A review of magnetic sensors

Abstract

The techniques used to produce magnetic sensors encompass many aspects of physics and electronics. Eleven of the most common technologies used for magnetic field sensing are described and compared. These are search coil, flux-gate, optically pumped, nuclear precession, SQUlD, Hall-effect, magnetoresis tive, magnetodiode, magnetotransistor, fiber optic, and magneto-optic. The usage of these sensors in relation to working with or around €arth's magnetic field is also presented. I. MAGNETIC SENSOR TECHNOLOGIESMagnetic sensors have assisted mankind in analyzing and controlling thousands of functionsfor many decades. Computers have unlimited memorythrough theuseof 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.Factories have higher productivity because of the precise stability and low cost of magnetic sensors.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. I n the second half, the major applications of magnetic sensors are discussed in relation to three categories: measuring fields stronger than Earth's field, measuring perturbations in Earth's field, and measuring gradients in generated or induced magnetic fields. A common theme among all applications is that magnetic sensors provide a more rugged, reliable, and maintenancefree technology compared t o other sensor technologies.Magnetic sensing techniques exploit a broad range of physics and chemistry disciplines. Eleven of the more common magnetic sensor technologies are listed in Fig. 1, which compares approximate sensitivity ranges. In some cases, projected ranges based on further improvements are indicated by dashes. It is important to note that the sensitivity range for each concept is influenced by the readout electronics. There are many other factors, particularly frequency response, size, and power, that affect what sensor is best suited for an application. A description of each of these concepts follows.

NMR probes for measuring magnetic fields and field dynamics in MR systems

“…In this case, they give rise to a certain bandwidth and distribution of Larmor frequencies in the probe's NMR response. Such inhomogeneous broadening violates Equation [1], which accounts only for a single relevant field strength at any given time. The resulting error in phase is difficult to estimate without making assumptions about the specific field distribution inside the droplet.…”

“…If interpolation is used this property can be readily recovered by resampling the unwrapped phase at the original sampling frequency. The continuous phase time course is the desired measure of magnetic field integral according to Equation [1], with its precision given by Equations [3] and [4]. For the probe sensitivity achieved in this work, ϭ 10 5 ͱHz, Equation [4] yields a phase resolution of / ͱBW ϭ 7.1 ϫ 10 Ϫ6 rad/ ͱHz.…”

“…1 For such measurements to enable the observation of field dynamics in MR systems the probes need to fulfill a set of performance requirements. The first requirement is sensitivity to phase accrual on the order of 10 Ϫ9 Ts, as discussed in the Introduction.…”

Ultrahigh magnetic field sensitivity in laminates of TERFENOL-D and Pb(Mg1/3Nb2/3)O3–PbTiO3 crystals

“…This is a significant achievement for a totally passive (ϳ0 W power consumed͒ and compact magnetic field sensor that has an output voltage linearly proportional to H. It offers opportunities in sensitive low-level magnetic field and/or electric current sensing. For example, the most sensitive magnetic field sensors are superconducting quantum interference devices ͑SQUIDs͒, which have a maximum sensitivity limit of Ϸ10 Ϫ14 T, when operated below 77 K. 22,23 The next best methods for ultralow magnetic field detection are optical pumping and nuclear precision, 22 offering sensitivity limits of Ϸ10 Ϫ11 T.…”

Magnetic Flux Leakage Method based Local Fault Detection for Inspection of Wire Rope

“…마그네틱 센서는 그 종류가 다양하며 대상 구조 물의 특성에 맞춰 사용하게 되는데 (Lenz, 1990) (Weischedel, 1985;Atherton, 1987;Weischedel et al, 1991;Coktepe, 2001;Mandache et al, 2005;Kim et al, 2009;Park et al, 2012Park et al, , 2014Kang et al, 2014;Kim et al, 2014). (Lenz, 1990). 아를 이용하여 누설자속의 세기를 계측 가능한 전압 값으로 변환하며 (Ramsden, 2006), 이렇 게 변환된 전압 값은 DAQ장비를 통해 계측되어진다.…”