Saturday, January 14, 2012

SCADA System Concept

The word of SCADA generally refers to the systems that centralized which monitor and control whole sites, or systems complexity extensive over great areas. Mainly the actions of control are run automatically by PLCs or by RTUs. The functions of host control are generally controlled to essential overriding or intervention of supervisory level. For instance, a PLC might control the cooling water flow through an industrial process part, but the system of SCADA may let operators to adjust the set points for the flow for example flow and high temperature loss, to be showed and documented. The control loop of feedback transfers through the PLC or RTU, while the system of SCADA checks the overall loop performance.

The acquisition of data starts at the level of PLC or RTU and comprises equipment status and meter readings reports that are corresponded to SCADA as needed. Then the data is collected and designed in such a system that the operator of control room using the HMI can build supervisory decisions to amend or supersede normal controls of RTU or PLC. The data may also be supplied to a Historian, frequently developed a Database Management System commodity, to enable trending and other diagnostic auditing.

The systems of SCADA characteristically execute a distributed database, usually referred to as a tag database, which includes elements of the data called points or tags. A point corresponds to a single value of input or output checked or managed by the system. A hard point stands for an real input or output within the system, while the results of soft point from math and logic operations implemented to other points. Points are usually saved as pairs of value-timestamp: a value, and the timestamp when it was documented or counted. A value-timestamp series pairs presents the point history. It's also familiar to save extra metadata with tags, for example the path to a PLC registers or field device, plan time comments, and information of alarm.

Common system components
A system of SCADA typically contains the subsystems as following:
• A HMI (Human Machine Interface) is the equipment which presents data of process to an operator, and the human operator observes and manages the process.
• A system of supervisor, gathering or acquiring data on the process and transferring commands to the process.
• The RTUs (Remote terminal units) linking to sensors in the process, translating the signals of sensor to digital data and transferring data digital to the system of supervisory.
• The PLCs (Programmable logic controller) utilized as devices field because they are more save cost, flexible, versatile.
• The infrastructure of communication joining the system of supervisory to the remote terminal units (RTUs).
• Various instrumentation of process and analytical.


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