Automation in Construction 30 (2013) 9–14

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Automation in Construction

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Fiber optic and KNX sensors network for remote monitoring a new buildingcladding system

R. Unzu a, J.A. Nazabal a, G. Vargas b,c, R.J. Hernández b,c, C. Fernández-Valdivielso a, N. Urriza a,M. Galarza a, M. Lopez-Amo a,⁎a Universidad Publica de Navarra, Spainb Alonso, Hernández & Associates Architects, Spainc University of the Basque Country, Spain

⁎ Corresponding author.E-mail address: mla@unavarra.es (M. Lopez-Amo).

0926-5805/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.autcon.2012.11.005

a b s t r a c t

a r t i c l e i n f o

Article history:Accepted 6 November 2012Available online 12 December 2012

Keywords:Fiber optic sensorsNew materialsBuilding Management SystemKNXMultiplexingStructural health monitoring

This paper shows the utilization of a fiber optic sensor network for monitoring the behavior of the enclosureof a telecommunication tower. Such enclosure is composed by double monolithic glass panels filled with analveolar type polycarbonate structure. These fiber optic sensors are also integrated with a fiber optic acceler-ator and various building automation KNX sensors into a custom developed Building Management System forremote monitoring the structural health of the building. Results related to the measurements recorded intothe glass alveolar panel have been used for assessing the structural reliability of the panel, under thermaland mechanical working conditions.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Optical sensors based on Fiber Bragg Gratings (FBGs) [5] havebeen widely used in monitoring of civil structures [1,2]. They showinteresting features and advantages such as the possibility of theirwavelength multiplexing using a single fiber optic cable, their reducedsize and lightness, and total immunity to electromagnetic interferences,among others [3–5]. The measured parameters by the FBG sensors arecodified in the light wavelength. This feature avoids the distortion ofinformation through the sensor network due to external effects. Theremote measurement of these sensors is also possible using an opticalfiber channel [6] or by a wireless link [2].

Remote Structural Monitoring Methodology (RSM) [7–9] requiresan interdisciplinary approach integrating areas such as materialsengineering, sensor technology, communication technology, statisticalmechanics, and information technology for online data transmissionand damage detection/health assessment of the structure.

RMS has many advantages such as continuous monitoring ofthe measured parameters, early alarm of any incipient damage, anddata acquisition even in adverse climatic/environmental conditions.

The aim of this work is to know the performance of the enclosurematerials in real time and under service conditions. Moreover, thesystem is intended to prevent severe alterations in the enclosure dueto external agents (temperature changes, wind loads, vibrations…).Themonitored building considered in this work is a telecommunication

rights reserved.

tower then the use of any metallic elements must be avoided. With theuse of conventional electric sensors, metallic cables can electromag-netically shield the signal of the antennas, and it can induce electriccurrents into the cable, distorting data. This is the main reason whyfiber optic sensors have been used.

In this work, FBG sensors with an optical accelerometer and someKNX sensors are integrated at the building's enclosures into a customsystem for remote monitoring building structural health. The ideais to extend the functionalities of an existing commercial BuildingManagement System (BMS) like KNX, with new capabilities (e.g. theuse of fiber optic sensors and remote monitoring). KNX has been usedbecause it is a leading building automation technology, with an impor-tant implantation level in many European countries.

2. Application description

In order to evaluate and quantify the behavior of building elementsunder real service conditions bymeans of FBG sensors, a singular towerdesigned by Alonso, Hernández & Associates Architects S.L. was chosen.Moreover, the building has an innovative enclosure system: doubleglass panels filled with polycarbonate core with alveolar structure.

2.1. Telecommunication tower

The chosen tower belongs to the headquarter of TRACASA, a publiccompany of the Government of Navarra, located in front of the Inno-vation Center and next to the Eco-city of Sarriguren (Navarra, Spain).The main building has a built surface of 20.600 m2 and disposes

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advanced technological solutions that allow for an efficient use of theenergy needs of the building, reducing the energetic consumption.

This building has a singular telecommunication tower that containsthe information systems and the communication antennas. Due to thisfunctional exclusivity of the tower, the construction needed an innova-tive glass enclosure solution, which is shown in Fig. 1.

2.2. Alveolar sandwich structure

A vertical façade of innovative panels was installed in the tower.These panels have a sandwich structure, composed by two externalglass skins filled with a central alveolar core. The two cladding glassesuse monolithic glass and the core has an alveolar/lattice structure(honeycomb configuration) of polycarbonate (see Fig. 1).

The main structural features of the panels are reached by theadhesive union between the glass claddings and the core, withoutthe need of any other kind of holder in the perimeter of the panel.In addition, the sandwich configuration provides outstandingmechanicalproperties to strain and compression. Besides, from an aesthetic pointof view there are several new design degrees.

The glass skins provide the panels with a translucent feeling,whereas the alveolar core allows the use of colored solutions and avariety of degrees of translucency. The translucency reaches its max-imum in the perpendicular direction with respect to the panel plane,and it decreases with increasing angles of observation.

3. Network description and installation

3.1. Network design

In order to monitor the mentioned tower, both commercial strainand temperature sensors have been installed. Strain sensors offer in-formation about contraction, dilatation and deformation of the glasspanels, and temperature sensors are necessary to compensate forthe deformation effects on strain sensors due to thermal changes onthe glass.

Moreover, the information provided by the temperature sensorsis required to establish the current temperature of the panels andit is essential to experimentally determine the coefficient of thermal

Fig. 1. Panel structure.

expansion (CTE) of the glass. As the glass alveolar panels have asandwiched structure, some sensor pairs have been placed in bothfaces of the panels, as is shown in Fig. 2. The aim of such placementis to record the differences in deformation and temperature in bothfaces of each investigated sandwiched glass, and to evaluate theirisolation capability.

The experimental setup system of the tower is complementedwith a vibration sensor located at the upper floor of the building. Itis a double terminal optical accelerometer, model GS 6500 by GaveaSensors. Three types of surface strain sensors have been used, dependingon their suitability to external use. Polyamide sensors provided byFiberSensing, for example, are easy and quick to install, and havebeen installed inside the building.

Sensors provided by MicronOptics are tougher and have betterweathering features, so they were installed outside. In order to definethe distribution of the sensors, the following aspects have been con-sidered. For measuring strain, FS 6200 sensor model, and for measur-ing temperature, FS 6300 sensor model, provided by FiberSensing,have been used. For measuring strain MicronOptics sensors OS 3100(epoxy bond style) and OS 3200 models have also been used.

3.1.1. Power limitationThe number of sensors that can be placed throughout the optic

channel is limited due to the losses produced along the network,such as loss insertion of the connectors, bend loss in the fiber, etc.

3.1.2. Wavelength limitationThe range of temperature sensors is ±1.5 nm (100 °C×10 pm/°C+

0.5 nm) and the range of strain sensors is ±1.1 nm. (500 με×1.2 pm/με+500 pm).

The measurement provided by the strain sensors has to be com-pensated with the temperature effect on deformation. Therefore, atemperature sensor is placed between each pair of strain sensors.The interrogator, model FS 5200 by FiberSensing, has four opticalchannels and interrogates each FBG sensor at a frequency of 1 Hz.

It has an additional special optic channel to interrogate the opticalaccelerometer with a customizable sample rate with a maximum rateof 20000 samples per second. This device is a composition of an opti-cal board designed and assembled by FiberSensing and a NationalInstruments' PXI DAQ Card (PXI6040). In this project we need to mea-sure vibrations of frequency components with 400 Hz maximum, soaccording to the sampling theorem, the sampling rate needed is of800 samples per second.

Fig. 2. Detail of one of the installed FBG sensors.

Fig. 4. Installation of the external optical sensors.

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The previous limitations define the number of sensors that can beinstalled on each optic channel and they are useful to determine themost suitable wavelength for each one.

The final system comprises a total of thirty one FBG sensors (20deformation sensors, 10 temperature sensors and 1 accelerometer)distributed in four branches or arrays that were located on differentpoints of the tower:

- Branch 1 Six sensors inside the building, and three sensors outside,located at the first and second floors.

- Branch 2 Six sensors inside the building at the third floor.- Branch 3 Nine sensors inside the building at the fourth floor.- Branch 4 Six sensors inside the building at the fourth floor.

Some of the installed sensors are shown in Fig. 3. All availablechannels of the interrogator were utilized.

3.2. Installation and assembly

Prior to the installation and interconnection of the sensors in thebuilding, we checked whether both the selected configuration of thesensors and the branches worked properly. Afterwards, the use of alift platform was necessary to place the branches in the outside ofthe tower, as shown in Fig. 4.

The measurement provided by the strain sensors has to be com-pensated with the temperature effect on deformation. Therefore, atemperature sensor is placed between each pair of strain sensors.

Since the aim of the work is the remote monitoring in real-time ofthe sensed parameters, it is necessary to interconnect the interrogatorof the sensing network with the general management system of thebuilding to extract all the data using a remote application.

The chosen building possesses an intelligent control system basedon KNX (European standard for Home and Building Control), whichprovides a high degree of flexibility and an important saving in ener-getic management and maintenance.

Fig. 3. Optical sensor distribution.

The utilization of FBG sensors working with KNX networks waspreviously proposed by part of us in [10–12]. In order to facilitatethe future management and maintenance of the monitoring system,we have developed a unique global management tool which controlsboth the KNX system and the sensor system. So, in the tower coexiststwo kinds of sensor networks, one optical and another KNX.

Making use of the network KNX it was installed a meteorologicalstation into the building's roof which will give more informationabout weather conditions that may affect the enclosure during the re-alization of this work. The station can measure in real-time the speedof the wind, the temperature outside and the brightness in threedirections. Furthermore, it has a rain sensor and a more accurate twi-light brightness sensor. The weather station used is the 2224 HWmodel by Jung.

The temperature range, for which the meteorological stationworks correctly, goes from −22 °C to 50 °C. The sensor of wind canmeasure winds from 0 m/s to 40 m/s with an accuracy of ±2 m/s;however it cannot give information about the direction of the wind.Similarly, the station detects if it is raining, even with thin drizzle,but it cannot measure the intensity of the rain. The brightness is mea-sured from 1 kLx to 110 kLx in the normal brightness sensors andfrom 0 to 60 kLx in the twilight one.

The developed management system software tool remotely ac-quires processes and stores data from these networks. As shown inFig. 5, each fiber optic sensor branch is connected to a different opticalchannel. The optical accelerometer is connected to the interrogator'sPXI module, also with optical fiber. The KNX/IP interface is connectedwith an Ethernet UTP cable to one of the two available interrogator'snetwork interfaces, and the other is used for connecting the 3 G router,also using an Ethernet UTP cable.

3.3. Software description

The software of the system consists of five separated modules,intercommunicated with UDP or TCP, depending on the nature of thedata exchanged.

Fig. 5. System's hardware scheme.

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Both protocol works above the Internet IP layer. TCP is a reliable,connection oriented protocol that will be used for exchanging criticaldata whereas the other is a non reliable, non connection oriented pro-tocol that will be used for sending “streaming” data. The system'ssoftware scheme used is shown in Fig. 6.

The module that obtains the data provided by the optical sensornetwork acquired by the interrogator, the BraggMeter module, hasbeen developed in LabVIEW.

This module communicates with the interrogator proprietary tool,also programmed in LabVIEW, via shared variables for accessing fiberoptic sensor data from other V.I.s. The sensor data includes the sensor'sname, units and the measured value and can be acquired anytime.

Fig. 6. System's software scheme.

The module that obtains the data provided by the optical acceler-ometer via the PXI optical card, the BraggScope module, has been alsodeveloped in LabVIEW. For accessing the data from the PXI card usingLabVIEW, the installation of the NI-DAQmx driver software is needed.This tool is a software interface between the hardware part (in thiscase the PXI card) and the software part (in this case, LabVIEW). Foraccessing the MySQL database from LabVIEW, “labSQL” [14] libraryhas been used but first ODBC (Open Database Connectivity) windowsdrivers must be installed and properly configured.

This module compresses the data to be sent, using LabVIEW's Zliblibrary [15], for making a more efficient use of the Internet connection.Zlib is free, lossless data compression library and is portable across plat-forms. The compression rate varies depending on encrypteddata,with amaximum compression factor exceeding 1000:1.

On the other hand, the module that obtains data from the KNXnetwork, the CalimeroDaemon module, has been developed in Java,for which the Java library “Calimero” developed in the Vienna TechnicalUniversity has been used. This library allows a bidirectional accessto KNX network through an IP interface using KNXnet/IP protocol(Eur. Family of Std. 13321, 2006).

A hardware KNXnet/IP interface is also needed, that will be usedby the “Calimero” library for interactuating with the KNX meteoro-logical station. The interface used is the N148/21model from Siemens.For data storage, we have used aMySQL server, a powerful open sourcedatabase. Finally this module has the functionality of connecting re-motely with the main module, the SensorViewer module, via Internetfor displaying the sensor networks data in real-time, as well asdisplaying the values stored in the database for a given time interval.

For the graphical representation of the sensor data, “JFreechart”library has been used, written in Java. It has a large variety of differenttypes of graphics and supports many output types, including Swingcomponents, image files and vector graphics file formats.

4. Experimental results and conclusions

The network of FBG sensors, the data processing, the data storageand the automatic remote monitoring of the sensors, are a completeand well defined systemwith the capability to perform simultaneouslymultiple sensor measurements and to provide both local and globalinformation about the behavior of the panel under different environ-mental parameters and operating conditions. Both, the data of the sen-sors and the information of themeteorological station, have been savedevery fifteen minutes during a year. The placement of the FBG sensorshaving different orientations and height positions, and located insideand outside the tower, allows monitoring the different temperaturesand deformations the panels go through.

As shown in the graph plotted in Fig. 7, panels reach temperatureshigher than outside temperature, understanding as outside tempera-ture the one measured by the meteorological station. This differenceincreases for higher registered outside temperatures. For example,considering the daily registered maxima temperatures, the averagedifference of the temperature inside the last floor with west orienta-tion and the outside temperature is about 14 °C. Considering the dailyminimum temperature, the average difference does not reach 6 °C.There is a typical period each day when the outside temperature asshown in the graph plotted in Fig. 7, panels reach temperatureshigher than outside temperature, understanding as outside tempera-ture the one measured by the meteorological station.

Considering the daily minimum temperature, the average differ-ence does not reach 6 °C.

There is a typical period each day when the outside temperatureis higher than the temperature of the panels: from 9:00 to 13:00(see Fig. 7). At that moment, the temperature difference is negative.The temperature in each panel depends on the following aspects:

The height: The last floor reaches higher temperatures.

Fig. 7. Strain and temperature measurements of the sensors in branch 3.

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The orientation: Depending on the hour of the day, there are sidesof the tower which suffer higher temperatures than the others.

Logically, the North side of the tower remains always the coldest.Panels in the South side of the building reach higher temperaturesthan the other sides from 17:00 to 10:00. This is due to the direct sun-light of this side during most part of the day.

Based on the simultaneous recording of the strain and the temper-ature it is possible to observe the deformation of the panels due to thechanges of temperature. Therefore, it is possible to estimate the coef-ficient of thermal expansion (CTE) of the panels. The CTE is defined asthe unitary thermal deformation of a material depending on thechanges of temperature. Mathematically:

Δε ¼ CTE ΔT : ð1Þ

This coefficient is a physical property of the material, which is usu-ally obtained by means of laboratory experimental setups. In furtherresearch, coefficient of thermal expansion will be determined inorder to compare it with in situ measurements. For the case of theglasses used in the panels, the coefficient of thermal expansion is:CTEGlass=9×10−6 °C−1.

In order to carry out the estimation of the deformation in thepanels, we established a reference temperature having zero strain

Fig. 8. Measured temperature of an inner sensor (R3t1) and an outer sensor

for T0=20 °C. This normalization is necessary because the tempera-ture in each panel was different in the moment the sensors wereinstalled.

εF ¼ ε0 þ CTE Tf−T0

� �ð2Þ

Results in Fig. 8 show the effect of the contraction and expansionthat the glasses suffer when the temperature decreases or increases,respectively. In Fig. 9, it is displayed the temperature that is reachedin both sides of a panel in the last floor of the building, as well astheir difference with room temperature. We confirm that the temper-ature inside the panel is always higher than that in the outside.

Fig. 10 shows the relationship between the temperature and thedeformation for several panels. The coefficient of determination, R2,between the temperature and the deformation is in all cases, greaterthan 0.9. R2 is a number among 0 and 1. When this value is near to 1,the acceptance of “lineal relationship” can be assumed.

Therefore, we can conclude that there exists a lineal relationshipbetween temperature and deformation [13]. Moreover, in this graphwe can observe that there are other factors that produce deforma-tions to the glass panels. The deformation of the panels depends onboth thermal conditions (changes of temperature) and mechanicalconditions (such as the load of the wind, vibrations, and strain due

(R4t1) of the same panel and their difference against roof temperature.

Fig. 9. Strain measurements of 3 sensors against temperature.

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to its own weight or mechanical supports used to fix the panels). Ithas not been possible to establish a clear relationship between theload of wind and the strain in the panels, due to the light wind thepanels went through during the measured period.

Figs. 9 and 10 show that panels placed higher show higher de-formation due to more critical thermal and mechanical conditions(remember that the range of temperature registered for these panelsis the largest).

Sensors installed in the outside recorded more unstable measure-ments compared to sensors in the inner side. The base packaging wasnot enough to avoid the hostile environment, such as rain, hailstone,birds, etc. Therefore, some ranges of measurements have not beenconsidered as reliable.

5. Conclusions

In this paper we have presented what it constitutes, to our knowl-edge, the first demonstration of in situ monitoring using optical fibersensors of new claddingmaterials for construction. The cladding is com-posed by double glass panels filled with a new alveolar type structure.

Fig. 10. Strain measurements excluding temperature effects.

The multiplexing system we have developed has monitored re-motely during a year, using a control system based on 6 KNX sensors,20 fiberoptic strain sensors, 10 temperature sensors and 1 acceler-ometer. Using this monitoring system we evaluated the behaviorof the new cladding system. Regarding structural reliability of thefaçade, strain measurements (between −300 and +400 με) indicatethat double glass panels suitably bear both thermal and mechanicalstresses.

Acknowledgments

This work was supported by the Spanish Government undergrants TEC2007-67987-C02-02 and TEC2010-20224-C02-01 andgrant IIM010566.RI1 from the Government of Navarra.

The authors are grateful to G. Beloki, A. Yoldi, and I.R. Matías,from UPNA; P. Branchi, and L. Borquez, from AH&A, S. Abad, fromFiberSensing and A. Méndez, from Micron Optics; for their help atthe beginning of the project. We also thank TRACASA for their avail-ability and help.

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