195 research outputs found

    Robust Onboard Visual SLAM for Autonomous MAVs

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    Abstract. This paper presents a visual simultaneous localization and mapping (SLAM) system consisting of a robust visual odometry and an efficient back-end with loop closure detection and pose-graph optimization. Robustness of the visual odometry is achieved by utilizing dual cameras pointing different directions with no overlap in their respective fields of view mounted on an micro aerial vehicle (MAV). The theory behind this dual-camera visual odometry can be easily ex-tended to applications with multiple cameras. The back-end of the SLAM system maintains a keyframe-based global map, which is used for loop closure detec-tion. An adaptive-window pose-graph optimization method is proposed to refine keyframe poses of the global map and thus correct pose drift that is inherent in the visual odometry. The position of each map point is then refined implicitly due to its relative representation to its source keyframe. We demonstrate the efficiency of the proposed visual SLAM algorithm for applications onboard MAVs in ex-periments with both autonomous and manual flights. The pose tracking results are compared with the ground truth data provided by an external tracking system.

    Topomap: Topological Mapping and Navigation Based on Visual SLAM Maps

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    Visual robot navigation within large-scale, semi-structured environments deals with various challenges such as computation intensive path planning algorithms or insufficient knowledge about traversable spaces. Moreover, many state-of-the-art navigation approaches only operate locally instead of gaining a more conceptual understanding of the planning objective. This limits the complexity of tasks a robot can accomplish and makes it harder to deal with uncertainties that are present in the context of real-time robotics applications. In this work, we present Topomap, a framework which simplifies the navigation task by providing a map to the robot which is tailored for path planning use. This novel approach transforms a sparse feature-based map from a visual Simultaneous Localization And Mapping (SLAM) system into a three-dimensional topological map. This is done in two steps. First, we extract occupancy information directly from the noisy sparse point cloud. Then, we create a set of convex free-space clusters, which are the vertices of the topological map. We show that this representation improves the efficiency of global planning, and we provide a complete derivation of our algorithm. Planning experiments on real world datasets demonstrate that we achieve similar performance as RRT* with significantly lower computation times and storage requirements. Finally, we test our algorithm on a mobile robotic platform to prove its advantages.Comment: 8 page

    Visual SLAM for Autonomous Navigation of MAVs

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    This thesis focuses on developing onboard visual simultaneous localization and mapping (SLAM) systems to enable autonomous navigation of micro aerial vehicles (MAVs), which is still a challenging topic considering the limited payload and computational capability that an MAV normally has. In MAV applications, the visual SLAM systems are required to be very efficient, especially when other visual tasks have to be done in parallel. Furthermore, robustness in pose tracking is highly desired in order to enable safe autonomous navigation of an MAV in three-dimensional (3D) space. These challenges motivate the work in this thesis in the following aspects. Firstly, the problem of visual pose estimation for MAVs using an artificial landmark is addressed. An artificial neural network (ANN) is used to robustly recognize this visual marker in cluttered environments. Then a computational projective-geometry method is implemented for relative pose computation based on the retrieved geometry information of the visual marker. The presented vision system can be used not only for pose control of MAVs, but also for providing accurate pose estimates to a monocular visual SLAM system serving as an automatic initialization module for both indoor and outdoor environments. Secondly, autonomous landing on an arbitrarily textured landing site during autonomous navigation of an MAV is achieved. By integrating an efficient local-feature-based object detection algorithm within a monocular visual SLAM system, the MAV is able to search for the landing site autonomously along a predefined path, and land on it once it has been found. Thus, the proposed monocular visual solution enables autonomous navigation of an MAV in parallel with landing site detection. This solution relaxes the assumption made in conventional vision-guided landing systems, which is that the landing site should be located inside the field of view (FOV) of the vision system before initiating the landing task. The third problem that is addressed in this thesis is multi-camera visual SLAM for robust pose tracking of MAVs. Due to the limited FOV of a single camera, pose tracking using monocular visual SLAM may easily fail when the MAV navigates in unknown environments. Previous work addresses this problem mainly by fusing information from other sensors, like an inertial measurement unit (IMU), to achieve robustness of the whole system, which does not improve the robustness of visual SLAM itself. This thesis investigates solutions for improving the pose tracking robustness of a visual SLAM system by utilizing multiple cameras. A mathematical analysis of how measurements from multiple cameras should be integrated in the optimization of visual SLAM is provided. The resulting theory allows those measurements to be used for both robust pose tracking and map updating of the visual SLAM system. Furthermore, such a multi-camera visual SLAM system is modified to be a robust constant-time visual odometry. By integrating this visual odometry with an efficient back-end which consists of loop-closure detection and pose-graph optimization processes, a near-constant time multi-camera visual SLAM system is achieved for autonomous navigation of MAVs in large-scale environments.Diese Arbeit konzentriert sich auf die Entwicklung von integrierten Systemen zur gleichzeitigen Lokalisierung und Kartierung (Simultaneous Localization and Mapping, SLAM) mit Hilfe visueller Sensoren, um die autonome Navigation von kleinen Luftfahrzeugen (Micro Aerial Vehicles, MAVs) zu ermöglichen. Dies ist noch immer ein anspruchsvolles Thema angesichts der meist begrenzten Nutzlast und Rechenleistung eines MAVs. Die dafür eingesetzten visuellen SLAM Systeme müssen sehr effizient zu sein, vor allem wenn parallel noch andere visuelle Aufgaben durchgeführt werden sollen. Darüber hinaus ist eine robuste Positionsschätzung sehr wichtig, um die sichere autonome Navigation des MAVs im dreidimensionalen (3D) Raum zu ermöglichen. Diese Herausforderungen motivieren die vorliegende Arbeit gemäß den folgenden Gesichtspunkten: Zuerst wird das Problem bearbeitet, die Pose eines MAVs mit Hilfe einer künstlichen Markierung visuell zu schätzen. Ein künstliches neuronales Netz wird verwendet, um diese visuelle Markierung auch in anspruchsvollen Umgebungen zuverlässig zu erkennen. Anschließend wird ein Verfahren aus der projektiven Geometrie eingesetzt, um die relative Pose basierend auf der gemessenen Geometrie der visuellen Markierung zu ermitteln. Das vorgestellte Bildverarbeitungssystem kann nicht nur zur Regelung der Pose des MAVs verwendet werden, sondern auch genaue Posenschätzungen zur automatischen Initialisierung eines monokularen visuellen SLAM-Systems im Innen- und Außenbereich liefern. Anschließend wird die autonome Landung eines MAVs auf einem beliebig texturierten Landeplatz während autonomer Navigation erreicht. Durch die Integration eines effizienten Objekterkennungsalgorithmus, basierend auf lokalen Bildmerkmalen in einem monokularen visuellen SLAM-System, ist das MAV in der Lage den Landeplatz autonom entlang einer vorgegebenen Strecke zu suchen, und auf ihm zu landen sobald er gefunden wurde. Die vorgestellte Lösung ermöglicht somit die autonome Navigation eines MAVs bei paralleler Landeplatzerkennung. Diese Lösung lockert die gängige Annahme in herkömmlichen Systemen zum kamerageführten Landen, dass der Landeplatz vor Beginn der Landung innerhalb des Sichtfelds des Bildverarbeitungssystems liegen muss. Das dritte in dieser Arbeit bearbeitete Problem ist visuelles SLAM mit mehreren Kameras zur robusten Posenschätzung für MAVs. Aufgrund des begrenzten Sichtfelds von einer einzigen Kamera kann die Posenschätzung von monokularem visuellem SLAM leicht fehlschlagen, wenn sich das MAV in einer unbekannten Umgebung bewegt. Frühere Arbeiten versutchen dieses Problem hauptsächlich durch die Fusionierung von Informationen anderer Sensoren, z.B. eines Inertialsensors (Inertial Measurement Unit, IMU) zu lösen um eine höhere Robustheit des Gesamtsystems zu erreichen, was die Robustheit des visuellen SLAM-Systems selbst nicht verbessert. Die vorliegende Arbeit untersucht Lösungen zur Verbesserung der Robustheit der Posenschätzung eines visuellen SLAM-Systems durch die Verwendung mehrerer Kameras. Wie Messungen von mehreren Kameras in die Optimierung für visuelles SLAM integriert werden können wird mathematisch analysiert. Die daraus resultierende Theorie erlaubt die Nutzung dieser Messungen sowohl zur robusten Posenschätzung als auch zur Aktualisierung der visuellen Karte. Ferner wird ein solches visuelles SLAM-System mit mehreren Kameras modifiziert, um in konstanter Laufzeit robuste visuelle Odometrie zu berechnen. Die Integration dieser visuellen Odometrie mit einem effizienten Back-End zur Erkennung von geschlossener Schleifen und der Optimierung des Posengraphen ermöglicht ein visuelles SLAM-System mit mehreren Kameras und fast konstanter Laufzeit zur autonomen Navigation von MAVs in großen Umgebungen

    Scalable Distributed Collaborative Tracking and Mapping with Micro Aerial Vehicles

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    This paper describes work on a distributed framework for collaborative multi-robot localisation and mapping with large teams of Micro Aerial Vehicles (MAVs). We demonstrate the benefits of running both image capture and frame-to-frame tracking on the same device while offloading the more computationally intensive aspects of map creation and optimization to an off-board computer. We show no impact on the accuracy of pose estimates of this distributed approach and indeed demonstrate a robustness to delay that improves localisation performance. The bandwidth requirements of our system are much lower than similar systems which enables us to accommodate larger teams of MAVs. In the results section we demonstrate the performance of our system in both simulated and real-world environments

    Scalable Distributed Collaborative Tracking and Mapping with Micro Aerial Vehicles

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    This paper describes work on a distributed framework for collaborative multi-robot localisation and mapping with large teams of Micro Aerial Vehicles (MAVs). We demonstrate the benefits of running both image capture and frame-to-frame tracking on the same device while offloading the more computationally intensive aspects of map creation and optimization to an off-board computer. We show no impact on the accuracy of pose estimates of this distributed approach and indeed demonstrate a robustness to delay that improves localisation performance. The bandwidth requirements of our system are much lower than similar systems which enables us to accommodate larger teams of MAVs. In the results section we demonstrate the performance of our system in both simulated and real-world environments
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