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DRV5055: The difference in output between when approaching and when moving away.

naoki numakura
Prodigy 10 points
Part Number: DRV5055

I measured the distance between the magnet and this part.
At that time, there is a difference in output between when approaching and when moving away.
I found that the value tends to be higher when moving away.
Could you please tell me the cause and countermeasures?

  • 0
    TI__Mastermind 20980 points

    Numakura-san,

    There are a few factors which may cause an expected shift in output performance.  First, if there are changes in Vcc, DRV5055 is configured to adjust the output ratiometrically.  That is, the output should remain a relative constant %FS across all VCC voltages with a fixed input.  If for any reason this voltage changed during your test, it is possible that you may observe shifts in output voltage for the resting position of your magnet. Using appropriate decoupling, and maintaining a constant VCC voltage will help reduce any impact this has on your measurement.

    A second possibility is that the device temperature changed while the temperature of the magnet did not.  DRV5055 also implements temperature compensation opposite that which is typical to neodymium magnets.  As a result, if there is heating of the sensor that is not experienced by the magnet, there can be a resulting unwanted shift in sensitivity.  In the case that the sensor does heat differently than the magnet, it would be beneficial to consider using one of the zero compensation variants of this device (i.e. DRV5055Z1).

    The third option, which sounds closest to your description is that there is some sort of magnetic hysteresis in your system.  Any nearby ferromagnetic material will experience some residual magnetization when a magnetic field is applied and then changed.  Any magnetization of this material that occurred when the magnet was placed close to this could interact with the magnetic field of the permanent magnet. Please check that there are no ferromagnetic materials near your sensor during your test.

    Thanks,

    Scott Bryson

  • 0
    Prodigy 10 points

    Dear Mr. Scott Bryson

    Thank you very much for your message.
    We conducted an experiment in a room at 20°C with a magnet attached to the Calipers.

    About VCC
     The output values are the same.
     The VCC when the magnet was moved was between 4.99V and 4.98V.
     There is no trend of voltage change.

    About temperature
     I think that it is not affected by the temperature.

    About the magnetic field
     The Calipers is made of aluminum.

    Please let me know if there are any other causes.

    Sincerely,

    Naoki Numakura

  • 0 in reply to naoki numakura
    TI__Mastermind 20980 points

    Numakura-san,

    If Vcc changes by 10 mV, then the Vq voltage should change by 5 mV.  The sensitivity of the device will also vary following the description below:

    So, in the example of DRV5055A1 with a 10 mT input the output with Vcc = 4.99 V should be:

    0.998 V + 2.495 V = 3.493V.

    At Vcc = 4.98 V this changes to:

    0.996 V + 2.49 V = 3.486 V

    The expected variance with respect to Vcc is 7 mV.  Additionally the input referred noise for this device is 1.2 mT, which could add some additional variance.

    The purpose of the ratiometry is so that when Vcc is used as a reference voltage for the ADC measuring the output of the device, the output code will remain constant as Vcc varies.

    Additionally, I would assume that you are measuring the exact position of the magnet relative to the sensor, but if there is possibility for the magnet to move relative to the measure point I suppose it could be possible that there is some mechanical hysteresis impacting the result.

    Thanks,

    Scott