Thomas Stastny

Last Name

Stastny

First Name

Thomas

ORCID

0000-0002-7244-7026

Organisational unit

03737 - Siegwart, Roland Y. / Siegwart, Roland Y.

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Publications 1 - 10 of 13

Low-Altitude Control and Local Re-Planning Strategies for Small Fixed-Wing UAVs
Stastny, Thomas (2020)
In recent years, small, easily manageable, operated, and maintained Unmanned Aerial Vehicles (UAVs) have become ubiquitous in an every-growing set of industrial, humanitarian, scientific, and commercial domains. For large-scale remote sensing and mapping applications, small fixed-wing platforms provide the advantages of longer range and higher speeds, with respect to their Micro-Aerial Vehicle (MAV) counterparts. However, today’s fixed-wing UAVs are largely limited to a primitive set of basic waypoint following and pre-programmed tasks, with little awareness of the environment in which they fly, adaptability to changing conditions, or higher-level decision making capabilities. Particularly relevant to small fixed-wing mission profiles, is the required ability to operate safely near uncertain terrain while disturbed by possibly strong and turbulent wind fields. Enabling these activities entails the design of efficient, robust, and more adaptable motion planning and control algorithms which moreover adhere to the vehicle’s restrictive dynamic flight envelope. The primary goal of this thesis is to develop practical control and local re-planning strategies for low-flying, small, fixed-wing UAVs with explicit awareness of these environmental hazards. Part A of this thesis addresses the challenge of guiding small, low-speed fixed-wing aircraft in strong winds. Our first contribution in this part is the unique consideration of excess winds, i.e. wind speeds that exceed the vehicle’s nominal airspeed, within a lateral-directional control law for fixed-wing UAVs. We develop a principled, nonlinear guidance law which guarantees convergence to a safe and stable vehicle configuration with respect to the wind field while preserving some tracking performance with respect to the path target. We then expand on this concept by including an energy efficient airspeed reference compensation logic, enabling not only mitigation, but also prevention or over-powering of excess winds which would otherwise cause the aircraft to "run-away". We emphasize heavily in this second iteration on field testing results, demonstrating track keeping errors of less than 1 meter consistently maintained during gusting excess winds over various mountainous regions in Switzerland. The third component of this part revisits the efficiency of the airspeed references. A coupled approach to airspeed and heading reference commands is developed with a more principled consideration of airspeed reference minimization. The coupled method is compared against the previous decoupled approach in simulations showing both increased power-efficiency and tracking performance in static and dynamic winds. Part B of this thesis operates as a road map to fast, environment-aware local re-planning for fixed-wing UAVs operating near terrain. We first delve into the practicalities of deploying a guidance level Nonlinear Model Predictive Controller (NMPC) on a small fixed-wing platform. We develop new control augment modeling methods featuring reduced order models of the underlying low-level autopilot response and quasi-steady forces, simple parameter identification procedures, and open loop predictability on the order of tens of seconds, making our modeling approach suitable for long horizon NMPC. Through flight experiments, we demonstrate Dubins aircraft path segment tracking in three dimensions with wind speeds exceeding 50% of the vehicle’s airspeed, and further show a mock motor failure scenario. A particular focus is further spent on soft constraint formulation. The third component of this part reworks the developments of the first two towards our local re-planning formulation. Wind-aware reference trajectory generation is developed from the guidance logic in Part A for lateral-directional states, and vertical wind is included in the longitudinal guidance. Vision-based elevation mapping is utilized to to provide a generalized 2.5D world representation to the aircraft. The map is bilinearly interpolated for height feedback, and we design an efficient ray casting approach for detection of forward (line of flight) and lateral occlusions. The occlusions are used to construct novel "relative" Euclidean Signed Distance Fields (RESDFs), which are a function of the relative velocity between the vehicle and obstacle. We further present a method of transforming the RESDFs into optimizable soft constraints for the objective function of the NMPC. A preliminary example of the full system acting to avoid an obstructing hillside is demonstrated in hardware-in-the-loop (HITL) simulation.
On Flying Backwards: Preventing Run-away of Small, Low-speed, Fixed-wing UAVs in Strong Winds
Stastny, Thomas; Siegwart, Roland (2019)
2019 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS)
Small, low-speed fixed-wing Unmanned Aerial Vehicles (UAVs) operating autonomously, beyond-visual-line-of-sight (BVLOS) will inevitably encounter winds rising to levels near or exceeding the vehicles' nominal airspeed. In this paper, we develop a nonlinear lateral-directional path following guidance law with explicit consideration of online wind estimates. Energy efficient airspeed reference compensation logic is developed for excess wind scenarios (i.e. when the wind speed rises above the airspeed), enabling either mitigation, prevention, or over-powering of excess wind induced run-away from a given path. The developed guidance law is demonstrated on a representative small, low-speed test UAV in two flight experiments conducted in mountainous regions of Switzerland with strong, turbulent wind conditions, gusts reaching up to 13 meters per second. We demonstrate track-keeping errors of less than 1 meter consistently maintained during a representative duration of gusting, excess winds and a mean ground speed undershoot of 0.5 meters per second from the commanded minimum forward ground speed demonstrated in over 5 minutes of the showcased flight results.
Sun2Ice: Monitoring calving glaciers from solar-powered UAVs
Jouvet, Guillaume; Stastny, Thomas; Oettershagen, Philipp; et al. (2018)
Geophysical Research Abstracts ~ Geophysical Research Abstracts
Nonlinear Model Predictive Velocity Control of a VTOL Tiltwing UAV
Rohr, David; Studiger, Matthias; Stastny, Thomas; et al. (2021)
IEEE Robotics and Automation Letters
This letter presents the modeling, system identification and nonlinear model predictive control (NMPC) design for longitudinal, full envelope velocity control of a small tiltwing hybrid unmanned aerial vehicle (H-UAV). A first-principles based dynamics model is derived and identified from flight data. It captures important aerodynamic effects including propeller-wing interaction and stalled airfoils, but is still simple enough for on-board online trajectory optimization. Based on this model, a high-level NMPC is formulated which optimizes throttle, tilt-rate and pitch-angle setpoints in order to track longitudinal velocity trajectories. We propose and investigate different references suitable to regularize the optimization problem, including both offline generated trims as well as preceding NMPC solutions. In simulation, we compare the NMPC with a frequently reported dynamic inversion approach for H-UAV velocity control. Finally, the NMPC is validated in flight experiments through a series of transition maneuvers, demonstrating good tracking capabilities in the full flight envelope.
Long-duration fully autonomous operation of rotorcraft unmanned aerial systems for remote-sensing data acquisition
Malyuta, Danylo; Brommer, Christian; Hentzen, Daniel; et al. (2020)
Journal of Field Robotics
Differential Sweep Attitude Control for Swept Wing UAVs
Harms, Marvin; Kaufmann, Noah; Rockenbauer, Friedrich M.; et al. (2020)
2020 International Conference on Unmanned Aircraft Systems (ICUAS)
A novel approach for attitude control of swept wing unmanned aerial vehicles (UAVs) is presented, involving the use of only differential wing sweep and rudder deflection. An analytic aerodynamic model of the aircraft based on simple sweep theory is derived in a first step. The prediction of a vortex lattice method is then compared to the initial model. Based on the body moment analysis of the two models, design constraints and a control structure are proposed and implemented on a small scale UAV with variable sweep wings. The control structure involves a cascaded PID controller with a nonlinear mapping from controller output to sweep angles. The obtained simulation results show that simultaneous bank and elevation inputs can be tracked successfully by the attitude controller. Tracking of step inputs and dynamic inputs in the roll direction using only wing sweep is demonstrated in flight tests. The results show that the nonlinear mapping achieves decoupling of the roll and pitch movement, but performance is limited by the inertia of the moving wings. © 2020 IEEE.
Soliro - a hybrid dynamic tilt-wing aerial manipulator with minimal actuators
Pantic, Michael; Hampp, Elias; Flammer, Ramon; et al. (2024)
Springer Proceedings in Advanced Robotics ~ Experimental Robotics. ISER 2023
The ability to enter in contact with and manipulate physical objects with a flying robot enables many novel applications, such as contact inspection, painting, drilling, and sample collection. Generally, these aerial robots need more degrees of freedom than a standard quadrotor. While there is active research of over-actuated, omnidirectional MAVs and aerial manipulators as well as VTOL and hybrid platforms, the two concepts have not been combined. We address the problem of conceptualization, characterization, control, and testing of a 5DOF rotary-/fixed-wing hybrid, tilt-rotor, split tilt-wing, nearly omnidirectional aerial robot. We present an elegant solution with a minimal set of actuators and that does not need any classical control surfaces or flaps. The concept is validated in a wind tunnel study and in multiple flights with forward and backward transitions. Fixed-wing flight speeds up to 10 m/s were reached, with a power reduction of 30% as compared to rotary wing flight.
WindSeer: real-time volumetric wind prediction over complex terrain aboard a small uncrewed aerial vehicle
Achermann, Florian; Stastny, Thomas; Danciu, Bogdan; et al. (2024)
Nature Communications
Real-time high-resolution wind predictions are beneficial for various applications including safe crewed and uncrewed aviation. Current weather models require too much compute and lack the necessary predictive capabilities as they are valid only at the scale of multiple kilometers and hours – much lower spatial and temporal resolutions than these applications require. Our work demonstrates the ability to predict low-altitude time-averaged wind fields in real time on limited-compute devices, from only sparse measurement data. We train a deep neural network-based model, WindSeer, using only synthetic data from computational fluid dynamics simulations and show that it can successfully predict real wind fields over terrain with known topography from just a few noisy and spatially clustered wind measurements. WindSeer can generate accurate predictions at different resolutions and domain sizes on previously unseen topography without retraining. We demonstrate that the model successfully predicts historical wind data collected by weather stations and wind measured by drones during flight.
Fisher Information Based Active Planning for Aerial Photogrammetry
Lim, Jaeyoung; Lawrance, Nicholas; Achermann, Florian; et al. (2023)
2023 IEEE International Conference on Robotics and Automation (ICRA)
Small uncrewed aerial systems (sUASs) are useful tools for 3D reconstruction due to their speed, ease of use, and ability to access high-utility viewpoints. Today, most aerial survey approaches generate a preplanned coverage pattern assuming a planar target region. However, this is inefficient since it results in superfluous overlap and suboptimal viewing angles and does not utilize the entire flight envelope. In this work, we propose active path planning for photogrammetric reconstruction. Our main contribution is a view utility function based on Fisher information approximating the offline recon struction uncertainty. The metric enables online path planning to make in-flight decisions to collect geometrically informative image data in complex terrain. We evaluate our approach in a photorealistic simulation. A viewpoint selection study shows that our metric leads to faster and more precise reconstruction than state-of-the-art active planning metrics and adapts to different camera resolutions. Comparing our online planning approach to an ordinary fixed-wing aerial survey yields 3.2 × faster coverage of 16 ha undulated terrain without sacrificing precision.
Locally Power-optimal Nonlinear Model Predictive Control for Fixed-wing Airborne Wind Energy
Stastny, Thomas; Ahbe, Eva; Dangel, Manuel; et al. (2019)
2019 American Control Conference (ACC)

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Thomas Stastny

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