Water resistance is a key factor in evaluating the performance of multi-touch capacitive screens. It might seem that mutual capacitance-based touchscreens inherently have good water resistance, making it less of a design challenge. But why is that the case? When using self-capacitance scanning, both water droplets and finger touches cause similar signal changes, making it hard to distinguish between them. However, with mutual capacitance scanning, the direction of the signal change caused by a finger touch is opposite to that caused by a water droplet. Specifically, a finger touch reduces mutual capacitance, while a water droplet increases it. This natural contrast makes it easier for the system to differentiate between the two, which gives the impression that mutual capacitance screens are naturally water-resistant without special waterproofing measures. However, the reality is more complex. While water droplets on a mutual capacitance screen may not cause false triggers initially, problems often arise when the droplets are wiped away. After removal, touching the same area with a finger may fail or produce intermittent false signals. In some cases, the system may eventually detect the touch, but in most situations, the sensitivity doesn't fully return to normal. For a high-quality product, such issues are unacceptable and cannot rely on luck. Therefore, addressing touch failure due to water and preventing false triggers remains a critical challenge in multi-point capacitive touchscreen design. This issue isn’t limited to water droplets; it also includes water films and large water areas. To better understand the problem, let’s first look at how self-capacitance and mutual capacitance work in touchscreens. Self-capacitance occurs between a sensing block (say A) and its adjacent block (say B), while mutual capacitance arises when one block acts as a transmitter and the other as a receiver. When a high-frequency AC signal is applied to block A and block B is grounded, the coupling is self-capacitance, with a value of Cs. If the signal is sent from A to B, the coupling becomes mutual capacitance, with a value of Cm. The size of Cs and Cm depends on the boundary length between the blocks, the dielectric constant of the medium, and the distance between the blocks. Typically, the potential of block A is higher than that of block B, so the electric field flows from A to B. Figures 1 and 2 illustrate this concept. Next, consider how a finger touch affects these capacitances. In self-capacitance coupling, where B is grounded, A serves as both the transmitter and receiver. In mutual capacitance, A transmits and B receives. When a finger touches the screen, it acts as a conductor, creating a path to ground. As a result, the finger's potential is approximately equal to the device ground, causing an increase in self-capacitance. In mutual capacitance, the finger introduces additional coupling capacitances between A and B, reducing the mutual capacitance. This is why we say that a finger touch decreases mutual capacitance. Now, let’s examine how water affects self and mutual capacitance. Water is conductive and can alter the electric field between sensing blocks. However, unlike a finger, the surface area of a water droplet is much smaller, and its potential is not fixed at ground. Instead, it lies between the potentials of A and B. In self-capacitance, water forms capacitors with both A and B, connected in series and then in parallel with Cs. This results in a smaller overall effect compared to a finger touch, but the direction of the signal change is the same. In mutual capacitance, water increases the mutual capacitance instead of decreasing it, leading to a different signal pattern. This distinction helps in identifying water droplets, even though the signal change is smaller than that of a finger. After water droplets are removed, mutual capacitance screens can still experience false triggers. This happens because the basic line value (used for comparison) may not update quickly enough, causing a mismatch with the actual AD conversion value. If the offset exceeds the threshold, a false touch may be registered. These errors can persist until the system resets, making it difficult to restore normal functionality. One solution to this problem is alternating scanning between self and mutual capacitance modes. By doing so, the system can detect the unique signal changes caused by water droplets and differentiate them from other environmental factors. Once detected, the basic line value remains unchanged until the water is gone. This allows the system to ignore the water-induced signal changes and respond correctly to finger touches. This approach enhances the reliability of mutual capacitance screens in wet environments. The difference in how water behaves on self and mutual capacitance screens is a key feature that enables effective waterproofing. By leveraging this behavior and implementing alternate scanning, designers can create more robust and reliable multi-touch capacitive touchscreens. Dot Matrix Led Display,Round Dot Matrix Led Display,5X7 Dot Matrix Led Display,1.2 Inch 5X7 Dot Matrix Display Wuxi Ark Technology Electronic Co.,Ltd. , https://www.arkledcn.com