SENSORS & SENSING SYSTEMS u ANALOG DEVICES
Magnet Design for Giant Magnetoresistance Multi-turn Position Sensors
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.
By Stephen Bradshaw, Product Applications Engineer, Christian Nau, Product Applications Manager, and Enda Nicholl, Strategic Marketing Manager, Analog Devices
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. Magnetic write errors may occur if the magnetic field is either too high or too low. It is 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 developed by ADI 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 per cent.
MAGNETIC CONSIDERATIONS When designing the magnet, there are some
Figure 1. A thermal coefficient comparison of the operating window vs. a typical SmCo magnet.
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); 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
30 September/October 2025 Irish Manufacturing
placement tolerance of the magnet with respect to the sensor. Low cost magnetic materials such as ferrites have a much higher TC than the GMR sensor, which would limit the operating temperature range compared with materials such as samarium-cobalt (SmCo) or neodymium-iron- boron (NeFeB). Understanding the TC of the chosen magnetic
material as well as the variation in the magnetic field strength due to manufacturing variations allows the required magnetic field strength at room temperature (25°C) to be determined. Design simulations may then be carried out at room temperature with a high degree of
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