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LDC0851: Coil Design for inductive sensing in PCB direction

Markus Neumair
Prodigy 165 points
Part Number: LDC0851

Hello,

I'm currently evaluating the benefits of the LDC0851 compared to a hall sensor in a motor speed detection circuit.

Because of the application mechanics, I have a fixed orientation of my PCB in relation to the motor shaft. The shaft is perpendicular to the PCBs X- and Y-axis, the shaft passes through the PCB.

Therefore, using a planar printed coil on the PCB, I would have to add some kind of tooths to the shaft to be able to detect rotation.

What I would like to know:

Can you provide a PCB coil design that enables the detection of the motor shaft rotation speed if the shaft has a flat area (shaft diameter is 25mm)?

If not, is there a technical reason that limits my application?

This is an example of what I could imagine:

Thanks,

Markus

  • 0
    Prodigy 165 points

    Sorry, adding the picture in my prior post via copy-paste didn't work. Here is the attached image...

  • 0 in reply to Markus Neumair
    TI__Genius 17440 points
    Hi Markus,

    The limiting factors for this application would be the minimum inductance that the LDC0851 can drive (heavily influenced by the space available for the coils), the reduced response from the coils based on their orientation in relation to the target, and the rotation speed of your motor. Unfortunately I’m writing this on a mobile right now, but as soon as I’m back in the office I’ll share more specific numbers for the limitations I mentioned, as well as point you to some of our sensor design resources.

    Regards,
  • 0 in reply to Kristin Jones93
    Prodigy 165 points

    Hello Kristin,

    thanks for your quick reply.

    I'm not really limited in boardspace, but the motor shaft has a fixed diameter of 25mm.

    Calculating the inductance of a coil with the shown structure on a 1.6mm PCB with a width of 20mm and 9 turns results in an inductance of up to 2.79µH.

    According to the datasheet, 1.8µH is enough at 3.3V supply. Anyway, I could add further turns to increase the inductance.

    The maximum motor speed in my application is 3500rpm. With one flat side on the shaft, I should get approx. 58Hz at the output of the LDC0851.

    Do you think this concept is worth to give it a try?

    Best regards,

    Markus

  • 0 in reply to Markus Neumair
    TI__Genius 17440 points

    Hi Markus,

    You may want to increase the coils' inductance so that the target interaction does not reduce the inductance below the specified value, but this should not be challenging if you aren't limited in board space.

    I think this application could work with the LDC0851. The largest challenge will likely be the sensitivity of the coils. The coils are most sensitive when the target is arranged in a parallel plane and directly above the coil, and the sensitivity is significantly reduced when the target is beside the coil (like it is in your picture). The target design will also heavily influence the sensitivity. I think it is worth testing, but the coil sensitivity may prevent you from using the LDC0851 for this application. 

    Here are some resources I would recommend:

    This reference design is similar to your application.

    These two application notes are helpful for LDC sensor design and LDC target design.

    This calculator tool is also especially helpful for any required calculations.

    Regards,