870 research outputs found
Dual Rate Control for Security in Cyber-physical Systems
We consider malicious attacks on actuators and sensors of a feedback system
which can be modeled as additive, possibly unbounded, disturbances at the
digital (cyber) part of the feedback loop. We precisely characterize the role
of the unstable poles and zeros of the system in the ability to detect stealthy
attacks in the context of the sampled data implementation of the controller in
feedback with the continuous (physical) plant. We show that, if there is a
single sensor that is guaranteed to be secure and the plant is observable from
that sensor, then there exist a class of multirate sampled data controllers
that ensure that all attacks remain detectable. These dual rate controllers are
sampling the output faster than the zero order hold rate that operates on the
control input and as such, they can even provide better nominal performance
than single rate, at the price of higher sampling of the continuous output
Target Assignment in Robotic Networks: Distance Optimality Guarantees and Hierarchical Strategies
We study the problem of multi-robot target assignment to minimize the total
distance traveled by the robots until they all reach an equal number of static
targets. In the first half of the paper, we present a necessary and sufficient
condition under which true distance optimality can be achieved for robots with
limited communication and target-sensing ranges. Moreover, we provide an
explicit, non-asymptotic formula for computing the number of robots needed to
achieve distance optimality in terms of the robots' communication and
target-sensing ranges with arbitrary guaranteed probabilities. The same bounds
are also shown to be asymptotically tight.
In the second half of the paper, we present suboptimal strategies for use
when the number of robots cannot be chosen freely. Assuming first that all
targets are known to all robots, we employ a hierarchical communication model
in which robots communicate only with other robots in the same partitioned
region. This hierarchical communication model leads to constant approximations
of true distance-optimal solutions under mild assumptions. We then revisit the
limited communication and sensing models. By combining simple rendezvous-based
strategies with a hierarchical communication model, we obtain decentralized
hierarchical strategies that achieve constant approximation ratios with respect
to true distance optimality. Results of simulation show that the approximation
ratio is as low as 1.4
Searching in Unstructured Overlays Using Local Knowledge and Gossip
This paper analyzes a class of dissemination algorithms for the discovery of
distributed contents in Peer-to-Peer unstructured overlay networks. The
algorithms are a mix of protocols employing local knowledge of peers'
neighborhood and gossip. By tuning the gossip probability and the depth k of
the k-neighborhood of which nodes have information, we obtain different
dissemination protocols employed in literature over unstructured P2P overlays.
The provided analysis and simulation results confirm that, when properly
configured, these schemes represent a viable approach to build effective P2P
resource discovery in large-scale, dynamic distributed systems.Comment: A revised version of the paper appears in Proc. of the 5th
International Workshop on Complex Networks (CompleNet 2014) - Studies in
Computational Intelligence Series, Springer-Verlag, Bologna (Italy), March
201
Enhancing the selective extracellular location of a recombinant E. coli domain antibody by management of fermentation conditions
The preparation of a recombinant protein using Escherichia coli often involves a challenging primary recovery sequence. This is due to the inability to secrete the protein to the extracellular space without a significant degree of cell lysis. This results in the release of nucleic acids, leading to a high viscosity, difficulty to clarify, broth and also to contamination with cell materials such as lipopolysaccharides and host cell proteins. In this paper, we present different fermentation strategies to facilitate the recovery of a V H domain antibody (13.1 kDa) by directing it selectively to the extracellular space and changing the balance between domain antibody to nucleic acid release. The manipulation of the cell growth rate in order to increase the outer cell membrane permeability gave a small ~1.5-fold improvement in released domain antibody to nucleic acid ratio without overall loss of yield. The introduction during fermentation of release agents such as EDTA gave no improvement in the ratio of released domain antibody to nucleic acid and a loss of overall productivity. The use of polyethyleneimine (PEI) during fermentation was with the aim to (a) permeabilise the outer bacterial membrane to release selectively domain antibody and (b) remove selectively by precipitation nucleic acids released during cell lysis. This strategy resulted in up to ~4-fold increase in the ratio of domain antibody to soluble nucleic acid with no reduction in domain antibody overall titre. In addition, a reduction in host cell protein contamination was achieved and there was no increase in endotoxin levels. Similar results were demonstrated with a range of other antibody products prepared in E. coli
An Information-Theoretic Analysis of Discrete-Time Control and Filtering Limitations by the I-MMSE Relationships
Fundamental limitations or performance trade-offs/limits are important
properties and constraints of control and filtering systems. Among various
trade-off metrics, total information rate, which characterizes the sensitivity
trade-offs and average performance of control and filtering systems, is
conventionally studied by using the (differential) entropy rate and
Kolmogorov-Bode formula. In this paper, by extending the famous I-MMSE (mutual
information -- minimum mean-square error) relationship to the discrete-time
additive white Gaussian channels with and without feedback, a new paradigm is
introduced to estimate and analyze total information rate as a control and
filtering trade-off metric. Under this framework, we enrich the trade-off
properties of total information rate for a variety of discrete-time control and
filtering systems, e.g., LTI, LTV, and nonlinear, and also provide an
alternative approach to investigate total information rate via optimal
estimation.Comment: Neng Wan and Dapeng Li contributed equally to this pape
Evaluation of options for harvest of a recombinant E. coli fermentation producing a domain antibody using ultra scale-down techniques and pilot-scale verification
Ultra scale-down (USD) methods operating at the millilitre scale were used to characterise full-scale processing of E. coli fermentation broths autolysed to different extents for release of a domain antibody. The focus was on the primary clarification stages involving continuous centrifugation followed by depth filtration. The performance of this sequence was predicted by USD studies to decrease significantly with increased extents of cell lysis. The use of polyethyleneimine (PEI) reagent was studied to treat the lysed cell broth by precipitation of soluble contaminants such as DNA and flocculation of cell debris material. The USD studies were used to predict the impact of this treatment on the performance and here it was found that the fermentation could be run to maximum productivity using an acceptable clarification process (e.g a centrifugation stage operating at 0.11 L per m(2) equivalent gravity settling area per h followed by a resultant required depth filter area of 0.07 m(2) per L supernatant). A range of USD predictions was verified at the pilot scale for centrifugation followed by depth filtration. This article is protected by copyright. All rights reserved
Optimal Control of Systems with Delayed Observation Sharing Patterns
Coordinated Science Laboratory was formerly known as Control Systems LaboratoryNational Science Foundatio
Tidal circulation and flushing characteristics of the Nauset Marsh System : report to the Town of Orleans
Various interested bodies (i.e., National Park Servce, Cape Cod Commssion, and the
Town of Orleans) charged with management of the Nauset Marsh system on Cape Cod, MA,
commissioned a study of the estuarine circulation within the Nauset system. Recent signficant
morphological changes in the system have changed mixing processes and residence times for the
embayment. This study specifically addressed the differing water circulation and residence times
arising from a migrating single inlet (dominant condition) and dual inlet (1992-1996) situations.
These residence times are to be used by the Cape Cod Commission to identify nitrogen-sensitive
sub-embayments based on various assumptions of build-out and nutrient loading. The Nauset
Marsh system has experienced considerable development in recent years; proper management of
this resource area requires knowledge of the consequences of such development.
Application of field observations of bathymetry, sea surface elevation, temperature,
salinity and currents, leads to better understanding the physics of the system. These data,
analyzed in various forms, served as input data for a numerical, two-dimensional circulation
model of the embayment. The circulation model provided flow and discharge data with which the
residence times were calculated. Bathymetric measurements defined the volumes of the various
sub-embayments to be used in the calculation of residence times.
Residence times were calculated for six sub-embayments of the system, defined on the
basis of their common hydrodynamic and morphologic characteristics. Two scenarios were
evaluated: one for the present single-inlet system, which is near typical for most system states,
and one for a dual inlet system such as existed for a period of time from 1992 through 1996.
Residence times were evaluated for twelve cases, to demonstrate the range of residence
times that can be defined based on varing assumptions. For instance, residence times can be
defined on the basis of mean low water volumes or mean water levels, the latter being the more
conservative (yielding a longer residence time). In addition, residence times depend on whether
spring tides, neap tides, or average tidal conditions are used. We provide data on all three
conditions: the neap tidal case is the most conservative in the sense of providing a longer
residence time. This case can serve as the basis for flushing if conservatism is desired. Finally,
residence time can be defined based on the amount of time it takes for water to renew itself with
water from adjacent sub-embayments, or more conservatively assuming renewal from the
offshore waters (which are presumed to be cleaner).
Based on these various inputs, assumptions and calculations, residence times for Salt and
Mill ponds under conditions of a single inlet are the longest of the various sub-embayments.
Town Cove is still relatively quickly renewed, though not as fast as the main channels serving the
system.
Flow pattern under dual-inlet condition does seem to be partioned well, with the
northern inlet serving the northern part of the system and the southern inlet serving the southern
part of the system, with litte hydrodynamc communication between the two divisions. This
new hydrodynamc behavior results in shorter residence times under dual inlets than under a
single inlet.
Calculations indicate that the slowest flushing occurs in Mill and Salt ponds. The main
body of the embayment, consisting of narrow channels between well-flushed salt marsh and tidal
flats, flushes rapidly. Two-dimensional calculations show that Town Cove also flushes
relatively rapidly, on average. However, its greater depth and occasional temperature
stratification create conditions which might accumulate nutrients in bottom sediments, which,
when released, can cause decrease in water quality (such as plankton blooms). A more
sophisticated low-trophic level ecosystem model combined with vertical hydrodynamic structure
could clarfy the dynamics of this process.
This study provides a defensible basis for evaluating nutrient loading and potential
eutrophication arising from development in the watershed around Nauset embayment. However,
since morphological changes occur on a rapid basis in this area, the issue of residence time should
be re-examined periodically. For instance, rapid onshore migration of the southern barrier beach
is threatening closure of the south chanel, a condition which could adversely affect water quality
in Nauset Harbor in the near futue. A process should be established to examine the sensitivity
of residence times for rapidly changing morphology.Funding was provided by the Town of Orleans, the National Park Service
and the Andrew W. Mellon Foundation
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