sensors Article
Wireless Metal Detection and Surface Coverage Sensing for All-Surface Induction Heating Veli Tayfun Kilic 1 , Emre Unal 1 and Hilmi Volkan Demir 1,2, * 1
2
*
Department of Electrical and Electronics Engineering, Department of Physics, UNAM-Institute of Materials Science and Nanotechnology, Bilkent University, Ankara TR-06800, Turkey;
[email protected] (V.T.K.);
[email protected] (E.U.) School of Electrical and Electronic Engineering, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 639798, Singapore Correspondence:
[email protected]; Tel.: +90-312-290-1021
Academic Editor: Vittorio M. N. Passaro Received: 6 February 2016; Accepted: 1 March 2016; Published: 11 March 2016
Abstract: All-surface induction heating systems, typically comprising small-area coils, face a major challenge in detecting the presence of a metallic vessel and identifying its partial surface coverage over the coils to determine which of the coils to power up. The difficulty arises due to the fact that the user can heat vessels made of a wide variety of metals (and their alloys). To address this problem, we propose and demonstrate a new wireless detection methodology that allows for detecting the presence of metallic vessels together with uniquely sensing their surface coverages while also identifying their effective material type in all-surface induction heating systems. The proposed method is based on telemetrically measuring simultaneously inductance and resistance of the induction coil coupled with the vessel in the heating system. Here, variations in the inductance and resistance values for an all-surface heating coil loaded by vessels (made of stainless steel and aluminum) at different positions were systematically investigated at different frequencies. Results show that, independent of the metal material type, unique identification of the surface coverage is possible at all freqeuncies. Additionally, using the magnitude and phase information extracted from the coupled coil impedance, unique identification of the vessel effective material is also achievable, this time independent of its surface coverage. Keywords: electromagnetic induction; metal detection; wireless sensing; all-surface heating; coils
1. Introduction Although the idea of power transmission via electromagnetic waves goes back to the early 20th century [1], induction systems have been most recently becoming increasingly more popular and crucial and their usage areas have been expanding because of their high efficiency and safety [2,3]. Today, in addition to melting, metal processing and wireless charging, induction cooking is one of the most important applications of induction systems. In induction hobs, coils are placed beneath the hob surface. These coils are fed by a supply circuit with an alternative current of frequency between 20 KHz and 100 KHz in general. Circulating currents on the coils produce magnetic fluxes and these magnetic fluxes couple to a vessel located on the hob surface. Therefore, in induction hobs, instead of conduction heat transfer, energy is transferred wirelessly from a coil to a vessel via electromagnetic fields. The transferred electromagnetic energy turns into heat on the vessel surfaces as a result of the induced eddy currents [3–6]. Accordingly, induction hobs are more efficient, non-hazardous and cleaner for users compared to conventional heaters including resistive and flame heaters [6]. In regular induction hobs, vessels should typically have similar sizes with those of coils and they should be placed right on top of a coil for efficient heating. To overcome these issues, induction hobs with an Sensors 2016, 16, 363; doi:10.3390/s16030363
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structure, efficient heating of vessels with different shapes and sizes located anywhere on the hob array of small-area coils have been proposed [7–9]. Thanks to the array structure, efficient heating of might be possible. Therefore, a very recent important implementation in induction hobs is vessels with different shapes and sizes located anywhere on the hob might be possible. Therefore, a all-surface heating [10,11]. Structure of an all-surface induction heating system is visualized in very recent important implementation in induction hobs is all-surface heating [10,11]. Structure of an Figure 1. all-surface induction heating system is visualized in Figure 1.
Figure 1. 1. Conceptual Conceptual view view of of an an all-surface all-surface induction induction heating heating system system structure, structure, where where system system parts parts Figure and the the pot pot are are illustrated illustrated with with red red arrows. arrows. and
In bebe identified together with its its sizesize in In all-surface all-surfaceinduction inductionhobs, hobs,location locationofofthe thevessel vesselshould should identified together with order to to power upup thethe corresponding in order power correspondingcoils coilsfor forefficient, efficient,safe, safe,and andlow-cost low-costheating. heating. In In commercial commercial induction induction hobs, hobs, pan pan detection detection is is made using the damping factor measurement [12–14]. In In these these systems, coil isisnot notloaded, loaded,then thenlong longoscillation oscillation signals detected. other hand, if coil the systems, if a coil signals areare detected. OnOn thethe other hand, if the coil is loaded a metal a few-cycle damped oscillation is observed. This is loaded with with a metal vessel,vessel, then athen few-cycle damped oscillation signal issignal observed. This method method for detecting vesselHowever, presence.this However, this approach cannot specifyexact the is usefulisforuseful detecting the vessel the presence. approach cannot specify the vessel’s vessel’s position of material. the hob or its material. positionexact on the surfaceon of the the surface hob or its Since Since an an all-surface all-surface induction inductionhob hob isis aa radically radically new newtopology, topology, there there are are no no previous previous reports reports in in the literature addressing addressingthe the problem of metal coverage sensing to identify which coils to be the literature problem of metal coverage sensing to identify which coils to be powered powered up in all-surface for heating. efficient In heating. In this we and propose and demonstrate up in all-surface inductioninduction for efficient this work, wework, propose demonstrate a simple aand simple and robust that solution that provides us ability with the vessel presence while robust solution provides us with the to ability detect to thedetect vesselthe presence while uniquely uniquely sensing surface and coverage and identifying itsmaterial effectivetype material the same time in sensing its surfaceitscoverage identifying its effective at thetype sameattime in all-surface all-surface induction heating systems. Here, we systematically study variations in the induction heating systems. Here, we systematically study variations in the inductance andinductance resistance and an all-surface induction coupled todifferent a metal locations plate at different locations with of anresistance all-surfaceofinduction coil coupled to acoil metal plate at with different coverage different In thismaps study, maps and of the magnitude andinangle variations in the areas. In coverage this study,areas. we obtain ofwe theobtain magnitude angle variations the coupled system’s coupled system’s impedance. Althoughreports some previous reports discuss variationsand of inductance impedance. Although some previous discuss variations of inductance resistance and of a resistance a coil inheater an induction heater for heating measurements [15,16], this work is theoffirst coil in an of induction for heating measurements [15,16], this work is the first account the account of the proof-of-concept of wireless metal detection and surface coverage proof-of-concept demonstration ofdemonstration wireless metal detection and surface coverage sensing independent sensing independent of the metallic material type in all-surface induction heating, which is of the metallic vessel material type invessel all-surface induction heating, which is essential to determining essential determining the coil vessel each coil for the powering and obtaining the material information the vesseltocoverage by each for coverage poweringby and obtaining information of the vessel and of thefor vessel material and size for efficient heating. size efficient heating. 2. Experiments Experiments 2. In our our experimental experimental study, study, aa plate plate was was used used to be placed over a conventional conventional circular circular coil coil and and In moved around around horizontally horizontally without without changing changing its its perpendicular perpendicular distance distance to to the coil. Inductance Inductance and and moved resistance values values of of the the studied studied coil coil were were measured measured for for each each position position of of the the plate plate at at two two different different resistance frequencies (20 (by means of an meter Agilent 4263B). Measurement setup is shown frequencies (20 and and100 100KHz) KHz) (by means of LCR an LCR meter Agilent 4263B). Measurement setup is in Figure 2a. In 2a. experiments whilewhile the coil the plate waswas systematically shifted on shown in Figure In experiments thewas coil immobile, was immobile, the plate systematically shifted on the x-y plane with the help of a computer-controlled mechanical stage. As a result, the changes of inductance and resistance values with an area of the coil covered by the plate were measured.
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the x-y plane with the help of a computer-controlled mechanical stage. As a result, the changes of Sensors 2016, 16, 363 3 of 10 inductance and resistance values with an area of the coil covered by the plate were measured.
Figure (a) measurement measurement setup setup and and its its parts, parts, where where components components are are pointed pointed out out and and indicated indicated with with Figure 2. 2. (a) red arrows; (b) coupled system from side-view together with its geometrical parameters; and (c) red arrows; (b) coupled system from side-view together with its geometrical parameters; and (c) the the coil coil structure structure used used in in measurements measurements and and its its geometrical geometrical sizes. sizes.
itsits geometrical parameters in Figure 2b. As A side-view side-view of of coupled coupledsystem systemisissketched sketchedwith with geometrical parameters in Figure 2b. represented in the figure, thethe coupled system is isconstructed As represented in the figure, coupled system constructedbybya acoil, coil,aaplate plateand and aa ferrite layer below the coil. The plate is a disk with a diameter of 180 mm and a thickness of 1 mm. This diameter this plate plate isisin inthe thestandard standardrange rangeofofvessel vesseldimensions, dimensions, and it selected is selected to larger be larger than of of this and it is to be than thatthat of the the coil to model coil array heating in all-surface application. The distance between theand coilthe and the coil to model coil array heating in all-surface application. The distance between the coil plate platewas (h1)setwas towhich 8 mm,iswhich to the typicalofthickness of a glass ceramic surface in (h1) to 8set mm, similaristosimilar the typical thickness a glass ceramic surface in conventional conventional During characterization, plate across was scanned across the planar surfacethe 8 mm hobs. During hobs. characterization, the plate was the scanned the planar surface 8 mm above coil. above the the coil.distance Similarly, the distance between the ferrite and set theto coil 0.4 mm. Similarly, between the ferrite layer and the coillayer (h2) was 0.4(h2) mm.was Thissetis to because of Thissubstrate is because of the substrate layer used inofthe of the coil.exists Another mica the (mica) layer used in(mica) the manufacturing themanufacturing coil. Another mica layer on top of layer exists on top coil as well, but its thickness the coil as well, butofitsthe thickness was included in h1. was included in h1. On the other hand, the coil used in these measurements has a conventional circular shape with a 75 mm outer diameter and a 30 mm inner diameter. The outer diameter was set to be the same with The coil consists of 44 the minimum minimum vessel vessel dimension dimension(e.g., (e.g.,aacoffee coffeepot) pot)accepted acceptedbybymanufacturers. manufacturers. The coil consists of circular turns in in total and, inin each turn, 44 circular turns total and, each turn,1313copper copperwires wireswith with0.25 0.25mm mmdiameter diameter exist. exist. Coil Coil structure structure were used used in in our our other other studies, studies, too too [17]. [17]. is shown in Figure 2c. These coils with the same structure were various materials, materials, measurements were repeated Because vessels used in daily life are made of various 430) and and aluminum aluminum plates. plates. These with stainless steel (AISI 430) These materials materials were were selected selected due to their opposite properties effective on the induction heating process. In contrast to aluminum’s very low magnetic permeability and electrical resistivity, stainless steel has huge permeability and relatively high resistivity. Although aluminum is not a proper vessel material to be used in induction heating, stainless steel is one of the most appropriate vessel materials. Therefore, the measurements were done with one of the most and least proper vessel materials.
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opposite properties effective on the induction heating process. In contrast to aluminum’s very low magnetic permeability and electrical resistivity, stainless steel has huge permeability and relatively high resistivity. Although aluminum is not a proper vessel material to be used in induction heating, stainless steel is one of the most appropriate vessel materials. Therefore, the measurements were done Sensorsone 2016, 363most and least proper vessel materials. 4 of 10 with of16, the 3. Results Resultsand andDiscussion Discussion First, the measurements measurements were obtained obtained using using aa stainless stainless steel steel plate. plate. Mappings of the system’s system’s inductance and and resistance resistancevalues valueswhile while plate is scanned across 20 are KHz are depicted in thethe plate is scanned across at 20at KHz depicted in Figure Figure 3. figure, In the each figure, eachrepresents point represents the center-to-center distances plate 3. In the point the center-to-center distances betweenbetween the platethe and the and coil the coil projections the plane and color the color data correspond themeasured measured resistance resistance and projections on the on plane and the bar bar data correspond to tothe and inductance values. values.
Figure 3. Measured inductance (a) and resistance (b) maps for the system where the coil is loaded by Figure 3. Measured inductance (a) and resistance (b) maps for the system where the coil is loaded by the steel plate as a function of its lateral position at a horizontal plane 8 mm above the coil. Here, the the steel plate as a function of its lateral position at a horizontal plane 8 mm above the coil. Here, the coil is centered at the (0,0) origin. In addition, the white dash circles represent the points where the coil is centered at the (0,0) origin. In addition, the white dash circles represent the points where the coil and the plate projections are tangential. End points of the circles are pointed out with pink ticks at coil and the plate projections are tangential. End points of the circles are pointed out with pink ticks ´127.5 mm and 127.5 mm on the x and y axis. at −127.5 mm and 127.5 mm on the x and y axis.
Inductance Inductance of of the the unloaded unloaded coil coil is is around around 165 165 µH μH (see (see Figure Figure 3a). 3a). However, However, as as the the plate plate and and the the coil closer, the the system’s equivalent inductance first increases and thenand decreases. Normally, coil centers centersget get closer, system’s equivalent inductance first increases then decreases. monotonic coveragewith area iscoverage expected area because of enhancement in coupling. The reason Normally, decreasing monotonicwith decreasing is expected because of enhancement in for unexpected inductance increase is induced currents on the top surface of the plate are in coupling. The reason for unexpected inductance increase is induced currents on the topthat surface of reverse currents on thewith bottom surface. case where theIncoil partially the platedirection that are with in reverse direction currents on In thethe bottom surface. theiscase wherecovered, the coil looping magnetic fields produced by the coil create such reverse currents on the plate’s top surface. is partially covered, looping magnetic fields produced by the coil create such reverse currents on the In addition, bendingIn of addition, the induced currents bottomcurrents surface of causes reverse currents. plate’s top surface. bending of on thethe induced onthe theplate bottom surface of the plate Since stainless steel has a huge permeability, the magnetic flux produced by the coil induces eddy causes reverse currents. Since stainless steel has a huge permeability, the magnetic flux produced by currents not on the plate’s top surface its bottom surface.but However, as the plate moves out of the the coil induces eddy currents not onbut theon plate’s top surface on its bottom surface. However, as coil’s projection, the of induced currents on thethe bottom surface goeson through the plate’s side edges and the plate moves out the coil’s projection, induced currents the bottom surface goes through flows on theside top surface as well. These currents on the bottom top currents surfaces are in reverse the plate’s edges and flows on the top surface as well.and These on the bottomdirections and top and thus they construct a complete loop. The induced current distribution on the steel plate’s current bottom surfaces are in reverse directions and thus they construct a complete loop. The induced and top surfaces calculated 3D full electromagnetic given in supplementary distribution on were the steel plate’susing bottom and top surfaces solutions were calculated using 3D full materials (see Figures S1–S3). On the other hand, resistance of the unloaded coil is the approximately electromagnetic solutions given in supplementary materials (see Figures S1–S3). On other hand, 0.5 Ω and increases monotonically with the plate’s (see Figure 3b). This iswith because of the resistance of the unloaded coil is approximately 0.5 coupling Ω and increases monotonically the plate’s
coupling (see Figure 3b). This is because of the fact that the induced eddy currents on the plate turn into heat and dissipate power independently from its direction. Moreover, in the figures it is observed that, in spite of their circular distribution, the inductance and the resistance plots are not purely symmetric. The reason for this measurement error is that the plate was not perfectly parallel to the coil plane.
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resistance is skin depth. Since the skin depth decreases with operational frequency, the resistance increases. These observations are clearly seen in Figure 4 where changes of the inductance and resistance Sensors 2016, values 16, 363 with a percentage area of the coil covered by the plate are presented. 5 of 10 The percentage area is simply ratio of the coil area covered by the plate to the coil’s total area. The total area and the partial area covered by the plate are calculated by Equation (1) [18]. fact that the induced eddy currents on the plate turn into heat and dissipate power independently r 2 from its direction. Moreover, in the figures itAiscoilobserved that, in spite of their circular distribution, the inductance and the resistance plots are not purely symmetric. The reason for this measurement error is 0 ,r R d that the plate was not perfectly parallel to the coil plane. (1) ,d R r cov ered r 2 Measurements were repeated at 100 KHz frequency. It was observed that the inductance values 2 2 2 2 2 d2 R2 r2 1 1 d r R 2 2 2 are lower atr 2100 than those was r that Rsystem d rthe cos KHz ( ) Robserved cos 1 ( at 20 KHz. ) Onthe d other r Rhand, R it d ralso ,Rseen 2dr 2dR 2 equivalent resistance increases with frequency. One of the effects for the enhancement in resistance is skin depth. theAskin decreases with operational frequency, the These coil and covereddepth are the coil’s total area and the intersection arearesistance between increases. the coil and the Here, ASince observations are clearly seen in Figure 4 where of the inductance and resistance values plate’s projection, respectively. In addition, d changes is the distance between the centers of the platewith (witha percentage area ofcoil the (with coil covered by the plate are presented. radius R) and the radius r).
Figure 4. Variation of the measured inductance (a) and resistance (b) as a function of the coil area Figure 4. Variation of the measured inductance (a) and resistance (b) as a function of the coil area covered by the steel plate at 20 and 100 KHz frequencies. covered by the steel plate at 20 and 100 KHz frequencies.
Thethe percentage is simply ratio of the coil area covered by the to the total area. In figure, it area is seen that the system’s equivalent resistance hasplate a trend ofcoil’s monotonically The total area partial area covered thefor plate calculated by [18]. increasing, butand thethe same behavior is not by held theare inductance. In Equation addition, (1) variations in the inductance and resistance are higher at 100 KHz than those obtained at 20 KHz. Therefore, it can be A “ πr2 $ for the steel plate’s detection, coil deduced that, the resistance should be considered rather than the 0 ,r`R ă d ’ ’ & 2 (1) inductance, and πr it is easier at high operation frequencies. In addition, in the, dfigures, ă R ´ r there exist Acovered “ c” ı” ı 2 2 2 2 ´ R2 2 ’ d ` r d ` R ´ r 1 ’ multiple inductance values that correspond As ´1 p % r2 cos´1 p and resistance q ` R2 cos q´ pd ` to rq2 constant ´ R2 R2 ´covered pd ´ rq2 area , R ´ rpercentages. ď d ď r`R 2dr 2dR 2 explained previously, the reason for this measurement error is the plate’s not being completely parallel to the coil. Here, Acoil and Acovered are the coil’s total area and the intersection area between the coil and Similar measurements were conducted usingd an aluminum whose geometry is the the plate’s projection, respectively. In addition, is the distanceplate between the centers of the same plate with that of the steel plate. Variations of the inductance and the resistance values with the coil area (with radius R) and the coil (with radius r). covered by the aluminum plate arethe shown in Figure 5. In the figure, it is seen that system’s equivalent resistance has a trend of monotonically increasing, but the same behavior is not held for the inductance. In addition, variations in the inductance and resistance are higher at 100 KHz than those obtained at 20 KHz. Therefore, it can be deduced that, for the steel plate’s detection, the resistance should be considered rather than the inductance, and it is easier at high operation frequencies. In addition, in the figures, there exist multiple inductance and resistance values that correspond to constant covered area percentages. As explained previously, the reason for this measurement error is the plate’s not being completely parallel to the coil. Similar measurements were conducted using an aluminum plate whose geometry is the same with that of the steel plate. Variations of the inductance and the resistance values with the coil area covered by the aluminum plate are shown in Figure 5.
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Figure 5. Variation of the measured inductance (a) and resistance (b) as a function of the coil area Figure 5. Variation of the measured inductance (a) and resistance (b) as a function of the coil area covered by the aluminum plate at 20 and 100 KHz frequencies. covered by the aluminum plate at 20 and 100 KHz frequencies.
different from thatthat observed withwith the steel expressed. Here instead In the the figure, figure,a abehavior behavior different from observed the plate steel isplate is expressed. Here of the resistance, system’s inductance has a monotonically decreasing trend trend with the area. instead of the resistance, system’s inductance has a monotonically decreasing withcovered the covered Therefore, for thefor aluminum plate’s coverage detection,detection, the inductance value should be considered area. Therefore, the aluminum plate’s coverage the inductance value should be rather than the resistance butresistance high operation frequencies do not make thisdo procedure easier. Figure 5, considered rather than the but high operation frequencies not make this In procedure the inductance change considerably withconsiderably frequency. with This frequency. is becauseThis of low magnetic easier. In Figuredoes 5, thenot inductance does not change is because of permeability the aluminum at 20 KHz andat100 fluxMagnetic producedflux by low magneticof permeability of the aluminum 20 KHz frequencies. and 100 KHzMagnetic frequencies. the coil couples the aluminum plate. produced by theless coilto couples less to the aluminum plate. These different different behaviors behaviors observed observed for for the the steel steel and and the the aluminum aluminum plates plates are are because because of their These magnetic and and electrical electrical properties. properties. For For the the aluminum aluminum plate plate (see (see Figure Figure 5b), 5b), the measured resistance magnetic first increases and then decreases as the coil moves moves out out of of the the plate’s plate’s projection. projection. Small increases initially observed observed are are due to the plate’s edges. Currents Currents induced induced on the aluminum plate encounter initially discontinuities at at the theedges, edges,which whichincreases increasesthe theequivalent equivalentresistance. resistance. However, at some point discontinuities However, at some point (at (at around at KHz 20 KHz at 100 KHz in Figure 5), the resistance decreases towards level around 75%75% at 20 andand 60%60% at 100 KHz in Figure 5), the resistance decreases towards thethe level of of unloaded coil’s resistance. This because thefact factthat, that,after aftersome somepoint, point, decrease decrease in in coupling unloaded coil’s resistance. This is is because ofofthe between the edge effects. Since thethe aluminum’s resistivity is very low, the coil coil and andthe theplate platedominates dominatesthe the edge effects. Since aluminum’s resistivity is very this effect is seen here,here, but itbut is negligible for stainless steel steel because of its of high low, this effect is seen it is negligible for stainless because its resistance. high resistance. Moreover, the a monotonically decreasing trend, which is different from Moreover, themeasured measuredinductance inductancehas has a monotonically decreasing trend, which is different that observed previously in Figurein4 for the steel plate. of its Because low magnetic the from that observed previously Figure 4 for the Because steel plate. of itspermeability, low magnetic induced currents the bottom surface of the aluminum plate couple to the top surface considerably. permeability, the on induced currents on the bottom surface of the aluminum plate couple to the top In otherconsiderably. words, because of thewords, coil’s magnetic fluxes, currents are induced on both bottom and top surface In other because of the coil’s magnetic fluxes, currents are induced on surfaces of the These currents flowThese in thecurrents same direction. Although there still exist reverse both bottom andplate. top surfaces of the plate. flow in the same direction. Although there currents the topcurrents surface on as athe result bending the currents on theofbottom surfaceon near plate still existon reverse top of surface as of a result of bending the currents thethe bottom edges and the looping magnetic fields, the total current on the top surface is in the same direction surface near the plate edges and the looping magnetic fields, the total current on the top surface is in withsame the current on with the bottom surface. continuous decreasea in the measured inductance the direction the current on Therefore, the bottoma surface. Therefore, continuous decrease in the is observedinductance with the covered area. with Current onCurrent the aluminum plate located different measured is observed thedistributions covered area. distributions on the at aluminum positions wereatalso calculated usingwere 3D full shown in supplementary plate located different positions also electromagnetic calculated usingsolutions 3D full electromagnetic solutions materials (see Figures S4–S6). shown in supplementary materials (see Figures S4–S6). As a result, by applying small signals at two different frequencies and measuring the system inductance and and resistance, resistance, the the fractional fractional area area of of the the coil covered by the vessel can be determined inductance To investigate its precisely. However, in all-surface heating, coils are located in an array architecture. To effect, another circular coil was brought near the coil that is connected to the LCR meter. expected, another circular coil was brought near the coil that is connected to the LCRAsmeter. As expected, since measurements were obtained with small signals and the second coil’s tips were open
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circuit, the inductance and resistance values were the same within those of the single coil since measurements were obtained with small signals and the second coil’s tips were open circuit, measurements. Therefore, by applying small signals to each of the array coils individually at two the inductance and resistance values were the same within those of the single coil measurements. different frequencies, in all-surface heaters, the location and the material of a vessel can be uniquely Therefore, by applying small signals to each of the array coils individually at two different frequencies, identified. To avoid a large database in the identification process, materials that have similar in all-surface heaters, the location and the material of a vessel can be uniquely identified. To avoid properties can be grouped together. Furthermore, measured data directly can be used to calculate a large database in the identification process, materials that have similar properties can be grouped power transmission from coils to the vessel. On the other hand, algorithms based on the coils’ together. Furthermore, measured data directly can be used to calculate power transmission from coverage can be created for easy determination of the vessel’s location. For instance, the center of a coils to the vessel. On the other hand, algorithms based on the coils’ coverage can be created for easy vessel can be found by considering the coils’ coverage by the vessel, which are placed side by side. determination of the vessel’s location. For instance, the center of a vessel can be found by considering By using distances of these coils to the identified center point, the vessel’s size can be determined the coils’ coverage by the vessel, which are placed side by side. By using distances of these coils to the together with its approximate shape. identified center point, the vessel’s size can be determined together with its approximate shape. Since the inductance and resistance values cannot be measured directly from the coil’s small Since the inductance and resistance values cannot be measured directly from the coil’s small signal response and their calculations require additional time and effort for the system, investigating signal response and their calculations require additional time and effort for the system, investigating other parameters that are directly measurable might be a better idea. To this end, we calculated other parameters that values are directly mightfrom be a the better idea. To this end, and we calculated magnitude and angle of themeasurable coil’s impedance measured inductance resistance magnitude and angle values the coil’s impedance from the inductance and resistance using Equations (2) and (3), of respectively. The magnitude andmeasured angle of the coil impedance can be using and (3), respectively. The magnitude of levels the coilthat impedance found foundEquations directly by(2)the amplitude ratio of signal's voltage and andangle current is sent tocan thebe coil and directly by the amplitude ratio of signal's voltage and current levels that is sent to the coil and the the phase difference between them, respectively. phase difference between them, respectively. 2 abs Z abs R jωL Ra ω 2 L2 (2) abs pZq “ abs pR ` jωLq “ R2 ` ω2 L2 (2) ˙ ˆ 1 ωL ωL angle Z angle R jωL tan ´ 1 angle pZq “ angle pR ` jωLq “ tan R R
(3) (3)
Here, coil’s impedance. Its magnitude and angle are represented with abs(Z) angle(Z), Here, ZZisisthethe coil’s impedance. Its magnitude and angle are represented withand abs(Z) and respectively. Similarly, L and R denote the measured inductance and resistance values, respectively. In angle(Z), respectively. Similarly, L and R denote the measured inductance and resistance values, addition, ω isIn the angularωfrequency. respectively. addition, is the angular frequency. Changes in the magnitude and angle of the coil’s coil’s impedance impedance as a function function of of the the steel steel plate’s plate’s loading loading are are shown shown in in Figure Figure 6. 6.
Figure 6. Changes of the impedance magnitude (a) and angle (b) at 20 and 100 KHz frequencies as a Figure 6. Changes of the impedance magnitude (a) and angle (b) at 20 and 100 KHz frequencies as a function of the coil area covered by the steel plate. Here, in part (a), the y axis is scaled nonuniformly function of the coil area covered by the steel plate. Here, in part (a), the y axis is scaled nonuniformly for better representation of the calculated magnitudes at different frequencies. for better representation of the calculated magnitudes at different frequencies.
Similar Similar plots plots are are given given in in Figure Figure 77 for for the the aluminum aluminum plate. plate.
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Figure 7. Changes of the impedance magnitude (a) and angle (b) at 20 and 100 KHz frequencies Figure 7. Changes of the impedance magnitude (a) and angle (b) at 20 and 100 KHz frequencies as a as a function of the coil area covered by the aluminum plate. Here, in part (a), the y axis is scaled function of the coil area covered by the aluminum plate. Here, in part (a), the y axis is scaled nonuniformly for better representation of the calculated magnitudes at different frequencies. nonuniformly for better representation of the calculated magnitudes at different frequencies.
From thatthat the coverage of a plate its and material can be identified uniquely From the thefigures, figures,it is it seen is seen the coverage of a and plate its material can be identified from the magnitude and angle ofand the angle coil’s impedance. addition, the measurements uniquely from the magnitude of the coil’sInimpedance. Inimpedance addition, the impedance should be carried out atbe different Otherwise, identification would be hard especially measurements should carriedfrequencies. out at different frequencies. Otherwise, identification would for be small covered area percentages because of the fact that impedance magnitudes are similar for both the hard especially for small covered area percentages because of the fact that impedance magnitudes steel and thefor aluminum whilethe thealuminum impedanceplates angleswhile vary the in small ranges angles at a single frequency. are similar both theplates steel and impedance vary in small ranges at a single frequency. 4. Conclusions
4. Conclusions In this work, a new wireless detection methodology for sensing metal coverage and identifying its material induction hobs is presented. for a coupled system, In this type work,inaall-surface new wireless detection methodology forResults sensingshow metalthat, coverage and identifying one can determine accurately the partial area of the coil covered by the vessel from the inductance its material type in all-surface induction hobs is presented. Results show that, for a coupled system, and variation. Material properties can alsoby bethe identified. However, wireless one resistance can determine accurately the partial areaofofthe thevessel coil covered vessel from the inductance detection is easier at higher frequencies because variations in the inductance and resistance and resistance variation. Material properties of the vessel can also be identified. However, increase wireless with operating frequency. detection is easier at higher frequencies because variations in the inductance and resistance increase metal detection is also presented here using the amplitude and angle of the coil’s withMoreover, operating frequency. impedance. By measuring voltage of each sequentially, identification Moreover, metal detection is and also current presented here coil using the amplitude and angleofofthe theplate’s coil’s coverage by coils together with its material is possible in all-surface heaters. In addition, impedance impedance. By measuring voltage and current of each coil sequentially, identification of the plate’s measurements at different make this process easier. These results areIn beneficial detecting coverage by coils togetherfrequencies with its material is possible in all-surface heaters. addition,for impedance vessels and determining which coils to power up in all-surface induction hobs. measurements at different frequencies make this process easier. These results are beneficial for
detecting vessels and determining which coils to power up in all-surface induction hobs.
Supplementary Materials: The following document is available online at http://www.mdpi.com/1424-8220/16/ 3/363/s1, Supplementary Materials.doc. Supplementary Materials: The following document is available online at http://www.mdpi.com/1424-8220/ Acknowledgments: We acknowledge Arcelik Household Appliances Company and TUBA for their support. 16/3/363/s1, Supplementary Materials.doc. Author Contributions: H.V.D. supervised the research. V.T.K. and E.U. designed and performed the experiments. Acknowledgments: We acknowledge Household Appliances Company TUBAthe formanuscript. their support. V.T.K, E.U. and H.V.D. interpreted and Arcelik analyzed the data. V.T.K., E.U. and H.V.D.and prepared
Author Contributions: supervised the research. V.T.K. and E.U. designed and performed the Conflicts of Interest: The H.V.D. authors declare no conflict of interest. experiments. V.T.K, E.U. and H.V.D. interpreted and analyzed the data. V.T.K., E.U. and H.V.D. prepared the manuscript. Abbreviations Conflicts of Interest: The authors declare no conflict of interest. The following abbreviation is used in this manuscript:
AISI Abbreviations
American Iron and Steel Institute
The following abbreviation is used in this manuscript:
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