• SCADA and Telemetry Basics - SCADA System Basics

SCADA Basics

SCADA basicsAs most of you probably know, SCADA stands for Supervisory Control and Data Acquisition. It’s a term that’s been around since the 60s and grew out of the utility power and natural gas industries where it was necessary to operate equipment at remote, unmanned substations. (Read a detailed history of SCADA ) In general terms, SCADA allows for monitoring of dispersed equipment and/or processes and initiation of control of those same dispersed elements.

Today, you can find SCADA systems of some type in virtually every industry, and most plants within an industry category. The efficiency benefits of a SCADA system compared to manually monitoring and adjusting equipment and processes are significant. Consider as an example, a municipal water distribution system where pre-SCADA operation involved a technician driving from water tank to water tank to read tank levels and initiate action accordingly.

SCADA Overview

A typical SCADA network consists of instrumentation located with each dispersed element, to monitor/measure its condition or operation. Instrumentation can be as simple as level and flow measurements to complex chemical analysis or material composition (think thickness and brightness measurement at multiple points across the width of a sheet of paper as it speeds off a paper machine). Modern manufacturing plants may employ vision systems to check quality. In essence, any parameter that can be recognized can be incorporated into a SCADA system.

Instrumentation is connected to a remote terminal unit (RTU) which converts the signals from the equipment into a data stream that is communicated to the central location, where a data concentrator collects the data and feeds it to the operator terminal. In most cases, this terminal is a computer that provides the human-machine-interface (HMI) which allows the operator to view the monitored data and initiate changes that may be necessary. Depending on the elements/parameters being monitored and the type of system, some responses to change information may occur automatically, while others may require operator intervention.

Telemetry Basics

The data stream from the RTUs to the central data concentrator constitutes the telemetry, which is the term generally applied to the monitoring information being sent from the RTU. Telemetry signaling techniques advanced over time to include pulse rate and variable frequency schemes. Technology advances also allowed for multiplexing telemetry from multiple RTUs over a single communications channel, which led to allowing operators to use selective telemetry systems to separate out the signals.

Automated data loggers helped capture large volumes of telemetry data without requiring constant operator attention. Solid state technology, improved recording systems, and computerized handling of the data have elevated telemetry into modern monitoring and response systems.

SCADA Communications

The communications link itself has undergone massive changes through the years in the technology employed. In the early years, telephone lines were used as the transmission medium, and telephone company step-x-step switching technology was employed for signaling. This amounted to little more than sequences of open-close electrical pulses created by the switches.

Advances in technology allowed improved signaling capabilities, first in half-duplex (one-way-at-a-time signaling), then in full-duplex (simultaneous transmission both directions) mode. Pulse modulation techniques and computerization have made possible today’s systems that can generate and receive the massive amounts of data that feed a modern SCADA system.

Wireless Within a SCADA Network

Over time, hardwire connections became less practical and more expensive. Radio technology evolved to be the preferred method of communicating telemetry signals. Licensed radio with limited bandwidth and power made its way into the telemetry field. After WW II, spread spectrum technologies became available to the factory floor and SCADA systems, allowing multiple users to operate in the same band.

In the 80s, the FCC stepped in and divided up frequency bands for different applications, and manufacturers developed a variety of proprietary systems operating in different frequency ranges. Eventually, standards were developed that made spread spectrum and licensed radio good options for implementing wireless communications links within a SCADA network.

But radio also has some disadvantages. Antenna placement and height are often difficult to optimize for signal strength, though radio surveys can minimize this problem by helping to determine the best of both parameters. Another challenge with radio, sometimes experienced in water/wastewater systems, is that antennas high enough to provide proper range and signal strength become lightning targets, causing equipment and system outages and chewing through maintenance budgets.

The current trend that we’re seeing in wireless communications for SCADA networks is cellular technology. Cellular networks now provide nearly universal geographic coverage. Cellular service is relatively inexpensive compared to other technologies. And cellular technology allows RTUs to be located at ground level, without the need for lightning attracting antennas.

Questions?

This has been a relatively simple overview of SCADA and its various elements. And for our many, more experienced readers, it would probably qualify as overly simplistic. But SCADA is a huge term, forming the heart of automation and control systems, and trying to address all of its elements in depth would go beyond the practicality of a single blog post. But if you have specific elements, aspects, or issues of SCADA that you would like us to address in more depth, please let us know. You many add comments below, use the Contact Us function of the web site, or email us directly at revere@reverecontrol.com. We look forward to providing more SCADA information at some point in the future.

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