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Performance comparison of two altitude-control algorithms for a fixed-wing UAV

Uninhabited aerial vehicles (UAV) have proven their tremendous capabilities in military and civil applications. In a UAV, the onboard autopilot autonomously controls the aircraft flight and navigation. The altitude acquire-and-hold is an important function of autopilot, implemented using a control design algorithm that flies the UAV to commanded altitude and maintains it. Most of the commercially available autopilots use Proportional-Integral-Derivative (PID) controllers for altitude and heading control. In this paper, we present a performance comparison of two altitude-controller design techniques, the PID controller and the Phase Lead compensator. We have used a nonlinear mathematical model of the UAV Aerosonde in our work. The nonlinear model is lineraized around a stable trim condition and decoupled for linear controller design. The designed controllers are tested with the nonlinear model in view of small perturbation control theory. The results for the compensated linear and nonlinear models are presented. Our investigation reveals that Phase Lead compensator has inherent strengths compared to PID controller for UAV altitude acquire- and-hold in terms of better transient response, thus improving the payload performance during an altitude-change maneuver. The findings may lead to an effective approach in UAV autopilot design.