Dialogue between people and computers--mutual capacitive touch technology

Since the advent of computers, people have been thinking about how to achieve a more effective way to communicate between people and computers, which is the so-called human-computer interaction technology. Capacitive touch technology, especially mutual capacitance technology, has greatly improved the efficiency and convenience of communication between people and computers due to its direct, efficient, accurate, smooth and fashionable characteristics. In the future, it will definitely replace the mouse and keyboard and become the mainstream of future consumption.

Projected capacitive screen touch detection principle

Projected capacitive screens can be divided into two types: self-capacitive screens and mutual-capacitive screens. ITO (a transparent conductive material) is used to make horizontal and vertical electrode arrays on the glass surface. These horizontal and vertical electrodes form capacitors with the ground respectively. This capacitance is usually called self-capacitance, that is, the capacitance of the electrode to the ground. When a 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.

During touch detection, the self-capacitive screen detects the horizontal and vertical electrode arrays in turn, determines the horizontal coordinates and vertical coordinates according to the changes in capacitance before and after the touch, and then combines them into the plane touch coordinates. The self-capacitive scanning method is equivalent to projecting the touch point 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 point.

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 touch points 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 respectively, and 4 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-point touch.

The mutual capacitance screen also uses ITO to make horizontal 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 sets of electrodes intersect, that is, the two sets of electrodes constitute 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 the two electrodes. When detecting the size of the mutual capacitance, the horizontal electrodes send out excitation signals in turn, and all the vertical electrodes receive the signals at the same time, so that the capacitance value of all the intersection points of the horizontal and vertical electrodes can be obtained, that is, the capacitance size of the two-dimensional plane of the entire 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 true coordinates of each touch point can be calculated.

Figure 1. The mechanism of self-capacitance ghosting.