Supervisory Control And Data Acquisition

What is SCADA?

Introduction
SCADA or Supervisory Control And Data Acquisition is a large scale control system for automated industrial processes like municipal water supplies, power generation, steel manufacturing, gas and oil pipelines etc. SCADA also has applications in large scale experimental facilities like those used in nuclear fusion. SCADA systems monitor and control these operations by gathering data from sensors at the facility or remote station and then sending it to a central computer system that manages the operations using this information.

The SCADA System
The sheer size and of the operations we saw earlier demands that the control system be equally elaborate to handle the requirements. This is where SCADA scores. The SCADA system is equipped to manage anything from a few thousands to a million input/output channels. The technology is still evolving and we can expect an expansion of the market for SCADA. A full fledged SCADA system is made up of signal hardware for input/ output, networks, control equipment, user interface (sometimes called the Human-Machine Interface or HMI), communication equipment and the software to go with it all. And here we are talking about the central command system of SCADA. The central system is often miles away from where the operations take place. Thus the system also needs on-site sensors to collect and monitor data.

How SCADA Works
The measurement and control system of SCADA has one master terminal unit (MTU) which could be called the brain of the system and one or more remote terminal units (RTU). The RTUs gather the data locally and send them to the MTU which then issues suitable commands to be executed on site. A system of either standard or customized software is used to collate, interpret and manage the data. SCADA as of now uses predominantly open-loop control systems, though some closed-loop characteristics are often built in. As this is an open-loop system, it means that SCADA system cannot use feedback to check what results its inputs have produced. In other words, there is no machine-learning.

SCADA and Other Control Systems
There are several other systems that are used for similar purposes as SCADA. These are generally called Distributed Control Systems (DCS). The functions are the same; the difference is in the range. Distributed Control Systems are typically effective within a confined area like a factory complex. Thus the communication is carried out through a local area network (LAN). These networks are highly dependable and fast, and can manage closed-loop (feedback based) control. DCS networks have their limitations. They cannot cover large territories. This is where SCADA comes in handy. However the communication systems are not as reliable as a LAN, and therefore it is not viable to implement closed-loop control. Right now, the size and scope is what mark SCADA.

Riktesh Srivastava
PhD Internet Technologies
MS (Electronics and Communicatios)

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