Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
1998-10-28
2001-01-09
Lee, John R. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S227140, C250S227250, C250S904000
Reexamination Certificate
active
06172377
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
None.
BACKGROUND OF THE INVENTION
This invention relates to liquid level sensors, typically the sending unit for a fuel gauge. More particularly, this invention relates to liquid level sensors that employ an optical waveguide that is immersed in the fluid to provide a continuous measurement of the liquid level.
The need for continuous liquid-level measurements exists in numerous commercial and military applications, such as in the fuel, oil and water tanks of aircraft, automobiles and trucks. Less mobile applications include storage tanks or fuel dispensing, waste water, home, chemical, and food processing purposes, to name but a few. Electrical sensors are of particular concern in many cases, particularly those involving flammable fluids. Hazards are apparent from electric sparks from such sensors in these potentially explosive environments. In other environments, electromagnetic interference may overwhelm the signals generated by these sensors. Conventionally, the level of a liquid in a vessel is detected using a float with a mechanical or magnetic coupling to an external gauge, an ultrasonic or optical transducer which measures time of flight to deduce the liquid level, or a parallel wire capacitance sensor that monitors the change in the dielectric constant between the wires associated with a change in liquid level. Capacitance sensors lose accuracy with increasing amounts of water in the fuel. Sensing of fluid level by visual inspection is only viable in limited circumstances since most usage's require remote and continuous sensing of fluid levels.
A number of fluid level sensors based on fiber optics have been proposed. They are attractive because they introduce no electrical energy into the tank, are insensitive to electromagnetic interference, have no moving parts and can provide a continuous measurement of fluid level. A number of older fiber optic fluid level sensors are described in the article “Experimental Investigations on Fibre Optic Liquid Level Sensors and Refractometers,” by K. Spenner et al. in IEE OFS 221 pages 96-99. These implementations employ two separate optical fibers and are primarily for the measurement of discrete levels in the vessel, e.g. to sense whether the vessel is completely full or completely empty or at some intermediate point. It would be more desirable to have a fiber optic sensor that could provide continuous measurements.
An early continuous measurement fiber optic sensor is described in “Fibre Optic Fluid Level Sensor,” by M. Belderdid, N. Ghanderharioun, and B. Brennan in the Proceedings of the SPIE Conference
566
Fibre Optic and Laser Sensors IlIl (1985) pages 153-158. This sensor is based on the bending or cladding loss principle, consisting of large loops of a single fibre, the loops being of ever increasing diameter and suspended in the liquid. U.S. Pat. No. 4,870,292 to Alpert et al., teaches a fluorescent doped detector fiber to collect light reflected from a source fiber in the presence of air; the light is refracted away when a fluid of higher refractive index is present. The output signal from the fluorescent doped fiber is analogous to the fluid level.
U.S. Pat. No. 4,942,306 builds upon the '292 patent above by embedding one end of the source optical fiber into a transparent substrate such that the injected light enters the substrate at such an angle that it will refract out of the substrate when the substrate is in the liquid but will continue to be totally internally reflected when the substrate is in the air above the liquid. The detecting fiber is again a separate fluorescent doped fiber that is placed against the transparent substrate, typically being wrapped in grooves around the outside of the substrate.
Nevertheless, there remains a need in the art for a simple, low cost liquid level sensor based on this fiber optic technology, preferably realized as a single optical element immersed in the fluid.
SUMMARY OF THE INVENTION
This invention comprises a single optical waveguide that contains fluorescent material therein that absorbs light at a first wavelength and emits light at a second wavelength. To one end of the waveguide is attached a light input means that introduces light of the first wavelength into the waveguide at an angle wherein total internal reflection will occur within the waveguide if the fluid adjacent to the waveguide is air. However, the refractive index of the waveguide and the angle of internal reflection within the waveguide are such that, if the waveguide is immersed in a liquid with a refractive index above a certain value, the light will dissipate into the liquid and no longer be retained within the waveguide. The light that stays within the waveguide will interact with the fluorescent material and be re-emitted as light at the second wavelength. The light at the one end of the waveguide is gathered by collection means for transfer to an optical sensor that is sensitive only to the second wavelength of light. A filter is interposed in order to block all light except that of the second wavelength from reaching this sensor. The amount of collected light at the second wavelength is a direct function of how much of the waveguide is in air rather than in the liquid. The longer the length of waveguide that supports total internal reflection of the light, the more of the fluorescent material that is exposed to the light at the first wavelength. Thus, the amount of light collected at the second wavelength monotonically increases with the length of the waveguide above the liquid if the concentration of the fluorescent material in the waveguide is constant. As a result, a relationship can be established between the strength of the received optical signal and the liquid level.
REFERENCES:
patent: H1364 (1994-10-01), Toeppen
patent: 4650992 (1987-03-01), Ruhrmann
patent: 4727247 (1988-02-01), Johnston
patent: 4870292 (1989-09-01), Alpert et al.
patent: 4880971 (1989-11-01), Danisch
patent: 4942306 (1990-07-01), Colbourne
patent: 4994682 (1991-02-01), Woodside
patent: 5164608 (1992-11-01), Vali et al.
Cone Gregory A.
Lee John R.
Sandia Corporation
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