JDT takes you to understand the principle of capacitive touch screen

Touch screen products were not very popular a few years ago. At that time, touch screens were only used in PDA, TablePC and other products. However, in recent years, as the application range of touch screens has gradually increased, whether mobile phones, cameras, or portable audio-visual players have been racing to launch products with touch screens. As people have more and more contact with touch screen products, touch screen products have been recognized by more people in the past two years, and the development speed has gradually accelerated.

　　The rapid growth of touch screens has not only aroused fiercer competition, but also indirectly promoted the development of technology. After the launch of Apple’s iPhone last year, its multi-touch operation has increased the influence of touch screen products to a new level, and the capacitive touch screen adopted by the iPhone has gradually attracted attention.

　The capacitive touch screen is very different from the traditional resistive touch screen. The resistive touch screen can only judge one touch point at a time when it is working. If there are more than two touch points, the correct judgment cannot be made. Therefore, the resistive touch screen is only suitable for simple clicks, drags, etc. Action judgment. The multi-touch of the capacitive touch screen can decompose the user's touch into two tasks: collecting multi-point signals and judging the meaning of the signals to complete the judgment of complex actions.

　　Using the stretching and transposition of two fingers, you can complete interesting operations such as zooming in and rotating on the screen, which was almost unimaginable before the appearance of capacitive touch screens. After Apple’s iPhone went on the market, it quickly created a wave of touch control. Soon after, Apple took advantage of the victory and launched the iPodtouch (in fact, it is equivalent to a simplified version of the iPhone) that also supports multi-touch. Sought after.

　　 Relatively speaking, domestic manufacturers have been slower to follow up with capacitive touch screen products. It was not until the recent launch of Teclast T50 that this gap was filled. However, due to the gap in manufacturing process and technology, the current domestic capacitive touch screen products are still slightly behind foreign manufacturers' products in terms of sensitivity and sense of operation.

  　Actually, the implementation principle of touch screens is roughly the same, which is to add a transparent touch panel to an ordinary LCD screen. The types of resistance and capacitance that we are talking about are divided according to their working principles. At present, the classification of touch screens mainly includes four types: resistive, capacitive, infrared, and surface acoustic wave. In real life, the resistive touch screen is the one we touch most. It has been widely used in mobile phones and portable digital products.

The working principle of resistive touch screen

  　Before analyzing the principle of capacitive touch screen, let's first understand the working principle of resistive touch screen.

　The resistive touch screen is mainly controlled by pressure sensing. Its composition is a display screen and a resistive thin film screen that closely fits the display screen. This resistive film screen is usually divided into two layers. One layer is a base layer made of glass or plexiglass, and its surface is coated with a transparent conductive layer; the base layer is pressed with a hardened and scratch-resistant plastic layer that we usually contact directly. There is also a conductive layer inside the plastic layer, and the two conductive layers are separated. When we touch the screen with a finger or other objects, the two conductive layers come into contact, and the resistance changes. The controller judges the coordinates of the contact point and performs corresponding operations based on the specific changes in resistance.

The working principle of capacitive touch screen

  　Different from resistive touch screens, capacitive touch screens use the current induction of the human body to work.

The sensing screen of the capacitive touch screen is a four-layer composite glass screen. The inner surface and interlayer of the glass screen are each coated with a conductive layer, and the outermost layer is a thin protective layer of silica glass. When we touch the sensor screen with a finger, the electric field of the human body makes the finger and the surface of the touch screen 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 these four electrodes is proportional to the distance from the finger to the four corners. The controller obtains the position of the touch point by accurately calculating the ratio of these four currents. .

Compared with traditional resistive touch screens, capacitive touch screens have the following advantages:

1. Operation is novel. The capacitive touch screen supports multi-touch, the operation is more intuitive and interesting.

  　2. Not easy to touch by mistake. Since the capacitive touch screen needs to sense the current of the human body, only the human body can operate it, and it will not respond when touched with other objects, so the possibility of accidental touch is basically avoided.

  　3. High durability. Compared with resistive touch screens, capacitive touch screens have better performance in terms of dust resistance, water resistance, and wear resistance.

　As the currently popular touch screen technology, although capacitive touch screen has the advantages of gorgeous interface, multi-touch, and only human body induction, it also has the following shortcomings:

　1. The accuracy is not high. Due to technical reasons, the accuracy of capacitive touch screens is still lacking compared to resistive touch screens. Moreover, only fingers can be used for input, and it is difficult to recognize more complicated handwriting input on a small screen.

　2. Easily affected by the environment. When environmental factors such as temperature and humidity change, it will also cause instability or even drift of the capacitive touch screen. For example, if the user moves his body close to the screen while using it, it may cause drift. Even operating in a crowded crowd may cause drift. This is mainly due to the working principle of capacitive touch screen technology. Although the user's finger is closer to the screen, there are many electric fields that are much larger than the finger near the screen, which will affect the judgment of the touch position.

　3. The cost is high. In addition, the current capacitive touch screen still has certain technical difficulties in the step of attaching the touch panel to the LCD panel, and the yield rate is not high, so the cost of the capacitive touch screen is also virtually increased.

Prospect 　In the past, due to factors such as cost and technology, the amount of resistive control panels used far exceeded capacitive touch technology. However, in the recent stage, with the advancement of technology and mass production, the price of capacitive touch screens is continuously falling, and the price gap with resistive touch screens is getting smaller and smaller, and the price gradually has the ability to compete with resistive touch screens. .

With the prosperous Apple iPhone, iPodTouch and other products, not only major manufacturers have begun the research and development of capacitive touch screen products, but also many touch screen manufacturers such as 3M and Synaptics have followed up in large numbers and invested in the research and development of capacitive touch screens. And in production. I believe that with the continuous escalation of competition, capacitive touch screens will have a great development in the short term; and the application of capacitive touch screens will not only be the existing mobile phones, portable video players, etc., but also cameras, GPS navigator, game console and other products are expanded.

　Finally, what we want to say is: Although capacitive touch screens have their own unique selling points, from the perspective of comprehensiveness and stability of functions, it is currently not enough to replace the deep-rooted position of resistive touch screens.