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LDC1614EVM: Inductive position sensor (differential)

Maurizio Liverani
Prodigy 40 points
Part Number: LDC1614EVM
Other Parts Discussed in Thread: LDC1614, LDC1101

I need to measure differential displacement as distance of 2 metal walls from two coils, either on X than Y axis (4coils as total)

1) Is the LDC1614 the correct choice for this purpose or is better use 4 separated LCD1101?

2) Is the firmware inside the microprocessor of the evaluation module available as open source in order to rearrange it as differential displacement sensor?

3) in order to get the best sensor displacement linearity, it is better use Rp measurement and than calculate the rate Rp/L , while the two characteristics has nearby the same behavior over Target distance/Sensor diameter?

Thanks of all,

Maurizio Liverani

stilsas@libero.it

  • 0
    TI__Expert 6625 points
    Hi Maurizio,
    I've got your request. Allow me some time to think about this and get back to you.

    Best Regards,
    Bala Ravi
  • 0 in reply to Bala Ravi
    Prodigy 40 points
    Hi Bala,
    I think that the first thing to ascertain is if LCD1614 has the same measuring registers found in LCD 1101: in particular if it is available, also in LCD1614 the register named:
    RPDATA: is the contents of RP_DATA_MSB and RP_DATA_LSB (registers 0x21 and 0x22).
    In other words: is LCD1614 capable to measure Rp of each multiplexed coil?
    Into the data sheet of this device this is not specified, also if Rp registers are present, but only for settling purpose (if I understood correctly).
    Thanks for your support,

    Best regards,
    Maurizio Liverani
  • 0
    TI__Expert 6625 points
    Hi Maurizio,
    Please find my answers below.

    1) Is the LDC1614 the correct choice for this purpose or is better use 4 separated LDC1101?
    I think LDC1614 will be a better choice as it also has 4 channels. I would only recommend LDC1101 if you're also planning to do Rp measurement.

    2) Is the firmware inside the microprocessor of the evaluation module available as open source in order to rearrange it as differential displacement sensor?
    Yes, if you go to our product page, at the bottom we have some TI designs along with some example code.

    3) in order to get the best sensor displacement linearity, it is better use Rp measurement and than calculate the rate Rp/L , while the two characteristics has nearby the same behavior over Target distance/Sensor diameter?
    The Rp value changes significantly with changes in temperature, where as L measurement is more stable across temperature. Hence, please always do L instead of Rp measurement.

    In response to your second post, LDC1614 is not capable of Rp measurement. If Rp measurement is a requirement, please go with LDC1101. I hope this helps!

    Best Regards,
    Bala Ravi
  • 0 in reply to Bala Ravi
    Prodigy 40 points

    Hi Bala,

    Thanks for your suggestions, but i have still some doubt.

    On LDC1101 datasheet, looking at the characteristic "Figure 49. RP vs Target Distance for a 14-mm Diameter Sensor" and "Figure 51. Inductance vs Normalized Target Distance for an Example Sensor" of LDC1101 I see that relative sensitivity of Rp/distance and L/distance has significant difference: The rp/dist. ranges from 2 to 17 Kohms over normalized distance change from 0.01 to 0.55,  while L/dist goes from 25 to 36 uH over normalized distance change from 0.07 to 0.7  This means that Rp sensitivity is much more higher than L sensitivity versus normalized distance.

    May be that higher Rp sensitivity compensates for higher temperature drift in calculating final distance precision.

    Furthermore, on LDC1614 datasheet, page 42, Application information, i read:

    Figure 54. Example RP vs. Distance with a 14-mm PCB Coil and 2mm Thick Stainless Steel Target.
    So it seems that also with LDC1614 you can made Rp measurements. But how? Through which registers?
    Is this just a mistake in data sheet editing or I can really measure Rp also with LDC1614?

    You say also that Rp measurement changes significantly with temperature; but also on LDC1614 data sheet, on first page, is reported the "Measurement Precision vs. Target Distance", that rises to 0.8um over the normalized distance. I do not know if this measurement precision figure is related to L or Rp measurements, neither i know if Rp measurement temperature drift is comparable with this precision data if relevant to L measurement.

    Either datasheets of LDC1101 and LDC1614 seems to be incomplete in this field.

    May be i can cooperate through lab tests in order to get higher confidence in distance measurements, if I get the two evaluation boards, either via Rp then via L measurement or using both in a mathematical combination of the two.
    Sorry, but until now I do not feel the question of the post as solved.

    Best regards,

    Maurizio Liverani

     

  • 0 in reply to Maurizio Liverani
    TI__Expert 6625 points
    Hi Maurizio,
    I completely understand. Allow me some time to think about your questions and get back to you.

    Best Regards,
    Bala Ravi
  • 0 in reply to Bala Ravi
    TI__Expert 6625 points
    Hi Maurizio,
    I really appreciate your patience. As I mentioned before L measurements are more stable across temperature. L measurement shifts ~ 30ppm/C with NPO/COG capacitors. Whereas, Rp shifts with copper targets ~ 3900ppm/C. For more information, you can check the following app note: www.ti.com/.../snaa212.pdf. Usually Rp measurements are used for material composition detection and analysis, very low frequency (<100kHz) measurement with steel, loss measurements (inductive cookware detection, inductive charging checks) etc. It's useful for applications where absolute measurements are not needed but only relative measurements.

    You're right about Rp having larger shifts for closer targets but closer targets also require more sensor drive current. If Rp is too low, sensor oscillation amplitude is also too low.

    Figure 51 in the datasheet is just showing Rp dependence to distance. The Q or Quality factor is still dependent on Rp, C and L. This does not mean LDC1614 can do Rp measurements.

    And again you can always order our EVM boards though our product folder from Ti.com and evaluate yourself. Finally, if your intend to do both L and Rp measurements, go with LDC1101. If not, LDC1614 will be a better option for you.

    Best Regards,
    Bala Ravi
  • 0 in reply to Bala Ravi
    Prodigy 40 points

    Many thanks Bala,

    I do specially found useful the appnote you suggest. ( www.ti.com/.../snaa212.pdf)

    In it you can read:

    The system should be designed such that the outputs of the sensors depend differently on the measured
    parameter (distance/position/angle of the target). For example, if the position of a target is to be
    measured, the sensors should be located on opposite sides of it. For “slider” designs, slides should be
    pointed to opposing sides, and so forth.

    While this is exactly what I intend to do (use differential measurement from two coils) this is the best answer to my doubts.
    I'll use LDC1101 and than a compensation strategy for measurements drifts with temperature.

    This solved definitively my problems.
     

    Thanks again and best regards,

    Maurizio Liverani

  • 0 in reply to Maurizio Liverani
    Prodigy 20 points
    Into the information sheet of this gadget this isn't indicated, additionally if Rp registers are available, however just to settle reason (on the off chance that I saw effectively).