314 research outputs found
A methodology for the concurrent design of products and their assembly sequence
No full textThis thesis reports on the development of a Two-Tier methodology that provides
support for assembly sequence construction, validation and evaluation in parallel with
the design. This facilitates the production of products that are optimised for
assemblability. The proposed approach diverges significantly from many of the
sequence generation methods developed to date, which assume that assembly
planning starts at the conclusion of the design process. It is believed that the latter
approach misses an important opportunity to concurrently implement design and
sequence improvements that would result in products inherently suited to assembly.
The industrial assembly planning process was found to be completely different from
the automatic sequence generation approach. The Two-Tier methodology has its
foundations in this manual process, which uses a breadth-first, depth-second search. A
constraint-based method is used to interactively validate the sequence. In direct
contrast to traditional sequence generators, the hard and soft constraints are invoked
throughout the process. A novel approach to sequence evaluation allows the user to
quantitatively determine the suitability of the sequence at any time during the
construction process.
However, designers are rarely assembly experts and it is unreasonable to expect
practical sequences to be generated without assistance. Thus, a set of generic
assembly planning rules was identified from industrial surveys by the author. These
were collaboratively implemented into an Expert Assembler, which currently consists
of two mini advisors. Support is available to identify the most suitable base
component and the most appropriate component to add next.
The Two-Tier methodology has been implemented into a computer-based system
called SPADE (Sequence Planning And Design Environment). A four-layer model
holds the product data that underpins this implementation. The methodology and
SPADE have been successfully tested using representative case studies and the results
are reported as part of this thesis
On the questions of local and global well-posedness for the hyperbolic PDEs occurring in some relativistic theories of gravity and electromagnetism
No full textThe two hyperbolic systems of PDEs we consider in this work are the source-free Maxwell-Born-Infeld (MBI) field equations and the Euler-Nordstr??m system for gravitationally self-interacting fluids. The former system plays a central role in Kiessling's recently proposed self-consistent model of classical
electrodynamics with point charges, a model that does not suffer from the infinities found in the classical Maxwell-Maxwell model with point charges. The latter system is a scalar gravity caricature of the incredibly more complex Euler-Einstein system. The primary original contributions of the thesis can be summarized as follows:
1) We give a sharp non-local criterion for the formation of singularities in plane-symmetric solutions to the source-free MBI field equations. We also use a domain of dependence argument to show that 3-d initial data agreeing with certain plane-symmetric data on a large enough ball lead to solutions that form singularities in finite time. This work is an extension of a theorem of Brenier, who studied singularity formation in periodic plane-symmetric solutions.
2) We prove well-posedness for the Euler-Nordstr??m system with a cosmological constant k (EN_k) for initial data that are an H^N perturbation (not necessarily small) of a uniform, quiet fluid, for N [greater than]= 3. The method of proof relies on the framework of energy currents that has been recently developed by Christodoulou. We turn to this method out of necessity: two common frameworks for showing well-posedness in H^N, namely symmetric hyperbolicity and strict hyperbolicity, do not apply to the EN_k system, while Christodoulou's techniques apply to all hyperbolic systems derivable from a Lagrangian, of which the EN_k system is an example.
3) We insert the speed of light c as a parameter into the EN_k system (and designate the family of systems EN_k^c) in order to study the non-relativistic limit c to infinity. Taking the formal limit in the equations gives the Euler-Poisson system with a cosmological constant (EP_k). Using energy currents, we prove that for fixed initial data, as c goes to infinity, the solutions to the EN_k^c system converge uniformly on a spacetime slab [0,T] x R^3 to the solution of the EP_k system.Ph.D.Includes bibliographical references (p. 140-143)
The effects of increased midsole bending stiffness of sports shoes on muscle-tendon unit shortening and shortening velocity: a randomized crossover trial in recreational male runners
No full textBackground: Individual compliances of the foot-shoe interface have been suggested to store and release elastic strain energy via ligamentous and tendinous structures or by increased midsole bending stiffness (MBS), compression stiffness, and resilience of running shoes. It is unknown, however, how these compliances interact with each other when the MBS of a running shoe is increased. The purpose of this study was to investigate how structures of the foot-shoe interface are influenced during running by changes to the MBS of sport shoes.
Methods: A randomised crossover trial was performed, where 13 male, recreational runners ran on an instrumented treadmill at 3.5 m center dot s(-1) while motion capture was used to estimate foot arch, plantar muscle-tendon unit (pMTU), and shank muscle-tendon unit (sMTU) behaviour in two conditions: (1) control shoe and (2) the same shoe with carbon fibre plates inserted to increase the MBS.
Results: Running in a shoe with increased MBS resulted in less deformation of the arch (mean +/- SD; stiff, 7.26 +/- 1.78 degrees; control, 8.84 +/- 2.87 degrees; p <= 0.05), reduced pMTU shortening (stiff, 4.39 +/- 1.59 mm; control, 6.46 +/- 1.42 mm; p <= 0.01), and lower shortening velocities of the pMTU (stiff, - 0.21 +/- 0.03 m center dot s(-1); control, - 0.30 +/- 0.05 m center dot s(-1); p <= 0.01) and sMTU (stiff, - 0.35 +/- 0.08 m center dot s(-1); control, - 0.45 +/- 0.11 m center dot s(-1); p <= 0.001) compared to a control condition. The positive and net work performed at the arch and pMTU, and the net work at the sMTU were significantly lower in the stiff compared to the control condition.
Conclusion: The findings of this study showed that if a compliance of the foot-shoe interface is altered during running (e.g. by increasing the MBS of a shoe), the mechanics of other structures change as well. This could potentially affect long-distance running performance.articl
Academy of Criminal Justice Sciences Doctoral Student Summit: A Model of Professional Development
No full textDoctoral student attrition has been a consistent and ongoing problem, with only half of Ph.D. students completing their degrees. Often generated by feelings of isolation and dissatisfaction during the Ph.D. process, attrition results in negative consequences for students, their departments and institutions, and the broader academic community. Students face substantial financial and psychological burdens, institutions lose out on their investments, and the larger community misses out on significant contributions to resolutions to social challenges. To make matters worse, attrition rates are highest among the most marginalized students. Effective strategies for reducing attrition involve aiding in the development of students’ academic identity and creating a supportive sense of community. Although many departmental and institutional efforts to support doctoral students exist, challenges remain. Further, the needs of students are far-reaching and arguably require interventions from the academic community as a whole. To confront these challenges, the Academy of Criminal Justice Science approved the development of the Doctoral Student Summit, a professional development seminar intended to support Ph.D. students in degree completion through providing opportunities to foster academic identities and develop extended academic communities. This manuscript outlines the origins and evolution of the Doctoral Summit and provides an outline of its model and potential benefits to the field of Criminal Justice and other disciplines.Heather L. Pfeifer, Deeanna M. Button, Monica E. Summers, Courtney M. Porter, Jared R. Dmell
Impact of load type on microgrid stability
No full textThesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2012.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.CD-ROM contains PDF of thesis and MDL file.Both MIT Institute Archives and Barker Library copy: with CD-ROM.Cataloged from student submitted PDF version of thesis.Includes bibliographical references.Microgrids show great promise as a means of integrating distributed generation sources into the public grid distribution system. In order to provide uninterrupted,high quality power to local loads, microgrids must have the ability to operate independently of or in parallel with the local utility. Transitioning between independent operation, also called "islanded" operation, and utility connected operation can induce stability problems in the microgrid, especially when islanding is fault induced. Software simulation suggests that induction motor loads on the microgrid significantly decrease stability during fault induced islanding. To validate the software simulations and to investigate the impact of load type on microgrid stability, we have built a hardware system that simulates the operation of a microgrid.by Jared P. Monnin.M.Eng
Knitting with directed graphs
No full textThesis: M. Eng., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2018.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 67-68).Knitting has historically been communicated by its means of construction. For hand knitting, this is typically a list of instructions or a pictorial grid with knitting symbols. For machine knitting, a similar pictorial grid is used to express needle-level instructions. However, these formats suffer by the nature of their tight coupling with the method used to construct the garments they represent. Alternatively, we use Knit Meshes, which represent knitting structures by their geometry separate from a directed graph description of their topology. This thesis presents an algorithm that can generate a natural, deformed two-dimensional layout of Knit Meshes as well as a conversion pipeline that converts written hand knitting instructions to and from Knit Meshes and an algorithm that converts certain Knit Meshes into knitting machine code.by Jared B. Counts.M. Eng
Advancing the Maximum Accelerating Gradient of Niobium-3 Tin Superconducting Radiofrequency Accelerator Cavities: RF Measurements, Dynamic Temperature Mapping, and Material Growth
No full text300 pagesNiobium-3 Tin (Nb3Sn) is the most promising alternative material for Superconducting Radiofrequency (SRF) particle accelerator cavities. Current SRF accelerators use superconducting niobium accelerator cavities, which are nearing their theoretical limits of performance. Nb3Sn promises increased quality factors, twice the operational temperature (4.2 K instead of 2 K}), and almost twice the theoretical accelerating gradient--96 MV/m in a TESLA elliptical style cavity. These advances can reduce the size and complexity of particle accelerators while simultaneously making them more efficient. The capability of operating at 4.2 K enables the creation of small-scale superconducting accelerators that are run off cryocoolers and could be used in research and industrial applications. Current Nb3Sn cavities achieve quality factors of 2*10^10 at 4.2 K. The accelerating gradient, however, is limited far below the theoretical potential of this material, with the best recorded reaching 24 MV/m. In this work we present studies on what is limiting the maximum accelerating gradients in these cavities. We study cavity performance under RF testing, make dynamic measurements of cavity heating during operation, study samples with microscopy, and develop models of Nb3Sn material growth. In the process we develop new diagnostic tools for SRF development: a dynamic/high speed temperature mapping system that measures the spatial heat distribution on a cavity at 50 ksps, and high-power test system for measuring the ultimate critical fields (theoretical limit of the accelerating gradient) of new materials. We explore models of cavity losses/heating that limit the accelerating gradient and propose modifications to the material growth process to create Nb3Sn cavities with higher accelerating gradients and quality factors. We conclude with results from a new cavity coating wherein we have suppressed multi-gap superconductivity that has been seen in these cavities and was a limitation to the quality factor
Changes in Achilles tendon stiffness and energy cost following a prolonged run in trained distance runners
No full textDuring prolonged running, the magnitude of Achilles tendon (AT) length change may increase, resulting in increased tendon strain energy return with each step. AT elongation might also affect the magnitude of triceps surae (TS) muscle shortening and shortening velocity, requiring greater activation and increased muscle energy cost. Therefore, we aimed to quantify the tendon strain energy return and muscle energy cost necessary to allow energy storage to occur prior to and following prolonged running. 14 trained male (n = 10) and female (n = 4) distance runners (24 +/- 4 years, 1.72 +/- 0.09 m, 61 +/- 10 kg, (V) over barO(2)max 64.6 +/- 5.8 ml.kg(-1).min(-1)) ran 90 minutes (RUN) at approximately 85% of lactate threshold speed (sLT). Prior to and following RUN, AT stiffness and running energy cost (E-run) at 85% sLT were determined. AT energy return was calculated from AT stiffness, measured with dynamometry and ultrasound and estimated TS force during stance. TS energy cost was estimated on the basis of AT force and assumed crossbridge mechanics and energetics. Following RUN, AT stiffness was reduced from 328 +/- 172 N.mm(-1) to 299 +/- 148 N.mm(-1) (p = 0.022). E-run increased from 4.56 +/- 0.32 J.kg(-1).m-1 to 4.62 +/- 0.32 J.kg(-1).m-1 (p = 0.049). Estimated AT energy return was not different following RUN (p = 0.99). Estimated TS muscle energy cost increased significantly by 11.8 +/- 12.3 J.stride(-1), (p = 0.0034), accounting for much of the post-RUN increase in E-run (8.6 +/- 14.5 J.stride(-1), r(2) = 0.31). These results demonstrate that a prolonged, submaximal run can reduce AT stiffness and increase E-run in trained runners, and that the elevated TS energy cost contributes substantially to the elevated E-run.articl
Practicing with gradual increases in contextual interference enhances visuomotor learning
No full textThe purpose of this study was to determine if practicing with gradual increases in contextual interference (CI) facilitated the learning of a continuous motor skill that required visuomotor tracking. We hypothesized the group that practiced with increasing amounts of CI would perform significantly better on a retention and transfer test compared to participants that practiced with blocked and random scheduling. A total of 78 participants were randomly assigned to one of three groups (i.e. Blocked, Increasing, Random). The level of CI was modified by varying the number of revolutions per minute (RPM) on a rotary pursuit tracker. Following the acquisition phase, participants returned after a 24-hour period and completed the 12-trial
retention and transfer test. The results of statistical analysis indicated that all three groups improved their performance during practice. The posttest analysis indicated the Increasing group was better than the Blocked and Random groups on the retention and transfer test
Effects of Temperature and Hydrology on Growth and Recruitment of Shovelnose Sturgeon in the Lower Mississippi River
No full textI evaluated the effects of thermal and hydrologic conditions on growth and recruitment of Shovelnose Sturgeon (Scaphirhynchus platorynchus) in the lower Mississippi River and assessed mortality. Duration of water temperatures 12-24°C had a positive influence and temperatures below 11°C had a negative influence on annual growth. Duration of water temperatures above 28°C, duration of floodplain inundation, duration of low water, and minimum and maximum river stage did not influence annual growth. Duration of water temperatures 18-20°C and 18-24°C had a positive influence on recruitment, and duration of temperatures at and below 10°C had a negative influence on recruitment. Duration of days above 5, 6, 7, 8, and 8.9 m on the Vicksburg, MS river gage did not influence recruitment. Annual mortality was 28%. Growth and recruitment of Shovelnose Sturgeon in the lower Mississippi River appear to be positively influenced by duration of moderate water temperatures and minimally influenced by hydrologic conditions
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