The principle and classification of mobile phone touch screen

Introduction: in the digital age, the upgrading of mobile phones is also faster and faster. It is absolutely necessary to understand the knowledge of mobile phone touch screens before you buy Android and other smart touch-screen phones. Here are two principles of mobile phone touch screens, so that you can know well when you buy them and avoid being fooled.


Mobile phone touch screen classification


Generally speaking, mobile phone touch screens are divided into two types: resistive screen and capacitive screen. At present, most of the most common touch screens in the market are what we call "lens" screens, that is, flat resistive and mirror capacitive screens. Most Nokia mobile phones that are gradually withdrawing from the market use resistive screen as their mobile phone touch screen. The representative of capacitive screen is iPhone, so with the more superior characteristics of capacitive screen, Apple mobile phones are also favored by most people. The difference between them is that resistive touch screen is commonly known as "soft screen", which is mostly used in mobile phones with windows mobile system. Some mobile phones such as Sony and Samsung use resistive touch screen, while capacitive touch screen is called "hard screen" by most people. Apple mobile phone is one of the most widely used mobile phones with capacitive touch screen.


Basic principle of touch screen


The working part of a typical touch screen is generally composed of three parts, as shown in Figure 1: two transparent resistive conductor layers, an isolation layer between two conductors, and an electrode. The resistive conductor layer is made of resistive materials, such as indium tin oxide (ITO) coated on the substrate. The upper substrate is made of plastic and the lower substrate is made of glass. The isolation layer is a viscous insulating liquid material, such as polyester film. The electrode is made of materials with excellent conductivity (such as silver powder ink), and its conductivity is about 1000 times that of ITO.


When the touch screen works, the upper and lower conductor layers are equivalent to a resistance network, as shown in Figure 2. When a certain layer of electrode is applied with voltage, a voltage gradient will be formed on the network. If an external force makes the upper and lower layers contact at a certain point, the voltage at the contact point can be measured at the other layer without voltage applied to the electrode, so as to know the coordinates at the contact point. For example, if a voltage is applied to the electrodes (x+, x-) on the top layer, a voltage gradient will be formed on the conductor layer on the top layer. When an external force makes the upper and lower layers contact at a certain point, the voltage at the contact point can be measured on the bottom layer, and then the X coordinate at that point can be known according to the distance relationship between the voltage and the electrodes (x+). Then, switch the voltage to the bottom electrode (y+, y-), and measure the voltage at the contact point on the top layer, so as to know the Y coordinate.


Now in many PDA applications, the touch screen is used as an input device, and there are special chips for the control of the touch screen. Obviously, the control chip of the touch screen needs to accomplish two things: one is to complete the switching of electrode voltage; The second is to collect the voltage value (i.e. a/d) at the contact point. Taking ADS7843 chip produced by BB (burr Brown) Company as an example, this paper introduces the realization of touch screen control.


Working principle of mobile phone resistance screen


The resistance screen of mobile phone is basically a kind of sensor. It accurately locates the touch point through pressure sensing. When we touch the screen with our fingers or other similar objects, the two conductive layers inside the resistance touch screen immediately contact, and the resistance value produces a series of changes, which can be detected by the controller at the other end, Thus, the coordinates of the contact point can be judged according to certain measurement methods. The disadvantage of resistive touch screen is that it can only react to one touch point at a time when it works. If the number of touch points is too large, it will slow down. Therefore, according to the characteristics of the resistive screen, it is not difficult for us to think that the resistive touch screen is only suitable for clicking, dragging and other actions, and will have a certain impact on the line of sight under the sun.


Working principle of mobile phone capacitive screen

The capacitive screen of mobile phone applies the working principle of human thermal induction. Generally speaking, it can only be touched by hot objects such as fingers. From the physical structure, it has four layers of composite glass screen. A thin layer of ITO is coated on the inner surface and interlayer of the glass screen. The outermost layer is a silica glass protective layer that we can hardly see. The inner layer of ITO is the screen layer to ensure the stability of the working environment of the entire capacitor screen. When people do not touch the touch screen, the electrode potentials in the four screens are the same. However, once you click the capacitive touch screen with your finger, a coupling capacitance will be generated among the human electric field, finger tip and conductor layer, and the electrodes at the four corners will emit current one by one and flow to the contact uniformly. The significance of capacitive touch screen in multi touch level completely exceeds the limit of resistive screen. It can effectively judge complex actions.

SCRCTECH mobile phone screen factory is located in Dongguan, a famous hometown of overseas Chinese, "the famous manufacturing city in China". It was invested and established by SCRCTECH CO., LTD in 2016. It is a mobile phone screen factory that is dominated by mobile phone screens and matched with mobile phone accessories, integrating R&D, production and sales. The factory covers an area of 5,000 square meters. There are more than 100 employees in the factory. There are 15 production lines with advanced technology in the world, and advanced technical equipment has been introduced, such as laminating machines, separators, and defoaming machines. Mobile phone screens have undergone strict quality inspection procedures before leaving the factory, and the defective rate is strictly kept below 0.63%, which is far lower than the industry average.
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