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FEATURE


SENSORS & SENSING SYSTEMS


Magnet Design for gia Multi-turn Pos


True power-on multi-turn sensors based on giant magnetoresistance (GMR) sensing technology are set to revolutionise the position sensing


market in both industrial and automotive use cases due to reduced system complexity and maintenance requirements compared to


existing solutions. As well as examining some of the key factors that must be considered when designing a magnetic system to ensure


reliable operation even in the most demanding applications, this article introduces a magnetic reference design that is available for early


adoption of the technology. By Analog Devices’ Stephen Bradshaw, product applications engineer, Christian Nau, product applications manager, and Enda Nicholl, strategic marketing manager


T


he multi-turn sensor is essentially a magnetic write and electronic read memory combined with a conventional


magnetic angle sensor to provide a highly accurate absolute position. The magnetic write process requires the incident magnetic field to be maintained with a specific operating window. However, magnetic write errors may occur


if the magnetic field is either too high or too low. It is therefore essential to design the system magnet carefully and to consider any stray magnetic fields that might interfere with the sensor as well as mechanical tolerances over the life of the product. Small stray magnetic fields could cause an error in the measured angle while larger stray magnetic fields could cause a magnetic write error leading to a gross turn count error.


Magnetic reference Design goals


A careful understanding of the system requirements is necessary to design the optimum magnet and shielding. Generally, the looser the system requirements, the larger and more expensive the magnet solution required to achieve the target specifications. Analog Devices is developing a series of


magnetic reference designs addressing various mechanical, stray field, and temperature requirements that can be adopted by customers of the ADMT4000 true power-on multi-turn sensor. The first design covers systems with relatively loose tolerances: sensor to magnet placement of 2.45mm ±1mm, a total displacement of the sensor to the axis of rotation of ±0.6mm, operating temperature range of –40˚C to +150˚C, and stray magnetic field shield attenuation of greater than 90%.


4 DESIGN SOLUTIONS FEBRUARY 2024 6


Figure 1. A thermal coefficient comparison of the operating window vs. a typical SmCo magnet


Magnetic consiDerations


When designing the magnet, there are some key considerations to take into account and the following section provides a high level view of the main aspects to consider when designing for the GMR sensor.


Magnet Material The GMR sensor operates in a defined magnetic window (16mT to 31mT). [The operating window


is subject to change pending the release of the ADMT4000]. In addition, the maximum and minimum operating range has a thermal coefficient (TC) as can be seen by the red traces in Figure 1. Selecting a magnet material with a TC that matches that of the GMR sensor will maximise the allowed variation of the operating magnetic field. This allows for greater variation in the strength of the magnet and/or the placement tolerance of the magnet with respect to the


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