The basic structure of capacitive touch screen is that the substrate is a single-layer organic glass, a layer of transparent conductive film is uniformly forged on the inner and outer surfaces of the organic glass, and a narrow electrode is respectively on the four corners of the transparent conductive film on the outer surface.
Its working principle is that when the finger touches the capacitive touch screen, the high-frequency signal is switched on at 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, when the finger touches, it absorbs a small current 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. The controller can obtain the contact point coordinate value by calculating the four current proportions.
Capacitive touch screen can be simply seen as a screen composed of four layers of composite screen. First, the outer layer is a glass protective layer, then a conductive layer, the third layer is a non-conductive glass screen, and the fourth layer behind is also a conductive layer. The inner 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 whole touch screen. There are direct leads on four corners or four sides, Be responsible for detecting the position of touch points.
The upper covering layer is toughened glass or polyethylene terephthalate (PET). The advantage of PET is that the touch screen can be thinner. On the other hand, it is cheaper than the existing plastic and glass materials. The insulation layer is glass (0.4~1mm), organic film (10~100um), adhesive and air layer.
One of the important layers is the indium tin oxide (ITO) layer. The typical thickness of ITO is 50~100nm, and its block resistance is about 100~300 ohms. The three-dimensional structure of ITO technology has a great impact on the capacitive touch screen, which is directly related to the two important capacitance parameters of the touch screen, the inductive capacitance (between the finger and the upper ITO) and the parasitic capacitance (between the upper and lower ITOs, and between the lower ITO and the display screen).
The structure of capacitive touch screen is mainly to coat a transparent film layer on the glass screen, and then add a piece of protective glass to the conductor layer. The double glass design can completely protect the conductor layer and inductor, while the light transmittance is higher, and it can better support multi-point touch.
The capacitive touch screen is coated with long and narrow electrodes on all sides of the touch screen, forming a low-voltage AC electric field in the conductive body. When touching the screen, due to the human electric field, a coupling capacitor will be formed between the fingers and the conductor layer, and the current from the four sides of the electrodes will flow to the contacts, and the strength of the current is inversely proportional to the distance between the fingers and the electrode.
The controller behind the touch screen 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 conductor and inductor, but also effectively prevent the impact of external environmental factors on the touch screen. If the screen is stained with dirt, dust or oil, the capacitive touch screen can still accurately calculate the touch position.
As the capacitance varies with the contact area and dielectric of the medium, its stability is poor, and drift often occurs. This kind of touch screen is suitable for the debugging stage of system development.