Abstract:
A capacitive stylus pen is provided that inputs an indicated position to a tablet by capacitive coupling. The capacitive stylus pen includes a signal generating circuit that generates a signal having the same frequency as a frequency of a signal to be transmitted from an electrode, a transformer that boosts the generated signal and that includes a primary winding and a secondary winding, and an analog switch for controlling connection between a first end of the secondary winding and the electrode to an on-state or an off-state. The capacitive stylus pen further includes a power extracting circuit that extracts power to be supplied to the analog switch from an electromotive force induced in the secondary winding, and a capacitor coupled to a second end of the secondary winding to set the second end at a fixed potential in an alternating-current manner.

Description:
FIELD OF THE INVENTION 
     The present disclosure relates to improvements in a position indicator (e.g., capacitive stylus pen) in which an indicated position on a position detecting device (e.g., tablet) is detected based on a capacitive system. 
     BACKGROUND 
     Japanese Patent Laid-Open No. 2007-164356, hereinafter JP 164356, discloses a position indicator in which a coil is provided at a position indicating part of the indicator to obtain a coordinate position by electromagnetic induction with a tablet. JP 164356 also discloses a position indicator in which an electrode (a conductor core) is provided at a position-indicating part to obtain an indicated position by capacitive coupling with a tablet. 
     In addition, JP 164356 discloses that a writing pressure detected inside a position indicator is converted to digital information, and a transmission signal is subjected to amplitude shift keying (ASK) modulation to transmit writing pressure information as the digital information. As a result, the position indicator can stably detect and transmit the writing pressure without suffering from external influences such as noise. 
     Moreover, JP 164356 discloses a position indicator that uses an electric double-layer capacitor as a power supply to extend the use of the position indicator with a single charge. 
     Furthermore, other position indicators using a capacitive system are disclosed in Japanese Patent Laid-Open No. Hei 07-295722 and Japanese Patent Laid-Open No. Hei 06-250772. 
     The above-described position indicators, which use a capacitive system, have a common characteristic that a tablet sensor is able to detect a position indicator (e.g., a stylus) and can be used also as a touch panel to detect an object (e.g., a finger). 
     SUMMARY 
     Although it may be advantageous for a tablet sensor to also serve as a touch panel, position detection based on a capacitive system has a problem of being more susceptible to the influence of noise than an electromagnetic induction system. The noise problem may be alleviated by increasing a transmission voltage output by the position indicator of the capacitive system. 
     A transmission signal of the above-described high voltage is needed, which may be modulated with information on a writing pressure or the like. This in turn requires that the position indicator be provided with a high-voltage power supply. 
     In the position indicators of the type described above, the capacitance and voltage of a battery or a capacitor used as a power supply are generally limited in order to improve their operability. Thus, to increase the transmission voltage as described above, the supply voltage needs to be boosted through a direct current-direct current (DC-DC) converter, or the like. This would lead to problems, such as an increase in power consumption by the position indicator and a decrease in usage time for which the position indicator can be operated with a single charge. 
     According to an aspect, the present disclosure presents solutions for the above-described problems. In one embodiment a capacitive stylus pen is provided, which includes a light-weight, small-size battery or capacitor as a power supply, and which is operable to transmit a signal having larger voltage amplitude than a voltage of the power supply. The transmission signal may be modulated with information such as a writing pressure. Thus, the capacitive stylus pen allows a tablet to obtain information on an indicated position, a writing pressure, and so forth, accurately and reliably. 
     According to one aspect of the present invention, a capacitive stylus pen is provided, which includes a light-weight, small-size battery or capacitor as a power supply and which has low power consumption so as not to require frequent charging or battery replacement. 
     The present disclosure provides the following configurations and embodiments of a capacitive stylus pen that inputs an indicated position to a table based on capacitive coupling with the tablet. 
     In one embodiment, the capacitive stylus pen includes an electrode provided at a position indicating part of the pen, and a signal generating circuit that generates a signal having the same frequency as a frequency of a signal to be transmitted from the electrode. The capacitive stylus pen also includes a transformer that boosts the signal generated by the signal generating circuit and that includes a primary winding and a secondary winding that are wound around a magnetic body. The capacitive stylus pen still further includes an analog switch for controlling a connection between a first end of the secondary winding of the transformer and the electrode to be in an on-state or an off-state. The capacitive stylus pen also includes a power extracting circuit that extracts power to be supplied to the analog switch from an electromotive force induced in the secondary winding of the transformer. The capacitive stylus pen further includes a capacitor coupled to a second end of the secondary winding of the transformer to set the second end of the secondary winding at a fixed potential in an alternating-current manner. 
     In one aspect, the analog switch is controlled according to information represented by a binary code, to transmit the information by ASK modulation. 
     In another aspect, writing pressure information is included in the information represented by the binary code. 
     In another aspect, the signal generating circuit is formed of a line control (LC) oscillation circuit including the primary winding of the transformer as a constituent element of the LC oscillation circuit. 
     According to embodiments of the present invention, amplitude of an alternating current (AC) signal generated by the signal generating circuit is boosted by the transformer. The boosted signal is supplied to the electrode provided at the position indicating part of the capacitive stylus pen via the analog switch. The capacitor is connected to the second end of the secondary winding of the transformer to set the second end at a fixed potential in an alternating-current manner. Power is extracted from the first end of the secondary winding and is supplied as power supply for the analog switch. Therefore, the connection of the first end of the secondary winding to the electrode can be switched between the on-state or the off-state without lowering the amplitude of the signal on the secondary winding side, which allows for the tablet to stably obtain the indicated position of the capacitive stylus pen. 
     Furthermore, information on the writing pressure and so forth is transmitted as digital information by ASK modulation. Therefore, the information transmission is less susceptible to the influence of noise and the information on the writing pressure and so forth can be accurately obtained by the tablet. 
     In addition, a signal with large amplitude can be generated and controlled without increasing the supply voltage and, thus, power consumption can be reduced. Accordingly, operation time of the capacitive stylus pen may be extended, even with a power supply that uses a light-weight, small-size battery or capacitor. 
     In one aspect, the signal generating circuit is formed by an LC oscillation circuit including the primary winding of the transformer as a constituent element. Therefore, signal generation with low power consumption becomes possible. Accordingly, operation time of the capacitive stylus pen may be extended, even with a power supply that uses a light-weight, small-size battery or capacitor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing an example of an internal structure of a capacitive stylus pen in accordance with an embodiment disclosed herein. 
         FIG. 2  is a diagram showing an example of a circuit configuration of the capacitive stylus pen in accordance with an embodiment disclosed herein. 
         FIG. 3  is a diagram showing an example of voltages of a circuit configuration of the capacitive stylus pen in accordance with an embodiment disclosed herein. 
         FIG. 4  is a diagram showing an example of signals of a circuit configuration of the capacitive stylus pen in accordance with an embodiment disclosed herein. 
     
    
    
     DETAILED DESCRIPTION 
     A capacitive stylus pen according to the present disclosure will be described below with reference to the drawings.  FIG. 1  is a diagram showing an internal structure example of the capacitive stylus pen according to one embodiment of the present disclosure.  FIG. 2  is one example of a circuit configuration diagram of the capacitive stylus pen according to one embodiment of the present disclosure. In  FIGS. 1 and 2 , the same constituent element is shown by the same numeral. 
     Numeral  10  denotes a printed board on which circuit elements, ICs, and so forth configuring the circuit of  FIG. 2  are disposed. Numeral  11  denotes a pen core that forms a position indicating part of the capacitive stylus pen. Numeral  12  denotes a capacitance-variable capacitor whose capacitance changes according to the writing pressure. Numeral  13  denotes an electrode provided inside the pen core  11 . Numeral  14  denotes a connection line between circuitry disposed on the printed board  10  and the electrode  13 . 
     In one embodiment, the capacitance-variable capacitor  12  may be a capacitance-variable capacitor that is disclosed in Japanese Patent Laid-Open No. 2011-186803. For example, the capacitance-variable capacitor  12  capacitance may change according to a writing pressure applied to the pen core  11 . A description of the configuration of a capacitance-variable capacitor is omitted in the present disclosure. 
     The pen core  11  is physically joined to the capacitance-variable capacitor  12  to transmit a writing pressure to the capacitance-variable capacitor  12 . In addition, the electrode  13  is electrically connected to the connection line  14  where the pen core  11  and the capacitance-variable capacitor  12  are joined. That is, the pressure applied to the pen core  11  is transmitted to the capacitance-variable capacitor  12  and a transmission signal generated in the printed board  10  is transmitted, via the connection line  14 , from the electrode  13 . 
     Numerals  15 ,  16 , and  17  denote a battery, an oscillation circuit, and an analog switch, respectively. Numerals  18 ,  19 , and  20  denote a diode, a capacitor, and a transformer, respectively. In one embodiment, the analog switch  17  is a semiconductor device, such as a field effect transistor (FET). The transformer  20  includes a primary winding L 1  and a secondary winding L 2 . The primary winding L 1  forms a resonant circuit and is included in the oscillation circuit  16 . 
     Numeral  23  (see  FIG. 1 ) denotes a shield electrode having a hollow cylindrical shape. The shield electrode  23  is disposed to surround an outer circumference of the pen core  11  except for a tip part of the electrode  13 . The shield electrode  23  is connected to a fixed potential in the circuit of  FIG. 2 , such as to a negative terminal of the battery  15 . 
     Typically, a signal generated by the oscillation circuit  16  can only achieve a voltage that is approximately twice the battery voltage for the voltage across the primary winding L 1 . For example, if the voltage of the battery  15  is set to 1.5 V, the oscillation circuit  16  may only achieve a voltage of about 3 V. However, by adjusting a turns ratio between the primary winding L 1  and the secondary winding L 2  to, for example, one to three, a signal with voltage that is up to approximately three times the voltage of the primary winding L 1  (e.g., 9 V) may be generated across the secondary winding L 2 . 
     One (first) end of the secondary winding L 2  is connected to the electrode  13  via the analog switch  17  and is also connected to an anode side of the diode  18 . Power is extracted from a voltage generated at the first end of the secondary winding L 2  and is accumulated in a capacitor  21 . The capacitor  21  is connected to a positive-side power supply terminal VCC of the analog switch  17 . As a result, power is supplied to the analog switch  17 . 
     With the above-described configuration, only a power of about 4.5 V can be extracted because the diode  18  allows the passage of only the positive half-cycles of the AC voltage generated in the secondary winding L 2 . To address this, the capacitor  19  is inserted between the other (second) end of the secondary winding L 2  and a negative-side power supply terminal VSS of the analog switch  17 . Further, the negative-side power supply terminal VSS of the analog switch  17  is connected to a reference potential terminal GND (negative terminal of the battery  15 ). As a result, in a steady state, the capacitor  19  is charged to about 4.5 V with respect to the amplitude 9 V of the secondary winding L 2 . Therefore, the voltage generated at the first end of the secondary winding L 2  reaches 9 V at its peak and, thus, power of about 9 V is extracted in the capacitor  21 . 
       FIG. 3  is a diagram showing one example of how voltages at point a, point b, and point c of  FIG. 2  may change in the period from activation of the oscillation circuit  16  to when a voltage of the capacitor  19  reaches a steady state. 
     Numeral  22  denotes a microprocessor that operates based on a predetermined program. As will be described later, the microprocessor  22  charges and discharges the capacitance-variable capacitor  12  by setting a terminal P 2  to a high-level output state or an input state based on the predetermined program, and obtains the value of the writing pressure applied to the pen core  11  as a digital value. 
     An AC signal generated at the first end (point a of  FIG. 2 ) of the secondary winding L 2  of the transformer  20  is subjected to ASK modulation by the analog switch  17  and an output signal thereof (point d of  FIG. 2 ) is supplied to the electrode  13  via the connection line  14 . 
       FIG. 4  is a diagram showing one example of how signals at point e, point f, and point d of  FIG. 2  may change. The microprocessor  22  carries out control to keep a terminal P 1  of  FIG. 2  (point e) at a high level for a certain period of time. See the signal at point e in  FIG. 4 . As a result, during this period of time, a signal is radiated from the electrode  13  continuously. See the continuously transmitted signal at point d during the “CONTINUOUS TRANSMISSION PERIOD” as shown in  FIG. 4 . 
     In this continuous transmission period, the microprocessor  22  controls the terminal P 2  to obtain a writing pressure applied to the capacitance-variable capacitor  12 . Specifically, the microprocessor  22  charges the capacitance-variable capacitor  12  by setting the terminal P 2  to a high-level output state. Subsequently, the microprocessor  22  switches the terminal P 2  to an input state. At this time, a charge accumulated in the capacitance-variable capacitor  12  is discharged by a resistor coupled in parallel to the capacitance-variable capacitor  12 . Thus, the voltage of the capacitance-variable capacitor  12  gradually decreases. See the signal at point f in  FIG. 4 . The time Tp of  FIG. 4  is the period from the switching of the terminal P 2  to the input state to when a voltage at point f is equal to or lower than a threshold. The time Tp is equivalent to the writing pressure to be obtained. In this particular embodiment, the microprocessor  22  obtains the writing pressure (based on Tp) as a 10-bit value. 
     After the continuous transmission period has ended, the microprocessor  22  carries out ASK modulation by controlling the terminal P 1  to be a high and low levels with a predetermined cycle Td. See the signal at point e of  FIG. 4 . At this time, in the first cycle, the microprocessor  22  may set the terminal P 1  to a high level. See the start signal of  FIG. 4 . The purpose of this is to allow the tablet side to accurately determine the subsequent data transmission timing. 
     Subsequent to the start signal, the microprocessor  22  sequentially transmits the writing pressure data of 10 bits obtained by the above-described operation. Specifically, the microprocessor  22  sets the terminal P 1  to a low level when the transmission data is 0 and sets the terminal P 1  to the high level when the transmission data is 1. See the writing pressure data transmission period of  FIG. 4 . For example,  FIG. 4  shows a case in which the writing pressure to be transmitted is “1010111010.” 
     In one embodiment, the operation of  FIG. 4  is repeatedly carried out. 
     In the above-described embodiments, the turns ratio of the transformer  20  is set to one to three and the signal with amplitude of 9 V is generated on the secondary side. However, the turns ratio of the transformer  20  may be lowered to generate amplitude of a higher voltage. Alternatively, the turns ratio may be increased. 
     In the above-described embodiments, the battery of 1.5 V is used as a power supply. However, a battery of a different voltage may be used or a chargeable secondary battery may be used. Furthermore, an electric double-layer capacitor may be used as a power supply. 
     In the above-described embodiments, the voltage of the battery is used as the power supply for the circuit as it is. However, the voltage may be used after being stabilized through conversion to a different voltage. 
     In the above-described embodiments, the first end of the secondary winding of the transformer  20  is connected to the anode side of the diode  18 , and, thereby, the voltage generated at the first end of the secondary winding is extracted as power supply in the positive direction. However, an orientation of the diode may be changed and the voltage may be extracted as power supply in the negative direction to be supplied to the negative-side power supply terminal of the analog switch  17 . 
     In the above-described embodiments, only the writing pressure is reported as the information transmitted by ASK modulation. However, other kinds of information, such as switch information and an ID code unique to the pen, may be transmitted. 
     It is to be noted that the embodiments of the present disclosure is not limited to the foregoing embodiments, and that various changes can be made without departing from the spirit of the present disclosure.