Marcos Singer

A computational study of branch and bound techniques for minimizing the total weighted tardiness in job shops

We present and compare a number of branch and bound algorithms for minimizing the total weighted tardiness in job shops. There are basically two types of branching schemes. The first one inserts operations in a partial schedule, while the second one fixes arcs in the disjunctive graph formulation of the problem. The bounding schemes are based on the analysis of precedence constraints, and on the solution of nonpreemptive single machine subproblems that are subject to so-called delayed precedence constraints. We obtain optimal solutions for all the instances with ten jobs and ten machines that we consider, including three tardiness versions of a well-known 10 × 10 instance introduced by Muth and Thompson [1] in 1963.

Decomposition methods for large job shops

Abstract A rolling horizon heuristic is presented for large job shops, in which the total weighted tardiness must be minimized. The method divides a given instance into a number of subproblems, each having to correspond to a time window of the overall schedule, which are solved using a shifting bottleneck heuristic. A number of rules for defining each time window are derived. The method is tested by using instances up to 10 machines and 100 operations per machine, outperforming a shifting bottleneck heuristic that has been shown to generate close to optimal results. Scope and purpose There has been a significant amount of research focused on the scheduling of a job shop, either minimizing the makespan or the tardiness. Although the results for small-size problems are satisfactory, there has been no approach as for yet middle- and large-size problems. This paper presents a heuristic that decomposes the problems on a time window basis, solving each subproblem using a shifting bottleneck heuristic. Its results for a due-date-related objective function are promising.

Internal supply chain management in the Chilean sawmill industry

Purpose – The existing business paradigm in the sawmill industry is that improvements are achieved by enhancements at each individual sawmilling plant. The authors investigate a collaborative approach, and propose a model for optimizing production and inventory planning decisions within a system of plants.Design/methodology/approach – The methodology models the sawmill production process in terms of two transformation stages and two inventory stages. The authors then extend the model to account for supply chain decisions consisting of timber transfers among plants. They apply the model at 11 Chilean sawmills to evaluate whether they can benefit from operating in a more integrated manner.Findings – The authors find an opportunity to increase profits by more than 15 percent through a higher utilization of the capacity and a better assignment of production orders.Research limitations/implications – The study neglects the possibility of exchanging raw material among plants. Also, the findings are derived from...

Assessing an ambulance service with queuing theory

We describe an ambulance service in terms of its main operation parameters and strategic decision variables. Assisted by queuing theory, we calculate the key performance indicators (KPI) that concern the manager and the KPI that concern the patient. We use them to improve the operation of a private ambulance service in Chile. First, we evaluate whether the historical performance is consistent with the resources deployed. Then, we estimate the impact of some operational enhancements, such as reducing the cycle time or enlarging the fleet. Finally, we optimize the geographical coverage of the bases. We conclude that simple mathematical expressions are very useful to evaluate and improve the operation.

DOES e-PROCUREMENT SAVE THE STATE MONEY?

Scientific literature reports scarce evidence of whether Internetbased procurement systems improve the efficiency of State purchases. We propose a methodology to estimate savings in: (i) the centralization of administrative tasks, and (ii) price differentials due to a larger number of contractors and suppliers bidding on contracts. We test our methods with ChileCompra, the Chilean e-procurement agency. During 2007, 885 Chilean State agencies used this system to purchase US

Who wants to break the hockey-stick sales pattern in the supply chain?

4.5 billion in products and services. Our preliminary results show price reductions of 2.65% and administrative cost savings of 0.28%-0.38% between 2006 and 2007.

Semi-autonomous planning using linear programming in the Chilean General Treasury

The hockey-stick pattern faced by suppliers consists of sales spikes at the end of each period. One of its causes is the information asymmetry that favors the retailer, who has better knowledge about the stochastic consumer demand. Because of delayed purchases, the supplier is induced to offer promotions, allowing the retailer to forward-buy at low prices. We model this situation as an infinitely repeated game, where each stage-game is subject to imperfect information. Drawing from the Nash equilibrium, we express sales and inventories in terms of demand, cost and the strategies players may adopt, and derive the conditions for a cooperative equilibrium.

Optimal Planning of a Multi-Station System with Sojourn Time Constraints

Abstract The Chilean General Treasury is the governmental institution that, among other functions, is in charge of collecting debts from taxes, credits, and fees. Until now the central office in Santiago has performed the planning, which has caused inefficiencies due to the lack of knowledge of the local realities of 35 offices across the country. In order to allow for more independence, and yet not lose control, we develop a system in a spreadsheet that allows each office to perform its own planning. The system consists of a multi-period linear program that considers a combination of parameters that depend on each region. The primal optimal variables provide the production goals for each period that allow an objective performance appraisal, while the dual variables allow comparing the marginal benefit of allocating additional resources to the different offices.

An Interactive MRP II — Scheduling System

This paper studies a dynamic production system where multiple products must visit stations where inventories are constrained by maximum and minimum sojourn times with neither negative flow nor backlog being allowed. A resource availability constraint limits the aggregate throughput of the stations. The objective is to minimize the sum of flow and inventory cost. The problem is broken down into several single-product serial systems that serve as subroutines of a Lagrangian relaxation routine. This model is implemented in a spreadsheet so that it can be used by the officials of a Chilean institution for planning the operations and defining the optimal allocation of resources.

A Shifting Bottleneck Heuristic for Minimizing the Total Weighted Tardiness in a Job Shop

We implement a Manufacturing Resources Planning (MRP II) system that considers the process that begins when an order is placed by a customer and ends with the production of the corresponding item. Two implementations, in a printing shop and in an appliance factory, show that the success of such a system depends on its capability to complement, rather than replace, the human planner. Such a capability is based on the system’s architecture: The data structure, the scheduling heuristic, and the graphic interface required by the user in the shop floor.