Control System and Automation
Control System And Automation
Selecting the Best Control System for Automation Applications
Two main types of platforms form the foundation of modern process automation: the distributed control system (DCS) and the programmable logic controller (PLC). Today’s decision process for a new or upgraded control system, however, requires more than simply choosing between the two. Contemporary control systems could also include other technologies, such as the safety instrumented system (SIS) and the supervisory control and data acquisition system (SCADA)—as well as Industrial Internet of Things (IIoT) functionality and other connectivity to enterprise business systems.
Taking these other technologies and required functions into account, along with additional factors as detailed below, will result in an improved decision-making process, leading to a successful implementation over the multi-decade lifecycle expected from modern control systems.
DCS vs. PLC: Factors to Consider
PLCs and DCSes were initially developed for entirely different applications. The PLC was created to control a large number of discrete I/O, with programming simulating hard-wired relay ladder logic. The DCS was developed to control large continuous processes requiring more sophisticated control schemes such as cascaded PID control loops, process optimization and advanced or multivariate process control.
The differences between PLCs and DCSes are less clear today, although significant differences remain, with mixed systems often appropriate. Some questions to be answered during the selection process include:
• What is the product value of a batch or the product stream over a shift?
• How complex is the plant start-up and shut-down sequence?
• What is the relative mix of discrete and analog I/O?
• Do the control requirements include advanced regulatory control, or only simple PID loops?
• Must the process adhere to ISA18.2 or EEMUA 191 alarm management standards?
• Will the process require a Safety Instrumented System (SIS)?