Selecting a PLC (Programmable Logic Controller) for a production line isn’t merely about matching input and output counts - it’s about choosing a control platform that ensures reliability, scalability, and integration across the plant floor. Below are key considerations and frequent pitfalls seen in retrofit or modernisation projects.
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1. Define Functional and Technical Requirements
Before selecting any controller, perform a detailed process analysis:
Input/Output capacity
Count all discrete and analogue I/O points, including sensors, valves, and control signals. Always plan a 20-30% margin for future expansion.
Performance and scan time
Applications such as conveyors, packaging, or synchronised drives require short, deterministic cycle times. Check CPU performance and response latency.
Advanced control functions
If your process involves PID loops, motion control, counters, or temperature regulation, ensure the PLC supports these via built-in libraries or expansion modules.
Communication and network protocols
Modern production lines demand integration with SCADA, MES, or ERP systems. Choose a PLC supporting open protocols like EtherNet/IP, Modbus TCP, Profinet, EtherCAT, or OPC UA.
Modularity and expandability
Modular PLCs are preferable for growing lines - they allow for easy addition of extra I/O, communication cards, or safety modules without replacing the base system.
Electrical and environmental specifications
Verify the power supply voltage (typically 24 V DC), electromagnetic compatibility, and IP protection rating. For harsh environments, opt for PLCs with enhanced vibration and temperature tolerance.
Safety and redundancy
For machinery subject to risk assessments, consider Safety-rated PLCs (SIL 2 / SIL 3 certified) to manage emergency stop circuits and safe zones.
Lifecycle support and ecosystem
Evaluate software availability, licensing model, technical support, and long-term component supply. A strong vendor ecosystem simplifies commissioning and maintenance.
Total Cost of Ownership (TCO)
The cheapest PLC may become the most expensive in the long run. Consider engineering time, integration costs, and spares availability alongside the purchase price.
3 Common Mistakes During Automation Modernisation Projects
1. Incomplete assessment of existing systems
The issue:
Many modernisation projects begin with the purchase of new hardware before a thorough analysis of the existing installation. Legacy interlocks, I/O lists, and field devices are often overlooked.
Consequences:
- Missing signals or functions after commissioning
- Compatibility issues with existing sensors and actuators
- Delays and cost overruns due to on-site modifications
How to avoid it:
Carry out a full as-is audit, including all I/O, network topologies, and control sequences. Develop a functional specification before choosing hardware.
2. Copy-and-paste programming without modular structure
The issue:
Reusing old PLC code without refactoring leads to monolithic logic, difficult to troubleshoot or modify.
Consequences:
- Longer scan times and unstable logic cycles
- Difficult testing and debugging
- Reduced maintainability
How to avoid it:
Apply a modular programming approach, use parameterised function blocks, and document your logic. Simulate critical functions before commissioning to avoid downtime.
3. Ignoring network integration and data exchange
The issue:
Focusing solely on machine-level control while neglecting communication with higher-level systems (SCADA, MES) leads to poor data visibility and inefficiencies.
Consequences:
- Incomplete production data
- Delayed reporting or loss of traceabilit
- Limited diagnostics and control
How to avoid it:
Design with data flow in mind from day one. Verify that the PLC supports all required protocols and has adequate processing capacity for network traffic. Test connectivity under realistic load conditions.
The Important Role of a Trusted Supplier
At Automation Trader, we offer a comprehensive range of industrial automation components - including compact and modular PLCs, expansion modules, power supplies, communication interfaces, and many other related products.
We invite you to explore our portfolio - the PLC examples above can easily be included in your online catalogue or linked directly from your e-commerce platform.
