Alberto Luaces

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.

A 3D Physics-Based Hydraulic Excavator Simulator

The actuation of hydraulic excavators is a complex and not intuitive task which requires long and costly training periods, since the qualification of the operator has a significant impact in productivity and safety. Simulation-based training combined with virtual reality is becoming a competitive alternative to traditional training to reduce costs and risks in the instruction of excavator operators. Several excavator training simulators have been developed, but none of them features a dynamic model of the machine complete enough to simulate all the maneuvers performed in the daily work of real excavators. The authors have applied real-time simulation techniques from multibody system dynamics to develop a full 3D physics-based excavator simulator made up of 14 rigid bodies with 17 degrees of freedom. The simulation engine includes a custom collision detection algorithm and detailed tire force and contact force models. Terrain excavation and bucket loading and unloading are also simulated. The resulting model delivers realistic real-time behavior and can simulate common events in real excavators: slipping on slope terrains, stabilizing the machine with the blade or the outriggers, using the arm for support or impulsion to avoid obstacles, etc. The simulator console has a semi-immersive virtual reality interface that emulates the excavator cabin. The operator console imitates most of the controls of the real machine cabin using low-cost standard USB input devices: steering wheel, 2 joystiks with the standard excavator functions and 2 pedals. A tactile screen replicates the digital control panel of the excavator, which lets the operator control different machine settings. A hard shell hemispherical dome of 2 m diameter is used to project the subjective view from the operator’s position. The resulting simulator, which can run in a standard PC due to its high computational efficiency, can reproduce almost all the maneuvers performed by real excavators.Copyright

A collaborative benchmarking framework for multibody system dynamics

Despite the importance given to the computational efficiency of multibody system (MBS) simulation tools, there is a lack of standard benchmarks to measure the performance of these kinds of numerical simulations. This works proposes a collaborative benchmarking framework to measure and compare the performance of different MBS simulation methods. The framework is made up of two main components: (a) an on-line repository of test problems with reference solutions and standardized procedures to measure computational efficiency and (b) a prototype implementation of a collaborative web-based application to collect, organize and share information about performance results in an intuitive and graphical form. The proposed benchmarking framework has been tested to evaluate the performance of a commercial MBS simulation software, and it proved to be an effective tool to collect and analyze information about the numerous factors which affect the computational efficiency of dynamic simulations of multibody systems.

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.

PARALLEL LINEAR EQUATION SOLVERS AND OPENMP IN THE CONTEXT OF MULTIBODY SYSTEM DYNAMICS

Computational efficiency of numerical simulations is a key issue in multibody system (MBS) dynamics, and parallel computing is one of the most promising approaches to increase the computational efficiency of MBS dynamic simulations. The present work evaluates two non-intrusive parallelization techniques for multibody system dynamics: parallel sparse linear equation solvers and OpenMP. Both techniques can be applied to existing simulation software with minimal changes in the code structure; this is a major advantage over MPI (Message Passing Interface), the de facto standard parallelization method in multibody dynamics. Both techniques have been applied to parallelize a starting sequential implementation of a global index-3 augmented Lagrangian formulation combined with the trapezoidal rule as numerical integrator, in order to solve the forward dynamics of a variable number of loops four-bar mechanism. This starting implementation represented a highly optimized code, where the overhead of parallelization would represent a considerable part of the total amount of elapsed time in calculations. Several multi-threaded solvers have been added to the original software. In addition, parallelizable regions of the code have been detected and multi-threaded via OpenMP directives. Numerical experiments have been performed to measure the efficiency of the parallelized code as a function of problem size and matrix filling ratio. Results show that the best parallel solver (Pardiso) performs better than the best sequential solver (CHOLMOD) for multibody problems of large and medium sizes leading to matrix fillings above 10 non-zeros per variable. OpenMP also proved to be advantageous even for problems of small sizes, in despite of the small percentage of parallelizable workload with respect to the total burden of the execution of the code. Both techniques delivered speedups above 70% of the maximum theoretical values for a wide range of multibody problems.Copyright

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

Design of an innovative Hybrid Fuzzy Controller for laser surface treatments

This article introduces an innovative fuzzy logic control system for laser surface treatments, which allows to increase significantly the uniformity and final quality of the process, reducing the rejection rate and increasing the productivity and efficiency of the treatment. The proposed hybrid structure combines a PD fuzzy logic controller, with a pure integral action, both fully decoupled, improving considerably the performances of the process with a reasonable design cost, since the system nonlinearities are fully compensated by the fuzzy logic component of the controller, while the integral action contributes to eliminate the steady state error.

APPLICATION CRITERIA FOR CONSERVING INTEGRATORS AND PROJECTION METHODS IN MULTIBODY DYNAMICS

This work presents the application to the dynamics of multibody systems of two methods based on augmented Lagrangian techniques, compares them, and gives some criteria for its use in realistic problems. The methods are an augmented Lagrangian method with orthogonal projections of velocities and accelerations, and an augmented Lagrangian energy conserving method. Both methods were presented by the authors in a very recent work, but it was not complete since the testing and the comparison of the methods was done by simulating a simple and academic example, and that was not sufficient to draw conclusions in terms of efficiency. For this work, the whole model of a vehicle has been simulated through both formulations, and their performance compared for such a large and realistic problem.Copyright