EP1344029A1 - Device for electromagnetic detection of the level of a material which is or has been made conductive, in particular molten glass - Google Patents

Abstract

The device comprises a field winding (32) generating an alternating current in the material and enclosing the container (24) of said material, at least a magnetic field sensor (52, 54) in the zone delimited by the field winding, said sensor comprising a pair of magnetic transducers for detecting the partial differential, along an axis of the inductor, of the field radial component, means (68, 70) for demodulating signals supplied by the sensor, the demodulation taking place with a phase shift substantially of π/2 relative to the current powering the field winding, and means (72, 74, 76) for processing the signals supplied by the demodulating means and for supplying a signal indicating that the material has reached the sensor level.

Description

 ELECTROMAGNETIC LEVEL DETECTION DEVICE

OF A MATERIAL WHICH IS CONDUCTIVE OR MADE CONDUCTIVE,

ESPECIALLY WELDED GLASS

DESCRIPTION

TECHNICAL AREA

The present invention relates to an electromagnetic device for detecting the level of a material, more precisely of a liquid or powdery or solid material in divided form (such as for example granules), which is, or is rendered, electrically conductive, or even of the position of an electrically conductive mobile solid (for example a piston). The invention applies in particular to the detection of the level of a liquid chosen from molten metals, cold liquid metals, electrolytes and molten glasses.

STATE OF THE PRIOR ART It is already known from the documents

US 5,103,893 A (Y. Naganuma et al.) And US 5,232,043 A

(J. Mosch et al.), Devices for detecting the level of a liquid. These devices use the axial component of an alternating magnetic field induced in the liquid.

Also known from documents US 4,138,888 A (S. Linder) and US 4,144,756 A (S. Linder), other devices for detecting the level of a liquid. These other devices use the radial component of an alternating magnetic field induced in the liquid.

The signals supplied by the sensor intended to detect this radial component are demodulated without phase shift with respect to the current which supplies the inductor generating the magnetic field.

STATEMENT OF THE INVENTION

The present invention solves the problem of detection, with greater precision than that which is permitted by the known devices, mentioned above, of the level of a liquid or pulverulent or solid material in divided form, which is, or is rendered , electrically conductive, or even from the position of an electrically conductive mobile solid. Specifically, the present invention relates to a device for detecting the level of a liquid or pulverulent or even solid material in divided form, which is, or is made, electrically conductive and of which a container is filled or emptied having a vertical axis, or the position of a solid monolithic and electrically conductive material which is movable in the container, this device being characterized in that it comprises:

An inductor which is designed to create an alternating electric current in the material and which surrounds the container and has the same axis as this container, the latter being substantially transparent to the magnetic field generated by the inductor, - an alternating current source provided to supply the inductor with current,

- at least one magnetic field sensor which is arranged between the inductor and the axis of the container and oriented along a substantially horizontal axis meeting the axis of the container, for detecting the partial differential, along the axis of the inductor, of the radial component of the magnetic field, the magnetic field sensor comprising a pair of magnetic transducers, mounted in a differential manner and arranged one above the other, the sensitive axes of the magnetic transducers being substantially parallel, the axis according to which the magnetic field sensor is oriented being in the plane defined by these two sensitive axes and being equidistant from them,

means for demodulating the signals supplied by the magnetic field sensor, the demodulation taking place with a phase shift of substantially π / 2 relative to the current supplied by the source, and

- Processing and signaling means provided for processing the signals supplied by the demodulation means and for providing a signal indicative of the material reaching the level of the magnetic field sensor.

By "sensitive axis of a magnetic transducer" is meant an axis (geometric) at the points at which the sensitivity of this magnetic transducer is maximum. For example, the sensitive axis of a coil is the axis of the latter.

According to a preferred embodiment of the device which is the subject of the invention, the magnetic transducers are identical.

The processing and signaling means preferably comprise electronic means for detecting zero crossing or changing the sign of the signals supplied by the demodulation means.

According to a first particular embodiment of the device o jet of the invention, the magnetic field sensor is disposed between the inductor and the container regardless of the state (liquid or solid) of the material in the container.

According to a second particular embodiment, the level detection takes place inside the liquid or pulverulent or even solid material in divided form, this embodiment not applying in the case of a monolithic solid material.

In this case, the magnetic field sensor is placed in an electrically insulating or very weakly conductive tube, closed at its lower end and arranged, preferably vertically, in the container, between the internal wall and the axis of this container.

This second particular embodiment is useful in the case where the first particular embodiment is difficult to implement, for example because of the existence of significant parasites related to the proximity of the inductor or because of the excessive shielding effect of the container.

In this case, the tube is preferably placed at a distance from the axis of the container, at which the quadrature component of the radial component of the magnetic field is maximum, which leads to maximum detection sensitivity.

The invention is particularly applicable to level measurement in a cold crucible, intended for direct induction melting:

The container can be a cold crucible, this crucible being formed of sections electrically insulated from each other and provided with cooling means, the inductor being used to heat the material placed in the container.

In this case, the material can be chosen from the group comprising molten glasses and molten metals.

According to a particular embodiment of the invention, the device comprises a plurality of magnetic field sensors which are arranged at a plurality of levels of the container, in order to detect the reaching, by the material, of the level of any one of these magnetic field sensors. In this case, the invention also makes it possible to measure the speed of movement of the level of the material:

The processing and signaling means can also be provided for measuring the time interval between successive levels reached by the material and for dividing the distance separating these successive levels by the time interval thus measured, the device then forming a device for measuring the speed of variation of the level of the material in the container. This speed measurement is favored by the precision of the measurements allowed by a device according to the invention and by the possibility of associating a large number of magnetic field sensors along a generator of a container. It should be noted that the measurement of time intervals and the measurement of distances separating successive levels use techniques known to those skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be better understood on reading the description of exemplary embodiments given below, by way of purely indicative and in no way limiting, with reference to the appended drawings in which: "FIG. 1 is a view schematic of a particular embodiment of the device which is the subject of the invention, "FIG. 2 is a schematic view of another particular embodiment, applied to a cold crucible for direct induction melting, and • FIG. 3 is a schematic and partial view of a variant of the device of Figure 2, using an insulating rod. DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

The device according to the invention, which is schematically shown in Figure 1, is intended to detect the level of an electrically conductive liquid 2 with which a container is filled 4. The arrow 6 symbolizes this filling. The reference X designates the vertical axis of the container 4.

The device of FIG. 1 comprises a magnetic coil 8 which is supplied by a source 10 of alternating current. This coil 8 surrounds the container 4 and has the same axis X as this container. It constitutes an inductor which creates a magnetic field, which generates an alternating current in the liquid.

The container 4 is made of a material chosen or arranged so as to limit the screen effect to the alternating magnetic field emitted by the inductor and intended to be transmitted to the liquid of the container.

This can be achieved in particular by using a container made of electrically conductive elements which are juxtaposed and electrically isolated from each other.

The device also comprises a pair of identical coils 11 and 12 forming a differential magnetic field sensor 13. This pair of coils 11 and 12 is intended to provide a signal proportional to the partial differential (along the vertical axis X) of the component substantially radial from the magnetic field.

In addition, this pair of coils 11 and 12 is disposed between the coil 8 and the container 4. It is oriented along a horizontal Y axis which substantially meets the X axis of the container.

It is specified that the axes (not shown) of the coils 11 and 12 are parallel and define a vertical plane and that the axis Y is contained in this plane and equidistant from the axes of the coils 11 and 12.

It should be noted that the invention is based on the detection, by the magnetic field sensor, of the passage through an extremum, at the interface between the conductive medium and the non-conductive medium, of the component substantially in quadrature with the induction current (aspect temporal field) of the radial component of the magnetic field (spatial aspect of the field).

The electrical voltage capable of being supplied by the pair of coils 11 and 12 is sent to means 14 for amplifying this voltage.

The device of FIG. 1 also comprises means 16 for synchronous demodulation of the voltage thus amplified. These means 16 are capable of demodulating substantially at π / 2 the voltage supplied by the output of the amplifier 14 so that the demodulation takes place with a phase shift of substantially π / 2 relative to the current supplied by the source 10. In the invention, as a demodulation reference, the inductor current is therefore not used directly, but a phase-shifted current of substantially π / 2 relative to the latter.

When the conductive liquid reaches the level of the sensor 13, i.e. the level defined by the Y axis, the pair of coils provides signals electric which are amplified by means 14 then demodulated by means 16.

The device of FIG. 1 also includes electronic means 18 for processing the signals supplied by the demodulation means 16.

These means 18 comprise for example a rocker (or a “trigger”) connected to a bulb (or to a light-emitting diode or LED) to supply a signal when the level of the Y axis is reached by the conductive liquid 2.

The signal supplied by the means 18 is therefore sent to signaling means 20 (for example bulb or LED) provided to provide a visual alarm when the level of the Y axis is reached. As a variant, the means 18 are connected to signaling means capable of providing an audible alarm when this level is reached.

The example of the invention, which is schematically represented in FIG. 2, is applied to a molten glass 22 whose electrical resistivity is between 0.001 Ω.m and 1 Ω.m. This molten glass is heated by direct induction in a cold crucible 24 whose axis, which is vertical, is denoted x. This cold crucible is sectorized; it is formed of metal tubes (not shown), for example stainless steel, which are juxtaposed and electrically isolated from each other.

We see in Figure 2 the sole 28 which constitutes the bottom of the crucible. This sole 28 and the rest of the crucible 24 are cooled by means of water circulation means 30.

Also seen in Figure 2 an inductor 32, powered by an AC source 34 and provided for heating the glass. This inductor surrounds the crucible 24 and the axis of the inductor is also the x axis.

The sectoring of the crucible makes it substantially transparent to the magnetic field generated by the inductor 32. We also see, in FIG. 2, a pouring nozzle 36 provided for recovering the molten glass when the shutter means 38 with which the pouring nozzles are retracted.

We also see: - a thin layer 40 of frozen glass, between 5 mm and 10 mm thick, which separates the molten glass from the cold metal of the internal wall of the crucible, and

- A layer 42 which is located above the molten bath and which is formed of glass being melted.

The arrow 44 symbolizes the filling of the crucible and the arrows 46 symbolize convection movements in the molten glass 22.

References 48 and 50 in Figure 2 respectively designate the top (horizontal) level and the bottom (horizontal) level of molten glass in the crucible. Once the fusion has started, care must be taken during pouring to keep a minimum bath level so that the fusion does not defuse. The device according to the invention of FIG. 2 is intended to detect these low levels and high, but can also be used to monitor changes in the level of the material during filling and emptying operations.

This device comprises the inductor 32 which is supplied by the alternating current source 34 as well as two magnetic field sensors 52 and 54 which are respectively arranged at levels 48 and 50 and provided for detecting the partial differential, along the axis of l 'inductor, of the radial component of the magnetic field.

More specifically, in the example of FIG. 2, these sensors 52 and 54 are arranged between the inductor 32 and the crucible 24 and each of these sensors comprises a pair of identical coils, mounted in a differential manner (in opposition) and arranged one above the other, the axes of these two coils being substantially horizontal and parallel.

The coils of the sensor 52 (respectively 54) have the references 56 and 58 (respectively 60 and 62) and the axis of this sensor, axis which is in the plane defined by the axes of these two coils, parallel to these axes and equidistant of these, meets the x axis of the crucible 24 and corresponds to the low level (respectively high) of the molten glass in the crucible.

The device of FIG. 2 also includes an operational amplifier 64 (respectively 66), the two inputs of which are respectively connected to the two terminals of the set of coils 56-58 (respectively 60-62) as seen in FIG. 2. The device also includes:

synchronous demodulation means 68 (respectively 70) whose signal input is connected to the output of the operational amplifier 64 (respectively 66) and whose reference input is connected to the source 34,

electronic means 72 (respectively 74) for detecting zero crossing or change of sign of the signals supplied by the demodulation means, the input of these electronic means 72 (respectively 74) being respectively connected to the output of the means of demodulation 68 (respectively 70), and

signaling means 76 which receive as input the signals supplied by the electronic means 72 and 74 which supply a signal during the passage through zero or the change of sign. For each level 48 or 50, the synchronous demodulation is carried out substantially in quadrature (phase shift of π / 2) relative to the inductor current.

The demodulation reference comes from a point of capture of the current flowing in the inductor.

It is specified that the shape of each of the (identical) coils 56 and 58 depends on the precision of the detection as well as on the sensitivity and the stability of the measurement and that it is the same for the coils 60 and 62.

The differential mounting of the coils 56 and 58 (respectively 60 and 62) makes it possible to obtain the partial derivative along the x axis of the radial component of the magnetic field.

The introduction of the signal recovered at the output of the demodulation means 68 (respectively

5 70) in electronic means 72 (respectively

74) makes it possible to obtain the triggering, thanks to the signaling means 76, of an alarm signal when the level of molten glass passes in front of the sensor 52

(respectively 54).

.0 In another particular embodiment, which is schematically and partially shown _'in Figure 3, the sensors 52 and 54 are no longer in the space between the inductor 32 and the crucible 24 they are arranged in a cane 78 is-

.5 i.e. a tube closed at its bottom.

This rod is electrically insulating

(for example in alumina or ceramic), cooled if necessary (by means not shown) and arranged vertically in the crucible 24, between the wall

10 internal of the latter and the x axis, so that it plunges into the molten glass 22 through the layer 42.

The dimensions of the rod 78 and the coils 56, 58, 60 and 62 are adapted according to the mechanical and thermal constraints.

! 5 Preferably, the rod 78 is disposed at a distance R from the axis x of the crucible at which the radial component of the magnetic field is maximum.

To determine this distance R, we use for example the following method: iO We calculate the magnetic field H inside the crucible, using, for example, a finite element simulation code into which all the constituent elements (geometric and physical) of the device are entered.

From the magnetic field H thus calculated, the quadrature component (phase shift of substantially π / 2 with the induction current) is extracted from the radial field Hr.

We then look for the distance at which this component is maximum. With the invention, it is possible to measure the high and low levels of the molten glass bath with an accuracy of the order of ± 10 mm, during the transfer phase

(where the rate of descent of the molten glass is approximately

40 mm per minute) as in the filling phase (where the rise speed is approximately 1 mm per minute).

With the device of FIG. 2, it is possible to detect the passage of the molten glass at more than two levels by providing one or more other assemblies of the kind of the assembly 54-66-70-74 of FIG. 2. Then, the sensor of each of these other assemblies is between the sensors 52 and 54.

The use of several such assemblies allows a measurement of the speed of movement of the level of molten glass in the crucible in addition to the measurement of the position of this level of molten glass.

To do this, the means 76 of FIG. 2 can also be provided:

- to measure the time interval between successive levels reached by the molten glass and - to divide the distance separating these successive levels by the time interval thus measured.

This distance is for example measured by the users and then stored in the means 76.

In the above examples, we considered the detection of the level of a molten glass.

The invention also makes it possible to detect the level of a molten metal or of a liquid metal or, more generally, of a conductive or made conductive liquid.

The invention even makes it possible to detect the level of a conductive pulverulent material such as, for example, pulverulent carbon or the “toner” of printers or photocopiers.

The invention also applies to the detection of the position of a monolithic conductive solid which is mobile.

This is schematically illustrated by the example of FIG. 1 where we see a monolithic solid conductor S which floats on the surface of a liquid 80 whose level, in the container 4, is made variable by means not shown. The solid S is detected, thanks to the sensor 13, when it arrives at the level of the axis Y (if the level of the liquid 80 rises sufficiently).

It should be noted that the detection of a relative level, identified by a horizontal axis, in a container, using a device according to the invention, allows the measurement of this level if the distance between the axis is known. horizontal and 'a point any mark, for example the bottom of the container. In this sense, the device which is the subject of the invention can also constitute an absolute level measurement device. It is further specified that it is possible to adapt the size of the turns of any coil used in the invention to capture the magnetic field, as a function of the constraints encountered.

One can, for example, widen the angular sector of the turns to obtain a better sensitivity of measurement or to make the average of local angular disturbances. In the examples of FIGS. 1 to 3, magnetic transducers made up of coils have been considered.

However, the invention is not limited to the use of such coils: it is possible to use, in place of these, magnetic transducers for example made up of magneto-resistors or Hall effect transducers by adapting the means amplification, demodulation and processing 14-16-18 or 64-68-72 and 66-70-74 to such magnetic transducers.

In addition, in the examples, pairs of identical magnetic transducers have been considered.

However, the invention is not limited to this: one or more magnetic field sensors can be used in the invention, each comprising a pair of magnetic transducers (for example example a pair of coils) different from each other and adapt the amplification, demodulation and processing means to such pairs of transducers.

Claims

1. Device for detecting the level of a liquid or pulverulent or even solid material in divided form (2; 22), which is, or is rendered, electrically conductive and from which a container is filled or emptied (4; 24 ) having a vertical axis, or the position of a solid monolithic and electrically conductive material which is movable in the container, this device being characterized in that it comprises: - an inductor (8; 32) which is designed to create an alternating electric current in the material and which surrounds the container and has the same axis as this container, the latter being substantially transparent to the magnetic field generated by the inductor, - an alternating current source (10; 34) intended to supply the inductor with current, - at least one magnetic field sensor (13; 52, 54) which is disposed between the inductor and the axis of the container and oriented along a substantially horizontal axis meeting the axis of the container, to detect the partial differential, according to l axis of the inductor, of the radial component of the magnetic field, the magnetic field sensor comprising a pair of magnetic transducers (11-12, 56-58, 60-62), mounted in a differential manner and arranged one at the above the other, the sensitive axes of the magnetic transducers being substantially parallel, the axis along which the magnetic field sensor is oriented being in the plane defined by these two sensitive axes and being equidistant from them, means (16; 68, 70) for demodulating the signals supplied by the magnetic field sensor, the demodulation taking place with a phase shift of substantially π / 2 relative to the current supplied by the source, and - processing and signaling means (18, 20; 72, 74, 76) provided for processing the signals supplied by the demodulation means and for providing a signal indicative of the material reaching the level of the magnetic field sensor . 2. Device according to claim 1, in which the magnetic transducers are identical. 3. Device according to any one of claims 1 and 2, wherein the processing and signaling means (72, 74) comprise electronic means for detecting zero crossing of the signals supplied by the demodulation means. 4. Device according to any one of claims 1 and 2, wherein the processing and signaling means (72, 74) comprise electronic means for detecting change of sign of the signals supplied by the demodulation means. 5. Device according to any one of claims 1 to 4, wherein the magnetic field sensor (13; 52, 54) is disposed between the inductor (8; 32) and the container (4; 24), whatever either the liquid or solid state of the material in the container. 6. Device according to any one of claims 1 to 4, in which the material is liquid or powdery or solid in divided form and the magnetic field sensor is placed in an electrically insulating or very weakly conductive tube (78), closed to its lower end and disposed in the container (24), between the internal wall and the axis of this container. 7. Device according to claim 6, in which the tube (78) is disposed at a distance from the axis of the container, at which the quadrature component of the radial component of the magnetic field is maximum. 8. Device according to any one of claims 1 to 7, wherein the container is a cold crucible (24), this crucible being formed of sections electrically isolated from each other and provided with cooling means (30), the inductor (32) being used to heat the material disposed in the container. 9. Device according to claim 8, in which the material is chosen from the group comprising molten glasses and molten metals. 10. Device according to any one of claims 1 to 9, comprising a plurality of magnetic field sensors (52, 54) which are arranged at a plurality of levels of the container, in order to detect the reaching, by the material, of the level of any of these magnetic field sensors. 11. Device according to claim 10, in which the means for processing and signaling (72, 74, 76) are further provided for measuring the time interval separating the successive levels reached by the material and for dividing the distance separating these successive levels by the time interval thus measured, the device then forming a device for measuring the speed of variation of the level of the material in the container. 12. Device according to any one of claims 1 to 11, in which the magnetic field transducers are chosen from the group comprising coils, magneto-resistors and Hall effect transducers.