Water will increase the capacitance, which will affect the capacitive touch screen to judge the signal, easy to lead to touch response speed drop, drift, ghost spots, in most cases, it is difficult to recover the original touch sensitivity. We know that a qualified product does not allow such a situation to happen, let alone allow luck. Therefore, how to solve the problem of finger touch failure and false trigger caused by water is a big challenge in the design of multi-point capacitive touch screen. When water falls on the touch screen, it changes the electric field coupling between the two sensing modules because the water conducts electricity. For a drop of water the diameter of a finger, the change in signal produced by it must be smaller than that produced by the touch of a finger. It is usually 1/4 the size of the finger touch signal, but it goes in the opposite direction of the finger touch signal.
Because finger touches reduce mutual capacitance, drops increase it. This makes it easy for designers to create the illusion that dripping water on the compatible screen won't be mistaken for finger touch, meaning the product won't be triggered by mistake. When the baseline present value offset due to water approaches or exceeds the set hand value threshold, a spurious trigger occurs at the instant of wiping the water. In many cases, this error trigger is not easily restored to normal because the offset baseline value is not easily updated back to the normal baseline value. Fake triggers can last a long time and even require a touchscreen system reset.
How to eliminate the false trigger caused by water drop wiping is a major challenge in the design of waterproof mutual capacitance screen.
To solve this problem, it is important to know when the droplets begin to arrive on the capacitor board. There are several possible reasons why the baseline value tilts in the opposite direction of the AD conversion value when the finger touches it. For example, changes in ambient temperature (high and low temperature test), humidity; Static interference; When the touch screen system starts up, the finger is just pressed on the touch screen, and when it starts up, the finger is removed, and so on.
How to distinguish the basic line value change caused by water droplets on the mutual screen from that caused by other conditions is the key to know that there are water droplets on the mutual screen.
The main feature of the capacitive touch screen is the different behavior caused by the effect of water on the self-capacitive and mutual capacitive screens. Taking full advantage of this feature and using alternate scanning makes it possible to design waterproof capacitive touch screens.
It requires the touch screen to be capable of both mutual capacitance scanning and self capacitance scanning. By alternating scanning, the droplet generated signals are detected in the signal changes caused by various factors. Once the signal generated by the drop is detected, the baseline value will remain unchanged until it knows who has been erased, and the baseline value will be updated in good faith according to the previous rules.