Annals of DAAAM for 2012 & Proceedings of the 23rd International DAAAM Symposium, Volume 23, No.1, ISSN 2304-1382 ISBN 978-3-901509-91-9, CDROM version, Ed. B. Katalinic, Published by DAAAM International, Vienna, Austria, EU, 2012 Make Harmony between Technology and Nature, and Your Mind will Fly Free as a Bird Annals & Proceedings of DAAAM International 2012 ROBUST TRACKING CONTROL OF A QUADROTOR HELICOPTER WITHOUT VELOCITY MEASUREMENT STEVANOVIC, S[tojan]; KASAC, J[osip] & STEPANIC, J[osip] Abstract: In this paper, a robust output tracking controller for quadrotor helicopter is proposed. The proposed controller requires measurement of only four state variables: positions in inertial coordinate frame and yaw angle. Also, the controller is robust to unmodeled dynamics and provides rejections of all external force and torque disturbances. The effectiveness of the proposed controller is tested on a simulation example of quadrotor tracking under wind influence which is modeled as unmatched external force disturbances in horizontal plane. Keywords: Quadrotor helicopter, tracking control, robust control, output control 1. INTRODUCTION The quadrotor helicopter is a small agile vehicle controlled by four rotors. Compared with other flying vehicles, quadrotors have specific characteristics that allow execution of applications that would be difficult or impossible otherwise. This superiority is due to their unique ability for vertical, stationary and low speed flight. The quadrotor also has higher payload capacity compared to conventional flying vehicles. The main disadvantage of quadrotor is its high energy requirement because it uses four motors. From the control point of view, quadrotor is a highly nonlinear, multivariable, strongly coupled and underactuated system which has six degrees of freedom and only four actuators. The low cost and simplicity of mechanical structure mean the quadrotor provides an excellent testing ground for application of advanced control techniques. Various advanced control methods have been developed, such as feedback linearization method [1, 2], adaptive control [3, 4], sliding-mode control [5], backstepping control [6], H ∞ robust control [7], etc. However, the most of proposed method require full information on the state that may limit their practical applicability, because the increased number of sensors makes the overall system more complex in implementation and expensive in realization. With the aim of reducing the number of sensors, several solutions were proposed. In [8], the authors proposed a velocities estimator for a tracking control of an under-actuated quadrotor UAV using only linear and angular positions. In [9] and [10], sliding-mode observers were proposed to estimate the effect of external perturbations using measurement of positions and yaw angle. The main practical difficulties of quadrotor tracking control are parametric uncertainties, unmodeled dynamics, and external disturbances. During the quadrotor flights, sudden wind–gusts can significantly affect flight performance and even cause instability [11, 12]. In order to accomplish high level tracking performances, robust flight control systems are required to track desired trajectories in the presence of wind or other disturbances. Recently, several solutions were proposed for reducing influence of wind disturbances on quadrotor tracking performances, mostly based on sliding-mode disturbance observers [13, 14]. The proposed control laws are based on full-state measurement, which require large number of sensors. Further, the outer feedback loop is based on feedback linearization of full translational dynamics of quadrotor, leading to computationally expensive control laws. In this paper we propose a robust output tracking controller, which requires measurement of quadrotor linear position and yaw angle only. The proposed controller provides rejection of external force and torque disturbances like wind gusts. The control design is based on a bilinear reduced model of quadrotor, which preserves the relative degree of the system. The resulting outer control-loop, which is based on feedback linearization, has very simple structure compared to controllers described in literature. The inner control-loop is based on sliding-mode control design with the aim of compensating all disturbances and unmodeled dynamics. Further, sliding-mode filters are used to estimates all derivatives in control law providing significant reduction of required sensors. Also, a smoothing nonlinear filter is used to prevent sudden jumps of control variables. The paper is organized as follows. In Section 2, the dynamic model of the quadrotor helicopter is developed. Based on this nonlinear dynamic model, in Section 3 is designed a robust output tracking controller. The simulation results are presented in Section 4. Finally, the concluding remarks are emphasized in Section 5. 2. QUADROTOR DYNAMIC MODEL The quadrotor helicopter is made of a rigid cross frame equipped with four rotors. The equations describing the altitude and the attitude motions of a quadrotor helicopter are basically same as those describing a rotating rigid body with six degrees of freedom [15, 16]. 2.1 Quadrotor attitude dynamics and kinematics The three-axis rotational dynamic of rigid body in body-fixed reference frame is given by        Iω ω Iω τ d , (1) - 0595 -