Autonomous navigation of ships by combining optimal trajectory planning with informed graph search

Autonomous trajectory generation plays an essential role in the navigation of vehicles in space as well as in terrestrial scenarios, i.e. in the air, on solid ground, or water. For the latter, the navigation of ships in ports has specific challenges since ship dynamics are highly nonlinear with limited agility, while the manoeuvre space in ports is limited. Nevertheless, for providing support to humanly designed control strategies, autonomously generated trajectories have not only to be feasible, i.e. collision-free but shall also be optimal with respect to manoeuvre time and control effort. This article presents a novel approach to autonomous trajectory planning on the basis of precomputed and connectable trajectory segments, the so-called motion primitives, and an A*-search algorithm. Sequences of motion primitives provide an initial guess for a subsequent optimization by which optimal trajectories are found even in terrains with many obstacles. We illustrate the approach with different navigation scenarios

Increased sensitivity in parameter identification problems for piezoelectrics

Für die Entwicklung neuer piezoelektrischer Geräte sind Computersimulationen unumgänglich geworden. Damit die Simulationen korrekte Vorhersagen treffen können ist es wichtig, dass diese mit den korrekten Materialparameterwerten des zu simulierenden Materials instanziiert werden. Materialparameterwerte werden üblicherweise durch den Hersteller oder durch neuere Methoden der Materialparametercharakterisierung quantisiert. Diese Werte sind häufig fehleranfällig mit Fehlern von bis zu 20%. Viele dieser Methoden basieren auf der Lösung eines sogenannten Inversen Problems bei dem Messungen und Simulationen der elektrischen Impedanz iterativ verglichen werden, um auf die ursächlichen korrekten Materialparameter zu schließen. Eines der Probleme dieses Ansatzes ist jedoch, dass die Sensitivität der Impedanz bezüglich einiger Materialparameter sehr klein oder gar Null beträgt. In dieser Dissertation wird die Möglichkeit der Steigerung der Sensitivität bezüglich der Materialparameter unter Verwendung von Piezokeramiken mit drei Elektrodenringen erörtert. Theoretische Aussagen bezüglich der Existenz, Eindeutigkeit und Regularität von Lösungen von Partiellen Dierentialgleichungen, die das Verhalten der Piezokeramik vorhersagen, werden gegeben. Die Sensitivität wird in einem Optimierungsproblem gesteigert. Verwendete Optimierungsverfahren werden durch präzise Ableitungen ergänzt welche mit Algorithmischem Dierenzieren berechnet wurden. Dies wird auch anhand zahlreicher numerischer Resultate dargestellt. Schließlich wurde im Rahmen dieser Dissertation eine optimale Geometrie für Piezokeramiken mit drei Elektrodenringen ermittelt. Diese Piezokeramik wird zur Zeit für reale Messungen, die zu neuen Materialparametercharakterisierungsmethoden führen, genutzt.For the development of new piezoelectric devices computer simulations have become an essential part of the process. In order for the computer simulations to give correct predictions it is of grave importance to know the correct material parameter values for the material that is simulated. However, material parameter values provided by the manufacturer or by common characterization methods are error-prone with errors ranging up to 20%. Many of these methods are based on a solution of an inverse problem where simulations of the electrical impedance and measurements thereof are compared in an iterative process to and the correct values. A common complaint for these methods is that the sensitivity of impedance with respect to certain material parameters is low or zero. In this thesis the possibility to increase the sensitivity of impedance with respect to material parameters using a triple-ring electrode setup is analyzed. Theoretical results concerning existence, uniqueness and regularity of solutions to the governing equations are given. The sensitivity is increased in an optimization problem where the optimization method is supplemented with accurate derivatives computed via Algorithmic Differentiation. Numerical results are presented. An optimized electrode geometry for a triple-ring electrode piezoceramic was developed in this thesis and is currently used for real physical measurements leading to novel material parameter characterization methods.von Benjamin Jurgelucks ; [Gutachter: Prof. Dr. Andrea Walther, Univ.-Prof. Dipl.-Ing. Dr. Barbara Kaltenbacher, Jun.-Prof. Dr. Tom Lahmer]Tag der Verteidigung: 29.01.2019Universität Paderborn, Dissertation, 201

A usability case study of algorithmic differentiation tools on the ISSM ice sheet model

Algorithmic differentiation (AD) based on operator overloading is often the only feasible approach for applying AD in complex C++ software environments. Challenges pertaining to the introduction of an AD tool based on operator overloading have been studied in the past. However, in order to assess possible performance gains or to verify derivative values, it is advantageous to be able to apply more than one AD tool to a given code. Hence, in this work, we investigate usability issues when exchanging AD tools. Our study is based on the NASA/JPL/UCI Ice Sheet System Model (ISSM) which currently employs the AD tool ADOL-C. We introduce CoDiPack to ISSM, a more recent AD tool offering a similar set of features while promising performance improvements. In addition to the obvious type change for the AD-augmented float type, this transition requires the change to a different adjoint MPI library, adaptation of the MUMPS solver wrapper, and changes to the derivative seeding and extraction routines. We believe that these issues are fairly generic for numerical simulation software, and the issues we report on provide a blueprint for similar undertakings. We also believe that our experiences provide guidance towards the development of AD interfaces that support AD tool interoperability. In addition, we improve upon the memory management of the existing ADOL-C instrumentation, which exhibited considerable runtime problems for higher mesh resolutions. We conduct serial and parallel ISSM model runs on a 2D mass transport benchmark as well as a model of the Pine Island Glacier to verify the derivatives computed by both tools and report on runtime performance and memory usage. In comparison, the CoDiPack AD variant of ISSM runs faster with less memory overhead than the ADOL-C variant and, thus, enables future model runs with an increased number of mesh elements. But the existence of two different AD implementations provides added confidence in the correctness of derivatives, in particular for future AD tool versions