Mechanical Behavior Of Structures Research Articles

Development of a high speed system to control dynamic behaviour of mechanical structures

Techniques for using real-time system control to modify the dynamics of mechanical structures have been developed and demonstrated on a two-stage mechanical positioning system. Methods for designing control algorithms for fast sampling rates were investigated, and simulations of the system which accounted for many of the nonlinearities of digital control were developed. The results were implemented on an experimental system which showed a dramatic improvement in performance over the uncontrolled case. A unique method of handling known decaying periodic disturbances to the system was also developed and tested. This method resulted in more than 70% reduction of the error due to the disturbance. A computer control system that uses a high speed Digital Signal Processor (DSP) chip to implement control algorithms with fast sampling rates was developed. There still remain, however, several approaches to be investigated before the immense capabilities of DSPs as compensators in digitally controlled mechanical systems can be realised.

Synthesis, experimentation, and the biomechanics of economical movement

An issue that should concern sports biomechanists is the need for more synthesis of ideas and experimental testing of those ideas. Descriptive analysis is overemphasized at present, and more stress on synthesis of existing data and experimental testing of new ideas should generate new theories that will give us insights into the mechanical behavior of structures and the kinetics and kinematics of sports movements. One area where this approach may be especially helpful is in studying the question of what makes movements energetically economical. Twenty-six behavioral and mechanical factors have been shown to have an association with the economy of movement. A synthesis should be undertaken which probes the meaning of these various data. Trends in these findings point to intra- and intersegmental energy transfer, center of mass excursion, and elastic energy storage as potential areas for expanding our understanding of the biomechanics of economical movement. These possibilities might be explored by enhancing link-segmental mathematical models to predict what makes movements economical and then testing these predictions by using biofeedback techniques to train subjects to perform the correct movements while being monitored. Optimization models might offer an additional strategy for developing a theory of mechanically and energetically economical movement.

Nonlinear Dynamic Behaviour of Typical Mechanical Structures

The aim of this paper is to emphasize nonlinear dynamic behaviour of some mechanical structures in order to illustrate that such behaviour is not only of academic importance. The assumption of linear behaviour has to be verified to validate consistency of results of a modal analysis test. Popular methods to check linearity are Maxwells reciprocity theorem and the principle of superposition. In this paper, tests on five mechanical structures are represented: checks on linearity indicated the presence of some typical nonlinear dynamic behaviour. These examples are selected to give a survey of common causes of nonlinear dynamic behaviour: backlash and friction in connections, nonlinear type of damping, nonlinear stiffness resulting from the geometrical shape of the structure or the dynamic properties of the material. Following structures have been tested: a longitudinal beam of a car-frame, a roundness measuring-instrument, the IGLOO-module of ESA-spacelab, the grinding wheel-workpiece contact area and the dynamical properties of rubber elements. These structures show a degree of nonlinearity too significant to be neglected if accurate measurements are desired.

Comparison of Modal Analysis and Holography

Modal analysis and holography are two powerful tools of modern measuring techniques. Both methods may be used to test the behaviour of mechanical structures in order to detect and remove possible weak points. Due to the fully different physical principles of these methods, the measuring procedures, the representation of results and their evaluation are quite different. The first part of this contribution describes the typical operation of both systems. The resulting main differences in the measurement and analysis are illustrated with the help of several practical examples. The possibilities and the specific advantages and disadvantages of each instrument are then compared in detail. Corresponding fields of applications, as well as combined testing procedures, can be deduced there from. Some expected future improvements are also mentioned.