This invention relates to a touch panel device employed by a computer system as an input system to locate a position pushed by a finger or a pen-like material. (2) Description of the Related Art
According to a well known touch panel, a fixed potential gradient is formed on a resistance material and a position to which a stylus pen or the like supplies external force is located with reference to the potential detected at the pushed position.
For example, the conventional touch panel in FIG. 1 comprises a quadrilateral planar resistance member 1 and electrodes 2 and 3 connected at ends of the resistance member 1, the ends opposing to each other. In this construction, potential difference between the electrode 2 and the stylus pen 11 is proportional to a physical distance therebetween. Based on a ratio of the potential difference to the distance, the position of the stylus pen 11 is detected when it contacts with the resistance member 1.
Another touch panel in FIG. 2 detects the pushed position in two dimensions. As shown in the figure, the touch panel includes electrodes 4 and 5 besides the components of the above touch panel. In this construction, potential is supplied to a first pair of electrodes and a second pair of electrodes mutually, the first pair including electrodes 2 and 3 while the latter pair including electrodes 4 and 5. Thus, X and Y coordinates of the pushed position are detected one after the other.
According to the touch panel where the planar resistance member 1 has the electrodes 2 through 5 at all ends thereof, some current leaks. For example, when the potential is supplied between the electrodes 2 and 3, some of the potential leaks via the electrodes 4 and 5. Such leak increases power dissipation noticeably. The power dissipation can be suppressed if the electrodes 2 through 5 have some resistances. In this case, the wiring of the touch panel will be constructed as shown in FIG. 3. Another wiring can be added in the figure to detect X coordinate of the pushed position.
However, when each of the electrodes 2 through 5 has some resistance, the potential cannot be applied to the whole electrode equally. For example, when the electrode 3 has some resistance, the potential at point A or B thereof may differ from the potential at point C. As a result, as shown in the figure the equipotential lines will not be straight and the proportional relation between the potential difference and the physical distance will be broken, so that the accuracy in the detection of the pushed position will be deteriorated. The proportional relation between the potential difference and the physical distance would be retained by an equation and a correction; however, this will hurt simple structure of the device.
A touch panel disclosed in U.S. Pat. No. 4,198,539 is designed to overcome the above problems. As shown in FIG. 4, the electrodes 2 through 5 are curved and they are connected via resistances 12 through 15.
The electrodes 2 through 5 and the resistances 12 through 15 have fixed resistances. In this construction, both equipotential lines extending along X and Y axes, the former shown by broken lines while the latter shown by two-dot chain lines in the figure, become straight since the potential drop between the points D and E is equal to the potential drop between the points D and F when the power source 16 is turned on. Also the same potential drop is applied between ends of the resistance 12. Similarly, when the power source 17 is turned on, the potential drop between the points G and H is equal to the potential drop between the points G and F. Also the same potential drop is applied to between ends of the resistance 12.
However, the conventional touch panel in FIG. 4 still has some drawbacks to be overcome. That is, the resistance member enlarges when it is surrounded by the curved electrodes. As apparent from the figure, the resistance member 1 surrounded by the curved electrodes 2 through 5 includes areas in which the pushed position is not detected besides an active sensing area. Hence, the touch panel tends to be oversized when it employs the curved electrodes.
The power dissipation of the touch panel will become high due to the leak of the potential via the electrodes 12 through 15, same as the above conventional touch panel, as well as due to the potential drops such as the one applied between ends of the resistance 12.
Another well known conventional touch panel prevents the leak of current and the non-straight equipotential lines by replacing each of the electrodes 2 through 5 with a plurality of short electrodes being apart from each other, each of the short electrodes having its own switch and diode. The switches and the diodes can be connected to the resistance member externally, or can be formed thereon. However, there still has problems in either case. That is, when they are connected to the resistance member externally, a connector for connecting the switches and diodes to the touch panel includes a large number of terminals, so that the connector will be oversized. Also the contact resistance of the connector deteriorate accuracy of the detection. When the switches are formed on the resistance member 1, the equipotential lines will not be straight enough since the number of the switches the resistance member 1 can accommodate thereon is limited.