We are putting the finishing touches on a white paper that addresses the question of control system platform selection, PLC vs. DCS. It will provide an in-depth discussion of this subject and will hopefully clear up many misconceptions and provide some meaningful guidance on a question that many end users are faced with. As a preview of the paper, we are providing the selection matrix that is part of it. We hope this is beneficial and gives you a sense of the way in which the subject is being addressed.
Key Considerations When Selecting a Control Platform
The following matrix provides general guidance for narrowing down the selection of controller platforms and is not intended to replace a consultation by a trained knowledgeable professional.
| Criteria | PLC | DCS | Notes: |
| Product Value – We know a pound of gold is worth more than a pound of coal, so if you lose a batch of product, will it be bad or catastrophic? | Good | Better | Is the added cost of redundancy, hardware and engineering worth the benefit? The DCS has redundancy capabilities from the processor down to the individual I/O point. |
| Production Startup – Does your process start with the push of a button and return to speed quickly, or do you have to stage and/or charge production units over a lengthy period of time? | Better choice on fast startups | Better choice on slower startups | PLCs are fast; they run an input, compute, output cycle in milliseconds. Slower processes typically require coordination across various production units, which matches the capabilities of the DCS. |
| Data Analysis – enterprise-wide data historization and manufacturing execution system (MES) | Good | Better | PLCs have begun to close the gap with SPC, historian and data storage capabilities; however, these functions are typically integrated tightly within the DCS platform. |
| Discrete I/O – motor controls, fans, interlocks and other on/off equipment | Better | Good | This is where PLCs first gained experience, and they remain very effective in this environment with fast scans and low-cost. DCS providers can offer micro-DCS to compete in this arena. |
| Analog I/O – pressure controls, flow rates and PID control loops | Good | Better | PLCs are fine for simple PID loops; however, DCSs have the built-in infrastructure to perform advanced regulatory control on a plant-wide scale. |
| Exception Notification – Alarming is based upon defined limits; limited operator interaction. | Better | Good | Many PLC applications can run for days or weeks with little if any operator interaction. |
| Centralized Control -Operations continuously monitor process conditions and change setpoints based upon condition changes. | Good | Better | DCSs provide a system-wide database and can handle large loop counts in a multi-tasking mode. |
| Maintainability – engineering and technician capabilities | Better | Good | PLCs utilize symbology that is common to electrical technicians, which typically requires less training. Many technicians are already familiar with common PLC ladder logic programming.Along with the added capabilities of the DCS comes the need for highly trained personnel. DCSs typically require more specialized training involving IT-related networking and database functions. |
Please let us know if you find this information helpful.