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Sensing position of a vibrating magnet using LDC1000?

Other Parts Discussed in Thread: LDC1101

I am looking to measure the position of a vibrating magnet (https://www.kjmagnetics.com/proddetail.asp?prod=D201-N52) that is attached to an object that is vibrating at up to 10 kHz.  There is a stainless steel plate between the magnet and the pickup coil, which is spaced ~1cm away.  The displacement of the magnet is a few microns.

Is the LDC1000 appropriate for this sensing operation?  Can it observe motion of a permanent magnet - I know the features say it doesn't require a magnet, but my application requires one for other reasons.

Is this frequency of vibration with small displacement resolvable?

Thanks,

Taylor

  • Hi Taylor,
    The LDC1101 is an improved LDC1000 which also offers higher sample rates and many other improvements: www.ti.com/.../LDC1101. It samples up to 180ksps. A few microns change may be ok. What sample rates are you looking for?

    Inductive sensors are generally not affected by magnetic fields, but the interaction between the magnetic field and some magnetic steels can actually be sensed. Refer to  for more details.

  • Thanks for the information Ben - I read the attached links and see what you are talking about.

    So if I were to replace the magnet with a piece of iron or steel that was vibrating, then I would have much better luck resolving its position vs time?

    What is the ideal target material to sense?

    Taylor

  • Hi Taylor,

    The best and most feasible sensing target material would be copper.

    But back to Ben's original questions:

    • What is the thickness of the stainless steel metal plate between the sensor and the metallic/magnetic target?
    • What kind of sampling rate would you need to account for the vibrating motion?

    Sensing through a stainless steel plate is possible if you operate at low LC resonant frequencies but the sensitivity is diminished due to the presence of the steel plate which is also conductive.

    Ben has an experiment in his blog where he shields the magnet with an Aluminum plate, in your case the stainless steel would act like a shield.

    Regards,

    Varn Khanna,

    Applications Engineer, Sensor Signal Path,
    Silicon Valley Analog,
    Texas Instruments.

  • So I am understanding that you sense materials with high electrical conductivity best.  I need the "target" to be magnetic, how would something like a permalloy work? (invar, kovar, etc.) It has similar conductivity as stainless steel.  

    Is the shield and target materials having similar properties problematic?

    Currently the thickness of the stainless plate between is 0.05" - but could be made thinner.

    The highest frequency I expect to resolve is 10 kHz - so I believe a sampling rate >20 kHz would be required. (I'm a mech. engr - I looked up sampling rates and found an article regarding the Nyquist frequency/sampling-rate, if this is incorrect please advise)

    Thanks,

    Taylor

  • Hello Taylor,
    in most applications, the sensor senses a moving target directly. Aluminum and copper are excellent target choices, but less conductive materials such as stainless steel can also be used (with reduced code change compared to aluminum or copper).

    Sensing through metal is possible under certain conditions. If you want to sense through metal (sheet of 'interfering' metal that is placed between the coil and the moving target), then the following factors are important:
    - Thickness of the interfering metal: the interfering metal would have to be very thin; the thinner the better.
    - Conductivity of the interfering metal: The interfering metal should be as little conductive as possible. For example, sensing an Aluminum target through SS304 works much better than the other way around.
    - Operating frequency: Because the skin depth of the metal is larger at lower frequencies, the interfering metal reduces the dynamic range less than at higher frequencies. Therefore, in this special case, I recommend operating the sensor frequency as low as possible (ideally tens of kHz).
    - Magnetic properties of the interfering metal: Magnetic materials are much better shields to magnetic fields than non-magnetic materials. Therefore, if an interfering metal in the system is unavoidable, it should not be a magnetic material. Alloys such as invar, kovar, or SS430 are very effective at shielding the sensor, and therefore very little of the actual target behind it can be seen by an inductive sensor.
  • Hi Taylor,

    If you have a strong vibrating magnet, you can use magnetic plate (iron, or SS400-series steel) between. Use a LC frequency below 1MHz.

  • I want to retain my SS304 shield, which is not strongly magnetic.  I will try to use copper as the target material.

    The next complication I have is that the target that I am looking to resolve is 1/8" diameter and 1/32" thick.

    I read that for sizing considerations you'd want the coil to be at most 50% larger than the target.  Has anyone tried to make such a small coil? Also should I only expect a sensing distance of the coil diameter?

  • Hello Taylor,
    a 3mm diameter target is difficult to sense at a distance of 10mm through another piece of metal. Is it possible to make your target larger? Please refer to: e2e.ti.com/.../inductive-sensing-target-size-matters
  • Hi Taylor,
    additionally, a 1cm target distance should ideally have a coil of >2cm diameter. You may need an even larger coil given that you are sensing through metal. Please also refer to e2e.ti.com/.../inductive-sensing-how-far-can-i-sense