Automatic generation of equations of motion for multibody system in discrete event simulation framework

AbstractIn this paper, the development of a simulation program that can automatically generate equations of motion for mutibody systems in the discrete event simulation framework is presented. The need to analyze the dynamic response of mechanical systems that are under event triggered conditions is increasing. General mechanical systems can be defined as multibody systems that are collections of interconnected rigid bodies, consistent with various types of joints that limit the relative motion of pairs of bodies. For complex multibody systems, a systematic approach is required to efficiently set up the mathematical models. Therefore, a dynamics kernel was developed to automatically generate the equations of motion for multibody systems based on multibody dynamics. The developed dynamics kernel also provides the numerical solver for the dynamic analysis of multibody systems. The general multibody dynamics kernel cannot deal with discontinuous state variables, event triggered conditions, and state triggered conditions, though. To enable it to deal with multibody systems in discontinuous environments, the multibody dynamics kernel was integrated into a discrete event simulation framework, which was developed based on the discrete event system specification (DEVS) formalism. DEVS formalism is a modular and hierarchical formalism for modeling and analyzing systems under event triggered conditions, which are described by discontinuous state variables. To verify the developed program, it was applied to an block-lifting and transport simulation, and dynamic analysis of the system is carried out

Middeck Active Control Experiment (MACE), phase A

A rationale to determine which structural experiments are sufficient to verify the design of structures employing Controlled Structures Technology was derived. A survey of proposed NASA missions was undertaken to identify candidate test articles for use in the Middeck Active Control Experiment (MACE). The survey revealed that potential test articles could be classified into one of three roles: development, demonstration, and qualification, depending on the maturity of the technology and the mission the structure must fulfill. A set of criteria was derived that allowed determination of which role a potential test article must fulfill. A review of the capabilities and limitations of the STS middeck was conducted. A reference design for the MACE test article was presented. Computing requirements for running typical closed-loop controllers was determined, and various computer configurations were studied. The various components required to manufacture the structure were identified. A management plan was established for the remainder of the program experiment development, flight and ground systems development, and integration to the carrier. Procedures for configuration control, fiscal control, and safety, reliabilty, and quality assurance were developed

A multibody approach to the contact dynamics: a knee joint application

In this thesis, a general approach for dynamic analysis of multibody systems with contact is presented, being a special attention given to the articular contact at the human knee joint. Two methodologies, in two- and three-dimensions, for knee contact modeling are proposed under the framework of multibody systems using generalized Cartesian coordinates. The development of the planar multibody knee model encompasses four steps: (i) geometrical representation of contacting profiles by means of curve fitting techniques based on spline interpolation functions; (ii) location of contact points and evaluation of the contact indentation; (iii) calculation of the contact forces by using an appropriate constitutive law; (iv) description of the ligament behavior by a quadratic stress-strain relation. The motion of the tibia relative to the femur is modeled combining the action of the knee ligaments with the potential contacts between the bones. The contact forces, together with the forces produced by the ligaments, are introduced into the Newton-Euler equations of motion as external generalized forces. Within the three-dimensional methodology, the contact surfaces are described by means of point-clouds extracted from parametric representations. The spatial formulation presents a pre-processing unit. This preprocessor allows for a significantly reduction of the amount of memory required for data storage and an improvement of the computational efficiency of the contact detection process. Computational simulations were performed with the aim of validating both proposed approaches, two-dimensional and three-dimensional. The behavior of the planar knee model resultant of the application of different contact force laws was studied. Moreover, the influence of the geometric and material properties on the dynamic response of the knee joint model was investigated. In a broad sense, the proposed methodologies demonstrated to be suitable for the analysis of the dynamic behavior of multibody models with contact, especially those biological systems such as the knee joint that involve complex geometries, a large range of motion and high dynamic loads.Nesta tese é proposta uma abordagem genérica para a análise dinâmica de sistemas de corpos múltiplos com contacto, dando um especial enfoque ao contacto articular no joelho humano. No âmbito da dinâmica de sistemas de corpos múltiplos são apresentadas duas metodologias, bidimensional e tridimensional, para a modelação do contacto no joelho usando coordenadas cartesianas generalizadas. O desenvolvimento do modelo bidimensional do joelho engloba quatro etapas: (i) representação geométrica dos perfis de contacto por meio de técnicas de ajuste de curva com base em funções de interpolação por splines, (ii) localização dos pontos de contacto e avaliação da indentação de contacto, (iii) cálculo das forças de contacto usando uma lei constitutiva apropriada, (iv) descrição do comportamento dos ligamentos através de uma relação quadrática de tensão-deformação. O movimento da tíbia em relação ao fémur é modelado como uma acção combinada entre os ligamentos do joelho e os potenciais contactos entre os ossos. As forças de contacto, juntamente com as forças produzidas pelos ligamentos, são introduzidas nas equações de movimento de Newton-Euler como forças externas generalizadas. Na metodologia tridimensional, as superfícies de contacto são descritas por meio de nuvens de pontos extraídas de representações paramétricas. No âmbito da formulação tridimensional é apresentada uma unidade de pré-processamento. Este pré-processador permite uma redução significativa da quantidade de memória necessária para o armazenamento de dados e, desta forma, melhora a eficiência computacional do algoritmo de deteção de contacto. Com o objetivo de validar as metodologias propostas, realizaram-se várias simulações computacionais. Os comportamentos do modelo bidimensional do joelho resultantes da aplicação de diferentes leis de força de contacto foram estudados. A influência das propriedades geométricas e de material na resposta dinâmica do modelo bidimensional do joelho foi investigada. De uma forma geral, as metodologias propostas demonstraram ser adequadas para a análise do comportamento dinâmico de modelos de corpos múltiplos com contacto, especialmente sistema biológicos, como o joelho humano, que envolvem geometrias complexas, uma grande amplitude de movimentos e elevadas cargas dinâmicas

A comparative study of the viscoelastic constitutive models for frictionless contact interfaces in solids

The nature of the constitutive contact force law utilized to describe contact-impact events in solid contact interfaces plays a key role in predicting the response of multibody mechanical systems and in the simulation of engineering applications. The goal of this work is to present a comparative study on the most relevant existing viscoelastic contact force models. In the sequel of this process, their fundamental characteristics are examined and their performances evaluated. Models developed based on the Hertz contact theory and augmented with a damping term to accommodate the dissipation of energy during the impact process, which typically is a function of the coefficient of restitution between the contacting solids, are considered in this study. In particular, the identified contact force models are compared in the present study for simple solid impact problems with the sole purpose of comparing the performance of the various models and examining the corresponding system behavior. The outcomes indicate that the prediction of the dynamic behavior of contacting solids strongly depends on the selection of the contact force model.Fundação para a Ciência e a Tecnologia (FCT