Rotating blade vibration analysis using photogrammetry and tracking laser Doppler vibrometry

Online structural dynamic analysis of turbomachinery blades is conventionally done using contact techniques such as strain gauges for the collection of data. To transfer the captured data from the sensor to the data logging system, installation of telemetry systems is required. This is usually complicated, time consuming and may introduce electrical noise into the data. In addition, contact techniques are intrusive by definition and can introduce significant local mass loading. This affects the integrity of the captured measurements. Advances in technology now allow for the use of optical non-contact methods to analyse the dynamics of rotating structures. These include photogrammetry and tracking laser Doppler vibrometry (TLDV). Various investigations to establish the integrity of photogrammetry measurements for rotating structures involved a comparison to data captured using accelerometers. Discrepancies that were noticed were attributed to the intrusive nature of the contact measurement technique. As an extended investigation, the presented work focuses on the validation of photogrammetry applied to online turbomachinery blade measurements, using TLDV measurements. Through a frequency based characterization approach of the dynamics of the two scanning mirrors inside the scanning head of a scanning laser Doppler vibrometer (SLDV), TLDV is employed in developing a system that can be used to achieve a perfect circular scan with a Polytec SLDV, (PSV 300). Photogrammetry out-of-plane displacements of a laser dot focused on a specific point on a rotating blade are compared to displacements captured by the laser scanning system. It is shown that there is good correlation between the two measurement techniques when applied to rotating structures, both in the time and frequency domains. The presence of slight discrepancies between the two techniques after elimination of accelerometer based errors illustrated that the optical system noise floor of photogrammetry does contribute to inconsistences between photogrammetry and other measurement techniques.http://www.elsevier.com/locate/jnlabr/ymssp2017-08-31hb2016Mechanical and Aeronautical Engineerin

Evaluation of optical techniques applied to online turbomachinery blade vibration measurements

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.Dissertation (MEng)--University of Pretoria, 2015.tm2015Mechanical and Aeronautical EngineeringMEngUnrestricte

Shape principal component analysis as a targetless photogrammetric technique for condition monitoring of rotating machines

Rotating machines are widely used in engineering for applications which include power generation and machine propulsion systems. These machines have to be accurately monitored and maintained to avoid system failures. Vibration analysis, which involves the use of contact and non-contact measurement techniques to capture vibrational data indicative of the condition of a machine, is normally used for this purpose. 3D Point Tracking (3DPT) and Digital Image Correlation (DIC) constitute photogrammetric-based optical non-contact measurement techniques that have proven to be efficient for the vibration analysis of rotating machinery. In addition to complex image processing software and tracking algorithms, these two approaches typically require surface preparation in the form of markers and speckle patterns. These requirements limit the applicability of photogrammetry as a condition monitoring tool, especially when it comes to industrial environments. This paper proposes 2D shape analysis for target-less non-contact measurement in condition monitoring of rotating machines. Through comparison to measurements captured using conventional proximity probes on an experimental test setup, it is also illustrated how different dynamic characteristics of a rotating system can be distinguished using this measurement approach.The Eskom Power Plant Engineering Institute (EPPEI) South Africa.http://elsevier.com/locate/measurementhj2020Mechanical and Aeronautical Engineerin