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MEMS Sensors Rev Their Engines
Sensors based on the technology of microelectromechanical systems (MEMS) are speeding their way into race cars and automobiles, and driving hard into consumer applications
Gary Legg -- Design News, May 5, 2003
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Fast Lane: Delphi ENgineer Bruce Natvig led the team that developed an earpiece sensor that measures dynamic forces impacting a race car driver's head when a crash occurs. The data will help engineers design better restraint systems. |
Drivers in the Indy Racing League have a new piece of gear this year, and it isn't under the hood, but in their ears. Embedded in the drivers' radio earpieces is a tiny MEMS sensor system developed by engineers at Delphi (www.delphi.com) that measures the dynamic forces applied to the driver's head during an accident. The g-force data collected will provide researchers a clearer picture of what happens in that split-second of time that it takes for a crash to occur, leading to better design of driver restraint systems and safety devices.
Size mattered, since the sensing system—consisting of 3 accelerometers, one for each axis, and two circuit boards—had to fit inside the earpiece alongside the transducer for the radio receiver. "Engineers first tried putting the sensors into the helmet, but discovered that the helmet moves relative to the head. It did not produce an accurate reading of head forces," says Bruce Natvig, an engineer at Delphi who headed up the development project.
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Small Stuff: The small size of MEMS accelerometers enables them to fit inany application. |
Fortunately, the tiny 4.5 x 4.5 x 2-mm sensor system fit comfortably inside an earpiece, sized even for the smallest ear because of the technology of microelectromechanical systems (MEMS), which has made sensors very small. That size opens up a host of new applications.
MEMS devices are essentially mechanical systems in extreme miniature. In their packages, they look like ordinary integrated circuits, but inside they're mechanical systems with extremely small masses, beams, and springs that move. The proof mass in Analog Devices' MEMS gyroscopes, for example, weighs only eight millionths of a gram. It's suspended only two microns (two millionths of a meter) over the device's electronic circuitry. The proof mass in accelerometers from MEMSIC beats even that. It's nothing but a gas that moves in a sealed chamber, and because gas is the only thing that moves, there are no parts that can break. MEMSIC claims its devices can withstand shocks of up to 50,000 g's.
The credit for enabling such small mechanical systems goes to semiconductor manufacturing technology. The same photolithographic processes that create super small transistors are just as capable of creating super small beams, springs, and other mechanical structures. In fact, these mechanical structures, tiny as they are, loom huge in comparison with electronic circuit components. That means that MEMS devices can be produced on trailing-edge semiconductor fabrication equipment, a financial bonus for semiconductor manufacturers. In addition, semiconductor manufacturing processes create hundreds or thousands of MEMS chips at a time on a single silicon wafer, bringing about enormous per-unit cost savings that will only increase as semiconductor manufacturing technology continues to improve.
Two Paths to Production
Two different approaches—bulk micromachining and surface micromachining—create different kinds of MEMS devices. The bulk micromachining process, normally used to make pressure sensors, creates mechanical structures by etching away material from both sides of a silicon slab that's relatively thick (typically 400 microns). The other approach, surface micromachining, adds the common IC-fabrication steps of depositing and then selectively etching multiple silicon layers that are very thin (typically 2 to 4 microns). Surface-micromachined MEMS devices are typically 20 times smaller, by volume, than bulk-micromachined devices, which are tiny themselves. Surface micromachining is the process used to make accelerometers, but Motorola also turned to surface micromachining to make the pressure sensor in its new tire-pressure monitoring system.
 
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How it works: In a MEMS accelerometer, a moving proof mass shifts capacitive sensing fingers that are attached to it. The resulitng change in the capacitance provides a measure of acceleration. In MEMs accelerometers from MEMSIC below, a heated gas changes position as the device moves. Temperature sensors measure the gas's shift to determine acceleration. |
Both bulk micromachining and surface micromachining capitalize on silicon's physical strength to make microscopic mechanical structures. According to Dave Monk, development engineering manager for Motorola's Sensor Products Division, a typical diaphragm thickness in a MEMS pressure sensor is 12 to 25 microns, compared to 75 microns for the thickness of a human hair. "This is silicon, but it can flex," Monk says. "You can flex it with a pencil and see it with the naked eye."
In surface-micromachined MEMS devices, mechanical structures are indeed microscopic. For example, the sensor in the ADXRS150 and ADXRS300 MEMS gyroscopes from Analog Devices measures only 1 x 0.5 mm and yet includes, among other things, 5,000 interleaved capacitive-sensing fingers (beams) that move. In a MEMS accelerometer, says Chris Lemaire, business development engineer at Analog Devices, the device's proof mass moves by mere Angstroms.
How do such small devices work? "Picture a trampoline," says Motorola's Monk. A MEMS accelerometer's proof mass, Monk says, is like the trampoline's surface. Silicon springs connect the proof mass to anchor points. The springs aren't coils, but serpentine cantilever beams, and even though they're made of silicon, they can stretch enough to do their intended job. When the proof mass moves, with control provided by the springs, interleaving capacitive-sensing fingers detect minuscule changes in capacitance, which then get converted electronically to an acceleration reading. In a MEMS gyroscope, or angular rate sensor, a vibrating mass replaces the spinning mass of a traditional gyroscope.
The changes in capacitance that a MEMS device detects are just as tiny as MEMS mechanical components. "We're measuring zeptofarads," says Paul Ganci, product line director for the Inertial MEMS Group at Analog Devices. A zeptofarad is 10-21 farads, Ganci notes—so small that having on-chip circuitry to measure it and process the reading is preferable to off-chip circuitry, which can affect readings. Analog Devices and MEMSIC both have such on-chip circuitry. Other MEMS manufacturers put two chips—one with MEMS mechanical components and one with circuitry—in each MEMS package.
Small Size, Big Market
MEMS applications may be growing, but the technology isn't exactly new. Motorola has sold over 300 million MEMS pressure sensors since 1980. Analog Devices has shipped over 100 million MEMS accelerometers since introducing them in 1991.
And now, MEMS devices are getting set to become even more numerous. Automotive applications, for example, already the largest market for MEMS devices, will use more of them—9.1 per vehicle in 2007, according to market research firm In-Stat/MDR (www.instat.com), up from 5.0 in 2002. MEMS pressure sensors are useful for engine management, braking, fuel measurement, suspension control, tire-pressure monitoring, and more. MEMS accelerometers and angular rate sensors are finding use in active suspensions, rollover detection, and automatic headlight leveling.
Falling prices are even pushing MEMS devices into consumer applications. MEMS accelerometers have recently become available for around $2.50 from Analog Devices and startup MEMSIC (www.memsic.com), an Analog Devices spinoff. STMicroelectronics (www.st.com), a big player in components for consumer electronics, is also getting into MEMS. It plans—along with other MEMS manufacturers—to put them into cell phones and PDAs to give engineers functions they never even knew they needed. Says Benedetto Vigna, manager of MEMS development for ST, "I believe this will be the decade of MEMS inertial sensors for consumer applications."
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The Skinny: The silicon diaphragm, left, in a mems snesor is typically 12-25 microns thick. |
Part of the impetus of new MEMS applications comes from increasingly sophisticated features. Low-g MEMS accelerometers, for example, now have measurement ranges as small as 1g (the acceleration of gravity), with resolution as fine as 0.001 of full scale. Performing as very sensitive motion and tilt sensors, they're starting to provide one-handed, keyless scrolling of displays on cell phones and PDAs. To scroll a mobile phone's tiny display, you just tilt the phone in the appropriate direction. Three-axis accelerometers are even adding zoom capabilities to the displays. To zoom in, you raise the phone or PDA a bit; to zoom out, you lower it. MEMS gyroscopes—more accurately described as angular rate sensors—are also recently available. One of their uses will be in cars for rollover detection.
MEMS sensors are also prized for their reliability and low-power consumption. The accelerometers from Analog Devices (www.analog.com) that deploy airbags in car crashes exhibit less than one failure per billion hours of operation. Motorola's tire-pressure monitor, including MEMS device and additional circuitry for remote wireless operation, will operate for seven to 10 years on a single, tiny button battery.
And then, there are the really esoteric MEMS devices. Researchers have created microscopic MEMS motors, for example, and minuscule mechanical manipulators that can grasp a single red blood cell. On a more practical level, fluidic MEMS actuators by the millions are serving as printheads in inkjet printers. They accurately dispense ink in amounts as small as four picoliters (four trillionths of a liter).
What's not small about MEMS is the growth potential. As automotive applications for MEMS devices increase, and as consumer applications get ready to take off, other areas also show promise. MEMS optical components, such as those in LCD projectors, are one promising area; another is healthcare, where MEMS blood pressure sensors are already widely used. Some observers even view MEMS as being positioned now where microelectronics was in the 1970s and early 1980s, with nowhere to go but up. If that's true, we may see another instance of something very small becoming very big.
MEMS-based system monitors tire pressure
MEMS-based system monitors tire pressure
From now on, even tires will contain electronic components. Motorola's MPXY8000 tire-pressure monitoring system goes inside a tire or onto a tire's valve stem to constantly check for dangerous tire deflation. The system wirelessly transmits information to a car's remote keyless entry receiver, where software can alert a driver to stop and add air. The system satisfies requirements of the TREAD Act (Transportation Recall Enhancement, Accountability, and Documentation), which mandates pressure monitoring in tires for vehicles with a gross weight of 10,000 lbs or less beginning in November of this year.
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Pressure Sensor: Motorola's new MEMS-based tire pressure monitoring system can mount on a wheel or in a valve stem. |
The MEMS-based system is so small, it can fit inside a tire's valve stem, says Motorola Application Engineer Jeff Burgess. Some tire makers install the device inside the stem, Burgess says; others install it either on the outside of the valve stem or on the wheel inside the tire. To make the system small enough to fit inside a valve stem, Motorola combined a MEMS pressure sensor, a MEMS temperature sensor, and interface circuitry with a power-saving wake-up feature on a single chip. To shrink the pressure sensor, in order to fit it into the valve stem, Motorola used surface micromachining instead of the bulk micromachining normally used to make pressure sensors.
A tiny button battery powers all of the electronics in the system. According to Burgess, the battery will last anywhere from seven to 10 years, longer than the life of a typical tire.
Gear for the ear
Gear for the ear
At an Indy race in Phoenix on March 23, 2003, rookie Gilde Ferran sustained a head injury in a crash. For the first time, researchers were able to collect data on the dynamic forces impacting a race car driver's head, thanks to a MEMS-based earpiece sensor system developed by Delphi engineers. The data will be used not only to diagnose and treat head injuries, but also to help engineers design better driver restraint systems and safety devices.
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Data Collector: The Delphi earpiece sensor system adds three tiny accelerometers to each of the race car driver's radios to measure the impact force on a driver's head. |
"The plan is to correlate the various g-forces to actual head injuries and then work to reduce those levels through development of better head and neck restraints, helmet design, padding around the drivers, and so on," says Bruce Natvig, the Delphi engineer who headed up the earpiece project.
Delphi, which since 1997 has been employing an accident data recorder for sensing and recording key vehicle parameters, began developing the earpiece (called DESS for Delphi Earpiece Sensor), two years ago in conjunction with Dr. Henry Bock, the racing league's medical director. This year, the earpiece is mandatory for drivers in the Indy Racing League and CART.
Packaging proved to be particularly challenging, as the sensing device (one left, one right) had to fit into the small cavity of the earpiece housing the radio receiver transducer. The three miniscule accelerometers (Analog Devices)—one each for the x, y, and z axis—weren't so much of a problem. It was the two circuit boards, which engineers had to figure out how to mount 90° to each other and in a position as close as possible to the driver's actual orientation.
Operating on a 5V supply, the system consumes very little power, which is a plus. And Natvig points out an important temperature-related benefit: "There's no temperature drift at all, since the system stabilizes at 98.6°."
DESS records the change in g force with a measurement range of ±250 g and a resolution of 0.6 g. The data collection rate of 500 Hz is "more than sufficient" for the application, says Natvig. "We have looked at video footage of accidents, and you can't see the driver's head move at more than 30 Hz. Even if we log at a faster rate of, say, 1,000 Hz, doctors are always going to filter to 100 Hz because of the noise." Engine vibrations in race cars can run as high as 50 gs.
DESS connects to Delphi's Accident Data Recorder (ADR2) which records not only driver head data, but also key vehicle parameters, just prior to, during, and after an accident-triggering event. If an impact exceeds a preset threshold, the system turns on a red light mounted on the outside of the car to inform doctors and paramedics that an injury may have occurred.
"We already know that if a driver experiences a certain amount of force, the likelihood for injury, such as head trauma, increases significantly," says Bock. "Knowing that a crash has exceeded a threshold, safety workers on the scene will adjust their mindset, anticipating that injury has occurred and tailoring their assessment and extrication plans to properly support the driver."
Delphi has produced 200 earpiece systems for the Indy Racing League and CART this year, with positive feedback from drivers. "The whole trick was to make this completely transparent to the driver," says Natvig. "Apparently that's working, because we had one driver who was happy to use this earpiece and throw his old one out."
