AUVSI SUAS Technical Design Paper. California State University, Fullerton s Titan UAV. T1tan

Transcription

1 AUVSI SUAS Technical Design Paper T1tan

2 I. Systems Engineering Approach i. Mission Requirement Analysis The missions outlined to be completed at the Mission Demonstration for the SUAS AUVSI Competition were rated based on several fundamental factors. These factors include the value of the task toward the total mission score, the time needed to test existing infrastructure and develop its performance, and the level of difficulty to successfully accomplish the task into Complexity and Priority categories. The rankings include numerical values of one, two, and three, where an increase in value correlates to an increase in its respective task analysis. Table 1 below demonstrates the task analysis. The table identifies the requirements the missions impose on the aircraft, potential trade-offs, and whether the task will be attempted. Table 1: Analysis of task requirements imposed on the aircraft and respective tradeoffs. Task Details Complexity Priority Planned Action Autonomous Flight Autonomous takeoff/flight/landing, waypoint navigation, flight boundaries 1 3 Will Attempt Obstacle Avoidance Object Detection, Classification, Localization Autonomous Upload Off-Axis Standard Object Air Delivery Upload telemetry data to Interoperability System, avoid virtual obstacles Autonomous target detection, localization, classification Image capture autonomous submission Identify one standard target out of flight boundary Electromechanical release and survival of urgent air supply 2 2 Will Not Attempt 2 2 Will Attempt 3 2 Will Attempt 2 2 Will Attempt 1 1 Will Attempt ii. Design Rationale This year s CSUF Titan UAV team is building on the experiences from the university s entry and will be using an existing team airframe. The team is also able to use readily available materials and systems, such as balsa wood, propulsion system, and an autonomous system. These systems were tested and evaluated comprehensively for flight performance. Components that failed the testing were replaced. The team considered possible manufacturing methods and plans, such as 3D printers for rapid prototyping, before establishing the budget. Table 2 below demonstrates the team budget showing materials, services, electronics, and travel estimated costs. 1

3 Table 2: CSUF Titan UAV s budget for the year displaying categorical subtotals. Materials Item Quantity Price Per Unit Item Quantity Price Per Unit Subtotal $587 Electronics Subtotal $949 Balsa Wood, 0.125inx4inx24in 105 $2 $210 RC Receiver 1 $70 $70 Monokote, Roll 3 $8 $24 RC Transmitter 1 $200 $200 Fiberglass, 8sq ft 6 $7 $42 Batteries 2 $10 $20 Carbon Fiber, 1/4in ID x72in 1 $40 $40 Electric Motor 1 $100 $100 High Desity Foam, 22inx22inx2in 6 $18 $108 Electronic Speed Controller 1 $50 $50 Landing Gears, Set 1 $35 $35 Wire Cables 5 $7 $35 Propeller, Unit 3 $7 $21 Servos 10 $16 $160 Screws, Nuts, Bolts 2 $10 $20 Accelerometer 3 $18 $54 Resin, qrt 3 $17 $51 Pixhawk Set 1 $200 $200 Hardener, 0.74fl. oz. 6 $6 $36 Raspberry Pi 1 $60 $60 Services Subtotal $420 Travel Subtotal $2,284 Laser Cutting (1 Hr) 6 $20 $120 Gas $0.15/Mile 3000 Miles $450 3D Printing (1 Hr) 5 $20 $100 Hotel $100/Room/Night 4 Nights, 3 Rooms $1,200 CNC Machining (1 Hr) 4 $50 $200 Car Rentals 3 $211.13/Car/Week $634 Total Budget Cost Design configurations were examined to establish a conceptual design that will establish a preliminary design leading to the final design. The main factors considered were flight performance and ease of manufacturing. Figures of Merit (FOM) were created to facilitate the flow of decisions. A multirotor configuration was discarded as the endurance penalty was perceived by the team as detrimental. Prior to establishing the FOM, several assumptions were made. The aircraft will have a monoplane configuration. The monoplane configuration was preferred to the flying wing configuration as ease of manufacturing was weighted higher than the parasitic drag penalty. Readily available materials such as a GoPro Hero 3+, Raspberry Pi 2, and the Pixhawk flight controller by previous teams established certain choices and allowed the team to only consider variable aspects of the aircraft. This includes the wing configuration, the type of propulsion system, and the empennage configuration. A tapered and swept wing were not considered when designing for simplicity in the manufacturing process. The following configurations were analyzed for the aircraft: high-wing, midwing, and low-wing. Table 3 below demonstrates the wing configuration consideration and selection. Table 3: Figure of Merit chart for wing configuration of the aircraft. $4,240 Factor Value High-wing Mid-wing Low-wing Complexity 40% Weight 30% Size 20% Flight Performance 10% Total

4 A high-wing is most beneficial when designing for lateral stability and controllability. Aircrafts with a high-wing configuration also require a larger bank angle to fight the natural tendency to stabilize. The high-wing configuration is also easy to integrate to the fuselage. A mid-wing configuration contains a weight penalty as reinforcement is need for the point of contact with the fuselage. Low-wing is beneficial for maneuverability and simplicity when integrating landing gear to the plane. Low-wing designs often have a dihedral angle to provide stability and control as this wing type has a low tendency to stabilize to its equilibrium. For the wing configuration, the team determined high-wing would be the best option for manufacturing, design and stability purposes. This wing placement on the design would also allow for ease of integration, preferred lateral stability, and ease of manufacturing. The motor placement and its configuration were the next priority for the conceptual design of the plane. An important consideration is the placement of the downward facing camera. For ease of packaging, the camera will be placed on the nose of the aircraft. The single pusher is selected despite an efficiency penalty due to its considerable vibrational effect on the camera system. After finalizing a conceptual design, the team investigated manufacturing methods and material section. These included 3D printing, foam composite, and/or built-up balsa. 3D Printing: Using Polylactic Acid (PLA) and Acrylonitrile-Butadiene Styrene (ABS) plastic for rapid prototype on the ribs for the wings and the fuselage. Foam: Pieces of the foam will be cut using a Computer Numerically Controlled (CNC) to accurately create the shapes needed. Balsa Wood: Majority of the balsa wood is going to be cut using a laser cutter, which would create the desired parts quickly and accurately. The rest will be cut into the desired shapes using various tools such as a box cutter and the scalpel. II. iii. The team decided to integrate both 3D printing and the built-up balsa approach for ease of manufacturing, existing resources, time constraints, and budgetary reasons. Programmatic Risks and Mitigations The team followed safety procedures established by the engineering college at the university to address potential safety and health risks. During propulsion test, all equipment was inspected of physical damage, proper insulation and connection. A dress code was followed and safety glasses were used during testing in an eye damaging environment. System Design i. Aircraft Several types of manufacturing methods were researched and analyzed while planning how to construct the aircraft to successfully complete the mission demonstration. During the selection process the team considered manufacturing that would require the least amount of funding, had required materials commercially available and easiest in manufacturability. 3

5 Table 4: Materials selected for the respective manufacturing component. Manufacturing Component Material Main Material Balsa Wood Aircraft Skin Monokote Secondary Material PLA and ABS Plastic Adhesive Loctite Epoxy Part Support Velcro Two critical factors were examined in the early phases of manufacturing of the aircraft. First, compiling a list of material tradeoffs reflecting the pros and cons of selecting one type of material over another. This is crucial when producing an aircraft that must be lightweight, maintain its structural integrity, and overcome loading conditions with a max payload capacity. Due to physical constraints of manual machining methods, certain design aspects may be unachievable due to a lack of precision. The list of potential materials included carbon fiber, acrylonitrile-butadiene styrene (ABS), polylactic acid (PLA), fiberglass, monokote, balsa wood, plywood. Figure 1: Airframe composed of 3D printed PLA bulkheads, plywood stringers, and balsa sheets. 4

6 Figure 2: Wing composed of MH114 3D printed PLA airfoil ribs and balsa spar. Expected flight conditions were simulated on the preliminary aircraft using Mark Drela s XFLR5, ANSYS, and SolidWorks static finite element (FEA) analysis. ANSYS provided aerodynamic coefficients that subsequently impose requirements to be met by the propulsion system. Several airfoils were considered for the preliminary prototype. The main airfoils were narrowed down to the MH114 and the ClarkY. Figure 3: XFoil Foil Analysis displaying lift coefficient against angle of attack. 5

7 Main airfoil characteristics were taken into consideration and are summarized below in Table 4 below. Despite a larger percent thickness, the MH114 airfoil was selected due to its max coefficient of lift of and a larger max lift coefficient to drag coefficient ratio of Table 5: Airfoil characteristics derived from the XFLR5 batch direct foil analysis. Airfoil Angle at Max Cl Cl Max Cd at Max Cl Cl/Cd Max Max Percent Thickness MH (at 28.1% chord) Clark-Y SM (at 30.9% chord) Figure 4: XFLR5 aircraft analysis at a 3 degree angle of attack. Figure 5: Finite Element Analysis of the aircraft wing displaying stress values. Stress analysis simulated on the wing display potential areas of reinforcement. ABS reinforcement will be added near the root attachment to the fuselage. Balsa sheets will be placed at the leading and trailing edge of the wing. Monokote will be used in between to add torsional rigidity and reduce drag. 6

8 Table 6: Aircraft main characteristic displaying dimensions. Wing Vertical Stabilizer Horizontal Stabilizer Airfoil MH114 Airfoil NACA 0010 Airfoil NACA 0010 Span 50.6 Span 10.2 Span 20.1 Chord 7 Chord 7.2 Chord 9 Aspect Ratio 8.43 When researching motors as to which would be optimal to power our plane, the team looked into restoring an E-flite Power 90 Brushless Outrunner motor that had been used by a previous years team. With a bent propeller shaft being one of a few components needing to be replaced or repaired, the team invested in purchasing a new Power 90 motor. This particular motor is often used in converting 90-size 2-stroke glow engine planes to electric, with the desired plane weight ranging from 8 to 13 lbs. The substantial amount of torque and power that can be provided with this motor must be accommodated with an electronic speed controller (ESC) that can regulate a considerable amount of current drawn from the power supply. The Phoenix Edge Lite 100 by Castle Creations was good match for both the motor and the project itself. The 100 amp resistance exceeds the required 85 amp specified for the motor, as well as coming equipped with data logging capabilities through the Castle Creations software. This data logging allowed several propellers to be tested and results to be compared when determining which provided the best efficiency for the motor at cruising speed. The static thrust was also measured using a static thrust rig consisting of two pieces of plywood fastened together in an L shape, with a motor attached to the top of the vertical piece and a scale placed under the horizontal piece. These segments were then mounted to a base allowing them to pivot, translating the thrust produced by the motor to the scale yielding a given weight. Below are two figures, the first is of the static thrust rig mounted with the E-flite Power 90 motor and an APC 20x10E propeller. The second is a graph depicting the minimum, average, and maximum for voltage, current draw, wattage, temperature, revolutions per minute, throttle rate in, power produced, and amp hour rate pulled from the battery. 7

9 Figure 6: Static thrust rig with E-flite Power 90 and APC 20x10 propeller combination. Figure 7: Data for E-flite Power 90 and APC 20x10 propeller combination. ii. Autopilot The team decided to use the current system inherited and use the Pixhawk autonomous flight controller. The Pixhawk is preferred for its low cost, high availability. Due to its open source 8

10 hardware, it is easy to use and integrate into the current fixed wing airframe. Several different modes, including auto, guided, and manual are available within the programming onboard the Pixhawk. Auto mode allows the UAS to follow a predetermined set of waypoints and is the basis for the autonomous flight that the system will be achieving. This will allow the team to complete the waypoint capture requirement of the mission demonstration. Guided mode functions as an interruption to auto mode, where the user is allowed to set a single waypoint that the aircraft will go to directly. This allows the easy adaption for potential flight errors or if it is required to attempt a waypoint multiple times. The Pixhawk communicates with electrical components through MAVlink. The flight mode, waypoints, and other parameters can be changed by sending MAVlink packages to the Pixhawk. The ground control station will be Mission Planner, displayed below in Figure 8.The interface s flight data window shows aircraft attitude, altitude, ground speed, heading, flight mode, and relevant waypoint information. The graphical user interface shows satellite imagery of the flight area also updates with the aircraft s position, waypoint plans, and flight boundaries. iii. iv. Figure 8: Mission Planner GUI associated with the selected Pixhawk autonomous system. Imaging System Imaging will be received from two potential sources for integration: an inherited GoPro Hero 3+ and a CMUcam5 Pixy. The Pixy is open-source and contains substantial documentation for integration with the Raspberry Pi. Both cameras will be tested to demonstrate they meet mission capabilities and fully integrated in May and are anticipated to provide sufficient performance to complete the mission successfully. Object Detection, Classification, Localization (ODCL) A wireless network connection will provide the basis for a Raspberry Pi 2, the Pixhawk, and a ground station to for interoperability tasks and ODCL. As the selected onboard processing unit, the Raspberry Pi runs MAVproxy to communicate with the Pixhawk through GPIO pins. Through the network connection, the Pixhawk uses UDP protocols to transfer MAVlink packages to the ground station. MAVproxy allows the user to setup custom time-based camera activation Python 9

11 modules for target localization based on the aircraft s position and orientation. A Python script using OpenCV software will provide image analysis to the system. The images will be transferred to the ground station where the Python script will use HSV color changing and the contour methods to isolate the targets. The program will use flight position and orientation at photo captures to find the location of the target. v. Communications The Pixhawk uses a satellite radio antenna to communicate with the RC transmitter at 2.4GHz, while it uses MAVlink to communicate with the Raspberry Pi 2. A Ubiquity Bullet M5 and Ubiquity Airgrid will establish a wireless connection operating at 5GHz. The Raspberry Pi 2 will use this band to communicate with the laptops at ground control. A backup telemetry radio operating at 915MHz will provide a connection to the Pixhawk and ground control if primary methods fail. vi. Figure 9: Block diagram of the communication system expected to operate. Air Delivery The cargo-drop mechanism is comprised of a polylactic acid (PLA) 3D printed holder containing a screwed down servo. The servo is manually actuated by the transmitter. Figure 9 below demonstrates a SolidWorks rendering of the mechanism. 10

12 Figure 10: The air delivery mechanism consisting of a servo motor and aluminum pinning arm. The mechanism designed for the air delivery of the payload, an 8-ounce water bottle, is an attachable cap-lock. This device consists of three subcomponents that are secured around the neck of the water bottle, attaching a nylon parachute to the payload. The two objectives considered when designing the mechanism were survivability and simplicity. The gadget was modeled for the payload to be stored safely within the aircraft in a fixed position, altering the drag forces and the planes center of gravity as least as possible before and after the air delivery. Utilizing a cap-lock mechanism was evaluated to be the most effective design, being made of light weight PLA and easily accessible when attaching and detaching it to the neck of the water bottle. The parachute is hexagonal in shape, with 24 inch chords that are attached to the caplock. Figure 11: 3D printed PLA mechanical lock and parachute pairing of the water bottle payload. 11

13 III. vii. Cyber Security The team s flight demonstration network used by the team will be encrypted using Secure Shell (SSH). SSH establishes a secure network service over an unsecured network. It also allows for the connection and communication of connected network users. It is based on a central key management system. Testing and Evaluation Plan i. Development Testing There are many things that can affect the development of the UAS, making testing a critical factor in the design of the system. Tests, such as a wingtip loading test, are done prior to the wings being mounted onto the aircraft. This test is performed by loading the tip of the wing with weight to see how much force the wings can withstand relative to weight of the wing itself. The use of computer generated software allows for testing of the airframe itself to understand how aerodynamic the system is going to be, as well as testing the airframes structure. Each component was tested to meet the desired performance for each aspect of every mission. To perform the wingtip loading test, each wing was placed onto the end of a table and clamped down. On the tip of the wing, weights were added individually. The test was concluded when there was a break in the wings. Each test was inspected and documented. For this wing strength testing, two wing configurations with different rib structures were tested. Figure 12: Half-span wing tip loading tests method at 5 pound load. Figure 13: Wing tip load test results, failing at a successful 6 pound load. From the results, both wings failed at roughly 6 pounds of weight shown above in Figure 12. Based on these results both of these wings fell short of the desired value of 8 pounds of weight. In order to meet this desired value, both of the wings would need a larger leading edge across the wings to increase strength. Another modification to help increase strength would be to use spruce hard wood instead of balsa wood for the spar box inside the wings since that particular part was what failed. 12

14 Team members conducted a dropping test, with the mechanism secured to the payload, from atop a building calculated to be slightly over 75 feet tall. Figure 14: Cargo drop survivability test consisting of dropping the urgent aid package. Theoretically, the terminal velocity of the payload with parachute deployed is m/s, reaching in 1.71 seconds, traveling meters, it takes approximately 4.05 seconds to hit the ground from meter above. The impact of the water bottle with a contact time of.05 seconds is an actual tested value that is greater than the theoretical, meaning that the drag force due to the parachute is greater than if the payload were without it. 13

15 Table 7: Survivability testing of cargo drop of urgent aid package. Trial Time (s) ii. Individual Component Testing iii. The Pixhawk will be tested extensively using an existing quadcopter. Waypoints Existing sensors will be evaluated to ensure performance in the field. Waypoints similar to the mission demonstration will be established for the quadcopter to perform, while the aircraft is being finalized. Sensors, such as GPS and airspeed, will be evaluated for integration to other systems. Algorithms being currently developed for imaging and ODCL are being tested in various cameras and ground stations for performance evaluations. Mission Testing Planning IV. The team will simulate an environment that will mimic the flight conditions to those expected during the actual flight. Titan is expected to fly at 34 kts. An additional airframe is reserved, a commercial RMRC Skyhunter, as previously used by the CSUF team. This airframe will be incorporated after two failed T1tan iterations. Safety, Risks, & Mitigations Safe practices, such as the use of heavy-duty gloves and safety glasses, were imposed during manufacturing. To mitigate structural failure, aircraft wings were design at a Factor of Safety (FoS) of 1.5. During flight tests, the pilot immediately takes manual control of the system if any possible failure is detected. For a loss of signal, return to home is imposed for thirty seconds and the land command is imposed if connection is not established. The following are items followed by all team members during testing and flights. Pre-Flight Inspection Items: Inspect propeller, engine, and engine mounts for security, damage and wear Inspect flight control servos, linkage, and surfaces for loose fasteners Inspect all payload modules and associated wiring for security Inspect pitot and static ports for obstruction Test fail-safe and termination functions Immediately Before Propulsion Start: Airframe 14

16 Ensure that all batteries are fully charged and secured Ensure all wings are secured Ensure all flight controls are free and move in the correct direction Ensure the payload is secured Ensure airspace is clear Autopilot Verify boundaries and waypoint paths are correctly loaded and set with correct altitudes Verify payload and control system voltages are normal Verify a GPS lock is acquired and strong 15