What is a Distributed Control System?

A distributed control system (DCS) is a specially designed computerized control system geographically distributed throughout the plant or control area. The difference between a DCS and a centralized control system is that the latter has a single controller at the central location who controls all functions, while in DCS, each element is directed by a dedicated controller. DCS encompass several local controllers across numerous units of plant control area and are interconnected through a high-speed communication network. 

Microprocessor-based units are responsible for data acquisition and control functions in a distributed control system. The microprocessor units are distributed functionally and geographically across the plant and are located within the area where data collection and control functions are performed. The controllers can communicate among themselves and with other extended controllers such as the supervisory terminals. 

The Architecture of Distributed Control System

Three fundamental aspects characterize the building structure of DCS. The first attribute is the distribution of control functions and their grouping into small sets of subsystems. The small subsystems are semiautonomous and are interlinked via a top-speed communication bus. Some of the functions of these subsystems are data acquisition, process control, supervision, information relay, storage and retrieval of information, data representation, etc. 

Automation of industrial processes through integrating advanced and innovative control strategies is the second aspect of DCS. The third main characteristic of DCS is the orderly arrangement of things as a complete system. The entire control system structure is organized as a single automation system in which subsystems are combined via a suitable command structure to enhance information flow. The fundamental components in DCS entail engineering workstation, operating station, process control unit, communication system, and smart devices.  

Components in Distributed Control System

Engineering Workstation of DCS

The engineering workstation of DCs is the supervisory controller within the overall distributed control system. A PC or any computer with a specified engineering software can make up the engineering workstation. The work of this station is to provide strong configuration tools to allow the user to accomplish engineering functions that include the creation of input and output points, modification of sequential and continuous control logic, creation of new loops, etc.  

Smart Devices

In DCS, the traditional subsystems I/O are replaced by intelligent field devices, which is an advanced technology. The use of smart devices enhances the intelligence needed for sensing and control techniques. The primary sensing and actuating devices replace the necessity for DCS controllers to perform the role of sensing and control. 

Operating Station

The operating station facilitates the operation of DCS. The process of DCS operation starts with sensors sensing the process information and relaying it to local I/O modules, to which the connected actuators control process parameters. The field bus then controls data from the remote modules to the control unit. The use of smart devices enhances direct transfer of information to process control unit through field bus. 

Process Control Unit of DCS

A DCS can have more than one process station extended through different I/O units. Process control units comprise a powerful CPU module and field bus with more field bus capability with direct or remotely connected I/Os. Sensors, actuators, and other field devices are then connected to I/O modules of the process control unit I/O modules. 

Communication System

The communication medium interconnects all elements of the DCS with one another. It conveys the information from one station to another. In information transfer, communication protocols utilized in DCS include Profibus, Foundation Field Bus, Ethernet, Modbus, etc. 

Use and Application

Uses and applications of DCS in the modern world include:

• Large-scale processing and manufacturing plants require DCS since many continuous control loops require constant monitoring and control.
• DCS is used in automobile engineering.
• They are applied in environmental management systems.
• They are used in nuclear power plants and petrochemical plants.