Disturbance observer based control of twin rotor aerodynamic system

Twin rotor aerodynamic system (TRAS) approximates the dynamics of helicopters and other vertical take off rotor crafts. The nonlinear nature with significant cross-coupling between the inputs and outputs of the main and tail rotors make the control of such system for either stabilization or reference tracking a challenging task. In this paper, the problem of disturbance rejection for TRAS is addressed by designing disturbance observers through H infinity based approach. The system is decoupled into main and tail rotors subsystems. For each subsystem, an inner loop disturbance observer is synthesized that provides disturbance rejection, whereas to ensure stability and performance an outer loop baseline feedback controller is designed. Two different cases are considered. In first case 2 proportional-integral-derivative controllers are designed to use as outer loop baseline feedback controllers with disturbance observers whereas in the second case linear quadratic Gaussian (LQG) controllers are designed. For both cases simulations are performed with nonlinear Matlab Simulink model of TRAS and results are compared to determine which approach delivers better performance. Simulation results show that the 2 conflicting requirements of reference tracking and disturbance rejection can be met simultaneously with the proposed approach increasing the disturbance rejection capability of the closed loop system.