Decentralized Multi-Agent Path Finding in Warehouse Environments for Fleets of Mobile Robots with Limited Communication Range

The 2D Multi-Agent Path Finding (MAPF) problem aims at finding collision-free paths for a number of agents, from a set of start locations to a set of goal positions in a known 2D environment. MAPF has been studied in theoretical computer science, robotics, and artificial intelligence over several decades, due to its importance for robot navigation. It is currently experiencing significant scientific progress due to its relevance in automated warehousing (such as those operated by Amazon) and in other contemporary application areas. In this paper, we demonstrate that some recently developed MAPF algorithms apply more broadly than currently believed in the MAPF research community. In particular, we describe the 3D Pipe Routing (PR) problem, which aims at placing collision-free pipes from given start locations to given goal locations in a known 3D environment. The MAPF and PR problems are similar: a solution to a MAPF instance is a set of blocked cells in x-y-t space, while a solution to the corresponding PR instance is a set of blocked cells in x-y-z space. We show how to use this similarity to apply several recently developed MAPF algorithms to the PR problem, and discuss their performance on real-world PR instances. This opens up a new direction of industrial relevance for the MAPF research community.

Multi-Agent Path Finding (MAPF) deals with finding conflict-free paths for a set of agents from an initial configuration to a given target configuration. The Lifelong MAPF (LMAPF) problem is a well-studied online version of MAPF in which an agent receives a new target when it reaches its current target. The common approach for solving LMAPF is to treat it as a sequence of MAPF problems, periodically replanning from the agents’ current configurations to their current targets. A significant drawback of this approach is that in MAPF the agents must reach a configuration in which all agents are at their targets simultaneously. Coordinating the agents’ paths such that they occupy their targets at the same time is needlessly restrictive for LMAPF. Techniques have been proposed to indirectly mitigate this drawback. In this position paper, we describe cases where these mitigation techniques fail. As an alternative, we propose to solve LMAPF problems by solving a sequence of modified MAPF problems, in which the objective is for each agent to eventually visit its target, but not necessarily for all agents to do so simultaneously. We refer to this MAPF variant as MAPF for Lifelong (MAPF4L) and propose how to solve it by modifying several existing MAPF algorithms. A limited experimental evaluation identifies some cases where using a MAPF4L algorithm can improve the system throughput significantly.

**Read paper on the following link:** https://ifaamas.org/Proceedings/aamas2022/pdfs/p1464.pdf **Abstract:** With the rising demand of deploying robot teams in autonomous warehouses and factories, the Multi-agent Path Finding (MAPF) problem has drawn more and more attention. However, the classical MAPF problem focuses on navigating agent teams to reach single goal locations while avoiding collisions, and thus it cannot plan the case in which each agent is associated with a sequence of goal locations, and precedence constraints across goal sequences should be respected. Planning with these goal sequences and precedence constraints is important in real-world multi-agent systems. For example, a mobile robot needs to move an inventory pod to several stations for delivering different packages, which is captured by a sequence of goal locations, and another robot can only pick up a package after it has been delivered to the corresponding station, which can be modeled as precedence constraints across goal sequences. In this paper, we formalize the Multi-Agent Path Finding with Precedence Dependency (MAPF-PD) problem along with two algorithms to systematically solve this problem: Conflict-Based Search with Precedence Dependency (CBS-PD) is a complete and optimal algorithm; and Priority-Based Search with Precedence Dependency (PBS-PD) is an incomplete and suboptimal algorithm but shows great scalability and obtains near-optimal solutions in practice. Our experimental results show that our algorithms can scale to dozens of agents with hundreds of goal locations and precedence constraints.

With the rising demand for deploying robot teams in autonomous warehouses and factories, the Multi-Agent Path Finding (MAPF) problem has drawn more and more attention. The classical MAPF problem and most of its variants focus on navigating agent teams to goal locations while avoiding collisions. However, they do not take into account any precedence constraints that agents should respect when reaching their goal locations. Planning with precedence constraints is important for real-world multi-agent systems. For example, a mobile robot can only pick up a package at a station after it has been delivered by another robot. In this paper, we study the Multi-Agent Path Finding with Precedence Constraints (MAPF-PC) problem, in which agents need to visit sequences of goal locations while satisfying precedence constraints between the goal locations. We propose two algorithms for solving this problem systematically: Conflict-Based Search with Precedence Constraints (CBS-PC) is complete and optimal, and Priority-Based Search with Precedence Constraints (PBS-PC) is incomplete but more efficient in finding near-optimal solutions in practice. Our experimental results show that CBS-PC scales to dozens of agents and hundreds of goal locations and precedence constraints, and PBS-PC scales to hundreds of agents, around one thousand goal locations, and hundreds of precedence constraints.

The multi-agent path finding (MAPF) problem asks to find a set of paths on a graph such that when synchronously following these paths the agents never encounter a conflict. In the most widespread MAPF formulation, the so-called Classical MAPF, the agents’ sizes are neglected and two types of conflicts are considered: occupying the same vertex or using the same edge at the same time step. Meanwhile in numerous practical applications, e.g. in robotics, taking into account the agents’ sizes is vital to ensure that the MAPF solutions can be safely executed. Introducing large agents yields an additional type of conflict arising when one agent follows an edge and its body overlaps with the body of another agent that is actually not using this same edge (e.g. staying still at some distinct vertex of the graph). Until now it was not clear how harder the problem gets when such conflicts are to be considered while planning. Specifically, it was known that Classical MAPF problem on an undirected graph can be solved in polynomial time, however no complete polynomial-time algorithm was presented to solve MAPF with large agents. In this paper we, for the first time, establish that the latter problem is NP-hard and, thus, if P≠NP no polynomial algorithm for it can, unfortunately, be presented. Our proof is based on the prevalent in the field technique of reducing the seminal 3-SAT problem (which is known to be an NP-complete problem) to the problem at hand. In particular, for an arbitrary 3-SAT formula we procedurally construct a dedicated graph with specific start and goal vertices and show that the given 3-SAT formula is satisfiable iff the corresponding path finding instance has a solution.

Multi-Robot-Arm Motion Planning (M-RAMP) is a challenging problem featuring complex single-agent planning and multi-agent coordination. Recent advancements in extending the popular Conflict-Based Search (CBS) algorithm have made large strides in solving Multi-Agent Path Finding (MAPF) problems. However, fundamental challenges remain in applying CBS to M-RAMP. A core challenge is the existing reliance of the CBS framework on conservative "complete" constraints. These constraints ensure solution guarantees but often result in slow pruning of the search space -- causing repeated expensive single-agent planning calls. Therefore, even though it is possible to leverage domain knowledge and design incomplete M-RAMP-specific CBS constraints to more efficiently prune the search, using these constraints would render the algorithm itself incomplete. This forces practitioners to choose between efficiency and completeness. In light of these challenges, we propose a novel algorithm, Generalized ECBS, aimed at removing the burden of choice between completeness and efficiency in MAPF algorithms. Our approach enables the use of arbitrary constraints in conflict-based algorithms while preserving completeness and bounding sub-optimality. This enables practitioners to capitalize on the benefits of arbitrary constraints and opens a new space for constraint design in MAPF that has not been explored. We provide a theoretical analysis of our algorithms, propose new "incomplete" constraints, and demonstrate their effectiveness through experiments in M-RAMP.

We address rule-based algorithms for multi-agent path finding (MAPF). MAPF is a task of finding non-conflicting paths connecting agents' initial and goal positions in a shared environment specified via an undirected graph. Rule-based algorithms use a fixed set of predefined primitive operations to move agents to their goal positions in a complete manner. We propose to apply spectral clustering on the underlying graph to decompose the graph into highly connected components and move agents to their goal cluster first before the rule-based algorithm is applied. The benefit of this approach is twofold: (1) the rule-based algorithms are often more efficient on highly connected clusters and (2) we can potentially run the algorithms in parallel on individual clusters.

The Multi-Agent Path Finding (MAPF) problem, i.e., the scheduling of multiple agents to reach their destinations, has been widely investigated. Testing MAPF systems is challenging, due to the complexity and variety of scenarios and the agents’ distribution and interaction. Moreover, MAPF testing suffers from the oracle problem, i.e., it is not always clear whether a test shows a failure or not. Indeed, only considering whether the agents reach their destinations without collision is not sufficient. Other properties related to the ‘quality’ of the generated paths should be assessed, e.g., an agent should not follow an unnecessarily long path. To tackle this issue, this article proposes MET-MAPF, a Metamorphic Testing approach for MAPF systems. We identified 10 Metamorphic Relations (MRs) that a MAPF system should guarantee, designed over the environment in which agents operate, the behaviour of the single agents and the interactions among agents. Starting from the different MRs, MET-MAPF automatically generates test cases addressing them, so possibly exposing different types of failures. Experimental results show that MET-MAPF can indeed find MR violations not exposed by approaches that only consider the completion of the mission as test oracle. Moreover, experiments show that different MRs expose different types of violations.

Multi-agent path finding (MAPF) has been widely used for many real-world problems, such as warehouse automation. Reinforcement learning (RL) has been employed to develop partially observable distributed MAPF algorithms that can be scaled to any number of agents or map sizes. However, the RL-based MAPF policy may cause oscillation behavior, deadlock and detours, which can lead to the failure of MAPF. This paper introduces R<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{3}$</tex-math></inline-formula>, a new hybrid policy framework for MAPF, in which the RL-based policy is incorporated with the random walk policy and the reverse resumable A<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{*}$</tex-math></inline-formula>-based policy. When an agent exhibits oscillation behavior or falls into a deadlock, the random walk policy helps the agent escape the trouble. When no other agents are within the agent's field of vision, we utilize the reverse resumable A<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{*}$</tex-math></inline-formula>-based policy to assist the agent in avoiding any potential detours. This paper uses the state-of-the-art RL-based MAPF algorithm, Decision Causal Communication, to do the simulation experiments on eight benchmark grid maps. The results show that R<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{3}$</tex-math></inline-formula> can significantly improve the performance of the baseline Decision Causal Communication algorithm.

Portfolio-based algorithm selection can help in choosing the best suited algorithm for a given task while leveraging the complementary strengths of the candidates. Solving the Multi-Agent Path Finding (MAPF) problem optimally has been proven to be NP-Hard. Furthermore, no single optimal algorithm has been shown to have the fastest runtime for all MAPF problem instances, and there are no proven approaches for when to use each algorithm. To address these challenges, we develop MAPFASTER, a smaller and more accurate deep learning based architecture aiming to be deployed in fleet management systems to select the fastest MAPF solver in a multi-robot setting. MAPF problem instances are encoded as images and passed to the model for classification into one of the portfolio's candidates. We evaluate our model against state-of-the-art Optimal-MAPF-Algorithm selectors, showing +5.42% improvement in accuracy while being 7.1× faster to train. The dataset, code and analysis used in this research can be found at https://github.com/jeanmarcalkazzi/mapfaster.

Multi-agent pathfinding (MAPF) holds significant utility within autonomous systems, however, the calculation and memory space required for multi-agent path finding (MAPF) grows exponentially as the number of agents increases. This often results in some MAPF instances being unsolvable under limited computational resources and memory space, thereby limiting the application of MAPF in complex scenarios. Hence, we propose a decomposition approach for MAPF instances, which breaks down instances involving a large number of agents into multiple isolated subproblems involving fewer agents. Moreover, we present a framework to enable general MAPF algorithms to solve each subproblem independently and merge their solutions into one conflict-free final solution, and avoid loss of solvability as much as possible. Unlike existing works that propose isolated methods aimed at reducing the time cost of MAPF, our method is applicable to all MAPF methods. In our results, we apply decomposition to multiple state-of-the-art MAPF methods using a classic MAPF benchmark\footnote{https://movingai.com/benchmarks/mapf.html}. The decomposition of MAPF instances is completed on average within 1s, and its application to seven MAPF methods reduces the memory usage or time cost significantly, particularly for serial methods. Based on massive experiments, we speculate the possibilty about loss of solvability caused by our method is $&amp;lt;$ 1\%. To facilitate further research within the community, we have made the source code of the proposed algorithm publicly available\footnote{https://github.com/JoeYao-bit/LayeredMAPF/tree/minimize\_dependence}.

The Multi-Agent Warehouse Rearrangement (MAWR) problem calls for computing agents plans such that they collectively rearrange a warehouse environment from a given layout which species the location of every movable obstacle in the environment, to a goal layout. It is a natural variant of the well-studied Multi-Agent Path Finding (MAPF) and Multi-Agent Pickup and Delivery (MAPD) problems, which have numerous applications in warehouse automation. Similar to MAPF and MAPD, the problem is computationally hard and existing methods forgo any optimality guarantees while computing solutions to the problem. In contrast, in this work we present the first fully-coupled search-based makespan-optimal approach for solving the MAWR problem. This is done by shifting the algorithmic viewpoint from being agent centric to obstacle centric: Common MAPF and MAPD algorithms are agent-centric wherein tasks are assigned to agents, and the agents’ paths are planned to fulfill these tasks. In contrast, the approach we present here is obstacle-centric: Our algorithm iterates between two phases: Ph1 planning optimal paths for the movable obstacles using an adaptation of the celebrated CBS MAPF algorithm and Ph2 attempting to realize the movable obstacles paths using a network-flow algorithm. We prove that our approach guarantees minimal-makespan solutions and empirically demonstrate that it achieves better plans than state-of-the-art approaches for MAWR.

Multi-Agent Path Finding (MAPF) is an important problem with a variety of applications. Its aim is to find collision-free paths for agents having separate start and goal positions. This work proposes a new lexicographic bi-objective MAPF considering different task types, where agents are divided into two kinds to perform critical and acritical tasks. This is common in practical intelligent warehousing scenarios where a critical/acritical-task-performing agent (called c-agent and a-agent, respectively) may represent a full-load/no-load one or the one conducting urgent/non-urgent tasks. The primary objective is to minimize the sum-of-costs of c-agents, while the secondary objective is to minimize the sum-of-costs of a-agents. Two MAPF algorithms are modified to fit and solve the concerned problem for the first time. Moreover, four improvement strategies are embedded to the proposed algorithms and proved to be effective in solving MAPF problems with different task types.

Multi-Agent Path Finding (MAPF) is the problem of moving multiple agents from starts to goals without collisions. Lifelong MAPF (LMAPF) extends MAPF by continuously assigning new goals to agents. We present our winning approach to the 2023 League of Robot Runners LMAPF competition, which leads us to several interesting research challenges and future directions. In this paper, we outline three main research challenges. The first challenge is to search for high-quality LMAPF solutions within a limited planning time (e.g., 1s per step) for a large number of agents (e.g., 10,000) or extremely high agent density (e.g., 97.7%). We present future directions such as developing more competitive rule-based and anytime MAPF algorithms and parallelizing state-of-the-art MAPF algorithms. The second challenge is to alleviate congestion and the effect of myopic behaviors in LMAPF algorithms. We present future directions, such as developing moving guidance and traffic rules to reduce congestion, incorporating future prediction and real-time search, and determining the optimal agent number. The third challenge is to bridge the gaps between the LMAPF models used in the literature and real-world applications. We present future directions, such as dealing with more realistic kinodynamic models, execution uncertainty, and evolving systems.

A common assumption made in multi-robot research is that robots operate in a fully networked environment, where any two robots can communicate reliably at any time. However, in real-world missions over large geographical areas robots have a limited communication range and signal propagation conditions may be unpredictable due to obstacles or interferences. In such cases, communication limitations must be taken explicitly into account. In this paper, we present a multirobot routing algorithmfor dense environments under limited communication range. We focus on the aspect of careful mission planning in order to guarantee continuous connectivity and mission completeness, while offering a guarantee on performance under a variety of team objectives. The proposed algorithms are demonstrated in a large-scale domain using realistic simulations on the Player/Stage platform.