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position sensor for harsh environment.

Discussion in 'Electronic Design' started by Pat Ford, Jan 31, 2005.

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  1. Pat Ford

    Pat Ford Guest

    Hi All;
    I'm looking at building a 4 wheel steering system for an offroad truck. The
    rack has ~ 9" travel and ~3.5 turns at the pinion. I was thinking about a
    pot for position sensor, BUT this truck current runs, on occassion with the
    rear axle 12" under water.
    I was also thinking LVDT, but that is expensive.
    What would you do?

    BTW this is for off highway use, so the truck won't lose control and knock a
    gas tanker into a school buss convoy.
    Pat
     
  2. Put a rotary encoder on the steering column shaft, above the operational
    water level limit. If the rack jumps a tooth you have more problems than
    just a wrong sensor reading!
     
  3. Pat Ford

    Pat Ford Guest

    There isn't a column, a worm gear driven by an electric motor is the input.
    I'm going to see if I can find a rotary encoder that is water proof. I'm
    hoping to come up with a simple voltage output device, then I can do the
    control system as a simple analog servo.

    Thanks John
    Pat
     
  4. Is the motor waterproof? Is the non-drive end above the water level?
     
  5. Tim Wescott

    Tim Wescott Guest

    If you're trying to drive something through a gearbox your control
    system won't be simple --
    http://www.wescottdesign.com/articles/Friction/friction.html. You'll
    use up less board space (and likely tear out less hair) if you use a PIC
    with PWM output.

    If it were me I'd do one of the following:

    1. Grit my teeth and spring for an LVDT. When you buy a commercial
    LVDT for $150 you're spending about $20 for the transformer, $70 for the
    rugged, and $60 for the advertising and to have it sitting on the shelf
    so's you can buy it and have it shipped today.

    2. Roll my own LVDT. This is not that bad if you don't need rugged,
    and you may be able to arrange for rugged with larger size and less
    accuracy for less money (assuming your time isn't worth squat).

    3. Use an optical encoder an John suggested, with a home switch and a
    PIC to make it all work. Of course, a nice rugged optical encoder and a
    home switch costs as much as a nice rugged LVDT.

    4. OTOH, coarse optical encoders can be whomped up fairly easily.

    5. If I can use a pair of gear-tooth sensors in quadrature, plus a home
    switch: http://www.allegromicro.com/.

    I think that in any of the encoder cases I'd put the "home" switch so
    that it's closed on one side of travel and open on the other, with the
    switch happening at the center of travel. This isn't tradition but it
    means that you start out by driving to the center, and it also means
    that you have a redundant sensor should your encoder crap out.
     
  6. Pat Ford

    Pat Ford Guest

    the motor is water proof, it started life as a trolling motor ( for a
    fishing boat)
    Pat
     
  7. Ben Bradley

    Ben Bradley Guest

    I like the idea of a quadrature detector using magnetic Hall-effect
    sensors. Dust, dirt and water generally don't affect magnetic fields
    (I presume the mud doesn't contain iron ore or filings), whereas
    optical sensors can be disabled by the dirty water or mud that is
    likely to get into such a device near a wheel.
    You can put each Hall sensor halfway into a long heat-shrink tube
    that folds back up so that both open ends are well above the water/mud
    line.

    And even after doing the best job you can to make it reliable, I'd
    feel a little more comfortable if there were also a backup consisting
    of two center-off spring-loaded toggle switches as no-feedback (except
    leaning over and looking at the actual angle of the wheels, or just
    seeing which way the vehicle is going!) steering control, each one
    switching power directly to each steering motor.
     
  8. Paul Burke

    Paul Burke Guest


    Whatever encoder you use, don't use an incremental one! Otherwise you'll
    have to re- initialise the thing when you turn on. If you've got any
    rotational element that makes a single turn, a resolver would be a good
    choice- very rugged, absolute and not tremendously expensive compared to
    your life.

    Talking of which, I wouldn't voluntarily hang mine on an electronic
    system anyway...

    Paul Burke
     
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