Abstract:
There is provided a position detector including a member having a surface and a plurality of vertically and horizontally arranged antennas; a controller for sequentially driving the vertically arranged antennas in vertical positive and vertical negative directions, and for sequentially driving the horizontally arranged antennas in horizontal positive and horizontal negative directions so that the antennas sequentially radiate radio waves; a receiver for receiving the sequentially radiated radio waves on the surface of the member; and a detector for detecting the position of the receiver on the surface of the member based on the levels of the received radio waves. The detector detects the position of the receiver as an average position for a first position detected based on the levels of the radio waves which are sequentially radiated in the vertical positive and horizontal positive directions, and a second position detected based on the levels of the radio waves which are sequentially radiated in the vertical negative and horizontal negative directions. Therefore, since the receiver position detected by the detector does not vary, no delay time adjustment circuit is required, and the position of the receiver can be accurately detected.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a position detector for detecting a designated position on a tablet in which a plurality of antennas are vertically and horizontally arranged, as a matrix. 
     2. Related Arts 
     A position detector is known which includes a plane member, called a tablet, in which a plurality of antennas are arranged as a matrix and a touch pen used for selecting a position on the tablet, and which can detect the position on the tablet which is selected by using the touch pen. 
     FIGS. 6A and 6B are diagrams showing an example where the position detector is applied for an electronic apparatus used as an educational toy for children. In FIG. 6A, a toy  1  includes a book tablet  2   a  and a front tablet  2   b ,and a specific picture book  4 , which is placed on the book tablet  2   a . When operating the toy, a player uses the touch pen  5  to select a FIGURE, character or symbol in the picture book  4 , pressing the touch pen  5  against or bringing it near an illustration. Then, as is shown in FIG. 6B, the selected figure, etc., is displayed on a monitor  6 . 
     Further, when the player uses the touch pen  5  to draw a desired FIGURE or character on the front tablet  2   b , the locus of the touch pen  5  is displayed on the monitor  6 . 
     FIG. 7 is a diagram for explaining the principle employed for the detection of a position on the tablet  2  selected by using the touch pen  5 . As is shown in FIG. 7, the tablet  2  includes a plurality of horizontally arranged antennas  20 X and a plurality of vertically arranged antennas  20 Y. Under the control of a detection controller  9 , an antenna driver  8  drives the horizontal antennas  20 X in the order from right to left, and the vertical antennas  20 Y in the order from top to bottom, which sequentially radiate radio waves. 
     The radio waves radiated by the antennas are received at a reception antenna (not shown) incorporated in the touch pen  5 . Of the level of the received radio waves, the strongest level is that radiated by the antennas which is nearest to the position selected using the touch pen  5 . By comparing the levels of received waves and identifying the strongest level in the vertical and the horizontal directions, the detection controller  9  can detect a selected position. 
     FIG. 8 is a block diagram illustrating the touch pen  5 . In FIG. 8, radio waves radiated by the antennas on the tablet  2  are received by a reception antenna  51  incorporated in the tip of the touch pen  5 . A reception signal is amplified by a buffer amplifier  52 , and the resultant signal is passed through a ceramic filter  53 . The ceramic filter  53 , which permits only a single frequency to pass and blocks all others, performs a noise removal function. 
     The reception signal is then converted by a comparator included in an analog circuit  54  into a level signal which is compared with a predetermined threshold level. The level signal is passed through a delay time adjustment circuit  55  and a connector to a calculation circuit (not shown) for detecting a position selected with the touch pen  5 . 
     The delay time adjustment circuit  55  is provided to perform following functions. A variable delay time is provided for the ceramic filter  53  incorporated in the touch pen  5 , and therefore, if a plurality of touch pens  5  are used to select the same position on the tablet  2 , the delay times for the respective incorporated ceramic filters  53  will vary, and different positions will be detected. For example, even when the center of the tablet  2  is selected by using the touch pens  5 , the detected position will be shifted in the vertical and horizontal directions in which the antennas are driven. Thus, since the delay times for the ceramic filters  53  vary, the distance shifted will differ for the individual touch pens  5 . In order to prevent such an inconvenience, the delay time adjustment circuits  55  make an adjustment for the a variance in the delay times for the ceramic filters  53 , so that a constant delay time is provided for all the touch pens  5 . 
     However, the provision of the additional delay time adjustment circuit  55  increases the cost of a touch pen  5 , and accordingly, the overall cost of a position detector is increased. 
     SUMMARY OF THE INVENTION 
     It is, therefore, one objective of the present invention to provide an inexpensive position detector for which the delay time for a ceramic filter need not be adjusted, i.e., an apparatus for which a delay time adjustment circuit is not required. 
     It is another objective of the present invention to provide a position detector which, without delay time adjustment circuits being required, can accurately detect a position when that position is simultaneously selected with a plurality of touch pens. 
     To achieve the above objectives of the present invention, there is provided a position detector comprising: 
     a member having a surface and a plurality of vertically and horizontally arranged antennas; 
     a controller for sequentially driving the vertically arranged antennas in vertical positive and vertical negative directions, and for sequentially driving the horizontally arranged antennas in horizontal positive and horizontal negative directions so that the antennas sequentially radiate radio waves; 
     a receiver for receiving the sequentially radiated radio waves on the surface of the member; and 
     a detector for detecting the position of the receiver on the surface of the member based on the levels of the received radio waves. 
     Preferably, the detector detects the position of the receiver as an average position for a first position detected based on the levels of the radio waves which are sequentially radiated in the vertical positive and horizontal positive directions, and a second position detected based on the levels of the radio waves which are sequentially radiated in the vertical negative and horizontal negative directions. 
     With this arrangement, variances relative to the position of a receiver which could be picked up by the detector are eliminated, without an adjustment by a delay time adjustment circuit being required, and the position of the receiver can be accurately detected. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram illustrating a position detector according to one embodiment of the present invention; 
     FIG. 2 is a block diagram illustrating a calculation driver; 
     FIGS. 3A to  3 E are a cross-sectional view of a tablet and diagrams of an example reception signal and an example level signal at a touch pen; 
     FIGS. 4A to  4 D are diagrams for explaining the distortion of a reception signal; 
     FIG. 5 is a partial cross-sectional view of a tablet where a spacer is provided between an antenna and a surface cover; 
     FIGS. 6A and 6B are diagrams showing an example where the position detector is applied to an electronic apparatus used as an educational toy for children. 
     FIG. 7 is a diagram for explaining the principle of the detection of a position on a tablet which is selected by using a touch pen; and 
     FIG. 8 is a block diagram illustrating the touch pen. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a block diagram illustrating a position detector according to one embodiment of the present invention. The position detector in FIG. 1 is applied to an electronic apparatus for use as an educational toy. In FIG. 1, in the position detector two tablets (a book tablet  2   a  and a front tablet  2   b ), on which antennas are arranged like a matrix and a touch pen  5  are connected to a calculation and driving unit  10 . The calculation and driving unit  10  drives the antennas of the tablet  2 , and calculates the position (coordinates) selected with the touch pen  5  based on the levels of signal received by the touch pen  5 , as will be described later in detail. 
     A cartridge ROM (not shown), in which, for example, an image processing program for a picture book placed on the book tablet  2   a  is stored, is loaded into a cartridge connector  12 , and this program is processed by a CPU  11 . 
     In accordance with the program and the coordinates of the touch pen  5  position obtained by the calculation and driving unit  10 , the CPU  11  performs corresponding image processing and generates an image signal. The image signal is output to a monitor, and a predetermined image, e.g., a FIGURE, character or symbol on the picture book, including the obtained coordinates, is displayed on the monitor screen. A RAM  13  is a memory for temporarily storing data during the processing performed by the CPU  11 . 
     FIG. 2 is a block diagram illustrating the calculation and driving unit  10 . FIGS. 3A to  3 E are a cross-sectional view of the tablet  2  and diagrams of an example reception signal and an example level signal at the touch pen  5 . Specifically, FIG. 3A is a cross-sectional view taken along line A-A′ in FIG.  1 . From among eight horizontally arranged antennas ( 20   a  to  20   h ), an antenna  20   d  is selected with the touch pen  5  on the surface cover  21  of the tablet  2 . FIGS. 3B to  3 E are diagrams showing a reception signal and a level signal at the touch pen  5  in the state depicted in FIG.  3 A. The operation of the calculation and driving unit  10  in FIG. 2 will now be described while referring to FIGS. 3A to  3 E. 
     A controller  101  in FIG. 2 controls up-count and down-count of an up-down counter (hereinafter referred to simply as a counter)  102 , detects the trailing edge and the leading edge of a level signal at the touch pen  5 , and synchronizes the level signal with a clock signal received from a clock circuit  103 . 
     A drive signal from the counter  102  is supplied to the tablet  2 . And as the counter  102  up-counts, the antennas  20   a  to  20   h  in FIG. 3A radiate radio waves sequentially in the order of the antenna  20   a ,  20   b ,  20   c , . . . to  20   h  (from the left to the right) at each time the count value is incremented a predetermined counts (e.g., 16 counts). In FIG. 3B, is shown a signal (solid line) received at the touch pen  5  at this time. Since the touch pen  5  is nearer the antenna  20   d , at the touch pen  5  the reception level for the radio wave radiated by the antenna  20   d  is the strongest. However, the reception signal is delayed when it is passed through the ceramic filter  53  in the touch pen  5 . As is indicated by broken line N, the signal passed through the ceramic filter  53  is shifted to the right from solid line M. Therefore, as is shown in FIG. 3C, the level signal, which is to be compared with a threshold level (th) and output by the comparator in the touch pen  5 , is also shifted. Based on a control signal received from the controller  101 , the counter values corresponding to trailing edge {circle around (2)} and leading edge {circle around (2)} of the level signal are respectively stored in X coordinate data latches  104   a  and  104   b.    
     When the radiation of radio waves progressing to the right is terminated, the radiation of radio waves progressing to the left is begun. That is, the counter  102  down-counts, the antennas  20   a  to  20   h  radiate radio waves sequentially in the order of the antenna  20   h ,  20   g ,  20   f , . . . to  20   a  (from the right to the left) at each time the count value is decremented a predetermined counts (e.g., 16 counts). In FIG. 3D, a signal (solid line) received by the touch pen  5  is shown. Since the touch pen  5  is nearer the antenna  20   d , at the touch pen  5  the reception level for the radio wave radiated by the antenna  20   d  is the strongest, as it was in FIG.  3 B. In addition, since the radio waves are radiated sequentially, progressing to the left, as is indicated by broken line N′, the received signal passed through the ceramic filter  53  is shifted to the left from solid line M′. Accordingly, as is shown in FIG. 3E, the level signal output by the comparator is shifted to the left. The counter values for the trailing edge {circle around (3)} and the leading edge {circle around (4)} of the level signal are respectively stored in X coordinate data latches  104   c  and  104   d.    
     The counter values stored in the X coordinate data latches  104   a ,  104   b ,  104   c  and  104   d  are transmitted to an X coordinate calculator  106   a . The X coordinate calculator  106   a  obtains X coordinate X(R) for radiation progressing to the right and X coordinate X(L) for the radiation progressing to the left by using the following equations: 
     
       
           X ( R )=(counter value for {circle around (1)}+counter value {circle around (2)})/2  (1) 
       
     
     
       
           X ( L )=(counter value for {circle around (3)}+counter value {circle around (4)})/2  (2) 
       
     
     That is, according to the calculations, the center position, between the trailing edge and the leading edge, is the position selected with the touch pen  5 . The obtained X coordinates X(R) and X(L) are stored in an X coordinate register  107   a.    
     However, as is described above, the obtained X coordinates X(R) and X(L) are shifted to the right and the left, and are not the true coordinates selected with the touch pen  5 . Therefore, the following additional calculation is performed to obtain true coordinate X(D): 
     
       
           X ( D )=( X ( R )+ X ( L ))/2  (3). 
       
     
     That is, since the distance shifted due to the right radiation is the same as that due to the left radiation, the true coordinate X(D) can be obtained as the average of the X coordinates X(R) and X(L), i.e., the coordinate positioned midway between the X coordinates X(R) and X(L). In this embodiment, the CPU  11  in FIG. 1 calculates equation (3); however, the calculation and driving unit  10  may perform the calculation. 
     The same process as is performed for the X coordinate is also performed for the Y coordinate. Then, when the horizontal radiation of radio waves is terminated, the vertical radiation of radio waves is begun. As the counter  102  up-counts and down-counts, the radio waves are sequentially radiated, bidirectionally, i.e., upward and downward, by the vertically arranged antennas. The leading edge and the trailing edge of the level signal at the touch pen  5  are stored in Y coordinate data latches  105   a ,  105   b ,  105   c  and  105   d , and a Y coordinate calculator  106   b  performs the same calculations as those in equations (1) and (2). As a result, Y coordinate Y(R) for the upward radiation and Y coordinate Y(L) for the downward radiation can be obtained. 
     The thus obtained Y coordinates Y(R) and Y(L) are stored in a Y coordinate register  107   b . Then, the CPU  11  performs the same calculation for these Y coordinates as that which was performed in equation (3), so that the true Y coordinate Y(D) is obtained. 
     As is described above, since the radio waves radiated by the antenna  20  are radiated bidirectionally instead of unidirectionally, as is done conventionally, the shifting of the coordinates due to a delay can be offset. Therefore, the correct coordinates can always be obtained, without any adjustments for delay being required. 
     Thus, the delay adjustment circuit conventionally incorporated in the touch pen  5  is not required, and the labor involved in providing the circuit and the manufacturing costs can be reduced. 
     Since the radio waves are sequentially radiated bidirectionally instead of unidirectionally, as is done conventionally, the time required for position detection is twice that which is conventionally required. However, if the number of antennas is substantially reduced to half the conventional number, the same position detection time can be provided as that which is required conventionally. 
     If the number of antennas is reduced, the interval between the antennas may be increased and accuracy of the position detection may deteriorate. In this embodiment, however, the level of the drive signal for the antenna is increased (e.g., doubled), so that both the output level of a radio wave and the level of a received signal are increased. Further, since the threshold level (th) of the comparator for converting a received signal into a level signal is increased, the conventional position detection accuracy can be maintained. Conventionally, since the position detection is performed only through analog processing, a radio wave is radiated by transmitting to the antennas a sine wave signal of 2.5V. In this embodiment, however, since position detection is performed through digital processing, a rectangular wave signal of 5V is transmitted to the antennas. 
     In addition, since the output level of a radio wave and the interval between antennas is increased, a remarkable distortion of the waveform of a received signal may appear when a position on the antenna is not selected with the touch pen  5 . FIGS. 4A to  4 D are diagrams for explaining the distortion of a received signal. In FIG. 4A is shown the waveform of a received signal when a position just on the antenna is selected with the touch pen  5 , and no distortion appears. In FIG. 4B is shown the waveform of a received signal when a position midway between antennas is selected with the touch pen  5 . Since the distances from both antennas are equal, no distortion occurs. In FIGS. 4C and 4D, however, signals are shown that are received when a position selected with the touch pen  5  is shifted slightly away from an antenna. The waveforms are so distorted that they are asymmetrical to the peak. And when the waveform of a received signal is thus distorted, the detected coordinate position is shifted away from the true. 
     A received signal is distorted because the difference in the distance from the touch pen  5  to the antenna on either side is increased, and a difference in the strength of the reception levels for the antenna on one side and for the antenna on the other side is increased. Conventionally, since the interval between the antennas is comparatively small (because a large number of antennas are employed) and the output level of a radio wave is low, the difference in the reception levels is small and the distortion of a received signal is insignificant and can be ignored. To reduce difference of distances between touch pen  5  and antennas on both sides, in this embodiment a spacer  22  is provided between the antennas and a surface cover  21 . FIG. 5 is a partial cross-sectional view of a tablet  2  for which a spacer  22  is provided between the antennas  20  and the surface cover  21 . Since the difference of the distances between the touch pen  5  and the antennas  20  on both sides is reduced, the distortion of a received signal can be suppressed. 
     The distortion of a received signal can also be prevented by sandwiching a radio attenuation sheet between the antennas and the surface cover. 
     With this arrangement, the number of antennas can be reduced without deterioration of the position detection accuracy occurring, and accordingly, manufacturing costs can be reduced. Therefore, a delay adjustment circuit is not necessary and the overall cost of manufacturing the position detector can be reduced. 
     As is described above, according to the present invention, since a position selected by using a touch pen can be accurately detected without an adjustment being needed for a delay caused by a ceramic filter, which is incorporated in the touch pen, a delay adjustment circuit is not required, and the cost of manufacturing the position detector can be reduced. 
     Since a predetermined gap is provided between the antennas and the tip of the touch pen, the distortion of a received signal, which tends to occur when the distance to the antennas is increased by reducing the number of antennas, can be prevented. 
     The present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by foregoing description and all change which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.