Heating apparatus

Abstract: A heating apparatus such as a microwave oven in which thermal energy of high-temperature vapor generated by heating an object (2) is detected by a pyroelectric element (5) and a resulting detection signal is used to control (stop) a heat source (3) such as a microwave source, etc. Among AC voltages generated by the pyroelectric element due to heat exchange of the pyroelectric element, a voltage of a polarity generated at the time of heat discharge (temperature decrease) of the pyroelectric element (5) is selectively eliminated through a circuit means (41). A voltage generated by heat exchange between air moving slowly in a heating chamber (1) and the pyroelectric element caused by the rotation of a turntable (33) or the like is eliminated by another circuit means (43). A control section (4) controls the operation of the heat source by deciding an output signal of the pyroelectric element by comparing it with a detection level determined in accordance with a noise level for a predetermined length of time after the start of the heating operation. Erroneous detection is thus prevented which otherwise might be caused by a noise voltage generated in the pyroeelctric element by the thermal energy of a hot air staying in the heating chamber after the energization of the heat source (3) or a second heat source (34, 35).

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to a heating apparatus comprising a detection system for detecting high temperature water vapor emanating from an object and controlling a heat source by using the detection signal. DESCRIPTION OF THE RELATED ART A heating apparatus having a system for detecting automatically a finished state of a heated object finds applications in various forms. A humidity sensor for detecting humidity changes is most widely used as a detector for the detection system of such an automatic heating apparatus. The humidity sensor is used to detect changes in electrical resistance of an element due to the water molecules adsorbed on the surface thereof. In order to prevent the deterioration in sensitivity due to the smear of the element surface and to maintain a stable performance over a long period, it is necessary to burn off the smear from the element surface or take any other complicated procedure at regular intervals of time. As disclosed in EP-A-0367186 on the other hand, the inventors are studying a system in which the water vapor or other vaporized substance of a high humidity emanating from an object being heated, is collected by way of a vent formed in the wall of a heating chamber and is applied against a pyroelectric element outside of the heating chamber to detect a finished state of heating through a voltage generated from the pyroelectric element. This system is based on a physical phenomenon of a detection mechanism exchanging heat between the pyroelectric element and the vapor, and therefore unlike in conventional humidity sensors, the sensitivity would not be substantially affected by the smear of the element surface, thereby leading to the advantage of constructing a detection system in a very simple manner in principle. EP-A-0198430 discloses a heating apparatus which utilizes a piezoelectric sensor in a similar way. However, because both these systems utilize temperature changes of a pyroelectric element caused by the heat of the vapor they both suffer the disadvantage that the pyroelectric element can be undesirably energized to generate a voltage not only by vapor generated from an object but also by high-temperature air, that is, hot air applied suddenly thereto. In the case of a microwave oven comprising an electric or gas heater as a secondary heat source other than the microwave, hot air from the heat source remains in substantial amounts immediately after a heating operation. If an object is heated with the microwave under this condition, the pyroelectric element responsive to the residual hot air can generate a voltage regardless of the temperature of the food, with the result that there is a failure to discriminate a voltage due to the vapor emanating from the heating of the food, which can lead to an erroneous detection. This problem is liable to be caused also after a long heating operation with a microwave alone, as well as after heating with an auxiliary heater of a microwave oven, because of a similar phenomenon to the one mentioned above due to an increased temperature of the heating chamber or the like, thus making it difficult to detect a heated condition of the object food (a finished condition by heat) with high accuracy without error. With the increase in the temperature of the pyroelectric element, the smaller temperature difference with the vapor generated from the food reduces the detection sensitivity. The detection sensitivity undergoing changes in various fashions in this way according to the operating conditions has presented the problem of the difficulty of securing stable detection accuracy. An example of a prior art arrangement which makes use of a pyroelectric element is described in EP-A-0353691. It is to be noted that both EP-A-0367186 and EP-A-0353691, despite having an earlier priority date than the present invention, were published after the priority date of the present invention. Therefore, these documents are not to be considered in deciding whether the present invention involves an inventive step. SUMMARY OF THE INVENTION In view of the foregoing background, an object of the present invention is to provide a heating system using a pyroelectric element which is capable of detecting a high vapor temperature accurately without erroneously detecting residual heat in the heating chamber in repeated continuous operations of the heating apparatus when the apparatus body is heated to some degree such as immediately after the completion of a heater operation and besides regardless of a change in the sensitivity. According to a first aspect of the present invention there is provided a heating apparatus comprising: a heating chamber (1) for accommodating an object (2) to be heated; a heating means (3) for heating the object; an air path (20) for leading air in the heating chamber (1) to the exterior of said chamber (1); a pyroelectric element (5) disposed in the air path (20) for sensing temperature fluctuations in said air; wherein said pyroelectric element (5) generates, in use, a sensor signal including a noise signal caused by residual heat in said air and a vapor signal caused by hot water vapor produced from the object (2); and a control section comprising: a comparison-decision means (9) for deciding whether said voltage has reached a predetermined detection level and outputting a result of the decision; and a drive means (11) for controlling the heating means (3) on the basis of said result; said apparatus being characterized in that s a i d control section further comprises: a measuring means for measuring said voltage; a recording means for recording a peak value of said voltage during a predetermined length of time after the commencement of heating; a threshold setting means for setting a threshold value corresponding to said peak value; wherein said threshold voltage is set in accordance with the signal voltage caused by the residual heat of said air when said heating is commenced with said air in a hot state; whereby even if said heating is commenced with said air in a hot state, said threshold is not exceeded before hot water vapor is produced from said object and hence premature de-energization is prevented. According to a second aspect of the present invention there is provided a method of controlling a heating apparatus according to the first aspect of the present invention characterised by the steps of: detecting a peak value of said sensor signal during a predetermined length of time and storing a first value representing said peak value, obtaining a second value representing a threshold level from a table which has been set beforehand in accordance with the first value and storing the second value, and deciding the value of at least a selected one of: a frequency at which the sensor signal continuously exceeds the second value for a length of time longer than a predetermined length of time; and an accumulated time during which the sensor signal continuously exceeds the second value; and controlling the operation of said heating means by detecting that the value thus decided has reached a predetermined value. One embodiment of the invention includes a section associated with sensor signal processing means for processing a voltage generated by the pyroelectric element and a control section for controlling various operations of the apparatus in response to a signal voltage processed by the sensor signal processing means. First, the part of the embodiment relating to the sensor signal processing means will be explained. A first feature of this part is that the sensor signal processing means is configured to selectively eliminate a voltage of the polarity generated by the discharge of heat from a pyroelectric element (such as when temperature drops) among the voltages generated with the heat exchange of the pyroelectric element. The sensitivity to heat is thus reduced below that of the sensitivity to vapor. A second feature lies in that, of all the voltages generated from the pyroelectric element, the comparatively low frequency components generated by the pyroelectric element as a result of heat exchange with the residual hot air in the heating chamber are eliminated to remove the factors of erroneous detection. This residual hot air is induced with the drive of a turntable or like means for assuring uniform microwave heating which operates slower than the heat exchange caused by fluctuations of vapor from the object to be heated. By use of the aforementioned two means, the voltage generated by the heat of the residual hot air or the like other than the vapor generated from the object of heating is greatly dampened and eliminated as compared with the signal voltage generated by the vapor from the object, thereby preventing erroneous detection. Now, an explanation will be made of the control section for receiving a signal voltage from the sensor signal processing means to effect detection and control. The voltage output of the pyroelectric element immediately after starting the heating operation is measured for a first predetermined length of time, and a threshold value providing a detection level is set from a formula based on the measured signal level as a noise level. The control section thus has a function to decide a finished condition by detecting a voltage output higher than the threshold produced by the pyroelectric element in response to the vapor emanating from the object food. As a result, erroneous detection, (premature de-energization) due to residual heat or detection result dispersion due to sensor sensitivity dispersion can be minimized. The invention will be described now by way of example only, with particular reference to the accompanying drawings. In the drawings: Fig. 1 is a perspective view of the external appearance of a heating apparatus according to an embodiment of the present invention. Fig. 2 is a diagram showing a system block configuration of the essential parts of the same heating apparatus. Figs. 3A and 3B are diagrams for explaining a pyroelectric element of the heating apparatus in detail, of which Fig. 3A is a plan view and Fig. 3B is a sectional view. Fig. 4 shows a configuration of the essential parts arranged around sensor processing means and a pyroelectric element of the heating apparatus. Figs. 5A, 5B, 5C and 5D show waveforms observed at specific points (a-a′, b-b′, c-c′) in the circuit of Fig. 4. Figs. 6A and 6B are diagrams showing the detection signal of the pyroelectric element produced through sensor signal processing means of the heating apparatus and changes with time thereof. Fig. 7 is a flowchart showing a program structure for the heating control and the detection operation of an embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS A microwave oven with a heater providing a heating apparatus according to an embodiment of the present invention will be explained below with reference to the accompanying drawings. As shown in Fig. 1, a microwave oven 30 comprises an operating section 13 for designating and applying an operation control command for the units on the front thereof, a body 31 on the outside thereof, and a freely openable door 32 in the opening of a heating chamber 1. It is seen from Fig. 2, on the other hand, that the heating chamber 1 has mounted on the walls thereof a magnetron 3 for supplying microwave energy for heating an object 2 to be heated, an upper heater 35 and a lower heater 34 making up a second heat source for heating the object 2, and a lamp 14 for illuminating the interior of the heating chamber 1. A turntable 33 carrying the object 2 in the heating chamber 1 is driven by a turntable motor 18 and rotates to assure uniform heating of the object 2 while being heated. A fan motor 16 produces an airflow for cooling a high-voltage transformer 15 for supplying a high voltage to the magnetron 3 and the lamp 14 and also generates an airflow supplied into the heating chamber 1 for exhausting the water vapor and the like generated from the object 2 out of the heating chamber. The direction and amountof the airflow generated is regulated by an orifice 17 formed beside the fan motor 16. The high-voltage transformer 15, the fan motor 16 and the turntable motor 18 are controlled by drive means 11, the operation of which is in turn controlled by a control signal generated by the control section 4. The air sent from the fan motor 16, after entering the heating chamber 1, is exhausted out of the apparatus containing a water vapor gas of the object 2 by way of two exhaust paths. A first exhaust path is formed of a route including a first exhaust port 1, a first exhaust guide 21 and a first vent 26 in that order, and a second exhaust path is formed of a route including a second exhaust port 20, a second exhaust guide 22, a vent pipe 23, exhaust guides A24 and B25 and a second vent 27 in that order. The heat-sensitive surface of a pyroelectric element having a pyroelectric characteristic is exposed from the interior wall surface of the second exhaust path. Figs. 3A and 3B are diagrams for explaining the pyroelectric element 5 in detail. The pyroelectric element 5 includes a flat ceramic plate 36 having a pyroelectric effect, electrodes 37 and 38 formed on the sides of the ceramic plate 36, and a metal plate 39 made of stainless steel or the like bonded to the surface of one of the electrodes 37 and 38. This metal plate 39 functions as a heat-sensitive surface of the pyroelectric element 5. When a high-temperature gas like water vapor comes into contact with the metal plate 39, heat is transmitted to the ceramic plate 36 through the metal plate 39, and the ceramic plate 36 generates a voltage by the pyroelectric effect. In the case of the pyroelectric element 5 shown in Fig. 3, the electrode 38 to which the metal plate 39 is bonded is partially extended to the opposite side of the ceramic plate 36 by way of a part of the periphery thereof, so that a lead wire 40 from the electrodes 37 and 38 may be taken out only by the side of the electrodes 37 and 38 to which the metal plate 39 is not bonded. The ceramic plate 36 may be composed of PZT (lead zirconate-titanate ceramics), for example. The pyroelectric element 5 is polarized in such a manner that the electrode 37 has a positive polarity and the electrode 38 a negative polarity. Under this condition of polarization, a positive (plus) voltage is generated across the electrode 37 with the increase in temperature of the pyroelectric element 5. As shown in Fig. 2, the object to be heated (food) 2 placed in the heating chamber 1 is heated dielectrically by the microwave (high-frequency wave) of 2450 MHz generated from the magnetron 3. The object 2 gradually increases in temperature, and when it reaches a temperature near the boiling point of water, generates a large amount of high-temperature vapor. This vapor is passed through the second exhaust vent 20 formed in the ceiling of the heating chamber 1 and is applied against the pyroelectric element 5 through a cylindrical ventilation pipe 23. The vapor brought into contact with the pyroelectric element 5 supplies a great amount of thermal energy to the pyroelectric element 5. This thermal energy of course contains a great amount of latent heat generated by the vapor dewing on the surface of the pyroelectric element 5. The sharp temperature increase of the pyroelectric element 5 disturbs the equilibrium of polarization in the pyroelectric element 5, and generates a pulse signal with sharp voltage changes at the electrodes on the surface of the element. A similar pulse signal, having opposite characteristics, also appears during a sharp temperature decrease such as when a heated pyroelectric element comes into contact with a cold air. The vapor generated from the object (food) 2 flows non-uniformly through the air lower in temperature than the vapor, and therefore the amount of the vapor coming into contact with the pyroelectric element 5 fluctuates with time and space. Even after the vapor comes to be generated steadily with the object 2 (food) increased beyond a certain temperture, temperature changes (fluctuations), that is, heat exchanges are repeated in which the pyroelectric element 5 increases in temperature due to a great amount of vapor at some moment while the temperature thereof decreases with the vapor amount decreased at a next moment, followed by a temperature increase due a great amount of vapor generated. As a result, the pyroelectric element 5 continues to generate an irregular pulse signal voltage (AC voltage) of positive and negative polarities in response to the heat exchange (temperature fluctuations) described above, while the object (food) 2 continues to generate a high-temperature vapor. In this way, as the temperature of the object 2 approaches the boiling point of water with the heating operation of the microwave oven, vapor is abruptly generated from the object 2, thereby causing generation of a pulse voltage (AC voltage) v (several mv) of positive and negative polarities in large amplitude corresponding to the fluctuations between the electrodes of the pyroelectric element 5. The voltage thus generated by the pyroelectric element 5 is transmitted through the sensor signal processing means 12 to the control section 4. If the object 2 is in a reheating (food-reheating) menu, for instance, a substantially sufficient temperature is reached for the purpose of heating when a great amount of vapor begins to emanate. When the voltage generated from the pyroelectric element 5 reaches a predetermined detection level (threshold value), therefore, the control section 4 decides the de-energization of the magnetron 3 and the cooling fan 16 as the basic principle of a detection system. Fig. 4 is a diagram showing a circuit configuration of the essential parts centered on the pyroelectric element 5 and the sensor signal processing means 12 of a heating apparatus, that is, a microwave oven according to an embodiment of the present invention, and Figs. 5A, 5B, 5C and 5D voltage waveforms observed at specific points (a-a', b-b', c-c') in the circuit configuration. Fig. 5A shows a waveform observed between the section a-a' when the microwave oven is energized a sufficient length of time after the previous use, that is, from a cold state, and Fig. 5B a waveform observed when the microwave oven is energized immediately after heating by the second heat source, that is, from a hot state. In Fig. 5A showing the case of energization in a cold state, the microwave oven is energized for heating at time t 0 , and a signal is generated after a time t 2 when a great volume of vapor emanates from the food, namely the object 2 to be heated. In Fig. 5B showing a case in which the second heat source including the upper heater 35 and the lower heater 34 has been used, a noise signal due to the residual vapor is generated and is mixed with the vapor signal requiring to be detected. The noise signal of Fig. 5B will be explained more in detail below. Simultaneously with the start of the heating operation of the microwave oven at time t 0 , the fan motor 15 is energized and the cold air generated thereby cools the pyroelectric element 5. As a result, the temperature of the pyroelectric element 5 is reduced to generate a positive voltage (on the electrode 38) immediately after the start of the microwave oven. The airflow from the cooling fan 16 then causes the hot air remaining in the heating chamber 1 to reach the pyroelectric element 5 through the air path and increases the temperature of the pyroelectric element 5. The voltage across the element swings greatly to a negative value, thus generating a maximum voltage. The voltage thus swung to the negative value is shifted to a positive value with the temperature of the pyroelectric element 5 reaching the ceiling and decreasing again. Zero voltage is subsequently reached at equilibrium. This process of change occurs during a short period of several to several tens of seconds immediately after the energization of the microwave oven and is finished substantially within first 30 seconds (before time t 1 ). Even after termination of this transient voltage generated immediately after starting, however, the hot air remaining in the heating chamber 1 causes a noise voltage unlike under a cold state so that it coexists with the vapor signal requiring to be detected (Fig. 5B). The voltage waveform in the time period from t 1 to t 0 is caused by such a residual vapor. In a circuit configuration including the sensor signal processing means 12 and the pyroelectric element 5 shown in Fig. 4 the signal is half-wave rectified by a rectification diode 41 before being read by the control sectin 4, and the polarity of the pyroelectric element 5 is selected in such a manner that the voltage (positive voltages in Figs. 5A - 5C) remains due to the negative temperature change of the pyroelectric element 5. As seen from Fig. 5B, therefore, the detecting operation is not affected by the negative voltage containing a maximum amplitude voltage generated by residual hot air and most liable to cause erroneous detection, among the voltages generated during a period of scores of seconds immediately following the start of the cooling fan 16. Further, the voltages generated due to vapor or hot air by the pyroelectric element 5 are different in their manner of response, though both are caused by heat. In response to vapor, a voltage of substantially the same positive or negative degree is generated either in the temperature rise time or in the temperature fall time, while, in response to hot air, a voltage comparatively lower is generated in the temperature fall time than in the temperature rise time. This is considered to be due to the fact that the temperature decrease is largely affected by the vaporization heat of waterdrops adhered when vapor is involved, while the voltage generation due to hot air is not accompanied by any similar physical change. The sensitivity characteristic of the pyroelectric element 5 is such that, in the process of vapor detection after the heater energization, the noise voltage generated by hot air has a small voltage amplitude as compared with that of the detection voltage due to vapor during a temperature decrease. The present circuit configuration and the polarity of connection of the pyroelectric element 5 reduces remarkably the possibility of detecting a noise voltage erroneously as a vapor signal. The sensor signal processing means shown in Fig. 4 further includes a high-pass RC circuit 43 having a capacitor 42 and a resistor 46 with a time constant thereof determined approximately as T = 0.5 or 1.0. The frequency components of the detection signal generated by the resistor 46 spread over a comparatively wide area up to the frequency range higher than 6 Hz, while the noise voltage due to hot air is mainly caused by the fluctuations of hot air induced by the revolution of the turntable 33 of one rotation for each ten seconds. The change in the noise voltage, therefore, is comparatively slow with the frequency components thereof distributed mainly in the range from 1/T 1 Hz (T 1 : Rotational period of the turntable 33) to two Hz. Even in the case where the noise is mixed under a hot state as mentioned above, the low-frequency control means including the high-pass RC circuit 43 attenuates the noise components due to the residual vapor mainly comprised-of low frequencies to a degree more than the signal components due to vapor. Determination of the frequency range to be suppressed depends on the relationship between the frequency components of the signal voltage to be detected and that of the noise voltage to attenuated. The above-mentioned conditions, however, make it a best solution to set the upper limit of the frequency to be dampened substantially in a range from two to 1/T 1 Hz, or more specifically, in a range from one to two Hz. The result is an improved signal-to-noise ratio of the vapor signal and a greatly reduced probability of erroneous detection. The high-pass RC circuit 43 of course functions also as a DC-cutting circuit for preventing the DC voltage from being applied to the pyroelectric element 5. The pyroelectric element 5 generally includes a silver electrode, and the application thereto of a DC voltage is required to be prevented to avoid the deterioration of insulation caused by migration of silver. Fig. 5C shows a voltage waveform passed through the high-pass RC circuit in this way, and Fig. 5D the same voltage waveform further half-wave rectified by the rectification diode 41 and applied to the control section 4. The noise voltage generated by residual vapor in the heating chamber 1 is thus greatly dampened by the sensor signal processing means 12 before being applied to the control section 4. The control section has the function of not only applying an indication output signal to the operating section 13 in response to an input signal from the input keyboard of the operating means 13 and producing a signal for driving the drive means 11 to heat the object 2 by energization of the magnetron 3 or rotating the turntable 3, but also making decisions for controlling various parts on the basis of a signal voltage transmitted from the pyroelectric element 5 through the sensor signal processing means 12. Now, a method of detection and control by the control section 4 will be explained with reference to Figs. 6A, 6B and 7. First, the sequence and method of heating and automatic detection according to the present embodiment will be explained with reference to the flowchart of Fig. 7. Upon depression of a heating start key with an object 2 placed in the heating chamber 1, a control signal from the control section 4 is applied to the drive means 11 which then causes the operations of the magnetron 3 (high-voltage transformer 15), the fan motor 16 and the turntable 18 to be started (step a). The counting of the heating time T is started in the control section 4 (step b). The next step is to wait until the heating time T reaches a starting time T 1 of a predetermined length of time (step c). The voltage value D of the signal voltage is read by measuring means 6 (step d). The voltage value D read is recorded by the recording means 7 and the recording means 7 determines the largest voltage value D as a maximum value D m , and the voltage value D read subsequently is assumed as a new maximum value D m if larger than the recorded maximum value D m (step e). The steps d and e are repeated until the time T 2 where the predetermined time elapses. A threshold value corresponding to the maximum value D m recorded in the recording means 7 is determined by threshold value-setting means 8 (step g). After time T 2 , comparison-decision means 9 adds "1" to the count N (N = N + 1), if the signal voltage exceeds the threshold value for a predetermined length of time (step h). This step h is repeated until the count N reaches a predetermined value (say, 5) (step i). When N reaches 5, T = t d is recorded as a detection time, and various parts including the magnetron are controlled accordingly (step j). A method of decision and control has been explained above with reference to a flowchart. Now, the relationship between an output signal and a decision will be explained mainly with reference to Figs. 6A and 6B. The maximum value D m among the voltage values D measured repeatedly during a predetermined period of time (from T 1 to T 2 ) after heating start by the measuring means 6 in the control section 4 is recorded by the recording means 7. The threshold value selection means 8 of the control section 4 determines a threshold value providing a detection level for the value D m recorded by the recording means 7. After time T 2 , the comparison-decision means 9 determines whether the detection signal has reached the threshold level, and if the threshold level is exceeded a predetermined number of times in succession, the count N of the counter in the comparison-decision means 9 is incremented by one (N = N + 1). When the count N of the counter reaches a predetermined number, say, five, the detection time td is recorded as a time when the object 2 has been heated optimally. In the process, the number of times the pulse signal exceeds the threshold value is counted in such a manner that when the threshold level is exceeded for a predetermined length of time or more, for example, 100 ms or more, one count is added. Table 1 shown below is an example of a classification table used for selecting a threshold value. D m Threshold value 0v ≦ D m < 0.3v 0.5v 0.3v ≦ D m < 2.5v D m + 0.4v 2.5v ≦ D m 3.0v In Table 1, three constants including 0.5, 0.4 and 3.0 are prepared for setting a threshold level respectively for the three ranges of D m , and a threshold value is determined according to this table. The relationship between the signal level and detection time t d based on Table 1 will be explained with reference to Figs. 6A and 6B. Fig. 6A shows an example of starting the heating operation from the cold state of the microwave oven (after being left to stand for at least a predetermined length of time from the preceding operation). Fig. 6B shows an example of starting a heating operation immediately after the heater operation when a great amount of residual hot air remains in the heating chamber 1. In the case of Fig. 6A, the signal level remains substantially zero during the period from the heating start to generation of vapor from the object 2, and the maximum value D m detected during a first predetermined period of time (T 1 to T 2 ) is 0.2v. The threshold value is thus set to 0.5v. As a result, the comparison-decision means 9 comes to determine a finish detection time as t d . In the case of Fig. 6B, on the other hand, the great amount of residual hot air in the heating chamber 1 causes a signal level of a considerable amplitude to be observed from the time immediately after starting the heating operation, and the maximum value D m is 0.7v. The threshold value is thus set to a level of 1.1v which is higher than the signal level (D m ) generated by the residual hot air, which level is of course higher than the level set for the cold start in Fig. 6A. As explained above, the threshold level for determining the finish detection time t d is set in accordance with a signal voltage due to the residual hot air or the like detected before generation of vapor from the object 2. Erroneous detection (premature de-energization) is therefore prevented which otherwise might be caused by a signal voltage due to the residual hot air such as when the apparatus is started in a hot condition immediately after the heating with the heater. In addition, in the case where the maximum value D m is larger than a predetermined value, or when 2.5 < D m as shown in Table 1, the threshold level is fixed at 3.0v regardless of the value D m for the purpose of preventing an excessive threshold level from leading to a detection failure (preventing the signal due to the vapor generated from the object from reaching a threshold level). The pyroelectric element 5 composed of a ceramic element having a pyroelectricity according to the present embodiment may have piezoelectricity at the same time. A piezoelectric buzzer or an ultrasonic microphone using the characteristics of a piezoelectric element, for example, is of course applicable to the present invention with equal effect to the extent that they have pyroelectricity. The advantages of the present heating apparatus are as follows. (1) The sensor signal processing means is so configured as to selectively eliminate a voltage of the polarity (half wave) generated during the temperature decrease of a pyroelectric element among the voltages generated by the pyroelectric element. Further, even when the apparatus is started with a great amount of hot air remaining in the heating chamber after the operation of the heater, the voltage of a large amplitude generated with a temperature rise of the pyroelectric element due to the residual hot air for several tens of seconds immediately after the start of operation can be removed thereby to prevent erroneous detection which otherwise might be caused by this type of residual hot air. Further, the pyroelectric element has such a sensitivity characteristic that the sensitivity thereof to the hot air fluctuations during temperature decrease is lower than that to the vapor fluctuations. Erroneous detection due to the residual hot air occurs less. (2) The sensor signal processing means comprise low-frequency wave dampening means for removing a slowly changing frequency component, and therefore it is adapted to remove the voltage component caused by heat exchange with the residual hot air induced by the operation of the turntable or the like having a lower change rate than the fluctuation signal of the vapor generated from the object. The probability of occurrence of erroneous detection due to the residual hot air is thus remarkably reduced. (3) In making a decision on the detection at the control section, a maximum value of the signal voltage is detected for a predetermined length of time (first predetermined time) after the heating operation has started. The number of times is counted by which a voltage pulse longer than a predetermined time width exceeds a threshold level set as a detection level for the maximum value according to a predetermined rule. The time when the count reaches a predetermined number (say, five) is regarded as a detection time point t d . This method permits detection of an effective voltage signal level and obviates the problem of premature de-energization by erroneously detecting a noise signal of a high level due to the vapor remaining in the heating chamber immediately after the heater operation. In particular, while the first discrimination means detects a substantial maximum value and determines a corresponding threshold level, the comparison-decision means counts signal pulses exceeding the threshold level for a predetermined length of time. By changing the method of deciding a signal voltage in this way, a noise signal and a vapor signal can be discriminated from each other with higher accuracy.

Claims: A heating apparatus comprising: a heating chamber (1) for accommodating an object (2) to be heated; a heating means (3) for heating the object; an air path (20) for leading air in the heating chamber (1) to the exterior of said chamber (1); a pyroelectric element (5) disposed in the air path (20) for sensing temperature fluctuations in said gas; wherein said pyroelectric element (5) generates, in use, a sensor signal including a noise signal caused by residual heat in said air and a vapor signal caused by hot water vapor produced from the object (2); and a control section comprising: a comparison-decision means (9) for deciding whether said voltage has reached a predetermined detection level and outputting a result of the decision; and a drive means (11) for controlling the heating means (3) on the basis of said result; said apparatus being characterized in that s a i d control section further comprises: a measuring means (6) for measuring said voltage; a recording means (7) for recording a peak value of said voltage during a predetermined length of time after the commencement of heating; a threshold setting means (8) for setting a threshold value corresponding to said peak value; wherein said threshold voltage is set in accordance with the signal voltage caused by the residual heat of said gas when said heating is commenced with said air in a hot state; whereby even if said heating is commenced with said air in a hot state, said threshold is not exceeded before hot water vapor is produced from said object and hence premature de-energization is prevented. A heating apparatus according to claim 1, further comprising: a sensor signal processing means (12) for suppressing said noise signal from said sensor signal; wherein at least one of said sensor signal processing means (12) and said control section (4) includes selection means (41) for selectively accepting a voltage of a preselected polarity of the sensor signal generated by said pyroelectric element (5), said voltage of the preselected polarity being generated when the temperature of said pyroelectric element (5) falls, thereby suppressing the noise signal having a peak value of the reverse polarity produced by a temperature rise of said pyroelectric element (5) owing to heat exchange between said residual hot air and said pyroelectric element (5) after a heating operation is started and before a substantial amount of hot water vapor is produced from the object (2). A heating apparatus according to claim 2, wherein said sensor signal processing means (12) includes a diode (41). A heating apparatus according to claim 1, further comprising a rotary means (33) for assuring uniform microwave heating and for mounting the object (2) thereon, said rotary means (33) rotating at a rotational period of T 1 which is longer than 0.5 seconds; wherein said sensor signal processing means (12) includes low-frequency suppressing means (43) for suppressing said noise signal, said low-frequency eliminating means (43) having an upper limit frequency in a range from 1/T 1 Hz to 2 Hz, thereby suppressing said noise signal having a low frequency in a range from 1/T 1 Hz to 2 Hz caused by fluctuations of said residual hot air caused by the rotation of said rotary means, said noise signal caused by the fluctuations of said residual hot air being generated subsequent to the generation of said noise signal having a peak value of a negative polarity produced by a temperature rise of said pyroelectric element (5) caused by heat exchange between said residual hot air and said pyroelectric element (5) after a heating operation is started and before a substantial amount of hot water vapor is produced from the object (2). A heating apparatus according to claim 4, wherein said low-frequency suppressing means is a high-pass RC circuit (43). A heating apparatus according to claim 1, wherein said threshold setting means (8) decides said threshold value by using a predetermined table on the basis of said peak value (Dm) and stores the threshold value thus calculated. A method of controlling a heating apparatus as set out in claim 1, said method being characterized by the steps of: detecting a peak value (Dm) of said sensor signal during a predetermined length of time and storing a first value representing said peak value (Dm), obtaining a second value representing a threshold level from a table which has been set beforehand in accordance with the first value and storing the second value, and a third step of deciding the value of at least a selected one of: a frequency at which the sensor signal continuously exceeds the second value for a length of time longer than a predetermined length of time; and an accumulated time during which the sensor signal continuously exceeds the second value; and controlling the operation of said heating means (3) by detecting that the value thus decided has reached a predetermined value. A method of controlling a heating apparatus according to claim 7, wherein the table which has been set beforehand for use in the second step is such that the second value is set to a predetermined fixed value regardless of the magnitude of the first value when the first value is larger than a predetermined value.

Inventors: AKIYOSHI MITSUO, KASAI ISAO, MURAKAMI SUSUMU, OTSUKA AKINORI, SAKAI SHINICHI, SANO MASAAKI, YAMAGUCHI KIMIAKI, AKIYOSHI, MITSUO, KASAI, ISAO, MURAKAMI, SUSUMU, OTSUKA, AKINORI, SAKAI, SHINICHI, SANO, MASAAKI, YAMAGUCHI, KIMIAKI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1990
Main classification: H05B6
IPC codes: H05B6