# Autonomous 3D Exploration of Large Structures Using an UAV Equipped with a 2D LIDAR
## 2 Related Work
[PERSON] [24] uses the concept of a surface frontier, a free voxel with both an unknown and an occupied neighbour. Surface frontiers are favoured by the utility function to complete the surface inspection without exploring the entire scenario. In this approach, the user defines regions of interest that contain only one connected surface volume. To generate the path in a deterministic way, the path planner SPARTAN creates a sparsely connected graph across a tangential surface around obstacles maintaining a minimal clearance. [PERSON] et al. [33] presents a comparative study of exploration strategies both for a single vehicle and for cooperative exploration. This study compares the results of the nearest frontier, the utility function maximization by [PERSON] and [PERSON] [26], and a behaviour-based approach by [PERSON] [34] for single-vehicle exploration. The nearest frontier is expanded to multiple vehicles inspired by the work of [PERSON] et al. [35]. Similarly, the utility function is expanded to multi-vehicle exploration using a market-based approach, as presented by [PERSON] et al. [36]. Additionally, two systems that integrate the localization uncertainty are included in the study. One system uses a utility-cost function from the work of [PERSON] et al. [37] and a hybrid approach by [PERSON] et al. [38] that uses behaviour to return to positions with low uncertainty. This study establishes the application's goal as the decisive factor to choose the exploration strategy. When a high map quality is sought, the integrated techniques achieve better results. If minimizing the exploration time is the decisive factor, cost-utility must be addressed, knowing that utility techniques have a high information acquisition rate at first but increase the total exploration time because small parts of the environment are only explored at the end. In multi-robot approaches, when the number of robots increases, both map quality and execution time improve. However, global optimisation is best if each robot chooses the target, as is the case in market-based approaches. When the objective of the UAV is to collect surface information, exploration must address the NBV problem in order to place the sensor favourably. [PERSON] et al. [39] compares the impact of the volumetric information method on the surface coverage for NBV for occlusion aware, unobserved voxel, rear side voxel, rear ride entropy, proximity count, area factor, and average entropy. Concluding that the reconstruction can achieve most of its model completion within less than ten well-chosen views, regardless of the choice of volumetric information formulation. Regarding the sensor adopted to sample the world, for exploration in two dimensions, 2D laser sensors have been extensively used. For instance, [PERSON] et al. [40] applies it in planetary exploration and [PERSON] et al. [41] equips an UAV with a laser for the purpose of generating a 3D map although the exploration is conducted in 2D. However, the most common sensor used for exploration with UAVs are depth cameras [14; 16; 19; 20; 27; 29; 32; 39]. There are also examples of using a 3D LIDAR, combined with depth cameras [24]. Another aspect that gains relevance in an industrial setting is the minimum distance to an obstacle. In the related work presented here, when declared, the minimum distance to an obstacle varies from none in [14; 16], to 0.6 m in [29], to 2 m in [24]. Additionally, some approaches optimistically consider the unknown space as free [39]. In this paper, we propose a deterministic, autonomous system for 3D exploration using a 2D laser sensor. The system is able to run online and on-board an UAV that guarantees a minimum distance to obstacles of 2.5 m, while conservatively considering the unknown space as an obstacle. The developed modular architecture applies the frontier algorithm both at local and global levels, expanding the concept of a frontier surface into a surface neighbourhood.