Microwave Frequency Control Algorithms for Use in a Solid-State System to Achieve Improved Heating Performance

Microwave is a popular food heating technique. Its unique volumetric heating principle enables fast heating while also leads to nonuniform heat distribution, where the standing wave patterns caused by the magnetron as microwave source is the main reason for the poor uniformity. Solid-state microwave generator is a promising solution to address the nonuniform heating, as it allows flexible microwaves, with frequencies in a range rather than fixed, and thereby, the varied thermal patterns by different frequencies could overcome the standing wave pattern issue. Previous studies on the frequency control strategy mainly focused on orderly shifting frequencies in range, while not fully utilized the information provided by frequency-related pattern variation, where the complementary patterns could be found. Besides, such in order frequency-shifting did not consider sample variation and only applied a common shifting strategy to all products. Hence, the objective of this project is to develop a dynamic microwave frequency shifting algorithm that makes use of complementary-frequency and can accommodatively shifts frequencies according to real-time collected information.

Preliminary experiments were first conducted to validate the correctness of the complementary-frequency concept, where the pre-collected thermal patterns under different frequencies were used as initial dataset to design the frequency shifting path. Compared with orderly shifting, the complementary-frequency shifting algorithm was demonstrated to have improved microwave heating performance. Based on the validated concept, and to eliminate the pre-collection of data for path design, the complementary-frequency shifting algorithm was improved to be a dynamic version with a thermal camera mounted to the oven that could monitor the heating results in real time. The dynamic complementary-frequency shifting algorithm was proven to compete the orderly shifting strategies. Furthermore, the dynamic heating was compared with the rotatory magnetron heating, i.e., the domestic oven heating, where the performance evaluation was conducted on various types of commercial or prepared foods. The comparison results showed that the proposed dynamic complementary-frequency shifting algorithm, realized in a solid-state microwave system, successfully competed with the rotatory magnetron heating in a domestic oven.

Conclusions from the current dissertation shall provide useful information for the future design and commercialization of the solid-state microwave ovens.