Real-Time Implementation of a Stable 2 DOF PID Controller for Unstable Second-Order Magnetic Levitation System with Time Delay

The design of a two degree of freedom (DOF) proportional-integral-derivative (PID) controller is proposed in this paper to stabilize an unstable second-order plant having time delay. The lower and upper bounds of the controller gains for which the compensated system will exhibit a stable response, is determined using Hermite-Biehler theorem. In order to determine the bounds of the controller gains, this work proposes an extension of the Hermite-Biehler theorem, suitable for the transfer function of the maglev plant, which contains a negative dc-gain and a missing 's' term in the denominator polynomial. The optimal values of the controller gains, from the range thus estimated, are found by a recent bio-inspired algorithm, namely symbiotic organisms search (SOS). The obtained results have been experimentally verified, and it has been validated that the 2-DOF PID controller exhibits better response. The robustness of the compensated system for 1-DOF and 2-DOF configurations, in the presence of external disturbance and measurement noise, has also been validated in real time, and it is observed that the 2-DOF PID controller improves the robustness of the system, as compared to its 1-DOF counterpart.