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Human body model, capacitive sensor

Discussion in 'Electronic Design' started by [email protected], Mar 1, 2007.

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  1. Guest

    Hi

    I have this capacitive sensor I have made that works by applying a 5V
    square wave onto a sensor plate. I then measure the current going to
    that plate to estimate the capacitance from a human finger touch.

    The funny thing is that when I touch it the first reading is higher
    than the subsequent ones.

    Is there a enhanced human body model beyond the simple one consisting
    of a resistor and a capacitance to earth that can explain what I am
    seeing?

    Thanks

    Klaus
     
  2. Joerg

    Joerg Guest

    I don't know how you drive it but could it be that some charge bleeds
    off or onto the plate?

    One thing that does have such an effect is accumulated charge on the
    body. Happens a lot on winter days and when wearing shoes where the
    underside isn't leather but some type of plastics or rubber. The first
    touch bleeds that off and when you don't move around too much between
    that and the following touches the next ones won't register a surge.
     
  3. Jim Thompson

    Jim Thompson Guest

    Is the average value of the 5V square zero? Or is it 2.5V?

    ...Jim Thompson
     
  4. PeterD

    PeterD Guest

     
  5. Guest

    The output is actually driven hard with a 2MHz square wave 10%
    dutycycle for only 5us (ten pulses). Furtheremore the sensor is only
    activated once every second. So normally the output is 0V hard driven
    (push-pull output drive) and once every second some fast pulses are
    present on the sensor plate. Output to the plate is isolated with some
    Y-capacitors to provide protective impedance for the user.

    Therefore it is likely the finger is discharged to ground when it
    touches the plate (sensor normally at ground), so I would assume the
    initial charge of the body would be insignificant which is why I have
    this question of the human body model (maybe some sort of dielectric
    absorbtion??). Of course charge may accumulate on the Y capacitors.

    see simplified schematics at:

    www.microdesign.dk/tmp/SimpleSensorOut.pdf


    Thanks

    Klaus
     
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