Trajectory Generation and Tracking for UAV Landing

Abstract

Landing is the most critical phase of aircraft flight and poses vulnerability. Automatic landing offers an added safety by minimizing undue human errors during this critical flight phase. Unmanned Air Vehicle reduces this risk by eliminating human loss but monetary aspect is still there. This makes landing methodology improvement, an active research field for safe recovery of small scale UAV’s in wake of constraints. Small scale UAV runway landing option offers research ground being equally viable for large scale UAV’s and civil airliners. Driving ground for active research being the reduction in required runway length while offering minimum impact velocity at touch down to increase landing feasibility at short length airfields. This results in lower structure stresses. Landing consist of glide slope capturing phase and flare phase. UAV maintains a constant descend rate during glide phase till a threshold altitude above the runway called Flare initiation height. After achieving this altitude, UAV gets into flare maneuver to touch down on runway with minimum vertical velocity in order to reduce impact at touch down. This is attained by shedding off the kinetic and potential energy in the form of increased drag through pitch up maneuver. The distance required during landing depends upon obstacle avoidance, engine control capabilities and permissible impact velocity. Extensive research is already being done on online/offline trajectory generation for runway landing of fixed wing UAV. This thesis is aimed at design and development of vertical plane trajectory generation algorithm and tracking control for autonomous runway landing of fixed wing UAV. The proposed algorithm is capable of touch down at intended point on runway with intended vertical velocity allowing reduced required distance during flare. The algorithm is capable of being computationally viable for inflight calculations. H∞ SISO control law for altitude profile tracking with cascaded inner loop H∞ SISO pitch rate stability augmented pitch loop are designed in continuous time domain for Glide phase. H∞ SISO control law for vertical velocity profile tracking with cascaded inner loop H∞ SISO pitch rate stability augmented pitch loop are designed in continuous time domain for flare phase. These controllers are evaluated for stability, performance and uncertainty over an array of linear models depicting landing flight envelope. Controller implementation is carried out in discretized format in 6dof simulation and evaluated to track the trajectory command for accurate landing on runway. FASER (Free‐flying Aircraft for Subscale Experimental Research) of UAV Laboratories, University of Minnesota is used as the subject platform. C‐language based Simulink® interfaced 6dof simulation is used to implement and evaluate the performance of developed algorithm and tracking controller.