Evaluation of optical techniques applied to online turbomachinery blade vibration measurements

Abstract

Understanding the dynamic characteristics of blades is important in the online condition monitoring of turbomachinery. Conventionally contact methods are used for this purpose. However improvements in technology now allow for the use of non-contact methods. Contact measurement techniques for turbomachinery blade vibration analysis typically involve the use of strain gauges and accelerometers. These present some complications when analyzing rotating machinery. Being contact in nature, mass loading can affect the integrity of measurements captured. Turbomachines typically operate under the adverse conditions of high temperatures, high flow rates and sometimes wet environments. This significantly reduces the life of contact transducers installed to capture the blade dynamics. Installation of telemetry systems for signal transmission is also necessary. In addition to being invasive and expensive, telemetry systems can introduce electrical noise. These factors make it desirable to explore the applicability of various optical non-contact methods for analyzing turbomachine blade vibrations, such as Laser Doppler Vibrometry (LDV) and photogrammetry. Both techniques have been successfully used to analyze vibrations of structures. Photogrammetry is a full-field measurement technique which allows for non-intrusive simultaneous measurement of vibrations at different locations on a blade. This is particularly important for the updating of numerically developed models of structures, investigation of structural global dynamics, and more effective localization of damage. Accelerometers have been used to validate a variation of photogrammetry, three dimensional point tracking (3DPT), for rotational applications and discrepancies attributed to the contact nature of accelerometers were observed. To build confidence in the use of 3DPT as a non-contact method for analyzing rotating machines, it is necessary to investigate how well it correlates with various non-contact methods. Through such an investigation aspects that need to be addressed when using 3DPT to analyse turbomachines can be identified. If reliable measurements can be obtained using this technique, further investigations such as online damage detection and characterization in rotating structures can be conducted. In this study Tracking Laser Doppler Vibrometry (TLDV) and 3DPT are used as non-contact methods to investigate the online vibrations of a turbomachine test rotor. To employ TLDV on the test rotor, the dynamics of the scanning mirrors of a Polytec Scanning Vibrometer (PSV) are characterized using a frequency response approach. Look-up tables are constructed to provide the necessary phase angle compensation for the two signals supplied to the mirrors, to obtain a circular scanning path. Photogrammetric 3DPT is then validated by tracking the TLDV laser spot focused on one of the test rotor blade using high speed cameras, and comparing the 3DPT measurements to TLDV blade out-of-plane vibration measurements. The correlation between the two non-contact measurement techniques is presented. This establishes the validity of the employed scanning system, and also serves to show how well the two non-contact methods correlate with each other, when investigating dynamics of turbomachinery blades. 3DPT is then used to analyze the responses of the test rotor blades under excitation. Various Operational Deflection Shapes (ODSs) of the blades are identified and the results obtained are presented. The use of ODSs obtained from 3DPT to investigate irregularities along turbomachinery blades is also presented. This information is used to show that ODSs captured using 3DPT can be used to online detect and localize blade damage in turbomachines.