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LDC0851: PCB coil configuration for "through hole" shaft proximity measurement

Corben Taylor
Prodigy 10 points
Part Number: LDC0851

Hi

I was wondering if rather than the standard planar problem shown by many of the design references for the LDC0851, it would be possible to use this sensor in to detect the presence of a stainless steel shaft going through a hole in the PCB like the diagram below?

In theory the inner coil should have a higher flux density axially through the centre hole and therefore induce more eddy currents on the shaft than the out reference coil. Have you heard of any similar applications used elsewhere? Or is the "flat plate target" the best method still rather than centre hole and shaft idea? Thanks

  • 0
    TI__Mastermind 40295 points

    Corben,

    Matching the coils is an important factor for LDC0851 applications, and that is easiest with the "flat plate target" approach where the sense coil has a stronger interaction with the target than the reference coil. 

    The challenge for the sense and reference coils with co-located centers like in your diagram, is that matching the coil behaviors will be a challenge.
    and adding to that, both coils will interact with the rod, and could form a sort of transformer with mutual coupling between the coils. 

    If your requirements permit it, one alternative may be to relocate your reference coil off to the side so the rod does not pass thru its center.
    Matching the coils will be easier and it may be easier to control coupling, and allow the LDC to distinguish between the two coils' responses.

    Regards,
    John

  • 0 in reply to John Miller - Sensing
    Prodigy 10 points

    Thanks John, appreciate your answer. Offsetting the sense and ref coils to one side is a good idea. Do you think that in this configuration the through-hole shaft detection might be feasible? If you look at any FEMM coil problem, you get a relatively strong flux density in the inner hole so this might induce eddy currents in the shaft. God bless you brother.

  • 0 in reply to Corben Taylor
    TI__Mastermind 40295 points

    Corben,

    The through-hole detection scenario you have proposed sounds reasonable.
    We don't have dedicated tools to support that scenario, but prototyping something with an EVM would be a good starting point.

    Regards,
    John