PLC vs. DCS: White Paper Preview

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.