Introduction to DCS


A distributed control system (DCS) is a computerized control system for a process or plant usually with a large number of control loops, in which autonomous controllers are distributed throughout the system, but there is central operator supervisory control.

DCS is the most commonly used control architecture in industrial process control.

Distributed Control System is most popular which is specially designed with redundancy and diagnostic capabilities to improve control reliability and performance. It gives greater flexibility to control distributed discrete field devices and its operating stations.

In this era of revolution technology, industrial automation systems deal with advanced automation control technologies to have better control performance over complex processes.

The DCS concept increases reliability and reduces installation costs by localizing control functions near the process plant, with remote monitoring and supervision.

Distributed control systems first emerged in large, high value, safety critical process industries, and were attractive because the DCS manufacturer would supply both the local control level and central supervisory equipment as an integrated package, thus reducing design integration risk.

To increase reliability, productivity and quality while minimizing the production cost, process control industries must be driven by integrated controllers with high distributed control capability.

History of Distributed Control System

Because of the reliability problems and high cost of the control process computer systems of the 1960s, there were few new process computer projects in the early 1970s. The rare projects that were started in this period were based on medium-priced minicomputers that were designed to be small in size. At the same time, two developments occurred in electronics that profoundly changed the application of digital computers to process control.

The first was the development of integrated circuits and microprocessors. The second was the release of the distributed control system (DCS) by Honeywell in 1969. widely distributing the control to computer modules. Each of these modules controlled several instrument loops, generally one to four. They were connected by a single high-speed data communications link, called a data highway, that made possible communications between each of the computer modules and the central operator console. This design allowed the operator to monitor the operation of each local process.

In the mid-1970s, microprocessor-based modules replaced hardwired computer modules. The typical DCS had the configuration shown in Figure Today’s distributed control systems are much more powerful and faster than the first systems because of improvements in microprocessors and other electronic circuits.

What is a Distributed Control System?

In recent years, the use of smart devices and field buses has made distributed control systems (DCS) prominent in large and complex industrial processes as compared to the former centralized control system. This distribution of control system architecture around the plant has led to produce more efficient ways to improve reliability of control, process quality and plant efficiency.

A distributed control system (DCS) is a specially designed automated control system that consists of geographically distributed control elements over the plant or control area.

It differs from the centralized control system wherein a single controller at central location handles the control function, but in DCS each process element or machine or group of machines is controlled by a dedicated controller. DCS consists of a large number of local controllers in various sections of the plant control area and are connected via a high speed communication network.

Distributed Control System is a specially designed control system used to control complex, large and geographically distributed applications in industrial processes. In this, controllers are distributed throughout the entire plant area.

These distributed controllers are connected to both field devices and operating PCs through high speed communication networks

Discrete Field devices such as sensors and actuators are directly connected to input and output controller modules through communication bus. These field devices or smart instruments are capable of communicating with PLC’s or other controllers while interacting with real world parameters like temperature, pressure, etc.

Controllers are distributed geographically in various sections of the control area and are connected to operating and engineering stations which are used for data monitoring, data logging, alarming and controlling purposes via another high speed communication bus.

These communication protocols are of different types such as foundation field bus, HART, Profibus, Modbus, etc. DCS provides information to multiple displays for user interface.

In the DCS control system, data acquisition and control functions are carried through a number of DCS controllers which are microprocessor based units distributed functionally and geographically over the plant and are situated near areas where control or data gathering functions are performed as shown in the figure above. These controllers are able to communicate among themselves and also with other controllers like supervisory terminals, operator terminals, historians, etc.

DCS is most suited for large-scale processing or manufacturing plants wherein a large number of continuous control loops are to be monitored and controlled. The main advantage of dividing control tasks for distributed controllers is that if any part of DCS fails, the plant can continue to operate irrespective of the failed section.

Distributed Control System is most popular which is specially designed with redundancy and diagnostic capabilities to improve control reliability and performance. It gives greater flexibility to control distributed discrete field devices and its operating stations.

Basic Elements of Distributed Control System

Distributed Control System continuously interacts with the processes in process control applications once it gets instruction from the operator. It also facilitates variable set points and opening and closing of valves for manual control by the operator. Its human machine interface (HMI), face plates and trend display gives the effective monitoring of industrial processes.

The typical sub systems of a DCS are Operator station, Engineering station, Process control networks, Controller and I/O modules.

Operator Station Operator station gives an intuitive view into the process with easy access to process graphics, faceplates, alarm summaries, alarm faceplates, trends, and display navigation. Operator stations extend to diagnose from traditional process monitoring and control. The diagnostics includes monitoring intelligent devices and machinery monitoring, cyber and network monitoring, thereby increasing process uptime and reducing unplanned shutdowns.

Engineering station Engineering station gives the flexibility to implement the system to meet project and process needs. Engineers can implement portions of the system on the fly in a modular fashion as process design and control requirements become available. Engineering can be done in a phased manner as the process needs are becoming clear in an agile mode. With minimal effort engineers can specify control loops and field devices and then quickly complete the configuration.

Process Control Network Process Control Network 11 the backbone on which the entire DCS is connected. The network runs across the plant connecting all intelligent devices in a reliable deterministic manner. Process control networks also called PCNs can be open protocols such as Ethernet or also can be proprietary. The PCNs are generally installed in a redundant monitor to improve the reliability and are installed away from regular power cables to reduce the impact of electromagnetic interference and thereby increasing the reliability of the overall system.

Controllers The controllers execute control strategies to help optimize the process. The controllers are available in redundant and non-redundant configurations. The speed of execution of the control strategy determines the quality of control. The controllers are provided with a rich set of libraries for various algorithms such as PID, Feed forward etc. The controller receives the input from the channel of the input module connected to it. Similarly the controller sends the output to the configured channel of the output module to drive the final control element.

I/O Module I/O module is a modular subsystem that is designed to be installed in the field near to the instrument. I/O modules participate in the network to communicate with the controller. The I/O module can also become a gateway for communication with intelligent field devices over fieldbus networks. The I/O modules can also be configured as redundant in the critical control loop.

What are 5 level systems in DCS?

The 5 levels are described below:

Level 0:- Field Level is the field device level. All the field devices such as control valves, transducers, transmitters etc, are at this level.

Level 1:- Direct Control This is the control level. The microcontroller takes data from the fieldbus to control individual control loops. At a time more than one can be controlled. All control actions are to be done with this level.

Level 2:- Plant Supervisory This is the plant supervisory unit. All the information about the control loop is displayed in this unit using Video display unit (VDU).Large control systems may have many workstations which display the distributed control units around the plant. From this unit, an operator can adjust the set point or he can change from manual to automatic etc.

Level 3:- Production Control This is the Production control level. The application processor takes some values from the microprocessor and sends them to the Central Control Room (CCR). Normally, you cannot change control operations from this level. It can only display information for management overview.

Level 4:- Production Scheduling This is the production scheduling or Group management level. The group management level. Some signals for the AP are converted so that they can be sent (by microwave link, satellite, etc.) to a distant headquarters The workstation at headquarters cannot make changes at plant level. However, the workstation displays up to date information on production operations for planning purposes.

Concept of Distributed Control System

DCS refers to a control system usually of a manufacturing system, process or any kind of dynamic system. DCS controllers are not central in location, but are distributed throughout the system with each component sub-system controlled by one or more controllers.

The entire system is connected by a communication network to control and monitor the system.

The basic architecture of the distributed control system, as shown:

The DCS consists of an operator station, a control station, and a communication system. The operator station collects data relating to the process operation and can display and manipulate that data. The control station contains control functions such as for the DDC.

Working & Operation of DCS System

The operation of DCS goes like this; Sensors senses the process information and send it to the local I/O modules, to which actuators are also connected so as to control the process parameters. The information or data from these remote modules is gathered to the process control unit via field bus. If smart field devices are used, the sensed information is directly transferred to the process control unit via the field bus.

The collected information is further processed, analyzed and produces the output results based on the control logic implemented in the controller. The results or control actions are then carried to the actuator devices via field bus. The DCS configuring, commissioning and control logic implementation are carried at the engineering station as mentioned earlier. The operator is able to view and send control actions manually at operation stations.

Components of a DCS

The distributed control system (DCS) consists of the four interfaces. The process interface is the interface between the distributed control system and the plant (i.e., measurement sensors and final control elements).

Field Control Station: The control stations receive measurement signals from sensors such as for temperature, pressure, and flow rate, and perform control calculations in accordance with the deviations from the set point values. Output signals are then sent to the final control element to perform compensatory actions.

Communication Bus: Are used to communicate between HMI stations and control interfaces. It permits system build-up and software maintenance in the distributed control system.

Control sub-interface: This connects the distributed control system to other types of instruments such as the PLC (programmable logic controller) or a composition analyzer to integrate plant operation.

Human Machine Interface station: This is the interface between the distributed control system and the operator. It does central monitoring of the plant and permits the operator to perform operations. The operator console is composed of a powerful microprocessor, CRT, and keyboard. Many different operations can be carried out by switching displays on the CRT.

Communication media and protocol

Protocol: A set of rules and regulations is called a protocol.

Communication: Exchange of information from one system to another system with a medium is called communication.

Communication Protocol: A set of rules and regulations that allow two electronic devices to connect to exchange the data with one and another.

Communication media consists of transmission cables to transmit the data such as coaxial cables, copper wires, fiber optic cables and sometimes it might be wireless. Communication protocols selected depend on the number of devices to be connected to this network.

For example, RS232 supports only for 2 devices and Profibus for 126 devices or nodes. Some of these protocols include Ethernet, DeviceNet, foundation fieldbus, modbus, CAN, etc.

In DCS, two or more communication protocols are used in between two or more areas such as between field control devices and distributed controllers and other one between distributed controllers and supervisory control stations such as operating and engineering stations.

Distributed Control System Software

Operating System Software

Sets the rules of how computer hardware and application software work together.

Controls the operation of the computer.

Example: Windows XP, Windows Vista, Windows 7.

Application Software

Lets you accomplish specific tasks based on your needs.

Example: MS Word, MS Excel, MS Power Point, MS Works.

Important features of DCS To handle complex processes:

In factory automation structure, PLC-Programming Logic Controller is used to control and monitor the process parameters at high speed requirements. However due to limitation of number of I/O devices, PLC’s cannot handle complex structure. Hence DCS is preferred for complex control applications with more I/O’s with dedicated controllers. These are used in manufacturing processes where designing of multiple products are in multiple procedures such as batch process control.

System redundancy: DCS facilitates system availability when needed by redundant features at every level. Resuming steady state operation after any outages, whether planned or unplanned is somewhat better compared to other automation control devices. Redundancy raises the system reliability by maintaining system operation continuously even in some abnormalities while the system is in operation.

Lot of Predefined function blocks: DCS offers many algorithms, more standard application libraries, pre-tested and pre-defined functions to deal with large complex systems. This makes programming to control various applications being easy and consuming less time to program and control.

Powerful programming languages: It provides a number of programming languages like ladder, function block, sequential, etc for creating the custom programming based on user interest.

More sophisticated HMI: Similar to the SCADA system, DCS can also monitor and control through HMI’s (Human Machine Interface) which provides sufficient data to the operator to charge over various processes and it acts as the heart of the system. But this type of industrial control system covers large geographical areas whereas DCS covers confined areas. DCS completely takes the entire process plant to the control room as a PC window. Trending, logging and graphical representation of the HMI’s give an effective user interface. Powerful alarming system of DCS helps operators to respond more quickly to the plant conditions.

Scalable platform: Structure of DCS can be scalable based on the number of I/O’s from small to large server systems by adding more clients and servers in the communication system and also by adding more I/O modules in distributed controllers.

System security: Access to control various processes leads to plant safety. DCS design offers a perfect secured system to handle system functions for better factory automation control. Security is also provided at different levels such as engineer level, entrepreneur level, operator level, etc.

Advantages of Distributed Control Systems

The advantages of Distributed Control Systems can be enumerated as follows-

A Distributed Control System (DCS) centralizes plant operations to provide flexibility and simplicity by allowing central control, monitoring and reporting of individual components and processes. A DCS is designed to control complex processes that can be geographically disseminated using networked control elements that are distributed throughout the system. These features, along with redundancy that is designed into the overall structure to facilitate high system availability and reliability, drive operators of large, complex facilities and processes, such as those used in nuclear power plants, to choose DCS.

Unlike Programmable Logic Controllers, DCS uses a suite of configuration tools to set up the database, control logic, graphics and system security. Control applications are distributed to system controllers that are dedicated to specific plant processes utilizing field devices. Control logic can be created and dispersed across the system controllers. This allows changes that must be made to meet new requirements and/or the addition of new controllers or field devices, to be made efficiently and accurately, easing and simplifying updates and new equipment integration.

The controllers and associated inputs/outputs are connected through a redundant communications network to operating and engineering stations. The stations have graphical, easy-to-use displays for data monitoring, data logging, alarming and control. Field devices, such as actuators and sensors, are directly connected to input/output modules that communicate with assigned controllers while reading and reporting real world information, such as pressure and temperature.

DCS are scalable. A DCS can be deployed in an initial installation as a large, integrated system, or as a standalone system that can be added to as planned or needed. New controllers and inputs/outputs can be added throughout a plant. When new systems are added, they become part of the integrated DCS and are automatically updated by the controlling database.

Data presentation is in a systematic format enabling easy comparison of various parameters and taking decisions by a printer.

Logging of data is done by a printer thereby eliminating human error.

It is possible to control through dynamic graphics.

Operator’s action can be logged, thereby eliminating confusion.

The controlling software used is very simple and the application is readily The software changed in one unit has no impact on other units and hence the system becomes very flexible. User’s risk in software is minimal.

Disadvantages of Distributed Control Systems

The disadvantages of Distributed Control Systems can be enumerated as follows-

All information and data though presented in a systematic format is hidden behind the CRT. Hence, it requires a skilled operator.

In an emergency, decisions have to be taken single handedly, as few & operators are there in the control room.

Failure of one controller effects more than one loop. Hence it calls for very high MTBF (Mean Time Between Failures) and high degree of redundancy.

Applications of Distributed Control System (DCS)

The distributed control system (DCS) has applications in various fields. Most importantly it is used in various manufacturing plants as an industrial control system. Other than this following are the added applications of DCS:

Environment control system -Traffic and radio signals -Oil Refining, metallurgical, pharmaceutical, chemicals plants. -Water management units -Sensor networks -Cargo ships -Chemical plants. -Petrochemical (oil) and refineries. -Pulp and Paper Mills (see also: quality control system QCS) -Boiler controls and power plant systems. -Nuclear power plants.

DCS Maintenance DCS Maintenance could be defined as: “All actions that are necessary to keep the DCS stay operational in its full functional state as the main objective.

There are 4 types of DCS maintenance:

Breakdown maintenance This is the most inefficient way of maintenance: a part get repaired when it breaks or fails.

Preventive maintenance With preventive maintenance, regular activity is performed in order to keep the functional condition. During this activity there will be cleaning, replacing before failing, etc. This preventive maintenance can be approached on two different ways: periodic maintenance and predictive maintenance. Periodic maintenance follows a predefined time schedule while predictive maintenance is based on working conditions like: 2500 hours running at 85% of load)

Corrective maintenance With corrective maintenance, we try to detect and improve weaknesses that might result in failures or breakdowns.

Maintenance prevention Maintenance prevention means performing actions like design actions, to reduce maintenance requirements, based on the experience and analysis of the past with similar equipment.

Difference between PLC and DCS

PLC Programmable Logic Controller

PLC stands for Programmable Logic Controller which can handle the binary input and output of the logic statement which are stored in its memory. its a digitally operating system which are logic stored in its memory by programming system.

Programmable Logic Controllers, are frequently used to synchronize the flow of inputs from sensors (Physical) and events with the flow of outputs to actuators and events. This leads to precisely controlled actions that permit a tight control of almost any industrial process.

This is actually a control device that consists of a programmable microprocessor, and is programmed using a specialized computer language.

Before, a programmable logic controller would have been programmed in ladder logic, which is similar to a schematic of relay logic.

A modern programmable logic controller is usually programmed in any one of several languages, ranging from ladder logic to Basic or C.

Typically, the program is written in a development environment on a personal computer (PC), and then is downloaded onto the programmable logic controller directly through a cable connection.

The program is stored in the programmable logic controller in non-volatile memory. Programmable logic controllers contain a variable number of Input/output (I/O) ports, and are typically Reduced Instruction Set Computer (RISC) based.

They are designed for real-time use, and often must withstand harsh environments on the shop floor.

DCS Distributed Control systems

DCS stands for Distributed Control systems. Distributed control system controls all types of process variables at a time. Through this we can control the whole plant, we can see the status of the plant, and the handling unit.

It is a system of dividing plant or process control into several areas of responsibility, each managed by its own controller, with the whole system connected to form a single entity, usually by means of communication buses.

(DCS) refers to a control system usually of a manufacturing system, process or any kind of dynamic system, in which the controller elements are not central in location (like the brain) but are distributed throughout the system with each component sub-system controlled by one or more controllers. The entire system of controllers is connected by networks for communication and monitoring.

A DCS typically uses custom designed processors as controllers and uses both proprietary interconnections and Communications protocol for communication.

Input & output modules form component parts of the DCS. The processor receives information from input modules and sends information to output modules.

The input modules receive information from input instruments in the process and transmit instructions to the output instruments in the field.

Computer buses or electrical buses connect the processor and modules through multiplexer or de-multiplexers.

Buses also connect the distributed controllers with the central controller and finally to the Human-Machine Interface (HMI) or control consoles.


DCS and SCADA are monitoring and control mechanisms that are used in industrial installations to keep track and control of the processes and equipment; to ensure that everything goes smoothly, and none of the equipment work outside the specified limits. The most significant difference between the two is their general design. DCS, or Data Control System, is process oriented, as it focuses more on the processes in each step of the operation. SCADA, or Supervisory Control and Data Acquisition, focuses more on the acquisition and collation of data for reference of the personnel who are charged with keeping track of the operation.

DCS is process state driven, while SCADA is event driven. DCS does all its tasks in a sequential manner, and events are not recorded until it is scanned by the station. In contrast, SCADA is event driven. It does not call scans on a regular basis, but waits for an event or for a change in value in one component to trigger certain actions. SCADA is a bit more advantageous in this aspect, as it lightens the load of the host. Changes are also recorded much earlier, as an event is logged as soon as a value changes state.

In terms of applications, DCS is the system of choice for installations that are limited to a small locale, like a single factory or plant, while SCADA is preferred when the entire system is spread across a much larger geographic location, examples of which would be oil wells spread out in a large field. Part of the reason for this is the fact that DCS needs to be always connected to the I/O of the system, while SCADA is expected to perform even when field communications fail for some time. SCADA does this by keeping a record of all current values, so that even if the base station is unable to extract new information from a remote location, it would still be able to present the last recorded values.