Magnetic tuning of a kinetic energy harvester using variable reluctance

In this paper we present a new technique for tuning the resonant frequency of an energy harvester using a variable reluctance device which changes the strain in a cantilever beam to alter its spring constant. In order for energy harvesting devices to be able to operate reliably in many applications they must be able to generate energy as the input excitation frequency changes. Most harvesters are resonantly tuned mass-spring-damper devices and therefore it is important that their resonant frequency is tuneable during operation. Several mechanical methods have previously been demonstrated for accomplishing this task which operates by altering the stress in a cantilever beam by altering the distance between a fixed tuning magnet and a magnet on the moving cantilever. Here, we demonstrate a new actuation method for manipulating the stress in a beam. The actuation mechanism alters the magnetic reluctance between the cantilever mounted and the fixed magnet. The investigation has highlighted the importance of the design of the magnetic circuit and choice of materials in order to avoid eddy current damping and asymmetrical forces. The method presented here has demonstrated a maximum tunable frequency range of 11.1 Hz and may be more suitable for microfabrication than the previously reported techniques due to the reduced tuning force which makes microfabricated actuators feasible.