PLCs (Programmable Logic Controllers)
Complete encyclopaedia entry for PLCs — what they are, how they work, common failures, maintenance, lifespan, and selection criteria.
What Is It?
A PLC (Programmable Logic Controller) is an industrial computer designed for reliable operation in harsh environments. It monitors inputs (sensors, switches), executes control logic, and drives outputs (valves, motors, indicators) to automate machines and processes.
How Does It Work?
A PLC operates on a scan cycle: (1) Read all input states, (2) Execute the user program (typically ladder logic) based on the input states, (3) Update all outputs based on the program results. This cycle repeats continuously — typically every 10–50 milliseconds. The program is stored in non-volatile memory and determines how inputs are processed and what outputs are activated.
Common Failures
- Input/output card failure: I/O cards fail from voltage spikes, overload, or thermal stress — one point or the entire card.
- Power supply failure: The PLC power supply fails from age, voltage spikes, or overheating.
- Memory corruption: Rare in modern PLCs, but can occur from electrical noise or battery failure.
- Communication failure: Network interface fails — PLC loses connection to other PLCs, SCADA, or I/O.
- Environmental damage: Dust, moisture, heat, or vibration damages the PLC electronics.
Maintenance
- Backup program: Keep an up-to-date backup of the PLC program. Store both on-site and off-site.
- Battery replacement: Replace the PLC battery (for program retention during power loss) every 2–3 years or per manufacturer's schedule.
- Panel cleanliness: Keep the PLC panel clean and dust-free. Verify door seals and cooling.
- Spare parts: Keep spare I/O cards, power supply, and CPU for critical systems.
- Firmware updates: Apply firmware updates during scheduled shutdowns — always test in a development environment first.
Typical Lifespan
PLCs typically last 15–20 years. The CPU may be upgraded or replaced every 10–15 years as technology advances. I/O cards can last the life of the PLC. Obsolescence is the most common reason for PLC replacement — the manufacturer may stop supporting old models.
Selection Criteria
- I/O count: Determine the number of digital and analog inputs and outputs. Add 20% spare capacity.
- Scan time: Verify the PLC can scan fast enough for the application — typically 10–50ms for most industrial uses.
- Communication: Select the required communication protocols (Ethernet/IP, Modbus, Profinet).
- Redundancy: For critical processes, select a PLC with redundant CPU and power supply.
- Programming: Consider the programming language (ladder logic, function block, structured text) and the availability of skilled programmers.
How CHI can help
Ask CHI about plcs (programmable logic controllers) — how it works, common failures, and maintenance requirements. The CHI assistant can explain how the equipment works, recommend maintenance tasks, help with selection criteria, and troubleshoot common failures based on the symptoms you describe.