1 June 2005
Driving data node to node
By Ellen Fussell Policastro
Wireless sensors gain ground in industrial settings.
A networking technology that ties sensor devices together, machine to machine, and aggregates data typically generated from sensors is finding a home in industrial automation space. Its role? Predictive maintenance, process monitoring, and process control; in fact, sensor networking enables the whole trend toward predictive maintenance rather than preventive maintenance, said Burlington, Mass.-based Millennial Net's vice president of marketing, Mark Pacelle.
"Predictive maintenance involves the monitoring of process conditions," and sensors that measure vibration, temperature, humidity, or acidity are attached to these machines to continuously monitor things like bearing temperature. "When the temperature goes out of an acceptable range, it triggers an alarm. When it goes through a wireless sensor network, it travels back to a control point. Likewise, if you don't want to send an alarm, it'll send a continuous hourly reading of that bearing and the application end of the network," Pacelle said. "And the network would keep a continuous log."
Mesh networking has been around for a long time, and in and of itself, it's not a new phenomenon. The vast field of applications for wireless technology range from dusting a process with tiny wireless sensors to more traditional deployment. At the low end, wireless sensors use mesh networking with auto-adaptive, self-healing capabilities. The news is the way it's being applied to sensor networks in wireless and to some extent to control networks. "It's a nice application for wireless because it allows for mobility," said Hesh Kagan, director of technology for a ventures group at Invensys in Foxboro, Mass.
Installing these wireless sensors on ships for the oil and gas industry to use for temperature and vibration measurement is one instance where mobility comes in handy, said Mike Horton president and CEO of Crossbow in San Jose, Calif. "The drive to go wireless is to eliminate the manual data collection," he said. Historically the process was completely manual. Now, data collectors can get readings automatically, and they automatically network back to the central depository. The advantage is the ship runs with a lean crew to maintain all the equipment. "Having someone walk around and collect data is cumbersome and expensive," Horton said. "With wireless, they can inexpensively install sensors and automatically collect data."
But while mobility is nice in the industrial process world, it's not a driving force for wireless, Kagan said. Mobility addresses a niche market in industrial process, "but it's just a piece of the pie." The bigger question is how wireless is being adapted in industry. What are the constraints? What are the driving forces? "Mesh is just a subset of that," Kagan said. "It's a nice way of bringing multiple things together that are scattered around and of having devices joining the network, removing themselves, and rejoining the network."
You might think one driving force is of course the luxury of not having the cost of running the wire. "The short term view is wireless is slick and can be significant in terms of dollars," he said. (Wiring costs range from hundreds to thousands of dollars.) But of course, that's still a short-term, immediate benefit.
The real driving force is having access to all the incremental data and being able to co-incorporate the data that can save you money day after day, Kagan said. Optimizing and preventive maintenance are big deals in the wireless game. "Most maintenance in a plant today is done in a break-fix modality," Kagan said. "People wait for something to break, and then they fix it." And in the short run, it's probably the cheapest way to do it. But it's "when you get into trouble, when that thing that breaks causes a shutdown, that you have bigger problems," he said. Preventive maintenance gets pretty expensive too because, "essentially, you're throwing away good equipment because a manual tells you after so many hours it needs to be replaced, and it's not necessarily broken," Kagan said.
But the wireless concept is a challenge to get people to adopt because of reliability and cost, said Horton. "The way a mesh network addresses reliability, every node acts as a router, and that's true mesh. That means when you deploy a set of sensors with the true mesh technology built into it, every node can relay messages to every other node," he said. "If a customer deploys 50 sensors in an industrial factory environment, any given sensor will be able to talk to four or five other sensors reliably with one radio jump."
Horton likened the mesh network to the all-too-familiar spider's web—"strong and reliable because it's not dependent," he said. Losing a connection, like you would using a cellular phone, is "totally unacceptable in the industrial environment," he said. The way mesh overcomes that is it doesn't talk to just one base station, it talks to four or five other nodes, and they relay the message back to a central controller. "So there's an increase in strength and reliability that allows technology to be deployed in an industrial environment," Horton said.
Measure for measure
So, reliable routes and mobility are great. But how are you going to measure it? The ultimate way of managing assets is through a predictive type of maintenance as opposed to preventive, Kagan said. "You have to get new measurements on how well that equipment is performing, and you want to bring that new data into an application that can intelligently determine how it's degrading or wearing." Based on that, you can determine what the equipment's maximum use is and how to schedule. "You have to, so to speak, Velcro on these sensors, and low-cost wireless sensors make it very convenient to do that." And it's the only way you can do it in a meaningful way because it's too expensive to run wires for these incremental sensors for asset management, he said.
"Look at the way pumps and valves run until it becomes painfully obvious they need to be maintained. If you sensor that valve, you can determine how it's behaving, so you can determine overhaul. There are innumerable opportunities for process equipment to be maintained in a more rational fashion; you can save tons of dollars. To rebuild a big valve might be $20,000. If you can maintain it in such a way as you prolong its life, there are lots of savings involved," Kagan said.
Temperature measurement is one common application in industrial processes on the factory automation and process automation side, Horton said. "We've built these wireless temperature modules that plug in to an existing system and replace hardwired RTD connections, or thermocouple connections. So instead of a wired connection from the temperature sensor back to the measurement system, you have a battery-powered node, completely wireless, and that takes the temperature measurement and reports it into the same type of control system people have used for years."
Pacelle said his company was working with Emerson Process Control for oil and gas exploration in petroleum plants and refineries. "Those types of customers are interested in implementing this notion of predictive maintenance to save money and to minimize downtime," he said. The main reason is "predictive maintenance is shown to be a more effective way to maintain your physical assets."
One field trial coming out now in a big petrochemical plant is equipment motor bearings (the motors driving big refinery fan assemblies). The motor bearings have a temperature sensor that senses the temperature of lubricant within the bearings. And as long as that bearing housing remains within an acceptable range, the motor is deemed healthy and running appropriately. If the temperature rises outside an acceptable range, "that's an indication something's wrong with that bearing," Pacelle said.
Because temperature is a leading indicator of failure, manufacturers could have perfect information on the bearing temperatures and motor assemblies, and thus, they could predict failure. Wireless allows economical installation of these sensors onto motor bearings. "So it's now economically feasible to send information back to a single control point with wireless networking," Pacelle said. "If they have to run wires to collect this data, then it becomes economically unfeasible because it's so expensive to run wires in an industrial plant. You have to run them through conduit or bury them in cement if it's an explosive environment, and that breaks the feasibility of implementing this type of predictive monitoring and maintenance."
One of the most immediate benefits of being able to use inexpensive wireless sensors is the manufacturer's ability to make inferential measurements—inferring the measurement you want by measuring as many things around the process as possible. Whenever you try to refine oil or make a better cookie, you need to control your process, that includes the temperature, the oven, the speed of the conveyor belt, Kagan said. "You're constantly measuring. If you measure everything around the oven, such as the cookie dough, the temperature, even the weather, and you understand what those other measurements are, you can determine what the moisture of the center of the cookie is."
Collecting data and building models is nothing new, Kagan said, "but you can always build a better model by having more data. If you ask a process operator what's the one measurement they wish they had to control their process better," Kagan said, "they'll say 'I wish I could measure the smell, the texture, the color of the flame'—measurements that are difficult to get."
In fact you could get previously impossible measures. "When you're running a plant, whether an oil refinery or cookie company, if you can improve your process by a tiny fraction, there are big dollars involved," he said. Real-time strategic measurements of asset availability, asset utilization, condition monitoring, smell, taste, turbidity, quality, value, and color would be all but impossible through a physical sensor. "But with enough data points in your model, you can infer real-time measurements," he said, and save a bundle through such inferential measurements.
When you can deploy sensors into a process and collect data inexpensively, it allows you to deploy more sensors in a process, Pacelle said. "You have better information about how a process is working. And because you have better and more information about the health of a process, the better you can tune the efficiency of that process. More theoretically, you have better visibility and better insight into the characteristics of a dynamic process. The dynamic process is what you're looking at in your refinery or manufacturing environment. The more visibility you have, the better decision you can make and the better adjustments you can make in the process to make it more efficient."
Flesh out the mesh
By Tim Cutler
The term mesh networking is generating tremendous interest in wireless industrial automation circles, and for good reason. By providing a means for data to be routed among network nodes, rather than having to pass through a base station, mesh networking creates a self-healing network compensating for obstructions and reducing transmission range requirements. These characteristics could prove valuable to design engineers, who must deal with inhospitable environments; whether their benefits' tradeoffs are worthwhile depends on environments and applications.
Traditions and challenges
Wireless networking has traditionally featured one of two topologies: point-to-point or star. In a point-to-point topology, data moves through the air between two components, such as between a sensor and a PLC. In a star topology, a central device communicates with multiple sensors in separate point-to-point links. This could be a PLC connected to a controlling computer in a plant manager's office. Because so many industrial automation applications are point-to-multipoint in nature, the star topology has become the de facto standard for industrial wireless networks.
While star topologies address most industrial networking needs, you have certain requirements when you use them in industrial settings:
While these characteristics may not be issues in all industrial environments, they could pose significant network design challenges in some environments. In such cases, using star topology may increase cost because you'll need radios with higher RF power. Depending on the application, you might do better with a mesh topology.
Industrial network challenges
Remote nodes can talk not only to a base station, but directly to each other with new low-cost devices, which are boosting the popularity of mesh networking. Whereas a star topology reflects the way cellular networks are constructed, a mesh topology is similar to the Internet in that it is self-healing, finding a path for communications when you lose a connection or a single piece of equipment fails.
These characteristics compensate for the challenges of star networks:
Trade-off challenges with mesh
Dynamic routing enables the node-to-node hopping that gives mesh networking its advantages. Each network device must communicate its routing information to every other device with which it connects; when you lose a communications link, each device must then determine how to pass the data it receives. Routing algorithms typically choose the most direct route to the data's destination; but in any case, dynamic routing will always introduce some degree of latency.
Whether the latency introduces issues depends on the application's tolerance for it. While latency is typically measured in only tens or hundreds of milliseconds, such a delay is not always acceptable. In an industrial automation system that monitors slowly changing temperatures or fluid levels, latency is not likely an issue. In an application that controls a real-time mill operation, any measurable introduction of latency may render the application ineffective.
You might be a mesh user
The industrial environment and the application's tolerance for latency are some things to consider if you think you might want to go with mesh. If a star topology will deliver the data you need, where you need it, then you should think about using it. Star topologies can be easy to implement and maintain. They encompass open-standard and nonstandard wireless technologies (and might therefore cost less). They also don't introduce routing-related latencies. If barriers to communications are limited, you can address existing barriers with a limited number of repeaters.
If you have a lot of barriers or some unexpectedly introduced into the environment, if latency is not an issue, and if the network had multiple nodes in close enough proximity to realize the benefits of node-to-node hopping, you should consider mesh networking. As a means to mesh networking without having to choose a nonstandard technology, design engineers should think about the new ZigBee open standard, a product of an association of companies called the ZigBee Alliance. The standard provides low-cost, open-standard mesh networking that can be embedded in a wide range of products and applications.
Behind the byline
Tim Cutler is vice president of marketing and OEM/industrial sales at Cirronet, Inc. in Norcross, Ga.
Market for mesh
By Ellen Fussell Policastro
Wireless LAN has grown from a doubtful and expensive curiosity into a practical and trendy networking technology in recent years. Predictions say next generation access points (APs) will use wireless communication to transport traffic and communicate with each other to provide a self-constructive, self-healing wireless infrastructure.
IEEE Task Group 802.11 named this adaptive wireless network WLAN mesh. The device that transports traffic through wireless links and provides access capability to its client stations is called Mesh AP. "More and more you can hear startup [companies] saying they have solutions," said Dr. Ted Kuo, vice president of advanced development at Accton Technology Corp. in Sunnyvale, Calif. "People are thinking hard about how to use this technology; some companies are focusing on public access, while other are trying to apply it in the home."
Mesh APs can connect isolated Ethernet LAN segments and extend to a mixed wired and wireless hybrid network. Mesh APs that connect to isolated LAN segments can behave as bridging devices among those LAN segments. The technology works well in large areas that need coverage and where cable runs are impractical. It operates without centralized access points. Instead, each Mesh AP accessing the network acts as a transmission/access point, making mesh networking very scalable and flexible, Kuo said.
Accton has been involved in developing and promoting the 802.11 wireless mesh networking standards from the beginning, and their support of WLAN mesh networking has grown from their belief in the viability and utility of wireless mesh networking.
Yet there is a down side to mesh technology because right now it lacks a security standard. Kuo said he doesn't think security is going to be an issue in terms of mesh. "We do a lot of original design manufacturing (ODM) with networking equipment vendors," he said. "These are the potentials, but it's too early to discuss."
Right now, the technologies being sold are primarily for vendor applications, said David Myers, the company's Taiwan-based director of business development and corporate marketing. "The challenge is it's still in the standardization phase, and you have a lot of hot-spot deployments. Standardization has some kind of effect on customer's buying decisions."
"We know a lot of our customers sell their wireless devices to customers to form a small network by using the wireless bridging," Kuo said. Today, bridging technology uses a core Ethernet switch that can connect all these islands together," Kuo said. "People buying the standard will also use those features."
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