Capacitive touch screen can be simply seen as a screen composed of four layers of composite screen: the outer layer is a glass protective layer, followed by a conductive layer, the third layer is a non-conductive glass screen, and the fourth layer at the innermost is also a conductive layer. The innermost conductive layer is a shielding layer, which plays a role in shielding internal electrical signals. The conductive layer in the middle is a key part of the entire touch screen. There are direct leads on four corners or four edges, which are responsible for detecting the position of the touch point. The capacitive screen works by using the current induction of the human body. When the finger touches the metal layer, due to the human body electric field, the user and the touch screen surface form a coupling capacitor. For high-frequency current, the capacitor is a direct conductor, so the finger absorbs a small current from the contact point. This current flows from the electrodes on the four corners of the touch screen, and the current flowing through the four electrodes is proportional to the distance from the finger to the four corners. The controller obtains the position of the touch point by accurately calculating the four current proportions.
Advantages: At present, there are mainly several types of touch screens: resistive (double-layer), surface capacitive and inductive capacitive, surface acoustic wave, infrared, bending wave, active digital converter and optical imaging. They can also be divided into two categories: one requires ITO, such as the first three touch screens, and the other does not require ITO, such as the last several screens.
At present, the resistive touch screen and capacitive touch screen using ITO materials are most widely used in the market. ITO is the abbreviation of indium tin oxide, which is a transparent conductor. The properties of this material can be adjusted by adjusting the proportion of indium and tin, deposition method, oxidation degree and grain size. Thin ITO materials have good transparency, but high impedance; Thick ITO materials have low impedance but poor transparency. When depositing on PET polyester film, the reaction temperature will drop to below 150 ° C, which will lead to incomplete oxidation of ITO. In subsequent applications, ITO will be exposed to air or air interlayer, and its impedance per unit area will change with time due to self oxidation. This makes the resistive touch screen need to be calibrated frequently.
The multi-layer structure of the resistive touch screen will cause great light loss. For handheld devices, it is usually necessary to increase the backlight source to compensate for the poor light transmission, but this will also increase the battery consumption. The advantage of resistive touch screen is that its screen and control system are relatively cheap, and its response sensitivity is also very good. The surface capacitance touch screen only uses a single layer of ITO. When a finger touches the surface of the screen, a certain amount of charge will be transferred to the human body. In order to recover these charge losses, the charge is added from the four corners of the screen. The amount of charge added in each direction is proportional to the distance of the touch point. We can calculate the position of the touch point from this. The surface capacitive ITO coating usually needs to add a linearized metal electrode around the screen to reduce the influence of corner/edge effects on the electric field. Sometimes there is an ITO shielding layer under the ITO coating to block the noise. The surface capacitance touch screen can only be used after being calibrated at least once.
