Knowledge · Architecture
Separate PLCs or a compact DCS?
There is no universally right answer. There are a handful of questions that decide it for your plant, and they are mostly about how your process changes, not about the hardware.
Three common architectures
| Architecture | What it is |
|---|---|
| A PLC per machine | Each machine or skid has its own controller and operator panel. Machines are coordinated by people, or by simple hard-wired signals. |
| PLCs plus a supervisory layer | The machines keep their PLCs. A separate supervisory software package collects their data, provides plant-wide screens and stores history. |
| An integrated DCS | Controllers, operator screens, alarms and history come from one system and are configured in one engineering environment. ABB Freelance is an integrated DCS sized for single-site, small and mid-size process plants. |
The questions that decide it
- How many recipes, and how often do they change? A few fixed products favour simplicity. Many products, frequent changes and seasonal variants favour structured recipe handling. ISA-88 →
- Do your records have to stand up to an audit? If a customer, auditor or regulator can ask what happened to one lot, records need to be complete, time-stamped and protected. Traceability →
- How many process units must work together? Receiving, processing, CIP and utilities that depend on each other are a coordination problem, not a machine problem.
- How often will the process change? Every change has to be made, kept consistent and, where required, re-validated.
- Who will maintain the configuration in five years? Your own team, an integrator, or the equipment builder.
Tradeoffs to compare
These depend on the application and on how each option is engineered, so treat them as questions to ask of any proposal, ours included.
- Where the configuration lives. In one engineering environment, or split between controller programs and a separate supervisory project. If it is split, ask how tags, alarms and screens are kept consistent when something changes.
- What a change costs. Compare the engineering effort to add a product, a unit or an alarm, and any revalidation it triggers, not only the purchase price.
- Recipes and records. Ask which recipe and record functions are standard product and which are custom engineering for your project.
- Support responsibility. Ask who supports each part of the system, under what agreement, and for how long.
- Skills. Consider which platforms your team, and the integrators near you, already know.
When separate PLCs are the right answer. A single machine, a simple line, a few stable products and no significant records burden. In those cases a PLC is usually the simpler choice, and we will say so.
Where Freelance fits
Freelance is aimed at the plants in the middle: more coordination, recipes and records than separate machines handle comfortably, but one site and a scale that doesn't call for a large enterprise system. Recipe parameter management and reporting are available through Freelance Formulation; a full batch-management integration is specified per project where the application needs it. What a Freelance system includes →