The basic construction principles of capacitive touch screens you should know
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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 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 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: first, 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 behind is also Conductive layer. The inner conductive layer is a shielding layer, which plays the role of shielding internal electrical signals. The middle conductive layer is a 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 upper 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 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 three-dimensional structure of the ITO process has a great influence on the capacitive touch screen, which is directly related to the two important capacitance parameters of the touch screen: inductive capacitance (finger and upper ITO) and parasitic capacitance (between the upper and lower ITO layers, and the difference between the lower ITO and the 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, a coupling capacitor is formed between the finger and the conductor layer. The current from the four-side electrode will flow to the contact. The strength of the current is inversely proportional to the distance between the finger and the electrode. The controller located after touching the screen The ratio and strength of the current will be calculated, and the position of the touch point will be accurately calculated. 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. The screen is dirty, dust or oily. 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 drift phenomenon tends to occur. This kind of touch screen is suitable for the debugging stage of system development.
The principle of capacitive touch screen
The capacitive touch screen technology uses the current induction of the human body 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. The outer layer is a thin layer of silica glass protective layer. The interlayer ITO coating is used as the working surface. Each electrode, the inner ITO is a shielding layer to ensure a good working environment. When a finger touches the metal layer, due to the electric field of the human body, a coupling capacitor is formed between the user and the surface of the touch screen. 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 these four electrodes is proportional to the distance from the finger to the four corners. The controller calculates the position of the touch point by accurately calculating the ratio of these four currents.
