Capacitive touch systems are clearly superior to resistive touch systems. Resistive touch systems break down and wear out due to their moving parts. The majority of resistive touch systems also can’t effectively distinguish multi-touch interaction with a user. Legacy capacitive touch systems used self-capacitance sensing (Figure 1). They don’t wear out, and they can support multi-touch gestures as long as you don’t rotate your touch points or get them too close together.
Figure 1. A self-capacitive touch sensing system measures the capacitance from a wire to earth ground. Your finger provides a parallel path and increases the effective capacitance.
After the iPhone popularized pinch and rotate gestures in 2005, system designers have used mutual capacitive sensing to determine multiple touch points and gestures (Figure 2). The drawback of mutual capacitive sensing is that it takes longer to do the measurement and, hence, uses more power. If you use a dual-architecture chip that can do both schemes, you can provide both lower power and good multi-touch accuracy. While self-capacitive systems are less affected when there’s a drop of water on the screen, mutual systems can be significantly affected by moisture. To get the best touch screen, you benefit from both sensing schemes.
Figure 2. A mutual capacitive touch sensing system measures the capacitance between the x-axis and y-axis wires. It’ is more accurate for multi-touch but takes longer to do.
Legacy capacitive touch screens rely on self-capacitance sensing. Any wire in space will have a capacitive coupling to earth ground. In one instantiation a self-capacitance touch IC will dump a fixed charge on all the wires that run in the X-direction. That charge reacts against the capacitance to earth and creates a voltage. The touch chip will then measure that voltage. If your finger is touching the display, those wires will have an additional capacitive path to earth ground. Now the effective capacitance of that wire is increased, and the resultant voltage for the fixed charge injection on the wire will be less (Figure 3).
Figure 3. A self- touch sensing IC integrated circuit connects to both the x-axis and y-axis wires on the display. These are clear wires formed from ITO (indium tin-oxide).
Help me understand, Shar or taimoortariq or anyone else. I had always thought that all the capacitance pixels of a phone or tablet were mapped to memory addresses. The article is telling me that there are only two circuits; the X and Y axes. Then there is some kind of capacitance profile that identifies the spot that is touched. What is that algorithm or neural network like?
A secondary question: Is it possible that a single break in either the X or Y axis could cause a single point of failure for the whole device?
Thanks for the feedback everyone and glad you enjoyed the article! Regarding the question from User 78RPM- In mutual capacitance you scan all of the 'pixels' (or nodes in industry parlance) and convert capacitive measurements into digital. These are all stored in memory so you can make decisions in firmware as to which nodes represent fingers touching the screen. You can almost think of it as a topographical or 3D contour map of the screen with the X and Y axes representing position and the 'Z height' representing the capacitive signal. So the tallest peaks on the map represent likely finger locations. Of course you get complexities introduced from water droplets, palms resting on the screen, or hovering fingers that you want to report as hovering objects rather than touches, and so on. All of these need good algorithmic techniques to effectively reject them
In self capacitance you take of all these same capacitive measurements in each X and Y axis and you have a profile or a single measurement per trace. You can think of it as a bar chart with the height of each bar symbolizing the capacitive measurement. And the number of bars is the total of number of X and Y traces. You can then use a center of mass style calculation (or similar algorithm) that computes the X-position and Y-position of the finger. As you can see you're looking at each axis independently so you don't have a datapoint per node as you do in mutual capacitance but as the article illustrates there are benefits from self capacitance for power consumption, moisture immunity and first touch latency.
To answer your other question a single broken trace will cause a dead spot along that particular trace where the touchscreen will become unresponsive. The further along the trace (i.e.- farther from the routing channel and closer to the end of the trace) that break is located, the smaller the dead zone will be. If the trace breaks right where it routes into the touchscreen from the bezel edge then effectively the entire trace will become a deadzone. The remaining part of the touchscreen will be usable though.
Very nice article. I replaced the touch screen on a cell phone ( Sony Xperia Z2)by using Optical Clear Adhesive( OCA) and it works in a strange ways. e.g I click on one letter and it writes different or writes more than one . When I test it from a service menu it draws different line than it is correct.
I had tested that touch screen before gluing( using OCA ) and it worked well.
So I separated the touch screen and LCD it again (by using a repair machine) and test it again and it worked ok.
Does it mean that by a pressure ( that OCA caused ),the function changed?
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