Now we will get to the core of this article the seven questions you should ask yourself before choosing a system. You should realize that we will be using the broad generalizations in the following analysis, and that every individual application will have exceptions to these rules; however, the logic is still sound. While the authors work on different sides of the PLC or DCS fence for a supplier that has delivered both DCS and PLC solutions to the market for over 25 years, we feel that we are in a unique position to deliver both sides of the story.
The seven questions are designed to make you think about your company's operating philosophy and application requirements, taking into account the point of view of all the major stakeholders in your plant (engineering, operations, maintenance, etc.).
1. What are you manufacturing, and how?
2. What is the product value being manufactured and the cost of downtime?
3. What do you view as the "heart" of the system?
4. What does the operator need to be successful?
5. What system performance is required?
6. What degree of customization is required?
7. What are your engineering expectations?
Note that a consolidated list of the questions and possible responses are presented at the back of this article the tear off page. One simple method for the gauging whether you should be using a PLC or DCS is to go through this survey form, checking all of the responses that apply. If all of your responses are in one column, then your application clearly calls for this one type of system. If you have multiple selections from both the PLC and DCS columns, then maybe you have a hybrid application which requires a process control system capable of delivering both PLC and DCS functionality.