1,721,071 research outputs found
A new result to generalize and extend a single machine scheduling problem solved by dynamic programming
No full textThe problem of scheduling jobs on an unreliable single machine is considered in this paper under a control-theoretical point of view. The scheduling problem is formalized as an optimal control problem and optimal control strategies (functions of the system state) are determined. In this way, optimal decisions can be taken also in presence of perturbations which affect the single machine (such as breakdowns and slowdowns). In this paper, a new result is proposed, which allows generalizing and extending the methodology proposed by the author in the recent past. With this result it is possible to remove the hypothesis that any unitary tardiness cost is greater than any unitary cost related to the deviation from the nominal processing time; moreover, it is possible to add the setup cost to the objective function which is minimized
Optimal control strategies for single-machine family scheduling with sequence-dependent batch setup and controllable processing times
No full textThe problem of scheduling jobs on an unreliable single machine is considered in this paper. The scheduling problem is characterized by the following features: jobs are grouped into classes of equivalent jobs; the generalized due date model is adopted for each class of jobs; it is possible to reduce the processing time of a job, at the price of the payment of an extra cost; a costly setup is required when switching between jobs of different classes. The scheduling problem is solved from a perspective which is different from the traditional determination of an optimal sequence of jobs; in fact, the objective of the paper is to determine optimal control strategies (functions of the system state) which allow generating the optimal decisions during the evolution of the system, taking into account the actual system state. In this way, optimal decisions can be promptly taken also in the presence of perturbations which affect the single machine (such as breakdowns and slowdowns). To this aim, a specific optimal control problem is stated and solved in the paper
Definitions and applications of deterministic-timed Petri nets (DTPN)
No full textTimed Petri nets are considered in this paper, with the aim of providing an efficient tool for the modelling and the analysis of timed models, such as automated manufacturing systems (AMS) and intelligent transportation systems (ITS). In the proposed model, timings are associated with transitions, and firing times are deterministic values. Such a model, the deterministic-timed Petri nets (DTPN), is characterized by a certain number of assumptions. In a DTPN the system state is represented by the joint information consisting of the marking of places and of the residual firing times of timed transitions, and, under the assumptions which characterize the proposed model, it is possible to describe the evolution of the system state in a relatively simple way. In the paper, three subclasses of DTPN are also introduced, and two examples relevant to the application of the proposed model to AMS e ITS are discussed
A Petri Net Model for an Open Path Multi-AGV System
No full textAutomated distribution warehouses in which pallet and roll pallet loads are transported by means of forklift AGVs are considered in this work, with the objective of defining a mathematical model which accurately represents the behaviour of AGVs in the system. AGVs can move freely in the warehouse (an open path AGV system is adopted), and their transportation activities can be modelled as a sequence of elementary or basic actions. In the paper, a coloured Petri net (CPN) model is proposed. It allows representing any sequence of elementary actions of AGVs (including pick-up and drop-off activities), and accurately models the interactions among AGVs, in order to guarantee the safety during the execution of activities. The CPN model can be used to analyse and implement deadlock prevention and deadlock recovery strategies, and it has been adopted in the building of a discrete-event simulator which is employed to analyse the system's performance and to evaluate scheduling policies for transportation tasks
Task scheduling for multiple forklift AGVs in distribution warehouses
No full textDistribution warehouses are considered in this paper with the aim of scheduling the transportation of pallet and roll pallet loads from the storage area to the gates at which the trucks arrive. Transportation activities are carried out by forklift AGVs that can move freely along the warehouse's aisles (guide paths are not considered). In this paper, an optimization procedure is proposed, which is based on three sequential phases. In the first phase, the number of AGVs to be allocated to each arrived truck is determined; in the second phase, an AGV-to-truck assignment problem is solved; in the third phase, the single tasks are assigned to and sequenced on the AGVs. All phases are based on the formalization and solution of a specific mathematical programming problem, and a heuristic procedure is also proposed to solve a part of the third phase
On determining and applying optimal control strategies for optimal delivery of goods to a network of retailers based on inventory levels
No full textThe problem of delivering goods from a central storage warehouse to a network of retailers distributed over the territory is considered in this paper. It belongs to the class of Inventory-Routing Problems (IRP), since decisions concerning the set of retailers to be visited, the sequence in accordance with the retailers are visited, and the amount of goods to be delivered to the retailers are taken on the basis of the stock of goods (inventory level) at the retailers. The proposed methodology allows determining optimal control strategies and updating them in real-time, on the basis of the actual measurements of inventory levels provided by the retailers. It belongs to the class of Vendor-Managed Inventory (VMI) approaches, as replenishment decisions are taken by the vendor, and consists of an "off-line phase", to be accomplished daily before the beginning of the delivery process, and of an "on-line phase", to be carried out at the start of the trip and at the end of each delivery
A Medium-Scale Network Model for Short-Term Traffic Prediction at Neighbourhood Level
No full textA macroscopic model for predicting the evolution of traffic on medium-scale networks is proposed in this paper. The model takes into consideration the flows of vehicles which move from some origins to some destinations, and it is based on the LWR discrete-time/discrete-space vehicle conservation equation, which leads to quite simple models that can be employed within optimization and control schemes, aimed at regulating traffic and mitigating congestions at neighbourhood level. In the proposed model, vehicles are not constrained to stay in a link for at least one time interval, as they are allowed to enter a link and exit from it within the same interval. This feature requires a particular property (upstream dependence) of the digraph which represents the traffic network, in any case, a modified version of the dynamic model is also proposed in the paper, in order to deal with complex networks which do not have such a property
A Mathematical Programming Formulation for Cost-oriented Multi-manned Assembly Line Balancing Problem
No full textMulti-manned assembly line balancing problems are usually occurred in plants producing large-sized high-volume products such as automobiles and trucks. In this paper, a cost- oriented objective function is presented for a multi-manned assembly line balancing problem. This kind of objective function may be used to balance final assembly lines of products in which manufacturing process is very labor intensive. A mixed-integer mathematical programming model is proposed to solve the problem optimally. The proposed formulation has been used to solve some small size problems by considering both time-oriented and cost-oriented objective functions. The experiments show that, given the same precedence graph of multi-manned assembly line with a same cycle time, two different optimal solutions can be actually found when switching from time-oriented to cost-oriented objective functions, and vice versa. This difficulty increases the complexity of the cost-oriented multi-manned assembly line balancing problems with respect to the multi-manned assembly line balancing problems addressed in the literature
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