Application of electronic speckle pattern interferometry to the study of three-dimensional mechanical vibrations

Electronic speckle pattern interferometry (ESPI) has become an established technique for mechanical vibration analysis, but in the past has been restricted to uniaxial measurements and has suffered from producing results which require skilled interpretation. The work reported here has extended the range of application of the technique to include three-dimensional vibration studies, and has made progress in automating the acquisition and processing of data. After establishing the importance of empirical vibration analysis and the practical advantages of ESPI, the theoretical requirements for measuring three-dimensional motion are considered. An experimental rig has been constructed using a continuous wave laser which has demonstrated that ESPI is capable of measuring time-averaged in-plane vibrations, an ability which was previously in some doubt. The rig has been used to study the three-dimensional resonant behaviour of simple structures and real engineering components in laboratory conditions. Some limitations were encountered and, in order to overcome these, a pulsed laser was introduced to the system. This has enabled the method to be extended to unstable objects, large amplitudes and non-resonant behaviour. Image processing and phase-stepping techniques have also been applied, enabling quantitative in-plane and out-of-plane displacement plots to be computed from the ESPI data. Experimental results are presented showing modal analyses of flat plates, an ultrasonic forming die, a turbocharger blade and an ultrasonic cutting system. The application of pulsed ESPI to the study of travelling waves, unstable objects and factory environment measurements is also demonstrated. The performance of three-dimensional ESPI is compared with alternative techniques, and the potential for further improving the technique is discussed. It is concluded that the method offers particular advantages for some types of study, and that it compliments the existing range of empirical techniques.