Optical device for liquid level measurement in a reservoir, using a reflective prism.

0001The subject of the present invention is a device for measuring the level of a liquid in a reservoir, of the known type which comprises an elongated transparent prism, the lateral faces of which extend over the height of the reservoir, said prism being provided with a lateral entry face illuminated by light emitting means, with a lateral face for reflecting the light coming from said entry face, partly immersed in said liquid and which, in its submerged part, a reduced reflection coefficient, and a lateral outlet face opposite which are arranged receiving means for determining the intensity of the reflected light, representative of the level of the liquid in the reservoir.

0002The invention applies in particular to the measurement of the fuel level on board a motor vehicle.

0003A device of the type defined above is already known, by French application No 2 628 836. In such a device, the non-submerged part of the prism behaves like a prism with total reflection and reflects towards the outlet face and the means receivers all the light it receives from the input face, that is to say from the emitting means.

0004On the other hand, in the submerged part of the prism, because the index of the liquid is close to the index of the prism, the reflection face lets most of the light energy pass from the interior of the prism towards the liquid , a very small part of this light energy being reflected towards the exit face. As a result, the measurement of the intensity of the reflected light received by the receiving means provides information on the level of the liquid in the reservoir.

0005In the French application N 2 628 836, already cited, the emitting means comprise a plurality of light-emitting diodes distributed vertically along the input face, controlled by an electronic circuit which lights them sequentially. The electronic circuit is connected to the receiving means and determines, for each diode, the intensity of the light received. If this intensity is high, this means that the part of the reflection face disposed at the level of the lit diode, on which the reflection takes place, is not immersed. On the other hand, if the intensity is low, it is because the liquid arrives at the part of the reflection face located opposite the lit diode. The electronic circuit begins, for example, by lighting the diode placed at the top of the input face and measures the light intensity received. If the intensity received is high, it means that the liquid is not at the maximum level, and the electronic circuit passes to the diode placed immediately below, and so on, until the intensity undergoes a strong decrease, which indicates that the lit diode is slightly below the level of the liquid.

0006Such a device is particularly advantageous because it has no moving parts, and is insensitive to aging of the components, or to fouling of the prism, because the electronic circuit must only detect a variation in the light intensity received when it goes from one diode to another, a variation which is very large and easy to detect. However, since the reflection face of the prism is directly in contact with the liquid, it is necessary that the material of the prism is not attacked, chemically, by the liquid. In the case where the liquid is fuel, this practically makes it necessary to produce the glass prism, since the known transparent plastics are generally attacked by current fuels. However, a glass prism, the faces of which must be polished, is relatively expensive.

0007The present invention aims to overcome this drawback.

0008To this end, it relates to a device of the type defined above, characterized in that said lateral reflection face is protected from the liquid by a protective layer and illuminated under the incidence of Brewster by the light coming from said input face, and that a direction polarizer is provided parallel to the plane of incidence of light on the lateral reflection face, between said emitting means and said receiving means.

0009In the device of the invention, the presence of the protective layer makes it possible to use a molded plastic prism, easier to manufacture, and at a lower cost price than that of a polished glass prism. However, the presence of the protective layer does not degrade the behavior of the device because the lateral reflection face of the prism is illuminated under the incidence of Brewster, and that only the component of the light polarized parallel to the plane of incidence on this reflection face, filtered by the polarizer, reaches the reception means.

0010In the preferred embodiment, said emitting means comprise a plurality of sources distributed along said inlet face, and means are provided for sequentially switching on at least some of the sources and determining the level of the liquid in response to the intensity of light received from these sources.

0011Advantageously, said receiving means comprise means for collecting light from said outlet face and means for measuring the light intensity collected by said collecting means.

0012Advantageously also, said prism is an integral part of a sealed protective casing for the device, and is formed by a prismatic excess thickness of the wall of said casing.

0013The present invention will be better understood on reading the following description of the preferred embodiment of the device of the invention, and one of its variants, made with reference to the accompanying drawings, in which,<ul id="ul0001" list-style="dash"><li>FIG. 1 represents a sectional view of a reservoir provided with a level measurement device according to the invention,</li><li>FIG. 2 represents a partial view, in exploded perspective, of the device of FIG. 1,</li><li>Figures 3<u style="single">at</u> and 3<u style="single">b</u> each represent a top view, in section, of the device of FIG. 12 in its non-submerged part and in its submerged part, respectively,</li><li>FIG. 4 represents a block diagram of the electronic circuit of the device of FIG. 1, and,</li><li>FIG. 5 represents a top view of a variant of the device of FIG. 1.</li></ul>

0014In Figure 1 is shown schematically a tank 1 containing a liquid 2. As has already been reported, the invention relates to a device 3 for measuring the level of liquid 2 in the tank 1. Here, the liquid 2 is fuel and the tank 1 is mounted on board a motor vehicle.

0015The device 3 comprises a sealed housing 31, intended to support and protect its constituents. In known manner, the housing 31 is made integral with the reservoir 1 by its upper part which adapts to an orifice provided for this purpose in the upper wall of the reservoir 1. The housing 31 extends over the entire height of the reservoir 1. Here it has the shape of a circular cylinder.

0016Between the upper part of the housing 31 and its bottom are notably arranged, as appears in FIG. 2, an elongated transparent prism 32, a printed circuit board 33 supporting a plurality of light-emitting diodes 331, a polarizer 34 and a section 35 d 'a light guide.

0017The prism 32 is of triangular base, and its three lateral faces here extend vertically over the entire height of the reservoir 1. The first lateral face 321 is called the entry face, because the light from the diodes 331 penetrates into the prism 32 by this entry face 321. The second lateral face 322 is called the reflection face because the light from the diodes 331 which has penetrated into the prism 32 through the entry face 321 is reflected on this reflection face 322, at least in part . The third lateral face 323 is called the exit face, because the light from the diodes 331 which is reflected by the reflection face 322 comes out of the prism 32 by this exit face 323.

0018As shown in FIGS. 3, the base of the prism 32 is an isosceles triangle, the two equal sides of which correspond to the inlet 321 and outlet 323 faces, and the third side of which corresponds to the reflection face 322. The two angles equals of this isosceles triangle are notes i_B.

0019The prism 32 is produced here in molded polymethyl methacrylate (PMMA), of index n = 1.49.

0020The reflection face 322 of the prism 32 is coated with a layer 4 of protective varnish, clearly visible in FIGS. 3 where its thickness has been considerably dilated, for the sake of clarity. Here, the protective varnish is that sold by the company NOVATRAN under the name Parylene C, of ​​index n_V = 1.64. The thickness of layer 4 is here substantially equal to 10 microns.

0021So that the reflection face 322 of the prism 32, protected by the layer 4, is immersed over a height corresponding to that of the liquid 2 in the reservoir 1, a wall 5 is sealed on the face 322, leaving an elongated volume free 41 which communicates with the volume of the reservoir 1 through an orifice 311 made in the bottom of the housing 31. Thus the level of the liquid 2 in the volume 41 which extends along the reflection face 322 is the same as that in the tank 1, the orifice 311 allowing the filtering of sudden variations in the level of the liquid 2 which result from the vehicle vibrations and accelerations.

0022On the printed circuit board 33, the diodes 331 are here aligned, and the plate 33 is arranged so that the light from the diodes 331 penetrates through the input face 321 under normal incidence, the diodes 331 being here regularly and vertically distributed along the input face 321. The diodes 331 are controlled by an electronic circuit 7.

0023The polarizer 34 is disposed along the output face 323. Its direction of polarization is parallel to the plane of incidence of the light from each diode 331 on the reflection face 322, here a horizontal plane. The polarizer 34 therefore allows itself to be traversed only by the component of the light polarized rectilinearly in the direction parallel to the horizontal plane of incidence.

0024The section 35 is also arranged along the outlet face 323, so that the polarizer 34 is located between the section 35 and the outlet face 323. The section 35 is here made of transparent material charged with a fluorescent substance. More particularly, and by way of example, the material in question here is an optically pure transparent polymer of the PMMA family colored by a fluorescent dye and marketed by the company BAYER under the registered trademark "Lisa".

0025Opposite the upper straight section of the section 35 is a photodetector 351, here a photodiode, connected to the electronic circuit 7.

0026The electronic circuit 7 is connected to a display device 8 to indicate the level measured.

0027The value of equal angles i_B of the isosceles triangle section of the prism 32 is equal to:<maths id="math0001" num="(1)"><math display="block"><mrow><msub><mrow><mtext>i</mtext></mrow><mrow><mtext>B</mtext></mrow></msub><msub><mrow><mtext> = Arc tg (n</mtext></mrow><mrow><mtext>V</mtext></mrow></msub><mtext>/not)</mtext></mrow></math><img file="EP0447728A2_D0001.tif" /></maths>

0028Before describing the electronic circuit 7 in more detail, the operating principle of the device is described, with particular reference to FIGS. 3<u style="single">at</u> and 3<u style="single">b</u>.

0029In figure 3<u style="single">at</u>shows the path of a light beam 332 from a diode 331 located at a height high enough that the part of the reflection face 322 located at its level is not submerged. This light beam 332 crosses the input face 321 under normal incidence, and arrives on the reflection face 322 at an angle of incidence i_B. The value i_B defined by the relation (1) is none other than that of the Brewster incidence for the prism-varnished diopter of the reflection face 323, when the light comes from inside the prism 32, which is the case for the beam 332.

0030Because the diopter of the reflecting face 323 is illuminated under the incidence of Brewster, the light energy which is reflected by this face is polarized rectilinearly in a direction perpendicular to the plane of incidence. It is therefore stopped by the polarizer 34 and everything therefore happens as if it did not exist. We can therefore consider that all the "useful" light energy, that is to say, taking into account the presence of the polarizer 34, that polarized rectilinearly parallel to the plane of incidence, crosses the reflection face 322. C has been shown in Figures 3<u style="single">at</u> and 3<u style="single">b</u> where the ray reflected by the reflecting face 322 has not been represented, a ray which in reality exists, but which does not intervene, since it does not cross the polarizer 34.

0031In the varnish layer 4, the useful part of the beam 332, polarized parallel to the plane of incidence, propagates and arrives on the varnish-air diopter at an angle of incidence i_V, not shown in FIGS. 3 for the sake of simplicity, the value of which is given by the following well-known formula:<maths id="math0002" num="(2)"><math display="block"><mrow><msub><mrow><mtext>n sin i</mtext></mrow><mrow><mtext>B</mtext></mrow></msub><msub><mrow><mtext> = n</mtext></mrow><mrow><mtext>V</mtext></mrow></msub><msub><mrow><mtext> sin i</mtext></mrow><mrow><mtext>V</mtext></mrow></msub></mrow></math><img file="EP0447728A2_D0002.tif" /></maths>

0032This angle iV is here greater than the total reflection angle of the varnish-air diopter, total reflection angle which is equal to:<maths id="math0003" num="(3)"><math display="block"><mrow><msub><mrow><mtext>i</mtext></mrow><mrow><mtext>VRT</mtext></mrow></msub><msub><mrow><mtext> = Arc sin 1 / n</mtext></mrow><mrow><mtext>V</mtext></mrow></msub></mrow></math><img file="EP0447728A2_D0003.tif" /></maths>

0033Indeed, if care is taken to choose a varnish whose index nV is relatively close to the index n of the prism 32, as is the case for the example described, the relation:<maths id="math0004" num="(4)"><math display="block"><mrow><msub><mrow><mtext>i</mtext></mrow><mrow><mtext>V</mtext></mrow></msub><msub><mrow><mtext> > i</mtext></mrow><mrow><mtext>VRT</mtext></mrow></msub></mrow></math><img file="EP0447728A2_D0004.tif" /></maths> is generally satisfied.

0034The useful part of the beam 332 therefore undergoes total reflection on the varnish-air diopter, and, following a path symmetrical to that which it had followed hitherto, successively crosses the reflection face 322, the exit face 323, the polarizer 34 and is collected by the section 35. In the section 35, there occurs, in a known manner, a conversion of the light energy of the beam 332 into fluorescence light, this fluorescence light then being guided towards the photodiode 351 with very good efficiency.

0035If, on the other hand, and as shown in Figure 3<u style="single">b</u>, the part of the reflection face 323 which is at the level of the diode 331 is immersed, the total reflection no longer occurs on the varnish-liquid diopter, due to the fact that the index of the liquid, here nc = 1.49 if it is diesel, is close to that of varnish. Most of the light energy of the useful component polarized parallel to the plane of incidence crosses the varnish-liquid diopter, and the reflection coefficient on the reflection face 322 is considerably reduced.

0036The calculation shows that, as a first approximation, the ratio between the light intensity collected by the section 35 in the case of FIG. 3<u style="single">b</u> and that collected in the case of Figure 3<u style="single">at</u> is worth:<maths id="math0005" num="(5)"><math display="block"><mrow><msub><mrow><mtext>(nV cos r - n</mtext></mrow><mrow><mtext>vs</mtext></mrow></msub><msub><mrow><mtext> cos i</mtext></mrow><mrow><mtext>V</mtext></mrow></msub><msub><mrow><mtext>) ² / (n</mtext></mrow><mrow><mtext>V</mtext></mrow></msub><msub><mrow><mtext> cos r + n</mtext></mrow><mrow><mtext>vs</mtext></mrow></msub><msub><mrow><mtext> cos i</mtext></mrow><mrow><mtext>V</mtext></mrow></msub><mtext>)²</mtext></mrow></math><img file="EP0447728A2_D0005.tif" /></maths>

0037In formula (5), the angle r is the angle at which the beam 332 emerges in the liquid 2, of value given by the following well-known formula:<maths id="math0006" num="(6)"><math display="block"><mrow><msub><mrow><mtext>not</mtext></mrow><mrow><mtext>V</mtext></mrow></msub><msub><mrow><mtext> sin i</mtext></mrow><mrow><mtext>V</mtext></mrow></msub><msub><mrow><mtext> = n</mtext></mrow><mrow><mtext>vs</mtext></mrow></msub><mtext> sin r</mtext></mrow></math><img file="EP0447728A2_D0006.tif" /></maths>

0038By way of example, the ratio defined by formula (5) takes the numerical value 5.10⁻⁷ for the example currently described.

0039The particularly low value of this ratio should be noted, and shows that the detection of the presence or absence of liquid at the level of a determined diode 331 is very easy and very reliable, since the intensity collected by the section 35 is practically zero in the presence of liquid. The detection can thus be done, in practice, by "all or nothing".

0040The device is relatively insensitive to variations in the index n_vs liquid to be measured. For example, if the device currently described, intended for a fuel of index n_vs = 1.49 is used with a fuel of index n ′_vs = 1.33, the ratio of formula (5) takes the value 2.10⁻², which is still relatively low and quite exploitable.

0041Referring to Figure 4, the electronic circuit 7 is now described. Here it includes a clock 71 which delivers a clock signal to the counting input of a counter 72. The digital output of the counter 72 is connected on the one hand to the input of a flip-flop circuit 76 and on the other hand to the control input of a multiplexer 73 provided moreover with a single input connected to a voltage source 74, and a plurality of outputs each of which is connected to each of the diodes 331, respectively.

0042The output of photodiode 351 is connected to an input of a comparator 75 whose other input receives a threshold signal S. The output of comparator 75 is connected to the clock input of flip-flop circuit 76, the output is connected to the display device 8.

0043The electronic circuit 7 operates as explained now. The digital output of the counter 72 increases by one unit at each clock stroke of the clock 71. It follows that each of the diodes 331 is successively connected to the source 74, designed to cause it to ignite. The diodes 331 are therefore lit sequentially one after the other.

0044The intensity of the light collected by the section 35 is therefore representative of the light reflected by the portion of the reflection face 322 which is at the level of the lit diode. Comparator 75 permanently compares this intensity with threshold S. Assuming the multiplexer 73 arranged so that the first lit diode is that disposed at the bottom of the tank, the second that immediately disposed above, and so on, comparator 75 generates a transition as soon as the output signal from photodiode 351 crosses the threshold S, that is to say as soon as the lit diode is located just above the level of the liquid 2 in the tank 1. The output signal of the comparator 75 "freezes" the output value of the flip-flop circuit 76 to a value which copies its input value, and which indicates the rank of the diode 331 which is located just above the level of the liquid. The indication of the display device 8 is therefore representative of the level of the liquid.

0045Thus the electronic circuit 7 sequentially turns on each of the sources and determines the level of the liquid in response to the intensity of the light collected. As already mentioned, the device is insensitive to aging or fouling of its constituents, since it is sufficient that the difference between two consecutive values ​​of the collected light intensity is detected, the determination of the level proper then starting from the row of diode 331 which gave rise to this difference.

0046In Figure 5 is shown a variant of the device of the invention. In the device of Figure 5, there are the same elements as in the device of Figures 3, except that the housing 31 and the prism 32 are replaced by a housing 31 'and a prism 32', respectively. The housing 31 ′ and the prism 32 ′ form a single, sealed part produced during a single molding operation. More specifically, the prism 32 ′ is an integral part of the housing 31 and is formed by an extra thickness, or protuberance, prismatic of the wall of the housing 31 ′, produced here in PPMA. A simpler and more compact structure is thus obtained, the cross section of which can have dimensions of the order of 20 mm. In addition, sealing problems are simplified. In fact, in the device of FIG. 5, the reflection face 322 is located outside the housing 31 ′, and is naturally in contact with the liquid to be measured without it being necessary to provide a volume similar to the volume 41 of FIGS. 3. In FIG. 5, the wall 5 which spares this volume 41 has however been maintained, so that the variations in the level in the volume 41 are filtered with respect to those of the reservoir 1. However, in this case, it is not necessary for the wall 5 to be sealed in a sealed manner, which is the case in the device of FIGS. 3. In FIG. 5, all the parts of the housing 31 ′ in contact with the liquid 2 are protected by the protective layer of varnish 4, shown in this figure on a more realistic scale than in figures 3, without however being correct, since the layer 4 is here of a thickness of 10 microns, as has been reported.

0047Naturally, many other variants, within the reach of those skilled in the art, are possible without departing from the scope of the invention.

0048For example, the electronic circuit 7 which has only been described by way of example can be improved to perform in particular a filtering of the measurements. Indeed, the sequential ignition of the diodes can be relatively rapid, which makes it possible to obtain a relatively large flow of measurements, measurements which can be filtered electronically. As the decrease in the level of the liquid is generally relatively slow, it is also possible, once a first value of the level has been detected, to limit the sequential scanning of the diodes 331 to those which are in the vicinity of the detected level, of which the variations are then followed.

0049The diodes 331 are distributed uniformly along the entry face of the prism in the case of a reservoir of regular shape, where the volume of the liquid is proportional to its level. In the case of an irregularly shaped reservoir, and where one is interested in the remaining volume of the liquid, the variations in volume as a function of the level can be compensated by a judicious distribution of the diodes. To increase the resolution of the device, it is necessary to increase the number of diodes, which can lead to having them staggered, or in any other way capable of solving the problems of congestion.

0050The diode 331 located at the bottom of the tank can be used for an empty tank warning device.

0051The polarizer 34 is not necessarily arranged between the output face 323 and the section 35. It suffices that it is disposed between the emitting diodes 331 and the section 35, and could therefore be placed between the diodes 331 and the face d 'entry 321.

0052To increase the value of the detected light intensity, it is possible to place a second photodetector, identical to photodetector 351, opposite the lower cross section of the section 35, since the light collected by this section 35 is guided there, after transformation into fluorescence light, both upwards and downwards. The output signals of the two photodetectors are, in this case, added before processing.

0053Regarding the choice of materials and their indices, it is important to first choose a material for the protective layer of index n_V as close as possible to the index n_vs some cash. Then, the index n of the prism is chosen so that the relation (4) is satisfied.

0054It is also possible to use the device of the invention by lighting all the diodes 331 at the same time, or by providing a single source illuminating the whole of the input face 321, and by measuring the intensity of the collected light. .