AN H∞ DESIGN METHOD FOR THE PITCH ATTITUDE HOLD AUTOPILOT OF A FLYING UAV

Abstract

The paper presents an H∞ loop shaping method for the design of the automatic flight control system of an unmanned air vehicle's (UAV's) longitudinal dynamics. The design objectives include robust stabilization with respect to modeling uncertainties, pitch angle tracking of an ideal model and reduced sensitivity with respect to low frequency measurement errors. The design is illustrate by a case study for a flying wing UAV. The applications based on Unmanned Air Vehicle's (UAV's) received much attention over the last decades. Among them, the flying wing configuration of the UAVs was intensively analyzed due to their advantages concerning the reduced drag and the fuel consumption. A disadvantage of this configuration with respect to the classical one is its instability which fact requires an automatic control system able to stabilize the aircraft and to provide acceptable maneuverability qualities. The design problem of such a system has been previously considered in the control literature (see e.g. (1), (2), (3)). The proposed design methods include both classical techniques based for instance on the well- known PID (Proportional Integral Derivative) control laws, optimal approaches based on the linear quadratic problem, the H∞ norm minimization and nonlinear methods including nonlinear inversion, etc ((4), (5), (6), (7), (8)). The methods mentioned above have advantages and drawbacks; the main difficulty in choosing the control design method is to ensure a trade-off between the complexity of the automatic flight control system and its performances. The actual applications require a wide spectrum of performances including not only the stabilization of the aircraft but also robustness with respect to modeling uncertainties and flying conditions changes, time response performances and reduced sensitivity to disturbances and measurement errors. The aim of this paper is to present a design methodology for the Pitch Attitude Hold (PAH) autopilot of a flying wing configuration. The design procedure is illustrated and investigated for the Hirrus UAV designed and manufactured by a private Romanian company in collaboration with academics from Faculty of Aerospace Engineering of University Politehnica of Bucharest. The design approach is based on a modified version of the so-called two degrees of freedom (2 DOF) H∞ loop-shaping able to accomplish simultaneously several objectives as robustness stability, model tracking and disturbances attenuation requirements. The original 2 DOF H∞ method presented in (9) have been previously used for the autopilot design of the aircraft longitudinal and lateral SCIENTIFIC RESEARCH AND EDUCATION IN THE AIR FORCE-AFASES 2016