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Conveyer belt "wander" auto-correction?

Discussion in 'Electronic Design' started by DaveC, Apr 25, 2007.

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  1. DaveC

    DaveC Guest

    A coating machine in a print shop utilizes a 1-meter-wide, 10-foot-long
    conveyer. The belt for this part of the machine is 1cm (approx) mesh of what
    appears to be carbon fiber. Drive is via 200mm rollers at either end of the
    conveyer section.

    The belt, as do all such mechanisms, doesn't want to stay centered and
    constantly wanders to one edge or the other. The operator must observe the
    location of the belt and manually adjust one of the roller's parallel-ism(?)
    via a knob-and-screw for this purpose. Of course, this isn't a perfect
    solution and the belt has many times wandered too far and frayed the edges
    which necessitates replacement far more frequently than should be required.

    How best to automate this process? I've seen high-speed belt sanders (1-meter
    width) that have a solenoid that "jogs" one of the rollers when an optical
    sensor is tripped which "jumps" the belt sideways, but this belt is traveling
    at high speed and is much shorter in length than the conveyer belt, so the
    conveyer will not benefit from the same solution, I think.

    Motor drive of the adjustment screw with optical sensors at the edges of the
    belt's limit? I see a microcontroller project in my future. Seems to call for
    some fuzzy logic or such, so that the controller can "learn" where the center
    is and apply just enough adjustment to limit its travel to the extremes...
    (more easily said than done, by a factor of a few orders of magnitude!).

    Other suggestions?

    Thanks,
     
  2. Palindrome

    Palindrome Guest

    Machine the rollers to put a very small taper into the centre. Slightly
    increase the tension at the centre line of the belt. The belt will then
    auto align itself onto the centre line and self-correct any tendency to
    wander off.
     
  3. Salmon Egg

    Salmon Egg Guest

    Offhand, you probably have to contract with a competent licensed engineer.
    My gues is that the rollers and the belt were not designed with the
    appropriate camber or other features that provide self-centering.

    Bill
    -- Fermez le Bush--about two years to go.
     
  4. You could try capacative sensing, since the belts are carbon fibre,
    and some sort of bridge detector


    martin
     
  5. John Larkin

    John Larkin Guest

    The usual fix is to "crown" the rollers so that they are bigger
    diameter in the middle and skinnier at the ends. Then the belt
    self-centers.

    John
     
  6. DaveC

    DaveC Guest

    Machine the rollers to put a very small taper into the centre. Slightly
    Thanks, Sue.

    This is a *larger* diameter at the center?

    Where might I find more information re. how much taper to apply? This is not
    a situation for trial-and-error (c;
     
  7. Robert Baer

    Robert Baer Guest

    Very simple!
    The drive roller, or largest roller, should have a curve in it so
    that the center is larger than the edges.
    Or pick two smaller rollers, spaced "far" apart in the loop.

    For a quick and dirty test, add some adhesive-backed tape onto a
    roller, centered and maybe a second, more narrow tape also centered (to
    get that curve).
     
  8. When I was in high school, the school had an old-fashioned type printing
    press with the giant cast-iron flywheel. It had an electric motor to keep
    it going, but you still controlled the speed pretty much by hand. The shop
    teacher's favorite way of getting the giant leather belt to track right was
    to build up the drive pully with masking tape, forming a crown. Seemed to
    do a good job.
     
  9. John Larkin

    John Larkin Guest

    Yes, larger in the center. If the belt tension isn't extreme, try
    using some temporary bands, hvac type aluminum tape or something, to
    fake a taper and see if it works. I bet you could google "tapered
    roller belt" or something for tips, too. In my (limited) experience,
    the taper is usually pretty significant, and sometimes just near the
    ends.

    I'm surprised that anybody would use a smooth roller for a belt like
    this. It's guaranteed to be unstable.

    Classic leather-belt-driven lathes used rollers that were very
    rounded, quarter-circle almost. It was uncanny to see a belt start at
    the very edge and walk right up to the crowned center.

    John
     
  10. Palindrome

    Palindrome Guest

    You can either put the larger diameter/lower tension at the centre or
    smaller diameter/higher tension. Both will auto-correct but the latter
    will tend to make the belt to bunch up in the middle, whereas the former
    can tend to make things fall off the edge of the belt..

    It's just a bit of maths to work out what corrective force is being
    applied, knowing the difference in tension and the angle of the inclined
    plane created by the taper(s).

    The alternative is to install two extra free-wheeling short rollers,
    tilted towards the centre and under the edges of the belt. As the belt
    tries to wander towards one edge, it has to go "uphill", up the
    free-wheeling roller. The advantage of this method is that you can
    adjust the position and angle of the rollers to achieve the centreing
    that you want and it doesn't require any modification to the existing
    equipment..
     
  11. Phil Hobbs

    Phil Hobbs Guest

    One common method is to flare the rollers slightly at the ends. That
    way if the belt wants to ride up on the roller, there's a centering
    force due to the stretching of the belt.

    Failing that, something like what you propose could probably be made to
    work--but I'd definitely avoid optical sensors for this if possible.
    The problem is that they get dirty and stop working, which in an
    automatic control situation like this would be dangerous.

    Another simple method might be to have guide rollers at the edges of the
    belt. Rather than encoders and motors and microcontrollers, you could
    have the guide roller turn the adjustment screw via gears or a chain
    drive. That way, any time the belt got far enough out of line to spin
    the guide roller, it would be gradually adjusted until it didn't spin it
    any more. That's sort of an integrating servo--if the gear ratio (i.e.
    gain) is too high, the control loop will oscillate, and the poor thing
    will bounce back and forth until it chews itself to bits.

    Since the situation doesn't require too many smarts from the controller,
    this would probably be easier and a good deal more reliable.

    Cheers,

    Phil Hobbs
     
  12. Rich Grise

    Rich Grise Guest

    You need your rollers crowned. I can't draw one in ASCII, but essentially,
    the rollers need to be thicker in the middle. Not by much; I don't know
    if there's a formula, but that's the standard way of keeping a belt
    centered.

    Let me know if you need some custom rollers built - that's one of the
    things that the company where I sit does.

    Good Luck!
    Rich
     
  13. gavin

    gavin Guest

    Whwn i used to work for a conveyor manufacture we used to crown one quarter
    of the width to create a 2mm drop from the diameter of the roller

    Try this it will help

    Gavin
     
  14. Jamie

    Jamie Guest

    Normally using a crown center roller would fix it how ever, since this
    is a printing machine, and the belt width isn't very wide (10 mm) I
    think you said?, If you want automation it can be done via using 2
    simple small light weight with following groves with rollers, that have
    a small spring to keep the two rollers tight on the edges of the belt.
    These 2 rollers connect to an arm which is connected to a potentiometer
    that will drive a simple geared head motor on the manual screw via a
    regen drive..
    We had a similar problem also, ours was a much wider belt but that is
    what we did. In our case we use 2 pots, one on each side of the belt
    that are join electronically to report a differential signal to an
    air servo piston that is attached on one side of the roller assembly.
    The output operates an IP value (Current 4..20 ma's), the electronics
    is nothing more than a voltage comparator that generates the current signal.
    normally we would use a gear head motor how ever, money was a problem
    at the time and this worked out just fine.
    You may find using an air cylinder and IP valve from a dancer control
    board may work fine..
     
  15. whit3rd

    whit3rd Guest

    And, the important rollers are the turnaround rollers, the end ones
    that the belt wraps 180 degrees around. The simple idler rollers
    and the drive roller (if it's a capstan drive and doesn't turn the
    belt around) can be simple cylinders.
     
  16. Palindrome

    Palindrome Guest

    It is generally the rollers where there is a change of direction that
    are important, because they can most easily translate changes in
    longitudinal tension forces into lateral movement (inclined planes).

    For an outward tapering roller, the belt must be capable of conforming
    to some extent to the profile of the shaped roller(s) as a result of the
    belt tension. Any decrease in tension, eg belt stretch, can greatly
    affect lateral stability.

    OTOH, for an inward tapering roller, the belt must be rigid, laterally,
    or it will bunch into the centre. Belt tension variations are generally
    not so critical.

    The nature of the belt thus determines the roller profile needed to
    self-centre.
     
  17. jntel

    jntel Guest

    Look for "spreader rollers" or "banana rollers" in your favorite search
    tool.
     
  18. ["Followup-To:" header set to sci.electronics.design.]
    How about a very simple electrical "middle of the road" thing? Mechanical
    switches with rollers that activate a geared-down motor. One switch on
    either side of the belt, each making the gear motor turn in opposite
    direction from the other. Rugged, simple, and a lot cheaper than an
    all-mechanical solution.

    robert
     
  19. ["Followup-To:" header set to sci.electronics.design.]
    Maybe it *has* to be flat? Who knows.

    robert
     
  20. jasen

    jasen Guest

    use (slightly) barrel shaped rollers.

    Bye.
    Jasen
     
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