JPH01253644A - Piezoelectric-element applying sensor - Google Patents

Abstract

PURPOSE:To stabilize the detection of moisture and to detect the cooking state of food in a simple constitution, by molding a piezoelectric element, a temperature correcting element and an impedance conversion circuit with a resin on a metal plate. CONSTITUTION:A pair of electrodes 3 are evaporated on a lead titanate based piezoelectric element 2. The electrodes are bonded on a metal plate 5 with a bonding agent 4. The piezoelectric element 2, a temperature correcting element 11, an impedance conversion circuit 6, the metal plate 5 and the lead-out parts of various lead wires 8, 9, 13 and 14 are sealed with a molding material having no moisture transmitting property as a unitary body. When this sensor is used for detecting the heated state of food, the detecting time is not extended due to the decrease in output caused by the temperature characteristic of the piezoelectric element. Dispersion due to the effect of noises and leaking due to moisture can be prevented, and stable cooking can be performed.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to the application of piezoelectric elements, and particularly to humidity sensors for cooking appliances such as microwave ovens.

2. Description of the Related Art Conventionally, a humidity sensor is comprised of a sensor chip 38, a heater 39, a resin base 40, a mesh cover 41, and a terminal 42, as shown in FIG. sensor chip 3
Since the resistance value of No. 8 changes depending on humidity, the cooking state of the microwave oven is detected by comparing a reference voltage with a voltage determined by the resistance value. The peta 39 is for refreshing the sensor chip 38 because it gets dirty due to food gas, oil, etc. when the humidity sensor is used in a microwave oven, and the mesh cover 41 is for windproofing.
This is used to save power of the heater 39. (National Technical L/Port vol.

29N○j JAN 1983) Also, as shown in FIG. 9, a case will be described in which an ultrasonic microphone is used as a humidity sensor. The ultrasonic microphone includes a piezoelectric element 43, a spacer 44, a terminal 45, a vibrating part 46, and a case 4.
7, Lee-1/wire 48, inner ring 49, terminal plate 50. It is made of silicone rubber 51. Detection of the cooking state of food is disclosed in Japanese Patent Application Laid-Open No. 112929/1983.

Problems to be Solved by the Invention However, when using a humidity sensor as described above, gas and oil in the food adhere to the sensor chip 38 during cooking and the detection sensitivity decreases. Refresh, - Sensor chip 38 with heater 39 for heat treatment, etc.
It is necessary to evaporate the deposits on the sensor chip 38, which generates extra power and cost, and the voltage across the resistor is used as a control signal. , other resistances, variations in power supply voltage, etc. lead to variations in the control voltage signal, making management difficult.

Furthermore, when an ultrasonic microphone is used, the shape of the case 47 is complicated due to consideration of resonance frequency, vibration, etc., and due to the temperature characteristics of the piezoelectric element 43, when food is repeatedly heated in a microwave oven, the piezoelectric element 43 may be damaged. As the temperature of the attached exhaust part rises, the polarization current of the piezoelectric element 43 also decreases due to the atmospheric temperature condition of the exhaust part, and even if the same amount of food is heated, the heating end time becomes longer, and the cooking time increases. There was a problem with the finishing temperature changing.

In addition, the polarization current that is the output of the piezoelectric element 43 is minute, and when controlling it, a circuit or amplifier with high input impedance is required, and if the lead part from the piezoelectric element 43 to the circuit is long, the influence of noise is large. There was a problem.

Furthermore, since the piezoelectric element 43 is close to an insulator, there is also the problem that the piezoelectric element 43 is not easily affected by environmental humidity, such as an increase in leakage current when the humidity is high.

Means for Solving the Problems In order to solve the above problems, the piezoelectric element applied sensor of the present invention includes a metal plate or a conductive plate, a piezoelectric element and a temperature correction element each having a pair of electrodes,
an impedance conversion circuit, one end of each of the electrodes of the piezoelectric element and the temperature correction element and the ground of the impedance conversion circuit are electrically connected on the metal plate or a conductive plate; The other end of the electrode and the input end of the impedance conversion circuit are electrically connected, and the metal plate or a conductive plate body;
Lead wires are led out from the other end of the electrode of the temperature correction element, the output end of the impedance conversion circuit, and the power supply section to the impedance conversion circuit, and the metal plate or the conductive plate and the The piezoelectric element, the temperature correction element, the impedance conversion circuit, and the lead-out portions of the lead wires are integrally molded with resin.

According to the present invention, a piezoelectric element is used, and when used in a cooking appliance such as a microwave oven, the temperature correction element and the impedance conversion circuit are integrally molded to easily control the cooking condition of food. can be detected accurately.

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

FIG. 1 is a sectional view of a piezoelectric element applied sensor 1 according to an embodiment of the present invention. A pair of vapor-deposited electrodes 3 are applied to a lead titanate-based piezoelectric element 2, and one end of the electrodes 3 is electrically contacted and bonded onto a metal plate 5 with an adhesive 4. The other end of the electrode 3 enters an input section 7 of an impedance conversion circuit 6. Furthermore, a lead 8 for a power supply section and a lead 9 for an output section are led out from the impedance conversion circuit 6, and a ground 10 for connection to earth is connected to the metal plate 5.
＼-It's gone. One end of the electrode 12 of the temperature correction element 11 is in electrical contact with and adhered to the metal plate 5 with an adhesive 4. A lead 13 is led out from the other end of the electrode 12 of the temperature correction element 11. A lead 14 for connection to ground is also led out from the metal plate 5. Note that the various lead wires are electrically insulated. In order to prevent the influence of environmental humidity, the resin 15 includes a piezoelectric element 2, a temperature correction element 11, an impedance conversion circuit 6, a metal plate 5, and various lead wires 8.9.
The lead-out portions 13 and 14 are integrally sealed with a non-moisture permeable molding material. These constitute the piezoelectric element application sensor 1.

FIG. 2 is a block diagram of the main body of a microwave oven having the piezoelectric element applied sensor 1 of the present invention. A heating chamber 17 into which the food 16 is taken in and taken out, and a magnetron 1 which heats the food 16 with high frequency.
8. Piezoelectric element application sensor 1 (
A cooling fan 20 that cools the McNetron 18 (which of course includes a piezoelectric element, an impedance conversion circuit, and a temperature correction element), a duct 21 that blows part of the air from the cooling fan 20 to the heating chamber 17, and a filter that filters the output signal 22 of the piezoelectric element. filter 23 (hereinafter referred to as filter), the signal 24 of the filter 23
The controller 28 includes an amplifier 25 that amplifies the signal 25, a controller 28 that corrects the signal 26 from the amplifier 25 with a signal 27 from the temperature correction element, and controls various equipment operations.

Detection of the completion of heating the food 16 is performed as follows. Food 16 is placed in heating chamber 17, and a portion of the cooling air from magnetron 18 is guided into heating chamber 17 via duct 21 by cooling fan 2o.

A portion of the cooling air is shown by a solid arrow line 30, and a gas such as moisture generated from the food 16 is shown by a dotted arrow line 31. Part 3 of cooling air
0 and gas 31 such as moisture generated from the food 16 are discharged to the outside of the heating chamber 17 through the exhaust section 19. At this time, heat is exchanged between the piezoelectric element applied sensor 1 and the gas 31 such as moisture generated from the food 16, and the piezoelectric element applied sensor 1 generates an output signal 22 in response. This output signal 22 is filtered by a filter 23, amplified by an amplifier 25, and enters a controller 28. When the output signal entering the controller 28 exceeds a certain set level, it is determined that heating has ended, and a control signal 29 is output from the controller 28, causing the magnetron 18 to oscillate and the cooling fan 2° to stop. This is a mechanism to stop the At this time, the temperature correction signal 27 is used to correct the decrease in the output 22 due to the temperature characteristics of the piezoelectric element, thereby stably detecting the end of heating.

FIG. 3 shows an example of the impedance conversion circuit 6. There are various impedance conversion circuits, but here we will use FET.
We will describe a source follower using Since the piezoelectric element itself has a high impedance, it is difficult to control unless it is supported by a high input impedance circuit, and is susceptible to noise caused by routing. So resistance 32, resistance 3
3. An impedance conversion circuit 6 consisting of a source follower of an FET 34 is used to maintain a low impedance without changing the output voltage and extract a stable output voltage Vout22. However, a DC voltage Vc35 is required as a power source.

Figure 4 shows the pressure of 10 when heating 300cc of water.
＼-7 Type element application sensor 1 signal (impedance conversion circuit 6
The results of spectrum analysis of the output voltage waveforms regarding noise (output) and noise are shown below. A indicates the signal after boiling, the mouth indicates before boiling, and C indicates the signal when the power is not turned on. It can be seen that a large signal is generated due to the hot air containing warm water vapor hitting the piezoelectric element applied sensor 1. The difference between A and mouth is about 3 at 4Hz.
5 dB, and the signal level is a voltage of 1 mV or less.

FIG. 5 shows a low-pass filter 23 (cutoff frequency 4 Hz) designed based on the frequency characteristics shown in FIG. 4. Two stages of low-pass filters 36 and 37 are used to improve the characteristics.

FIG. 6 shows a characteristic diagram of the ambient temperature (temperature of the exhaust section 19) and the output voltage of the piezoelectric element application sensor 1. From this figure, when there is no temperature correction, the output voltage decreases as the ambient temperature rises, as shown in a. Therefore, by correcting the output signal with the temperature correction element 11 configured on a metal plate, even if the same amount of food is repeatedly heated as shown in b, the output will be the same and cooking can be completed in the same time. It shows 11 ・-.

FIG. 7 shows a characteristic diagram of frequency and output voltage of the piezoelectric element application sensor 1. From this figure, when the impedance conversion circuit 6 is present, α increases the output significantly more than when the impedance conversion circuit 6 is not present, β. Since the impedance after the impedance conversion circuit 6 is low, if we consider the noise level γ on the output signal, α with the impedance conversion circuit 6 is higher than β without the impedance conversion circuit 6. It is clear that things get better. In other words, the influence of noise on the lead section from the piezoelectric element application sensor 1 to the circuit is reduced.

Note that the piezoelectric element 2 and the temperature correction element 11 are bonded to the metal plate 5 and the adhesive 4 and are electrically short-circuited, which has the effect of simplifying the configuration and improving thermal responsiveness.

Further, by using materials having substantially the same coefficient of thermal expansion for the metal plate 5 and the piezoelectric element 2, there is an effect of preventing error voltages generated from unnecessary distortion due to a difference in coefficient of thermal expansion.

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

(1) Since the piezoelectric element, temperature correction element, and impedance conversion circuit are molded with resin on a metal plate, it has a simple configuration when used to detect the heating state of food, and it depends on the temperature characteristics of the piezoelectric element. There is no possibility that the detection time will be extended due to a decrease in the output and the cooking quality will change, and a signal with a good S/N ratio will be obtained, there will be little detection variation due to the influence of noise, and leaks due to moisture will be prevented, resulting in very stable cooking results. state.

(2) Since the impedance conversion circuit is integrated,
Since the output is increased and the influence of noise is suppressed, there is no need to touch shielded wires, etc., and lead terminal processing is easy.

(3) Since the heating state of the food is detected based on the response of the thermal change of the piezoelectric element, it is extremely flexible in terms of the shape of the metal plate, the piezoelectric element, the temperature correction element, and the arrangement of the impedance conversion circuit. It is easy to design, does not require a heater to remove dirt such as oil, which is essential for humidity sensors and gas sensors, and can be made lighter, thinner, and more compact.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing a piezoelectric element applied sensor showing one embodiment of the present invention, Fig. 2 is a block diagram showing the same piezoelectric element applied sensor when used in a microwave oven, and Fig. 3 is a block diagram showing the same piezoelectric element applied sensor. Figure 4 is a diagram showing the output and noise characteristics of the piezoelectric sensor, and Figure 5 is a diagram of the filter that filters the output of the piezoelectric sensor. The circuit configuration diagram, Figure 6 is a temperature characteristic diagram showing the output voltage of the piezoelectric element applied sensor depending on the ambient temperature,
Fig. 7 is a frequency characteristic diagram showing the output of a piezoelectric element application sensor depending on frequency, Fig. 8 is a perspective view of the structure of a conventional humidity sensor, and Fig. 9 is a cross-sectional view of the structure of a humidity sensor using a conventional ultrasonic microphone. be. 1 piezoelectric element applied sensor, 2 piezoelectric element, 5 metal plate, 6 impedance conversion circuit, 14..., 11 temperature correction element, 15...resin. Name of agent: Patent attorney Toshio Nakao and 1 other person/-
4-electronic fart sensor s-, Shudan Fig. 1, Fig. 2, Fig. 3, Fig. 4] Used LHz'J Fig. 6 Nei: A friend C'c3 Fig. 7

Claims (3)

[Claims] (1) A metal plate or a conductive plate, a piezoelectric element and a temperature correction element each having a pair of electrodes, and an impedance conversion circuit, and one end of each of the electrodes of the piezoelectric element and the temperature correction element. The ground of the impedance conversion circuit is electrically connected to the metal plate or a conductive plate, the other end of the electrode of the piezoelectric element and the input end of the impedance conversion circuit are electrically connected, and the Lead wires are led out from a plate or a conductive plate body, the other end of the electrode of the temperature correction element, the output end of the impedance conversion circuit, and the power supply section to the impedance conversion circuit, and the metal A piezoelectric element application sensor having a structure in which a plate or a conductive plate body, the piezoelectric element, the temperature correction element, the impedance conversion circuit, and the lead-out portion of each lead wire are integrally molded with resin. (2) The piezoelectric element applied sensor according to claim 1, wherein the piezoelectric element and the temperature correction element are bonded to a metal plate or a conductive plate with an adhesive. (3) The piezoelectric element applied sensor according to claim 1, wherein the metal plate or the conductive plate is made of a material having substantially the same coefficient of thermal expansion as the piezoelectric element.