Wireless media can also be a communication medium for the master unit and the remote unit. Systems using this type of media are termed "wireless SCADA systems." A few examples of wireless media are explained below.
- Spread Spectrum Radio - The frequency band for this is 900 MHz to 5.8GHz and is free for general pubic use. Spread spectrum radio modems are used to ensure efficient network communication.
- Microwave Radio - In this case signals are transmitted at high frequencies using parabolic dishes installed on towers or on the tops of buildings. However, one disadvantage of this communication is that transmission may get interrupted due to misalignment and/or atmospheric conditions.
- VHF/UHF Radio - This is an electromagnetic transmission with frequencies of 175MHz-450MGz-900MHz. Special antennas are required to receive these signals.
Benefits of a Wireless SCADA system
A perfectly designed wireless SCADA system offers the following benefits:
- Monitors in real time
- Minimizes the operational costs
- Provides direct information of system performance
- Improves system efficiency and performance
- Increases equipment life
- Reduces labor costs required for troubleshooting or servicing the equipment
- Automated report generation reduces errors in calculations and interpretations
- Uses advanced technologies
A typical SCADA system comprises of i/o signal hardware, controllers, software,networks and communication. SCADA system is normally used to monitor and control a remote site or a distribution that is spread out for a long distance. An RTU (Remote Terminal Unit) or a PLC (Programmable Logic Controller) is usually used to control a site automatically. The SCADA system also provides a host control functions for the supervisor to control and define settings.
For example, in a SCADA system a PLC can be used to control the flow of cooling water as part of an industrial process. At the same time the supervisor can use the Host control function to set the temperature for the flow of water. It can also have alarms and can record the flow of water temperature and report back to the SCADA system.
The RTUs and PLCs are responsible for data collection such as meter readings, equipment status etc and communicate back to the SCADA system. This data can be stored in a database for later analysis or monitored by a supervisor to take appropriate actions if required.
SCADA systems typically implement a distributed database, commonly referred to as a tag database, which contains data elements called tags or points. A point represents a single input or output value monitored or controlled by the system. Points can be either "hard" or "soft". A hard point is representative of an actual input or output connected to the system, while a soft point represents the result of logic and math operations applied to other hard and soft points. Most implementations conceptually remove this distinction by making every property a "soft" point (expression) that can equal a single "hard" point in the simplest case. Point values are normally stored as value-timestamp combinations; the value and the timestamp when the value was recorded or calculated. A series of value-timestamp combinations is the history of that point. It's also common to store additional metadata with tags such as: path to field device and PLC register, design time comments, and even alarming information.
SCADA is an acronym that stands for Supervisory Control and Data Acquisition. SCADA refers to a system that collects data from various sensors at a factory, plant or in other remote locations and then sends this data to a central computer which then manages and controls the data
SCADA systems are used not only in industrial processes: e.g. steel making, power generation (conventional and nuclear) and distribution, chemistry, but also in some experimental facilities such as nuclear fusion. The size of such plants range from a few 1000 to several 10 thousands input/output (I/O) channels. However, SCADA systems evolve rapidly and are now penetrating the market of plants with a number of I/O channels of several 100 K: we know of two cases of near to 1 M I/O channels currently under development.
There are many parts of a working SCADA system. A SCADA system usually includes signal hardware (input and output), controllers, networks, user interface (HMI), communications equipment and software. All together, the term SCADA refers to the entire central system. The central system usually monitors data from various sensors that are either in close proximity or off site (sometimes miles away).
Systems similar to SCADA systems are routinely seen in factories, treatment plants etc. These are often referred to as Distributed Control Systems (DCS). They have similar functions to SCADA systems, but the field data gathering or control units are usually located within a more confined area. Communications may be via a local area network (LAN), and will normally be reliable and high speed. A DCS system usually employs significant amounts of closed loop control.
SCADA systems on the other hand generally cover larger geographic areas, and rely on a variety of communications systems that are normally less reliable than a LAN. Closed loop control in this situation is less desirable.