configuration of a correction system of the rotation angle detection device in accordance with the third exemplary embodiment of the present invention. 
         FIG. 17  is a graph showing output signals from the first rotation angle detection unit in accordance with the third exemplary embodiment of the present invention. 
         FIG. 18  is a graph showing a relation between a rotation mechanical angle and a rotation electrical angle of an axis to be detected in accordance with the third exemplary embodiment of the present invention. 
         FIG. 19  is a view of the principle for calculating the rotation mechanical angle of the axis to be detected in accordance with the third exemplary embodiment of the present invention. 
         FIG. 20  is a graph showing an example of an error included in a rotation mechanical angle of the calculated axis to be detected in accordance with the third exemplary embodiment of the present invention. 
         FIG. 21  shows a method for obtaining a corrected approximate line from a rotation mechanical angle error in accordance with the third exemplary embodiment of the present invention. 
         FIG. 22  shows an example of a rotation mechanical angle error after it is corrected with an average value of the rotation mechanical angle error of each magnetic pole in accordance with the third exemplary embodiment of the present invention. 
         FIG. 23  shows a method for obtaining a corrected approximate linear property for correcting with the average value of the rotation mechanical angle error of each magnetic pole in accordance with the third exemplary embodiment of the present invention. 
         FIG. 24  is a perspective view showing a target in accordance with a fourth exemplary embodiment of the present invention. 
         FIG. 25  is a perspective view showing a target in accordance with a fifth exemplary embodiment of the present invention. 
         FIG. 26  shows a conventional rotation angle detection device. 
     
    
    
     REFERENCE MARKS IN THE DRAWINGS 
     
         
           101 ,  203  first rotor 
           102  input axis 
           103  target 
           108 ,  210  second rotor 
           109  magnet 
           110  first magnetic detection element 
           111  second magnetic detection element 
           114  microcomputer (CPU) 
           115  nonvolatile memory (EEPROM) 
           116  amplifier 
           119  rotation angle operation output signal from first magnetic detection element 
           120  rotation angle operation output signal from second magnetic detection element 
           121  calculated absolute rotation angle of rotation angle detection device 
           122  ideal absolute angle 
           123  sine wave signal 
           124  cosine wave signal 
           126  sine wave signal level 
           127  cosine wave signal level 
           128  specified range 
           129  switch 
           131  certain position determination signal line 
           132  output signal line 
           204  input axis 
           205  first target 
           210  second rotor 
           211  third rotor 
           212  second target 
           213  first detection unit 
           214  fourth rotor 
           215  third target 
           216  second detection unit 
           217  third detection unit 
           219 ,  220  substrate 
           301  axis to be detected 
           302  multipole ring magnet 
           303  first rotation angle detection unit 
           304  worm gear 
           305  wheel gear 
           306  magnet 
           307  second rotation angle detection unit 
           308  rotation angle detection device 
           309  motor 
           310  encoder 
           311  nonvolatile memory (EEPROM) 
           312  CPU 
           313  serial communication line 
           314  motor controller 
           315  Sin signal 
           316  Cos signal 
           327 ,  328  target 
         θe, θe 1  rotation electrical angle 
         θm, θm 1 , θm 2 , θm 3 , θm 4  rotation mechanical angle 
         Δθm 1 , Δθm 2  rotation mechanical angle error 
         Δθm 1 Av average value of rotation mechanical angle error 
       
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Exemplary Embodiment 
     Hereinafter, the first exemplary embodiment of the present invention is described with reference to  FIGS. 1 to 7 . 
       FIG. 1  is a diagram showing a basic configuration of an absolute rotation angle detection device in accordance with the first exemplary embodiment of the present invention;  FIG. 2  is a view showing a rotation angle detection signal of a first magnetic detection element;  FIG. 3  is a graph showing a rotation angle detection signal of a second magnetic detection element; and  FIG. 4  is a circuit block diagram showing an absolute rotation angle detection device.  FIG. 5  is a graph showing an ideal value and an actual value of an absolute rotation angle of the first and second rotors;  FIG. 6  is a graph showing a rotation angle operation output signal and an absolute rotation angle in a CPU; and  FIG. 7  is a graph showing output signals output from first and second magnetic detection elements. 
     In  FIG. 1 , first rotor  101  is a rotor having a multi-rotatable gear fitted into and connected to input axis  102 . First rotor  101  holds targets  103  and has an outer circumferential surface to which magnetic poles of alternate polarities are magnetized at an identical interval. Second rotor  108  is provided in a way in which it is engaged with the gear of first rotor  101  and has magnet  109  disposed in the center portion thereof. First magnetic detection element (detection unit)  110  is disposed in a position facing target  103 , and second magnetic detection element (detection unit)  111  is disposed in a position facing magnet  109 , so that the magnetic field direction is detected. First and second magnetic detection elements  110  and  111  are disposed on substrate  113 . The gear of first rotor  101  and a gear of second rotor  108  are connected to each other. When first rotor  101  rotates, second rotor  108  is rotated in accordance with the speed in response to the ratio of the number of teeth of respective gears. 
     First and second magnetic detection elements  110  and  111  are described in a case where a magnetoresistive element (hereinafter, referred to as “MR element”) is used. Magnetic detection elements  110  and  111  output a sine wave signal and a cosine wave signal in a form of an analog signal in response to the change of the magnetic field. When the change of the magnetic field of target  103  is detected by first magnetic detection element  110 , one cycle of sine wave signal and cosine wave signal are output to one pole. Therefore, it is possible to obtain sine wave signals and cosine wave signals by the number of magnetic poles each rotation. These output signals are amplified to a specified amplitude by an amplifier and are subjected to operation processing via an A/D converter (not shown) built in microcomputer (hereinafter, referred to as CPU)  114  so as to calculate the rotation of target  103 , that is, an absolute rotation angle of first rotor  101 . 
       FIG. 2A  shows a rotation angle detection signal output from first magnetic detection element  110 . The abscissa indicates a rotation angle (mechanical angle) of input axis  102 , and the ordinate indicates sine wave signal  123  and cosine wave signal  124  output from first magnetic detection element  110 , respectively. 
       FIG. 2B  shows a rotation angle (electrical angle) of first rotor  101  with respect to input axis  102 . 
     Second magnetic detection element  111  detects the change of the magnetic field of magnet  109  disposed in the center portion of second rotor  108 . Output signals of two cycles of sine wave signal and cosine wave signal are output with respect to one rotation of magnet  109 . These output signals are subjected to operation processing in CPU  114  so as to calculate an absolute rotation angle of second rotor  108 . 
       FIG. 3  shows a rotation angle detection signal of second magnetic detection element  111 . The abscissa indicates a rotation angle (mechanical angle) of input axis  102 , and the ordinate indicates sine wave signal  123  and cosine wave signal  124  output from second magnetic detection element  111 , respectively. Furthermore, the ordinate indicates rotation angle (electrical angle) θe 108  in the operation process in the CPU of second rotor  108 . 
       FIG. 4  is a circuit block diagram showing a rotation angle detection device. In  FIG. 4 , output signals output from first and second magnetic detection elements  110  and  111  are input into CPU  114  via amplifiers  116   a  and  116   b , respectively, and are subjected to operation processing. Thus, an absolute rotation angle is output. Furthermore, to CPU  114 , EEPROM  115  is coupled. 
     In  FIG. 5 , the abscissa indicates an absolute rotation angle of input axis  102  and the ordinate indicates a fine absolute rotation angle obtained from first rotor  101 . Characteristic  502  (solid line) shows an actual value of the absolute rotation angle obtained from first rotor  101 , and characteristic  504  (broken line) shows an ideal value of the absolute rotation angle of first rotor  101 , respectively. 
     In the lower segment of  FIG. 5 , the ordinate indicates rough absolute rotation angles (from 0° to 180°) obtained from second rotor  108 . Characteristic  506  (broken line) shows an ideal value of an absolute rotation angle of second rotor  108 , and characteristic  508  (solid line) shows an actual value of the absolute rotation angle obtained from second rotor  108 , respectively. Furthermore,  FIG. 5  shows absolute rotation angle detection range  510 . 
     Next, a rotation angle detection method of a rotor is described. In  FIG. 1 , when first rotor  101  rotates, second rotor  108  is rotated by a gear of second rotor  108  coupled to a gear of first rotor  101 . When the number of teeth of the gear of first rotor  101  is denoted by “a” and that of second rotor  108  is denoted by “b”, second rotor  108  rotates at a speed that is a/b times as that of first rotor  101 . At this time, by appropriately selecting numbers of teeth of gear “a” and “b”, second rotor  108  can be rotated at sufficiently lower speed than that of first rotor  101 . 
     With first magnetic detection element  110  disposed in the position facing target  103  held by first rotor  101 , the change of the magnetic field with respect to the rotation of first rotor  101  is detected so as to change the output signal. On the other hand, second magnetic detection element  111  disposed in a position facing second rotor  108  having magnet  109  in the center portion detects the change of the magnetic field penetrating second magnetic detection element  111  when second rotor  108  is rotated, so that output signal is changed. 
     Output signals from first magnetic detection element  110  and second magnetic detection element  111  are input via an A/D converter built in CPU  114 . With the output signal from second magnetic detection element  111 , an absolute angle detection is roughly carried out so as to detect that the angle at which second rotor  108  is disposed as compared with the initial position. Then, with the output signals from first magnetic detection element  110 , an absolute angle of the rotation angle of first rotor  101  is finely carried out. With the output signal, the absolute rotation angle is calculated and output. In  FIG. 6 , rotation angle detection range  510  is shown. 
       FIG. 6  shows correlation characteristic between rotation angle operation output signals from first and second magnetic detection elements  110  and  111  and an absolute rotation angle of the rotation angle detection device in CPU  114 . Rotation angle operation output signal  119  from first magnetic detection element  110 , rotation angle operation output signal  120  from second magnetic detection element  111 , calculated absolute rotation angle  121  and ideal absolute rotation angle  122  of the rotation angle detection device are shown, respectively. 
     Next, a method for suppressing variation in sensitivity of first and second magnetic detection elements  110  and  111  and amplifiers  116   a  and  116   b  and preventing the occurrence of an error in the detection of a rotation angle during operation is described with reference to  FIGS. 1 ,  4  and  7 . 
     In  FIG. 1 , when first rotor  101  rotates, target  103  also rotates. With the rotation of target  103 , the magnetic field changes. This change of the magnetic field is detected by first magnetic detection element  110 . First magnetic detection element  110  outputs sine wave signal  123  and cosine wave signal  124  with respect to this change of the magnetic field.  FIG. 7  shows these output signals. These output signals are input into CPU  114  via amplifiers, and inverse arc tangent signals are calculated from sine wave signal  123  and cosine wave signal  124 . However, as shown in  FIG. 7 , when sine wave signal level  126  and cosine wave signal level  127  are slightly different because of variation in sensitivity of the magnetic detection element or the amplifier, the accuracy of the calculated inverse arc tangent signal is lowered. 
     Then, when switch  129  shown in  FIG. 4  is turned on so as to set to a sensitivity memory mode, first rotor  101  is rotated so that second rotor  108  is rotated by 180° or more. Then, the maximum and minimum levels of sine wave signal  123  and cosine wave signal  124  are calculated, and each signal level (sensitivity) is stored in EEPROM  115 . Next, when switch  129  is turned off and a rudder angle value is calculated, the maximum and minimum levels of sine wave signal  123  and cosine wave signal  124  are operated to coincide with each other and an inverse arc tangent signal is calculated. Thus, the rudder angle is obtained. 
     Furthermore, in the case where the maximum value and minimum value of the output signals of first and second magnetic detection elements  110  and  111  do not fall within specified range  128 , the output signals do not change or necessary resolution cannot be obtained due to the temperature characteristic and the like. Therefore, by providing some means (not shown) for confirming that the maximum value and the minimum value of each output signal shown in  FIG. 7  fall in specified range  128 , an output error can be prevented. When a signal amplitude detection unit (not shown) for detecting a center of the amplitude of the output signals from first and second magnetic detection elements  110  and  111  is provided, an output error due to characteristic variation can be prevented. Furthermore, at this time, for example, by executing input at a plurality of times so as to take an average, or by taking an average excluding the maximum and minimum values, an output error can be prevented with a higher accuracy. 
     Furthermore, by storing output signals from first magnetic detection element  110  and second magnetic detection element  111  at a certain position, it is possible to detect an absolute rotation angle from a certain position. Furthermore, at this time, when a signal showing a certain position is sent by an electrical signal as shown by certain position determination signal line  131  in  FIG. 4 , it is possible to confirm the certain position without carrying out a mechanical operation. Furthermore, by reading an electrical signal at a plurality of times so as to be checked or by sending a signal by a serial signal, even if a wrong signal is input due to a noise, such unnecessary signals can be eliminated. Note here that the same effect can be obtained even when certain position determination signal line  131  uses the same terminal as that of output signal line  132  by switching input and output. 
     Second Exemplary Embodiment 
     A second exemplary embodiment is described with reference to  FIG. 8A  to  FIG. 14 .  FIGS. 8A ,  8 B and  8 C show a basic configuration of a rotation angle detection device in accordance with the second exemplary embodiment;  FIG. 9  is a circuit block diagram showing a rotation angle detection device;  FIGS. 10A and 10B  show an output signal from a first detection unit;  FIGS. 11A and 11B  show an output signal from a third detection unit;  FIGS. 12A and 12B  show an output signal from a fourth detection unit;  FIG. 13  shows rotation angles of third and fourth rotors used for detecting a rotation angle of the first and second rotors; and  FIG. 14  is a view to illustrate a method for preventing the occurrence of a detection error of a rotation angle. 
     In  FIG. 8A to 8B , multi-rotatable first rotor  203  is fitted into input axis  204 . First target  205  held by first rotor  203  has an outer circumferential surface to which magnetic poles of alternate polarities are magnetized at an identical interval. Second rotor  210  has a multi-rotatable gear fitted into first rotor  203 . Third rotor  211  is engaged with the gear of second rotor  210 , and second target (single pole magnet)  212  is disposed in the center portion of third rotor  211 . Second detection unit (magnetic detection element)  216  is disposed in a position facing second target  212  so as to detect the magnetic field direction. Fourth rotor  214  is engaged with a gear of third rotor  211  and third target (single pole magnet)  215  is disposed in the center portion of fourth rotor  214 . Third detection unit (magnetic detection element)  217  is disposed in a position facing third target  215  so as to detect the magnetic field direction. First detection unit (magnetic detection element)  213  is disposed in a position facing first target  205  so as to detect the magnetic field direction. Substrate  219  is provided with first detection unit  213  (magnetic detection element), and substrate  220  is provided with second and third detection units  216  and  217  (magnetic detection elements), respectively. 
     The number of magnetized magnetic poles of first target  205  is decided to be 30 poles (north pole: 15, and south pole: 15) with a margin. In this case, the degree per pole is 12°. 
     Next, a case where an MR element is used for first, second and third detection units  213 ,  216  and  217  is described. The MR element used for each detection unit detects the magnetic field direction respectively, and outputs a sine wave signal and a cosine wave signal in a form of an analog signal. 
     When the change of the magnetic field direction of first target  205  is detected by first detection unit  213 , one cycle of sine wave signal and cosine wave signal are output with respect to a pole with one magnetic pole. When first target  205  is rotated once, it is possible to obtain sine wave signals and cosine wave signals for the number of magnetized magnetic poles. 
       FIG. 9  is a circuit block diagram showing a rotation angle detection device in accordance with the second exemplary embodiment of the present invention. As shown in  FIG. 9 , an output signal from first detection unit  213  is amplified to a specified amplitude by amplifier  221 , input into an A/D converter (not shown) built in CPU  223 , and subjected to operation processing. Then, a rotation angle of first target  205 , that is, first rotor  203  is calculated. Furthermore, second and third detection units  216  and  217  are coupled to CPU  223  via amplifiers  230  and  231 , respectively. On the other hand, a rotation angle calculated in CPU  223  is output via output signal line  232 . In  FIG. 9 , EEPROM  251  calculates and stores certain positions of first detection unit  213 , second detection unit  216  and third detection unit  217 , signal levels (sensitivities), maximum and minimum levels and amplitude center level of a sine wave signal and the cosine wave signal output therefrom. 
       FIG. 10A  shows an output signal output from first detection unit  213 . The abscissa indicates a rotation angle of first rotor  203  fitted into input axis  204 . The ordinate indicates sine wave signal  224  and cosine wave signal  225  output from first detection unit  213 . 
       FIG. 10B  shows an electrical angle of an output signal output from first detection unit  213 . The abscissa indicates a rotation angle of first rotor  203 . The ordinate indicates a rotation angle (electrical angle) of first rotor  203  calculated in CPU  223  based on sine wave signal  224  and cosine wave signal  225 , respectively. 
     On the other hand, a gear of third rotor  211  is connected to a gear of second rotor  210  and rotates at a speed ratio by the ratio between the number of teeth of third rotor  211  and that of second rotor  210 . 
     Second detection unit  216  detects a magnetic field direction of second target (single pole magnet)  212  disposed in the center portion of third rotor  211 , and outputs one cycle of sine wave signal and cosine wave signal with respect to 0.5 rotations of second target (single pole magnet)  212 . This output signal is subjected to operation processing in CPU  223  so as to calculate a rotation angle of third rotor  211 . 
     In  FIG. 11A , the abscissa indicates a rotation angle of second rotor  210 , and the ordinate indicates sine wave signal  226  and cosine wave signal  227  output from second detection unit  216 , respectively. In  FIG. 11B , similar to  FIG. 11A , the abscissa indicates a rotation angle of second rotor  210 , and the ordinate indicates electrical angle θe 104  obtained by operating a rotation angle of third rotor  211  based on sine wave signal  226  and cosine wave signal  227  in CPU  223 . 
     A gear of fourth rotor  214  is connected to second rotor  210  via a gear of third rotor  211 . Fourth rotor  214  rotates at a speed ratio by the number of teeth of each gear when second rotor  210  is rotated. 
     Third detection unit  217  detects a magnetic field direction of third target (single pole magnet)  215  disposed in the center portion of fourth rotor  214 , and outputs one cycle of sine wave signal and cosine wave signal with respect to 0.5 rotations of third target (single pole magnet)  215 . This output signals are subjected to operation processing in CPU  223  so as to calculate a rotation angle of fourth rotor  214 . 
     In  FIG. 12A , the abscissa indicates a rotation angle of second rotor  210 , and the ordinate indicates sine wave signal  228  and cosine wave signal  229  output from third detection unit  217 , respectively. Similar to  FIG. 12A , in  FIG. 12B , the abscissa indicates a rotation angle of second rotor  210 , and the ordinate indicates electrical angle θe 214  calculated by operating a rotation angle of fourth rotor  214  based on sine wave signal  229  and cosine wave signal  229  in CPU  223 . 
     In  FIG. 13A , the abscissa indicates a rotation angle of second rotor  210  fitted into input axis  204 , and the ordinate indicates rotation angle of third rotor  211  calculated from a signal obtained by second detection unit  216 , respectively. 
     In  FIG. 13B , the abscissa indicates a rotation angle of second rotor  210  fitted into input axis  204 , and the ordinate indicates a rotation angle of fourth rotor  214  calculated from a signal obtained from third detection unit  217 , respectively. Since the number of teeth of a gear mounted on third rotor  211  is different from the number of teeth of a gear mounted on fourth rotor  214 , the rotation cycles with respect to the rotation angle of second rotor  210  are different. 
     In  FIG. 13C , the abscissa indicates a rotation angle of second rotor  210  fitted into input axis  204 , and the ordinate indicates a difference in the rotation angle between third rotor  211  and fourth rotor  214  calculated from a signal obtained from second detection unit  216  and a signal obtained from third detection unit  217 . 
     In  FIG. 13D , the abscissa indicates a rotation angle of first rotor  203  fitted into input axis  204 , and the ordinate indicates a rotation angle of first target  205  calculated from a signal obtained from first detection unit  213 , respectively. 
     Next, a method for detecting a multiple rotation angle of a rotor is described with reference to  FIGS. 8A to 8C . When second rotor  210  fitted into first rotor  203  shown in  FIG. 8A  rotates, third rotor  211  is rotated by a gear of third rotor  211  coupled to a gear of second rotor  210 . At the same time, fourth rotor  214  is rotated by a gear of fourth rotor  214  coupled to the gear of third rotor  211 . When the numbers of teeth of the gears of second rotor  210 , third rotor  211  and fourth rotor  214  are denoted by “a”, “b”, and “c”, third rotor  211  rotates at a speed that is a/b times as that of second rotor  210 , and fourth rotor  214  rotates at a speed that is a/c times as that of second rotor  210 . At this time, by appropriately selecting numbers of teeth of gears, “a,” “b” and “c”, the multiple rotation angle of second rotor  210  can be obtained from the difference in the rotation angle between third rotor  211  and fourth rotor  214 . 
     Second detection unit  216  disposed facing second target (single pole magnet)  212  disposed in the center portion of third rotor  211  detects a magnetic field direction penetrating second detection unit  216  so as to detect a rotation angle of third rotor  211 . 
     On the other hand, third detection unit  217  disposed facing third target (single pole magnet)  215  disposed in the center portion of fourth rotor  214  detects a magnetic field direction penetrating third detection unit  217  so as to detect a rotation angle of fourth rotor  214 . The output signals from second detection unit  216  and third detection unit  217  are input via an A/D converter (not shown) built in CPU  223 . The multiple rotation angle of second rotor  210  is calculated from the difference of a rotation angle calculated from output signals from second detection unit  216  and third detection unit  217 . By estimating the position of the magnetic pole of first target  205  from this multiple rotation angle, the multiple rotation angle of first target  205  is calculated with a high accuracy. 
       FIGS. 13A to 13D  show rotation angles calculated in CPU  223  based on the output signals from the first, second and third detection units  213 ,  216  and  217 . Rotation angle  235  of third rotor  211  is operated based on an output signal from second detection unit  216 , and rotation angle  236  of fourth rotor  214  is operated based on the output signal of third detection unit  217 , respectively. Rotation angle difference  237  shows a difference of rotation angle between third rotor  211  and fourth rotor and fourth rotor  214 , which is calculated from the output signals from second detection unit  216  and third detection unit  217 . Rotation angle difference  237  changes linearly from 0° to 180° in the rotation detection range of 0° to 1800° of second rotor  210 . This means that the multiple rotation angle of second rotor  210  can be uniquely defined in the rotation detection range of 0° to 1800° with rotation angle difference  237 . 
     On the other hand, rotation angle  233  of first target  205  (multipole ring magnet) calculated based on the signal from first detection unit  213  linearly changes from 0° to 180° in the electrical angle in the rotation angle between the magnetized poles (in this case, 12°). This means that the rotation angle of first rotor  203  holding first target  205  can be uniquely defined in the rotation angle between magnetized poles with rotation angle  233 . Since second rotor  210  and first rotor  203  holding first target  205  are fitted to the same axis, the position of the magnetic pole of first target  205  is estimated from the multiple rotation angle of second rotor  210  so as to calculate the multiple rotation angle of first target  205  with a high accuracy. 
     Next, a method for detecting abnormality of the rotation angle detection device by comparing the rotation angle of first rotor  203  and the rotation angle of third rotor  211  is described with reference to  FIGS. 9 ,  10 A,  10 B,  11 A,  11 B and  13 . 
     In  FIG. 9 , when first rotor  203  rotates, first target  205  held by first rotor  203  rotates. If thirty poles are magnetized on the surface of first target  205 , the output signal shown in  FIG. 10A  is obtained from first detection unit  213 . Every time first rotor  203  rotates at 12°, sine wave signal  224  and cosine wave signal  225  change in one cycle. An electrical angle calculated from these signals changes by 180°. That is to say, the rotation angle of first rotor  203  can be obtained uniquely in the range of 12° of the rotation angle. When the ratio of the number of teeth of a gear of second rotor  210  and that of third rotor  211  is provisionally set to ⅓, as shown in  FIG. 11A , every time second rotor  210  rotates in 60°, sine wave signal  226  and cosine wave signal  227  change in one cycle, and an electrical angle calculated from these signals changes by 180°. 
     In  FIGS. 13A and 13D , the difference between rotation angle  233  of first target  205  calculated from first detection unit  213  and rotation angle  235  of third rotor  211  calculated from second detection unit  216  is a value that is not more than the specified value unless inconvenience occurs in the rotation angle detection device, when the gradient of rotation angle  233  and rotation angle  235  is corrected with a rotation angle ratio (12:60=1:5) of one cycle by using a certain rotation angle as an original angle. That is to say, abnormality is determined by calculating a difference between a value that is made to be five times of rotation angle  235  and rotation angle  233 . 
     Next, a method for preventing a rotation detection error from occurring due to variation in sensitivity of first, second and third detection units (magnetic detection elements)  213 ,  216  and  217 , amplifiers  221 ,  230  and  231 , and the like, is described. 
     In  FIG. 8A , when first rotor  203  rotates, first target  205  also rotates. With the rotation of first target  205 , the magnetic field direction changes. This change of the magnetic field direction is detected by first detection unit  213 . From first detection unit  213 , with respect to this change of the magnetic field direction, sine wave signal  224  and cosine wave signal  225  are output. 
     In  FIG. 10A , the abscissa indicates a rotation angle of first rotor  203  and the ordinate indicates sine wave signal  224  and cosine wave signal  225 . These signals are input into CPU  223  via amplifier  221 . An inverse arc tangent signal is calculated based on sine wave signal  224  and cosine wave signal  225 . 
     However, as shown in  FIG. 14 , when sine wave signal level  245  and cosine wave signal level  246  are slightly different from each other due to variation in sensitivity of a magnetic detection element and an amplifier, the accuracy of the calculated inverse arc tangent signal is lowered. Then, when switch signal  250  shown in  FIG. 9  is turned on so as to set to be sensitivity memory mode, first rotor  203  is rotated by 12° or more and signal levels (sensitivities)  245  and  246  of sine wave signal  244  and cosine wave signal  243  are calculated, which are stored in EEPROM  251 . When the rotation angle is calculated, switch signal  250  is turned off, with stored signal levels (sensitivities)  245  and  246 , correction is carried out so that the maximum and minimum levels of sine wave signal  243  and cosine wave signal  244  coincide with each other, from which the inverse arc tangent signal from the corrected signal is calculated and the rotation angle is obtained. 
     Furthermore, second rotor  210  is rotated so that third and fourth rotors  211  and  214  are rotated by 180° or more as shown in  FIG. 8 , and the signal levels (sensitivities) of sine wave signals  226  and  228  and cosine wave signals  227  and  229  shown in  FIGS. 11A and 12A  are calculated and stored in EEPROM  251 . As shown in  FIG. 14 , with stored signal levels (sensitivities)  245  and  246 , correction is carried out so that the maximum and minimum levels of sine wave signal  243  and cosine wave signal  244  coincide with each other, from which the inverse arc tangent signal from the corrected signal is calculated and the rotation angle is obtained. 
     Furthermore, when the maximum value and the minimum value of the output signals from first, second and third detection units  213 ,  216  and  217  shown in  FIG. 14  are not present within specified range  247 , the output signal does not change due to the temperature characteristic or necessary resolution cannot be obtained. 
     Therefore, by providing a means (not shown) for detecting that the maximum value and the minimum value of the output signal are present in specified range  247 , it may be possible to prevent the detection error of the rotation angle from increasing. 
     A signal amplitude detector (not shown) for detecting amplitude centers  248  and  249  of the output signals from first, second and third detection units  213 ,  216  and  217  is used so as to confirm whether or not signals fall within a predetermined range. By making a correction so that amplitude centers  248  and  249  coincide with each other, it is possible to prevent the inconvenience that detection error of the calculated rotation angle is increased. Furthermore, at this time, for example, by executing input at a plurality of times so as to take an average, or by taking an average excluding the maximum and minimum values, an output error can be prevented with a higher accuracy. 
     Furthermore, by storing output signals from first, second and third detection units  213 ,  216  and  217  at a certain position or the rotation angle calculated from these output signals, it is possible to uniquely detect a rotation angle from the certain position. Furthermore, by reading an electrical signal at a plurality of times so as to be checked or by sending a signal by a serial signal, even if a wrong signal enters due to a noise or the like, such entering can be eliminated. The same effect can be obtained even when certain position determination signal line  252  uses the same terminal as that of output signal line  232  by switching input and output. 
     Third Exemplary Embodiment 
     Next, a third exemplary embodiment is described with reference to  FIGS. 15 to 23 . The third exemplary embodiment relates to a highly accurate rotation angle detection device for correcting a mechanical error of a gear or an electrical error in a rotation angle detection unit, and a method for correcting the rotation angle. 
       FIG. 15  is a diagram showing a configuration of a rotation angle detection device in accordance with the third exemplary embodiment of the present invention. Multipole ring magnet  302  that is a target is connected to axis to be detected  301  (hereinafter, also referred to as “axis  301 ”). First rotation angle detection unit  303  is disposed in a position facing multipole ring magnet  302 . Worm gear  304  is connected to axis  301 . With worm gear  304 , wheel gear  305  is engaged. In the center portion of wheel gear  305 , magnet  306  is disposed. In a position facing magnet  306 , second rotation angle detection unit  307  for detecting a rotation angle is disposed. Motor  309  is attached to an end face of axis  301 . Encoder  310  detects a mechanical angle of axis  301  of rotation by motor  309 . 
       FIG. 16  is a circuit block diagram showing a correction system of a rotation angle detection device. EEPROM  311  stores a corrected angle, and the like. CPU  312  is coupled to EEPROM  311  and rotation angle detection units  303  and  307 , and calculates a rotation angle. Furthermore, CPU  312  and motor controller  314  are linked to each other by serial communication line  313  for sending/receiving an angle signal or an instruction signal, so that signals can be sent and received. To axis  301 , motor  309  is attached. The rotation of motor  309  is driven and controlled by motor controller  314  with a high accuracy. The rotation angle of axis  301  is detected by encoder  310  with a high accuracy and the detected rotation angle is sent to motor controller  314 . 
       FIG. 17  shows signals from the first rotation angle detection unit disposed in a position facing the multipole ring magnet. In  FIG. 17 , the abscissa indicates a rotation mechanical angle of axis  301 . The ordinate indicates output signal from first rotation angle detection unit  303 . Sine wave signal  315  and cosine wave signal  316  are output in accordance with the rotation of axis  301 . 
       FIG. 18  shows a correlation characteristic between a rotation electrical angle obtained from one cycle of sine wave signal and cosine wave signal from the first rotation angle detection unit and a rotation mechanical angle of an axis to be detected. In  FIG. 18 , the abscissa indicates a rotation mechanical angle of axis  301  and the ordinate indicates a rotation electrical angle obtained by sine wave signal  315  and cosine wave signal  316  shown in  FIG. 17 . 
       FIG. 19  is a view showing the principle for calculating a multiple rotation mechanical angle of an axis to be detected from a signal of the first rotation angle detection unit and a signal of the second rotation angle detection unit. In  FIG. 19 , the abscissa indicates a rotation mechanical angle of axis  301  in rotation angle detection range  301 R. The ordinate indicates rotation electrical angle θe 303  obtained by first rotation angle detection unit  303  in the upper segment, rotation electrical angle θe 307  obtained by second rotation angle detection unit  307  in the middle segment, and θm 301   a  and θm 301   b  of axis  301  calculated by combining rotation electrical angles calculated from signals from first rotation angle detection unit  303  and second rotation angle detection unit  307 , respectively. θm 301   a  denotes an ideal value and θm 301   b  denotes an actually measured value, respectively. 
     Next, a method for detecting a rotation angle of axis  301  with the above-mentioned configuration is described. 
     In  FIG. 15 , when axis  301  is rotated, multipole ring magnet  302  connected to axis  301  is rotated. From first rotation angle detection unit  303 , it is possible to obtain an output signal corresponding to a rotation angle of multipole ring magnet  302 . In the case of the third exemplary embodiment, since the number of magnetic poles of multipole ring magnet  302  is selected to be 30, a rotation mechanical angle per magnetic pole is 12° (360°/30 poles=12°). 
     With respect to 12° that is the rotation mechanical angle per magnetic pole of multipole ring magnet  302  attached to axis  301 , sine wave signal  315  and cosine wave signal  316  that are signals from first rotation angle detection unit  303  change in one cycle (corresponding to 180° of the rotation electrical angle). In  FIG. 18 , an ideal rotation mechanical angle obtained from rotation electrical angle θe calculated based on the signal of first rotation angle detection unit  303  shown in  FIG. 17  changes linearly as shown in rotation mechanical angle θm. However, due to the effect of magnetization variation or deviation of multipole ring magnet  302 , variation in sensitivity, variation in position, or the like, of first rotation angle detection unit  303 , the rotation mechanical angle obtained from rotation electrical angle θe includes error like rotation mechanical angle θm 1  with respect to an ideal rotation mechanical angle θm. As shown in the upper segment of  FIG. 19 , from the rotation electrical angle θe calculated from first rotation angle detection unit  303 , rotation mechanical angle θm 1  of axis  301  (0° to 12°) can be obtained with a high accuracy and a high resolution. 
     On the other hand, worm gear  304  connected to axis  301  rotates and wheel gear  305  also rotates at a constant reduced ratio. In this case, the reduced ratio is set to ¼. The rotation angle of wheel gear  305  is calculated from second rotation angle detection unit  307  for detecting the magnetic field direction of magnet  306 . As shown in the middle segment of  FIG. 19 , from rotation electrical angle θe 2  obtained from Sin signal and Cos signal of second rotation angle detection unit  307 , it is possible to obtain rotation mechanical angle θm 2  in the range from 0° to 720° that is a detection range of axis  301 . As shown in the lower segment of  FIG. 19 , by determining what cycle the value of rotation mechanical angle θm 1  obtained from rotation angle detection unit  303  belongs from rotation mechanical angle θm 2  obtained by second rotation angle detection unit  307 , rotation mechanical angle θm 3  of axis  301  is obtained. Also in the lower segment of  FIG. 19 , due to the same effect described in  FIG. 18 , the calculated rotation mechanical angle θm 3  includes an error with respect to an ideal rotation mechanical angle θm 4 . 
     Next, a method for improving the detection accuracy (for reducing errors) of axis  301  in the above-mentioned configuration is described. 
       FIG. 20  shows an example of data obtained per rotation mechanical angle corresponding to a magnetic pole pitch of multipole ring magnet included in the calculated rotation mechanical angle of the axis to be detected. In  FIG. 20 , the abscissa indicates rotation mechanical angle θm 1  calculated from rotation electrical angle θe, which has been obtained by inverse transforming a tangent wave signal (=sine wave signal/cosine wave signal) calculated from a sine wave signal and a cosine wave signal that are signals from first rotation angle detection unit  303 . The ordinate indicates rotation mechanical error Δθm 1  that is a difference between rotation mechanical angle θm by which axis  301  is actually rotated and rotation mechanical angle θm 1 . In motor controller  314 , rotation mechanical angle θm of axis  301  detected by encoder  310  and rotation mechanical angle θm 1  of axis  301  calculated by CPU  312  built in rotation angle detection device  308  obtained via serial communication line  313  can be synchronized with each other and stored. In other words, in motor controller  314 , rotation mechanical angle error Δθm 1  can be determined from the following equation (1) with respect to rotation mechanical angle θm 1  of axis  301  calculated by rotation angle detection device  308 . When a rotation mechanical angle error is denoted by Δθm 1 , a calculated rotation mechanical angle of axis  301  is denoted by θm 1 , and a rotation mechanical angle by which axis  301  actually rotates is denoted by θm, Δθm 1  is represented by the following equation (1):
 Δθ m 1 =θm 1 −θm   (1) 
       FIG. 21  shows a method for obtaining a corrected approximate line from the rotation mechanical angle error. In  FIG. 21 , the abscissa indicates calculated rotation mechanical angle θm 1 , and the ordinate indicates rotation mechanical angle error Δθm 1 . A sampled average value is denoted by Δθm 1   a , and a not-sampled average value is denoted by Δθm 1   b , respectively. Approximate line y shows characteristic linking values of sampled rotation mechanical angle errors Δθm 1   a . Data examples of the actually rotation mechanical angle error Δθm 1  are shown. Motor controller  314  sends this rotation mechanical angle error Δθm 1  to CPU  312  by serial communication line  313 . CPU  312  stores this rotation mechanical angle error Δθm 1  with respect to rotation mechanical angle θm 1  in EEPROM  311 . Therefore, CPU  312  can always correct calculated rotation mechanical angle θm 1  of axis  301  by using rotation mechanical angle error Δθm 1  from equation (2). 
     That is to say, equation (2) is obtained by modifying equation (1) as follows:
 
θ m=θm 1 −Δθm 1  (2)
 
     However, in order to store rotation mechanical angle error Δθm 1  over the entire range of the rotation detection, large capacity EEPROM  311  is required. When the rotation detection range is 720° and the resolution is 1°, necessary EEPROM capacity is 720 bytes. 
     If rotation mechanical angle error Δθm 1  obtained every predetermined rotation mechanical angle (every three degrees in the example of  FIG. 21 ) is stored in EEPROM  311 , the capacity can be reduced to 240 bytes (one-third of 720 bytes). Error Δθm 1  of a rotation mechanical angle between predetermined rotation mechanical angles can be estimated from an approximate line obtained from rotation mechanical angle error Δθm 1  every three degrees. 
     Herein, when a rotation mechanical angle in a rotation angle range in three degrees is denoted by x, and the smallest rotation mechanical angle every 3° that is smaller than rotation mechanical angle x is denoted by c. In other words, rotation mechanical angle c&lt;rotation mechanical angle x&lt;rotation mechanical angle (c+3) is satisfied. Furthermore, rotation mechanical angle error m is a rotation mechanical angle error when the rotation mechanical angle is (c+3), and rotation mechanical angle error n is a rotation mechanical angle error when the rotation mechanical angle is c. Based on these values, when approximate line characteristic y of rotation mechanical angle error Δθm 1  is calculated, approximate line characteristic y is represented by equation (3).
 
 y =( m−n )·( x−c )/3 +n   (3)
 
     Motor controller  314  rotates motor  309  and allows encoder  310  to synchronize every three degrees of rotation mechanical angle θm 1  obtained from serial communication line  313  with rotation mechanical angles θm of axis  301  and gains thereof. In  FIG. 21 , when rotation mechanical angle θm 1  is 0°, rotation mechanical angle error Δθm 1 ( n ) is 0.001°, and when rotation mechanical angle θm 1  (c+3) is 3°, rotation mechanical angle error Δθm 1 ( m ) is 0.012°. As an equation for calculating the rotation mechanical angle error every 0.5° when rotation mechanical angle θm 11  is from 0° to 3°, by substituting the above-mentioned values in equation (3), equation (4) is obtained.
 
 y =(0.012−0.001)·( x− 0)/3+0.001=0.0036 ·x+ 0.001  (4)
 
     For example, when rotation mechanical angle θm 1  is 1°, rotation mechanical angle error Δθm 1  is 0.0046° from equation (4). The rotation mechanical angle error Δθm 1  at every one degree when rotation mechanical angle θm 1  is in the range from 3° to 6° is obtained by the same method. The rotation mechanical angle θm 1  of axis  301  calculated with rotation mechanical angle error Δθm 1 , which is obtained in the above, is corrected by equation (1). 
     Furthermore, in order to reduce the capacity of this EEPROM, as shown in  FIG. 20 , average value Δθm 1 Av of rotation mechanical angle error Δθm 1  of each magnetic pole is calculated from equation (5) at every one magnetic pole pitch (in this exemplary embodiment, every 12°) with respect to calculated rotation mechanical angle θm 1 . That is to say, when an average value of the rotation mechanical angle error is denoted by Δθm 1 Av and a sum of rotation mechanical angle error Δθm 1  of 1st to N-th magnetic poles in a certain rotation mechanical angle θm 1  is denoted by ΣΔθm 1 , the average value Δθm 1 Av of the rotation mechanical angle error can be obtained from equation (5).
 
Δθ m 1 Av=ΣΔθm 1 /N   (5)
 
     Rotation mechanical angle error Δθm 2  is obtained from average value Δθm 1 Av from equation (6). That it to say, the average value of the rotation mechanical angle error is denoted by Δθm 1 Av, the rotation mechanical angle error is denoted by Δθm 2 , and the calculated rotation mechanical angle of axis  301  is denoted by θm 1 , equation (6) is represented as follows:
 
Δθ m 2 =θm 1 −θm−Δθm 1 Av   (6)
 
       FIG. 22  shows an example of the rotation mechanical angle error data corrected by the average value of the rotation mechanical angle error of magnetic poles.  FIG. 22  plots rotation mechanical angle errors Δθm 2  calculated by substituting data of  FIG. 20  into equation (6). Since the correlation is observed in the generation trend of rotation mechanical angle error Δθm 1  of each magnetic pole shown in  FIG. 20 , with respect to the variation of rotation mechanical angle error Δθm 1  of ±0.2°, the variation of rotation mechanical angle error Δθm 2  shown in  FIG. 22  is reduced to ±0.1° or less. 
     Since in motor controller  314 , rotation mechanical angle θm of axis  301  detected by encoder  310  and rotation mechanical angle θm 1  of axis  301  obtained via serial communication line  313  and calculated by CPU  312  built in rotation angle detection device  308  are synchronized with each other and stored, average value Δθm 1 Av of rotation mechanical angle error Δθm 1  of each magnetic pole, which is calculated from equation (1), can be calculated from equation (5). Average value Δθm 1 Av of the rotation mechanical angle errors is sent to CPU  312  by serial communication line  313 , and stored in EEPROM  311  by CPU  312 . CPU  312  can always correct calculated rotation mechanical angle θm 1  of axis  301  by equation (7) by using average value Δθm 1 Av of rotation mechanical angle error although rotation mechanical angle error Δθm 2  is included. That is to say, when a rotation mechanical angle actually rotated by axis  301  is denoted by θm, a rotation mechanical angle error is denoted by Δθm 2 , and a rotation mechanical angle of calculated axis  301  is denoted by θm 1 , equation (7) is expressed as follows.
 
θ m+Δθm 2 =θm 1 −Δθm 1 Av   (7)
 
       FIG. 23  is an enlarged view showing a part of  FIG. 22 .  FIG. 23  shows a method for obtaining a corrected approximate line from average values of rotation mechanical angle error of each magnetic pole. A method for reducing the capacity of EEPROM  311  is described with reference to  FIG. 23 . If average value Δθm 1 Av of the rotation mechanical angle error in each magnetic pole as plotted in  FIG. 23  is stored in EEPROM  311  at 0.5° intervals of the rotation mechanical angle θm 1  in the range from 0° to 12°, the capacity of 24 is required. When average value Δθm 1 Av of the rotation mechanical angle error of each magnetic pole, which is calculated every predetermined rotation mechanical angle (every two degrees, in the example of  FIG. 23 ), is stored in EEPROM  311 , the capacity can be reduced to 6 (i.e. 12/2). Average value Δθm 1 Av between predetermined rotation mechanical angles can be estimated by an approximate line obtained by average value Δθm 1 Av every two degrees. 
     A certain rotation mechanical angle between two degrees is denoted by x, and the smallest rotation mechanical angle every two degrees that is smaller than rotation mechanical angle x is denoted by c 1 . In other words, rotation mechanical angle c 1 &lt;rotation mechanical angle x&lt;rotation mechanical angle (c 1 +2) is satisfied. Furthermore, average value Δθm 1 Av in rotation mechanical angle (c 1 +2) is m 1  and average value Δθm 1 Av in rotation mechanical angle c 1  is n 1 . Approximate line y 1  of average value Δθm 1 Av based on these values is represented by equation (8).
 
 y 1=( m 1− n 1)·( x−c 1)/2 +n 1  (8)
 
     Motor controller  314  rotates motor  309 , gains rotation mechanical angle θm 1  every two degrees from serial communication line  313 , and gains rotation mechanical angle θm of axis  301  from encoder  310 . From rotation mechanical angle θm, rotation mechanical angle θm 1  and equation (1), rotation mechanical angle error Δθm 1  is obtained. Furthermore, by using equation (5), average value Δθm 1 Av of the rotation mechanical angle error is obtained. 
     In  FIG. 23 , when the value of rotation mechanical angle θm 1  (c 1 ) is 0°, average value Δθm 1 Av of the rotation mechanical angle error is 0.031°. When the value of rotation mechanical angle θm 1  (c 1 +2) is 2°, average value Δθm 1 Av (m 1 ) of the rotation mechanical angle error is 0.042°. 
     As the equation for obtaining average value Δθm 1 Av every 0.5° in the rotation mechanical angle θm 1  in the range from 0° to 2°, by substituting the above-mentioned values into equation (8), equation (9) is obtained. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     Herein, for example, when the value of rotation mechanical angle θm 1  is 1°, average value Δθm 1 Av is 0.0365° from the equation (9). Values of average value Δθm 1 Av every 0.5° in the rotation mechanical angle θm 1  in the range from 2° to 4° are also obtained by the same method. With the thus obtained average value Δθm 1 Av of the rotation mechanical angle error, rotation mechanical angle θm 1  of axis  301  is calculated, and rotation mechanical angle θm 1  is corrected by equation (7). When average value Δθm 1 Av of the rotation mechanical angle error shown in  FIG. 20  is estimated by an approximate line of equation (8) and rotation mechanical angle error Δθm 2 Av is calculated from equation (6), substantially the same result as rotation mechanical angle error Δθm 2  shown in  FIG. 22  is obtained. 
     Fourth Exemplary Embodiment 
     Hereinafter, a fourth exemplary embodiment of the present invention is described with reference to  FIG. 24 . 
       FIG. 24  is a perspective view showing a target in accordance with the fourth exemplary embodiment. On the outer circumferential surface of target  327 , convex portions made of a magnetic substance are disposed at an identical interval. The rotation angle detection device including target  327  has the same signal shape as that of the first rotation angle detection unit in accordance with the third exemplary embodiment. With this signal, a rotation angle can be calculated. Since the rotation angle detection device using the target in accordance with the fourth exemplary embodiment has the same configuration and operation as those in the rotation angle detection device in accordance with the above-mentioned third exemplary embodiment, the description thereof is omitted herein. 
     Fifth Exemplary Embodiment 
     Hereinafter, a fifth exemplary embodiment of the present invention is described with reference to  FIG. 25 . 
       FIG. 25  is a perspective view showing a target in accordance with the fifth exemplary embodiment. Target  328  has a cylindrical portion and concave portions  328   a  and not-concave portions  328   b  are disposed in identical intervals on the outer circumferential surface of the cylindrical portion. This rotation angle detection device including target  328  has the same signal shape as that of the first rotation angle detection unit in accordance with the third exemplary embodiment. With this signal, a rotation angle can be calculated. Since the rotation angle detection device using the target in accordance with the fifth exemplary embodiment has the same configuration and operation as those in the rotation angle detection device in accordance with the above-mentioned third exemplary embodiment, the description thereof is omitted herein. 
     As mentioned above, the rotation angle detection device in accordance with the fifth exemplary embodiment has an advantage that in the detection range or in the rotation range corresponding to each magnetic pole width of a multipole ring magnet, by a method for storing the corrected angle with respect to the calculated rotation angle of axis  301  in EEPROM  311 , or by a method for storing the corrected angle in EEPROM  311 , reduction of the accuracy in rotation angle detection due to a magnetic error, a mechanical error, and an electrical error of the multipole ring magnet or rotation angle detection unit is corrected by using an EEPROM with less capacity, and thereby the accuracy of the detected rotation angle of the axis to be detected can be improved. 
     INDUSTRIAL APPLICABILITY 
     An absolute rotation angle detection device of the present invention can detect an absolute rotation angle with a simple configuration and with a high accuracy and a high resolution. Therefore, the absolute rotation angle detection device is useful for application in an absolute rotation angle used in vehicle power steering, and the like. 
     Furthermore, a rotation angle detection device of the present invention is used in, for example, a vehicle power steering and can detect a multiple rotation angle with a simple configuration and with a high accuracy and a high resolution. 
     In addition, a rotation angle correction method of the rotation angle detection device in accordance with the present invention has an advantage that multiple rotation of an axis to be detected can be detected with a simple configuration using an EEPROM with less capacity with a high accuracy. The method is suitable to be used as a rotation angle correction method of a rotation angle detection device used in, for example, a vehicle power steering. Therefore, industrial applicability thereof is high.