PLC-Based Multi-Crane Anti-Collision Dispatching Control System
Our collaborative dispatching system for multiple overhead cranes uses real-time PLC-to-PLC communication to deliver anti-collision interlocking and task allocation, supporting up to six cranes working together on the same crane bayther on the same crane bay. Anti-collision protection is built on a three-tier strategy: zone interlock, real-time distance calculation, and priority interlocklation, and priority interlock. Task allocation supports three scheduling algorithms—round-robin time slicing, shortest-path-first, and priority queuing. The system runs on either a Profinet ring network with redundancy or an EtherCAT distributed-clock architecture, with PLC-to-PLC data exchange latency below 2 ms.
When multiple overhead cranes operate in the same bay, collision risk rises exponentially with each additional crane. Traditional approaches rely on operator visual judgment and experience, which is inefficient and leaves significant room for safety gaps. Our intelligent collaborative dispatching system enables safe multi-crane operation in a shared bay through real-time PLC communication, a three-tier anti-collision interlock strategy, and dynamic task allocation algorithms. This article provides a complete engineering breakdown of the system—covering architecture, anti-collision strategy, task allocation algorithms, communication networking, and hardware configuration.
Key Challenges in Multi-Crane Collaborative Dispatching
When multiple overhead cranes operate on the same factory-building crane rail, three core challenges emerge: Collision risk—cranes can interfere with each other in both the crane bridge (long travel) and main girder directionser directions, and conflict probability rises significantly once more than three cranes share a bay; Scheduling efficiency—task allocation directly impacts overall throughput when multiple cranes work simultaneously, with average efficiency dropping 30–50% without algorithm-based dispatching; Communication reliability—PLC-to-PLC data exchange must be real-time and stable, as any communication failure creates an immediate safety hazard.
The design targets for the collaborative dispatching system: the minimum distance between any two cranes must never fall below the safety threshold (1.5 m default stop distance), ensuring zero collision risk; average task allocation wait time must not exceed 30 seconds; and system availability must remain at or above 99.9% (with automatic fallback to zone-interlock mode if PLC communication is interrupted). Kelude has deployed multiple six-crane collaborative systems at an automotive stamping workshop and a steel structure fabrication facilityollaborative systems at an automotive stamping workshop and a steel structure fabrication facility, with over 10,000 hours of cumulative operation and zero accidents (Source: Kelude 2025 After-Sales Service Statistical Report KL-2025-AFTERSALES-012~015).
Three-Tier Anti-Collision Interlock Strategy
Anti-collision interlocking is the most critical safety layer in the collaborative dispatching system, structured in three tiers: Tier 1—Zone Interlock: The crane rail is divided into fixed zones (each zone length ≥ crane length + 2× safety distance), and only one crane is permitted in a zone at any time. Zone boundaries are defined by virtual soft limits inside the PLC. When a crane occupies Zone A, cranes in adjacent zones may operate normally but must not approach Zone A within less than the safety threshold. Zone interlock is the foundational protection layer—it does not rely on sensors and offers the highest reliability.
Tier 2—Distance Interlock: Laser distance sensors (or encoder-based cumulative position tracking) calculate the actual gap between adjacent cranes in real time. When the gap falls below the warning threshold (configurable, 5 m default), the system triggers an audible and visual alarm; below the deceleration threshold (3 m default), the crane automatically slows to 30% of rated speed; below the stop threshold (1.5 m default), power to the travel mechanism is cut immediately. Distance interlock provides precise quantitative protection on top of zone interlock, preventing unnecessary stops at zone boundaries.
Tier 3—Priority Interlock: When a collision becomes unavoidable (e.g., multiple cranes receive urgent tasks simultaneously), the system arbitrates by task priority: the crane handling the higher-priority task (such as furnace charging) gets right-of-way, while lower-priority cranes automatically yield or stop and wait. Priority is assigned by the MES system when dispatching tasks, and the PLC automatically sorts the task queue accordingly. Kelude's three-tier interlock system has been validated across multiple projects, with a false-alarm rate below 0.1%.
PLC Task Allocation Algorithm Comparison
| Comparison Parameter | zone interlock(Basic Guarantee) | distance interlock(Quantitative Protection) | priority interlock(Arbitration Mechanism) |
|---|---|---|---|
| Principle | Crane Rail Zone-based Limitation1Vehicle | Laser Distance Measurement Real-time Distance Calculation | Priority-based Task Sorting |
| Sensor | None(PLCVirtual Soft Limit) | laser distance sensor | None(MESPriority Tag) |
| Threshold | Zone Leader=overhead crane Zone Leader+2×Safety Distance | Warning5m/Deceleration3m/Stop1.5m | Priority1~10Priority |
| Advantages | Independent of Sensor Highest Reliability | Precise Quantification Reduces Unnecessary Stops | Emergency Task Priority Handling |
| PLCCode | Low(Logic Interlock) | Medium(Distance Calculation+Comparison) | Highest Reliability(Priority Queue) |
When the MES issues a lifting task, the dispatching PLC (or the master crane PLC) must assign it to the most suitable overhead crane. The three algorithms below each serve distinct application scenarios, and production systems typically combine them based on operating conditions:
The three assignment algorithms show markedly different performance in collaborative dispatching of overhead cranes. Evaluation indicators include fairness (whether workload is balanced across cranes), average waiting time (from task issuance to execution), emergency task responsiveness, and PLC code implementation complexity. The comparison below covers seven dimensions. In practice, most systems adopt a hybrid "priority queue + weighted round-robin" approach—urgent tasks preempt the queue, while non-urgent tasks are assigned by shortest-path weighting, balancing efficiency and fairness. Kelude's collaborative dispatching system defaults to this hybrid algorithm, with weight parameters flexibly adjustable to match the customer's production process.
| Comparison Parameter | Round Robin | Shortest Path First | Priority Queue+Weighted Round Robin |
|---|---|---|---|
| Allocation Principle | Round-robin Task Assignment Among Vehicles | Nearest Lifting Nearest Vehicle Execution | High Priority+Shortest Distance Weighted |
| Fairness | Highest Reliability(Balanced Workload Among Vehicles) | Low(Nearest Vehicle Overload) | Medium(Balancing Fairness and Efficiency) |
| Average Waiting Time | 30~60Seconds | 15~30Seconds | 20~40Seconds |
| Emergency Task Response | Slow(Requires Waiting for Rotation) | Faster(Nearest Response) | Fastest(Priority Preemption) |
| PLCCode Complexity | Low(Simple Counter) | Medium(Distance Calculation+Comparison) | Highest Reliability(Priority Queue+Weight Adjustment) |
| Application Scenarios | Balanced Task Load Standard Workshop | Single-end Loading/Unloadingassembly line Scenario | Multi-variety/High-mix/Emergency-heavy Scenarios |
| Kruud Recommendation | Small to Medium Workshop(≤3 Cranes) | assembly line/Continuous Production Scenario | Large Workshop/Multi-variety Production(Kruud Recommendation) |
| Three-level Interlock zone interlock(Basic) distance interlock(Precise Quantification) priority interlock(Arbitration).False Alarm Rate<0.1%, Zero Collision Incidents. | Communication Real-time Performance Profinet IRTPeriod250μs/Jitter1μs.Ether CATPeriod100μs/Jitter1μs.PLCLatency<2ms. | Task Allocation Efficiency Weighted Round Robin Average Waiting20~40Seconds.Emergency Task Preemption Response.Efficiency Improvement vs. Non-algorithmic Scheduling40~60%. |
| Maximum Coordinated Vehicles Standard System Supports≤6 Cranes for Cross-zone Coordinationoss-zone Coordination.System Expansion(Additional Communication Switch)Supports≤12 Cranesnnetwork switch)System Supports≤12Vehicles.Applicable30mand Below Span. | Fail-safe Degradation Automatic Degradation on Communication Losszone interlock.3Safety Stop Triggered by Heartbeat Loss.Automatic Recovery After Communication Restored Synchronization Position. | Hardware Configuration Per Vehicle PLC S7-1200/1500+Network Managernetwork switch+Laser Distance Sensor / Laser Rangefinder.Master-Slave Mode, Slave Takes Over on Master Failure. |
Communication Network Architecture & Real-Time Performance Analysis
The collaborative dispatching system imposes strict real-time requirements on PLC-to-PLC communication: anti-collision interlock data exchange cycles must not exceed 10ms, and task assignment command latency must stay below 50ms. For industrial Ethernet deployments, the two dominant fieldbus choices are Profinet IRT (Isochronous Real-Time) and EtherCAT.
Profinet IRT Ring Redundancy Architecture: PLCs form a ring network via SCALANCE X network switches, leveraging Media Redundancy Protocol (MRP) for self-healing within ≤200ms after a single point of failure. In IRT mode, the data cycle can be as short as 250μs with jitter of ≤1μs. This approach is ideal for factories standardized on the Siemens ecosystem, offering native compatibility with S7-1200/1500 controllers.
EtherCAT Solution: Utilizes Distributed Clock (DC) technology to keep slave-to-slave clock synchronization jitter below 1μs. Data cycles can be reduced to as little as 100μs. It supports hot-swapping and line/tree topologies without requiring expensive managed switches. This is the preferred choice for applications demanding extreme real-time performance, such as servo positioning. Kelude's collaborative system supports both protocols, with recommendations tailored to the customer's existing PLC brand and real-time requirements.
Kelude Overhead Crane Collaborative Dispatching Service Advantages
Kelude Heavy Industry has accumulated extensive engineering expertise in collaborative dispatching, with multiple systems already deployed across automotive manufacturing, steel structure fabrication, and non-ferrous metal smelting industries. Our service advantages include:
Proven Track Record: Delivered 4 six-crane collaborative systems and 12 four-crane systems, with over 10,000 cumulative operating hours and zero collision incidents.
Flexible Architecture: Supports dual protocol stacks (Profinet/EtherCAT), compatible with the full range of Siemens, Beckhoff, and Mitsubishi PLCs, and offers MES/ERP interface integration.
End-to-End Delivery: From system design and configuration programming to cabinet integration, on-site commissioning, TSG acceptance, and operator training — delivered in ≤30 days.
Remote Operation & Maintenance: Standard remote monitoring gateway with 4G/5G remote diagnostics, online program upgrades, and operational data analytics.
Frequently Asked Questions
Q: How much additional investment is required for multi-crane collaborative dispatching compared to independent single-crane operation?
A: The main cost additions include PLC communication modules (Profinet CP cards at approximately $300 per crane), managed network switches at roughly $740 per system, laser distance sensors at about $440 per crane, and collaborative dispatching programming and commissioning fees of approximately $2,960–$5,920. Total added investment works out to roughly $2,220–$4,440 per crane (based on a 4-crane system), which is typically recovered within 1–2 years through a 40–60% improvement in operational efficiency.
Q: Can a collaborative dispatching system be retrofitted onto older overhead cranes?
A: Yes, provided the following prerequisites are met: ① The crane must have PLC control (relay-based control must first be upgraded to PLC); ② The crane bridge and trolley travel mechanisms must be equipped with variable-frequency drives (VFDs) — direct contactor start/stop cannot achieve the precise speed control and positioning required; ③ The PLC must have at least one available Profinet or Ethernet port. Kelude Heavy Industry offers retrofit assessment services, including on-site inspection followed by a detailed retrofit proposal and quotation.
Q: How does the system ensure safety during communication failures?
A: The system implements a three-tier fail-safe mechanism: ① PLCs exchange heartbeat messages every 100ms; communication is declared lost after three consecutive missed heartbeats; ② Upon communication loss, each crane's PLC automatically switches to zone interlock degraded mode (operating within preset fixed zones without relying on real-time distance data); ③ If a crane's position cannot be determined, it automatically decelerates to 30% of rated speed with an audible and visual alarm, and executes a safe shutdown if communication is not restored within 10 seconds. Once communication is re-established, all cranes automatically exchange position information and reallocate zones based on their actual current positions.
Q: How does the collaborative dispatching system integrate with MES/ERP?
A: Integration is achieved via an OPC UA server bridge or MQTT gateway. Tasks issued by the MES include lifting point coordinates, destination coordinates, cargo ID, priority level, and estimated weight. The dispatching PLC receives these tasks, automatically queues them, and reports completion status (completion time, actual start/end points, load data) back to the MES. Kelude Heavy Industry provides a standard OPC UA interface template, with typical integration timelines of 3–7 days, compatible with all major MES platforms.
