Source: http://www.google.com/patents/US5378875?dq=7125605
Timestamp: 2016-02-11 09:24:11
Document Index: 743570801

Matched Legal Cases: ['art 11', 'art 11', 'art 11', 'arts 11', 'arts 11', 'arts 11', 'arts 11', 'Application No. 1']

Patent US5378875 - Microwave oven with power detecting device - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inPatentsA microwave oven has only a microwave sensor or both microwave sensor and temperature sensor. The microwave sensor has a wave absorber to generate heat through absorption of microwave energy and a thermistor to detect temperature of this wave absorber. The temperature sensor has a thermistor to detect...http://www.google.com/patents/US5378875?utm_source=gb-gplus-sharePatent US5378875 - Microwave oven with power detecting deviceAdvanced Patent SearchPublication numberUS5378875 APublication typeGrantApplication numberUS 07/989,173Publication dateJan 3, 1995Filing dateDec 11, 1992Priority dateDec 25, 1991Fee statusLapsedAlso published asCA2085527A1, CA2085527C, DE4243597A1, DE4243597C2Publication number07989173, 989173, US 5378875 A, US 5378875A, US-A-5378875, US5378875 A, US5378875AInventorsMasahiro Hirama, Masami Koshimura, Sakae Mori, Jiro YoshidaOriginal AssigneeMitsubishi Materials CorporationExport CitationBiBTeX, EndNote, RefManPatent Citations (5), Referenced by (14), Classifications (11), Legal Events (6) External Links: USPTO, USPTO Assignment, EspacenetMicrowave oven with power detecting device
US 5378875 AAbstract
1. A microwave oven comprising:a wall defining a microwave chamber; a microwave energy source for supplying microwave energy to the chamber; a microwave sensor comprising:a wave absorber for generating heat through absorption of microwave energy; and a first thermistor for detecting the temperature of said absorber, said first thermistor having a temperature sensing part positioned adjacent to said wave absorber and positioned to avoid microwave energy from the microwave energy source; a temperature sensor comprising a second thermistor for detecting the ambient temperature immediate said wave absorber; and computing means for computing microwave power as a function of time, according to the expression: P=C�d&#920;/dt+&#948;�&#920;; where P represents microwave power absorbed by the wave absorber, Θ=Θ1 -Θ2, where Θ1 represents temperature rise detected by the first thermistor and Θ2 represents temperature rise detected by the second thermistor, C represents heat capacity of the microwave sensor, and δ represents a thermal radiation constant of the microwave sensor. 2. The microwave oven as defined in claim 1, wherein said temperature sensor has the same shape, size, and heat capacity as said wave absorber and has a wave reflector which reflects microwave energy, and wherein said second thermistor detects the temperature of said wave reflector and has the same construction as said first thermistor.
&#920;=P�t/c                                     (2)
In an actual heated object, however, heat radiated outside cannot be ignored when it receives microwave power. If the heated object has a heat radiation constant δ, the work P�dt which the object receives for a very short time dt is represented by the following expression (3).
P�dt=C�d&#920;+&#948;�&#920;�dt(3)
where dΘ is the temperature rise of the object during a very short time, C�dΘ is a heat energy stored in the object during a very short time, and δ�Θ dt is a heat energy radiated outside during a very short time. From the above-mentioned expression (3), the temperature rise value Θ of the object is represented by the following expression (4) when the electric power P is constant. This relation is shown in FIG. 27.
&#920;=(P/&#948;)�[1-exp(-t/&#964;)]             (4)
In this expression, τ is a thermal time constant ant has a relation of C=τ�δ. As seen in FIGS. 26 and 27, the difference grows between the temperature rise rates in the two cases as the temperature rise value Θ becomes larger.
d&#920;/dt=(P/&#948;/&#964;)�exp(-t/&#964;)       (5)
d2 &#920;/dt2 =(-P/&#948;/&#964;2)�exp(-t/&#964;)(6)
P=C�d&#920;/dt+&#948;�&#920;          (1)
The thermistor 11 is a MELF (Metal Electrode Face) type device of 1.35 mm in diameter and 1.45 mm in thickness, has the temperature sensing part 11a made of sintered metal oxide comprising Mn, Co, and Ni as its main ingredients, and is formed by soldering lead wires 11c onto terminal electrodes 11b of both ends of the temperature sensing part 11a. Resistance of the thermistor 11 at 25� C. is 100 kΩ and its B constant is 3965 K.
The wave absorber 12 of sintered SiC is 12 mm in diameter and 1 mm in thickness. One face 12a of the wave absorber 12 is a microwave absorbing face. The temperature sensing part 11a of the thermistor 11 with leads is adhered to the central part of the other face 12b of the wave absorber 12 using epoxy resin 10a. Heat radiation constant δ of the microwave sensor 10 including the thermistor 11, wave absorber 12 and epoxy resin 10a is 6 mW/� C. and its thermal time constant τ is 40 seconds.
Thermistors 11 and 51 are respectively a MELF (Metal Electrode Face) type device of 1.35 mm in diameter and 1.45 mm in thickness, have temperature sensing parts 11a and 51a made of sintered metal oxide comprising Mn, Co, and Ni as the main ingredients, and are formed by soldering the lead wires 11c and 51c respectively onto terminal electrodes 11b and 51b of both ends of the temperature sensing parts 11a and 51a. Resistance values of both thermistors 11 and 51 at 25� C. are respectively 100 KΩ and their B constants are 3965 K, respectively.
Wave absorbers 12 and 52 of sintered SiC are respectively 12 mm in diameter and 1 mm in thickness, and have the same heat capacity, respectively. One side faces 12a and 52a of the wave absorbers 12 and 52 are microwave absorbing faces, and, to their central parts of the other side faces 12b and 52b respectively, are adhered the temperature sensing parts 11a and 52a of the thermistors 11 and 51 with leads using epoxy resin 10a and 50a. The microwave receiving face 52a of the wave absorber 52 of the temperature sensor 50 is printed with Ag paste (H-5723 made by Shoei Kagaku) on it and is sintered keeping the maximum temperature of 800� C. for 10 minutes to make a metal coating 53. This metal coating 53 may be made by means of a thin film forming method such as vaporization, sputtering, and the like.
The heat radiation constant δ and thermal time constant τ of the microwave sensor 10 including the thermistor 11, wave absorber 12 and epoxy resin 10a are 6 mW/� C. and 40 seconds respectively. The heat radiation constant δ and thermal time constant τ of the temperature sensor 50 including the thermistor 51, wave absorber 52, and epoxy resin 50a are also 6 mW/� C. and 40 seconds, respectively.
Some organic materials, other than epoxy resin, to fix the thermistor 11 include phenol resin, silicone resin, polyimide resin, and the like. Fixing methods using inorganic material include a method in which after molding the thermistor with the paste which is made by mixing with water the material having silica and alumina as its main ingredients into a pasty state, the water is evaporated at about 80� C. and then heat treated at about 150� C.
The temperature sensing parts 11a and 51a of the thermistors 11 and 51 are respectively made of sintered material of metal oxide comprising Mn, Co, and Ni as its main ingredients, and are formed by soldering lead wires 11c and 51c onto both ends thereof, respectively. Resistance values of both thermistors 11 and 51 at 25� C. are respectively 100 kΩ, and their B constants are respectively 3965 K.
The heat radiation constant δ and thermal time constant τ of the microwave sensor 10 comprising the thermistor 11, wave absorber 12, and epoxy resin 10a are 6 mW/� C. and 40 seconds respectively.
As the thermistor 112, a publicly known device can be used such as bead, disk, rod, thick film, thin film, chip integrated-with-electrodes type, or the like. As a device with lead wires, a device of glass-coated bead type which is coated with glass or glass-sealed type which is sealed into a glass tube is desirable because of its heat resistance of about 200� to 400� C. A thermistor of glass-coated bead type is made by coating the thermistor body with melted glass after welding two fine dumet wires onto a thermistor body of bead type.
The wave absorbing layer 114 is provided on the surface of the metal cover 113. This wave absorbing layer 114 is made of one or both of organic and inorganic materials containing wave absorbing powder as filler. This wave absorbing powder is ceramic powder having one or both of magnetism and dielectricity. Wave absorbing powder with magnetism includes ferrite powder or ceramic powder containing ferrite as its main ingredient, and wave absorbing powder having dielectricity includes one or more of ceramic powder selected from the group SiC, Al2 O3, B4 C, SrTiO3, ZrO2, Y2 O3, PZT, and PLZT. Wave absorbing powder having both magnetism and dielectricity includes a ceramic powder having both magnetic loss and very large dielectric loss which has a reaction phase formed among ferrite particles or among ferrite particles and perovskite-type compound particles, and made by means of sintering at 1000� to 1500� C. a mixed material which is obtained by mixing magnetic material powder comprising fine ferrite particles of 50 μm or less in diameter and dielectric material powder comprising perovskite-type compound particles such as BaTiO3 particles of 10 μm or larger in diameter, which was disclosed in the Unexamined Published Japanese Patent Application No. 1-291406.
The base material of the wave absorbing layer 114 is used as an adhesive agent of the wave absorbing powder onto the metal cover 113. A material of high heat resistance and high heat conductivity is desirable for the wave absorbing layer 114. As an organic base material, for example, heat-resistant resin resistive to 200� to 300� C. is used, such as epoxy resin, phenol resin, silicone resin, fluororesin, or the like, or heat-resistant resin resistive to 300� to 400� C., such as polyimide resin, or the like is used. For an inorganic base material, for example, glass paste is used. Other materials can also be used, such as a composite material ("Chirano Polymer Coat AL-15" made by Ubekosan, Inc.) obtained by mixing inorganic fiber of Si-Ti-C-O compounds ("Chirano Fiber" by Ubekosan, Inc.) with the above-mentioned heat-resistant resin, such as epoxy resin or the like. This composite material has heat resistance of about 800� C. Epoxy resin and the above-mentioned composite material are desirable in view of their high heat resistance and high heat conductivity.
For use as the thermistor 312 composing the microwave sensor 10, a publicly known device can be utilized of such type as bead, disk, rod, thick film, thin film, chip, integrated-with-electrodes type, or the like. As a device with leads, a device of glass-coated bead type coated with glass or glass-sealed type sealed into glass tube is desirable because of its heat resistance of about 200� to 400� C. A thermistor of glass-coated bead type is made by coating the thermistor body with melted glass after welding two fine dumet wires onto the thermistor body of bead type.
In a manufacturing process of the microwave sensor 10, the thermistor 312 with leads 311 is inserted into the wave absorber 313, and then is fixed with filler 316. A material of high heat resistance and high thermal conductivity is desirable as the filler. For use as such kind of material, for example, heat-resistant resin resistive to 200� to 300� C. such as epoxy resin, phenol resin, silicone resin, fluororesin, or the like and another heat-resistant resin resistive to 300� to 400� C. such as polyimide resin or the like are utilized. Ceramic having silica and alumina as its main ingredient is mentioned as filler of a higher heat resistance. The filler is desirable to be an inorganic material since ceramic which is inorganic material having generally higher thermal conductivity. In order to fix the thermistor 312 in the wave absorber 313 with the filler 316, a method may be used where the dip-coated thermistor 312 may be inserted into the wave absorber 313, after the filler is melted and the thermistor 312 is dip-coated with this filler solution, or the melted filler may be poured into the wave absorber 313 after the thermistor 312 is inserted into the wave absorber 313.
The thermistor 416 is desired to be small-sized and heat-resistant, and as shown in FIG. 21, a glass-coated bead type coated with glass is thus desirable because of its heat resistance being about 200� to 400� C. A thermistor of glass-coated bead type is made by coating the thermistor body with melted glass after welding two fine lead wires 422 on both ends of the thermistor body 416 of bead type. In this example, the lead wires 422 are coated in the vicinity of the thermistor body 416 with an insulating material 420 having heat resistance, such as Teflon, polyimide, or the like.
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