CA1299389C - Microbend fiber optic strain gauge - Google Patents
Microbend fiber optic strain gaugeInfo
- Publication number
- CA1299389C CA1299389C CA000544687A CA544687A CA1299389C CA 1299389 C CA1299389 C CA 1299389C CA 000544687 A CA000544687 A CA 000544687A CA 544687 A CA544687 A CA 544687A CA 1299389 C CA1299389 C CA 1299389C
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- Canada
- Prior art keywords
- plates
- optical
- signal
- fiber
- strain gauge
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Fee Related
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- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 claims abstract description 6
- 229910052782 aluminium Inorganic materials 0.000 claims abstract description 6
- PCHJSUWPFVWCPO-UHFFFAOYSA-N gold Chemical compound [Au] PCHJSUWPFVWCPO-UHFFFAOYSA-N 0.000 claims abstract description 6
- 239000010931 gold Substances 0.000 claims abstract description 6
- 229910052737 gold Inorganic materials 0.000 claims abstract description 6
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- VYPSYNLAJGMNEJ-UHFFFAOYSA-N Silicium dioxide Chemical compound O=[Si]=O VYPSYNLAJGMNEJ-UHFFFAOYSA-N 0.000 claims description 7
- 238000005253 cladding Methods 0.000 claims 6
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- 229910000967 As alloy Inorganic materials 0.000 description 1
- 239000004952 Polyamide Substances 0.000 description 1
- XUIMIQQOPSSXEZ-UHFFFAOYSA-N Silicon Chemical compound [Si] XUIMIQQOPSSXEZ-UHFFFAOYSA-N 0.000 description 1
- 229910001069 Ti alloy Inorganic materials 0.000 description 1
- 238000004026 adhesive bonding Methods 0.000 description 1
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Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
- G01L1/00—Measuring force or stress, in general
- G01L1/24—Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infrared, visible light, ultraviolet
- G01L1/242—Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infrared, visible light, ultraviolet the material being an optical fibre
- G01L1/243—Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infrared, visible light, ultraviolet the material being an optical fibre using means for applying force perpendicular to the fibre axis
- G01L1/245—Measuring force or stress, in general by measuring variations of optical properties of material when it is stressed, e.g. by photoelastic stress analysis using infrared, visible light, ultraviolet the material being an optical fibre using means for applying force perpendicular to the fibre axis using microbending
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B11/00—Measuring arrangements characterised by the use of optical techniques
- G01B11/16—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge
- G01B11/18—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge using photoelastic elements
Landscapes
- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Length Measuring Devices By Optical Means (AREA)
- Optical Transform (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
A microbend strain gauge comprises a pair of plates having facing offset corrugations which clamp a signal optical fiber therebetween, The optical fiber is coated and a light signal is supplied to one end of the fiber which is read at an opposite end of the fiber by an optical sensor.
Modulations in the light are primarily due to a difference in pressure being applied to the fiber by the plates, A second optical fiber, the reference fiber, which is near the signal optical fiber, is subjected to the same thermal condition and its light signal compared to the light, signal through the signal optical fiber to offset any temperature error introduced into the signal by changes in temperature.
Aluminum, polyimide or gold coating increases temperature resistance for the fibers.
A microbend strain gauge comprises a pair of plates having facing offset corrugations which clamp a signal optical fiber therebetween, The optical fiber is coated and a light signal is supplied to one end of the fiber which is read at an opposite end of the fiber by an optical sensor.
Modulations in the light are primarily due to a difference in pressure being applied to the fiber by the plates, A second optical fiber, the reference fiber, which is near the signal optical fiber, is subjected to the same thermal condition and its light signal compared to the light, signal through the signal optical fiber to offset any temperature error introduced into the signal by changes in temperature.
Aluminum, polyimide or gold coating increases temperature resistance for the fibers.
Description
The present invention relates in general to strain gauges, and in particular to a new and useful microbend fiber optic strain gauge which utilizes a coated optical fiber held and hent between corrugated plates, and a reference optical fiber which is exposed to the same thermal condition but which is not held between the corrugated plates.
Strain gauges have been developed to measure structural loads to verify proper design of both individual components and the overall structure. Strain gauges now include foil, thin film, or wire resistance devices which are bonded or welded to the test piece to be measured. Loads applied to the test piece can cause it and the bonded gauge to extend, compress, or twist. The resulting strains induced in the gauge change its resistance. If the gauge resistor forms one leg of a Wheatstone bridge, the bridge will become unbalanced and a voltage developed in proportion to the amount of strain induced in the gauge. This approach is the basis of most strain gauge measurements performed today.
Difficulties are encountered when strain measurements are to be made at elevated temperatures. For example, differential expansion between the gauge and test piece induces strain in the gauge, using up a substantial portion of its range and masking the load-induced strain to be measured. Furthermore, for accurate and reliable measurement, resistance stroin gauges ore generally limited 3!~
to temperatures below about 315C (about 600F~. Above this temperature, physical and metallurgical effects such as alloy segregation, phase changes, selective oxidation and di~fusion result in large non-repeatable and unpredictable changes in the gauge output, and often in premature failure of the gauge or leadwire system.
Currently, no satisfactory method exists to perform accurate and reliable strain measurements at temperatures exceeding about 315C. A reliable, stable strain gauge is needed that will work at these elevated temperatures and which will match the thermal expansion of the test piece to enable the gauge to be bonded at low temperatures.
The measurement of the elongation of a structural member such as a long strut, presents several problems similar to those encountered in strain measurement. In a relatively benign environment which is free of vibration, the elongation may be slowly varying with time. This situation requires that an elongation sensor be capable of essentially D.C. measurements. As a consequence the sensor must exhibit extremely low drift.
This is further complicated when the structural member is in a hostile environment.
Instrumentation for in-flight monitoring of inlet and outlet engine conditions is needed for high-performance aircraft to improve fuel efficiency, engine performance, and overall reliability. This instrumentation must withstand the hostile engine environment which includes the high-lZ~3~
temperature operatiny conditions and vibrations Opticalfibers and optical sensing methods have been applied to a number of measurements in hostile environments including displacement, velocity, strain, flow, temperature, particle size distribution, gas composition and fluorescence. These optical sensing methods can also be used to measure pressure in the hostile environment.
Optical sensors can also be designed to operate at high temperatures and in regions of high electromagnetic fields.
The present invention is drawn to a strain gauge which utilizes a pair of corrugated plates having corrugations that face each other and which are offset with respect to each other, and includes a coated optical fiber engaged between said facing corrugated surfaces and bent by said corrugations by amounts which depend on a biasing force pushing the plates together, whereby light moving through the optical fiber is modulated depending on the amount of pressure applied to the plates.
Accordingly, this invention provides a strain gauge comprising a pair of plates having facing corrugated surfaces with the corrugations o~ one plate being offset with the corrugations of the other plate. A coated optical fiber is clamped between the corrugations of the plates for being bent to a greater and lesser extent depending on pressure exerted on the plates for moving the plates together. Optical signal applying means is connected to one end of the optical fiber for applying an optical signal to the optical fiber. An optical detector means is o3~
connected to an opposite end of the optical fiber ~or reading the optical signal and modulations in the optical signal which corresponds to pressures applied to the plates. The optical fiber comprises a signal fiber for transmitting the optical signal.
One aspect of the invention is to include, as part of the strain gauge, an additional optical fiber which is identical in construction to the first mentioned optical fiber but which is not engaged between the plates, the second optical fiber being near the first mentioned optical fiber so as to be exposed to the same temperature condition, light / :
3a ~Z~3~3t3~
passing through and being modulated by the second optical fiber being used in conjunction with the light passing through and being modulated by the first mentioned optical fiber to produce a thermo-mechanical offset correction value.
By coating a glass optical fiber with aluminu~ or polyimide, a strain gauge which is useful up to about 427C
(about 800F) is obtained. By coating the glass fiber with gold, the useful temperature range can be expanded up to about 540C. (about 1000F).
Another aspect of the present invention is to provide a strain gauge which is simple in design, rugged in construction and economical to manufacture, and one which can withstand severe environmental conditions.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure.
For a better understanding of the invention, its operating advantages and specific results attained by its uses, reference is made to the accompanying drawings and descriptive matter ln which preferred embodiments of the invention are illustrated.
In the drawinqs:
Fig. 1 is a side view in section showing the strain~gauge of t-e present invention in its si-plest form;
:
.~" :
, Fiy. 2 is a block diagram showing the strain yauge of the present invention used with a reference fiber in addition to the signal fiber;
Fig. 3 is a yraph plottiny load versus displacement for the optical fiber of the invention with two spatial bends;
Fig. 4 is a graph plottiny the strain gauge output voltage versus displacement of the plates in the strain gauge;
Fig. 5 is a graph showing calibration of the microbend strain gauge of the present invention relative to a reference gauge; and Fig. 6 is a side view in section showing the strain gauge of the present invention in a slot formed in the surface of a test piece whose strain is to be measured.
A microbend fiber optic strain gauge 1 is diagrammed in Fig. 1. A glass-on-glass optical fiber 10 is used with the following nominal characteristics:
Core diameter 125 ~m;
Clad diameter 170 ~m;
Numerical aperture 0.2;
Buffer coating 40 ~m thick aluminium or polyamide;
:
.
3~3~
and Overall diameter 250 ~m where 1 ~m = 10-6 meters.
Fibers with the mentioned coatings are strong and rugged with tensile strengths exceeding 100,000 psi. The microbend sensor is a light intensity sensor, and as such, uses simple opto-electronic components. The strain gauge comprises the above fiber 10 clamped between corrugated plates 12 and 14 made from material identical to that of a test piece. Changes in strain of the test piece change the plate separation and in turn the light intensity transmitted at the point of clamping. The corrugation spacing is about 3 mm. Two corrugations 16 are on one plate 12 and three corrugations 18 are on the opposite plate 14 to provide two spatial sinusoidal bends in the fiber 10. The fiber is preloaded (bias compression) between the plates such that the peak-to-peak fiber bend amplitude is approximately 300 ~m. In this configuration the sensitivity and repeatability of a microbend sensor has been demonstrated to be .006 ~m.
At these preloads the change in corrugated plate displacement with load is very nearly linear as shown in Fig~ 3. Also note from Fig. 4 the microbend sensor linearity of output signal versus displacement of the corrugated plates.
Performance data has been obtained on the microbend fiber optic strain gauge and is shown in Fig. 5.
The microbend strain gauge was calibrated relative to a reference gauge.
The microbend sensor plates 12 and 14 may be attached ~L29~3~7 to the test piece in several different ways. These include welding or gluing the ends 21 and 22 to th~ surface of the test piece. ~ less obtrusive method would be to slot the surface and insert the plates into the slot. Fig. 6 shows a test piece 20 with slot 26 in which plates 12 and 14 are engaged. The plates are urged toward each other by their back surfaces 23 and 24. The method of attachment will be chosen to minimize alterations in the test piece structural properties and static and dynamic response.
Accelerated dynamic life tests have been performed on the microbend sensor and have demonstrated lifetime in excess of one million cycles with peak displacements of 25 ~m. These tests were performed at 2OkHz cycling frequencies, which also demonstrated the high frequency response capability of the microbend sensor.
The microbend sensor uses inexpensive conventional opto-electronic components including a light emitting diode (LED), shown in Fig. 2 at 30, and silicon photodetector 40.
By pulsing the LED and using CMOS integrated circuits to detect and amplify the photodetector signal, an average electronic power drain of less than 12 milliwatts per sensor has been demonstrated.
As described previously and shown in Fig. 1, the microbend sensor may be preloaded by bias displacement of the plates so that the corrugations 16, 18 overlap by an amount greater than or equal to the fiber diameter or maximum expected elongation. When the plates are heated, the corrugation peak separation with temperature may be lZg~3~9 -- 8 ~
calculated. It is also straight~orward to show that for each plate the change in peak-to-peak corrugation spacing with temperature has a negligible effect on the sensor output signal. It is anticipated in practice that the microbend corrugated plates can be properly aligned so that the corrugation peaks are within ~ 13 ~m of the desired preloaded displacement. In this case, the worst thermally induced elongation (~L) T caused by positioning error is given by:
(~L) T = L ~ ~T
Substituting for ~ T the required thermal operating range of 400C, for ~ a value of 8.5 x 10-6 /C for a typical titanium alloy, and for L the position error of 13 ~m, the thermally induced elongation error is:
(~L) = (13)(8.5 x 106) (400) = 0.04 ~m.
Thus, for a gauge length of lcm, the resulting thermally induced error is (4 ~) strain, where 1 ~ strain = 1 ~m/m.
In addition to compensation of the thermo-mechanical offset just described, changes in optical fiber light transmission must be compensated as well as changes in light source intensity and drift of photodetector output sensitivity. The invention has successfully compensated these changes using the approach diagrammed in Fig. 2. As shown in Fig. 2, second opticaI fiber 11 (reference fiber) is co-located with the signal optical fiber 10 clamped between the corrugated plates (not shown in Fig. 2). The reference optical fiber 11 is unclamped, but sees the same thermal environment along its length as the signal fiber.
~LZ~93~
g environment along its length as the signal fiber.
A major advantage of the subject invention is that the microbend strain gauge allows the matching of the thermal expansion coefficient of the plate 12 and 14 with that of the substrate material to be tested. This cannot be done with conventional strain gauges, such as resistance strain gauges, and has the effect of 1) improving the range at temperature and 2) reducing the thermal output of the gauge.
The test data shown in Figures 3-5 were obtained using stainless steel plates. In general, the plate material would be chosen to match the thermal expansion coefficient of the underlying material. As an alternative, if the predominant strain direction is known, the thermal expansion coefficients of the plates and substrate can be initially mismatched, i.e~, biased against one another so as to increase the range of the strain gauge while maintaining the same sensitivity.
Plates 12 and 14 can also be made of fused silica or other similar ceramics to increase resistance to thermal effects such as thermal degrading of the plates and the thermal expansion and contraction effect.
The advantages of the microbend fiber optic gauge of the invention are listed as follows:
Operating temperatures above 427 C (800 F);
Lightweight, compact and non-obtrusive, especially if the structural member is slotted to accept the corrugated 3~9 microbend sensor platesi Accuracy of 0.005 ~m at frequencies from D.C. to 20kHz;
The microbend sensor may be mechanically and electronically compensated with temperature, and electronics signal processing may be used to eliminate drift;
Compatible with composite and mètallic materials, this requirement being met by making the corrugated microbend sensor plates from material identical to the strut material or test piece;
Immune to electromagnetic interference and electromagnetic pulse;
Since the sensor uses non-polarized light energy to operate, spark hazards are non-existent, and remote mounted sensors are locatable in explosion hazard environments; and Inert glass optical fiber material is r0sistant to corrosion.
To increase the useful range of the present invention up to about 540~C (about 1,000 F), a gold coated sio2 optical fiber can be utiliæed in place of the aluminum or polyimide coated glass fiber. Both signal fiber 10 and reference fiber 1~ can be constructed in this way. A strain gauge according to the invention and having this temperature resistance can-be useful for long-term measurements of creep strains on reheat or main steam lines in boilers.
Field installation of such gauges would be by capacitive discharge spot welding, thus requiring only local descaling and grinding for surface preparation. Insulation ~ ,`j..
~z~
which is normally used over pipes to be out~itted with the inventive strain gauge need only be removed in the i~mediate area of the gauge. A plug of insulation which is, for example, two to three inches in diameter, could be removed, the gauge installed, and the plug replaced. The optical fiber leads would be brought ouk through the insulation at the plug for connection to extension fibers and stra'in readout equipment.
Returning now to Fig. 2, both signal and reference fibers 10 and 11 are connected through known optical splices 42 to a fiber optic coupler 44.
The light output from LED 30 is split into two parts by the three dB coupler 44, and the now split output is coupled through splicers 42 to the signal fiber 10 and the reference fiber 11. These multimode optical fibers then supply their output signals to the dual photodetector 40 and its associated output circuitry 46. Signals A and B are digitized and converted in converter circuit 46 to form values (~-B)/(A+B) for the compensated sensor signal.
While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.
:: `
`
Strain gauges have been developed to measure structural loads to verify proper design of both individual components and the overall structure. Strain gauges now include foil, thin film, or wire resistance devices which are bonded or welded to the test piece to be measured. Loads applied to the test piece can cause it and the bonded gauge to extend, compress, or twist. The resulting strains induced in the gauge change its resistance. If the gauge resistor forms one leg of a Wheatstone bridge, the bridge will become unbalanced and a voltage developed in proportion to the amount of strain induced in the gauge. This approach is the basis of most strain gauge measurements performed today.
Difficulties are encountered when strain measurements are to be made at elevated temperatures. For example, differential expansion between the gauge and test piece induces strain in the gauge, using up a substantial portion of its range and masking the load-induced strain to be measured. Furthermore, for accurate and reliable measurement, resistance stroin gauges ore generally limited 3!~
to temperatures below about 315C (about 600F~. Above this temperature, physical and metallurgical effects such as alloy segregation, phase changes, selective oxidation and di~fusion result in large non-repeatable and unpredictable changes in the gauge output, and often in premature failure of the gauge or leadwire system.
Currently, no satisfactory method exists to perform accurate and reliable strain measurements at temperatures exceeding about 315C. A reliable, stable strain gauge is needed that will work at these elevated temperatures and which will match the thermal expansion of the test piece to enable the gauge to be bonded at low temperatures.
The measurement of the elongation of a structural member such as a long strut, presents several problems similar to those encountered in strain measurement. In a relatively benign environment which is free of vibration, the elongation may be slowly varying with time. This situation requires that an elongation sensor be capable of essentially D.C. measurements. As a consequence the sensor must exhibit extremely low drift.
This is further complicated when the structural member is in a hostile environment.
Instrumentation for in-flight monitoring of inlet and outlet engine conditions is needed for high-performance aircraft to improve fuel efficiency, engine performance, and overall reliability. This instrumentation must withstand the hostile engine environment which includes the high-lZ~3~
temperature operatiny conditions and vibrations Opticalfibers and optical sensing methods have been applied to a number of measurements in hostile environments including displacement, velocity, strain, flow, temperature, particle size distribution, gas composition and fluorescence. These optical sensing methods can also be used to measure pressure in the hostile environment.
Optical sensors can also be designed to operate at high temperatures and in regions of high electromagnetic fields.
The present invention is drawn to a strain gauge which utilizes a pair of corrugated plates having corrugations that face each other and which are offset with respect to each other, and includes a coated optical fiber engaged between said facing corrugated surfaces and bent by said corrugations by amounts which depend on a biasing force pushing the plates together, whereby light moving through the optical fiber is modulated depending on the amount of pressure applied to the plates.
Accordingly, this invention provides a strain gauge comprising a pair of plates having facing corrugated surfaces with the corrugations o~ one plate being offset with the corrugations of the other plate. A coated optical fiber is clamped between the corrugations of the plates for being bent to a greater and lesser extent depending on pressure exerted on the plates for moving the plates together. Optical signal applying means is connected to one end of the optical fiber for applying an optical signal to the optical fiber. An optical detector means is o3~
connected to an opposite end of the optical fiber ~or reading the optical signal and modulations in the optical signal which corresponds to pressures applied to the plates. The optical fiber comprises a signal fiber for transmitting the optical signal.
One aspect of the invention is to include, as part of the strain gauge, an additional optical fiber which is identical in construction to the first mentioned optical fiber but which is not engaged between the plates, the second optical fiber being near the first mentioned optical fiber so as to be exposed to the same temperature condition, light / :
3a ~Z~3~3t3~
passing through and being modulated by the second optical fiber being used in conjunction with the light passing through and being modulated by the first mentioned optical fiber to produce a thermo-mechanical offset correction value.
By coating a glass optical fiber with aluminu~ or polyimide, a strain gauge which is useful up to about 427C
(about 800F) is obtained. By coating the glass fiber with gold, the useful temperature range can be expanded up to about 540C. (about 1000F).
Another aspect of the present invention is to provide a strain gauge which is simple in design, rugged in construction and economical to manufacture, and one which can withstand severe environmental conditions.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure.
For a better understanding of the invention, its operating advantages and specific results attained by its uses, reference is made to the accompanying drawings and descriptive matter ln which preferred embodiments of the invention are illustrated.
In the drawinqs:
Fig. 1 is a side view in section showing the strain~gauge of t-e present invention in its si-plest form;
:
.~" :
, Fiy. 2 is a block diagram showing the strain yauge of the present invention used with a reference fiber in addition to the signal fiber;
Fig. 3 is a yraph plottiny load versus displacement for the optical fiber of the invention with two spatial bends;
Fig. 4 is a graph plottiny the strain gauge output voltage versus displacement of the plates in the strain gauge;
Fig. 5 is a graph showing calibration of the microbend strain gauge of the present invention relative to a reference gauge; and Fig. 6 is a side view in section showing the strain gauge of the present invention in a slot formed in the surface of a test piece whose strain is to be measured.
A microbend fiber optic strain gauge 1 is diagrammed in Fig. 1. A glass-on-glass optical fiber 10 is used with the following nominal characteristics:
Core diameter 125 ~m;
Clad diameter 170 ~m;
Numerical aperture 0.2;
Buffer coating 40 ~m thick aluminium or polyamide;
:
.
3~3~
and Overall diameter 250 ~m where 1 ~m = 10-6 meters.
Fibers with the mentioned coatings are strong and rugged with tensile strengths exceeding 100,000 psi. The microbend sensor is a light intensity sensor, and as such, uses simple opto-electronic components. The strain gauge comprises the above fiber 10 clamped between corrugated plates 12 and 14 made from material identical to that of a test piece. Changes in strain of the test piece change the plate separation and in turn the light intensity transmitted at the point of clamping. The corrugation spacing is about 3 mm. Two corrugations 16 are on one plate 12 and three corrugations 18 are on the opposite plate 14 to provide two spatial sinusoidal bends in the fiber 10. The fiber is preloaded (bias compression) between the plates such that the peak-to-peak fiber bend amplitude is approximately 300 ~m. In this configuration the sensitivity and repeatability of a microbend sensor has been demonstrated to be .006 ~m.
At these preloads the change in corrugated plate displacement with load is very nearly linear as shown in Fig~ 3. Also note from Fig. 4 the microbend sensor linearity of output signal versus displacement of the corrugated plates.
Performance data has been obtained on the microbend fiber optic strain gauge and is shown in Fig. 5.
The microbend strain gauge was calibrated relative to a reference gauge.
The microbend sensor plates 12 and 14 may be attached ~L29~3~7 to the test piece in several different ways. These include welding or gluing the ends 21 and 22 to th~ surface of the test piece. ~ less obtrusive method would be to slot the surface and insert the plates into the slot. Fig. 6 shows a test piece 20 with slot 26 in which plates 12 and 14 are engaged. The plates are urged toward each other by their back surfaces 23 and 24. The method of attachment will be chosen to minimize alterations in the test piece structural properties and static and dynamic response.
Accelerated dynamic life tests have been performed on the microbend sensor and have demonstrated lifetime in excess of one million cycles with peak displacements of 25 ~m. These tests were performed at 2OkHz cycling frequencies, which also demonstrated the high frequency response capability of the microbend sensor.
The microbend sensor uses inexpensive conventional opto-electronic components including a light emitting diode (LED), shown in Fig. 2 at 30, and silicon photodetector 40.
By pulsing the LED and using CMOS integrated circuits to detect and amplify the photodetector signal, an average electronic power drain of less than 12 milliwatts per sensor has been demonstrated.
As described previously and shown in Fig. 1, the microbend sensor may be preloaded by bias displacement of the plates so that the corrugations 16, 18 overlap by an amount greater than or equal to the fiber diameter or maximum expected elongation. When the plates are heated, the corrugation peak separation with temperature may be lZg~3~9 -- 8 ~
calculated. It is also straight~orward to show that for each plate the change in peak-to-peak corrugation spacing with temperature has a negligible effect on the sensor output signal. It is anticipated in practice that the microbend corrugated plates can be properly aligned so that the corrugation peaks are within ~ 13 ~m of the desired preloaded displacement. In this case, the worst thermally induced elongation (~L) T caused by positioning error is given by:
(~L) T = L ~ ~T
Substituting for ~ T the required thermal operating range of 400C, for ~ a value of 8.5 x 10-6 /C for a typical titanium alloy, and for L the position error of 13 ~m, the thermally induced elongation error is:
(~L) = (13)(8.5 x 106) (400) = 0.04 ~m.
Thus, for a gauge length of lcm, the resulting thermally induced error is (4 ~) strain, where 1 ~ strain = 1 ~m/m.
In addition to compensation of the thermo-mechanical offset just described, changes in optical fiber light transmission must be compensated as well as changes in light source intensity and drift of photodetector output sensitivity. The invention has successfully compensated these changes using the approach diagrammed in Fig. 2. As shown in Fig. 2, second opticaI fiber 11 (reference fiber) is co-located with the signal optical fiber 10 clamped between the corrugated plates (not shown in Fig. 2). The reference optical fiber 11 is unclamped, but sees the same thermal environment along its length as the signal fiber.
~LZ~93~
g environment along its length as the signal fiber.
A major advantage of the subject invention is that the microbend strain gauge allows the matching of the thermal expansion coefficient of the plate 12 and 14 with that of the substrate material to be tested. This cannot be done with conventional strain gauges, such as resistance strain gauges, and has the effect of 1) improving the range at temperature and 2) reducing the thermal output of the gauge.
The test data shown in Figures 3-5 were obtained using stainless steel plates. In general, the plate material would be chosen to match the thermal expansion coefficient of the underlying material. As an alternative, if the predominant strain direction is known, the thermal expansion coefficients of the plates and substrate can be initially mismatched, i.e~, biased against one another so as to increase the range of the strain gauge while maintaining the same sensitivity.
Plates 12 and 14 can also be made of fused silica or other similar ceramics to increase resistance to thermal effects such as thermal degrading of the plates and the thermal expansion and contraction effect.
The advantages of the microbend fiber optic gauge of the invention are listed as follows:
Operating temperatures above 427 C (800 F);
Lightweight, compact and non-obtrusive, especially if the structural member is slotted to accept the corrugated 3~9 microbend sensor platesi Accuracy of 0.005 ~m at frequencies from D.C. to 20kHz;
The microbend sensor may be mechanically and electronically compensated with temperature, and electronics signal processing may be used to eliminate drift;
Compatible with composite and mètallic materials, this requirement being met by making the corrugated microbend sensor plates from material identical to the strut material or test piece;
Immune to electromagnetic interference and electromagnetic pulse;
Since the sensor uses non-polarized light energy to operate, spark hazards are non-existent, and remote mounted sensors are locatable in explosion hazard environments; and Inert glass optical fiber material is r0sistant to corrosion.
To increase the useful range of the present invention up to about 540~C (about 1,000 F), a gold coated sio2 optical fiber can be utiliæed in place of the aluminum or polyimide coated glass fiber. Both signal fiber 10 and reference fiber 1~ can be constructed in this way. A strain gauge according to the invention and having this temperature resistance can-be useful for long-term measurements of creep strains on reheat or main steam lines in boilers.
Field installation of such gauges would be by capacitive discharge spot welding, thus requiring only local descaling and grinding for surface preparation. Insulation ~ ,`j..
~z~
which is normally used over pipes to be out~itted with the inventive strain gauge need only be removed in the i~mediate area of the gauge. A plug of insulation which is, for example, two to three inches in diameter, could be removed, the gauge installed, and the plug replaced. The optical fiber leads would be brought ouk through the insulation at the plug for connection to extension fibers and stra'in readout equipment.
Returning now to Fig. 2, both signal and reference fibers 10 and 11 are connected through known optical splices 42 to a fiber optic coupler 44.
The light output from LED 30 is split into two parts by the three dB coupler 44, and the now split output is coupled through splicers 42 to the signal fiber 10 and the reference fiber 11. These multimode optical fibers then supply their output signals to the dual photodetector 40 and its associated output circuitry 46. Signals A and B are digitized and converted in converter circuit 46 to form values (~-B)/(A+B) for the compensated sensor signal.
While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.
:: `
`
Claims (12)
1. A strain gauge operable in hostile environments comprising:
a pair of plates made of material having a temperature expansion coefficient similar to that of a material to be tested in a high temperature environment and having facing and offsetting corrugated surfaces and wherein at least one plate is attached to the material to be tested;
a buffer coated first optical signal fiber clamped between the corrugations of the plates for being bent to a greater or lesser extent depending on pressure exerted on the plates for moving the plates together;
a buffer coated reference optical signal fiber located in the vicinity of the plates so as to be simultaneously exposed to the same thermal and other conditions along its length as the first optical fiber;
optical signal applying means including a light source and light splitting means connected to one end of each optical fiber for simultaneously applying an optical signal to both optical fibers; and optical detector means connected to the opposite ends of both optical fibers for measuring the modulations in the optical signal transmitted through the first optical fiber which modulations correspond to pressures applied to the plates and for reading the modulations in the optical signal transmitted through the reference optical fiber.
a pair of plates made of material having a temperature expansion coefficient similar to that of a material to be tested in a high temperature environment and having facing and offsetting corrugated surfaces and wherein at least one plate is attached to the material to be tested;
a buffer coated first optical signal fiber clamped between the corrugations of the plates for being bent to a greater or lesser extent depending on pressure exerted on the plates for moving the plates together;
a buffer coated reference optical signal fiber located in the vicinity of the plates so as to be simultaneously exposed to the same thermal and other conditions along its length as the first optical fiber;
optical signal applying means including a light source and light splitting means connected to one end of each optical fiber for simultaneously applying an optical signal to both optical fibers; and optical detector means connected to the opposite ends of both optical fibers for measuring the modulations in the optical signal transmitted through the first optical fiber which modulations correspond to pressures applied to the plates and for reading the modulations in the optical signal transmitted through the reference optical fiber.
2. A strain gauge according to claim 1, wherein said signal and reference fibers have a glass core and cladding and an aluminum coating.
3. A strain gauge according to claim 1, wherein said signal and reference fibers have a glass core and cladding and a polyimide coating.
4. A strain gauge according to claim 1, wherein said signal and reference fibers have a core of SiO2, and a coating of gold.
5. A strain gauge operable in hostile environments, comprising:
a pair of plates made of material having a temperature expansion coefficient similar to that of the material to be tested in a high temperature environment of above about 315°C
and having facing and offsetting corrugated surfaces and wherein at least one plate is attached to the material to be tested;
a buffer coated first optical signal fiber clamped between the corrugations of the plates for being bent to a greater or lesser extent depending on pressure exerted on the plates for moving the plates together;
a buffer coated reference optical signal fiber located in the vicinity of the plates so as to be simultaneously exposed to the same thermal and other conditions along its length as the first optical fiber;
optical signal applying means including a light source and light splitting means connected to one end of each optical fiber for simultaneously applying an optical signal to both optical fibers; and optical detector means connected to the opposite ends of both optical fibers for measuring the modulations in the optical signal transmitted through the first optical fiber which modulations correspond to pressure applied to the plates and for reading the modulations in the optical signal transmitted through the reference optical fiber.
a pair of plates made of material having a temperature expansion coefficient similar to that of the material to be tested in a high temperature environment of above about 315°C
and having facing and offsetting corrugated surfaces and wherein at least one plate is attached to the material to be tested;
a buffer coated first optical signal fiber clamped between the corrugations of the plates for being bent to a greater or lesser extent depending on pressure exerted on the plates for moving the plates together;
a buffer coated reference optical signal fiber located in the vicinity of the plates so as to be simultaneously exposed to the same thermal and other conditions along its length as the first optical fiber;
optical signal applying means including a light source and light splitting means connected to one end of each optical fiber for simultaneously applying an optical signal to both optical fibers; and optical detector means connected to the opposite ends of both optical fibers for measuring the modulations in the optical signal transmitted through the first optical fiber which modulations correspond to pressure applied to the plates and for reading the modulations in the optical signal transmitted through the reference optical fiber.
6. A strain gauge according to claim 5, wherein said signal and reference fibers have a glass core and cladding and an aluminum coating.
7. A strain gauge according to claim 5, wherein said signal and reference fibers have a glass core and cladding and a polyimide coating.
8. A strain gauge according to claim 5, wherein said signal and reference fibers have a core of SiO2 and a coating of gold.
9. A strain gauge operable in hostile environments comprising:
a pair of plates made of a material having a temperature expansion coefficient dissimilar to that of a material to be tested for the purpose of increasing the range of the sensor and having facing and offsetting corrugated surfaces and wherein at least one plate is attached to the material to be tested;
a buffer coated first optical signal fiber clamped between the corrugations of the plates for being bent to a greater or lessor extent depending on pressure exerted on the plates for moving the plates together;
a buffer coated reference optical signal fiber located in the vicinity of the plates so as to be simultaneously exposed to the same thermal and other conditions along its length as the first optical fiber;
optical signal applying means including a light source and light splitting means connected to one end of each optical fiber for simultaneously applying an optical signal to both optical fibers; and optical detector means connected to the opposite ends of both optical fibers for measuring the modulations in the optical signal transmitted through the first optical fiber which modulations correspond to pressures applied to the plates and for reading the modulations in the optical signal transmitted through the reference optical fiber.
a pair of plates made of a material having a temperature expansion coefficient dissimilar to that of a material to be tested for the purpose of increasing the range of the sensor and having facing and offsetting corrugated surfaces and wherein at least one plate is attached to the material to be tested;
a buffer coated first optical signal fiber clamped between the corrugations of the plates for being bent to a greater or lessor extent depending on pressure exerted on the plates for moving the plates together;
a buffer coated reference optical signal fiber located in the vicinity of the plates so as to be simultaneously exposed to the same thermal and other conditions along its length as the first optical fiber;
optical signal applying means including a light source and light splitting means connected to one end of each optical fiber for simultaneously applying an optical signal to both optical fibers; and optical detector means connected to the opposite ends of both optical fibers for measuring the modulations in the optical signal transmitted through the first optical fiber which modulations correspond to pressures applied to the plates and for reading the modulations in the optical signal transmitted through the reference optical fiber.
10. A strain gauge according to claim 9, wherein said signal and reference fibers have a glass core and cladding and an aluminum coating.
11. A strain gauge according to claim 9, wherein said signal and reference fibers have a glass core and cladding and a polyimide coating.
12. A strain gauge according to claim 9, wherein said signal and reference fibers have a core of SiO2, and a coating of gold.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US92496286A | 1986-10-30 | 1986-10-30 | |
| US924,962 | 1986-10-30 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1299389C true CA1299389C (en) | 1992-04-28 |
Family
ID=25450986
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000544687A Expired - Fee Related CA1299389C (en) | 1986-10-30 | 1987-08-17 | Microbend fiber optic strain gauge |
Country Status (6)
| Country | Link |
|---|---|
| JP (1) | JPS63117205A (en) |
| CN (1) | CN1016100B (en) |
| AU (1) | AU598858B2 (en) |
| CA (1) | CA1299389C (en) |
| GB (1) | GB2196735B (en) |
| IN (1) | IN167564B (en) |
Families Citing this family (19)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2650670A1 (en) * | 1989-08-02 | 1991-02-08 | Fiori Costantino | System for detecting a fire or any other phenomenon generating an abnormal rise or fall in temperature with respect to a fixed reference |
| US5039218A (en) * | 1990-03-28 | 1991-08-13 | Hughes Aircraft Company | Testing of optical fiber by introducing multiple simulated peel location bends |
| GB2250813A (en) * | 1990-12-15 | 1992-06-17 | British Aerospace | Weighing apparatus for vehicles |
| JP3410101B2 (en) | 1994-11-29 | 2003-05-26 | ユナイテッド テクノロジーズ コーポレイション | Coating annihilation detection by optical fiber Bragg grating |
| EP1709416B1 (en) | 2004-01-23 | 2018-03-07 | LM Wind Power International Technology II ApS | Device including a system adapted for use in temperature compensation of strain measurements in fibre-reinforced structures |
| CN1300571C (en) * | 2004-08-24 | 2007-02-14 | 西安科技大学 | Snake type fiber-optical sensor burying and detecting method and its snake type fibre-optical sensor |
| CN101881633B (en) | 2010-04-06 | 2012-11-28 | 西安金和光学科技有限公司 | Spring type high-precision optical fiber sensor based on optical fiber bending loss |
| CN102221374B (en) * | 2010-04-19 | 2015-07-15 | 西安金和光学科技有限公司 | Fiber sensing device for sensing torque parameters |
| CN102829901A (en) * | 2011-06-14 | 2012-12-19 | 西安金和光学科技有限公司 | Optical fiber sensing device waveform grooves and cylinder body |
| WO2012010099A1 (en) * | 2010-07-21 | 2012-01-26 | 西安金和光学科技有限公司 | Optic fiber cylinder sensor |
| EP2962080B1 (en) * | 2013-02-28 | 2021-05-19 | Osmos Group | Optical measurement device having a reference channel and a measurement channel, and related method |
| CN103604540B (en) * | 2013-11-13 | 2015-06-10 | 中铁四局集团第一工程有限公司 | Photoelectric stressometer |
| CN104359653B (en) * | 2014-10-23 | 2015-08-12 | 河海大学 | A kind of optical fiber single-direction and dual-direction bending curvature binary channels tracking and monitoring instrument and monitoring method |
| CN105606276B (en) * | 2015-12-24 | 2018-06-12 | 中北大学 | MEMS optical fiber micro-bending pressure sensors and preparation method thereof |
| CN106441661A (en) * | 2016-12-02 | 2017-02-22 | 西南交通大学 | Fiber grating force-measuring gasket and anchor rod force-measuring system |
| US20200012058A1 (en) * | 2017-02-02 | 2020-01-09 | Fujikura Ltd. | Photodetector and method for manufacturing photodetector |
| CN109620186A (en) * | 2019-01-30 | 2019-04-16 | 福州新易达光电科技有限公司 | A kind of optical fiber micro-bending sensor for monitoring human vital sign parameter |
| GB2586974B (en) * | 2019-09-06 | 2022-12-28 | Nuron Ltd | System for producing strain in a fibre |
| CN111707206B (en) * | 2020-06-04 | 2022-03-18 | 哈尔滨工程大学 | Quantum dot optical fiber micro-bending sensor with position detection function |
Family Cites Families (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0027540A3 (en) * | 1979-09-11 | 1981-10-07 | Hydroacoustics Inc. | Optical sensor and transducer array system |
| US4436995A (en) * | 1981-06-29 | 1984-03-13 | General Electric Company | Fiber optics transducers for sensing parameter magnitude |
| GB2125161A (en) * | 1982-07-21 | 1984-02-29 | Gen Electric Co Plc | Optical fibre sensors |
| JPS5946805A (en) * | 1982-09-11 | 1984-03-16 | Diesel Kiki Co Ltd | Displacement detector |
| IN165010B (en) * | 1986-02-03 | 1989-07-29 | Babcock & Wilcox Co | |
| AU579041B2 (en) * | 1986-05-09 | 1988-11-10 | Thomas & Betts Corporation | Method of and apparatus for fiber optic sensing |
| GB8704540D0 (en) * | 1987-02-26 | 1987-04-01 | Bicc Plc | Optical sensors |
-
1987
- 1987-08-17 CA CA000544687A patent/CA1299389C/en not_active Expired - Fee Related
- 1987-08-17 GB GB8719390A patent/GB2196735B/en not_active Expired - Fee Related
- 1987-08-17 IN IN645/CAL/87A patent/IN167564B/en unknown
- 1987-09-23 AU AU78894/87A patent/AU598858B2/en not_active Ceased
- 1987-10-29 CN CN 87107210 patent/CN1016100B/en not_active Expired
- 1987-10-29 JP JP27198387A patent/JPS63117205A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| GB8719390D0 (en) | 1987-09-23 |
| GB2196735A (en) | 1988-05-05 |
| AU598858B2 (en) | 1990-07-05 |
| CN1016100B (en) | 1992-04-01 |
| AU7889487A (en) | 1988-05-05 |
| IN167564B (en) | 1990-11-17 |
| JPS63117205A (en) | 1988-05-21 |
| CN87107210A (en) | 1988-08-03 |
| GB2196735B (en) | 1991-01-23 |
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