Reconfiguration of a four-bar mechanism using phase space connections

Linkage mechanisms are perhaps the simplest mechanical structures in engineering, but they can exhibit significant nonlinearity which can in principle be exploited. In this paper a simple smart structure model is developed based on such nonlinearity to investigate the reconfiguration of a four-bar mechanism through phase space connections. The central idea is based on heteroclinic connections in the mechanism phase space between equal-energy unstable equilibria. It is proposed that transitions between such equal-energy unstable (but actively controlled) equilibria in principle require zero net energy input, compared to transitions between stable equilibria which require the input and then dissipation of energy. However, it can be difficult to obtain such heteroclinic connections numerically in complex dynamical systems, therefore an objective function approach is used to seek transtions between unstable equilibria which approximate true heteroclinic connections. The instability inherent in the model is therefore actively utilised to provide energy-efficient transitions between configurations of the mechanism. It will be shown that the four-bar mechanism then forms the basis for an elastic model of a smart buckling beam

Replication Control in Distributed File Systems

We present a replication control protocol for distributed file systems that can guarantee strict consistency or sequential consistency while imposing no performance overhead for normal reads. The protocol uses a primary-copy scheme with server redirection when concurrent writes occur. It tolerates any number of component omission and performance failures, even when these lead to network partition. Failure detection and recovery are driven by client accesses. No heartbeat messages or expensive group communication services are required. We have implemented the protocol in NFSv4, the emerging Internet standard for distributed filing.http://deepblue.lib.umich.edu/bitstream/2027.42/107880/1/citi-tr-04-1.pd

Reliable Replication at Low Cost

The emerging global scientific collaborations demand a scalable, efficient, reliable, and still convenient data access and management scheme. To fulfill these requirements, this paper describes a replicated file system that supports mutable (i.e., read/write) replication with strong consistency guarantees, small performance penalty, high failure resilience, and good scaling properties. The paper further evaluates the system using a real scientific application. The evaluation results show that the presented replication system can significantly improve the application's performance by reducing the first-time access latency to read the input data and by distributing the verification of data access to a nearby server. Furthermore, the penalty of file replication is negligible as long as applications use synchronous writes at a moderate rate.http://deepblue.lib.umich.edu/bitstream/2027.42/107950/1/citi-tr-06-2.pd

Naming, Migration, and Replication for NFSv4

In this paper, we discuss a global name space for NFSv4 and mechanisms for transparent migration and replication. By convention, any file or directory name beginning with /nfs on an NFS client is part of this shared global name space. Our system supports file system migration and replication through DNS resolution, provides directory migration and replication using built-in NFSv4 mechanisms, and supports read/write replication with precise consistency guarantees, small performance penalty, and good scaling. We implement these features with small extensions to the published NFSv4 protocol, and demonstrate a practical way to enhance network transparency and administerability of NFSv4 in wide area networks.http://deepblue.lib.umich.edu/bitstream/2027.42/107939/1/citi-tr-06-1.pd