Recent Developments in Aerial Robotics: A Survey and Prototypes Overview <s> B. Hexacopters <s> In this paper, we propose a novel inertial-assisted visual odometry system intended for low-cost micro aerial vehicles (MAVs). The system sensor assembly consists of two downward facing cameras and a inertial measurement unit (IMU) with three-axis accelerometers/gyroscopes. Real-time implementation of the system is enabled by a low-cost embedded system via two important features: first, simple pixel-level algorithms are integrated in a low-end FPGA and accelerated via pipeline and combinational logic techniques; second, a novel outlier rejection algorithm is implemented which is based on probability-based predetermined operations rather than hypothesis testing iterations. We illustrate the performance of our system by hovering a MAV in a GPS-denied environment. Its feasibility and robustness is also illustrated in a complex outdoor environment. <s> BIB001 </s> Recent Developments in Aerial Robotics: A Survey and Prototypes Overview <s> B. Hexacopters <s> This paper presents a method for localizing an Unmanned Aerial Vehicle (UAV) using georeferenced aerial images. Easily maneuverable and more and more affordable, UAVs have become a real center of interest. In the last few years, their utilization has significantly increased. Today, they are used for multiple tasks such as navigation, transportation or vigilance. Nevertheless, the success of these tasks could not be possible without a highly accurate localization which can, unfortunately be often laborious. Here we provide a multiple usage localization algorithm based on vision only. However, a major drawback with vision-based algorithms is the lack of robustness. Most of the approaches are sensitive to scene variations (like season or environment changes) due to the fact that they use the Sum of Squared Differences (SSD). To prevent that, we choose to use the Mutual Information (MI) which is very robust toward local and global scene variations. However, dense approaches are often related to drift disadvantages. Here, we solve this problem by using georeferenced images. The localization algorithm has been implemented and experimen- tal results are presented demonstrating the localization of a hexarotor UAV fitted with a downward looking camera during real flight tests. <s> BIB002 </s> Recent Developments in Aerial Robotics: A Survey and Prototypes Overview <s> B. Hexacopters <s> As the applications of Micro Aerial Vehicles (MAVs) get more and more complex, and require highly dynamic motions, it becomes essential to have an accurate dynamic model of the MAV. Such a model can be used for reliable state estimation, control, and for realistic simulation. A good model requires accurate estimates of physical parameters of the system, which we aim to estimate from recorded flight data. In this paper, we present a detailed physical model of the MAV and a maximum likelihood estimation scheme for determining the dominant parameters, such as inertia matrix, center of gravity (CoG) with respect to the IMU, and parameters related to the aerodynamics. To incorporate all information given by the IMU and the physical MAV model, we propose to use two process models in the optimization. We show the effectiveness of the method on simulated data, as well as on a real platform. <s> BIB003 </s> Recent Developments in Aerial Robotics: A Survey and Prototypes Overview <s> B. Hexacopters <s> The article proposes a solution allowing a multicopter to generate and follow a path while taking into account the obstacles in the environment. <s> BIB004 </s> Recent Developments in Aerial Robotics: A Survey and Prototypes Overview <s> B. Hexacopters <s> We propose a novel multi-rotor flying platform, ODAR (omni-directional aerial robot), which is fully-actuated (i.e., can assume arbitrary motion in SE(3) or generate arbitrary control wrench in se(3)) by six opportunistically distributed rotors, each driven by reversible ESC (electronic speed controller) for bi-directional thrust generation. Due to this omni-directional wrench generation, the ODAR system can realize such powerful behaviors impossible with conventional multi-rotor flying platforms as: 360° photo/video shooting while holding its position for VR scene generation; or resisting sideway gust while keeping its attitude and exerting downward pushing force larger than its own weight for aerial manipulation applications. This paper presents optimal mechanical design, modeling and control, hardware and software implementation, and experimental verification of the ability of the ODAR system to attain those behaviors impossible with the conventional drones as stated above. <s> BIB005
A hexacopter (Fig. 8 (a)-(f) ) BIB003 - BIB004 is a UAV with six rotors. Burri et al. BIB003 use a hexacopter (Fig. 8 (a) ) to perform system identification study. Specifically, they collect information from an onboard IMU sensor, motor speeds, and hexacopter's pose to estimate the complex dynamic model (required for accurate positioning flight). In addition to the IMU sensor, Zhou et al. BIB001 combine visual information from two downward-facing cameras to perform visual odometry in their hexacopter (Fig. 8 (b) ). On the other hand, Yol et al. BIB002 demonstrate a hexacopter (Fig. 8 (c) ) that could perform vision-based localization by using a downward-looking camera and geo-referenced images. Navigation and obstacle avoidance are also important topics for UAVs. Nguyen et al. BIB004 demonstrate their real-time path planning and obstacle avoidance algorithms with a commercial hexacopter ( Fig. 8 (d) ). Similar to a conventional quadcopter, a conventional hexacopter is a non-holonomic aircraft, which cannot move horizontally without changing its attitude. Ryll et al.  present a hexacopter (Fig. 8 (e) ) that could transform itself from a conventional hexacopter to a holonomic hexacopter, i.e., able to move horizontally without tilting the aircraft, by using a servo to tilt the six rotors simultaneously. Similarly, Park et al. BIB005 design a special hexacopter with six asymmetrically aligned and bi-directional rotors, which enable the hexacopter (Fig. 8 (f) ) to perform fully-actuated flight. While The holonomic capability of a hexacopter is not as energy-efficient as a conventional hexacopter, it has several merits such as robust to wind disturbance, precision flight, and intuitive human-drone interaction.