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LDC1314: LINEAR EXTENDED (30cm) POSITION SENSOR

Part Number: LDC1314
Other Parts Discussed in Thread: LDCCOILEVM

Hi,

I've already used with success LDC1314 as metal target proximity detector.

My new project is a linear position sensor with long travel: around 30cm

1) Evaluation coils and application notes put a limit at 10cm

Why?Is it possible to reach longer length?

2) In any case I could put coils in "series" to reach the desired detecting lenght. 

What about this?

3) I also have some doubts when the metal target covers two closed coils; is the sensor reading correct and linear in this particular position too? Some "crosstalk" bad effects due to parasitic mutual inductance?

Thank you in advance

Andrea Regis

  • Andrea,

    You have asked some great questions.
    1. Evaluation coils and application notes put a limit at 10cm.
        This is what we have found based on a single coil & target with nominal dimensions and parameters, interfaced with one of our LDC devices. 
         The "10cm" is intended to provide a reference point. It may be possible that one of ultrasonic sensors or radar-based sensors may be more appropriate,depending on the end application and its requirements.

    2. In any case I could put coils in "series" to reach the desired detecting length.
        We have no experience with a sensor arrangement like you have described.
        Having said that, your "series" coils would certainly be worth a try as long as each coil is:
          - Treated as a separate sensor channel by the LDC device(s) (e.g. none of the coils are connected to the same LDC input).
          - The resonant frequency of each resonant coil is different and non-harmonically related to the other two. 
             For example, picking resonant frequencies of 1MHz, 2MHz, and 4MHz would probably not a be effective. 
          -  Sensors can be placed in such a way as to minimize cross-talk. 
          -  You have not mentioned the target requirements, so I am assuming the target will "slide" along the three coils, at a small distance above them.

    3.  I also have some doubts when the metal target covers two closed coils; 
         I'm not sure what this means. Are they two separate co-located coils connected in series or parallel?

    Regards,
    John

  • HI Miller,

    thank you first for your support.

    1) I understand different kind of sensors could be more appropriate; anyway as a specification we would investigate inductive technology

    2)I use the tem "series" but i do not mind "electrical series connection" (sharing current...).

    I would simply put colis one close to another in order to cover the long travel in case one single coil can not be so long.

    The target is to be defined yet (aluminum?)

    3) I refear to the setup described in the drawing and at point 2 above; coils are close but connected each to independent LDC ouput. I'm wondering if the target , which is conductive, could produce a crosstalk effect between two coils while covering the end of coil A and the beginning of next coil B.

    I think the eddy currents produced on the target covering two independent, but close (LDC ouputs) coils could have a bad effect on the slide position reading.

    Thank you!

  • Andrea,

    I was about to say we have no experience coil & target arrangement like you have proposed, but then it occurred that we have  the TIDA-00508  EVM.

    If you haven't seen it already, please take a look at the user guide for the TIDA-0508.
    It is a dial application with 1 degree accuracy which uses four coils sharing a single target shaped as a bent double-taper.

    It is different than your application, but it shows that is some cases, coil sensors can share a target. 
    I hope it has some useful information.

    Please let me know if you have any questions.

    Regards,
    John

  • Dear John,

    thank you for the suggested link. I read TIDA-00508 and found very useful.

    In the past I also observed ferromegnetic target present on both coils could affect readings.

    I'll anyway test different target on two adiacent  LDCCOILEVM coils.

    Regards

  • Thank you Andrea.
    I will close the thread for now, but please feel free to update this thread with any of your findings, or with any additional questions you may have,

    Regards,
    John

  • Thank you; I'll test and I'll send our results. 

    It could be useful to indicate how a common target  affects reading of different coils (TIDA-00508  does not explain the parasitic mutual inductance....; the target should have poor magnetic property anyway).

    In case Texas has other detailed technical notes please share.

    Thank you for your good support.

  • Andrea,

    We don't have any info on multiple coils sharing a target, but I will add it to our growing list of app notes we are working on. 
    Thank you for your patience.

    Regards,
    John