Department of Informatics

Large-scale infrastructures are prone to deterioration due to age, environmental influences, and heavy usage. Ensuring their safety through regular inspections and maintenance is crucial to prevent incidents that can significantly affect public safety and the environment. This is especially pertinent in the context of electrical power networks, which, while essential for energy provision, can also be sources of forest fires. Intelligent drones have the potential to revolutionize inspection and maintenance, eliminating the risks for human operators, increasing productivity, reducing inspection time, and improving data collection quality. However, most of the current methods and technologies in aerial robotics have been trialed primarily in indoor testbeds or outdoor settings under strictly controlled conditions, always within the line of sight of human operators. Additionally, these methods and technologies have typically been evaluated in isolation, lacking comprehensive integration. This paper introduces the first autonomous system that combines various innovative aerial robots. This system is designed for extended-range inspections beyond the visual line of sight, features aerial manipulators for maintenance tasks, and includes support mechanisms for human operators working at elevated heights. The paper further discusses the successful validation of this system on numerous electrical power lines, with aerial robots executing flights over 10 kilometers away from their ground control stations.

Drones have the potential to revolutionize power line inspection by increasing productivity, reducing inspection time, improving data quality, and eliminating the risks for human operators. Current state-of-the-art systems for power line inspection have two shortcomings: (i) control is decoupled from perception and needs accurate information about the location of the power lines and masts; (ii) collision avoidance is decoupled from the power line tracking, which results in poor tracking in the vicinity of the power masts, and, consequently, in decreased data quality for visual inspection. In this work, we propose a model predictive controller (MPC) that overcomes these limitations by tightly coupling perception and action. Our controller generates commands that maximize the visibility of the power lines while, at the same time, safely avoiding the power masts. For power line detection, we propose a lightweight learning-based detector that is trained only on synthetic data and is able to transfer zero-shot to real-world power line images. We validate our system in simulation and real-world experiments on a mock-up power line infrastructure.

Recently, neural control policies have outperformed existing model-based planning-and-control methods for autonomously navigating quadrotors through cluttered environments in minimum time. However, they are not perception aware, a crucial requirement in vision-based navigation due to the camera's limited field of view and the underactuated nature of a quadrotor. We propose a method to learn neural network policies that achieve perception-aware, minimum-time flight in cluttered environments. Our method combines imitation learning and reinforcement learning (RL) by leveraging a privileged learning-by-cheating framework. Using RL, we first train a perception-aware teacher policy with full-state information to fly in minimum time through cluttered environments. Then, we use imitation learning to distill its knowledge into a vision-based student policy that only perceives the environment via a camera. Our approach tightly couples perception and control, showing a significant advantage in computation speed (10x faster) and success rate. We demonstrate the closed-loop control performance using a physical quadrotor and hardware-in-the-loop simulation at speeds up to 50 km/h.

Considering visual localization accuracy at the planning time gives preference to robot motion that can be better localized and is of benefit to vision-based navigation. To integrate the knowledge about localization accuracy in planning, a common approach is to compute the Fisher information of the pose estimation process from a set of sparse landmarks. However, this approach scales linearly with the number of landmarks and introduces redundant computation. To overcome these drawbacks, we propose the first dedicated map for evaluating the Fisher information of 6 degree-of-freedom visual localization for perception-aware planning. We separate and precompute the rotational invariant component from the Fisher information and store it in a voxel grid, namely the Fisher information field. The Fisher information for arbitrary poses can then be computed from the field in constant time. Experimental results show that the proposed Fisher information field can be applied to different planning algorithms and is at least 10 times faster than using the point cloud. Moreover, the proposed map is differentiable, resulting in better performance in trajectory optimization algorithms.

Most existing exploration approaches either assume a perfect state estimate, or, with a heavy computational burden, establish a globally consistent map representation through map optimization. In this research, we investigate whether the task of exploration (fully cover a previously unknown volume) can be solved without either of these two requirements. Our approach is to propose a novel representation which allows to resolve whether there is space left to explore in a globally topological and locally metric manner, exploiting place recognition.

For mobile robots to localize robustly, actively considering the perception requirement at the planning stage is essential. In this paper, we propose a novel representation for active visual localization. By formulating the Fisher information and sensor visibility carefully, we are able to summarize the localization information into a discrete grid, namely the Fisher information field. The information for arbitrary poses can then be computed from the field in constant time, without the need of costly iterating all the 3D landmarks. Experimental results on simulated and real-world data show the great potential of our method in efficient active localization and perception- aware planning. To benefit related research, we release our implementation of the information field to the public.

We present a 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 sensing 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, together with a visual-inertial odometry pipeline. We validate our approach in experiments demonstrating (I) the contradiction between perception and action objectives, and (I) improved behavior in extremely challenging lighting conditions.

To reach a given destination safely and accurately, a micro aerial vehicle needs to be able to avoid obstacles and minimize its state estimation uncertainty at the same time. To achieve this goal, we propose a perception-aware receding horizon approach. In our method, a single forward- looking camera is used for state estimation and mapping. Using the information from the monocular state estimation and mapping system, we generate a library of candidate trajectories and evaluate them in terms of perception quality, collision probability, and distance to the goal. The best trajectory to execute is then selected as the one that maximizes a reward function based on these three metrics. To the best of our knowledge, this is the first work that integrates active vision within a receding horizon navigation framework for a goal reaching task. We demonstrate by simulation and real-world experiments on an actual quadrotor that our active approach leads to improved state estimation accuracy in a goal-reaching task when compared to a purely-reactive navigation system, especially in difficult scenes (e.g., weak texture).

In this paper, we address one of the main challenges towards autonomous quadrotor flight in complex environments, which is flight through narrow gaps. We present a method that allows a quadrotor to autonomously and safely pass through a narrow, inclined gap using only its onboard visual-inertial sensors and computer. Previous works have addressed quadrotor flight through gaps using external motion-capture systems for state estimation. Instead, we estimate the state by fusing gap detection from a single onboard camera with an IMU. Our method generates a trajectory that considers geometric, dynamic, and perception constraints: during the approach maneuver, the quadrotor always faces the gap to allow state estimation, while respecting the vehicle dynamics; during the traverse through the gap, the distance of the quadrotor to the edges of the gap is maximized. Furthermore, we replan the trajectory during its execution to cope with the varying uncertainty of the state estimate. We successfully evaluate and demonstrate the proposed approach in many real experiments, achieving a success rate of 80% and gap orientations up to 45 degrees. To the best of our knowledge, this is the first work that addresses and successfully reports aggressive flight through narrow gaps using only onboard sensing and computing.

We address the problem of planning a path for a ground robot through unknown terrain, using observations from a flying robot. In search and rescue missions, which are our target scenarios, the time from arrival at the disaster site to the delivery of aid is critically important. Previous works required exhaustive exploration before path planning, which is time-consuming but eventually leads to an optimal path for the ground robot. Instead, we propose active exploration of the environment, where the flying robot chooses regions to map in a way that optimizes the overall response time of the system, which is the combined time for the air and ground robots to execute their missions. In our approach, we estimate terrain classes throughout our terrain map, and we also add elevation information in areas where the active exploration algorithm has chosen to perform 3D reconstruction. This terrain information is used to estimate feasible and efficient paths for the ground robot. By exploring the environment actively, we achieve superior response times compared to both exhaustive and greedy exploration strategies. We demonstrate the performance and capabilities of the proposed system in simulated and real-world outdoor experiments. To the best of our knowledge, this is the first work to address ground robot path planning using active aerial exploration.

While most of the existing work on path planning focuses on reaching a goal as fast as possible, or with minimal effort, these approaches disregard the appearance of the environment and only consider the geometric structure. Vision-controlled robots, however, need to leverage the photometric information in the scene to localize themselves and perform egomotion estimation. In this work, we argue that motion planning for vision-controlled robots should be perception-aware in that the robot should also favor texture-rich areas to minimize the localization uncertainty during a goal-reaching task. Thus, we describe how to optimally incorporate the photometric information (i.e., texture) of the scene, in addition to the the geometric information, to compute the uncertainty of vision-based localization during path planning.

The Information Gain Based Active Reconstruction Framework is a modular, robot-agnostic, software package for performing next-best-view planning for volumetric object reconstruction using a range sensor. Our implementation can be easily adapted to any mobile robot equipped with any camera-based range sensor (e.g stereo camera, structured light sensor) to iteratively observe an object to generate a volumetric map and a point cloud model. The algorithm allows the user to define the information gain metric for choosing the next best view, and many formulations for these metrics are evaluated and compared in our ICRA paper. This framework is released open source as a ROS-compatible package for autonomous 3D reconstruction tasks.

Download the code from GitHub.

The estimation of the depth uncertainty makes REMODE extremely attractive for motion planning and active-vision problems. In this work, we investigate the following problem: Given the image of a scene, what is the trajectory that a robot-mounted camera should follow to allow optimal dense 3D reconstruction? The solution we propose is based on maximizing the information gain over a set of candidate trajectories. In order to estimate the information that we expect from a camera pose, we introduce a novel formulation of the measurement uncertainty that accounts for the scene appearance (i.e., texture in the reference view), the scene depth, and the vehicle pose. We successfully demonstrate our approach in the case of realtime, monocular reconstruction from a small quadrotor and validate the effectiveness of our solution in both synthetic and real experiments. This is the first work on active, monocular dense reconstruction, which chooses motion trajectories that minimize perceptual ambiguities inferred by the texture in the scene.