Muhammad A. Qidwai

Energy harvesting concepts for small electric unmanned systems

In this study, we identify and survey energy harvesting technologies for small electrically powered unmanned systems designed for long-term (>1 day) time-on-station missions. An environmental energy harvesting scheme will provide long-term, energy additions to the on-board energy source. We have identified four technologies that cover a broad array of available energy sources: solar, kinetic (wind) flow, autophagous structure-power (both combustible and metal air-battery systems) and electromagnetic (EM) energy scavenging. We present existing conceptual designs, critical system components, performance, constraints and state-of-readiness for each technology. We have concluded that the solar and autophagous technologies are relatively matured for small-scale applications and are capable of moderate power output levels (>1 W). We have identified key components and possible multifunctionalities in each technology. The kinetic flow and EM energy scavenging technologies will require more in-depth study before they can be considered for implementation. We have also realized that all of the harvesting systems require design and integration of various electrical, mechanical and chemical components, which will require modeling and optimization using hybrid mechatronics-circuit simulation tools. This study provides a starting point for detailed investigation into the proposed technologies for unmanned system applications under current development.

MULTIFUNCTIONAL STRUCTURE-BATTERY MATERIALS FOR ENHANCED PERFORMANCE IN SMALL UNMANNED AIR VEHICLES

Aircraft design and manufacturing have been in a state of constant technological evolution over the last 100 years. Considerable effort has been focused on improving performance, durability, and reliability, and lowering costs. This is being accomplished today using cutting-edge design methodology that incorporates multidisciplinary design optimization of complex systems in place of older methods that independently optimized local subsystems and iterated between designs to satisfy global design constraints. Air vehicles are designed to move payload between two points, hence increasing the payload capacity or increasing the flight time endurance or range are important system-level goals in the design process. For winged aircraft, a large percentage of total weight is taken up by the structure (~37%) and fuel (~34%) (Thomas et al., 2002). Decreasing the weight of these subsystems or increasing the fuel weight fraction can improve aircraft performance, and this can be accomplished through structure-power multifunctionality. This abstract reports on the design and use of a multifunctional structure- battery (power) material to increase the flight endurance time of a small electric-propelled unmanned air vehicle (UAV). Flight endurance time is related, in Eq. (1), to the available battery energy, subsystem weights, and aerodynamic parameters. As can be seen from this equation, modifications in the available battery energy or sub-system weights (structure and battery) will affect system performance. Increases in the flight time are sought through a reduction of redundancy between the structure and battery subsystem materials and functions (shape and power). We can accomplish this by using a multifunctional structure-battery material that stores electrical energy while a carrying part of the mechanical load.

Structure-battery multifunctional composite design

In multifunctional material design, two or more functions performed by distinct system components or materials are incorporated into a single component or material system to improve system performance. The aim of this paper is to present a framework for the design of structure-battery (power) multifunctional composite materials for unmanned air vehicle (UAV) applications. The design methodology is based on optimization of composite material performance indices and the use of material design selection charts introduced by Ashby and coworkers in a series of papers for homogeneous and two-phase composite materials. Performance indices are derived for prismatic structure-battery composites under various loading conditions. The development of simple design tools in the form of spreadsheet templates is also discussed. Finally, results based on the above-mentioned framework and actual material properties will be presented for structure-battery circular and square struts.

Multifunctional Applications of Thin Film Li Polymer Battery Cells

Commercial off-the-shelf (COTS) thin-film solid-polymer Li-ion battery cells appear to posses the requisite physical characteristics for dual use as both electrical energy-storage devices and structural members under a finite load. One realistic application could be small electric unmanned vehicles where the power requirements are in the range of 10 to 100 watts and the mechanical loads are relatively small. We tested the multifunctional feasibility of COTS battery cells by designing a specific mechanical testing protocol based on realistic use in unmanned vehicles. Our characterization protocol included randomized bending and shear testing and generation of energy-power relation (Ragone) plots of the COTS cells. The results indicate that multifunction applications of COTS Li polymer battery cells are feasible; however, battery packaging geometry and bonding are critical design issues.

Autophagous structure-power systems

Novel autophagous (self-consuming) systems combining structure and power functionalities are under development for improved material utilization and performance enhancement in electric unmanned air vehicles (UAVs). Much of the mass of typical aircraft is devoted separately to the functions of structure and fuel-energy. Several methods are proposed to extract structure function from materials that can also serve as fuel for combustion or as a source of hydrogen. Combustion heat is converted to electrical energy by thermoelectric generation, and hydrogen gas is used in fuel cells to provide electrical energy. The development and implementation of these structure-fuels are discussed in the context of three specific designs of autophagous wing spars. The designs are analyzed with respect to mechanical performance and energy storage. Results indicate a high potential for these systems to provide enhanced performance in electric UAVs.

Structure-Power Multifunctional Materials for UAV's

This paper presents multifunctional structure-plus-power developments being pursued under DARPA sponsorship with the focus on structure-battery components for unmanned air vehicles (UAV). New design strategies, analysis methods, performance indices, and prototypes for multifunctional structure-battery materials are described along with the development of two UAV prototypes with structure-battery implementation.

Expanding Mission Capabilities of Unmanned Systems through the Collection of Energy in the Field

*† ‡ We report in this study on the feasibility of scavenging energy from a broad spectrum of terrestrial energy sources to enhance the operational capabilities of small electric unmanned vehicles by recharging the system’s energy while in the field. Five major sources of terrestrial energy: solar, kinetic-flow (wind), thermal, electromagnetic, and autophagous (self-consuming) structure-battery are considered in this work. Notional designs for each scavenging concept are developed and evaluated with regard to their net power collection potential. Major components such as collector elements, energy conversion hardware and process control hardware are identified. For the small-scale implementations we have examined, solar harvesting appear to be most advanced with existing prototypes capable of supplying energy at 1-10 W levels. Small-scale scavenging of other forms of energy is in the early stages of development. Analyses of kinetic flow and thermal scavenging indicate limitations to lower powers (e.g., 10W). Our notional designs provide the basis for a more advanced design and implementation analysis of each technology and support the concept of scavenging and converting energy from the environment for increasing the time-on-station of small electric unmanned vehicles.

Mitigation of Free-Edge Effects by Meso-Scale Structuring

We are developing a new class of fiber-reinforced polymer composite materials to facilitate the embedment of multifunctional features and devices in material systems and to manage interlaminar stresses at the external free edges and internal free surfaces of holes and cut-outs in composite laminates. The idea is centered on the introduction of one or more additional dimensions of design space by a tessellation of individual laminae into sets of discrete tiles, each possessing the same levels of design freedom normally associated with an entire lamina (material constituents, fiber orientation, and so on). In this work, we have focused on the development of tiling schemes that will allow blending of disparate laminates (lay-ups), where a lay-up suitable for suppressing interlaminar stresses could be substituted at necessary locations in place of another lay-up that may be more suitable for the global structural loads. This technique results in the inclusion of possibly detrimental matrix-rich tile-to-tile interface pockets in the plane of each lamina. Mechanical testing has shown that uniaxially reinforced tiled composites maintain stiffness levels near those of their traditional continuously reinforced counterparts, yet with a potential degradation of strength. We have used the finite element method to investigate the effects of resin-rich pocket size, the use of supporting continuous layers, tile size, and tile overlapping schemes (interface stacking geometry) on the distribution of stress and transfer of load around interfaces in uniaxially reinforced tiled composites. This was done with the aim to identify parameters controlling overall strength. We discovered that alignment of the resin-rich pockets through the thickness exacerbates stress-concentration and that outer continuous layers on the composite may help in better load transfer and more efficient material utilization. Failure analyses of the finite element results using three-dimensional Hashin-Rotem failure criteria [1] have shown the concept to be effective in the suppression of free-edge delamination in traditional quasi-isotropic and angle-ply laminates under tensile loading. Although each meso-scale structured solution must be tailored to the exact structural geometry and anticipated loads, the technique shows promise to have broad application

A Preliminary Study on the Mechanical Performance of Tiled Polymer Composites

We are developing and exploring the concept of in-plane tiling of composite laminates, called MOSAIC, to mitigate or control delamination at free edges due to interlaminar stresses. Initial mechanical testing has shown that MOSAIC composites with uniaxial graphite-fiber reinforced tiles retain the stiffness levels of traditional uniaxially reinforced composites but with reduced strength. The reduction in strength is attributed to the formation of resin-rich pockets between adjacent tiles. In this study, we have performed detailed finite element analyses to identify the critical design parameters that affect the mechanical performance of uniaxially reinforced MOSAIC composites. We have found that using continuous laminae on the outer surfaces significantly increases the overall loadcarrying capacity. Increasing aspect ratio of the pocket and decreasing material property differences between resin and tiles also cause better load transfer between tiles but may not necessarily improve overall strength due to increasing stress concentration. The tiling scheme and density of pocket placement influence the interaction of local stress concentrations. Consequently, a novel composite joint is proposed and found to have superior performance.Copyright

Suppression of Edge Delamination Through Meso-Scale Structuring

We are developing a new class of fiber-reinforced polymer composite materials to facilitate imbedding multifunctional features and devices in material systems, and to manage interlaminar stresses at free edges and cut-outs. The idea is centered on introducing one more level of design space by composing plies with individual tiles possessing the same degrees of design freedom that are associated with individual plies. In this work, we have focused on tiling schemes that will allow blending of laminates (lay-ups), where a lay-up suitable for suppressing interlaminar stresses could be placed at necessary locations whereas another lay-up could be used for the main objective. This results in the introduction of matrix-rich tile-to-tile interface pockets in the blending region. Preliminary mechanical testing shows that uniaxially reinforced tiled composites attain stiffness levels near those of their traditional counterparts, yet with a potential degradation of strength. We used the finite element method to investigate the effects of resin-rich pocket size, the use of supporting continuous layers, tile size, and tile overlapping (interface stacking) schemes on stress distribution around interfaces in uniaxially reinforced tiled composites, with the aim to identify parameters controlling overall strength. We discovered that alignment of the resin-rich pockets through the thickness exacerbates stress-concentration and that outer continuous layers on the composite may help in better load transfer. As a first step in the application of this technique for the suppression of delamination at the free edges of holes in laminates, a bilaminate material was modeled, and the concept was shown to be effective in the suppression of edge delamination.Copyright