ROBUST MODBUS-RTU COMMUNICATION DESIGN FOR SAFETY-CRITICAL COMPRESSOR VFD CONTROL IN SUB-ZERO COLD STORAGE
DOI:
https://doi.org/10.62985/j.huit_ojs.vol26.no2E.422Từ khóa:
Modbus-RTU, safety-critical control, VFD compressor, RS-485 communication, industrial refrigeration, fault toleranceTóm tắt
This paper presents a robust Modbus-RTU communication design for safety-critical variable-frequency drive (VFD) control in an industrial sub-zero cold storage refrigeration system. The target platform is a Siemens S7-1200 PLC with RTD temperature acquisition and an RS-485 Modbus-RTU network interfacing VFD-driven compressors over a 100-meter shielded cable installation. The proposed design integrates (i) deterministic polling and command scheduling under a fixed 250 ms communication cycle, (ii) bounded retry with exception-aware error handling distinguishing four fault types, and (iii) a watchdog-based safe-fallback stra tegy with temperature-threshold control to preserve thermal safety during communication degradation. A communication state machine is formulated to classify failures into transient timeouts, CRC errors, bus-off conditions, and Modbus exception responses, each mapped to a specific mitigation and recovery procedure. In addition, practical RS-485 physical-layer configurations-including baud rate selection (9600 bps), parity settings (8N1), termination (120Ω), biasing, bus topology, and grounding-are investigated through design analysis and trade-off evaluation to determine optimal parameters for frame error minimization and recovery performance. Field deployment observations, including cable disconnection, power supply disturbances, and electromagnetic interference scenarios, are presented to evaluate detection latency, recovery time, and temperature stability. Operational experience since November 2025 indicates that the proposed communication framework improves resilience and supports stable refrigeration operation with temperature control within ±2°C during transient industrial disturbances.
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