Taming tension9 May 2012
Colin Keating, a design engineer at Optima Control Solutions, discusses closed-loop tension control in the production process
Converters and printers know how critical good tension control is to their process and that tension can represent a single point of failure during production. Good web tension control systems ensure that no matter how demanding the production process is, the correct web tension is maintained for any type of material, at any point of the machine and at any speed.
Open-loop vs closed-loop
In an open-loop control system, operating on a predictive load basis, there is no direct tension feedback signal, so controlled corrective action for any web tension variance is not possible.
In contrast, a closed-loop tension system uses a signal (generally an electrical one) that is proportional to the position of a feedback transducer (ie, loadcell or dancer mechanism) that is pre-tensioned as determined by the main process requirements. The control system uses a PID – proportional, integral and derivative controller.
There is a cost-performance payoff between the two alternatives; high performance closed-loop systems provide consistent, accurate tension control. Two popular types of closed-loop controller are dancer mechanisms and electrical loadcell transducers. The tables summarise the pros and cons of the two types.
Dancer control systems
¦ Easy to set up
¦ Ability to accumulate web; more ‘forgiving’ of variation in speed
¦ Easy to commission and ‘debug’
¦ Difficult to design mechanically
¦ No direct capability of measuring and responding to tension values
¦ Low tensions are problematic – the dancer induces more tension from mechanics
¦ Accurate flexible measurement and control of tension
¦ Low maintenance requirement due to simple arrangement
¦ Compatible with digital communication networks Disadvantages
¦ Not responsive to disturbances in the production process.
¦ Harder to commission and ‘debug’
Loadcells are mechanically simpler. The only moving (wearing) part is a bearing that is quite easy to replace. With a more complex dancer system, one needs to maintain whatever the pivot point is (whether it is a cylinder, or chains and weights). Another more technical issue in dancer systems is the inherent non-linearity of the feedback signal that the mechanical experts need to consider in their designs. The mechanical parts must not tighten the web.
One project handled by Optima where dancers were the right solution was for CCL Decorative Sleeves. Their press had a flying splice winder section. Someone had removed the original dancer mechanism and replaced it with a loadcell arrangement, believing it would provide better accuracy. The machine would then only splice for one out of three attempts!
Our experience told us that the application needed a dancer mechanism, but in a much more modern arrangement. The difficulties in using loadcells on a flying splice were then all too apparent to CCL. They found that when switching the tension control from one roll to the next in the splicing sequence, the whole control system was disturbed within a fraction of a second and using the loadcell lost tension.
We engineered a new dancer system with very low friction components and achieved reliable splicing for them with ease. Most of the flying splices we have handled have dancers because of their accumulation advantage. If a loadcell is used for this type of process, the disturbance created by other splicing component rolls and knives can easily cause the web to jump off the roll. The dancer absorbs the shock and allows the control system to overcome the short disruption.
Loadcells are required when certain pieces of equipment need the material they are handling to be at a guaranteed tension. A printing press, for example, will need accurate and stable tension control. A print register system will only work well if the substrate being printed on is stable and running at the correct tension value.
Another common requirement is for tension levels to be different for various sections on a machine. A web transported or wound at too low a tension can wander from side to side, causing print registration issues or poor winder performance. Conversely, too much tension can deform a substrate or cause problems on the surface of the material.
One obvious advantage of using loadcells is their operational flexibility. Operators change the web tension simply by changing a number on a HMI (human-machine-interface) or a potentiometer dial. It is possible to make adjustments to a dancer system in a similar way using E-P transducers, but still the whole system is not as flexible.
The most significant innovation in loadcell control technology is the shift towards digital controls. The latest loadcell amplifiers employ intelligent tuning techniques and digital communications networks such as ProfiBus.
Optima’s clients experience a range of web tension control issues. Improving process and machine reliability is one; increasing production speed is another. Most of our solutions use loadcells, though understanding which applications necessitate one or the other (or a combination of the two) means we still employ dancers when required.
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