Sensor Technology
Figure 3. Magnetic field distribution with an air gap of 1.42mm and 2.45mm.
(size, material) without considering the surroundings other than the assumption it is operating in air. This is a quick calculation and useful when there are no adjacent ferromagnetic materials. FEA can model the effects of ferrous material in a larger magnetic system, which is essential when combining the magnet with a stray magnetic field shield or ferromagnetic materials close to the magnet or sensor. FEA is a time-consuming process, so usually a starting point for this would be a basic magnet design from an analytical analysis. FEA was used in the simulation of the reference design for the magnet and stray field shield.
Magnet design feature
The reference design magnet resulting from the simulation consists of a SmCo magnet
with an integrated steel stray field shield, as shown in Figure 2. The magnet is designed to be injection moulded so it is capable of being mass produced. Injection moulding of SmCo magnets is common due to the ability for it to produce complex shapes and it is used widely in automotive and industrial applications. The assembly is designed to be an interference fit with a 9mm diameter shaft; however, modifications to the bushing are possible to allow attachment to shafts of different sizes.
Magnet characterisation Careful characterisation of the magnet assemblies has been carried out to demonstrate the robust magnetic solution for the GMR sensor. The key to characterisation is the ability to perform detailed maps of the
magnetic field strength across an extended magnet to sensor placement window in a control environment. Key to the success of characterisation is a good understanding and calibration of the magnetic field probes used. Figure 3 shows an example of the measured field strength at two different air gaps, repeating these measurements across the full operating temperature range and air gap range is time consuming but is essential to understanding the performance of the magnet to ensure it operates under the required conditions.
Conclusion
In summary, the reference design magnet has been shown to meet the requirement for operation with a temperature of –40°C to +150°C, with an air gap of 2.45mm
±1mm, and an axial to sensor placement tolerance ±0.6mm.
The ADMT4000 is the first integrated true power-on multi-turn position sensor and is set to significantly reduce system design complexity and effort, ultimately resulting in smaller, lighter, and lower cost solutions. The reference design will be available to ADI’s customers to enable designers with and without magnetic design capability to add new and improved functionality to current applications and open the door to many new applications. To find out more about the ADMT4000 and the magnetic reference design, please visit
analog.com or contact your local ADI’s sales team who will be happy to discuss your requirements and applications.
analog.com
www.cieonline.co.uk
Components in Electronics
October 2024 19
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