FCN autonomous controllers, including the FCN-RTU version for low-power applications, are an integral part of Yokogawa’s “STARDOM” network-based control system.

A remote terminal unit (RTU) is a microprocessor-controlled device installed at a location such as a wellsite that collects data from sensing, metering, or other instruments; codes it; and transmits it back to a central station, typically a supervisory control (SCADA) or distributed control system (DCS). The RTU may also receive data from the central station for process execution.

Today’s RTUs go a step further in that they combine in a single unit advanced capabilities for automation, monitoring, and telemetry, i.e., data transmission to a distant point. RTU evolution has to be seen as part of SCADA market development, said Russ Novak, a senior research analyst with ARC Advisory Group. “For years, SCADA was seen primarily as a technology for the midstream. But when oil prices rose, value was seen in automating older fields. SCADA was the platform to hand for doing that. As the digital oilfield concept has taken hold, SCADA becomes one means for getting real-time information to centrally located operations centers.”

As for RTUs themselves, Novak classifies them as either “dumb, smart, PLCs [programmable logic controllers], flow controllers, or other.” Besides these general-purpose RTUs, oil industry service companies market dedicated hardware devices for things like control at the wellhead. And a whole new class of suppliers is emerging for subsea applications.

“When RTUs are replaced,” Novak said, “it’s not because they wear out. Rather it’s because the enhanced intelligence capabilities that result from the incorporation into RTUs of information technology and software are needed. For the same reason, IT departments are increasingly involved in their specification.”

Distance, power, and the weather

According to a recent report from the ARC Advisory Group entitled, “Oil & Gas Industry Growth Fuels SCADA Opportunities and Drives Requirements,” by Dave Woll, “There is an ongoing discussion about whether DCS controllers and PLCs will replace RTUs….[while] much of the technology is the same, we have yet to be convinced that DCS or PLC controllers can effectively replace RTUs for harsh remote environments, stingy power budgets, or utilize the many communication protocols found in SCADA applications.”

Woll also makes a number of predictions when it comes to RTU evolution. He doesn’t see them as at any time having a dedicated human interface, but rather that service or interrogation will take place by means of a wireless handheld device. Further, “Foundation Fieldbus” with “High-Speed Ethernet” capability will have a major impact on their future deployment — lowering costs, easing integration, and leading to even more field intelligence.

Foundation Fieldbus is an all-digital, serial, two-way communications system that serves as the base-level network in automation environments. Its development springs largely from the process-industries world, where proprietary “fieldbus” wars raged amongst the major automation vendors for years, as specifications for the open standard were debated and proprietary standards proliferated.

High-Speed Ethernet works at 100 Mbit/s and generally connects input/output subsystems, host systems, linking devices, gateways, and field devices using standard Ethernet cabling. It doesn’t currently provide power over the cable, although work is under way to address this.

The advent of industrial wireless technologies will only add further impetus to the trends noted above.

Combined capabilities

Semaphore is an Australian-based developer of SCADA, RTU systems, and telemetry solutions. In a recent white paper, “Infrastructure Management and Mobile Asset Management,” the company outlined its take on why RTUs remain a relevant technology.

At issue is the need to balance priorities that include remote site automation, monitoring, and telemetry, as are often needed in oil or gas fields. Data communication equipment (DCE) such as a cellular modem for telemetry; a data logger for monitoring; and a PLC for automation each accomplish their assigned task well, but Semaphore notes the following:
• DCEs provide telemetry, but only limited data logging capability;
• A data logger doesn’t provide automation capability; and
• PLCs today do integrate with DCEs, but pain comes from having to integrate multiple DCEs.

Thus, in a given instance, an automation team might find itself having to install all three pieces, and from different vendors. In contrast, Semaphore said, the latest RTUs combine PLC automation, data logging monitoring, and multiple DCE telemetry capabilities.

Another differentiator, also alluded to by Woll, is that to be suitable for use in the field, RTUs typically operate using battery or solar power.

One further innovation should be mentioned. RTUs today are often equipped with Internet protocol (IP)/Web capabilities. Semaphore said its RTUs include an “integral Web server, IP communications, and “push” messaging via e-mail, FTP, and SMS text.” The gist is that users can access site information by cellular phone, PDA, or laptop, i.e., with Web pages served via the Internet or intranet, email, and text messaging.

Proof in the product

Recent product introductions from two major automation vendors, Yokogawa and Rockwell Automation, further illustrate RTU progress.

In January, Yokogawa introduced new low-power consumption modules for use with its field-control node FCN autonomous controllers, resulting in the FCN-RTU low-power autonomous controller. The controller line is a core element of the “STARDOM” network-based control system. The RTUs include a CPU module with embedded I/O and communication ports; a power supply module; and a base module that is half the size of its predecessor.

Moreover, the company said, with autonomous capabilities that combine the features of control, monitoring, data logging, and transmission, FCN controllers are suitable for use in geographically distributed SCADA applications, and for PLC-type control and monitoring tasks. Both real-time and on-site data recorded on the controllers can be transmitted via a mobile phone network or other technology used to transmit field data.

The low-power consumption module is suitable for temperatures as extreme as -40°F to 158°F (-40°C to 70°C) and at altitudes up to 9,842 ft (3,000 m). The power supply runs on a wide range of voltages, supplied by solar batteries. Advanced control applications can be programmed using IEC 61131-3 languages. A Web server, FTP, and logging functions are embedded.

In February, Rockwell Automation introduced the Allen-Bradley DataSite remote terminal unit controller, a low-power controller for harsh, remote locations subject to extreme temperatures. The company describes it as “a PLC that processes a user program, and controls and monitors the operation of other equipment.”

Rockwell is an automation powerhouse in the discrete manufacturing world, but for the last several years has been making inroads into process industries. An electronic flow meter version of the DataSite RTU, the company said, is tailored for natural gas wellhead monitoring and includes firmware for calculating natural gas flow. Embedded algorithms allow gas flow measurement consistent with American Gas Association standards.

The embedded Web server allows users to display device data and act on controller data remotely over the Internet, without connecting directly to the device.

Final words

In a number of respects, the on-going RTU evolution bears resemblance to what’s happened over the years to its better-known cousin, the PLC. But besides obvious differences related to ruggedization and telemetry, there are some other, more subtle differences. For example, it is said use of the industry standard IEC 61131 programming tool is more widespread in the PLC community, with proprietary development environments persisting with RTU users.

Moore’s Law, telecommunications advances, and the demand for more oil and gas field intelligence have conspired to bring about changes in what makes an RTU an RTU. The result is an increase in the amount of information that can be gleaned from remote sites, and the amount of processing that can be accomplished at the site, with the RTU executing simple programs autonomously, without involvement of the host.