How Much Do We Know About Touch Screens?

Posted by sere on February 26th, 2021

The application of touch screens is becoming more and more widespread. How much do we know about touch screens?

Differentiating touch screens from technical principles can be divided into five basic categories: vector pressure sensing technology touch screens, resistive technology touch screens, capacitive technology touch screens, infrared technology touch screens, and surface acoustic wave technology touch screens.

Among them, the vector pressure sensing technology touch screen has withdrawn from history; the infrared technology touch screen is cheap, but its frame is fragile, prone to light interference, and distortion in the case of curved surfaces; the capacitive technology touch screen has a reasonable design concept, but its image distortion problem is difficult to obtain Fundamental solution; resistive technology touch screen is accurate in positioning, but its price is quite high, and it is afraid of being scratched easily; surface acoustic wave touch screen solves various defects of the previous touch screen, clear and not easily damaged, suitable for various occasions, the disadvantage is the screen surface If there are water droplets and dust, the touch screen will become dull and will not even work.

According to the working principle of the touch screen and the medium for transmitting information, the touch screen is divided into four types, which are resistive, infrared, capacitive induction, and surface acoustic wave. There are four types, they are resistance type, capacitance induction type, infrared type and surface acoustic wave type. Each type of touch screen has its own advantages and disadvantages. The key to understanding which type of touch screen is suitable for that occasion is to understand the working principle and characteristics of each type of touch screen technology.

The following briefly introduces the above-mentioned various types of touch screens:

1. Resistive touch screen

This touch screen uses pressure sensing for control. The main part of the resistive touch screen is a resistive film screen that fits very well with the surface of the display. This is a multi-layer composite film. It uses a layer of glass or hard plastic as the base layer and is coated with a layer of transparent metal oxide Resistance) conductive layer, covered with a layer of hardened outer surface, smooth and scratch-resistant plastic layer, its inner surface is also coated with a layer of coating, there are many small (less than 1/1000 inch) transparent between them The isolation point separates the two conductive layers. When a finger touches the screen, the two conductive layers are in contact at the touch point, the resistance changes, signals are generated in both the X and Y directions, and then sent to the touch screen controller. The controller detects this contact and calculates the position of (X, Y), and then operates according to the method of simulating a mouse. This is the most basic principle of resistive technology touch screen.

Its characteristics are:

① They are all a completely isolated working environment, not afraid of dust, water vapor and oil stains.

②Can be touched with any object, can be used for writing and drawing, which is their big advantage.

③The accuracy of the resistive touch screen depends only on the accuracy of the A / D conversion, so it can easily reach 4096 * 4096 • In comparison, the five-wire resistor is superior to the four-wire resistor in ensuring the resolution accuracy, but the cost is high. Therefore, the selling price is very high.

1.1 Four-wire resistive screen

The two-layer transparent metal layer of the four-wire resistance analog technology adds 5V constant voltage to each layer when working: one vertical direction and one horizontal direction. A total of four cables are required. Features: High resolution, high speed transmission response. Surface hardness treatment to reduce scratches, scratches and chemical treatment. With glossy and matte surface treatment. Once corrected, it has high stability and never drifts.

1.2 Five-wire resistive screen

The base layer of the five-wire resistance technology touch screen adds the voltage field in both directions to the conductive working surface of the glass through a precision resistance network. We can simply understand that the voltage field in both directions works on the same working surface in a time-sharing manner. The outer nickel-gold conductive layer is only used as a pure conductor, and the position of the touch point is measured by the method of detecting the X-axis and Y-axis voltage values of the inner layer ITO contact point after time-sharing. The inner layer ITO of the five-wire resistive touch screen requires four leads, and the outer layer is only a conductor, and the touch screen has a total of five lead wires. Features: High resolution and high-speed transmission response. High surface hardness, reduce scratches, scratches and chemical treatment. It can still be used with 30 million contacts at the same point. Conductive glass is the medium of the substrate. Once corrected, it has high stability and never drifts. The five-wire resistive touch screen has the disadvantages of high price and high environmental requirements.

1.3 Limitations of the resistive screen

Whether it is a four-wire resistive touch screen or a five-wire resistive touch screen, they are a completely isolated working environment from the outside world. They are not afraid of dust and moisture. It can be touched with any object and can be used for writing and drawing. It is more suitable for use in the field of industrial control and limited people in the office. The common disadvantage of the resistive touch screen is that the outer layer of the composite film is made of plastic material. If the person who does not know uses too much force or touches with a sharp object, it may scratch the entire touch screen and cause scrapping. However, within the limits, the scratches will only hurt the outer conductive layer. The scratches on the outer conductive layer are irrelevant to the five-wire resistive touch screen, but fatal to the four-wire resistive touch screen.

2. Capacitive touch screen

2.1 Capacitive technology touch screen works by using the current induction of the human body.

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 outermost layer is a thin layer of silica glass protective layer. The interlayer ITO coating is used as the working surface. Lead out four electrodes, the inner layer of ITO as a shielding layer to ensure a good working environment. When the finger touches the metal layer, a coupling capacitor is formed between the user and the touch screen surface due to the human electric field. For high-frequency current, the capacitor is a direct conductor, so the finger draws a small current from the contact point. This current flows out 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 proportion of these four currents accurately to get the position of the touch point .

2.2 Defects of capacitive touch screens

The transmittance and clarity of capacitive touch screens are better than those of four-wire resistive screens. Of course, they cannot be compared with surface acoustic wave screens and five-wire resistive screens. Capacitive screens have serious reflections, and the four-layer composite touch screen of capacitive technology has uneven transmittance of light of various wavelengths, and there is a problem of color distortion. Due to the reflection of light between the layers, image characters are also blurred. The capacitive screen uses the human body as an electrode of a capacitor element in principle. When a conductor with a sufficient capacitance is coupled between the conductor and the interlayer ITO working surface, the current flowing away is enough to cause the capacitive screen to malfunction. . We know that although the capacitance value is inversely proportional to the distance between the poles, it is directly proportional to the relative area, and also related to the insulation coefficient of the medium. Therefore, when the palm of a large area or the conductor held by it is close to the capacitive screen instead of touching it, it can cause malfunction of the capacitive screen. This situation is especially serious in wet weather. The hand holds the display and the palm is close to the display 7 cm. Within the body or within 15 cm of the display close to the display can cause malfunction of the capacitive screen. Another disadvantage of the capacitive screen is that it does not respond when touched with gloved hands or non-conductive objects. This is due to the addition of a more insulating medium. The main disadvantage of the capacitive screen is drift: when the environmental temperature and humidity change, and the environmental electric field changes, it will cause the capacitive screen to drift, causing inaccuracy. For example: the temperature rise of the display after powering on will cause drift: when the user touches the screen while the other hand or side of the body is close to the display, it will drift; when a large object near the capacitive touch screen is moved back to drift, if someone touches around to watch It will cause drift; the cause of the drift of the capacitive screen is a technical innate deficiency. Although the environmental potential surface (including the user's body) is far away from the capacitive touch screen, it is much larger than the area of the finger. They directly affect the measurement of the touch position. . In addition, in theory, many relationships that should be linear are actually non-linear, such as: the total current drawn by people with different weights or different finger wetness is different, and the change in total current and the change in four sub-currents It is a non-linear relationship. The custom polar coordinate system of the four corners used by the capacitive touch screen has no origin on the coordinates. The controller cannot detect and recover after the drift. Moreover, after the four A / Ds are completed, there are four points The calculation process of the X and Y coordinate values of the flow value to the touch point on the rectangular coordinate system is complicated. Since there is no origin, the drift of the capacitive screen is cumulative, and calibration is often required at the work site. The outermost silica protective glass of the capacitive touch screen is very scratch-resistant, but it is afraid that a small hole will be hit by nails or hard objects, and it will hurt the interlayer ITO, whether it is the interlayer ITO or the installation and transportation process. ITO layer on the inner surface,

The capacitive screen will not work properly.

3. Infrared touch screen.

Infrared touch screen is easy to install, only need to add a frame frame on the display, no need to add a coating or connect the controller to the screen surface. Infrared emitting tubes and receiving tubes are arranged on the four sides of the frame from the light spot to form an infrared network on the screen surface. When the user touches a certain point on the screen with his finger, the two infrared rays passing through the position are blocked, and the computer can immediately calculate the position of the touched point. Any touching object can change the infrared on the contact point to realize the touch screen operation. In the early conception, infrared touch screens have technical limitations such as low resolution, limited touch methods, and susceptibility to environmental interference and misoperation, which once faded out of the market. Since then, the second-generation infrared screen has partially solved the problem of anti-light interference. The third- and fourth-generation infrared screens have also improved in terms of improving resolution and stability, but neither has a qualitative leap in key indicators or overall performance. However, everyone who knows touch screen technology knows that infrared touch screens are not disturbed by current, voltage and static electricity, and are suitable for bad environmental conditions. Infrared technology is the ultimate development trend of touch screen products. Touch screens that use acoustics and other materials technology have their insurmountable barriers, such as damage and aging of a single sensor, fear of contamination of the touch interface, destructive use, and complicated maintenance. As long as the infrared touch screen truly achieves high stability and high resolution, it will certainly replace other technical products and become the mainstream of the touch screen market. In the past, the resolution of the infrared touch screen was determined by the number of infrared pairs in the frame, so the resolution was lower. The main domestic products on the market were 32x32 and 40X32.

In addition, the infrared screen was more sensitive to the lighting environment factors, and the change in lighting will misjudge or even crash. These are the weaknesses of the infrared screens sold by domestic agents of foreign non-infrared touch screens. The latest technology The resolution of the fifth generation infrared screen depends on the number of infrared tubes, scanning frequency and difference algorithm. The resolution has reached 1000X720. As for the infrared screen is unstable under lighting conditions, from the second generation infrared touch screen From the beginning, the weakness of anti-light interference has been overcome. The fifth-generation infrared touch screen is a new generation of intelligent technology products. It realizes 1000*720 high resolution, multi-level self-adjustment and self-recovery hardware adaptability and highly intelligent discrimination and recognition, which can be used in various harsh environments for a long time Any use below. And can be customized for users with extended functions, such as network control, sound sensing, human body proximity sensing, user software encryption protection, infrared data transmission and so on. Another major shortcoming of the original infrared touch screen promoted by the media is its poor anti-riot performance. In fact, the infrared screen can choose any anti-riot glass that the customer thinks is satisfactory without adding too much cost and affecting the performance. This is unmatched by other touch screens. Infrared touch screens are cheap, easy to install, and can sense light touch and quick touch better. However, since the infrared touch screen relies on infrared sensing action, changes in external light, such as sunlight and indoor spotlights, will affect its accuracy. In addition, the infrared touch screen is not waterproof and is afraid of dirt. Any small foreign objects will cause errors and affect its performance. It is not suitable for outdoor and public use.

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