Supervisory Control And Data Acquisition is what the acronym stands for, but what does that mean? The data acquisition part is simply acquiring data. Most SCADA systems are acquiring data from some type of sensors. Sensors vary from industry to industry, but can generally be broken into two categories:
- Analog Sensors – these are sensors that have values vary. At a high level analog sensors could show values like weight, height, tank level, pump RPM, current draw, phase frequency, flow rate, and many others. At a lower level these sensors would output signals such as 4-20ma, 0-5v, 0-10v, 0-24v, or pulses for counting.
- Status Sensors – these are simply on / off, true / false sensors that are tied to some type of switch. Higher level status indicators could have the on/off, true/false states mapped to labels such as “Running”/”Stopped” for a pump motor, or “Open”/”Closed” for a door. The lower level of status indicators may map a status from an analog value specifying a threshold at which the status shows True. So if the analog is above the threshold value, the status is true. More efficient systems typically use bitmaps to represent multiple status values in a single value.
The Supervisory Control definition seems to vary from SCADA provider to SCADA provider, some aren’t actually supervisory in my opinion. This aspect of SCADA for some providers works at a site level, and in their opinion by turning a pump on or off in response to a tank level or other event qualify as Supervisory Control. I tend to think of Supervisory aspect in terms of monitoring. Supervisory to me indicates that the system is constantly monitored.
So in addition to receiving all the data when the system is connected and operating properly, I also want to ensure that I get notified if the system is no longer communicating. This is really a mission critical aspect of SCADA to me. Even in monitoring something as simple as a sewer lift station, typical alerts for pumps running too long from a stuck float, or a high level alerts are always important. What if the power goes out to the lift station or the pump controller and SCADA take a lightening hit. A true supervisory system will let you know in a reasonable amount of time that the system is offline in addition to all the typical alerts. The reasonable amount of time really depends on the application, but it should be configurable to fit the application.
Now for the Control aspect, I like to think that a SCADA system is going to give me a Supervisory view of the system, and control of the system. Control really varies widely with the different industries, and could be anything from adjusting the speed of a huge motor via a VFD, turning a pump on or off, telling a pump control system to fill a tank, or making a chemical feeder spit out more chemicals.
Hopefully all this helps you understand what SCADA is all about, and if you understand scads, you also understand DAQ Data Acquisition, DAC – Data Acquisition and Control, Remote Monitoring, and Remote Control. SCADA is simply the acronym typically used the Utility Industries – Water, Gas, Oil, Electric, and Wastewater.