Cluster tools are the key equipment in semiconductor manufacturing. For high-end chip manufacturing processes, to ensure the wafer quality, periodical cleaning operations for process chambers in such ...tools are required for eliminating the residual gas and chemicals. In operating a tool, the wafer processing time in a process chamber may vary within a range. It is meaningful to predict the cycle time of cluster tools with chamber cleaning operations and processing time variation. By doing so, automation material handling system can be told when a lot is completed by a cluster tool such that it can assign an overhead hoist transporter to transport this lot at the right time. This plays an important role in improving the productivity of a whole semiconductor manufacturing system. This work conducts the cycle time analysis of cluster tools with chamber cleaning operations and processing time variation. Specifically, it proposes a novel method to calculate the average cycle time of a single-arm cluster tool under which an optimal schedule can be obtained. Then, for a dual-arm cluster tool, an efficient algorithm is developed to approximate the average system cycle time under which an optimal schedule can be obtained. Experiments show that the gap between the average system cycle time obtained by simulations and the average system cycle time obtained by the proposed method is no more than 0.1% which demonstrate the effectiveness of the proposed method.
This paper develops a local projection estimator for estimating impulse responses in the presence of time variation. Importantly, we allow local instabilities in both slope coefficients and ...variances. Monte Carlo simulations illustrate that the method performs well in practice. Using our proposed estimator, we shed new light on the effects of fiscal policy shocks and the size of government spending multipliers. Our analysis uncovers the existence of instabilities that were unaccounted for in previous studies, and links time variation in the multipliers to the size of government debt.
•A numerical method of grouting considering rheological characteristics is proposed.•A set of visual parallel plate crack polymer grouting test device is established.•The time-varying viscosity of ...slurry is the cause of pressure lag effect.
As a new type of chemical grouting material, polymers have been widely used in fractured rock masses, but the understanding of the diffusion characteristics of polymers still needs to be further improved. Due to the expansion of polymer slurry, the change of grouting pressure is not synchronized with the time change of diffusion radius, leading to the diffusion law of polymer slurry more complicated. Therefore, based on the rheological properties of the slurry and the theory of viscous hydrodynamics, a polymer fracture grouting diffusion model was established. Through the finite element level set method and adjusting the liquid level according to the fluid velocity, it can be obtained the diffusion interface of the fluid at the corresponding point in time and the change law of the pressure at the corresponding diffusion interface with time. Finally, the validity of the model is verified by the polymer slurry parallel plate grouting diffusion experiment. The research results indicated that: (1) The diffusion mode of the polymer fracture grouting diffusion model is basically consistent with the test results. (2) The grouting pressure gradually decreases along the direction of the diffusion radius, increases with the decrease of the crack width, and increases with the time, but the change of the grouting pressure lags behind the change of the diffusion radius. (3) The results are consistent with the pressure field curve measured by the experiment, indicating that the main factor causing the hysteresis effect of the pressure field change is the time-varying viscosity of the polymer slurry.
We identify the relative importance of changes in the conditional variance of fundamentals (which we call “uncertainty”) and changes in risk aversion in the determination of the term structure, ...equity prices, and risk premiums. Theoretically, we introduce persistent time-varying uncertainty about the fundamentals in an external habit model. The model matches the dynamics of dividend and consumption growth, including their volatility dynamics and many salient asset market phenomena. While the variation in price–dividend ratios and the equity risk premium is primarily driven by risk aversion, uncertainty plays a large role in the term structure and is the driver of countercyclical volatility of asset returns.
We address a robotic flow shop scheduling problem where two part types are processed on each given set of dedicated machines. A single robot moving on a fixed rail transports one part at a time, and ...the processing times of the parts vary on the machines within a given time interval. We use a reinforcement learning (RL) approach to obtain efficient robot task sequences to minimise makespan. We model the problem with a Petri net used for a RLenvironment and develop a lower bound for the makespan. We then define states, actions, and rewards based on the Petri net model; further, we show that the RL approach works better than the first-in-first-out (FIFO) rule and the reverse sequence (RS), which is extensively used for cyclic scheduling of a robotic flow shop; moreover, the gap between the makespan from the proposed algorithm and a lower bound is not large; finally, the makespan from the RL method is compared to an optimal solution in a relaxed problem. This research shows the applicability of RL for the scheduling of robotic flow shops and its efficiency by comparing it to FIFO, RS and a lower bound. This work can be easily extended to several other variants of robotic flow shop scheduling problems.
The population dynamics of forage fish are often ‘boom or bust’, and variation in recruitment may be a contributing factor to changes in abundance. Here we applied several methods for identifying ...stock recruit relationships (SRR) to 52 forage fish stocks: a time-invariant Ricker model and two time-varying methods (dynamic linear models and regime-based models). A positive relationship between spawning biomass and recruitment existed for 28 stocks, with 14 of those being best fit by a Ricker curve. Time-varying models were preferred for 26 stocks over a time-invariant Ricker curve. We also showed through simulation that spurious, yet significant stock-recruit relationships can be generated if recruitment was environmentally driven, the lag between recruitment and spawning biomass was small, and autocorrelation in recruitment was high—a combination of characteristics that many forage fish stocks display. Based on our results, observed time-invariant SRRs can result from a reversed causality of ‘boom and bust’ dynamics in situations in which life histories are short (i.e., changes in recruitment are driving changes in spawning biomass). ‘Hidden’ SRRs are also possible because of time-variation in SRR parameters. Understanding when each of these potential failings of fitting SRRs can occur is a key challenge in forage fish management.
Using daily data over the period August 5, 2013 - September 27, 2019, this study investigates the dynamic spillovers between international monetary policies across four major economies (i.e. ...Eurozone, Japan, UK and US) and three key cryptocurrencies (i.e. Bitcoin, Litecoin and Ripple). In doing so, we apply a Time-Varying Parameter Vector Auto-Regression (TVP-VAR) model, a dynamic connectedness approach and network analysis. The empirical results indicate that cryptocurrency returns and monetary policy spillovers were particularly large when shadow policy rates became negative, moderated during the Fed's 'tapering process', and sharpened again more recently as cryptocurrency buoyancy returned. Gross directional spillovers suggest that shadow policy rates have more 'to give than to receive', while those from and to cryptocurrency returns are naturally volatile. There is also strong interconnectedness between monetary policy in either the US or the Eurozone and the UK, and between Bitcoin and Litecoin. However, the spillovers across monetary policy and cryptocurrencies tend to be muted. Finally, spillovers were only slightly larger during the Fed's 'unconventional' policy compared to the 'standard' era, but their composition qualitatively changed over time.
Abstract
We present a new framework to study the time variation of fundamental constants in a model-independent way. Model independence implies more free parameters than assumed in previous studies. ...Using data from atomic clocks based on
87
Sr,
171
Yb
+
and
133
Cs, we set bounds on parameters controlling the variation of the fine-structure constant,
α
, and the electron-to-proton mass ratio,
µ
. We consider variations on timescales ranging from a minute to almost a day. In addition, we use our results to derive some of the tightest limits to date on the parameter space of models of ultralight dark matter and axion-like particles.
El golfo de Batabanó (GB), es la región de la plataforma marina cubana de mayor extensión e importancia pesquera. Afectaciones al hábitat de especies marinas han sido señaladas como consecuencia de ...factores naturales y antrópicos. El objetivo fue determinar variaciones espacio-temporales significativas en la hidrología del GB en diferentes periodos y situaciones ambientales. Para ello se analizó la variabilidad de la lluvia y la temperatura del aire en 1975-1995 y 1996-2015 periodos de baja y alta actividad ciclónica, las características termohalinas del GB anterior (1959-1970) y posterior (2006-2016) al represamiento fluvial, y el efecto de dos huracanes casi simultaneaos en 2008 (“Gustav” e “Ike”). Diferencias significativas en la variabilidad interanual de parámetros meteorológicos entre el norte y el sur del GB respondieron a características locales, y ocurrieron cambios en la distribución de los meses de máximas precipitaciones. El represamiento fluvial, sumado a déficits de precipitación en primavera, verano e invierno, provocaron aumentos termohalinos en 2006-2016, con incremento de 0,7 °C en la temperatura del agua y de 2,3 en la salinidad promedio del GB, respecto a 1959-1970. La salinidad mostró una ampliación en la distribución de valores hipersalinos, y media > 36 en meses de altas precipitaciones, con promedio general de 35,4 en 1959-1970 y 37,7 en 2006-2016, esto sugiere que el represamiento ha sido un factor principal en las variaciones de salinidad. El impacto termohalino en el GB de dos huracanes en 2008, se restableció en corto plazo, con mayor inercia en la turbidez.
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