Article

Basic analysis of a metal detector

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Abstract

Metal detectors are extensively used to find undesirable metal objects in processed food. In such a typical metal detector, the coils are coaxially arranged with the transmitting coil in the center and two receiving coils on the sides. The receiving coils are connected to a differential amplifier. When the magnetic field generated in the transmitting coil is disturbed by metal objects, the amplitude and phase of the output voltage of the differential amplifier change, and, thus, the existence of foreign metal pieces is detected. The relationship between the amplitude and phase of the output and the electromagnetic properties of the metal objects, however, has only been discussed experimentally so far. The authors have already developed the SRPM method to simultaneously estimate the electrical and magnetic properties of a spherical sample by vectorially measuring the difference in the impedance of two circular solenoid coils, one with and the other without a sample. An attempt is made to theoretically analyze the properties, such as size, conductivity, and permeability of the metal objects from the output, i.e., amplitude and phase of the metal detector. Based on this method, an equation to estimate the vector voltage induced in the receiving coil by the metal object is derived by using a spherical sample to simplify the analysis.

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... Currently there are several commercial metal detectors [13][14][15][16][17][18][19], mostly operating in low radio frequency (RF) ranges, 5.5 kHz-800 kHz [14]. These metal detectors based on eddy current-induced coil [19] typically consist of two loops, i.e., a transmitting loop to generate an eddy current and a receiving loop to detect the magnetic field of the eddy current [13]. ...
... Currently there are several commercial metal detectors [13][14][15][16][17][18][19], mostly operating in low radio frequency (RF) ranges, 5.5 kHz-800 kHz [14]. These metal detectors based on eddy current-induced coil [19] typically consist of two loops, i.e., a transmitting loop to generate an eddy current and a receiving loop to detect the magnetic field of the eddy current [13]. The swept-frequency excitation method was introduced to improve the sensitivity to detect the eddy current within a wide range of frequencies [15]. ...
... The swept-frequency excitation method was introduced to improve the sensitivity to detect the eddy current within a wide range of frequencies [15]. Several RF frequency metal detectors are also used to detect unnecessary metals in food industry [13,16]; however, they do not classify metal types. An approach based on the air-coil inductance change was implemented to detect ferrite existence in copper (Cu), iron (Fe), and aluminum (Al) [17,18], by comparing the amplitude and phase angle of the received signal. ...
... As we can see, MD application is extensive and consists of finding undesirable (or desirable) metal objects [5]- [10]. The possibility of differentiating between metallic and nonmetallic samples, or even classifying different types of metal, is important for discriminating between potential threat and non-threat objects or targets [6], [11]. For example, where there is a risk of finding active landmines, or even in the food industry where pieces of metal can fall into the food. ...
... The magnetic field generated from the coil -proportional to the number of turns of wire, frequency and the current passing through them -induces an eddy current in the target, causing a magnetic field to be generated from the metal. As a result, the presence of a metal object can be detected as variations in the induced voltage amplitude and phase with respect to a reference signal [2], [3], [6], [7]. ...
Conference Paper
Metal detection is present in more daily life applications than one can imagine. This is due to the fact metal detectors (MD) are simple and effective sensing instruments. These devices are mostly based on Ampere's and Faraday's induction laws and the majority only indicates the presence of a metal object or target, not differentiating its type. In this paper, we present a different approach to metal detection and classification. Our MD consists of a single copper coil and simple electronic components. The method is based on the fact that when a metal object approaches an air core coil, the surrounding magnetic permeability is affected. This changes the coil inductance and, consequently, its inductive reactance. Such a phenomenon can be used to detect and classify ferrous and non-ferrous metal objects based on the phase measurements. Preliminary results show that such an approach allows for a simple, cheap and accessible electronic design of a metal detector capable of detecting and classifying ferrous and non-ferrous materials.
... Metallic objects in the vicinity of the transmitter will change the measured mutual inductance due to ferromagnetic and non-ferromagnetic (eddy-current) effects, as they act to distort shape and strength of the surrounding magnetic field. Detecting these changes in inductance is a commonly found design in very-low-frequency metal detecting circuits [24,25]. Measuring this change in inductive coupling is the foundation of this work. ...
... Clearly the phase angle and permeability of the material have a positive correlation and can be used to determine whether the distorter is ferromagnetic or purely conductive in nature. These results compare favourably with those of [24] which showed that purely conductive materials introduce opposing phase to that of ferrous materials. ...
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... Denna konstruktion bygger på samma princip som ovan, men med ett balanserat spolsystem som mottagare, se figur 5. [5] Mottagarspolarna är motkopplade vilket innebär att det inte genereras någon utsignal från mottagarspolarna, förutsatt att inget metallföremål finns i systemet. [6] När ett metallföremål befinner sig vid någon av spolarna erhålles en skillnad i utsignalen, i fas och/eller amplitud, som är beroende av vad det är för metall och dess utformning samt metallföremålets placering i spolsystemet. Allt i förhållande till matarspolens matningsspänning. ...
... Spolarna distanserades med ett avstånd på 29,9 mm. Detta då denna distansering, vilket är 0,65 gånger spolarnas radie, är optimal.[6] Vidare gäller för denna distansering att mottagarspolarnas utsignal är fasoberoende oavsett spolens diameter, förutsatt att man distansierar med 0,65 gånger spolens radie. ...
... However the method using electromagnetic wave is dominant, and recently the method using superconducting coil has been attempted [8,9]. Theoretical analysis [10,11] and sensitivity analysis [12,13] along with the shape of metal sensor have been performed to explore the sensitivity of electromagnetic metal sensor. On the other hand, there were attempts [14] to enhance visibility of metal piece through signal processing. ...
... Likewise all mutual inductances including 11 and 21 are expressed in a similar way as is shown in 11 = 4 × 10 −7 ln √ 11 12 ...
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... In particular, because the mixing of insects can cause not only sanitary problems but also a sense of fear, it is crucial to consider the implementation of a monitoring and control system in food manufacturing and distribution spaces. Although foreign substance accidents in food are traditionally prevented by using X-rays [2], metal detectors [3], and optical detectors [4] to monitor the mixing of foreign substances, it is very difficult to detect insects which have penetrated into foods, because insects are usually small in size and have a soft body that is unlike metal. Moreover, it is more difficult to detect insects infiltrating the process of distribution or storage, compared to that of food manufacturing. ...
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... 2 systems: balanced coil (greater sensitivity); magnetic field (inspection of aluminum foil) -Limited to metals and conductive materials, sensitive to the FMD size. -Improvement made on operating conditions (vibration, moisture, heat) [25,26] X-ray Commercially available and widely used in several fields, namely the food industry where it is used for FMD (Plastic, glass, bones, wood, organic materials). ImprovedImproveddetection capabilities are possible with multi-modal equipment (absorption, phase contrast, dark field) -Low detection capability when the sample and FMD have similar absorption capabilities. ...
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... Due to mutual inductance of the solenoid and sample under test, the magnetic field produced by the eddy currents thus excited in the sample (typically metal) have a phase shift relative to the AC input to the solenoid. The phase shift is proportional to the conductivity of the sample [11], [12] and can thus be used to distinguish non-corroded rebars (higher conductivity) from corroded rebars (lower surface conductivity due to presence of rust i.e. iron oxide). Moreover, the eddy currents are localized to the external surface of the sample (limited by skin depth). ...
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... For example, continuous exposures of X-Ray radiation will cause health problems to the line operators [3], while a metal detector has limitations in detecting the existence of ferrous or non-ferrous metals. Also, metal detectors are believed to provide inaccurate information of dynamic process line in detecting foreign metals [4]. ...
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... Inductive proximity sensor based on the eddy current principle is fixed in the inlet part to identify the presence of metals in the waste [11]. The NPN sensor gives a logical 0 output in the presence of metal and logical 1 output in the absence of metal. ...
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... When some metal is coming close to the alternating magnetic field, the metal changing the field, and the inductance in the coil changes a little, and then the frequency. Active detectors uses the coil to transmit a pulse or a continually waveform, some uses the same coil to receive with, and others have 1 or 2 receiving coils [10]. The PI loads the coil with some current in a narrow pulse, and when it releases the coil it make a reflective pulse the duration of the reflected pulse is only a few μS, and the pulse can be several 100v high [11]. ...
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... Tumanski (2007) gives a useful review of coil-based sensors [4]. Yamazaki, Nakane, and Tanaka (2002) cover basic MD theory [5]. ...
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... (Brighton and English, 1993;Gray, 1994;Liu and Zhou, 2011;Yamazaki et al., 2002) (Clarke, 1994;Flind, 1987;Sharawi and Sharawi, 2007) 2 2 o o o o R C C R  (1) ซึ ่ งจะมี ผลท าให้ ได้ สั ญญาณรู ปไซน์ V o ที ่ มี ค่ าความถี ่ f o ประมาณเท่ ากั บ T eq o C L f  2 1  (2) เมื ่ อ C T = C o1 C o2 /(C o1 +C o2 ) และ LA 1 A 2 A 3 A 4 V ref1 V ref2 V 1 V 2 V 3 V 4 V 5 V 6 V 7 V cc R 1 R 2 R 3 R 4 R 5 D 1 A 0 C o1 C o2 R o2 R o1 (ก) วงจรรวมของระบบ R f1 A f1 A f2 R f2 V 3 V 2 R f3 R f4 R p2 A p1 A p2 V p V o R p1 A sh A c1 A c1 A c3 R c1 C c1 Q 1 R c2 R c3 R c4 R c5 G 1 V 1 V c1 V c2 C p C sh D p1 D p2 V cc V 4 R b1 R b2 C b1 (ข) วงจรตรวจวัV o V ref1 V 1 V 2 V o V ref1 V 1 V 2 V o V 3 V 4 V 2 V o V 3 V 4 V 2 V ref2 V 5 V 7 V o V ref2 V 5 V 7 V o( ...
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... According to the signal, the size of the metal is determined. [5][6] GPR Sensor GPR receiver is capable to find the discontinuities up to 30 cm in deep from the ground surface. The GPR controller produce trigger signal with the starting signal sent from the computer. ...
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The wireless charging time of mobile phone will increase greatly if the metal objects mix in the magnetic field coupling area. In addition, the fire may be caused as for the high temperature of metal objects. The paper proposed an improved detecting method based on balance coil for mobile phone wireless charging system according to comparing the advantages and disadvantages of traditional metal detection methods. The circuit model was established, and hardware and software were optimized. At last, experimental results verified the theoretical analysis.
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In metal detection systems, the response of a detector to a metal object may be approximated from the electromagnetic polarizability tensor of the object. Conversely, the tensors may be determined from multiposition measurements as the detector and object are moved relative to each other. This paper introduces and sets out the general approach to determining the tensor during in-line scanning. Two common application scenarios are considered, which share a similar consideration in the calculation of the tensor components. The first is the case for detectors of landmines or explosive remnants of war, where the detector is scanned on a surface above the object. The condition of this inverse problem depends on the geometry of the coil(s) and the measurement protocol, which at present is not fully understood. Our results consider two cases, namely, a single line scan over the object as two extreme cases. The results suggest that tensor inversion is possible for the 2-D raster scan, but not for a single line scan. The second application is a conveyor-type metal detector, which is used with a typical detector for industrial process lines. Here, a new rotation measurement method is proposed and examined for the case of simple coaxial sensor coils and in-line scanning. Finally, different inverse methods are analyzed for the new rotation measurement method.
Article
A new type of magnetic field detection system, which can operate at a frequency below 1 kHz, is proposed and developed. The system consists of a 40-turn-coil to generate the magnetic field and a magneto resistive sensor to detect both the amplitude and the phase signal of the magnetic field induced by eddy-current and magnetization of the samples. The magnetic field detection of ferro- and non-magnetic samples using the system is demonstrated and discussed.
Article
To investigate the relationship between the quality factor of a receiving coil made from specified superconducting material and resonant frequency in MRI system, a simulation model was developed using the HF software Ansoft HFSS and according to the size of the experimental setup of Bi2223/Ag superconducting receiving coil. Analyzing the frequency characteristics of resonance loop, it was found that the simulation results are basically in accord with the experimental results. The relationships between the quality factor of superconducting receiving coil and different resonant frequencies were obtained by adjusting the resonance capacitance of resonance loop to change its resonant frequency. The results indicated that only if the RF receiving coil is made from superconducting material and operating at low frequency, the signal noise ratio (SNR) in MRI system can be effectively improved.
Article
Purpose: Spatial and temporal resolutions are two of the most important features for quality assurance instrumentation of motion adaptive radiotherapy modalities. The goal of this work is to characterize the performance of the 2D high spatial resolution monolithic silicon diode array named “MagicPlate-512” for quality assurance of stereotactic body radiation therapy (SBRT) and stereotactic radiosurgery (SRS) combined with a dynamic multileaf collimator (MLC) tracking technique for motion compensation.
Article
A liquid material identification system based on electrical capacitance tomography technique is proposed. The structure of the sensor is discussed, and the parameters of designed capacitance tomography technique sensor are listed. The key parameters affecting the sensor performance and sensitivities are analysed. The performances of the sensor are simulated by COMSOL, and the practical performance of the capacitance tomography technique sensor is demonstrated in the experiments. The image reconstruction results are given which indicate the feasibility of the capacitance tomography technique system applied in liquid material identification.
Article
Shrapnel injury is a major cause of death in victims of bomb blast, land mines, and gun fire. Currently, doctors rely on imaging systems to locate the shrapnel before surgery. But, since these images do not provide any real-time information of the location of the shrapnel, effectiveness of surgery solely depends on the doctors' skill to trace them. Therefore, in some cases, the shrapnel, in spite of being visible in the images, may become untraceable during surgery. Hence, an online tool that can help the surgeon in finding the location of the shrapnel during the surgery will be very effective. A prototype of such a tool has been realized using an inductive proximity sensor (IPS). The new IPS is small enough for inserting into the victim's body. Since the detection depth (DD) of an IPS decreases with its diameter, this IPS uses a differential sensing scheme for detection. Hence, the new sensor achieves DDs longer than its diameter. The design of the sensor takes care of the effects of parasitic parameters that become predominant as the size gets smaller and ensures repeatable results. The tool also features a special excitation scheme that has multiple advantages when compared with existing schemes. The details of the sensor, its excitation and performance against different targets under various conditions are presented in this paper.
Article
This paper considers a dipole solution to the low-frequency electromagnetic responses of a typical in-line metal detector to metal contaminants. This solution is determined by the characteristics of metal targets and incident magnetic fields, which are treated separately as two independent factors. For the metal targets, the responses of sphere and wire samples are determined. The electromagnetic polarizability matrix of a metal sphere is directly computed from a spherical response function. The electromagnetic polarizability tensor of metal wire is derived from a measured eigenvalue matrix and a rotation matrix. The approximated responses of sphere and wire samples from the proposed solutions agree well with the measured responses from in-line metal detectors. In theory, the approximation method on metal wire samples is also applicable to the metal contaminants of other shapes.
Article
A method using a solenoid coil for simultaneously estimating the electric and magnetic properties of a spherical conductor was studied. These properties are estimated by finding the difference in the complex impedance of the coil with and without a sample to find out the measuring value that best coincides with the theoretical value. A new formula applicable to a nonmagnetic, a magnetic or a superconductive spherical sample was derived. The conductivities a and permeabilities μ measured by this method and by the conventional methods were compared using various samples. The deviations were no larger than 3% for a nonmagnetic samples, and 1.5% for μ of magnetic samples