Key Features and Applications of Remote Terminal Units (RTU)

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:

•  Electrical Power Distribution and Automation System
• The Transition from Relays to PLCs explained

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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How Digital Communication Technology is applied in Measurement and Control Systems

The continued advancement in digital technology has revolutionized how data is acquired in measurement and control instruments. Data acquisition in instrumentation systems encompasses the measurement and recording of process data.

The ability of being able to relay or communicate large amounts of data over a limited number of channels gives digital technology an upper hand over Analog technology. For example in Analogy technology where  4-20 mA or 3-15 PSI signals are used, each pair of wires can communicate only one variable whereas in digital networks, one pair of wires can communicate a limitless number of variables, with the only limit to it being the speed of that data communication.

From the above, 4-20 mA analog signals can be expensive to utilize in especially for an instrument generating multiple variables of measurement like Coriolis mass flowmeters that measures Flow rate, density, and temperature at the same time. In this case you will need dedicated wire pair for each process variable. Digital Technology overcomes these shortcomings of 4-20 mA analog signals.

One solution to this problem is using HART digital signals superimposed on 4-20 mA signals; we normally call this 4-20 mA plus HART. With this, you retain the analog signals while at the same time enjoying the multi-variable communication benefits that comes with digital technology however wired-HART communication is rather slow by any standard, and this restricts its use to maintenance {range changes, diagnostic data polling} and used only for slow process control processes.

Examples of digital communication standards include:

• Modbus
•  HART
•  FOUNDATION Fieldbus
• Profibus PA
• Profibus DP
• Profibus FMS
• AS-I
•  ControlNET
•  DeviceNet
• BACnet
• LonWorks
• CANbus

Some of the digital communication instruments find common use in distributed control systems (DCS) applications.

SCADA (Supervisory, Control and Data Acquisition) systems use digital communication technology. SCADA is similar to DCS, but it is spread over a large geographical area whereas DCS may cover only a plant floor. You will find SCADA systems applied in areas like:

•  Gas and oil exploration and distribution (pipeline) systems
• Electric power generation and distribution (power line, substation) systems.
• Water and wastewater treatment and distribution lines (water line, pumping stations) systems.
• Large irrigation or harvesting systems.

The process data in a SCADA system is sensed by various measurement devices (transmitters) converted to digital form by an RTU (Remote Terminal Unit), and communicated to one or more MTUs (Master Terminal Units) at a central location where we have human operators monitoring the data and at the same time make command decisions.

In a system where we have the flow of information just in one way (simplex) from the measurement devices to human operators, the system may be referred to as a Telemetry system rather than a SCADA system.

SCADA implies two-way (Duplex) information flow, where human operators not only monitor the process data but also issue commands back to the remote terminal units to effect change.

Actually the need of remote monitoring and control of electric power distribution systems lead to the development of power line carrier analog telemetry systems. These systems superimposed high-frequency (50 kHz to 150 kHz) carrier signals on a low-frequency (50 Hz and 60 Hz power line conductors to communicate basic information like human voice (telephone network dedicated to power system operators), power flow (MVAR meter, Wattmeter) monitoring and protective relay (automatic trip) controls. These are examples of telemetry systems that were among the first to benefit from digital technology.

Large scale power systems cannot be operated safely and with efficiency without the use of remote data monitoring and control systems.

You can also read: How to integrate PLC into a Control System

The bottom line

From the above discussion, you can see that digital communication technology forms an essential part of any modern measurement and control system, and as more research and development is being done in the digital field, industries will continue to use this technology to improve efficiency in their industrial production and processing systems.

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