Planar Variable Inductor Controlled by Ferrofluid Actuation

This paper reports a micropatterned variable inductor that utilizes ferrofluid as the movable magnetic core. Ferrofluid is displaced over the planar inductor using the magnetic field provided by another "actuation" coil aligned to the inductor, varying permeability distribution on the inductor to control its inductance. Superimposing a bias field on the varying magnetic field uniquely enables repelling of the fluid from the inductor, as well as switching between repelling and attracting motions simply by changing the direction of the driving current to the actuation coil. Microfabricated devices are tested to reveal that the repelling-mode operation provides 6.1 x larger inductive changes than those available with the attracting mode. The continuous modulation of inductance is demonstrated with a tuning range of 16% using input currents up to 1.2 A. Frequency tuning using a resonant tank formed with the device is also demonstrated to show a 76-ppm/mA frequency sensitivity to the input current.