Magnetic Proof Mass Configurations to Reduce Spatial Dependency of Piezomagnetic AC Current Sensors

AC sensors for current carrying wires that combine a piezoelectric cantilever with a magnetic proof mass have been developed to act as a dual functional zero power sensor and energy harvesting device to power wireless sensor networks. However, practical limitations reduce the sensors accuracy due to the high spatial location dependency between the magnetic proof mass and the wire. A misalignment along the lateral direction leads to a significant change in voltage output and current measurements, which decreases the accuracy and repeatability of the sensor. This paper investigates methods to increase sensor accuracy by varying the magnetic proof configurations to reduce the demand for precise lateral spatial dependency. This was accomplished by altering the shape of the magnetic proof mass to a triangular shape to create a more uniform torque distribution on the cantilever resulting in a consistent strain regardless of lateral spatial location of the proof mass to the wire. The results demonstrate that the magnetic proof mass shapes significantly affected the broadness of output voltage as a function of lateral location resulting in increased sensing accuracy for triangular shaped proof mass. The paper validated the concept both numerically through multiphysics finite element analysis as well as experimentally by creating unique shaped magnetic proof mass using neodymium iron boron powders with a parylene-N capping layer. The sensor with a long triangular magnetic proof mass demonstrated a 1% error due to 1 mm spatial misalignment compared to a rectangular magnet that demonstrated an error of 28.9% under the same conditions.