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SENSORS & SENSING SYSTEMS
FEATURE
ant Magnetoresistance sition sensors
reduced from the maximum and minimum operating window of the GMR sensor due to variations in the manufacturing process of the magnetic material. The green dotted lines show the maximum and minimum expected magnetic field due to a typical manufacturing variation of >5%.
Magnet Simulation The simulation of the magnet within the mechanical operating environment can take different forms. There are two types of simulation commonly used to design the magnet: an analytic simulation or finite element analysis (FEA). The analytic simulation solves the magnetic field using the bulk parameters of the magnet being simulated (size, material) without
considering the surroundings other than the Figure 2. The reference design magnet
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 confidence that the system will operate as expected over the full temperature range. In Figure 1, the solid green traces represent a window of the magnetic field strength that the magnet should be designed to produce over the active area of the GMR sensor. This window is
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 themeasured 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 the website or contact the ADI sales team.
Analog Devices
analog.com
Figure 3. Magnetic field distribution with an air gap of 1.42mm and 2.45mm FEBRUARY 2024 DESIGN SOLUTIONS 47
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