Mohammad Asefi

A 3-D Dual-Polarized Near-Field Microwave Imaging System

A novel 3-D dual-polarized microwave imaging system based on the modulated scattering technique (MST) is presented. The system collects the magnitude and phase of the scattered field using 120 MST probes and 12 transmitter/collector antennas distributed around an object-of-interest in the near-field region. The 12 antennas form a middle circumferential layer while the printed MST probes are arranged on three circumferential layers including the middle layer. The antennas are linearly polarized double-layer Vivaldi antennas, each fixed inside its own cylindrical conducting cavity and slanted with respect to the vertical axis of the imaging chamber. The MST probes are etched on both sides of a thin substrate and loaded with five evenly distributed p-i-n diodes along their length. These are positioned vertically and horizontally so that the z- and φ-components of the electric field is measured. Field data are collected using MST, calibrated, and then inverted using a multiplicatively regularized finite-element contrast source inversion algorithm. The system performance is evaluated by collecting and inverting data from different 3-D targets.

Grain bin monitoring via electromagnetic imaging

Feasibility of using electromagnetic waves to monitor stored grain was investigated.Successful synthetic analysis of a 2500 bushels grain bin was performed.A measurement device to find the electrical properties of bulk grain was built and successfully tested.A lab-scale measurement system capable of detecting spoilage region within grain was built and successfully tested.Results show the feasibility of applying EMI to grain bin monitoring. Stored grain monitoring is an important post-harvest stage of the food production chain. Grains are usually stored in large metal containers referred to as bins or silos. During storage, there is a possibility for grain to spoil and become unusable. Therefore, monitoring of grain bins is essential to detect conditions leading to spoilage within the bin. Most current grain bin monitoring techniques lack sensitivity as they require conditions leading to spoilage to surpass a certain limit before detection is possible, and consequently a large amount of stored grain is lost during monitored storage. This paper presents the advances in developing a novel grain-monitoring technique using electromagnetic imaging, a modality that can provide global, quantitative images of grain properties throughout the bin. Side-mounted antennas illuminate the contents of the bin and a set of receivers measures the electromagnetic energy within the bin at discrete locations. Using these measurements an optimization algorithm attempts to reconstruct the contents of the bin - herein a finite-element contrast source inversion (FEM-CSI) algorithm was used. The result is a global map of the electrical properties of the grain throughout the bin. In this work we first present a synthetic validation of the proposed method for a model of a full scale hopper bin using simulations to produce the electromagnetic field data. Next, a scaled experimental system was used to collect data from grain that contained regions of induced contamination. This data was used to produce images that show the applicability of the method in practice. Results suggest that this technology has potential to provide farmers with a reliable and robust method to remotely monitor stored grain, preserving stored food resources and increasing their revenue.

Surface-Current Measurements as Data for Electromagnetic Imaging Within Metallic Enclosures

A 3-D microwave imaging method within metallic enclosures is investigated and improved. This method uses the components of the surface-current vector at receiver points on the enclosure wall as data. At the metallic wall, the normal component of the magnetic field as well as the tangential components of the electric field is negligible, whereas the vectorial surface current, which is directly related to the tangential components of the magnetic field, is dominant. After presenting the results of a numerical investigation based on synthetic data, the method is validated using an experimental system comprised of 24 co-resident shielded, coaxial half-loop antennas, distributed in four layers, within a cylindrical metallic enclosure. These antennas are used in receiver-transmitter pairs to introduce an electromagnetic field into the chamber and collect the magnetic field at the receiver points. The measured data are used as input to a multiplicatively regularized finite-element contrast source inversion algorithm. Due to their relatively small size and minimal protrusion into the chamber, these antennas minimally perturb the field distribution inside the chamber and thereby allow the use of a simple numerical inversion model, which does not need to account for the passive antennas. These attributes are especially useful for large computationally intensive industrial applications. The experimental system described herein is a laboratory-scale prototype for a stored-grain imaging application where metallic silos are utilized.

Microwave Imaging Using Normal Electric-Field Components Inside Metallic Resonant Chambers

A novel 3-D microwave imaging approach performed within a resonant air-filled metallic chamber is introduced and investigated. The new method utilizes the measurements of normal electric-field components at discrete points along the metallic chamber’s wall—near the chamber-wall boundary, the normal-field components are dominant, while the tangential components vanish. The inversion algorithm fully incorporates the resonant features of the low-loss chamber. A numerical study is used to quantify the imaging performance of using this technique compared with the traditional unbounded domain imaging. An experimental system is presented where the electric field is collected using 24 antennas distributed in three circumferential layers around an object of interest located inside the circular-cylindrical metallic chamber. For collecting the normal component of the field, two types of linearly polarized antennas are investigated:

Analysis of a 3D microwave imaging system

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Use of Field-Perturbing Elements to Increase Nonredundant Data for Microwave Imaging Systems

monopole antennas and specially designed reconfigurable antennas (RAs), both projecting perpendicularly out from the chamber walls into the enclosure. The measured data are calibrated and then inverted using a multiplicatively regularized finite-element contrast source inversion algorithm. Using 3-D reconstructions of simple dielectric targets, it is shown that utilizing the RAs improves imaging performance due to a reduction in the modeling error introduced in the inversion algorithm.

Electromagnetic imaging inside metallic enclosures using the normal boundary field components

In this paper a full vectorial 3D microwave imaging system consisting of three layers is analyzed numerically. The system is capable of acquiring scattered fields in two different polarizations using modulated scattering technique. A comparison between the full system simulation and measurement data is also presented.

Industrial scale electromagnetic grain bin monitoring

Field-perturbing elements (FPEs) are introduced for microwave imaging. These elements affect the imaging performance by increasing the amount of nonredundant data. Although this technique can be implemented in nonmetallic chambers, it is especially effective inside metallic enclosures where small perturbations can change the interrogating fields significantly. Results of simulations and a numerical investigation based on synthetic data are presented. The method is validated using an experimental system comprised of 24 coresident radially oriented monopoles that collect the normal component of the electric field on the inside surface of the enclosure. The measured data are used as input to a finite-element contrast source inversion algorithm. To investigate the effectiveness of the approach, a second experimental example is presented where a simplistic breast phantom with a tumor inclusion is imaged inside a smaller cylindrical chamber with 18 radially oriented monopoles and a single FPE. Because FPEs are easy to manufacture and are low costs, they can reduce the cost of an imaging system significantly by reducing the number of required RF ports, as well as reducing the system complexity and modeling error.

A faceted magnetic field probe resonant chamber for 3D breast MWI: A synthetic study

Summary form only given. In recent years, electromagnetic imaging inside chambers with metallic walls has been investigated both theoretically and experimentally. Potential advantages in utilizing these chambers include: (i) shielding the inside of the imaging chamber from outside noise, (ii) better signal-to-noise ratio that may improve the resolution of the imaging modality, (iii) easier system modeling in comparison to open-boundary problems, (iv) the ability to use a lossless matching medium which means more energy is delivered to the target.First, an overview of the research conducted in the area of electromagnetic imaging inside metallic enclosures at the University of Manitoba is presented. This includes two-dimensional transverse magnetic (TM) and transverse electric (TE) microwave tomography (MWT) inside metallic chambers with different boundary shapes (A. Zakaria et al., IEEE TAP, 59, 2012); MWT inside rotating conductive chambers (P. Mojabi and J. LoVetri, IEEE TAP, 59, 2011); MWT using different metallic enclosures simultaneously; and methods and algorithms used to ease studying and performing electromagnetic imaging inside metallic enclosures (A. Zakaria et al., Inverse Problems, 26, 2010). Next, a novel approach for electromagnetic imaging in metallic enclosures is introduced and investigated. The new method utilizes normal field component measurements near the metallic chamber walls to perform imaging; near the chamber boundary the normal electric field components are dominant while the the tangential components vanish. The study is performed both synthetically and experimentally. Using an in-house parallelized full-vectorial electromagnetic finite-element solver, various chamber configurations are modeled and used to collect synthetic datasets (A. Zakaria et al., PIER, 147, 2013). The data are inverted using the finite-element contrast source inversion (FEMCSI) method. The goal of the synthetic study is to understand the effects of frequency-selection, number of observation points, and transceiver modeling on the imaging results. Experimentally, normal electric field data are collected from two configurations. The first setup consists of an air-filled circular metallic chamber with an open-top; within the chamber 24 antennas distributed in three layers are used to measure the normal electric field component near the chamber walls. In the second experiment, electromagnetic imaging inside a grain-bin storage facility is performed. The bin is an enclosed metallic chamber of 4.7 m radius and 7.5 m height. The data are collected using 12 monopole antennas normal to the bin walls.

Grain bin monitoring via microwave imaging

Presentation of first industrial EM imaging system for grain storage bins using wheat.Detection and 3D imaging of realistic grain spoilage spots.Presentation of antennas which withstand the forces generated during bin loading/unloading. We present an Electromagnetic Imaging (EMI) system capable of detecting spoiled grain regions inside a large-scale grain storage bin. Stored grain represents significant economic and nutritional value to humankind, but despite this value, storage losses are common (estimated to vary from 2% to 30%). While there are many mechanisms that cause storage losses, virtually all of them involve higher temperature and/or moisture content of the stored grain. Increases in temperature and/or moisture both raise the complex permittivity of the grain. Our EMI system creates a 3D image of the complex permittivity through 24 antennas mounted on the side of the bin operating at a frequency of 93MHz, combined with a 3D Finite-Element inversion/imaging code.The antennas are designed to have both the desired electrical characteristics, as well as withstand the significant forces caused by the loading and unloading of the grain. Results with 55 tonnes of hard-red winter wheat in a 2500 bushel (80 tonne) bin show that our system is capable of detecting a small spoilage region (0.24% of total grain volume, 2/5 of a wavelength in size) inside dry bulk grain. The 3D EMI system is a viable method of detecting spoiled grain in industrial grain storage facilities.