How to Resolve AGV Deadlocks on Shared Access Systems
📋 Key Summary
When dozens of AGVs share a single access system, the biggest fear is deadlock—AGV A waits for AGV B to yield, while AGV B waits for AGV A. Neither can move, and the entire access system grinds to a halt. Once a deadlock occurs, dispatching collapses. This article explains how deadlocks happen, how path planning enables advance avoidance, how traffic control ensures orderly passage, and the underlying logic of deadlock detection and recovery.
📌 Core Logic
Deadlock = multiple vehicles waiting on each other, none able to move.
Solution = path planning avoidance + traffic control and guidance + deadlock detection and recovery.
When dozens of AGVs operate inside the same factory building, sharing limited access systems, the one thing you fear most is a single word—deadlock. AGV A needs to pass through an access system but is blocked by AGV B; AGV B needs to pass through the same access system but is blocked by AGV A. Each vehicle waits for the other to yield, and neither can move.
Once a deadlock occurs, the access system locks up, AGVs behind pile up, and dispatching comes to a standstill.
Here's how to break deadlocks.
How Deadlocks Occur: Resource Contention and Circular Wait
A deadlock is the result of multiple AGVs waiting on one another.
Resource contention happens because all AGVs need the same limited access systems, intersections, and positions. When resources run short, AGVs must queue and compete.
Circular wait occurs during resource contention when AGV A occupies the path AGV B needs, while AGV B occupies the path AGV A needs—each waits for the other to yield first, and the system locks up.
At its core, a deadlock is a circular wait of "I wait for you, you wait for me." The narrower the access system and the more AGVs in operation, the higher the deadlock risk. When more than 20 AGVs share the same access system, deadlock risk rises significantly; deadlock recovery typically completes within 30 seconds. GB/T 28264-2017 Safety Monitoring and Management System for Lifting Appliances specifies requirements for operation status recording.
Path Planning: Advance Avoidance to Prevent Conflicts
The first line of defense against deadlock is path planning.
Path planning means mapping a conflict-free route for every AGV. When the dispatching system assigns tasks, it determines which route each AGV will take, avoiding paths that would collide.
Advance avoidance is the heart of path planning—preventing two AGVs from competing for the same access system segment at the same time. When routes are staggered, the root cause of deadlock is eliminated.
Kelude's dispatching system performs path planning at the moment tasks are assigned, keeping AGV routes separated and reducing deadlock risk at the source.
Traffic Control: Orderly Passage Through Intersections
The second line of defense against deadlock is traffic control.
Traffic control works like intersection traffic lights, governing the order in which AGVs proceed through intersections and access systems. Who goes first, who goes next—the dispatching system makes the call through unified dispatching.
Orderly passage through intersections is the key to traffic control. When multiple AGVs approach the same intersection, they pass through one by one according to priority, without competing or forcing their way—and no deadlock occurs.
Kelude applies traffic control to keep AGVs moving through intersections in an orderly manner, preventing deadlocks caused by simultaneous right-of-way conflicts.
Deadlock Detection and Recovery: Breaking the Gridlock
Even with solid path planning and traffic control in place, deadlocks are greatly reduced but not completely eliminated. That's why deadlock detection and recovery are essential.
Deadlock detection works by having the dispatching system continuously monitor the operating status of each AGV in real time. When several AGVs are found waiting on each other and remain stationary for an extended period, the system flags it as a deadlock. As Kelude's technical manager puts it: "A deadlock is fundamentally 'I wait for you, you wait for me.' Path planning enables advance avoidance, traffic control ensures orderly guidance, and deadlock recovery serves as the safety net—only by using all three layers together can you truly hold the line."
Deadlock recovery kicks in once a deadlock is detected. The dispatching system intervenes, directing one AGV to reverse or reroute, breaking the circular wait.
Detection plus recovery is the final safety net against deadlock. Kelude's dispatching system is equipped with deadlock detection and recovery, and FEM 1.001 Crane Design Standard specifies requirements for operation scheduling.
Most Common Mistakes in Dispatching Configuration
Mistake one: assigning tasks without planning paths. The dispatching system only allocates tasks but does not plan routes, leaving each AGV to find its own way—a recipe for head-on conflicts and deadlocks. Path planning must be built into dispatching.
Mistake two: no traffic control at intersections. When multiple AGVs approach an intersection and each tries to push through on its own, they end up blocking each other. Intersections need unified traffic control.
Mistake three: no deadlock recovery mechanism. When a deadlock occurs, the system cannot detect it or resolve it, forcing manual intervention. Kelude's dispatching system includes deadlock detection and recovery, so gridlocks resolve themselves.
Element Comparison: Deadlock Resolution in Dispatching
| means | function | keypoint | coveragestage |
|---|---|---|---|
| Path Planning | route offset | advance avoidance | prevention |
| traffic control | orderly passage | intersectionpriority | diversion |
| deadlock recovery | break deadlock | reverse detour | fallback |
Quick Reference of Standard Clauses on Scheduling Deadlock
| Standard | key provisions | interface with dispatching |
|---|---|---|
| GB/T 28264 Safety Monitoring and Management System-2017 | operation status recording | operation logrequirements |
| FEM 1.001 Crane Design Standard-2008 | operation schedulingrequirements | dispatching safety criteria |
| ISO 24445 | smart sensortechnical specification | position sensingIdentification |
FAQ: Deadlock in Dispatching Systems
Q: How does a deadlock occur?
A: When multiple AGVs compete for limited access systems and intersections, a deadlock can occur when AGV A occupies the path AGV B needs, while AGV B blocks AGV A's route—each waiting for the other to yield. This circular wait—"you wait for me, I wait for you"—is the core issue. The narrower the access system and the more vehicles in operation, the higher the deadlock risk.
Q: How can deadlocks be prevented at the source?
A: Through path planning combined with traffic control. When dispatching tasks, the system pre-plans each AGV's route to stagger movements and prevent two vehicles from simultaneously entering the same access segment. At intersections, traffic control enforces orderly passage based on priority. The key is advance avoidance and orderly flow, which eliminates most deadlock root causes before they arise.
Q: How is an existing deadlock resolved?
A: Through deadlock detection and recovery. The dispatching system performs real-time monitoring to identify when multiple AGVs are stuck in a mutual wait state. Once a deadlock is detected, the system intervenes—directing one vehicle to reverse or take an alternate route—thereby breaking the cycle. This detection-plus-recovery approach enables self-resolution without manual intervention.
Q: At what AGV count does deadlock become likely?
A: It depends on the number and width of access routes. The deadlock probability rises with more AGVs, narrower aisles, and a higher density of intersections. Generally, when more than 20 AGVs share the same access system, the operation enters a high-risk zone. Kelude Heavy Industry assesses deadlock risk during the planning phase based on AGV count and access layout, implementing traffic control and path planning measures in high-risk areas in advance.
Deadlock resolution shares common ground with fleet scheduling. For a deeper look at multi-crane collision avoidance, see the comparison in "Overhead Crane Fleet Scheduling Algorithms: Engineering Implementation of Multi-Crane Collision Avoidance and Task Allocation".
Access system gridlocks are solved through intelligent dispatching. Kelude Heavy Industry combines path planning for advance avoidance, traffic control for orderly flow, and deadlock detection and recovery as a safety net—keeping dozens of AGVs moving smoothly through shared access systems without stoppages.
