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TMAG5173EVM: Off-axis magnetic absolute encoder

Aldo De Carolis
Prodigy 10 points
Part Number: TMAG5173EVM

Tool/software:

Hi! I'm looking for an off-axis magnetic absolute encoder and the customer support suggested this product to me.
I would like to know how to install this kit on our motor shaft to perform an absolute position measurement, because I'm not sure how to do that. 
My idea is to have a magnet installed on our shaft (24.95 mm diameter) which will rotate and generate a rotating magnetic field, whereas the sensor will be fixed and will measure the magnetic field produced, which will be changing since the magnet on the shaft is spinning, allowing us to obtain the shaft angular position.
I see that there's a magnet (1/8-inch diameter x 1-inch thick N42 magnet) in this kit (TMAG5173EVM - https://www.ti.com/tool/TMAG5173EVM). How should I install the magnet on the shaft? Please know that we don't have access to the end of the shaft, that's why we can't use the on-axis configuration. We only have a space of 25 mm available on the shaft to install the components. I thought the best solution was to install a magnetic disc on the shaft, but we are open to alternatives, as long as they're feasible.
So, could you please explain to us how we can install that magnet on our shaft? If it's not possible, could you give us a solution that we can install on our shaft?
Thank you very much for your time.
  • 0
    TI__Mastermind 21340 points

    Aldo,

    Thanks for reaching out on E2E.  An off axis measurement is certainly possible using TMAG5173.  Without access to the end of the shaft, you would likely want a diametric ring magnet mounted onto an open section of your motor shaft.  

    The magnet in the kit is axially magnetized, and not very ideal for rotation applications. See below to see the difference between Diametric and Axial polarizations.

    Diametric magnet

    Axial magnet

    If the axial magnet were to spin about the Z-axis (your motor shaft), then the sensor gets exposed to an unchanging magnetic field vector due to the relative symmetry.  However in the case of the diametric magnet, the radial and tangential components of the field vector will oscillate.  The resulting sine and cosine of the X and Y field components should allow an angle calculation using arctangents.

    The magnet in the kit has no center bore either, so it might only be attached to the side of the shaft and this would create and imbalanced load for the rotor.

    To test out different magnet orientations and rotations, I would recommend that you try the TIMSS tool.

    https://www.ti.com/tool/TI-MAGNETIC-SENSE-SIMULATOR

    You might also explore different magnet options from a magnet vendor to see about what might be a practical choice for your system.  I have used this distributor before:

    https://www.kjmagnetics.com

    Thanks,

    Scott