Document Type : Research Article
School of Mechanical Engineering, University of Tehran
This paper presents a new hierarchical robust algorithm to solve the position tracking problem, in presence of exogenous disturbances and modeling uncertainties, of a quadrotor helicopter. The suggested control scheme includes a nonlinear H_∞ algorithm to track the reference flight path in the outer loop and a nonlinear H_∞ controller to stabilize the rotational movements in the inner loop. The resultant controller consists of three important parts to regulate tracking errors for translational and rotational motions, maintain robust performance confronting random disturbances and modeling uncertainties and reject the sustained disturbances from the system to vanish the steady-state errors. Analytical study on the stability of the cascade system is mentioned to verify the compatibility of two controllers considering coupling terms. Moreover, numerical performance analysis is accomplished using Monte-Carlo simulation. Statistical results which are obtained from 1000 simulations considering environmental disturbances and modeling uncertainties depict less than 5cm for position tracking error and less than 2 deg for attitude tracking error in steady state performance. The closed-loop performance of the controller is also compared with two previous algorithms by determining two ISE (Integrated Square Error) and IADU (Integral of the Absolute value of the control Derivative) indexes for state tracking performance and control efforts, respectively. The simulation results of the suggested control algorithm depict a significant reduction in both indexes in a similar mission.