ODS measurement on a
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ODS vs. Modal Analysis
ODS is easier to perform and far more accessible than modal analysis. It is, however, not at the same level of sophistication : while ODS applies to observe a machine’s workload induced movements, modal analysis leads to a full structural analysis. As such, ODS is often a viable step enabling better understanding of the machine’s performance and the evaluation of feasibility or necessity of going onwards to modal techniques.
Modal techniques proceed from the modelization of the structural responses to the application of a calculated or measured force through the concept of mobility whereas ODS remains strictly in the operations generated dynamic loads domain.
PHASE (Time Domain Context)
ODS is particularly useful since PHASE, the relative measurement of HOW (or when) various parts move in relation to one another, generally remains poorly understood. Reliance on this concept will enable an experienced user to determine the relationships involved in the measured vibration BUT a full comprehension and a sound and methodical approach are then necessary.
FINE TUNE thus provides an extra tool to play with this concept. In its simplest expression, the comparison of two parallel linear movements, we find that most people fully understand the concept. More so when the movement is perfectly opposed, meaning that the phase measurement shows a 180 degrees discrepancy.
Phase Fig. 1 / Two identical springs are dropped at the same instant. Their vertical oscillating movements are perfectly synchronized and therefore are said to be synchronous. There is no relative movement.
Phase Fig. 2 / The same springs are dropped at different moments in time. At this point, we need to define a REFERENCE SYSTEM. Since this is a very simple case, the ZERO moment in time can readily be defined as the moment when the first spring (on the left hand) reaches the top of its vertical course.
Phase Fig. 3 / The perfectly opposite movement of the previous example enabled the definition of an ultra SIMPLE system of reference. Here, things get a little more complicated...
Rotational Movement & Phase
Within the context of a rotating machine analysis, the simplest reference system is the rotation of the machine itself : some EVENT becomes our point of departure, such as the MOMENT IN TIME when a given part of the machine (or something we’ve added to the machine) passes in a specific point of space...
The former can be the passage of the keyway at the top of the vertical axis ( zero degrees or 12:00 noon). The moment when a tape glued to the machine passes in front of photocell and thus triggers the signal is also acceptable.
NB : the choice of the point of reference is really more one of convenience than an absolute... The important factor is the CONSISTENCY of maintaining that point of reference for all phase readings collected throughout a series of measurements used as a foundation for an ODS analysis.
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Operating Deflective Shape Analysis