WO2006075972A1

WO2006075972A1 - Bending sensor arrangement

Info

Publication number: WO2006075972A1
Application number: PCT/SG2005/000007
Authority: WO
Inventors: Swee Chuan Tjin
Original assignee: Sif Universal Pte Ltd
Priority date: 2005-01-14
Filing date: 2005-01-14
Publication date: 2006-07-20

Abstract

In this invention, a bending sensor arrangement (2) for sensing at least one force (4) comprises of at least two fiber Bragg grating (FBG) sensors (12) attached at substantially perpendicular axes to each other on a surface of a longitudinal member. The bending sensor arrangement (2) also includes a first system and a second system. The first system measures and interprets Bragg wavelength information from the at least two FBG sensors (12) respectively. The second system serves to provide user friendly data viewing of the results obtained from the first system. As mentioned earlier, the force (4) is exerted upon the longitudinal member causing the FBG sensors (12) to experience strain whereby said strain is detected by the first system. This first system measures, interprets and relates the Bragg wavelength information to determine the direction of the force (4) and the spatial displacement of the longitudinal member about a fixed end. This information is then processed and displayed by the second system in a user friendly format.

Description

BENDING SENSOR ARRANGEMENT

FIELD OF THE INVENTION

The present invention relates to a Fiber Bragg Grating (FBG) sensor arrangement for monitoring civil structures. More particularly, this invention relates to a FBG sensor arrangement to detect bending force experienced by civil structures.

BACKGROUND OF THE INVENTION

Nowadays, it is often reported that land sliding causes great loss in human life and fortune due to unexpected earth movement. At the same time, this unpredictable phenomenon has caused substantial damages to civil structures. In order to measure the constant changes in earth or soil movement, a bending sensor arrangement is required to monitor the bending forces exerted upon these civil structures. As an example, a beam being used to support a beam or bridge has a higher tendency of breaking due to a bending force compared to a compression or tension force. In order to overcome this problem, a Geotechnical

Inclinometer (Gl) is used.

A Geotechnical Inclinometer is used for monitoring and detecting any possible failure of a material due to bending force generated by soil movements. Mechanical sensors are deployed along a pipe, said pipe being planted into the ground to detect soil movement according to point- by- point sensing. In this method, it is assumed that the shape of pipe change in between two points follows a linear function. This assumption usually causes inaccuracy in results. Generally, a Geotechnical Inclinometer comprising of a longitudinal PVC pipe is introduced into the ground and anchored at its base. When there is soil movement, said movements will cause the pipe to bend polynomially. This movement will be registered by a sensor unit which will be manually lowered into the PVC guide pipe to measure the longitudinal deformation of the said pipe at specific vertical intervals.

However, there are many disadvantages in using Geotechnical Inclinometers. First, a Geotechnical Inclinometer assumes the section in between two vertical sensing points to be linear. The accuracy of this measurement is very much dependent on the individual performing the operation. Due to unforeseen errors or human errors, the measurements obtained will not be able to submit an accurate conclusion on the conditions in the civil structure. Scientists and engineers have realized that many cases of land slides and collapses of civil structures can be avoided if a distortion rate of structure and earth movement trend can be monitored regularly and remotely. A Geotechnical Inclinometer is able to monitor and determine real time information only, thus not able to predict continuous movement of soil in relation with time to detect any possible land slides or collapses.

There is a need to explore an efficient way to detect any possible land slides or collapses early so that remedial actions can be taken in advance. A new technique of measuring bending force, detecting force direction and displaying pipe bending distortion is required to overcome these problems of measuring force parameters.

Any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the invention. It should not be taken as an admission that any of the material forms a part of the prior art base or the common general knowledge in the relevant art in Singapore or elsewhere on or before the priority date of the disclosure and claims herein. All statements as to the date or representation as to the contents of these documents is based on the information available to the applicant and does not constitute any admission as to the correctness of the dates or contents of these documents.

SUMMARY OF THE INVENTION

In this invention, a bending sensor arrangement for sensing at least one force comprises of at least two fiber Bragg grating (FBG) sensors attached at substantially perpendicular axes to each other on a surface of a longitudinal member. The bending sensor arrangement also includes a first system and a second system. The first system measures and interprets Bragg wavelength information from at least two FBG sensors respectively. The second system serves to provide user friendly data viewing. As mentioned earlier, the force is exerted upon the longitudinal member causing the FBG sensors to experience strain whereby said strain is detected by the first system. This first system measures, interprets and relates the Bragg wavelength information to determine the direction of the force and the horizontal displacement of the longitudinal member about a fixed end. This information is then processed and displayed by the second system in a user friendly format.

The longitudinal member in the bending sensor arrangement above is positioned in soil or within a structure to monitor the conditions of said soil and structure. The first system as mentioned earlier refers to an interrogation system. This interrogation system basically comprises of a black box "FBG-IS" (fiber Bragg grating - Interrogation System), which includes a broadband source, coupler, filter, a photodetector and an analog to digital converter. More particularly, in this invention the filter used is an F-P filter. The Bragg wavelength information that is measured and interpreted involves peak Bragg wavelength shifts in the FBG sensors. The second system in this bending sensor arrangement is a Graphical User Interface (GUI), said (GUI) being connected to an electronic display device. Among the strains experienced by these FBG sensors are compression and/or tension strains.

In this bending sensor arrangement, a method can be used to detect the direction of at least one force being exerted on a structure. The method involves attaching at least two FBG sensors at substantially perpendicular axes to each other on a surface of a longitudinal member, measuring and interpreting Bragg wavelength information and displaying interpreted Bragg wavelength information, and force direction in a user friendly format. As stated earlier, the measuring and interpreting of Bragg wavelength information is done by the first system, and displaying of the interpreted information is done by the second system. Assume at least one force being exerted on a structure whereby said force causes the FBG sensors to experience strain. This strain causes change in Bragg wavelength information. The relationship between the Bragg wavelengths and direction of the exerted force is equivalent to measuring a peak wavelength shift of the FBG sensors.

A method involving determining a spatial displacement of a longitudinal member when at least one force is exerted on said tube can be done using the bending sensor arrangement. The method comprises the steps of at least two FBG sensors at substantially perpendicular axes to each other on a surface of a longitudinal member, measuring and interpreting Bragg wavelength information, and displaying interpreted Bragg wavelength information and force direction in a user friendly format. Assume at least one force being exerted on a structure whereby said force causes the FBG sensors to experience strain. This strain causes change in Bragg wavelength information. The relationship between the Bragg wavelengths and spatial displacement of the longitudinal member is summing the respective Bragg wavelengths shifts of the FBG sensors.

Referring to the two methods as mentioned above, the longitudinal member is positioned in soil and/or within a structure. This enables the longitudinal member attached with FBG sensors to detect any movement in the soil and/or structure. The longitudinal member used is preferably a PVC pipe, or a solid elastic pole. The first and second system refers to an interrogation system and Graphical User Interface (GUI), respectively. The strains experienced by the FBG sensors are compression and/or tension strains. The structures in this embodiment refer to civil structures and/or industrial structures. This invention will be described in greater detail in the following sections.

Other aspects and preferred aspects are disclosed in the specification and / or defined in the appended claims, forming a part of the description of the invention.

Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. BRIEF DESCRIPTION OF DRAWINGS

Further disclosure, objects, advantages and aspects of the present application may be better understood by those skilled in the relevant art by reference to the following description of preferred embodiments taken in conjunction with the accompanying drawings, which are given by way of illustration only, and thus are not limitative of the present invention, and in which:

Figure 1 shows a bending sensor arrangement;

Figure 2 illustrates the top view of the pipe disposed with two FBG sensors at perpendicular axes to each other on a surface of the PVC pipe;

Figure 3 discloses all parts in the black box fiber Bragg grating - Interrogation System (FBG-IS);

Figure 4 is an interrogation system to monitor Bragg wavelength information of fiber Bragg grating (FBG) sensors;

Figure 5 illustrates the direction of a force being exerted on a PVC pipe above and beyond a pre-selected rigid point at one end of the said pipe;

Figure 6 shows four fundamental regions A, B, C, D in cross section view of the pipe;

Figure 7 shows forces being exerted at different spaced apart points along a PVC pipe; Figure 8 is the spatial displacement of a PVC pipe bending due to a single force being exerted on the said pipe;

Figure 9 shows multi forces acting at different planes on a PVC pipe; and

Figure 10 shows multi forces being exerted on a PVC pipe is resolved into a same plane.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Fiber Bragg grating (FBG) is one of the new emerging technology platforms that has generated vast interest in research and industrial applications in recent years. The output of the FBG sensor has a number of advantages over other sensing schemes available in the market. Its inherent wavelength - encoded output is substantially accurate compared to output generated from other sensing devices. Besides this, the FBG is low cost, insensitive to electromagnetic fields and corrosion, and measurable for different parameters. Its small size and high density of information bring along new ranges of innovations and applications from pressure monitoring in civil structures, chemical process reactors and field of smart structures which includes buildings, bridges and aircraft which requires a nerve system capable of sensing changes in material. The advantages of FBG are generating great interest and demand in the industry.

The creation of new type of FBG bending sensor arrangement to replace

Geotechnical Inclinometer is crucial to the evolution in civil engineering. Referring to Figure 1 , there is shown a bending sensor arrangement (2). The bending sensor arrangement (2) serves to measure magnitude and direction of a force (4) exerted on a civil structure wherein said force (4) is caused by soil movement. Any other cause of nature or human action that causes exertion of forces on a structure can also be taken into consideration. The invention also has an object to determine spatial displacement of a longitudinal member when the force (4) is exerted on said longitudinal member. The longitudinal member in this embodiment can be a flexible rod whereby one end of the said member is permanently secured. This enables the longitudinal member to be displaced from its secured end. The longitudinal member used in this invention is preferably a cylindrical tube. In this preferred embodiment, the cylindrical tube is a PVC pipe (6). The PVC pipe (6) can be positioned in the soil or within the structure whereby said pipe is attached with sensors. Information is gathered from these sensors, whereby said information is interpreted and related by a system. This system will be described later in the description. The results obtained are displayed on a Graphical User Interface (GUI) (8) to ensure user friendliness in viewing relevant information on conditions of the structure and soil.

As shown in Figure 1, the invention involves the implementation of a bending sensor arrangement (2) comprises of three main parts. A first section (10) shows an arrangement whereby the PVC pipe (6) attached with FBG sensors (12) is positioned upright such that the force (4) is exerted on the said pipe (6). The force (4) in Figure 1 is a representative for forces being exerted on PVC pipes (6) positioned in soil or within a structure, in a real environment. The PVC pipe (6) is preferably flexible and is easily obtainable at low cost. Alternatively, conventional materials can also be used to substitute the PVC pipe (6). In this embodiment, bare FBG sensors cannot be directly disposed on the PVC pipe (6) to monitor soil movement and/or civil structures. These bare FBG sensors can be damaged easily due to their brittleness. This can cause many problems in the process of monitoring results regarding soil movement and/or civil structures. In response to this, carbon fiber is preferably used as an embedding material to embed these bare FBG sensors, thus protecting said sensors from being damaged easily. The method used for embedding the bare FBG sensors is known in the art.

Referring to Figure 1 , a second section (14) involves setup of an interrogation system (16) that measures Bragg wavelengths sourcing from the FBG sensors (12). The Bragg wavelengths obtained are sent to a computer, where a Data Acquisition Card (DAQ) (18) is installed. The DAQ (18) is configured to receive the Bragg wavelengths whereby a shift in these Bragg wavelengths will be detected and stored. Finally, a third section (20) in this embodiment involves design of a computer control Graphical User Interface (GUI) (8) preferably written in computer program, Labview 6.1. Alternatively, other computer programs that are compatible with the GUI (8) can be used, some of these computer programs include visual basic and C-programming language. The GUI (8) processes the Bragg wavelengths stored in the computer and displays user friendly information to the end user. The information displayed is relevant to the conditions of the soil and/or civil structures.

The constructing of the FBG sensor (12) originates from bare FBG fibers. These bare FBG fibers are often fabricated using Excimer Laser or UV laser with different phase masks to generate different peak wavelengths. This method of fabrication is known in the art. These bare FBG fibers are then embedded into a carbon composite material to form a reinforced laminate. Alternatively, other conventionally available embedding methods can be used. A mould (not shown) is designed to standardize and reduce the experimental discrepancy of the embedded FBG sensors (12). The mould serves to fabricate the embedded FBG sensors (12) is preferably semicircular in shape. In this preferred embodiment, the FBG sensors (12) are attached to a surface of the PVC pipe (6) by means of epoxy. The FBG sensors (12) being semicircular in shape will correspond to the shape of the PVC pipe (6), thus easing attachability of said sensors onto the said pipe. In this embodiment, two types of FBG sensors (12) are fabricated preferably to 26.6 mm and 33.4 mm in diameter, respectively. However, different types of FBG sensors (12) can be fabricated to suit the requirement of the bending sensor arrangement (2).

Having described the FBG sensors (12), we will now describe the attachment of these FBG sensors (12) to the surface of the PVC pipe (6). Figure 2 shows a top view of the PVC pipe (6) whereby at least a pair of FBG sensors (12) are positioned at perpendicular axes to each other. As mentioned earlier, these embedded FBG sensors (12) are attached to the surface of the PVC pipe (6). According to this invention, more than one pair of FBG sensors (12) are preferably disposed at spaced apart points along the surface of the PVC pipe (6). The positioning of these FBG sensors (12) enables said sensors to experience different types of strains, either compression or tension strains. These strains are caused by a bending moment of the PVC pipe (6), said bending moment is due to the force (4) exerted on the said pipe. For simplicity purposes, the FBG sensors (12) will be denoted as sensor one (24) and sensor two (26) in this description. The positioning of sensors one and two (24 and 26) will enable said sensors to transmit Bragg wavelength information accurately to the interrogation system (16).

The interrogation system (16) will be described now. As shown in Figures 3 and 4, the interrogation system (16) comprises of a black box "Fiber Bragg Grating - Interrogation system" ("FBG - IS") (28). In this preferred embodiment, the ("FBG-IS") includes a broadband source (30), coupler (32), fiber F-P filter (34), photo detector (36) and an analog to digital converter (38). All devices included in the black box ('FBG-IS") (28) is known in the art. Alternative filters are available such as acousto filters and FBG - based filters. According to this invention, the interrogation system (16) tracks a signal from the FBG sensors (12). The signal involves Bragg wavelength shifts, in the FBG sensors (12) caused by the bending moment of the PVC pipe (6). The interrogation system (16) relates and converts all Bragg wavelength information. As illustrated in Figure 4, when force (4) is exerted upon the FBG sensors (12), there will be a wavelength shift (A₁ to A₂). Alternatively, other interrogation systems (16) known in the art can be used to achieve this goal.

In this invention, a mathematical model is formulated to illustrate the relationship between force direction, pipe displacement and Bragg wavelength information. A few assumptions are brought into consideration in formulating the mathematical model. The assumptions include ignoring temperature effects, PVC pipe (6) being in a good condition and homogenous in every part, one single horizontal force (4) is exerted at a point wherein the said force (4) points towards the center of the pipe and the force (4) used for testing must be controlled to act horizontally, as illustrated in Figure 5. According to this invention, the PVC pipe (6) with outer diameter 26.8mm, and an inner diameter 20mm, and a length of 1005mm is used. In this description, these dimensions will be used as reference. In a real environment, a proper PVC pipe (6) can be configured to suit the surroundings. The PVC pipe (6) has an anchored base, similar to the like of the earlier discussed Geotechnical Inclinometer. Two bare FBG sensors are attached to the surface of the PVC pipe (6) using epoxy and positioned 90° away from each other. The bare FBG sensors are only used for formulating the mathematical model, but in the real environment the bare FBG sensors are embedded for protection purposes, preventing them from being damaged. The earlier mentioned sensor one and two (24 and 26) will be adopted into this section of the description. Both sensors (24 and 26) are 15mm long, said sensors are positioned 10mm away from a pre-selected rigid point (40). The pre-selected rigid point (40) refers to the base of the PVC pipe (6) that is anchored.

As shown in Figure 5 and 6, a single force (4) is exerted onto the PVC pipe (6). In this preferred embodiment, sensor one (24) is placed at 0°. In respect to a clock wise rotation, sensor two (26) is disposed at 270° away from sensor one (24). The positioning of sensor one and two (24 and 26) enables said sensors to experience tension or compression. Two imaginary neutral lines (42, 44) runs through the center of the PVC pipe (6). As shown in Figure 6, the imaginary neutral lines (42, 44) divide the PVC pipe (6) into four regions wherein each region occupies one quarter of the circle area, thus forming quadrants. Each region is denoted with alphabets A, B, C, and D respectively for ease of this description. The force (4) is exerted individually on each of these quadrants, thus generating the results below:

Table 1 Force Direction Encoded into Peak Wavelength Change

*K1 represents sensor one (24) Bragg Wavelength change slope *K2 represents sensor two (26) Bragg Wavelength change slope *+ slope of the Bragg Wavelength change value increases *- slope of the Bragg Wavelength change value decreases

*Model fit is described in detail below. Due to the characteristics and the analysis shown in Table 1 , we can model using trigonometric properties:

K1 = a sin(x) (1)

K2 = b cos(x) (2) where x is angle, a and b are constants. Based on Equation (1) and (2), the ratio and sum of the slope of two sensors (24 and 26) experiencing Bragg wavelength shifts are shown in the following Δ equations,

AWl

Kl = -^jα Λ^Ln^L-= AWl =— a t,an ,(x _Λ) _i (3_os)

Kl ΔW2 AW2 b AD

Kl+K2 = ^<Ja²+b² sin(x + 0) (4) where θ = tan^"1 (a/b), ΔW1 and ΔW2 are the Bragg wavelength shifts, ΔD denotes the horizontal spatial displacement at the certain distance above the pre-selected rigid point (40) of the PVC pipe (6).

Basically in determining the direction of the force (4), the two sensors (24 and 26) is positioned 90° with respect to each other at a quadrant and must be attached to the surface of the PVC pipe (6). The PVC pipe (6) can actually be represented by four regions. In real life situations, more than one force (4) is exerted on the PVC pipe (6) from various directions simultaneously. Because of the positioning of the sensors (24 and 26), the tension and compression strains will be detected by the said sensors due to bending moment of the PVC pipe (6). When force (4) is exerted onto the PVC pipe (6), there will be a change in reflected peak Bragg wavelengths in the two sensors (24 and 26). Thus, the two peak Bragg wavelengths change has four different combinations in terms of peak wavelength increase or decrease depending on the location of the force (4). A curve can be expressed using Nelder and DoseResp equations, said equations to be known to one skilled in the art. The relationship between the peak Bragg wavelength information which follows this curve, and force direction, is actually measuring the peak Bragg wavelength shifts in sensors one and two (24 and 26) respectively. The concept of interpreting the Bragg wavelength shifts is applicable in the real environment. However, all dimension and specifications given above can be modified to suit the environment.

Referring to Figures 7 and 8, formulation of a mathematical model for the spatial displacement of pipe bending by a single force wherein said displacement of the pipe above its pre-selected rigid point (40) can be determined. This can be achieved by manipulating equation (4) stated above, thus generating equation,

AKUAK2= ^AW1+AW2 - (5)

AD

Hence, ΔD can be determined.

Force (4) is exerted at different points as shown in Figure 7, thus values of K1 + K2 will vary, but they still follow a sinusoidal curve except that the magnitudes of K1 and K2 are different. Hence, the force (4) is tested at different points to determine the relationship among these magnitudes. The value of K1 + K2 can be calculated at these different spaced apart points, above the preselected rigid point (40) of the PVC pipe (6) as shown in Figures 7 and 8. To determine the spatial displacement of the pipe, the Bragg wavelength shifts of both sensors (24 and 26) are summed. Again, the concept of interpreting the Bragg wavelength shifts is applicable in the real environment. However, all dimensions and specifications given above can be modified to suit the environment. These concepts are applicable when multiple forces being exerted on the PVC pipe (6) from different directions. One single force (4) is seldom seen in the real environment. Instead, force array, multiple point forces and irregular force directions often occur in real life situations. Referring to Figures 9 and 10, the multiple forces from different directions were simplified into one single force for the purpose of formulating the mathematical model.

In the final course of this invention, a Graphical User Interface (GUI) (8), which is a computer control program written in Lab view 6.1 is preferably used.

However, the GUI (8) is compatible with other programming languages, such as visual basic and C-programming language, and provides a user friendly format for viewing information. The information viewed in GUI (8) is interpreted Bragg wavelength information from the interrogation system (16) and computer that is installed with a Data Acquisition Card (DAQ) (18). The (GUI) (8) is a known technology and can be configured to perform specific displaying operations.

As shown in Figure 1 , the Graphical User Interface (GUI) (8) communicates with a Chatillon (46) which is then connected to a force gauge (48), thereon communicating the data with a computer. This force gauge (48) represents the force (4) that is exerted on the PVC pipe (6) in real life situations. Two semicircular shaped FBG sensors (12) are positioned at perpendicular axes to each other at a quadrant and attached to the surface of the PVC pipe (6). When the force (4) is exerted on the PVC pipe (6) and FBG sensors (12), the peak Bragg wavelengths of FBG sensors (12) change. The next phase involves the interrogation system (16) whereby Bragg wavelength information is interpreted. A computer containing Data Acquisition Card (DAQ) (18) is connected to the interrogation system (16) enabling the measurement readings to be displayed and be saved as data files. These files will then be processed and displayed as user friendly information on the GUI (8), which is connected to an electronic display device.

Although the present invention has been described related to particular embodiments thereof, there will be many other modifications and variations taken upon the present invention by one skilled in the art. It is preferred therefore, that the present invention be not only by the specific disclosure herein, but only the appended claims.

While this invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modification (s). This application is intended to cover any variations uses or adaptations of the invention following in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the invention pertains and as may be applied to the essential features hereinbefore set forth.

As the present invention may be embodied in several forms without departing from the spirit of the essential characteristics of the invention, it should be understood that the above described embodiments are not to limit the present invention unless otherwise specified, but rather should be construed broadly within the spirit and scope of the invention as defined in the appended claims. Various modifications and equivalent arrangements are intended to be included within the spirit and scope of the invention and appended claims. Therefore, the specific embodiments are to be understood to be illustrative of the many ways in which the principles of the present invention may be practiced. In the following claims, means-plus-function clauses are intended to cover structures as performing the defined function and not only structural equivalents, but also equivalent structures. For example, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface to secure wooden parts together, in the environment of fastening wooden parts, a nail and a screw are equivalent structures. "Comprises/comprising" when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof."

Claims

1. A bending sensor arrangement (2) for sensing at least one force (4) comprising of:

at least two fiber Bragg grating (FBG) sensors (12) attached at substantially perpendicular axes to each other on a surface of a longitudinal member,

a first system that measures and interprets Bragg wavelength information from the at least two FBG sensors (12) respectively,

a second system that provides user friendly data viewing,

wherein said force (4) is exerted upon the longitudinal member causing the FBG sensors (12) to experience strain whereby said strain is detected by the first system, said first system measures the Bragg wavelength information, interprets and relates said Bragg wavelength information to determine the direction of the force (4) and the horizontal displacement of the longitudinal member about a fixed end, said Bragg wavelength information is then processed and displayed by the second system in a user friendly format.

2. A bending sensor arrangement (2) as claimed in Claim 1 wherein the longitudinal member is positioned in soil and/or within a structure to monitor conditions of the said soil and/or structure.

3. A bending sensor arrangement (2) as claimed in Claim 1 wherein the longitudinal member is a PVC pipe (6) or a solid elastic pole.

4. A bending sensor arrangement (2) as claimed in Claim 1 wherein the first system is an interrogation system (16).

5. A bending sensor arrangement (2) as claimed in Claim 1 or 4 wherein the interrogation system (16) comprises of a black box (fiber Bragg grating -

Interrogation System) "FBG-IS" (28) which includes a broadband source (30), coupler (32), filter, a photodetector (36) and an analog to digital converter (38).

6. A bending sensor arrangement (2) as claimed in Claim 5 wherein the filter is an F - P filter (34).

7. A bending sensor arrangement (2) as claimed in Claim 1 wherein the measurement and interpretation of the Bragg wavelength information involves peak Bragg wavelengths shifts in the FBG sensors (12).

8. A bending sensor arrangement (2) as claimed in Claim 1 wherein the second system is a Graphical User Interface (GUI) (8) connected to an electronic display device.

9. A bending sensor arrangement (2) as claimed in Claim 1 wherein the strain experienced by the FBG sensors (12) are compression and/or tension strains.

10. A method of detecting the direction of at least one force (4) being exerted on a structure, using a bending sensor arrangement (2) comprising the steps of:

attaching at least two FBG sensors (12) at substantially perpendicular axes to each other on a surface of a longitudinal member, measuring and interpreting Bragg wavelength information from FBG sensors (12) through a first system,

displaying interpreted Bragg wavelength information and force (4) direction in a user friendly format on a second system,

wherein said force (4) causes the FBG sensors (12) to experience strain, said strain causing change in Bragg wavelength information whereby the relationship between the Bragg wavelengths and direction of the exerted force (4) is equivalent to measuring a peak Bragg wavelength shift of the said FBG sensors (12).

11. A method as claimed in Claim 10 wherein the longitudinal member is positioned in soil and/or within a structure whereby movements in the said soil and/or structure can be detected by said longitudinal member.

12. A method of detecting a direction of at least one force (4) being exerted on a structure as claimed in Claim 10 wherein the longitudinal member is a PVC pipe (6) or a solid elastic pole.

13. A method of detecting a direction of at least one force (4) being exerted on a structure as claimed in Claim 10 wherein the first system is an interrogation system (16).

14. A method of detecting a direction of at least one force (4) being exerted on a structure as claimed in Claim 10 wherein the second system is a Graphical User Interface (GUI) (8).

15. A method of detecting a direction of at least one force (4) being exerted on a structure as claimed in Claim 10 wherein the strain experienced by the FBG sensors (12) are compression and/or tension strains.

16. A method of detecting a direction of at least one force (4) being exerted on a structure as claimed in Claim 10 wherein the structure is civil structures and/ or industrial structures.

17. A method of determining a spatial displacement of a longitudinal member when at least one force (4) is exerted on said, using a bending sensor arrangement (2) tube comprising the steps of:

attaching at least two FBG sensors (12) at substantially perpendicular axes to each other on a surface of a longitudinal member,

measuring and interpreting Bragg wavelength information from FBG sensors (12) through a first system,

wherein said force (4) causes the FBG sensors (12) to experience strain, said strain causing change in Bragg wavelength information whereby the relationship between the Bragg wavelengths and spatial displacement of the longitudinal member is summing the respective Bragg wavelengths shifts of the FBG sensors (12).

18. A method as claimed in Claim 17 wherein the longitudinal member is positioned in soil and/or within a structure whereby movements in the said soil and/or structure can be detected by said longitudinal member.

19. A method of determining a spatial displacement of a longitudinal member as claimed in Claim 17 wherein the longitudinal member is a PVC pipe (6) or a solid elastic pole.

20. A method of determining a spatial displacement of a longitudinal member as claimed in Claim 17 wherein the first system is an interrogation system (16).

21. A method of determining a spatial displacement of a longitudinal member as claimed in Claim 17 wherein the second system is a Graphical User Interface (GUI) (8).

22. A method of determining a spatial displacement of a longitudinal member claimed in Claim 17 wherein the structure is civil structures and/ or industrial structures.

23. A method of determining a spatial displacement of a longitudinal member claimed in Claim 17 wherein the strains experienced by the FBG sensors (12) are compression and/or tension strains.

24. A Fiber Bragg Grating (FBG) sensor (12) as claimed in either Claims 1 to 23 wherein said FBG sensors (12) are embedded with carbon composite material, and semicircular in shape to ease attachment of said sensors to the longitudinal member.

25. A Fiber Bragg Grating (FBG) sensor (12) as claimed in Claim 24 wherein the longitudinal member is a cylindrical tube.