SCADA Systems Tutorial
1 - Data Acquisition.
SCADA systems have to monitor hundreds or perhaps thousands of single sensors. Some sensors are put in place to measure inputs into the system, while others measure outputs.
Some sensors (known as discrete sensors) are used to monitor very simple “binary” events. These events are either “on” or “off”. For example, every time a particular piece of production gear in a manufacturing plant completes a process, it may output an electrical signal via a contact closure. A discrete sensor will detect this electrical signal and report it back to you at the control console of your SCADA system.
Other sensors measure more complicated values where it’s vital to know the exact value. These are called analog sensors, and they measure continuous changes within a possible range of values.
A simple mercury thermometer is a great example of an analog sensor - whereas a simple thermostat is the discrete form of temperature sensor. With the mercury thermometer, you know exactly what the temperature is (within a single degree of accuracy, of course). With the thermostat, you only know that the temperature is either above or below the value that you preset.
Obviously, analog measurement is important in SCADA systems where you need to keep track of fluid levels in water and fuel tanks, voltages of batteries, temperature, humidity, and other values that are most appropriately measured with a continuous range “analog” sensor.
To make it simpler for a human operator to interact with analog sensors, the best SCADA systems allow you to define a normal range for an analog value. For instance, you might specify that the temperature in your server room should remain between 60 and 79 degrees Fahrenheit. If the temperature in the server room goes outside this range, your SCADA system will provide an automated alert - either at a control console or directly to you via cell phone or e-mail.
2 - Data Communication.
SCADA systems involve monitoring a lot of processes and pieces of gear from a single location. To do this, you have to have a communications network to bring remotely collected data to your screen.
Data in modern SCADA systems is typically transported via ethernet or IP over SONET. It is important, however, to keep SCADA traffic off of the public Internet. This is an important security measure against both the real and perceived threats of terrorism. Public infrastructure, utilities and manufacturing facilities are valuable targets for attacks. This makes it very important to take at least basic security measures.
Fortunately, the trend in SCADA systems today is toward open protocols and data formats. While older systems locked you into a single manufacturer to maintain compatibility, today you have many options based on DNP3 and MODBUS Protocols. If you buy a piece of DNP3 MODBUS gear today, you can buy compatible gear tomorrow from one of many other manufacturers. This protects you from the trap of only having a single source for expanding your SCADA system.
In order for your central SCADA console to receive information from sensors, which are very simple devices, you need to install an RTU (Remote Telemetry Unit) at each monitored location. An RTU collects data from sensors and converts the readings into a protocol, such as MODBUS or DNP3, that can be transported across your communications network and back to you.
The same communication works in reverse (from you to your RTU) for control commands. In this scenario, you would issue a command from your central SCADA console. That command would be encoded into the SCADA protocol you are using and sent out across your network. The appropriate RTU would receive and decode your command, then respond by latching a control relay. This command process tells the gear that you have wired into your RTU to perform a specific action.
Issuing commands remotely provides the benefit of not having to drive out to distant sites every time you receive a SCADA alarm or other alert. In many cases (if you prepared during your installations), you can skip the drive time and simply issue a remote command.
3 - Data Presentation.
As a human being, you can’t just sit down to read SCADA data in its raw format (at least, you’d never want to). In order to provide at-a-glance status information and make it easier to train new SCADA staff, SCADA systems display information in human readable format at central consoles and via remote alerts.
The central computer in SCADA systems is known as a master station, a HMI (Human-Machine Interface), or in HCI (Human-Computer Interface), depending on who you’re talking to. All of these terms mean the same thing: a computer console that sorts and recaps data from your SCADA system and offers the ability to issue controls.
Part of sorting the status of your actions and processes is sorting alarms the staff doesn’t need to see. In any operation of a large size, you run the risk of overloading your staff with frequent, meaningless alerts that they’ll quickly learn to ignore. The first time a “real” alarm comes in, it’s likely to be missed in the noise of unimportant alarms.
That’s exactly why quality SCADA systems allow you to choose which alarms your staff should see. You can filter on location, severity, or the amount of time alarm condition has existed. Just a few carefully designed filters will hide “nuisance” Alarms from view. All the data is still there, but the operator you hired last week won’t be overloaded with worthless information.
After sorting alarms, SCADA systems have to present the data that remains. This can come in all sorts of formats, but the best systems have graphical interfaces that are easy to see and figure out. You want a system that offers a lot of display options, including geographic maps, blueprints and floor plans, photographs of rack-mounted and other gear, color-coded lists, and sorted lists (see T/Mon for an example of a SCADA master/HMI).
Also, if you don’t want to be stuck at a central console all the time, you need to choose a SCADA master station that can be accessed by a lot of users via remote network connection.
Choose SCADA systems that can send out automatic e-mail and pager/cell phone alerts. This also helps increase your mobility. These updates also provide faster alert of emerging problems that you can tackle from the field without returning to your central office.
4 - Control.
As said earlier, one key use of SCADA systems is to control gear remotely. It’s just not smart to go to the site of the problem every time you get an alarm. In SCADA, if it seems like there must be a better way, there likely is.
When it comes to controlling gear remotely, that better way is control relay commands issued from your SCADA master station and transmitted to your RTU’s via your network. In this way, you can control gear as if you were there - without any time traveling.
Even better, advanced SCADA systems allow you to pre-specify responses to single alarms, combos of alarms, or preset events. Once you’ve completed this databasing, your SCADA system will respond within seconds when automatic control condition is triggered. This is an good way to switch to a backup system in the event of a main system failure, especially in public safety, telecom, transit, and manufacturing environments.