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Capacitive touch screen technology uses the human body's current sensing to work. The capacitive touch screen is a four-layer composite glass screen. The inner surface and the interlayer of the glass screen are each coated with a layer of ITO, and the outer layer is a thin layer of silica glass protective layer. The interlayer ITO coating is used as the working surface. Electrodes, the inner layer of ITO is a shielding layer to ensure a good working environment. 耦合电容 ,对于 高频电流 来说,电容是直接导体,于是手指从接触点吸走一个很小的电流。 When a finger touches the metal layer, due to the electric field of the human body, the user and the touch screen surface form a coupling capacitor . For high-frequency current , the capacitor is a direct conductor, so the finger draws 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 calculates the position of the touch point by calculating the proportion of these four currents.

basic structure

The basic structure of a capacitive touch screen is: the substrate is a single layer of plexiglass, a transparent conductive film is evenly forged on the inner and outer surfaces of the plexiglass, and a narrow and long cone is formed on the four corners of the transparent conductive film on the outer surface. electrode. The working principle is that when a finger touches the capacitive touch screen, a high-frequency signal is turned on 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 is a high-frequency signal on the working surface. A small current is drawn at the touch point. This small current flows from the electrodes on the four corners of the touch screen. The current flowing through the four electrodes is proportional to the straight line distance from the finger to the four corners. Calculate the coordinate value of the contact point.

The capacitive touch screen can be simply regarded as a four-layer 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 of the inner layer is also Conductive layer. The inner conductive layer is a shielding layer, which plays a role of shielding the internal electrical signals. The middle conductive layer is a key part of the entire touch screen. There are direct leads at four corners or four sides, which are responsible for detecting the position of the touch point.
The upper cover is tempered glass or polyethylene terephthalate (PET). The advantage of PET is that touch screens can be made thinner. On the other hand, they are also cheaper than existing plastic and glass materials. The insulating layer is a glass (0.4 to 1 mm), an organic thin film (10 to 100 um), an adhesive, and an air layer. The more important layer is an indium tin oxide (ITO) layer. The typical thickness of ITO is 50-100 nm, and its block resistance is about 100-300 ohms. The three-dimensional structure of the ITO process has a great impact on the capacitive touch screen. It is directly related to the two important capacitance parameters of the touch screen: the inductive capacitance (finger and upper ITO) and the parasitic capacitance (between the upper and lower ITO, the lower ITO and the display between).
The structure of the capacitive touch screen is mainly a transparent thin film layer on the glass screen, and a protective glass is added outside the conductor layer. The double-glass design can completely protect the conductor layer and the sensor. At the same time, the transmittance is higher. Better support for multi-touch.
Capacitive touch screens are coated with narrow electrodes on all sides of the touch screen to form a low-voltage AC electric field in the conductive body. 成一个耦合电容,四边电极发出的电流会流向触点,而电流强弱与手指到电极的距离成反比,位于触摸屏幕后的控制器便会计算电流的比例及强弱,准确算出触摸点的位置。 When the screen is touched, due to the electric field of the human body, a coupling capacitance is formed between the finger and the conductor layer. The current from the four electrodes will flow to the contact, and the strength of the current is inversely proportional to the distance from the finger to the electrode. The controller located behind the touch screen Then it will calculate the proportion and strength of the current to accurately calculate the position of the touch point. The double glass of the capacitive touch screen not only protects the conductors and sensors, but also effectively prevents external environmental factors from affecting the touch screen. Even if the screen is stained with dirt, dust or oil, 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 poor, and drifting tends to occur. This touch screen is suitable for the debugging phase of system development.


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