Effects of parameter uncertainties on longitudinal web dynamics

Web handling systems are very common in industry for metal, paper, textile and polymer material treatments. The key variables to monitor and control are the web speed and web tension in each span. The objective is to reach the expected web speed while maintaining web tension in an acceptable range around the tension reference. Nevertheless, the longitudinal web dynamic behavior is sensitive to parameter variations or parameter uncertainties, as for example the web elasticity, the web speed, the roll radius.The implemented control strategy is a classical one: a first loop ensures roller speed control whereas the external loop ensures web tension control. The web tension controllers are automatically synthesized using an optimization approach. The influences of parameter variations are studied firstly on the open-loop system and then on the closed-loop behavior

Control design for longitudinal web dynamics: Benefits and drawbacks of robust control approaches

Web tension and speed are two key variables to be monitored and controlled in order to achieve the expected final product quality. One of the main objectives in web handling plants is to reach an expected web speed while maintaining the web tension within an acceptable range around the tension reference in the entire processing line. In the recent years, many works have focused on the topic of web tension control and have proposed various ways to enhance the performance: H-infinity, optimal state feedback, neural network, etc. But the common practice in industrial web transport systems remains the use of decentralized PI-type controllers.An improved design methodology of these PI controllers with fixed -order and -structure synthesis approaches has been made. Nevertheless, despite high performances for a nominal working point, it has been noticed that the closed-loop system performances depend on the web elasticity since the dynamic behavior is strongly affected by the Young's modulus. Consequently the emphasis of this contribution is on the automatic tuning of PID (or PI) controllers for web processing plants that guaranty good performances of the closed-loop system

Modeling and frequency response of web tension with a pendulum dancer, and comparison of load-cell and dancer based tension control systems

In web processing lines, web tension is typically regulated using an outer loop that provides a trim to the velocity reference of the inner velocity loop. The feedback signal for the outer tension loop is either a position signal from a dancer or tension signal from load cells mounted on a roller. Both these strategies are used extensively in the web processing industry, but a systematic analysis, based on mathematical models and experimental observations, on the benefits and limitations of the strategies is lacking. The paper will report two investigations. First, a model that describes the action of a pendulum dancer on web tension will be developed, and frequency response of web tension in the presence of the pendulum dancer will be discussed. Second, a comparison of tension control strategies based on force feedback from load cells and position feedback from dancer motion will be given.Mechanical and Aerospace Engineerin

Systematic method for determining the controller gains in a multi-span web line

A web line may have multiple locations where feedback systems are used to control web and roller speeds, and web tension. The Euclid Web Line (EWL) in the Web Handling Research Center at Oklahoma State University is such a line. The EWL has four sections - unwind, S-wrap, process, and rewind section. The S-wrap establishes the web speed. There are five speed controllers and two tension controllers. For the studies reported in this paper, all controllers are assumed to be Proportional (P) + Integral (I) controllers. A systematic method for finding the controller gains is the primary objective of this paper. The method involves first simplifying the model for each section using a Routh Approximation, and then determining the controller gains based on selected performance criteria. Experimental studies on the Euclid line with the determined gains are presented.Mechanical and Aerospace Engineerin

Modeling slip between a web and a roller

In roll-to-roll processes, slip between a web and a roller may be considered a defect. Slip can cause scratches which can be detrimental to a product, if not catastrophic. As line speeds increase to meet demands for more product per unit of operating time, the likelihood of slip increases. Slip may be partial or total, or a combination of both. In this paper we consider total slip only, i.e. slip over the entire area of contact between a web and a roller.Whitworth [1] defined criteria for when full slip initiates, and developed a model for how slip affects the tension in the spans on either side of a roller where slip occurs. A Sliding Friction Driven Roller (SFDR) model is developed in this paper to account for the case where the web slides on the roller. The SFDR model uses the Whitworth criteria and span tension model. Both the Whitworth and SFDR models use the Capstan equation to determine if slip is occurring. With the Whitworth model, full adhesion of the web on the roller occurs until the torque which drives the roller, due to the tensions in the span on either side of the roller, reaches a critical level and slip initiates. But, with the SFDR model the torque which drives the roller is due to sliding friction between the web and roller.The Euclid Web Line (EWL) in the Web Handling Research Center at Oklahoma State University was used to study slip both analytically and experimentally. A nonlinear dynamic model of the EWL was developed. Measured physical characteristics of the elements in the EWL were used in the analytical studies. Simulations for the case of a startup with an industrial ramp input in speed, showed that the Whitworth model is valid when the web and roller are moving at almost the same rotary speeds. However, simulations showed that the SFDR model covers the total slip situation when the tangential velocity of the roller and web velocity are distinctly different.The simulations also showed that the torque due to bearing friction at a roller had to be equal to the torque due to the difference in tensions in the spans on either side of the roller in order for the initiation of slip to occur. Calculations using the measured bearing friction on the roller of interest show that slip would not occur at speeds the EWL could attain.In the experimental studies, a parasitic torque was applied to a roller mounted on load cells to create a slip condition. Hanging weights were used to apply the parasitic torque. An encoder was mounted on this roller to measure rotary velocity of the roller. Results from the experimental studies showed that the Whitworth model was valid only when the parasitic torque was small. In contrast, experimental studies showed that the SFDR model was valid only when the parasitic torque was large.Mechanical and Aerospace Engineerin

Modeling web handling systems and a method for determining feedback controller gains

Modeling the longitudinal dynamics and control of web handling systems requires accurate models of the primitive elements involved, and an understanding of the processes used for speed and tension control. Primitive elements in a web line are the motors, the rolls of material, the idle rollers, and the web itself. Each primitive element has a simplified mathematical model at its heart. Modeling feedback control systems used for accurate control of speed and longitudinal tension in a web line require models of the web line. A systematic method for determining the controller gains for the speed and tension control systems in a web handling system is discussed along with the gain calculation method residing in the web line software, the Rockwell method. The Rockwell and the Routh Approximation methods are compared and found to have similar performance with Rockwell having better tension control while the Routh Approximation method has better speed control.Results from experiments on the Euclid and High-Speed Web Lines are used to validate the models of the primitive elements. Simulations of experiments are used to validate the simulation tool. Simulation is used to show the effect on tension from a parameter study on span lengths and the effect on tension of changing a feedback device from a load cell to a dancer.Slip is not distinctly a roller or a web problem, but a problem at the interface of the web and roller, which affects the assumptions used to derive primitive elements. Experiments show the presence of slip in certain circumstances and that different mechanisms of slip occur depending on operating conditions and certain parameters. Simulations of the Euclid Web Line show the effect of including the Ducotey-Good traction model. Experiments show the Ducotey-Good traction model is the appropriate mechanism of slip based on the speed of the roller at certain conditions, while at other conditions the Sliding-Friction Driven Roller model is the mechanism. The research finds a unified slip model is needed

Design and analysis of feedback and feedforward control systems for web tension in roll-to-roll manufacturing

In Roll-to-Roll (R2R) manufacturing, efficient transport of flexible materials (webs) on rollers requires simultaneous control of web speed and tension. Webs experience disturbing forces during transport due to nonideal machine elements and processes such as printing, coating, lamination, etc. Since rotating machine elements are employed, these disturbances are in the form of periodic oscillations in web tension and speed. Design of efficient model-based web tension and speed control systems employing both feedback and feedforward actions that can adapt to changes in parameters and reject periodic disturbances were investigated in this research. Tools from adaptive and robust control theory and singular perturbation method were utilized for the design and analysis of these control systems.Model reference and relay feedback based adaptive Proportional-Integral (PI) tension control schemes were developed to regulate web tension; these schemes overcome the tedious tuning procedures required for fixed gain PI schemes when process parameters and conditions change. To directly control the roll speed when belt-pulley and gear transmissions are employed, a control scheme that uses both motor and load speed feedback is developed. In the presence of a compliant transmission system, it is shown that using pure load speed feedback must be avoided as it results in an unstable system. In situations where linearization of the nonlinear web tension governing equation is not possible due to changes in operating conditions, a nonlinear tension regulator is designed via a solution method employed in the nonlinear servomechanism problem. The feedforward action is synthesized by considering a discretized form of the tension governing equation in conjunction with adaptive estimation of periodic disturbance parameters. It is also shown that interaction between different subsystems of the R2R system may be minimized by employing feedforward action. The strategy of utilizing tension signal from the web tension zone downstream of the driven roller is shown to result in minimization of propagation of disturbances into further downstream tension zones. For each of the developed designs, experiments were conducted on a large R2R platform for different web materials and transport conditions to evaluate and compare their performance. Implementation guidelines are provided for ease of applying the designs to other industrial R2R machines

Analysis of the Unwind Section of an Industrial Web Processing Line

The purpose of this thesis is to analyze the control strategies and tension performance of the unwind section of an industrial web processing line that manufactures composite flooring materials. The main elements of the unwind section of the web line include a torque controlled unwind roll, two driven rollers, an accumulator, and a dancer. Dynamic models describing the interaction and control of these web line elements are derived and used to generate improvements to the existing control strategies. The controller structures used are the Proportional-Integral-Derivative type. Experiments are conducted on the improved strategies for the unwind roll to verify their performance. A comparison between a torque controlled unwind roll and a velocity controlled unwind roll is performed. The basis of comparison is the stability regions in their corresponding controller parameter spaces. Comparing model simulation results and measured data verifies that the model adequately predicts trends measured in the unwind section. Model simulations and experimentation using the improved unwind roll strategies show improved tension regulation performance. However, it is seen that the variability and inconsistency in the braking mechanisms may limit the effectiveness of these improvements. Simulation results indicate that the modified control strategy for the first pull roll and accumulator also increased tension regulation performance. The analysis of the stability regions for the torque controlled unwind roll and velocity controlled unwind roll show that there is a greater flexibility in selecting the controller gains for the velocity controlled unwind.Mechanical & Aerospace Engineerin