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Elliptic coil

Sami Lakka
Intellectual 315 points
Other Parts Discussed in Thread: LDC1000EVM, LDC1612, LDC1614, MSP430G2553

Hi

I need to sense a stainless vibrating metal piece with height of 6 mm and width of 2 mm. The distance varies between 6-10 mm and I essentially need to see the vibration frequency. This metal piece is immersed in water and the electronics is behind a plastic sheet.

Because the surface area is pretty small the signal is not too strong. I can see the vibrations with LDC1000EVM test coil but I would like to strengthen the signal. Because the metal piece is attached by two very narrow metal legs (length of 20 mm and width of 0.2 mm) If I would make the coil ellipsoidal, say one diameter of 9 mm and the other diameter 15 mm, I probably could couple to signal to the metal legs as well. Are there any considerations that I should take into account if I use ellipsoidal coils?

  • 0
    TI__Expert 6550 points
    Hello Sami,

    First please make sure that the plastic material between the target and the coils is not a dialectric (some plastics are). Second, I assume you are using L measurement only. In the case, LDC1612 would have a larger response than LDC1000.

    Best Regards,
    Natallia Holubeva
  • 0 in reply to Natallia Holubeva1
    Intellectual 315 points

    I'm currently using proximity data values and compensating with external temperature sensor. The change with the LDC1000EVM PCB coil is around 100 units while the noise level is around 20 units. I cannot see practically any change in the frequency data when using LDC1000EVM GUI. Would the increased resolution in LDC1612 allow me to see changes in the L values? Currently the frequency data varies between 765-767 when the metal plate vibrates which is too small for measurement.

  • 0 in reply to Sami Lakka
    TI__Genius 14270 points

    Please check the following:

    1) are you using a response time setting of 6144? This is the highest setting and will provide the lowest measurement noise.

    2) The LDC1000EVM has a CLKIN frequency of 6MHz, which is less than the 8MHz maximum frequency. Using 8MHz for the CLKIN frequency will increase the resolution by 33%. Additional techniques to improve the frequency measurement resolution on the LDC1000 is to use a lower frequency sensor and also to ensure you are using the response time setting of 6144.

    As for the LDC1612, yes, you will have significantly improved measurement resolution - more than 8x improvement in frequency measurement (the L measurement - note that the LDC1612 and LDC1614 only measure L and do not measure RP). The LDC1612 can also support a much longer measurement interval, which should also provide more a better meaurement resolution.

    ChrisO

  • 0 in reply to Chris Oberhauser66
    Intellectual 315 points
    Thanks Chris, I will increase the frequency to 8 MHz in our test setup. I will also test if LDC1612 has enough resolution so that we could go to L measurement instead of Rp.
  • 0 in reply to Sami Lakka
    TI__Expert 6550 points
    Sami,

    I was wondering what is the reson to use proximity data in your application. L measurement is more stable versus temperature so maybe you could switch to L only. What is the oscillation frequency of the sensor? Thank you very much.

    Best Regards,
    Natallia Holubeva
  • 0 in reply to Natallia Holubeva1
    Intellectual 315 points
    I am using proximity data because L data does not show any change when target vibrates, neither in the EVM GUI or in our custom PCB. I am using the coil from the LDC1000EVM and the LDC1000 parameters are below:

    MSP430G2553 clock frequency 12Mhz and LDC1000 CLK 6 MHz
    #define VIV_RPMIN 0x38
    #define VIV_RPMAX 0x17
    #define VIV_SENSORFREQ 0x64
    #define VIV_LDC 0x15
    Target size = 5mm x 2 mm. Distance to target about 8mm.

    Our target is vibrating with varying frequency range between 20 Hz to over 200Hz. I see the vibrations better with LDC of 0x15 when the frequency is higher.
  • 0 in reply to Sami Lakka
    TI__Expert 6550 points
    Sami,

    Thank you very much for your explanation. Your work is important to us.

    Best Regards,
    Natallia Holubeva