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LDC1000: Minimizing the impact of distance on Rp

Ali Goname
Prodigy 70 points

Part Number: LDC1000

Hi,

Background information: I would like to determine the composition of a particular metal+non-metal material. Due to chemistry I do not fully understand, when a block of it is cast, more non-metals at the top than at the bottom. The change in composition will have a significant and measurable impact on the resistivity according to literature I've read and some preliminary tests.

My setup: I have a coil facing down at a fixed height. Below it, I have the block on a motorized platform. I have laid the block on its side, such that the variation in composition, and thereby in resistivity, is now on the x axis. As the platform moves left or right, I am able to observe a change in Rp.

My problem: I believe in order for this to work with any accuracy, the distance between the block and the coil has to be uniform. As the blocks are not perfectly flat, there can be up to 1.5mm of variation in the distance between the block and the coil.

My question: What steps, if any, can I take to minimize the effect of the change in distance on the Rp value? 

Note: I do not care for the value of Rp in absolute terms, only in its variation along the block.

  • 0
    TI__Mastermind 23810 points
    Ali,

    You are correct, variations in distance will also look like variations in Rp. Note that Rp will also be affected by temperature, although since you only care about relative Rp, that should be less of a concern.

    Also note that skin depth is a factor here. Eddy currents will mostly form on the surface of the metal block, meaning that metal composition changes further in may not be seen by the inductive sensor. You can calculate this by referring to page 4 of the following app note:
    www.ti.com/.../snoa957a.pdf

    It sounds like you have already taken some measurements, so can you answer the following questions?
    - What variation in Rp do you need to be able to detect?
    - What is the minimum and maximum Rp that you expect in this system?
    - What are the dimensions of the metal you are measuring?
  • 0 in reply to Clancy Soehren
    Prodigy 70 points

    Hi Clancy,

    Thank you for your reply. I have added a diagram below that may help.

    The metal block is 156x156x300mm.

    I have not yet designed a coil (using the pcb coil) or chosen a capacitor value so I did not save the results. I can run some tests tonight and report back. I was planning on designing the coil and choosing the capacitor value based on any information you could provide to me on how to design them so that the distance variation has minimal impact. If I am going to change the coil and capacitor, would the results I'm getting now be relevant? 

    If I am not mistaken, the skin depth should not be an issue here as the change in metal composition only occurs along the 300mm length where I intend to do the line scan. The surface should have the same resistivity (Ω . m) as the inside. I have calculated the skin depth to be somewhere in between 20.8μm and 6.59mm depending on the resistivity at a frequency of 5kHz, the minimum frequency as per the datasheet. Are there any disadvantages to using such a low frequency? Thank you,

    Ali Goname

  • 0 in reply to Ali Goname
    TI__Mastermind 23810 points
    Your skin depth calculations aren't matching mine for 0.01 Ohms-cm to 1000 Ohms-cm. I am calculating that this would put the skin depth between 71mm and 22m at 500kHz... This is using the equation:

    skin depth = sqrt((2*p)/(2*pi*f*u_0*u_r))

    where :
    u_0 = 4*pi*10^-7
    u_r = 1

    Can you verify the resistivity of the material? I am running some simulations to see how much the 1.5mm distance variation will impact you. I suspect that using a racetrack shape will give you the best performance since you care about the Rp variation along the x-axis.
  • 0 in reply to Clancy Soehren
    TI__Mastermind 23810 points
    Ali,

    I haven't heard from you in a few weeks. Were you able to get this working?