| 1. |
- Rodriguez-Deniz, Hector, et al.
(författare)
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Urban Network Travel Time Prediction via Online Multi-Output Gaussian Process Regression
- 2017
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Ingår i: 2017 IEEE 20TH INTERNATIONAL CONFERENCE ON INTELLIGENT TRANSPORTATION SYSTEMS (ITSC). - : IEEE. - 9781538615263
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Konferensbidrag (refereegranskat)abstract
- The paper explores the potential of Multi-Output Gaussian Processes to tackle network-wide travel time prediction in an urban area. Forecasting in this context is challenging due to the complexity of the traffic network, noisy data and unexpected events. We build on recent methods to develop an online model that can be trained in seconds by relying on prior network dependences through a coregionalized covariance. The accuracy of the proposed model outperforms historical means and other simpler methods on a network of 47 streets in Stockholm, by using probe data from GPS-equipped taxis. Results show how traffic speeds are dependent on the historical correlations, and how prediction accuracy can be improved by relying on prior information while using a very limited amount of current-day observations, which allows for the development of models with low estimation times and high responsiveness.
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| 2. |
- Torres, Hector, et al.
(författare)
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Self-lubricating laser claddings for friction control during press hardening of Al-Si-coated boron steel
- 2019
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Ingår i: Journal of Materials Processing Technology. - : Elsevier. - 0924-0136 .- 1873-4774. ; 269, s. 79-90
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Tidskriftsartikel (refereegranskat)abstract
- In recent years, the use of Al/Si coatings has become widespread in hot stamping in order to protect the work piece from detrimental mechanisms such as scale formation or decarburisation affecting the quality of the finished product. However, the formation of Al-Fe intermetallics due to diffusion at high temperature can lead to unstable friction and damage both the tool and the work piece.In the present study, self-lubricating coatings with the addition of silver and MoS2 have been prepared by means of laser cladding deposition, aiming at their use in hot stamping in order to decrease friction and wear. The coatings were evaluated at high temperatures against Al-Si-treated boron steel using two different testing configurations featuring open and closed tribosystems. A significant reduction in friction for the self-lubricating claddings were observed along with decreased material transfer. This could be beneficial for hot stamping applications as it can ensure the stability of the process while preventing surface damage to the work piece. Additionally, closed configuration tribotesting has been found to underestimate friction and wear of the tool/work piece system, thus making it less suited for the lab-scale simulation of hot metal forming compared to open configuration tribometers. This finding has been considered relevant as many references in the available literature still report the use of closed configuration tribometers.
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| 3. |
- Young, Hector, et al.
(författare)
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Model-free Predictive Torque Control of an Induction Machine Based on Parameter Estimation
- 2021
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Ingår i: 6Th IEEE International Conference On Predictive Control Of Electrical Drives And Power Electronics (PRECEDE 2021). - : Institute of Electrical and Electronics Engineers (IEEE). ; , s. 725-731
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Konferensbidrag (refereegranskat)abstract
- The uncertainty or variation of the electric machine parameters in predictive torque control (PTC) has a noticeable impact on the controller's performance. This paper proposes a model-free PTC strategy based on the estimation of the prediction model parameters using input and output data of the controlled system, applied to an induction machine. This approach has the advantage of not requiring a detailed previous knowledge of the system, with a high robustness to mismatch in the inductance parameters of the machine. The stator resistance is identified as a critical parameter for PTC, therefore an adaptation mechanism based on support vector regression is proposed to increase the robustness of the system. Simulation tests are carried out to validate the effectiveness of the proposed strategy.
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| 4. |
- Rodriguez-Deniz, Hector, et al.
(författare)
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Robust Real-Time Delay Predictions in a Network of High-Frequency Urban Buses
- 2022
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Ingår i: IEEE transactions on intelligent transportation systems (Print). - 1524-9050 .- 1558-0016. ; 23:9, s. 16304-16317
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Tidskriftsartikel (refereegranskat)abstract
- Providing transport users and operators with accurate forecasts on travel times is challenging due to a highly stochastic traffic environment. Public transport users are particularly sensitive to unexpected waiting times, which negatively affect their perception on the system's reliability. In this paper we develop a robust model for real-time bus travel time prediction that departs from Gaussian assumptions by using Student-t errors. The proposed approach uses spatiotemporal characteristics from the route and previous bus trips to model short-term effects, and date/time variables and Gaussian processes for long-run forecasts. The model allows for flexible modeling of mean, variance and kurtosis spaces. We propose algorithms for Bayesian inference and for computing probabilistic forecast distributions. Experiments are performed using data from high-frequency buses in Stockholm, Sweden. Results show that Student-t models outperform Gaussian ones in terms of log-posterior predictive power to forecast bus delays at specific stops, which reveals the importance of accounting for predictive uncertainty in model selection. Estimated Student-t regressions capture typical temporal variability between within-day hours and different weekdays. Strong spatiotemporal effects are detected for incoming buses from immediately previous stops, which is in line with many recently developed models. We finally show how Bayesian inference naturally allows for predictive uncertainty quantification, e.g. by returning the predictive probability that the delay of an incoming bus exceeds a given threshold.
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| 5. |
- Ruiz, Héctor A., et al.
(författare)
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Severity factor kinetic model as a strategic parameter of hydrothermal processing (steam explosion and liquid hot water) for biomass fractionation under biorefinery concept
- 2021
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Ingår i: Bioresource Technology. - : Elsevier BV. - 0960-8524. ; 342
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Forskningsöversikt (refereegranskat)abstract
- Hydrothermal processes are an attractive clean technology and cost-effective engineering platform for biorefineries based in the conversion of biomass to biofuels and high-value bioproducts under the basis of sustainability and circular bioeconomy. The deep and detailed knowledge of the structural changes by the severity of biomasses hydrothermal fractionation is scientifically and technological needed in order to improve processes effectiveness, reactors designs, and industrial application of the multi-scale target compounds obtained by steam explosion and liquid hot water systems. The concept of the severity factor [log10 (Ro)] established>30 years ago, continues to be a useful index that can provide a simple descriptor of the relationship between the operational conditions for biomass fractionation in second generation of biorefineries. This review develops a deep explanation of the hydrothermal severity factor based in lignocellulosic biomass fractionation with emphasis in research advances, pretreatment operations and the applications of severity factor kinetic model.
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| 6. |
- Suau-Sanchez, Pere, et al.
(författare)
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Regulatory airport classification in the US: The role of international markets
- 2015
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Ingår i: Transport Policy. - : Elsevier. - 0967-070X .- 1879-310X. ; 37, s. 157-166
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Tidskriftsartikel (refereegranskat)abstract
- In a context of debate over the future of the US Federal Aviation Administrations (FAA) funding model, this paper revisits the current system of airport classification used for the allocation of public funding for capacity developments. Previous papers have already addressed the limitations of the FAAs uni-dimensional method, and proposed new approaches that take into account the two dimensions of "hubbing" activity, i.e., traffic generation and connectivity. However, these studies are biased by the lack of detailed demand data on international connections. Using an MIDT dataset comprising a sample of domestic and international markets served by US airports during the first quarter of 2013, this paper aims at providing a full picture on the pitfalls of the existing FAA method, as well as addressing the impact of international connectivity in characterising the airports hubbing profiles. Hierarchical clustering is used to provide alternative criteria for hub classification within the context of US National Plan of Integrated Airport Systems (NPIAS). This new typology of primary US airports can help to optimize AIP funding by allowing for further differentiation in the FAA allocation criteria.
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| 7. |
- Tovar, Beatriz, et al.
(författare)
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Classifying Ports for Efficiency Benchmarking: A Review and a Frontier-based Clustering Approach
- 2015
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Ingår i: Transport reviews. - : Taylor and Francis (Routledge): SSH Titles. - 0144-1647 .- 1464-5327. ; 35:3, s. 378-400
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Tidskriftsartikel (refereegranskat)abstract
- Port efficiency and port clustering are two aspects that have received different degrees of attention in the existing literature. While the actual estimation of port efficiency has been extensively studied, the existing literature has paid little attention to developing robust methodologies for port classification. In this paper, we review the literature on classification methods for port efficiency, and present an approach that combines stochastic frontier analysis, clustering and self-organized maps (SOM). Cluster methodologies that build on the estimated cost function parameters could group ports into performance metrics categories. This helps when setting improvement targets for ports as a function of their specific cluster. The methodology is applied to a database of Spanish port authorities. The dendrogram features three clusters and five outlier Spanish Port Authorities. SOM are employed to track the temporal evolution of Spanish Port Authorities that are of special interest for some reasons (i.e. outliers). Results show that use of a combination of cost frontier and cluster methods to define robust port typology and SOMs, jointly or in isolation, offers useful information to the decision-makers.
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| 8. |
- Tovar, Beatriz, et al.
(författare)
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Container port competitiveness and connectivity: The Canary Islands main ports Case
- 2015
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Ingår i: Transport Policy. - : Elsevier. - 0967-070X .- 1879-310X. ; 38, s. 40-51
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Tidskriftsartikel (refereegranskat)abstract
- The Canary Islands economy is extremely dependent on sea transport. Since accessibility and connectivity are major determinants of international transport costs, the analysis of their main ports connectivity is crucial for keeping costs under control. Since different port authorities manage the major ports of the Canary Islands, they could be tempted to compete for transshipment cargoes, instead of working together to facilitate supply chain integration that would increase their competitive standing. The aim of the paper is twofold. First, the infrastructure and superstructure endowment of the main Canarian ports and their accessibility, by evaluating site and situation factors, is documented. Secondly, the connectivity of the main Canarian ports is assessed by means of graph theory. This provides important measures that define a ports competitiveness, and its potential to achieve or keep regional or global hub status, and also to follow its evolution. A brief review of papers measuring port connectivity based on graph theory is included to illustrate the current approaches in port network analysis, and to justify our methodological framework. A sub-network of 53 ports directly related with Las Palmas and Tenerife ports has been selected for this purpose. Our findings are mainly related to the connections among the nodes in the sample network, and to the position that the targeted ports hold. Additionally, some policy recommendations, regarding how to improve the connectivity and competitiveness of the Canarian ports, are also enumerated. Previous analysis indicates that, at present, the Las Palmas port is the only regional hub in the Canaries. Both Canarian port authorities should differentiate themselves by specializing in certain valued added services and increasing traffic in these services. This would avoid the danger of a destructive competition between them to attract transit traffic. In summary they should be proactive in maintaining and improving the main Canarian ports connectivity.
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| 9. |
- Heydari, Rasool, et al.
(författare)
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Model-Free Predictive Control of Grid-Forming Inverters With LCL Filters
- 2022
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Ingår i: IEEE transactions on power electronics. - : Institute of Electrical and Electronics Engineers (IEEE). - 0885-8993 .- 1941-0107. ; 37:8, s. 9200-9211
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Tidskriftsartikel (refereegranskat)abstract
- With the increasing penetration of distributed energy resources into islanded microgrids, a grid-forming inverter (GFI) has become the key element interfacing renewable energy sources. Usually, the GFI is employed with an output filter to minimize the harmonic content achieving high-quality output voltage regulation. To this end, model-predictive control (MPC) has been widely proposed to control the output voltage of GFI systems due to the high-quality performance, fast control response, and straightforward handling of constraints. However, an accurate model of the system is required for the conventional MPC to avoid a suboptimal performance under uncertainties. To overcome this known drawback, a novel model-free predictive control is proposed in this article. Consequently, the output voltage of an LC L-filtered GFI is regulated without the knowledge of the physical model.
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| 10. |
- Martín, Juan Carlos, et al.
(författare)
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Determinants of airport cost flexibility in a context of economic recession
- 2013
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Ingår i: Transportation Research Part E. - : Elsevier. - 1366-5545 .- 1878-5794. ; 57, s. 70-84
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Tidskriftsartikel (refereegranskat)abstract
- The recent economic downturn led to a significant contraction in the global demand for air travel and cargo. In spite of that, airports’ operating costs did not mirror the traffic trends and kept increasing during the same period, showing evident signs of lack of flexibility. With this background, this paper aims at identifying the drivers of airport cost flexibility in a context of economic recession. This is done by estimating a short-run stochastic cost frontier over a balanced pool database of 194 airports worldwide between 2007 and 2009. Using the total change in cost efficiency during the sample period as a proxy for cost flexibility, the impact of variables such as ownership, outsourcing, airline dominance, low-cost traffic, and revenue diversification is tested in a second-stage regression. Contrary to the existing literature, a higher level of outsourcing is shown to reduce cost flexibility. Results also indicate that low-cost traffic, diversification, and corporatization increase the airports’ ability to control costs. The negative impact of airline dominance suggests the need for more stringent regulations on slot allocation at congested airports in order to ensure optimal infrastructure usage.
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