Active Vertical Takeoff of an Aquatic UAV
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IEEE Robotics and Automation Letters (RAL) paper presented at the 2020 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS) October 25-29, 2020, Las Vegas, NV, USA (Virtual) Active Vertical Takeoff of an Aquatic UAV Etienne´ Tetreault,´ David Rancourt and Alexis Lussier Desbiens Abstract— To extend the mission duration of smaller un- The Sherbrooke University Water-Air VEhicle (SUWAVE) manned aerial vehicles, this paper presents a solar recharge is an aquaUAV with autonomous VTOL and flight capa- approach that uses lakes as landing, charging, and standby ar- bilities, able to perform a solar recharge between mission eas. The Sherbrooke University Water-Air VEhicle (SUWAVE) is a small aircraft capable of vertical takeoff and landing on cycles (Fig 1). The flying wing design makes for a low water. A second-generation prototype has been developed with profile on water, which prevents capsizing while retaining new capabilities: solar recharging, autonomous flight, and a the endurance characteristic of fixed-wing aircraft. The single larger takeoff envelope using an actuated takeoff strategy. A propeller can reorient itself to execute vertical takeoff, and 3D dynamic model of the new takeoff maneuver is conceived to water re-entry is done by diving nose-first [13]. Both these understand the major forces present during this critical phase. Numerical simulations are validated with experimental results maneuvers were implemented to circumvent undesired wave from real takeoffs made in the laboratory and on lakes. The interactions associated with traditional runway takeoffs and final prototype is shown to have accomplished repeated cycles landings on water. of autonomous takeoff, followed by assisted flight and landing, without any human physical intervention between cycles. I. INTRODUCTION Flight The many advantages of bigger unmanned aerial vehicles Vertical (UAVs) are not always enough to compensate for their high Takeoff Vertical price and complexity of use. On the other hand, one of the Landing main drawbacks of smaller platforms is their short flight duration. Solar panels may mitigate this issue, but at a small Solar recharge scale they are not sufficient to enable continuous flight [1], + flotation so there is still the need to land and recharge. Numerous laboratories have developed diverse landing approaches, such as using vertical walls [2]–[5], electric power lines [6], and moving platforms [7]. Another interesting solution is the use Fig. 1. Proposed mission cycle (left), and time lapse of autonomous takeoff of bodies of water, which offer some unique advantages: they from lake (right). form a large flat surface with no obstacle, their location is known, and they are quite common in places like Canada. SUWAVE’s vertical takeoff starts by tilting the propulsion An aquatic unmanned aerial vehicle (aquaUAV) can use system vertically and applying full thrust so that the nose water as a safe landing spot, but also has its own set of rapidly rises from the water. This pulls the whole wing challenges [8]. Seaplanes are the traditional aquatic-aerial out before aligning the two bodies for normal flight. In the platforms. The Oregon Ironworks “SeaScout”, Warrior Aero- earlier prototype, this alignment was performed passively marine’s “Gull” and the University of Michigan’s “Flying using counterweight and latches. This required a fine bal- Fish” are aquaUAVs with autonomous takeoff and landing ance of gravity, buoyancy and inertial forces for successful capabilities [9]–[11]. Those are large aircraft, the lightest takeoffs [13]. This passive strategy was sensitive to wave having a mass of 18 kg. Numerous problems arise as stan- perturbations and strong wind could prevent the latching of dard seaplane configurations are scaled down. Waves are a both bodies. This version of the SUWAVE could also only concern for runway takeoff and shallow descent landing [12]. takeoff from water as it needed free space under the wing to The risk of capsizing while resting on water also increases allow for the counterweight motion. with smaller size [13]. Multicopters have also been adapted The new SUWAVE presented in this paper uses an actuated to aquatic operation [14], [15], using vertical takeoff and joint to tilt the propeller. It also includes a rudder and a landing (VTOL) to leave water. However, their range remains controller that takes advantage of these two new actuators shorter than that of their fixed-wing counterpart, though some to orient the SUWAVE during takeoff. Takeoff in wind and new designs may mitigate that [16]. from solid ground are now possible with these actuators that are effective even in the first few seconds of takeoff at low This work was supported by FRQNT and NSERC. airspeed [17]. The propeller tilt and the rudder immersed in The authors are with the Createk Design Lab at the Universite´ the prop wash control pitch and yaw movement respectively, de Sherbrooke, Canada, J1K 2R1 fetienne.tetreault, david.rancourt2, alexis.lussier.desbiensg leaving roll rate unchecked. During vertical takeoffs, the spin @usherbrooke.ca. of the plane around itself does not affect the trajectory and Copyright ©2020 IEEE the thrust is aligned to fight gravity without the need for A. Airframe Configuration aerodynamical lift. This vertical trajectory is maintained until The aircraft has a total mass of 865 g and a wingspan sufficient airspeed and altitude are reached. Then, a transition of 1240 mm. Its main feature is the tilt mechanism on the phase smoothly brings the nose down to transition to normal nose, which is activated by an RC servomotor and used to level flight. tilt the propeller up or down. In the previous generation, The main contribution of this paper consists in the new this pivot was a free hinge that passively rotated under its SUWAVE prototype and actuated takeoff strategy to perform own weight [13]. The new actuated version enables control robust vertical takeoffs, as opposed to the preceding passive of the thrust orientation in order to generate the pitching approach. The new configuration allows the SUWAVE to moment and correct the aircraft’s attitude. We also added takeoff vertically from water or land. Another contribution of a rudder to the flying wing. By being immersed in the this paper is the extension of the 2D takeoff model presented air flow induced by the propeller (prop wash), the rudder in [13] to explain the undesirable gyroscopic effects caused can effectively control yaw movement during the initial low by the rapid tilt actuation. The structure of this paper reflects airspeed of takeoff. those novelties. Section II presents the new aircraft, with the avionics and the actuators required for autonomous active B. Electronics and Power Systems takeoff. The Section III describe the 3D numeric model used SUWAVE has its own custom autopilot board, derived to represent the new takeoff sequence while the controller from the Pixhawk® open standards autopilot [18] with added for this maneuver is explained in Section IV. Lastly, the solar recharge capabilities. With good solar exposure, the paper presents some experimental results demonstrating the charging current is 1.5 A. The flight stack is the open-source new autonomous vertical capabilities of the prototype in Sec- PX4 [19], which was also modified to include solar recharge, tion V. Those results are also demonstrated in the attached VTOL from water, and control of the new actuators. The au- video. topilot includes an inertial measurement unit (IMU) and the software performs the necessary sensor fusion to estimate the II. PROTOTYPE VEHICLE attitude, position, and velocity of the aircraft. The autopilot and all other sensitive electronics are coated in a waterproof Compared to the first generation of SUWAVE, the new conformal coating. model is larger and heavier, mostly due to the added so- lar panels, protective fiberglass and avionics required for III. TAKEOFF MODEL autonomous flight. Fig. 2 depicts the aircraft, and Table I The new generation of SUWAVE is almost 50% heavier presents its mass budget. than its predecessor, with a very different mass distribution. This causes the two versions to have very distinct trajectories while using a similar takeoff strategy. The takeoffs of the new rotating rudder wing with solar panels active and solar version are much more tridimensional, and the path taken is no longer held in a single plane. One of the main challenges is the high positive yaw rate occurring once the aircraft fully elevates its nose. Without a rudder, this uncontrolled movement leads to takeoff failure. To under- tilting actuator pivoting thruster stand the origin of this dynamic phenomenon, we developed a 3D model which we used to validate the effectiveness of the rudder to counter this undesired movement. The various bodies, forces, and points of interest of the model are shown in Fig. 3. Fig. 2. Global view of the second-generation SUWAVE prototype. A. Reference Frames, Bodies and Motion The various frames and their orientation follow aeronauti- TABLE I cal standards [20]. The model uses three of them: the inertial SUWAVE’S MASS BUDGET frame N , and two others affixed to the rigid bodies of the Empty aircraft: Foam, solar panels, fiber- 404 g flying wing W and thruster P respectively. The inertial frame glass, epoxy, 4 x RC servomotors N is of type north-east-down (NED), meaning that its x axis Propulsion: Motor 300 W, and folding 117 g points north, y points east, and z points vertically downward. propeller 12 x 6 Its origin is named N0. The flying wing W rigid body has Battery: 3S-1000 mAh 90 g its origin at its center of mass, Wcm. From there, the body x Avionic: Autopilot board, ESC 30 A, RC 129 g axis points toward the nose of the aircraft, the y points toward receiver, Telemetry and GPS antenna the starboard wing (the right-hand wing when viewed from Other 125 g the rear), and the z exits the plane through its belly.