Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C65/00—Joining or sealing of preformed parts, e.g. welding of plastics materials; Apparatus therefor
  • B29C65/02—Joining or sealing of preformed parts, e.g. welding of plastics materials; Apparatus therefor by heating, with or without pressure
  • B29C65/04—Dielectric heating, e.g. high-frequency welding, i.e. radio frequency welding of plastic materials having dielectric properties, e.g. PVC
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C66/00—General aspects of processes or apparatus for joining preformed parts
  • B29C66/90—Measuring or controlling the joining process
  • B29C66/91—Measuring or controlling the joining process by measuring or controlling the temperature, the heat or the thermal flux
  • B29C66/912—Measuring or controlling the joining process by measuring or controlling the temperature, the heat or the thermal flux by measuring the temperature, the heat or the thermal flux
  • B29C66/9131—Measuring or controlling the joining process by measuring or controlling the temperature, the heat or the thermal flux by measuring the temperature, the heat or the thermal flux by measuring the heat or the thermal flux, i.e. the heat flux
  • B29C66/91311—Measuring or controlling the joining process by measuring or controlling the temperature, the heat or the thermal flux by measuring the temperature, the heat or the thermal flux by measuring the heat or the thermal flux, i.e. the heat flux by measuring the heat generated by Joule heating or induction heating
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C66/00—General aspects of processes or apparatus for joining preformed parts
  • B29C66/90—Measuring or controlling the joining process
  • B29C66/91—Measuring or controlling the joining process by measuring or controlling the temperature, the heat or the thermal flux
  • B29C66/914—Measuring or controlling the joining process by measuring or controlling the temperature, the heat or the thermal flux by controlling or regulating the temperature, the heat or the thermal flux
  • B29C66/9161—Measuring or controlling the joining process by measuring or controlling the temperature, the heat or the thermal flux by controlling or regulating the temperature, the heat or the thermal flux by controlling or regulating the heat or the thermal flux, i.e. the heat flux
  • B29C66/91651—Measuring or controlling the joining process by measuring or controlling the temperature, the heat or the thermal flux by controlling or regulating the temperature, the heat or the thermal flux by controlling or regulating the heat or the thermal flux, i.e. the heat flux by controlling or regulating the heat generated by Joule heating or induction heating
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C66/00—General aspects of processes or apparatus for joining preformed parts
  • B29C66/90—Measuring or controlling the joining process
  • B29C66/91—Measuring or controlling the joining process by measuring or controlling the temperature, the heat or the thermal flux
  • B29C66/914—Measuring or controlling the joining process by measuring or controlling the temperature, the heat or the thermal flux by controlling or regulating the temperature, the heat or the thermal flux
  • B29C66/9161—Measuring or controlling the joining process by measuring or controlling the temperature, the heat or the thermal flux by controlling or regulating the temperature, the heat or the thermal flux by controlling or regulating the heat or the thermal flux, i.e. the heat flux
  • B29C66/91651—Measuring or controlling the joining process by measuring or controlling the temperature, the heat or the thermal flux by controlling or regulating the temperature, the heat or the thermal flux by controlling or regulating the heat or the thermal flux, i.e. the heat flux by controlling or regulating the heat generated by Joule heating or induction heating
  • B29C66/91653—Measuring or controlling the joining process by measuring or controlling the temperature, the heat or the thermal flux by controlling or regulating the temperature, the heat or the thermal flux by controlling or regulating the heat or the thermal flux, i.e. the heat flux by controlling or regulating the heat generated by Joule heating or induction heating by controlling or regulating the voltage, i.e. the electric potential difference or electric tension
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C66/00—General aspects of processes or apparatus for joining preformed parts
  • B29C66/90—Measuring or controlling the joining process
  • B29C66/91—Measuring or controlling the joining process by measuring or controlling the temperature, the heat or the thermal flux
  • B29C66/919—Measuring or controlling the joining process by measuring or controlling the temperature, the heat or the thermal flux characterised by specific temperature, heat or thermal flux values or ranges
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C66/00—General aspects of processes or apparatus for joining preformed parts
  • B29C66/90—Measuring or controlling the joining process
  • B29C66/96—Measuring or controlling the joining process characterised by the method for implementing the controlling of the joining process
  • B29C66/961—Measuring or controlling the joining process characterised by the method for implementing the controlling of the joining process involving a feedback loop mechanism, e.g. comparison with a desired value
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B6/00—Heating by electric, magnetic or electromagnetic fields
  • H05B6/46—Dielectric heating
  • H05B6/48—Circuits
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C66/00—General aspects of processes or apparatus for joining preformed parts
  • B29C66/90—Measuring or controlling the joining process
  • B29C66/94—Measuring or controlling the joining process by measuring or controlling the time
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C66/00—General aspects of processes or apparatus for joining preformed parts
  • B29C66/90—Measuring or controlling the joining process
  • B29C66/94—Measuring or controlling the joining process by measuring or controlling the time
  • B29C66/949—Measuring or controlling the joining process by measuring or controlling the time characterised by specific time values or ranges

Definitions

• the invention relates to a method and an apparatus for controlling transfer of high frequency power from an electric AC generator to an item to be treated; the method is of the type stated in the introductory clause of claim 1, and the apparatus is of the type stated in the intro ⁇ ductory clause of claim 4.
• Treatment of items with high frequency electric power can be used in processing - such as melting, heating, welding, tacking and hardening - and surface treatment of plastic materials, etc.; in heating, pasteurizing and ste ⁇ rilizing foodstuff and pharmaceuticals; and in drying of wood, etc., and drying and hardening of glue in wooden pro ⁇ ducts, etc.
• Treatment of items with high frequency electric power may also be used for melting and thermal treatment of metals, the power being here inductively transferred, i.e. the item forms part as loss inducing material in the mag ⁇ netic field of a coil being part of a circuit which may be a resonance circuit tuned to resonance at the AC frequency.
• This appa ⁇ ratus may also be re-tuned for maintaining the resonance during the treatment of the item; this tuning is made by changing the capacitance of a variable capacitor on basis of measurement of the deviation of the load from ohmic load, detected as phase difference between current and vol ⁇ tage in a transmission line between the apparatus and the generator supplying power to the apparatus.
• an impedance transformation takes place in this apparatus between the generator and the item to be treated, the generator being connected to a tap on the coil of the resonance circuit.
• no actual adjustment of the impedance transforming ratio can be made in this appa- ratus, as besides the coil is a fixed component.
• the value of only one component can be continu ⁇ ously adjusted, viz. the capacitor used in tuning to reso ⁇ nance, and adjustment of one component is not sufficient for adjusting the impedance transforming ratio to a desired value.
• the impedance transforming ratio is a two-di ⁇ mensional quantity. This ratio may thus be expressed as a complex quantity, which is the ratio between two complex quantities (two impedances) , each of which is determined by their real part and their imaginary part (the ohmic resis ⁇ tance and the reactance) .
• the impedance transforming ratio in this known apparatus is not variable means that the appara ⁇ tus only has a limited working area, such as correction for the settling of one and the same tool during a sealing ope- ration, or a change between several, substantially iden ⁇ tical tools.
• EP-A2-0 546 502 it is known that an impedance matching should be made between the generator and the elec ⁇ trodes of the tool, and it is stated to be the object of the apparatus and the method according to that publication to attain impedance matching and to attain low reflected power from the tool back towards the generator.
• the published publication likewise mentions that the circuit, of which the electrodes of the tool form part, should be in resonance.
• EP-A2-0 546 502 only mentions the use of fixed (non-variable) impedances for the impedance matching, even though the publication mentions the use of a continu ⁇ ously variable capacitor for the tuning to resonance. This is a drawback of the apparatus and the method according to that publication, as it is not possible with fixed impe ⁇ dances to attain a correct impedance matching.
• the formation of standing waves in the transmission lines of the known apparatuses is a particular drawback when using the apparatuses for treatment of big items, for instance for drying of big wooden items, such as whole planks or logs.
• the standing waves invariably spread to the treatment tool, and the tools for treatment of such big items may have dimensions of several times the half wave length at the frequencies used.
• the standing waves on the tool will thus have one or more nodes, opposite which only quite a small power will be deposited in the item, while a big effect will be deposited opposite the antinodes of the waves. Consequently, no even distribution of power can take place along a tool with dimensions nearing the wave length at the frequency used, when the standing wave ratio is high.
• variable capacitor is present in paral- lei with the tool, as the voltage over the tool should be able to rise considerably at resonance; in practice volt ⁇ ages of up to 20-30 kV RMS are needed for instance for weld ⁇ ing of plastic materials with low losses or glass.
• a vari ⁇ able capacitor is to carry the full voltage from plate to plate, and only variable capacitors up to 7 kV p are avail ⁇ able on the market .
• the object of the present invention is to provide a method and an apparatus which are of the types mentioned by way of introduction and which make it possible to control transfer of high frequency power from an electric AC gene ⁇ rator to an item to be treated in such a way that the above drawbacks are avoided; to provide treatment of items which have a dimension which is comparable with the wave length of the frequency used; and to measure exactly and repro- ducibly the power transferred to the treated item, as this among others is desirable for controlling the process and for quality control.
• the load impedance constituted by the component, of which the item to be treated forms part normally varying and in cer ⁇ tain cases, such as when drying wood, varying considerably during the course of the treatment .
• the set impedance transforming ratio detected according to claim 3 is an ex- pression of the load impedance of the item, the generator impedance being known.
• the invention provides an appa ⁇ ratus which is characteristic in the subject matter of the characterizing clause of claim 4.
• the subject matter of ' claim 5 it is possible to tune sufficiently accurately to resonance and sufficiently accurately to adjust the impedance trans ⁇ forming ratio at the preferred working frequencies.
• a par ⁇ ticularly advantageous embodiment of a variable coil is attained, which makes it possible to apply particularly high voltages across the tool.
• the coil according to claim 7 can be designed in such a way that it only needs one sup ⁇ port in the high voltage side.
• Fig. 1 schematically shows a preferred embodiment of an apparatus according to the invention, connected to a high frequency generator and registration and controlling devices, in connection with the treatment of an item,
• Fig. 2 schematically shows a particular connection of a tool for the treatment of extensive items to the appa ⁇ ratus according to the invention
• Fig. 3 is a partial sectioned longitudinal view of a variable coil according to claim 7, and
• Fig. 4 is a longitudinal section in a detail in the coil in Fig. 3.
• an apparatus 1 according to the invention is supplied with high frequency power from an electric AC generator 2 through a transmission line 3 which here, has the form of a coaxial cable.
• the generator 2 has an output impedance corresponding to the characteristic impedance of the cable 3, and there is thus correct impedance matching at this place.
• the power is transferred on through a high fre ⁇ quency wattmeter 4 and along a transmission line 7 which is likewise in form of a coaxial cable with the same charac ⁇ teristic impedance as the output impedance of the generator 2.
• the high frequency wattmeter may be of a type known per se, several of which are commonly sold on the market; it consists in this connection of a so-called line section 5, which is inserted in the transmission line 3, 7 and forms part thereof, with the same characteristic impedance and thus a correct impedance matching, and a calculation and display unit 6.
• a high frequency wattmeter forward power and reflected power and figures calculated on basis thereof, in particular standing wave ratio, can nor ⁇ mally be measured.
• An example of such an apparatus is Bird "Thruline" Model 4385 from Bird Electronic Corporation, USA. It is shown in Fig. 1 that from the calculation and display unit 6 through the connection 27 a signal is trans ⁇ mitted which corresponds to the forward power, and through the connection 26 a signal corresponding to the standing wave ratio.
• the power is con ⁇ ducted along the transmission line 7 to the circuit 8 which comprises two continuously variable impedances, here a con- tinuously variable capacitor 9 and a continuously variable coil 10.
• the circuit 8 acts as an impedance transformer between the characteristic impedance of the transmission line 7, which towards the circuit 8 acts like a generator impedance Z G , and the load impedance Z B , to which the cir- cuit 8 delivers the power.
• the load impedance Z B is here constituted by a cir ⁇ cuit comprising a connection 11 and a tool 12, for instance for welding or drying cf an item 14.
• the tool 12 substan ⁇ tially consists of two plates 13, 15 which in combination constitute a capacitor, in which the item 14 forms part as a loss inducing dielectric.
• the circuit 8 acting as impedance transformer may be designed in many other ways commonly known within the field of radio engineering than those shown here.
• the con- figuration of the circuit shown in Fig. 1 is, however, par ⁇ ticularly advantageous when transforming into high vol ⁇ tages. In practice voltages up to 20-30 kV ⁇ - is needed for instance for welding of low loss plastic materials or glass, and this high voltage is to be present on the output terminal of the circuit 8.
• the coil 10 constitute a connection to the output ter- minal 8 of the circuit 8 and to let the capacitor 9 be con ⁇ nected between the transmission line 7, where the voltage is low on account of the low impedance of the coaxial cable, and ground.
• the impedance transforming ratio of the circuit 8 is set by means of two mechanical adjustment means, which are in Fig. 1 indicated by 16 and 17, and by means of which the two continuously variable impedances can be adjusted.
• the adjustment means 16 and 17 may be adapted to be manu ⁇ ally operated, but in Fig.
• the regulator 25 is preferably designed for several different regulating purposes, which will be seen from the following. It may in this connection be designed to work according to one or more servo principles, many of which are per se known and used within the art of servo tech ⁇ nique.
• the regulator 25 receives via the connection 27 an error signal which is a signal from the calculation and display unit 6 of the high frequency wattmeter. This signal is used, when the regulator 25 is to tune to resonance and to adjust the impedance transforming ratio, when the treat ⁇ ment of an item is to be initiated.
• the signal is prefer ⁇ ably the value calculated by the high frequency wattmeter of the power brought forward through the transmission line 3, 5, 7.
• the regulator 25 receives over the connection 25 a second error signal which is likewise a signal from the calculation and display unit 6 of the high frequency wattmeter 4. This second signal is used when the regulator 25 is to correct the tuning to resonance and the adjustment of the impedance transforming ratio during the treatment of an item.
• the signal is preferably the value of the standing wave ratio on the transmission line 3, 5, 7 calculated by the high frequency wattmeter, but may also be any other measured or derived signal, for instance the reflected power running along the transmission line 3, 5, 7 from the circuit 8 in the direction towards the generator 2.
• the regulator 25 is by said correction to control two variables, viz. the value of the two continuously variable impedances 9 and 10 on basis of the value from a single error signal, viz. the standing wave ratio, which the regu-.
• the impedance transforming ratio of the circuit 8 is adjusted, said ratio being as said above expressible as a complex quantity and therefore a two-dimensional quan ⁇ tity.
• the deviation from correct impedance matching is likewise a two-dimensional quantity, whereas, however, the error signal is a one-dimensional quantity.
• Control of a two-dimensional quantity on basis of the deviation of a one-dimensional quantity cannot be attained straightaway by means of traditional regulators, but principles are known within the servo technique which allows such a control.
• a comparatively simple principle like for instance an alternating dither control, will provide a fully satisfactory control.
• a dither control the controlled quantity is allowed to vary periodically around the set value, while it is detected if the variation of the set value and the resulting variation in the error signal is in phase or in opposite phase; are they in phase, the controlled quantity is to be diminished to diminish the error, and are they in opposite phase, the controlled quantity is to be increased to diminish the error.
• Such a dither control can without any difficulty be performed alternately for several controlled quantities, and the regulation can be stopped, as long as the error is below a certain value to avoid unnecessary mechanical wear on the servo mechanisms.
• a further servo loop is shown in Fig. 1, viz. for the control of the power transferred from the generator 2 to the tool 12, said power being here termed the forward power.
• a signal corresponding to the forward power is trans ⁇ mitted to a regulator 28 which can likewise be designed to function according to one definite or one of several known servo principles; this regulator 28 controls through a con ⁇ nection 29 the high frequency power supplied by the gene ⁇ rator 2.
• This power is at correct impedance matching trans- ferred to the tool without substantial losses or reflec ⁇ tions, and by controlling this power, the power deposited in the item may be controlled in a safe and reproducible way.
• a registration unit 18 collects signals from the two servo mechanisms 21 and 22. These signals are preferably signals corresponding to the adjustments executed by the servo mechanisms of the two variable impedances 9 and 10. Two such signals will define the resulting impedance transforming ratio of the circuit 8, and consequently indirectly the load impedance Z B of the tool 1 (the impedance transforming ratio and the load impedance being as previously mentioned two-dimensio ⁇ nal quantities) , and by means of this signal capture it is possible to record the changes of the load impedance.
• the following course is an example of an advantage ⁇ ous regulation strategy for the two regulators 25 and 28 in controlling the impedance transforming ratio for the at ⁇ tainment of resonance and correct impedance matching:
• the regulator 28 adjusts the power of the gene- rator 2 to a low value, for instance a few percent of the value which is to be used in the treatment of the item 14,
• the servo mechanism 22 lets the coil 10 run through the whole of its regulation span, and the regulator 25 measures hereby through the connection 27 the forward power and registers the value of the setting of the coil 10 which gives the biggest forward power,
• the regulator 25 adjusts via the servo mechanism 22 the coil 10 to this setting, 5) the regulator 25 adjusts (in the same way as described below) through the servo mechanisms 21, 22 the capacitor 9 and the coil 10 to values which give the lowest standing wave ratio, measured through the connection 26, 6) the regulator 28 adjusts the power of the gene ⁇ rator to the value to be used at the treatment of the item 14.
• the regulator 25 receives through the connection 26 an error signal corresponding to the value of the stand ⁇ ing wave ratio on the transmission line 3, 5, 7, measured by the high frequency wattmeter 4; the regulator 25 is pas ⁇ sive, as long as the standing wave ratio is below a certain value, for instance 1.2,
• the standing wave ratio exceeds 1.2
• the setting of the coil 10 is varied according to the dither principle for determination of the direction in which to adjust. When this has been determined, adjustment is made in this direction until a minimum has been passed, which is indicated thereby that the standing wave ratio begins to increase again.
• the standing wave ratio exceeds for instance
• the adjustment of the capacitor 9 is varied in the same way, and is varied, until a minimum has been passed.
• steps 2) and 3) are repeated until the standing wave ratio is below 1.1.
• steps 2) - 4) are repeated, but this time the adjust ⁇ ment starts with varying of the setting of the capacitor 9.
• a computer 31 or a similar control appa ⁇ ratus is shown, which by data circuits 30, which as indi ⁇ cated by arrows may be bi-directed, may be connected with one or more of the elements of the apparatus according to the invention.
• the computer may for instance execute the various steps in the above-mentioned sequences; it may for instance col ⁇ lect data about the course of the treatment as expressed by the signals captured by the registration unit 18 from the two servo mechanisms 21 and 22, and it may take care of the communication with the operating staff or itself control the apparatus according to the invention, for instance by letting means not shown change to the next item 14 to be treated.
• Fig. 2 an advantageous way of connecting a tool with high capacitance to the apparatus according to the invention is shown.
• a tool for instance for thermal harden ⁇ ing of glue in whole plywood sheets or for welding of very big items of plastic foil will have a considerable plate area in relation to the plate spacing and consequently a capacitance which is substantially larger than the capaci ⁇ tance of common tools for welding or for thermal treatment.
• a tool for drying of wet wood will have a big capacitance as long as the wood is wet on account of the high dielec- trie constant of water.
• Fig. 3 shows an advantageous embodiment of a -coil
• the coil 10 has a winding 46, which is helically wound and which at its first end has a terminal 51, from where the coil is connected to the output terminal 34 of the apparatus by means of a flexible wire 52.
• the winding 46 can be moved forwards and backwards along its longitudinal axis as indicated by the arrow T and is at the same time secured against turning in that the terminal 51 is carried by two electrically insulating bars 50, 53 extending from the terminal 51 and being slidable in guides 48, 55 by means of sliding shoes 49, 54.
• the winding is from its other end carried by an electrically conductive tube 44, which constitutes the se ⁇ cond terminal of the coil, from where the coil is connected with the rest of the circuit 8 by means of a wire 58.
• the tube 44 is journalled with a shaft 40 in a bearing 43, thereby being pivotal about the longitudinal axis of the winding as indicated by the arrow 41, but secured against axial displacement by means 42.
• a screw thread 45 is provided, said thread corresponding to the helical shape of the winding 46 and into which the winding is screwed in as shown in Fig. 3.
• the winding 46 will be screwed into or out of the tube 44, whereby the terminal 51 is displaced in the direction shown by the arrow 56, the sliding shoes 49, 54 being displaced along the guides 48, 55.
• the inductance of the coil 10 is determined by the number of free turns 47, the inductance may be thus adjusted by turning the shaft 40.
• the windings 57, which are not free, are short-circuited by the tube 44 and are thus ineffective.
• the coil cf this design makes it possible to use particularly high voltages across the tool, the winding 46 being self-supporting and only one support being necessary at the high voltage terminal 51, where a support may be provided in the simple way shown in Fig. 3, said support allowing axial displacement but securing at the same time against turning.
• the more complicated parts of the variable coil 10, including the tube 44, the journalling mechanism ⁇ herefor, and a somewhat higher capacitance to the sur- roundings are all present at the low voltage terminal 58.
• Fig. 4 it is shown that the screw thread 45 in the tube 44 is designed with a bigger depth in radial di ⁇ rection, whereby a radial interspace 62 is created between the turn 57 and the bottom of the thread 45. This has been made to prevent the winding 46 and the tube 44 from being
• a tool 12 for the treatment of an item 14 takes the form of a capacitor with two horizontal plates 13, 15.
• the plates 13, 15 may simultaneously constitute the planes of a press, whereby it becomes possible to keep the wood in press during drying in order to reduce warping of the wood during the drying.
• the space between the plates 13, 15 and which is substantially filled out by the wood has the dimensions length 5 m x width 1 m x height 1 m; the length x _he width corresponds to the dimensions of the plates, and the height corresponds to the plate spacing.
• this tool 12 has a capacitance C ⁇ of
• the space between the plates 13, 15 is filled up with wooden planks which are stacked.- They may be stacked with intermediates, like -when being air-dried, but it is, however, not necessary to use intermediates as it has turned out that the water mainly escapes through the end surfaces of the wood. The wood is not stacked quite to the edges of the plates in order to avoid too heavy marginal effects. If the plates 13, 15 are planes in a press, said press is tightened to attain a suitable pressing of the planks.
• the continuously variable coil 10 used has an inductance L of approx. 120 nH per turn of winding; the coil therefore has to be adjusted in such a way that at the start of the treatment approx. VA- winding 47 is clear of the tube 44. When using the apparatus 1 according to the invention these adjustments take place automatically as described above.
• the specific heat capacity of water is 335 kJ/kg and the heat of evaporation at 100°C is 2.25 MJ/kg, and the specific heat capacity of wood is left out of regard, this lot of wood will theoretically be heatable from 20-100°C in less than 4 ⁇ hours and subsequently water will be evaporable down to a water content of 20% in the course of 22 hours with the apparatus according to the in ⁇ vention and with a power of 50 kW.
• the coil 10 is in the drawing shown with a tube 44 with an internal thread 45. However, there is nothing to prevent using instead of the tube 44 a tube with smaller diameter and with an external thread to be screwed into the winding 46. 17
• the apparatus 1 is in the drawing shown with two regulators 25, 28 and two servo mechanisms 21, 22.
• the ne ⁇ cessary regulating tasks may, however, very well be carried out with a different number of regulators, for instance three or with regulators being incorporated in the servo mechanisms.
• circuit 8 of the apparatus 1 is shown with two-pole components, but there is nothing to prevent the apparatus 1 from being designed with one or more components with more than two poles.
• a three-pole component a butterfly variable capacitor with a terminal on the rotable plate set may be mentioned, and as an example of a four-pole component a variable trans ⁇ former with two separate windings with variable degree of coupling can be mentioned.
• the circuit 8 of the apparatus 1 is in the drawing shown as an unbalanced circuit, i.e. all voltages are related to ground potential, and all return currents run in a common chassis. There is, however, nothing to prevent the circuit 8 in the apparatus according to the invention from being designed as a balanced circuit with or without ground connection at centre taps. This only requires the insertion of a balancing transformer or the like between the trans ⁇ mission line 7 and the circuit 8, and the design of the components of the circuit as balanced or doubled compo ⁇ nents, respectively.
• the capacitor 9 may for instance be a butterfly capacitor, - and the coil 10 may be replaced by two coils which are mutually mechanically interconnec- ted.
• the part of the circuit between the generator 2 and the balancing transformer may also be balanced, but that would hardly be advantageous.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Physics & Mathematics (AREA)
• Thermal Sciences (AREA)
• Electromagnetism (AREA)
• Measurement Of Resistance Or Impedance (AREA)
• Discharge Heating (AREA)
• Control Of High-Frequency Heating Circuits (AREA)

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• 1994
  • 1994-06-06 DK DK63694A patent/DK63694A/da not_active IP Right Cessation
• 1995
  • 1995-06-02 CA CA 2192082 patent/CA2192082A1/en not_active Abandoned
  • 1995-06-02 AU AU26119/95A patent/AU2611995A/en not_active Abandoned
  • 1995-06-02 WO PCT/DK1995/000219 patent/WO1995034945A2/en not_active Ceased
  • 1995-06-02 EP EP19950920794 patent/EP0764361A2/de not_active Withdrawn

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