For manufacturers in the process industries, the procedure for selecting the best automation technology is not as easy as it once was. In the past it was fairly easy to determine whether a PLC or a DCS was right for your application, because their strengths and weaknesses were well understood. In recent years this has become more difficult, thanks primarily to the advancement of the microprocessor, which has allowed the technologies to merge. With the trend toward flexible manufacturing in industry, many of the applications in the process
industries now share the requirements traditionally thought to be exclusive to either DCS or PLC. These hybrid applications typically require a process control system that can deliver both PLC and DCS capabilities. Thus understanding the merging of PLC and DCS functionality is important for selecting the best system for your company.
The convergence of PLC and DCS technologies has created a situation where it is more challenging than ever for process manufacturers to select the best technology for their application. A successful evaluation should start by developing a clear picture of the requirements of your application and the needs of your engineering, maintenance, and operations personnel. To help clearly define these requirements and needs for your company, this paper outlines the seven key questions that will lead you to making the right choice.
Benefits of Selecting the "Right" Automation Technology:
In this era of global competition, manufacturers in the process industries are being driven to achieve operational excellence to secure their place ahead of their competition now and in the future. Selection of new automation technologies impacts this goal. Consequently,the selection process is more important to a company's staying power than ever before. In fact, the importance of the selection of technology far outweighs the cost of the automation investment itself.
Selecting the right technology and the right supplier can help your company:
- respond quickly to changing market conditions in a way that creates a sustainable competitive advantage.
- minimize Total Cost of Ownership (TCO) over the life of your plant.
- create a system which is easily maintainable/ upgradeable for the long-term.
- achieve its future goals and vision.
Let's look at the components of a DCS or PLC based system to see how different (or similar) they really are.At first glance, the pictured system architectures look very similar. Both systems share the following components:
- Field devices
- Input/output modules
- Controllers.
- Human machine interface (HMI).
- Engineering.
- Supervisory control.
- Business integration.
As you look at the following system architectures, you should note that the technologies used in each system are in fact, very similar; the difference becomes more apparent when you consider the nature and requirements of the application.
For example, in the DCS architecture diagram, redundancy is often employed for I/O, controllers, networks, and HMI servers. Since redundancy adds cost and sometimes complexity, DCS users must carefully evaluate their need for redundancy in order to achieve their required system availability and to prevent unplanned downtime.The PLC architecture illustrates one of its most common applications, the control of discrete field devices such as motors and drives. To effectively control motors and drives requires that the controller be able to execute at high speeds (typically a 10–20 msec scan rate), and that the electrical technician responsible for maintaining it be able to read and troubleshoot the configuration in a language that he is familiar with (relay ladder logic).
From a technology point of view, one can see that PLC and DCS are not that different, which has paved the way for them to merge. Therefore, we must look beyond technology to the application expertise and domain knowledge that is built in to these systems by the supplier, so that we can better understand the "sweetspots" where each is best applied.
Classic Stereotypes of PLC vs. DCS
Characteristic PLC DCS
Market Introduction 1960s 1975
Replacement of . . . Electromechanical Relays Pneumatic & Single-Loop Controllers
Products Manufactured . . . “Things” “Stuff”
Classic Application Automotive Refining
Type of Control Discrete Regulatory
Redundancy “Warm” Backup “Hot” Backup
Engineering Mindset "Programming" "Configuration"
Operator Interaction Exception Basis Man in the Loop
Operator Interface Simple Graphics Sophisticated Graphics
Size/Footprint Compact Large
Up-front cost $$ $$$$
System "Open" "Closed" (Proprieta)
