SCADA: Systems Concept

Composition

Input/output signal hardware, network, human machine interface, controllers, database, communication and software constitute a SCADA system. SCADA comes under the scope of Instrumentation Engineering.

Real Time

There are real time automated and integrated control systems like its cooling by the computer itself for quick responding to the process changes within the processes’ own time-frame. But SCADA is not critical to controlling real time process.

Control/Monitor Function

A system or a complete site is usually spread over a long geographical distance. SCADA is the central system for control and monitor of such a site or system. A Programmable Logic Controller (PLC) or a Remote Terminal Unit (RTU) automatically performs the main site control process. The host control functions in most cases are limited to the capability of supervisory level or site override. For example, in an industrial process, flow of cooling water through a part of the process may be controlled by PLC but the control set point for the water flow can be allowed to be changed by an operator by the SCADA system. Also any alarm condition like high temperature or flow loss may be recorded and displayed by SCADA. Through PLC the feedback control loop is closed but the total performance of the loop is monitored by SCADA. In this example of control of set point for water flow, the measured flow/level of water is read by the SCADA system before sending the set points to the two PLCs. The PLC for flow after comparing the measured flow to the set point sets the sped pump to match the flow with the set point. The other PLC for level after comparing the measured level to the set point controls the flow through a valve in a way as to match level with the set point. This is how the SCADA system controls and monitors the overall system without performing the main site control process that is done through PLC.