Urbano Lugrís

Performance and Application Criteria of Two Fast Formulations for Flexible Multibody Dynamics

Abstract The performance of the simulation of flexible multibody systems can be improved by means of the use of topological formulations, which have provided good results in the simulation of large rigid multibody systems. In this work, a topological formulation for rigid bodies is extended to the flexible case, and tests are carried out in order to compare its performance with that of a global formulation. Three systems are simulated: a double four-bar mechanism, a vehicle suspension, and a full vehicle. As it happens in the rigid case, the topological formulation is faster than the global one only for large mechanisms.

Efficient and accurate simulation of the rope–sheave interaction in weight-lifting machines

This article presents different approaches that can be used for modelling wire ropes in weight-lifting machines. It is shown that modelling the rope as a linear spring, although very simple and efficient, is energetically inconsistent and produces spurious terms in the equations of motion if the rope deformation along the segment in contact with the sheave is not considered. In order to overcome this problem and obtain an efficient yet accurate method for the simulation of such systems, a semi-analytical method is derived by introducing an analytical model of the rope–sheave interaction in the system, and the obtained results are compared with a finite-element numerical model. The semi-analytical model is based on a continuum mechanics approach of the rope; it assumes Coulomb friction between the sheave and the rope and neglects the centrifugal force of the segment of rope in contact with the sheave while accounting for tangential inertia forces in the rope. The numerical model is based on the Absolute Nodal Coordinate Formulation, and accounts for both the inertia forces and the bending and axial deformation of the rope.

Gait analysis system for spinal cord-injured subjects assisted by active orthoses and crutches

The inverse dynamics of human gait from motion capture data is an already mature discipline. The present work addresses the problems that arise when assistive devices such as crutches and active orthoses are added to the analysis. The objective is to provide an analysis tool for the gait of spinal cord-injured subjects, since these patients always require the help of assistive devices to walk. A gait analysis system for subjects walking with the aid of crutches and active knee–ankle–foot orthoses is presented. The assistive devices are introduced both at the experimental and computational levels. The required sensors and actuators are incorporated to the system, and the measurements are used to solve the inverse dynamics problem in order to calculate the joint motor torques produced by the subject during gait. Such analysis can be greatly helpful for comparing the performance of passive and active orthoses, evaluating and improving the controllers in the latter, monitoring the adaptation of the patients to the orthoses and their rehabilitation level, and improving the understanding of the interaction between active orthoses and the muscular system.

Efficient calculation of the inertia terms in floating frame of reference formulations for flexible multibody dynamics

Abstract One of the characteristics of floating frame of reference (FFR) formulations for flexible multibody dynamics is the fact that the inertia terms are highly non-linear. At every time-step, both the mass matrix and the velocity-dependent forces vector must be updated, and this can become the most CPU intensive task. This work studies the efficiency of two different methods for performing this operation, when applied to both a formulation in absolute coordinates and another in relative coordinates. The first method calculates the inertia terms by projecting the finite element (FE) mass matrix into the generalized coordinates, by means of a variable projection matrix. The second one calculates the inertia shape integrals at a preprocessing stage and uses them for obtaining the inertia terms in a more efficient way, at the cost of a more involved implementation. Both methods have been tested when used in combination with either the FFR absolute or relative formulation, by simulating a vehicle with 12 flexible elements. The results show that the performance can be considerably increased by means of the preprocessing method, especially in the case of large FE models, whereas, for small models, the projection method can be more convenient due to its simplicity.

IRK VS STRUCTURAL INTEGRATORS FOR REAL-TIME APPLICATIONS IN MBS

Recently, the authors have developed a method for real-time dynamics of multibody systems, which combines a semi-recursive formulation to derive the equations of motion in dependent relative coordinates, along with an augmented Lagrangian technique to impose the loop closure conditions The following numerical integration procedures, which can be grouped into the socalled structural integrators, were tested trapezoidal rule, Newmark disstpative schemes, HHT rule, and the Generalized-α family It was shown that, for large multibody systems, Newmark dissipative was the best election since, provided that the adequate parameters were chosen, excellent behavioi was achieved in terms of efficiency and lobustness with acceptable levels of accuracy In the present paper, the performance of the described method in combination with another group of integrators, the Implicit Runge-Kutta family (IRK), is analyzed The purpose is to clarify which kind of IRK algorithms can be more suitable for real-time applications, and to see whether they can be competitive with the already tested structural family of integrators The final objective of the work is to provide some practical criteria for those interested in achieving real-time performance for large and complex multibody systems

Design and Experimental Evaluation of a Low-Cost Robotic Orthosis for Gait Assistance in Subjects with Spinal Cord Injury

Robotic gait training after spinal cord injury (SCI) is of high priority to maximize independence and improve the living conditions of these patients. Current rehabilitation robots are expensive and heavy, and are generally found only in the clinic. To overcome these issues, we present the design of a low-cost, low-weight robotic orthosis for subjects with SCI. The paper also presents a preliminary experimental evaluation of the assistive device on a subject with SCI. Results show that gait velocity, stride length and cadence of walking increased (24.11, 7.41 and 15.56 %, respectively) when wearing active orthoses compared to the case with standard passive orthoses.

Evaluation of Motion/Force Transmission Between Passive/Active Orthosis and Subject Through Forward Dynamic Analysis

Forward dynamic analysis of the acquired gait of subjects assisted by either passive or active knee-ankle-foot orthoses and crutches is used to evaluate the motion and force transmission between orthosis and subject depending on the connecting stiffness. Unlike inverse dynamic analysis, this approach allows to consider the subject’s limbs and the assistive devices as different entities, so that their relative behavior may be studied. The quality of motion transmission and the intensity of interface forces are evaluated for a range of connecting stiffness values, so that those providing the best trade-off between both aspects can be identified.

Calibration and Validation of a Skeletal Multibody Model for Leg-Orthosis Contact Force Estimation

Estimation of contact forces between lower limb and orthosis during gait is useful to prevent skin issues in subjects wearing this type of assistive devices. While inverse-dynamics based gait analysis of multibody models is difficult to apply due to the limited accuracy of motion capture systems, a forward-dynamics based analysis in which leg and orthosis are considered as independent entities is shown to provide acceptable results. Contact model parameters are calibrated through comparison of measured and calculated bending torque at the orthosis location where a load cell is installed, and the attained correlation allows to validate the model.

Human Gait Analyses Using Multibody Systems Formulation: Normal and Pathological Scenarios

The main goal of this work is to present planar biomechanical multibody model, suitable to be used in inverse dynamic analyses. The proposed approach is straightforward and computationally efficient for the study of different human gait scenarios e.g. normal and pathological. For this, a biomechanical model of the lower limb of the human body was considered. The model consists of three rigid bodies (thigh, calf and foot), corresponding to relevant anatomical segments of lower limb. The three bodies are connected by revolute joints and described by eight natural coordinates, which are the Cartesian coordinates of the basic points located at the joints (hip, knee, ankle, metatarsal-phalangeal). The anthropometric dimensions of the model correspond to those of a normal male of 1.77 m and 80.0 kg and a poliomyelitis (polio) patient of 1.78 m and 92 kg. The total biomechanical system encompasses 5 degrees-of-freedom: 2 degrees-of-freedom for hip trajectory, 1 degree-of-freedom for hip flexion-extension motion, 1 degree-of-freedom for knee flexion-extension and 1 degree-of-freedom for ankle plantarflexion-dorsiflexion. The developed model was applied to solve an inverse dynamics problem of human motion. Therefore, the main objective of this study is to determine the joint kinematics, moments-of-force and reaction forces during an entire gait cycle.

CONSIDERATION OF ASSISTIVE DEVICES IN THE GAIT ANALYSIS OF SPINAL CORD-INJURED SUBJECTS

The gait of spinal cord–injured subjects can be improved by means of properly designed active orthoses. Since the gait pattern varies greatly among different patients, the orthoses and their corresponding controllers must be carefully customized, and the joint motor torques obtained from inverse dynamic analysis constitute a useful input for this task.Nowadays, the analysis of standard gait by using motion capture data is a mature discipline. However, the problem becomes more complex in the case of spinal cord–injured subjects wearing active orthoses: in addition to the presence of the orthoses, these patients always require the help of supplementary assistive devices, such as crutches, in order to stabilize their gait.In this work, the gait analysis of a subject walking with the aid of crutches and active knee–ankle–foot orthoses is performed, by introducing the assistive devices both at the experimental and computational levels. The required sensors and actuators are incorporated to the system, and the measurements are used to solve the inverse dynamics problem in order to calculate the joint motor torques produced by the subject during gait.Copyright