ODS is the measurement of dynamic deformation patterns of a structure while under operation. This means that your machine is running and creating the modal shapes without the use of a person swinging an impaq hammer to excite the structure. With the correct vibration ratio and phase data, one can directly show the modal shape without curve fitting mathematics like modal analysis requires.
There are several reasons why ODS measurement is preferred to Modal Analysis:
ODS measurement requires no external excitation. For heavy structures, this is an important advantage, for it requires a very large energy source to excite the structure.
ODS reflects the real deformation of a structure, even if the structure is operating in a nonlinear manner, while modal testing typically investigates the linear behavior of the structure.
ODS testing is easier, more straightforward for both testing and analysis.
ODS testing can be conducted without shutting down the machine.
A reciprocating gas compressor ha sever piping vibration of 28 mm/s@13Hz.
This is due to the resonance of the piping structure.
With ODS analysis, the vibration shape was investigated, and the weak point of the structure was found
To perform an ODS measurement, an accelerometer is mounted at a fixed position defined as the reference point while other accelerometers are roving at different positions throughout the measurement. Because ODS does not require an impaq hammer to excite the structure, all you need at this point is an analyzer that can measure cross channel analysis for measurement of the test data. This FRF (frequency response function) or cross power spectrum of each point is measured and then processed in post analysis software for mode shape animation.
Benstone Instruments recommends: Impaq Elite or Impaq 4 channel analyzer for this function and a triaxial accelerometer. For simple ODS measurements you can use the Fieldpaq 2 +1 channel analyzer.
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