JPS5949679B2 - microwave oven - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a microwave oven, and particularly to a microwave oven that streamlines cooking by controlling heating time.

A microwave oven is a cooking appliance that heats food in a short time by projecting microwave power generated from a microwave power generation source such as a magnetron onto the food being heated, and using the power loss that occurs inside the food. It has the characteristics of being able to heat food from the inside.

Conventionally, the following methods have been proposed for controlling heating in a microwave oven.

First, a method is known in which the cooking time is set using a timer.

However, it is practically difficult to estimate the heating time required for each food and set a timer. Another (7) $ control method is a known method in which a humidity sensor is placed in the exhaust duct of the air passing through the microwave, and heating is stopped when it detects that the moisture in the heated object has reached its boiling point. There is.

This method is effective when heating an object to the boiling point of water, but it is not suitable when thawing frozen food or simply warming soup. Another conventional technique is
A humidity sensor is installed in the exhaust duct of a microwave oven, focusing on the fact that when the heated object reaches its boiling point, the surrounding water vapor pressure rises rapidly.By measuring the humidity in the exhaust gas, it is possible to detect humidity from the time the microwave oven is turned on until the boiling point is reached. technology that measures the amount of water in the heated object based on this time, calculates the additional heating time required for the specified type of dish, and controls the microwave oven according to this time. It has been known.

However, this technique cannot be applied to cooking methods in which the object to be heated does not reach its boiling point, such as reheating, boiling down sake, or thawing frozen foods. SUMMARY OF THE INVENTION Therefore, the present invention aims to improve the above-mentioned drawbacks of the prior art and provides a microwave oven that can be precisely controlled even for non-boiling foods.

The feature of the present invention is that accurate input power is determined from the difference between microwave incident power and reflected power, while
The purpose of this method is to determine the heat capacity of the object to be heated by measuring the value of , and to control the power generated by the microwave power generation source according to a heating program predetermined for each object to be heated. Examples will be described below with reference to the drawings. FIG. 1 is a block diagram of a microwave oven according to the present invention, in which reference number 1 is a cooking chamber, 2 is a waveguide for power supply, 3 is a magnetron that functions as a microwave power generation source, and 4 is a driving power source for the magnetron. , 5 is a blower, 6 is an exhaust duct, 7 is a coupling window, 10 and 10a are microwave level detection loops,
11 is a directional coupler for measuring incident power, and 12 is a directional coupler for measuring reflected power.

Sensors (not shown) for measuring power are coupled to the tips Ila and I28 of the directional couplers 11 and 12, respectively. The cooking chamber 1 is designed as a microwave cavity resonator in order to better concentrate energy on the object to be heated. Here, the Q value of chamber 1 when chamber 1 is empty is QO, and the Q value of chamber 1 when chamber 1 contains the object to be heated is Qm. The Q of a chamber is proportional to the ratio of energy stored within the chamber to energy dissipated within the chamber. On the other hand, the energy stored in the chamber is determined by the size of the chamber and is a constant value. Therefore, 1/Q is proportional to the energy consumed within the chamber. Therefore, the loss L due to the presence of the heated object is as follows. On the other hand, since the value of QO is sufficiently larger than Qm and does not change depending on usage conditions, it is sufficient to measure it immediately after manufacturing the microwave oven.

Therefore, 1/QO is a constant ν, and the loss L is determined only by the value of the η river. Therefore, it is only necessary to measure the energy consumption when the object to be heated is placed in the chamber, but it is impossible to directly measure it. Here, the value of Q in the cavity resonator is proportional to the enclosure field strength (or magnetic field strength) in the portion where the displacement current flows. Therefore, the value of Q in the chamber is Q. It is determined by measuring the electromagnetic field within the par. Therefore, since the oscillation frequency of the magnetron approximately matches the resonant frequency of chamber 1, the value of Qm is determined by the applied electric field strength (or magnetic field strength) and the electric field strength within chamber 1 (
or magnetic field strength). On the other hand, most of the composition of food is water, and the loss L is determined approximately by the amount of water in the object to be heated, so the amount of the object to be heated itself can be estimated from the value of loss L. However, since the load loss due to the excitation side waveguide is included in Qm, it is necessary to correct for this amount. Now, if the ratio of the applied electric field strength to the electric field strength inside the chamber is KQ'Tn, (k is a constant E specific to the chamber, and Qm is given by the formula {2).

Qm=(1+β)Q'm (2) However, the sign of PIN, incident power PRET, and reflected power Γ is positive if it is tightly coupled and negative if it is loosely coupled, but the value of Γ becomes sufficiently small when the range is used. Since it is designed to have a loose coupling when there is no object to be heated inside, there is no problem in practical use if the sign of Γ is set to negative.

Considering that the strength of the electric field inside the chamber is the square root of the power inside the chamber, the output from the loop 10, 10a that detects the microwave level inside the chamber is PCH.
As AN, equation (1) can be rewritten as follows. here(
) is the correction by the driving side waveguide, and ? is a constant as mentioned before.

It can be assumed that all the microwave power input into the chamber is consumed by the object to be heated, so in order to know the total energy input to the object to be heated, we can calculate the time integration of the input power and the loss L according to equation (3). The temperature rise of the object to be heated can be estimated from the required input energy.

On the other hand, input power P is given by equation (4)゜P=P
IN-PRET (4) PIN, PRET as above
By measuring and PCHAN, it is possible to accurately estimate the temperature rise of the heated object.

Therefore, it is possible to appropriately set a heating program depending on the type of food to be cooked, and to control the microwave oven so as to perform heating according to the program according to the above three values. In order to obtain the loss L, it is possible to use only the initial detection values from the power measurement sensor and the microwave level detection loops 10 and 10a, or it is possible to detect these values at regular time intervals. The loss L may be determined sequentially, or it is also possible to detect these values continuously to continuously determine the loss L. Further, the magnetron can be easily controlled by controlling its arc angle using a thyristor, or by controlling the energization time to the magnetron using a timer.
The microwave oven according to the present invention is particularly effective in cooking that does not involve boiling, such as thawing and heating frozen foods.

Therefore, thawing and accompanying heating will be described in detail below. It is known that the dielectric constants (complex dielectric constants) ε7 and ε7 of water and ice are given by the following equations.

where ω is the angular frequency and the other constants are given in the table below.

Here, f = the frequency normally used in microwave ovens.
^i2.45GHzVC} When calculating the dielectric loss of water and ice, each is O. It can be seen that the dielectric loss of water is 2500 times greater than that of ice.

Immediately after frozen food is placed in the chamber, the dielectric loss is small, so PRET/P1N is close to 1. As thawing progresses and the loss of water increases, this ratio decreases, and when all the water becomes water, the ratio of PRET/P1N becomes smaller. Changes will be minimal. Since the temperature of the ice at this time is 00C, it is possible to heat the ice to the required temperature by controlling the microwave power input according to the amount of water D calculated by measuring Qm. The ratio between these (P
FIG. 2 shows the change in RET/PIN over time. Here a
Point b indicates the start time of thawing, point b indicates the end time of thawing the fish, and point c indicates the end time of heating. When the chamber is empty, the ratio (PRET/
4) Since the value of 1N) is close to 1 (almost all of the incident power is reflected), dry firing can be prevented by turning off the power supply to the magnetron when this value exceeds a predetermined value. You can also do it. By measuring the temperatures at the inlet and outlet of the air in the chamber and simultaneously processing them using a microcomputer, it is possible to control the microwave oven with even greater precision.

As detailed above, the microwave detection loop installed inside the cooking chamber measures the amount equivalent to the heat capacity of the food, and the input power is determined by measuring the incident power and reflected power into the chamber. By controlling the microwave power by data processing the operating value, it becomes possible to cook food more appropriately using a microwave oven.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a microwave oven according to the present invention, and FIG. 2 is a curve showing a change over time in the ratio of incident power to reflected power during thawing of frozen food. 1... Cooking chamber, 2... Power supply waveguide, 3... Microwave power generation source,
4... Drive power supply, 5... Blower, 6...
...exhaust duct, 7...combination window, 10.
...Microwave level detection loop, 11...
... Directional coupler for detecting incident power, 12... Directional coupler for detecting reflected power.

Claims (1)

[Claims] 1. In a microwave oven having a cooking chamber and a microwave power generation source coupled to the cooking chamber via a power supply waveguide, the power supply waveguide transmits power incident to the cooking chamber and reflected power. A sensor for measuring the level is installed via a directional coupler, and a loop for detecting the microwave level is provided in the cooking chamber, and the input power is determined from the incident power and reflected power detected by the sensor, and the input power is determined by the loop. Based on the electric field in the cooking chamber and the incident power and reflected power detected by the sensor, determine the energy loss corresponding to the amount of the heated object, and calculate the temperature rise of the heated object estimated from these,
A microwave oven characterized by controlling the power generated by a microwave power generation source so as to perform optimal cooking according to a heating program determined for each object to be heated.