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Combining Self- & Mutual-Capacitive Sensing for Distinct User Advantages
1/31/2014

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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.
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.

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taimoortariq
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Gold
Capacitive Sensing
taimoortariq   1/31/2014 3:46:16 PM
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I still remember the amount of effort we had to do with resistive touches, Thanks for the elaboration of the sentive screen Shar, The science behind the touch screens is indeed fascinating.

78RPM
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Platinum
Re: Capacitive Sensing
78RPM   2/2/2014 1:15:57 PM
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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?

TJ McDermott
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Blogger
A VERY educational primer
TJ McDermott   2/5/2014 12:55:00 AM
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Thank you for the lesson covering capacitive sensing touch screens.

Shar N.
User Rank
Iron
Re: Capacitive Sensing
Shar N.   2/19/2014 4:39:46 PM
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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. 

 

Shar Narasimhan (author)

Atmel Corporation

 

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