In Part 4 of the Web Guiding Systems Series we discussed Unwind and Rewind Guides in Roll-to-Roll Winding Systems.
Winding System Guide Accuracy and Response
You will want your guide to move an edge towards its setpoint with great accuracy and repeatability. Indeed, some are asking for edge position tolerances of less than 5 mils. Also, in general, you would want the guide to be as fast and responsive as possible. (An exception is to avoid following a ragged or scalloped edge.) The combination of accuracy and response will improve product appearance and reduce trim waste.
To achieve accuracy, repeatability and response, it is important to follow guidelines as given here or by the builder. Bending these rules will usually result in sluggish response or hunting of the guide. Breaking these rules can degrade function to the point of inoperability. However, there are other considerations when designing guides to tight tolerance applications. The sum of edge scallop, sensor resolution and actuator hysterisis must be no greater than the total allowable edge variation. However, we must also avoid structural flexibility and play.
Flexibility of the actuator or framework will require detuning the response of the guide in order to avoid oscillation and other instabilities. The element with the greatest flexibility will be the weak and, thus, determining link in the system. In many machines, it is the hydraulic cylinder that is softest. If so, one might consider an electric ballscrew as an alternative if it has sufficient speed and load rating. However, also keep in mind that the mounting of the actuator can needlessly soften the dynamic system if it is not stout and rigid. While decreasing the guides’ mass can also improve response, it is difficult to achieve significant gains there. It should not be surprising, then, that winder and unwind guides might be more sluggish than other styles because of their greater mass than steering or displacement guides.
In Part 6 of the Web Guiding Series we discuss Acceleration Offset in Roll to Roll Web Systems.
Written for Faustel by David R. Roisum, Ph.D.