A microwave oven as a representative high frequency heating apparatus has conventionally been constructed as shown in FIGS. 1-7.
A microwave oven of FIG. 1 is of a general structure employing a turntable 1. In this microwave oven, electromagnetic waves emitted from a magnetron 2 as an electromagnetic wave emission means are transmitted via a waveguide 3 to a heating chamber 4, where the waves are distributed as standing waves that are determined by the shape of the heating chamber 4 and the position of an opening 5 through which the electromagnetic waves are radiated into the heating chamber 4. A food 6 generates heat correspondingly to an electric field component of the standing waves and a dielectric loss of the food 6. The electric power P [W/m.sup.3 ] absorbed per unit volume of the food 6 is expressed by the intensity of an applied electric field E [V/m], the frequency f [Hz], the dielectric constant .epsilon.r and the dielectric tangent tan.delta. of the food 6 in an expression (1) below. The heating distribution of the food 6 is generally determined by the distribution of the standing waves of the electromagnetic waves and, hence, the heating distribution on concentric circles is uniform due to a rotating of the turntable 1. EQU P=(5/9)..epsilon.r.tan .delta..f.E.sup.2 .times.10.sup.-10 [W/m.sup.3 ](1)
In FIG. 1, reference numeral 19 denotes a control means, 22 denotes a motor, 23 denotes a weight sensor, and 27 denotes a fan.
As other examples of the uniforming means, a stirrer system has been employed in which electromagnetic waves are stirred by a constant rotation of a metallic plate inside the heating chamber. Electromagnetic waves are also taken out from the waveguide 3 by a rotary waveguide (emission part) 8 having a coupling part 7 and are emitted through an emission port 9, as shown in FIG. 2, in other words, the opening part itself has been rotated constantly. In this case, the rotary waveguide 8 has been built on a bottom face of the heating chamber 4 and rotated constantly at all times by a motor 10, and the whole of a bottom part of the heating chamber 4 has been covered with a cover 11 of a material allowing the electromagnetic waves to pass therethrough.
Actually, however, most of the apparatuses in the market are of the turntable type.
Some apparatuses are provided with a plurality of opening parts, wherein an exit for the electromagnetic waves is switched to provide a uniform a heating distribution. FIG. 3 shows an apparatus of the kind having two openings 5 defined in a side wall of the heating chamber 4 (Japanese Patent Laid-Open Publication No. 4-319287).
A plurality of magnetrons and a plurality of waveguides are installed in some cases to constitute a plurality of opening parts (Japanese Patent Laid-Open Publication Nos. 61-181093 and 4-345788).
Alternatively, one waveguide is branched in many directions to form a plurality of waveguides while there is arranged a single magnetron, thereby constituting a plurality of opening parts (Japanese Patent Laid-Open Publication No. 61-240029 and Japanese Utility Model Laid-Open Publication No. 1-129793).
In a different constitution, end faces 14 of two sub waveguides 13 are moved at positions facing a plurality of openings 5, as indicated in FIG. 4, so that the electromagnetic waves may be directed to one opening 5 which apparently is easy for the electromagnetic waves to pass through, to thereby uniform the heating distribution (Japanese Patent Laid-Open Publication No. 5-74566).
In a system of FIG. 5, a metallic part 12 is moved within the single waveguide 3 having a plurality of openings 5, so that the opening 5, which apparently is easy for the electromagnetic waves to pass through, is selected to thereby uniform the heating distribution (Japanese Patent Laid-Open Publication Nos. 3-11588 and 5-121160).
In FIGS. 6 and 7, a plurality of openings are formed at upper and lower parts of the heating chamber, and the openings 5 at the lower part are switched to thereby provide a uniform heating distribution (Japanese Utility Model Laid-Open Publication No. 1-129793).
A feedback control is also executed in some apparatuses by detecting the weight, shape, temperature or dielectric constant of the food 6 or the temperature, humidity or electric field in the heating chamber by sensors.
According to the above-described conventional arrangements, however, in the case where the waveguide and the heating chamber are connected to guide the electromagnetic waves into the heating chamber, all kinds of food could not be heated uniformly by a single opening part, because the position of the optimum opening part to obtain a uniform heating distribution was different for every material or shape of the food.
For example, when a flat food is heated by the conventional microwave oven, the heating proceeds from an edge portion of the food, resulting in a large heating irregularity with a central portion of the food left cold.
Considering the position of the opening part, if the opening part is formed near the center of the bottom face of the heating chamber and when a bottom face of the food is heated, the food is uniformly heated if it is a liquid one allowing convection, whereas only the bottom face of the food is raised in temperature if the food is a solid one allowing no convection. In this case, while the concentric heating distribution is made uniform with the use of a turntable, the heating distribution in a radial direction or a vertical direction as viewed from a rotational center of the turntable cannot be improved in spite of the rotation of the turntable.
When the stirrer or rotary waveguide is used to stir the electromagnetic waves, the electric field distribution is changed in such a manner as to switch the opening part in accordance with the rotation of the stirrer or rotary waveguide and, hence, the concentration of electromagnetic waves can be avoided to some extent in the case of defrosting or the like manner of heating requiring the avoidance of the concentration. However, due to the stirring caused by a constant rotation without regard to the kind of food, any kind of food is heated by repeating the same electric field distribution for each rotation of the stirrer or rotary waveguide, thus making it difficult to achieve a perfectly uniform heating distribution.
Even when a plurality of openings are formed, a certain fixed electric field is constituted if the openings are simply opened at the same time. Accordingly, it is hard to provide a uniform heating distribution for every kind of food. Therefore, there is actually no large difference of the heating distribution between the microwave oven of FIG. 1 and the microwave oven of FIG. 3. A satisfactory cooking result cannot be expected unless the optimum opening is switched or selected for each individual kind of food.
Meanwhile, in the apparatus provided with a plurality of magnetrons and a plurality of waveguides, the control of oscillation of each magnetron is followed by a switching of the waveguides and, hence, the opening through which the electromagnetic waves are to be emitted is switched. Although this arrangement is slightly effective to provide a uniform heating distribution, the increased number of magnetrons raises costs and makes the apparatus heavy and inconvenient to carry.
When a plurality of waveguides are branched in many directions from one waveguide, the opening easy for the electromagnetic waves to pass through cannot be switched perfectly, i.e., a certain amount of electromagnetic waves leak also from the openings not selected. Moreover, a large quantity of sheet metal is needed for the waveguides, causing the apparatus to be expensive and hard to manufacture.
As a solution to the above, end faces 14 of the sub waveguides 13 are moved at positions facing the openings 5, as shown in FIG. 4, to thereby select the opening 5 which apparently is easy for the electromagnetic waves to pass through. Although the heating distribution is provided more or less effectively uniform according to this method, the space for the plurality of sub waveguides 13 and the space for a plurality of shields to prevent the leak of electromagnetic waves when the end faces 14 of the sub waveguides 13 are moved are required in practice. As a result, the whole microwave oven becomes bulky or the effective volume of the heating chamber to the whole apparatus is reduced, leading to such a user's dissatisfaction that the apparatus occupies a considerable space or affords to contain only small food. At the same time, the apparatus becomes heavy and hard to carry. An amount of power is probably consumed to move the end faces 14 of the sub waveguides including the shields at a plurality of positions.
As shown in FIG. 5, even if the metallic part is moved within one waveguide 3 having a plurality of openings 5, it is impossible to completely switch to select the opening easy for the electromagnetic waves to pass. Unrequested openings 5 are also open, through which the electromagnetic waves leak.
In the constitutions of FIGS. 1, 3, 4 and 5, the openings 5 are formed only at a side face of the apparatus, in other words, separated far from the food 6.
If the distance between the opening 5 and the food 6 is large, the rate of the electromagnetic waves not only entering the food 6 directly from the opening 5, but entering the food 6 after reflected at the wall face of the heating chamber 4, etc. is increased. In consequence of this, the heating distribution of the food 6 is disadvantageously changed large by the size of the heating chamber 4, or the position or shape of the food 6.
From the same reason as above, a peripheral portion of the general food 6 tends to be heated more easily.
The arrangement of FIG. 6 or 7 is more useful to provide a uniform heating distribution than the other conventional arrangements. However, the peripheral portion of the food is still easy to heat because of the electromagnetic waves always radiated from the upper part of the heating chamber and, a portion of the food between one and the other openings adjacent to each other at the lower part of the heating chamber is hard to heat.
What is common to these conventional arrangements of FIGS. 1, 3, 4, 5, 6 and 7 is an undesirable probability that the electromagnetic waves will be concentrated only where the openings 5 are formed, causing a heating irregularity.
In the arrangements of FIGS. 3 and 5-7, the distance from the magnetron 2 to the opening 5 is not taken into account.
Generally, whether or not it is easy for the electromagnetic waves to enter the heating chamber 4 is determined by matching of the heating chamber and openings, and is changed depending on the position of the openings 5 in the heating chamber 4, the length of the waveguide 3, the distance between the magnetron 2 and the opening 5, etc. Particularly, the ease at which electromagnetic waves come out from the waveguide 3 varies with a cycle of .lambda.g/2 wherein .lambda. is the guide wavelength of the electromagnetic waves. Therefore, when a plurality of openings 5 are present, the matching should be adjusted for each opening 5 so as to emit the electromagnetic waves equally from all the openings 5.
Unless the matching is achieved by determining the position of the opening 5 solely to lengthen the distribution it becomes, the electromagnetic waves to enter the heating chamber, whereby the heating efficiency is worsened. In addition, an increased amount of reflecting waves enter the magnetron 2, which necessitates countermeasures to prevent a temperature rise or generation of unnecessary radiation noises.
In the feedback control by detecting the state of food, an initial heating state or a state change from a heating start has been detected or heating completion has been detected with the use of a weight sensor, a humidity sensor, a temperature sensor, an electromagnetic field detection sensor, a steam detection sensor, an alcohol detection sensor or the like. Any of the aforementioned sensors have not been practically designed to carry out such feedback control as to detect the heating distribution or correct the heating irregularity.