Performance of Optical Structural Vibration Monitoring Systems in Experimental Modal Analysis

Abstract

Image-based optical vibration measurement is an attractive alternative to the conventional measurement of structural dynamics predominantly relying on accelerometry. Although various optical vibration monitoring systems are now readily available, their performance is currently not well defined, especially in the context of experimental modal analysis. To this end, this study provides some of the first evidence of the capability of optical vibration monitoring systems in modal identification using input–output measurements. A comparative study is conducted on a scaled model of a 3D building frame set in a laboratory environment. The dynamic response of the model to an impulse excitation from an instrumented hammer, and an initial displacement, is measured by means of five optical motion capture systems. These include commercial and open-source systems based on laser Doppler velocimetry, fiducial markers and marker-less pattern recognition. The performance of these systems is analysed against the data obtained with a set of high-precision accelerometers. It is shown that the modal parameters identified from each system are not always equivalent, and that each system has limitations inherent to its design. Informed by these findings, a guidance for the deployment of the considered optical motion capture systems is given, aiding in their choice and implementation for structural vibration monitoring.

Performance of Camera-Based Vibration Monitoring Systems in Input-Output Modal Identification Using Shaker Excitation

“…This implies that the potential error from the optical MCS is within the identification uncertainty bounds imposed by the excitation type and intensity in the case of accelerometry. In general, the errors in modal frequencies are negligible for all modes and all modal results closely follow the qualitative and quantitative outcomes obtained in [46]. An impulse excitation delivered with an instrumented hammer was used therein to mobilise a simpler and lighter structure of which response was measured with a great variety of instrumentation systems.…”

“…The error magnitudes in modal frequency, damping and mass fall in this case below 0.65%, 9% and 12%, respectively. As could be expected [46], the most challenging parameter to capture in experimental studies is the modal mass. The deviation from the baseline values from accelerometry reaches in this case up to 74% for mode 3 and TM during T3-the test yielding the worst results overall.…”

“…Therefore, classical experimental modal analysis (EMA), relying on the measured input force and output response, is favoured in the full dynamic characterisation of structures. Within this context, the only study known to the authors reporting the modal mass based on measurements from camera-based MCS is that of Kalybek et al 2021 [46]. An instrumented hammer was used to provide excitation force to a simple spatial structure monitored with various optical MCS.…”

“…The main reason for that is the significant uncertainty in the determination of modal damping and mass associated with the variety of damping mechanisms and modelling simplifications, and sensitivity to the vibration amplitudes and parameter extraction methods, in particular the type and arrangement of instrumentation systems and data processing algorithms [42,48]. This uncertainty reveals itself in the discrepancy between the estimated modal damping and mass relative to the benchmark, e.g., an instrumentation system of higher accuracy and sensitivity, reaching several dozen percent [46,48,49].…”

“…Considering the above points and building on the work presented in [46], the purpose of this study was to assess the performance of optical MCS in experimental modal analysis using shaker excitation. To this end, the behaviour of a scaled model of a cable-stayed bridge set in a laboratory environment was investigated subjected to pseudo-random white noise-type and sine chirp excitation.…”