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LCD touch screen has gradually replaced the keyboard as a commonly used human-computer interaction tool in embedded systems due to its advantages of lightweight, less space occupation, convenience and flexibility. Touch screens can be divided into five categories according to their technical principles: vector pressure sensing, resistive, capacitive, infrared and surface acoustic wave. Among them, resistive touch screens are mostly used in embedded systems.
LCD Touch Screen, also known as Touch Panel, is an inductive LCD device that can receive input signals such as contacts. When touching the graphic buttons on the screen, the tactile feedback system on the screen can drive various connecting devices according to the pre-programmed program, which can be used to replace the mechanical button panel and be displayed by LCD. Display pictures create vivid audio and video effects.
The working part of a typical touch screen generally consists of three parts: two transparent resistive conductor layers, isolation layer between two conductors and electrodes. Resistive conductor layers are made of resistive materials, such as indium tin oxide (ITO) coated on the substrates, plastic on the upper substrates and glass on the lower substrates. The isolation layer is a viscous insulating liquid material, such as polyester film. The electrodes are made of materials with excellent conductivity, such as silver inks, which are about 1000 times as conductive as ITO.
In order to operate conveniently, people use touch screen instead of mouse or keyboard. When working, we must first touch the touch screen installed at the front of the display with finger or other objects, and then the system locates and selects information input according to the position of the icon or menu touched by finger. Touch screen is composed of touch detection component and touch screen controller. Touch detection component is installed in front of the display screen to detect the user's touch position and receive the backward touch screen controller. The main function of touch screen controller is to receive touch information from touch point detection device and convert it into contact coordinates. It can also receive commands from CPU and execute them.
From the technical principle to distinguish touch screen, it can be divided into five basic categories: vector pressure sensing technology touch screen, resistance technology touch screen, capacitance technology touch screen, infrared technology touch screen, surface acoustic technology touch screen. Among them, the vector pressure sensing technology touch screen has withdrawn from the historical stage; the infrared technology touch screen is cheap, but its outer frame is fragile, easy to produce light interference, distortion in the case of curved surface; the capacitance technology touch screen design is reasonable, but its image distortion problem is difficult to solve fundamentally; the resistance technology touch screen positioning accuracy Sure, but its price is quite high, and it is afraid of scratching and fragility. Surface acoustic wave touch screen solves all kinds of defects of the previous touch screen. It is clear and not easy to be damaged. It is suitable for various occasions. The disadvantage is that if there are water droplets and dust on the screen surface, the touch screen will become dull or even not work. According to the working principle of touch screen and the medium of transmitting information, we divide the touch screen into four types: resistance type, capacitance induction type, infrared type and surface acoustic wave type. Each kind of touch screen has its own advantages and disadvantages. To understand which kind of touch screen is suitable for which occasion, the key is to understand the working principle and characteristics of each kind of touch screen technology.
Following is a brief introduction to the various types of touch screens mentioned above:
1. Resistance touch screen (working principle diagram of resistance touch screen) This kind of touch screen uses pressure induction to control. The main part of the resistive touch screen is a resistive film screen which matches the surface of the display very well. It is a multi-layer composite film. It uses a glass or hard plastic plate as the base, and the surface is coated with a transparent metal oxide (transparent conductive resistance) conductive layer, which is covered with an outer surface hardening treatment. Smooth anti-friction plastic layer, its inner surface is also coated with a layer of coating, between them there are many small (less than 1/1000 inch) transparent isolation points to insulate the two conductive layers. When the finger touches the screen, the two conductive layers touch at the touch point, and the resistance changes. The two layers generate signals in the X and Y directions, and then send them to the touch screen controller. The controller detects this contact and calculates the position of (X, Y) and then operates in the way of simulating the mouse. This is the basic principle of resistance technology touch screen. The key of resistive touch screen lies in material science and technology. The common transparent conductive coating materials are:
A, ITO, indium oxide, weak conductor, the characteristic is that when the thickness falls below 1800 A (E = 10-10 meters), it suddenly becomes transparent, the transmittance rate is 80%. When it is thinner, the transmittance decreases, and when it reaches 300 A, it rises to 80%. ITO is the main material used in all resistance technology touch screen and capacitance technology touch screen. In fact, the working surface of resistance and capacitance technology touch screen is ITO coating.
B. Nickel-gold coating, the outer conductive layer of five-wire resistance touch screen uses nickel-gold coating material with good ductility. Because of frequent touch, the outer conductive layer uses nickel-gold material with good ductility in order to prolong its service life, but the process cost is relatively high. Although nickel-gold conductive layer has good ductility, it can only be used as transparent conductor, and it is not suitable for working face of resistance touch screen because of its high conductivity, and the metal is not easy to achieve very uniform thickness, so it is not suitable for voltage distribution layer, and can only be used as a sounding layer.