Waterproof Touchscreens, Cont'd.
Putting Mutual and Self-Capacitance Together
Self-capacitance sensing with a driven shield has very good Water Rejection capabilities, but does not allow true multitouch. From a high-level point of view, mutual-capacitance sensing should also work well for Water Rejection because adding water to the touchscreen surface produces a change in mutual capacitance that is in opposite polarity to a finger touch. But this also means that the removal of water can look identical to a finger touch.
Putting Mutual and Self-Capacitance Sensing Together
Self-cap with shield works well with water but it doesn’t allow for true multi-touch performance. Conversely, mutual cap allows multi-touch performance but has trouble with water. For Water Rejection, the most robust solution uses both mutual-cap and self-cap sensing. For this solution to be practical, the touchscreen controller must have the capability to dynamically switch pin function between TX, RX, and Shield.
The combination of self plus mutual sensing for Wet Finger Tracking provides less of an advantage. When a finger touches water on the touchscreen, the water becomes conductively tied to the finger. The capacitance decreases because the finger steals the fringe field current and diverts it to ground. Depending on how much water there is and what shape it holds on the surface, the touch profile will look similar to a large thumb or even a face that covers the entire screen during a phone call. Without any special algorithms to deal with this large signal profile, the touchscreen controller will try and resolve the center of the water shape, which may or may not be the location of the actual finger. Figure 6 illustrates the possible accuracy error.
There is information contained in the large signal profile that allows improvement in accuracy, but relying on Wet Finger Tracking accuracy to be the same as normal finger tracking without water is not realistic with capacitive sensing technology.
The term ”waterproofing” is not well defined in the capacitive touchscreen industry. Although some general standards exist, they mostly apply to destructive tests and do not cover touchscreens specifically. Water Rejection and Wet Finger Tracking are two subset definitions of waterproofing that are gaining the most traction. Self and mutual capacitance are affected differently with water; intelligent touch screen controller architectures and mature intellectual property can be used to exploit this phenomenon.
About the Author
Erik Anderson is a Staff Systems Engineer for TrueTouch products at Cypress Semiconductor. He works in new product development with an emphasis on capacitive sensing technology. He graduated from Western Washington University (WWU) in 2004 with a B.S. in EET. He can be reached at email@example.com.