What is the principle of LCD touch screen?

Type of touch screen

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1, resistive touch screen

This touch screen is controlled by pressure sensing. The main part of the resistive touch screen is a resistive film screen that is very compatible with the surface of the display. It is a multi-layer composite film with a layer of glass or hard plastic as the base layer and a transparent oxide metal on the surface (transparent conductive). Resistor) Conductive layer, covered with a layer of outer surface hardened, smooth anti-scratch plastic layer, coated with a coating on its inner surface, with many fine (less than 1/1000 inch) transparency between them The isolation point separates the two conductive layers from each other. When the finger touches the screen, the two conductive layers have contact at the touch point position, the resistance changes, and 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), which then operates according to the way the mouse is simulated. This is the most basic principle of the resistive touch screen. The key to resistive touch screens lies in material technology. Commonly used transparent conductive coating materials are:

A, ITO, indium oxide, weak conductor, the characteristic is that when the thickness drops below 1800 angstroms (A = 10-10 meters), it will suddenly become transparent, the light transmittance is 80%, and then the light transmittance is reduced. It drops to 80% when it reaches 300 angstroms. ITO is the main material used in all resistance technology touch screens and capacitive technology touch screens. In fact, the working surface of the resistive and capacitive technology touch screen is ITO coating.

B. Nickel-gold coating, the outer conductive layer of the five-wire resistive touch screen uses a nickel-gold coating material with good ductility. The outer conductive layer is frequently touched, and the nickel-gold material with good ductility is used for the purpose of prolonging the service life. , but the process cost is relatively high. Although the nickel-gold conductive layer has good ductility, it can only be used as a transparent conductor. It is not suitable as a working surface of a resistive touch screen because it has high conductivity and the metal is not easy to be very uniform in thickness. It is not suitable for voltage distribution layer. Floor.

1.1 four-wire resistive screen

The four-layer transparent metal layer of the four-wire resistance analog technology works with a constant voltage of 5V per layer: 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 smooth and matte finish. One calibration, high stability, never drift.

1.2 five-wire resistive screen

The base layer of the five-wire resistive touch screen puts the voltage fields in both directions on the conductive working surface of the glass through the precision resistor network. We can simply understand that the voltage field time division work in both directions is added to the same working surface. The outer layer of nickel-gold conductive layer is only used as a pure conductor, and the position of the touch point is measured by a method of detecting the X-axis and Y-axis voltage values of the inner ITO contact point after touch. The inner layer of ITO of the five-wire resistive touch screen requires four leads, the outer layer is only used as one conductor, and the touch screen has five lead wires. Features: high resolution, high speed transmission response. High surface hardness reduces scratches, scratches and chemical resistance. 30 million contact points at the same point are still available. The conductive glass is the medium of the substrate. One calibration, high stability, never drift. Five-wire resistive touch screen has high price and high environmental requirements

1.3 Limitations of the resistance screen

Whether it is a four-wire resistive touch screen or a five-wire resistive touch screen, they are a completely isolated working environment for the outside world. They are not afraid of dust and moisture. They can be touched by any object. They can be used for writing and drawing, which is more suitable for industrial control. And the use of limited people in the office. The common disadvantage of resistive touch screens is that because the outer layer of the composite film is made of plastic material, an unknown person who is too hard or uses a sharp touch may scratch the entire touch screen and cause scrapping. However, within the limits, scratching only hurts the outer conductive layer. The scratch of the outer conductive layer is irrelevant for a five-wire resistive touch screen and fatal for a four-wire resistive touch screen.

2, capacitive touch screen

2.1 Capacitor technology touch screen

It uses the current sensing 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 coated with a layer of ITO. The outermost layer is a thin layer of bauxite glass protective layer. The interlayer ITO coating is used as the working surface at the four corners. Four electrodes are taken out, and the inner layer of ITO is a shielding layer to ensure a good working environment. When the finger touches the metal layer, the user and the surface of the touch screen form a coupling capacitor due to the electric field of the human body. For high-frequency current, the capacitor is a direct conductor, and the finger sucks a small current from the contact point. This current is discharged from the electrodes on the four corners of the touch screen, and the current flowing through the 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 the four currents. .

2.2 Capacitive touch screen defects

The transmittance and clarity of the capacitive touch screen is superior to that of the four-wire resistive screen, and certainly cannot be compared with the surface acoustic wave screen and the five-wire resistive screen. The capacitive screen is seriously reflective. Moreover, the four-layer composite touch screen of the capacitor technology has uneven transmittance for light of various wavelengths, and there is a problem of color distortion. Due to the reflection of light between layers, image characters are also blurred. In principle, the capacitive screen uses the human body as an electrode of a capacitor element. When a conductor is close to a capacitor with a sufficient capacitance between the working surface of the ITO and the 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 proportional to the relative area and also to the insulation coefficient of the medium. Therefore, when a large area of the palm or hand-held conductor is close to the capacitive screen instead of touching, it can cause the malfunction of the capacitive screen. In humid weather, this situation is particularly serious. The hand holds the display and the palm is close to the display 7 cm. Within 15 cm of the display or the body close to the display can cause malfunction of the capacitive screen. Another disadvantage of capacitive screens is that they do not respond when touched with a gloved hand or a non-conductive object, because of the addition of a more insulating medium. The main disadvantage of the capacitive screen is drift: when the ambient temperature and humidity change, the environmental electric field changes, causing the drift of the capacitive screen, resulting in inaccuracy. For example, the temperature rise of the display after power-on will cause drift: when the user touches the screen, the other hand or the side of the body will drift near the display; the larger object near the capacitive touch screen will move back and drift, and if someone touches it, It will cause drift; the drift of the capacitive screen belongs to the technical congenital 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 finger area, and they directly affect the measurement of the touch position. . In addition, in theory, many linear relationships should be nonlinear. For example, the total amount of current drawn by people with different body weights or different finger wetness is different, and the change of total current and the change of four partial currents. It is a non-linear relationship. The custom polar coordinate system of the four corners used in the capacitive touch screen does not have an origin on the coordinates. After the drift, the controller cannot detect and recover, and after the completion of the four A/Ds, four points are obtained. The calculation process of the value of the flow rate to the X, Y coordinate value of the touch point in the Cartesian coordinate system is complicated. Since there is no origin, the drift of the capacitive screen is cumulative and calibration is often required at the job site. The outermost bauxite protection glass of the capacitive touch screen is very scratch-resistant, but it is afraid of the nail or hard object knocking. If a small hole is knocked out, the interlayer ITO will be damaged, whether it is the injury of the sandwich ITO or the installation and transportation. The inner surface of the ITO layer, the capacitive screen will not work properly.

3, infrared touch screen

The infrared touch screen uses an infrared matrix densely arranged in the X and Y directions to detect and locate the user's touch. The infrared touch screen is provided with a circuit board outer frame on the front side of the display, and the circuit board arranges an infrared transmitting tube and an infrared receiving tube on four sides of the screen, and one-to-one correspondingly forms an infrared matrix which is horizontally and vertically crossed. When the user touches the screen, the finger blocks the two infrared rays passing through the position, so that the position of the touch point on the screen can be judged. Any touch object can change the infrared light on the contact to achieve touch screen operation. In the early concept, infrared touch screens had technical limitations such as low resolution, limited touch mode, and susceptibility to environmental interference, 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 generations have also improved the resolution and stability, but they have not made a qualitative leap in key indicators or comprehensive performance. However, those who know the touch screen technology know that the infrared touch screen is not affected by current, voltage and static electricity, and is suitable for harsh environmental conditions. Infrared technology is the final development trend of touch screen products. Touch screens using acoustics and other materials science techniques have insurmountable barriers, such as damage and aging of a single sensor, and the touch interface is subject to contamination, destructive use, and maintenance. As long as the infrared touch screen truly achieves high stability and high resolution, it will become the mainstream of the touch screen market instead of other technology products. The resolution of the past infrared touch screen is determined by the number of infrared pairs in the frame, so the resolution is low. The main domestic products on the market are 32x32 and 40X32. In addition, the infrared screen is sensitive to the lighting environment, and the illumination changes. When you are big, you will misjudge or even crash. These are the weaknesses of the infrared screens sold by domestic agents of non-infrared touch screens abroad. The resolution of the latest technology fifth-generation infrared screen depends on the number of infrared pairs, the scanning frequency and the difference algorithm. The resolution has reached 1000X720. As for the infrared screen, the infrared screen is unstable under the illumination conditions, from the second generation infrared touch screen. At the beginning, it has already better overcome the weakness of anti-light interference. The fifth-generation infrared touch screen is a new generation of intelligent technology products, which realizes 1000*720 high-resolution, multi-level self-adjusting and self-recovering hardware adaptability and highly intelligent discriminative identification, which can be used in various harsh environments for a long time. Any use. And can customize extension functions for users, such as network control, sound induction, human proximity sensing, user software encryption protection, infrared data transmission and so on. Another major drawback of the original infrared touch screen promoted by the media is the poor resistance to violent. In fact, the infrared screen can completely select any riot glass that the customer considers satisfactory without increasing the cost and affecting the performance. This is something that other touch screens cannot follow.

4, surface acoustic wave touch screen

4.1 Surface acoustic waves

Surface acoustic wave, a type of ultrasonic wave, a mechanical energy wave propagating in a shallow layer on the surface of a medium such as a rigid material such as glass or metal. Through the wedge-shaped triangular base (strictly designed according to the wavelength of the surface wave), the surface acoustic wave energy emission of the orientation and small angle can be achieved. The surface acoustic wave performance is stable, easy to analyze, and has very sharp frequency characteristics in the process of shear wave transmission. In recent years, the application in the direction of non-destructive testing, angiography and de-waveper has developed rapidly. Theoretical research on surface acoustic wave, semiconductor materials, sound The materials such as guiding materials and detection technology are quite mature. The touch screen portion of the surface acoustic wave touch screen can be a flat, spherical or cylindrical glass plate mounted in front of a CRT, LED, LCD or plasma display screen. Vertical and horizontal ultrasonic transducers are fixed in the upper left and lower right corners of the glass screen, and two corresponding ultrasonic receiving transducers are fixed in the upper right corner. The four perimeters of the glass screen are engraved with a 45° angle from the sparse to densely spaced reflective stripes.

4.2 Surface acoustic wave touch screen works

Take the X-axis transmit transducer in the lower right corner as an example: the transmit transducer converts the electrical signal sent by the controller through the touch screen cable into acoustic energy that is transmitted to the left surface, and then a set of precision reflective stripes on the underside of the glass plate. The sound energy is reflected into an upward uniform surface, and the sound energy passes through the surface of the screen, and then the upper reflective strips are gathered into a rightward line to the X-axis receiving transducer, and the receiving surface acoustic wave that the transducer will return. The energy becomes an electrical signal. When the transmitting transducer emits a narrow pulse, the acoustic energy reaches the receiving transducer through different routes, the earliest arrival on the far right, the latest arrival on the far left, and the arrival of these early arrival and late arrivals. For a wider waveform signal, it is easy to see that the received signal is a collection of all the acoustic energy returned by different paths in the X-axis direction. The distance traveled on the Y-axis is the same, but on the X-axis, the farthest ratio The most recent has gone twice the maximum distance of the X-axis. Therefore, the time axis of this waveform signal reflects the position before the original waveform is superimposed, that is, the X-axis coordinate. When the transmitted signal and the received signal waveform are not touched, the waveform of the received signal is exactly the same as the reference waveform. When a finger or other object capable of absorbing or blocking the energy of the sound wave touches the screen, the sound energy of the X-axis traveling upward through the finger portion is partially absorbed, and the reaction has a decaying gap on the receiving waveform at a certain moment. The receiving waveform corresponding to the finger blocking part signal attenuates a gap, and the position of the notch is calculated, that is, the touch coordinate controller analyzes the attenuation of the received signal and determines the X coordinate from the position of the notch. The same process of the Y axis then determines the Y coordinate of the touch point. In addition to the X, Y coordinates that the general touch screen can respond to, the surface acoustic wave touch screen also responds to the Z-axis coordinate of the third axis, that is, the value of the user's touch pressure can be perceived. The principle is calculated from the amount of attenuation at the attenuation of the received signal. Once the three axes are determined, the controller passes them to the host.

4.3 surface acoustic wave touch screen features

High definition and good light transmission. Highly durable, good scratch resistance (surface film with respect to resistance, capacitance, etc.). Responsive. Not affected by environmental factors such as temperature and humidity, high resolution, long life (50 million times in good maintenance); high light transmittance (92%), able to maintain clear and translucent image quality; no drift, just install One correction; there is a third axis (ie pressure axis) response and is currently used more in public places. The surface acoustic wave screen needs frequent maintenance, because dust, oil stains and even liquid of the beverage are stained on the surface of the screen, which will block the waveguide groove on the surface of the touch screen, so that the wave cannot be normally emitted, or the waveform is changed and the controller cannot recognize it properly, thus affecting For the normal use of the touch screen, the user must pay strict attention to environmental sanitation. The surface of the screen must be wiped frequently to keep the screen clean and to be completely wiped out at regular intervals. ”