CN114485452B - Athermal optical fiber strain gauge - Google Patents
Athermal optical fiber strain gauge Download PDFInfo
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- CN114485452B CN114485452B CN202210146419.2A CN202210146419A CN114485452B CN 114485452 B CN114485452 B CN 114485452B CN 202210146419 A CN202210146419 A CN 202210146419A CN 114485452 B CN114485452 B CN 114485452B
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- 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
- G01B11/165—Measuring arrangements characterised by the use of optical techniques for measuring the deformation in a solid, e.g. optical strain gauge by means of a grating deformed by the object
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Abstract
The athermal fiber strain gauge of the present invention allows the second FBG to be used for temperature sensing while being easy to manufacture and assemble and very compact. The present invention can change the measuring range by changing the overall size or just the size ratio of its internal parts, maintaining the same overall length and using the same standard design for the FBG itself. Thus, performance and form factor can be made similar to commercial resistive strain gauges, with other inherent advantages of fiber optic sensors, such as resistance to electromagnetic interference and insensitivity to lead resistance variations. In addition, the FBG of the invention has short length and small maximum strain, greatly reduces the possibility of fiber breakage, improves the long-term reliability, eliminates the need of recoating the FBG as much as possible, and simplifies the manufacturing process. The standardized FBG designs of all gauges of the present invention, including their length and period, also facilitate mass production, thereby helping to reduce manufacturing costs.
Description
Technical Field
The invention relates to an optical fiber sensor, in particular to an athermal optical fiber strain gauge.
Background
Strain gauges are a ubiquitous tool in many applications, for example: weighing, stress analysis, force sensing, pressure sensing, etc. The most widespread technique for strain sensing is resistive strain gauges, which are based on printed electrode patterns on plastic film substrates, whose resistance changes when stretched. The strain gauge is inexpensive and has good performance, but is fine to install and operate. The small resistance change to be measured requires sophisticated electronics and the measurement is sensitive to disturbances in the wires leading to the gauge and resistance changes. They are also temperature sensitive and many techniques can be used to reduce this dependence.
Strain gauges using optical fibers have also been in existence for many years. The main technology is Fiber Bragg Grating (FBG), which is a small length of optical fiber with longitudinal periodic perturbations of refractive index printed on it, that can reflect light within a narrow bandwidth around a specific wavelength (Bragg wavelength). Thus, stretching or compressing the fiber cross-section containing the FBG affects the period and causes a shift in the bragg wavelength, which can be detected by various methods.
One feature of fiber grating sensors is that wavelength division multiplexing can measure multiple sensors along one fiber by having each sensor with a different Bragg wavelength. So far, most applications of FBG strain sensors use this approach.
On the other hand, it is not uncommon to use a single FBG strain gauge (a single wire connected to the interrogator) as a resistive strain gauge. Due to the cost of the measuring device being too high (only for a large number of sensors) and the high cost of the sensors themselves. However, an optical strain gauge having the same performance as a resistive strain gauge would have significant advantages because of its immunity to interference and insensitivity to electrical and electromagnetic interference.
But FBG-based strain measurement has some problems in addition to cost. Typical resistive strain gauges range from 2000 to 5000 mu epsilon. The 5000. Mu.. Epsilon. Strain causes a 6048pm shift in the Bragg wavelength and a 0.4% elongation of the fiber. In addition, a typical operating temperature range of-20 to 50 ℃ will also result in a wavelength shift of about 700 pm. This means that for a wavelength division multiplexing scheme, any sensor should be allocated a spectral bandwidth of at least 7nm to avoid overlapping with sensors of other wavelengths. Since a typical light source used in commercial interrogators is in the range of 40-50nm, the number of sensors is limited to around 7. If a second set of FBGs is used to measure temperature, this is sometimes done to compensate for temperature because of the temperature dependence of the strain sensor. They will also occupy valuable bandwidth and further limit the number of sensors of an instrument, which in turn increases the cost of measurement per sensor.
Second, care must be taken to ensure that the optical fiber has sufficient mechanical reliability to maintain a draw ratio of 0.4% or higher over a longer period of time. The writing process of FBGs typically weakens the fiber, and the steps or stripping after FBG writing and recoating the fiber can complicate the manufacturing process. There are some techniques to write FBGs through the fiber coating, but the bonded coated fiber slips more easily over time than the uncoated fiber, which can affect the measurement. The probability of fiber failure is also proportional to the length of the fiber being stretched, which is typically about 10-20mm for FBG strain sensors.
Another problem with FBG strain sensors is their temperature dependence. Many schemes for temperature compensation have been proposed. FBGs can be fixed to mounts on which the optical fiber is connected to two different materials having different coefficients of thermal expansion, the materials being expanded in opposite directions so that an increase in temperature results in a weakening of the tension applied to the fiber, the tension applied to the fiber by the thermo-optic effect eliminating wavelength shifts due to expansion of the fiber and its refractive index changes, such devices being disclosed in U.S. patent No. 5,042,898. A similar approach using aluminum and invar is proposed in us patent 7116846. A geometric device for a pressure sensor is also proposed in us patent 790,072. Patent WO2009128040 describes a similar athermal sensor with a mechanism to amplify the strain response. European patent 2295946 describes a athermal strain sensor with a similar cantilever Duan Buzhi using a single cantilever section made of a different material than the support material of the strain measured. All of these techniques use a combination of materials with high and low coefficients of thermal expansion to eliminate temperature induced drift. Another major technique proposed is to use a second strain-independent FBG to measure temperature, as proposed in us patent 8,282,276 and 6,668,105.
None of these patents address the problems associated with large strain measurement ranges (e.g., 5000 μ epsilon). Patent WO2009128040 actually makes the problem worse by amplifying strain sensitivity. The reason is that most commercial FBG sensor interrogators have a limited fixed resolution, on the order of 1pm. But a typical resolution of a typical resistive strain gauge is 0.01% of its measurement range, with a corresponding strain of 0.5 mu epsilon and a wavelength shift of 0.41pm for a range of 5000 mu epsilon. Thus, even at a maximum range of 5000 μs, commercial interrogators cannot achieve a resolution that matches typical resistance strain gauges. For smaller ranges (e.g. 2000 mu epsilon) the problem is even more serious, as they require a resolution of 0.16 pm.
The prior art invention (us patent 9810556) describes a different method of measuring FBG sensors. Using this approach, the light source does not scan the wavelength spectrum, but instead emits a pair of fixed wavelengths that are spaced apart by a fraction of the FBG spectral bandwidth. This technique is recently referred to as "dual wavelength differential detection" (DWDD), which we will use here. Using DWDD technology, the measurement range is limited by the FBG bandwidth. Thus, in that case, a larger bandwidth is preferred, while for the spectral scanning measurement method, a smaller FBG bandwidth is preferred due to its improved resolution. But the spectral bandwidth of the FBG is inversely proportional to the bandwidth, so in this case a shorter FBG can be used. Empirically, the bandwidth of a 1mm long FBG is about 1000pm. FBG sensing systems typically use FBGs of 10mm length with a bandwidth of about 100pm.
DWDD technology uses time division multiplexing instead of wavelength division multiplexing to interrogate multiple identical sensors simultaneously. The sensor may be along a single optical fiber, but may also be in multiple parallel optical fibers, with similar performance in both schemes. The cost of an interrogator using DWDD can be very low because the light source is a single inexpensive commercial laser diode and the circuitry uses widely available electronic components for signal digitizing and processing. Furthermore, as discussed in Ouellette et al in 2020, the resolution may be less than 0.01% of the measurement range, ultimately limited to around 0.02 pm. Thus, even a resolution of 0.1pm, similar to a resistive strain gauge, a 1000pm range corresponding to 827 μs can provide a resolution of 0.01%. On the other hand, using DWDD technology, FBG sensors with a wider spectrum are used, resulting in a larger measurement range and a gradual decrease in resolution. Thus, a range of about 1000pm appears to be a good compromise, maximizing the range to resolution ratio and achieving the 0.01% resolution typical of resistive strain gauges. In this case, the required 1mm long FBG is attractive because the shorter length makes the meter mount more compact.
However, a range of 1000pm only resulted in a strain range of 827 mu epsilon. In addition, the temperature induced displacement also requires a range of 700pm, so the range available for strain sensing is only 127 μ. In order to take advantage of the high resolution of DWDD instruments, there is therefore a need for a strain gauge design that has two functions: (1) Subjecting the FBG to only a portion of the strain measured by the entire strain gauge; (2) Temperature compensation is performed to eliminate the measurement range caused by the temperature-induced wavelength shift. Since the temperature compensation is rarely 100%, it is also a desirable feature to allow the second FBG to take a temperature measurement.
The prior art has defects and needs to be solved urgently.
Disclosure of Invention
In order to achieve the above purpose, the present invention adopts the following technical scheme.
The invention discloses an athermal optical fiber strain gauge, comprising:
two end parts are respectively provided with an attachment point for being fixed on a measured material;
the detection part is positioned between the two end parts, and the height of the detection part is smaller than that of the two end parts;
the detection part is sequentially provided with a first part and a second part;
the first part is provided with a passing part;
The second part is provided with a cantilever beam section, and the size of the cantilever beam section is smaller than that of the passing part;
The detection part is also provided with an optical fiber Bragg grating sensor and a second sensor for measuring strain;
The fiber Bragg grating sensor for measuring strain and the second sensor are positioned between the two attachment points;
the fiber Bragg grating sensor for measuring strain and the second sensor are positioned on two sides of the cantilever beam section.
In this scheme, include:
The second sensor is a fiber Bragg grating.
In this scheme, include:
A third portion is also provided, the second portion being disposed between the first portion and the third portion.
In this scheme, include:
the third portion comprises one or more components, and the cross-sectional area of the third portion is smaller than the cross-sectional areas of the first portion and the second portion.
In this scheme, include:
the end material is the same material as the material to be tested or a material that is stiffer than the material to be tested.
In this scheme, include:
The material of the cantilever beam section has a higher thermal expansion coefficient than the material of the first portion
In this scheme, include:
the first portion is of the same material as the two end portions.
In this scheme, include:
also included is a seal having a structure for passing the optical fiber therethrough.
In this scheme, include:
The gap between the two end parts is filled with waterproof sealant.
In this scheme, include:
The cross-sectional areas of the first portion and the second portion are determined according to preset calculation rules.
The athermal fiber strain gauge of the present invention allows the second FBG to be used for temperature sensing while being easy to manufacture and assemble and very compact. The present invention can change the measuring range by changing the overall size or just the size ratio of its internal parts, maintaining the same overall length and using the same standard design for the FBG itself. Thus, performance and form factor can be made similar to commercial resistive strain gauges, with other inherent advantages of fiber optic sensors, such as resistance to electromagnetic interference and insensitivity to lead resistance variations. In addition, the FBG of the invention has short length and small maximum strain, greatly reduces the possibility of fiber breakage, improves the long-term reliability, eliminates the need of recoating the FBG as much as possible, and simplifies the manufacturing process. The standardized FBG designs of all gauges of the present invention, including their length and period, also facilitate mass production, thereby helping to reduce manufacturing costs.
Drawings
FIG. 1 illustrates a general schematic of a structural strain relief;
FIG. 2 shows a general schematic diagram of temperature compensation using two materials with different coefficients of thermal expansion;
FIG. 3 shows a block diagram of an athermal fiber optic strain gauge of the present invention;
FIG. 4 shows an arrangement of the present invention with multiple sensors along one fiber;
FIG. 5 shows a block diagram of another embodiment of an athermal fiber optic strain gauge of the present invention;
Numbering in the figures: the device comprises an end part 1, an end part 2, an attachment point 3, a measured material 4, a first part 5, a first part center 6, a second part 7, a cantilever beam section 8, a third part 9, a through hole 10, a cantilever beam section through hole 11, a fiber Bragg grating sensor 12, a second sensor 13 and an optical fiber 14.
Detailed Description
The following description of the embodiments of the present invention will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments of the present invention, but not all embodiments.
In the description of the present invention, it should be understood that the terms "upper," "lower," "front," "rear," "left," "right," "top," "bottom," "inner," "outer," and the like indicate or are based on the orientation or positional relationship shown in the drawings, merely to facilitate description of the present invention and to simplify the description, and do not indicate or imply that the devices or elements referred to must have a specific orientation, be configured and operated in a specific orientation, and thus should not be construed as limiting the present invention.
As shown in fig. 1, to maintain the desired strain range of the fiber bragg grating sensor while providing a greater strain range throughout the strain gauge, the sensor assembly is constructed such that the strain distribution is uneven in different parts of the assembly and the FBG sensor is mounted to the less strained parts. This is possible because the strain of a material caused by an applied longitudinal force is a function of its young's modulus and cross section according to well known formulas:
Where F is force, Y is Young's modulus, and A is cross section. Thus, if a given force is applied to both ends of a structure that includes different portions having different cross sections and different Young's moduli, the strain affecting each cross section will be different. Furthermore, the elongation δl of each portion depends on its length, according to the definition of strain ε=δL/L. If the structure is attached to the material such that the total elongation of the structure is constrained to be equal to the elongation of the material itself, the different sections will have different elongations, the sum of the elongations of all parts L T being equal to the elongation of the material to which it is attached. The elongation of a section relative to the total elongation can be found by considering the same stress along the structure. The strain seen by the section can be obtained by dividing the elongation by the length of the section itself. The ratio of this strain to the strain seen throughout the structure. In the case of fig. 1, for both materials, the ratio of strain seen by section 1 made of material of young's modulus Y 1, cross section a 1 and length L 1 to total strain is:
In particular, if Y 2 and a 2 are less than Y 1 and a 1, and L 2 is greater than L 1, the ratio may be less than 1. This can be intuitively understood by the example of elasticity on a block of rigid material fixed on the other side. The pull-out elasticity stretches it while the block itself is hardly stretched. The use of a structure similar to that of fig. 1 is the first basic module for making a strain gauge with strain reduction and temperature compensation functions.
The second component is to incorporate temperature compensation. As shown in fig. 2, where one end of the FBG is connected to a material having a lower coefficient of thermal expansion and the other end is connected to one end made of a material having a higher coefficient of thermal expansion, the cantilever section is connected to the first material at the other end. When the temperature change is ΔT, the cantilever section expands to the left because it is free to move in that direction, while the overall expansion of the support material moves the right end of the cantilever section an equal amount to the right. Thermal expansion of the support material. The difference between these two different expansions in opposite directions results in a decrease in tension on the fiber, which can result in a shift in the Bragg wavelength that is equal to and opposite to the shift typically caused by temperature changes. The equation for the ratio of the length of cantilever beam Duan Jiemian L B to the length L 1 can be derived to achieve perfect temperature compensation:
Where α f is the coefficient of thermal expansion of the fiber and β ε is the stress optical coefficient of the fiber. The coefficients of thermal expansion of the support material and cantilever beam Duan Cailiao are α M and α B, respectively.
Fig. 3 shows a block diagram of an athermal fiber optic strain gauge of the present invention.
As shown in fig. 3, the present invention discloses an athermal optical fiber strain gauge, comprising:
Two end parts 1 and 2 are respectively provided with an attachment point 3 for being fixed on a measured material 4;
a detection part located between the two end parts 1 and 2, the height of the detection part being smaller than the height of the two end parts;
the detection part is sequentially provided with a first part 5 and a second part 7;
the first part is provided with a passing part;
the second part is provided with a cantilever beam section 8, and the size of the cantilever beam section is smaller than that of the passing part;
The detection part is also provided with a fiber Bragg grating sensor 12 and a second sensor 13 for measuring strain;
The fiber Bragg grating sensor for measuring strain and the second sensor are positioned between the two attachment points;
the fiber Bragg grating sensor for measuring strain and the second sensor are positioned on two sides of the cantilever beam section.
The two ends (1) and (2) have a length L e, a width W e, and a height H e. The end portion acts as an attachment point (3) to the surface of the material M (4) to which the strain is being measured and should be glued or attached in any other way (e.g. soldered, welded or screwed). The central part of the assembly is not in contact with the surface of the material M and is therefore constrained from expanding or compressing by the ends (1) and (2). The end portion is preferably made of the same material as the material M or another material having similar or greater rigidity, that is, the end portion material is made of the same material as the material to be measured or of a material having greater rigidity than the material to be measured. Thus, with strain applied, the total elongation δl M of the central portion of the total length L M is the same as the elongation of the material M over the same length, the measured strain being δl M/LM. The first part (5) has a U-shape and is also cut off with space in the centre (6) of the first part to allow the fibre formed by the fibre-optic grating sensor to expand or compress freely over its length, so that compression or expansion will only be limited by the attachment points of the fibres on both sides.
It should be noted that the height H of all other portions intermediate the two ends is slightly less than H e to ensure that its movement is not constrained or limited by friction with the underlying material M, i.e. the portion intermediate the two ends is less than the height of the ends. The detection part is further provided with a fiber bragg grating sensor 12 for measuring strain and a second sensor 13, and the second sensor 13 may be a fiber bragg grating sensor. The fiber Bragg grating sensor for measuring strain and the second sensor are positioned on two sides of the cantilever beam section.
The detecting part is positioned between the two end parts, and the height of the detecting part is smaller than that of the two end parts; the detection part is provided with a first part and a second part in sequence. Wherein a through portion is provided in the first portion, said through portion allowing the cantilever beam section 8 and the fiber bragg grating sensor to pass through and be accessible by means of embedding or the like, the access portion of which may be in a gap with the first portion. The cantilever beam Duan Bufen serves as a temperature compensation device. The material of the cantilever beam section has a higher coefficient of thermal expansion than the material of the first portion. The first portion is of the same material as the two end portions.
According to an embodiment of the invention, the second sensor (13) is a fiber bragg grating sensor.
According to an embodiment of the invention, a third portion 9 is also provided, said second portion being arranged between the first portion and the third portion 9. The third portion 9 comprises one or more components, the cross-sectional area of which is smaller than the cross-sectional areas of the first and second portions.
It should be noted that the second portion (7) may be made of the same material as the first portion (5) or of the same or different material as the third portion (9), mainly serving as a structural support for the cantilever beam segments (8) extending to the left in the open space of the U-shaped cross-section (5), and thus having a width and a length slightly smaller than the opening. The material of the cross section of the cantilever beam section (8) has a higher thermal expansion coefficient than the material of the cross section (5). In this embodiment, the third portion (9) may comprise one or more members. Preferably, the total cross-section a 9 of the third portion (9) is smaller than the cross-section of the first portion (5) and the second portion (7), and the young's modulus Y 9 is preferably smaller than the cross-section of the first portion (5), and the length L 9 is such that the product of L 9/A9Y9 is smaller than the sum of the products of L 5/A5Y5 and L 7/A7Y7 in the first portion (5) and the second portion (7). The fiber cross section includes a fiber Bragg grating sensor attached to the left-to-end (1), right-to-cantilever beam Duan Jiemian (8), using an adhesive or glass solder, or other prior art technique, to provide strong and stable adhesion with minimal slippage and variation over time. The fiber extends to the left outside the mounting structure to connect to the measuring instrument and can be cut past the connection point on the cantilever beam Duan Bufen (8) or further extends and has another bragg grating sensor (13) that can move freely on the right so that it is only temperature responsive. Still alternatively, the optical fiber may extend to the right-side exterior of the mounting structure, where one or more other sensors may be included, with similar or different mounting structures, as shown in FIG. 4.
According to an embodiment of the present invention, there is also included a seal having a structure for passing an optical fiber therethrough.
It should be noted that the whole structure can be closed by a cover, the bottom of which can be made of the same material M, the rest covering the structure and having a structure for the passage of the optical fibers, without interfering with the movement of the central section and without affecting the overall elongation of the structure under strain. The sealing cover may protect the internal structure from adverse environmental factors such as humidity.
According to an embodiment of the invention, the gap between the two ends is filled with a waterproof sealant.
Fig. 5 shows a block diagram of another embodiment of an athermal fiber optic strain gauge of the present invention.
As shown in fig. 5, the two ends (1) and (2) have a length L e, a width W e, and a height H e. The ends serve as attachment points (3) for the material M (4) whose strain is being measured and are bonded or attached thereto in any other way (e.g. soldered, welded or screwed) that they are not in contact with the surface of the material M and are therefore limited by the expansion or compression of the end portions (1) and (2). The end portions are preferably made of the same material as material M. Thus, the total elongation δl M of the central portion of the total length L M is the same as the material M over the same length under the applied strain, the measured strain being δl M/LM.
Between the end portions are three other portions, all of which have a height H slightly less than H e, for example less than 0.5mm, to ensure that their movement is not constrained or limited by friction with the base material M. The total width of these three portions is W. The first portion (5) has a length L 1 and is divided into two sub-portions, one of length L f and the other of length L B. The portion is preferably made of the same material as the end portion and material M and thus has a young's modulus Y 1＝YM. Furthermore, it is made of two sides with an opening of width W o in the center. Thus, the total cross-section of this portion is a 1＝(W-Wo) H. Thus, the portions (1) and (5) can be cut into U-shaped individual portions from the same material.
The second portion (7) is made of a different material, has a young's modulus Y 2 that is significantly smaller and has a length L 2, the same height H and width W, but no gap in the center, and therefore has a cross section a 2 = W H. A cantilever beam portion (7) of length L B made of the same material is attached in the centre of the portion and protrudes to the left, the cantilever beam section (8) extending in the opening of the pass-through between the two side cantilever beams of the first portion, so that the width W b is slightly smaller than W o. The width of the cantilever beam segments should be substantially greater than the width of the optical fibers.
Thus, the second portion (7) and the cantilever section (8) can be cut out of a single piece of material and bonded to the left first portion (5) and the right end section (2), leaving the cantilever section free to expand under the influence of temperature variations without itself being affected by strain affecting the whole structure.
The optical fiber is connected to the left end portion, and a V-groove may be cut thereon to conveniently guide the position of the optical fiber 14. An optical fiber 14 is also attached to the cantilever portion (10) and a fiber bragg grating sensor for measuring strain is located between these two attachment points. The connection may be made by means of an adhesive or a so-called glass solder or any other method providing a firm and stable connection. It should be noted that a small hole may be provided, and the optical fiber may pass through the small hole, through the left end, or through the cantilever beam, or both.
According to an embodiment of the present invention, the cross-sectional areas of the first portion and the second portion are determined according to a preset calculation rule.
In order to achieve strain reduction and temperature compensation, the dimensions of the sections must be selected according to the following formula.
To reduce the strain, it was found that the soft material would absorb a significant portion of the total elongation. The ratio R s of the strain ε f experienced by the fiber cross-section to the strain ε M experienced by material M is given by:
This is slightly different from equation (3) because the length of the optical fiber portion L f subjected to deformation is different from L 1. For temperature compensation there is a ratio L B/L1 that perfectly compensates the arrangement of the protruding cantilever with coefficient of thermal expansion a 2, the protruding cantilever with effective coefficient of thermal expansion a eff at its end point and the optical fiber itself with coefficient of thermal expansion a f, and its stress optical coefficient is β ε:
In the schematic diagram shown in fig. 2, with a high coefficient of expansion cantilever beam supported by a lower coefficient of thermal expansion material, the coefficient α eff is simply the coefficient of the lower coefficient of expansion material. However, in fig. 3, the total expansion of the parts (5) and (6) is limited by the expansion of the material M, but a part thereof is absorbed by the soft material of the part (6). As a result, the effective expansion coefficient α eff will be slightly less than that of material M, and can be calculated by the following equation:
It should be noted that, the preset calculation rule in the present invention may also be dynamically changed by a neural network model. The neural network can be a pre-trained neural network model, and is obtained by training a large amount of historical data and result data, and the specific training method can be an existing neural network training method. When the sectional areas of the first part and the second part are calculated, various parameters of the strain gauge and specific application material parameters can be input into the neural network model, and data of the sectional areas of the first part and the second part can be output.
In order to better illustrate embodiments of the present invention, a specific embodiment is described below.
It is considered to use an FBG sensor having a length of 1mm, and a half-peak spectral width of the sensor is about 1000pm. For DWDD measuring instruments, a useful measuring range is corresponded. The gauge factor associated with strain and wavelength shift is typically 1.21 pm/. Mu.epsilon.and therefore the strain measurement range of the FBG sensor is 821. Mu.epsilon. In order to have an effective range on a 5000 mu epsilon gauge to use it in stainless steel materials with a coefficient of thermal expansion alpha Μ＝15x10-6℃-1 and young's modulus Y M =180 GPa, a glass reinforced plastic such as Tecatron GF-40 can be used, with a young's modulus of 6.5GPa and a coefficient of thermal expansion alpha 2＝40x10-6℃-1. A temperature compensation ratio L B/L1 =0.47 is obtained. The following dimensions will then provide a ratio R s =0.164, so that a range of 5000 μ epsilon will produce a strain of 821 μ epsilon on the FBG and thermal compensation:
For the ends (1) and (2) of length 2mm respectively, if a gap of 0.5mm is left under the first portions (5) and (6), the total length is 9mm, while the width is 8mm and the total height is 1.5mm. The length of the fiber segment L f is large enough to accommodate a 1mm long FBG.
In order to obtain a 2000 [ mu ]. Epsilon range with the same FBG length, the same total length L M can be maintained for the sections (5) and (6) while only their relative lengths are changed. In this case, the dimensions are:
It can be seen that there is enough room to extend the fiber to its attachment point on the beam cross section to add additional length to the second FBG sensor that is not attached at its right end. The temperature measuring FBG sensor is denoted by (13) in fig. 5, and its bragg wavelength should be different from that of the strain measuring FBG sensor. Such additional temperature measurements may be added to ensure more perfect temperature compensation. This is because the thermal expansion coefficients used in equations (5) and (6) generally vary slightly with temperature, and it is impossible to completely eliminate this effect. This does not mean that temperature compensation itself is not useful, since one of the functions of temperature compensation is also to reduce the spectral range of the FBG sensor due to the possible operating temperature range.
Variations of this basic design are conceivable, all using the same basic principle for strain reduction and temperature compensation, and possibly making the final device more or less reliable, more or less compact and more or less easy to manufacture. Various combinations of lengths and other dimensions may provide the desired R s values as well as temperature compensation.
The athermal fiber strain gauge of the present invention allows the second FBG to be used for temperature sensing while being easy to manufacture and assemble and very compact. The present invention can change the measuring range by changing the overall size or just the size ratio of its internal parts, maintaining the same overall length and using the same standard design for the FBG itself. Thus, performance and form factor can be made similar to commercial resistive strain gauges, with other inherent advantages of fiber optic sensors, such as resistance to electromagnetic interference and insensitivity to lead resistance variations. In addition, the FBG of the invention has short length and small maximum strain, greatly reduces the possibility of fiber breakage, improves the long-term reliability, eliminates the need of recoating the FBG as much as possible, and simplifies the manufacturing process. The standardized FBG designs of all gauges of the present invention, including their length and period, also facilitate mass production, thereby helping to reduce manufacturing costs.
The foregoing is only a preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art, who is within the scope of the present invention, should make equivalent substitutions or modifications according to the technical scheme of the present invention and the inventive concept thereof, and should be covered by the scope of the present invention.
In the present invention, the term "plurality" means two or more, unless explicitly defined otherwise. The terms "mounted," "connected," "secured," and the like are to be construed broadly, and may be, for example, fixedly connected, detachably connected, or integrally connected; "coupled" may be directly coupled or indirectly coupled through intermediaries. The specific meaning of the above terms in the present invention can be understood by those of ordinary skill in the art according to the specific circumstances.
In the description of the present specification, the terms "one embodiment," "some embodiments," "particular embodiments," and the like, mean that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the present invention. In this specification, schematic representations of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or more embodiments or examples.
The above description is only of the preferred embodiments of the present invention and is not intended to limit the present invention, but various modifications and variations can be made to the present invention by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (10)
1. An athermal fiber optic strain gauge, comprising:
The two end parts, the first end part and the second end part are respectively provided with an attachment point for being fixed on a measured material;
the detection part is positioned between the two end parts, and the height of the detection part is smaller than that of the two end parts;
The detection part is sequentially provided with a first part and a second part; the first part is made of a material with a larger thermal expansion coefficient, and the second part is made of a material with a lower thermal expansion coefficient;
the first part is U-shaped and is provided with a passing part;
The center of the second part is provided with a cantilever beam section in a protruding way towards the passing part of the first part, the size of the cantilever beam section is smaller than that of the passing part, one end of the cantilever beam section is connected to a material with a lower thermal expansion coefficient, and the other end of the cantilever beam section extends in a U-shaped opening of the passing part of the first part;
Two ends of the second part are respectively adhered to the first part on the left side and the second part on the right side;
The detection part is also provided with an optical fiber Bragg grating sensor and a second sensor for measuring strain; one end of the fiber Bragg grating sensor is connected to a material with a lower thermal expansion coefficient, and the other end of the fiber Bragg grating sensor is connected to a material with a higher thermal expansion coefficient;
The fiber Bragg grating sensor for measuring strain and the second sensor are positioned between the two attachment points;
the fiber Bragg grating sensor for measuring strain and the second sensor are positioned on two sides of the cantilever beam section.
2. An athermal optical fiber strain gauge according to claim 1, comprising:
The second sensor is a fiber Bragg grating sensor.
3. An athermal optical fiber strain gauge according to claim 1, comprising:
A third portion is also provided, the second portion being disposed between the first portion and the third portion.
4. A non-thermal optical fiber strain gauge according to claim 3, comprising:
the third portion comprises one or more components, and the cross-sectional area of the third portion is smaller than the cross-sectional areas of the first portion and the second portion.
5. An athermal optical fiber strain gauge according to claim 1, comprising:
the first end and the second end are made of the same material as the tested material or a material with rigidity higher than that of the tested material.
6. An athermal optical fiber strain gauge according to claim 1, comprising:
the material of the cantilever beam section has a higher coefficient of thermal expansion than the material of the first portion.
7. An athermal optical fiber strain gauge according to claim 1, comprising:
the first portion is of the same material as the two end portions.
8. An athermal optical fiber strain gauge according to claim 1, comprising:
also included is a seal having a structure for passing the optical fiber therethrough.
9. An athermal optical fiber strain gauge according to claim 1, comprising:
The gap between the two end parts is filled with waterproof sealant.
10. An athermal optical fiber strain gauge according to claim 1, comprising:
The cross-sectional areas of the first portion and the second portion are determined according to preset calculation rules.
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