The debate over the virtues of the distributed control system (DCS) vs. the programmable logic controller (PLC) has been ongoing since these two architectures came into existence 40 years ago. But as functionality differences narrow and price points align, the arguments are getting more intense than ever.
The DCS has always focused on distributing control on a network so operators can monitor and interact with the entire scope of the plant. PLCs have emphasized flexible, fast local control with primarily a controller centric view.
When PLCs and HMI software packages are integrated, the result looks a lot like a DCS. But this is very much a “Do It Yourself” (DIY) approach with plenty of technical risk as well as added costs that are not always immediately obvious.
Here are the Top 10 issues to consider when evaluating a DCS vs. building your own distributed control system using a PLC-based architecture:
- Network Performance: DCS suppliers provide best practice information so users start with a sound network design for their control system including fault coverage so you never lose view of the process or access to the final control device. Contrast this to the DIY world where the application engineer must design the network topology and consider the various fault modes and coverage.
- Control Performance: While the PLC runs “as fast as it can,” the process controller in a DCS favors repeatability. The DCS also tightly coordinates the control that generates alarms, as well as the alarm and event subsystems that collect, store and report those alarms.
- HMI Graphics: With a system like a DCS where the control and operator environments are designed and built together, up to 90 percent of what is needed to run a process is available standard out of the box, pre-built ready to go.
- Control Algorithms: By creating function blocks with a complete set of parameter-based functions, the DCS user can develop and fine-tune control strategies without designing control functions, delivery high fidelity, reliable control and condition handling. Not so with a PLC.
- Application Software: In the world of DIY, one can find all of the applications required to run a process plant. But isn’t it easier to have everything built into the system – designed to work together and maintained and tested from a single DCS supplier – than purchasing different software packages that you must put together and maintain?
- Data Management: When piece parts are used to form a DIY system, the various data models must be synchronized and maintained. A burden exists on engineers and administrators to accomplish this task of configuring and maintaining multiple databases in the PLCs and HMI. With a DCS, the entire data model is based on a single data owner, so it is created once and available throughout the system for use, and if needed can be easily modified in one location. The DCS data model has been conceived to cover all parts of the system.
- Batch Automation: The various aspects of batch automation can now be captured in a single DCS data model. All elements necessary for batch management and execution run in the process controller without the need for a PC operating as a batch server. Batches can be run faster and more reliably by eliminating a separate batch server and all of the server-controller communications for processing the various batch steps.
- Open Connectivity: The DCS architecture brings third-party devices into a common data model. This incorporation of existing controllers means operators can view information from various brand controllers in a consistent fashion.
- Simulation Technology: Some DCS suppliers offer simulator technology to support improved performance throughout the lifecycle of a plant – from off-line use in steady-state design simulation, control check-out and operator training, to online use in control and optimization, performance monitoring, and business planning.
- Process History: Robust process history functionality built directly into the modern DCS enables engineers and plant management to analyze the performance of the entire operation from a single location.
Industrial operations can’t go wrong with the new breed of scalable DCS solutions, which involve less engineering effort to configure and are easier to maintain than a PLC-based system. With out-of-the-box functionality and flexibility, they also have lower implementation cost and require less ongoing maintenance.
About the Author
Bill Stevens, global product marketing manager for Honeywell Process Solutions, is responsible for all automation products sold through channels (distributors, system integrators and OEMs). He has 28 years of experience at Honeywell and has worked in various positions in marketing, sales and business development, strategic planning, product management, and channel development and management. He also served as a control system engineer for a major consulting engineering firm on power industry projects. Bill holds a degree in mechanical engineering and controls from Rensselaer Polytechnic Institute.