A New Direction for Digital Compasses

The advance could lead to motion sensors showing up in running shoes and tennis rackets.

Cell phones and many other mobile devices now come packed with sensors capable of tracking them as they move. The digital compasses, gyroscopes, and accelerometers embedded in such devices have spawned a wide range of location-based services, as well as novel ways of controlling mobile gadgets—for instance, with a shake or a flick. Now a new way of making these sensors could make such technology smaller and cheaper.

The advance could also result in motion sensors appearing in many more devices and objects, including running shoes or tennis rackets, says Nigel Drew of the Barcelona, Spain-based Baolab Microsystems, which developed the new technology.

Baolab has made a new kind of digital compass using a simpler manufacturing method. The technology will appear in GPS devices next year, says Drew. The company has also made prototype accelerometers and gyroscopes, and plans to combine all three types of sensor on the same chip.

Conventional digital compasses are made using what's called complementary metal-oxide-semiconductor manufacturing, the most common method for making microchips and electronic control circuitry. But such compasses include structures such as magnetic field concentrators that need to be added after the chip is made, which adds complexity and cost. "The fundamental difference is that [our compass is] made entirely within the standard CMOS," says Drew.

This is possible because the compass exploits a phenomenon called the Lorentz force. Most commercial digital compasses rely on a different phenomenon, called the Hall Effect, which works by running a current through a conductor and measuring changes in voltage caused by the Earth's magnetic field.

The Lorentz force, in contrast, happens when a magnetic field generates a force on a conducting material when a current is flowing through it. A device can determine the magnetic field by measuring the displacement of an object upon which this force is acting.

In Baolab's chips, a nanoscale micro-electromechanical system (MEMS) is etched out of a conventional silicon chip. This nano-MEMS device consists of an aluminum mass suspended by springs. When a device drives a current through the mass, any magnetic fields present will exert a force on the mass and affect its resonance. A pair of metal plates that flank the mass will detect these changes. A device can they measure the magnetic field in one direction by noting minuscule changes in the capacitance of these plates. Using a set of three of these sensors, the device can determine direction of the Earth's magnetic field, and hence it's orientation.

"This sort of MEMS-CMOS co-integration technology will improve the sensitivity and enable smaller, and therefore cheaper, sensor chips compared to the conventional ones," says Hiroshi Mizuta, a professor of nanoelectronics at Southampton University's NANO Group.

Each of Baolab's nano-MEMS sensors is less than 90 microns long. Drew says it should be possible to integrate all three types of sensors into a single chip just three millimeters long.