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Driveshafts - Part II

Discussion in 'Electronic Design' started by mpm, Jul 12, 2007.

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  1. mpm

    mpm Guest

    I give up. What is the "best" way to detect driveshaft speed and
    direction?
    We are only concerned with about +/- 5mph, and axel gear ratios from
    the mid 2's to high-5's generally. I haven't plotted these yet, but I
    suspect it will be in the under 300 Hz range, for mid-30" size tires.

    Problem: Are there any Hall Effect (or whatever?) sensors that can
    detect the south-approaching pole of a magnet AND have a "reasonable"
    sensing distance?? So far, all I've found are lots (and lots) of spec
    sheets like this one:

    http://www.cherrycorp.com/english/sensors/sd1002.htm

    Seems horribly scant on information, though I admit I don't engineer
    Hall sensors very much.
    I want to avoid unbalancing the shaft, or having to use multiple
    (oddly spaced) magnets to detect direction of spin. Anybody got any
    clever ideas? Thanks!!! -mpm
     
  2. James Arthur

    James Arthur Guest

    Use multiple sensors, spaced 90 deg, and two magnets, spaced so as to
    maintain the shaft's balance?

    Cheers,
    James Arthur
     
  3. PeterD

    PeterD Guest


    Generally one magnet won't affect balance, but if you are worried, use
    two.

    Mount two sensors, offsetted from each other by a few degrees. The
    phase between the two sensor outputs will give you operating
    direction. Simple application, I bet there are chips to do it already
    on the market...
     
  4. kell

    kell Guest

    that one's for sensing gear teeth, not magnets
    maybe this:
    http://sccatalog.honeywell.com/pdbdownload/images/103sr13a-1.pdf

    here's one that I have used in the past:
    http://sccatalog.honeywell.com/pdbdownload/images/sr13c-a1.pdf
    but it might not be suitable for installing in the undercarriage of a
    vehicle.

    I had a very small neodymium magnet positioned about a quarter inch or
    5/16" away from the sensor IIRC.
    I'm sure you could have the magnet farther away than that, just don't
    know exactly how far. You might be able to do some research and
    figure it out from the gauss figures on the data sheet.
     
  5. Joerg

    Joerg Guest

    I am no expert on this but have done similar things in industrial
    control. A very long time ago. One important aspect is to deal with all
    the crud that can accumulate down there over time. Either it must not
    matter or the rotation of the shaft has to move enough of it out of the
    way, all the time. What we used were sensors that had an oscillator and
    receiver coil, arranged in a U-shape. IOW there was a large slit in
    this potted device. The output was digital, a sharp drop in current when
    interrupted because that stalls the internal oscillator. These devices
    would keep working even when badly covered up with oil sludge.

    This device was bolted to some flange on the frame and a partial disk
    would interrupt that on every rotation. Basically a piece of metal
    fastened on the shaft. In your case you would need two such sensors next
    to each other and the partial disk would have to cover both but not much
    more. Then you can detect which one is interrupted first so you also
    have the direction. You'll need some (simple) logic to cover the case
    where the shaft reverses direction while being sensed.

    Anyhow, if this is for heavy duty industrial use and reliability is
    paramount I suggest to talk to these guys and AFAIR they have a US
    subsidiary:

    http://www.turck.de/en/
     
  6. Tim Wescott

    Tim Wescott Guest

    Why don't you just tap off of the speed sensor in the transmission?

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Do you need to implement control loops in software?
    "Applied Control Theory for Embedded Systems" gives you just what it says.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     

  7. Or one of the ABS speed sensors...
     
  8. Jasen Betts

    Jasen Betts Guest

    two sensors, or cheat and tap the reverse-light switch.

    Bye.
    Jasen
     
  9. mpm

    mpm Guest

    This does seem to be the right approach.
    I was thinking multiple magnets, but multiple sensors helps with the
    bigger problem of shaft balancing! And, this approach should be
    fairly "easy" to ruggedize.

    It also solves the problem of trying to find a single sensor with 2
    pickups and a reasonable sensing distance.

    Believe it or not, a lot of off-road construction type vehicles don't
    have backup lamps, so that's no help. Also, as there are several
    model trucks involved with this project, we want to stay away from
    tapping into the vehicle's speedo - which may not function in reverse
    anyway(?). In fact, some of the vehicle's don't even have speedos, so
    that's out too.

    Originally, I wanted to use an accelerometer. This way, we don't have
    to "touch" the vehicle at all, which saves installation labor.
    However, the software development time to use such a sensor and get it
    working right looks like it will exceed our window for
    deliverables..., unless there were an off the shelf solution which so
    far, we have not located. Plus, it looks like those sensors require
    substantial calibration post-installation, which is not very
    appealing.

    Thanks for the help. I will check the links out and give 'em a try.
    -mpm
     
  10. Joerg

    Joerg Guest

    Many vehicles for heavy duty use don't have any of this, especially
    older ones.
     
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