Fletcher and his team decided on a multidrop configuration with existing wired HART transmitters, but connected to MACTek "Bullet" WirelessHART adapters, since each Bullet can handle up to eight HART transmitter inputs in multidrop mode.
"Far fewer cables have to be deployed now, and situations that placed cables at risk of damage (i.e., crossing roads) can now be handled by radio links instead," Fletcher reports. "Now, we are collecting the data in real time directly into an Excel spreadsheet via Matrikon Analytic's Excel Reporter plug-in for OPC data."
This application is a classic example of how WirelessHART and wired HART work together. Both are HART, and HART is robust enough to operate in the challenging environment of a power plant and have the devices and gateways continue to communicate reliably.
How Do I?
Wired HART installations are very straightforward, whether they are 4-20mA home-runs or multidrop installations. WirelessHART is somewhat less so for two basic reasons. First, WirelessHART is new, and not everybody understands how to install and commission working (or wireless) systems, and second, the fact that it is a radio-based system means that WirelessHART is a little trickier to install than a standard
In a brownfield application, of course, you can apply trial and error like the Verizon man ("Can you hear me now?") and end up with a correctly sited WirelessHART device communicating to a gateway. In a greenfield application, things may be a little different.
Pepperl +Fuch's Gerrit Lohmann says, "There are some rules one has to consider when installing a WirelessHART Network," he says. "Generally, when the devices can ‘see' each other in a line of sight, they will communicate.
Emerson's AMS Snap-On tool for designing
This would be the key when designing a network with no existing tanks or structures. Depending on the packing density of an area—which determines the maximum communication range—one can estimate if the devices can talk to each other over the distance. Finally, a check on the number of instrument ‘neighbors' is necessary. There should be two network neighbors at a minimum in a mesh network, while three is better. If a device has only one neighbor, or the distance between them exceeds the site constraints, a router may be required."
Endress + Hauser's vice president of solutions, Victor Wolowec, is not so sanguine. "A site survey of an existing facility should always be made before any WirelessHART device is installed. This is a standard part of Endress + Hauser's solution offering. The site survey determines the best position for the gateway relative to the devices and determines whether any repeaters are necessary. The adapter concept has a certain advantage here, since it allows the dipole antenna to be placed at some distance from the measuring point."
Vendors have started to include wireless network managers in their product portfolios for WirelessHART. Since late in 2008, Emerson Process Management has been including an AMS "Snap-On" that helps users design WirelessHART networks.
Bob Karschnia says, "AMS Wireless Snap-On is a tool that can be used with plot plans or simple scaled drawings to layout the network. AMS Wireless Snap-On can be used as a stand-alone application for site planning and in a real-time mode to monitor the network. This tool incorporates the best practices recommended for planning a network to minimize the chance that a device won't be able to join the network."
The self-organizing mesh capability of WirelessHART (IEC 62591) does not require a line of sight for a robust network. If devices do not join the network, there are several options, including changing the gateway placement, changing the height of the device to get above obstacles, using an antenna with a longer range or adding strategically placed repeaters. Karschnia says the AMS tool can help identify "stranded" devices and visualize how to remedy the issue.
"By either following these best practices or using the tool, EPCs can be assured that the wireless will work with no issue. More important, documents and procedures that are needed by EPCs are now available to help them execute projects with wireless content. The bottom line is that wireless has been made easy," Karschnia concludes.
With the new features of HART 7 and the wireless capabilities of WirelessHART, it is easy to see why HART remains the world's leading field communication protocol for the process industries.
HART Plant of the Year Award
The HART Plant of the Year Award is presented annually by the HART Communication Foundation to recognize innovative use of HART Communication in real-time industrial process plant applications. It is the only public award presented to end user companies to recognize ingenuity in the application of the HART Field
The best candidates are plants that use the full range of HART capabilities—that have taken the capabilities of HART instruments beyond configuration and calibration, or are using real-time diagnostics and process variables of HART-enabled devices integrated with control, information and safety systems.
To nominate your plant, go to www.hartcomm.org.