Vitória Pureza

Vehicle routing with multiple deliverymen: Modeling and heuristic approaches for the VRPTW

In real life distribution of goods, relatively long service times may make it difficult to serve all requests during regular working hours. These difficulties are even greater if the beginning of the service in each demand site must occur within a time window and violations of routing time restrictions are particularly undesirable. We address this situation by considering a variant of the vehicle routing problem with time windows for which, besides routing and scheduling decisions, a number of extra deliverymen can be assigned to each route in order to reduce service times. This problem appears, for example, in the distribution of beverage and tobacco in highly dense Brazilian urban areas. We present a mathematical programming formulation for the problem, as well as a tabu search and an ant colony optimization heuristics for obtaining minimum cost routes. The performance of the model and the heuristic approaches are evaluated using instances generated from a set of classic examples from the literature.

Waiting and Buffering Strategies for the Dynamic Pickup and Delivery Problem with Time Windows

Abstract We investigate the impact of two strategies for dynamic pickup and delivery problems on the quality of solutions produced by insertion heuristics: (a) a waiting strategy that delays the final assignment of vehicles to their next destination, and (b) a request buffering strategy that postpones the assignment of some non-urgent new requests to the next route planning. In this study, the strategies are tested in a constructive-deconstructive heuristic for the dynamic pickup and delivery problem with hard time windows and random travel times. Comparisons of the solution quality provided by these strategies to a more conventional approach were performed on randomly generated instances up to 100 requests with static and dynamic (time-dependent) travel times and different degrees of dynamism. The results indicate the advantages of the strategies both in terms of lost requests and number of vehicles.

Some experiments with a simple tabu search algorithm for the manufacturer's pallet loading problem

The manufacturers pallet loading problem consists in arranging, orthogonally and without overlapping, the maximum number of boxes with dimensions (l,w) or (w,l) onto a rectangular pallet with dimensions (L,W). This problem has been successfully handled by block heuristics, which generate loading patterns composed by one or more blocks where the boxes have the same orientation. A common feature of such methods is that the solutions provided are limited to the so-called first order non-guillotine patterns. In this paper we propose an approach based on the incorporation of simple tabu search (without longer-term memory structures) in block heuristics. Starting from an initial loading pattern, the algorithm performs moves that increase the size of selected blocks in the current pattern; as a result, other blocks are decreased, eliminated or created. Computational results indicate that the approach is capable of generating superior order optimal patterns for difficult instances reported in the literature.

Combining Process Selection and lot sizing Models for Production Scheduling of Electrofused Grains

This work presents an optimization model to support decisions in the production planning and control of the electrofused grain industry. A case study was carried out in a Brazilian company with the aim of helping to increase productivity and improve customer service concerning meeting deadlines. A mixed integer linear programming model combining known models of process selection and single-stage lot sizing were applied to the production scheduling of electrofused grains. Optimizing this scheduling is not a simple task mainly because of the scale of the equipment setup times, the diversity of the products and the deadlines of the order due dates. A constructive heuristic is also proposed as an alternative solution method, particularly for large-sized instances. The results show that the model and the heuristic can produce better solutions than the ones currently used by the company.

Geração de padrões de cortes bidimensionais guilhotinados restritos via programação dinâmica e busca em grafo-e/ou

A heuristic method for generating constrained two-dimensional guillotine cutting patterns based on the exact method by Christofides and Hadjiconstantinou (1995) was presented in Silveira and Morabito (2002). The method combines a state space relaxation of a dynamic programming formulation, a subgradient optimization procedure and an inner heuristic that turn infeasible solutions provided in each step of the optimization procedure into feasible solutions. In this work, the method of Silveira and Morabito is modified by using a more effective inner heuristic, and an and/or-graph search approach in order to generate good initial solutions. Results for benchmark and randomly generated instances indicate that the refined methods performance is superior to the previous one, and it is competitive face to other methods proposed in the literature.

Uma heurística de busca tabu simples para o problema de carregamento de paletes do produtor

The manufacturer’s pallet loading problem consists in arranging, orthogonally and without overlapping, the maximum number of boxes with dimensions (l,w) or (w,l) onto a rectangular pallet with dimensions (L,W). This problem has been handled successfully by block heuristics, which generate loading patterns composed by one or more blocks where the pieces have the same orientation. A feature of such methods is that the patterns are limited to the so called 1 st order non-guillotine patterns. In this work we propose an approach based on the incorporation of simple tabu search (without longer term memory structures) in block heuristics. Starting from an initial pattern, the algorithm performs moves that increase the size of selected blocks in the current pattern; as a result, other blocks are decreased, eliminated or created. Computational results indicate that the approach is capable of generating superior order optimal patterns for difficult instances reported in the literature.

Some experiments with a savings heuristic and a tabu search approach for the vehicle routing problem with multiple deliverymen

In this work we consider a variant of the vehicle routing problem that allows the assignment of multiple deliverymen to one or more routes. A practical motivation for this variant arises, for example, in the distribution of beverages in highly dense urban areas, characterized by the difficulty in serving daily requests within regular working day hours with a single deliveryman per vehicle. We present a mathematical model and a savings algorithm in order to generate low cost routes that maximize the number of requests served in compliance with the maximum route time. The impact of the extra deliverymen on the solutions provided by the proposed heuristic is assessed by means of sets of generated examples based on classical instances of literature. It is also presented the results obtained by an adaptation of a tabu search approach from the literature.

Um refinamento do algoritmo tabu de Dowsland para o problema de carregamento de paletes do produtor

The Manufacturers Pallet Loading Problem (MPLP) consists in arranging, orthogonally and without overlapping, the maximum number of rectangular and identical pieces of sizes (l, w) onto a pallet (L, W). The MPLP has been successfully handled by block heuristics, which generate loading patterns with one or more blocks where the pieces have the same orientation. The resulting patterns produced by these methods are limited to the so-called 1st order non-guillotine patterns. In this work we present a tabu search implementation based on the algorithm proposed by Dowsland to solve the MPLP; differently than block heuristics, the solutions are not limited to particular loading patterns. Computational experiments with a set of 34 instances extracted from the literature as well as from practical contexts indicate that this approach is capable to solve most of the examples. For the unsolved instances, we propose a simple additional procedure, which resulted in optimal patterns.

Um algoritmo de busca tabu para o carregamento de contêineres com caixas idênticas

Este trabalho aborda o problema do carregamento de conteineres, cuja forma mais geral consiste em arranjar itens (por exemplo, produtos embalados em caixas) de varios tamanhos dentro de objetos maiores (por exemplo, conteineres) com maximo aproveitamento do volume disponivel. Em particular, e considerado o caso especial em que as caixas a serem arranjadas sao retangulares e identicas. Com o objetivo de resolver este problema, estendemos a heuristica de busca tabu de Pureza e Morabito, originalmente projetada para o problema de carregamento de paletes do produtor. Partindo-se de uma solucao inicial gerada por uma heuristica de blocos, sao realizados movimentos de troca de orientacao e/ou expansao de blocos selecionados, resultando na geracao de novos padroes de carregamento. Resultados computacionais obtidos com 22 exemplos da literatura sugerem que a abordagem proposta e promissora.

Um estudo de impactos do roteamento dinâmico de veículos em atividades de prestação de serviço

In this paper we evaluate the impacts resulting from the incorporation of dynamic vehicle routing and scheduling in service production systems where due dates for service are a priority issue. We propose a constructive/deconstructive heuristic, which is activated by a dispatcher module in order to obtain routes in real time. The relative impact of the heuristic application to other methods is analyzed by means of sets of randomly generated instances, based on the data supplied by a drinks company in Sao Paulo State. The results indicate significant gains in terms of service level.