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Sensing through different materials

mark ward
Prodigy 20 points

I have the LDC1000 evaluation kit and have done a very basic test to confirm my suspicion about sensing through different materials. Would I be correct in my observation that any conducting material placed between the coil and, say a moving metalic object would block the variations made by the latter?

Or would it be possible to sense through a conducting material by using a different type of coil and/or lowering the operating frequency?

  • 0
    TI__Genius 9300 points

    Hello,

    Indeed, to sense through the metal you will need to lower LC tank frequency.

    Eval kit LC tank frequency is around 3.5MHz, so the skin depth is very small, on the order of 50um, depending on the metal.

    You need to achieve the skin depth comparable to the thickness of of the blocking object.

  • 0 in reply to Evgeny Fomin
    Prodigy 20 points

    Thanks for that reply.

    Could you offer a 'rule of thumb' guide to the thickness of aluminium that could be penetrated in this way?

    If the lowest frequency of operation is 2.5KHz would that penetrate 3mm?

  • 0 in reply to mark ward
    TI__Genius 9300 points

    Hello,

    Sampling rate is the king here.

    Once you know the required acquisition rate, you can calculate the minimum LC frequency = rate x 192.

    Now that you know the frequency, you calculate the skin depth = 503*root(r/f/mu), where r is resistivity of material to look through, mu its permeability, f – LC frequency.

     That will be roughly (give or take a factor of 2, app specific) the max thickness of material to look through.

    For Al it gives 1.7mm. Thus, magnetic field behind Al shield will be reduced roughly 10 times.

    It should still be sufficient to detect a presence of your target, but may not be enough for high-precision measurements.

  • 0 in reply to Evgeny Fomin
    Prodigy 30 points

    I have a similar need to sense through a conductor (titanium) so I need to lower the frequency.  Suppose I want to use 10KHz. and would like to increase both the L and C in the sensor.  What are the guide lines for increasing C from the 100pf on the eval board?

  • 0 in reply to David DAlfonso
    TI__Genius 9300 points

    You will need a coil with a very big inductance to Rs (@10kHz) ratio, or with very big L to keep C small.

    Rp = L/Rs/C whas to be above 800 Ohms. As L and C go up, Rp usually degrade, as with larger L, Rs increases as well.