A Microwave Sensor for Evaluation of Plastic Wall Thickness

Transcription

1 ELECTRONICS AND ELECTRICAL ENGINEERING ISSN No. 7(3) ELEKTRONIKA IR ELEKTROTECHNIKA HIGH FREQUENCY TECHNOLOGY, MICROWAVES AUKŠTŲJŲ DAŽNIŲ TECHNOLOGIJA, MIKROBANGOS A Micowave Senso fo Evaluation of Plastic Wall Thickness A. Janeliauskas Depatment of Electonics Engineeing, Kaunas Univesity of Technology, Studentų st Kaunas, Lithuania, phone: , atuas.janeliauskas@ktu.lt Intoduction Non destuctive measuement of the thickness and monitoing its vaiation of lossy dielectic slab poducts is of geat inteest in many facets of industy. Methods ae based on measuements of tansmitted and/o eflected electomagnetic powe fom a dielectic mateial unde test by an incident electomagnetic wave. The ecoded amplitude and phase of the tansmitted o eflected wave is analyzed and pocessed to ceate appopiate coelations with physical and/o dimensional vaiations of a mateial. Many techniques (tansmission/eflection line, fee space, open ended coaxial o waveguide pobe) have been developed to measue these popeties such as techniques in time domain o fequency domain with one pot o two pots, etc. Evey technique is limited to specific fequencies, mateials and applications [ 3]. Howeve, in some industial pocesses used mateials have a low pemittivity o high loss; opposite wall is not metal backed and only one side of mateials is accessible. Reflectivity signal fom the opposite wall is low, these methods unsuitable in industy. Resonant measuements ae the most accuate. To educe the senso size, it is appopiate to use the esonance popeties of micostip antennas [4]. The lightweight constuction and the suitability fo integation with MICs (Micowave Integated Cicuits) ae two moe of thei numeous advantages. Micostip pinted antenna technology is suitable fo low cost manufactuing. This is impotant, since MICs ae much easie to handle and less expensive than the altenative waveguides. Suitability of micostip antennas fo low fequency applications have been demonstated in [5]. Although micostip antennas ae widely used, thee is a little eseach on the design of nea field micostip antennas fo evaluation of plastic wall thickness and monitoing its vaiation fo industial applications. Theoy The elative complex pemittivity of an isotopic mateial is dependent on seveal vaiables, such as fequency, tempeatue, density, and moistue content. Fom a view of slab thickness measuement, these vaiables must be fixed o appopiate coelations must be established. The loss facto, is often used to study the stuctue and composition of mateials. The depth of penetation d of micowaves into a dielectic is a function of the total dielectic loss tangent and the vacuum wavelength [6] d, () tan whee is the eal pat of dielectic pemittivity, tan is the total dielectic loss tangent, tan imaginay pat of dielectic pemittivity. Micowave has been low dept of penetation inside Cabon Fibe Reinfoced Plastics (CFRP) and esults fom measuement can be ambiguous to intepet. Lowe fequency wave penetates deepe (Fig. ). Howeve, the size of the antenna inceases and the esolution degades apidly. Micowave penetation dept d, mm ,5 GHz,45 GHz Imaginay pat of dielectic constant " Fig.. The depth of penetation of micowaves into a dielectic f 433,9 MHz Poblem can be solved using a substate integated waveguide antennas o low fequency micostip antennas 69

2 [5]. The substate mateial (fille) is selected with the neaest dielectic pemittivity as the measuement slab. The filles can significantly educe the fequency of opeation and keeping the small size of antennas, educing eflection fom a measuement wall, inceases the penetation depth, maintains the esolution, and inceasing the detection sensitivity and measuement accuacy. Polaizability of micowave signals enables the study of fibe bundle oientation o misalignment duing manufactuing and povides infomation about cut o boken fibe bundles. To eliminate the influence of anisotopy fo wall thickness measuement should be used micowaves of cicula polaization. Assuming a pependicula plane wave taveling though a laye of low-lossmateial ( " << ) with thickness h, the eal and imaginay components of the elative complex pemittivity ae detemined as follows []:, () h A, (3) h whee m ( m ) is the phase shift; m tunc ( h / ) the intege pat of h / ; A is the attenuation in decibels. Eq. () shows that phases infomation is necessay to detemine the sample thickness h, when is known the numeical value. At a given fequency, sample thickness should fall within the following inteval [3] maximum adiated field nomal to the suface of the patch, ceate fa field and penetate into a dielectic deepe. Nomal and tangential fields ae complementay fo senso applications and allow measuement fom low to highest thickness. Senso design To measue the thickness and popeties of the low pemittivity o high loss composites, when only one side of mateials is accessible, most suitable ae micostip antennas. They ae naowband antennas compaed to conventional micowave antennas, since the adiation is a consequence of a esonance [4]. Fo esonant measuement antenna (senso), a special feeding technique (poximity coupling) was used. This antenna uses electic and magnetic coupling, in which no physical connection is made between the feed line and the dipole [7]. Poximity coupled micowave thickness senso antenna have many advantages ove end fed and coaxial fed antenna. Some advantages include: no galvanic contact between feed line and adiating element, low pemittivity patch substate with a lage thickness, no dilling equied, less spuious adiation, bette esonance sensitivity. The antenna (Fig. ) consists of two stacked substates 3, 4 with a gound plane 5 undeneath and two collinea micostiplines, at diffeent levels. The feed line is located between the two substates and ovelaps the dipole, located on top of the substate 3. W f L d W d m h ( m ), (4) 3 L f whee m is an intege to be detemined. In any industial pocess contol, the phase ambiguity poblem has to be solved in a way that can be easily automated. If a dielectic slab thickness h, the intege m can be detemined as well as Nicholson Ross Wei convesion technique by analysis of goup delay. Delay though the mateial is a function of the mateial total length and can be used to esolve this ambiguity. Measuements at two diffeent fequencies can also be used to solve the phase ambiguity poblem [, 3]. Fo one side thickness measuement, wave eflected fom the opposite side must pass the same distance as the incident wave. If the incident wave popagation vecto is pependicula to the dielectic suface, the penetation depth should be at least twice highe. In this case, bette suited the patch antenna [4]. The fields at the end of the patch can be split into tangential and nomal components with espect to the gound plane. Nomal field components ceate only nea field. Thei contibution to the fa field in boadside diection cancels each othe. The tangential field components, which ae in phase, combine to give the t b H t H b 4 5 Fig.. The expeimental poximity fed antenna geometic: dipole; feed line; 3 top substate; 4 bottom substate; 5 gound plane The theoy behind poximity feed patches is quite complex and only design guidelines will be pesented [7]. ANTENNA MAGUS antenna design softwae could easily calculate specifications (geometic dimensions, effective dielectic pemittivity, etun loss and attenuation). Final coections ae then made fo coupling fom numeical evaluation with CST MICROWAVE STUDIO (CST MWS). Tune the length vaiable of the patch antenna in the model such that the tuned patch adiates at the desied fequency. Tune the patch ovelaps distance in the model such that the patch will give maximum paallel esonant quality. Geometic dimensions and paametes of the expeimental poximityfed antenna ae given in Table. L 7

3 Table. Geometic dimensions and paametes of the expeimental poximity fed antenna Cente fequency f,45 GHz Input impedance (at f ) Z 5 Ohms Dipole length L d 49,7 mm Dipole width W d,5 mm Feedline length L f 8 mm Feedline width Ovelap length L Top substate height H t Top substate elative dielectic pemittivity Bottom substate height Bottom substate elative dielectic pemittivity W f 8 mm 5 mm 6,5 mm t,3 H b 3, mm b,35 The antenna adiation chaacteistic at cente fequency is shown in Fig. 3. n = n = n = 5 n = n = n = 5 Fig. 4. Electic and magnetic field distibution on the feed line at TLR esonant states of n =,, 5 The input impedance and admittance of TLR cicuit in fequency ange to 5 GHz ae shown in Fig. 5. As we can see in Fig 5 in fequency ange to 5 GHz gets tee shunt (a) and two seies (b) esonance. Maximum shunt non loaded esonance quality is 5 at,863 GHz. Additional investigation showed that the lowest shunt esonance occus at.9365 GHz. At this fequency, eal pat of impedance Z = 7 Ohms and non-loaded esonance quality ae 5. Input impedance Z, Ohms Re(Z) Im(Z) Fig. 3. Antenna adiation chaacteistics at cente fequency The appoach hee is based on the idea that fo sensing applications exploited esonance popeties of this antenna. An equivalent cicuit of the antenna shown in [7]. The coupling capacitance between the feed line and the patch is a distibuted element, which with feed line and equivalent shunt capacitance and inductance of patch foms tansmission line esonato (TLR) cicuit. Input impedances of low loss tansmission line esonatos ae geneally expessed in tems of tanscendental functions, so it is difficult to quantify analytically. CST MWS allows to evaluate impedance and admittance of TLR in boad fequency ange. Fig. 4 shows the field distibutions at feed line (the TLR suface) as some esonant states of n =,, 5 fo compaison. A change in the pemittivity of the patch envionment causes a change in its capacitance and hence a shift of the esonato fequency. This cicumstance allows to evaluate the thickness of the high loss composites via ecoding of esonant fequency o phase deviation. Input admittance Y, S Fequency f, GHz a) Re(Y) Im(Y) b) Fig. 5. Input impedance (a) and admittance (b) of TLR cicuit in boad fequency ange Fo sensing applications, due to high quality most suitable shunts esonance of TLR. In ode to incease patch tangential electic field component influence, TLR shunt esonance opeating fequency aea must be close as possible to antenna cente fequency. 7

4 Senso substate mateials The most citical paamete fo patch antennas is the dielectic pemittivity, which must be contolled to within +/, % to achieve a esonant fequency accuacy of +/. %. [8]. The top substate must be made with a lowe pemittivity to poduce loosely bound finging fields, yielding bette adiation and inceases the sensitivity. Fo a foam substate, thee is no suface wave excitation losses and highe efficiency ae obtained by making the substate thicke. A highe value of pemittivity of bottom substate causes tightly coupled field, that dont poduce spuious adiation. Afte analysis, the most appopiate mateials fo top and bottom substate of poposed micowave senso ae pesented in the Table and 3. Table. Extemely low dielectic pemittivity foam mateials fo the top substate of expeimental poximity fed antenna Top substate mateial Polyuethane foam block Coss linked hydocabon closed cell foam block ROHACELL 3 HF tg T max, º C WA, %,4.5,,5 35 3,3;,6, 85,4,5 <. 3 Note: depending on density; T max maximum continuous opeating tempeatue; WA 4 hou wate absoption. ROHACELL 3 HF at Evonik Industies AG is a % closed cell igid foam plastic based on PMI (Polymethacylimid) with a fine cell, excellent bonding popeties, extemely low dielectic pemittivity and paticulaly favoable tansmission popeties in the high fequency egion. Owing to its excellent ceep compession esistance (dimensional change unde the influence of pessue and tempeatue), ROHACELL 3 HF is ideally suited fo micowave sensos in industy and allows measuements unde hostile envionments and elative high tempeatues. Table 3. High fequency laminate fo the bottom substate of expeimental poximity fed antenna Laminate (bottom substate) Roges RT/duoid 587 Roges RT/duoid 588,33. tg,5,,4,9 T max, º C WA, % 5, 5, Roges RT/duoid 587 and 588 high fequency laminates at Roges Copoation ae PTFE composites einfoced with glass mico fibes. They have the lowest dielectic pemittivity and loss of any einfoced PTFE mateial, ae isotopic, have unifom electic popeties ove fequency, extemely low wate absoption chaacteistics and esistant to all solvents and eagents nomally used in etching pinted cicuits o plating edges. Paamete stability in boad tempeatue ange makes them well suitable fo micowave senso. Senso numeical evaluation The CST MICROWAVE STUDIO evaluation model of plastic slab thickness measuement is shown in Fig. 6. Measued vaiable depends mainly on slab mateials, thickness, opeating fequency and on the distance between the sample and the senso (gap size). Dielectic popeties ove a tempeatue ange of common plastics fo numeical evaluation can find in [9]. To simplify the measuement at fist thee wee only flat polypopylene plates with thickness d between mm and mm used. Senso shunt esonant fequency was measued fo vaious thickness and gap sizes h at seveal fequency anges. Plastic slab Micowave thickness senso XZ plane cut Fig. 6. CST MWS evaluation model of plastic slab thickness measuement The best othogonal behaviou was located in a fequency ange of,8 GHz to,9 GHz when the opposite wall is fee. The dielectic loading of senso affects both its esonant fequency and impedance (Fig. 7). Real pat of impedance Z, Ohms d=,, 3,..., mm Fig. 7. Real impedance and esonance fequency vaiation vesus on the polypopylene slab thickness d. Opposite wall is not metal backed, gap size, mm As the dielectic thickness of the substate inceases fom to 4 mm, the antenna shunt esonant fequency deceases fom,849 to,833 GHz, which inceases the eal pat of impedance Z fom 5 to 375 Ohms. Fo thickness fom 4 to mm fequencies decease moe slowly to,83 GHz and Z deceases to 65 Ohms. The following esults focus on a measuement fequency of,8 GHz to,9 GHz, whee the best esults wee achieved (Fig. 8 a). Simila measuements wee also pefomed at fequencies of 4,5 to 4,7 GHz (Fig. 8 b). As shown in Fig. 8 esonant fequency deviation of the polypopylene slab thickness vaiation is geate at lowe opeating fequency aea. At highe fequencies, the 7

5 deviation is smalle. This confims, that to incease the sensos sensitive need to select opeating aea close to the cente fequency of the antenna. Real impedance and esonant fequency ae almost independent of bounday conditions on the othe side of the plastic wall (Fig. 8, a) Thickness d, mm a) Thickness d, mm Fig. 8. Resonance fequency deviation vesus of the polypopylene thickness vaiation: a when the opposite wall is fee () and metal backed (); b at highe esonant fequencies, when the opposite wall is fee. Gap size is, mm Duing the measuement at manufactuing pocess is difficult to ensue a fixed minimal gap between the antenna and the measued object. To examine the accuacy of method, it is necessay to evaluate the influence of the gap to esonant fequency change. The influence of the gap size to esonant fequency is shown in Fig Thickness d, mm b),75,5,5, h= mm Fig. 9. Resonance fequency deviation vesus of the gap size h vaiation Poducts of plastics ae fomed at highe than ambient tempeatues, in addition the poduct tempeatue is changing duing the technological pocess and is needed to evaluate the tempeatue effect to the esonant fequency dift. Thee is a need to contol polypopylene poduct manufactuing pocess at the tempeatue of (4 6) C. Fig. shows the esonance fequency dift due to tempeatue effect. Fo the fomation of poducts fom plastics uses vaious fillings, which changes the dielectic popeties. Filles fo micowave incu additional losses, which educe the esonance cuve paametes and educe the potential to measue the thickness of plastic in this way. Fig. shows CST MWS deived cuves fom which we can see of dielectic loss impact on esonance paametes of the antenna O Tempeatue, K d= mm h=, mm Fig.. Resonance fequency dift vesus of the polypopylene slab tempeatue vaiation Real pat of impedance Z, Ohms tg d<, --- tg d=, 3--- tg d=, 4--- tg d= d= mm h=, mm Fig.. The antenna impedance changes vesus of the loss tangent As shown in Fig., the losses have a geate impact on tan close to. When tan <, (cuves 3,, ) loaded esonance quality Q vaies in ange fom 35 to 98. Phase detecto cicuit would achieve highest senso sensitivity. Fo a constant fequency measuement, the phase shift as a function of the coupled with composite thickness pemittivity change and can be appoximated by [] Q, when, (5) whee Q is the loaded esonance quality, and is absolute changes of the detected pemittivity. A senso phase sensitivity can be defined as s Q. (6) 4 Sensitivity facto is in ange 85 to 77, depending of loss. So the phase detecto allows the design sensitive slab thickness sensos fo high lossy composite if the absolute changes of the thickness (and pemittivity also) ae small. Conclusions A poximity fed patch antenna senso fo evaluation 73

6 is used. The main element of the senso is electomagnetic coupled patch feed tansmission line esonato, which allows to evaluate the thickness of the high loss composites via ecoding of esonant fequency o phase deviation. Fo sensing applications, due to high esonance quality (up to 5 at,863 GHz), most suitable shunt (paallel) esonance of TLR. When composite tan <, and wall thickness is mm, loaded esonance quality vaies in ange fom 35 to 98. Phase detecto cicuit would achieve highest senso sensitivity. Senso phase sensitivity facto is in ange fom 85 to 77, depending of loss. Method is suited when the fixed ai gap up to mm between the wall and sensos occus, and total loss tangent of dielectic is up to,. With this method was evaluated polypopylene wall thickness measuement fom to mm. Showed good spatial esolution, penetation depth and detection sensitivity. Refeences. Kaatze U. Techniques fo measuing the micowave dielectic popeties of mateials // Metologia,. No. 47. P DOI:.88/6 394/47//S.. Tabelsi S., Meinbei S., Kaszewski A. W., Nelson S. O. Phase Shift Ambiguity in Micowave Dielectic Popeties Measuements // IEEE Tansaction on Instumentation and Measuement,. Vol. 49. No.. P DOI:.9/ Hasa U. C. A new method fo evaluation of thickness and monitoing its vaiation of medium and low loss mateials // Pogess In Electomagnetics Reseach, 9. Vol. 94. P DOI:.58/PIER Yang Li, Bowle N., Johnson D. B. A Resonant Micowave Patch Senso fo Detection of Laye Thickness o Pemittivity Vaiations in Multilayeed Dielectic Stuctues // IEEE Sensos Jounal,. Vol.. No.. P DOI:.9/JSEN Aaneo R., Celozzi S.A New EMC Antenna fo the low fequency SE measuement of small enclosues // IEEE Intenational Symposium on Electomagnetic Compatibility,. Vol.. P DOI:.9/ISEMC Diene L. Micowave nea field imaging with open ended waveguide compaison with othe techniques of nondestuctive testing // Reseach in NDE, 995. Vol. 7. No. 3. P DOI:.7/BF Vajha S., Pasad S. N. Design and Modeling of Poximity Coupled Patch Antenna // IEEE APS Confeence on Antennas and Popagation fo Wieless Communications,. P DOI:.9/APWC Cuickshank D. G. Micowave Mateials fo Wieless Applications. Atech House Publishes,. 48 p. 9. Riddle B., Bake Javis J., Kupka J. Complex Pemittivity Measuements of Common Plastics Ove Vaiable Tempeatues // IEEE Tansaction on Micowave Theoy and Techniques, 3. Vol. 5. No 3. P DOI:.9/TMTT Puentes M., Penischke A., Schussle M., Jakoby R. Micowave Mass Flow Mete fo Industial Applications based on a Lefthanded Tansmission Line Resonato // Geman Micowave Confeence, Tech. Univ. Damstadt, 9. P. 4. DOI:.9/GEMIC Received 3 Accepted afte evision 4 A. Janeliauskas. A Micowave Senso fo Evaluation of Plastic Wall Thickness // Electonics and Electical Engineeing. Kaunas: Technologija,. No. 7(3). P A poximity fed electomagnetic coupled patch micowave senso fo evaluation of plastic wall thickness is pesented in this aticle. Senso is suitable to measue the thickness of the low pemittivity and elative high total loss mateials, when the opposite wall is not o metal backed and only one side of mateials is an accessible. The main element of the senso is electomagnetic coupled patch feed tansmission line esonato, which allows to evaluate the thickness of composites via ecoding of esonant fequency o phase deviation. It showed, that phase detecto would achieve highest senso sensitivity. Loaded esonance quality vaies in ange fom 35 to 98 depending of measuement object loss. Senso phase sensitivity facto ange is fom 85 to 77. Method is suited when the fixed ai gap up to mm between the wall and senso occus, and total loss tangent of dielectic is up to,. With this method was evaluated polypopylene wall thickness measuement fom to mm. Showed good spatial esolution, micowave penetation depth and detection sensitivity. Ill., bibl., tabl. 3 (in English; abstacts in English and Lithuanian). A. Janeliauskas. Mikobanginis plastiko sienelių stoio matavimo jutiklis // Elektonika i elektotechnika. Kaunas: Technologija,. N. 7(3). P Naginėjamas atimuoju elektomagnetiniu lauku susietas mikobanginis juostelinis plastiko stoio matavimo jutiklis. Paodyta, kad jutiklis tinka mažos dielektinės skvabos i palyginti didelių nuostolių medžiagų stoiui matuoti, kai piešinga sienelės pusė ya laisva aba padengta metalu, o piėjimas ya tik iš vienos pusės. Pagindinis jutiklio elementas ya elektomagnetiškai susietas juostelinių pedavimo linijų ezonatoius, kuis leidžia įvetinti kompozitų stoį, stebint ezonasinio dažnio a fazės pokyčius. Paodyta, kad naudojant fazės detektoių galima padidinti jautumą. Apkauto ezonatoiaus kokybė, piklausomai nuo nuostolių matuojamame objekte dydžio, kinta nuo 35 iki 98. Jutiklio fazės jautumo faktoius ya nuo 85 iki 77. Matuoti galima, kai tap sienelės i jutiklio ya iki mm pastovus oo tapelis, o suminių nuostolių kampo tangentas nevišija,. Pateikti polipopileno sienelės stoio nuo iki mm matavimų modeliavimo ezultatai. Modeliavimo ezultatai paodė geą skiiamumą, mikobangų įsiskvebimo gylį i matavimo jautumą. Il., bibl., lent. 3 (anglų kalba; santaukos anglų i lietuvių k.). 74