When faced with automating a process application, many automatically think of a Distributed Control System (DCS). While DCS platforms are powerful, they’re often unnecessary—and too costly—for smaller-scale projects. In these cases, a modern Programmable Logic Controller (PLC) can be a smarter, more efficient choice. 

What Is a Process Application?

Process applications rely heavily on analog I/O—measuring variables like pressure, temperature, level, flow, and other continuous signals. Control is typically achieved using modulating valves or variable-speed motors and is governed by PID (Proportional, Integral, Derivative) algorithms. In contrast, discrete applications involve simpler on/off logic, such as controlling lights, push buttons, or moving a machine between fixed positions. While most real-world systems contain a mix of both, process applications lean much more heavily on analog I/O. Common examples include refineries, chemical plants, air separation units, and water treatment facilities.

PLC vs. DCS: The Historical Divide

When automation was in its infancy, PLCs were designed for discrete processes with high-speed processing but had zero or very limited capability to handle analog I/O. DCS platforms, on the hand, were built for large-scale analog control using function block languages, but with slower scan times.

Today, that gap has closed. Modern PLCs now support extensive analog I/O, while DCSs have incorporated more discrete control features. Key differences now exist in an integrated platform, supplied libraries, and scalability.

When a PLC Is the Right Fit

For smaller process applications—where a DCS may be cost-prohibitive—a PLC often makes the most sense. Consider a small but critical asset like an instrument air compressor. It may only require a handful of I/O points (e.g., tank pressure, filter pressure drop, dew point, and temperature readings) with some basic discrete control. If the compressor isn’t integrated into the plant’s central DCS, a PLC is likely the optimal solution.

Key PLC Selection Criteria for Process Applications

Redundancy and Availability
If the process is critical, look for PLC platforms that offer redundancy—redundant CPUs, power supplies, and I/O modules. Support for resilient network architectures (like ring topologies) ensures the system can continue running even after a single point of failure.

Diagnostics and Monitoring
High diagnostic capability is essential for continuous operation. Features like HART communication enable access to detailed device diagnostics, allowing maintenance teams to detect and resolve issues before they escalate. Robust diagnostics also speed up recovery in the event of failure.

Flexible I/O Support
Modern PLCs support a wide array of analog signals—4-20mA, 0-5VDC, ±5VDC, 0-10VDC, thermocouples, RTDs, and both loop- and field-powered devices. Many vendors now support fieldbus communication (e.g., Ethernet/IP, Modbus TCP, Profinet) natively or via expansion cards, making integration with smart instruments easier than ever.

Advanced Programming Capabilities
Today’s PLCs include comprehensive math functions and advanced control capabilities, including PID, feedforward, dead time compensation, and more. Many platforms support multiple programming languages, including ladder logic, function block diagrams, and sequential function charts. Tag-based naming and reusable code modules streamline configuration and HMI integration.

Online Changes and Scalability
Process applications often run continuously, so the ability to make online program and hardware changes is essential. Many PLC platforms allow for hot-swapping I/O and modifying logic without stopping the process—if planned properly during initial setup.

Memory and Performance
Although historically a limitation, PLC memory has expanded significantly. Most modern platforms can now support complex control logic, batch operations, and data-intensive applications.

Final Thoughts

Modern PLCs offer the flexibility, power, and reliability needed to handle many process applications, especially those where a DCS would be too expensive or excessive. Choosing the right platform requires thoughtful planning, and an experienced system integrator can provide guidance to ensure success.

Need expert guidance?
Reach out to Applied Control Engineering to find the right PLC solution for your next process application.