According to the working principle of the touch screen and the medium for transmitting information, we divide the touch screen into four types, which are resistive, capacitive inductive, infrared, and surface acoustic wave. Each type of touch screen has its own advantages and disadvantages. To understand the type of touch screen that is suitable for that occasion, the key is to understand the working principle and characteristics of each type of touch screen technology. The following briefly introduces the various types of touch screens described above:
1, resistive touch screen (resistive touch screen working principle) 1.1 four-wire resistive screen 1.2 five-wire resistive screen 2, capacitive touch screen, infrared touch screen (infrared touch screen working principle) 4, surface acoustic wave touch screen (surface acoustic wave The working principle of the touch screen).3 Surface acoustic wave touch screen features high definition and good light transmittance. Highly durable, good scratch resistance (surface film with respect to resistance, capacitance, etc.). Responsive. Not affected by environmental factors such as temperature and humidity, high resolution, long life (50 million times in good maintenance); high light transmittance (92%), able to maintain clear and translucent image quality; no drift, just install One calibration; there is a third axis (ie pressure axis) response and is currently used more in public places. The surface acoustic wave screen needs frequent maintenance, because dust, oil stains and even liquid of the beverage are stained on the surface of the screen, which will block the waveguide groove on the surface of the touch screen, so that the wave cannot be normally emitted, or the waveform is changed and the controller cannot recognize it properly, thus affecting For the normal use of the touch screen, the user must pay strict attention to environmental sanitation. The surface of the screen must be wiped frequently to keep the screen clean and regularly wiped thoroughly.
The following is a detailed description of the resistive touch screen. Now all the modified navigation uses a four-wire resistive touch screen.
A resistive touch screen is a sensor that converts the physical position of a touch point (X, Y) in a rectangular area into a voltage representing an X coordinate and a Y coordinate. Many LCD modules use resistive touch screens, which can use four-wire, five-wire, seven-wire or eight-wire to generate the screen bias voltage and read back
The resistive touch screen is basically a film-plus-glass structure. The adjacent side of the film and glass is coated with ITO (Indium Tin Oxides) coating. ITO has good conductivity and transparency. . When touched, the ITO under the film will contact the ITO on the upper layer of the glass, and the corresponding electrical signal will be transmitted through the inductor, sent to the processor through the conversion circuit, and converted into X and Y values on the screen by calculation, and the point is completed. The selected action is presented on the screen.
Touch screen principle
The touch screen includes two transparent layers stacked on top of each other. The four-wire and eight-line touch screens are composed of two transparent resistive materials having the same surface resistance. The five-wire and seven-wire touch screens are composed of a resistive layer and a conductive layer, usually Use an elastic material to separate the two layers. When the pressure on the surface of the touch screen (such as pressing through a pen tip or finger) is large enough, contact is made between the top layer and the bottom layer. All resistive touch screens use a voltage divider principle to generate voltages that represent the X and Y coordinates. As shown in Figure 1, the voltage divider is implemented by connecting two resistors in series. The upper resistor (R1) is connected to the positive reference voltage (VREF) and the lower resistor (R2) is connected to ground. The voltage measurement at the junction of the two resistors is proportional to the resistance of the resistor below.
In order to measure a coordinate in a particular direction on a resistive touch screen, a resistive layer needs to be biased: one side of it is connected to VREF and the other side is grounded. Also, connect the unbiased layer to the high impedance input of an ADC. When the pressure on the touch screen is large enough to make contact between the two layers, the resistive surface is divided into two resistors. Their resistance is proportional to the distance from the touch point to the offset edge. The resistance between the touch point and the ground side is equivalent to the one below the voltage divider. Therefore, the voltage measured on the unbiased layer is proportional to the distance from the touch point to the ground side.
Four-wire touch screen
The four-wire touch screen contains two resistive layers. One of the layers has a vertical bus on the left and right edges of the screen, and the other layer has a horizontal bus at the bottom and top of the screen.
. To make measurements in the X-axis direction, the left bus is biased to 0V and the right bus is biased to VREF. Connect the top or bottom bus to the ADC and make a measurement when the top and bottom layers are in contact. /I="+
Figure 1 shows the top bus biased to VREF and the bottom bus biased to 0V for measurement in the Y-axis direction. The ADC input is terminated to the left or right bus, and the voltage is measured when the top layer is in contact with the bottom layer. Figure 2 shows a simplified model of a four-wire touch screen when two layers are in contact. For a four-wire touch screen, the ideal connection method is to connect the bus biased to VREF to the positive reference input of the ADC and the bus set to 0V to the negative reference input of the ADC. sY=$\hj
Five-wire touch screen
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The five-wire touch screen uses a resistive layer and a conductive layer. The conductive layer has a contact, usually at the edge of one side. There is one contact at each of the four corners of the resistive layer. To measure in the X-axis direction, the upper left and lower left corners are biased to VREF, and the upper right and lower right corners are grounded. Since the left and right corners are the same voltage, the effect is similar to that of the bus connected to the left and right sides, similar to the method used in the four-wire touch screen.
To measure along the Y-axis, the upper left and upper right corners are offset to VREF, and the lower left and lower right corners are offset to 0V. Since the upper and lower corners are respectively the same voltage, the effect is substantially the same as the bus connecting the top and bottom edges, similar to the method used in the four-wire touch screen. The advantage of this measurement algorithm is that it keeps the voltages in the upper left and lower right corners constant; but if grid coordinates are used, the X and Y axes need to be reversed. For a five-wire touch screen, the best way to connect is to connect the upper left corner (offset to VREF) to the positive reference input of the ADC and the lower left corner (offset to 0V) to the negative reference input of the ADC.
Seven-line touch screen
The seven-line touch screen is implemented in the same way as the five-line touch screen except that one line is added to the upper left and lower right corners. When performing a screen measurement, connect one line in the upper left corner to VREF and the other line to the positive reference end of the SAR ADC. At the same time, one line in the lower right corner is connected to 0V, and the other line is connected to the negative reference end of the SAR ADC. The conductive layer is still used to measure the voltage of the voltage divider. :
Eight-line touch screen
In addition to adding one line to each bus, the eight-wire touch screen is implemented in the same way as a four-wire touch screen. For the VREF bus, one line is used to connect VREF and the other line is used as the positive reference input for the DAC ADC's digital-to-analog converter. For the 0V bus, one line is used to connect 0V, and the other line is used as the negative reference input for the DAC ADC's digital-to-analog converter. Any of the four wires on the unbiased layer can be used to measure the voltage of the voltage divider.
Check for contact
All touch screens can detect if a touch has occurred by pulling one of the layers with a weak pull-up resistor and pulling the other layer with a strong pull-down resistor. If the measured voltage of the pull-up layer is greater than a certain logic threshold, it indicates that there is no touch, and vice versa. The problem with this approach is that the touch screen is a huge capacitor and it may be necessary to increase the capacitance of the touch screen leads in order to filter out the noise introduced by the LCD. A weak pull-up resistor connected to a large capacitor can lengthen the rise time and may result in the detection of a false touch.
