Towards Topological Exploration of Abandoned Mines

Towards Topological Exploration of Abandoned Mines Aaron Morris, David Silver, Dave Ferguson, and Scott Thayer Robotics Institute Carnegie Mellon University Pittsburgh, PA, USA acmorr, dsilver, dif, sthayer ¡ @cmu.edu Abstract— The need for reliable maps of subterranean spaces too hazardous for humans to occupy has motivated the use of robotic technology as mapping tools. As such, we present a systemic approach to autonomous topological exploration of a mine environment to facilitate the process of mapping. This approach focuses upon the interaction of three high-level processes: topological planning, intersection identification and local navigation. Topological planning tasks the robot to investigate stretches of mine corridor for the purpose of collecting data. Intersection identification converts sensory input into topological components used to construct an online topological map and provide the robot with a global sense of position. Local navigation transforms topological exploration objectives into robot actuation enabling traversal of mine corridors. These processes are described in detail with results presented from experiments conducted at a research coal mine near Pittsburgh, PA. I. INTRODUCTION Fig. 1. Groundhog, our current robotic mine mapping platform As discussed in previous mine mapping work [1], [2], [3], [4], abandoned mines pose a significant threat to both surface life and nearby subterranean operations. Mine no external communication (including GPS) makes it easy maps, the primary source of information on mine struc- for a robot to become lost. Therefore, along with physical tures, are key components in resolving this threat and robustness, an autonomous mine mapping robot must be preventing mine-related disasters; however, in the case equipped with a highly reliable navigational framework. of abandoned mines, such maps are often inaccurate and Our current research is aimed at addressing each of unreliable if they exist at all. the aforementioned challenges. We are interested in both The components necessary to verify old maps or to the creation of systems that can robustly explore mine produce new ones are (1) data on void structure and (2) environments and the development of algorithms that can a means of acquiring this data. To date, direct observation generate accurate maps of these environments. Previous from within the void has been the most reliable method for work in this area [3] has alleviated some of the major obtaining quality data. The caveat, however, to establishing exploration challenges by dealing primarily with the task of a physical presence within the mine is that human contact portal inspection. Portal inspection restricts exploration to is often precluded due to the hostile nature of mine en- the primary entrances of an abandoned mine and confines vironments, especially when upkeep has been abandoned. the robot into traversing a single corridor. This simplifies As a result, robotic technologies have been applied to navigation and eliminates issues concerning global local- the domain of abandoned coal mines to offer nonhuman ization and loop closure. The push for more intricate mine methods of observation and data collection. [2], [3], [5]. inspection tasks, however, demands that these issues be While robots can physically endure the harsh conditions resolved. To enable functionality such as perimeter discov- of abandoned mines, mobility and autonomy for a robotic ery or full exploration, more capable navigation and global system is a significant challenge. Mine floors, often cov- positioning are required if the robot is to successfully ered with fallen timbers, roof bolts, collapsed rock, and explore multiple corridors. countless other mine-related artifacts, exhibit rugged terrain In this paper, we present a systemic approach to robotic that is difficult to negotiate. Mine walls and ceilings also exploration of a room-and-pillar [6] mine. This approach is possess projecting obstacles that challenge robot maneu- based upon the interaction of three core concepts: topologi- verability. In addition, the room-and-pillar mine structure cal planning, intersection detection, and corridor navigation prevalent in abandoned coal mines is problematic to ex- (Figure 2). Topological planning maintains a topologi- ploration; the combination of a highly cyclic design with cal representation of the mine and generates navigation Fig. 3. This map was generated from data acquired during experimen- tation and utilizes our offline globally consistent mapping techniques. It shows the highly cyclic nature of room-and-pillar mines. Fig. 2. System overview. Groundhog from portal inspection to multi-corridor explo- objectives using this topological framework. Intersection ration. These requirements include (1) a representation of detection monitors sensory input and identifies corridor the mine for global localization, (2) a task-level planner that intersections, which create the nodes of the topological can decompose an exploration objective into intermediate representation. Local navigation decomposes navigation navigation goals, and (3) a navigation system that can objectives into motion planning goals that direct the robot realize the navigation goals through path planning. along corridors for the exploration of new areas. The remainder of this paper discusses the concepts of our Topological representations [7] coincide nicely with the topological exploration method. In Section II we discuss inherent structure of room-and-pillar mines. Room-and- related research in this area. We then present our system pillar mines consist almost exclusively of narrow corridors in Section III and results from an experimental mine in and corridor intersections (see Figure 3). A topological Section IV. We conclude with discussion and extensions. map is a graph representation of a given environment where the nodes of the graph correspond to distinct lo- II. MOTIVATION FOR TOPOLOGICAL EXPLORATION cations in the environment and the edges correspond to Towards achieving our mine mapping goal, we have a direct path between two such locations. For mines, constructed a 700 kg custom-built ATV-type robotic plat- nodes and edges correspond to intersections and corridors, form known as Groundhog (Figure 1) that is physically respectively. In addition, topological representations exhibit tailored for operation in the harsh conditions of abandoned several advantages over grid or feature-based approaches. mines [3]. Groundhog has been used extensively in both Such advantages include a compact data representation test and abandoned mine environments, accruing hundreds and a reduction in processing time for the construction, of hours of mine navigation with 8 successful portal manipulation, and utilization of these maps [8]. entry experiments in the abandoned Mathies mine outside Implicit in representing a mine as a topological map of Pittsburgh, PA. From these experiments, log data has is that the robot has some way of determining node generated globally consistent large-scale maps (Figure 3) locations from its sensor data, as well as the edges leaving using offline techniques [1], [2]. Over the course of its each node. Once on an edge, the robot must be able to lifetime, Groundhog has evolved into a system that is follow that edge to the next node. A topology that is highly proficient at autonomously traversing and mapping detectable from local sensor data is said to be embedded isolated mine corridors, successfully navigating over 2 km in the robot’s environment. Embedded topologies exhibit of abandoned mine. useful properties in the context of task-level planning for For the next phase of Groundhog’s development, we exploration. wish to extend exploration tasks beyond isolated corridors For task-level planning, topological maps have proven and towards a complete cyclic mine structure. In this paper, to be very useful in robotic exploration tasks [9] [10]. we have taken the first step towards full exploration by Unexplored edges in a topological map correspond to un- performing acyclic or tree exploration. The challenges in- explored regions of the robot’s environment. If the chosen volved in cyclic and acyclic exploration are nearly identical topology is a roadmap [11], exploring every edge in a with the exception of loop closure. Loop closure is still a topological map will allow the robot to explore every part difficult problem in globally consistent mapping research of the freespace. and one we are currently investigating separately. Instead, If this roadmap should be a tree (a condition that is this paper addresses the pertinent requirements that take enforced in the context of this paper), then the environment can be fully explored by maintaining a list of nodes that have unexplored edges. A sequence of edge traversals can then be planned [8] to direct the robot to traverse an unexplored edge. This process repeats until all edges are explored. For navigation, three dimensional motion planning ( 2D position and 1D orientation) is necessary given that the robot must choose between different corridors at each intersection. To further complicate 3D motion planning, the robot must cope with the complex arrangement of obstacles inherent in these environments. As described in [3], obstacles can project from the ceiling and walls of mines, which places additional constraints on such a path planner. Terrain analysis and select aspects of path generation (a) Strong Node were covered in previous work [1], [2]. The navigation system utilized in our

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