Component classification of capacitive touch screen

There are two types of capacitive touch screens: surface capacitive touch screens and projection capacitive touch screens.
Surface capacitive touch screen:
The commonly used type is surface capacitive touch screen, which has a simple working principle, low price, and simple designed circuit, but it is difficult to achieve multi touch.
Projection capacitive touch screen:
The projected capacitive touch screen has the function of multi finger touch. Both types of capacitive touch screens have advantages such as high transmittance, fast response speed, and long lifespan. However, the disadvantage is that the capacitance value changes with temperature and humidity, resulting in poor stability and frequent drift. It is necessary to frequently calibrate the screen and do not wear ordinary gloves for touch positioning.
Projection capacitive screens can be divided into two types: self capacitive screens and mutual capacitive screens. The more common mutual capacitive screen is an example, which is composed of a driving electrode and a receiving electrode. The driving electrode emits a low-voltage high-frequency signal and projects it to the receiving electrode to form a stable current. When the human body comes into contact with 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 line through this equivalent capacitance. In this way, the amount of charge received by the receiving end decreases. As the finger approaches the transmitting end, the charge decreases more significantly. Finally, the touched point is determined based on the intensity of the current received by the receiving end.
On the surface of glass, ITO is used to create horizontal and vertical electrode arrays, which respectively form capacitors with the ground. This capacitor is commonly referred to as self capacitance, which is the capacitance between the electrodes and the ground. When the finger touches the capacitive screen, the capacitance of the finger will be superimposed on the capacitance of the screen body, increasing the capacitance of the screen body.
In touch detection, the self capacitive screen sequentially detects the horizontal and vertical electrode arrays, determines the horizontal and vertical coordinates based on the changes in capacitance before and after touch, and then combines them into a planar touch coordinate. The scanning method of self capacitance is equivalent to projecting the touch points on the touch screen separately in the X-axis and Y-axis directions, then calculating the coordinates in the X-axis and Y-axis directions, and finally combining them into the coordinates of the touch points.
If it is a single touch, the projections in both the X and Y directions are unique, and the combined coordinates are also unique. If there are two touches on the touch screen and these two points are not in the same X or Y direction, then there are two projections in the X and Y directions, and four coordinates are combined. Obviously, only two coordinates are real, and the other two are commonly known as "ghost points". Therefore, self capacitive screens cannot achieve true multi touch.
Mutual capacitance screen is also made of ITO on the glass surface to form horizontal and vertical electrodes. The difference between it and self capacitance screen is that the intersection of the two sets of electrodes will form a capacitor, that is, these two sets of electrodes respectively form the two poles of the capacitor. When a finger touches the capacitive screen, it affects the coupling between the two electrodes near the touch point, thereby changing the capacitance between these two electrodes. When detecting the mutual capacitance, the horizontal electrodes sequentially emit excitation signals, and all vertical electrodes simultaneously receive signals. This can obtain the capacitance values at the intersection of all horizontal and vertical electrodes, that is, the capacitance size of the entire two-dimensional plane of the touch screen. Based on 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 true coordinates of each touch point can be calculated.
The advantage of mutual capacitance screen is that it has fewer wiring and can simultaneously identify and distinguish the differences between multiple contacts. Self capacitance screen can also sense multiple contacts, but due to the fuzzy signal itself, it cannot be distinguished. In addition, the induction scheme of mutual capacitance screen has the advantages of fast speed and low power consumption, as it can simultaneously measure all nodes on a driving line, thus reducing the number of acquisition cycles by 50%. This dual electrode structure has the function of self shielding external noise and can improve signal stability at a certain power level.
In any case, the touch position is determined by measuring the distribution of signal changes between the X and Y electrodes, and then using mathematical algorithms to process these changed signal levels to determine the XY coordinates of the touch point.