Weldable FBG strain sensor arrangement

Abstract

A FBG strain sensor arrangement includes a sensor carrier of steel sheet to which a FBG strain sensor is fastened. A protective cover has a first part configured thin and narrow and bonded to the sensor carrier along an optical fiber so that the optical fiber lying underneath is fixed with a fiber Bragg grating on the sensor carrier. The protective cover is enlarged at each end portion of the optical fiber to thereby form a cavity underneath, with edges of the enlarged second part of the protective cover being bonded to the sensor carrier. Arranged in the cavity is an elastic filler which embeds the coupling points in a vibration damping manner. The protective cover with elastic filler accommodates thermal expansions and functions for dynamic measurements by the vibration damping.

Claims

1. A fiber Bragg grating (FBG) strain sensor arrangement, comprising: a sensor carrier of steel sheet; a FBG strain sensor fastened to the sensor carrier, said FBG strain sensor including an optical fiber having two end portions; a Bragg grating provided between the end portions; optical signal lines coupled to the end portions, respectively, for transmission of a measurement signal; a protective cover having a first part which is configured thin and narrow and bonded to the sensor carrier along the optical fiber, so that the optical fiber lying underneath is fixed with the Bragg grating on the sensor carrier, said protective cover having an enlarged second part at each of both end portions of the optical fiber at coupling points of the end portions with the optical signal lines to thereby form a cavity underneath, with edges of the enlarged second part of the protective cover being bonded to the sensor carrier; and an elastic filler arranged in the cavity and embedding the coupling points in a vibration damping manner.

2. The FBG strain sensor arrangement of claim 1, wherein the sensor carrier is configured narrower in a region of the optical fiber than in a region of the enlarged second part of the protective cover.

3. The FBG strain sensor arrangement of claim 1, wherein the protective cover is made of epoxy resin.

4. The FBG strain sensor arrangement of claim 1, wherein the protective cover is configured to be semicircular.

5. The FBG strain sensor arrangement of claim 1, wherein the optical signal lines comprise a steel reinforcement.

6. The FBG strain sensor arrangement of claim 1, wherein the protective cover is maximal 3 times as wide and maximal 2.5 times as thick as a diameter of the optical fiber with the Bragg grating at least in a region of the Bragg grating.

7. The FBG strain sensor arrangement of claim 1, wherein the sensor carrier is configured as a triangle, with each side of the triangle having fastened thereon one of said FBG strain sensor.

8. The FBG strain sensor arrangement of claim 1, wherein the sensor carrier is configured as a rectangle, with each side of the rectangle having fastened thereon one of said FBG strain sensor.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) The invention will be explained in greater detail hereinafter with reference to exemplary embodiments and drawings:

(2) FIG. 1 shows a perspective view of a weldable strain sensor and two sectional views A-A and B-B thereof.

(3) FIG. 2 shows a triangular sensor carrier with 3 FBG strain sensors mounted thereon.

(4) FIG. 3 shows a square sensor carrier with 4 FBG strain sensors mounted thereon.

(5) FIG. 4 shows symbolically the safety of the FBG strain sensor arrangement against foot traffic.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(6) FIG. 1 shows a perspective view of a weldable strain sensor arrangement 1. An optical fiber with an FBG strain sensor 2 is shown as a dashed line and mechanically connected at its two end portions for transmitting an optical measurement signal in signal communication with steel-reinforced signal lines 3a, 3b. The steel reinforcement is provided to render the strain sensor arrangement mechanically robust. The optical fiber 2 with the FBG strain sensor is bonded onto a sensor carrier 4 of steel sheet. In this exemplary embodiment, the steel sheet has a thickness of 0.1 mm and a tensile strength of 884 N/mm.sup.2.

(7) The optical fiber with the FBG strain sensor 2 and coupling points to the signal lines 3a, 3b are continuously covered with a protective cover 5 of solid plastic. In the present exemplary embodiment, epoxy resin is used because it is particularly strong and resistant to aging. The protective cover 5 is firmly connected to the sensor carrier 4 at each point.

(8) In the region of the optical fiber with the FBG strain sensor 2, the protective cover 5 is configured narrow and flat. In the present exemplary embodiment, the protective cover 5 has in this region a width of 2 mm and a thickness of 0.5 mm.

(9) In the region of the optical fiber with the FBG strain sensor 2, the sensor carrier 4 is configured narrower than at its end portions. In the present exemplary embodiment, the end portions of the sensor carrier have a width of 23 mm, whereas the section between the end portions is only 11 mm wide. In this way, a good transfer of the elongation of the workpiece onto the FBG sensor is realized.

(10) At each of both end portions of the optical fiber, i.e. at the coupling points with the signal lines 3a and 3b, the protective cover has an enlarged configuration and is bonded to the sensor carrier as well. As is apparent from the sectional views in FIG. 1 the protective cover is configured semicircular and thin-walled. In this exemplary embodiment, the wall thickness W in the region of the two end portions is 0.5 mm, i.e. It is of same size as the thickness D of the protective cover in the region of the optical fiber with the FBG strain sensor 2. Arranged in the cavity underneath the semicircular protective cover is an elastic filler 6, which assumes the function of a vibration damper. Vibration damping is particularly advantageous when strain measurements are involved, which are additionally superimposed by mechanical vibrations.

(11) The sectional views A-A and B-B show the cross-section of the semicircular protective cover along the section lines A-A and B-B in the perspective view of the strain sensor arrangement of FIG. 1. It can be seen that the filler 6 fills the cavity of the semicircular protective cover.

(12) Reference numeral 7 indicates fictitious welding points, which would be created when the sensor carrier 4 is mounted onto a workpiece surface through spot welding.

(13) Reference number 8 designates an angled sealing zone which connects the thin semicircular protective cover 5 to the sensor carrier 4 and ensures that no leakage occurs.

(14) The particular advantage of this FBG strain sensor arrangement resides in the very simple structure and its easy variability in shape. The sensor carrier 4 can be produced inexpensively by laser cutting in a variety of shapes. Likewise, the cover, which is made entirely of the same material, preferably of durable plastic, can be easily manufactured. FIGS. 2 and 3 show only two of the many design options of the sensor carrier and thus the FBG strain sensor arrangement.

(15) The steel reinforcement of the signal lines 3a, 3b and the semicircular curvature of the cover provide good protection of the sensor against a flat mechanical contact, e.g. by shoes as shown symbolically in FIG. 4. Therefore, the need for additional and elaborate protective measures may oftentimes be eliminated.