Selcuk Karabati

Assembly line balancing in a mixed-model sequencing environment with synchronous transfers

Abstract We consider the assembly line balancing problem in a mixed-model line which is operated under a cyclic sequencing approach. We specifically study the problem in an assembly line environment with synchronous transfer of parts between the stations. We formulate the assembly line balancing problem with the objective of minimizing total cycle time by incorporating the cyclic sequencing information. We show that the solution of a mathematical model that combines multiple models into a single one by adding up operation times constitutes a lower bound for this formulation. As an approximate solution to the original problem, we propose an alternative formulation that suggests to minimize the maximum subcycle time. We also develop a simple heuristic approach for this alternative problem. We provide computational results that compare the various approaches we discuss.

A bicriteria approach to the two-machine flow shop scheduling problem

In this paper we address the problem of minimizing makespan and sum of completion times simultaneously in a two-machine flow shop environment. We formulate the problem as a bicriteria scheduling problem, and develop a branch-and-bound procedure that iteratively solves restricted single objective scheduling problems until the set of efficient solutions is completely enumerated. We report computational results, and explore certain properties of the set of efficient solutions. We then discuss their implications for the Decision Maker.

Assigning cross-trained workers to departments: A two-stage optimization model to maximize utility and skill improvement

Abstract We develop a general framework that is applicable in both manufacturing and service settings for assigning cross-trained workers across departments. The framework consists of a two-stage optimization model where two objective functions, departmental utility and skill improvement, are considered sequentially. Departmental utility is a function of departmental labor shortage and the first-stage optimization model maximizes total departmental utility subject to typical assignment constraints. The second stage model seeks to maximize total skill improvement, which is quantified by a hyperbolic learning curve, while trying not to deviate from the utility level obtained during the first stage optimization. Our computational experiments suggest that incorporating the skill improvement function explicitly in the model results in significant improvement in the total skill level of the workforce and thus leads to more effective worker assignments.

The two-machine flowshop total completion time problem: Improved lower bounds and a branch-and-bound algorithm

Abstract This paper presents a branch-and-bound algorithm for the two-machine flowshop scheduling problem with the objective of minimizing the sum of completion times. The main feature of the branch-and-bound algorithm is a new lower bounding scheme that is based on a network formulation of the problem. With extensive computational tests, we demonstrate that the branch-and-bound algorithm can solve problems with up to 60 (45) jobs, where processing times are uniformly distributed in the [1,10] ([1,100]) range.

Single-supplier/multiple-buyer supply chain coordination : Incorporating buyers' expectations under vertical information sharing

Abstract We address the coordination problem in a single-supplier/multiple-buyer supply chain. The supplier wishes to coordinate the supply chain by offering quantity discounts. To obtain their complete cost information, the supplier exchanges his own cost parameters with buyers leading to vertical information sharing. The supplier thinks that the buyers, as they have access to supplier’s setup and holding cost information, may demand a portion of the anticipated coordination savings based on the partial information they hold about the cost structure of the entire supply chain. We model each buyer’s expectations based on her limited view of the entire supply chain which consists of herself and the supplier only. These expectations are then incorporated into the modeling of the supply chain, which results in a generalization of the traditional Stackelberg type models. We discuss alternative efficiency sharing mechanisms, and propose methods to design the associated discount schemes that take buyers’ expectations into account. In designing the discount schemes, we consider both price discriminatory and non-price discriminatory approaches. The study adds to the existing body of work by incorporating buyers’ expectations into a constrained Stackelberg structure, and by achieving coordination without forcing buyers to explicitly comply with the supplier’s replenishment period in choosing their order quantities. The numerical analysis of the coordination efficiency and allocation of the net savings of the proposed discount schemes shows that the supplier is still able to coordinate the supply chain with high efficiency levels, and retain a significant portion of the net savings.

Cyclic scheduling in synchronous production lines

In This paper we address the scheduling problem in uppaced synchronous mixed-model production lines operated under a cyclic scheduling policy. Synchronous lines are widely used in the production and assemply of various products such as automobiles and household part.

Cyclic scheduling in flow lines: Modeling observations, effective heuristics and a cycle time minimization procedure

In this paper we address the cyclic scheduling problem in flow lines. We develop a modeling framework and an integer programming formulation of the problem. We subsequently present exact and approximate solution procedures. The exact solution procedure is a branch-and-bound algorithm which uses Lagrangian and station-based relaxations of the integer programming formulation of the problem as the lower bounding method. Our heuristic procedures show a performance superior to the available ones in the literature. Finally, we address the stability issue in cyclic scheduling, demonstrate its relationship to the work-in-progress inventory control of a flow line, and present a very simple procedure to generate stable schedules in flow lines.

A location problem on unicyclic networks: Balanced case

Abstract The problem of locating n stations on a unicyclic unidirectional network is considered. We formulated the problem as an integer program for the case of a balanced flow matrix, i.e., the total inflow to a station equals the total outflow from that station. The integer programming formulation reduces the problem to a station sequencing problem regardless of the actual station locations on the network. Furthermore, the empirical evidence suggests that the LP solutions to the relaxed IP formulations are optimal.

A Min-Max-Sum Resource Allocation Problem and Its Applications

In this paper we consider a class of discrete resource-allocation problems with a min-max-sum objective function. We first provide several examples of practical applications of this problem. We then develop a branch-and-bound procedure for solving the general case of this computationally intractable problem. The proposed solution procedure employs a surrogate relaxation technique to obtain lower and upper bounds on the optimal objective function value of the problem. To obtain the multipliers of the surrogate relaxation, two alternative approaches are discussed. We also discuss a simple approximation algorithm with a tight bound. Our computational results support the effectiveness of the branch-and-bound procedure for fairly large-size problems.

Flow-line scheduling problem with controllable processing times

Abstract In this paper we address the simultaneous scheduling and optimal-processing-times selection problem in a multi-product deterministic flow line operated under a cyclic scheduling approach. The selection of processing times plays an important role in achieving the desired production rate with the least possible operating cost. We first formulate the important subproblem of optimal-processing-times selection for different objectives, when the sequence of jobs is fixed, and then develop an efficient solution procedure for it. The fast solution of the fixed sequence problem is necessary for the development of efficient approximate solution procedures for the simultaneous scheduling and optimal-processing-times problem. A computational study on the effectiveness of the proposed solution procedure is presented. For the solution of the simultaneous scheduling and optimal-processing-times problem we suggest an iterative solution procedure, and report our computational experience with this procedure. For t...