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What is Modal Analysis?

(1) An understanding of the dynamic behaviour of mechanical systems is an increasingly important aspect of the design process. A demand for high performance has produced a trend towards increased power and higher machine speeds which have given rise to an increase in noise, vibration and dynamic loading. At the same time, improved manufacturing techniques and a closer control over material quality and working tolerances have made available lightweight products fabricated in a continuous structure. This form of construction tends to have a low damping and therefore may respond significantly to the surrounding vibration environment, particularly at resonant frequencies.

(2) In order to predict the dynamic behaviour of a given product in service, it is usual to simulate the intended operating environment by means of an electrodynamic or hydraulic shaker. Although such testing can identify potential vibration problem areas and give an important practical demonstration of product survival, it is of limited use in the design process. This form of qualification testing often leads to quick-fix modifications; the application of a bracket or strut, the introduction of a reinforcing plate, damping pads, etc. The complexity of built-up structures frequently inhibits a clear understanding of the problem cause, and may give no clue as to the remedial action which should be taken.

(3) Clearly, an analytical approach applied to components and sub-assemblies can have a more significant impact on the product design and development cycle. A well formulated design will consider theoretical predictions of vibration response at the outset and these will be supported by experimental techniques. One of the most popular approaches to provide theoretical prediction takes the form of finite element modelling, for which a number of commercially available software packages are available. Experimentation is usually concerned with the characterisation of a structure by its modes of vibration, associated natural frequencies and damping.

(4) This is achieved from a measurement of a structure's response to a given excitation (vibration test), followed by an extraction of the modal properties from the measured response characteristics (modal analysis). It is unfortunate that the techniques of finite element and experimental modal analysis have polarised within many organisations.

In many cases this results from a demarcation between advanced design by keyboard skills and practical engineering, (often reinforced by an educational system which separates mathematicians from engineers). Prototypes and engineering facilities are perceived as expensive, and it is convenient for management to place a high degree of confidence in mathematical models.

(5) In practice, both design tools are capable of significant error, and an integrated approach is more likely to succeed. Once a prototype becomes available, a modal analysis may be carried out, either as a stand-alone experimental tool or as confirmation of a theoretical model. Modal analysis can be very time consuming (and hence expensive) if a high degree of accuracy is required. Hence, it is important to establish the objectives of the testing programme before commencing the work.

Require further training? Consider a short course on Practical Modal Analysis.   Delivered on your company premises or at a convenient local hotel, the course will emphasise the practical issues of the subject and can be delivered over one, two or three days for 5 or more delegates.

(6) The results obtained from an experimental modal analysis can be employed to:

  • predict the response of a structure to various excitation conditions which the product may see in service.
  • enable verification of, and possible refinement to, an existing theoretical model.
  • allow the effect of changes in the mass, stiffness and damping properties to be examined without the need for the construction of expensive prototypes and subsequent testing for each configuration.
  • obtain a mathematical description of one particular component which forms part of a complete assembled structure and which may not be amenable to a theoretical analysis.