Bernhard G. Zagar

Acoustic Microscopy Technique to Precisely Locate Layer Delamination

In this paper, the synthetic aperture focusing technique (SAFT) is applied to extend the depth of focus for spherically focused ultrasound transducers. This technique uses a virtual source element in the geometrical focus of the transducer. Initial experiments with a 90-mum copper wire are conducted to investigate the efficacy of SAFT processing for positive and negative defocus. Furthermore, delamination of two glued together transparent Perspex plates is investigated. Compared to the common B-mode ultrasonic imaging that can only detect the presence of layer delamination, the proposed SAFT can accurately locate the position of delamination and visualize its extension.

Estimation of the surface normal velocity of high frequency ultrasound transducers

This paper is concerned with the characterization of the true locally resolved surface normal velocity of an assumed piston-type ultrasonic transducer. Instead of involving a very complicated direct pointwise measurement of the velocity distribution, an inverse problem is solved which yields a spatially discretized weighting vector for the surface normal velocity of the transducer. The study deals with a spherically focused high frequency transducer, which is driven in pulse-echo mode. As a means of posing the inverse problem, the active transducer surface is divided into annuli of equal surface so that for each annulus the spatial impulse response can be calculated. An acrylic glass plate acts as a simple structured target. The resulting ill-posed nonlinear inverse problem is solved with an iterative regularized Gauss-Newton algorithm. The solution of the inverse problem yields an estimated weight for the surface normal velocity for each annulus. Experimental results for a thin copper wire target are compared to simulation results for both uniform and estimated surface normal velocities.

Image processing to estimate the ellipticity of steel coils using a concentric ellipse fitting algorithm

Measuring the ellipticity is an important task to ensure the quality of steel coils before they are shipped out. In this paper a new algorithm to measure the ellipticity of steel coils using a machine vision system is presented. Traditional algorithms for ellipse detection based on least squares fitting can not be directly applied to this problem, since they lead to ellipses with a very high ellipticity if the amount of fitting data is small. In our new concentric ellipse fitting least squares algorithm the additional information, that the elliptic input curves are concentric, is used to increase the amount of available input data, to estimate the ellipticity even if the curve segments are very short. In this paper the results of a traditional ellipse fitting algorithm are compared to those of the new concentric ellipse fitting algorithm. Furthermore the preprocessing tasks (Hough transform, edge detection, curve filtering), which lead to the fitting data, are described in detail. Finally a simulation of the radius dependency of the ellipticity is done, which is compared to the results of the algorithm.

Optical tomography imaging based on higher order Born approximation of diffuse photon density waves

In this paper, we introduce a tomographical system suitable for imaging absorbers in strongly scattering media using a single light source-detector pair. Their reflective alignment would make it possible to implement both source and detector into a compact scan-head. Suitable probes would be soft tissue, some strongly scattering biological probes, or translucent technical materials, for example. An intensity-modulation technique is utilized to take advantage of the amplitude and delay alterations between the source and the detector. The propagation of the photons is modeled by the Helmholtz equation and then appropriate inverse calculations of several measurements reveal the distribution of the absorption coefficient inside the investigated volume. Therefore a reconstruction algorithm employing a Born approximation of higher order and a Tikhonov-regularization scheme were used to solve the deduced ill-posed and underdetermined system of equations. In this paper, we present results of simulations as well as of measurements.

Sub-surface defect detection in a steel sheet

In recent years, the focus on quality control in the steel industry has shifted from offline to inline non-destructive testing in order to detect defects at the earliest possible stage in the production process. The detection and elimination of such defects is vital for sustaining product quality and reducing costs. Various measurement principles (e.g. ultrasonic testing, electromagnetic acoustic transducer, x-ray inspection) were analyzed and their advantages and disadvantages are discussed regarding their usability in a steel plant. Based on these findings a magnetic method combined with a new sensor concept was chosen. By using highly sensitive sensors based on the giant magnetoresistive effect, it is possible to detect magnetic flux leakage variations on the surface of a magnetized steel strip caused by defects or inhomogeneities inside the material. Based on promising measurement results of preliminary tests and simulation results obtained by finite element method-models, a prototype is now being built for offline measurements and the optimization of the measurement method. In the event that the development of this second prototype is successful, an inline configuration will be implemented.

Analysis and Validation of a Planar High-Frequency Contactless Absolute Inductive Position Sensor

This paper presents a low cost precise and reliable inductive absolute position measurement system. It is suitable for rough industrial environments, offers a high inherent resolution (0.1%-0.01% of measurement range), can measure target position over a wide measurement range, and can potentially measure multiple target locations simultaneously. The basic position resolution is improved by adding two additional finer pitched receive channels. The sensor works on principles similar to contactless resolvers. It consists of a rectangular antenna printed circuit board and a passive LC resonant circuit. A mathematical model and the equivalent circuit of this kind of sensor are explained in detail. Such sensors suffer from transmitter to receiver coil capacitive crosstalk, which results in a phase sensitive offset. This crosstalk will be analyzed by measurements. Moreover, the mechanical transducer arrangement, the measurement setup, and the measured results will be presented.

Phase contrast method for measuring ultrasonic fields

Pulsed acoustic waves in water generated by ultrasonic transducers with power levels in the medical diagnostic range are characterized by analyzing the optical diffraction patterns of short laser pulses due to the pressure waves to be analyzed. The diffracted laser light is filtered in the Fourier plane of an optical system and projected onto a charge-coupled-device camera to be grabbed and further processed. Using this technique, it is possible to measure and characterize the ultrasound field in minutes time, which would otherwise last hours or days using the standard method of measuring the intensity on a fine three-dimensional grid with a hydrophone. In addition, the phase information of the ultrasonic wave is acquired easily.

High dynamic range imaging by varying exposure time, gain and aperture of a video camera

To generate high dynamic range (HDR) images usually the exposure time is the only parameter varied when acquiring the low dynamic range (LDR) image series. In this paper the f-number and the gain of the camera are also considered to be variable as exposure defining parameter. This enables HDR imaging for image acquisition systems with limited exposure time range to prevent motion blur. The impacts of the added parameters gain and f-number are analyzed. Since common HDR image generating algorithms require the exposure time as input, an approach to estimate an equivalent exposure time by the used parameter set (exposure time, f-number and camera gain) is shown. Further this equivalent exposure time can also be estimated without a priori knowledge of the parameter set. At the end of the paper a quality assurance application is shown, where this extended parameter set was used and proved necessary for the HDR image acquisition.

Application of a locally operating laser-speckle strain sensor

In this paper, we report on both the theory and applicability of a novel laser-speckle shift strain measurement system. Based on the theoretical sensitivity of the system to strain, some sources of error due to imperfections in the geometrical setup are discussed. Some references to the digital signal processing involved are given. We conclude with some experimental results from testing fibers, foils, and standardized specimen that can only be obtained in a noncontacting and preferably optical way because of the fragility and the small physical dimensions of the specimen.

Measurement of Paint Coating Thickness by Thermal Transient Method

This paper is concerned with the accurate laminary determination of the thickness of coatings on metallic surfaces. The development goal was to devise a system that allows for highly spatially resolved full-frame 2-D scans that conventional systems are not able to provide. Furthermore, it is able to allow for a sufficiently large physical separation between the measuring head and the inspected surface to accommodate industrial environments that provide a high operating velocity and is able to detect quality problems of square millimeter in size. The system consists of a heat source that introduces a thermal transient to the surface layer of the specimen, a thermal imaging system to acquire the time-dependent cooling behavior, and an image processing system to determine thermal parameters indicative of surface quality and/or coating thickness. The results clearly show that a thickness resolution of better than 50 mum can reliably be obtained at a scanning velocity of 0.37 m/s, covering a field of view (FOV) of approximately 200 times 140 mm2.