COORDINATE READING DEVICE

Abstract: A coordinate reading device comprises a coordinate indicator (6), a drive circuit (5) for driving the coordinate indicator, sense lines (1) for detecting a signal generated by the coordinate indicator and an analogue circuit (7) for amplifying and shaping the waveform of a signal detected by the sense lines. An A/D converter (8) converts an analogue signal from the analogue circuit into a digital signal and scanning circuitry (4a,4b) scans the sense lines. A control circuit (2) sends the scanning address to the scanning circuitry and determines the position of the coordinate indicator (6) from the digital signal. The analogue circuit (7) includes an amplifier (10) and an analog switch (14) for varying the amplification factor of the amplifier in dependence upon the scanning address received from the control circuit (2).

Description: This invention relates to coordinate reading devices for use, for example, as input devices such as computers. Figure 3 shows the construction of a known coordinate reading device. A sense line 1 is scanned successively through X- and Y-axis scanning circuits 4a,4b in response to a scanning address 3 sent from a control circuit 2. The signal generated from a coordinate indicator 6 by a drive circuit 5, is fed to the control circuit 2, after being detected by the sense line 1, then amplified and waveformed by an analogue circuit 7, and converted into a digital signal 9 by an A/D converter 8. It will be appreciated that, in the control circuit 2, the digital signal 9 generated from the scanning address 3 and the A/D converter 8, operates the position of the coordinate indicator 6. An example of this type of conventional coordinate reading device is disclosed in the Patent Publications Gazette Showa No. 55-99683. In the conventional coordinate reading device, the sense line 1 is constituted by, for example, a printed baseboard comprising a sense line 1a in the X-axis direction and a sense line 1b in the Y-axis direction mounted on opposite faces of an electrically insulating board 1c, as shown in Figure 4. Accordingly, the level of signals detected at the X-axis sense line 1a and the Y-axis sense line 1b differ because of the difference between the distance ℓa,ℓb from a signal source 6a of the coordinate indicator 6 to each of the sense lines 1a,1b, respectively. It is known that the levels of these detecting signals is an inverse ratio to the square of the distances ℓa,ℓb. Therefore, as shown in Figure 4, when the X-axis sense line 1a is mounted on the side of the insulating board 1c closer to the signal source 6a, the detecting signal at each sense line is attenuated greatly when it is inputted into the A/D converter 8 through the analogue circuit 7, even when the input level of the A/D converter 8 is appropriate for the level of the detecting signal from the X-axis sense line 1a, the detecting signal from the Y-axis sense line 1b will be small and insufficient and therefore the analysis ability to detect the Y-axis sense line will be poor. Also when trying to obtain an input level of the detecting signal from the Y-axis sense line 1b sufficient for the A/D converter 8, the level of the detecting signal from the X-axis sense line 1a will be too great for the A/D converter 8. In the conventional coordinate reading device to obtain analysis ability sufficient for both the X-axis sense line and the Y-axis sense line a very thin insulating board 1c (thickness t = 0.4 - 0.5 mm) must be used to reduce the distance difference |ℓa-ℓb| between the signal source 6a to the respective sense lines 1a,1b. Consequently the conventional coordinate reading device must be treated with much care and manufacturing yield is low. To reduce the difference in level of the detecting signals detected by the respective sense lines 1a,1b, the distance between the signal source 6a and the X-axis sense line 1a can be increased. Therefore a ratio K of the levels of the detecting signals, when t = ℓb-ℓa is given by: and it will readily be appreciated that when ℓa is large compared to t, the value of K in the above formula tends to unity. However, increase in the distance between the signals source 6a from the sense lines 1a,1b will cause a fall in the signal-to-noise (S/N) ratio, and naturally will cause a fall of the analysis ability. To maintain the analysis ability, the signal generated from the signal source 6a must be increased, and the analogue circuit 7 and the A/D converter 8 shown in Figure 3 must be made precisely to operate with great accuracy. This has the disadvantage that the conventional coordinate reading device is of large size and is very expensive to manufacture. The present invention seeks to provide a coordinate reading device which has sufficient analysis ability in both X- and Y-axis, even when an insulating board on opposite sides of which sense lines are provided, has a thickness of 1.6 mm. According to the present invention there is provided a coordinate reading device comprising: a coordinate indicator; a drive circuit for driving the coordinate indicator; sense lines for detecting a signal generated by the coordinate indicator; an analogue circuit for amplifying and shaping the waveform of a signal detected by the sense lines; an A/D converter for converting an analogue signal from the analogue circuit into a digital signal; a scanning circuit for scanning the sense lines; and a control circuit for sending scanning addresses to the scanning circuit for determining the position of the coordinate indicator from the digital signal, characterised by the analogue circuit including amplifier means and switch means for varying the amplification factor of the amplifier means in dependence upon a scanning address received from the control circuit. In the preferred embodiment the switch means is an analogue switch. The analogue circuit may include resistance elements that are, in operation, switched into and out of a circuit including the amplifier means to vary the amplification factor of the amplifier means. In a coordinate reading device according to the present invention because the amplification factor of the amplifier means is determined by a scanning address inputted to the amplifier means, different levels of signals detected by the sense lines have the same signal level when inputted into the A/D converter. Thus it is possible to obtain substantially constant analysis ability in both X- and Y- axes. The invention is illustrated, merely by way of example, in the accompanying drawings, in which:- Figure 1 is a block diagram of one embodiment of a coordinate reading device according to the present invention; Figure 2 is a circuit diagram of an amplifier of an analogue circuit of the coordinate reading device of Figure 1; Figure 3 is a block diagram of a conventional coordinate reading device; and Figure 4 is a sectional view showing the relationship between a sense line and a coordinate indicator of the conventional coordinate reading device of Figure 3. Throughout the drawings like parts have been designated by the same reference numerals. Referring to Figure 1, a sense line 1 of one embodiment of a coordinate reading device according to the present invention is scanned successively by a scanning address 3 sent from a control circuit 2 through an X-axis scanning circuit 4a and a Y-axis scanning circuit 4b. For example, when 128 lines of the sense line 1 are provided for the X-axis and Y-axis, the scanning address 3 needs 7 lines (SCA 0-6) to scan the 128 lines of each axis. Moreover, one line (SCA 7) as a scanning address 3a is required to select the X-axis and Y-axis, and therefore a total of 8 lines are necessary. The scanned sense line detects the signal generated from a coordinate indicator 6 driven by a drive circuit 5, and the output from the sense line is inputted to an analogue circuit 7 through the scanning circuits 4a,4b. A detecting signal 15 detected by the analogue circuit 7 by the sense line 1 is amplified and waveformed or shaped and sent to an A/D converter 8, then converted into a digital signal 9. The control circuit 2 scans successively the sense line 1 by the scanning address 3 (SCA 0-6), and when the detecting signal 15 detected at the sense line 1 respectively compares the value converted into the digital signal 9, the coordinate value of the X-axis and the Y-axis is determined. At this point, the scanning address 3a (SCA 7) and one of the scanning addresses 3 (SCA 0-6) for selecting the X- and Y-axis sense line, is taken into the analogue circuit 7. Figure 2 shows an embodiment of an amplification portion of the analogue circuit 7. This amplification portion is an ordinary non-inversion amplifier, and comprises an operational amplifier 10, resistance elements R1,R2,R3 which determine the amplification rate of the amplifier and an analogue switch 14 which is turned ON/OFF by the scanning address 3A to switch the resistance element into and out of the circuit of the amplifier 10 to vary the amplification factor thereof. The amplification factor A ON when the analogue switch is ON, and the amplification factor A OFF when the analogue switch 14 is OFF are given by: Generally, the distance ℓa in Figure 4 is about 5 mm. In the conventional coordinate reading device the printed baseboard carrying the sense line is 0.4 mm in thickness which is very thin for a baseboard. When a board of a regular thickness of about 1.6 mm is used, the ratio K referred to above of the levels of detecting signals detected by the X- and Y-axis sense lines 1a,1b will be: Therefore, in Figure 2, if the constants of the resistance elements R1,R2,R3 are selected to be A ON/A OFF = 0.57, and if the logic is matched so that the analogue switch 14 is ON when the scanning address 3a (SCA 7) is scanning the X-axis and the analogue switch is OFF when scanning the Y-axis, an output signal 17 of the amplifier of Figure 2 can decrease the difference of the level of the detecting signals 15 detected from the X- and Y-axis sense lines, and this output signal 17 can give a definite substantially constant level of signal to the A/D converter shown in Figure 1. In this embodiment, as the thickness of the printed baseboard is 1.6 mm compared to 0.4 mm for the conventional coordinate reading device, the distance ℓb in Figure 4 is increased from 5.4 mm to 6.6 mm, and the level of the signal detected by the Y-axis sense line 1b is 5.4²/6.6² = 0.67, a 30% decrease but this can show sufficient analysis ability with the analogue circuit of the present invention compared to that of the conventional coordinate reading device. The amplifier shown in Figure 2 uses the scanning address 3a and controls the amplification factor of the amplifier 10. However, it will be appreciated that there are many alternative ways of achieving the same end other than the one shown in Figure 2. As stated above, by inputting the scanning address to the analogue circuit, and by adding a simple circuit comprising one analogue switch, without destroying the analysis ability of the conventional coordinate reading device, it is possible to use a baseboard for the sense lines of regular thickness, e.g. 1.6 mm which has the effect of improving manufacturing yield and reducing cost. The increased thickness of the baseboard compared to that of the conventional coordinate reading device means that there is no need to strengthen the sense lines and the resulting coordinate reading device is vibration­ proof and robust.

Claims: 1. A coordinate reading device comprising: a coordinate indicator (6); a drive circuit (5) for driving the coordinate indicator; sense lines (1) for detecting a signal generated by the coordinate indicator; an analogue circuit (7) for amplifying and shaping the waveform of a signal detected by the sense lines; an A/D converter (8) for converting an analogue signal from the analogue circuit into a digital signal; a scanning circuit (4a,4b) for scanning the sense lines; and a control circuit (2) for sending scanning addresses to the scanning circuit for determining the position of the coordinate indicator from the digital signal, characterised by the analogue circuit (7) including amplifier means (10) and switch means (14) for varying the amplification factor of the amplifier means in dependence upon a scanning address (SCA7) received from the control circuit. 2. A coordinate reading device as claimed in claim 1 characterised in that the switching means (14) is an analogue switch. 3. A coordinate reading device as claimed in claim 2 characterised in that the analogue circuit (7) includes resistance elements (R1,R2,R3) that are, in operation, switched into and out of a circuit including the amplifier means (10) to vary the amplification factor of the amplifier means. 4. A coordinate reading device comprising: a coordinate indicator which generates certain signals; a drive circuit which drives the said coordinate indicator; several sense- lines which detect the signal generated from said coordinate indicator; an analogue circuit which amplifies and wave­ forms the signal detected on the said senselines; A/D converter which converts the analogue signal from the said analogue circuit into digital signals; a scanning circuit which scans the said senselines; and a control circuit which sends the scanning address to said scanning circuit, and also operates the position of the said coordinate indicator with the signal from said A/D converter; wherein switching elements like analogue switches are provided at the amplification portion of the said analogue circuit, and has the feature of control­ ling the said control circuit with the scanning address and varying the amplification rate of the said amplification portion.

Inventors: SAEKI, SHINJI
Applicants: SEIKO INSTRUMENTS INC.
Application year: 1985
Main classification: G06K11
IPC codes: G06K11, G06F3