ODS (operating deflection shape) analysis provides information for evaluating dynamic behavior of structures for the purpose of reducing sound and vibration. An ODS represents the motion (deflection shape) a structure exhibits at a particular frequency when subjected to dynamic forces. Dynamic forces can be generated internally by rotating or reciprocating mechanisms, or dynamic forces can originate from external sources. The ODS occurs as a combination of the forced response and the natural (resonance) response. The ODS is graphically depicted by the complex relationship between the responses at discrete points on the structure.
In practice, data for conducting ODS analysis are acquired by measuring transfer functions (transmissibility) between discrete response points and a reference response point. The points (and distances between points) at which responses are measured are selected to visually represent the deflection shapes (noting structural wavelengths and impedance). The transfer functions are complex vector quantities in that each has magnitude & phase and a direction (axis of the accelerometer). Mapping the amplitudes and phases of the transfer functions reveal the operating deflection shapes.
The RION SA-78 two-channel, handheld FFT analyzer allows for transfer function measurements in the field. Data is stored and then can be easily transferred via USB to a computer. The operating deflection shapes are displayed by post-processing the data by one of a number of available graphics programs. The SA-78 also offers octave analysis, waveform analysis, and measurement up to 80 kHz to support ultrasonic analysis. Measurement by both accelerometers and microphones are supported by the RION SA-78.
A simple application for ODS analysis is the process of locating node points on a beam or node lines on a panel (see figure). Nodes are locations where vibration response is zero at the frequency of operation. Nodes provide ideal locations for attaching additional structure or mechanisms in order to minimize vibration of the structure. To identify node lines, a response accelerometer is initially located near a reference accelerometer. The cross-phase between the two points will be 0° since the accelerometers move together. The response accelerometer is then incrementally moved away from the reference accelerometer. At the point that the response accelerometer crosses a node line, the cross-phase will suddenly shift from 0° to 180°.
The SA-78 can also be utilized for mapping the ODS over complex, 3-dimensional structures. One case study involved high-amplitude, low-frequency sound radiation from an air-conditioning condensing unit. In the following figures, the ODS for an A/C condensing unit is shown at the frequency of compressor rotation. High amplitude vibration is revealed on the side panel adjacent the compressor. The operating deflection shape depicted in the figure is evaluated for designing stiffening elements or for applying damping treatments.