research article two-channel metal detector using two ...two-channel metal detector, having two sets...

Research ArticleTwo-Channel Metal Detector UsingTwo Perpendicular Antennas

Kyoo Nam Choi

Department of Information and Telecommunication Engineering Incheon National University Incheon 406-772 Republic of Korea

Correspondence should be addressed to Kyoo Nam Choi knchoiincheonackr

Received 6 March 2014 Revised 14 June 2014 Accepted 23 June 2014 Published 10 July 2014

Academic Editor Ignacio R Matias

Copyright copy 2014 Kyoo Nam Choi This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Two-channel metal detector having two sets of perpendicularly oriented sensor antennas is proposed to expand detectable sizeranging from mm through cm scale of metal sensor while conventional metal sensor is dedicated for detection only in mm orcm scale The characteristics of the two metal detection sensor channels were investigated respectively and the interference effectwhile in simultaneous operation between two sensor channels was discussed Metal detection channel having sensitivity in mmscale showed detectable sensitivity to moving ferrous sphere with diameter down to 07mm at 50 kHz exciting frequency andenhanced sensitivity distribution And metal detection channel having sensitivity in cm scale showed more uniform sensitivitydistribution with the flexibility for future modular construction The effect of interference while in simultaneous operation of twosensors resulted in reduced output response but still within usable detection range Thus it was feasible to operate two sensorshaving different sensitivity range simultaneously and to extend detection range frommm to cm scale within practically acceptableinterference

1 Introduction

The metal detection sensor is now widely used in not onlyfood processing industry but also defense industry [1 2] todetect metal pieces in surrounding object and considerableefforts have been devoted to enhance sensitivity and selec-tivity Foreign materials are not only metal but also woodceramics [3] and germs [4] which are not detectable by usingmetal sensor Other methods are envisaged to detect thesematerials which are not detectable by using metal sensorAmong these foreignmaterials the detection formetal piecesis important [5] in food industry and various researches areconcentrating on this The demand for metal detection ishigh in food industry and recently the detection methodsby using X-ray [6] and light [7] have been already appliedfor this However the method using electromagnetic wave isdominant and recently the method using superconductingcoil has been attempted [8 9] Theoretical analysis [10 11]and sensitivity analysis [12 13] along with the shape of metalsensor have been performed to explore the sensitivity ofelectromagnetic metal sensor On the other hand there were

attempts [14] to enhance visibility of metal piece throughsignal processing The parent body containing metal pieceis important from the point of selectivity and the effort todetect foreign pieces in powder has been attempted [15]The structure of sensor head in metal detector plays animportant role in terms of sensitivity and relevant studies[16] have been conducted in this regard Active researcheshave been performed for the various kinds and shapes ofmetal in metal sensor head [17] However the single channelmetal detection sensor has not shown sensitivity resolutionthrough wide range of metal sizes from mm to cm scaleThus there was a need to cascade the sensors having differentsensitivity resolutions Metal detection sensor using elec-tromagnetic wave usually is strongly influenced by nearbyelectromagnetic wave which is within several times of sensorwidth thus it is not feasible to place a second sensor inthe vicinity of the first sensor This paper is concernedwith the experimental development of two channel metalsensors by cascading two metal sensors having differentsensitivity resolution with minimum interference with eachother Two-perpendicular-antennas model is introduced to

Hindawi Publishing CorporationJournal of SensorsVolume 2014 Article ID 412621 11 pageshttpdxdoiorg1011552014412621

2 Journal of Sensors

minimize physical interference and the optimal signal detec-tionmethod is investigated to reject the interference betweenmetal sensors having different sensitivity resolution

2 Two-Antenna Model

21 Single Channel Model The conventional metal detectionsensorwith sensitivity resolution inmmscale has the antennaset one transmitting antenna and two receiving antennaswhich are connected in opposite polarity to cancel receptionsignal while in steady state In case when an object containinga nonspherical metal piece is fed through hollow antennacross-sectional space as shown in Figure 1 then the sensorshows good sensitivity only for one directional positionbetween metal piece and antenna where the perturbationin electromagnetic flux linkages becomes maximum In thesingle channel model the moving object containing metalpiece perturbs gradually from first electromagnetic fluxlinkages between TX antenna 1 and RX antenna 1 tosecond electromagnetic flux linkages between TX antenna1 and RX antenna 2 The amounts of these spatiallyvarying electromagnetic flux linkages induce currents intoRX antennas 1 and 2 and the current difference betweenRX antennas 1 and 2 becomes output current whichrepresent the electromagnetic flux unbalance of antenna set1

The equivalent circuit of an antenna set one transmittingand two receiving antennas is shown as in Figure 2 Theinstantaneous output voltage V

10 in no load condition can be

expressed as the difference of mutual inductance1198721minus 1198722

between transmitting and respective receiving antenna asshown in (1) and is also proportional to exciting frequency asshown in (3) In this model the distance between transmit-ting antenna and receiving antenna is closer than the distanceto passing object containingmetal piece thus antenna induc-tances are larger than mutual inductance 119872

11and 119872

12 If

transmitting antenna is excited using sinusoidal signal 1198901119904=

11989010sin(120596119905+120575) then antenna current becomes as shown in (2)

Consider

V10= V11minus V12= (11987211minus11987212)

1198891198941199011

119889119905 (1)

1198941199011= 119895

120596119871119901111989010

radic(1206031198711199011)2

+ 1198772

1199011

sin (120596119905 + 120575) (2)

V10= 119895

1205962119871119901111989010(11987211minus11987212)

radic(1206031198711199011)2

+ 1198772

1199011

cos (120596119905 + 120575) (3)

When the phasor expression is used for the flux linkage120595then the phasor voltage119881 due to flux linkage can be expressedas in (4) where 119868 denotes phasor current

11988110= 119895120596120595

10= 119895120596 (119872

11minus11987212) 1198681199011 (4)

It is noted that mutual inductance variance plays a keyrole in determining sensitivity of metal detection sensor

The mutual inductance is expressed as in (5) in which 11989611

and 11989612

represent coupling coefficient between transmittingand receiving antennas respectively And these couplingcoefficients have the value 0 le 119896

11 11989612le 1

11987211= 11989611radic1198711199011119871 11990412 119872

12= 11989612radic1198711199011119871 11990412 (5)

In steady state of initial measurement11987211

and11987212

areadjusted to119872

11= 11987212 and output voltage becomes V

10asymp 0

When 120596 is higher and11987211minus11987212are bigger for the same size

of metal piece then it is advantageous in terms of sensitivity

22 Two-Channel Model In the two antenna sets as shownin Figure 3 the moving object after passing antenna set 1perturbs gradually from third electromagnetic flux linkagesbetween TX antenna 2 and RX antenna 4 to fourthelectromagnetic flux linkages between TX antenna 2 andRXantenna 3 Also the amounts of spatially varying electromag-netic flux linkages induce currents into RX antennas 4 and3 and the current difference between RX antennas 4 and3 becomes output current of antenna set 2

It is difficult to calculate the variance of mutual induc-tance for the moving object thus experimental method isattempted By using the model in Figure 4 the output voltagein each antenna set becomes the product of mutual induc-tance matrix and instantaneous current of each transmittingantenna as in

[11988110

11988120

] = 119895120596 [11987211minus1198721211987221minus11987222

11987213minus1198721411987223minus11987224

] [1198681199011

1198681199012

] (6)

Assume 11986811987911

is phasor current in one conductor ofTX antenna 1 119863 is the distance 119875 in subscript is theremote point outside antenna 119879 in first subscript alphabet istransmitting antenna119877 in first subscript alphabet is receivingantenna the number in second subscript numeric is antennanumber the number in third subscript numeric is conductornumber of antenna and 4th through 6th alphabet andnumeric follow the same convention as in 1st through 3rdalphabet and numeric Then 119863

1198771111987911is the distance between

one conductor of RX antenna 1 and one conductor of TXantenna 1 and119863

11987911119875is the distance between one conductor

of TX antenna 1 and a remote point 119875 The flux linkage1205951198771111987911

with one conductor of RX antenna 1 due to oneconductor of TX antenna 1 can be expressed as in

1205951198771111987911

= 2 times 10minus7(11986811987911

ln 11986311987911119875

1198631198771111987911

) (7)

Journal of Sensors 3

RX antenna 1

Objectpassage

TX antenna 1

RX antenna 2

Figure 1 Conventional antenna set of metal detection sensor

TX+

RXOut

RX antenna 2

TX antenna 1 RX antenna 1

minus

(a)

+

++


RX antenna 2

minusminus

minus

ip1

Rp1

Lp1

Ls12

Ls12

Rs12

Rs12

is1

M11

M12

E1s

10

11

12

(b)

Figure 2 Equivalent circuit of antenna set (a) schematic diagram and (b) equivalent circuit

Objectpassage

Antenna set 2

Antenna set 1

X

Y

RX antenna 1

RX antenna 3

RX antenna 2

TX antenna 1TX antenna 2

RX antenna 4

Figure 3 Two antenna sets of metal detection sensor

Objectpassage

P

Y

M12

M11

M13

M14

M21

M22

M23

M24

Figure 4 Sensor layout for two antenna sets


If we consider flux linkage to one conductor of receivingantenna 1 from two transmitter antennas this flux linkage12059511987711

can be expressed as in

12059511987711

= 1205951198771111987911

+ 1205951198771111987912

+ 1205951198771111987921

+ 1205951198771111987922

= 2 times 10minus7(11986811987911

ln 11986311987911119875

1198631198771111987911

+ 11986811987912

ln 11986311987912119875

1198631198771111987912

+11986811987921

ln 11986311987921119875

1198631198771111987921

+ 11986811987922

ln 11986311987922119875

1198631198771111987922

)

= 2 times 10minus7(11986811987911

ln 1

1198631198771111987911

+ 11986811987912

ln 1

1198631198771111987912

+ 11986811987921

ln 1

1198631198771111987921

+ 11986811987922

ln 1

1198631198771111987922

+ 11986811987911

ln11986311987911119875

+ 11986811987912

ln11986311987912119875

+11986811987921

ln11986311987921119875

+ 11986811987922

ln11986311987922119875

)

(8)

In this configuration the sum of two currents in TXantenna 1 is zero 119868

11987911+11986811987912

= 0 also likewise 11986811987921+11986811987922

= 0

for TX antenna 2 Let the point 119875 move infinitely far awayso that the set of terms containing logarithms of ratios ofdistances from 119875 becomes infinitesimal then the ratio ofthe distances approaches 1 Substituting these into (8) andrecombining some logarithmic terms we have (9) with theunit weber-turnsmeter

12059511987711

= 2 times 10minus7(11986811987911

ln1198631198771111987912

1198631198771111987911

+ 11986811987921

ln1198631198771111987922

1198631198771111987921

) (9)

Thus flux linkage 1205951198771

in RX antenna 1 becomes sum of12059511987711

and 12059511987712

as shown in

1205951198771= 2 times 10

minus7(11986811987911

ln1198631198771111987912

1198631198771111987911

+ 11986811987911

ln1198631198771211987912

1198631198771211987911

+11986811987921

ln1198631198771111987922

1198631198771111987921

+ 11986811987921

ln1198631198771211987922

1198631198771211987921

)

= 2 times 10minus7((ln

1198631198771111987912

1198631198771111987911

+ ln1198631198771211987912

1198631198771211987911

) 11986811987911

+(ln1198631198771111987922

1198631198771111987921

+ ln1198631198771211987922

1198631198771211987921

) 11986811987921)

= 4 times 10minus7(ln

radic1198631198771111987912

1198631198771211987912

radic1198631198771111987911

1198631198771211987911

11986811987911

+ lnradic1198631198771111987922

1198631198771211987922

radic1198631198771111987921

1198631198771211987921

11986811987921)

(10)

Likewise all mutual inductances including11987211

and11987221

are expressed in a similar way as is shown in

11987211= 4 times 10

minus7 lnradic1198631198771111987912

1198631198771211987912

radic1198631198771111987911

1198631198771211987911

11987221= 4 times 10

minus7 lnradic1198631198771111987922

1198631198771211987922

radic1198631198771111987921

1198631198771211987921

11987212= 4 times 10

minus7 lnradic1198631198772111987912

1198631198772211987912

radic1198631198772111987911

1198631198772211987911

11987222= 4 times 10

minus7 lnradic1198631198772111987922

1198631198772211987922

radic1198631198772111987921

1198631198772211987921

11987213= 4 times 10

minus7 lnradic1198631198773111987912

1198631198773211987912

radic1198631198773111987911

1198631198773211987911

11987223= 4 times 10

minus7 lnradic1198631198773111987922

1198631198773211987922

radic1198631198773111987921

1198631198773211987921

11987214= 4 times 10

minus7 lnradic1198631198774111987912

1198631198774211987912

radic1198631198774111987911

1198631198774211987911

11987224= 4 times 10

minus7 lnradic1198631198774111987922

1198631198774211987922

radic1198631198774111987921

1198631198774211987921

(11)

When a foreign object for example a metal cube passesthrough the above flux linkage then the above mutualinductance 119872 will be perturbed and the perturbed fluxlinkage is converted to voltage at sensor output port Forthe metal sphere having radius 119886 which is 119909 meters fromthe center of antenna conductor the field intensity becomes119867119909= 1198682120587119909 and accordingly the flux density at the distance119909

is 119861119909= 1205831198682120587119909webersm2Thus flux linkage by metal sphere

becomes as in (12) and this amount perturbs the mutualinductances in steady state

120595119909=

4

sum

119894=1

41205871198862120583119868119894

2120587119909119894

(12)

3 Signal Detection Method

31 Noise Rejection Using BPF Two-channel metal detectionsensor uses two different frequencies between antenna sets1 and 2 to avoid interference In frequency domain thisinterference can be minimized by raising the frequencyselectivity of receiver Assume the characteristic of band-pass filter (BPF) in receiver 1 is |119867

1198771(119891)| and |119867

1198772(119891)| for

receiver 2 and power spectral density of incoming signalis 1198781 In(119891) for receiver 1 and 1198782 In(119891) for receiver 2 Then

power spectral densities 1198781BPF(119891) of receiver 1 and 1198782BPF(119891)

of receiver 2 become as in

[1198781BPF (119891)1198782BPF (119891)

] = [

10038161003816100381610038161198671198771(119891)1003816100381610038161003816

2 10038161003816100381610038161198671198771(119891)1003816100381610038161003816

2

10038161003816100381610038161198671198772(119891)1003816100381610038161003816

2 10038161003816100381610038161198671198772(119891)1003816100381610038161003816

2] [1198781 In (119891)1198782 In (119891)

] (13)

As the bandwidth of BPF becomes narrower the systemwill bemore immune to ambient noise However this will also


increase instability in maintaining center frequency of BPFbecause component values are subject to change along withtemperature change So there exists the optimum 119876 value ofBPF from the practical point of view which is needed to bedetermined by experiment

32 Noise Rejection Using PSD The input signal after BPF isfed into phase sensitive detector (PSD) to increase selectivityagainst interfering noise as in Figure 5 In time domainthe output signal from two receiving antennas which isconnected to cancel each other is fed to receiver as 119881In AfterBPF it is filtered to119881BPF and the product of119881BPF with119881REF isagain filtered through LPF resulting in119881LPF In case the inputsignal119881In is themixture of signals from transmitting antennas1 and 2 and noise 119899(119905) then 119881In is expressed as in (14) and119881OP as in (15) Hence

119881In = 1198641 sin (1205961119905 + 1205791) + 1198642 sin (1205962119905 + 1205792) + 119899 (119905) (14)

119881BPF = 119881In lowast ℎBPF (119905)

= [1198641sin (120596

1119905 + 1205791) + 1198642sin (120596

2119905 + 1205792)

+119899 (119905) + 1198642sin (120596

2119905 + 1205792)] lowast ℎBPF (119905)

119881OP = 119881BPF times 119881REF

= 119864REF sin (1205961119905 + 1205791)

times [1198641sin (120596

1119905 + 1205791) lowast ℎBPF (119905)

+ 1198642sin (120596

2119905 + 1205792) lowast ℎBPF (119905)

+119899 (119905) lowast ℎBPF (119905) ]

(15)

In case the band-pass filter is ideally tuned to 1205961and low-

pass filter ideally cut off unnecessary frequency componentthen the output of sensor signal detector 119881LPF becomes as in(16) This output signal 119881LPF in (16) shows DC signal levelwhich is proportional to receiver input signal with minimuminterference

119881LPF = 119881OP lowast ℎLPF (119905)

= 119864REF sin (1205961119905 + 1205791)

times [1198641sin (120596

1119905 + 1205791) lowast ℎBPF (119905)

+ 1198642sin (120596

2119905 + 1205792) lowast ℎBPF (119905)

+119899 (119905) lowast ℎBPF (119905)] lowast ℎLPF (119905)

asymp119864REF11986412

+ 120572 sdot 119899 (119905)

(16)

In case the interfering signal has the same frequency assensor detection signal but with different phase relationshipthen the signal output after LPF in Figure 5 will be shifted inDC level depending on the degree of interference in an idealcase

VIn VOPVBPF

VREF

VLPF(hBPF(t)) (hLPF(t))

BPF LPF

Figure 5 Block diagram of sensor signal detector

Enclosure

Entry

Figure 6 Photo of outer metal enclosure shielding antenna set

4 System Design

41 Metal Detection Sensor Having Sensitivity in mm ScaleThree frequencies as 50 kHz 200 kHz and 400 kHz weredesigned to be injected into the experimental sensor setuphaving sensitivity in mm scale to investigate frequencydependant sensitivity while in simultaneous operation withthe sensor having sensitivity in cm scale Antenna set 1was contained in metal enclosure having outer dimension ofW55 times H29 times D23 cm with the opening and W30 times H10 timesD23 cm for sensing entry to shield outer interfering noise aswas shown in Figure 6 Single turn antennas were used foreasy balancing between antennas

The output voltage from receiving antenna pair whichwas connected in opposite polarity was adjusted to producenearly zero voltage by using two screws as shown in Figure 7And the degree of null output signal was measured asCMRR (common mode rejection ratio) After adjusting nullposition internal cavity of antenna set 1 was filled withepoxy to be resistant to outer shock or vibration

The transmitter block diagram for antenna set 1 isshown in Figure 8 The 8MHz crystal oscillator was usedfor temperature stability and fundamental frequency wasdivided into desired frequencies Time division switch wasfacilitated to select single or mixed frequencies A part ofthis transmitter signal was fed into phase sensitive detectorof receiver as synchronous trigger signal source Antennamatching circuit was used to match antenna impedance withtransmitter impedance

The receiver block diagram for antenna set 1 is shown inFigure 9 The input signal from receiving antenna pair wasfed into antenna matching circuit not only for impedancematching but also for boosting voltage After amplification inPRE AMP block and filtering interfering signal in BPF blockthe phase of input signal was compared with synchronoustrigger signal in PSD block Finally high frequency compo-nent was filtered out in LPF block and only DC componentproportional to phase difference appeared and was amplifiedin AMP block as signal outputThe microprocessor was used


Adjustingscrew

Figure 7 Adjusting screws to produce zero voltage for antenna set1

OSC Divider(120)

Divider(12)

Divider(14)

Time division switch

Poweramplifier

Antennamatching

Transmittingantenna 1

Synch signal

Control

8MHz

400kHz 200 kHz 50kHz

(400kHz 200 kHz and 50kHz)

(8MHz)

Figure 8 Block diagram of transmitter for antenna set 1

to control frequency selection and other control parametersin MICOM block

42 Metal Detection Sensor Having Sensitivity in cm ScaleSingle 20 kHz frequency was injected into the experimentalsensor setup having sensitivity in cm scale to investigateinterfering effect with the former sensor having sensitivity inmm scale while in simultaneous operationThe arrangementof antenna set 2 was devised to be perpendicular withantenna set 1 to avoid interference Multiturn antennas wereused to compensate sensitivity deficiency due to relativelylong distance between transmitting and receiving antennasas shown in Figure 10 Transmitting antenna was fabricatedby using CNC machine to obtain sufficient thickness andreduce resistance and receiving antennas were fabricated bypatterning on PCB Receiving antenna pair is connected inopposite polarity and adjusted to near zero offset

The transmitter block diagram is shown in Figure 11 Theexciting frequency was adjustable by using potentiometer toselect frequency for optimum operation

In the receiver side as shown in Figure 12 the inputsignal from receiving antenna pair was directly amplifiedin PRE AMP block without antenna matching circuit After

Sig out MICOM

PRE AMP BPF PSD

Synch signal

AMP

Antennamatching

Receivingantenna

1

2

Receivingantenna

LPF

TX control

Control

Figure 9 Block diagram of receiver for antenna set 1

Pattern

Base

(a)

PatternBase

(b)

Figure 10 Photo of antenna set 2 (a) transmitting antenna and (b)receiving antennas

filtering interference signal in BPF block the phase of inputsignal was compared with synchronous trigger signal fromtransmitter in PSD block Finally high frequency componentwas filtered out in LPF block and amplified in AMP block assignal output Only DC component proportional to phasedifference between input signal and synchronous triggersignal appeared as signal output

5 Measurement

51 Metal Detection Sensor Having Sensitivity in mm ScaleFor this sensing channel the sensitivity is the key part of thesensorTheminimum size of detectable metal piece is relatedto the degree of canceling signals from receiving antennapair because this limits the maximum amplification ratioAnd this figure of merit (FOM) is represented by CMRRwhich is the logarithmic value of differential output 2mVpp(before amplification) over single channel output 10VppThemeasured CMRR was minus74 dB as shown in

CMRR [dB] = 20 log(02Vgain10V

) = minus74 [dB] (17)

The dependency of output voltage on ferrous metal sizewas investigated using ferrous test balls having diameter of08mm 10mm and 12mm at 50 kHz exciting frequencyand 33Hz LPF (low-pass filter) cutoff frequency after PSD(phase sensitive detector) Measurement data showed that


Poweramplifier

Antennamatching


Synch signal

Bufferamplifier

OSCTune

(20kHz)


PRE AMP BPF PSD LPF

AMP Sig out

Synch signal

Receivingantenna

Receivingantenna

3

4


output voltagewas almost linearly proportional to the volumeas in Figure 13 The minimum detectable size was up to thediameter of 08mm using 33Hz LPF (low-pass filter) cutofffrequency

The dependency of output voltage on applying frequency(50 kHz 200 kHz and 400 kHz) was investigated using Feball of 12mm diameter as shown in Figure 14 Measurementdata showed that output voltage was increasing with applyingfrequency and this was closely matched if we multiplied thefrequency characteristics of antenna matching circuit to thetheoretically expected value

The metal detection sensor using differential loop anten-nas usually suffers a nonuniform sensitivity distributioninside hollow center area of coil which is used for samplepassage due to the nature of loop coil It is necessaryideally to maintain equal sensitivity throughout sensing areaOtherwise the sample metal ball will be undetectable whenpassing the center area even if it was detectable when passingedge area Therefore it is necessary to compensate sensitivitydistribution for close to equal sensitivity as possible Thesensitivity distribution was measured for the above sensingentry W300mm times H100mm by applying 50 kHz and usingFe test ball of 12mm diameter as shown in Figure 15(a) Twosmall copper plates on center of receiving antenna coil inhorizontal direction were patched to compensate sensitivityin sensing entry by providing more electromagnetic fluxlinkages Measurement data showed that the sensitivity wasminus6 dB at the center of sensing entry as shown in Figure 15(b)which was +4 dB enhancement in comparison to the casewithout patches

The bandwidth of LPF after PSD is critical for sensitivityenhancement In the conventional LPF the narrower thebandwidth is the lower the noise level is However for themetal sensor for moving object detection the sensitivity isdegraded if the bandwidth of LPF is too narrow becausefrequency component due tomoving object is attenuated Onthe contrary the noise level soars if the bandwidth of LPF is

0

200

400

600

800

1000

0 02 04 06 08 1

(mV

)

Volume (mm3)

D = 08mm

D = 10mm

D = 12mm

Figure 13 Signal output versus Fe ball volume

0

4

8

12

16

0 100 200 300 400 500

Out

put (

V)

Frequency (kHz)

Measured

Theoretically expected

Figure 14 Signal output versus applying frequency using Fe ball119863 = 12mm

too wide resulting in degraded SNR (signal to noise ratio)Also the improved SNR by narrowing the bandwidth of LPFdoes not mean increasing sensitivity unless it is amplifiedTherefore the signal of this ultrahigh sensitivity metal sensoroperating close to detection limit is only able to be amplifiedafter lowering noise level without sacrificing usable frequencycomponent of moving object The effect of LPF bandwidthwas measured by varying cutoff frequency to find optimumsensitivity when exciting frequency of 50 kHz is appliedDuring the experiment the signal was amplified for the levelwhich was equivalent to decreasing noise floor while keepingoverall system gain because there did not exist a margin forsignal amplification due to inherent noise level for ultrahighsensitivitymetal sensor operating close to detection limitThecutoff frequencies of LPF from 33Hz to lower than 11Hzwere attempted and the frequency below 11Hz resulted inweaker signal output due to too deep attenuation of signalfrequency component The cutoff frequency of 11Hz showedthe best performance for themoving object whichwas similarto practical application The signal responses at 11 Hz cutofffrequency together with 33Hz cutoff frequency are shown inFigure 16 for comparison purpose

Further test is conducted by applying 50 kHz for ferroustest sample ball having 08mm and 07mm diameter usingthe same LPF119891co = 11Hz to find detection limit as shownin Figure 17 Measurement showed that this metal sensorwas able to detect ferrous test sample ball down to 07mm


Y (mm)

X (mm)

Out

putV

pp

050

10006

04

02

000100200300

(a)

Y (mm)

X (mm)

Out

putV

pp

050

10006

04

02

000100200300

(b)

Figure 15 Sensitivity plot of sensing entry (output voltage versusposition in sensing entry using Fe 119863 = 12mm ball) (a) beforepatching and (b) after patching

fco = 11Hz

fco = 33Hz

Figure 16 The effect of cutoff frequency of LPF for 119863 = 12mmferrous test ball 119884 = 200mVdiv119883 = 100msdiv

diameter Thus the minimum detectable size of Fe ball wasextended from 08mm diameter to 07mm diameter byoptimizing the cutoff frequency of LPF

52 Metal Detection Sensor Having Sensitivity in cm ScaleMeasured data showed that the previous ultrahigh sensitivitymetal sensor had the linear response range of about 07mmsim4mm of Fe ball diameter when the exciting frequency was50 kHz and cutoff frequency of LPF was 11Hz Over thissize limit the sensor detects the presence of metal but theoutput response becomes saturated and unable to set further

Signal

(a)

Signal

(b)

Figure 17 Detection limit of metal sensor (a) 119863 = 08mm ferroustest ball (119884 = 50mVdiv 119883 = 200msdiv) (b) 119863 = 07mm ferroustest ball (119884 = 20mVdiv119883 = 200msdiv)

threshold point for different size of Fe test ball Of coursethe detectable size can be altered by varying the sensor gainhowever the detection range remains similar to the aboveThemetal sensor having sensitivity in cm scale was devised withthe flexibility to locate the other sensor in adjacent axis tocompensate direction dependent sensitivity Thus this sensorenables sensing throughout wide range of metal sizes frommm to cm scale if it is combined with the previous ultrahighsensitivity metal sensor

The single channel response was measured with reducedgain to characterize the frequency characteristics of sensorby using TX antenna 2 and RX antenna 3 as shown inFigure 18 In the experiment test metal plate occupying 66of antenna surface was inserted to find the effect of metalin sensitivity through projected frequency range The highpeak around 50 kHz was observed and this was regardedas frequency matching characteristics between antenna andtransmitter However the effect of metal piece was similarwithin the range 40sim60 which was defined as outputvoltage after inserting versus output voltage before insertingas shown in Figure 19 It is noted that the sensitivity is increas-ing along with the frequency increase as expected even ifthe absolute value of output voltage is decreasing in higherfrequency region due to frequency matching characteristics


0

1

2

3

4

0 20 40 60 80 100

Out

put (

V)

Frequency (kHz)

With metal

Without metal

Figure 18 Signal output of receiver 2 for single channel using RXantenna 3119883 = frequency 119884 = AU

0

20

40

60

80

100

0 20 40 60 80 100

Ratio

()

Frequency (kHz)

Figure 19 Effect of frequency in metal sensing output voltage afterinserting versus before inserting119883 = frequency 119884 = AU

The effect of distance ldquo119889rdquo between transmitting antennaand receiving antenna was investigated by separating thedistance by two times and the ratio was calculated the outputvoltage at 119871 = 2119889 over the output voltage at 119871 = 119889 as shownin Figure 20 The measured result shows the tendency closeto the rule of prop 1119889

2 The deviation between the ideal dataand measured data is regarded due to leaked electromagneticfluxes as the distance is farther

The sensitivity distribution in horizontal direction wasinvestigated by gradually covering the antenna along themidline between transmitting antenna and receiving antennawith test metal plate The measured data showed fairly goodhorizontal linearity only 65 nonlinearity The sensitivityvariation in vertical direction was measured as sim13 alongthe 20sim80 line connecting between transmitting antennaand receiving antenna This implies that the loop antennaconfiguration facing each other provides more uniformdistribution in sensitivity

Following the single channel experiment two receivingantennas were connected in differential mode and the gainwas increased accordingly The CMRR was measured in a

0

10

20

30

40

50

0 20 40 60 80 100

Ratio

()

Frequency (kHz)

Ideal

Measured

Figure 20 Effect of distance between transmitting and receivingantennas119883 = frequency 119884 = AU

Table 1 Sensitivity comparison along with test sample layingdirection

SensitivityDirection

119883 direction(mV)



Peak output voltage (AU) 110 80 210

similar way to the former sensor channel and was calculatedas ndash52 dB less than 22 dB in comparison to the former sensorThus this sensor channel is regarded as adequate for metalobject having dimension in cm scale Ferrous test sample ballshaving dimension of 10 times 5 times 5 (mm) were tested to findthe sensitivity in 3 axes And the result is summarized inTable 1 If we extend the concept of antenna configuration inhorizontal direction instead of the present vertical directionthen we can obtain rotated sensitivity distribution In casewe cascade these two antenna configurations then we areable to obtain more uniform sensitivity distribution bycompensating the sensitivity to each other

53 Simultaneous Operation of Two Sensors The objectiveof this research is to obtain wide range of sensor responsesthroughout the object from mm scale to cm scale by cascad-ing two sensors having different sensitivity and distributionHowever metal detection sensor operating in differentialmode maintains extremely high gain to increase sensitivitythus it is strongly influenced by nearby electromagneticwaves Therefore the effect from the other sensor is mainlyinvestigated through measurement using the present experi-mental setup to examine the feasibility of this concept Sincethe experimental setup was extremely sensitive to outsidevibration and electromagnetic waves all antennaswere firmlymounted adjusted and fixed using epoxy resin thus it wasnot feasible to change parameters such as dimensions anddistances The response from the sensor having sensitivityin mm scale showed lower response of sim30 as shown inFigure 21 while in simultaneous operation with the sensorhaving sensitivity in cm scale in comparison to standaloneoperation This is considered from the characteristics of PSDbecause the output from PSD has the tendency of decreasing


0

200

400

600

800

1000

0 02 04 06 08 1

Out

put (

mV

)

Standalone operation

Simultaneous operationD = 08mm

D = 10mm

D = 12mm

Volume (mm3)

Figure 21 Effect of simultaneous operation of two sensors

along with the increasing outside noise However it was notpossible to decouple each effect individually such as BPFeffect shielding effect and PSD effect due to the delicatelyintegrated sensor system

On the contrary case the response from the sensorhaving sensitivity in cm scale showed a little less responseof sim85 while in simultaneous operation with the sensorhaving sensitivity in mm scale in comparison to standaloneoperation This is considered also from the characteristics ofPSD but with less effect due to lower gain level of the sensorhaving sensitivity in cm scale

6 Conclusion

The characteristics of metal detection sensor having two setsof perpendicularly oriented sensor antennas were investi-gated to extend the sensing range from mm scale to cmscale with less interference by cascading two sensors Metaldetection sensor having sensitivity in mm scale had superiorsensitivity to ferrous sphere with diameter down to 07mmusing 50 kHz exciting frequency in standalone operationSensor response was proportional to the exciting frequencyand the volume of ferrous test sample as expected The sen-sitivity distribution at the object passage showed enhanceduniformity by attaching copper patch on thewinding coilThebandwidth around 11Hz of LPF after phase sensitive detectorwas found to be optimum for sensitivity enhancement Metaldetection sensor having sensitivity in cm scale showed moreuniform sensitivity distribution but with order of lowersensitivity which was suited to extend sensing range to cmscale with minimum interference This antenna structurefacing each other has the advantage of adding more axesin a simple way thus it enables modular construction toachieve near uniform sensitivity distribution without direc-tion dependent sensitivity The effect of interference whilein simultaneous operation of two sensors was investigatedand the measured result showed reduced output responsebut still within usable detection range Thus it was feasible

to operate two sensors having different sensitivity rangesimultaneously and to extend detection range from mm tocm scale within practically acceptable interference

Conflict of Interests

The author declares that there is no conflict of interestsregarding the publication of this paper

Acknowledgment

This work was supported by Incheon National UniversityResearch Grant in 2013

References

[1] I T McMichael E C Nallon V P Schnee W R Scott andM S Mirotznik ldquoEBG antenna for GPR colocated with a metaldetector for landmine detectionrdquo IEEE Geoscience and RemoteSensing Letters vol 10 no 6 pp 1329ndash1333 2013

[2] M S Sharawi and M I Sharawi ldquoDesign and implementationof a low costVLFmetal detectorwithmetal-type discriminationcapabilitiesrdquo in Proceedings of the IEEE International Conferenceon Signal Processing and Communications (ICSPC rsquo07) pp 480ndash483 November 2007

[3] T Miyakawa and K Honjo ldquoDevelopment of instrumentdetecting nonmetal foreign bodies in food materialrdquo IEEETransactions on Instrumentation and Measurement vol 43 no2 pp 359ndash363 1994

[4] M Zourob S Mohr and N J Goddard ldquoIntegrated deep-probe optical waveguides for label free bacterial detectionrdquo inProceedings of the International Symposium on Signals Systemsand Electronics (ISSSE 07) pp 49ndash52 Montreal CanadaAugust 2007

[5] B Liu and W Zhou ldquoThe research of metal detectors using infood industryrdquo in Proceedings of the International Conference onElectronics and Optoelectronics (ICEOE 11) vol 4 pp V4-43ndashV4-45 Dalian China July 2011

[6] J Kwon J Lee and W Kim ldquoReal-time detection of foreignobjects using x-ray imaging for dry food manufacturing linerdquoin Proceedings of the 12th IEEE International Symposium onConsumer Electronics (ISCE rsquo08) pp 1ndash4 April 2008

[7] W SHua J RHooksW JWu andWCWang ldquoDevelopmentof a polymer based fiberoptic magnetostrictive metal detectorsystemrdquo in Proceedings of the International Symposium onOptomechatronic Technologies (ISOT rsquo10) pp 1ndash5 TorontoCanada October 2010

[8] T Nagaishi F Kamitani H Ota et al ldquoFirst practical high TcSQUID system for the detection of magnetic contaminants incommercial productsrdquo IEEE Transactions on Applied Supercon-ductivity vol 17 no 2 pp 800ndash803 2007

[9] H Krause G I Panaitov N Wolters et al ldquoDetection ofmagnetic contaminations in industrial products using HTSSQUIDsrdquo IEEE Transactions on Applied Superconductivity vol15 no 2 pp 729ndash732 2005

[10] S Yamazaki H Nakane and A Tanaka ldquoBasic analysis of ametal detectorrdquo in Proceedings of the 18th IEEE Instrumentationand Measurement Technology Conference vol 1 pp 474ndash477May 2001


[11] S Yamazaki H Nakane and A Tanaka ldquoBasic analysis ofa metal detectorrdquo IEEE Transactions on Instrumentation andMeasurement vol 51 no 4 pp 810ndash814 2002

[12] P P Silvester and D Omeragic ldquoSensitivity of metal detectorsto spheroidal targetsrdquo IEEE Transactions on Geoscience andRemote Sensing vol 33 no 6 pp 1331ndash1335 1995

[13] Z Tang andLCarter ldquoMetal detector head analysisrdquo inProceed-ings of the 5th International Conference on Sensing Technologypp 93ndash96 Palmerston North New Zealand 2011

[14] K R Blay F Weiss D A Clark G J J B de Groot M Bickand D Sen ldquoSignal processing techniques for improved perfor-mance of a SQUID-basedmetal-detectorrdquo IEEE Transactions onApplied Superconductivity vol 19 no 3 part 1 pp 812ndash815 2009

[15] M Herrmann and K Sakai ldquoObjects in powders detected andimaged with THz radiationrdquo in Proceedings of the IEEE Con-ference on Lasers and Electro-Optics (CLEO rsquo00) San FranciscoCalif USA May 2000

[16] M Brighton andM J English ldquoCalculation of optimumspacingfor a three coil axially symmetric metal detectorrdquo ElectronicsLetters vol 29 no 10 pp 838ndash839 1993

[17] P P Silvester and D Omeragic ldquoSensitivity maps for metaldetector designrdquo IEEE Transactions on Geoscience and RemoteSensing vol 34 no 3 pp 788ndash792 1996

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014


Active and Passive Electronic Components

Control Scienceand Engineering

Journal of



RotatingMachinery


Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design



Shock and Vibration


Civil EngineeringAdvances in

Acoustics and VibrationAdvances in



Electrical and Computer Engineering

Journal of

Advances inOptoElectronics


Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of


Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014


Chemical EngineeringInternational Journal of Antennas and

Propagation




Navigation and Observation



DistributedSensor Networks



minimize physical interference and the optimal signal detec-tionmethod is investigated to reject the interference betweenmetal sensors having different sensitivity resolution

2 Two-Antenna Model

21 Single Channel Model The conventional metal detectionsensorwith sensitivity resolution inmmscale has the antennaset one transmitting antenna and two receiving antennaswhich are connected in opposite polarity to cancel receptionsignal while in steady state In case when an object containinga nonspherical metal piece is fed through hollow antennacross-sectional space as shown in Figure 1 then the sensorshows good sensitivity only for one directional positionbetween metal piece and antenna where the perturbationin electromagnetic flux linkages becomes maximum In thesingle channel model the moving object containing metalpiece perturbs gradually from first electromagnetic fluxlinkages between TX antenna 1 and RX antenna 1 tosecond electromagnetic flux linkages between TX antenna1 and RX antenna 2 The amounts of these spatiallyvarying electromagnetic flux linkages induce currents intoRX antennas 1 and 2 and the current difference betweenRX antennas 1 and 2 becomes output current whichrepresent the electromagnetic flux unbalance of antenna set1

The equivalent circuit of an antenna set one transmittingand two receiving antennas is shown as in Figure 2 Theinstantaneous output voltage V

10 in no load condition can be

expressed as the difference of mutual inductance1198721minus 1198722

between transmitting and respective receiving antenna asshown in (1) and is also proportional to exciting frequency asshown in (3) In this model the distance between transmit-ting antenna and receiving antenna is closer than the distanceto passing object containingmetal piece thus antenna induc-tances are larger than mutual inductance 119872

11and 119872

12 If

transmitting antenna is excited using sinusoidal signal 1198901119904=

11989010sin(120596119905+120575) then antenna current becomes as shown in (2)

Consider

V10= V11minus V12= (11987211minus11987212)

1198891198941199011

119889119905 (1)

1198941199011= 119895

120596119871119901111989010

radic(1206031198711199011)2

+ 1198772

1199011

sin (120596119905 + 120575) (2)

V10= 119895

1205962119871119901111989010(11987211minus11987212)

radic(1206031198711199011)2

+ 1198772

1199011

cos (120596119905 + 120575) (3)

When the phasor expression is used for the flux linkage120595then the phasor voltage119881 due to flux linkage can be expressedas in (4) where 119868 denotes phasor current

11988110= 119895120596120595

10= 119895120596 (119872

11minus11987212) 1198681199011 (4)

It is noted that mutual inductance variance plays a keyrole in determining sensitivity of metal detection sensor

The mutual inductance is expressed as in (5) in which 11989611

and 11989612

represent coupling coefficient between transmittingand receiving antennas respectively And these couplingcoefficients have the value 0 le 119896

11 11989612le 1

11987211= 11989611radic1198711199011119871 11990412 119872

12= 11989612radic1198711199011119871 11990412 (5)

In steady state of initial measurement11987211

and11987212

areadjusted to119872

11= 11987212 and output voltage becomes V

10asymp 0

When 120596 is higher and11987211minus11987212are bigger for the same size

of metal piece then it is advantageous in terms of sensitivity

22 Two-Channel Model In the two antenna sets as shownin Figure 3 the moving object after passing antenna set 1perturbs gradually from third electromagnetic flux linkagesbetween TX antenna 2 and RX antenna 4 to fourthelectromagnetic flux linkages between TX antenna 2 andRXantenna 3 Also the amounts of spatially varying electromag-netic flux linkages induce currents into RX antennas 4 and3 and the current difference between RX antennas 4 and3 becomes output current of antenna set 2

It is difficult to calculate the variance of mutual induc-tance for the moving object thus experimental method isattempted By using the model in Figure 4 the output voltagein each antenna set becomes the product of mutual induc-tance matrix and instantaneous current of each transmittingantenna as in

[11988110

11988120

] = 119895120596 [11987211minus1198721211987221minus11987222

11987213minus1198721411987223minus11987224

] [1198681199011

1198681199012

] (6)

Assume 11986811987911

is phasor current in one conductor ofTX antenna 1 119863 is the distance 119875 in subscript is theremote point outside antenna 119879 in first subscript alphabet istransmitting antenna119877 in first subscript alphabet is receivingantenna the number in second subscript numeric is antennanumber the number in third subscript numeric is conductornumber of antenna and 4th through 6th alphabet andnumeric follow the same convention as in 1st through 3rdalphabet and numeric Then 119863

1198771111987911is the distance between

one conductor of RX antenna 1 and one conductor of TXantenna 1 and119863

11987911119875is the distance between one conductor

of TX antenna 1 and a remote point 119875 The flux linkage1205951198771111987911

with one conductor of RX antenna 1 due to oneconductor of TX antenna 1 can be expressed as in

1205951198771111987911

= 2 times 10minus7(11986811987911

ln 11986311987911119875

1198631198771111987911

) (7)


RX antenna 1

Objectpassage

TX antenna 1

RX antenna 2


TX+

RXOut

RX antenna 2


minus

(a)

+

++


RX antenna 2

minusminus

minus

ip1

Rp1

Lp1

Ls12

Ls12

Rs12

Rs12

is1

M11

M12

E1s

10

11

12

(b)


Objectpassage

Antenna set 2

Antenna set 1

X

Y

RX antenna 1

RX antenna 3

RX antenna 2


RX antenna 4


Objectpassage

P

Y

M12

M11

M13

M14

M21

M22

M23

M24





12059511987711

= 1205951198771111987911

+ 1205951198771111987912

+ 1205951198771111987921

+ 1205951198771111987922

= 2 times 10minus7(11986811987911

ln 11986311987911119875

1198631198771111987911

+ 11986811987912

ln 11986311987912119875

1198631198771111987912

+11986811987921

ln 11986311987921119875

1198631198771111987921

+ 11986811987922

ln 11986311987922119875

1198631198771111987922

)

= 2 times 10minus7(11986811987911

ln 1

1198631198771111987911

+ 11986811987912

ln 1

1198631198771111987912

+ 11986811987921

ln 1

1198631198771111987921

+ 11986811987922

ln 1

1198631198771111987922

+ 11986811987911

ln11986311987911119875

+ 11986811987912

ln11986311987912119875

+11986811987921

ln11986311987921119875

+ 11986811987922

ln11986311987922119875

)

(8)


11987911+11986811987912

= 0 also likewise 11986811987921+11986811987922

= 0


12059511987711

= 2 times 10minus7(11986811987911

ln1198631198771111987912

1198631198771111987911

+ 11986811987921

ln1198631198771111987922

1198631198771111987921

) (9)



and 12059511987712

as shown in

1205951198771= 2 times 10

minus7(11986811987911

ln1198631198771111987912

1198631198771111987911

+ 11986811987911

ln1198631198771211987912

1198631198771211987911

+11986811987921

ln1198631198771111987922

1198631198771111987921

+ 11986811987921

ln1198631198771211987922

1198631198771211987921

)


1198631198771111987912

1198631198771111987911

+ ln1198631198771211987912

1198631198771211987911

) 11986811987911

+(ln1198631198771111987922

1198631198771111987921

+ ln1198631198771211987922

1198631198771211987921

) 11986811987921)


radic1198631198771111987912

1198631198771211987912

radic1198631198771111987911

1198631198771211987911

11986811987911

+ lnradic1198631198771111987922

1198631198771211987922

radic1198631198771111987921

1198631198771211987921

11986811987921)

(10)


and11987221


11987211= 4 times 10

minus7 lnradic1198631198771111987912

1198631198771211987912

radic1198631198771111987911

1198631198771211987911

11987221= 4 times 10

minus7 lnradic1198631198771111987922

1198631198771211987922

radic1198631198771111987921

1198631198771211987921

11987212= 4 times 10

minus7 lnradic1198631198772111987912

1198631198772211987912

radic1198631198772111987911

1198631198772211987911

11987222= 4 times 10

minus7 lnradic1198631198772111987922

1198631198772211987922

radic1198631198772111987921

1198631198772211987921

11987213= 4 times 10

minus7 lnradic1198631198773111987912

1198631198773211987912

radic1198631198773111987911

1198631198773211987911

11987223= 4 times 10

minus7 lnradic1198631198773111987922

1198631198773211987922

radic1198631198773111987921

1198631198773211987921

11987214= 4 times 10

minus7 lnradic1198631198774111987912

1198631198774211987912

radic1198631198774111987911

1198631198774211987911

11987224= 4 times 10

minus7 lnradic1198631198774111987922

1198631198774211987922

radic1198631198774111987921

1198631198774211987921

(11)




120595119909=

4

sum

119894=1

41205871198862120583119868119894

2120587119909119894

(12)



1198771(119891)| and |119867

1198772(119891)| for




[1198781BPF (119891)1198782BPF (119891)

] = [

10038161003816100381610038161198671198771(119891)1003816100381610038161003816

2 10038161003816100381610038161198671198771(119891)1003816100381610038161003816

2

10038161003816100381610038161198671198772(119891)1003816100381610038161003816

2 10038161003816100381610038161198671198772(119891)1003816100381610038161003816

2] [1198781 In (119891)1198782 In (119891)

] (13)





119881In = 1198641 sin (1205961119905 + 1205791) + 1198642 sin (1205962119905 + 1205792) + 119899 (119905) (14)


= [1198641sin (120596

1119905 + 1205791) + 1198642sin (120596

2119905 + 1205792)

+119899 (119905) + 1198642sin (120596

2119905 + 1205792)] lowast ℎBPF (119905)

119881OP = 119881BPF times 119881REF

= 119864REF sin (1205961119905 + 1205791)

times [1198641sin (120596

1119905 + 1205791) lowast ℎBPF (119905)

+ 1198642sin (120596

2119905 + 1205792) lowast ℎBPF (119905)

+119899 (119905) lowast ℎBPF (119905) ]

(15)




= 119864REF sin (1205961119905 + 1205791)

times [1198641sin (120596

1119905 + 1205791) lowast ℎBPF (119905)

+ 1198642sin (120596

2119905 + 1205792) lowast ℎBPF (119905)


asymp119864REF11986412

+ 120572 sdot 119899 (119905)

(16)


VIn VOPVBPF

VREF


BPF LPF


Enclosure

Entry


4 System Design






Adjustingscrew


OSC Divider(120)

Divider(12)

Divider(14)


Poweramplifier

Antennamatching


Synch signal

Control

8MHz



(8MHz)






Sig out MICOM

PRE AMP BPF PSD

Synch signal

AMP

Antennamatching

Receivingantenna

1

2

Receivingantenna

LPF

TX control

Control


Pattern

Base

(a)

PatternBase

(b)



5 Measurement



) = minus74 [dB] (17)



Poweramplifier

Antennamatching


Synch signal

Bufferamplifier

OSCTune

(20kHz)


PRE AMP BPF PSD LPF

AMP Sig out

Synch signal

Receivingantenna

Receivingantenna

3

4






0

200

400

600

800

1000

0 02 04 06 08 1

(mV

)

Volume (mm3)

D = 08mm

D = 10mm

D = 12mm


0

4

8

12

16

0 100 200 300 400 500

Out

put (

V)

Frequency (kHz)

Measured






Y (mm)

X (mm)

Out

putV

pp

050

10006

04

02

000100200300

(a)

Y (mm)

X (mm)

Out

putV

pp

050

10006

04

02

000100200300

(b)


fco = 11Hz

fco = 33Hz




Signal

(a)

Signal

(b)





0

1

2

3

4

0 20 40 60 80 100

Out

put (

V)

Frequency (kHz)

With metal

Without metal


0

20

40

60

80

100

0 20 40 60 80 100

Ratio

()

Frequency (kHz)






0

10

20

30

40

50

0 20 40 60 80 100

Ratio

()

Frequency (kHz)

Ideal

Measured











0

200

400

600

800

1000

0 02 04 06 08 1

Out

put (

mV

)



D = 10mm

D = 12mm

Volume (mm3)




6 Conclusion





Acknowledgment


References





















RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










RX antenna 1

Objectpassage

TX antenna 1

RX antenna 2


TX+

RXOut

RX antenna 2


minus

(a)

+

++


RX antenna 2

minusminus

minus

ip1

Rp1

Lp1

Ls12

Ls12

Rs12

Rs12

is1

M11

M12

E1s

10

11

12

(b)


Objectpassage

Antenna set 2

Antenna set 1

X

Y

RX antenna 1

RX antenna 3

RX antenna 2


RX antenna 4


Objectpassage

P

Y

M12

M11

M13

M14

M21

M22

M23

M24





12059511987711

= 1205951198771111987911

+ 1205951198771111987912

+ 1205951198771111987921

+ 1205951198771111987922

= 2 times 10minus7(11986811987911

ln 11986311987911119875

1198631198771111987911

+ 11986811987912

ln 11986311987912119875

1198631198771111987912

+11986811987921

ln 11986311987921119875

1198631198771111987921

+ 11986811987922

ln 11986311987922119875

1198631198771111987922

)

= 2 times 10minus7(11986811987911

ln 1

1198631198771111987911

+ 11986811987912

ln 1

1198631198771111987912

+ 11986811987921

ln 1

1198631198771111987921

+ 11986811987922

ln 1

1198631198771111987922

+ 11986811987911

ln11986311987911119875

+ 11986811987912

ln11986311987912119875

+11986811987921

ln11986311987921119875

+ 11986811987922

ln11986311987922119875

)

(8)


11987911+11986811987912

= 0 also likewise 11986811987921+11986811987922

= 0


12059511987711

= 2 times 10minus7(11986811987911

ln1198631198771111987912

1198631198771111987911

+ 11986811987921

ln1198631198771111987922

1198631198771111987921

) (9)



and 12059511987712

as shown in

1205951198771= 2 times 10

minus7(11986811987911

ln1198631198771111987912

1198631198771111987911

+ 11986811987911

ln1198631198771211987912

1198631198771211987911

+11986811987921

ln1198631198771111987922

1198631198771111987921

+ 11986811987921

ln1198631198771211987922

1198631198771211987921

)


1198631198771111987912

1198631198771111987911

+ ln1198631198771211987912

1198631198771211987911

) 11986811987911

+(ln1198631198771111987922

1198631198771111987921

+ ln1198631198771211987922

1198631198771211987921

) 11986811987921)


radic1198631198771111987912

1198631198771211987912

radic1198631198771111987911

1198631198771211987911

11986811987911

+ lnradic1198631198771111987922

1198631198771211987922

radic1198631198771111987921

1198631198771211987921

11986811987921)

(10)


and11987221


11987211= 4 times 10

minus7 lnradic1198631198771111987912

1198631198771211987912

radic1198631198771111987911

1198631198771211987911

11987221= 4 times 10

minus7 lnradic1198631198771111987922

1198631198771211987922

radic1198631198771111987921

1198631198771211987921

11987212= 4 times 10

minus7 lnradic1198631198772111987912

1198631198772211987912

radic1198631198772111987911

1198631198772211987911

11987222= 4 times 10

minus7 lnradic1198631198772111987922

1198631198772211987922

radic1198631198772111987921

1198631198772211987921

11987213= 4 times 10

minus7 lnradic1198631198773111987912

1198631198773211987912

radic1198631198773111987911

1198631198773211987911

11987223= 4 times 10

minus7 lnradic1198631198773111987922

1198631198773211987922

radic1198631198773111987921

1198631198773211987921

11987214= 4 times 10

minus7 lnradic1198631198774111987912

1198631198774211987912

radic1198631198774111987911

1198631198774211987911

11987224= 4 times 10

minus7 lnradic1198631198774111987922

1198631198774211987922

radic1198631198774111987921

1198631198774211987921

(11)




120595119909=

4

sum

119894=1

41205871198862120583119868119894

2120587119909119894

(12)



1198771(119891)| and |119867

1198772(119891)| for




[1198781BPF (119891)1198782BPF (119891)

] = [

10038161003816100381610038161198671198771(119891)1003816100381610038161003816

2 10038161003816100381610038161198671198771(119891)1003816100381610038161003816

2

10038161003816100381610038161198671198772(119891)1003816100381610038161003816

2 10038161003816100381610038161198671198772(119891)1003816100381610038161003816

2] [1198781 In (119891)1198782 In (119891)

] (13)





119881In = 1198641 sin (1205961119905 + 1205791) + 1198642 sin (1205962119905 + 1205792) + 119899 (119905) (14)


= [1198641sin (120596

1119905 + 1205791) + 1198642sin (120596

2119905 + 1205792)

+119899 (119905) + 1198642sin (120596

2119905 + 1205792)] lowast ℎBPF (119905)

119881OP = 119881BPF times 119881REF

= 119864REF sin (1205961119905 + 1205791)

times [1198641sin (120596

1119905 + 1205791) lowast ℎBPF (119905)

+ 1198642sin (120596

2119905 + 1205792) lowast ℎBPF (119905)

+119899 (119905) lowast ℎBPF (119905) ]

(15)




= 119864REF sin (1205961119905 + 1205791)

times [1198641sin (120596

1119905 + 1205791) lowast ℎBPF (119905)

+ 1198642sin (120596

2119905 + 1205792) lowast ℎBPF (119905)


asymp119864REF11986412

+ 120572 sdot 119899 (119905)

(16)


VIn VOPVBPF

VREF


BPF LPF


Enclosure

Entry


4 System Design






Adjustingscrew


OSC Divider(120)

Divider(12)

Divider(14)


Poweramplifier

Antennamatching


Synch signal

Control

8MHz



(8MHz)






Sig out MICOM

PRE AMP BPF PSD

Synch signal

AMP

Antennamatching

Receivingantenna

1

2

Receivingantenna

LPF

TX control

Control


Pattern

Base

(a)

PatternBase

(b)



5 Measurement



) = minus74 [dB] (17)



Poweramplifier

Antennamatching


Synch signal

Bufferamplifier

OSCTune

(20kHz)


PRE AMP BPF PSD LPF

AMP Sig out

Synch signal

Receivingantenna

Receivingantenna

3

4






0

200

400

600

800

1000

0 02 04 06 08 1

(mV

)

Volume (mm3)

D = 08mm

D = 10mm

D = 12mm


0

4

8

12

16

0 100 200 300 400 500

Out

put (

V)

Frequency (kHz)

Measured






Y (mm)

X (mm)

Out

putV

pp

050

10006

04

02

000100200300

(a)

Y (mm)

X (mm)

Out

putV

pp

050

10006

04

02

000100200300

(b)


fco = 11Hz

fco = 33Hz




Signal

(a)

Signal

(b)





0

1

2

3

4

0 20 40 60 80 100

Out

put (

V)

Frequency (kHz)

With metal

Without metal


0

20

40

60

80

100

0 20 40 60 80 100

Ratio

()

Frequency (kHz)






0

10

20

30

40

50

0 20 40 60 80 100

Ratio

()

Frequency (kHz)

Ideal

Measured











0

200

400

600

800

1000

0 02 04 06 08 1

Out

put (

mV

)



D = 10mm

D = 12mm

Volume (mm3)




6 Conclusion





Acknowledgment


References





















RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation












12059511987711

= 1205951198771111987911

+ 1205951198771111987912

+ 1205951198771111987921

+ 1205951198771111987922

= 2 times 10minus7(11986811987911

ln 11986311987911119875

1198631198771111987911

+ 11986811987912

ln 11986311987912119875

1198631198771111987912

+11986811987921

ln 11986311987921119875

1198631198771111987921

+ 11986811987922

ln 11986311987922119875

1198631198771111987922

)

= 2 times 10minus7(11986811987911

ln 1

1198631198771111987911

+ 11986811987912

ln 1

1198631198771111987912

+ 11986811987921

ln 1

1198631198771111987921

+ 11986811987922

ln 1

1198631198771111987922

+ 11986811987911

ln11986311987911119875

+ 11986811987912

ln11986311987912119875

+11986811987921

ln11986311987921119875

+ 11986811987922

ln11986311987922119875

)

(8)


11987911+11986811987912

= 0 also likewise 11986811987921+11986811987922

= 0


12059511987711

= 2 times 10minus7(11986811987911

ln1198631198771111987912

1198631198771111987911

+ 11986811987921

ln1198631198771111987922

1198631198771111987921

) (9)



and 12059511987712

as shown in

1205951198771= 2 times 10

minus7(11986811987911

ln1198631198771111987912

1198631198771111987911

+ 11986811987911

ln1198631198771211987912

1198631198771211987911

+11986811987921

ln1198631198771111987922

1198631198771111987921

+ 11986811987921

ln1198631198771211987922

1198631198771211987921

)


1198631198771111987912

1198631198771111987911

+ ln1198631198771211987912

1198631198771211987911

) 11986811987911

+(ln1198631198771111987922

1198631198771111987921

+ ln1198631198771211987922

1198631198771211987921

) 11986811987921)


radic1198631198771111987912

1198631198771211987912

radic1198631198771111987911

1198631198771211987911

11986811987911

+ lnradic1198631198771111987922

1198631198771211987922

radic1198631198771111987921

1198631198771211987921

11986811987921)

(10)


and11987221


11987211= 4 times 10

minus7 lnradic1198631198771111987912

1198631198771211987912

radic1198631198771111987911

1198631198771211987911

11987221= 4 times 10

minus7 lnradic1198631198771111987922

1198631198771211987922

radic1198631198771111987921

1198631198771211987921

11987212= 4 times 10

minus7 lnradic1198631198772111987912

1198631198772211987912

radic1198631198772111987911

1198631198772211987911

11987222= 4 times 10

minus7 lnradic1198631198772111987922

1198631198772211987922

radic1198631198772111987921

1198631198772211987921

11987213= 4 times 10

minus7 lnradic1198631198773111987912

1198631198773211987912

radic1198631198773111987911

1198631198773211987911

11987223= 4 times 10

minus7 lnradic1198631198773111987922

1198631198773211987922

radic1198631198773111987921

1198631198773211987921

11987214= 4 times 10

minus7 lnradic1198631198774111987912

1198631198774211987912

radic1198631198774111987911

1198631198774211987911

11987224= 4 times 10

minus7 lnradic1198631198774111987922

1198631198774211987922

radic1198631198774111987921

1198631198774211987921

(11)




120595119909=

4

sum

119894=1

41205871198862120583119868119894

2120587119909119894

(12)



1198771(119891)| and |119867

1198772(119891)| for




[1198781BPF (119891)1198782BPF (119891)

] = [

10038161003816100381610038161198671198771(119891)1003816100381610038161003816

2 10038161003816100381610038161198671198771(119891)1003816100381610038161003816

2

10038161003816100381610038161198671198772(119891)1003816100381610038161003816

2 10038161003816100381610038161198671198772(119891)1003816100381610038161003816

2] [1198781 In (119891)1198782 In (119891)

] (13)





119881In = 1198641 sin (1205961119905 + 1205791) + 1198642 sin (1205962119905 + 1205792) + 119899 (119905) (14)


= [1198641sin (120596

1119905 + 1205791) + 1198642sin (120596

2119905 + 1205792)

+119899 (119905) + 1198642sin (120596

2119905 + 1205792)] lowast ℎBPF (119905)

119881OP = 119881BPF times 119881REF

= 119864REF sin (1205961119905 + 1205791)

times [1198641sin (120596

1119905 + 1205791) lowast ℎBPF (119905)

+ 1198642sin (120596

2119905 + 1205792) lowast ℎBPF (119905)

+119899 (119905) lowast ℎBPF (119905) ]

(15)




= 119864REF sin (1205961119905 + 1205791)

times [1198641sin (120596

1119905 + 1205791) lowast ℎBPF (119905)

+ 1198642sin (120596

2119905 + 1205792) lowast ℎBPF (119905)


asymp119864REF11986412

+ 120572 sdot 119899 (119905)

(16)


VIn VOPVBPF

VREF


BPF LPF


Enclosure

Entry


4 System Design






Adjustingscrew


OSC Divider(120)

Divider(12)

Divider(14)


Poweramplifier

Antennamatching


Synch signal

Control

8MHz



(8MHz)






Sig out MICOM

PRE AMP BPF PSD

Synch signal

AMP

Antennamatching

Receivingantenna

1

2

Receivingantenna

LPF

TX control

Control


Pattern

Base

(a)

PatternBase

(b)



5 Measurement



) = minus74 [dB] (17)



Poweramplifier

Antennamatching


Synch signal

Bufferamplifier

OSCTune

(20kHz)


PRE AMP BPF PSD LPF

AMP Sig out

Synch signal

Receivingantenna

Receivingantenna

3

4






0

200

400

600

800

1000

0 02 04 06 08 1

(mV

)

Volume (mm3)

D = 08mm

D = 10mm

D = 12mm


0

4

8

12

16

0 100 200 300 400 500

Out

put (

V)

Frequency (kHz)

Measured






Y (mm)

X (mm)

Out

putV

pp

050

10006

04

02

000100200300

(a)

Y (mm)

X (mm)

Out

putV

pp

050

10006

04

02

000100200300

(b)


fco = 11Hz

fco = 33Hz




Signal

(a)

Signal

(b)





0

1

2

3

4

0 20 40 60 80 100

Out

put (

V)

Frequency (kHz)

With metal

Without metal


0

20

40

60

80

100

0 20 40 60 80 100

Ratio

()

Frequency (kHz)






0

10

20

30

40

50

0 20 40 60 80 100

Ratio

()

Frequency (kHz)

Ideal

Measured











0

200

400

600

800

1000

0 02 04 06 08 1

Out

put (

mV

)



D = 10mm

D = 12mm

Volume (mm3)




6 Conclusion





Acknowledgment


References





















RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation












119881In = 1198641 sin (1205961119905 + 1205791) + 1198642 sin (1205962119905 + 1205792) + 119899 (119905) (14)


= [1198641sin (120596

1119905 + 1205791) + 1198642sin (120596

2119905 + 1205792)

+119899 (119905) + 1198642sin (120596

2119905 + 1205792)] lowast ℎBPF (119905)

119881OP = 119881BPF times 119881REF

= 119864REF sin (1205961119905 + 1205791)

times [1198641sin (120596

1119905 + 1205791) lowast ℎBPF (119905)

+ 1198642sin (120596

2119905 + 1205792) lowast ℎBPF (119905)

+119899 (119905) lowast ℎBPF (119905) ]

(15)




= 119864REF sin (1205961119905 + 1205791)

times [1198641sin (120596

1119905 + 1205791) lowast ℎBPF (119905)

+ 1198642sin (120596

2119905 + 1205792) lowast ℎBPF (119905)


asymp119864REF11986412

+ 120572 sdot 119899 (119905)

(16)


VIn VOPVBPF

VREF


BPF LPF


Enclosure

Entry


4 System Design






Adjustingscrew


OSC Divider(120)

Divider(12)

Divider(14)


Poweramplifier

Antennamatching


Synch signal

Control

8MHz



(8MHz)






Sig out MICOM

PRE AMP BPF PSD

Synch signal

AMP

Antennamatching

Receivingantenna

1

2

Receivingantenna

LPF

TX control

Control


Pattern

Base

(a)

PatternBase

(b)



5 Measurement



) = minus74 [dB] (17)



Poweramplifier

Antennamatching


Synch signal

Bufferamplifier

OSCTune

(20kHz)


PRE AMP BPF PSD LPF

AMP Sig out

Synch signal

Receivingantenna

Receivingantenna

3

4






0

200

400

600

800

1000

0 02 04 06 08 1

(mV

)

Volume (mm3)

D = 08mm

D = 10mm

D = 12mm


0

4

8

12

16

0 100 200 300 400 500

Out

put (

V)

Frequency (kHz)

Measured






Y (mm)

X (mm)

Out

putV

pp

050

10006

04

02

000100200300

(a)

Y (mm)

X (mm)

Out

putV

pp

050

10006

04

02

000100200300

(b)


fco = 11Hz

fco = 33Hz




Signal

(a)

Signal

(b)





0

1

2

3

4

0 20 40 60 80 100

Out

put (

V)

Frequency (kHz)

With metal

Without metal


0

20

40

60

80

100

0 20 40 60 80 100

Ratio

()

Frequency (kHz)






0

10

20

30

40

50

0 20 40 60 80 100

Ratio

()

Frequency (kHz)

Ideal

Measured











0

200

400

600

800

1000

0 02 04 06 08 1

Out

put (

mV

)



D = 10mm

D = 12mm

Volume (mm3)




6 Conclusion





Acknowledgment


References





















RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










Adjustingscrew


OSC Divider(120)

Divider(12)

Divider(14)


Poweramplifier

Antennamatching


Synch signal

Control

8MHz



(8MHz)






Sig out MICOM

PRE AMP BPF PSD

Synch signal

AMP

Antennamatching

Receivingantenna

1

2

Receivingantenna

LPF

TX control

Control


Pattern

Base

(a)

PatternBase

(b)



5 Measurement



) = minus74 [dB] (17)



Poweramplifier

Antennamatching


Synch signal

Bufferamplifier

OSCTune

(20kHz)


PRE AMP BPF PSD LPF

AMP Sig out

Synch signal

Receivingantenna

Receivingantenna

3

4






0

200

400

600

800

1000

0 02 04 06 08 1

(mV

)

Volume (mm3)

D = 08mm

D = 10mm

D = 12mm


0

4

8

12

16

0 100 200 300 400 500

Out

put (

V)

Frequency (kHz)

Measured






Y (mm)

X (mm)

Out

putV

pp

050

10006

04

02

000100200300

(a)

Y (mm)

X (mm)

Out

putV

pp

050

10006

04

02

000100200300

(b)


fco = 11Hz

fco = 33Hz




Signal

(a)

Signal

(b)





0

1

2

3

4

0 20 40 60 80 100

Out

put (

V)

Frequency (kHz)

With metal

Without metal


0

20

40

60

80

100

0 20 40 60 80 100

Ratio

()

Frequency (kHz)






0

10

20

30

40

50

0 20 40 60 80 100

Ratio

()

Frequency (kHz)

Ideal

Measured











0

200

400

600

800

1000

0 02 04 06 08 1

Out

put (

mV

)



D = 10mm

D = 12mm

Volume (mm3)




6 Conclusion





Acknowledgment


References





















RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










Poweramplifier

Antennamatching


Synch signal

Bufferamplifier

OSCTune

(20kHz)


PRE AMP BPF PSD LPF

AMP Sig out

Synch signal

Receivingantenna

Receivingantenna

3

4






0

200

400

600

800

1000

0 02 04 06 08 1

(mV

)

Volume (mm3)

D = 08mm

D = 10mm

D = 12mm


0

4

8

12

16

0 100 200 300 400 500

Out

put (

V)

Frequency (kHz)

Measured






Y (mm)

X (mm)

Out

putV

pp

050

10006

04

02

000100200300

(a)

Y (mm)

X (mm)

Out

putV

pp

050

10006

04

02

000100200300

(b)


fco = 11Hz

fco = 33Hz




Signal

(a)

Signal

(b)





0

1

2

3

4

0 20 40 60 80 100

Out

put (

V)

Frequency (kHz)

With metal

Without metal


0

20

40

60

80

100

0 20 40 60 80 100

Ratio

()

Frequency (kHz)






0

10

20

30

40

50

0 20 40 60 80 100

Ratio

()

Frequency (kHz)

Ideal

Measured











0

200

400

600

800

1000

0 02 04 06 08 1

Out

put (

mV

)



D = 10mm

D = 12mm

Volume (mm3)




6 Conclusion





Acknowledgment


References





















RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










Y (mm)

X (mm)

Out

putV

pp

050

10006

04

02

000100200300

(a)

Y (mm)

X (mm)

Out

putV

pp

050

10006

04

02

000100200300

(b)


fco = 11Hz

fco = 33Hz




Signal

(a)

Signal

(b)





0

1

2

3

4

0 20 40 60 80 100

Out

put (

V)

Frequency (kHz)

With metal

Without metal


0

20

40

60

80

100

0 20 40 60 80 100

Ratio

()

Frequency (kHz)






0

10

20

30

40

50

0 20 40 60 80 100

Ratio

()

Frequency (kHz)

Ideal

Measured











0

200

400

600

800

1000

0 02 04 06 08 1

Out

put (

mV

)



D = 10mm

D = 12mm

Volume (mm3)




6 Conclusion





Acknowledgment


References





















RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










0

1

2

3

4

0 20 40 60 80 100

Out

put (

V)

Frequency (kHz)

With metal

Without metal


0

20

40

60

80

100

0 20 40 60 80 100

Ratio

()

Frequency (kHz)






0

10

20

30

40

50

0 20 40 60 80 100

Ratio

()

Frequency (kHz)

Ideal

Measured











0

200

400

600

800

1000

0 02 04 06 08 1

Out

put (

mV

)



D = 10mm

D = 12mm

Volume (mm3)




6 Conclusion





Acknowledgment


References





















RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










0

200

400

600

800

1000

0 02 04 06 08 1

Out

put (

mV

)



D = 10mm

D = 12mm

Volume (mm3)




6 Conclusion





Acknowledgment


References





















RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation



















RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation









research article two-channel metal detector using two ...two-channel metal detector, having two sets...

Documents