Methods of sensing humidity, and controlling a humidity sensor, in a microwave oven

Abstract: In a microwave oven, input power is applied to a high voltage transformer (12) and the elevated voltage of the secondary winding is applied to a magnetron (14) by way of an amplifying section (13) having a high voltage diode (H.V.D.). The high voltage diode (H.V.D.) interrupts the power supply to the magnetron (14) so that the applied microwave power has alternating oscillation rest modes. The read-time of a humidity sensor of the microwave oven is controlled so as to avoid interference. One oscillation of the applied microwave power is divided into first and second sections and a number of humidity readings are taken during each said section. A noise count is increased in a section when the difference between a maximum reading value and a minimum reading value is larger than a predetermined reference value. The obtained noise counts are compared, and then the one of said first and second sections having the lower noise count is selected as the section from which the humidity readings are taken.

Description: The present invention relates to a method of sensing the humidity of foodstuffs subjected to microwave power in a microwave oven, and to apparatus utilising the method. In general, automatic cooking methods used in the microwave oven are classified largely as a program type automatic cooking method and a sensor type automatic cooking method. In a sensor type automatic cooking method, a humidity sensor in the microwave oven senses the humidity value of moisture emitted from foodstuffs heated in the heating chamber, and other sensors monitor other surrounding conditions such as temperature. The cooking is controlled by automatic setting of the heating time according to the humidity sensing. A great deal of research and development has been concentrated on ways to sense the humidity. However, in the known humidity sensing arrangements, the humidity values determined generally differ from practical humidity values. This is because, in a microwave oven, food is heated by microwaves generated by a magnetron, and leakage of microwaves, which necessarily happens in the course of heating the food, has an effect on the humidity sensing performed. For example, there is a possibility that microwaves leak and flow through wires and nodes in the circuit and then function as noise to the sensed humidity value. One proposal to overcome the problem is to incorporate a plurality of noise-absorbing capacitors in the humidity sensing circuit so as to reduce the effect of the leakage. However, it has been found that such installations cannot entirely remove the effect of the leakage of microwaves. On the contrary, the voltage charge of the capacitors may have an adverse effect on the sensed humidity values and thereby cause the value to be more inaccurate. Further, the capacitors necessarily increase the number of parts in the circuit so as to make the circuit more complicated. In an alternative method, the humidity values are sensed several times and then a mean value of the sensed humidity values is adopted as a resultant sensed humidity value. Whilst this method may bring about some improvement, it cannot remove entirely the bad effect of the leakage of microwaves. It has been proposed to drive a magnetron to have alternating oscillation modes and rest modes to enable the humidity value to be read without hindrance by the leakage of microwaves in the rest mode during which the oscillation of microwaves by the magnetron is interrupted. However, it is not always possible to ensure that the humidity readings are reliably taken during the rest periods. There may also be circumstances in which errors lead to incorrect operation. For example, in an actual assembling process of a microwave oven, electric wires in the microwave oven are discriminated only by colour and black and white wires have respective black and white electric terminals. When the black and the white wires are connected to their correct terminals, the electric phases of the parts in the microwave oven will coincide with their own phases. However, even if the wires are not connected to their correct terminals, the operation of all the parts can be normal. Thus if an incorrect winding happens in the course of manufacture, the microwave oven may still operate correctly making it difficult to recognise that a fault exists. Furthermore, it is not easy to find the wrong winding after manufacture of the microwave oven. Of course, if the phase of the electric power is inverted due to an incorrect winding, when the humidity value is sensed supposedly in the microwave-rest mode it will, in fact, be sensed at a time when the humidity is effected by the leakage of microwaves and thereby a contrary effect results. CN-A-1090047, which corresponds to US-A-5445009, describes a method of sensing the humidity of foodstuffs subjected to microwave power in a microwave oven, wherein the applied microwave power is oscillated such that each oscillation has a first period during which the microwave power is applied and a second period during which the microwave power is interrupted, wherein, to provide that humidity readings are only selected from second periods when the microwave power is interrupted, said method comprises the steps of sensing the humidity a plurality of times during each of said first and second periods, determining a noise count for the humidity readings of each said period, comparing the determined noise counts, identifying the period with the lowest determined noise count as said second period of the oscillation and selecting the sensed humidity readings of each said identified second period. The method described in US-A-5445009 reduces or eliminates the affect of microwave noise on humidity readings by calculating cumulative voltage difference values from a humidity sensor making a number of readings for each half period of a commercial alternating current frequency. The comparison between the cumulative differences is used to determine oscillating and non-oscillating terms of the magnetron. It is an object of the present invention to provide an alternative method to reduce the affect of microwave noise on humidity readings. According to the present invention a method of sensing the humidity of foodstuffs subjected to microwave power in a microwave oven, as defined above, is characterised in that the method further comprises the steps of, for each of said periods determining the maximum and minimum humidity readings, forming the difference, between the maximum and minimum humidity readings, comparing said difference with a predetermined reference value, and adding a noise count in the first or second period when said difference in larger than said predetermined value whereby the noise count for each said period is determined. A method of the invention enables the periods when the microwave power is interrupted to be accurately determined. Generally, the applied microwave power is output from a magnetron which is oscillated by applied electric power, each frequency period of said applied electric power being divided into said first and second periods by an interrupt signal. Preferably, the steps of sensing the humidity a plurality of times during said first and second periods, and determining the noise count of the humidity readings of each said period, are repeated during a predetermined determining time. Embodiments of the present invention will hereinafter be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a circuit diagram of a substantially conventional humidity sensing circuit for a microwave oven; Figure 2 shows a circuit diagram of a general control circuit for a microwave oven; Figure 3 shows a flow chart illustrating the determination of the read-time of an humidity sensor of a microwave oven of the present invention; and Figures 4A and 4B show wave forms of an outer interrupt signal and an output of a humidity sensor respectively. A substantially conventional humidity sensing circuit used for a microwave oven will be described with reference to Figure 1. The humidity sensing circuit of Figure 1 has a humidity sensing section 1 for sensing humidity, an amplifying section 2 for differentially amplifying the output of the humidity sensing section 1, a microcomputer 3 for outputting a control signal for controlling the humidity according to the amplified signal from the amplifying section 2, and an equilibrium control section 4 for controlling the equilibrium of the humidity sensing section 1 according to the humidity control signal from the microcomputer 3. The humidity sensing circuit shown in Figure 1 operates as follows. At an initial stage of sensing the humidity, since an exact read-time for sensing humidity has not yet been set, the microcomputer 3 sends an humidity control signal having a predetermined value through output terminals P0 through P4 and the equilibrium control section 4 and thereby presets the humidity sensing section 1. Then, voltages of two nodes a and b of the humidity sensing section 1 are input to the non-inverting terminal and to the inverting terminal respectively of an amplifier OP1 of the amplifying section 2 and are differentially amplified thereby. The amplified voltages are input to an humidity value input terminal A/D of the microcomputer 3. In this case, the input voltages corresponding to the humidity value have analog forms, and are input to an analog/digital converter of the microcomputer 3 and converted thereby to digital values. However, the humidity values differ from the practical humidity value. This is because, in a microwave oven, food is heated by a microwave generated by a magnetron installed in the microwave oven so that leakage of the microwave, which necessarily happens in the course of heating the food, has an effect on the humidity sensing performed by the humidity sensing section 1. A more detailed description of the leakage of the microwave will be given with reference to Figure 2 which shows a general control circuit for a microwave oven. As is shown in Figure 2, the control circuit of the microwave oven comprises a high voltage transformer 12 for elevating the voltage of the inputted electric power to a predetermined value, an amplifying section 13 connected to the high voltage transformer 12 so as to amplify the elevated voltage, and a magnetron 14 for generating microwaves utilising the voltage amplified in the amplifying section 13 as a driving power. The control circuit also comprises a low voltage transformer 11, and a power supply section 15 for supplying electrical power into a control circuit board using the voltage received from low voltage transformer 11. The control circuit board comprises an interrupt signal generating section 16 for generating an interrupt signal according to the power supply from the power supply section 15, an humidity sensing section 17 for sensing the humidity, an amplifying section 18 for amplifying the sensed humidity value, and a microcomputer 19 for generally controlling various parts of the microwave oven in accordance with signals received from the interrupt signal generating section 16 and the amplifying section 18. When the control circuit of microwave oven is operated, an input power of 110/220V and 60 Hz is applied to the high voltage transformer 12 under the control of a door switch and relay switches for driving the magnetron 14, which switches are not shown. Then, a voltage elevated to about 2000V is applied from the secondary windings of the high voltage transformer 12 to the amplifying section 13 having a high voltage condenser H.V.C. and a high voltage diode H.V.D. and is thereby doubled to about 4000V. The doubled voltage is applied to the magnetron 14 as a driving voltage so as to make the magnetron 14 oscillate and generate microwaves. Since the electric current is interrupted for half-periods due to the characteristic of the high voltage diode H.V.D. in the amplifying section 13, an oscillation mode and a rest mode alternate corresponding to the frequency of the input power during the whole oscillation. At the same time, input power is supplied by way of the low voltage transformer 11 to the power supply section 15 in the control circuit board. The power supply section 15 transforms the input power into a direct current power and then supplies the direct current power to the microcomputer 19, to the humidity sensing section 17, and to other load driving relays (not shown). The power supplied to the microcomputer 19 is applied by way of the interrupt signal generating section 16 which applies an outer interrupt signal as a pulse signal by a zero-crossing detection of the frequency of an electric power of the power supply section 15. Generally, the interrupt signal generating section 16 is used to enable the microcomputer 19 to determine whether the frequency of an electric power is a predetermined frequency such as 50 Hz or 60 Hz, or is used for generating an interrupt signal for a specific object such as a time-count, in a conventional control circuit of a microwave oven. As described above, in the conventional control circuit of a microwave oven, there is a possibility that the microwaves oscillated in the magnetron 14 leak and flow through wires and nodes into the circuit and then function as noise to the sensed humidity value. To overcome this problem, various methods for minimising the leakage of microwaves have been proposed. An example of such methods is shown in Figure 1 in which a plurality of noise-absorbing capacitors C1 to C4 are connected to the humidity sensing section 1 and to the amplifying section 2 so as to reduce the effect of the leakage of microwaves. However, such an installation cannot entirely remove the effect of the leakage of microwave. On the contrary, the voltage charge on the condensers can have bad effect on the sensed humidity value so as to cause the value to be more inaccurate. Further, the condensers necessarily invite increase in the number of parts of the circuit so as to make the circuit more complicated. In the control circuit as shown in Figure 2 and described above, the voltage elevated up to about 2000 V by the high voltage transformer 12 is doubled to about 4000 V by the amplifying section 13 having the high voltage condenser H.V.C and the high voltage diode H.V.D. and this voltage is then applied to the magnetron 14 as the main driving voltage of the magnetron. The magnetron is driven to have an oscillation mode and an alternate rest mode corresponding to the frequency of input power, such as 50 Hz or 60 Hz, during the whole oscillation of the magnetron since the input power is interrupted during half-periods thereof due to the characteristic of the high voltage diode H.V.D. in the amplifying section 13. Therefore, the humidity value can be read without hindrance by the leakage of microwave in the rest mode at which the oscillation of microwave by the magnetron is instantly interrupted. This solution can be very effective so long as it is accurately determined that the humidity value is read during the rest mode. Figure 3 shows a flow chart for determining the read-time of the humidity sensor of the microwave oven. As is indicated by the flow chart of Figure 3, cooking of food in the microwave oven is started according to conditions set by a user (step 100). After the cooking is started, a predetermined time for stabilising the oscillation of the magnetron (about 1-2 seconds) is waited for (step 110). The microcomputer 19 checks whether the stabilising time has passed at every predetermined time interval (step 120), and it proceeds to step 130 when the stabilising time has passed. In step 130, as indicated in Figure 4B, an outer interrupt signal generated by one time at every one period of a frequency of an electric power is divided into two half-periods of time as a first section RT1 and a second section RT2. In step 140, the routine of the microcomputer 19 is arranged to continually sense whether the outer interrupt signal according to the frequency of the electric power is generated. When the outer interrupt signal is generated, the routine proceeds to step 150. In this case, as shown in Figure 4A, the outer interrupt signal is a pulse signal the edge of which is the zero-crossing point of the input power, and the falling edge is recognised as the interrupt. In step 150, it is decided whether a predetermined read time determining time, which corresponds to about 10 seconds after the stabilisation of the oscillation, has passed. When the predetermined read time determining time has not yet been passed, the humidity sensing value is read by a predetermined times, such as four times, at regular intervals during the first section (step 160). A maximum value RT1 max and a minimum value RT1 min are found out among the values read in step 160, and it is decided whether the difference between the maximum value RT1 max and a minimum value RT1 min is larger than a predetermined noise-determining reference value A (step 170), a noise count in the first section is added one by one when the difference is larger than the value A (step 180). Also, in the same way, the humidity sensing value is read at predetermined times, such as four times, at regular intervals during the second section (step 190). A maximum value RT2 max and a minimum value RT2 min are found out among the values read in the second section at step 190, and it is decided whether the difference between the maximum value RT2 max and the minimum value RT2 min is larger than the predetermined noise-determining reference value A (step 200), a noise count in the second section is added one by one when the difference is larger than the value A (step 210). Steps 140 through 210, at which an outer interrupt signal generated by one time at every one period of a frequency of an electric power is divided into two half-periods of time and the noise count is added according to the difference between the maximum and the minimum values in each section, are repeated according to the above described process. In the course of the above repetition, if the read time determining time has passed, the noise count value RT1 count in the first section RT1 and the noise count value RT2 count in the second section RT2 are compared with each other (step 220). As a result of the comparison, if the noise count value RT1 count in the first section RT1 is larger than the noise count value RT2 count in the second section RT2, it is interpreted that a leakage of microwaves has happened and thus that noise has been generated in the first section. Therefore, the second section RT2 is determined as the humidity sensing read time (step 230). On the contrary, if the noise count value RT2 count in the second section RT2 is larger than the noise count value RT1 count in the first section RT1, it is interpreted that a leakage of microwaves has happened and thus that noise has been generated in the second section. Therefore, the first section RT1 is determined as the humidity sensing read time (step 240). Therefore, the rest section in which no microwave is oscillated is accurately found by determining a half-period of a frequency of an electric power with less noise as the read time, and then the humidity is sensed by reading the humidity sensing value in the read time section decided as above according to the outer interrupt signal after the preset is completed. Thus, and as is described above, a noise section and a read time section are decided based on the noise-generating frequency, and the humidity sensing value is read only in the read time section. Accordingly, the humidity value can be sensed without being influenced by the leakage of microwave, so that the humidity is accurately sensed. By this means, variances in cooking performance are reduced and so the reliability of the microwave oven is increased. Whilst the present invention has been shown and described with reference to a particular embodiment thereof, it will be understood that variations and alterations may be made to the embodiment described within the scope of the appended claims.

Claims: A method of sensing the humidity of foodstuffs subjected to microwave power in a microwave oven, wherein the applied microwave power is oscillated such that each oscillation has a first period (RT1, RT2) during which the microwave power is applied and a second period (RT2, RT1) during which the microwave power is interrupted, wherein, to provide that humidity readings are only selected from second periods (RT2, RT1) when the microwave power is interrupted, said method comprises the steps of sensing the humidity (R1-R8) a plurality of times during each of said first and second periods, determining a noise count (RT1 count , RT2 count ) for the humidity readings of each said period (RT1, RT2), comparing the determined noise counts, identifying the period with the lowest determined noise count as said second period of the oscillation and selecting the sensed humidity readings (R1-R8) of each said identified second period (RT1, RT2), characterised in that the method further comprises the steps of, for each of said periods determining the maximum and minimum humidity readings (RT1 max , RT1 min ; RT2 max , RT2 min ), forming the difference (RT1 max -RT1 min ; RT2 max -RT2 min ), between the maximum and minimum humidity readings, comparing said difference with a predetermined reference value (A), and adding a noise count in the first or second period when said difference is larger than said predetermined value whereby the noise count for each said period (RT2, RT1) is determined. A method as claimed in Claim 1, wherein the applied microwave power is output from a magnetron (14) which is oscillated by applied electric power, each frequency period of said applied electric power being divided into said first and second periods (RT1, RT2) by an interrupt signal (Figure 4A). A method as claimed in Claim 1 or Claim 2, wherein the steps of sensing the humidity (R1-R8) a plurality of times during said first and second periods, and determining the noise count (RT1 count , RT2 count ) of the humidity readings of each said period, are repeated during a predetermined determining time. A method as claimed in Claim 3, wherein the determined noise counts (RT1 count , RT2 count ) are compared at the end of said predetermined determining time. A method as claimed in any preceding claim, wherein said first period (RT1) is equal to said second period (RT2). A method as claimed in any preceding claim, wherein humidity readings (R1-R8) are only read after a predetermined read time determining time has passed such that the oscillations of the applied microwave power have stabilised. Apparatus for sensing the humidity of foodstuffs in a microwave oven utilising a method as claimed in any of Claims 1 to 6.

Inventors: KIM SEONG JAE, KIM, SEONG JAE
Applicants: DAEWOO ELECTRONICS CO LTD, DAEWOO ELECTRONICS CO., LTD
Application year: 1995
Main classification: H05B6
IPC codes: F24C7, H05B6