Operating Deflection Shape Analysis

During Operation When structures have excessive vibration it is often possible to learn much about the vibration characteristics of the structure by measuring the vibration during operation.

 

The measurement procedure is similar to the procedure for single input modal analysis. Instead of using the excitation force as the reference for the remaining measurements, vibration from one location is used as the reference. Roving accelerometers are used to collect the vibration with respect to the reference location. The relative amplitude and phase is calculated for each of the roving response accelerometer locations with respect to the reference accelerometer. This relative amplitude and phase is animated on a stick figure of the structure.

 

If a single excitation force excites the structure with the reference accelerometer representing that force, the operating deflection shape will be a good representation of the mode shape.

 

With a random excitation source such as the turbulence generated by the wind, the shape can also be a good representation of the mode shape. In general, with multiple excitation sources, and the excitation frequency somewhere between resonant frequencies, the shape will be a combination of several mode shapes. The results from the operating deflection shape analysis show where the vibration is the most severe.