JPH01267446A - Piezo-electric element applied sensor - Google Patents

Abstract

PURPOSE:To detect cooking condition of foods accurately with a simple construction, by using a piezo-electric element to increase an output of an impedance with a conversion circuit along with an integral molding thereof. CONSTITUTION:One end of an electrode 3 deposited on a lead titanate based piezo-electric element 2 is bonded on a metal plate 5 and the other end is put into an input section 7 of an impedance conversion circuit 6. A lead 8 at a power source supply section and a lead 9 at an output section are led out of the impedance conversion circuit 6 and a gland 10 to be connected to the earth is connected to a metal plate 5. A lead 11 to be connected to the earth is also led out of the metal plate 5. A resin 12 seals up a piezo-electric element 12, the impedance conversion circuit 6, the metal plate 5 and leaders of various leads 8-11 integral, thereby constructing a piezo-electric element applied sensor 1.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a piezoelectric element, and more particularly to a humidity sensor for cooking appliances such as microwave ovens.

2. Description of the Related Art A conventional humidity sensor of this type includes a sensor chip 34, a heater 35, a resin base 36, a mesh cover 37, and a terminal 38, as shown in FIG. Since the resistance value of the sensor chip 34 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. When the humidity sensor is used in a microwave oven, the heater 35 detects food gas,
This is for refreshing the sensor chip 34 since it gets dirty with oil etc., and the mesh cover 37 is for wind protection and is used to save the wax power of the heater 35. (National Technical Report vol. 1.29 No. 3
JAN 1983) Also, as shown in FIG. 8, a case will be described in which an ultrasonic microphone is used as a humidity sensor. The ultrasonic microphone includes a piezoelectric element 2, a spacer 39, a terminal 40, a vibrating section 41, a case 42, a lead wire 43. It is composed of an inner ring 44, a terminal plate 45, and silicone rubber 46. Regarding the detection of the cooking state of food, Japanese Patent Application Laid-Open No. 62-11292
9@ Shown in the official bulletin.

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 humidity sensor during cooking, reducing the detection sensitivity, so it must be refreshed every - time. The dirt on the humidity sensor must be evaporated using a heater for heat treatment, which generates extra power and cost.Since the voltage across the resistor is used as a control signal, there are many
During production, variations in the resistance of the humidity sensor, other resistances, power supply voltage, etc. that are each component are linked to variations in the control voltage signal, making management difficult.

Further, when an ultrasonic microphone is used, there is a problem in that the shape of the case is complicated due to consideration of resonance frequency, vibration, etc.

In addition, the polarization current that is the output of the piezoelectric element is minute, so a high input impedance circuit or amplifier is required to control it, and if the lead from the piezoelectric element to the circuit is long, the effect of noise is large. was there.

Furthermore, since piezoelectric elements are close to insulators, they have the problem of being easily affected by environmental humidity, such as increased leakage current when humidity increases.

Means for Solving the Problems In order to solve the above problems, the piezoelectric element application sensor of the present invention first uses a piezoelectric element in order to prevent the generation of excess power and cost of the humidity sensor and to facilitate management. I am using it.

Furthermore, the piezoelectric element application sensor of the present invention is equipped with an impedance conversion circuit that suppresses the influence of noise and facilitates terminal processing in addition to the piezoelectric element adhered to a metal plate or a conductive plate, and In order to prevent this effect, it is configured to be integrally sealed with a molding material that is not moisture permeable.

Function According to the present invention, when using a piezoelectric element in a cooking appliance such as a microwave oven, it is possible to accurately control the cooking condition of food with a simple configuration by increasing the output by an impedance conversion circuit and by integrating the mold. It has a detectable effect.

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. Further, a power supply section glue portion 8 and an output section lead 9 are led out from the impedance conversion circuit 6, and a ground 10 for connection to earth is connected to the metal plate 5. A lead 1 is connected to the ground from the metal plate 5.
1 has also been derived. Note that the various lead wires are electrically insulated. The resin 12 is made of a piezoelectric element 2, an impedance conversion circuit 6, and a metal plate 5 in order to prevent the influence of environmental humidity.
, the lead-out portions of the various lead wires 8, 9, and 11 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 14 into which food 13 is placed and taken out, and a magnetron 1 which heats food 1a at high frequency.
5. Piezoelectric element application sensor 1 (
Of course, the piezoelectric element and impedance conversion circuit), cooling fan 17 that cools the magnetron 15, cooling fan 17
duct 18 that blows a portion of the air into the heating chamber 14, a filter 20 (hereinafter referred to as a filter) that filters the output signal 19 of the piezoelectric element, and an amplifier 22 that amplifies the signal 21 of the filter 20.
, a controller 24 (control signal 25), etc., which controls the operation of various devices according to signals 2-3 from the amplifier 22.

Detection of the completion of heating the food 13 is performed as follows. Food 13 is placed in heating chamber 14 , and a portion of the cooling air from magnetron 15 is guided into heating chamber 14 through duct 18 by cooling fan 17 .

A portion of the cooling air is shown by a solid arrow line 26, and a gas such as moisture generated from the food 13 is shown by a dotted arrow line 27. Part of cooling air 2
6 and gas 27 such as moisture generated from the food 13 are discharged to the outside of the heating chamber 14 through the exhaust section 16 . At this time, heat is exchanged between the piezoelectric element applied sensor 1 and the gas 27 such as moisture generated from the food 13, and the piezoelectric element applied sensor 1 generates an output signal 19 in response. This output signal 19 is filtered by a filter 20, amplified by an amplifier 22, and enters a controller 24. When the output signal entering the controller 24 exceeds a certain set level, it is determined that heating has ended, and a control signal 25 is output from the controller 24 to stop the oscillation of the magnetron 15 and the cooling fan 17.

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 2 itself has a high impedance, it is difficult to control and easily affected by noise caused by routing unless it is supported by a high input impedance circuit. Therefore, an impedance conversion circuit 6 consisting of a resistor 2, a resistor 29, and a source follower of an FET 30 is used to lower the impedance without changing the output voltage, and extract a stable output voltage Vout19. However, a DC voltage Vc31 is required as a power source.

Fig. 4 shows the signal of the piezoelectric element applied sensor 1 (output of the impedance conversion circuit 6) when 300cc of water is heated.
The results of spectrum analysis of the output voltage waveforms for noise and noise are shown. 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 the a and the mouth is about 35 dB at 4Hz.
The signal level is a voltage of 1 mV or less.

FIG. 5 shows a low-pass filter 20 (cutoff frequency 4 Hz) designed based on the frequency characteristics shown in FIG. 4. Two stages of low-pass filters 32 and 33 are used to improve the characteristics.

FIG. 6 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 r on the output signal, the S/N is higher than α when there is an impedance conversion circuit 6, and β when there is no 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 applied sensor 1 to the circuit is reduced.

Note that the piezoelectric element 2 is bonded to the metal plate 5 with an adhesive 4 and 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.

(Since the re-impedance conversion circuit is integrated,
Since the output is increased and the influence of noise is suppressed, there is no need to use shielded wires, and lead terminal processing is easy.

(2) Since the piezoelectric element and the impedance conversion circuit are molded with resin on a metal plate, when used to detect the heating state of food, the configuration is simple and a signal with good S/N can be obtained. There is little variation in detection due to the influence of noise, and leaks due to moisture can be prevented, resulting in extremely stable cooking results.

(3) Since the heating state of the food is detected by the response of the thermal change of the piezoelectric element, the shape of the metal plate, the arrangement of the piezoelectric element, and the impedance conversion circuit are extremely flexible and easy to design. There is no need for a heater to remove dirt such as oil, which is essential for humidity sensors and gas sensors, and it can be made lighter, thinner, and more compact.

(4) Similar to (3), it is power saving because no heater is required.

(5) Similar to (3) and (4), since no heater is required, there is no need for control parts for maintaining the accuracy of the heat-retaining heater power or a special transformer for the heater power, and the cost is low. When used in cooking appliances such as microwave ovens, it can significantly reduce costs.

[Brief explanation of the drawing]

Fig. 1 is a cross-sectional view of a piezoelectric element applied sensor showing an embodiment of the present invention, Fig. 2 is a main body configuration diagram when the piezoelectric element applied sensor is used in a microwave oven, and Fig. 3 is a cross-sectional view of a piezoelectric element applied sensor showing an embodiment of the present invention. A circuit diagram showing an example of the included impedance conversion circuit. Figure 4 is a characteristic diagram of the output and noise of the piezoelectric element applied sensor when 300 cc of water is heated. Figure 5 is a filtered output of the piezoelectric element applied sensor. Fig. 6 is a frequency characteristic diagram showing the output of a piezoelectric element applied sensor depending on frequency, Fig. 7 is a partially cutaway perspective view of a conventional absolute humidity sensor, and Fig. 8 is a diagram of a conventional ultrasonic microphone. FIG. 1... Piezoelectric element applied sensor, 2... Piezoelectric element, 5... Metal plate, 6... Impedance conversion circuit, 12... resin. Name of agent: Patent attorney Toshio Nakao and 1 other person1-
Piezoelectric photon applied senna? = Piezoelectric baby 3-t pole 5- meeting A 6- impedance f conversion circuit 12- resin Fig. 1 Fig. 2 18 Do8R-11 stroke a Fig. 4 Hamaaki [1-izl Fig. 6 Fig. 7 34-de Nsa + Soge? ? --Spanner 柘-Hatoko Φ1 Transferring part 42- Case No. 81Z43-Lee): ``A-・Inner ring

Claims (4)

[Claims] (1) A metal plate or a conductive plate body, a piezoelectric element having a pair of electrodes, and an impedance conversion circuit, and one end of the electrode of the piezoelectric element and the ground of the impedance conversion circuit are connected to the metal plate or the conductive plate. electrically connecting the other end of the electrode of the piezoelectric element and the input end of the impedance conversion circuit;
Lead wires are led from the metal plate or the conductive plate, the output end of the impedance conversion circuit, and a power supply section to the impedance conversion circuit,
A piezoelectric element application sensor having a structure in which the metal plate or conductive plate body, the piezoelectric 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 is 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. (4) The piezoelectric element applied sensor according to claim 1, wherein the resin is non-moisture permeable and is integrally molded.