Parameters of capacitive touch screen
1. Point reporting rate: if it is too fast, the CPU cannot respond in time, which may cause point dropping and easy line breaking
2. Accuracy: 99% accuracy
3. Sensitivity: less than two ounces of force can be sensed, and less than 3MS of rapid response
4. Resolution: it is related to the number of channels of X and Y axes. The more channels, the higher the resolution. The higher the resolution, the higher the touch accuracy
5. Reporting range: This is generally the same as the resolution setting of LCD.
6. Linearity: linearity defines the proximity between the actual output of the device and the ideal straight line in the whole operating range.
7. Anti interference ability: high temperature; Electromagnetic field; Oil stain, water; static electricity.
8. Hardness, compressive capacity: completely scratch resistant glass material (Mohs hardness 7h), not easy to be scratched and worn by sharp objects.
9. Power consumption:
10. Clarity: the clarity will be affected if the glass material is not good enough.
11. Point reporting rate: the number of contact information reported to the host per second.
Classification of capacitive touch screen
Capacitive touch screen can be divided into surface capacitive touch screen and projection capacitive touch screen.
Surface capacitive touch screen
Surface capacitive touch screen is commonly used. It has the advantages of simple working principle, low price and simple circuit design, but it is difficult to realize multi touch.
projected capacitive touch
The projection capacitive touch screen has the function of multi finger touch. Both capacitive touch screens have the advantages of high light transmittance, fast response speed and long service life. The disadvantages are that the capacitance value will change with the change of temperature and humidity, resulting in poor working stability and frequent drift. The screen needs to be calibrated frequently, and ordinary gloves cannot be worn for touch positioning.
Projection capacitance screen can be divided into self capacitance screen and mutual capacitance screen. For example, the more common mutual capacitance screen is composed of driving electrode and receiving electrode. The driving electrode sends low-voltage and high-frequency signals to the receiving electrode to form a stable current. When the human body contacts the capacitance screen, due to the grounding of the human body, the finger and the capacitance screen form an equivalent capacitance, The high-frequency signal can flow into the ground through this equivalent capacitance. In this way, the amount of charge received by the receiving end is reduced. When the finger is closer to the transmitting end, the charge reduction is more obvious. Finally, the touch point is determined according to the current intensity received by the receiving end.
Precautions for capacitive touch screen
When the traditional resistive touch panel senses a finger or a stylus, the flexible transparent material on the top layer is pressed down and contacts the conductive material layer below; The projection capacitor screen has no movable parts. In fact, the projected capacitive sensing hardware consists of a glass top layer, followed by X and y-axis components, and an indium tin oxide (ITO) insulating layer covering the glass substrate. Some sensor suppliers will make a single-layer sensor with embedded X and y-axis sensors and small bridging components in a single-layer ITO. When fingers or other conductive objects approach the screen, a capacitance will be generated between the sensor and fingers. Compared with the system, this capacitance is quite small, but it can be measured by a variety of techniques.
One technology is to use truetouch components, including rapidly changing the capacitance and using a discharge resistance to measure the discharge time. This all glass touch surface gives users a smooth touch. Terminal product manufacturers also prefer glass screens, because glass materials will give terminal products a sense of industrial design with beautiful lines and provide high-quality capacitive signals for measurement touch. Finally, it is important not only to consider the appearance of the touch panel, but also to understand its operation mode. In order to design touch screen products with excellent performance, the following parameters must be paid attention to.
Accuracy: accuracy can be defined as the maximum positioning error in a predefined touch screen area, in the unit of the linear distance between the actual position of the finger and the measured position. When measuring accuracy, an analog or mechanical finger is used. Place the finger at an accurate position on the panel, and then compare the actual position of the finger with the measured position. Accuracy is very important. Users want the system to find the finger position accurately. One of the most criticized disadvantages of resistive touch screen is low accuracy, and the accuracy will gradually weaken over time. The accuracy of capacitive touch screens has created many new applications, such as virtual keyboards and handwriting recognition without a stylus. Figure 1 shows the data of a touch panel with incomplete structure, showing that the finger position moves, but in fact, the simulated finger moves in a straight line.
Finger spacing: finger spacing is defined as the shortest distance between the center points of two fingers on the screen when the touch screen controller measures the position of two fingers. The finger spacing measurement method is to place two simulated or mechanical fingers on the panel, and then gradually shorten the distance between the two fingers until the system measures that the two fingers are one finger. Some touch screen suppliers’ finger spacing refers to the distance from edge to edge, while others refer to the distance between center points. The 10mm finger spacing of 10mm mechanical fingers indicates that multiple fingers touch the screen, or the distance between fingers is 10mm. The actual situation depends on the specification definition of the touch controller. Without good finger spacing, it is impossible to design a multi touch solution. For a simulated keyboard, finger spacing is particularly important because when using a simulated keyboard, the finger spacing on the screen is usually very short.