JPH0240546A - boiling sensor - Google Patents

Abstract

PURPOSE:To stabilize an output in a simple constitution having a small number of parts by outputting a voltage obtained by converting a polarized current of a piezoelectric element from an impedance converting circuit, and comparing the output with the output of a setting means. CONSTITUTION:A pair of electrodes are evaporated on both surfaces of a piezoelectric element 1. One electrode is electrically bonded to a metal case 3. A polarized current is generated with heat at the time of boiling. A resistor 6 having high resistance, a capacitor 7 and an impedance converting circuit 8 are attached to a substrate 11 and electrically connected to the element 1 in parallel. Those parts are molded with a resin 12 and sealed together with a case 3 as a unitary body. Therefore, intrusion of moisture from the outside is prevented. The change in high resistance of the resistor 6 is prevented. The output of a sensor 13 is stabilized. The output signal from the element 1 is increased in the resistor 6. Noises are decreased in the capacitor 7. The boiling state can be detected from the output voltage from the circuit 8 and the preset voltage from a setting means 14 only with a circuit constitution having the comparing means for the values from the outside. At this time, an integrating circuit comprising the capacitor 7 and the internal resistance of the element 1 is used.

Description

[Detailed Description of the Invention] Industrial Application Field The present invention relates to a boiling sensor, and in particular to a boiling sensor used in a microwave oven to detect and control water vapor generated from an object to be heated (hereinafter referred to as food). The present invention relates to a boiling sensor using a piezoelectric element as a detection element.

2. Description of the Related Art Some conventional boiling sensors utilize humidity sensors. One such humidity sensor includes a sensor chip 23, a heater 24, a resin base 25, a mesh cover 26, and a terminal 27, as shown in FIG.

The resistance value of the sensor chip 23 changes depending on the humidity.

Based on this change in resistance value, the cooking status of the microwave oven is detected by a controller (not shown) or by a voltage divided by a reference voltage and a resistor.

Furthermore, as a simple method for detecting a boiling state, there has conventionally been a method of detecting a polarization current generated by transfer of heat from water vapor to a piezoelectric element. The circuit configuration for detection is such that the output of the piezoelectric element is passed through a filter circuit, an amplifier circuit, and a smoothing circuit, and then compared with a setting means (for example, Japanese Patent Laid-Open No. 61-269890).

Problems to be Solved by the Invention However, when a humidity sensor is used as described above, gas, oil, etc. in the food adhere to the humidity sensor during cooking, resulting in a decrease in detection sensitivity. For this reason, it is necessary to evaporate the deposits on the humidity sensor using a heater for refresh heat treatment or the like after each cooking cycle, resulting in extra power and cost. Furthermore, since the voltage across the resistor is used as the control signal, when producing a large number of products, variations in the resistance of each component such as the humidity sensor, the resistance of the resistor, the voltage of the power supply, etc. This leads to signal variations and is difficult to manage.

Further, in the configuration of the detection circuit using the pressure 'fti, element, there are problems such as a large number of parts, the need to adjust the gain of the amplifier circuit, and variations in the cutoff frequency of the filter circuit.

SUMMARY OF THE INVENTION An object of the present invention is to solve these problems and provide a boiling sensor that uses a piezoelectric element, has a simple configuration with a small number of parts, and can provide a stable output signal.

Means for Solving the Problems In order to achieve the above object, the boiling sensor of the present invention includes a piezoelectric element that generates a polarized current upon receiving heat during boiling, and a piezoelectric element that is electrically connected in parallel with the piezoelectric element and made of resin. a high resistance, capacitance and impedance conversion circuit molded and enclosed in a case together with the piezoelectric element; a setting means capable of setting a value corresponding to the output of the impedance conversion circuit at the time of boiling; It is composed of comparison means for comparing the output with the output of the setting means.

Operation According to the above configuration, the polarization current of the piezoelectric element is converted into voltage with high resistance. Furthermore, the capacitance and the internal resistance of the piezoelectric element constitute an integrating circuit, which serves as a filter whose cutoff frequency is determined. The impedance conversion circuit outputs the voltage detected at the high resistance as it is, and by comparing this with the output of the setting means, boiling can be detected with a simple configuration. High resistance, capacitance and impedance conversion circuits are molded with resin.
Since it is enclosed in a case together with the piezoelectric element, it prevents moisture from entering from the outside and stabilizes the output signal.

EXAMPLE Hereinafter, an example of the present invention will be described based on the accompanying drawings.

FIG. 1 is a configuration diagram of a boiling sensor 13 according to an embodiment of the present invention. A pair of electrodes 2 are deposited on both sides of a lead titanate-based piezoelectric element l, and itr! One side of @2 is bonded to the metal case 3 with an adhesive 4 so as to make electrical contact. Reference numeral 11 denotes a board, which includes a lead wire 5 connected to the board ttr:Fi, a metal case 3, a high resistance 6, a capacitor fk (hereinafter referred to as a capacitor) 7, and an impedance conversion circuit 8 composed of an FET. is installed. As shown in Figure 2,
These high resistance 6, capacitor 7, and impedance conversion circuit 8 are electrically connected in parallel with the piezoelectric element 1. 9 is a lead wire that supplies power to the impedance conversion circuit 8, and is an output lead wire of the toh impedance conversion circuit 8. The components on the substrate 11 are molded with a resin 12, and the substrate 11 and the metal case 3 are integrally sealed.

FIG. 2 is a circuit diagram of boil sensor 130. here 14
ti is a setting means for setting a signal value of a boiling end state.

15 is a comparison means, and F which is an impedance conversion circuit 8
The output of ET and the output of setting means 14 are compared. A resistor 16 connected in series with the impedance conversion circuit 8 is provided inside the boiling sensor 13 .

FIG. 3 is an overall configuration diagram of a microwave oven using the boiling sensor 13. This microwave oven has a heating chamber 17, a magnetron 18, a cooling fan 19, and an exhaust path 20. 21 is a food product, and 22 is a control means.

The food 21 is heated with high frequency by a magnetron, and eventually boils to generate water vapor, which hits the metal case 3 of the boiling sensor 13 by the wind of the cooling fan 19, causing a thermal change to the piezoelectric element 1.

Figure 4 shows the pressure v! 20Hz from direct current (DC) to L element l
It shows the change in the output signal at Six and the output signal before boiling when a capacitor (CG) 7e is added and when it is not added when a voltage in the band up to AC is applied.

When capacitor 7 is added, the signal level before boiling is lower (about 10 dB) than when it is not added.
Qualitatively, it can be seen that the S ratio with the signal level at the time of IJ15 is improved.

In other words, since the capacitor 7 and the internal resistance of the piezoelectric element 1 form an integrating circuit, the cutoff frequency of the filter is determined by selecting the capacitance of the capacitor to an appropriate value (for example, 0.001 μF), and the noise component is reduced. This is something that can be reduced.

FIG. 5 shows the impedance conversion circuit 8 and 11.
! It is a figure which shows the change of the output signal before and after boiling.

It can be seen that as the resistance value of the high resistance (RG) 6 connected in parallel to the piezoelectric element 1 increases, the voltage vs of the output signal increases. In addition, the resistance value of high resistance 6 is 100M
Approximately ρ is appropriate.

This is because when the piezoelectric element 1 undergoes a thermal change of water vapor, the following equation (11) is derived.

V5=α・Ro・IP(ΔT)−−−−−−m Here,
α is a proportionality constant. IP is the current generated due to thermal change (ΔT) and is a function of temperature. Also, the temperature of boiling water vapor is 100 degrees Celsius, and the thermal change is very large, so FM! IC-style IP will also grow. Therefore, it is observed as a sudden change in ■s.

FIG. 6 is a diagram showing the state of boiling detection by the comparing means 15. The output signal voltage v5 of the impedance conversion circuit 8 and the set voltage of the setting means 14 are determined by the comparison means 15.
When the output signal voltage v5 exceeds the set voltage of the setting means 14, the comparing means 15 is inverted. Based on this inversion signal, the control means 22 controls the energization of the magnetron 18 and the cooling fan 19.

Note that the impedance conversion circuit 8 may be any conversion circuit for high input impedance, such as a voltage follower of an operational amplifier, and is not limited to the above-described embodiment of the present invention.

However, the impedance conversion circuit 8 may be provided externally, and the lead wire may be a shield wire.

Effects of the Invention As described above, according to the present invention, the following effects can be obtained.

(1) High resistance, capacitance, and impedance conversion circuits are molded with resin and sealed in a case together with piezoelectric elements.
This prevents moisture from entering from the outside and provides high resistance resistance due to leakage.

This prevents changes in the contact value and stabilizes it, thereby stabilizing the output signal of the sensor.

(2) The high resistance connected in parallel to the piezoelectric element increases the output signal, and the capacitance reduces noise. This capacitance and the internal resistance of the piezoelectric element form an integrating circuit. Figure 5 is a diagram showing signal changes before and after boiling of the boiling sensor, Figure 6 is a diagram showing the state of boiling detection by the boiling sensor, and Figure 7 is a configuration diagram of a conventional humidity sensor for detecting boiling. be.

DESCRIPTION OF SYMBOLS 1... Piezoelectric element, 3... Metal case, 6... High resistance, 7... Capacitance, 8... Impedance conversion circuit,
12... Resin, 13... Boiling sensor, 14... Setting means, 15... Comparing means.

Agent Hiroshi Mori Moto Boiling conditions can be detected on roads.

Eight

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a boiling sensor according to an embodiment of the present invention, and FIG.
The figure is a detection circuit diagram of the same boiling sensor, Figure 3 is an overall configuration diagram when the same boiling sensor is applied to a microwave oven, and Figure 4 is a frequency characteristic diagram of the same boiling sensor. 4 dozen digits - - High th technique - Elephant quantity - Impedance Mugineia Road 2nd / 4 - 1 Watari 1 Negative Office, - Hisoft J 歿 4th figure Zhou Age (Ht) Fig. 5 Second, the gate lock

Claims (1)

[Claims] 1. A piezoelectric element that generates a polarized current when it receives heat during boiling, and a high resistance and capacitor that is electrically connected in parallel with this piezoelectric element, molded with resin, and enclosed in a case together with the piezoelectric element. and a boiling sensor comprising an impedance conversion circuit, a setting means capable of setting a value corresponding to the output of the impedance conversion circuit during boiling, and a comparison means for comparing the output of the impedance conversion circuit and the output of the setting means. .