According to the working principle of the touch screen and the medium of transmitting information, we divide the touch screen into four types: resistive type, capacitive type, infrared type and surface acoustic wave type. Each type of touch screen has its own advantages and disadvantages, to understand the kind of touch screen for that occasion, the key is to understand the working principle and characteristics of each type of touch screen technology. Below a brief introduction to the various types of touch screen described above:
1, resistive touch screen
This touch screen uses pressure sensing for control. The main part of a resistive touch screen is a resistive film screen that works well with the display surface. This is a multilayered composite film that uses a layer of glass or hard plastic plate as the substrate, coated with a layer of transparent metal oxide (transparent conductive Resistance) conductive layer, covered with a layer of an outer surface hardened, smooth rubbed plastic layer, the inner surface of which is also coated with a coating of many small (less than 1/1000 inch) transparent Isolation points separate the two conductive layers from each other. When the finger touches the screen, the two conductive layers touch at the touch point, the resistance changes, producing a signal in both the X and Y directions, which is 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 simulated mouse. This is the most basic principle of resistive touch screen technology. Resistance type touch screen is the key to materials science and technology, commonly used transparent conductive coating materials are:
A, ITO, indium oxide, weak conductor, the characteristic is that when the thickness drops below 1,800 Angstroms (A = 10 to 10 meters), it suddenly becomes transparent with a light transmittance of 80% Dropped to another 80% at a thickness of 300 angstroms. ITO is the main material used in all resistive touch screens and capacitive touch screens. In fact, the working surface of resistive and capacitive touch screens is ITO coating.
B, nickel-gold coating, the outer conductive layer of the five-wire resistive touch screen uses a good ductility nickel-gold coating material, the outer conductive layer due to frequent touch, the use of good ductility nickel-gold material purpose is to extend the service life , But the cost of the process is relatively high. Nickel gold conductive layer although scalability, but only for transparent conductors, 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 voltage distribution layer, only as a probe Floor.
1.1 four-wire resistance screen
Four-wire resistive analog technology, two layers of transparent metal layer work to increase the constant voltage of 5V each layer: a vertical direction, a horizontal direction. A total of four cables are required. Features: High resolution, high speed transfer reaction. Surface hardness treatment to reduce abrasion, scratching and chemical treatment. With smooth and matte finish. A calibration, high stability, never drift.
1.2 five-wire resistance screen
Five-wire resistive touch screen of the grass-roots level in both directions of the voltage field through precision resistor networks are added to the conductive glass surface, we can simply understand the two directions of the voltage field work added to the same work surface, The outer layer of nickel gold conductive layer is only used as a pure conductor, after touching the inner layer ITO touch points to detect X-axis and Y-axis voltage value of the method of measuring the location of the touch point. Five-wire resistive touch screen inner ITO need four leads, the outer layer only as a conductor, the touch screen leads a total of five. Features: high resolution, high-speed transmission reaction. High surface hardness, reduce scratches, scratches and chemical treatment. The same point of contact with 30 million can still be used. Conductive glass substrate for the media. A calibration, high stability, never drift. Five-wire resistive touch screen has high price and high environmental shortcomings
1. 3 resistance screen limitations
Whether it is four-wire resistive touch screen or five-wire resistive touch screen, they are a complete isolation of the working environment, not afraid of dust and water vapor, it can touch with any object, can be used to draw pictures, more suitable for industrial control field And the limited use of the office. The common disadvantage of resistive touch screens is that the outer layers of the composite film are made of plastic material, and people who do not know too hard or may touch the touch screen with scratches may cause scrapping. However, within the limits, the scratch will only hurt the outer conductive layer. The scratching of the outer conductive layer has no relation to the five-wire resistive touch screen, but is fatal to the four-wire resistive touch screen. 2, capacitive touch screen
2.1 capacitive technology touch screen
Is to use the human body’s current sensing work. Capacitive touch screen is a four-layer composite glass screen, the inner surface of the glass screen and the interlayer coated with a layer of ITO, the outermost layer is a thin layer of silica glass protective layer, laminated ITO coating as a working face, the four corners Leads to four electrodes, the inner layer of ITO as a shield 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 capacitance due to the human body electric field. For high-frequency current, the capacitance is a direct conductor and the finger sucks a small current from the contact point. The current flows out of 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 corners. The controller calculates the position of the touch point by accurately calculating the four current ratios .
2.2 capacitive touch screen defects
Capacitive touch screen transmittance and clarity better than four-wire resistive screen, of course, can not and surface acoustic wave screen and five-wire resistive screen compared. Capacitive screen reflective serious, and capacitive technology, four-layer composite touch screen light transmittance of light at different wavelengths, there is the problem of color distortion, due to the reflection of light in the layers, but also cause fuzzy image characters. Capacitive screen in principle, the human body as a capacitor element of an electrode to use, when there is a conductor close to the mezzanine ITO surface coupling capacitance sufficient capacitance value, the flow of current is sufficient to cause capacitive screen malfunction . We know that although the capacitance value is inversely proportional to the distance between the electrodes, it is proportional to the relative area, and is also related to the dielectric constant of the medium. Therefore, when a large area of ​​the palm or hand-held conductor close to the capacitive screen rather than touch can cause capacitive screen malfunction, especially in wet weather, holding the monitor, the palm of your hand close to the display 7 cm Within 15 cm of the body or near the display can cause capacitive screen malfunction. Another disadvantage of the capacitive screen is that there is no reaction when touched with a gloved hand or a hand-held non-conductive object because of the addition of a more insulating medium. The main disadvantage of capacitive screen is drift: When the ambient temperature, humidity changes, the electric field changes, will cause the capacitive screen drift, resulting in inaccurate. For example, the temperature of the display increases after the power is turned on, which may cause drift. When the user touches the screen, the other hand or the body side will drift near the display. The larger object near the capacitive touch screen moves back and drifts backwards. Will cause drift; capacitance screen drift due to technical inherent deficiencies, environmental potential surface (including the user’s body) and capacitive touch screen even farther, but much larger than the finger area, they directly affect the determination of the touch position . In addition, in theory, many should be linear relationship is actually non-linear, such as: different body weight or finger wetting different people sucked the total amount of current is different, and the total amount of current changes and changes in the amount of four sub-current Is a non-linear relationship between the capacitive touch screen using the four corners of the custom polar coordinate system does not have the origin on the coordinates, drift controller can not detect and recover, and after the completion of A / D by the four sub-flow The value of the touch point in Cartesian coordinates on the X, Y coordinates of the calculation process is complicated. Since there is no origin, capacitive screen drift is cumulative, and often require calibration at the job site. Capacitive touch screen, the outermost silica protection glass scratch resistance is good, but the fear of nail or hard object percussion, knock out a small hole will hurt mezzanine ITO, whether it is injured mezzanine ITO or during installation and transportation injuries The inner surface of the ITO layer, capacitive screen can not work properly.
3, infrared touch screen
Infrared touch screen is the use of X, Y direction of the dense infrared matrix to detect and locate the user’s touch. Infrared touch screen in the front of the monitor to install a circuit board frame, the circuit board in the four sides of the screen are arranged infrared emission tube and infrared receiver tube, one by one corresponding to form a horizontal and vertical crossed infrared matrix. When the user touches the screen, the finger will block the two infra-red lines passing through the position, so that the position of the touch point on the screen can be judged. Any touch of an object can change the touch on the infrared and touch-screen operation. Early concept, the infrared touch screen has low resolution, touch mode is limited and vulnerable to environmental disturbances and other technical limitations, 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 also improved their resolution and stability, but none of them made any qualitative leap in key indicators or overall performance. However, anyone who knows touch screen technology knows that infrared touch screens are immune to current, voltage and static electricity and are suitable for harsh environmental conditions. Infrared technology is the ultimate trend of touch screen products. Touches that use acoustics and other materials science techniques have insurmountable barriers such as damage to a single sensor, aging, fear of contaminated touch interfaces, disruptive use, maintenance and more. As long as the infrared touch screen really high stability and high resolution, will replace other technology products and become the mainstream of the touch screen market. In the past, the resolution of the infrared touch screen is determined by the number of infrared tubes in the frame. Therefore, the resolution is relatively low. The major domestic products on the market are 32 × 32 and 0 × 32. In addition, the infrared screen is also said to be sensitive to the light environment factors. Big mistake or even crash. These are the foreign non-infrared touch screen sales agents propaganda infrared screen weaknesses. The latest technology, the resolution of the fifth generation of infrared screen depends on the number of infrared tubes, scanning frequency and the difference algorithm, the resolution has reached 1000X720, as to say that the infrared screen is unstable in light conditions, from the second generation of infrared touch screen The beginning, it has been better to overcome the weaknesses of light interference. The fifth generation of infrared touch screen is a new generation of intelligent technology products, which achieved 1000720 high-resolution, multi-level self-regulation and self-recovery hardware adaptability and highly intelligent discrimination can be a long time in a variety of harsh environments use. And customized extensions for users, such as network control, acoustic sensors, human proximity sensor, user software encryption protection, infrared data transmission. The original infrared media touch screen advertising another major drawback is the riot of violence is poor, in fact, the infrared screen can choose any customer that is satisfied with the anti-riot glass without adding too much cost and affect the use of performance, which is the other touch screen can not follow.
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4, surface acoustic wave touch screen
4.1 Surface acoustic waves
A surface acoustic wave, a type of ultrasonic wave that propagates shallowly on the surface of a medium (eg, a rigid material such as glass or metal). Through the wedge-shaped triangular base (based on the surface wave wavelength strictly designed), can do directional, small-angle surface acoustic wave energy emission. Surface acoustic wave is stable, easy to analyze and has very sharp frequency characteristics in the process of transverse wave propagation. In recent years, its applications in non-destructive flaw detection, imaging and wave-tracing have been rapidly developed. Surface acoustic wave theory, semiconductor materials, Guide materials, testing technology and other technologies have been quite mature. The touch screen portion of a surface acoustic wave touch screen can be a flat, spherical, or cylindrical glass plate mounted on the front of a CRT, LED, LCD, or plasma display screen. Upper and lower right corner of the glass screen are fixed vertical and horizontal ultrasonic transducer, the upper right corner is fixed two corresponding ultrasonic receiver transducer. The four sides of the glass screen are engraved with very precise reflections of 5 ° from sparse to densely spaced.
4.2 surface acoustic wave touch screen works
Take the X-axis transducer in the lower right corner as an example: The transducer transforms the electrical signal sent by the controller through the touch screen cable into acoustic energy and transmits it to the left-hand surface. Then a set of finely-reflected stripes The acoustic energy reflected upward uniform surface transmission, the sound energy through the screen surface, and then from the top of the reflection stripes converge to the right line of transmission to the X-axis receiving transducer, receiving transducer will return to the surface acoustic wave Energy becomes electrical signal. When the transmitting transducer emits a narrow pulse, the acoustic energy reaches the receiving transducer in different ways, and the earliest arrivals to the far right, the leftmost arrivals, the early arrivals and late arrivals are superimposed into one It is not difficult to see that the received signal is a collection of all the acoustic energy that has traveled along the length and the short path in the X-axis direction. The distance traveled by the Y-axis is the same. However, in the X-axis, the farthest ratio More recently walk twice the maximum distance of the X-axis. Therefore, the time axis of this waveform signal reflects the position of each original waveform before superimposing, that is, the X-axis coordinate. Transmitting and Receiving Waveforms When there is no touch, the received signal has exactly the same waveform as the reference waveform. When a finger or other object capable of absorbing or blocking sonic energy touches the screen, the sound energy of the X-axis traveling upward through the finger portion is partially absorbed and the waveform has an attenuation notch at the receiving waveform, ie, at a certain moment. Receive waveform corresponding finger blocking part of the signal attenuation of a gap, the calculation of the notch position that touch coordinate controller to analyze the attenuation of the received signal and determine the position of the gap X coordinate. After Y axis the same process determines the Y coordinate of the touch point. In addition to the general touch screen can respond to the X, Y coordinates, the surface acoustic wave touch screen also respond to the third axis Z-axis coordinates, which is able to perceive the value of the user touch the pressure value. The principle is calculated by the amount of attenuation at the receive signal attenuation. Once the three axes have been determined, the controller sends them to the host.
4.3 surface acoustic wave touch screen features
High definition, good light transmittance. Highly durable, good scratch resistance (relative to the resistance, capacitance, etc. have a surface film). fast reaction. Not subject to environmental factors such as temperature and humidity, high resolution, long life (50 million times in good condition); high light transmittance (92%), can maintain clear and translucent image quality; no drift, just install A correction; a third axis (ie pressure axis) response, the current use of more public places. Surface acoustic wave screens need frequent maintenance because dirt, grease and even liquid stains on the screen can clog the guide grooves on the surface of the touch screen, causing the waves to not fire properly or causing the waveform to change without the controller recognizing, thus affecting The normal use of the touch screen, users should pay strict attention to environmental health. The surface of the screen must always be rubbed to keep the screen clean and regularly erased once and for all.

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