Technical note | Modelling of a Bi-axial Vibration Energy Harvester
This report fully details the techniques involved in the modelling of a nonlinear and bi-axial vibration energy harvesting device. The device utilises a wire-coil electromagnetic (EM) transducer within a nonlinear oscillator created with a permanent-magnet/ball-bearing arrangement. The mechanical oscillations of the ball-bearing in response to bi-axial vibrations in a host structure induce a voltage across the coil, and therefore energy to power an attached device - such as an in-situ structural health monitoring system on an aircraft platform. Modelling of the mechanical dynamics and the electromechanical transduction of the harvester is undertaken by: means of finite element analysis (FEA), the homotopy analysis method (HAM), a novel probability-of-existence approach to vibro-impact, and numeric EM calculations. The models produced demonstrate high accuracy in comparison to a laboratory prototype.
Continuing developments in the Structural Health Monitoring (SHM) domain hold great promise for the application of in-situ devices on air platforms, enabling the Australian Defence Force (ADF) to move from the current expensive time-based maintenance approach, to a more cost-effective condition-based approach. DSTO is investigating vibration energy harvesting (VEH) which is a foundation technology for powering many in-situ SHM applications (for example, autonomous SHM systems that are designed to be retro-fitted onto a vehicle to monitor structural and/or corrosion hotspots). While commercial VEH solutions are available, the environment specific to air platforms inhibit their use – that is, commercial devices are often heavy, respond to only uni-axial vibrations, do not operate well at high accelerations, and produce output power at a very narrow frequency bandwidth. Recent work at the DSTO has resulted in the development of a bi-axial VEH device, capable of harvesting useable energy from wide-band bi-axial excitations within a small device footprint, and therefore proving appropriate for integration into an in-situ SHM system for air platforms. The device operates as a permanent-magnet/ball-bearing mechanical oscillator, free to respond to host structure excitations. An electromagnetic wire coil transducer (between the magnet and ball-bearing) produces electrical output due to a changing magnetic field distribution as the ball-bearing oscillates. This Technical Note provides a brief summary on the functioning of the VEH device, and details the modelling work undertaken at the DSTO to investigate the device. The complex nonlinear dynamics and electromagnetic properties of the device require numerical computation tools such as finite element analysis (FEA), and advanced mathematical techniques pertaining to nonlinear oscillations and discontinuous systems. The outcome of the modelling work is a full description of the mechanical dynamics and electrical transduction of the VEH device, and a capability for optimisation work and guidance for design decisions.