A gap-varying electrostatic transducer utilizing ferrofluid-based actuation for motion harvesting

This paper provides the electrical characterization of the gap-varying ferrofluid-based electrostatic springless proximity inertial harvester (SPIH). The SPIH is a multi-axis motion harvesting structure capable of three-dimensional low-frequency operation from human or environmental application scenarios among others. The device structure consists of an array of electrostatic transducers that are interconnected and filled with a magnetic fluid. A spherical magnet serves as the proof mass. Mechanical energy is transferred to each transducer magnetically. A hydrostatic pressure in the magnetic fluid actuates the top plate of each variable capacitor. Each 2-mm-diameter transducer is capable of producing between 0.05-4.2 nJ of energy per actuation cycle at bias voltages of 10-100 V under controlled experiments. Harvesting multi-axial motion from random hand movements (including x- and y-axis translation and rotation) is demonstrated to produce peak power levels as high as 3 nW (with only one transducer from the array connected) and by using a 10 V initial bias.