Principle of capacitive screen

There are two types of capacitive touch screens: surface capacitive touch screens and projected capacitive touch screens.

Surface Capacitive Touch Screen

The commonly used surface capacitive touch screen has simple working principle, low price and simple circuit design, but it is difficult to realize multi-touch.

Projected capacitive touch screen

Projected capacitive touch screen has the function of multi-finger touch. Both of these capacitive touch screens have the advantages of high light transmittance, fast response speed, and long life. The disadvantage is: with the change of temperature and humidity, the capacitance value will change, resulting in poor working stability and often drift phenomenon. The screen needs to be calibrated frequently, and ordinary gloves cannot be worn for touch positioning.

Projected capacitive screens can be divided into two types: self-capacitance screens and mutual-capacitance screens. The more common mutual-capacitance screen is an example. The interior is composed of driving electrodes and receiving electrodes. The driving electrodes send low-voltage high-frequency signals to the receiving electrodes to form a stable When the human body touches the capacitive screen, due to the grounding of the human body, the finger and the capacitive screen form an equivalent capacitance, and the high-frequency signal can flow into the ground wire through this equivalent capacitance, so that the amount of charge received by the receiving end is reduced. When the finger is closer to the transmitting end, the charge decreases more obviously, and finally the touched point is determined according to the current intensity received by the receiving end.

The horizontal and vertical electrode arrays are made of ITO on the glass surface. These horizontal and vertical electrodes respectively form capacitors with the ground. When the finger touches the capacitive screen, the capacitance of the finger will be superimposed on the capacitance of the screen body, which increases the capacitance of the screen body.

During touch detection, the self-capacitance screen detects the horizontal and vertical electrode arrays in turn, determines the horizontal and vertical coordinates respectively according to the change of capacitance before and after the touch, and then combines them into a plane touch coordinate. The self-capacitance scanning method is equivalent to projecting the touch points on the touch screen to the X-axis and Y-axis directions respectively, and then calculating the coordinates in the X-axis and Y-axis directions respectively, and finally combining them into the coordinates of the touch points.

If it is a single-point touch, the projections in the X-axis and Y-axis directions are unique, and the combined coordinates are also unique. If there are two touches on the touch screen and the two points are not in the same X direction or the same Y direction, then There are two projections in the X and Y directions respectively, then 4 coordinates are combined. Obviously, only two coordinates are real, and the other two are commonly known as "ghost points". Therefore, self-capacitance screens cannot achieve true multi-touch.

The mutual capacitance screen also uses ITO to make horizontal electrodes and vertical electrodes on the glass surface. The difference between it and the self-capacitance screen is that a capacitor will be formed where the two groups of electrodes intersect, that is, the two groups of electrodes constitute the two poles of the capacitor respectively. When a finger touches the capacitive screen, the coupling between the two electrodes near the touch point is affected, thereby changing the capacitance between the two electrodes. When the mutual capacitance is detected, the horizontal electrodes send out excitation signals in turn, and all the vertical electrodes receive signals simultaneously, so that the capacitance values ​​of all the intersections of the horizontal and vertical electrodes can be obtained, that is, the capacitance of the entire two-dimensional plane of the touch screen. According to the two-dimensional capacitance change data of the touch screen, the coordinates of each touch point can be calculated. Therefore, even if there are multiple touch points on the screen, the real coordinates of each touch point can be calculated.

The advantage of the mutual capacitive screen is that there are fewer wirings, and it can identify and distinguish the difference between multiple contacts at the same time. The self capacitive screen can also sense multiple contacts, but because the signal itself is fuzzy, it cannot be distinguished. In addition, the sensing scheme of the mutual capacitive screen has the advantages of high speed and low power consumption, because it can measure all nodes on a driving line at the same time, so it can reduce the number of acquisition cycles by 50%. This two-electrode structure has the function of self-shielding external noise, which can improve the signal stability at a certain power level.

In any case, the touch location is determined by measuring the distribution of signal changes between the X and Y electrodes, and then using a mathematical algorithm to process these changed signal levels to determine the XY coordinates of the touch point.