What is SCADA? Data Acquisition Systems Explained

What does SCADA stand for? SCADA stands for Supervisory Control And Data Acquisition.

What Is SCADA and How Does It Work?

The definition of SCADA is pretty self-explanatory:

The function of a SCADA system is to Supervise, Control, and manage Data Acquisition.

A SCADA system may include:

At least one computer or server. The computers are connected to each other over a network to perform supervisory functions, while also implementing human-machine interfaces (HMIs).
A series of peripheral devices including but not limited to RTUs (remote terminal units), Input and Output Modules, and others intelligent devices that interface the machinery, plant, and processes via actuators and sensors.
A communication network that includes a variety of communication protocols and transmission media. This network functions to ensure the correct transmission of information between peripheral devices and computers.

And the following subsystems are usually present in a SCADA system:

Programmable Logic Controllers (PLCs) that are deployed as field devices
Interface (i.e. display) used by an operator. The data acquired from the process is displayed in this interface
A supervisory system that collects all the data from and about the process
Remote Terminal Units (RTUs) connected to sensors. They help to convert the signals gathered by the sensors to digital data to be processed by the supervisory system.
A communication network (i.e. LAN), connecting RTUs to the supervisory system

SCADA software is an integrated development environment that provides all the required tools to develop SCADA applications and software.

These applications and software are designed to run on supervisory computers that perform the core Supervision, Control, and Data Acquisition functions of a SCADA system, which we will discuss below.

Supervision

The function that allows operators to have an immediate view of the process at hand so they can control the progress of the processes over time by monitoring and analyzing the sequence of states.

The main objective of Supervision is to provide an HMI (human-machine interface) or multiple HMIs.

To be effective, the HMI must provide the operator with a complete and accurate visualization of the whole process, allowing the operator to monitor the status, the evolution of states, and unexpected deviations (potential issues and bottlenecks).

For an effective HMI, graphic visualization of the process is very important. Translating the relevant information of the process to an easy-to-understand visual language is paramount.

For instance, the software can display the status of manufacturing equipment through an easy-to-use graphic symbol complete with color codes.

When an unexpected deviation occurs, the interface will display an alarm notification via a pop-up window.

Control

The Control function in a SCADA system refers to the ability of the system to interact with a controlled process so it can modify the evolution of states according to pre-established policies or manual decisions taken by the operator.

It’s important to note that ‘control’ in a SCADA system doesn’t refer to real-time process control typical to PLCs, but rather emphasizes the ability to modify the evolution of states in a process.

To better understand the difference, let’s use an example of air pressure in a process.

In PLC real-time control, the objective is to keep this pressure unchanged throughout the whole process, for example by controlling the appropriate actuators.

On the other hand, the objective of SCADA control is to modify the pressure in which the process should work by sending an appropriate command through the SCADA software.

Data Acquisition

Data Acquisition in a SCADA system is arguably the main function, and arguably outweighs the other two functions just discussed.

Data Acquisition refers to the two-way transfer of information from field devices and the supervisory computer. This allows the supervisory computer to control the processes as discussed above in the Control section.

By putting the Data Acquisition system in constant communication with the Supervision system, the supervisory system can collect all the necessary information to allow monitoring and observation of the process as a whole.

The objective of Data Acquisition is to ensure the error-free transmission of data and information between processes to allow for complete supervision.

How a SCADA System Works

To really understand how a SCADA system work, we have to understand how the system’s hardware and software aspects work.

SCADA Hardware Architecture

When discussing the hardware aspect of the SCADA system, generally we can divide it into two different layers: the client layer and the data server layer.

The client layer, as the name suggests, handles the human-machine interactions of the SCADA system, while the data server layer handless data acquisition and data processing.

The SCADA station (a single PC) connects directly to a database server. This database server communicates with devices in the field through PLCs, RTUs, or other controllers. These controllers will then connect to the field devices like sensors.

In a SCADA system, we can connect PLCs to the data servers directly (i.e. via ethernet connection) or via networks. The RTUs convert the sensor signals to digital data so it can be read and processed by the supervisory system.

According to the feedback received by the RTU or PLCs, it will then apply electrical signals to control the process.

SCADA Software Architecture

The software aspect of the SCADA system is responsible for monitoring changes in the process and perform analysis to find trends and patterns. The software will then manage information such as logistics data, scheduled processes, and troubleshooting. The software system allows the operator to properly monitor a visual representation of the data provided by the process.

SCADA Operation Workflow

In a SCADA system; sensors, PLCs (or other controllers), RTUs, and a communication network (i.e. LAN or WAN) are working together to perform the following functions:

Data Acquisition
Data Communication
Data Presentation
Monitoring of Data

To be more precise, the sensors collect data/information and RTUs will then send this data to the PLC controller so the interface can display the status of the system.

According to the information displayed in the interface, the operator can give a command to other system components. All of these are made possible by the communication network.

Different Types of SCADA System

In its history, there have been four different types, or generations:

Early or Monolithic SCADA systems (first-gen)
Distributed SCADA systems (second-gen)
Networked SCADA systems (third-gen)
IoT SCADA systems (fourth-gen)

Early SCADA Systems

In the first generation of SCADA systems, minicomputers are used to connect directly to RTUs.

During this time network services are not yet available, so the early SCADA systems are mostly independent without much connectivity to other systems.

All RTUs in an early SCADA system would connect to a back-up mainframe system, which is very important to achieve redundancy.

The early SCADA systems are mainly limited to monitoring sensors and sending flags to operators in cases when a certain threshold has been detected by the sensors.

Distributed SCADA Systems

In a second-generation, distributed SCADA system, LAN (Local Area Network) is used to share the control functions across multiple systems.

Individual SCADA stations were used to share real-time information and command processing for controlling processes.

With this model, the cost and overall size of the system were reduced compared to the first-generation systems.

However, network protocols were not standardized in this period, so the security of the SCADA system was mainly ignored.

Networked SCADA Systems

Most SCADA systems at the moment communicate via WAN (Wide Area Network) via Ethernet, Fiber Optic, or WiFi. Third-generation SCADA systems also utilize PLCs for monitoring and controlling purposes.

While the first and second-generation of SCADA systems are mainly limited to a single-site network as a closed system, we can now have several parallel-working SCADA systems under a single operator in the network architecture.

IoT SCADA Systems

The latest generation of SCADA system now makes use of the concept of the Internet of Things (IoT).

IoT provided a mechanism to scale SCADA systems to be much larger than the previous generations. At the same time, it reduced the overall cost of implementing a SCADA system by utilizing cloud computing.

Maintenance, implementation, and integration are also much easier in an IoT SCADA system compared to earlier generations.

By utilizing IoT concepts, the SCADA system can now report the evolution of process states in real-time by using cloud computing, which also allows more complex control algorithms to be implemented on the PLCs.

Security-wise, we can also use open network protocols (like TLS) to replace proprietary network protocols in previous generations, which will also allow more manageable security functions.

Popular Software Used for SCADA

Ignition SCADA

Ignition SCADA by Inductive Automation offers an industry-leading toolset for SCADA functionalities on a scalable universal platform.

The software comes with an excellent set of data acquisition tools like OPC UA that can connect to virtually any PLC, and can seamlessly connect to any SQL database.

With Ignition SCADA, you can also seamlessly connect to IoT devices via MQTT open messaging.

Conclusion

As you can imagine, this was a very introductory article on what SCADA is and what makes up a Supervisory Control and Data Acquisition system.

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