| This dissertation presents the research work on the development of a variable frequency microwave processing system for polymer and composite materials, with an emphasis on achieving uniform temperature distribution using intelligent process control. A variable frequency microwave material processing system was constructed based on the existing fixed frequency microwave processing technology with the use of a variable-frequency microwave power source. Data acquisition and control hardware was implemented for process monitoring, measurement, and control. Software programs were developed in LabVIEW for data acquisition, system characterization, and process control. The control objective is to achieve efficient, uniform, and controlled heating, which was realized by mode tuning, intelligent mode switching, on-line mode characterization, and effective power control.; Two uniform processing techniques were developed and evaluated. They are mode sweeping heating, and intelligent mode switching heating. Mode sweeping heating proved to be very effective for small size samples. Intelligent mode switching heating optimizes the sequence of the modes used for heating, by comparing the mode heating characteristics with measured temperature distributions and selecting the mode that will alleviate the temperature gradients the most. Using intelligent mode switching heating, great improvement of temperature uniformity was achieved over single mode heating and mode sweeping heating. An on-line mode characterization technique was developed to enable the process control system to adjust to process condition changes. With the addition of on-line mode characterization capability, consistent and good performance was ensured for the variable frequency microwave processing system, as demonstrated by the uniform and stable processing of composite parts with complex geometry.; During the mode selection process in the intelligent mode switching heating, modes were compared by their ability to decrease the temperature gradients and generate the most uniform temperature distribution within a desired period of time. Temperature uniformity was measured by the standard deviation of the temperatures. Therefore, the optimal mode would decrease the temperature standard deviation the most within the specified period of time. Two power control algorithms were designed to achieve the objectives of providing fast heating, reducing temperature overshoot, and maintaining constant curing temperature. Consequently, a simple parabolic power controller and a multi-staged PID controller were designed for microwave power control. The former needed no controller tuning, while the latter required proper tuning of the control parameters but provided more stable and accurate control performance.; The experimental results proved the variable frequency microwave processing system successful in achieving uniform processing with consistent performance. A viable variable frequency microwave processing system for industrial applications can be developed based on this newly developed system. The full automation of the hardware and the flexibility of the software ensure easy implementation and make the system adaptable to different types of applications. |
