Sensors have made serious inroads into automotive, medical, industrial, and aerospace applications. But you ain't seen nothin' yet. Rising concerns for safety, convenience, entertainment, and efficiency factors, coupled with worldwide government mandates, will see sensor usage swell to unprecedented levels.

Add to that the predicted explosion in wireless and consumer applications, and one can see why sensor manufacturers anticipate quickly developing huge markets and applications through the end of this decade. Most of these sensors will be of the microelectromechanical-system (MEMS) and microsystem-technology (MST) type, with nanosensors showing great promise.

Mention automotive systems, and sensor manufacturers can easily see a host of sensing possibilities for measuring not only pressure, but also inertia, position, proximity, temperature, flow rate, force, strain, torque, vibration, and tilt. And the sensing technologies used to measure these parameters are just as varied (see "The Business Of Sensors," Drill Deeper 8325 at www.elecdesign.com). According to Alex Cade, a technical fellow at the General Motors Technology Center (www.gm.com), "sensing needs for automobiles are growing by leaps and bounds." He cited several growth areas for chassis controls, vehicle positioning/location, object detection, vision enhancement, auto environment heating, ventilation, and air conditioning, as well as engine and transmission controls. Vehicle stability enhancement was just one of the many examples he cited.

A recent U.S. National Highway Transportation Administration (NHTSA) proposal for side-impact airbags would add two to six sensors to every automobile. Even though the proposal doesn't mandate their use, U.S., European, and Japanese auto manufacturers indicated that they would supply side-impact airbags in all of their vehicles by the end of this decade for safety reasons. Some automotive suppliers like TRW (www.trw.com) and Delphi (www.delphi.com) use a combination of accelerometer and pressure sensors (the latter having a faster response than accelerometers) in side-impact airbags.

Inertial sensing in cars has become another hot topic. In fact, Motorola (www.motorola.com) and Analog Devices (www.analog.com) propose the use of inertial sensing modular clusters to manage the vast number of sensing functions that will be required for vehicle dynamics, navigation, safety, and steer-by-wire applications (Fig. 1).

"The interaction between anti-lock braking, electronic brake-force distribution, traction control, and active yaw control systems allows the achievement of dynamic stability in an automobile," says Harvey Weinberg, a senior applications engineer for Analog Devices. Motorola's John P. Schuster adds that "the modular approach allows multiple applications to be supported by using a core platform. It builds on aerospace gyro applications that can be adapted for automotive applications at a lower cost and smaller size."

OPTICAL SENSING IS IN
One novel approach developed by Optrand (www.optrand.com) to measure engine pressure involves a multifunctional device that combines a fiber-optic-based pressure sensor with the glow plug used in passenger diesel engines. The PressureSense glow plug, comprising a sensing head, a fiber-optic cable, and signal-conditioning electronics, offers a total accuracy of 62% against a water-cooled reference transducer at pressures above 5 bar and less than 0.2 bar error for pressures below 5 bar. The company foresees the first use of this device by 2007.

Honeywell (www.honeywell.com) proposes the use of optical sensing for a low-cost passive keyless entry system, parts of which can be embedded within a car's door handle. The sensor would consist of a key-like optical enclosure that houses a transceiver. To gain entry, the vehicle owner places the key-like enclosure between the door handle and the car's body.

Hall effect sensors will find homes in a vast range of automotive functions, including sensing throttle and brake pedal position, camshaft position and speed, barometric air pressure, and manifold absolute pressure (MAP). According to Infineon Technologies AG (www.infineon.com) applications engineer Werner Roessler, active Hall effect sensors can be put to use in power-train control and cam and crankshaft applications. "This provides more accuracy, better startup strategies, and the ability to detect a crankshaft's starting point position, \[versus\] a passive sensor approach," he says. Another advocate of the Hall effect sensor approach, Melexis Inc. (www.melexis.com), proposes using this technology for contactless position sensing.

A NEW SENSING PARADIGM
Electric field or E-field sensing uses electrodes and the electric field between them. It's another option for sensors in airbags and other applications, according to Freescale Semiconductor.

"This method of sensing makes for a smarter airbag, in which the bag will not deploy prematurely, by taking into consideration not only the passenger's head position (i.e., has it moved or not?), but also the passenger's size and weight," says Don Laybourn, applications engineer for Freescale.

Such sensors can be set up on steering wheels with electrodes around the rim and other points, allowing them to determine when a wheel is released (such as when a driver falls asleep or suffers a disabling medical condition), which will then produce a warning signal. This method can also be used to gently bring the vehicle to a halt.

Seat electrodes in a vehicle could also apply the brakes through the anti-lock braking system if it's determined that no one is in the driver's seat if a vehicle is moving. This would prevent runaway conditions, such as when a car is parked on a slope. Car-window rain and frost sensing is yet another application.

Another huge arena for sensors is the wireless sector. Wireless sensors grab a $500 million share of the current $40 billion sensor market, says MEMS pioneer Janusz Bryzek (janusz@bryzek.com), who is also managing partner of BN Ventures, a venture capital firm. By the end of the decade, that number should climb to over $10 billion.

Bryzek notes that only a small fraction of these wireless sensors operates from a battery. Yet given present wireless standards activities, there's a growing need for wireless sensors that can operate from battery voltages of 2 to 3.6 V. On the other hand, power consumption must be minimized, which is not an easy task for wireless sensing systems.

One of the hottest areas in the world of wireless sensors is automotive tire-pressure monitoring systems (TPMSs), with government mandates in the wings acting as a spurring agent. Present-day tire-pressure monitoring systems are indirect yet low in cost. but the more expensive direct tire-pressure monitoring method—where every tire has a sensor and transceiver—is favored by many sensor and automotive subsystem OEMs. Of course, such systems require batteries to operate. Battery-less direct systems have been developed, but they've yet to be proven in the field.

According to Dirk Leman, TPMS product manager for Melexis, the direct method will likely prevail, being phased in over the next few years. He notes that "battery-based technology is here to stay for some more years, and passive technologies are mature enough to penetrate niche applications in the near future. However, for a Tier 1 supplier implementing passive technologies, the road map, business model, and leveraging of existing competencies are at least as important as the individual product cost when selecting a single technology."

In the long run, he cautions that intelligent indirect technologies may generate a bigger growth potential for MEMS sensors than direct TPMSs. That's because they can offer increased safety and comfort in addition to pressure monitoring.

Medical applications are numerous. The potential for wireless sensing also extends to the medical field, where wearable electronics will make a huge impact on remote patient bio-monitoring and bio-chemical detection. For instance, wireless strain gauge sensors can be used to measure in-vivo total knee-replacement joint loads in humans. According to Steven Arms, president of Microstrain Inc. (www.microstrain.com), "We've shown that this can be done in a collaborative project with the Scripps Institute (www.scripps.edu) and Johnson & Johnson (www.jnj.com)."

In another development, Microstrain and the Department of Veterinary Medicine at the University of Wisconsin in Madison (www.wisconsin.edu) measured bone plate growth in-vivo in lambs. The company also performed in-vivo vertebral spine bone strains in humans in collaboration with the Department of Orthopedics at the University of Arizona (www.arizona.edu) and re-animated paralyzed human limbs in a cooperative project with the Bioengineering Deptartment of Case Western University (www.cwru.edu) using control systems based on strain gauges.

The plain magnet is still being used, in combination with a new sensing methodology, to hold together large broken bones more accurately and safely in humans. Present methods use X-rays and a surgeon's skill to do this, but it's more costly, less safe, and less accurate and takes a longer time. So in a joint project with Virginia Tech (www.vt.edu), the University of Virginia (www.virginia.edu), the Carilion Biomedical Institute (www.biomedicalinstitute.org), and Carilion Health Systems (www.carilion.com), Triad Semiconductor (www.triadsemi.com) developed intramed-ullary nails (IMNs) that allow orthopedic surgeons to safely, quickly, and accurately hold broken bones together. The group's prototype magnetic targeter system uses a target magnet on a wand within the IMN and an LED display (Fig. 2).

Sensors of all types will also find uses in many industrial applications for monitoring the health of rotating machines and motors and to track the health of our nation's infrastructure. Radatec (www.radatec.com) reports on a microwave-based proximity/displacement sensor that allows the monitoring of machinery in extremely harsh environments at high rotating speeds and very high temperatures.

The aforementioned medical applications from Microstrain are also being used to keep track of the structural integrity of bridges, tunnels, highway overpasses, buildings, and other civil structures. Their EmbedSense strain-gauge-based wireless-transmission modules provide low-power, battery-operated sensing nodes at high data-acquisition rates required by civil authorities. Scalable arrays of passive strain gauge sensors can be interrogated remotely. Switched-reactance methods eliminate many components such as the RF oscillator, crystals, and feed-through antennas.

USHERING IN NANO
Nanotechnology will no doubt play an important role in sensing. At last month's Sensors Expo, held in Detroit, Mich., Dean Aslam, associate director at Michigan State University (www.egr.msu.edu), presented "multi-walled carbon nanotubes" (CNTs), which were grown to form a preconcentrator section of a micro gas chromatograph for sensing chemicals. Featuring wall layers ranging from 5 to 30 µm and lengths up to 500 µm, the CNTs offer an ultra-high surface area and low energy consumption, as well as superior adsorption/desorption characteristics compared to other known materials. The work was supported by the Engineering Research Centers Program of the National Science Foundation (NSF) and is part of the research work being performed at the University of Michigan's Center for Wireless Integrated Microsystems (WIMS).

No matter what type of nano-device emerges, nanotechnology itself needs lots more research and must be mastered before it can be realized. Professor Ahmed Busnaina, a leading nanotechnology researcher at Northeastern University (www.northeastern.edu) and a keynote speaker at Sensors Expo, holds this opinion. He says the industry needs to develop nanotemplates as nanomanufacturing tools for nanodevices and sensors as a bare minimum before the technology can flourish.