AMR sensor array design for permanent magnet 3D motion tracking

State-of-the-art tactile sensors based on magnetic transduction principles measure the change in flux density resulting from the applied force and often exploit conventional Hall-effect sensors. Compared to the latter, magnetoresistive (MR) sensors exhibit enhanced signal-to-noise ratio, larger sensitivity and increased thermal stability. In this context, anisotropic MR (AMR) sensors are appealing due to their relatively simple and cheap fabrication process, which makes them easily prone to miniaturization thus enabling the achievement of high sensitivity at low cost in a compact footprint. Here, we combine numerical methods and analytical calculations to design planar AMR sensor arrays capable of tracking the 3D motion of a permanent magnet, thereby laying the foundation for realizing miniaturized 3D tactile sensors suitable to detect both normal and shear force components.
The proposed tactile sensor concept includes a permanent magnet embedded in a deformable membrane moving relative to a fixed planar array of AMR sensors. The latter are made of thin Permalloy (Ni80Fe20) stripes whose MR response is linearized via barber-pole biasing. For modelling purposes, the permanent magnet’s magnetic field is calculated by means of analytical expressions via the Magpylib Python package and used as input in the finite-difference micromagnetic simulations (MuMax3) computing the AMR sensor response: the system ground state is determined by partitioning the Permalloy stripes into a finite number of regions, each with a different external magnetic field, which accounts for the field inhomogeneities at the AMR sensor position. The barber pole geometry is finally optimized via finite-element simulations (Ansys Maxwell).
Two AMR sensor array design concepts have been found which are suitable for tracking the 3D motion of sub-mm NdFeB magnets by solving a magnetic inverse problem. In both cases, the Permalloy stripes are arranged in Wheatstone bridges. The first design enables the reconstruction of the magnet position with ca. 10-µm accuracy within a motion range of 600 µm along x and y and of 300 µm along z, while the second one covers a wider motion range in all directions but with a 5-10 times lower accuracy.
These results demonstrate the possibility to track the 3D movement of a permanent magnet via properly designed and monolithically fabricated planar arrays of AMR sensors. Future work will be devoted to sensor design optimization and experimental validation. The versatility of the device concept discussed here holds promise for the manufacture of not only tactile sensors, but also of a large spectrum of other easy-to-fabricate, miniaturized and low-cost sensors detecting a wide variety of observables (e.g., pressure, fluid flow, acceleration).