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LDC1000 EVM with Steel

shabaz
Prodigy 65 points

Hi,

We've been trying to analyze the LDC1000 behavior with steel present, and we've noticed that the measured inductance drops when in the presence of steel.

I'm no expert on inductor cores, but it was surprising - I thought the inductance should go up.

As a check, we snapped off the LC circuit on the LDC1000 eval board, and subjected it to a VNA. The results are in the comments here. 

The whole topic resulted from this thread on element 14.

Does anyone know why the behaviour with steel results in what appears to be a lower measured inductance?

Thanks!

 

  • 0
    TI__Genius 14270 points

    Hello Shabaz,

    At the higher frequencies (>500kHz) that LDC devices typically operate, the eddy currents on the target surface overwhelm the permeability and hence the inductance drops. If you change the sensor to one that operates at a much lower frequency (~40kHz), then you will see an increase in inductance when the steel is brought into the field. There is a transition region where you will see a minimal change in inductance due to a steel target position; this frequency range will vary based on the type of steel.

    The Rp measurement will still show a shift due to target presence.

    Is there a specific reason that you need to have an increase in inductance for your system?
     
    Regards,

    ChrisO

  • 0 in reply to Chris Oberhauser66
    Prodigy 65 points
    Hi Chris,
    Thank you for the detailed answer! That helps loads. There was no reason we were looking for an increase or decrease, we were just curious about the behaviour because some of us had been experimenting with the device.

    Many thanks,

    Shabaz.