SCADA (supervisory control and data acquisition system) refers to the combination of
telemetry and data acquisition. SCADA encompasses the collecting of the information via
a RTU (remote terminal unit), transferring it back to the central site, carrying out any
necessary analysis and control and then displaying that information on a number of
operator screens or displays. The required control actions are then conveyed back to
the process.
In the early days of data acquisition relay logic was used to control production and plant
systems. With the advent of the CPU (as part of the microprocessor) and other electronic
telemetry and data acquisition. SCADA encompasses the collecting of the information via
a RTU (remote terminal unit), transferring it back to the central site, carrying out any
necessary analysis and control and then displaying that information on a number of
operator screens or displays. The required control actions are then conveyed back to
the process.
In the early days of data acquisition relay logic was used to control production and plant
systems. With the advent of the CPU (as part of the microprocessor) and other electronic
devices, manufacturers incorporated digital electronics into relay logic equipment,
creating the PLC or programmable logic controller, which is still one of the most widely
used control systems in industry. As needs grew to monitor and control more devices in
the plant, the PLCs were distributed and the systems became more intelligent and smaller
in size. PLCs and/or DCS (distributed control systems) are used as shown below. Although
initially RTU was often a dedicated device, PLCs are often used as RTUs these days.
creating the PLC or programmable logic controller, which is still one of the most widely
used control systems in industry. As needs grew to monitor and control more devices in
the plant, the PLCs were distributed and the systems became more intelligent and smaller
in size. PLCs and/or DCS (distributed control systems) are used as shown below. Although
initially RTU was often a dedicated device, PLCs are often used as RTUs these days.
PC to PLC or DCS with a fieldbus and sensors
The advantages of the PLC/DCS/SCADA system are:
• The computer can record and store a very large amount of data
• The data can be displayed in any way the user requires
• Thousands of sensors over a wide area can be connected to the system
• The operator can incorporate real data simulations into the system
• Many types of data can be collected from the RTUs
• The data can be viewed from anywhere, not just on site
The disadvantages are:
• The system is more complicated than the sensor to panel type
• Different operating skills are required, such as system analysts and
programmer
• With thousands of sensors there is still a lot of wire to deal with
• The operator can see only as far as the PLC
As the requirement for smaller and smarter systems grew, sensors were designed with
the intelligence of PLCs and DCSs. These devices are known as IEDs (intelligent
electronic devices). The IEDs are connected on a fieldbus such as Profibus, DeviceNet or
Foundation Fieldbus to the PC. They include enough intelligence to acquire data, communicate
to other devices and hold their part of the overall program. Each of these super
smart sensors can have more than one sensor on board. Typically an IED could combine
an analog input sensor, analog output, PID control, communication system and program
memory in the one device.
The advantages of the PLC/DCS/SCADA system are:
• The computer can record and store a very large amount of data
• The data can be displayed in any way the user requires
• Thousands of sensors over a wide area can be connected to the system
• The operator can incorporate real data simulations into the system
• Many types of data can be collected from the RTUs
• The data can be viewed from anywhere, not just on site
The disadvantages are:
• The system is more complicated than the sensor to panel type
• Different operating skills are required, such as system analysts and
programmer
• With thousands of sensors there is still a lot of wire to deal with
• The operator can see only as far as the PLC
As the requirement for smaller and smarter systems grew, sensors were designed with
the intelligence of PLCs and DCSs. These devices are known as IEDs (intelligent
electronic devices). The IEDs are connected on a fieldbus such as Profibus, DeviceNet or
Foundation Fieldbus to the PC. They include enough intelligence to acquire data, communicate
to other devices and hold their part of the overall program. Each of these super
smart sensors can have more than one sensor on board. Typically an IED could combine
an analog input sensor, analog output, PID control, communication system and program
memory in the one device.
PC to IED using a fieldbus
The advantages of the PC to IED fieldbus system are:
• Minimal wiring is needed
• The operator can see down to the sensor level
• The data received from the device can include information such as serial
numbers, model numbers, when it was installed and by whom
• All devices are plug and play; so installation and replacement are easy
• Smaller devices mean less physical space for the data acquisition system
The disadvantages of a PC to IED system are:
• The more sophisticated system requires better trained employees
• Sensor prices are higher (but this is offset somewhat by the lack of PLCs)
• The IEDs rely more on the communication system
The advantages of the PC to IED fieldbus system are:
• Minimal wiring is needed
• The operator can see down to the sensor level
• The data received from the device can include information such as serial
numbers, model numbers, when it was installed and by whom
• All devices are plug and play; so installation and replacement are easy
• Smaller devices mean less physical space for the data acquisition system
The disadvantages of a PC to IED system are:
• The more sophisticated system requires better trained employees
• Sensor prices are higher (but this is offset somewhat by the lack of PLCs)
• The IEDs rely more on the communication system
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