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What kind of coil is needed to sense a metal object 4-6'' (or more) away from the LDC?

Other Parts Discussed in Thread: LDC1000EVM

Hi TI,


I am using the LDC1000EVM and clearly the PCB coil has a limited sensing distance.  I am plan to use a PCB coil large enough to sense a metal object that could be 6'' away that has a target diameter between 2 and 3.5 feet.  Using the Webench tool (www.ti.com/.../inductive-sensing.page), I am unable to input diameters greater than 7cm.  It seems I would need a coil that has an inductance greater than 300uF and ~170 turns, best guess.  Is there another Webench tool I can use to make the calculation for me?

Would a 100nF capacitor suffice for a design of this type?

  • Hello Sam,

    Ufortunately there is no other WEBENCH tool that can simulate it. However, I can do an approximate calculation for you. I would need the following information:

    1. Coild inner and outer diameter (mm).
    2. Number of layers on the PCB the coil will have.
    3. Trace and width space of the coils (mils).
    4. Sensor frequency you are planning to use (optional).

    Best Regards,
    Natallia Holubeva
  • Hi Natallia,


    I am only able to answer questions 2 & 3.

    2.  PCB layers can be either 2 or 4.

    3.  1oz copper will be used.  What do you recommend for trace width and spacing?

    Thank you,


    Sam

  • Hello Sam,

    I need the outer diameter spec. I usually use 4x4 mils or 6x6 mils. You could contact your PCB manufacturer on pricing.

    Best Regards,

    Natallia Holubeva

  • Sam,

    I would like to give you an example coil for your application.

    If the target is 6 inches away, ideally your coil should be 12 inches in diameter. You could get away with a smaller coil size if you use LDC161x devices. I would design the coil with 6x6 trace and space widths because small size is not an issue and 6x6 spec is more cost efficient. Also, I would go for a 2 layer PCB due to the same constrains. If you need a 4 layer PCB for other reasons (other devices on the same PCB), we could do it as well. The outer diameter of 12 inch leads me to 4 inch inner diameter. The rule of a thum is for the inner diameter to be 1/3 of the outer one.

    The specs are as described:
    1. Outer d = 12'', inner d = 4 ''
    2. 2 layers PCB
    3. 6x6 trace an space
    4. Lets take 1MHz oscillation frequency

    Now the results:

    30 turns would give you 2.5mH (a pretty significant inductance) and 100pF cap in parallel would lead you to 300kHz oscillation frequency. Rp of the sensor is also quiet large. Note that for such a large coil the self resonant frequency is 1.4MHz, and you want to operate at least 20% below it.

    I would recommend 20 turns , leading to a large hole aka larger inner diameter. Total inductanceis ~1.3uH, Rp ~ 81kOhms, Q factor ~13, self resonant frequency ~2MHz.

    Best Regards,
    Natallia Holubeva