2. introduction of touch screen technology principle
3. excellent quality touch screen-e1otoouch product
5. typical application of touch screen
2. introduction of touch screen technology principle
touch screen is the most convenient, simple and natural input means, and people who don't know anything about computers can operate computers when they get up. The user looks at the displayed content and simply touches what he wants to choose. Through the touch screen, people can enjoy your application software and query the information they are interested in.
since the touch screen is the most suitable input device for information inquiry, all developed countries are actively developing the touch screen, just as the PC developed from 286 and 386 to Pentium, the touch screen also developed from low-end to high-end, from infrared and resistive to capacitive induction, and now it has developed into surface acoustic wave touch screen and five-wire resistive touch screen. The performance is more and more reliable, and the technology is more and more advanced. For example, the EloTouch surface acoustic wave touch screen in the United States is installed with a piece of pure glass without any film coating, which indicates the arrival of the mature product era of touch screen in terms of clarity and durability.
this chapter mainly introduces the technologies of surface acoustic wave touch screen, resistance touch screen, capacitance induction touch screen and infrared touch screen in the domestic market one by one. Although the news that the famous overseas TPIS infrared touch screen was discontinued came at the beginning of 1995, in a short time, all domestic military forces will coexist in the domestic market for a period of time with balanced strength at price.
due to the characteristics of the touch screen itself, the requirements for the touch screen are not only very transparent and accurate positioning, but also accurate for a long time, stable and reliable in operation, not affecting the appearance and not easy to be damaged. Therefore, judging the advantages and disadvantages of a touch screen technology is mainly from these points.
Firstly, the characteristics of various touch screens are listed as follows:
Category
Performance
Surface acoustic wave
Capacitance
Infrared
Five-wire resistance
Four-wire resistance
Good definition, blurred character image, blurred character image
Less serious reflectivity and less light transmittance
92% (. Distortion is absent
Resolution is 496× 496 124× 124 4× 32 496× 496 124× 124
Influence of pressure axis is absent
Drift is absent
Scratch resistance is very good and it is not afraid of hard objects, and it is afraid of hard objects hitting. Main defects of sharp instruments
Rough use is not afraid that the general frame is fragile. However, it is afraid of sharp instrument difference
reaction speed 1ms15-24ms5-3ms1-2ms
material pure glass four-layer composite film light-transmitting shell is plated on glass and plated on plexiglass
multi-touch intelligent judgment center point
light interference does not have this problem, it cannot be out of range, it does not have this problem, it does not have this problem, it does not have this problem, the electromagnetic field is dry. Do you have this problem? Do you have this problem? Do you have this problem? Do you have this problem? Do not have this problem
Not afraid of dust, not afraid of blocking the light transmission part, not afraid of
Life > > 5 million times semi-permanent 2 million times too many sensor damage probability is 35 million times 1 million times
Installation risk is not fragile, fragile and easy to break, and the shell is not fragile and easy to be damaged. ITO
Market repair rate 21% 4% 5% 3%
Appearance does not affect, appearance does not affect, unevenness does not affect
On-site maintenance does not need to be calibrated frequently, cleaning the shell does not need
1. surface acoustic wave touch screen
in this paper, the e1otoouch surface acoustic wave touch screen of e1otoouch systems company of the United States is taken as an example.
surface acoustic wave, a kind of ultrasonic wave, is a mechanical energy wave that propagates shallowly on the surface of a medium (such as rigid materials such as glass or metal). Surface acoustic wave (SAW) is stable and easy to analyze, and has very sharp frequency characteristics in the process of transverse wave transmission. In recent years, it has developed very well in nondestructive testing, imaging and filter.
the touch screen part of the surface acoustic wave touch screen can be a flat, spherical or cylindrical glass plate, which is installed in front of the screen of CRT, LED, LCD or plasma display. This glass plate is just a piece of pure tempered glass, which is different from other touch screen technologies in that there is no film and cover.
the vertical and horizontal ultrasonic transmitting transducers are fixed in the upper left corner and the lower right corner of the glass screen, and two corresponding ultrasonic receiving transducers are fixed in the upper right corner. The four edges of the glass screen are engraved with reflective stripes with a 45-degree angle from sparse to dense. See figure 1.
figure: the InteIliTouch touch screen includes a strengthened glass plate with surface acoustic waves intersecting in the x and y directions
Taking the y axis as an example, the transmitting transducer converts the 5MHz electrical signal generated by the controller into ultrasonic energy and emits it. The design of the transducer base makes it have a narrow direction angle to spread the acoustic surface adhesive energy to the left, and in the process of transmission, it is reflected upward by the 45-degree reflection stripes at the bottom to spread to a uniform surface in the vertical direction of the screen surface, and then it is converged into a line by the reflection stripes at the top to spread to the Y-axis receiving transducer, and finally it is converted into an electrical signal and transmitted back to the controller.
the received signal attenuates at the corresponding touch position
when there is no touch, the waveform of the received signal is exactly the same as the reference waveform. When the finger touches the screen, the finger absorbs some acoustic energy, and the controller detects the attenuation of the received signal at a certain moment, so that the position of the touch point on the Y axis can be calculated, and the position of the touch point on the X axis can be obtained by the same principle. (See Figure 2). In addition to the X and Y coordinates that can be responded by the general touch screen, the SAW touch screen also responds to its unique third axis Z axis coordinates, that is, it can sense the user's touch pressure. Its principle is calculated from the attenuation of the received signal. Once the three axes are determined, the controller transmits them to the host computer. Because the surface acoustic wave technology is very stable, and the controller of the surface acoustic wave touch screen calculates the touch position by measuring the position of the decay time on the time axis, the surface acoustic wave touch screen is very stable and the accuracy is also very high. At present, the accuracy of the surface acoustic wave technology touch screen is usually 496×496.
the advantages of the surface acoustic wave touch screen mainly include: the longest service life (according to the report of the authoritative magazine of electronic engineers in the United States, touching the same position for 5, times without failure), semi-permanent product, excellent scratch resistance, the highest light transmittance (> 92%) and clarity, clear and bright image quality, and no color distortion. These advantages come from the fact that its touch screen is pure glass without any film and coating, and unlike coated glass. In addition, the surface acoustic wave touch screen technology has absolutely no drift, so it is not necessary to calibrate after installation, and the Ducal rectangular coordinate system is directly adopted, and the data conversion is undistorted.
As mentioned earlier, the surface acoustic wave touch screen also has a third axis Z axis, that is, the pressure axis response, because the greater the user touches the screen, the wider and deeper the attenuation gap on the received signal waveform. Among all touch screens, only the surface acoustic wave touch screen has the ability to sense the touch pressure. With this function, each touch point is not only a digital switch state with touch and no touch, but also an analog switch with perception. This function is very useful, for example, in multimedia information query software, a button can control the playback speed of animation or image.
see table 1 for the comparison of the above characteristics of the surface acoustic wave touch screen with other types of touch screens, and we can see that it has great advantages, especially the touch screen application that can withstand all kinds of rough touches and is most suitable for public places. Public places are very destructive and cannot be looked after by special personnel. Therefore, we must choose a durable touch screen.
. resistive touch screen:
The main part of resistive touch screen is a resistive film screen which is very compatible with the display surface. This is a multi-layer composite film, which is made of a layer of glass or plexiglass as the base layer, and the surface is coated with a transparent conductive layer called ITo, which is covered with a smooth scratch-resistant plastic layer with hardened outer surface, and its inner surface is also coated with a conductive layer (ITO or nickel gold). When a finger touches the screen, there is a contact between the two conductive layers at the touch point, and the controller detects this connection and calculates the positions of the X and Y axes, which is the most basic principle of all resistive touch screens. See figure 3.
fig. 3. Structure of resistive touch screen and principle of analog resistive screen
The two layers of ITO working surfaces of resistive touch screen must be complete, and a uniform and continuous parallel voltage distribution can be formed in one direction of the working surface by applying a silver glue on two side lines of each working surface, with a voltage of 5V at one end and V at the other end. After the touch is detected, the analog voltage value of the contact point is measured by A/D conversion immediately, and the position of the touch point in this direction can be calculated according to its proportional formula with 5V.
here it is necessary to mention two transparent conductive coating materials: ①ITO, oxidized steel and weak conductor, whose characteristics are that when the thickness drops below 18 Angstroms (Angstroms = 1-1m), it suddenly becomes transparent, with a light transmittance of 8%. When it is thinner, the light transmittance will decrease, and it will rise to 8% when it reaches a thickness of 3 Angstroms. Unfortunately, ITO is very brittle at this thickness, and it is easy to break and crack. 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. (2) Nickel-gold coating. The outer conductive layer of five-wire resistive touch screen is made of nickel-gold coating material with excellent ductility. Because of frequent touching, the outer conductive layer is made of nickel-gold material with good ductility in order to prolong its service life, but the cost is relatively high. Although the nickel-gold conductive layer has good ductility, it can only be used as a transparent conductor, so it is not suitable for the working face of resistive touch screen, because it is too conductive to make precise resistance measurement, and the thickness of metal is not easy to achieve very uniform.
the two layers of ITO in the first generation of four-wire resistance technology all add a uniform voltage distribution field of 5V to V when working: one working surface adds vertical direction and the other working surface adds horizontal direction. There are always four cables from the lead to the controller. Because the outer layer of plastic and ITO coating on the four-wire resistive touch screen is often touched, after a period of time, the thin ITO coating on the outer layer will have tiny cracks. Obviously, once there is a crack on the conductive working surface, the current will bypass it, and the voltage field distribution on the working surface will no longer be uniform. In this way, the touch screen drifts seriously near the cracks, and after the cracks increase, some areas of the touch screen may no longer be touched.
The base of four-wire resistive touch screen is mostly plexiglass, which not only has the problems of low light transmittance, weathering and aging, but also has the risk of installation. This is because plexiglass has poor rigidity, so you can't pinch the silver glue on the edge during installation to avoid the thin ITO and relatively thick silver glue from cracking, and you can't press or pull the touch screen hard to avoid breaking the ITO layer. It is for this reason that some four-wire resistive touch screens appear uneven after installation.
ITO is an inorganic substance, and plexiglass is an organic substance. Organic substances and inorganic substances can't be well combined, and they will peel off easily after a long time. If a curved glass plate can be produced, which is inorganic and can be well combined with ITO to form conductive glass, won't the life of the resistive touch screen be greatly prolonged?
this kind of conductive glass is used at the base of the second-generation five-wire resistance technology touch screen. Moreover, the five-wire resistance technology applies voltage fields in two directions to the conductive working surface of the glass through a precise resistance network. We can simply understand that the voltage fields in two directions are applied to the same working surface in a time-sharing manner, while the outer nickel-gold conductive layer is only used as a pure conductor. After touching, the position of the touch point is measured by detecting both the voltage of the inner ITO contact point and the conduction current. The ITO in the inner layer of the five-wire resistive touch screen needs four leads, and the outer layer only needs one conductor, so it needs five cables to the controller. Because the outer nickel-gold conductive layer of the five-wire resistance screen is not only malleable, but also only used as a conductor, as long as it is constantly split in half, it can still continue to fulfill its mission as a conductor, while the inner layer 1TO, which bears the heavy responsibility, is directly combined with the base glass to become conductive glass, which naturally has no disadvantages of plexiglass as the base. Therefore, Compared with the four-wire resistance screen, the service life and light transmittance of the five-wire resistance screen have made a leap: the touch life of the five-wire resistance screen is 35 million times, while that of the four-wire resistance screen is less than 1 million times, and there is no installation risk of the five-wire resistance touch screen. At the same time, the ITO layer of the five-wire resistance screen can be made thinner, so the light transmittance and clarity are higher, and there is almost no color distortion.
whether it is a four-wire resistive touch screen or a five-wire resistive touch screen, they are all completely isolated from the outside world. They are not afraid of dust, water vapor and oil stains. They can be touched by any object, used for writing and drawing, and are suitable for industrial control fields and limited people in offices. The disadvantage of resistive touch screen is that the outer layer of composite film is made of plastic material, and people who don't know it may scratch the whole touch screen too hard or touch it with sharp instruments, resulting in scrapping. However, within the limit, scratches will only hurt the outer conductive layer, which is irrelevant to the five-wire resistive touch screen, but fatal to the four-wire resistive touch screen.
capacitive touch screen:
capacitive touch screen is a four-layer composite glass screen. The inner surface and interlayer of the glass screen are coated with a layer of ITO, and the outermost layer is a silica glass protective layer with a thickness of only .15mm. The interlayer ITO coating is used as the working surface, and four electrodes are led out at the four corners. The inner ITO is the shielding layer to ensure a good working environment.
when the user touches the capacitive screen, a coupling capacitor is formed between the user's finger and the working surface due to the electric field of the human body. Because the working surface is connected with high-frequency signals, the finger absorbs a small current. This current flows out from the electrodes at the four corners of the touch screen, and theoretically 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 through precise calculation of these four current ratios, as shown in Figure 4.
Figure 4. Calculating the touch point position by comparing four currents
The light transmittance and clarity of capacitive touch screen are better than those of four-wire resistance screen, but they can't be compared with those of surface acoustic wave screen and five-wire resistance screen. See Table 1 and Figure 5. The capacitive screen reflects light seriously, and the four-layer composite touch screen with capacitive technology transmits light of various wavelengths.