Modal Shape and Operation Deflection Shape (ODS) Analysis

ODS and Modal Shape analyses are advanced diagnostic tools that are employed to isolate the source of vibration in a dynamic fashion on complex systems such as those found on ships.

These tests may be required if there are inherent anomalies which are possibly the result of design flaws, machines that display chronic or objectionable vibration patterns or where conventional corrective measures have failed.

These tests reveal structural resonance's and bending moments in multiple degrees of freedom of the machine, foundation and understructure as well as influences from other machines and shipboard vibrations.

Corrective actions can include adding/removing masses, changes in the support/components or changing or avoiding frequencies that result in these offensive patterns.

Both Mode Shape and Operation Deflection Shape (ODS) display the vibration on a drawing or picture of a machine.

Mode shape (based on the inherent resonance of a structure) can be determined if a hammer or bump test can produce those frequencies on the machine when not in use.

ODS (based on the source frequencies and the resonance frequencies) can be determined from roving and reference measurements on the machine while in actual use.

The methods are useful in determining where the vibration appears on a machine, where the nodes and anti-nodes are, where is the source of the vibration, and where the structure should be re-enforced.

Is It a Mode Shape, or an Operating Deflection Shape?
Mark H. Richardson, Vibrant Technology, Inc., Jamestown, California
Sound & Vibration Magazine 30th Anniversary Issue March 1997

Abstract
Mode shapes and operating "deflection" shapes are related to one another. In fact, one is always measured in order to obtain the other. Yet, they are quite different from one another in a number of ways. This article discusses the relationships between modal testing, modal analysis and operating deflection shape measurements.

Introduction
The question, "Is it a mode shape, or an operating deflection shape?" is probably asked more often than any other when testing structures, especially when attempting to identify their resonant or modal properties. Another way that it is asked is, "When the excitation changes, the mode shape changes. What's going on here?"

The subject of mode shapes versus operating deflection shapes has certainly been written about before. In fact, a previous Sound and Vibration magazine article [1] covered them quite extensively. I recommend that you read that article, because it provides valuable insight and contains a number of examples. To shed more light on this subject, I will point out other similarities and differences between the two types of shapes, and discuss the measurements required to obtain each of them.

Over the past 20 years, the number of ways in which modal testing has been done has proliferated greatly. Traditionally, most modal testing was done using sine wave based methods and analog instrumentation. During the late 1960s however, the discovery of the Fast Fourier Transform (FFT) algorithm and the use of digital computers in laboratory testing systems allowed experimentalists to begin exploring the use of new excitation and signal processing techniques for modal testing.

Because the FFT provides the frequency spectrum of a signal in fractions of a second, various kinds of broad band random, swept sine, and transient signals, which excite many frequencies at once, could be used to excite structures and measure their responses. Impact testing has become the most popular modal testing method today. It can be done rather quickly and inexpensively using an instrumented hammer, an accelerometer, a 2 channel FFT analyzer, and post processing software. Also, the availability of lower cost transducers, PC based data acquisition systems, portable data collectors, desktop and notebook computers, and more powerful software have all helped to put modal testing into the hands of more practitioners.

Nevertheless, modal analysis has often been shrouded in a veil of mystery, while the concept of an operating deflection shape has remained relatively straightforward. Ole Dossing began his article with the statements,

"Operational deflection shapes (ODSs) can be measured directly by relatively simple means. They provide very useful information for understanding and evaluating the absolute dynamic behavior of a machine, component or an entire structure.''

This suggests that maybe mode shapes are not so easy to measure. If not, then why not.

For the complete article please click here.

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