Resistive touch screen working principle


Release time:

2023-12-29

Resistive touch screen working principle

Basic Principles

The working principle of the resistive touch screen is mainly to realize the operation and control of the screen content through the principle of pressure sensing. The screen body of this touch screen is very close to the display surface.

 

Resistive touch screen

With the multi-layer composite film, the first layer is glass or organic glass bottom layer, the second layer is the interlayer, the third layer is the multi-resin surface layer, the surface is also coated with a layer of transparent conductive layer, and then covered with a layer of outer surface hardened, smooth scratch-resistant plastic layer. The conductive layer on the surface of the multi-component grease layer and the glass layer sensor are separated by many tiny spacers. The current passes through the surface layer. When the surface layer is pressed lightly, it touches the bottom layer. The controller simultaneously reads the commensurate current from the four corners and calculates the distance of the finger position. The touch screen uses two highly transparent conductive layers to form a touch screen, and the distance between the two layers is only 2.5 microns. When a finger touches the screen, the two conductive layers normally insulated from each other have a contact at the position of 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 detecting this connection, the controller performs A/D conversion and compares the obtained voltage value with 5V to obtain the Y-axis coordinate of the touch point, in the same way, the coordinates of the X axis are obtained, which is the most basic principle common to all resistive technology touch screens.

 

circuit realization

The touch screen consists of two transparent layers stacked on top of each other. The four-wire and eight-wire touch screens consist of two layers of transparent resistive materials with the same surface resistance. The five-wire and seven-wire touch screens consist of a resistive layer and a conductive layer. 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 a finger) is large enough, contact between the top layer and the bottom layer will occur. All resistive touch screens use the voltage divider principle to generate voltages representing the X and Y coordinates. 3, the voltage divider is implemented by connecting two resistors in series. The upper resistor (R1) is connected to a 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 lower resistor.

 

In order to measure a coordinate in a specific 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 connected to ground. At the same time, the unbiased layer is connected to the high impedance input of an ADC. When the pressure on the touch screen is great 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 ground edge is equivalent to the lower resistance in the voltage divider. Thus, the voltage measured on the unbiased layer is proportional to the distance between the touch point and the ground edge.

 

The transparent ITO conductive film coated inside the resistive touch screen has process requirements. The coating should not be too thick, otherwise it will not only reduce the light transmittance, but also form an internal reflection layer and reduce the clarity; the coating should not be too thin, otherwise it is easy to break. In the process of use, because the working accuracy of the touch screen needs to be realized by the precision of the resistance network, if there is a failure of 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 will have small cracks, which will also lead to touch failure; The outer layer of the transparent ITO conductive film is made of plastic material without protective layer, so the 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, water vapor, oil, etc., and is suitable for wearing gloves or can not be directly touched by hands, so it can work normally in harsh environments and is suitable for aviation airborne display systems.

Keywords: membrane switch, membrane panel, touch screen