McGill Robotics is developing a fixed-wing autonomous aircraft – Martlet-1 – to compete for its first time in the Association for Unmanned Vehicle Systems International (AUVSI) Seafarer’s Student Unmanned Aerial Systems (SUAS) competition. The team is focusing on developing a solid software foundation and has mostly opted for off-the-shelf components to accelerate development. Martlet-1 is aimed to be fully autonomous and to be capable of completing selected competition tasks.
As Martlet-1 will be competing for its first time, the design is focused on the reliable execution of the core tasks, the respect of competition rules, the time constraints, and the safety considerations of the AUVSI Seafarer’s SUAS Competition.
Mission Requirement Analysis
Our team identified four high-level mission requirements that the system should perform, each with its own set of sub-requirements.
The first is the ability to autonomously navigate the aircraft to predefined way-points placed throughout the competition area. Reliability is the primary focus of this task, as a manual takeover would result in significant penalties. A secondary focus is accuracy. In order to maximize points earned, the UAV should fly as close as possible to the waypoint.
Secondly, the UAV must perform an aerial survey of a predefined area. The primary objective is to photograph targets placed throughout the area. The targets take the form of coloured shapes with letters or numbers in the middle. Additional points are awarded for autonomous identification and geolocation of the targets. Target data must then be uploaded to the competition’s interoperability servers.
The third task is the UAV’s ability to avoid virtual obstacles. The competition’s interoperability server will provide the location of various static or mobile obstacles that the UAV must avoid. Points are awarded separately for static obstacles and mobile ones, each of these being either a pass or a fail. For points to be awarded, the UAV must not cross any of the obstacles.
The fourth task requires the UAV to autonomously deliver an 8 oz water bottle as close as possible to a predetermined location.
The initial aircraft design decisions of Martlet-1 involved consideration of the type of airframe to be used. Primary categories of aircraft compared were multirotors, helicopters, and fixed wing aircraft. Research indicated that multirotors and helicopters were extremely agile, and were capable of hovering and achieving vertical takeoff and landing (VTOL). Additionally, it was found that multirotors tended to have better crash resistance than helicopters. However, fix winged aircraft, both conventional and flying wing, were found to have better flight endurance and payload capacity than their rotary wing counterparts. As the competition is held at an airfield, VTOL and vertical agility were not critical features. Additionally, the ability to loiter was not necessary as all targets were static. Given these findings, a fixed-wing airframe was chosen.
After the type of airframe had been selected, the project entered the next phase of research: exploration of custom designs for the airframe and suitable commercial off-the-shelf products. As this project is McGill Robotics’ first aerial vehicle, a commercial off-the-shelf airframe was chosen to maximize the available time for full system testing.
An FPVmodel electric swallow was chosen as the the competition airframe, as it met the desired payload capacity and monetary constraints. A ventral compartment provides ample space for the custom 2 degree of freedom (DOF) camera gimbals. Other components were placed with two goals in mind: electromagnetic interference reduction and centre of gravity position.
In previous robots, we have found that high power motor lines have the capability of interfering with digital signal lines. This was most noticeable when the digital lines were in close proximity to the high power ones. Therefore, efforts were made to separate the battery and motor lines from the regulated and signal lines.
The centre of gravity must be maintained at one third of the chord of the wing. The battery was found to be the heaviest component and was therefore attached to an adjustable plate. This allowed for variations in the flight payload arrangement by shifting the battery, allowing custom components to be added throughout the year.
In addition to Martlet-1, the mechanical division has maintained a fleet of smaller UAVs, which we have all named Gooney. These UAVs are used to quickly test software features without risking Martlet-1. The use of smaller UAVs also allowed for much simpler testing logistics. The use of Gooney for testing had allowed for much shorter software iteration cycles and had provided valuable operational knowledge, which were used to streamline procedures during subsequent tests.
Gooney-3, the current Gooney model, is based on the HobbyKing Bixler v1.1 airframe which the team modified to meet the testing needs. The top portion of the nose of the fuselage was removed to make room for the autopilot unit. A hatch was then cut into its bottom to allow access to the interior, which was reinforced. Easily replaceable 3D-printed and laser cut parts were used to mount the autopilot unit as well as various sensors, hold the battery in place, and attach the GoPro Hero 4 camera that allows the Gooney-3 to gather video footage.
The Martlet-1 contains a total of 7 custom designed printed circuit boards (PCB) generously sponsored by Elecrow.com. Each PCB was designed by our team members using DipTrace software.
1) Power System: Martlet-1 is powered by a six-cell 22,000 mAh lithium polymer battery to ensure 45 min of flight time. Four physical switches are used to determine Martlet-1’s power state: an emergency switch immediately disconnects the battery from the rest of the system; a computer on-off switch is used to switch the computer system on and off; a motor kill switch which turns off the motor to allow safe manipulation of the robot; and lastly, a restart switch which forces a computer reboot. The first three switches are each connected to a high-side P-channel metaloxidesemiconductor field-effect transistor (MOSFET) to control the connections electrically and can allow up to 90 A of continuous current draw. The last switch is directly connected to the computer motherboard.
2) On-Board Computer: The on-board computer is built from a NUC7I7 computer kit with an Intel i7 7567U CPU, 16 GB DDR4-2133 RAM and a 525 GB M.2 SSD. The power to the computer is provided by a DCDC-NUC power supply unit specifically designed for the NUC computer to allow its safe and reliable operation. A custom designed hot-swap controller is used to allow the computer to run with a backup power supply and to allow swapping batteries without turning off the computer.
3) Camera Gimbal Controllers: The cameras of Martlet-1 are stabilized by two custom designed gimbal and uses STorM32 gimbal controllers. The gimbal controllers are powered by a custom designed buck converter that steps down the battery voltage to the 12V required by the gimbal motor. Each gimbal communicates with the computer using a custom serial driver and is able to stow its camera during take-off and landing, stablize the camera, and set its camera to specific orientation during flight.
4) Interfaces: Martlet-1 uses two types of connectors for electrical connections: XT-90 connectors for power connections and DF13 connectors for signal connections. This unified connector scheme allows each submodule to be added or removed without needing a complete system redesign.
For user interactions, two boards were used: the battery indicator board and the plane control panel board. The battery indicator board indicates the battery level. With a single button press, the power remaining is displayed on the six tri-color light-emitting diodes (LED). The UAV control panel acts as a mechanical structure to host all the fore-mentioned mechanical switches. In addition, it serves as a carrier for multiple LED indicators such as battery insertion, power supply status, computer power state, and software state.
The system also allows for a hot-pluggable High-Definition Multimedia Interface (HDMI) screen to be connected to the computer, enabling extended on-board debugging.
The ground station is composed of two major subsections: the ground station computer (GSC) and the ground station gimbal (GSG).
1) Ground Station Computer: The GCC is composed of a Gigabyte BRIX miniature computer kit with Intel 4770R CPU, 8 GB DDR3-1600 RAM, and 480 GB SATA SSD. Regulated by a DCDC-NUC computer PSU, the ground station can be powered by battery or by a DC power supply. The power source can be changed dynamically without interrupting the GSC’s operation. The GSC will be directly connected with the interoperability server and will communicate with the rest of the system via Wi-Fi connection provided by the ground station gimbal.
2) Ground Station Gimbal: The main function of the GSG is to orient the antenna toward the Martlet-1. To do so, a highly integrated PCB, named PiHAT, is designed as a shield for a Raspberry Pi 3 (RPi3) to form an 1 DOF gimbal. The PiHAT is composed of a buck voltage regulator used to power the RPi3, a Global Position System (GPS) module, and a 9 DOF Inertial Measurement Unit (IMU) that is used to obtain the absolute position and orientation of the gimbal, and a motor controller to control the gimbal motor. An Unscented Kalman Filter (UKF) fuses the IMU data, the GPS data, and the angular-only odometry data to compute an accurate estimate of the gimbal’s current heading. A PID controller then simply minimizes the difference between the heading and the desired azimuth. The GSG also hosts a modular router system (RouterBOARD), a 2.4/5.8 GHz high gain patch antenna, a 2.4 GHz omnidirectional antenna, and 900 Mhz omnidirectional antenna. The GSG is powered by a 3-cell 8,000 mAh lithium polymer battery and it is completely wireless to allow continuous rotation.