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This paper presents the integration of electrical capacitance tomography (ECT) with a moisture controller for the microwave drying of polymer foam. The proportional–integral (PI) control and the linear quadratic Gaussian (LQG) control are employed in designing the controller. The control objective in this process is that the moisture of polymer foam after the drying process reaches the desired set point. The permittivity distribution of polymer foam after the drying process is estimated in real-time using a designed ECT sensor and transferred as feedback to the controller. Since the permittivity and the moisture are strongly correlated, the material moisture can be controlled by controlling the permittivity. A state-space model is derived for the microwave drying process based on a system identification approach using the experimental data from the process. The derived model is employed in designing the LQG controller and adjusting the parameters of the PI controller. The designed controllers are implemented on a testbed microwave oven, and the experimental results show that the designed controllers are able to follow the desired set point moisture. The performance of the system with both controllers is compared, and their advantages and disadvantages are discussed. Moreover, the benefits of having a moisture controller for the microwave drying process are shown in simulation studies compared to an uncontrolled system. • Electrical capacitance tomography (ECT) to estimate the moisture distribution • Modeling the microwave drying process using the system identification approach • Combining the process tomography and control to achieve the desired moisture • Implementing a proportional–integral (PI) controller to track the set point moisture • Designing a linear quadratic Gaussian (LQG) controller using a black-box model [ABSTRACT FROM AUTHOR]

Moisture distribution information is a critical element in drying processes. The drying of products by employing high-power microwave (MW) technology is widely used in the industry. Although microwaves allow volumetric and selective heating resulting in a significant reduction of processing time and energy consumption, there is always a risk of non-uniform moisture distribution in the final product. This paper investigates the capability of a designed electrical capacitance tomography (ECT) sensor to estimate the moisture distribution of polymer foams in a microwave drying process. The moisture distribution is estimated based on the non-intrusive contactless measurements of the electrical capacitances between the electrodes mounted on a frame around the target polymer foam. The obtained moisture information can be employed as feedback to a controller to adjust the power level of each microwave source in the microwave system to reduce or eliminate the non-homogeneity of the moisture distribution inside the polymer foam. In a series of experiments, we first examine the capability of the ECT sensor in estimating the moisture distribution in a stationary foam. We extend the tests to estimating the moisture distribution in a case where the foam is moving on a conveyor belt. Several study variables are taken, including the sample size, the sample location, the moisture percentage, the conveyor belt speed, and the microwave power. These experiments show that the sensor has a satisfactory accuracy in estimating the moisture distribution of the foam, and the ECT measurements can be further used in a closed-loop control system. [ABSTRACT FROM AUTHOR]

The microwave drying process has a wide application in industry, including drying polymer foams after the impregnation process for sealings in the construction industry. The objective of the drying process is to reach a certain moisture in the foam by adjusting the power levels of the microwave sources. A moisture controller can be designed to achieve this goal; however, a process model is required to design model-based controllers. Since complex physics governs the microwave drying process, system identification tools are employed in this paper to exploit the process input and output information and find a simplified yet accurate model of the process. The moisture content of the foam that is the process output is measured using a designed electrical capacitance tomography (ECT) sensor. The ECT sensor estimates the 2D permittivity distribution of moving foams, which correlates with the foam moisture. Experiments are conducted to collect the ECT measurements while giving different inputs to the microwave sources. A state-space model is estimated using one of the collected datasets and is validated using the other datasets. The comparison between the model response and the actual measurements shows that the model is accurate enough to design a controller for the microwave drying process. [ABSTRACT FROM AUTHOR]