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Coil design for maximum sensing range with small target (LDC0851)

Jesse Davidson
Prodigy 30 points
Other Parts Discussed in Thread: LDC0851, LDC1614, LDC1612

I have an application where I need to switch in the presence of  a ~8mm diam x 8 L stainless cylinder at the largest range possible.

I know that larger coil diameters tend toward greater sensing range, but I have been unable to find any discussion of optimizing design for small targets. Is it better to continue the spiral all the way to the middle (choosing line and space to achieve the right inductance) or make a few tightly spaced turns at the largest diameter possible? It seems like, all else being equal, a single layer coil would be better than a 2 layer or 4 layer coil, because the target it as close as possible to the majority of the turns.

I've experimented with using a fixed, off-the-shelf reference coil which has seemed to help compared to a stacked coil configuration. What effect does LC resonant frequency have on sensitivity to small targets? The Excel tools for the LDC0851 do not take target diameter into account at all.

Thanks very much for any guidance you can offer.

Jesse

  • 0
    TI__Genius 13335 points
    Hi Jesse,
    Your assertions are correct. A single layer coil with many inner turns is the best way to sense small targets. Also, matching will help you a lot. If both the sense and reference are perfectly matched inductances then you will get the longest range. However, if you are using ADJ = 0 V, then we recommend either a small mismatch between the coils ~0.4% or by using a fixed target on the reference coil. Also, note that increasing coil diameter in this scenario doesn't help you get further range. Take a look at this blog post where we investigate various target sizes for a single coil diameter: e2e.ti.com/.../inductive-sensing-target-size-matters
    Regards,
    Luke
  • 0 in reply to Luke LaPointe
    Prodigy 30 points
    Hello Luke,
    Thanks very much for your response. I had looked at that blog post but was having a hard time drawing conclusions about coil design from it. Just to clarify, does resonant frequency play any role in sensing range?
  • 0 in reply to Jesse Davidson
    TI__Genius 13335 points
    Hi Jesse,
    The main conclusion I would say is concerning the outer coil diameter. When the target area / coil area >= 1 then you get the most sensitivity. For target area / coil area < 1, the response drops off dramatically. This data was collected for the LDC1614 so it doesn't directly translate to the LDC0851 switching distance. However, if you have an 8mm diameter target you would have the maximum response from an 8mm coil. Figure 5 of the LDC0851 datasheet shows the longest possible sensing ranges for target area / coil area >= 1. Depending on the range you need to hit you may need to increase the coil outer diameter to get the range but lose some of the effectiveness of the coil. So, there will be a tradeoff here to find the optimal point in your system. I would recommend fabricating a PCB with several coils on it starting with an 8mm coil, then add more coils each with one more outer turn.
    Regarding the frequency planning, we typically suggest a higher sensing frequency to take advantage of skin depth relationship to frequency. If you are using a material like copper or aluminum we expect that you get very good switching distances even at low frequencies, but for materials like Stainless steel we recommend increasing the sensor frequency to get the best response.
    Regards,
    Luke
  • 0 in reply to Luke LaPointe
    Prodigy 30 points
    Hi Luke,
    Thanks again for your detailed response to my questions. I think you're right - in the end I'll have to do some experiments with coil geometry but it's nice to know where to start. You mentioned some data for the LDC1614, but I'm so far unable to find any figures showing the effect of target diameter vs coil diameter.
    Thanks again,
    Jesse
  • 0 in reply to Jesse Davidson
    TI__Genius 13335 points
    Hi Jesse,
    Yes, I think prototyping is the right choice. The data for the LDC1614 I was referring to was for the blog. Actually it was the LDC1612, but the only difference is the number of channels. Everything was normalized to the largest target 50x50 at the closest distance and represented as % frequency shift.
    Regards,
    Luke