In modern industrial lifting applications, the PLC (programmable logic controller) has become the core brain of crane control systems. Compared with traditional relay-contactor controls, PLC-based solutions deliver a quantum leap in reliability, maintainability, and functional scalability. For a 20t double-girder overhead crane, the electrical control system must coordinate multi-axis motion—hoisting, long travel, and cross travel—within milliseconds while continuously monitoring critical signals such as travel limits, overload protection, and emergency stop circuits.

Designing a crane PLC control system is far more than selecting and stacking hardware components. It is a systems engineering exercise that begins with signal mapping, progresses through I/O point allocation, and culminates in a robust communication architecture. In accordance with the electrical control system requirements of ISO 4301 (Crane Design Standard) and the mandatory monitoring functions outlined in GB/T 28264 Safety Monitoring and Management System for Lifting Appliances, a compliant PLC control system must meet industrial-grade standards across three dimensions: safety integrity, real-time responsiveness, and multi-crane coordination. This article walks through the complete design of a PLC control system for a 20t double-girder crane—from PLC selection to multi-crane communication—using a real-world design example.

PLC Selection Guide: Siemens S7-1200 vs. Mitsubishi FX5U vs. Inovance AM600

Selecting a PLC for crane applications requires evaluating five key dimensions: I/O capacity, motion control capability, communication interfaces, safety functions, and environmental adaptability. A typical 20t double-girder crane demands approximately 200 digital I/O points, 16 analog channels, 8 high-speed counters, and at least two communication ports (Profinet/EtherNet/IP for HMI and CANopen for VFDs), with support for safety PLC expansion. The following comparison examines three leading small-to-medium PLCs currently used in crane control applications.

PLC Selection Core Parameter Comparison

From a practical engineering standpoint in crane control, the Siemens S7-1200 holds the largest market share in the crane industry, thanks to the convenience of the TIA Portal fully integrated engineering platform and native Profinet RT support for distributed I/O. The Mitsubishi FX5U offers strong value in cost-sensitive projects, though its communication ecosystem limits scalability on larger cranes. The Inovance AM600 is rapidly gaining ground in the mid-range market with its EtherCAT high-speed bus and highly competitive pricing, making it particularly well-suited for projects with demanding multi-axis synchronization requirements.

I/O Point Allocation Example: Complete I/O Map for a 20t Double Girder Crane

I/O point allocation is the first step in PLC control system design—and the easiest place to make mistakes. Over-allocating leaves no room for future expansion, while under-allocating wastes hardware budget. For double-girder cranes, we recommend organizing I/O modules by functional area rather than physical location, which keeps main program and subroutine calls cleaner. The table below shows a complete I/O allocation map for a 20t double-girder crane, totaling 128 digital inputs, 64 digital outputs, 16 analog inputs, and 4 analog outputs.

Complete I/O Point Allocation Map for a 20t Double Girder Crane

I/O allocation follows three principles: first, signals from the same functional area are assigned to consecutive addresses for easier programming and maintenance; second, safety circuit signals (emergency stop, redundant limit switch channels, safety gates) are isolated on dedicated I/O modules of the safety PLC, physically separated from the main control PLC; third, analog channels are grouped by signal type—4~20mA current-type sensors (load cells, anemometers, laser distance measurement) are kept separate from 0~10V voltage-type signals to minimize common-mode interference. A total of six ET 200SP distributed I/O stations are used, mounted at both ends of the crane bridge, on the trolley platform, and inside the operator cabin, communicating with the main control PLC via a Profinet RT ring network.

Key Performance Parameters and Technical Specifications

The performance of a crane PLC control system depends not only on CPU processing speed but also on communication cycles, I/O refresh times, and safety response times. The six technical indicators below provide a full-chain performance assessment—from program execution to communication latency—and serve as the core benchmarks for determining whether a PLC control system meets specification.

Safety Circuit Design: Dual-Channel Redundancy for Emergency Stop, Limit Switches, and Overload Protection

The safety circuit is the highest-priority and most rigorously designed part of a crane PLC control system. In accordance with ISO 13849-1 Safety of machinery—Safety-related parts of control systems and IEC 62061 Safety of machinery—Functional safety of safety-related electrical, electronic and programmable electronic control systems, the emergency stop, hoisting limit switches, and overload protection on a crane must achieve SIL2 (Safety Integrity Level 2) or PLd (Performance Level d). This means a single fault must not result in loss of the safety function.

The system in this example uses a Siemens S7-1200F (fail-safe CPU) running independently from the main control PLC, creating a physically isolated dual-channel redundant architecture. Each safety input signal (e.g., emergency stop button, upper limit switch, overload detection switch) is wired to two independent channels on the F-DI module. The F-CPU performs voting on the dual-channel signals during every scan cycle—a signal is only considered valid when both channels agree; any discrepancy triggers an immediate safe shutdown and lockout. Safety outputs use a dual-contactor series configuration, so even if one contactor welds or jams, the other can still interrupt the power circuit.

Safety Circuit Configuration Details

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One critical aspect of the safety circuit design that cannot be overlooked is the "heartbeat" monitoring between the main control PLC and the safety PLC. The main control PLC sends a heartbeat pulse to the safety PLC via a hardwired signal every 10 ms. If the safety PLC fails to receive the heartbeat for three consecutive cycles (30 ms), it automatically triggers a safe shutdown. This mechanism prevents the extreme risk of safety functions being bypassed in the event of a main control PLC crash or communication failure. In actual engineering practice, it is also necessary to verify that the PFHd (probability of dangerous failure per hour) of the safety circuit is less than 1×10⁻⁷ to meet the quantitative requirements of SIL2.

Multi-Crane Synchronization: Profinet vs. CANopen vs. EtherCAT

In large steel structure workshops or shipyard block assembly shops, it is common to use 2–4 overhead cranes in tandem to hoist oversized and overweight components. The core technical challenge of multi-crane synchronization lies in real-time position synchronization and load distribution among the cranes, all of which depend on a reliable workshop-level communication network. Three communication protocols dominate the industrial landscape today—Profinet RT, CANopen, and EtherCAT—each with its own strengths and trade-offs in multi-crane synchronization scenarios.

Communication Protocol Comparison for Multi-Crane Synchronization

In practical engineering applications, a hybrid "Profinet + CANopen" architecture is recommended: Profinet RT serves as the workshop backbone network, handling data exchange and I/O communication between each crane's PLC and the central HMI; CANopen acts as the dedicated bus for multi-crane synchronization, leveraging its native multi-master capability and SYNC message mechanism to broadcast position and synchronize speed across four overhead cranes within a 1–2 ms cycle. EtherCAT is better suited for scenarios requiring extremely high acquisition rates (e.g., vibration monitoring, dynamic weighing), but it is less flexible than CANopen for multi-master coordination.

EMI Protection Design: Shielding, Grounding, and Filtering

Crane working environments rank among the harshest for electromagnetic interference in industrial settings—strong EMI radiation from VFD PWM outputs, grid voltage fluctuations caused by large motor start/stop cycles, and continuous arcing between conductor rails and current collectors are all interference sources the PLC control system must contend with. A well-designed immunity solution requires a systematic approach across three layers: shielding, grounding, and filtering.

Cable Shielding — All analog signal cables (4–20 mA sensors, encoder feedback) use double-shielded cable: the inner aluminum foil shield is single-ended grounded (at the PLC cabinet side), while the outer copper braid shield is grounded at both ends, creating a "single-ended for low frequency, double-ended for high frequency" frequency-selective shielding strategy. Digital signal cables use single-layer shielding with both ends grounded, and communication cables (Profinet/CANopen) strictly use manufacturer-supplied shielded cable.

Grounding System — A three-tier grounding network is established: cabinet protective grounding (PE) and shield grounding (FE) are consolidated on a grounding copper busbar inside the PLC cabinet, then connected to the workshop earth grid at a single point, with grounding resistance ≤ 4 Ω. Special attention must be paid to keeping the VFD cabinet's PE and the PLC cabinet's PE on independent runs to the main grounding point, preventing VFD leakage current from coupling into the PLC system through a shared PE conductor.

Power Filtering — Two-stage EMI filters (common-mode + differential-mode) are installed upstream of the PLC control power supply, with varistors on the input side to absorb lightning surges and switching overvoltages. Input reactors (3% impedance) and harmonic filters are fitted on the VFD input side to suppress 5th and 7th harmonic pollution on the grid. Critical sensor power supplies use DC/DC isolation modules (isolation voltage 1500 VDC) to block ground-loop interference.

PLC Programming Framework: Main Routine, Subroutines, and Alarm Handling

The crane PLC program follows a three-layer software architecture—"main program scheduling + functional subroutines + centralized alarm handling"—to maximize code readability, testability, and maintainability. The main program (OB1) executes on a fixed 10 ms cycle, calling functional subroutines in priority order.

Functional subroutines are divided into six modules based on crane operating modes: Long Travel Control (FC1), Cross Travel Control (FC2), Hoisting Control (FC3), Rail Clamp and Anchor Control (FC4), Safety Interlock Handling (FC5), and Communication Data Packaging (FC6). Each subroutine uses a Sequential Function Chart (SFC) structure, breaking a complete crane motion cycle into discrete state steps with clearly defined, traceable transition conditions.

The alarm handling module (FC100) runs independently of the functional programs, polling 200+ alarm conditions across the crane and classifying them into three severity levels: Level 1 (yellow warning) displays on the HMI only without affecting operation—for example, motor bearing temperature exceeding 75°C but below the 85°C threshold; Level 2 (orange power derating) automatically limits the speed and acceleration of the affected mechanism to 50% of rated values, such as when a single VFD faults and runs in derated mode; Level 3 (red shutdown) immediately triggers the safety contactor to open and latch, requiring manual reset—for example, emergency stop activation, simultaneous dual upper limit switch trips, or overload exceeding 110% of rated load. All alarm events are logged with timestamps and key operating parameters to the CPU's retentive memory, ensuring data survives power loss for post-incident fault analysis.

Frequently Asked Questions

Kelude specializes in the development and integration of electrical control systems for industrial cranes, with extensive engineering experience in PLC control, inverter drive technology, and multi-crane synchronization. The company's technical team delivers full-cycle project execution—from I/O point allocation and safety circuit design through to programming and commissioning—and provides customized electrical control system solutions for a wide range of bridge and gantry cranes. For technical consultation or system design, contact the Kelude engineering team for detailed configuration lists and project case studies.