Automatic defrost sensing arrangement for microwave oven

A microwave oven operable in a defrost mode in which objects in a frozen state are heated until converted to a thawed state, said oven comprising:

• a cooking cavity for receiving objects to be heated therein;
• a source of microwave energy;
• waveguide means for coupling microwave energy from said source to said cavity, said cavity waveguide and source being configured such that in normal operation the magnitude and phase of the electromagnetic field supported in said waveguide is substantially unchanged by the transition of an object being heated in said cavity from its frozen state to its thawed state;
• sensing means for monitoring the field strength of the electromagnetic field supported in said waveguide at a predetermined location in the waveguide and generating a field strength signal indicative thereof;
• means for periodically introducing an electromagnetic field discontinuity into said waveguide effective to substantially change the phase and magnitude of the electromagnetic field in the waveguide for an object in said cavity in its frozen state relative to that for the object in its thawed state, thereby providing a readily detectable difference in said field strength signal between the signals corresponding to the object in its frozen and thawed states, respectively;
• means for sampling said field strength signal when said electromagnetic field discontinuity is present in said waveguide to detect a field strength signal indicative of the object having assumed its thawed state;
• and
• means for controlling oven operation in the defrost mode in response to detection of such a signal.

A microwave oven operable in a defrost mode to convert food objects from a frozen state to a thawed state, said microwave oven comprising:

• a cooking cavity for heating objects received therein;
• a source of microwave energy;
• first radiating means for radiating energy into said cavity adjacent one wall of said cavity;
• second radiating means for radiating energy into said cavity adjacent a second wall of said cavity.,
• waveguide means for coupling energy from said source to said first and second radiating means, said waveguide means including first and second guide sections for coupling energy from said source to said first and second radiating means, respectively;
• field strength detection means responsive to the field strength at a predetermined location in said first guide section operative to generate a field strength signal indicative of the field strength at said location,
• discontinuity means selectively movable between a neutral position having no significant effect on energy propagating in said waveguide means and an active position in said second guide section in which said discontinuity means is effective to substantially alter the phase and standing wave ratio of the standing wave supported in said waveguide means for an object in the cavity in its frozen state relative to the phase and standing wave ratio for the same object in its thawed state;
• means for sampling said field strength signal when said discontinuity means is in its active position to detect a signal level indicative of the object having assumed its thawed state;
• and
• means for controlling oven operation in the defrost mode responsive to said sampling means.

A method for defrosting objects in a microwave oven operable in a defrost cycle and having a cooking cavity for receiving objects to be heated therein, a source of microwave energy, and a waveguide coupling the microwave energy from the source to the cooking cavity, said method comprising the steps of:

• (a) placing a frozen object in the cooking cavity;
• (b) periodically introducing an electromagnetic field discontinuity into the waveguide for relatively short sampling periods, which discontinuity is effective to substantially increase the difference in the phase and magnitude of the electromagnetic field supported in the waveguide between the object in its frozen state and that for the object in its thawed state, relative to the difference which exists in the absence of the discontinuity;
• (c) sampling the field strength of the electromagnetic field of the microwave energy in the waveguide at a predetermined location along the length of the waveguide to detect a field strength level indicative of the object assuming its thawed state;
• (d) de-energizing the energy source, thereby terminating the defrost operating mode upon detection of the object assuming its thawed state.

A method for defrosting frozen objects in a microwave oven having a cooking cavity for receiving objects to be heated therein, a source of microwave energy and a waveguide comprising the steps of:

• (a) operating the microwave energy source at a first power level;
• (b) periodically introducing an electromagnetic field discontinuity is effective to substantially increase the change in the phase and magnitude of the electromagnetic field support in the waveguide for an object in its frozen state being heated in the cavity relative to that object in its thawed state;
• (c) sampling the field strength of the electromagnetic field in the waveguide during each sampling period to detect a field strength level indicative of the object assuming its thawed state;
• (d) measuring a first time period from the beginning of the defrost cycle to the first detection of a field strength level indicative of the object assuming its thawed state;
• (e) reducing the output power level of the microwave source to a second relatively lower power level upon such detection;
• (f) operating the oven at the second power level for a second time period of a duration calculated as a function of the first time period;
• (g) terminating oven operation at the end of the second time period.