ICAS 2002 CONGRESS AN AUTONOMOUS CONTROL TECHNIQUE FOR LAUNCHING SHIP BASED UNMANNED AIR VEHICLES (UAVS) IN EXTREME CONDITIONS M.R. Crump , C. Bil Sir Lawrence Wackett Center for Aerospace Design Technology, RMIT University GPO Box 2476V Melbourne 3001, Vic. Australia. Keywords: UAV, launch, control, N4SID Abstract intensive and prone to both technical and hu- man factor complications which can result in This paper presents a solution to the au- the complete loss of the aircraft and full mis- tonomous control problem for launching un- sion failure. This situation is exacerbated manned air vehicles from maritime platforms by extreme environmental conditions (both in extreme conditions. A controller is de- oceanic and atmospheric). Autonomous con- veloped utilising both linear robust modern trol of the UAV during the launch procedure control theory (H1 methods) and a new ap- has been identi¯ed as a key technological re- proach involving trajectory optimisation and quirement for ship based ¯xed wing UAV op- system identi¯cation. The resulting controller erations. is found to allow the aircraft to fly to its phys- The major problem identi¯ed when ical limits. This controller has been found to launching a small UAV from a ship deck have very good robustness properties through results when a large tailwind is present. This extensive simulation. large tailwind reduces the aircraft airspeed during the launch process, resulting in both 1 Introduction loss of controllability and aircraft stall, either of which can be catastrophic during launch. The ability to launch ship based Unmanned As opposed to launch from conventional Air Vehicles (UAVs) in extreme sea conditions runways, where multidirectional launch capa- would greatly extend the usefulness of these bility is generally present, a relatively small aircraft for both military and civilian mar- ship deck presents limited launch directions itime purposes. Currently the majority of ship and it is undesirable to manoeuvre the ship based UAVs are either VTOL aircraft or ¯xed to allow a feasible launch direction. Thus, wing aircraft remotely launched under limited launch in these tailwind conditions becomes conditions. The advantages of ¯xed wing air- necessary in some situations. craft over VTOL aircraft in terms of flight Other problems identi¯ed include large speed, range and endurance are well known, amounts of turbulence through the ship air- and can be of great bene¯t to various missions. wake presenting large disturbances to the air- Thus there exists a need for ¯xed wing UAV craft which a controller (or human pilot) must operation o® maritime platforms. be able to cope with. Remote launch procedures involving an ex- This paper presents a control and guid- ternal human pilot are generally very labour 562.1 M.R. CRUMP , C. BIL ance strategy that allows autonomous launch located on the twin vertical tails. The simula- of a ¯xed wing UAV under a large set of en- tion model has been slightly modi¯ed to give vironmental conditions, including conditions a slight increase in aerodynamic performance exhibiting the turbulence and tailwind prob- around the high angle of attack region. The lems. It is found that the limiting conditions modi¯ed model includes hypothetical aircraft for launch when implementing this controller stall and post-stall e®ects, nonlinear actuator are physical limits that are set by the air- e®ects (including rate and positional satura- craft dynamics and not those set by the con- tion) and sensor limitations. troller. Thus, the controller allows aircraft The ship model is based upon seakeeping launch through the entire physically possible theory [10] in conjunction with measured data aircraft launch envelope. [6] and some results of simulations based upon This controller is developed using a com- a generic frigate type vessel. The simulation bination of both modern linear robust con- allows prerecorded (or arti¯cially generated) trol techniques (H1 ) and a newly proposed motion data to be used, or simulates generic control method. The robust linear controller ship motion using a six degree of freedom sim- controls the aircraft through the majority of ple harmonic motion model. The model takes the launch phase, guiding the aircraft along account of the ship heading with respect to a nominal trajectory whilst rejecting external wave heading, sea state, ship speed and ba- disturbances in the form of turbulence from sic ship characteristics (such as length). The the ship airwake. The controller has been de- ship model is that of a generic frigate, typical signed using a variation of the H1 mixed sen- of the Australian designed ANZAC Frigate or sitivity approach. the Oliver Hazard Perry Class FFG's in ser- Control for high tailwind launches is pro- vice with the Australian Navy. vided initially by a Trajectory Optimisation The launcher model uses linear dynamics with Inverse System Identi¯cation (TOISI) to accelerate the aircraft while constraining controller. This controller guides the aircraft aircraft rotation in all dimensions and motion through a minimum altitude loss trajectory, in the ramp normal directions. This simulates whilst maintaining a nominated climb angle. an attachment between the launcher and air- Once a safe climb regime has been obtained, craft, that is released at the desired launch the controller switches to the linear robust point. The model can also simulate a 'free' controller, which guides the aircraft to a safe launcher, in which the aircraft is free to lift o® altitude where a mission controller may oper- the ramp when it has attained su±cient speed. ate. The simulation model also has the capability This technique has been tested comprehen- to simulate rocket assisted takeo®, removing sively in nonlinear simulations of the aircraft the need for a launch ramp. and has been found to follow the physically The atmospheric model contains steady optimal launch trajectory in the presence of state wind components, turbulence spectra both uncertainty in the simulation models and (based upon Dryden spectra shown in MIL-F- external disturbances due to turbulence. 8785C [1]), gust components and windshear el- ements which are based upon the e®ects of the 2 Dynamic Models. ship airwake and ocean surface e®ects. These approximations are included as there is lit- The aircraft used for development of the con- tle published data on ship airwakes (however troller is based upon the University of Sydney there are current studies [5, 7]). UAV Ariel [9]. This aircraft is a conventional Further details of all the models used may ¯xed wing UAV with four main control inputs, be found in Crump [4, 2, 3]. throttle, elevators, ailerons and twin rudders 562.2 An Autonomous Control Technique for Launching Ship Based Unmanned Air Vehicles (UAVs) in Extreme Conditions 3 Control Objectives needs to be controlled. the main objective is to maintain a positive altitude. This implies When launching any aircraft, several objec- that altitude needs to be controlled, however tives arise. The ¯rst and main objective is the accuracy of various altitude measuring de- to maintain a positive altitude. Several other vices is somewhat questionable, so controlling sub-objectives may also be speci¯ed. The ¯rst altitude using these devices may not be fea- is to gain altitude as rapidly as is safely pos- sible. Instead, the altitude can be indirectly sible. The second is to maximise the distance controlled by maintaining a positive rate of between the aircraft and the ship. climb. Whilst it is possible that the UAV mission Whilst maintaining a climb angle, it is also controller is capable of launching the aircraft important to maintain the aircraft's airspeed, autonomously, it is unlikely that it will suc- and as such a control on the aircraft's velocity cessfully ful¯ll the launch objectives in an op- is also important. To simplify the flight condi- timal manner. Considering the fact that the tion, it is also desired that the aircraft is flying majority of flight control in today's aircraft is in the conventional upright position with lit- digital, the addition of another controller to tle bank angle and sideforce. To this end, a speci¯cally control the aircraft during launch simple bank angle control will also be imple- should not prove too cumbersome in terms of mented with a sideforce regulator (if desired physical implementation. these two controllers may be changed into a The ¯rst step in the design of a launch con- heading hold controller). troller is to determine the desired launch tra- The control of a UAV during catapult jectory that will be followed in the nominal launch is a problem involving rapidly chang- case (this trajectory will change according to ing dynamics over a short period of time, ambient conditions). This trajectory should corresponding to the ¯rst few seconds after maintain a positive altitude at all times, with ramp departure, followed by a longer period as high a rate of climb (ROC) as possible dur- of slowly changing dynamics during the air- ing the early stages of the flight. Maintain- craft climb. This problem suggests itself to a ing this high rate of climb will require a high dual control solution. angle of attack, so in order to prevent stall A highly robust nominal controller de- occurring regularly (due to gusts and turbu- signed using linear robust modern control the- lence), this rate of climb should be reduced ory is implemented for use when the aircraft is slightly to give a margin of safety. This rate flying well within the linear flight regime. This of climb should also be a sustainable rate of controller should be capable of guiding the air- climb, corresponding to a trimmed condition craft through the launch phase in nominal or of the aircraft. near nominal conditions, however it is unrea- For the case of the UAV Ariel, a launch sonable to expect this controller to be capable speed of 30 m/s with a rate of climb of climb of of handling the rapidly changing nonlinear be- 6.2m/s is chosen for the launch trajectory, giv- haviour when the aircraft is well out of its lin- ing a climb angle of 12±.