Helicopter Unmanned Aircraft...

Virginia Commonwealth University Helo UAS Team of 19

2008 AUVSI Student Competition Helicopter Unmanned Aircraft System

Authors: Robert C. DeMott II Joshua S. Grinnan Jose E. Ortiz

Advisor: Dr. Robert H. Klenke

Submitted: May 28, 2008

Abstract The following paper discusses Virginia Commonwealth University's design and implementation of an unmanned aircraft system (UAS) to compete in the 2008 Association for Unmanned Vehicle Systems International (AUVSI) UAV competition. The system is able to autonomously fly a series of waypoints to reach a search area. Once in the search area, the vehicle is able to locate ground targets and identify target characteristics, communicating this information to operators in near realtime. The vehicle platform is an electrically powered, radio controlled helicopter. The complete solution includes a custom developed autopilot, or flight control system (FCS), an ISR payload featuring a gimbalstabilized, high resolution digital camera, and a ground control system (GCS) that provides mission planning, control, and analysis capabilities. The system has been designed with utmost importance placed on operator, observer, and vehicle safety. Testing, operating, and failure procedures have been implemented to ensure safety throughout development and competition.


Table of Contents Abstract................................................................................................................................................... 1 Table of Contents .................................................................................................................................... 2 Design Overview..................................................................................................................................... 3

Custom Flight Control System ........................................................................................................ 3 Vehicle.................................................................................................................................................... 4 Flight Control System.............................................................................................................................. 5 Software.............................................................................................................................................. 5 Hardware ............................................................................................................................................ 6 Main Control Unit ........................................................................................................................... 6 Inertial Measurement Unit............................................................................................................... 6 Lateral Velocity and Altitude Sensor ............................................................................................... 7 Communication............................................................................................................................... 8 UAV Safety Switch......................................................................................................................... 8 Overall FCS Architecture ................................................................................................................ 9

Ground Control System ........................................................................................................................... 9 GCS Server ......................................................................................................................................... 9 GCS Control Client ........................................................................................................................... 10 Map Display ................................................................................................................................. 10 Telemetry Panel ............................................................................................................................ 10 Telemetry Gauges ......................................................................................................................... 11 Parameter Control Forms .............................................................................................................. 11

Intelligent Reconnaissance Imaging System........................................................................................... 12 Camera.............................................................................................................................................. 12 Video System.................................................................................................................................... 13 Video Tx/Rx ................................................................................................................................. 13 Digital Video Recorder ................................................................................................................. 13

Gimbal .............................................................................................................................................. 13 Control Unit ...................................................................................................................................... 13 Wireless Image Transfer.................................................................................................................... 14 IRIS Workstations and Software........................................................................................................ 14

Power System........................................................................................................................................ 15 Operational Setup.................................................................................................................................. 16 Flight Control Operator ..................................................................................................................... 16 Video System Operator ..................................................................................................................... 17 Intelligence Analysts ......................................................................................................................... 17

Safety.................................................................................................................................................... 17 Failsafe flight modes ........................................................................................................................ 17 Return Home................................................................................................................................. 17 Stability Augmentation Mode........................................................................................................ 17 Flight Terminate ........................................................................................................................... 18

Batteries............................................................................................................................................ 18 Transmission Loss............................................................................................................................. 18 Failure Modes Analysis ..................................................................................................................... 18

Conclusion ............................................................................................................................................ 19 Acknowledgements ............................................................................................................................... 19


Design Overview The decision of a rotary wing aircraft was unanimous among the VCU Helo UAS Team. The ability to maneuver in confined spaces, loiter in a location for extended periods of time and perform vertical takeoff and landings are features that fixed wing aircraft do not possess.

Several different design approaches were utilized to develop and integrate the various sub systems. Based upon previous fixedwing unmanned aircraft systems (UAS) research, the VCU UAS group developed a custom flight control system (FCS). Highresolution imagery and 802.11 Wifi communications technologies have continued to evolve for the VCU UAS program. Commercialofftheshelf (COTS) components have been utilized for digital imaging and wireless communications hardware. These systems have been designed, integrated, and tested under a thorough review process focused on mission functionality and overall safety. The Helo Unmanned Aircraft System (Helo UAS) is designed to meet all requirements to compete in the 2008 AUVSI UAV Competition.

System Architecture Overview

Custom Flight Control System In general, systems utilizing COTS products realize a savings in development time and cost. However, a custom FCS was developed in favor of using a commercially available unit for several reasons. Previous experience and research of unmanned systems at VCU has been utilized in the creation of custom FCS hardware and software for the Helo UAS. When considering the "blackbox" nature of many commercial systems and issues such as


expandability and debugging, a custom FCS was the logical design choice. Additionally, many commercial systems do not easily adapt to various helicopter airframes. The Helo FCS has been tailored to the unique flight characteristics of the helicopter, but can be adjusted to autonomously control other airframes without difficulty. Lastly, the Helo FCS is produced at a fraction of the cost of comparable commercial systems.

Vehicle The vehicle chosen is a Miniature Aircraft XCell IonX electrically powered helicopter. To maximize flight time and payload capacity, the helicopter is equipped with longer rotor blades (800mm vs. stock 690mm) and a longer tail boom. The helicopter is also upgraded with a Hacker motor using single stage gear reduction, improving performance and reducing vibration. With two 8 AH lithium polymer motor batteries, the helicopter, equipped with a ten pound payload, still achieves a fifteen minute flight time. If the helicopter is configured with four 8AH batteries, the maximum flight time increases to 25 minutes. This capability coincides with the competition's twenty minute mission objective.

Although a gas powered helicopter of similar size provides a longer flight time and increased payload weight, electric power is used for several reasons. Electric motors produce less vibration than their gas equivalents. Highfrequency vibrations have proven to be detrimental to sensors used in the flight control and imaging systems. Electric motors also do not produce exhaust, which can be problematic for optical sensors, such as cameras.

XCell IonX Helicopter (Shown With Video Gimbal)


Flight Control System The AUVSI mission rules require the flight control system to be capable of the following abilities: 1) Navigate a predetermined flight path consisting of a series of GPS coordinates and altitudes. 2) Allow for ground station operators to dynamically change the vehicle’s target altitude and

airspeed in real time. 3) Allow for ground station operators to specify new waypoints and search areas in real time.

In order to meet these requirements, a custom rotorwing based autopilot system was developed by VCU students. This system is based on previous work conducted at VCU in the area of fixed wing UAVs and adapted to handle the intricacies of a rotorwing aircraft. In addition to meeting the above requirements, the FCS is capable of adjusting control algorithm parameters in real time. Autonomous take off and landing technologies are still under development and will be fully integrated into the FCS in the near future.

Software The FCS software consists of a main control program that runs at 50Hz and is responsible for reading from all sensors, running the control algorithms and handling radio communication with the ground station. This 50Hz operation allows for smooth control of the helicopter and rapid response to both sensor readings and ground station commands.

The flight control algorithms are based on standard PID (Proportional, Integral, and Derivative) control loops using feedback from both onboard and external sensors. Autonomous flight of the helicopter is divided into the following aspects, each being controlled by one or more PID loops: Heading, Altitude, Lateral Movement, and Forward Flight.

Standard PID Control Algorithm

Because of this division of control, each algorithm could be thoroughly tested and tuned independent of the other algorithms. This was achieved by selectively toggling autonomous


control of each algorithm, with the RC pilot controlling the remaining aspects of flight. Since each algorithm was tuned both individually and in combination with the others, more optimum flight performance was achieved.

Hardware

Main Control Unit The FCS software runs onboard an embedded computer known as the Suzaku. The Suzaku features a 48MHz softcore processor, 16MB of RAM and a 1 million gate FPGA. Since the Suzaku features an FPGA, it can easily be reprogrammed to communicate with multiple sensors and I/O devices through its onboard peripheral bus (OPB). In addition to interfacing with sensors and communication equipment, the Suzaku is also capable of monitoring and generating the PWM signals necessary for controlling the helicopter servos. This unit is mounted on a custom designed PCB motherboard along with the barometric sensors and the radio modem.

FCS Main Control Unit

Inertial Measurement Unit For attitude, position and velocity information, the MIDG II INS/GPS unit from Microbotics was used. The MIDG II is a compact, lightweight unit that combines a 5Hz GPS, 3Axis Rate Gyro, 3Axis Accelerometer and 3Axis Magnetometer with an advanced Kalman filter capable of operating at 50Hz. With proper mounting and vibration dampening, this unit has proven extremely reliable, providing the FCS with accurate and precise sensor readings. Attitude accuracy is within 0.4 degrees and position accuracy is within 2 meters. Because the MIDG II can run at 50 Hz (same as FCS), it will always provide uptodate information to the FCS algorithms, improving system response.


Microbotics MIDG II INS/GPS Unit

Although the MIDG II also provides altitude information, a temperature compensated barometric pressure sensor was chosen as the source for this information. This choice was necessary due to the fact that the MIDG II relies on its GPS to determine altitude. In the event that GPS communication is lost, this data would no longer be valid.

Lateral Velocity and Altitude Sensor Autonomous takeoff and landings require precise and uptodate position and velocity information. Because the MIDG II uses accelerometers, it has difficulty detecting motion at a constant velocity. To aid in with autonomous takeoff and landings, a vision based velocity censor has been developed inhouse. This sensor uses a small on board computer, video camera, and a laser diode or ultrasonic ranger to determine a three dimensional velocity vector and provide altitude information during the landing approach. The sensor velocity accuracy is within 2cm/s at an altitude of 1 meter and improves at lower altitudes and has a data update rate of 30Hz.

Lateral Velocity and Altitude Sensor


Communication The Communication System provides a link to transfer commands, parameters, and sensor data between the FCS and the GCS. A MaxStream 900Mhz OEM RF module was selected for the FCS side of the link and communicates at 9600 baud. This OEM module was chosen for its ability to mount directly on the FCS motherboard, significantly reducing the total size and weight of the FCS when compared to the use of an external modem.

MaxStream OEM RF Module

UAV Safety Switch The AUVSI mission rules state that the vehicle must be capable of manual override by the safety pilot at any time. To achieve this objective, a UAV safety switch from Electro Dynamics is utilized. This device is essentially a 5 channel, optically isolated switch for PWM signals. One set of inputs is from the RC receiver (safety pilot) and the other set is from the FCS. The set of signals that are forwarded to the servos is controlled by the auto/manual channel from the RC receiver. This allows control of the helicopter to be selected independently of the FCS. Even if the FCS fails completely, the safety pilot can regain control. This device operates on a separate power supply from the FCS to further minimize risk of failure.

UAV Safety Switch


Overall FCS Architecture The overall configuration of the FCS components is shown below.

FCS Architecture

Ground Control System The AUVSI rules state that the GCS must be capable of the following features: 1) The system will display all no fly zones and search areas to the operators and judges. 2) The system will display vehicle position with respect to no fly zones and search areas. 3) The system will display current vehicle altitude above mean sea level. 4) The system must be capable of dynamically adding waypoints and modifying the vehicle’s

flightpath.

The ground control system software has been designed to meet these requirements in a user friendly manner.

GCS Server The Ground Control System consists of a server component and a client component. The server connects to a radio modem on the ground to communicate with the helicopter FCS. The server component is responsible for sending all commands and parameters to the FCS and receiving and processing all sensor readings/data. The server allows a potentially unlimited number of specialized Ground Control clients to connect to the system and coordinates connections between the clients and the helicopter FCS.


GCS Control Client The control client consists of the following components:

• Map Display • Telemetry Panel • Telemetry Gauges • Parameter Adjustment Forms

GCS Control Client

Map Display The map display features a satellite image of the flight area overlaid with the vehicle’s current position, heading and flight path, the competition search areas, no fly zones and identified targets. Flight control operators can add new waypoints and modify the flight path using a simple point and click interface. Search areas and no fly zones can be modified in a similar manner. When entering waypoints, the interface will take no fly zones into account to prevent the operator from planning routes that would lead the vehicle out of the competition area. Waypoint changes will automatically be uploaded to the helicopter and confirmed on the map display.

Telemetry Panel The left hand side of the control client displays the following telemetry data:

• Mode of vehicle operation (Manual/Autonomous) • Vehicle Altitude (above sea level and above ground level) • Vehicle Position (Latitude, Longitude) • Vehicle Orientation (Pitch, Roll, Yaw) • Vehicle Velocity Info (Air Speed, Ground Speed, Ground Track) • Vehicle Battery Levels and approximate flight time remaining


• Status of wireless communications link • Diagnostic information

Units of measure can also be toggled between English, Metric and Nautical where appropriate.

Telemetry Gauges Selected telemetry data is also shown in the form of graphical instrument gauges for easier reading by the operators and judges. The following gauges are available:

• Altimeter: Displays both current altitude and target altitude • Speedometer: Displays both current air speed and target air speed. • Compass: Displays vehicle’s current heading and ground track. • Attitude: Displays vehicle’s current pitch and roll.

Gauge position, size and units of measure can also be adjusted by the user.

Parameter Control Forms The control client also features collapsible parameter entry forms along the right hand side of the display. This allows for quick access to flight parameters without cluttering the display area. In addition to allowing adjustments to target air speed and altitude, the flight operator can make adjustments to PID control parameters and servo trim values if necessary.


Intelligent Reconnaissance Imaging System

The AUVSI rules state that the Imagery System must be capable of the following features: 1) The system will locate and display images of targets to the judges. 2) The system will be able to identify target parameters from various altitudes. 3) The system will be able to capture images and identify target parameters up to sixty degrees

from directly below the vehicle. 4) The system will be able to provide actionable intelligence: imagery, location, and parameters

to the judges.

The requirements above dictate that the system must be equipped with an imaging system that is capable of locating targets and identifying unique features of those targets. Furthermore, the system must be highresolution, be able to be pointed independent from the vehicle’s orientation, and ideally, be able to provide images in realtime. To provide this realtime, actionable intelligence, the team has developed an Intelligent Reconnaissance Imaging System, or IRIS. The IRIS components include a digital camera, gimbal, video system, control unit, and ground station software, as shown in the figure below.

Intelligent Reconnaissance Imaging System Architecture

Camera The team elected to use the Canon S2 IS digital camera. The S2, known as an “advanced” point and shoot camera, was selected over smaller point and shoot cameras and more sophisticated SLR cameras for its greater zoom capability, faster lens and shutter speed, and its lighter weight and lower cost when compared to SLR cameras. The camera’s image resolution is capable of identifying target parameters from high altitudes; from 500’, the image will have a pixel


resolution of less than 1.5”. The table below illustrates the effective distance per pixel for images captured at various altitudes.

Altitude Horizontal Vertical Dist/Pixel X Dist/Pixel Y (Feet) Feet Inches Feet Inches Feet Inches Feet Inches 100 62.5 750 41.6 499.2 0.024 0.289 0.021 0.257 200 125 1500 83.3 999.6 0.048 0.579 0.043 0.514 300 187.5 2250 125 1500 0.072 0.868 0.064 0.772 400 250 3000 166.6 1999.2 0.096 1.157 0.086 1.028 500 312.5 3750 208.3 2499.6 0.121 1.447 0.107 1.286

Video System

Video Tx/Rx The camera is utilized twofold; in addition to taking high resolution images, the camera’s composite video out (TV connection) is used to transmit what the camera sees to the GCS using the 1.2 GHz video transmitter. The ground station portion of this link consists of a wireless video receiver connected to a high gain flat panel antenna. The video system aids operators with target identification and discrimination.

Digital Video Recorder To assist the video system operator in his or her task of reviewing the incoming images in real time, the video system includes a commercial Digital Video Recorder (DVR). The DVR allows the operator to not only record the video of the entire flight, but also to pause, rewind, and fast forward the live video stream without disturbing the recording. Thus, at any time, when the operator sees something that may be of interest, he or she can stop the playback and rewind to the point where the object was seen. Then when identification of the object is complete, the operator can fastforward through the recorded video to resync with the live stream.

Gimbal A unique gimbal system is constructed of a lightweight, highstrength material known as G10. The gimbal provides two axes of movement, pan and tilt, with 360 and 180 degrees of motion, respectively. The gimbal structure also acts as the landing gear, elevating the aircraft and allowing the camera to be mounted directly under the drive shaft, the area of least vibration and instability. Additionally, the entire landing gear pans with the camera, ensuring that the landing gear never obstructs the camera’s field of view.

Control Unit The IRIS control unit (ICU) is a small single board computer (SBC) which includes a 400 MHz processor running a Linux distribution. The ICU provides serial and USB communication ports, and is outfitted with a 2.4 GHz WiFi communication package. The FCS communicates with the ICU through the serial port, sending ground station commands and telemetry data. Communicating with the camera through USB, the ICU sends commands and can also modify


camera stored images to embed telemetry data in the image EXIF data. This is essential for imagery analysis on the ground.

Wireless Image Transfer The Wireless Image Transfer system is capable of transmitting highresolution images from the aircraft to the ground station in real time. The 802.11g 2.4GHz system is capable of transferring large amounts of data over large distances. Airborne components of the system include the integrated Wifi on the ICU, a 250mW amplifier, and a highgain omnidirectional antenna. A directional antenna, amplifier, and COTS wireless router are the ground based portion of the system. The router is configured with custom software that optimizes transfer parameters, such as packet size and transmit power based on link status. The system is designed to be capable of image transfer from any location within the mission area of operation.

IRIS Workstations and Software Imagery collection and analysis computers are organized in a serverclient architecture much like the Ground Control system. The imagery server (which doubles as an analysis workstation) is connected to the Wireless Image Transfer system and collects images from the IRIS Control Unit. As images are loaded into the server, custom software parses the EXIF telemetry and location data. Using this information, the image is orthographically rectified and geographically located to allow for easy interpretation by the analysts.

The imagery analysis client software displays highresolution images from the server and is capable of displaying the realtime video feed. In addition to these displays, analysts are provided an overview map of the mission area and several pointandclick tools. Analysts can pan, rotate, and scale images, measure features, and process and classify targets. Automated target filing and error checking ensure that analysts provide the most accurate and complete intelligence, even when facing the stress from the mission. The software is capable of creating individual “target reports” on the fly, as well as producing a final report of all target locations and parameters.

Imagery Analysis Client


Power System In order to power the Helo UAS, several different batteries are required. The most critical of these are the 18.5 volt lithium polymer batteries used to power the helicopter’s motor and the 5 volt nickel metal hydride receiver battery. The 5V NiMH battery also powers the helicopter servos and the UAV safety switch. The FCS, including the radio modem and sensors, are run off of an 11.1V LiPo battery. The IRIS system is powered from a 14.8V LiPo and uses two battery elimination circuits (BECs) to step down the voltage to the required levels. The overall configuration of the power system is shown below.

Helo UAS Power System


Operational Setup In order to increase efficiency, team members will be given specific tasks and work in parallel to complete the mission. In addition to the RC Safety Pilot, the mission team will consist of the following members:

• Flight Control Operator • Video System Operator • Two Intelligence Analysts

During the competition they will work in and around the team’s trailer as shown below.

Flight Control Operator The flight control operator will be in charge of running the GCS Control Client and communicating with the RC safety pilot. He will be responsible for entering waypoints, adjusting target altitude and airspeed and plotting flight paths through the search area. Additionally, he will make any changes to the nofly zones and search areas as required by the judges during the mission. The flight control operator will be located closest to the RC safety


pilot to allow for efficient communication about the status of the helicopter. Should a problem be detected with the FCS or wireless link, he can quickly notify the RC safety pilot to take manual control. A second monitor will be connected to the flight control operator’s computer in order to allow the judges to easily view the current status of the mission at any time.

Video System Operator The video system operator will be located inside the trailer and be in charge of running the video system. He will be able to control the level of autonomy of both the camera aiming process and the image capture process. By analyzing the live video feed, he will be able to flag certain locations as high or low priority based on the presence of potential targets. By flagging these locations, the flight control operator and the intelligence analysts will be automatically notified through their respective GCS clients. The flight control operator can then modify the flight path to get a closer look at the targets if necessary.

Intelligence Analysts Two intelligence analysts will also be located inside the trailer and be in charge of analyzing the high resolution images captured by the IRIS. When they locate a target, they will record all the relevant parameters (size, shape, location, etc) and prepare actionable intelligence for the judges. Like the video system operator, they will also have the ability to flag certain areas for further analysis.

Safety

Failsafe flight modes The flight control system incorporates additional flight modes that ensure maximum safety in the event of a system failure. These flight modes can also be activated by the safety pilot or the Flight Control Operator.

Return Home The return home system allows the aircraft to safely fly to a set of GPS coordinates designated as the home location. Home coordinates are set automatically during autonomous takeoff unless manually entered by the Flight Control Operator before flight. The coordinates can also be modified after takeoff if necessary.

Stability Augmentation Mode Stability augmentation is an additional mode of operation intended to assist the RC safety pilot in returning the helicopter safely, should it be necessary. In this mode of operation, the FCS will hold the last set altitude, heading and orientation set by the RC pilot. The RC pilot will be able to command changes in the FCS parameters using the RC transmitter. For instance, increasing the throttle/collective stick will command the FCS to increase the target altitude. Returning the stick to center will lock in the helicopter’s current altitude. Similar adjustments can be made to the helicopter’s heading and orientation. This is similar to the stability augmentation mode of the Yamaha RMAX Helicopter.


Flight Terminate The exact definition of flight termination depends on status of available sensors and batteries. In any flight termination event, the aircraft will conduct a minimum energy landing within the specified 500 ft radius.

Batteries Battery voltages are continuously monitored by the FCS (and transmitted to the GCS) to ensure they remain in safe operating regions. Safe operating thresholds are calculated based upon the current aircraft distance from the home location, with an additional safety margin. If any battery voltage drops below this safety threshold, the FCS enters Return Home mode. In the event that the flight critical motor batteries become depleted before the aircraft is able to return home, the aircraft will perform a minimum energy landing.

Transmission Loss Transmission loss events have been divided into two categories depending on the type of signals lost. Loss of signal from the Safety Pilot RC transmitter results in the UAV switch entering FCS mode. This event also triggers the FCS to enter Return Home mode. Transmission loss from the GCS can be detected by both the FCS and GCS. In this event, the FCS enters Return Home mode and the Flight Control Operator is notified.

Failure Modes Analysis

Failure Response Loss of RC Communication (Initial)

Ground operators are notified and FCS enters return home mode.

Loss of RC Communication (30 sec)

Aircraft will execute flight termination procedure.

Loss of GCS Communication (30 sec)

Ground operators are notified and FCS enters return home mode.

Loss of GPS Link (Initial) Aircraft will hold zero degrees pitch and roll and attempt to maintain a hover. If helicopter begins to drift into a no fly zone, RC safety pilot will take manual or stability augmented control.

Loss of GPS Link (1 min) RC safety pilot will take manual or stability augmented control. Loss of IRIS Link Image system will continue to capture images automatically.

Images will be analyzed after landing. FCS, Receiver or Motor Battery approaches minimum safety level

Ground operators are notified and FCS enters return home mode. RC safety pilot may choose to take manual or stability augmented control.

Motor or Receiver Battery voltage falls below critical level

Aircraft will execute flight termination procedure.

FCS battery dies RC safety pilot will take manual control.


Conclusion Based upon performance of the 2007 AUVSI competition, a helicopter was chosen again as the ideal vehicle for the mission, primarily for its ability to loiter in a given location and efficiently gather actionable intelligence. The flight control system is capable of autonomously flying the aircraft, while the imagery system provides actionable intelligence to ground control operators and judges. Furthermore, an iterative testing process with emphasis on safety has served to minimize the risk of failure. Improvements to the flight control system with the addition of new features have continued to make the system more robust. The team has recognized the educational value of the project and is proud to be participating in the 2008 AUVSI competition. In conclusion, the VCU Helo UAS Team believes they have been successful in creating a complete Unmanned Aircraft System capable of meeting the objectives of the AUVSI competition.

Acknowledgements The Helo UAS Team would like to acknowledge the following individuals for their support.

Dr. Robert H. Klenke, Advisor Mr. Mark Sternheimer Mr. Mark Schon Mr. Brian Thomas

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