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:
- It helps ensure that all the wireless devices will be within a reasonable signal range of each other, since they’ll be placed in a relatively limited area.
- With several devices in the same area, there are more available communication paths for routing messages around obstructions or other interference.
- It’s easier to integrate the data into information systems if all data sources follow the same organizational structure—which in most plants is based on process units.
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.
Plan Locations, Validate Design
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.
Gateway First, then Add Devices
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.
Fortify and Expand
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.
Overall, the wireless devices in your self-organizing network will have good connections if it meets the following criteria:
- At least 99% of messages sent by each device reach the assigned gateway.
- At least 70% of transmissions between two nodes (one “hop”) are successful.
- Device batteries last as long as expected.
- Radio signal strength in the gateway diagnostics is good. This check can be misleading on its own (weak signals can still get through if the path is stable), but it can help identify a problem when it arises.
When it comes to adding devices to your first WirelessHART self-organizing wireless network, remember that, in general, bigger really is better. In fact, the more wireless nodes in the network, the easier it is to expand. It really is that easy.
Rules of Thumb for Manual Network Design
Although a new breed of automated tools is available to streamline wireless network design (see sidebar, p7) manual rules of thumb can also be used. Starting with a scale layout of the process unit or area, draw connecting lines between each planned wireless device and neighboring wireless devices that meet any of the following criteria:
As a best practice during the design phase, each wireless device should be connected to three other wireless devices, even though the wireless connection distances may vary by direction. Having three connections during the design phase ensures each device has two alternate connections after installation.
If a wireless device does not have three connections during the design phase, then add additional measurement points or use a range extender to fortify connectivity.
There should not be any connectivity lines between wireless devices in the following situations:
Australian Terminal Keeps Bitumen Flowing
At Terminals Pty.’s facility in Geelong, Victoria, Australia, bitumen is unloaded from ships through a pipeline 3,000 ft (900 m) long and 8 in. (200 mm) in diameter. Because bitumen solidifies at ambient temperature, electric heaters operate all along the pipeline to keep the bitumen hot (160 ºC) and fluid. If a heater fails, a cold spot could form, causing the bitumen to solidify and plug the line, an expensive problem.
The terminal chose wireless technology, Siklic says, for its lower initial cost and minimal maintenance as compared with hard wiring. Eight Rosemount wireless temperature transmitters are evenly spaced along the pipeline, sending temperature readings on one-minute intervals to a Smart Wireless Gateway on shore that channels data to the AMS Suite predictive maintenance software used for instrument configuration and performance monitoring. The collected data also are forwarded to a SCADA system in the terminal control center via fiber-optic cable.
Due to the self-organizing nature of this technology, each wireless device acts as a router for other nearby devices, passing the signals along until they reach their destination. If there is an obstruction, transmissions simply are rerouted along the mesh network until a clear path to the Smart Wireless Gateway is found. All of this happens automatically, without any involvement by the user, providing redundant communication paths and better reliability than direct, line-of-sight communications between individual devices and their gateways.
“This is an ideal application for wireless,” Siklic says. ”Since numerous paths exist to carry the transmissions, the network would easily compensate for a transmitter failure, and the operators would be warned. This wireless network has proved to be reliable, compatible with existing control equipment and cost-effective.”
New Tool Eases Wireless Network Design
While there are well-developed rules of thumb for manually validating that a WirelessHART network configuration will provide adequate connectivity (see sidebar, p5), a new engineering tool from Emerson Process Management now makes the job even easier.
Called the AMS Wireless SNAP -ON, the tool allows users to drag and drop devices and gateways onto a plant layout, then easily validate and optimize the network design against known best practices. 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.
To design a wireless network using the AMS Wireless SNAP -ON, the user first imports an image of the process area where the network will operate (top image). Then he or she sets the image’s scale by drawing a line across any two points and typing in the distance. The user then designates whether the process area represents an environment of high, medium or low density of process equipment.
The user then drags and drops desired WirelessHART devices and gateway(s) onto the plant layout. Then the user automatically validates the design against best practices planning parameters (middle image). In this image, the red circle indicates a violation of best practices that might be addressed by adding another measurement point or wireless repeater to complement current communication paths.
For wireless networks already in operation, the user is able to see the device icons from the HART Device Descriptor (DD) and the self-organizing network communication pathways (bottom image).
LAPEM Streamlines Efficiency Testing
On behalf of Mexico’s Federal Electrical Commission (CFE), wireless technology is helping to streamline the measurement of thermal efficiencies at power generating units throughout the country.
The ease of use and the reliable performance of Emerson’s Smart Wireless system resulted in a decision by the Laboratory Analysis group to equip all five of its analytical teams with wireless instrumentation. Their productivity is expected to increase by another 40 percent with faster turnaround time between services. As a result, all five teams should perform 25 more assessment services per year, producing an extra US $1,375,000 annually without adding personnel. Each of the 140 power units can now be visited and analyzed every other year.
“In the past, we could only cover about 50 plants per year,” said Oscar Martinez Mejia of LAP EM. “We needed to reduce turnaround time at each plant in order to reach every plant on a two-year cycle. Emerson’s Smart Wireless made it possible for the team equipped with wireless devices to cut their on-site time by one-third, enabling them to complete more services in a year’s time and proving the value of wireless.”
“It takes 15 days to install and commission wired instruments, take the readings, and tear down the setup,” Martinez Mejia said. “Then, another week is needed for reporting and other activities before a team can move on to the next plant. In the future, they will be able to cover 75 plants per year, because the on-site work can be done in just 10 days using wireless devices.”