REMOTE TERMINAL UNIT
FUNCTION
A Remote Terminal Unit (RTU) is a microprocessor controlled
electronic device that interfaces instruments and electrical
devices in the physical world to a SCADA or telemetry system.
It uses messaging over a telemetry medium to transmit data
and receive commands. The telemetry medium can be radio,
phone line, satellite or mobile phone through to hardwired links
like Ethernet cable or fibre. The hardware for messaging is
generally built into the unit, meaning they are ‘one-box’ solutions. The RTU may be ’dumb’ (i.e. have no programming capability and merely be a conduit for sending and receiving signals), or may have built in, programmable capabilities. Generally, they are not as sophisticated as a Programmable Logic Controller (PLC), having limited programming capabilities.
RTUs have generally been used in the water industry for remote site applications including:
Pump Station Control
The RTU may control one or more pumps either in direct response to commands or if programmable, may have simple logic to respond to level or pressure signals to start and stop pumps locally. This makes them robust in situations where remote communications is unreliable.
Storage Tanks
RTUs are ideal where very low I/O and limited data requirements exist, such as a storage tank where only level, overflow and possibly intruder security alerts are required. Low capacity radio links can be used where only a few bytes of data are transmitted back to a central SCADA system each time the site is polled. To reduce data transmission even further, Report by Exception is often used, where the RTU will only send data when a change is detected.
Control Valves
Control valves may be installed at sites where they operate entirely hydraulically and no electrical power is present. Very low powered RTUs using batteries or solar power can be used very effectively to permit remote monitoring and control.
Drinking Water Distribution Network and Wastewater Collection System Monitoring
Monitoring of water quantity and water quality in distributed pipe networks is challenging because at the monitoring locations power and communication lines are often unavailable and too expensive to install. Therefore, sensors and monitors at such locations are connected to the SCADA or other central data collection systems through wireless telemetry solutions. In such a case, an RTU often provides the necessary telemetry interface. Some sensors have the RTU functionality integrated in the sensor package, but in many cases a stand-alone, third-party RTU is used.
BENEFITS
RTUs are cost-effective solutions for data gathering and control over wide distances. They are often a single box solution with all power, communications, radios etc., built into a rugged unit that can survive in harsh conditions. Often, they have flexible parallel communications methods so that, for example, loss of a radio link to SCADA is detected and a backup dial-up link used instead.
SYSTEM REQUIREMENTS
RTUs can be installed almost anywhere, including on the side of water tanks, at the top of power poles and inside valve pits. They do require power but this can be supplied with batteries or solar installations where main power is not available. This allows them to even be used in temporary installations (e.g. short-term pressure and flow monitoring during maintenance events). RTUs do not generally require a cabinet and can operate in most weather conditions.
