Quantitative modal analysis using electronic speckle pattern interferometry

Existing modal analysis techniques based on pointwise methods such as accelerometers and laser vibrometers, suffer from the compromises required to infer whole field vibration behaviour from a predetermined number of discrete measurement data points. The measurement grid is normally chosen before the most sensitive areas of the test piece have been identified and this can lead to a requirement for further grid refinement, with the consequent spatial and temporal disadvantages. Whole-field modal analysis using electronic speckle pattern interferometry (ESPI) has been developed, and focuses on the manipulation of optical information to provide a grid of data points which is transferred to a modal analysis software package for comparison with traditional point wise modal data sets. The advantages of obtaining the initial whole-field vibration picture in real time and subsequently overlaying a chosen measurement data grid, are demonstrated. Modal analysis of a steel plate is presented, which demonstrates single displacement vector measurements. Data is transferred into modal analysis software allowing ESPI mode shape information to be directly compared with measurements using other pointwise techniques and finite element analysis (FEA), through a modal assurance criterion (MAC) calculation. Further results are presented for multiple displacement vector analysis of high-power ultrasonic components, demonstrating the ability to accurately measure the modal characteristics of complex dynamic components.