Closed-loop control improves processes

Colin Keating, a design engineer at Optima Control Solutions discusses the pros and cons of dancer mechanisms and electrical load cell transducers for tension control

Experts in the converting and printing processes know how critical good tension control is to their process and that tension can represent a single point of failure during production. Accurate and stable tension control on driven material ensures higher quality product; less scrap and therefore improved productivity; higher production speeds; improved downtime and lower operating costs. 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 versus closed loop

In an open-loop control system there is no direct tension feedback signal so controlled corrective action for any variance is not possible; it operates on a predictive load basis. In contrast, a closed loop tension system uses a signal that is proportional to the position of a feedback transducer (load cell or dancer mechanism) that is pretensioned as determined by the main process requirements. High performance closed-loop systems provide accurate, consistent tension control.

Load cells are mechanically simpler than dancers as the only moving (wearing) part is a bearing that it is quite easy to replace. With a more complex dancer system, one needs to maintain whatever the pivot point is (whether a cylinder or chains and weights). A further technical issue that occurs in dancer systems is the inherent non-linearity of the feedback signal that needs to be considered in the design. The mechanical parts must not tighten the web.

For example, a printing machine at CCL Decorative Sleeves had a flying splice winder section. The original dancer mechanism (using weights) had been replaced with a load cell arrangement in the belief that it would provide better accuracy. However, the machine would then only splice for one out of three attempts – clearly unsustainable. Experience told us that the application needed a dancer mechanism but in a much more modern arrangement. The difficulties in using load cells on a flying splice were then all too apparent to CCL.

CCL 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 load cell meant it lost tension. We engineered a dancer system with very low friction components and achieved reliable splicing for them with ease. Most of the flying splices we have done have dancers because of the accumulation advantage. If a load cell 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.


Flexibility of load cells

Certain pieces of equipment such as a printing press need the material to be at a guaranteed tension. A print register system ensures that all the colours are printed accurately with respect to one another. It will only work well if the substrate being printed on is stable and running at the correct tension value. The printer will need to control his substrate flexibly and accurately to accommodate different materials. A common requirement is for the tension levels to be different for various sections on a machine. If the web is transported or wound at too low a tension it can wander from side to side causing process problems or poor winder performance. Conversely, too much tension can deform a substrate or cause surface problems. One obvious advantage with load cells is their flexibility. Operators alter the web tension simply by changing a number on a HMI or potentiometer dial. It is possible to adjust a dancer system in a similar way using E-P transducers but the whole system is not as flexible.

Load cell technology is so widely developed that dancers have become the more costly solution. Furthermore, in applications where low web tension is required, dancers are unsuitable – they contain inherent mass and inertia that the web must overcome. The latest load cell amplifiers employ intelligent tuning techniques and digital communications networks such as ProfiBus.

Our clients experience a range of issues including improving process and machine reliability and increasing production speed. Projects have involved straightforward control system upgrade through to removing existing line shaft arrangements and introducing individual section controls providing faster setup times and better flexibility for the operators.

Optima Control Solutions

T: +44 (0)1254 272829