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Multi-agent pathfinding for unmanned aerial vehicles
Unmanned aerial vehicles (UAVs), commonly known as drones, have become more and
more prevalent in recent years. In particular, governmental organizations and companies
around the world are starting to research how UAVs can be used to perform tasks such
as package deliver, disaster investigation and surveillance of key assets such as pipelines,
railroads and bridges. NASA is currently in the early stages of developing an air traffic
control system specifically designed to manage UAV operations in low-altitude airspace.
Companies such as Amazon and Rakuten are testing large-scale drone deliver services in
the USA and Japan.
To perform these tasks, safe and conflict-free routes for concurrently operating UAVs must
be found. This can be done using multi-agent pathfinding (mapf) algorithms, although
the correct choice of algorithms is not clear. This is because many state of the art mapf
algorithms have only been tested in 2D space in maps with many obstacles, while UAVs
operate in 3D space in open maps with few obstacles. In addition, when an unexpected
event occurs in the airspace and UAVs are forced to deviate from their original routes
while inflight, new conflict-free routes must be found. Planning for these unexpected
events is commonly known as contingency planning. With manned aircraft, contingency
plans can be created in advance or on a case-by-case basis while inflight. The scale at
which UAVs operate, combined with the fact that unexpected events may occur anywhere
at any time make both advanced planning and planning on a case-by-case basis impossible.
Thus, a new approach is needed. Online multi-agent pathfinding (online mapf) looks to
be a promising solution. Online mapf utilizes traditional mapf algorithms to perform path
planning in real-time. That is, new routes for UAVs are found while inflight.
The primary contribution of this thesis is to present one possible approach to UAV
contingency planning using online multi-agent pathfinding algorithms, which can be used
as a baseline for future research and development. It also provides an in-depth overview
and analysis of offline mapf algorithms with the goal of determining which ones are likely
to perform best when applied to UAVs. Finally, to further this same goal, a few different
mapf algorithms are experimentally tested and analyzed
Improving Continuous-time Conflict Based Search
Conflict-Based Search (CBS) is a powerful algorithmic framework for optimally
solving classical multi-agent path finding (MAPF) problems, where time is
discretized into the time steps. Continuous-time CBS (CCBS) is a recently
proposed version of CBS that guarantees optimal solutions without the need to
discretize time. However, the scalability of CCBS is limited because it does
not include any known improvements of CBS. In this paper, we begin to close
this gap and explore how to adapt successful CBS improvements, namely,
prioritizing conflicts (PC), disjoint splitting (DS), and high-level
heuristics, to the continuous time setting of CCBS. These adaptions are not
trivial, and require careful handling of different types of constraints,
applying a generalized version of the Safe interval path planning (SIPP)
algorithm, and extending the notion of cardinal conflicts. We evaluate the
effect of the suggested enhancements by running experiments both on general
graphs and -neighborhood grids. CCBS with these improvements significantly
outperforms vanilla CCBS, solving problems with almost twice as many agents in
some cases and pushing the limits of multiagent path finding in continuous-time
domains.Comment: This is a pre-print of the paper accepted to AAAI 202
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