What is the principle of LCD touch screen?

Types of touch screens

1, Resistive touch-screen

This touchscreen is controlled using pressure sensing. The main part of the resistive touch screen is a resistive film screen with the display surface, which is a multilayer composite film, it is a layer of glass or hard plastic plate as the base, coated with a layer of transparent oxide metal (transparent conductive resistance) conductive layer, covered with a layer of 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 inches) of the transparent isolation point between the two layers of conductive layer separated insulation. When the finger touches the screen, the two layers of conductive layer have contact at the touch point position, the resistance changes, the signal is generated in the X and y two directions, and then the touch screen controller is sent. The controller detects this contact and calculates the position (x, y), and then operates according to the simulated mouse mode. This is the basic principle of resistive technology touch screen. The key to resistive touch screen is material technology, commonly used transparent conductive coating materials are:

A, ITO, indium oxide, weak conductive body, the characteristic is when the thickness drops to 1800 (Egypt = 10-10 meters) will suddenly become transparent, light transmittance of 80%, and then thin down the transmittance decreases, to 300 of the thickness of the rise to 80%. Ito is the main material used in all resistive technology touch screen and capacitive technology touch screens, in fact the face of the resistive and capacitive technology touchscreen is ITO coating.

B, nickel gold coating, five-wire resistive touch screen of the outer conductive layer is a good ductility of nickel-gold coating material, external conductive layer due to frequent touch, the use of ductile nickel gold material is designed to prolong the service life, but the process cost is high. Although the nickel conductive layer is good ductility, but only transparent conductor, not suitable as a resistive touch screen face, because of its high conductivity, and the metal is not easy to achieve a very uniform thickness, not suitable for the voltage distribution layer, can only be used as the exploration layer.

1.1 Four-wire resistive screen

The two-layer transparent metal layer of the four-wire resistor analogue technique increases the 5V constant voltage per layer: one vertical direction, one horizontal direction. A total of four cables are required. Features: High-resolution, high-speed transmission of the reaction. Surface hardness treatment, reduce scratches, scratches and chemical treatment. With smooth and matte finish. One correction, high stability, never drift.

1.2 Five-wire resistive screen

Five wire resistance technology touch screen of the base of the two-direction voltage field through the precision resistor network is added to the glass conductive face, we can simply understand that the voltage field in two directions work on the same working face, and the outer nickel-gold conductive layer is only used as a pure conductor, The position of the touch point is measured by means of a touch-time tick detection of the x-axis and y-axis voltage values of the inner Ito contact point. Five-wire resistive touch screen inner Ito need four lead, the outer layer only as a conductor only one, touch screen has a total of 5 lines. Features: High resolution, high speed transmission reaction. High surface hardness, reduce scratches, scratches and chemical treatment. The same point of contact 30 million times is acceptable to use. The conductive glass is the substrate medium. One correction, high stability, never drift. Five-wire resistive touchscreen with high price and high environmental requirements

3 Limitations of the resistive screen

Whether it is a four-wire resistive touch screen or five-wire resistive touch screen, they are a completely isolated work environment, not afraid of dust and water vapor, it can be touched by any object, can be used to write and draw, more suitable for industrial control field and the use of limited people in the office. Resistive touch screen common disadvantage is because the composite film of the outer layer of plastic materials, do not know people too hard or use sharp touch may scratch the entire touch screen and lead to scrap. However, within the limits, scratches will only hurt the outer conductive layer, the outer conductive layer scratches for the five-wire resistive touch screen is not related to the four-wire resistive touch screen is fatal.

2. Capacitive Touch-screen

2.1 Capacitive Technology Touch screen

is to use the human body's current sensing to work. Capacitive touch screen is a four-layer composite glass screen, the inner surface of the glass screen and the interlayer are coated with a layer of ITO, the outermost layer is a thin layers of silica glass protective layer, the sandwich Ito coating as a working face, four angle leads to four electrodes, the inner layer Ito as a shielding layer to ensure a good working environment. When the finger touches the metal layer, because the human field, the user and the touch screen surface to form a coupling capacitor, for high-frequency current, the capacitance is a direct conductor, so the finger from the contact point suction away a very small current. The current is drawn from the electrodes in the four corners of the touchscreen, and the current flowing through the four electrodes is proportional to the distance between the fingers and the four corners, and the controller obtains the position of the touch point by accurately calculating the four current ratios.

2.2 Defects of Capacitive touch screen

Capacitive touch screen transmittance and clarity is better than four-wire resistive screen, of course, and can not be compared with the surface acoustic wave screen and five-wire resistive screen. Capacitive screen reflective serious, moreover, the capacitive technology of the four-layer composite touch screen for each wavelength of light transmittance is uneven, there is the problem of color distortion, due to light in the reflection between the layers, but also cause the image character blur. Capacitive screen on the principle of the human body as a capacitor element of the use of an electrode, when there is a conductor close to the sandwich Ito face coupled with a sufficient amount of capacitance, the flow of current is sufficient to cause the capacitive screen error action. We know that although the capacitance value is inversely proportional to the distance between the poles, it is in direct proportion to the relative area and is also related to the dielectric coefficient. Therefore, when the large area of the palm or hand-held conductors near the capacitive screen and not touch can cause the wrong action of the capacitive screen, in humid weather, this situation is particularly serious, hand-held display, palm close to the monitor 7 cm or the body close to the monitor within 15 centimeters can cause the wrong action of the capacitive screen. Another disadvantage of the capacitive screen is that there is no reaction when touching with a gloved hand or holding a non-conductive object because of the addition of more insulating media. Capacitive screen is the main drawback is the drift: when the ambient temperature, humidity changes, the environmental electric field changes, will cause the drift of the capacitive screen, resulting in inaccurate. For example: The display temperature rise after the boot will cause drift: the user touch the screen while the other hand or side of the body near the monitor will drift; capacitive touch screen near the large objects moved back drift, you touch if someone around to watch will also cause drift; the drift of capacitive screen belongs to the technical congenital deficiency, environmental potential surface ( Including the user's body) although far from the capacitive touch screen, but larger than the size of the finger, they directly affect the touch position measurement. In addition, theoretically many of the relationships that should be linear are actually non-linear, such as: different weight or the degree of wet fingers of different people suck the total amount of electricity flow is different, and the total electrical flow and the change of four separate flow is nonlinear relationship, capacitive touch screen using this four-angle custom polar coordinate system has no coordinates on the origin , the controller can not detect and recover after drift, moreover, after 4 A/d is completed, it is complicated to calculate the x and Y coordinate values of the value of four points to the touch point in the right angle coordinate system. Since there is no origin, the drift of the capacitive screen is cumulative and calibration is often required on the job site. Capacitive touch screen The most outside of the silica protection glass scratch resistance is very good, but afraid of nails or hard objects, tapping a small hole will hurt the sandwich Ito, whether it is injured and laminated ito or the installation of the transport process of slander and the inner surface ITO layer, capacitive screen will not work properly.

3. Infrared Type touch screen

The infrared touchscreen is used to detect and locate the user's touch using an infrared matrix that is dense in the X and y directions. Infrared touch screen in front of the display to install a circuit board outside the frame, the circuit board in the four sides of the screen layout of the infrared emitter tube and infrared receiver tube, each corresponding to the formation of a cross-border infrared matrix. When the user touches the screen, the finger blocks the two infrared rays that pass through the position, thus judging the position of the touch point on the screen. Any touch object can change the infrared on the contact to achieve touch screen operation. Early on, the infrared touch screen has low resolution, limited touch mode and interference by the environment and other technical limitations, and thus once faded out of the market. Since then the second generation of infrared screen to solve the problem of anti-optical interference, the third and fourth generation in the improvement of resolution and stability can also be improved, but none of the key indicators or comprehensive performance has a qualitative leap. However, understand the touch screen technology people know that the infrared touch screen is not affected by current, voltage and electrostatic interference, suitable for harsh environmental conditions, infrared technology is the final development trend of touch screen products. The use of acoustic and other materials technology touch screen has its insurmountable barrier, such as the damage of a single sensor, aging, touch interface fear of pollution, destructive use, maintenance complex and so on. Infrared touch screen as long as the real realization of high stability and high resolution, will replace other technology products and become the mainstream touch screen market. The resolution of the infrared touch screen in the past is determined by the number of infrared tubes in the frame, so the resolution is lower, the main domestic products in the market are 32x32, 40X32, and also said that the infrared screen is sensitive to light environment factors, will be misjudged or even panic when the light changes greatly. These are foreign non-infrared touch screen domestic agents sales propaganda of the weakness of the infrared screen. And the latest technology of the fifth-generation infrared screen resolution depends on the number of infrared, scanning frequency and difference algorithm, the resolution has reached 1000x720, as to say that the infrared screen in the light condition is not stable, from the second generation of infrared touch screen start, it has been better to overcome the vulnerability of light interference. The fifth generation of infrared touch screen is a new generation of intelligent technology products, it realizes the 1000*720 high resolution, multi-level self-tuning and self-recovery of hardware adaptability and highly intelligent identification, can be used for a long time in various harsh environments. And can be customized for user expansion functions, such as network control, sound induction, human proximity sensor, user software encryption protection, infrared data transmission and so on. The original media promotion of the infrared touch screen Another major drawback is uprising poor, in fact, the infrared screen can be used by any customer is satisfied with the anti-riot glass without adding too much cost and impact on the use of performance, which other touch screen can not be emulated.

4. Surface Acoustic Wave touch screen

4.1 Surface Acoustic Wave

A mechanical energy wave that propagates in a shallow layer on a surface, such as a rigid material such as glass or metal. Through the wedge-shaped triangular base (according to the wavelength of the surface wave strictly designed), can be directed, small angle of the surface acoustic wave energy emission. The surface acoustic wave performance is stable, easy to analyze, and in the shear wave transmission process has very sharp frequency characteristics, in recent years in nondestructive testing, imaging and the direction of the application of the development of a rapid, surface acoustic wave-related theoretical research, semiconductor materials, acoustic guidance materials, detection technology and other technologies have been quite mature. The touchscreen part of the surface acoustic wave touchscreen can be a flat, spherical or cylindrical glass plate mounted in front of a CRT, LED, LCD or Plasma display screen. The upper left and right corner of the glass screen are fixed in the vertical and horizontal direction of the ultrasonic transmitter transducer, the upper right corner is fixed two corresponding ultrasonic receiver transducer. The four periphery of the glass screen is engraved with a very precise reflective stripe at a 45° angle from the sparse to dense interval.

4.2 How the surface acoustic wave touch screen works

Take the X-axis emission transducer in the lower right corner as an example: the transmitting transducer converts the signal sent by the controller through the touch screen cable into the sound energy to the left surface, and then the acoustic energy is reflected by a group of precise reflective stripes underneath the glass plate to transmit the sound waves to an upward uniform surface, And then the reflective stripe on the top of the line to the right to transmit to the x-axis receiving transducer, the receiving transducer will return the surface acoustic energy into an electrical signal. When the transmitting transducer emits a narrow pulse, the sound wave energy goes through different ways to receive the transducer, taking the earliest arrival at the far right, taking the nearest to the left, arriving early, and arriving late with the sound wave energy superimposed into a wider waveform signal, it is not difficult to see, The receiving signal sets all the acoustic energies that have gone through different paths in the x-axis, and they travel the same way along the y-axis, but on the x-axis, the farthest is twice times the maximum distance from the nearest x-axis. So the time axis of this waveform signal reflects the position of the original waveform before the superposition, that is, the x-axis coordinates. Transmitting signal and receiving signal waveform when there is no touch, the waveform of receiving signal is exactly the same as the reference waveform. When a finger or other object that absorbs or blocks the wave energy touches the screen, the energy of the x-axis that travels upward through the finger is partially absorbed, reacting to a decay gap in the waveform on the receiving waveform at a certain point in time. The receiving waveform corresponds to the signal of the finger blocking position attenuation a notch, the calculation of the gap location is the touch coordinate controller analysis to receive signal attenuation and the position of the gap to determine the x-coordinate. The same process of the y-axis then determines the y-coordinate of the touch point. In addition to the X-and y-coordinates that the general touchscreen can respond to, the surface acoustic touch screen responds to the third-axis z-coordinate, which is the value of the user's touch pressure. The principle is calculated by attenuation at the attenuation of the received signal. Once the three axes are determined, the controller transmits them to the host.

4.3 Surface acoustic wave touch screen features

High clarity, good transmittance. High durability, good scratch resistance (relative to resistance, capacitance, etc. with surface film). Sensitive to reactions. Not affected by temperature, humidity and other environmental factors, high resolution, long life (50 million times in good condition), high transmittance (92%), can maintain a clear and transparent image quality; no drift, only one correction at the time of installation, there is a third axis (that is, the pressure axis) response, is currently used in public places more. Surface Acoustic wave screen needs to be maintained frequently, because dust, oil and even drink liquid stains on the surface of the screen, will block the surface of the touch screen guide groove, so that the wave can not be normal launch, or change the waveform and the controller can not be recognized, thus affecting the normal use of the touch screen, users need to pay strict attention to environmental health. The surface of the screen must be wiped regularly to keep the screen smooth and clean, and to be thoroughly wiped out regularly. ”

What is the principle of the

LCD touch screen?