Rapid-fire advances in "sensor nets"—wireless sensor modules consisting of some combination of a sensor, controller, transceiver, battery, and antenna—are now yielding initial commercial implementations. Progress in hardware device miniaturization, tiny software operating systems, and lower power consumption levels have led the way.
Some industry experts call these modules integrated on-chip radios. They act as intelligent nodes within a larger network comprising a few or many more nodes. The potential for sensor nets is limited only by the imagination. Once certain technical challenges are overcome, they will ultimately become a regular part of our lives.
And these challenges are being met every day. They include designing cost-effective sensor net nodes that dissipate very little power, choosing the right modulation and demodulation scheme, getting the right sensor transmission range, having a better understanding of RF transmissions on silicon die, and working out the right communications protocols.
Consumer electronics is experiencing a huge influx of wireless sensor applications. Just look at the explosive growth rate of cell phones with cameras. Add to that a host of consumer electronics items like laptops, notebooks, PDAs, DVD players, and digital cameras, and the vast potential for wireless sensor applications becomes obvious.
"Wireless sensor nets will become most ubiquitous in commercial markets for the near future, with applications ranging from security and bio-detection to building and home automation, industrial control, pollution monitoring, and agriculture," explains Bar-Giora Goldberg, chief technology officer for Avaak Inc. and a well-known expert in the field of radio communications systems.
The sensor measures real-world variables like pressure, temperature, heat, flow, force, vibration, acceleration, position, shock, torque, strain, motion, humidity, and images. Although a number of sensors can be considered microelectromechanical systems (MEMS), many other conventional sensors have been around for decades.
The sensor is nothing more than one element of a more complex sensor net system. By itself, it can't work properly in a wireless net environment if it's incompatible with the RF data-transmission circuitry. The sensor also must comply with the right communications protocols (see "Choosing The Right Communications Protocol," p. 66).
Working with researchers at the University of California at Berkeley, Intel scientists have already examined a "mote" research project. Motes—tiny, self-contained, battery-powered computers—have radio links that let them communicate and exchange data with one another as well as self-organize into ad hoc networks. Motes form the building blocks of wireless sensor networks.
The Intel Mote project team seeks to create a new platform design that delivers a high level of integration plus low-power operation in a small physical size. Features include modular hardware and software design, system power management, and low-cost, high-volume production potential.
FROM SOUP TO NUTS
Some companies supply total turnkey sensor net solutions, including the hardware and software. Others supply some combination of the sensor chips, the power source, a controller, and a transceiver. Many also specialize in supplying the RF transmission link.
Such companies include Avaak, Chipcon, Crossbow Technology, Intel, Maxim Integrated Products, Melexis, Microchip Technology, Millennial Net, Nordic, RF Micro Devices, and Xemics. Depending on the application, the total cost per sensor net node now ranges from $50 to $100. In a couple of years, look for prices to drop to about $25.
The availability of wireless sensing technology makes once unachievable applications now practical. It eliminates miles of bulky cables on the factory floor and allows for signal monitoring in hard-to-reach locations. For example, putting a sensor atop a construction crane eliminates the need for a bulky and lengthy cable that's prone to strain and failure.
In another application, sensors on everyday motors measure the strain on spinning flywheels. Typically, slip-rings on these motors introduce noise and offset errors, fouling up meter readings. Mounting a wireless sensor directly to the flywheel eliminates this by transmitting error results to the monitoring electronics.
Wireless sensor nets on the factory floor help keep track of processes and inventories. In the medical arena, they're used for diagnostic imaging, drug delivery, and patient monitoring applications. And in home and building automation, they're watching security systems, energy management, home appliances, and entertainment systems.
One particularly hot area for sensors is automatic remote-meter-reading applications. Some of the latest applications include environmental monitoring, avionics, and land-vehicle and off-road-vehicle automation.
Sensors also are strong in the automotive field when it comes to tire-pressure monitoring systems (TPMSs). Presently, sensor manufacturers are trying to bring down the costs per TPMS node, making possible cost-effective systems that dissipate very little power (Fig. 1). Under the Transportation Recall Enhancement, Accountability and Documentation (TREAD) Act issued by the U.S. National Highway Transportation and Safety Administration (NHTSA), U.S. automakers had to install TPMSs on at least 10% of last year's model vehicles. That figure will rise to 65% in 2006.
Auto manufacturers can opt for an indirect means of tire-pressure monitoring by piggybacking atop a vehicle's existing anti-lock braking system (ABS). This method is less costly but also not as accurate as the direct TPMS method. The latter method has been gaining favor with automotive manufacturers as well as sensor manufacturers who are readying sensor products compatible with popular communications protocols, particularly the ZigBee protocol.
Sensor manufacturers are going to great lengths to fit the TPMS module within an existing tire's structure. One solution is to make the module fit inside the tire's valve stem assembly, something already accomplished by Infineon (Fig. 2).
Civil and building structure strain monitoring with power-efficient, high-speed wireless sensor networks is now possible. For example, Microstrain installed its system on a heavily trafficked bridge in Vermont. Displacement sensors are attached to steel girders for static and dynamic strain measurement. Strain data is acquired via a wireless link. The wireless system can remain on the bridge for long-term interrogation under normal and controlled operating conditions (Fig. 3).
UBIQUITOUS IMAGE SENSORS
Imaging sensors are everywhere, from consumer products and the medical field to security systems and environmental monitoring. Thanks to their versatility, image sensors represent the fastest growing segment within the entire sensor market. Semiconductor-based image sensors are rapidly penetrating a host of applications, due to advances in low power dissipation, higher resolution, and lower costs.
Who's manufacturing these imagers? Among the players are Kodak, Micron Technology, Mitsubishi, OmniVision, Philips Semiconductor, Samsung Electronics, Sanyo Electric, SMaL Camera Technologies, STMicroelectronics, Toshiba America Electronic Components Inc., and Zarlink Semiconductor.
"The introduction of vision sensing adds a completely new dimension to the equation. Vision is by far the most desired sense," explains Avaak's Bar-Giora Goldberg. "Such networks can improve productivity and profitability in process control, work flow, inventory management, asset tracking, and identification. Applications are numerous in the commercial market. We can expect continuous improvement in performance and price due to integration, shrinking geometries, and on-board image processing," he adds.
Most of these sensors are made on CMOS processes, and they historically have been less accurate than charge-coupled device (CCD) sensors. But that's changing. The density of CMOS image sensors is now approaching 5 Mpixels—a number that continues to climb.
Agilent Technologies recently unveiled a CMOS color image sensor that measures 5 by 5 by 1 mm. According to the company, the HDJD-S72QR999 is the smallest CMOS color image sensor available. This 5-V device integrates color filters, photodiode arrays, amplifiers, and gain-selection circuitry.
But don't think CCD image sensors are standing still. At this year's International Solid State Integrated Circuits Conference (ISSCC), scientists from Sanyo Electric presented a 1/4.5-in., 3.1-Mpixel frame-transfer CCD with a pixel size of just 1.56 µm. That's the smallest size yet, making it useful for mobile imaging applications.
Medical imaging often relies on an in vivo implantable capsule that monitors blood glucose levels at regular intervals. Such a device not only notifies the patient via an alarm of abnormal blood sugar levels, it also can be programmed to deliver medications.
A custom transmitter chip from Given Imaging combines ultra-low-power wireless technology with clever design techniques to realize a tiny swallowable camera capsule for gastrointestinal-tract diagnosis (Fig. 4). The semiconductor camera is made by Zarlink Semiconductor. Patients swallow Given Imaging's PillCam, a vitamin-sized capsule that broadcasts pictures as it passes through the esophagus, stomach, and small intestine. The capsule includes a camera, LEDs, batteries, the custom chip, and an antenna.
Homeland security is another promising application niche for wireless sensor nets. Avaak, a supplier of solutions for video monitoring and surveillance, has an autonomous product that measures just one cubic inch (Fig. 5). This miniature wireless video (camera) platform includes a battery, radio, camera, (color imager plus lens), controller, antenna, and temperature sensor.
We're only at the wee stages of the wireless sensor net's maturation. Continued innovation in sensor, transceiver, battery, antenna, and controller technologies, coupled with improved communication protocols and topologies, is sure to increase their potential.
Power sources like solar energy will one day eliminate the need for batteries and battery chargers, as well as power monitoring and regulating circuits. Regardless of where the data is coming from—medical, industrial, automotive, consumer appliances and gadgets, and seismic applications—wireless-sensor-net technology will become ubiquitous in nearly every facet of our lives.
Even antennas are starting to improve for maximum sensor-net performance. Designers now realize that this vital component can go a long way toward enhancing sensor-net capacity, range, power consumption, and reliability. Combining techniques help mitigate multipath, shadowing, and fading effects.
MEMS technology might even come to the rescue for antennas, much like they've done for most sensors. Work at the John H. Glenn Research Center for NASA has produced a MEMS-based printed antenna that employs MEMS switches (Fig. 6). These RF switches are more linear and consume less power than microwave antennas that use printed parts.
In another coming trend, it's anticipated that sensors the size of dust particles will become the norm. Crossbow Technology has developed tiny sensors known as smart dust that can detect everything from light to vibrations. The sensors, which come complete with a computer chip, battery, and radio, pass information to each other and send the information back to a main computer. The smart-dust network is being tested to fight forest fires by alerting emergency crews when a fire starts in a remote area.
Research into smart-dust sensing is ongoing at laboratories like the University of California at San Diego and Sandia National Laboratories. Because chemical dust sensors can change colors and pattern in the presence of certain chemicals, they should be highly effective in homeland-security, industrial, and medical applications.
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Maxim Integrated Products
OmniVision Technologies Inc.
RF Micro Devices
Sandia National Laboratories
SMaL Camera Technologies
Toshiba America Electronics Corp
University of California at Berkeley
University of California at San Diego