Agile Drone Flight

We work on perception, learning, planning, and control strategies to enable extremely agile maneuvers, which reach the limits of the actuators. While pushing the boundaries of our quadrotors, we also enable them to recover from difficult conditions in case of a failure.


Learning High-Speed Flight in the Wild

Science21_Loquercio

Quadrotors are agile. Unlike most other machines, they can traverse extremely complex environments at high speeds. To date, only expert human pilots have been able to fully exploit their capabilities. Autonomous operation with onboard sensing and computation has been limited to low speeds. State-of-the-art methods generally separate the navigation problem into subtasks: sensing, mapping, and planning. While this approach has proven successful at low speeds, the separation it builds upon can be problematic for high-speed navigation in cluttered environments. Indeed, the subtasks are executed sequentially, leading to increased processing latency and a compounding of errors through the pipeline. Here we propose an end-to-end approach that can autonomously fly quadrotors through complex natural and man-made environments at high speeds, with purely onboard sensing and computation. The key principle is to directly map noisy sensory observations to collision-free trajectories in a receding-horizon fashion. This direct mapping drastically reduces processing latency and increases robustness to noisy and incomplete perception. The sensorimotor mapping is performed by a convolutional network that is trained exclusively in simulation via privileged learning: imitating an expert with access to privileged information. By simulating realistic sensor noise, our approach achieves zero-shot transfer from simulation to challenging real-world environments that were never experienced during training: dense forests, snow-covered terrain, derailed trains, and collapsed buildings. Our work demonstrates that end-to-end policies trained in simulation enable high-speed autonomous flight through challenging environments, outperforming traditional obstacle avoidance pipelines. We release the code open source.


References

Science21_Loquercio

A. Loquercio*, E. Kaufmann*, R. Ranftl, M. Müller, V. Koltun, D. Scaramuzza

Learning High-Speed Flight in the Wild

Science Robotics, 2021.

Project Webpage and Datasets PDF YouTube Code


Range, Endurance, and Optimal Speed Estimates for Multicopters


Multicopters are among the most versatile mobile robots. Their applications range from inspection and mapping tasks to providing vital reconnaissance in disaster zones and to package delivery. The range, endurance, and speed a multirotor vehicle can achieve while performing its task is a decisive factor not only for vehicle design and mission planning, but also for policy makers deciding on the rules and regulations for aerial robots. To the best of the authors' knowledge, this work proposes the first approach to estimate the range, endurance, and optimal flight speed for a wide variety of multicopters. This advance is made possible by combining a state-of-the-art first-principles aerodynamic multicopter model based on blade-element-momentum theory with an electric-motor model and a graybox battery model. This model predicts the cell voltage with only 1.3% relative error (43.1 mV), even if the battery is subjected to non-constant discharge rates. Our approach is validated with real-world experiments on a test bench as well as with flights at speeds up to 65 km/h in one of the world's largest motion-capture systems. We also present an accurate pen-and-paper algorithm to estimate the range, endurance and optimal speed of multicopters to help future researchers build drones with maximal range and endurance, ensuring that future multirotor vehicles are even more versatile.


References

A Quadrotor Tracking Racing Trajectories

L. Bauersfeld, D. Scaramuzza

Range, Endurance, and Optimal Speed Estimates for Multicopters

Arxiv Preprint, 2021.

PDF


Performance, Precision, and Payloads: Adaptive Nonlinear MPC for Quadrotors

Agile quadrotor flight in challenging environments has the potential to revolutionize shipping, transportation, and search and rescue applications. Nonlinear model predictive control (NMPC) has recently shown promising results for agile quadrotor control, but relies on highly accurate models for maximum performance. Hence, model uncertainties in the form of unmodeled complex aerodynamic effects, varying payloads and parameter mismatch will degrade overall system performance. In this paper, we propose L1-NMPC, a novel hybrid adaptive NMPC to learn model uncertainties online and immediately compensate for them, drastically improving performance over the non-adaptive baseline with minimal computational overhead. Our proposed architecture generalizes to many different environments from which we evaluate wind, unknown payloads, and highly agile flight conditions. The proposed method demonstrates immense flexibility and robustness, with more than 90% tracking error reduction over non-adaptive NMPC under large unknown disturbances and without any gain tuning. In addition, the same controller with identical gains can accurately fly highly agile racing trajectories exhibiting top speeds of 70 km/h, offering tracking performance improvements of around 50% relative to the non-adaptive NMPC baseline.


References

A Quadrotor Tracking Racing Trajectories

D. Hanover, P. Foehn, S. Sun, E. Kaufmann, D. Scaramuzza

Performance, Precision, and Payloads: Adaptive Nonlinear MPC for Quadrotors

Arxiv Preprint, 2021.

PDF YouTube


A Comparative Study of Nonlinear MPC and Differential-Flatness-Based Control for Quadrotor Agile Flight

Accurate trajectory tracking control for quadrotors is essential for safe navigation in cluttered environments. However, this is challenging in agile flights due to nonlinear dynamics, complex aerodynamic effects, and actuation constraints. Our work empirically compares two state-of-the-art control frameworks: the nonlinear-model-predictive controller (NMPC) and the differential-flatness-based controller (DFBC), by tracking a wide variety of agile trajectories at speeds up to 72km/h. The comparisons are performed in both simulation and real-world environments to systematically evaluate both methods from the aspect of tracking accuracy, robustness, and computational efficiency. We show the superiority of NMPC in tracking dynamically infeasible trajectories, at the cost of higher computation time and risk of numerical convergence issues. For both methods, we also quantitatively study the effect of adding an inner-loop controller using the incremental nonlinear dynamic inversion (INDI) method, and the effect of adding an aerodynamic drag model. Our real-world experiments, performed in one of the world's largest motion capture systems, demonstrate more than 78% tracking error reduction of both NMPC and DFBC, indicating the necessity of using an inner-loop controller and aerodynamic drag model for agile trajectory tracking.


References

A Quadrotor Tracking Racing Trajectories

S. Sun, A. Romero, P. Foehn, E. Kaufmann, D. Scaramuzza

A Comparative Study of Nonlinear MPC and Differential-Flatness-Based Control for Quadrotor Agile Flight

Arxiv Preprint, 2021.

PDF YouTube


Model Predictive Contouring Control for Near-Time-Optimal Quadrotor Flight

We tackle the problem of flying time-optimal trajectories through multiple waypoints with quadrotors. State-of-the-art solutions split the problem into a planning task - where a global, time-optimal trajectory is generated - and a control task - where this trajectory is accurately tracked. However, at the current state, generating a time-optimal trajectory that takes the full quadrotor model into account is computationally demanding (in the order of minutes or even hours). This is detrimental for replanning in presence of disturbances. We overcome this issue by solving the time-optimal planning and control problems concurrently via Model Predictive Contouring Control (MPCC). Our MPCC optimally selects the future states of the platform at runtime, while maximizing the progress along the reference path and minimizing the distance to it. We show that, even when tracking simplified trajectories, the proposed MPCC results in a path that approaches the true time-optimal one, and which can be generated in real-time. We validate our approach in the real-world, where we show that our method outperforms both the current state-of-the-art and a world-class human pilot in terms of lap time achieving speeds of up to 60 km/h.


References

MPCC for Quadrotors

A. Romero, S. Sun, P. Foehn, D. Scaramuzza

Model Predictive Contouring Control for Near-Time-Optimal Quadrotor Flight

Arxiv Preprint, 2021.

PDF YouTube


Time-Optimal Planning for Quadrotor Waypoint Flight

Quadrotors are among the most agile flying robots. However, planning time-optimal trajectories at the actuation limit through multiple waypoints remains an open problem. This is crucial for applications such as inspection, delivery, search and rescue, and drone racing. Early works used polynomial trajectory formulations, which do not exploit the full actuator potential because of their inherent smoothness. Recent works resorted to numerical optimization but require waypoints to be allocated as costs or constraints at specific discrete times. However, this time allocation is a priori unknown and renders previous works incapable of producing truly time-optimal trajectories. To generate truly time-optimal trajectories, we propose a solution to the time allocation problem while exploiting the full quadrotor’s actuator potential. We achieve this by introducing a formulation of progress along the trajectory, which enables the simultaneous optimization of the time allocation and the trajectory itself. We compare our method against related approaches and validate it in real-world flights in one of the world’s largest motion-capture systems, where we outperform human expert drone pilots in a drone-racing task.


References

Time-Optimal Quadrotor Trajectories

P. Foehn, A. Romero, D. Scaramuzza

Time-Optimal Planning for Quadrotor Waypoint Flight

Science Robotics, July 21, 2021.

PDF YouTube Code


NeuroBEM: Hybrid Aerodynamic Quadrotor Model

NeuroBEM: Hybrid Aerodynamic Quadrotor Model

Quadrotors are extremely agile, so much in fact, that classic first-principle-models come to their limits. Aerodynamic effects, while insignificant at low speeds, become the dominant model defect during high speeds or agile maneuvers. Accurate modeling is needed to design robust high-performance control systems and enable flying close to the platform's physical limits. We propose a hybrid approach fusing first principles and learning to model quadrotors and their aerodynamic effects with unprecedented accuracy. First principles fail to capture such aerodynamic effects, rendering traditional approaches inaccurate when used for simulation or controller tuning. Data-driven approaches try to capture aerodynamic effects with blackbox modeling, such as neural networks; however, they struggle to robustly generalize to arbitrary flight conditions. Our hybrid approach unifies and outperforms both first-principles blade-element theory and learned residual dynamics. It is evaluated in one of the world's largest motion-capture systems, using autonomous-quadrotor-flight data at speeds up to 65km/h. The resulting model captures the aerodynamic thrust, torques, and parasitic effects with astonishing accuracy, outperforming existing models with 50% reduced prediction errors, and shows strong generalization capabilities beyond the training set.


References

RSS21_Bauersfeld

L. Bauersfeld*, E. Kaufmann*, P. Foehn, S. Sun, D. Scaramuzza

NeuroBEM: Hybrid Aerodynamic Quadrotor Model

Robotics: Science and Systems (RSS), 2021.

PDF YouTube Project Page


Autonomous Drone Racing with Deep Reinforcement Learning

Arxiv21_Yunlong

In many robotic tasks, such as drone racing, the goal is to travel through a set of waypoints as fast as possible. A key challenge for this task is planning the minimum-time trajectory, which is typically solved by assuming perfect knowledge of the waypoints to pass in advance. The resulting solutions are either highly specialized for a single-track layout, or suboptimal due to simplifying assumptions about the platform dynamics. In this work, a new approach to minimum-time trajectory generation for quadrotors is presented. Leveraging deep reinforcement learning and relative gate observations, this approach can adaptively compute near-time-optimal trajectories for random track layouts. Our method exhibits a significant computational advantage over approaches based on trajectory optimization for non-trivial track configurations. The proposed approach is evaluated on a set of race tracks in simulation and the real world, achieving speeds of up to 17 m/s with a physical quadrotor.


References

Arxiv21_Yunlong

Y. Song*, M. Steinweg*, E. Kaufmann, D. Scaramuzza

Autonomous Drone Racing with Deep Reinforcement Learning

IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS), Prague, 2021.

PDF YouTube


AutoTune: Controller Tuning for High-Speed Flight

RAL21_Pfeiffer

Due to noisy actuation and external disturbances, tuning controllers for high-speed flight is very challenging. In this paper, we ask the following questions: How sensitive are controllers to tuning when tracking high-speed maneuvers? What algorithms can we use to automatically tune them? To answer the first question, we study the relationship between parameters and performance and find out that the faster the maneuver, the more sensitive a controller becomes to its parameters. To answer the second question, we review existing methods for controller tuning and discover that prior works often perform poorly on the task of high-speed flight. Therefore, we propose AutoTune, a sampling-based tuning algorithm specifically tailored to high-speed flight. In contrast to previous work, our algorithm does not assume any prior knowledge of the drone or its optimization function and can deal with the multi-modal characteristics of the parameters' optimization space. We thoroughly evaluate AutoTune both in simulation and in the physical world. In our experiments, we outperform existing tuning algorithms by up to 90\% in trajectory completion. The resulting controllers are tested in the AirSim Game of Drones competition, where we outperform the winner by up to 25\% in lap-time. Finally, we show that AutoTune improves tracking error when flying a physical platform with respect to parameters tuned by a human expert.


References

RAL21_Pfeiffer

A. Loquercio, A. Saviolo, D. Scaramuzza

AutoTune: Controller Tuning for High-Speed Flight

Arxiv Preprint, 2021.

PDF Code YouTube


Data-Driven MPC for Quadrotors

Aerodynamic forces render accurate high-speed trajectory tracking with quadrotors extremely challenging. These complex aerodynamic effects become a significant disturbance at high speeds, introducing large positional tracking errors, and are extremely difficult to model. To fly at high speeds, feedback control must be able to account for these aerodynamic effects in real-time. This necessitates a modelling procedure that is both accurate and efficient to evaluate. Therefore, we present an approach to model aerodynamic effects using Gaussian Processes, which we incorporate into a Model Predictive Controller to achieve efficient and precise real-time feedback control, leading to up to 70% reduction in trajectory tracking error at high speeds. We verify our method by extensive comparison to a state-of-the-art linear drag model in synthetic and real-world experiments at speeds of up to 14m/s and accelerations beyond 4g.


References

RAL21_Torrente

G. Torrente*, E. Kaufmann*, P. Foehn, D. Scaramuzza

Data-Driven MPC for Quadrotors

IEEE Robotics and Automation Letters (RA-L), 2021.

PDF YouTube Code


Flightmare: A Flexible Quadrotor Simulator

Currently available quadrotor simulators have a rigid and highly-specialized structure: either are they really fast, physically accurate, or photo-realistic. In this work, we propose a paradigm-shift in the development of simulators: moving the trade-off between accuracy and speed from the developers to the end-users. We release a new modular quadrotor simulator: Flightmare. Flightmare is composed of two main components: a configurable rendering engine built on Unity and a flexible physics engine for dynamics simulation. Those two components are totally decoupled and can run independently from each other. Flightmare comes with several desirable features: (i) a large multi-modal sensor suite, including an interface to extract the 3D point-cloud of the scene; (ii) an API for reinforcement learning which can simulate hundreds of quadrotors in parallel; and (iii) an integration with a virtual-reality headset for interaction with the simulated environment. Flightmare can be used for various applications, including path-planning, reinforcement learning, visual-inertial odometry, deep learning, human-robot interaction, etc.


References

Flightmare_Yunlong

Y. Song, S. Naji, E. Kaufmann, A. Loquercio, D. Scaramuzza

Flightmare: A Flexible Quadrotor Simulator

Conference on Robot Learning (CoRL), 2020

PDF YouTube CoRL 2020 Pitch Video Website


Deep Drone Acrobatics

Performing acrobatic maneuvers with quadrotors is extremely challenging. Acrobatic flight requires high thrust and extreme angular accelerations that push the platform to its physical limits. Professional drone pilots often measure their level of mastery by flying such maneuvers in competitions. In this paper, we propose to learn a sensorimotor policy that enables an autonomous quadrotor to fly extreme acrobatic maneuvers with only onboard sensing and computation. We train the policy entirely in simulation by leveraging demonstrations from an optimal controller that has access to privileged information. We use appropriate abstractions of the visual input to enable transfer to a real quadrotor. We show that the resulting policy can be directly deployed in the physical world without any fine-tuning on real data. Our methodology has several favorable properties: it does not require a human expert to provide demonstrations, it cannot harm the physical system during training, and it can be used to learn maneuvers that are challenging even for the best human pilots. Our approach enables a physical quadrotor to fly maneuvers such as the Power Loop, the Barrel Roll, and the Matty Flip, during which it incurs accelerations of up to 3g.


References

RSS20_Kaufmann

Elia Kaufmann*, Antonio Loquercio*, René Ranftl, Matthias Müller, Vladlen Koltun, Davide Scaramuzza

Deep Drone Acrobatics

Robotics: Science and Systems (RSS), 2020.

PDF YouTube RSS2020 Pitch Video Code


AlphaPilot: Autonomous Drone Racing

We present a novel system for autonomous, vision-based drone racing combining learned data abstraction, nonlinear filtering, and time-optimal trajectory planning. The system has successfully been deployed at the first autonomous drone racing world championship: the 2019 AlphaPilot Challenge. Contrary to traditional drone racing systems, that only detect the next gate, our approach makes use of any visible gate and takes advantage of multiple, simultaneous gate detections to compensate for drift in the state estimate and build a global map of the gates. The global map and drift-compensated state estimate allow the drone to navigate through the race course even when the gates are not immediately visible and further enable to plan a near time-optimal path through the race course in real time based on approximate drone dynamics. The proposed system has been demonstrated to successfully guide the drone through tight race courses reaching speeds up to 8m/s and has led to rank second at the 2019 AlphaPilot Challenge.


References

RSS20_Foehn

P. Foehn*, D. Brescianini*, E. Kaufmann*, T. Cieslewski, M. Gehrig, M. Muglikar, D. Scaramuzza

AlphaPilot: Autonomous Drone Racing

Autonomous Robots (AuRo), 2021

PDF YouTube RSS2020 Pitch Video

RSS20_Foehn

P. Foehn, D. Brescianini, E. Kaufmann, T. Cieslewski, M. Gehrig, M. Muglikar, D. Scaramuzza

AlphaPilot: Autonomous Drone Racing

Robotics: Science and Systems (RSS), 2020

Best Systems Paper Award!

PDF YouTube RSS2020 Pitch Video


Towards Low-Latency High-Bandwidth Control of Quadrotors using Event Cameras

Event cameras are a promising candidate to enable high speed vision-based control due to their low sensor latency and high temporal resolution. However, purely event-based feedback has yet to be used in the control of drones. In this work, a first step towards implementing low-latency high-bandwidth control of quadrotors using event cameras is taken. In particular, this paper addresses the problem of one-dimensional attitude tracking using a dualcopter platform equipped with an event camera. The event-based state estimation consists of a modified Hough transform algorithm combined with a Kalman filter that outputs the roll angle and angular velocity of the dualcopter relative to a horizon marked by a black-and-white disk. The estimated state is processed by a proportional-derivative attitude control law that computes the rotor thrusts required to track the desired attitude. The proposed attitude tracking scheme shows promising results of event-camera-driven closed loop control: the state estimator performs with an update rate of 1 kHz and a latency determined to be 12 ms, enabling attitude tracking at speeds of over 1600 degrees per second.


References

Towards Low-Latency High-Bandwidth Control of Quadrotors using Event Cameras

R. Sugimoto, M. Gehrig, D. Brescianini, D. Scaramuzza

Towards Low-Latency High-Bandwidth Control of Quadrotors using Event Cameras

IEEE International Conference on Robotics and Automation (ICRA), 2020

PDF YouTube ICRA2020 Video Pitch


Dynamic Obstacle Avoidance for Quadrotors with Event Cameras

Today's autonomous drones have reaction times of tens of milliseconds, which is not enough for navigating fast in complex dynamic environments. To safely avoid fast moving objects, drones need low-latency sensors and algorithms. We departed from state-of-the-art approaches by using event cameras, which are bioinspired sensors with reaction times of microseconds. Our approach exploits the temporal information contained in the event stream to distinguish between static and dynamic objects and leverages a fast strategy to generate the motor commands necessary to avoid the approaching obstacles. Standard vision algorithms cannot be applied to event cameras because the output of these sensors is not images but a stream of asynchronous events that encode per-pixel intensity changes. Our resulting algorithm has an overall latency of only 3.5 milliseconds, which is sufficient for reliable detection and avoidance of fast-moving obstacles. We demonstrate the effectiveness of our approach on an autonomous quadrotor using only onboard sensing and computation. Our drone was capable of avoiding multiple obstacles of different sizes and shapes, at relative speeds up to 10 meters/second, both indoors and outdoors.


References

Science20_Falanga

Davide Falanga, Kevin Kleber, and Davide Scaramuzza

Dynamic Obstacle Avoidance for Quadrotors with Event Cameras

Science Robotics, March 18, 2020.

PDF Supplementary Material YouTube


The Role of Latency in High-Speed Sense and Avoid

In this work, we study the effects that perception latency has on the maximum speed a robot can reach to safely navigate through an unknown cluttered environment. We provide a general analysis that can serve as a baseline for future quantitative reasoning for design trade-offs in autonomous robot navigation. We consider the case where the robot is modeled as a linear second-order system with bounded input and navigates through static obstacles. Also, we focus on a scenario where the robot wants to reach a target destination in as little time as possible, and therefore cannot change its longitudinal velocity to avoid obstacles. We show how the maximum latency that the robot can tolerate to guarantee safety is related to the desired speed, the range of its sensing pipeline, and the actuation limitations of the platform (i.e., the maximum acceleration it can produce). As a particular case study, we compare monocular and stereo frame-based cameras against novel, low-latency sensors, such as event cameras, in the case of quadrotor flight. To validate our analysis, we conduct experiments on a quadrotor platform equipped with an event camera to detect and avoid obstacles thrown towards the robot. To the best of our knowledge, this is the first theoretical work in which perception and actuation limitations are jointly considered to study the performance of a robotic platform in high-speed navigation.


References

How Fast is Too Fast? The Role of Perception Latency in High-Speed Sense and Avoid

D. Falanga, S. Kim, D. Scaramuzza

How Fast is Too Fast? The Role of Perception Latency in High-Speed Sense and Avoid

IEEE Robotics and Automation Letters (RA-L), 2019.

PDF YouTube


The UZH-FPV Drone Racing Dataset




Despite impressive results in visual-inertial state estimation in recent years, high speed trajectories with six degree of freedom motion remain challenging for existing estimation algorithms. Aggressive trajectories feature large accelerations and rapid rotational motions, and when they pass close to objects in the environment, this induces large apparent motions in the vision sensors, all of which increase the difficulty in estimation. Existing benchmark datasets do not address these types of trajectories, instead focusing on slow speed or constrained trajectories, targeting other tasks such as inspection or driving.

We introduce the UZH-FPV Drone Racing dataset, consisting of over 27 sequences, with more than 10 km of flight distance, captured on a first-person-view (FPV) racing quadrotor flown by an expert pilot. The dataset features camera images, inertial measurements, event-camera data, and precise ground truth poses. These sequences are faster and more challenging, in terms of apparent scene motion, than any existing dataset. Our goal is to enable advancement of the state of the art in aggressive motion estimation by providing a dataset that is beyond the capabilities of existing state estimation algorithms.

Information field illustration

J. Delmerico, T. Cieslewski, H. Rebecq, M. Faessler, D. Scaramuzza

Are We Ready for Autonomous Drone Racing? The UZH-FPV Drone Racing Dataset

IEEE International Conference on Robotics and Automation (ICRA), 2019.

PDF YouTube Project Webpage and Datasets Code


Accurate Tracking of High-Speed Trajectories




In this work, we prove that the dynamical model of a quadrotor subject to linear rotor drag effects is differentially flat in its position and heading. We use this property to compute feed-forward control terms directly from a reference trajectory to be tracked. The obtained feed-forward terms are then used in a cascaded, nonlinear feedback control law that enables accurate agile flight with quadrotors. Compared to state-of-the-art control methods, which treat the rotor drag as an unknown disturbance, our method reduces the trajectory tracking error significantly. Finally, we present a method based on a gradient-free optimization to identify the rotor drag coefficients, which are required to compute the feed-forward control terms. The new theoretical results are thoroughly validated trough extensive comparative experiments.




Quadrotors are well suited for executing fast maneuvers with high accelerations but they are still unable to follow a fast trajectory with centimeter accuracy without iteratively learning it beforehand. In this work, we present a novel body-rate controller and an iterative thrust-mixing scheme, which improve the trajectory-tracking performance without requiring learning and reduce the yaw control error of a quadrotor, respectively. Furthermore, to the best of our knowledge, we present the first algorithm to cope with motor saturations smartly by prioritizing control inputs which are relevant for stabilization and trajectory tracking. The presented body-rate controller uses LQR-control methods to consider both the body rate and the single motor dynamics, which reduces the overall trajectory-tracking error while still rejecting external disturbances well. Our iterative thrust-mixing scheme computes the four rotor thrusts given the inputs from a position-control pipeline. Through the iterative computation, we are able to consider a varying ratio of thrust and drag torque of a single propeller over its input range, which allows applying the desired yaw torque more precisely and hence reduces the yaw-control error. Our prioritizing motor-saturation scheme improves stability and robustness of a quadrotor's flight and may prevent unstable behavior in case of motor saturations. We demonstrate the improved trajectory tracking, yaw-control, and robustness in case of motor saturations in real-world experiments with a quadrotor.


RAL18_Faessler

M. Faessler, A. Franchi, and D. Scaramuzza

Differential Flatness of Quadrotor Dynamics Subject to Rotor Drag for Accurate Tracking of High-Speed Trajectories

IEEE Robotics and Automation Letters (RA-L), 2018.

PDF YouTube


RAL16_Faessler

M. Faessler, D. Falanga, and D. Scaramuzza

Thrust Mixing, Saturation, and Body-Rate Control for Accurate Aggressive Quadrotor Flight

IEEE Robotics and Automation Letters (RA-L), Vol. 2, Issue 2, pp. 476-482, Apr. 2017.

PDF YouTube




Optimal and Perception Aware Control

Optimal control and model-based predictive control are extremely powerful methods to control systems or vehicles. However, most control approaches for MAVs use simple PID control in a cascaded structure and strictly split estimation, planning, and control into separate problems. With optimal control methods one could not only simplify the control architecture, task description, interfacing, and usability, but also take into account dynamic perception objectives and solve planning and control in one single step. Our control architectures abstract the underlying model and provide optimal control and receding horizon predictions at real-time with computation on low-power ARM processors. We also investigate the advantages of perception-aware control, where the robot's perception restrictions are taken into account in the control and planning stage and are used to improve perception performance. While such control pipelines are great for systems with known and simple dynamics, recent advances in machine learning (especially deep neural networks) have shown superior performance in very difficult high-level tasks and high-dimensional data processing. We strongly belief that both model-based and learning-based approaches should work as a union to fully exploit the advantages of both worlds. We intend to provide strong controllers as the foundation for neural-network-based high-level control and sensor data abstraction.

PAMPC

D. Falanga, P. Foehn, P. Lu, D. Scaramuzza

PAMPC: Perception-Aware Model Predictive Control for Quadrotors

IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS), Madrid, 2018.

PDF YouTube Code


Onboard State Dependent LQR for Agile Quadrotors

P. Foehn, D. Scaramuzza

Onboard State Dependent LQR for Agile Quadrotors

IEEE International Conference on Robotics and Automation (ICRA), 2018

PDF Video ICRA18 Video Pitch PPT


RSS17_Foehn

P. Foehn, D. Falanga, N. Kuppuswamy, R. Tedrake, D. Scaramuzza

Fast Trajectory Optimization for Agile Quadrotor Maneuvers with a Cable-Suspended Payload

Robotics: Science and Systems (RSS), Boston, 2017.

PDF PPT YouTube


Agile Drone Flight through Narrow Gaps

with Onboard Sensing and Computing




References

ICRA17_Falanga

D. Falanga, E. Mueggler, M. Faessler, D. Scaramuzza

Aggressive Quadrotor Flight through Narrow Gaps with Onboard Sensing and Computing using Active Vision

IEEE International Conference on Robotics and Automation (ICRA), 2017.

PDF YouTube


In this work, we address one of the main challenges towards autonomous drone flight in complex environments, which is flight through narrow gaps. Indeed, one day micro drones will be used to search and rescue people in the aftermath of an earthquake. In these situations, collapsed buildings cannot be accessed through conventional windows, so that small gaps may be the only way to get inside. What makes this problem challenging is that a gap can be very small, such that precise trajectory-following is required, and can have arbitrary orientations, such that the quadrotor cannot fly through it in near-hover conditions. This makes it necessary to execute an agile trajectory (i.e., with high velocity and angular accelerations) in order to align the vehicle to the gap orientation.

Previous works on aggressive flight through narrow gaps have focused solely on the control and planning problem and therefore used motion-capture systems for state estimation and external computing. Conversely, we focus on using only onboard sensors and computing. More specifically, we address the case where state estimation is done via gap detection through a single, forward-facing camera and show that this raises an interesting problem of coupled perception and planning: for the robot to localize with respect to the gap, a trajectory should be selected, which guarantees that the quadrotor always faces the gap (perception constraint) and should be replanned multiple times during its execution to cope with the varying uncertainty of the state estimate. Furthermore, during the traverse, the quadrotor should maximize the distance from the edges of the gap (geometric constraint) to avoid collisions and, at the same time, it should be able to do so without relying on any visual feedback (when the robot is very close to the gap, this exits from the camera field of view). Finally, the trajectory should be feasible with respect to the dynamic constraints of the vehicle. Our proposed trajectory generation approach is independent of the gap-detection algorithm being used; thus, to simplify the perception task, we used a gap with a simple black-and-white rectangular pattern.

We successfully evaluated our approach with gap orientations of up to 45 degrees vertically and up to 30 horizontally. Our vehicle weighs 830 grams and has a thrust-to-weight ratio of 2.5. Our trajectory generation formulation handles trajectories up to 90-degree gap orientations although the quadrotor used in these experiments is too heavy and the motors saturate for more than 45-degree gap orientations. The vehicle reaches speeds of up to 3 meters per second and angular velocities of up to 400 degrees per second, with accelerations of up to 1.5 g. We can pass through gaps 1.5 times the size of the quadrotor, with only 10 centimeters of tolerance. Our method does not require any prior knowledge about the position and the orientation of the gap. No external infrastructure, such as a motion-capture system, is needed. This is the first time that such an aggressive maneuver through narrow gaps has been done by fusing gap detection from a single onboard camera and IMU.



We challenged two Swiss drone-racing pilots to demonstrate FPV flight through narrow gaps. It turned out not to be that easy.. but after some a few attempts they managed quite well!





Automatic Re-Initialization and Failure Recovery




High-resolution photos can be found here.


With drones becoming more and more popular, safety is a big concern. A critical situation occurs when a drone temporarily loses its GPS position information, which might lead it to crash. This can happen, for instance, when flying close to buildings where GPS signal is lost. In such situations, it is desirable that the drone can rely on fall-back systems and regain stable flight as soon as possible.

We developped a new technology to automatically recover and stabilize a quadrotor from any initial condition. On the one hand, this new technology can allow a quadrotor to be launched by simply tossing it in the air, like a "baseball". On the other hand, it allows a quadrotor to recover back into stable flight after a system failure. Since this technology does not rely on any external infrastructure, such as GPS, it enables the safe use of drones in both indoors and outdoors environments. Thus, our new technology can become relevant for commercial use of drones, such as parcel delivery.

Our quadrotor is equipped with a single camera, an inertial measurement unit, and a distance sensor (Teraranger One). The stabilization system of the quadrotor emulates the visual system and the sense of balance within humans. As soon as a toss or a failure situation is detected, our computer-vision software analyses the images looking for distinctive landmarks in the environment, which it uses to restore balance.

All the image processing and control runs on a smartphone processor onboard the drone. The onboard sensing and computation renders the drone safe and able to fly unaided. This allows the drone to fulfil its mission without any communication or interaction with the operator.

The recovery procedure consists of multiple stages, in which the quadrotor, first, stabilizes its attitude and altitude, then, re-initializes its visual state-estimation pipeline before stabilizing fully autonomously. To experimentally demonstrate the performance of our system, in the video we aggressively throw the quadrotor in the air by hand and have it recover and stabilize all by itself. We chose this example as it simulates conditions similar to failure recovery during aggressive flight. Our system was able to recover successfully in several hundred throws in both indoor and outdoor environments.


References

M. Faessler, F. Fontana, C. Forster, D. Scaramuzza

Automatic Re-Initialization and Failure Recovery for Aggressive Flight with a Monocular Vision-Based Quadrotor

IEEE International Conference on Robotics and Automation (ICRA), Seattle, 2015.

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M. Faessler, F. Fontana, C. Forster, E. Mueggler, M. Pizzoli, D. Scaramuzza

Autonomous, Vision-based Flight and Live Dense 3D Mapping with a Quadrotor Micro Aerial Vehicle

Journal of Field Robotics, 2016.

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