Breaking the Circle: An Autonomous Control-Switching Strategy for Stable Orographic Soaring in MAVs

TL;DR

This work addresses the persistent circling observed in autonomous MAV orographic soaring by introducing SAOS, a switched-control strategy that toggles between longitudinal and vertical axis control to circumvent underactuation near zero throttle. By incorporating an angle-of-attack sensor and AoA-based force estimation, along with a drag term in the outer-loop model, the method improves directionality and reduces energy waste. The SAOS approach is validated through comprehensive simulations (300 randomized references across six configurations) and wind-tunnel experiments on two MAV platforms, showing improved convergence to feasible updraft regions, reduced roll oscillations, and lower throttle usage. The findings indicate substantial gains in stability and energy efficiency in constrained soaring environments, with future work focusing on outdoor trials, gain scheduling, and enhanced aerodynamic-angle integration.

Abstract

Orographic soaring can significantly extend the endurance of micro aerial vehicles (MAVs), but circling behavior, arising from control conflicts between the longitudinal and vertical axes, increases energy consumption and the risk of divergence. We propose a control switching method, named SAOS: Switched Control for Autonomous Orographic Soaring, which mitigates circling behavior by selectively controlling either the horizontal or vertical axis, effectively transforming the system from underactuated to fully actuated during soaring. Additionally, the angle of attack is incorporated into the INDI controller to improve force estimation. Simulations with randomized initial positions and wind tunnel experiments on two MAVs demonstrate that the SAOS improves position convergence, reduces throttle usage, and mitigates roll oscillations caused by pitch-roll coupling. These improvements enhance energy efficiency and flight stability in constrained soaring environments.

Figures (15)

• Figure 1: Wind tunnel test trajectories of the Seal plane over 10 seconds using SAOS (proposed control switching method) and BASE (previous INDI-based controller from AOSoar hwang2023aosoar without any modifications) control methods. (a) SAOS: minimal circling and stable soaring flight. (b) BASE: pronounced circling behavior and trajectory divergence.
• Figure 2: Illustrative example of MAV flight trajectory near a ramp during orographic soaring. Colored streamlines represent wind flow, with a gradient from gray to red indicating increasing wind speed. Airplane icons show the MAV’s position and pitch angle at different time steps along the path. The yellow dot marks the predefined reference position.
• Figure 3: Time-overlapped image extracted from outdoor flight footage of the Seal G-1500 MAV, demonstrating circling behavior over a sand dune.
• Figure 4: Flight trajectory of the Seal G-1500 MAV exhibiting circling behavior during orographic soaring over a sand dune. The trajectory is reconstructed from position data recorded during an outdoor flight test.
• Figure 5: The interaction between the soaring search algorithm and the INDI-based soaring controller. The diagonally striped area is newly introduced in this paper. In the controller, two control allocations are employed to handle cases where the thrust (T) is negatively saturated, switching to a reduced 2-axis control while limiting pitch by angle of attack (AoA).