Remote Terminal Units also called Remote Telemetry
Units or Remote Telecontrol Units are Microprocessor controlled devices that
interfaces in the physical world to either SCADA (Supervisory Control and Data
acquisition) system or DCS (Distributed and Control system). They
transmit data to a master system and uses messages from the master supervisory
system to control objects connected to the system.
They are designed for use in applications in remote
locations unattended. These locations may have limited to no power, hence RTUs
are designed to consume low power than DCS and PLC & this enables operation
on solar power and batteries.
In application where supervision is done from
distant central location, the SCADA software sits in the central office
connected over a backhaul network typically using radio communication to the
RTUs located far away and in most cases geographically spread out. The
communication may be interrupted for long periods of time therefore RTUs have
on-board data storage continuing local data collection for more than a
month if backhaul communication is lost as well as “history backfill’’
uploading this data once the connection is established again. Report by exception
communication mechanisms are often used to minimize backhaul communication
using Wide Area Networks e.g. Mobile, Microwave, Satellite.
Remote Terminal Units
(RTU) Configuration
The RTU configuration software is separate from the
HMI (Human Machine Interface) software from a third-party manufacturer i.e. two
separate databases. RTU is configured first; next the OPC server is configured.
For a native OPC server this happens automatically, but for OPC server from a
third-party, manual data mapping is required which can be time-consuming
and error prone requiring thorough testing. In most cases native OPC server is
preferred. To finalize, the HMI database has to be configured for graphics,
alarms, and trends etc.
The 4-20 mA AI and AO cards for a RTU optionally
support native HART pass through hence separate HART multiplexer (MUX) hardware
and associated work is not required. Native HART pass through AI and AO cards
are much easier to integrate and should be specified if 4-20 mA is used. Since
RTUs are generally used in very slow monitoring applications that don’t require
fast control, some applications do not use the real-time analog 4-20 mA but
only the digital HART communication multi-drop topology. This means the field
instruments draw less than 4 mA instead of up to 20 mA hence further reducing
the overall power consumption.
You can also read:
Applications of Remote Terminal Units (RTU)
Remote terminal units are commonly used in the
following applications:
- Electrical Power Transmission Networks and Associated Equipment.
- Remote Monitoring of Functions and the whole Instrumentation Network in Oil and Gas (offshore platforms, onshore oil wells, Pump Stations on Pipelines)
- Water and Wastewater collection and supply networks including the Pumping stations.
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