First, the touch screen principle


The basic principle of the touch screen is that when the touch screen installed on the front of the display is touched with a finger or other object, the touched position (in coordinate form) is detected by the touch screen controller and sent to the CPU through an interface (such as an RS-232 serial port) , To determine the input information.

The touch screen system generally includes two parts: a touch screen controller (card) and a touch detection device. Among them, the main function of the touch screen controller (card) is to receive touch information from the touch point detection device, and convert it into contact coordinates, and then sent to the CPU, it can also receive the command from the CPU and execute: touch detection The device is generally installed in the front of the monitor, the main role is to detect the user’s touch location, and sent to the touch screen control card.


Second, the type of touch screen


Touch screen is generally divided into: resistive touch screen, infrared touch screen, capacitive touch screen, surface acoustic touch screen


1. Resistive touch screen

The body part of the resistive touch screen is a multi-layer composite film matched with the surface of the display. It is composed of a layer of glass or plexiglass as the base layer. The surface is coated with a transparent conductive layer, which is covered with a layer of external surface hardening treatment and smooth protection. The plastic layer is shaved, and its inner surface is also coated with a transparent conductive layer. There are many tiny (less than one-thousandth of an inch) transparent isolation points between the two conductive layers to insulate them.

When the finger touches the screen, two layers of conductive layers, which are usually insulated from each other, have a contact point at the touch point. Because one of the conductive layers is connected to a 5-volt uniform voltage field in the Y-axis direction, the voltage of the detection layer changes from zero to Non-zero, this on-state is detected by the controller, the A / D conversion, and the voltage value obtained compared with the 5V can get the Y-axis touch point coordinates, the same way to get the X-axis coordinates , Which is the most basic principle of all resistance technology touch screen common. The key to resistive touch screens is material technology. The resistance screen is divided into four-wire, five-wire, six-wire and multi-wire resistive touch screens according to the number of lead wires. Resistive touch screen in the tempered glass surface were coated with two layers of OTI transparent metal oxide conductive layer, the outermost layer of OTI coating as the conductor, the second layer OTI is precision network attached both horizontal and vertical +5 V to 0V voltage field between two layers of OTI separated by small transparent isolation points. When the finger touches the screen, two layers of OTI conductive layer will appear a contact point, the computer detects the voltage and current, calculate the touch position, the reaction speed of 10-20ms.

The outer conductive layer of the five-wire resistive touch screen uses a ductile nickel-gold coating material. Since the outer conductive layer touches frequently, the purpose of using the ductile nickel-gold material is to prolong the service life, but the process cost 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.

Resistive touch screen is a complete isolation of the working environment, not afraid of dust and water vapor, it can touch with any object, can be used for drawing pictures, more suitable for industrial control and office limited personal use. 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, scratches will only hurt the outer conductive layer, scratches of the outer conductive layer have no relation to the five-wire resistive touch screen, and are fatal to the four-wire resistive touch screen


2. Infrared touch screen

Infrared touch screen installation is simple, just add the spotlight on the monitor frame, without the coating on the screen surface or connect the controller. The light spot is arranged on the four sides of the frame with an infrared transmitting tube and a receiving tube, and forms an infrared network on the surface of the screen. Users touch the screen with a finger at a point, it will block the location of the two vertical and horizontal infrared, the computer can instantly calculate the location of the touch point. Any touch object can change the infrared on the touch point to achieve touch screen operation. In the early conception, infrared touch screens had technical limitations such as low resolution, limited touch methods, and being susceptible to environmental interference and misoperation, and thus 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. The resolution of the infrared touch screen in the past is determined by the number of infrared pairs in the frame, so the resolution is relatively low. The major domestic products on the market are 32×32 and 40×32. There are also said that the infrared screen is more sensitive to the light environment and the light changes. Large misjudgment even * machine. These are the foreign non-infrared touch screen sales agents propaganda infrared screen weaknesses. The latest technology of the fifth generation of infrared screen resolution depends on the number of infrared tubes, scanning frequency and difference algorithm, the resolution has reached 1000X720, as the infrared screen is not stable under light conditions, from the second generation of infrared touch screen At the beginning, the weak point of resisting light interference has been overcome. The fifth-generation infrared touch screen is a whole new generation of intelligent technology products. It realizes 1000*720 high-resolution, multi-level self-adjusting and self-healing hardware adaptability and highly intelligent identification and identification, and can be used in various harsh environments for a long time. Under arbitrary use. And customized extensions for users, such as network control, acoustic sensors, human proximity sensing, user software encryption protection, infrared data transmission. Another major drawback of the original infrared touch screen media is that the anti-vibration is poor. In fact, the infrared screen can completely use any anti-riot glass that the customer thinks is satisfactory without increasing the cost and affecting the performance. This is not possible with other touch screens.

Infrared touch screen is cheap, easy to install, can better sense light touch and quick touch. However, due to infrared touch screen infrared sensing action, the outside light changes, such as sunlight, indoor lighting, etc. will affect its accuracy. In addition, the infrared touch screen is not waterproof and is resistant to dirt. Any small foreign object can cause errors and affect its performance. It is not suitable for outdoor use and public places.


3. Capacitive touch screen

Capacitive touch screen structure is mainly in the glass screen coated with a transparent film body layer, and then in the conductor layer of a protective glass, double glass design can completely protect the conductor layer and the sensor.

In addition, all four sides of the additional touch screen are coated with long, narrow electrodes to form a low voltage AC electric field in the conductive body. When the user touches the screen, a coupling capacitor is formed between the body electric field and the finger and the conductor layer, and the current from the four-sided electrode flows to the contact. The strength of the four-sided electrode is proportional to the distance between the finger and the electrode, and the controller located behind the touch screen Will calculate the current ratio and strength, accurate calculation of the location of the touch point. The double glass of the capacitive touch screen not only protects the conductor and the sensor, but also effectively prevents external environmental factors from affecting the touch screen. Even if the screen is stained with dirt, dust, or oil, the capacitive touch screen can still accurately calculate the touch position.

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 near the mezzanine ITO coupling between the capacitive capacity of a sufficient capacity, 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, in wet weather, this situation is particularly serious, holding the display with the palm of your hand close to the display 7 cm Within or within 15cm of the display, the capacitive screen can 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, an increase in the temperature of the monitor after power on causes drift: When the user touches the screen, the other hand or the side of the body moves closer to the monitor. The larger object near the capacitive touch screen moves back and drifts. When you touch it, if someone looks around, Can cause drift; capacitive screen drift is a technical congenitally deficient cause, environmental potential surface (including the user’s body), although far from the capacitive touch screen, but much larger than the size of the finger, 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 restore, and, after the completion of 4 A / D by four points The process of calculating the X and Y coordinate values ​​on the Cartesian coordinate system from the value of the flow to the touch point 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 ITo layer, capacitive screen can not work properly.


4. Surface acoustic wave touch screen

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. This piece of glass plate is just a pure tempered glass, different from other types of touch screen technology is no foil and coating. The vertical and horizontal ultrasonic transmission transducers are fixed at the upper left corner and the lower right corner of the glass screen respectively, and two corresponding ultrasonic receiving transducers are fixed at the upper right corner. The four perimeters of the glass screen are engraved with 45 corners with very precise reflections from the sparse to dense spacing.

The working principle is the X-axis transmit transducer in the lower right corner as an example:

Transmitting transducer converts the electric signal sent from the controller through the touch screen cable into acoustic energy and transmits it to the left surface. Then a set of precise reflecting stripes under the glass plate reflects the acoustic energy into an upward uniform surface transmission. The energy of the acoustic wave passes through. The surface of the screen body is then converged by a reflection stripe on the upper side to a right line to the receiving transducer of the X-axis, and the receiving transducer converts the returned surface acoustic wave energy into an electric 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 the 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 The most recent walked twice the X-axis maximum distance. 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 gap location that touch coordinate controller to analyze the attenuation of the received signal and determine the position of the gap X coordinate. After the same process on the Y axis, the Y coordinate of the touch point is determined. In addition to the X and Y coordinates that can be responded to by a general touch screen, 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 sensed. The principle is calculated from the attenuation of the received signal attenuation. Once the three axes have been determined, the controller passes them to the host.

The surface acoustic wave touch screen is characterized by resistance to violence, because the working surface of the surface acoustic wave touch screen is a layer of invisible, unbreakable sound wave energy, the touch screen base glass without any sandwich and structural stress (surface acoustic wave touch screen can be developed directly on the surface of the CRT So there is no “screen”) and is therefore very resistant to violence and suitable for use in public places.

The second characteristic surface acoustic wave reaction speed, all the touch screen in the fastest response, feel very smooth when used.

The third characteristic of surface acoustic wave is stable performance, because the principle of surface acoustic wave technology is stable, and the controller of the surface acoustic wave touch screen calculates the touch position by measuring the position of the attenuation time on the time axis, so the surface acoustic wave touch screen is very stable and has high precision. At present, the accuracy of the surface acoustic wave technology touch screen is usually 4096× 4096×256 levels.

The fourth characteristic of the surface acoustic wave touch screen is that the control card can know what is the dust and water droplets, what is the finger, and how much in the touch. Because: Our finger touch accuracy of 4096 × 4096 × 256 level of intensity, 48 times per second touch data can not be unchanged, and dust or water droplets will not change at all, the controller found a “touch “After the appearance of the filament unchanged for more than three seconds that is automatically identified as interference.

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