Novel techniques for laser doppler vibrometry for vibrating shiny and mirror surfaces

• Main Author: Kusuma Agusanto
• Other Authors: -
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Electrical and electronic engineering::Optics, optoelectronics, photonics
• Online Access: https://hdl.handle.net/10356/137796
• Institution: Nanyang Technological University

A laser Doppler vibrometer (LDV) is a versatile non-contact optical instrument used to measure the velocity at a single point on the surface of a moving object. The instrument works most efficiently when used for an object whose surface roughness is sufficient to allow scattered light to be received at the device sensor head. For an object with a mirror-like surface, which allows only specular reflection, precise alignment is a necessity, as a small tilt change can cause the light to diverge away from the sensor. Surface enhancements such as retroreflective tapes can mitigate the loss of light signal detection by improving the light scattering from the surface. This solution does not work for delicate or small objects, as altering them may change their measurement properties. This study addresses these gaps in LDV-based methods by introducing novel external retroreflection techniques. These new methods allow recovery of the light signal lost due to a vibrating mirror with a large tilt or rotation angle. However, the Doppler-shifted frequency observed using this new method does not directly yield the target velocity without correction, because the oblique retroreflection itself also contributes to the Doppler frequency shift. In this study, an equation describing the contribution of the oblique retroreflection to the detected Doppler frequency shift in the signal is derived. With the help of this equation and the oblique retroreflection method, a conventional LDV can be used to characterize a vibrating mirror with a large tilt angle. Furthermore, the new technique has also been used for unprecedented measurements of large deflections of a cantilever mirror with dynamic rotation. For a scanning mirror with a large rotational angle, the oblique retroreflection model is inadequate as it does not consider the translation of the target point during the rotational motion. Therefore, an in-line velocity model is developed to overcome this limitation and enable the LDV with the external retroreflection method to characterize the velocity of the dynamic target point of a scanning mirror along the direction of the laser beam. In addition, the in-line velocity model includes a unique trough-shaped retroreflector design, superior to the design used in the external retroreflection method. With the trough design, the retroreflector can recover reflected light from the mirror at a wider angle of rotation. This second technique has been used successfully to characterize a rotating mirror driven by a torsional spring at a large rotational angle (up to 30°), a performance beyond the capability of a conventional LDV by itself. Additionally, an off-axis detection using an in-plane LDV is developed and tested to measure the in-plane motion vibration of shiny metallic samples, which a conventional in-plane LDV is not capable of doing. This off-axis detection method offers a great potential value for the in-plane motion characterization of micro-electro- mechanical systems (MEMS) devices with shiny metallic surfaces.