On the electromagnetic imaging using linear reconstruction techniques

The feasibility of using the presently available linear reconstruction techniques [1], [2] in microwave imaging is examined using a two-dimensional cross section model of the thorax. The electromagnetic field problem describing the wave transmission between a transmitter and a receiver located across the thorax was solved numerically using the method of moments. The obtained results, particularly at higher frequencies, suggest the possible use of the linear reconstruction algorithms in microwave imaging. As expected better resolution is obtained at higher frequencies.     

A solution of electromagnetic imaging using pseudoinverse transformation

A new reconstruction algorithm as applied to electromagnetic imaging is proposed. It is aimed at reconstructing constitutive parameter distributions of infinitely long dielectric cylinders with arbitrary cross section, from the scattered fields they produce. Computer simulations show that the implementation of the pseudoinverse transformation in this algorithm yields excellent results for thin cylinders irradiated by transverse magnetic (TM) waves, even in the presence of realistic uncertainties in the scattered field measurements.     

An intuitive electromagnetic approach to MIMO communication systems

The aim of this paper is to introduce MIMO propagation in a simple and intuitive way, which exalts the role of the laws of electromagnetism in the channel capacity of space-time communication systems. The approach takes advantage of mode expansions in waveguides and spherical-harmonics expansions in free space to clarify the physical limitations in the channel capacity of MIMO systems.     

An approach to radar imaging using the spherical projection density concept*

High resolution radar imaging is used to perform target signature analysis. In this paper, we present a comparative study of diverse 2- D imaging methods on spherical waves, such as sar processing and its modes, ssar and isar as well as holography, whose mathematical formulation is identical by using the spherical projection density function, although the measurement technique are quite different. We show that azimuth processing is unified, and under some conditions range processing can be unified. In order to reconstruct the fixed target side looking radar image, we have developed an idea based on the transformation of antenna lateral displacement as a function of equivalent angular displacement, and then the 1- D images coherent summation algorithm of classical holography can be used.     

An inchworm mobile robot using electromagnetic linear actuator

The capability of motion is one of the important aspects for a micro-robot to fulfill its given tasks. Micro-autonomous systems usually require large force, large displacement and less power consumption. Among different actuation schemes, electromagnetic actuator shows the benefit in a combination of force, displacement and cost effective control. A bristle-based inchworm mobile robot using a short stroke electromagnetic linear actuator is described in the paper. The main body and movable unit of the robot are joined by using a sealed bellows and the bristle legs are designed so that it can operate both on plane surfaces and in liquid. The actuator designed for the robot is a tubular type linear machine with an overall size of Φ7 × 10 mm. The key dimensions of the actuator were determined through magnetic field analysis to achieve optimum force output and necessary travel stroke within the limited space. The predicted actuation force of the actuator is 20 mN and the stroke length is 1.2 mm. Two working prototypes of the actuator were constructed and the performance tests show the effectiveness of the design. A sensorless control scheme with a novel start-up strategy for the designed actuator was developed based on the robotic system modeling and the analyzed results show the satisfactory performance of the system.     

Fast analysis of 2-D electromagnetic crystal devices using a periodic Green function approach

A novel integral equation-based method for simulating wave propagation in two-dimensional (2-D) electromagnetic crystal (EC) devices is presented. A small number of irregular defects aside, the targeted devices are obtained by removing cylinders from infinite, doubly periodic, and defectless electromagnetic crystals. Integral equations in terms of equivalent currents that reside on the surfaces of the voids left by the removed cylinders are constructed by using Green functions innate to the defectless electromagnetic crystal. The sparse system of equations that results upon discretizing these integral equations is solved efficiently by a multifrontal method. The scheme is ideally suited to extract electromagnetic crystal device S parameters as it permits imposing modal excitations and exact absorbing boundary conditions. The scheme is applied to the analysis of two multiplexer-demultiplexer devices, a filter, and a bended EC waveguide, thereby demonstrating its versatility and computational efficiency.     

Microwave imaging using the finite-element method and a sensitivity analysis approach

A method for reconstructing the constitutive parameters of two-dimensional (2-D) penetrable scatterers from scattered field measurements is presented. This method is based on the differential formulation of the forward scattering problem, which is solved by applying the finite-element method (FEM). Given a set of scattered field measurements, the objective is to minimize a cost function which consists of two terms. The first is the standard error term, which is related to the measurements and their estimates, while the second term, which is related to the Tikhonov regularization, is used to heal the ill posedness of the inverse problem. The iterative Polak-Ribière nonlinear conjugate gradient algorithm is applied to the minimization of the cost function. During each iteration of the algorithm, the direction of correction is computed by using a sensitivity analysis approach, which is carried out by an elaborate finite-element scheme. The adoption of the finite-element method results in sparse systems of equations, while the computational burden is further reduced by applying the adjoint state vector methodology. Finally, a microwave medical imaging application, which is related to the detection of proliferated bone marrow, is examined, while the robustness of the proposed technique in the presence of noise and for different regularization levels is investigated.     

An optimization approach to time-domain electromagnetic inverseproblem for a stratified dispersive and dissipative slab

The one-dimensional electromagnetic inverse problem for a stratified dispersive and dissipative slab is considered in the time domain. An optimization approach is used to reconstruct the parameters using the measured reflected field and/or transmitted field. Wave-splitting is used, and an explicit expression for the gradient is derived by introducing dual functions. The reconstruction algorithm is tested with both synthetic and experimental data     

An All-MOS Analog-to-Digital converter using a constant slope approach

A novel circuit configuration for indirect or slope-type analog-to-digital converters (ADC's) is described. Due to the simplicity of the analog requirements, this technique lends itself to implementation in a single low-cost single-polarity MOS chip. A breadboard version has been operated with 11 bit accuracy at a sample rate of 20/s.     

An enhanced Gauss-Newton inversion algorithm using a dual-optimal grid approach

We developed two algorithms for solving the nonlinear electromagnetic inversion problem in the Earth. To achieve a balance between efficiency and robustness, both algorithms employ the Gauss-Newton inversion method. Moreover, to speed up the inversion's computational time, the so-called optimal grid technique is utilized. The first algorithm uses a forward solver with a very coarse optimal grid to calculate the Jacobian matrix. Hence, in this scheme we employ two different sets of optimal grids. One set is used to compute the data mismatch to be minimized and the other set is used to construct the Jacobian matrix. In the second approach we use a fixed-point iteration process where the inverse kernel is approximated on a coarse optimal grid that does not significantly compromise accuracy. The advantage of these optimal grids is that they considerably reduce the computation time without compromising accuracy. Numerical examples for two-dimensional axially symmetric and three-dimensional anisotropic configurations are used to demonstrate the advantage of using both algorithms over the standard Gauss-Newton inversion method.     

Current controller design for an electromagnetic actuator using anonline parameter optimization approach

The design is presented of a digital proportional-plus-integral current controller for a nonlinear electromagnetic actuator using an online parameter optimization approach. The minimization algorithm of J.A. Nelder and R. Mead (1965) is used to calculate the proportional and integral controller gain such that a design objective function is minimized. At each iteration of the minimization procedure, the objective function is calculated by sampling the actual current and voltage signals of the actuator     

An effective wavelet matrix transform approach for efficientsolutions of electromagnetic integral equations

An effective numerical method based on wavelet matrix transforms for efficient solution of electromagnetic (EM) integral equations is proposed. Using the wavelet matrix transform produces highly sparse moment matrices which can be solved efficiently. A fast construction method for various orthonormal or nonorthonormal wavelet basis matrices is also given. It has been found that using nonsimilarity wavelet matrix transforms such as nonsimilarity nonorthonormal cardinal spline wavelet (NSNCSW) transform, one can obtain a much higher compression rate and much better accuracy of the approximate solutions than using similarity wavelet transforms such as Daubechies' (1992) orthonormal wavelet (DOW) transform. Numerical examples are given to show the validity and effectiveness of the method     

Planetary exploration using a small electromagnetic sensor

A prototype broadband electromagnetic (EM) sensor, GEM-5, has been built and tested as a possible new probe for the future Mars rover to seek an ice-bonded layer at a given depth below the Martian surface. The sensor, with a vertical coaxial coil configuration, will measure the terrain resistivity and susceptibility to determine lateral variations in resistivity and magnetic susceptibility. The lateral variations will indicate regions of resistivity/susceptibility anomalies that may contain ice or water at depth. The forward solution for the sensor geometry over a layered formation and inverse algorithms to convert the EM data into the apparent susceptibility and resistivity are developed to investigate the ability of the sensor in detecting and resolving a buried (wet) ice layer in Mars-like geologic formations. Based on the simulated study, we find that the prototype sensor design should be able to resolve the lateral variations in resistivity/susceptibility under conditions of the Martian subsurface.     

Transform approach to electromagnetic scattering

In this paper, we present a comprehensive review of the Fourier transform technique as applied to the problem of high-frequency scattering and introduce the concepts of the spectral theory of diffraction (STD). In contrast to the more commonly employed ray-optical method for high-frequency scattering, viz. the geometrical theory of diffraction (GTD), the STD approach interprets the scattered field as the spectrum, or the Fourier transform of the induced current on the scatterer. Such an interpretation offers several important advantages: uniform nature of representation, capacity to improve and extend the ray-optical formulas in a systematic manner, and convenient accuracy tests for the results. Methods for combining integral equation methods with the Galerkin procedure and asymptotic techniques in the transform domain are described, and representative examples illustrating the application of the spectral approach are included.     

New defect detection approach using near electromagnetic field probing of high density PCBAs

With the density increase of today's printed circuit board assemblies (PCBA), the electronic fault detection methods reached their limits. In the same time the requirements of high reliability and robustness are greater. Industrials are obliged to find better-adapted test methods. Current test methods must be rethought to include a large panel of physical phenomena that can be used to detect electrical defects of components, absence, wrong value, and shorts at component level on the board under test (BUT).We will present the possibility of using electromagnetic signature to diagnose faulty components contactlessly. The technique consists in using magnetic field probes, which detect the field distribution over powered sensitive components. Reference EM signatures are extracted from a fault-free circuit, which will be compared to those extracted from a sample PCBA in which we introduced a component level defect by removing or changing the value of critical components to evaluate the relevance of the method.     

A moment-method-based approach to electromagnetic scatteringcomputation by a perturbation technique

A modified version of the moment-method is proposed for scattering computations. The approach is based on the assumption that the bodies under test partially differ from known standard models. The numerical computation is developed only for the body parts that exhibit physical and geometrical differences. This can be very useful in several cases, for example, when many partially different scatterers have to be analyzed or when a scatterer may assume different spatial positions whereas a part of the scatterer stays unchanged. The proposed method is described and some considerations are made concerning the offered computational advantages     

NEW APPROACH TO ELECTROMAGNETIC SCATTERING FROM A CONDUCTIVE DISC-RING STRUCTURE

A one-dimensional modified integral equation for electromagnetic scattering from a metallic concentric disc-ring structure is derived by using Fourier expansion and separating the contribution of charge density from conventional EFIE. In the case of normal incidence, the derived equation is very simple and easy to be solved numerically regardless of the electric dimension of the structure. The comparison between the data of RCS of a disc by using this method and analytical method shows the effectiveness of this approach. Numerical results are also given for current distributions and the scattered fields for a disc-ring structure.     

Inertial vibration control using a shunted electromagnetic transducer

Inertial drives and passive tuned-mass dampers utilize a suspended mass to reduce the vibration experienced by a host structure. Active vibration control systems typically include a voice coil type electromagnetic actuator to develop the required reaction forces. In this paper, the technique of sensorless active shunt control is applied to inertial vibration absorption. An electrical impedance is designed and connected to an electromagnetic coil with a view to minimizing structural vibration. Standard optimal control tools can be applied to design the required shunt impedance. This technique requires no additional feedback sensors. Vibration in an experimental structure is heavily attenuated by the application of an active shunt impedance.     

An electromagnetic analysis of a cylindrical homogeneous lens

Radially symmetric lenses are ideally suited for use as multiple-beam antennas. In recent years, several investigations using a geometrical optics approach have indicated that simple lens structures such as homogeneous lenses might be suitable antennas. This paper presents an electromagnetic analysis of a cylindrical homogeneous lens as well as examining in detail the effect upon performance of parameter variations such that engineering and design evaluations can be made. It is shown that a lens with a relative dielectric constant of 3.5 gives the best performance. With the use of simple matching techniques, overall performance comparable to the more complex Luneburg lens is obtained.     

A new formulation of electromagnetic wave scattering using an on-surface radiation boundary condition approach

A new formulation of electromagnetic wave scattering by convex, two-dimensional conducting bodies is reported. This formulation, called the on-surface radiation condition (OSRC) approach, is based upon an expansion of the radiation condition applied directly on the surface of a scatterer. Past approaches involved applying a radiation condition at some distance from the scatterer in order to achieve a nearly reflection-free truncation of a finite-difference time-domain lattice. However, it is now shown that application of a suitable radiation condition directly on the surface of a convex conducting scatterer can lead to substantial simplification of the frequency-domain integral equation for the scattered field, which is reduced to just a line integral. For the transverse magnetic (TM) case, the integrand is known explicitly. For the transverse electric (TE) case, the integrand can be easily constructed by solving an ordinary differential equation around the scatterer surface contour. Examples are provided which show that OSRC yields computed near and far fields which approach the exact results for canonical shapes such as the circular cylinder, square cylinder, and strip. Electrical sizes for the examples areka = 5andka = 10. The new OSRC formulation of scattering may present a useful alternative to present integral equation and uniform high-frequency approaches for convex cylinders larger thanka = 1. Structures with edges or corners can also be analyzed, although more work is needed to incorporate the physics of singular currents at these discontinuities. Convex dielectric structures can also be treated using OSRC. These will be the subject of a forthcoming paper.