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FLEXO Magazine : Maxcess University Special
WEIGHTLESS LOAD CELL CALIBRATION Calibrating load cells has always been a cumbersome job at best. It requires removing the web, threading a rope through the machine and then hanging a known weight from the rope and adjusting the gain accordingly---a fairly straightforward process on a cantilevered machine with load cells of less than 50 lbs. Difficulty increases exponentially as the load cell rating increases and, as a result, more weight is required for calibration. Newer systems automatically calibrate the load cells without hanging weights. Instead, the task can be completed by software in the control with the entry of wrap angle, load cell type and rating. AUTO TUNING The PID (proportional gain, integrator and derivative) loop is still the heart of the closed loop tension control. Simply stated, it measures how much, how often and how fast a control output is given in response to an error in tension. In the past, the PID loop was the realm of control engineers and theorists who worked their black magic to tune and stabilize the system. Today's controls allow anyone to tune the system with a few simple commands through the keypad. The systems can automati- cally run to optimum settings based on the dynamics of the machine. GAIN SCHEDULING Every machine is designed with certain pa- rameters in mind. It is the job of the end user to push those limits and get everything he/she can from that machine. One of those areas commonly pushed is roll build. In the past, a machine may have been designed to run with 30-in. O.D. (outer diameter) full rolls with a 3- in. O.D. core. One common way to increase ef- ficiency is to reduce the number of roll changes by increasing roll sizes. It is not uncommon today to see the above machine modified to accept 40-in., 48-in. or even 60-in. O.D. rolls. One threat to the increases in efficiency is the limitation of the older control loops (PID). These older systems employed a single PID loop that was a compromise giving you the best control from full roll to core (because a larger roll requires more gain than a smaller roll). If the system worked at a 10:1 roll build as it was designed (30-in. full roll and 3-in. core), odds are it will not work for the larger 20:1 roll build of the modification (60-in. full roll and 3-in. core). Even retuning the PID loop will most likely result in very sluggish control at full roll and instability at core. The result will be the in- ability to run all the way to core before the web becomes unstable. If you are throwing away material because you cannot run all the way to the core, you need to keep reading. The answer is gain scheduling. Today's closed-loop tension controls offer the ability to have PID parameters at full roll with enough gain to deal with any transients in tension and a com- pletely different set of control parameters at core with enough reduction in gain to keep your system stable. These parameters automatically adjust to give you optimal control all the way through the roll as the diameter changes. The technology of tension control has changed greatly and will continue to evolve as will the needs of the market. The capabili- ties of the newest tension controls have made them easier to use and easier for customers to run the best product possible as ef- ficiently as possible. ABOUT THE AUTHOR: Darrell Whiteside is product manager at Magnetic Power Systems Inc. (MAGPOWR.) He joined the company in August of 1999, assuming responsibilities for international sales. In May of 2001, he was promoted to his current position. Whiteside holds a BS in Industrial Science, with an emphasis on electronics and robot- ics, from Northeast Missouri State University. FIGURE 1. Dancer systems. FIGURE 2. Load cell-based systems. www.flexomag.com FLEXO SUPPLEMENT/MAXCESS UNIVERSITY FLEXO 7 EDUCATION & TRAINING
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