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Cost savings are an oft-cited advantage to using wireless networks in industrial environments. Eliminate the wire and conduit or armored cable—together with the I/O and engineering costs associated with them—and, presto! Costs for an incremental process measurement plummet by as much as a factor of 10.
But for a growing roll call of leading process manufacturers, it’s not the cost-saving aspects of wireless networks that are the primary driver for adoption. Rather, it’s the easy part. Indeed, all wireless field networks are easier and less costly to install than traditional wired systems, simply because they’re wireless.
But unlike line-of-sight or point-to-point wireless approaches, self-organizing mesh networks, such as those based on the WirelessHART standard, don’t require detailed site surveys or specialized equipment to implement.
With a brand new generation of planning and management tools, it’s easier than ever to ensure that your first wireless effort performs optimally from the start.
For your first WirelessHART self-organizing wireless network, it’s best to focus on a logical plant area or single processing unit, such as a tank farm or distillation unit. Doing so has three primary benefits:
If a process unit is of complex design, for example, an enclosed multiple-floor manufacturing facility, then it may be optimal to scope a wireless network to each floor. For process facilities that are extremely compartmentalized by steel and concrete, you may want to treat each large enclosure as a process unit.
The wireless devices’ role as routers in a self-organizing network requires enough devices in proximity to each other to support reliable communication paths.
The first step in planning your first wireless network is to obtain or create a scaled drawing of the process unit or area where the network will be installed. (For an outdoor facility, the images available on Google Earth—http://earth.google.com—can be used to create one.) Within the scoped area, identify the measurement points that satisfy current and future application needs. With the scaled drawing completed, you have two choices going forward. Wireless device locations can be plotted by hand, and the anticipated reliability of network communication gauged by the guidelines listed in the sidebar, “Rules of Thumb for Manual
Network Design,” at the end of this article.
Another option is to use Emerson Process Management’s new AMS Wireless SNAP-ON tool to validate a planned network’s design easily and automatically (see sidebar at the end of this article, “New Tool Eases Wireless Network Design”), and to optimize the network’s ongoing operation.
Once the device locations are plotted and validated, choose a location for the wireless gateway that provides power, is convenient for the physical (or possibly wireless) connection to the host control or information system and, ideally, provides a direct wireless connection (without a “hop”) to 25% of the wireless devices in the network.
For even the smallest networks, have at least two devices that communicate directly with the gateway. For larger networks, a useful rule of thumb is one additional directly connected device for every eight devices in the network.
You must follow two essential rules when you install your first self-organizing network: Install and power up the gateway first; then the wireless devices nearest the gateway.
For the highest signal quality, install the gateway outdoors (minimum rating of Class I Div II or Zone 2) at least 3 ft (1 m) above other canopy structures, such as above the roof of a control room. If outdoor mounting is not an option, connect the gateway to a remote omni-directional antenna using a cable no longer than 20 ft (6 m).
Once the gateway is up, start with the field devices that are closest to the gateway. Most WirelessHART devices, including Emerson Process Management’s Smart Wireless instruments, have process connections and mounting engineered to the same practices and systems that govern wired instrumentation today, with the exception of the loop wiring.
Once the first devices are working, you can be confident of a reliable communication path for the others and a solid foundation for expanding the network. You can use repeaters to temporarily strengthen the network until all the devices are installed or until the network surrounds an entire process unit completely.
Once you’ve verified that each device has joined the network and is communicating properly, identify any “pinch points,” where messages from several wireless devices must all pass through a single device or repeater at any point on their way to the gateway. Use additional repeaters or measurement devices to eliminate this vulnerability.
Emerson’s new AMS Wireless SNAP-ON tool (see sidebar "New Tool Eases Wireless Network Design" at the end of this article,) makes this task especially easy by graphically displaying network traffic patterns. Further, once the network is up and running, the tool allows the user to maintain the network easily by graphically displaying network traffic and diagnostic data.
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