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arxiv: 2509.14127 · v2 · submitted 2025-09-17 · 💻 cs.RO · cs.MA

Relay-Based Coordination for Energy-Efficient Multi-Robot Pickup and Delivery

Pith reviewed 2026-05-18 15:50 UTC · model grok-4.3

classification 💻 cs.RO cs.MA
keywords multi-robot coordinationpickup and deliveryrelay networksVoronoi diagramsSteiner treesenergy efficiencytask allocation
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The pith

Relay-based coordination through Voronoi-Steiner networks reduces multi-robot delivery travel by 31% on average

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper proposes VCST-RCP, a framework that builds a shared relay backbone using Voronoi-derived exchange interfaces and Steiner tree optimization for multi-robot package pickup and delivery. Instead of direct transport, packages move through this relay network with handoffs between robots. This reduces total fleet travel distance by an average of 31%, with peaks near 50%, outperforming Hungarian assignment and OR-Tools CVRP solvers. The gains lead to over 50% higher delivery efficiency in packages per kilometer. An ablation study shows that relay placement optimization is the main driver of these improvements rather than just spatial partitioning.

Core claim

VCST-RCP operates in two stages: first constructing a sparse relay backbone by combining Voronoi-derived exchange interfaces with Steiner tree optimization, and second synthesizing robot-level pickup, relay, and delivery schedules under capacity and service-time constraints. This relay-based approach reduces redundant long-haul motion and achieves an average 31% reduction in total fleet travel distance compared to Hungarian assignment, with statistically significant improvements.

What carries the argument

The VCST-RCP framework that explicitly treats inter-robot relays as a design primitive and organizes package flow through a shared relay network constructed from Voronoi interfaces and Steiner trees.

If this is right

  • Relay optimization provides substantially larger gains than adapting spatial partitioning alone.
  • Total fleet travel distance drops by 31% on average and up to nearly 50% in tested instances.
  • Delivery efficiency rises by over 50% when measured as packages delivered per kilometer.
  • Statistically significant improvements hold across multiple problem scales with p less than 10 to the minus 3.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Extending the relay concept to dynamic environments could allow robots to adapt the backbone as new packages arrive.
  • Adding modest handoff costs in the model might still retain most benefits if relay points are chosen to minimize extra motion.
  • Physical robot tests could confirm if simulation gains persist when communication and synchronization delays are real.

Load-bearing premise

The assumption that inter-robot package handoffs at the computed relay points incur no extra time, energy, or failure costs beyond the modeled capacity and service-time constraints.

What would settle it

Running physical experiments with actual robots performing handoffs and measuring total energy consumption and delivery times to check if the 31% travel reduction holds under real-world conditions.

Figures

Figures reproduced from arXiv: 2509.14127 by Alkesh K. Srivastava, Jared Michael Levin, Philip Dames.

Figure 1
Figure 1. Figure 1: Conceptual comparison of classical planners and our approach. [PITH_FULL_IMAGE:figures/full_fig_p001_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Minimum Spanning Tree vs. Steiner tree for three terminals forming [PITH_FULL_IMAGE:figures/full_fig_p002_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: VCST-RCP system overview. Left— Robots induce a Voronoi partitioning of the workspace; each shared boundary yields a relay candidate (orange diamond). Middle—Stage 1: A Steiner relay trunk is built to connect the depot to all goals with minimal travel-time cost. Right—Stage 2: Robots are assigned pickup/receiver roles and goals, and an asynchronous schedule (drop now, pick up later) is compiled under capac… view at source ↗
Figure 4
Figure 4. Figure 4: Illustration of Steiner Trunk Construction (Stage 1). (a) Terminals [PITH_FULL_IMAGE:figures/full_fig_p004_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Scenario-level performance comparison across small, medium, large, and capacity-stress categories. Columns report [PITH_FULL_IMAGE:figures/full_fig_p005_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Large-scale makespan comparison between VCST-RCP and [PITH_FULL_IMAGE:figures/full_fig_p006_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Scalability of fleet travel distance with increasing problem size [PITH_FULL_IMAGE:figures/full_fig_p006_7.png] view at source ↗
read the original abstract

We consider the problem of delivering multiple packages from a single depot to distinct goal locations using a homogeneous fleet of robots with limited carrying capacity. We propose VCST-RCP, a Voronoi-Constrained Steiner Tree Relay Coordination Planning framework that explicitly treats inter-robot relays as a design primitive. The approach operates in two stages: (i) constructing a sparse relay backbone by combining Voronoi-derived exchange interfaces with Steiner tree optimization, and (ii) synthesizing robot-level pickup, relay, and delivery schedules under capacity and service-time constraints. Unlike traditional methods that rely on direct source-to-destination transport, our framework organizes package flow through a shared relay network, reducing redundant long-haul motion. Extensive experiments across multiple scales show that VCST-RCP reduces total fleet travel distance by an average of 31% (up to nearly 50%) compared to Hungarian assignment and significantly outperforms OR-Tools CVRP, with statistically significant improvements (p < 10^{-3}). These gains translate into over 50% higher delivery efficiency (packages per kilometer), directly improving energy utilization. An ablation study further reveals that optimizing relay placement yields substantially larger improvements than adapting spatial partitioning alone, establishing relay design as the dominant factor governing system performance. Overall, the results demonstrate that relay-based coordination provides a scalable and effective framework for energy-aware multi-robot delivery in real-world logistics settings.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript proposes VCST-RCP, a two-stage Voronoi-Constrained Steiner Tree Relay Coordination Planning framework for multi-robot pickup and delivery from a depot to distinct goals with capacity-limited homogeneous robots. Stage (i) builds a sparse relay backbone via Voronoi-derived exchange interfaces combined with Steiner tree optimization; stage (ii) synthesizes per-robot pickup-relay-delivery schedules respecting capacity and service-time constraints. Experiments across scales report an average 31% (up to ~50%) reduction in total fleet travel distance versus Hungarian assignment, statistically significant outperformance of OR-Tools CVRP (p < 10^{-3}), and >50% higher delivery efficiency (packages per km), with an ablation attributing gains primarily to relay placement rather than spatial partitioning alone.

Significance. If the travel-distance reductions are shown to persist once synchronization and transfer costs are modeled, the work would offer a concrete, scalable primitive for energy-aware multi-robot logistics that moves beyond direct-assignment baselines. The explicit treatment of relays as a design variable and the ablation isolating its contribution are positive features; reproducible code or parameter-free derivations would further strengthen the contribution.

major comments (2)
  1. [Abstract / Experiments] Abstract and experimental section: the headline claim of 31% average travel-distance reduction (p < 10^{-3}) and the associated efficiency gains are presented without reported trial counts, variance or standard deviations, exact instance sizes (number of packages, robots, map scale), or explicit description of how capacity constraints were enforced during schedule synthesis. These omissions make the statistical significance and generalizability of the central performance result difficult to verify.
  2. [Method / Problem formulation] Two-stage construction (Voronoi-Steiner backbone followed by schedule synthesis): the model treats inter-robot handoffs at computed relay points as incurring zero extra time, energy, or motion cost beyond the stated capacity and service-time constraints. Because total fleet travel distance is the primary metric, any realistic synchronization (arrival-time windows, physical transfer, or forced waiting) can induce additional motion or longer paths that compound across multi-hop flows and may shrink or reverse the reported advantage over Hungarian assignment and OR-Tools CVRP.
minor comments (2)
  1. [Abstract] The abstract uses the phrase 'Voronoi-Constrained Steiner Tree' without a short inline definition or pointer to the specific Steiner-tree formulation employed; a one-sentence clarification would aid readers unfamiliar with the geometric construction.
  2. [Figures / Tables] Figure captions and table legends should explicitly state the number of random seeds or map realizations used for each reported average.

Simulated Author's Rebuttal

2 responses · 1 unresolved

We are grateful to the referee for the constructive and detailed feedback on our manuscript. We address each major comment point by point below, indicating the revisions we plan to make.

read point-by-point responses
  1. Referee: [Abstract / Experiments] Abstract and experimental section: the headline claim of 31% average travel-distance reduction (p < 10^{-3}) and the associated efficiency gains are presented without reported trial counts, variance or standard deviations, exact instance sizes (number of packages, robots, map scale), or explicit description of how capacity constraints were enforced during schedule synthesis. These omissions make the statistical significance and generalizability of the central performance result difficult to verify.

    Authors: We acknowledge these omissions in the presentation of our experimental results. In the revised manuscript, we will expand the experimental section to include the number of independent trials conducted for each configuration, standard deviations and variances for the reported metrics (including the 31% average reduction), exact instance sizes (number of packages, robots, and map scales), and a clear description of how capacity constraints were enforced during schedule synthesis. These details will be added via an expanded table and accompanying text to improve verifiability and allow better assessment of generalizability. revision: yes

  2. Referee: [Method / Problem formulation] Two-stage construction (Voronoi-Steiner backbone followed by schedule synthesis): the model treats inter-robot handoffs at computed relay points as incurring zero extra time, energy, or motion cost beyond the stated capacity and service-time constraints. Because total fleet travel distance is the primary metric, any realistic synchronization (arrival-time windows, physical transfer, or forced waiting) can induce additional motion or longer paths that compound across multi-hop flows and may shrink or reverse the reported advantage over Hungarian assignment and OR-Tools CVRP.

    Authors: The referee is correct that our model assumes zero additional costs for inter-robot handoffs at the relay points, beyond the capacity and service-time constraints already stated. This is an explicit modeling choice to focus on the travel-distance savings enabled by the Voronoi-constrained Steiner tree backbone as the primary objective. We agree that realistic synchronization, waiting, or transfer costs could introduce overhead that affects the net gains. In the revision, we will add a limitations paragraph discussing this assumption, its rationale, and how it positions the reported results as an idealized baseline. We will also outline extensions for incorporating time windows and transfer costs in future work. revision: partial

standing simulated objections not resolved
  • Empirically demonstrating whether the travel-distance reductions persist once synchronization and transfer costs are explicitly modeled would require new experiments and extensions beyond the scope of the current manuscript.

Circularity Check

0 steps flagged

No significant circularity: empirical evaluation against external baselines

full rationale

The paper presents VCST-RCP as a two-stage algorithmic procedure (Voronoi-Steiner relay backbone followed by capacity-constrained schedule synthesis) whose performance is measured via simulation against independent external solvers (Hungarian assignment, OR-Tools CVRP). The reported 31% travel-distance reduction and ablation results are direct outputs of these comparisons on generated problem instances, not re-expressions of fitted parameters or self-citations that define the metric by construction. No equations or load-bearing self-references collapse the claimed savings into the input data or prior author work; the derivation chain remains self-contained as a constructive planner evaluated externally.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The approach rests on standard geometric constructions and optimization routines already present in the literature; no new physical entities are postulated and no free parameters are explicitly fitted to the target performance metric in the abstract description.

axioms (2)
  • domain assumption Voronoi diagrams produce suitable exchange interfaces for multi-robot handoffs.
    Stage (i) of the framework invokes this geometric partition to locate relay points.
  • standard math Steiner tree optimization yields a sparse, low-cost relay backbone under the problem constraints.
    The same stage combines the Voronoi interfaces with Steiner-tree minimization.

pith-pipeline@v0.9.0 · 5776 in / 1543 out tokens · 51741 ms · 2026-05-18T15:50:23.224049+00:00 · methodology

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Reference graph

Works this paper leans on

39 extracted references · 39 canonical work pages · 2 internal anchors

  1. [1]

    Robots are feeding the pack!,

    J. Jordan, “Robots are feeding the pack!,” Mar 2023

  2. [2]

    A review on energy efficiency in autonomous mobile robots,

    M. Wu, C. F. Yeong, E. L. M. Su, W. Holderbaum, and C. Yang, “A review on energy efficiency in autonomous mobile robots,”Robotic Intelligence and Automation, vol. 43, no. 6, pp. 648–668, 2023

  3. [3]

    The hungarian method for the assignment problem,

    H. W. Kuhn, “The hungarian method for the assignment problem,” Naval research logistics quarterly, vol. 2, no. 1-2, pp. 83–97, 1955

  4. [4]

    On the capacitated vehicle routing problem,

    T. K. Ralphs, L. Kopman, W. R. Pulleyblank, and L. E. Trotter, “On the capacitated vehicle routing problem,”Mathematical programming, vol. 94, no. 2, pp. 343–359, 2003

  5. [5]

    Amazon has more than 750,000 robots that sort, lift, and carry packages-see them in action,

    T. Greenawalt, “Amazon has more than 750,000 robots that sort, lift, and carry packages-see them in action,” Oct 2024

  6. [6]

    Steiner tree problems,

    F. K. Hwang and D. S. Richards, “Steiner tree problems,”Networks, vol. 22, no. 1, pp. 55–89, 1992

  7. [7]

    Capacitated vehicle routing problem (cvrp) – google or-tools

    Google Developers, “Capacitated vehicle routing problem (cvrp) – google or-tools.”https://developers.google.com/opti mization/routing/cvrp, 2025. Accessed: 2025-09-13

  8. [8]

    The traveling salesman problem,

    M. J ¨unger, G. Reinelt, and G. Rinaldi, “The traveling salesman problem,”Handbooks in operations research and management science, vol. 7, pp. 225–330, 1995

  9. [9]

    The multiple traveling salesman problem: an overview of formulations and solution procedures,

    T. Bektas, “The multiple traveling salesman problem: an overview of formulations and solution procedures,”omega, vol. 34, no. 3, pp. 209– 219, 2006

  10. [10]

    Toth and D

    P. Toth and D. Vigo,The vehicle routing problem. SIAM, 2002

  11. [11]

    The vehicle routing problem: A taxonomic review,

    B. Eksioglu, A. V . Vural, and A. Reisman, “The vehicle routing problem: A taxonomic review,”Computers & Industrial Engineering, vol. 57, no. 4, pp. 1472–1483, 2009

  12. [12]

    Conflict-based search for optimal multi-agent pathfinding,

    G. Sharon, R. Stern, A. Felner, and N. R. Sturtevant, “Conflict-based search for optimal multi-agent pathfinding,”Artificial intelligence, vol. 219, pp. 40–66, 2015

  13. [13]

    The increasing cost tree search for optimal multi-agent pathfinding,

    G. Sharon, R. Stern, M. Goldenberg, and A. Felner, “The increasing cost tree search for optimal multi-agent pathfinding,”Artificial intel- ligence, vol. 195, pp. 470–495, 2013

  14. [14]

    Efficient sat approach to multi-agent path finding under the sum of costs objective,

    P. Surynek, A. Felner, R. Stern, and E. Boyarski, “Efficient sat approach to multi-agent path finding under the sum of costs objective,” inProceedings of the twenty-second european conference on artificial intelligence, pp. 810–818, 2016

  15. [15]

    Multi-agent path planning and network flow,

    J. Yu and S. M. LaValle, “Multi-agent path planning and network flow,” inAlgorithmic Foundations of Robotics X: Proceedings of the Tenth Workshop on the Algorithmic Foundations of Robotics, pp. 157– 173, Springer, 2013

  16. [16]

    A review of graph-based multi-agent pathfinding solvers: From classical to beyond classical,

    J. Gao, Y . Li, X. Li, K. Yan, K. Lin, and X. Wu, “A review of graph-based multi-agent pathfinding solvers: From classical to beyond classical,”Knowledge-Based Systems, vol. 283, p. 111121, 2024

  17. [17]

    Research challenges and opportunities in multi-agent path finding and multi-agent pickup and delivery problems,

    O. Salzman and R. Stern, “Research challenges and opportunities in multi-agent path finding and multi-agent pickup and delivery problems,” inProceedings of the 19th International Conference on Autonomous Agents and MultiAgent Systems, pp. 1711–1715, 2020

  18. [18]

    Suboptimal variants of the conflict-based search algorithm for the multi-agent pathfinding problem,

    M. Barer, G. Sharon, R. Stern, and A. Felner, “Suboptimal variants of the conflict-based search algorithm for the multi-agent pathfinding problem,” inProceedings of the International Symposium on Combi- natorial Search, vol. 5, pp. 19–27, 2014

  19. [19]

    Ddm: Fast near-optimal multi-robot path planning using diversified-path and optimal sub-problem solution database heuristics,

    S. D. Han and J. Yu, “Ddm: Fast near-optimal multi-robot path planning using diversified-path and optimal sub-problem solution database heuristics,”IEEE Robotics and Automation Letters, vol. 5, no. 2, pp. 1350–1357, 2020

  20. [20]

    Task and path planning for multi- agent pickup and delivery,

    M. Liu, H. Ma, J. Li, and S. Koenig, “Task and path planning for multi- agent pickup and delivery,” inProceedings of the International Joint Conference on Autonomous Agents and Multiagent Systems (AAMAS), 2019

  21. [21]

    Lifelong Multi-Agent Path Finding for Online Pickup and Delivery Tasks

    H. Ma, J. Li, T. Kumar, and S. Koenig, “Lifelong multi-agent path finding for online pickup and delivery tasks,”arXiv preprint arXiv:1705.10868, 2017

  22. [22]

    Rolling horizon path planning of an autonomous system of uavs for persistent cooperative service: Milp formulation and efficient heuristics,

    B. D. Song, J. Kim, and J. R. Morrison, “Rolling horizon path planning of an autonomous system of uavs for persistent cooperative service: Milp formulation and efficient heuristics,”Journal of Intelligent & Robotic Systems, vol. 84, no. 1, pp. 241–258, 2016

  23. [23]

    Rolling horizon planning for a dynamic collaborative routing problem with full-truckload pickup and delivery requests,

    X. Wang and H. Kopfer, “Rolling horizon planning for a dynamic collaborative routing problem with full-truckload pickup and delivery requests,”Flexible Services and Manufacturing Journal, vol. 27, no. 4, pp. 509–533, 2015

  24. [24]

    Multi-robot coordination through dynamic voronoi par- titioning for informative adaptive sampling in communication- constrained environments,

    S. Kemna, J. G. Rogers, C. Nieto-Granda, S. Young, and G. S. Sukhatme, “Multi-robot coordination through dynamic voronoi par- titioning for informative adaptive sampling in communication- constrained environments,” in2017 IEEE International Conference on Robotics and Automation (ICRA), pp. 2124–2130, IEEE, 2017

  25. [25]

    Distributed multi-robot information gathering using path-based sensors in entropy- weighted voronoi regions,

    A. K. Srivastava, G. P. Kontoudis, D. Sofge, and M. Otte, “Distributed multi-robot information gathering using path-based sensors in entropy- weighted voronoi regions,” inInternational Symposium on Distributed Autonomous Robotic Systems, pp. 286–299, Springer, 2022

  26. [26]

    Anchor-oriented localized voronoi partitioning for gps-denied multi-robot coverage,

    A. Munir, E. Latif, and R. Parasuraman, “Anchor-oriented localized voronoi partitioning for gps-denied multi-robot coverage,” in2024 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS), pp. 3395–3402, IEEE, 2024

  27. [27]

    V oronoi- based multi-robot autonomous exploration in unknown environments via deep reinforcement learning,

    J. Hu, H. Niu, J. Carrasco, B. Lennox, and F. Arvin, “V oronoi- based multi-robot autonomous exploration in unknown environments via deep reinforcement learning,”IEEE Transactions on Vehicular Technology, vol. 69, no. 12, pp. 14413–14423, 2020

  28. [28]

    V oronoi-based coverage control with connec- tivity maintenance for robotic sensor networks,

    W. Luo and K. Sycara, “V oronoi-based coverage control with connec- tivity maintenance for robotic sensor networks,” in2019 International Symposium on Multi-Robot and Multi-Agent Systems (MRS), pp. 148– 154, IEEE, 2019

  29. [29]

    A voronoi diagram-based workspace partition for weak cooperation of multi-robot system in orchard,

    J. Kim and H. I. Son, “A voronoi diagram-based workspace partition for weak cooperation of multi-robot system in orchard,”IEEE Access, vol. 8, pp. 20676–20686, 2020

  30. [30]

    A path planning strategy for multi-robot moving with path-priority order based on a generalized voronoi diagram,

    S.-K. Huang, W.-J. Wang, and C.-H. Sun, “A path planning strategy for multi-robot moving with path-priority order based on a generalized voronoi diagram,”Applied Sciences, vol. 11, no. 20, p. 9650, 2021

  31. [31]

    Adaptive centroidal voronoi tessellation with agent dropout and reinsertion for multi-agent non-convex area cover- age,

    K. Lee and K. Lee, “Adaptive centroidal voronoi tessellation with agent dropout and reinsertion for multi-agent non-convex area cover- age,”IEEE Access, vol. 12, pp. 5503–5516, 2024

  32. [32]

    Gm-vpc: An algorithm for multi- robot coverage of known spaces using generalized voronoi partition,

    V . G. Nair and K. Guruprasad, “Gm-vpc: An algorithm for multi- robot coverage of known spaces using generalized voronoi partition,” Robotica, vol. 38, no. 5, pp. 845–860, 2020

  33. [33]

    Distributed voronoi partitioning for multi-robot systems with limited range sensors,

    K. Guruprasad and P. Dasgupta, “Distributed voronoi partitioning for multi-robot systems with limited range sensors,” in2012 IEEE/RSJ international conference on intelligent robots and systems, pp. 3546– 3552, IEEE, 2012

  34. [34]

    Distributed multi-target search and tracking using the phd filter,

    P. M. Dames, “Distributed multi-target search and tracking using the phd filter,”Autonomous robots, vol. 44, no. 3, pp. 673–689, 2020

  35. [35]

    The zermelo–voronoi diagram: A dynamic partition problem,

    E. Bakolas and P. Tsiotras, “The zermelo–voronoi diagram: A dynamic partition problem,”Automatica, vol. 46, no. 12, pp. 2059–2067, 2010

  36. [36]

    DELIVER: A system for llm-guided coordinated multi-robot pickup and delivery using voronoi-based relay planning,

    A. K. Srivastava, J. M. Levin, A. Derrico, and P. Dames, “DELIVER: A system for llm-guided coordinated multi-robot pickup and delivery using voronoi-based relay planning,”arXiv preprint arXiv:2508.19114, 2025

  37. [37]

    Anytime multi-task multi-agent pickup and delivery under energy constraint,

    F. Kudo and K. Cai, “Anytime multi-task multi-agent pickup and delivery under energy constraint,”IEEE Robotics and Automation Letters, vol. 9, pp. 10145–10152, Nov 2024

  38. [38]

    Review of autonomous mobile robots for the warehouse environment,

    R. Keith and H. M. La, “Review of autonomous mobile robots for the warehouse environment,”arXiv preprint arXiv:2406.08333, 2024

  39. [39]

    Solving High Volume Capacitated Vehicle Routing Problem with Time Windows using Recursive-DBSCAN clustering algorithm

    K. Bujel, F. Lai, M. Szczecinski, W. So, and M. Fernandez, “Solving high volume capacitated vehicle routing problem with time win- dows using recursive-dbscan clustering algorithm,”arXiv preprint arXiv:1812.02300, 2018