Winder - Web Failure Considerations

In control design, most engineers focus on programming a proper functioning machine but many times neglect to look at regulation during failures. This disregard could create not only safety issues, but machine and product damage as well.

In a recent winder analysis of a Rockwell / Reliance Automax system, a serious programming oversight was found: Upon any drive failure, the machine would decelerate under a fast deceleration rate. Normally this would be OK, However... sometimes the sets bounce and dislodge from the drum pocket causing binding in the drums as well as other serious problems. What was found was that, when the 'fast-decel' happened, and in the case where the wound sets dissasociated from each other, the drums would actually go to a motoring torque trying to maintain the "fast decel rate"! In other words, the paper was binding in the drum nip and acting as a brake, and was trying to stop the drums faster than their commanded speed reference. This resulted in paper fires, horrendous set bounce and shredding, and even set throw outs.

The correction was to implement a simple control limitation that SDS has been using for years: namely, implementing a small positive torque limit during web break. This allows mechanical failures to stop the drums faster than the control system commanded reference.

An augmentation of this idea could be carried even further. For instance, the control system could be programmed to note when both drum drives were in positive torque limit during web break. When this occurs, there is obviously something forcing the drums to a stop, and the drives could actually go into a coast stop, removing their influence on deceleration.

Robk
http://www.sdscontrols.com
robk@sdscontrols.com