sensors Article

Magnetoelectric Current Sensors Mirza Bichurin *, Roman Petrov, Viktor Leontiev, Gennadiy Semenov and Oleg Sokolov Department of Design and Technology of Radioequipment, Novgorod State University, Veliky Novgorod 173003, Russia; [email protected] (R.P.); [email protected] (V.L.); [email protected] (G.S.); [email protected] (O.S.) * Correspondence: [email protected]; Tel.: +7-8162-974-267 Academic Editor: Nian X. Sun Received: 1 March 2017; Accepted: 30 May 2017; Published: 2 June 2017

Abstract: In this work a magnetoelectric (ME) current sensor design based on a magnetoelectric effect is presented and discussed. The resonant and non-resonant type of ME current sensors are considered. Theoretical calculations of the ME current sensors by the equivalent circuit method were conducted. The application of different sensors using the new effects, for example, the ME effect, is made possible with the development of new ME composites. A large number of studies conducted in the field of new composites, allowed us to obtain a high magnetostrictive-piezoelectric laminate sensitivity. An optimal ME structure composition was matched. The characterization of a non-resonant current sensor showed that in the operation range to 5 A, the sensor had a sensitivity of 0.34 V/A, non-linearity less than 1% and for a resonant current sensor in the same operation range, the sensitivity was of 0.53 V/A, non-linearity less than 0.5%. Keywords: current sensor; magnetoelectricity; magnetoelectric effect; sensor design

1. Introduction Current sensors are very important types of devices. There are a large number of current sensors operating on various physical principles. The most common current sensors are current transformers, magnetoresistance and Hall sensors [1,2]. Despite the fact that magnetoelectric (ME) current sensors have the small size, weight and a high sensitivity they have received relatively little attention in the publications, compared with the ME magnetic field sensors [3]. The first attempt to measure the direct current (DC) based on the ME effect was made in [4]. There are also a number of articles about ME current sensors [5,6]. Evaluation of the value of current flowing in a conductor is possible by measuring the corresponding magnetic field. The ME current sensor uses the ME effect as a basis of its measurements. As is well-known, the ME effect is a polarization response to an applied magnetic field, or conversely a magnetization response to an applied electric field. ME behavior exists as a composite effect in multiphase systems of piezoelectric and magnetostrictive materials. In a magnetostrictive-piezoelectric layered structure the interaction between magnetic and electric subsystems occurs through mechanical deformation. It means that the ME effect is much stronger at frequencies corresponding to the electromechanical resonance range. In current sensor applications the induced ME voltage coefficient is more important than the induced ME electric field coefficient, as voltage is the physical quantity measured. ME current sensors can work in different regimes. In the first case, the ME element works in a non-resonant mode regime, and in the second case, in the resonant one. The same ME element can be used as the sensitive element of the sensor in both cases. The properties of non-resonant ME current sensors were considered in [7,8], and of the resonant ones in [9]. In the paper we consider the basic principles of work of ME DC sensors of non-resonant type based on using the low-frequency ME effect and next the resonant type, working in one excited mode in the piezoelectric phase of magnetoelectric material. It should be noted that the paper describes the construction and characteristics of the ME current sensors used in practice. Sensors 2017, 17, 1271; doi:10.3390/s17061271

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2. Principle of Operation 2. Principle of Operation Magnetoelectric current sensor consists of a ME element made in the form of the piezoelectric current consists of a ME element madeon. in The the form of the plateMagnetoelectric and the side surfaces of sensor the magnetostrictive plates are glued principle of piezoelectric operation of plate and the side surfaces of the magnetostrictive plates are glued on. The principle of operation of ME sensor is shown in Figure 1. The figure on the left shows the dependence of dc magnetic field on ME sensor is shown in Figure 1. The figure on the left shows the dependence of dc magnetic field on the ME coefficient. The zero sensitivity of the sensor corresponds to a dc magnetic field equal to zero. the ME The zero sensitivity of the sensor corresponds to a dc magnetic field equal to zero. The MEcoefficient. coefficient α ME is determined by the following expression: The ME coefficient αME is determined by the following expression: α ME = f ( H0 + H_ + H∼ ) (1) αME = f(H0 + H_ + H~) (1) Passing the initial nonlinear part, we get to the linear section. In this linear section, the current m q , where K is the gain factor, andm m q sensor is working. The term αME canbe bewritten writtenas as ααME ME can , where K is the gain factor, and q31 31 is ME==KKmq31 31 is the piezomagnetic module of the magnetic phase. The figure on the right shows the output the piezomagnetic module of the magnetic phase. The figure on the right shows characteristic indicated by by thethe redred lineslines on the The tuning of the of linear characteristic of ofthe thecurrent currentsensor, sensor, indicated on left thefigure. left figure. The tuning the section occurs occurs by introducing a magnetic field H0field . Here conductor or a current replaces linear section by introducing a magnetic Ha0. current Here a current conductor or acoil current coil areplaces bias magnetic for inducing theinducing ME effect. The modulating coil is needed for of the a bias field magnetic field for the ME effect. The modulating coildefinition is needed for frequency Then everyone can choose to operate the linear portion of ME portion voltage coefficient definition range. of the frequency range. Then everyone can choose to operate thethe linear of the ME dependence and getdependence a result of the sensitivity dependence for the ME sensor, that theME dependence of voltage coefficient and get a result of the sensitivity dependence foristhe sensor, that the output voltage of onthe theoutput input measuring is the dependence voltage on current. the input measuring current.

Figure 1. Magnetoelectric current sensor: principle of operation.

3. Non-Resonant 3. Non-Resonant Current Current Sensor Sensor 3.1. Sensor Design The ME non-resonant current sensor consists of three major parts: the the sensing sensing head head (generator, (generator, inductance coil, ME element, current coil), power supply and the signal processing (amplifier, peak detector) part. Each part affects the performance of the current sensor. The block diagram of the ME non-resonant current currentsensor sensorisisshown shownininFigure Figure2.2.The Theprinciple principle operation sensor is based non-resonant ofof operation of of thethe sensor is based on on measuring the electromotive appearing the output of sensitive dueME to effect the ME measuring the electromotive forceforce appearing at the at output of sensitive elementelement due to the as asof a result of the influence of the alternating a bias magnetic field. aeffect result the influence of the alternating magneticmagnetic field and field a biasand magnetic field.

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Figure 2. Block diagram of ME non-resonant current sensor. Figure 2. Block diagram of ME non-resonant current sensor. Figure 2. Block diagram of ME non-resonant current sensor.

Figure 2. Block

3.2. Sensing Head 3.2. Sensing Head 3.2. Sensing Head 3.2. Sensing Head The ME element, shown in Figure 3, is the primary part of the whole currentThe sensor, its shown in Figu ME and element, The ME element, shown in Figure 3, is the primary part of the whole current sensor, and its function is to convert the measuring current into the voltage signal that can be measured easily. The function is to convert The ME element, shown in Figure 3, is the primary part of the whole current sensor, and its the measuring function is to convert the measuring current into the voltage signal that can be measured easily. The ME element is the sensitive part ofcurrent a ME into current sensor and inthat our can case, consists of piezoelectric ME element is the function is to convert the measuring the voltage signal beitmeasured easily. The MEsensitive part of a ME element is the sensitive part of a ME current sensor and in our case, it consists of piezoelectric and magnetostrictive in Figure 3. and The in layered structure based on a piezoceramic and magnetostrictive element is the sensitivelayers part ofasa shown ME current sensor our case, it consists of piezoelectric and layers as show and magnetostrictive layers as shown in Figure 3. The layered structure based on a piezoceramic PZT plate in our caseashad thickness of 0.38 mm, length of 10 mm and of 1plate mmPZT [8,9]. The PZT in our case had thickness magnetostrictive layers shown in Figure 3. The layered structure based on width a piezoceramic plate PZT plate in our case had thickness of 0.38 mm, length of 10 mm and width of 1 mm [8,9]. The polarized in the thickness Thewidth electrodes are[8,9]. applied on two sideswas of the piezoelectric polarized in the thi inpiezoelectric our case hadwas thickness of 0.38 mm, length ofdirection. 10 mm and of 1 mm The piezoelectric was piezoelectric was polarized in the thickness direction. The electrodes are applied on two sides of the magnetostrictive plates. Thickness of one layer ofare Metglas was (1) and PZT plates. (3) magnetostrictive Thickness o polarized in the thickness direction. The electrodes applied on 0.02 two mm. sides Metglas of the magnetostrictive magnetostrictive plates. Thickness of one layer of Metglas was 0.02 mm. Metglas (1) and PZT (3) plates Thickness are connected bylayer gluing Different of glue can be The thickness the adhesive by gluing (2). Di platesof are connected plates. of one of (2). Metglas was types 0.02 mm. Metglas (1)used. and PZT (3) plates are connected plates are connected by gluing (2). Different types of glue can be used. The thickness of the adhesive layer does exceedtypes 10 microns. numbers of Metglas layers layer does exceed 10 microns. T by gluing (2).not Different of glue To canincrease be used. the Thesensitivity, thickness ofdifferent the adhesive layer does notnot exceed layer does not exceed 10 microns. To increase the sensitivity, different numbers of Metglas layers may be used. The electrical signal is taken from the electrodes (4) and then passes on to the signal may be used. The 10 microns. To increase the sensitivity, different numbers of Metglas layers may be used. The electricalelectrical signal is may be used. The electrical signal is taken from the electrodes (4) and then passes on to the signal processing part of the of the device. signal is taken from thedevice. electrodes (4) and then passes on to the signal processing processing part of the part device. processing part of the device.

Figure3.3. Magnetoelectric Magnetoelectric element: element: 1—magnetostrictive 1—magnetostrictive plates plates (Metglas); (Metglas); 2—adhesive 2—adhesive layer; Figure 3. Magnetoelectric elemen Figure layer; Figure 3. Magnetoelectric element: 1—magnetostrictive plates (Metglas); 2—adhesive layer; 3—piezoelectricplate plate(PZT); (PZT);4—electrodes. 4—electrodes. 3—piezoelectric plate (PZT); 4—elec 3—piezoelectric 3—piezoelectric plate (PZT); 4—electrodes.

The sensitive element of the sensor is a magnetostrictive-piezoelectric composite and itselement of the s The sensitive The The sensitive sensitive element element of of the the sensor sensor isis aa magnetostrictive-piezoelectric magnetostrictive-piezoelectric composite composite and and its its magnetostriction constant is a function of magnetic field and in general can bemagnetostriction written as λ = f(H 0) constant is a functi magnetostriction of of magnetic field andand in general can can be written as λ =asf (H [10]. magnetostrictionconstant constantisisa afunction function magnetic field in general be written λ =0 )f(H 0) [10]. In turn the piezomagnetic module depends on AC magnetic field as follows: [10]. In turn the piezomagnetic modu In turn module depends on AConmagnetic field as follows: [10]. In the turnpiezomagnetic the piezomagnetic module depends AC magnetic field as follows: q = dλ/dH (2) q q= = dλ/dH (2) dλ/qH (2) For the transverse ME voltage coefficient of layered composite the following For dependence on the transverse ME voltage For the transverse ME voltage coefficient of layered composite the following dependence on material parameters was obtained [10,11] material parameters was obtained [10 For the transverse voltage[10,11] coefficient of layered composite the following dependence on material parameters wasME obtained material parameters was obtained [10,11] E V (1 V )(mm q11 mm q21 )pp d 31 E E , 31 E33 α E , 31 =(3)3 = V (1 pV )( qp11 qp21 ) d 31 p m m p 2 p m m p E , 31 H H V()( q11p+s11q)( (3) 1 E31 33 (m s12 m s11−)VV(p1 − 33 V31) 2p d231 (1 V ) ε 33 ( m s12 + 211) d p ss12 α E,31 = H1 = pp 33 (3) ( s s ) V ( s )( 1 V ) 2 d ( 1 V ) 12+ m s11 )V + p ε33 ( p s12 + p s11 )(1 − V ) − 2 p 31 H1 ε 33 (m s12 d231 (1 − V ) 33 11 12 11 is a coefficient where E3 and H1 are the intensities of the electric and magnetic fields, psiiwhere E3 and H1 ofare the intensitie psii is a coefficient of where E3 and H1 are the intensities of the electric and magnetic msii isp safields, compliance phase electric field,fields, coefficient of compliance of compliance of piezoelectric phase a where E3 and of H1 piezoelectric are the intensities of at theconstant electric and magnetic is a coefficient of compliance ii compliance of piezoelectric phase at constantp melectric field, msii is a coefficient of compliance of p phase at constant magnetic dielectric permittivity, 31 a piezoelectric phase at constant magnet ofmagnetic piezoelectric phase at constant electricfield, field, ε33siiisis aa coefficient of compliancedmagnetic of is magnetic phase magnetic phase constant field, pε33 is a dielectric permittivity, pd31 is a piezoelectric p ε magnetic p d fraction mpm mqij at pVaa+ piezomagnetic m is a field, piezomagnetic module, V is the volume of the piezoelectric, V = V/( module, and q ij is atmodule, constantand magnetic is a dielectric permittivity, is a piezoelectric module, and q is 33 31 ij mqij is a piezomagnetic module, V is the volume fraction of the piezoelectric, V = pV/(pV + module, mV), pV иand mV are mV the volumes the piezoelectric phases, piezomagnetic module, V is theofvolume fraction ofand the magnetic piezoelectric, V = prespectively. V/(p V + mmV), ppVVи m V are are the the volumes of the pi mV), pV и mV are the volumes of the piezoelectric and magnetic phases, respectively. TheofME is placed the current and inductance coils, where a bias magnetic and is placed into th The MEfield element volumes theelement piezoelectric andinto magnetic phases, respectively. The ME element is placed into the current and inductance coils, where a bias magnetic field and a variable modulation magnetic field are created, respectively. a variable modulation magnetic field a variable modulation magnetic field are created, respectively.

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The ME is placed into the current and inductance coils, where a bias magnetic field and a Sensors 2017, 17,element 1271 4 of 13 Sensors 2017, 17, 1271 4 of 13 variable modulation magnetic field are created, respectively. An important important part part is is the the fixation fixation of of ME ME element element in in the the inductance inductance coil. coil. The The ME ME element element was was An An important part is the fixation of ME element in the inductance coil. The ME element was fixed by by the the glue glue on on one one end end of of sample sample in in order order to to avoid avoid jamming jamming the the ME ME element element surface, surface, as as shown shown fixed fixed by the glue on one end of sample in order to avoid jamming the ME element surface, as shown in Figure Figure 4. 4. in in Figure 4.

Figure 4. 4. Design Design of of the the hard hard core core of of aamagnetoelectric magnetoelectric sensor: sensor: 1—inductance 1—inductance coil; 2—ME element; Figure Figure 4. Design of the hard core of a magnetoelectric sensor: 1—inductance coil; 2—ME element; 3—leading-out 3—leading-out wire wire of of ME ME element; element; 4—glue; 4—glue;5—current 5—currentcoil. coil. 3—leading-out wire of ME element; 4—glue; 5—current coil.

3.3. Signal Processing Processing 3.3. 3.3. Signal Processing The signal signal processing part part is used to to process the the voltage signal signal from the the sensing head, head, the block block The The signalprocessing processing partisisused used toprocess process thevoltage voltage signalfrom from thesensing sensing head,the the block scheme is shown in Figure 5. In the non-resonant case a current sensor signal processing scheme scheme schemeisisshown shownininFigure Figure5.5. Inthe thenon-resonant non-resonantcase caseaacurrent currentsensor sensorsignal signalprocessing processingscheme scheme consists of of aa generator generator that that is is tuned tuned to to the the frequency frequency at at 500 500 Hz. Hz. The The generator generator is is connected connected to the consists consists of a generator that is tuned to the frequency at 500 Hz. The generator is connectedtotothe the inductance coil then the output output signal signal from the the ME ME element element is is amplified amplified and and provided provided to to the the peak peak inductance inductance coil then the output signal from the ME element is amplified and provided to the peak detector.The The currentcoil coil creates a bias magnetic proportional the current theFor coil. For detector. creates a bias magnetic fieldfield proportional to theto in the in coil. signal detector. Thecurrent current coil creates a bias magnetic field proportional tocurrent the current in the coil. For signal conversion the microprocessor can be used. conversion the microprocessor also canalso be used. signal conversion the microprocessor also can be used.

Figure 5. Schematic diagram of the signal processing. Figure5.5.Schematic Schematicdiagram diagramofofthe thesignal signalprocessing. processing. Figure

In the resonant case the signal processing scheme of the current sensor is similar, but the In the resonant case the signal processing scheme of the current sensor is similar, but the In the resonant case the signal processing of thetimes. current sensor is output similar, signal but theissensitivity sensitivity of the scheme increases from ten toscheme a hundred If the level sufficient sensitivity of the scheme increases from ten to a hundred times. If the level output signal is sufficient of the scheme increases from ten to a hundred times. If the level output signal is sufficient for estimation for estimation and father processing, then it is possible to work without the amplifier. for estimation and father processing, then it is possible to work without the amplifier. and father processing, then it is possible to work without the amplifier. 3.4. Construction 3.4.Construction Construction 3.4. The ME current sensor is a system consisting of the ME composite (3), a generator (4), an TheME MEcurrent current sensor a system consisting the ME composite (3), a generator (4), an The sensor is a is system consisting the of ME (3),aa body. generator (4), an amplifier, amplifier, a peak detector, two coils (1, 2) placedofinto one composite another and Amplifier and peak amplifier, a peak detector, two coils (1,into 2) placed into one another and a body. Amplifier and peak adetector peak detector, coils (1, 2) placed one another and a body. Amplifier peak detector attachedtwo to the back side of the generator. The construction of the DC MEand sensor is shown in detector attached to the back side of the generator. The construction of the DC ME sensor is shown in attached side of thethe generator. The the DCtoME Figure 6. Figure 6.to It the canback be noted that design of anconstruction AC sensor isofsimilar thesensor designisofshown a DC in sensor and Figure 6.noted It canthat be the noted that of the of anisAC sensor is similar to the design ofand a DC sensor the and It can bein design andesign AC sensor similar to the design ofsensor a DC sensor differs the presence of the conductor near the ME element. An AC can also bediffers used in like a differs in the presence of the conductor near the ME element. An AC sensor can also be used like a presence of the conductor the ME element. An AC sensor alsosensitivity be used like DCsensors sensor only DC sensor only without near the generator and modulating coil.can The of aME will DC sensor only without the generator and modulating coil. The sensitivity of ME sensors will without generator coil. The of ME sensors will should mainly depend on the mainly the depend on and the modulating ME properties andsensitivity the maximal sensitivity occur at the mainly depend on the ME properties and the maximal sensitivity should occur at the ME properties and resonance the maximal sensitivity should occur at the electromechanical resonance frequency. electromechanical frequency. electromechanical resonance frequency.

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Figure 6. Design of a magnetoelectric non-resonant current sensor prototype: (a) non-resonant Figure 6. Design of a magnetoelectric non-resonant current sensor prototype: (a) non-resonant current current sensor; (b) current sensor in the body. sensor; (b) current sensor in the body.

3.5. Characteristics 3.5. Characteristics The theoretical curve and the experimental points of the output characteristic of ME The theoretical curve and the experimental points of the output characteristic of ME non-resonant non-resonant current sensor are shown in Figure 7. The output voltage of the sensor was calculated current sensor are shown in Figure 7. The output voltage of the sensor was calculated by the by the following equation: following equation: I− m p L q I+ K H d31 nUin mKp ν d amp 0 11 K K Lq11 H 0 d 31nU in Uout =amp (4) (4) m s p s εε dc R U out s 0 11 11 m p 11 s11 s11 0 c11 Rs where, Kamp is a coefficient of amplifier, K is a coefficient due to manufacturing technology, Uin is where, Kamp is a coefficient of qamplifier, K is a coefficient due cto technology, U in is an an input amplitude voltage, module, is an effective longitudinal stiffness 11 is a piezomagnetic 11 manufacturing p v/p s + m v/m s module, p s , m sc11 are input amplitude voltage, q11 cis a piezomagnetic is an effective longitudinal stiffness coefficient of the composite, = , the coefficients of compliance for 11 11 11 11 11 pv/ps11 + mv/ms11, ps11, ms11 are the m p coefficient of the composite, c 11 = coefficients of compliance for piezoelectric and magnetostrictive phase at constant electric field, v, v are the volume fractions of piezoelectric and and magnetostrictive phase atrespectively. constant electric field, mv, pv are the volume fractions of magnetostrictive piezoelectric phases, m m p m p magnetostrictive The term vand = piezoelectric L/( L + L)phases, and vrespectively. = p L/(p L + m L), where m L, p L are the thicknesses of mL/(pL + mL) and pv = pL/(pL + mL), where mL, pL are the thicknesses of magnetostrictive The term mv =and magnetostrictive piezoelectric layers, respectively, d31 is a piezoelectric module, n is a number and piezoelectric layers, respectively, 31 is a piezoelectric module, n is a of number windingsε per of windings per unit length of the ACdsolenoid, Rs is an active resistance the ACofsolenoid, is a unit length of the ACofsolenoid, Rs is an active of the AC ε isshow a dielectric dielectric permittivity piezoelectric phase, ε0 is anresistance electric constant. Thesolenoid, calculations suitable permittivity of piezoelectric phase, ε0 is an electric constant. matching matching between theory and experiment. Consequently, allThe thecalculations parameters show of the suitable sensor are chosen between theory and experiment. Consequently, all the parameters of the sensor are chosen quite quite optimally. optimally. In our case, the magnetostrictive material Metglas based on iron alloy was used, and the In our case, thecoefficient magnetostrictive material Metglas based in onFigure iron alloy was used, and the dependence of ME on bias magnetic field is shown 1. The plot of the curve max can be dependence coefficient onused bias magnetic field is shown with in Figure The plot Measurements of the curve from from αME startoftoME αME for designing a sensor high1.linearity. of start to МЕmax can be used for designing a sensor with high linearity. Measurements of Metglas МЕ Metglas parameters and calculations necessary to determine the values of bias magnetic field and the parameters and calculations necessary to sensitivity determineofthe values of bias magnetic field andrange the maximal measured current were made. The sensor reached 0.34 V/A. The working maximal measured current made. The sensitivity of sensoroutput reached 0.34 V/A.1%. The working of currents was set up to 5 A.were The nonlinearity of the characteristic was within range of currents was set up to 5 A. The nonlinearity of the characteristic output was within 1%.

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Figure 7. Output characteristics of the ME non-resonant current sensor.

Figure Figure 7. 7. Output Output characteristics characteristics of of the the ME ME non-resonant non-resonant current current sensor. sensor.

4. Resonant Current Sensor

4. 4. Resonant Current Sensor Sensor 4.1. Sensor Design

4.1. Design 4.1. Sensor SensorThe Design resonant current sensor has a similar design as the non-resonant one. It also consists of a generator, inductance MEhas composite, permanent magnet, current coil (sensing head), power of The resonant current sensor aa similar design the one. consists The resonant currentcoil, sensor has similar design as as the non-resonant non-resonant one. It It also also consists of aa supplyinductance and rectifiercoil, (signal processing). The block diagram of thecurrent resonantcoil current sensorhead), is shown generator, ME composite, permanent magnet, (sensing power generator, inductance coil, ME composite, permanent magnet, current coil (sensing head), power in Figure 8. supply supply and and rectifier rectifier (signal (signal processing). processing). The The block block diagram diagram of of the the resonant resonant current current sensor sensor is is shown shown in Figure 8. in Figure 8.

Figure 8. Block diagram of resonant current sensor.

The principle of operation of the sensor is based on measuring the electromotive force appearing at the outputFigure of sensitive element dueoftoresonant the ME current effect assensor. a result of the influence of the 8. Block diagram Figure Block diagram of resonant currentissensor. alternating magnetic field and a8.bias magnetic field. The amplifier not applicable here because the output signal level enough forof estimation. In this the rectifier (diode bridge) is sufficient. The principle of isoperation the sensor is case based on measuring the electromotive force

The principle of operation of the element sensor isdue based appearing appearing at the output of sensitive to on themeasuring ME effect the as aelectromotive result of the force influence of the 4.2. Construction at the output of sensitive element due to the ME effect as a result of the influence of the alternating alternating magnetic field and a bias magnetic field. The amplifier is not applicable here because the The sensing head is similar as in the sensor.(diode We used the same composite, magnetic field and aisbias magnetic field. Thenon-resonant amplifier iscurrent not here because the output signal output signal level enough for estimation. In this case theapplicable rectifier bridge) is sufficient. but with a slight difference. It consists of piezoelectric and magnetostrictive layers as shown in level is enough for estimation. In this case the rectifier (diode bridge) is sufficient. Figure 3. The ME current sensor shown in Figure 9 consists of the ME composite (3), the generator

4.2. Construction 4.2. Construction The sensing head is similar as in the non-resonant current sensor. We used the same composite, The sensing head is similar as in the non-resonant current sensor. We used the same composite, but with a slight difference. It consists of piezoelectric and magnetostrictive layers as shown in but with a slight difference. It consists of piezoelectric and magnetostrictive layers as shown in Figure 3. Figure 3. The ME current sensor shown in Figure 9 consists of the ME composite (3), the generator

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The ME current sensor shown in Figure 9 consists of the ME composite (3), the generator (4), the rectifier (4), the rectifier (5), permanent magnet, the inductance (2) and current (1) coils (coils placed into one (5), permanent magnet, the inductance (2) and current (1) coils (coils placed into one another). another). The ME composite is a layered structure which includes a thin piezoceramic plate of PZT placed The ME composite is a layered structure which includes a thin piezoceramic plate of PZT between two magnetostrictive layers of Metglas performing the function of electrodes. The final placed between two magnetostrictive layers of Metglas performing the function of electrodes. The dimensions of the composite were 6were ×1× withwith a PZT concentration final dimensions of the composite 6 ×0.62 1 × mm 0.62 mm a PZT concentrationofof0.8. 0.8.The ThePZT PZT layer was poled in an electric in the thickness direction. The METhe laminate operates in a transverse layer was poled in anfield electric field in the thickness direction. ME laminate operates in a mode regime in which thein applied magnetic (H~fields , H0 )(H are parallel each each other andand areare located transverse mode regime which the applied fields magnetic ~, H 0) are parallel other located in the plane of the laminate. The sensing head system comprises the generator, the in the plane of the laminate. The sensing head system comprises the generator, the inductance and inductance coilsThe andgenerator ME element. generatorcoincidedent had a frequency coincidedent a current coils andand MEcurrent element. hadThe a frequency with a resonancewith frequency resonance frequency of the ME composite. The coil is wound on the composite and designed to of the ME composite. The coil is wound on the composite and designed to induce an AC magnetic an AC magnetic field. The rectifier is a diode bridge and converts an AC to a DC. Such field.induce The rectifier is a diode bridge and converts an AC to a DC. Such sensors can detect the AC and sensors can detect the AC and DC in the range from 0.01 A to 100 A depending on the design DC in the range from 0.01 A to 100 A depending on the design parameters. parameters.

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Figure 9. Design of magnetoelectric resonant current sensor prototype: (a) view from above; (b)

Figure 9. Design of magnetoelectric resonant current sensor prototype: (a) view from above; bottom view. (b) bottom view.

4.3. Characteristics

4.3. Characteristics

Resonant ME current sensors were designed. The sensors work in the electromechanical range with the resonance frequency = 168 kHz. TheThe sensor sensitivity ME voltage coefficientrange Resonant ME current sensorsfreswere designed. sensors work and in the electromechanical dependencies were investigated using the designed measurement setup. The measurement stand with the resonance frequency fres = 168 kHz. The sensor sensitivity and ME voltage coefficient includes two power supplies an APS-7315 (Aktakom, Moscow, Russia) and a HMP4040 (HAMEG dependencies were investigated using the designed measurement setup. The measurement stand Instruments, Mainhausen, Germany), a HM 8112-3 multimeter (HAMEG Instruments, Mainhausen, includes two power supplies an APS-7315 (Aktakom, Moscow, Russia) and a HMP4040 (HAMEG Germany), a HMO722 oscilloscope (HAMEG Instruments, Mainhausen, Germany) and a GMW 5403 Instruments, Mainhausen, Germany), a HM 8112-3 multimeter (HAMEG Instruments, Mainhausen, electromagnet (Magnet Systems, CA 94070, USA). The graph below shows the output characteristics Germany), a HMO722 oscilloscope (HAMEG Instruments, Mainhausen, Germany) and a GMW 5403 of the resonant current sensor. electromagnet CA 94070, USA). The graph below the output characteristics Figure(Magnet 10 shows Systems, the theoretical curve and the experimental pointsshows of the characteristic output of of thethe resonant resonantcurrent current sensor. sensor. As can be seen from the graph, the calculated and experimental results Figure 10 shows the theoretical andselected the experimental points ofparameters the characteristic output of are in good agreement. This meanscurve that the sensor and materials are optimal. The maximum linearity is reached at the additional bias field of 10 Oe. Also it is known that the resonant current sensor. As can be seen from the graph, the calculated and experimentalthe results sensitivity is describedThis by the slope of thethe characteristic output.and The sensitivity the sensor reached are in good agreement. means that selected sensor materials of parameters are optimal. 0.53 V/A. The current working range at 5 A. Thebias nonlinearity output was The maximum linearity is reached atwas the set additional field of of 10the Oe.characteristic Also it is known that the within 0.5%. The resonant frequency of composite for the first axial mode was calculated by the sensitivity is described by the slope of the characteristic output. The sensitivity of the sensor reached following equation:

0.53 V/A. The current working range was set at 5 A. The nonlinearity of the characteristic output was within 0.5%. The resonant frequency 1of composite s11p rs11mfor the first axial mode was calculated by the fres (5) p m following equation: 2L v s11s11 (r p m ) u p m rs11 1 u t ofs11 where L is the plate length, pρ, mρ are the+piezoelectric and magnetic phases, r is the (5) f res the = density p m p 2L s11 s11 (r ρ + m ρ ) ratio of thicknesses, r = mL/pL. where L is the plate length, p ρ, m ρ are the density of the piezoelectric and magnetic phases, r is the ratio of thicknesses, r = m L/p L.

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For calculation of the characteristic output the following equation was obtained: For calculation of the characteristic output the following equation was obtained:

I I− d31 nUin m m H 8KQ ν p L q11 0 + dd nU H 8 KQ p Lq 31 in 11 0 Uout = m p d U out m s11 s11 εε 0 c211 π 2 ( Rs + i2π f res Ls ) p s11 s11 0 c11 Rs i 2 f res Ls

(6)

(6)

where, Q isQthe quality is resonance frequency, Ls inductance is the inductance where, is the qualityfactor factorof of resonance, resonance, fresfres is resonance frequency, Ls is the of the ACof the AC solenoid. solenoid.

Figure 10. Characteristic output of the ME resonant current sensor.

Figure 10. Characteristic output of the ME resonant current sensor.

5. Magnetoelectric Equivalent Circuits

5. Magnetoelectric Equivalent Circuits 5.1. Magnetoelectric Equivalent Circuits: Non-Resonant Current Sensor

5.1. Magnetoelectric Equivalent Circuits: Non-Resonant Current Sensor

For a better understanding of the current processes it is suggested to consider the magneto-electro-mechanical circuit of (Figure equivalent to a non-resonant current sensor [12,13]. the For a better understanding the 11), current processes it is suggested to consider The magnetic circuit (I) comprises an AC voltage source and an inductance coil. Flowing in the magneto-electro-mechanical circuit (Figure 11), equivalent to a non-resonant current sensor [12,13]. magnetic circuit (I) an electric current by means of an ideal transformer causes in mechanically The magnetic circuit (I) comprises an AC voltage source and an inductance coil. Flowing in the circuit (II) the occurrence of mechanical current ῡ. Mechanical circuit (II) is connected with the magnetic circuit (I) an electric current by means of an ideal transformer causes in mechanically second ideal transformer with an electric circuit (III), thus the mechanical current results in flow of ¯ Mechanical circuit (II) is connected with the second circuit (II) the occurrence of mechanical current v. electric current i in an electric circuit. The output voltage V is taken from the capacitor C0 of electrical idealcircuit. transformer with an electric circuit (III), thus the mechanical current results in flow of electric current i in circuit. The output voltage V is taken (L from the capacitor C active of electrical circuit. Foran theelectric calculation we used the value of coil inductance s = 5.83 × 10−3 H) and 0 resistance −3 H) and active resistance For the calculation we used the value of coil inductance (L = 5.83 × 10 (Rs = 510 Ohm) obtained from the experimental data. It is necessary to make the resistance Z as high s (Rs =as510 Ohm) then obtained from the circuit experimental data. It isinput necessary toofmake the resistance Z as as high possible, the mechanical will not distort the current the solenoid circuit. So not to distort the mechanical thenot output circuit C0 mustofbethe reduced by the same So as as possible, then the mechanicalconnection circuit will distort thecapacity input current solenoid circuit. amount have increased connection the resistancethe Z: output circuit capacity C0 must be reduced by the same not to distortwe the mechanical

amount we have increased the resistance Z:1 kl Z jM vA1 cot 2 1 2 kl Z = − jMρvA1 cot 2 2 d312 lb 33 p s11 d231 − ps C0 lb ε 33 p 11 C0 = M t p M t M >> 1

(7)

(7) (8)

(8) (9)

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M >> 1

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(I) (I) Magnetic Magnetic section

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(II) (II)

(III) (III)

Mechanical Mechanical section

Electric Electric section

(9)

Figure 11. The equivalent circuit of ME non-resonant DC sensor. Figure 11. The equivalent circuit of ME non-resonant DC sensor.

In [13], authors used the input section with the magnetic field source. In our case the input In [13], authorsan used the input section with thethe magnetic field source. In our case the section section contains electric current. Therefore, ideal ratio of transformation hasinput the following contains form: an electric current. Therefore, the ideal ratio of transformation has the following form: mm νq q1111 A n ϕm = 1 m mm s A1n s 11

(10) (10)

11

For this equivalent circuit the equations areare asas follows: For this equivalent circuit the equations follows: .

Rss JJ + jωL j Ls JJ = U u u R Uinin +ϕmm . Zu Zum = J ϕmpJV+ ϕ p V . Jdisp = jωC0 V − ϕ p u

(11) (11)

J disp jC0V p u

Using the open-circuit conditions Jdisp = 0, where Jdisp = (δD3 /δt) = 0, the following equation for Using the open-circuit conditions Jdisp = 0, where Jdisp = (δD3/δt) = 0, the following equation for the the output voltage was obtained: output voltage was obtained: m ϕp ϕ Uout = U (12) U out jωC0 [ Z ( Rs + jωLs )m− pϕ22m ] − ϕ2 2p ( Rs + jωLs )U inin (12)

jC0 Z Rs j Ls m p Rs j Ls

In order to match the theoretical and experimental data, we introduced a coefficient due to In order technology to match the and experimental data, we and introduced a coefficient dueofto manufacturing K =theoretical 0.087. In Figure 12 the theoretical curve the experimental points manufacturing K = 0.087. Figure 12 the theoretical curve and the experimental points output voltage fortechnology low frequency sensorInare presented. of output voltage for low frequency sensor are presented. Study of the equivalent circuit for the low-frequency mode of operation, allowed us to find the Study oftothe equivalent circuit for theK, low-frequency of operation, allowed us to find the coefficient due manufacturing technology necessary for mode the further consideration of the resonant coefficient due to manufacturing technology K, necessary for the further consideration of the mode of operation. resonant mode of operation.

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Figure 12. Output voltage depending on the frequency for the non-resonant current sensor. Figure12. 12.Output Outputvoltage voltagedepending dependingon onthe thefrequency frequencyfor forthe the non-resonant non-resonant current current sensor. sensor. Figure

5.2. Magnetoelectric Equivalent Circuit for Resonant Current Sensor 5.2. Magnetoelectric Magnetoelectric Equivalent Equivalent Circuit Circuit for for Resonant Resonant Current Current Sensor Sensor 5.2. When considering the ME equivalent circuit, we generally followed the method developed in When considering considering the ME circuit, wewe generally followed thethe method developed in [10]. MEequivalent equivalent generally developed in [10].When In that paper, thethe authors used thecircuit, magnetic circuit of followed the magneticmethod field source directly, In that paper, the authors used the magnetic circuit of the magnetic field source directly, without [10]. In that paper, how the authors used field the magnetic of thewe magnetic fieldthe source directly, without specifying the magnetic is created.circuit In contrast, considered appearance of specifying how the magnetic field is created. Increated. contrast,In wecontrast, considered the appearance of the magnetic without specifying how the magnetic field is we considered the appearance the magnetic field during the flow of electric current through the inductor. This allowed us of to fieldmagnetic during the flowduring of electric current through the inductor. This allowed us to consider the resulting the field the flow of electric current through the inductor. This allowed us to consider the resulting frequency response of the sensor to the complex resistance of the AC coil. frequency response of frequency the sensorresponse to the complex resistance of the AC coil. consider the resulting of the the sensor sensor to the complex resistanceelectrical of the ACequivalent coil. In the study of the resonant mode of a completely simplified In the study of the resonant mode of the sensor a completely simplified electrical electrical equivalent equivalent In the study of the resonant mode of the sensor a completely simplified circuit can be considered (Figure 13). The input voltage equals Uin, whereby the current through the circuit can can be considered considered (Figure 13). 13). The input input voltage voltage equals equals U Uinin,,whereby whereby the the current current through through the the circuit coil flows. be This gives rise(Figure to a currentThe in the resonant circuit containing the dynamic inductance of L, coil flows. This gives rise to a current in the resonant circuit containing the dynamic inductance of coil flows. This gives rise to a current the resonant containing of L, resistance R and capacitance C. The in output voltagecircuit is removed fromthe thedynamic capacitorinductance C1 constituting L, resistance R and capacitance C. Theoutput outputvoltage voltageisisremoved removedfrom fromthe the capacitor capacitor C C11 constituting constituting resistance R and The together with thecapacitance C2 capacity,C. full operating capacity. together with the C capacity, full operating capacity. 2 together with the C2 capacity, full operating capacity.

Figure 13. The equivalent circuit of ME resonant DC sensor. Figure13. 13.The Theequivalent equivalentcircuit circuitof ofME ME resonant resonant DC DC sensor. sensor. Figure

ME voltage coefficient is given by the following equation: ME voltage coefficient is given by the following equation:

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11 of 13 m p q11d31 p s11 tan E E m h1 s11 0 p s112 c11 d312 tan c11 p s11

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(13)

ME voltage coefficient is given by the following equation: where: αE =

m ν p νq d p s tan( η ) E kl 11 31 11 = −m 2 2 p h1 s11 εε 0 s11 c11 2 η + d31 tan(η ) − c11 p s11 η

The magnetic field inside a long solenoid: where:

(13) (14)

kl

= h1 η nJ 2

(14) (15)

Then magnetic field of inside long solenoid: where, is the number coilsaper unit length, J is a current. Current in the solenoid: h1 = nJ

(15)

U J J is ina current. where, n is the number of coils per unit length, Rs j Ls Current in the solenoid:

J=

Output voltage:

Output U out voltage: E p L m L E h1 Uout

p

L mL

(16)

Uin Rs + jωLs

(16)

p Lq11d31 p s11 tan

m

nJ s11 0 p s112 c11 d312 tan c11 p s11 m ν P L q d p s tan( η ) pL + ML = − M 11 31 11 nJ = E p Lm+ pα E h1 p s2 c η + d2 tan( η )− c p s η m p LqL11d= s n tan U s εε in 11 11 ]) 31 11 11 ( 0 11 11 31 [ m νp L q

d

ps

m

n tan(η )U

31 11 p 2 =s11− s11cp11 d312d112 tan c c11p s p sinη11])(R +RiωL m s 0 ( εε s )i Ls s2 c η + [tan(η )−

m

11

0

11 11

31

11

11

s

(17) (17)

s

Taking into into account account the the calibration calibration coefficient coefficientK, K,the thefollowing followingratio ratioof ofvoltages voltageswas wasfound: found: Taking p K mKpmLq L qd 31d s11 p n tan U out Uout ν p11 31 s11 n tan( η ) 11 = − 2p 2 2 2 U in Uin m s11 m s110 pεεs11 d31 tan c11 ηd+ Ls s) (η) −c11c11p sp11s11η R(Rs s+iiωL 0 cs11 11 31 tan

(18)

The The calculation calculation and and experimental experimental data data results results are are presented presented in in Figure Figure14. 14.

Figure 14. Amplitude frequency characteristics of the resonant current sensor. Figure 14. Amplitude frequency characteristics of the resonant current sensor.

The value of the resonant frequency calculated by Equation (4) is of fres = 168 kHz, which agrees value of the resonant frequency by line Equation of fres =of168 which agrees with The the experimental data. The solid linecalculated and dotted show (4) theisresults thekHz, calculation by the with the experimental data. The solid line and dotted line show the results of the calculation by the equation and the equivalent circuit, respectively. The considered equivalent circuit adequately

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describes the current operation of the sensor in the resonant mode, as the frequency response of the equivalent circuit is in good agreement with theoretical frequency response and accurate experimental data. 6. Performance Comparison of Existing Current Sensors Table 1 shows the comparative characteristics of DC current sensors. The information is about sensors produced by LEM Holding SA (HO8-NP, Freiburg, Switzerland), Honeywell Inc. (CSLW6B5, Morristown, NJ, USA), Allegro MicroSystems (ACS712ELCTR-05B-T, Worcester, MA, USA). As can be seen from the table, the ME current sensor has higher sensitivity and lower current consumption. Table 1. Performance comparison of current sensors. Sensors

HO8-NP

CSLW6B5

ACS712ELCTR-05B-T

Magnetoelectric Sensor

Measuring principle

Hall effect measuring principle

Miniature ratiometric linear Hall effect sensor

Hall effect sensor

Magnetoelectric effect

Primary current, measuring range, Ipm (A)

0–20

±5

±5

0–5

Sensitivity (V/A)

0.1

0.2

0.185

0.53

Supply voltage (V)

5 ± 10%

4.5–10.5

5 ± 10%

5 ± 10%

Current consumptions (mA)

19

9

10

2.5

Accuracy (%)

1

0.5

1.5

0.5

Output voltage range Uout (V)

2.5−0.5

2.7–3.7

2.5–4.5

1.5–4.5

Size (mm)

24 × 12 × 12

16.2 × 14 × 10

6 × 5 × 1.75

30 × 20 × 10

Known current transformers (CTs), with high breakdown voltage, are the most widely applied devices for AC sensing in traditional power systems. CTs can’t be used as DC sensors because they are based on Faraday’s law of induction. Disadvantages of CTs are: large size, high price, limited bandwidth and large consumption of metal resources, so they are only used in power stations and substations. Shunts are more often applied in DC converter stations and power electronics. Disadvantages of shunts are: the measured current has to be interrupted into the sensor, an overcurrent may permanently damage it, and the intrinsic inductance limits the accuracy and bandwidth. Hall effect current sensors are mainly applied in non-contact current measurements. The problems of the Hall effect sensors are: low sensitivity, low breakdown voltage, and susceptibility to temperature, which limits them to applications in high voltage power systems [2]. Compared with the current sensors presented above, the ME current sensor has advantages of higher sensitivity, higher linearity, lower cost, simple structure of sensor, which make it most promising for current measurements. 7. Conclusions In this paper the principle of operation of ME current sensors was considered. Theoretical and experimental investigations of both low frequency and resonant current sensors were discussed. The optimal materials for sensitive elements were chosen and PZT and Metglas were used, respectively in the the piezoelectric and magnetostrictive phases. The equivalent circuit method was used to calculate the characteristics of the sensors. The calculation results allowed us to estimate the internal parameters. The design and development of non-resonant and resonant current sensors were discussed.

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The characterization of the non-resonant current sensor showed that in the operation range up to 5 A, its sensitivity was 0.34 V/A, and its non-linearity was less than 1% and for the resonant one in the same operation range, the sensitivity was 0.53 V/A, and the non-linearity less than 0.5%. The resonant current sensor has higher sensitivity compared with the non-resonant current sensor, therefore it is recommended for the detection of very low currents, for example, leakage currents. Finally, ME current sensors can be used for measuring equipment, power nets and control systems, security systems and safety systems; in metal detectors and the automotive industry, rail transport; in wireless electricity metering systems and space technology and robotics. Acknowledgments: The research was supported by the Russian Science Foundation, grant No. 15-19-10036. Author Contributions: Mirza Bichurin conceived the project, organized the paper content, and edited the manuscript. Roman Petrov designed magnetoelectric current sensors. Viktor Leontiev performed the experimental work and wrote the manuscript. Gennadiy Semenov performed the experimental work and wrote the manuscript. Oleg Sokolov performed theoretical calculations and modeling of equivalent circuits. Conflicts of Interest: The authors declare no conflict of interest.

References 1. 2. 3.

4. 5. 6. 7.

8.

9. 10. 11. 12. 13.

Pavel, R.; Alois, T. (Eds.) Modern Sensors Handbook; ISTE Ltd.: London, UK, 2007; p. 518. Ouyang, Y.; He, J.; Hu, J.; Wang, S.X. A current sensor based on the giant magnetoresistance effect: Design and potential smart grid applications. Sensors 2012, 12, 15520–15541. [CrossRef] [PubMed] Bichurin, M.I.; Petrov, V.M.; Petrov, R.V.; Tatarenko, A.S. Magnetoelectric magnetometers. In High Sensitivity Magnetometers; Grosz, A., Michael, J.H.-S., Subhas, C.M., Eds.; Springer International Publishing: Cham, Switzerland, 2016; pp. 127–166. Bichurin, M.I.; Petrov, V.M.; Petrov, R.V.; Kiliba, Y.V.; Bukashev, F.I.; Smirnov, A.Y.; Eliseev, D.N. Magnetoelectric sensor of magnetic field. Ferroelectrics 2002, 280, 199–202. [CrossRef] Palneedi, H.; Annapureddy, V.; Priya, S.; Ryu, J. Status and Perspectives of Multiferroic Magnetoelectric Composite Materials and Applications. Actuators 2016, 5, 9. [CrossRef] Lu, C.; Li, P.; Wen, Y.; Yang, A.; Yang, C.; Wang, D.; He, W.; Zhang, J. Magnetoelectric Composite Metglas/PZT-Based Current Sensor. IEEE Trans. Magn. 2014, 50, 1–4. [CrossRef] Petrov, R.V.; Yegerev, N.V.; Bichurin, M.I.; Aleksi´c, S.R. Current sensor based on magnetoelectric effect. In Proceedings of the 18th International Symposium on Electrical Apparatus and Technologies (SIELA), Bourgas, Bulgaria, 29–31 May 2014. Solovyev, I.N.; Solovyev, A.N.; Petrov, R.V.; Bichurin, M.I.; Vuˇckovi´c, A.N.; Raiˇcevi´c, N.B. Sensitivity of Magnetoelectric Current Sensor. In Proceedings of the 11th International Conference on Applied Electromagnetics—ΠEC 2013, Niš, Serbia, 1–4 September 2013; pp. 109–110. Petrov, R.V.; Solovyev, I.N.; Soloviev, A.N.; Bichurin, M.I. Magnetoelectic current sensor. In Proceedings of the PIERS Proceedings, Stockholm, Sweden, 12–15 August 2013; pp. 105–108. Bichurin, M.I.; Petrov, V.M. Modeling of Magnetoelectric Effects in Composites. Springer Ser. Mater. Sci. 2014, 201, 108. Magnetoelectricity in Composites; Bichurin, M.; Viehland, D. (Eds.) Pan Stanford Publishing: Singapore, 2011; p. 286. Balabanian, N. Fundamentals of Circuit Theory; Literary Licensing, LLC: Whitefish, MT, USA, 2012; p. 570. Dong, S.X.; Zhai, J.Y. Equivalent circuit method for static and dynamic analysis of magnetoelectric laminated composites. Chin. Sci. Bull. 2008, 53, 2113–2123. [CrossRef] © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

Magnetoelectric Current Sensors Mirza Bichurin *, Roman Petrov, Viktor Leontiev, Gennadiy Semenov and Oleg Sokolov Department of Design and Technology of Radioequipment, Novgorod State University, Veliky Novgorod 173003, Russia; [email protected] (R.P.); [email protected] (V.L.); [email protected] (G.S.); [email protected] (O.S.) * Correspondence: [email protected]; Tel.: +7-8162-974-267 Academic Editor: Nian X. Sun Received: 1 March 2017; Accepted: 30 May 2017; Published: 2 June 2017

Abstract: In this work a magnetoelectric (ME) current sensor design based on a magnetoelectric effect is presented and discussed. The resonant and non-resonant type of ME current sensors are considered. Theoretical calculations of the ME current sensors by the equivalent circuit method were conducted. The application of different sensors using the new effects, for example, the ME effect, is made possible with the development of new ME composites. A large number of studies conducted in the field of new composites, allowed us to obtain a high magnetostrictive-piezoelectric laminate sensitivity. An optimal ME structure composition was matched. The characterization of a non-resonant current sensor showed that in the operation range to 5 A, the sensor had a sensitivity of 0.34 V/A, non-linearity less than 1% and for a resonant current sensor in the same operation range, the sensitivity was of 0.53 V/A, non-linearity less than 0.5%. Keywords: current sensor; magnetoelectricity; magnetoelectric effect; sensor design

1. Introduction Current sensors are very important types of devices. There are a large number of current sensors operating on various physical principles. The most common current sensors are current transformers, magnetoresistance and Hall sensors [1,2]. Despite the fact that magnetoelectric (ME) current sensors have the small size, weight and a high sensitivity they have received relatively little attention in the publications, compared with the ME magnetic field sensors [3]. The first attempt to measure the direct current (DC) based on the ME effect was made in [4]. There are also a number of articles about ME current sensors [5,6]. Evaluation of the value of current flowing in a conductor is possible by measuring the corresponding magnetic field. The ME current sensor uses the ME effect as a basis of its measurements. As is well-known, the ME effect is a polarization response to an applied magnetic field, or conversely a magnetization response to an applied electric field. ME behavior exists as a composite effect in multiphase systems of piezoelectric and magnetostrictive materials. In a magnetostrictive-piezoelectric layered structure the interaction between magnetic and electric subsystems occurs through mechanical deformation. It means that the ME effect is much stronger at frequencies corresponding to the electromechanical resonance range. In current sensor applications the induced ME voltage coefficient is more important than the induced ME electric field coefficient, as voltage is the physical quantity measured. ME current sensors can work in different regimes. In the first case, the ME element works in a non-resonant mode regime, and in the second case, in the resonant one. The same ME element can be used as the sensitive element of the sensor in both cases. The properties of non-resonant ME current sensors were considered in [7,8], and of the resonant ones in [9]. In the paper we consider the basic principles of work of ME DC sensors of non-resonant type based on using the low-frequency ME effect and next the resonant type, working in one excited mode in the piezoelectric phase of magnetoelectric material. It should be noted that the paper describes the construction and characteristics of the ME current sensors used in practice. Sensors 2017, 17, 1271; doi:10.3390/s17061271

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2. Principle of Operation 2. Principle of Operation Magnetoelectric current sensor consists of a ME element made in the form of the piezoelectric current consists of a ME element madeon. in The the form of the plateMagnetoelectric and the side surfaces of sensor the magnetostrictive plates are glued principle of piezoelectric operation of plate and the side surfaces of the magnetostrictive plates are glued on. The principle of operation of ME sensor is shown in Figure 1. The figure on the left shows the dependence of dc magnetic field on ME sensor is shown in Figure 1. The figure on the left shows the dependence of dc magnetic field on the ME coefficient. The zero sensitivity of the sensor corresponds to a dc magnetic field equal to zero. the ME The zero sensitivity of the sensor corresponds to a dc magnetic field equal to zero. The MEcoefficient. coefficient α ME is determined by the following expression: The ME coefficient αME is determined by the following expression: α ME = f ( H0 + H_ + H∼ ) (1) αME = f(H0 + H_ + H~) (1) Passing the initial nonlinear part, we get to the linear section. In this linear section, the current m q , where K is the gain factor, andm m q sensor is working. The term αME canbe bewritten writtenas as ααME ME can , where K is the gain factor, and q31 31 is ME==KKmq31 31 is the piezomagnetic module of the magnetic phase. The figure on the right shows the output the piezomagnetic module of the magnetic phase. The figure on the right shows characteristic indicated by by thethe redred lineslines on the The tuning of the of linear characteristic of ofthe thecurrent currentsensor, sensor, indicated on left thefigure. left figure. The tuning the section occurs occurs by introducing a magnetic field H0field . Here conductor or a current replaces linear section by introducing a magnetic Ha0. current Here a current conductor or acoil current coil areplaces bias magnetic for inducing theinducing ME effect. The modulating coil is needed for of the a bias field magnetic field for the ME effect. The modulating coildefinition is needed for frequency Then everyone can choose to operate the linear portion of ME portion voltage coefficient definition range. of the frequency range. Then everyone can choose to operate thethe linear of the ME dependence and getdependence a result of the sensitivity dependence for the ME sensor, that theME dependence of voltage coefficient and get a result of the sensitivity dependence foristhe sensor, that the output voltage of onthe theoutput input measuring is the dependence voltage on current. the input measuring current.

Figure 1. Magnetoelectric current sensor: principle of operation.

3. Non-Resonant 3. Non-Resonant Current Current Sensor Sensor 3.1. Sensor Design The ME non-resonant current sensor consists of three major parts: the the sensing sensing head head (generator, (generator, inductance coil, ME element, current coil), power supply and the signal processing (amplifier, peak detector) part. Each part affects the performance of the current sensor. The block diagram of the ME non-resonant current currentsensor sensorisisshown shownininFigure Figure2.2.The Theprinciple principle operation sensor is based non-resonant ofof operation of of thethe sensor is based on on measuring the electromotive appearing the output of sensitive dueME to effect the ME measuring the electromotive forceforce appearing at the at output of sensitive elementelement due to the as asof a result of the influence of the alternating a bias magnetic field. aeffect result the influence of the alternating magneticmagnetic field and field a biasand magnetic field.

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Figure 2. Block diagram of ME non-resonant current sensor. Figure 2. Block diagram of ME non-resonant current sensor. Figure 2. Block diagram of ME non-resonant current sensor.

Figure 2. Block

3.2. Sensing Head 3.2. Sensing Head 3.2. Sensing Head 3.2. Sensing Head The ME element, shown in Figure 3, is the primary part of the whole currentThe sensor, its shown in Figu ME and element, The ME element, shown in Figure 3, is the primary part of the whole current sensor, and its function is to convert the measuring current into the voltage signal that can be measured easily. The function is to convert The ME element, shown in Figure 3, is the primary part of the whole current sensor, and its the measuring function is to convert the measuring current into the voltage signal that can be measured easily. The ME element is the sensitive part ofcurrent a ME into current sensor and inthat our can case, consists of piezoelectric ME element is the function is to convert the measuring the voltage signal beitmeasured easily. The MEsensitive part of a ME element is the sensitive part of a ME current sensor and in our case, it consists of piezoelectric and magnetostrictive in Figure 3. and The in layered structure based on a piezoceramic and magnetostrictive element is the sensitivelayers part ofasa shown ME current sensor our case, it consists of piezoelectric and layers as show and magnetostrictive layers as shown in Figure 3. The layered structure based on a piezoceramic PZT plate in our caseashad thickness of 0.38 mm, length of 10 mm and of 1plate mmPZT [8,9]. The PZT in our case had thickness magnetostrictive layers shown in Figure 3. The layered structure based on width a piezoceramic plate PZT plate in our case had thickness of 0.38 mm, length of 10 mm and width of 1 mm [8,9]. The polarized in the thickness Thewidth electrodes are[8,9]. applied on two sideswas of the piezoelectric polarized in the thi inpiezoelectric our case hadwas thickness of 0.38 mm, length ofdirection. 10 mm and of 1 mm The piezoelectric was piezoelectric was polarized in the thickness direction. The electrodes are applied on two sides of the magnetostrictive plates. Thickness of one layer ofare Metglas was (1) and PZT plates. (3) magnetostrictive Thickness o polarized in the thickness direction. The electrodes applied on 0.02 two mm. sides Metglas of the magnetostrictive magnetostrictive plates. Thickness of one layer of Metglas was 0.02 mm. Metglas (1) and PZT (3) plates Thickness are connected bylayer gluing Different of glue can be The thickness the adhesive by gluing (2). Di platesof are connected plates. of one of (2). Metglas was types 0.02 mm. Metglas (1)used. and PZT (3) plates are connected plates are connected by gluing (2). Different types of glue can be used. The thickness of the adhesive layer does exceedtypes 10 microns. numbers of Metglas layers layer does exceed 10 microns. T by gluing (2).not Different of glue To canincrease be used. the Thesensitivity, thickness ofdifferent the adhesive layer does notnot exceed layer does not exceed 10 microns. To increase the sensitivity, different numbers of Metglas layers may be used. The electrical signal is taken from the electrodes (4) and then passes on to the signal may be used. The 10 microns. To increase the sensitivity, different numbers of Metglas layers may be used. The electricalelectrical signal is may be used. The electrical signal is taken from the electrodes (4) and then passes on to the signal processing part of the of the device. signal is taken from thedevice. electrodes (4) and then passes on to the signal processing processing part of the part device. processing part of the device.

Figure3.3. Magnetoelectric Magnetoelectric element: element: 1—magnetostrictive 1—magnetostrictive plates plates (Metglas); (Metglas); 2—adhesive 2—adhesive layer; Figure 3. Magnetoelectric elemen Figure layer; Figure 3. Magnetoelectric element: 1—magnetostrictive plates (Metglas); 2—adhesive layer; 3—piezoelectricplate plate(PZT); (PZT);4—electrodes. 4—electrodes. 3—piezoelectric plate (PZT); 4—elec 3—piezoelectric 3—piezoelectric plate (PZT); 4—electrodes.

The sensitive element of the sensor is a magnetostrictive-piezoelectric composite and itselement of the s The sensitive The The sensitive sensitive element element of of the the sensor sensor isis aa magnetostrictive-piezoelectric magnetostrictive-piezoelectric composite composite and and its its magnetostriction constant is a function of magnetic field and in general can bemagnetostriction written as λ = f(H 0) constant is a functi magnetostriction of of magnetic field andand in general can can be written as λ =asf (H [10]. magnetostrictionconstant constantisisa afunction function magnetic field in general be written λ =0 )f(H 0) [10]. In turn the piezomagnetic module depends on AC magnetic field as follows: [10]. In turn the piezomagnetic modu In turn module depends on AConmagnetic field as follows: [10]. In the turnpiezomagnetic the piezomagnetic module depends AC magnetic field as follows: q = dλ/dH (2) q q= = dλ/dH (2) dλ/qH (2) For the transverse ME voltage coefficient of layered composite the following For dependence on the transverse ME voltage For the transverse ME voltage coefficient of layered composite the following dependence on material parameters was obtained [10,11] material parameters was obtained [10 For the transverse voltage[10,11] coefficient of layered composite the following dependence on material parameters wasME obtained material parameters was obtained [10,11] E V (1 V )(mm q11 mm q21 )pp d 31 E E , 31 E33 α E , 31 =(3)3 = V (1 pV )( qp11 qp21 ) d 31 p m m p 2 p m m p E , 31 H H V()( q11p+s11q)( (3) 1 E31 33 (m s12 m s11−)VV(p1 − 33 V31) 2p d231 (1 V ) ε 33 ( m s12 + 211) d p ss12 α E,31 = H1 = pp 33 (3) ( s s ) V ( s )( 1 V ) 2 d ( 1 V ) 12+ m s11 )V + p ε33 ( p s12 + p s11 )(1 − V ) − 2 p 31 H1 ε 33 (m s12 d231 (1 − V ) 33 11 12 11 is a coefficient where E3 and H1 are the intensities of the electric and magnetic fields, psiiwhere E3 and H1 ofare the intensitie psii is a coefficient of where E3 and H1 are the intensities of the electric and magnetic msii isp safields, compliance phase electric field,fields, coefficient of compliance of compliance of piezoelectric phase a where E3 and of H1 piezoelectric are the intensities of at theconstant electric and magnetic is a coefficient of compliance ii compliance of piezoelectric phase at constantp melectric field, msii is a coefficient of compliance of p phase at constant magnetic dielectric permittivity, 31 a piezoelectric phase at constant magnet ofmagnetic piezoelectric phase at constant electricfield, field, ε33siiisis aa coefficient of compliancedmagnetic of is magnetic phase magnetic phase constant field, pε33 is a dielectric permittivity, pd31 is a piezoelectric p ε magnetic p d fraction mpm mqij at pVaa+ piezomagnetic m is a field, piezomagnetic module, V is the volume of the piezoelectric, V = V/( module, and q ij is atmodule, constantand magnetic is a dielectric permittivity, is a piezoelectric module, and q is 33 31 ij mqij is a piezomagnetic module, V is the volume fraction of the piezoelectric, V = pV/(pV + module, mV), pV иand mV are mV the volumes the piezoelectric phases, piezomagnetic module, V is theofvolume fraction ofand the magnetic piezoelectric, V = prespectively. V/(p V + mmV), ppVVи m V are are the the volumes of the pi mV), pV и mV are the volumes of the piezoelectric and magnetic phases, respectively. TheofME is placed the current and inductance coils, where a bias magnetic and is placed into th The MEfield element volumes theelement piezoelectric andinto magnetic phases, respectively. The ME element is placed into the current and inductance coils, where a bias magnetic field and a variable modulation magnetic field are created, respectively. a variable modulation magnetic field a variable modulation magnetic field are created, respectively.

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The ME is placed into the current and inductance coils, where a bias magnetic field and a Sensors 2017, 17,element 1271 4 of 13 Sensors 2017, 17, 1271 4 of 13 variable modulation magnetic field are created, respectively. An important important part part is is the the fixation fixation of of ME ME element element in in the the inductance inductance coil. coil. The The ME ME element element was was An An important part is the fixation of ME element in the inductance coil. The ME element was fixed by by the the glue glue on on one one end end of of sample sample in in order order to to avoid avoid jamming jamming the the ME ME element element surface, surface, as as shown shown fixed fixed by the glue on one end of sample in order to avoid jamming the ME element surface, as shown in Figure Figure 4. 4. in in Figure 4.

Figure 4. 4. Design Design of of the the hard hard core core of of aamagnetoelectric magnetoelectric sensor: sensor: 1—inductance 1—inductance coil; 2—ME element; Figure Figure 4. Design of the hard core of a magnetoelectric sensor: 1—inductance coil; 2—ME element; 3—leading-out 3—leading-out wire wire of of ME ME element; element; 4—glue; 4—glue;5—current 5—currentcoil. coil. 3—leading-out wire of ME element; 4—glue; 5—current coil.

3.3. Signal Processing Processing 3.3. 3.3. Signal Processing The signal signal processing part part is used to to process the the voltage signal signal from the the sensing head, head, the block block The The signalprocessing processing partisisused used toprocess process thevoltage voltage signalfrom from thesensing sensing head,the the block scheme is shown in Figure 5. In the non-resonant case a current sensor signal processing scheme scheme schemeisisshown shownininFigure Figure5.5. Inthe thenon-resonant non-resonantcase caseaacurrent currentsensor sensorsignal signalprocessing processingscheme scheme consists of of aa generator generator that that is is tuned tuned to to the the frequency frequency at at 500 500 Hz. Hz. The The generator generator is is connected connected to the consists consists of a generator that is tuned to the frequency at 500 Hz. The generator is connectedtotothe the inductance coil then the output output signal signal from the the ME ME element element is is amplified amplified and and provided provided to to the the peak peak inductance inductance coil then the output signal from the ME element is amplified and provided to the peak detector.The The currentcoil coil creates a bias magnetic proportional the current theFor coil. For detector. creates a bias magnetic fieldfield proportional to theto in the in coil. signal detector. Thecurrent current coil creates a bias magnetic field proportional tocurrent the current in the coil. For signal conversion the microprocessor can be used. conversion the microprocessor also canalso be used. signal conversion the microprocessor also can be used.

Figure 5. Schematic diagram of the signal processing. Figure5.5.Schematic Schematicdiagram diagramofofthe thesignal signalprocessing. processing. Figure

In the resonant case the signal processing scheme of the current sensor is similar, but the In the resonant case the signal processing scheme of the current sensor is similar, but the In the resonant case the signal processing of thetimes. current sensor is output similar, signal but theissensitivity sensitivity of the scheme increases from ten toscheme a hundred If the level sufficient sensitivity of the scheme increases from ten to a hundred times. If the level output signal is sufficient of the scheme increases from ten to a hundred times. If the level output signal is sufficient for estimation for estimation and father processing, then it is possible to work without the amplifier. for estimation and father processing, then it is possible to work without the amplifier. and father processing, then it is possible to work without the amplifier. 3.4. Construction 3.4.Construction Construction 3.4. The ME current sensor is a system consisting of the ME composite (3), a generator (4), an TheME MEcurrent current sensor a system consisting the ME composite (3), a generator (4), an The sensor is a is system consisting the of ME (3),aa body. generator (4), an amplifier, amplifier, a peak detector, two coils (1, 2) placedofinto one composite another and Amplifier and peak amplifier, a peak detector, two coils (1,into 2) placed into one another and a body. Amplifier and peak adetector peak detector, coils (1, 2) placed one another and a body. Amplifier peak detector attachedtwo to the back side of the generator. The construction of the DC MEand sensor is shown in detector attached to the back side of the generator. The construction of the DC ME sensor is shown in attached side of thethe generator. The the DCtoME Figure 6. Figure 6.to It the canback be noted that design of anconstruction AC sensor isofsimilar thesensor designisofshown a DC in sensor and Figure 6.noted It canthat be the noted that of the of anisAC sensor is similar to the design ofand a DC sensor the and It can bein design andesign AC sensor similar to the design ofsensor a DC sensor differs the presence of the conductor near the ME element. An AC can also bediffers used in like a differs in the presence of the conductor near the ME element. An AC sensor can also be used like a presence of the conductor the ME element. An AC sensor alsosensitivity be used like DCsensors sensor only DC sensor only without near the generator and modulating coil.can The of aME will DC sensor only without the generator and modulating coil. The sensitivity of ME sensors will without generator coil. The of ME sensors will should mainly depend on the mainly the depend on and the modulating ME properties andsensitivity the maximal sensitivity occur at the mainly depend on the ME properties and the maximal sensitivity should occur at the ME properties and resonance the maximal sensitivity should occur at the electromechanical resonance frequency. electromechanical frequency. electromechanical resonance frequency.

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1

2

3

4 (a)

(b)

Figure 6. Design of a magnetoelectric non-resonant current sensor prototype: (a) non-resonant Figure 6. Design of a magnetoelectric non-resonant current sensor prototype: (a) non-resonant current current sensor; (b) current sensor in the body. sensor; (b) current sensor in the body.

3.5. Characteristics 3.5. Characteristics The theoretical curve and the experimental points of the output characteristic of ME The theoretical curve and the experimental points of the output characteristic of ME non-resonant non-resonant current sensor are shown in Figure 7. The output voltage of the sensor was calculated current sensor are shown in Figure 7. The output voltage of the sensor was calculated by the by the following equation: following equation: I− m p L q I+ K H d31 nUin mKp ν d amp 0 11 K K Lq11 H 0 d 31nU in Uout =amp (4) (4) m s p s εε dc R U out s 0 11 11 m p 11 s11 s11 0 c11 Rs where, Kamp is a coefficient of amplifier, K is a coefficient due to manufacturing technology, Uin is where, Kamp is a coefficient of qamplifier, K is a coefficient due cto technology, U in is an an input amplitude voltage, module, is an effective longitudinal stiffness 11 is a piezomagnetic 11 manufacturing p v/p s + m v/m s module, p s , m sc11 are input amplitude voltage, q11 cis a piezomagnetic is an effective longitudinal stiffness coefficient of the composite, = , the coefficients of compliance for 11 11 11 11 11 pv/ps11 + mv/ms11, ps11, ms11 are the m p coefficient of the composite, c 11 = coefficients of compliance for piezoelectric and magnetostrictive phase at constant electric field, v, v are the volume fractions of piezoelectric and and magnetostrictive phase atrespectively. constant electric field, mv, pv are the volume fractions of magnetostrictive piezoelectric phases, m m p m p magnetostrictive The term vand = piezoelectric L/( L + L)phases, and vrespectively. = p L/(p L + m L), where m L, p L are the thicknesses of mL/(pL + mL) and pv = pL/(pL + mL), where mL, pL are the thicknesses of magnetostrictive The term mv =and magnetostrictive piezoelectric layers, respectively, d31 is a piezoelectric module, n is a number and piezoelectric layers, respectively, 31 is a piezoelectric module, n is a of number windingsε per of windings per unit length of the ACdsolenoid, Rs is an active resistance the ACofsolenoid, is a unit length of the ACofsolenoid, Rs is an active of the AC ε isshow a dielectric dielectric permittivity piezoelectric phase, ε0 is anresistance electric constant. Thesolenoid, calculations suitable permittivity of piezoelectric phase, ε0 is an electric constant. matching matching between theory and experiment. Consequently, allThe thecalculations parameters show of the suitable sensor are chosen between theory and experiment. Consequently, all the parameters of the sensor are chosen quite quite optimally. optimally. In our case, the magnetostrictive material Metglas based on iron alloy was used, and the In our case, thecoefficient magnetostrictive material Metglas based in onFigure iron alloy was used, and the dependence of ME on bias magnetic field is shown 1. The plot of the curve max can be dependence coefficient onused bias magnetic field is shown with in Figure The plot Measurements of the curve from from αME startoftoME αME for designing a sensor high1.linearity. of start to МЕmax can be used for designing a sensor with high linearity. Measurements of Metglas МЕ Metglas parameters and calculations necessary to determine the values of bias magnetic field and the parameters and calculations necessary to sensitivity determineofthe values of bias magnetic field andrange the maximal measured current were made. The sensor reached 0.34 V/A. The working maximal measured current made. The sensitivity of sensoroutput reached 0.34 V/A.1%. The working of currents was set up to 5 A.were The nonlinearity of the characteristic was within range of currents was set up to 5 A. The nonlinearity of the characteristic output was within 1%.

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Figure 7. Output characteristics of the ME non-resonant current sensor.

Figure Figure 7. 7. Output Output characteristics characteristics of of the the ME ME non-resonant non-resonant current current sensor. sensor.

4. Resonant Current Sensor

4. 4. Resonant Current Sensor Sensor 4.1. Sensor Design

4.1. Design 4.1. Sensor SensorThe Design resonant current sensor has a similar design as the non-resonant one. It also consists of a generator, inductance MEhas composite, permanent magnet, current coil (sensing head), power of The resonant current sensor aa similar design the one. consists The resonant currentcoil, sensor has similar design as as the non-resonant non-resonant one. It It also also consists of aa supplyinductance and rectifiercoil, (signal processing). The block diagram of thecurrent resonantcoil current sensorhead), is shown generator, ME composite, permanent magnet, (sensing power generator, inductance coil, ME composite, permanent magnet, current coil (sensing head), power in Figure 8. supply supply and and rectifier rectifier (signal (signal processing). processing). The The block block diagram diagram of of the the resonant resonant current current sensor sensor is is shown shown in Figure 8. in Figure 8.

Figure 8. Block diagram of resonant current sensor.

The principle of operation of the sensor is based on measuring the electromotive force appearing at the outputFigure of sensitive element dueoftoresonant the ME current effect assensor. a result of the influence of the 8. Block diagram Figure Block diagram of resonant currentissensor. alternating magnetic field and a8.bias magnetic field. The amplifier not applicable here because the output signal level enough forof estimation. In this the rectifier (diode bridge) is sufficient. The principle of isoperation the sensor is case based on measuring the electromotive force

The principle of operation of the element sensor isdue based appearing appearing at the output of sensitive to on themeasuring ME effect the as aelectromotive result of the force influence of the 4.2. Construction at the output of sensitive element due to the ME effect as a result of the influence of the alternating alternating magnetic field and a bias magnetic field. The amplifier is not applicable here because the The sensing head is similar as in the sensor.(diode We used the same composite, magnetic field and aisbias magnetic field. Thenon-resonant amplifier iscurrent not here because the output signal output signal level enough for estimation. In this case theapplicable rectifier bridge) is sufficient. but with a slight difference. It consists of piezoelectric and magnetostrictive layers as shown in level is enough for estimation. In this case the rectifier (diode bridge) is sufficient. Figure 3. The ME current sensor shown in Figure 9 consists of the ME composite (3), the generator

4.2. Construction 4.2. Construction The sensing head is similar as in the non-resonant current sensor. We used the same composite, The sensing head is similar as in the non-resonant current sensor. We used the same composite, but with a slight difference. It consists of piezoelectric and magnetostrictive layers as shown in but with a slight difference. It consists of piezoelectric and magnetostrictive layers as shown in Figure 3. Figure 3. The ME current sensor shown in Figure 9 consists of the ME composite (3), the generator

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The ME current sensor shown in Figure 9 consists of the ME composite (3), the generator (4), the rectifier (4), the rectifier (5), permanent magnet, the inductance (2) and current (1) coils (coils placed into one (5), permanent magnet, the inductance (2) and current (1) coils (coils placed into one another). another). The ME composite is a layered structure which includes a thin piezoceramic plate of PZT placed The ME composite is a layered structure which includes a thin piezoceramic plate of PZT between two magnetostrictive layers of Metglas performing the function of electrodes. The final placed between two magnetostrictive layers of Metglas performing the function of electrodes. The dimensions of the composite were 6were ×1× withwith a PZT concentration final dimensions of the composite 6 ×0.62 1 × mm 0.62 mm a PZT concentrationofof0.8. 0.8.The ThePZT PZT layer was poled in an electric in the thickness direction. The METhe laminate operates in a transverse layer was poled in anfield electric field in the thickness direction. ME laminate operates in a mode regime in which thein applied magnetic (H~fields , H0 )(H are parallel each each other andand areare located transverse mode regime which the applied fields magnetic ~, H 0) are parallel other located in the plane of the laminate. The sensing head system comprises the generator, the in the plane of the laminate. The sensing head system comprises the generator, the inductance and inductance coilsThe andgenerator ME element. generatorcoincidedent had a frequency coincidedent a current coils andand MEcurrent element. hadThe a frequency with a resonancewith frequency resonance frequency of the ME composite. The coil is wound on the composite and designed to of the ME composite. The coil is wound on the composite and designed to induce an AC magnetic an AC magnetic field. The rectifier is a diode bridge and converts an AC to a DC. Such field.induce The rectifier is a diode bridge and converts an AC to a DC. Such sensors can detect the AC and sensors can detect the AC and DC in the range from 0.01 A to 100 A depending on the design DC in the range from 0.01 A to 100 A depending on the design parameters. parameters.

1

(a)

2

3

4

5

(b)

Figure 9. Design of magnetoelectric resonant current sensor prototype: (a) view from above; (b)

Figure 9. Design of magnetoelectric resonant current sensor prototype: (a) view from above; bottom view. (b) bottom view.

4.3. Characteristics

4.3. Characteristics

Resonant ME current sensors were designed. The sensors work in the electromechanical range with the resonance frequency = 168 kHz. TheThe sensor sensitivity ME voltage coefficientrange Resonant ME current sensorsfreswere designed. sensors work and in the electromechanical dependencies were investigated using the designed measurement setup. The measurement stand with the resonance frequency fres = 168 kHz. The sensor sensitivity and ME voltage coefficient includes two power supplies an APS-7315 (Aktakom, Moscow, Russia) and a HMP4040 (HAMEG dependencies were investigated using the designed measurement setup. The measurement stand Instruments, Mainhausen, Germany), a HM 8112-3 multimeter (HAMEG Instruments, Mainhausen, includes two power supplies an APS-7315 (Aktakom, Moscow, Russia) and a HMP4040 (HAMEG Germany), a HMO722 oscilloscope (HAMEG Instruments, Mainhausen, Germany) and a GMW 5403 Instruments, Mainhausen, Germany), a HM 8112-3 multimeter (HAMEG Instruments, Mainhausen, electromagnet (Magnet Systems, CA 94070, USA). The graph below shows the output characteristics Germany), a HMO722 oscilloscope (HAMEG Instruments, Mainhausen, Germany) and a GMW 5403 of the resonant current sensor. electromagnet CA 94070, USA). The graph below the output characteristics Figure(Magnet 10 shows Systems, the theoretical curve and the experimental pointsshows of the characteristic output of of thethe resonant resonantcurrent current sensor. sensor. As can be seen from the graph, the calculated and experimental results Figure 10 shows the theoretical andselected the experimental points ofparameters the characteristic output of are in good agreement. This meanscurve that the sensor and materials are optimal. The maximum linearity is reached at the additional bias field of 10 Oe. Also it is known that the resonant current sensor. As can be seen from the graph, the calculated and experimentalthe results sensitivity is describedThis by the slope of thethe characteristic output.and The sensitivity the sensor reached are in good agreement. means that selected sensor materials of parameters are optimal. 0.53 V/A. The current working range at 5 A. Thebias nonlinearity output was The maximum linearity is reached atwas the set additional field of of 10the Oe.characteristic Also it is known that the within 0.5%. The resonant frequency of composite for the first axial mode was calculated by the sensitivity is described by the slope of the characteristic output. The sensitivity of the sensor reached following equation:

0.53 V/A. The current working range was set at 5 A. The nonlinearity of the characteristic output was within 0.5%. The resonant frequency 1of composite s11p rs11mfor the first axial mode was calculated by the fres (5) p m following equation: 2L v s11s11 (r p m ) u p m rs11 1 u t ofs11 where L is the plate length, pρ, mρ are the+piezoelectric and magnetic phases, r is the (5) f res the = density p m p 2L s11 s11 (r ρ + m ρ ) ratio of thicknesses, r = mL/pL. where L is the plate length, p ρ, m ρ are the density of the piezoelectric and magnetic phases, r is the ratio of thicknesses, r = m L/p L.

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For calculation of the characteristic output the following equation was obtained: For calculation of the characteristic output the following equation was obtained:

I I− d31 nUin m m H 8KQ ν p L q11 0 + dd nU H 8 KQ p Lq 31 in 11 0 Uout = m p d U out m s11 s11 εε 0 c211 π 2 ( Rs + i2π f res Ls ) p s11 s11 0 c11 Rs i 2 f res Ls

(6)

(6)

where, Q isQthe quality is resonance frequency, Ls inductance is the inductance where, is the qualityfactor factorof of resonance, resonance, fresfres is resonance frequency, Ls is the of the ACof the AC solenoid. solenoid.

Figure 10. Characteristic output of the ME resonant current sensor.

Figure 10. Characteristic output of the ME resonant current sensor.

5. Magnetoelectric Equivalent Circuits

5. Magnetoelectric Equivalent Circuits 5.1. Magnetoelectric Equivalent Circuits: Non-Resonant Current Sensor

5.1. Magnetoelectric Equivalent Circuits: Non-Resonant Current Sensor

For a better understanding of the current processes it is suggested to consider the magneto-electro-mechanical circuit of (Figure equivalent to a non-resonant current sensor [12,13]. the For a better understanding the 11), current processes it is suggested to consider The magnetic circuit (I) comprises an AC voltage source and an inductance coil. Flowing in the magneto-electro-mechanical circuit (Figure 11), equivalent to a non-resonant current sensor [12,13]. magnetic circuit (I) an electric current by means of an ideal transformer causes in mechanically The magnetic circuit (I) comprises an AC voltage source and an inductance coil. Flowing in the circuit (II) the occurrence of mechanical current ῡ. Mechanical circuit (II) is connected with the magnetic circuit (I) an electric current by means of an ideal transformer causes in mechanically second ideal transformer with an electric circuit (III), thus the mechanical current results in flow of ¯ Mechanical circuit (II) is connected with the second circuit (II) the occurrence of mechanical current v. electric current i in an electric circuit. The output voltage V is taken from the capacitor C0 of electrical idealcircuit. transformer with an electric circuit (III), thus the mechanical current results in flow of electric current i in circuit. The output voltage V is taken (L from the capacitor C active of electrical circuit. Foran theelectric calculation we used the value of coil inductance s = 5.83 × 10−3 H) and 0 resistance −3 H) and active resistance For the calculation we used the value of coil inductance (L = 5.83 × 10 (Rs = 510 Ohm) obtained from the experimental data. It is necessary to make the resistance Z as high s (Rs =as510 Ohm) then obtained from the circuit experimental data. It isinput necessary toofmake the resistance Z as as high possible, the mechanical will not distort the current the solenoid circuit. So not to distort the mechanical thenot output circuit C0 mustofbethe reduced by the same So as as possible, then the mechanicalconnection circuit will distort thecapacity input current solenoid circuit. amount have increased connection the resistancethe Z: output circuit capacity C0 must be reduced by the same not to distortwe the mechanical

amount we have increased the resistance Z:1 kl Z jM vA1 cot 2 1 2 kl Z = − jMρvA1 cot 2 2 d312 lb 33 p s11 d231 − ps C0 lb ε 33 p 11 C0 = M t p M t M >> 1

(7)

(7) (8)

(8) (9)

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M >> 1

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(II) (II)

(III) (III)

Mechanical Mechanical section

Electric Electric section

(9)

Figure 11. The equivalent circuit of ME non-resonant DC sensor. Figure 11. The equivalent circuit of ME non-resonant DC sensor.

In [13], authors used the input section with the magnetic field source. In our case the input In [13], authorsan used the input section with thethe magnetic field source. In our case the section section contains electric current. Therefore, ideal ratio of transformation hasinput the following contains form: an electric current. Therefore, the ideal ratio of transformation has the following form: mm νq q1111 A n ϕm = 1 m mm s A1n s 11

(10) (10)

11

For this equivalent circuit the equations areare asas follows: For this equivalent circuit the equations follows: .

Rss JJ + jωL j Ls JJ = U u u R Uinin +ϕmm . Zu Zum = J ϕmpJV+ ϕ p V . Jdisp = jωC0 V − ϕ p u

(11) (11)

J disp jC0V p u

Using the open-circuit conditions Jdisp = 0, where Jdisp = (δD3 /δt) = 0, the following equation for Using the open-circuit conditions Jdisp = 0, where Jdisp = (δD3/δt) = 0, the following equation for the the output voltage was obtained: output voltage was obtained: m ϕp ϕ Uout = U (12) U out jωC0 [ Z ( Rs + jωLs )m− pϕ22m ] − ϕ2 2p ( Rs + jωLs )U inin (12)

jC0 Z Rs j Ls m p Rs j Ls

In order to match the theoretical and experimental data, we introduced a coefficient due to In order technology to match the and experimental data, we and introduced a coefficient dueofto manufacturing K =theoretical 0.087. In Figure 12 the theoretical curve the experimental points manufacturing K = 0.087. Figure 12 the theoretical curve and the experimental points output voltage fortechnology low frequency sensorInare presented. of output voltage for low frequency sensor are presented. Study of the equivalent circuit for the low-frequency mode of operation, allowed us to find the Study oftothe equivalent circuit for theK, low-frequency of operation, allowed us to find the coefficient due manufacturing technology necessary for mode the further consideration of the resonant coefficient due to manufacturing technology K, necessary for the further consideration of the mode of operation. resonant mode of operation.

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Figure 12. Output voltage depending on the frequency for the non-resonant current sensor. Figure12. 12.Output Outputvoltage voltagedepending dependingon onthe thefrequency frequencyfor forthe the non-resonant non-resonant current current sensor. sensor. Figure

5.2. Magnetoelectric Equivalent Circuit for Resonant Current Sensor 5.2. Magnetoelectric Magnetoelectric Equivalent Equivalent Circuit Circuit for for Resonant Resonant Current Current Sensor Sensor 5.2. When considering the ME equivalent circuit, we generally followed the method developed in When considering considering the ME circuit, wewe generally followed thethe method developed in [10]. MEequivalent equivalent generally developed in [10].When In that paper, thethe authors used thecircuit, magnetic circuit of followed the magneticmethod field source directly, In that paper, the authors used the magnetic circuit of the magnetic field source directly, without [10]. In that paper, how the authors used field the magnetic of thewe magnetic fieldthe source directly, without specifying the magnetic is created.circuit In contrast, considered appearance of specifying how the magnetic field is created. Increated. contrast,In wecontrast, considered the appearance of the magnetic without specifying how the magnetic field is we considered the appearance the magnetic field during the flow of electric current through the inductor. This allowed us of to fieldmagnetic during the flowduring of electric current through the inductor. This allowed us to consider the resulting the field the flow of electric current through the inductor. This allowed us to consider the resulting frequency response of the sensor to the complex resistance of the AC coil. frequency response of frequency the sensorresponse to the complex resistance of the AC coil. consider the resulting of the the sensor sensor to the complex resistanceelectrical of the ACequivalent coil. In the study of the resonant mode of a completely simplified In the study of the resonant mode of the sensor a completely simplified electrical electrical equivalent equivalent In the study of the resonant mode of the sensor a completely simplified circuit can be considered (Figure 13). The input voltage equals Uin, whereby the current through the circuit can can be considered considered (Figure 13). 13). The input input voltage voltage equals equals U Uinin,,whereby whereby the the current current through through the the circuit coil flows. be This gives rise(Figure to a currentThe in the resonant circuit containing the dynamic inductance of L, coil flows. This gives rise to a current in the resonant circuit containing the dynamic inductance of coil flows. This gives rise to a current the resonant containing of L, resistance R and capacitance C. The in output voltagecircuit is removed fromthe thedynamic capacitorinductance C1 constituting L, resistance R and capacitance C. Theoutput outputvoltage voltageisisremoved removedfrom fromthe the capacitor capacitor C C11 constituting constituting resistance R and The together with thecapacitance C2 capacity,C. full operating capacity. together with the C capacity, full operating capacity. 2 together with the C2 capacity, full operating capacity.

Figure 13. The equivalent circuit of ME resonant DC sensor. Figure13. 13.The Theequivalent equivalentcircuit circuitof ofME ME resonant resonant DC DC sensor. sensor. Figure

ME voltage coefficient is given by the following equation: ME voltage coefficient is given by the following equation:

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11 of 13 m p q11d31 p s11 tan E E m h1 s11 0 p s112 c11 d312 tan c11 p s11

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(13)

ME voltage coefficient is given by the following equation: where: αE =

m ν p νq d p s tan( η ) E kl 11 31 11 = −m 2 2 p h1 s11 εε 0 s11 c11 2 η + d31 tan(η ) − c11 p s11 η

The magnetic field inside a long solenoid: where:

(13) (14)

kl

= h1 η nJ 2

(14) (15)

Then magnetic field of inside long solenoid: where, is the number coilsaper unit length, J is a current. Current in the solenoid: h1 = nJ

(15)

U J J is ina current. where, n is the number of coils per unit length, Rs j Ls Current in the solenoid:

J=

Output voltage:

Output U out voltage: E p L m L E h1 Uout

p

L mL

(16)

Uin Rs + jωLs

(16)

p Lq11d31 p s11 tan

m

nJ s11 0 p s112 c11 d312 tan c11 p s11 m ν P L q d p s tan( η ) pL + ML = − M 11 31 11 nJ = E p Lm+ pα E h1 p s2 c η + d2 tan( η )− c p s η m p LqL11d= s n tan U s εε in 11 11 ]) 31 11 11 ( 0 11 11 31 [ m νp L q

d

ps

m

n tan(η )U

31 11 p 2 =s11− s11cp11 d312d112 tan c c11p s p sinη11])(R +RiωL m s 0 ( εε s )i Ls s2 c η + [tan(η )−

m

11

0

11 11

31

11

11

s

(17) (17)

s

Taking into into account account the the calibration calibration coefficient coefficientK, K,the thefollowing followingratio ratioof ofvoltages voltageswas wasfound: found: Taking p K mKpmLq L qd 31d s11 p n tan U out Uout ν p11 31 s11 n tan( η ) 11 = − 2p 2 2 2 U in Uin m s11 m s110 pεεs11 d31 tan c11 ηd+ Ls s) (η) −c11c11p sp11s11η R(Rs s+iiωL 0 cs11 11 31 tan

(18)

The The calculation calculation and and experimental experimental data data results results are are presented presented in in Figure Figure14. 14.

Figure 14. Amplitude frequency characteristics of the resonant current sensor. Figure 14. Amplitude frequency characteristics of the resonant current sensor.

The value of the resonant frequency calculated by Equation (4) is of fres = 168 kHz, which agrees value of the resonant frequency by line Equation of fres =of168 which agrees with The the experimental data. The solid linecalculated and dotted show (4) theisresults thekHz, calculation by the with the experimental data. The solid line and dotted line show the results of the calculation by the equation and the equivalent circuit, respectively. The considered equivalent circuit adequately

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describes the current operation of the sensor in the resonant mode, as the frequency response of the equivalent circuit is in good agreement with theoretical frequency response and accurate experimental data. 6. Performance Comparison of Existing Current Sensors Table 1 shows the comparative characteristics of DC current sensors. The information is about sensors produced by LEM Holding SA (HO8-NP, Freiburg, Switzerland), Honeywell Inc. (CSLW6B5, Morristown, NJ, USA), Allegro MicroSystems (ACS712ELCTR-05B-T, Worcester, MA, USA). As can be seen from the table, the ME current sensor has higher sensitivity and lower current consumption. Table 1. Performance comparison of current sensors. Sensors

HO8-NP

CSLW6B5

ACS712ELCTR-05B-T

Magnetoelectric Sensor

Measuring principle

Hall effect measuring principle

Miniature ratiometric linear Hall effect sensor

Hall effect sensor

Magnetoelectric effect

Primary current, measuring range, Ipm (A)

0–20

±5

±5

0–5

Sensitivity (V/A)

0.1

0.2

0.185

0.53

Supply voltage (V)

5 ± 10%

4.5–10.5

5 ± 10%

5 ± 10%

Current consumptions (mA)

19

9

10

2.5

Accuracy (%)

1

0.5

1.5

0.5

Output voltage range Uout (V)

2.5−0.5

2.7–3.7

2.5–4.5

1.5–4.5

Size (mm)

24 × 12 × 12

16.2 × 14 × 10

6 × 5 × 1.75

30 × 20 × 10

Known current transformers (CTs), with high breakdown voltage, are the most widely applied devices for AC sensing in traditional power systems. CTs can’t be used as DC sensors because they are based on Faraday’s law of induction. Disadvantages of CTs are: large size, high price, limited bandwidth and large consumption of metal resources, so they are only used in power stations and substations. Shunts are more often applied in DC converter stations and power electronics. Disadvantages of shunts are: the measured current has to be interrupted into the sensor, an overcurrent may permanently damage it, and the intrinsic inductance limits the accuracy and bandwidth. Hall effect current sensors are mainly applied in non-contact current measurements. The problems of the Hall effect sensors are: low sensitivity, low breakdown voltage, and susceptibility to temperature, which limits them to applications in high voltage power systems [2]. Compared with the current sensors presented above, the ME current sensor has advantages of higher sensitivity, higher linearity, lower cost, simple structure of sensor, which make it most promising for current measurements. 7. Conclusions In this paper the principle of operation of ME current sensors was considered. Theoretical and experimental investigations of both low frequency and resonant current sensors were discussed. The optimal materials for sensitive elements were chosen and PZT and Metglas were used, respectively in the the piezoelectric and magnetostrictive phases. The equivalent circuit method was used to calculate the characteristics of the sensors. The calculation results allowed us to estimate the internal parameters. The design and development of non-resonant and resonant current sensors were discussed.

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The characterization of the non-resonant current sensor showed that in the operation range up to 5 A, its sensitivity was 0.34 V/A, and its non-linearity was less than 1% and for the resonant one in the same operation range, the sensitivity was 0.53 V/A, and the non-linearity less than 0.5%. The resonant current sensor has higher sensitivity compared with the non-resonant current sensor, therefore it is recommended for the detection of very low currents, for example, leakage currents. Finally, ME current sensors can be used for measuring equipment, power nets and control systems, security systems and safety systems; in metal detectors and the automotive industry, rail transport; in wireless electricity metering systems and space technology and robotics. Acknowledgments: The research was supported by the Russian Science Foundation, grant No. 15-19-10036. Author Contributions: Mirza Bichurin conceived the project, organized the paper content, and edited the manuscript. Roman Petrov designed magnetoelectric current sensors. Viktor Leontiev performed the experimental work and wrote the manuscript. Gennadiy Semenov performed the experimental work and wrote the manuscript. Oleg Sokolov performed theoretical calculations and modeling of equivalent circuits. Conflicts of Interest: The authors declare no conflict of interest.

References 1. 2. 3.

4. 5. 6. 7.

8.

9. 10. 11. 12. 13.

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