Before we talk about DCS (distributed control system) we will look over the two different types of process control systems.
1)ANALOG CONTROL SYSTEM:
In an analog control system,The controller consists of analog devices & circuits i.e. Amplifiers. The signals coming from the process will be analog in nature(i.e. voltage or current signal).
In an analog control system any change in either setpoint or feedback is sensed immediately and the amplifiers adjust their output accordingly.
In the diagram, we can see that transmitter is sending an voltage signal to the operational amplifier & operational amplifier is sending its command to FCE(final control element) in current signal.
Both the signals sending and receiving by the amplifier is analog in nature and there are no signal converters in this control system.
2)DIGITAL CONTROL SYSTEM:
In an digital control system, The controller use microprocessors to do the control function.
Digital control systems are preferred over analog control systems because it is easy to interface with computers for data analysis.
From the above diagrammatical presentation we can understand the working principle of digital control system & we can understand that most control systems are controlling analog control processes.
This means that in many cases the digital control system must be first convert analog input data into digital form before it can be processed and similarly the output from the controller must be converted into the digital form to analog form.
Digital Control Systems are Classified into two types,
1)CENTRALISED CONTROL SYSTEM
In this type of control system the monitoring functions and control functions are centralised.
In the diagram, we can see that all the inputs from the field and all the set points of different processes are given to a single processor or cpu and all the commands to final control elements are sending through the same cpu.
DISADVANTAGES OF CCS
1)If the CPU fails the entire plant will get affected.
2)Redundancy concept is not available in this type of control system. Redundancy is having two controllers. One would be active and the other would be standby. If the active controller fails, the standby controller takes over.
2)DISTRIBUTED CONTROL SYSTEMS(DCS)
In this type of control system control action is distributed and monitoring is centralised. The entire system of controllers is connected by networks for communication and monitoring. World’s first DCS has been introduced by Yokogawa in the year 1975.
DCS uses processors as controllers and communications protocol for communication. DCS is used in a variety of industries, to monitor and control distributed equipment.
.1)Electrical power grids and electrical generation plants
2)Environmental control systems.
3)Water management systems.
4)On shore & Off shore oil refining plants.
5)Metallurgical process plants.
7)Dry cargo and bulk oil carrier ships.
10)Paper & Pulp industries etc.
BASIC COMPONENTS OF DCS:
1)CPU:Central Processing Unit also called as Field Control Station
It is used to control the process. All the instruments and interlocks created by software reside in the memory of this unit & all the field instruments like transmitters and control valves are wired to the FCS.
Here we can see that every component(i.e. power supply, processor cards)of the FCS is redundant in nature. One would be active and the other would be standby. If the active one fails the standby takes over the action.
The power supply unit is designed to supply power to the CPU cards, and up to five I/O module nests.
The processor receives information from input modules and sends information to output modules.
The input modules receive information from field instruments in the process and transmit instructions to the output instruments in the field.
This is also called as Engineering station. It is a PC loaded with engineering software & HIS. So that it can be used as HIS as well as ENG Station.
In ENG Station we program or writes a command as per the client requirement and downloads them to CPU.
CPU performs the the control action according to the program downloaded in it. By having HIS we can monitor and control the process from operator station.
It is used to communicate between the FCS and the OPS. It plays an very important role in the DCS because, It determines the superiority of the entire control system.
But also the Uploading & Dowloading of the programs and accessing the data & process of the field can be done through Communication bus only.
This, Diagrammatical presentation will give you a clear picture about the role of communication bus in DCS.
ADVANTAGES OF DCS:
1)Control function is distributed among multiple CPUs (Field Control Stations). Hence failure of
one FCS does not affect the entire plant.
2)Redundancy is available at various levels, Instruments and interlocks are created by software.
3)Generation and modifications of the interlocks are very flexible and simple.
4)Information regarding the process is presented to the user in various formats.
5)Field wiring is considerably less. Maintenance and trouble shooting becomes very easy.
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