R. Snoeys

Trends in experimental modal analysis

Abstract The scope of this paper is to comment on current trends and new developments in the field of experimental modal analysis. The first section covers modal measurement and estimation procedures, with special emphasis on the use and limitations of recent techniques such as multiple input processing, total least square, global time- and frequency domain parameter estimation. In the second section reference is made to applications and use of modal parameters in techniques such as structural modification, fatigue and acoustic analysis. Emphasis is put on applications used at the Katholieke Universiteit, Leuven.

Accurate modal analysis measurements with programmed sine wave excitation

Abstract Due to the development of several new dynamic analysis methods which use experimental modal analysis results as input data, these results need to be subjected to stringent accuracy demands. One of the techniques to achieve this goal is the use of stepped since excitation. Although the principle of this technique has been known for a long time, a feasible implementation, suited for testing large structures or non-linear systems, has only been possible by making full use of todays techniques in digital signal processing. In this paper, the characteristics of the method are briefly reviewed, for both single input and multiple input testing. An implementation with response adaptive definition of excitation frequency and amplitude is presented and illustrated with some examples. The impact of this method on the estimation of the modal parameters is discussed.

The complex stiffnes method to detect and identify non-linear dynamic behaviour of SDOF systems

Abstract A new method to detect and identify non-linear dynamic behaviour of single-degree-of-freedom (SDOF) systems is presented. The method is based on the characteristic evolution of the least squares equivalent damping and stiffness as a function of the velocity and the displacement respectively. With these two functions, it is possible to detect the non-linearity, to identify its basic type and to obtain quantitative information on the system parameters. The method is evaluated by means of several examples and compared to other detection and identification methods. Extensions to multi-degree-of-freedom (MDOF) systems and to modeling non-linear systems are outlined.

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.

A programmable function generator for dynamic analysis testing

A flexible function generator with high-resolution programmability of frequency, amplitude, and DC offset has been developed for structural dynamics testing. The generator consists of a clock generator which defines the signal frequency and memory tables which define the signal functions. In order to obtain a fast-settling, stable, accurate variable clock signal, a frequency-locked-loop circuit was developed.

Design of multiple input/output modal analysis instrumentation

A flexible measurement system has been developed to implement recently developed experimental dynamic analysis techniques. The system consists of a modular front-end and a mini- or microcomputer. Several modules for automated signal conditioning and data acquisition as well as for programmable signal generation are available. A software interface package simplifies the development of application programs. The system is compatible with existing signal and system analysis software.