Jolanda Coppola

A Hybrid Model of Complex Automated Warehouse Systems—Part I: Modeling and Simulation

An automated warehouse system has two main components: an automated storage and retrieval subsystem consisting of a number of aisles, each one served by a crane, and a picking area which is formed by bays where stock units coming from the aisles are partially emptied by human operators. These two components are connected via an interface area consisting of carousels, conveyors, and buffers. This area is usually modeled as a discrete event system, while the overall system performance depends also on continuous time phenomena. Part I presents a hybrid model based on a new Petri net formalism that merges the concepts of Hybrid Petri Nets and Colored Petri Nets to obtain modular and compact models for these systems. An example is discussed in detail to motivate the introduction of a new formalism. A control oriented simulation tool is also presented. Part II will focus on the application of this formalism to automated warehouse systems analysis and performance evaluation. Finally, a real case study is considered to show the effectiveness of the approach.

A Hybrid Model of Complex Automated Warehouse Systems—Part II: Analysis and Experimental Results

An automated warehouse system has two main components: an automated storage and retrieval subsystem consisting of a number of aisles, each one served by a crane, and a picking area which is formed by bays where stock units coming from the aisles are partially emptied by human operators. These two components are connected via an interface area consisting of carousels, conveyors and buffers. This area is usually modeled as a discrete event system, while the overall system performance depends also on continuous time phenomena. In Part I, a hybrid modeling approach based on a new Petri net formalism and a freeware simulation tool have been presented. The concepts of Hybrid Petri Nets and Colored Petri Nets are merged to obtain modular and compact models for automated warehouse systems. Part II now focuses on the application of this formalism to automated warehouse systems analysis and performance evaluation. Liveness analysis is performed by means of a hybrid automaton obtained from the net model. A deadlock prevention policy is synthesized working on an aggregated model. Finally, a real case study is considered to show the effectiveness of the approach.

Identification of Time Petri Net Models

This paper deals with the identification of time Petri net systems. An identification algorithm for timed net systems must take into account that the firing of a transition requires not only that the enabling condition is met, as in untimed net systems, but it is also required that the firing interval of a transition is congruent with the observed firing instant times. The key idea behind the approach is to express these conditions by a set of logical propositions that can be directly transformed into linear mixed-integer inequalities. The identification algorithm consists of building the logical propositions from the observed behavior and solving a mixed-integer linear programming problem.

A Novel Model Repair Approach of Timed Discrete-Event Systems With Anomalies

In this paper, the model repair of timed discrete-event systems where anomalies may occur is considered. The nominal model is assumed to be known, and a set of observed timed sequences is given. The approach works with time Petri net models and is based on the formulation of a mixed-integer linear programming problem. The repaired model is obtained from the nominal one by adding fault transitions as well as by extending the firing interval of transitions. Note to Practitioners-The data collected from the observation of a discrete-event system are usually given in terms of behavioral sequences that may be fixed or may be increased in the course of the system operation (e.g., due to new experiments or simply to the system running). If this occurs, the nominal model of a system may reveal not consistent with these additional observations if they include anomalies. This paper presents an approach to repair the model of the system in order to make it able to also generate the observed timed faulty behavior. The repaired model can be used to perform fault diagnosis.

An approach for the identification of time Petri net systems

This paper deals with the identification of time Petri net systems. The proposed algorithm identifies a time Petri net model on the basis of the observed behavior, extending existing approach for untimed systems. A mixed-integer programming problem is formulated to take into account that the firing of a transition requires that the enabling condition is met but, for time net systems, it is also required that the firing interval of a transition is congruent with the observed firing instant times.

Colored Hybrid Petri-nets for modeling material handling systems

Material handling systems are usually modeled as discrete event systems. However, when the size of these systems grows, the overall system performances depend also on continuous time phenomena. We present a hybrid model based on a new Petri net formalism that merges the concepts of Hybrid Petri Nets and Colored Petri Nets to obtain compact models for these systems. An example is discussed in detail to motivate the introduction of a new formalism.

A discrete event model for the control and analysis of complex automated warehouse systems

In the last twenty years, a big effort has been made to find optimal strategies for planning and control of automated warehouse systems. An automated warehouse system has two main components: an automated storage and retrieval subsystem consisting of a number of aisles, each one served by a crane, and a picking area which is formed by bays where stock units coming from the aisles are partially emptied by human operators. These two components are connected via an interface area consisting of carousels, conveyors and buffers. The role of this interface area is usually neglected, while the overall system performance depends on how this subsystem is controlled. A model focusing on the interface area is proposed in this paper to improve the control and the analysis of complex automated warehouse systems. A real case study is considered to show the effectiveness of the approach.

A cyber-physical view of automated warehouse systems

Automated warehouse systems play a key role in manufacturing systems and are usually controlled using hierarchical and centralized control architectures and conventional automation programming techniques. Cyber-physical systems are driving a significative revolution in manufacturing systems. In our opinion, they are suitable to be adopted in many transportation and material handling systems, in particular automated warehouse systems. In this paper, we present from a cyber-physical perspective a model oriented to the analysis, control and performance evaluation of decentralized warehouse control systems.

Real time identification of Time Petri net faulty models

This paper considers the problem of the identification of a faulty model for timed discrete event systems when the nominal model is known and a set of faulty observed timed sequences is given. The faulty behavior is modeled by adding fault transitions. The approach works with Time Petri net models and is based on the formulation of a Mixed-Integer Linear Programming Problem.

Identification of labeled Time Petri nets

In a previous paper we presented an approach to identify a Time Petri net system, given a set of observed timed sequences. The set of transitions, which coincides with the set of events, is assumed to be known, while the net structure, the initial marking and the firing duration of transitions are computed in one stage solving a mixed-integer linear programming problem derived from a set of logical predicates. In this paper we extend this approach by allowing that two transitions may share the same event label. Then, the identified model is a labeled Time Petri net system. The identification problem can still be solved via a mixed-integer linear programming problem but the constraint set previously determined is modified.