For air platforms, the installation of Structural Health Monitoring (SHM) 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 generally not feasible. For example, replacing batteries on many SHM devices deployed across a fleet would be impractical, and accessing an onboard power system to supply SHM devices may lead to flight worthiness and certification issues.

A schematic of a wireless structural health monitoring system concept with the sensing unit, Energy Harvester, and wireless data and power transfer capability.
To address this powering issue, the use of vibration energy harvesting (VEH) has been investigated. Two unresolved scientific issues that inhibit the use of VEH on aircraft are: (1) the need for a wide operational frequency bandwidth to permit harvesting from the frequency- rich vibration that can be present on airframes, and (2) the need for a multi-axial harvesting approach, since aircraft vibrations are typically not uni-axial. Previous work addressed the first issue by developing the vibro-impacting energy harvesting approach that 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.

A bi-axial approach represents a significant advancement in VEH; specifically, the approach increases the operational directionality from single-axis to 360 degrees in a plane. Furthermore, this design uses a magnet/bearing cantilever analogue (replacing the cantilever design used by many harvesters), potentially allowing a significant reduction in harvester volume. This design also uses an oscillating ball bearing to create magnetic flux steerage through a magnetoelectric laminate transducer to generate harvestable electrical power.

The concept involves three main components: (1) a sensor mounted inside the aircraft at a difficult-to-access location is monitoring in-flight mechanical loads on an airframe, (2) with the sensor utilizing energy that is parasitically harvested from local airframe vibrations by an energy harvester, (3) when the aircraft is on the ground, a wireless link— the acoustic electric feedthrough—is used to download sensor data and simultaneously provide additional energy to the sensor unit.

The bi-axial vibration energy harvesting approach can harvest energy from the multi-axis accelerations experienced by an aircraft. A bi-axial oscillator was created using a permanent-magnet/ballbearing arrangement. The magnet produces a bi-axial restoring force on the bearing, and as the bearing oscillates, it steers magnetic field through a magnetostrictive/ piezoelectric laminate transducer, thereby producing an oscillating charge that can be harvested.

Modeling was used to make a qualitative assessment of the magnetic flux changes in the ME transducer as the bearing oscillates, which indicated that large flux variations occur as the bearing moves from the magnet’s central-line towards the edge. A simple laboratory demonstrator of a biaxial ME energy harvester was created using a Terfenol-D/lead zirconate titanate/Terfenol-D transducer. Harvester output was measured as a function of drive-angle, host acceleration, and load resistance. The harvester produced a peak rms power of 121 mW from an rms host acceleration of 61 mG at 9.8 Hz.

This work was done by Scott Moss, Joshua McLeod, Ian Powlesland, and Steve Galea of the Defence Science and Technology Organisation. DSTO-0002

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Bi-Axial Vibration Energy Harvesting

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This article first appeared in the February, 2013 issue of Defense Tech Briefs Magazine.

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