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detecting metal on human skin

John Noel
Prodigy 90 points
Other Parts Discussed in Thread: LDC1000EVM, LDC1614EVM, LDC1612, LDC1614

Hello,

I'm interested in using the sensor as a metal detector for detecting presence of a small stainless steel needle sitting on top of human skin. Proximity of both the human and steel signal change together because they are fixed together. Is there a way to reject the skin signal?

I have the LDC1000EVM and LDC1614EVM. I improved dynamic range by adjusting rpmax to 12.12 kHz and rpmin to 9.235 kHz, however, the proximity data of the control (human skin alone) remains nearly identical to that with skin+steel.

Any suggestions?

Thanks!

  • 0
    TI__Genius 14270 points
    Hello John,

    How large is the needle? You may be able to use the LDC1614 (or LDC1612) with 2 channels, where one sensor provides a reference to remove the skin signal and the second sensor looks for the needle.

    Regards,

    ChrisO
  • 0 in reply to Chris Oberhauser66
    Prodigy 90 points
    Haha! Yes, I realized this same thing after my post.

    The needle is typically 5 mm long but only 0.5 mm wide. For an optimal LC tank design in webbench, should I use the small dimension (0.5mm?).
  • 0 in reply to John Noel
    TI__Genius 14270 points
    Hi John,

    The 0.5mm dimension will result in an inductance that is too low for the LDC to effectively drive. In addition, you will have a very limited sensing range. I think the 5mm dimension is better; but even a 5mm sensor is quite small. If you want to make the sensor with a 2 layer PCB, you would need 4mil trace width and spacing - but this sensor will only have ~0.9uH ( I would recommend increasing the size to 5.5mm). If you use a 4 layer PCB you would be able to use 6mil trace width and spacing and manufacture a sensor with an inductance of 1.3uH.

    Because of the small size of the target, you will have a smaller response than to a larger target.

    Regards,

    ChrisO