Method and apparatus for sensing a measurand

• US 4,713,540 A
• Filed: 07/16/1985
• Issued: 12/15/1987
• Est. Priority Date: 07/16/1985
• Status: Expired due to Fees

• Current Assignee: Foxboro Company The Foxboro Massachusetts
• Original Assignee: The Foxboro Company (Schneider Electric SE)
• Inventors: Gilby, Anthony C., Ihnat, Dale E.
• Primary Examiner(s): Westin, Edward P.
• Application Number:
  US06/755,646
• Time in Patent Office: 882 Days
• Field of Search: 250/227, 250/231 R, 250/231 P, 73/862.41, 73/862.59, 73/655, 73/705, 73/800, 350/96.29
• US Class Current: 250/227.21
• CPC Class Codes: G01D 5/268 using optical fibres G01D5/28 - G01D5/38 take precedence
• CPC Class Codes: G01L 9/0014 Optical excitation or measuring of vibrations
• CPC Class Codes: G01L 9/0022 of a piezoelectric element
• CPC Class Codes: G10K 15/046 using optical excitation, e.g. laser bundle

Abstract

Vibrating resonant mechanical structures are excited optically by photokinetic energy transfer and detected optically. In particular, a tuning fork sensor measuring a physical parameter is excited into oscillation by radiant energy and the frequency of vibration is detected by radiant energy which may be communicated via an optical fiber. These vibrations are communicated via a fiber optic to a detection location.

Claims

1. Apparatus for use in the measurement of a physical parameter by photokinetic stimulation and optical detection of resonant mechanical structures, comprising:

(a) resonant mechanical structure means for vibration;

(b) means for communicating the physical parameter to said resonant mechanical structure means;

(c) radiant energy source means for providing at least one radiant energy beam;

(d) radiant energy pathway means for communicating said radiant energy to a first area of said resonant mechanical structure;

(e) means for directly converting said radiant energy to thermal energy conducted within at least a portion of the resonant mechanical structure to stimulate motion thereof at a frequency related to the physical parameter value; and

(f) means for returning at least a portion of the radiant energy along at least a portion of said pathway means as a modulated signal related to the resonant structure vibration.

2. Apparatus as in claim 1, further including means for detection of the returned modulated signal, means for providing an output signal related to the returned modulated signal, and means for closing an oscillation loop formed by the radiant energy source means, the radiant energy pathway means, the resonant mechanical structure means, the radiant-to-thermal conversion means, the means for returning a modulated signal, and said means for detection of the returned modulated signal and said means for providing an output signal.

3. Apparatus as in claim 1, wherein the resonant mechanical structure means is a beam structure.

4. Apparatus as in claim 3, wherein the beam structure comprises a single beam.

5. Apparatus as in claim 3, wherein the beam structure is a multiple beam.

6. Apparatus as in claim 1, wherein the resonant mechanical structure is a tuning fork.

7. Apparatus as in claim 1, further including a resonator surface coating which enhances heat absorption.

8. Apparatus as in claim 3, wherein the resonant mechanical structure is hollow.

9. Apparatus as in claim 1, wherein said mechanical structure is an elongated structure including a face surface, a rear surface, and at least one area on the face surface thereof adapted to receive radiant energy and convert said radiant energy to a stress within the resonant mechanical structure.

10. Apparatus as in claim 1, wherein said first area of said resonant mechanical structure is an excitation area and wherein said radiant energy pathway means includes a first fiber optic for communicating excitation energy to the excitation area.

11. Apparatus as in claim 1, wherein said resonant mechanical structure includes an optical modulation sense area and wherein said radiant energy pathway means communicates an optical sense signal to the optical modulation sense area.

12. Apparatus as in claim 11, wherein said first area of said resonant mechanical structure is an excitation area and wherein said sense area and said first area are the same.

13. Apparatus as in claim 1, in which said resonant mechanical structure is adapted for intermittent interruption of said at least one radiant energy beam along said radiant energy pathway means and further including additional radiant energy pathway means for conveying interrupted radiant energy to said first area of said resonant mechanical structure.

14. Apparatus as in claim 1, which is optically excited and is self-oscillating.

15. Apparatus as in claim 1, wherein said first area of said resonant mechanical structure is an excitation area and further including shuttering means for interruption of a portion of the radiant energy, and wherein a single steady radiant energy beam can be communicated along a single fiber optic pathway means to said means for shuttering optical energy, further including means to communicate a portion of the shuttered radiant energy to said excitation area.

16. Apparatus as in claim 11, wherein said first area of said resonant mechanical structure is an excitation area and wherein said sense area and said first area are the same.

17. Apparatus as in claim 15, wherein said resonant mechanical structure further includes means for reflecting radiant energy back along said fiber optic pathway means substantially only during interruption of said radiant energy.

18. Apparatus as in claim 1, wherein said resonant mechanical structure is a multiple beam device having at least two beams, a first one of which includes means for reflecting radiant energy, said radiant energy pathway means is adapted to direct radiant energy to said first area of said resonant mechanical structure, and said means for returning at least a portion of the radiant energy is said means for reflecting radiant energy.

19. Apparatus as in claim 1, wherein said means for communicating radiant energy to said first area of said resonant mechanical structure is terminated by a pattern of alternating reflection and transmission areas, wherein said first area of said resonant mechanical structure includes a matching pattern of alternating areas, one of which group of areas is absorptive, and wherein the two patterns are aligned to facilitate photokinetic stimulation of the resonant structure into vibration and optical reflection of the vibrating frequency.

20. A method for the photokinetic stimulation and optical detection of a resonant mechanical structure sensing at least one physical parameter, comprising the steps of:

(a) generation of radiant energy from at least one first radiant energy source;

(b) communicating said radiant energy along at least one radiant energy communicating pathway to a resonant mechanical structure;

(c) converting at least a portion of the radiant energy to thermal energy;

(d) dispersing the thermal energy within a portion of the resonant mechanical structure as stress to excite vibration of the resonant mechanical structure;

(e) altering the frequency of the resonant mechanical structure as a function of the measured physical parameter;

(f) modulating a portion of the radiant energy according to the frequency of vibration;

(g) returning at least a portion of the radiant energy along said radiant energy communicating pathway as a modulated sense signal; and

(h) detecting said modulated sense signal to provide a signal related to the physical parameter.

21. The method according to claim 20, further including the step of feeding back a signal derived from the detected modulated sense signal to the first radiant energy source.

22. The method according to claim 20, further including generation of first radiant energy for photokinetic excitation of the resonant mechanical structure.

23. The method of claim 22, wherein the radiant energy for sensing the vibrating frequency is modulated by the resonant mechanical structure.

24. The method according to claim 22, further including generation of radiant energy for optically sensing the vibrating frequency of the resonant mechanical structure.

25. The method of claim 22, wherein the first radiant energy is of a first wavelength and the second radiant energy is of a second, discernibly different wavelength.

26. The method of claim 25, wherein the first radiant energy and the radiant energy for sensing the vibrating frequency are carried on a single optical fiber.

27. The method of claim 20, wherein step (c) comprises:

(c) shuttering the radiant energy by interruption thereof by a portion of the resonant mechanical structure and conveying the shuttered radiant energy to the portion of the resonant mechanical structure for excitation thereof by converting the shuttered radiant energy to time-varying thermal energy in order to produce self-oscillation.