"Operational deflection shapes (ODS) 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.'' Ole Døssing, 1988.
What is an Operating Deflection Shape?
An Operating Deflection Shape or ODS is in the literature defined as the deflection of a structure at a particular frequency relative to a specific point on the structure. This specific point is called the reference point. This definition is referring to the frequency domain representation of the ODS.
However, it is also useful to observe the ODS in time domain, where the deflection is animating in time with absolute scaling, i.e. not relative to a reference point. In this representation the deflection is composed of all frequency content between 0 and Nyquist frequency at a certain time instant.
So ODS is not only one technology but rather two, Time Domain ODS (TODS) and Frequency Domain ODS (FODS).
Why Measure Operating Deflection Shapes?
ODS measurements can be helpful answering vibration related questions such as,
In general, all vibration is a combination of both forced vibration and modal response of the structure. Forced vibration can be due to;
An Operating Deflection Shape contains the overall vibration on a structure. In other words, the ODS contains both forced and modal response components. On the other hand, a mode shape characterizes only the modal response. If a structure is excited dynamically around the resonance frequencies of one or more of the structural modes, the resulting resonant vibration typically amplifies the vibration response of a structure far beyond the design levels for static loading. Resonant vibration is typically a significant contributing factor to many of the vibration related problems that occur in operating structures.
By using both ODS and modal analysis tools it is much easier to determine the cause of a vibration problem. The two types of tools let's you judge if a vibration problem is linked to the forced vibrations or to the structural system itself.