Uncertainty due to misalignment in Laser Vibrometry
thesisposted on 06.07.2011, 14:54 authored by Mario Tirabassi
Laser Doppler Vibrometry (LDV) is a well established technique used for non intrusive velocity measurements in fluid flows and on solid surfaces. Unlike traditional contacting vibration transducers, laser vibrometers require no physical contact with the test object. The ability to combine advanced mirror systems together with the laser source allows automated scanning LDV (SLDV) measurements, where a high number of measurement points can be measured consecutively. Non-contact vibration measurements with very high spatial resolution are possible with such a scanning system and can lead to a significantly more detailed analysis from vibration tests. One of the main limitations of Laser Vibrometry is the difficulty to realize a perfect alignment between the investigated target and the laser beam. Frequently, for engineering applications, it is desirable to investigate different points on a target using the LDV system and, in this case, accurate knowledge of the measuring point position is required. Misalignments associated with the laser beam or the optics used to deflect the beam introduce deviation from the desired position and uncertainties in the measured velocity. All optical configurations are sensitive to misalignments, especially scanning systems able to move the laser beam around static or rotating targets. This thesis describes advances in the application and interpretation of such measurements using Laser Vibrometry and concentrates on the analysis of the uncertainties due to the inevitable misalignments between the laser beam and the investigated target in vibration measurements on rotating components. The work is divided into three main sections. The first part proposes a novel method to model any kind of LDV optical arrangement suitable for vibration measurements. This model has been developed with scanning LDV systems in mind but it can be used for any optical configuration. The method is based on a vector approach and integrates directly with the Velocity Sensitivity Model to determine the velocity measured by a single incident beam. The resulting mathematical models describe completely the beam path, the scan pattern and the measured velocity in the presence/absence of target vibrations and misalignments without any kind of approximation. The mathematical expressions derived are complex but easily implemented in software such as Matlab. The models are an important tool for LDV because they help the user to have a better understanding of measured data and to make the best alignment possible. The second part of the thesis concentrates on the modelling of different optical systems using the new method. Different systems from the simplest to the most complex have been analysed using the method. For some arrangements, mathematical models have been formulated for the first time such as for the newly proposed single and dual wedge SLDV systems and for the recently introduced Dove prism SLDV system. These systems are compared to the dual mirror SLDV system. In particular, for the single and the dual wedge SLDV systems, experimental tests have been performed to validate theoretical predictions. The results confirm the validity of the models and show the potential of these systems. Established systems such as the dual mirror and the self-tracking SLDV systems, for which generally less comprehensive models can be found in literature, have been re-analysed with the new method and theoretical predictions have been compared to the data from literature in order to confirm the validity of the new models and also to investigate for the first time some details that have previously been neglected. The models enable identification of the main characteristics of any arrangement, in particular the sensitivity to typical misalignments and target vibration components. For tracking applications on rotating targets, the presence of misalignments causes measured velocities at DC and the first target rotation harmonic whose values depend on how the misalignments combine. The analysis of misalignment effects enables identification of the optical device(s) with the most critical alignment and supplies an initial estimation of the level of uncertainty affecting typical, practical applications. Investigation shows as the self-tracking scanning systems are much sensitive to misalignments and target vibrations than the other scanning systems. The third part of the thesis concentrates on effects on radial and pitch/yaw vibration measurements on rotating targets of both misalignments and surface roughness of the test rotor. It is known that radial and pitch/yaw vibrations taken directly from a rotor using LDV are affected by a cross-sensitivity to the orthogonal vibration component. Resolution of the individual radial or pitch and yaw components is possible via a particular arrangement of the laser beams and using a dedicated resolution algorithm. Error sources such as instrument misalignments, rotation speed measurement error and introduce uncertainties in the resolution algorithm output. Research has quantified these uncertainties when radial vibrations with different or equal amplitude are applied to the target. Particular attention has been given to the effects that surface roughness has on the cross-sensitivity encountered in these measurements. From the tests, it is possible to identify three different ranges of surface roughness. For very smooth circular rotors, the cross-sensitivities are negligible and measurements can be made directly on the rotor without the need for a resolution algorithm. For very rough surfaces including surfaces coated in retro-reflective tape, the measurements have to be resolved to remove the cross-sensitivity. For surface roughnesses between the very smooth and the very rough, reliable measurements cannot be made because levels of the cross-sensitivity cannot be predicted making correct resolution impossible. The significant developments in the use of Laser Vibrometry for different optical configurations and quantification of the uncertainties expected for typical applications on rotating components realised during this research project make this work a practical and important tool for the user.
- Mechanical, Electrical and Manufacturing Engineering