Could innovations in power electronics be used to make DC circuit breakers feasible for high-power electrical systems? And what can very small, rudimentary robots teach us about how to control their movements? These are some of the questions currently being explored by Georgia Tech, Florida State, and Northwestern University researchers, whose work could lead to new components and systems for the energy and defense industries, among others. John Toon, director of research news at Georgia Institute of Technology, outlines their research in the following two reports.
Hybrid Breakers Could Make Direct Current Practical in High Power Applications
Direct current (DC) powers flashlights, smartphones, and electric cars, but major power users depend on alternating current (AC), which cycles on and off 60 times per second. Among the reasons: AC is simple to turn off when there’s a problem — known as a fault — such as a tree falling on a power line.
But DC has inherent advantages over its alternating cousin, among them higher efficiency and the ability to carry more power over longer distances. That could be increasingly important as wind farms in rural areas produce power needed in population centers. And future electric aircraft and ships are likely to be powered by high-power-density DC systems.
Alternating current can be shut down when the power level hits zero during a cycle — the zero-crossing point of a sine wave — which is the basis for breakers that protect modern power systems everywhere from substations to home installations. Without these alternating cycles, however, direct current has no opportune time to turn off the power.
New technology funded by a $3.3 million award from ARPA-E’s BREAKERS program could help solve that problem using innovations in power electronics, piezoelectric actuators, and new insulation materials to make high-power DC circuit breakers feasible. Researchers from the Georgia Institute of Technology and Florida State University (FSU) expect to enable breaker switching speeds ten times faster than existing equipment and commercialize the technology through a consortium of industry partners.
“The transition from AC to DC, which is already happening, will open up a new paradigm for efficiently and controllably managing power in future electrical systems and military platforms,” said Michael “Mischa” Steurer, a research faculty member at Florida State University’s Center for Advanced Power Systems. “This will be enabled by the amazing developments that have happened over the past two decades in power electronics.”
The hybrid circuit breaker under development by the research team will use stacks of very large transistors to switch off the DC when necessary. Semiconductors are less efficient at conducting current than conventional mechanical switches, so under ordinary conditions, the current will flow through mechanical switches. But when the power must be turned off, current will be briefly routed through the power electronics until the mechanical breakers can be opened.
“We are proposing a hybrid DC circuit breaker in which the current will have two paths,” explained Lukas Graber, an assistant professor in the School of Electrical and Computer Engineering at Georgia Tech. “One path will be through the semiconductors, which can interrupt the current when needed. The second path will be through mechanical switches, which will provide a much less resistive path that will be more efficient for normal operations.”
In common consumer electronics applications, transistors are too small to see and handle just a few volts. The transistors that will be used in DC switching are much larger — a square centimeter — and dozens or hundreds of them would be combined in serial or parallel to provide enough capacity for switching thousands of volts. After the current has been moved to the solid-state transistor pathway, piezoelectric actuators will quickly separate the contacts in the mechanical switches before current rises too high in the transistors. Once separated, the current through the transistors can be switched off.
“We need to be extremely fast,” Graber said. “We have to separate the contacts within 250 microseconds and to completely break the current within 500 microseconds — just half a millisecond. For that reason, we cannot use spring-loaded or hydraulic actuators common to AC breakers. Devices that rely on the piezoelectric effect can do that for us.”
The Georgia Tech and FSU researchers have developed intellectual property for components of the proposed DC breakers, and will work together to combine the technologies. The project is known as Efficient DC Interrupter with Surge Protection (EDISON).
“We will combine the strengths of significantly different technologies — solid state and mechanical — into a system that functions better overall than its individual components,” said Steurer. “The pieces of the system have to work together seamlessly within half a millisecond to achieve our goal.”
The researchers include Associate Professor Maryam Saeedifard, VentureLab Principal Jonathan Goldman, and Postdoctoral Fellow Chanyeop Park at Georgia Tech, as well as Professor Fang Peng, Research Faculty Karl Schoder, and Assistant Professor Yuan Li at FSU. They expect to build a prototype that will be tested at FSU’s five-megawatt test facility within three years. The development and testing will be done in collaboration with a team of industrial partners who will ultimately transition the DC breakers to commercial use.
Direct current could be particularly useful as more renewable energy comes online. Photovoltaics in the West may still be generating power after the sun sets in the East. Wind turbines may be producing power in the midsection of the country while clouds cover other parts of the country. Transmitting power from one location to another could therefore become more important.
“There are large distances to be bridged with renewables,” Graber said. “When we rethink what the next grid is going to be like, DC may play a larger role.”
For those who know the history of electrical power, the work opens a new chapter of a story that goes back almost a century and a half to two of the most celebrated inventors of all time.
The relative merits of DC versus AC provided the basis for the “War of Current” between inventors Thomas Edison and Nickolas Tesla in the 1880s. Edison, a proponent of DC, ultimately lost out to Tesla’s AC. But had Edison been able to use modern power electronics, the story might have turned out differently.
“Edison was right, but at the time he was wrong,” Graber said. “DC is coming back strong, and we will be a part of making it practical.”
Funding for the work is from ARPA-E’s Building Reliable Electronics to Achieve Kilovolt Effective Ratings Safely (BREAKERS) program. The project was among eight funded to support the development of medium-voltage devices for grid, industry, and transportation applications.
Writer: John Toon
Reprinted with permission of Georgia Tech Research News.
Shape-Shifting Robot Built from ‘Smarticles’ Shows New Locomotion Strategy
Building conventional robots typically requires carefully combining components like motors, batteries, actuators, body segments, legs, and wheels. Now, researchers have taken a new approach, building a robot entirely from smaller robots known as “smarticles” to unlock the principles of a potentially new locomotion technique.
The 3D-printed smarticles — short for smart active particles — can do just one thing: flap their two arms. But when five of these smarticles are confined in a circle, they begin to nudge one another, forming a robophysical system known as a “supersmarticle” that can move by itself. Adding a light or sound sensor allows the supersmarticle to move in response to the stimulus — and even be controlled well enough to navigate a maze.
Though rudimentary now, the notion of making robots from smaller robots — and taking advantage of the group capabilities that arise by combining individuals — could provide mechanically based control over very small robots. Ultimately, the emergent behavior of the group could provide a new locomotion and control approach for small robots that could potentially change shapes.
“These are very rudimentary robots whose behavior is dominated by mechanics and the laws of physics,” said Dan Goldman, a Dunn Family Professor in the School of Physics at the Georgia Institute of Technology. “We are not looking to put sophisticated control, sensing, and computation on them all. As robots become smaller and smaller, we’ll have to use mechanics and physics principles to control them because they won’t have the level of computation and sensing we would need for conventional control.”
The research, which was supported by the Army Research Office and the National Science Foundation, was reported September 18 in the journal Science Robotics. Researchers from Northwestern University also contributed to the project.
The foundation for the research came from an unlikely source: a study of construction staples. By pouring these heavy-duty staples into a container with removable sides, former Ph.D. student Nick Gravish — now a faculty member at the University of California San Diego — created structures that would stand by themselves after the container’s walls were removed.
Shaking the staple towers eventually caused them to collapse, but the observations led to a realization that simple entangling of mechanical objects could create structures with capabilities well beyond those of the individual components.
“A robot made of other rudimentary robots became the vision,” Goldman said. “You could imagine making a robot in which you would tweak its geometric parameters a bit and what emerges is qualitatively new behaviors.”
To explore the concept, graduate research assistant Will Savoie used a 3D printer to create battery-powered smarticles, which have motors, simple sensors, and limited computing power. The devices can change their location only when they interact with other devices while enclosed by a ring.
“Even though no individual robot could move on its own, the cloud composed of multiple robots could move as it pushed itself apart and shrink as it pulled itself together,” Goldman explained. “If you put a ring around the cloud of little robots, they start kicking each other around, and the larger ring — what we call a supersmarticle — moves around randomly.”
The researchers noticed that if one small robot stopped moving, perhaps because its battery died, the group of smarticles would begin moving in the direction of that stalled robot. Graduate student Ross Warkentin learned he could control the movement by adding photo sensors to the robots that halt the arm flapping when a strong beam of light hits one of them.
“If you angle the flashlight just right, you can highlight the robot you want to be inactive, and that causes the ring to lurch toward or away from it, even though no robots are programmed to move toward the light,” Goldman said. “That allowed steering of the ensemble in a very rudimentary, stochastic way.”
School of Physics Professor Kurt Wiesenfeld and graduate student Zack Jackson modeled the movement of the these smarticles and supersmarticles to understand how the nudges and mass of the ring affected overall movement. Researchers from Northwestern University studied how the interactions between the smarticles provided directional control.
“For many robots, we have electrical current move motors that generate forces on parts that collectively move a robot reliably,” said Todd Murphey, a professor of mechanical engineering who worked with Northwestern graduate students Thomas Berrueta and Ana Pervan. “We learned that although individual smarticles interact with each other through a chaos of wiggling impacts that are each unpredictable, the whole robot composed of those smarticles moves predictably and in a way that we can exploit in software.”
In future work, Goldman envisions more complex interactions that utilize the simple sensing and movement capabilities of the smarticles. “People have been interested in making a certain kind of swarm robots that are composed of other robots,” he said. “These structures could be reconfigured on demand to meet specific needs by tweaking their geometry.”
The project is of interest to the U.S. Army because it could lead to new robotic systems capable of changing their shapes, modalities, and functions, said Sam Stanton. He is program manager of complex dynamics and systems at the Army Research Office, an element of U.S. Army Combat Capabilities Development Command’s Army Research Laboratory.
“Future Army unmanned systems and networks of systems are imagined to be capable of transforming their shape, modality, and function. For example, a robotic swarm may someday be capable of moving to a river and then autonomously forming a structure to span the gap,” Stanton said. “Dan Goldman’s research is identifying physical principles that may prove essential for engineering emergent behavior in future robot collectives as well as new understanding of fundamental tradeoffs in system performance, responsiveness, uncertainty, resiliency, and adaptivity.”
In addition to those already mentioned, the research also included Georgia Tech graduate student Shengkai Li.
This material is based upon work supported by the Army Research Office under award W911NF-13-1-0347 and by the National Science Foundation under grants PoLS-0957659, PHY-1205878, DMR-1551095, PHY-1205878. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the sponsoring agencies.
CITATION: William Savoie, et al., “A robot made of robots: emergent transport and control of a smarticle ensemble,” (Science Robotics 2019).
Writer: John Toon
Reprinted with permission of Georgia Tech Research News.