Let Your Mobile Do the Pointing
By Mark Frauenfelder, Wed Sep 15 13:15:00 GMT 2004

A tiny magnetic sensor in your mobile phone could turn the planet into a point-and-click Yellow Pages directory.

We'd be lost without the Earth's magnetic field -- literally. But thanks to the liquefied iron and nickel swirling around in our planet's core, people and migratory animals have learned to use the resultant magnetic field to help them figure out where they need to go. Birds have built-in magnetic sensory apparatus that they use for navigation, but people must resort to a tool called a compass. Compasses have been used for navigation for over 2,000 years, and the basic design hasn't changed much -- a magnet on a freely rotating mount that swivels to stay parallel with the Earth's magnetic flux lines -- until the 1960s, when electronic magnetic sensors were developed.

From Lodestone to Silicon

Today, electronic compasses have become small enough that manufacturers are considering them for use in mobile phones and other wireless devices. In August, the leader in anisotropic magnetoresistive (AMR) sensors, Honeywell International, announced that it was making millions of tiny sensors for a number of major mobile phone manufacturers. The chip, called the HMC6052, measures 4 by 4 by 0.9 mm, and is being targeted by Honeywell for use in wristwatches, mobile phones and PDAs. (The company has not disclosed the names of the companies that will use the chips, but the phones are for the Asian and European markets.)

While there are many ways to make an electronic compass, AMR sensors are ideal for mobile applications because they're inexpensive, have high sensitivity, are immune to background noise and come in small packages.

Honeywell's magnetic sensors exploit a property of magnetic materials, called anisotropic magnetoresistance. When a magnetic field passes though a thin strip of ferrous material that has a current running through it, the resistance of the material changes. By arranging strips of the material to form a "Wheatstone Bridge" (a 150-year-old design for measuring very small changes in resistance), it's possible to determine the orientation of the device with respect to the Earth's magnetic field. Honeywell's magnetoresistive sensing elements are made from a nickel-iron alloy called Permalloy, which is deposited as a thin film on a silicon substrate.

When GPS Is Not Enough

What good is a compass in a mobile phone? For one thing, it will enhance location-based GPS services. GPS alone is fine for telling you where you are, but unless you're moving quickly enough for the system to be able to determine you're heading, it can't figure out which direction you're pointed. A magnetic "heading" sensor in a phone can solve that problem.

A magnetic sensor in a phone or car navigation system can also help people get around in "urban canyons," where GPS satellites can't reach a GPS unitís antenna. As long as the carís odometer is accurate and is tied into the system, a navigational system based on magnetic sensing could do a decent job of guiding you to your destination.

The most exciting mobile application for magnetic sensors is the capability to map an online "Yellow Pages" on top of the real world, allowing users to point their phones in the direction of a building or other public area and get information about it. For example, say you're driving down the street and see a bookstore you'd like to visit later. You could simply point your phone at the store and press a button on your phone, sending the GPS coordinates and direction information to a service that returns the operating hours and additional information about the store, along with a coupon for 10% off your purchase. If you point it at a restaurant, you could get the Zagat rating, the menu and the opportunity to make a reservation.

How much will it cost to put a magnetic sensor in a phone or car? Mark Amundson, an applications engineer for magnetic sensors at Honeywell Internationalís Solid State Electronics Center, believes the "cost of integrating electronic compasses into handheld and vehicle platforms can range from a couple of dollars to several tens of dollars per unit, depending on the scale of manufacturing."

The additional cost could either be paid for by advertisers, or by increasing the price of the phone by a few bucks. Who cares? Either way, magnetic sensors are pointing the mobile market in a cool new direction.