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Low-cost magnetic field sensors

June 26, 2002

People who can claim possession of the latest generation of outdoor wristwatch may rightly assume that the conventional compass with its magnetic needle has served its time. Thanks to modern, space-saving chip technology, new features are constantly being integrated: altimeters, cameras, or even electronic compasses can be conveniently worn on the wrist. Yet today, large numbers of magnetic field sensors are also required for less obvious technical applications: They measure position, torsion and the number of revolutions of shafts and wheels, as well as monitoring the operational status of robots and machine tools. Integrated in joysticks, the tiny sensors "see" where computer game players are aiming at.

When magnetic materials such as steels change their spatial position, they influence the geomagnetic field surrounding them. This fact makes the sensors suitable for all kinds of non-contact measuring and monitoring systems. For this reason, the Fraunhofer Institute for Integrated Circuits IIS has developed a new type of chip which will soon be produced cost-efficiently and on a large scale thanks to standard CMOS technology. The scientists are now hoping that industrial companies will integrate the chips in their devices.

"Admittedly, the chips don't feature particularly high resolution", says group manager Dr. Hans-Peter Hohe. "However, an accuracy of one microtesla is more than adequate for most applications - and this in all spatial dimensions. By comparison, the Earth's magnetic field measures approximately 40 to 60 microtesla." In addition to keeping costs to a minimum, researchers are also concerned with developing a chip that digitally processes its signals, since digital sensors feature certain advantages over purely analog sensors, such as a significantly lower degree of drift. The researchers were able to reduce this offset to such an extent that they can measure fields of a few microtesla to several tesla, such as those produced by MRI scanners. Furthermore, the new sensors are less sensitive to temperature than their predecessors.

"In general, the trend is shifting towards combining all sorts of digital functions on one chip," summarizes Dr. Hohe. "This is also the case with our circuits: We integrate functions for failure recognition and self-monitoring directly on the chips." The digital signal chain to the computer is continuous and unreliable analog data transmission is no longer necessary.

Fraunhofer-Gesellschaft