Sealing method of fiber bragg grating strain assembly

Abstract

The invention relates to the technical field of fiber bragg gratings, in particular to a sealing method of a fiber bragg grating strain assembly. The embodiment of the invention provides a sealing method of a fiber bragg grating strain assembly. The sealing method comprises the following steps: placing an optical fiber at the axis of a V-shaped groove extending along the axis on two metal columns; filling the V-shaped groove in which the optical fiber is placed with low-temperature glass solder, and heating and melting the low-temperature glass solder; after the low-temperature glass solder is cooled, the metal column is connected to the edge of the U-shaped groove of the strain substrate in a spot welding mode. The embodiment of the invention provides a sealing method of a fiber bragg grating strain assembly. The sealing method can provide a fiber bragg grating strain structure which adapts to extremely high and low temperature environments of aerospace and is high in measurement precision.

Description

Technical Field

[0001] This invention relates to the field of fiber Bragg grating technology, and in particular to a sealing method for a fiber Bragg grating strain assembly. Background Technology

[0002] Fiber Bragg grating strain sensors are a type of fiber optic sensor that primarily acquires sensing information by modulating the Bragg wavelength of an optical fiber using external physical parameters. Due to their advantages such as immunity to electromagnetic interference, large signal bandwidth, high sensitivity, and resistance to high and low temperatures, fiber Bragg grating strain sensors are widely used for measuring data such as strain, displacement, and vibration.

[0003] Fiber Bragg grating (FBG) strain gauge components are the core sensing components of FBG strain sensors. Currently, civilian FBG strain gauge components employ adhesive bonding or glass solder bonding to seal the fiber to the strain substrate. Adhesive bonding, due to the organic nature of the adhesive, is prone to aging, decomposition, and creep, resulting in poor stability and unreliable lifespan under the long-term high and low temperature environments of aerospace. Glass solder bonding, on the other hand, can withstand the high and low temperature environments of aerospace. When a pre-tension is applied to the FBG, causing the center wavelength to shift to the design wavelength, the FBG and substrate are heated and welded together to a sealing temperature above 350°C, requiring 90-120 seconds of heating time. The residual stress generated during welding leads to a significant drift in the center wavelength, affecting measurement accuracy and making it difficult to guarantee product consistency, which does not meet the high reliability and high precision micro-strain measurement requirements of aerospace applications. Summary of the Invention

[0004] This invention provides a sealing method for a fiber optic strain gauge assembly, which can provide a fiber optic strain gauge structure that is adaptable to the extreme high and low temperature environments of aerospace and has high measurement accuracy.

[0005] In a first aspect, embodiments of the present invention provide a sealing method for a fiber optic strain gauge assembly, comprising: The optical fiber is placed at the axis of a V-shaped groove extending along the axis of two metal pillars. The V-shaped groove containing the optical fiber is filled with low-temperature glass solder, and the low-temperature glass solder is heated to melt it. After the low-temperature glass solder cools, the metal pillar is spot-welded to the edge of the U-shaped groove of the strain substrate.

[0006] In one possible design, after the cryogenic glass solder cools, the metal pillar is spot-welded to the edge of the U-shaped groove of the strain gauge substrate, including: After the low-temperature glass solder cools, the openings of the V-groove and the U-groove are placed opposite each other; One of the metal pillars is spot-welded to the edge of the U-shaped groove of the strain substrate; Apply tension to the optical fiber to shift the center wavelength to the design wavelength; While maintaining tension, spot weld another of the metal pillars to the strain substrate to keep the center wavelength of the grating at the design wavelength.

[0007] In one possible design, the opening width of the U-shaped groove is smaller than the diameter of the metal column, but larger than the maximum opening width of the V-shaped groove.

[0008] In one possible design, the metal pillar is made of the same material as the strain substrate.

[0009] In one possible design, both the metal pillar and the strain substrate are plated with a gold layer of less than 0.3 μm.

[0010] In one possible design, prior to placing the optical fiber at the axis of the V-groove extending along the axis on the two metal pillars, the following is also included: Ultrasonic cleaning of metal columns and optical fibers; Dry in an oven at 104℃±2℃ for more than 30 minutes.

[0011] In one possible design, the low-temperature glass solder is heated to molten welding using a high-frequency electromagnetic induction heating device, and the spot welding is laser spot welding.

[0012] In one possible design, the metal column is prepared to form a V-groove by wire cutting, and the strain substrate is machined to form a U-groove by milling.

[0013] In one possible design, the strained substrate is annealed after processing.

[0014] Secondly, embodiments of the present invention provide a fiber optic grating strain assembly, which is prepared by any of the methods described above.

[0015] Compared with the prior art, the present invention has at least the following beneficial effects: (1) The sealing method of the present invention, which combines high-frequency electromagnetic induction heating welding and laser spot welding, uses a transition metal pillar made of Kovar alloy as an intermediary. The fiber grating and the strain substrate are not heated at the same time, which realizes low residual stress sealing between the fiber grating and the strain substrate. The wavelength drift after sealing is less than 0.2nm, which is suitable for the extreme high and low temperature environment of aerospace. At present, the fiber grating strain components of civilian products adopt adhesive bonding or glass solder sealing process. The fiber grating and the substrate are heated as a whole to the sealing temperature. The residual stress generated by sealing is large, the stability is poor, and the measurement error is large.

[0016] (2) The sealing method combining high-frequency electromagnetic induction heating welding and laser spot welding used in this invention provides high reliability for fiber optic grating assemblies. Welding strength tests were conducted on the high-frequency electromagnetic induction welding of fiber optic gratings and transition metal pillars. The fiber optic cable fractured under tensile stress, while the weld joint remained undamaged, confirming that the tensile strength was greater than 60N (the fiber optic grating's own tensile strength is approximately 60N). Tensile strength tests were also conducted on the laser spot welding of transition metal pillars and strain gauges, showing tensile strength greater than 70N, exceeding the tensile strength of the fiber optic cable. This confirms the sealing reliability, ensuring high-reliability applications in aerospace. Currently, civilian fiber optic grating strain gauge assemblies use a glass solder integral sealing process, with sealing strength tests showing less than 20N. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 Schematic diagram of the V-groove in the transition metal tube; Figure 2 Schematic diagram of strain gauge shape; Figure 3 Schematic diagram of the assembly of optical fiber, transition metal tube, and glass solder; Figure 4 Schematic diagram of the assembly of the transition metal tube and the strain gauge.

[0019] In the picture: 1-Metal pillar; 2-Strain substrate; 3-Grate. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0021] Please refer to Figures 1 to 4 This invention provides a sealing method for a fiber optic strain gauge assembly, comprising: The optical fiber is placed at the axis of a V-shaped groove extending along the axis of two metal pillars. The V-groove containing the optical fiber is filled with low-temperature glass solder, and the low-temperature glass solder is heated to melt it. After the low-temperature glass solder cools, the metal pillars are spot-welded to the edge of the U-shaped groove of the strain substrate.

[0022] In some embodiments of the present invention, after the low-temperature glass solder cools, the metal pillar is spot-welded to the edge of the U-shaped groove of the strain gauge, including: After the low-temperature glass solder cools, place the openings of the V-groove and U-groove opposite each other; A metal pillar is spot-welded to the edge of the U-shaped groove of the strain gauge. Apply tension to the optical fiber to shift the center wavelength to the design wavelength; While maintaining tension, spot weld another metal pillar to the strain substrate to keep the center wavelength of the grating at the design wavelength.

[0023] In some embodiments of the present invention, the opening width of the U-shaped groove is smaller than the diameter of the metal column, but larger than the maximum opening width of the V-shaped groove.

[0024] In some embodiments of the present invention, the materials used to prepare the metal pillars are the same as those used to prepare the strain substrates.

[0025] In some embodiments of the present invention, both the metal pillar and the strain substrate are coated with a gold layer of less than 0.3 μm.

[0026] In some embodiments of the invention, before placing the optical fiber at the axis of the V-groove extending along the axis on the two metal pillars, the method further includes: Ultrasonic cleaning of metal columns and optical fibers; Dry in an oven at 104℃±2℃ for more than 30 minutes.

[0027] In some embodiments of the present invention, the low-temperature glass solder is heated to molten welding by a high-frequency electromagnetic induction heating device, and the spot welding is laser spot welding.

[0028] In some embodiments of the present invention, the metal column is prepared to obtain a V-shaped groove by wire cutting, and the strain substrate is processed to obtain a U-shaped groove by milling.

[0029] In some embodiments of the present invention, the strain substrate is annealed after processing.

[0030] This invention provides a fiber optic grating strain assembly, which is prepared by any of the methods described above.

[0031] Specifically, the sealing method for a fiber optic strain gauge assembly proposed in this invention includes the following steps: Step 1: Prepare two transition metal pillars; like Figure 1As shown, the transition metal pillar 1 is cylindrical, with a central hole matching the diameter of the high-temperature optical fiber. A V-shaped through-slot is cut using a wire cutting method to serve as a container for the low-temperature glass solder. The surface treatment of the part involves a gold plating layer of less than 0.3 μm.

[0032] Step 2: Prepare strain substrates made of the same material as the transition metal pillars; like Figure 2 As shown, the strain gauge 2 has a U-shaped through-slot at the location where the transition metal pillar is mounted. The through-slot is machined using milling, and its width should be smaller than the diameter of the transition metal pillar. After machining, the strain gauge undergoes stress-relief annealing. The surface of the part is treated with a gold plating layer less than 0.3 μm.

[0033] Step 3: Before high-frequency electromagnetic induction heating welding, clean and bake the optical fiber and transition metal pillar; The two transition metal pillars were ultrasonically cleaned with acetone, and the optical fiber used for welding was ultrasonically cleaned with anhydrous ethanol to remove surface oil, dust, etc., and then dried in an oven at 104℃±2℃ for more than 30 minutes. In particular, the optical fiber coating is made of polyimide, which is a moisture-sensitive material. After baking, moisture is removed to avoid the generation of a large number of bubbles in the low-temperature glass solder during welding.

[0034] Step 4: Install the optical fiber along the axis of the transition metal post; Place the two transition metal pillars to be welded with their V-grooves facing upwards in the fixed position of the welding fixture on the high-frequency electromagnetic induction heating welding table. Clamp the two ends of the fiber optic grating onto the Z-axis lifting platform and adjust the height of the fiber optic grating to the axis of the V-groove. For example... Figure 3 As shown, the grating 3 should be adjusted to be at the symmetrical center of the two transition metal pillars, the optical fiber should be straightened, and the optical fibers at both ends should be clamped and fixed.

[0035] Step 5: High-frequency electromagnetic induction welding of optical fiber and transition metal pillar; Low-temperature glass solder is spread throughout the length of the V-shaped groove. The high-frequency electromagnetic induction heating equipment is activated, and the temperature is raised above the melting point of the low-temperature glass solder. The solder fully melts in approximately 10 seconds, completing the welding process. Allow to cool naturally. Figure 3 As shown, welding is complete.

[0036] Step 6: Laser spot welding of the transition metal pillar and the strain substrate.

[0037] The strain gauge substrate is first mounted on the welding positioning fixture of the laser spot welding equipment welding platform. The metal column fiber optic assembly is then mounted on the strain gauge substrate. One end of the transition metal column, with its V-groove facing downwards, is placed at the required position in the U-groove of the strain gauge substrate, and the middle of the transition metal column is temporarily fixed with tape. The laser spot welding machine is started, and the symmetrical weld points at both ends of the transition metal column are first spot welded, a total of 4 weld points. After removing the tape, the two symmetrical weld points at the center of the transition metal column are spot welded. There are a total of 6 weld points between the transition metal column and the substrate.

[0038] The other end of the transition metal column is placed downwards in the U-shaped groove of the strain substrate. After applying a counterweight to the fiber tail and applying pre-tension, the transition metal columns at both ends are centrally symmetrical about the strain zones of the grating and the substrate. The other end of the transition metal column is then spot-welded to the substrate using the same laser spot welding method, with six weld points applied in the same manner.

[0039] This invention employs transition metal pillars to first weld a fiber optic grating to two metal pillars using low-temperature glass solder and high-frequency electromagnetic induction, forming a fiber optic assembly with metal pillars. When the center wavelength of the fiber optic grating shifts to the design wavelength after applying pre-tension, the metal pillars are then laser-spot-welded to a strain gauge substrate. This process transforms the initial overall heating and sealing into two independent welding operations. The advantage of this two-stage independent welding over the initial heating and sealing is that, through the use of transition metal pillars and two specific high-energy welding methods with minimal heat-affected zones, the grating region does not participate in the heating process, and the strain gauge substrate and fiber optic grating are not heated simultaneously, thus avoiding additional wavelength drift caused by the heating and sealing process. The center wavelength drift after sealing is less than 0.2 nm. For aerospace applications, the reliability of the sealing operations—high-frequency magnetic induction heating welding of the metal pillars to the fiber optic grating and laser spot welding of the metal pillars to the strain gauge substrate—was tested, achieving sealing strengths greater than 60 N and 70 N respectively, demonstrating low residual stress and high reliability.

[0040] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A sealing method for a fiber Bragg grating strain gauge assembly, characterized in that, include: The optical fiber is placed at the axis of a V-shaped groove extending along the axis of two metal pillars. The V-shaped groove containing the optical fiber is filled with low-temperature glass solder, and the low-temperature glass solder is heated to melt it. After the low-temperature glass solder cools, the metal pillar is spot-welded to the edge of the U-shaped groove of the strain substrate.

2. The sealing method according to claim 1, characterized in that, After the low-temperature glass solder cools, the metal pillar is spot-welded to the edge of the U-shaped groove of the strain gauge, including: After the low-temperature glass solder cools, the openings of the V-groove and the U-groove are placed opposite each other; One of the metal pillars is spot-welded to the edge of the U-shaped groove of the strain substrate; Apply tension to the optical fiber to shift the center wavelength to the design wavelength; While maintaining tension, spot weld another of the metal pillars to the strain substrate to keep the center wavelength of the grating at the design wavelength.

3. The sealing method according to claim 1, characterized in that, The opening width of the U-shaped groove is smaller than the diameter of the metal column, but larger than the maximum opening width of the V-shaped groove.

4. The sealing method according to claim 1, characterized in that, The metal pillar is made of the same material as the strain substrate.

5. The sealing method according to claim 1, characterized in that, Both the metal pillar and the strain substrate are coated with a gold layer of less than 0.3 μm.

6. The sealing method according to claim 1, characterized in that, Before placing the optical fiber at the axis of the V-groove extending along the axis on the two metal pillars, the method further includes: Ultrasonic cleaning of metal columns and optical fibers; Dry in an oven at 104℃±2℃ for more than 30 minutes.

7. The sealing method according to claim 1, characterized in that, The low-temperature glass solder is heated to molten welding using a high-frequency electromagnetic induction heating device, and the spot welding is laser spot welding.

8. The sealing method according to claim 1, characterized in that, The metal column is prepared into a V-shaped groove by wire cutting, and the strain substrate is machined into a U-shaped groove by milling.

9. The sealing method according to claim 8, characterized in that, The strain substrate is annealed after processing.

10. A fiber optic strain gauge assembly, characterized in that, Prepared by any of the methods in claims 1-9.

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