Connect with us

Magnetic Pickup output signal problem

Discussion in 'Electronic Design' started by lissy6098, Jul 23, 2007.

Scroll to continue with content
  1. lissy6098

    lissy6098 Guest

    I am observing the voltage output of a magnetic pickup transducer.
    When I maintain a constant rpm (speed) it seems that the signal shows
    that the voltage varies alot in between peak points. Is there a reason
    for a voltage in the signal and how can I solve this problem.
  2. Eeyore

    Eeyore Guest

    I though that's what they're supposed to do.

    Did you expect a constant DC voltage ?


  3. This question really belongs in or for google
    group users.

    How are you reading the voltage? If it is with a digital meter, the
    sample rate will cause the meter to read at different points of the
    waveform generated by the sensor. You need a scope to see what the
    waveform looks like, and to compare the waveform. Also, what are you
    sensing? a single point, or gear teeth/multiple points?

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  4. Guest

    The most common reason for mag pickup signal amplitude variation is
    run out in the speed wheel (tone wheel excitation ring etc.) This
    effectively changes the gap which is relatively proportional to the
    signal amplitude. If you look at the signal on a scope set with the
    sweep equal to one revolution, you can see the signal amplitude
    variation for one revolution. Other physical factors can cause
    amplitude variation, like inconsistant tooth profile, damaged or
    missing teeth. Usually the rotation speed is determined by frquency
    to voltage converter measuring the time between signal zero crossing,
    or averaging of fixed size pulses, derived from each cycle of the
  5. "A lot" means what? How precise IS the mechanical distance from pickup
    to 'points'?
    BTW what did you expect, and what setup you provided?

    When "testing" it is very good to rely on known AND steady settings.


  6. The gear teeth (or other objects being sensed by the pickup)
    are not well centered. Some are passing closer to the
    sensor than others are.
  7. I work on Sony analog Betacam tape machines that use Hall sensors to
    read the tape reel rotation info. When operating properly, the
    variations are less than 10% but as the reel tables age and the
    adhesive that hold the magnet onto the wheel shift a bit, the
    variations can get up to 50%. Replacing the reel turntable corrects
    this. Output is a pair of quadrature sines that get squared up in

  8. lissy6098

    lissy6098 Guest

    The voltage output goes directly into a Labview DAS. Once we have the
    data in the computer, the data is plotted using Matlab. Where sensing
    the voltage vs time at a constant rpm. Every time one of the teeth
    passes the pickup a pulse is outputted, and it should be a sinusoidal
    waveform but instead of a perfect sinusoid we get one that in between
    peaks there are a variety of different peaks. Is there anyway to
    insert a picture of the waveform, this is my first time on here

  9. Like I said, you need to look at it with a (Preferably a triggered
    analog) scope to see the real waveform. I doubt that the shape of the
    tooth will allow a clean sine wave. The precision of the machining of
    both the teeth and of the sensor will affect the amplitude and waveform,
    but unless you have a very high sampling rate, you can't really see the
    true waveform. You will have aliasing unless the rate is high enough.

    Pictures are not allowed on this newsgroup. We use a binaries
    newsgroup, that you can't access through Google. Put it on a website
    and post the link.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  10. Guest

    You would need a proper tooth profile to approch a sinusoidal
    waveform. In most cases the output is quasi sinusoidal, with all
    kinds of distortions due to tooth profile and other factors. If your
    DAS is sampling fast enough, you maybe able to determine zero crossing
    with enough resolution to measure signal crossing intervals or
    crossings per time. With the crossing interval method, a rolling
    average of succesive intervals will stablize the reading and provide
    more acurracy. Counting crossings per time is good when the speed is
    stable and you can count for 1 to many seconds.

  11. neon


    Oct 21, 2006
    magntics lines follow the law of thr the square of the distance
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day