Technical report | Bi-axial Vibration Energy Harvesting
This report describes a vibration energy harvesting approach that uses a magnetoelectric (ME) transducer to harvest energy from bi-axial vibrations. The approach is being explored as a potential means of powering in situ structural health monitoring systems embedded within aircraft and other high value engineering assets that experience mechanical vibration. A bi-axial oscillator is created using a permanent-magnet/ball-bearing arrangement, which has the added benefit of permitting a relatively compact design. The magnet produces a bi-axial restoring force on the bearing, and as the bearing oscillates it steers a magnetic field through a magnetostrictive/piezoelectric laminate transducer thereby producing an oscillating charge that can be harvested. A simple laboratory demonstrator of a bi-axial ME harvester was created using a Terfenol-D/lead zirconate titanate/Terfenol-D transducer, and was shown to produce a peak rms power of 121 μW from an rms acceleration of 61 mG at 9.8 Hz.
In-situ Structural Health Monitoring (SHM) devices allow the Australian Defence Force to move from expensive time-based maintenance approaches for ageing platforms to cost-effective condition-based approaches. For air platforms the installation of these systems is complicated by the fact that the majority of SHM devices need to be fitted on internal aircraft structure, underneath the aircraft's skin. If the SHM device is in a location that is difficult to access, then powering the device may be problematic because traditional powering methods are in general not feasible. For example, replacing batteries on many SHM devices deployed across a fleet would be impractical, and accessing an on-board power system to supply SHM devices may lead to flight worthiness and certification issues. To address this powering issue the Australian Defence Science and Technology Organisation (DSTO) is investigating the possible use of vibration energy harvesting (VEH). Two unresolved scientific issues that inhibit the use of VEH on aircraft are: (i) the need for a wide operational frequency bandwidth to permit harvesting from the frequency-rich vibration that can be present on airframes, and (ii) the need for a multi-axial harvesting approach, since aircraft vibrations are typically not uni-axial. Previous collaborative work between the DSTO and the Active Materials Laboratory at UCLA addressed the first issue by developing the vibro-impacting energy harvesting approach which produced VEH over a broader operational bandwidth compared with many other harvester approaches, including harvesters that are currently commercially available. The second fundamental issue with most VEH approaches (again including all known commercial vibration energy harvesters) is that they are uni-directional, and hence can only harvest vibrational energy from host accelerations along a single axis. Therefore, while a considerable amount of scientific literature exists on the topic of VEH, none to date reports on a technique to effectively harvest from bi-axial host accelerations. This report describes a bi-axial approach that represents a significant advancement in VEH, specifically the approach increases the operational directionality from single-axis to 360 degrees in a plane. Furthermore, to the author's knowledge this is the first harvester design that uses a magnet/bearing cantilever analogue (replacing the cantilever design used by many harvesters described in the literature) potentially allowing a significant reduction in harvester volume. Finally, to the authors' knowledge the harvester described in this report is the first that uses an oscillating ball-bearing to create magnetic flux steerage through a magnetoelectric laminate transducer to generate harvestable electrical power. This report will describe modelling of the uses an oscillating ball-bearing to create magnetic flux steerage through a magnetoelectric laminate transducer to generate harvestable electrical power., and will also report on a simple laboratory demonstrator that was developed as a proof of concept.