The basic structure of the capacitive touch screen is: the substrate is a single-layer plexiglass, a layer of transparent conductive film is uniformly forged on the inner and outer surfaces of the plexiglass, and a narrow and long cone is placed on the four corners of the transparent conductive film on the outer surface. electrode. Its working principle is: when a finger touches the capacitive touch screen, a high-frequency signal is connected to the working surface. At this time, the finger and the working surface of the touch screen form a coupling capacitor, which is equivalent to a conductor, because there are high-frequency signals on the working surface. A small current is drawn at the touch point. This small current flows out from the electrodes on the four corners of the touch screen. The current flowing through the four electrodes is proportional to the linear distance from the finger to the four corners. By calculation, the coordinate value of the contact point can be obtained.
The capacitive touch screen can be simply regarded as a screen composed of four layers of composite screens: the outermost layer is a glass protective layer, followed by a conductive layer, the third layer is a non-conductive glass screen, and the fourth innermost layer It is also a conductive layer. The innermost conductive layer is the shielding layer, which plays the role of shielding the internal electrical signals. The middle conductive layer is the key part of the entire touch screen. There are direct leads on the four corners or sides to detect the position of the touch point.
The uppermost covering layer is tempered glass or polyethylene terephthalate (PET). The advantage of PET is that the touch screen can be made thinner, and on the other hand, it is cheaper than the existing plastic and glass materials. The insulating layer is glass (0.4-1mm), organic film (10-100um), adhesive, and air layer. The most important layer is the indium tin oxide (ITO) layer. The typical thickness of ITO is 50-100nm, and its sheet resistance is about 100-300 ohm range. The process and three-dimensional structure of ITO has a great influence on capacitive touch screens. It is directly related to the two important capacitance parameters of touch screens: inductive capacitance (finger and upper ITO) and parasitic capacitance (between upper and lower ITO layers, between lower ITO and display screen). between).
The structure of the capacitive touch screen is mainly to plate a transparent film body layer on the glass screen, and then add a protective glass outside the conductor layer. The double glass design can completely protect the conductor layer and the sensor, and the light transmittance is higher. Can better support multi-touch.
The capacitive touch screen is plated with long and narrow electrodes on all four sides of the touch screen to form a low-voltage AC electric field in the conductive body. When touching the screen, due to the electric field of the human body, there will
Capacitive touch screen becomes a coupling capacitor. The current from the four-side electrodes will flow to the contacts, and the current strength is inversely proportional to the distance between the finger and the electrode. The controller located behind the touch screen will calculate the current ratio and strength To accurately calculate the location of the touch point. The double glass of the capacitive touch screen not only protects the conductors and sensors, but also prevents external environmental factors from affecting the touch screen. Even if the screen is dirty, dust or oil stains, the capacitive touch screen can still accurately calculate the touch position.
Since the capacitance varies with the contact area and the dielectric of the medium, its stability is relatively poor, and drift phenomenon often occurs. This kind of touch screen is suitable for the debugging stage of system development.
