Navigation is an essential capability for autonomous robots. In particular, visual navigation has been a major research topic in robotics because cameras are lightweight, power-efficient sensors that provide rich information on the environment. However, the main challenge of visual navigation is that it requires substantial computational power and memory for visual processing and storage of the results. As of yet, this has precluded its use on small, extremely resource-constrained robots such as lightweight drones. Inspired by the parsimony of natural intelligence, we propose an insect-inspired approach toward visual navigation that is specifically aimed at extremely resource-restricted robots. It is a route-following approach in which a robot's outbound trajectory is stored as a collection of highly compressed panoramic images together with their spatial relationships as measured with odometry. During the inbound journey, the robot uses a combination of odometry and visual homing to return to the stored locations, with visual homing preventing the buildup of odometric drift. A main advancement of the proposed strategy is that the number of stored compressed images is minimized by spacing them apart as far as the accuracy of odometry allows. To demonstrate the suitability for small systems, we implemented the strategy on a tiny 56-gram drone. The drone could successfully follow routes up to 100 meters with a trajectory representation that consumed less than 20 bytes per meter. The presented method forms a substantial step toward the autonomous visual navigation of tiny robots, facilitating their more widespread application.

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Obstacle avoidance is an important capability for flying robots. But for robots with limited resources, such as small drones this becomes particularly challenging. Bug algorithms have been proposed to solve path planning with only minimal resources. And stereo vision provides a rich description of the world for limited weight but typically has a limited Field of View (FoV) and is fixed to the drone frame to further reduce weight. Based on these, a computationally light 3D path-planning algorithm is proposed. The proposed algorithm is called Frustumbug and is based on the Wedgebug algorithm since this algorithm copes well with a limited FoV. Since Wedgebug only addresses 2D problems, the Local-ϵ-Tangent-Graph (LETG) is used to extend the path planning to 3D. Disparity images are obtained through an optimized stereo block matching algorithm. Frustumbug copes well with noisy range sensor data and includes 3D trajectories like reversing, climbing and descending maneuvers to avoid or escape local minima. The algorithm has been tested with 225 flights in two challenging simulated environments and achieved a success rate of 96%. Here, 3.6% did not reach the goal and 0.4% collided. Frustumbug has been implemented on a 20-gram stereo vision system and was tested in the real world on a MAV. This shows the potential for small drones to reach their targets fully autonomously based on very limited resources. @en

To investigate how an unmanned air vehicle can detect manned aircraft with a single microphone, an audio data set is created in which unmanned air vehicle ego-sound and recorded aircraft sound are mixed together. A convolutional neural network is used to perform air traffic detection. Due to restrictions on flying unmanned air vehicles close to aircraft, the data set has to be artificially produced, so the unmanned air vehicle sound is captured separately from the aircraft sound. They are then mixed with unmanned air vehicle recordings, during which labels are given indicating whether the mixed recording contains aircraft audio or not. The model is a convolutional neural network that uses the features Mel frequency cepstral coefficient, spectrogram or Mel spectrogram as input. For each feature, the effect of unmanned air vehicle/aircraft amplitude ratio, the type of labeling, the window length and the addition of third party aircraft sound database recordings are explored. The results show that the best performance is achieved using the Mel spectrogram feature. The performance increases when the unmanned air vehicle/aircraft amplitude ratio is decreased, when the time window is increased or when the data set is extended with aircraft audio recordings from a third party sound database. Although the currently presented approach has a number of false positives and false negatives that is still too high for real-world application, this study indicates multiple paths forward that can lead to an interesting performance. Finally, the data set is provided as open access@en

In the field of robotics, a major challenge is achieving high levels of autonomy with small vehicles that have limited mass and power budgets. The main motivation for designing such small vehicles is that compared to their larger counterparts, they have the potential to be safer, and hence be available and work together in large numbers. One of the key components in micro robotics is efficient software design to optimally utilize the computing power available. This paper describes the computer vision and control algorithms used to achieve autonomous flight with the ∼30g tailless flapping wing robot, used to participate in the International Micro Air Vehicle Conference and Competition (IMAV 2018) indoor microair vehicle competition. Several tasks are discussed: line following, circular gate detection and fly through. The emphasis throughout this paper is on augmenting traditional techniques with the goal to make these methods work with limited computing power while obtaining robust behavior.

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With a growing number of drones, the risk of collision with other air traffic or fixed obstacles increases. New safety measures are required to keep the operation of Unmanned Aerial Vehicles (UAVs) safe. One of these measures is the use of a Collision Avoidance System (CAS), a system that helps the drone autonomously detect and avoid obstacles. The design of a Collision Avoidance System is a complex task with many smaller subproblems, as illustrated by Albaker and Rahim [1]. How should the drone sense nearby obstacles? When is there a risk of collision? What should the drone do when a conflict is detected? All of these questions need to be answered to develop a functional Collision Avoidance System. However, all of these subproblems – except the sensing of obstacles – only concern the behavior of the vehicle. They can be solved independently of the target platform as long as it can perform the required maneuvers; it does not matter whether it is a UAV or a larger vehicle. The sensing of the environment, on the other hand, is the only subproblem that places requirements on the hardware, specifically the sensors that should be carried by the UAV. It is the hardware that sets UAVs apart from other vehicles. Unlike autonomous cars, other groundbased vehicles or larger aircraft, UAVs have only a small payload capacity. It is therefore not practical to carry large or heavy sensors such as LIDAR or radar for obstacle avoidance. Instead, obstacle avoidance on UAVs requires clever use of lightweight sensors: cameras, microphones or antennae. This research will therefore focus on the sensing of the environment. Out of the sensors mentioned above – cameras, microphones and antennae – cameras are the only ones that can detect nearly all groundbased obstacles and other air traffic; microphones and antennae are limited to detection of sources of noise or radio signals1. Therefore, this research will focus on the visual detection of obstacles. The field of computer vision is welldeveloped; it may already be possible to find an adequate solution for visual obstacle detection using existing stereo vision methods like Semiglobal Matching (SGM) [23]. These methods, however, only use a fraction of the information present in the images to estimate depth – the disparity. Other cues such as the apparent size of known objects are completely ignored. The use of appearance cues for depth estimation is a relatively new development driven largely by the advent of Deep Learning, which allows these cues to be learned from large, labeled datasets. As long as the UAV’s operational environment is similar to this training dataset it should be possible to use appearance cues in a CAS. However, this is difficult to guarantee and may require a prohibitively large training set. SelfSupervised Learning may provide a solution to this problem. After training on an initial dataset, the UAV will continue to collect new training samples during operation. This allows it to ‘adapt’ to its operational environment and to learn new depth cues that are relevant in that environment. SelfSupervised Learning for depth map estimation is a young field, the first practical examples started to appear around 2016 (e.g. [17]). Most of the current literature is focused on automotive applications @en

In the field of robotics, a major challenge is achieving high levels of autonomy with small vehicles that have limited mass and power budgets. The main motivation for designing such small vehicles is that, compared to their larger counterparts, they have the potential to be safer, and hence be available and work together in large numbers. One of the key components in micro robotics is efficient software design to optimally utilize the computing power available. This paper describes the computer vision and control algorithms used to achieve autonomous flight with the _30-gram tailless flapping wing robot, used to participate in the IMAV 2018 indoor micro air vehicle competition. Several tasks are discussed: line following, and circular gate detection and fly-through. The emphasis throughout this paper is on augmenting traditional techniques with the goal to make these methods work with limited computing power while obtaining robust behaviour. @en

To investigate how an Unmanned Air Vehicle (UAV) can detect manned aircraft with a single microphone, an audio data set is created in which UAV ego-sound and recorded aircraft sound are mixed together. A convolutional neural network is used to perform the air traffic detection. Due to restrictions on flying UAVs close to aircraft, the data set has to be artificially produced, so the UAV sound is captured separately from the aircraft sound. They are then mixed with UAV recordings, during which labels are given indicating whether the mixed recording contains aircraft audio or not. The model is a CNN which uses the features MFCC, spectrogram or Mel spectrogram as input. For each feature the effect of UAV/aircraft amplitude ratio, the type of labeling, the window length and the addition of third party aircraft sound database recordings is explored. The results show that the best performance is achieved using the Mel spectrogram feature. The performance increases when the UAV/aircraft amplitude ratio is decreased, when the time window is increased or when the data set is extended with aircraft audio recordings from a third party sound database. Although the currently presented approach has a number of false positives and false negatives that is still too high for real-world application, this study indicates multiple paths forward that can lead to an interesting performance. In addition, the data set is provided as open access, allowing the community to contribute to the improvement of the detection task. @en

Deep neural networks have lead to a breakthrough in depth estimation from single images. Recent work shows that the quality of these estimations is rapidly increasing. It is clear that neural networks can see depth in single images. However, to the best of our knowledge, no work currently exists that analyzes what these networks have learned. In this work we take four previously published networks and investigate what depth cues they exploit. We find that all networks ignore the apparent size of known obstacles in favor of their vertical position in the image. The use of the vertical position requires the camera pose to be known; however, we find that these networks only partially recognize changes in camera pitch and roll angles. Small changes in camera pitch are shown to disturb the estimated distance towards obstacles. The use of the vertical image position allows the networks to estimate depth towards arbitrary obstacles - even those not appearing in the training set - but may depend on features that are not universally present.@en