Magnetic Sensors and Their Applications
James Lenz and Alan S. Edelstein
Abstract—Magnetic sensors can be classiﬁed according to whether they measure the total magnetic ﬁeld or the vector components of the magnetic ﬁeld. The techniques used to produce both types of magnetic sensors encompass many aspects of physics and electronics. Here, we describe and compare most of the common technologies used for magnetic ﬁeld sensing. These include search coil, ﬂuxgate, optically pumped, nuclear precession, SQUID, Hall-effect, anisotropic magnetoresistance, giant magnetoresistance, magnetic tunnel junctions, giant magnetoimpedance, magnetostrictive/piezoelectric composites, magnetodiode, magnetotransistor, ﬁber optic, magnetooptic, and microelectromechanical systems-based magnetic sensors. The usage of these sensors in relation to working with or around Earth’s magnetic ﬁeld is also presented. Index Terms—Anisotropic magnetoresistance, ﬂuxgate, giant magnetoresistance, magnetic, magnetic tunnel junctions, magnetoresistance, microelectromechanical systems (MEMS), optically pumped, sensor, spin valves, superconducting quantum interference device (SQUID).
Fig. 1. Estimate of sensitivity of different magnetic sensors. The symbols E and GMN are used to indicate the strength of the Earth’s magnetic ﬁeld and geomagnetic noise, respectively.
I. INTRODUCTION TO MAGNETIC SENSOR TECHNOLOGIES HIS paper discusses and reviews magnetic sensors and their applications. Magnetic sensors have assisted mankind in analyzing and controlling thousands of functions for many decades. Computers have nearly unlimited memory through the use of magnetic sensors in magnetic storage disks and tape drives. Airplanes ﬂy with higher safety standards because of the high reliability of noncontact switching with magnetic sensors. Automobiles use magnetic sensors to determine position in several places such as the engine crank shaft and wheel braking. Factories have higher productivity because of the stability and low cost of magnetic sensors. The subject has been reviewed previously –. There are many ways to sense magnetic ﬁelds, most of them based on the intimate connection between magnetic and electric phenomena. In the ﬁrst half of this paper, the more popular sensor technologies will be described with examples of products. In the second half, the major applications of magnetic sensors are discussed in relation to four categories: measuring ﬁelds stronger than the Earth’s ﬁeld, measuring perturbations in the Earth’s ﬁeld, measuring small changes or gradients in generated or induced magnetic ﬁelds, and medical/biological applications. A common theme among all applications is that magnetic sensors provide a very rugged, reliable, and maintenance-free technology. Manuscript received October 1, 2004; revised December 6, 2005. This work was supported by the ONR and DARPA. The associate editor coordinating the review of this paper and approving it for publication was Dr. Usha Varshney. J. Lenz is with the University of Minnesota, Minneapolis, MN 55454 USA (e-mail: firstname.lastname@example.org). A. S. Edelstein is with the U.S. Army Research Laboratory, Adelphi, MD 20783-1197 USA (e-mail: email@example.com). Digital Object Identiﬁer 10.1109/JSEN.2006.874493
Magnetic sensing techniques exploit a broad range of ideas and phenomena from the ﬁelds of physics and material science. Most of the more common magnetic sensor technologies are listed in Fig. 1, which compares approximate sensitivity ranges. In Fig. 1, the symbols E and GMN are used to indicate the strength of the Earth’s magnetic ﬁeld and geomagnetic noise, respectively. Because of the large magnitude of the Earth’s magnetic ﬁeld, sensitive sensors must either have a large dynamic range or use a coil to decrease the ﬁeld at the position of the sensor. Geomagnetic noise is spatially correlated over distances of the order of kilometers because...