A. Di Febbraro

Urban traffic control structure based on hybrid Petri nets

An urban network of signalized intersections can be suitably modeled as a hybrid system, in which the vehicle flow behavior is described by means of a time-driven model and the traffic light dynamics are represented by a discrete event model. In this paper, a model of such a network via hybrid Petri nets is used to state and solve the problem of coordinating several traffic lights with the aim of improving the performance of some classes of special vehicles, i.e., public and emergency vehicles. The proposed model has been validated using real traffic data relevant to the city of Torino, Italy. Some relevant experimental results are reported and discussed.

Nonlinear optimization for freeway control using variable-speed signaling

A traffic control problem with a dynamic macroscopic model is considered by means of simulation. An optimal control problem is stated for variable-speed signaling in order to improve traffic behavior near congestion. A traffic state estimator based on the extended Kalman filter is designed to generate real-time estimates of the traffic density and, by means of these, to activate speed signaling. The variable-speed signaling control law is closed loop and is set by minimizing (or maximizing) a performance criterion. The optimization procedure is based on Powells method, and its off-line execution is computationally tractable on low-cost computers too. Simulation results demonstrate the efficacy of the proposed approach for preventing and reducing congestion.

On applying Petri nets to determine optimal offsets for coordinated traffic light timings

Significant classes of transportation systems can be represented by Petri net models. In this paper, Petri nets are applied to provide a modular representation of urban traffic systems regulated by signalized intersections. The basic idea is to consider such systems to be composed of elementary structural components, namely, intersections and road stretches. In order to describe thoroughly the movement of vehicles in the traffic network, a microscopic representation is adopted and realized via timed Petri nets. An interesting feature of the designed model consists in the possibility of representing the offsets among different traffic light cycles as embedded in the structure of the model itself. In turn, that allows us to simply implement the coordination among traffic light timings, which facilitates the description and solution of the relevant optimization problems.

Modular representation of urban traffic systems based on hybrid Petri nets

The problem of providing a modular representation of urban traffic and roadways by means of Petri nets is addressed. A modular modelling methodology is presented. After defining the urban traffic model and identifying each one of its components, it is possible to define for each element a hybrid Petri net module (HPNM), and provide an internal representation of it at a desired level of detail. A synthesis procedure allows one to join the single modules and provide the representation of the whole urban roadway considered in terms of hybrid Petri nets. An interesting characteristic proper to the proposed methodology is the complete independence of HPNMs from their internal representations. This allows one to change the model without any effort by increasing/decreasing the specified level of detail of some HPNMs making up the HPN-model of the whole system.

A petri net modelling approach of intermodal terminals based on Metrocargo system

To cope with the growing demand of moving freight, an innovative system for easily and quickly manage containers moved by trains, named Metrocargoreg, has been designed. The leading idea of such a system is to speed up the operations of loading/unloading containers on/from trains, with the goal of assimilating goods to passengers in transportation, thus increasing the flexibility of the whole system and, possibly, optimising the space allocation inside each terminal. In this framework, whereas the performances of such a system appear to be promising, no models to evaluate them have been proposed yet. Then, to achieve this task, in the paper a model of Metrocargoreg intermodal terminals based on timed Petri nets is presented, which allows to easily define a modular, performance-evaluation oriented representation of the considered system

On representing signalized urban areas by means of deterministic-timed Petri nets

Signalized urban areas are microscopically represented by means of deterministic-timed Petri nets (DTPNs), with the purpose of providing a suitable modelling tool for traffic management and signalling control. The model described in this paper consists of signalized intersections and roads. In order to properly model traffic congestion, intersections are divided into several crossing sections. An intersection always includes a multi-stage traffic signal, whose stage matrix (i.e., the sequence of signal stages) is a-priori known. The DTPN representing the traffic model, intended to traffic management and signalling control, is proposed and fully described in the paper.

Performance optimization in manufacturing systems by use of max-plus algebraic techniques

A class of manufacturing systems that can be modelled by use of timed event graphs is considered in this paper. The interest for considering such a subclass of timed Petri nets is due to the possibility of applying a set of mathematical results, mainly based on the use of max-plus algebra, for performance analysis. Basing on these results, some significant performance indexes can be computed as functions of a certain set of decision parameters. Then, the system throughput optimization problem can be stated as a linear fractional programming problem.<<ETX>>

Traffic-responsive signalling control through a modular/switching model represented via DTPN

Urban signalized traffic areas are considered in this paper, with the aim of minimizing congestion situations via a traffic-responsive signalling control procedure founded on a hierarchical Petri net (PN) representation of the system. The higher level of the PN representation consists of net modules, each one representing an intersection, a road, a signal staging, etc.; the description of each module in terms of deterministic-timed Petri nets (DTPN) is given at the lower level. Such a representation leads to a corresponding two-level control procedure. The high-level control system, which acts over the modular representation, switches among internal module structures so as to modify some parts of the model of the traffic system (e.g., signal plans, turning rates, etc.), depending on both state and time. The low-level control system, which acts over the DTPN representation, optimizes the performances of the traffic system, by solving a mathematical programming problem which minimizes the number of vehicles in the system. In the paper, the adopted model of the signalized urban area is briefly presented, and the two-level representation and the control system are described in details

Optimal control laws for lot-sizing and timing of jobs on a single production facility

The optimal control of a single machine processing a certain number of jobs and modeled as a discrete-event dynamic system is considered. The number of jobs and their sequence are fixed, whereas their timing and sizes represent the control variables of the system. The objective function to be optimized is a weighted sum of the quadratic earliness and tardiness of each job, and of the quadratic deviations of job lot sizes and actual machine service speeds from those specified by the production demand and by the regular machine speeds. An optimization problem with quadratic cost function and nonlinear constraints is stated and formalized as a multistage optimal control problem. Necessary conditions to be satisfied by an optimal control sequence are derived. A simpler model is also considered in which the machine speeds are fixed; in this case, the control problem is solved by a procedure making use of dynamic programming techniques. The optimal control laws at each stage are thus obtained.

Fault diagnosis in an intermodal container terminal

Petri nets are used to define a hierarchical monitoring structure intended for real-time fault diagnosis and recovery in an intermodal terminal where freight only arrives and departs stored into containers. An intermodal container terminal can be modelled as a discrete event system prone to unforeseeable events resulting in deviations (delays) from the a-priori planned schedule. Whenever a failure, i.e., a delay in some operation, is detected, a diagnosis procedure is triggered accordingly. A real case study relevant to a terminal in the port of Genova, Italy, is introduced to show a practical application of the proposed model.