{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,3,20]],"date-time":"2026-03-20T15:46:28Z","timestamp":1774021588119,"version":"3.50.1"},"reference-count":38,"publisher":"MDPI AG","issue":"5","license":[{"start":{"date-parts":[[2017,5,7]],"date-time":"2017-05-07T00:00:00Z","timestamp":1494115200000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/501100007273","name":"Comisi\u00f3n Interministerial de Ciencia y Tecnolog\u00eda","doi-asserted-by":"publisher","award":["TRA2013-48314-C3-1-R"],"award-info":[{"award-number":["TRA2013-48314-C3-1-R"]}],"id":[{"id":"10.13039\/501100007273","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100007273","name":"Comisi\u00f3n Interministerial de Ciencia y Tecnolog\u00eda","doi-asserted-by":"publisher","award":["TRA2015-63708-R"],"award-info":[{"award-number":["TRA2015-63708-R"]}],"id":[{"id":"10.13039\/501100007273","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/501100007273","name":"Comisi\u00f3n Interministerial de Ciencia y Tecnolog\u00eda","doi-asserted-by":"publisher","award":["TRA2016-78886-C3-1-R"],"award-info":[{"award-number":["TRA2016-78886-C3-1-R"]}],"id":[{"id":"10.13039\/501100007273","id-type":"DOI","asserted-by":"publisher"}]},{"DOI":"10.13039\/100012818","name":"Comunidad de Madrid","doi-asserted-by":"publisher","award":["S2013-MIT-2713"],"award-info":[{"award-number":["S2013-MIT-2713"]}],"id":[{"id":"10.13039\/100012818","id-type":"DOI","asserted-by":"publisher"}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>One of the most challenging problems in the domain of autonomous aerial vehicles is the designing of a robust real-time obstacle detection and avoidance system. This problem is complex, especially for the micro and small aerial vehicles, that is due to the Size, Weight and Power (SWaP) constraints. Therefore, using lightweight sensors (i.e., Digital camera) can be the best choice comparing with other sensors; such as laser or radar.For real-time applications, different works are based on stereo cameras in order to obtain a 3D model of the obstacles, or to estimate their depth. Instead, in this paper, a method that mimics the human behavior of detecting the collision state of the approaching obstacles using monocular camera is proposed. The key of the proposed algorithm is to analyze the size changes of the detected feature points, combined with the expansion ratios of the convex hull constructed around the detected feature points from consecutive frames. During the Aerial Vehicle (UAV) motion, the detection algorithm estimates the changes in the size of the area of the approaching obstacles. First, the method detects the feature points of the obstacles, then extracts the obstacles that have the probability of getting close toward the UAV. Secondly, by comparing the area ratio of the obstacle and the position of the UAV, the method decides if the detected obstacle may cause a collision. Finally, by estimating the obstacle 2D position in the image and combining with the tracked waypoints, the UAV performs the avoidance maneuver. The proposed algorithm was evaluated by performing real indoor and outdoor flights, and the obtained results show the accuracy of the proposed algorithm compared with other related works.<\/jats:p>","DOI":"10.3390\/s17051061","type":"journal-article","created":{"date-parts":[[2017,5,8]],"date-time":"2017-05-08T11:45:16Z","timestamp":1494243916000},"page":"1061","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":102,"title":["Obstacle Detection and Avoidance System Based on Monocular Camera and Size Expansion Algorithm for UAVs"],"prefix":"10.3390","volume":"17","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-0212-6400","authenticated-orcid":false,"given":"Abdulla","family":"Al-Kaff","sequence":"first","affiliation":[{"name":"Intelligent Systems Lab, Universidad Carlos III de Madrid, Leganes, 28911 Madrid, Spain"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-6291-5009","authenticated-orcid":false,"given":"Fernando","family":"Garc\u00eda","sequence":"additional","affiliation":[{"name":"Intelligent Systems Lab, Universidad Carlos III de Madrid, Leganes, 28911 Madrid, Spain"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-3764-5083","authenticated-orcid":false,"given":"David","family":"Mart\u00edn","sequence":"additional","affiliation":[{"name":"Intelligent Systems Lab, Universidad Carlos III de Madrid, Leganes, 28911 Madrid, Spain"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-2618-857X","authenticated-orcid":false,"given":"Arturo","family":"De La Escalera","sequence":"additional","affiliation":[{"name":"Intelligent Systems Lab, Universidad Carlos III de Madrid, Leganes, 28911 Madrid, Spain"}]},{"given":"Jos\u00e9","family":"Armingol","sequence":"additional","affiliation":[{"name":"Intelligent Systems Lab, Universidad Carlos III de Madrid, Leganes, 28911 Madrid, Spain"}]}],"member":"1968","published-online":{"date-parts":[[2017,5,7]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"32","DOI":"10.1109\/MAES.2013.6516147","article-title":"An experimental UAV system for search and rescue challenge","volume":"28","author":"Erdos","year":"2013","journal-title":"IEEE Aerosp. Electron. Syst. Mag."},{"key":"ref_2","unstructured":"Kamel, B., Santana, M.C.S., and De Almeida, T.C. (2010, January 29\u201331). Position estimation of autonomous aerial navigation based on Hough transform and Harris corners detection. Proceedings of the 9th WSEAS International Conference On Circuits, Systems, Electronics, Control & Signal Processing, Athens, Greece."},{"key":"ref_3","doi-asserted-by":"crossref","unstructured":"Fraundorfer, F., Heng, L., Honegger, D., Lee, G.H., Meier, L., Tanskanen, P., and Pollefeys, M. (2012, January 7\u201312). Vision-based autonomous mapping and exploration using a quadrotor MAV. Proceedings of the 2012 IEEE\/RSJ International Conference on Intelligent Robots and Systems (IROS), Vilamoura, Algarve, Portugal.","DOI":"10.1109\/IROS.2012.6385934"},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"73","DOI":"10.1007\/s10514-010-9205-0","article-title":"Cooperative manipulation and transportation with aerial robots","volume":"30","author":"Michael","year":"2011","journal-title":"Auton. Robots"},{"key":"ref_5","unstructured":"Eschmann, C., Kuo, C.M., Kuo, C.H., and Boller, C. (2012, January 2\u20136). Unmanned aircraft systems for remote building inspection and monitoring. Proceedings of the 6th European Workshop on Structural Health Monitoring, Dresden, Germany."},{"key":"ref_6","doi-asserted-by":"crossref","unstructured":"Choi, S., and Kim, E. (2015). Image Acquisition System for Construction Inspection Based on Small Unmanned Aerial Vehicle. Advanced Multimedia and Ubiquitous Engineering, Springer.","DOI":"10.1007\/978-3-662-47487-7_40"},{"key":"ref_7","doi-asserted-by":"crossref","unstructured":"Bills, C., Chen, J., and Saxena, A. (2011, January 9\u201313). Autonomous MAV flight in indoor environments using single image perspective cues. Proceedings of the 2011 IEEE International Conference on Robotics and Automation (ICRA), Shanghai, China.","DOI":"10.1109\/ICRA.2011.5980136"},{"key":"ref_8","doi-asserted-by":"crossref","unstructured":"Blosch, M., Weiss, S., Scaramuzza, D., and Siegwart, R. (2010, January 3\u20138). Vision based MAV navigation in unknown and unstructured environments. Proceedings of the 2010 IEEE International Conference on Robotics and Automation (ICRA), Anchorage, AK, USA.","DOI":"10.1109\/ROBOT.2010.5509920"},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"27","DOI":"10.1109\/MRA.2006.1678136","article-title":"Conflict-free navigation in unknown urban environments","volume":"13","author":"Shim","year":"2006","journal-title":"IEEE Robot. Autom. Mag."},{"key":"ref_10","unstructured":"Luo, D., Wang, F., Wang, B., and Chen, B. (2012, January 25\u201327). Implementation of obstacle avoidance technique for indoor coaxial rotorcraft with Scanning Laser Range Finder. Proceedings of the 2012 31st Chinese Control Conference (CCC), Hefei, China."},{"key":"ref_11","doi-asserted-by":"crossref","unstructured":"Shang, E., An, X., Li, J., and He, H. (2014, January 8\u201311). A novel setup method of 3D LIDAR for negative obstacle detection in field environment. Proceedings of the 2014 IEEE 17th International Conference on Intelligent Transportation Systems (ITSC), Qingdao, China.","DOI":"10.1109\/ITSC.2014.6957888"},{"key":"ref_12","unstructured":"Ariyur, K., Lommel, P., and Enns, D. (2005, January 8\u201310). Reactive inflight obstacle avoidance via radar feedback. Proceedings of the American Control Conference, Portland, OR, USA."},{"key":"ref_13","doi-asserted-by":"crossref","unstructured":"Broggi, A., Cattani, S., Patander, M., Sabbatelli, M., and Zani, P. (2013, January 6\u20139). A full-3D voxel-based dynamic obstacle detection for urban scenario using stereo vision. Proceedings of the 2013 16th International IEEE Conference on Intelligent Transportation Systems (ITSC), The Hague, Netherlands.","DOI":"10.1109\/ITSC.2013.6728213"},{"key":"ref_14","doi-asserted-by":"crossref","unstructured":"Gao, Y., Ai, X., Rarity, J., and Dahnoun, N. (2011, January 9\u201311). Obstacle detection with 3D camera using U-V-Disparity. Proceedings of the 2011 7th International Workshop on Systems, Signal Processing and their Applications (WOSSPA), Tipaza, Algeria.","DOI":"10.1109\/WOSSPA.2011.5931462"},{"key":"ref_15","unstructured":"Na, I., Han, S.H., and Jeong, H. (2011, January 19\u201322). Stereo-based road obstacle detection and tracking. Proceedings of the 2011 13th International Conference on Advanced Communication Technology (ICACT), PyeongChang, South Korea."},{"key":"ref_16","unstructured":"Li, J., and Li, X.-M. (2011, January 9\u201311). Vision-based navigation and obstacle detection for UAV. Proceedings of the 2011 International Conference on Electronics, Communications and Control (ICECC), Ningbo, China."},{"key":"ref_17","doi-asserted-by":"crossref","unstructured":"Saha, S., Natraj, A., and Waharte, S. (2014, January 13\u201314). A real-time monocular vision-based frontal obstacle detection and avoidance for low cost UAVs in GPS denied environment. Proceedings of the 2014 IEEE International Conference on Aerospace Electronics and Remote Sensing Technology (ICARES), Yogyakarta, Indonesia.","DOI":"10.1109\/ICARES.2014.7024382"},{"key":"ref_18","doi-asserted-by":"crossref","unstructured":"Majumder, S., Shankar, R., and Prasad, M. (2015, January 19\u201320). Obstacle size and proximity detection using stereo images for agile aerial robots. Proceedings of the 2015 2nd International Conference on Signal Processing and Integrated Networks (SPIN), Noida, Delhi-NCR, India.","DOI":"10.1109\/SPIN.2015.7095261"},{"key":"ref_19","doi-asserted-by":"crossref","first-page":"13","DOI":"10.1117\/12.571554","article-title":"Obstacle avoidance for unmanned air vehicles using optical flow probability distributions","volume":"5609","author":"Merrell","year":"2004","journal-title":"Mob. Robots XVII"},{"key":"ref_20","doi-asserted-by":"crossref","unstructured":"Hrabar, S., Sukhatme, G., Corke, P., Usher, K., and Roberts, J. (2005, January 2\u20136). Combined optic-flow and stereo-based navigation of urban canyons for a UAV. Proceedings of the 2005 IEEE\/RSJ International Conference on Intelligent Robots and Systems (IROS 2005), Edmonton, AB, Canada.","DOI":"10.1109\/IROS.2005.1544998"},{"key":"ref_21","doi-asserted-by":"crossref","unstructured":"Beyeler, A., Zufferey, J.C., and Floreano, D. (2007, January 10\u201314). 3D Vision-based Navigation for Indoor Microflyers. Proceedings of the 2007 IEEE International Conference on Robotics and Automation, Rome, Italy.","DOI":"10.1109\/ROBOT.2007.363170"},{"key":"ref_22","doi-asserted-by":"crossref","unstructured":"Celik, K., Chung, S.J., Clausman, M., and Somani, A. (2009, January 10\u201315). Monocular vision SLAM for indoor aerial vehicles. Proceedings of the IEEE\/RSJ International Conference on Intelligent Robots and Systems, St. Louis, MO, USA.","DOI":"10.1109\/IROS.2009.5354050"},{"key":"ref_23","unstructured":"Chavez, A., and Gustafson, D. (December, January 30). Vision-based obstacle avoidance using SIFT features. Proceedings of the 5th International Symposium on Advances in Visual Computing: Part II, ISVC\u201909, Las Vegas, NV, USA."},{"key":"ref_24","doi-asserted-by":"crossref","unstructured":"Mori, T., and Scherer, S. (2013, January 6\u201310). First results in detecting and avoiding frontal obstacles from a monocular camera for micro unmanned aerial vehicles. Proceedings of the 2013 IEEE International Conference on Robotics and Automation (ICRA), Karlsruhe, Germany.","DOI":"10.1109\/ICRA.2013.6630807"},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"1003","DOI":"10.1016\/j.visres.2003.07.012","article-title":"Asymmetry in the perception of motion-in-depth","volume":"44","author":"Shirai","year":"2004","journal-title":"Vis. Res."},{"key":"ref_26","doi-asserted-by":"crossref","unstructured":"Gibson, J.J. (2014). The Ecological Approach to Visual Perception: Classic Edition, Psychology Press.","DOI":"10.4324\/9781315740218"},{"key":"ref_27","doi-asserted-by":"crossref","unstructured":"De Croon, G., de Weerdt, E., De Wagter, C., and Remes, B. (2010, January 14\u201318). The appearance variation cue for obstacle avoidance. Proceedings of the 2010 IEEE International Conference on Robotics and Biomimetics (ROBIO), Tianjin, China.","DOI":"10.1109\/ROBIO.2010.5723570"},{"key":"ref_28","doi-asserted-by":"crossref","first-page":"96","DOI":"10.1109\/MRA.2008.919023","article-title":"Optic-Flow-Based Collision Avoidance","volume":"15","author":"Green","year":"2008","journal-title":"IEEE Robot. Autom. Mag."},{"key":"ref_29","doi-asserted-by":"crossref","first-page":"397","DOI":"10.1108\/00022661111173270","article-title":"Obstacle avoidance for small UAVs using monocular vision","volume":"83","author":"Lee","year":"2011","journal-title":"Aircr. Eng. Aerosp. Technol."},{"key":"ref_30","doi-asserted-by":"crossref","unstructured":"Ma, Z., Hu, T., Shen, L., Kong, W., and Zhao, B. (2015, January 24\u201328). A detection and relative direction estimation method for UAV in sense-and-avoid. Proceedings of the 2015 IEEE International Conference on Information and Automation, Gothenburg, Sweden.","DOI":"10.1109\/ICInfA.2015.7279738"},{"key":"ref_31","doi-asserted-by":"crossref","unstructured":"Byrne, J., and Taylor, C.J. (2009, January 6\u20137). Expansion segmentation for visual collision detection and estimation. Proceedings of the International Conference on Robotics and Automation (ICRA\u201909), Kobe, Japan.","DOI":"10.1109\/ROBOT.2009.5152487"},{"key":"ref_32","doi-asserted-by":"crossref","first-page":"911","DOI":"10.1016\/j.proeng.2012.07.262","article-title":"Moving Obstacle Avoidance of a Mobile Robot Using a Single Camera","volume":"41","author":"Kim","year":"2012","journal-title":"Procedia Eng."},{"key":"ref_33","doi-asserted-by":"crossref","first-page":"315","DOI":"10.1002\/rob.20414","article-title":"A Survey of Advances in Guidance, Navigation and Control of Unmanned Rotorcraft Systems","volume":"29","author":"Kendoul","year":"2012","journal-title":"J. Field Rob."},{"key":"ref_34","doi-asserted-by":"crossref","first-page":"152","DOI":"10.1016\/j.paerosci.2015.01.001","article-title":"Sense and avoid technologies with applications to unmanned aircraft systems: Review and prospects","volume":"74","author":"Yu","year":"2015","journal-title":"Prog. Aerosp. Sci."},{"key":"ref_35","doi-asserted-by":"crossref","first-page":"1","DOI":"10.1016\/j.paerosci.2015.10.002","article-title":"A survey of autonomous vision-based See and Avoid for Unmanned Aircraft Systems","volume":"80","author":"Mcfadyen","year":"2016","journal-title":"Prog. Aerosp. Sci."},{"key":"ref_36","doi-asserted-by":"crossref","unstructured":"Krajnik, T., Vonasek, V., Fiser, D., and Faigl, J. (2011). AR-drone as a platform for robotic research and education. Research and Education in Robotics-EUROBOT, Springer.","DOI":"10.1007\/978-3-642-21975-7_16"},{"key":"ref_37","first-page":"11","article-title":"The comparison of point feature detectors and descriptors in the context of robot navigation","volume":"7","author":"Schmidt","year":"2013","journal-title":"J. Autom. Mob. Robot. Intell. Syst."},{"key":"ref_38","doi-asserted-by":"crossref","unstructured":"Jeong, C.Y., and Choi, S. (August, January 31). A comparison of keypoint detectors in the context of pedestrian counting. Proceedigns of the 2016 International Conference on Information and Communication Technology Convergence (ICTC), Jeju Island, Korea.","DOI":"10.1109\/ICTC.2016.7763399"}],"container-title":["Sensors"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/1424-8220\/17\/5\/1061\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,11]],"date-time":"2025-10-11T18:34:57Z","timestamp":1760207697000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/1424-8220\/17\/5\/1061"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2017,5,7]]},"references-count":38,"journal-issue":{"issue":"5","published-online":{"date-parts":[[2017,5]]}},"alternative-id":["s17051061"],"URL":"https:\/\/doi.org\/10.3390\/s17051061","relation":{},"ISSN":["1424-8220"],"issn-type":[{"value":"1424-8220","type":"electronic"}],"subject":[],"published":{"date-parts":[[2017,5,7]]}}}