The Definition of Projected Capacitive Touch Screen ( PCAP Touch)

- Aug 06, 2018-

           The Definition of Projected Capacitive Touch Screen(PCAP Touch screen)


The projected capacitive touch screen (PCAP) has the function of multi-finger touch. Both capacitive touch screens have the advantages of high light transmittance, fast response speed, long life, etc. The disadvantage is that as the temperature and humidity change, the capacitance value will change, resulting in poor working stability and frequent drift. You need to proofread the screen frequently, and you can't wear ordinary gloves for touch positioning.

PCAP can be divided into two types: self-capacitive screen and mutual capacitive screen. The common mutual capacitive screen is an example. The internal part is composed of a driving electrode and a receiving electrode. The driving electrode emits a low-voltage high-frequency signal and is projected onto the receiving electrode to form a stable state. Current, when the human body touches the capacitive screen, the finger and the capacitive screen form an equivalent capacitance due to the grounding of the human body, and the high-frequency signal can flow into the ground through the equivalent capacitance, so that the amount of charge received by the receiving end is reduced. Small, and the closer the finger is to the transmitting end, the more obvious the charge reduction, and finally the point touched is determined according to the current intensity received by the receiving end.

ITO is used to form a horizontal and vertical electrode array on the surface of the glass. These lateral and longitudinal electrodes respectively form a capacitance with the ground. This capacitance is known as the self-capacitance, that is, the capacitance 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, which increases the capacitance of the screen.

In the touch detection, the self-capacitance screen sequentially detects the horizontal and vertical electrode arrays respectively, and determines the lateral coordinates and the longitudinal coordinates according to the change of the capacitance before and after the touch, and then combines them into planar touch coordinates. 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 the coordinates of the touch points.

If it is a single 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, and four coordinates are combined. Obviously, only two coordinates are true, and the other two are commonly known as "ghost points." Therefore, the self-capacitive screen cannot achieve true multi-touch.

The mutual capacitance screen (PCAP touch) is also made of ITO with lateral electrodes and vertical electrodes on the surface of the glass. The difference from the self-capacitance screen is that the capacitors are formed at the intersection of the two sets of electrodes, that is, the two sets of electrodes respectively 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 mutual capacitance, the lateral electrodes sequentially emit excitation signals, and all the longitudinal electrodes receive signals at the same time, so that the capacitance values of all the intersections of the lateral and longitudinal electrodes can be obtained, that is, the capacitance 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.

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

In any case, the touch position is determined by measuring the distribution of the amount of signal change between the X and Y electrodes, which are then processed using a mathematical algorithm to determine the XY coordinates of the touch point.

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