The present invention relates generally to apparatus and methods for the purpose of determining the progress of a food load being cooked or otherwise heated in a microwave oven. More particularly, the invention relates to such apparatus and methods responsive to changes in electrical characteristics of the food load as it is heated, particularly changes as a result of variations in state and quantity of moisture content.
It is desirable in a cooking microwave oven to be able to monitor or determine the progress of food being cooked or heated. While microwave ovens typically include viewing windows, such windows provide limited visibility, and often little information is obtained concerning the progress of food cooking by observing the appearance in any event. One general method, useful particularly in the case of large pieces of meat being cooked, is to employ a temperature-sensing probe assembly inserted into the food being cooked, such as is disclosed in the Chen et al. U.S. Pat. No. 3,975,720 and in the Meek et al. U.S. Pat. No. 4,086,813. However, the use of such a probe is not always convenient or possible, and it desirable to provide further alternative approaches. (As employed herein, the term "cooking" is employed in a broad sense to mean the heating or thermalization of food placed in a microwave oven, regardless of the particular temperature range over which the heating occurs and regardless of the particular chemical or physical change occurring within the food.)
Another general method of providing information about the progress of food cooking in a microwave oven relies upon changes in the characteristics of the food as an electrical load or a microwave absorber during cooking. For example, conductivity and dielectric properties of food change during cooking or heating, particularly as water content is affected by the microwave heating. One known approach relying upon such changes monitors the standing wave ratio or a related property within a feed waveguide or other form of transmission line supplying the microwave cooking cavity. This general approach is disclosed for example in the Moe U.S. Pat. No. 3,813,918.
Although not directly responsive to changes in the load properties of individual items of food as cooking progresses, similar sensing principles have been proposed for controlling the length of cooking time as a function of energy delivered to a food load or available energy apportioned between a plurality of individual items of food. For example, in the system of the Schroeder U.S. Pat. No. 2,744,990, cooking time is related to net energy supplied to the microwave cooking cavity as determined by a directional coupler in a feed transmission line, net power delivered being sensed forward power minus sensed reflected power. Similarly, in the systems described in the Moore U.S. Pat. No. 3,999,027 and Tallmadge et al. U.S. Pat. No. 4,009,359, microwave field strength is sensed within the cooking cavity itself, rather than in a feed waveguide, for the purpose of controlling the time duration of operation to achieve a desired temperature within a food load material. With a higher microwave field strength, it is assumed that the cooking effect is greater, and the time duration is accordingly shortened. Specifically, electromagnetic field strength is integrated with respect to time as an indicator of overall cooking effect.
Another condition which may be sensed using related techniques, particularly for protective purposes, is the absence of any food load whatsoever within the microwave cooking cavity. Under such conditions, the standing wave ratio within the feed waveguide, as well as the field strength within the cavity, are higher than normal. Various systems have been proposed for sensing such conditions, and automatically turning off the microwave generator in response. For example, the system of the Meissner et al. U.S. Pat. No. 3,281,567 directly senses field strength within a cooking cavity. In a somewhat similar fashion, the system of the Haagensen et al. U.S. Pat. No. 3,527,915 indirectly responds to field strength within a cooking cavity by sensing the temperature rise of an element placed at the bottom of the cavity and which absorbs microwave energy. Other protective systems respond to conditions within the feed waveguide, for example, as disclosed in the Anderson U.S. Pat. No. 4,412,227, the Kohler et al. U.S. Pat. No. 3,491,222, the Jones et al. U.S. Pat. No. 3,662,140, and the Bucksbaum U.S. Pat. No. 3,670,134.
In a somewhat different vein, a related principle of operation is utilized in an automatic control system for a continuously-moving type microwave dryer disclosed in the Kashyap et al. U.S. Pat. No. 4,035,599. In the Kashyap et al. system, microwave energy is passed through a moving web load, such as paper. Input power and output power are separately sensed by means of directional couplers. In order to control microwave input power to maintain a constant drying effect, microwave input power is varied as a function of sensed input and output power, as well as of web velocity.
The effect of varying quantities of food and changes within a food load as cooking progresses is recognized in the systems of the Sawada U.S. Pat. No. 3,104,304 and the Stecca et al. U.S. Pat. No. 3,321,604, each attempting to maintain optimum conditions as the food changes. In Sawada, the oscillator frequency is varied to maintain resonance. In Stecca et al., the cavity itself is tuned to maintain resonance.
Various non-electrical approaches to the problem of monitoring cooking progress in a microwave oven have also been proposed. For example, the Smith U.S. Pat. No. 3,467,804 proposes a sensor for steam or other vapors which may be emitted when an article of food is heated, or has reached a predetermined temperature. In the Ueno U.S. Pat. No. 4,049,938, an infrared radiation detector is proposed.
While the various approaches described above for indirectly obtaining information concerning the progress of food being cooked in a microwave oven, particularly those which rely upon a change in the load properties of the food itself, do function to some extent, greater sensitivity is desirable. This greater sensitivity is provided by the apparatus and method of the present invention. In particular, the sensing effected by the present invention provides a relatively large percentage change in the sensed parameter depending upon the dielectric properties of the food load, particularly as a result of the state and quantity of its moisture content.