Capacitive level sensor

Dear all,

     I need to design a capacitive continous level sensor for a fuel tank. The sensor it self is a rod placed inside tube having empty capacitance of 47pF and with the tank filled about 150pF. The usual method is to use the capacitance in an RC oscillator and than measure the frequency than convert it to volumetric leveling. The success is unfortunately temperature dependent and should be compensated. Is there any better way to build up this sensor? I would need at least 10bit over 100pF so a definition of 0.1pF is requested. One important remark is, that one of the measuring capacitors armature is grounded to the tank (tank is made out of regular steel sheets). I have looked some of the CDC (AD7151) chips but those seems to work only for feed-in capacintances from excitation to the CDC and the output is only treshold based.

Any hint or advice is welcome! 

Thank you , Endre

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  • Sean,

    I want to use AD7148 to measure the position of a metal plate, like a metallic "robot finger". I was thinking I should ground this robot finger, as the human is considered to be a ground in the models I see in the AD7148 data sheet.

    But if the robot finger is grounded, then the AD7148 would be measuring the "capacitance to ground", which you say is only allowed on the AD7747, not the other capacitor sensors. And if "capacitance to ground" measurement isn't allowed on this AD7148 sensor, just what model and potential is the human, if not a ground?

    So I'm confused, is the human not a ground? Should I let my robot finger (a small metal plate) be floating (not grounded)?

    BTW, I calculate a maximum of a couple pF for the size and closest position of the small plate relative to the sensor pad, so I'm good there.

    Thanks,

    -Larry

Reply
  • Sean,

    I want to use AD7148 to measure the position of a metal plate, like a metallic "robot finger". I was thinking I should ground this robot finger, as the human is considered to be a ground in the models I see in the AD7148 data sheet.

    But if the robot finger is grounded, then the AD7148 would be measuring the "capacitance to ground", which you say is only allowed on the AD7747, not the other capacitor sensors. And if "capacitance to ground" measurement isn't allowed on this AD7148 sensor, just what model and potential is the human, if not a ground?

    So I'm confused, is the human not a ground? Should I let my robot finger (a small metal plate) be floating (not grounded)?

    BTW, I calculate a maximum of a couple pF for the size and closest position of the small plate relative to the sensor pad, so I'm good there.

    Thanks,

    -Larry

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