Capacitive touch screen classification

The types of capacitive touch screens are divided into surface capacitive touch screens and projection capacitive touch screens.

Surface-type capacitive touch screen

Often used is the surface capacitive touch screen, which is simple in working principle, cheap, simple to design the circuit, but difficult to achieve multi-touch. 

Projection-type capacitive touch scree.

The projection capacitive touch screen has the function of multiple finger touch control. These two capacitive touch screens have the advantages of high light transmittance, fast reaction speed, long life, and so on. The disadvantages are: with the change of temperature and humidity, the capacitance value will change, resulting in poor work stability, often drift phenomena, the need to frequently touch and locate the screen, and not wearing ordinary gloves.
Projection electrical capacitor screen can be divided into two types from the capacitor screen and mutual capacitor screen, more common mutual capacitor screen, for example, internal composed of drive electrode and receiving electrode, drive electrode from low voltage high frequency signal to the receiving electrode to form a stable current, when the human body contact with the capacitor screen, because the body ground, fingers and capacitor screen to form an equivalent capacitor, and high frequency signal can flow through the equivalent capacitor into the ground, so the receiving charge decreases, and when the finger closer to the transmitting end, the charge decreases more obvious, finally according to the electricity received by the receiving end Flow intensity to determine the point touched.
On the glass surface with ITO made into an array of horizontal and vertical electrodes, these transverse and longitudinal electrodes, respectively, constitute the capacitor. This capacitor is commonly known as the self-capacitor, that is, the capacitor of the electrode to the ground. When the finger touches the capacitive screen, the capacitance of the finger will be superimposed on the screen capacitance, increasing the screen capacitance.
During touch detection, the self-capacitance screen detects the transverse and longitudinal electrode arrays in turn, determines the transverse and longitudinal coordinates, respectively, according to the changes in capacitance before and after touch, and then combines them into planar touch coordinates. The scanning mode of the self-contained capacitor is equivalent to the touch The touch points on the screen are projected in the X axis and Y axis directions, respectively, and then the coordinates are calculated in the X axis and Y axis directions, respectively, and finally combined into the coordinates of the touch points.
If it is a single point of touch, the projection is unique in the X axis and Y axis directions, 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, 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 spots". Therefore, the self-capacitive screen cannot achieve true multi-touch.
The mutual capacitance screen is also in the field. The glass surface is made of ITO to make horizontal and longitudinal electrodes. The difference between it and the self-capacitor screen is that the capacitor will form where the two groups of electrodes cross; that is, the two sets of electrodes constitute the two poles of the capacitor. When the finger touches the capacitive screen, it affects the coupling between the two electrodes near the touch point, thus changing the capacitance between the two electrodes. When the mutual capacitance is detected, the transverse electrodes send the excitation signal in turn, and all the longitudinal electrodes receive the signal at the same time. In this way, we can get the capacitance value size of all transverse and longitudinal electrodes at their meeting point, that is, the capacitance size of the two-dimensional plane of the whole touch screen. According to the touch screen 2D capacitor change quantity data, you can calculate the coordinates of each touch point. Therefore, even with multiple touch points on the screen, the true coordinates of each touch point can be calculated.
The advantage of the mutual capacitive screen is that it has less wiring and can identify and distinguish the differences 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 induction scheme of the mutual capacitor screen has the advantages of fast speed and low power consumption because it can measure all the nodes on a drive line at the same time, reducing the number of acquisition cycles by 50%. This two-electrode-type structure has the function of self-shielding the external noise, and the signal stability can be improved at a certain power level.
In any case, the touch position is determined by measuring the amount of signal change between the X and Y electrodes, and these signals are then processed using mathematical algorithms to determine the X and Y coordinates of the touch point.