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LDC1101: Custom Sensor Design and Increasing Distance

Skylar Sang
Prodigy 40 points
Part Number: LDC1101

Hello,

I am currently trying to use the LDC1101 to sense the presence of a metal object passing over it at high speeds. It seems to be the only board that can achieve the ksps that I need. In order to fit the project that we are working on we are trying to create new sensors that can sense changes at an effective distance of 1.5 inches or more.

Here are the specs of the current spiral coils that we are using:

Capacitance: 200pf (the results seem to be within the frequency range of the sensor, but this can be changed as needed)

Turns/Layer: Roughly 18

Outer Diameter: 1.03" copper edge to edge

Trace Spacing and Width: Both 10 mils

Separation Thickness: FR4 62 mils

Copper thickness: 1 oz-cu

After some basic tests at high resolution we have started to see an unusual trend in the data (see attached image). At around 1 inch the raw counts begins to decrease, then suddenly increases again within 0.4 inches of the sensor. This trend typically stays consistent when changing the minimum frequency, RP_MAX/MIN, and rcounts. The effects seem to be less intense at low rcounts, but I am guessing that's just due to a loss in resolution.

This brings up a couple of questions and concerns for me:

Has this behavior been seen by anyone before? I think it may be a resonance issue and can ask for a higher capacitance but I want to be sure.

The sensor currently only sees changes within ~1 inch. We have read through the LDC sensor design guides and it seems like the only way to be able to increase this distance would be to increase the diameter of the sensor (I do not think I can increase the size by much), as the sensing distance is approximately the same radius of the sensor according to the document. One the other hand, the sensor that comes attached to the LDC1101EVM performs better than our current sensor in both accuracy and precision, even though it is smaller. Are there any changes we can make to our own coil design to be able to surpass the performance of the EVM sensor?

Just for sensing presence of an object moving horizontally over the sensor, is axial sensing with spiral coils fine for this purpose?

Thank you so much for your time,

S

  • 0
    TI__Mastermind 23810 points

    Increasing the diameter of the coil typically does help with sensing distance. Are you measuring Rp or are you using the HRL mode? The HRL mode is probably more appropriate for this application.

    Can you try looking at the oscillation waveforms when you run this test? Are the amplitudes reasonable?

    Also, axial sensing with spiral coils should be fine for this application.

  • 0 in reply to Clancy Soehren
    Prodigy 40 points

    Dear Clancy,

    Thank you for your prompt reply. I am indeed using the LHR mode on the LDC1101.

    If the amplitude is too high or too low, do you have any advice for fixing the issue and on how to improve the sensor design?

    Thank you,

    S