It may surprise you to know that PLC, HMI and SCADA implementations today are consistently proving more expensive than DCS for the same process or batch application. CEE finds out more…
Traditionally, DCSs were large, expensive and very complex systems that were considered as a control solution for the continuous or batch process industries. In large systems this is, in principle, still true today, with engineers usually opting for PLCs and HMIs or SCADA for smaller applications, in order to keep costs down.

So what has changed? Integrating independent PLCs, the required operator interface and supervisory functionality, takes a lot of time and effort. The focus is on making the disparate technology work together, rather than improving operations, reducing costs, or improving the quality or profitability of a plant.

Yet a PLC/ SCADA system may have all or part of the following list of independent and manually coordinated databases.

* Each controller and its associated I/O
* Alarm management
* Batch/recipe and PLI
* Redundancy at all levels
* Historian
* Asset optimisation
* Fieldbus device management

Each of these databases must be manually synchronised for the whole system to function correctly. That is fine immediately after initial system development. However, it becomes an unnecessary complication when changes are being implemented in on-going system tuning and further changes made as a result of continuous improvement programmes.

Making changes
Every time a change is made in one database, the others usually need to be updated to reflect that change. For example, when an I/O point and some control logic are added there may be a need to change or add a SCADA element, the historian and the alarm database. This will require the plant engineer to make these changes in each of these databases, not just one – and get it right.

In another scenario, a change may be made in an alarm setting in a control loop. In a PLC implementation there is no automatic connection between the PLC and the SCADA/ HMI. This can become a problem during start up of a new application, where alarm limits are being constantly tweaked in the controller to work out the process, while trying to keep the alarm management and HMI applications up to date with the changes and also being useful to the operator.

Today’s DCS, which are also sometimes called ‘process control systems,’ are developed to allow a plant to quickly implement the entire system by integrating all of these databases into one. This single database is designed, configured and operated from the same application.

This can bring dramatic cost reductions when using DCS technology, when compared with PLC/ SCADA (or HMI): at least in the cost of engineering. DCS hardware has always been considered as being large and expensive. This is certainly no longer the case today. DCS hardware even looks like a PLC, and the software runs on the same specification PC, with the same networking – so why the extra cost? Is it the software? Although it is true to say that DCS software can be made to be expensive – but only by buying all of the many advanced functional features that are available – and often that you would not use or need!

Where smaller and medium systems are concerned, then price comparisons on acquiring hardware and software are comparable to PLC/SCADA. So, the real difference is actually in the costs associated with the workflow – which is enhanced and simplified by the single database at the heart of a DCS.

At this point one may think that DCS functionality is biased towards control loops, whilst PLCs are biased towards discrete sequential applications and that this, therefore, is not a like-for-like comparison. This is an