EP0484210B1 - High frequency resonance or microwave applicator for the thermal treatment of flat continuously moving material - Google Patents

Description

The present invention relates to a high-frequency or microwave resonant applicator, that is to say a device capable of applying electromagnetic radiation in the frequency range of high frequencies or microwaves to a planar material in movement. continuous, in particular a textile material, a paper or a nonwoven. It relates more particularly to an applicator comprising a capacitor between the plates from which the planar material moves.

It is already known to subject a planar material in movement to the action of electromagnetic radiation by passing it between the plates of a capacitor, for example to dry or heat it. However, the use of such applicators raises difficulties; in particular, it is difficult to obtain good uniformity of treatment in the transverse direction of the material; moreover, treatment regulation is very delicate, especially when the characteristics of the material vary as a function of the temperature of the material.

An applicator of this type is supplied by a generator whose operating parameters are imposed by the manufacturer. To obtain sufficient stability conditions, it is always necessary to have between the generator supplying the electromagnetic energy and the applicator an additional electromagnetic circuit, commonly called adapter box. Most often these adapter boxes are made up of inductors and capacitors and do not include resistive elements which are capable of dissipating heat and therefore reduce the total energy efficiency of the installation.

However, the applicant has noticed that, despite the presence of an adaptation box intended to regulate the adaptation of the impedances of the generator and of the applicator, there could be a phenomenon of thermal runaway. This phenomenon occurs produced in particular when the dielectric constant of the material to be treated exhibits a large variation as a function of temperature, even when the intensity of the electric field applied to said material is constant.

We have already sought to avoid this thermal runaway phenomenon by measuring the temperature of the product and by reducing the applied electric field. However, this solution comes up against implementation difficulties due to the inertia of the servo mechanisms which are necessary.

The aim which the applicant has set for himself is to propose an applicator which overcomes all the drawbacks noted, in that it makes it possible to obtain uniformity of treatment in the transverse direction of the material and in that it is self- regulating, that is to say that it corrects by itself the conditions of its operation due to variations in the characteristics of the material to be treated.

Document US.A.3,532,848 has already proposed a high-frequency or microwave resonant applicator for the treatment of a flat material whose operation is independent of its width. This applicator comprises, in known manner, a flat capacitor between the plates from which the material to be treated moves. Typically, it comprises, on one side of the material, two trays which follow one another and are interconnected by a plurality of inductors or by a single inductor, continuous over their entire width.

dans laquelle R est le rayon du bobinage , l la longueur de la bobine et µ_o la perméabilité du vide. On sait aussi que la capacité d'un condensateur plan dont les plateaux sont rectangulaires est donnée par la formule : $${\text{C = ε}}_{\text{o}}\frac{\text{.l.d}}{\text{e}}$$

dans laquelle ε_o est la permettivité du vide, l et d respectivement la largeur et la longueur d'un plateau et e l'écartement entre les deux plateaux.The inductor constitutes with the capacity of the flat capacitor an oscillating circuit whose operation is independent of the width of the applicator. Indeed, we know that the value of the air choke or self-inductance of a winding with contiguous turns is given by the formula: $${\text{L = µ}}_{\text{o}}\text{·}\frac{\text{πR²}}{\text{1}}$$

where R is the radius of the winding, l the length of the coil and µ _o the permeability of the vacuum. We also know that the capacity of a planar capacitor whose plates are rectangular is given by the formula: $${\text{C = ε}}_{\text{o}}\frac{\text{.ld}}{\text{e}}$$

in which ε _o is the permissibility of the vacuum, l and d respectively the width and the length of a plate and e the spacing between the two plates.

In the present case, the choke and the capacitor are connected in a distributed manner over the entire length l of the choke, according to the width of the same dimension l of the capacitor.

par le calcul on en déduit que : $$\text{ω² =}\frac{\text{1}}{{\text{µ}}_{\text{o}}{\text{.ε}}_{\text{o}}\text{π}}\frac{\text{e}}{\text{d.R²}}$$

The square of the resonance frequency of the oscillating circuit is equal to the inverse of the LC product. $$\text{ω² =}\frac{\text{1}}{\text{LC}}$$

by calculation we deduce that: $$\text{ω² =}\frac{\text{1}}{{\text{µ}}_{\text{o}}{\text{.ε}}_{\text{o}}\text{π}}\frac{\text{e}}{\text{d.R²}}$$

It can therefore be seen that the resonant frequency of the applicator is, in this type of applicator, independent of the width l of the plates.

However, an applicator as described in document US.A.3,532,848 is not self-regulating.

However, an applicator has been found, and this is the subject of the invention, which makes it possible to obtain uniformity of treatment in the transverse direction of the material and which is self-regulating.

It is an applicator which comprises, in known manner, a flat capacitor between the plates of which said material moves and a choke constituting with the capacitance of the capacitor an oscillating circuit. Typically, according to the invention, the inductor is constituted by a metal strip extending transversely across its entire width the plate of the capacitor acting as a hot electrode; moreover, the internal face of the capacitor plate serving as a ground electrode is divided into elementary sections by a set of longitudinal grooves, in the direction of movement of the material.

In view of this structural feature, it can be considered that the longitudinal grooves define sections elements which are elementary applicators of small dimensions, having the same resonant frequency and having no interactions between them. This structure provides the desired self-regulation.

Indeed, when the characteristics of the material, passing between the plates of the capacitor, fluctuate, it follows variations in the value of the capacitance of the capacitor and consequently in the resonance frequency of the oscillating circuit. Correlatively the electric voltage applied to the terminals of the capacitor will vary and compensate for the fluctuations considered, as will be explained in the description below.

Preferably the elementary sections have a width which is at most 5 cm. Advantageously, it is around or less than a centimeter.

Preferably the internal faces of the plate of the capacitor, acting as a hot electrode, and of the inductor are divided into elementary sections by a set of longitudinal grooves arranged opposite the grooves formed in the plate of the capacitor acting as electrode of mass. This grooving of the other elements makes it possible to reinforce the independence of the elementary applicators.

Preferably the inductor consists of a metal strip in the shape of an arc of a circle, transversely extending the plate of the capacitor acting as a hot electrode.

The metal strip curved in the shape of an arc of a circle can be compared to a contiguous coil.

According to the preferred embodiment of the invention, the applicator comprises two flat capacitors, the plates of which acting as hot electrodes are in the same plane and connected by the ends of the metal strip in an arc, constituting the inductor.

In the case where the inductor consists of a metal strip in an arc, it is possible to adjust the resonance frequency of the applicator by means of deformation means. the metal strip, making it possible to vary the internal section of the choke. This is for example a metal plate which is in contact with the external face of the metal strip and which is equipped with a sliding system, capable of exerting a stress on the strip such that the arc of a circle is uniformly deforms over the entire length of the choke.

The resonant frequency of the applicator can also be adjusted by means of adjusting the spacing of the plates of the capacitor.

Another means of adjusting the electromagnetic field on the material may consist of bars mounted transversely on the faces of the plates of the capacitor facing the material; such bars are capable of varying the intensity of the electromagnetic field and of concentrating it on the product to be treated. They are particularly useful when the material to be treated is thin.

According to the invention and in order not to create transverse interactions, the transverse bars are themselves divided into elementary sections by longitudinal grooves, arranged opposite the grooves formed in the plate of the capacitor acting as a ground electrode. The elementary sections can be separated from each other by strips of insulating material.

Preferably, the applicator is supplied with power by a generator connected by means of a coaxial flange on the one hand to a first metal strip of progressive section extending the plate of the capacitor acting as ground electrode and on the other hand to a second metal strip of progressive section, the end of which forms a coupling strip is placed parallel to and opposite another coupling strip extending in elevation the plate of the capacitor acting as a hot electrode, the two coupling strips acting as coupling capacity.

This particular arrangement makes it possible to obtain a distributed coupling of great homogeneity in the transverse direction.

In the preferred embodiment, the applicator comprises two capacitors, the plates of which acting as hot electrodes are connected by a choke; it is possible to put several applicators of this type in series, and also to increase the number of successive inductors with capacities in the direction of travel of the material. This solution has the advantage of increasing the processing time and distributing in the direction of travel the total available energy, especially if one is limited by the maximum applied voltage.

• La figure 1 est une vue schématique en perspective de l'applicateur selon l'invention traversé par un matériau plan ,et
• La figure 2 est un diagramme représentant le schéma électrique correspondant.
• La figure 3 est une vue schématique en coupe de l'applicateur selon le plan AA′ de la figure 1.

The invention will be better understood on reading the description which will be given of the preferred embodiment of an applicator resonating with two capacities connected by a self distributed over the same length in the transverse direction, illustrated by the appended drawing in which :

• FIG. 1 is a schematic perspective view of the applicator according to the invention traversed by a planar material, and
• Figure 2 is a diagram showing the corresponding electrical diagram.
• FIG. 3 is a schematic sectional view of the applicator according to the plane AA ′ of FIG. 1.

The high-frequency or microwave resonant applicator 1 consists of an upper plate 2 and a lower plate 3 which are parallel and facing each other, between which a flat material 4 moves. This material is for example a fabric, a knitted fabric, a nonwoven, a paper, a plastic film ...

At the inlet and at the outlet of the applicator 1, the material 4 is supported by two rollers 5,6 respectively, positioned so that the material is flat between these two rollers 5,6 and without contact with the two trays 2,3 and parallel to these.

The upper plate 2 is a rectangular metal plate whose width l, that is to say the dimension measured in the transverse direction of movement of the material according to arrow D, is at least equal and preferably greater than the width of the material 4.

The internal face of the upper plate 2, turned towards the flat material 4, has longitudinal grooves, illustrated in FIG. 1 by dotted lines 7.

These grooves 7 are such that they delimit the elementary sections 19 of small dimension, of the order of a centimeter in width, independent of each other. The spacing between two sections 19, corresponding to the width of a groove 7, is small, of the order of one to a few millimeters. It is understood that the maintenance in position of the various elementary sections 19 may result from an assembly using an insulating material.

The lower plate 3 of the same overall dimension as the upper plate 2 is composed of two rectangular portions 8, 9 connected between two by a metal strip 10 curved in an arc of a circle towards the outside of the plate 3.

The internal face of the two portions 8, 9 of the lower plate 3 has longitudinal grooves, illustrated in FIG. 1 by dotted lines 7 ′, and arranged opposite the grooves 7 made in the upper plate 2. The grooves 7 ′ delimit elementary sections 20.

Crossbars 18 are fixed, by screwing, to the internal faces, facing the flat material 4, of the upper plate 2 and of the lower portions 8, 9 either facing one another, or in staggered rows as shown in Figure 1.

The transverse bars 18 are also grooved, in the same way as the plates 2,3, so as to delimit elementary sections 22, which are separated from each other by blades 23 of an insulating material.

The front transverse edge of the upper plate 2 is extended by a first metal strip 11 whose width gradually decreases to a dimension allowing connection to a coaxial flange 12 connected to a high frequency generator or microwave not shown. It is the mass of the coaxial flange 12 which is connected to the first metal strip 11.

The central electrode 13 of the coaxial flange 12 is in contact with an upper coupling strip 14 by through a second metal strip 15. The upper coupling strip 14 is a rectangular metal plate placed in front of the upper plate 2, of short length and width equivalent to that of the upper plate 2. It is flush with the flat material 4, parallel to it. The second metal strip 15 connects the central electrode 13 to the upper coupling strip 14. Its width gradually decreases from the upper coupling strip 14 which it extends to the central electrode 13.

The lower front portion 8 is extended by a lower coupling strip 16 which is a metal plate raised relative to the internal face of said portion 8. This lower coupling strip 16 faces the upper coupling strip 14, being separated of the latter by a spacing sufficient to allow the passage of the plane material 4.

Thus the two lower portions 8, 9 are the hot electrodes of two capacitors, the ground electrodes of which are constituted by the upper plate 2; the metal strip in an arc 10 constitutes the inductor of the oscillating circuit.

The internal face of the metal strip arc of a circle 10 has longitudinal grooves 7˝, arranged opposite the grooves 7 made in the upper plate 2.

The hot electrodes are supplied with energy by coupling between the upper coupling strip 14 supplied by the generator and the lower coupling strip 16, the two upper and lower strips 14, 15 forming a coupling capacity.

The applicator 1 has two systems for adjusting the resonant frequency. The first is a system for deformation of the metal strip in an arc 10 forming the choke; it comprises a metal plate 17 bearing on an external generator of the circular arc, and sliding means, for example sets of threaded rods and nuts making it possible to vertically move said plate 17 by a determined and adjustable height . Such displacement results in correlatively a deformation of the arc of circle and consequently a variation of the cross section of the inductor. It is understood that this variation of the surface of said section of the inductor varies the resonance frequency.

The second system for adjusting the resonant frequency of the applicator is a system for adjusting the spacing of the upper plate 2 relative to the two fixed lower portions 8, 9. This is for example a set of jacks fixed to a frame and whose rods are integral with the upper plate 2, having several possible adjustment positions.

On either side of the applicator 1, facing the open ends of the strip 10, are placed two vertical metal plates, not shown in FIG. 1 for the sake of clarity; these plates have dimensions such that they extend well beyond the choke; they are connected to the upper plate 2. These plates have the role of stopping the magnetic field created by the self.

The operation of the applicator is explained by the equivalent electrical diagram shown in Figure 2, in which we find the actual applicator consisting of the self L corresponding to the metal strip in a circular arc 10, and the two capacities C capacités and C₂, corresponding to the two portions 8, 9 and to the upper plate 2, connected between the ends of the inductor and the ground.

The applicator is connected to the generator G by the coupling capacity C₃ corresponding to the two upper and lower coupling strips 14, 16. The resistors R₁ and R₂ represent the transformation of the material during the treatment, for example the heating of the material corresponds to the power dissipated in each of the resistors R₁ and R₂. The power dissipated in the resistor r, in series of the inductor L, represents the power dissipated in the applicator and the losses thereof.

It is easy for a person skilled in the art to determine the value of each of the equivalent elements of the circuit, from measurements made with an impedance meter and a network analyzer.

For example, to determine the value of the coupling capacitance C₃, it suffices to measure the impedance at the input of the applicator when the first capacitor is short-circuited by a metal plate which connects the two electrodes of the capacitor.

For example, to determine the value of the capacities C₁ and C₂, it suffices to short-circuit the choke by connecting the two sections 8, 9 of the lower plate 3.

For example, to determine the value of the self L, it is obtained by measuring the resonant frequency. For example, to determine the value of the resistance of the inductor, it is obtained by determining the value of the overvoltage factor of the oscillating circuit in the absence of planar material 4.

It is necessary to carry out such measurements in the event of modifications to the setting of the applicator: spacing of the plates, number of transverse bars, deformation of the inductor.

From the values thus measured, and from the equivalent electrical diagram, it is possible to calculate by simple application of Ohm's law the average electric voltage which appears between the electrodes of the capacitors when the generator is connected, the latter delivering a known electrical voltage. In addition, the equivalent diagram makes it possible to carry out the power balance and thus calculate the power transferred to the product, the power consumed in the applicator and the total reflected power by the applicator.

For the proper functioning of the applicator, it is good to know how the planar material 4 to be treated behaves during treatment, and in particular how its dielectric constant will change as a function of temperature. Indeed the adjustment of the resonance frequency will be determined taking into account this information so as to obtain the desired self-regulating effect. The resonant frequency will be chosen to be slightly higher The resonant frequency will be chosen either slightly higher than the generator frequency or lower than this.

All of the elementary sections 19, 20, 21, delimited by the vertical planes 24, 25 passing through two successive grooves, respectively in the upper plate 2, in the lower plate 3, in the transverse bars 18 and in the metal strip in circular arc 10 forms a basic, independent applicator.

Suppose that a variation in the characteristics of the material occurs locally in an elementary applicator If the resonance frequency has been chosen slightly higher than the generator frequency and this variation tends to decrease the value of the capacity of the oscillating circuit, this will lead to this elementary applicator an increase in the resonance frequency of the oscillating circuit and a decrease in the voltage applied to the terminals of the capacitor. The power supplied to the material in this elementary applicator will decrease.

On the contrary if the resonant frequency of the oscillating circuit has been chosen lower than the generator frequency, the same fluctuation tends to reduce the difference between the two frequencies and leads to supplying the capacitor with a higher electric voltage; that is to say to increase the power supplied to the material in this elementary applicator.

Each elementary applicator having an independent functioning, the regulation effect is local and does not modify the treatment of the material located in the neighboring elementary applicators.

It is up to the person skilled in the art, depending on the physical and dielectric characteristics of the planar material to be treated, to determine the operating conditions of the applicator 1 according to the invention: choice of the resonant frequency relative to the frequency of the generator, adjusting the section of the choke and / or the spacing between the upper plate 2 and the lower sections 8.9 to obtain the adequate resonant frequency, presence or absence of transverse bars 18 allowing the electric field to be concentrated on the planar material 4, in particular when the latter is not very thick.

It is understood that due to the self-regulating effect of the applicator according to the invention, it is always possible to subsequently adjust the value of the capacitor C3 in order to adjust the overall impedance of the applicator and make it equal to that of generator G. This makes it unnecessary to use an adapter box, as is generally provided between the generator and the conventional applicators.

The invention is not limited to the preferred embodiment which has been described by way of non-exhaustive example, but covers all variants thereof. In particular, the applicator may include several inductors, for example the lower plate will then be composed of three lower portions as described above, separated and linked together by two metal bands in an arc forming a circle acting as inductors. This version increases the processing time and distributes the total available energy in the direction of travel of the planar material; it is interesting when one is limited by the maximum applied voltage.

Furthermore, the arc shape of the inductor is not limiting; it can for example have a substantially square shape. In this case the variation of the internal section of the choke will be obtained by displacement of its mobile base along its fixed lateral sides.

Finally preferably the applicator according to the invention will be installed inside a box electrically connected to the ground of the generator and acting as electromagnetic shielding.

It should be noted that thanks to the progressive section of the metal bars (11,15), all the elementary applicators are supplied with current under the same conditions. This homogeneous distribution of the current in the transverse direction further improves the self-regulation performance.

Claims (9)

1. Resonant high-frequency or micro-wave applicator for the treatment of a flat material (4) comprising a flat capacitor between the plates of which said material moves and an inductor, constituting with the capacitance of the capacitor an oscillating circuit, characterized in that the inductor is constituted by a metal band (10) transversely extending the plate of the capacitor acting as hot electrode (8 or 9), and in that the inner face of the plate (2) of the capacitor, acting as ground electrode, is divided into elementary sections (19) by an assembly of longitudinal grooves (7) in the direction of displacement of the material.
2. Applicator according to claim 1, characterized in that the elementary sections (19) have a width which is 5 centimetres maximum and preferably around or less than 1 centimetre.
3. Applicator according to one of claims 1 or 2, characterized in that the inner faces of the plate (3) of the capacitor, acting as hot electrode, and of the metal band (10) acting as inductor are divided into elementary sections (20, 21) by an assembly of grooves (7′, 7˝) disposed opposite the grooves (7) made in the plate (2) of the capacitor acting as ground electrode.
4. Applicator according to claim 1, characterized in that it comprises two flat capacitors, of which the plates (8, 9) acting as hot electrodes are in the same plane and connected by the ends of an arcuate metal band (10), constituting the inductor.
5. Applicator according to claim 4, characterized in that it comprises means for deforming the metal band, which means are adapted to vary the inner section of the inductor.
6. Applicator according to claim 5, characterized in that the deformation means consist in a metal plate (17) which is in contact with the outer face of the metal band (10) and which is equipped with a system of slide, adapted to exert a stress on the band such that the arc of circle is deformed uniformly over the whole length of the inductor.
7. Applicator according to claim 1, characterized in that it comprises bars (18) mounted transversely on the inner faces of the plates (2, 3) of the capacitor, adapted to vary the intensity of the electro-magnetic fields and to concentrate it on the flat material (4), and divided into elementary sections (22) by an assembly of grooves disposed opposite the grooves (7) made in the plate (2) of the capacitor acting as ground electrode.
8. Applicator according to claim 1, characterized in that it comprises, for supplying the applicator (1), a generator connected via a coaxial flange, on the one hand, to a first metal band (11) of progressive section extending the plate of the capacitor (2) acting as ground electrode and, on the other hand, to a second metal band (15) of progressive section of which the end forming coupling band (14) is placed parallel to and opposite another coupling band (16) extending in superelevation the plate (8) of the capacitor acting as hot electrode, the two coupling bands (14, 16) acting as coupling capacitance.
9. Applicator according to claim 1, characterized in that it comprises a plurality of grooved inductors alternating with capacitor plates in the direction of advance of the material.

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