SCADA
SCADA is the acronym for Supervisory Control And Data Acquisition.
SCADA may be called Human-Machine Interface(HMI) in Europe. The term refers to a
large-scale, distributed measurement (and control) system. SCADA systems are used
to monitor or to control chemical, physical or transport processes. SCADA is an
industrial measurement and control system consisting of a central host or master
(usually called a master station, master terminal unit or MTU); one or more field
data gathering and control units or remotes (usually called remote stations, remote
terminal units, or RTU's); and a collection of standard and/or custom software used
to monitor and control remotely located field data elements. Contemporary SCADA systems
exhibit predominantly open-loop control characteristics and utilize predominantly
long distance communications, although some elements of closed-loop control and/or
short distance communications may also be present.
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.
The term SCADA usually refers to a central system that monitors and controls a
complete site. The bulk of the site control is actually performed automatically
by a Remote Terminal Unit (RTU) or by a Programmable Logic Controller(PLC).
Host control functions are almost always restricted to basic site over-ride
or supervisory level capability.
Data acquisition begins at the RTU or PLC level and includes meter readings and
equipment statuses that are communicated to the SCADA as required. Data is then
compiled and formatted in such a way that a control room operator using the HMI
can make appropriate supervisory decisions that may be required to over-ride
normal RTU (PLC) controls. (A SCADA system includes all the pieces, HMI,
controllers, I/O devices, networks, software, etc.)
SCADA systems typically implement a distributed database which contains data
elements called 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. The 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 possible to purchase a SCADA system, or Distributed Control System (DCS)
from a single supplier. It's also possible to assemble a SCADA system from
components like Wonderware HMI, Allen-Bradley & GE PLCs, Ethernet
communication devices, etc.
