Experimental Modal Analysis Techniques Research Articles

An experimental modal analysis technique for large-scale structures

Using the theory of stationary random vibrations an experimental modal analysis technique, previously suggested and applied by Luz [2–4], is derived. This method does not require an artificial excitation of the structure under investigation and is based on measurements of the system's velocity only. It is well suited for applications to large-scale structures such as high-rise buildings, towers or bridges.

Evaluation of Advanced Cementitious Composites for Machine-Tool Structures

Reported are the results of an investigation conducted on the suitability of advanced cementitious materials, like ferrocement, fibrous ferrocement and polymer impregnated ferrocement as a replacement of cast iron for machine tool structures. Prototype beds for a typical lathe machine have been designed and fabricated, and an experimental modal analysis technique is used to study the dynamic characteristics of these beds. Ferrocement beds are found to be dynamically more stable than the parent cast iron bed. Addition of discrete fibres further Improves the performance. However, the most significant improvement is observed when polymer is impregnated into the ferrocement matrix.

Experimental modal analysis of non-linear systems: A feasibility study

Experimental modal analysis techniques are based on the theory of linear systems. In this paper the feasibility of their application to non-linear systems is examined. A three degree of freedom non-linear mechanical system was modeled on an analog computer and system responses due to band limited random excitation were examined by using a two channel FFT analyzer. Frequency response functions, natural frequencies and model responses were studied experimentally for a number of example cases. This study shows that the measured transfer function plots can alert one to the fact that the structure is non-linear; measured modal response data base can still give a rough idea about the system behavior. In addition, measured coherence function estimates can be used as the “warning singls” for inherent non-linearities.

Experimental modal analysis technique for three-dimensional acoustic cavities

This paper substantially advances Nieter and Singh’s acoustic modal analysis technique [J. Acoust. Soc. Am. 72, 319–326 (1983)] which used the coincident-quadrature response method to extract acoustic global properties of one-dimensional systems. Here a technique to identify modal parameters including damping of a three-dimensional cavity is proposed. The cavity is excited by a convertible acoustic driver whose volume velocity is monitored. Acoustic transfer impedance database is obtained for a number of cavity boundary locations which are chosen such that all modes of interest can be adequately described. The vector diagram method is used to fit the measured database in the neighborhood of each resonance frequency with a circle. The experimental technique proposed here has been verified by applying it first to a cylindrical cavity whose exact eigensolutions are known. Then for an annularlike cavity, experimentally extracted natural frequencies and pressure mode shapes are found to agree well with the results of a finite element model. The significance of this paper lies in the successful demonstration of an experimental technique which can be used for diagnosing the acoustics of irregular shaped cavities. The proposed technique also offers an alternative to the finite element method for acoustic modal analysis of three-dimensional ducts and resonators of irregular shapes and/or with complicated boundary conditions.

On predicting point mobility plots from measurements of other mobility parameters

Experimental modal analysis techniques are designed to exploit a property of linear vibration theory in order to construct a mathematical model of a structure from the minimum amount of measured mobility data. This property, derived from the orthogonality of normal modes, is that, in principle, all the elements of the full N × N mobility matrix can be derived from measurement of the elements in just one row or column of that matrix. The property is also used in procedural quality “checks”, in which one measures some of the derived mobilities as well as predicting them. The derivation process itself, however, gives rise to an inherent error when the frequency range covered does not include all the natural frequencies of the structure. The source of this error is discussed and some illustrations of its significance are quoted, demonstrating the need for caution when applying modal analysis methods to practical structures.