Patent Publication Number: US-7210451-B2

Title: Throttle control devices

Description:
This application claims priorities to Japanese patent application serial number 2003-130434, the contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to throttle control devices for controlling a flow rate of intake air supplied to an engine, e.g., an internal combustion engine of an automobile, and in particular to throttle control devices that are electrically or electronically controlled. 
   2. Description of the Related Art 
   Japanese Laid-Open Patent Publication No. 2001-59702 teaches a known throttle control device that includes a throttle valve disposed within an intake air channel formed in a throttle body. The throttle valve is rotatably driven by a motor in order to open and close the intake air channel, so that the flow rate of the intake air is controlled. The throttle control device further includes a throttle sensor (also known as “throttle position sensor”) that detects the degree of opening of the throttle valve. The throttle sensor includes a pair of magnets and a magnetic detecting element, such as a Hall element. The magnets are attached to a support member. The support member is mounted to at throttle shaft that rotates in unison with the throttle valve, so the magnets are positioned to oppose to each other with respect to the rotational axis of the support member. The magnetic detecting element is mounted to the throttle body. The magnetic detecting element detects the intensity of the magnetic field produced by the magnets and outputs the detected intensity as signals that represent the degree of opening of the throttle valve. 
   However, because the magnetic detecting element detects the intensity of the magnetic field produced by the pair of magnets, the magnetic detection element may output incorrect signals if the pair of magnets has been offset from their initially set positions relative to the magnetic detection element. The offset could be due to possible displacement of the throttle shaft during a long period of use or due to thermal expansion of the molded resin that incorporates the magnets through an insert molding process. Such incorrect signals also may be outputted if the level of intensity of the magnetic field has been changed due to temperature-dependent characteristics of the magnets. For these reasons among others, the detection accuracy of the degree of opening of the throttle valve may be lowered, and therefore, the accuracy of the control of the flow rate of the intake air may also subsequently be lowered. This problem becomes more significant if the throttle body is made of a synthetic resin that has a large coefficient of thermal expansion or if the throttle body is made of a material that cannot be accurately formed or machined. Therefore, it has been desired to improve the known throttle control devices and reduce these problems. 
   To this end, Japanese Laid-Open Patent Publication No. 8-35809 has proposed a device  101   a  for detecting a rotational angle, as shown in  FIG. 6  of the publication, in which a pair of stators  160  and  161 , each having a semi-circular cross section, are disposed within the yoke  110 . A gap  162  is formed between the stators  160  and  161 . The sensor  170  is positioned within the gap  162  for detecting the strength of a magnetic field. With this arrangement, the direction of the magnetic field is directed in primarily one direction, i.e., a direction indicated by the arrows shown across the gap  162  as viewed in  FIG. 6  of the publication. The magnetic field is most unidirectional particularly where the magnetic field lines intersect the sensor  170 , throughout a change in the rotational angle of the yoke  110 . Therefore, the sensor  170  can properly detect the rotational angle of the rotary shaft over the entire range of rotation. 
   However, incorporation of the stators  160  and  161  may increase the total number of parts required for a device used in detecting rotational angles and therefore may increase the overall manufacturing cost. In addition, an increase in the number of parts may consequently demand increased accuracy in the assembling operation. 
   SUMMARY OF THE INVENTION 
   It is accordingly an object of the present invention to teach improved throttle control devices that can accurately detect the degree of opening of a throttle valve. 
   According to one aspect of the present teachings, throttle control devices are taught that include a throttle body defining an intake air channel. The throttle control device also includes a throttle shaft that is able to rotate about a rotational axis. A throttle valve is mounted to the throttle shaft and disposed within the intake air channel. A motor is coupled to the throttle shaft, so that the throttle valve rotates to incrementally open and close the intake air channel so as to control the flow rate of intake air. A detection device serves to detect the degree of opening of the throttle valve and may include at least two magnets and a sensor. The magnets are mounted to the throttle shaft via a magnet support. In addition, the magnets are positioned to oppose to each other across the rotational axis, so as to produce a magnetic field. The sensor is mounted to the throttle body and serves to detect the direction of the magnetic field produced by the magnets, so that the detection device outputs a signal representing the degree of opening of the throttle valve. 
   Because the sensor detects the direction of the magnetic field produced by the magnets, the output signal may not be substantially influenced by the potential offset of the magnets from their set positions or by the potential change of the strength of the magnetic field of the magnets. Therefore, the degree of opening of the throttle valve can be accurately detected. For example, the magnets may be offset from their initial set positions when the position of the throttle shaft has been offset due to wear during a long period of use. In the case where the magnet support is made of resin and integrally molded containing the magnets via an insert molding process, the magnets may be offset from their initially set positions due to thermal expansion of the resin. In addition, the strength of the magnetic field may change due to the temperature characteristics of the magnets. 
   In another aspect of the present teachings, the throttle control device further includes a ring-shaped yoke that is made of magnetic material and is mounted to the magnet support. The yoke has substantially the same axis as the rotational axis of the throttle shaft. The magnets are attached to an inner peripheral surface of the yoke. The magnets are magnetized to produce a substantially uniform magnetic field represented by substantially parallel, unidirectional, magnetic field lines. 
   The production of substantially parallel, unidirectional magnetic field lines by the magnets, improves the accuracy of the detection of the direction of the magnetic field. 
   In another aspect of the present teachings, each of the magnets extends along an angle measured about the rotational axis. The angle is determined such that an error in the sensor output signal due to an offset away from the ideal set positions of the magnets or detection device, is such that the error is less than a predetermined value. The error in the outputted signal may be due to an offset of a position of at least one of the magnets and the detection device relative to at least one of the other of the magnets and the detection device, away from ideal set positions. 
   This arrangement may further improve the detection accuracy. 
   In another aspect of the present teachings, the magnet support comprises a throttle gear mounted to the throttle shaft. No separate magnet support is required for the magnets. 
   In another aspect of the present teachings, the sensor comprises a holder attached to the throttle body and a sensing element disposed within the bolder. For example, the sensing element may be a magnetoresistive element or a Hall element. 
   In another aspect of the present teachings, the holder has a bottomed tubular configuration having an open end, The sensing element is fixed in position within the holder by filling resin into the holder. Therefore, the sensing element can be reliably maintained in the set position. 
   In another aspect of the present teachings, the sensing element comprises a sensing section and a computing section that are integrated with one another. The result is a compact construction for the sensing element. 
   In another aspect of the present teachings, the sensing element has a substantially square configuration. The sensing element is positioned on the rotational axis of the throttle shaft. 
   In another aspect of the present teachings, the sensor further includes a circuit board. The circuit board is electrically connected to the sensing element. The circuit board is positioned so as to substantially close the open end of the holder. 
   In another aspect of the present teachings, the throttle body includes a removable cover. The sensor is mounted to the removable cover. This aspect facilitates the assembly operation of the sensor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectional plan view of a representative throttle control device; 
       FIG. 2  is a cross sectional view taken along line II—II in  FIG. 1 ; and 
       FIG. 3  is a side view of the throttle control device with a cover removed; and 
       FIG. 4  is an exploded view of a sensor assembly; and 
       FIG. 5  is a schematic view showing magnets of a detecting device; and 
       FIG. 6  is a cross sectional view taken along line VI—VI in  FIG. 5 ; and 
       FIG. 7  is a cross sectional view showing magnetic field lines that may be produced when the angular range of the magnets are appropriately determined; and 
       FIG. 8  is a view similar to  FIG. 7  but showing the magnetic field lines that may be produced when the angular range of the magnets are tool small; and 
       FIG. 9  is a view similar to  FIG. 7  but showing the magnetic field lines that may be produced when the angular range of the magnets are too large; and 
       FIG. 10  is a graph illustrating the relation between the angular range of the magnets and possible maximum error of the detected angle when the position of the sensor has been offset from the center; and 
       FIG. 11  is a cross sectional view similar to  FIG. 6  but showing an alternative embodiment of magnets. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Each of the additional features and teachings disclosed above and below may be utilized separately or in conjunction with other features and teachings to provide improved throttle control devices and methods of using such improved throttle control devices. Representative examples of the present invention, which examples utilize many of these additional features and teachings both separately and in conjunction with one another, will now be described in detail with reference to the attached drawings. This detailed description is merely intended to teach a person of skill in the art further details for practicing preferred aspects of the present teachings and is not intended to limit the scope of the invention. Only the claims define the scope of the claimed invention. Therefore, combinations of features and steps disclosed in the following detailed description may not be necessary to practice the invention in the broadest sense, and are instead taught merely to particularly describe representative examples of the invention. Moreover, various features of the representative examples and the dependent claims may be combined in ways that are not specifically enumerated in order to provide additional useful embodiments of the present teachings. 
   A representative embodiment will now be described with reference to the drawings. First, the construction of a representative throttle control valve will be described in brief. Referring to  FIGS. 1 and 2 , the throttle control valve includes a throttle body  1  that is made of resin. The throttle body  1  has a bore portion  20  and a motor housing portion  24  that are formed integrally with one another. As shown in  FIG. 1 , a substantially cylindrical intake air channel  1   a  is formed in the bore portion  20  and extends vertically as viewed in  FIG. 2  through the bore portion  20 . An air cleaner (not shown) may be connected to the upper part of the bore portion  20 . An intake manifold  26  is connected to the lower part of the bore portion  20 . In the drawings, only a connecting portion of the manifold  26  is shown. A metal throttle shaft  9  is disposed within the bore portion  20  and extends across the intake air channel  1   a  in the diametrical direction. 
   As shown in  FIG. 1 , left and right support portions  21  and  22  rotatably support the throttle shaft  9  via respective left and right bearings  8  and  10 . The support portions  21  and  22  are formed integrally with the bore portion  20  of the throttle body  1 . Preferably, the left bearing  8  is a thrust bearing and the right bearing  10  is a radial ball bearing. The throttle shaft  9  is press fitted into an inner race  10   a  of the right bearing  10 . The outer race  10   b  of the right bearing  10  is fitted with clearance into the support portion  22  of the resin throttle body  1 . The loose fitting of the outer race  10   b  has been incorporated in order to avoid cracking of the support portion  22 . The dimensional tolerance of the diameter of the inner peripheral surface of the support portion  22  is relatively large because the throttle body  1  is made of resin. In addition, the thermal linear expansion coefficient of the support portion  22  is considerably different from that of the bearing  10 . Therefore, when the outer race  10   b  has been press fitted into the support portion  22 , the press-fitting force may possibly crack the support portion  22 . On the other hand, in the case where the throttle body  1  is made of metal, such as aluminum alloy for example, the inner peripheral surface of the support portion  22  may be machined (cut) to within a relatively small dimensional tolerance. The metal throttle body  1  may also have a relatively small difference in the thermal linear expansion coefficients between the support portion  22  and the bearing  10 . Therefore, in such a case, the outer race  10   b  may be press fitted into the support portion  22  without causing any cracking problem. 
   As shown in  FIG. 1 , a throttle valve  2  made of resin is secured to the throttle shaft  9  by rivets  3 , and the throttle valve  2  is adapted to open and close the intake air channel  1   a  (see  FIG. 2 ) as it rotates with the throttle shaft  9 . The motor  4  rotatably drives the throttle shaft  9 , so that the throttle valve  2  rotates to incrementally open and close the intake air channel  1   a . The throttle valve  2  rotates in order to control the flow rate of the intake air within the intake air channel  1   a . In the state shown  FIG. 2 , the throttle valve  2  is in a fully closed position. The throttle valve  2  may rotate in a counterclockwise direction as viewed in  FIG. 2  (“Open” direction as indicated by an arrow shown in  FIG. 2 ) to open the intake air channel  1   a.    
   As shown in  FIG. 1 , a plug  7  is fitted into the support portion  21  that forms a first end  9   a  (left end as viewed in  FIG. 1 ) of the throttle shaft  9 . The plug  7  serves to scal the first end  9   a  within the bore portion  20 . A second end  9   b  (right end as viewed in  FIG. 1 ) of the throttle shaft  9  extends through the support portion  22 . A throttle gear  11  is secured to the second end  9   b  and does not rotate relative to the throttle shaft  9 . The throttle gear  11  is made of resin and is configured as a sector gear. A return spring  12  is interposed between the throttle body  1  and the throttle gear  11  in order to normally bias the throttle valve  2  toward the fully closed position. Although not shown in the drawings, a stopper device is provided between the throttle body  1  and the throttle gear  11  in order to prevent the throttle valve  2  from rotating further beyond the fully closed position. 
   As shown in  FIG. 1 , the motor housing portion  24  of the throttle body  1  is configured as a bottomed hollow cylindrical member that has a central axis parallel to a rotational axis L of the throttle shaft  9 . As shown in  FIG. 2 , a motor accommodating space  24   a  is defined within the motor housing portion  24  and is open on a right side as viewed in  FIG. 1 . The motor  4  is inserted into the motor accommodating space  24   a . For example, the motor  4  may be a DC motor. In the accommodated state, the motor  4  is positioned such that the longitudinal axis of the motor  4  extends substantially parallel to the rotational axis L of the throttle shaft  9 . The output shaft  4   a  (see  FIG. 3 ) of the motor  4  is oriented rightward as viewed in  FIG. 1  (i.e., a direction opposite to the inserting direction of the motor  4  into the motor accommodating space  24   a ). As shown in  FIG. 1 , a mount flange  29  is formed on the right end (one end opposite to the motor insertion direction) of a motor casing  28 , i.e., an outer hull, of the motor  4 . The mount flange  29  is secured to the motor housing portion  24 , by means of screws  5  for example. 
   As shown in  FIG. 3 , a motor pinion  32  is secured to the output shaft  4   a  of the motor  4 . The motor pinion  32  may be made of resin. As shown in  FIG. 1 , a countershaft  34  is mounted to the throttle body  1  in a position between the bore portion  20  and the motor housing portion  24 . The countershaft  34  extends parallel to the rotational axis L of the throttle shaft  9 . A counter gear  14 , made of resin, is rotatably supported on the countershaft  34 . The counter gear  14  includes a first gear portion  14   a  and a second gear portion  14   b , having different outer diameters from one another. The first gear portion  14   a , having a relatively larger outer diameter, engages the motor pinion  32 . The second gear portion  14   b , having a smaller outer diameter, engages the throttle gear  11  (see  FIG. 3 ). The motor pinion  32  and the counter gear  14  constitute a speed reduction gear mechanism  35 . 
   As shown in  FIG. 1 , a cover  18  is mounted to the right side of the throttle body  1  in order to cover the reduction gear mechanism  35  and other associated mechanisms from the outside. The cover  18  may be fixed in position relative to the throttle body  1  by an appropriate mounting device, for example, such as a snap-fit device, a screw device, and a clamp device, among others. An O-ring  17  is interposed between the throttle body  1  and the cover  18  in order to provide a hermetic seal therebetween. In this way, the cover  18  may serve as a component of the throttle body  1 . Two motor terminals  30  (only one terminal  30  is shown in  FIG. 1 ) extend from the mount flange  29  of the motor  4  and are electrically connected to respective relay connectors  36  mounted to the cover  18 . Although not shown in the drawings, connecting terminals are integrated with the cover  18  via an insert molding process of the cover  18 . One end of each connecting terminal is electrically connected to the corresponding relay connector  36 . The other end of each connecting terminal extends into a connector formed on the cover  18 . 
   The motor  4  may be controlled based on signals from a control unit, such as an ECU (engine control unit), of an internal combustion engine of an automobile. The control unit may output signals to the motor  4  in order to control the opening degree of the throttle valve  2 . For example, the output signals may include an accelerator signal with regard to the depression amount of an accelerator pedal, a traction control signal, a constant-speed travelling signal, and an idling speed control signal. The rotation or the driving force of the motor  4  may be transmitted to the throttle shaft  9  via the reduction gear mechanism  35  (i.e., the motor pinion  32  and the counter gear  14 ) and the throttle gear  11 . 
   As shown in  FIG. 1 , the throttle gear  11  has a substantially cylindrical tubular portion  11   a  that is positioned to extend rightward of the right end surface of the throttle shaft  9 . The tubular portion  11   a  has the same axis as the rotational axis L of the throttle shaft  9 . A yoke  45  is formed integrally with the inner peripheral surface of the tubular portion  11   a  through an insertion molding process of the tubular portion  11   a . The yoke  45  is made of magnetic material and has a ring-shaped configuration substantially about the rotational axis L of the throttle shaft  9 . A pair of magnets  47  and  48  (permanent magnets) is attached to the inner peripheral surface of the yoke  45 . Magnets  47  and  48  are positioned to symmetrically oppose each other with respect to the rotational axis L of the throttle shaft  9 . The magnets  47  and  48  are simultaneously integrated with the tubular portion  11   a  and the yoke  45  during the insertion molding process of the tubular portion  11   a . Therefore, the yoke  45  and the magnets  47  and  48  are embedded within the tubular portion  11   a  in such a way that only the inner peripheral surfaces of magnets  47  and  48  are exposed to or communicate with the inside of the tubular portion  11   a . In this way, throttle gear  11  serves as a support means for supporting the yoke  45  and the magnets  47  and  48 . 
   A sensor assembly  50  is disposed inside of the cover  18  and is positioned opposing the right end of the throttle shaft  9 . As shown in  FIG. 4 , the sensor assembly  50  includes a holder  52 , a sensor IC  54 , and a circuit board  59 . The yoke  45 , the magnets  47  and  48 , and the sensor assembly  50 , constitute a detection device  44  (see  FIG. 1 ) that may serve as a throttle sensor. 
   As shown in  FIG. 4 , the holder  52  has a bottomed tubular portion  52   a  and is made of resin. The sensor IC  54  is a sensing element disposed within the tubular portion  52   a  of the holder  52 . The holder  52  may be joined to the cover  18  (see  FIG. 1 ) by an appropriate joining technique, such as crimping under beat, heat welding, and adhesion. The tubular portion  52   a  has a central axis that is along the same central axis of the yoke  45  and the rotational axis L of the throttle shaft  9 . A resin, such as UV curable resin (not shown), may be filled within the tubular portion  52   a  in which the sensor IC  54  is disposed. The sensor IC  54  includes a sensing section  55  and a computing section  56  that are connected to each other. The sensing section  55  and the computing section  56  are electrically connected by means of terminals  57  (four terminals  57  are provided in the representative embodiment). The sensing section  55  may have a magnetoresistive element accommodated therein. 
   The sensing section  55  of the sensor IC  54  has a substantially square plate-like configuration. The computing section  56  has a substantially rectangular plate-like configuration. The terminals  57  are bent at substantially right angles, so that the sensor IC  54  has a substantially L-shaped configuration as shown in  FIG. 4 . The sensing section  55  of the sensor IC  54  serves to detect the direction of the magnetic field produced between the magnets  47  and  48 . To this end, the sensing section  55  is positioned between the magnets  47  and  48  on the rotational axis L of the throttle shaft  9  such that the square surface of the sensing section  55  extends perpendicular to the rotational axis L. In addition, the tubular portion  52   a  of the holder  52  is disposed coaxially with the tubular portion  11   a  of the throttle gear  11  and in an intermediate position between the magnets  47  and  48 . 
   The sensor IC  54  includes a full-bridge circuit (not shown) that includes a pair of magnetoresistive elements (not shown) disposed within the detecting section  55  and displaced from each other in the circumferential direction by an angle of 45°. The computing section  56  may calculate the arctangent of the output from the full-bridge circuit so as to produce linear output signals that correspond to the direction of the magnetic field. The linear output signals are supplied to the control unit. With this arrangement, the direction of the magnetic field can be detected without being influenced by change of strength of the magnetic field. As a result, the degree of opening of the throttle valve  2  can be detected as signals outputted from the sensor IC  54 . The signals represent the direction of the magnetic field. The direction is obtained as a magnetic physical quantity of the magnets  47  and  48 . In this way, the sensor IC  54  serves as a magnetic-field direction detecting device. 
   Based on the following, signals representing the degree of opening of the throttle valve  2  and outputted from the sensor IC  54 , signals representing a travelling speed of the automobile and outputted from a speed sensor (not shown), signals representing the rotational speed of the engine and outputted from a crank angle sensor (not shown), signals representing a depression amount of an accelerator pedal and outputted from an accelerator pedal sensor, signals from an O 2  sensor (not shown), and signals from an airflow meter (not shown) among others, the control unit, i.e., an Engine Control Unit (ECU), may serve to adjust and control various parameters such as fuel injection control, correction control of the degree of opening of throttle valve  2 , and variable speed control of an automatic transmission. 
   The circuit board  59  of the sensor assembly  50  (see  FIG. 4 ) may be mounted to the holder  52  such that the open end of the holder  52  is closed by the circuit board  59 . Preferably, a mount mechanism utilizing resilient deformation, such as a snap-fit mechanism, may be used for mounting the circuit board  59  to the holder  52 . In addition, connecting terminals  54   a  of the sensor IC  54  are electrically connected to the circuit board  59  by soldering. Four terminals  60  (only two terminals  60  are shown in  FIG. 1 ) are integrated with the cover  18  through an insertion molding process of the cover  18 . The terminals  60  are electrically connected to the circuit board  59 . The terminals  60  have connecting ends that extend into a connector portion (not shown) formed integrally with the cover  18 . 
   Next, the arrangement of the magnets  47  and  48  will be described in detail. As shown in  FIGS. 5 and 6 , each of the magnets  47  and  48  has an arc-shaped configuration along the inner peripheral surface of the yoke  45 . The magnets  47  and  48  are positioned symmetrically with respect to the rotational axis L of the throttle shaft  9 . The magnets  47  and  48  are magnetized such that the magnetic lines of the magnetic field extend substantially parallel to each other in the vertical direction as viewed in  FIGS. 5 and 6 . In other words, the magnets  47  and  48  produce parallel magnetic lines within the inner space of the yoke  45 . 
   Preferably, the magnets  47  and  48  may be made of ferritic magnet material. The ferritic magnetic material is advantageous for use because the ferritic magnetic material can be more easily formed to have an arc-shaped configuration than in comparison with rare earth magnet material. In general, ferritic magnet material is relatively soft but has a better toughness than rare earth magnet material. In addition, ferritic magnet material can typically be purchased at a lower cost than rare earth magnet material. 
   As shown in  FIG. 6 , each of the magnets  47  and  48  has an outer peripheral surface S 1  and an inner peripheral surface S 2 . Both peripheral surfaces have arc-shaped configurations about the rotational axis L of the throttle shaft  9 . In addition, each of the magnets  47  and  48  has a thickness d in the radial direction about the rotational axis L. The outer peripheral surface S 1  has a radius or curvature that is substantially equal to the radius of curvature of the inner peripheral surface of the yoke  45 . Further each of the magnets  47  and  48  has opposing circumferential end surfaces S 3  that extend along the radial direction about the rotational axis L. 
   Furthermore, as shown in  FIG. 6 , each of the magnets  47  and  48  has a circumferential length defined by an angle θ 1  about the rotational axis L of the throttle shaft  9 . In other words, circumferential edges P of the inner peripheral surface S 2  are spaced from each other by an angle θ 1  about the rotational axis L. The angle θ 1  is chosen to minimize the possible error of the output signals to a predetermined value. The possible error from the sensor IC  54  may be caused due to displacement away from an ideal location of the magnets  47  and  48  in the radial direction, relative to the sensor IC. Thus, by choosing an appropriate angle value of the angle θ 1 , almost all of the magnetic lines (indicated by arrows in  FIG. 7 ) may extend parallel to each other in the magnetic field produced by the magnets  47  and  48 . However, if the angle θ 1  is too small, as shown in  FIG. 8 , magnetic lines Y 1  on both sides of the magnetic field may not extend parallel to central magnetic lines. Resulting in a potentially reduced region of parallel magnetic lines. On the other hand, if the angle θ 1  is too large, as shown in  FIG. 9 , magnetic lines Y 2  on both sides of the magnetic field also may not extend parallel to central magnetic lines. Again resulting in a potentially reduced region of parallel magnetic lines. 
   By choosing an appropriate angle θ 1  such that almost all of the magnetic lines of the magnetic field produced by the magnets  47  and  48  may extend parallel to each other, as shown in  FIG. 7 , the output signals from the sensor IC  54  are consistent across some deviations of the positional relationships between the magnets  47 , and  48 , and the sensor IC  54 . In other words, relatively large amounts of displacement of the sensor IC  54  relative to the magnets  47 , and  48 , is allowed without resulting in a significant error in the readings of the sensor IC  54 . On the other hand, if the angle θ 1  is not appropriately chosen, the region of parallel magnetic lines may be limited to a relatively narrow range. Therefore, if the positional relationship between the magnets  47 , and  48 , and the sensor IC  54 , is offset from ideal to even a small extent, an error may be present in the output signals of the sensor IC  54 . For an inappropriate angle θ 1 , there is a small allowable tolerance in the amount of displacement of the sensor IC  54  relative to the magnets  47 , and  48 . 
     FIG. 10  is a graph showing experimental results of the relationship between a maximum potential error E (°) of the output signals of the sensor IC  54  and the angle θ 1  (°) of the magnets  47  and  48 . The maximum possible error E (°) has been measured by deviating the position of the sensor IC  54  away from an ideal set position shown in  FIG. 6 . The ideal set position is where the sensor IC  54  is centered on the rotational axis L and in the intermediate position of the magnets  47  and  48  located about the rotational axis L. The magnets  47  and  48  used in the experiments are made of ferritic magnetic materials. Each magnet  47  and  48  has an inner radius r (radius of the inner peripheral surface S 2 ) of 10 mm and a thickness d of 3 mm. In an attempt to determine the maximum possible signal error E (°), the sensor IC  54  has been shifted by a distance of 0.75 mm from the ideal set position respectively in an X-direction (left and right directions as viewed in  FIG. 6 ), a Y-direction (vertical direction as viewed in  FIG. 6 ) and a Z-direction (left and right directions as viewed in  FIG. 5 ). The characteristic line A indicates the results of the experiments in  FIG. 10 . 
   According to the characteristic line A shown in  FIG. 10 , if the desired maximum threshold value of possible error E is set to be 2.5°, an appropriate value of the angle θ 1  is within the range of 80° to 130°. If the upper limit of possible error E in the detected rotation angle is set to be 0.4°, an appropriate value of the angle θ 1  is within the range of 95° to 102°. The reverse is also true, by selecting a desired value of the angle θ 1 , the corresponding maximum possible error E can be determined. For example, if the angle θ 1  is set within a range of 95° to 102°, the upper limit of permissible error may be 0.4°. 
   In operation of the representative throttle control device, when the engine is started the control unit, i.e., an ECU, may output control signals to the motor  4  in order to control the degree of rotation of the motor  4 . As described previously, the rotational force of the motor  4  may be transmitted to the throttle valve  2  via the speed reduction mechanism  35 . The throttle valve  2  is subsequently rotated to open or close the intake air channel  1   a  of the throttle body  1 . As a result, the flow rate of the intake air through the intake air channel  1   a  is controlled. In addition, as the throttle shaft  9  rotates, the throttle gear  11  rotates together with the yoke  45  and the magnets  47  and  48  attached thereto. The direction of the magnetic field produced by the magnets  47  and  48  across the sensor IC  54  is altered in relation to the rotation of the magnets  47  and  48 . Therefore, the output signals of the sensor IC  54  may be also be altered. The control unit may receive the output signals from the sensor IC  54 . The control unit may then determine the rotational angle of the throttle shaft  9  based on the output signals. Because the sensor IC  54  detects the change of direction of the magnetic field, the output signals may not be substantially influenced by the displacement of the magnets  47  and  48  due to displacement of the throttle shaft  9  or the displacement of the sensor  55 . In addition, the output signals may not be substantially influenced by a change of strength of the magnetic field due to various temperature characteristics of the magnets  47  and  48 . Here, the displacement of the throttle shaft  9  means the displacement relative to the sensor IC  54 . Such displacement may be caused by various reasons, such as an error in mounting the throttle shaft  9 , differences in thermal expansion coefficients between the throttle body  1  and the cover  18 , vibration of the throttle shaft  9  and/or the bearings  8  and/or  10  due to wear, and thermal expansion of the resin (i.e., throttle gear) that is insert molded containing the magnets  47  and  48 , among other reasons. 
   Therefore, the sensor IC  54  can accurately detect the direction of the magnetic field, improving the accuracy of detection of the degree of opening of the throttle valve  2 . This feature is particularly advantageous if the throttle body  1  is made of a resin that cannot be accurately molded. This feature is also advantageous if the throttle body  1  and the cover  18  are made of different materials from one another, such as the case in which the throttle body  1  is made of metal and the cover  18  is made of resin. 
   In addition, the magnets  47  and  48  are attached to the inner peripheral surface of the ring-like yoke  45 . The yoke  45  is made of magnetic material and is mounted to the throttle gear  11  so as to have the same central axis as the rotational axis L of the throttle shaft  9 . Furthermore, the magnets  47  and  48  are magnetized such that the magnetic lines of the magnetic field produced by the magnets  47  and  48  extend substantially parallel to one another. The magnets  47  and  48 , and the yoke  45 , may form a magnetic circuit such that almost all of the magnetic lines produced by the magnets  47  and  48  extend parallel to each other as shown in  FIG. 7 , further improving the detection accuracy of the sensor IC  54  of the direction of the magnetic field. 
   The angle θ 1  of the magnets  47  and  48  around the rotational axis L is chosen in order to keep the error in the output signals of the sensor IC  54  (due to displacement of the magnets  47  and  48  from their ideal set positions relative to the sensor IC  54 ) below a predetermined value. The detection accuracy of the sensor IC  54  in determining the direction of the magnetic field can also be improved in this respect. 
     FIG. 11  shows alternative configurations of the magnets  47  and  48 , in which each of end surfaces S 3  in the circumferential direction includes a first end surface S 3   a  and a second end surface S 3   b . The first end surface S 3   a  extends in a direction perpendicular to the magnetizing direction of the magnets  47  and  48 . The second end surface S 3   b  extends parallel to the magnetizing direction. An angle θ 2 , defined between both ends (edges) Pa of the inner peripheral surface S 2  about the rotational axis A, is determined in the same manner as the angle θ 1  of the above representative embodiment. 
   According to an alternative configuration of the magnets  47  and  48 , the inner peripheral surface S 2  and the first end surface S 3   a ; intersect at a corner C 1  at an obtuse angle. The outer peripheral surface S 1  and the second end surface S 3   b  intersect at a corner C 2 , also by an obtuse angle. Therefore, potential damage of the corners C 1  and C 2  may be minimized during the machining or forming operation of the magnets  47  and  48  due to the lack of a relatively thinner, more acute corner. In addition, the first and second end surfaces, S 3   a  and S 3   b , may be easily formed by a simple machining operation such as a cutting operation. With this embodiment it is possible to minimize the potential damage of the corners C 1  and C 2  due to possible impacts that may be applied during the assembly operation, for example, when the magnets  47  and  48  are mounted to the yoke  45 . The assembly operation of the magnets  47  and  48  can be more readily facilitated. 
   The present invention may not be limited to the above representative embodiments but may be modified in various ways. For example, although the throttle body  1  is made of resin in the representative embodiment, the throttle body  1  may be made of metal, such as aluminum alloy. Although the throttle valve  2  is preferably made of resin, the throttle valve  2  may be made of metal, such as aluminum alloy and stainless steel. In addition, the magnets  47  and  48  may be made of any magnetic material other than ferritic magnetic materials. Although the detecting section  55  and the computing section  56  of the sensor IC  54  are integrally connected to each other, lead wires, flexible terminals, or printed circuit boards among other known electrical connection techniques, may connect them. Furthermore, the sensor IC  54  may be replaced with any other detection device as long as such a detection device can detect the direction of the magnetic field formed between the magnets  47  and  48 .