Optimal maintenance policies for machines subject to deterioration and intermittent breakdowns

Optimal preventive maintenance policies, for a machine subject to deterioration with age and intermittent breakdowns and repairs, are derived using optimal control theory. The optimal policies are shown to be of bang-bang nature. The extension to the case when there are a large number of identical machines and several repairmen in the system is considered next. This model takes into account the waiting line formed at the repair facility and establishes a link between this problem and the classical ``repairmen problem.''

Effect of gas injection on drag and surface heat transfer rates for a 30 degrees semi-apex angle blunt body flying at Mach 5.75

Effect of coolant gas injection in the stagnation region on the surface heat transfer rates and aerodynamic drag for a large angle blunt body flying at hypersonic Mach number is reported for two stagnation enthalpies. A 60° apex-angle blunt cone model is employed for this purpose with air injection at the nose through a hole of 2mm diameter. The convective surface heating rates and aerodynamic drag are measured simultaneously using surface mounted platinum thin film sensors and internally mounted accelerometer balance system, respectively. About 35–40% reduction in surface heating rates is observed in the vicinity of stagnation region whereas 15–25% reduction in surface heating rates is felt beyond the stagnation region at stagnation enthalpy of 1.6MJ/kg. The aerodynamic drag expressed in terms of drag coefficient is found to increase by 20% due to the air injection.

System and Method for Temporal Datamining

A system for temporal data mining includes a computer readable medium having an application configured to receive at an input module a temporal data series and a threshold frequency. The system is further configured to identify, using a candidate identification and tracking module, one or more occurrences in the temporal data series of a candidate episode and increment a count for each identified occurrence. The system is also configured to produce at an output module an output for those episodes whose count of occurrences results in a frequency exceeding the threshold frequency.

System and Method for mining of Temporal Data

A system for temporal data mining includes a computer readable medium having an application configured to receive at an input module a temporal data series having events with start times and end times, a set of allowed dwelling times and a threshold frequency. The system is further configured to identify, using a candidate identification and tracking module, one or more occurrences in the temporal data series of a candidate episode and increment a count for each identified occurrence. The system is also configured to produce at an output module an output for those episodes whose count of occurrences results in a frequency exceeding the threshold frequency.

New Insights into Optimal Discrete Rate Adaptation for Average Power Constrained Single and Multi-Node Systems

The throughput-optimal discrete-rate adaptation policy, when nodes are subject to constraints on the average power and bit error rate, is governed by a power control parameter, for which a closed-form characterization has remained an open problem. The parameter is essential in determining the rate adaptation thresholds and the transmit rate and power at any time, and ensuring adherence to the power constraint. We derive novel insightful bounds and approximations that characterize the power control parameter and the throughput in closed-form. The results are comprehensive as they apply to the general class of Nakagami-m (m >= 1) fading channels, which includes Rayleigh fading, uncoded and coded modulation, and single and multi-node systems with selection. The results are appealing as they are provably tight in the asymptotic large average power regime, and are designed and verified to be accurate even for smaller average powers.

Spectroscopic Characterization and Modeling of Quadrupolar Charge-Transfer Dyes with Bulky Substituents

Joint experimental and theoretical work is presented on two quadrupolar D-pi-A-pi-D chromophores characterized by the same bulky donor (D) group and two different central cores. The first chromophore, a newly synthesized species with a malononitrile-based acceptor (A) group, has a V-shaped structure that makes its absorption spectrum very broad, covering most of the visible region. The second chromophore has a squaraine-based core and therefore a linear structure, as also evinced from its absorption spectra. Both chromophores show an anomalous red shift of the absorption band upon increasing solvent polarity, a feature that is ascribed to the large, bulky structure of the moleCules. For these molecules, the basic description of polar solvation in terms of a uniform reaction field fails. Indeed, a simple extension of the model to account for two independent reaction fields associated with the two molecular arms quantitatively reproduces the observed linear absorption and fluorescence as well as fluorescence anisotropy spectra, fully rationalizing their nontrivial dependence on solvent polarity. The model derived from the analysis of linear spectra is adopted to predict nonlinear spectra and specifically hyper-Rayleigh scattering and two-photon absorption spectra. In polar solvents, the V-shaped chromophore is predicted to have a large HRS response in a wide spectral region (approximately 600-1300 nm). Anomalously large and largely solvent-dependent HRS responses for the linear chromophores are ascribed to symmetry lowering induced by polar solvation and amplified in this bulky system by the presence of two reaction fields.

Flow-enhanced pairing and other unusual effects in Fermi gases in synthetic gauge fields

Recent experiments on fermions in synthetic gauge fields result in systems with a spin-orbit coupling along one spatial axis, a detuning field, and a Zeeman field. We show theoretically that the presence of all three results in interesting and unusual phenomena in a system of interacting fermions (interactions described by a scattering length). For two fermions, bound states appear only over a certain range of the center-of-mass momenta. The deepest bound state appears at a nonzero center-of-mass momentum. For center-of-mass momenta without a bound state, the gauge field induces a resonance-like feature in the scattering continuum resulting in a large scattering phase shift. In the case of many particles, we demonstrate that the system, in a parameter range, shows flow-enhanced pairing, i.e., a Fulde-Farrell-Larkin-Ovchnnikov superfluid state made of robust pairs with a finite center-of-mass momentum. Yet another regime of parameters offers the opportunity to study strongly interacting normal states of spin-orbit-coupled fermionic systems utilizing the resonance-like feature induced by the synthetic gauge field.

A two-axis in-plane motion measurement system based on optical beam deflection

Measurement of in-plane motion with high resolution and large bandwidth enables model-identification and real-time control of motion-stages. This paper presents an optical beam deflection based system for measurement of in-plane motion of both macro- and micro-scale motion stages. A curved reflector is integrated with the motion stage to achieve sensitivity to in-plane translational motion along two axes. Under optimal settings, the measurement system is shown to theoretically achieve sub-angstrom measurement resolution over a bandwidth in excess of 1 kHz and negligible cross-sensitivity to linear motion. Subsequently, the proposed technique is experimentally demonstrated by measuring the in-plane motion of a piezo flexure stage and a scanning probe microcantilever. For the former case, reflective spherical balls of different radii are employed to measure the in-plane motion and the measured sensitivities are shown to agree with theoretical values, on average, to within 8.3%. For the latter case, a prototype polydimethylsiloxane micro-reflector is integrated with the microcantilever. The measured in-plane motion of the microcantilever probe is used to identify nonlinearities and the transient dynamics of the piezo-stage upon which the probe is mounted. These are subsequently compensated by means of feedback control. (C) 2013 AIP Publishing LLC.

Influence of exhaust gas heating and L/D ratios on the discharge efficiencies for an activated carbon natural gas storage system

A transient 2D axi-symmetric and lumped parameter (LP) model with constant outflow conditions have been developed to study the discharge capacity of an activated carbon bed. The predicted discharge times and variations in bed pressure and temperature are in good agreement with experimental results obtained from a 1.82 l adsorbed natural gas (ANG) storage system. Under ambient air conditions, a maximum temperature drop of 29.5 K and 45.5 K are predicted at the bed center for discharge rates of 1.0 l min(-1) and 5.0 l min(-1) respectively. The corresponding discharge efficiencies are 77% and 71.5% respectively with discharge efficiencies improving with decreasing outflow rates. Increasing the LID ratio from 1.9 to 7.8 had only a marginal increase in the discharge efficiency. Forced convection (exhaust gas) heating had a significant effect on the discharge efficiency, leading to efficiencies as high as 92.8% at a discharge of 1.0 l min(-1) and 88.7% at 5 l min(-1). Our study shows that the LP model can be reliably used to obtain discharge times due to the uniform pressure distributions in the bed. Temperature predictions with the LP model were more accurate at ambient conditions and higher discharge rates, due to greater uniformity in bed temperatures. For the low thermal conductivity carbon porous beds, our study shows that exhaust gas heating can be used as an effective and convenient strategy to improve the discharge characteristics and performance of an ANG system. (C) 2013 Elsevier Ltd. All rights reserved.

Design and Modeling of an Active Five-Axis Compliant Micromanipulator

This paper presents the design and modeling of an active five-axis compliant micromanipulator whose tip orientation can be independently controlled by large angles about two axes and the tip-position can be controlled in three dimensions. These features enable precise control of the contact point of the tip and the tip-sample interaction forces with three-dimensional nanoscale objects, including those features that are conventionally inaccessible. Control of the tip-motion is realized by means of electromagnetic actuation combined with a novel kinematic and structural design of the micromanipulator, which, in addition, also ensures compatibility with existing high-resolution motion-measurement systems. The design and analysis of the manipulator structure and those of the actuation system are first presented. Quasi-static and dynamic lumped-parameter (LP) models are then derived for the five-axis compliant micromanipulator. Finite element (FE) analysis is employed to validate these models, which are subsequently used to study the effects of tip orientation on the mechanical characteristics of the five-axis micromanipulator. Finally, a prototype of the designed five-axis manipulator is fabricated by means of focused ion-beam milling (FIB).

Organic photovoltaics: key photophysical, device and design aspects

Key aspects of Organic Photovoltaics (OPVs) have been reviewed in this tutorial. Issues pertaining to the choice of materials, fabrication processes, photophysical mechanisms, device characterization, morphology of active layers and manufacturing are discussed. Special emphasis has been given to recent developments in large-area modules. Current strategies in enhancing the performance using external optical engineering approaches have also been highlighted. OPVs as a technology combine low weight, flexibility, low cost, good form factor and high-throughput processing; making them a promising PV technology for the future.

Discrete-Rate Adaptation and Selection in Energy Harvesting Wireless Systems

In a system with energy harvesting (EH) nodes, the design focus shifts from minimizing energy consumption by infrequently transmitting less information to making the best use of available energy to efficiently deliver data while adhering to the fundamental energy neutrality constraint. We address the problem of maximizing the throughput of a system consisting of rate-adaptive EH nodes that transmit to a destination. Unlike related literature, we focus on the practically important discrete-rate adaptation model. First, for a single EH node, we propose a discrete-rate adaptation rule and prove its optimality for a general class of stationary and ergodic EH and fading processes. We then study a general system with multiple EH nodes in which one is opportunistically selected to transmit. We first derive a novel and throughput-optimal joint selection and rate adaptation rule (TOJSRA) when the nodes are subject to a weaker average power constraint. We then propose a novel rule for a multi-EH node system that is based on TOJSRA, and we prove its optimality for stationary and ergodic EH and fading processes. We also model the various energy overheads of the EH nodes and characterize their effect on the adaptation policy and the system throughput.

Airblast spray in crossflow - Structure, trajectory and droplet sizing

This study reports results of an experimental investigation of airblast spray of water and ethanol in crossflow. Laser shadowgraphy and Particle/Droplet Imaging Analysis (PDIA) are used to derive spray trajectory and drop size information while Particle Tracking Velocimetry (PTV) is used to measure droplet velocities. A new phenomenon of spray bifurcation is observed for low Gas to Liquid Ratio (GLR) cases. The reasons for the spatial bifurcation can be attributed to a combination of reasons. These are (a) presence of large ligaments and droplets in the near-nozzle region for low GLRs (b) secondary breakup experienced by ligaments/droplets leading to formation of a large number of small droplets, and (c) the crossflow causing differential dispersion of the small and large droplets. A novel correlation for spray trajectory is proposed incorporating the momentum ratio and liquid surface tension. This correlation is shown to be effective in predicting the non-linear spray trajectory over a large range of conditions for not only water but ethanol and Jet-A also. It is observed that the larger droplets penetrate further into the crossflow, in the direction of injection. Thus, with increase in height of the measurement location from the injection plane, the droplet Sauter Mean Diameter (SMD) is found to increase. Moreover, as the droplets travel downstream in the crossflow direction, the droplet SMD is observed to decrease. The effect of drag is assessed by comparing velocity of different sizes of droplets at various locations. Smaller droplets are entrained into the crossflow at much lower elevations, whereas larger droplets tend to penetrate further into the crossflow. (C) 2015 Elsevier Ltd. All rights reserved.

Air microjet system for non-contact force application and the actuation of micro-structures

We demonstrate a non-contact technique to apply calibrated and localized forces in the micro-Newton to milli-Newton range using an air microjet. An electromagnetically actuated diaphragm controlled by a signal generator is used to generate the air microjet. With a nozzle diameter of 150 mu m, the microjet diameter was maintained to a maximum of 1 mm at a distance of 5 mm from the nozzle. The force generated by the microjet was measured using a commercial force sensor to determine the velocity profile of the jet. Axial flow velocities of up to 25 m s(-1) were obtained at distances as long as 6 mm. The microjet exerted a force up to 1 mu N on a poly dimethyl siloxane (PDMS) micropillar (50 mu m in diameter, 157 mu m in height) and 415 mu N on a PDMS membrane (3 mm in diameter, 28 mu m thick). We also demonstrate that from a distance of 6 mm our microjet can exert a peak pressure of 187 Pa with a total force of about 84 mu N on a flat surface with 8 V operating voltage. Out of the cleanroom fabrication and robust design make this system cost effective and durable.

Nonlinear Dynamics of a Rotating Flexible Link

This paper deals with the study of the nonlinear dynamics of a rotating flexible link modeled as a one dimensional beam, undergoing large deformation and with geometric nonlinearities. The partial differential equation of motion is discretized using a finite element approach to yield four nonlinear, nonautonomous and coupled ordinary differential equations (ODEs). The equations are nondimensionalized using two characteristic velocities-the speed of sound in the material and a velocity associated with the transverse bending vibration of the beam. The method of multiple scales is used to perform a detailed study of the system. A set of four autonomous equations of the first-order are derived considering primary resonances of the external excitation and one-to-one internal resonances between the natural frequencies of the equations. Numerical simulations show that for certain ranges of values of these characteristic velocities, the slow flow equations can exhibit chaotic motions. The numerical simulations and the results are related to a rotating wind turbine blade and the approach can be used for the study of the nonlinear dynamics of a single link flexible manipulator.