Emerging Trends

Industry developments such as lighter materials, faster speeds and quality mandates place tough demands on tension system performance levels. Such demands leave little room for error and require a fine degree of tension regulation. Speaking in numerical terms, older web processing machines could perhaps live with tension inconsistencies deviating as much as 5-20% from set point. Today, however, it is not uncommon to see tension set points controlled to within 1-2%. Going forward, this number will eventually reach a fraction of a percent.

Tension fluctuations, typically those greater than 10%, will likely manifest in the form of web instability. In the machine direction, such instability leads to inconsistent processing problems. Common problems resulting from inadequate tension control range from poor print registration in printing presses to web curl in laminating operations. The magnitude of the problem, however, is largely dependent upon the elastic modulus of the material. Paperboard, for example, can typically withstand greater tension fluctuations than lighter weight fine paper and/or thin films.

Of the two most common closed loop tension control systems, dancer roll systems have long been accepted as suitable for web fed machines, especially when processing out of round rolls. The dancer system’s “storage” capability also lends itself well in high accel/decel and stop/start applications. Load cell systems, however, have distinct advantages and seem to be gaining in popularity. Perhaps of greatest significance, load cells can be fitted to most any idler roll in the machine, thus making load cell systems readily adaptable to most machines.

Relative to controls, with the advent of digital controls technology came sweeping changes to the controls industry. Present day digital tension controls now make it possible to program various parameters, track roll diameter with no external sensors and store settings to name just a few. They can also “automate” tasks such as calibration and tuning procedures. Performance benefits include the use of control algorithms to ensure uniform control response regardless of roll diameter. Thanks to flexibility, performance and set-up features, digital tension controls are fast becoming the industry standard.

Analog systems are still preferred by many operators, especially those comfortable making adjustments with potentiometers instead of keypads. In time, however, analog controls will become outdated. Newer controls offering the best of both worlds, a combination of both analog and digital technologies, have recently been introduced. These systems offer the flexibility afforded by digital systems while retaining the simplicity of analog potentiometer settings.

Further, advances in drives technology have significantly improved over the years. Speed regulation capability offered by top of the line drives is reportedly at .001%. This capability represents the cornerstone of future tension system development. With such a fine degree of speed regulation, newer digital and AC vector drives can significantly improve tension regulation tolerances.

Especially in larger applications, with the cost of energy continually escalating, conventional braking systems will eventually be replaced by regenerative systems employing motors. Although this change will not take place overnight, advances in drives/electric motor technology continue to bring prices down while performance levels improve. Brakes, unfortunately, generate wasteful heat whereas regenerative systems generate electricity in their “braking” mode. Not only will there be a savings in energy, but performance will be improved as well. Regenerative systems can drive the rolls which is helpful at start up to overcome the inertia in unwind applications.

International quality mandates have proven to be an influential factor in tension system development. Electrical devices conforming to CE criteria undergo rigorous test procedures to meet European Low Voltage Directives. Such tests may include EMI (electromagnetic interference), RFI (radio frequency interference) and ESD (electrostatic discharge).

Products carrying the CE mark offer the customer better performance and ensures better compatibility with other electrical devices such as drives.

Determining the appropriate tension system for your application is not a cut and dry process. Without exception, there are merits and limitations to every method of tension control. Tension control selection should be a step by step process first considering the application in which control is to be employed.
When retrofitting existing equipment, one important consideration should be kept in mind: the machine operator. By including the operator in the decision making process, you are in essence ensuring the success of the control.

Combining advantages of both a dancer roll and load cell system, Montalvo’s DTI Danacer Controller with Tension Indication offers the “storage” capability of a dancer system and tension read-out/setting feature of load cells. Numerous inputs and outputs allow the controller to be interfaced with a PLC. Additional features include soft-start, anti-coast, E-stop, range expansion and fully adjustable parameters, thanks to it’s micro-processor front end.

One of the most important considerations in any tension system is the degree of control or tension regulation tolerance. Unfortunately, in most cases this is unknown, as tension indication equipment is non-existent, hence, the advantage of and recommendation to include tension indication equipment at every unwind. Tension indication should be viewed as a tool that provides vital, real time statistical process information which may be used in a variety of ways.

It is difficult to make specific tension control system recommendations without specific application parameters. As a rule of thumb, dancer roll systems are recommended when processing out of round rolls and when tension transients are common. Load cell control systems are recommended in zones free from tension transients and in retrofit applications.

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