Choosing Wireless Sensor Networks

Proprietary wireless sensor networks have been with us for more than a decade. The standards-based wireless network, IEC 62591 WirelessHART has now been available for more than 4 years. In an Industrial Automation Asia article, Choose Wireless Wisely, Emerson’s Jonas Berge describes some of the background and strengths of this wireless technology for the process industries.

Jonas highlights how 4-20mA wired instrumentation communications with digital protocols evolved from propriety to standards-based:

…every DCS manufacturer used a different protocol and only supported their own devices, locking the plant in. Multiple handheld terminals were required. In the end, the HART protocol prevailed, and thanks to 4-20 mA/HART, most DCS can now communicate with any 4-20 mA device, and only a single field communicator is required.

Jonas describes how the WirelessHART standard went through end user testing as part of the NAMUR NE 124 wireless automation requirements process, where a:

…field trial was organised by NAMUR at the BASF plant in Ludwigshafen, Germany in 2009. The trial included gateways and devices from ABB, Emerson, Endress+Hauser, MACTek, Pepperl+Fuchs, and Siemens. The trial proved that WirelessHART meets process user requirements.

Through tests like these as well as trial and proof in plant applications, he notes that the WirelessHART technology is now in operation in more than 2,100 plants around the globe.

Elements designed into the standard to provide reliability include the ability for the wireless sensor communications to coexist in the same wireless frequencies as other wireless networks. Jonas describes it:

WirelessHART operates in the same license-free 2.4 GHz ISM-band as Wi-Fi and other wireless technologies, but since WirelessHART uses channel hopping and channel black listing, WirelessHART is able to coexist with these technologies.

I highlighted these coexistence techniques designed into the standard in an earlier post, Coexistence and Diversity Techniques for Reliable Wireless. Another way the standard addresses reliability is through the adoption of mesh technology. Jonas explains:

Mesh-topology is more robust because devices establish multiple paths among themselves, routing messages at device level, in a “mesh”. If one of the paths is disrupted, the network automatically switches to another path, thus maintaining a reliable connection.

That is, redundant data pathways eliminate single points of failure. Actual installations consistently demonstrate greater than 99 percent data reliability.

One reoccurring area of conversation around wireless instrumentation is the battery, which powers these devices. Jonas clarifies [hyperlink added]:

One misconception is that some wireless protocols use standard D-cell batteries. However, although the size is the same, no wireless devices use regular carbon-zinc or alkaline batteries. All wireless transmitters use Lithium Thionyl Chloride batteries, which have very high power, long life, and wide temperature range but cannot be found in regular stores. They must be purchased from a specialty supplier or through wireless device suppliers.

He highlights how the standard was designed to permit wireless instrumentation to be used in control loops:

WirelessHART is time synchronised and scheduled with a precisely periodic macrocycle (called superframe), and makes use of publisher/subscriber (called burst mode and catch) communication similar to Foundation fieldbus. The result is deterministic communication.

Another area of conversation is the distances between wireless devices. Wireless sensor networks use the IEEE 802.15.4, which is the:

…standard which specifies the physical layer and media access control for low-rate wireless personal area networks (LR-WPANs).

Jonas describes the communication range as a function of:

  1. Transmission power
  2. Antenna gain
  3. Receiver sensitivity

The same IEEE 802.15.4 chip can be used for WirelessHART or any other protocol and they will achieve the same distance so there is no range difference between protocols. An unobstructed range of several hundred metres is possible with raised antennas. However, this is line-of-sight (ie: no obstruction) as in an open field which you may find in a flat desert oil field.

Give the article a read to understand the elements of the WirelessHART standard and how it helps support reliable, rapidly deployable installations.