Machine Design

Industrial Sensing the wireless way

Sensors that talk to each other over wireless links capture data that was impractical to collect before.

Industrial sensing the wireless way

Mark Pacelle
Millenial Net Inc.
Burlington, Mass.

Mesh network datapaths

In mesh networks, each wireless sensor acts as a router, sending and receiving data from other sensors or the gateway. Selfconfiguring networks automatically determine the best path for data to take from sensor to gateway. Data is automatically sent around failed sensor routers.

Star-network datapaths

Using star topology, each wireless sensor end point sends data directly to the gateway. From there data is sent on to other systems. Star networks offer the fastest data gathering speed.

Star-mesh network datapaths

Star-mesh networks combine star and mesh topologies to gain the speed of the star network with the self-repairing capability of the mesh network. Sensors may be either end points or routers, depending on where they are used in the system.

Information is power. This is not news. Organizations understand the value of information and invest heavily in gathering, integrating, and analyzing data to reduce costs, refine quality, boost revenue, and improve customer satisfaction. Enterprises constantly look for ways to create more "eyes" and "ears" that gather data and delve ever deeper into processes.

There is, however, a point of diminishing returns where the cost of getting more data exceeds the value of the information it would provide. That is where wireless-sensor networks come in. They constitute a breakthrough technology that can overcome existing barriers to make the data-gathering process more economical.

Consider the following scenarios. A manufacturer able to sense bearing wear in the electric motors driving its assembly lines could keep production running through preemptive maintenance. Petroleum refiners that knew how full their far-flung tanks were could save money by ensuring nearly full tanks before sending out trucks to empty them. A building manager could monitor room occupancy and remotely control temperature and humidity in real time to cut energy costs but still give occupants superior comfort. And all accomplished without running one additional foot of wiring.

The technological innovation that makes these scenarios possible and practical is self-organizing, wirelesssensor networking. The cost of implementing a wireless-sensor network can vary widely but the benefits are clear. According to the market research firm ARC Advisory Group, the cost of installing a wireless network is roughly 10% that of a hard-wired design. And new networks can be up and running in as little as a few hours.

All self-organizing, wireless-sensor networks share three common features: small form factor, long battery life, and a robust and efficient networking protocol. Form factors as small as 25 15 mm let these devices fit inside or attach easily to other equipment. An ability to operate from as low as 40 A at 3 V lets such devices get power from small, coin-size batteries for extended periods of time.

Several network properties should be examined while planning a wireless installation. Latency is the time required between sensing a change in the acquired data and taking action on that change. Scalability identifies how many components the network can support in any given installation. And a final factor is how well a network responds to changes in operation because of interference,-malfunctioning devices, or other nonstandard conditions. A robust networking protocol provides low latency responses measured in milliseconds, high scalability supporting hundreds to thousands of networked components, and fast network response in correcting errors and acquiring signals. This last property is particularly important when sensor nodes are mobile as in medical and industrial instrumentation.

Wireless sensor networks have immediate utility in various industrial, medical, consumer, and homeland-security applications. The architectures for the networks include star, mesh, and star-mesh hybrid topologies. Which is appropriate for the tasks at hand depends on how much and how fast data get transmitted, transmission distances involved, battery life, and the mobility and degree of change in the sensor nodes.

A star topology is a single-hop system in which all wireless sensor nodes are within direct communication range (usually 30 to 100 m) of a base or monitoring station called a gateway. Among wireless-networking topologies, the star consumes the least overall power but is limited by how far the radio transmitter in each node can send. This topology suits installations that need the lowest power consumption over limited geographic range.

Mesh topologies are multihopping systems. Here wireless sensor nodes called routers "hop" data to each other and to a base station. The network is self-configuring for the optimum data path of each node. A node failure makes the network automatically reconfigure itself sending data around the problem. A mesh network is highly fault tolerant because each sensor node has multiple paths by which it can get data back to the base station (gateway) and to other nodes. The multihop technique gives much longer range than a star topology but consumes more power. This is a consequence of the network's higher duty ratio. Sensor nodes must always "listen" for messages or for changes in the prescribed routes through the mesh. Depending on the number of nodes and the distances between them, the network may also experience high latency as sensor data hops node-to-node on its way to the base station. This topology is best when there is a premium on high redundancy but node power and battery life are not major concerns.

A star-mesh hybrid topology combines a star network's low power and simplicity with the extended range and self-healing property of mesh networks. A star-mesh hybrid organizes sensor nodes in a star topology around routers or repeaters which, in turn, organize themselves in a mesh network. The routers serve both to extend the range of the network and to provide fault tolerance. Because wireless-sensor nodes can communicate with multiple routers, the network reconfigures itself around the remaining routers if one fails or if a radio link experiences interference. A star-mesh network offers the highest degree of sensor-node mobility and flexibility for rapid changes to the network. Overall, it consumes the least power for networks that must stretch beyond 30 to 100 m. This topology offers redundant routes all the way to the end points while minimizing end-point power.

Among various industrial, building and home-wireless applications, " periodic sampling," "event driven," and "store-and-forward" represent the most common choices for acquiring sensor data.

Periodic sampling serves in applications where a certain condition or process needs constant monitoring, as with temperature in a conditioned space or pressure in a process pipeline. Sensor data are acquired from remote points and forwarded to a data-collection center periodically. The sampling period mainly depends on how fast the condition or process changes and the intrinsic qualities to be captured. In many cases, the dynamics of the condition or process to be monitored can slow down or speed up over time. Sensors that are dynamically adjusting transmit readings more often during rapid changes in the process and less often during slow changes prolonging battery life.

Event-driven sensors transmit only when a threshold is reached. Common examples include fire alarms, door and window sensors, and instruments that are used intermittently.

In still other applications, a remote node can capture, store, and even process sensor data forwarding it to the base station. Remote nodes can aggregate and process data instead of transmitting it instantaneously. This consolidation can potentially cut network power consumption and boost bandwidth efficiency.

Wireless-sensor networking is already a reality, though continued innovation will further increase its potential. Improvements in the cards include smaller components, faster data-exchange rates, longer ranges, better battery life, and even batteryless sensor nodes that run from energy in their environment.

A new wireless standard, IEEE 802.15.4, now addresses the power, transmission distance, and data-rate requirements for wireless devices. A growing number of consultants, systems integrators, sensor, and other hardware manufacturers will follow to support wireless-sensor networking applications as part of their business offerings and product lines.

End points End points directly interface to sensors and actuators
Gateway The gateway, which aggregates data from the network, interfaces the sensor network to the host, LAN, or the Internet.
Routers Routers extend network area coverage, route around obstacles, and provide backup routes in case of network congestion or device failure.
System software The system software provides the core protocol and network routing and optimization logic that enables the hardware modules to configure themselves into a robust ad-hoc network and route data to the gateway in an efficient way.
Network monitoring Provides a configuration interface to the sensor and a and configuration software view into the operational activity within the network.
API Application Programming Interface: a robust set of software interfaces provide integration capabilities between the end point and the sensor and between the gateway and the application. APIs are the primary integration facility for developers and OEMs.

ARC Advisory Group Inc.,

(781) 471-1000,
Millenial Net Inc.,

(781) 222-1030,

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