Abstract:
A capacitive stylus pen that inputs an indicated position by capacitive coupling with a tablet is provided. The capacitive stylus pen includes a power supply that is charged in a contactless manner, a coil provided at a position indicating part of the capacitive stylus pen, an electrode that penetrates through the coil and protrudes to form a tip part, and an alternating-current signal generating circuit that supplies an alternating-current signal to the electrode. During contactless charging, the coil is magnetically coupled to an external charge device that has a penholder shape, to thereby charge the power supply, which supplies power to the alternating-current signal generating circuit.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a capacitive stylus pen that serves as a position indicator, which can be used to indicate a position on a tablet based on a capacitive coupling system. The capacitive stylus pen includes a transmission circuit that transmits information regarding the position indicator, such as writing pressure information. 
     BACKGROUND 
     Patent Document 1 (Japanese Patent Laid-open No. 2007-164356) co-assigned to the applicant, discloses a position indicator of an electromagnetic induction system. The position indicator of the electromagnetic induction system includes, at its position indicating part, a coil for indicating a coordinate position by electromagnetic induction with a tablet. The position indicator also includes an electric double layer capacitor that is used as a power supply. Patent Document 1 describes contactless charging by electromagnetic induction between the coil and a charger shaped in the form of a penholder. According to Patent Document 1, high-speed charging is performed when a user places the position indicator in the penholder, and thus the position indicator has superb operability which does not require the user to consciously perform charging operation. 
     In the case of the position indicator of the electromagnetic induction system of the type described above, a coordinate position is obtained by electromagnetic induction with the tablet. In general it is difficult to use the tablet sensor of the electromagnetic induction type also as a capacitive touch panel. 
     Patent Document 1 further discloses an example of a capacitive position indicator that can be used with a touch panel. Additional capacitive position indicators using a capacitive system are disclosed in Patent Document 2 (Japanese Patent Laid-open No. Hei 07-295722), Patent Document 3 (Japanese Patent Laid-open No. Hei 06-250772), etc. 
     Prior Art Documents 
     Patent Document 1: Japanese Patent Laid-open No. 2007-164356 
     Patent Document 2: Japanese Patent Laid-open No. Hei 07-295722 
     Patent Document 3: Japanese Patent Laid-open No. Hei 06-250772 
     SUMMARY 
     Technical Problem 
     In the example of the capacitive position indicator described in Patent Document 1, an electric double layer capacitor is used as a power supply, but a contact point that connects to an external power supply needs to be provided to charge the double layer capacitor. This renders charging operation of the position indicator cumbersome and, also, a contact failure may occur. 
     Furthermore, in the capacitive position indicator of Patent Document 1, an electrode for position detection, a shield electrode, a writing pressure sensor, and so forth are all positioned at the position indicating part of the position indicator, which is near the tip part of the position indicator and which is brought into contact with or close to the tablet. Therefore, there is no sufficient space for providing a coil for the purpose of contactless charging. 
     The present invention suggests a solution to the above-described problems by providing a capacitive stylus pen, which can be contactlessly charged by a charger in the shape of a penholder and thus has improved operability. 
     Technical Solution 
     In the invention of claim  1 , a capacitive stylus pen is provided that inputs an indicated position by capacitive coupling with a tablet. The capacitive stylus pen includes a power supply that is charged in a contactless manner, a coil that is located at a position indicating part, an electrode that penetrates through the coil for electrical connection and is so disposed as to protrude to a tip part, and an alternating-current signal generating circuit that supplies an alternating-current signal to the electrode. The coil is magnetically coupled to an external charge device to charge the power supply at a time of charge operation. 
     The invention of claim  2  is characterized in that a writing pressure detecting sensor is provided in the capacitive stylus pen, and the electrode or a line connecting to the electrode is provided in a core body that transmits a pressure applied at the tip part of the capacitive stylus pen to the writing pressure detecting sensor. 
     The invention of claim  3  is characterized in that an engaging part between the writing pressure detecting sensor and the core body serves as a contact point for supplying the alternating-current signal to the electrode. 
     It is preferable that the coil is wound around a hollow magnetic core as in the invention of claim  4 . 
     The invention of claim  5  is characterized in that the coil is set at a fixed potential to be used as a shield electrode at a time of normal operation, as opposed to the time of charge operation described above. 
     Advantageous Effect 
     According to this invention, a capacitive stylus pen is provided, which can be contactlessly charged by a charger in the shape of a penholder and thus has improved operability. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an internal structure diagram of a position indicating part of a capacitive stylus pen according to an embodiment of the present invention. 
         FIG. 2  is a circuit configuration diagram of the capacitive stylus pen of the embodiment of  FIG. 1 . 
         FIG. 3  is a diagram used for explaining the operation of the capacitive stylus pen of the embodiment of  FIG. 1 .  FIG. 4  is a diagram for explaining other embodiments of a capacitive stylus pen according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a diagram showing an internal structure of a position indicating part of a capacitive stylus pen according to an embodiment of the present invention.  FIG. 2  is a circuit configuration diagram of an embodiment example. In  FIGS. 1 and 2 , the same constituent element is shown by the same symbol. Numeral  11  denotes an electrode core. Numeral  12  denotes a coil wound around a ferrite core  21 , wherein the ferrite core  21  is as an example of a magnetic core in which a penetration hole  21   a  is opened along the axial direction. Numeral  13  denotes a variable-capacitance capacitor whose capacitance changes depending on writing pressure. The diameter of the penetration hole  21   a  made in the ferrite core  21  is sufficiently larger than the outer diameter of the electrode core  11 . In this embodiment, the electrode core  11  is a bar-shaped conductor and is obtained by integrally forming an electrode and a core body that presses the variable-capacitance capacitor  13 . 
     The variable-capacitance capacitor  13  is one example of a writing pressure detecting sensor and can be formed, for example, by a variable-capacitance capacitor as described in Japanese Patent Laid-open No. Hei 4-96212. Specifically, the variable-capacitance capacitor  13  may be composed of two electrodes opposed to each other, and a dielectric provided between the two electrodes. The contact area between one of the two electrodes and the dielectric is changed depending on the applied pressure. It should be noted, however, that the variable-capacitance capacitor  13  is not limited to such a variable-capacitance capacitor. For example, the variable-capacitance capacitor  13  may be a capacitive pressure sensing semiconductor device, where capacitance changes depending on the applied pressure, as described in Japanese Patent Application No. 2012-15254. 
     In  FIG. 2 , numeral  15  denotes an electric double layer capacitor and numeral  16  denotes a diode for rectification. Numeral  17  denotes a voltage conversion circuit and numeral  18  denotes an oscillation circuit as an alternating-current signal generating circuit. As shown in  FIG. 2 , in this example, one end of the coil  12  is connected to the anode of the diode  16  and the other end is grounded (GND). Furthermore, one terminal of the electric double layer capacitor  15  is connected to the cathode of the diode  16  and the other end is grounded. 
     The electrode core  11  penetrates through the penetration hole  21   a  of the ferrite core  21 , around which the coil  12  is wound, and is physically coupled (engaged) with the variable-capacitance capacitor  13 . In addition, the electrode core  11  is electrically connected to a connection line  14  where the electrode core  11  and the variable-capacitance capacitor  13  are coupled. The connection line  14  electrically connects the electrode core  11  to the oscillation circuit  18 . Therefore, a pressure (writing pressure) applied to the electrode core  11  is transmitted to the variable-capacitance capacitor  13  due to the physical coupling between the electrode core  11  and the variable-capacitance capacitor  13 , and a transmission signal from the oscillation circuit  18  is transmitted to the electrode core  11  via the connection line  14 . 
     The oscillation circuit  18  generates a signal whose frequency changes depending on the capacitance of the variable-capacitance capacitor  13 , and supplies the generated signal to the electrode core  11 . The signal from the oscillation circuit  18  is radiated from the electrode core  11  as an electric field based on the signal. In one embodiment, the oscillation circuit  18  is formed by an LC oscillation circuit that utilizes resonance by a coil and a capacitor. 
       FIG. 3  is a diagram showing the relationship between a pressure applied to the variable-capacitance capacitor  13  and a frequency of a signal from the oscillation circuit  18 . In a tablet that detects the coordinate position of the capacitive stylus pen of this embodiment, a writing pressure applied to the electrode core  11  can be obtained from a frequency of a signal from the oscillation circuit  18 . 
     The voltage conversion circuit  17  converts a voltage accumulated in the electric double layer capacitor  15  to a threshold voltage and supplies the converted voltage as a power supply to the oscillation circuit  18 . In one embodiment, the voltage conversion circuit  17  is a buck-type circuit, where an output voltage is lower than a voltage across the electric double layer capacitor  15 . In another embodiment, the voltage conversion circuit  17  is a boost-type circuit, where an output voltage is higher than a voltage across the electric double layer capacitor  15 . In a further embodiment, the voltage conversion circuit  17  is a circuit of a boost-buck type that operates as a buck circuit when a voltage across the electric double layer capacitor  15  is higher than the threshold voltage, and operates as a boost circuit when a voltage across the electric double layer capacitor  15  is lower than the threshold voltage. 
     When the capacitive stylus pen of this embodiment is mounted on a charger (not shown; see Patent Document 1), an induced electromotive force is generated in the coil  12  by an alternating magnetic field generated by the charger such that the electric double layer capacitor  15  is charged via the diode  16 . 
     When the capacitive stylus pen of this embodiment is carrying out a normal operation (when a charge operation is not being carried out), the coil  12  is at a fixed potential, such as a ground potential (GND), and, therefore, acts as a shield electrode provided around the electrode core  11 . It should be noted that the fixed potential of the coil  12 , when the capacitive stylus pen is carrying out the normal operation, is not limited to a ground potential. For example, the fixed potential of the coil  12  may be a positive-side potential of the power supply, or may be an intermediate potential between the positive-side potential of the power supply and the ground potential. 
     Effects of Embodiment 
     According to the capacitive stylus pen of the above-described embodiment, because the electrode core  11  is so disposed as to penetrate through the charge coil  12  to protrude the tip part thereof, the writing pressure applied to the electrode core  11  is transmitted to the variable-capacitance capacitor  13  without being inhibited. 
     Furthermore, according to the capacitive stylus pen of the above-described embodiment, because the electrode core  11  is so disposed as to penetrate through the charge coil  12 , a cross-sectional area of the charge coil  12  can be large. Therefore, charging operation can be performed efficiently at high speed. 
     In addition, according to the capacitive stylus pen of the above-described embodiment, because the electrode core  11  is so disposed as to penetrate through the charge coil  12 , the charge coil  12  and the electrode core  11  can be disposed in a compact space and the capacitive stylus pen can be made with a small diameter. 
     Moreover, according to the capacitive stylus pen of the above-described embodiment, because the charge coil  12  acts as a shield electrode at a time of normal operation (when a charge operation is not being carried out), even when the position indicating part of the capacitive stylus pen is held by a user&#39;s hand the signal output from the electrode core  11  is not affected. 
     Furthermore, according to the capacitive stylus pen of the above-described embodiment, because the charge coil  12  is provided around the electrode core  11 , contactless charging becomes possible by a charger provided in the shape of a penholder, thus improving operability of the capacitive stylus pen. 
     Other Embodiments or Modification Examples 
     In the above-described embodiment, a frequency of a signal generated by the oscillation circuit  18  is changed by changing the capacitance of the variable-capacitance capacitor  13  depending on the writing pressure. However, the frequency of the signal may be changed by changing the inductance of an LC oscillation circuit based on a stroke change of the electrode core  11  depending on the writing pressure. 
     As the circuit to perform charging operation in the above-described embodiment, a half-wave rectification circuit based on the coil  12  and the diode  16  is used. However, a full-wave rectification circuit made by providing an intermediate terminal to the coil  12  and connecting it to the GND may be used, or a bridge rectification circuit may be used. 
     In the above-described embodiment, the electrode core  11  has a form of a bar-shaped conductor made by monolithically forming an electrode and a core body. However, the electrode core  11  may have a form obtained by forming an electrode and a core body separately and coupling them together.  FIGS. 4(A)  to (C) show various examples of an electrode coupled to a core body that transmits a pressure (writing pressure) to a writing pressure detecting sensor, such as a variable-capacitance capacitor. 
     In a first example shown in  FIG. 4(A) , a core body  31 A is formed of a bar-shaped body made of an insulating material, such as resin, has a circular or polygonal column shape, and has an end part, in the axial core direction, that is formed into a tapered shape to be employed as a pen tip of a stylus pen. Furthermore, an electrode  32 A, formed of a conductor with a thin bar shape, is disposed at the center axis position of the core body  31 A. In the case of this first example, the electrode  32 A is also exposed at a coupling part (engaging part) between the core body  31 A and the variable-capacitance capacitor  13 , which is an example of a writing pressure detecting sensor, and is electrically connected to the connection line  14  directly. That is, the coupling part between the variable-capacitance capacitor  13 , as a writing pressure detecting sensor, and the core body  31 A serves as a contact point for supplying an alternating-current signal to the electrode  32 A. 
     In a second example shown in  FIG. 4(B) , a core body  31  B is formed of a bar-shaped body made of an insulating material, such as resin, and has a circular or polygonal column shape. An electrode  32 B, which is formed of a conductor with a substantially triangular pyramid shape, is coupled to an end part of the core body  31 B in the axial core direction by an adhesive or the like. Furthermore, a connection part (connecting part)  33 B, formed of a line-shaped or bar-shaped conductor, is included at a center axis position of the core body  31 B, and is electrically connected to the electrode  32 B. In the case of this second example, the electrode  32 B is electrically connected to the connection line  14  via the connection part  33 B. 
     In a third example shown in  FIG. 4(C) , a core body  31 C is formed of a bar-shaped body made of an insulating material, such as resin, and has a circular or polygonal column shape. A spherical electrode  32 C is buried at an end part of the core body  31 C in the axial core direction. Furthermore, a connection part (connecting part)  33 C formed of a line-shaped or bar-shaped conductor is included at the center axis position of the core body  31 C, and is electrically connected to the electrode  32 C. In the case of this third example, the electrode  32 C is electrically connected to the connection line  14  via the connection part  33 C. 
     In the above-described embodiments, a writing pressure detected by a writing pressure detecting sensor is converted to a frequency to be supplied to an electrode. However, a signal attribute associated with the writing pressure is not limited to frequency. For example, the writing pressure may be associated with a phase of a signal, or a number of times of connection/disconnection of a signal. 
     In the above-described embodiments, an alternating-current signal generating circuit sends out an alternating-current signal according to a writing pressure detected by a writing pressure detecting sensor via an electrode. However, the alternating-current signal generated from the alternating-current signal generating circuit is not limited to the signal according to the writing pressure. For example, a tilt detected by a tilt detecting sensor that detects the tilt of a stylus pen relative to an indication input surface may be supplied to the electrode as the alternating-current signal. 
     DESCRIPTION OF REFERENCE SYMBOLS 
     
         
         
           
               11  Electrode core, 
               12  Coil, 
               13  Variable-capacitance capacitor, 
               14  Connection line, 
               15  Electric double layer capacitor, 
               16  Diode, 
               17  Voltage conversion circuit, 
               18  Oscillation circuit, 
               21  Ferrite core.