EP0000957A1

EP0000957A1 - Humidity controlled microwave oven and method of cooking

Info

Publication number: EP0000957A1
Application number: EP78100791A
Authority: EP
Inventors: Ronald G. Buck
Original assignee: Litton Systems Inc
Current assignee: Northrop Grumman Guidance and Electronics Co Inc
Priority date: 1977-08-30
Filing date: 1978-08-30
Publication date: 1979-03-07
Also published as: AU522240B2; CA1109529A; AU3930578A; US4162381A

Abstract

A microwave oven sensing system having a humidity sensor and a temperature sensor at an exit ventilation port of a microwave oven heating cavity to sense the exiting humidity and the air temperature of the microwave oven heating cavity is disclosed. The sensors connect to a programmable controller and input the sensed humidity and temperature of the microwave oven heating cavity to a programmable controller. Characteristic humidity curves for microwave cooking of different types of foods are stored in the memory of the programmable controller and the programmable controller controls the microwave cooking in accordance with a selected algorithm for the sensed environmental conditions. A numeric keyboard control panel inputs to the programmable controller the microwave cooking parameters and selects the desired characteristic humidity curve. The programmable controller may control a microwave power source control circuit and an air exchange circuit according to the selected algorithm in the programmable controller.

Description

Technical Field

The present invention relates generally to improvements in a microwave oven and more particularly, pertains to a new and improved microwave oven sensing system wherein humidity and temperature sensors are positioned to sense the humidity and temperature environmental conditions in the microwave oven heating cavity of the microwave oven and wherein the cooking of food in the oven is controlled in response to the sensed humidil and temperature.

Background of Prior Art

In the field of microwave ovens it has been known to sense environmental conditions of the microwave oven heating cavity in which foods are cooking with sensing devices which have been positioned external to the cavity, usually in an exhaust duct exiting from the microwave oven heating cavity to prevent electromagnetic interference to the sensors. Such sensing devices have been unsatisfactory in that the sensors, to operate properly, require a continuous flow of gas in the duct and therefore suffer from the dynamics of the gas flow in the duct system. Also, the response time and accuracy of the measurement by the sensors depended upon the degree of equilibrium reached in the exhaust duct system. Further, other factors affected the accuracy of the sensors such as the gas pressure and flow rate from the microwave oven heating cavity through the duct system to the outside environment, the diameter and length of the duct system itself, and the susceptibility of the sensor itself to large concentrations of pollutants in the gases being expelled from the oven cavity.
Prior art sensing systems have not been able to accurately measure temperature and more importantly, humidity, in an "in-situ" environment as the sensors which were located in the ducts for electromagnetic interference considerations were subject to large numbers of water molecules carrying grease and carbon particles from the wet, moist environment of the microwave oven heating cavity through the exhaust duct system to the outside environment. The sensors were not able to endure the water molecules and the exhausted gases from the hostile environment of the microwave oven heating cavity over extended time periods as sometimes the temperature of the exhausted gases exceeded sixty to seventy degrees centigrade, further degrading the sensors. As a result of the severe environmental conditions in the microwave oven heating cavity, the sensors would go out of calibration in addition to suffering from sensor degradation.
Past prior art humidity sensors have been of the variety such as lithium chloride humidity sensors which are affected by continuous abrupt humidity and temperature changes in addition to exposure to humidity saturation conditions. These sensors are further subject to concentration of gases, freons, sulfides, ozones, dioxides and monoxides, nitrogen, as well as saturated water vapor. These prior art humidity sensors further did not have long term stability in use and storage in addition to being subject to degradation from continued cycling over wide humidity ranges.
Moreover, while the prior art shows examples of sensing humidity in a microwave oven duct, the humidity information thus generated was of very limited usefulness and generally was used to simply detect a boiling point temperature and to shut the oven off when such condition was indicated. Control of the oven operation to both control the humidity conditions and to accurately determine a doneness condition for various foods through such non-contact sensing has not been generally known in the art.
This invention, a humidity controlled microwave oven and method of cooking, overcomes the disadvantages of the prior art by providing an accurate sensing system of the humidity and temperature environmental conditions of the microwave oven heating cavity.

Brief Summary of the Invention

The present invention avoids the disadvantages of the prior art by providing a microwave oven sensing system to sense the time dependent "in-situ" humidity and temperature environmental conditions in the microwave oven heating cavity and control the microwave cooking in accordance with cooking algorithms of characteristic humidity curves stored in a programmable controller according to the selected algorithm for the sensed time dependent "in-situ" humidity and temperature environmental conditions by the sensors.
The term "in-situ" as used in this application is defined as the actual time dependent environmental conditions which exist in the environment surrounding the food product which is located in and cooked in a microwave oven heating cavity. Although in the present invention, a humidity sensor and a temperature sensor may be positioned outside of the microwave oven heating cavity, the sensors are configured to sense the "in-situ" environmental conditions of the microwave oven heating cavity and provide signal information of the environmental conditions of the microwave oven heating cavity to a programmable controller controlling the microwave oven.
According to a preferred embodiment of the present invention, there is provided a microwave oven sensing system having humidity and temperature sensors positioned to sense time dependent environmental conditions of the microwave oven heating cavity, a programmable controller having a memory which stores a microwave cooking algorithm of a "characteristic humidity curve" connected between the sensors and the microwave power supply control circuit whereby the programmable controller receives signals from the sensors of sensed environmental conditions of the microwave oven heating cavity and controls the microwave power supply circuit according to the microwave cooking algorithm for the sensed time dependent environmental conditions. The sensor for sensing the humidity is an aluminum oxide sensor. The sensors and associated circuitry connect to the programmable controller through analog signal conditioner interface circuitry. The program mable controller has stored microwave cooking algorithms in the memory corresponding to "characteristic humidity curves" for different types of foods. Each "characteristic humidity curve" for a particular type of food is selected by a numeric keyboard on a control panel of the microwave oven from the memory of the programmable controller. The programmable controller also controls the air exchange rate, and increases or decreases the incoming air temperature by preheating the air with a coil of an infrared lamp or a resistance heater or other heating element.
A further significant aspect and feature of the presen invention is a humidity control on a numeric keyboard control panel to preset a predetermined humidity to be maintained within the microwave oven heating cavity during microwave cooking.

Brief Description of the Drawings

Many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, in which like reference numerals designate like elements throughout the figures thereof and wherein:

FIGURE 1 illustrates a front plan view of a preferred embodiment of a microwave oven in accordance with the present invention;
FIGURE 2 illustrates a top plan view of the microwave oven
FIGURE 3 illustrates a side and top perspective view of the microwave oven with a portion of a housing wrap removed and illustrating a cutaway portion of a ventilation hood;
FIGURE 4 illustrates a side view of the microwave oven with the housing wrap removed and showing the cutaway portion of the vent hood;
FIGURE 5 illustrates a top plan view of the microwave oven with a portion of the top of the housing wrap cutaway and also showing a top cutaway portion of the ventilation hood;
FIGURE 6 illustrates a top enlarged and expanded view of a humidity sensor and a temperature sensor positioned adjacent to and by an exit ventilation port of the microwave oven with the housing wrap removed;
FIGURE 7 illustrates a section of the present invention taken on line 7-7 of Figure 6 looking in the direction of the arrows showing the humidity and temperature sensor positioned by the exit ventilation port;
FIGURE 8 illustrates an electrical circuit schematic block diagram for the present invention, and;
FIGURE 9 illustrates a typical "characteristic humidity curve" for the present invention.

Detailed Description of the Invention

Figure 1 which illustrates a front plan view of a preferred embodiment of a microwave oven 10 in accordance with the present invention shows an oven door 12, a numeric keyboard control panel 14, and a push bar door release handle 16. A three sided housing wrap cover 18 having a top 20, a right side 22 and a left side 24 enclose: the top, the right hand side and the left hand side respectively of the microwave oven 10 frame structure. Upper and lower hinges 26 and 27 respectively hinge the door 12 to the left hand side 24 of the microwave oven 10 frame structure. The oven door 12 includes a window 28 consisting of a microwave radio frequency screen sandwiched between two panes of door glass to enable an operator to view cooking in the microwave heating cavity 30, the outline of which is shown in dashed lines. A humidity sensor 32 and a temperature sensor 34 are positioned adjacent to and by a plurality of exit port ventilation holes 36 in the top of the microwave heating cavity 30 and below a plurality of ventilation holes 38 in the top 20 of the housing wrap 18. The humidity sensor 32 and the temperature sensor 34 connect to a programmable controller 40 as shown in Figures 3-5 located behind the control panel 14 by electrical cables and interface with analog signal conditioner circuitry as required. A ventilation hood 42 shown in dashed lines is positioned between the plurality of exit port ventilation holes 36 in the top of the heating cavity 30 and the plurality of ventilation holes 38 in the top 20 of the housing wrap 18.
The numeric keyboard control panel 14 which connects to the programmable controller 40 is similar to the microwave oven control panel of the Litton Model 420 Microwave Oven, assignee of the present invention. The control panel 14 has a lighted digital display countdown timer and numeric controls to input cooking parameters to the programmable controller 40. In addition to the existing numeric controls on the numeric keyboard control panel 14, a "Humidity" control 44 to set a predetermined humidity in the microwave oven heating cavity 30, a "Turn Food Over" control 45 to indicate that the food is to be turned over, a "Time Out" control 46 to indicate that the door 12 is to be closed after turning food over after a predetermined time has elasped, and a "Done" control 47 to indicate internal doneness of the food are positioned on the control panel 14.
Figure 2 illustrates a top plan view of the microwave oven 10 showing the door 12, the numeric keyboard control panel 14, the push bar door release handle 16, an upper hinge 26, the plurality of ventilation holes 38 in the top 20 of the housing wrap 18, the outline of the ventilation hood 42 as shown in dashed lines, the humidity sensor 32 and the temperature sensor 34. A waveguide 52 couples energy from a microwave power source 50, a magnetron for way of example and purposes of illustration only, past a mode stirrer 54 driven by a mode stirrer motor 56 to the microwave heating cavity 30, these elements being illustrated in imaginary lines.
Figure 3 illustrates a side and top perspective view of the microwave oven 10 with a portion of the housing wrap 18 removed and illustrating a cutaway portion of the ventilation hood 42 showing the top side 20 and the right side 22 of the housing wrap 18, the door 12, the control panel 14, the push bar door release handle 16, the upper hinge 26, the exterior side of the exit ventilation port 36 of the microwave heating cavity 30, the humidity sensor 32 and the temperature sensor 34, both shown in imaginary lines, positioned adjacent to and by the exterior side of the plurality of exit ventilation port holes 36 and connected to the programmable controller 40, the magnetron 50, the waveguide 52, and the mode stirrer 54 driven by the mode stirrer motor 56. A circulation fan 58 draws air through the lower portion of a rear intake ventilation port 60, past the underside of a diode rectifier 62 and turns the air around to circulate past the microwave power source 50 to cool the electrical components, finally exhausting the air out the upper portion of the rear intake ventilation port 60. A diverter 64 as shown in Figure 4 further divert: the air flow into a plurality of entrance port ventilation holes 66 to introduce air through the side of and into the microwave heating cavity 30. The air circulates in the microwave heating cavity 30 around the food being cooke< and is exhausted through the exit port ventilation holes 36, past the humidity sensor 32 and the temperature sensor 34 which are positioned adjacent to and by the plurality of exit port ventilation holes 36 in the top of the microwave heating cavity 30, and through the plurality of the ventilation holes 38 in top 20 of the housing wrap 18 via way of the ventilation hood 42.
Figure 4 illustrates the side view of the microwave oven 10 with the right side 22 of the housing wrap 18 removed illustrating the air circulation fan 58 which also draws air up through a bottom intake ventilation port 67, past a power transformer 68 and a capacitor 70, and around horizontal frame member 72 which divides the microwave power source 50 - diode bridge 62 compartment from the power transformer 68 - capacitor 70 compartment. The air from the bottom intake ventilation port 67 joins the air circulation path of the air pulled in through the lower portion of the rear intake ventilation port. This air circulation is also introduced into the microwave heating cavi _' 30 through the entrance port ventilation holes 66, which after circulating within the heating cavity 30, is exhauste< through the plurality of exit ventilation port holes 36 past the humidity sensor 32 and the temperature sensor 34 into the outside environment through the plurality of exit ventilation holes 36 via way of the ventilation hood 42 which is partially cutaway. The described path of air circulation in the microwave oven heating cavity is for way of example and purpose of illustration of the invention.
Figure 5 illustrates a top plan view of the microwave oven showing a portion of the top 20 of the housing wrap 18 cutaway and showing a portion of the ventilation hood 42 cutaway. The humidity sensor 32 and the temperature sensor 34 are shown positioned adjacent to the plurali- of exit port ventilation holes 36 in the top of the microwave heating cavity 30. Electrical cables connect the humidity and temperature sensors 32 and 34 respectively to the programmable controller 40 positioned behind the numeric keyboard control panel 14 through the analog signal conditioner interface circuitry. The air flow circulation path is shown by the arrows in the figure corresponding to the air flow circulation path through the rear intake ventilation port 60 and through the microwave oven heating cavity 30, the air being exhausted through the plurality of the exit port ventilation holes 36 to the plurality of ventilation holes 38 via way of the ventilation hood 42. All other numerals correspond to those elements previously described.
Figure 6 illustrates a top enlarged and expanded view of the humidity sensor 32 and the temperature sensor 34 as shown in Figure 5 positioned adjacent to and by the plurality of exit port ventilation holes 38 in the top of the microwave heating cavity 30 with the top 20 of the housing wrap 18 and the ventilation hood 42 partially cutaway. All other numerals correspond to those elements previously described.
Figure 7 illustrates a section of the present invention taken on line 7-7 of Figure 6 looking in the direction of the arrows showing the humidity sensor 32 and the temperature sensor 34 held by friction in a bracket 74 positioned adjacent to and by the plurality of exit port ventilation holes 36 in the top of the microwave heating cavity 30 with the bracket 74 being affixed to the oven frame structure 76 by any structure in the art such as adhesive. The humidity sensor 32 and the temperature sensor 34 can affix directly to the oven frame 76 by any suitable structure instead of being retained in the bracket 74.
Figure 8 illustrates an electrical circuit schematic block diagram of the present invention showing the humidity sensor 32 connected to a humidity circuit 78. The humidity sensor 32 and the humidity circuit 78 consist of a commercially available package such as a Thunder Scientific Corporation PC-2000 Humidity Measurement module for the humidity circuit 78 and the BR-101B Aluminum Oxide humidity sensor 32. The temperature sensor 34 and the temperature circuit 80 consist of a commercially available package such as a National Semiconductor Corporation LX 5700 Temperature Transducer. The humidity sensor 32 and the temperature sensor 34 can be integrated into a single unitary package such as an integrated circuit also containing the humidity circuitry 78 and the temperature circuitry 80 forming a single sensor package and eliminating the need for separate sensor elements and associated circuitry.
The humidity sensor 32 through the humidity circuit 78 and the temperature sensor 34 through the temperature circuit 80 connect to the programmable controller 40 through a series connection of an analog multiplexer 82, a sample and hold gate 84, and an analog to digital converter 86. The analog mulitplexer 82, the sample and hold gate 84, and the analog to digital converter 86 may consist of an analog signal conditioner circuit to properly interface the sensed temperature and humidity signals to the programmable controller 40. In the alternative, the interface circuitry can be directly incorporated in the programmable controller 40 to provide interfacing for input of the sensed and sampled humidity and the temperature signals.
A microwave control circuit 88 such as a silicon controlled rectifier power supply for the microwave power source 50 is controlled by the programmable controller 40. An air exchange control circuit for the fan 58 is also controlled by the programmable controller 40. The air exchange control circuit can further consist of an infrared lamp or a resistance heating coil not shown in the drawings to preheat the air temperature of the air being introduced at the entrance ventilation port 66 of the microwave oven heating cavity 30 during microwave cooking.

Preferred Mode of Operation

Time dependent "in-situ" environmental conditions of the microwave oven heating cavity 30 are sensed during microwave cooking of foods such as hot dogs, hamburgers, pork roasts, beef roasts, beef steaks, pork chops, poultry, fish, french fries, cakes, cookies, meat loafs, bread, vegetables, etc. The time dependent environmental conditions are defined as the sensed humidity and temperature environmental conditions of the microwave heating cavity 30 by humidity and temperature sensors. During the process of cooking foods with microwave energy, various energy conversions occur that modify the combined physical and chemical state of the food. These state changes directly determine the "doneness" or acceptable "palatability" of the cooked food. Since these states are electronically sensed and pragmatically correlated to the doneness of the food, then a microwave oven sensing system of the time dependent environmental conditions of the microwave oven heating cavity is achieved to control the microwave cooking of the food. The purpose of implementing the microwave oven sensing system with the programmable controller having stored algorithms of "characteristic humidity curves" for different foods is to improve the "convenience and efficiency" of the microwave oven as well as the "palatability" of the food.
The humidity sensor 32 and the temperature sensor 34 are appropriately positioned as described to sense the environmental conditions of the microwave oven heating cavity 30. Statically and dynamically, the exhaust air through the exterior of the exit ventilation port of the microwave oven heating cavity 30 is most depictive of the time dependent humidity and temperature environmental conditions of the microwave oven heating cavity 30. The environment on the exterior side of the exit ventilation port holes 36 for all practical considerations is equal to the environment on the interior side of the exit ventilation port holes 36 of the microwave oven heating cavity 30 permitting sensing of the time dependent environmental conditions of the microwave oven heating cavity 30 even though the humidity sensor 32 and the temperature sensor 34 are not located within the microwave oven heating cavity 30.
While the humidity sensor 32 and the temperature sensor 34 have been illustrated in Figures 1-7 as being positioned adjacent to and by the exterior side of the exit ventilation port holes 36 of the microwave oven heating cavity 30 for way of example and purposes of illustration only, the sensors 32 andy-34 can be positioned directly above the exit port ventilation holes 36 of the microwave oven heating cavity 30 so long as the surface of the humidity sensor 32 is shielded against condensation preventing saturation of the sensor 32 from the steam and condensation gases being vented from the microwave oven heating cavity 30. In the alternative, if the humidity sensor 32 is positioned above the exit port ventilation holes 36 of the microwave oven heating cavity 30 and not shielded against condensation, then it is necessary to program the programmable controller 40 to account for saturated humidity sensor 32 conditions resulting from condensation on the humidity sensor 32.
The descriptive "words and phrases" in describing the positioning of the humidity sensor 32 as illustrated in Figures 1-7 of the drawings, also includes the terms near, at, on top of, to one side of, in the general vicinity of, etc., the exit port ventilation holes 36 of the microwave oven heating cavity 30.
The main thrust of placement of the sensors is to position the humidity sensor 32 and the temperature sensor 34 to sense and sample the time dependent "in-situ" environ mental conditions of the microwave oven heating cavity 30, and one of the preferred positions is where the humidity and temperature sensors 32 and 34 respectively are positioned adjacent to and by the exit port ventilation holes 36 of the microwave oven heating cavity 30 as being most depictive and in equilibrium with the microwave oven heatir. cavity 30. The humidity sensor 32 and the temperature sensor 34 are-positioned on the exterior side of the exit port ventilation holes 36 so as not to be subjected to electromagnetic interference and because the environment on the exterior side of the exit port ventilation holes 36 is for all practical considerations is equal to the environment of the interior of the microwave oven heating cavity 30 permitting time dependent sensing of the humidity and temperature environment conditions of the microwave oven heating cavity 30.
The humidity and the temperature sensors 32 and 34 respectively can also be positioned within the interior confine of the ventilation hood 42 but yet near enough to the exit port ventilation holes 36 of the microwave oven heating cavity 30 to sense the environmental condi_- tions of the microwave oven heating cavity 30.
The humidity sensor 32 and the temperature sensor 34 can further be positioned anywhere in the interior of the microwave oven heating cavity 30 instead of positioning the sensors 32 and 34 at the exterior of the exit ventilation port 36 so long as the sensors have proper electromagnetic interference shielding to isolate the sensors 32 and 34 from interference by the microwave energy source 50.
The humidity sensor 32 and the temperature sensor 34 can additionally be positioned in an exhaust duct not shown in the drawings of the microwave oven 10 if utilized to exhaust the steam and gases from the microwave oven heating cavity 30 through the frame structure to the front, back, bottom, side or top of the microwave oven 10. The humidity sensor 32 and the temperature sensor 34 can be positioned directly in the exhaust duct structure of the microwave oven 10 but the humidity and temperature equilibrium conditions in the duct should be compensated for either by second humidity and temperature sensors located at a distance from the first humidity and temperature sensors or by programming equilibrium constant conditions into cooking algorithms stored in memory of the programmable controller 40.
The microwave oven sensing system derives from the most fundamental microwave cooking principles. That is, as microwave energy is converted to thermal energy, the internal temperature of the food increases. As the internal temperature of the food increases, some of the thermal energy is used to break the bonding forces holding the water molecules to the food's cell structure. When the latent heat of vaporization has been added to the food, these free water molecules are vaporized and released locally. Thus, the rate of evaporation is directly proportional to the rate of temperature rise, and the integration of the rate of evaporation is proportional to the weight loss of the food. By monitoring the humidity and the temperature in the microwave oven heating cavity 30, the food's internal temperature, surface temperature, and weight loss can be determined.
The absolute humidity in grams per cubic meter in the microwave oven heating cavity 30 as a function of time in seconds yields a "Characteristic Humidity Curve" 92 as illustrated in Figure 9 for the microwave cooking of foods which is expressed by the equation 1 as
where h_o is the initial absolute humidity, h_x is the peak absolute humidity, B is a constant, in one over seconds, related to food characteristics, t is time in seconds and n is an integer. Absolute humidity is also known as vapor density. The characteristic humidity curve is determined by sensing the time dependent humidity and temperature environmental conditions of the microwave oven heating cavil 30 for a product being heated.
The characteristic humidity curve 92 of the Figure 9 of the absolute humidity in grams per cubic meters versus time in seconds can be approximated as a three segment 94, 96 and 98 piecewise linear curve prior to time t_x. The slope of the first segment 94 of the three segment piecewise linear curve is proportional to the rise in surface temperature of the food up to one hundred degrees centigrade. Once the hottest section of the surface has reached one hundred degrees centigrade at the change of slope 100, the humidity increases rapidly as illustrated by the slope of the second segment 96 of the three segment piecewise linear curve which is due to the "latent heat of vaporization" of the food. The change of slope 102 is indicative of the beginning of the surface temperature of the food reaching equilibrium. The slope of the third segment 98 of the three segment piecewise linear curve from point 102 to 104 is equilibrium occuring of the surface temperature of the food. The characteristic humidity curve 92 plateaus at point 104 indicating that the surface temperature of the food has reached equilibrium of one hundred degrees centigrade. While the peaked out plateau portion of the curve 92 is represented as a smooth curve, in reality there are "noise-like" perturbations on the curve due to surface boiling of water of the food. Any point on the characteristic humidity curve 92 is indicative of the surface temperature of the food while the integral of the area under the curve 92 at that point is indicative of the "internal doneness" of the food being cooked in the microwave oven heating cavity 30. Each particular type of food has its own individual "characteristic humidity curve" and the food during microwave cooking follows the characteristic humidity curve for that particular food.
In utilizing the microwave oven sensing system in the microwave oven 10, the interfacing circuitry of analog multiplexer 82, sample and hold gate 84, and analog digital converter 86 may not be required depending upon the type of programmable controller 40 utilized. The humidity sensor 32 and the temperature sensor 34 can connect directly to the programmable controller 40 obviating any need for analog signal conditioner interface circuitry including the need for a humidity sensor circuit 78 and temperature sensor circuit 80. The programmable controller 40 can be a commercially available microprocessor such as an Intel 8080 or TMS 1000 with a read only memory (ROM) storing characteristic humidity curves as cooking algorithms in the memory.
The programmable controller 40 is programmed with cooking algorithms based on characteristic humidity curves for different types of foods such as meats, etc. to be cooked during microwave cooking. For example, different types of meats have different characteristic humidity curves which follow equation 1 and are implemented as cooking algorithms which are stored in the memory of the programmable controller. For example, it is known that meats brown when the surface temperature is rising at a certain rate and the ambient humidity at the surface region of the meat is at a certain humidity level. During microwave cooking, the programmable controller 40 receives the humidity and temperature signals from the humidity and temperature sensors 32 and 34 respectively positioned at the exit ventilation port 36 of the microwave heating cavity 30 sensing the "in-situ" environmental conditions, and follows a selected cooking algorithm of the "characteri tic humidity curve" for the particular type and thickness of meat. being cooked, thereby optimizing the cooking and browning of the meat. Further, the selected cooking algorithm for the meat being cooked through the programmable controller 40 can indicate to the cook as to the proper time to turn the meat over and the meat is cooked to donene
The programmable controller 40 is also programmed to make electronic decisions as to increasing or decreasing the air exchange rate of the air exchange control circuit 90 which consists of the fan 58; increasing or decreasing the microwave power source duty cycle, in this example, the magnetron control circuit 88, or; increasing or decreasing the incoming air temperature such as controlling preheating of the incoming air temperature with an infrarec lamp or by passing the air over a resistance heater such as a Calrod.
Individual cooking algorithms for characteristic humidity curves for different types of foods stored in the programmable controller 40 can be selected for the type of food being cooked in the microwave oven 10 through the numeric inputs on the numeric keyboard control panel 14. This control panel contains numeric input operator feature: in addition to the standard microwave oven control panels currently on the market such as a "Humidity" control 44, a "Turn Food Over" control 45, a "Time Out" control 46 and a "Done" control 47. The "Humidity" keyboard control 44 permits the input of the humidity cooking parameter for the selected cooking algorithm stored in the programmable controller 40.
By controlling the humidity in the microwave oven heating cavity 30, the internal temperature of the food, the surface temperature of the food, browning of the food, and the weight loss of the food can all be controlled according to the input humidity parameter by the "Humidity" control 44 on the numeric keyboard control panel 14 or to the selected cooking algorithm called from storage in the memory of the programmable controller 40. The "Humidity" control 44 permits the presetting of a predetermined humidity to be maintained within the microwave oven heating cavity 30 during microwave cooking.
The humidity sensor 32 sensing the time dependent humidity at the exit ventilation port 36 of the microwave oven heating cavity 30 inputs the sensed humidity to the programmable controller 40 which according to the selected stored cooking algorithm of the characteristic humidity curve for the particular type of food being cooked determines how the food is cooked, at what time the food is turned over, and at what time the food is done. The programmable controller 40 receives the sensed time dependent humidity from the humidity sensor 32 which is sensed absolute humidity and also receives the sensed time dependent temperature from the temperature sensor 34 to convert the sensed absolute humidity to relative humidity for the cooking algorithm program stored in the programmable controller 40. The programmable controller 40 can then compute when the food will reach the desired doneness temperature by continually comparing the sensed humidity points with the characteristic humidity curve for that food type, which curve is stored in the memory of the controller. When the point on the curve equivalent to doneness is reached, the controller can cause the microwave energy source to be shut off.
The programmable controller 40 also indicates to the cook when the optimum time is reached to turn a food over such as meat and activates a "Turn Food Over" control 45 such as illuminating a control light as well as sounding an audible alarm. This point occurs where the characteristic humidity curve 92 of Figure 9 reaches peak absolute humidity h at point 104 as determined by the programmable controller 40. If the cook fails to close the door 12 within a predetermined time as determined by the programmable controller 40 after turning the food over, the "Time Out" control 46 such as an illuminated control light as well as an audible alarm are activated by the programmable controller 40. The programmable controller also indicates to a cook when the food is done by activating the "Done" control 47 such as illuminating a control light as well as sounding an audible alarm.
In commercial microwave cooking applications such as fast food franchises, it may be desired to cook food samples exactly the same way each time for identical food samples such as hamburgers, bread, pastry, etc. To accomplish the identical microwave cooking of food samples, the desired characteristic humidity curve determined empirically or through a previous cooking cycle for a food sample is stored in the memory of the programmable controller 40 as the cooking algorithm for that food. The programmable controller 40 then compares the stored cooking algorithm for the food being cooked to the sensed relative humidity at a plurality of time spaced points to maintain microwave cooking of the food sample in the microwave oven cavity 30 until the food reaches the point or the humidity curve indicating that the preselected degree of doneness has been reached.
An additional humidity sensor and associated interface circuitry which connects to the programmable controller 40 can be positioned in a nonhostile environment of the frame structure of the microwave oven 10 to provide calibration for the humidity sensor 32 located adjacent to and by the exterior of the exit ventilation port 36.
Various modifications can be made to the microwave oven sensing system of the present invention without departing from the apparent scope of this invention.
Specifically, the programmable controller 40 can store cooking algorithms to maintain the humidity at a predetermined level to brown foods, to indicate the temperature and internal doneness of food being cooked, to indicate the optimum time to turn food over, to control the cooking of juiced foods, to detect a fire in the cavity, etc.
The invention has a wide variety of industrial applications as a microwave oven control and method of cooking food. The invention can be applied to microwave ovens for domestic use as well as to those used in commercial applications such as institutions, restaurants, vending locations, and the like.

Claims

1. A microwave oven control system comprising:

(a) 'microwave oven heating cavity means;

(b) microwave power means coupled to said cavity; and

(c) sensor means positioned to sense time dependent environmental conditions of microwave cooking in said microwave oven heating cavity means;

(d) control means responsive to said sensor means and coupled to said microwave power means to control said power means in response to sensed time dependent environmental conditions in said microwave oven heating cavity means.

2. In a microwave oven of the type having a cooking cavity in which food to be cooked is placed, a source of microwave energy coupled to said cavity, a user operated controller having a programmable memory portion and a keyboard control panel wherein the user can selectively operate the microwave energy source to obtain desired cooking times or results, the improvement characterized by:

(a) sensing means positioned to sense the humidity of the cooking environment surrounding the food and to provide a signal indicative of said humidity to said controller, whereby the food is cooked according to a humidity dependent algorithm stored in the programmable memory portion of said controller.

3. The apparatus according to claim 2 wherein said humidity sensing means comprises an aluminum oxide sensor positioned adjacent the exit ventilation port of said cavity.

4. The apparatus of claim 3 further comprising a temperature sensor co-located with said humidity sensor adjacent said exit ventilation port.

5. In a microwave oven of the type which includes a cooking cavity, a source of microwave energy coupled to the cooking cavity, an air ventilation system whereby air is circulated through the cooking cavity and exhausted therefrom through a suitable ventilation port and a controller having a programmable memory, the controller being capable of receiving operating instructions and controlling oven components in accordance with those instructions, the improvement in such oven characterized by:

(a) a humidity sensor located adjacent said ventilation port for detecting the humidity of the cooking cavity environment;

(b) a temperature sensor located adjacent said ventilation port for measuring the temperature of the exhaust air from the cooking cavity;

(c) means for energizing and de-energizing the microwave energy source and means for adjusting the flow rate or temperature of the air circulated through the cooking cavity, whereby the microwave energy source and the ventilation air are controlled by said controller in response to the humidity and temperature measured by said sensors.

6. The apparatus of claim 5 wherein the programmable memory portion of said controller has stored therein a characteristic humidity curve for a food to be cooked in said oven.

7. The apparatus of claim 5 wherein the means for energizing the microwave energy source comprises an electrical circuit including said controller whereby the microwave energy source may be repeatedly turned on and off by said controller.

8. The apparatus of claim 5 wherein said means for adjusting the flow rate of air circulated through the cooking cavity includes a variable speed fan electrically connected to said controller whereby said controller may vary the fan speed.

9. The apparatus of claim 5 including an electrical resistance heater positioned in the ventilation air stream for heating the ventilation air before it is introduced into said cooking cavity.

1₀. The apparatus of claim ₅ wherein said humidity sensor is an aluminum oxide sensing device.

11. A method for cooking food in a microwave oven comprising the steps of:

(a) storing a predetermined humidity curve for the food in the memory portion of a programmable controller;

(b) heating the food in said oven and measuring the humidity and temperature of the air exhausted from said oven as the food is heated and providing signals indicative of said humidity and temperature to said controller;

(c) comparing the measured humidity and temperature with the stored, predetermined humidity curve;

(d) terminating the heating of food when the humidity and temperature reach a selected point on said predetermined humidity curve.

12. The method of claim 11 including the additional step of adjusting the rate of air flow through said oven as the food is heated to maintain a predetermined desired humidity in said oven.