Tips and Tricks in Deploying Wireless Field Networks

Since the WirelessHART standard was ratified in 2007 and approved as the global IEC 62591 standard in 2010, its use among process manufacturers has continued to grow. A release from last fall noted:

IMS Research reports that worldwide shipments of industrial products that are wireless-enabled are set to grow from an estimated 1.2 million in 2009 to more than 3 million in 2015, at an average annual growth rate of 18 percent.

With this as background, I wanted to highlight a Control Engineering Asia magazine article by Emerson’s Jonas Berge, Thinking Through Wireless, which shares tips & tricks for deploying wireless in process automation applications.

Jonas highlights the need to select a wireless protocol that has wide supplier support and tested interoperability. Beyond the actual wireless field devices, this interoperability should extend to:

…handheld field communicators, laptop software, and documenting calibrators your plant already has and your technicians are familiar with, you can simplify work and avoid specialized software and tools.

Network planning is a consideration for wireless networks. Networks based on wireless meshes can adapt to changing plant radio frequency (RF) conditions. Jonas notes:

Pumps, motors, cars, and the weather can change the RF patterns and will vary unpredictably. Instead, use network planning tools that can account for the total mesh network and varying obstruction density. Mesh networks will adapt more easily to changing RF conditions than other networks.

Jonas counsels to spread out the network for high availability and reliability:

…do not lump all these transmitters onto the same network. Instead, create one smaller dedicated network for each plant area, each gateway associated with that area’s DCS [distributed control system] controller. This fits nicely with responsibilities divided by plant area.

For more critical wireless measurements:

…consider use of redundant gateways to increase availability. Remember to include UPS or redundant power supplies and transient protectors (if required). Redundancy doesn’t help if the power is lost to both.

Unlike wired field devices that are powered by the I/O card to which they are attached, wireless devices require batteries. The life of the battery is a function of the device’s update communications rate, and the amount of wireless connections it has to other devices to relay communications back to the wireless gateway. Jonas clarifies the relative impact of each:

The biggest impact on battery life is measurement updates, since the sensor must be powered. Although the radios in a mesh network may be powered more frequently to enable mesh routing, this has minimal impact on battery life. Take a moment to decide what update rate is really required. Depending on the application, a temperature check once every minute is often sufficient, and vibration checks every half-hour are appropriate.

Depending on the communications update rate, up to 10-year battery life is possible in a mesh network topology:

…because the routing function only turns on the radio, not the sensor, measurement electronics or display, all of which consume the most power.

Jonas highlights another concern often expressed around wireless networks—security. He lists important security requirements including: encryption, authentication, verification, anti-jamming, key rotation, and sequence number. In an earlier post, Coexistence and Diversity Techniques for Reliable Wireless, I shared the number of wireless diversity techniques to establish secure, reliable communications between the wireless devices as they create the wireless field network. In this article, Jonas adds:

You should also let the gateway periodically rotate (change) the encryption keys automatically to make hacking more difficult. And to avoid the security risk of “sticky notes” with passwords written down, consider using intelligent device management software which can automatically generate the network join key (password) and automatically transfer it into the wireless transmitter using a wire modem.

Finally, with regard to using wireless devices in PID control loops, Jonas offers:

For PID control, although no network currently has a wireless valve positioner, highly reliable wireless networks can be used for measurement input into PID loops.

Give the article a read if you are considering applications with wireless field devices.

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Posted Tuesday, January 31st, 2012 under Interoperability, Wireless.

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