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# Shake, Rattle, and RollPUBLIC ACCESS

Thanks to the Airbag, MEMS Accelerometers are Getting to Explore New Territory.

[+] Author Notes

Associate Editor

Mechanical Engineering 123(11), 56-58 (Nov 01, 2001) (3 pages) doi:10.1115/1.2001-NOV-2

## Abstract

This article highlights that airbag has been a boon to MEMS business. Sales of the tiny accelerometers that sense when the bags should deploy have helped to drive down prices significantly since the devices were first implemented. Now, the high volumes, low costs, and dependable performance of micro devices are opening the way to new applications. Sneaker companies are looking at MEMS accelerometers in running shoes to act as speedometers of sorts. Advantage of the MEMS accelerometer is that it has a wide bandwidth, capable of reading high as well as low frequencies. High-frequency data provides information about thin reservoir zones, faults, and changes that are taking place as fluids are being drained from pores in the rock, said Denver. Higher frequency signals are critical to accurate interpretation. Low-frequency signals are useful in identifying the type of rock, be it sandstone, shale, or carbonate, for example. The VectorSeis is as rugged as a conventional geophone and can be successfully deployed in down-hole environments to get a closer reading of a reservoir.

## Article

The airbag has been a boon to MEMS business. Sales of the tiny accelerometers that sense when the bags should deploy have helped to drive down prices significantly since the devices were first implemented. Now, the high volumes, low costs, and dependable performance of micro devices are opening the way to new applications.

To be sure, transportation will continue to play a role as cars incorporate more safety features, such as side airbags or safety curtains, in the event of rollovers. But microelectromechanical accelerometers are also finding their way into handheld electronic gadgets as inexpensive and ubiquitous as cellphones. The trend, according to some suppliers, is one example of a successful push into the consumer electronics market, which holds a large number of potential applications for micro devices.

## Going Mainstream

MEMS suppliers see big potential in consumer applications. “Probably the most significant trend is the advent of these devices being considered and designed into consumer electronics applications,” said Tom Cunneen, vice president of marketing and sales at Memsic Inc. in Andover, Mass. The company has fielded inquiries from cellphone manufacturers about using MEMS accelerometers to measure shock, apparently to give a sense of mistreatment of phones returned under warranty.

Another novel application may be on the horizon: Sneaker companies are looking at MEMS accelerometers in running shoes to act as speedometers of sorts. A device could relay information on rate and impact through a radio frequency output to a wrist-watch that would display a readout during training.

Micro accelerometers have been tested by Input/Output and Veritas DGC, a provider of geophysical services, in land surveys for oil exploration.

The other big potential application, in Cunneen’s view, is gaming. MEMS accelerometers can be found in joysticks and handheld games, but it is still a leading-edge development for those applications, he said. MEMS accelerometers are also being considered for user interfaces for computers, eliminating buttons and mice, and making possible more intuitive controls.

Cunneen said the company’s MEMS accelerometers have lowered the price significantly, making the accelerometers competitive for new applications. He said that the MEMS accelerometer is generally more expensive than piezoelectric devices, but inexpensive enough to be considered for many consumer applications.

Bob Suloff, director of business development and marketing of the microma-chined products division of Analog Devices in Norwood, Mass., is seeing more applications in using MEMS accelerometers for measuring tilt in handheld consumer electronics devices, such as cellphones and TV remotes.

“You make a movement, which is very intuitive, and translate that movement into an electrical signal that allows you to interact with a video game or controlling channels or volume.” It’s an area he calls “gesture recognition.”

Suloff added that gesture recognition could play a role in security by recognizing and identifying certain movements. MEMS accelerometers can be used in gesture recognition that will render a laptop useless if stolen, or even identify its location the next time the modem is used, he said.

## What Lies Beneath

At the same time, improved performance capabilities have pointed MEMS accelerometers in an entirely different direction as well, toward high-demand seismic applications. A new MEMS accelerometer has been developed that is being used by at least one company for oil and gas exploration. The application is significant, because it is an alternative to coil-based geophones, a mature technology that has been used in the field for 50 years. This is not as high a volume application as automotive or, potentially, consumer electronics, but it seriously pushes the envelope in terms of MEMS accelerometer performance.

Drilling for oil and gas is not for the fainthearted. Drilling an oil well in deep water, for instance, can cost between $20 million and$50 million. Making what is already a risky venture even more so, oil and gas exploration is inexorably moving to ever more remote and harsh terrain and deeper oceans. Thus, any tool that can help energy companies reduce their risk is vital.

Exploration companies routinely pound the Earth with acoustic waves, and then pore over seismic data to predict where the oil and gas deposits are most likely to be found, some 15,000 feet or so below the surface.

Input/Output says that its tiny VectorSeis accelerometer has a very low noise floor, which is comparable to conventional geophones and is well suited to collecting multicomponent data.

Traditionally, this work is done with geophones, coil-based sensors that are rugged and very sensitive to ground motion, according to Input/Output, which is a Stafford, Texas, supplier of seismic instrumentation and geophones to the oil exploration industry. A typical land survey entails placing an array of geophones in the ground. An impulse, delivered either by a dynamite charge set off underground or by large vibrator trucks, sends a seismic wave into the ground, which is then recorded by the geophones as it is reflected back to the surface.

A survey consists of thousands of geophone arrays spread out over several square miles. To get the truest picture of what is underground, the geophones should measure the complete motion of the ground. This means collecting multicomponent data—that is, recording ground vibrations in the X, y, and 2 directions. To maintain performance, each geophone must be placed within about 2 degrees of level, a tight tolerance that is difficult to accomplish out in the field.

The signal that is reflected back to the surface is very faint—not much above the ambient noise at the surface that might result from wind or trees. The receiver has to have a low “noise floor,” so it can collect these small signals over a wide frequency range.

## Digital Alternative

Having evolved over a half-century, geophones have performed the task well, according to Jeffery Gannon, technology development manager of Applied MEMS Inc., a unit of Input/Output. “Geophones are a mature technology,” he said. About two years ago, Applied MEMS developed a micro accelerometer for oil and gas exploration. Gannon added that there are other applications for this outside of oil exploration. These include strong motion sensors for earthquake, volcano, bridge, and dam monitoring.

Part of the spring structure of Analog Devices' ADXL202 accelerometer, which has been used in consumer electronics applications.

The device, called VectorSeis, has undergone field trials since last February. The company is currently developing a second-generation commercial version of the device that should be available by January 2002, according to Peter Maxwell, commercialization manager for the VectorSeis product line. Input/Output has been working with Veritas DGC Inc., a Houston-based provider of geophysical services for the petroleum industry, in conducting the field trials.

## Intriguing Capabilities

Maxwell said that geophones will be used in oil and gas exploration for many years to come. Still, the new VectorSeis product offers some intriguing capabilities. For one thing, it is better suited for multicomponent data acquisition. This is partly because the accelerometer can compensate if it is not leveled exactly in the ground, so vertical orientation becomes less of an issue. He added that the accelerometer provides a very high accuracy of the ground motion measurements, providing a clearer seismic image. This can potentially make it easier to interpret, he said.

The VectorSeis also has a very low noise floor—comparable to conventional geophones and adequate to record seismic data, he said. The low noise floor results from a combination of the accelerometer’s design and the use of feedback control.

The accelerometer consists of a “proof mass,” or inertial mass, suspended on springs and surrounded by a frame structure. One of the unique aspects of its design is that, although the sensor is very small, it has a relatively large proof mass, Maxwell said. The proof mass is also packaged in a vacuum, which reduces damping and lowers noise, according to the company.

The sensor uses a feedback loop that serves as an error correction signal that keeps the output proportional to the acceleration, said Maxwell. The sensor uses a mixed-signal application specific integrated circuit, or ASIC, to convert the force measured on the proof mass to a digital output. The sensor and ASIC are spaced less than a half-inch apart, minimizing the chance for the signal to be corrupted, said Maxwell.

Larry Denver, vice president of reservoir applications at Input/Output, said that the VectorSeis accelerometer has potential not only for the exploration phase, but also for the life of the field production phase. During the exploration phase, sensors are deployed in land arrays or, in the case of ocean deposits, towed behind ships in what are known as streamers, to collect data. After the deposit is located, the task becomes more of a development exercise in figuring out how to drill the best well and exploit the field.

Lower prices have helped to make MEMS accelerometers competitive for high-volume consumer applications, according to device maker Memsic.

Higher-quality data is needed to do that, and this is where multicomponent data plays an important role, Denver said. The rock matrix and fluid can change rapidly in the horizontal and vertical directions. Instead of just collecting compression information about rocks, multicomponent data can result in more information about the fluids that are trapped in the gaps, Denver said. This additional information helps to mitigate “dry hole” risk and optimize drilling locations, he said.

Another advantage of the MEMS accelerometer is that it has a wide bandwidth, capable of reading high as well as low frequencies. High-frequency data provides information about thin reservoir zones, faults, and changes that are taking place as fluids are being drained from pores in the rock, said Denver. Higher-frequency signals are critical to accurate interpretation. Low-frequency signals are useful in identifying the type of rock, be it sandstone, shale, or carbonate, for example. Maxwell said the VectorSeis is as rugged as a conventional geophone and can be successfully deployed in down-hole environments to get a closer reading of a reservoir.

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