Low Work-in-process Research Articles

An efficient constructive heuristic to balance trade-offs between makespan and flowtime in permutation flow shop scheduling

Balancing trade-offs between production cost and holding cost is critical for production and operations management. Utilization of a production line affects production cost, which relates to makespan, and work-in-process (WIP) inventories in a production line affect holding cost, which relate to flowtime. There are trade-offs between two objectives, to minimize makespan and to minimize flowtime. Without addressing trade-off balancing issues in flow shop scheduling, WIP inventories are still high in manufacturing, generating unnecessary holding cost. However, utilization is coupled with WIP inventories. Low WIP inventory levels might lower utilization and generate high production cost. Most existing constructive heuristics focus only on single-objective optimization. In the current literature, the NEH heuristic proposed by Nawaz, Enscore, and Ham (1983) is the best constructive heuristic to minimize makespan, and the LR heuristic proposed by Liu and Reeves (2001) is the best to minimize flowtime. In this paper, we propose a current and future deviation (CFD) heuristic to balance trade-offs between makespan and flowtime minimizations. Based on 5400 randomly generated instances, 120 instances in Taillard’s benchmarks, and one-year historical records of operating room scheduling from University of Kentucky HealthCare (UKHC), our CFD heuristic outperforms the NEH and LR heuristics on trade-off balancing, and achieves the most stable performances from the perspective of statistical process control (SPC).

Research on DBR Production Scheduling System Application Based on Simulation

Traditional planning methods ignore the enterprise production capacity,the paper depend on the specific case in enterprise production to build a Production Scheduling System which applies Drum-Buffer-Rope theory. This system proposes the high output, low work in process and on time delivery as the goals, considering the bottleneck resource as well as providing the master production plan with the bottleneck resource, in addition employing the simulation optimization software-Simio to optimize the bottleneck process scheduling and raise the utilization ratio of bottleneck resource.

A multiple-logistic-objective-optimized manufacturing planning and control system

A multiple-logistic-objective-optimized (MLOO) manufacturing planning and control approach is developed in an attempt to optimize the logistics objectives of a short throughput time, a low work in process, and high schedule reliability. The proposed approach also needs to effectively cope with disturbances including internal disturbances (equipment failure, rework, etc.) and external disturbances (variations in demand patterns, unsatisfied raw material delivery, etc.). This paper focuses on make-to-order manufacturing systems with routing flexibility. Simulation experiments are conducted using industrial data in order to compare the MLOO approach with several existing methods. The simulation results indicate that MLOO is able to optimize the target logistics objectives and effectively cope with disturbances.

A WIP control policy for tandem lines

The control of manufacturing systems has many similarities with the management of inventories in warehouses. Based on the detailed analysis of these similarities, a new control policy is proposed in this paper. Simulation studies demonstrate that this new policy performs better than several established control policies by providing a higher service level with a lower work in process (WIP). This policy is in its infant stage of development and only single part type tandem lines (transfer lines) are studied in this paper. Studies of re-entrant production systems or multiple part type systems are the focus of future research.

Assessment of Best Scheduling Practice in Continuous Casting and Hot Rolling of Stainless Steel Strip by System Dynamics Simulation

A rapid flow of materials with little intermediate buffering between steel mill and hot strip mill has many benefits. One is energy savings due to raised charging temperature in the reheat furnaces of the hot strip mill. Another is that tied capital is freed up, thereby improving mill economy. Still, it is not unusual that average lead-time is in the order of days, or even weeks. The aim of the present work was to show how lead-times from casting to rolling could be improved by changes in the scheduling function. A System Dynamics model of a stainless steel strip production facility with continuous caster and hot rolling mill was created. The model was used to study the dynamics of the system in response to changes in parameters that defined the scheduling configuration. More frequent schedule updating generally resulted in less work in process (WIP) and shorter lead times from casting to rolling, with resulting higher charging temperatures. The amount of oscillation in the system was also reduced. More frequent work roll changes were required when scheduling frequency increased, resulting in an increased fraction of setup time in relation to total processing time. Therefore, a development towards increased scheduling frequency may have to be complemented by efforts to reduce changeover times. The conclusion was that dynamic scheduling routines with frequent schedule updating result in better overall performance of the system due to lower WIP and better heat utilization. Dynamic scheduling routines with frequent updates make the system respond better to changes in the system and give better overall performance. The result is lower WIP, increased energy efficiency and less oscillation in the system.

Operation of Manufacturing Systems with Work-in-process Inventory and Production Control

The operation of large manufacturing systems with buffers has two goals: to meet production target and minimize the work-in-process (WIP) inventory. This paper introduces a novel approach, based on optimal control theory, to achieve both goals simultaneously by on-line adjustment of the production rate of each machine. In this method the state variables are the buffer levels, the control variables are the machine production rates, and the output variable is the target production (the demand). The method is evaluated through simulations under various conditions, and compared with other methods in the literature. The results demonstrate that the proposed method can successfully produce low WIP inventory than other methods, while the required production demand is still fulfilled. It is also shown that the method is capable of providing feasible solutions for large manufacturing systems - a goal that is harder to achieve with the current known method.

Multi-agent systems in production planning and control: An application to the scheduling of mixed-model assembly lines

This work deals with production smoothing, one of the keys of success of Just In Time and Lean Production. By levelling the load of the workstations, production smoothing allows a regular material flow, shorter manufacturing lead times, and lower work in process. Different solutions to the mixed-model assembly lines sequencing problem have been proposed in literature. In this paper, a Multi-Agent System is presented, which solves this problem according to the theory of autonomous agents. The experimental results show that this innovative approach has a good performance if compared with the traditional ones.

A workload balancing model for determining set-up time and batch size reductions in GT flow line workcells

Set-up time and batch size reductions cornerstones of Quick Response Manufacturing (QRM) are often viewed as pathways leading to lower workin-process (WIP) inventory levels and reduced manufacturing lead times. However, set-up time and batch size reductions may not always yield such improvements. Increased workload due to the additional set-ups can lead to a violation of the system's capacity constraints and cause system instability, which results in increased manufacturing lead times and WIP inventory levels. A workload balancing model in conjunction with a heuristic model is presented to determine the required set-up time reduction schemes for different levels of batch size reduction. The model is based on the Theory of Constraints to balance the flow line and limit the workload of each workstation such that each workstation meets the reserved capacity constraint. It is shown that a reduction plan determined by the workload balancing model will maintain the cell's system stability and improve its performance. Incorporating the workload balancing and simulation models, a general approach for selecting the most appropriate reduction plan is illustrated through an example of a gear manufacturing cell. The implications for management are: (1) that pre-established levels for set-up time and batch size reductions are not always effective; and (2) that effective reduction plans need to be preceded by system capacity and performance analysis to determine the effects of such reductions prior to implementation.

An effective optimization-based algorithm for job shop scheduling with fixed-size transfer lots

Abstract Effective scheduling of production lots is of great importance for manufacturing medium to high-volume products that require significant setup times. Compared to traditional entire-lot production, lot splitting techniques divide a production lot into multiple smaller sublots so that each sublot can be “transferred” from one stage of operation to the next as soon as it has been completed. “Transfer lots,” therefore, significantly reduce lead times and lower work-in-process (WIP) inventory. The mathematical modeling, analysis, and control of transfer lots, however, is extremely difficult. This paper presents a novel integer programming formulation with separable structure for scheduling job shops with fixed-size transfer lots. A solution methodology based on a synergistic combination of Lagrangian relaxation, backward dynamic programming (BDP), and heuristics is developed. Through explicit modeling of lot dynamics, transfer lots are handled on standard machines, machines with setups, and machines requiring all transfer lots within a production lot to be processed simultaneously. With “substates” and the derivation of DP functional equations considering transfer lot dynamics, the standard BDP is extended to solve the lot-level subproblems. The recently developed “time step reduction technique” is also used for increased efficiency. It implicitly establishes two time scales to reduce computational requirements without much loss of modeling accuracy and scheduling performance, thus enabling resolution of long-horizon problems within controllable computational requirements. The method has been implemented using object-oriented programming language C++, and numerical tests show that high-quality schedules involving transfer lots are efficiently generated to achieve on-time delivery of products with low WIP inventory.