Unmanned Aerial Vehicle Design, Development, and Implementation

Transcription

1 Unmanned Aerial Vehicle Design, Development, and Implementation Faculty Advisor Dr. David Schmidt Team Members Patrick Herklotz, Shane Kirkbride, Mike Kopps, Mark Kraska, John Ordeman, Erica Rygg, Matt Spears, Sam Szarka, and Benjamin Young Special Thanks to: Our sponsors: And especially: IEEE Pikes Peak Section Executive Committee, Dr. Terrance E. Boult, Ben Udhall, Brandon Garrison, Daniel Ruiz, Delphine Humphries, Jim Fournier, Ryan Thomas, Ski Munch, and Katie Collins

2 Abstract: The Near Space Unmanned Aerial Vehicle Team (NSUAV) at University of Colorado at Colorado Springs (UCCS) designed and constructed an Unmanned Aerial Vehicle (UAV). This project was inspired by the Near Space Initiative which is an agreement between the UCCS and the Air Force Research and Army Research Labs. As a first step toward understanding the complexities of a near space environment the NSUAV team chose to build a basic UAV. The mission and goals of the AUVSI Seafarer s Student Unmanned Aerial Vehicle competition are consistent with the NSUAV team goals. The NSUAV team chose to enter the initial project into the competition to gain an objective mark on their progress. The process of designing a low cost unmanned aerial vehicle with commercially available parts is summarized in this paper. The implementation of a navigation system with capability to follow and fly through dynamic waypoints carrying a two to three pound payload while taking in video imagery is also discussed. UCCS 2

3 Table of Contents I. Introduction 4 II. Design Process and Overview... 4 III. Autonomous Navigation 6 IV. Platform Design..9 V. Power System VI. Target Recognition System.14 VII. General Safety and Procedures..15 VIII. Conclusion.16 UCCS 3

4 Introduction The NSUAV Team's entry into the AUVSI competition was initiated with the purpose of submitting a UAV entry that meets the AUVSI competition specifications and the following goals as stated on the competition rules an unmanned, radio controllable aircraft to be launched and transition or continue to autonomous flight, navigate a specified course, use onboard payload sensors to locate and assess a series of man made objects prior to returning to the launch point for landing. In addition to this goal, the NSUAV had further goals of constructing a UAV with off the shelf parts, keeping the cost under 10,000 dollars, and to design a robust UAV with the simplest possible architecture to keep complexity low and system malfunctions to a minimum. Design Process and Overview The engineering process begins with the identification of specific goals. In this step the goals of the product to be designed are discussed and settled upon. Next, a system level breakdown of the individual systems occurs. Here the different tasks or systems of the product are determined and defined. An initial system break down is shown in Figure 1. After the design is made and the systems specifications are determined and defined. Action is taken to implement the design. Lastly, the designed product is tested to determine whether the product meets the goals and specifications. The individual systems may be tested independently at first in order to isolate system specific faults. Later, the entire product design/prototype is tested to isolate any Figure 1: Systems Breakdown for a UAV remaining errors or design faults. Design Overview Figure 1 shows the four subsystems which make up the NSUAV design. The autonomous navigation subsystem is responsible for the actual flight of the UAV. This system must be robust, contain failsafe capabilities, lightweight, low power, and low cost The target recognition system was developed in accordance with the AUVSI competition standards and had low power, low weight and size requirements in addition to minimum video resolution requirements. The power system powers the electronics both on the vehicle and on the ground and the batteries within it were required to be as small and lightweight as possible. The platform system is defined as the mechanical and structural parts of the design. Each of these systems will be discussed with emphasis on their construction, components and the specifications which determined the selection of the components used. UCCS 4

5 Autonomous Navigation Autonomous navigation can be achieved with a complex flight control system more commonly referred to as an autopilot. A basic autopilot will be able to take latitudinal and longitudinal coordinate readings of the current position and altitude of the aircraft. It should take these readings and use them to control the flight of the aircraft in such a manner as to steer the aircraft toward the desired target destination. At the same time the autopilot will accept the current thrust measurement and three dimensional velocity vector (yaw, pitch, roll) readings and make adjustments to the aircraft control surfaces in such a manner as to keep the aircraft in a stable and level flight pattern. Some autopilots also have the ability to take action in the event GPS data is lost or a communication link breaks Figure 2: Entire UAV system with starter kit down. Procerus Kestrel Autopilot After considering the complex control system engineering involved in the design of an autonomous navigation unit, it was decided the purchase of a commercially available autopilot was to be the practical time and cost efficient solution. The Kestrel Autopilot from Procerus Technologies, shown in Figure 3, was selected from among a number of different commercially available products due to its ease of configuration, small size, weighing 16.7 grams and 1.29in2 in volume, low voltage and power requirements (5VDC), and cost. In addition, the Kestrel Figure 3: Kestrel 2.2 autopilot with Aerocomm AC4490 makes communications simple modem attached with its piggyback modem, and plug-and-play GPS receiver. UCCS 5

6 Among the Kestrel's features is an Inertial Measure Unit (IMU) which is composed of 3-axis rate gyros and accelerometers. Absolute and differential pressure sensors provide barometric pressure and aircraft air speed. Three temperature sensors combined with a 20 point temperature Figure 4: Kestrel 2.x block diagram compensation algorithm reduce sensor drift improving aircraft state measurement and estimation. Figure 4 shows how all of these components work together on the Kestrel. The Kestrel was originally designed for a foam delta wing aircraft powered with a lightweight electric motor. However, the NSUAV team platform is a gas powered trainer plane. The nitro 2stroke engine vibrations created a problem which manifested itself as an intermittent ground control to Kestrel communications link. This problem was resolved by designing and constructing a vibration dampening mount for Figure 5: the autopilot vibration dampening mount for the the craft and the Kestrel. This is Kestrel shown in figure 5. The design is intended to minimize the amount of vibrations while still holding the autopilot as stable as possible during flight. Autopilot Communications Aircraft coordinate and altitude telemetry is transmitted and received via a pair of Aerocomm AC MHz radio modems. The Ground Control station consists of a notebook computer and a device called the Commbox. The Commbox is a small communications hub. It contains the modem needed to communicate with the Kestrel Autopilot and the serial port connector to interface with a notebook computer The notebook computer along with the Virtual Cockpit Figure 6: UAV in flight UCCS 6

7 software from Procerus Technologies lets a user upload waypoints, tune PID gains, calibrate sensors, and monitor the flight. Waypoints can be defined as latitude, longitude, and altitude coordinates which are points in space through which the flight coordinator or mission control operator wishes to fly the UAV to, or through. Mission Control/ Navigation System for the autopilot The mission control system consists of a graphical user interface (GUI) built into a software package called the Virtual Cockpit. A basic screen shot of the Virtual Cockpit is shown in figure 7. The Virtual Cockpit runs on a notebook PC and is designed to command the autopilot to fly through a predetermined waypoint sequence. The commands are issued from the software to the Commbox via a serial port and are then transmitted to the autopilot. Mission control also has the ability to accept and Figure 7: A screen shot of the Virtual Cockpit Software submit a change in the waypoint GUI. This software is responsible for the ground sequence in mid-flight if a change in control data management mission objectives occurs in mid-flight. Figure 8: The entire ground control station including the Commbox and the Futaba RC transmitter unit. The Virtual Cockpit software contains a GUI which makes current altitude, attitude, and elevation information immediately available to the mission control operator. The Virtual Cockpit s graphical display shows the aircraft's vital signs in an obvious way so the flight coordinator and backup pilot would be able to make rapid decisions in the event an emergency was to occur. The flight telemetry data can then be saved to a file and evaluated if needed. UCCS 7

8 Platform Design The platform for the UAV is the Sig Kadet LT-40 ARF. This model aircraft has proven to be an easy flier for beginner RC pilots and is highly recommended as a trainer by the RC community for its excellent stability. An unmodified Sig Kadet LT-40 ARF is shown in figure 9. The entire platform with the payload is 8 lbs. Figure 9: An unmodified ARF SIG LT-40 Airframe Selection The Kadet LT-40 has a large wing surface area of 900in2 with a wingspan of 70in. The large wing surface lift area is a must for heavier payloads and uncompromised flight stability. The standard airfoil design has a flat bottom with airfoil top side. The tricycle style landing gear of the Kadet provides take-off and landing stability while also remaining maneuverable on the ground. Propulsion Propulsion is provided by an O.S nitro RC engine. This is a 2 stroke engine with a horsepower of 1.90 BHP at 16,000 RPMs. The engine was chosen for use in combination with the Kadet LT-40 platform to allow enough power to sustain normal flight with the increased payload weight of the autonomous flight electronics. The altitude of UCCS is 6,300 ft. above sea level; the thinner atmosphere was a factor in sizing the engine to an above average displacement. Payload Placement The payload must be carefully distributed to maintain the center of gravity within acceptable limits. The vehicle's center of gravity is determined by payload placement, engine size, location and weight among other factors. It was determined that the center of gravity must be kept a few inches behind the airfoil's leading edge in order to keep the lift vector and center of gravity as close together as possible. Since the majority of the payload area is located within the fuselage, re-balancing of Figure 10: The target recognition (external) loads lateral to the midline was unnecessary. payload was placed directly under the Figure 10 shows the placement of the center of gravity to maintain stable flight external payload. Special care was taken in the dynamics. design of this payload mount to ensure a minimal amount of change occurred in the flight dynamics of the platform. The weight of the payload is about 1.5lbs. UCCS 8

9 Power System Overall Power System Structure Figure 11, below, shows the block diagram outline of the entire power system used in the UAV. Figure 11: Power Block Diagram The UAV has two power sections, the first is on the onboard system and the second is the ground station. As seen in figure 11, the laptop unit, video receiver, and the Commbox for the Kestrel, will be powered using a separate DC to AC power inverter will be powered by a 12V car battery. These are the three most critical devices outside of the autopilot itself. The other devices are powered by their own internal batteries. UCCS 9

10 On-board Power System Figure 12 outlines the power system on-board the platform. On Board Power Supply System for UAV Charge Port 5V 4.8 V 700 mah Servo Battery 11.1 V Li PO 2300 mah Kestrel Battery 9.6 V 600 mah NiCd Gimble Battery Charge Port 12V Charge Port 10V Servos Switch 1 Kestrel GPS Switch 2 Gimble Power Switch 3 Video Transmitter Camera Gimble Servo Radio Reciever Figure 12: On-board Power Scheme Outline The entire system is powered by three primary batteries: one Lithium Polymer battery for the Kestrel Auto Pilot, and two NiCd batteries for the airplane servos and the gimbal. These three batteries are attached to three switches which switch between a charging supply and the component. An interface board with the three charge ports is attached to one end of the switches along with the batteries. When the switches are in the off position, the charge ports are attached directly to the batteries themselves. This allows the batteries to charge without removing them from the platform, and allows for quick voltage checks in the field. The 4.8V NiCd for the servo control does not connect directly to the servos. Rather, the connection is routed through the Kestrel unit, and multiplexed with the servo controls on each channel. This is shown as a loop through in figure 11. The 11.1V Li-PO is the most powerful battery, at 2300 mah, used to power the Kestrel unit itself. This also routes outward to power the GPS receiver unit, and the 900 MHz modem attachment. Because of the volatile nature of the Li-PO, it is removable so it can be charged separate of the platform. Figure 13 shows the way the three batteries are mounted inside the platform. The 9.6V gimbal battery is mounted beneath the fuel tank, and the Velcro strips outline where the Li-PO is mounted. UCCS 10

11 Figure 13: Battery Mounts The gimbal unit has a custom-built power supply system powered off of the 9.6V NiCd. Figure 14 shows the power board schematics and board layout diagrams for the main board. Figure 14 shows the schematic drawing of the power kill daughter board (PKDB). Figure 14: Gimbal Power Supply Main PCB Drawing Figure 15: Schematic of the power supply with all components shown UCCS 11

12 Figure 16: Power Kill Daughter Board In Figure 15, JP2 is the 9.6V battery input. JP1 is the interface with the PKDB seen in Figure 16. All other connectors on the board output 5V, which power the video transmitter, the gimbal radio receiver, and the camera. Figure 17 shows the layout of the PKDB, originally designed for use with the Li-PO battery in the earlier stages of the project. IC1 is an Atmel TINY13S microcontroller, which senses the incoming voltage from the battery. When the voltage drops below the programmed threshold voltage, it disables the switching regulator on the main board. JP2 is a 3x2 header connector used to program the part. Ground Station Power Most units on the ground are powered using internal batteries. A separate radio transmitter, powered by a 9V internal battery array, controls the servo attached to the camera on the gimbal, as seen in Figure 11. The Commbox unit has a12v rechargeable battery inside. The vital importance of this unit and the laptop computer necessitates a power inverter be used so the power loss issues will not be a factor. A 12V adapter plugged into the inverter must power the video receiver. The Futaba radio transmitter does not need external power supply unit. UCCS 12

13 Target Recognition System Design Image Recognition Design Overview This system contains software that processes the images received from the plane and has the sole purpose of finding target shapes/objects/colors in the pictures, and determining their locations via the GPS information received from the plane at the time of the photo being taken. The video processing software will give a signal to mission control to take a picture. At the same time, the GPS coordinates and heading of the plane will be sent to the image processing software. The GPS coordinates are needed as this allows pictures to be matched up with the locations where they were taken. The process of matching pictures with their GPS coordinates must have a low time delay, since longer the time delays will introduce larger errors into the data. Hardware Design There are five main components in the hardware design; the components are the camera, transmitter, servo, power unit and receiver. First, the Black Widow camera is responsible for actually capturing the image. Then, signal is digitized and sent to the 2.4GHz Black Widow transmitter. The receiver and servo are connected so the camera can be pointed in any given Figure 17: The hardware for the imaging system. direction along one axis. The ability for the camera to turn compensates for the image distortion which occurs as the plane rolls in a turn. The power supply ties the unit together with a 9.8V power supply as mentioned above. Software Design Figure 18: The data flow upon clicking on a target This section describes how the data imaging software is intended to work and describes the data flow within the software. First the target image is received from a 2.4GHz 0.6Watt transmitter via a 2.4GHz video receiver and its high gain 2.4GHz antenna. Data is translated and stored to an MPEG file using a video capture device. The software displays the video and when the user sees a desirable target, the user places a mouse cursor over the target and clicks. The click's UCCS 13

14 pixel coordinates are used by the software to identify the location of the target. Telemetry from the UAV at the time of the video frame is matched up with the pixel coordinates and the exact coordinates of the target are mathematically extrapolated. The program then logs the information to a file for future reference. The data flow for this program is shown in Figure 19. Figure 19: Software Block Diagram Procedures and General Safety General Safety UAV safety precautions fall under 5 stages: initial safety check, starting the engine, taking off, flight and landing. Aircraft Safety: Check/Walk around Walk Around - Plug aileron and pitot tube** into designated attachments; attach wing and secure wing bolts; ensure no wires hanging out. - Check all hatches to confirm 4 screws attaching them to aircraft; tighten any loose screws - Rack all control surfaces gently; ensure no loose screws on control horn attachments or broken hinges - Gently shake aircraft and listen for loose parts tumbling around in aircraft; remove any loose parts and confirm parts origination before continuing - Turn all switches on and weight for ground controller sensor check, COMM Check, RC COMM check, pressure check, altimeter check, and zero all control surfaces check - Throw all control surfaces in designated directions with manual control unit to ensure they move in the correct direction; reverse any control surfaces that do not and confirm changes with ground controller - Gain Clearance to begin engine startup procedures and proceed to next section ** Pitot Tube A tube used to measure air pressure due to forward velocity of an aircraft. A pressure tube is routed from outside the aircraft to a pressure sensor. Starting Engine Fuel Engine UCCS 14

15 - Command full throttle to open carburetor assembly. Clear area within 6 feet diameter around aircraft; pilot plugs carburetor and whorls prop until fuel drawn into engine and engine audibly starts. - Command idle throttle setting. - Confirm throttle cut closes carburetor assembly. Start Engine - Pilot assistant attaches glow clip to engine glow plug. - Pilot announces CLEAR PROP. - Clear bystanders from 15 feet on either side of line of prop (See Diagram on following page.) Airplane: Top View Kill Zone Red: DANGER ZONE: clear bystanders from prop if prop shatters during startup No Standing Figure 20: Diagram of danger zones for propeller safety - Apply electric starter and whorl engine until it runs Move behind engine; pull glow clip off of engine and ensure engine runs on its own Announce full throttle; pilot assistant holds aircraft vertical to ensure RPM stays constant; richen or lean engine as needed Announce idle and prepare aircraft for taxing Takeoff Gain clearance to takeoff from ground controller - Pilot look both ways on Runway and ensure no obstacles/bystanders on runway or tarmac; ground crew check also and confirm clearance status - Announce TAKING OFF followed by FULL THROTTLE - Allow aircraft to come up to sufficient airspeed for takeoff roll - Fly aircraft to designated AGL and activate UAV systems when instructed to do so by ground controller Landing Gain clearance to land from ground controller - Pilot glance both ways on runway to check for obstacles/bystanders on runway or tarmac; ground crew check also and confirm clearance status - Pilot announce LANDING proceeded by THROTTLING DOWN - Fly aircraft into landing pattern and announce BASE LEG followed by FINAL TURN INTO RUNWAY followed by FINAL APPROACH - Land aircraft gently within flight envelope - Gain clearance to taxi on to tarmac and into pit area; shutdown engine - Turn off all switches on aircraft and transmitter - Move all equipment into competition impound area UCCS 15

16 Failsafe The failsafe is designed to gently land plane in case of loss of transmission. The following commands are executed in case of loss of transmission. - Throttle Closed - Elevator full up - Full right rudder - Full right aileron Procedures There are 4 initial flights before the system can be perfectly configured. These procedures must be conducted after any change in platform or system. They are the procedures recommend by Procerus for proper use of the Kestrel system. Flight 1: Launch, flight, landing The entire flight will be conducted in Manual Mode. In this mode, the control inputs on the RC controller go directly to the control surfaces. Rate damping PID loops can be enabled in this mode to make the aircraft easier to fly, however, because the rate loops have not been tuned, they should be disabled for this flight. The goals for this flight are trimming the aircraft and finding reasonable values for trim airspeed, trim throttle, and trim angle of attack. Flight 2: Tuning Rate Damping Servo PID loops The purpose of the second flight is to tune the rate damping servo loops. The rate damping PID loops damp the aircraft rotation around the pitch, roll, and yaw axis if the aircraft has a rudder. Familiarity with the control algorithms of the autopilot is very useful for this stage. Flight 3: Tuning the inner attitude hold loops, the outer airspeed and altitude hold PID loops The purpose of the third flight is to tune the inner attitude hold loops and the outer airspeed and altitude hold PID loops. Because the rate loops were tuned in Flight 2, the pilot can now utilize rate damping in Manual Mode if desired. Again, keep the aircraft close enough such that the pilot can easily switch to manual mode to save the aircraft. Flight 4: Re-verification of waypoint navigation and proper loitering The purpose of the fourth flight is to verify waypoint navigation and loiter work correctly. At this point, it is assumed flights one, two, and three have been completed and the aircraft is tuned to fly in Altitude and Speed Modes. Use the Failsafe Setup Screen to enable the fail-safes. The user may also choose to enable the joystick or keyboard inputs for roll and altitude control in Speed and Altitude modes. UCCS 16

17 Conclusion In conclusion, the target goals of designing a robust low cost UAV with a low system complexity to minimize system malfunctions was met with great success. Test flights confirm that the use of external charging ports, a small and efficient autopilot with plug-and-play peripherals, and other equally effective systems has benefited in a low time to flight and low incidence of operator errors. However, whether or not the NSUAV design can meet AUVSI specifications and accomplish the mission goals is yet to be determined. UCCS 17