Knowledge about capacitive touch screens

Make clear

Capacitive touch screen technology uses the human body's current induction to work. The capacitive touch screen is a four-layer composite glass screen. The inner surface of the glass screen and the interlayer are coated with one layer of ITO, and the outermost layer is a thin silica glass protective layer. The interlayer ITO coating is used as the working surface, and four electrodes are drawn from the four corners, while the inner ITO is the shielding layer to ensure a good working environment. When the finger touches the metal layer, a coupling capacitor is formed between the user and the touch screen surface due to the human electric field. For high-frequency current, the capacitor is a direct conductor, so the finger draws a small current from the contact point. The current flows from the electrodes at the four corners of the touch screen, respectively, and the current flowing through the four electrodes is proportional to the distance between the finger and the four corners. The controller obtains the position of the touch point through the accurate calculation of the ratio of the four currents.

Principle overview

A capacitive touch screen is a transparent layer of special metal-conductive material on the glass surface. When the finger touches the metal layer, the capacitance of the contact changes, causing the frequency of the oscillator connected to it to change. By measuring the frequency change, the position of the touch can be determined. Because capacitors vary with temperature, humidity, or ground conditions, their stability is poor, often resulting in drift.

Multi-touch is to divide the screen into blocks and set a group of mutual capacitance modules in each area to work independently, so the capacitor screen can independently detect the touch condition of each area, and after processing, simply realize multi-touch control. Capacity touch panel The CTP (Capacity Touch Panel) works by sensing currents in the human body. The capacitor screen is a four-layer composite glass screen. The inner surface of the glass screen and the interlayer are each coated with one layer of ITO (nanoindium tin metal oxide), and the outermost layer is a silica glass protective layer only 0.0015mm thick. The interlayer ITO coating is used as the working surface, and four electrodes are drawn from four corners. When the user touches the capacitor screen, a coupling capacitor is formed between the user's finger and the working face due to the human electric field. Because the working surface is connected with high-frequency signals, the finger absorbs a small current, which flows out of the electrodes at the four corners of the screen, respectively. Theoretically, the current flowing through the four electrodes is proportional to the distance between the finger and the four corners. The controller obtains the position through a precise calculation of the ratio of four currents. It can achieve 99% accuracy and has a response speed of less than 3 ms.