JRoss Posted April 5, 2012 Report Posted April 5, 2012 (edited) I'm posting this in the general forum, as I'm looking for more process oriented advice, but for those who care I'm using an AB ControlLogix L3 processor and Kinetix 6500 servos. My application is a metering system for feeding stacks of product into a wrapper. I have to line the product up with the lugs on the flighted conveyor of the wrapper, so we're using servos to do this. This system is using six servo driven conveyors in series. The final one in the sequence is speed-matched to the wrapper, and the other five are used to gradually meter the product so that the spacing matches the flights on the wrapper. My plan is to build on a similar system that I've done which only had a single servo following an encoder directly mounted to the flighted conveyor. In that case, I used the encoder as a master axis and "geared" the servo to it starting with a 1:1 gearing ratio. Then each time the product made a metering eye on the downstream end of the metering conveyor, I would calculate the lead/lag and adjust the gearing ratio up or down to compensate for error. The first couple of products would invariably be off, but product would quickly be adjusted to the correct position. This system needs multiple servos because we are dealing with stacked product instead of single product, so I can't adjust things very fast, or product will go flying. But I can't wrap my head around the order that things should be chained together. Should I slave Stage 5 to Stage 6 (final conveyor), Stage 4 to Stage 5 and on back the line? Should I slave all the stages to Stage 6? Should I do something else entirely? Any advice would be much appreciated. EDIT: See the sister thread on PLCS.net: http://www.plctalk.net/qanda/showthread.php?t=70777 Edited April 6, 2012 by JRoss Quote
RonHowell.com Posted May 23, 2012 Report Posted May 23, 2012 What you are re-inventing is called a smart belt. Google Smart belt and/or check out PMMI for smart belt vendors. Looking at the existing designs will give you a lot of good ideas for what to incorporate into yours. (I've run product into wrapper pin conveyors using smart belts at over to 1000/minute). Many of the systems do not only rely on the servos but also have the ability to adjust the lengths of the middle conveyor sections which allows tunning for the product length. Fast response times are critical for all elements of the system (photoeyes, motion controls, servo controls, network, etc.) Running everything sycronously on the same real time clock is very important. SERCO was always the best choice for this. I believe the Kinetix platform has a SERCOS option if you need to bridge Kinetix backplanes. Also check to make certain that the Kinetix backplane is sycronous and deterministic. Any bumps in timing will get amplifed in a high gain system like a smart belt and can result in real big problems. I like to use a virtual encoder as the master that drives all motion instead of a real encoder. Speed fluctuations of the master will get amplified in the slaves in high gain motion systems. The virtual master settles this down. Quote
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