|Posted by ash.finest on April 4, 2012 at 2:20 AM|
A SCADA system performs four functions:
Networked data communication
These functions are performed by fourkinds of SCADA components:
Sensors (either digital or analog) and control relays that directly interface with the managed system.
Remote telemetry units (RTUs). These are small computerized units deployed in the field at specific sites and locations. RTUs serve as local collection points for gathering reports from sensors and delivering commands to control relays.
SCADAmaster units. These are larger computer consoles that serve as the central processor for the SCADA system. Master units provide a human interface to the system and automatically regulate the managed system in response to sensor inputs.
The communications network that connects the SCADA master unit to the RTUs in the field.
The World’s Simplest SCADA System
The simplest possible SCADA system wouldbe a single circuit that notifies you of one event. Imagine a fabricationmachine that produces widgets. Every time the machine finishes a widget, itactivates a switch. The switch turns on a light on a panel, which tells a humanoperator that a widget has been completed.
Obviously, a real SCADA system does morethan this simple model. But the principle is the same. A full-scale SCADA systemjust monitors more stuff over greater distances.
Let’s look at what is added to oursimple model to create a fullscale SCADA system:
First, the systems you need to monitor aremuch more complex than just one machine with one output. So a real-life SCADAsystem needs to monitor hundreds or thousands of sensors. Some sensors measureinputs into the system (for example, water flowing into a reservoir), and somesensors measure outputs (like valve pressure as water is released from thereservoir).
Some of those sensors measure simpleevents that can be detected by a straightforward on/off switch, called adiscrete input (or digital input). For example, in our simple model of thewidget fabricator, the switch that turns on the light would be a discrete input.In real life, discrete inputs are used to measure simple states, like whetherequipment is on or off, or tripwire alarms, like a power failure at a criticalfacility.
Some sensors measure more complexsituations where exact measurement is important. These are analog sensors, whichcan detect continuous changes in a voltage or current input. Analog sensors areused to track fluid levels in tanks, voltage levels in batteries, temperatureand other factors that can be measured in a continuous range of input.
For most analog factors, there is a normalrange defined by a bottom and top level. For example, you may want thetemperature in a server room to stay between 60 and 85 degrees Fahrenheit. Ifthe temperature goes above or below this range, it will trigger a thresholdalarm. In more advanced systems, there are four threshold alarms for analogsensors, defining Major Under, Minor Under, Minor Over and Major Over alarms.
In our simple model of the widgetfabricator, the “network” is just the wire leading from the switch to thepanel light. In real life, you want to be able to monitor multiple systems froma central location, so you need a communications network to transport all thedata collected from your sensors.
Early SCADA networks communicated overradio, modem or dedicated serial lines. Today the trend is to put SCADA data onEthernet and IP over SONET. For security reasons, SCADA data should be kept onclosed LAN/WANs without exposing sensitive data to the open Internet.
Real SCADA systems don’t communicatewith just simple electrical signals, either. SCADA data is encoded in protocolformat. Older SCADA systems depended on closed proprietary protocols, but todaythe trend is to open, standard protocols and protocol mediation.
Sensors and control relays are very simpleelectric devices that can’t generate or interpret protocol communication ontheir own. Therefore the remote telemetry unit (RTU) is needed to provide aninterface between the sensors and the SCADA network. The RTU encodes sensorinputs into protocol format and forwards them to the SCADA master; in turn, theRTU receives control commands in protocol format from the master and transmitselectrical signals to the appropriate control relays.
The only display element in our modelSCADA system is the light that comes on when the switch is activated. Thisobviously won’t do on a large scale — you can’t track a lightboard of athousand separate lights, and you don’t want to pay someone simply to watch alightboard, either.
A real SCADA system reports to humanoperators over a specialized computer that is variously called a master station,an HMI (Human-Machine Interface) or an HCI (Human-Computer Interface).
The SCADA master station has severaldifferent functions. Themaster continuously monitors all sensors and alerts theoperator when there is an “alarm” — that is, when a control factor isoperating outside what is defined as its normal operation. The master presents acomprehensive view of the entire managed system, and presents more detail inresponse to user requests. The master also performs data processing oninformation gathered from sensors — it maintains report logs and summarizeshistorical trends.
An advanced SCADA master can add a greatdeal of intelligence and automation to your systems management, making your jobmuch easier.
Unfortunately, our miniature SCADA systemmonitoring the widget fabricator doesn’t include any control elements. So let’sadd one. Let’s say the human operator also has a button on his control panel.When he presses the button, it activates a switch on the widget fabricator thatbrings more widget parts into the fabricator.
Now let’s add the full computerizedcontrol of a SCADA master unit that controls the entire factory. You now have acontrol system that responds to inputs elsewhere in the system. If the machinesthat make widget parts break down, you can slow down or stop the widgetfabricator. If the part fabricators are running efficiently, you can speed upthe widget fabricator.
If you have a sufficiently sophisticatedmaster unit, these controls can run completely automatically, without the needfor human intervention. Of course, you can still manually override the automaticcontrols from the master station.
In real life, SCADA systems automaticallyregulate all kinds of industrial processes. For example, if too much pressure isbuilding up in a gas pipeline, the SCADA system can automatically open a releasevalve. Electricity production can be adjusted to meet demands on the power grid.Even these real-world examples are simplified; a full-scale SCADA system canadjust the managed system in response to multiple inputs.