Optical Fiber Sensors and Conventional Sensors

ABSTRACT

This study deals with the comparison of the two types of sensors which are widely used in civil engineering, namely, conventional sensors and optical fiber sensors. Temperature and displacement are the two principal parameters which are measured with the aid of Fiber optic sensors. Bragg Grating, Interferometric, Intensity Sensors, and optical time domain reflectometry (OTDR) are some of the techniques which are used for sensing. In this study, various case studies have been undertaken and have been analyzed. With the aid of these case studies, a detailed analysis and comparison of the sensors is carried out.

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Chapter 1: INTRODUCTION

In the last two decades, the world has witnessed a revolution in the sectors of optoelectronics and fiber optic communications. Various products such as laser printers and bar code scanners which have become a part of our daily usage, are a result of this technical revolution only. The reasons for the phenomenal growth of the fiber optics are many. The most conspicuous reason being the ability of the fiber optics to provide high performance and highly reliable communication links and that too at a very low bandwidth cost. As we see that optoelectronic and fiber communications industry has progressed a lot, and along with these industries fiber optic sensors have also benefited a lot from these developments. Due to the mass production in these industries, availability of fiber optic sensors at a low cost has been made possible 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 traditional sensors.

In recent years, the demand for the development of new materials to strengthen, upgrade and retrofit existing aged and deteriorated concrete structures has increased rapidly. The continuing deterioration and functional deficiency of existing civil infrastructure elements represents one of the most significance challenges facing the world’s construction and civil engineers. Deficiencies in existing concrete structures caused by initial flawed design due to insufficient detailing at the time of construction, aggressive chemical attacks and ageing of structural elements enhance an urgent need of finding 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 number 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 period of time.

Moreover, the increase of traffic volume and population in many developing countries is causing the demand to upgrade existing concrete structures to increase. The damage of reinforced concrete (RC) structures through reinforcement corrosion and residual capacity are the most important issues that concern engineers. These problems occur not only in constructed concrete structures but also in structures strengthened by externally bonded steel reinforcements.

In the past, the external steel plate bonding method 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 volumetric

change in the corroded 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 interest 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 can strengthen the structure with minimum changes to its weight and

dimensions. FRP offers substantial improvement in solving many practical problems that conventional materials cannot solve to provide a satisfactory service life of the structure. Unlike the conventional steel materials, FRP is corrosion resistant. The beneficial characteristics of using 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 FRP-plate bonding techniques in strengthening civil concrete structures, the interest in finding a suitable means of monitoring the structural health conditions of these strengthened structures has therefore increased substantially. Since strengthened structures are covered by the FRP plates, the mechanical properties of the concrete may not be measured or detected easily through conventional nondestructive evaluation (NDE) methods, such as strain measurements using surface mounted strain gauges or extensometers, radiography, thermography and acoustic emission methods, particularly in areas with microcracks

and debonds underneath the externally-bonded plate. Besides, these structural inspection technologies, in certain cases, require special surface preparations or a high degree of flatness in the concrete surface. These requirements may be hard to achieve, particularly

for an area that is exposed to a harsh environment. During the 1990s, a multi-disciplinary field of engineering known as ‘Smart Structures’ has developed as one of the most important research topics in the field.The structure is formed by a marriage of engineering materials with structurally-integrated sensor systems. The system is capable of assessing damage and warning of impending weakness in the structural integrity of

the structure. Fibre-optic sensor technology is a most attractive device currently used in the aerospace and aircraft industry for on-line monitoring of large-scale FRP structures. The development of distributed fibreoptic sensors, which provides information on a large

number 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 its

health status and send response signals to operators during any marginal situation during service. The embedding sensor, due to its extremely small physical size, can provide the information to a high accuracy and resolution without influencing the dimension and

mechanical 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 source; (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 mass

production; 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 host’s physical and mechanical properties such as temperature and

strain, during service is appreciated as a ‘Smart structural health monitoring system’. The term smart 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 structure’s possibilities created by the confluence of the four disciplines is shown. In the figure, a structure invested with actuating, sensing and neutral networking systems to form a new class of adaptive structures is shown. A structure with integrated sensor or actuator systems is able to provide a self-structural health monitoring or actuating response, 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 systems

into 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 STUDY

There has been an unprecedented development in the fields of optoelectronics and fiber optic communications. This in turn, has brought about a revolution in the sectors of telecommunication 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 electromagnetic 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 modern 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 STATEMENT

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.

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 mostly 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 structures.

1.3 OBJECTIVES

There are a few objectives that are planned to be achieved at the end of this project, these are:

1. A general discussion on the present state of structural monitoring and the need of fiber optic sensors in this field

2. A general study on Comparison between Conventional Sensors and Optical Fiber Sensors

3. Review of Case Studies on Fiber Optic Sensors application in Civil Engineering Structures

1.4 WORK PLAN

Discussion, reading and observation

• Problem identification through reading, discussion and observation of the area studied

• Understand and identify the background of problem

• Studying feasibility and needs to carry out the investigation

• Identification of the Title for the project

• Identify the aim, objective and scope of the project

Literature Review

• Understanding the background of the problem

• Understanding the history of the sensor technology in structural monitoring

• Carrying out literature survey on generic technologies of sensors for concrete structures

• Identify the types of sensor involved in monitoring the structural in civil engineering

• Identify the technique used and the working principle for each type of sensors (in particular optical fiber sensors)

Case Study

• Choose the relevant and related case study for discussion

• Describe important aspects of case study

• Analyze the use of sensors in the case study

Discussion, Conclusion and Recommendations

• Discuss the similarities and differences

• Discuss the technical facets of sensor application

• Draw the overall conclusion for this project

• Give some recommendation for future

Chapter 2: APPLICATIONS

These days the fiber optic sensors are being used for a variety of applications, the most prominent of them being:

• Measurement of rotation and acceleration of bodies

• Measurement of electric and magnetic fields

• Measurement of temperature and pressure of bodies

• Measurement of acoustics and vibrations of various bodies

• Measurement of strain, viscosity and chemical properties of materials

• Measurement of surface condition and tactile sensing

• Measurement of sound , speed and proximity of bodies

• Determination of color and weight of different objects

• Measurement of linear and angular positions and this is widely utilized in civil engineering structures

2.1 ADVANTAGES OF FIBER OPTIC SENSORS

Like with any other technology, there are both advantages and disadvantages using fiber optic sensors. The prominent advantages being:

• Fiber optic sensors are lightweight and this is of great importance in case of engineered structures

• Fiber optic sensors are of smaller size as compared to the traditional sensors

• Also, fiber optic sensors consume less power as compared to the traditional sensors

• Along with this, these sensors show high resistance to electromagnetic interference as compared to the traditional sensors

• On top of this, fiber optic sensors have enjoy high bandwidth and high sensitivity as compared to their traditional counterparts

• Fiber 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 sensors

• Also, these sensors are accurate over a greater dynamic range as compared to the traditional sensors

• Fiber optic sensors are also capable of being multiplexed which again is a further advantage over their traditional counterparts

• Also, fiber optic sensors are capable of distributed measurements which gives them an edge over and above the traditional sensors

• Last but not the least, they also show greater environmental ruggedness as compared to the traditional sensors

2.2 DISADVANTAGES OF FIBER OPTIC SENSORS

But 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 today’s world has been somewhat curtailed. The major disadvantages of fiber optic sensors are:

• Fiber optic sensors are quite costly 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 educated 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 whole 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 conditions

• Also with the fiber optic sensors there exists the inherit ingress/egress difficulty

• Fiber optic sensors usually have a non-linear output which is a cause for concern in some applications

From 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 sensors. 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.

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2.3 APPLICATIONS IN CIVIL ENGINEERING

Now 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 today’s 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 give 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, namely:

• Material 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 observe the behavior of the construction materials under the conditions of load, temperature etc. In this approach, short base length 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 only 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 account 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 SENSORS

Fiber optic sensors are of many kinds, but they can be broadly classified into two types, namely, extrinsic fiber optic sensors and intrinsic fiber optic sensors. There is a great deal of difference between these two types of fiber optic sensors and this difference is discussed in detail below.

3.1 EXTRINSIC FIBER OPTIC SENSORS

This type of fiber optic sensor is also known as hybrid fiber optic sensor.

As we can see in the figure above that there is a black box and an input 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 FIBER OPTIC SENSORS

The 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 modulation 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 SENSORS

While 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 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 enters from one side. And when this light exits from the other side, it exits in the form of a cone and the angle of this cone depends on two parameters. The two parameters upon which the angle of this cone depends are:

• Firstly, it depends on the index of refraction of the core

• Secondly, it depends on the cladding of the optical fiber

Also, 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 are:

• It depends on the acceptance angle

• It also depends on the distance “d” between the optical fibers

Another type of fiber optic sensor is the flexible mounted mirror sensor. The important characteristics of this sensor are:

• In 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 strip is used.

• These sensors are used to measure small variations and displacements

3.4 LINEAR POSITION SENSORS

In today’s 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 division multiplexing is used. An illustration of the linear position sensor is shown in the figure below.

The various components of this linear position sensor are:

1. It consists of a broadband light source

2. It consists of various detectors as shown in the figure above

3. It also consists of wavelength division multiplexing element which acts as the principal component of this instrument.

4. It also consists of an encoder card

In 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 linear 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 are:

• In commercial aircrafts

• In military aircrafts

In 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 SENSORS

This is another type of intensity 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 occupy the pivotal role.

These sensors can be utilized for a variety of purposes. The most prominent of its applications are:

• Measurement of pressure changes in gels

• Measurement of pressure changes in various liquids

• Measurement of refractive index changes in gels

• Measurement of refractive index changes in different types of liquids

• Measurement of the level of a liquid in a vessel and this application is utilized in various industries to measure liquid levels

These sensors have an accuracy of about 5 percent and are gaining importance in various industries for their usefulness.

3.6 SOFO SENSORS

These are fiber optic sensors which are utilized for strain measurement. These sensors have become quite popular owing to their innate merits. Out of all the fiber optic sensors, these sensors are the ones 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 are:

• These fiber optic sensors enjoy a high resolution. The resolution of these sensors is 2 µm

• These 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 compensated

The SOFO system setup consists of a number of equipments. The main components of the SOFO system setup are:

• It 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 chain with partial reflectors.

• One terminal of this sensor is connected to the coupler

• Another 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, external anchoring is used.

Chapter 4: CASE STUDIES

Case study 1: Monitoring of San Giorgio pier

San Giorgio pier is a massive concrete structure. Its length is about 400metres. It