24 research outputs found
PAMPC: Perception-Aware Model Predictive Control for Quadrotors
We present the first perception-aware model predictive control framework for
quadrotors that unifies control and planning with respect to action and
perception objectives. Our framework leverages numerical optimization to
compute trajectories that satisfy the system dynamics and require control
inputs within the limits of the platform. Simultaneously, it optimizes
perception objectives for robust and reliable sens- ing by maximizing the
visibility of a point of interest and minimizing its velocity in the image
plane. Considering both perception and action objectives for motion planning
and control is challenging due to the possible conflicts arising from their
respective requirements. For example, for a quadrotor to track a reference
trajectory, it needs to rotate to align its thrust with the direction of the
desired acceleration. However, the perception objective might require to
minimize such rotation to maximize the visibility of a point of interest. A
model-based optimization framework, able to consider both perception and action
objectives and couple them through the system dynamics, is therefore necessary.
Our perception-aware model predictive control framework works in a
receding-horizon fashion by iteratively solving a non-linear optimization
problem. It is capable of running in real-time, fully onboard our lightweight,
small-scale quadrotor using a low-power ARM computer, to- gether with a
visual-inertial odometry pipeline. We validate our approach in experiments
demonstrating (I) the contradiction between perception and action objectives,
and (II) improved behavior in extremely challenging lighting conditions
Faster than FAST: GPU-Accelerated Frontend for High-Speed VIO
The recent introduction of powerful embedded graphics processing units (GPUs)
has allowed for unforeseen improvements in real-time computer vision
applications. It has enabled algorithms to run onboard, well above the standard
video rates, yielding not only higher information processing capability, but
also reduced latency. This work focuses on the applicability of efficient
low-level, GPU hardware-specific instructions to improve on existing computer
vision algorithms in the field of visual-inertial odometry (VIO). While most
steps of a VIO pipeline work on visual features, they rely on image data for
detection and tracking, of which both steps are well suited for
parallelization. Especially non-maxima suppression and the subsequent feature
selection are prominent contributors to the overall image processing latency.
Our work first revisits the problem of non-maxima suppression for feature
detection specifically on GPUs, and proposes a solution that selects local
response maxima, imposes spatial feature distribution, and extracts features
simultaneously. Our second contribution introduces an enhanced FAST feature
detector that applies the aforementioned non-maxima suppression method.
Finally, we compare our method to other state-of-the-art CPU and GPU
implementations, where we always outperform all of them in feature tracking and
detection, resulting in over 1000fps throughput on an embedded Jetson TX2
platform. Additionally, we demonstrate our work integrated in a VIO pipeline
achieving a metric state estimation at ~200fps.Comment: IEEE International Conference on Intelligent Robots and Systems
(IROS), 2020. Open-source implementation available at
https://github.com/uzh-rpg/vili
Model Predictive Contouring Control for Time-Optimal Quadrotor Flight
In this article, 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 considers the full quadrotor model requires solving a difficult time allocation problem via optimization, which is computationally demanding (in the order of minutes or even hours). This is detrimental for replanning in the presence of disturbances. We overcome this issue by solving the time allocation problem and the control problem 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
Beauty and the Beast: Optimal Methods Meet Learning for Drone Racing
Autonomous micro aerial vehicles still struggle with fast and agile
maneuvers, dynamic environments, imperfect sensing, and state estimation drift.
Autonomous drone racing brings these challenges to the fore. Human pilots can
fly a previously unseen track after a handful of practice runs. In contrast,
state-of-the-art autonomous navigation algorithms require either a precise
metric map of the environment or a large amount of training data collected in
the track of interest. To bridge this gap, we propose an approach that can fly
a new track in a previously unseen environment without a precise map or
expensive data collection. Our approach represents the global track layout with
coarse gate locations, which can be easily estimated from a single
demonstration flight. At test time, a convolutional network predicts the poses
of the closest gates along with their uncertainty. These predictions are
incorporated by an extended Kalman filter to maintain optimal
maximum-a-posteriori estimates of gate locations. This allows the framework to
cope with misleading high-variance estimates that could stem from poor
observability or lack of visible gates. Given the estimated gate poses, we use
model predictive control to quickly and accurately navigate through the track.
We conduct extensive experiments in the physical world, demonstrating agile and
robust flight through complex and diverse previously-unseen race tracks. The
presented approach was used to win the IROS 2018 Autonomous Drone Race
Competition, outracing the second-placing team by a factor of two.Comment: 6 pages (+1 references
AlphaPilot: Autonomous Drone Racing
This paper presents 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, which 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 ranked
second at the 2019 AlphaPilot Challenge.Comment: Accepted at Robotics: Science and Systems 2020, associated video at
https://youtu.be/DGjwm5PZQT
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.Comment: 8 page
Nonlinear MPC for Quadrotor Fault-Tolerant Control
The mechanical simplicity, hover capabilities, and high agility of quadrotors
lead to a fast adaption in the industry for inspection, exploration, and urban
aerial mobility. On the other hand, the unstable and underactuated dynamics of
quadrotors render them highly susceptible to system faults, especially rotor
failures. In this work, we propose a fault-tolerant controller using nonlinear
model predictive control (NMPC) to stabilize and control a quadrotor subjected
to the complete failure of a single rotor. Differently from existing works,
which either rely on linear assumptions or resort to cascaded structures
neglecting input constraints in the outer-loop, our method leverages full
nonlinear dynamics of the damaged quadrotor and considers the thrust constraint
of each rotor. Hence, this method could effectively perform upset recovery from
extreme initial conditions. Extensive simulations and real-world experiments
are conducted for validation, which demonstrates that the proposed NMPC method
can effectively recover the damaged quadrotor even if the failure occurs during
aggressive maneuvers, such as flipping and tracking agile trajectories.Comment: 9 pages, 13 figure
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 letter, 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
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. In this article, we empirically compare 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 20 m/s (i.e., 72 km/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 the 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 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