Classification and Principle of Touch Screen

Typically, the touch screen system consists of two parts: the touch detection sensor and the touch screen controller. The former acquires the touch information of the user and transmits it to the controller. The latter processes the received information to obtain the touch position of the user and sends the location information to the host of the upper layer, and receives the control command sent by the host carried out.
The main classification of the touch screen
From the technical principle of distinction, touch screen can be divided into four basic categories: infrared technology touch screen, surface acoustic wave touch screen, resistive touch screen, capacitive touch screen. The following four touch screen technology will be a brief introduction.
1, infrared technology touch screen
The touch screen is composed of an infrared emitting and receiving device mounted on the outer frame of the touch screen. The emitter device forms an infrared detection network on the surface of the screen, and any object can change the infrared of the contact to achieve touch detection. Infrared touch screen from current, voltage and electrostatic interference, suitable for poor working conditions, low prices, easy installation, fast response. Infrared is now widely used in the application, are generally used for large equipment, such as TV host touch large TV, life in general, high accuracy, support more, the best light transmission, up to 100%.
2, the surface acoustic wave touch screen
The surface acoustic wave is a mechanical wave propagating along the surface of the medium. Such touch screen by the touch screen, sound wave generator, reflector and acoustic receiver components. Wherein the sound wave generator generates a high frequency sound wave across the screen surface, and when the finger touches, the sound waves on the electric shock are prevented, and the acoustic wave receiver thus determines the coordinate position. Surface acoustic wave touch screen from the temperature, humidity and other environmental factors, high resolution, excellent scratch resistance, long life, good light transmission, no drift, the surface is not afraid of planning, the disadvantage is afraid of water and oil , Dirty to maintain.
3, resistive touch screen
Resistive touch screen is a multi-layer composite film that matches the surface of the display. The structure of a layer of glass as a base layer, the surface coated with a layer of transparent conductive layer (ITO, indium oxide), the upper layer and then covered with a layer of scratch-resistant plastic layer, its inner surface is also coated with a layer of ITO, The touch screen consists of two 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, usually between the two layers of conductive layers, which are small (less than one thousandth of an inch ) Of the transparent isolation points to separate them. When the pressure on the surface of the touch screen (such as by pressing the pen or finger) is large enough, there is a contact between the top and bottom layers. All resistive touch screens use a voltage divider principle to produce a voltage representative of the X and Y coordinates.
In order to measure a coordinate in a specific direction on a resistive touch screen, it is necessary to bias a resistive layer by connecting it to VREF and the other to the 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 allow 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 edge is equivalent to the resistance below the voltage divider. Thus, the voltage measured on the unbiased layer is proportional to the distance between the touch point and the ground edge. Resistive touch screen has the advantage of its screen and control systems are relatively cheap, the reaction sensitivity is also very good, and whether it is four-wire resistive touch screen or five-wire resistive touch screen, they are a completely isolated working environment, not afraid of dust And water vapor, can adapt to a variety of harsh environments. It can use any object to touch, stable performance is better. The disadvantage is that the outer layer of the resistive touch screen is easily scratched to cause the touch screen to be unusable, and the multilayer structure can cause significant light loss. For handheld devices, it is often necessary to increase the backlight to compensate for poor light transmission problems. Will increase the battery consumption.
4, capacitive touch screen
Capacitive touch screen structure is mainly coated with a layer of transparent conductive layer (ITO), and then covered with a protective glass in the outer layer, double glass structure can more effectively protect the conductor layer and the sensor.
According to the different working principle, capacitive touch screen can be roughly divided into surface capacitive touch technology (surface capacitive touch) and projection capacitive touch technology (projected capacitive touch).
Surface Capacitance Touch Technology (SCT):
The SCT panel is a distributed ITO layer with four leads each connected to the SCT controller. SCT controller internal drive circuit to charge the panel to form a uniform electric field, when the finger touches, the electric field changes, causing a trace of current flow. At this point the detection circuit within the IC were analyzed four lines of current, and according to the amount of data points to calculate the X, Y coordinates. In order to overcome the interference, you can use the hardware filter or software filter on the calculated coordinates of the processing.
Projected Capacitive Touch Technology (PCT):
PCT is built on the concept of matrix. In the touch screen production section, IT panel ITO through etching to produce a specific pattern to improve the signal to noise ratio of each contact to enhance the recognition accuracy. According to the scanning method, generally can be divided into self-capacitance and mutual capacitance two:
Self-capacitance, usually refers to the capacitance of the scanning electrode and ground. In the glass surface, with ITO made horizontal and vertical scanning electrodes, these electrodes and the ground between the two poles constitute a capacitor, when the human finger touch, the body will be generated in parallel to the capacitor capacitance caused by changes in capacitance, capacitance control The unit coordinates the coordinates of the contact by detecting this change.
The mutual capacitance generally refers to the coupling capacitance formed between the lateral and longitudinal ITO electrodes of the glass surface. The mutual capacitance detection method is to determine the position of the touch point by detecting the capacitance change at each intersection. When the human body touches, the electric field of the electrode is affected by the human body and decreases, thus affecting the size of the coupling capacitor. The capacitance controller can detect changes in capacitance values, and by processing these changes, accurate coordinate information can be obtained. Because the mutual capacitance is scanned by detecting the change in the capacitance value at each intersection, it requires a longer scan time and requires X * Y data to be detected, but it is precisely because of this detection that it can achieve true multipoint Touch detection.
Comparison of the above several types of touch screen technology can be found for the general public information and industrial control field, the use of touch screen size is relatively large, the stability of the higher requirements, so the infrared touch screen and surface acoustic wave touch screen due to its strong anti- Stability, with a larger market. And for portable devices (mobile phones, MP4 and other entertainment devices), resistive and capacitive touch screen is a better choice.
Today, touch technology has been popular in the mobile phone market, touch screen mobile phone has become the focus of competition. In the development of this touch-screen mobile phone, due to capacitive touch screen touch can sense, easy to use. And fingers and touch screen contact almost no wear, stable performance, the mechanical test life of up to 30 years. With the improvement of manufacturing technology, as well as the development of capacitive touch technology, capacitive touch screen module prices gradually decline, resistive touch screen because of its long-term people criticized the life and light transmission and other issues, will gradually be capacitive touch screen Replaced.