UC Berkeley

As the electric power grid incorporates an increasing number of renewable energy sources and large, variable loads, better electricity monitoring at increased resolution becomes essential. Simultaneously, the rise of wireless sensor nodes, a.k.a. "motes," has made larges-cale, low-power data collection increasingly-feasible. The existing technologies for AC current sensing, however, are less than optimally-suited for this sort of application; current transformers are large and bulky, and most proximity-based magnetic field sensors consume too much power for extended operation on batteries. In the work that follows, we will explore the modeling, design, prototyping, and testing of a new, novel type of proximity-based current sensor that is both small and ultra-low-power.

By using a pair of interconnected piezoelectric cantilevers with permanent magnets mounted on their free ends, it is possible to sense AC current by proximity while simultaneously rejecting ambient mechanical vibrations. Starting from an analytic system-as-a-circuit model, a design strategy is implemented using dynamic range as a primary figure of merit to develop an aluminum nitride, MEMS-based candidate sensor. Primary production of this candidate design is completed by MEMSCAP Inc, and final assembly and integration of the designs is completed at UC Berkeley.

Using the 11 mm-long MEMSCAP-produced aluminum nitride dies and two 750-micron magnets, a dual-cantilever current sensor with a sensitivity of 6.41 mV per gauss has been produced. Due to non-idealities in fabrication, this output voltage is less than originally modeled but still close to the design target of 7 mV per gauss . The vibration-cancellation technique functions as-intended for frequencies below 150 Hz or above 185 Hz. Proof of concept testing of this technique is demonstrated for 1-35 A of 60 Hz AC current with the sensor in proximity to a vibrating motor generating 50 mG of 120 Hz sinusoidal acceleration.