444 A. Noth, W. Engel, and R. Siegwart 3 Navigation and Control System In order to reach the goal of the project, the autopilot design phase followed those principles: • select components not only according to criteria of precision and resolution, but as well of weight and power consumption to be suitable for the targeted application. • use as much as possible digital output and calibrated sensor to reduce the development time and avoid additional need of A/D converter, interface microcontroller, etc. • interface the sensors so that the central processor doesn’t have to wait on them but can access directly and rapidly to the information on request. This applies for example to the GPS. 3.1 Computer and Interfaces Sky-Sailor will fly autonomously using an onboard autopilot, only high level orders being given from the ground. The system is mainly based on a single board computer, the X-board < 861> which is a compact embedded PC design for low power consumption. Fig. 3. X-board < 861> single board computerfrom Kontron It includes aGeode SC1200 Processor,upto128 MbyteofDRAM and up to 128 Flash storage mediaonboard.Despite the compact size of abusiness card,itoffersalot of interfaces:integrated Graphics, Ethernet, USB, RS 232, I 2 C, audio The OS running on it is areduced Linuxdistribution,based on Debian, that onlycontains thenecessary features. 3.2 Sensors In Fig. 4, one can see the powergenerator system andthe autopilot, with all sensors and their interfaces to theX-board. Fig. 4. Schematic view of the powerand control partsofSky-Sailor Attitude The attitude and angle rate of the airplaneare given by the MT9-BIMU 7 at afrequency of up to 512 Hz. Such alow-costsensor is perfectly suffi- cient to performinertial navigation comparedtoheavier one[6]. It contains accelerometers,magnetometers, gyroscopes and communicatesthrough serial port (RS232)with the X-board on whichdata fusion is executed. In the future versionofthis device,the sensor fusion will be done by aDSP chip, reducing the computationalcost on thecentral processor of the autopilot. VGAcamera One direction of the projectistoachieveautonomous navigation based on vision, using SLAM techniques as shownin[1] [2].One or more lightweight VGAcameras willgive680 x480 images of thelandscapeand allow localiza- tion and mappingofthe terrain. Efforts will be done in thisdirection in the following month.Cameras are connected to thecentral processor via USB. Absolutex-y position andaltitude The absolute position is given by an ultralow powerGPS sensor withpatch antennafrom Nemerix. This sensorconsumes only61mWfor aweightof12.36 gr. In terms of positionaccuracy,95%/99.7 %ofthe time, the estimated positionlies within 2.7933 m/4.2028 mrespectivelyofthe actualposition. 7 Inertial Measurement Unit Design of an Ultra-lightweight Autonomous Solar Airplane 445 446 A. Noth, W. Engel, and R. Siegwart A future version will accept WAAS/EGNOS correction for more precise mea- surements. The data are sent on a serial port at a fixed rate of 1 Hz to a microcontroller that decodes the NMEA protocol, stores the value internally and sends them on demand to the main processor via I 2 C. The same microcontroller interfaces the altitude pressure sensor MS5534. Pressure and temperature values, as well as four calibration factors allow the computation of the altitude with a resolution of 1 m. The relation between pressure and altitude being variant with the atmospheric condition, the mi- crocontroller will achieve data fusion, using the GPS altitude as an absolute value to correct the drift of the MS5534. Airspeed The airspeed sensor DSDX is a differential pressure sensor, with digital I 2 C readout and temperature compensated. It is connected to a Pitot tube fixed at the attack border of the wing. 3.3 Ground Control Station The control of the airplane is executed onboard but there is a link to a ground control station through a serial radio modem that allows a baudrate of 9600 bps. The goal is to: • download and upload airplane and control parameters, but as well the flight plan, before the takeoff, • get a visual feedback of the state of the airplane once airborne, modify flight plans on-the-fly, • retrieve and record the telemetry for flight analysis, system identification, etc. Fig. 5. Ground control station and it’s graphical user interface The GUI 8 wasdevelopedwithQTgraphical libraries under Linux (Fig.5). It is composed of three main layers which ensures modularity: 8 Graphical UserInterface • the graphical interface, that allows a visual overview of the state of the airplane and its position on a 3D map of the terrain. • a second layer which processes data and control the GUI • a communication module that receives and sends the data in packets to the airplane through the serial port connected to the radio modem. Control of the airplane from the ground As shown in Fig. 4, the commands given to the servos can come from the au- topilot or a human pilot on the ground using an RC transmitter. The ”servo board” decodes the PPM 9 from the RC source and get the value given by the autopilot through the I 2 C bus. Based on one additional channel on the RC remote, it switches from one source to the other. It is also possible, for control tuning purpose, to mix sources and, for example, allow the autopi- lot to command only the elevator while the other actuators are commanded manually. 3.4 Autopilot Design Results The final design leads us to a navigation and control system with a total mass of 140 g for a consumption of around 4 W. One can see that 6/8 of the power is used by the X-board and 1/8 for the transmission, the rest being used by the sensors. Table 1. Autopilot power and mass distribution Part Weight[g] Powerconsumption [W] X-board 22 3.00 Mother Board 22 - IMU 14.5 0.21 VGACamera 0.55 0.1 GPS 12.4 0.061 Altitude sensor board 20.03 Airspeed sensor board 30.03 Radio-modem240.5 Antenna 19.6 - Cables, connectors 20 - Total 140g 3.93 W Globally, the autopilot represents 5% of the total mass of theairplaneand uses 20% of the power. 9 Pulse PeriodModulation Design of an Ultra-lightweight Autonomous Solar Airplane 447 448 A. Noth, W. Engel, and R. Siegwart 4 Simulation of the Solar Flight For the validation of a long endurance solar flight, a simulation was real- ized under Matlab Simulink. Fig.6 represents the schematic of the model that includes first the irradiance model based on [12] and depending on the geo- graphic position, time and solar panels orientation. We then take into account the surface of solar cells, their electrical efficiency and the efficiency of the con- nection configuration. For the MPPT, the electrical and algorithm efficiencies are taken into account. The power consumption is the addition of the autopi- lot power and the power needed for flight, which was measured in the case of levelled flight and climbing phase. Depending on the irradiance conditions and the consumption, the battery is charged or discharged, taking into account the efficiency of the energy transfer. Fig.6. Schematic of the simulation model under Matlab Simulink 4.1 Study of Various Scenarios The simulation environment allows to test different flight strategies in order to accomplish a long endurance flight and analyze the benefit of a climbing phase or the influence of the other parameters on the feasibility of a multi-days flight. We will present here two scenarios. In the first simulation, Sky-Sailor starts a flight at EPFL location on the 21th of June with an empty battery, keeping always the same altitude. The two graphs below show the evolution of the power distribution during 48 hours. With good sun conditions, the battery is fully charged at 13h30. At this moment, the MPPT measures that the battery voltage reaches the maximum Fig. 7. Powerdistribution on Sky-Sailor duringlevelled flight Fig. 8. Battery charge/discharge current and energy during levelled flight voltageof33.7 [V] and adapts the maximumpowerpointtoavoid overcharge. In this phase, the total amountofenergy that is not used but that could be retrievedfromthe solar panels reaches 92.5 [Wh]. During the night, the battery suppliesthe all airplanebut at 5h10 it is totally discharged. Another strategy is to better use the energyafterthe battery charge by increasing altitude. Fig. 9, 10 and 11 showthe same scenario presented before but with aclimbingphase until 2000 [m]. Basically,Sky-Sailor usesthe additional energy to gainaltitude at 0.3 [m/s] using an electrical powerof40[W]. Having reached 2000 [m],itstays at this altitudeuntil the energy is not sufficientanymore for levelled flight. At this point, the motor is turned off and thedescentstarts. Finally,atthe most critical pointat6h13 in the next morning, the batterystill hasacapacity of 4.7 [Wh] and thecharging process starts again. Globally,the unused energy during the dayis61.5 [Wh]. Design of an Ultra-lightweight Autonomous Solar Airplane 449 450 A. Noth, W. Engel, and R. Siegwart Fig. 9. Powerdistribution on Sky-Sailor duringflightwith climbing phase Fig.10. Altitude during flightwithclimbing phase Fig. 11. Battery charge/discharge currentand energy during flightwith climbing phase 5 Status of the Project and Future Work The mechanical structure of the airplane is actually ready, it has been success- fully tested and validated in terms of power and stability. The solar generator, composed of the solar modules and the MPPT, is in the integration phase on the wing. Concerning the autopilot, the different parts of the system are being as- sembled and all functionalities will be tested during the first half of this year. In the summer, we should have achieved many flights and experiments to clearly evaluate the capabilities of our UAV. 6Potential Applications Small andhigh endurance UAVs find uses in alot of variedfields, civilian or military. The civil applications,leaving sidethe military ones, could include coast or border surveillance, atmosphericaland weather researchand predic- tion,environmental, forestry,agricultural,and oceanic monitoring, imaging forthe media and real-estateindustries,and alot of others. The target mar- ketfor thefollowing years is extremely important[11]. The great advantagesofSky-Sailor compared to othersolutions would be withoutany doubt its capabilitytoremain airborne for avery longperiod,its lowcost and thesimplicitywith whichitcan be used and deployed, without anyground infrastructure for thelunchsequence. As an example, in thehypotheticalcase of forest firerisks during awarm period, adozen Sky-Sailor,easily launched withthe hand, could efficiently monitor an extended surface, looking for fire starts. Afast report would allow arapidintervention and thus reduce the cost of suchdisaster, in termsof human and materiallosses. Sky-Sailor would be as well avery interesting platformfor academicre- search,inaerodynamics or control. 7Conclusion In this paper, thedesign of an ultra-lightweightUAV waspresented, including details about it’s mechanicalstructure, the solar generatorand the autopilot system. The approachadopted doesn’t aimonly at building an efficientautopi- lot, but alsokeeps in mindit’s futureapplication.This is done by designing and selectingall the parts to obtain alightweightand low-powerairplane.We plantoperform the firstexperiments withthe autonomous airplane during the first half of this year and along endurance flightthis summer. Design of an Ultra-lightweight Autonomous Solar Airplane 451 452 A. Noth, W. Engel, and R. Siegwart Acknowledgement The authors would like to thank all the people who contributed to the defi- nition study, Samir Bouabdallah for fruitful discussions and advices on flying robots, Walter Engel for the realization of the mechanical structure and all the students who worked or are working on this project. References 1. DavisonAJ(2003) Real-time simultaneous localization and mapping witha single camera, IEEEInt.Conf. on ComputerVision, ICCV-2003, pp. 1403-1410, Nice (France), October2003 2. LacroixS,Kung IK(2004) High resolution 3D terrainmapping withlow alti- tude imagery, 8th ESA Workshop on Advanced Space Technologies for Robotics and Automation (ASTRA’2004), Noordwijk(Pays-Bas),2-4 Novembre 2004 3. Eisenbeiss H(2004) Amini unmannedaerialvehicle(UAV): systemoverview andimage acquisition, International Workshop on ”Processingand visualization using high-resolution imagery” 18-20 November 2004,Pitsanulok, Thailand 4. Kim J H, Sukkarieh S(2002) FlightTest Results of GPS/INS Navigation Loop for an Autonomous Unmanned AerialVehicle(UAV), The 15th International Technical Meeting of the Satellite Division of the Institute of Navigation (ION) 24-27 September, 2002, Potland, OR,USA 5. KimJ H, Wishart S, Sukkarieh S(2003) Real-time Navigation, Guidanceand Control of aUAV using Low-cost Sensors.InInternationalConference of Field and Service Robotics (FSR03), Japan, July 2003. 6. Brown AK,LuY(2004) Performance Test Results of an Integrated GPS/MEMS Inertial Navigation Package, Proceedings of ION GNSS 2004, Long Beach, CA, Sept. 2004 7. AtkinsEMetal. (1998) Solus:AnAutonomous Aircraft for FlightControl and TrajectoryPlanning Research, Proceedings of the AmericanControl Con- ference, Pennsylvania, June 1998 8. Johnson ENetal. (2004) UAVFlight Test Programs at Georgia Tech,Pro- ceedingsofthe AIAA UnmannedUnlimitedTechnical Conference, Workshop, and Exhibit, 2004. 9. Granlund G(2000) Witas: An intelligent autonomous aircraftusing activevi- sion. In Proceedings of the UAV2000 International Technical Conference and Exhibition, Paris, France, June 2000. EuroUVS 10. DeGarmo M, Nelson GM(2004) Prospective Unmannedaerial vehicleopera- tionsinthe future national airspace system, AIAA 4th Aviation Technology, Integration and Operations (ATIO)Forum,20-23Sept 2004,Chicago 11. Wong K.C, Bil C(1998) UAVs overAustralia -Market And Capabilities,Paper No. 4, Proceedings of the 13th Bristol International ConferenceonRPVs/UAV s, Bristol, UK, 1998 12. DuffieJA,BeckmanWA(1991) Solar Engineering of Thermal Processes, SecondEdition. NewYork: Wiley-Interscience. Control and Guidance for a Tail-Sitter Unmanned Air Vehicle R. Hugh Stone School of Aerospace, Mechanical and Mechatronic Engineering, Building, J07, University of Sydney, NSW, Australia 2006 hstone@aeromech.usyd.edu.au Summary. This paper details the control and guidance architecture for the T-Wing tail-sitter unmanned air vehicle, (UAV). The vehicle uses a mixture of classical and LQR controllers for its numerous low-level and guidance control loops. Different controllers are used for the vertical, horizontal and transition flight modes, glued together with supervisory mode-switching logic. This allows the vehicle to achieve autonomous waypoint navigation throughout its flight-envelope. The control design for the T-Wing is complicated by the large differences in vehicle dynamics between vertical and horizontal flight; the difficulty of accurately predicting the low-speed vehicle aerodynamics; and the basic instability of the vertical flight mode. This paper considers the control design problem for the T-Wing in light of these factors. In particular it focuses on the integration of all the different types and levels of controllers in a full flight-vehicle control system. Keywords: UAVs, Guidance, Control, VTOL, Tail-sitter 1 Introduction The T-Wing is a VTOL UAV that aims to combine the greater efficiency of wing-born flight with the operational flexibility offered by VTOL configura- tions such as the helicopter. The T-Wing is a tail-sitter twin-engined vehicle and is controlled during vertical flight via propeller-wash over its wing and fin-mounted control surfaces. In this respect it is similar to the early manned tail-sitter vehicles of the 1950s, the Lockheed XF-V1 and the Convair XF-Y1 [1, 2]. This allows the T-Wing to be substantially less complicated than other “convertiplane” configurations such as the tilt-wing, tilt-rotor or tilt-body. In overall configuration the T-Wing is most similar to the Boeing Heliwing of the early 1990s [3]. This was also a twin-engined vehicle but unlike the T-Wing used helicopter cyclic and collective pitch controls during vertical flight. A picture of the T-Wing vehicle during fully autonomous vertical mode flight is shown in 1(a), while a diagram of its typical flight profile is given in 1(b). P. Corke and S. Sukkarieh (Eds.): Field and Service Robotics, STAR 25, pp. 453–464, 2006. © Springer-Verlag Berlin Heidelberg 2006 [...]... Flight profile Fig 1 T-Wing Vehicle in full autonomous hover flight and Flight Profile Styrofoam balls attached to fins are for tip-over protection during initial vertical flight testing 2 Basic Vehicle Model For the purposes of simulation and control design, The T-Wing vehicle is modeled as a standard 6-DOF non-linear rigid-body aircraft model The modeling is done in standard body-axes centered at the... modular solution to deal with the different aspects of control The focus of this P Corke and S Sukkarieh (Eds.): Field and Service Robotics, STAR 25, pp 465–476, 2006 © Springer-Verlag Berlin Heidelberg 2006 466 D.T Cole et al paper will be on the current demonstrations of the Mission, Guidance, and Low-Level Control Modules in particular Although the algorithms presented are generic in terms of hardware... 2002 [5] J How, E King, and Y Kuwata Flight demonstrations of cooperative control for uav teams In AIAA 3rd ”Unmanned Unlimited” Technical Conference, Workshop and Exhibit, Chicago, Illinois, September 2004 [6] J.H Kim, S Wishart, and S Sukkarieh Real-time navigation, guidance, and control of a uav using low-cost sensors In The 5th International Conference of Field and Service Robotics, pages 95–100,... vehicle, and are then used (with judicious saturation limits) to generate appropriate W and V velocity commands for the elevon and rudder controllers respectively Height errors are similarly used to generate vertical velocity commands to the throttle controller, while pointing errors are used to supply vertical roll-rate commands to the aileron control-circuit 5 Horizontal Flight Control and Guidance... pitch and yaw rate controllers for its elevator and rudder control surfaces as well as a roll-rate controller for the ailerons Speed control is via throttle All these controllers are designed using classical SISO root-locus techniques The form of the pitch and yaw rate compensators are as given in equation (5) for the pitch-rate controller δe = K(s + z1 )(s + z1 ) (Qcommand − Q) s2 As the pitch and yaw-rate... with the next waypoint and the off-axis error is converted into a desired sideways acceleration (in the LVLH frame) which in turn corresponds to a bank-angle command to turn the vehicle towards the waypoint This is similar to well-known missile “ proportional navigation guidance” coupled with bank-to-turn algorithms [?] While performing a banked turn, yaw rate and pitch rate are commanded to match that... discuss the on-board sensors and filters Sections 4 and 5 will introduce some aspects of the control and guidance of the vehicle for vertical and horizontal flight phases respectively, while Section 6 will deal with the transition modes Section 7 considers the flight-state logic and controller transition issues, before looking at the actual implementation of the control system and some flight-test results... in Matlab to auto-generate C code for integration into the State Machine 5 Testing and Evaluation This section outlines the various degrees of testing which occur for any change to a module within the system Generally there are 3 stages which are used for verification and validation: A C++ real-time simulator, a HardWare-Inthe-Loop simulation and actual flight tests 5.1 Real-Time Multi-UAV Simulator (RMUS)... This is useful for controldesign and is in-line with standard aircraft control practice The rationale for using Euler angles for control is threefold Firstly, their use allows the state variables (including attitude states) and controls to be separated into distinct longitudinal and lateral partitions Secondly, within these partitions, Control and Guidance for a Tail-Sitter Unmanned Air Vehicle 457... the most general problem of trajectory generation between two arbitrary postures Among other motivations, the appearance of higher P Corke and S Sukkarieh (Eds.): Field and Service Robotics, STAR 25, pp 479–490, 2006 © Springer-Verlag Berlin Heidelberg 2006 480 T.M Howard and A Kelly derivatives in the constraints allows for smoother transitions between adjacent trajectory segments 1.1 Motivation Future . the propeller generated forces in comparison to those due to the free-stream dynamic pressure. This change occurs as the vehicle goes from low-speed vertical flight (propeller and propeller slipstream forces. includes aGeode SC1200 Processor,upto128 MbyteofDRAM and up to 128 Flash storage mediaonboard.Despite the compact size of abusiness card,itoffersalot of interfaces:integrated Graphics, Ethernet, USB,. of its typical flight profile is given in 1(b). P. Corke and S. Sukkarieh (Eds.): Field and Service Robotics, STAR 25, pp. 453–464, 2006. © Springer-Verlag Berlin Heidelberg 2006 454 R.H. Stone The