It's pretty easy to find the "working principle" of surface capacitive touchscreen like this one from elotouch: http://www.elotouch.com/Technologies/SurfaceCapacitive/howitworks.asp.
Roughly speaking, I know that
- A surface capacitive touchscreen has a conductive layer to which some electrodes are attached
- The electrodes apply voltage across the layer
- When some conductive material, e.g. finger touches the layer a portion of current is drawn yielding numbers' changes in the sensors attached to the electrodes
- According to the numbers read from sensors, one calculates the touch location
Here're the questions:
- Should the electrodes apply DC or AC signals to the layer?
- How is the "conductive layer" modeled, i.e. is it regarded as a 2D conductive plane ? Or is it regarded as a 3D conductive block? In either case, how to calculate or approximately guess the charge/voltage/current distribution when the layer is idle/touched?
- What are proper sensors to attach to the electrodes, e.g. voltage metres or current metres or sth else? Why?
First question: it could be both DC or AC voltage. It is important to have electrostatic charge over the active sensing layer.
Second question: The conductive layer is not a uniformed structure. Active layer has grid structures so that row and column arrays are coupled in points with a dielectric layer between them.
Third question: When the human body touches the screen, it disturbs capacitance over each low-column overlapped pixels, resulting in change in the output voltage. Therefore, by voltage meter you will be able to detect the signal.
Hi @Morteza Amjadi, thanks for answering! I'm not sure whether we're talking about the same kind of capacitive touchscreen bcz you mentioned "grid". In wikipedia of capacitive sensing, there is a type named "projected capacitance touchscreen"( https://en.wikipedia.org/wiki/Capacitive_sensing#Projected_capacitance) which is with a grid-layer and it's not the one I'm asking for. Would you please confirm that?
Hi Wingfong, I totally misunderstood your question. I just took a look at surface capacitive touchscreen and what I could understand is:
There are 4 electrodes at the edges which produce positive charges over active conductive layer (DC voltage). When human body touches the screen, it disturb the charge balance since human body is negatively charged. This charge distribution changes the current flow over electrode sides. In my view, the DC voltage is always constant and current changes are measured for localization.
how does change in capacitance on touch screen is measured for mobile phone?
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