Regular ArticleMODAL TESTING USING A SCANNING LASER DOPPLER VIBROMETER
Abstract
A number of vibration mode-shape measurement techniques are described in which the measurement point of a laser Doppler vibrometer (LDV) is continuously scanned over the surface of a sinusoidally excited structure. The LDV vibration output is an amplitude-modulated sine wave and mode shapes, defined along the scan line, may therefore be established by demodulation. Alternatively, in the frequency domain, the LDV output is a line spectrum, with sidebands centred on the excitation frequency and spaced at the scan frequency. The sideband amplitudes give series coefficients for the mode shape defined along the scan line—Fourier coefficients for a circular scan or, via a simple transform, polynomial coefficients for a sinusoidal straight line or area scan. Useful measurements can also be made, directed at a single point on a structure: a small-diameter circular scan will give the angular vibration of the structure at the centre of the circle; a conical scan, produced with a short-focus lens, will measure the translational vibration at the point of focus, in magnitude and direction.
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A novel demodulation method with a reference signal for operational modal analysis and baseline-free damage detection of a beam under random excitation
2024, Journal of Sound and VibrationA novel demodulation method with a reference signal is developed for operational modal analysis and damage detection of a beam structure under random excitation. The novel demodulation method can process measurements of the beam by a continuously scanning laser Doppler vibrometer (CSLDV) system and measurements of a reference point on the beam by a single-point laser Doppler vibrometer to estimate its modal parameters, such as damped natural frequencies and undamped mode shapes. Damped natural frequencies of the beam are estimated from fast Fourier transforms of measurements of the CSLDV system. A cross-correlation function between a measurement of the CSLDV system and a measurement of a single-point laser Doppler vibrometer is calculated, and the cross-correlation function is multiplied by a sinusoidal signal whose frequency is an estimated damped natural frequency of the beam. The processed cross-correlation function is filtered by a low-pass filter to obtain the undamped mode shape of the beam that corresponds to the estimated damped natural frequency of the beam. Smooth polynomials are used to fit estimated undamped mode shapes, which can be considered as undamped mode shapes of an undamaged beam. Curvatures of estimated undamped mode shapes and polynomials are compared by curvature damage indices to determine the location of a damage in the beam. The novel demodulation method with a reference signal is investigated for baseline-free damage detection from both finite element simulation and experiment. Modal parameters of a finite element model of the damaged beam and a damaged beam specimen, which are under random excitation, are successfully estimated, and locations of damages in the beam model and beam specimen are accurately determined.
Into a rapid polymer characterization employing optical measurement systems and high-power ultrasonic excitation
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Interpretation of bimodal interference in and optimized operational modal analysis for long-range continuously scanning laser Doppler vibrometer measurements with a beam under white noise excitation
2023, Measurement: Journal of the International Measurement ConfederationThis paper explains the phenomenon of a dominant frequency in the spectrum of a raw response measured by a long-range continuously scanning laser Doppler vibrometer (LCSLDV) system being symmetrically shifted into two sharp peaks. This phenomenon is referred to as bimodal interference in this work, which gradually becomes obvious when the scanning length is greater than 0.5 m. Bimodal interference is significant because it makes natural frequencies unable to be estimated in LCSLDV measurements and this paper analyzes its nature. Natural frequencies and mode shapes of a structure under white noise excitation can be estimated by analysis of its responses measured by the LCSLDV system. The effect of bimodal interference has been ignored in previous LCSLDV and continuously scanning laser Doppler vibrometer (CLSDV) measurement studies, but its appearance makes the value of the dominant frequency not to be accurately identified. By deducing mathematical interpretation of bimodal interference with analysis of incidence and reflection of a continuously scanning laser beam in a tiny time interval and comparing experimental results with 21 groups of different conditions, it is concluded that bimodal interference is caused by signal coupling between the scanning frequency of the mirror and the vibration frequency of the measured structure. There is a positive correlation between the scanning frequency and the magnitude of the symmetric frequency shifts centered at the dominant frequency generated by bimodal interference. In experimental results of LCSLDV measurements with a distance of 60 m, the frequency shift ratio compared to the dominant frequency can be reduced to 0.06% when the scanning frequency is lower than 0.01 Hz. In this case, bimodal interference is not noticeable in the spectrum. The experiment was carried out on a cantilever beam machined based on a prescribed design. Modal parameters of the cantilever beam estimated by white noise excitation were successfully estimated by the LCSLDV system with a distance of 60 m. The maximum difference between the first four natural frequencies of the cantilever beam from the finite element model and the LCSLDV system was 2.97% and the minimum difference was 1.1%.
A novel robust edge detection method for full-field modal parameter estimation using an image-based tracking continuously scanning laser Doppler vibrometer system
2023, Mechanical Systems and Signal ProcessingA novel robust edge detection method is developed for an image-based tracking continuously scanning laser Doppler vibrometer (CSLDV) system to track a rotating structure without attaching any encoder or mark to it and scan its whole surface. The robust edge detection method can extract edges of the rotating structure from its complex background by processing grayscale images of the rotating structure. Once edges of the rotating structure are clearly shown in the processed image, their positions can be easily determined. To track a rotating structure like a rotating blade of a wind turbine with multiple blades, a distance condition is used to determine the position of an edge of the structure that needs to be tracked and estimate its real-time rotation speed. Once the position of the edge of the rotating structure is determined, the image-based tracking CSLDV system can scan the whole surface of the rotating structure via a two-dimensional scan scheme. An improved demodulation method is used to process measured data of response of the rotating structure subject to random excitation and estimate its modal parameters including damped natural frequencies and full-field undamped mode shapes. The robust edge detection method is investigated by tracking and scanning a rotating fan blade subject to random excitation with a complex background using the image-based tracking CSLDV system, and estimating modal parameters of the rotating fan blade with different constant speeds.
Identification of modal parameters of a model turbine blade with a curved surface under random excitation with a three-dimensional continuously scanning laser Doppler vibrometer system
2023, Measurement: Journal of the International Measurement ConfederationModal parameters of a model turbine blade with a curved surface excited by white noise are identified by a novel general-purpose 3D continuously scanning laser Doppler vibrometer (CSLDV) system via an extended demodulation method (EDM). The proposed general-purpose system is calibrated to synchronously and continuously move three laser spots on the curved surface of the blade along the same scan trajectory and capture its 3D vibrations. The EDM is used to estimate its damped natural frequencies (DNFs) and 3D full-field undamped mode shapes. Identified modal parameters are compared with those from a commercial 3D scanning laser Doppler vibrometer (SLDV) system. Differences between their first six DNFs are less than 1.5%; their first six mode shapes are highly correlated as their modal assurance criterion values all exceed 95%. However, the 3D CSLDV system has much higher efficiency in obtaining 3D mode shapes of the blade than the 3D SLDV system.
Unmanned aerial vehicle-based computer vision for structural vibration measurement and condition assessment: A concise survey
2023, Journal of Infrastructure Intelligence and ResilienceWith the rapid advance in camera sensor technology, the acquisition of high-resolution images or videos has become extremely convenient and cost-effective. Computer vision that extracts semantic knowledge directly from digital images or videos, offers a promising solution for non-contact and full-field structural vibration measurement and condition assessment. Unmanned aerial vehicles (UAVs), also known as flying robots or drones, are being actively developed to suit a wide range of applications. Taking advantage of its excellent mobility and flexibility, camera-equipped UAV systems can facilitate the use of computer vision, thus enhancing the capacity of the structural condition assessment. The current article aims to provide a concise survey of the recent progress and applications of UAV-based computer vision in the field of structural dynamics. The different aspects to be discussed include the UAV system design and algorithmic development in computer vision. The main challenges, future trends, and opportunities to advance the technology and close the gap between research and practice will also be stated.