A Foucault Pendulum on a Chip

1 February 2011—A new type of microscopic gyroscope could lead to better inertial guidance systems for missiles, better rollover protection in automobiles, and balance-restoring implants for the elderly.

Researchers from the MicroSystems Laboratory at the University of California, Irvine (UCI), described what they’re calling a Foucault pendulum on a chip at last week’s IEEE 2011 conference on microelectromechanical systems (MEMS) in Cancun, Mexico. A Foucault pendulum is a large but simple mechanism used to demonstrate Earth’s rotation. The device the UCI engineers built is a MEMS gyroscope made of silicon that is capable of directly measuring angles faster and more accurately than current MEMS-based gyroscopes.

”Historically it has been very pie-in-the-sky to do something like this,” says Andrei Shkel, professor of mechanical and aerospace engineering at UCI.

Today’s MEMS gyroscopes don’t measure angles directly. Instead, they measure angular velocity, then perform a calculation to figure out the actual angle. When something is in motion, such as a spinning missile, keeping track of its orientation requires many measurements and calculations, and each new calculation introduces more error. Shkel says his gyroscope is more accurate because it measures the angle directly and skips the calculation. ”You’re pretty much eliminating one step,” he says.

The gyroscope works on the same principle as does the Foucault’s pendulum you’d find in many museums, demonstrating Earth’s spin. The plane on which the pendulum oscillates stays in one position relative to the fixed stars in the sky, but its path over the floor gradually rotates as the world turns. Similarly, the oscillation of a mass in the gyroscope stays the same with respect to the universe at large, while the gyroscope spins around it.

Of course, the pendulum in Shkel’s two-dimensional device is not a bob on a string. Instead, four small masses of silicon a few hundred micrometers wide sit at the meeting point of two silicon springs that are at right angles to each other. A small electric current starts the mass vibrating in unison. As the gyroscope spins, the direction of the vibrational energy precesses the same way a swinging pendulum would.

The gyroscope operates with a bandwidth of 100 hertz and has a dynamic range of 450 degrees per second, meaning it detects as much as a rotation and a quarter in that time. Many conventional microgyroscopes (at least those of the ”mode matching” variety) operate at only 1 to 10 Hz and have a range of only 10 degrees per second. But inertial guidance systems—such as those that stabilize an SUV when it hits a curb or keep a rapidly spinning missile on track—require both high dynamic range and high-measurement bandwidth to accurately and quickly measure directional changes in such moving objects.

Shkel described and patented the concept for a chip-scale Foucault pendulum back in 2002, but the device’s architecture requires such precise balance among its elements that it is too hard to manufacture, even nine years later. But last week, Shkel’s colleague Alexander Trusov presented a new design, which Shkel says is more complicated in concept but easier to make, requiring standard silicon processes and only a single photolithographic mask.

But it’s just one possible design. Shkel is on leave from his academic post and currently working with the U.S. Defense Advanced Research Projects Agency (DARPA), which has launched a program to create angle-measuring gyroscopes for better inertial guidance systems. Three-dimensional designs that use concepts other than the one behind his 2-D device might be preferable for DARPA’s needs because they’ll take up less space, Shkel says. He hopes the DARPA program will also improve manufacturing processes in general, giving conventional microgyroscopes higher precision for applications that don’t require the bandwidth and dynamic range of a chip-scale Foucault pendulum.

”We will have a new class of devices,” he says, ”but we will also help existing devices.”