Fibre optic discrete or continuous liquid level sensor

0001This invention relates to a method and apparatus for optically detecting a liquid level or other physical parameters.

<u style="single">Background of the Invention</u>

0002Conventionally, 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 which monitors the change in the dielectric constant between the wires associated with a change in liquid level.

0003The application of fibre optics to level sensing in liquids is well documented. The principal advantages of this type of level sensor are its passivity, i.e., no moving or mechanical parts, and its intrinsic dielectric properties which mean no spark hazard when used with combustible fluids and virtually zero susceptibility to electromagnetic interference. The more common implementations use small prisms mounted at the end of two optical fibres, a conically shaped optical fibre tip, or a U-shaped bent optical fibre [Canadian Patent 1102151] (devices of these types are described for example in the article "Experimental Investigations on Fibre Optic Liquid Level Sensors and Refractometers" by K. Spenner et al. - IEE OFS 221 pages 96-99). In all instances, the optical fibre probe or sensor is suspended or made to protrude into the vessel, and the potentially fragile sensor is exposed to damage by floating debris, vibration, and dynamic effects during filling. The potential for damage is increased if routine maintenance of the sensor is required due to biological or chemical fouling of the optical surface.

0004Other level sensors utilizing fibre optics are found, for example, in U.S. Patents 3,995,168 to Neuscheler et al. and 4,440,022 to Masom.

0005The fibre optic sensors just described are primarily for discrete level measurement, eg., to sense whether the vessel is empty or full at some intermediate point. A multiplicity of such point sensors is operable and is suggested but generally represents an impractical configuration for a continuous liquid level measure. A continuous measure is desirable, however, for improved resolution in many applications. Consider the case of an aircraft fuel gauging system. The dielectric properties of the optical fibre sensor are desirable from the point of view of safety with respect to spark hazard and lightning strikes but high resolution and accuracy are also desirable so that excess fuel quantities would not have to be carried thus reducing aircraft weight and consequently fuel consumption. Present day aircraft fuel level sensors are for the most part capacitance type sensors which lose accuracy when the fuel becomes laden with water and the dielectric constant is changed significantly.

0006Only one continuous fibre optic liquid level sensor has been found reported thus far. It is "Fibre Optic Fluid Level Sensor" by M. Belkerdid, N. Ghanderharioun, and B. Brennan in the Proceedings of SPIE Conference 566 Fibre Optic and Laser Sensors III (1985) pages 153-158. Based on the bending or cladding loss principle, it consists of large loops of a single fibre, the loops being of ever increasing diameter, which are suspended in the liquid. Here again, the sensor packaging is not sufficiently robust for most applications.

0007Another patent of interest is U.S. Patent 4 403 826 which teaches a detection optical fibre, the core of which contains fluorescing material. On exposure to ultraviolet light, depending on the position of a sleeve during calibration, or some other means or parameter during actual measurement, the fluorescent material will fluoresce and conduct light to a detector. The intensity and distribution of ultraviolet light can be used, for example, in industrial monitoring of materials and processing, etc.

Summary of the Invention

0008The present invention as claimed in claim 1 overcomes the problems of the prior art in that the optical fibres are embedded in an optically transparent substrate material that provides for protection of the sensor and also provides a surface which can be cleaned. This same packaging geometry can be applied both to a discrete (i.e., point) liquid level sensor and to a continuous or analog liquid level sensor.

0009It is well known that the relative refractivity and hence the critical angle of total reflection at an interface is dependent on the indices of refraction of the two media forming the interface. This is more often referred to as Snell's law, n₁sinϑ₁ = n₂sinϑ₂, which describes the relationship between the angles of incidence ϑ₁ and refraction ϑ₂ for a light ray propagating in a medium of refractive index n₁, incident on a medium of refractive index n₂. Using this equation it is evident that for n₁ > n₂ there is a limiting angle of incidence called the critical angle, given by the arcsine of the ratio of the refractive indices, beyond which total internal reflection occurs.

0010In this discussion, the terms "liquid" and "air" are used to refer to any two fluids of sufficiently different refractive indices to be sensed by this refractometric technique. Hence "air" might equally well refer to the gaseous phase of the sensed liquid or to another liquid as in the case of oil and water. Similarly, the term "optical fibre" is used to describe any elongated light conducting body or light guide capable of conveying light by multiple internal reflections.

0011The continuous liquid level sensors described herein consist of two optical fibres, a source or transmit optical fibre having one end adapted to be optically connected to an external light source, and a detector or receive optical fibre having one end adapted to be optically connected to an external light detector. The source and detector optical fibres each have at least the other end thereof embedded in an optically clear substrate material or window that is in contact with the liquid to be sensed such that the light exiting the source fibre is incident on the substrate to liquid or air interface at an angle between the critical angle for the liquid and the critical angle for air. The substrate or window material has a refractive index equal or nearly equal to that of the core of the optical fibre so that the maximum refractive index mismatch will occur at the interface to the liquid. The detector fibre is doped so as to fluoresce when light of specific wavelength(s) impinges thereon. The fluorescent detector fibre is mounted within the substrate so that it can receive any light from the source which is internally reflected from the interface. Hence a received light signal will occur at the external light detector only when a portion of the sensor is exposed to air. When a fluid of higher refractive index such as water or gasoline is present at the liquid sensing surface, the substrate - liquid interface becomes primarily transmissive and only a small amount of light is reflectively coupled from the source fibre to the detector fibre. In this way, the optical signal detected from the detector fibre is related to the height of the liquid sensing surface which is immersed.

0012In any case, a mechanism can be easily included to provide for detection of any failure in the sensor's optical path from and including the light source to the detector.

0013The device may be made to measure parameters other than fluid level (such as displacement, temperature, pressure, etc.) by the addition of a sleeve external to the substrate, the motion of which is driven by the change in the parameter to be measured. The sleeve would cause variation in the output signal by the same reflection/refraction principle described above.

0014In summary of the above the present invention may be considered as providing an indicating device for indicating the level of fluids in a container, comprising at least two optical fibres having at least a portion thereof embedded in a transparent substrate material of similar refractive index, one of the fibres being optically connected to a light source and being capable of emitting a particular wavelength or range of wavelengths of light along its length, the other fibre being optically connected to a light detector and being doped so as to fluoresce when illuminated by light at the wavelength(s) emitted by the source fibre, the two optical fibres being orientated with respect to the substrate-fluid interface so that light exiting the source fibre is coupled into the detector fibre when total internal reflection occurs at the substrate-fluid interface in the presence of a first fluid, and coupling does not occur in the presence of a second fluid of higher refractive index than the first fluid.

<u style="single">Brief Description of the Drawings</u>

0015This invention will be described in more detail hereinafter in connection with the accompanying drawings in which: <ul id="ul0001" list-style="none"><li><b>Figure 1</b> is a perspective view of a discrete or point sensor illustrating principles on which the invention is based;</li><li><b>Figures 2A and 2B</b> are side views of the discrete fibre optic liquid level sensor shown in Figure 1, showing the cases of total internal reflection (Fig. 2A) and refraction (Fig. 2B) in the presence of air and liquid respectively;</li><li><b>Figure 3A</b> is a plan view of the fluorescent-fibre continuous liquid level sensor of the present invention;</li><li><b>Figure 3B</b> is a perspective view of the invention of Fig. 3A;</li><li><b>Figure 4</b> describes the principle of operation of the fluorescent detector optical fibre shown in Figure 3;</li><li><b>Figure 5</b> shows the cases of total internal reflection and refraction in the presence of air and liquid respectively for the invention depicted in Figure 3;</li><li><b>Figures 6, 7 and 8</b> show additional embodiments of the fluorescent-fibre configuration;</li><li><b>Figure 9</b> shows a general view of the fluorescent-fibre continuous level sensor configured as a cylindrical probe;</li><li><b>Figure 10</b> is similar to Figure 9 except that the source optical fibre is replaced with a bundle of optical fibres;</li><li><b>Figure 11</b> shows a level sensor using two sources and two detectors; and</li><li><b>Figure 12</b> shows a sensor in which a plurality of source fibres is arranged at different levels within the substrate.</li></ul>

<u style="single">Description of the Preferred Embodiment</u>

(a) Discrete Level Sensor

0016Referring to the drawings, in the discrete or point sensor shown in Figures 1 and 2, a source optical fibre 1 is optically coupled to a light source 5 such as a light emitting diode or laser diode and a detector optical fibre 2 is optically coupled to a light detector 6 such as a photodiode or phototransistor. The fibres 1 and 2 are mounted in an optically transparent substrate 3 in the same plane normal to the planar interface surface 4 and at an equal angle ϑ to the normal such that the angle of incidence equals the angle of reflection. The optical fibre ends may be polished using standard lapping film techniques or may be simply cleaved or cut, and mounted in blind holes in which some fluid or epoxy, of a refractive index which matches that of the optical fibre core material and the substrate material, has been placed to minimize any optical loss due to the refractive index mismatch, i.e., Fresnel loss. The blind end of each hole is normally as close to the interface surface 4 as possible to minimize the optical loss of the effectively unguided light due to the effective end separation of the optical fibres. In the case of a substrate material with a refractive index 1.5 as would by typical of glass or nylon or some other plastic, and where the liquid is water with a refractive index of 1.33 above which is air with a refractive index of 1.00, the critical angles for total internal reflection are 41.8 degrees and 62.5 degrees for air and water respectively. Hence, if the optical fibres are positioned between these two critical angles, e.g., at 45 degrees with respect to the normal and in the same plane such that the angle of incidence of the light exiting the source optical fibre onto the window to liquid interface equals the angle of reflection and of the detector optical fibre, then in the presence of air, total internal reflection will occur and the light detector 6 will be illuminated signifying an "on" or low level condition (Fig. 2A). When the liquid is present, the light exiting the source fibre will be largely refracted into the liquid and the light detector 6 will be dark, indicating an "off" or high level condition (Fig. 2B).

0017A multiplicity of such point detectors can be ganged together to approximate continuous level detection.

(b) Continuous Level Sensor Using Fluorescence

0018A truly continuous liquid level sensor can be fabricated with a somewhat different optical fibre arrangement based on the same optical principle and a similar packaging arrangement. In the continuous liquid level sensor as shown in Figures 3A and 3B, the source optical fibre 14 and the detector optical fibre 13 are embedded longitudinally in the optically transparent substrate material 7. The two optical fibres are parallel or nearly parallel to one another and to the planar sensor window 17. In addition, a radius from each of the two fibres intersects at or near the sensor window 17 according to the basic laws of reflection. Referring now to Figure 4, the core 8 of the detector optical fibre 13 is doped with a fluorescent substance which when irradiated or excited with a short wavelength optical source, re-emits light of a longer wavelength. Normally, light passing transverse to and through an optical fibre, that is an unguided light, is not coupled into the optical waveguide because its angle of incidence exceeds the critical angle defined by the core and cladding refractive indices. However, in the fluorescent optical fibre, the fluorescent sites (10) in Figure 4 emit spherically; a portion of this longer wavelength emission as denoted by the cones of light 9 in Figure 4 will remain as guided light in the detector optical fibre's core, and will propagate to both ends of the optical fibre, at least one end of which has been adapted to optically connect to an external light detector (16 in Figure 3B) such as a photodiode.

0019Referring again to Figure 3B, the source fibre 14 is optically connected at one or both ends to an external light source 15 which may be a laser diode or flash tube such as a xenon strobe light, either of which has some wavelength or colour component matched to the fluorescence excitation. The source fibre is configured so that light is lost or emitted along its length thus creating an optical line source. This can be achieved by having the fibre abraded or scratched or modified to include a longitudinal strip of higher refractive index than that of the core. Another means of "leaking" the light from the fibre is to arrange it in a series of microbends. With mirrors or reflective gratings 11 and lenses 12 to redirect the light exiting the source fibre into a more radial and focused direction, that direction being between the critical angles for air and the liquid to be sensed and along the intersecting radii noted previously, the line source fibre 14 illuminates the fluorescent detector optical fibre 13 via total internal reflection as in the previous embodiment, the incident light being proportional to the length of the sensor face which is exposed to the air as shown in Figure 5. This incident light stimulates a proportional fluorescence signal in the detector optical fibre which is then guided to the external light detector. The light from the line source optical fibre emitted below the liquid's level is refracted into the liquid and does not propagate to the fluorescent fibre where it would contribute to the fluorescence signal. The resulting optical signal received at the light detector corresponds inversely with the liquid level.

0020The lenses 12 as shown in Figure 3 are convex or cylindrical positive, which is suitable for the case where the refractive index of the lens is greater than that of the medium. If the reverse were true the lenses would be concave or cylindrical negative.

0021If a portion of the continuous sensor can be protected from other than total internal reflection such as with a mirrored reflecting surface (18 in Figure 3B) on the window interface to the liquid, then a light signal offset will always be present at the light detector which can be used to indicate the integrity of the optical link from and including the light source to the light detector. Any failure in this link will be instantly detectable thus providing a self checking feature. Placement of the reflector 18 at the base of the assembly permits it to monitor the integrity of the sensor as well as that of the optical link.

0022Performance of the sensor assembly in Figure 5 can be improved by constructing the substrate 17 in a series of horizontal slices or laminates 19 coated with an opaque material 20 as seen in Figure 6. The result is alternating layers of clear and opaque material such that only light rays propagating near the horizontal direction are allowed to travel to the detector fibre, giving enhanced accuracy of level detection.

0023Another improvement is the addition of a vertically mounted spacer 21 of opaque material placed between the source 14 and detector 13 fibres in order to prevent direct propagation of light between them. Hence the only light that propagates to the detector is that which has been reflected from the window 17 (see Figure 7).

0024The sensor can be made to measure parameters other than liquid level. If the vertical spacer 21 described above is allowed to slide vertically in response to an externally applied force, the light output signal will be analogous to the amplitude of the force or displacement. Parameters such as temperature and pressure could be measured by causing the motion of the slide in response to the deflection of a bimetallic element or diaphragm.

0025If it were desired to measure liquid quantity as opposed to level in odd shaped tanks, the relative positions of the source and detector fibres could be arranged such as to give a stronger signal (e.g., fibres closer together) in parts of the tank having larger surface area and hence representing greater liquid quantity.

0026A stepwise or "digital" output could be achieved by replacing the detector fibre 13 with a fibre made up of a series of fluorescent fibre segments 22 joined by non-fluorescent fibre segments 23 as shown in Figure 8. Such an arrangement would be tolerant to signal variations imposed by varying environmental conditions, as the presence of a signal within a certain range would be known to indicate a specific level. A stepwise output could also be achieved by retaining the fluorescent detector fibre but replacing the continuous source fibre with a series of fibres of different lengths (one terminating at each "step"). The fibres could share a common optical source. The output would give a "count" of the number of source fibres exposed and hence indicate the liquid level. Either of these step-output arrangements could be arranged non-linearly to provide liquid quantity measurement (as opposed to level) in irregularly shaped tanks as described above.

(c) Cylindrical Probe Using Fluorescence

0027Figure 9 shows an embodiment of the fluorescent-fibre sensor wherein the fibres are built into a cylindrical waveguide to provide a rugged and easily-cleaned probe. The source optical fibre 24 is inserted in the substrate at an angle ϑ₁ to illuminate the inner surface of the substrate 25. The numerical aperture (NA) of the source fibre is given by n_ssinϑ₂ where n_s is the refractive index of the substrate. The source fibre NA should be chosen so that 2ϑ₂ ≦ [ (ϑ_c)_liquid - (ϑ_c)_air], for maximum sensor efficiency. Then the angle ϑ₁ is chosen such that ϑ₁ - ϑ₂ is greater than the critical angle for air and ϑ₁ + ϑ₂ is less than the critical angle for the liquid to be sensed.

0028If air is present at the surface of the substrate, the light is totally internally reflected as in any optical waveguide and propagates toward the opposite end of the sensor. If liquid is present at the surface, some or all of the light is refracted out and lost.

0029The fluorescent detector optical fibre 26 intercepts a fraction of the light which is totally internally reflected resulting in an optical intensity output at the end of the detector fibre which increases continuously according to the length of the substrate which is above the liquid level. Although the sensor may have a square or rectangular cross section, in the preferred embodiment, the substrate waveguide is circular and a bundle of optical source fibres is splayed radially as seen in Figure 10 to illuminate the core of the sensor as uniformly as possible and the fluorescent fibre 26 is embedded in the substrate near its surface, along the sensor's length. This ensures that the predominant skew rays are detected unless refracted away into the liquid. A detector fibre along the axis of the sensor would be susceptible only to the fewer meridional rays and the sensor would not be as sensitive. The detector fibre should be shrouded as at 26a in Figure 9 near the top of the sensor in the region where the substrate surface is not yet fully and uniformly illuminated.

0030If the detector fibre 26 is laid in a helical groove G in the substrate as seen in Figure 10, the pitch of the helix can be varied, i.e., decreased, along the length of the sensor to offset any attenuation of the optical source along the sensor's length or to provide a linear response for irregular tank shapes. A longitudinal linear lay of the detector fibre as shown in Figure 9 may, however, facilitate manufacturability of the sensor.

0031If the end 25a of the substrate 25 is made to be reflecting, then when the fluid level falls below the end 25a of the substrate 25, the light from the fibre or fibres 24 will propagate along the entire length of the substrate 25, be reflected by the mirrored end 25a, and propagate back up the substrate 25. The fluorescence induced in the fibre 26 will thus increase abruptly due to illumination by both the downward propagating and upward propagating light in the substrate 25 when the entire substrate 25 is out of the fluid. If any portion of the substrate 25 is surrounded by fluid, the light will be refracted out of the substrate 25 and will not reach the end 25a. The mirrored end 25a thus can provide a large "empty tank" signal when the sensor is used to measure fluid level in a tank.

0032If the end 25a is made to be non-reflecting, then the induced fluorescence will continue to increase smoothly near the bottom of the sensor. This would simplify linearization of the sensor output.

0033A stepwise or "distal" output could be obtained by retaining the fluorescent detector fibre but changing the arrangement of the source fibres in the bundle 24 such that each source fibre 24a, b, c, d... terminates at a different distance along the length of the substrate (Figure 11). The output would thereby give a "count" of the number of source fibres exposed and hence indicate the level. Either of those step-output arrangements could be arranged non-linearly to provide liquid quantity measurement (as opposed to level) in irregularly shaped tanks.

0034In the course of presenting the preferred embodiment of the invention, certain background assumptions have been made regarding components that are common to fibre optic devices. For example, as discussed earlier, the light source may be a light emitting diode, laser diode, flash tube such as a xenon strobe or a more conventional source. The detector may be a photodiode, phototransistor or photomultiplier. Some fluid or epoxy, of a refractive index which matches that of the optical fibre core material and the substrate material, may be placed in the holes or grooves in the substrate which contains the fibres, thereby minimizing any optical loss due to refractive index mismatch (Fresnel loss).

(d) Lead Fibre Insensitivity

0035The continuous level sensor described herein is an intensity-type sensor, since the intensity of the fluorescent signal indicates the liquid level. Intensity type sensors are sensitive to changes in source intensity, detector responsivity, or attenuation in the optical fibres and connectors leading to and from the sensor, all of which will affect the detected optical intensity. It is often impossible to tell the difference between one of the changes listed and a real change of the measured quantity. However, variations in lead fibre attenuation cannot be tolerated by the sensor, as previously described, if the light source and light detector are remote from the sensor. Special techniques must be used to eliminate the sensitivity to lead fibre attenuation.

0036Techniques which have been used in the past to reduce lead fibre sensitivity include multiple pass pulses, wavelength referencing, polarization, Doppler few mode, and reference path techniques. Of these, only wavelength referencing can be easily applied to the present invention with the penalty being the addition of various couplers and wavelength multiplexers. A reflection/transmission technique using two light sources and two detectors has recently been reported by Beheim and Anthan (<u style="single">Applied Optics</u>, vol.27.No.13 1 July, 1988, pages 2767,2775). This technique can be modified for use with the fluorescent fibre fuel probe as follows.

0037In this method, two outputs are required which have different intensity vs. liquid level curves. For the rod type probe of Figure 9 or 10, suitable outputs are the top and bottom ends of the fluorescent fibre. These two outputs behave differently because of the self-attenuation of the fluorescent fibre (the attenuation of the fluorescence signal as it propagates in the fluorescent fibre). In addition, two inputs are required which case different responses at the outputs. One input can be the normal source fibre. The second input can be a fibre simply butted perpendicularly against the fluorescent fibre outside the liquid-sensing portion of the probe. The resulting configuration is shown in Figure 12.

0038To take a reading, the procedure is to first turn on source B only, and measure P_1B and P_2B, the optical powers at detectors 1 and 2 respectively. Then turn on source A and measure P_1A and P_2A. The quantity<maths id="math0001" num="(Equation I)"><math display="block"><mrow><mtext>Y = </mtext><mfrac><mrow><msub><mrow><mtext>P</mtext></mrow><mrow><mtext>1B</mtext></mrow></msub><msub><mrow><mtext> P</mtext></mrow><mrow><mtext>2A</mtext></mrow></msub></mrow><mrow><msub><mrow><mtext>P</mtext></mrow><mrow><mtext>1A</mtext></mrow></msub><msub><mrow><mtext> P</mtext></mrow><mrow><mtext>2B</mtext></mrow></msub></mrow></mfrac></mrow></math><img file="EP0334533B1_D0001.tif" /></maths> is then calculated, which varies monotonically with the fuel level x and is independent of the attenuations in all four lead fibres. The theoretical expression for Y is,<maths id="math0002" num="(Equation II)"><math display="block"><mrow><mtext>Y = ( </mtext><mfrac><mrow><msub><mrow><mtext>α </mtext></mrow><mrow><mtext>T</mtext></mrow></msub><msub><mrow><mtext> + α</mtext></mrow><mrow><mtext>F</mtext></mrow></msub></mrow><mrow><msub><mrow><mtext>α</mtext></mrow><mrow><mtext>T</mtext></mrow></msub><msub><mrow><mtext> - α</mtext></mrow><mrow><mtext>F</mtext></mrow></msub></mrow></mfrac><mtext> ) ( </mtext><mfrac><mrow><mtext>1 - e</mtext><msup><mrow><mtext>​</mtext></mrow><mrow><msub><mrow><mtext>-(α</mtext></mrow><mrow><mtext>T</mtext></mrow></msub><msub><mrow><mtext> - α</mtext></mrow><mrow><mtext>F</mtext></mrow></msub><mtext>)x</mtext></mrow></msup></mrow><mrow><mtext>1 - e</mtext><msup><mrow><mtext>​</mtext></mrow><mrow><msub><mrow><mtext>-(α</mtext></mrow><mrow><mtext>T</mtext></mrow></msub><msub><mrow><mtext> + α</mtext></mrow><mrow><mtext>F</mtext></mrow></msub><mtext>)x</mtext></mrow></msup></mrow></mfrac></mrow></math><img file="EP0334533B1_D0002.tif" /></maths> where α_T is the attenuation coefficient for pump light travelling down the probe, and α_F is the self-attenuation coefficient of the fluorescent fibre. As the level x changes so will the quantity Y and it is an easy enough exercise, perhaps accomplished by a microcomputer, to calculate and display the fuel level x, using the equations above.