Abstract:
A position detecting device is provided, which is configured to minimize leakage of magnetic flux in an electromagnetic induction system. The position detecting device includes: a sensor unit including a plurality of first loop coils arranged in a first direction and a plurality of second loop coils arranged in a second direction intersecting with the first direction; a yoke sheet provided on a side of the sensor unit that is opposite to a side that faces a position indicator; an auxiliary loop coil provided at a corner part of the sensor unit; a signal transmitter configured to transmit a signal to one of the coils in order to generate a magnetic field to induce an induced current in a coil of the position indicator; and a controller configured to select one of the coils, and to control whether to transmit a signal from the signal transmitter to the selected one of the coils or to make the selected one of the coils receive a signal from the position indicator.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims priority under 35 U.S.C. §119(a) of Japanese Application No. 2010-069427, filed Mar. 25, 2010, the entire content of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to a position detecting device of an electromagnetic induction system and a position input device including this position detecting device. 
         [0004]    2. Description of Related Art 
         [0005]    As disclosed in e.g. Japanese Patent Laid-Open No. 2006-309308, a position input device composed of a pen-type position indicator and a position detecting device is known. The position detecting device includes a sensor unit for detecting the position indicated by the position indicator. 
         [0006]    This sensor unit generally includes a sensor substrate of a flat plate shape and an indication detecting plane to detect the input position indicated by the position indicator. In the case of a position detecting device of an electromagnetic induction system, on the sensor substrate, a large number of long thin loop coils are so provided as to be arranged in each of the X-axis direction and the Y-axis direction. The position indicator includes a resonant circuit composed of a coil and a capacitor. 
         [0007]    Furthermore, the position detecting device makes a current of a specific frequency (i.e., a transmission current for excitation) flow through the respective loop coils of the sensor unit, one coil by one coil, to generate a magnetic field from the loop coils. When the position indicator is close to the loop coil generating the magnetic field, the resonant circuit of the position indicator resonates due to electromagnetic induction to generate an induced magnetic field. Next, the generation of the magnetic field from the loop coil is stopped. Subsequently, the induced magnetic field generated from the resonant circuit of the position indicator is received by the loop coil and a signal current (i.e., a reception current) flowing through the loop coil is detected. The position detecting device carries out this operation for each one of the loop coils and detects the position of the position indicator based on the reception current. 
         [0008]    Directly beneath the sensor substrate, on which a large number of loop coils are formed, a magnetic path sheet (i.e., a yoke sheet) to stabilize the magnetic characteristics of the loop coil is provided. If a conductor plate such as an earth substrate is provided directly beneath the sensor substrate, on which a large number of loop coils are formed, an eddy current flows through the conductor plate due to the influence of an alternating magnetic field from the loop coil, and this eddy current adversely affects the operation of the device. Therefore, the magnetic path sheet (the yoke sheet) is so configured as to work also as a shield member (refer to, e.g., Japanese Patent Laid-Open No. 2007-157107). 
       SUMMARY OF THE INVENTION 
       [0009]    When the transmission current for excitation is made to flow through the loop coil at the peripheral part of the sensor unit (at the vicinity part of the end part (end edge) of the sensor substrate), the amount of leakage of magnetic flux to the external is larger compared with the loop coil at the center part of the sensor unit (at the part surrounded by the peripheral part), and possibly the magnetic flux leaks to the external as unnecessary radiation. 
         [0010]    Specifically, as shown in  FIGS. 16A and 16B , a yoke sheet  200  is disposed directly beneath the sensor unit on which plural loop coils  100  are formed. As shown in  FIG. 16A , magnetic flux  301 , which is generated when the transmission current for excitation flows through the loop coil  100  at the center part inside the end part of the sensor unit among the plural loop coils  100 , passes through the yoke sheet  200  and hardly leaks to the external. 
         [0011]    In contrast, as shown in  FIG. 16B , magnetic flux  302 , which is generated when the transmission current for excitation flows through the loop coil  100  at the peripheral part of the sensor unit, leaks from the end part of the yoke sheet  200  to the external. Furthermore, because the peripheral part of the sensor unit involves many factors interfering with transmission/reception to/from the position indicator, a larger current is made to flow through the loop coil  100  at the peripheral part compared with the other part. This also causes the large leakage of magnetic flux at the peripheral part of the sensor unit. 
         [0012]    There is a need to provide a position detecting device and a position input device capable of reducing leakage of magnetic flux. 
         [0013]    According to an aspect of this invention, there is provided a position detecting device including a sensor unit including a plurality of first loop coils that are so provided as to be arranged in a first direction and a plurality of second loop coils that are so provided as to be arranged in a second direction intersecting with the first direction, a yoke sheet provided on the opposite side to a position indicator across the sensor unit, and an auxiliary loop coil provided at a corner part of the sensor unit. The position detecting device further includes a signal transmitter configured to transmit a signal to one of the first loop coil, the second loop coil, and the auxiliary loop coil in order to generate a magnetic field to induce an induced current in a coil of the position indicator, and a controller configured to select one of the first loop coil, the second loop coil, and the auxiliary loop coil, and to control whether to transmit a signal from the signal transmitter to one of the first loop coil, the second loop coil, and the auxiliary loop coil or to make one of the first loop coil, the second loop coil, and the auxiliary loop coil receive a signal from the position indicator. 
         [0014]    According to exemplary embodiments of this invention with the above-described configuration, due to the provision of the auxiliary loop coil for transmission at the corner of the sensor unit, the first loop coils at the peripheral parts of the sensor unit that extend perpendicular to the first direction and the second loop coils at the peripheral parts of the sensor unit that extend perpendicular to the second direction can be used only as the coils for reception to detect a signal dependent on the magnetic field from the position indicator by electromagnetic induction, without being used as the coils for transmission. Therefore, leakage of magnetic flux from the end part of the sensor unit can be reduced. 
         [0015]    According to exemplary embodiments of this invention, the provision of the auxiliary loop coil for transmission at the corner of the sensor unit allows for an operation in which the first loop coils are not used as the coils for transmission at the peripheral parts that extend perpendicular to the first direction and the second loop coils are not used as the coils for transmission at the peripheral parts that extend perpendicular to the second direction. Thus, leakage of magnetic flux from the end part of the sensor unit can be reduced. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a diagram showing the appearance of a position input device of an embodiment of this invention; 
           [0017]      FIG. 2  is an exploded perspective view for explaining a configuration example of a position detecting device of the embodiment of this invention; 
           [0018]      FIG. 3  is a diagram for explaining a configuration example of a yoke sheet in the position detecting device of  FIG. 2  in the embodiment; 
           [0019]      FIG. 4  is a diagram for explaining a configuration example of a sensor unit in the position detecting device of  FIG. 2  in the embodiment; 
           [0020]      FIG. 5  is a diagram for explaining a configuration example of the sensor unit in the position detecting device of  FIG. 2  in the embodiment; 
           [0021]      FIG. 6  is a diagram showing the unnecessary radiation characteristic of leaking magnetic flux by a loop coil at the peripheral part of the sensor unit; 
           [0022]      FIG. 7  is a diagram for explaining the provision position of an auxiliary coil used in the position detecting device of the embodiment of this invention; 
           [0023]      FIG. 8  is a diagram for explaining the shape and size of the auxiliary coil used in the position detecting device of the embodiment of this invention; 
           [0024]      FIGS. 9A and 9B  are diagrams showing examples of the shape and size of the auxiliary coil used in the position detecting device of some embodiments of this invention; 
           [0025]      FIG. 10  is a diagram for explaining setting of transmission and reception regarding the loop coils in the position detecting device of the embodiment of this invention; 
           [0026]      FIG. 11  is a block diagram showing a configuration example of a signal processor in the position detecting device of the embodiment of this invention; 
           [0027]      FIG. 12  is a flowchart for explaining one example of the flow of processing of detecting the position indicated by a position indicator in the position detecting device of the embodiment of this invention; 
           [0028]      FIG. 13  is a diagram for explaining another configuration example of the sensor unit used in the position detecting device of the embodiment of this invention; 
           [0029]      FIGS. 14A and 14B  are diagrams for explaining another configuration example of the sensor unit used in the position detecting device of an embodiment of this invention; 
           [0030]      FIG. 15  is a diagram for explaining another configuration example of the sensor unit used in the position detecting device of an embodiment of this invention; and 
           [0031]      FIGS. 16A and 16B  are diagrams for explaining unnecessary radiation of leaking magnetic flux from a position detecting device. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0032]    A position detecting device and a position input device according to an embodiment of this invention will be described below with reference to the drawings. 
       [Position Input Device] 
       [0033]      FIG. 1  is an appearance perspective view of the position input device according to the embodiment of this invention. A position input device  10  of this embodiment is composed of a pen-type position indicator  1  and a position detecting device  2 . The position detecting device  2  includes a position detecting plane  2   a  and detects the position indicated by the position indicator  1  on this position detecting plane  2   a  to output the detected position data to an external apparatus such as a personal computer. In the following description, an X-Y rectangular coordinate system is assumed to be defined by the horizontal direction of this position detecting plane  2   a  as the X axis and the vertical direction as the Y axis, and it is assumed that the position indicated by the position indicator  1  is detected as X and Y coordinates. 
       [Position Detecting Device] 
       [0034]      FIG. 2  is an exploded perspective view schematically showing the configuration of the position detecting device  2 . The position detecting device  2  of this example has a structure in which a sensor unit  23 , a yoke sheet  24 , and a signal processor  25  are housed in a housing composed of an upper case  21  and a lower case  22 . 
         [0035]    The upper case  21  serves as the upper-side part of a housing  20  of the position detecting device  2 . This upper case  21  includes the position detecting plane  2   a  formed of an insulating plate of, for example, plastic or glass. 
         [0036]    The sensor unit  23  is disposed directly beneath the position detecting plane  2   a  of this upper case  21 . This sensor unit  23  is formed of a rectangular printed wiring board (hereinafter referred to as the sensor substrate) on which plural loop coils are formed as a wiring pattern as described later. 
         [0037]    The yoke sheet  24  is disposed directly beneath the sensor unit  23 . This yoke sheet  24  has substantially the same size as that of the sensor unit  23 . As shown in a sectional view of  FIG. 3 , this yoke sheet  24  is obtained by forming a high magnetic layer  242  of, for example, an amorphous magnetic metal on a shield sheet  241  formed of, for example, an aluminum foil. The high magnetic layer  242  is formed by depositing, for example, a large number of amorphous magnetic metal ribbons on the shield sheet  241  of the aluminum foil. 
         [0038]    The signal processor  25  is connected to the sensor unit  23 . As described later, this signal processor  25  includes a signal generator to generate an AC (alternate current) signal of a predetermined frequency to be supplied to the plural loop coils of the sensor unit  23 , and a position detector to detect a signal current arising in the loop coil due to electromagnetic induction with the position indicator  1  to thereby detect the position indicated by the position indicator  1 . 
         [0039]    This signal processor  25  outputs position data of the position indicator  1  to the external apparatus such as a personal computer via, for example, a USB interface cable  26 . 
         [0040]    As shown in  FIG. 2 , the components except the upper case  21  formed of an insulator, i.e. all of the sensor unit  23 , the yoke sheet  24 , and the lower case  22  are grounded. 
       [Sensor Unit  23  of Position Detecting Device  2 ] 
       [0041]    The sensor substrate included in the sensor unit  23  is formed of a two-layer printed wiring board in this example.  FIG. 4  shows part of a sectional view of this sensor substrate  230  along the X-axis direction. The sensor substrate  230  is obtained by stacking a first layer substrate  231  and a second layer substrate  232  in the Z-axis direction perpendicular to the X axis and the Y axis. 
         [0042]    On the surface of the first layer substrate  231  on the side of the upper case  21 , plural first loop coils  233  are so provided as to be arranged in the X-axis direction at every predetermined interval. Hereinafter, the first loop coil will be referred to as the X coil. 
         [0043]    On the surface of the second layer substrate  232  on the side of the lower case  22 , plural second loop coils  234  are so provided as to be arranged in the Y-axis direction at every predetermined interval. Hereinafter, the second loop coil will be referred to as the Y coil. 
         [0044]    Furthermore, plural auxiliary loop coils  235  are provided between the opposing surfaces of the first layer substrate  231  and the second layer substrate  232 . Hereinafter, the auxiliary loop coil will be abbreviated as the auxiliary coil. 
         [0045]      FIG. 5  is a diagram showing the arrangement state of the X coils  233 , the Y coils  234 , and the auxiliary coils  235  provided on the sensor substrate  230 . 
         [0046]    As shown in  FIG. 5 , in the X-axis direction, plural X coils  233 - i  (i=1, 2, 3 . . . N (N is an integer equal to or larger than two)) are arranged. The X coils  233 - i  are each a one-turn loop coil extended along the Y-axis direction and are so arranged as to be in parallel to each other and overlap with each other. Each X coil  233 - i  includes a longer side  233   a  along the Y-axis direction and an end part  233   b  as the loop bent part. The length of the longer side  233   a  is set almost equal to the distance between both ends of the sensor substrate  230  in the Y-axis direction. That is, the longer side  233   a  of the X coil  233 - i  is extended to both end parts of the sensor substrate  230  in the Y-axis direction. 
         [0047]    Although loop coils with shapes having the longer sides  233   a  and  234   a  and the end parts  233   b  and  234   b  are employed as the X coil  233  and the Y coil  234 , loop coils having, for example, any rectangular shape, a square shape, an oblong shape or another shape may be employed as well. 
         [0048]    In the Y-axis direction, plural Y coils  234 - j  (j=1, 2, 3 . . . M (M is an integer equal to or larger than two)) are arranged. The Y coils  234 - j  are each a one-turn loop coil extended along the X-axis direction and are so arranged as to be in parallel to each other and overlap with each other. Each Y coil  234 - j  includes the longer side  234   a  along the X-axis direction and the end part  234   b  as the loop bent part. The length of the longer side  234   a  is set almost equal to the distance between both end edges of the sensor substrate  230  in the X-axis direction. That is, the longer side  234   a  of the Y coil  234 - j  is extended to both end parts of the sensor substrate  230  in the X-axis direction. 
         [0049]    In the whole range of the area in which the plural X coils  233  and the plural Y coils  234  are provided in the above-described manner, a signal transmitted from the position indicator  1  can be received by the sensor unit  23  as described later. That is, the whole of the area in which the X coils  233  and the Y coils  234  are provided can be made to function as the effective area capable of detecting the position of the position indicator  1 . The position detecting plane  2   a  of the position detecting device  2  is so configured as to overlap with this effective area. 
         [0050]    The X coils  233  and the Y coils  234  are formed from a printed pattern of one or plural layers on the first layer substrate  231  and the second layer substrate  232  of the sensor substrate  230 . The X coils  233  and the Y coils  234  are not limited to coils configured with one turn and may have a plural-turn configuration as need. 
         [0051]    At four corners of the sensor substrate  230 , auxiliary coils  235 - 1  to  235 - 4  are provided. Specifically, at four corners of the whole area of the rectangular shape in which the X coils  233  and the Y coils  234  are provided, four auxiliary coils  235 - 1  to  235 - 4  are so provided as to overlap with the X coils  233  and the Y coils  234 . 
       [Avoidance of Problem of Leakage of Magnetic Flux and Auxiliary Coil  235 ] 
       [0052]    The reason for the provision of the auxiliary coils  235 - 1  to  235 - 4  will be described below. 
         [0053]    The X coil  233  is extended along the Y-axis direction and includes the longer side  233   a  along the Y-axis direction and the end part  233   b  along the X-axis direction. When the transmission current for excitation is made to flow through this X coil  233 , magnetic flux interlinked with this X coil  233  is generated. The yoke sheet  24  is provided directly beneath the sensor substrate  230 . Therefore, when the transmission current for excitation is made to flow through the X coil  233  located at one of both peripheral parts of the sensor substrate  230  in the X direction, magnetic flux interlinked with the longer side  233   a  of the X coil  233  leaks to the external from the end part of the yoke sheet  24  in the X direction as described with use of  FIGS. 16A and 16B  and this leakage of magnetic flux becomes a problem. At this time, magnetic flux interlinked with the end part  233   b  of the X coil  233  also leaks to the external from the peripheral end part of the yoke sheet  24  in the Y direction. However, because the end part  233   b  is shorter than the longer side  233   a , this leakage of magnetic flux is less and causes no problem. 
         [0054]    Similarly, the Y coil  234  is extended along the X-axis direction and includes the longer side  234   a  along the X-axis direction and the end part  234   b  along the Y-axis direction. When the transmission current for excitation is made to flow through this Y coil  234 , magnetic flux interlinked with this Y coil  234  is generated. Magnetic flux interlinked with the longer side  234   a  of the Y coil  234  leaks to the external from the peripheral end part of the yoke sheet  24  in the Y direction and this leakage of magnetic flux becomes a problem. At this time, magnetic flux interlinked with the end part  234   b  of the Y coil  234  also leaks to the external from the end part of the yoke sheet  24  in the X direction. However, because the end part  234   b  is shorter than the longer side  234   a , this leakage of magnetic flux is less and causes no problem. 
         [0055]    For example, the transmission current for excitation was made to flow through the Y coil  234  and leakage magnetic flux was measured. As a result, the result shown in  FIG. 6  was obtained. This example of  FIG. 6  shows the characteristic obtained when a 60-mA alternating current with a frequency of 667 kHz was made to flow through the Y coil  234  as the transmission current for excitation. The abscissa indicates the distance from the end part of the sensor substrate  230  in the Y direction to the Y coil  234 , and the ordinate indicates the unnecessary radiation level. 
         [0056]    From  FIG. 6 , it is discerned that no problem is caused as long as the Y coil  234  is distant from the end part of the sensor substrate  230  in the Y direction by about 1 cm or longer if the threshold of the allowable unnecessary radiation level is 15 dB. 
         [0057]    From the above-described point, to avoid the problem of leakage of magnetic flux, it is sufficient that, as shown in  FIG. 7 , the X coil  233  to which the transmission current for excitation is supplied is distant from each of both ends of the sensor substrate  230  in the X direction by a predetermined distance dx or longer. Furthermore, it is sufficient that the Y coil  234  to which the transmission current for excitation is supplied is distant from each of both ends of the sensor substrate  230  in the Y direction by a predetermined distance dy or longer. The predetermined distances dx and dy are each the distance, from the end part of the sensor substrate  230 , at which the unnecessary radiation level of leaking magnetic flux is equal to or lower than a threshold. 
         [0058]    In the following description, of the whole area of the sensor substrate  230 , the areas that are within the predetermined distance dy from both ends in the Y direction and are outside the areas within the predetermined distance dx from both ends in the X direction will be referred to as the area A as shown in  FIG. 7 . 
         [0059]    Furthermore, of the whole area of the sensor substrate  230 , the areas that are within the predetermined distance dx from both ends in the X direction and are outside the areas within the predetermined distance dy from both ends in the Y direction will be referred to as the area B. 
         [0060]    In addition, the areas that are at four corners of the sensor substrate  230  and are within the predetermined distance dx from both ends in the X direction and within the predetermined distance dy from both ends in the Y direction will be referred to as the area D. Moreover, of the whole area of the sensor substrate  230 , the center area outside the areas A, the areas B, and the areas D will be referred to as the area C. 
         [0061]    As one countermeasure to solve the problem of the unnecessary radiation level of leaking magnetic flux, it will be effective to employ the following configuration. Specifically, the size of the yoke sheet  24  is set to the size of the whole area shown in  FIG. 7 . In addition, the size of the sensor substrate  230  is set to the size of the area C shown in  FIG. 7 , and the X coils  233  and the Y coils  234  are disposed only in this area C. If this configuration is employed, the X coils  233  and the Y coils  234  of the sensor substrate  230  exist only at positions distant from the end parts of the yoke sheet  24  by at least the distance dx and the distance dy, so that the problem of leaking magnetic flux is eliminated. 
         [0062]    However, in this case, the size of the position detecting plane  2   a , whose size is substantially equal to that of the sensor substrate  230 , is substantially equal to that of to the area C. In contrast, the size of the yoke sheet  24  needs to be set to a size including the areas A, the areas B, and the areas D around the area C. This causes a problem that the housing  20  of the position detecting device  2  needs to include a large part as the part surrounding the position detecting plane  2   a  and thus its size inevitably becomes large compared with the size of the position detecting plane  2   a.    
         [0063]    To avoid this problem, in this embodiment, the sizes of the sensor substrate  230  and the yoke sheet  24  are set almost equal to each other as described above. Furthermore, as described with use of  FIG. 5 , the X coils  233  and the Y coils  234  are provided across the area of the sensor substrate  230  to both ends in the X direction and both ends in the Y direction similarly to the related art. 
         [0064]    However, in this embodiment, to avoid the problem of unnecessary radiation of leaking magnetic flux, the transmission current for excitation is not supplied to the X coils  233  existing in the areas within the distance dx from both end parts of the sensor substrate  230  in the X direction (the areas B and the areas D vertically adjacent to the areas B). Furthermore, the transmission current for excitation is not supplied to the Y coils  234  existing in the areas within the distance dy from both end parts of the sensor substrate  230  in the Y direction (the areas A and the areas D horizontally adjacent to the areas A). 
         [0065]    As described later, in this embodiment, the X coil  233  and the Y coil  234  to which the transmission current for excitation is not supplied are used as the loop coil to detect the reception current. 
         [0066]    As just described, in this embodiment, the transmission current for excitation is not supplied to the X coils  233  at both end parts in the X direction and the Y coils  234  at both end parts in the Y direction. Even though such a configuration is employed, because the longer side  233   a  of the X coil  233  is formed across the whole length of the sensor substrate  230  along the Y direction, a magnetic field can be generated also in the area A with the same intensity as that in the area C by making the transmission current for excitation flow through the X coil  233 . Similarly, because the longer side  234   a  of the Y coil  234  is formed across the whole length of the sensor substrate  230  along the X direction, a magnetic field can be generated also in the area B with the same intensity as that in the area C by making the transmission current for excitation flow through the Y coil  234 . 
         [0067]    However, the transmission current for excitation is supplied to neither the X coil  233  nor the Y coil  234  in the areas D at four corners in  FIG. 7 , and thus the magnetic field in these areas D is weak. 
         [0068]    So, in this embodiment, the auxiliary coil  235  is provided in order to generate a magnetic field with intensity that is high enough to detect the position indicated by the position indicator  1  in the area D. Specifically, four auxiliary coils  235 - 1  to  235 - 4  shown in  FIG. 5  are provided corresponding to the areas D at four corners in  FIG. 7 , respectively. In the example shown in  FIG. 5 , each of the auxiliary coils  235 - 1  to  235 - 4  is a rectangular loop coil surrounding the area D. 
       [Size and Shape of Auxiliary Coil  235 ] 
       [0069]    The amount of unnecessary radiation of leaking magnetic flux from the sensor substrate  230  due to a loop coil depends on the length of the closest part of this loop coil to the end part of the sensor substrate  230 . The end parts  233   b  and  234   b  of the X coil  233  and the Y coil  234  are shorter than the longer sides  233   a  and  234   a  of the X coil  233  and the Y coil  234 . Therefore, the amount of unnecessary radiation of leaking magnetic flux by the X coil  233  and the Y coil  234  depends on the longer sides  233   a  and  234   a  of them. 
         [0070]    On the other hand, the auxiliary coil  235  of this example is provided at four corners of the sensor substrate  230  and therefore two sides of the auxiliary coil  235  are the closest to the X-direction end part and the Y-direction end part of the sensor substrate  230 . Consequently, the amount of unnecessary radiation of leaking magnetic flux by the auxiliary coil  235  depends on the sum of the lengths of two sides of the auxiliary coil  235  closest to the X-direction end part and the Y-direction end part of the sensor substrate  230 . 
         [0071]    The amount of unnecessary radiation of leaking magnetic flux by the auxiliary coil  235  needs to be smaller than that of unnecessary radiation of leaking magnetic flux by the loop coil whose longer side ( 233   a ,  234   a ) is shorter, of the X coil  233  and the Y coil  234 . 
         [0072]    In this example, the longer side of the X coil  233  is shorter than that of the Y coil  234 . Therefore, the sum of the lengths of two sides of the auxiliary coil  235  closest to the X-direction end part and the Y-direction end part of the sensor substrate  230  needs to be smaller than the length of the longer side  233   a  of the X coil  233 . 
         [0073]    Specifically, as shown in  FIG. 8 , when the length of the longer side  233   a  of the X coil  233  is defined as La and the lengths of two sides of the auxiliary coil  235  closest to the X-direction end part and the Y-direction end part of the sensor substrate  230  are defined as Lb and Lc, respectively, the size and shape of the auxiliary coil  235  are so decided as to satisfy at least the following expression. 
         [0000]        Lb+Lc&lt;La   (Equation 1)
 
         [0074]    The shape of the respective auxiliary coils  235 - 1  to  235 - 4  may be any as long as they can generate magnetic flux that allows supply of an induced magnetic field with proper intensity to the position indicator  1  above the area D. Thus, the shape is not limited to a rectangular shape like that shown in this example and may be a polygonal shape or a circular shape. 
         [0075]    For example, as shown in  FIG. 9A , each of the auxiliary coils  235 - 1  to  235 - 4  at four corners may have a circular shape. If the loop coil has a circular shape, the length of the closest part of the auxiliary coil  235  to the X-direction end part and the Y-direction end part of the sensor substrate  230  is small as shown in  FIG. 9A . 
         [0076]    Alternatively, as shown in  FIG. 9B , each of the auxiliary coils  235 - 1  to  235 - 4  at four corners may have a triangular shape. 
         [0077]    It is enough for the auxiliary coil  235  to be capable of generating magnetic flux that allows supply of a sufficient induced magnetic field to the position indicator  1  that indicates a position above the area D. Therefore, the size of this auxiliary coil may be larger than that of the area D or may be smaller. 
         [0078]    Although all of the auxiliary coils  235  are a one-turn loop coil in the above-described examples, it is obvious that they may be a plural-turn loop coil. 
       [Setting of Transmission and Reception of Loop Coil in Respective Areas of Sensor Substrate  230 ] 
       [0079]    In the position detecting device of this embodiment, for each of the area A, the area B, the area C, and the area D shown in  FIG. 7 , the loop coil to which the transmission current for excitation is supplied (i.e., a transmitting coil) is set and the loop coil that receives the reception current generated due to electromagnetic induction coupling with the position indicator  1  (i.e., a receiving coil) is set. 
         [0080]      FIG. 10  shows an example of the transmitting coil and the receiving coil set for each of the area A, the area B, the area C, and the area D. In this case, the receiving coil is the same for all of the area A, the area B, the area C, and the area D. Specifically, the X coil  233  is used as the coil for detecting the X coordinate and the Y coil  234  is used as the coil for detecting the Y coordinate. 
         [0081]    However, the transmitting coil differs for each of the area A, the area B, the area C, and the area D. Specifically, for the area A, the plural X coils  233  existing in the area distant from both ends of the sensor substrate  230  in the X direction by the distance dx or longer can be used as the transmitting coils for detecting the X coordinate and the Y coordinate because the longer side  233   a  of the X coil  233  is extended to the areas A. 
         [0082]    For the area B, the plural Y coils  234  distant from both ends of the sensor substrate  230  in the Y direction by the distance dy or longer can be used as the transmitting coils for detecting the X coordinate and the Y coordinate because the longer side  234   a  of the Y coil  234  is extended to the areas B. 
         [0083]    For the area C, both of the X coil  233  and the Y coil  234  can be used as the transmitting coils because the problem of unnecessary radiation of leaking magnetic flux does not occur in the area C. Therefore, for the area C, either one of the X coil  233  and the Y coil  234  can be used as each of the transmitting coils for detecting the X coordinate and the Y coordinate. 
         [0084]    For the area D, neither the X coil  233  nor the Y coil  234  is used as the transmitting coil because both coils cause unnecessary radiation of leaking magnetic flux. Instead, the auxiliary coil  235  is used as the transmitting coils for detecting the X coordinate and the Y coordinate for this area D. 
       [Configuration Example of Signal Processor  25 ] 
       [0085]      FIG. 11  is a diagram showing a configuration example of the signal processor  25 . In  FIG. 11 , the connection relationship between the signal processor  25  and the sensor unit  23  is also shown. However, in  FIG. 11 , the X coils  233  and the Y coils  234  of the sensor unit  23  are schematically represented as straight lines and the auxiliary coils  235  are represented as rectangles. As shown in  FIG. 11 , the signal processor  25  is connected to the X coils  233 , the Y coils  234 , and the auxiliary coils  235  included in the sensor unit  23 . For convenience of understanding, the position indicator  1  is also shown in  FIG. 11 . 
         [0086]    The position indicator  1  has a resonant circuit  103  including a coil  101  and a capacitor  102 , and an integrated circuit (IC)  104 . The resonant circuit  103  is connected to the IC  104 . The position indicator may have a simple structure that is not controlled by the IC. 
         [0087]    The signal processor  25  includes a control circuit  301 , a selecting circuit  302 , a transmission/reception switch circuit  303 , an amplifier circuit  304 , a band-pass filter  305 , a detection circuit  306 , a sample hold circuit  307 , an analog-to-digital (A/D) conversion circuit  308 , a signal generating circuit  309 , and an amplifier circuit  310 . 
         [0088]    The control circuit  301  includes, for example, a microcomputer and executes control processing for the respective necessary units in this signal processor  25 . 
         [0089]    The selecting circuit  302  is connected to each of the plural loop coils composed of N X coils  233 , M Y coils  234 , and four auxiliary coils  235  included in the sensor unit  23 , and selects one loop coil from these plural loop coils in accordance with a control command from the control circuit  301 . 
         [0090]    The transmission/reception switch circuit  303  switches the operation mode between the transmission mode in which a signal is sent out to the loop coil selected by the selecting circuit  302  and the reception mode in which a signal from the position indicator  1  is received by the loop coil selected by the selecting circuit  302 , in accordance with a control command from the control circuit  301 . 
         [0091]    The amplifier circuit  304  amplifies a signal output from the transmission/reception switch circuit  303  and outputs the amplified signal to the band-pass filter  305 . For the signal amplified by the amplifier circuit  304 , the band-pass filter  305  allows the passage of only the signal component in a predetermined frequency band and outputs the signal component to the detection circuit  306 . The detection circuit  306  converts the signal component that has passed through the band-pass filter  305  to a voltage value and outputs the voltage value to the sample hold circuit  307 . 
         [0092]    The sample hold circuit  307  holds the voltage value from the detection circuit  306  at a predetermined timing, specifically at a predetermined timing in the reception mode, based on a clock signal for sample hold from the control circuit  301 , and sends out the voltage value to the A/D conversion circuit  308 . The A/D conversion circuit  308  converts the analog output of the sample hold circuit  307  to a digital signal and outputs the digital signal to the control circuit  301 . 
         [0093]    The signal generating circuit  309  generates an AC signal of a predetermined frequency and supplies the AC signal to the amplifier circuit  310  in accordance with control by the control circuit  301 . The amplifier circuit  310  amplifies the AC signal from the signal generating circuit  309  and converts the AC signal to a current to send out the current to the transmission/reception switch circuit  303 . The transmission/reception switch circuit  303  supplies this current from the amplifier circuit  310  to the loop coil selected by the selecting circuit  302 . 
         [0094]    The signal processor  25  detects position input operation by the position indicator  1  in the following manner. 
         [0095]    First, the control circuit  301  controls the selecting circuit  302  to make it select one loop coil. In addition, the control circuit  301  controls the transmission/reception switch circuit  303  to make it switch the operation mode to the transmission mode. 
         [0096]    Subsequently, the control circuit  301  controls the signal generating circuit  309  to make it generate an AC signal of a predetermined frequency. This AC signal is amplified by the amplifier circuit  310  and input to the selecting circuit  302  via the transmission/reception switch circuit  303 . In this manner, the current dependent on this AC signal flows through the loop coil selected by the selecting circuit  302  and a magnetic field (alternating magnetic field) interlinked with this loop coil is generated. 
         [0097]    In the position indicator  1  located near the sensor unit  23 , an induced current is induced in its resonant circuit  103  due to the magnetic field generated by this loop coil, and the IC  104  starts operation based on this induced current. The IC  104  makes the resonant circuit  103  generate a signal of a predetermined frequency and transmits this signal of the predetermined frequency from the resonant circuit  103  to the sensor unit  23 . 
         [0098]    After continuing the operation of the above-described transmission mode for a predetermined time, the control circuit  301  controls the transmission/reception switch circuit  303  to switch the operation mode to the reception mode. This switching stops the input of the signal from the signal generating circuit  309  to the selecting circuit  302 . In the case of changing the loop coil selected in the reception mode to a loop coil different from the loop coil selected in the immediately-previous transmission mode, the control circuit  301  sends a command of the change control to the selecting circuit  302  to control the selecting circuit  302  so that the selected loop coil may be changed. If the same loop coil as that selected in the immediately-previous transmission mode is used in the reception mode, the control circuit  301  does not carry out this change control of the loop coil. 
         [0099]    In this reception mode, a signal transmitted from the position indicator  1  by the operation of the IC  104  is received by the loop coil selected by the selecting circuit  302 . Specifically, a signal current flows through the resonant circuit  103  in the position indicator  1  and whereby a magnetic field is generated around this resonant circuit  103 . Due to this magnetic field, an induced current flows through the loop coil near the position indicator  1  among the plural loop coils of the sensor unit  23 . The induced current flowing through the loop coil selected by the selecting circuit  302  is supplied to the amplifier circuit  304  via the transmission/reception switch circuit  303  and amplified therein. 
         [0100]    The output signal from this amplifier circuit  304  is supplied to the band-pass filter  305  and subjected to band limiting, and a component in a predetermined frequency band from this band-pass filter  305  is input to the detection circuit  306 . This component is converted to a voltage value by the detection circuit  306  and held by the sample hold circuit  307 . The voltage value held by the sample hold circuit  307  is converted to digital data by the A/D conversion circuit  308  and output to the control circuit  301 . 
         [0101]    The control circuit  301  temporarily stores the digital data from the A/D conversion circuit in a buffer memory. Subsequently, the control circuit  301  changes the receiving coil by selection control of the loop coil in the selecting circuit  302 , and captures the digital data input from the A/D conversion circuit  308  in the reception mode with this receiving coil to temporarily store the digital data in the buffer memory. 
         [0102]    Subsequently, the control circuit  301  executes calculation processing of the digital data about the respective loop coils, temporarily stored in the buffer memory, to thereby specify the loop coil close to the position indicated and input by the position indicator  1  and obtain the coordinates of the position indicated by the position indicator  1 . 
         [0103]    The position indicator  1  includes a core (rod) for sensing the writing pressure and has a mechanism to vary the inductance of the coil  101  or the capacitance of the capacitor  102  depending on the writing pressure applied to this core (rod), although not illustrated in  FIG. 11  (see, for example, Japanese Patent Laid-Open No. Hei 4-96212). In the position indicator  1 , the resonant frequency of the resonant circuit  103  changes in linkage with change in the writing pressure. 
         [0104]    The control circuit  301  detects this change in the resonant frequency (phase change) by executing calculation processing of the digital data from the A/D conversion circuit  308 , and senses also the writing pressure applied to the position indicator  1 . 
       [Selection Control of Loop Coil] 
       [0105]    A description will be made below about one example of the selection control of the loop coil by the control circuit  301  until detection of the position indicated by the position indicator  1 . In the example described below, first, the control circuit  301  sequentially searches the X coils or the Y coils skippingly and carries out rough detection of the indicated position (hereinafter referred to as a global scan). Subsequently, the control circuit  301  sequentially searches, one coil by one coil, the X coils or the Y coils in the vicinity area of the loop coil from which a significant reception result is obtained in the global scan and carries out refined detection of the indicated position (hereinafter referred to as a sector scan). Furthermore, in this embodiment, position detection in the areas at four corners by use of the auxiliary coil  235  as the transmitting coil is carried out. Also for these areas at four corners, the control circuit  301  carries out rough detection of the indicated position (hereinafter, referred to as a rough corner scan) and refined detection of the indicated position (hereinafter referred to as a refined corner scan). 
         [0106]      FIG. 12  is a flowchart showing one example of the flow of processing of detecting the indicated position by the control circuit  301 . 
         [0107]    In this example, first, about the area composed of the areas A (upper and lower areas A) at both peripheral parts of the sensor substrate  230  in the Y direction and the area C, the control circuit  301  carries out a global scan by using the X coils  233  existing in this area as the transmitting/receiving loop coil (step S 1 ). 
         [0108]    Specifically, the control circuit  301  makes the selecting circuit  302  select the X coil  233 , e.g. the X coil  233 - k , at a position distant from one end part of the sensor substrate  230  in the X direction, for example, the left end, by the distance dx or longer as the first loop coil in the global scan of this step S 1 . k is an integer equal to or larger than {(the number of X coils  233  within the distance dx from the end part of the sensor substrate  230  in the X direction)+1}. k′ to be described later is also a similar integer. Furthermore, the control circuit  301  switches the transmission/reception switch circuit  303  to the transmission mode and makes the signal current from the signal generating circuit  309  flow through this X coil  233 - k.    
         [0109]    Next, upon the elapse of a predetermined time, the control circuit  301  switches the transmission/reception switch circuit  303  to the reception mode. In this reception mode, the control circuit  301  captures, by the sample hold circuit  307 , the induced current flowing through the X coil  233 - k  depending on the magnetic field from the position indicator  1 , and receives the digital data thereof from the A/D conversion circuit  308 . Subsequently, the control circuit  301  temporarily stores the captured digital data in the buffer memory as the reception result about the X coil  233 - k  in this reception mode. 
         [0110]    After the transmission and reception modes about the X coil  233 - k  are completed in the above-described manner, the control circuit  301  controls the selecting circuit  302  so that the selecting circuit  302  may select the second or n-th adjacent (n is an integer equal to or larger than three) X coil  233 , for example, the fourth adjacent X coil  233 -( k+ 4). Subsequently, the control circuit  301  repeats the above-described transmission mode and reception mode about the newly-selected transmitting/receiving X coil  233 -( k+ 4). Also in the subsequent operation in this example, similarly, the control circuit  301  switches the transmitting/receiving X coil to every fourth X coil  233 -( k+ 8), 233-( k+ 12), . . . sequentially until the X coil  233 -(N−k′) at a position distant from the right end of the sensor substrate  230  by dx or longer is selected, and repeats the above-described transmission mode and reception mode about each selected transmitting/receiving X coil. 
         [0111]    After the global scan in the step S 1  is completed in the above-described manner, in this example, next for the area composed of the areas B (left and right areas B) at both peripheral parts of the sensor substrate  230  in the X direction and the area C, the control circuit  301  carries out a global scan by using the Y coils  234  existing in this area as the transmitting/receiving loop coil (step S 2 ). 
         [0112]    Specifically, the control circuit  301  makes the selecting circuit  302  select the Y coil  234 , for example, the Y coil  234 - p , at a position distant from one end part of the sensor substrate  230  in the Y direction, for example, the upper end of the sensor substrate  230 , by the distance dy or longer as the first loop coil in the global scan of this step S 2 . p is an integer equal to or larger than {(the number of Y coils  234  within the distance dy from the end part of the sensor substrate  230  in the Y direction)+1}. p′ to be described later is also a similar integer. Furthermore, the control circuit  301  switches the transmission/reception switch circuit  303  to the transmission mode and makes the signal current from the signal generating circuit  309  flow through this Y coil  234 - p.    
         [0113]    Next, upon the elapse of a predetermined time, the control circuit  301  switches the transmission/reception switch circuit  303  to the reception mode. In this reception mode, the control circuit  301  holds, by the sample hold circuit  307 , the induced current flowing through the Y coil  234 - p  depending on the magnetic field from the position indicator  1 , and receives the digital data thereof from the A/D conversion circuit  308 . Subsequently, the control circuit  301  temporarily stores the received digital data in the buffer memory as the reception result about the Y coil  234 - p  selected in this reception mode. 
         [0114]    After the transmission and reception modes about the Y coil  234 - p  are completed in the above-described manner, the control circuit  301  controls the selecting circuit  302  so that the selecting circuit  302  may select the second or n-th adjacent (n is an integer equal to or larger than three) Y coil  234 , for example, the fourth adjacent Y coil  234 -( p+ 4). Subsequently, the control circuit  301  repeats the above-described transmission mode and reception mode about the newly-selected transmitting/receiving Y coil  234 -( p+ 4). Also in the subsequent operation in this example, similarly, the control circuit  301  switches the transmitting/receiving Y coil to every fourth Y coil  234 -( p+ 8),  234 -( p+ 12), . . . sequentially until the Y coil  234 -(M−p′) at a position distant from the lower end of the sensor substrate  230  by dy or longer is selected, and repeats the above-described transmission mode and reception mode about each selected transmitting/receiving Y coil. 
         [0115]    After the global scan in the step S 2  is completed in the above-described manner, the control circuit  301  refers to the reception results about the respective X coils  233 , temporarily stored in the global scan of the step S 1 , and the reception results about the respective Y coils  234 , temporarily stored in the global scan of the step S 2 , and determines whether or not the reception result indicating a significant value is present (step S 3 ). 
         [0116]    If the control circuit  301  determines that the reception result indicating a significant value does not exist in this step S 3 , the control circuit  301  carries out rough corner scan by use of the auxiliary coil  235  as the transmitting coil about the areas D at four corners of the sensor substrate  230  (step S 4 ). 
         [0117]    Specifically, in this step S 4 , the control circuit  301  makes the selecting circuit  302  select, for example, the auxiliary coil  235 - 1  at the upper left corner as the first loop coil in the rough corner scan of this step S 4 . Subsequently, the control circuit  301  switches the transmission/reception switch circuit  303  to the transmission mode and makes the signal current from the signal generating circuit  309  flow through this auxiliary coil  235 - 1 . 
         [0118]    Next, upon the elapse of a predetermined time, the control circuit  301  switches the transmission/reception switch circuit  303  to the reception mode, and controls the selecting circuit  302  so that the selecting circuit  302  may select one X coil  233  or Y coil  234  passing through the area D at this upper left corner. Furthermore, in this reception mode, the control circuit  301  holds, by the sample hold circuit  307 , the induced current flowing through the X coil  233  or the Y coil  234  selected by the selecting circuit  302  depending on the magnetic field from the position indicator  1 , and receives the digital data thereof from the A/D conversion circuit  308 . Subsequently, the control circuit  301  temporarily stores the received digital data in the buffer memory as the reception result about the X coil  233  or the Y coil  234  selected in this reception mode. 
         [0119]    In the rough corner scan of this step S 4 , after the end of the reception mode, the control circuit  301  controls the selecting circuit  302  so that the selecting circuit  302  may select the auxiliary coil  235 - 1  again, and carries out the above-described transmission mode. After the end of the transmission mode, the control circuit  301  controls the selecting circuit  302  so that the selecting circuit  302  may select one different X coil  233  or Y coil  234  passing through the area D at this upper left corner, and carries out the above-described reception mode. 
         [0120]    After repeating the transmission and reception modes in which the auxiliary coil  235 - 1  is selected as the transmitting coil and a respective one of the different X coils  233  or Y coils  234  passing through the area D at the upper left corner is selected as the receiving coil, the control circuit  301  carries out rough corner scan about, for example, the area D at the upper right corner. Specifically, the control circuit  301  carries out control to repeat the transmission and reception modes in which the auxiliary coil  235 - 2  is selected as the transmitting coil and a respective one of the different X coils  233  or Y coils  234  passing through the area D at the upper right corner is selected as the receiving coil. 
         [0121]    After the end of the rough corner scan about the area D at this upper right corner, about the area D at the lower left corner, the control circuit  301  carries out a rough corner scan of repeating the transmission and reception modes in which the auxiliary coil  235 - 3  is selected as the transmitting coil and a respective one of the different X coils  233  or Y coils  234  passing through the area D at the lower left corner is selected as the receiving coil. 
         [0122]    Moreover, after the end of the rough corner scan about the area D at this lower left corner, about the area D at the lower right corner, the control circuit  301  carries out a rough corner scan of repeating the transmission and reception modes in which the auxiliary coil  235 - 4  is selected as the transmitting coil and a respective one of the different X coils  233  or Y coils  234  passing through the area D at the lower right corner is selected as the receiving coil. Through the above-described process, the rough corner scan of this step S 4  is completed. 
         [0123]    In the above description, in the rough corner scan of the step S 4 , the transmission mode and the reception mode are repeated plural times with switching of the receiving coil for each of the areas D at four corners. However, because the size of the area D is comparatively small, it is also possible that, in the rough corner scan of the step S 4 , only one set of the transmission mode and the reception mode is carried out for each area D instead of the multiple repetition of these modes. 
         [0124]    After the rough corner scan in the step S 4  is completed in the above-described manner, the control circuit  301  refers to the reception results about the respective X coils  233  or Y coils  234 , temporarily stored in the buffer memory, and determines whether or not the reception result indicating a significant value is present (step S 5 ). 
         [0125]    If the control circuit  301  determines that the reception result indicating a significant value does not exist in this step S 5 , the control circuit  301  returns to the step S 1  and repeats the processing of this step S 1  and the subsequent steps. 
         [0126]    On the other hand, if the control circuit  301  determines that the reception result indicating a significant value exists in the step S 3 , the control circuit  301  determines the X coil  233  and/or the Y coil  234  whose reception result indicates the significant value to thereby determine the area for which a sector scan should be carried out and carry out the sector scan (step S 6 ). 
         [0127]    When the loop coils whose reception result indicates a significant value are both of the X coil  233  and the Y coil  234 , at least the area C is included in the area for which a sector scan should be carried out. 
         [0128]    If the reception results of both of the X coil  233  and the Y coil  234  indicate a significant value and the X coil  233  whose reception result indicates a significant value is near one of the areas B, the control circuit  301  determines that the area including the area C and this one area B is the area for which a sector scan should be carried out. In the area B, the Y coil  234  should be selected as the transmitting coil as shown in  FIG. 10 . Thus, the control circuit  301  carries out a sector scan in the determined area in such a manner that only the Y coil  234  is selected as the transmitting coil and the X coil  233  and the Y coil  234  are switched as the receiving coil, to thereby detect the position indicated by the above-described position indicator  1 . 
         [0129]    If the reception results of both of the X coil  233  and the Y coil  234  indicate a significant value and the Y coil  234  whose reception result indicates a significant value is near one of the areas A, the control circuit  301  determines that the area including the area C and this one area A is the area for which a sector scan should be carried out. In the area A, the X coil  233  should be selected as the transmitting coil as shown in  FIG. 10 . Thus, the control circuit  301  carries out a sector scan in the determined area in such a manner that only the X coil  233  is selected as the transmitting coil and the X coil  233  and the Y coil  234  are switched as the receiving coil, to thereby detect the position indicated by the above-described position indicator  1 . 
         [0130]    If the reception results of both of the X coil  233  and the Y coil  234  indicate a significant value and the X coil  233  showing the determination result that its reception result is a significant value is near one of the areas B and the Y coil  234  showing the determination result that its reception result is a significant value is near one of the areas A adjacent to this one area B, the control circuit  301  determines that the area including the area D adjacent to both of this one area B and this one area A is the area for which a sector scan should be carried out. In the area D, the auxiliary coil  235  should be selected as the transmitting coil as shown in  FIG. 10 . Thus, the control circuit  301  carries out a sector scan in the determined area in such a manner that only the auxiliary coil  235  is selected as the transmitting coil and the X coil  233  and the Y coil  234  are switched as the receiving coil, to thereby detect the position indicated by the above-described position indicator  1 . 
         [0131]    Moreover, if the reception results of both of the X coil  233  and the Y coil  234  indicate a significant value and a state other than the above-described states is obtained regarding the position of the coil whose reception result indicates a significant value, the control circuit  301  determines that only the area C is the area for which a sector scan should be carried out. In the area C, there is no limit to the transmitting coil and both of the X coil  233  and the Y coil  234  can be used as the transmitting coil as shown in  FIG. 10 . Thus, for the determined area, the control circuit  301  first carries out the transmission and reception modes about each X coil  233  and thereafter carries out the transmission and reception modes about each Y coil  234 , for example. Subsequently, the control circuit  301  creates and outputs the position data of the position indicator  1  based on the temporarily-stored reception results of the respective X coils  233  and the respective Y coils  234 . 
         [0132]    After the end of this step S 6 , the control circuit  301  returns to the step S 1  and repeats the processing of this step S 1  and the subsequent steps. 
         [0133]    If the control circuit  301  determines that the reception result indicating a significant value exists in the step S 5 , the control circuit  301  determines the X coil  233  and/or the Y coil  234  whose reception result indicates a significant value to thereby determine the area for which a refined corner scan should be carried out and carry out the refined corner scan (step S 7 ). At this time, first, either one of the areas D at four corners is determined, and then the area for which a refined corner scan is carried out is further determined for this determined one area D. It is also possible to determine that the whole area of the determined area D is the area for which a refined corner scan should be carried out. 
         [0134]    In the refined corner scan in this step S 7 , the auxiliary coil is always selected as the transmitting coil and the receiving coil is switched to the X coil  233  and the Y coil  234 . Subsequently, the control circuit  301  creates and outputs the position data of the position indicator  1  based on the reception results of the respective X coils  233  and the respective Y coils  234 , temporarily stored in the buffer memory. 
         [0135]    The above-described selection control of the loop coil is one example and the selection control is not limited thereto. For example, the global scan of the step S 2  in the above-described example may be carried out earlier than the global scan of the step S 1 . 
         [0136]    In the above-described example, the shift to a sector scan is allowed after the global scan of the step S 1  and the global scan of the step S 2  are carried out. However, the following way is also possible. Specifically, after the global scan of the step S 1  and the step S 2  and the rough corner scan of the step S 4  are sequentially carried out, the area for which a sector scan or a refined corner scan is carried out is decided by comprehensively determining the reception results of the respective X coils  233  and the respective Y coils  234 , temporarily stored in the global scan and the rough corner scan of these steps, and the sector scan or the refined corner scan is carried out. 
         [0137]    The rough corner scan of the step S 4  may be carried out before the global scan of the step S 1  or the global scan of the step S 2 . Alternatively, the rough corner scan of the step S 4  may be carried out in the middle of the global scan of the step S 1  or the global scan of the step S 2 . 
         [0138]    The above-described example employs the method of combining a global scan and a sector scan and combining a rough corner scan and a refined corner scan. However, it is also possible to carry out detection of the indicated position by sequentially switching the loop coil, without carrying out the global scan and the rough corner scan. In this case also, it is obvious that the transmitting coil that can be used for the respective areas shown in  FIG. 10  needs to be carefully selected. 
       [Other Configuration Examples of Sensor Unit  23 ] 
       [0139]    In the sensor unit  23  of the position detecting device  2  of the above-described embodiment, the auxiliary coils are provided in the middle of the two-layer printed wiring board as shown in  FIG. 4  and  FIG. 5 . However, the configuration of the sensor unit  23  is not limited to the above-described example. 
         [0140]      FIG. 13  shows one of other configuration examples of the sensor unit  23 . In this example, plural X coils  233 - 1 ,  233 - 2 , . . .  233 -N are formed from a printed pattern of one or plural layers on the front surface side of one double-sided printed wiring board  236 , and plural Y coils  234 - 1 ,  234 - 2 , . . .  234 -M are formed from a printed pattern of one or plural layers on the back surface side of this printed wiring board  236 . 
         [0141]    Furthermore, in this example, the auxiliary coils  235 - 1 ,  235 - 2 ,  235 - 3 , and  235 - 4  are formed from a printed pattern on the surfaces of small printed wiring boards  237 - 1 ,  237 - 2 ,  237 - 3 , and  237 - 4 , respectively. In addition, four small printed wiring boards  237 - 1 ,  237 - 2 ,  237 - 3 , and  237 - 4  on which the auxiliary coils  235 - 1 ,  235 - 2 ,  235 - 3 , and  235 - 4  are formed are bonded to four corners of the double-sided printed wiring board  236 . 
         [0142]    Instead of four small printed wiring boards, a printed wiring board having a frame shape corresponding to the peripheral part of the double-sided printed wiring board  236  may be used.  FIGS. 14A and 14B  are diagrams showing a configuration example of the sensor unit  23  of this case.  FIG. 14A  is a plan view of the sensor unit  23  of this example, and  FIG. 14B  is a sectional view along line X-X in  FIG. 14A . 
         [0143]    As shown in  FIGS. 14A and 14B , the sensor unit  23  of this example includes the double-sided printed wiring board  236  having the plural X coils  233  on its front surface and the plural Y coils  234  on its back surface. Furthermore, a printed wiring board  238  having a frame shape corresponding to the peripheral part of the sensor unit  23  is bonded onto, for example, the front surface of this double-sided printed wiring board  236 . At four corners of this printed wiring board  238  having the frame shape, the auxiliary coils  235 - 1 ,  235 - 2 ,  235 - 3 , and  235 - 4  are formed from a printed pattern. 
         [0144]    In  FIGS. 14A and 14B , the part of the double-sided printed wiring board  236  protruding toward the outside from the printed wiring board  238  having the frame shape corresponds to the part at which lead terminals of the X coils  233  and the Y coils  234  are provided. 
         [0145]    In the above-described embodiment, the auxiliary coils are provided only in the areas D at four corners of the sensor unit  23 . However, the auxiliary coils may be provided also in the areas A and the areas B at the peripheral part of the sensor unit  23 .  FIG. 15  shows one example of the sensor unit  23  in which the auxiliary coils  235  are provided also in the areas A and the areas B at the peripheral part. 
         [0146]    Specifically, the sensor unit of the example of  FIG. 15  includes a sensor substrate formed by bonding the printed wiring board  238  having a frame shape onto the surface of the double-sided printed wiring board  236  on which the X coils  233  and the Y coils  234  are formed, similarly to the example of  FIGS. 14A and 14B . On the printed wiring board  238  having the frame shape, the auxiliary coils having such shape and size as to satisfy the above-described (Equation 1) are formed. 
         [0147]    In the example of  FIG. 15 , on the printed wiring board  238  having the frame shape, the above-described auxiliary coils  235 - 1 ,  235 - 2 ,  235 - 3 , and  235 - 4  are formed in the areas D at four corners of the sensor unit  23 . In addition, auxiliary coils  239  having the same shape and size as those of the auxiliary coils  235 - 1  to  235 - 4  are formed in the areas A and the areas B at predetermined intervals. 
         [0148]    For example, in the above-described embodiment, the X coil  233  is used as the transmitting coil in the area A and the Y coil  234  is used as the transmitting coil in the area B not only in global scan but also in sector scan. In contrast, in the case of the sensor unit of the example of  FIG. 15 , the auxiliary coil  239  is used as the transmitting coil in sector scan in the area A and the area B. 
         [0149]    Specifically, in the sector scan, one of the upper and lower areas A or one of the left and right areas B can be set as the search area. In the example of  FIG. 15 , the sector scan can be efficiently carried out by using the auxiliary coils  239  provided in the areas A and the areas B as the transmitting coil. 
         [0150]    Furthermore, as described above, predetermined circuits (IC and so forth) are often provided around the periphery of the sensor unit  23  and coupling by an induced magnetic field is often weak at the peripheral part of the sensor unit  23  due to the influence of noise from these circuits. For example if a liquid crystal display (LCD) is provided at the part of the position detecting plane, a backlight drive circuit is provided around the periphery of the sensor unit  23 . 
         [0151]    Therefore, in the related art, the transmission current for excitation needs to be set large if the X coil and the Y coil at the peripheral part of the sensor unit are used as the transmitting coil. This is one of the factors causing the problem of unnecessary radiation of leaking magnetic flux. 
         [0152]    However, in the example of  FIG. 15 , the auxiliary coil  239  smaller in size than the X coil and the Y coil is used as the transmitting coil. Therefore, even when the large transmission current for excitation set in consideration of the influence of external circuits around the sensor unit  23  is made to flow through the auxiliary coil  239 , the amount of leakage of magnetic flux is small and can be suppressed to an unproblematic level. 
         [0153]    In seeking of the position indicated by the position indicator in the area A and the area B, the auxiliary coil  239  in addition to the X coil  233  and the Y coil  234  may be used in combination, like the above-described example, of course. In such a case, a proper magnetic field designed in consideration of the influence of external circuits around the sensor unit  23  can be generated by the auxiliary coil  239 . 
         [0154]    The auxiliary coil  239  at the peripheral part of the sensor unit, such as the areas A and the areas B, may be provided only in the area in which the influence of external circuits around the sensor unit  23  is strong, instead of being provided across the entire areas A and the entire areas B as shown in  FIG. 15 . 
         [0155]    Plural auxiliary coils are provided in the sensor unit in the above-described embodiment. However, the number of auxiliary coils may be one.