How SCADA Systems Work?

Introduction:

SCADA stands for supervisory control and data acquisition. It is a type of software application program for process control.  SCADA is a central control system which consist of controllers network interfaces, input/output, communication equipments and software. SCADA systems are used to monitor and control the equipments in the industrial process which include manufacturing, production, development and fabrication. The infrastructural processes include gas and oil distribution, electrical power, water distribution.  Public utilities include bus traffic system, airport.  The SCADA system takes the reading of the meters and checks the status of sensors in regular interval so that it requires minimal interference of human.

General SCADA Network

A large number of processes occur in large industrial establishment. Every process you need to monitor is very complex because each machine gives different output. The SCADA system used to gather the data from sensors and instruments located at remote area. The computer then processes this data and presents in a timely manner. The SCADA system gathers the information (like leak on a pipeline occurred) and transfer the information back to the system while giving the alerts that leakage has occurred and displays the information in a logical and organized fashion. The SCADA system used to run on DOS and UNIX operating systems.

Architecture:

Generally SCADA system is a centralized system which monitors and controls entire area. It is purely software package that is positioned on top of hardware. A supervisory system gathers data on the process and sends the commands control to the process. The SCADA is a remote terminal unit which is also known as RTU. Most control actions are automatically performed by RTUs or PLCs. The RTUs consist of programmable logic converter which can be set to specific requirement. For example, in the thermal power plant the water flow can be set to specific value or it can be changed according to the requirement.

 The SCADA system allows operators to change the set point for the flow, and enable alarm conditions incase of loss of flow and high temperature and the condition is displayed and recorded. The SCADA system monitors the overall performance of the loop. The SCADA system is a centralized system to communicate with both wire and wireless technology to Clint devices. The SCADA system controls can run completely all kinds of industrial process.

EX: If too much pressure in building up in a gas pipe line the SCADA system can automatically open a release valve.

1. Hardware Architecture:

 The generally SCADA system can be classified into two parts:

  • Clint layer
  • Data server layer

The Clint layer which caters for the man machine interaction.

The data server layer which handles most of the process data activities.

The SCADA station refers to the servers and it is composed of a single PC.  The data servers communicate with devices in the field through process controllers like PLCs or RTUs. The PLCs are connected to the data servers either directly or via networks or buses. The SCADA system utilizes a WAN and LAN networks, the WAN and LAN consists of internet protocols used for communication between the master station and devices. The physical equipments like sensors connected to the PLCs or RTUs. The RTUs convert the sensor signals to digital data and sends digital data to master. According to the master feedback received by the RTU, it applies the electrical signal to relays. Most of the monitoring and control operations are performed by RTUs or PLCs as we can see in the figure.

SCADA Diagram

2. Software Architecture:

Most of the servers are used for multitasking and real time database. The servers are responsible for data gathering and handling. The SCADA system consists of a software program to provide trending, diagnostic data, and manage information such as scheduled maintenance procedure, logistic information, detailed schematics for a particular sensor or machine and expert system troubleshooting guides. This means the operator can sea a schematic representation of the plant being controlled.

EX: alarm checking, calculations, logging and archiving; polling controllers on a set of parameter, those are typically connected to the server.

Architecture of SCADA system

Working Procedure of SCADA system:

The SCADA system performs the following functions:

  • Data Acquisitions
  • Data Communication
  • Information/Data presentation
  • Monitoring/Control

These functions are performed by sensors, RTUs, controller, communication network.  The sensors are used to collect the important information and RTUs are used to send this information to controller and display the status of the system. According to the status of the system, the user can give command to other system components. This operation is done by the communication network.

Data Acquisitions:

Real time system consists of thousand of components and sensors. It is very important to know the status of particular components and sensors. For example, some sensors measure the water flow from the reservoir to water tank and some sensors measure the value pressure as the water is release from the reservoir.

Data Communication:

The SCADA system uses wired network to communicate between user and devices. The real time applications use lot of sensors and components which should be control remotely. The SCADA system uses internet communications. All information is transmitted through internet using specific protocols. Sensor and relays are not able to communicate with the network protocols so RTUs used to communicate sensors and network interface.

Information/Data presentation:

The normal circuit networks have some indicators which can be visible to control but in the real time SCADA system, there are thousand of sensors and alarm which are impossible to be handled simultaneously. The SCADA system uses human machine interface (HMI) to provide all of the information gathered from the various sensors.

Human machine interface:

The SCADA system uses human machine interface. The information is displayed and monitored to be processed by the human. HMI provides the access of multiple control units which can be PLCs and RTUs. The HMI provides the graphical presentation of the system. For example, it provides the graphical picture of the pump connected to the tank. The user can see the flow of the water and pressure of the water. The important part of the HMI is an alarm system which is activated according to the predefined values.

SCADA Experimental diagram

For example: The tank water level alarm is set 60% and 70% values. If the water level reaches above 60% the alarm gives normal warning and if the water level reach above 70% the alarm gives critical warning.

Monitoring/Control:

The SCADA system uses different switches to operate each device and displays the status at the control area. Any part of the process can be turned ON/OFF from the control station using these switches. SCADA system is implemented to work automatically without human intervention but at critical situations it is handled by man power.

SCDA for Remote Industrial plant:

In large industrial establishments many process occur simultaneously and each needs to be monitored, which is actually a complex task. The SCADA systems are used to monitor and control the equipments in the industrial processes which  include water distribution, oil distribution and power distribution. The main aim of this project is to process the real time data and control the large scale remote industrial environment. In the real time scenario, a temperature logging system for a remote plant operation is taken.

Temperature Control industrial plant

Block Diagram by Edgefx Kits

The temperature sensors are connected to the microcontroller , which is connected to the PC at the front end and software is loaded on the computer. The data is collected from the temperature sensors. The temperature sensors continuously send the signal to the microcontroller which accordingly displays these values on its front panel. One can set the parameters like low limit and high limit on the computer screen. When the temperature of a sensor goes above set point the microcontroller send a command to the corresponding relay. The heaters connected through relay contacts are turned OFF and ON.

For example SCADA for Remote Industrial Planet:

This is a temperature logging System. Here 8 temperature sensors in multiplexing mode are connected to the microcontroller through ADC 0808. Then the values of all the sensors are sent serially by microcontroller through Max 32 to the com port of the PC. A Software “DAQ System” loaded on the PC takes these values and show them on its front panel, and also logs them to the data base “daq.mdb” .One can set by interactive way some parameters like set point , low limit ,and high limit on the computer screen . When temperature of some sensor increases beyond set point, the microcontroller sends commands to relay driver IC. The heaters connected through relay contacts are (specific for that sensor) turned OFF (or ON in opposite case).High limit and low limits are for alarm. When temperature goes above high limit or below low limit the alarm will be turned on.

SCADA for Remote Industrial Planet

 Applications:

  • Power generation, transmission and distribution
  • Water distribution and reservoir system
  • Public buildings like electrical heating and cooling system.
  • Generators and turbines
  • Traffic light control system

Advantages:

  • The SCADA system provides onboard mechanical and graphical information
  • The SCADA system is easily expandable. We can add set of control units and sensors according to the requirement.
  • The SCADA system ability to operate critical situations.

Photo Credit:

  • General SCADA Network by mycpanel
  • SCADA Diagram by eeweb
  • Architecture of SCADA system by eeweb
  • SCADA Experimental diagram by wikimedia

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