Abstract:
A rotation angle detection device capable of easy selection of detection gears is provided to detect a rotation angle of a main gear. The device includes first and second detection gears each rotatable in conjunction with the main gear, a first magnetic sensor for detecting a rotation angle of the first detection gear, a second magnetic sensor for detecting a rotation angle of the second detection gear, and a microcomputer for detecting the rotation angle of the main gear on the basis of the rotation angle detected by the first magnetic sensor and the rotation angle detected by the second magnetic sensor. The periodicity from the first magnetic sensor and the periodicity of the second magnetic sensor are set to be prime to each other. Further, the teeth number of the first magnetic sensor is set to be a numeric value obtained by multiplying the periodicity of the first magnetic sensor by a predetermined integral number. As well, the teeth number of the second magnetic sensor is set to be a numeric value obtained by multiplying the periodicity of the second magnetic sensor by a predetermined integral number.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a rotation angle detection device equipped with a rotating main gear, for detecting a rotation angle of the main gear. 
   As a prior art, there is a known technology that allows a steering angle of a steering wheel to be detected uniquely even if the steering angle of the steering wheel exceeds 360 degrees (see Japanese Patent Application Laid-open No. 2002-531858). In detail, the above technology enables the steering angles of 370 degrees and 10 degrees to be distinguished from each other. 
   In this technology, a rotation angle detection device disclosed therein includes a main gear, first and second detection gears, first and second detection magnets, first and second magnetic sensors and a microcomputer. 
   In the rotation angle detection device, the main gear rotates integrally with a steering shaft of a vehicle, while the first and second detection gears rotate in conjunction with the main gear. 
   The first detection magnet, which is a magnet magnetized with two poles, rotates together with the first detection gear. Similarly, the second detection magnet also magnetized with two poles rotates together with the second detection gear. 
   The first magnetic sensor detects the directions of magnetic lines of the first magnet fixed on the first detection gear, that is, the rotation angle of the first detection gear and further outputs a detection signal in accordance with the detection result (i.e. detection angle). Similarly, the second magnetic sensor detects the directions of magnetic lines of the second magnet fixed on the second detection gear, that is, the rotation angle of the second detection gear and further outputs a detection signal in accordance with the detection result (i.e. detection angle). 
   The microcomputer calculates the rotation angle of the main gear based on the detection signals outputted from the first and second magnetic sensors. 
   Here noted that the number of teeth (or “teeth number”) m 1  of the first detection gear and the number of teeth (teeth number) m 2  of the second detection gear are respectively established so as to meet a condition 1 or a condition 2 as follows:
 
 m 2= m 1+1  (condition 1)
 
| m 1− m 2|&gt;1 ( m 1 and  m 2 are prime to each other)  (condition 2)
 
   When the numbers of teeth m 1 , m 2  meet the above condition 1 or the condition 2, the combinations of detection signals from the first magnetic sensor with detection signals from the second magnetic sensor differ from each other so long as the rotation angles of the main gear corresponding to the combinations differ from each other, within the rotating range of the main gear, in other words, the steering range of a steering wheel. Therefore, if only given the detection signals from the first and second magnetic sensors, then it becomes possible to determine the rotation angle of the main gear uniquely. 
   That is, according to the above technology, even if the steering angle of the steering wheel exceeds 360 degrees in such a system that detects the rotation angle of the main gear on the ground of the rotation angles detected by the first magnetic sensor and the second magnetic sensor, it is possible to detect the rotating angle of the main gear, namely, the steering angle of the steering wheel uniquely. 
   However, the following problem resides in the above-mentioned technology. That is, as the detection gears used in the technology are respectively limited to ones meeting the above condition 1 or the above condition 2, the manufacturing of a device according to the technology requires the selection of gears that can be accommodated in such a device and further meet the condition 1 or the condition 2. 
   Thus, if there is a limitation in size of the device, it is complex to select appropriate gears. 
   SUMMARY OF THE INVENTION 
   Under the circumstances, it is therefore an object of the present invention to provide a rotation angle detection device that allows detection gears to be easily selected. 
   According to the first aspect of the present invention, the above object of the present invention described above can be accomplished by a rotation angle detection device for detecting a rotation angle of a main gear, comprising: first and second detection gears rotatable in conjunction with the main gear, respectively; first detecting unit configured to detect a rotation angle of the first detection gear and obtain a first periodicity that is the number of revolutions of the first detection gear rotating during the main gear&#39;s rotation from one limit value up to the other limit value within a rotatable range of the main gear; second detecting unit configure to detect a rotation angle of the second detection gear and obtain a second periodicity of the second detecting unit that is the number of revolutions of the second detection gear rotating during the main gear&#39;s rotation from one limit value up to the other limit value within the rotatable range of the main gear; and third detecting unit configured to detect the rotation angle of the main gear on a basis of the rotation angle detected by the first detecting unit and the rotation angle detected by the second detecting unit, wherein the first periodicity from the first detecting unit and the second periodicity from the second detecting unit are set to be prime to each other, wherein a teeth number of the first detection gear is set to be a numeric value obtained by multiplying the second periodicity from the second detecting unit by a predetermined integral number, and wherein a teeth number of the second detection gear is set to be a numeric value obtained by multiplying the first periodicity from the first detecting unit by the predetermined integral number. 
   In the above-constructed rotation angle detection device, since the first and second periodicities of the first and second detecting unit are set to be prime to each other, the third detecting unit can determine the rotation angle of the main gear uniquely. Further, regarding the teeth numbers of the first and second detection gears, the teeth numbers of the first and second detection gears can have any numeric values as long as meeting the condition described above. 
   In a preferred embodiment of the second aspect of the present invention, the teeth number of the first detection gear and the teeth number of the second detection gear may have a common divisor which is different from a divisor of the teeth number of the main gear. 
   Further, the main gear may be configured so as to rotate integrally with a steering shaft of a vehicle. 
   Further, the main gear may be configured so as to rotate in conjunction with a steering shaft of a vehicle. 
   Further, in another embodiment, the rotation angle detection device further comprises a first magnet arranged at the center of the first detection gear and magnetized with two poles and a second magnet arranged at the center of the second detection gear and magnetized with two poles, wherein the first detecting unit has a first magnetic sensor arranged in the vicinity of the first magnet to detect the rotation angle of the first detection gear within a range from 0 to 360 degrees in cooperation with the first magnet, and the second detecting unit has a second magnetic sensor arranged in the vicinity of the second magnet to detect the rotation angle of the second detection gear within a range from 0 to 360 degrees in cooperation with the second magnet. 
   Further, the third detecting unit may have a microcomputer configured to input respective signals from the first magnetic sensor and the second magnetic sensor. 
   These and other objects and features of the present invention will become more fully apparent from the following description and appended claims taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic structural view of a rotation angle detection device in accordance with one embodiment of the present invention; 
       FIG. 2  is a block diagram showing a control system of the rotation angle detection device; 
       FIG. 3  is a characteristic diagram showing a relationship between the rotation angles of a main gear and the values of detection signals from a magnetic sensor; 
       FIG. 4  is a characteristic diagram showing a part of  FIG. 3  in enlargement; 
       FIG. 5  is a characteristic diagram showing a relationship between the rotation angles of a main gear and the values of detection signals from a magnetic sensor; and 
       FIG. 6  is a characteristic diagram showing a part of  FIG. 5  in enlargement. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Embodiments of the present invention will be described with reference to the drawings. 
   First, referring to  FIGS. 1 and 2 , the structure and function of constituents of a rotation angle detection device  1  in accordance with the first embodiment will be described. Noted that  FIG. 1  is a schematic structural view of the rotation angle detection device  1  and  FIG. 2  is a block diagram showing a control system of the rotation angle detection device  1 . 
   As shown in  FIG. 1 , the rotation angle detection device  1  is accommodated in a casing  10  and includes a main gear  2 , a detection gear  3  (as the first detection gear of the invention), another detection gear  4  (as the second detection gear), two magnets  31  and  41 , a magnetic sensor  32  (as the first detecting unit), another magnetic sensor  42  (as the second detecting unit) and a microcomputer (as the third detecting unit)  5 . 
   The main gear  2  rotates integrally with a steering shaft of a vehicle (not shown). Accordingly, the steering angle of the steering shaft (or a steering wheel) coincides with the rotation angle of the main gear  2 . Noted that the main gear  2  may be an element rotatable in conjunction with the steering shaft. 
   For instance, the rotatable range of the main gear  2  is from 0 to 1,440 degrees. Of course, the rotatable range of the main gear  2  may be different from the above range. 
   The detection gears  3 ,  4  rotate in conjunction with the main gear  2 . In this embodiment, the number of teeth of the detection gear  4  is established larger than that of the detection gear  3 . Therefore, the detection gear  4  rotates at a rotating speed smaller than that of the detection gear  3 . 
   The magnet  31  (as the first magnet of the invention) is a magnet magnetized with two poles and is arranged at the rotational center of the detection gear  3 . The magnet  31  rotates together with the detection gear  3 . 
   The magnetic sensor  32  is arranged in the vicinity of the magnet  31  to detect the directions of magnetic lines of the magnet  31 , in other words, the rotation angle of the detection gear  3  within a range from 0 to 360 degrees. Additionally, the magnetic sensor  32  produces a detection signal of n-bit (e.g. 6 to 10 bit) corresponding to the detected rotation angle and outputs the detection signal to the microcomputer  5 . The magnetic sensor  32  is constructed so as to output a detection signal having a larger value as the detected rotation angle gets larger. 
   Similarly, the magnetic sensor  42  (as the second magnet) is arranged in the vicinity of the magnet  41  to detect the directions of magnetic lines of the magnet  41 , in other words, the rotation angle of the detection gear  4  within a range from 0 to 360 degrees. Additionally, the magnetic sensor  42  produces a detection signal of n-bit (e.g. 6 to 10 bit) corresponding to the detected rotation angle and outputs the detection signal to the microcomputer  5 . The magnetic sensor  42  is constructed so as to output a detection signal having a larger value as the detected rotation angle gets larger. 
   In order to allow the microcomputer  5  to determine the rotation angle of the main gear  2  uniquely even if it exceeds 360 degrees, it is established that the cycle c 1  of the magnetic sensor  32  and the cycle c 2  of the magnetic sensor  42  meet the following expression (1). Noted that the cycle c 1  of the magnetic sensor  32  indicates a rotation angle of the main gear  2  rotating during one revolution of the detection gear  3 , while the cycle c 2  of the magnetic sensor  42  indicates a rotation angle of the main gear  2  rotating during one revolution of the detection gear  4 .
 
A least common multiple of the cycles c1 and c2≧rotatable range of the main gear 2  (1)
 
   Therefore, according to the embodiment, the cycles c 1  and c 2  are respectively established so that the least common multiple is more than 1,440 degrees. 
   As shown in  FIG. 2 , the microcomputer  5  includes a control unit  51 . Based on the detection signals from the magnetic sensors  32  and  42 , the control unit  51  does determine or detect the rotation angle of the main gear  2  uniquely. In succession, the control unit  51  produces an angular signal corresponding to the so-determined rotation angle and further outputs the angular signal to a not-shown instrument requiring the data of the rotation angle of the main gear  2  (i.e. steering shaft). 
   Referring to  FIGS. 3 and 4 , we now describe the reason why the above establishment of the cycles c 1  and c 2  in accordance with the expression (1) allows the rotation angle of the main gear  2  to be determined uniquely.  FIG. 3  is a characteristic view showing the relationship among the rotation angle of the main gear  2  (i.e. steering angle of the steeling wheel), the values of the detection signals outputted from the magnetic sensors  32 ,  42  under condition that the cycles c 1  and c 2  meet the expression (1).  FIG. 4  is a characteristic view showing a part of  FIG. 3  in enlargement. Noted that the number n of bits is equal to ten in  FIGS. 3 and 4  (i.e. n=10). 
   A kinked solid line  20  of  FIG. 3  designates the relationship between the steering angle (horizontal axis) and the rotation angle (vertical axis) of the main gear  2  within the range from 0 to 360 degrees. On the other hand, a dotted line  30  designates the relationship between the rotation angle (horizontal axis) of the main gear  2  and the value (vertical axis) of an output signal from the magnetic sensor  32 . Similarly, a broken line  40  designates the relationship between the rotation angle (horizontal axis) of the main gear  2  and the value (vertical axis) of an output signal from the magnetic sensor  42 . 
   As shown in  FIG. 3 , the cycle c 1  is set to approximately 100 degrees, while the cycle c 2  is set to approximately 128 degrees. Therefore, the least common multiple of the cycles c 1  and c 2  is approximately 3,200 degrees. Then, the values of cycles c 1  and c 2  meet the expression (1). 
   In the rotating range (from 0 to 1,440 degrees) of the main gear  2 , as shown in  FIG. 3 , the combinations of the detection signals from the magnetic sensor  32  with the detection signals from the magnetic sensor  42  differ from each other if the rotation angles of the main gear  2  corresponding to the combinations differs from each other, despite that the rotation angles of the main gear  2  are equal to each other within the range from 0 to 360 degrees. Therefore, if the detection signals from the magnetic sensors  32  and  42  are obtained respectively, it is possible for the control unit  51  to determine the rotation angle of the main gear  2  uniquely. 
   For instance, as shown in  FIGS. 3 and 4 , when two rotation angles of the main gear  2  are 180 degrees and 540 degrees respectively (each corresponding to 180 degrees in the range from 180 to 360 degrees), the values of the detection signals from the magnetic sensor  32  respectively exhibit different values a 1 , a 2 , while the values of the detection signals from the magnetic sensor  42  respectively exhibit different values b 1 , b 2 . Therefore, when the rotating angle of the main gear  2  is 180 degrees, there is established a combination consisting of the values a 1  and b 1 . While, when the rotating angle of the main gear  2  is 540 degrees, there is established another combination consisting of the values a 2  and b 2 . In this way, two combinations are different from each other. 
   In other words, if the combination of two detection signals from the magnetic sensors  32  and  42  is formed by the values (a 1 , b 1 ), the control unit  51  can determine that the rotation angle of the main gear  2  is equal to 180 degrees. Similarly, if the combination of two detection signals from the magnetic sensors  32  and  42  is formed by the values (a 2 , b 2 ), the control unit  51  can determine that the rotation angle of the main gear  2  is equal to 540 degrees. 
   In this case, the number of teeth z 1  of the detection gear  3  and the number of teeth z 2  of the detection gear  4  are respectively established so as to meet the following expressions (2) and (3).
 
 z 1= f×x 2  (2)
 
 z 2= f×x 1  (3)
         Note; x 2 : first periodicity of magnetic sensor  42     x 1 : second periodicity of magnetic sensor  32     f: an integral number more than 1       

   Here, we explain the reason why the teeth numbers z 1  and z 2  can be established in the above way. If the cycle c 1  of the magnetic sensor  32  and the cycle c 2  of the magnetic sensor  42  meet the expression (1), the first periodicity x 1  of the magnetic sensor  32  and the second periodicity x 2  of the magnetic sensor  42  are prime to each other. Here noted that the first periodicity x 1  is the number of revolutions of the detection gear  3  during the rotation of the main gear  2  from one limit value within its rotatable range up to the other limit value. Similarly, the second periodicity x 2  is the number of revolutions of the detection gear  4  during the rotation of the main gear  2  from one limit value within its rotatable range up to the other limit value. 
   Additionally, the following expressions (4), (5) and (6) come into existence.
 
(rotatable range of main gear 2)= c 1× x 1= c 2× x 2  (4)
 
 c 1=360 ×z 1/ z 0  (5)
 
 c 1=360 ×z 2/ z 0  (6)
         Note; z 0 : number of main gear  2         

   When substituting the expressions (5) and (6) into the expression (4), there is obtained the following expression (7).
 
 z 1× x 1= z 2× x 2  (7)
 
   Additionally, the following expressions (8) and (9) are obtained by the above expression (7).
 
 z 1= z 2× x 2/ x 1  (8)
 
 z 2= z 1× x 1/ x 2  (9)
 
   By the following facts: the first and second periodicities x 1  and x 2  are prime to each other; the teeth numbers z 1  and z 2  are integral numbers more than 1; and the expressions (8) and (9), the following expressions (10) and (11) are obtained. Here, both alphabets “d” and “e” are integral numbers more than 1.
 
 z 1= d×x 2  (10)
 
 z 2= e×x 1  (11)
 
   When substituting the expressions (10) and (11) into the expression (7), there is obtained the following expression (12). Here, alphabet “f” is an integral number more than 1.
 
d=e=f  (12)
 
   Then, when substituting the expression (12) into the expressions (10) and (11), the above-mentioned expressions (2) and (3) are obtained. 
   Therefore, if only establishing the teeth numbers z 1  and z 2  so as to meet the expressions (2) and (3), the control unit  51  can determine the rotation angle of the main gear  2  uniquely since the cycles c 1  and c 2  meet the expression (1) immediately. In the example shown in  FIG. 3 , the teeth numbers z 0 , z 1  and z 2  are  90 ,  25  and  32 , respectively while meeting the expressions (2) and (3). 
   As for the cycles c 1  and c 2  meeting the expression (1), the cycles shown in  FIGS. 5 and 6  will be quoted besides those of  FIGS. 3 and 4 . Here,  FIGS. 5 and 6  are characteristic views similar to  FIGS. 3 and 4 . 
   In an example of  FIG. 5 , the cycle c 1  is set to approximately 90.7 degrees, while the cycle c 2  is set to approximately 128 degrees. Therefore, the least common multiple of the cycles c 1  and c 2  is approximately 2,176 degrees. Then, the values of cycles c 1  and c 2  meet the expression (1). In this example, the teeth numbers z 0 , z 1  and z 2  are  135 ,  34  and  48 , respectively while meeting the expressions (2) and (3). 
   In this way, according to the embodiment, since the first periodicity x 1  of the magnetic sensor  32  and the second periodicity x 2  of the magnetic sensor  42  are prime to each other, the control unit  51  can determine the rotation angle of the main gear  2  uniquely. 
   As for the teeth number z 1  of the detection gear  3  and the teeth number z 2  of the detection gear  4 , any numbers will do so long as they meet the above expressions (2) and (3). Thus, since the range of choice in the teeth numbers of the detection gears  3  and  4  is broadened in comparison with the conventional art, it is possible to select the detection gears  3  and  4  easily in even a situation where there exists a limitation in the size of the rotation angle detection device  1 . As the combinations of the teeth numbers that are not included in the prior art, it is noted that there are the following combinations (z 0 , z 1 , z 2 ) of: ( 135 ,  38 ,  48 ); ( 135 ,  24 ,  62 ); ( 135 ,  24 ,  58 ); ( 135 ,  34 ,  48 ); ( 135 ,  48 ,  26 ); and ( 135 ,  50 ,  24 ). 
   Further, regarding the combinations of the main gear  2  and the detection gears  3  and  4 , when there exists a common divisor between the teeth number z 1  of the detection gear  3  and the teeth number z 2  of the detection gear  4 , there can be selected the above-mentioned combinations where the above common divisor is different from divisors of the teeth number z 0  of the main gear  2 . 
   Again, as the main gear  2  rotates integrally with the steering shaft of the vehicle, it is possible to determine the steering angle of the steering wheel uniquely. Additionally, even if the size of the rotation angle detection device  1  is limited in view of mounting the device  1  on the vehicle, it is possible to easily select the detection gears  3 ,  4 . 
   Although the above embodiment is described on the assumption that the main gear  2  rotates integrally with the steering shaft of the vehicle, the main gear  2  may be constructed to rotate in conjunction with the steering shaft of the vehicle in the modification. Alternatively, the main gear  2  may be constructed to rotate in conjunction with the other rotating element (not shown). 
   Finally, it will be understood by those skilled in the art that the foregoing descriptions are nothing but one embodiment of the disclosed rotation angle detection device and therefore, various changes and modifications may be made within the scope of claims.