Defect Detection using Power Spectrum of Torsional Waves in Guided-Wave Inspection of Pipelines

Ultrasonic Guided-wave (UGW) testing of pipelines allows long-range assessment of pipe integrity from a single point of inspection. This technology uses a number of arrays of transducers separated by a distance from each other to generate a single axisymmetric (torsional) wave mode. The location of anomalies in the pipe is determined by inspectors using the received signal. Guided-waves are multimodal and dispersive. In practical tests, nonaxisymmetric waves are also received due to the nonideal testing conditions, such as presence of variable transfer function of transducers. These waves are considered as the main source of noise in the guided-wave inspection of pipelines. In this paper, we propose a method to exploit the differences in the power spectrum of the torsional wave and flexural waves, in order to detect the torsional wave, leading to the defect location. The method is based on a sliding moving window, where in each iteration the signals are normalised and their power spectra are calculated. Each power spectrum is compared with the previously known spectrum of excitation sequence. Five binary conditions are defined; all of these need to be met in order for a window to be marked as defect signal. This method is validated using a synthesised test case generated by a Finite Element Model (FEM) as well as real test data gathered from laboratory trials. In laboratory trials, three different pipes with defects sizes of 4%, 3% and 2% cross-sectional area (CSA) material loss were evaluated. In order to find the optimum frequency, the varying excitation frequency of 30 to 50 kHz (in steps of 2 kHz) were used. The results demonstrate the capability of this algorithm in detecting torsional waves with low signal-to-noise ratio (SNR) without requiring any change in the excitation sequence. This can help inspectors by validating the frequency response of the received sequence and give more confidence in the detection of defects in guided-wave testing of pipelines.

Sensors Network for Ultrasonic Ranging System

The spectrum matching and correlation characteristic are both important in the multiple-user ultrasonic ranging system. As people know, an ultrasonic ranging system, which has a bell-shaped magnitude spectrum, acts like a band-pass filter. If the spectrum of the excitation signal does not match that of the ultrasonic ranging system, some of energy cannot be transmitted by the ultrasonic system. In other words, it does not make full use of the bandwidth of the ultrasonic ranging system. The good correlation characteristics can eliminate cross-talk among multichannel ultrasonic sensors firing simultaneously. To the authors’ knowledge, not many researchers considered how to make the spectrum of the excitation sequence match to that of the ultrasonic ranging system as well as improve the correlation characteristics. In this paper, the chaotic frequency modulation (CFM) excitation sequences are proposed for multiple-user ultrasonic ranging system. To obtain the excitation sequences which are spectrally matched to the ultrasonic ranging system as well as have the best correlation characteristic, the NSGA-II is applied to optimize the CFM excitation sequences.

A Novel Coded Excitation Sequence for Ultrasonic Life Location System

A simple and feasible modulation scheme called Continuous Pulse Sequence Width Modulation (CPSWM) is proposed to eliminate the crosstalk among multiple ultrasonic sensors in ultrasonic life location system. To improve the performance of multi-life location system (i.e. sharp autocorrelation and flat cross-correlation characteristics), both the continuous pulse sequence (CPS) width and the duration between two CPSs can be modulated using chaotic, pseudorandom or other methods. The duty cycle of each pulse is fixed to be 50%. To adapt to the intrinsic inertia property of ultrasonic transducers, only central work frequency is used in the CPSWM sequence scheme. The CPSWM scheme only uses digital signal generated by a FPGA processor, no D/A converter is used. The experimental results of ultrasonic multi-life location system consisting of two-channel SensComp 600 series electrostatic sensors with the proposed excitation sequences demonstrate that the ultrasonic multi-life location system with the proposed method can realize multi-life location simultaneously.