Principle of capacitive touch screen and troubleshooting

- Dec 04, 2019-

When the user touches the capacitive screen, due to the electric field of the human body, the user's finger and the working surface form a coupling capacitor. Because the working surface is connected to high-frequency signals, the finger absorbs a small current, which is drawn from the four corners of the screen. The upper electrode flows out, and in theory the current flowing through the four electrodes is proportional to the distance from the finger to the four corners. The controller calculates the position by precise calculation of the four current ratios. Can reach 99% accuracy, with a response speed of less than 3ms.


Touch technology of projected capacitive panel Projected capacitive touch screen is to etch different ITO conductive circuit modules on two layers of ITO conductive glass coating. The patterns etched on the two modules are perpendicular to each other, and they can be regarded as X and Y Continuously changing slider. Since the X and Y structures are on different surfaces, a capacitor node is formed at the intersections. One slider can be used as a drive line, and the other slider can be used as a detection line. When a current passes through one of the driving wires, if there is a signal of a change in capacitance, it will cause a change in the capacitance node on the other layer of wire. The change in the detection capacitance value can be measured by the electronic circuit connected to it, and then converted to a digital signal by the A / D controller to allow the computer to perform arithmetic processing to obtain the (X, Y) axis position, thereby achieving the purpose of positioning.


Capacitive touch screens have better light transmittance and clarity than four-wire resistive screens, and of course they cannot be compared with surface acoustic wave screens and five-wire resistive screens. Capacitive screens have serious reflections. In addition, the four-layer composite touch screen of capacitive technology has non-uniform transmittance of light at various wavelengths, and there is a problem of color distortion. Due to the reflection of light between the layers, the image characters are blurred.


The capacitive screen uses the human body as an electrode of a capacitor element in principle. When a sufficient amount of capacitance is coupled between the conductor and the sandwich ITO working surface, the flowing current is sufficient 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 related to the dielectric coefficient of the dielectric. Therefore, when a large area of the palm or a hand-held conductive object approaches the capacitive screen instead of touching it, it can cause the capacitive screen to malfunction. This situation is particularly serious in humid weather. Hold the display with your hand and the palm 7 cm away from the display Within or within 15 cm of the body near the display can cause malfunction of the capacitive screen.




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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 more major disadvantage of capacitive screens is drift: when the ambient temperature and humidity change, and the environmental electric field changes, it will cause the capacitive screen to drift, causing inaccuracies. The outermost silicon protective glass of the capacitive touch screen has good scratch resistance. However, it is afraid that a small hole will hurt the sandwich ITO if it is struck by nails or hard objects. The inner surface of the ITO layer will not work properly.


If the user is using a capacitive touch screen, it is recommended that the user first properly install the driver required for the capacitive touch screen according to the requirements of the relevant manuals when using it for the first time, and then click on the screen with "Start" "/" Program "/" Microtouch Touchware "to run the screen calibration program. After the calibration is completed, the system automatically stores the calibrated data in the register of the controller. After restarting the system in the future, there is no need to calibrate the screen.


If you change the display resolution or display mode of the touch screen when you operate the capacitive touch screen midway, or if you adjust the refresh frequency of the touch screen controller yourself, and you feel that the cursor does not correspond to the touch point, you must perform the touch screen system again. Calibration operation. In order to ensure the normal operation of the touch screen system, in addition to ensuring the correct installation of the system software, you must also remember not to install two or more touch screen drivers on one host. This will easily lead to conflicts during system operation. As a result, the touch screen system cannot be used normally.


When using a resistive touch screen, if it is found that the cursor does not move or can only move in a local area, the user can check whether the touch area of the touch screen is always pressed by other touching objects, such as once the touch screen is pressed by the display case or the cabinet case It is equivalent to a certain point being touched all the time, so the coordinate position feedback to the controller is not accurate.


As mentioned earlier, once the system changes the display resolution, adjusts the screen size, and the first time installation, there will be click inaccuracy or drift, you need to start the positioning program that comes with the application to reposition, but when positioning , It is better to use a thinner pen or fingertip for positioning so that it is more accurate. The surface acoustic wave touch screen requires a higher working environment. It must be operated in a clean and dust-free environment, and the dust on the surface of the touch screen must be cleaned regularly. Otherwise, the dust in the air will cover the reflection stripes around the touch screen. Or the transducer will affect the correct positioning of the system.


Do not allow water droplets or other soft things to stick to the surface of the touch screen, otherwise the touch screen is likely to mistakenly believe that the surface acoustic wave screen is inaccurate due to hand touch. In addition, when removing dirt on the surface of the touch screen, the user can carefully wipe away from the center of the screen with a soft dry cloth or detergent, or clean the surface of the touch screen with a dry soft cloth dipped in industrial alcohol or glass cleaning solution.


If you touch the surface acoustic wave touch screen with your hand or other touching objects, the touch screen response is very slow, which indicates that the touch screen system is probably outdated, the internal clock frequency is too low, or the water droplets on the touch screen surface are moving. The touch screen resumes fast response, and the system must be replaced or upgraded again, or the water droplets on the surface of the touch screen must be dried with a rag.


When a finger touches a certain position of the capacitive touch screen, when the touch screen does not respond, it is likely that the corresponding touch position is inaccurate, and the cursor cannot be positioned correctly, of course. If the cabinet shell presses the touch area, the user can increase the distance between the cabinet and the monitor screen. If the monitor shell presses the touch area, the user can try to loosen the screws of the monitor shell.