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REG: Proximity detection

Yeshwanth Kumar Jayakrishnan
Prodigy 150 points

Hello everyone, I have been trying to detect non conductive objects using FDC1004EVM to test  the Capacitance-to-Digital Converter FDC1004 but but I haven't been able to detect a significant change in capacitance (My setup is as in figure.).

The copper foil are is 3x3cm.The plastic cover thickness is around 3mm. The cable length is around 5~10cm.

When I tried to detect an plastic object the change in capacitance is around 0.2pf. I unsure as to how stable this value will remain.

Can you help me to have a better stable difference. what changes should I do to get better results.

Best Regards

Yesh

  • 0
    TI__Expert 7400 points
    Hi Yesh:

    The change in capacitance that you are seeing is reasonable. It's because the dielectric constant is very small, not too much difference between air. To get higher sensitivity, make sure the sampling rate is at 100S/s and you can probably have the gain setting at it's highest. The only issue with the gain setting is that it's post operation, in the sense that both the signal and noise will be amplified.

    Stability is dictated by system environment, and unfortunately, I cannot comment on this since I do not know where your system will be used in.

    Thanks
    -David Wang
    Capacitive Sensing Applications
  • 0 in reply to David Wang11
    Prodigy 150 points

    HI David,

    Though 0.2pf may be reasonably good, the problem is that every time the base reference keeps changing. So I dont get how should I fix my reference value to enable detection. Like the base reference value changes with the movement of cable. That has been the challenge for quite some time. I tried different shelding techniques nothing seems to work.

    If you need more information on my log data I can share. 

    If you could share your ID I can share my schematics as well as my application images so that you may suggest any corrections if required in it.

     Best Regards

    Yesh

  • 0 in reply to Yeshwanth Kumar Jayakrishnan
    TI__Expert 7400 points

    The issue you are seeing is a mechanical/setup issue not an electrical issue so changes in schematic will not make a difference. It's very typical for system setups to experience capacitance drift. You need to compensate for this in software....

    Please refer to http://www.ti.com/lit/an/snoa939/snoa939.pdf

    It can help eliminate the drift you are seeing, but it is also dependent on the rate of change of the target itself relative to the noise/drift you are seeing.

    Thanks

    -David Wang

    Capacitive Sensing Applications

  • 0 in reply to David Wang11
    Prodigy 150 points
    Hello David,
    You were correct , I have been to work with it, the change in the capacitance value is consistence.
    Since I have started this query I am posting 1 of my observation here itself.
    When the sensor plate is touched with bare hands the capacitance value reaches 16pf, but the same doesn't happen when i short the sensor plate with ground. Shouldn't the value read 16pf?(going on the principle that when human touch comes in contact with ground??)
    I don't get this behavior. Is my testing procedure correct??


    Other than that the system seems to work fine, Now Ill head to testing in noisy environment.

    Thanks for your help.
    '

    Best Regards
    Yesh
  • 0 in reply to Yeshwanth Kumar Jayakrishnan
    TI__Expert 7400 points

    You will not see the same thing happen when you touch the sensor compared to connecting the sensor plate to ground. You actually should not be shorting the sensor plate to ground because it will damage the part. The reason why you don't see the same thing is that the human body also is seen as a high impedance (high resistance) so technically you are not "shorting" the sensor when you touch it. The human body can be seen as a floating conductor, there is still a dielectic material between the sensor and "human ground".

    Thanks

    -David Wang

    Capacitive Sensing Applications