US3189722A - Microwave oven apparatus - Google Patents

Description

5 415 D 0 @mgg ga ggwggmm mm Jiine 15,1965

INVENT .KARL FRITZ" BY ATTORNEYS June 15, 1965 Tz 3,189,722

- MICROWAVE OVEN APPARATUS Filed Sept. 21, 1962 2 Sheets-Sheet 2 v 0 0 //42 Z o I 0 4 1p /5 24 /Zb' F I G. 8 /2 Z5 INVENT KARL FRI BY Wa- L,M4#M

ATTORNEYS United States Patent 3,189,722 MICROWAVE OVEN APPARATUS Karl Fritz, Freibnrg im Breisgau, Germany, assignor to MIWAG Mikrowellen Aktien Gesellschaft, a corporation of the Swiss Confederation Filed Sept. 21, 1962, Ser. No. 225,225 2 Claims. (Cl. 219-1055) The present invention relates in general to heating by electrical energy and more particularly to ovens utilizing microwave energy for heating purposes.

In order to obtain an even distribution of heat through an object or material heated by microwave energy, it is common practice to provide apparatus for ensuring a regular distribution of microwave energy in a metallic enclosure and to compensate for residual inequalities therein. In the past, devices such as a rotating radiator or reflector, sometimes termed a mode mixer, have been used to cause the standing wave pattern set up by the microwave field to move over the object being heated while devices such as a rocking or rotating support or a conveyor belt or chain have been used to move the heated object in the microwave field.

In practice, it has been found, where the microwave energy in the heating enclosure assumes a stationary and regular pattern, that causing relative movement of the field and the object is not enough to give a sufficiently even rate of heat-ing throughout the volume of some substances. This is especially true With electrically nonisotropic substances. For example, frozen organic materials (e.g., frozen foods) usually consist of an agglomeration of irregularly oriented ice crystals and particles of other matter. The electrical propert es (absorption factor, loss angle, dielectric constant) and other physical characteristics, such as thermal conductivity, of certain electrically non-isotropic substances are different from particle to particle and variable to a marked degree with temperature during treatment when the substances are changes from the frozen to the thawed state, or during the process of desiccation. Moreover, some substances have properties which vary within the particles themselves.

Even in a microwave field of homogeneous amplitude distribution where relative movement of the field and object is utilized in order to equalize heating, there can still be unequal distribution of the rate of thawing when certain materials are treated, because the rate of heating of diiferent parts of the material will depend to a considerable degree on the plane of polarization of the microwave field to which they are subjected. Thus, when heating is to be carried out in a short time by use of a high microwave power level, hot spots may develop in the material at a time when other parts of same are still cold. For certain materials, this result is of little practical importance. For example, deep-frozen raw foods are customarily thawed and cooked in one continuous operation, and as they are kept at the boiling point much longer than the time spent in thawing, heat conduction within the material equalizes final temperature soon enough for all parts to be adequately done.

In other situations, such as in industrial and scientific applications, hot spots resulting from inequalities in the rate of thawing cannot be tolerated. For example, in the case of many chemical products and biological Patented June 15, 1965 materials, no part of the material must, at any time during treatment, even temporarily be heated above the temperature, which is often quite low, at which chemical decomposition, inactivation of ferments or antibodies, denaturizing of proteins, or other deleterious act on, can occur.

The principal object of the present invention is to obtain an equal rate of heating throughout an object heated in a microwave oven. An advantage of the present invention is that uniform heating is achieved in materials which are non-homogeneous or anistr-opic or both, wherein the heating effect produced in the material depends not only on the distribution of the microwave field but also on the plane of polarization of the incident microwave energy. Further, relative movement of the microwave field pattern and the object being heated serves not only to average the field amplitude in which the object is exposed over the time of heating, but such relative movement also serves at the same time to expose the object to waves of a different polarization so that different responses of different parts of the object to waves polarized in different planes are averaged out as well. By employing the invention, non-homogeneous objects are heated as evenly as homogeneous objects, even where the heating period is so short that heat conduction within the material cannot play an important role in equalizing temperature. A power level which might cause hot spots in non-isotropic materials with conventional apparatus may be used without detrimental effects in apparatus constructed according to the invention; therefore, when large quantities of foods are to be heated in a short interval, as in .a restaurant during peak business hours, valuable time can be saved. Finally, in the case of anistropic or non-homogeneous materials which are sensitive to rapid heating to the extent that the permissible power level with conventional apparatus is so low as to render microwave heating parctically useless, by employing equipment and procedures according to the invention, enough power can be used to heat such mateterials rapidly without special regard to the r physical and electrical configuration.

The invention resides in an apparatus having a metallically confined heating chamber into which microwave energy is fed. A pair of polarizing grids are arranged perpendicular to the direction of wave propagation at a distance from the front and rear walls of the chamber corresponding to a quarter wavelength of the energy of the enclosure. Further, the planes of polarization of the grids are at right angles to each other. The incoming wave energy is split into two orthogonally polarized components, one component setting up a pattern of standing waves between one wall and the opposing grid from which it is reflected and the other component an identical pattern between the other wall and the other grid. As both patterns are at right angles to each other, there is no mutual interference between the two polarized wave patterns, and as they are displaced axially by a quarter wavelength, a maximum of one pattern coincides with the minimum of the other. The wave pattern of the mutual interpenetration area of the heating chamber is not only subject to periodic variations of field strength, but also to periodic changes in polarization, so that an object in the chamber is heated evenly. In one embodiment of the invention, microwave energy is fed into the heating enclosure through a horizintal coupling slot, at an angle of 45 to the polarizing grids. In another embodiment of the invention, the energy is fed into the heating enclosure through a pair of slots whose planes of polarization are perpendicular to each other, each slot being parallel to one of the polarizing grids. In a further embodiment of the invention, energy is fed into the heating enclosure by means of a half-dipole antenna. The two polarizing grids are rotated synchronously. As the grids rotate, they alternately reflect or pass polarized radiation from the antenna so that a field pattern is alternately set up between one of the walls and one of the grids. In still another embodiment of the invention, the plane of polarization of the field is moved by means of a rotating antenna, alternately setting up field patterns between one of the walls and one of the grids.

Referring now to the drawings,

FIG. 1 is a schematic representation of a heating chamber having polarizing grids adjacent its end walls;

FIGS. 2 and 3 show the standing wave patterns set up in the heating chamber by the two orthogonally polarized components of the microwave energy;

FIG. 4 shows the resultant distribution of microwave energy in the heating chamber;

FIG. 5 schematically depicts an embodiment of the in vention in which wave energy is fed into the working space within the chamber through orthogonally disposed coupling slots;

FIG. 6 depicts a type of polarizing grid which may be used in the invention;

FIG. 7 illustrates another embodiment of the invention in which polarizing grids in a working space are continuously rotated; and

FIG. 8 depicts still another embodiment of the invention wherein an antenna feeding wave energy into a working space is continuously rotated.

Referring now to FIG. 1, which depicts en ambodiment of the invention in schematic form, there is shown a metallically closed heating chamber 2 of an oven into which microwave energy from a Waveguide 4 is fed through a radiating element such as a horizontal extending slot 6 in the end wall of the chamber. The polarization of the wave emanating from the slot 6 is perpendicular to the plane in which the length of the slot 6 extends. A pair of polarizing grids 8 and 12 are arranged perpendicularly to the direction of wave propagation in the heating chamber at a distance from the front and rear walls thereof, corresponding to a quarter wavelength of the energy in the enclosure. The planes of polarization of the grids 8 and 12 are at right angles to each other and at an angle of 45 to the slot 6. The incoming wave energy is split into two components whose planes of polarization are at right angles to each other, one component setting up the standing wave pattern depicted in FIG. 2 between one wall and the grid from which it is reflected and the other component setting up a similar pattern depicted in FIG. 3 between the other wall and the other grid. As each pattern is constituted by wave energy which is polarized at right angles with respect to the Wave energy of the other pattern, there is no mutual interference between the two polarized Wave patterns, and as they are displaced axially by a quarter Wavelength, a maximum of one pattern coincides with the minimum of the other pattern.

FIG. 4 shows the microwave energy distribution, which is the resultant of the wave patterns of FIGS. 2 and 3, in the mutual interpenetration area which is the actual working space of the enclosure 2. An object moved in this area is not only subject to periodic variations of field strength, but also to periodic changes in polarization, so that the object is heated evenly in all of its parts, regardless of any direction of electrical predilection; viz., electrical anisotropy.

Referring now to FIG. 5, there is shown an alternative embodiment of the device of FIG. 1. In the apparatus of FIG. 5, the microwave energy available from waveguides 22 and 24 is fed into the heating enclosure through a pair of radiating elements, such as slots 26 and 28, Whose planes of energy polarization are at right angles to each other. Slot 26 coincides with and is, preferably, an aperture in end wall 30; its radiation passes freely through grid 32, the bars of which are set at right angles to the plane of polarization of the energy emanating from slot 26, that is, parallel to the length of the slot 26. Slot 28, which is at right angles to slot 26, lies in the plane of the grid 32. The latter grid, as well as grid 34, are mounted one-quarter wavelength from the end walls of the chamber. Grid 34, the bars of which are set at right angles to those of grid 32, allows the radiated polarized energy from slot 28 to pass, but reflects the radiated energy from slot 26. The device of FIG. 5 operates so that the two separate interpenetrating field patterns are set up as in FIGS. 2 and 3, each offset by a quarter wavelength so that the maxima of one standing wave falls on the minimum of the other standing wave, while their planes of polarization are at right angles to each other. The arrangement of FIG. 5 allows a somewhat more even distribution of field energy than the device of FIG. 1.

In the embodiments of FIGS. 1 and 5, one of the ends of the enclosures, preferably, is the door of the oven, and the door is made approximately a quarter wavelength deep so that it carries the adjacent grid. In the device of FIG. 1, the grid 12 and the portion of the chamber to its right is hinged to swing downwardly as a unit to form the ovens door, whereas in the device of FIG. 5 the grid 34 and the portion of the chamber to its right is similarly hinged to provide a door for that embodiment. In this manner, when the door is opened, the grid swings with the door so that access to the interior of the oven is unhindered.

Referring now to FIG. 6, there is shown a ditfcrent type of grid construction which can be used to form the grids 12 and 34 in the embodiments of FIGS. 1 and 5. When higher order modes occur in the heating chamber of the devices of FIGS. 1 and 5, the subdivision of the grids is made according to the mode which carries a greater amount of energy. The simple grids shown in FIG. 5 are used with the fundamental or lower order mode. FIG. 6 indicates a grid 42 intended to be used with a higher order mode.

As previously discussed, the invention contemplates including provisions for relative movement of the object heated in the oven and the field patterns therein. While mechanical movement of the object may be used to obtain even heating since the loci of equal field intensity and corresponding polarization would be parallel to the three dimensions of the enclosure, best results are obtained when the object is moved in a direction which is not parallel to any of the three spatial coordinates of the enclosure, in a way sometimes referred to as wind-blown. The actual mechanical arrangement most suitable in a given case is described in copending application No. 225,222. On the other hand, methods of moving the field pattern about a stationary object is thought to be the most suitable way and are contemplated by the invention. One obvious way is to move the polarization grids; another, to move the radiating elements; a third, to move thed end wall surfaces which reflect energy back to the gri s.

FIGS. 7, 7a, and 7b depict an embodiment of the invention wherein the polarizing grids are continuously rotated. In this embodiment, the metallic enclosure 52 is a cylinder. Microwave energy is fed into the enclosure 52 from a generator 54 by means of a half dipole antenna 56 At a distance of a quarter wavelentgh from the end faces 58 and 62 of the cylindrical enclosure are mounted polarizing grids 64 and 66, the bars of which are perpendicular to each other. The two grids 64, 66 are mounted on a pair of gears 68, 72, respectively. The gears 68, 72

mesh with a second pair of gears 74, 76, respectively. In order to rotate the grids 64, 66 synchronously, a motor 78 drives a pair of shafts 82, 84. The shaft 82 is connected through a bearing 86 to the gear 74.

The end portion 88 of enclosure 52 containing the grid 66 is slidably positioned as can be seen by the dotted lines in FIG. 7, thus allowing access to the enclosure 52. The shaft 84 passes through a bearing 92 and is connected to a fork 93. The gear 70 is connected to the fork 93 by means of a rod 94 which contains a pin 96 at one end thereof. The pin 96 is movable in a slot 98 cut in the fork. The end portion 88 is pivotally mounted on brackets 100 so that the end portion may be supported when opened. As the grids rotate, they will alternately reflects or pass the polarized radiation from the antenna so that a field pattern is alternately set up between wall 58 and grid 66, or wall 62 and grid 64. Not only is the field thus moved continually from left to right and vice versa, but its plane of polarization will vary through 90 at the same time so that the object being heated in the oven is exposed to microwave fields of varying intensity and polarization, averaging over the time of treatment. Thus, the device of FIG. 7 provides even heating through all parts of the object and in all directions, even in the presence of pronounced standing wave formation. It will be noted that, while the field pattern is moved, the electrical length of the enclosure remains unchanged, an obvious advantage when designing for a particular configuration of field patterns.

Referring now to FIG. 8, there is shown an embodiment for obtaining a moving field with variation of the plane of polarization. In this embodiment, a pair of polarization grids 112 and 114 are stationary, and the plane of polarization of the field is moved by means of a rotating antenna 116. The polarizing grids 112 and 114 are mounted a quarter wavelength away from opposite end walls 118 and 122 of the enclosure 124 and have their polarizing elements at right angles to each other. The enclosure is cylindrical in shape, one of its end faces constituting a door such as end wall 122 to which the grid 114 is fixed. The member 122 is pivotally mounted on a bracket 126 and contains a flange 128 which mates with flange 132. The antenna consists of an L-shaped rod 134 which forms one dipole element, and at the same time, the coupling member to a waveguide 136 onto which microwave energy is fed from a generator 138, and a second L-shaped rod 142, one end of which is mechanically and electrically connected to a metal disk 144. The two rods 134 and 142 are secured together by an insulating member 146. The disk 144 is insulated from the wall of waveguide 136, thereby forming a grounding capacitor. Rod 134 carries an insulating extension 148, which is rotated mechanically, for example, by means of a driving pulley 152. A characteristic feature of the arrangement of FIG. 8 is that the antenna rotates inside a cylindrical waveguide cavity 153 which is coaxial with the cylindrical wall working enclosure 124.

The operation of the device of FIG. 8 is as follows: As the antenna is rotated, field patterns are alternately set up between polarizing grid 112 and end wall 122, or polarizing grid 114 and end wall 118. The working space between the two grids is thus the interpenetration area of two similar field patterns, offset by a quarter wavelength, moving back and forth with continuous variation of the direction of polarization. The object being heated is thus exposed, over the time of treatment, to microwave energy of varying amplitude and polarization. Thus, inequalities in heating due to anisotropy in the material are averaged out. As can be readily seen, the electrical length of the enclosure 124 is held constant.

The arrangements described provide polarization only in a plane symmetrical about the longitudinal axis of the enclosure so that additional provision would seem necessary in order to obtain field pattern polarization in all directions. It is found, however, that higher modes of 6 oscillation occur in the enclosure of the usual dimensions. These modes have longitudinal field components, the total intensity of which is suflicient to insure that the object is exposed to substantially equal amounts of microwave energy polarized in every direction.

It is to be understood that although thawing of frozen foods is the application most frequently referred to in the present application, this is merely because it is the problem most frequently encountered. The invention is, obviously, useful for any product in which the material to be heated is, or becomes, anisotropic during any stage of treatment, such as freeze-drying, vacuum drying, crystallization from liquids, melting of crystals, treatment of Wood and fibers, drying of emulsions or suspensions, curing of chemical mixtures, and numerous others.

It is also obvious that apparatus according to the invention is by no means restricted in its application to treatment of anisotropic materials, but can also be used equally well for heating any kind of material, homogeneous or not, to any desired temperature. In other words, While conventional apparatus may constitute a good heating oven, but not be quite satisfactory when used for thawing, apparatus incorporating the characteristic features of the invention will constitute a most satisfactory heating oven, and in addition, be able to handle frozen foods much better than ordinary equipment. The invention is thus of universal application in extending the range of usefulness of any kind of microwave heating apparatus.

Obviously, many other modifications and variations of the present invention are possible in the light of the foregoing teachings. It is to be understood, therefore, that the invention is not limited in its application to the details of construction and arrangement of parts specifically described or illustrated, and that within the scope of the appended claims, it may be practiced otherwise than as specifically described or illustrated.

I claim:

1. In a microwave oven of the type having:

an enclosure for confining microwave energy, the enclosure having a pair of oppositely disposed wave energy reflecting walls;

a source of microwave energy;

apparatus for coupling energy from the source to the enclosure;

and a pair of polarizing grids in the enclosure, each grid reflecting wave energy polarized in a first plane and being transparent to wave energy polarized in a second plane perpendicular to the first plane; the grids being disposed at right angles to one another, the grids being spaced from the reflecting walls to cause the establishment in the enclosure of at least two, non-coincident, similar, standing wave patterns; the improvement comprising:

an antenna forming a part of the apparatus for coupling energy from the source to the enclosure, the antenna being of the type radiating linearly polarized waves;

means for rotating the antenna during operation of the oven;

and means for causing radiation from the antenna to be directed toward the polarizing grids.

2. In a microwave oven of the type having:

an enclosure for confining microwave energy, the enclosure having a pair of oppositely disposed wave energy reflecting walls;

a source of microwave energy;

apparatus for coupling energy from the source to the enclosure;

and a pair of polarizing grids in the enclosure, each grid reflecting wave energy polarized in a first plane and being transparent to wave energy polarized in a second plane perpendicular to the first plane, the grids being spaced from the reflecting walls to cause the establishment in the enclosure of at least two non-coincident similar standing wave patterns;

the improvement comprising:

an antenna for radiating polarized waves, the antenna forming a part of the apparatus for coupling energy from the source to the enclosure, the antenna being situated between the two polarizing grids whereby polarized wave energy is simultaneously directed toward the two grids;

and means for synchronously rotating the polarizing grids.

References Cited by the Examiner UNITED STATES PATENTS Wolff 343786 Blass et a1 219-10.55 Weil 343-756 Guanella 21910.55 Donnellan et al 343756 RICHARD M. WOOD, Primary Examiner.

Claims (1)

1. IN A MICROWAVE OVEN OF THE TYPE HAVING: AN ENCLOSURE FOR CONFINING MICROWAVE ENERGY, THE ENCLOSURE HAVING A PAIR OF OPPOSITELY DISPOSED WAVE ENERGY REFLECTING WALLS; A SOURCE OF MICROWAVE ENERGY; APPARATUS FOR COUPLING ENERGY FROM THE SOURCE TO THE ENCLOSURE; AND A PAIR OF POLARIZING GRIDS IN THE ENCLOSURE, EACH GRID REFLECTING WAVE ENERGY POLARIZED IN A FIRST PLANE AND BEING TRANSPARENT TO WAVE ENERGY POLARIZED IN A SECOND PLANE PERPENDICULAR TO THE FIRST PLANE; THE GRIDS BEING DISPOSED AT RIGHT ANGLES TO ONE ANOTHER, THE GRIDS BEING SPACED FROM THE REFLECTING WALLS TO CAUSE THE ESTABLISHMENT IN THE ENCLOSURE OF AT LEAST TWO, NON-COINCIDENT, SIMILAR, STANDING WAVE PATTERNS; THE IMPROVEMENT COMPRISING: AN ANTENNA FORMING A PART OF THE APPARATUS FOR COUPLING ENERGY FROM THE SOURCE TO THE ENCLOSURE, THE ANTENNA BEING OF THE TYPE RADIATING LINEARLY POLARIZED WAVES; MEANS FOR ROTATING THE ANTENNA DURING OPERATION OF THE OVEN; AND MEANS FOR CAUSING RADIATION FROM THE ANTENNA TO BE DIRECTED TOWARD THE POLARIZING GRIDS.

Images