Distributed Control Systems (DCS)

Distributed control systems (DCS) use decentralized elements or subsystems to control distributed processes or complete manufacturing systems. They do not require user intervention for routine operation, but may permit operator interaction via a supervisory control and data acquisition (SCADA) interface. Distributed control systems (DCS) consist of a remote control panel, communications medium, and central control panel. They use process-control software and an input/output (I/O) database. 

Some suppliers refer to their remote control panels as remote transmission units (RTU) or digital communication units (DCU). Regardless of their name, remote control panels contain terminal blocks, I/O modules, a processor, and a communications interface. The communications medium in a distributed control system (DCS) is a wired or wireless link which connects the remote control panel to central control panel, SCADA, or human machine interface (HMI). Specialized process-control software is used to read an I/O database with defined inputs and outputs.

Selecting distributed control systems (DCS) requires an analysis of network protocols. Ethernet is a local area network (LAN) protocol that uses a bus or star typology and supports data transfer rates of 10 Mbps. To handle simultaneous demands, Ethernet uses carrier sense multiple access / collision detection (CSMA/CD) to monitor network traffic. Fieldbus is a bi-directional communications protocol used for communications among field instrumentation and control systems. Network protocols for distributed control systems (DCS) also include controller area network bus (CANbus), control network (ControlNet), DeviceNet, INTERBUS (Phoenix Contact GmbH & Co), and PROFIBUS (PROFIBUS International). The process fieldbus (PROFIBUS®) is a popular, open communication standard used in factory automation, process automation, motion control, and safety applications.

Distributed control systems (DCS) differ in terms of complexity and applications. Smaller implementations may consist of a single programmable logic controller (PLC) connected to a computer in a remote office. Larger, more complex DCS installations are also PLC-based, but use special enclosures for subsystems that provide both I/O and communication. In terms of applications, some distributed control systems (DCS) are used in electrical power grids or electrical generation facilities. Others are used in environmental control systems, wastewater treatment plants, and sensor networks. Distributed control systems (DCS) for petroleum refineries and petrochemical plants are also common. Fully-distributed systems enable remote nodes to operate independently of the central control. These nodes can store all of the process data necessary to maintain operations in the even of a communications failure with a central facility.