This manuscript describes the development of a new MEMS sensor for the measurement of AC electric current. The sensor is comprised of a MEMS piezoelectric cantilever with a microscale permanent magnet mounted to the cantilever’s free end. When placed near a wire carrying AC current, the magnet couples to the oscillating magnetic field surrounding the wire, causing the cantilever to deflect, and piezoelectric coupling produces a sinusoidal voltage proportional to the current in the wire. The sensor is itself passive, requiring no power supply to operate. It also operates on proximity and need only be placed near a current carrier in order to function. The sensor does not need to encircle the current carrier and it therefore can measure current in two-wire zip-cords without necessitating the separation of the two conductors. Applications for this sensor include measuring residential and commercial electricity use and monitoring electric power distribution networks.
An analytical model describing the behavior of the current sensor was developed.This model was also adapted to describe the power output of an energy scavenger coupled to a wire carrying AC current. A mesoscale sensor exhibited a sensitivity of 75 mV/A when measuring AC electric current in a zip-cord. A mesoscale energy scavenger produced 345μW when coupled to a zip-cord carrying 13 A.
MEMS current sensors were fabricated from aluminum nitride piezoelectric cantilevers and composite permanent magnets.The cantilevers were fabricated using a four-mask process. Microscale permanent magnets were dispenser-printed using NdFeB magnetic powder with an epoxy binder. The MEMS AC current sensor was interfaced with amplification circuitry and packaged inside an aluminum enclosure. The sensor was also integrated with a mesoscale energy scavenger and power conditioning circuitry to create a fully self-powered current sensor. Unamplified sensitivity of the sensor was 0.1–1.1 mV/A when measuring currents in single wires and zip-cords. The self-powered current sensor operated at a 0.6% duty cycle when coupled to the zip-cord of a 1500 W space heater drawing 13 A. The self-powered sensor’s energy scavenger transferred energy to a 10 mF storage capacitor at a rate of 69μJ/s.
December 31, 2009
Leland, E. S. (2009). A MEMS Sensor for AC Electric Current. (n.p.): University of California, Berkeley.