Conveyer belt "wander" auto-correction?

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,
--
DaveC
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DaveC wrote:

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.
--
Sue


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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;
--
DaveC
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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
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["Followup-To:" header set to sci.electronics.design.] John Larkin wrote:

Maybe it *has* to be flat? Who knows.
robert
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DaveC wrote:

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..
--
Sue





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DaveC wrote:

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..
--
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seems kind of counter intuitive doesn't it :) but that's how I recall it being done.

sure it is! apply some packing tape to the centre of both rollers and see how it goes, if good have a permanent fix done.
Bye. Jasen
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jasen wrote:

Think of two independent belts, one each side of the centre line. They would both tend to move away from the centre and run off the edge of the rollers.
Now tie the two belts together. As one tries to run off, it pulls the other towards the centre. Which creates a restorative force, as that belt's tension increases due to the increased running length.

The self-centreing mechanism does rely on the tension in one section of the width of the belt being able to change wrt the other. If the tension across the belt is always uniform (belt is very stiff laterally) then no corrective force is created. Similarly, if the belt is not under tension, no corrective force is created. Also, if the running length does not increase (eg a mid roller, not an end roller) then no corrective force is generated.
--
Sue





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Again, IIRC, it's a convex profile for elastic belts (eg; leather, neoprene), & concave for other materials (eg; chain, fibre-reinforced).
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us:

Go away, troll. Your grasp of mechanical engineering is on par with your level of maturity in your behavior in usenet, and you have proven that to be nil.
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IIRC, it depends on the belt & roller material, also on whether the roller is powered, or an idler.
--
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It is an arc and it is less than a couple degrees across the entire face.
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Or, just the opposite by placing a "dip" in the center, the belts edges are kept within the ends of the roller.
Some cases concave works, and some cases convex works. That roller goes at one end, and the adjusting roller goes at the other. It may or may not already have an arced (convex) face. The overall taughtness of the belt is also a factor.
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snipped-for-privacy@ssiveBlackHoleAtTheCenterOfTheMilkyWayGalaxy.org wrote:

There are two different mechanisms that can be at work. One is the "inclined plane". The other is "longitudinal displacement", caused by part of the belt having a longer path (ie over the larger roller diameter(s)).
--
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There is one more. The belt flexes, and then rebounds from the flexure. The taughtness of the belt in its center on the domed set up creates a line of tension down the length of the conveyor. The perpendicularity of the other end (the adjustment face/roller) with respect to that line of tension, is what keeps things centered. The less taught sides are where the entire "system" looks for a symmetry in the forces that would shove the belt end off the edge of a roller.
The goods being placed on the belt also become a factor.
So, the tension is really one of the most important factors for making the domed roller have enough "affect" to exert continuous control.
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On 4/25/07 7:41 AM, in article snipped-for-privacy@news.sf.sbcglobal.net, "DaveC"

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.
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On Wed, 25 Apr 2007 07:41:57 -0700, in sci.electronics.design DaveC

You could try capacative sensing, since the belts are carbon fibre, and some sort of bridge detector
martin
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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
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John Larkin wrote:

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.
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