Abstract:
A rotation angle sensor wherein when code plates rotate as a result of rotation of rotary members, detecting elements detect information on information recording portions of the code plates in order to detect the rotation angles of the rotary members. Therefore, the rotation angles of the rotary members can be detected with high precision. In conventional rotation angle sensors, a rotary drum, being a magnetic medium, is mounted directly to a rotary shaft. Therefore, when two such conventional rotation angle sensors are mounted to a rotary shaft, and a rotational torque is applied to a drive shaft portion in order to rotate a load shaft portion, so that a large load is applied to the load shaft portion, the drive shaft portion and the load shaft portion may become decentered. This causes the gap between one of the two rotary drums and its associated detecting sensor as well as the gap between the other of the two rotary drums and its associated detecting sensor to vary, making it impossible to precisely detect the difference between the rotation angles of the two rotary drums. The rotation angle sensor of the invention overcomes this problem.

Description:
This application is a continuation of application Ser. No. 09/227,054 filed Jan. 7, 1999, abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a rotation angle sensor, a torque sensor incorporating the rotation angle sensor, and an electrically driven power steering apparatus using the torque sensor, and, more particularly, to a rotation angle sensor which can detect rotational angles with high precision, and apparatuses to which the rotation angle sensor is applied. 
     2. Description of the Related Art 
     FIG. 12 illustrates a conventional rotation angle sensor, wherein a rotary drum  2 , having disk-shaped magnetic portions, is affixed to a rotary shaft  1 . A magnetic code, consisting of a plurality of magnetic north-south (N-S) poles, is formed along the entire outer periphery of the rotary drum  2 . 
     A magnetic detecting sensor  4  is provided so as to be spaced from the outer periphery of the magnetic drum  2  by a predetermined gap P that is disposed therebetween, whereby the conventional rotation angle sensor is formed. 
     Such a conventional rotation angle sensor is constructed such that when the magnetic drum  2  is rotated as a result of rotation of the rotary shaft  1 , the magnetic sensor  4  detects analog changes, that is changes in the magnetic forces of the magnetic poles, in order to detect the angle of rotation of the magnetic drum  2 . 
     A description will now be given of a conventional torque sensor, wherein two such rotation angle sensors described above are mounted to a rotary shaft  1  having a drive shaft portion la and a load shaft portion  1   b  illustrated in FIG.  13 . 
     The drive shaft portion  1   a  and the load shaft portion  1   b  of the rotary shaft are connected together by a resilient member (not shown), being a torsion bar. 
     The two rotary drums  2  and  3  are affixed to the drive shaft portion  1   a  and the load shaft portion  1   b , respectively. They are connected towards an end of the drive shaft portion  1   a  and an end of the load shaft portion  1   b  that is connected to the drive shaft portion  1   a , and are separated by a distance L. 
     A pair of magnetic sensors  4  and  5  are provided such that the magnetic sensor  4  is separated from the outer periphery of the rotary drum  2  by a predetermined gap P 1  and the magnetic sensor  5  is separated from the outer periphery of the rotary drum  3  by a predetermined gap P 2 . 
     In such an operating shaft  1 , by applying a torque to the drive shaft portion  1   a  that is greater than the torque applied to the load shaft portion  1   b , the drive shaft portion  1   a  and the load shaft portion  1   b  can be rotated. 
     When the rotary shaft  1  is rotated, the load shaft portion  1   b  starts to rotate slightly later than the drive shaft portion  1   a , due to the resilient member. 
     A slightly delayed rotation of the load shaft portion  1   b  results in a difference between the rotational angle of the drive shaft portion  1   a  and that of the load shaft portion  1   b . The difference in the rotational angles is proportional to the rotational torque on the drive shaft portion  1   a , so that when the difference in the rotational angles is large, the rotational torque on the drive shaft portion  1   a  is large, whereas when the difference in the rotational angles is small, the rotational torque on the drive shaft portion  1   a  is small. 
     Such a conventional torque sensor can detect the rotational torque on the drive shaft portion  1   a  by computing the difference between the rotational angles of the drive shaft portion  1   a  and the load shaft portion  1   b  through an integrated circuit (IC), which is not shown. 
     However, in such a conventional rotation angle sensor and torque sensor, rotational drums, being magnetic media, are directly mounted to the rotary shaft  1 , so that when a large load torque is applied to the load shaft portion  1   b , the drive shaft portion  1   a  and the load shaft portion  1   b  may become decentered. When decentering occurs, the amount of gap P 1  between the rotary drum  2  and the magnetic detecting sensor  4  and the amount of gap P 2  between the rotary drum  3  and the magnetic detecting sensor  5  change, so that the magnetic detecting sensors  4  and  5  cannot detect the strength of the magnetic field between the two rotary drums  2  and  3  with precision. This makes it difficult to make precise detections of the difference between the rotational angles. 
     Therefore, when such a conventional torque sensor is used in an electrically driven power steering apparatus of, for example, an automobile, the shafts  1   a  and  1   b  must be formed precisely and made highly durable, in order for the power steering apparatus to provide highly reliable power steering properties. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to make it possible to overcome the above-described problems in order to provide a high-precision angle sensor. 
     To this end, according to a first aspect of the present invention, there is provided a rotation angle sensor comprising a rotary member having a gear portion at the outer peripheral portion thereof, the rotary member having a shaft-inserting hole at the center of rotation thereof; a code plate which engages the gear portion of the rotary member, the code plate having an information recording portion which rotates in response to the rotation of the rotary member; and a detecting element for detecting information written on the information recording portion; wherein when the code plate rotates as a result of rotation of the rotary member, the detecting element detects the information on the code plate in order to detect the rotation angle of the rotary member. 
     Although not exclusive, in a preferred form of the invention, the rotary member may comprise a first rotary member portion and a second rotary member portion, the first rotary member portion and the second rotary member portion being separately rotatable and having the same center of rotation; the code plate may comprise a first code plate portion and a second code plate portion, each having a gear portion which engages the rotary member and being separately rotatable; and the detecting element may comprise a first detecting element portion and a second element portion for detecting information on the first code plate portion and the second code plate portion, respectively. In this structure, when the first rotary member portion and the second rotary member portion are rotated by engaging the gear portion of the first code plate portion with a gear portion of the first rotary member portion, and by engaging the gear portion of the second code plate portion with a gear portion of the second rotary member portion, the information on the first code plate portion is detected by the first detecting element portion, and the information on the second code plate portion is detected by the second detecting element portion, whereby the rotation angle of the first rotary member portion and the rotation angle of the second rotary member portion are separately detected. 
     Although not exclusive, in a preferred form of the invention, the first code plate portion and the second code plate portion may have the same center of rotation, with one side of the gear portion of the first code plate portion and one side of the gear portion of the second code plate portion being disposed such that they face each other, and the other side of the gear portion of the first code plate portion having formed thereat the information portion associated thereto and the other side of the gear portion of the second code plate portion having formed thereat the information recording portion associated thereto, the outside diameter of each information recording portion being larger than the outside diameter of the gear portion associated thereto. In addition, the first rotary member portion and the second rotary member portion may be rotatably interposed between the information recording portions. 
     Although not exclusive, in a preferred form of the invention, the rotary member, the code plate, and the detecting element may be accommodated in a box-shaped housing, and the code plate may be disposed between the detecting element and the rotary member. 
     Although not exclusive, in a preferred form of the invention, the information recording portion of the code plate may be composed of a magnetic material with a plurality of magnetic poles, and the detecting element may comprise a magnetic sensor which reacts with the magnetic field of the magnetic material. 
     According to a second aspect of the present invention, there is provided a torque sensor comprising a rotation angle sensor including a first rotary member and a second rotary member being separately rotatable and having the same center of rotation, each having a gear portion at the outer peripheral portion thereof and a shaft-inserting hole at the center of rotation thereof. In addition, the rotation angle sensor includes a first code plate and a second code plate being separately rotatable, the first code plate having a gear portion which engages the first rotary member and the second code plate having a gear portion which engages the second rotary member, the first code plate having an information recording portion which rotates in response to the rotation of the first rotary member and the second code plate having an information recording portion which rotates in response to the rotation of the second rotary member. Further, the rotation angle sensor includes a first detecting element for detecting information written on the first information recording portion, and a second detecting element for detecting information written on the second information recording portion. In the rotation angle sensor, when the first rotary member and the second rotary member rotate to rotate the first code plate and the second code plate, respectively, the first detecting element detects the information on the first code plate and the second detecting element detects the information on the second code plate, whereby the rotation angle of the first rotary member and the rotation angle of the second rotary member are detected. The torque sensor further comprises a first operating shaft and a second operating shaft, an end of the first operating shaft and an end of the second operating shaft being abutted against each other and connected by a resilient member, being a torsion bar. In the torque sensor, the first rotary member is supported by the end of the first operating shaft, and the second rotary member is supported by the end of the second operating shaft, in order to detect the rotation angle of the first operating shaft by the first detecting element and the rotation angle of the second operating shaft by the second detecting element, whereby the rotational torque on the first operating shaft is detected from the difference between the rotation angle of the first operating shaft and the rotation angle of the second operating shaft. 
     Although not exclusive, in a preferred form of the invention, a spring member may be provided at the inner peripheral surface of the edge of the shaft-inserting hole of the first rotary member and at the inner peripheral surface of the edge of the shaft-inserting hole of the second rotary member, the spring members resiliently pressing against the first and the second operating shafts in order to support the first rotary member by the first operating shaft and the second rotary member by the second operating shaft. 
     According to a third aspect of the present invention, there is provided an electrically driven power steering apparatus comprising a rotary angle sensor including a first rotary member and a second rotary member being separately rotatable and having the same center of rotation, each having a gear portion at the outer peripheral portion thereof and a shaft-inserting hole at the center of rotation thereof. In addition, the rotary angle sensor includes a first code plate and a second code plate being separately rotatable, the first code plate having a gear portion which engages the first rotary member and the second code plate having a gear portion which engages the second rotary member, the first code plate having an information recording portion which rotates in response to the rotation of the first rotary member and the second code plate having an information recording portion which rotates in response to the rotation of the second rotary member. Further, the rotation angle sensor includes a first detecting element for detecting information written on the first information recording portion, and a second detecting element for detecting information written on the second information recording portion. In the rotation angle sensor, when the first code plate and the second code plate are rotated as a result of rotation of the first rotary member and the second rotary member, respectively, the first detecting element detects the information on the first code plate and the second detecting element detects the information on the second code plate, whereby the rotation angle of the first rotary member and the rotation angle of the second rotary member are detected. The electrically driven power steering apparatus also comprises a vehicle handle side steering shaft for supporting the first rotary shaft, and a vehicle wheel side steering shaft for supporting the second rotary member, an end of the vehicle handle side steering shaft and an end of the vehicle wheel side steering shaft being abutted against each other and connected by a resilient member, being a torsion bar. The electrically driven power steering apparatus further comprises a motor used for providing assistance in turning a handle. In the apparatus, the rotation angle of the handle side steering shaft is detected by the first detecting element, and the rotation angle of the wheel side steering shaft is detected by the second detecting element, in order to detect the rotational torque on the first operating shaft from the difference between the rotation angle of the handle side steering shaft and the rotation angle of the wheel side steering shaft, whereby when the rotational torque exceeds a predetermined value, the motor starts to operate to provide assistance in turning the handle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a rotation angle sensor in accordance with the present invention, without a cover. 
     FIG. 2 is a sectional side view of the main portion of the rotation angle sensor in accordance with the present invention. 
     FIGS. 3A and 3B are external views of the rotation angle sensor in accordance with the present invention. 
     FIG. 4 is a plan view of a first rotary member of the rotation angle sensor in accordance with the present invention. 
     FIG. 5 is a sectional side view of the main portion of the first rotary member of the rotation angle sensor in accordance with the present invention. 
     FIG. 6 is a plan view of the portions of a spring member, in the process of being formed, of the rotation angle sensor in accordance with the present invention. 
     FIG. 7 is a plan view of the spring member of the rotation angle sensor in accordance with the present invention. 
     FIG. 8 is a side view of the spring member of the rotation angle sensor in accordance with the present invention. 
     FIGS. 9A,  9 B, and  9 C are a top view, a sectional side view, and a bottom view of a code plate of the angle sensor in accordance with the present invention. 
     FIG. 10 is a side view of the main portion of the steering shaft to which the rotation angle sensor of the present invention is mounted. 
     FIG. 11 is a schematic view of the rotation angle sensor of the present invention mounted to the steering shaft. 
     FIG. 12 is a schematic view of a conventional rotation angle sensor. 
     FIG. 13 is a schematic view of a torque sensor formed by mounting two conventional rotation angle sensors to the rotary shaft. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A description will now be given of a rotation angle sensor of the present invention with reference to FIGS. 1 to  11 . As shown in FIGS. 3A and 3B, which are external views of the rotation angle sensor of the present invention, the rotation angle sensor is formed by molding such that its inside is hollow. A substantially D-shaped housing  10  forms the outer portion of the rotation angle sensor. 
     A plate-shaped cover  11 , which has the same external form as the housing  10 , is placed onto the top portion of the housing  10 . It is affixed to the housing  10  with a plurality of screws  12 , whereby the top portion of the housing  10  is covered by the cover  1 . 
     As shown in FIG. 2, a circular opening  10   b  is formed in substantially the center portion of a bottom wall  10   a  of the housing  10 . A circular guide wall  10   c  is formed along the circumference of the opening  10   b  so as to protrude upwardly by a predetermined height. 
     Similarly, a circular opening  11   a  is formed in substantially the center portion of the cover  11 . As shown in FIG. 2, a guide wall  11   b  is formed along the circumference of the opening  11   a  so as to protrude downwardly by a predetermined height. 
     A first rotary member  13  is inserted into the opening  11   a  of the cover  11 , and is made of, for example, a molding material. FIG. 5 is a sectional side view of the main portion of the first rotary member  13 . The first rotary member  13  has a flange  13   a , shown at the bottom side of FIG. 5, and a gear portion  13   b , with a predetermined number of teeth and modules, shown in FIG. 4 that is a top view of the first rotary member  13 . 
     The first rotary member  13  has a bearing  13   c  formed above the flange  13   a . The bearing  13   c  is formed to a predetermined height and has a substantially flange-like external form. 
     As shown in FIG. 4, a circular, shaft-inserting hole  13   d  is formed in the first rotary member  13 , at the center of rotation thereof, and a plurality of grooves  13   e , formed to a predetermined depth and width, are formed in an inner peripheral surface  13   h  of the edge of the shaft-inserting hole  13   d , in the axial direction thereof. A plurality of rectangular protrusions  13   f  are formed on the top surface of the bearing  13   c  so as to protrude a predetermined height from the top surface of the bearing  13   c.    
     An annular protrusion  13   g  is formed along the circumference of a portion, below the flange  13   a  of FIG. 5, of the shaft-inserting hole  13   d , so as to protrude by a small height. 
     The bearing  13   c  of the first rotary member  13  is inserted into the opening  11   a  of the cover  11 , and is guided along the guide wall  11   b , formed along the circumference of the opening  11   a , such that it can rotate freely. 
     A first engaging spring  14 , such as that shown in FIGS. 6,  7 , and  8 , whose external shape is annular, is mounted to the top surface of the bearing  13   c  of the first rotary member  13 . The first engaging spring  14  is formed of, for example, a springy stainless steel. As shown in FIG. 6, it has an annular frame portion  14   a , formed at the outer periphery of the first engaging spring  14 , and a plurality of spring portions  14   b , which extend from the frame portion  14   a  into an opening  14   e  of the first engaging spring  14 . The annular frame portion  14   a  and the spring portions  14   b  are formed by punching out, for example, stainless steel, by, for example, a pressing operation. 
     A plurality of square holes  14   c  are formed in the frame portion  14   a  by punching out a portion of the frame portion  14   a , and a square hole  14   d  is formed towards an end of each spring portion  14   b  by punching out a portion of the end of each spring portion  14   d . Each spring portion  14   b  is formed such that its top portion, which is substantially cone shaped, includes its associated square hole  14   d.    
     As shown in FIG. 8, which is a sectional side view of the base portion of some of the spring portions  14   b , the first engaging spring  14  is bent downward. As shown in FIG. 7, which is a top view of the first engaging spring  14 , the first engaging spring  14  has a substantially circular opening  14   e.    
     As shown in FIG. 2, the square holes  14   c  of the frame portion  14   a  are inserted onto the protrusions  13   f , formed on the top surface of the bearing  13   c  of the first rotary member  13 , and the ends of the protrusions  13   f  protruding above their respective square holes  14   c  are, for example, caulked by heating, whereby the first rotary member  13  and the first engaging spring  14  are integrally formed. 
     The downwardly bent spring portions  14   b  are positioned in the grooves  13   e  of the first rotary member  13 . The substantially cone-shaped top portion, including its associated square hole  14   d , of each spring portion  14   b  protrudes from the inner peripheral surface  13   h  of the edge of the shaft-inserting hole  13   d  into the shaft-inserting hole  13   d  of the first rotary member  13 . 
     At the lower side of FIG. 2 that opposes the first rotary member  13  is disposed a second rotary member  15  having substantially the same form as the first rotary member  13 . It comprises, for example, a gear portion  15   b , a bearing portion  15   c , and a shaft-inserting hole  15   d , formed at an inner peripheral surface  15   h  of the second rotary member  15 . 
     A second engaging spring  16 , which has substantially the same form as the first engaging spring  14  and comprises a spring portion  16   b , etc., is mounted to the second rotary member  15  by using the same method as that used for mounting the first engaging spring  14  to the first rotary member  13 . 
     In other words, the first and second engaging springs  14  and  16 , serving as spring members, are disposed at the inner peripheral surfaces  13   h  and  15   h  of the edges of the shaft-inserting holes  13   d  and  15   d  of the first rotary member  13  and the second rotary member  15 , respectively. 
     The bearing portion  15   c  of the second rotary member  15  is inserted into the opening  10   b  of the housing  10 , and is guided along the guide wall  10   c , formed along the circumference of the opening  10   b , so that it can rotate freely. 
     The first rotary member  13  and the second rotary member  15 , mounted to the cover  11  and the housing  10 , have the same center of rotation and can rotate separately. 
     At the lower left side of the housing  10  of FIG. 1 are disposed a first code plate  17 , which engages the gear portion  13   b  of the first rotary member  13 , and a second code plate  18 , which engages the gear portion  15   b  of the second rotary member  15 . 
     As shown in FIG. 9, the first code plate  17  comprises a gear portion  17   a  and an information recording portion  17   b . The gear portion  17   a  engages the gear portion  13   b  of the first rotary member  13  and is made of, for example, resinous material. The information recording portion  17   b  is mounted to a side of the gear portion  17   a  and is made of a magnetic material with a plurality of magnetic north-south (N-S) poles. 
     The information recording portion  17   b  is formed into the shape of a disk with an outside diameter which is greater than the outside diameter of the gear portion  17   a . A protruding boss portion  17   c  is formed at a side of the gear portion  17   a.    
     The boss portion  17   c , formed at a side of the gear portion  17   a , is press-fitted or bonded to a boss hole  17   d  in the information recording portion  17   b , whereby the gear portion  17   a  and the information recording portion  17   b  are integrally formed together. 
     A shaft-inserting hole  17   e  is formed through the first code plate  17 , at the center of rotation of the first code plate  17 . At a side surface  17   f  of the gear portion  17   a  are formed a spring groove  17   g , having a predetermined depth, and a groove  17   h  for stopping rotation of a torsion coil spring (not shown) which is inserted into the spring groove  17   g.    
     A protrusion  17   j , which protrudes slightly from the side surface  17   f , is formed at the inner peripheral side of the edge of the spring groove  17   g.    
     As shown in FIG. 2, a second code plate  18 , with the same form as the first code plate  17 , is disposed at the lower side in FIG. 2 which opposes the gear portion  17   a  of the first code plate  17 . 
     The second code plate  18  comprises a gear portion  18   a , which engages the gear portion  15   b  of the second rotary member  15 , an information recording portion  18   b , a shaft-inserting hole (not shown), a protrusion (not shown), etc. A metallic supporting shaft is inserted into the shaft-inserting hole  17   e  of the first code plate  17  and the shaft-inserting hole (not shown) of the second code plate  18 , and one side of the gear portion  17   a  and one side of the gear portion  18   a  are brought into contact with each other, in order to allow the code plates  17  and  18  to rotate separately. 
     The supporting shaft has its top end affixed to the cover  11  side and its bottom end affixed to the housing  10  side in order to accommodate the first code plate  17  and the second code plate  18  in the housing  10 . 
     The spring groove  17   g  has inserted therein a torsion coil spring (not shown), which prevents backlash from occurring at the two code plates  17  and  18 . 
     A holder  22  is disposed at the left lower corner of the housing  10  of FIG.  1 . To the holder  22  are mounted first detecting elements  20  and second detecting elements  21 , being, for example, hole elements, for detecting information, that is magnetic information, written on the information recording portions  17   b  and  18   b  of the code plates  17  and  18 , respectively. 
     The portion of the holder  22  to which the detecting elements  20  and  21  are mounted has two nonparallel sides that incline towards each other. Two first detecting elements  20  are mounted to one of the nonparallel sides of the holder  22 , while two second detecting elements  21  are mounted to the other nonparallel side of the holder  22 , whereby they are mounted separately and vertically to the holder  22 . The first detecting elements  20  are mounted at a location opposing the information recording portion  17   b  of the first code plate  17 , while the second detecting elements  21  are mounted at a location opposing the information recording portion  18   b  of the second code plate  18 . 
     The holder  22 , to which the first detecting elements  20  and the second detecting elements  21  are mounted, has a flat back surface, which is, for example, bonded to a substrate  23  behind the back surface. 
     An integrated circuit (IC)  24 , which is used to perform computations on the information sent from the detecting elements  20  and  21 , is mounted to the insulating substrate  23 , and a lead wire  25 , for transmitting the information processed by the IC  24  to an external device, is mounted to the insulating substrate  23 , by soldering or the like. 
     A description will now be given of the case where the rotation angle sensor of the present invention is used to form a torque sensor and is applied to an automobile steering shaft. 
     As shown in FIG. 10, the automobile steering shaft  26  comprises, for example, a first operating shaft portion  27  and a second operating shaft portion  28 , with T-shaped grooves  27   b  and  28   b  being formed in ends  27   a  and  28   a , respectively. These ends  27   a  and  28   a  are abutted against each other. A resilient member  29 , shown in black in FIG. 10, is inserted into the grooves  27   b  and  28   b , whereby the first operating shaft portion  27  and the second operating shaft portion  28  are connected together. The resilient member  29  is, for example, a torsion bar. 
     The first rotary member  13  of a rotation angle sensor S of the present invention is inserted into the end  27   a  of the first operating portion shaft portion  27 , and the second rotary member of the rotation angle sensor S is inserted into the end  28   a  of the second operating shaft portion  28 . 
     As described above, the spring portions  14   b  and  16   b  of the first and second engaging springs  14  and  16 , being spring members, are disposed at the inner peripheral surfaces  13   h  and  15   h  of the edges of the shaft-inserting holes  13   d  and  15   d  of their respective first rotary member  13  and the second rotary member  15 . The spring portions  14   b  and  16   b  resiliently press against the first operating shaft portion  27  and the second operating shaft portion  28 , respectively, in order for the first rotary member  13  and the second rotary member  15  to be supported by the first operating shaft portion  27  and the second operating shaft portion  28 , respectively, whereby the rotation angle sensor S is mounted to the steering shaft  26 . 
     As shown in FIG. 11, the steering shaft  26  is constructed such that a handle  30  is mounted to the first operating shaft portion  27 , and the second operating shaft portion  28  is mounted to a wheel (not shown). When the handle  30  is turned to rotate the second operating shaft portion  28 , the rotational torque on the second operating shaft portion  28  becomes large due to, for example, the condition of the road surface (not shown) with which the vehicle wheels are in contact. In this case, when the first operating shaft portion  27  is rotated as a result of turning the handle  30 , the second operating shaft portion  28 , due to the resilient member  29 , is rotated later than the first operating shaft portion  27 , causing the rotation angles of the first operating shaft portion  27  and the second operation shaft portion  28  to differ. 
     This difference in rotation angles causes the rotation angles of the first code plate  17  and the second code plate  18  to differ. The rotation angle of the first code plate  17  and the rotation angle of the second code plate  18  are detected by the first detecting element  20  and the second detecting element  21 , respectively. The difference in the rotation angles of the code plates  17  and  18  are computed by means of the IC  24 , thereby allowing the rotational torque at the first operating shaft portion  27  side to be detected. Accordingly, the rotation angle sensor S of the present invention can be used to form a torque sensor. 
     The electrically driven power steering apparatus of the present invention comprises an electric motor (not shown), such as a motor which assists the operator in operating the handle  30 . When the handle  30  is turned, the torque sensor detects the rotational torque on the first operating shaft portion  27 . When the rotational torque on the first operating shaft portion  27  exceeds a predetermined value, an operation command is sent forth from the IC  24  towards the electric motor, through a driver, thereby actuating the electric motor. 
     The actuating force of the electric motor is used to assist the operator, who is turning the handle  30  with a certain turning force, in turning the handle  30 , whereby less rotational torque is exerted on the handle  30 . 
     Although in the foregoing description of the rotation angle sensor S of the present invention the information recording portions  17   b  and  18   b  of the code plates  17  and  18  were described as magnetic media, and the detecting elements  20  and  21  were described as magnetic sensors, the information recording portions  17   b  and  18   b  may be identification marks identifiable by, for example, an optical sensor, and the detecting elements may be, for example, optical sensors consisting of a light emitter and a light receiver. 
     According to the rotation angle sensor of the present invention, when the code plates are rotated as a result of turning the rotary members, the detecting elements detect information on their associated code plates in order to detect the rotation angles of their associated rotary members. Therefore, the code plates and the rotary members can be formed as separate members. Even when the rotary members are slightly displaced as a result of undue load on the rotary members, the undue load is not exerted onto the code plates, so that the amount of gap between the code plates and their associated detecting elements does not change. 
     Consequently, it is possible to provide a rotation angle sensor which can detect the angle of rotation of a rotary member precisely, even when an undue load is exerted onto the rotary member. 
     In addition, the rotation angle sensor comprises first and second rotary members which can rotate separately and which have the same center of rotation; first and second code plates which have gear portions that engage their respective rotary members and which can rotate separately; and first and second detecting elements for detecting information on their associated first code plate and second code plate. When the first rotary member and the second rotary member are rotated by engaging the gear portion of the first code plate with the gear portion of the first rotary member, and by engaging the gear portion of the second code plate with the gear portion of the second rotary member, the information on the first code plate is detected by the first detecting element, and the information on the second code plate is detected by the second detecting element, so that the rotation angles of the first rotary member and the second rotary member can be separately detected. Therefore, the code plates and the rotary members can be formed as separate component parts through the gear portions, so that even when an undue load is exerted onto the rotary members, it is possible to support the code plates without backlash. The rotation angle sensor can be used for a torque sensor which can detect the rotational torque on the first rotary member and the second rotary member, from the difference between the rotation angles of the two rotary members detected by their associated two code plates. 
     Further, according to the rotation angle sensor, the first and second code plates have the same center of rotation, with the gear portions being disposed such that one side of one of the gear portions and one side of the other of the gear portions oppose each other. The information recording portions are formed on the other sides, not facing each other, of the gear portions, such that their outside diameters are larger than the outside diameters of their respective gear portions. The first and second rotary members are rotatably interposed between the information recording portions. By virtue of such a structure, the outside dimensions of the information recording portions of the code plates can be made large, thereby allowing the rotation angles to be detected with high precision. 
     Still further, since the rotary members are interposed between the information recording portions, the outside dimensions of the rotation angle sensor do not become large, even when the outside dimensions of the information recording portions are large. 
     Still further, the rotary members, the code plates, and the detecting elements are accommodated in a box-shaped housing such that the detecting elements are disposed at a corner of the housing, and the code plates are disposed between the detecting elements and their associated rotary members. Therefore, the outside dimensions of the rotation angle sensor can be made small. 
     Still further, the code plates each have an information recording portion made of magnetic material with a plurality of magnetic poles, and the detecting elements are magnetic sensors which react to the magnetic field of the magnetic material. Therefore, when the magnetic field is varied as a result of rotating the code plates, the magnetic field variation can be detected by the detecting elements with high precision. Consequently, it is possible to provide a rotation angle sensor which can detect the rotation angle of a rotary member with high precision. 
     Still further, since the magnetic members around the shaft-inserting holes of the rotary members do not have to be formed to large diameters, the rotation angle sensor can be formed to the minimum size required. Therefore, a cheap rotation angle sensor can be provided. 
     The torque sensor of the present invention comprises a rotation angle sensor including a first rotary member and a second rotary member being separately rotatable and having the same center of rotation, each having a gear portion at the outer peripheral portion thereof and a shaft-inserting hole at the center of rotation thereof. In addition, the rotation angle sensor includes a first code plate and a second code plate being separately rotatable, the first code plate having a gear portion which engages the first rotary member and the second code plate having a gear portion which engages the second rotary member, the first code plate having an information recording portion which rotates in response to the rotation of the first rotary member and the second code plate having an information recording portion which rotates in response to the rotation of the second rotary member. Further, the rotation angle sensor includes a first detecting element for detecting information written on the first information recording portion, and a second detecting element for detecting information written on the second information recording portion. In the rotation angle sensor, when the first rotary member and the second rotary member rotate to rotate the first code plate and the second code plate, respectively, the first detecting element detects the information on the first code plate and the second detecting element detects the information on the second code plate, whereby the rotation angle of the first rotary member and the rotation angle of the second rotary member are detected. The torque sensor also comprises a first operating shaft and a second operating shaft, an end of the first operating shaft and an end of the second operating shaft being abutted against each other and connected by a resilient member, being a torsion bar. In the torque sensor, the first rotary member is supported by the end of the first operating shaft, and the second rotary member is supported by the end of the second operating shaft, in order to detect the rotation angle of the first operating shaft by the first detecting element and the rotation angle of the second operating shaft by the second detecting element, whereby the rotational torque on the first operating shaft is detected from the difference between the rotation angle of the first operating shaft and the rotation angle of the second operating shaft. By virtue of this structure, the rotation angle sensor can detect with high precision the difference between the rotation angle of the first operating shaft and the rotation angle of the second operating shaft. An IC performs computations on the difference of the rotation angles in order to convert it to a torque value, whereby the rotational torque on the first operating shaft can be detected with high precision. 
     In the torque sensor, a spring member is provided at the inner peripheral surface of the edge of the shaft-inserting hole of the first rotary member and at the inner peripheral surface of the edge of the shaft-inserting hole of the second rotary member, the spring members resiliently pressing against the first and the second operating shafts in order to support the first rotary member by the first operating shaft and the second rotary member by the second operating shaft. Therefore, the rotary members can be supported by their respective operating shafts, while the spring members resiliently press against the operating shafts, by merely fitting the rotary members into their respective operating shafts. 
     Consequently, it is possible to provide a torque sensor which allows the rotary members to be easily mounted to the operating shafts, without slippage of the rotary members with respect to the operating shafts. 
     The electrically driven power steering apparatus comprises a rotation angle sensor including a first rotary member and a second rotary member being separately rotatable and having the same center of rotation, each having a gear portion at the outer peripheral portion thereof and a shaft-inserting hole at the center of rotation thereof. In addition, the rotation angle sensor includes a first code plate and a second code plate being separately rotatable, the first code plate having a gear portion which engages the first rotary member and the second code plate having a gear portion which engages the second rotary member, the first code plate having an information recording portion which rotates in response to the rotation of the first rotary member and the second code plate having an information recording portion which rotates in response to the rotation of the second rotary member. Further, the rotation angle sensor includes a first detecting element for detecting information written on the first information recording portion, and a second detecting element for detecting information written on the second information recording portion. In the rotation angle sensor, when the first code plate and the second code plate are rotated as a result of rotation of the first rotary member and the second rotary member, respectively, the first detecting element detects the information on the first code plate and the second detecting element detects the information on the second code plate, whereby the rotation angle of the first rotary member and the rotation angle of the second rotary member are detected. The electrically driven power steering apparatus also comprises a vehicle handle side steering shaft for supporting the first rotary shaft, and a vehicle wheel side steering shaft for supporting the second rotary member, an end of the vehicle handle side steering shaft and an end of the vehicle wheel side steering shaft being abutted against each other and connected by a resilient member, being a torsion bar. The apparatus further comprises a motor used for giving assistance in turning a handle. In the apparatus, the rotation angle of the handle side steering shaft is detected by the first detecting element, and the rotation angle of the wheel side steering shaft is detected by the second detecting element, in order to detect the rotational torque on the first operating shaft from the difference between the rotation angle of the handle side steering shaft and the rotation angle of the wheel side steering shaft, whereby when the rotational torque exceeds a predetermined value, the motor starts to operate for giving assistance in turning the handle. By virtue of this structure, it is possible to provide an electrically driven power steering apparatus which can detect the rotational torque on an operating shaft with high precision and which can provide high performance utilizing the rotational torque detected with high precision.