Station Service Time Research Articles

An extensive evaluation of CONWIP-card controlled and scheduled start time based production system designs

Order release policies such as a card-controlled CONWIP policy aim to improve system responsiveness and minimize system-wide inventory levels. It is not clear if order release policies (without card control) can be equally effective under certain settings of the production system design parameters. In this research, we study the performance of alternate order release (material control policies) such as CONWIP and scheduled start time policy under a variety of design parameter settings such as number of stations, station service time characteristics, availability of raw material, and the location of the bottleneck station using queuing network models. Using an exhaustive numerical study, we observe that throughput times in CONWIP systems are about 2–50% less compared to open control systems; however, for a balanced system, the Deterministic scheduled start time policy performs equally well for a large set of production system designs. We further identify that the number of stations and the location of the bottleneck station affect the choice of the order release policy. Additionally, we also analyze the system for a variety of demand inter-arrival times and check its effect on the expected number of backlogs and system-wide expected waiting times.

Throughput Matching Algorithm and Stochastic Models for Analysis of Open and Closed Manufacturing Systems

Order release policies such as card-controlled CONWIP policy aim at improving system responsiveness and minimizing system-wide inventory levels. It is not clear if order release policies (without card control) can be equally effective under certain settings of the production system design parameters. In this research, we study the performance of alternate order release (material control policies) under a variety of design parameter settings such as number of stations, station service time characteristics, and the location of bottleneck station using queuing network models. To compare CONWIP policy (closed system) with open control policies (such as deterministic start times), we develop a throughput matching algorithm. The throughput times in CONWIP system is about 2%-50% less in comparison to open control systems; however, we show that the number of stations and the location of the bottleneck station affects the choice of the order release policy, and the benefit of card-controlled policies diminishes for a system with large number of stations (the throughput time percentage benefit using CONWIP in comparison to open control is 1/K, where K is the number of stations). We also analyze the system for a variety of demand inter-arrival times and check its effect on the expected number of backlogs and system wide expected waiting time.

Queueing network modelling of flexible manufacturing system using mean value analysis

In the present investigation, we develop queueing model for the performance prediction of flexible manufacturing systems (FMSs) with a multiple discrete material-handling devices (MHD). An iterative method has been suggested using mean value analysis (MVA) for the state-dependent routing. Two queueing network models are considered to determine the material-handling device interference. In the first one, we model the interference from the MHD by inflating the station service times but neglect queueing at the MHD. In another network, the queueing for the MHD is taken into consideration. The performance of FMS configuration is obtained by iterating between two networks. The suggested algorithms demonstrate better results than the algorithm used by earlier workers for single MHD. Some performance indices viz. throughput, mean service time, mean waiting time, etc. are obtained. Numerical results are provided to highlight the effect of the system parameters on performance indices, which are further evaluated by using neuro-fuzzy controller system to validate the tactability of soft computing approach.

A spreadsheet method for calculating work completion time probability distributions of paced or linked assembly lines

This study presents a spreadsheet method for computing the work completion time probability distribution of a linked assembly line with variable station service times. Once developed, this distribution can be used to analyse the probability of completing all work at all stations within a given cycle time on either a linked or mechanically paced assembly line. The method can be used to determine what cycle time to set on a paced line, given a desired workload at each station. Alternately, given a desired cycle time and probability that all work has been completed at each station within the cycle time, the method can be used to determine how much work to allocate to each station on the paced line. The method is simple, easy to use, and works with any continuous or discrete station service time distribution that can be specified in a spreadsheet. Comparisons between the output of the spreadsheet method and simulations of the same system indicate the spreadsheet method is quite accurate. Hence, the method can be useful to both researchers and practitioners working with the design and/or behaviour of linked or paced lines, and it provides an easy-to-use teaching tool for illustrating characteristics of linked or paced assembly lines.

Performance Analysis of Flexible Manufacturing Systems with a Single Discrete Material-Handling Device

We present an analytical model for performance prediction of flexible manufacturing systems (FMSs) with a single discrete material-handling device (MHD). This configuration of FMS is significant for many reasons: it is commonly found in industry, it simplifies material-handling control, it is amenable to analytical modeling, and it forms a building block for more complex systems. Standard queueing models are inadequate to analyze this configuration because of the need to take into consideration many nontrivial issues such as state-dependent routing, interference from the MHD, and the analysis of the MHD. To account for state-dependent routing, we develop an iterative method that is built around mean value analysis. To analyze the MHD interference, we use two queueing network models. In the first, we ignore queueing at the MHD but model the interference from the MHD by inflating the station service times. The second network models the queueing for the MHD and estimates the blocking (inflation) times needed for the first model. By iterating between the two networks, we are able to predict the performance of this configuration of FMS. Our analytical estimates are validated against discrete event simulation and shown to be quite accurate for initial system design.

Stochastic unpaced line design: A reply

Executive SummaryIn the May 1985 issue of the Journal of Operations Management, we published a paper containing two major segments (Smunt and Perkins (1985)). The first segment provided a comprehensive review of previously published research on unpaced assembly lines. Two of the primary references in this review were path‐breaking studies by Hillier and Boling (1966) and Hillier and Boling (1979). These studies introduced the idea of the “bowl phenomenon,” which suggests that line output can be increased (compared to a balanced line) by unbalancing the line with high service times placed at the beginning and end of the line and low service times placed in the middle of the line. This pair of studies also verified the existence of the bowl phenomenon for exponential and Erlang service times with line lengths up to five stations and buffer capacities between stations from zero to four units.The second segment of our 1985 paper reported on a set of experiments to investigate the “robustness” of the bowl phenomenon under varying service time and buffer capacity assumptions. We tested and confirmed three primary hypotheses: The use of the bowl distribution to unbalance an assembly line will not significantly increase the output rate as compared to a balanced configuration when normally‐distributed work station service times are employed with less variance than the exponential distribution. Minimal increases of buffer storage capacity will significantly reduce or will negate the benefit of using the bowl distribution. Output rate is not highly sensitive to moderate variations from optimal task time allocation for stochastic unpaced lines.In other words, we found that the bowl distribution does not have advantages over a balanced line if either more realistic normally‐distributed service times are used or larger buffer capacities are available. Thus the bowl phenomenon, while interesting, does not appear to exist in most real world unpaced lines.In the “Note” which immediately precedes this “Reply,” the authors take issue with our conclusions, based primarily on their view that our 1985 experiments were “flawed.” While the “Note” authors provide some valuable arguments and correctly point out a weakness concerning the number of simulation repetitions in our 1985 study, in general it is their arguments that are “flawed,” as we will establish in this paper. In responding to the preceding “Note,” we have conducted a very extensive simulation experiment—much larger than either our 1985 study or the small study conducted as part of the preceding paper. The results of this enhanced study provide strong support for the conclusions of our 1985 paper.

OPTIMIZATION OF THE HYDROMETRIC NETWORK OPERATION BY A HEURISTIC TRAVELING SALESMAN ALGORITHM1

ABSTRACT: The problem of determining the optimal route to operate a network of hydrometric gauges within a given area is formulated and solved by an original heuristic traveling salesman algorithm. The algorithm used for solving the problem takes into account the variation of station service time, the eight‐hour maximum working day, existence of more than one home base, and the budget constraint. The traveling salesman algorithm is used to solve the optima! assignment problem within a one‐day time limit, locating a starting base, stations to visit, and the closest home base. The ending base for the previous day becomes the starting base for the next day, and the assignment procedure is repeated. The computation proceeds from day to day, until all the stations in the considered region are serviced. The solution obtained presents the optimal route to operate a network of hydrometric gauges. The optimization procedure has been applied to the Dauphin hydrometric field area in Manitoba.

The Throughput Rate of Multistation Production Lines with Stochastic Servers

A serial production line with an arbitrary number of stations is analyzed using the holding-time model. In contrast to Markovian state models, the holding-time model does not require the service times to be exponentially distributed. The distribution of the interdemand time at the first station is obtained as a function of the distribution of station service times which are of a general nature. The throughput rate of the line is the reciprocal of the mean interdemand time. For service times of phase type, two numerical solution methods, one exact and the other approximate, are presented. Numerical results are given for up to ten stations.

The bowl phenomenon revisited

A novel method of analysing serial production lines has been developed. This method lets one compute the throughput rate of lines composed of dissimilar stations, as well as for a large class of distributions of station service times. Several distribution-free models of 3-station lines are presented. These models are used to compute the throughput rate of unbalanced lines in which the sum of the mean service times is constant. Results are shown as contour plots of constant throughput rate. The bowl phenomenon is reviewed in the light of this capacity to model with a greater degree of freedom.

The Design of Imbalanced Series Queue Flow Lines

Series queue operator paced flow lines are simulated to study the effect on mean line idle time of line imbalance due to differences in individual station service time variability and/or different buffer storage capacities. Ten line design hypotheses are tested.

The effects of performance time variance on a balanced, four-station manual assembly line

Production for a four-station manual assembly line, notionally balanced with respect to station service time means and unpaced in the mechanical sense is simulated in order to determine the effects of station service time variance on operating characteristics of the line. A deeper insight into this problem than has been reported in previous research is presented for small assembly line operation. Details of operating characteristics, including production rate, idle time and buildup of inter-process stock, are measured against station service time variance over a wide spectrum of levels and varying conditions.

A note on the operating characteristics of‘ balanced’ and ‘ unbalanced’ production flow lines

Abstract Computer simulation is used to investigate the idle time and maximum queue occurring at stations on flow lines with ‘balanced’ independently normally distributed station service times. The effect of ‘inbalance’ in station service time mean and variability, on idle time, maximum queues and output is also investigated.