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Mouser Electronics White Paper A designer may also use a single-turn angle sensor combined with a Wiegand sensor, which can harvest energy from a reversing magnetic field while counting turns. The Wiegand sensor can generate sufficient energy to power a nonvolatile ferroelectric RAM (FRAM) without the need for a battery. Like the backup- battery method, this approach is smaller than the mechanical gear solution, yet it still imposes size penalties and can present complex integration challenges. These incumbent solutions present a variety of challenges, including increased risk of mechanical or electrical component failure, loss of absolute position information in the event of equipment failure or a power outage, and the imposition of rehoming or recalibration requirements after the loss of position information. These challenges call for a simpler, more compact, and more robust approach to ensure that equipment comes back online and becomes fully operational with minimal disruption to production. Understanding Contactless Multiturn Sensing The modern, alternative approach to rotary position sensing overcomes the challenges of these incumbent solutions by combining GMR and AMR technologies in a single, compact, integrated device. Within the device, AMR sensing can measure fine- angle position within a single rotation. AMR technology uses a phenomenon discovered by William Thomson (later Lord Kelvin) in the 1850s: The resistance of a current-carrying ferromagnetic material depends on the angle between the direction of current flow and the direction of an externally applied magnetic field, in accordance with the following equation: R=R 0 +∆Rcos(2a) Here, R is the resistance when the angle between the current flow and the magnetic field directions is a, R 0 is the base resistance when current flow and the applied magnetic field are perpendicular, and ∆R is the maximum change in resistance over the allowable values of a. The application of Thomson's discovery was not practical for more than a century, until the appearance of thin-film deposition technology. Today, an integrated AMR sensor can simply measure the internal resistance of integrated Wheatstone bridges in the presence of an applied magnetic field and then interpret the differential bridge outputs to derive angular position. AMR sensing can be applied to linear as well as rotational displacement measurements, as shown in Figure 2, which uses the Analog Devices ADA457x AMR device as an example. Engineers can achieve turn-count measurement in addition to angle measurement by adding a GMR sensor. These sensors use a GMR process, which sandwiches a nonmagnetic material between layers of ferromagnetic material. The bottom ferromagnetic layer is fixed, and the top ferromagnetic layer floats on the nonmagnetic material. As Figure 3 displays, when the magnetization of the two ferromagnetic layers aligns (left), resistance is low, but when the top layer magnetization opposes that of the bottom layer (right), resistance is relatively high. Turn-count information is encoded and retained through shape anisotropy and the propagation of magnetic domain walls within a spiral nanowire, a form of Figure 2: An AMR sensor used for linear displacement measurement (top left), off-shaft rotation measurement (top right), and end-of-shaft rotation measurement (bottom). (Source: Analog Devices)

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