On the complexity of computation in the presence of link failures: the case of a ring

Summary We investigate the message complexity of distributed computations on rings of asynchronous processors. In such computations, each processor has an initial local value and the task is to compute some predetermined function of all local values. Our work deviates from previous works concerning the complexity of ring computations in that we consider the effect oflink failures. A link is said to fail if some message sent through it never reaches its destination. We show that the message complexity of any function, which is sensitive to all its inputs, is (n logn) whenn, the number of processors, is a-priori known; and is (n ² ) whenn is not known. Interestingly, these tight bounds do not depend on whether the identity of a leader is a-priori known before the computation starts. These results stand in sharp contrast to the situation in asynchronous rings with no link failures, where the message complexity is affected by the a-priori knowledge of a leader but is not affected by the knowledge ofn. Oded Goldreich was born in Tel-Aviv, Israel, on February 4th 1957. Received B.A., M.Sc., and D.Sc. in Computer Science from the Technion — Israel Institute of Technology, Haifa, Israel, in 1980, 1982, and 1983, respectively. He is currently an Associate Professor of Computer Science in the Technion. From 1983 to 1986, he was a postdoctoral fellow in MIT's Laboratory for Computer Science. His research interests include cryptography and related areas, relation between randomness and algorithms, and distributed computation. Luiba Shrira was born in Vilnius, Lithuania. Received B.A., M.Sc., and D.Sc. in Computer Science from the Technion—Israel Institute of Technology, Haifa, Israel in 1977, 1980, and 1985, respectively. from 1986 to 1989 she was a postdoctoral fellow at Laboratory for Computer science at MIT, where she is currently a Research Associate. Her research interests include highly-available and reliable distributed algorithms and systems, persistent object systems, and programming methodology.Part of the work has been done while the first author was in the Laboratory for Computer Science of MIT and the second author was in the Computer Science Department of the Technion. First author was partially supported by a Weizmann Postdoctoral Fellowship, an IBM Postdoctoral Fellowship, and Albert Einstein Research Fund (through Technion's V.P.R. Fund)     

Zoom tracking and its applications

We present a new active vision technique called zoom tracking. Zoom tracking is the continuous adjustment of a camera's focal length in order to keep a constant-sized image of an object moving along the camera's optical axis. Two methods for performing zoom tracking are presented: a closed-loop visual feedback algorithm based on optical flow, and use of depth information obtained from an autofocus camera's range sensor. We explore two uses of zoom tracking: recovery of depth information and improving the performance of scale-variant algorithms. We show that the image stability provided by zoom tracking improves the performance of algorithms that are scale variant, such as correlation-based trackers. While zoom tracking cannot totally compensate for an object's motion, due to the effect of perspective distortion, an analysis of this distortion provides a quantitative estimate of the performance of zoom tracking. Zoom tracking can be used to reconstruct a depth map of the tracked object. We show that under normal circumstances this reconstruction is much more accurate than depth from zooming, and works over a greater range than depth from axial motion while providing, in the worst case, only slightly less accurate results. Finally, we show how zoom tracking can also be used in time-to-contact calculations. Received: 15 February 2000 / Accepted: 19 June 2000     

Visual analysis of quality-related manufacturing data using fractal geometry

Improving manufacturing quality is an important challenge in various industrial settings. Data mining methods mostly approach this challenge by examining the effect of operation settings on product quality. We analyze the impact of operational sequences on product quality. For this purpose, we propose a novel method for visual analysis and classification of operational sequences. The suggested framework is based on an Iterated Function System (IFS), for producing a fractal representation of manufacturing processes. We demonstrate our method with a software application for visual analysis of quality-related data. The proposed method offers production engineers an effective tool for visual detection of operational sequence patterns influencing product quality, and requires no understanding of mathematical or statistical algorithms. Moreover, it enables to detect faulty operational sequence patterns of any length, without predefining the sequence pattern length. It also enables to visually distinguish between different faulty operational sequence patterns in cases of recurring operations within a production route. Our proposed method provides another significant added value by enabling the visual detection of rare and missing operational sequences per product quality measure. We demonstrate cases in which previous methods fail to provide these capabilities.     

Achieving stress-constrained topological design via length scale control

A new suite of computational procedures for stress-constrained continuum topology optimization is presented. In contrast to common approaches for imposing stress constraints, herein it is proposed to limit the maximum stress by controlling the length scale of the optimized design. Several procedures are formulated based on the treatment of the filter radius as a design variable. This enables to automatically manipulate the minimum length scale such that stresses are constrained to the allowable value, while the optimization is driven to minimizing compliance under a volume constraint – without any direct constraints on stresses. Numerical experiments are presented that incorporate the following : 1) Global control over the filter radius that leads to a uniform minimum length scale throughout the design; 2) Spatial variation of the filter radius that leads to local manipulation of the minimum length according to stress concentrations; and 3) Combinations of the two above. The optimized designs provide high-quality trade-offs between compliance, stress and volume. From a computational perspective, the proposed procedures are efficient and simple to implement: essentially, stress-constrained topology optimization is posed as a minimum compliance problem with additional treatment of the length scale.     

Non-linear QFT synthesis by local linearization

Existing non-linear quantitive feedback theory (QFT) techniques are based on some form of linearization of the non-linear plant, where the quality of the controller is a function of the equivalent linearized plant. We propose a new approach to non-linear QFT, based on local linearization of the non-linear plant about closed-loop acceptable outputs. Here, acceptable outputs appear as uncertain parameters in an equivalent linear family, being a generalization of the more traditional linearization about equilibrium points. In some applications, this technique may give better results and is an alternative when the other existing techniques fail. A comparison of this technique to previous ones using an uncertain Van der Pol plant considered previously in the QFT literature, and a pH control system, is presented.     

Analysis of Multi-Robot Systems

This work introduces and analyzes the concept of a multi-robot cell. The paper discusses the advantages of such a cell, suggests a classification scheme and points out the specific activity control issues to achieve conflict free (e.g., no collision) operations. Finally a methodology to aluate the advantages of such a cell is presented and illustrated by an example.     

Bioinspired Flexible and Tough Layered Peptide Crystals

One major challenge of functional material fabrication is combining flexibility, strength, and toughness. In several biological and artificial systems, these desired mechanical properties are achieved by hierarchical architectures and various forms of anisotropy, as found in bones and nacre. Here, it is reported that crystals of N‐capped diphenylalanine, one of the most studied self‐assembling systems in nanotechnology, exhibit well‐ordered packing and diffraction of sub‐Å resolution, yet display an exceptionally flexible nature. To explore this flexibility, the mechanical properties of individual crystals are evaluated, assisted by density functional theory calculations. High‐resolution scanning electron microscopy reveals that the crystals are composed of layered self‐assembled structures. The observed combination of strength, toughness, and flexibility can therefore be explained in terms of weak interactions between rigid layers. These crystals represent a novel class of self‐assembled layered materials, which can be utilized for various technological applications, where a combination of usually contradictory mechanical properties is desired.     

Exploring motivations for contributing to open source initiatives: The roles of contribution context and personal values

We explore contextual and dispositional correlates of the motivation to contribute to open source initiatives. We examine how the context of the open source project, and the personal values of contributors, are related to the types of motivations for contributing. A web-based survey was administered to 300 contributors in two prominent open source contexts: software and content. As hypothesized, software contributors placed a greater emphasis on reputation-gaining and self-development motivations, compared with content contributors, who placed a greater emphasis on altruistic motives. Furthermore, the hypothesized relationships were found between contributors’ personal values and their motivations for contributing.     

Reinforcement layout design for concrete structures based on continuum damage and truss topology optimization

This article presents a new procedure for the layout design of reinforcement in concrete structures. Concrete is represented by a gradient-enhanced continuum damage model with strain-softening and reinforcement is modeled as elastic bars that are embedded into the concrete domain. Adjoint sensitivity analysis is derived in complete consistency with respect to path-dependency and the nonlocal model. Classical truss topology optimization based on the ground structure approach is applied to determine the optimal topology and cross-sections of the reinforcement bars. This approach facilitates a fully digital work flow that can be highly effective, especially for the design of complex structures. Several test cases involving two- and three-dimensional concrete structures illustrate the capabilities of the proposed procedure.