WO2007007123A1

WO2007007123A1 - Radio frequency power apparatus

Info

Publication number: WO2007007123A1
Application number: PCT/GB2006/050164
Authority: WO
Inventors: David John Smith; Magnus Loutit; Mark Stuart Cornell; Ryan Grimes
Original assignee: Stanelco Rf Technologies Limited
Priority date: 2005-07-14
Filing date: 2006-06-20
Publication date: 2007-01-18

Abstract

An apparatus incorporates means (32) to feed radio frequency power to electrical terminals at least one of which is movable, wherein an electrical conductor connected to at least one of the electrical terminals comprises a tubular structure (40) of variable length of an electrically-conducting material. The apparatus may be for dielectric heating or welding of a material (14, 34), with an opposed pair of electrodes (18, 24) between which is a gap into which the material can be placed, wherein the electrodes are connected to the said electrical terminals. The RF feed may be a coaxial structure, with an outer tube (42) of bellows construction.

Description

Radio Frequency Power Apparatus

This invention relates to an apparatus utilising radio frequency power, for example to an apparatus for performing dielectric welding, for example for welding sheets of a polymeric material, or for dielectric heating.

The welding of thermoplastic polymeric films using dielectric heating (which may also be referred to as radio frequency heating or high frequency heating) has been known for many years. In this process the two films of thermoplastic material are positioned between opposed electrodes (or one electrode and a base plate) , the electrodes are pressed together, and a radio frequency voltage is applied between the electrodes. This process is applicable to materials which have a significant dielectric loss index at an appropriate RF frequency. Suitable thermoplastic materials are polyvinyl chloride and polyurethane, amongst many others. For example the use of radio frequency for welding plastics is described in EP 0 026 330. The radio frequency signals have typically been supplied to the live welding electrode using a flexible strip of copper, as such a strip can be cut to a suitable size to carry the necessary electrical power, and can accommodate the necessary movements of the electrode. However with repeated flexing, fractures may occur across the foil; movement of the foil relative to the adjacent parts of the welding equipment can alter the tuning characteristics of the equipment; the foil may come into contact with surrounding metal components as it flexes, causing a short-circuit; and it is difficult to enclose such a foil with screening to prevent transmission of radio waves into the environment.

Power radio frequency signals are also required in performing induction heating (which may be at frequencies in the kHz range) , and for microwaves (at frequencies around 1 GHz to 20 GHz, for example) .

According to the present invention there is provided an apparatus incorporating means to feed radio frequency power, the feed means incorporating electrical conductors connected to electrical terminals at least one of which is movable, wherein an electrical conductor connected to at least one of the electrical terminals comprises a tubular structure of variable length of an electrically- conducting material .

In a common arrangement one electrical terminal is stationary, and the other is movable, and the tubular structure of variable length would be used to make contact with the movable terminal .

It will be appreciated that the nature of the electrical terminals will depend upon the process that is being performed. In particular the apparatus may be an apparatus for dielectric heating or welding of a material, the apparatus comprising an opposed pair of electrodes between which is a gap, such that the material to be heated or welded may be placed in the gap between the electrodes, wherein the electrodes are connected to the said electrical terminals. The electrodes may, for example, consist of parallel flat plates; for forming long welds the electrodes might consist of rollers; for forming narrow welds at least one electrode may consist of a narrow bar. The electrodes, whether of the flat plate, narrow bar, or roller design, may be shaped so as to form a plurality of welds simultaneously.

The radio frequency supply for dielectric heating may in principle be at a frequency between 1 MHz and 200 MHz, usually between 10 MHz and 100 MHz, but stringent limits are imposed on any emitted radio waves . In practice therefore the choice of frequency may be more limited. For example the supply frequency may be 27.12 MHz, or 40.68 MHz. Preferably the radio-frequency signal generator is a solid-state device, and the signals are supplied via a matching network. The matching network preferably is an active matching network, incorporating a reactive network with at least one variable capacitor controlled by a motor; it monitors the radio frequency power, and adjusts the value of the or each variable capacitor accordingly. This may for example be such that the impedance presented to the generator remains at a constant value such as 50 Ω.

The tubular structure of variable length may incorporate one or more sliding contacts, but a preferred structure is a bellows, as this can ensure good electrical conduction throughout its length, and does not require any sliding contacts. The wall of the bellows is of zigzag or wavy shape in cross-section, and is of integral structure (for example with welded edges, or formed from a continuous tube) . It may be made of beryllium copper or another good electrical conductor, and is springy. Alternatively the tubular structure may be formed of strips, with gaps between them, the length being variable as the strips bend and the gaps vary in width.

Preferably the tubular structure of variable length which provides electrical connection is at least partly of coaxial structure. It may for example comprise two concentric bellows, or a bellows connected to the welding electrode and surrounded by a coaxial tube of fixed length. The impedance of such a pair of concentric tubes (which acts as a transmission line) depends on the ratio between the internal diameter of the outer tube, and the external diameter of the inner tube (i.e. the inner bellows) . A desired transmission line impedance (for example of 50 Ω) can be ensured by suitable selection of a value for this ratio. It will be appreciated that the diameters of the bellows may vary along their length; the transmission line impedance would remain at the desired value as long as this ratio remains constant.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings in which:

Figure 1 shows a diagrammatic side view of a dielectric welding apparatus, partly in section, incorporating a tubular electrical connection;

Figure 2 shows a side view of a modification of the tubular electrical connection of the apparatus of figure 1;

Figure 3 shows a side view of an alternative modification of the tubular electrical connection of the apparatus of figure 1;

Figure 4 shows a perspective view of an alternative construction of part of the tubular electrical connection of the apparatus of figure 1;

Figure 5 shows a perspective view of another alternative construction; and

Figure 6 shows a sectional view of another alternative construction.

Referring to figure 1, a welding apparatus 10 is shown, partly diagrammatically, for packaging a food product 12 (such as a ready meal) in a stiff, generally rectangular tray 14 of a dielectrically weldable polymeric material, that has rounded corners and a peripheral rim 16. The tray 14 is of thickness (say in the range 300 to 600 μm) to ensure it is stiff and an adequate oxygen barrier; and the rim 16 is of width 4 mm. The apparatus 10 includes a lower aluminium die 18 which defines a generally rectangular aperture 20 into which the tray 14 locates, and the upper surface of the die 18 is coated with a 50 μm thick layer 22 of a dielectric barrier material (e.g. PTFE-loaded alumina), so that when the tray 14 is located in the aperture 20 its rim 16 is supported by the upper surface of the layer 22 on the die 18. An upper aluminium die 24 has a recess 25 of the same shape as the aperture 20, surrounded by a ridge 26 with a lower surface which is also coated with a dielectric barrier material 28. The upper surface of the die 18 and the lower surface of the ridge 26 are each of the same width, such that the rim 16 projects just beyond their edge.

The upper die 24 is connected via a conductor 36 through an active matching network 30 to a solid-state RF generator 32, so that the conductor 36 carries the RF feed current, while the lower die 18 provides part of the current return path to the matching network 30. The matching network 30 incorporates variable capacitors controlled by servomotors which are operated so that the impedance presented to the generator 32 remains at a constant value such as 50 Ω.

In use of the apparatus 10, a tray 14 containing a food product 12 is located into the aperture 20. A film 34 of suitable polymeric material is placed on top of the tray 14, and the upper die 24 is lowered so that the film 34 and the rim 16 are sandwiched between the barrier layers 22 and 28 on the dies 18 and 24. The generator 32 is then activated (for example for 1.5 seconds), such that the film 34 is welded to the rim 16 of the tray 14. The upper die 24 is then lifted up, and the sealed tray 14 bonded to the film 34 is removed. The movements of the upper die 24 are indicated by the double headed arrow P. It will thus be appreciated that in operation the dies 18 and 24 act as electrodes, and they will be referred to as such in the context of the electrical operation.

Considering the electrical connections in more detail, the generator 32 is connected to the matching network 30 by a coaxial cable 38 with an impedance of 50 Ω (to match the output impedance of the generator 32), and the matching network 30 is connected to the conductor 36 by another coaxial cable 39. The conductor 36 is itself of coaxial construction, consisting of two concentric bellows 40 and 42 described in more detail below. The outer bellows 42 is electrically connected to the outer conductor of the coaxial cable 39, and to the lower electrode 18, so as to form part of the return path.

The bellows 40 and 42 in this example are each made of edge-welded flat metal rings, so that each are good conductors; alternatively one or both of the bellows 40 and 42 might be made of a tube of metal whose wall is deformed into a wavy shape. The inner bellows 40 is fixed to the upper electrode 24, and is sufficiently flexible to accommodate its motion P. The outer bellows 42 terminates close to the upper electrode 24, 12 mm above it in this example, being attached to a plate 43 (shown only in part) which defines a hole of the same diameter as the inner diameter of the bellows 42, this plate 43 being 12 mm above the top of the electrode 24 when the electrode 24 is in its raised position. The space between the bellows 40 and 42 is empty, apart from air. The inner bellows 40 is of external diameter sufficiently large to carry the feed current required.

For example the external diameter of the inner bellows 40 may be 65 mm; in this case the outer bellows 42 would be of internal diameter 149.5 mm if the conductor 36 is to have a characteristic impedance of 50 Ω. In this example the inner bellows 40 is 120 mm long when extended, and 60 mm long when compressed.

In this example the plate 43 is electrically connected to the lower electrode 18 (as indicated by 27), and during operation the plate 43 may be moved vertically (in addition to the movement P of the upper electrode 24) . The outer bellows 42 accommodates such vertical movements .

It will be appreciated that the impedance Z of the conductor 36, considered as a transmission line, depends upon the ratio of inductance L to capacitance C of the conductors per unit length, in accordance with:

Taking the radii of the inner and outer conductors as a and b, then

Z = (V(μ_o/ε_o)) (l/2π) (ln(b/a))

in which εO is the permittivity of vacuum and μO is the permeability of vacuum.

In this situation there is no dielectric material (apart from air) between the conductors. If we wish to have an impedance that is matched to the impedance of the generator, i.e. 50 Ω, then the ratio b/a must equal 2.3.

If no such characteristic impedance is required, then the ratio can have other values, providing that the distance between the outer diameter of the inner bellows 40 and the inner diameter of the outer bellows 42 is sufficiently large to ensure that dielectric breakdown of the air between them does not occur.

It will be appreciated that the apparatus 10 may be modified in various ways while remaining within the scope of the present invention. In particular, the electrical circuitry and the use of the bellows are not specific to welding food trays; they would be applicable in any power RF apparatus, and in particular in any dielectric welding apparatus in which one or both of the electrodes have to move apart and then together. The shape of the electrodes would depend upon the task that is to be achieved. Where a dielectric barrier 22 or 28 is provided on the electrodes, this may be of a different thickness, and of a different material.

In one modification the coaxial cable 39 may be omitted, the bellows 40 and 42 being connected directly to the output of the matching network 30. In some situations the outer bellows 42 may be omitted; in this case the external shielding conductor of the coaxial cable 39, or the current return path to the matching network 30 if there is no coaxial cable 39, would be connected directly or indirectly to the lower electrode 18. In situations where there is no requirement for the outer tube to be of variable length, the outer bellows 42 may be replaced by a cylindrical tube of conducting material. Referring now to figure 2 (in which identical components are referred to by the same reference numerals) , in another modification the electrical connection to the upper die 24 is made through a conductor 46, again of coaxial construction. The conductor 46 consists of two concentric tubes 50 and 52 each of which is partly in the form of a cylindrical tube, and partly in the form of bellows. The inner tube 50 has a cylindrical part 53 above a bellows part 54, whereas the outer tube 52 has a bellows part 55 above a cylindrical part 56. As in the apparatus of figure 1, the bellows parts 54 and 55 are each of edge-welded beryllium-copper discs, so that each are good conductors; alternatively one or both of the bellow parts 54 and 55 might instead be made of a tube of beryllium-copper (or another suitable good conductor) whose wall is deformed into a wavy shape. The bellows part 54 of the inner tube 50 is fixed to the upper electrode 24, and is sufficiently flexible to accommodate its motion P; the cylindrical part 53 is longer then the maximum length of the bellows part 52 during operation. The outer tube 52 is attached to the plate 43. The space between the tubes 50 and 52 is empty, apart from air. The inner tube 50 is sufficiently large to carry the feed current required. The conductor 46 is thus arranged such that each of the tubes 50 and 52 is of variable length, yet the bellows parts 54 or 55 of one are always adjacent to cylindrical parts 53 or 56 of the other.

Referring now to figure 3 (in which identical components are again referred to by the same reference numerals) , in another modification to the apparatus 10 of figure 1, the outer conductor is in the form of bellows 42, and this carries the return current. The inner conductor 60, which carries the live feed, consists of two end plates 62 each of dodecagonal shape and of width 50 mm to which are fixed twelve rectangular foil strips 64 of beryllium copper, each 12 mm wide and 150 mm long, and of thickness 0.1 mm. When the inner conductor 60 is at its maximum length the strips 64 are almost straight, with a slight bend at their mid points, so that the overall shape of the conductor 60 is a hollow dodecagonal tube with a bulge at its midpoint, and with narrow gaps between adjacent foil strips 64. When the inner conductor 60 becomes shorter, the strips 64 bend outwardly more at their mid points and become slightly further apart.

It will be appreciated that this conductor 60 is given by way of example only, and may be modified in various ways . It will be appreciated that the material of which the strips 64 are made should be both resilient and a good electrical conductor, but that various different materials may be used. The end plates might be of a different polygonal or circular shape, for example the polygon might be one having between four and twenty sides, and the number of sides of the polygon is preferably equal to the number of strips . The strips are preferably flat in cross-section, at least in the section which has to undergo bending. For example the strips might be joined to each other to form a short length of cylindrical tube at each end, these cylindrical sections being linked by the middle sections of the strips . Indeed, as shown a figure 4, a conductor of this sort may be made from a thin-walled tube 70 of a suitable material provided with slits 72 along its length, the sections between the slits 72 defining strips 74, and being bent outwardly near their midpoint.

Clearly the strips 64 or 74 must be shaped so that they can readily flex to accommodate changes in length of the conductor, but they may be bent in a variety of different ways. If adjacent strips are sufficiently far apart they may be bent inwardly rather than outwardly. If there is an even number of strips then strips may be bent alternately inwardly and outwardly near their mid points; alternatively each strip may be bent inwardly near one end and outwardly near the other end, adjacent strips having inward bends at opposite ends. It will also be appreciated that the strips need not necessarily be rectangular: the only requirement is that they should extend from one end of the conductor to the other, and that they can readily bend. As shown in figure 5, for example, an alternative conductor 80 comprises two polygonal end plates 82 (hexagonal in this case) linked by flexible strips 84 which extend along part-helical paths between the end plates 82, and which bend outwardly around their middle.

Referring now to figure 6, an alternative conductor 90 consists of two cylindrical tubes 92 and 93 of different diameters, which are linked by flexible strips 94. Each strip 94 is bent into a U-shape 95 at an intermediate position along its length, and its ends are welded to the inside of the wider tube 92 and to the outside of the narrower tube 93 respectively. It will be appreciated that the conductor 90 is generally tubular; as the conductor 90 changes length, the position of the bend 95 in each strip 94 moves along the strip 94. In a modification, the strips 94 are not welded to the tube 93 but make contact with it through sliding contacts (not shown) .

Claims

1. An apparatus incorporating means to feed radio frequency power, the feed means incorporating electrical conductors connected to electrical terminals at least one of which is movable, wherein an electrical conductor connected to at least one of the electrical terminals comprises a tubular structure of variable length of an electrically-conducting material .

2. An apparatus as claimed in claim 1 wherein the apparatus is an apparatus for dielectric heating or welding of a material, the apparatus comprising an opposed pair of electrodes between which is a gap, such that the material to be heated or welded may be placed in the gap between the electrodes, wherein the electrodes are connected to the said electrical terminals.

3. An apparatus as claimed in claim 1 or claim 2 wherein the tubular structure of variable length is a bellows.

4. An apparatus as claimed in any one of the preceding claims wherein the feed means comprises a coaxial tubular structure, at least the inner tubular structure being of variable length.

5. An apparatus as claimed in claim 4 wherein the both the inner and outer tubular structures are bellows of variable length.

6. An apparatus as claimed in claim 1 or claim 2 wherein the tubular structure of variable length comprises a multiplicity of flexible strips .

7. An apparatus as claimed in claim 1 or claim 2 wherein the feed means comprises a coaxial tubular structure in which the inner tubular structure comprises a multiplicity of flexible strips, and the outer tubular structure comprises a bellows.