Dominique Knittel

Tension control for winding systems with two-degrees-of-freedom H/sub /spl infin// controllers

The plant considered in this paper is a web system with winder, traction motor and unwinder. Transport system dynamics change dramatically due to the wide variation in roll radii and inertia. The effect of these changes is reduced by gain scheduling control. Industrial systems typically use decentralized PID controllers. In this paper, the authors present multivariable H/sub /spl infin// robust control with two degrees of freedom and gain scheduling. This controller significantly improves disturbance rejection performance and significantly reduces coupling between web tension and velocity. Performances of this controller and classical H/sub /spl infin// controllers are compared.

Fixed-Order

Flexible materials such as textiles, papers, polymers, and metals are transported on rollers during their processing. Maintaining web tension in the entire processing line under changing web speed is a key factor in achieving good final product quality. Many industrial applications use dancer position feedback to indirectly regulate tension. Although widely used in the industry, pendulum dancers (rotational motion of the dancer roller) have received very little attention in the literature compared to linear ones (translational motion). The lack of clearly identified controllers synthesis methods can thus be noticed, as industry typically uses hand-tuned decentralized PI controllers. An improved alternative based on H∞ methods is proposed in this paper to provide a systematic framework. The focus in this study is the unwind section of a processing line that contains a pendulum dancer (PD). The nonlinear and linear phenomenological models of the unwind section containing the PD are discussed first. The position controller based on dancer position feedback is synthesized using the standard H∞ approach with mixed sensitivity. Because of the high order of the controllers synthesized with this approach, techniques to generate reduced-order controllers are used to calculate a fixed-order controller resembling standard industrial practice. The performance of the proposed controllers is demonstrated by carrying out experiments on a large experimental web handling platform containing four driven rollers, many idle rollers, and a PD in the unwind section. To the best of our knowledge, these are the first published results of successful application of an H∞ controller to a real plant containing a PD.

H_{\infty}

Demand for improved performance under a wide variety of dynamic conditions and web materials are placing additional emphasis on developing new advanced control strategies. This paper presents decentralized Hinfin controllers with model based feedforward for web winding systems which provide improved web tension and velocity regulation in the presence of uncertainties. First, mathematical models of fundamental elements in a web process line are presented. A new state space model is developed which enables calculation of the phenomenological model feedforward signals and helps in the synthesis of decentralized Hinfin controllers around the set points given by the reference signals. Different Hinfin control strategies with additive feedforward have been validated on a nonlinear simulator identified on a 3-motor winding test bench

Tension Control in the Unwinding Section of a Web Handling System Using a Pendulum Dancer

In web transport systems, the main concern is to control independently speed and tension in spite of perturbations such as radius variations and changes of setting point. Industrial systems typically use decentralized PID controllers but these techniques do not achieve good decoupling. Multivariable centralized controllers have been shown to provide better results but they are not suitable for large systems due to their high orders. We present a multivariable decentralized control strategy applied to a large scale winding system. After introducing the overlapping decomposition method, the decomposition step is discussed for our special case. Simulation results using H/sub /spl infin// controllers show that disturbance rejection and coupling between web tension and velocity are significantly improved.

Decentralized Robust Control Strategies with Model Based Feedforward for ElasticWebWinding Systems

In this paper, we address the problem of the detection of out-of-plane web vibrations by means of a single camera and a laser dots pattern device. We have been motivated by the important economical impact of web vibrations phenomena which occur in winding/unwinding systems. Among many sources of disturbances, out-of-plane vibrations of an elastic moving web are well-known to be one of the most limiting factors for the velocity in the web transport industry.The new technique we proposed for the contact-less estimation of out-of-plane web vibration properties and during the winding process is the main contribution of this work. As far as we know, this is the first time a technique is proposed to evaluate the vibrations of a moving web with a camera. Vibration frequencies are estimated from distance variations of a web cross-section with respect to the camera.Experiments have been performed on a winding plant for elastic fabric with a web width of 10 cm. Distances from the web surface to the camera have been estimated all along an image sequence and the most significant frequencies have been extracted from the variations of this signal (forced and free vibrations) and compared to those provided with strain gauges and also with a simple elastic string model, in motion.

Robust decentralized overlapping control of large scale winding systems

The tension measurement is a key point in the operation of web systems handling thin materials such as textile, paper, polymer or metal. An experimental procedure is proposed to compute the web tension using a contactless sensor, which can be implemented on existing industrial web handling machines. A laser sensor measures the position of the web during the process and identifies out of plane vibrations. Using a sliding FFT, those vibrations can be separated into forced vibrations created by the process and machines, and free vibrations. It is then possible to compute the web tension as the frequencies of free vibrations are directly related to web tension. A test bench has been set up to represent a classical web handling systems and measurements have been made to identify free and forced vibrations. The procedure to compute web tension from measurements is then detailed.

A structured light vision system for out-of-plane vibration frequencies location of a moving web

In web transport systems, the main concern is to control independently speed and tension in spite of perturbations such as radius variations and set point changes. In this paper we present multivariable H/sub 2/ controllers applied to winding systems. Two controller structures are considered: a centralized for a three-motor winding system and a semi-decentralized controller without overlapping for a large scale system, materialized here by a nine-motor winding benchmark. These controllers, based on state feedback, are designed with bilinear matrix inequality (BMI) optimization. In order to cancel steady state errors in response to constant reference signals or constant disturbances two solutions are considered: the use of a static gain compensation matrix and of full or partial integral action. Simulation results are given for both of these systems, based on models identified on an experimental bench.

Moving Web Tension Determination by Out of Plane Vibrations Measurements Using a Laser

In web transport systems, the main concern is to control independently speed and tension in spite of perturbations, such as radius variations and changes of set points. In this paper we present multivariable Hinfin controllers, applied to winding systems and determined by a bilinear matrix inequality (BMI) approach. The design relies on full state feedback, with or without addition of integrators. The use of partial integral action for error-free reference tracking and disturbance rejection is established theoretically and discussed in this paper. Simulation results are given based on a nonlinear model identified on a 3-motor winding test bench as well as on its 9-motor expansion

State feedback control with full or partial integral action for large scale winding systems

The plant is a web transport system with winder and unwinder. Due to a wide-range variation of the radius and inertia of the rollers the system dynamics change considerably. Two different control strategies for web tension and linear transport velocity are presented. The first is an H/sub /spl infin// robust control strategy with varying gains based on a particularity of the plant. The second is an LPV control strategy with smooth scheduling of controllers synthesized for different operating points. The quadratic stability and the quadratic performance of the closed loop system are analyzed. The LPV control strategy gives better results on an experimental setup, for the rejection of the disturbances introduced by velocity variations.

H~~-Feedback Decentralized Control by BMI Optimization for Large Scale Web Handling Systems

Abstract A web transport system including an unwinder, a winder and a traction motor has been modeled from the laws of mechanics and identified by parameter optimization. During the winding process the radius and the inertia of the rollers change in a large scale. The effect of these changes is minimized by a gain scheduling approach using a particularity of the plant. The type of controller appearing classically in industrial systems is usually decentralized and using PID controllers. In this paper we present a multivariable H ∞ robust control with gain scheduling that improves significantly the performances concerning the disturbance rejection and decouples web tension and web velocity. These results are validated on an experimental setup. A µ -analysis shows a good robustness to the elasticity modulus uncertainty of the web.