How does a SCADA system work?
The term SCADA usually refers to centralized systems which monitor and control entire sites system complexes spread across big fields (anything between an industrial plant and a nation).
Remote terminal devices (“RTUs”) or programmable logic controllers (“PLCs”) perform most control activities automatically. Host control functions are generally limited to fundamental overriding or interference at the supervisory level.
Working of SCADA with the example shown below:
Consider an example of PLC controlling flow of cooling water:
PLC may regulate the flow of cooling water through a portion of an industrial process, but the SCADA scheme may allow operators to alter the flow setpoints and allow alarm conditions to be displayed and registered, such as flow loss and elevated temperature.
The feedback control loop passes through the RTU or PLC, while the SCADA system monitors the overall performance of the loop.
Data acquisition starts at the RTU or PLC stage and involves meter measurements and status reports of facilities that are transmitted as needed to SCADA. Data is then compiled and formatted so that supervisory decisions can be made by a control room operator using the HMI to adjust or override normal controls of the RTU (PLC).
Data may also be fed to a Historian, often built on a commodity Database Management System, to allow trending and other analytical auditing.
Tags in SCADA:
Typically, SCADA systems introduce a distributed database, frequently referred to as a tag database, containing tags or points. A point reflects a single system-monitored or controlled input or output value. Points can be "Hard or “soft.”
A hard point represents an actual input or output within the system, whereas the result of logic and math operations applied to other points is a soft point. (Most implementations remove the distinction conceptually by making each property a “soft” point expression that, in the simplest case, could be equal to a single hard point.)
Difference between SCADA and DCS:
There is, in several industries, considerable confusion over the differences between SCADA systems and Distributed control systems (DCS).
A SCADA system generally refers to a scheme that coordinates, but in real time does not regulate procedures. Newer telecommunications technology muddles the debate of real-time control, allowing for reliable, low latency, high-speed communications across broad fields.
Most of the differences between SCADA and DCS are determined culturally and can generally be overlooked. The difference between SCADA and DCS will fade as communication infrastructures with greater ability become accessible