JPS60240093A - High frequency heater having wireless temperature probe - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION P "Field of Industrial Application" The present invention relates to a high-frequency heating device that captures the temperature of an object to be heated using a wireless temperature glove and automates cooking control.

``Prior art'' In conventional high-frequency heating equipment, as a method of automating heating control, heat, water vapor, or gas generated from the object to be heated is detected using a thermistor, humidity sensor, or gas sensor, respectively, and the temperature of the object to be heated is determined. There are some that detect and automatically control heating.

In addition, the surface temperature of the heated object is detected by an infrared sensor.

There is also a method that detects the finish of the heated object and performs automatic control.

In both of these methods, the sensor is installed outside the heating chamber to prevent it from being affected by strong electric fields and high frequencies.

Cooking operability is good, but the temperature of the atmosphere inside the heating chamber is poor.

It is inevitable that some errors will occur when detecting humidity and gas, so please check if the detection accuracy decreases due to repeated heating or if there is a difference in the amount of the heated object. P There is a decrease in detection accuracy. However, it has been difficult to constantly detect and control any heated object with high precision. Among these, the method using an infrared sensor.

Since the area to be detected, that is, the field of view, is narrow, it is impossible to detect if the object to be heated is removed from that area, and there are drawbacks such as the need to devise the installation position of the object to be heated and the shape of the container.

To solve these problems, there is a method in which a glove equipped with a temperature sensor such as a thermistor is inserted into the object to be heated, and the temperature signal is taken out of the heating chamber by wire to control heating.

In the above case, the temperature inside the object to be heated can be accurately determined, and the heating can be controlled at the temperature desired by the user. However, since the probe is connected by wire to a control circuit outside the heating chamber, it is mechanically and electrically difficult to use it in combination with a turntable, which is effective in preventing uneven heating of the heated object. . It was also inconvenient to use because it was wired.

For this purpose, a glove is used as a wireless temperature probe, and a resonant circuit is constructed with a temperature-sensing capacitor and a coil antenna. Radio waves containing the resonant frequency are transmitted from a transmitting antenna inside the heating chamber, and are received by a receiving antenna and placed on the heated object. A device has been developed to detect the temperature of However, there was a problem in that the operation of the temperature probe became unstable due to the high frequency in the heating chamber.

``Means for Solving Problems'' The present invention consists of a wireless temperature probe consisting of an LC resonant circuit consisting of a coil antenna and a temperature-sensing capacitor, which is brought into contact with the object to be heated. A choke structure is used between the coil antenna and the capacitor as a globe structure in a device that detects the temperature of the heated object by transmitting changes in the resonant frequency of the probe from the transmitting antenna and capturing the resonance point of the probe in the receiving circuit. Two chokes are connected in series, and a coil is connected between the chokes.

``Function'' The amount of attenuation increases in a wider band than in the case of a single-stage choke structure, and high frequencies are prevented, so the temperature-sensing capacitor is protected and its operation becomes stable.

``Embodiment'' Two embodiments of the present invention will be described below with reference to the drawings, including a description of a conventional device.

FIG. 1 is a perspective view of a high frequency heating device using a probe. 1 is a main body of the high-frequency heating device, 2 is a control panel, 6 is a heating chamber, and 4 is a door. 5 is an object to be heated, and 6 is a turntable. 7 is a glove, 8
is a transmitting antenna, and 9 is a receiving antenna. FIG. 2 is a sectional view of FIG. 1. 10 is a motor that drives the turntable.

11 is a magnetron that oscillates a high frequency. In FIGS. 1 and 2, the transmitting antenna 8 is installed above or below the heated object 3, and the wireless temperature globe 7 and the heated object 5 are sandwiched between the transmitting antenna 8 and the receiving antenna 9. do. Note that it is also possible to use a sheathed heater as both the transmitting and receiving antennas.

Next, a block diagram of this device is shown in FIG. The outline is that two loop antennas are installed, these P loop antennas are electromagnetically coupled, and the coil antenna of the globe is electromagnetically coupled to these loop antennas so that the resonant frequency of the probe changes depending on the temperature. I made it. The resonant frequency is then captured and the temperature of the heated object detected by the probe is detected.

This will be explained in more detail below.

The probe 7 electrically consists of a coil antenna and a temperature-sensing capacitor, which will be described later. The capacitor is provided at the tip of the probe 7 and detects the temperature inside the object to be heated 5. Generally, the capacitance C of a capacitor is.

When expressed as a function of temperature, it becomes the following equation.

C= (1+A(T To )] C6-(1) where A is the temperature coefficient, T is the measured temperature, and TO is the reference temperature (20
°C) + Co is the capacitance value of the capacitor at the reference temperature of 20 °C.

Further, the resonant frequency fO of the probe is given by the inductance of the coil antenna being L.

It is expressed as Therefore, if the inductance L is fixed at 7P, the resonant frequency fo changes in proportion to the reciprocal of the square root with respect to the temperature T, but in reality, the capacitance co of the capacitor is 220 pF and the temperature coefficient A is -750 PPM/deg.
year.

When the impedance L of the coil antenna is 1.0 μH, the relationship between temperature and resonance frequency is as shown in FIG. 4.

From this characteristic, when the temperature T is 0 to 100°C, the resonant frequency f
o changes almost linearly. However, as can be seen from equation (2), the resonant frequency f with respect to the temperature T.

is a quadratic curve, but because the temperature range used is narrow, it can be regarded as a straight line.

The signal shown in FIG. 5G) generated by the transmitting circuit 12 is transmitted intermittently or continuously from the transmitting antenna 8 in the heating chamber 6. This signal is a signal swept 12 times from a certain frequency f1. The frequency width of the oxtail is determined so as to include the resonant frequency fo of the probe.

When the signal of the receiving antenna 9 is amplified, the received signal shown in FIG. 5(1)) is obtained. In this received signal, a resonance characteristic is obtained in which the voltage at the frequency fo tuned to the probe 7 becomes significantly large. Furthermore, in order to make this resonance characteristic more noticeable, the Q of the probe 7 circuit is increased.

The receiving circuit 14 captures this resonant frequency fo,
A big blow to the microcomputer 16. The microcomputer 13 knows the resonant frequency fo, derives the temperature of the probe tip, displays the temperature on the display 16, and controls the power supply circuit 15 of the magnetron 11 which oscillates a high frequency when the temperature set by the user is reached. Reference numeral 17 indicates a keyboard of the control panel 2, through which the user sets the temperature.

In the receiving circuit 14, there are two methods for detecting the temperature of the probe 7. For example, the resonance point is determined by detecting the peak value of the received signal, and time is measured using another stable high frequency signal at the resonance point from the start of the sweep.

There is a method of determining the position of the resonance point and then detecting the temperature of the tip of the probe, that is, the temperature of the heated object.

Next, the structure of the glove 7 is shown in FIG. Probe 7 is
A temperature-sensing capacitor 21 for detecting temperature is enclosed in a protrusion inserted into the heated object 5, and a band rejection filter is provided to protect it from the high frequency of the strong electric field in the heating chamber entering from the coil antenna 18. As a band rejection filter.

There is a choke structure 21 which has a simple structure and a large passing loss. This is formed from a metal cylindrical conductor, and its structure will be explained in detail later. The coil antenna 18 has a structure in which a distance is provided between the wires of the coil or an insulating material 22 is sandwiched between the wires of the coil to prevent discharge due to high frequency waves of a strong electric field within the heating chamber. The coil antenna part is a trench.

It is coated with a non-conductive material 23 to make it easier for the user to grip.

Furthermore, as can be seen in Fig. 6, the prologue 7 has no so-called electronic parts except for the temperature-sensing capacitor 21, so the ambient temperature inside the heating chamber 6 during oven cooking is 200 to 250°C.
It is not affected and can be used.

FIG. 7 is a cross-sectional view of the choke structure 19.

If the length of the choke is 428, then the choke opening 66
Karamida impedance zo is.

Zo = jRotanβ4 (8) It is expressed as Here, Ro is the characteristic resistance of the choke.

β is a phase constant.

In equation (8), the choke length t, 28 is expressed as 4(λ
:Free space wavelength of radio wave used for 0P), impedance zo
becomes infinity, ie, the choke opening becomes open.

4 from the point of the choke opening on a transmission line with a choke
The impedance associated with a point separated by the length of is 0, that is, a short circuit occurs.

However, this holds true for radio waves of a single frequency, and actually has frequency characteristics as shown in FIG. A characteristic 51 in FIG. 9 indicates that in the choke structure in FIG. 7, the outer diameter of the inner conductor 3o is
Inner diameter b1 = 3 wn + outer diameter c1 of inner conductor filter 2 of choke = 5w + 1. Inner diameter d+”18m+ of outer conductor 35
++ binding, chalk length 428 is 30.6 van
, when the gap ct of the choke opening, 29, is set to 3. With this characteristic, the peak value of passing loss is at frequency 245 [1
μH2, which is the frequency used in high-frequency heating equipment.

There is also another choke structure as shown in FIG. In order to shorten the overall length, a partition plate 48 is provided inside the choke, and the folded length t345 is set to η. In the choke structure 11 P of FIG. 8, the characteristic impedances of the chokes are made equal. That is, since the choke is in a coaxial mode, the dimensions are set to satisfy the condition 60, L-60t-■ □ (4). Here, e is the inner diameter of the inner choke. f is the outer diameter of the inner choke, 39°, and the inner diameter of the outer choke, 40. n is the outer diameter 41 of the outer choke.

In this folded choke structure, outer diameter a2 of inner conductor 48 = 2 mm + inner diameter 1)2 of outer conductor 58 = 3
tMl+ Inner diameter of the inner choke e25, outer diameter of the inner choke f = 9 tan, inner diameter of the outer choke m = 10 mm.

Outer diameter of the outer choke n=18 mm, gap g2-6 between the choke opening, gap g3-6 between the partition plate 48 inside the choke
The frequency characteristic at the beginning of the second generation is a characteristic curve 81 in FIG. When compared with the characteristic 51 of the structure in FIG.

The characteristic 81 of the folded choke is that when it starts to deviate from the resonance point, the loss level changes to 9 (', 2450±5QMH
The attenuation effect is about 3Q dB in z. Further, the characteristic 51 of the structure shown in FIG. 7 is an attenuation of about 40 dB at 2450±50 MHz, which is about 10 dB different from that of a folded choke.

This difference in attenuation level is due to the shorter portion of the transmission line A34 and the difference in characteristic impedance of the choke.

Therefore, it can be seen that although the folded choke structure can reduce the overall size, the performance is slightly lower.

To increase the effect of passing loss in the choke structure shown in Figure 7, it is necessary to use trenches.

1) Characteristic resistance Rc of the transmission line between the inner conductor 60 and the outer conductor 32
Reduce l.

2) Increase the characteristic resistance Rco of the choke.

Two points can be derived from the theoretical formula. Specifically, 1) is
Characteristic resistance Rc1 is expressed by the following equation.

Here, ε is the spacer 31 (
dielectric constant). Therefore, in order to reduce the characteristic resistance Rc4, it is sufficient to reduce the value of l2i-, that is, to reduce the gap between the inner conductor 30 and the outer conductor 32. However, in reality, if the gap is too small, discharge due to the 3P high frequency is likely to occur, so it cannot be made too small.

In the case of 2), the characteristic resistance Rc6 of the choke is.

It is expressed as RC (+ = 60 in (6), but if the inner diameter d of the outer conductor is increased, it is 1%
The sexual resistance Rco increases. However, the inner diameter d of the outer conductor cannot be made too large from the viewpoint of usability.

Therefore, the performance of the one-stage choke structure cannot be expected to be much more effective than the characteristics shown in FIG.

The above can naturally be said about the folded choke structure shown in FIG.

Looking at the characteristics of the first stage of the Ushida choke structure, the Q is large, so the operating frequency of the high frequency heating device (2450±50MHz
) must match the frequency of the peak loss value. Therefore, dimensional accuracy and assembly accuracy of the choke are required.

As an example of solving these problems, there is a choke structure shown in FIGS. 10 and 11. That is, the choke structures described above are connected in a two-stage series, and the coil 524P is connected between the choke structures.

FIG. 10 shows the choke structure shown in FIG. 7 connected in a two-stage series. The two choke structures 21 are housed in a metal cylindrical body 59, and high frequency waves are emitted from the gap between the cylindrical body 59 and the choke outer conductor 35. Contact and secure to prevent leakage. Furthermore, the inner conductor 60, which is a high-frequency transmission line, has a non-contact part near the middle between the choke structures, and the coil 5
Connect 2.

In the above structure, the dimensions of the single choke structure are as described above, the length t451 between the choke structures is 10 mm, the coil 52 is fm diameter 0.3 amφ, and the coil diameter is 3
Wnφ.

When the number of differential turns is 3, the 2-frequency characteristic becomes a characteristic curve 56 in FIG. 12. Therefore, compared to the characteristic curve 51 in FIG. 9, high frequencies can be prevented over a considerably wide band. Also, the attenuation value of 65 dB to 70 dB is a measurable value,
The actual attenuation amount is estimated to be larger than this value. Even if the length t451 between the chokes is shorter than 10 lengths, the frequency characteristics do not change. However, the choke structure shown in FIG. 10 tends to be a little longer because the overall length t755 needs to be about 60 holes or more.

15p The choke structure shown in FIG. 11 is approximately half the length of the choke structure shown in FIG. 10. This is the 8th
The folded choke structures shown in the figure are connected in a two-stage series, and an unconnected part is provided near the middle between the choke structures, and the coil 52 is connected to this point. The two choke structures are then housed and fixed in the metal cylinder 59.

In FIG. 11, each dimension is as described above, the length t654 between the choke structures is 10 mm, and the frequency characteristic when the coil described above is used is the characteristic curve 59 in FIG. 12.

Looking at this characteristic curve 57, in terms of performance it is equivalent to the characteristic shown in FIG. That is, the attenuation amount is 60 dB or more at OOMH2 of 2450 mm, so there is no problem.

The structure shown in FIG. 11 has an overall dimension t755 of 40 cm, which is practical.

Next, the frequency characteristics when two stages of choke structures are provided are determined using a theoretical formula and compared with experimental results.

FIG. 16 shows an equivalent circuit diagram when chokes are connected in a two-stage series. This transmission line will also be discussed as a lossless line. In the circuit, the thick black line is the choke point.

The characteristic impedance Z0161 of the choke part is.

As mentioned above, it is expressed by a quadratic equation.

Zo, = 607n −-□ (7) Here, C is the inner diameter 26 of the choke, and d is the outer diameter 27 of the choke.

The impedance seen from the choke opening is jzol
−β4o, the load impedance is zt
Then, the impedance z, 63 in Figure 2 is the following formula % formula % (8) The impedance Zoo64 of the trench transmission line A34 has the outer diameter of the inner conductor 30 of a, 2A, and the inner diameter of the outer conductor 32 of b125
Therefore, it becomes a quadratic equation.

Here, ε is the relative dielectric constant of the spacer 61. Also, the line length of impedance Zoo is t1165.

7P Impedance Z266 can be expressed by the following equation.

Next, Z is the impedance of the transmission line between the chokes. 267, zo267 is a coaxial system, and the inner diameter is "
Because I+outer diameter is K.

Zo2-60 tn H’ 1 It is expressed as

If its length is t1□68, impedance Z36
9 is the following formula.

Here, W is the angular frequency and L is the inductance of the coil 52.

Next, the length of Cho) is 7,371. Assuming that the impedance is Zo370, Zc3 has the same value as l201.

Impedance Z472 id in the circuit diagram.

Z4 = Z3 + jtan Zc3・-β right, □0
The impedance Z047 of the transmission line A is equal to the impedance Zoo, which is expressed by the equation (9). Since the length of the transmission line A is t1474, the impedance z5 seen from the inlet of the choke is expressed as t1474.

When the impedance on the power supply 77 side is Zc 76, the 2 reflection coefficient γ becomes a linear expression.

Therefore, the passing loss (Loss) is expressed by the following formula.

Loss (dB) = 10 log(1-γ2)
By actually substituting each dimension, load, and input impedance into the above equation and changing the inductance L of the coil 52 between the chokes, a frequency characteristic as shown in FIG. 14 is obtained. In FIG. 14, the characteristic curve 78 represents the inductance L
This is a case where is O, that is, a case where they are directly connected. The characteristic curve 79 shows the case when the inductance is L -0, 1 μH, and the characteristic curve 80 shows the case when the inductance L -〇, 25 μH. It can be seen that the band becomes broadband and the amount of attenuation also increases.

19P Therefore, in the actually measured characteristics shown in FIG. 12, if the inductance of the coil 52 is increased, the frequency characteristics become even wider.

"Effects of the Invention" As described above, according to the present invention, a filter with a simple structure and a broadband attenuation effect is constructed, which can block almost all high frequencies, protect the temperature probe, and eliminate fluctuations caused by leaked radio waves. External view of a high-frequency heating device using a low-temperature probe.

FIG. 2 shows a sectional view of FIG. FIG. 6 is a block diagram of the wireless temperature probe. Figure 4 is a characteristic diagram showing the relationship between temperature and resonant frequency of the probe, and Figure 5 (
FIG. 11A is a waveform diagram of a transmitted signal, and FIG. 7B is a waveform diagram of a received signal.

FIG. 6 is a cross-sectional view of the probe with a single choke structure. Figure 7 is a cross-sectional view of the same choke structure.
The figure is a cross-sectional view of the choke structure in which the folding length is λ7. Figure 9 is.

FIG. 8 is a diagram of actually measured frequency characteristics in the choke structures shown in FIGS. 7 and 8. FIG. FIG. 10 is a sectional view of a two-stage choke structure according to an embodiment of the present invention.

FIG. 11 is a sectional view of the choke structure of the included embodiment. FIG. 12 is an actually measured frequency characteristic diagram of the same choke structure. FIG. 13 is an equivalent circuit diagram of the same choke structure, and FIG. 14 is a frequency characteristic diagram of the choke structure based on the same theoretical calculation.

Ro...Heating chamber, 5...Object to be heated.

7...Wireless temperature probe.

8... Transmitting antenna, 9... Receiving antenna.

18...Coil antenna, 19...Choke structure.

21... Temperature sensing capacitor.

52...Coil.

Applicant: Hitachi Thermal Appliances Co., Ltd. Figure 7 Figure 8 Figure 9 Monthly wave number (G)lZ) <gp>’t Beast glance Zheng

Claims (1)

[Claims] A wireless temperature probe consisting of a resonant circuit composed of a temperature-sensing capacitor and a coil antenna that detects the temperature of the object to be heated in the heating chamber is inserted into the object to be heated, and a resonant frequency corresponding to the temperature of the temperature probe is set. A wireless temperature sensor that transmits radio waves with a frequency range including the above from a transmitting antenna inside the heating chamber, receives the signal with a receiving antenna placed between the above temperature probes, captures the resonance point of the above probes, and detects the temperature of the heated object. In a high-frequency heating device equipped with a probe, a choke structure (two 2+1s are provided in series, and the choke structure 01 is installed between the coil antenna and the capacitor so that the temperature-sensing capacitor is not affected by the high frequency waves in the heating chamber). ) A high-frequency heating device equipped with a wireless temperature probe, characterized in that a coil (54) is connected between the coils (54).