such as dams, bridges, buildings and composite material structures. ..... A.L. Kalamkarov, S.B. Fitzgerald, D.O. Macdonald, âOn the processing and evaluation of ...
New generation of Fabry-Perot fiber optic sensors for monitoring of structures Pierre Choqueta, François Juneaua, John Bessetteb a
Roctest Ltd, 665 Pine Ave, St-Lambert, Quebec, J4P 2P4, Canada b
Roctest Inc, 94 Industrial Blvd, Plattsburgh, NY, 12901 ABSTRACT
This paper reports on a new generation of Fabry-Perot fiber optic sensors to be used in parallel or in replacement of conventional instruments for monitoring of structures. The new generation of sensors is based on a unique fiber optic strain sensor that represents a breakthrough in fiber optic sensing. The novel technique is based on extrinsic Fabry-Perot white-light interferometry which offers outstanding accuracy and repeatability. Furthermore, all sensors are completely immune to lightning surcharges which opens new possibilities in the field of reliable long term structure monitoring. Instruments such as piezometers, embedment strain gages, surface strain gages, temperature sensors and displacement transducers are all available in Fabry-Perot fiber optic version. Furthermore, all these instruments use a common multimode optical fiber to carry the signal to the readout units and multi-channels dataloggers. Both static and dynamic measurements are possible with this technology. The paper presents results of laboratory and field studies on fiber optic sensors mentioned above including integrated fiber optic sensors in carbon and glass fiber reinforced polymer. A case study of bridge strain monitoring with Fabry-Perot sensors is also reported. Keywords: Fiber-optic sensor, Fabry-Perot interferometer, structural instrumentation, bridge monitoring, fiber reinforced polymer
1. INTRODUCTION Many research programs are in progress in the field of fiber optic sensors for structural monitoring of large civil structures such as dams, bridges, buildings and composite material structures. These sensors can be attached to the surface or embedded into materials and structures to continuously monitor conditions such as damage, strain, stress, crack formation, pore pressure, temperature, etc. The absolute extrinsic Fabry-Perot interferometer (EFPI) is one of these sensors. This sensor has the inherent advantages of many fiber optic sensors like small size, light weight, non conductivity, fast response, resistance to corrosion, immunity to electromagnetic noise and radio frequency interferences eliminating the need for costly and bulky shielding and lightning protection accessories. They can sense a variety of physical effects such as pressure, strain, temperature and displacement. Their small size allows them to be incorporated in composite material without sacrificing structural integrity. This paper presents some results of laboratory experiments and field tests for different types of Fabry-Perot sensors in order to verify their behaviour, their performance and their applicability for smart structures. In these experiments, the sensors are subjected to mechanical and thermal strains. The Fabry-Perot sensors and readout equipments are from Roctest Ltd. and Fiso Technologies Inc. An explanation of the Fabry-Perot sensor technology and signal processing is also presented.
2. FABRY-PEROT SENSOR TECHNOLOGY The Fabry-Perot interferometric principle presented in this paper makes use of a broadband white light source instead of laser light. This highly sensitive measurement technique can make precise, absolute and linear measurements without stabilisation means such as preheating time of light source and is quit insensitive to environmental temperature changes. Several EFPI designs are used in the family of sensors presented in this paper. As an example, the EFPI design used to manufacture a strain sensors is illustrated in Figure 1. It consists of two semi-reflective mirrors facing each other. These mirrors are deposited on the tips of multimode optical fibers and these fibers are spot fused into a capillary. Published in :
Proceedings of SPIE’s 7th Annual International Symposium on Smart Structures and Materials, 5-9 March 2000, Newport Beach, CA.
The air gap between the mirrors is called the Fabry-Perot cavity length (lcavity) and the distance separating the fused spots is called the gage length (Lg) and dictates the gage operating range and sensitivity. Optical fiber 200 µm
Few tens of microns
Gage length (Lg)
25 mm Fusion spots
mirror
Capillary tube
Light reflected from the Fabry-Perot
Fabry-Perot cavity length (0 to few tens of microns) Linear CCD array
Figure 1: Extrinsic Fabry-Perot fiber optic sensor for the measurement of strain
Fizeau Interferometer
Figure 2: Fizeau interferometer and CCD array
A portion of the white light is launched by a readout unit into one end of a fiber optic cable and travels toward the FabryPerot sensor. One part of the light is reflected by the first semi-reflective mirror. The remaining light travels through the Fabry-Perot cavity and is partially reflected, a second time, by the next semi-reflective mirror. The light from the two reflections interfere and travels back to the readout unit toward a detector. Cavity length (lcavity) is determined instantaneously by means of an optical white light cross-correlator (Fizeau interferometer) contained in the readout unit. The next section will explain this device and how the signal is analysed. When the sensor is bonded to a substrate, the strain transferred to the sensor is converted into cavity length (lcavity) variation and the strain is given by the following equation:
Strain (ε ) =
∆l cavity Lg
Let’s mention that the Fabry-Perot sensor is mounted at the end of a fiber optic cable to form a point sensor but it is also possible to construct Fabry-Perot cavity in different ways in order to develop a wide range of instruments.
3. SIGNAL PROCESSING The conversion of the optical signal into measurement of a physical value is accomplished by means of a Fizeau interferometer and a linear CCD (Charge Couple Device) array combination as shown in Figure 2. Theoretical considerations is covered in detail in a paper by Belleville and Duplain [2]. The light signal reflected back by the Fabry-Perot strain sensor illuminates the complete width of the Fizeau interferometer, which consists in a spatially-distributed interferometer whose thickness varies from almost zero to a few tens of microns, namely exactly the same values as the minimum and maximum values of the Fabry-Perot cavity length. Light is transmitted maximally at the exact location along the Fizeau interferometer where his thickness is equal to the Fabry-Perot cavity length of the sensor. The Fizeau interferometer makes an instantaneous correlation of the signal for all spacing values of the cavity. Further processing detects and locates the position of the light power peak response obtained by the linear CCD array and determines precisely the absolute cavity length. This cavity length is said absolute because it corresponds to the true cavity length of the Fabry-Perot interferometer at the time at which the optical signal was measured, as opposed to a relative measurement of the cavity length in which case it is determined in relation to an arbitrary zero value. The absolute measurement is very important in applications where long term or static measurements are required as in structural monitoring. The optical signal is converted in cavity length at a frequency given by the sampling rate of the readout unit. At this time, technology advancement allows sampling rate up to 1000 Hz with a resolution slightly better than 0.01% full scale and a precision of 0.025% full scale.
4. READOUT UNITS AND DATALOGGERS All readout units for the Fabry-Perot sensors have a white light as light source. The white light system does not need preheating time before to be used. Different readout units from 1 to 32 channels have been developed for static and dynamic
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applications. Laboratory and field versions of readout unit with sampling rate varying between 10 to 1000 Hz according to the readout unit were used. Gages connected to the readout unit may be disconnected at any time with no loss of measurement reference. For each readout, the strain data can be taken manually or the unit can be permanently connected to a computer through a RS-232 link. The information extracted by a datalogger can also be transmitted over a phone line toward a central monitoring station. This application can be very useful for structural monitoring. A Windows based data acquisition software called FISO Commander can been used to setup the datalogger and retrieve data.
Figure 3: Thirty-two channels datalogger
Figure 4: Single-channel readout unit
One advantage of each readout is their universality because they allow to read dissimilar transducer types, all based on the Fabry-Perot principle. The user only needs to enter a gage factor for each instrument in the permanent memory of the readout. This factor is used to define the gage type, range, sensitivity and to display the reading directly in engineering units.
5. STRAIN SENSOR 5.1 Surface mounted sensor subjected to mechanical strain Many mechanical tests were carried out to assess and characterize the behaviour of the Fabry-Perot strain sensor illustrated in Figure 1. This sensor has a small diameter of 0.20 mm and is available in various ranges. Fabry-Perot strain sensors were bonded on steel bars and composite materials using suitable industrial quality adhesives. In order to investigate their strain sensitivity, linearity and repeatability, a sensor was bonded in the middle section of a steel bar of 19.5 mm of diameter with a conventional resistive foil gage on the opposite side as a reference strain signal. In the first test, the steel bar was loaded up to 1600 microstrains. 100
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Figure 5: Comparison of the strain measured on a steel bar by a conventional electrical strain gage and a Fabry-Perot fiber optic sensor.
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Figure 6: Loading and unloading response of a Fabry-Perot fiber optic strain sensor bonded to composite material and tested five times.
Figure 5 shows the comparison for both the Fabry-Perot sensor and the electrical strain gage. The graphic shows a perfect correlation (R2=0.9999) between the measurements obtained from the Fabry-Perot sensors and those from the electrical strain gage; the strain response is very linear. The strain resolution of the sensor tested was 0.01% full scale or 0.5 microstrain. The experiment was repeated several times and different bars were also tested. Similar results were obtained. To effectively transfer strain from structure to the fiber optic sensor, a good installation to host material is required. However the bonding technology used for conventional electrical strain gage can be transferred easily to fiber optic sensors. Another test was done to verify the repeatability and the hysteresis during loading and unloading cycles. In this case the Fabry-Perot sensor was bonded to a composite material, more specifically an Isorod bar of 15 mm of diameter. Five load and unload cycles have been done and each time the load was increased higher until reaching about 2000 microstrains. Figure 6 shows that strain response is repeatable and it is free of hysteresis during loading and unloading. This experiment was also repeated with two other bars and similar results were obtained. Fabry-Perot strain sensor can also be embedded within several different composite materials and many experiments on this subject have been reported by Lawrence et al.[5] and Kalamkarov et al.[6]. Figure 7 shows three Fabry-Perot strain sensors, as illustrated in Figure 1, embedded in carbon fiber reinforced polymer (CFRP) Nefmac grid. This grid was installed in the deck of Joffre bridge, in Sherbrooke, Canada. Detailed information about using of FRP reinforcement integrated with fiber optic sensors for concrete construction are given by Benmokrane et al. [7]. Other Fabry-Perot sensors were installed between two plies of carbon fiber sheet for the reinforcement of an overpass in Singapore. 5.2 Fabry-Perot strain sensor subjected to thermal strain Two advantages of the Fabry-Perot sensors are their insensitivity to thermal variations and to transverse strains. Thirty strain sensors, as illustrated in Figure 1, were placed in a temperature controlled oven and subjected to thermal variations from 20 to 80oC in increments of 15oC. Figure 8 shows the temperature test for three sensors. The readings are stable with temperature variations. The average sensitivity for the 30 sensors are very small (i.e. -0.1 microstrain/oC) and this is due to the slight mismatch in the thermal expansion coefficients of the optical fiber and the capillary of glass. This value is negligible when compared with the coefficient of expansion of steel (11.5 microstrain/oC) and aluminium (23 microstrain/oC). 6500 FOS #1
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Figure 7: Fiber optic sensors embedded in composite materials
Figure 8: Temperature test with Fabry-Perot strain sensors
The thermal insensitivity of the strain sensor opens an interesting possibility for strain sensing with embedded and surfacewelded strain gages of the next section. Temperature compensated and non-compensated strain gages can be manufactured, allowing to read strain due to both thermal and mechanical effects in a structure, or solely strain due to mechanical effects (Choquet et al. [1] ).
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6. EMBEDDED STRAIN GAGE FOR CONCRETE 6.1 Embedded strain gage subjected to mechanical strain The embedability of conventional optical fibers with standard polymeric coatings in concrete structures is often poor because of their fragility. The process of concrete mixture causes many agitations and presence of aggregates exerts severe stress on fiber optic sensors. Then they must be properly protected if long term monitoring is expected. For this reason, the FabryPerot strain sensor of Figure 1 is mounted inside a stainless steel envelope ensuring its protection (Figure 9). This more rugged sensor becomes an Embedment Fiber Optic strain gage (EFO) that can be used for structural and geotechnical monitoring. It is about 75 mm long and is provided with two end flanges for better adherence to concrete. The embeddable fiber optic sensor illustrated in Figure 9 was tested in compression in a load frame. Figure 10 shows that the sensitivity of the sensor is the same during the loading and the unloading path and it is free of hysteresis. The non-linearity error was under 0.5% full scale for each path test with a resolution of 0.5 microstrain. An analysis was also performed with a finite element software to verify the influence of the stiffness of the sensors in concrete. Experimental tests in concrete demonstrate the good response of the embedded strain gage to static and dynamic loading. The conclusion and details of these tests are reported by Quirion et al. [4]. Several sensors were embedded in concrete of different field projects such as the Joffre bridge, a multiple-floor parking lot, a retaining wall, an airport runway, a sea wharf, a test pavement and a telecommunication tower. 250 Loading
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Figure 9: Fiber optic Fabry-Perot strain gage for embedment in concrete (EFO)
Figure 10: Strain response of a Fabry-Perot embedment sensor (EFO) subjected to compression load.
7. SURFACE-WELDED STRAIN GAGE The fiber optic surface-welded strain gage (SFO-W) illustrated in Figure 11 is a 25 mm long spot-weldable gage consisting in a small diameter stainless steel tube welded to a 4.5 x 25 mm steel foil. This sensor can be installed easily on steel structure with a portable spot weldable machine and it does not require an experimented welder. The Fabry-Perot strain sensor of Figure 1 is mounted inside of the steel tube and strains are transferred from the structure to the small tube. The high frequency response of the Fabry-Perot fiber optic sensor allows dynamic measurements such as traffic monitoring on bridge. Many fiber optic strain gages have been used to monitor the internal strain changes within the deck of the Joffre bridge in Sherbrooke, Canada. Several sensors were embedded in reinforcement composite materials and in concrete of the bridge. Dynamic tests using calibrated heavy trucks were done in order to evaluate the stress level in FRP reinforcement, concrete deck and steel girders. Two SFO-W sensors were welded at the middle and bottom of one steel girder beam web of the bridge as illustrated in Figure 12. Dynamic measurements at 100 Hz have been taken while three heavy trucks were passing over the bridge at a constant speed and Figure 12 presents the result obtained from such tests. The higher values obtained from the bottom sensor is as expected and justify the reliability of the fiber optic instruments used in this test. The maximum strain recorded for this path was about 25 microstrains and modal excitations of the bridge induced by the transient loading event can also be seen. Further details about this field test are given by Benmokrane et Al. [7].
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Figure 11: Spot-weldable Fabry-Perot fiber optic strain gage (SFO-W)
Figure 12: Strain response versus time of three loaded trucks passing over a bridge recorded by a Fabry-Perot fiber optic strain gage.
8. PRESSURE SENSOR AND PIEZOMETER The EFPI design for the fiber optic pressure sensor (FOP), from which piezometers can readily be built, is based on a noncontact measurement of the deflection of a stainless steel diaphragm, as opposed to more conventional measurement of diaphragm deformation. When the gage is under pressure, there is a variation of the Fabry-Perot cavity length made by the inner surfaces of the stainless steel diaphragm on one side and the tip of an optical fiber on the other side as illustrated in Figure 13. The geometry and material of the transducer are selected in order to obtain a linear relationship between the deflection of the diaphragm and the applied pressure. The pressure transducer comes in two different versions: gage or absolute. Filter
Stainless steel housing
Diaphragm
Optical fiber Fabry-Perot cavity length
Stainless Feed-through steel connector diaphragm Fiber optic cable
Figure 13: Operating principle of a Fabry-Perot fiber optic pressure sensor
Figure 14: Fabry-Perot fiber optic pressure sensor (FOP)
Many ranges from 50 to 70 000 kPa were tested in laboratory. Mechanical robustness of the sensor is ensured by all welded stainless steel construction and there is no use of epoxy, sealing rubber, or other kind of polymeric materials. Figure 14 illustrates the fiber optic pressure sensor and a calibration curve is presented in Figure 15. This figure presents the applied pressure on the diaphragm versus the fiber optic pressure sensor reading. The maximum non-linearity error of the sensor tested is 0.04% full scale and is also presented on Figure 15. The same sensor was also tested in the field in a water column of 13 meters. Figure 16 presents results obtained from this test and a perfect correlation (R2 = 0.9999) can be seen between the fiber optic pressure sensor reading and the head of water. Similar results were obtained with others sensors tested.
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Figure 15: Calibration of a fiber optic pressure sensor
Figure 16: Fiber optic sensor tested in a water column.
9. DISPLACEMENT TRANSDUCER The EFPI design for an absolute fiber optic displacement sensor (FOD) is based on a spatially-distributed Fabry-Perot interferometer. A detailed explanation of the working principle can be found in Duplain et al. [3]. Figure 17 shows a calibration curve obtained with a fiber optic displacement transducer using a digital micrometer table as reference. The result shows an excellent agreement (R2=1.0000) between the reference reading and the response of the fiber optic transducer. The maximum non-linearity error observed is lower than 0.1% full scale. The resolution of the sensor is 0.002 mm. This sensor illustrated in Figure 18 compares favorably in performance with more conventional transducers like LVDTs. 1.50
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Figure 17: Calibration of a displacement fiber optic transducers.
Figure 18: Fiber optic displacement transducer (FOD).
10. TEMPERATURE SENSOR 10.1 Temperature sensor tested in laboratory The EFPI design for a fiber optic temperature sensor (FOT) consists in a Fabry-Perot sensor similar to Figure 1 housed in a rigid protective housing. Different configurations are available depending on the application, the time response, the temperature range and the level of accuracy. Figure 19 presents a calibration curve of a temperature gage between -40 and 125oC. The accuracy is better than 0.1% over the full calibrated range. The result of Figure 19 shows a perfect correlation (R2=1.0000) between the reference thermometer and the fiber optic temperature sensor.
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10.2 Temperature sensor tested in field application
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Two fiber optic temperature sensors were embedded in concrete of the Joffre Bridge in Sherbrooke, Canada. Figure 20 shows the variation of the temperature during 50 minutes with a reading acquisition every minute. The difference in readings from the two sensors can be explained by the fact that one sensor is embedded in the middle of concrete and the other one is nearer of the surface. This temperature will be used as reference to measure real strain in the structure after the fiber optic strain gage will be corrected in temperature to eliminate thermal strain due to the coefficient of expansion of concrete. These sensors have been installed in concrete for two years and no problem has been encountered.
75 50 25 y = 0.9997x - 0.0245
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Figure 19: Calibration of a temperature fiber optic sensor.
Figure 20: Response of a temperature fiber optic sensor in a concrete of Joffre Bridge.
11. CONCLUSION The characteristics and the behaviour of different kinds of fiber optic sensors based on the Fabry-Perot interferometers manufactured by Roctest and Fiso Technologies have been presented in this paper. Sensors available cover a range of requirements for structural and geotechnical monitoring in general. They include strain gages, pressure sensors, displacement transducers and temperature sensors. The characteristics of this family of instruments, together with the calibration results, show that they can be used for monitoring safety and performance of large structures such as dams, tunnel, bridges. Their intrinsic immunity to lightning strikes should also be considered in the evaluation of the potential of this new technology. The sensors were subjected to mechanical and thermal strains in laboratory and in field. Comparative measurements with conventional electrical strain gage were in good agreement with the Fabry-Perot sensor. The insensitivity of the Fabry-Perot sensor to thermal variations has been presented. The sensors show no hysteresis and they are very repeatable. The results indicate that these sensors accurately measure strain and they are suitable for static and dynamic measurements with a resolution of 0.5 microstrain and a non-linearity error better than 0.5% full scale. The pressure, displacement and temperature sensors tested present a non-linearity error better than 0.1% full scale. The universal readout units are able to achieve a frequency response between 10 and 1000 Hz according to the type of readout used. Single channel and multi-channels dataloggers have been developed and field tested with satisfactory results.
REFERENCES 1. 2. 3.
P. Choquet, R. Leroux, F. Juneau, “New Fabry-Perot Fiber Optic Sensors for Structural and Geotechnical Monitoring Applications”, Transportation Research Record, no 1596, pp.39-44, 1997. C. Belleville, G. Duplain, “White-light interferometric multimode fiber-optic strain sensor”, Optics Letters, 18(1) pp.7880, 1993. G. Duplain, C. Belleville , S. Bussière, P.A. Bélanger, “Absolute Fiber-Optic Linear Position and Displacement Sensor”, 12th International Conference on Optical Fiber Sensor, Williamsburg, VA, 1998.
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4.
5. 6. 7.
M. Quirion, G. Ballivy, P. Choquet, D. Nguyen, “Behaviour of Embedded Fiber-Optic Strain Gauge in concrete: Experimental and Numerical Simulations”, International Symposium on High Performance and Reactive Powder Concretes. Sherbrooke, 1998. C.M. Lawrence, D.V. Nelson, J.R. Spingarn, T.E. Bennett, “Measurement of process-induced strains in composite materials using embedded fiber-optic sensors”, SPIE, Vol. 2718, San Diego, pp.60-68, 1996. A.L. Kalamkarov, S.B. Fitzgerald, D.O. Macdonald, “On the processing and evaluation of smart composite reinforcement”, SPIE, Vol. 3241, pp.338-346, 1997. B. Benmokrane, H. Rahman, P. Mukhopadhyaya, R. Masmoudi, M.Chekired, J.F. Nicole, “Use of FRP reinforcement integrated with fibre optic sensors for concrete bridge construction”, submitted to ACI Concrete International Journal.
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