978-1-4244-9005-9/10/$26.00 ©2010 IEEE 367 Developing a Flight Control Strategy for a Four-rotor Under-actuated UAV S. Akash SRM University, Chennai, India s.akash279@gmail.com S.S. Sridhar SRM University, Chennai, India sridhar.srm@gmail.com Bibek Kabi SRM University, Chennai, India bibek.kabi@yahoo.in N. Chellammal SRM University, Chennai, India chellamal_venkat@rediffmail.com Abstract— This paper provides a theoretical approach in the stabilization of a four rotor UAV using a dynamics model. We performed various simulations in open and closed loop platforms and implemented several experiments on the miniature VTOL system. The vehicle feedback system uses an Inertial Measurement Unit (IMU).A state space variable model of the vehicle dynamics is presented here in the literature. In order to explain this system, we have developed a simulink based model for the PID controller. Also various control techniques like Neural Networks which will enhance the system performance are proposed. Keywords- UAV; VTOL system; IMU; state space variable model; PID controller; Neural Networks. I. INTRODUCTION The first unmanned helicopter was the one built by Forlanini in 1877. It was neither actively stabilized nor steerable. With the outstanding technological advancements after WW II it became possible to build and control unmanned helicopters. The development of full-scale quadrotors experienced limited interest in the past. Nevertheless, the first manned short flight in 1907 was on a quadrotor. Present quadrotors have four fixed propellers in cross configuration. [6]These are mechatronic systems in which the two pairs of propellers, driven in opposite directions thus removing the need for a tail rotor. This is a major difference between quadrotors and helicopters. [7]While the front and the rear motor rotate clockwise, the left and the right motor rotate counterclockwise which nearly cancels the horizontal forces. This is an important condition for stable hovering. One additional advantage of the quadrotor compared to a conventional helicopter is the simplified rotor mechanics. By varying the speed of the single motors, the lift force can be changed and vertical and/or lateral motion can be created. Pitch movement is created by creating a difference in the speeds of the front and the rear motor whereas a similar difference in left and right motors creates roll movement. [10]A yaw movement occurs when there is a difference between the pitch movement and the roll movement (front-rear and left-right motors), while keeping collective lift of vehicle constant. This model is classified as an under-actuated system as the number of actuators is less than the number of degrees of freedom. A degree of freedom is change in position (translation, rotation). Another advantage of this system is that it is capable of vertical take-off and landing. This is possible because of the use of thrusters hence classifying it as a VTOL (vertical take-off and landing) system. The IMU forms the feedback system for the closed loop model. Using modeling concepts, we derived the state space model which was implemented in the Simulink block diagram in Matlab. Experiments were performed in open loop and closed loop platforms and it was observed that the closed loop provided higher accuracy and control over disturbance rejection. Currently PID controller was used as an error compensator. However system performance can be enhanced using a RBF Network. Some promising areas of applications for UAV’s include surveillance, fire fighting, aerial photography, [2]traffic control updates, relay positioning between two points. II. QUADROTOR CONFIGURATION VTOL systems are considered to be highly unstable. Here in this paper, we consider an ideal model as shown below. It has got an earth frame of reference E and a body frame of reference. Using Euler angles parameterization, the airframe orientation in space is given by a rotation R from B to E, where R is the rotation matrix. Figure1. SMART flyer