Three Considerations in Choosing a DCS or a PLC Control System Platform

by Rod Smith

In the past, control system selection between PLC and DCS platforms was generally done according to the application specifics, mostly involving digital (on/off) and continuous (4-20ma) processes.  In some cases, it was the difference in programming expertise at the plant (ladder logic or function blocks). Today, PLC systems and DCS systems play in both applications. Another consideration was the number of interfaces to other control systems, business systems, maintenance systems and digital control networks (ProfiBus, FieldBus, etc.). Again, in today’s platforms, both systems are capable.

The question becomes which platform fits the individual plant and/or corporate needs. The items listed below are derived from several project evaluations, and are generally the basis of the control system platform selection. The priority level of the items below differs from one end user to the next.

  • The long term look at the plant or corporate operations is important. To select the proper control system platform, you must know how you intend to use it, both in the short term and for its life cycle. This applies to not only the process application but also the roles and responsibilities of the management, engineering, operations, maintenance, and IT groups at the plant. The capabilities and size of each group that have ownership of, or interface to, the control system must be considered. We are in an era of doing more with less and at the same time losing expertise to retirement, so it is important to look at the long term.
    • Whether a plant intends to self-perform or outsource all or part of the control system work is an important consideration. This item is often not discussed as part of the decision, but it can be a major factor in the long-term success.
  • Which platform and manufacturer has the history of maintaining the system migration path, support structure and local support to the plant, is a major consideration. For the reasons detailed above, outsourcing is more of a necessity than an option for most companies, so having local access to parts, training, and technical support is a requirement. All systems are subject to software revisions, and the frequency is getting faster every day, so the history of migration path is also very important. All systems have obsolescence issues, which goes to the history of system migrations. We have looked at companies with several plants around the world, and while we might agree on the platform (PLC or DCS) for all sites, we might also select a different manufacturer for each site, with very different levels of local support.
  • Cost is always an issue but the evaluation should be based on life-cycle cost, not just initial project cost. Because of the short-term software revision or obsolescence migration, long-term cost must be evaluated. This is difficult because no manufacturer wants to tell you when the system will require modification, but you know it will happen. The other problem is that you want to select a field proven system and not be serial #1, but you do not want a system that will be obsolete by the time you implement the project. This is a risk analysis based on the history of the system’s life cycle, and it differs between PLC and DCS systems.

In a recent evaluation performed with a chemical customer, the customer selected a PLC platform because of the following factors.

The customer outsourced most the programming work and needed several sources for  local support that could be responsive within a day.

The existing DCS platform was obsolete and the customer was not satisfied with the support, cost, or responsiveness of the supplier.

The PLC could be programmed using function blocks, not ladder logic, which was preferred by the plant for their mostly continuous process.

The selected manufacturer (Rockwell Automation) had local support from direct employees, distributor, and several system integrators.

The cost of installing the PLC system verses the DCS was substantially lower. The DCS cost was direct from the manufacturer while the PLC cost was from a local system integrator.

While DCS and PLC platforms both can perform in multiple applications, choosing the right platform for your particular plant and particular circumstances requires looking at the above key considerations. It’s too important a decision to make without weighing each issue carefully. One role of a control system integrator is to help clients identify and analyze these considerations to assist in making the best choice. To find an integrator that fits your particular needs and locale, go to the CSIA Exchange.