4-20 mA, Fieldbus, or Wireless Technology — There Are Pros and Cons Either Way for Transmitting Field Data

Related Vendors

DÜPERTHAL Sicherheitstechnik GmbH & Co. KG

TURCK GmbH

EagleBurgmann Germany GmbH & Co. KG

Phoenix Contact Electronics GmbH

WirelessHart and ISA100.11A Have a Limited Range

Various wireless systems are suited for use in process engineering. A comparison of the WirelessHart and ISA100.11A standards shows that both systems promise the same performance. Differences become apparent when integrating them into existing applications. WirelessHart will gain a market foothold more quickly here, since Hart communication already forms an integral component of many 4-20 mA field devices. These devices can thus be integrated into the wireless network using wireless data adapters. The field variables can still be transmitted via the 4-20 mA signal.

New wireless field devices have an antenna and batteries for transmitting the variables wirelessly. Since the field device antenna is screwed securely into the housing, it cannot be oriented optimally or installed in a better position. Before mounting, the user should therefore measure the field strength at the installation site.

WirelessHart requires the installation of an extensive wireless infrastructure. So-called wireless gateways are installed in the field for this purpose that act as interfaces between the wireless field devices or wireless adapters and the process control system. The wireless gateways are equipped with an antenna over which they exchange data with the field devices. The gateways have a copper-based Ethernet connector for connecting to the process control system LAN.

Phoenix Contact’s WirelessHart gateway offers both a LAN and a WLAN interface, making LAN cable installation unnecessary (Figure 3). However, WirelessHart and ISA100.11A have the drawback of being limited to 10 mW of transmitting power, which restricts their range. For longer ranges of up to three kilometers, proprietary wireless solutions having 100 mW of transmitting power, such as Trusted Wireless, have to be used.

Conclusion: Wireless technology will not supplant 4-20 mA field devices or fieldbus technology. Facilities that have evolved over a period of years most typically use a combination of all three solutions. New facilities will presumably use the fieldbus instead of 4-20 mA technology. Wireless systems find use in areas where it is not possible to install cabling.

* The author is International Project Manager Process, Phoenix Contact GmbH & Co. KG, Blomberg/Germany.

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