1. Field of the Invention
The present invention relates to an input apparatus and a display system that includes the input apparatus. More particularly, the present invention relates to a screen input type image display system that is built by combining a capacitive touch sensor and a display device.
2. Description of the Related Art
An image display system that has a sensor function in a display screen such as a function (screen input function) of inputting information by a touching operation using a finger or a stylus pen is used for a mobile electronic device such as a PDA or a portable terminal, various home electric appliances, or a stationary customer guidance terminal such as an automatic reception machine. As a sensing method used for the image display system which has such a screen input function, there are known a method of detecting changes in resistance value of a touched portion, a method of detecting changes in capacitance thereof, a method of detecting changes in light intensity of a portion shielded by touching, and the like. Among these methods, the method of detecting changes in capacitance (capacitive method) has particularly grown in usage because of little influence on how a displayed image is seen and high durability. The capacitive method is roughly divided into a projected type in which an electrode for detecting the capacitance is broken into pieces that are arranged in a matrix pattern and a surface capacitive type in which a single-piece sheet electrode is used.
U.S. Pat. No. 6,466,036 B1 discloses an example of the projected type method of detecting changes in capacitance. In this method, electrode pieces are sequentially charged/discharged to detect the point of contact with a finger or the like from a difference in the amount of charge or discharge among the electrode pieces. JP 2006-146895 A discloses an example of the surface type method of detecting changes in capacitance. In this method, signals are input from the four corners of a single rectangular electrode to detect the point of contact with a finger or the like from a difference in the amount of output current among sources of the signals.
The projected type method described in U.S. Pat. No. 6,466,036 B1 uses an electrode broken into pieces and accordingly has an advantage in that a high point detection resolution is obtained with a sensor that is small in area such as the one for a portable terminal. A problem of this method is that the need to form a complicated electrode structure raises the manufacture cost.
The surface type method described in JP 2006-146895 A uses a simple electrode structure and accordingly has an advantage of being low in manufacture cost. A drawback of this method is that obtaining a high point detection resolution is difficult with a sensor that is small in area. The application of the surface type method to a small-area sensor such as the one for a portable terminal is therefore difficult while the surface type method is easily applied to cases where a large-sized sensor is needed as in a stationary customer terminal.