Overhead Crane & AGV Interlock: Smart Factory Dispatching System

When overhead cranes and AGVs/RGVs operate in the same workspace, equipment interlocking is the first line of defense against collisions. The interlock system must cover five combination types: crane-to-crane, crane-to-AGV, AGV-to-AGV, crane-to-RGV, and AGV-to-RGV. This article breaks down the engineering implementation across four interlock design layers, the status-sharing data structure, three-level safety thresholds, docking station coordination, and communication protocol mapping.

Crane–AGV Shared-Aisle Interlock Scenarios and Priority Logic

Collision risk in shared-aisle crane and AGV operations comes from multiple dimensions. Interlock priority across the five scenarios is ranked as follows: crane-to-crane ranks highest (head-on encounters on the same runway have the most severe consequences), while AGV-to-AGV ranks lowest (low speed and light mass). The scheduling PLC receives status broadcasts from all equipment every 100 ms, calculates the three-dimensional distance for each pair (X/Y/hoist load height), and executes the three-tier response of warning, deceleration, and stop. With 12 devices, 66 pair distances must be computed; the interlock logic runs approximately 200 lines of SCL and executes in 1–2 ms.

Crane–AGV Docking Station Design and Positioning Accuracy

The docking station where crane and AGV meet is the critical node in coordinated operations—docking accuracy directly determines the reliability and efficiency of load handover. Key design requirements: AGV positioning accuracy in the docking zone is ±10 mm (achieved via laser reflector or QR-code correction), while the crane's load-lowering position accuracy is ±5 mm (using fused laser and encoder positioning). The crane hook-to-AGV carrier interface uses a tapered guide design—the hook bottom has a 60° cone angle, and the AGV carrier locating hole has a 45° chamfer. This taper guidance automatically corrects horizontal deviation up to ±15 mm.

Docking sequence parameters: The AGV enters the docking station and stops at the precise position (3–5 s), then broadcasts "docking ready." Upon receipt, the crane begins lowering; when the load bottom is 500 mm above the AGV carrier, speed drops to 20% of rated; over the final 100 mm, speed reduces to 5% of rated. A travel switch confirms full seating, the hook disengages, and the crane hoists to a safe height (≥ AGV height + 500 mm) and broadcasts "docking complete." The AGV verifies safety and departs. The full docking cycle takes approximately 20–30 s, with the crane lowering phase accounting for 12–18 s. Kelude Heavy Industry measured a 99.7% docking success rate at an automotive plant (3 failures out of 1,000 dockings, all caused by AGV positioning timeouts triggering retries).

Safety Threshold Configuration and Emergency Stop Circuitry

Safety distances for shared-aisle crane and AGV operations are set in three threshold tiers. Using a steel mill continuous-casting bay as an example: 12 overhead cranes and 8 AGVs operate simultaneously, with safety distances configured separately for each of the five scenarios—crane-to-crane, crane-to-AGV, AGV-to-AGV, crane-to-RGV, and AGV-to-RGV. Real-time distances are reported to the PLC by a UWB positioning system (accuracy ±10 cm, 10 Hz update rate), which triggers the three-tier response per the threshold table below:

AGV Interlock Retrofit for Aging Overhead Cranes

For overhead cranes in service for 5 to 15 years, achieving AGV interlocking requires a tailored retrofit strategy based on the existing electrical control system type. The upgrade is carried out on three levels: the sensing layer (adding UWB positioning tags and distance sensors), the control layer (PLC upgrade or addition), and the communication layer (Ethernet networking and interlock logic programming). The following outlines the approach for each crane electrical control type.

Relay-Controlled Cranes (Approx. 30% of Existing Fleet). These cranes have no PLC and rely solely on contactor-relay control circuits, necessitating a complete upgrade. Recommended solution: Install a new Siemens S7-1200 control cabinet (including CPU 1212C, SM1222 DO module ×1, SM1231 AI module ×1, 24V switching power supply, and enclosure), and replace the master switches and pendant station. Wiring essentials: Hoist and travel contactor coil signals are sourced from the S7-1200 DO module (isolated via intermediate relays), and existing thermal overload relay signals are wired into DI channels. Cost: approximately $2,400 per unit (including control cabinet, sensors, installation & commissioning), with a project timeline of 5–7 days (including on-site electrical wiring during shutdown and programming/commissioning).

Cranes with Existing PLCs (Approx. 50%). Most of these cranes are already equipped with a PLC (Siemens S7-200/S7-300 or Mitsubishi FX series). The original PLC is retained, and only a communication module and interlock logic are added. For the S7-200, a CP243-1 Ethernet module is used (approx. $220); for the S7-300, a CP343-1 module is used (approx. $370). An AGV interlock subroutine is added to the existing PLC program: the PLC receives position coordinates and safety zone status from the AGV dispatching system via Ethernet, compares them with its own position, and outputs interlock signals. The ladder logic program consists of approximately 80–120 lines, including heartbeat detection (500ms cycle, with emergency shutdown triggered after 3 consecutive missed heartbeats). Cost: approximately $440–$740 per unit (including modules, programming, and commissioning), with a project timeline of 2–3 days.

Smart Cranes (Approx. 20%). Cranes manufactured after 2019 typically come standard with a Siemens S7-1200/S7-1500 PLC and industrial Ethernet interface. In these cases, only an AGV interlock function block (FB) needs to be added to the existing program. Kelude Heavy Industry provides a standard interlock function block (compatible with both S7-Graph and LAD). Programming essentials: Add an FC_AGV_Interlock call in OB1, with input parameters including the crane ID and UWB coordinates, and output parameters including the interlock status (0=Normal / 1=Warning / 2=Deceleration / 3=Stop). The interlock status is written to the preceding interlock bit of the original PLC safe stop routine. Cost: approximately $300–$440 per unit, with a 1-day timeline (including remote commissioning). After any of these upgrades, the crane PLC connects to the factory Ethernet via Profinet, with a data exchange cycle of ≤200ms with the AGV dispatching system.

Post-Retrofit Verification Steps: ① First, verify interlock logic under no-load conditions: simulate the three-level response (warning, deceleration, stop) for the crane and AGV at various distances, testing each level at least 10 times; ② Next, verify under load: re-test under actual lifting conditions to confirm braking distance compliance; ③ Finally, endurance testing: run continuously for 24 hours with no abnormal alarms. All test data is recorded in the retrofit delivery report. Kelude Heavy Industry provides a turnkey service covering on-site surveys, solution design, installation, and after-sales maintenance.

Crane-AGV Communication Protocol & Data Mapping

Data exchange between the crane PLC and the AGV dispatching system uses standard industrial protocols—Profinet RT or Modbus TCP. The crane sends a data packet (16 bytes) to the AGV dispatching system every 100ms: Crane ID (2B), Bridge Position (mm, 4B), Hoist Load Height (mm, 4B), Operating Status (1B, 0=Idle / 1=Transporting / 2=Standby / 3=Fault), Docking Station Flag (1B, 0=No / 1=Yes), Reserved (4B). The AGV dispatching system broadcasts a data packet (20 bytes: 4 AGVs × 5 bytes/AGV) to the crane every 100ms: AGV ID (1B), Position X (2B), Position Y (2B), Speed (1B), Direction Angle (1B), Crane Docking Status (1B, 0=Not Relevant / 1=Ready / 2=Handover in Progress / 3=Complete), Reserved (2B).

Data mapping is configured in TIA Portal: Create a global data block, DB_AGV_Interface (200 bytes), on the crane PLC side to map all data written by the AGV dispatching system. The AGV dispatching system (HMI or PLC) writes to the corresponding addresses in DB_AGV_Interface via Profinet IO or as a Modbus TCP Client. After configuration, use Siemens PRONETA software to verify the communication link: data refresh cycle ≤100ms and packet loss rate ≤0.01%. Kelude Heavy Industry provides a standard communication configuration template, including the AGV interface DB block for the crane PLC (one set for S7-1200/1500 each) and the data mapping configuration file for the AGV dispatching system.

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