Autonomous Ledge Detection and Landing with Multi-rotor UAV Swee King Phang 1 , Mohamed Redhwan Abdul Hamid 2 , Xudong Chen 2 , Feng Lin 2 Abstract—As surveillance and reconnaissance utilizing UAVs become more prominent today thanks to the advancement in MEMS sensors and small yet powerful microprocessors, vertical-take-off-and-landing (VTOL) vehicles such as multi- rotor UAVs dominate this area due to its capability of hovering in the air. The endurance, however, is a downside of such op- eration. In this manuscript, we propose a surveillance solution with multi-rotor UAV by perching at the edge of roof near the target-of-interest, enabling long hour monitoring capability. Challenges on the mechanical design and autonomous ledge detection of the UAV will be addressed and possible solutions will be discussed. Flight experiments were conducted and positive results will be published in this manuscript. I. I NTRODUCTION Over the years, research on unmanned aerial vehicles (UAVs) has been one of the leading topics in universities and research institutions. With the introduction of multi-rotor UAV in early year 2000, the ease of mechanical design of such UAV has led to multiple research on the control of such UAV, for example in [1], [2]. Then, as the control of multi-rotor system has reached a mature stage, researchers has switched their focuses to vision-based UAV localization methods [3], [4], [5], and towards LiDAR-based localization and mapping on UAV [6], [7], [8]. Besides the fundamental development of the UAVs as described above, many UAV systems were developed to perform specific operations, such as vertical replenishment of goods [9], recovery of UAVs on ships [10], and many more. One particular operation that is useful for surveillance that has not been widely explored by the researcher is the perching of small scale UAV on the edge of building roof. Specifically in Singapore, most of the high-rise building has ledges on their roof, as shown in Fig. 1. As surveillance and reconnaissance using UAVs becoming more prominent today, vertical-take-off-and-landing (VTOL) vehicles such as multi-rotor UAVs dominate this area due to its capability of hovering in the air [11]. The endurance, however, is the downside of such operation. A typical mutli- rotor UAV can last approximately 20 to 30 minutes while staying stationary in the air, while usually a surveillance job requires much longer period than this. In 2012, DARPA has launched a UAV related challenge called UAVForge, with one of the mission on the surveillance of a remote target area for a duration of 3 hours [12], [13]. It is almost impossible 1 Swee King Phang is with School of Engineering, Taylor’s Uni- versity, 1 Jalan Taylors, 47500 Subang Jaya, Selangor, Malaysia. sweeking.phang@taylors.edu.my 1 Mohamed Redhwan, Xudong Chen, and Feng Lin are with Temasek Laboratories, National University of Singapore, Singapore {tslmor,tslcx,linfeng}@nus.edu.sg Fig. 1. Typical roof top with ledge of buildings in Singapore for a multi-rotor UAV to last this long in the air. Instead, the UAV can be commanded to land and perch along the edge of roof of the buildings around the area of interest. In this manuscript, the development of a small scale multi- rotor UAV for the application of perching at ledges on the roof will be discussed. The area of discussion includes UAV platform modification for secure landing on ledge in Section II, autonomous flight control design for such a UAV in Section III, robust real-time ledge detection with LiDAR sensor in Section IV, and some of the flight trial results with the proposed algorithm in Section V. Finally concluding remarks to be made in the last section. II. HARDWARE PLATFORM This section will discuss and provide the specifications of the UAV chosen for the task of autonomous ledge detection and landing. The decision was based on the the following criteria: 1) A UAV capable to implement our proposed flight controller (i.e., self developed flight control software); 2) A UAV with sufficient load carrying capacity for essential sensors for perching operation to be carried in flight; and 3) Sufficient flight endurance (> 10 minutes) for the whole flight profile. The custom built multi-rotor platform from the National University of Singapore, codenamed T-Lion, was chosen as the platform for the mentioned task. It had a flight endurance of up to 15 minutes on a single 8000 mAh Lithium polymer (LiPo) battery, a payload carrying capacity of up to 2 kg with a bare 3 kg of fuselage weight. To enable physical landing on the ledge, a landing device was designed for the specific case of landing on a rectangular 2018 IEEE 14th International Conference on Control and Automation (ICCA) June 12-15, 2018. Anchorage, Alaska, USA 978-1-5386-6088-1/18/$31.00 ©2018 IEEE 42