Application of parallel computing to robot dynamics

In this paper an approach for the application of parallel processing to the dynamic analysis of robots based on the multibody system method is presented. The inherent structure of the symbolic equations of motion is used for partitioning those into independent modules for concurrent evaluation. The applied strategies for parallelization include the parallel evaluation of subsystem equations and the parallel computation of the inertia matrix along with its factorization, and of the force vectors and control inputs. The implementation of the parallel structures is discussed with respect to optimal utilization of transputer resources. The presented approach yields a strong reduction of computing time and supports real-time simulations of controlled robots, which is also shown in an example

Parallel algorithms and architecture for computation of manipulator forward dynamics

Parallel computation of manipulator forward dynamics is investigated. Considering three classes of algorithms for the solution of the problem, that is, the O(n), the O(n exp 2), and the O(n exp 3) algorithms, parallelism in the problem is analyzed. It is shown that the problem belongs to the class of NC and that the time and processors bounds are of O(log2/2n) and O(n exp 4), respectively. However, the fastest stable parallel algorithms achieve the computation time of O(n) and can be derived by parallelization of the O(n exp 3) serial algorithms. Parallel computation of the O(n exp 3) algorithms requires the development of parallel algorithms for a set of fundamentally different problems, that is, the Newton-Euler formulation, the computation of the inertia matrix, decomposition of the symmetric, positive definite matrix, and the solution of triangular systems. Parallel algorithms for this set of problems are developed which can be efficiently implemented on a unique architecture, a triangular array of n(n+2)/2 processors with a simple nearest-neighbor interconnection. This architecture is particularly suitable for VLSI and WSI implementations. The developed parallel algorithm, compared to the best serial O(n) algorithm, achieves an asymptotic speedup of more than two orders-of-magnitude in the computation the forward dynamics

Efficient Implementations and Co-Simulation Techniques in Multibody System Dynamics

[Abstract] Current research in simulation of multibody systems (MBS) dynamics is focused on two main objectives: the increase of the computational efficiency of the software that carries out the simulations, and the diversification of the problems this software is able to tackle, sometimes through the inclusion in the calculations of non purely mechanical phenomena. This work deals with these two objectives, studying the effect of source code implementation in software performance, as well as the different communication methods with external software that can contribute the interaction of the MBS code with elements of a non mechanical nature. The first Chapter of this thesis contains a brief introduction to the present state of the art of MBS simulation software. It introduces the research lines this thesis forms part of and outlines its main objectives. The second Chapter describes the software architecture for MBS simulation that has been developed for this research. The C++ language has been used for its implementation, according to the object–oriented programming paradigm. This Chapter also enumerates the programming tools employed in the process and draws general conclusions about the development of MBS programs. The third and fourth Chapters introduce efficient implementation techniques in the field of linear algebra operations, and in the parallelization of the code, respectively. The obtained improvements in performance have been quantified, and the range of application of each technique has been delimited. The fifth and sixth Chapters deal with the communication of the developed software with external simulation software packages. A comparative study between the different ways in which the coupling can be performed has been carried out and, besides, the impact on the efficiency and accuracy of the use of multirate co–simulation techniques has been assessed. A generic interface for multirate integration has been designed to link the MBS software with MATLAB/Simulink, a mathematical and block diagram package very popular in the multibody community. Finally, the seventh Chapter summarizes the conclusions of the present work, and proposes future research lines that can be derived from it.[Resumen] La investigación actual en simulación dinámica de sistemas multicuerpo (MBS) gira en torno a dos objetivos principales: el incremento de la eficiencia computacional del software que lleva a cabo las simulaciones y la diversificación de las tareas que este es capaz de realizar, a veces mediante la inclusión en los cálculos de fenómenos no puramente mecánicos. Este trabajo aborda ambos objetivos, estudiando el efecto de la implementación del código fuente en el rendimiento del software, así como las diferentes estrategias de comunicación con programas externos que puedan aportar a la simulación multicuerpo la interacción con elementos de naturaleza no mecánica. El primer capítulo de esta tesis consiste en una breve introducción al estado actual del software para simulaci´on de sistemas multicuerpo. En él se muestran también las líneas de investigación en las que se enmarca el proyecto y se señalan sus objetivos principales. El segundo capítulo describe la arquitectura del software para la simulación de sistemas multicuerpo que se ha creado en esta investigación. Para su implementación se ha utilizado el lenguaje C++, dentro del paradigma de programación orientada a objetos. En este capítulo se enumeran también las herramientas de programación utilizadas en el proceso y se obtienen conclusiones de validez general para la generación de programas multicuerpo. Los capítulos tercero y cuarto presentan técnicas de implementación eficiente de las operaciones de álgebra lineal y en la paralelización del código, respectivamente. Se han cuantificado las mejoras en el tiempo de ejecución obtenidas y se han delimitado los campos de aplicación de cada estrategia. En los capítulos quinto y sexto se aborda la comunicación del software desarrollado con otros programas de simulación externos. Se ha realizado un estudio comparativo de los diversos modos posibles en que se puede realizar esta unión y, además, se ha evaluado el impacto del empleo de técnicas de cosimulación multirate sobre la eficiencia y la precisión de los cálculos. Se ha diseñado para ello una interfaz genérica entre el software MBS y MATLAB/Simulink, una aplicación matemática y de diagramas de bloques de gran aceptación entre la comunidad de investigación en sistemas multicuerpo. Por último, en el capítulo séptimo se resumen las conclusiones del presente trabajo y se proponen líneas de investigación futuras que pueden derivarse de él

Proceedings of the Fifth NASA/NSF/DOD Workshop on Aerospace Computational Control

The Fifth Annual Workshop on Aerospace Computational Control was one in a series of workshops sponsored by NASA, NSF, and the DOD. The purpose of these workshops is to address computational issues in the analysis, design, and testing of flexible multibody control systems for aerospace applications. The intention in holding these workshops is to bring together users, researchers, and developers of computational tools in aerospace systems (spacecraft, space robotics, aerospace transportation vehicles, etc.) for the purpose of exchanging ideas on the state of the art in computational tools and techniques

Proceedings of the 3rd Annual Conference on Aerospace Computational Control, volume 1

Conference topics included definition of tool requirements, advanced multibody component representation descriptions, model reduction, parallel computation, real time simulation, control design and analysis software, user interface issues, testing and verification, and applications to spacecraft, robotics, and aircraft

a review of particle damping modeling and testing

Abstract This survey provides an overview of the different approaches seen in the literature concerning particle damping. The emphasis is on particle dampers used on beams vibrating at frequencies between 10 Hz and 1 kHz. Design examples, analytical formulations, numerical models, and experimental setups for such dampers are gathered. Modeling approaches are presented both for particle interaction and for systems equipped with particle dampers. The consequences of the nonlinear behavior of particle dampers are brought to attention. As such, the apparent contradictions of the conclusions and approaches presented in the literature are highlighted. A list of particle simulation software and their use in the literature is provided. Most importantly, a suggested approach to create a sound numerical simulation of a particle damper and the accompanying experimental tests is given. It consists of setting up a discrete element method simulation, calibrating it with literature data and a representative damper experiment, and testing it outside of the range of operation used for the tuning

Shear Bands in Granular Materials: Formation and Persistence at Smooth Walls

This thesis contains numerical studies of rheology and shear characteristics of dense assemblies of granular materials. Beside the various experimental and theoretical studies, which deal with these materials, there is also a wide variety of simulation methods, which are used to study the flow behavior, compaction and other characteristics of granular materials. In this work, the contact dynamics method (CD) has been used to study two-dimensional systems of hard, dry disks. The particles interact by Coulomb friction forces parallel to, and volume exclusion forces normal to the contact surfaces, with collisions being fully inelastic. The shear flow is confined between two parallel, smooth, frictional walls, moving with opposite prescribed velocities. Discrete element simulations, carried out in samples with prescribed normal stress reveal that, unlike rough walls made of strands of particles, absolutely smooth but frictional ones can lead to inhomogeneous shear rate and shear strain localization in boundary layers. These are both caused by slip at smooth walls. Three shear regimes associated with different shear velocity intervals are identified and studied in this work. The transitions between these regimes are essentially independent of system size and occur for specific values of shear velocity. Applying constitutive laws deduced both for the bulk material and the boundary regions supplemented by an elementary stability analysis, the occurrence of both transitions, as well as the characteristic transient times are predicted. Investigating the role of the rotational degrees of freedom of round frictional particles and their microscopic contact properties at smooth walls, a critical microscopic friction coefficient at the walls is identified, below which the walls are unable to shear the system. New distinctive features are observed at this critical point. To perform a finite-size-analysis, simulations with very large systems have been frequently necessary during this thesis. To afford large scale simulations with CD, which are more comparable to real granular systems, within a conceivable time, a fully parallel version of CD is presented in this work. For large enough systems, 100% efficiency is achieved for up to 256 processors using a hierarchical domain decomposition with dynamic load balancing. Compared to the sequential implementation, no influence of the parallelization on simulation results is found.Scherbänder in granularer Materie: Entstehung und Stabilität an glatten Wänden Diese Arbeit behandelt die numerische Untersuchung der Rheologie und Schereigenschaften granularer Materie aus runden Teilchen. Neben den vielfältigen experimentellen und theoretischen Arbeiten, die sich mit dieser Materie beschäftigen, gibt es unterschiedliche Simulationsmethoden mit denen das Fließverhalten, die Kompaktierung und andere Eigenschaften granularer Materie untersucht werden. In dieser Arbeit wurde die Kontakt-Dynamik-Methode (CD) zur Untersuchung eines zweidimensionalen Systems aus granularer Materie angewandt. Die Teilchen sind starre Scheiben und die einzigen Kontaktkräfte zwischen diesen sind die Coulombsche Reibungskraft parallel und Volumenausschluss-Kräfte senkrecht zur Kontaktfläche. Die Teilchen befinden sich in einem System mit planarer Geometrie, das von oben und unten durch zwei parallele Wände begrenzt ist. Der Druck und die Schergeschwindigkeit sind in jeder Simulation fest vorgegeben und bleiben während der gesamten Simulation konstant. In dieser Arbeit werden, im Gegensatz zu vielen aktuellen Untersuchungen, absolut glatte, mit Reibung versehene Wände zur Scherung benutzt. Diese führen zu sehr inhomogenen Scherraten im System mit deutlicher Scherlokalisierung an den Wänden, die durch den Schlupf an diesen verursacht wird. Drei unterschiedliche Scherregime werden hierbei beobachtet. Jedes dieser Regime gehört zu einem wohldefinierten Intervall der Schergeschwindigkeit, das hauptsächlich von der Systemgröße unabhängig ist. Sowohl die Eigenschaften dieser drei Regime als auch die beiden Übergänge zwischen Ihnen werden detailliert in Kapitel 6 behandelt. In Kapitel 7 werden die konstitutiven Gesetze separat im Bulk und in den Grenzgebieten zu den Wänden hergeleitet. Anhand dieser konstitutiven Gesetze und ergänzender elementarer Stabilitätsanalysen wird das Vorkommen beider Übergänge, sowie charakteristische Transientenzeiten vorausberechnet. In Kapitel 8 wird eine kritische Mindestgröße des Reibungskoeffizienten an glatten Wänden festgestellt, die das Scheren ermöglicht. Bei diesem kritischen Reibungskoeffizienten wird ein besonderes Verhalten des Systems im quasistatischen Regime beobachtet, über welches zuvor noch nicht in der Literatur berichtet worden ist. In Kapitel 9 wird über eine erfolgreiche Parallelisierung der CD berichtet. Diese ermöglicht Simulationen in größeren Systemen, eher vergleichbar zur realen Systemen, die ebenso für die ``Finite-Size-Analyse'' notwendig sind

Fourth NASA Workshop on Computational Control of Flexible Aerospace Systems, part 1

The proceedings of the workshop are presented. Some areas of discussion are as follows: modeling, systems identification, and control of flexible aircraft, spacecraft, and robotic systems