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Principle Of Capacitive Touch Panel
Aug 15, 2018

Capacitive touch screens are widely used for people's friendly human-machine interface. Its low power consumption and long life, as well as smooth operation performance, make capacitive touch screens popular in the market. Various capacitive touch screen products are available. As the process progresses and batches, its cost continues to drop, gradually replacing the resistive touch screen.

The surface capacitive touch screen uses only a single layer of ITO. When a finger touches the surface of the screen, a certain amount of charge is transferred to the human body. In order to restore these charge losses, the charge is replenished from the four corners of the screen, and the amount of charge replenished in each direction is proportional to the distance of the touch point, from which we can derive the position of the touch point.

Surface Capacitive ITO coatings typically require linearized metal electrodes on the perimeter of the screen to reduce the effect of corner/edge effects on the electric field. Sometimes there is an ITO shield under the ITO coating to block noise. The surface capacitive touch screen needs to be calibrated at least once to be used.

Compared to surface capacitive touch screens, inductive capacitive touch screens can penetrate thicker overlays without the need for correction. Inductive Capacitance etches different ITO modules on two ITO coatings, taking into account the total impedance of the module, the impedance of the connection between the modules, and the parasitic capacitance generated at the intersection of the two ITO modules. Moreover, in order to detect finger touch, the area of the ITO module should be smaller than the finger area, and when a diamond pattern is used, the diagonal length is usually controlled to 4 to 6 mm.

The touch position detected by the inductive capacitive touch screen corresponds to the intersection point at which the maximum capacitance change value is sensed. For the X axis or the Y axis, the signal amount of different ITO modules is weighted and averaged to obtain the position amount, and the system is then under the touch screen. Touch points or tracks are displayed on the LCD.

When there are two finger touches (two points in red), there will be two maximum values on each axis. There are two possible combinations, and the system cannot accurately determine the judgment. This is what we usually call the mirror. Point (two points in blue).

In addition, the underside of the touch screen is the LCD display, the surface of which is also conductive, which will produce parasitic capacitance with the ITO-coated ITO module. We usually need to keep a certain air layer between the two layers. Reduce the effects of parasitic capacitance.

In the design of touch screen products, there is a trade-off between performance and cost. Resistive touch screens are less expensive, competitive, and have limitations in performance and application.

1. The capacitive touch screen requires only touch and does not require pressure to generate a signal.

2. Capacitive touch screens require only one or no correction after production, while resistor technology requires routine calibration.

3. The life of the capacitor solution will be longer because the components in the capacitive touch screen do not require any movement. In the resistive touch screen, the upper ITO film needs to be thin enough to be elastic so as to be bent downward to contact the underlying ITO film.

4. Capacitor technology is superior to resistor technology in terms of optical loss and system power consumption.

5. Whether to choose capacitive or resistive technology depends primarily on the object that touches the screen. If it is a finger touch, a capacitive touch screen is a good choice. If you need a stylus, whether it's plastic or metal, a resistive touch screen can do the job. A capacitive touch screen can also use a stylus, but a special stylus is required to match.

6. Surface capacitive type can be used for large-size touch screen, and the phase cost is also low, but gesture recognition cannot be supported at present; inductive capacitive type is mainly used for small and medium size touch screens, and can support gesture recognition.

7. Capacitive technology is resistant to wear and has a long service life. The maintenance cost is low when the user uses it, so the overall operating cost of the manufacturer can be further reduced.

The development trend of capacitive touch screen

Capacitive touch screens have been applied to iPhones and other handheld devices. Positioning single-point tracks/simulating mouse double-clicks is its basic function, and the identification and application of multi-finger gesture operations has become a hot spot in the current market. In portable applications, the user can only use the other hand to hold the device, so it is especially important to recognize multi-finger grab/panning, stretching/compression, rotation, page turning and other gestures. The PSoC inductive capacitive touch screen enables multi-point detection to support two-finger gesture recognition. It is foreseeable that capacitive touch screens that support gesture recognition will shine in the market.

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