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Plant brochure designed by Charles L.
Woodruff 1999 and revised in June 2004.
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Kruger SCADA System Overview |
The Kruger SCADA plant is a graphical
presentation of the physical plant where the monitored components, such as
pumps, motors, valves and measuring equipment are in focus. The SCADA plant
allows for effective optimisation of the process and energy consumption,
better overview, quick transmission of alarms, flexible control and quick
and easy reporting.
Kruger's SCADA plant has built-in process knowledge, logically designed
systems, standardised products with uniform and well-arranged user interface
as well as professional graphics with moving elements. |
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The SCADA system
automates the entire process to improve operation efficiency and effluent
quality.
The major components of SCADA system are:
PC's — Personal Computers which run operator interface software. PCs provide
a graphical view of the process to the operator, which enables him to
monitor and control the process. Computers display historical data for
analysis at any time.
PLC's – Programmable Logic Controllers which runs a ladder logic program.
They control the outputs based on the inputs being monitored, in the desired
sequence. The PLC also communicates with other computers.
Radio modem or Telephone modems - which are used to communicate from the
plant to the remote site. This system is more reliable and faster than the
old telemetry system.
RTU's - Remote Telemetry (or Terminal) Units, rugged industrial computers
which provides intelligence in the field, and allows the central SCADA
master to communicate with the field instruments. It is a stand alone data
acquisition and control unit. Its function is to control process equipment
at the remote site, acquire data from the equipment, and transfer the data
back to the central SCADA system. |
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| The illustration below
shows the major components of the Kruger SCADA System installed at the
Princeton Wastewater Treatment Plant and the communications connections
linking the units. |
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Kruger SCADA System
Communications Overview |
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So then, just
what
is SCADA?
It is used to monitor and control the plant and its equipment.
SCADA (supervisory control and data acquisition): 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 utilise predominantly long distance
communications, although some elements of closed-loop control and/or short
distance communications may also be present. |
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 The
primary interface to the operator is a graphical display (mimic) which shows
a representation of the plant or equipment in graphical form. Live data is
shown as graphical shapes (foreground) over a static background. As the data
changes in the field, the foreground is updated. E.g. a valve may be shown
as open or closed. Analogue data can be shown either as a number, or
graphically. The system may have many such displays, and the operator can
select from the relevant ones at any time.
And the power of the computer can be used
to perform complex sequencing of operations. e.g. OPEN a valve, then START a
pump, but only if the pressure is greater than 50.
The computer can be used to summarise and display the data it is processing.
Trends (graphs) of analogue values over time are very common. Collecting the
data and summarising it into reports for operators, and management are
normal features of a SCADA system.
Pictured below is just one example of the many such displays available to
the operator.
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One of The SCADA System
Overview Screens |
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There are two basic types
of RTU - the "single board RTU" which is compact, and contains all I/O on a
single board, and the "modular RTU" which has a separate CPU module, and can
have other modules added, normally by plugging into a common "backplane" (a
bit like a PC motherboard and plug in peripheral cards).
The single board RTU normally has fixed I/O e.g. 16 digital inputs, 8
digital outputs, 8 analogue inputs, and say 4 analogue outputs. It is
normally not possible to expand its capability.
The modular RTU is designed to be expanded by adding additional modules.
Typical modules may be a 8 analogue in module, a 8 digital out module. Some
specialised modules such as a GPS time stamp module may be available.
The RTU will operate scanning its inputs, normally at a fairly fast rate. It
may do some processing such as change of state processing, time stamping of
changes, and storage of the data awaiting polling from the SCADA master.
Some RTU's have the ability to initiate reporting to the SCADA master,
although more common is the situation where the SCADA master polls the RTU's
asking for changes. The RTU may do some alarm processing. When polled by the
SCADA master, the RTU must respond to the request, which may be as simple as
"give me all your data", to a complex control function to be executed.
PLC's vs. RTU's
A PLC (programmable logic controller) is a small industrial computer which
originally replaced relay logic. It had inputs and outputs similar to those
an RTU has. It contained a program which executed a loop, scanning the
inputs and taking actions based on these inputs. Originally the PLC had no
communications capability, but they began to be used in situations where
communications was a desirable feature. So communications modules were
developed for PLC's, supporting Ethernet (for use in distributed control
systems) and the Modbus communications protocol for use over dedicated
(wire) links.
Pictured below is a simple animated
example of a PLC at work, showing a tank being filled, and a pump starting
and emptying it when sensors indicate the tank is full, then another pump
starting to fill it again when the sensors indicate the tank is empty. This
example does not involve telemetry as the pump is close to the tank.
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Animation Showing a PLC
controlling the emptying & Filling of a Tank |
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| A more detailed
description of the operation of the PLC's and RTU's installed in the
Princeton Plant can be found on the SCADA PLC page in this section. |
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