High-Precision Positioning of Laser Beams for Vibration Measurements

Predicting the vibratory response of structures with complex geometry can be challenging especially when their properties (geometry and material properties) are not known accurately. These structures can suffer also from high modal density, which can result in small changes in structural properties creating large changes in the resonant response. To address this issue, structural properties could be accurately identified, or the structural response could be experimentally measured. Both these approaches require collecting measurements of higher order vibration modes, which have complicated shape. Consequently, high-accuracy positioning of laser beams is necessary for vibrometers based on laser Doppler velocimetry. This paper presents a methodology to address this challenge. The architecture involves a single-point vibrometer, a motion controller, translating/rotating stages, and special application software for alignment and edge detection. A key novelty of this technology is that the beam of the vibrometer is used for both detecting the edges and for measuring the vibration. Using a motion controller, the system can automatically place/scan and measure the surface of the structure with a positioning resolution of 1 μm. Experimental results are provided to demonstrate the new technique.