SCADA is a software application for the supervision and control of automated processes from a centralized location, both locally and remotely. The heart of a SCADA system is its modular and distributed software architecture. Control systems, which are typically PLCs (but not always), have the responsibility of feeding the SCADA system with the values of the process variables to monitor the machine and the process behavior. The SCADA server nodes have the necessary drivers to communicate with the field devices either through their legacy communication protocols or often through OPC or a fieldbus protocol. At the server level, the data is gathered on a regular basis or, when changes occur, processed to filter noise, and then stored in a real-time database. There are two different types of data collection:
- Polling: The signal is sampled on a regular basis. The polling time is the time interval between the sampled values of the same signal. Even so, the sample of an analogical signal is only gathered by SCADA if its value changes by more than a predefined dead-band compared to the value of the previous sample collected.
- Unsolicited: The data is gathered when its value changes. The data collection interval is the shortest time interval in which a variation of the value of the signal is caught by the data acquisition system.
The data acquisition interval depends on the following factors:
- The time taken for the communication channel and the related protocol to talk to the device from which the data is being gathered
- The performance of the driver in terms of how long it takes to refresh the data of the device from which the data is gathered
- The amount of data to be collected from the same driver and data source
Typical data acquisition intervals for SCADA applications are in a range of between half a second and one minute. It is not very common for SCADA to carry out data acquisition sampling in under half a second unless you have dedicated hardware specifically for this purpose.
Client nodes are mainly dedicated to data visualization. The communication between the server and the client nodes is typically based on the Ethernet protocol. The presence of multiple server nodes makes it possible to process data in a distributed way.
The core of a SCADA system is the process database to which all other modules that make up the system refer to. A small or medium sized control system typically handles thousands of process signals. It adopts a common approach to process information based on a set of parameters that are necessary for the correct management of the information. These include the tag name, the description, the sampling time, the minimum or maximum value, the engineering units, and so on. This piece of information is based on the asset registry of the device to which the signals belong. The asset registry and the related asset model are very important for industrial companies because they are used for measuring performance, planning maintenance, and designing, setting, and running analytics. We will explain these in detail later. The user interface of a SCADA system allows the operator to access the tag name database and send commands to the system through graphic pages and synoptic panels of either the whole system or parts of it. This is also called the Human Machine Interface (HMI). Given the importance of the graphic representations that are used in the operator user interface, SCADA systems provide a library of graphic objects that allow you to develop complex synoptics easily.
SCADA also allows you to set thresholds and configure events and conditions for detecting anomalies and generating alarms. Alarms are classified by type and priority.
Another important task that is carried out by the SCADA system is tracking production sequences and volumes. It does this both for past data, by using the stored time series of the values of the process variables, and for the current moment, through the real-time graphical representation of the evolution of these values. The storage of this information takes place either through specialized databases for managing time series (such as Historian) or through relational databases that are managed directly by the SCADA application.
SCADA applications provide both dedicated programming languages and common scripting languages such as VBScript or JavaScript to program and customize tasks according to the requirements of the process. They also often provide integration capabilities with the most common programming frameworks like .NET or Java. Some of the most common SCADA applications on the market are iFiX, InTouch, WinCC, FactoryTalk, and Iconics.