FLUID LEVEL SENSING APPARATUS

Abstract: Fluid level sensing apparatus includes a light projecting and collecting head (4) which is mounted above the maximum permitted fluid level in a fluid container. The head (4) comprises beam splitting means (6) for projecting a beam of light derived from an amplitude modulated light source (1) via an optical fibre (3), onto the surface (7) of the fluid and lens means (8) for collecting the light reflected from the fluid surface. The reflected light and the unreflected light transmitted by the beam splitting means (6) are focussed into optical fibres (9, 10) which conduct the reflected and unreflected light along paths of substantially equal length to comparing means (12, 13, 17) which compares the optical phase of the reflected light with that of the unreflected light in order to detect the phase difference and, hence, monitor the level of the fluid surface (7) below the head (4).

Description: FLIΠD LEVEL SENSING APPARATUS The present invention relates to fluid level sensing apparatus and, more particularly, to such appa¬ ratus for sensing or measuring the level of the surface of a fluid in a tank, reservoir or other container relatively to a reference point or datum located above the fluid. For exampl , the invention may be used for sensing or measuring the level of oil or other liquid contained in storage tanks, ships tanks or the tanks of other vehicles. There are many well-known installations available for measuring the level of a fluid surface relative to some fixed reference or datum point. Most, although not all, of these installations require the use of some form of sensing device in physical contact with the surface of the fluid. If the sensing device requires for its operation, or if it produces, an electri¬ cal signal there is always the risk of an explosion when the device is used in an hazardous environment. An object of the present invention is to provide fluid level sensing apparatus which utilises an optical system for sensing the level of a fluid surface. Another object is to provide such apparatus may be designed so as to require no electrical energy or signals in the vicinity of a fluid container, thereby making the apparatus inherently safe or use with any type of luid. The present invention consists in fluid level sensing apparatus comprising means for projecting a beam of light derived from an amplitude modulated light source onto the surface of a fluid, and means for collecting the light reflected from said fluid surface and comparing the optical phase of the reflected modu¬ lated light with that of unreflected light from said source, whereby to monitor the level of the fluid surface relatively to the projecting means. The invention utilises a method of measuring the level of a fluid surface which is an absolute method and consequently provides considerably more accurate results than currently available fluid level sensors. The amplitude modulation of the light source effectively changes the wavelength of the light and the modulation wavelength utilised depends on the range of measurement desired. For example, if the modulation frequency is 100 MHz, the wavelength is 3 metres and because the light traverses the distance between the reference position, that is, the projecting and collecting means, and. the fluid surface twice, such a wavelength is effective to sense a fluid level which is not greater than 1.5 metres below the reference position. If the fluid surface is at a greater distance from the reference position than 1.5 metres, the distance cannot be measured uniquely -with an effective -wavelength of 3 metres. This problem is overcome by reducing the modulation frequency. Hence, at 10 MHz the range becomes 1 metres and at 1 MHz it is 150 metres. The precision of the level measurement depends on the accuracy with which the difference in optical phase can be electronically measured and, in one embodiment of the invention, this has been found to be of the order of 0.1 . The latter is equivalent to an accuracy of 0. mm at 100 MHz and cms at 1 MHz. Thus, to ensure both accuracy and non- fJR£ OKPI ambiguity in the range of measurement, the apparatus is preferably designed to operate at two different modu¬ lation frequencies. Conveniently, the projecting means projects a beam of light onto the fluid surface substantially perpendicularly to the surface, and the collecting means includes lens means located coaxially with the projecting means. The latter may, for example, comprise a beam splitter which directs a part of an input beam of light from the modulated light source onto the fluid surfa and transmits another unreflected part of the input beam Both the reflected light and the transmitted or un¬ reflected part of the input beam may be focussed into output optical fibres which transmit the reflected and unreflected light to comparing means along light paths of substantially equal length. The lens means for collecting the reflected light may be a relatively large aperture lens disposed substantially coaxially with respect to the beam splitter and arranged to focus the light into the associated output fibre. Preferably, the input beam of modulated light is transmitted to the beam splitter by means of an input optical fibre. The use of optical fibres to transmit the input and output light beams rto- ' and from the optical projecting and collection means enables the electrical power source and components of the apparatus to be disposed remotely from the optical projecting and collection system so that there is no requirement for the former to be in the vicinity of a fluid container, and the apparatus of the invention is therefore especially suitable for use in monitoring the level of hazardous fluids, such as oil. The projecting and collection optics may be designed to be small in weight and dimensions and may be incorporated in a single robust maintenance—free unit or head which may be located at or adjacent the top of the fluid container. In applications, such as, ships and other vehicles, where the fluid level may not be normal to the projected light beam, the optical unit or head may be mounted in a suitable form of gimbal which is free to move under gravitational forces to ensure that the projected beam is normal to the fluid surface to be detected. The electronic components of the comparing means may comprise photodetector means for producing electrical signals corresponding to the reflected and unreflected light, and means for processing the resulting electrical signals to determine the phase difference and, hence, the level of the fluid surface below the projecting means. Such processing means may comprise a first oscillator for modulating the light source, a second oscillator for producing a second signal having a. frequency close to the modulating frequency of the first oscillator, means for mixing the second signal with the electrical signals produced y the photodetector means, whereby to preserve the phase difference between the detected signals whilst converting the latter into lower frequency signals, and comparator means for comparing the mixed detected signals with a reference signal derived from mixing the two oscillator signals. Preferably, the oscillators are selectively adjustable to permit alteration of the modulation frequency in dependence upon the range of fluid levels to be sensed. If the fluid surface to be sensed is insufficiently reflective, a satisfactory reflected beam may be obtained with the aid of a float incorporating reflecting means. This float is floated on the fluid surface in a suitable position below the projecting and collection means and is constrained to move in a substantially vertical direction as the fluid level changes, for example, by being guided in a„tube. If the fluid to be sensed is transparent or translucent to the projected beam, the invention can also be used to detect the level of an interface between the fluid and a denser fluid, such as, a contaminating fluid, below the first-mentioned fluid. To this end, the apparatus may include at least one additional projecting means for projecting a second beam of ampli¬ tude modulated light towards the fluid surface, and additional means or collecting the light reflected from an interface between the first-mentioned fluid and the second denser fluid, the comparing means being arranged to compare the optical phase of the modulated light reflected from the interface with that of the unreflected light from the light source so as to monitor the level of the second fluid relatively to the additional pro¬ jecting means. The reflection of light at the interface may be enhanced by the use of a reflective float adapted to float on the second fluid. In order that the invention may be more - 3RE OMPI readily understood, reference will now be made to the accompanying drawings, in which Figures 1 and 2 sche¬ matically illustrate two different embodiments of the invention. Referring to Figure 1 of the drawings, the apparatus comprises a light source 1 , such as a laser light source or light emitting diode, which is amplitude modulated, as will hereinafter be more fully described, at a maximum frequency of 50 MHz, thereby to give the light source an effective wavelength of 6 _7 metres rather than its true wavelength of 8 x 10 metres. The output from the light source 1 is coupled into an optical fibre power dividing switch 2 which is used selectively to couple the light into several input multi ode optical fibres 3. The opposite ends of these fibres are coupled to optical projecting and collecting heads which are positioned in a plurality of liquid tanks, reservoirs or other containers, above the maximum permitted liquid levels, for monitoring the respective liquid levels, and which constitute the reference points from which the respective levels are measured. For simplicity, only one of the heads is illustrated, in Figure 1 and it will be appreciated that the other heads are identical. The input light beam emitted from the input optical fibre 3 associated with the head is converted into a substantially parallel light beam by a lens 5 and is directed towards a small beam splitter 6 which projects a part of the input light beam towards the fluid surface 7 to be sensed. The light reflected OMP from the fluid surface is collected by a suitably large aperture lens 8 mounted coaxially with the beam splitter and is focussed into an output optical fibre 9» The lens and beam splitter assembly 8,6 are designed so that no light can be reflected directly into the fibre 9 * To alleviate errors in the phase measurement due to arbitary path length variations in the input light path, a second output fibre 10 is used to trans¬ mit unreflected light from the head h for subsequent comparison.with the reflected light. Hence, the part of the input beam transmitted by the beam splitter 6 is focussed into the second output fibre 10 by a lens 11. The output fibres 9,10 are closely coupled together and routed through common trunking. The projecting .and collection head h is constructed so as to be small in weight and dimensions, robust and essentially maintenance-free. By reason of the use of the optical fibres 3,9,10 to transmit light to and from the head h , the electronic control and sensing components of the apparatus may be located remotely from the head. The light from the light source 1 , which is modulated at 50 MHz and is transmitted along the output fibres 9,10, has travelled different distances, the difference being equal to twice the distance from the reference point, that is the beam splitter 6 , to the surface 7 of the liquid. The optical signals returning along the output fibres 9,10 are transmitted through a variable optical attenuator switch 12 which enables the optical signals to be sequentially sampled by a photodetector 13 which, in this embodiment, is an avalanche photo—diode. The purpose of the variable attentuator switch 12 is to adjust the level of the optical signal produced by the unreflected light in the output fibre 10 to the same intensity as that of the reflected beam transmitted by the output fibre 9 so as to reduce errors in the phase measurement. The relative phase between these electrical signals is measured in order to measure thelevel of the liquid surface 7. The light source 1 is modulated by a crystal controlled programmable oscillator , the output from this oscillator being fed to the light source 1 and also to a double balanced mixer 15. The output from a second crystal controlled programmable oscillator 16 is .connected to the mixer 15 and also to the output of the photodetector 13 for mixing with the output current of the latter. The connection to the photo¬ detector output may either be effected directly or via another double balanced mixer. The maximum frequency used for the oscillators 1 ,16 is 0 MHz and 50.01 MHz, although higher frequencies may be employed if improved precision is required. Mixing of the electrical signals detected at the output of the photodetector 13 with the output signal of the oscillator 16 preserves the relative phase information whilst translating the electrical signal frequency to 10 KHz. The mixed detected signals are fed to a phase comparator 17 and are sequentially compared with the mixed oscillator signals supplied to the comparator via the mixer 1 in order to provide the measurement - URE 4 OMPI in orma ion. The phase comparator 17 is a conventional gated counter using the mixed oscillator signals as a stable oscillator reference. Using such a comparator, it has been found that the measurement of the relative phase between two corresponding points on signals derived from the output fibres 9,10 can be measured to an accuracy of 1 part in -10 . This corresponds to a distance accuracy of 6 x 10~. metres, that is, 0.6 mm. Typically, a microcomputer 18 is used to store the data from the phase comparator 17 sund. this microcomputer is also programmed to control the position of the variable attenuator switch 12 and the two .fre¬ quencies at which the oscillators 14,1 operate, the lowest frequency being determined from the known depth of the tank associated with the head 4. Figure 2 illustrates a modification which can be used when the liquid 19 to be monitored is transparent to the projected light beam and it is desirable also to detect both the presence and approxi- mate level (+ or - 0.5 cms) of a contaminating liquid 20, such as, water. In this eπ-bodiment, the projecting head 4 is modified so that two light beams are projected from the head. Hence, the part of the input beam transmitted by the beam splitter 6 is transferred to a second beam splitter 21 which projects part of this light beam towards the surface of the liquid. The remaining part of the beam passes through the beam splitter 21 and is coupled into the output optical fibre 10 by the lens 11, as in the previous embodiment. Reflected light resulting from the beam projected towards O PI 4 wι*o the liquids by the beam splitter 21 is collected by a suitably large aperture lens 22 disposed coaxially with the beam splitter 21 and is focussed into a third output optical fibre 23. The beam splitter and lens assembly 21,22 is similar to the assembly 6,8. The first beam projected by the splitter 6 is used, as before, to measure the level of the surface 7 of the liquid 19 and the second beam produced by the splitter 21 is directed towards a float 24 designed to float only in the contaminated liquid 20 so that it is stable at the liquid-contaminant interface 25. The float is fitted with a retro-prism 26 and is constrained to move substantially vertically under the associated projected beam. The retro-prism reflects the beam back to the collecting lens 22 which transfers the optical signal along the optical fibre 23. This optical signal is processed in a similar manner to that described in the previous embodiment to allow the position of the float 24 relative to the beam splitter 21 to be deter- mined from the optical phase difference between the light beams in the optical fibres 7,23, thus enabling the height of the water contamination to be determined. The variable attenuator switch 12 in this embodiment is designed to accommodate three fibres instead of two, the position of the switch again being controlled by the microcomputer 18. In applications such as oil storage tank farms, the microcomputer 18 may also be used to switch the optical power sequentially to the other optical input fibres 3 of the optical fibre switch 2 to enable the levels of other storage tanks to be sequentially measured. Whilst particular embodiments have been described, it will be understood that modifications can be made without departing from the scope of the invention as defined by the appended claims. For example, whilst the optical signals returning along the output fibres 9,10 are preferably transmitted through a variable optical attenuator switch 12, as described, enabling the optical signals to be sequentially sampled by a photodetector 13, alternatively, these optical signals may be directly detected by photo-diodes associated respectively with the fibres 9,10. Whilst a float 24 is only shown as being used at the interface between two liquids 19,20, it will be appreciated that where unsatisfactory reflection of light is being obtained from the free surface of a liquid, a reflecting float, suitably constrained, may also be used at this surface to assist the sensing operation. O MPI.

Claims: CLΛIMS 1. Fluid level sensing apparatus characterised by means (6) for projecting a beam of light derived from an amplitude modulated light source (1 ) onto the surface (7) of a fluid, means (8-17) for collecting the light reflected from said fluid surface and comparing the optical phase of the reflected modulated light with that of the unreflected. light from said source, whereby to monitor the level of the fluid surface relatively to the projecting means. 2. Apparatus according to claim 1 , characterised in that the projecting means (6) projects a beam of light onto the fluid surface (7) substantially perpen¬ dicularly to said surface, and the collecting means is located coaxially with respect to the projecting means. 3« Apparatus according to claim 2, characterised in that the projecting means comprises a beam splitter (6) which directs a part of an input beam of light from the modulated light source (1 ) onto the luid surface (7) and transmits another part of said input beam, and in that the reflected light and the transmitted part of the input beam are ocussed.into output optical fibres (9,10) which transmit the reflected and unreflected light to comparing means (12-17) along light paths of substantially equal length. 4. Apparatus according to claim 3, characterised in that the collecting means comprises lens means (8) for collecting the reflected light, said lens means being a relatively large aperture lens disposed substantially coaxially with respect to the beam splitter (6) and arranged to focus the light into the associated output fibre (9) • 5. Apparatus according to any preceding claim, characterised in that the light from the modulated light source (l) is transmitted to the projecting means (6) by an input optical fibre (3). 6. Apparatus according to any preceding claim, characterised by photodetector means (13) for producing electrical signals corresponding to the reflected and unreflected light, means (14-17) for processing the resulting electrical signals to determine the phase difference and, hence, the level of the fluid surface below the projecting means. 7. Apparatus according to claim 6, characterised in that the processing means comprises a first oscillator (1 ) for modulating the light source (l), a second oscillator (16) for producing a second signal having a frequency close to the modulating frequency of the first oscillator, means for mixing the second signal with the electrical signals produced by the photodetector means (13), whereby to preserve the phase difference between the detector signals whilst converting the latter into lower frequency signals, and comparator means (17) for comparing the mixed detector signals with a reference signal derived from mixing the two oscillator signals. 8. Apparatus according to any preceding claim, characterised by at least one additional projecting means (21 ) for projecting a second beam of amplitude modulated light towards the fluid surface (7) and additional means (22) for collecting the light reflected from an interface (25) between the fluid (19) an a second denser fluid (20) , said comparing means (17) being arranged to compare the optical phase of the modulated light reflected from the interface (25) with that of unreflected light from the light source (1 ) so as also to monitor the level (25) of the second fluid relatively to the additional projecting means. 9. Apparatus according to any preceding claim, characterised by a float (24) having reflecting means (26) and arranged to float on the fluid surface (7) or at the fluid interface (25) in a position below the associated projecting and collection means (6,8 or 21,22) -and constrained to move in a substantially vertical direction. O PI

Inventors: JACKSON DAVID ALFRED, JACKSON, DAVID, ALFRED
Applicants: KENT SCIENT IND PROJECTS, KENT SCIENTIFIC AND INDUSTRIAL PROJECTS LIMITED
Application year: 1983
Main classification: G01F23
IPC codes: G01F23