Optical Fiber Sensors and Conventional Sensors

Optical Fiber Sensors and Conventional SensorsABSTRACTThis interpret deals with the semblance of the deuce types of detectors which argon widely apply in urbane engineer, namely, schematic sensors and visualal type sensors. Temperature and displacement ar the two principal parameters which are measured with the aid of Fiber optic sensors. Bragg Grating, Interferometric, Intensity Sensors, and optical time surface area reflectometry (OTDR) are some of the techniques which are utilise for sensing. In this study, non-homogeneous case studies dedicate been undertaken and have been analyzed. With the aid of these case studies, a detailed analysis and comparison of the sensors is carried out.Chapter 1 INTRODUCTIONIn the last two decades, the world has witnessed a revolution in the sectors of optoelectronics and type optic communications. Various products such as laser printers and bar code s natesners which have become a part of our daily usage, are a case of this technic al revolution only. The reasons for the phenomenal growth of the role optics are m whatever. The about conspicuous reason being the ability of the character optics to provide lofty proceeding and highly reliable communication links and that excessively at a very dispirited bandwidth cost. As we see that optoelectronic and persona communications industry has progressed a lot, and along with these industries fiber optic sensors have as well benefited a lot from these developings. Due to the mass production in these industries, availability of fiber optic sensors at a dispirited cost has been made realizable in recent years. With their availability at affordable costs, fiber optic sensors have been able to enter the domain which was otherwise being ruled by the tralatitious sensors.In recent years, the demand for the development of new clobbers to qualificationen, upgrade and retrofit existing aged and deteriorated concrete complex body parts has increased rapidly. The continuing deterioration and running(a) deficiency of existing complaisant infra expression elements represents one of the most signifi whoremasterce challenges facing the worlds construction and civil engineers. Deficiencies in existing concrete structures caused by initial flawed design due(p) to insufficient detailing at the time of construction, aggressive chemical attacks and ageing of geomorphologic elements enhance an urgent need of purpose an effective means to improve the performance of these structures without additionally increasing the overall weight, maintenance cost and time. In the last 50 years, a large moment of civil concrete structures have been built many of these structures, particularly in off-shore regions have now deteriorated and require repair in a short full point of time.Moreover, the increase of traffic volume and population in many developing countries is ca utilize the demand to upgrade existing concrete structures to increase. The damage of rei nforced concrete (RC) structures through reinforcement corrosion and residual capacity are the most principal(prenominal) issues that concern engineers. These problems occur not only in constructed concrete structures notwithstanding in like manner in structures strengthened by externally bonded sword reinforcements.In the past, the external steel plate bonding regularity has been used to improve strength in the tensile region of concrete structures with an epoxy adhesive and has proved to be successful over a period of 20 years. However, the use of steel reinforced plates and bars has its disadvantages including high corrosion rates, which could adversely affect the bond strength and cause surface spalling of the concrete, due to volumetricchange in the eat steel reinforcements. Since the early 1980s, fibre-reinforced plastic (FRP) materials have been used as a replacement for conventional steel materials for concrete strengthening applications. In recent years, the absorb in utilizing FRP materials in the civil concrete industry in forms of rods, plates, grid and jacket has grown increasingly. When an FRP plate with high tensile strength properties bonds on the concrete surface, it bay window strengthen the structure with minimum changes to its weight anddimensions. FRP offers substantial improvement in solving many practical problems that conventional materials cannot solve to provide a satisfactory attend life of the structure. Unlike the conventional steel materials, FRP is corrosion resistant. The beneficial feature of speechs of exploitation the FRP in concrete construction include its high strength-to-weight ratio, low labour requirement, ease of application, reduced traffic interruption during repair, cost reductions in both transportation and in situ maintenance for a long-term strategy. Its high damping characteristic also attracts more structural engineers to use these materials for seismic retrofitting. Due to the increasing use of FR P-plate bonding techniques in strengthening civil concrete structures, the interest in finding a suitable means of admonishering deviceing the structural health conditions of these strengthened structures has therefore increased substantially. Since strengthened structures are covered by the FRP plates, the automatonlike properties of the concrete may not be measured or detected easily through conventional nondestructive evaluation (NDE) methods, such as accent measurements using surface mounted strain gauges or extensometers, radiography, thermography and acoustic emission methods, particularly in areas with microcracksand debonds underneath the externally-bonded plate. Besides, these structural inspection technologies, in certain cases, require limited surface preparations or a high degree of flatness in the concrete surface. These requirements may be hard to achieve, particularlyfor an area that is exposed to a harsh surround. During the 1990s, a multi-disciplinary field of engineering crawl inn as sharp Structures has developed as one of the most pregnant research topics in the field.The structure is formed by a marriage of engineering materials with structurally-integrated sensor frames. The system is capable of assessing damage and warning of impending weakness in the structural ace ofthe structure. Fibre-optic sensor technology is a most attractive device currently used in the aerospace and aircraft industry for on-line observe of large-scale FRP structures. The development of distributed fibreoptic sensors, which provides information on a largenumber of continuously distribution parameters such as strain and temperature is of great interest in most engineering applications.11,12 The sensors are embedded into a structure to form a novel self-strainmonitoring system, i.e. the system can self-detect itshealth status and send response signals to operators during any marginal situation during service. The embedding sensor, due to its extremely s mall physical size, can provide the information to a high accuracy and resolution without influencing the dimension and mechanized properties of the structure. Fibre-optic sensors present a number of advantages over the conventional strain measuring devices (a) providing an absolute measurement that is sensitive to fluctuation in irradiance of the illuminating witness (b) enabling the measurement of the strain in different locations in only one single optical fibre by using multiplexing techniques(c) having a low manufacturing cost for massproduction and (d) its ability to be embedded inside a structure without influencing the mechanical properties of the host material.A new development of Smart materials and structures was driven by a strong demand for high performance over recent years. A system integrated into structures and being able to monitor its hosts physical and mechanical properties such as temperature andstrain, during service is appreciated as a Smart structural health monitoring system. The term able material and structure is widely used to describe the unique marriage of material and structural engineering by using fibre-optic sensors and actuation control technology. The smart structure is constructed of materials that can continuously monitor their own mechanical and physical properties, and thereby, be capable of assessing damage and warning of impending weakness in structural integrity. This design concept results in improved safety and economic concerns regarding the weight saving and avoidance of over-designing of the structure in the long run. In Fig. 1, a schematic illustration of the structures possibilities created by the confluence of the four disciplines is shown. In the range, a structure invested with actuating, sensing and neutral networking systems to form a new class of reconciling structures is shown. A structure with integrated sensor or actuator systems is able to provide a self-structural health monitoring or actuating r esponse, respectively. If both systems are integrated together into a structure, the sensor and actuators can act as nervous and muscular systems, like a human body, to sense the conditions such as mechanical strain and temperature of the structure(a smart structure) and to provide control of such changes of stiffness, shape and vibration mode (a controlled structure). The combination of these two systemsinto one structure is called a Smart adaptive structure. This structure with a built-in neural networking system, like a brain, is then able to self evaluate the conditions, which are based on changes of structural parameters, thermal conditions and ambient environments to give an appropriate mechanical adjustment. This structure is commonly called an Intelligent adaptive structure.1.1 BACKGROUND OF THE STUDYThere has been an unprecedented development in the field of optoelectronics and fiber optic communications. This in turn, has brought about a revolution in the sectors of tele communication and various other industries. This has been made possible with the aid of high performance and reliable telecommunication links which have low bandwidth cost.Optical fibers have numerous advantages and some disadvantages. The advantages include their small size, resistance to electro magnetised interference and high sensitivity. On the other hand, some of its disadvantages are their high cost and unfamiliarity to the end user. But its great advantages completely overshadow its minor disadvantages. So, in this study an attempt is being made to compare the unexampled age fiber optic sensors with the conventional sensors. Also, with the aid of the case studies, the impact of fiber optic sensor technology on monitoring of civil structures is studied (McKinley and Boswell 2002).1.2 PROBLEM STATEMENTIn the past various kinds of sensors have been used in civil engineering for measuring temperature, pressure, stress, strain etc. And as the optical fiber sensors spread their w ings, the civil engineering is bound to gain a lot from these modern sensors.Presently, there exist a number of problems with the existing civil infrastructures. These civil infrastructures such as bridges etc. have a pretty long service period which may amount to several decades or maybe even hundred years. Thus, during this time period, these structures suffer from corrosion, fatigue and extreme loading. Since concrete is used more often than not in these civil infrastructures, it degradation is a major issue all over the world.The amount of degradation and the time when the degradation starts depends on various factors and is inevitable and unavoidable. Thus, in order to keep these civil structures in good condition, it becomes necessary that their condition be monitored and adequate steps be taken. Thus, we need sensors which can monitor these structures throughout the life of these structures. Thus, in this study the impact of fiber optic sensors is studied on civil structure s.1.3 OBJECTIVESThere are a few objectives that are planned to be achieved at the end of this pick up, these areA general discussion on the present state of structural monitoring and the need of fiber optic sensors in this fieldA general study on Comparison between Conventional Sensors and Optical Fiber SensorsReview of Case Studies on Fiber Optic Sensors application in Civil Engineering Structures1.4 WORK PLAN establishion, reading and cardProblem identification through reading, discussion and observation of the area studiedUnderstand and identify the background of problemStudying feasibility and needs to carry out the investigationIdentification of the Title for the projectIdentify the aim, objective and scope of the projectLiterature ReviewUnderstanding the background of the problemUnderstanding the history of the sensor technology in structural monitoringCarrying out literature survey on generic technologies of sensors for concrete structuresIdentify the types of sensor involv ed in monitoring the structural in civil engineeringIdentify the technique used and the working principle for each type of sensors (in particular optical fiber sensors)Case StudyChoose the applicable and related case study for discussionDescribe important aspects of case studyAnalyze the use of sensors in the case studyDiscussion, Conclusion and RecommendationsDiscuss the similarities and differencesDiscuss the technical facets of sensor applicationDraw the overall conclusion for this projectGive some recommendation for futureChapter 2 APPLICATIONSThese days the fiber optic sensors are being used for a variety of applications, the most prominent of them beingMeasurement of rotation and acceleration of bodiesMeasurement of electric and magnetic fieldsMeasurement of temperature and pressure of bodiesMeasurement of acoustics and vibrations of various bodiesMeasurement of strain, viscosity and chemical properties of materialsMeasurement of surface condition and tactile sensingMeasureme nt of effectual , speed and proximity of bodiesDetermination of color and weight of different objectsMeasurement of linear and angular military postures and this is widely utilized in civil engineering structures2.1 ADVANTAGES OF lineament OPTIC SENSORSLike with any other technology, there are both advantages and disadvantages using fiber optic sensors. The prominent advantages beingFiber optic sensors are lightweight and this is of great splendour in case of engineered structuresFiber optic sensors are of smaller size as compared to the traditional sensorsAlso, fiber optic sensors drink down less power as compared to the traditional sensorsAlong with this, these sensors show high resistance to electromagnetic interference as compared to the traditional sensorsOn top of this, fiber optic sensors have wonder high bandwidth and high sensitivity as compared to their traditional counterpartsFiber optic sensors are usually embedded in objects and due to this, these sensors can gain access to areas which till date remain inaccessible with the aid of traditional sensorsAlso, these sensors are accurate over a greater energising range as compared to the traditional sensorsFiber optic sensors are also capable of being multiplexed which again is a further advantage over their traditional counterpartsAlso, fiber optic sensors are capable of distributed measurements which gives them an edge over and above the traditional sensorsLast but not the least, they also show greater environmental ruggedness as compared to the traditional sensors2.2 DISADVANTAGES OF eccentric OPTIC SENSORSBut all this is just one side of the coin. Though on seeing these advantages, it might appear that fiber optic sensors are way too advanced as compared to the traditional ones, but it is not exactly true. These fiber optic sensors also have some disadvantages due to which their advancement in todays world has been somewhat curtailed. The major disadvantages of fiber optic sensors areFiber optic sensors are kinda dear(p) as compared to the traditional sensors. Due to this, many people still consider traditional sensors to be a better option in cases where cost is a major consideration.Secondly, these sensors have come into prominence only in the last two decades. Due to this, people appear to be somewhat less educate regarding their usage and operations. And this unfamiliarity with the usage of these sensors, has proved to be a major hurdle in being able to capture the totally market.Also, these sensors are considered to be more fragile as compared to the traditional sensors which raises a question over their adaptability in extreme conditionsAlso with the fiber optic sensors there exists the inherit opening/egress difficultyFiber optic sensors usually have a non-linear output which is a cause for concern in some applicationsFrom the above discussion, we can see that as is the case with any other new technology, there are both merits and demerits of fiber optic s ensors. But, what is worth considering here is that the advantages of this technology are much more than its disadvantages and are able to outweigh them. Also, from the demerits which are mentioned here, it is clear that these demerits are bound to wither away as this technology develops and gains more prominence.2.3 APPLICATIONS IN CIVIL ENGINEERINGNow we come to the discussion of the need and applications of the fiber optic sensors in the field of civil engineering structures. The monitoring of civil structures has a great significance in todays world. Today, we not only need to construct reliable and strong civil structures, but we also need to monitor these structures in order to ensure their proper functioning and their safety. Also, with the aid of the monitoring of various parameters of the structures, we can get knowledge about state of the building and by using this data, we can in turn plan the maintenance schedule for the structure (Mckinley, 2000). Also, this data can gi ve us an insight into the real behavior of the structure and can thus take make important decisions regarding the optimization of similar structures which are to b e constructed in future.The maintenance of the structures can be approached in one of the two ways, namelyMaterial point of view- In this approach, monitoring is concentrated on local properties of the materials which are used in the construction. In this approach, we notice the behavior of the construction materials under the conditions of load, temperature etc. In this approach, short base space sensors are usually utilized. Also, it is possible to get the information about the whole structure with the aid of extrapolation of the data obtained from these sensors.Structural point of view- In this approach of measurement, the structure is viewed from a geometrical point of view. In this approach, long gauge length sensors appear to be the ideal choice. In this approach, we will be able to detect material degradation onl y if this material degradation has an impact on the form of the structure.In the recent years most of the research work which has been carried out in field of optic sensors has been in the field of material monitoring rather than structural monitoring. It is also worth mentioning here that, more sensors are required in the case of material monitoring as compared to structural monitoring.We know that civil engineering requires sensors that can be embedded in the concrete, mortars, steel, rocks, soil, road pavements etc. and can measure various parameters reliably. Also what should be taken into sum up is that these sensors should be easy to install and should not hamper the construction work or the properties of the structure in any derogatory manner. Also, it is common knowledge that at the sites of civil engineering, there exist the unavoidable conditions of dust, pollution, electromagnetic disturbances and of unskilled labor. Thus, the sensors to be used in these cases need to be rugged, should be inert to harsh environment conditions and should be easy to install and their installation could be carried out by unskilled labor. Along with all these things, it is imperative that these sensors are able to survive a period of at least ten years so that they can allow for a constant monitoring of the aging of the structure. Thus, we see that the fiber optic sensors can prove to be quite handful in civil engineering applications and structures. In the past various kinds of sensors have been used in civil engineering for measuring temperature, pressure, stress, strain etc. And as the optical fiber sensors spread their wings, the civil engineering is bound to gain a lot from these modern sensors (Vurpillot et al., 1998).Chapter 3 LITERATURE REVIEW ON FIBER OPTIC SENSORSFiber optic sensors are of many kinds, but they can be broadly assort into two types, namely, extrinsic fiber optic sensors and internal fiber optic sensors. There is a great deal of difference bet ween these two types of fiber optic sensors and this difference is discussed in detail below.3.1 EXTRINSIC FIBER OPTIC SENSORSThis type of fiber optic sensor is also known as crossbreed fiber optic sensor.As we can see in the figure above that there is a portentous box and an comment fiber enters into this black box. And from this input fiber, information is impressed upon light beam. There can be various ways by which the information can be impressed upon. usually this information is impressed upon the light beam in terms of frequency or polarization. This light which then posses the information is carried away by the optical fiber. The optical fiber now goes to an electronic processor. (Vurpillot et al., 1998) Here, in the electronic processor the information which is brought along by the fiber is processed. Though we can have separate input fiber and output fiber, but in some cases it is preferred to have the same fiber as the input fiber and the output fiber.3.2 INTRINSIC FI BER OPTIC SENSORSThe other type of optic fiber sensors is the intrinsic fiber sensors. An example of an intrinsic fiber sensor is shown in the figure below. The working of the intrinsic fiber sensors is somewhat different from the working of the extrinsic fiber sensors. In the intrinsic fiber sensors, the light beam is modulated and we rely on this flexion in the fiber in order to carry out the measurement.In the figure above, we can see an intrinsic fiber sensor or what is also known as all fiber sensor.3.3 INTENSITY BASED FIBER OPTIC SENSORSWhile there exist various kinds of fiber optic sensors today, but the most common of these sensors is the hybrid type fiber optic sensor which depends upon intensity level modulation in order to carry out the measurements (Zako et al., 1995)In the figure below, we can see a vibration sensor. In this vibration sensor, there exist two optical fibers.The functioning of this fiber optic sensor is quite simple. In this fiber optic sensor, light en ters from one side. And when this light exits from the other side, it exits in the form of a cone and the careen of this cone depends on two parameters. The two parameters upon which the angle of this cone depends areFirstly, it depends on the index of refraction of the coreSecondly, it depends on the cladding of the optical fiberAlso, the amount of light captured by the second optic fiber depends on a number of factors.The prominent factors on which the amount of light captured depend areIt depends on the acceptance angleIt also depends on the distance d between the optical fibersAnother type of fiber optic sensor is the flexible mounted mirror sensor. The important characteristics of this sensor areIn this case, a mirror is mounted which is used to respond to external parameters such as pressure.The modulation in intensity is caused the shifts in the mirror position.These sensors are used in a variety of applications such as door closures. In a door closure, a reflective strippe r is used.These sensors are used to measure small variations and displacements3.4 LINEAR POSITION SENSORSIn todays world, linear position sensors have become widely applicable. They are being used for various purposes (Zako et al., 1995). In many of the linear positioning sensors, wavelength parting multiplexing is used. An illustration of the linear position sensor is shown in the figure below.The various components of this linear position sensor areIt consists of a broadband light sourceIt consists of various detectors as shown in the figure aboveIt also consists of wavelength division multiplexing element which acts as the principal component of this instrument.It also consists of an encoder cardIn the example above, a broadband light source is utilized. The light from this broadband source is carried to a wavelength division multiplexing system with the aid of a single optic fiber. The wavelength division multiplexing system is used to determine the linear position.Another line ar motion sensing method which is very widely used today and is quite similar to the method discussed above is known as the time division multiplexing method. This method is illustrated with the aid of a figure shown below.In this method instead of a broadband light source a light pulse is used. Here, the combination of the returned signals takes place. As a result of this combination of the returned signals, the net signal which is produced moves onto the position of the encoder card.The main areas in which these intensity based fiber optic sensors have found application areIn commercial aircraftsIn military aircraftsIn these applications these modern sensors have performed quite well and are at par with the performance of the conventional sensors. But, because of the various advantages these sensors enjoy over and above the conventional sensors, these modern sensors are bound to replace the conventional sensors in the years to come.3.5 LIQUID LEVEL SENSORSThis is another type of i ntensity based fiber optic sensor. In the functioning of this sensor, the principle of total internal reflection is utilized. Thus, in these sensors the refraction index of the glass and the fiber study the pivotal role.These sensors can be utilized for a variety of purposes. The most prominent of its applications areMeasurement of pressure changes in gelsMeasurement of pressure changes in various liquidsMeasurement of refractive index changes in gelsMeasurement of refractive index changes in different types of liquidsMeasurement of the level of a liquid in a vessel and this application is utilized in various industries to measure liquid levelsThese sensors have an accuracy of about 5 percent and are gaining importance in various industries for their usefulness.3.6 SOFO SENSORSThese are fiber optic sensors which are utilized for strain measurement. These sensors have become quite popular owing to their innate merits. step up of all the fiber optic sensors, these sensors are the on es which are being used most extensively today. These sensors are being used to measure curvature and various other parameters in giant civil structures. These sensors form a part of the interferometric system (Vurpillot et al., 1998). Also, these sensors have the ability of measuring the parameters in an absolute manner using low-coherent light. The important properties of these sensors areThese fiber optic sensors enjoy a high resolution. The resolution of these sensors is 2 mThese sensors can be of varied lengths. Their length can be as small as 0.2m or can be as large as 20m.Also, these sensors have the property of being temperature compensatedThe SOFO system setup consists of a number of equipments. The main components of the SOFO system setup areIt consists of a fiber optic sensor which forms the crux of this monitoring system. It is the most important component of the monitoring system. It consists of a sensor cooking stove with partial reflectors.One terminal of this sensor is machine-accessible to the couplerAnother terminal of the sensor chain with partial reflectors is connected to the LED.The coupler in turn is connected to the photo diode and a mobile mirror.This whole portable reading unit is connected to portable computer terminal. This ensures that that the whole monitoring system can be taken to the location and can be directly used at site.These sensors can be utilized in two ways. They can either be embedded in the structure at the time of the construction of the structure. Or, they can used to measure the various parameters externally.Though in both the cases, that is, in case of embedding or in the case of external anchoring, the performance of the sensors remains the same, but still, in modern smart structures, embedding is preferred (Perez 2001).This is because, in the case of embedded sensors, the sensors continuously measure the parameters and are easy to manage. Whereas in the older structures, where embedding is not preferred, exte rnal anchoring is used.Chapter 4 CASE STUDIESCase study 1 Monitoring of San Giorgio pierSan Giorgio pier is a massive concrete structure. Its length is about 400metres. It