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Two EVMs and mutual interference between coils

Torgeir Skomedal
Prodigy 195 points

Hello

I now try to use two EVMs simultaniously. I observe that the data are affected when they are approximately 5 cm and closer apart. Is the data still realiable (non correlated) or are they correlated?

The data is affected more the closer the coils are.

I wonder how this will affect Applications With more coils for temperature Independence or "Lateral position sensing with z-axis insensitivity"

What I saw in my trails was that even if the coils had fixed distance the samplevalues decreased at some distances even if the coil traveled towards the metal Object. And the noise could be huge and/or uniform.

Regards Torgeir

  • 0
    TI__Expert 4890 points

    Hi,

    You can try to use different fresquency for closer coils.

    Regards, Sunny.

  • 0 in reply to Sunny Qin
    Prodigy 195 points

    The Challenge is that the frequency of the sensor/coil changes on both EVMs as they move relativ to their target. And at some point the frequencies of the sensors cross. They vary between 3,5 MHz and 4,6 MHz for my Application.

    As you can see the the band is 1,1 MHz. The bands must be very thoughtful calculated to never overlap.

    Just a warning for everybody thinking of an array of coils. The stray Field of the coils may be larger than you initially thought of...

    Regards Torgeir

  • 0 in reply to Torgeir Skomedal
    TI__Genius 9300 points

    Yes, that's a valid concern, especially when the inductance variance is large.

    Another way of avoiding the crosstalk is to physically separate the coils. 1-2 coil diameters should be sufficient to greatly reduce it.

  • 0 in reply to Evgeny Fomin
    Guru 22270 points

    Hello Evgeny,

    I have a similar question about multiple coils.  I was thinking of creating a grid array of coils to put in a shoe for measuring contact and pressure on the soles of the feet.  My idea was to actually multiplex the reading of the coils and use only one LDC.

    Would I run into the same problem as above?  I would want to make the coil spacing as small as possible to generate a 3D Image from the data.

    Has multiplexing of coils been done using a single LDC?

    What would be the minimum time between each read?   Since it is a grid array, two adjacent coils do not have to be read.   A patern for maximum distance between coils can be used.

  • 0 in reply to Greenja
    TI__Genius 9300 points

    If you multiplex, you do not have cross-talk issue.

    You will need to drop up to 1st four samples right after the switch moment, as they may be invalid.

  • 0 in reply to Sunny Qin
    Expert 1070 points

    Hi, Sunny,

    what frequency difference is appropriate? I suppose that LDC1000 can suppress difference frequencies

    in some range...

    Best wishes,

         Igor Gorbounov

  • 0 in reply to Igor Gorbounov
    TI__Genius 9300 points

    Hello Igor,

    15-20% of frequency difference is normally sufficient.