High-speed scanning of laser doppler vibrometer for rotating object vibration and angular scan pulse-echo ultrasonic sensing

Abstract

Laser Doppler vibrometer (LDV) is widely used as a tool for non-contact velocity measurement in aerospace as well as automotive industry because it offers remote operation, high frequency measurement, and high spatial resolution. Due to these benefits, laser Doppler vibrometer (LDV) is also an effective optical sensing tool that can be used for non-destructive evaluation (NDE) applications. Applying point-by-point scanning using LDV can be very effective for vibration analysis of a moving object and for the ultrasonic testing of various structures. This dissertation presents the development of high-speed scanning laser Doppler vibrometer (SLDV) that is optimized and tested for the two real-world applications. Firstly, it meets the demand for a tracking system to apply for the vibration measurement of a rotating object underwater and secondly, it offers high-speed scanning of up to 10 kHz for pulse-echo ultrasonic testing in a novel system called angular scan pulse-echo ultrasonic wave propagation imager (A-PE-UPI). The scanning laser Doppler vibrometer consists of a scanning head, controller unit, and data acquisition (DAQ) system. In vibration measurement system of a rotating object, the scanning head is comprised of Galvanomotor-based mirror scanner and 633 nm helium-neon laser Doppler Vibrometer. Whereas, in angular scan pulse-echo ultrasonic wave propagation imager, scanning head also includes a 1064 nm Q-switched diode-pumped solid state (DPSS) laser for the generation of ultrasonic wave, and optical mirrors to combine the sensing and generation laser beam. The data acquisition was integrated and graphical user interface (GUI) application was developed to perform real-time measurement and result generation. In addition, the scanning LDV was first tested on an aluminum propeller inside a water tank. Circular scanning function was implemented synchronized with the propeller rotation to track a single point in each revolution. During the implementation phase, the major challenges that were investigated and solved are misalignment between rotational axis and scanning LDV and synchronization between propeller, DAQ, and sensing beam. The bolt impact test was performed on propeller blade at 60 RPM while changing the conditions such as in air and underwater. The results were acquired and processed to observe in both time domain and frequency domain. In all conditions, a good agreement in results was observed in the frequency components for each case. In addition, this PhD study also presents the integration of scanning laser Doppler vibrometer in angular scan pulse-echo ultrasonic wave propagation imager (A-PE-UPI) development. A-PE-UPI employs pulse-echo method using a sensing and a generation laser beam to obtain the thickness measurement and damage detection of a structure. In this system, raster scan method was implemented in scanning LDV for the scanning of specimen under test using combined sensing and generation laser beams. Multiple damage detection algorithms namely pulse-echo wave propagation imaging (PE-UWPI) and variable time window amplitude map (VTWAM) were implemented in the system to identify damages at a very high scan speed. The main objective in the development of this system was to achieve 10 kHz scan speed and on-the-fly signal processing to produce real-time result visualization. A-PE-UPI was applied on various real-world structures to demonstrate high-speed inspection. A-PE-UPI results produced for the inspection of 160,000 points at 10 kHz scan speed takes only 16 seconds which saves up to 96.3% inspection time as compared to mechanical scanning based linear pulse-echo ultrasonic propagation imaging (L-PE-UPI) system. Moreover, the secondary objective was to enhance the flexibility and portability of the scan head for the localized inspection which was achieved by employing a rotational stage to rotate the mirror scanner for the inspection of angled surfaces. Also, thermal elements were installed in the scan head and the controller to operate A-PE-UPI at challenging low temperature conditions. As for the future works, to enhance the accessibility of A-PE-UPI to perform inspection of in-situ large scale complex structures, a robotic arm will be considered and integrated in to A-PE-UPI.