HEATING APPARATUS WITH PIEZOELECTRIC DEVICE SENSOR

Abstract: In an exhaust passage (37) of a heating apparatus, a pie­ zoelectric device sensor (26) is provided for detecting heating condition of an object (32), such as food in a microwave oven heating chamber (31). A piezoelectric device of the sensor may be received in a metal casing (53) and be hermetically sea­ led. An output signal from the piezoelectric device sensor is processed by circuitry including an amplifier (27) having a pre­ determined frequency response so that only particular fre­ quency components lower than that of commercial a.c. supply are amplified and then compared with a threshold to detect whether sufficient amount of moisture or hot steam is emitted from the object in the heating chamber of the heating appara­ tus. The invention may be applied to the prevention of overheat­ ing in a microwave oven, temperature control of chemical pro­ cesses or the like to control heating time.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to heating apparatus, and particularly to a sensor for detecting heating condition of an object to be heated. 2. Prior Art In a heating apparatus, such as a microwave oven used for cooking, food to be cooked is apt to be overheated because of very high heating speed. Therefore, it is necessary to detect the heating condition of the heating object, such as food, so as to stop heating or reduce heating power. One conventional example of a sensor used for detecting heating condition is shown in United States Patent 3,839,616. However, the technique disclosed in this patent requires relatively complex structure since the variation in resistance of a sensing element is detected for the detection of heating condition. Furthermore, the sensing element has to be kept clean because it is apt to become dirty. Another conventional example is disclosed in United States Patent 3,467,804. However, this technique also requires complex structure since acoustic impedance is detected by way of an acoustic transmitter and an acoustic receiver, while it is necessary to involve complex improvement so as to detect various pieces of food having different shapes and dimensions. SUMMARY OF THE INVENTION The present invention has been developed in order to remove the above-described drawbacks inherent in the conventional heating condition detecting devices. It is, therefore, an object of the present invention to provide a new and useful heating apparatus with an improved heating condition detecting sensor. According to a feature of the present invention a piezoelectric device is used as a heating condition sensor so as to detect hot steam emitted from heating object, such as food in a microwave oven. According to another feature of the present invention, heating condition is effectively detected using a new principle, which is found by the present inventors, that low frequency components around 10 Hz of an output signal from a piezoelectric device has correlation with the amount of hot steam emitted from heating object. In accordance with the present invention there is provided a heating apparatus as defined in the appended claim 1. BRIEF DESCRIPTION OF THE DRAWINGS The object and features of the present invention will become more readily apparent from the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings in which: Fig. 1 is a cross-sectional view of a coventional microwave oven; Fig. 2 is a cross-sectional view of another conventional microwave oven; Fig. 3 is a schematic view of an embodiment of heating apparatus according to the present invention; Fig.; 4 shows how a piezolelectric device sensor is installed in the microwave oven shown in Fig. 3; Fig. 5 is a cross-sectional view of the piezoelectric device sensor shown in Figs. 3 and 4; Figs. 6A and 6B show characteristics of output voltage from the piezoelectric device sensor of Fig. 5; Fig. 7 is a circuit diagram of an amplifier shown in Fig. 3; and Fig. 8 is diagram showing an output signal from the amplifier shown in Fig. 3. The same or corresponding elements and parts are designated at like reference numerals throughout the drawings. DETAILED DESCRIPTION OF THE INVENTION Prior to describing a preferred embodiment of the present invention, the above-mentioned conventional arrangements will be described for a better understanding of the present invention. Fig. 1 shows a microwave oven disclosed in the above-mentined United States Patent 3,839,616. The microwave oven comprises a casing 1 and a door 2 attached thereto to define a heating chamber 3 in which food (no numeral) is shown to be received. As the food is heated, steam is emitted therefrom which is exhausted together with air in the heating chamber 3 via an exhaust fan 4 and a exhaust passage 5. The reference 6 designates a sensing element whose resistance varies as ambient humidity changes. In order to detect the variation in resistance of the sensing element 6, it is necessary to apply a reference a.c. voltage to a series circuit of the sensing element 6 and a standard or reference resistor so that the voltage across the reference resistor is detected. In order to accurately detect the heating condition, the reference voltage should be stabilized while it is necessary to pay attention to reduce variation in the resistance of the reference resistor throughout a number of products. Furthermore, it is necessary to reduce thermal time constant of the sensing element 6. Since the sensing element 6 is apt to become dirty due to particles from food or the like, it is arranged to clean the same by heating the same to a high temperature. Therefore, complex structure for such heating of the sensing element 6 is needed while additional power consumption is required. Fig. 2 shows a microwave oven disclosed in the above-mentioned another United States Patent 3,467,804. A heating chamber 7 of a microwave oven is formed of an upper wall 8, a rear wall 9, a lower wall 10, side walls 12 and a door 11. Microwave energy generated in a magnetron 15 is supplied from an antenna 16 via a waveguide 14 and a slot 13 made in the upper wall 8 to the inside of the heating chamber 7. Within the heating chamber 7 is provided a shelf 17. A dish 19 containing food 18 to be cooked is shown to be placed on the shelf 17. Openings 24 and 25 are made in the rear wall 9, and an acoustic transmitter 20 provided at the exterior side of the rear wall 9 behind the opening 24, while an acoustic receiver 21 is proivided on the same behind another opening 25. A pad 22 is interposed between the receiver 21 and the rear wall 9, while a soundproof cover 23 is provided around the receiver 21 and the pad 22. In this arrangement, in order to detect the amount of steam emitted from the food 18 sound waves are emitted from the acoustic transmitter 20 and are received by the acoustic receiver 21. More specifically, the magnitude of sound waves propagated through the heating chamber 7 is detected to detect the amount of steam emitted from the food 18 using a principle that an acoustic impedance of the air or other cases in the heating chamber 7 varies due to the increase in steam amount. However, since acoustic resonating state canges due to the dimension of the space in the heating chamber 7, it is difficult to accurately detect the amount of steam when the shape or dimension of the food 18 changes. As a result, further complex improvement is required. Referring now to Fig. 3 an embodiment of the present invention will be described in connection with an example that the present invention is applied to a microwave oven. In a heating chamber 31 of a microwave oven generally denoted at the reference 100, a piezoelectric device sensor 26 is provided in an exhaust pipe or passage 37 so as to be exposed to exhaust gases from the heat chamber 31. The piezoelectric device sensor 26 is connected to an input terminal of an amplifier 27 used for voltage amplification, which amplifier is of the type arranged to pass only a given frequency range signal. An output terminal of the amplifier 27 is connected to an input terminal of a comparator 28 whose output terminal is connected to an input terminal of a control circuit 29 which controls energization of a motor 38 driving a fan 33 with which cooling air is applied to a magnetron 30. Food 32 to be cooked is shown to be placed in the heating chamber 31. The cooling air passed through the magnetron 30 is led via a duct 34 into the heating chamber 31. The cooling air is shown by way of arrows 35 while hot gases including moisture or steam emitted from the food 32 is shown by dotted lines with arrows 36. The cooling air 35 and the above-mentioned gases 36 including moisture emitted from the food 32 are exhausted from the heating chamber 31 via the exhaust pipe 37 to outside. The fan 33 is driven by a motor M whose first and second windings 41 and 42 are wound around a motor core 38. The first winding 41 is connected via a power switch 40 and a plug 39 to an unshown commercial a.c. supply line. The second winding 42 constitutes a constant-voltage power source circuit together with a rectifying diode bridge 43, a capacitor 44, a resistor 45, and a zenner diode 46. This constant-voltage power source circuit supplies the above-mentioned amplifier 27, comparator 28 and control circuit 29 with power, and no transformer is required because the voltage fed to the rectifying diode bridge 43 is much lower than that fed to the first winding 41. A buzzer 47 is connected to an output terminal of the control circuit 29 to alert that a sufficient amount of moisture is emitted from the food 32 when the voltage of the output signal from the amplifier 27 exceeds a threshold or reference voltage 48 fed from an unshown reference voltage source. In the illustrated embodiment, the control circuit 29 may be a switching circuit such as a power transistor for energizing the buzzer 47 in response to the output signal from the comparator 28. Although the buzzer 47 is used in the illustrated embodiment, this buzzer 47 may be replaced with other alarming device if desired. Moreover, the output signal from the comparator 28 may be used for controlling heating output by cutting off or reducing the same. In the case it is intended to control the output power of the magnetron 30, any of known control circuits for the same purpose may be used as the control circuit 29. Figs. 4 and 5 show an example of a moisture-proof ultrasonic microphone for 40kHz used as the piezoelectric device sensor 26 of Fig. 3. More specifically, Fig. 4 shows how to install the ultrasonic microphone 26 and Fig. 5 shows the structure of the same by way of a cross-section. As shown in Fig. 5, the ultrasonic microphone 26 comprises a piezoelectric device 49, a silicone rubber spacer 50, terminals 51, a diaphragm 52, a casing 53, lead wires 54, an inner ring 55, a terminal plate 56, and a silicone rubber supporting member 57. As shown in Fig. 4, the ultrasonic microphone 26 of Fig. 5 is attached to a circuit board 58 which is secured to a wall of the exhaust pipe 37 by way of bolts 59. As shown in Fig. 5, the piezoelectric device 49 is placed in the casing 53 which is hermetically sealed so that no moisture from the food 32 in the heating chamber 31 enters to deteriorate the piezoelectric device 49. In place of such a.hermetic sealing, the piezoelectric device 49 may be covered by an epoxy resin or other synthetic resin through dipping. Alternately, the piezoelectric device 49 may be placed in a sealed glass tube. Figs. 6A and 6B are diagrams showing characteristics of a signal and noise of the above-mentioned piezoelectric device sensor 26 which characteristics are obtained when water positioned in the heating chamber 31 as load is boiled. More particularly, Fig. 6A shows variation in voltage between terminals of the piezoelectric device sensor 26 as time goes, while Fig. 6B shows a frequency spectrum of the output voltage of the same. In Fig. 6B, waveform (i) shows spectrum before the microwave oven is operated, waveform (ii) shows spectrum immediately after power is supplied to the microwave oven, and waveform (iii) shows spectrum at the time water in the heating chamber 31 boils. As will be understood from the comparison between these three waveforms (i), (ii) and (iii), the output signal from the ultrasonic microphone or piezoelectric device sensor 26 includes a low frequency component, which is between 0 and 50 Hz, whose amplitude is large when wind including hot steam is directed to the ultrasonic microphone 26. In Fig. 6B, a peak in waveform (iii) around 60 Hz is noise caused from commercial supply line. The reason why such low frquency components can be detected by using the ultrasonic sensor 26 may be as follows: When gases passing through the exhaust pipe 37 includes warm steam having high calorie, the piezoelectric device 49 receives mechanical forces due to thermal change caused from such high calorie. As a result, polarization occurs in the piezoelectric device 49 to develop a voltage. As such piezoelectric device 49 may be used quartz, Rochelle salt crystal (KNaC₄H₄O₆), lead titanate (PbTiO₃), barium titanate (BaTiO₃) or the like. Fig. 7 shows circuit arrangement of the amplifier 27 used in the embodiment of Fig. 3. An output terminal of the piezoelectric device sensor 26 is processed so that frequency components above 3 Hz are selected by way of a high-pass filter including an operational amplifier 61, and subsequently another frequency components below 15 Hz are selected by way of a low-pass filter including an operational amplifier 63. As a result, an output signal within a frequency range between 3 and 15 Hz is obtained at an output terminal 60. Fig. 8 shows voltage variation of an output signal at the output terminal 60 of the amplifier 27 shown in Fig. 7 which output signal is obtained through an experiment in which 400 milliliter of water is placed in the heating chamber 31 to be heated by way of microwave to perform dielectric heating. Although the output signal magnitude is small and substantially constant at the beginning of heating, the magnitude increases after four minitues or so. It has been confirmed through the experiment that this timing of signal level increase substantially corresponds to timing where the water starts boiling. Therefore, if we detect the increase in magnitude of the output signal from the piezoelectric sensor 26, we can know the timing that water in the heating chamber 31 boils. Using this principle it is now possible to perform automatic cooking through the detection of the output voltage from the piezoelectric device sensor 26. For instance, the heating output of the microwave oven may be reduced or stopped so as to prevent undesirable overheating. Furthermore, the output signal from the piezoelectric device sensor 26 may be used so that additional heating is performed for a given period of time after the output voltage from the piezoelectric device sensor 26 increases rather than simply terminating heating. In this way, microwave oven can be controlled so that sophisticated cooking is now performed. To this end, various constants including zero are stored in the control circuit 29 in advance to correspond to various groups of food, and one of these constants is selected by a user before cooking through a switch or the like. Then the microwave oven starts cooking with its power switch being turned on. As time goes, an output signal from the comparator 28 changes in response to the level increase of the output signal from the amplifier 27. A time period required until the output signal from the comparator 28 changes is detected and is multiplied by the constant stored in the control circuit 29 to obtain a product indicative of a time period. Using this time period, the microwave oven may be continuously energized for this additional time period to complete cooking since the detection of moisture by the piezoelectric device sensor 26 does not necessarily indicate the completion of cooking because it is necessary to apply heat not only to the surface or skin portion of food body but also to the very inside of the same. Moreover, another function may be added if desired. More specifically, it is desired that the level increase indicative of the detection of moisture from food is distinguished from noise voltage appearing immediately after the power switch of the microwave oven is turned on. To this end, a timer may be provided to the control circuit 29 so as to mask the same for a given period of time after the power switch is turned on. With this arrangement malfunction due to noise voltage appearing immediately after the power switch is turned on can be effectively prevented. The present invention has the following advantages in addition to its original function of detecting moisture so as to know cooking condition of food in the heating chamber 31. (1) Since polarization voltage developed in a piezoelectric device is detected and processed, complex circuit arrangement involving an a.c. reference voltage source and a standard resistor required in one of the aforementioned conventional examples is now unnecessary. Thus high accuracy of parts is not required so that detection circuit can be manufactured at low cost which is suitable for mass production. (2) Since the detection circuit is of passive type such that only a detection element is provided, the number of required parts is reduced to less than one half that of a conventional arrangement using a transmitter and a receiver. (3) Since the detection signal from the piezoelectric device senosr has low frequency components, the magnitude of noise on operation of the microwave oven is relatively small, and thus detection can be done with high signal-to-noise ratio. (4) In the conventional arrangement utilizing a humidity-sensitive resistor, since crystal grain bounday of an element is mainly used, it is necessary that gases including moisture directly acts on the grain boundary. As a result, such sensor is apt to become dirty. On the contrary, the piezoelectric device sensor used in the present invention does not suffer from such problem of becoming dirty since sufficient sensitivity is ensured even if the diaphragm of the sensor is covered and sealed. It has been recognized in an experiments, that the signal level hardly changes in connection with a frequency range from 5 to 10 Hz even if silicone grease is painted on the surface of the diaphragm at a thickness of 0.5 millimeters. Thererfore, no heater for burning dirty stuff is needed, while it is not required to consider thermal time constant. (5) When the amplifier 27 responsive to the piezoelectric device sensor 26 is constructed such that no d.c. Voltage is applied to the piezoelectric device sensor 26, the polarization characteristic can be used with high stability for a long period of time. Furthermore, when using an amplifier which amplifies components whose frequency is lower than that of commercial a.c. supply, then signal-to-noise ratio is extremely high. (6) When the present invention is applied to a microwave oven as shown in Fig. 3, even in the event that boiling timing cannot be detected for some reasons, the piezoelectric device sensor 26 produces an output signal of high voltage when the food in the heating chamber 31 starts smoking. This is because exhaust gases including smoke from the food has a heat amount or carorie which is higher than that of the above-mentioned warm steam therefrom. Therefore, burning of the food can be effectively prevented and thus it is also prevented that the exhaust pipe 37 catches on fire. In this way, when the present invention is applied to microwave ovens or other heating apparatus, overheating as well as firing is effectively prevented so that the apparatus is very safe. Although the embodiment of the present invention has been described with reference to a microwave oven used for cooking food, the present invention may be applied to any other heating apparatus so as to prevent overheating and to control heating period of time. For instance, the present invention may be applicable to temperature control of chemical processes. The above-described embodiment is just an example of the present invention, and therefore, it will be apparent for those skilled in the art that many modifications and variations may be made without departing from the scope of the present invention.

Claims: A heating apparatus comprising: [a] a heating chamber (31) for receiving an object (32) to be heated; [b] a heat source (30) for heating said object (32); [c] an exhaust passage (37) through which gases in said heating chamber (31) are exhausted; [d] and sensing means for producing a signal which is indicative of the temperature of said object; characterized in that said sensing means is a piezoelectric device sensor (26) mounted in said exhaust passages (37) for producing an output signal in response to said exhaust gases, said piezoelectric device sensor (26) [d1] including a piezoelectric material (49), [d2] electrodes (51, 54) being provided at both sides of said piezoelectric material (49) and [d3] a diaphragm (52) being attached to one of said electrodes, [d4] wherein said output signal is further filtered in an amplifier (27), comprising at least a low-pass filter, thereby extracting an a.c. signal in the frequency range below 50 Hz indicative of the temperature of said object. A heating apparatus according to claim 1, characterized in that said amplifier is further comprising a high-pass filter, thereby extracting an a.c. Signal in the frequency range between 3 and 15 Hz. A heating apparatus according to claim 1 or 2, characterized in that said piezoelectric device sensor (26) is hermetically covered by a casing (53) made of a heat-conductive material. A heating apparatus according to one of claims 1 to 3, characterized by: [a] a comparator (28) for producing an output signal when the output voltage from said amplifier (27) exceeds a threshold voltage; and [b] an alarm device (47) responsive to said output signal from said comparator (28) for indicating a predetermined temperature of said object. A heating apparatus according to one of claims 1 to 4, characterized by: [a] a comparator (28) for producing an output signal when the output voltage from said amplifier (27) exceeds a threshold voltage; and [b] a control circuit (29) responsive to said output signal from said comparator (28) for controlling said heat source (30).

Inventors: KUSUNOKI, SHIGERU, MAEHARA, NAOYOSHI
Applicants: MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1986
Main classification: H05B6
IPC codes: G01K7, F24C7, H05B6, B01J19