Real-Time Optimization via Directional Modifier Adaptation, with Application to Kite Control

by Sean Costello

Institution: EPFL
Year: 2015
Keywords: Real-Time Optimization; Control; Airborne Wind Energy; Kite Power
Record ID: 1091735
Full text PDF: http://infoscience.epfl.ch/record/206238


The steady advance of computational methods makes model-based optimization an increasingly attractive method for process improvement. Unfortunately, the available models are often inaccurate. The traditional remedy is to update the model parameters, but this generally leads to a difficult parameter estimation problem that must be solved on-line, and the resulting model may still poorly predict the process optimum. An iterative real-time optimization method called Modifier Adaptation overcomes these obstacles by directly incorporating plant measurements into the optimization framework, in the form of constraint values and plant-gradient estimates. Experimental gradient estimation is the main difficulty encountered when applying Modifier Adaptation. The experimental effort required to estimate plant gradients increases along with the number of plant inputs. This tends to make the method intractable for processes with many inputs. The main methodological contribution of this thesis is a new algorithm called ‘Directional’ Modifier Adaptation, which handles the gradient-estimation problem by estimating plant derivatives only in certain privileged directions. By spending less effort on gradient estimation, the algorithm can focus on optimizing the plant. A ‘Dual’ Directional Modifier Adaptation is proposed, which estimates these ‘directional’ derivatives using past operating points. This algorithm exhibits fast convergence to a neighborhood of the plant optimum, even for processes with many inputs. Modifier Adaptation also makes use of an approximate process model. Another difficulty which may be encountered is that this model’s inputs differ from those of the real process. The second methodological contribution is ‘Generalized’ Modifier Adaptation, a framework for dealing with the case where the model’s inputs differ from those of the plant. This approach circumvents remodeling the system. For example, Generalized Modifier Adaptation allows an open-loop process model to be used to optimize a closed-loop plant, without having to model the controller. The Dual Directional Modifier Adaptation method is applied to a purpose-built experimental kite system. Kites are currently being developed into a radical new renewable energy technology. Large-scale applications include pulling ships and generating electricity from wind at altitudes beyond the reach of conventional wind turbines. While kites were traditionally manually controlled, these new applications require autonomous operation. The first challenge is to design reliable control algorithms for kites, capable of dealing with noise, wind disturbances, and time delays. The control algorithm keeps the kite flying a periodic path, at very high speeds. The second challenge is to choose this path in order to maximize the energy extracted from the wind. During this thesis, a small autonomous kite system was constructed. Thirty days of experimental testing were carried out, over the space of two years. A new modeling hypothesis was validated, linking steering deflections to a…