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Accurate Axial Distance Sensing, LDC0851 or LDC1101?

Other Parts Discussed in Thread: LDC1612, LDC0851

I've got a somewhat uniques application and at the last minute I remembered that the inductance sensors might be a perfect fit.

I'm building an automated PCB test station. The boards will be on a panel arranged in a 5x5 array. The board has 50mil diameter test point (8 total spaced on a 100mil 2x4 grid). My goal is to move the probing head down towards the PCB panel and use the sensor to know how far down the PCB is. I'm thinking of using this for other applications so adding a kind of non-contact Z-Axis probe could be a beneficial feature.

I'm completing a 2-layer board design that will be fitted onto the probing head. I've got some room to add an inductive sensor circuit but the smaller the better. This might be a crazy idea but I thought I'd post and get some feedback and guidance to see if I could add in a circuit.

Thanks,

George

P.S. Even if the idea doesn't work out well at this point I'd like to know what the best part and coil geometry would be. That way I can add the circuit and if it doesn't end up working I've only lost some PCB space.

  • Hello George,
    if I understand the application correctly, then I would suggest two possible approaches (depending on which one is more mechancially feasible for you):
    1) detect a piece of metal at the same level as the PCB. The LDC1612 may be a good device choice because it has very high performance and dual channel (If you like to probe how level the PCB is by measuring the base plate at more than one location). The sensor size depends on the maximum distance that you want to detect: e2e.ti.com/.../inductive-sensing-how-far-can-i-sense
    Here is an apps note on more sensor design details. www.ti.com/.../snoa930.pdf
    2) detect the position of the mechanism that raises/lowers the probe head. www.ti.com/.../snoa931.pdf shows examples of linear position sensors.

    Note that the LDC0851 is a switch and only has a 1-bit output. If you only need to detect a single threshold, then the LDC0851 is a good choice; otherwise the LDC1612 is more suitable.
  • Hi Ben,

    Thanks for answering, especially on the weekend! Sounds like you have a grasp on what I'm trying to do. In a nutshell I want to accurately sense the distance down to the top of the PCB. What would be easiest as far as setup goes, is if we can sense the 0.050" diameter pads on the board that we want to contact but they may be too small? 

    Those pads are spaced on 0.100" centers. Since the board has copper in it covered by soldermask I would the sensor would see more than the pads I'm trying to make contact with. What I'm thinking is I move the test head down and when I reach a certain distance away from these pads, say 10.0mm, I'll use the sensor output to then know how far more I'll need to probe

    Does the LDC0851 have the same or similar accuracy? It's fine for this application for the single bit since I'm just trying to get a known reference distance away from the board. Although being able to use the LDC1612 and report back the actual distance could be a nice feature now that I think of it.

    Thanks,

    George