Manufacturing automation engineers want customizable control platforms, which offer the individual components that can be programmed quickly together to accomplish the task at hand. Often engineers and integrators open the PLC toolkit, roll up their sleeves and start programming. Typically the tools provided by a PLC are optimized to support a bottom-up approach to engineering, which works well for smaller applications.
On the other hand, DCS engineers are typically most effective using a top-down approach for engineering, which forces them to put significant effort into the upfront design. This focus on upfront design is a key to compressing the project schedule, and minimizing costs, creating an application that can be maintained by plant personnel over the long term. Since DCS applications are larger and plant wide in scope, the ability to propagate libraries and templates throughout the application is very important to minimize rework and promote the use of standards.
Think about it this way, the PLC is controlling a machine, while the DCS is controlling the plant. For instance, pencil manufacturer is producing an incredible amount of pencils at an extremely rapid speed using a PLC. Engineers could be able to squeeze another 10 milliseconds out of the machine by programming in machine code, which is now capable of punching out even more profits and pencils.
The PLC engineer demands that kind of open architecture and flexibility. The process engineers are controlling entire plants with a DCS require more intuitive programming platforms, which utilize pre-tested and predefined functions to drive repeatability and save time.
Getting the right tool for the job is also critical. Ladder logic is the preferred and ideal configuration language for many discrete control applications, such as the control of motors and drives or high-speed interlocking. On the other hand, function block diagram is preferred for continuous control and for implementation of alarming schemes.