Abstract:
A position indicator contains a resonant circuit. A first resistor having a minimum resistance, a second resistor having a maximum resistance, and a variable resistor whose resistance varies within the range of minimum to maximum resistances are connected to the resonant circuit at first, second, and third specific times based on predetermined timing information supplied from a tablet. Signal levels detected by the tablet at the first and second specific times are used as a lower limit and an upper limit of an operation quantity. Within this range, a signal level detected at the third specific time is converted into an operation quantity. Thus, the continuous quantity can be detected at a fast sampling rate without being influenced by the height and tilt of the position indicator.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to position detectors and position indicators therefor for use in inputting graphics and characters to computers.  
           [0003]    2. Description of the Related Art  
           [0004]    One type of known position detector is disclosed in Japanese Unexamined Patent Application Publication No. Kokai Hei 1-53223 (earlier patent application  1 )(U.S. Pat. No. 5,134,689).  
           [0005]    In this position detector, a position indicator is provided with a resonant circuit containing a coil and a capacitor. Electromagnetic waves are transmitted between a tablet and the position indicator, whereby coordinate values of a position indicated by the position indicator are detected. Of a plurality of loop coils which are provided in the tablet and which are parallel to position detecting directions, the loop coils are sequentially selected, and the selected loop coil emits electromagnetic waves. The electromagnetic waves which are reemitted from the resonant circuit in the position indicator are received, and coordinate values of the position indicated are thereby detected on the basis of a distribution of reception signal strengths.  
           [0006]    In this type of position detector, there are demands for inputting, in addition to the coordinate values of the position indicated, information which indicates the state in which the position indicated (which should be actually input) is specified. Examples of such information include information for continuously varying the line thickness, hue, and luminance.  
           [0007]    In the earlier patent application  1 , the coil which forms the resonant circuit is a coil whose inductance continuously varies in accordance with the writing force. Thus, the resonance frequency continuously varies in accordance with the writing force. A continuous variation in the resonance frequency is detected as a continuous variation in phase angle, and the writing force is detected thereby.  
           [0008]    The range of phase angles, which corresponds to the range of writing forces to be detected, varies as a result of factors such as the distance from the tablet to the position indicator, the tilt of the position indicator with respect to the tablet, etc. Also, the range of phase angles (writing forces) varies in accordance with the aging of the inductance of the coil. It is thus impossible to accurately detect writing forces.  
           [0009]    In Japanese Unexamined Patent Application Publication No. Kokai hei 7-225644 (earlier patent application 2), the applicant of the present invention has proposed a pen in which the writing force and the continuous quantities of the three RGB colors are set, thereby allowing information such as color hue and color strength to be input with the pen. More specifically, four variable capacitors are provided in order to set the writing force and the quantities of three colors. At four different times, the four variable capacitors are sequentially connected to the resonant circuit, whereby the four continuous quantities are independently detected.  
           [0010]    With the foregoing pen, the phase of a signal when no variable capacitors are connected is detected beforehand, thereby suppressing effects caused by the aging of the coil inductance as in the earlier patent application  1 . In contrast, as in the earlier patent application  1 , the range of phase angles corresponding to the range of continuous quantities to be detected varies as a result of factors such as the distance from the pen to the tablet, the tilt of the pen with respect to the tablet, etc. It is thus impossible to obtain the correct continuous quantities.  
           [0011]    The applicant of the present invention has proposed in Japanese Unexamined Patent Application Publication No. 5-313439 (earlier patent application 3)(U.S. Pat. No. 5,679,930) a position indicator which converts therein a continuous quantity into binary code and which transmits back the binary code to the tablet, thereby detecting the continuous quantity.  
           [0012]    With this position indicator, an accurate continuous quantity can be detected while factors such as the height and tilt of the position indicator with respect to the tablet have no influence on the position indicator. In contrast, the structure of the position indicator becomes complex. The position indicator has another disadvantage in that the sampling rate decreases due to the necessity for repeating transmission/reception for each bit because a single continuous quantity is transmitted back in terms of binary code.  
         SUMMARY OF THE INVENTION  
         [0013]    Accordingly, it is an object of the present invention to provide a position detector and a position indicator therefor for accurately detecting a continuous quantity in accordance with an operation, at a low cost, without causing the structure of the position indicator to become complex.  
           [0014]    It is another object of the present invention to provide a position detector and a position indicator therefor for accurately detecting a continuous quantity in accordance with an operation at a fast sampling rate.  
           [0015]    According to the present invention, the foregoing objects are achieved through provision of a position detector including a position indicator including a resonant circuit which includes at least a coil and a capacitor; and a tablet for transmitting/receiving electromagnetic waves to/from the position indicator, whereby coordinate values of a position indicated by the position indicator are detected. Electromagnetic waves which include predetermined timing information are generated by the tablet. The predetermined timing information is extracted from a dielectric voltage generated at the resonant circuit in the position indicator which has received the electromagnetic waves. A resonance characteristic of the resonant circuit in the position indicator is controlled in accordance with operations at specific times based on the predetermined timing information. The phase or strength of electromagnetic waves which are generated at the specific times by the resonant circuit in the position indicator is detected.  
           [0016]    The position indicator further includes a first resonance characteristic control unit for causing the resonance characteristic of the resonant circuit at a first specific time based on the predetermined timing information to be a value within a predetermined variable range of the resonance characteristic in accordance with a continuous operation quantity; a second resonance characteristic control unit for causing the resonance characteristic of the resonant circuit at a second specific time based on the predetermined timing information to be a maximum within the predetermined variable range of the resonance characteristic; and a third resonance characteristic control unit for causing the resonance characteristic of the resonant circuit at a third specific time based on the predetermined timing information to be a minimum within the predetermined variable range of the resonance characteristic.  
           [0017]    The tablet includes a detection value obtaining unit for obtaining the phase or strength of the electromagnetic waves generated by the resonant circuit in the position indicator at the first, second, and third specific times as first, second, and third detection values, respectively; and a processor for detecting a continuous operation quantity by relating the first detection value to a range from an upper limit to a lower limit, the upper limit being one of the second and third detection values and the lower limit being the other detection value.  
           [0018]    According to the present invention, even when the level and phase of a signal vary because of the height and tilt of a position indicator, a continuous quantity in accordance with an operation can be accurately detected. Also, detection of a single continuous quantity and the maximum reference value and the minimum reference value of the continuous quantity can each be performed by one transmission/reception. As a result, a continuous quantity in accordance with an operation can be accurately detected at a high sampling rate.  
           [0019]    One aspect of the present invention comprises a stylus for use with a sensor where the stylus includes a resonant circuit having a resonance characteristic with a variable value that is affected by a condition of the stylus. The variable value is a first value when the condition of the stylus is at a maximum level and a second value when the condition of the stylus is at a minimum level. The stylus also includes a first resonance control circuit for causing the resonance characteristic to be the first value and a second resonance control circuit for causing the resonance characteristic to be the second value. The condition of the stylus is then determined by detecting the variable value, the first value, and the second value.  
           [0020]    Another aspect of the invention comprises a method of determining a level of a condition of a stylus that requires providing a resonant circuit in a stylus that has a resonance characteristic with a variable value that is affected by the condition of the stylus. The condition has a minimum level and a maximum level, and the resonance characteristic has a first value when the level of the condition of the stylus is at the maximum level and a second value when the level of the condition of the stylus is at the minimum level. A first resonance control circuit is also provided for causing the variable value to be the first value and a second resonance control circuit is provided for causing the variable value to be the second value. The variable value, first value and second value are detected, and the level of the condition is calculated by comparing the variable value to the first value and the second value.  
           [0021]    A further aspect of the invention comprises a method of determining a level of pressure applied to a tip of a stylus where the stylus has a resonant circuit with a resonance characteristic having a variable value affected by pressure applied to the tip of the stylus. The pressure can vary between a minimum level and a maximum level, and the resonance characteristic has a first value when the level of the pressure is at the maximum level and a second value when the level of the pressure is at the minimum level. A first resonance control circuit is used to cause the variable value to be the first value, and a second a second resonance control circuit is used to cause the variable value to be the second value. The variable value, the first value and the second value are determined, and the level of pressure is calculated by comparing the variable value to the first value and the second value. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1 is a circuit diagram of a position indicator according to a first embodiment of the present invention;  
         [0023]    [0023]FIG. 2 is a block diagram of a tablet according to the first embodiment of the present invention;  
         [0024]    [0024]FIG. 3 includes waveform diagrams showing the operation of the first embodiment of the present invention;  
         [0025]    [0025]FIG. 4 is a flowchart showing a process of detecting a position in the first embodiment;  
         [0026]    [0026]FIG. 5 is a circuit diagram of a position indicator according to a second embodiment of the present invention;  
         [0027]    [0027]FIG. 6 is a block diagram of a position detector according to the second embodiment of the present invention;  
         [0028]    [0028]FIG. 7 includes waveform diagrams showing the operation of the second embodiment of the present invention;  
         [0029]    [0029]FIG. 8 is a circuit diagram of a position indicator according to a third embodiment of the present invention;  
         [0030]    [0030]FIG. 9 is an external view of the position indicator according to the third embodiment of the present invention: and  
         [0031]    [0031]FIG. 10 includes waveform diagrams showing the operation of the third embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]    First Embodiment  
         [0033]    Structure of First Embodiment  
         [0034]    [0034]FIG. 1 shows the structure of a position indicator according to a first embodiment of the present invention.  
         [0035]    Referring to FIG. 1, a coil  11   a  and a capacitor  11   b  form a resonant circuit  11  which resonates at a predetermined frequency f o . A power supply circuit  12  extracts power from a high-frequency voltage generated at the resonant circuit  11 .  
         [0036]    A detector circuit  13  detects timing in accordance with transmission/reception based on a signal (b) generated at the resonant circuit  11 . An output port of the detector circuit  13  is connected to a comparator  14 . The comparator  14  extracts a clock signal (c) which corresponds to transmission/reception of electromagnetic waves by a tablet. The clock signal (c) is connected to a clock input terminal of a shift register  15 .  
         [0037]    The clock signal (c) is also connected to an integrating circuit  16 , and the integrating circuit  16  generates a signal (d) which generates a voltage only in a continuous transmission period (described below). The signal (d) is supplied to a reset terminal of the shift register  15 . The signal (d) passes through an inverter  17 , a differentiating circuit  18 , and a comparator  19 , thereby generating a signal (e) which is outputted at a high level only in a period from the end of the continuous transmission to the end of the subsequent transmission. The signal (e) is connected to a data input terminal of the shift register  15 .  
         [0038]    Three outputs (Q 0  to Q 2 ) of the shift register  15  are connected to differentiating circuits  20 ,  21 ,  22 , respectively, all of which have the same time constant. The differentiating circuits  20 ,  21 , and  22  are connected to inverters  23 ,  24 ,  25 , respectively.  
         [0039]    A variable resistor  28  formed of pressure-sensitive conductive rubber or the like is connected to the inverter  25 . The variable resistor  28  is connected to the resonant circuit  11  through a diode  31 .  
         [0040]    The resistance of the variable resistor  28  varies in accordance with a pressure. In the first embodiment, the variable resistor  28  is employed to detect a writing force applied with a pen. The range of writing forces to be detected is from 0 g to 500 g. Under a load within this range, the resistance of the variable resistor  28  varies within the range of Rmax to Rmin.  
         [0041]    A resistor  27  which has the same resistance as the maximum resistance Rmax available to the variable resistor  28  is connected to the inverter  24 . The resistor  27  is connected to the resonant circuit  11  through a diode  30 . A resistor  26  which has the same resistance as the minimum resistance Rmin available to the variable resistor  28  is connected to the inverter  23 . The resistor  26  is connected to the resonant circuit  11  through a diode  29 .  
         [0042]    [0042]FIG. 2 shows portions of a position detector, excluding the position indicator, according to the first embodiment of the present invention. The structure of the tablet according to the first embodiment of the present invention is illustrated in FIG. 2.  
         [0043]    Referring to FIG. 2, a group of loop coils  41  consisting of X 1  to X 40  and Y 1  to Y 40  is provided in the X-axis direction and the Y-axis direction, respectively. These loop coils  41  are connected to a selector circuit  42  which selects the loop coils  41  one at a time.  
         [0044]    An oscillation circuit  43  oscillates at the same frequency as the resonant frequency f o  of the position indicator. The oscillation circuit  43  is connected at a transmitting side (T) of a transmission/reception switching circuit  45  through a electric current driver  44 . The transmission/reception switching circuit  45  is connected to the selector circuit  42 , and electromagnetic waves at the frequency f o  are radiated from a selected loop coil towards the position indicator.  
         [0045]    A receiving side (R) of the transmission/reception switching circuit  34  is connected to an amplifier circuit  46 , and the amplifier circuit  46  is connected to a detector circuit  47 . The detector circuit  47  is connected to a low pass filter  48 , and the low pass filter  48  is connected to an integrating amplifier  49 . The integrating amplifier  49  stores and holds a reception signal for a predetermined period of time. The voltage stored by the integrating amplifier  49  is output to an AD converter circuit  50 , and an output of the AD converter circuit  50  is connected to a CPU  51 .  
         [0046]    The CPU  51  transmits control signals (information) to the selector circuit  42 , the transmission/reception switching circuit  45 , the integrating amplifier  49 , and the AD converter circuit  50 .  
         [0047]    Operation of First Embodiment  
         [0048]    Referring to FIGS. 1 and 2, the operation of the first embodiment will now be described.  
         [0049]    In the first embodiment, as in a known position indicator, overall scanning is performed to roughly detect the position at which the position indicator is placed on a position detecting surface formed by the group of loop coils  41 .  
         [0050]    Overall scanning is performed in the following manner: All of the forty loop coils in the X-axis direction and the Y-axis direction are selected one at a time, and electromagnetic waves are transmitted/received. It is checked whether a signal level greater than or equal to a predetermined value is received. A loop coil which generates a signal with the highest signal level (hereinafter referred to as a peak coil) is detected.  
         [0051]    The operation subsequent to detecting the approximate position of the position indicator by the foregoing overall scanning will now be described. In the first embodiment, a case in which the position indicator is placed near the intersection of the loop coils X 7  and the Y 5  is used for explanation.  
         [0052]    [0052]FIG. 3 includes schematic waveform diagrams showing the operation of portions after the approximate position of the position indicator has been detected. Referring to FIG. 3, traces (a) to (j) show waveforms of portions denoted by the same reference symbols in FIGS. 1 and 2.  
         [0053]    The CPU  51  applies a control signal to the selector circuit  42  so that the selector circuit  42  selects the loop coil X 7  (peak coil) and a control signal to the transmission/reception switching circuit  45  so that the transmission/reception switching circuit  45  is switched to the transmitting side (T). The loop coil X 7  emits electromagnetic waves at the frequency f o . The emission of electromagnetic waves is continued for a relatively long period of time (continuous transmission period) such as 1 mS. Subsequently, the CPU  51  applies a control signal to the transmission/reception switching circuit  45  so that the transmission/reception switching circuit  45  is switched to the receiving side (R).  
         [0054]    After the transmission of the electromagnetic waves ends, the signal remains in the resonant circuit  11  for a while. Therefore, the tablet detects this signal as the signal (i). A receiving period for receiving the signal (i) is continued for a period in which the signal in the resonant circuit  11  is sufficiently attenuated (such as 100 μS).  
         [0055]    When the receiving period subsequent to the continuous transmission ends, a relatively brief transmission period (such as 50 μS) and a receiving period (such as 100 μS) are each repeated three times in order to detect a writing force applied to the variable resistor  28 .  
         [0056]    In the first transmission/reception, as shown in FIG. 3, the terminal Q 0  of the shift register  15  rises to the high level. Thus, the signal f becomes the low level at the rising edge and remains the low level for a period determined by the time constant of the differentiating circuit  20 . In the meantime, the signal b at both ends of the resonant circuit  11  is attenuated by the diode  29  and the resistor  26 .  
         [0057]    Similarly, in the second transmission/reception, the signal b at both ends of the resonant circuit  11  is attenuated by the diode  30  and the resistor  27 .  
         [0058]    Similarly, in the third transmission/reception, the signal b at both ends of the resonant circuit  11  is attenuated by the diode  31  and the variable resistor  28 .  
         [0059]    The strengths of the signals received by the tablet for the first to third times correspond to the values of the resistors  26  and  27  and the variable resistor  28 , respectively. In these three periods, the levels of integration outputs j are denoted by Va, Vb, and Vp, respectively. As shown in FIG. 3, Va is lower than Vb, and Vp is between Va and Vb. Specifically, when a load applied by the writing force is zero, then Vp=Vb. Under the maximum load (500 g), Vp=Va. By computing the level of Vp between Va and Vb based on proportions, the writing force value within the range of loads to be detected can be accurately computed. This operation constitutes a feature of the present invention.  
         [0060]    When the writing force detecting period ends, the CPU  51  performs partial scanning to detect the Y-coordinate value and the X-coordinate value. This operation is performed in a manner similar to a known position indicator.  
         [0061]    [0061]FIG. 4 shows a process of detecting a position in the first embodiment. Referring to FIG. 4, the number of times electromagnetic wave transmissions/receptions are performed in the writing force detecting period is indicated by i. Specifically, V 1 =Va, V 2 =Vb, and V 3 =Vp.  
         [0062]    Extended First Embodiment  
         [0063]    Although a variable resistor formed of pressure-sensitive conductive rubber has been used to detect a continuous quantity, the present invention can be implemented by other methods.  
         [0064]    When a load of 0 g is applied to the variable resistor  28 , the resistance of the variable resistor  28  becomes substantially maximum. In such a case, the differentiating circuit  21 , the inverter  24 , the resistor  27 , and the diode  30  in the position indicator, and the second transmission/reception during the writing force detecting period can be omitted, and the reception level immediately after the continuous transmission can be used as Vb.  
         [0065]    Second Embodiment  
         [0066]    Structure of Second Embodiment  
         [0067]    [0067]FIG. 5 shows the structure of a position indicator according to a second embodiment of the present invention. Referring to FIG. 5, the same reference numerals are given to components corresponding to those in the first embodiment. The structure shown in FIG. 5 differs from that of the first embodiment (FIG. 1) in the circuit configuration among the differentiating circuits  20 ,  21 , and  22  and the resonant circuit  11 .  
         [0068]    Referring to FIG. 5, the differentiating circuits  20 ,  21 , and  22  are connected to comparators  61 ,  62 , and  63 , respectively. The comparators  61 ,  62 , and  63  are connected to analog switches  67 ,  68 , and  69 , respectively.  
         [0069]    The capacitance of a variable capacitor  66  varies in accordance with a writing force. The variable capacitor  66  is connected in parallel to the resonant circuit  11  through the analog switch  69 . It is assumed that a writing force to be detected ranges from 0 g to 500 g and that, within this range of loads, the electrostatic capacitance of the variable capacitor  66  varies within the range of Cmin to Cmax.  
         [0070]    A capacitor  64  has the same capacitance as the minimum capacitance Cmin available to the variable capacitor  66 . The capacitor  64  is connected in parallel to the resonant circuit  11  through the analog switch  67 . A capacitor  65  has the same capacitance as the maximum capacitance Cmax available to the resonant capacitor  66 . The capacitor  65  is connected in parallel to the resonant circuit  11  through the analog switch  68 .  
         [0071]    [0071]FIG. 6 shows the structure of a tablet according to the second embodiment of the present invention. The same reference numerals are given to components corresponding to those in the first embodiment. The structure shown in FIG. 6 differs from that of the first embodiment (FIG. 2) in that a second receiving system is provided to detect the phase of a signal which returns from the position indicator and to detect a wiring force.  
         [0072]    Referring to FIG. 6, a synchronous detector  71  is connected to the amplifier circuit  46 . The oscillation circuit  43  is connected to the synchronous detector  71  and supplies a clock which is used as a reference for phase detection. The synchronous detector  71  is connected to a low pass filter  72 , and the low pass filter  72  is connected to an integrating amplifier  73 . The integrating amplifier  73  stores and holds a reception signal for a predetermined period of time. The voltage held by the integrating amplifier  73  is output to an AD converter circuit  74 , and an output of the AD converter circuit  74  is connected to the CPU  51 .  
         [0073]    Operation of Second Embodiment  
         [0074]    Referring to FIGS. 5 and 6, the operation of the second embodiment will now be described.  
         [0075]    As in the first embodiment, in the second embodiment, overall scanning is performed to detect an approximate position at which the position indicator is placed on the position detecting surface formed by the group of loop coils  41 .  
         [0076]    The operation subsequent to detecting the approximate position of the position detector by overall scanning will now be described. A case in which the position indicator is placed at the intersection of the loop coils X 7  and Y 5  is used for explanation.  
         [0077]    [0077]FIG. 7 includes schematic waveform diagrams showing the operation of portions after the approximate position of the position indicator has been detected. Referring to FIG. 7, traces (a) to (k) show waveforms of portions denoted by the same reference symbols in FIGS. 5 and 6.  
         [0078]    As shown in FIG. 7, the second embodiment is similar to the first embodiment in that the writing force detecting period, Y-coordinate detecting period, and X-coordinate detecting period are repeated subsequent to the continuous transmission period. Also, the signals a to j are substantially the same as those in the first embodiment. In contrast, the second embodiment differs form the first embodiment in that the level of the integration output (j) in the writing force detecting period does not vary whereas the level of the integration output (k) by the signal from the synchronous detector  71  varies.  
         [0079]    Referring to FIG. 7, when the continuous transmission and the subsequent reception period end, a relatively brief transmission period (for example, 50 μS) and a reception period (for example, 100 μS) are each repeated three times in order to detect a writing force applied to the variable capacitor  66 .  
         [0080]    In the first transmission/reception, the signal f becomes the high level, and the analog switch  67  is turned ON. As a result, the capacitor  64  is connected to the resonant circuit  11 , thereby slightly changing the resonant frequency of the resonant circuit  11 . The synchronous detector  71  detects a variation in frequency of a signal returning from the resonant circuit  11  and uses this frequency shift as a phase shift. In response, the integrating amplifier  73  outputs the signal k at a level in accordance with the phase.  
         [0081]    Similarly, in the second transmission/reception, the capacitor  65  is connected to the resonant circuit  11 , and the signal k in accordance with the electrostatic capacitance of the capacitor  65  is output.  
         [0082]    Similarly, in the third transmission/reception, the variable capacitor  66  is connected to the resonant circuit  11 , and the signal k in accordance with the writing force at that time is output.  
         [0083]    The strengths of the signals k output from the integrating amplifier  73  for the first to third times correspond to the values of the capacitors  64  and  65  and the variable capacitor  66 , respectively. If the strengths of the signals k are denoted by Va, Vb, and Vp, respectively, as shown in FIG. 6, Vp is within Va and Vb. Specifically, when a load applied by the writing force is zero, then Vp=Va. Under the maximum load (500 g), Vp=Vb. By computing the level of Vp between Va and Vb based on proportions, the writing force value within the range of loads to be detected can be accurately computed. This operation constitutes a feature of the present invention.  
         [0084]    When the writing force detecting period ends, as in the first embodiment, the CPU  51  performs an operation to detect the Y-coordinate value and the X-coordinate value.  
         [0085]    The process of detecting a position in the second embodiment is basically the same as that in the first embodiment.  
         [0086]    Extended Second Embodiment  
         [0087]    Although analog switches are used to connect the capacitors  64  and  65  and the variable capacitor  66  to the resonant circuit  11 , the capacitors  64  and  65  and the variable capacitor  66  can be connected to the resonant circuit  11  through diodes instead, as in the first embodiment.  
         [0088]    Although the output value of the integrating amplifier  73  decreases as the electrostatic capacitance of the variable capacitor  66  increases, the synchronous detector  71  can be configured in order that this relationship can be reversed.  
         [0089]    Third Embodiment  
         [0090]    Structure of Third Embodiment  
         [0091]    [0091]FIG. 8 shows the structure of a position indicator according to a third embodiment of the present invention. Referring to FIG. 8, the same reference numerals are given to components corresponding to those in the first embodiment.  
         [0092]    Referring to FIG. 8, the coil  11   a  and the capacitor  11   b  form the resonant circuit  11  which resonates at the predetermined frequency f o . The power supply circuit  12  extracts power from the high-frequency voltage generated at the resonant circuit  11 .  
         [0093]    The detector circuit  13  detects timing in accordance with transmission/reception based on the signal (b) generated at the resonant circuit  11 . The output port of the detector circuit  13  is connected to the comparator  14 . The comparator  14  extracts the clock signal (c) which corresponds to transmission/reception of electromagnetic waves by the tablet.  
         [0094]    The clock signal (c) is connected to the integrating circuit  16 , and the integrating circuit  16  generates the signal (d) which generates a voltage only in the continuous transmission period (described below). The clock signal (c) is also connected to a first input port of an AND gate  80  and to a one-shot monostable multivibrator circuit  81 .  
         [0095]    An output port of the AND gate  80  is connected to a clock input terminal of a counter circuit  82 . An output of the integrating circuit  16  is connected to a reset terminal of the counter circuit  82 . Output terminals Q 0  to Q 2  for three lower bits to be output from the counter circuit  82  are connected to select terminals of an analog multiplexer  83 . An output terminal Q 3  of the counter circuit  82  is connected to a second input port of the AND gate  80  through an inverter  93 .  
         [0096]    An output port of the one-shot monostable multivibrator circuit  81  is connected to an enable terminal of the analog multiplexer  83 . A common terminal of the analog multiplexer  83  is connected to the resonant circuit  11  at the ground side. In accordance with the three-bit input signal value Q 0  to Q 2 , resistors and variable resistors are selected and the selected resistors are thus connected to the resonant circuit  11 . When the three-bit input signal value Qo to Q 2  is (000) or (111), no resistors are connected to the resonant circuit  11 .  
         [0097]    The resistance of each of variable resistors  84  to  87  varies in accordance with the operation. Resistors  88  and  92  are also provided.  
         [0098]    [0098]FIG. 9 shows the external view of the position indicator according to the third embodiment of the present invention. The position indicator is operated to input a writing force and three types of analog quantities. Referring to FIG. 9, the resistance of the variable resistor  84  varies in accordance with a load applied to a writing force detector  84 ′. Dials  85 ′,  86 ′, and  87 ′ are operated to change the resistances of the variable resistors  85 ,  86 , and  87 , respectively.  
         [0099]    In the third embodiment, three dials  85 ′,  86 ′, and  87  are provided so that the allocation of the three primary colors (red, blue, and green) can be set. Thus, the position indicator can be used as an electronic pen capable of inputting a writing force and hue.  
         [0100]    A load applied to the writing force detector  84 ′ is detected within the range of, for example, 0 to 500 g. When no load is applied, the resistance of the variable resistor  84  becomes infinite. With application of 500 g load, the resistance of the variable resistor  84  becomes RAmin.  
         [0101]    The resistances of the variable resistors  85  to  87  are changed by operating the dials  85 ′ to  87 ′, respectively.  
         [0102]    The resistances of the resistors  88  to  91  are the same as minimum resistance RAmin of the variable resistor  84 . The resistance of the resistor  92  is the sum of the minimum resistance RAmin of the variable resistor  84  and the maximum resistance RBmax of the variable resistors  85  to  87 , that is, RAmin+RBmax.  
         [0103]    Alternatively, the tablet for use in connection with the position indicator of the third embodiment can be structured as shown in FIG. 2.  
         [0104]    Operation of Third Embodiment  
         [0105]    Referring to FIG. 8 which shows the position indicator and to FIG. 2 which shows the tablet used therefor, the operation of the position indicator is described.  
         [0106]    In the third embodiment as in the first and second embodiments, overall scanning is performed to roughly detect the position at which the position indicator is placed on the position detecting surface formed by the group of loop coils  41 .  
         [0107]    The operation after detecting the approximate position of the position indicator by overall scanning will now be described. In the third embodiment, a case in which the position indicator is placed near the intersection of the loop coils X 7  and Y 5  is used for explanation.  
         [0108]    [0108]FIG. 10 includes schematic waveform diagrams showing the operation of portions after the approximate position of the position indicator has been detected. Referring to FIG. 10, traces a to j indicate the waveforms at the portions denoted by the same reference symbols in FIGS. 8 and 2.  
         [0109]    The CPU  51  applies a control signal to the selector circuit  42  so that the selector circuit  42  selects the loop coil X 7  and a control signal to the transmission/reception switching circuit  45  so that the reception/transmission switching circuit  45  is switched to the transmitting side (T). In response, the loop coil X 7  emits electromagnetic waves at the frequency f o . The emission of electromagnetic waves is continued for a relatively long period of time (continuous transmission period) such as 1 mS. Subsequently, the CPU  51  applies a control signal to the transmission/reception switching circuit  45  so that the transmission/reception switching circuit  45  is switched to the receiving side (R).  
         [0110]    Since a signal remains in the resonant circuit  11  for some time after the electromagnetic wave transmission has ended, the tablet detects this signal as the signal (i). The reception period is continued for a period in which the signal in the resonant circuit  11  is sufficiently attenuated (such as 100 μS). Signal level V 0  detected at this time is a value in a state in which no resistors are connected to the resonant circuit  11 .  
         [0111]    When the reception period subsequent to the continuous transmission ends, a relatively brief transmission period (such as 50 μS) and a reception period (such as 100 μS) are each repeated six times in order to detect the load applied to the writing force detector  84 ′ and the preset values of the dials  85 ′ to  87 ′.  
         [0112]    In the first transmission/reception, as shown in FIG. 10, the analog multiplexer  83  selects the first terminal, and hence the resistor  91  is connected to the resonant circuit  11 . Thus, signal level V 1  detected by the tablet in this period is a value which corresponds to the resistance RAmin.  
         [0113]    In the second transmission/reception, the analog multiplexer  83  selects the second terminal, and hence the resistor  92  is connected to the resonant circuit  11 . Thus, signal level V 2  detected by the tablet in this period is a value which corresponds to the sum of the resistances RBmax and RAmin.  
         [0114]    In the third transmission/reception, the analog multiplexer  83  selects the third terminal, and hence the variable resistor  84  is connected to the resonant circuit  11 . Thus, signal level V 3  detected by the tablet in this period is a value which corresponds to the preset value of the variable resistor  84 .  
         [0115]    This value indicates the load applied to the writing force detector  84 ′. The load varies within the range of V 0  to V 1 , which has already been determined within the range of 0 to 500 g. Even when signal level V 3  at this time varies because of the height and tilt of the position indicator, V 0  and V 1  also vary in accordance with the variation in signal level V 3 . By computation based on proportions, the more accurate load value can be detected. This operation constitutes a feature of the present invention.  
         [0116]    The computation can be performed by the following equation:  
         load=(V 0 −V 3 )×500 ( g )/(V 0 −V 1 )  
         [0117]    In the fourth transmission/reception, the analog multiplexer  83  selects the fourth terminal, and hence the variable resistors  85  and  88  are connected to the resonant circuit  11 . Thus, signal level V 4  detected by the tablet in this period is a value which corresponds to the preset value of the variable resistor  85 .  
         [0118]    This value indicates the setting state of the dial  85 ′. Based on the setting, V 4  varies within the range of V 1  to V 2 . Even when signal level V 4  varies because of the height and tilt of the position indicator, V 1  and V 2  also vary in accordance with the variation in signal level V 4 . By computation based on proportions, the setting state of the dial  85 ′ can be reliably computed.  
         [0119]    A case in which the dial  85 ′ is used to input a red quantity (R) which can vary within the range of 0 to 100 is used for explanation. The red quantity (R) can be computed by the following equation:  
           R =(V 4 −V 1 )×100/(V 2 −V 1 )  
         [0120]    Similarly, the preset values of the dials  86 ′ and  87 ′ can be computed by the following equations, respectively:  
           G =(V 5 −V 1 )×100/(V 2 −V 1 )  
           B =(V 6 −V 1 )×100/(V 2 −V 1 )  
         [0121]    After the writing force detecting period ends, as in the first and second embodiments, the CPU  51  performs an operation to detect the Y-coordinate value and the X-coordinate value.  
         [0122]    The foregoing process of detecting a position in the third embodiment is basically the same as that in the first embodiment except for the fact that signal detection is also performed in the continuous transmission period and that transmissions/receptions of electromagnetic waves are performed six times in an operation information detecting period (which corresponds to the writing force detecting period in the first embodiment).  
         [0123]    Extended Third Embodiment  
         [0124]    In the third embodiment, the resistors each having the resistance RAmin are connected in series with the variable resistors  85  to  87 , respectively, and hence the reference value when the dial setting is minimum is commonly used as V 1  which is the reference value for detecting the writing force. Alternatively, resistors having different resistances can be employed, and reference values can be computed at different times.  
         [0125]    [0125]FIG. 1 
         [0126]    [0126] 100 : power supply  
         [0127]    [0127] 15 : shift register  
         [0128]    [0128]FIG. 2 
         [0129]    [0129] 47 : detector  
         [0130]    [0130] 49 : integrator  
         [0131]    [0131]FIG. 3 
         [0132]    [0132] 101 : coil number  
         [0133]    transmission signal (a)  
         [0134]    voltage of resonant circuit (b)  
         [0135]    shift register clock (c)  
         [0136]    shift register reset (d)  
         [0137]    shift register D terminal (e)  
         [0138]    shift register output Q 0    
         [0139]    shift register output Q 1    
         [0140]    shift register output Q 2    
         [0141]    resonant circuit control signal (f)  
         [0142]    resonant circuit control signal (g)  
         [0143]    resonant circuit control signal (h)  
         [0144]    tablet reception signal (i)  
         [0145]    tablet integration output (j)  
         [0146]    [0146] 102 : operation mode  
         [0147]    [0147] 103 : continuous transmission period  
         [0148]    [0148] 104 : writing force detecting period  
         [0149]    [0149] 105 : Y-coordinate detecting period  
         [0150]    [0150] 106 : X-coordinate detecting period  
         [0151]    [0151]FIG. 4 
         [0152]    [0152] 107 : Start  
         [0153]    S 1 : Select all loop coils one at a time and transmit/receive electromagnetic waves  
         [0154]    S 2 : Is there any voltage greater than or equal to threshold? 
         [0155]    S 3 : Detect peak coil  
         [0156]    S 4 : Select peak coil  
         [0157]    S 5 : Transmit/receive electromagnetic waves (continuous transmission period)  
         [0158]    S 6 : i=1  
         [0159]    S 7 : Transmit/receive electromagnetic waves (writing force detecting period)  
         [0160]    S 8 : Detect Vi  
         [0161]    S 9 : i=3  
         [0162]    S 10 : Compute writing force from V 1  to V 3  (Va, Vb, and Vp)  
         [0163]    S 11 : i=i+1  
         [0164]    S 12 : Select some of the loop coils one at a time and transmit/receive electromagnetic waves  
         [0165]    S 13 : Is there any voltage greater than or equal to threshold? 
         [0166]    S 14 : Compute coordinate values  
         [0167]    [0167]FIG. 5 
         [0168]    [0168] 108 : power supply  
         [0169]    [0169] 15 : shift register  
         [0170]    [0170]FIG. 6 
         [0171]    [0171] 109 : detection clock  
         [0172]    [0172] 71 : synchronous detector  
         [0173]    [0173] 73 : integrator  
         [0174]    [0174] 47 : detector  
         [0175]    [0175] 49 : integrator  
         [0176]    [0176]FIG. 7 
         [0177]    [0177] 110 : coil number  
         [0178]    transmission signal (a)  
         [0179]    voltage of resonant circuit (b)  
         [0180]    shift register clock (c)  
         [0181]    shift register reset (d)  
         [0182]    shift register D terminal (e)  
         [0183]    shift register output Q 0    
         [0184]    shift register output Q 1    
         [0185]    shift register output Q 2    
         [0186]    resonant circuit control signal (f)  
         [0187]    resonant circuit control signal (g)  
         [0188]    resonant circuit control signal (h)  
         [0189]    tablet reception signal (i)  
         [0190]    output of integrating amplifier  49  (i)  
         [0191]    output of integrating amplifier  73  (k)  
         [0192]    [0192] 111 : operation mode  
         [0193]    [0193] 112 : continuous transmission period  
         [0194]    [0194] 113 : writing force detecting period  
         [0195]    [0195] 114 : Y-coordinate detecting period  
         [0196]    [0196] 115 : X-coordinate detecting period  
         [0197]    [0197]FIG. 8 
         [0198]    [0198] 116 : power supply  
         [0199]    [0199] 82 : counter  
         [0200]    [0200]FIG. 9 
         [0201]    [0201]FIG. 10 
         [0202]    [0202] 117 : coil number  
         [0203]    transmission signal (a)  
         [0204]    voltage of resonant circuit (b)  
         [0205]    comparator output (c)  
         [0206]    counter reset (d)  
         [0207]    analog multiplexer EN (e)  
         [0208]    counter output Q 0    
         [0209]    counter output Q 1    
         [0210]    counter output Q 2    
         [0211]    [0211] 118 : analog multiplexer selected number  
         [0212]    tablet reception signal (i)  
         [0213]    tablet integration output (j)  
         [0214]    [0214] 119 : operation mode  
         [0215]    [0215] 120 : continuous transmission period  
         [0216]    [0216] 121 : operation information detecting period  
         [0217]    [0217] 122 : writing force  
         [0218]    [0218] 123 : Y-coordinate detecting period  
         [0219]    [0219] 124 : X-coordinate detecting period