V. John Mathews

Fast imaging in cannula microscope using orthogonal matching pursuit

Fluorescent miscroscopy is a state-of-the-art method for creating high contrast and high resolution images of microscopic structures and has found wide application in microendoscopy (i.e., imaging cellular information from an optical probe within an animal). Cannula based microscopy methods have recently shown great promise for efficient microendoscopy imaging. Yet, performing real-time imaging with cannula methods have yet to be achieved due to the high computational complexity of the algorithms used for image reconstruction. We present an approach based on compressive sensing to improve computational speed and image reconstruction quality. We compare our approach with the state-of-the-art implementation based on direct binary search, a non-linear optimization technique. Results demonstrating up to 70 times improvement in the computation time and visual quality of the image over the direct binary search method are included in the paper.

Blind identification of QAM signals using a likelihood-based algorithm

This paper presents a method for automatically identifying different QAM modulations. This method identifies the modulation type as the hypothesis for which the likelihood function of the amplitudes of the received signal is the maximum. The derivation of the likelihood functions assumes additive white Gaussian noise and no pulse shaping. In order to accommodate pulse shaping in the received signal, the system sub-samples the incoming signals non-uniformly so that the distribution of the amplitudes of the sub-sampled signals approximately matches that of QAM signals without pulse shaping. This method does not need prior knowledge of carrier frequency and baud rate and can identify modulation types at relatively low SNRs and with relatively few symbols. Simulation results demonstrating accurate modulation identification in the presence of additive noise are included in the paper. Results presented in the paper with non-Gaussian noise indicate that the system is robust to variations from the assumed noise model.

Numerical simulation and experimental validation of Lamb wave propagation behavior in composite plates

Structural Health Monitoring (SHM) based on Acoustic Emission (AE) is dependent on both the sensors to detect an impact event as well as an algorithm to determine the impact location. The propagation of Lamb waves produced by an impact event in thin composite structures is affected by several unique aspects including material anisotropy, ply orientations, and geometric discontinuities within the structure. The development of accurate numerical models of Lamb wave propagation has important benefits towards the development of AE-based SHM systems for impact location estimation. Currently, many impact location algorithms utilize the time of arrival or velocities of Lamb waves. Therefore the numerical prediction of characteristic wave velocities is of great interest. Additionally, the propagation of the initial symmetric (S0) and asymmetric (A0) wave modes is important, as these wave modes are used for time of arrival estimation. In this investigation, finite element analyses were performed to investigate aspects of Lamb wave propagation in composite plates with active signal excitation. A comparative evaluation of two three-dimensional modeling approaches was performed, with emphasis placed on the propagation and velocity of both the S0 and A0 wave modes. Results from numerical simulations are compared to experimental results obtained from active AE testing. Of particular interest is the directional dependence of Lamb waves in quasi-isotropic carbon/epoxy composite plates. Numerical and experimental results suggest that although a quasi-isotropic composite plate may have the same effective elastic modulus in all in-plane directions, the Lamb wave velocity may have some directional dependence. Further numerical analyses were performed to investigate Lamb wave propagation associated with circular cutouts in composite plates.Structural Health Monitoring (SHM) based on Acoustic Emission (AE) is dependent on both the sensors to detect an impact event as well as an algorithm to determine the impact location. The propagation of Lamb waves produced by an impact event in thin composite structures is affected by several unique aspects including material anisotropy, ply orientations, and geometric discontinuities within the structure. The development of accurate numerical models of Lamb wave propagation has important benefits towards the development of AE-based SHM systems for impact location estimation. Currently, many impact location algorithms utilize the time of arrival or velocities of Lamb waves. Therefore the numerical prediction of characteristic wave velocities is of great interest. Additionally, the propagation of the initial symmetric (S0) and asymmetric (A0) wave modes is important, as these wave modes are used for time of arrival estimation. In this investigation, finite element analyses were performed to investigate asp...

Impact location estimation in anisotropic structures

Impacts are major causes of in-service damage in aerospace structures. Therefore, impact location estimation techniques are necessary components of Structural Health Monitoring (SHM). In this paper, we consider impact location estimation in anisotropic composite structures using acoustic emission signals arriving at a passive sensor array attached to the structure. Unlike many published location estimation algorithms, the algorithm presented in this paper does not require the waveform velocity profile for the structure. Rather, the method employs time-of-arrival information to jointly estimate the impact location and the average signal transmission velocities from the impact to each sensor on the structure. The impact location and velocities are estimated as the solution of a nonlinear optimization problem with multiple quadratic constraints. The optimization problem is solved by using first-order optimality conditions. Numerical simulations as well as experimental results demonstrate the ability of the a...

Impact location estimation in anisotropic media

Impact location estimation techniques are important components of Structural Health Monitoring systems. This paper considers impact location estimation in composite structures using acoustic emission signals arriving at a passive sensor array attached to the structure. Because composite structures are anisotropic, the wave propagation properties depend both on the direction of propagation and the location on the structures. As a result, this is a substantially more difficult problem than that of impact location estimation in isotropic media. The algorithm presented in this paper uses three sensor clusters and formulates the impact location estimation problem as one of minimizing a quadratic objective function. Unlike many published location estimation algorithms, the algorithm in this paper does not require the waveform velocity profile for the structure. Experimental results demonstrating the ability of the algorithm to accurately estimate the impact location using acoustic emission signals is included in the paper.

Acoustic emission based damage characterization in composite plates using low-velocity impact testing

Acoustic Emission (AE) based Structural Health Monitoring (SHM) systems are important for impact damage detection and characterization in composite structures. Low velocity impact events can induce internal damage without producing visual indications, compromising the materials structural performance while being very difficult to detect and assess by traditional visual inspection method. Over the years, AE sensor networks have been developed to provide real-time monitoring and detect impact events from low velocity impacts over a large area with minimal intrusion to the composite structure. Yet, most AE methods have focused on detecting and locating damage, not characterization and assessment. This paper develops a preliminary framework for characterization of damage resulting from impacts based on AE signal features. Specialized drop-weight impact experiments were designed to study two particular damage states: delamination with minimal fiber damage and fiber-breakage with minimal delamination. Impacted test panels were inspected using ultrasonic Cscans and recorded waveform signals using piezoelectric sensors were characterized to analyze damage response in the time and frequency domains with assistant parameters such as root mean square (RMS) of the waveform doi: 10.12783/SHM2015/185

A comparative evaluation of piezoelectric sensors for acoustic emission-based impact location estimation and damage classification in composite structures

Acoustic Emission (AE) based Structural Health Monitoring (SHM) is of great interest for detecting impact damage in composite structures. Within the aerospace industry the need to detect and locate these events, even when no visible damage is present, is important both from the maintenance and design perspectives. In this investigation, four commercially available piezoelectric sensors were evaluated for usage in an AE-based SHM system. Of particular interest was comparing the acoustic response of the candidate piezoelectric sensors for impact location estimations as well as damage classification resulting from the impact in fiber-reinforced composite structures. Sensor assessment was performed based on response signal characterization and performance for active testing at 300 kHz and steel-ball drop testing using both aluminum and carbon/epoxy composite plates. Wave mode velocities calculated from the measured arrival times were found to be in good agreement with predictions obtained using both the Disperse code and finite element analysis. Differences in the relative strength of the received wave modes, the overall signal strengths and signal-to-noise ratios were observed through the use of both active testing as well as passive steel-ball drop testing. Further comparative is focusing on assessing AE sensor performance for use in impact location estimation algorithms as well as detecting and classifying damage produced in composite structures due to impact events.Acoustic Emission (AE) based Structural Health Monitoring (SHM) is of great interest for detecting impact damage in composite structures. Within the aerospace industry the need to detect and locate these events, even when no visible damage is present, is important both from the maintenance and design perspectives. In this investigation, four commercially available piezoelectric sensors were evaluated for usage in an AE-based SHM system. Of particular interest was comparing the acoustic response of the candidate piezoelectric sensors for impact location estimations as well as damage classification resulting from the impact in fiber-reinforced composite structures. Sensor assessment was performed based on response signal characterization and performance for active testing at 300 kHz and steel-ball drop testing using both aluminum and carbon/epoxy composite plates. Wave mode velocities calculated from the measured arrival times were found to be in good agreement with predictions obtained using both the Dispe...