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Walk-through metal detectors are used at check points for preventing personnel and passengers from carrying threatening metallic objects, such as knives and guns, into a secure area. These systems are capable of detecting small metallic items, such as handcuff keys and blades, but are unable to distinguish accurately between threatening objects and innocuous items. This paper studies the extent to which a K-nearest-neighbour classifier can distinguish various kinds of metallic objects, such as knives, shoe shanks, belts and containers. The classifier uses features extracted from the magnetic polarizability tensor, which represents the electromagnetic properties of the object. The tests include distinguishing threatening objects from innocuous ones, classifying a set of objects into 13 classes, and distinguishing between several similar objects within an object class. A walk-through metal detection system is used as source for the test data, which consist of 835 scans and 67 objects. The results presented show a typical success rate of over 95% for recognizing threats, and over 85% for correct classification. In addition, we have shown that the system is capable of distinguishing between similar objects reliably. Overall, the method shows promise for the field of security screening and suggests the need for further research. [ABSTRACT FROM AUTHOR]

A completely contactless flow measurement technique based on the principle of EM induction measurements-contactless inductive flow tomography (CIFT)-has been previously reported by a team based at Forschungszentrum Dresden-Rossendorf (FZD). This technique is suited to the measurement of velocity fields in high conductivity liquids, and the possible applications range from monitoring metal casting and silicon crystal growth in industry to gaining insights into the working of the geodynamo. The forward problem, i.e. calculating the induced magnetic field from a known velocity profile, can be described as a linear relationship when the magnetic Reynolds number is small. Previously, an integral equation method was used to formulate the forward problem; however, although the sensitivity matrices were calculated, they were not explicitly expressed and computation involved the solution of an ill-conditioned system of equations using a so-called deflation method. In this paper, we present the derivation of the sensitivity matrix directly from electromagnetic field theory and the results are expressed very concisely as the cross product of two field vectors. A numerical method based on a finite difference method has also been developed to verify the formulation. It is believed that this approach provides a simple yet fast route to the forward solution of CIFT. Furthermore, a method for sensor design selection based on eigenvalue analysis is presented. [ABSTRACT FROM AUTHOR]

Specific conductance and frequency-dependent resistance (impedance) data are widely utilized for understanding the physicochemical characteristics of aqueous and non-aqueous fluids and for evaluating the performance of chemical processes. However, the implementation of such an in-situ probe in high-temperature and high-pressure environments is not trivial. This work provides a description of both the hardware and software associated with implementing a parallel-type in-situ electrochemical sensor. The sensor can be used for in-line monitoring of thermal desalination processes and for impedance measurements in fluids at high temperature and pressure. A comparison between the experimental measurements on the specific conductance in aqueous sodium chloride solutions and the conductance model demonstrate that the methodology yields reasonable agreement with both the model and literature data. A combination of hardware components, a software-based correction for experimental artifacts, and computational fluid dynamics (CFD) calculations used in this work provide a sound basis for implementing such in-situ electrochemical sensors to measure frequency-dependent resistance spectra. [ABSTRACT FROM AUTHOR]

Magnetic induction tomography (MIT) is a new and emerging type of tomography technique that is able to map the passive electromagnetic properties (in particular conductivity) of an object. Because of its non-invasive feature, it becomes a suitable technique for many industries, such as metal processing and mining. This paper presents a volumetric MIT (VMIT) system based on an existing measurement setup in our 2D system (MIT Mk-I). By increasing the number of sensors in the axial direction, volumetric imaging can be realized and hence can improve the spatial resolution of the reconstructed images. All of the system control, data acquisition and signal demodulation are accomplished by a commercial data acquisition card and the National Instruments graphical programming language. In this paper, both the system architecture and the forward 3D sensitivity model will be presented. The image reconstruction scheme is modified by introducing a 3D sensitivity map to replace the previous 2D sensitivity map used for the MIT Mk-I system. The iterative Landweber technique was implemented as the inverse solver to reconstruct the images. Several laboratory-based experimental results are demonstrated in this paper, with different shapes of imaging objects. The reconstructed images are satisfactory showing for the first time volumetric conductivity reconstruction using a multi-layer MIT system. The results indicate the high-quality image reconstruction using our novel VMIT system for potential use in industrial applications, such as metal flow imaging. [ABSTRACT FROM AUTHOR]

Image reconstruction of electromagnetic tomography (EMT) is often ill-posed due to the limited prior information about imaging features, leading to low reconstruction accuracy. In this paper, a novel image reconstruction method based on frequency domain feature extraction for EMT is proposed to improve the reconstruction accuracy. The two-dimensional EMT image is first modeled as a one-dimensional finite-length streams of Dirac signal, which is a typical signal with finite rate of innovation (FRI). Then the reconstruction results of the recent algorithms, such as the LBP, Landweber and TV algorithms, can be also modeled as FRI signals. The frequency domain feature of such signals can be extracted with a distributed FRI sampling system. Based on the obtained FRI samples, a new EMT measurement equation is established, and a joint image reconstruction algorithm is proposed, to improve the quality of the reconstructed image. Finally, simulation results have shown that the proposed method outperforms the related methods with better indicators such as image error and correlation coefficient. [ABSTRACT FROM AUTHOR]

In high-frequency eddy-current testing, the coil impedance is often seriously affected by the capacitance effect, while the existing calibration method only corrects the capacitive couplings within the coil itself and ignores the coupling between the coil and the conductor. Combined with the equivalent circuit, a correction method for both the capacitive couplings is proposed using single-plate calibration. Furthermore, considering the skin effect at high frequencies, which leads to the calculation error of coil impedance, a modified impedance is introduced in the equivalent circuit and the double-plate calibration is presented. Using the two proposed methods and the existing calibration method, the experimental values of the coil impedance were respectively corrected when the coil was on a single-coated plate (titanium coating and TiAl6V4 titanium alloy substrate), and compared with the theoretical values between 1 and 100 MHz. The results show a sequentially improved correction effect of the existing calibration method, single-plate calibration, and double-plate calibration. Combined with the Levenberg–Marquardt method, the double-plate calibration was applied to the parameter measurement of the conductive plate with tens of micrometer coating. Through analyzing the sensitivity function of each tested parameter, we employed the inductance to calculate the substrate conductivity at hundreds of kilohertz and the resistance to calculate the coating thickness and conductivity at tens of megahertz. The parameters of three single-coated plates with coating thicknesses of 23 μm, 32 μm and 52 μm were respectively tested, and the results show that the relative measurement errors of all parameters are less than 15%. [ABSTRACT FROM AUTHOR]

Non-destructive monitoring of metal hardness is important for process monitoring and product quality assessment. In this paper, a partially hardened metal sample is assessed in regards to physical effects using multi-frequency inductance spectroscopy. For this purpose, the influence of change of conductivity, permeability and stress on the inductance spectra are analyzed. To extract the numerical values of permeability, the measured inductance spectra are compared to the analytical inductance model. Statistical analysis with principal component analysis is conducted to fuse the spectral data into a single representative value. The results show the possibility to obtain information from different layers of the partially hardened sample, thus indicating a strong relationship between the measured inductance and restraint hardening. [ABSTRACT FROM AUTHOR]

We address the problem of extracting the position and the Euler angles of an underground object from measurements of the magnetic field above that object. The traditional Euler inversion positioning method requires us to make use of the high-order magnetic gradient tensor system and extract the second-order tensor data, but at the same time it will increase the sensitivity to errors and measurement noise, which reduces the positioning accuracy. To solve this problem, we propose a method that uses only first-order magnetic gradient tensor data and supplements the tensor spacial invariant relations to achieve a higher-precision positioning performance for magnetic object. We analyze the tensor invariants and the eigenvalues of the tensor matrix under a magnetic dipole source field, and derive two tensor spacial invariant relations: (1) the angle between magnetic moment and position vector is constant and can be represented by the tensor eigenvalues, and (2) the eigenvector of the absolute minimum eigenvalue is perpendicular to the magnetic moment and position vector, as well as the eigenvectors of remaining eigenvalues are coplanar to the magnetic moment and position vector. Hence, we compute a total four possible solutions of the position vector with respect to the four quadrants of a plane over the measuring center, and finally determine the true one based on the actual detection direction and measured magnetic field data. The results show that the proposed method has significantly higher detection accuracy and larger range under a same level noise condition than the Euler inversion positioning method. When positioning small-scale magnets (e.g. about 5 cm in diameter and 0.5 cm in thickness), we control the tri-axial coordinates accuracy with just the first-order magnetic gradient tensor system within 1 cm root mean square error. [ABSTRACT FROM AUTHOR]

The solidification process in continuous casting is a critical part of steel production. The speed and quality of the solidification process determines the quality of the final product. Computational fluid dynamics (CFD) simulations are often used to describe the process and to design its control system but, so far, there has been no tool that provides an online measurement of the solidification front of hot steel during the continuous casting process. This paper presents a novel magnetic induction tomography (MIT) solution, developed in the EU-funded SHELL-THICK project, to work in a real casting setting and to provide a real-time and reliable measurement of the shell thickness in a cross section of the strand. The new MIT system was installed at the end of the secondary cooling chamber of a casting unit and tested over several days in a real production process. MIT is able to create an internal map of the electrical conductivity of hot steel deep inside the billet. The image of electrical conductivity is then converted to a temperature profile that allows the measurement of the solid, mushy and liquid layers. In this study, such a conversion is done by synchronizing in one time step the MIT measurement and the thermal map generated with the actual process parameters available at that time. The MIT results were then compared with the results obtained with the CFD and thermal modelling of the industrial process. This is the first time in situ monitoring of the interior structure has been carried out during a real continuous casting. [ABSTRACT FROM AUTHOR]

Magnetic induction tomography (MIT) is a tomographic technique capable of imaging the passive electromagnetic properties of an object. It has the advantages of being contact-less and non-invasive, as the process involves interrogating the electromagnetic field of the imaging subject. As such, the potential applications of MIT are broad, with various domains of operation including biomedicine, industrial process tomography and non-destructive evaluation. Consequently, there is a rich—yet underexplored—research landscape for the practical applications of MIT. The aim of this review is to provide a non-exhaustive overview of this landscape. The fundamental principles of MIT are discussed, alongside the instrumentation and techniques necessary to obtain and interpret MIT measurements. [ABSTRACT FROM AUTHOR]

Carbon fibre reinforced polymer (CFRP) materials pose new challenges to the non-destructive evaluation (NDE) techniques. This study addresses the issue of large defect identification in CFRP plates using electromagnetic measurements. A dual plane magnetic induction tomography (MIT) technique is proposed as a method for damage localization in composite parts, where two arrays of planar sensors are utilized to measure the changes in induced voltages due to the changes in electrical conductivity properties. This geometry meets the requirements of damage inspection in plate structures and thus makes the imaging process feasible. The electrical voltage measurements are used as input to inversely map the spatial resolution of the samples in the region of interest. The stability and detectability of the dual plane system is examined using small metallic cubes. Both individual and multiple instances of damage embedded in CFRP samples are created as a representation of the possible manufacturing defects. Experimental study shows that the presence of damage can be identified in both cases using the dual plane MIT system. With advanced sensing design, rapid data collection unit and improvement in resolution, MIT could become a rapid NDE technique for the integrity inspection of composite structures. [ABSTRACT FROM AUTHOR]

Utilizing electrically conductive nanocomposites for integrated self-sensing and health monitoring is a promising area of structural health monitoring (SHM) research wherein local changes in conductivity coincide with damage. In this research we conduct proof of concept investigations using electrical impedance tomography (EIT) for damage detection by identifying conductivity changes and by imaging conductivity evolution in a carbon nanofiber (CNF) filled epoxy composite. CNF/epoxy is examined because fibrous composites can be manufactured with a CNF/epoxy matrix thereby enabling the entire matrix to become self-sensing. We also study the mechanisms of conductivity evolution in CNF/epoxy through electrical impedance spectroscopy (EIS) testing. The results of these tests indicate that thermal expansion is responsible for conductivity evolution in a CNF/epoxy composite. [ABSTRACT FROM AUTHOR]

The PDF file provided contains web links to all articles in this volume. [ABSTRACT FROM AUTHOR]

Measurement of water concentration of oil–water mixtures in a pipeline is important in the oil industry. Capacitance, γ-ray and ultrasonic techniques have been developed for the purpose of water-cut measurement for years. In this paper, we demonstrate a new non-contact method, which uses an eddy current sensor to monitor the flow in a pipeline based on the relationship between the water-cut of the flow and the inductance of the sensor. A simulation model and a simple experiment setup are presented. The experimental results have shown that the measurement error for water-cut over the range of 0–100% is less than 4.5%. [ABSTRACT FROM AUTHOR]

Universal exhaust gas oxygen sensors (UEGOs) are in widespread use in internal combustion engines where they are used to measure lambda (the non-dimensional air-fuel ratio) and oxygen concentration (Xo2). The sensors are used on production engines and for research and development. In a previous paper, a model of the UEGO sensor was presented, based on a solution of the Stefan-Maxwell equations for an axisymmetric geometry, and it was shown that for a known gas composition, predictions of the sensor response agreed well with experiment. In the present paper, the more 'practical' problem is addressed: how well can such a model predict λ and Xo2 based on the sensor response? For IC engine applications, a chemistry model is required in order to predict λ, and such a model is also desirable for an accurate prediction of Xo2. A fast (matrix exponential) method of solving the Stefan-Maxwell equations is also introduced, which offers the possibility of a near real-time computation of λ and Xo2, with application, for example, to bench instruments. Extensive results are presented showing how the interpretation of the UEGO response may be compromised by uncertainties. These uncertainties may relate not only to the sensor itself, such as temperature, pressure and mean pore diameter, but also the chemistry model. [ABSTRACT FROM AUTHOR]

Magnetic induction tomography (MIT) is an imaging technique based on the measurement of the magnetic field perturbation due to eddy currents induced in conducting objects exposed to an external magnetic excitation field. In MIT, current-carrying coils are used to induce eddy currents in the object and the induced voltages are sensed with the receiving coils. When the driving frequency is significantly high relative to the frequency range in which MIT normally operates, metallic targets with high conductivity between the coils can be treated as perfect electric conductors (PEC) with negligible errors. In this scenario, the penetration depth of the magnetic field into the target is extremely small and the traditional versions of the finite element method (FEM) are not efficient for the calculation of the sensitivity and the forward problem due to the requirement for large number of elements to reach an acceptable computational precision. Other versions of FEMs (such as Hp-FEM), which have higher discretization efficiency and more advanced elements to satisfy the requirement, are exceptions. Nevertheless, the discretization regions for all FEMs have to extend beyond the region that contains the conducting object and volumetric elements are generally required for 3D problems. In contrast, the boundary element method (BEM) based on integral formulations becomes an effective way to analyze this kind of scattering problem since meshes are only required on the surface of the object. By point collocation, the boundary integral equations can be transformed into linear equations. Numerical methods are used to solve the linear equations and the solution of the original integral equations can be obtained. In this paper, we compute four typical sensitivity maps between the coil pairs in high-frequency MIT system due to a PEC perturbation. The magnetic scalar potential is used to improve the efficiency. Five PEC objects of different shapes are used in the simulation. The results have been compared with the experimental results and that obtained from the H · H formulations. We can know that the sensitivity maps derived by BEM are in good agreement with that from the experiment and theoretical solution. Overall, BEM is an effective way to calculate the sensitivity distributions of a high-frequency MIT system. [ABSTRACT FROM AUTHOR]

An on-line inspection system for stainless steel strips has been developed on an annealing and pickling line at the Acciai Speciali Terni S.p.A. steel mill. Besides a machine vision apparatus, the system contextually exploits a magnetic imaging system designed and realized for the specific application. The main goal of the research is represented by the fusion of the information provided by the two apparatuses that can improve the detection and classification tasks by enlarging the set of detectable defects. In this paper, the development, the calibration and the characteristics of the magnetic imaging apparatus are detailed and experimental results obtained both in laboratory and in situ are reported. A comparative analysis of the performances of the two devices is also reported based on preliminary results and some conclusions and perspectives are drawn. [ABSTRACT FROM AUTHOR]

A look at the history of this journal reveals some significant trends in the evolution of measurement science and shows how key papers published in the journal have influenced these developments. [ABSTRACT FROM AUTHOR]

In magnetic induction tomography (MIT), an array of excitation coils is typically used to apply time-varying magnetic fields to induce eddy currents in the material to be studied. The magnetic fields from the eddy currents are then detected by an array of sensing coils to form an image of passive electromagnetic properties (i.e. conductivity, permittivity and permeability). Increasing the number of transmitters and receivers can provide a better image quality at the expense of a larger and more expensive MIT system. Instead of increasing the number of coils, this study investigates the possibility of rotating a single transmit-receive coil to image the electrical properties of the sample, by emulating an array of 200 transmit-receive coils by time-division multiplexing. Engineering details on the electromechanical design and development of a rotating MIT system are presented. The experimental results indicate that representative images of conductive samples can be obtained at 5 MHz by rotating a single transmit-receive coil. [ABSTRACT FROM AUTHOR]