Abstract:
A position detection apparatus includes a position pointer having a coil and a sensor unit for detecting the position of the position pointer. The sensor unit has a sensor board including a plurality of loop coils juxtaposed and extending in a predetermined direction, and the sensor unit detects a signal generated in the loop coils by electromagnetic induction between the coil of the position pointer and the loop coils, to thereby detect the position of the position pointer. The sensor unit further includes a shield member disposed on the sensor board remotely from the position pointer for reducing noise, and a magnetic path sheet formed from a plurality of magnetic path members of a substantially rectangular shape having a higher magnetic permeability than that of the shield member and disposed between the sensor board and the shield member. The magnetic path members have mutually contacting portions disposed in an inclined relationship by a predetermined angle relative to said predetermined direction of the sensor board.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     The present application claims priority to Japanese Patent Application No. 2009-164890, filed Jul. 13, 2009, the entire content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a technique suitable to be applied to a position detection apparatus and a sensor unit, and more particularly to a technique for enhancing the position detection performance of an electromagnetic induction type digitizer. 
     2. Description of the Related Art 
     Various apparatus are available as an inputting apparatus for applying position information to a computer. One of such apparatus is a position detection apparatus generally called a digitizer. 
     The position detection apparatus is an inputting apparatus having a detection plane for detecting two-dimensional position information, on which a position pointer in the form of a pen is operated, in such a manner as to carry out drawing. The position detection apparatus has been widely popularized as an inputting apparatus suitable for the user who uses an application program in which drawing must be carried out quickly and accurately in the fields of design, art and so forth. The position detection apparatus is configured from a position pointer in the form of a pen for pointing to a position and a sensor unit for detecting a point pointed to by the position pointer and converting the detected information into two-dimensional position information which can be utilized by a computer and then outputting the resulting information. 
     The sensor unit includes a position detection plane for detecting the position of the position pointer. A printed board wherein loop coils extending in an X-axis direction and a Y-axis direction is embedded in the position detection plane. 
     The loop coils are each supplied with current having a specific frequency so that a magnetic field is generated therefrom. As a result, a resonance circuit included in the position pointer resonates when the position pointer is positioned in the proximity of a loop coil which is generating the magnetic field. Then, an induced magnetic field generated from the resonance circuit of the position pointer is received by a loop coil. The position of the position pointer is detected by carrying out the operation described above for each of the loop coils. 
     Japanese Patent Laid-Open No. 2007-166147 assigned to the assignee of the present patent application is listed as a prior art document. 
     SUMMARY OF THE INVENTION 
     A position detection apparatus includes a built-in printed board or sensor board, having the above-described loop coils formed thereon, disposed directly below the position detection plane. In order to stabilize a magnetic characteristic of the sensor board, a metal plate of a ferromagnetic material such as a silicon steel lamination is laid directly below the sensor board. However, since the silicon steel lamination is heavy in weight, it is not preferable to apply the silicon steel lamination to a portable PC or a portable telephone set which have begun to be used in recent years. Therefore, a material called magnetic path sheet formed from a thin film of amorphous magnetic metal, which is lighter in weight and better in magnetic characteristic than a silicon steel lamination, has been placed into use. 
     However, it is difficult to acquire a thin film of amorphous magnetic metal which is sufficiently wide to cover over a wide position detection plane of a position detection apparatus, and accordingly, it is difficult to form a position detection plane which is uniform in quality. Therefore, in some cases, a plurality of belt-shaped ribbons of amorphous magnetic metal are adhered to each other to form a single sheet for covering the position detection plane. 
     However, if a magnetic path sheet is formed in this manner, then the distribution of magnetic fluxes passing through the magnetic path sheet becomes non-uniform, particularly at a joining location of the magnetic metal ribbons. Therefore, at the joining location of the magnetic metal ribbons, the position detection accuracy of the position detection apparatus sometimes deteriorates to such a degree that position detection data may be output representing that the position pointer exists at a position different from the position at which the position pointer actually exists. 
     According to one aspect of the present invention, a position detection apparatus is provided, which can be configured at a reduced cost and is improved in position detection accuracy, and a sensor unit is provided which is used to form the position detection apparatus. 
     According to one embodiment of the present invention, a position detection apparatus is provided, including a position pointer having at least one coil, and a sensor unit having a sensor board including a plurality of loop coils juxtaposed and extending in a predetermined direction. The sensor unit is configured to detect a signal generated in the loop coils by electromagnetic induction between the coil in the position pointer and the loop coils, to thereby detect the position of the position pointer. The sensor unit further includes a shield member disposed on the side of the sensor board opposite to that of the position pointer for reducing noise, and a magnetic path sheet formed from a plurality of magnetic path members of a substantially rectangular shape having a higher magnetic permeability than that of the shield member and disposed between the sensor board and the shield member. The magnetic path members have contacting portions at which the magnetic path members contact with each other, and the contacting portions are disposed in an inclined relationship by a predetermined angle relative to said predetermined direction of the sensor board. 
     In short, in one embodiment, a magnetic path sheet disposed directly below a sensor board in a position detection apparatus of the electromagnetic induction type is configured by superposing a plurality of magnetic ribbons such that contacting portions between them are disposed in an inclined relationship by a predetermined angle relative to the predetermined direction of the sensor board, in which the plurality of loop coils extend. By configuring the magnetic path sheet in this manner, a variation of the sensitivity of a loop coil in the proximity of a joining portion between the magnetic ribbons can be suppressed. Accordingly, according to one embodiment of the present invention, a position detection apparatus can be implemented which is superior in detection sensitivity of a position pointer, position detection accuracy, noise resistance, and performance. 
     According to an embodiment of the present invention, a position detection apparatus can be provided which can be configured at a reduced cost and is improved in position detection accuracy. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing an appearance of a position detection apparatus to which the present invention is applied; 
         FIG. 2  is an exploded perspective view schematically showing an internal configuration of a sensor unit of the position detection apparatus; 
         FIG. 3  is a circuit diagram of a position pointer shown in  FIG. 1 ; 
         FIG. 4  is a block diagram of the sensor unit; 
         FIG. 5  is a schematic plan view of a sensor board shown in  FIG. 2 ; 
         FIG. 6  is a schematic view showing the shape of wires (or loop coil windings) on the sensor board; 
         FIGS. 7A ,  7 B and  7 C are schematic views showing a structure of a magnetic path sheet shown in  FIG. 2 ; 
         FIG. 8  is a schematic view illustrating the sensor board and the magnetic path sheet; 
         FIGS. 9A ,  9 B and  9 C are diagrammatic views illustrating a relationship between the position pointer and loop coil windings and a principle of detection of the position of the position pointer; 
         FIG. 10A  is a perspective view of a metal ribbon shown in  FIG. 7A  or  7 C and  FIG. 10B  is a diagrammatic view illustrating a magnetic characteristic of the metal ribbon; 
         FIGS. 11A and 11B  are diagrammatic views illustrating magnetic characteristics where two metal ribbons shown in  FIG. 7A  or  7 B are abutted with each other and where two metal ribbons are partially superposed with each other, respectively; 
         FIG. 12A  is a graph illustrating a magnetic flux distribution of the position pointer and  FIG. 12B  is a schematic view illustrating an arrangement relationship of the position pointer, the center line of a loop coil winding, and a metal ribbon; 
         FIGS. 13 ,  14  and  15  are graphs showing signal levels when a signal sent from the position pointer is detected by the loop coils; and 
         FIG. 16  is a schematic view of a notebook type personal computer to which the position detection apparatus is applied. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the following, a preferred embodiment of the present invention is described in detail with reference to  FIGS. 1 to 15 . 
     Referring first to  FIG. 1 , there is shown a position detection apparatus to which the present invention is applied. 
     The position detection apparatus  101  is of the type usually called a digitizer and includes a position pointer  102  and a sensor unit  103  for detecting the position of the position pointer  102 . The sensor unit  103  outputs information regarding the presence and the position of the position pointer  102  on a position detection plane  104  to an external apparatus. 
       FIG. 2  is an exploded perspective view schematically showing an internal configuration of the sensor unit  103 . 
     The sensor unit  103  includes an upper case  202  which forms an upper face portion of a housing of the position detection apparatus  101 . The upper case  202  includes the position detection plane  104  in the form of a flat plate of plastic, glass or the like. 
     A sensor (or sensor board)  203  is disposed immediately below the position detection plane  104  of the upper case  202 . 
     A magnetic path sheet  204  formed from an amorphous magnetic metal ribbon is disposed immediately below the sensor  203 . 
     A shield sheet  205  formed from aluminum foil is disposed immediately below the magnetic path sheet  204 . 
     The sensor  203 , magnetic path sheet  204  and shield sheet  205  and a signal processing section  206  are accommodated in a housing formed from the upper case  202  and a lower case  207 . 
     The signal processing section  206  outputs information regarding the presence and the position of the position pointer  102  to an externally-connected apparatus such as a personal computer through a USB interface cable  208 . 
     The sensor  203  is a double-sided printed board and has a wire pattern of coils of a quadrangular shape in a longitudinal direction and a transverse direction. Details of the shape of the coils wired on the sensor  203  are hereinafter described with reference to  FIG. 4  and so forth. 
       FIG. 3  shows an electric circuit of the position pointer  102 . It is to be noted that a power supply and like elements are omitted in  FIG. 3 . 
     A button switch  306  is provided on a side face of a housing of the position pointer  102 . 
     A penpoint  309  of the position pointer  102  transmits pen tip pressure to a variable capacitor C 308  provided inside of the position pointer  102 . The variable capacitor C 308  has capacitance which varies in response to the pen tip pressure applied to the penpoint  309 . 
     Referring to  FIG. 3 , a coil L 302 , a capacitor C 303  and a semi-fixed capacitor C 304  are connected in parallel to configure a resonance circuit. The coil L 302  receives an electromagnetic wave from a loop coil provided on the sensor  203  of the sensor unit  103  hereinafter described. The coil L 302  generates induced electromotive force when it receives an electromagnetic wave. The induced electromotive force is accumulated as charge in the coil L 302  and the capacitor C 303 . The induced electromotive force generated at the coil L 302  repetitively resonates and is accumulated as charge in the capacitor C 303 . When the supply of electromagnetic wave from a loop coil stops, the accumulated charge generates an ac magnetic field passing through the coil L 302  of the resonance circuit back and forth, and supplies the electromagnetic wave to the loop coil provided on the sensor  203  of the sensor unit  103 . 
     A capacitor C 305  and the button switch  306  are connected in series to each other and connected in parallel to the capacitor C 303  and the semi-fixed capacitor C 304  through a changeover switch  307 . The variable capacitor C 308  is connected to the other terminal of the changeover switch  307 , and also the variable capacitor C 308  is connected in parallel to the capacitor C 303  and the semi-fixed capacitor C 304  through the changeover switch  307 . 
     A diode D 311  half-wave rectifies the ac current generated from the coil L 302 . 
     An integrator  314  formed from a resistor R 312  and a capacitor C 313  is connected to the cathode of the diode D 311 . 
     Current flowing from the diode D 311  charges the capacitor C 313  through the resistor R 312 . Consequently, the voltage across the capacitor C 313  gradually rises in accordance with a time constant defined by the resistor R 312  and the capacitor C 313 . 
     The capacitor C 313  is connected to the negative input terminal of a comparator  310 . A reference voltage Vref is applied to the positive side input terminal of the comparator  310 . The reference voltage Vref is obtained by dividing a power supply voltage +Vcc by resistors R 315  and R 316  and is, for example, equal to one half the power supply voltage +Vcc. 
     The voltage across the capacitor C 313  is compared with the reference voltage Vref by the comparator  310 . 
     If a result of the comparison with the reference voltage Vref indicates that the voltage across the capacitor C 313  exceeds the reference voltage Vref, then the potential of a control signal to be outputted from the comparator  310  varies from a high potential to a low potential. The changeover switch  307  is controlled by the control signal. 
     The diode D 311 , the integrator  314 , and the comparator  310  control the changeover switch  307  to be switched when a predetermined period of time elapses. The predetermined period of time here is a period of time required after a point of time at which an electromagnetic wave is provided from the sensor unit  103  to the coil L 302  and induced electromotive force is generated in the coil L 302  until the voltage across the capacitor C 313  exceeds the reference voltage Vref. In short, the diode D 311 , the integrator  314 , and the comparator  310  implement the function of a timer which does not use a clock. 
     If the button switch  306  is controlled between on and off in a state wherein the changeover switch  307  is selectively connected to the capacitor C 305 , then the combined capacitance of the capacitors which form the resonance circuit varies. Accordingly, the resonance frequency of the resonance circuit varies in response to the state of the button switch  306 . 
     Similarly, in another state wherein the changeover switch  307  is selectively connected to the variable capacitor C 308 , the combined capacitance of the capacitors which form the resonance circuit varies in response to the variation of the capacitance of the variable capacitor C 308 . Accordingly, the resonance frequency of the resonance circuit varies in response to the variation of the capacitance of the variable capacitor C 308 . 
       FIG. 4  shows a block configuration of the sensor unit  103 . 
     Referring to  FIG. 4 , the sensor unit  103  includes an oscillator  402  which generates a sine wave ac signal of a frequency substantially equal to the resonance frequency of the resonance circuit of the position pointer  102  described above. The generated sine wave ac signal is supplied to a current driver  403  and a synchronous detector  404 . 
     The current driver  403  amplifies the current of the sine wave ac signal inputted thereto from the oscillator  402  and signals the amplified sine wave ac signal to a transmission/reception changeover switch  405 . 
     The transmission/reception changeover switch  405  exclusively connects one of an output terminal of the current driver  403  and an input terminal of a reception amplifier  406  to a selection switch  407 . 
     The selection switch  407  selects one of a plurality of loop coils  408   a ,  408   b ,  408   c  and  408   d  and connects the selected loop coil to the transmission/reception changeover switch  405 . 
     When the transmission/reception changeover switch  405  connects the selection switch  407  and the current driver  403  to each other, sine wave ac current supplied from the current driver  403  is supplied to one of the loop coils  408   a ,  408   b ,  408   c  and  408   d  selected by the selection switch  407 . 
     When the transmission/reception changeover switch  405  connects the selection switch  407  and the reception amplifier  406  to each other, a signal outputted from one of the loop coils  408   a ,  408   b ,  408   c  and  408   d  selected by the selection switch  407  is inputted to the reception amplifier  406 . 
     It is to be noted that the transmission/reception changeover switch  405  and the selection switch  407  are controlled by respective control signals supplied thereto from a control section  409  hereinafter described. 
     The loop coils  408   a ,  408   b ,  408   c  and  408   d  of a substantially rectangular shape are juxtaposed substantially in parallel to each other on the sensor  203  disposed immediately below the position detection plane  104  of the sensor unit  103 . The loop coils  408   a ,  408   b ,  408   c  and  408   d  are connected at one of terminals thereof to the selection switch  407  and grounded at the other terminal thereof. 
     When one of the loop coils  408   a ,  408   b ,  408   c  and  408   d  selected by the selection switch  407  is connected to the output terminal of the current driver  403  through the transmission/reception changeover switch  405 , an ac magnetic field is generated from the loop coil selected by the selection switch  407  by sine wave ac current supplied thereto from the current driver  403 . 
     When the position pointer  102  is positioned in the proximity of a loop coil which is generating an ac magnetic field, electromotive force is generated in the coil L 302  inside the position pointer  102 , and charge based on the electromotive force is accumulated in the resonance circuit. 
     Then, if the transmission/reception changeover switch  405  is controlled by a control signal from the control section  409 , then the loop coil selected by the selection switch  407  is connected to the input terminal of the reception amplifier  406 . Consequently, since the supply of sine wave ac current to the loop coil selected by the selection switch  407  from the current driver  403  stops, the ac magnetic field from the loop coil disappears. On the other hand, since charge based on the induced electromotive force described hereinabove is accumulated in the resonance circuit in the position pointer  102 , an ac induced magnetic field of the resonance frequency of the resonance circuit is generated from the coil L 302  by the charge. When the loop coil receives this induced magnetic field, weak ac current is generated in the loop coil. The reception amplifier  406  voltage-amplifies the current and supplies a resulting voltage to the input terminal of an envelope detector  410  and the synchronous detector  404 . 
     It is to be noted that, for the convenience of description, a state wherein the transmission/reception changeover switch  405  connects the selection switch  407  and the current driver  403  to each other is defined as transmission state, and another state wherein the transmission/reception changeover switch  405  connects the selection switch  407  and the reception amplifier  406  to each other is defined as reception state. Further, operation of the sensor unit  103  in the transmission state is defined as transmission operation, and operation of the sensor unit  103  in the reception state is defined as reception operation. 
     In the transmission operation, an ac magnetic field is generated from the loop coils  408   a ,  408   b ,  408   c  and  408   d  so that induced electromotive force is generated in the position pointer  102  positioned in the proximity of the loop coils  408   a ,  408   b ,  408   c  and  408   d.    
     In the reception operation, an ac induced magnetic field generated from the position pointer  102  is received by any of the loop coils  408   a ,  408   b ,  408   c  and  408   d  which are positioned in the proximity of the position pointer  102 . 
     When the position pointer  102  is not positioned in the proximity of the loop coil from which an ac magnetic field is generated, no induced electromotive force is generated in the coil L 302  in the position pointer  102 , and charge based on the induced electromotive force is not accumulated in the resonance circuit. Accordingly, even if the transmission/reception changeover switch  405  is changed over to the reception side, that is, to the reception amplifier  406  side, the loop coil selected by the selection switch  407  does not receive any ac induced magnetic field generated from the position pointer  102 , and no ac current is generated in the loop coil. 
     A large number of such groups of the loop coils  408   a ,  408   b ,  408   c  and  408   d  are juxtaposed in the longitudinal direction and the transverse direction of the position detection plane  104  and changed over (switched) one by one by means of the selection switch  407 , so that transmission operation and reception operation as described above are repeated among them. Consequently, a signal can be extracted from a loop coil positioned in the proximity of the position pointer  102 . The position of the position pointer  102  on the position detection plane  104  is calculated based on the position of the loop coil from which the signal is obtained and also on the level of the signal. 
     The controlling operation of changing over (switching) the loop coils  408   a ,  408   b ,  408   c  and  408   d  one by one by means of the selection switch  407 , the controlling operation of changing over (switching) the transmission/reception changeover switch  405 , and the operation of calculating the position of the position pointer  102  based on the received signal are executed by the control section  409  which is formed from a well-known microcomputer. 
     Circuit blocks interposed between the output terminal of the reception amplifier  406  and the control section  409  are provided for changing over (switching) reception signals obtained from the loop coils  408   a ,  408   b ,  408   c  and  408   d.    
     The envelope detector  410  carries out full-wave rectification of an ac reception signal. 
     An integration circuit  411   a  integrates the full-wave rectified reception signal. The integration circuit  411   a  integrates a weak signal on the time axis to obtain a high signal level while maintaining the S/N ratio. 
     A sample hold circuit  412   a  holds an output voltage of the integration circuit  411   a  at a certain point of time. 
     An A/D conversion circuit  413   a  converts an output voltage of the sample hold circuit  412   a  into digital data. 
     Through the circuits described, data corresponding to the level of the signal based on the ac induced magnetic field generated from the position pointer  102  is obtained. 
     The synchronous detector  404  is a well-known analog multiplication section and outputs a signal obtained by multiplying an ac signal of the oscillator  402  and the reception signal. The integration circuit  411   b , sample hold circuit  412   b  and A/D conversion circuit  413   b  at the succeeding stage to the synchronous detector  404  are the same as the circuit blocks disposed following the envelope detector  410  described hereinabove. 
     The synchronous detector  404  carries out an operation substantially the same as that of a full-wave rectification circuit where the phases of the ac signal and the reception signal are identical to each other. However, if the phases of the ac signal and the reception signal are different, then the signal level drops. The variation of the phase in the reception signal is caused by variation of the resonance frequency of the resonance circuit of the position pointer  102 . When the capacitance of the variable capacitor C 308  varies in response to pressure applied to the position pointer  104  (pen tip pressure) or when the combined capacitance of the capacitors which form the resonance circuit varies as a result of incorporation of the capacitor C 305  into the resonance circuit by activation of the button switch  306 , the level of the signal obtained from the synchronous detector  404  varies in response to the variation of the capacitance or of the combined capacitance. 
     If predetermined mathematical operation processing is applied to the level of the signal obtained from the envelope detector  410  and the level of the signal obtained from the synchronous detector  404 , then data corresponding to the phase of a signal based on the ac induced magnetic field generated from the position pointer  102  is obtained. Then, a state of the position pointer  102  having a shape of a pen can be determined from the variation of the phase. 
     Within a period of time within which the sensor unit  103  is in the reception state wherein one of the loop coils  408   a ,  408   b ,  408   c  and  408   d  is selected, if the button switch  306  is depressed, then the control signal outputted from a comparator  310  of the position pointer  102  is switched and, then, the sensor unit  103  can detect two states, that is, a state wherein the changeover switch  307  selects the capacitor C 305  and another state wherein the changeover switch  307  selects the variable capacitor  308 , within a period of time of the reception state. In other words, the sensor unit  103  can detect whether or not the button switch  306  is depressed or whether the pen tip pressure can be detected. 
     Now, general operation of the sensor unit  103  and the position pointer  102  is described. 
     The position pointer  102  will be moved to a position in the proximity of the sensor  203  in which the loop coils are provided, that is, in the proximity of the position detection plane  104 . In the position pointer  102 , the resonance circuit is built which is designed such that the coil L 302 , capacitor C 303  and so forth are connected in parallel and resonate at approximately 562.5 kHz in one embodiment. 
     If the resonance circuit receives an ac magnetic field generated from the loop coil, then induced electromotive force is generated in the resonance circuit. Immediately after this, the transmission/reception changeover switch  405  is changed over to connect the loop coil to the succeeding sensor reception function section so that an ac induced magnetic field from the position pointer  102  by the induced electromotive force generated in the resonance circuit is received and the signal level is converted into data. 
     The procedure described is repetitively carried out for the loop coils juxtaposed in the X axis direction and the loop coils juxtaposed in the Y axis direction. Consequently, a coil which exhibits a maximum value of the detected signal level is found in regard to both of the X axis direction and the Y axis direction. Consequently, the sensor unit  103  can detect the position of the position pointer  102  on the position detection plane  104 . 
     Structure of the Sensor  203   
     In the following, the structure of the sensor  203  is described with reference to  FIGS. 5 and 6 . 
       FIG. 5  schematically shows part of the sensor  203  in a form significantly shortened in the vertical (e.g., X-axis) direction in order to clearly indicate a wiring pattern of the loop coils. 
     Referring first to  FIG. 5 , the sensor  203  is a printed board having wiring patterns formed on the opposite faces thereof. In particular, longitudinal wires  502  are formed on the front face while transverse wires  503  are formed on the rear face. 
     The longitudinal wires  502  are connected at the opposite ends thereof to the wires on the rear face of the sensor  203  through through-holes  504 . 
       FIG. 6  shows a shape of the wires on the sensor  203 . Particularly,  FIG. 6  shows one of the loop coils shown in  FIG. 5  in a simplified form. 
     Referring to  FIG. 6 , a loop coil  602  is configured from loop coil windings  603   a ,  603   b ,  603   c ,  603   d  and  603   e  provided on the front side of the sensor  203  and inter-winding lead wires  604   a ,  604   b ,  604   c  and  604   d  provided on the rear side of the sensor  203 . 
     The loop coil windings  603   a ,  603   b ,  603   c ,  603   d  and  603   e  and the inter-winding lead wires  604   a ,  604   b ,  604   c  and  604   d  which are provided on the rear side of the sensor  203  are connected to each other through through-holes  605   a ,  605   b ,  605   c ,  605   d ,  605   e ,  605   f ,  605   g  and  605   h.    
     When the position pointer  102  exists within the range of an effective position detection region  606  in the position detection plane  104 , the loop coil  602  resonates with the position pointer  102 . 
     In  FIG. 6 , the loop coils are disposed on the sensor  203  such that each two thereof are displaced from each other. In particular, of adjacent ones of the loop coil windings shown in  FIG. 6 , those loop coil windings in the same winding direction belong to the same loop coil. Simply speaking, the sensor  203  has a large number of duplex winding loop coils wired thereon. 
     Magnetic Path Sheet  204   
       FIGS. 7A ,  7 B and  7 C show a structure of the magnetic path sheet  204 . 
     Particularly,  FIG. 7A  shows the magnetic path sheet  204  as viewed from above. 
     Referring to  FIG. 7A , the magnetic path sheet  204  is formed in the same shape as the sensor  203  as shown in  FIG. 2 . The magnetic path sheet  204  includes a large number of metal ribbons  702  formed in a belt-like shape and partly superposed with each other while extending obliquely at a predetermined angle with respect to the outer profile (outline) of the magnetic path sheet  204 . 
     The metal ribbons  702  are made of amorphous metal such as, for example, iron-silicon-boron alloy. Since amorphous magnetic metal is light in weight and exhibits superior soft magnetism, it is used for the core of a transformer for a power supply in place of an iron core or for the yoke. 
     The magnetic path sheet  204  is used to reinforce a magnetic field formed by the sensor  203  making use of the above-described characteristic of an amorphous magnetic metal. In other words, the magnetic path sheet  204  plays a role of a yoke for effectively introducing a magnetic flux generated from the resonance circuit of the position pointer  102  to a loop coil. 
       FIG. 7B  shows a cross section of the magnetic path sheet  204  taken along line A-A′ of  FIG. 7A  as viewed from the side. Although the metal ribbons  702  which form the magnetic path sheet  204  are actually formed very thin, in  FIG. 7B , they are shown with an exaggerated thickness for the convenience of illustration. 
     The metal ribbons  702  are disposed such that adjacent ones thereof are partly superposed with each other. In other words, the metal ribbons  702  are superposed such that no gap is formed therebetween when the magnetic path sheet  204  is viewed from above. 
     In  FIG. 7A , as an example, the metal ribbons  702  are shown disposed such that they are inclined by 45° with respect to the longitudinal direction of the magnetic path sheet  204 . This angle and the superposition are used for a particular reason. The reason for the angle, width, and superposition of the metal ribbons  702  which form the magnetic path sheet  204  is hereinafter described. 
       FIG. 7C  shows another example of the magnetic path sheet  204 . Referring to  FIG. 7C , where an amorphous magnetic metal used to form the metal ribbons  702  is available as a sheet having a large width, the magnetic path sheet  204  can be formed by adhering a smaller number of such sheets so that the width of the sheet can be utilized effectively to the utmost. While details are hereinafter described, since a magnetic characteristic suffers from some disorder at the adhesion location of the metal ribbons  702 , preferably the number of places at which the sheets are adhered to each other is minimized. 
       FIG. 8  schematically illustrates a relationship between the sensor  203  and the magnetic path sheet  204 . In particular,  FIG. 8  shows loop coil windings  802  of the sensor  203  and the magnetic path sheet  204  in a superposed state. 
     The boundary portions at which the metal ribbons  702  are adhered to each other are inclined with respect to the wiring direction of the loop coil windings  802  of the sensor  203 . 
       FIGS. 9A ,  9 B and  9 C illustrate a relationship between the position pointer  102  and a loop coil  901  and a principle of detection of the position of the position pointer  102 . 
     In particular,  FIG. 9A  schematically illustrates a positional relationship between the loop coil  901  and the position pointer  102 . It is to be noted that, in  FIG. 9A , the loop coil  901  is shown as a simplex winding coil for the convenience of illustration. The loop coil  901  here may be superposed on an adjacent loop coil. 
     The position pointer  102  outputs maximum current when it is positioned on the center line  902  of the loop coil  901 . 
       FIG. 9B  schematically illustrates a relationship between the center line  902  and the position pointer  102 . 
     It is assumed now that the position pointer  102  exists at a point P shown in  FIG. 9B . 
     The sensor unit  103  finds that the position pointer  102  is positioned in the proximity of a coil which exhibits the highest signal level detected in the loop coil group. However, in terms of the resolution of the sensor unit  103  and so forth, it is difficult to closely arrange such loop coils  901  on the sensor  203 . 
     Therefore, the control section  409  shown in  FIG. 4  carries out an interpolation mathematical operation process. 
       FIG. 9C  illustrates an outline of the interpolation mathematical operation process. 
     The magnetic flux density of magnetic fluxes generated from the position pointer  102  indicates a distribution generally in accordance with a Gaussian curve with respect to the distance from the position pointer  102 . Therefore, the control section  409  refers to the signal intensity of the loop coil  901  on the opposite sides of the center of the loop coil  901  at which the highest signal intensity is exhibited. Then, the control section  409  applies this to the Gaussian curve to calculate the true position of the position pointer  102 . In other words, the output level of the loop coil is applied to a Gaussian curve to calculate the point P at the apex of the Gaussian curve. 
     The foregoing description given with reference to  FIGS. 9A ,  9 B and  9 C is based on an assumption that the signal intensity obtained from such loop coils  901  has a uniform characteristic. Naturally, it is apparent from  FIG. 9C  that the loop coils  901  of the sensor unit  103  must have a uniform sensitivity. 
     If only a particular loop coil  901  has a high sensitivity or low sensitivity, then upon interpolation mathematical operation, a wrong position will be detected as the true position of the position pointer  102 . 
     The characteristic of the magnetic path sheet  204  contributes much to the sensitivity of the loop coil  901 . In particular, the magnetic characteristic of the magnetic path sheet  204  must originally be uniform. However, although it is not impossible to form an amorphous magnetic metal sheet having a large area, the magnetic path sheet  204  requires high production cost. Further, if the sensor unit  103  has an increased size, then there is the possibility that a single amorphous magnetic metal sheet may not have a large area sufficient to cover the position detection plane  104 . 
     Accordingly, as realistic designing, a large number of metal ribbons  702  each formed as a belt-shaped amorphous magnetic metal sheet are adhered to each other to form the magnetic path sheet  204  having an area necessary to cover the position detection plane  104 . 
     Reason of the Superposition 
       FIGS. 10A and 10B  schematically illustrate a magnetic characteristic of a metal ribbon  702 . 
     If the magnetic characteristic of a metal ribbon  702  used for the magnetic path sheet  204  is viewed from the side in a state in which the metal ribbon  702  is cut along line B-B′ as seen in  FIG. 10A , then such a magnetic characteristic as seen in  FIG. 10B  is obtained. What is to be noticed is that, at the opposite ends of the metal ribbon  702 , the magnetic flux density indicates a rise because magnetic fluxes are led to the opposite ends of the metal ribbon  702  and concentrated there such that the opposite ends act as exits of the magnetic fluxes. 
     For adhesion of the metal ribbons  702 , two methods are available including a method wherein they are abutted with each other and another method wherein they are partly superposed with each other.  FIG. 11A  illustrates a magnetic characteristic where two metal ribbons  702  are abutted with each other, and  FIG. 11B  illustrates a magnetic characteristic where two metal ribbons  702  are partly superposed with each other. 
     More particularly,  FIG. 11A  shows a cross section of two metal ribbons  702  in the state wherein they are abutted with each other and a magnetic characteristic of the metal ribbons  702 . 
     As seen in  FIG. 10B , the magnetic flux density indicates a rise at the opposite end portions of the metal ribbon  702 . Accordingly, if two metal ribbons  702  are abutted with each other, then the magnetic flux density further increases. The rise of the magnetic flux density is a cause of non-uniformity of the magnetic characteristic. In particular, if the metal ribbons  702  are abutted with each other, then a greater amount of magnetic fluxes leaks out at the abutted location. This is not suitable for the magnetic path sheet  204 . 
       FIG. 11B  shows a cross section of two metal ribbons  702  in a state wherein they are partly superposed with each other and a magnetic characteristic of the metal ribbons  702 . 
     If two metal ribbons  702  are partly superposed with each other, then a rise of the magnetic flux density at the end portions can be suppressed. It is considered that this arises from the fact that, although, where two metal ribbons  702  are abutted with each other, magnetic fluxes flow out from a gap formed between the abutting ends of the metal ribbons  702 , where two metal ribbons  702  are partly superposed with each other, such flowing out portion is not concentrated at a single point but is dispersed. In other words, it can be recognized that, if two metal ribbons  702  are partly superposed with each other, then the resulting magnetic path sheet  204  exhibits a good characteristic although it is not as good as that which is achieved by the magnetic path sheet  204  where it is formed from a single metal sheet. 
     Reason of the Oblique Superposition 
       FIG. 12A  illustrates a magnetic flux density from the position pointer  102 , and  FIG. 12  illustrates a positional relationship of the position pointer  102 , the center line  902  of a loop coil  901  (see  FIG. 9A ), and metal ribbons  702 . 
     In particular,  FIG. 12A  illustrates a magnetic flux density from the position pointer  102  placed on the position detection plane  104 . A curve of the graph of  FIG. 12A  is analogous to a Gaussian curve. The range of values equal to one half the value of the apex of the magnetic density distribution indicated by a Gaussian curve is defined as half-value width. This half-value width Al 202  is defined as effective position detection range. 
       FIG. 12B  illustrates a circle corresponding to (i.e., having a diameter equal to) the half-value width shown in  FIG. 12A  and a relationship of a loop coil  901  and a boundary portion of a superposed portion of metal ribbons  702 . 
     A principle of moderation of disorder of a magnetic characteristic by oblique arrangement of metal ribbons  702  with respect to a loop coil  901  to form an angle therebetween is described. It is to be noted that, in order to simplify the description, it is assumed that the loop coil  901  has a simplex winding and is not superposed with an adjacent loop coil and that a signal is transmitted from the position pointer  102  to the loop coil  901 , that is, the sensor unit  103  is in a reception state. 
     It is well known that the intensity of an electric field or a magnetic field generated from a point on a certain plane is distributed in accordance with a Gaussian distribution. To detect a magnetic field of a Gaussian distribution, an intensity of the magnetic field sufficient for distinction from peripheral noise is demanded. Where the position detection apparatus is of the electromagnetic type, it is demanded to detect a position pointer within a range of a substantially half-value width of the magnetic field distribution as an index when a weak magnetic field generated from the position detector is to be picked up. 
     The range of a signal from the position pointer, that is, the range in which the signal can be detected, is determined as a range of a circle having a diameter equal to the half-value width of the magnetic flux density distribution, and the diameter of the circle is represented by φ. If the width of the loop coil  901  is represented by d, then the range within which the loop coil can detect a magnetic field is considered a region Al 203  of the width d surrounded by the loop coil  901 . It is contemplated that the influence upon a signal to be detected varies depending upon the length of the boundary portion, which disorders the magnetic flux density distribution, included in the region Al 203 . 
     In other words, as the length of the boundary portion included in the region Al 203  increases, the influence on the detection signal increases. 
     On the contrary, if the extension direction of the boundary portion is close to a direction parallel to the center line of one loop coil, then only the loop coil including the boundary portion will be substantially influenced. Therefore, the metal ribbons  702  are arranged in an inclined relationship such that the boundary portion, which is a boundary line of the metal ribbons  702 , crosses the loop coil  901  within a circle of the half-value width, which is the effective position detection range of the position pointer  102 . Consequently, the metal ribbons  702  come to have an influence on the sensitivity of a plurality of adjacent loop coils. Accordingly, the possibility of erroneous detection can be reduced as compared to the case wherein the sensitivity variation appears only with a single loop coil. 
     If an angle θ by which the metal ribbons  702  must be inclined in the minimum is calculated in accordance with the study given above, then the inclination angle θ necessary for the metal ribbons  702  can be represented by the following expression:
 
θ=sin −1 ( d /φ)
 
     The inclination angle φ is set equal to or greater than 12°. 
     As an example, where φ is 30 mm and d is 6.4 mm, θ is approximately 12.32°. 
     On the contrary, if the inclination angle θ of the metal ribbons  702  is determined, then the length of the boundary portion which traverses the region Al 203  can be calculated. Where the length of the boundary portion is represented by φ′, it can be represented by the following expression:
 
φ′= d /sin θ
 
     As an example, where d is 6.4 mm, the length p′ of the boundary portion when the inclination angle θ is 15° is 24.72 mm; the length p′ of the boundary portion when the inclination angle θ is 45° is 9.05 mm; and the length p′ of the boundary portion when the inclination angle θ is 90° is 6.4 mm. 
     However, where the inclination angle θ is greater than 45°, the boundary portion comes to have an influence upon other sensor coils extending in an orthogonal direction. 
     From the foregoing, 45° is an angle which minimizes the influence of the boundary portion. 
     While the foregoing description is given assuming that the loop coil has a simplex winding, the basic way of thinking applies similarly also where the loop coil has duplex windings as described hereinabove with reference to  FIGS. 6 and 8  or has a greater number of windings. Calculation similar to that described hereinabove may be executed to determine that, from among a plurality of loop coil windings  802  which belong to the same loop coil, those loop coil windings  802  having the same winding direction may be regarded as a virtual single loop coil winding. 
       FIG. 13  illustrates the signal level when a signal sent from the position pointer  102  is detected by the loop coils. In particular,  FIG. 13  illustrates the signal level in those cases wherein the inclination angle θ of the boundary portion with respect to the longitudinal direction of the loop coils is 0°, 15° and 45° where metal ribbons  702  are abutted with each other without being superposed on each other. 
     The position of a peak at the center of the curve of 0° corresponds to the boundary portion where the metal ribbons  702  are abutted with each other. It can be seen that magnetic fluxes flow out from a very small gap formed at the boundary portion at which the metal ribbons  702  are abutted with each other, and portions at which the magnetic flux density is comparatively low appear on the opposite sides of the gap. 
     If the curve of 0° is compared with the curve of 15° or of 45°, then it can be observed apparently that the difference between the peak and the bottom of the signal level of the latter curve is significantly small. 
     From  FIG. 13 , it can be recognized that, by forming the boundary portion at which the metal ribbons  702  are abutted with each other such that it is inclined with respect to the loop coils, the influence of the boundary portion upon the loop coil concerning signal detection can be reduced. 
       FIG. 14  illustrates the signal level when a signal sent from the position pointer  102  is detected by the loop coils. In particular,  FIG. 14  illustrates the signal level where metal ribbons  702  are abutted with each other without being superposed on each other and the signal level where metal ribbons  702  are partly superposed with each other over 3 mm. It is to be noted that, in both cases, the boundary portion is inclined with respect to the loop coil windings  802 . 
     If the curve of the signal level where the metal ribbons  702  are abutted with each other is compared with the curve of the signal level where the metal ribbons  702  are superposed on each other, then it can be recognized apparently that the difference between the peak and the bottom of the signal level of the latter case is significantly small. 
     From  FIG. 14 , it can be recognized that, by superposing the metal ribbons  702  on each other, the influence of the boundary portion between the metal ribbons  702  upon signal detection by the loop coils can be reduced. 
       FIG. 15  illustrates the signal level when a signal sent from the position pointer  102  is detected by the loop coils. In particular,  FIG. 15  illustrates the signal level where metal ribbons  702  are superposed on each other over 3 mm and besides the boundary portion of the metal ribbons  702  is disposed at the angles of 0°, 15° and 45° with respect to the longitudinal direction of the loop coils. 
     If the curve of the signal level where the inclination angle is 0° is compared with the curve of the signal level where the inclination angle is 15° or 45°, then it can be recognized apparently that the difference between the peak and the bottom of the signal level of the latter case is significantly small. 
     From  FIG. 15 , it can be recognized that, by superposing the metal ribbons  702  on each other when additionally the boundary portion of the metal ribbons  702  is inclined with respect to the loop coils  802 , the influence of the boundary portion upon the loop coil concerning signal detection can be reduced to the minimum. 
     The present embodiment may have such applications as described below. 
     (1) The sensor unit  103  may be configured such that the upper case  202  thereof includes a liquid crystal panel, which has a protective glass plate on the surface thereof, and which may be provided in place of the upper case  202  or formed integrally with the upper case  202 . 
     (2) The position detection apparatus described above in the description of the present embodiment can be incorporated as part of an electronic computer. 
       FIG. 16  schematically shows a notebook type personal computer. 
     Referring to  FIG. 16 , the notebook type personal computer  1602  which is a kind of well-known electronic computer includes a sensor unit  1604  disposed on the rear face of an LCD unit  1603 . The sensor unit  1604  is configured such that the sensor  203 , magnetic path sheet  204  and shield sheet  205  described hereinabove with reference to  FIG. 2  are superposed on each other. 
     The signal processing section  206  described hereinabove with reference to  FIGS. 2 and 4  is connected to the sensor  203 , and the signal processing section  206  is incorporated in the inside of the notebook type personal computer  1602 . 
     It is to be noted that the control section  409  in the signal processing section  206  of  FIG. 4  can be replaced by a mathematical operation function of the notebook type personal computer  1602 . In particular, A/D converters  413   a  and  413   b  are connected to an internal bus of the notebook type personal computer  1602  while the control section  409 , which is a microcomputer, is omitted. 
     The present embodiment disclosed herein is directed to a position detection apparatus. 
     The magnetic path sheet disposed immediately below the coil board of the electromagnetic induction type position detection apparatus is configured by superposing ribbons of amorphous magnetic metal such that they extend obliquely. By configuring the magnetic path sheet in this manner, the variation of the sensitivity of coils in the proximity of a joining portion of the metal ribbons can be suppressed. Accordingly, a position detection apparatus can be implemented which is superior in detection sensitivity of a position pointer, position detection accuracy, noise resistance, and performance. 
     While a preferred embodiment of the present invention has been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.