Wireless sensor networks are taking over.(DATA ACQUISITION)

For industrial applications, Accutech, a division of Adaptive Instruments, provides a range of integrated sensing and signal-conditioning field nodes based on FHSS technology. The battery-powered nodes are FM/CSA/ATEX rated for Class 1, Division 1 hazardous environments with ambient temperatures from -40[degrees]C to +85[degrees]C, and up to 100 can form a LAN with a base radio node. Up to 255 base radios can be deployed in overlapping LANs. Figure 3 shows a field node attached to a range-extending antenna.

According to Wallace Lueders, the company's vice president of sales and marketing, Accutech focuses on process measurement and industrial automation in continuous and batch manufacturing. A large number of industries as well as municipalities and hospitals are served by the more than 2,000 networks installed to date.

A fast and deterministic response time is ensured by the direct communications from field node to end PC or programmable logic controller (PLC) via a single base radio node. It also is possible to add base radio repeater nodes to create redundant paths for higher reliability. In addition, the repeater nodes can create an intermediate network layer by concentrating data from a number of base radio LANs. In these cases, the repeaters would connect directly to the network PC or PLC.

Millennial Net WSNs are based on the MeshScape network protocol that features persistent dynamic routing. This approach is advantageous in systems with rapidly changing topologies. In contrast to the static network representation that might be drawn on paper, actual WSN communications are dynamic, taking various routes through a mesh network as local conditions change.

The basic star, mesh, and star-mesh hybrid physical configurations are supported as are several data models. Periodic sampling is a good solution in temperature or pressure monitoring applications. Event-driven operation better suits alarm conditions that arise when preset limits are exceeded. Controller-based installations can use serial polling. And, when a portable gateway must be added to a network for ad hoc data access and analysis, an on-demand capability also is available.

Millennial Net recently introduced longer range options with output power up to 100 mW, the maximum level permitted in North America, compared to the previous 10-mW limit, still the globally permitted maximum. These new products allow network designers to trade range and power against number of nodes. Where previously many nodes were required to cover a large geographical distance in a hop-by-hop mode, 10x higher power can reduce node count and data latency at the expense of battery life for the high-power node.

GreenPeak features low-power routing, which means that the network does not depend on a backbone of line-powered wireless routers as do some ZigBee networks. Both end devices and routers can sample sensors and control actuators. However, the main distinction is the low power and associated low maintenance requirements of the entire network.

In addition to obvious design considerations that support low-power operation, GreenPeak has developed synchronization techniques. Nodes only wake up during their assigned time slot. Time slots overlap so that communications are possible between adjacent nodes, for example, but not simultaneously among a large group of nodes. This approach strictly enforces hop-by-hop message passing because only a small number of nearby nodes can be listening for messages at any time.

In this case, low power and long battery life have been emphasized at the expense of message latency. However, because the power required is so low, it is possible to use energy-scavenging devices as power sources, totally eliminating battery replacement maintenance.

The zSeries Wireless Sensor System from Omega Engineering features a number of sensors and battery-powered end devices for monitoring barometric pressure, relative humidity, and temperature. As many as 32 end devices can be located up to 300 ft from a line-powered coordinating node connected to an Ethernet network and the Internet.

Data is accessed from a Web browser via the IP address of the coordinating node. The system is low cost and easy to install and doesn't need a PC for operation. With a multipoint-to-single-point architecture, the zSeries does not provide redundancy. The wireless end devices operate at 2.4 GHz and comply with the IEEE 802.15.4 radio specification.

Mesh Networks

Three companies support WSNs in which all the wireless nodes can send and receive information among themselves. The network implementations are proprietary but architecturally similar in having few restrictions on node capabilities.

Arch Rock has adopted IP-based connectivity throughout the WSN. The description of a new product, Primer Pack/IP, given by Brian Bohlig, vice president of marketing, indicates why this is relevant: "Primer Pack/IP extends the original Primer Pack by making it the first commercial implementation of the Internet Engineering Task Force (IETF) 6Lo WPAN standard (RFC 4944) for IPv6 communications over low-power IEEE 802.15.4 wireless radio.

"Primer Pack/IP runs native IP end-to-end, taking the IP protocols beyond their current boundary at the WSN gateway and out to the individual sensor nodes," he continued. "Because of IP's pervasiveness as a global communications standard across industries, Primer Pack/IP sensor nodes will be able to communicate directly with other IP devices.... Network managers will gain direct, real-time access to sensor nodes and the ability to apply a broad range of Internet management and security tools."

At the network layer, distinct protocols enable triple redundancy in the data-collection path, directed routing to task a specific node with an action, density-aware dissemination for reliable propagation of small objects throughout the network, and over-the-air programming and provisioning for disseminating large objects such as major system software updates.

Arch Rock technology has been incorporated in the Advanced Incident Response System (AIRS), an IPv6-enabled network that allows personnel from various local, municipal, and government agencies to communicate seamlessly during an event while minimizing dependencies on the fixed infrastructure of the disaster site.

Dust Networks was one of the first companies developing WSN products and continues to favor mesh networks with no special routing or infrastructure nodes. Data is encrypted for security, and all the nodes are both low power and routing-capable. Robert Shear, the company's director of corporate marketing, focused on the bottom line: "The emphasis needs to be on the payback of the solution--period. Small size is not a primary goal, but ease of use, flexibility, and reliability are.

"Any viable WSN solution must be simple to install by instrument technicians, extremely reliable, and self powered." He contended, "Networks that require extensive RF expertise to deploy or power supply cable runs will not be viable. The revolution with WSNs is not that instruments are now wireless. The revolution is that the networks just work. Now, you can put nodes places previously infeasible with wired solutions."

BBN's WSN experience has a large military element, which necessarily puts higher value on small size and low energy requirements than most commercial applications. Principal Scientist Jason Redi said, "We support full mesh networks with equal peers. Years ago, we used a dynamic hierarchy for routing but found that the complexity in maintaining the hierarchy was a problem. Hierarchy can be used to distinguish different kinds of nodes, but most of our customers have homogeneous systems so peer-to-peer meshes work better for them.

"Many of our sensor technologies are used in military applications," he explained. "Examples include soldiers dropping sensors while clearing a building to determine if someone else enters the structure or watching for patterns of vehicles at certain intersections. These applications usually require multihopping of information back to places where humans can observe it."

For military sensors, small size and low energy requirements are more important than ease of integration. Wireless interoperability in the commercial world wasn't possible until recently, but as people become used to it, size and energy may again become primary considerations because they affect weight and logistics.

Testing WSNs

Test and measurement applications typically involve higher data rates than those compatible with ultralow-power WSNs. The 4-kHz rate of MicroStrain's V-Link instrument is adequate for many mechanical or electromechanical tests, but battery life is relatively short. There no doubt are similar types of test or troubleshooting applications for which higher data rates and shorter battery life would be a good fit.

From a test and measurement point of view, ultralow-power WSNs are important as products that need to be tested rather than a means of performing a test. These networks are proliferating, and being wireless, their performance is subject to degradation from many sources. For example, adding or relocating large equipment in an industrial plant could affect signal propagation. Similarly, extending a WSN or relocating its routers may affect latency.

It's logical then to question how extensively a WSN has been tested before deployment and by what means this was accomplished. As an example, Sun Labs, the applied research and advanced development arm of Sun Microsystems, has developed a software application that supports interactions among real and virtual nodes on a simulated network. The real Java-based nodes, Sun small programmable object technology, or Sun SPOTs, together with development software, can be purchased by anyone who wishes to experiment with this WSN technology.

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