US3662140A - High frequency electronic heating apparatus - Google Patents

Abstract

A no-load sensor in the form of a ferrite circulator is provided in the launching section of high frequency electronic heating apparatus with the body of a ferrimagnetic material oriented to display the field-displacement effect and yield nonreciprocal attenuation of electromagnetic energy. Launched energy initially is propagated to a heating enclosure with low loss. Reflected energy above a predetermined level resulting from the absence of a load or any malfunction is diverted to an energy absorber and thermal relay to terminate operation of the energy generator.

Description

United States Patent Jones et al.

[ 1 May 9, 1972 [54] HIGH FREQUENCY ELECTRONIC HEATING APPARATUS [72] Inventors: William C. Jones, Natick, Mass; Dan R.

McConnell, Iowa City, Iowa [73] Assignee: Raytheon Company, Lexington, Mass.

[22] Filed: Oct. 7, 1970 [21] Appl. N0.: 78,713

[52] US. Cl... ..2I9/10.55, 333/17 [5 1] Int. Cl. ..H05b 5/00 [58] FieldoiSearch ..2l9/l0.55;333/1.l, l7;

[56] References Cited UNITED STATES PATENTS 3,238,475 3/1966 DeVitaetal ..333/l7 GENERATOR SUPP LY 3,437,777 4/1969 Nagai et a1 ..2l9/l0.55

Primary Examiner-J. V. Truhe Assistant Examiner-Hugh D. Jaeger Attorney-Harold A, Murphy, Joseph D. Pannone and Edgar 0. Rest [5 7] ABSTRACT A no-load sensor in the form of a ferrite circulator is provided in the launching section of high frequency electronic heating apparatus with the body of a ferrimagnetic material oriented to display the field-displacement effect and yield nonreciprocal attenuation of electromagnetic energy. Launched energy initially is propagated to a heating enclosure with low loss. Reflected energy above a predetermined level resulting from the absence of a load or any malfunction is diverted to an energy absorber and thermal relay to terminate operation of the energy generator.

7 Claims, 7 Drawing Figures P'ATE'N'TEnm 9:972 3.662.140 SHEET 1 OF 2 HIGH 44 42 FREQUENCY GENERATOR N 2/ 25 Li l2 46 VOLTAGE j SUPPLY \R Q KKK N \\\NN-X N\-\NQW 50 @7777 P'A'TENTEnm 9 I972 SHEEIEUFZ HIGH FREQUENCY GENERATOR SUPPLY I l L I (104 60 /58 {k .REVERSE ELECTRIC K FIELD I FORWARD I Q \0 74 Y X 9 GENERATOR SUPP LY GENERATOR SUPPLY 7' 1 6 HIGH FREQUENCY ELECTRONIC HEATING APPARATUS BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates generally to high frequency electronic heating apparatus and specifically to no-load sensing means.

2. Description of the Prior Art Electronic heating apparatus of the type under consideration commonly utilizes electromagnetic wave energy directed by waveguide or other suitable propagating means within a conductive enclosure. An exemplary electrical generator of such high frequency waves is the magnetron oscillator of World War II radar system fame. The text Microwave Magnetrons, Radiation Laboratory Series, Vol. 6, by G. B. Collins, McGraw-Hill Book Company, Inc., 1948, provides a detailed description of the construction and operation of such devices. The energy generators operate at frequencies within the electromagnetic spectrum and for electronic ovens have allocated operating frequency bands of 915 or 2,450 megahertz. Other high frequency energy generators include vacuum tube oscillators and klystrons. The foregoing generators are intricate and expensive structures operated by high voltage circuits carrying many thousands of volts of rectified electrical energy.

The high frequency waves are absorbed and rapidlyheat any materials disposed within the enclosure when the generator is suitably matched to the load to be heated. The absence of a load or other mismatch conditions such as the placing of a metallic object in the oven results in a high degree of reflected energy back to the high frequency source. The reflected energy presents a potential catastrophic failure problem to the generator. A need arises, therefore, in all such high frequency electronic heating apparatus for automatic means to protect the generator and accompanying circuits by sensing of the noload or severe mismatch conditions.

Numerous prior art attempts to provide such protective means will now be reviewed. U.S. Pat. No. 2,498,719 issued to P. L. Spencer provides a circuit-controlling means for detecting and continuously monitoring the standing wave amplitudes within the launching waveguide section between the enclosure and the generator. The occurrence of standing wave peaks within the transmission line in excess of a predetermined value in combination with gas-filled electrical discharge devices results in the energizing of a control relay opening the contacts serially connecting the generator to the voltage source.

Another suggested solution is disclosed in U.S. Pat. No. 2,498,720 issued to N. R. Wild et al. wherein the'electron source for the high frequency generator is automatically decoupled upon the incidence of an unduly high ratio between the standing wave maxima and minima values. Again, gaseous electron discharge devices are utilized in combination with the waveguide transmission line. A further prior art embodiment is noted in U.S. Pat. No. 2,679,595 issued to P. L. Spencer again monitoring the voltage standing wave ratio as an indication of a match or mismatch condition of the load to the generator. When a load change creates a mismatch condition with the resultant reflected energy a crystal semiconductor detects such a circuit fluctuation and automatically reduces the cathode filament temperature to thereby protect the high frequency generator. Still another suggested embodiment is noted in U.S. Pat. No. 3,412,227 issued to Carl L. Anderson wherein a directional coupler is physically associated with the waveguide transmission line and contains energy absorbing material at one end. A neon bulb sensor and photocell transducer at the other end of the directional coupler varies the energization current of the generator tube control relay as a function of any reflected high frequency energy.

All of the foregoing prior art devices operateon the principle of sensing and monitoring the voltage standing wave ratios and have the disadvantage of being both expensive to install as well as being inconsistent with the efficient operation of the electronic heating apparatus.

SUMMARY OF THE INVENTION In accordance with the teachings of the present invention sensing and protective means are provided within the transmission line coupling the generator tothe heating apparatus. The device disclosed comprises a circulator of a ferrimagnetic material disposed in such a manner as to display nonreciprocal field-displacement characteristics to the energy propagating within the line. Launched energy in the forward direction enters the heating enclosure with little loss while any high power reflected energy is diverted perpendicularly to the propagation path into the energy absorbing means. Thermally actuated means coupled to the absorber provide a signal to terminate the operating voltages and deenergize thegenerator. The ferrimagnetic circulator and absorbing means do not absorb full reflected power'over extended periods but function solely as sensor means to activate the thermal relay as soon as a predetermined threshold temperature level is exceeded. The circulator sensor is oriented in a direction parallel to the electric fieldE vector of the electromagnetic energy at a point offset from the longitudinal axis of, illustratively, a waveguide transmission line. The coupling of the thermal relay to the absorbing means in the overall sensor results in a ferrimagnetic element which is not required to dissipate large amounts of high frequency energy. The structure disclosed herein gives maximum protection for load mismatch conditions within the enclosure and is relatively simple, efficient and inexpensive to incorporate within such apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS The invention, as well as the details for the provision of a preferred embodiment, will be readily understood after consideration of the following detailed description and reference to the accompanying drawings, wherein:

FIG. 1 is a perspective view of the embodiment of the invention with a portion of the upper wall broken away to reveal internal structure;

FIG. 2 is a vertical cross-sectional view'of the electronic heating apparatus embodying the present invention;

FIG. 3 is a diagrammatic representation of conventional coordinate axes terminology in waveguide transmission lines;

FIG. 4 is a diagrammatic representation of the field-displacement effect in ferrimagnetic loaded waveguide;

FIG. 5 is a top view of the embodiment-with a portion of the walls removed;

FIG. 6 is a detailed cross-sectional view taken along the line 6-6 in FIG. 5; and

FIG. 7 is the diagrammatic view illustrative of the operation of the embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT framing a perforated panel 22. The perforations extend over a major portion of the panel to prevent the escape of electromagnetic energy during the operation of the apparatus.

Handle 24 provides for manual opening and closing of the door. Control panel 26 is disposed adjacent to the enclosure access opening with the electromagnetic energy generation means and accompanying circuitry disposed behind.

Referring now to FIG. 2 an energy generator, illustratively a magnetron of the type well known in the art indicated generally by block v28 is coupled to a voltage supply and electrical controls indicated generally by block 30. The electromagnetic energy is fed from generator 28 by means of a radiating probe 32 in a dielectric dome member 34 to a launching section of waveguide transmission line 36 adapted to propagate the desired frequency into the heating enclosure 14. The waveguide line is short-circuited at one end by wall member 38 and is open at the inner end 40. The electromagnetic energy radiated within the enclosure is uniformly distributed by means of a stirrer 42 of the well-known type described in detail in US. Pat. No. 2,813,185 issued Nov. 12, 1957 to Robert V. Smith. The stirrer 42 has a plurality of vane members 44 rotatably actuated by fractional horsepower motor means. Articles to be heated are supported on dielectric member 46 spanning wall arrangement 48 defining a dimpled section with shoulders 50. The dielectric member is permeable to electromagnetic energy and facilitates the heating of the articles on all sides by reflection from the surrounding conductive walls. Such dielectric members also provide minimal protection to absorb some of the electromagnetic energy in the event that the generator is operated into a mismatch condition.

In accordance with the present invention a no-load sensing means is provided by a rod-shaped ferrimagnetic element 52 extending partially or entirely within waveguide launching section 36 and having appended thereto an externally mounted integral magnetizing member 54. An electromagnetic energy absorber 56 is disposed within one of the narrow side walls 36a of waveguide section 36 and is oriented to intercept any high power energy propagated in a reverse direction.

Before proceeding to describe further specific details of the embodiment, it will be of assistance to review a few of the fundamental and underlying principles relative to the art of ferrimagnetics and ferrites, as well as propagation of electromagnetic energy in the fundamental TE mode in rectangular waveguide. The designations are shown in FIG. 3 for the system of coordinates conventionally referred to in this field. The Z axis extends perpendicular to the direction of propagation of energy and parallel to the narrow walls of a rectangular waveguide. The X axis extends along the longitudinal axis and parallel to the broad walls of the rectangular waveguide section. The Y axis extends perpendicular to the X axis. It is well known in the art that rectangular waveguide transmission lines containing bodies of ferrimagnetic material magnetized in the Z axis can propagate electromagnetic energy in various modes. In these modes the electric field associated with the propagating energy is aligned with the Z axis while the magnetic associated with the propagating energy is oriented in the plane of the X and Y axes. It is also well known in the field that the response of a magnetized ferrimagnetic body is inherently nonreciprocal unless the body is located at the center of the waveguide. The phase shift experienced by a wave propagating in the direction of the positive X axis is different from the phase shift experienced by the wave of the same frequency propagating in the reverse direction or negative X axis.

The next consideration involves the so-called field-displacement effect in ferrite isolators fully described in the text Microwave Ferrites and Ferromagnetics" by B. Lax and K. 1. Button, McGraw-Hill Book Company, Inc., 1962, pp. 362-372. This effect considers the distribution of the alternating fields associated with high frequency electromagnetic energy propagating in a rectangular waveguide line containing an asymmetrically disposed magnetized ferrite slab. Referring to FIG. 4 a rectangular waveguide 58 is loaded with ferrite slab 60 magnetized in a transverse direction indicated by the arrow and symbol H In unloaded waveguide the maximum electric field intensity conventionally occurs at the center of the waveguide along its longitudinal axis. Curve 62 depicts the electric field intensity pattern in the forward direction of propagation. It will be noted that the electric field distribution follows the approximate pattern of unloaded waveguide with the maximum point near the center. Relatively low loss, therefore, will occur in the propagation of energy in the forward or launching direction due to the presence of the ferrimagnetic body.

Conversely, a wave propagated in the reverse direction assumes the profile indicated by curve 64. It will be noted that the maxima point of the electric field intensity has been shifted to one side or offset from the longitudinal axis of the waveguide. In the aforereferenced text, the authors note that a field-displacement ferrite isolator may be realized by the placement of a resistance card on the face of the slab to pro vide for attenuation of the reverse wave where the electric field is at a maximum. In the present invention the disposition of the ferrimagnetic body within the launching section of the high frequency electronic heating apparatus is in the offset manner to take advantage of the aforementioned field-displacement effect in the nonreciprocal propagation of the energy. The reverse-forward ratio of attenuation, therefore, is nonreciprocal to result in the distortions of the electric field. Further information regarding the field-displacement isolator may also be obtained in an article by S. Weisbaum and H. Boyet in the I.R.E. Transactions on Microwave Theory and Techniques, Vol. MT'F- 5, No. 3, July, 1957, pp. 194-198. An excellent means, therefore, exists in the art providing for nonreciprocal transmission of electromagnetic energy utilizing a magnetized body of ferrimagnetic material displaced with relation to the longitudinal axis of a transmission line.

FIGS. 5 and 6 explicitly detail the orientation of the sensor element comprising a magnetized body of ferrimagnetic material. One such material which has been noted to provide superior performance is of a nickel-aluminum-ferrite composition or magnesium-ferrite materials in combination with a magnetizing member 54 of a material commercially available and referred to as Alnico V or VIII. The ferrimagnetic element is displaced with respect to longitudinal axis 66 of waveguide section 36 in a direction toward the narrow side wall 36a with the energy absorbing means 56. The ferrimagnetic element 52, therefore, by reason of the nonreciprocal energy propagation characteristics will act as a circulator and direct any reflected electromagnetic energy into the absorbing means 56. A circulator is thereby provided which is readily distinguishable from the prior art resonance isolators or Faraday rotator type ferrite devices which absorb the full energy incident thereupon. Such energy absorption requires large masses of ferrite elements, as well as heat exchange or cooling means in combination with the ferrimagnetic element. In the present invention the field-displacement circulator results in a substantially lower mass of ferrimagnetic material with accompanying savings in expense as well as space. Further, in accordance with the teachings of the present invention, the energy absorbing means 56 is provided in the narrow side wall surface at the point where the approximate electric field maximum of the reverse propagated wave occurs. An excellent material for the energy absorbing means is silicon carbide which also has a certain degree of thermal lag and, therefore, will not automatically terminate operation of the heating apparatus by the reflection of electromagnetic energy below a predetermined threshold value.

Adjacent to the energy absorbing means 56 a thermal relay member 68 is disposed having a predetermined actuating level. An illustrative structure for the thermal relay would be a bimetallic element of the type used in sensing safety and control means for domestic and industrial heaters, as well as many appliances including electric dryers or the like. Wires 70 and 72 couple the thermal sensing relay means 68 to the energy generator supply 30. A solenoid, circuit breaker or relay may be incorporated within the voltage supply 30 to automatically open the circuit and deenergize the energy generator upon receipt of an appropriate electrical signal.

In FIG. 6 an intermediate conductive member has been il' lustrated disposed between the ferrimagnetic element and magnetizing means 54. To assist in the magnetization of the ferrimagnetic material a substantially dome-shaped member 74 is provided which can be considered to be comparable to a pole piece in the magnetizing of crossed field oscillator devices to assist in the better coupling of the magnetic field to the desired member. Many other variations for enhancement of the magnetizing arrangement will readily occur to those skilled in the art.

Referring now to FIG. 7 the operationof the illustrative embodiment of the invention will be described. The appropriate energy at the predetermined frequency level generated by oscillator source 80 is fed through launching waveguide section indicated by line 82 and contacts the ferrimagnetic sensing element 84. Due to the favorable disposition in accordance with the field-displacement effect, the sensing element functions as a circulator as indicated by the arrow 86. The launched energy is propagated with very little loss out of the launching section into the heating enclosure designated generally by the box 88. Excessive mismatch by reason of a no-load or a malfunction will result in the reflection of substantial electromagnetic energy in a reverse direction indicated by line 90. The element 84 will divert the reflected energy as indicated by arrow 92 into the combined energy absorber and thermal relay 94. A signal is thereby impressed upon line 96 to deenergize the generator supply 98 and prevent any further egress of energy through output coupler 100 of the generator.

A very simple and inefficient no-load sensing means is thereby disclosed which is relatively inexpensive to implement in high frequency electronic heating apparatus. No cooling fans or heat exchange means are required to remove the heat generated by the absorption of the electromagnetic energy. Another feature will be noted in that the implementation of the embodiment is achieved without the use of any branch waveguide lines which are both cumbersome and expensive to incorporate in the applicable apparatus. The absorbing means heretofore enumerated have been illustrated as of a square configuration, however, a circular configuration may also be advantageously employed. Other modifications or alterations will also be apparent. It is intended, therefore, that the foregoing embodiment of the invention shown and described herein be considered as illustrative only and not in a limiting sense.

What is claimed is:

1. A protective system comprising:

a source of high frequency electromagnetic energy;

means for energizing said source;

waveguide means for coupling said source to a load:

means disposed within said waveguide means for sensing a mismatch condition;

said sensing means being disposed in any offset manner relative to the waveguide means longitudinal axis;

means for absorbing energy reflected from the load; and

means in thermal contact with said absorbing means for generating a deenergizing signal after absorption of a predetermined level of reflected energy.

2. A protective system comprising:

a source of high frequency electromagnetic energy;

controllable means for energizing said source;

a single rectangular waveguide transmission line for coupling both forward and reflected energy between said source and a load;

a magnetized ferrimagnetic material disposed within said line for sensing a load mismatch condition;

said sensing means being displaced laterally relative to the longitudinal axis of said line;

means for absorbing energy reflected from the load; and

means in thermal contact with said absorbing means to generate a signal for said controllable means to deenergize said source after a predetermined temperature rise.

3. A protective system according to claim 1 wherein said load comprises a heating enclosure.

4. High frequency electronic heating apparatus comprising:

a conductive enclosure;

an electromagnetic energy generator;

waveguide means for coupling said energy into said enclosure;

energy sensor and nonreciprocal transmission means including a magnetized ferrimagnetic material disposed within said waveguide means in an offset manner with respect to its longitudinal axis to receive and propagate in a direction away from said generator energy reflected from said enclosure;

means for absorbing said reflected energy; and

means in thermal contact with said absorbing means for deenergizing said generator after incidence of a predetermined level of reflected energy.

5. High frequency electronic heating apparatus comprising:

a conductive enclosure;

a source of electromagnetic energy;

a single rectangular waveguide transmission line for coupling both forward and reflected energy between said source and said enclosure;

an element of a magnetized ferrimagnetic material disposed within said waveguide line at a point offset from its longitudinal axis to sense and propagate in a direction away from said source energy reflected from said enclosure;

means for absorbing said reflected energy; and

thermally actuated means in contact with said absorbing means to deenergize said source after the occurrence of a predetermined temperature rise.

6. Heating apparatus according to claim 5 wherein said ferrimagnetic material element is displaced laterally in a direction toward said energy absorbing means.

7. Heating apparatus according to claim 5 wherein said energy absorbing means is mounted on a wall of said waveguide adjacent to said ferrimagnetic rod.

Claims (7)

1. A protective system comprising: a source of high frequency electromagnetic energy; means for energizing said source; waveguide means for coupling said source to a load: means disposed within said waveguide means for sensing a mismatch condition; said sensing means being disposed in any offset manner relative to the waveguide means longitudinal axis; means for absorbing energy reflected from the load; and means in thermal contact with said absorbing means for generating a deenergizing signal after absorption of a predetermined level of reflected energy.

2. A protective system comprising: a source of high frequency electromagnetic energy; controllable means for energizing said source; a single rectangular waveguide transmission line for coupling both forward and reflected energy between said source and a load; a magnetized ferrimagnetic material disposed within said line for sensing a load mismatch condition; said sensing means being displaced laterally relative to the longitudinal axis of said line; means for absorbing energy reflected from the load; and means in thermal contact with said absorbing means to generate a signal for said controllable means to deenergize said source after a predetermined temperature rise.

3. A protective system according to claim 1 wherein said load comprises a heating enclosure.

4. High frequency electronic heating apparatus comprising: a conductive enclosure; an electromagnetic energy generator; waveguide means for coupling said energy into said enclosure; energy sensor and nonreciprocal transmission means including a magnetized ferrimagnetic material disposed within said waveguide means in an offset manner with respect to its longitudinal axis to receive and propagate in a direction away from said generator energy reflected from said enclosure; means for absorbing said reflected energy; and means in thermal contact with said absorbing means for deenergizing said generator after incidence of a predetermined level of reflected energy.

5. High frequency electronic heating apparatus comprising: a conductive enclosure; a source of electromagnetic energy; a single rectangular waveguide transmission line for coupling both forward and reflected energy between said source and said enclosure; an element of a magnetized ferrimagnetic material disposed within said waveguide line at a point offset from its longitudinal axis to sense and propagate in a direction away from said source energy reflected from said enclosure; means for absorbing said reflected energy; and thermally actuated means in contact with said absorbing means to deenergize said source after the occurrence of a predetermined temperature rise.

6. Heating apparatus according to claim 5 wherein said ferrimagnetic material element is displaced laterally in a direction toward said energy absorbing means.

7. Heating apparatus according to claim 5 wherein said energy absorbing means is mounted on a wall of said waveguide adjacent to said ferrimagnetic rod.

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