Resistive touch panel technology introduction

sentiment:324 Published time :2016-07-16 20:11

Resistive touch panel (RTP) is a kind of sensor, which is basically a structure of thin film and glass. The adjacent sides of the thin film and the glass are coated with ITO (Nano Indium Tin Metal Oxide) coating. ITO has good conductivity and transparency. When touch operation, the ITO of the lower layer of the film will contact the ITO of the upper layer of the glass, and the corresponding electrical signal will be transmitted through the sensor, and then sent to the processor through the conversion circuit, which is converted into the X and Y values on the screen through calculation to complete the point. The selected action is displayed on the screen.

1.     Working principle of RTP

The working principle of the RTP is mainly through the principle of pressure sensing to realize the operation and control of the screen content. The screen body of this touch panel is a multi-layer composite film that matches the surface of the display. The first layer is glass or plexiglass. The bottom layer, the second layer is the interlayer, and the third layer is the surface layer of multi-element resin. The surface is also coated with a transparent conductive layer, and the surface is covered with a hardened, smooth and scratch-resistant plastic layer. The conductive layer and the glass layer sensor on the surface of the multi-element resin are separated by many tiny interlayers. The current passes through the surface layer. When the surface layer is touched lightly, it touches the bottom layer. The controller reads out the corresponding current and Calculate the distance of the finger position. This 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 that are usually insulated from each other make a contact at the touch point. Because one conductive layer is connected to a 5V uniform voltage field in the Y-axis direction, the voltage of the detection layer changes from zero to zero. Non-zero, after the controller detects this is turned on, 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 can be obtained. It is the most basic principle common to all resistive technology touch panels.

2.     Circuit realization

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. Use an elastic material 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 resistor (R1) is connected to the positive reference voltage (VREF), and the lower resistor (R2) is grounded. 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: 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 bias edge. The resistance between the touch point and the ground edge is equivalent to the lower resistance in the voltage divider. Therefore, the voltage measured on the unbiased layer is proportional to the distance from the touch point to the ground edge.

3.  Advantage and disadvantage

a. Advantage

The advantage of the resistive touch screen is that its screen and control system are relatively cheap, and the response sensitivity is very good, and whether it is a four-wire resistive touch screen or a five-wire resistive touch screen, they are a working environment that is completely isolated from the outside world and are not afraid of dust and Water vapor can adapt to various harsh environments. It can be touched with any object and has good stability.

The advantages of resistive touch screens can be classified as:

1). RTP has high accuracy, which can reach the level of pixels, and the maximum applicable resolution can reach 4096x4096.

2). RTP is not affected by dust, moisture and oil, and can be used in a lower or higher temperature environment.

3). RTP uses pressure sensing, which can be touched with any object, even with gloves, and can be used for handwriting recognition.

4). RTP are relatively inexpensive due to mature technology and low threshold.

b. Disadvantage

The disadvantage is that the outer film of the RTP is easily scratched and the touch screen is unusable. The multi-layer structure will cause a lot of light loss. For handheld devices, it is usually necessary to increase the backlight source to compensate for the poor light transmission problem, but this also will increase battery consumption.

The disadvantages of RTP can be classified as:

1). RTP can be designed to be multi-touch, but when two points are pressed at the same time, the pressure on the screen becomes unbalanced, resulting in errors in touch, so the realization of multi-touch is more difficult.

2). RTP are more likely to damage the touch parts of the screen due to scratches.