APPARATUS AND METHOD FOR DETECTING THE LEVEL OF A LIQUID

Abstract: Light from a light source (2) is conducted through a light conducting element (4) which is partially submerged in a liquid (12). Light escaping from the light conducting element (4) is scattered by the meniscus (14) of the liquid and the scattered light is collected by a lens (6) and focused to form an image at a detector (8). The meniscus appears as a bright band in the image formed at the detector (8) and the position of the band in the image is used to determine the level of the liquid.

Description: Apparatus and method for detecting the level of a ligtn'H The present invention relates to apparatus and a method for detecting the level of a liquid. Level detecting systems are known in which the optical properties of the body of liquid to be measured are used to alter the path of a beam of light so that a light detector generates a signal indicative of the level of the liquid. In U.S. Patent 3,511,572 interference fringes generated by light reflected from the meniscus of a column of liquid are observed through a movable viewer mounted on a scale to accurately locate the level of the meniscus. According to the present invention, there is provided apparatus for detecting the level of a liquid comprising a light source, a light conducting element arranged to conduct light from the light source and detection means arranged to detect, in use, light emerging from the light conducting element and being deflected by the meniscus of the liquid. The invention operates according to the principle that some light which emerges from the light conducting element in the region of the meniscus is deflected, for example by refraction or total internal reflection, at the meniscus surface. The position of the meniscus and hence the level of the liquid may be determined by detecting this deflected light. Viewed from a further aspect the invention provides a method for detecting the level of a liquid, wherein light is conducted through a light conducting element and light emerging from the light conducting element and being deflected by the meniscus of the liquid is detected. A preferred embodiment of the invention comprises a compact and self-contained level measuring device which has no moving parts. In general, the light conducting element is arranged to contact the liquid to be measured so that a meniscus is formed at the interface between the liquid and the light conducting element. For example, the light conducting element may be partially submerged in the liquid so that the liquid contacts an external surface of the light conducting element. In one possible arrangement, the light conducting element forms a wall or part of a wall of a vessel containing the liquid. However, the light conducting element itself does not have to contact the liquid provided that an optically suitable, liquid-contacting intermediate member is provided. The refractive indexes of the intermediate member and the light conducting element are preferably similar, or most preferably the same. For example, the level of a liquid contained in a vessel which has at least one transparent wall may be detected by arranging the light conducting member in contact with the outer surface of the transparent wall. According to this example, a single set of the apparatus of the invention may be used successively to detect the level of the liquid in a plurality of such vessels. Furthermore, the apparatus is able to detect the level of the liquid without entering the vessel. The light conducting element may be of any suitable geometry. In a preferred embodiment, the light conducting element is elongate, more preferably cuboidal. In this way, the light conducting element may extend into the liquid to give a large measurement range without displacing a large volume of the liquid. For effective detection according to the invention, it is desirable for the meniscus to be concave, i.e. for the outer edges of the meniscus to be at a higher level than the centre of the meniscus. In this way, light emerging from the light conducting element in the region of the meniscus passes through a small amount of the liquid before reaching the boundary of the meniscus. If the liquid is of a higher optical density than its surroundings, total internal reflection will occur at the meniscus, depending on the angle of incidence of the emerging light. In order to achieve a concave meniscus, the material of the light conducting element, or the intermediate member contacting the liquid, should be chosen to attract, rather than repel, the liquid to be measured. In a preferred embodiment, polyethersulphone, for example BASF ultrasonic E3010, has been found to be a suitable material for use in measuring the level of hydraulic oil, diesel oil and petrol. For the measurement of other liquids, such as water, other materials, such as glass, may be used to construct the light conducting element. In general, the meniscus will be formed at the upper interface between the liquid and the surrounding atmosphere, for example air. However, according to the invention it would be equally possible to determine the level of a meniscus at the interface of two liquids, for example oil floating on water. In such a case, the degree of reflection due to total internal reflection from the meniscus would depend on the relative optical densities of the two liquids. Furthermore, the positions of a plurality of menisci, for example at an oil-water interface and at an oil-air interface, may be determined in accordance with the invention. The light source may be of any suitable type. In the preferred embodiment, the light source comprises one or more light emitting diodes (LEDs) . The light source may generate light of any suitable wavelength, for example visible light, ultra violet light or infra red light. The light need not be visible to the human eye. Thus, any suitable electromagnetic radiation may be employed in accordance with the invention. The detection means may comprise one or more detectors for generating an electrical signal indicative of the liquid level. In a simple arrangement, a single detector may be positioned to generate a signal when a predetermined liquid level is reached, for example a minimum or maximum level. Preferably, however, the detector (s) are arranged to provide an output indicative of values over a range of liquid levels. For example, a series of sensors may be provided, each responsive to a predetermined level of the liquid. Alternatively, a detector may be arranged to provide an output indicative of the liquid level over a continuous range. In a preferred embodiment, the light is reflected at the meniscus back towards the light conducting element. It can then pass through the element and be detected on a side thereof remote from the meniscus. The detector (s) may be positioned adjacent the light conducting element to receive directly the light deflected by the meniscus. In a preferred embodiment, however, the detection means comprises optics for directing the deflected light towards a detector. In this way, greater flexibility is possible in the physical arrangement of the apparatus, such that a compact device may be achieved. The optics may comprise refractive or reflective elements. In one arrangement a series of optical waveguides of gradually increasing length is provided to guide scattered light to the detector. In another arrangement, an angled mirror is provided to direct scattered light towards the detector. Desirably, the angle between the mirror and the light conducting element is relatively small, in order to maintain a compact arrangement of the level measuring device. The detection means may comprise optics for forming an image of at least a portion of the light conducting element at a detector. In this way, the light deflected by the meniscus can be identified in the image to determine the level of the liquid. In a preferred arrangement, an image of the plane (the "object plane") at which the light is deflected by the meniscus is formed in an image plane by a lens. Advantageously, the object plane, the image plane and the lens plane are oriented according to Scheimpflug geometry, so that the image of the meniscus is in focus at the image plane for all positions along the length of the light conducting element. Preferably, the detector is aligned with the image plane so oriented. Preferably, the image is a demagnified image, so that the dimensions of the detector may be smaller, preferably much smaller, than those of the light conducting element. A suitable detector may comprise an array of light sensitive elements or a continuous light sensitive element . Examples of suitable detectors include charge coupled device (CCD) arrays or CMOS imaging devices. The detector may be arranged to generate a digital image of the light conducting element, or a part thereof, and suitable image processing algorithms may be used to identify the light scattered by the meniscus in the digital image. Some embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a schematic view of a level detecting apparatus according to an embodiment of the invention; Figure 2 is an enlarged view of the meniscus of the liquid in figure 1; Figure 3 shows the response of the detector of the embodiment of figure 1 over a range of fluid levels,- Figure 4 shows the linearised response of figure 3 ; Figure 5 is a schematic view of a level detecting apparatus according to a further embodiment of the invention; Figure 6 is a schematic side view of a level detecting apparatus according to a yet further embodiment of the invention; and Figure 7 is a schematic front view of the apparatus of figure 6. Figure 1 shows a level detecting apparatus according to a first embodiment of the invention. The apparatus comprises a light source 2, a light conducting element 4, a lens 6 and a detector 8, all contained in a housing 10. The apparatus is partially submerged in a fluid 12, such as hydraulic oil, diesel oil or petrol. The light conducting element 4 forms part of one wall of the housing 10 and protects the other components of the apparatus from contamination by the fluid 12. The light source 2 comprises five Kondenshi gallium arsenide LEDs of type OPE5594 arranged in a horizontal linear array across the top of the light conducting element 4. The light conducting element 4 is a planar sheet of BASF ultrasonic E3010 polyethersulphone 3mm thick and having a height of at least 75mm. The lens 6 has a focal length of 14 mm and a diameter of 15mm. The detector 8 is a Hamamatsu position sensitive detector of type 53931 which has an active area of 6mm by 1mm. The detector 8 is connected to a Hamamatsu type C3683-01 processing unit (not shown) for processing the output from the detector. As shown in figure 1, light from the light source 2 is coupled into the light conducting element 4 and propagates therethrough. The bottom horizontal surface of the light conducting element 4 acts as a reflector which reflects the light back upwards through the light conducting element 4. This reflective surface is not essential to the invention, but considerably enhances the contrast of the image of the meniscus 14 formed at the detector 8. As shown in detail in figure 2, light travelling upwardly through the light conducting element 4 and emerging (coupling) therefrom in the region of the meniscus 14 of the fluid 12 is reflected by the meniscus 14 back through the light conducting element 4. This reflected light is focused by the lens 6 onto the detector 8 at which an image of the usable area of the light conducting element 4 is formed. Referring again to figure 1, the optical arrangement of the detector 8 and lens 6 relative to the light conducting element 4 is shown in detail. The arrangement of the light conducting element 4, lens 6 and detector 8 follows the Scheimflug geometry, which is described, for example, in "Scheimpflug' s Patent", Harold M. Merklinger, Photo Techniques Nov/Dec 1996 and GB 1196/1904. In figure 1, the edges of the planes of the lens 6 ("lens plane") , the detector 8 ("image plane") and the surface of the light conducting element 4 at which the meniscus is formed ("object plane") are represented as dashed lines, and the planes are considered to be parallel in the direction running into the plane of figure 1. The line at which these planes intersect is represented in figure 1 by point A. Points B, C and D represent other intersections, with point C also representing the optical centre of lens 6. Points F and F' are, respectively, the front and rear focal points of lens 6. According to the Scheimflug geometry, the image plane AB, the object plane AD and the lens plane AC intersect at the point A. Furthermore, the plane BC which is parallel to the object plane and passes through the centre C of the lens 6, the rear focal plane BF 1 of the lens 6 and the image plane AB also intersect at the point B. As a consequence of these two requirements, the plane CD through the centre of the lens 6 parallel to the image plane AB, the front focal plane DF of the lens 6 and the object plane AD intersect at point D. Thus, it will be seen that the lens plane AC is at an angle α to the object plane AD and that the image plane AB is at an angle β to the lens plane AC. The consequence of the Scheimflug geometry is that the image of an object located in the object plane AD at any position below point D will be in focus in the image plane AB at a position to the left of point B. In an ideal system, the distance y of the image from point B along image plane AB for an object in object plane AD at a distance x below point D is given by: y= x sinα sinβ where f is the focal length of the lens 6. It will be seen therefore that using this relationship, the position x of the meniscus 14 in the object plane AD, and thus the height of the liquid, can be calculated readily from the position y of the image in the image plane AB for the fixed optical geometry shown in figure 1. The detector 8 is aligned with the image plane AB such that the image of the meniscus 14 is in focus across the full detection field of the detector 8. The reflected light from the meniscus 14 appears as a bright band in the dark image of the light conducting element 4 at the detector 8. Thus, a measurement of the level of the liquid is produced by the processing unit by reference to the position of the bright band in the dark image recorded by the detector 8. Figure 3 shows the results of a test of the apparatus according to figure 1. The x-axis represents the position of a plunger which was progressively immersed in the fluid to raise the fluid level. The plunger was then progressively withdrawn and further readings were taken. The y-axis represents the pixel number on the CCD array which was used as a detector 8 for this test at which the bright image of the meniscus was located. The upper curve in figure 3 represents the response of the apparatus when the position of the meniscus image is quantified as the zero-crossing of the derivative of the light intensity distribution across the detector. The lower curve in figure 3 represents the response of the apparatus when the position of the meniscus image is quantified as the centroid of the light intensity distribution. Figure 4 represents the measured height of the fluid (in mm) using the upper curve data of Figure 3 against the actual height of the fluid (in mm) . A best cubic fit is used to map the pixel number to the measured fluid height and it will be appreciated that after an initial calibration a linearised relationship can be achieved, as shown in figure 4. Figure 5 shows a further embodiment of the invention which corresponds substantially to that of figure 1. Like reference numerals are used in figure 5 for those items which have already been described in relation to the embodiment of figure 1. The embodiment of figure 5 differs from that of figure 1 in that a mirror 16 is provided at an acute angle to the light conducting element 4 for directing the light deflected by the meniscus 14 of the fluid 12 to the detector 8. Figures 6 and 7 show a yet further embodiment of the invention. Again like reference numerals are used for those items which have already been described in relation to the preceding embodiments. According to this embodiment, the light deflected by the meniscus 14 of the fluid 12 is directed towards the detector (not shown in figures 6 and 7) by a series of light guides 18 of increasing length and each having a 45° prismatic face at their lower end. As shown clearly in figure 7 deflected light from the meniscus 14 is bent through a right angle by the prismatic face of the light guide 18 proximate the meniscus and directed vertically through the light guide 18 to the detector (not shown) .

Claims: Claims 1. Apparatus for detecting the level of a liquid comprising a light source, a light conducting element arranged to conduct light from the light source and detection means arranged to detect, in use, light emerging from the light conducting element and being deflected by the meniscus of the liquid. 2. Apparatus as claimed in claim 1, wherein, in use, the light conducting element is interposed between the detection means and the meniscus of the liquid. 3. Apparatus as claimed in claim 1 or 2 , wherein the detection means comprises optics for directing the deflected light towards a detector. 4. Apparatus as claimed in claim 3 , wherein the detection means comprises optics for forming an image of at least a portion of the light conducting element at the detector. 5. Apparatus as claimed in claim 4 , wherein the detector is arranged such that the image of the meniscus is in focus over the field of view of the detector. 6. Apparatus as claimed in claim 5, wherein the detector and optics are arranged according to Scheimpflug geometry. 7. Apparatus as claimed in any of claims 3 to 6, wherein the detector comprises an array of light sensitive elements. 8. Apparatus as claimed in any of claims 3 to 6, wherein the detector comprises a continuous light sensitive element. 9. A method for detecting the level of a liquid, wherein light is conducted through a light conducting element and light emerging from the light conducting element and being deflected by the meniscus of the liquid is detected. 10. A method as claimed in claim 9, wherein an image of at least part of the light conducting element is formed at a detector. 11. Apparatus ■ for detecting the level of a liquid substantially as hereinbefore described with reference to the accompanying drawings . 12 ' . A method of detecting the level of a liquid substantially as hereinbefore described with reference to the accompanying drawings .

Inventors: JONES ROBERT, JONES, ROBERT
Applicants: CASCO PRODUCTS CORP, JONES ROBERT, CASCO PRODUCTS CORPORATION, JONES, ROBERT
Application year: 1999
Main classification: G01F23
IPC codes: G01F23