Overhead Crane PLC Control System: S7-1500, PROFINET & G120

Three-Tier Architecture for Overhead Crane Control

The PLC control system for unmanned overhead cranes adopts a classic three-tier distributed architecture, comprising from bottom to top: the field level, the control level, and the management level. The field level includes VFD drive units (G120/SINAMICS S120 series), encoders, laser distance sensors, limit switches, light grid sensors, safety door locks, and other field devices, all exchanging real-time data with the control-level PLC via PROFINET. The control level is built around a Siemens S7-1500F safety PLC as the core controller, equipped with TM PosInput 2 positioning modules and technology modules. It handles closed-loop positioning control for crane travel, hoisting speed control, automatic grab/release logic, and fault diagnosis logic. The management level connects the PLC to the workshop MES system and SCADA/HMI monitoring stations via industrial Ethernet, enabling remote monitoring of crane status, production task dispatch, operational data archiving, and alarm log management.

The key advantages of this three-tier design are clear separation of concerns, fault isolation, and ease of maintenance — a field-level device failure does not affect management-level communications; a control-level PLC fault leaves field devices in a safe state; and a management-level network issue does not interrupt automatic crane operation. Network isolation across the three tiers must be carefully planned: the field-level PROFINET IRT real-time network (cycle time 1–4 ms), the control-level plant Ethernet (non-real-time, used for HMI and SCADA communications), and the management-level office network (isolated via firewall). Field experience shows that when PROFINET IRT shares a network switch with the plant office network, VLAN isolation and QoS priority tagging must be enabled (IRT data assigned the highest priority, VLAN Tag 4); otherwise, IRT communication jitter can increase by 50–200 µs.

PLC Selection Strategy and I/O Point Allocation

The S7-1500F safety PLC is the preferred controller for overhead crane safety control systems. When selecting the CPU, both performance class and F-safety level must be considered. For a single 50 t foundry crane (with one drive each for the crane bridge X-axis, trolley Y-axis, and hoist Z-axis), the recommended configuration is either a CPU 1515F-2 PN (6ES7 515-2FM01-0AB0) or a CPU 1517F-3 PN (6ES7 517-3FP00-0AB0, for complex cranes with more than 256 I/O points). The 1515F supports PROFINET IRT with a 4-port switch, a processing speed of 65 ns per bit operation, and an F-safety level of SIL3 — sufficient for all unmanned overhead crane control requirements.

Estimated I/O point allocation (per crane): Digital inputs total approximately 48 points — hoist upper/lower limit switches (4 points), bridge left/right limits (4 points), trolley left/right limits (4 points), rail clamp status (3 points), brake open/close status (6 points), operator station pushbuttons (12 points), safety doors/light grids (4 points), encoder SSI interfaces (3 channels × 4 wires = 12 wires, connected via TM PosInput modules), plus 12 spare points (20% margin). Digital outputs total approximately 24 points — hoist/bridge/trolley VFD enable (3 points), brake control (3 points), alarm indicator lights (4 points), fans/lubricating oil pumps (4 points), plus 10 spare points. Analog inputs total approximately 8 points — laser rangefinder 4–20 mA backup (3 channels), temperature sensors (2 channels), and 3 spare points for current/voltage feedback. Recommended modules: 2 × DI 32×24V (6ES7 521-1BL00-0AB0), 2 × DQ 16×24V/0.5A (6ES7 522-1BH01-0AB0), 1 × AI 8×U/I (6ES7 534-1HE01-0AB0), and 3 × TM PosInput 2 (6ES7 551-1AB00-0AB0) — one per axis.

PROFINET IRT Ring Topology Configuration

The crane safety control system uses a PROFINET IRT (Isochronous Real-Time) ring topology to ensure deterministic real-time communication between field devices and the PLC. The network topology forms a closed ring: PLC → Hoist Inverter → Bridge Drive VFD → Trolley VFD → HMI → PLC. The IRT communication cycle is set to 2 ms, satisfying the 5× sampling requirement for speed closed-loop control (encoder position sampling at 400 µs, speed loop at 2 ms, and current loop at 125 µs handled internally by the G120). MRP (Media Redundancy Protocol, per IEC 62439-2) is enabled in the ring; in the event of a cable break anywhere in the ring, redundancy switchover completes in ≤200 ms without losing a single IRT data packet.

Configuration steps (TIA Portal V18+): First, in the device view, add the PLC and all IO devices to the same IRT domain, setting all devices to the "Sync Slave" synchronization role and the PLC as "Sync Master." Second, in the device properties under "PROFINET interface → Advanced options → Real-time settings," set the communication cycle to 2 ms and the IRT send clock to an integer multiple of 1 ms (2 ms is recommended, equal to the send clock). Third, create an MRP ring in the "MRP domain" section, adding both PLC ports (Port 1 and Port 2) and all IO devices to the ring domain. Set the ring role to "Client" for all IO devices, while the PLC also serves as "MRM" (Media Redundancy Manager), responsible for ring monitoring and redundancy switching. Fourth, assign a fixed IP address to each IO device (e.g., Hoist Inverter IP 192.168.1.11, Bridge Drive VFD 192.168.1.12, Trolley VFD 192.168.1.13, HMI 192.168.1.20), and set the DeviceName to the corresponding device abbreviation. Once the ring is configured, the ring status can be verified in TIA Portal under "Online PROFINET topology," confirming that all devices are online and IRT communication is healthy.

G120 VFD Parameter Tuning for Hoist, Bridge, and Trolley

SINAMICS G120 VFD parameter tuning differs significantly across the three crane mechanisms, and a one-size-fits-all parameter set is not acceptable. The hoisting mechanism (Z-axis) is a constant-torque, potential-energy load. The VFD must be sized at 120% of the motor's rated current (G120 PM240-2 power module, heavy-duty 125% overload for 60 s). Key parameters: P1300=22 (torque control with speed controller — closed-loop control is mandatory for hoisting to prevent load slipping), P1082=50 Hz / Max speed=3000 rpm, P1240=1 (Vdc control enabled to prevent overvoltage trips when lowering heavy loads), and brake control P1215=3 (standard brake / Motor Holding Brake enabled — the hoist inverter automatically closes the brake when output frequency falls below P2162=1.5 Hz, preventing load slipping).

The bridge mechanism (X-axis) and trolley mechanism (Y-axis) are translational drives with high load inertia but no potential-energy load. Bridge drive VFD parameters: P1300=22 (closed-loop vector control — mandatory for precise crane travel positioning), P1082=40 Hz / Max speed=2400 rpm, acceleration time P1120=8 s (high bridge inertia requires gradual ramp-up/ramp-down to prevent load swing), deceleration time P1121=8 s. The trolley VFD, given its short travel and frequent acceleration/deceleration cycles: acceleration time P1120=4 s, deceleration time P1121=4 s (lower trolley inertia allows shorter ramps for higher throughput), and P1496=5 (acceleration feed-forward coefficient, reducing speed-following error for more precise positioning). Brake control logic is configured identically across all three mechanisms: VFD P1215=3 (standard brake / Motor Holding Brake enabled). The brake-open condition is set to VFD output frequency ≥3 Hz with flux fully established (P1216 brake-open delay = 0.1 s). The brake-close condition is set to VFD output frequency ≤1 Hz (P1217 brake-close delay = 0.3 s, preventing premature brake engagement that could cause abrupt stopping and load swing). Note: for the hoisting mechanism, the brake-close delay P1217 should be set to 0.5 s (0.2 s longer than for translational mechanisms) to ensure the load has come to a complete stop before the brake engages, preventing load-slip incidents. After commissioning, a loaded load-slip test is recommended — with the VFD stopped at rated load, hoist lowering displacement must be ≤20 mm to pass.

SIL3 Safety Functions with PROFIsafe Integration

The S7-1500F safety PLC, combined with the integrated safety functions of the G120 VFD, enables a SIL3-rated crane safety control system. Safety functions are implemented via the PROFIsafe safety communication profile (the safety profile running on PROFINET), where safety telegrams are transmitted in parallel with standard process data over IRT — no separate safety bus is required. The core safety functions include STO (Safe Torque Off) and SS1 (Safe Stop 1).

STO (Safe Torque Off) is a VFD-side safety function that cuts the drive pulses to the power stage via an F-DI (Fail-safe Digital Input) signal, bringing the motor to a coast-to-stop state with zero torque output. The S7-1500F F-DQ module sends the STO command to the G120 via PROFIsafe (enable by setting P9601=C01800 in the VFD parameters — STO via PROFIsafe is not enabled by default and must be manually activated). In the crane safety logic, STO trigger conditions include: safety door opened, light grid interrupted, Emergency Stop Button pressed, and overspeed detection triggered (STO is triggered immediately when hoisting speed exceeds 110% of rated speed; hoist overspeed can be detected either by the PLC calculating encoder speed or via the G120's own speed monitoring at P2159).

SS1 Safe Stop 1 is implemented via a safety-rated PLC controlling the VFD to decelerate the crane at a predefined safety deceleration rate. Once the speed drops below the safe threshold, STO (Safe Torque Off) is triggered to cut torque. In the S7-1500F, SS1 is realized through the F-function block "DRV_SS1" (integrated in the Drive-Lib safety library within TIA Portal). Key parameters include: maximum deceleration (2.5 m/s² recommended for overhead crane travel mechanisms to prevent load swing; 1.5 m/s² recommended for the hoisting mechanism to avoid wire rope shock during emergency stops), and safe speed monitoring thresholds (hoisting ≤ 50 rpm, approximately 1% of rated speed; travel ≤ 100 rpm). The advantage of SS1 over a direct STO stop is its gentler stopping profile — when a light curtain is obstructed, the crane first decelerates at the safe rate before torque is cut, preventing load swing or wire rope damage caused by abrupt stops. After an SS1 trip, the operator must perform a manual reset to re-enable the VFD, ensuring that the drive cannot resume operation before the safety event has been acknowledged.

Network Deployment and Commissioning Checklist

Network deployment follows a sequential process: Electrical cabinet assembly and wiring — connect the PLC cabinet GND busbar to the PE busbar using a 10 mm² copper braided strap (resistance ≤ 0.1 Ω); route SSI encoder cables and PROFINET cables in separate conduits with a minimum spacing of 200 mm to prevent electromagnetic interference. PROFINET ring network cabling and continuity testing — verify wiring diagrams, cable length, and attenuation for each segment using a Fluke Networks MicroScanner; terminate all RJ45 connectors with PROFINET-grade metal shielded connectors (FIT 180°). TIA Portal hardware configuration and network assignment — import GSDML files for all I/O devices and VFDs, verify topology connections, and assign IP addresses. Safety program development and F-CPU compilation/download — the initial download of the safety program requires entering the F-target password and confirming the F-frame CRC check passes before proceeding.

On-site commissioning checklist:

• Power supply verification — PLC 24 V supply within ±5%; G120 DC bus voltage stable (PM240 module rated at DC 540 V); PROFINET switch port Link LEDs normal (solid green = link established, flashing yellow = data communication active).
• I/O signal testing — test all 48 DI points individually (short the signal and verify the PLC online variable updates correctly); test all 24 DQ points (force outputs and verify corresponding relay actuation); calibrate all 8 AI analog channels at 4 mA, 12 mA, and 20 mA.
• VFD commissioning — use STARTER software or the G120 BOP-2 operation panel to perform static and rotating identification on all three drives (P1910 static identification measures motor winding resistance; P1960 rotating identification measures flux and speed loop PI parameters).
• Positioning system verification — encoder position values must change continuously without jumps (rotate the encoder shaft 360° and confirm the PLC reading changes smoothly and corresponds to the pulse count); laser rangefinder readings must match actual distances (compare against tape measurements at 10 m, 20 m, and 50 m with a deviation ≤ 5 mm).
• Safety function acceptance — trigger each STO and SS1 condition individually (e-stop, safety gate, light curtain, overspeed simulation) and confirm the safety function responds correctly and that stopping time and deceleration meet design requirements.

Each item on this checklist should be signed off and archived as part of the overhead crane control system acceptance documentation.

Frequently Asked Questions