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capacitive sensing

Hussein A
Prodigy 80 points
Other Parts Discussed in Thread: FDC1004

Hi,

I have questions about capacitive sensing : 

  • First, I would like to use capacitive sensing in order to detect a non-organic object. If this is possible, what kind of object (plastic, wood...?) Can it detect?
    Is it realistic or is it mandatory to use a human touch with a capacitive sensor?

  • Second question is about the use of an electrode matrix with capacitive sensing (organic / non-organic object). 
    According to the Texas Guide " capacitive sensing basics", there is a method to create a matrix of capacitance detection. 
    Does each electrode need to be wired with one capacitive sensor or is there a capacitive sensor that could withstand the information of multiple electrodes at once (with multiplexing technique)?

    Thanks
    ZA+ HA
  • 0
    TI__Mastermind 38435 points

    Hussein,
    The goal is for the object to change the sensor's nominal capacitance to a degree that it can be detected.
    So capacitive sensing works best detecting objects with a permittivity greater than free space.

    According the google, water has a relative permittivity of ∈r ≈ 80, while wood is ∈r ≈ 2-3.
    This implies, with all other things being equal, that sensors for detecting water level could have a much smaller area than sensors for wood.

    An array of regularly shaped sensors (e.g circular, rectangular or square copper plates) would need to provisioned as one sensor per capacitive sensing channel to create an array.
    A capacitive sensing device like the FDC1004 can support four channels, which translates into four sensors. 

    You might be able to support more that one sensor per channel by multiplexing multiple sensors onto a single channel.
    How well this would work would depend on your application, your requirements and the devices.
    You would no doubt need to pick multiplexers with very low parasitic resistance and capacitance, and which have the bandwidth need to support the sensor signals.
    Your PCB layout supporting these same requirements would also be critical.

    regards,
    John