What is a Distributed Control System?

• A distributed control system is a particularly designed control system used to control complex, large, and geographically distributed applications in industrial processes. In this system the controllers are distributed over the entire plant area.
• These distributed controllers are connected with field devices and operating computers as well through high-speed communication networks.
• Separate field devices like sensors and actuators are directly connected to input and output controller modules through a communication bus. These field devices are capable of communicating with PLC’s or other controllers while interacting with actual world parameters like temperature, pressure, etc.
• These controllers are distributed in various sections of the plant and they 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 like foundation filed bus, HART, Profibus, Modbus, etc. distributed control system provides information to multiple displays for the user interface.

4 Basic Elements of Distributed Control System

Distributed Control System constantly interacts with the processes in process control applications once it gets instructed by the operator. It also gives a facility of variable set points and opening and closing of valves for manual control by the operator. It’s a human machine interface, faceplates, and trend display give the powerful monitoring of industrial processes.

Engineering PC or controller

This s the supervisory controller over all the distributed processing controllers. In this controller all various device control algorithms and configuration are executed. Network communication between engineering and processing computers can be implemented by simplex or redundant configurations.

Distributed controller or Local control unit

It can be placed near field devices (sensors and actuators) or certain locations where these field devices are connected via the communication link.

It can control and sense both digital and analog inputs/outputs by digital and analog Input modules. These modules can be extended according to the number of inputs and outputs. It will collect the information from discrete field devices and sends the information to engineering and operating stations.

Controllers AC 700F and AC 800F act as a communication interface between engineering station and field devices. In most of the cases these act as local control for field instruments.       

Operating station or Human machine interface

It’s mainly used to monitor whole plant parameters graphically and to log the data in plant database systems. The trend display of various process parameters provides dominant display and easy monitoring.

Some operating stations are from different types such like operating stations used to monitor only parameters, some for only trend display, some for data logging, and alarming requirements.

Communication media and protocol

Communication media consists of transmission cables to transmit the data like coaxial cables, copper wires, fiber-optic cables and sometimes it might be wireless. Communication protocols devices are selected upon number of devices to be connected to this network.

In distributed control system, two or more communication protocols are used within two or more areas such as between distributed controllers and field control device and another one between distributed controllers and supervisory control stations like operating and engineering stations.

 

7 Important Features of distributed control system

• To handle complex processes:

In the industry for the automation structure, PLC-Programming Logic Controller is used to control and monitor the process parameters at high-speed requirements. But, due to the limitation of a number of input devices, PLCs cannot handle complex structures.

Hence distributed control system is preferred for complex control applications with a greater number of inputs with dedicated controllers. These are used in manufacturing processes where designing multiple products are in multiple procedures such as batch process control.

• System redundancy:

Distributed control system gives a facility to the system availability when needed by redundant features at every level.

Continue the steady-state operation after any outages, whether planned or unplanned is somewhat better than other automation control devices.

Redundance raises the system reliability by maintaining the system operation continuously even in some irregularity while the system is in operation.

• A lot of Predefined function blocks:

Distributed control system gives numerous algorithms, standard application libraries, pre-tested and pre-defined functions to deal with large complex systems. This makes programming to control various applications for being easy and less time consuming to program and control.

• Powerful programming languages:

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

• More sophisticated HMI:

Similar to the supervisory control and data acquisition (SCADA) system, distributed control system can also monitor and control through Human Machine Interface’s which provides sufficient data to the operator to charge over various processes and it behave as the heart of the system. Even so, this type of industrial control system covers large geographical areas whereas distributed control system covers the confined areas.

• Sophisticated HMI

Distributed control system completely takes the entire process plant to the control room in the computer window. Trending, logging, and graphical representation of the human machine interfaces (HMI) give an effective user interface. The effective alarming system of distributed control system helps operators to respond more quickly to the plant conditions

• Scalable platform:

The structure of distributed control system can be scalable based on the number inputs from small to large server systems by adding more clients and servers in the communication system and also by adding more input modules in distributed controllers.

• System security:

Plant safety is leads to control the various process in the plant. Distributed control system design offers an entire secured system to handle system functions for better industry automation control. Various security system is also provided at the different levels such as engineer level, entrepreneur level, operator level, etc.

 

Application of Distributed Control System

• A distributed control system can be used in a like load management system using a network of microcontrollers.
• Here the input is given from a keypad to a microcontroller, which communicates with the other two microcontrollers. Some of the microcontrollers are used to display the status of the process as well as the loads, while the other microcontroller controls the relay driver.
• I hope you have understood the concept of the distributed control system and its importance.