Influence of water on capacitive touch screen and waterproof strategy in design
Time:2022-07-12
Views:1940
1、 Introduction
Waterproof performance is a landmark index to measure the design performance of multi-point capacitive touch screen. It seems that the multi-point capacitive touch screen using mutual capacitance scanning has natural waterproof ability, and it does not pose a design challenge. But why do you say that? Because using the touch screen with self capacitance scanning, the signal change direction produced by water drops and finger touch is the same, it takes a lot of trouble to distinguish water drops from finger touch. The signal change direction of the touch screen water droplets scanned by mutual capacitance and the finger touch is exactly the opposite, because the finger touch reduces the mutual capacitance, but the water droplets increase the mutual capacitance. This gives people the feeling that the multi-point capacitive touch screen using mutual capacitance scanning has natural waterproof ability without special measures. The real situation is not so simple. When the water drops onto the mutual capacitance screen, it does not and does not produce false triggering, but when the water drops are wiped off, it will not work if you touch the original place with your fingers, or false touch signals will appear from time to time when there is no finger touch. When you are lucky, you can detect finger touch after a period of time. In most cases, it is difficult to restore the original touch sensitivity. We know that such a situation is not allowed for a qualified product, and we will not rely on good luck. Therefore, how to solve the problem of finger touch failure and false trigger caused by water is a challenge in the design of multi-point capacitive touch screen. In fact, the problem of touch failure caused by water not only refers to water droplets, but also includes water films and large pieces of water.
2、 Influence of water on self capacitance and mutual capacitance
1. First, let‘s understand the self capacitance and mutual capacitance of the touch screen.
On the touch screen, self capacitance or mutual capacitance can be generated between an induction block (assumed to be a) and its adjacent induction block (assumed to be b). If the high-frequency AC signal TX is applied to the induction block a, the induction block B is grounded. The coupling between the induction blocks a and B is self capacitance coupling, and the size of the self capacitance is CS. If the high-frequency AC signal TX is applied to the induction block a, the induction block B receives the signal RX from the induction block a as the induction electrode. At this time, the coupling between the induction block a and B is mutual capacitance coupling, and the mutual capacitance is cm. The size of self capacitance CS and mutual capacitance cm are proportional to the boundary length between sensing blocks a and B and the dielectric constant of the medium, and are in the opposite direction with the distance between sensing blocks a and B. Generally speaking, the potential on the induction block a is higher than that on the induction block B, so the direction of the electric field is always from the induction block a to the induction block B.
2. Next, look at the influence of finger touch on self capacitance and mutual capacitance.
In self capacitive coupling, since the B sensing block is grounded, the a sensing block is both the transmitter of the excitation signal and the receiver of the measurement signal; In mutual capacitance coupling, a and B sensing blocks are the transmitter of excitation signal and the receiver of measurement signal respectively. When the finger touches the cover on the sensing block, because the finger and the human body can be considered as conductors, the capacitance cbody between the human body and the earth and the capacitance cboard between the equipment ground and the earth are large enough to have only a very small capacitive reactance to the high-frequency AC excitation signal, it can be considered that the potential of the finger in the touch system is approximately the potential GND of the equipment ground. Refer to figures 3 and 4. Thus, in the electric field coupling of self capacitance, finger touch is equivalent to a capacitor cft/fr connected in parallel on the self capacitance CS. Therefore, finger touch increases the self capacitance. In the electric field coupling of mutual capacitance, since the potential of the finger in the touch system is approximately the potential GND of the device ground, the potential of the induction block a and B is higher than that of the finger, and there will be electrical coupling between the induction block a and B and the finger, which means that there will be coupling capacitors CFT and CFR between the induction block a and B and the finger. When the high-frequency AC excitation signal is applied to the induction block a, part of the current from the mutual capacitance cm to Rx is shunted by the finger capacitance CFT and CFR, and the current of Rx at the receiving end will be smaller than the original, so it is equivalent to the reduction of the mutual capacitance cm. Therefore, we usually say that the finger touch reduces the mutual capacitance.
3. Let‘s look at the influence of water on self capacitance and mutual capacitance.
When water falls on the touch screen, because it is conductive, it can be considered as a conductor, which will also change the electric field coupling between the sensing block a and the sensing block B. However, the surface area of water is much smaller than that of human body. The capacitance cbody between human body and the earth no longer exists, and the capacitance between water droplets and the earth is very small, as small as almost zero. The potential of water droplets in the touch system can no longer be approximately the potential GND of the equipment ground, and its potential is determined by the potential of the sensing block a and the sensing block B. Based on common sense, its potential should be between the potential of induction block a and that of induction block B. Referring to figure 5 and Figure 6, in the electric field coupling of self capacitance, water droplets form capacitors cwt/wr and CWS with induction block a and induction block B respectively. It can be considered that these two capacitors are connected in series and then connected in parallel at both ends of CS, so for the screen of self capacitance, water droplets will increase the self capacitance like fingers, but since the two capacitors generated by water droplets are connected in series and then connected in parallel at both ends of CS, the value of series capacitance will be less than that of any capacitance in the series capacitance, For a water drop with the size and diameter of a finger, the signal change generated by it will certainly be less than that generated by finger touch, but they are in the same direction. Similarly, in the electric field coupling of mutual capacitance, water droplets form capacitors CWT and CWR with sensing block a and sensing block B respectively. It can be considered that these two capacitors are connected in series and then connected in parallel at both ends of CM. Therefore, for the screen of mutual capacitance, water droplets do not reduce the mutual capacitance like touching with fingers, but increase the mutual capacitance! Similarly, since the two capacitors generated by the water drop are connected in series and then connected in parallel at both ends of CM, the value of the series capacitance will be less than any capacitance in the series capacitance. For a water drop with the size and diameter of the finger, the signal change generated by it will certainly be less than the signal change generated by the finger touch. Usually, it is 1/4 the size of the finger touch signal, but it is in the opposite direction with the signal change generated by the finger touch.
3、 Alternating scanning to realize waterproof of multi-point capacitive touch screen
How to eliminate the false triggering of mutual capacitance screen after water droplets are erased is a challenge for the waterproof design of mutual capacitance screen. To solve this problem, we must first know when the water droplets began to appear on the capacitive screen. There may be many reasons for the basic line value to shift in the opposite direction of the AD conversion value when the finger touches. For example, changes in ambient temperature (high and low temperature test) and humidity; Electrostatic interference; When the touch screen system is started, the finger is just pressed on the touch screen, and the finger is removed after starting, etc. How to distinguish the basic line value change caused by water droplets on the mutual capacitance screen from the basic line value change caused by other conditions is the key to know that there are water droplets on the mutual capacitance screen at present.
In fact, we have previously discussed the different behaviors of water droplets on the mutual capacitance screen and the self capacitance screen. Water droplets on the self capacitance screen make the AD conversion value change in the same direction as the AD conversion value when touching with fingers, while water droplets on the mutual capacitance screen make the AD conversion value change in the opposite direction when touching with fingers. This feature itself provides us with an effective method to identify water droplets on the capacitive screen. However, it requires that the touch screen system can not only scan the mutual capacitance of the same capacitive screen, but also scan the self capacitance. Through alternating scanning, the signal produced by water droplets can be detected in the signal changes caused by various factors. Once the signal generated by the water drop is detected, the basic line value will remain unchanged until the water is erased, and the basic line value will not be updated according to the previous rules. When there is water, touch the touch screen with your fingers. Because the signal change generated by your fingers is much larger than that generated by water in the opposite direction, the touch of your fingers on the touch screen with water droplets will not be greatly affected. So as to realize the effective waterproof of mutual capacitance touch screen.
4、 Concluding remarks
The different behaviors caused by the influence of water on self capacitance and mutual capacitance become the main characteristics of water on capacitive touch screen. Making full use of this feature and using alternating scanning makes the waterproof design of mutual capacitance touch screen possible.
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