Abstract:
Disclosed is a coordinates input system having a tablet constituting a coordinates input portion, a position designating device such as a stylus pen, and a position detection circuit adapted to drive said tablet and detect a position at which coordinates are input by said position designating device. The system comprises: an antenna coil disposed around a coordinates input range of said tablet; and a tuning circuit disposed in said position designating device and including a coil and a capacitor, wherein radio waves are generated by said antenna coil by application of an AC signal of a predetermined frequency thereto, and the status of said tuning circuit is discriminated by a signal of said antenna coil at the time when the transmission of said radio waves is suspended, thereby detecting the status of said position designating device. Hence, this coordinates input system is capable of detecting the status of the position designating device without connecting the position designating device and other circuits by means of a cord and without providing the position designating device with a conventionally employed complicated signal generating circuit, a battery, and the like.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a coordinate input system and, more particularly, to a coordinate input system which is capable of detecting the status of a position designating device which has designated only the position of coordinates to be input as well as the status of various types of operation designated by the same. 
     2. Statement of the Related Art 
     Hitherto, a system has been known for detecting the status (hereafter referred to as the &#34;pen-down status&#34;) of a position designating device which has designated only the position of coordinates to be input on a tablet. This system is arranged such that a switching means is provided in the position designating device and is turned ON (or OFF) only in the pen-down status, and timing signals based on the ON (or OFF) status of the switching means are transmitted to a position detecting circuit via a cord or by the use of ultrasonic waves or infrared ways. 
     However with a system of the type in which timing signals are transmitted from the position designating device via a cord, there has been a drawback in that the cord causes a hindrance to the operating efficiency of the position designating device. In addition, with a system of the type in which timing signals are transmitted by the use of ultrasonic waves or infrared rays, a transmitter, a signal generating circuit, a battery, and the like must be provided in the position designating device per se. Hence, there has been a drawback in that the arragement of the position designating device becomes complicated and large in size and weight, thereby aggravating the operating efficiency of the position designating device. 
     SUMMARY OF THE INVENTION 
     Accordingly, a primary object of the present invention is to provide a coordinate input system which is capable of detecting the status of a position designating device without deteriorating the operating efficiency of the position designating device. 
     To this end, according to the present invention, there is provided a coordinates input system having a tablet constituting a coordinates input portion, a position designating device such as a stylus pen, and a position detection circuit adapted to drive the tablet and detect a position at which coordinates are input by the position designating device, the system comprising: an antenna coil disposed around a coordinates input range of the tablet; and a tuning circuit disposed in the position designating device and including a coil and a capacitor, wherein radio waves are generated by the antenna coil by application of an AC signal of a predetermined frequency thereto, and the status of the tuning circuit is discriminated by a signal of the antenna coil at the time when the transmission of the radio waves is suspended, thereby detecting the status of the position designating device. 
     According to this aspect of the invention, the tuning circuit which has received radio waves from the antenna coil resonates or does not resonate with a substantially identical frequency or an identical or different phase in correspondence with its status, reflects or does not reflect radio waves whose frequency is substantially identical and whose phase is identical or different. From the fact that signals are or are not generated by the antenna coil which has suspended the transmission of radio waves on the basis of the reflected radio waves, the status of the tuning circuit can be discriminated and the status of the position designating device can be detected. Accordingly, no cord is required for connecting the position designating device and other circuits, and it suffices only to provide the position designating device with a tuning circuit, including a coil and a capacitor. Therefore, a conventionally employed complicated signal generating circuit, a battery and the like become unnecessary, so that it is possible to provide a position designating device which excels in operating efficiency, and its status can be detected accurately. 
     A second object of the present invention is to provide a coordinate input system which is capable of detecting the positional status of a position designating device as well as the status of various types of operation designated by the same. 
     To this end, according to another aspect of the present invention, there is provided a coordinates input system having a tablet constituting a coordinates input portion, a position designating device such as a stylus pen, and a position detection circuit adapted to drive the tablet and detect a position at which coordinates are input by the position designating device, the system comprising: a plurality of tuning circuits disposed in the position designating device, each of the plurality of tuning circuits including a coil and a capacitor to constitute a set and being adapted to transmit radio waves with mutually equivalent tuning frequencies and different phases in reponse to an external signal; switching means disposed in the position designating device and adapted to turn ON and OFF the connection between the coil and the capacitor of each of the tuning circuits; and an antenna coil disposed around a coordinates input range of the tablet; wherein radio waves are generated by the antenna coil when an AC signal of a frequency identical with that of the tuning frequency is intermittently applied to the antenna coil, the status of the position and operation of the position designating device is detected when signals responded to by the tuning circuits during suspension of transmission of the radio waves are received by the antenna coil, and the ON-OFF status of the switching means with respect to the tuning circuits is discriminated by means of input signals having mutually different phases with respect to output signals of the antenna coil. 
     According to this aspect of the invention, when the switches of the tuning circuits are ON, the tuning circuits resspectively transmit signals with peculiar phase differences with respect to signals transmitted by the antenna coil, and the antenna coil receives the same. The positional and operational status of the position designating device can be detected by reception of the signals. Discrimination can be made as to which of the switches of the position designating devices has been turned ON by input signals having mutually different phases. Accordingly, no cord is required for connecting the position designating device and other circuits, and it suffices only to provide the position designating device with a plurality of tuning circuits each including a coil and a capacitor. Therefore, a conventionally employed complicated signal generating circuit, a battery and the like become unnecessary, and the operational status of the plurality of switches provided in the position designating device can be discriminated without using a cord, thereby permitting color designation and erasure designation for the input pen by means of the switches. Thus, it is possible to provide a position designating device which excels in operating efficiency. 
     A third object of the present invention is to provide a coordinate input system which is capable of detecting the positional status of a position designating device, the status of its use, and the like without causing any hindrance to the operating efficiency thereof. 
     To this end, according to still another aspect of the present invention, there is provided a coordinates input system having a tablet constituting a coordinates input portion, a position designating device such as a stylus pen, and a position detection circuit adapted to drive the tablet and detect a position at which coordinates are input by the position designating device, the system comprising: a tuning circuit disposed in the position designating device, the tuning circuit including a set of a coil, a capacitor, and/or a resistor and being adapted to generate radio waves with mutually equivalent tuning frequencies and a change in the phase in response to an external signal by changing any of the values of the coil, the capacitor, and/or the resistor in correspondence with the status of use of the position designating device; and an antenna coil disposed around a coordinates input range of the tablet; wherein radio waves are generated by the antenna coil when an AC signal of a frequency identical with that of the tuning frequency is intermittently applied to the antenna coil, the status of the position and operation of the position designating device is detected when signals responded to by the tuning circuit during suspension of transmission of the radio waves are received by the antenna coil, and the status of use of the position designating device is discriminated by means of the input signals having mutually different phases with respect to output signals of the antenna coil. 
     According to this aspect of the invention, any of the values of the coil, the capacitor, and the resistor of the tuning circuit changes in accordance with the status of use of the position designating device, which in turn causes the phase of the tuning circuit to undergo change with respect to an input signal of the antenna coil, and the tuning circuit thereby responds to the same and transmits a signal. The antenna coil receives that signal. The  positional and operational status of the position designating device can be detected by reception of the signal. The status of use of the position designating device can be discriminated in accordance with a change in the phase of the input signal. Accordingly, no cord is required for connecting the position designating device and other circuits, and it suffices only to provide the position designating device with a tuning circuit, including a coil and a capacitor, Therefore, a conventionally employed complicated signal generating circuit, a battery and the like become unnecessary. In addition, signals corresponding to the status of use of the position designating device can be received without using a cord, and the operational status of the switch provided in the position designating device can be discriminated without using a cord. Hence, it becomes possible to designate, for instance, the size of a line drawn in correspondence with a pressing force of an input pen. Thus, it is possible to provide a position designating device which excels in operating efficiency. 
     The above and other objects, features, and advantages of the present invention will become apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view illustrating an outline of a first embodiment of the present invention; 
     FIG. 2 is a cross-sectional view of an input pen; 
     FIG. 3 is a block diagram of a timing control circuit; 
     FIG. 4 is a diagram illustrating wavefoms of various sections shown in FIG. 3; 
     FIG. 5 is a perspective view of a position designating device illustrating a second embodiment; 
     FIG. 6 is a cross-sectional view of the input pen; 
     FIG. 7 is a block diagram of the timing control circuit in a case where two switches are provided; 
     FIG. 8 is a diagram illustrating waveforms of various sections shown in FIG. 7; 
     FIG. 9 is a diagram illustrating the operation of a phase detector shown in FIG. 7; 
     FIG. 10 is a block diagram of the timing control circuit in a case where four switches are provided; 
     FIG. 11 is a waveform diagram illustrating the operation of the phase detector shown in FIG. 10; 
     FIG. 12 is a cross-sectional view of the input pen illustrating a third embodiment of the present invention; 
     FIG. 13 is a block diagram of a timing control circuit thereof; 
     FIG. 14 is a diagram illustrating waveforms of various sections shown in FIG. 13; 
     FIGS. 15 and 16 are waveform diagrams illustrating the operation of phase detectors shown in FIG. 13; and 
     FIG. 17 is a diagram illustrating relationships between a pressing force of a core member and the size of a line. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a first embodiment of the present invention. In the drawings, reference numeral 1 denotes a tablet; 2, a position designating device (hereafter referred to as the &#34;input pen&#34;); 3, a position detection circuit; and 4, a timing control circuit. 
     The tablet 1 is arranged such that a tablet body 12 and an antenna coil 13 are accommodated in a casing 11 which is made of a non-metallic material such as a synthetic resin. The tablet body 12 is connected to the position detection circuit 3, while the antenna coil 13 is connected to the timing control circuit 4. 
     The tablet body 12 is driven by the position detection circuit 3 and constitutes a detection section for detecting a position designated by the input pen 2. The tablet body 12 is disposed substantially in the center of the casing 11. Incidentally, a frame 14 drawn on an upper panel 11a of the casing indicates a range of the input of coordinates thereof. 
     As for the tablet body 12 and the position detection circuit 3, it is possible to use, for instance, those described in Japanese Patent Application No. 32244/1984 &#34;Position Detection Device&#34; (see Japanese Patent Laid-Open No. 176133/1985) and Japanese Patent Application No. 238532/1983 &#34;Coordinate Position Detection Device&#34; (see Japanese Patent Laid-Open No. 129616/1985), both filed by the present applicant. The former device is arranged such that a multiplicity of magnetostrictive transmitting media are disposed parallel with the surface of the tablet body 12 such that some of the magnetostrictive transmitting media are disposed at right angles with others, and magnetostrictive vibrations are imparted periodically from one end to the other. When the input pen 2 approaches the same, the magnetostrictive vibration at that location is enhanced by means of a bar magnet provided thereto. By making use of this phenomenon, the position detection circuit 3 detects X-Y coordinates thereof through the time duration of propagation to that location. On the other hand, the latter device is arranged such that magnetostrictive media, some of which are disposed at right angles with others, are excited by an AC current, and induced voltages thereof are fetched by detection coils. X-Y coordinates are detected by making use of the phenomenon in which, when a similar input pen approaches the same, the permeability of the magnetostrictive media changes locally with a resultant change in induced voltages. 
     The antenna coil 13 is arranged such that a conductive wire provided with an insulation coating such as polyvinyl chloride is disposed around the periphery of the coordinates inputting range of the tablet body 12, i.e., in the casing of this embodiment, on the rear surface of the upper panel 11a of the casing 11 around the frame 14. Incidentally, although the conductive wire is given one turn in the illustrated example, a plurality of turns may be provided, as required. 
     The input pen 2 incorporates a tuning circuit 22 which includes a magetism generator for designating a position, e.g., a bar magnet 21, a coil, and a capacitor. 
     FIG. 2 illustrates a detailed structure of the input pen 2 which is arranged as follows: A core member 24, such as a ball-point pen, the bar magnet 21 having a through-hole capable of slidably accommodating the core member 24, a coil spring 25, and a tuning circuit 22 constituted by a switch 221, a coil 222 with a core, a capacitor 223, and a variable capacitor 224 are incorporated, in that order starting from a tip of the input pen 2, as an integral combination inside a pen shaft 23 which is constituted by a non-magnetic material, such as a synthetic resin, or, for instance, aluminum. A cap 26 is installed at a rear end thereof. 
     The switch 221 is arranged such that, when the core member 24 is pressed into the inside of the pen shaft 23 by, for instance, pressing a tip thereof against the surface of the tablet, the switch 221 is turned ON by being pressed from a rear end thereof via the coil spring 25. In addition, as is also shown in FIG. 3, the capacitor 223 and the variable capacitor 224 are connected to each other in parallel. One end of the coil 222 is connected to ends of the capacitor 223 and the variable capacitor 224 via the switch 221, while the other end of the coil 222 is connected to the other ends thereof, thereby constituting a known parallel resonance circuit. 
     It should be noted that values of the coil 222, the capacitor 223, and the variable capacitor 224 are selected in such a manner as to resonate (to be tuned) with the frequency of radio waves transmitted from the antenna coil 13. 
     FIG. 3 shows a detailed arrangement of the timing control circuit 4. In the drawing, reference numeral 401 denotes an oscillator; 402, a frequency demultiplication counter; 403, 404 denote NAND gates; 405 denotes a transmission terminal; 406, a reception terminal; 407, 408, denote reception changeover switches; 409, 410, 411, amplifiers; 412 denotes a filter; 413, a phase detector; 414, a low-pass filter; 415, a comparator; and 416, an output terminal. 
     FIG. 4 is a waveform diagram of signals in each section shown in FIG. 3. Hereafter, a detailed description will be made of operation. 
     A clock pulse of, say, 910 kHz generated by the oscillator 401 is divided into 1/2 and 1/32 by the frequency demultiplication counter 402. A pulse signal A of 455 kHz obtained by dividing the frequency into 1/2 is input to one input terminal of the NAND gate 403, while a pulse signal of 28.44 kHz obtained by dividing the frequency into 1/32 is input to the other input terminal. Its output is further sent to the NAND gate 404, and becomes a signal B in which a 455 kHz pulse signal is sent or is not sent for each 28.44 kHz, as shown in FIG. 4. 
     The signal B is sent to the antenna coil 13 via the transmission terminal 405 and is transmitted as radio waves. At that juncture, if the switch 221 is ON in the tuning circuit 22 of the input pen 2, the tuning circuit 22 resonates with radio waves that are transmitted. Since the tuning circuit 22 continues to resonate while being attenuated while the transmission on the transmission side is stopped, the tuning circuit 22 generates a signal C as shown in FIG. 4, and the signal C is transmitted as radio waves by the coil 222 and is received by the antenna coil 13. 
     Since the reception changeover switches 407, 408 have already been changed over by the aforementioned 28.44 kHz pulse signal, the reception changeover switches 407, 408 receive signals from the reception terminal 406 only during the period when transmission is suspended. The input signal becomes a signal D as shown in FIG. 4 if the switch 221 of the tuning circuit 22 is ON, while the input signal becomes a signal D&#39; if it is OFF. The input signal D is amplified into a signal D by the amplifiers 409, 410, a component of noise is eliminated through a mechanical filter 412 having a resonance frequency of 455 kHz, and the signal is then is transmitted to the phase detector 413 via the amplifier 411. 
     The 455 kHz pulse signal A has already been input to the phase detector 413. At this juncture, if the phase of the input signal E coincides with the phase of the pulse signal A, a signal F, in which a lower half of the signal E is inverted, as shown in FIG. 4, is output. 
     The signal F is converted into a flat signal by means of a low-pass filter 414 having a sufficiently low cut-off frequency, and is input to one input terminal of the comparator 415. A predetermined threshold voltage VT has been input to the other input terminal of the comparator 415, the output of the low-pass filter 414 is compared with the threshold voltage VT, and a high (H) level signal G is output to the output terminal 416. 
     Incidentally, in the case of the signal D&#39;, the signal level is &#34;0&#34;, including those of both signals E and F, and the level of the signal G becomes low (L). 
     If the signal G is transmitted to the position detection circuit 3 on the basis of a definition that when the signal G is at high level, the status is that of pen down, and that when it is at low level, the status is not that of pen down, the input of a position can be effected simply by pressing the tip of the input pen 2 against the tablet at a position where coordinates are to be input, by operating the input pen 2 on the tablet 1. 
     Incidentally, although, in the foregoing embodiment, a changeover is effected by the turning ON and OFF of the switch in the tuning circuit 22 to determine whether or not it is a pen-down status, it is possible to provide an alternative arrangement in which the capacity of the capacitor in the tuning circuit is varied on the basis of the operation of the input pen 2, and the phase of the reflected signal is changed to alter the waveform of the signal F, thereby making it possible to vary an output level of the low-pass filter. Thus, the switch per se is not essential. 
     FIG. 5 shows a second embodiment of the present invention. In the drawing, reference numeral 1 denotes the tablet; 2, the position designating device (hereafter referred to as the &#34;input pen&#34;); and 3, the timing control circuit. The details of these components are the same as those of the above-described first embodiment except for the portions that are described below. 
     The input pen 2 incorporates the magnetism generator for designating a position, such as the bar magnet 21, and two sets of tuning circuits 22a, 22b, each set including the coil and the capacitor. 
     FIG. 6 shows a detailed structure of the input pen 2 which is arranged as follows: The core member 24, such as a ball-point pen, the bar magnet 21 having a through-hole capable of slidably accommodating the core member 24, the coil spring 25, and two sets of tuning circuits 22a, 22b, constituted by switches 221a, 221b, the coil 222 with an iron core, two capacitors 223a, 223b, and two variable capacitors 224a, 224b for fine adjustment, as shown in FIG. 7, are incorporated, in that order starting from a tip of the input pen 2, as an integral combination inside the pen shaft 23 which is constituted by a non-magnetic material, such as a synthetic resin. The cap 26 is installed at a rear end thereof. 
     The switch 221a is arranged such that, when the core member 24 is pressed into the inside of the pen shaft 23 by, for instance, pressing a tip thereof against the surface of the tablet, the switch 221 is turned ON by being pressed from a rear end thereof via the coil spring 25. The switch 221b sends a signal to a separate host computer (not shown) each time the switch 211b is turned ON by a pressing operation when the switch 221a is ON, and is adapted to designate, for instance the changeover of color or coloring of a certain portion in correspondence with the number of ON operations thereof. In addition, as is also shown in FIG. 7, the capacitor 223a and the variable capacitor 224a are connected to each other in parallel. One end of the coil 222 is connected to ends thereof via the switch 221a, while the other end of the coil 222 is connected to the other ends thereof, thereby constituting one parallel resonance circuit 22a. The capacitor 223b, the variable capacitor 224b, the coil 222, and the switch 221b are similarly connected to each other. In this case, the arrangement is such that when both switches 221a, 221b are turned ON, the other parallel resonance circuit 22b is formed. 
     It should be noted that the resonance circuits 22a, 22b are set in such a manner that the resonance circuit 22a has the same phase as that of radio waves transmitted from the antenna coil 13 and resonates (is tuned) with the frequency thereof, while the resonance circuit 22b  resonates with a phase difference of 180° therebetween. Since the resonance circuits 22a, 22b are set as described above, selectivity Q=R/(ω 0  L) (where ω 0  is an angular velocity of resonance; R is a value of resistance; and L is an inductance) changes, so that a phase shift occurs. At the same time, although a resonance frequency also changes, resonance is made possible. 
     FIG. 7 shows a detailed arrangement of the timing control circuit 4. In the drawing, reference numeral 401 denotes the oscillator (OSC); 402, the frequency demultiplication counter; 403, 404 denote NAND gates; 405 denotes the transmission terminal; 406, the reception terminal; 404, 408, denote reception changeover switches; 409, 410, 411, amplifiers; 412 denotes the filter; 413, the phase detector (PSD); 414, the low-pass filter (LPF); 415a, 415b denote comparators; and 416a, 416b, output terminals. 
     FIG. 8 is a waveform diagram of signals of various sections shown in FIG. 7. Hereafter, a detailed description will be made of operation. 
     A clock pulse of, say 910 kHz generated by the oscillator 401 is divided into 1/2 and 1/32 by the frequency demultiplication counter 402. The pulse signal A of 455 kHz obtained by dividing the frequency into 1/2 is input to one input terminal of the NAND gate 403, while a pulse signal of 28.44 kHz (with a pulse width of 17.6 μs) obtained by dividing the frequency into 1/32 is input to the other input terminal. Its output is further sent to the NAND gate 404, and becomes the signal B in which a 455 kHz pulse signal is sent or is not sent for each 17.6 μs, as shown in FIG. 8. 
     The signal B is sent to the antenna coil 13 via the transmission terminal 405 and is transmitted as radio waves. At that juncture, for example, if the switch 221a is ON in the tuning circuit 22a of the input pen 2, the tuning circuit 22a resonates with the transmitted radio waves. Since the tuning circuit 22a continues to resonate while being attenuated while the transmission on the transmission side is stopped, the tuning circuit 22a generates the signal C as shown in FIG. 8, and the signal C is transmitted as radio waves by the coil 222 and is received by the antenna coil 13. 
     The reception changeover switches 407, 408, which have been changed over for each 17.6 μs by the aforementioned 28.44 kHz pulse signal, receive signals from the reception terminal 406 only during the period of suspension of transmission. The input signal becomes the signal D as shown in FIG. 8 if the switch 221a of the tuning circuit 22a is ON, while the input signal becomes the signal D&#39; if it is OFF. The input signal D is amplified into the signal D by the amplifiers 409, 410, a component of noise is eliminated through the mechanical filter 412 having a resonance frequency of 455 kHz, and the signal is then is transmitted to the phase detector 413 via the amplifier 411. Incidentally, the amplifier 410 has an automatic level control function to set the signal E to a fixed amplitude. 
     The 455 kHz pulse signal A has already been input to the phase detector 413. At this juncture, since the phase of the input signal E is made to coincide with the phase of the pulse signal A, the signal F, in which a lower half of the signal E is inverted, as shown in FIG. 8, is output. 
     The signal F is converted into a flat signal by means of the low-pass filter 414 having a sufficiently low cut-off frequency, and is input to input terminals of the the comparators 415a, 415b. A predetermined threshold voltage +VT has been input to the other input terminals of the comparator 415a, the output of the low-pass filter 414 is compared with the threshold voltage +VT, and the high (H) level signal Ga is output to the output terminal 416a. 
     Incidentally, in the case of the signal D&#39;, the signal level is &#34;0&#34;, including those of both signals E and F, and the level (not shown) of the signal Ga becomes low (L). 
     If the signal Ga is transmitted to the position detection circuit 3 on the basis of a definition that when the signal Ga is at high level, the status is that of pen down, and that when it is at low level, the status is not that of pen down, the input of a position can be effected simply by pressing the tip of the input pen 2 against the tablet at a position where coordinates are to be input, by operating the input pen 2 on the tablet 1. 
     In giving a description of a case where the switch 221b is operated, description will be made of the operating waveform of the phase detector 413 including a case where the above-described switch 221a alone is operated. 
     If it is assumed that a signal obtained by the frequency demultiplication by 1/2 of the frequency demultiplication counter 402 is e R  as the input of the phase detector 413, and, referring to FIG. 9(A), since this is a square wave having an amplitude of 1, if an angular velocity thereof is assumed to be ω R , we have from the Fourier expansion the following formula: 
     
         e.sub.R =(4/π){sin ω.sub.R t+(1/3) sin 3ω.sub.R t+(1/5) sin 5ω.sub.R t+ . . . }                                 (1) 
    
     Furthermore, if it is assumed that a signal from the amplifier 411 is e i , and that this signal is constituted by a synchronous component e S  determined by a maximum value E S  and an angular velocity ω S  as well as a non-synchronous component e N  which is noise and determined by a maximum value E N  and an angular velocity ω N , since the angular velocity ω S  =ω R , we have 
     
         e.sub.i =e.sub.S +e.sub.N =E.sub.S sin ω.sub.R t+E.sub.N sin ω.sub.N t                                           (2) 
    
     At tis juncture, if the switch 221a alone is ON, the phase of the signal e R  and that of e S  are identical, and if an output of the phase detector 413 is assumed to be e 0 , we have ##EQU1## Here, since a DC component is included only in the first term sin 2  ω R  t, and the remainder is an AC component, if we focus our attention only on the DC component as the output of the low-pass filter 414 to which the output of the phase detection is imparted, and if that output is assumed to be e 0 , from sin 2  ω R  t=(1/2){1-cos 2ω R  t}, we have 
     
         e.sub.0 =(2/π)E.sub.S                                   (4) 
    
     Formula (4) shows a mean value of the signal e 0  in FIG. 9 (A). 
     Next, when the switches 221a, 221b are turned ON, if it is assumed that a phase difference between the signal e R  and the signal e S  is φ, then φ=180°, and if the first term of Formula (3) is assumed to be e&#39; 0 , we have 
     
         e&#39;.sub.0 =(4/π)E.sub.S {sin (ω.sub.R t-φ) sin ω.sub.R t}=(4/π)E.sub.S (1/2){cos φ-cos (2ω.sub.R t+φ)}(5) 
    
     Since the second term is an AC component, the DC output e 0  becomes as follows: 
     
         e.sub.0 =(2/π)E.sub.S cos φ                         (6) 
    
     Since φ=180°, we have 
     
         e.sub.0 =-(2/π)E.sub.S 
    
     Formula (6) shows a mean value of the signal e 0  in FIG. 9 (C), the signal e 0  is input to the comparators 415a, 415b. The comparator 415b compares the signal with a predetermined threshold voltage -VT, and a signal Gb of low (L) level is output to the output terminal 416b. 
     It should be noted that when the switch 221b is OFF, the signal e 0  is constantly greater than the threshold voltage -VT, and its output signal Gb is constantly set to high (H) level (not shown). 
     FIG. 10 illustrates an input pen which is used in place of the input pen 2 shown in FIG. 6, and is provided with four sets of tuning circuits RE 1  -RE 4  and four switches SW 1  -SW 4  for selectively turning them ON by an operation. In this input pen, a shifter (SHIFT) 417 for advancing by 90° the signal e R , which is obtained by dividing the frequency by 1/2 by the frequency demultiplication counter 402, is added to the arrangement shown in FIG. 7. Furthermore, also added to the same are a phase detector 413&#39; for receiving signals from the shifter 417 and those from the amplifier 411 as well as a low-pass filter 414&#39; for fetching a DC component from its output, so as to fetch operation signals for the switches SW 1  to SW 4  from four comparators 415a-415d. 
     Incidentally, the above-described tuning circuits RE 1  -RE 4  are made to cope with, for instance, designation of various colors, the aforementioned coloring designation, and erasure designation by making the portion of the cap 26 of the input pen 2 a rotary type. 
     Description will be given hereafter of the operation of the circuit shown in FIG. 11, centering on portions that differ from those of FIG. 7. 
     Referring again to FIG. 9, the signals e S  obtained by the tuning circuits RE 1  -RE 4  respectively have phase differences of 0°, 90°, 180°, and 270° with respect to the signal e 0 , as shown in (A), (B), (C), and (D). With respect to the phase differences of 90° and 270°, for instance, e 0  equals zero if is assumed that φ=90° and φ=270° in Formula (6). Accordingly, in order to detect the signal e 0  when φ=90° and φ=270°, the signal e R  is advanced 90°, as shown in FIG. 11. Consequently, cos φ is replaced by sin φ, and the output of the low-pass filter 414&#39; in this case becomes the mean average e 0  =(2/π)E S  of the signal e 0  of FIG. 11 (B) when φ=90°. When φ=270° , said output becomes the mean value e 0  =-(2/π)E S  of the signal shown in FIG. 11 (D). Similarly, output signals Gc, Gd are obtained by the comparators 415c, 415d. 
     FIGS. 12 to 17 show a third embodiment of the present invention. The tablet 1, the position designating device 2, the position detection circuit 3, and the timing control circuit 4 are the same as those of the first embodiment except for the portions which will be described below. 
     The input pen 2 incorporates the tuning circuit 22 which includes the magnetism generator for designating a position, e.g., the bar magnet 21, the coil, the capacitor, and a resistor. 
     FIG. 12 illustrates a detailed structure of the input pen 2 which is arranged as follows: The core member 24, such as a ball-point pen, the bar magnet 21 having a through-hole capable of slidably accommodating the core member 24, the coil spring 25, and the tuning circuit 22 constituted by the switch 221, the coil 222 with a core, the capacitor 223, and the variable capacitor 224 for fine adjustment, and a pressure variable capacitor 225 are incorporated, in that order starting from the tip of the input pen 2, as an integral combination inside the pen shaft 23 constituted by a non-magnetic material, such as a synthetic resin. The cap 26 is installed at a rear end thereof. 
     The switch 221 is arranged such that, when the core member 24 is pressed into the inside of the pen shaft 23 by, for instance, pressing a tip thereof against the surface of the tablet, the switch 221 is turned ON by being pressed from a rear end thereof via the coil spring 25. In addition, the capacity of the pressure capacitor is adapted to change by a pressing force of the core member 24, and is adapted to designate the size of a line to be drawn on the basis of a change in the capacity corresponding to the pressing force. In addition, as is also shown in FIG. 13, the capacitor 223, the variable capacitor 224, and the pressure variable capacitor 225 are connected to each other in parallel. One end of the coil 222 is connected to ends thereof via the switch 221, while the other end of the coil 222 is connected to the other ends thereof, thereby constituting the parallel resonance circuit 22. 
     It should be noted that the resonance circuit 22 is set by being adjusted by the variable capacitor 224 so as to resonate (to be tuned) with the frequency of radio waves transmitted from the antenna coil 13. Since the capacity of the pressure variable capacitor 225 changes, selectively Q=R/(ω 0  L) (where ω 0  is an angular velocity of resonance; R is a value of resistance; and L is an inductance) changes, so that a phase shift occurs in the resonance circuit 22. At the same time, although a resonance frequency also changes, resonance is made possible. 
     FIG. 13 shows a detailed arrangement of the timing control circuit 4. In the drawing, reference numeral 401 denotes the oscillator (OSC); 402, the frequency demultiplication counter; 403, 404 denote NAND gates; 405 denotes the transmission terminal; 406, the reception terminal; 404, 408 denote reception changeover switches; 409, 410, 411, amplifiers; 412 denotes the filter; 413, 413&#39; denote phase detectors (PSD); 414 denotes the phase shifter (SHIFT); 415, 415&#39; denote the low-pass filters (LPF); 416 denotes a phase angle computing device (PHASE); and 417 an output terminal. 
     FIG. 14 is a waveform diagram of signals in each section shown in FIG. 13. Hereafter, a detailed description will be made of operation. 
     A clock pulse of, say, 910 kHz generated by the oscillator 401 is divided into 1/2 and 1/32 by the frequency demultiplication counter 402. The pulse signal A of 455 kHz obtained by dividing the frequency into 1/2 is input to one input terminal of the NAND gate 403, while a pulse signal of 28.44 kHz (with a pulse width of 17.6 μs) obtained by dividing the frequency into 1/32 is input to the other input terminal. Its output is further sent to the NAND gate 404, and becomes the signal B in which a 455 kHz pulse signal is sent or is not sent for each 17.6 μs, as shown in FIG. 8. 
     The signal B is sent to the antenna coil 13 via the transmission terminal 405 and is transmitted as radio waves. At that juncture, for example, if the switch 221a is ON in the tuning circuit 22a of the input pen 2, the tuning circuit 22a resonates with the transmitted radio waves. Since the tuning circuit 22a continues to resonate while being attenuated while the transmission on the transmission side is stopped, the tuning circuit 22a generates the signal C as shown in FIG. 8, and the signal C is transmitted as radio waves by the coil 222 and is received by the antenna coil 13. 
     The reception changeover switches 407, 408, which have been changed over for each 17.6 μs by the aforementioned 28.44 kHz pulse signal, receive signals from the reception terminal 406 only during the period of suspension of transmission. The input signal becomes the signal D as shown in FIG. 8 if the switch 221a of the tuning circuit 22a is ON, while the input signal becomes the signal D&#39; if it is OFF. The input signal D is amplified into the signal D by the amplifiers 408, 410, a component of noise is eliminated through the mechanical filter 412 having a resonance frequency of 455 kHz, and the signal is then is transmitted to the phase detector 413 via the amplifier 411. Incidentally, the amplifier 410 has an automatic level control function to set the signal E to a fixed amplitude. 
     The 455 kHz pulse signal A has already been input to the phase detector 413. At this juncture, since the phase of the input signal E is made to coincide with the phase of the pulse signal A, the signal F, in which a lower half of the signal E is inverted, as shown in FIG. 14, is output. 
     The shifter 414 has already advanced the pulse signal A by 90° and has imparted that signal to the phase detector 413. After receiving the signal E and the signal in which the signal A is advanced 90°, the phase detector 413 delivers an output to a signal F&#39; which will be described below. 
     The signals F, F&#39; are converted into flat signals by means of the filters 415, 415&#39; having sufficiently low cut-off frequencies, and are input to the phase angle computing device 416. 
     Now, if it is assumed that a signal obtained by the frequency demultiplication by 1/2 of the frequency demultiplication counter 402 is e R  as the input of the phase detector 413, and, referring to FIG. 15 (A), since this is a square wave having an amplitude of 1, if an angular velocity thereof is assumed to be ω R , we have from the Fourier expansion the following formula: 
     
         e.sub.R =(4/π){sin ω.sub.R t+(1/3) sin 3ω.sub.R t+(1/5) sin 5ω.sub.R t+ . . . }                                 (7) 
    
     Furthermore, if it is assumed that a signal from the amplifier 411 is e i , and that this signal is constituted by a synchronous component e S  determined by a maximum value E S  and an angular velocity ω S  as well as a non-synchronous component e N  which is noise and determined constituted by a maximum value E N  and an angular velocity ω N , since the angular velocity ω S  =ω R , we have 
     
         e.sub.i =e.sub.S +e.sub.N =E.sub.S sin ω.sub.R t+E.sub.N sin ω.sub.N t                                           (8) 
    
     When the signals e R  and e S  are of the same phase, if it is assumed that the output of the phase detector is e 0  (the signal F in FIGS. 13 and 14), we have ##EQU2## Here, since a DC component is included only in the first term sin 2  ω R  t, and the remainder is an AC component, if we focus our attention only on the DC component as the output of the low-pass filter 414 to which the output of the phase detection is imparted and if that output is assumed to be e 0 , from sin 2  ω R  t=(1/2){1-cos 2ω R  t}, we have 
     
         e.sub.0 =(2/π)E.sub.S                                   (10) 
    
     Formula (10) shows a mean value of the signal e 0  in FIG. 15 (A). 
     Next, when the capacity of the pressure variable capacitor 225 changes, and when the phase difference φ occurs between the signals e R  and e S , and if the first term of Formula (9) is assumed to be e&#39; 0 , we have 
     
         e&#39;.sub.0 =(4/π)E.sub.S {sin (ω.sub.R t=φ) sin ω.sub.R t}=(4/π)E.sub.S (1/2){cos φ-cos (2ω.sub.R t+φ)}(11) 
    
     Since the second term is an AC component, the DC output e 0  becomes as follows: 
     
         e.sub.0 =(2/π)E.sub.S cos φ                         (12) 
    
     If it is assumed that φ=180°, we have 
     
         e.sub.0 =-(2/π)E.sub.S 
    
     Formula (12) shows a mean value of the signal e 0  in FIG. 15 (C), and the signal e 0  is input to the phase angle computing device 416. 
     Considered next are cases where the phase differences of the signal e S  with respect to the signal e R  have become 0°, 90°, 180°, and 270°, respectively, as shown in FIG. 15 (A), (B), (C), and (D). For instance, with respect to the phase differences of 90° and 270°, E 0  equals zero if it is assumed that φ in Formula (12) equals 90° or 270°. Accordingly, in order to detect the signal e 0  including the region of these phase differences, the signal e R  is advanced 90° by the shifter 414, as shown in FIG. 16. Consequently, cos φ in Formula (12) is replaced by sin φ, and if the mean value of a signal e 90  (the signal F&#39; in FIG. 13) shown in FIG. 16 is this case is assumed to be e 90 , the following formula is derived 
     
         e.sub.90 =(2/π)E.sub.S sin φ                        (13) 
    
     and the signal e 90  is input to the phase angle computing device 417. The phase angle computing device 416 computes the phase angle from Formulae (12) and (13). Since e 90  /e 0  =sin φ/cos φ, we have 
     
         φ=tan.sup.-1 (e.sub.90 /e.sub.0)                       (14) 
    
     thus a signal with a phase angle φ of Formula (14) is output from the output terminal 417 to a microcomputer (not shown) accommodated in the tablet 1. 
     It should be noted that although, in this embodiment, the responding phase of the tuning circuit 22 is altered by the pressure variable capacitor 225, an alternative arrangement may be provided such that variable resistor by the use of pressure sensitive rubber or the like may be employed, or the inductance of the coil 222 may be made variable. 
     FIG. 17 is a diagram illustrating the relationships between a pressing force (g) of the core member 24 and codes for designating the size of a line to be drawn. A signal representing a phase angle φ and output from the output terminal 417 in correspondence with an analog-like pressing force is converted by the microcomputer into a digital code which designates the size of a line, and the size is thus designated. Incidentally, when the pressing force is zero, both signals e 0 , e 90  in Formula (14) become zero, so that the phase angle φ becomes inconstant. The phase angle computing device 416 detects that inconstancy, and discrimination is made that the switch 221 is OFF. In carrying out this discrimination, thresholds which slightly exceed zero may be provided for the signals e 0 , e 90 , and a logical sum of the result of comparison with the thresholds  may be monitored. If a definition is given that when the phase angle φ is thus detected, it is a pen-down state, and that when it is inconstant, it is not a pen-down state, and if the discriminated signal is sent to the phase detection circuit 3, it is possible to input a position by operating the input pen 2 on the tablet 1 and simply by pressing the tip of the input pen 2 against the tablet at a position where coordinates are to be input.