Design and Implementation of a High Quality Multi-Touch Sensing System for Capacitive Touch Screen Panel

Abstract

Recently, more and more researches have been conducted to develop better-input methods for the emotional use of mobile devices. Particularly, a gesture-based touch input method is considered most intuitive and user-friendly, which made it one of the most widely used input methods in mobile devices. The method of sensing touch based on small-sized touch screen panel (TSP) is typically categorized into two types. The first method is resistive type, which recognizes the contact of the upper and lower layers. The second method is capacitive type, which measures the change in capacitance. Between them, the capacitive TSP has been more popular method since being adopted by smart phones, due to its multi-touch capability, superior durability, better transmittance, and higher sensitivity compared to the resistive type. However, since the capacitive type is based on the change of capacitance in the touch panel, the capacitive type is vulnerable to noises that may affect the capacitance, such as white noise, AC noise or noises from the sync signals generated by the display panel. Even though a large amount of research has been done to find a precise measurement of the change in capacitance, it still needs lots of improvement in terms of sensitivity and accuracy. This thesis proposes a high quality multi-touch sensing system for capacitive TSP that minimizes the irritation from noises and improves its touch-sensitivity. The proposed multi-touch sensing system consists of the analog and digital circuits, and was designed on the basis of ARM Cortex Platform. When it senses touch on the panel, the analog circuit amplifies the voltage generated by the change in capacitance by a differential amplifier, transforming the analog signal into the digital one, and sends it outside. Particularly, to minimize the distortion of output voltage caused by the external factor, a rectifier controlling the drift of the output voltage was included. The digital circuit consists of two parts, the touch controller and the touch engine. The touch controller controls the analog circuit, and the touch engine senses the touch. The digital circuit receives the digital signal from the analog circuit, eliminating the noises using a digital filter and the touch algorithms, which finally sends out the calculated coordinates of the sensed touch. In order to adapt it to both the mid-sized capacitive TSP and the small-sized high-resolution capacitive TSP, the proposed multi-touch sensing system is designed to 16 RX channels and 26 TX channels. And, it works regardless of the order and numbers of channel connections. Additionally, proposed system also supports the capacitive stylus touch. As a result of its performance evaluation based on on-cell TSP module, the proposed system was capable of supporting smooth drawing with minimum touch tip of Φ6-pitch, and showed outstanding results in accuracy, linearity and speed evaluation of sensing touch.