A substantial correction to this article was made on 7 March; to read it, click here.
Nintendo’s Wii is the hottest computer game and arguably the only one that’s good for you. Its two wireless remote controls track any movement, encouraging players to engage opponents with a heart-pounding physicality that is already melting fat off overfed children. Yet although detecting motion is critical to the success of the US $250 game, the job depends on $3 sensors the size of shirt buttons.
The supplier of the sensors, STMicroelectronics, got into the business a decade ago in order to squeeze a few more dollars out of an obsolescent chip-making plant. ”We wanted something good for it that didn’t require deep submicron technology,” says Benedetto Vigna, the Italian physicist who developed the sensor.
In 1995, a month after Vigna joined the Geneva-based chip maker, his boss asked him if he was interested in MEMS. Sure, Vigna said, what’s MEMS? His boss could only spell out ”microelectromechanical systems” and explain that the technology could use outdated photolithographic tools, because it sculpted silicon into things measured in micrometers, not nanometers. It made mechanical the beams, levers, and springs of real, moving machines—the kind a watchmaker would recognize if he had a microscope.
Vigna learned what he could on his own, and a year later he went off to the University of California at Berkeley to study MEMS and do some work-study stints at local companies. Then ST reeled him back, assigned him a staff, and invested heavily in their research. In 2001, three years into the project, Vigna hit on the ideal mass-market product: a chip that could detect motion in three dimensions.
There were already tiny, cheap MEMS devices that could detect motion in two dimensions. That is all that some applications really need—the airbag in a car, for instance, which inflates in the same direction as the collision that it cushions. And there were already big, bulky sensors costing up to $30 000 for airplanes and rockets. Nobody, however, had bothered to fill the gap between the market’s high and low ends.
One D, Two D
Motion detection begins with a device called an accelerometer, a cantilever hewn from silicon and teetering between two electrodes. Apply a 1-volt field, and the cantilever’s beam will vibrate; accelerate the package, either by pushing it in one dimension or by rotating it, and the beam’s tip will trace an ellipse. The eccentricity of the ellipse measures acceleration. Place two such accelerometers at right angles, and you can track acceleration in a plane—add a third, and you can track it in space.
ST’s earliest device occupied a cubic inch, a lot smaller than the brick-size gizmos in aircraft, but still way too big for a consumer product. So ST’s elves set about shrinking it.
In the photo at right, Vigna [center] shows editors Erico Guizzo [standing] and Philip E. Ross a series of his company’s three-dimensional motion sensors. For a close-up of the series, click on the image.
Vigna's team played with various configurations of the basic design—basically, a mass and a spring—and refined the accompanying electronic circuit until it could discern the displacement of fewer than 10 electrons. That way, it could detect very slight motion. The researchers also had to tweak everything so that the sensor would work at both high and low acceleration. ”We can measure a flick of the wrist or a big movement of the arm, which wasn’t true before at this price level,” Vigna says.
To keep costs down, ST found a way to pack the entire mechanical system in plastic rather than in metallic ceramic, as in high-end accelerometers. ”Plastic had been considered too weak,” Vigna says, ”but we designed the whole system to eliminate the package’s parasitic effects,” that is, its tendency to vibrate along with the sensor.
You might wonder why a device so critical to an expensive product has to sell at $3 rather than $4 or $10. The answer is simple: in the consumer electronics business, every penny counts. Steve Jobs may have convinced millions of people to download songs at 99 cents a pop, but if he’d set the price at $1.25, he’d be known for only two fabulous, world-beating products.
One of the first applications of the 3-D sensor was in laptops, where sensors guard against damage from a fall. In the split second of free fall that comes before the collision with the floor, the sensor tells a controller to park the read/write head safely away from the hard drive.
Another application came in 2003, in a Maytag washing machine that uses a somewhat smaller sensor—14 millimeters by 7 mm by 4 mm—to detect vibrations due to an unbalanced load and to adjust the washer’s speed to dampen them.
Another ST product enables the user of a cellphone or a PDA to adjust the display of images or retrieve data from memory by just tilting the device. It sure beats trying to use grown-up fingers to punch commands on baby-size buttons.
Core of the product
Games were already on Vigna’s to-do list when he discovered that Nintendo, in Kyoto, was ahead of him. ”We met Nintendo in March of 2005—our vision was in line with their vision, and we got married,” he says. Two months later, ST delivered a prototype sensor, and 16 months after that, Nintendo launched worldwide sales. Since then, the demand for the game has strained production. IEEE Spectrum could find no Wii boxes at the listed price; to get our test version, we paid a $100 markup to an enterprising reseller on eBay.
Vigna says the Wii has been the biggest application of all ”because the MEMS chip is the core of the product.” (To be sure, the motion detector doesn’t do the whole job by itself; an infrared system helps, by setting the player’s initial position.)
In recognition of his work, ST recently made Vigna the general manager of its MEMS product division. That means he has to plan for the long term.
First he wants to make the sensor even smaller, even cheaper, even tougher. ”I want it to fit in all kinds of places—shoes and textiles, for instance, where it might be useful for medical monitoring,” he says.
”Then I want to make a three-dimensional gyroscope, to measure rotation around three different axes. Today, such products are quite big, a cube 10 centimeters on a side. We want to do this in less than a 30-millimeter cube, to serve as an image stabilizer in cameras and to track a person’s position in the intervals when he can’t get a GPS signal.”
Better still, he adds, would be to throw in a magnetic detector, freeing the navigator from GPS altogether. It would be yet another marvel from Lilliput—the smallest compass ever sold.