Shape descriptors for mode-shape recognition and model updating

The most widely used method for comparing mode shapes from finite elements and experimental measurements is the Modal Assurance Criterion (MAC), which returns a single numerical value and carries no explicit information on shape features. New techniques, based on image processing (IP) and pattern recognition (PR) are described in this paper. The Zernike moment descriptor (ZMD), Fourier descriptor (FD), and wavelet descriptor (WD), presented in this article, are the most popular shape descriptors having properties that include efficiency of expression, robustness to noise, invariance to geometric transformation and rotation, separation of local and global shape features and computational efficiency. The comparison of mode shapes is readily achieved by assembling the shape features of each mode shape into multi-dimensional shape feature vectors (SFVs) and determining the distances separating them. © 2009 IOP Publishing Ltd

Robust passivity-based continuous sliding-mode control for under-actuated nonlinear wing sections

The stability of an under-actuated nonlinear aeroelastic wing section is addressed using a robust passivity-based continuous sliding-mode control approach. The controller is shown to be capable of stabilising the system in the presence of large matched and mismatched uncertainties and large input disturbance. It is demonstrated in theory that within known bounds on the input disturbance and nonlinearity uncertainty, the controller is able to stabilise the system globally. A numerical example, based on the Texas A&M University experimental rig, is used to demonstrate the stabilisation of the system with a fully-developed limit cycle oscillation and a flap deflection limited to 20 degrees. This is of practical interest because it shows that the system is at least stabilised locally, whereas global stability is a concept limited to theoretical studies and is impossible to demonstrate in practice

Nonlinear control of a flexible aeroelastic system

Although it is a common practice in the field of Dynamics to treat a system as being linear, the assumption of linearity is only valid in situations where the effect of any nonlinearities is minimal. Significant nonlinear behaviour (such as Limit Cycle Oscillations) has been observed in many practical manifestations of aeroelastic systems, highlighting the need to account for system nonlinearities. A consequence of incorporating nonlinearity into the model is that the application of linear control methods becomes inadequate when the system operates in a substantially nonlinear regime. Thus, the present work addresses both these concerns by applying nonlinear control on an aeroelastic system consisting of a flexible wing with a structural nonlinearity. The Feedback Linearisation method is employed to render the system linear, such that linear control methods are applicable. The utility of the Small Gain Theorem and Adaptive Feedback Linearisation in situations where errors in the parameters describing the nonlinearities are present is demonstrated

Feedback Linearization in Systems with Nonsmooth Nonlinearities

This paper aims to elucidate the application of feedback linearization in systems having nonsmooth nonlinearities. With the aid of analytical expressions originating from classical feedback linearization theory, it is demonstrated that for a subset of nonsmooth systems, ubiquitous in the structural dynamics and vibrations community, the theory holds soundly. Numerical simulations on a three-degree-of-freedom aeroservoelastic system are carried out to illustrate the application of feedback linearization for a specific control objective, in the presence of dead-zone and piecewise linear structural nonlinearities in the plant. An in-depth study of the arising zero dynamics, based on a combination of analytical formulations and numerical simulations, reveals that asymptotically stable equilibria exist, paving the way for the application of feedback linearization. The latter is demonstrated successfully through pole placement on the linearized system

Quantifying and managing uncertainty in operational modal analysis

This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this record.Operational modal analysis aims at identifying the modal properties (natural frequency, damping, etc.) of a structure using only the (output) vibration response measured under ambient conditions. Highly economical and feasible, it is becoming a common practice in full-scale vibration testing. In the absence of (input) loading information, however, the modal properties have significantly higher uncertainty than their counterparts identified from free or forced vibration (known input) tests. Mastering the relationship between identification uncertainty and test configuration is of great interest to both scientists and engineers, e.g., for achievable precision limits and test planning/budgeting. Addressing this challenge beyond the current state-of-the-art that are mostly concerned with identification algorithms, this work obtains closed form analytical expressions for the identification uncertainty (variance) of modal parameters that fundamentally explains the effect of test configuration. Collectively referred as ‘uncertainty laws’, these expressions are asymptotically correct for well-separated modes, small damping and long data; and are applicable under non-asymptotic situations. They provide a scientific basis for planning and standardization of ambient vibration tests, where factors such as channel noise, sensor number and location can be quantitatively accounted for. The work is reported comprehensively with verification through synthetic and experimental data (laboratory and field), scientific implications and practical guidelines for planning ambient vibration tests.This work is funded by the UK Engineering and Physical Sciences Research Council (EP/N017897/1 & EP/N017803/1). The support is gratefully acknowledged

Seismic assessment of the Matera cathedral

This paper presents the seismic assessment of the Cathedral of Matera, in southern Italy, to determine the capacity of the structure when subjected to earthquakes. This church dates back to the 13th century and is one of the most representative monuments of the Apulian Romanesque architecture. Within the context of the evaluation of the seismic response of the cathedral, modal identification tests were performed in order identify and characterize the main dynamic properties of the structure. The results of these tests were used to develop a representative finite element model, which is able to provide the response to seismic actions. A pushover analysis was performed to characterize the seismic behavior of the structure. The results of the seismic analyses on the cathedral show that its vulnerability is high, being the transversal direction the less stiff and resistant. Elements as the nave and the façade, along with the bell tower, might be the most vulnerable to seismic actions. Additionally, it was observed that components as the trusses of the central nave strongly modify the seismic response and capacity of the structure. Apparently, the structure might not be able to withstand a strong earthquake from the region or might present several damage after one. Hence, it is recommendable to perform further studies about the seismic behavior, especially of the most vulnerable elements.The authors would like to acknowledge the University of Minho for supporting the experimental campaign. Thanks is also extended to Dr. Nuno Mendes, University of Minho, for his guidance and help for performing the in-situ tests on the cathedral. The authors would also like to thank to the ELARCH project number 552129-EM-1-2014-1-IT-ERASMUS MUNDUS-EMA 21 for funding the graduate studies of the first author

A Computer Vision-Based Approach for Non-contact Modal Analysis and Finite Element Model Updating

Computer vision-based techniques for modal analysis and system identification are rapidly becoming of great interest for both academic research and engineering practice in structural engineering. For instance, this is particularly relevant in fields such as bridge or tall building monitoring, where the large size of the structure would require an expensive sensor network, and for the characterisation of very slender, highly-flexible structural components, where physically-attached sensors cannot be deployed without altering the mass and stiffness of the system under investigation. This study concerns the latter case. Here, an algorithm for the full-field, non-contact extraction and processing of useful information from vibrational data is applied. Firstly, video acquisition is used to capture rapidly very spatially- and temporally-dense information regarding the vibrational behaviour of a high-aspect-ratio (HAR) prototype wing, with high image quality and high frame rate. Video processing is then applied to extract displacement time histories from the collected data; in turn, these are used to perform Modal Analysis (MA) and Finite Element Model Updating (FEMU). Results are benchmarked against the ones obtained from a single-point laser Doppler vibrometer (LDV). The study is performed on the beam-like spar of the wing prototype with and without the sensors attached to appreciate the disruptive effects of sensor loading. Promising results were achieved