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increasing sensitivity for capacitive sense touch by means of software

Krishna Thallapaka
Intellectual 770 points

Hello,

As in subject, can it be done when I pad the cap sense with extra material on top. Currently, the hardware though, is not responding with this addendum.

Regards,

Krishna.

 

 

  • 0
    Expert 1710 points

    http://focus.ti.com/general/docs/techdocsabstract.tsp?abstractName=slaa363a

    http://focus.ti.com/general/docs/litabsmultiplefilelist.tsp?literatureNumber=slaa379

     

    Those are worthwhile reading about designing the sense areas and padding / covering materials as well as the thickness versus type of those covering materials to get the best results in msp430 capacitive sensing.

     

    In general you will have higher sensitivity when you use thinner covering materials rather than thicker ones, and the higher the dielectric constant covering materials will affect the sensitivity moreso than the lower dielectric constant materials, as the application notes reveal.  Of course as they also suggest you can use more signal averaging techniques to improve your sensitivity accuracy versus noise and signal strength as may be needed depending on your sensor design, but in general there isn't often much margin of signal versus noise for this type of sensor as is revealed by the small number of bits of signal change that are generated by a touch event, so it does not take much change in sensor design or covering material thickness/type to go from a working detector to a non working one at least unless you recalibrate your detection threshold to adjust for a slightly weaker sensing signal delta if needed.

     

     

  • 0 in reply to C. Hayes
    Intellectual 770 points

    Thank you Hayes.

    I filtered the signal using low pass. I increased the delay to accommodate for the increased capacitance(thus discharge time). I also changed the threshold a bit, though I don't know if it made any difference.

     

    Regards,

    Krishna.

  • 0 in reply to Krishna Thallapaka
    Guru 227245 points

    The capacitive sensors reach (hence the name) on the change of the capacitance of the pad.
    There is no ways to increase sensitivity by software. Sensitivity is raised if the change of capacitance raises in relation to the untouched base capacitance. This can be done by lowering the base capacitance or raising the touched capacitance. There is no way to do this in software.

    You can, however, try to increase the probability or accuracy of the readings (and the interpretation of the result). by any type of digital filter, probability check, autocalibration or whatever you can think of. But this does not increase sensitivity, only increase quality of the detection, which is desireable too, but a completely different thing.

    One drawback of this kind of capacitive sensors (isolated one-point sensor) doe snot mak a difference between a childs finger pressed on it or a grownups thumb hovering over it. Both cause the same capacity change. Higher sensitivity will detect smaller fingers ont he pad as well as bigger fingers further from the pad. Physically the same.
    Other touch sensors work differently. There a electrical connection is needed between the sensor and the touching finger. Then the whole body of the acting person acts as capacitor (or even resistor) against ground, resulting in a really huge change in teh moment of the touch. This is easy to detect, but then there is no isolation anymore. This is why you cannot call the elevator with your gloves on (with gauntlets, however, it would work :) )

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