David M. Harvey

Sparse signal representation and its applications in ultrasonic NDE.

Many sparse signal representation (SSR) algorithms have been developed in the past decade. The advantages of SSR such as compact representations and super resolution lead to the state of the art performance of SSR for processing ultrasonic non-destructive evaluation (NDE) signals. Choosing a suitable SSR algorithm and designing an appropriate overcomplete dictionary is a key for success. After a brief review of sparse signal representation methods and the design of overcomplete dictionaries, this paper addresses the recent accomplishments of SSR for processing ultrasonic NDE signals. The advantages and limitations of SSR algorithms and various overcomplete dictionaries widely-used in ultrasonic NDE applications are explored in depth. Their performance improvement compared to conventional signal processing methods in many applications such as ultrasonic flaw detection and noise suppression, echo separation and echo estimation, and ultrasonic imaging is investigated. The challenging issues met in practical ultrasonic NDE applications for example the design of a good dictionary are discussed. Representative experimental results are presented for demonstration.

Advanced acoustic microimaging using sparse signal representation for the evaluation of microelectronic packages

Acoustic microimaging (AMI) has been widely used to nondestructively evaluate microelectronic packages for the presence of internal defects. To detect defects in small devices such as /spl mu/BGA, flip-chip, and chip-scale packages, high acoustic frequencies are required for the conventional AMI systems. The acoustic frequency used in practice, however, is limited by its penetration through materials. In this paper, a novel acoustic microimaging technique, which utilizes nonlinear signal processing techniques to improve the resolution and robustness of conventional AMI, is proposed and investigated. The technique is based on the concept of sparse signal representations in overcomplete time-frequency dictionaries. Simulation and experimental results show its super resolution and high robustness.

Interrogation of extrinsic Fabry-Perot interferometric sensors using arrayed waveguide grating devices

In this paper we present details of a solid state interrogation system based on a 16-channel arrayed waveguide grating (AWG) for interrogation of extrinsic Fabry-Perot interferometric (EFPI) sensors. The sensing element is configured in a reflecting mode and is illuminated by a broad-band light source through an optical fiber. The spectrum of light reflected from the sensor is analyzed using an AWG device acting as a coarse spectrometer. Using measurement points from the AWG channels, the original spectrum of the sensing element is reconstructed by a means of curve fitting. This allows sufficient information for the position of the reflection peak (or inverted peak) to be uniquely determined and the value of a measurement quantity obtained. In addition to the theoretical simulations of the proposed measurement system, we provide details of the laboratory evaluation using an EFPI strain sensor.

Instant center of rotation estimation using the Reuleaux technique and a Lateral Extrapolation technique.

A mathematical model of a rolling wheel was used to investigate the errors encountered when the Reuleaux technique is employed to estimate planar instant centers of rotation (ICRs). The investigation showed that large errors can result when this pole measurement technique is applied to objects rotating more than 12 degrees. The investigation also showed that these errors can be substantially reduced by applying a new Lateral Extrapolation technique to the pole data. When the Reuleaux technique is applied to marks on a 10cm radius wheel, the resulting offset errors from the ICR are 3.96cm for a 45 degrees roll and 1cm for a 12 degrees roll. Following lateral extrapolation, these offset errors reduce to 0.52cm for the 45 degrees roll and less than 0.04cm for the 12 degrees roll. Thus, the extrapolation technique is over seven times more accurate for a 45 degrees roll, and over 25 times more accurate for a 12 degrees roll. The extrapolation technique has been validated with the model for joints that exhibit both slip and roll, such as the knee. As joint ICR pathway measurement can be used to detect pathology, these accuracy improvements offer potential benefits for clinical applications.

Signal denoising and ultrasonic flaw detection via overcomplete and sparse representations

Sparse signal representations from overcomplete dictionaries are the most recent technique in the signal processing community. Applications of this technique extend into many fields. In this paper, this technique is utilized to cope with ultrasonic flaw detection and noise suppression problem. In particular, a noisy ultrasonic signal is decomposed into sparse representations using a sparse Bayesian learning algorithm and an overcomplete dictionary customized from a Gabor dictionary by incorporating some a priori information of the transducer used. Nonlinear postprocessing including thresholding and pruning is then applied to the decomposed coefficients to reduce the noise contribution and extract the flaw information. Because of the high compact essence of sparse representations, flaw echoes are packed into a few significant coefficients, and noise energy is likely scattered all over the dictionary atoms, generating insignificant coefficients. This property greatly increases the efficiency of the pruning and thresholding operations and is extremely useful for detecting flaw echoes embedded in background noise. The performance of the proposed approach is verified experimentally and compared with the wavelet transform signal processor. Experimental results to detect ultrasonic flaw echoes contaminated by white Gaussian additive noise or correlated noise are presented in the paper.

Contemporary ultrasonic signal processing approaches for nondestructive evaluation of multilayered structures

Various signal processing techniques have been used for the enhancement of defect detection and defect characterisation. Cross-correlation, filtering, autoregressive analysis, deconvolution, neural network, wavelet transform and sparse signal representations have all been applied in attempts to analyse ultrasonic signals. In ultrasonic nondestructive evaluation (NDE) applications, a large number of materials have multilayered structures. NDE of multilayered structures leads to some specific problems, such as penetration, echo overlap, high attenuation and low signal-to-noise ratio. The signals recorded from a multilayered structure are a class of very special signals comprised of limited echoes. Such signals can be assumed to have a sparse representation in a proper signal dictionary. Recently, a number of digital signal processing techniques have been developed by exploiting the sparse constraint. This paper presents a review of research to date, showing the up-to-date developments of signal processing techniques made in ultrasonic NDE. A few typical ultrasonic signal processing techniques used for NDE of multilayered structures are elaborated. The practical applications and limitations of different signal processing methods in ultrasonic NDE of multilayered structures are analysed.

Through lifetime monitoring of solder joints using acoustic micro imaging

Purpose – The purpose of this paper is to evaluate the application of an acoustic micro‐imaging (AMI) inspection technique in monitoring solder joints through lifetime performance and demonstrate the robustness of the monitoring through analysis of AMI data.Design/methodology/approach – Accelerated thermal cycling (ATC) test data on a flip chip test board were collected through AMI imaging. Subsequently, informative features and parameters of solder joints in acoustic images were measured and analysed. Through analysing histogram distance, mean intensity and grey area of the solder joints in acoustic images, cracks between the solder bump and chip interface were tracked and monitored. The results are in accord with associated Finite Element (FE) prediction.Findings – At defective bumps, the formation of a crack causes a larger acoustic impedance mismatch which provides a stronger ultrasound reflection. The intensity of solder joints in the acoustic image increase according to the level of damage during th...

Low cost scaleable parallel image processing system

Abstract The design and application of a DSP-based Parallel Image Processing system is presented. A scaleable system based around a TMS320C40 DSP Master–Slave architecture is shown to be a suitable vehicle for industrial inspection problems. Custom vision bus and parallel communication channels are used to pass data between local and shared memory in the image processing system. The target application of SMT solder bond inspection required the fast processing of 2-D FFTs. Analysis of the systems suitability to this task is presented followed by actual results from a three-processor system. The target inspection rate of 100 SMT solder joints per second can be met.

Resolution improvement of acoustic microimaging by continuous wavelet transform for semiconductor inspection

Abstract Acoustic microimaging (AMI) is used as an important non-destructive tool in semiconductor reliability evaluation and failure analysis. As advanced microelectronic packages are being produced smaller and thinner, detection of the internal features and defects in the packages is approaching the resolution limits for conventional AMI. To meet this challenge, an acoustic time–frequency domain imaging technique is proposed in this paper, which utilizes the excellent time–frequency localization characteristics of the continuous wavelet transform (CWT) to improve the axial resolution of AMI, without increasing acoustic frequencies. The proposed technique is compared to time domain AMI, frequency domain AMI and sparse signal representation based AMI (SSRAMI) with respect to both axial resolution and robustness. Simulation results show that the proposed technique has superior performance compared to time domain and frequency domain AMI techniques, and has close performance to SSRAMI but with less computation load.

Fibre reflection mach-zehnder interferometer

Abstract A novel design of two-port, all-fibre reflection Mach-Zehnder interferometer is demonstrated. We calculate the theoretical frequency variation of output power and demonstrate close agreement with experimental observations. The device has potential application as a reflective modulator for a fibre laser and as a fibre sensor.