Capacitive screen Principle

Capacitive Screen 
The capacitive touch screen is a layer-4 composite glass screen, as shown in. The inner surface and interlayer of the glass screen are coated with an ITO conductive layer, and the outermost layer is only a 0.0015mm thick glass protective layer. The inner ITO layer is used as the shielding layer to ensure a good working environment. The inner layer Ito coating is used as the working layer for detection and positioning, and four electrodes are extracted on four edges or four edges.

The initial idea of the basic working principle of the capacitive screen is that a person is an imaginary Ground Object (zero potential body) and a very low voltage is given to the working face. When the user touches the screen, the finger absorbs a very small current which flows out from the four edges or four edges of the touch screen, and theoretically the current flowing through these four electrodes is proportional to the distance from the finger to the four corners, the Controller accurately calculates the ratio of the four currents to obtain the position of the touch point.

This idea is good. However, according to this idea, we have encountered insurmountable obstacles: the current transparent conductive material Ito-oxide metal is very fragile and will be damaged after a few clicks, it cannot be directly used as a work layer. The problem of materials was hard to solve at the moment, so we had to seek perfection by adding a very thin hard glass to the outside.

This layer of glass is obviously non-conductive, and DC conductive is not good. Instead, the high-frequency AC signal is switched, and the coupling capacitor formed by the working face (through thin glass) is used to suck a AC current, this is the origin of the capacitive screen "Capacitor" Name: working by coupling capacitor.

The problem is solved, but the cost is very high: first, "drift", because the coupling capacitor method is unstable, it is directly affected by the temperature, humidity, moisture of the fingers, body weight, and dryness of the ground. It is also greatly affected by the interference of large external objects, resulting in unstable results, these are directly against the basic requirements of the absolute coordinate system, such as the touch screen, and inevitably lead to drift. Some capacitive touch screens want to solve the Drift Problem Through the 25-point calibration method or even the 96-point calibration method, in fact, it is impossible. The drift is determined by the way the capacitor works, even in the Controller's MicrocontrollerProgramDynamic computation and empirical data table can only be a temporary cure. The multi-point calibration method was first used by the large screen projection touchpad to eliminate linear Distortion corresponding to coordinates, and the linear Distortion of capacitive touch screen is also very severe, this is mainly because the calculation of the capacitive screen is based on the ideal state in which the four electrical flows are proportional to the distance from the touch point to the Four electrodes. In reality, due to the influence of the Environmental capacitance, line parasitic capacitance, and the use of different people, this proportional relationship cannot be completely linear. The multi-point calibration method can only solve the linear problem of local distribution and cannot solve the overall drift.

Another cost of the capacitor method is: the outermost layer of the extremely thin glass, under normal circumstances, anti-scratch performance is very good, but the process requires manufacturing in a vacuum, because it is afraid of hydrogen, even a little hydrogen will be combined into a brittle broken glass, and a small hole will be made upon a slight tap, which is terrible for capacitive touch screens: the area with a diameter of 5cm cannot be used. The actual vacuum is impossible. This layer of extremely thin glass has a 5% probability of hitting a product with holes.

Capacitive touch screens have better light transmittance and clarity than four-line resistive screens, especially some new products.
Source:Http://bbs.meizu.com/thread-1104407-1-1.html