Optical EngineeringFast and full-field measurement of brake squeal using pulsed ESPI technique
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In the field of structural testing, optical techniques have already been successfully applied for the determination of the dynamic structure response. Recently electronic speckle pattern interferometry (ESPI) systems using pulsed lasers are increasingly used to replace conventional double pulse holography interferometers. ESPI takes over not only the main benefits of holography toward conventional measuring techniques, such as full-field, noncontact measurement and sensitivity, but also uses modern camera and computer techniques for image capturing and processing. The capabilities of pulsed ESPI have already been further extended to 3-D measurements of the dynamic response and modal analysis of vibrations. An important area of application has already been demonstrated in the field of brake disks analysis, where pulsed ESPI systems provide complete maps of the vibration of any component of brakes during testing (brake disks, drums, calipers, and pads). Using the 3-D technique, completely automatic brake squeal inspection systems have been designed, which capture the squealing signal, automatically set and fire the laser, and provide a complete full-field deformation map of the component under test. We describe recent developments of the 3-D ESPI technique using pulsed lasers, and give examples of applications in the fields of dynamic structure response, with an emphasis in the field of brake squeal analysis. The application for the determination of the complex amplitude of vibration is demonstrated, which opens further potential into the field of operational deflection measurement and modal analysis. Conventional techniques using accelerometers and vibrometers have a very high temporal resolution and therefore are predestined to measure the frequency response of the vibrating object. ESPI overcomes this feature with very high spatial resolution, allowing a full-field measurement also for transient events.