Adaptive inertia tuning of an energy harvester for increasing its operational bandwidth

Abstract A rotational energy harvesting system comprises a sprung mass coupled to an electrical generator through a motion transmission system such as a ball screw. In this paper, the operational bandwidth of a rotational energy harvester is expanded by varying its moment of inertia and load resistance of the generator. It is shown that the resulting tuneable device produces significantly higher amounts of harvested power. In addition to mass and stiffness, the natural frequency of a rotational device is defined by its moment of inertia, an additional design parameter that enables implementing the approach presented here. This parameter also determines the apparent mass (inertance) of the device, an important factor that allows a small additional mass to increase the apparent mass hugely and hence increase the overall power density of the harvester. It is shown that the system with variable load resistance shows a good performance at frequencies around the natural frequency of the device whereas away from resonance frequencies the system with variable moment of inertia produces more power. The approach described in this paper is a first step in the direction of having an autonomous energy harvester with a wide operational bandwidth. One of the advantages of the presented method is that, unlike some other methods, changing the adjustable parameters (i.e., moment of inertia and load resistance) can be conducted intermittently. In other words, this approach only consumes power during the tuning operations and does not use energy once the harvester is tuned at its optimum conditions. These tuneable rotational systems should be used where the excitation frequency varies slowly (e.g., in marine environment) as any sudden changes in frequency would result in an instantaneous change in the apparent mass and the device may even stall. To implement the device effectively, some kind of predictive control may need to be used that can detect frequency variations fast enough for the inertia to change in a timely manner. This aspect that is outside the scope of this paper is currently under investigation.