EP0277342B1 - Method and device to homogenize the heating of food in a cooking oven heated by microwaves - Google Patents

Method and device to homogenize the heating of food in a cooking oven heated by microwaves Download PDF

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Publication number
EP0277342B1
EP0277342B1 EP87118985A EP87118985A EP0277342B1 EP 0277342 B1 EP0277342 B1 EP 0277342B1 EP 87118985 A EP87118985 A EP 87118985A EP 87118985 A EP87118985 A EP 87118985A EP 0277342 B1 EP0277342 B1 EP 0277342B1
Authority
EP
European Patent Office
Prior art keywords
rotating element
rear wall
wave guide
microwaves
cooking chamber
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
Application number
EP87118985A
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German (de)
French (fr)
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EP0277342A3 (en
EP0277342A2 (en
Inventor
Gerd Dr. Naumann
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Electrolux Juno Kuchentechnik GmbH
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Electrolux Juno Kuchentechnik GmbH
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Publication date
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Priority to AT87118985T priority Critical patent/ATE91375T1/en
Publication of EP0277342A2 publication Critical patent/EP0277342A2/en
Publication of EP0277342A3 publication Critical patent/EP0277342A3/en
Application granted granted Critical
Publication of EP0277342B1 publication Critical patent/EP0277342B1/en
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    • 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
    • H05B6/64—Heating using microwaves
    • H05B6/70—Feed lines
    • H05B6/707—Feed lines using waveguides
    • 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
    • H05B6/64—Heating using microwaves
    • H05B6/74—Mode transformers or mode stirrers
    • H05B6/745—Rotatable stirrers

Definitions

  • the invention relates to a microwave oven according to the preamble of the main claim and a method for operating a microwave oven according to the preamble of claim 4.
  • the microwaves generated by a magnetron are fed into a cooking chamber which acts as a resonance chamber and is enclosed by the metallic walls of the cooking muffle. It is possible to feed the magnetron output probe directly into the cooking space.
  • a cooking chamber which acts as a resonance chamber and is enclosed by the metallic walls of the cooking muffle. It is possible to feed the magnetron output probe directly into the cooking space.
  • rotary antennas which move the microwave field relative to the food to be cooked at rest.
  • the direct feeding of the milk waves into the cooking space causes great difficulties for the magnetron to adapt to different loading of the cooking space, such a construction is very complex. Because of the different resonance structure of the cooking chamber, there is also an uneven heating.
  • a microwave oven for uniform heating of food which has a microwave generator and an elongated waveguide leading to the cooking space.
  • the waveguide is closed at its end facing away from the microwave generator with a rear wall arranged perpendicular to a coupling opening between the waveguide and the cooking space.
  • a rotating body is arranged in the waveguide.
  • a microwave oven which is also provided with a rotating body in the waveguide.
  • the rotating body consists of a pin and a rectangular loop attached to the pin. Due to this special design of the rotating body, the phases of the incident wave and the wave reflected from the rear wall are shifted against each other.
  • the object of the present invention is to improve a microwave oven of the type mentioned and a method such that a simple and effective coupling of the microwaves into the cooking space is achieved and at the same time the material to be heated can be homogeneously heated due to a uniform field distribution.
  • the microwave oven according to the invention is designed in the manner characterized in the main claim.
  • the method for solving the problem is characterized in claim 4.
  • the undesired change in the load impedance of the magnetron with different loading conditions is used to bring about a change in the frequency of the magnetron and thereby achieve a uniform heating of the food.
  • the periodic change in the wave resistance results in a periodic change in the output frequency of the magnetron and this change in frequency causes the excitation of different forms of resonance in the cooking chamber serving as a resonator.
  • the superimposition of different forms of resonance leads to an overall uniform heating result.
  • the waveguide 3 is arranged between the magentron 1 generating the microwaves and the cooking chamber 2.
  • the rotating body 5 In front of the rear wall 4 of the end of the waveguide 3 facing away from the magnetron, the rotating body 5 is arranged, which consists of a plate running parallel to the rear wall and rotatable about its central longitudinal axis.
  • the plate is made of an electrically conductive or dielectric material.
  • the wave resistance of the magnetron 1 through the waveguide 3 via the coupling opening 6 into the cooking space can be changed periodically by the rotating body 5. This change in the input resistance of the furnace affects the magnetron 1 by periodically changing its output frequency.
  • the change at an output frequency of 2.450 GHz can be up to ⁇ 10 MHz, which results in a superposition of different resonance forms. This leads to an even heating of the food.
  • the effect of the device according to the invention is not based on the deflection of the microwaves by a complicated "wave stirrer", but on the "change in length" of the waveguide, which couples the microwaves into the cooking space. This is done using a simple sheet metal part. While the waveguide is shortened in the position of the plate of the rotating body shown, this plate is practically ineffective in the dashed position. When the plate rotates in the direction of the arrow, the wave resistance changes constantly and periodically.
  • the coupling opening 6 is covered in a known manner by a wall 7 which is permeable to microwaves and which prevents harmful influences from the cooking chamber from acting on the magnetron.
  • a wall 7 which is permeable to microwaves and which prevents harmful influences from the cooking chamber from acting on the magnetron.
  • High-temperature resistant dielectric materials such as ceramics or borosilicate glass are common for this.
  • the rotating body 5 has a good effect if it is located at a distance between ⁇ / 4 and ⁇ / 2 from the rear wall 4 and above the coupling opening 6. Good results are achieved in particular if the rotating body 5 is arranged at a distance of 3 ⁇ 8 ⁇ from the rear wall.

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  • Physics & Mathematics (AREA)
  • Electromagnetism (AREA)
  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Constitution Of High-Frequency Heating (AREA)
  • Electric Ovens (AREA)
  • General Preparation And Processing Of Foods (AREA)

Abstract

The known methods for homogenising the field distribution in a cooking space heated by microwaves, such as rotating antennas, rotating plates and wave stirrers, are relatively expensive. The characteristic impedance is periodically changed by means of a rotating element (5) arranged in the waveguide (3) between the magnetron (1) and the injection aperture (6) into the cooking space (2). In this way, the output frequency of the magnetron is also changed, resulting in homogenisation of the field. <IMAGE>

Description

Die Erfindung betrifft einen Mikrowellenofen nach dem Oberbegriff des Hauptanspruches sowie ein Verfahren zum Betrieb eines Mikrowellenofens nach dem Oberbegriff von Anspruch 4.The invention relates to a microwave oven according to the preamble of the main claim and a method for operating a microwave oven according to the preamble of claim 4.

Bei einem Mikrowellenofen werden in einen als Resonanzkammer wirkenden Garraum, welcher von den metallischen Wänden der Garmuffel umschlossen wird, die von einem Magnetron erzeugten Mikrowellen eingespeist. Es ist dabei möglich die Ausgangssonde des Magnetrons unmittelbar in den Garraum zu führen. Zur Verbesserung der Gleichförmigkeit der Erwärmung des Gargutes ist es bereits bekannt sogenannte Drehantennen vorzusehen, welche das Mikrowellenfeld relativ zum ruhenden Gargut bewegen. Abgesehen davon, daß die direkte Einspeisung der Milkrowellen in den Garraum große Anpassungsschwierigkeiten des Magnetrons an unterschiedliche Beschickung des Garraumes mit sich bringt, ist eine solche Konstruktion sehr aufwendig. Auch verbleibt wegen der unterschiedlichen Resonanzstruktur des Garraumes eine Ungleichmäßigkeit der Erwärmung.In a microwave oven, the microwaves generated by a magnetron are fed into a cooking chamber which acts as a resonance chamber and is enclosed by the metallic walls of the cooking muffle. It is possible to feed the magnetron output probe directly into the cooking space. In order to improve the uniformity of the heating of the food to be cooked, it is already known to provide so-called rotary antennas which move the microwave field relative to the food to be cooked at rest. In addition to the fact that the direct feeding of the milk waves into the cooking space causes great difficulties for the magnetron to adapt to different loading of the cooking space, such a construction is very complex. Because of the different resonance structure of the cooking chamber, there is also an uneven heating.

Es ist heute allgemein üblich, die Antennensonde des Magnetrons in eine Wellenleitung einzusetzen, wobei die Antennensonde im Abstand von einem Viertel der Wellenlänge von einem Kurzgeschlossenen Ende der Wellenleitung eingesetzt ist, so daß möglichst die gesamte Mikrowellenenergie in die andere Richtung ausgekoppelt wird. Das dem kurzgeschlossenen Ende der Wellenleitung gegenüberliegende Ende mündet im allgemeinen über eine Einkopplungsöffnung zwischen Wellenleiter und Garraum in die Garmuffel. Zur Vergleichmäßigung der Erwärmung ist es dabei üblich, in der Nähe der Einkopplungsöffnung einen sogenannten "Wellenrührer" anzuordnen. Die DE-PS 26 22 363 zeigt einen solchen Wellenrührer, dessen Rührflügel unter dem Magnetron aber über dem Garraum angeordnet und vom Garraum durch eine Wand getrennt sind. Eine ähnliche Ausführung zeigt die DE-OS 29 21 266 in Fig. 2. Die Wellenrührer haben die Aufgabe mit ihren sich drehenden Flügeln die Mikrowellen unterschiedlich zu reflektieren und so ein Streufeld in den Garraum einzuspeisen. Auch solche Konstruktionen sind aufwendig, ebenso wie die bekannten Drehteller, bei denen das Gargut in dem Garraum stetig bewegt wird, um eine gleichmäßige Erwärmung zu erzielen.It is now common practice to insert the antenna probe of the magnetron into a waveguide, the antenna probe being inserted at a quarter of the wavelength from a short-circuited end of the waveguide, so that as much of the microwave energy as possible is coupled out in the other direction. The end opposite the short-circuited end of the waveguide generally opens into the cooking muffle via a coupling opening between the waveguide and the cooking space. To make the heating more uniform, it is customary to arrange a so-called "wave stirrer" in the vicinity of the coupling opening. DE-PS 26 22 363 shows such a wave stirrer, the stirring blades of which are arranged under the magnetron but above the cooking space and are separated from the cooking space by a wall. A similar design is shown in DE-OS 29 21 266 in FIG. 2. The wave stirrers have the task of reflecting the microwaves differently with their rotating blades and thus feeding a stray field into the cooking space. Such constructions are also complex, as are the known turntables, in which the food to be cooked is constantly moved in the cooking space in order to achieve uniform heating.

Aus der US-A-4 336 434 ist ein Mikrowellenofen zur gleichmäßigen Erwärmung von Speisen bekannt, der einen Mikrowellenerzeuger und einen länglichen, zum Garraum führenden Hohlleiter aufweist. Der Hohlleiter ist an seinem dem Mikrowellenerzeuger abgewandten Ende mit einer senkrecht zu einer Einkopplungsöffnung zwischen Hohlleiter und Garraum angeordneten Rückwand abgeschlossen. Im Hohlleiter ist ein Drehkörper angeordnet.From US-A-4 336 434 a microwave oven for uniform heating of food is known, which has a microwave generator and an elongated waveguide leading to the cooking space. The waveguide is closed at its end facing away from the microwave generator with a rear wall arranged perpendicular to a coupling opening between the waveguide and the cooking space. A rotating body is arranged in the waveguide.

Aus der US-A-4 301 347 ist ein Mikrowellenofen bekannt, der ebenfalls mit einem Drehkörper im Hohlleiter versehen ist. Der Drehkörper besteht aus einem Stift und einer am Stift befestigten rechteckförmigen Schleife. Durch diese besondere Ausgestaltung des Drehkörpers werden die Phasen der einfallenden und der von der Rückwand reflektierten Welle gegeneinander verschoben.From US-A-4 301 347 a microwave oven is known which is also provided with a rotating body in the waveguide. The rotating body consists of a pin and a rectangular loop attached to the pin. Due to this special design of the rotating body, the phases of the incident wave and the wave reflected from the rear wall are shifted against each other.

Aufgabe der vorliegenden Erfindung ist es, einen Mikrowellenofen der eingangs genannten Art und ein Verfahren derart zu verbessern, daß eine einfache und wirkungsvolle Einkopplung der Mikrowellen in den Garraum erzielt und gleichzeitig das Erwärmungsgut aufgrund einer gleichmäßigen Feldverteilung homogen erwärmt werden kann.The object of the present invention is to improve a microwave oven of the type mentioned and a method such that a simple and effective coupling of the microwaves into the cooking space is achieved and at the same time the material to be heated can be homogeneously heated due to a uniform field distribution.

Zur Lösung dieser Aufgabe ist der erfindungsgemäße Mikrowellenofen in der in dem Hauptanspruch gekennzeichneten Weise ausgebildet. Das Verfahren zur Lösung der Aufgabe ist in Anspruch 4 gekennzeichnet. Die an sich unerwünschte Änderung der Belastungsimpedanz des Magnetrons bei unterschiedlicher Beschickungsbedingung wird dazu benutzt, eine Frequenzänderung des Magnetrons herbeizuführen und dadurch eine gleichmäßige Erwärmung des Gargutes zu erzielen.To achieve this object, the microwave oven according to the invention is designed in the manner characterized in the main claim. The method for solving the problem is characterized in claim 4. The undesired change in the load impedance of the magnetron with different loading conditions is used to bring about a change in the frequency of the magnetron and thereby achieve a uniform heating of the food.

Durch die periodische Veränderung des Wellenwiderstandes ergibt sich eine periodische Veränderung der Ausgangsfrequenz des Magnetrons und diese Frequenzänderung bewirkt in dem als Resonator dienenden Garraum die Anregung unterschiedlicher Resonanzformen. Die Überlagerung verschiedener Resonanzformen führt zu einem in der Summe gleichmäßigen Erwärmungsergebnis.The periodic change in the wave resistance results in a periodic change in the output frequency of the magnetron and this change in frequency causes the excitation of different forms of resonance in the cooking chamber serving as a resonator. The superimposition of different forms of resonance leads to an overall uniform heating result.

In der Zeichnung ist ein Ausführungsbeispiel der Erfindung dargestellt. Es zeigen:

Fig. 1
schematisch die Vorrichtung zur Mikrowelleneinspeisung
Fig. 2
den periodischen Verlauf der Eingangsimpedanz eines Mikrowellenofens im Smith-Diagramm.
In the drawing, an embodiment of the invention is shown. Show it:
Fig. 1
schematically the device for microwave feeding
Fig. 2
the periodic course of the input impedance of a microwave oven in the Smith chart.

Zwischen dem die Mikrowellen erzeugenden Magentron 1 und dem Garraum 2 ist der Hohlleiter 3 angeordnet. Vor der Rückwand 4 des dem Magnetron abgewandten Ende des Hohlleiters 3 ist der Drehkörper 5 angeordnet, welcher aus einer parallel zur Rückwand verlaufenden und um seine Mittellängsachse drehbaren Platte besteht. Die Platte ist aus einem elektrisch leitenden oder dielektrischen Material hergestellt. Der Wellenwiderstand der vom Magnetron 1 durch den Hohlleiter 3 über die Einkopplungsöffnung 6 in den Garraum kann durch den Drehkörper 5 periodisch verändert werden. Diese Änderung des Eingangswiderstandes des Ofens wirkt auf das Magnetron 1 zurück indem dessen Ausgangsfrequenz periodisch verändert wird. Wie aus dem Diagramm nach Fig. 2 zu entnehmen ist, kann die Änderung bei einer Ausgangsfrequenz von 2,450 GHz bis zu ± 10 MHz betragen, wodurch sich eine Überlagerung verschiedener Resonanzformen ergibt. Dies führt zu einer gleichmäßigen Erwärmung des Gargutes. Die Wirkung der erfindungsgemäßen Vorrichtung beruht nicht auf der Umlenkung der Mikrowellen durch einen komplizierten "Wellenrührer", sondern durch die "Längenänderung" des Hohlleiters, der die Mikrowellen in den Garraum einkoppelt. Dies erfolgt durch ein einfaches Blechteil. Während in der gezeigten Stellung der Platte des Drehkörpers der Hohlleiter verkürzt ist, ist in der gestrichelten Stellung diese Platte praktisch nicht wirksam. Bei Drehung der Platte in Pfeilrichtung ändert sich somit ständig und periodisch der Wellenwiderstand. Die Einkopplungsöffnung 6 ist in bekannter Weise durch eine für Mikrowellen durchlässige Wand 7 abgedeckt, welche verhindert, daß aus dem Garraum schädliche Einflüsse auf das Magnetron einwirken können. Üblich sind hierfür hochtemperaturbeständige dielektrische Materialien wie Keramik oder Borsilikatglas.The waveguide 3 is arranged between the magentron 1 generating the microwaves and the cooking chamber 2. In front of the rear wall 4 of the end of the waveguide 3 facing away from the magnetron, the rotating body 5 is arranged, which consists of a plate running parallel to the rear wall and rotatable about its central longitudinal axis. The plate is made of an electrically conductive or dielectric material. The wave resistance of the magnetron 1 through the waveguide 3 via the coupling opening 6 into the cooking space can be changed periodically by the rotating body 5. This change in the input resistance of the furnace affects the magnetron 1 by periodically changing its output frequency. As from the diagram according to FIG. 2 can be seen, the change at an output frequency of 2.450 GHz can be up to ± 10 MHz, which results in a superposition of different resonance forms. This leads to an even heating of the food. The effect of the device according to the invention is not based on the deflection of the microwaves by a complicated "wave stirrer", but on the "change in length" of the waveguide, which couples the microwaves into the cooking space. This is done using a simple sheet metal part. While the waveguide is shortened in the position of the plate of the rotating body shown, this plate is practically ineffective in the dashed position. When the plate rotates in the direction of the arrow, the wave resistance changes constantly and periodically. The coupling opening 6 is covered in a known manner by a wall 7 which is permeable to microwaves and which prevents harmful influences from the cooking chamber from acting on the magnetron. High-temperature resistant dielectric materials such as ceramics or borosilicate glass are common for this.

Eine gute Wirkung des Drehkörpers 5 ist gegeben, wenn er sich im Abstand zwischen λ/4 und λ/2 von der Rückwand 4 und oberhalb der Einkopplungsöffnung 6 befindet. Gute ergebnisse werden insbesondere dann erzielt, wenn der Drehkörper 5 im Abstand von ⅜ λ von der Rückwand angeordnet wird.The rotating body 5 has a good effect if it is located at a distance between λ / 4 and λ / 2 from the rear wall 4 and above the coupling opening 6. Good results are achieved in particular if the rotating body 5 is arranged at a distance of ⅜ λ from the rear wall.

Claims (4)

  1. Microwave oven for the uniform heating of foodstuffs with an elongate wave guide (3) which leads from the microwave generator (1) to the cooking chamber (2) and is closed off at its end remote from the microwave generator (1) by a rear wall (4) which is arranged perpendicular to a coupling aperture (4), between the wave guide (3) and the cooking chamber (2), wherein a rotating element (5) which is periodically moved is arranged in the wave guide (3), characterised in that the rotating element (5) consists of a rotatable plate, which is produced from an electrically conductive or a dielectric material, in that the rotating element (5) is arranged at a distance of Lambda/4 to Lambda/2 from the rear wall (4) and screens this in a varying manner owing to its rotational movement, and in that the coupling aperture (6) is located under the rotating element (5).
  2. Device according to Claim 1, characterised in that the plate is rotatable about its horizontal central axis.
  3. Device according to either of Claims 1 and 2, characterised in that the rotating element is arranged at a distance of 3 x Lambda/8 from the rear wall (4).
  4. Method for the operation of a microwave oven for the uniform heating of foodstuffs, wherein the microwaves generated by a microwave generator (1) are conducted into the cooking chamber (2) via an elongate wave guide (3), which is closed off at its end remote from the microwave generator (1) by a rear wall (4) which is arranged perpendicular to a coupling aperture (6) and between the wave guide (3) and the cooking chamber (2), and the wave resistance of the microwaves conducted through the wave guide (3) to the cooking chamber (2) is altered by a periodically moved rotating element (5) arranged in the wave guide (3), characterised in that the microwaves are introduced directly into the cooking chamber via the coupling aperture (6) located below the rotating element (5) by the rotating element (5) which consists of a rotatable plate, which is produced from a dielectric or electrically conductive material, and in that the rotating element, which is arranged at a distance of Lambda/4 to Lambda/2 from the rear wall (4), screens the rear wall (4), in a varying manner as it rotates.
EP87118985A 1987-02-03 1987-12-21 Method and device to homogenize the heating of food in a cooking oven heated by microwaves Expired - Lifetime EP0277342B1 (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AT87118985T ATE91375T1 (en) 1987-02-03 1987-12-21 METHOD AND APPARATUS FOR EQUALIZING THE HEATING OF FOOD IN A MICROWAVE ENERGY HEATED COOKING APPLIANCE.

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
DE19873703133 DE3703133A1 (en) 1987-02-03 1987-02-03 METHOD AND DEVICE FOR COMPARISONING THE HEATING OF FOOD IN A COOKER HEATED WITH MICROWAVE ENERGY
DE3703133 1987-02-03

Publications (3)

Publication Number Publication Date
EP0277342A2 EP0277342A2 (en) 1988-08-10
EP0277342A3 EP0277342A3 (en) 1988-08-31
EP0277342B1 true EP0277342B1 (en) 1993-07-07

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Application Number Title Priority Date Filing Date
EP87118985A Expired - Lifetime EP0277342B1 (en) 1987-02-03 1987-12-21 Method and device to homogenize the heating of food in a cooking oven heated by microwaves

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AT (1) ATE91375T1 (en)
DE (2) DE3703133A1 (en)

Families Citing this family (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE3818490A1 (en) * 1988-05-31 1989-12-07 Bosch Siemens Hausgeraete MICROWAVE HOUSEHOLD APPLIANCE
DE3843039A1 (en) * 1988-12-21 1990-06-28 Bosch Siemens Hausgeraete Domestic appliance
WO1997047161A2 (en) * 1996-06-03 1997-12-11 Matsushita Electric Industrial Co., Ltd. Microwave heating apparatus
EP2187702A1 (en) * 2008-11-17 2010-05-19 Topinox Sarl Cooking device with microwave heating

Family Cites Families (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2761942A (en) * 1952-06-06 1956-09-04 Raytheon Mfg Co Heating apparatus
GB752206A (en) * 1953-11-12 1956-07-04 Raytheon Mfg Co Improvements in or relating to high frequency dielectric heating apparatus
FR1208182A (en) * 1957-12-10 1960-02-22 Miwag Mikrowellen A G Device for generating a defined primary field distribution in a cavity for heating organic substances by means of microwave frequencies
CH393577A (en) * 1961-12-13 1965-06-15 Patelhold Patentverwertung Device for dielectric heating of materials
US3521019A (en) * 1968-02-19 1970-07-21 Varian Associates Microwave heating cavity with a venetian blind mode stirrer
SE345903B (en) * 1970-12-21 1972-06-12 Philips Svenska Ab
US4301347A (en) * 1980-08-14 1981-11-17 General Electric Company Feed system for microwave oven
US4336434A (en) * 1980-08-15 1982-06-22 General Electric Company Microwave oven cavity excitation system employing circularly polarized beam steering for uniformity of energy distribution and improved impedance matching

Also Published As

Publication number Publication date
DE3703133A1 (en) 1988-08-11
DE3786451D1 (en) 1993-08-12
EP0277342A3 (en) 1988-08-31
EP0277342A2 (en) 1988-08-10
ATE91375T1 (en) 1993-07-15

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