WO1999026044A1

WO1999026044A1 - A level detector

Info

Publication number: WO1999026044A1
Application number: PCT/SE1998/001959
Authority: WO
Inventors: Rolf Westerlund
Original assignee: Rolf Westerlund
Priority date: 1997-10-30
Filing date: 1998-10-29
Publication date: 1999-05-27
Also published as: AU1057599A; SE9703980L; SE9703980D0; EP1032807A1; SE511605C2

Abstract

A detector for detecting the level of a boundary surface or interface between a first medium (A) and a second medium (B) in a tank (1) includes a radiation emitter (3) and a radiation sensor. A reflective device (2) is disposed in the tank for irradiation by the emitter (3). The reflector device (2) is chosen to change a property of the radiation emitted by the emitter when reflecting said radiation, in relation to the extent to which the two medium (A, B) are present at the reflective device (2). The sensor (4) is adapted to sense said change as an indication of the position of the interface surface relative to the reflective device, which is retroreflexive and which is chosen to impart essentially different angles of scatter to the reflected radiation depending on whether said reflection takes place in the presence of one of said media (A, B).

Description

A LEVEL DE ECTOR

The present invention relates to a level detector for detecting the position of the interface or boundary surface between a first medium and a second medium contained in a tank, said detector including a radiation emitter and a radiation sensor and being of the kind defined in the preamble of the accompanying Claim 1.

Although many different types of detectors that can be used, for instance, with level meters or level monitors for material-containing tanks are well known to the art, there is nevertheless a desire to provide a detector that can be based on a few simple components which are both robust and available commercially at a low price.

The object of the present invention is to provide a low-cost detector which comprises only a few simple, robust components and which can be fitted to the tank at low cost, and which will deliver an output signal that provides a good indication of the level of the material sensed or detected or of the corresponding volume of material in said tank, either, directly or in response to a simple signal processing operation.

This object is achieved in accordance with the invention with a detector according to Claim 1.

Further embodiments of the inventive detector are set forth in the dependent Claims and will be apparent from the following description.

The invention is based on the realisation that a retroreflector can be used beneficially for determining the level of liquid contained in a tank, when an emitter and a sensor are mounted in close proximity to one another in the upper part of the tank. A typical example of a retroreflector is found in a conventional reflex tape or ribbon that is sewn to^' the clothing of a person so that, said person will more visible in the dark when walking or travelling along trafficked highways, roads, etc.. Such reflex tape is commercially available and consists, for instance, of small glass spheres that are attached to a reflective substrate. One problem with reflex tape of this kind is that its retroreflection is greatly impaired when the tape becomes wet with rainwater. It is precisely this feature of reflex tape of the retroreflector type that is utilised beneficially in the invention. For instance, such retroreflector tape can be fitted in the tank and the emitter and sensor positioned so that the emitter is able to irradiate the tape with light, so as to enable the sensor to determine the amount of light that is retroreflected. This retroreflection means that a light beam directed towards the reflex tape will be reflected onto the light source with a relatively small angle of scatter. However, the angle of scatter will be much wider if the reflex tape becomes/is immersed in liquid. Only an insignificant part of the light reflected from the retroreflecting device beneath the level of liquid will be detected by the sensor, even when the liquid concerned is highly transparent. It will be understood from this that the invention is not contingent on the transparency of the material whose level in the tank shall be measured.

The inventive concept, however, is more general than the illustrative example.

The emitter, the sensor and the reflector are thus adapted generally for electromagnetic radiation. The term "light" is used here for the sake of simplicity and shall therefore be interpreted as consisting of electromagnetic radiation of any wavelength whatsoever.

The reflector is generally of the kind that changes a property of the "light" that it receives from the emitter, in accordance with the extent to which the reflector covered by one or the other medium in the tank. The sensor is located so as to be able to receive the light emitted by the emitter and reflected by the reflector onto the sensor, so as to enable the sensor to detect the property change concerned. The sensor is adapted to establish an indication of the position of the interface or boundary surface between the media concerned, on the basis of this property change.

The emitter and the sensor may be incorporated with the tank, or may be placed inside said tank.

The invention may, of course, be used to measure the levels of materials that do not flow/ are not liquid. For instance, when the material concerned is sand, the level of the sand will be uneven. However, the skilled person will be able to limit this drawback, by placing several reflecting devices in spaced relationship in the tank.

This arrangement will, of course, be of interest in those applications in which the tank slopes or is tilted in use, or when the surface of the liquid in the tank slopes in relation to the horizontal.

Measuring of the height/level of the interface between a medium A and another medium B in a tank enables the volume of, e.g., medium A in said tank to be evaluated. For instance, when the tank has a constant cross-sectional area along its height, the detected level will be directly proportional to the volume of the medium A in said tank, such that the output signal of the sensor will be directly indicative of the volume of said medium A, without needing to process the output signal of the light sensor in order to calculate said volume. According to the present invention, the width of the retroreflective device can be varied in a vertical direction, such that the relative width of said retroreflective device at one level will correspond to the relative area of the tank at said level, whereby the amount of light detected by the sensor will be proportional to the tank volume above said level. Alternatively, the effectiveness of the screen can be varied to achieve the same result.

When the reflector (the screen) has an effectiveness that is dependent on the angle at which it is illuminated or irradiated, this can be compensated for either by varying the width of the screen or by varying the transmittance, such that the measured reflection will be uniform over the whole of the screen when the tank is empty. By effectiveness is meant the degree of retroreflectance or absorbence of the screen. If more than one screen is installed, or if all sides of the tank are constructed as screens or provided with screens, reading will be independent of the orientation of the tank or its movement. The strength of the measured radiation will be a measurement of the level of material in the tank.

In one aspect of the invention, that part of the screen which is surrounded by a medium A beneath the interface or boundary surface between said medium A and a medium B in the tank will send light from the emitter in all directions. The irradiance on the detector from this part of the screen will be small, even when the medium A is transparent.

That part of the screen which is located in medium B above said boundary surface will function as a retroreflector and send back almost all light to an area in the close proximity of the radiation emitter. The irradiance on the detector from this part of the screen will be high. The total irradiance on the detector will therewith depend on the level of the material/medium A. It is assumed that medium A and medium B have mutually different refractive indexes. The invention will now be described in more detail with reference to exemplifying embodiments . thereof and also with reference to the accompanying drawings.

Figs. 1-4 illustrate schematically different embodiments of an inventive level detector.

Fig. 5 illustrates a variant of the level detectors shown in Figs. 1.4.

Fig. 6 illustrates an embodiment of a retroreflective screen that can be used in a tank in accordance with the invention.

Fig. 7 illustrates inventive level detector means having two detectors.

Fig. 8 illustrates how the reflectance of the retroreflector varies per unit of area with the viewing angle in a wet and a dry state respectively.

Fig. 9 illustrates how the quotient between the signals from the detectors in the detector means of Fig. 3 varies with the extent to which the tank is filled, with the retroreflector being dirtied or soiled to different extents.

Shown in Fig. 3 is a tank 1 which contains in its bottom part a medium A, for instance a transparent liquid, and in its upper part above said medium A contains a medium B, for instance air. The refractive index of medium A differs from that of medium B.

The tank 1 has a retroflective screen 2 mounted on one side- wall thereof, for instance. In the illustrated case, the screen/retroreflector 2 is a commercially available, flexible retroreflector. The screen is retroreflective provided that it is surrounded by air/gas, i.e. when impinged upon by a light beam from a light source, it will reflect said beam back to the region of the light source with only a small angle of scatter.

Arranged in the upper part of the tank 1 is a light source 3 which is able to irradiate or illuminate the screen 2. A light sensor 4 is positioned to shield the retroreflective light from the screen 2.

Fig. 3 shows a retroreflector 2 which when surrounded by the medium A reflects the light from the light source 3 at a relatively wide angle of scatter, meaning that only a negligible part of the light reflected from that part of the reflector 2 located in the medium A will reach the sensor. The light intensity detected by the sensor 4 will thus constitute a measurement of the total area of the reflex tape 2 that lies above the upper surface of the medium A. When the screen 2 has a uniform width along its length, that part of the light measured by the sensor 4 will constitute an indication of the length or the extent of the tape 2 above the surface of the liquid A. This enables the level of liquid A in the tank to be readily assessed. This liquid level corresponds to the volume of the liquid in the tank when the horizontal cross-sectional area of the tank is constant in all vertical or height levels of the tank. When the horizontal cross-sectional area of the tank varies along the height of the tank, this can be compensated for by, e.g., giving the retroreflector screen (which is assumed to have uniform retroreflectance per unit area) a width that varies vertically in accordance with variations in the cross- sectional area of the tank in a vertical direction, as illustrated in Fig. 6 where the outer contour 11 indicates the cross-section of the tank and the inner contour 21 indicates a corresponding shape of the screen 2.

Fig. 1 illustrates an embodiment in which the screen 2 provides, instead, a mirror reflection in the presence of medium B and a retroreflection or lambert reflection in the presence of medium B, where the sensor 4 is placed in the bottom part of the tank to detect reflection from the screen in medium B.

Fig. 2 illustrates an embodiment in which the screen, instead, provides retroreflection or lambert reflection in medium B and mirror reflection in medium A, said sensor being placed to shield the mirror reflection light. Fig. 4 illustrates an embodiment in which the screen has, instead, lambert reflection or mirror reflection in medium B and retroreflective reflection in medium A, said sensor 4 being placed in the proximity of the light source 3.

The retroreflection screen may, of course, have a uniform width although with varying reflectance in a vertical direction. If the light source 3 or sensor 4 has an intensity/sensitivity that varies in different directions, this can be compensated for in corresponding manners.

When the level detector is to be used solely as a level monitor, the elongated screen 2 can be replaced with a small retroreflection screen at the level to be monitored.

Fig. 5 illustrates an embodiment in which the tank 1 includes two mutually separate screens 2. These screens can both be illuminated by one and the same light source 3, wherewith retroreflected light from both screens can be sensed or detected by one single sensor 4.

An embodiment according to Fig. 5 will compensate for movement of the tank 1 to an inclined position during operation, or for the situation when the surface of the liquid A deviates from the horizontal plane.

An embodiment corresponding to Fig. 5 can also be used beneficially when the medium A is not liquid, but has a varying upper surface. For instance, when the medium A is o sand the intensity of the light detected by the sensor 4 will constitute some sort of mean value for the level of the sand in the tank.

It will also be apparent that the inventive level detector is operable even when the material A is non-transparent.

The invention finds general application for measuring the level and/or the volume of a medium in a tank. The media concerned will preferably have mutually different refractive indexes when the reflector 2 is retroreflective. The invention can be applied particularly readily when the media concerned are such that the boundary surface or interface therebetween will remain flat and horizontal when the tank and media are at rest. However, the invention can also be applied when the inclination of the tank is changed and when the tank is accelerated, rotated or similarly moved, by appropriate positioning of the retroreflector and/or by using several inventive devices in one and the same tank, so as to establish measurement values of the interface or boundary surface in different regions of the tank.

Medium and medium B in the above described are mutually exchangeable. Although the emitter 3 and the sensor 4 are shown placed in the tank in the embodiments of Figs. 1-4, it will be understood that said emitter and said sensor may alternatively be placed outside the tank provided that said tank is provided with suitable windows or when using fibre optics. Moreover, the sensor need not, of course, be placed in the proximity of the emitter but may be placed conveniently in a position in which the reflected radiation can best be detected. The inner surface of the tank may be made retroreflective, either totally or partially.

The proposed detector (the level meter and the level monitor) is sensitive to a dirty screen 2. Correct interpretation of the detector signal would be difficult to achieve when the y extent to the screen has been dirtied or soiled is unknown when using a detector means that includes only one single detector 4. When the level meter is used as a level monitor, dirtying of the screen 2 would also cause the level monitor to cut-out at the wrong criterion.

Fig. 7 illustrates schematically a further development of the level detector which, in addition to the sensor 4 (which is also indicated at Dl, also includes a further sensor D2, said sensors Dl and D2 being placed such as to have mutually different viewing angles αl, α2 as seen from the reflector 2. The sensors Dl, D2 may be spaced at different distances from the reflector or screen 2. Apertures 31, 32 are placed between the light source and the sensors Dl and D2, such as to allow a given amount of "stray light" to come directly from the light source 3 to the sensors. The reading error obtained with a full tank can be limited by appropriate selection of aperture size.

The arrangement also includes a damping screen 33 which dampens the light reflected from the screen 2 to the sensor Dl in a manner such as to enable the sensor signals Di, D₂ to be made roughly equally to one another.

As will be seen from Fig. 8, the reflectance of the screen per unit of surface area (A) varies with the viewing angle α when the screen 2 is wet (R„) and dry (R_t, R*) respectively. By viewing angle is meant the angle between incident light and reflected light.

The embodiment according to Fig. 7 conveniently enables the quotient of the signals from Dl and D2 to be formed. This quotient will vary with the extent to which the tank is filled, as indicated by the curve C in Fig. 9. Also shown is the extent to which the relationship is influenced by various degrees of screen dirtying. The curve C illustrates the ratio or relationship when the screen 2 is not dirtied (S = 0 along the full height of the screen 2). The curve a denotes the relationship when the top of the screen 2 is clean, i.e. s = 0.95 in its uppermost part and S = 0.05 in its lower part. The relationship b illustrates the ratio when the bottom of the screen 2 is clean and S therewith equal to 0.95 in the bottom part of the screen and S = 0.05 in the upper part thereof. The curve R denotes the relationship when dirtying of the screen varies randomly between S = 1.0 and S = 0.0, wherewith the mean value of dirtying of the screen is about 0.4.

As will be apparent from Fig. 9, the signal quotient Dι/D₂ = 1.0 will always be obtained when the liquid level in the tank lies somewhere between the upper and lower edges of the screen 2.

It is possible to obtain a level monitor function for a tank- filling level that lies between the upper and lower edges of the screen 2.

The amount of light delivered to D_t and D_λ respective can be described as

where

B,H = The width and height of the screen.

C = Damping of light to sensor Dl (damping factor for the damping shield 33) .

T = Transmittance of the liquid.

S(x) = Dirtying on level x (x=1.0 for the screen). i Light intensity.

L = Current liquid level. R, Screen reflectance to sensors Dl and D2 when the screen is dry, Rι and R.j are. chosen respectively with the aid of the positioning of the sensors according to Fig. 1. _v Screen reflectance when the screen is wet. R can be made different for the two sensors, by dissimilar spacing of Dι and D_z from said screen. (It is assumed in the above formulae that the sensors are placed equidistantly from the screen) . Fι,F₂= The size of apertures 31, 32.

Damping of sensor Di is chosen, for instance, so that D_x = D₂ when L = H/2 and when the screen is clean.

An output signal which jumps from 0 to 1 when Dl is greater than D2 and vice versa can be obtained by comparing Dl and D2 in a simple comparator. This output signal then forms the output signal of the level monitor, and is essentially independent of the extent to which the reflector or screen 2 is dirtied.

It will be apparent that damping of the sensor Dl can be chosen so that Dl will equal D2 at some other liquid level that lies between the upper and lower edges of the screen.

The sensor signals Dl, D2 given above may, of course, be corrected with respect to respective distances of the sensors from the screen, for instance by dividing R_v with the distance concerned.

Claims

1. A level detector for detecting the position of a boundary surface or interface between a first medium (A) and a second medium (B) contained in a tank (1), said indicator comprising a radiation emitter (3) and a radiation sensor (4) , and a reflective device (2) which is placed in the tank for irradiation by said emitter (3) , wherewith the reflective device (2) is chosen so as to change a property of the radiation emitted by the emitter when reflecting said radiation relative to the extent of the presence of said two media (A, B) in respect to said reflective device (2) , and wherewith the sensor (4) is adapted to detect the change as an indication of the position of said interface surface relative to said reflective device, characterised in that the reflective device (2) is retroreflective and is chosen to impart essentially different angles of scatter to said reflected radiation depending on whether or not said reflection occurs in the presence of one of said media (A, B) .

2. A detector according to Claim 1, characterised in that the retroreflective device has a small vertical extension, such that said detector will function as a level monitor.

3. A detector according to Claim 1, characterised in that the retroreflective device has a vertical extension in excess of a chosen level measuring region so as to enable the volume of the medium to be evaluated.

4. A detector according to any one of Claims 1 and 3, characterised in that the detector includes several mutually separated retroreflective devices that are irradiated by one or more emitters and the reflected light of which is detected or sensed by one or more sensors.

5. A detector according to any one of Claims 1, 3 and 4, characterised in that the retroreflective device has in its vertical direction a reflective distribution that corresponds to variation in the free horizontal area of the tank.

6. A detector according to any one of Claims 1, 3, 4 and 5, characterised in that the reflective device has a uniform reflectivity per unit of surface area; and in that the width of the device varies vertically in correspondence with the vertical variation of the free horizontal area of the tank.

7. A detector according to any one of Claims 1, 3, 4, 5 and 6, characterised in that the retroreflective device has an inclination or curvature that promotes a reflectivity that corresponds to vertical variation in the free horizontal area of the tank.

8. A detector according to any one of Claims 1-7, characterised in that in addition to said sensor (4, Dl) the detector includes a further sensor (D2) which has from the screen (2) and relative to the direction of incident light a different viewing angle than the viewing angle of the first sensor (Dl) ; and in that the detector also includes signal processing means for dividing output signals from the sensors (Dl, D2), wherewith the result of said division is essentially independent of a dirty retroreflector (2) .

9. A detector according to Claim 8, characterised in that the sensors are disposed so that the result of said division will be 1.0 at a chosen fluid level between the top and bottom edges of the retroreflector screen.