Inside the Otis Air National Guard Base—in Cape Cod, Mass.—the commercial DJI Flamewheel drone zipped down a row lined with cardboard boxes and tarps. At the end of the row, it smacked against the aircraft hangar’s floor, bounced, and tumbled to a stop.
The Defense Advanced Research Project Agency (DARPA) is playing with commercial drones. Well, not so much playing as experimenting.
Recently, DARPA’s Fast Lightweight Autonomy (FLA) program completed their first-flight data collection. In it, the program’s three performer teams demonstrated the commercial drone’s capability of reaching manned speeds up to 20 m/s, or 45 mph, and successfully navigating obstacles at slower speeds without human aid.
“Very lightweight UAVs (Unmanned Aerial Vehicles) exist today that are agile and can fly faster than 20 m/s, but they can’t carry sensors and computation to fly autonomously in cluttered environments,”said the program’s manager Mark Micire. “And large UAVs exist that can fly high and fast with heavy computing payloads and sensors on board. What makes the FLA program so challenging is finding the sweetspot of a small size, weight and power air vehicle with limited onboard computing power to perform a complex mission completely autonomously.”
The drone—outfitted with E600 motors, 12 in. propellers, and a 3DR Pixhawk autopilot—carried a variety of high-definition cameras and sensors, such as LIDAR, sonar, and inertial measuring instruments.
The three performances teams included Draper, which teamed with Massachusetts Institute of Technology; Univ. of Pennsylvania; and Scientific Systems Company, Inc., which teamed with AeroVironment.
According to Defense News, DARPA was offering $5.5 million in research funding for the program.
“We’re excited that we were able to validate the airspeed goal during the first-flight data collection,” said Micire. “The fact that some teams also demonstrated basic autonomous flight ahead of schedule was an added bonus. The challenge for the teams now is to advance the algorithms and onboard computational efficiency to extend the UAV’s perception range and compensate for the vehicles’ mass to make extremely tight turns and abrupt maneuvers at high speeds.”
Once fully developed, these drone systems will aid the military in surveillance operations, either patrolling hazardous urban environments or responding to disasters. As trials continue, the testing environment will grow more complex, with more obstacles added.
01 Oct 2015
Using nanometer-scale components, researchers have demonstrated the first optical rectenna, a device that combines the functions of an antenna and a rectifier diode to convert light directly into DC current.
Based on multiwall carbon nanotubes and tiny rectifiers fabricated onto them, the optical rectennas could provide a new technology for photodetectors that would operate without the need for cooling, energy harvesters that would convert waste heat to electricity — and ultimately for a new way to efficiently capture solar energy.
In the new devices, developed by engineers at the Georgia Institute of Technology, the carbon nanotubes act as antennas to capture light from the sun or other sources. As the waves of light hit the nanotube antennas, they create an oscillating charge that moves through rectifier devices attached to them. The rectifiers switch on and off at record high petahertz speeds, creating a small direct current.
Billions of rectennas in an array can produce significant current, though the efficiency of the devices demonstrated so far remains below one percent. The researchers hope to boost that output through optimization techniques, and believe that a rectenna with commercial potential may be available within a year.
“We could ultimately make solar cells that are twice as efficient at a cost that is ten times lower, and that is to me an opportunity to change the world in a very big way” said Baratunde Cola, an associate professor in the George W. Woodruff School of Mechanical Engineering at Georgia Tech. “As a robust, high-temperature detector, these rectennas could be a completely disruptive technology if we can get to one percent efficiency. If we can get to higher efficiencies, we could apply it to energy conversion technologies and solar energy capture.”
The research, supported by the Defense Advanced Research Projects Agency (DARPA), the Space and Naval Warfare (SPAWAR) Systems Center and the Army Research Office (ARO), is scheduled to be reported September 28 in the journal Nature Nanotechnology.
Developed in the 1960s and 1970s, rectennas have operated at wavelengths as short as ten microns, but for more than 40 years researchers have been attempting to make devices at optical wavelengths. There were many challenges: making the antennas small enough to couple optical wavelengths, and fabricating a matching rectifier diode small enough and able to operate fast enough to capture the electromagnetic wave oscillations. But the potential of high efficiency and low cost kept scientists working on the technology.
“The physics and the scientific concepts have been out there,” said Cola. “Now was the perfect time to try some new things and make a device work, thanks to advances in fabrication technology.”
Using metallic multiwall carbon nanotubes and nanoscale fabrication techniques, Cola and collaborators Asha Sharma, Virendra Singh and Thomas Bougher constructed devices that utilize the wave nature of light rather than its particle nature. They also used a long series of tests — and more than a thousand devices — to verify measurements of both current and voltage to confirm the existence of rectenna functions that had been predicted theoretically. The devices operated at a range of temperatures from 5 to 77 degrees Celsius.
Fabricating the rectennas begins with growing forests of vertically-aligned carbon nanotubes on a conductive substrate. Using atomic layer chemical vapor deposition, the nanotubes are coated with an aluminum oxide material to insulate them. Finally, physical vapor deposition is used to deposit optically-transparent thin layers of calcium then aluminum metals atop the nanotube forest. The difference of work functions between the nanotubes and the calcium provides a potential of about two electron volts, enough to drive electrons out of the carbon nanotube antennas when they are excited by light.
In operation, oscillating waves of light pass through the transparent calcium-aluminum electrode and interact with the nanotubes. The metal-insulator-metal junctions at the nanotube tips serve as rectifiers switching on and off at femtosecond intervals, allowing electrons generated by the antenna to flow one way into the top electrode. Ultra-low capacitance, on the order of a few attofarads, enables the 10-nanometer diameter diode to operate at these exceptional frequencies.
“A rectenna is basically an antenna coupled to a diode, but when you move into the optical spectrum, that usually means a nanoscale antenna coupled to a metal-insulator-metal diode,” Cola explained. “The closer you can get the antenna to the diode, the more efficient it is. So the ideal structure uses the antenna as one of the metals in the diode — which is the structure we made.”
The rectennas fabricated by Cola’s group are grown on rigid substrates, but the goal is to grow them on a foil or other material that would produce flexible solar cells or photodetectors.
Cola sees the rectennas built so far as simple proof of principle. He has ideas for how to improve the efficiency by changing the materials, opening the carbon nanotubes to allow multiple conduction channels, and reducing resistance in the structures.
“We think we can reduce the resistance by several orders of magnitude just by improving the fabrication of our device structures,” he said. “Based on what others have done and what the theory is showing us, I believe that these devices could get to greater than 40 percent efficiency.”
- Asha Sharma, Virendra Singh, Thomas L. Bougher, Baratunde A. Cola. A carbon nanotube optical rectenna. Nature Nanotechnology, 2015; DOI: 10.1038/nnano.2015.220
That’s what Northeastern Univ. physicists Swastik Kar and Srinivas Sridhar found during their four-year project to modify graphene, a stronger-than-steel infinitesimally thin lattice of tightly packed carbon atoms. Primarily funded by the Army Research Laboratory and Defense Advanced Research Projects Agency, or DARPA, the researchers were charged with imbuing the decade-old material with thermal sensitivity for use in infrared imaging devices such as night-vision goggles for the military.
What they unearthed, published in Science Advances, was so much more: an entirely new material spun out of boron, nitrogen, carbon and oxygen that shows evidence of magnetic, optical and electrical properties, as well as DARPA’s sought-after thermal ones. Its potential applications run the gamut: from 20-megapixel arrays for cellphone cameras to photo detectors to atomically thin transistors that when multiplied by the billions could fuel computers.
“We had to start from scratch and build everything,” says Kar, an assistant professor of physics in the College of Science. “We were on a journey, creating a new path, a new direction of research.”
An artistic ren¬dering of novel mag¬netism in 2D-BNCO sheets, the new mate¬rial Swastik Kar and Srinivas Sridhar cre¬ated. Image: Northeastern Univ.
The pair was familiar with “alloys,” controlled combinations of elements that resulted in materials with properties that surpassed graphene’s—for example, the addition of boron and nitrogen to graphene’s carbon to connote the conductivity necessary to produce an electrical insulator. But no one had ever thought of choosing oxygen to add to the mix.
What led the Northeastern researchers to do so?
“Well, we didn’t choose oxygen,” says Kar, smiling broadly. “Oxygen chose us.”
Oxygen, of course, is everywhere. Indeed, Kar and Sridhar spent a lot of time trying to get rid of the oxygen seeping into their brew, worried that it would contaminate the “pure” material they were seeking to develop.
“That’s where the Aha! moment happened for us,” says Kar. “We realized we could not ignore the role that oxygen plays in the way these elements mix together.”
“So instead of trying to remove oxygen, we thought: Let’s control its introduction,” adds Sridhar, the Arts and Sciences Distinguished Professor of Physics and director of Northeastern’s Electronic Materials Research Institute.
Oxygen, it turned out, was behaving in the reaction chamber in a way the scientists had never anticipated: It was determining how the other elements—the boron, carbon and nitrogen—combined in a solid, crystal form, while also inserting itself into the lattice. The trace amounts of oxygen were, metaphorically, “etching away” some of the patches of carbon, explains Kar, making room for the boron and nitrogen to fill the gaps.
“It was as if the oxygen was controlling the geometric structure,” says Sridhar.
They named the new material, sensibly, 2D-BNCO, representing the four elements in the mix and the two-dimensionality of the super-thin lightweight material, and set about characterizing and manufacturing it, to ensure it was both reproducible and scalable. That meant investigating the myriad permutations of the four ingredients, holding three constant while varying the measurement of the remaining one, and vice versa, multiple times over.
After each trial, they analyzed the structure and the functional properties of the product—electrical, optical—using electron microscopes and spectroscopic tools, and collaborated with computational physicists, who created models of the structures to see if the configurations would be feasible in the real world.
Next they will examine the new material’s mechanical properties and begin to experimentally validate the magnetic ones conferred, surprisingly, by the intermingling of these four nonmagnetic elements. “You begin to see very quickly how complicated that process is,” says Kar.
Helping with that complexity were collaborators from around the globe. In addition to Northeastern associate research scientists, postdoctoral fellows, and graduate students, contributors included researchers in government, industry, and academia from the U.S., Mexico and India.
“There is still a long way to go but there are clear indications that we can tune the electrical properties of these materials,” says Sridhar. “And if we find the right combination, we will very likely get to that point where we reach the thermal sensitivity that DARPA was initially looking for as well as many as-yet unforeseen applications.”
Source: Northeastern Univ.