Attitude Control of Quad-Rotor UAVs Using An Intuitive Kinematics Model Dongming Gan, Guowei Cai, Jorge Dias and Lakmal Seneviratne Robotics Institute Khalifa University of Science, Technology and Research Abu Dhabi, UAE E-mail: {dongming.gan, guowei.cai, jorge.dias, lakmal.seneviratne}@kustar.ac.ae Jorge Dias Institute of Systems and Robotics University of Coimbra Coimbra, Portuga E-mail: jorge@isr.uc.pt Lakmal Seneviratne School of Natural and Mathematical Sciences King’s College London London WC2R2LS, UK E-mail: lakmal.seneviratne@kcl.ac.uk Abstract—This paper presents the work on attitude control of quad-rotor UAVs applying an intuitive kinematics representation, called rotation vector. There are three elements in the rotation vector which has clear physical meaning of the rotations and avoids the singularity problem of Euler angles and the unity norm constraint problem of quaternions. Basic definition of the rotation vector and its relation with the object body angle velocity is introduced and used in the 6DOF quad- rotor dynamics. Based on the property that the rotation vector rate is equivalent to the body angle velocity when the rotation is small, a simple and intuitive attitude reference is proposed. A proportional-derivative (PD) law is used by integrating the new attitude reference for the attitude control of quad-rotor UAVs. Simulation results prove the efficiency of the new method which provides a new model with intuitive physical meaning for quad- rotor UAVs. I. INTRODUCTION Unmanned aerial vehicles (UAVs) have been prevalently employed in numerous defense- and civil-related applications over the last two to three decades. Among various types of UAVs, miniature quad-rotor UAV (an example, constructed at the Robotics Institute at Khalifa University, is shown in Fig. 1) has gained particular popularity due to its unique features such as compact size, good agility, high maneuverability and vertical take-off and landing (VTOL). For now they are widely studied and used in surveillance, first responder for fire and civil accidents, inspection, photography and mapping [1]. Quad-rotor UAVs generally have a cross shape with four rotors arranged at the four ends of the cross. By controlling speeds of the four rotors in variable combinations, under controlled strategy is applied to control four degrees of freedom (DOFs) in realizing the 6-DOF translations and orientations. Quad-rotor UAVs are single rigid bodies when considering their dynamics which provides a good platform for applying various control laws from linear methods including LQR [2, 3], LQG [4], PD [5] and PID [6], to non- linear ones using hierarchical controllers with Lyapunov method [7-9], sliding-mode technique [10, 11], and predictive strategy [12, 13]. However, most of the above work used Euler angle model which is one of the two most popular orientation representation models including Euler angles and quaternions. The main disadvantages of Euler angles are [14] that they have singularities in angular velocity calculation and that they are less accurate than unit quaternions when used to integrate incremental changes in attitude over time. For unit quaternions [14], their four parameters do not have intuitive physical meanings and they must have unity norm to represent a pure rotation. The quadratic unity norm constraint is particularly problematic in optimization. The rotation vector used in this paper lacks both the singularities of the Euler angles and the quadratic constraint of the unit quaternion. In addition, it also shows intuitive physical meaning in the attitude representation of quad-rotor UAVs, which results in a new attitude reference in the quad-rotor UAV control. Figure 1. Khalifa University UAV (KUAV) 978-1-4799-2452-3/13/$31.00 ©2013 IEEE 597