Wireless Act

Combine sensors with tiny radio transmitters, and interesting applications result.

In a bridge, an embedded strain gauge wirelessly alerts engineers that heavy trucks are exceeding the bridge's safe capacity. In a football helmet, embedded wireless accelerometers send data that helps medical researchers learn how concussions occur. Soon, implanted sensors will wirelessly inform surgeons if a newly transplanted organ is getting inadequate blood flow. Wireless sensors are literally enabling applications that would otherwise be impractical or even impossible.

Choosing the right wireless sensor for an application can be difficult, however. Some wireless technologies don't work well in rain or fog. Others are susceptible to disabling interference from radio noise sources like motors, welding equipment, and cordless phones. Some don't have enough range. Still others are power hogs, making battery operation impractical.

To complicate matters, there's no wireless sensor standard. The Institute of Electrical and Electronics Engineers is working on one, IEEE 1451.5, but it isn't likely to gain formal approval until early 2005. In the meantime, proponents of various wireless technologies are jockeying for inclusion in the standard. Some of these technologies have technical advantages; others have marketing muscle. Few have both, and all are incompatible.

So, if you want to use wireless sensors, you face confusing options from the technologies that are available.

Some sensors go wireless with Bluetooth, the consumer-oriented technology designed to eliminate computer cables. Others use 802.11b, or Wi-Fi, the wireless LAN technology used in offices and increasingly in airports, hotels, and restaurants. A technology known as ZigBee, based on the HomeRF technology, is also starting to receive serious consideration for sensor use. Currently, however, most wireless sensors incorporate proprietary technologies, which their providers say deliver a variety of technical advantages.

Many technology trade-offs

Wireless technologies have different strengths and weaknesses, each of which affects a sensor's suitability for different applications. Sensor-system company Crossbow Technology, Inc. (San Jose, CA), which two years ago developed a Bluetooth-based wireless data logger, has since augmented its product line with other wireless technologies to suit different customers' needs. "What we found, as we got into a variety of sensor applications," says Crossbow Vice President John Crawford, "is that one size doesn't fit all. Bluetooth was fairly expensive, and its architecture is very complex. Also, the power consumption was pretty significant."

Wireless sensors: Sending Data where you need it to go.

Many sensor experts say that Bluetooth and Wi-Fi simply aren't well suited for industrial applications. For example, says Steven Arms, president of sensor company MicroStrain (Williston, VT), "Bluetooth tends to be rather power hungry for a very short communications range." Wi-Fi modules have better range, Arms acknowledges, "but the 'gotcha' is that you have to plug them in. Those receivers are quite power hungry." Crawford says that Crossbow, after consulting with numerous customers in many different application areas, concluded that, "Wireless really means wireless. If you have to run wire for power, you might as well run the signal, too."

Bluetooth does have its proponents, however, with most citing potentially low costs due to its huge consumer market.

And Bluetooth does have features that make it appealing for industrial applications. Bluetooth nodes can detect other Bluetooth nodes, for example, and automatically form networks. Bluetooth is also fairly immune to electrical noise.

Bluetooth can, in some circumstances, interfere with Wi-Fi, however, and Wi-Fi is increasingly appearing on factory floors. In most cases, the two technologies coexist well, but an optional and still largely unimplemented higher-powered version of Bluetooth-which could alleviate Bluetooth's range limitations-could upset that balance.

To guard against interference from industrial and other noise sources, many wireless technologies, including Bluetooth and Wi-Fi, incorporate spread-spectrum technology. A spread-spectrum transmitter spreads a wireless signal over a range of frequencies rather than a single frequency, thus making it more immune to interference from a narrowband noise source, such as a motor.

Running interference

Wi-Fi's direct-sequence spread spectrum (DSSS) technology spreads a Wi-Fi signal across a 22-MHz wide channel. Bluetooth's frequency-hopping spread spectrum (FHSS) technology causes a Bluetooth signal to "hop" around over a much wider band, but the signal occupies a very narrow band at any one time. Both DSSS and FHSS provide considerable protection against electrical interference.

But spread spectrum is no cure-all. Wi-Fi, for example, was designed for a high throughput of 11 Mbps, says Wayne Manges, program manager of industrial wireless programs at Oak Ridge National Laboratory (Oak Ridge, TN), and that led to some performance compromises. In particular, it reduced Wi-Fi's process gain-the ability to pull a weak signal out of noise-and therefore its interference immunity. Bluetooth, Manges says, has no process gain at all. "Its noise immunity comes simply from brute force. If it gets clobbered at a particular frequency, it just doesn't go to there anymore." In the presence of broadband noise, such as from welding equipment, that could leave Bluetooth with nowhere to go.

Bluetooth and Wi-Fi aren't the only wireless technologies used with sensors, however. Others are available from a handful of companies that include Axonn (New Orleans, LA), Crossbow, Endevco (San Juan Capistrano, CA), Graviton (San Diego, CA), Microprocessor Designs, MicroStrain, and Phoenix Contact (Harrisburg, PA). Most of these technologies are proprietary, not standard, but they provide options for range, interference immunity, power consumption, and cost.

For example, says Finkelstein of Microprocessor Designs, you might spend $100 on a Wi-Fi module, but only $10 or $12 for the components to implement a wireless surface acoustic wave (SAW) sensor. There are tradeoffs, of course, says Finkelstein. In the SAW solution, "The power is only 1/100, but you're going 1000 times slower, and you're not going as far, and you're going at a fixed frequency." Furthermore, says Finkelstein, the fixed frequency-as opposed to spread spectrum-leaves you susceptible to interference. "If you don't have more power than everybody else who's transmitting, you lose."

Alternatives

Between overkill and overly simple, however, are some alternative wireless technologies. MicroStrain and Crossbow, for example, both offer 916-MHz and 418-MHz technologies, as opposed to the 2.4-GHz band for Bluetooth and Wi-Fi. Signals at 418 MHz are relatively free of interference, both companies claim, because they don't face competition from devices such as cordless phones. They also propagate well through rain, says Crossbow's Crawford, something that signals at some other frequencies don't do. The 900-MHz band, says MicroStrain's Arms, also provides a good tradeoff between range and power consumption and is well suited to high data throughput-for example, for acquiring waveforms from accelerometers.

Just which wireless technologies will become sensor standards is still unclear, however. Oak Ridge's Manges, a member of the IEEE 1451.5 standard committee, says the committee's goal is to include several different technologies, each best suited to a particular application need, such as cost or reliability. Those technologies, he says, will likely include at least one with consumer roots and one uniquely suited to more demanding industrial applications.

From the consumer technologies, Manges says, the 1451.5 committee will probably embrace Wi-Fi, but more for its market presence than technical merit.

Bluetooth, also under consideration by the committee, seems to be losing ground to ZigBee, a variation of HomeRF technology based on the IEEE 802.15.4 wireless standard. ZigBee has lower data rates than Bluetooth, but has longer range and uses much less power. Its proponents say ZigBee devices can operate on batteries for several years.

Reliable, low-powered

A major goal for the industrial wireless sensor technology, according to Manges, is to reduce the broadcast power to the bare minimum while maintaining a reliable connection. "It won't have a high data rate," Manges says. "In our experience, high data rates are not necessary in hard industrial environments." Another goal, he adds, is to keep the price per wireless node down to about $5.

Applications for wireless sensors are advancing in the absence of a standard.

One wireless sensor application, HITS (for Head Impact Telemetry System), goes into sports helmets to help medical researchers learn how concussions occur. Sponsored by the National Institute of Health (NIH) and developed jointly by Simbex LLC (Lebanon, NH) and Microprocessor Designs, HITS incorporates six accelerometers and a temperature sensor, all of which connect to a battery-operated wireless transceiver that communicates with a sideline controller.

Other medical applications are progressing, too. Oak Ridge National Laboratory, for example, has developed an implantable wireless sensor that measures blood flow to transplanted organs, where early detection of a blood-flow problem-otherwise impossible-can prevent irreversible organ damage. In another application, MicroStrain is developing a wireless sensor that detects minuscule motions inside an artificial hip. Analysis of these motions can indicate that the hip is wearing out and needs replacement.

Until wireless sensors incorporate a single, dominant wireless technology, however, mass-market applications are likely to grow slowly. Rapid growth depends on wireless sensors that are reliable, power-miserly, and reasonably priced, and price depends largely on production volumes that only a widely accepted technology can achieve. That chicken-and-egg situation puts wireless sensor development largely on hold, and leaves many innovative applications for wireless sensors just waiting for all the right conditions.