DK149245B - HF RESONATOR WITH ADJUSTABLE RESONANCE FREQUENCY - Google Patents

Description

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149245

The invention relates to an adjustable resonant frequency HF resonator for sealing or welding thin, especially thermoplastic materials such as packaging foils, and in which a resonator cavity contains ferromagnetic bodies that determine the resonant frequency and whose magnetic permeability can be altered by the magnetic field. for the resonator located electrical coil.

HF resonators with adjustable resonant frequency 0 0 are already known, for example from DE-AS 11 86 119 and DE-PS 937 475.

A resonator circuit always includes an inductance and a capacitance, which inductance resonates at a given characteristic value for the current value of the inductor and capacitance, which means, for example, a parallel resonant circuit, that the impedance of the circuit has a maximum at the resonant frequency while the impedance in a series resonant circuit has a minimum at this resonant frequency.

Cavity and coaxial line resonators also have an inductance and a capacitance, the size of which depends on the dimensions of the resonator. Such a resonator can be tuned to a certain frequency by changing the resonator's volume or its height. However, it is mechanically complicated to design a resonator which can be adjusted in such a way that its volume or height can be changed to adjust the resonant frequency.

30 It is known to use resonators of the above-mentioned type in connection with apparatus for sealing thin packaging material in packaging machines, and SE-PS 396 033 discloses a sealing device incorporating a resonator. It is known that by using resonators, a highly concentrated heat release in insulating materials can be obtained by providing dielectric losses in these materials when the material is exposed to a high frequency electric field.

In the known resonator devices, and in particular the resonators included in sealing devices where the packaging laminate is sealed by heat provided in the laminate by the resonator, it has heretofore been a disadvantage that the characteristic properties of the packaging laminate, e.g. . material thickness, humidity, plastic quality, etc. vary, which affects the resonant frequency of the sealing apparatus, which therefore does not remain constant despite the fact that the geometric parameters of the resonator do not change. Since the generator supplying the resonator usually operates at a fixed frequency for the feed current, this means that the resonator is output at resonance if the resonator frequency changes, which in turn implies that the energy provided in the material used for sealing does not. 20 becomes a maximum. If the resonator frequency changes too much, the energy may become insufficient to produce an acceptable seal.

Another aspect is that changing the resonant frequency of the resonator e.g. due to the use of a greater material thickness packaging material can also affect the frequency of the generator, and since the generator frequency, which is generally in the range of 400-500 MHz, e.g. 433.43 MHz, according to the applicable regulations must be kept within ± 0.2%, it is important to ensure that the resonator of the sealing device does not affect the generator frequency in such a way that this frequency will be outside the frequency range. permissible tolerances.

Thus, there is a need to be able to adjust the feed rate of the generator or resonator frequency in such a way that the resonator always operates at its resonant frequency, and since the generator generally operates at a preset frequency, it is desirable to be able to tune the resonator 5 in such a way that it always operates at its resonant frequency.

The present invention provides a solution to this problem, and an HF resonator according to the invention is characterized in that the resonator is divided into two cavities by means of a middle wall, that in these cavities and in the region of them which are located. perpendicular to the end wall of the resonator, ferromagnetic bodies are inserted and a series of electrical coils extend over the entire length of the resonator.

In such a resonator, the magnetic fields which serve to control the resonant frequency not only concentrate in the ferromagnetic bodies, which is known per se, but are also caused to exert their main effect in that part of the resonator. which is furthest away from the actual sealing area, ie. the "open" end of the cavity.

Indeed, some separation occurs between the optimum magnetic field and the optimum electric field and the division into two chambers or cavities favors this effect. In addition, the foil material no longer needs to be welded over a wide area, and the welding can actually be limited to two virtually parallel, narrow zones. This conveniently affects the duration of the welding operation, contributes to increased heating and protects the foil material.

The fact that the coils and ferromagnetic bodies increase the inductance also causes the volume of the resonator to be reduced, which can be an advantage when the resonator is to be built into a welding apparatus, where welding space is limited.

4 149245

The invention is explained in more detail below with reference to the schematic drawing, in which fig. 1 shows a section through a known plastic laminate resonator; FIG. 2 is a section through a resonator equipped with an inlay body of ferromagnetic material; FIG. 3 shows the synthetic curve of a ferromagnetic material; and FIG. 4-9 three different embodiments of a resonator equipped with magnet coils for pre-magnetizing the ferromagnetic material embedded in the resonator.

As previously mentioned, at a resonator of the embodiment shown in FIG. 1, it is found that it is difficult to precisely match the resonant frequency of the resonator to the frequency of the food generator, and since the material used for sealing has a certain influence on the resonant frequency, such a resonator does not always work with maximum energy yield, 20 ie. at its resonant frequency, which can cause the energy release to become too weak and the sealing result unsatisfactory. In addition, in FIG. 1 shows the disadvantage that the resonator housing has a relatively high height, which in the illustrated embodiment is denoted h. The present invention may apply to different types of resonators, but for the sake of clarity, the following description concentrates on the so-called coaxial kidney resonator, which is the type of resonator most commonly used for sealing in packaging machines.

As shown in FIG. 1, the resonator 1 consists of two parallel metal side walls 4 connected to each other by means of a metal wall 5, on the central part of which is placed an intermediate metal plate 3. The spaces 2 between the side walls 4 and the central plate 3 form the resonator cavity, and the design of this cavity and the height and spacing of the side walls and the central plate are critical to the resonant frequency of the resonator, which should be adjusted to coincide with the frequency of operation of the food generator. The current and energy of the high frequency generator not shown here can be transmitted to the resonator by capacitive or inductive coupling or as shown in the drawing in that the center conductor 8 of a coaxial cable 6 connected to the generator is directly connected to the central plate 3 of the resonator 1, while the outer shield 7 of the coaxial cable is connected to one of the sidewalls 4. As mentioned in the preamble, a resonator can be considered a parallel resonant circuit, and at the resonant frequency, the inductor of the resonator and thus the magnetic field can be considered to be concentrated in the region 11 near the wall of the resonator. 5, while the capacitance of the resonator and its maximum electric field 20 can be considered to be concentrated at the open part of the resonator 12. When the packaging material 9 consisting of a laminate comprising a material with a suitable loss factor is placed at the open part of the resonator 1 and preferably pressed against it. part by means of an insulating counterplate 10, heat is generated in one or more of the laminate layer of the packaging material when portions of the packaging material are exposed to a highly concentrated, high frequency electric field which arises between the lower end 13 of the sidewalls 4 and the lower part 14 of the center plate 3. This strong and concentrated electric field produces such a large dielectric losses in one or more of the laminate material 9 layers, that the thermoplastic layers contained in the laminate material can be melted and fused together to form concentrated sealing zones U9245 6 16, where the layers of the laminate material are firmly connected to one another in a mechanical solid. joining.

In order to provide a sufficiently large heat release in the laminate material in a short time 5, a high frequency electric field is required. Despite this, the resonator height h in many cases becomes too large, which is a disadvantage ia. by incorporating the device into automatic packaging machines.

10 The high altitude of the resonator of FIG. 1 can be substantially reduced if the resonator of FIG. 2 is provided with an inlay body 15 of ferromagnetic material, preferably ferrite. Such a body 15 of ferromagnetic material can increase the inductance of the resonator and if the body 15 is placed at the location where the magnetic field, i.e. The H field is greatest, the contribution of the ferromagnetic body to the inductance changes the resonant frequency of the resonator in such a way that the height h of the resonator must be reduced so that the resonant frequency of the resonator can again correspond to the frequency of the food generator.

Thus, the advantage is obtained that the height h of the resonator can be limited, which, as previously mentioned, is an advantage with regard to the machine construction.

A further advantage, as previously mentioned, which constitutes the real object of the present invention is that by altering the pre-magnetization in the ferromagnetic body 15 it is possible to change the inductance and thus also change the resonant frequency of the resonator in such a way that it always is adapted to the frequency of the food generator regardless of whether the characteristics or dimensions of the materials used for sealing affect the resonator in such a way that its resonant frequency tends to change. The shaping of the body 15 may e.g. is done by means of a coil introduced into the body which is fed from an adjustable direct current source.

In order to pre-magnetize the ferromagnetic body 15, the resonator should consist of non-magnetic material such as aluminum, brass or stainless steel, and in order to further reduce the dimensions of the resonator, it may be appropriate in some cases to fill the cavities 2 with e.g. oil, in which case the open part of the resonator must be sealed with a material which has such a low loss factor that the electric field occurring at the open part of the resonator does not generate heat in this material. A further advantage of using an oil-filled resonator is that condensation or vapor formation within the resonator is avoided, which is an advantage, since condensor vapor formation on the inner surfaces of the resonator alters the electrical properties of the resonator.

As mentioned above, it may be advantageous to use ferrite for the body 15, which may consist of rods 20 of rectangular or circular cross-section, which rods may again be composed of shorter pieces. To hold the ferrite body 15 in place in the resonator, it may conveniently be secured by a suitable binder. The ferrite materials have the property that their permeability or μ value is a function of the magnetic induction B and the magnetic field strength H. The permeability is different for different materials and Figs. 3 shows a characteristic hysteresis curve indicating the relationship between the magnetic induction B deposited on the ordinate axis and the magnetic field strength H deposited on the abscissa axis.

By the device described herein, the ferromagnetic body or bodies are magnetized either by a direct current component emanating from said coils or electromagnets fed from an adjustable direct current source and partly by an alternating current component emanating from the high frequency magnet. By changing the DC component, ie. the pre-magnetization by means of coils embedded in the ferrite or by external electromagnets, different working points I, II, III can be selected on the magnetization curve, cf. 3. At each work point there is a so-called side loop 26 which graphically represents the AC magnetization superimposed on the DC magnetization and the slope of these side loops 26 is a measure of the permeability which is µ = tg ti.

At point I, the permeability is μχ, which is greater than the permeability Mjj * which in turn is greater than the permeability Pjjj ·

Thus, since the inductance is increased by the magnetic induction in a circuit, by changing the magnetic induction B of the inlaid ferrite rods 20, the inductance addition of the resonator equipped with ferrite rods can also be changed. This can be done as shown by pre-magnetizing the ferrite rods by means of coils or DC-fed electromagnets placed relative to the resonator and ferrite rods as shown in FIG. 4-9. By pre-magnetizing the ferrite rods in the above manner, the permeability of the material may be altered. In other words, the permeability of the material can be altered by means of the pre-magnetization chosen in such a way that an appropriate working point is obtained on the magnetization curve. Thus, with an external influence from DC-fed electromagnets, it is possible to change the magnetic induction in the inserted ferrite rods in such a way that the inductance changes, and it is therefore possible in each case to adjust the inductance value of the resonator to a such that its resonant frequency matches the frequency of the food generator, although other external factors, e.g. the laminate material determined for sealing at the open portion of the resonator affects and changes the resonant frequency of the resonator.

As previously mentioned, FIG. 4-9 three practical embodiments of a tunable resonator according to the invention, and for the sake of clarity 10, FIG. 4-9 used the same reference numerals as in FIG. 1 and 2. The embodiment of FIG. 4, like the other embodiments, is provided with inlaid rods 15 of ferrite material in the interior of the resonator at the point where the magnetic field strength is maximal. On the outer sides of the resonator, as shown in FIG. 4, electromagnets 17 are arranged as horseshoes, which electromagnets have magnetizing windings 18. FIG. 5, which is a section along section line A-A of FIG. 4, shows how the magnets 20 are arranged along the sides of the resonator, and the magnet coils 18 can be connected in series or in parallel, but must at least be connected to a direct current source which, through a resonant frequency regulator, can tune the inductor of the resonator to such way that the resonant frequency of the resonator always matches the frequency of the food generator.

FIG. 6, which is a section along section line B-B of FIG. 7 shows another embodiment of the resonator, wherein the inserted ferrite rods 15 have a circular cross section and the electromagnet is located centrally on the resonator wall 5 by means of a plurality of pins 21 of magnetic material which are fixed to and preferably drilled. into the wall 5.

Said leg 21 carries a common yoke 19 on which 14 9 2 A 5 10 is arranged a plurality of magnet coils 20 which can be connected through a regulator to a DC power source.

The FIG. 8 can almost be characterized as a U-shaped magnetic body which is so long that its leg 22 encloses the resonator 1, which is furthermore equipped with inlaid ferrite rods 15. The yoke 23 of the electromagnet carries a number of magnetizing coils 24 which, in the same way as in the foregoing cases, they may be connected in series or in parallel 10 and are connected to a regulator which serves to regulate the current through the magnet coils in such a way that the ferrite rods 15 inserted in the resonator can be pre-magnetized to control the inductance of the resonator and thus adaptation 15 of its resonant frequency to the frequency of the food generator.

It has been found in experiments that the resonator according to the invention functions satisfactorily and that it is possible to adjust with great accuracy the frequency of the resonator in such a way as to obtain a maximum energy yield and thus a satisfactory sealing result.

In the embodiments described herein, only such resonators have been treated as used in 25. connection with sealing devices by packaging machines and only coaxial resonator type resonators. However, the idea of the invention may apply to other types of resonators, e.g. cavity resonators, since the location of the ferrite elements 30 can be adapted to the physical properties of the resonator and localized to those parts of the resonator where the magnetic field strength and hence the inductance are maximal. The invention may also be used in conjunction with resonators which are not used for sealing the packaging material or for generating heat in thin laminate layers, but for other purposes. That idea,

Claims (4)

Thus, the 1492A5 underlying the invention is not limited to the above embodiments and can be adapted to all types of resonators for all possible applications where it is desired to continuously tune and adapt a resonator frequency to the frequency of the food generator. An HF resonator (1) with adjustable resonant frequency for sealing or welding thin, especially thermoplastic materials (9), such as packaging foils, and in which a resonator cavity contains ferromagnetic bodies co-determining the resonant frequency permeability can be changed by the magnetic field from an electric coil located outside the resonator, characterized in that the resonator is divided into two cavities (2) by means of a middle wall (3), that in the regions (11) thereof cavities located at the end wall (5) of the resonator (1) are provided with ferromagnetic bodies (15) and a series of electrical coils (18, 20, 24) extend over the entire length of the resonator (1). Resonator according to claim 1, characterized in that the external electrical coils (18, 25 20, 24) in the series are connected under DC voltage with adjustable current or voltage. Resonator according to claim 2, characterized in that a regulator is provided for supplying the external coils (18, 20, 24) in said row of voltage or current in such a way that the resonator (1) is always in operation. is in resonance. Resonator according to any one of the preceding claims, characterized in that in connection with the electrical coils (18, 20, 24) a