FIBER OPTIC STRAIN GAGE AND CARRIER

• US 20090126501A1
• Filed: 11/15/2007
• Published: 05/21/2009
• Est. Priority Date: 11/15/2007
• Status: Active Grant

• Current Assignee: Luna Innovations, Inc.
• Original Assignee: Micron Optics Incorporated (Luna Innovations, Inc.)
• Inventors: Ferguson, Stephen K.
• Granted Patent: US 7,856,888 B2
• Time in Patent Office: Days
• US Class Current: 73/800
• CPC Class Codes: G01B 11/18 using photoelastic elements
• CPC Class Codes: G01L 1/246 using integrated gratings, e.g. Bragg gratings
• CPC Class Codes: G01M 11/085 the optical fiber being on or near the surface of the DUT

Abstract

The current invention relates to optical gages designed to measure strain on the surface of a test specimen. The gages of this invention is designed to be installed and used in a manner similar to conventional electronic foil strain gages, but to have the advantages of an all-optical gage. The gage of this invention is constructed to allow surface strain on the test specimen to be transferred to a length of optical fiber containing a fiber Bragg grating (FBG). As strain is applied to the fiber, the optical spectrum center wavelength reflected by the Bragg grating shifts in wavelength. This shift in wavelength can be converted directly to units of strain. The current invention provides a gage carrier design for use with fiber optic strain sensors comprising one or more FBGs which provide the benefits of a carrier for ease of handling and mounting without degrading gage performance. The gage carrier provides features that allow the FBG to be precisely positioned and securely bonded to the test specimen in a controlled manner. The carrier additionally provides fiber protection to the installed FBG.

Claims

1. A gage carrier for a fiber optic strain gage comprising:

a body having a passageway extending along its length for receiving an optical fiber containing one or more fiber Bragg gratings, a central portion of said body intersecting at least a portion of said passageway forming a mold cavity for receiving a fiber bonding material, said mold cavity positioned in the body, with respect to the passageway and the optical fiber inserted into the passageway, such that the optical fiber is in contact with the test specimen when the gage carrier is positioned on the test specimen;

a filling hole passing from the outside surface of the carrier into the mold cavity for introduction of fiber bonding material into the mold cavity and one or more vent holes passing from the outside surface of the carrier into the mold cavity and spaced apart from said filling hole to allow air to escape the mold cavity as fiber bonding material is introduced into the mold cavity; and

a mounting surface for attaching the carrier to a test specimen.

2. The gage carrier of claim 1 further comprising one or two fastening points for attachment of fiber protection buffer tubing.

3. The gage carrier of claim 1 further comprising a pre-applied adhesive on the mounting surface for attachment of the carrier to the test specimen.

4. The gage carrier of claim 1 further comprising one or more fiber guides to center the fiber within the passageway.

5. The gage carrier of claim 1 wherein the carrier is fabricated using injection molding.

6. The gage carrier of claim 1 wherein the carrier is fabricated from an elastomeric material, a thermoplastic material or a thermoset material.

7. The gage carrier of claim 1 wherein the carrier is fabricated from metal, glass, or ceramic.

8. The gage carrier of any one of claims 1-7 further comprising a Transducer Electronic Data Sheet (TEDS) implemented by adding an EEPROM or other electronic memory device during manufacture.

9. The gage carrier of claims 1-7 further comprising a Radio Frequency Identification (RFID) inlay molded in during manufacture or added after manufacture.

10. The gage carrier of claims 1-7 further comprising a virtual Transducer Electronic Data Sheet (TEDS) implemented by adding a unique machine readable identifier.

11. The gage carrier of claims 1-7 that remains in place after installation providing fiber protection and attachment points for protective buffer tubing to protect fiber pigtails.

25. A method for measuring strain in a test specimen which comprises the steps of:

(a) attaching one or more strain gages each comprising a carrier of any one claims 1-7 to the test specimen;

(b) introducing light of appropriate wavelength into the optical fiber of the strain gage;

(c) detecting light reflected from the FBG of the optical fiber to detect changes in the wavelength of the FBG and thereby detect strain in the test specimen.

26. The carrier of claim 1 wherein the mold cavity has a depth such that when the carrier is attached to the test specimen, the optical fiber is held in place against the test specimen.

27. The carrier of claim 1 wherein the test specimen forms a portion of the mold cavity when the carrier is attached to the test specimen.

28. The carrier of claim 1 further comprising a pre-applied adhesive for attachment to the test specimen and one or more fiber guides to center the fiber within the passageway, wherein the mold cavity has a depth such that the when the carrier is attached to the test specimen, the optical fiber is held in place against the test specimen and wherein the test specimen forms a portion of the mold cavity when the carrier is attached to the test specimen.

34. The method of claim 25 wherein the step of attaching one or more strain gages comprises the steps of:

attaching the gage carrier of the one or more strain gages to the test specimen and

injecting bonding material into the mold cavity through the filling hole to fill the mold cavity with adhesive and bond the optical fiber to the test specimen.

35. The method of claim 34 wherein the step of attaching one or more strain gages further comprises the step of removing the gage carrier of the one or more strain gages after the optical fiber is bonded to the test specimen.

12. A fiber optical strain gage for measurement of strain in a test specimen comprising:

an optical fiber fastened to the longitudinal axis of a gage carrier wherein the gage carrier comprises;

a body having a passageway extending along its length for receiving the optical fiber, a central portion of said body intersecting at least a portion of said passageway forming a mold cavity for receiving a fiber bonding material, said mold cavity positioned in the body, with respect to the passageway and the optical fiber inserted into the passageway, such that the optical fiber is in contact with the test specimen when the gage carrier is positioned on the test specimen;

a filling hole passing from the outside surface of the carrier into the mold cavity for introduction of fiber bonding material into the mold cavity and one or more vent holes passing from the outside surface of the carrier into the mold cavity and spaced apart from said filling hole to allow air to escape the mold cavity as fiber bonding material is introduced into the mold cavity; and

a mounting surface for attaching the carrier to a test specimen, wherein the optical fiber contains one or more fiber Bragg gratings at least one of which fiber Bragg gratings is in the length of fiber contained within the carrier.

13. The strain gage of claim 12 further comprising a single fiber connection or a double ended fiber connection extending from the gage.

14. The strain gage of claim 12 further comprising a strain relief element to protect the optical fiber on entrance, exit or both from the gage carrier.

15. The strain gage of claim 12 wherein the optical fiber containing one or more fiber Bragg gratings is directly incorporated into the carrier during the injection molding process.

16. A strain gage kit comprising a strain gage of any one of claims 12-15 and a bonding material for introduction into the mold cavity of the strain gage.

17. A strain gage kit comprising a strain gage of any one of claims 12-15, epoxy adhesive, syringe with applicator tip, and installation instructions.

18. An optical fiber sensing system comprising one or more of the strain gages of any one of claims 12-15.

19. An optical fiber sensing system comprising one or more of the strain gages of any one of claims 12-15 and a displacement transducer.

20. A device for measurement of displacement which comprises a strain gage of any one of claims 12-15.

21. A device for measuring force comprising the strain gage of any one of claims 12-15 and a load cell responsive to the force to be measured wherein the strain gage is attached to the load cell.

22. A device for measuring pressure comprising the strain gage of any one of claims 12-15 and a diaphragm that is responsive to the pressure to be measured wherein the strain gage is attached to the diaphragm.

23. An accelerometer comprising a strain gage of any one of claims 12-15 and a mass which is responsive to the acceleration to be measured wherein the strain gage is attached to the mass.

24. A temperature measurement device which comprises a strain gage of any one of claims 12-15.

29. The strain gage of claim 12 wherein the mold cavity of the gage carrier has a depth such that the when the carrier is attached to the test specimen, the optical fiber is held in place against the test specimen.

30. The strain gage of claim 12 wherein the test specimen forms a portion of the mold cavity when the gage carrier is attached to the test specimen.

31. The strain gage of claim 12 wherein the carrier further comprising a pre-applied adhesive for attachment to the test specimen and one or more fiber guides to center the fiber within the passageway of the carrier, wherein the mold cavity of the carrier has a depth such that the when the carrier is attached to the test specimen, the optical fiber is held in place against the test specimen and the test specimen forms a portion of the mold cavity.

32. A gage carrier for a fiber optic strain gage comprising:

a body having a passageway extending along its length for receiving an optical fiber containing one or more fiber Bragg gratings, a central portion of said body intersecting at least a portion of said passageway forming a mold cavity for receiving a fiber bonding material, said mold cavity positioned in the body, with respect to the passageway and the optical fiber inserted into the passageway, such that the optical fiber is in contact with the test specimen when the gage carrier is positioned on the test specimen;

a filling hole passing from the outside surface of the carrier into the mold cavity for introduction of fiber bonding material into the mold cavity and one or more vent holes passing from the outside surface of the carrier into the mold cavity and spaced apart from said filling hole to allow air to escape the mold cavity as fiber bonding material is introduced into the mold cavity; and

a mounting surface for attaching the carrier to a test specimen, wherein the mold cavity has a depth equal to the diameter of the optical fiber.

33. A fiber optical strain gage for measurement of strain in a test specimen comprising:

an optical fiber fastened to the longitudinal axis of a gage carrier of claim 32, wherein the optical fiber contains one or more fiber Bragg gratings at least one of which fiber Bragg gratings is in the length of fiber contained within the carrier.