Improvement of control of a web accumulator

Abstract

Previous analysis of the dynamics of a conventional accumulator (no driven rollers within the accumulator, with all rollers identical) was published by Shelton [1]. This 1999 analysis showed that such accumulators commonly suffer from an excessive difference of tension between the entry and exit, often leading to excessive local changes in tension as well as slackness and/or slippage between the web and specific rollers. The analysis also showed that the inertia of the carriage, because of its usually large mechanical advantage, often is negligible in comparison to the total inertia of the rollers. Additional conclusions were that (1) J/R2, not the inertia J by itself, is a governing parameter, (2) as the operating velocity is increased, the required height (distance of travel) of the accumulator may become excessive, and (3) the force required for counterbalancing the carriage can be calculated for specific goals of optimization of operating conditions. Specific schemes of control were not addressed in the 1999 IWEB paper.

This paper does not repeat derivations or results of the 1999 paper; instead, it examines schemes for control of tension within an accumulator, with emphasis on control of velocities instead of the less desirable control of forces and torques. Equations for velocities during steady-state running, stoppage, constant acceleration or deceleration of the web, and filling or emptying of the accumulator are derived.

Lateral errors, sometimes great enough to cause a failure, are commonly caused by the long multiple spans of an accumulator in combination with excessive flexibility of the locating elements of the carriage and the imperfection of the web; hence, schemes are presented for automatic leveling of the carriage, or tilting of the carriage for incremental correction of errors caused by camber or other imperfections.

Driving all the rollers in an accumulator with precise control of their velocities along with synchronized control of the velocity of the carriage, should almost eliminate the tension variations inherent in current practice, and should almost eliminate the carriage travel now required for the acceleration and deceleration modes of the rollers.