Detection Of Metallic Objects Research Articles

Metal Object Detection in a Wireless Power Transfer System Based on Double-Layer Symmetric Sensing Coils

The intrusion of a metal object into a wireless charging system could cause safety issues, such as temperature rise and even combustion. This paper proposes a double-layer symmetric sensing coil design for metal object detection (MOD) of wireless electric vehicle charging to prevent the detection blind area. First, the magnetic effect and the eddy current effect of a metal object in high-frequency magnetic fields are analyzed. Second, the principle of the proposed double-layer symmetric sensing coils is detailed. Finally, the finite element simulation is implemented in order to test the performance of the proposed sensing coil design. The simulation results show that the proposed double-layer symmetric sensing coils can effectively detect the presence and position of the metal object simultaneously.

Cavity resonator sensor and temporal signals analysis for object detection in granular flows

We present a microwave measurement system for detection of objects in granular media flowing through pipes. The system comprises a resonant cavity sensor operated in several high-order modes that yield spatially periodic sensitivities, and a microwave transceiver for fast S-parameter measurements. The temporal signatures from the passage of an object and the temporal correlations of the noise caused by flow inhomogeneities are incorporated in a matched filter detector, which is shown to perform uniformly better than two reference power detectors. The detection capabilities of the system are evaluated based on many object passage events in granulates flowing down a vertical pipe. We study the detection of dielectric and metal objects in microcrystalline cellulose pellets and glass beads, where the minimum detectable object size is 2–4 mm for metal objects and 4–6 mm for dielectric objects at a mass flow rate of 100 kg/h.

Improved Capacitive Proximity Detection for Conductive Objects through Target Profile Estimation

The accuracy of a capacitive proximity sensor is affected by various factors, including the geometry and composition of the nearby object. The quantitative regression models that are used to seek out the relationship between the measured capacitances and distances to objects are highly dependent on the geometrical properties of the objects. Consequently, the application of capacitive proximity sensors has been mainly limited to detection of objects rather than estimation of distances to them. This paper presents a capacitive proximity sensing system for the detection of metallic objects with improved accuracy based on target profile estimation. The presented approach alleviates large errors in distance estimation by implementing a classifier to recognize the surface profiles before using a suitable regression model to estimate the distance. The sensing system features an electrode matrix that is configured to sweep a series of inner-connection patterns and produce features for profile classification. The performance of the sensing modalities is experimentally assessed with an industrial robot. Two-term exponential regression models provide a high degree of fittings for an object whose shape is known. Recognizing the shape of the object improved the regression models and reduced the close-distance measurement error by a factor of five compared to methods that did not take the geometry into account. The breakthroughs made through this work will make capacitive sensing a viable low-cost alternative to existing technologies for proximity detection in robotics and other fields.

Symmetric Sensing Coil Design for the Blind-Zone Free Metal Object Detection of a Stationary Wireless Electric Vehicles Charger

In this article, a simple and cost-effective symmetric sensing coil set to be used for the metal object detection (MOD) in a stationary wireless electric vehicle charger is newly proposed. Compared to the conventional nonoverlapping sensing coil using the magnetic field change detection method, the proposed symmetric sensing coil set design offers the considerable advantage of eliminating all blind zones, i.e., regions where metal is undetectable in the conventional method. Moreover, by reducing the number of layers to one, the fabrication of the proposed sensing coil is easier and more cost effective. The design of the proposed sensing coil is optimized for highly sensitive MOD based on a sensitivity equation and on the finite element method simulations. An experiment conducted with a metal object 40 × 40 mm2 in size and a 20 A transmitting (Tx) coil current verifies the reliability and the blind-zone elimination feature of the proposed sensing coil.

A review of foreign object detection (FOD) for inductive power transfer systems

Abstract Wireless power transfer (WPT) has a promising application prospect due to its convenience and feasibility to special occasions. Inductive power transfer (IPT) uses the magnetic field for power transfer and it is currently the most popular and mature WPT technology. However, the strong magnetic field will heat up metal objects falling in the charging area due to eddy currents generated in the objects. It can also harm animals or toddlers staying in the charging area. To deal with these issues, foreign object detection (FOD), including metal object detection (MOD) and living object detection (LOD), should be developed for the safe operation of IPT systems. FOD can be divided into system parameter detection methods, wave-based detection methods, and field-based detection methods. System parameter detection methods are normally used in low-power systems for MOD. Wave-based detection methods are suitable for high-power applications, for both MOD and LOD. Field-based detection methods work for both high-power and low-power applications, for both MOD (in an inductive way) and LOD (in a capacitive way). This paper reviews and summarizes the state-of-the-art development of FOD technology in IPT systems.

A time-reversal imaging system for buried objects in layered media using complex images Green’s functions

In this paper, the three-dimensional problem of detection and localization of buried metallic objects inside a multilayered medium is studied. Decomposition of the time-reversal operator (DORT) is used as the inversion algorithm in the imaging method. In this approach, one has to analyze the time-reversal (TR) operator obtained by using the multistatic data matrix (MDM) of a TR array. To attain MDM in the forward problem, full-wave simulation results from a commercial simulation software are used. For synthetic retransmission of signals in this inverse problem, determination of the appropriate medium Green’s function is required. Here, for the first time the complex images technique is employed in calculating the required Green’s functions. The complex images technique gives a closed-form Green's function, instead of evaluating time-consuming numerical integrals resulting in a fast and accurate Green’s function calculation. Using singular value decomposition of the MDM at a frequency range of 1–3 GHz and synthesized DORT method, images are constructed. Numerical results show the accuracy of the proposed approach in the detection and localization of buried objects. Furthermore, noise effect and sensitivity analysis on layers’ permittivity values are studied. The results confirm the robustness of the method against noise and uncertain layers parameters.

Self-Inductance-Based Metal Object Detection With Mistuned Resonant Circuits and Nullifying Induced Voltage for Wireless EV Chargers

In this paper, a metal object detection (MOD) system, a kind of foreign object detection (FOD), which is based on mistuned resonant circuits and utilizes the variation of self-inductance of a sensing pattern, is newly proposed for wireless electric vehicle (EV) chargers. The sensing pattern that consists of multiple loop coil sets is mounted on the transmitting (Tx) pad of an EV charger, where a loop coil set has two coils connected in series with the opposite polarity to cancel out the induced voltage generated by the Tx coil. Variation of self-inductance of the loop coil set is detected by a parallel-resonant circuit, driven by a current source and operating at near 1 MHz, in order to enhance the resolution of the proposed MOD system. To increase the detection sensitivity of the proposed MOD system, instead of an exact resonant frequency, a mistuned operating frequency near the –3 dB point is utilized for the parallel-resonant circuit. In this way, the proposed MOD system can detect very small metal objects regardless of their position and orientation on the Tx coil without any blind zone. Through simulations and experiments, it is found that the proposed MOD system detects not only horizontal but also standing upright metal objects. A prototype MOD system, operating at 85 kHz to satisfy the standard J2954, was fabricated to verify its feasibility. The results showed that output voltage change of the proposed MOD system becomes 22.7% for a piece of the aluminum foil of 3 × 3 cm2 and 40.9% for 100 Korean Won coin.

Dual-Purpose Nonoverlapping Coil Sets as Metal Object and Vehicle Position Detections for Wireless Stationary EV Chargers

For commercialization of wireless stationary electric vehicles (EV) chargers, metal object detection (MOD) on a power supply coil and detection of position (DoP) of EVs are needed. In this paper, dual-purpose nonoverlapping coil sets for both MOD and DoP, which detect a variation of magnetic flux on the power supply coil, are newly proposed, where the proposed MOD and DoP methods make no contribution to any power losses. The existence of metal objects on the power supply coil is determined by an induced voltage difference of the nonoverlapping coil sets, whereas the position of the EV is determined by an induced voltage of the nonoverlapping coil sets. A sensing circuit, which has a variable resistor that is different from the conventional overlapping coil for MOD, can make the induced voltage difference zero even when the magnetic flux distribution is distorted by moving the pick-up coil. The proposed nonoverlapping coil sets with the sensing circuit have been demonstrated by simulations and experiments. When metallic coins and aluminum sheets are located on the power supply coil, the induced voltage difference of the coil sets, which is ideally zero without metal objects, significantly increases to 62.8 and 450 mV, respectively, which is more than ten times the value without metal objects throughout experiments. In addition, when the pick-up coil approaches the power supply coil, induced voltage of each coil set increased roughly 1.6 times at 10 cm air gap.

Application of spectral analysis for differentiation between metals using signals from eddy-current transducers

The authors theoretically and experimentally substantiated the use of the spectral method for processing a signal of the vortex-current metal detector for dichotomous differentiation between metals. Results of experimental research that prove the possibility of using spectral analysis for differentiation between metals were presented. The vortex-current method for detection of hidden metal objects was analyzed. It was indicated that amplitude of output VCD signal is determined by electric conductivity of material of a hidden object and its magnetic permeability. It was shown that the spectral density of a signal can be an informative feature. The authors designed and fabricated a mockup of the vortex-current device, the special features of which include modularity, which, if necessary, makes it possible to replace quickly each of the modules. The developed algorithm of normalization of signals allows an operator to choose freely a scan mode and compare correctly VCD signals with reference signals. Research results show that it is possible to distinguish easily between the spectra of ferrous metals and those of non-ferrous metals. Spectral methods can be applied both for dichotomous analysis of hidden metal objects and for analysis of the type of metal in the subgroup of non-ferrous metals under condition of using highly sensitive spectroanalyzers with measurement error not exceeding 1 %. It was shown that advantage of the spectral method for analysis of signals of vortex-current transducer is identification of hidden objects by type of metal. The use of spectral methods for detection of hidden metals offers a new property – distant analysis of composition of detected metal objects.

A Three-Dimensional Surface Imaging Method Using THz Dual-Frequency Interferometry

A novel 3-D surface imaging method based on terahertz (THz) dual-frequency interferometry is proposed for the detection and imaging of metal objects. The interferometer is formed first in the range direction with two azimuth–elevation images at different THz frequencies. Then, the 3-D surface image can be obtained by interference processing of two azimuth–elevation imaging results. Compared with the traditional wideband 3-D imaging, the proposed method can avoid the nonlinear error caused by the signal nonlinearity. In addition, the image registration for the interference processing is evitable, when the frequency difference satisfies certain conditions. Based on the electromagnetic calculation data, the 3-D surface imaging results of both rough cone and rough cube validate the effectiveness of the proposed method. Furthermore, the reconstruction error increases slightly with the increasing of the error of frequency difference, which indicates that the proposed method has a good tolerance with respect to frequency instability.

Advanced Instrumentation for Polyharmonic Metal Detectors

This paper describes the application of polyharmonic sinc signals in devices for metal object detection. A basic problem of conventional eddy current metal detectors (induction devices in general) lies in their limited possibilities of discrimination or identification of detected objects. To better characterize the detected object, the excitation signal and the following signal processing in the detector should be done. The application of polyharmonic excitation signal and its processing could bring an opportunity to improve the determination ability. This paper is focused on the polyharmonic sinc signal. Spheres of different diameters and from different ferrous and non-ferrous materials were used as specimens. Experimental measurements were processed in frequency-domain amplitude, and phase spectra were calculated and presented. As part of the work, classifications of ferrous and non-ferrous materials were done based on measured data as well as the classification of individual ferrous, including non-ferrous materials and estimation of the size of the classified object. Support vector classifier was primarily used for data classification.

Detection of deep buried metal objects with the UltraTEM

The UltraTEM Deep metal detection system combines a rugged, fast-switching transmitter with a powerful custom designed generator and one to ten three-component receiver coils that collect time-domain electromagnetic induction data across a wide time-range. The roving receiver is operated inside a fixed loop of copper cable, typically 30-150 m long and 5-50 m wide. The large size of the loop results in a slow falloff in primary field with depth and effective excitation of deep buried unexploded ordnance as well as other metal objects such as Ground Engaging Tools (GET). We describe the results of an extensive trial of the UltraTEM system at a magnetically challenging site in Laos. The UltraTEM was able to detect all seeded items buried at depths down to 5 m across a wide range of site conditions. Subsequent work with the system at a different site demonstrated an ability to detect GET to depths of at least 3.0 m in a magnetite stockpile.

2-D Imaging of Concealed Objects using Circular Co-Arrays at MM Wave Frequencies: Simulation and Experimental Results

Simulation and experimental results at 35 GHz, for a single target of unit reflectivity, a concrete pillar and a gun shaped object placed in the central region of 2-D scene , have been presented using circularly co-located transmit and receive co-arrays. Simulation program has been written in MATLAB to reconstruct the raw image using an improvised algorithm. Simulation and experimental results confirm the method of reconstruction of image. Reconstruction of a Gun-shaped object has also been shown with help of simulation and experimental results. Reconstruction of image has been performed using post data acquisition processing on the received scattered data. Experimental results confirm that the method can be used to reconstruct quality image for detection of concealed metallic objects using MM wave frequencies.

System scheme of radiofrequency radar detection for short-range targets using a self-rotating antenna architecture at millimeter wave

A new scheme of a pulse-Doppler radar for the effective detection of metal objects in a short range of millimeter waves is presented. An omnidirectional transmitting antenna (TXA) is used to provide full angular coverage while a directional receiving antenna (RXA) collects the coded signals for the determination of range and angle of the arriving signal. A Doppler shift is created by making RXA horizontally rotate along the axis of TXA, which creates a relative velocity between the RXA and the target under detection. It is shown that the system may detect targets as small as 0.2 m 2 with a high resolution.

A magnetic-induction introscope for flaw detection of metal objects

An apparatus for obtaining two-dimensional images of sheet-metal products that is based on a scanning matrix of spiral induction coils has been designed. It was experimentally shown that it is possible to detect flaws and objects that are hidden behind metallic barriers. The scanning area was 37 × 32 cm and the measurement step was 5 mm. Special treatment allows one to restore the distribution of induction currents in conductive objects from the remote measurement of the normal-component distribution of the magnetic induction vector on a plane.

Detection of buried objects using reflected GNSS signals

The use of reflected Global Navigation Satellite System (GNSS) signals for sensing the Earth has been growing rapidly in recent years. This technique is founded on the basic principle of detecting GNSS signals after they have been reflected off the Earth's surface and using them to determine the properties of the reflecting surface remotely. This is the so-called GNSS reflectometry (GNSS-R) technique. In this paper, a new application regarding the detection of metallic buried objects is analyzed and it is validated through several experimental campaigns. Although the penetration depth of GNSS signals into the ground is not optimal and depends on the soil moisture, GNSS signals can likely interact approximately with the first 10 cm of the ground and therefore can be reflected back by any metallic object buried on the first terrain layer. A very light and low-cost GNSS receiver prototype based on a software-defined radio approach was developed. This receiver can be used as a payload on board small drones or unmanned aerial systems to detect metallic objects (mines or other explosive devices). A signal processing tool based on an open-loop GNSS signal acquisition strategy was developed. The results of two experiments which show the possibility of using GNSS-R signals to detect buried metallic objects and to provide an estimate of their dimensions are discussed.

A Partially Supervised Approach for Detection and Classification of Buried Radioactive Metal Targets Using Electromagnetic Induction Data

The analysis of the data obtained from electromagnetic induction (EMI) sensors is one of the most viable tools for the detection of metallic objects buried under soil. The existing detection methods usually consist of sophisticated EM modeling of the source/target geometry to build suitable discriminators. The major technical challenge in this field is the reduction of false alarms with an increase of the detection probability. In this paper, we propose a partially supervised approach to detect buried radioactive targets, i.e., depleted uranium, without sophisticated EM modeling. Using the EMI data obtained by a GEM-3 sensor for a field survey, our proposed algorithm can successfully detect and discriminate the targets from nontarget metals, compared to other unsupervised and supervised approaches.

Method of radar detection and identification of metal and dielectric objects with resonant sizes located in dielectric medium

Mathematical model of a radar detection and identification system to be used for finding different objects buried in the ground or in other dielectric media is considered. In the suggested model scattering characteristics of 3D resonant objects are calculated by solving 2nd kind Fredholm integral equations for equivalent current densities on the object’s surface. Based on the obtained results we derive a method of detection and identification for specific types of objects which incorporates analysis of their natural resonant frequencies. Potentialities of the developed method for detection and identification of different types of mines are estimated.

Enhanced Detection of On-Metal Retro-Reflective Tags in Cluttered Environments Using a Polarimetric Technique

The detection and tracking of targets in highly cluttered environments poses a challenge due to the strong clutter backscatter present in the scene. A design concept based on a planar retro-reflective array to aide in the standoff detection and tracking of large metallic objects in a warehouse setting is presented. While traditional methods to achieve enhanced detection involve an increase in gain of the target, this retro-reflector design seeks to minimize the footprint of the tag by utilizing contrasting polarization properties of the target compared to the surrounding clutter. This allows enhanced detection from a reduced size by maximizing the ratio of target signal to that of the clutter. To accomplish this, a unique series-fed dual polarized antenna is designed and implemented with minimal cross-coupling among elements. The isolation allows for the integration of amplifiers with switchable gains into the design architecture to further reduce size, enhance detection and provide the array with unique polarization properties further contrasting those of the clutter. These retro-reflector tags are provided a unique identification by modulating their radar cross section in a prescribed manner. This compact array is implemented at 26 GHz and design details including measured performance for this cross-polarized retro-reflective tag are presented.

A weak approximation of stochastic time domain electromagnetic response of a conductive permeable sphere

Time domain electromagnetic (TDEM) response of a conductive permeable sphere is a main topic for good understanding and in developing past studies for detection and discrimination of buried metallic objects. Stochastic differential equation model is a valuable tool for stimulating real experience. In this paper, according to an Ito integral, we have obtained a weak approximation of stochastic TDEM response of permeable and nonpermeable sphere. We have used the deterministic solution of TDEM response of conductive permeable sphere by changing one of the variables that have been obtained from the boundary conditions of problem to random variable. By adding white noise to random variable and using stochastic integral, we have displayed stochastic time domain response of conductive permeable sphere. The Ito integral includes a factor that shows infirmity and intensity of noise which has been simply considered constant. Accordingly, we have showed the effect of noise for magnetic and nonmagnetic spheres. Numerical results from step and impulse response of TDEM have shown that the amount of this factor is different for the two types of spheres.