Terminal iterative learning for cycle-to-cycle control of industrial processes.

摘要

The objective of this thesis is to study a cycle-to-cycle control approach called Terminal Iterative Learning Control (TILC) and apply it to the process of plastic sheet heating in a thermoforming oven. Until now, adjustments to the oven heater temperature setpoints have been made manually by a human operator following a trial and error approach. This approach causes financial losses, because plastic sheets are wasted during the period of time when the adjustments are made at the beginning of a production run. Worse, the heater setpoints are subject to modification because of variation in the ambient temperature, which has an important impact on the sheet reheat process.;The stability and rate of convergence of the TILC algorithm can be analyzed through the location of the closed-loop system poles in the cycle domain. This analysis is relatively easy for a first-order TILC but becomes more complex for a higher-order TILC algorithm. The singular value decomposition (SVD) is used to simplify the convergence analysis while decoupling the system in the cycle domain. The SVD technique can be used to facilitate the design of higher-order TILC algorithms.;Internal Model Control (IMC) is another approach that can make the ILC design easier, because there is only one parameter per filter to adjust. The IMC technique has an interesting feature. In the case where the system is nominal, the closed-loop transfer function of the system is the same as the IMC filter's transfer function. Therefore, the adjustment of the filter parameter allows the designer to select the desired system response.;For industrial processes such as thermoforming ovens, it is important that the systems controlled by TILC algorithms are stable and have good performance. For thermoforming ovens, the terminal sheet temperature response must not be too oscillatory from cycle to cycle, since this may lead to high heater temperature setpoints. In the most serious case, high heater temperatures can cause the sheet to melt and spill on the heating elements at the bottom of the oven.;The TILC approach is analyzed by studying the closed-loop system in the discrete cycle domain through the use of the z-transform. The system, which has dynamic behaviour in the time domain, becomes a static linear mapping in the cycle domain. One can then apply on this equivalent system a traditional control approach, while considering that the system output is sampled once at the end of the cycle. On the other hand, from the standpoint of the real system, this control approach can be viewed as cycle-to-cycle control.;The performance aspect must not be neglected, since it is important to minimize the number of wasted plastic sheets, particularly at process startup. To avoid such waste of time and material, it is necessary that the TILC algorithm converge as quickly as possible. However, the robustness and performance objectives are conflicting and an acceptable compromise must be achieved. The control engineer must define specifications to describe these two constraints. Tools such as the Hinfinity Mixed-Sensitivity Analysis and mu-Analysis can be used to check the compliance of a given TILC algorithm with the robustness and performance specifications defined before the analysis. One can therefore compare various TILC algorithms quantitatively, through a computed measure obtained with one of the two approaches. These same tools can be used for the design of TILC algorithms, using weighting functions representing the specifications.;Simulation and experimental results obtained on industrial thermoforming machines show the effectiveness of the various approaches in this thesis. Many examples are also presented throughout the chapters.