How Sensors Connect with Low-Bandwidth Wireless Nets

The wireless revolution is changing the way that many small sensors and other components communicate. Low-bandwidth networks are being used to gather inputs from many areas where it’s difficult to string wires.

SKF Insight uses a small sensor and wireless technology to make a “smart bearing.”
SKF Insight uses a small sensor and wireless technology to make a “smart bearing.”

As with Wi-Fi, network managers are turning to IEEE standards that ensure compatibility and a range of product offerings. IEEE 802.15.4 transmitters typically consume much less power than Wi-Fi. The tradeoffs are that these networks have much lower data rates and shorter distances.

Battery-operated nodes can gather data, send it and then sit in sleep mode for long periods. That makes it possible for batteries to run a few years.  These capabilities fit especially well for sensors that must be located in hard-to-reach spots.

“One application is at the top of a smokestack where a sensor measures pollutants. It’s very difficult to run wires to the top of some industrial smokestacks,” says Doug Bellin, global senior manager for manufacturing and energy at Cisco. “We have a partnership with Honeywell for products that collect data using wireless 802.15.4 to collect data. After it’s gathered, it’s converted to 802.11 and sent along using Wi-Fi.”

Distances can be extended by creating mesh networks, in which nodes send data to a neighboring module that will relay it to its destination. Properly configured, mesh networks are quite fault tolerant, since new routes can be created when a node fails. This technique can also be used to increase battery lifetimes.

“The transmit power of a radio is directly related to the amount of power the radio draws,” says Justin Shade, wireless product marketing specialist at Phoenix Contact. “Since low power devices cannot transmit as far, they use other nodes as a repeater and send their information through those devices to get it to its destination.  Each device has multiple paths back to the host, and therefore is very reliable.”

Sensors inside
Wireless nodes also make it possible to put sensors inside equipment. Early this year bearing maker SKF (www.skf.com) unveiled an industrial bearing that includes a sensor and wireless transmitter for collecting and sending vibration data to systems that can analyze changes. Getting vibration information from deep inside the machine lets technicians see potential reliability problems far earlier than traditional vibration sensors that are farther away from the bearings, according to an SKF spokesman.

While mesh network topologies have many benefits, they also pose some challenges. One is that they don’t interact well with tablets and smartphones, which are increasingly being used to gather and display the types of diagnostic input collected by wireless nodes. “802.15.4 is very neat because it automatically starts a mesh when it’s configured” Bellin says. “However, most handhelds don’t run any mesh protocols, so people need to do something to make them work with tablets.”

Mesh networks may face some competition from Wi-Fi in applications where battery lifetimes are a critical factor. Semiconductors for the ubiquitous network are constantly being improved as competitors vie for an edge. Trimming power requirements is an important focus for many consumer products.

“Wi-Fi has the perception that there are issues regarding power consumption,” says Brian Bedrosian, senior director for embedded wireless at Broadcom. “We’re making progress with system-on-chip technology that will make it possible to see more battery-operated Wi-Fi."

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