US3118040A

US3118040A - H. f. dielectric heating apparatus

Info

Publication number: US3118040A
Application number: US127081A
Authority: US
Inventors: Stanley Edmund Christopher
Original assignee: Radio Heaters Ltd
Current assignee: Radio Heaters Ltd
Priority date: 1957-07-02
Filing date: 1961-07-17
Publication date: 1964-01-14
Anticipated expiration: 1981-01-14
Also published as: GB839487A; FR1207655A; DE1121246B

Description

1964 E. c. STANLEY H.F. DIELECTRIC HEATING APPARATUS 3 Sheets-Sheet 1 Filed July 17, 1961 Inventor fdmund C. t n 6y y M Mttorneys Jan. .14, 1964 E. c. STANLEY 3,113,040

mi DIELECTRIC HEATING APPARATUS Filed July 17, 1961 s Sheets-Sheet 2 Edmund C Szan/ey A Home y Jan. 14,1964 E. c. STANLEY 3,113,040

m". DIELECTRIC HEATING APPARATUS Filed July 17, 1961 3 Sheets-Sheet 3 Inventor E C. 5 fan /ey 3,ll8,ll i@ Patented Jan. 14, 1964 lice 3,113,040 HJF. DEELEQTRlQ HEATING APPARATUS Edmund tlhristopher Stanley, Woltingham, England, assignor to Radio Heaters Limited Filed Italy 17, E61, Ser. No. 127,081 @Eaisns priority, application Great Britain July 2, 1957 20 Claims. {Qt 2l9--lll.7)

This invention relates to high frequency dielectric heating apparatus.

In apparatus employing large electrodes, the connections from the generator to the live electrode or electrodes are usually comparatively long, and this results in a loss of efliciency in feeding power to the electrodes, or in tuning difficulties, or both. A further undesirable effect occurring with large electrodes is the production of standing waves on the electrodes, resulting in an uneven electrode voltage pattern. The present invention has for its object to reduce these difficulties.

Dielectric heating apparatus is known in which a coaxial feeder line usually of characteristic impedance of the order of 60 or 80 ohms and having a length equal to a fraction of a wavelength or a multiple of such a fraction, is used to connect the high frequency generator into an electrode circuit, and hitherto a resonant electrode circuit tuned to the generator frequency has been used. The use of a resonant circuit as load, by providing a load of low reactance at the generator frequency, ensures that the peak voltage is produced at the load. However, a sharply tuned arrangement of this kind does not provide a uniform voltage over the electrode even when the termination of the feeder line is connected to more than one point on the electrode. It is in fact necessary to use tuning loops between the electrodes or between the live electrode and the base plate in order to obtain a more-even voltage distribution over the electrode and to assist in tuning the electrode system to match the generator frequency. These tuning loops not only complicate the apparatus-but also obstruct the passage of material through the apparatus when it is fed continuously into one side of the electrode system and out of the other side. We have found that this uneven voltage distribution is at least partly due to the use of high impedance transmission lines which necessitate tuning the electrode system as a matching load into which the tuned transmission lines feed.

It has also been proposed to use two generators operating at different frequencies feeding opposite ends of a single electrode through tuned feeder sections each being of a multiple of a quarter wavelength of their respective generator working frequencies.

In apparatus embodying the present invention, tuned lines are deliberately avoided. An electrical connection between the enerator output connection and the work electrode is split into at least two balanced conductive paths which connect to the electrode at widely separated points, and a substantial part of each path is in the form of a section of low-impedance high-frequency transmission line consisting of a central conductor within an outer sheath, the total electrical length from the generator along each transmission line to the furthest point of the electrode from its connection to the line being approximately one-quarter of the fundamental operating wavelength of the apparatus. This means that in practice the transmission line has a length which is less than one-eighth of the fundamental operating wavelength.

Thus, the idea of treating the electrode system as a tuned-circuit load at the end of a transmission line is replaced, according to the invention, by the idea of bulking the inductance and capacitance of the electrode with that of the short sections of low-impedance duct 0 avoided.

between the electrodes and the generator, so that the whole of the electrical system from the generator up to and including the electrodes becomes a flatly tuned load which does not require any critical matching between the feeders and the electrodes. This is possible owing to the use of low-impedance transmission lines, which enable the line and the electrode to be treated as an undivided quarter-wave system, and enable the requirement for accurate matching of the tuning of an electrode system to be By the use of the section of low-impedance transmission line between the generator output terminal and the point of connection to the work electrode, a greater physical distance between the generator terminal and the end of the electrode is possible than would be the case if an open electrical connector were used of which the effective inductance might be so great that it would be impossible to tune the electrode system and the feed connection from the generator as a whole, to the required resonance with the generator working frequency. With the arrangement according to the invention, each transmission line provides across the electrode a voltage which increases slightly towards the side of the electrode remote from the transmission line, but the result of the electrode voltages provided by the individual lines is the production of a substantially even voltage over the whole of comparatively large electrodes. The effective impedance of the transmission lines should be as low as possible and is preferably less than 20 ohms, but satisfactory results can be obtained with transmission lines having an effective impedance of as much as 30 or even 35 ohms. The central conductor, which should have a comparatively large surface area, may conveniently be of wide copper strip.

In the preferred form of the invention, the electrode or electrodes are suspended from a transverse bar between vertical supports and the balanced conductive paths from a generator located below the electrode system pass through or adjacent to the vertical supports on their way to the electrode or electrodes, so that material fed through the apparatus between the supports is not obstructed by the connections from the generator to the electrodes. According to a subsidiary feature of the invention, the electrode is offset with respect to a line joining the two supports, so that material to he heated can also be fed through the apparatus in a direction parallel to this line without obstruction from the supports or the connections from the generator to the electrode.

In order that the invention may be better understood, several embodiments thereof will now be described with reference to the accompanying drawings in which:

FIGURE 1 shows a simple form of dielectric welding apparatus embodying the present invention;

FIGURE 2 shows an open-span dielectric welding press in accordance with the invention;

FIGURE 3 is a plan view, partly in cross-section, of the apparatus of FIGURE 2;

FIGURE 4 shows the application of the invention to a conveyor band system. employing balanced dielectric heating electrodes;

FIGURE 5 shows the addition of a tuning circuit to the output circuit of the dielectric heating apparatus; and

FIGURE 6 is a diagram illustrating the operation of the apparatus of FIGURE 1.

in FIGURE 1 the upper electrode 6 is connected through an insulated mount 8 to a shaft ltl which is arranged for vertical movement to allow the electrode 6 to be raised or lowered with respect to the press bed 12 which constitutes the lower electrode. The casing of a generator M- is connected to two transmission ducts l6 and 18, the central conductors 20 of which are connected to the output circuit of the generator, here represented diagrammatically by the variable capacitor 22. and the inductance 2-4. The transmission ducts l6 and. 18 with their central conductors 20, which are preferably of wide strip copper, constitute low impedance transmission lines of length less than one-eighth of the wavelength of operation of the apparatus. The ends of the conductors which are remote from the generator are connected through flexible conductors 26 to opposite sides of the electrode 6. The press bed 12 is connected to the

ducts

16 and 18 through connections 28. The conductive path between the generator and the electrode 6 is thus split into two balanced paths, the greater part of each of which is formed by a low impedance transmission line, the tuning of which is not critical. As a result a much more uniform voltage is produced over the electrode 6 than has been possible hitherto.

The manner in which the apparatus of FIGURE 1 produces a more uniform voltage distribution over the electrode will be better understood from the explanatory diagram which constitutes FIGURE 6. In FIGURE 6 there is shown diagrammatically the electrode 6 with its two ends connected through conductors 20 to opposite terminals of the generator 14. The conductors 2%) pass through the two transmission ducts l6 and 18. Although, in the diagram, the ends of the transmission ducts are connected by dotted lines to the generator 14, it will be seen from FIGURE 1 that in fact the outer ends of the transmission lines terminate at the output of the generator 14. As shown in FIGURE 6, each transmission line, the connection between the transmission line and the electrode, and the electrode itself constitute a quarterwave system. The voltage distribution over the electrode which would result from the use of only the transmission duct 16 is shown by the curve V and the voltage dis tribution due to the transmission duct is alone is shown by the curve V It will be seen that the voltage due to the transmission duct 16 would be less at A than at B, which is the end of the electrode remote from the feed of the generator, but that this difference is not too great owing to the fact that this portion of the sine wave constituting the voltage curve is relatively flat. The portion of the sine wave between points C and B is less steep than that between points A and C, and the present invention takes advantage of this feature by feeding opposite ends of the electrode, or widely spaced points on the electrode through similar transmission lines of the low impedance kind so that the efiective voltage between C and B is that due to the transmission duct 16 and the effective voltage between points A and C is that due to the transmission duct 13. The effective voltage along the length of the electrode is represented in FIGURE 6 by a heavy line.

If desired more than two transmission lines can be connected to the output of the generator, the ends of these lines remote from the generator being connected to widely separated points on the electrode.

FIGURE 2 shows the application of the invention to an open-span press. In conventional open-span presses, however, the electrode or electrodes are supported from a horizontal bar connected between two vertical pillars, which arrangement enables the material which is to be welded to be passed from front to back of the apparatus, between the vertical pillars. In the apparatus shown in FIGURE 2 the longitudinal axis of the electrode is offset with respect to a line joining the front ends of the two vertical supports 32, so that the material to be welded can be passed continuously through the apparatus from front to back, between the supports 32, or from side to side, in which case no obstruction is caused by the supports 32 because of the offsetting of the electrode 39. To prevent obstruction of the material to be welded by the connections from the electrode 3th to the generator 34 in the base of the apparatus, these connections are led away diagonally from the electrode 30, as will now be explained. As in the case of FIGURE 1, the output circuit of the generator is connected to a pair of balanced transmission lines, the ends 38 of which project through the bedplate bridge members 42 at opposite ends of the electrode 30.

Each of the bridge members 42 is connected to the electrode at two points, one at the front of the electrode and one at the back. The oblique arrangement of the flexible strips 4% to prevent obstruction of the workpiece by the connection from the electrode to the generator is seen more clearly in the plan view of FTGURE 3.

The bridges 42 constitute in effect a pair of parallel inductances connected between the ends of the flexible conductors 40 and the electrode 3%. These additional inductances permit adjustments of the voltage at the electrode and enable a greater elficiency to be obtained in the feeding of RF. power from the generator to the electrode. They also enable a more uniform voltage distribution to be obtained over the surface of the electrode.

As in the case of FIGURE 1 the electrode 30 is connected to a vertical shaft 44 which is supported from a top plate 46 projecting forward from the vertical pillars 4?, and is movable in the vertical direction.

In FIGURE 4 there is shown diagrammatically a balanced electrode system in which the two similar electrodes 43 are located above a conveyor band 50 on which material to be heated passes through the electrode system. The ends of the output coil 52 of the generator are connected to the central conductors 54 of two transmission lines, the outer conductors 56 of which are of hollow rectangular cross-section. Each of these transmission lines splits into two further transmission lines 58 which include a right-angle bend enabling them to rise above the level of the conveyor band and electrodes. The ends of the central conductors 60 in these transmission lines are connected through flexible connectors 62 to conductive bridge members 64 on opposite sides of the electrode. The ends of the bridge members are connected to the electrodes 48 at Widely spaced points. Both ends of the output coil 52 are at a high radio frequency potential with respect to the earthed conveyor band. In FIGURE 4, the pick-up coil 52 is shown at the side of the conveyor band, for clarity, but in practice it would be located beneath or above the conveyor band. In many cases, the bridge members 64 can be omitted, and the flexible strips 62 connected directly to the electrodes immediately beneath their points of connection to the bridge members in FIGURE 4. With or without the bridge members, as in the case of the apparatus shown in FIGURE 1, the electrical length of the path from the generator along each transmission line to the furthest point of the electrode from the connection of the feeder 62 to the bridge 64 (or to the electrode when the bridge member is omitted) is approximately one-quarter wavelength at the fundamental operating frequency of the apparatus. In the case of the apparatus of FIGURE 4, the voltage distribution cannot be illustrated as simply as in the case of the apparatus of FIGURE 1, but it remains true that the voltage variation over the electrode is reduced firstly by treating the whole of the electrical system from the generator up to and including the electrodes as an undivided quarter-wave system, and secondly by using balanced feeder paths which connect to the electrode at widely separated points, so that points of low voltage for one feeder do not correspond with points of low voltage for the other feeder.

The central conductors of the transmission lines may be of box form instead of strip form.

It may in some cases be desirable to provide means for tuning the ends of the transmission lines remote from the generator in order to ensure that the required electrode voltage is applied to the electrode in spite of the different electrical capacitances of different electrodes used in the system. In this case a section of inductance 66 can be connected between the end of the central conductor 6 8 of the transmission line and earth. This additional inductance 66 is effectively in series with the fiexible conductor 70 which connects the centre conductor 68 of the transmission line with the electrode 72, the ratio of the inductance formed by the connecting loop to the electrode and the added inductance between the end of the transmission line and ground forming in effect a matching transformer permitting the RF. voltage on the work-plate or electrode to be altered. The arrangement thus permits the electrical loading on the transmission line to be altered in order to achieve the optimum power and optimum voltage transmission to the work electrode.

If desired, an electrical capacitor 74, for example an air-spaced or ceramic dielectric condenser, may be connected in series with the inductance 66, as shown in FIG- URE 5, to serve as a blocking condenser for DC. or low frequency voltages in cases Where such voltages are applied to the electrode by arc-limiting circuits. The connection of the arc limiting devices in this way would not be possible if the inductance 66 were provided without the capacitor 74.

If desired, a number of pairs of balanced transmission lines can be used to connect a series of electrode plates to the output terminal of a common RF. generator. As an example, it might be desired to use an electrode for each section of an oven, or for each drawer of a drying oven of the type comprising a series of drawers. The pairs of transmission lines may be connected directly to the output terminal of the generator, or alternatively they may be connected to a common section of transmission line which itself terminates at the output terminal of the generator.

This application is a continuation-in-part of application Serial No. 744,739, filed June 26, 1958, and now abandoned.

i claim:

1. Dielectric heating apparatus comprising a source of high frequency current, a work electrode, and electrical connecting means between the source and the electrode, said connecting means consisting of an even number of balanced conductive paths connected to the electrode at widely separated points to supply in-phase voltage thereto, a substantial part of each path being in the form of a section of low-impedance high-frequency transmission line consisting of a central conductor within an outer sheath, the electrical length of the path from the generator along each transmission line to the furthest point of the electrode from its connection to said line being approximately one-quarter wavelength at the fundamental operating frequency of the apparatus.

2. Apparatus according to claim 1, in which a series of different electrodes are fed with power from a common RF. generator and in which each individual electrode is connected to the output terminal of the generator by means of a pair of low-impedance transmission lines. t

3. Apparatus according to claim 2, in which each or" a pair of transmission line sections connected to the output terminals of the generator is common to a number of lowimpedance transmission lines connecting corresponding electrodes to the generator.

4-. Apparatus according to claim 1, in which said central conductors of the transmission lines are of strip metal and the outer sheaths are ducts of rectangular crosssection, and in which said connecting conductors between said transmission lines and said electrode are of flexible strip metal.

5. Apparatus according to claim 1, in which the output ends of the centre conductors in the low-impedance transmission lines are tuned by means of a section of inductance connected between the output end of the centre conductor of each transmission line and ground.

6. Apparatus according to claim 5, in which an electrical capacitor is connected in series with the additional inductance between ground and the output end of the 6 conductor in the transmission line to block DC. or loW= frequency voltages applied to the electrode by arc-limiting circuits.

7. Dielectric heating apparatus comprising a surface on which the work to be heated is placed, a pair of electrodes spaced from said surface, a high-frequency generator having an output coupling coil both ends of which are at high. radio-frequency potential during thte operation of the apparatus, and electrical connections between the two ends of said output coil and said two electrodes, respectively, each of said connections consisting of an even number of balanced conductive paths. which connect said generator to supply in-phase voltage to widely separated points on the corresponding electrode, a substantial part of each path being in the form of a section of low-impedance high-frequency transmission line consisting of a. central conductor within an outer sheath, the electrical length of the path from thegenerator along each transmission line to the furthest point of the electrode from its connection to said line being approximately one-quarter wavelength at the fundamental operating frequency of the apparatus.

8. Apparatus according to claim 7, comprising a first pairof sections of low-impedance transmission line having central conductors to which the two ends of the output coil are connected, respectively, each of said lowimpedance transmission line sections terminating in a further pair of low-impedance transmission line sections, and electrical connections between the ends of said further pairs of, transmission line sections and said widely separated points on the corresponding electrodes.

9. Dielectric heating apparatus for welding thermoplastic material comprising a high-frequency generator, a bed plate constituting a lower electrode, an upper electrode, a mounting for said upper electrode comprising vertical support members, a transverse bar between said support members and an electrode carrier unit mounted on said transverse bar, said electrode carrier unit being so arranged that said upper electrode is offset with respect to a line joining said support members, whereby material to be welded can be fed through the press in a direction parallel to a line joining said support members without being obstructed by the latter, said apparatus further comprising an electrical connection between said generator and said electrode, said connection consisting of an even number of balanced conductive paths connected to said electrode at widely separated points to supply inphase voltage thereto, a substantial part of each path being in the form of a section of low-impedance highfrequency transmission line consisting of a central conductor within an outer sheath, the electrical length of the path from the generator along each transmission line to the furthest point of the electrode from its connection to said line being approximately one-quarter wavelength at the fundamental operating frequency of the apparatus.

10. Apparatus according to claim 9, in which said generator is located below said bed plate, said connections extending adjacent to said support members to prevent obstruction to material passing between said electrodes and through said apparatus.

11. Dielectric heating apparatus comprising a highfrequency generator, a work electrode, and an electrical connection between said generator and electrode, said connection consisting of an even number of balanced conductive paths which connect to the electrode at widely separated points to supply in-phase voltage thereto, a substantial part of each path being in the form of a section of low-impedance high-frequency transmission line consisting of a central conductor within an outer sheath, the electrical length of the path from the generator along each transmission line to the furthest point of the electrode from its connection to said line being approximately one-quarter wavelength at the fundamental operating frequency of the apparatus, said apparatus further comprising an additional inductance connected between the end 7 of each of said central conductors of said transmission line sections and said electrode.

12. Apparatus according to claim 11, comprising a number of parallel-connected additional inductances connected between the end of each central conductor and said electrode.

13. Apparatus according to claim 12, in which the end of each central conductor is connected to one end of a flexible connector, from the other end of which additional inductances extend to opposite sides of the electrode.

14. Dielectric heating apparatus comprising a surface on which the Work to be heated is placed, a pair of electrodes spaced from said surface, a high-frequency generator having an output coupling coil both ends of which are at high radio-frequency potential during the operation of the apparatus, and electrical connections between the two ends of said output coil and said electrodes, respectively, each of said connections consisting of an even number of balanced conductive paths which connect said generator to the corresponding electrode at widely separated points to supply in-phase voltage thereto, a substan tial part of each path being in the form of a section of low-impedance high-frequency transmission line of length less than one-eighth of the wavelength of the apparatus and consisting of a central conductor within an outer sheath.

15. Apparatus according to claim 14, comprising a first pair of sections of low-impedance transmission line having central conductors to which the two ends of the output coil are connected, respectively, each of said lowimpedance transmission line sections terminating in a further pair of low-impedance transmission line sections, and electrical connections between the ends of said further pairs of transmission line sections and said widely separated points on the corresponding electrodes.

16. Dielectric heating apparatus for welding thermoplastic material comprising a high frequency generator, a bed plate constituting a lower electrode, an upper electrode, a mounting for said upper electrode comprising vertical support members, a transverse bar between said support members and an electrode carrier unit mounted on said transverse bar, said electrode carrier unit being so arranged that said upper electrode is otlset with respect to a line joining said support members, whereby material to be welded can be fed through the press in a direction ,llaoso parallel to a line joining said support members without being obstructed by the latter, said apparatus further comprising an electrical connection between said generator and said electrode, said connection consisting of an even number of balanced conductive paths *which connect to said electrode at widely separated points to supply in-phase voltage thereto, a substantial part of each path being in the form of a section of low-irnpedance high-frequency transmission line of length less than one-eighth of the wavelength of the apparatus and consisting of a central conductor within an outer sheath.

17. Apparatus according to claim 16, in which said generator is located below said bed plate, said connections extending adjacent to said support members to prevent obstruction to material passing between said electrodes and through said apparatus.

18. Dielectric heating apparatus comprising a high frequency generator, a Work electrode, and an electrical connection between said generator and electrode, said connection consisting of an even number of balanced conductive paths which connect to the electrode at widely separated points to supply in-phase voltage thereto, a substantial part of each path being in the form of a section of low-impedance high-frequency transmission line of length less than one-eighth of the wavelength of the apparatus and consisting of a central conductor within an outer sheath, said apparatus further comprising an additional inductance connected between the end of each of said central conductors of said transmission line sections and said electrode.

19. Apparatus according to claim 18, comprising a number of parallel-connected additional inductances connected between the end of each central conductor and said electrode.

20. Apparatus according to claim 19, in which the end of each central conductor is connected to one end of a flexible connector, from the other end of which additional inductances extend to opposite sides of the electrode.

Brown Apr. 14, 1953 Amundsen et a1. May 24, 1960