Polarization Threshold Research Articles

Split-spectrum processing technique for SNR enhancement of ultrasonic guided wave

Ultrasonic guided wave (UGW) systems are broadly used in several branches of industry where the structural integrity is of concern. In those systems, signal interpretation can often be challenging due to the multi-modal and dispersive propagation of UGWs. This results in degradation of the signals in terms of signal-to-noise ratio (SNR) and spatial resolution. This paper employs the split-spectrum processing (SSP) technique in order to enhance the SNR and spatial resolution of UGW signals using the optimized filter bank parameters in real time scenario for pipe inspection. SSP technique has already been developed for other applications such as conventional ultrasonic testing for SNR enhancement. In this work, an investigation is provided to clarify the sensitivity of SSP performance to the filter bank parameter values for UGWs such as processing bandwidth, filter bandwidth, filter separation and a number of filters. As a result, the optimum values are estimated to significantly improve the SNR and spatial resolution of UGWs. The proposed method is synthetically and experimentally compared with conventional approaches employing different SSP recombination algorithms. The Polarity Thresholding (PT) and PT with Minimization (PTM) algorithms were found to be the best recombination algorithms. They substantially improved the SNR up to 36.9dB and 38.9dB respectively. The outcome of the work presented in this paper paves the way to enhance the reliability of UGW inspections.

Split-Spectrum Signal Processing for Reduction of the Effect of Dispersive Wave Modes in Long-range Ultrasonic Testing

This paper presents a Split-Spectrum Signal Processing (SSP) with applications to Long Range Ultrasonic Testing (LRUT). The problem of coherent noise due to Dispersive Wave Modes (DWM) in the context of ultrasonic scattering is addressed and a novel solution by utilizing the SSP technique is proposed for reduction of the effects of DWM in the received signal. The proposed technique investigates the sensitivity of SSP performance to the filter bank parameter values such as processing/filter bandwidth, and filter overlap. Therefore, as a result the optimum values are introduced that improve the signal to noise ratio (SNR) significantly. The proposed method has been compared with conventional approaches for synthesized signals for a 6 inch pipe by applying the different recombination SSP techniques. The Polarity Thresholding (PT) and PT with Minimization (PTM) methods were found to give the best result and substantially improve the SNR performance by an average of 10dB.

Nonlinear clutter suppression using split spectrum processing and optimal detection

Nonlinear processing of outputs from a filter bank to reduce ultrasonic clutter is often referred to as split spectrum processing (SSP). In this work, the optimal detector (OD) for a known transient in additive Gaussian noise is formulated as a filter bank followed by a nonlinear operation (decision). Then, the new formulation is used for several purposes. The first purpose is to present a simple framework for comparison of SSP based on the polarity thresholding (PT) method with SSP based on the OD. The second purpose is to point out that although the OD relies on a test statistic which is a linear function of the observed samples, it can be used for nonlinear clutter suppression. This is because the OD output is used to gate (mask) the original ultrasonic signal and the OD output is a nonlinear function of the filter bank outputs. The third purpose is to identify practical advantages of OD-based SSP, including fewer demands on the experimental data used for parameter tuning, a better understanding of the processing performed, improved resolution, and fewer parameters to tune. The fourth purpose is to discuss the difference between contrast enhancement in clutter suppression and receiver operating characteristics in detection theory. Statistical and geometrical interpretations of both PT and OD are also presented, as well as experimental results from simulated and real ultrasonic data.

41257 Research on the statistics of grain lattice echoes and their use in grain size estimation and grain echo suppression : Karpur, P.; Newhouse, V.L.; Disque, R.C. Drexel University, Philadelphia, Pennsylvania, (United States), AD-A199 811/1/GAR, 170 pp. (19 Aug. 1988)

Abstract : Split spectrum processing (SSP) was developed in the late seventies for ultrasonic applications as a convenient method of introducing frequency diversity in ultrasonic signals for the purpose of the reduction of coherent material noise. Although the technique is very powerful, the use of SSP remained somewhat limited because of the ambiguity that prevailed in the selection of suitable processing parameters. The ambiguity was mainly because of a lack of a physical model of the spectral splitting process. The need for such a physical modeling has been fulfilled as a part of this dissertation. An ultrasonic signal has been modeled as a time limited process which facilitates the use of the frequency sampling theorem to describe the frequency splitting process. Further, the theory of Fourier integrals is used to determine the bandwidth of the theoretical SINC filters obtained as per the frequency sampling theorem. Experimental results have been provided to corroborate the theory. Another aspect of SSP addressed by this thesis is a detailed theoretical analysis of a newly developed algorithm called the Polarity Thresholding (PT) algorithm. Yet another important aspect of SSP addressed by this dissertation is the demonstration of the utility of SSP in many engineering materials such as centrifugally cast stainless steel (CCSS), composites and interface between bimetallic alloys. The potential of SSP as a tool for material characterization is also experimentally demonstrated. Theses.