T. Stepinski

Publications (3)0 Total impact

• Conference Proceeding: Optimized algorithm for synthetic aperture imaging

  T. Stepinski, F. Lingvall
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  ABSTRACT: We present a novel synthetic aperture imaging algorithm based on concepts used in synthetic aperture radar (SAR) and sonar (SAS). The algorithm, based on a convolution model of the imaging system developed in the frequency domain, accounts for the beam-pattern of the finite sized transducer used in the synthetic aperture. A 2D Fourier transform is used for the calculation of the 2D spectrum of the ultrasonic data. The spectrum is then interpolated to convert the polar coordinate system used for the acquisition of ultrasonic signals to rectangular coordinates. After windowing, the transformed spectrum is subjected to the 2D inverse Fourier transform to get the time domain image again. Performance of the proposed algorithm and the classical time-domain SAFT (synthetic aperture focusing technique) are compared using both simulated and real ultrasonic data.
  Ultrasonics Symposium, 2004 IEEE; 09/2004
• Conference Proceeding: Compensating transducer diffraction effects in synthetic aperture imaging for immersed solids

  F. Lingvall, T. Stepinski
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  ABSTRACT: One of the fundamental requirements for the successful application of the classical synthetic aperture focusing technique (SAFT) is the use of a transducer that emits spherical (cylindrical) waves. For a planar transducer, the performance of the SAFT algorithm will deteriorate if its active area becomes too large comparing to the wavelength. This is due to the spatial impulse responses (SIRs) associated with the transducer that no longer resemble Dirac functions since the emitted waves is not spherical. Therefore, to achieve a high resolution or finite-sized transducers, the SIRs must be taken into consideration. Here, we propose a method that is based on a discrete linear model of the imaging system. The method uses a spatio-temporal deconvolution technique designed to minimize the mean squared reconstruction error of the imaging system. To demonstrate the performance of the proposed method we present experiments using a phased array for the inspection of a copper specimen. The results obtained using the deconvolution method for finite apertures are compared to those obtained with a time-domain SAFT algorithm and a focused phased array.
  Ultrasonics Symposium, 2002. Proceedings. 2002 IEEE; 11/2002
• Article: Automatic detecting and classifying defects during eddy current inspection of riveted lap-joints

  F. Lingvall, T. Stepinski
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  ABSTRACT: This article presents a novel method for automatic detection and classification of cracks located in the second lower layer of the aircraft lap-joints during Eddy Current (EC) inspection. The cracks originating from the rivet holes were detected using a tailor-made deep penetrating EC probe. The proposed method consists of three steps: pre-processing, feature extraction and classification. The pre-processing, performed before the feature extraction included median filtering, rotation and de-biasing of the EC patterns. The rotation of the patterns was performed so that energy of the responses to the rivets was maximized along the quadrature direction, while the defect responses were maximized in the in-phase direction in the impedance plane. Feature extraction was then performed using four different methods: discrete wavelet transform, Fourier descriptors, principal component analysis (PCA) and block mean values. The classification was performed using a standard multi-layer perceptron (MLP) neural network. All the pre-processing methods showed similar classification performance on the used data set, but the PCA method compressed the data best.
  NDT & E International.