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FDC1004: Sensor Pad Design Support

Steve Patterson
Intellectual 570 points
Part Number: FDC1004

We will be using the FDC1004 for indirect water level sensing so are following the concept described in AN TIDU736A, "Capacitive-Based Liquid Level Sensing Sensor Reference

Design."  In our case, we really just want to know if the water level is at or above 5mm from the bottom of the sensor pad board.  Our sensor flex board will be 33mm tall by 60mm wide with the target water level at 5mm.  The "always wet" sensor is a problem because it will see between 0 and 5mm of water depending on the fill level and pad height, so won't always be under the water level.  The dry sensor will always be dry, so we'll just match the wet sensor pad size so it should work as intended.  We've done some prototyping using copper tape and wires on perf board with limited success, but the lack of precision is limiting useful conclusions.  With such a "shallow" reservoir, how should the pad sizes and locations be adjusted?  We'd like to avoid trial and error due to the time and cost.  Thanks for any guidance/suggestions.

  • 0
    TI__Genius 17440 points

    Hi Steve,

    The reference liquid sensor needs to be completely covered by liquid in order to accurately measure the liquid level. Have you considered other liquid level sensing approaches, like ultrasonic sensing? This method does not have any minimum liquid level requirements. https://www.ti.com/lit/an/snaa220a/snaa220a.pdf

    Best Regards,

  • 0 in reply to Kristin Jones93
    Intellectual 570 points

    Kristin,

    I agree that the AN TIDU736A implementation cannot be followed exactly, but should work with pad geometry and SW processing changes.  Note that the "wet" pad is underwater when the water level is at the minimum, so adjusting this method should work.  For example, how short can the wet pad be made and still serve its purpose?  Its capacitance should stop rising once it's underwater.  Can TI supply any guidance on the best pad size/geometries for our application?

    Thanks for the ultrasound suggestion.  It should work, but we don't have access to the bottom of the reservoir so we would have to measure the air between the sensor and water surface, which would be difficult with the product geometry.  It's also a 2 chip + transducer solution, so is 3-4x as expensive and board space used (and power), so we chose the capacitance approach.

    - SteveP

  • 0 in reply to Steve Patterson
    Intellectual 570 points

    Any follow-up on my application questions?  Is David Wang available to comment on my application from his experiences developing AN TIDU736A?

  • 0 in reply to Steve Patterson
    TI__Genius 17440 points

    Hi Steve,

    Thanks for your patience while we were out of office for the holidays. Unfortunately David Wang isn't available to comment.

    We haven't tested a sensor geometry like the one you've described, but realistically the minimum sensor size is one that can measure a significant change in capacitance between dry and wet. Your best bet is to do some testing with just the wet sensor to make sure that you can measure a significant difference between wet and dry. Otherwise it will not be an effective reference sensor. Section 4.4 of the reference design you mentioned has some additional recommendations for sensor design optimization.

    Best Regards,