PLC (PLC)

A Programmable Logic Controller (PLC) is a ruggedised industrial computer designed to automate electromechanical processes by executing user-defined logic against real-time inputs from sensors and switches, then driving outputs to motors, valves, actuators, alarms, and displays. PLCs are the central nervous system of every modern processing plant in India — biogas, plastic recycling, e-waste, lithium-ion battery, and pyrolysis facilities all rely on them for sequencing, interlocking, safety shutdowns, and data logging.

The architecture of a PLC system has three primary elements:

• CPU module: executes the program logic in scan cycles of 10-50 milliseconds.
• I/O modules: digital inputs (limit switches, pushbuttons), digital outputs (relays, contactors), analog inputs (4-20 mA from pressure, temperature, level transmitters), and analog outputs (control valve positioners, VFD speed references).
• HMI (Human-Machine Interface): touchscreen panels showing process states and allowing operator interaction.

Programming follows IEC 61131-3 standards in five languages — Ladder Diagram (most common in India for retrofit-friendly maintenance), Function Block Diagram, Structured Text, Instruction List, and Sequential Function Chart. Major PLC brands deployed in Indian plants include Siemens (S7-1200, S7-1500), Allen-Bradley (CompactLogix, ControlLogix), Schneider (Modicon M340), Mitsubishi, and Delta. A small CBG plant uses one PLC with 100-200 I/O points; a large recycling plant may distribute logic across 5-10 networked PLCs with 2,000+ points.

The trade-off versus computer-based DCS (Distributed Control System) is scale: PLCs are simpler, cheaper, and faster for discrete and small-batch process control but lack the redundancy and advanced control libraries of a DCS, which dominates above 5,000 I/O. PLC capex in India runs 1.5-5 lakh INR per panel for small plants and 15-50 lakh INR for fully integrated mid-size installations. The biggest operational risk is cybersecurity — networked PLCs without segmented Operational Technology (OT) networks have been targeted by ransomware, and CEA (Central Electricity Authority) cybersecurity guidelines and IS/IEC 62443 increasingly govern industrial control system protection in India.