240 research outputs found
Bound on the graviton mass from Chandra X-ray cluster sample
We present new limits on the graviton mass using a sample of 12 relaxed
galaxy clusters, for which temperature and gas density profiles were derived by
Vikhlinin et al (astro-ph/0507092) using Chandra X-ray observations. These
limits can be converted to a bound on the graviton mass, assuming a non-zero
graviton mass would lead to a Yukawa potential at these scales. For this
purpose, we first calculate the total dynamical mass from the hydrostatic
equilibrium equation in Yukawa gravity and then compare it with the
corresponding mass in Newtonian gravity. We calculate a 90 % c.l. lower/upper
limit on the graviton Compton wavelength/ mass for each of the 12 clusters in
the sample. The best limit is obtained for Abell 2390, corresponding to
km or eV. This is
the first proof of principles demonstration of setting a limit on the graviton
mass using a sample of related galaxy clusters with X-ray measurements and can
be easily applied to upcoming X-ray surveys such as eRosita.Comment: 6 pages, 1 figur
A Conformal Mapping Based Fractional Order Approach for Sub-optimal Tuning of PID Controllers with Guaranteed Dominant Pole Placement
A novel conformal mapping based Fractional Order (FO) methodology is
developed in this paper for tuning existing classical (Integer Order)
Proportional Integral Derivative (PID) controllers especially for sluggish and
oscillatory second order systems. The conventional pole placement tuning via
Linear Quadratic Regulator (LQR) method is extended for open loop oscillatory
systems as well. The locations of the open loop zeros of a fractional order PID
(FOPID or PI{\lambda}D{\mu}) controller have been approximated in this paper
vis-\`a-vis a LQR tuned conventional integer order PID controller, to achieve
equivalent integer order PID control system. This approach eases the
implementation of analog/digital realization of a FOPID controller with its
integer order counterpart along with the advantages of fractional order
controller preserved. It is shown here in the paper that decrease in the
integro-differential operators of the FOPID/PI{\lambda}D{\mu} controller pushes
the open loop zeros of the equivalent PID controller towards greater damping
regions which gives a trajectory of the controller zeros and dominant closed
loop poles. This trajectory is termed as "M-curve". This phenomena is used to
design a two-stage tuning algorithm which reduces the existing PID controller's
effort in a significant manner compared to that with a single stage LQR based
pole placement method at a desired closed loop damping and frequency.Comment: 23 pages, 7 figures, in press; Communications in Nonlinear Science
and Numerical Simulations, 201
Optimum Weight Selection Based LQR Formulation for the Design of Fractional Order PI{\lambda}D{\mu} Controllers to Handle a Class of Fractional Order Systems
A weighted summation of Integral of Time Multiplied Absolute Error (ITAE) and
Integral of Squared Controller Output (ISCO) minimization based time domain
optimal tuning of fractional-order (FO) PID or PI{\lambda}D{\mu} controller is
proposed in this paper with a Linear Quadratic Regulator (LQR) based technique
that minimizes the change in trajectories of the state variables and the
control signal. A class of fractional order systems having single non-integer
order element which show highly sluggish and oscillatory open loop responses
have been tuned with an LQR based FOPID controller. The proposed controller
design methodology is compared with the existing time domain optimal tuning
techniques with respect to change in the trajectory of state variables,
tracking performance for change in set-point, magnitude of control signal and
also the capability of load disturbance suppression. A real coded genetic
algorithm (GA) has been used for the optimal choice of weighting matrices while
designing the quadratic regulator by minimizing the time domain integral
performance index. Credible simulation studies have been presented to justify
the proposition.Comment: 6 pages, 5 figure
Privacy-Preserving Secret Shared Computations using MapReduce
Data outsourcing allows data owners to keep their data at \emph{untrusted}
clouds that do not ensure the privacy of data and/or computations. One useful
framework for fault-tolerant data processing in a distributed fashion is
MapReduce, which was developed for \emph{trusted} private clouds. This paper
presents algorithms for data outsourcing based on Shamir's secret-sharing
scheme and for executing privacy-preserving SQL queries such as count,
selection including range selection, projection, and join while using MapReduce
as an underlying programming model. Our proposed algorithms prevent an
adversary from knowing the database or the query while also preventing
output-size and access-pattern attacks. Interestingly, our algorithms do not
involve the database owner, which only creates and distributes secret-shares
once, in answering any query, and hence, the database owner also cannot learn
the query. Logically and experimentally, we evaluate the efficiency of the
algorithms on the following parameters: (\textit{i}) the number of
communication rounds (between a user and a server), (\textit{ii}) the total
amount of bit flow (between a user and a server), and (\textit{iii}) the
computational load at the user and the server.\BComment: IEEE Transactions on Dependable and Secure Computing, Accepted 01
Aug. 201
Estimation, Analysis and Smoothing of Self-Similar Network Induced Delays in Feedback Control of Nuclear Reactors
This paper analyzes a nuclear reactor power signal that suffers from network
induced random delays in the shared data network while being fed-back to the
Reactor Regulating System (RRS). A detailed study is carried out to investigate
the self similarity of random delay dynamics due to the network traffic in
shared medium. The fractionality or selfsimilarity in the network induced delay
that corrupts the measured power signal coming from Self Powered Neutron
Detectors (SPND) is estimated and analyzed. As any fractional order randomness
is intrinsically different from conventional Gaussian kind of randomness, these
delay dynamics need to be handled efficiently, before reaching the controller
within the RRS. An attempt has been made to minimize the effect of the
randomness in the reactor power transient data with few classes of smoothing
filters. The performance measure of the smoothers with fractional order noise
consideration is also investigated into.Comment: 6 pages, 6 figure
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