PLC vs DCS – What solution to choose?

When we want to integrate several software applications, use advanced control algorithms and have future expansion plans, the PLC / SCADA solution may not be adequate. Currently the differences between both solutions have decreased, but they are still important, especially in systems with more than 500 I / O and many analog outputs.

What is better to automate my plant, a DCS or a system based on PLC / PACs + SCADA?

Years ago it was much easier to answer this question than now. If we were talking about large continuous processes with analog signals and with more than one controller the answer was DCS, if it was mainly discrete signals the solution was the PLC. Today the boundary for decision making is much more diffuse and no longer depends so much on hardware (such as processor power or the ability to handle many analog signals) but rather on software and other non-technical factors related to the whole life cycle of the plant.

PLCs have evolved tremendously and have similar processing capabilities to DCS controllers. In fact, there are manufacturers of DCS and PLCs that use the same controllers, such as Siemens with the PSC7 and the S7-400. The SCADA have also evolved a lot and allow us to develop custom objects and libraries that give us features similar to those of the DCS.

DCS is associated to the large facilities where it is very beneficial to distribute the controllers and I / O by zones of the plant, and with several operating stations located in the control room. In addition, the integration of other applications into a single database is required. All this together with the cost and level of specialization that some DCS have in specific applications (refining, electricity generation, pharmacy, etc.) is what makes the difference.

The complete plant life cycle is another important aspect to consider in deciding for a DCS or PLC / SCADA. If we plan to grow, gradually integrate other applications, optimize the control algorithms and making other changes then possibly our solution goes through the DCS because managing all this with PLC / SCADA, having several databases, can be complicated and expensive.

PLC vs DCS – When is the DCS better?

In what cases is it clear that we should go to the DCS solution? If we meet the following requirements the solution will probably be the DCS:

-We have many I / O (> 2000) and a high number of analog outputs (> 200).
-We have multiple distributed controllers that communicate with each other.
-We use transmitters and intelligent valves that parameterize and diagnose in remote.
-We need to integrate the control system with a MES system or with other automation and / or information systems (safety system, electrical control system, ERP, asset management, data analytics, etc.)
-We need a system with a high level of redundancy (CPUs, I / O, bus).
-It is a batch application with many recipes and complex.
-We need a multi-operator HMI and advanced alarm management.
-The Plant will probably be expanded in less than five years.
-We have other similar plants with DCS and advanced control algorithms that we have optimized over time.

PLC vs DCS – When is the PLC / SCADA better?

In what cases is it clear that we should go to the PLC solution? If we meet the following requirements the solution will probably be PLC + SCADA:

-We have less than 300 I / O and few analog outputs.
-We only need one or two PLCs.
-We do not have special or complex HMI / SCADA requirements. The SCADA alarm management system is sufficient.
-This is a plant or package that we do not plan to grow in the next few years.
-We already use this type of PLC and SCADA and we do maintenance ourselves.
-We do not need the system to be redundant.

The decision will not be so clear if we have more than 300 or 400 I / O and we do not fulfill all the previous requirements. In this case we will have to do an analysis of our case and evaluate our standards and requirements. In addition, there may be other factors that tip the scale to one side or the other, such as the following:

– Who is going to program and configure the system? (Local integrator, manufacturer, ourselves).
– Who will perform the maintenance of the system?
– Are we familiar with the system and have a significant stock of spare parts?
– Is it a new system? Does it already have proven references in similar applications?