Basic principle of resistance touch screen
working principle
Fundamentals
The working principle of the resistance touch screen is mainly to realize the operation and control of the screen content through the principle of pressure induction. The screen body of the touch screen is a multilayer composite film that is very compatible with the display surface. The first layer is the bottom layer of glass or plexiglass, the second layer is the spacer layer, and the third layer is the surface layer of multi-component resin. The surface is also coated with a transparent conductive layer, and the upper layer is covered with a layer of external surface that has been hardened Smooth, scratch resistant plastic layer. The conductive layer and glass layer inductor on the surface of the polybasic grease layer are separated by many tiny spacers, and the current passes through the surface layer. When touching the surface layer gently, the sensor contacts the bottom layer. The controller reads the corresponding current from four corners and calculates the distance of the finger position at the same time. The touch screen is composed of two layers of highly transparent conductive layers, and the distance between the two layers is only 2.5 microns. When a finger touches the screen, two normally insulated conductive layers have a contact at the touch point. Because one of the conductive layers is connected to a 5V uniform voltage field in the Y-axis direction, the voltage of the detection layer changes from zero to non-zero. After the controller detects this connection, it performs A/D conversion and compares the voltage value obtained with 5V to obtain the Y-axis coordinate of the touch point. Similarly, the X-axis coordinate is obtained, This is the basic principle shared by all resistance technology touch screens.
Circuit implementation
The touch screen consists of two transparent layers stacked up and down. The four wire and eight wire touch screens are composed of two layers of transparent resistive materials with the same surface resistance. The five wire and seven wire touch screens are composed of a resistive layer and a conductive layer. Usually, an elastic material is used to separate the two layers. When the pressure on the surface of the touch screen (such as pressing with a pen tip or finger) is large enough, contact will occur between the top layer and the bottom layer. All resistive touch screens use the voltage divider principle to generate voltages representing X and Y coordinates. As shown in Figure 3, the voltage divider is realized by connecting two resistors in series. The upper resistance (R1) is connected to the positive reference voltage (VREF), and the lower resistance (R2) is grounded. The voltage measurement at the two resistance connection points is proportional to the resistance of the following resistance.
In order to measure a coordinate in a specific direction on the resistive touch screen, it is necessary to bias a resistive layer: connect one side of it to VREF and the other side to ground. At the same time, 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 separated 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 grounding edge is equivalent to the lower resistance in the voltage divider. Therefore, the voltage measured on the unbiased layer is proportional to the distance between the touch point and the grounding edge.
The transparent ITO conductive film coated inside the resistive touch screen has process requirements. The coating shall not be too thick, otherwise it will not only reduce the light transmittance, but also form an internal reflection layer, reducing the clarity; The coating shall not be too thin, or it may break easily. In the process of use, the working accuracy of the touch screen depends on the precision of the resistance network. If there is a fault in the resistance network somewhere, the touch screen will fail to touch: the surface of the touch screen is often touched, and a thin layer of transparent ITO conductive film on the surface will appear small cracks, which will also lead to touch failure; The outer layer of transparent ITO conductive film is made of plastic material without a protective layer, so its safety is poor. However, from the structural point of view, the resistive touch screen is a relatively closed system, so compared with other touch screens, it is not affected by external pollutants, such as dust, moisture, oil, etc., and is suitable for occasions with gloves or where it cannot be directly touched by hand. Therefore, it can work normally in harsh environments and is suitable for airborne display systems.
