Temperature insensitive fiber-optic torque and strain sensor

• US 6,513,390 B1
• Filed: 11/01/2000
• Issued: 02/04/2003
• Est. Priority Date: 11/01/2000
• Status: Expired due to Term

• Current Assignee: Mcmaster University
• Original Assignee: Mcmaster University
• Inventors: De La Puente, Gonzalo, Jessop, Paul E.
• Primary Examiner(s): Oen, William
• Assistant Examiner(s): Dickens, Charlene
• Application Number:
  US09/703,230
• Time in Patent Office: 825 Days
• Field of Search: 738/00, 738/623.24, 738/626.24, 737/62, 737/05, 356/33, 356/364, 356/34, 356/35, 356/366, 356/367, 250/231.19, 250/225
• US Class Current: 73/800
• CPC Class Codes: G01B 11/165 by means of a grating deformed by the object
• CPC Class Codes: G01B 11/168 by means of polarisation
• CPC Class Codes: G01B 11/18 using photoelastic elements
• CPC Class Codes: G01L 1/241 by photoelastic stress analysis

Abstract

The present invention provides a simple design for a temperature-insensitive extrinsic polarimetric strain sensor. The sensing element is a thin sheet of photoelastic material that is bonded to the test object. It is illuminated with linearly polarized light with the polarization direction at 45 degrees relative to the strain-induced fast and slow axes in the photoelastic material. The sensor measures the difference between the strains along these two orthogonal directions. The reduced sensitivity of the sensor to temperature results from the fact that the illumination is perpendicular to the surface of the test object. All polarization components that are parallel to the surface will experience identical refractive index changes due to thermal effects. Consequently, a measurement of the difference in strains along two directions in the surface plane is insensitive to temperature.

Claims

1. A temperature insensitive strain sensor, comprising:

a strain-sensitive material adapted to be physically contacted to a surface of a workpiece;

means for illuminating said strain-sensitive material with a beam of linearly polarized light at a selected polarization direction relative to strain-induced fast and slow orthogonal axes in said strain-sensitive material, said beam of linearly polarized light being substantially perpendicular to the surface of the workpiece; and

detection means positioned with respect to the workpiece for measuring an intensity of at least one of a first polarization component that is parallel to said selected polarization direction and a second polarization component that is perpendicular to said selected polarization direction of said beam of linearly polarized light making at least one pass through said strain-sensitive material and processing means connected to said detection means for calculating from said intensity a difference between strains along the strain-induced fast and slow orthogonal axes in said strain-sensitive material, said difference being substantially independent of fluctuations in ambient temperature.

2. The sensor according to claim 1 wherein said strain-sensitive material is a photoelastic sheet having first and second planar sides and having strain-induced fast and slow orthogonal x- and y-axes, said photoelastic sheet having its second planar side bonded to a surface of a workpiece, and wherein said means for illuminating said strain-sensitive material with linearly polarized light includes a waveguide optically coupled at one end thereof to a first surface of said photoelastic sheet and optically coupled at another end thereof to a light source, a first polarizing means optically coupled to said waveguide for polarizing light that enters said photoelastic sheet at said selected polarization direction, and a second polarizing means for analysing a polarization of light that makes at least one pass through said photoelastic sheet.

3. The sensor according to claim 2 wherein said selected polarization direction is about 45 degrees.

4. The sensor according to claim 2 wherein said second planar side of said photoelastic sheet bonded to said surface of said workpiece is coated with a reflective coating, wherein said detection means is located to detect light reflected back through said photoelastic sheet from said reflective coating.

5. The sensor according to claim 4 wherein said first polarizing means is a polarizing plate bonded to said first planar side of said photoelastic sheet and oriented to give said selected polarization direction of about 45 degrees, and wherein said second polarizing means is said polarizing plate, and wherein said polarizer plate is oriented with respect to said photoelastic material in such a way that a field of the lightwave, as it enters the photoelastic material, may be written as $E=\frac{E_o}{\sqrt{2}}$

[x^+y^]

where x and y specify the directions that are parallel and perpendicular to a known direction of the applied strain that is to be measured in said test object, and wherein said detection means includes means for monitoring an incident intensity I_o, and wherein said intensity of light is given by $I=\frac{I_o}{2}<br><br>\left[1+\cos <br><br>\left({\mathrm{\phi <br><br>}}_y-{\mathrm{\phi <br><br>}}_x\right)\right]$

and wherein the phase difference, φ

_y−

φ

_x, is given by ${\mathrm{\phi <br><br>}}_y-{\mathrm{\phi <br><br>}}_x=\frac{4<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\pi <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>C_{\mathrm{\varepsilon <br><br>}}<br><br>t}{\mathrm{\lambda <br><br>}}<br><br>\left({\mathrm{\varepsilon <br><br>}}_y-{\mathrm{\varepsilon <br><br>}}_x\right)$

where C_ε

is a strain-optic coefficient of the photoelastic sheet, t is its thickness, λ

is the wavelength of the light, and ε

_y−

ε

_x is a difference in strains along the y- and x-directions which is calculated from said phase difference.

6. The sensor according to claim 4 wherein said waveguide and said first polarizing means optically coupled to said waveguide includes a polarization maintaining optical fiber optically coupled at one end thereof to said photoelastic sheet, the other end of said polarization maintaining optical fiber being connected to a polarization splitter, wherein said detection means includes a first detector, wherein said first detector and said light source are optically coupled to a 2×

2 fiber-optic coupler, wherein said second detector and said 2×

2 fiber-optic coupler are optically coupled to a fiber-optic polarization splitter, and one output from said polarization splitter is optically coupled to said photoelastic sheet by means of polarization maintaining optical fiber.

7. The sensor according to claim 6 wherein said first detector measures the intensity (I) of the parallel polarized component of light given by $I=\frac{I_o}{2}$

[1+cos

(φ

y-φ

x)]

and wherein said second detector measures an intensity (I′

) of the perpendicular polarized component of light given by $I^{\mathrm{\prime <br><br>}}=\frac{I_o}{2}<br><br>\left[1-\cos <br><br>\left({\mathrm{\phi <br><br>}}_y-{\mathrm{\phi <br><br>}}_x\right)\right],$

and wherein said means for determining a strain applied to said test object from said measured light intensity calculates an output signal given by $S=\frac{I-I^{\mathrm{\prime <br><br>}}}{I+I^{\mathrm{\prime <br><br>}}}=\cos <br><br>\left({\mathrm{\phi <br><br>}}_y-{\mathrm{\phi <br><br>}}_x\right).$

and wherein the phase difference φ

_y−

φ

_x, is given by ${\mathrm{\phi <br><br>}}_y-{\mathrm{\phi <br><br>}}_x=\frac{4<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\pi <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>C_{\mathrm{\varepsilon <br><br>}}<br><br>t}{\mathrm{\lambda <br><br>}}<br><br>\left({\mathrm{\varepsilon <br><br>}}_y-{\mathrm{\varepsilon <br><br>}}_x\right),$

therein C_ε

is a strain-optic coefficient of the photoelastic sheet, t is its thickness, λ

is the wavelength of the light, and ε

_y−

ε

_x is a difference in strains along the y- and x-directions which is calculated from said phase difference.

8. The sensor according to claim 7 wherein said polarization maintaining optical fiber optically coupled to said photoelastic sheet delivers polarized light from said light source parallel to one of a principal axes of said polarization maintaining optical fiber so that the light output from said optical fiber into said photoelastic sheet is linearly polarized at a direction substantially 45 degrees relative to a direction of maximum applied strain.

9. The sensor according to claim 8 including means for measuring optical power optically coupled to said 2×

2 fiber-optic coupler for measuring an output power of said light source.

10. The sensor according to claim 9 wherein said polarization maintaining optical fiber is optically coupled to one end of a graded index lens and the other end of said graded index lens is bonded to said photoelastic sheet.

11. The sensor according to claim 8 including a birefringent wave plate bonded between said photoelastic sheet and said graded index lens attached to the end portion of said polarization maintaining optical fiber, wherein said birefringent wave plate is oriented with one its principal axes parallel to a direction of maximum applied strain for adding a fixed phase delay to a strain-induced phase delay between the x- and y-polarization components.

12. The sensor according to claim 4 wherein said selected polarization direction is about 45 degrees, and wherein said first and second polarizing means is a polarizing beamsplitter having first and second faces disposed at 90 degrees to each other, said polarizing beamsplitter having a third face perpendicular to said first face and being bonded to said photoelastic sheet, wherein said detection means includes a first detector optically coupled to a first face of said polarizing beamsplitter for detecting the intensity (I) of the parallel polarized component of light reflected from said reflective coating back through said photoelastic sheet, and a second detector optically coupled to a second face of said polarizing beamsplitter for measuring the intensity (I′

) of the perpendicularly polarized component of light reflected from said reflective coating back through said photoelastic sheet.

13. The sensor according to claim 12 wherein said light source and said first detector are optically coupled to an optical coupler by optical fibers, wherein said optical coupler is optically coupled to said first face of said polarizing beams splitter by another optical fiber, and wherein said second detector is optically coupled to said second face of said polarizing beams splitter by means of an optical fiber.

14. The sensor according to claim 13 wherein said intensity (I) of parallel polarized component of light is given by $I=\frac{I_o}{2}$

[1+cos

(φ

y-φ

x)]

and wherein said intensity (I′

) of the perpendicularly polarized component of light is given by $I^{\mathrm{\prime <br><br>}}=\frac{I_o}{2}<br><br>\left[1-\cos <br><br>\left({\mathrm{\phi <br><br>}}_y-{\mathrm{\phi <br><br>}}_x\right)\right],$

and wherein said means for determining a strain applied to said workpiece from said measured light intensity calculates an output signal given by $S=\frac{I-I^{\mathrm{\prime <br><br>}}}{I+I^{\mathrm{\prime <br><br>}}}=\cos <br><br>\left({\mathrm{\phi <br><br>}}_y-{\mathrm{\phi <br><br>}}_x\right)$

and wherein the phase difference φ

_y−

φ

_x, is given by ${\mathrm{\phi <br><br>}}_y-{\mathrm{\phi <br><br>}}_x=\frac{4<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\pi <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>C_{\mathrm{\varepsilon <br><br>}}<br><br>t}{\mathrm{\lambda <br><br>}}<br><br>\left({\mathrm{\varepsilon <br><br>}}_y-{\mathrm{\varepsilon <br><br>}}_x\right),$

wherein C_ε

is a strain-optic coefficient of the photoelastic sheet, t is its thickness, λ

is the wavelength of the light, and ε

_y−

ε

_x is a difference in strains along the y- and x-directions which is calculated from said phase difference.

15. The sensor according to claim 14 wherein each optical fiber optically coupled to said first and second faces of said polarizing beamsplitter have end portions which are bonded to an associated graded index lens, and wherein one of said graded index lenses with one of said optical fibers attached thereto is bonded to said first face and a second of said graded index lenses with the other optical fiber attached thereto is bonded to said second face.

16. The sensor according to claim 15 wherein said optical coupler is a 3 dB optical coupler.

17. The sensor according to claim 2 wherein said workpiece includes a hole extending therethrough and said photoelastic sheet is attached to the surface of the test piece covering said hole, and wherein said second polarizing means for analysing a polarization of light that makes at least one pass through said photoelastic sheet is located on an opposite side of said test workpiece to the side on which said photoelastic sheet is attached.

18. The sensor according to claim 17 wherein said first polarizing means is a polarizing plate bonded to said photoelastic sheet and oriented to give said selected polarization direction of about 45 degrees, and wherein said second polarizing means is a second polarizer plate optically coupled to said detection means, and wherein said first polarizer plate is oriented with respect to said photoelastic material in such a way that a field of the lightwave, as it enters the photoelastic material, may be written as $E=\frac{E_o}{\sqrt{2}}$

[x^+y^]

where x and y specify the directions that are parallel and perpendicular to a known direction of the applied strain that is to be measured in said workpiece, and wherein said detection means includes means for monitoring an incident intensity I_o, and wherein said intensity of light is given by $I=\frac{I_o}{2}<br><br>\left[1+\cos <br><br>\text{\hspace{1em}<br><br>}<br><br>\left({\mathrm{\phi <br><br>}}_y-{\mathrm{\phi <br><br>}}_x\right)\right].<br><br>\text{\hspace{1em}<br><br>}$

and wherein the phase difference, φ

_y−

φ

_x, is given by ${\mathrm{\phi <br><br>}}_y-{\mathrm{\phi <br><br>}}_x=\frac{4<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\pi <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>C_{\mathrm{\varepsilon <br><br>}}<br><br>\text{\hspace{1em}<br><br>}<br><br>t}{\mathrm{\lambda <br><br>}}<br><br>\text{\hspace{1em}<br><br>}<br><br>\left({\mathrm{\varepsilon <br><br>}}_y-{\mathrm{\varepsilon <br><br>}}_x\right)<br><br>\text{\hspace{1em}<br><br>}$

where C_ε

is a strain-optic coefficient of the photoelastic sheet, t is its thickness, λ

is the wavelength of the light, and ε

_y−

ε

_x is a difference in strains along the y- and x-directions which is calculated from said phase difference.

19. The sensor according to claim 18 wherein said detection means is a detector, including an optical fiber having one end connected to said detector and another end bonded to a graded index lens, said graded refractive index lens being attached to said second polarizer plate.

20. The sensor according to claim 17 wherein said first polarizing means is a polarizer plate bonded to said first planar side of said photoelastic sheet and oriented to give said selected polarization direction of about 45 degrees, wherein said light source and said polarizer plate are optically coupled together by an optical fiber, and wherein said second polarizing means is a polarizing beamsplitter having first and second planar faces disposed at 90 degrees to each other and a third planar face parallel to said first face positioned so that light passing through said work piece is incident upon said third planar face, wherein said detection means includes a first detector optically coupled to said first planar face of said polarizing beamsplitter for detecting the intensity (I) of a component of light polarized parallel to the polarization of the light transmitted through said polarization plate, and a second detector optically coupled to said second planar face of said polarizing beamsplitter for measuring the intensity (I′

) of the component of light polarized perpendicular to the polarization of the light transmitted through said polarization plate.

21. The sensor according to claim 20 wherein said intensity (I) of parallel polarized component of light is given by $I=\frac{I_o}{2}$

[1+cos

 



(φ

y-φ

x)]

 

and wherein said intensity (I′

) of the perpendicularly polarized component of light is given by $I^{\mathrm{\prime <br><br>}}=\frac{I_o}{2}<br><br>\left[1-\cos <br><br>\text{\hspace{1em}<br><br>}<br><br>\left({\mathrm{\phi <br><br>}}_y-{\mathrm{\phi <br><br>}}_x\right)\right],$

and wherein said means for determining a strain applied to said workpiece from said measured light intensity calculates an output signal given by $S=\frac{I-I^{\mathrm{\prime <br><br>}}}{I+I^{\mathrm{\prime <br><br>}}}=\cos <br><br>\text{\hspace{1em}<br><br>}<br><br>\left({\mathrm{\phi <br><br>}}_y-{\mathrm{\phi <br><br>}}_x\right).$

and wherein the phase difference φ

_y−

φ

_x, is given by ${\mathrm{\phi <br><br>}}_y-{\mathrm{\phi <br><br>}}_x=\frac{4<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\pi <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>C_{\mathrm{\varepsilon <br><br>}}<br><br>\text{\hspace{1em}<br><br>}<br><br>t}{\mathrm{\lambda <br><br>}}<br><br>\text{\hspace{1em}<br><br>}<br><br>\left({\mathrm{\varepsilon <br><br>}}_y-{\mathrm{\varepsilon <br><br>}}_x\right),$

wherein C_ε

is a strain-optic coefficient of the photoelastic sheet, t is its thickness, λ

is the wavelength of the light, and ε

_y−

ε

_x is a difference in strains along the y- and x-directions which is calculated from said phase difference.

22. The sensor according to claim 21 wherein each optical fiber optically coupled to said first and second faces of said polarizing beamsplitter have end portions which are bonded to an associated graded index lenses, and wherein one of said graded index lenses with one of said optical fibers attached thereto is bonded to said first face and a second of said graded index lenses with the other optical fiber attached thereto is bonded to said second face.

23. The sensor according to claim 2 wherein said light source is a superluminescent diode that emits light at a wavelength of about 850 nm.

24. A method of measuring strain in a workpiece, comprising the steps of:

illuminating a strain-sensitive material with a beam of linearly polarized light with a selected polarization direction relative to strain-induced fast and slow orthogonal axes in the strain-sensitive material, the strain-sensitive material being in physical contact with a surface of a workpiece, the beam of linearly polarized light being substantially perpendicular to the surface of the workpiece; and

measuring an intensity of at least one of a first polarization component that is parallel to the selected polarization direction and a second polarization component that is perpendicular to the selected polarization direction of the beam of linearly polarized light making at least one pass through the strain-sensitive material and calculating from the intensity a difference between strains along the strain-induced fast and slow orthogonal axes in the strain-sensitive material, the difference being substantially independent of fluctuations in ambient temperature.

25. The method of measuring strain according to claim 24 wherein said selected polarization direction is substantially at 45 degrees, and wherein the step of measuring said intensity of one or both of the polarization components that are parallel and perpendicular to the selected input polarization direction in light and calculating from said intensity a difference between strains includes calculating from said measured intensity a phase shift φ

_y−

φ

_x accumulated in said linearly polarized light and calculating the difference in strains from an equation ${\mathrm{\phi <br><br>}}_y-{\mathrm{\phi <br><br>}}_x=\frac{4<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\pi <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>C_{\mathrm{\varepsilon <br><br>}}<br><br>t}{\mathrm{\lambda <br><br>}}<br><br>\left({\mathrm{\varepsilon <br><br>}}_y-{\mathrm{\varepsilon <br><br>}}_x\right)$

wherein C_ε

is a strain-optic coefficient of the strain-sensitive element, t is its thickness, λ

is the wavelength of the light, and ε

_y−

ε

_x is the difference in strains along the y and x directions.

26. The method according to claim 25 wherein said strain-sensitive element is a photoelastic sheet bonded to said surface of said workpiece.

27. The method according to claim 24 wherein said selected polarization direction is substantially at 45 degrees, and wherein said intensity of both said parallel and perpendicular components of light are measured, and wherein said intensity (I) of parallel polarized component of light is given by $I=\frac{I_o}{2}$

[1+cos

 



(φ

y-φ

x)]

and wherein said intensity (I′

) of the perpendicular component of light is given by $I^{\mathrm{\prime <br><br>}}=\frac{I_o}{2}<br><br>\left[1-\cos <br><br>\text{\hspace{1em}<br><br>}<br><br>\left({\mathrm{\phi <br><br>}}_y-{\mathrm{\phi <br><br>}}_x\right)\right],$

and wherein said step of claculating a strain includes calculating an output signal given by $S=\frac{I-I^{\mathrm{\prime <br><br>}}}{I+I^{\mathrm{\prime <br><br>}}}=\cos <br><br>\text{\hspace{1em}<br><br>}<br><br>\left({\mathrm{\phi <br><br>}}_y-{\mathrm{\phi <br><br>}}_x\right).$

and wherein the phase difference φ

_y−

φ

_x, is given by ${\mathrm{\phi <br><br>}}_y-{\mathrm{\phi <br><br>}}_x=\frac{4<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\pi <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>C_{\mathrm{\varepsilon <br><br>}}<br><br>\text{\hspace{1em}<br><br>}<br><br>t}{\mathrm{\lambda <br><br>}}<br><br>\text{\hspace{1em}<br><br>}<br><br>\left({\mathrm{\varepsilon <br><br>}}_y-{\mathrm{\varepsilon <br><br>}}_x\right),$

wherein C_ε

is a strain-optic coefficient of the photoelastic sheet, t is its thickness, λ

is the wavelength of the light, and ε

_y−

ε

_x is a difference in strains along the y- and x-directions which is calculated from said phase difference.

28. The method according to claim 24 wherein said light source is a laser that emits at a wavelength of about 850 nm.