Tech Briefs

This technology can be used to provide energy to micro air vehicles.

The goal of this work was to investigate the concept of using harvested energy to directly control the vibration response of flexible aerospace systems. Small, lightweight, flexible micro air vehicles (MAVs) operate near flutter, providing both harvesting opportunities and vibration suppression requirements. The possibility that the ambient energy might be harnessed and recycled to provide energy to mitigate the vibrations through various control laws was investigated. The goal was to integrate harvesting, storage, control, and computation into one multifunctional structure.

Schematic of a Multifunctional Structure containing harvesting, control, energy storage, and computing.
As ambient energy is relatively low level and the hope was to run vibration suppression systems off of harvested ambient energy, feedback control laws were sought that used a minimum amount of energy. Such control laws did not exist, so ways to minimize control effort for vibration suppression had to be discovered. Basic control laws were tuned to achieve the same performance. The required amount of energy in each case was calculated and compared.

It was found that as much as two-thirds of the required energy can be saved by using a saturation control. This reduction makes running a control law off of harvested energy possible. In implementing these control laws, it was discovered that the high voltages commanded by the control laws result in the piezoelectric coupling coefficient being non-constant. An adaptive control law (exponential actually) was implemented to account for the change in coupling coefficient as the control voltage demand increased. The next major result was to integrate harvesting and storage into the same package with a control actuator and a control law (i.e. the circuitry) all embedded in a multifunctional composite structure as illustrated in the accompanying schematic.

A multifunctional system was fabricated, modeled, and tested, and was capable of energy harvesting, sensing, energy storage, vibration suppression using active control, embedded computing (providing energy management and control laws), and structural integrity. Before proceeding, the harvesting, sensing, and control authority of several different types of piezoelectric material were considered in order to choose the best components for each task. Macro fiber composites form the best control actuation devices, and monolithic piezoceramic forms the best sensing and harvesting device.

Following these initial results, the concept of a multifunctional composite beam was applied to a problem prevalent in unmanned air vehicles (UAVs). UAVs tend to be light and travel near their flutter speed, which means that they are susceptible to instabilities caused by gusts. While the UAV is in normal flight, its wing vibrates. The multifunctional wing spar, modeled after the schematic, would transfer the wing vibration into electrical energy and store it in the embedded battery. When the UAV hits a gust, the sensor function of the multifunctional spar would then see the increased strain, and turn on the active control system embedded in the PCB part of the spar.

The resulting feedback control law would then quiet the gust response and keep the vibration suppressed during the period of the gust. The laboratory results show great agreement with the theoretical models and numerical simulations.

Simulations were then used to predict how the system would behave as a gust suppression system for a small UAV. The gust and clear sky condition (the condition of vibration induced during normal flight) were simulated using the Dryden PSD signal for both clear sky and gust. The simulations were fed into the model of the multifunctional wing spar. The response of the wing to a gust shows a large tip deflection. The response of the wing tip with the controller turned on and the gust as input shows substantial vibration reduction.

There are applications where harvested energy can be of use, even when the energy requirements exceed those that are required, if there is not a constant need for that energy. This is surely the case illustrated here with the gust alleviation example. Many other examples exist in the area of structural health monitoring. The main work here shows that closed loop control can be accomplished with harvested energy.

This work was done by Daniel J. Inman and Pablo Tarazaga of Virginia Tech for the Air Force Office of Scientific Research. AFOSR-0005

This Brief includes a Technical Support Package (TSP).

Simultaneous Vibration Suppression and Energy Harvesting (reference AFOSR-0005) is currently available for download from the TSP library.

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