Operating Deflection Shape Testing

We have performed ODS testing on a huge variety of systems including many delicate semiconductor fabrication and inspection tools, with sub-micron motions, to entire 6 story building structures for the mining and mineral processing industry. A vibration consultants and experts in structural dynamics testing and analysis, we use ODS testing as one of our diagnostic tool within a large skill set, to bring a larger picture of the issues at hand.

 

Modal analysis and ODS testing go hand in hand to create the full picture of the forced response of a system with structural resonances below and near the operating frequency. These resonances amplify motion at some frequencies and attenuate it at others. The mode shapes from modal analysis also give us the physical, geometric, dependence of the structural response to a force at a given location on the structure acting through a specific axis. Some times the force levels are perfectly reasonable but the resonances have changed to become problematic making modal testing very important. Unfortunately, quite often we cannot shut down the vibrating structure to preform high quality modal testing to characterize the structural resonances because shutting the equipment in questing means shutting down the whole factory. This is costly (although sometimes we perform limited modal tests with the structure in operation).

 

Often it is the forced shape during operation that we want to see, as when we characterize an issue with a unstable control system as the ODS shape is often non-linear. Thus, ODS testing alone, however, still allows us to visualize the way the structure deforming due to the forces inherent in operation. This is very valuable and can give us very important clues as to where the structure is weak, where the structure is moving abnormally, and suggesting where we might consider modifications to the structure. We would still like to see the resonant modes that are found using experimental modal analysis but this will often have to wait until there is a scheduled plant shutdown.

 

For instance a fan motor may drive a machine to vibrate at a frequency that is between two resonant frequencies. In that case the response of the structure will be dominated by the two adjacent modes of the structure and will be a super position of the two modes weighted by the modal response at the fan input location. A single mode animation from a modal analysis, in this case, would not show the actual operating deformed shape. With the ODS we see the deformed shape that comes from the excitation of two modes that dominate this operating deformation shape. If we want the "bottom line" deformation shape the ODS analysis is the best approach. Most often, however, we can get to a solution with either technique.

 

Often the item under test can not be turned off, like the turbine support structure of a power plant, and ODS analysis is the best we can do to characterize the vibration problem. There are also cases where we just can not get the input force applied in the right way, at the right location, to excite the modes of interest like in the case of a system driven by equal and opposite magnetic forces applied at the center of a compact machine.

 

Whether using a modal analysis of ODS analysis we have the skill set, the experience, and the instrumentation to be efficient and thorough in the de-bugging process of test design, data collection, analysis and laying out a set of solution options.

 

Take the example of the engine/gen set (See Fatigue Cracks on Engine/Gen Set) ODS testing was performed on an engine generator set to find out how the structure was moving to cause cracks in the steel support structure. Since both the gas engine and generator have the potential to produce large forces the operating deflection shape (ODS) was important, not just the mode shapes. The mode shapes showed that indeed the structure was operating near a resonance of the system involved in bending at the part of the steel skid that was cracking. We built an FEA model and "tuned" the model using realistic adjustments of the poorly defined contacts between parts (inherent in most bolted structures) and matched the existing structural dynamics. We then explored various structural modifications using FEA to find the best structural modification option for moving the resonances away from the operating speed and to decouple given the geometry of the forces acting of the structure. We have performed ODS testing on hundreds of structures large and small to characterize the vibration problem and engineer a solution.