Tiny MEMS Sensors Hold Big Potential
By Mike Sutton WardsAuto.com, Oct 22, 2008 11:14 AM

DETROIT – As computer chips and electronic components advance in capability and shrink in size, parts almost too small to see are playing an increasingly important role in the performance of vehicles and their systems.

“The key (for electronics) is to optimize the performance of all types of systems,” Steve Smith, Custom Sensors & Technologies (CST) director-transportation sales-Western Region and Asia, says at the Convergence Transportation Electronics Conference here.

“Anything we can do (as sensor developers) to help the processes is a big benefit.”

Moorpark, CA-based CST, along with its Kavlico, Crouzet, Crydom, Newall and Systron Donner operations, is a unit of Schneider Electric SA that focuses on advanced pressure, position, force and other types of specialized sensors.

The company relies on established, thin-film titanium-oxynitride (TiON) and ceramic-capacitive technologies for many of its sensing systems, which are at the heart of some of the most advanced vehicle functions, such as in-cylinder, combustion-pressure monitoring, gasoline direct injection-pressure sensing and urea-aftertreatment dosing in heavy trucks.

However, it is the micro-electromechanical system technology that is among the most amazing and promising, Smith says, calling MEMS a “game changer” in sensors.

CST sCap1 MEMS sensors.

MEMS essentially are miniature machines about 0.4 in. (1 mm) or less in size that have components, such as microsensors, a microprocessor and a central control unit integrated into a tiny aluminum or silicon die.

While not new (MEMS appeared several years ago), the technology holds promise because of its miniscule size and ability to operate without an electrical current. Drawing on faint sources of power from the environment, such as acoustical and/or heat energy, MEMS can be isolated from other systems yet still perform vital tasks.

“The industry initially introduced MEMS for TPMS (tire-pressure-monitoring systems), but we should have just brought it out and showed the world what it could do,” Smith says, such as MEMS’ potential in small medical devices, as well as other automotive uses.

“Imagine a device that harvests the energy from the sound of your voice to give you data,” he adds.

CST currently uses its sCap range of pulse-width modulated MEMS for manifold-absolute-pressure sensors that monitor air/fuel ratios, as well as in modules that smooth out the operation of 6-speed automatic transmissions and improve overall efficiency.

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Other applications include hydraulic (braking) and refrigerant-pressure sensing, Smith says, adding MEMS also are capable of transmitting temperature readings.

For this, the units must be robust.

Operating temperatures range from - 40° F (- 40° C) to 302° F (150° C), he says, with pressure thresholds exceeding that of the most-powerful 29,000-psi (2,000-bar) diesel-fuel injectors.

“They are orders of magnitude stronger” than larger TiON and ceramic-capacitive sensors in over-pressure measurement, which is the point where readings spike well above normal operating ranges, Smith says.

However, MEMS can be expensive for certain applications, which is why CST uses ceramic-capacitive units for its urea-dosing sensors.

Another downside of MEMS is their vulnerability to hostile environments.

This makes it necessary to protect the small dies with stainless-steel “pills” or silicone coatings, Smith says, noting urea solutions, which reduce oxides of nitrogen emissions in diesel exhaust, are particularly nasty environments for electronics.

Despite this, MEMS hold other advantages.

While many sensor systems accumulate information in analog form, which must then be converted to digital data by a computer, MEMS sensors are advanced enough to produce a digital signal on their own, eliminating costs and complexity.

Manufacturing techniques currently can produce MEMS sensors smaller than a grain of rice, but advancements in nanotechnology will allow them to become even smaller in the future.

By tightening the wavelength of visible light, which is used to photo-etch the dies, MEMS units only will be visible under a microscope., Smith says.

With this ability, the sensors could be injected into the human body to monitor and assist with vital functions, as well as be integrated into various automotive coatings, fluids and miniaturized components.

msutton@wardsauto.com