Modeling Advanced Temperature Measurement And Control Algorithms In Thermoforming PDF Download

"Thermoforming is a plastic manufacturing process consisting of three major phases: heating, forming, and cooling. During the heating phase, the plastic sheet has to be heated to a precise temperature set-point (profile) at which it is flexible enough to be molded. Often, because of the complexity of the final product's shape, uneven temperature set-points are required across the plastic sheet. Therefore, it is crucial to systematically control the heaters' temperatures so that the exact temperature profile is achieved across and through the depth of the plastic sheet. In the first part of this thesis, advanced temperature measurement algorithms are developed, namely model-based virtual sensors (MBVS) and core-temperature observers. The concept of MBVSs allows for extra surface-temperature measurement points in addition to the existing infrared sensors. This leads to improved observability of the sheet's temperature and increased accuracy in achieving uneven temperature profiles, thus eliminating the use of extra infrared sensors and significantly reducing the cost of the control system. Additionally, core-temperature observers are developed in order to precisely monitor the core-temperature of the plastic sheet, as it is crucial for the center-plane of the sheet to be within the forming temperature window at the end of the cycle. In the second part of this thesis, the application of the Watanabe-modified Smith predictor control technique (an internal-model control technique) is studied for the thermoforming heating phase in order to reduce the heating-cycle time of the process. The third part of the thesis addresses the temperature control of multilayered plastic sheets during the heating phase. Thermoforming of multilayered sheets has proven to be quite challenging since different plastic materials contain their own distinctive rheological properties and forming temperatures. In this thesis, a dynamical temperature evolution model of multilayered plastic sheets is presented, followed by a proposed discrete-time model predictive control (DTMPC) algorithm to solve, for the first time, the non-uniform temperature tracking problem of all the layers, while incorporating the nonlinear dynamics of the actuators in the design. Finally, the last part of this thesis covers the important problem of parameter variations in thermoforming. During the thermoforming heating phase, temperature evolution models of plastic sheets consist of nonlinear temperature-dependent parameters, which have yet to be accounted for when solving the temperature tracking problem. These equations are subsequently modeled in the hybrid systems framework based on the segmentation of the parameter varying elements. Employing a proposed constrained hybrid optimal control (HOC) algorithm based on the Hybrid Minimum Principle (HMP), which contains the nonlinear actuator constraints of the heating phase, the temperature tracking problem is solved for this parameter varying system, for the first time. The HOC algorithm is also designed to minimize the energy consumption of the heaters during the heating phase. Moreover, a closed-loop hybrid controller (CLHC) is developed, based on the proposed HOC, to provide robustness against perturbations. Successful application of the proposed HOC algorithm also serves as a proof of concept to show that HMP based HOCs can be implemented on large-scaled nonlinear industrial processes containing parameter variations and nonlinear actuator constraints." --