FR2503520A1 - High frequency induction system with self tuning resonant balance - to maintain heat input to successive welds or adhesive joints with varying dielectric properties - Google Patents

Abstract

The heat input to items (1) undergoing a high frequency field from a mutual inductance circuit is kept at maximum efficiency by feedback control of a rotary electromechanical device in the applicator circuit to keep the resonant frequency of the applicator circuit in tune with that of the primary circuit. Suitable for systems for heating a series of items which may differ in their dielectric properties due to differences in dimensions and/or temperature. Esp. applicable to processes for e.g. welding a series of thermoplastic joints or heating adhesives in situ, e.g. between saw tooth joint profile surfaces for successive pieces of timber. Ensures that successive targets absorb equivalent amounts of heat from similar induction heating cycles.

Description

 Process aimed at keeping constant the thermal energy dissipated in a product placed in an applicator supplied by a high frequency field, implementation devices, application to certain continuous manufacturing: wood butting, bonding.

 The present invention relates to the sector of industrial applications of HF. It takes advantage of the fact that HF makes it possible to supply calories by dielectric losses within materials to accelerate certain industrial processes such as the welding of plastics, the polymerization of adhesives, etc. . The invention is all the more interesting as it relates to materials with low dielectric losses.

Currently when you want to treat a material with HF. it is placed in an applicator powered by an HF generator. Depending on the geometric shape of the material, the applicator is of the flat capacitor type, in a daisy chain, etc. To connect the applicator
C to the HF generator (of the quartz piloted or self-oscillating type), either direct coupling is used: FIG. 1, or indirect coupling by mutual induction Figure 2. Currently, taking into account the frequency stability standards which the generator must meet, the Figure 1 assembly is practically abandoned. (Assembly 1 is as simple as possible,
The sample is placed directly in the oscillating circuit capacitor. It is certain that such an assembly will work the first time, without problem. Unfortunately, if the dielectric properties of the material come to vary the frequency of the oscillator will be modified. This is inevitable. Even if the material always had the same texture, the same degree of humidity, the frequency would still be modified if only because of the polymerization of the glue, The law imposing strict standards of frequency stability (for the frequency band centered on 27.12 MHZ a drift of - 0.6% is tolerated) it results that this assembly is rejected). To achieve an optimal transfer of HF energy from the generator to the material, it is necessary that the resonant frequency f of the applicator circuit is equal to that f of the generator and that
o the input impedance of the applicator seen from points A and B, ie the complex combined with that of output seen from points A and B of the generator, Figure 2. To achieve these two conditions, we play on the number of turns of the output transformer and place as shown in Figure 3 capacitors C1 and C2 in series or in parallel with the applicator. These capacitors are sometimes replaced by inductors. By successive trial and error, the energy transfer from the HF generator is best adapted. Indeed, when the applicator is not at resonance its impedance is very high and the consumption of the oscillator or amplifier tube is very low. , that of the agreement, it is maximum. We therefore have at their disposal an indication enabling this agreement to be checked. Currently, HF industrial application technology faces two challenges. The first results from the fact that during treatment the material subjected to waves heats up which is of course the desired goal. Unfortunately this heating leads to a variation of its electrical properties in particular of its dielectric constants # 'r and # "r. This results in a disagreement of the applicator. On the other handXfor treatment at the parade, when passing from one sample to another, if the next one is not strictly the same as the previous one, the agreement of the applicator is again lost. To these two troubles it is necessary to add the instabilities in frequency of the generator itself. instabilities are caused by fluctuations in the supply voltage, thermal expansions and especially the reaction of the applicator circuit on the oscillating circuit of the self-oscillator.This difficulty could be eliminated by using an HF generator controlled by quartz. In any case, the user of an HF machine is then led to touch up manual settings, which is disastrous for an automatic machine. To overcome these pitfalls, some have recommended devices using a servo capacitor (placed as
C2 of Figure 3), capacitor driven by a motor so that the current delivered by the tube of the HF generator remains constant. This path is difficult and has not led to industrial applications.

 The method and the devices according to the invention make it possible to avoid these drawbacks. In effect even if the properties of the sample due to its heating come to vary during processing, or if one passes during continuous manufacturing from one sample to another of slightly different dimensions or electrical properties the device which is the subject of the invention makes it possible to always dissipate the same power in the sample. The invention therefore also makes it possible, for the first time, to envisage continuous production lines, in certain applications.

These applications are all the applications where it is desired to heat materials by high frequency, such as for the welding of plastics (placing in a carton of a liquid, manufacture of certain synthetic fabrics, production of PVC leather goods), gluing or butt-jointing of wood pieces by glue polymerization or any other energy application of HF

Other characteristics and advantages of the invention will be better understood on reading the description which will follow and with reference to the appended drawing in which - FIG. 1 represents a known assembly with coupling
direct, - Figure 2 shows a known assembly with coupling
indirect by mutual induction, - Figure 3 shows a variant of a known arrangement, - Figure 4 shows the block diagram of a
assembly for implementing the method according to
the invention, - Figure 5, which consists of Figures 5a, b, c and d,
represents an embodiment of the set C of
v
Figure 4 (5b) and a top view of the part
active of this set (5a), the representative symbol
of this set (5c), and the variation of the capacity
C of this set C as a function of time (5d).

v - Figure 6, which consists of graphs 6a, b and c,
represents in the context of the invention the tension
across the applicator (-), the tube current
transmitter (6c), and the recall of the graph represented on
Figure 5d, as a function of time - Figure 7, which consists of Figures 7a and b
represents the voltage across the applicator,
respectively according to the state of the art and according to the invention - Figure 8 shows an improved variant of
assembly for implementing the method according to
the invention; - Figure 9 shows the scanning system used
according to the invention corresponding to example 1 of execution
given.

- Figure 10 shows an example of mounting allowing
wood butting - Figure 11 shows a production line in
continuous roof panels comprising machine
butt joint by HF shown in Figure 10.

In these drawings, the same references have the same meanings, which are as follows 1. material to be treated (for example, butting area
rafters).

r cathode resistance.

C applicator.

C modulation element according to the invention.

 v 2 movable blade.

3 fixed blades.

M 4-conductor motor.

5 insulator.

G designates, for the purpose of simplification, the generator
HF comprising element C according to the invention and
v
feeding the applicator C from the side where the latter
is not earthed (see Figures 4 and 8).

8. Mobile carriage on the rails 13, carrying the
generators G and applicators C.

10. reset wood.

11. wood blocking cylinders.

12. advancement cylinder.

13. guide rails.

14. Continuation of the production line.

(a) and (b): series of chevrons processed in parallel,
in the nonlimiting example of
string considered, "to give directly in
end of chain) s roof panels targeted.

15. complementary cutting of two rafters to be
butt-jointed, obtained with a tufting machine, after gluing
and nesting (see Figure 11).

 The electrical symbols are used in the accompanying drawing in their usual sense.

The method which is the subject of the invention is characterized in that it comprises a frequency sweeping system making it possible to modulate the series resonance frequency f of the applicator on either side of that f of the
o generator. Figure 4 gives the block diagram of the assembly carried out. In this figure, in parallel, on the applicator C, which can be of any type (capacitor, plane, stray field, etc.), there is a set C which is part of the invention and can be
v in a first version consisting of a butterfly capacitor whose movable blades are driven in a rotational movement by a motor. Figures 5a, 5b, 5c, 5d respectively give the block diagram of this capacitor, an embodiment, its representative symbol and the variation of its capacity C as a function of time. If it is arranged so that the resonance of the secondary circuit containing the applicator occurs af (generator frequency) when the
o movable blade is half engaged between the fixed blades, it is understood that the voltage across the terminals of the applicator will change as indicated in graph 6b.

The graph 6a identical to 5d is only there to show the temporal link existing between the evolution of the value of the capacity C
v and the voltage V across the terminals of the applicator as well as the current of the transmitter tube. This current in FIG. 6c has the same shape, except for the continuous background near Io, which remains even if the secondary circuit containing the applicator is not tuned. 10 varies in opposite direction to the overvoltage coefficient of the oscillating power circuit of the HFO generator. produced for slightly higher or lower frequencies f. This variation in capacity will therefore have no effect
Cmax. + Cmin.

at least as long as it stays below
2
To set the resonant frequency of the applicator circuit in the vicinity of f0 it is necessary to provide for this purpose a calibration capacitor Ct placed in parallel on the applicator. The device which is the subject of the invention therefore makes it possible to overcome fluctuations in the resonance frequency of the applicator circuit. In the classic case, in FIG. 7a1, a voltage is obtained at the terminals of the applicator, the amplitude of which depends on the agreement of the applicator. If we use our device, we obtain voltage pulses in figure 7b. The average power dissipated in the sample therefore goes, if f varies, from something very variable for a conventional installation to something practically constant when using our invention which is well îà the desired goal. It should be noted that for a conventional HF generator without scanning there is no dead time, the applicator being supplied with permanent power. The presence of the sweep leads to a pulse supply giving a lower utilization factor. This fact could prove to be a drawback for the invention of the present application in practice, it does not lead to any loss of efficiency because when the circuit of the applicator is not tuned the HF generator consumes negligible power.

To compensate for this lower utilization factor, it suffices to increase the instantaneous power of the generator.
HF It can be done without damage by keeping the same tube as that of a conventional installation of the same power since seuX counts for the dimensioning of this component the average power delivered. A kind of generator with controlled carrier is then produced. It is possible to regulate around a set point or even to change, as a function of time, according to a predetermined program, the power dissipated in the product placed in the applicator. Indeed, the current of the generator power tube (therefore the power
HF dissipated in the product placed in the applicator) changes in the same way as the tension of the applicator.

 It is therefore possible by comparing the average value of the voltage appearing across the resistance placed in the cathode circuit of the emitting tube, FIG. 8, with a reference voltage to obtain a positive or negative error signal. By using a controlled supply for the H.F. generator (by thyristors or by saturable inductance) it is possible, thanks to the error signal previously developed, to control the supply of the H.F. generator and therefore to regulate the power dissipated in the sample. If the reference voltage changes according to a certain law, the power dissipated in the sample follows the same law. This possibility can be taken advantage of for the welding of plastic material, thus making it possible to use a lower power at the start of welding when the sample is cold, thereby reducing the risks of breakdown. Note that the applicator always works in impulse fashion. , the risk of breakdown is reduced even if the instantaneous power is slightly increased. It is the phenomenon which makes that with equal current a switch supports an alternating current much better than a direct current. Note that in the case of a controlled H.F. generator, the power control will not be carried out on the power stage but on the "exciter". The power to be controlled is then much lower, which reduces the cost of the control.

Note that in the latter case the power stage of the H.F. generator must necessarily operate as a linear amplifier. The description of the capacitor scanning system has been given above.

In practice, we will use a professional butterfly capacitor. The moving blades will be driven at constant speed by a synchronous motor so as to describe for example 50 times per second the resonance curve of the applicator circuit. A variant of the system can be made using a variable choke. I1 suffices for this to use a kind of U in coarse wire in front of which a disk is made up comprising insulating sectors (phenolic resins or epoxy) and conductive sectors in copper, figure 9. The passage of a copper strip in parallel in the plane of the pin modifies in large proportions the self view of points A and B of the pin. In this case, the scanning device is not placed in parallel on the applicator but in series.

 The following examples illustrate the invention without, however, limiting its scope.

EXAMPLE 1: wood butting
Joining wood consists of making complementary cuts at the end of the rafters, by passing through a tufting machine, gluing these cuts and nesting the rafters to obtain a sufficient length of wood.

 This technique, as well as its imperatives and the drawbacks of the known methods, are described in detail in French patent application No. 81/06670 filed by the Applicant on the same day as the present application, and to which reference may be made.

 / The adhesives which can be used in the process according to the invention for butt jointing by H.F. are all the adhesives which react at high frequencies. These adhesives are known. However, use will preferably be made, among these adhesives, of resorcinol, which is commonly used in the butting technique, made sensitive to H.F. by the addition of 5 to 20% of phenol7. The adhesive "PLACURITE 76-476" was used here (resorcinol + 15% phenol).

 The butted rafters have a thickness (e) of 85 mm.

 The applicator C consists of two aluminum plates (16) with a side of 90 mm (this dimension is largely sufficient to ensure a homogeneous distribution of the high frequency power density at the level of the butting zone (15) / see figure 197.

We used the assembly of Figure 4, in which the element C (mounted in series and not in parallel)
v was as shown in Figure 9. Care must be taken to center the butting area between the plates (maximum tolerance + 1 cl).

 An application of the HF energy for 30 s makes it possible to obtain a completely sufficient bonding of the rafters, corresponding to the requirements of the professionals in the matter (resistance to bending at least equal to that of the rafters i in traction, the wood breaks before joining).

 HF standards. were met without difficulty.

EXAMPLE 2: Continuous finger joint -
The results of Example 1 show that continuous butting by HF is possible.

 The chain shown in Figure II was used.

 This chain is identical to that described in the aforementioned French patent application filed the same day by the Applicant. except that the microwave device has been replaced by an H.F.

according to the present invention.

 For the general operation of the chain, reference will therefore be made to said request.

 The essential characteristic of this chain is that the chevron trains move at, for example, 3 m / min., While the applicators C accompany this movement so that the jointing zone remains substantially facing the plates 16.

 In order to respect the time required for the application of the H.F. energy determined in Example 1, it can be seen that it is sufficient for the applicator to accompany the butting zone over approximately 1.5 m.

 The finger-jointed timber obtained meets the two criteria used in the profession to assess the quality of butting, that is to say that 1) the% tearing of the butting is very close to 100% at the end of the chain and 2) the resistance of the wood to bending is at least equal to the resistance before finger jointing.

EXAMPLE 3
In Example 2, the modulation element according to FIG. 9 has been replaced by that according to FIGS. 5 (a) and Çb), mounted in parallel.

 The same results were obtained.

Claims (24)

1. Process for the thermal treatment of products using an applicator supplied by a high frequency field, in an assembly with indirect coupling by mutual induction, applicator in which the products to be treated are placed, characterized in that, instead of granting the resonance frequency f of the applicator to keep it equal to that of the HF generator (f), we bypass a rapid scan of f around f thanks  o a rotary electromechanical device arranged in parallel or in series with respect to the applicator. 2. Method according to claim 1, characterized in that said frequency modulation device is mounted in parallel and is of the variable capacitor type driven by a motor. 3. Method according to claim 2, characterized in that said motor is a synchronous motor. 4. Method according to claim 3, characterized in that said device consists of a butterfly capacitor whose movable blades driven by a synchronous motor allow to obtain a few tens of pulses per second of H.F. in the sample under treatment. 5. Method according to claim 4, characterized in that the number of pulses is 50 / s. 6. Method according to claim 1, characterized in that said device consists of a variable inductor placed in series in the circuit of the applicator, inductor produced by a conductive pin in "U" in front of the plane of which rotates a disc provided of conductive sectors alternating with insulating sectors. 7. Method according to claim 6, characterized in that the conductive sectors are made of copper and the insulating sectors made of phenolic or epoxy resins. 8. Method according to any one of claims 1 to 7, characterized in that, taking as information the average voltage present at the terminals of the cathode resistance of the transmitter tube, voltage directly proportional to the power dissipated in the placed sample in the applicator, this power is controlled by a given reference which can be fixed or variable over time. 9. Method according to claim 8, characterized in that the control is carried out, in the case of an HF generator self-oscillator type, by directly controlling the supply of the power tube through a supply controlled by thyristors or inductance saturable.  10. Method according to claim 8, characterized in that, in the case of a controlled HF generator (quartz for example), the power control will not be done on the power tube of the HF generator but on the exciter . 11. Method according to any one of claims 1 to 10, characterized in that, between two pulses of H.F. elapses relatively long, allowing better resistance of the material to the risk of flasch. 12. The method of claim 11, characterized in that, the generator consuming practically no energy during idle times, the power tube is supercharged which safely allows to obtain in the sample practically the same energy as that which a conventional assembly without scanning would provide. 13. Device for scanning the resonance frequency f of the HF applicator around the frequency f0 of the generator, in a high-frequency circuit With indirect coupling by mutual induction, intended to be mounted in parallel or in series with respect to the applicator, characterized in that it consists of a rotary electromechanical device. 14. Device according to claim 13, charac terized in that it is of the variable capacitor type driven by a motor, to be mounted in parallel. 15. Device according to claim 14, characterized in that the motor is synchronous. 16. Device according to claim 15, characterized in that it consists of a butterfly capacitor whose moving blades driven by a synchronous motor make it possible to obtain a few tens of pulses per second of H.F. in the sample under treatment. 17. Device according to claim 16, characterized in that the number of pulses is 50 / s. 18. Device according to claim 13, characterized in that it consists of a variable inductor placed in series in the circuit of the applicator, inductor produced by a conductive pin in "U" in front of the plane of which rotates a disc provided of conductive sectors alternating with insulating sectors. 19. A method of continuous thermal treatment of products according to which said products are placed between the metal plates of a high frequency applicator integrated into an HF assembly of the type with indirect coupling by mutual induction, characterized in that it is mounted in parallel or respectively in series of the applicator, in said assembly, a device of the variable capacitor or self type according to any one of claims 13 to 18, the products are scrolled continuously, and the applicator and the scrolling system are mechanically coupled of the products so that the area to be treated with the products remains substantially between the plates of the applicator for the time necessary for the treatment. 20. Method according to claim 19, characterized in that the treatment of the products consists of the joining of wooden rafters. 21. The method of claim 20, characterized in that the adhesive used is resorcinol in a mixture with 5-20, preferably 15% of phenol, and the time necessary for the HF treatment is approximately 30 s for rafters of thickness 85 mm. 22. Method according to any one of claims 19 to 21, characterized in that the plates of the applicator are made of aluminum and of dimensions approximately 90 x 90 mm. 23. Continuous chevron butting joint, characterized in that it implements the method according to any one of claims 19 to 22. 24. Butted rafters, characterized in that they have been butted by means of the production line according to claim 23.