Bragg grating pressure sensor for industrial sensing applications

• US 20020154860A1
• Filed: 11/08/2001
• Published: 10/24/2002
• Est. Priority Date: 12/04/1998
• Status: Active Grant

• Current Assignee: Cidra Corporate Services, Inc.
• Original Assignee: Cidra Sarl (Nordic Cultural & Educational Foundation)
• Inventors: Kersey, Alan D., Fernald, Mark R., Bailey, Timothy J., Putnam, Martin A., Davis, Michael A., Sanders, Paul E., Dunphy, James R., Wright, Christopher J., Brucato, Robert N., Sullivan, James M., Miller, Matthew B.
• Granted Patent: US 6,776,045 B2
• Time in Patent Office: Days
• US Class Current: 385/37
• CPC Class Codes: G01L 11/025 using a pressure-sensitive optical fibre

Abstract

A fiber grating pressure sensor for use in an industrial process includes an optical sensing element 20,600 which includes an optical fiber 10 having a Bragg grating 12 impressed therein which is encased within and fused to at least a portion of a glass capillary tube 20 and/or a large diameter waveguide grating 600 having a core and a wide cladding and which has an outer transverse dimension of at least 0.3 mm. Light 14 is incident on the grating 12 and light 16 is reflected from the grating 12 at a reflection wavelength λ1. The sensing element 20,600 may be used by itself as a sensor or located within a housing 48,60,90,270,300. When external pressure P increases, the grating 12 is compressed and the reflection wavelength λ1 changes. The shape of the sensing element 20,600 may have other geometries, e.g., a “dogbone” shape, so as to enhance the sensitivity of shift in λ1 due to applied external pressure and may be fused to an outer shell 50. A temperature grating 270 may be used to measure temperature and allow for a temperature-corrected pressure measurement. The sensor may be suspended within an outer housing 112, by a fluid, spacers, or other means. The sensor may also be combined with an instrument, an opto-electronic converter and a controller in an industrial process control system.

Claims

1. A pressure sensor for use in an industrial process, comprising:

an optical sensing element, having at least one pressure Bragg Grating disposed therein, said pressure Bragg Grating having a pressure reflection wavelength;

said sensing element being axially strained due to a change in external pressure, said axial strain causing a change in said pressure reflection wavelength, and said change in said pressure reflection wavelength being indicative of said change in pressure; and

at least a portion of said sensing element having a transverse cross-section which is contiguous and made of substantially a single material and having an outer transverse dimension of at least 0.3 mm.

2. The apparatus of claim 1 wherein said sensing element comprises:

an optical fiber, having said Bragg Grating embedded therein; and

a tube, having said optical fiber and said Bragg Grating encased therein along a longitudinal axis of said tube, said tube being fused to at least a portion of said fiber.

3. The pressure sensor of claim 1 wherein said sensing element comprises:

a tube fused to at least a portion of an optical fiber along a longitudinal axis of said tube;

a large diameter optical waveguide having an outer cladding and an inner core disposed therein; and

said tube and said waveguide being axially fused and optically coupled together.

4. The pressure sensor of claim 3 wherein said Bragg Grating is embedded in said fiber and encased in said tube along said longitudinal axis of said tube.

5. The pressure sensor of claim 3 wherein said Bragg Grating is disposed in said optical waveguide.

6. The pressure sensor of claim 1 wherein said material comprises a glass material.

7. The pressure sensor of claim 1 further comprising a housing attached to at least a portion of said sensing element which applies an axial strain on said sensing element due to said change in pressure.

8. The pressure sensor of claim 1 wherein said sensing element is strained in compression.

9. The pressure sensor of claim 2 wherein said tube is fused to said optical fiber where said Bragg Grating is located.

10. The pressure sensor of claim 2 wherein said tube is fused to said optical fiber on opposite axial sides of said Bragg Grating.

11. The pressure sensor of claim 1 wherein said Bragg Grating is a Bragg grating.

12. The pressure sensor of claim 1 wherein said Bragg Grating has a characteristic wavelength and wherein said sensing element comprises a shape that provides a predetermined sensitivity to a shift in said wavelength due to a change in force on said tube.

13. The pressure sensor of claim 12 wherein said sensing element comprises a dogbone shape.

14. The pressure sensor of claim 1 wherein said sensing element comprises a dogbone shape and comprises an outer tube fused to at least a portion of large sections of said dogbone shape on opposite axial sides of said Bragg Grating.

15. The pressure sensor of claim 1 further comprising a temperature Bragg Grating disposed in said sensing element in thermal proximity to said pressure Bragg Grating, and having a temperature reflection wavelength that changes with temperature.

16. The pressure sensor of claim 15 wherein said temperature reflection wavelength does not change in response to a change in said pressure wavelength due to a change in said pressure.

17. The pressure sensor of claim 1, further comprising an outer housing, surrounding said sensing element and suspension means disposed between said sensing element and said outer housing for suspending said sensing element within said housing.

19. The method of claim 18 wherein said sensing element comprises:

an optical fiber, having said pressure Bragg Grating embedded therein; and

a tube, having said optical fiber and said Bragg Grating encased therein along a longitudinal axis of said tube, said tube being fused to at least a portion of said fiber.

18. A method for sensing pressure in an industrial process, comprising:

obtaining an optical sensing element having at least one pressure Bragg Grating disposed therein along a longitudinal axis of said sensing element, said pressure Bragg Grating having a pressure reflection wavelength;

axially straining said sensing element due to a change in pressure, said axial strain causing a change in said pressure reflection wavelength, and said change in said pressure reflection wavelength being indicative of said change in pressure; and

at least a portion of said sensing element having a transverse cross-section which is contiguous and made of substantially a single material and having an outer transverse dimension of at least 0.3 mm.

20. An industrial process control system, comprising:

a pipe having a fluid flowing therein, said fluid having a process pressure;

a pressure sensor disposed within said pipe in fluid communication with said fluid, said pressure sensor comprising;

an optical sensing element, having at least one pressure Bragg Grating disposed therein, said pressure Bragg Grating having a pressure reflection wavelength;

said sensing element being axially strained due to a change in external pressure, said axial strain causing a change in said pressure reflection wavelength, and said change in said pressure reflection wavelength being indicative of said change in pressure; and

at least a portion of said sensing element having a transverse cross-section which is contiguous and made of substantially a single material and having an outer transverse dimension of at least 0.3 mm;

an optical instrument providing a light signal to said pressure sensor; and

said sensor providing a return signal to said instrument wherein said signal is indicative of said process pressure of said fluid.

21. The industrial process control system in accordance with claim 21 wherein said instrument provides an optical output signal indicative of said process pressure, said system further comprising:

an opto-electrical converter receiving said optical output signal and converting said optical output signal into an electrical signal; and

a control device receiving said electrical signal and capable of controlling said process pressure to a predetermined pressure level.