Michael D. Todd

Flexural beam-based fiber Bragg grating accelerometers

A flexural beam is utilized as the primary transduction mechanism for demonstrating a series of fiber Bragg grating (FBG) accelerometers. The fully packaged FBG accelerometers reported in this work have many desirable features including good acceleration sensitivity (212.5 /spl mu//spl epsiv//g), high resonant frequencies (on the order of 1 kHz), very low cross-axis sensitivity (<1% of the primary axial responsivity), and low noise (/spl sim/1 mg//spl radic/(Hz) near 1 Hz). Arrays of such devices can be utilized in a variety of applications, including structural monitoring. Closed-form analytical formulas describing the resonant frequency and responsivity of the FBG accelerometer are provided which may be utilized to tailor the sensor performance to specific applications.

A novel Bragg grating sensor interrogation system utilizing a scanning filter, a Mach-Zehnder interferometer and a 3×3 coupler

This work describes a new technique for fibre Bragg grating (FBG) sensor interrogation and multiplexing. The technique combines a scanning Fabry-Perot (SFP) bandpass filter used to wavelength-multiplex multiple gratings in a single fibre, and an unbalanced Mach-Zehnder fibre interferometer made with a 3×3 coupler to detect strain-induced wavelength shifts. A passive technique for interferometer drift compensation using non-sensing FBGs is included in the system. A prototype complete system interrogates four gratings in a single fibre at a Nyquist sampling rate up to 10 kHz, with a noise floor measured near 4 n Hz-1/2 above 0.1 Hz. The inclusion of the interferometer drift compensation technique is shown to make quasi-static measurements feasible.

Structural health monitoring through chaotic interrogation

The field of vibration based structural health monitoring involves extracting a ‘feature’ which robustly quantifies damage induced changes to the structure in the presence of ambient variation, that is, changes in ambient temperature, varying moisture levels, etc. In this paper, we present an attractor-based feature derived from the field of nonlinear time-series analysis. Emphasis is placed on the use of chaos for the purposes of system interrogation. The structure is excited with the output of a chaotic oscillator providing a deterministic (low-dimensional) input. Use is made of the Kaplan–Yorke conjecture in order to ‘tune’ the Lyapunov exponents of the driving signal so that varying degrees of damage in the structure will alter the state space properties of the response attractor. The average local attractor variance ratio (ALAVR) is suggested as one possible means of quantifying the state space changes. Finite element results are presented for a thin aluminum cantilever beam subject to increasing damage, as specified by weld line separation, at the clamped end. Comparisons of the ALAVR to two modal features are evaluated through the use of a performance metric.

Deployment of a fiber Bragg grating-based measurement system in a structural health monitoring application

The development and maturation of fiber optic sensor technology has been an increasingly important component of the structural health monitoring field. A strain sensor system has been developed to provide high-resolution, low-noise sets of useful data which can be analyzed and processed with a number of existing damage detection techniques. Recent research at the Naval Research Laboratory has also begun in combining vibration-based damage detection with statistical methods, and some preliminary results are included.

EMD AND INSTANTANEOUS PHASE DETECTION OF STRUCTURAL DAMAGE

In this chapter, a new structural health-monitoring and damage-detection method is presented. A general time-frequency data analysis technique (empirical mode decomposition and the Hilbert-Huang spectrum) in conjunction with a wavemechanics-based concept is developed to provide a diagnostic tool for detecting and interpreting adverse changes in a structure. Sets of simple basis function components, known as intrinsic mode functions (IMF), are extracted adaptively from the measured structural response time series data. These IMFs are amplitudeand phase-modulated signals and are used to define the instantaneous phases of structural waves. The state of a structure is evaluated, and damage is identified based on these instantaneous phase features. Furthermore, fundamental relationships are developed connecting the instantaneous phases to a local physics-based structural representation in order to infer damage in terms of physical parameters, such as structural mass, stiffness, and damping. Damage-detection applications are investigated by using numerical simulations and a variety of laboratory experiments with simple structures. Several different types of excitation mechanisms are used for dynamic input to the structures. The time series output of the structural response is then analyzed by using the new method. The instantaneous phase relationships are extracted and examined for changes which may have occurred due to damage. These results are compared to those from other newly developed detection methods, such as an algorithm based on the geometric properties of a chaotic attractor. The studies presented here show that our method, without linear-system or stationary-process assumptions, can identify and locate structural damage and permit the further development of a reliable real-time structural health-monitoring and damage-detection system.

Geometric time-domain methods of vibration-based damage detection

We present a new methodology for vibration based damage detection derived from the characterization of changes in the geometric properties of the time domain response of a structure. Many new features present themselves when the geometry of attracting objects in phase space are considered. The most promising avenue of study are metrics that describe changes to the attractor shape or dimension. In particular, the utility of a feature consisting of the ratio of average local variance (or spatial dispersion) of the input to the average local variance of the response is assessed. Presenting the results of the geometric time domain method in a statistical framework highlights the methods increased sensitivity to subtle damage-inflicted changes to the structure when compared to more traditional modal based methods. In addition the geometric method demonstrates a more robust handling of changes due to ambient environmental fluctuation. Results are presented from a finite element model of a thin plate with weld line damage implemented through a relaxation of a boundary condition.

Excitation considerations for attractor property analysis in vibration-based damage detection

In past work, we have presented a methodology for vibration based damage detection derived from the characterization of changes in the geometric properties of the time domain response of a structure. In brief, input forcing signals and output response signals can be transformed into state space geometrical representations. When allowed to evolve to a steady state, the geometric object is called an attractor. Certain properties of the attractor, such as the local variance of neighborhoods of points or prediction errors between attractors, have been shown to correlate directly with damage. While most inputs will generate some type of attracting geometric object, prescribing a low dimensional input forcing signal helps to maintain a low dimensional output signal which in turn simplifies the calculation of attractor properties. Work to date has incorporated the use of a chaotic input forcing signal based on its low dimensionality yet useful frequency content. In this work we evaluate various forms of shaped noise as alternative effectively low dimensional inputs. We assess whether the intrinsic properties of the chaotic input leads to better damage detection capabilities than various shaped noise inputs. The experimental structure considered is a thin plate with weld line damage.

The effects of thermal and polarization fluctuations on 3/spl times/3 coupler performance

For the Gould 3/spl times/3 coupler studied here, changes in input SOP produce measurable changes in the power transfer properties of the coupler while temperature-induced changes are almost negligible over the range 0-100/spl deg/C. When used for passive demodulation in interferometric systems, changes in SOP will cause noise and inaccuracies in the calculated phase, even in systems employing Faraday rotator mirrors to prevent fringe visibility fading. Hence, the effects of polarization must be controlled by, for example, using a depolarized source or by realtime monitoring of the coupler phase angles. Future studies will extend the investigation to a variety of coupler types and to an investigation of the dependence of the complex scattering matrix on wavelength.

Measurement and analysis of complex modulated motions in a weakly nonlinear system

Abstract The two-timing dynamics of a forced, weakly nonlinear system are considered. Experimental results on a roll-forced spherical pendulum verify the occurrence of slow-time, subharmonically and chaotically modulated oscillations in a frequency band near resonance; the modulatory behavior follows the occurrence of a Hopf bifurcation in the slow-time coordinates. A phase-sensitive detection method for experimentally isolating the slow-time behavior is described. A Lagrangian model for the pendulum is developed, and slow-time equations are presented. A two-thirds power scalling law relating roll angle to the appearance of the modulations is derived from the slow-time equations and tested with experimental results.

Transient dynamics of a lightly damped, roll-forced pendulum

A roll-forced pendulum is studied as a rudimentary model for a sea-state driven ship crane in this work. Centripetal acceleration measurements were made in a number of non-resonant forcing conditions. In addition to large-amplitude resonant responses, a variety of beating responses were found in the transient mode; this behavior is significant due to the light damping and thus long influence of the transient dynamics. The (transient as well as steady-state) frequency content of the centripetal acceleration spectrum is predicted and compared to experiment using power spectral methods and spectrograms.