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Basic structure of capacitive touch screen

The basic structure of capacitive touch screen is: the substrate is a single-layer plexiglass, and a layer of transparent conductive film is evenly forged on the inner and outer surfaces of the plexiglass, and a narrow and long cone is taped 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 capacitance, which is equivalent to a conductor. Because there is a high-frequency signal on the working surface, when the finger touches A small current is sucked at the touch point, and the small current flows out from the electrodes on the four corners of the touch screen respectively. The current flowing through the four electrodes is proportional to the straight-line distance from the finger to the four corners. Calculate the contact point coordinates.

capacitive touch screen
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 innermost layer is a fourth 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 four sides, which are responsible for the detection of the touch point position.
The top cover 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, 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 ohms. The three-dimensional structure of the ITO process has a great influence on the capacitive touch screen, which is directly related to two important capacitance parameters of the touch screen: inductive capacitance (finger and upper ITO) and parasitic capacitance (between the upper and lower ITO, between the lower ITO and the display screen) between).
The structure of the capacitive touch screen is mainly to coat a layer of 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 at the same time, the light transmittance is higher. Better support for multi-touch.
The capacitive touch screen is plated with long and narrow electrodes on the four sides of the touch screen to form a low-voltage AC electric field in the conductor. When touching the screen, due to the electric field of the human body, a coupling capacitance will be formed between the finger and the conductor layer, and the current from the electrodes on the four sides will flow to the contacts, and the strength of the current is inversely proportional to the distance from the finger to the electrode. The controller behind the touch screen is then It will calculate the proportion and strength of the current, and accurately calculate the position of the touch point. The double glass of the capacitive touch screen can not only protect the conductors and sensors, but also effectively prevent external environmental factors from affecting the touch screen. Even if the screen is stained with dirt, dust or oil stains, the capacitive touch screen can still accurately calculate the touch position.
Capacitive touch screen manufacturers tell you that because the capacitance changes with the contact area and the dielectric of the medium, its stability is poor and drifting often occurs. This kind of touch screen is suitable for the debugging stage of system development.

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