Margarida F. Machado

A Parametric Study on the Baumgarte Stabilization Method for Forward Dynamics of Constrained Multibody Systems

This paper presents and discusses the results obtained from a parametric study on the Baumgarte stabilization method for forward dynamics of constrained multibody systems. The main purpose of this work is to analyze the influence of the variables that affect the violation of constraints, chiefly the values of the Baumgarte parameters, the integration method, the time step and the quality of the initial conditions for the positions. In the sequel of this process the formulation of the rigid multibody systems is reviewed. The generalized Cartesian coordinates are selected as the variables to describe the bodies’ degrees of freedom. The formulation of the equations of motion uses the Newton-Euler approach that is augmented with the constraint equations that lead to a set of differential algebraic equations. Furthermore, the main issues related to the stabilization of the violation of constraints based on the Baumgarte approach are revised. Special attention is also given to some techniques that help in the selection process of the values of the Baumgarte parameters, namely those based on the Taylor’s series and Laplace transform technique. Finally, a slider crank mechanism with eccentricity is considered as an example of application in order to illustrated how the violation of constraints can be affected by different factors such as the Baumgarte parameters, integrator, time step and initial guesses.© 2009 ASME

Influence of the contact model on the dynamic response of the human knee joint

The goal of this work is to study the influence of the contact force model, contact geometry, and contact material properties on the dynamic response of a human knee joint model. For this purpose, a multibody knee model composed by two rigid bodies, the femur and the tibia, and four non-linear spring elements that represent the main knee ligaments, is considered. The contact force models used were the Hertz, the Hunt–Crossley, and the Lankarani–Nikravesh approaches. Results obtained from computational simulations show that Hertz law is less suitable to describe the dynamic response of the cartilage contact, because this pure elastic model does not account for the viscoelastic nature of the human articulations. Since knee can exhibit conformal and non-conformal contact scenarios, three different geometrical configurations for femur–tibia contact are considered, that is convex–convex sphere contact, convex–concave sphere contact, and convex sphere–plane contact. The highest level of contact forces is obtained for the case of convex–convex sphere contact. As far as the influence of the material contact properties is concerned, the dynamic response of a healthy and natural knee is analysed and compared with three pathological and two artificial knee models. The obtained results demonstrate that the presence of the cartilage reduces significantly the knee contact forces.

Investigation on the Baumgarte Stabilization Method for Dynamic Analysis of Constrained Multibody Systems

This paper presents an investigation on the Baumgarte stabilization method for dynamic analysis of constrained multibody systems. The purpose of this work is to study the influence of the main variables that affect the constraints violation, namely, the values of the Baumgarte parameters. In the process, the formulation of the dynamic equations of motion of constrained multibody systems and the main issues of the Baumgarte stabilization method are revised. Attention is given to the techniques to help in the Baumgarte parameters selection. A demonstrative example is presented and the results of some simulations are discussed.

A Lookup-Table-Based Approach for Spatial Analysis of Contact Problems

The aim of this work is to present an efficient methodology to deal with general 3D-contact problems. This approach embraces three steps: geometrical definition of 3D-surfaces; detection of the candidate contact points; evaluation of the contact forces. The 3D-contact surfaces are generated and represented by using parametric functions due to their simplicity and easiness to handle freeform shapes. This task is carried in preprocessing, performed preliminarily to the implementation of the multibody code. The preprocessing procedure can be condensed into four steps: a regular and representative surface collection of points is extracted from the 3D-parametric surface; for each point the tangent vectors to the u and v directions of the parametric surface and the normal vector are computed; the geometrical information on each point is saved in a lookup table, including the parametric point coordinates, the corresponding Cartesian coordinates and the Cartesian components of the normal, tangent and binormal vectors; the lookup table is rearranged such that the u-v mapping is converted into a 3D-matrix form. In the last step, the surface data is saved as a direct access file. Regarding the detection of the contact points, the relative distance between the candidate contact points are computed and used to check if the bodies are in contact. The actual contact points are selected as those that correspond to the maximum relative indentation. The contact forces are determined as functions of the indentation, impact velocity and geometric and material properties of the contacting surfaces. In general, lookup tables are used to reduce the computation time in dynamic simulations. However, the application of these schemes involves an increase of memory needs. Within the proposed approach, the amount of memory used is significantly reduced, as a result of a partial upload into memory of the lookup table. A slider-crank mechanism with a cup on the top of the slider and a marble ball is used as demonstrative example. A contact pair is considered between a cup and a marble ball, being the contact forces computed using a dissipative contact model.Copyright

A novel continuous contact force model for multibody dynamics

A general and comprehensive analysis on the continuous contact force models in multibody dynamics is presented and a novel contact force model is proposed. The force models are developed based on the foundation of the Hertz law together with a hysteresis damping parameter that accounts for the energy dissipation during the contact process. In a simple way, these contact force models are based on the analysis and development of three main issues: (i) the dissipated energy associated with the coefficient of restitution that includes the balance of kinetic energy and the conservation of the linear momentum between the initial and final instant of contact; (ii) the stored elastic energy, representing part of initial kinetic energy, which is evaluated as the work done by the contact force developed during the contact process; (iii) the dissipated energy due to internal damping, which is evaluated by modeling the contact process as a single degree-of-freedom system to obtain a hysteresis damping factor. This factor takes into account the geometrical and material properties, as well as the kinematic characteristics of the contacting bodies. The proposed contact force model has the great merit that can be used for contact problems involving materials with low or moderate values of coefficient of restitution. This contact force model is suitable to be included into the equations of motion of a multibody system and contributes to their stable numerical resolution. Two demonstrative examples of application are used to provide the results that support the analysis and discussion of procedures and methodologies adopted in this work.Copyright

ON DYNAMIC ANALYSIS OF CONTACT PROBLEMS WITH FREEFORM SURFACES: A KNEE JOINT 3D-STUDY

Fundacao para a Ciencia e a Tecnologia (FCT) - DACHOR (MIT-Pt/BSHHMS/ 0042/2008), BIOJOINTS (PTDC/EME-PME/ 099764/2008), SFRH/BD/40164/ 2007, SFRH/BD/47750/2008

On the Contact Modeling and Analysis of the Human Knee Joint

The goal of this work is to study the influence of the contact force model and contact material properties on the dynamic response of a human knee joint. For this purpose, a multibody knee model composed by two rigid bodies, the femur and the tibia, and four nonlinear spring elements that represent the main knee ligaments, is considered. The contact geometrical profiles are extracted from medical images and fitted using spline functions. The tibia motions are modeled, not using a conventional kinematic joint, but rather in terms of the action of the ligaments and potential contact between the bones. Besides, an external force is applied on the center of mass of the tibia in order to simulate the force of the quadriceps muscle group. When a contact is detected, a continuous contact force law is applied. The contact force laws studied are the Hertz, the Hunt-Crossley and the Lankarani-Nikravesh models. Results obtained from computational simulations show that Hertz law is less suitable to describe the dynamic response of the cartilage contact, because this pure elastic model does not account for the viscoelastic nature of the human articulations. Moreover, the effect of the amplitude of the external applied force on the dynamic response of the knee joint model is also evaluated. The obtained results show that the increase of the amplitude of the external applied force increases the contact indentations and lead to an earlier first impact. As far as to the influence of the material contact properties is concerned, the dynamic response of a healthy and natural knee is analyzed and compared with three pathological and two artificial knee models. Results demonstrate that the presence of the cartilage reduces significantly the knee contact forces.Copyright

Spatial Multibody Systems with Lubricated Spherical Joints: Modeling and Simulation

The dynamic modeling and simulation of spatial multibody systems with lubricated spherical joints is the main purpose of the present work. When the spherical clearance joint is modeled as dry contact; i.e., when there is no lubricant between the mechanical elements which constitute the joint, a body-to-body (typically metal-to-metal) occurs. The joint reaction forces in this case are evaluated through a Hertzian-based contact law. The presence of a fluid lubricant avoids the direct metal-to-metal contact. In this situation, the squeeze filmaction, due to the relative approaching motion between the mechanical joint elements, is considered utilizing the lubrication theory associated with the spherical bearings. In both cases, the intra-joint reaction forces are evaluated as functions of the geometrical, kinematical and physical characteristics of the spherical joint. These forces are then incorporated into a standard formulation of the system‘s governing equations of motion as generalized external forces. A spatial four bar mechanism that includes a spherical clearance joint is considered here as example.