A custom-made Peltier-based thermal device is designed to perform miniaturized bio-molecular reactions in a microfluidic platform for medical diagnostic tests, especially the polymerase chain reaction for DNA amplification. The cascaded two-stage device is first approximated by multiple local linear models whose parameters are obtained by system identification. A decentralized switching controller is proposed, where two internal model-based PI controllers are used in local stabilizations and PD and PI controllers are applied during transitions respectively. Couplings and drift are further reflected into the controllers. Desired temperature tracking performance on the transition speed and overshoot is achieved, and the feasibility of the Peltier device in a microfluidic platform is further validated by the successful applications of viral detection. To achieve fast and smooth transition while avoiding tuning by trial-and-errors, a nonlinear model is developed based on the first principles, whose parameters are partially calculated from empirical rules and partially determined by open-loop and closed-loop experimental data. Two novel nonlinear controllers are designed based on the nonlinear model. The first controller extends the input-to-state feedback linearization technique to a class of nonlinear systems that is affine on both the control inputs and the square of control inputs (including the Peltier system). Additional local high gain controllers are introduced to reduce the steady-state errors due to parameter uncertainty. The second controller is a time-based switching controller which switches between nonlinear pseudo-PID/ state feedback controllers and local PI controllers. Calculation burden is reduced and steady-state error is minimized using a PI controller locally, while fast and smooth transition is achieved by the nonlinear counterpart. The robustness of the controller is verified in simulation under worse case scenarios. Both simulation and experimental results validated the effectiveness of the two nonlinear controllers. The proposed linear/ nonlinear, switching/ non-switching controllers provide different options for the Peltier-based thermal applications. The scalability and the parameter updating capability of the nonlinear controllers facilitate the extension of the Peltier device to other microfluidic applications. / Controls

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