Capacitive Touch Sensor?

Would it be feasible to allow capacitive touch sensors, such as those sold by Quantum, to detect non-conductive objects by buffering the touch
plate with foam and then overlayed with some conductive film or foil. The idea being if a nonconductive object depresses the foam, the foil would be detected by the sensor. Is this even necessary? What range of materials are able to be detected capacitively?
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Ingo Cyliax wrote: [...] > As capacitor, any large metallic surface will work. It helps to > have a large ground counterpoise. Your idea of using foam/foil > should work well. Capacitive sensor will sense many materials, > the measure the e-field and thus how materials change this from > air, but water or humans work best.
I'm not sure I've explained correctly. Basically, I'd like to use capacitive elements as force sensors by capacitively sensing the proximity of the foil. The actual capacitor/sensor-pad would be a separate metallic sheet or interdigitated pattern on the opposite side of the foam, which could have it's own grounding plane to prevent the EM field from extending backwards. I'm simply trying to get around the "water or humans work best" limitation. Also, when used as a skin, the upper layer of foam/foil would have the benefit of being easily repairable, while maintaining full sensitivity of the sensor.
Is this approach practical? It seems simple enough that if it were, someone else would already be using it in a robotic application.
And please, respond to the group for everyone's benefit.
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On Sun, 18 Jul 2004 23:26:09 -0400, "Chris S."

In my previous life, I worked for a prosthetic firm in the robotic Research and Developement department. From personal experience, it would be easier to implement this sensor approach inductively rather than capacitively. Several methods could then be applied...
i.e.
1) A "free-running" L-C oscillator - measure the number of pulses over a fixed or 'known' amount of time
2) Use a PI (pulse induction) method similar to how some metal detectors work.
3) BFO (beat frequency oscillator) ... a derivative of example #1 above... again a method some metal detectors use.
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Beau Schwabe wrote:

What about implementing an inductive sensor is easier than a capacitive sensor? From what I can tell, most inductive proximity sensors require a complex physical device, as opposed to a simple sheet of metal for capacitive sensors. Also, ICs incorporated the signal processing end appear to be about 5 times more expensive for inductive sensors over capacitive.
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On Mon, 19 Jul 2004 15:23:21 -0400, "Chris S."

Are you making your capacitor? ...if so then you can just as easily make your own inductor. A simple sheet of metal as opposed to a few turns of wire or an etched PCB? Capacitors are more subject to parasitic dielectric variations from external sources that may not be beneficial to your application. Inductors are not impervious to external noise, but tend to be more stable for this type of application.
complex physical device? - in most cases just a coil of wire.
Signal processing for a capacitor *can be* just as complex. For your purpose it should be about the same either way for the capacitor or inductor.
There are definitely pros and cons to one approach over the other, I am simply offering another design angle for you to consider.
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