GB2147697A

GB2147697A - Level measurement method and apparatus

Info

Publication number: GB2147697A
Application number: GB08422571A
Authority: GB
Inventors: Hans-Peter Kugler; Norbert Eisenreich
Original assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Current assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Priority date: 1983-10-05
Filing date: 1984-09-06
Publication date: 1985-05-15
Also published as: GB8422571D0; GB2147697B; FR2553190B1; FR2553190A1

Abstract

An optical method for measuring the level 6 of a filling material 1 proposes that at least one light beam 3 is directed on to the filling material at an angle inclined to the normal, the secondary beam 3', 3'' emanating from the filling material following a path whose position is dependent upon the level 6, 6'. The position of the secondary beam is detected by a photosensor array 21. Alternatively the secondary beam may be passed through a device 8 having a variable attenuation characteristic in the direction of displacement of the beam 3', 3'', so that an intensity analogue of the surface level 6, 6' is generated. <IMAGE>

Description

are required.

According to the invention the aforementioned problem is solved by a method, in which a coherent light beam is directed on to the filling material surface and the seconardy beam is detectable by a row of detectors which are at least juxtaposed in the said extension direction, or in which a coherent light beam is directed on to the filling material surface and the reflected beam is initially transmitted through a medium with an optical permeability for the light ofthe light source which is continuously variable in the plane covered by the incident beam and the secondary beam and which is at a finite angle to the direction of the secondary beam and that the light beam intensity is measured by means of the detector arrangement.For performing the method, an apparatus is provided, in which either the light source is a laser and the reception arrange ment has a row of juxtaposed detectors, or in which the light source is a laser and the reception arrangement has a medium arranged in the path of the beam of the secondary light emitted by the filling material upstream of the detector arrangement with a variable optical permeability in the plane covered by the axis of the light source and the perpendicular of the surface, but under a finite angle to the direction of the secondary beam.

The term secondary beam is generally understood to mean the beam coming from the filling material and whose optical position is influenced thereby.

The serial detector arrangement, detector row or detector array can in particular be constituted by image sensors in the form of serial, but also flat, matrix-like arrangements (whose largest dimension then extends in the said priveledged dimension) of optoelectronic semiconductor components as photoelectric receivers.

According to a preferred embodiment, the secondary beam coming from the filling material is a beam reflected by the filling material surface, or in the case of a transparent filling liquid material, the secondary beam coming from the filling material is refracted at least in the latter, the secondary beam then being more particularly reflected on the bottom of the filling material container.

In the construction with the upstream, partially permeable medium, the actual reception and the conversion into electrical signals can then be brought about by a row of detectors positioned downstream of the medium. However, in preferred manner, the beam downstream of the medium can be directed with variable permeability on to a detector.

The optical permeability of a body is determined by the ratio of the light intensity entering and leaving the same and is consequently inversely proportional to the opacity. The method according to the invention makes it possible to detect the level of a filling material in a continuous analog manner. The detectors used are preferably photodiodes, secondary electron multipliers, phototransistors, particularly photo-FET's or the like.The spatial resolution when using the method according to the invention is a function of the geometry and intensity of the light beam, the gradient of the permeability of the SPECIFICATION Level measurement method and apparatus The invention relates to a method for measuring the level of a filling material, such as for measuring the level of a liquid in a container or a pourable material in a silo or the like, with at least one light source and a detector arrangement, at least one primary beam being directed on to the surface of the filling material at an angle differing from 90 and the secondary beam from the filling material being detected along an extension direction in the plane covered by the incident beam and the secondary beam, at a finite angle to the direction of the reflected beam, as well as a level measurement apparatus, particularly for performing the method with at least one light source and at least one detector, the light source being arranged above the surface of the filling material in such a way that its light strikes the surface at an angle differing from 90" and that a reception arrangement is arranged in the beam path of the secondary light emanating from the filling material in the plane covered by the axis of the light source and the perpendicular of the surface, but extending under a finite angle to the direction of the reflected beam.

Numerous level measurement methods are known, including optical methods using light sources and detector arrangements. The known methods either involve considerable and particularly electronic expenditure and have only a narrow measuring range, or are not sufficiently accurate.

There is a risk of the measured results being falsified. The methods are often sensitive to environmental influences. There are often restrictions with regards to the measuring range. For example, methods and apparatuses are known, which only perform a limit value measurement. German Patent 1207103 describes a procedure with a limited measuring range, but acceptable sensitivity. In this case the light source comprises two substantially identical, light-emitting objects, such as the two electrodes of a neon tube which alternately emit light in a predetermined frequency. When the reflected surface differs from a predetermined position, one of the images of the light-emitting objects is covered to a greater or a lesser extent.An electronic device must be provided for determining such a coverage and responds to the frequency of the alternate light emission, so that then the position of the reflected surface, i.e. the filling level can be determined. This apparatus is mainly intended for maintaining a predetermined level, but is unable to measure level changes over a wider range, unless complicated refilling arrangements are provided.

Quite apart from this, a complicated frequencyselected electronics is necessary.

The problem of the invention is to provide a simple level measurement method, which still ensures a high resolution and a simple relationship between the filling level and the measurement information obtained, so that despite the high precision which can be obtained, no complicated detection and evaluation equipment or electronics medium and the sensitivity of the detector as well as optionally the dimensions of a diaphragm which may be provided in the path of the beam. A very high resolution can be obtained. Whereas in the case of large deflections, the intensity obtained is exponentially dependent on the absorption coefficients and thickness of the material, the transmitted intensity can be linearized in the range of the light beam thickness in the case of small deflections.Within the scope of the method according to the invention, the light beam can be limited by diaphragms or guided by imaging optics and particularly after passing from the medium can be directed on to a detector with variable permeability, so that it can in particular be provided that the reflected beam is directed with variable permeability on to a detector downstream of the medium.

It is readily possible within the scope of the present method to work with a constant permeability gradient of the medium in one direction, which forms a finite angle to the emission direction of the secondary beam. However, in preferred manner, the medium has a variable permeability gradient and in particular logarithmically changes in the direction of movement. The latter being with respect to the aforementioned exponential dependence of the ratio of transmitted intensity to incident intensity on the physical thickness of the medium. As a result it is possible to achieve a linear relationship between the filling level and the intensity obtained. In the case of a constant thickness of the medium, which is e.g.

shaped like a parallelepiped, this can be achieved in that the absorption coefficient logarithmically changes, advantageously in a direction perpendiculay to the secondary beam direction. However, the medium itself may have a logarithmically varying physical thickness in said direction.

Further advantages and features of the invention can be gathered from the claims and the following description of exemplified embodiments and with reference to the drawings, wherein show: Figure 1 a diagrammatic view of a first arrange mentfor performing the method according to the invention in side view with reflected secondary beam.

Figure 2 an other arrangementwith a reflected secondary beam, also in side view.

Figure 3 another arrangement with a secondary beam refracted in the filling material.

Figure 4 an arrangement, optionally with are fracted secondary beam.

For the purpose of measuring the level of a liquid 1 in a container 2, a light beam 3 of a laser 4 is directed on to the surface 6 of liquid 1 at an angle differing from the perpendicular, imaging optics 7 being generally interposed between laser 4 and surface 6.

The light beam 3 from laser 4 striking the surface 6 of liquid 2 is reflected from surface 6 as a reflected beam 3'. In the plane covered by the incident and reflected beam 3, 3', which is defined by the light source axis and the perpendicular to the liquid surface 6, is arranged in so-called neutral wedge 8, which, in the represented embodiment, comprises a right-angled triangular prism 9 made from neutral glass and a corresponding right-angled triangular prism 11, made from completely transparent material assembled to form a parallelepiped. Coangular prism 9 tapers in a direction enclosing a finite angle to the reflected beam 3' and located in the plane covered by beam 3, 3'. There is no need for the surface 12 of triangular prism 9 directed opposite to the directed beam 3, 3' to form a right angle with the direction of the reflected beam 3, 3'.The neutral wedge 8 formed by the two triangular prisms 9 and 11 forms a partially permeable medium with a variable light permeability at a finite angle to the reflected beam 3', i.e. in the represented embodiment neutral wedge 8 has a higher light permeability at the passage point 13 of the beam 3' reflected by the liquid surface 6 than at the passage point 14 of a beam 3" reflected by a surface 6' in the case of a lower level of liquid 1.

On the side of neutral wedge 8 remote from the liquid surface 6, 6' is provided a detector 16, on to which the reflected beams 3', 3" are directed by imaging optics 17 in the embodiment according to Figure 1. The light received by detector 16 is then converted into an electrical signal.

If the level in container 2 has the height required by surface 6, then the incident light beam 3 of laser 4 is deflected by the surface into the reflected beam 3', which passes through neutral wedge 8 at passage point 13, where it is attenutated only to a limited extent and is then directed on to detector 16. If there is little liquid in container 2, this has a lower level than surface 6', so that the incident light beam 3 of the laser is reflected at a point displaced with respect to the reflection point on surface 6 into beam 3", which is parallel to beam 3'. Reflected beam 3" passes through the neutral wedge at passage point 14 and, as the neutral glass prism 9 is much thicker here, is attenuated to a much greater extent than the reflected beam 3' at its passage point 13. Reflected beam 3" is then also directed on to detector 16.As a function of the passage point 13, 14 of reflected beams 3', 3", the passage point being determined by the filling level corresponding to the surfaces 6, 6' of liquid 1, a different light fraction passes through neutral wedge 8 and is directed on to detector 16.

Due to the intensity of the light passing through neutral wedge 8, it is possible to easily, but very accurately determine the filling level in container 2, because as a result of the arrangement according to the invention, the fraction of the transmitted light is a function of the level. As the fraction of the transmitted light in the case of the given material absorption coefficient decreases exponentially with the thickness, e.g. the change of the partially permeable part of a neutral wedge corresponding to prism 9 could take place logarithmically and not linearly as in the case of the prism, so that then the change to the transmitted light fraction would be proportional to the level change.

The arrangements of a Figure 1 with a detector and an imaging optics can preferably be used e.g. for determining the filling level of very fine bulk material in a silo or the like, because such bulk material has a fixed angle of repose, at least under constant environmental conditions, such as in particular humidity. The level of such bulk material can also be satisfactorily determined in the case of a suitable orientation of the arrangement, imaging optics 17 only focusing on to the detector the main fraction of the reflected light reflected by the surface of the poured pile of material at an angle corresponding to the angle of incidence of the incident beam. However, due to the granularity of the bulk material, diffusely reflected light is refracted in such a way that it does not strike the detector.

In the embodiment according to Figure 2, a detector row 21 is positioned downstream of neutral wedge 8, in place of imaging optics 7 and a single detector 16. Such a detector row 21, apart from the continuous or analog intensity determination, the quasi-continuous location of the individual detectors of row 21 can be used for determining and checking the level. In principle, only the detector row need be provided, i.e. the level measurement takes place without a neutral wedge, so as to permit a quasicontinuous level determination with high resolution.

In place of a geometrical change to the permeability of the irradiated, partially permeable medium or body over the thickness thereof, or the thickness of a more absorbent part of such a medium or body in the form of a neutral wedge 8, as shown in Figures 1 and 2, another possibility of keeping the variability of the permeability of the medium at a finite angle to the reflected beam 3', 3" in one direction consists of taking a medium or body through which irradiation is to take place, the absorption coefficient only varying in one direction with a finite angle to the reflected beam 3', 3" and then in particular the thickness of the irradiated body can be constant. The change to the absorption coefficient can also take place either linearly or in some other way, so that the absorption coefficient gradient can be constant or variable.

According to the embodiments of Figures 3 and 4, once again a liquid 1 is provided in a container 2 and its level is measured by means of a light beam 3 of a laser 4. As in the previous embodiment, laser 4 is directed on to the surface 6 of liquid 1 at an angle differing from the perpendicular. The light beam 3 from laser 4 striking surface 6 of liquid 1 penetrates the same, which has an adequate transparency and is refracted by it to a beam 3'. The mirror 23 arranged on the bottom of container 2 and reflects the refracted beam 3' into the liquid, so that it finally emerges therefrom again as a secondary beam.

Secondary beam 3' is reflected by a mirror 23 on to a photodiode row 24, which is connected downstream of the signal evaluation arrangement 25. If the level in container 2 has the height required by surface 6, then the incident light beam 3 of laser 4 is refracted in liquid 1 in such a way that after reflection on mirror 23, it passes out of the liquid again as secondary beam 3 and is subsequently directed onto photodiode row 24 at a point 25.If container 2 contains less liquid 1, i.e. the latter has a lower level with surface 6', then the incident light beam of laser 4 is refracted later, namely at surface 6' and consequently at a passage point displaced along surface 6, so that after reflection on mirror 23, a secondary beam 3" passes out of the liquid with filling level 6', runs parallel to beam 3' and consequently strikes the photodiode row 24 at a point 26 displaced with respect to the impact point 25 of beam 3'. The signal evaluation means can then determine the height of the surface of the liquid or the filling level. In this embodiment, like that of Figure 1 and a further embodiment to be described hereinafter, the arrangement with a photodiode row could be replaced by a neutral wedge with optics imaging the light beam on a single photodiode.In the embodiment of Figure 4, the incident primary beam 3 is also refracted on the surface 6 of liquid 1 in container 2, and as a secondary beam 3' passes out of the liquid 1 again through the bottom of container 2. In the manner described relative to Figure 1, the secondary beam 3' then passes in a similar manner through the neutral wedge 8 and is focused by a iens 17 on to a photodetector 16.

If the liquid has a different level with surface 6', then the incident primary beam is refracted at a displaced point and passes out of the bottom of container 2 as a secondary beam 3" at a point displaced with respect to secondary beam 3", radiates through the neutral wedge 8 at a different point from secondary beam 3' and is thereby attenutated in a different way and less than secondary beam 3' in the represented embodiment. After passing through neutral wedge 8, the secondary beam 3" is again focused on to the photodetector 16 by the imaging arrangement 17. The filling level can then again be determined by the different intensity of the incident beams.

The features of the invention disclosed in the above description, drawings and claims, can be essential for the realization of the invention in its various embodiments, either singly or in random combinations.

Claims

1. A method for measuring the level of a filling material, such as for measuring the level of a liquid in a container or a pourable material in a silo or the like, with at least one light source and a detector arrangement, at least one primary beam being directed on to the surface of the filling material at an angle differing from 90" and the secondary beam from the filling material being detected along an extension direction in the plane covered by the incident beam and the secondary beam, at a finite angle to the direction of the reflected beam, wherein a coherent light beam is directed on to the filling material surface and the secondary beam can be detected by a row of detectors, which are at least juxtaposed in said extension direction.

2. A method for measuring the level of a filling material, such as for measuring the level of a liquid in a container or a pourable material in a silo or the like, with at least one light source and a detector arrangement, at least one primary beam being directed on to the surface of the filling material at an angle differing from 90" and the secondary beam from the filling material being detected along an extension direction in the plane covered by the incident beam and the secondary beam, at a finite angle to the direction of the reflected beam, wherein a coherent light beam is directed on to the filling material surface and the reflected beam is initialiy transmitted through a medium with an optical permeability for the light of the light source which continuously varies, in the plane covered by the incident and the secondary beam under a finite angle to the direction of the secondary beam, the intensity of the light beam being measured by means of the detector arrangement.

3. A method according to claim 2, wherein downstream of the medium, the secondary beam is directed with variable permeability on to a detector.

4. A method according to one of the claims 2 or 3, wherein the medium has a variable optical permeability gradient.

5. A method according to claim 4, wherein the permeability logarithmically varies at right angles to the direction of the secondary beam.

6. A method according to one of the claims 2 to 5, wherein the medium is parallelepipedic.

7. A method according to one of the claims 2 to 6, wherein the medium with variable optical permeability has a variable absorption coefficient under a finite angle to the direction ofthe reflected beam.

8. A method according to claim 7, wherein the absorption coefficient logarithmically changes.

9. A method according to one of the claims 2 to 8, wherein the medium tapers from one end to the other in a direction forming a finite angle to the direction of the reflected beam.

10. A method according to claim 9, wherein the irradiated medium is wedge-shaped.

11. A method according to claim 9, wherein the irradiated medium has a logarithmically varying physical thickness.

12. A method according to one of the claims 1 to 11, wherein the secondary beam from the filling material is reflected bythefilling material surface.

13. A method according to one of the claims 1 to 11, wherein in the case of a transparent, liquid filling material the secondary beam from the latter is a beam which is at least refracted in the filling material.

14. A method according to claim 13, wherein the secondary beam is reflected on the bottom of the filling material container.

15. An apparatus for measuring the filling level, particularly for performing the method according to one of the claims 1 to 14, with at least one light source and at least one detector, the light source being arranged above the surface of the filling material in such a way that its light strikes the surface at an angle differing from 90" and that a reception arrangement is arranged in the beam path of the secondary light emanating from the filling material in the plane covered by the axis of the light source and the perpendicular of the surface, but extending under a finite angle to the direction of the reflected beam, wherein the lightsource is a laser and the reception arrangement has a row of juxtaposed detectors in the said extension direction.

16. An apparatus for measuring the filling level particularly for performing the method according to one of the claims 1 to 14, with at least one light source and at least one detector, the light source being arranged above the surface of the filling material in such a way that its light strikes the surface at an angle differing from 90" and that a reception arrangement is arranged in the beam path of the secondary light emanating from the filling material in the plane covered by the axis of the light source and the perpendicular of the surface, but extending under a finite angle to the direction of the reflected beam, wherein the light source is a laser and the reception arrangement has a medium arranged in the beam path of the secondary light emanating from the filling material and upstream of the detector arrangement, and with an optical permeability varying in the plane covered by the axis of the light source, as well as the perpendicular of the surface, but under a finite angle to the direction of the secondary beam.

17. An apparatus according to claim 16, in conjunction with claim 13, wherein the detector arrangement is a single detector, imaging optics being arranged between the medium with the variable permeability and the detector, which directs the secondary light beam on to the detector, independently of the filling level.

18. An apparatus according to one of the claims 15 to 17, wherein the detector arrangement is arranged in the path of the secondary beam reflected by the filling material surface.

19. An apparatus according to one of the claims 15 to 17, wherein in the case of a liquid transparent filling material, the detector arrangement is arranged in the beam path of the secondary beam refracted in the filling material.

20. An apparatus according to claim 19, wherein a mirror is arranged in the beam path on the bottom ofthefilling material container.

21. A method for measuring the level of a filling material substantially as herein described with reference to the accompanying drawings.

22. An apparatus for measuring the filling level constructed, arranged and adapted to operate substantially as herein described with reference to, and as shown in the accompanying drawings.