mit-6
http://dspace.mit.edu:80
The DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.2019-11-13T14:15:25ZAn End-to-End Approach to Self-Folding Origami Structures
https://hdl.handle.net/1721.1/122923
An End-to-End Approach to Self-Folding Origami Structures
An, Byoungkwon; Miyashita, Shuhei; Ong, Aaron; Tolley, Michael T.; Demaine, Martin L; Demaine, Erik D; Wood, Robert J.; Rus, Daniela L
This paper presents an end-to-end approach to automate the design and fabrication process for self-folding origami structures. Self-folding origami structures are robotic sheets composed of rigid tiles and joint actuators. When they are exposed to heat, each joint folds into a preprogrammed angle. Those folding motions transform themselves into a structure, which can be used as body of 3-D origami robots, including walkers, analog circuits, rotational actuators, and microcell grippers. Given a 3-D model, the design algorithm automatically generates a layout printing design of the sheet form of the structure. The geometric information, such as the fold angles and the folding sequences, is embedded in the sheet design. When the sheet is printed and baked in an oven, the sheet self-folds into the given 3-D model. We discuss, first, the design algorithm generating multiple-step self-folding sheet designs, second, verification of the algorithm running in O(n² ) time, where n is the number of the vertices, third, implementation of the algorithm, and finally, experimental results, several self-folded 3-D structures with up to 55 faces and two sequential folding steps. Keywords: actuators; control engineering computing; design engineering; grippers; mobile robots; shape memory effects; sheet materials; solid modelling; structural engineering computing; multiple-step self-folding sheet designs; sequential folding steps; end-to-end approach; self-folding origami structures; fabrication process; robotic sheets; 3-D origami robots; layout printing design; sheet design; 3D model; walkers; analog circuits; rotational actuators; microcell grippers; Mobile robots; Smart Devices; Actuators; Fabrication; Solid modeling; Three-dimensional displays; Geometry; Three-dimensional printing; Cellular and modular robots; printable origami robots; self-folding; smart actuators
2018-12-01T00:00:00ZWork-education mismatch: An endogenous theory of professionalization
https://hdl.handle.net/1721.1/122922
Work-education mismatch: An endogenous theory of professionalization
Ghaffarzadegan, Navid; Xue, Yi; Larson, Richard Charles
We model the education-workforce pipeline and offer an endogenous theory of professionalization and ever-higher degree attainment. We introduce two mechanisms that act on the education enterprise, causing the number of educated people to increase dramatically with relatively short-term changes in the job market. Using our illustrative dynamic model, we argue that the system is susceptible to small changes and the introduced self-driving growth engines are adequate to over-incentivize degree attainment. We also show that the mechanisms magnify effects of short-term recessions or technological changes, and create long-term waves of mismatch between workforce and jobs. The implication of the theory is degree inflation, magnified pressures on those with lower degrees, underemployment, and job market mismatch and inefficiency. Keywords: System dynamics; Education policy; Inefficiency; Education mismatch; Public policy
2017-03-01T00:00:00ZUltrafast Graphene Light Emitters
https://hdl.handle.net/1721.1/122921
Ultrafast Graphene Light Emitters
Kim, Young Duck; Gao, Yuanda; Shiue, Ren-Jye; Wang, Lei; Aslan, Ozgur; Bae, Myung-Ho; Kim, Hyungsik; Seo, Dongjea; Choi, Heon-Jin; Kim, Suk Hyun; Nemilentsau, Andrei; Low, Tony; Tan, Cheng Hock; Efetov, Dmitri; Taniguchi, Takashi; Watanabe, Kenji; Shepard, Kenneth L.; Heinz, Tony F.; Englund, Dirk R.; Hone, James
Ultrafast electrically driven nanoscale light sources are critical components in nanophotonics. Compound semiconductor-based light sources for the nanophotonic platforms have been extensively investigated over the past decades. However, monolithic ultrafast light sources with a small footprint remain a challenge. Here, we demonstrate electrically driven ultrafast graphene light emitters that achieve light pulse generation with up to 10 GHz bandwidth across a broad spectral range from the visible to the near-infrared. The fast response results from ultrafast charge-carrier dynamics in graphene and weak electron-acoustic phonon-mediated coupling between the electronic and lattice degrees of freedom. We also find that encapsulating graphene with hexagonal boron nitride (hBN) layers strongly modifies the emission spectrum by changing the local optical density of states, thus providing up to 460% enhancement compared to the gray-body thermal radiation for a broad peak centered at 720 nm. Furthermore, the hBN encapsulation layers permit stable and bright visible thermal radiation with electronic temperatures up to 2000 K under ambient conditions as well as efficient ultrafast electronic cooling via near-field coupling to hybrid polaritonic modes under electrical excitation. These high-speed graphene light emitters provide a promising path for on-chip light sources for optical communications and other optoelectronic applications. Keywords: graphene; ultrafast light emitter; thermal radiation; van der Waals heterostructure; optoelectronics
2018-01-22T00:00:00ZSharper p-Values for Stratified Election Audits
https://hdl.handle.net/1721.1/122920
Sharper p-Values for Stratified Election Audits
Higgins, Michael J.; Rivest, Ronald L; Stark, Philip B.
Vote-tabulation audits can be used to collect evidence that the set of winners of an election (the outcome) according to the machine count is correct — that it agrees with the outcome that a full hand count of the audit trail would show. The strength of evidence is measured by the p-value of the hypothesis that the machine outcome is wrong. Smaller p-values are stronger evidence that the outcome is correct. Most states that have election audits of any kind require audit samples stratified by county for contests that cross county lines. Previous work on p-values for stratified samples based on the largest weighted overstatement of the margin used upper bounds that can be quite weak. Sharper p-values can be found by solving a 0-1 knapsack problem. For example, the 2006 U.S. Senate race in Minnesota was audited using a stratified sample of 2–8 precincts from each of 87 counties, 202 precincts in all. Earlier work (Stark 2008b) found that the p-value was no larger than 0.042. We show that it is no larger than 0.016: much stronger evidence that the machine outcome was correct. We also give algorithms for choosing how many batches to draw from each stratum to reduce the counting burden. In the 2006 Minnesota race, a stratified sample about half as large — 109 precincts versus 202 — would have given just as small a p-value if the observed maximum overstatement were the same. This would require drawing 11 precincts instead of 8 from the largest county, and 1 instead of 2 from the smallest counties. We give analogous results for the 2008 U.S. House of Representatives contests in California. Keywords: post-election audits; knapsack problem
2011-10-01T00:00:00Z