Control of a Quadrotor Helicopter using Visual Feedback

We present control methods for an autonomous four-rotor helicopter, called a quadrotor, using visual feedback as the primary sensor. The vision system uses a ground camera to estimate the pose (position and orientation) of the helicopter. Two methods of control are studied one using a series of mode-based, feedback linearizing controllers, and the other using a backstepping-like control law. Various simulations of the model demonstrate the implementation of feedback linearization and the backstepping controllers. Finally, we present initial flight experiments where the helicopter is restricted to vertical and yaw motions. 1 I n t r o d u c t i o n The purpose of this study is to explore the control methodologies that will make an unmanned aerial vehicle (UAV) autonomous. An autonomous UAV will be suitable for applications like search and rescue, surveillance and remote inspection. Rotary wing aerial vehicles have distinct advantages over conventional fixed wing aircrafts on surveillance and inspection tasks, since they can takeoff/ land in limited spaces and easily hover above the target. A quadro tor is a four rotor helicopter. One example is shown in Figure 1. The idea of using four rotors is not new. A full-scale four-rotor helicopter was built by De Bothezat in 1921 [11. Other examples are the Mesicopter [21 and Hoverbot [3]. Also, related models for controlling the VTOL aircraft are studied by Hauser et al [41 and in [51. Helicopters are dynamically unstable and therefore suitable control methods are needed to make them stable. Although unstable dynamics is not desirable, it is good for agility. The instability comes from the changing helicopter parameters and the disturbances such as wind. A quadrotor helicopter is controlled by varying the rotor speeds, thereby changing the lift forces. It is an underactuated, dynamic vehicle with four input forces and six output coordinates. One of the advantages of using a multi-rotor helicopter is the increased payload capacity. It has more lift therefore heavier weights can be carried. Quadrotors are highly maneuverable, which enables vertical take-off/landing, as well as flying into hard to reach areas. Disadvantages are the increased helicopter weight and increased energy consumption due to the extra motors. Since it is controlled with rotor-speed changes, it is more suitable to electric motors, and large helicopter engines which have slow response may not be satisfactory without a proper gear-box system. The main concentration of this study is using non-linear control techniques to stabilize and perform output tracking control of a helicopter. In Section 2 the helicopter model and dynamics of quad-rotor is described. The equation of motion of a simplified quadrotor is given here. Feedback linearization and backstepping controllers are described and simulation results are introduced in Section 3. Real-time control and the vision system which is responsible for pose estimation and real-time control are described in Section 4. Experiments on a real quadrotor test-bed are given in Section 5. 2 H e l i c o p t e r M o d e l Unlike regular helicopters that have variable pitch angles, a quadrotor has fixed pitch angle rotors and the rotor speeds are controlled to produce the desired lift forces. Basic motions of a quadrotor can be described using Figure 1. Vertical motion of the helicopter can be achieved by changing all of the rotor speeds at the same time. Motion along the x-axis is related to tilt around the y-axis. This tilt can be obtained by decreasing the speeds of rotors 1 and 2 and by increasing speeds of rotors 3 and 4. This tilt also produces acceleration along the x-axis. Similarly y-motion is the result of the tilt around the x-axis. The yaw motions are obtained using the moments that are created as the rotors spin. Conventional helicopters have the tail rotor in order to balance the moments created by the main rotor. With the four-rotor case, spinning directions of the rotor are set to balance and cancel these moments. This is also used to produce the desired yaw motions. To turn in a clock-wise direction, the speeds of rotor 2 and 4 should be increased to overcome the moments created by rotors 1 and 3. A good controller should be able to reach a desired yaw angle while keeping the tilt angles and height constant. 2.1 D y n a m i c s of Quadrotor He l i copter A body fixed frame is assumed to be at the center of gravity of the quadrotor, where the z-axis is pointing upwards. This body axis is related to the inertial frame by a position vector (x,y,z) and 3 Euler angles, (0,~,¢), representing pitch, roll and yaw respectively. A ZYX-Euler