Tailless aircraft control using output subspace stabilization

This paper presents the application of Ontpnt Subspace’ !&abilization and Linear Adaptive Control Methdology to controlling tailess aircmft. The absence of a vertical stabilator and of a rudder reqnires combining the operation of several control surfaces to control each channel conversely each actuator contriie to the controlling of several control channels. As a result, the control problem is a Multiple InpntMultiple Ontpnt control problem, with very significant couplings between pit& roll and yaw channels. Application of Linear Adaptive Control Methodollogy’” circumvents the use of complicated nonlinear control techniques, and the attendsnt analysis uncertainties. The use of real-time dkhnbanceobserver~‘~ allows treating the entire response of the sy9teq with the exceptiou of the control term as a disturbance, and’to use the estimauon of the disturbance to “cancel-out” this ~esponse~~ and thereby effectively de-coupling the control channels while avoiding to have to model inter-chatmels coupling terms. Subspace st&ilizatiod” control allow to using the effects of one or of several actuators to steer the system error-state to a certain s&qace S (hyperplane) Which represents the prescribed ideal tracking error behavior, while suitably bounding the !abequent motion on the s&space S. Simulation results for a worked example demonstrate that the pre controller tracks prescribed trajectories accurately. 1. Introdnction Flying wings and tailess aimaft are design attempts to enhance combat ai.rm& operational performances, by the removing of vertical stabilators and the blending of the wing and fuselage. The downside of such design is the considerable redaction ofahcra8transversalliflslopeanddutchrolldamping New aim& aw beiug equipped with multiple redundmt control su&ceq that properly deflected can produc& requimi pitch, roll, and yaw moments. This 1Copyright01999bytheAmericanInstituteof Aeronautics and Astronautics, Inc. All rights reserved C. D. Johnson” The University of Alabama in Huntsville, Huntsville, AL 35899 cmflguration exhibits a considerable coupling of the control channels. Any pitch, roll, and yaw maneuver requires deflecting multiple aerodynamic ontrols such as elevons, pitch flaps, all moving tips, and eventual thmt vectoring. Conversely the deflection of any control actuator, afhxts multiple control channels. A solution to this problem” was presentedbased in H, and ~synthesis, which is the application of References12‘14 to the case of tailess aircmfl. Bufi@ton’5 has studied the opt&z&ion of the utilization of multiple rcxlmbt control smfaces. Several researcherPg have investigated the application of Sliding Mode Control as a means to achieve the desired robustness. Reference’g investigated thedesignofatailessaircraftcontroldesignbasedon sliding mode control. It does not require a detailed knowledge of the plant, which makes this technique robust. It is based on the property that daring high &eqnency switching (chattering) of the control, motion of the system state in the sliding surface remains insensitive to certain parameter TGW.illtiq nonlinearities and dktmbances. Discontimtous, bangbang control is used to move the system to the switching surface, and high-freqnency bang-bang control is used to keep it an-the sliding.su&ice thereafter. The ideal high-tkquency control is supposed filtered b the low-pass actuators, i.e. hydraulic jacks. The imperfect filtering of the high tiqnncy switching may excite vehicle structural modes, which is serious enough to prechtde the use of disconthmous relay controllers. The so-called smoothing sliding mode controls approach used in RefercncePg consists in substituting the relay discontinuous control by a satmation discontimmus control law. The linear porti0r.k of the control law corresponds to the opxation of the system in the close boundary layer of the surface. i Senior Scientist, Senior Member, AIAA ii Professor, Department of Electrical and Computer Engineering