web tension

Web

Web is a material that is thin, flexible and continuous in nature. Web materials are transported within a machinery to convert them into finished products. A wide variety of consumer products are made from web materials such as paper, plastics, films, foils and metals, into finished products such as diapers, female hygiene products, labels, tapes, magazines, aluminum cans, and even some electronics.

In roll-to-roll processes the presence of non-ideal elements, such us out-of-round or eccentric rollers is fairly common. Periodic oscillations in web tension and web velocity are observed because of the presence of such non-ideal elements. Models of web transport on rollers based on the ideal behavior of various machine elements are not able to reproduce these oscillations in model simulations but can only follow the average of the measured tension and velocity signals. In order to reproduce the tension oscillations the models have to be modified to include the mechanism that creates the oscillations.

Periodic oscillations in the tension signal are frequently observed in roll-to-roll manufacturing due to the presence of many rotating elements which are often non-ideal, such as out-of-round material or eccentric rolls. In certain situations the amplitude of the oscillations is large enough to affect normal operation of the web line. The proportional-integral-derivative (PID) feedback control algorithms that are commonly used for tension regulation do not have the dynamic complexity to compensate for such periodic disturbances. In this paper we investigate a two-degree-of-freedom controller which has two control actions, feedback and feedforward. The feedforward part is adaptive and is designed to provide control actions to compensate for periodic oscillations. Several issues must be considered when designing a control algorithm for the attenuation of periodic oscillations. First, since the control algorithm is executed in real-time using a real-time system which may have restrictions on the sampling period, the complexity of the algorithm must be such that the control action can be computed in a time period that is less than the sampling period, and the sampling period for most systems is typically in the range of tens of milliseconds. Second, it is desirable to have a feedforward algorithm that can be implemented in parallel with an existing feedback control scheme for tension and speed regulation without the need to retune and redesign the existing scheme. Further, it is desirable to have an algorithm that is understandable to practicing engineers who may have limited or no advanced controls background other than an undergraduate course in control systems. Considering the aforementioned issues, an adaptive feedforward (AFF) algorithm that can work in parallel to an existing feedback control systems is developed for control of web tension and to attenuate periodic oscillations. The essential ingredient of the AFF algorithm is the estimation of amplitude and phase of the periodic oscillations based on which a feedforward compensating control action is generated. The action of the AFF algorithm is such that retuning or redesign of the existing feedback controller is not required. Several different configurations of the AFF for different scenarios in terms of where to apply the feedforward action in the control system are investigated. Extensive experiments are conducted on a large web platform with different scenarios and by transporting two different web materials at various speeds. Results from these experiments are presented and discussed. Experimental results show the effectiveness of the proposed AFF algorithm to attenuate tension oscillations.

In roll-to-roll (R2R) manufacturing the presence of non-ideal elements, such as out-of-round or eccentric rolls, induces periodical oscillations in the web tension signal. Model simulations based on ideal elements do not exhibit these tension oscillations but can only follow the measured tension signal in an average sense. In order for the models to predict these measured tension oscillations due to non-ideal elements, the derivation of governing equations must consider a mechanism to include the correct behavior of the non-ideal transport elements. Continuing with our previous work on this topic presented at previous IWEBs, we present additional results that provide improvements to the web span tension governing equation which can better predict measured tension signals. In particular, this work is useful for tension control in the unwind section of the web line when the unwind material roll is often out-of-round. The governing equation for web span tension is typically derived using the law of conservation of mass by considering a control volume enclosing the web span, i.e., at any instant of time the variation of web mass in the control volume is equal to the difference of the incoming and outgoing material flow rates. If the web span is between two ideal elements the only way to induce changes in web span tension is with an imbalance in the web material flow. For ideal elements it is easily shown that the material flow rate is proportional to the difference of the peripheral velocities of the web on the surface of the rolls adjacent to the web span. When an out-of-round roll is at one end of the web span, two aspects make the derivation of the web tension governing equation different from the ideal case. First, because of the out-of-roundness of the roll, the span length adjacent to the roll is time-varying; variations in the span length induce web tension variations that are not associated with an imbalance in material flow. Second, the material flow rate is not proportional to the peripheral velocity of the web on the out-of-round roll and must be computed explicitly. Given a measure of out-of-roundness of the roll, due to the complexity of the problem it is difficult to derive a closed form expression for the material flow rate as a function of the roll position and velocity. A numerical algorithm for the computation of the material flow rate is presented in the paper. Based on the computation of the material flow rate and the algorithm for the computation of the span length adjacent to an out-of-round roll which was presented in the previous IWEB, a new governing equation for web tension is developed. Using this new governing equation a dynamic model for an experimental web line is developed and model simulations are conducted. To corroborate the model, experiments are conducted on the web line with an out-of-round unwind material roll. Comparison of the results from model simulations and experiments are presented and discussed.

Since rotating machinery is used to transport flexible materials (commonly known as webs) on rollers, it is common to observe periodic oscillations in measured signals such as web tension and web transport velocity. These periodic oscillations are more prevalent in the presence of nonideal elements such as eccentric rollers and out-of-round material rolls. One of the critical needs in efficient transport of webs is to maintain web tension at a prescribed value. Tension regulation affects almost all key processes involved during web transport including printing, registration, lamination, winding, etc. Governing equations for web tension and transport velocity that can accurately predict measured behavior in the presence of nonideal rollers are beneficial in understanding web transport behavior under various dynamic conditions and the design of suitable web tension and speed control systems. The focus of this paper is on modeling the effect of eccentric rollers and out-of-round material rolls on web tension and web transport velocity. The new governing equations for web velocity on an eccentric roller and web tension in spans adjacent to the eccentric roller are presented and discussed; a web span is the free web between two consecutive rollers. To solve these governing equations, the location of the entry and exit point of the web on the eccentric roller as it rotates and the length of the web spans adjacent to the eccentric roller are required; a procedure for obtaining this information is described. To corroborate the models and the developed approach, data from experiments on a large experimental web platform are compared with data from model simulations, and a representative sample of the results are presented and discussed.