Papers by Apostolos Kotsialos
Swarm Intelligence Algorithms for Macroscopic Traffic Flow Model Validation with Automatic Assignment of Fundamental Diagrams
Applied Soft Computing, 2015
Computational Management Science, 2010
This paper is concerned with a fluid approach towards modelling of production networks. It focuse... more This paper is concerned with a fluid approach towards modelling of production networks. It focuses on developing a macroscopic modelling framework that is able to describe the average dynamics of the manufactured items' flow in a production network taking under consideration possible non-linearities that occur due to the operation and interaction of workcells. The notion of clearing functions is used in a similar manner as the fundamental diagram of traffic engineering is used for modelling vehicular traffic flow. The system dynamics are represented by the aggregate variables of items' density, flow, speed and queue length, which are associated 2 Apostolos Kotsialos through the use of a conservation law. These aggregate variables are further differentiated with respect to the customer order they belong to, providing this way a more detailed description of system dynamics. The suggested approach is a computationally efficient scheme that can be used for modelling and control purposes in a manner similar to macroscopic freeway traffic flow models.
Control of Set-Up Dominant Multivariate Manufacturing Processes
A practical control chart is introduce, called multivariate Set-Up Process Algorithm (m-SUPA), wh... more A practical control chart is introduce, called multivariate Set-Up Process Algorithm (m-SUPA), which can be used to signal when a process is statistically off-target. This control chart uses a traffic light system to provide simple information to an operator about how close a measured part is to its global target. The chart works with a simple rule set resulting in process adjustments at a calculated point, rather than relying on rule-of-thumb methods. A final consideration is calculating the size of process adjustment, when one control adjustment has multiple effects on different design features. Simple feedback controllers are suggested for calculating process adjustments, providing consistency to an action taken. Simulation results suggest that m-SUPA with adjustments based on this kind of controllers is able to steer the process to a desired performance region.
Macroscopic Modelling of Long Manufacturing Lines
Non-smooth optimization based on resilient backpropagation search for unconstrained and simply bounded problems
Optimization Methods and Software, 2013
ABSTRACT This paper describes a non-smooth optimization method based on a backpropagation search ... more ABSTRACT This paper describes a non-smooth optimization method based on a backpropagation search method. More specifically, the Resilient backPROPagation (RPROP) algorithm, used extensively for neural network training, is employed here. RPROP is recast in terms of numerical optimization and used as a step-finding method. The result is the fast search algorithm RPROP, which avoids expensive line searches and performs one function and one gradient evaluation per iteration. Furthermore, only the gradient's sign is used rather than its value. RPROP is applied to a set of 10 test problems, which have unconstrained and simply bounded constrained versions. The results are discussed and assessed against a set of reported results based on bundle methods. It is shown that RPROP is able to deal efficiently with very large non-smooth problems.
Nonlinear optimisation using directional step lengths based on RPROP
Optimization Letters, 2014
Computational Management Science, 2010
This paper is concerned with a fluid approach towards modelling of production networks. It focuse... more This paper is concerned with a fluid approach towards modelling of production networks. It focuses on developing a macroscopic modelling framework that is able to describe the average dynamics of the manufactured items' flow in a production network taking under consideration possible non-linearities that occur due to the operation and interaction of workcells. The notion of clearing functions is used in a similar manner as the fundamental diagram of traffic engineering is used for modelling vehicular traffic flow. The system dynamics are represented by the aggregate variables of items' density, flow, speed and queue length, which are associated 2 Apostolos Kotsialos through the use of a conservation law. These aggregate variables are further differentiated with respect to the customer order they belong to, providing this way a more detailed description of system dynamics. The suggested approach is a computationally efficient scheme that can be used for modelling and control purposes in a manner similar to macroscopic freeway traffic flow models.
Modelling of the large scale motorway network around Amsterdam
Set-up Dominated Low Volume Process Capability Estimation Using a Pre-Control Method
This paper seeks to highlight often overlooked techniques of process control, Pre-Control, Pairs ... more This paper seeks to highlight often overlooked techniques of process control, Pre-Control, Pairs and Acceptance Control Charts, which are becoming increasingly relevant in the context decreasingly small production runs and an emphasis on high precision and high value product. A methodology is built on these techniques, known as Set-up Process Algorithm (SUPA), to assist operators make adjustments to Set-up dominant processes in a statistically valid framework. The performance of these methods is then assessed analytically and through simulation to demonstrate their respective sensitivity at detecting process deviations from the specified target.
Journal of Applied Physics, 2015
We report on the use of a liquid crystalline host medium to align single-walled carbon nanotubes ... more We report on the use of a liquid crystalline host medium to align single-walled carbon nanotubes in an electric field using an in-plane electrode configuration. Electron microscopy reveals that the nanotubes orient in the field with a resulting increase in the DC conductivity in the field direction. Current versus voltage measurements on the composite show a nonlinear behavior, which was modelled by using single-carrier space-charge injection. The possibility of manipulating the conductivity pathways in the same sample by applying the electrical field in different (in-plane) directions has also been demonstrated. Raman spectroscopy indicates that there is an interaction between the nanotubes and the host liquid crystal molecules that goes beyond that of simple physical mixing. V C 2015 AIP Publishing LLC. [http://dx.
Systems and Control: Foundations & Applications, 2005
Recurrent and non-recurrent congestions on freeways may be substantially reduced if today's "spon... more Recurrent and non-recurrent congestions on freeways may be substantially reduced if today's "spontaneous" infrastructure utilisation is replaced by an orderly, controllable operation via comprehensive application of ramp metering and freeway-to-freeway control, combined with powerful optimal control techniques. This paper first explains why ramp metering can lead to a dramatic amelioration of traffic conditions on freeways. Subsequently, a large-scale example demonstrates the high potential of advanced ramp metering approaches. It is demonstrated that the proposed control scheme is efficient, fair and real-time feasible.
Discrete release rate impact on ramp metering performance
IEE Proceedings - Intelligent Transport Systems, 2006
ABSTRACT
Transportation Research Part C: Emerging Technologies, 2010
A nonlinear model-predictive hierarchical control approach is presented for coordinated ramp mete... more A nonlinear model-predictive hierarchical control approach is presented for coordinated ramp metering of freeway networks. The utilized hierarchical structure consists of three layers: the estimation/prediction layer, the optimization layer and the direct control layer. The previously designed optimal control tool AMOC (Advanced Motorway Optimal Control) is incorporated in the second layer while the local feedback control strategy ALINEA is used in the third layer. Simulation results are presented for the Amsterdam ring-road. The proposed approach outperforms uncoordinated local ramp metering and its efficiency approaches the one obtained by an optimal open-loop solution. It is demonstrated that metering of all on-ramps, including freeway-to-freeway intersections, with sufficient ramp storage space leads to the optimal utilization of the available infrastructure.
Integrated optimal control of motorway traffic networks
Proceedings of the 1999 American Control Conference (Cat. No. 99CH36251), 1999
ABSTRACT
Proceedings of the 2005, American Control Conference, 2005., 2005
A nonlinear model-predictive control approach to the problem of coordinated ramp metering is pres... more A nonlinear model-predictive control approach to the problem of coordinated ramp metering is presented. The previously designed optimal control tool AMOC is used within the framework of a hierarchical control structure which consists of three control layers: the estimation/prediction layer, the optimization layer and the direct control layer. More emphasis is given to the last two layers where the control actions on a network-wide and on a local level, respectively, are decided. The hierarchical control strategy combines AMOC's coordinated ramp metering control with local feedback (ALINEA) control in an efficient way. Simulation investigations for the Amsterdam ring-road are reported whereby the results are compared with those obtained by applying ALINEA as a stand-alone strategy. It is demonstrated that the proposed control scheme is efficient, fair and real-time feasible.
Efficiency versus fairness in network-wide ramp metering
ITSC 2001. 2001 IEEE Intelligent Transportation Systems. Proceedings (Cat. No.01TH8585), 2001
ABSTRACT
11th IFAC Symposium on Control in Transportation Systems, 2006, 2006
Optimal Coordinated Ramp Metering with Advanced Motorway Optimal Control
Transportation Research Record, 2001
ABSTRACT
Procedia CIRP, 2013
The main focus of this paper is to use discrete-event simulation models, to test the robustness o... more The main focus of this paper is to use discrete-event simulation models, to test the robustness of two process control methods against processes with different statistical distributions. The two methods under scrutiny are the Small-Batch X & R chart and the Set-Up Process Algorithm (SUPA). These have been developed for 'setup dominant processes', were the major source of product variation is detected between batches. Minimizing this type of variation is critical to ensure spare parts produced at a later date will fit in operating assemblies, maintaining a Through-life Engineering Service. This paper shows their suitability to industry.
Proceedings of the 16th IFAC World Congress, 2005, 2005
The problem of medium to long term sales forecasting raises a number of requirements that must be... more The problem of medium to long term sales forecasting raises a number of requirements that must be suitably addressed in the design of the employed forecasting methods. These include long forecasting horizons (up to 52 periods ahead), a high number of quantities to be forecasted, which limits the possibility of human intervention, as well as frequent introduction of new articles (for which no past sales are available for parameter calibration) and withdrawal of running articles. The problem has been tackled by use of a modified Holt-Winters method as well as Feedforward Multilayer Neural Networks (FMNN) applied to sales data from two German companies.
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Papers by Apostolos Kotsialos