Distributed Control System
DCS is the most commonly used control architecture in industrial process control.
DCS can be defined as an architecture where the sub systems are geographically distributed and functionally integrated.The DCS is the most cost effective and scalable solutions ranging from small plants to large refineries.Components of a DCS
The typical sub systems of a DCS are Operator station,Engineering station,Process control networks,Controller and I/O modules.
Operator station gives an intuitive view into the process with easy access to process graphics,faceplates,alarm summaries,alarm faceplates,trends,display navigation.Operator stations extends 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 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 a agile mode. With minimal effort engineers can specify control loops and field devices and then quickly complete the configuration.
Process Control Network 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 network also called PCNs can be open protocol such as Ethernet or also can be proprietary.The PCNs are generally installed in a redundant monitor to improve the reliability and is installed away from regular power cables to reduce the impact of electromagnetic interference and thereby increasing the reliability of the overall system.
The controllers execute control strategies to help optimizing process.The controllers are available in a redundant and non redundant configurations. The speed of execution of the control strategy determines the quality of control.The controllers are provided with rich set of library for various algorithms such as PID,cascade,Feed forward etc. The controller receives the input from the channel of input module connected to it. Similarly controller sends the output to the configured channel of the output module to drive the final control element.
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 critical control loop.
Advantages of DCS
Higher overall availability,as single controller failure does not lead to a total automation system failure.
Each controller being smaller in size,handling,troubleshooting maintenance are relatively convenient.
Disadvantages of DCS
Technically more complicated.