and other expensive materials. Because these tools can cost thousands of dollars, the engineers want technology such as RFID to help them to get the greatest return from their investments.

Devices talk by phone

RFID is not the only technology connecting smart field devices to distributed control systems and other computer networks. Mobile phone technology is another way that field devices are communicating, as the governmental agency in Denmark responsible for managing the streetlights discovered. The technology helped the government to monitor and adjust the lights to match actual conditions, and, in the process, reduce the power consumption by 35 percent.

Generating that much savings required more than simply paying careful attention to altering the times that the lights come on as the seasons change. It also needed to account for current weather conditions, the location of the lights and time of day. Not only were the lights supposed to turn on when storms darkened the skies, but the plan also called for dimming the lights at various locations as the traffic becomes less busy. Clearly, a simple timer, photoelectric switch or human intervention was insufficient.

Moreover, the lights and other devices on the network also needed the ability to keep a central control apprised of their status. and to send diagnostic information about themselves. For instance, management wanted reports of any faults immediately so it could generate reports on the status, dead bulbs, and other problems, and dispatch maintenance crews to fix the problems. This meant establishing two-way communications over a network capable of transmitting data in real time.

Building such a network that connects the control points physically by wire would have been too expensive and unwieldy. So, Amplex, the Aarhus, Denmark-based contractor responsible for the integration work, decided to exploit a network that already exists everywhere in the country—the mobile phone system. It installed a network of terminals using the general packet radio service (GPRS), a wireless communications protocol for transmitting as fast as 115 kilobits per second over the global system for mobile communications (GSM) networks used by most European mobile phone companies.

Amplex’s engineers also specified the Siemens MC 35 mobile radio module, which weighs just 16 grams and is only 6 millimeters (mm) thick. At only 54-by-36 mm, it’s smaller than a business card.

Is telemetry suitable for industrial and other commercial applications? It depends. “If you are in a fixed, permanent location that already has telephone lines and internet hookups, then you wouldn’t use cellular,” admits Peter Fowler, general manager at Siemens Wireless Modules. He also says that the technology is inappropriate for any links that cannot endure the short interruptions that cellular phone services experience occasionally.

Despite those limitations, there are many commercial applications that can benefit from telemetry. “These are sites that need monitoring, but either don’t have communications networks or move around,” says Fowler. “Chemical plants, for example, often have a lot of facilities with tanks and pumps in remote locations. Cellular technology can help them to monitor fluid levels in the tanks and the condition of the pumps.” The alternative is to send a driver to check manually—but that wouldn’t be a very smart use of intelligent field devices.  

For more information, search keywords “sensing” and “smart sensors” at www.automationworld.com.