Dancer Speed follower circuit before i build one

This is the scenario: Lets say we have a catenary arm with a pot on it that monitors the catenary position. This arm has a chive (roller) where we wrap wire around
it to serve as a pully. As the wire starts to be pulled away from the machine, this arm will move thus moving the POT which signals the drive to give the reel of wire a push to maintain the catenary arm in a desired position.
Now, we use regen drives for this application (Penta KB series) to be exact, and here is the problem.. The load on the shaft is heavy and what happens is when the catenary arm moves in position to signal the drive, there may be a delay of response due to the extra swing load of weight and thus the arm might end up in a position where high bias voltage is now at its point and when the shaft finally gets going in the other direction, it goes to fast! And thus, this extra weight generates a lot of it's own momentum. (Swing load).
This is what i want to know before i go and build it. cause if it's on the market i would rather buy it.
I need to detect the speed of which the catenary arm is moving, not the reel speed! I do not want to employ encoders on the shaft because of expense reasons. It must generate a negative response to what the POT wants the drive to do as the POT starts to change position and as soon as the pot stops, the POT's real signal will be reported at the position it is currently in. I need this type of control to not allow the drive to accelerate so fast that the catenary arm whiplashes. My thoughts on this for ideal thinking, is something like a voltage follower that mirrors the POT's signal, a capacitive system will create an inhibiting signal as the voltage changes but not inhibit when the voltage from the POT is stable or changing very slowly.
The end results is, the POT's signal to the drive will change slowly to get the catenary arm into position but not too slow where it will cause it to stall in position. Lead, Lags, Delays etc found on most Dancer and drive controls does not do what i need, due to the fact that a lot of these circuits tend to memorize last time effects and just compound the problem.
I hope i have made this clear enough, if there something on the market in like a mini board form that runs off lets say the REF's supply voltage of around 10 or 15 volts that conditions the signal it would be nice.. Other wise, i will have to make my own debouncer circuit..
P.S. we have already tried outboard dancer boards like the old Reflex series and the drive it self has an extensive set of controls which serve their purpose just fine how ever, none of them give me this kind of control.
I have seen this in eurotherm digital drives where the max speed can be retarded by the rate in which the POT value changes. The faster the POT changes, the slower the drive/motor attempts to go. but in our case, we are using non-digital drives ( a lot of them) with Armature feed back!.. Thanks for any input.
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Jamie wrote:

This is a solved problem. In fact, it sounds very much like you're asking how to design a PD controller with your dancer arm as an input.
Many motor drives have built-in PID controllers; you can often dial in the gains without having to get an external board. I don't know if this is possible with your particular drive, however. I have looked for 'just' PID controllers, but there don't seem to be any available that operate at this sort of speed. In theory you could use a PLC, but they tend to operate much too slow for motion control.
I can tell you all about building a custom controller for any given application, but I am very weak indeed when it comes to knowing what parts to buy off the shelf to do this sort of job. So I'm taking the liberty of cross-posting question onto sci.engr.control, where there are people who _do_ know the answer to this sort of question.
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In another life we had these damn things...you can fiddle with PID loops and stuff all you want, which is what we did.You have sereral error terms, which interact to cause you misery. Here's what we finally did to get some peace.
The dancer pot was replaced with a gray-code absolute encoder, and connected to a GE DV-series drive (reel drive) operating in position control mode, namely "control the position of that f#$#@ dancer." It has a PID loop, but you don't have to fiddle with it. Later on, we added an ultrasonic sensor to measure the diameter of the material on the reel and used that signal to tweak the gain of the system as the reel (and therefore the "gear ratio") increased/decreased. That was in 1997 and it runs still today. If you do that, do yourself a favor and write down a calibration procedure at the build and keep it forever.
There is some minor stuff to do, such as setting speed limits and faults such as a material break. The install is costly, but if you ask me it is cheap at twice the price.
Seems better to think in servo terms rather than speed control terms.
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Steve Cothran wrote:

Thanks for your input. I'm glad to read the views of others that understand exactly where I am coming from :) Using any sort of encoder set up is out of the question due to cost over head. This project originally was suppose to be quick and dirty with very little unexpected undesirable results. The project was done by a contractor due to the work load we already have in the plant. After a few Penta Drives (KB regen series) going belly up on initial power up of the line. I started to curl my brows a little bit at the contractor's abilities. I say this because, the week before this company diagnosed a problem on one of our lines that coast us $7k which didn't solve the problem at hand! It was one of our young engineers that suggested to have them look at the problem when i had already give them my opinion of what it was, and it sure wasn't a drive, which is what they replaced. Any ways, after they left that day, i decided to inspect a couple of the drives that burned up.. What i found was soot in side with no FUSE protection at all per drive. The only protection they were relying on was the main line fuses that feed all the drives on that circuit, that issue has been corrected. Now the part with the Dancer which i have come up with something now and testing it. I know an encoder be it an incremental of Gray code would do the job how ever, like i said COST!.
What I'm doing is using an AtTiny uC with ADC inputs and PWM outputs to regenerate the +/- volts.
I use one ADC input that has a POT on it to adjust my max dancer speed movement before the circuit starts to retard the ref signal. One ADC to read the Dancer POT. One Input to select 0 or 5 volts as a center ref. I was able to put it on a mini board and stick it in side the NEMA 4x box using the 15V +/- reference pot leads as the supply to operate it. I'm using the internal OSC of this AVR so that i have extra legs to play with. I have 2 legs more to play with for other options as this grows. It seems to work , we'll see in actual long term use.
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On Sun, 10 Dec 2006 20:55:00 -0500, Jamie
world:

Regen drives often have pots to adjust min speed, max speed, max accel and max decel. They often also have a "comp" adjustment which controls boost or sag in the response. Better regen drives have PID built in. I have found that you can often get a dancer control stable by adjusting the max accel and decel to match the inertial load. Cut off all but the proportional portion of PID and adjust it as wide as possible eg min speed at pot arm travel high and max speed at pot low. It should work.
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Paul M wrote:

Yes, these drives have all of those controls how ever, they do not correct for the problems totally. These pay off units use a catenary arm that has a short travel to them. If it wasn't for this short travel, we wouldn't be having so many problems. We can balance the units fine on lite loads but on heavy loads, they start to whiplash due to the swing load that is taking place. making the DB (dead band) at it's widest helps the problem greatly other wise, you get out of control rocking, because the drive is regening and we need this function to maintain tension at idle.
Any ways, the Sampling of readings using the AVR seems to work in detecting the rate of POT value changing. it's kind of strange looking when you watch this in action. The Catenary moves about 2 inches back and forth slowly as it runs, but no jerking and slamming the arm on it's stops. We might apply this same little unit on some over head accumulators so that the operator does not have to manual nurse the dollies in place for dance mode after a real chance on the fly.
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On Sun, 10 Dec 2006 22:07:15 -0500, Jamie
world:

I don't know what your process is, but I have never seen a system that works well by tensioning with a drive. I would abandon that idea and tension with a weighted roller arm, using the drive to maintain the arm position only. You can vary the tension by using a movable weight on the tension arm.
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On Mon, 11 Dec 2006 00:55:23 -0500, Paul M <PaulMatWiredogdotcom> wrote:
. I would abandon that idea and

Yeah, and even better if he festooned the stuff on weighted rollers.
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Paul M <PaulMatWiredogdotcom> wrote in

hrm.. There are tons of them that work just dandy in the paper industry.
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Anthony wrote:

What do You think about such tension control I found in flexographic printer controlled by Digitrac by MAGPOWR?
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proclaimed to the world:

You are right, I should not have made such a blanket statement. Let me qualify this a little. Using the controls and motors he indicated and/or I am assuming, the systems I have seen do not work as well as a dancer roll simply because each part of the system has to be matched exactly to the characteristics of parts in the system which change with speed and material. Inertia varies with product weight. Product properties differ. If you want to make a line flexible, then it needs to be able to compensate for those things. Dancers can do this because they make slack in the web while tensioning at the same time. Take a hypothetical line that rolls out dough. As the does is calendared and pulled to a roller conveyer it stretches. A dancer roll is actually measuring that stretch and adjusting the roll speed as product varies. The arm can be adjusted to pull on the web only slightly where a motor tensioning system must overcome its on friction, which varies as the machine ages and bearings get dirty. Adjusting the controls so they work today will not insure they work next week.
So on a line that needs minimum tension and the material properties vary, a dancer works much better.
I am working now on designing a new type of tensioning system for a machine that wraps a material under tension onto a spool using a regen drive system. In this situation I am looking for obtaining consistent high tension values. I believe that this will work better than the systems out there now. It's ironic that I would be working on designing a drive tension system and make the statement that they don't work well, but it really depends on the product properties. The paper industry needs either a follower system with no tension or it is tensioning a product with little to no stretch.
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Paul M wrote:

Do you recall the vacuum columns on the old reel-to-reel computer tape drives? Constant negative pressure in the column maintained constant tape tension no matter which reel held the most tape, and photocells sensed the loop positions, adjusting the servoed speed of the reel motors.
Jerry
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proclaimed to the world:

No I never got to be around any of that gear except for what I happened to pick up in a lot of surplus. I used to buy stuff from government and university surplus auctions. Some of the stuff I rebuilt and used in my labs or savaged for parts to build custom lab equipment. I've still got a junk yard in a warehouse I take from. Saved a lot of money on variac autotransformers. I once bought a bomb calorimeter and a microwave spectrum analyzer for $5 bucks. Somewhere in there were some old tape drives but never tried to do anything with them. I could never have afforded the equipment otherwise.
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I do, IBM ones, although I think the ones I saw had a line of holes in the tape column and sensed the tape loop position by looking at which (or perhaps how many) holes saw vacuum. The real advantage of this 'dancer' system is its effectively zero weight.
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Paul M wrote:

I have corrected the problem by using an AVR "atmini" processor , i simply pass the Pot Analog in, process it, pass the results back out to recreate the analog signal but only conditioned, to correct for whiplash control. The system we have only uses a catenary arm roller with aprox 15" in length and has no more than aprox 70 degrees max in swing. It would have been simpler if we had an accumulator, but that isn't the case here. I gotta say, this has turned out much better than I thought it would have. I was able to get the unit to track it so even at faster speeds, the unit would condition the signal to force the arm into the near 0 volt position of the POT. For most types of dancer controls, usually faster speeds means the resting point of the arm will tend to be in different places at various speeds. Yesterday, we applied this circuit to an over head accumulator, to control the fill speed until it reached resting dancing position. By using a precision high resolution multiturn pot, we were able to get the processor board to monitor and bias the slip clutch drive to hold a crawl speed to where the operator could simply walk away from it with out the need to nurse it using only a POT and no encoders for speed control etc.. I'm very pleased with what i was able to do with this tiny little 8 pin uC..
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On Thu, 14 Dec 2006 22:08:46 -0500, Jamie
world:

That's great. AGM makes a similar signal conditioner which I used to trim a follower system out. I hope it continues to work well for you.
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Paul M <PaulMatWiredogdotcom> wrote in

Actually there can be considerable stretch, even intermittent growth or shrink due to heat/cool cylcing as the material gets printed, dried and printed again going through the press. Even in the paper industry, there many printers, etc that run plastic as well as paper, on the same machines and many plastics stretch like heck. You can't have a loose web while printing, it will walk all over the place. You can't re-roll a loose web either....well....not on a 3000 lb roll anyway. It's done with a dancer and servo system. Check out the Bobst Registron systems.
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