WIRELESS SENSORS EMBEDDED IN CONCRETE

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1 7th Euopean Wokshop on Stuctual Health Monitoing July 8-11, La Cité, Nantes, Fance Moe Info at Open Access Database WIRELESS SENSORS EMBEDDED IN CONCRETE Amal Abbadi 1 Phd Student, IRCICA, IEMN, Univesité Lille1, 50 Avenue du Halley, Villeneuve d Ascq Amal.abbadi@etudiant.univ-lille1.f ABSTRACT Efficient embedded antennas ae needed fo futue wieless stuctual health monitoing. The popeties of patch antennas with concete ae investigated at 860MHz. Simulations fo diffeent cases (diffeent concete pemittivity and tangent loss) with and without the pesence of steel einfocements ae pefomed using ANSYS HFSS softwae fo 3-D full-wave electomagnetic field simulation and some expeimental esults ae pesented. KEYWORDS : Wieless senso netwoks, concete attenuation, embedded antenna, concete chaacteization, einfocement bas. INTROD CTION SHM (Stuctual Health Monitoing) is a system that diagnoses the condition of stuctues such as buildings, bidges, oads, and so on, to epai and pevent the potential dange of collapse by detecting it in advance. Cuent stuctual health monitoing systems ae usually based on wied measuement equipment. The main dawback of such systems is that the numbe of sensos is limited, educing the extensibility of the system significantly. Especially, wied systems equie a non-tivial amount of time and cost fo the initial deployment. Recently thee has been consideable inteest among eseaches on distibuted wieless sensos to monito the health of infastuctues. Of paticula impotance ae embedded sensos which can be implemented duing the constuction phase of bidges and buildings. The fundamental poblems of any wieless senso embedded in concete ae the wieless communication and powe consumption. Electomagnetic waves suffe fom attenuation in concete depending on the moistue content [3], the heteogeneity of concete, the pesence of metal [4] and othe factos. Efficient embedded antennas ae needed fo futue wieless stuctual health monitoing. Since the integation of wieless communication emoves the need fo tansmitting data fom one point to anothe with cables, the lack of cables equies in situ powe. Cuently, batteies epesent the most common potable powe souce fo wieless senso. In this pape the chaacteistics of an embedded patch antenna is studied to investigate the data communication. Numeical simulations wee conducted using HFSS to study the etun loss and tansmission loss of an embedded patch antenna within concete. 1 DIELECTRIC P P IES OF CONCRETE To design efficient embedded antennas, we need to know the elative pemittivity (dielectic constant) and conductivity of concete. It is well known that the popagation of electomagnetic waves will be affected by the pesence of moistue in the concete. The eal pat of the complex pemittivity and the effective conductivity of concete have a fundamental ole in assessing the ability of concete to inhibit the popagation of electomagnetic waves. The inheently lossy natue of concete is taken into account by a complex pemittivity (1): Copyight Inia (2014) 1317

2 * ' j " (1) Whee and ae the eal and imaginay pats of the concete dielectic constant, espectively. In geneal both and can depend on fequency in complicated ways and to some degee on moistue content. The eal pat of the elative pemittivity is pesented in equation (2): ( ) (2) Whee stands fo the diffeence between the values of the eal pat of the elative pemittivity at low and high fequencies, and is the elaxation time. The eal pat of the complex elative pemittivity epesents the ability of the medium to stoe electical enegy. The imaginay pat of the complex elative pemittivity epesents the enegy losses due to dielectic elaxation as follows in equation (3):, ( ) ( ) dc eff eff (3) 0 Whee dc is the DC electical conductivity of concete and eff () is the effective conductivity. 2 CHOICE O UCY AND ANTENNA Attenuation in concete incease with fequency [1]. Moistue and metal inceases adio attenuation in concete, an effect which is moe ponounced at highe fequencies. The 860MHz ISM band is chosen as the tansmission fequency fo sensos embedded in einfocement concete, since antennas woking in this band ae less sensitive to the effects of vaying humidity as well as eba configuation. Ou objective is to design and develop an antenna embedded in concete in the 860MHz band that can suppot the functions of data communication. Since the dielectic constant and loss tangent of concete vay depending on the moistue content we investigate the chaacteistics of a patch antenna as function of those paametes. Patch o pinted antenna is a type of antenna which can be used fo tansmitting and eceiving signals. They ae low pofile, small size and light weight. In this pape, the pototype patch antennas wee simulated, designed, fabicated and tested within concete. 3 ANTENNA CONCN Efficient embedded antennas ae needed fo futue wieless stuctual health monitoing. The input etun loss and tansmission losses of a patch antenna ae studied at aound 860 MHz when these antennas ae embedded inside a concete box. The etun loss S₁₁ is the loss of signal powe esulting fom the eflection caused at a discontinuity in a tansmission line. Usually expessed in decibels (db), it is a measue of how well devices ae matched. A match is good if the etun loss is low [2]. The tansmission coefficient S₂₁ is the fowad gain o loss. Antenna pefomance is investigated in fee-space, in ai died concete and in satuated concete without the pesence of steel einfocements. A pobe-fed patch antenna measuing 7.014cm by 7.11cm by 2.8 mm was designed on Taconic RF-60(tm) substate ( =6.15 and tan =0.0028). 3.1 M tu: Two pais of pobe-fed patch antennas wee buied in (40x60x100cm³) boxes of: fee space, dy and satuated concete espectively with a distance of D=20cm between the two antennas fo each case. In Fee space: =1 and tan =0, Dy concete: =4.5 and tan =0.0111, Satuated concete: 1318

3 =7.5 and tan = Simila geometies ae used fo all thee cases. Expeimentally, the senso has a cente fequency at 860 MHz when matched to a 50 ohm tansmission line. The electomagnetic popeties of concete a t diffeent elative pemittivity detemine the powe loss level of the electomagnetic wave and the pefomance of the antenna buied inside it. Figue 1: HFSS geometic configuations of 2 patch antennas in fee space 3.2 Simls: Simulated etun loss S₁₁ and tansmission S₂₁ plots ae shown in figue 2, 3 and 4. As seen in figue 2 and 3; the incease in loss tangent and pemittivity degades the eflection coefficient fom S₁₁ = db in fee space to S₁₁ = dB in dy concete and S₁₁ = dB in satuated concete. When the loss tangent is inceased while keeping the same elative pemittivity, thee is a shift in the cente fequency and the antennas ae less matched. When embedded in concete the esonant fequencies deceased fom thei fee space values of 860MHz to 857.5MHz in died concete to 859MHz in satuated concete. Figue 2: Retun loss of patch antennas embedded in died concete by vaying the loss tangent 1319

4 Figue 3: Retun loss of patch antennas embedded in satuated concete by vaying the loss tangent As shown in figue 4, the tansmission coefficients S₂₁ vaied fom db fo satuated concete to db fo died concete and to db fo vacuum. We can say that the tansmission loss fo satuated concete is 2 db less than the tansmission loss fo dy concete. Figue 4: Tansmission coefficient of patch antennas embedded in vacuum, dy and satuated concete 4 ANTENNA REALIZATION Measuements have been pefomed using Agilent Netwok Analyze. The antennas wee placed on a polystyene box to potect the antennas fom hash envionmental condition. The oute dimensions of the polystyene box ae 15x15x3.5cm³. The measued eflection coefficients of antenna in fee space is S₁₁ =-20.99dB. 1320

5 Figue 5: Realized Patch antenna A concete mold of size (55x36x20) cm ³ (LxWxH) was then used, the two patch antenna wee placed in the cente of the mold and the concete has been added. The antennas matching is degadated in concete with a etun loss S₁₁ = dB. Figue 6: Expeimental setup fo two patch antenna embedded in concete slab L=55cm, W=36cm, H=20cm Figue 7: Tansmission Loss S₂₁ (db) afte placing Concete Afte adding the concete, the tansmission coefficient inceases with the pogession of time with S₂₁ =-53.34dB immediately afte the concete has been poued and S₂₁ = dB afte fou month of concete cuing. These esults confim the effect of humidity on concete attenuation. Wate content deceases ove the concete cuing cycle (weeks o even months). 1321

6 5 CONCRETE ARACTERI!#TION By vaying the elative pemittivity without dielectic losses we obtain a cental fequency of 855MHz with =5 as shown in figue 8. We can see in Figue 9 that a compatible simulated S₂₁ plot and expeimental S₂₁ plot ae obtained with tan =0.4. Figue 8: Retun loss of 2 patch antennas in simulation and expeimentation Figue 9: Tansmission coefficient of 2 patch antennas in simulation and expeimentation 6 E$$%CT O$ REIN$&RCEMENT BARS The main impediment to adio waves is the use of eba in the concete. The natue of popagation loss is affected by the width of a einfocement ba lattice and diamete of steel elements. In this section, we have studied the influence of these citical paametes on the tansmission chaacteistics of the antennas. 6() +o,i-io.i.n of /0i.fo/10m0.- 23/,7 Patch antenna is positioned below, between and above einfocement bas in ode to identify the best position in concete box. The esults ae shown in figue 11. We can see that positioning the patch antenna between o above the eba layes povide bette signal tansmission. 1322

7 Figue 10: HFSS geometic configuation of patch antenna positioned a) below, b) between and c) above einfocement bas Figue 11: Tansmission coefficients of patch antenna positioned below, between and above einfocement bas 89: S;uae gid side length: The tansmitting antenna will be installed on the outside suface and the eceiving antenna attached between two eba layes. In pactice, the eba gid mesh and diamete in concete slabs ae always changing. Some of the effects of these eba geometical paametes ae studied hee and the esults ae shown in figue 12. The tansmission coefficients ae calculated fo a mesh having a squae side length vaying between 11cm, 15cm and 20cm. This study is made at 860 MHz. The steel diamete is 10mm. Results ae given in figue 12.We can see that the influence of the gid can neve be negligible fo the chosen dimension with S₂₁ vaying fom -13dB fo a side length of 11cm to dB fo a side length of 15cm. 1323

8 Figue 12: Tansmission coefficients at 860MHz fo a gid having a squae side length of 11cm, 15cm, and 20cm 6.3 Steel diamete: Fo a squae gid of side length of 15cm, steel diametes of 4mm, 8mm, 12mm and 16mm ae consideed. The esults obtained ae shown in figue 14 and table I. We can see that the steel diamete has negligible affect at 860MHz. Table I: Steel diamete effects on tansmission coefficients Steel diamete <mm) =>? <@B) CONCLUSION SHM is a poweful tool to know the behavio of the stuctues ove time. Conventional sensos ae being used in a successfully way fo ambient monitoing puposes. Howeve, nowadays a new technology based on wieless systems is becoming inteesting fo SHM. In this pape, we have expessed test esult concening the eflection and tansmission coefficients in concete. We have seen that the influence of the gid cannot be neglected. It was assumed that einfocing bas will affect the communications distance in the same manne as the concete. Futhe studies in hadwae development and pocessing tools ae also pat of this wok. REFERENCES [1] Donnacha Daly, Thomas Melia and Gead Baldwin navigation Concete embedded RFID fo waypoint Positioning Intenational confeence on indoo positioning and indoo Zuich, [2] Tevo S. Bid Definition and Misuse of Retun Loss IEEE Antennas and Popagation Magazine, Vol. 51, No.2, Austalia, Apil [3] Ade Ogunsola, Ugo Reggiani and Leonado Sandolini Modelling shielding popeties of concete 17th Intenational Zuich Symposium on electomagnetic compatibility,2006. [4] Roge A. Dalke, Chistophe L. Holloway, Paul McKenna, Matin Johansson, and Aza S. Ali Effects of Reinfoced Concete Stuctues on RF Communications IEEE Tansactions on electomagnetic compatibility, vol. 42, N. 4, NOVEMBER