Continuous strain measurement along a span

• US 4,734,577 A
• Filed: 07/28/1987
• Issued: 03/29/1988
• Est. Priority Date: 01/30/1986
• Status: Expired due to Fees

• Current Assignee: Grumman Aerospace Corporation (Northrop Grumman Corporation)
• Original Assignee: Grumman Aerospace Corporation (Northrop Grumman Corporation)
• Inventors: Szuchy, Nicholas C.
• Primary Examiner(s): Westin, Edward P.
• Assistant Examiner(s): Wieland, Charles
• Application Number:
  US07/080,231
• Time in Patent Office: 245 Days
• Field of Search: 250/227, 250/231 P, 356/32, 356/34, 356/35, 73/800, 73/802
• US Class Current: 250/227.16
• CPC Class Codes: G01B 11/18 using photoelastic elements
• CPC Class Codes: G01L 1/243 using means for applying force perpendicular to the fibre axis
• CPC Class Codes: G01L 1/245 using microbending

Abstract

A fiber optic load measuring system, and method of constructing the same, are disclosed which comprises a light source, a fiber optic attenuator, a photodetector, and a signal processor. The fiber optic attenuator was formed to have a curved portion along the length thereof disposed in low transfer relation with the surface where the load is to be measured. The signal processor is adapted to receive signals from the photodetector and measure variations attributable to the application of a load on the surface. The processor is further adapted to linearly translate the detected variations to compute deformation of the curved portion in response to an applied load.

Claims

1. A fiber optic load measuring system comprising:

(a) an light source;

(b) a fiber optic attenuator in optical communication with said light source, said attenuator having a selected thickness and formed to have at least one curved portion along the length thereof, said curved portion having a curve radius and a curve period, said curved portion being disposed in load transfer relation with a surface where the load is to be measured so that the curve radius and curve period are variable in response to an applied load, said curved portion being dimensioned so that light is attenuated in direct linear relation to deformation of said curved portion in response to an instantaneous applied load over a substantial range of applied load;

(c) a photodetector in optical communication with said attenuator, said photodetector being operative to receive light signals reflected by said attenuator; and

(d) a signal processor connected to said photodetector and operative to detect and measure reflected light signals attributable to the application of the instantaneous load on the surface, said processor being further operative to translate the reflected light signals to compute the instantaneous applied load.

2. The system as recited in claim 1 wherein said light source comprises a steady state light source.

3. The system as recited in claim 1 wherein said light source comprises a pulsed light source.

4. The system as recited in claim 1 further comprising an optical pulse delay element connected to said light source and said fiber optic attenuator.

5. The system as recited in claim 4 wherein said delay element comprises:

(a) a spool; and

(b) a predetermined length of optical material wound about said spool, said length of optical material being operative to delay light signals communicated between said light source and said fiber optic attenuator.

6. The system as recited in claim 1 wherein said fiber optic attenuator is formed to have a plurality of curved portions.

7. The system as recited in claim 1 wherein said fiber optic attenuator is formed to have a serpentine shape.

8. The system as recited in claim 1 wherein said fiber optic attenuator curved portion has a curve radius dimensioned so as to provide static attenuation of light passing therethrough, said curved portion also being dimensioned to provide variable dynamic attenuation of the light passing therethrough, said dynamic attenuation being linearly related to the instantaneous deformation of said curved portion in response to the load applied to the surface.

9. The system as recited in claim 1 wherein said attenuator curved portion has a curve radius dimensioned so as to attenuate the optical signal received from said optical signal generator in linear relation to the instantaneous applied load.

10. The system as recited in claim 1 wherein said attenuator is formed as an optical fiber of approximately 400 microns thickness.

11. The system as recited in claim 1 wherein said attenuator is formed as an optical fiber of approximately 600 microns thickness.

12. The system as recited in claim 1 wherein said attenuator is formed as an optical fiber of approximately 1000 microns thickness.

13. The system as recited in claim 1 wherein said attenuator is embedded in the surface where the load is to be measured.

14. The system as recited in claim 1 wherein said attenuator is disposed upon the surface where the load is to be measured.

15. The system as recited in claim 1 wherein said attenuator is formed as a circular optical fiber having a uniform refractive index.

16. The system as recited in claim 1 wherein said processor is operative to indicate the decay of the reflected return pulse as a function of distance along the length of said attenuator.

17. The system as recited in claim 1 wherein said processor is operative to determine the rate of change of the applied load.

28. The system as recited in claim 10 wherein said attenuator is formed to have a bend diameter of between one quarter and three quarter inches.

29. The system as recited in claim 11 wherein the attenuator is formed to have a bend diameter of between one eighth and one quarter inches.

30. The system as recited in claim 1 wherein said attenuator comprises a length of fiber optic material having at least one permanently curved portion along the length thereof.

31. The system as recited in claim 30 wherein said attenuator further comprises a substrate for supporting the length of fiber optic material, said substrate being in direct contact with and adhered to the surface.

18. A method for measuring a load applied to a surface comprising:

(a) bending an optical fiber having a selected thickness to form at least one curved portion along the length thereof said optical fiber being formed to have a curve radius and a curve period, said curve radius and said curve period being variable in response to an applied load, said curve radius and said fiber thickness being selected so light passing through said curved portion is attenuated in linear relation to the deformation of said curved portion in response to an instantaneous applied load over a substantial range of applied load;

(b) disposing said optical fiber curved portion in load transfer relation to the surface where a load is to be applied;

(c) communicating a light signal to the optical fiber curved portion;

(d) receiving a return light signal reflected from said optical fiber curved portion; and

(e) processing said return light signal to determine the load applied, said processing comprising linearly translating deviations in the return signal to determine the instantaneous applied load.

19. The method as recited in claim 18 wherein said step of bending an optical fiber comprises the step of forming a serpentine shaped fiber optic attenuator.

20. The method as recited in claim 18 wherein said step of communicating a light signal comprises the step of generating a series of pulsed light signals.

21. The method as recited in claim 18 wherein said step of communicating a light signal comprises the step of generating a series of pulsed light signals.

22. The method as recited in claim 18 further comprising the step of delaying the light signal communicated to the optical fiber.

23. The method as recited in claim 22 further comprising the step of delaying the return light signal reflected from the optical fiber.

24. The method as recited in claim 18 further including the step of comparing deviations in the return signal over a period of time.

25. The method as recited in claim 18 wherein said step of bending an optical fiber comprises bending the optical fiber to form a curved portion, said curved portion being formed to have a curve radius dimensioned such that light passing therethrough is attenuated in linear relation to the deformation of said curved portion in response to the load applied to the surface.

26. The method as recited in claim 18 wherein said step of bending an optical fiber comprises bending the optical fiber to form a curved portion, said curved portion being formed to have a curve radius dimensioned such that light passing therethrough is attenuated in linear relation to the load applied to the surface.

27. The method as recited in claim 18 wherein said step of bending an optical fiber comprises bending the optical fiber to form a curved portion, said curved portion being formed to have a curve radius dimensioned so as to provide static attenuation of the light passing therethrough, said curved portion also being dimensioned to provide variable dynamic attenuation of light passing therethrough, said dynamic attenuation being linearly related to the deformation of said curved portion in response to the load applied to the surface.