Distributed Control Systems

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 system deals with advanced automation control technologies to have better control performance over complex processes.

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.

What is Distributed Control System

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 as shown in figure.

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 section of control area and are connected to operating and engineering stations which are used for data monitoring, data logging, alarming and controlling purpose via another high speed communication bus.

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

4 Basic Elements of Distributed Control System

Distributed Control System continuously interacts with the processes in process control applications ones it gets instruction from the operator. It also facilitates to 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.

Engineering PC or controller

This controller is the supervisory controller over all the distributed processing controllers. Control algorithms and configuration of various devices are executed in this controller. Network communication between processing and engineering PC can be implemented by simplex or redundant configurations.

Distributed controller or Local control unit

It can be placed near to field devices (sensors and actuators) or certain location where these field devices are connected via communication link. It receives the instructions from the engineering station like set point and other parameters and directly controls field devices.

It can sense and control both analog and digital inputs / outputs by analog and digital I/O modules. These modules are extendable according to the number of inputs and outputs. It collects the information from discrete field devices and sends this information to operating and engineering stations.

Operating station or HMI

It is used to monitor entire plant parameters graphically and to log the data in plant database systems. Trend display of various process parameters provides the effective display and easy monitoring.

These operating stations are of different types such as some operating stations (PC’s) used to monitor only parameters, some for only trend display, some for data logging and alarming requirements. These can also be configured to have control capabilities.

Communication media and protocol

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 depends 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 filed bus, 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.

7 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 number of 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 feature at every level.

Resuming of the 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 system is in operation.

  • Lot of Predefined function blocks:
  • Powerful programming languages:

It provides more 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 heart of the system. But this type of industrial control system covers large geographical areas whereas DCS covers confined area.

DCS completely takes the entire process plant to control room as a PC window. Trending, logging and graphical representation of the HMI’s give 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 I/O’s from small to large server system by adding more number of clients and servers in 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 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

Application of Distributed Control System

DCS system can be implemented in a simple application like load management using network of microcontrollers.

Application of DCS
Application of DCS

Here the input is given from a keypad to a microcontroller, which communicates with the other two microcontrollers.  One of the microcontrollers is used to display the status of the process as well as the loads, while the other microcontroller controls the relay driver. The relay driver in turn drives the relay to operate the load.

Hope you have understood the concept of distributed control system and its importance. Here is a basic question for you – Give any application of a DCS, you are aware of.