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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.
|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.
|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.|
Kruger SCADA System Communications Overview
|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.
|[ندعوك للتسجيل في المنتدى أو التعريف بنفسك لمعاينة هذه الصورة]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.
One of The SCADA System Overview Screens
|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.
Animation Showing a PLC controlling the emptying & Filling of a Tank
|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.|