by Introduction To SCADA and Telemetry
Definition – SCADA
What is SCADA?
SCADA (Supervisory Control And Data Acquisition) system refers to the combination of telemetry and data acquisition.
It consists of:
• Collecting information
• Transferring it back to a central site
• Carrying out necessary analysis and control
• Displaying this data on a number of operator screens
* The SCADA system is used to monitor and control a plant or equipment.
* Control may be automatic or can be initiated by operator commands.
Definition - Telemetry
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What is telemetry?
• Telemetry is usually associated with SCADA systems. It is a technique used in transmitting and receiving information or data over a medium.
• The information can be measurements, such as voltage, speed or flow.
• These data are transmitted to another location through a medium such as cable, telephone or radio. Information may come from multiple locations. A way of addressing these different sites is incorporated in the system.
Definition – Data Acquisition
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What is data acquisition?
• Data acquisition refers to the method used to access and control information or data from the equipment being controlled and monitored.
• The data accessed are then forwarded onto a telemetry system ready for transfer to the different sites.
• They can be analog and digital information gathered by sensors, such as flowmeter, ammeter, etc.
• It can also be data to control equipment such as actuators, relays, valves, motors, etc.
Components of SCADA System
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• Remote Station
• Communication Network
• Central Monitoring Station (CMS)
• Field Instrumentation
Remote Station
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• Field instrumentation connected to the plant or equipment being monitored and controlled are interfaced to the remote station to allow process manipulation at a remote site.
• It is also used to gather data from the equipment and transfer them to the central SCADA system. The remote station may either be an RTU (Remote Terminal Unit) or a PLC (Programmable Logic Controller).
Communication Network
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• The Communication Network refers to the communication equipment needed to transfer data to and from different sites.
• The medium used can either be cable, telephone or radio.
• Remote sites are usually not accessible by telephone lines. The use ofradio offers an economical solution.
• Radio modems are used to connect the remote sites to the host. An on-line operation can also be implemented on the radio system. For locations wherein a direct radio link cannot be established, a radio repeater is used to link these sites.
Central Monitoring Station (CMS)
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• The Central Monitoring Station (CMS) is the master unit of the SCADA system
• It is in charge of collecting information gathered by the remote stations and of generating necessary action for any event detected.
• The CMS can have a single computer configuration or it can be networked to workstations to allow sharing o information from the SCADA system
Field Instrumentation
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• Field Instrumentation refers to the devices that are connected to the equipment or machines being controlled and monitored by the SCADA system
• These are sensors for monitoring certain parameters; and actuators for controlling certain modules of the system
• Field instruments are usually either discrete or analog
Field Instrumentation (Analog Devices)
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- These instruments convert physical parameters (i.e., fluid flow, velocity, fluid level, etc.) to electrical signals (i.e., voltage or current)
- Signals using current (mA) are more common because they can be used for longer distances with less error due to signal loss
Field Instrumentation (Discrete Devices)
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• Digital inputs and outputs are used to differentiate the discrete status of the equipment
• Usually, <1> is used to mean EQUIPMENT ON and <0> for EQUIPMENT OFF status. This may also mean <1> for FULL or <0> for EMPTY
Field Instrumentation (Digital or Smart Devices )
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• Instruments that provide more than one variable on one pair of wires (turbidity, temp,pH, etc.)
• Usually wired on a bus network
• Saves installation cost in wiring
• Provides diagnostic information of instruments
Typical System Configurations
Three Basic Types
• Point-to-Point
• Point-to-Multipoint
• Peer-to-Peer
System Configurations Point-to-Point
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• Most simple type of communications
• One station talks directly to the other (ie: water tank to well pump)
System Configurations Point-to-Multipoint
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• Usually has one master polling location
• Sometimes CMS
• Most common type
System Configurations Peer-to-Peer
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• All sites talk directly with each other
• Communications protocol has to be able to support this type of communication
• Manufacturing facilities and treatment plants are good examples that use this type of communications
SCADA HMI Examples Remote Stations
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SCADA HMI Examples Central Monitoring Station
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Human Machine Interface (HMI)
• Graphical representation of a process
• Usually a software application on a PC at CMS to let the operator interface with control system
• Has control functionality
• Alarm capabilities
• Data Historian and trending
• Typical software packages: Tetragenics MC3000, Wonderware, Intelution and RSView32
Tips in Planning SCADA System
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• Get involved with your engineer for vendor and equipment selection
• Check references of integrators’ past installations
• Select vendor that can respond quickly if needed
• Select systems that can be serviced by another
• integrator in the event of having to change integrators
• Hire integrator that has background with your process requirement
• If using radios, a path study is highly recommended
• Bury fiber optic cable when possible (this is fairly inexpensive during new construction)
• Enter a yearly maintenance contract with the vendor
Maintenance
• Check antenna connections and other communications equipment yearly
• Calibrate instruments regularly, as per manufactures recommendations (at least quarterly)
• Backup your data at least once a month
• Have backups of HMI and other software applications
• Have a spare hard drive formatted and ready to go
• Keep spare parts for field instruments in inventory
