Average Buffer Level Research Articles

Multi-objective optimization of simultaneous buffer and service rate allocation in manufacturing systems based on a data-driven hybrid approach

The challenge presented by simultaneous buffer and service rate allocation in manufacturing systems represents a difficult non-deterministic polynomial problem. Previous studies solved this problem by iteratively utilizing a generative method and an evaluative method. However, it typically takes a long computation time for the evaluative method to achieve high evaluation accuracy, while the satisfactory solution quality realized by the generative method requires a certain number of iterations. In this study, a data-driven hybrid approach is developed by integrating a tabu search–non-dominated sorting genetic algorithm II with a whale optimization algorithm–gradient boosting regression tree to maximize the throughput and minimize the average buffer level of a manufacturing system subject to a total buffer capacity and total service rate. The former algorithm effectively searches for candidate simultaneous allocation solutions by integrating global and local search strategies. The prediction models built by the latter algorithm efficiently evaluate the candidate solutions. Numerical examples demonstrate the efficacy of the proposed approach. The proposed approach improves the solution efficiency of simultaneous allocation, contributing to dynamic production resource reconfiguration of manufacturing systems.

The impact of unequal processing time variability on reliable and unreliable merging line performance

Research on merging lines is expanding as their use grows significantly in the contexts of remanufacturing, reverse logistics and developing economies. This article is the first to study the behavior of unpaced, reliable, and unreliable merging assembly lines that are deliberately unbalanced with respect to their coefficients of variation (CV). Conducting a series of simulation runs with varying line lengths, buffer storage capacities and unbalanced CV patterns delivers intriguing results. For both reliable and unreliable lines, the best pattern for generating higher throughput is found to be a balanced configuration (equal CVs along both parallel lines), except for unreliable lines with a station buffer capacity of six. In that case, the highest throughput results from the descending configuration, i.e. concentrating the variable stations close to the beginning of both parallel lines and the steady stations towards the end of the line. Ordering from the least to most steady station also provides the best average buffer level. By exploring the experimental Pareto Frontier, this study shows the combined performance of unbalanced CV patterns for throughput and average buffer level. Study results suggest that caution should be exercised when assuming equivalent behavior from reliable and unreliable lines, or single serial lines and merging lines, since the relative throughput performance of some CV patterns changed between the different configurations.

The operating behaviour of unbalanced, unpaced merging assembly lines

Unbalanced assembly line research has grown in importance because of its increasing applications in emerging economies, reverse logistics and remanufacturing. This paper examines the performance of numerous simulated patterns for reliable unbalanced manual merging assembly lines. The contribution of this study to the literature is that imbalance does not always negatively impact efficiency and that it can improve merging line performance when compared to a corresponding balanced merging line. The best performance was found to be a balanced line configuration and a monotone decreasing order for both parallel merging lines, with the former generally resulting in a lower throughput and the latter resulting in a lower average buffer level than that of a balanced line.

Influence of unbalanced operation time means and uneven buffer allocation on unreliable merging assembly line efficiency

Unbalanced, unreliable (UR), unpaced, merging assembly lines are simulated in this study with varying line lengths, buffer storage capacities, imbalance degrees and unequal mean operation time configurations and uneven buffer capacity (BC) allocation. This paper contributes to the literature by suggesting that, in many cases, imbalance can improve merging lines’ performance, as compared to a corresponding balanced merging line. It was found that an inverted bowl or descending patterns for mean operation times (MTs), and an inverted bowl (concentrating BC towards the centre of the line) or an ascending pattern for buffer allocation, result in higher throughput (TR). In terms of average buffer level (ABL), the best pattern is a monotone decreasing order regarding MTs and a monotone increasing order with respect to BC allocation. Additionally, it was found that when considering a profit function, the best performing patterns for UR lines tend to be the patterns that reduce ABL, even when considering very low inventory holding costs; contrary to the behaviour of the profit function in reliable lines, which suggests that either patterns that increase TR or reduce ABL can lead to a good performance, depending on the values of the unitary inventory holding costs.

Studying unbalanced workload and buffer allocation of production systems using multi-objective optimisation

Numerous studies have investigated the effects of unbalanced service times and inter-station buffer sizes on the efficiency of discrete part, unpaced production lines. There are two main disadvantages of many of these studies: (1) only some predetermined degree of imbalance and patterns of imbalance have been evaluated against the perfectly balanced configuration, making it hard to form a general conclusion on these factors; (2) only a single objective has been set as the target, which neglects the fact that different patterns of imbalance may outperform with respect to different performance measures. Therefore, the aim of this study is to introduce a new approach to investigate the performance of unpaced production lines by using multiple-objective optimisation. It has been found by equipping multi-objective optimisation with an efficient, equality constraints handling technique, both the optimal pattern and degree of imbalance, as well as the optimal relationship among these factors and the performance measures of a production system can be sought and analysed with some single optimisation runs. The results have illustrated that some very interesting relationships among the key performance measures studied, including system throughput, work-in-process and average buffer level, could only be observed within a truly multi-objective optimisation context. While these results may not be generalised to apply to any production lines, the genericity of the proposed simulation-based approach is believed to be applicable to study any real-world, complex production lines.

Unpaced Merging Lines With Uneven Buffer Allocation

As a common production practice in many developing economies and in reverse logistics, unbalanced merging assembly lines are a research area which is re-emerging in prominence. This paper aims to study the performance of reliable, unpaced merging assembly lines that are unbalanced in terms of their buffer storage sizes. Lines are simulated with different values of line length, mean buffer storage capacity and configuration of uneven buffer allocation. The best patterns in terms of generating higher throughput and lower average buffer level as compared to a balanced merging line are those where total available buffer capacity is allocated as evenly as possible between workstations and to concentrate more buffer capacity towards the end of the line, respectively.

Asymmetrical buffer allocation in unpaced merging assembly lines

Asymmetrical merging assembly lines are a research area which is rising in prominence due to increasing use in reverse logistics, remanufacturing, and developing economies. This paper studies the performance of reliable, unpaced merging assembly lines with asymmetric buffer storage sizes. Lines are simulated with varying line lengths, mean buffer storage capacities and uneven buffer allocation configurations. Contrary to typical manufacturing expectations, results indicate that production line imbalances do not always result in detrimental performance. Higher throughput and lower average buffer levels, as compared to a balanced merging line, are found where total available buffer capacity is allocated as evenly as possible, and with a higher buffer capacity concentration towards the end of the line. This paper contributes to total production line knowledge by providing performance improvement methods for unpaced merging assembly lines with asymmetrical buffer allocation, and inexpensive or no cost managerial options to increase productivity and resource utilization, and decrease waste, in asymmetrical merging assembly lines.

The performance of unpaced production lines with unbalanced mean operation times and unreliability patterns

This paper reports on the performance of unbalanced non-automated production lines, whose workstations differ in terms of their mean operation times and their degree of unreliability (failure and repair rates). Simulations are done of 5- and 8-station lines with 1, 2 and 6 buffer units. Statistical analysis shows that breakdown patterns have a significant effect in terms of throughput and average buffer level, leading to the conclusion that the study of unreliable unbalanced lines needs not only take into account overall line breakdown, but should consider where in the line failure occurs.

The impact of failure, repair and joint imbalance of processing time means and buffer sizes on the performance of unpaced production lines

This article studies the performance of unpaced serial production lines that are subject to breakdown and are imbalanced in terms of both of their processing time means (MTs) and buffer storage capacities (BCs). Simulation results show that the best pattern in terms of throughput is a balanced line With respect to average buffer level; the best configuration is a monotone decreasing MT order, together with an ascending BC arrangement. Statistical analysis shows that BC, combined patterns of MT and BC imbalance, line length and degree of imbalance all contribute significantly to performance. [...] Sabry Shaaban, Tom McNamara and Sarah Hudson

Mean time imbalance effects on unreliable unpaced serial flow lines

This paper investigates the benefits of deliberately unbalancing operation time means for unreliable non-automated production lines. The lines were simulated with various line lengths, buffer capacities, degrees of imbalance and patterns of imbalance. Data on two performance measures, namely throughput and average buffer level were gathered, analyzed and compared to a balanced line counterpart. A number of conclusions were made with respect to the ranking of configurations, as well as to the relationships among the independent design parameters and the dependent variables. It was found that the best configurations are a balanced line arrangement and a monotone decreasing order, with the first generally resulting in lower throughput and the second leading to lower average buffer levels than those of a balanced line. Preliminary results show that unbalanced lines cope well with unreliability.

Simulation of unbalanced buffer allocation in unreliable unpaced production lines

This article presents the results of a study investigating the performance of unpaced unreliable production lines (i.e. subject to breakdown) that are unbalanced in terms of their buffer storage sizes. Simulation is carried out for five, eight and 10 station lines with mean buffer space set at two, four and six units. Buffer capacity is allocated in different configurations for each of these lines. Performance indicators on throughput, idle time and average buffer level are analysed using a range of statistical tools, and relationships between the independent and dependent variables are determined. Overall results show that the best patterns for unreliable lines in terms of generating higher throughput rates (or lower idle times) as compared to a balanced line are those where total available buffer capacity is allocated as evenly as possible between workstations. In contrast, concentrating more buffer capacity towards the end of the line gives best average buffer level results.

The effects of unbalancing operation time variability on the performance of unreliable lines

PurposeThe purpose of this paper is to investigate the performance of unpaced unreliable production lines that are deliberately unbalanced in terms of their coefficients of variation (CVs).Design/methodology/approachA series of simulation experiments were carried out for five and eight station lines with mean buffer space set at one, two, four and six units. CVs were allocated in 12 different configurations for each of these lines.FindingsThe results show that the best unbalanced CV patterns in terms of throughput rates or idle times as compared to a balanced line counterpart are those where the steadiest stations are concentrated near the centre of the line. On the other hand, either concentrating the steadier operators towards the centre or close to the end of the line gives best average buffer level results.Practical implicationsThe results provide guidelines for production line managers when designing unpaced unbalanced lines depending on their performance aims.Originality/valueThe investigation of the effects of unbalancing CVs in unreliable lines has not previously been studied and can provide insights into how best to place workstations with differing variability along the line.

An improved decomposition method for evaluating the performance of transfer lines with unreliable machines and finite buffers

In this paper, we consider transfer lines consisting of a series of machines separated by finite buffers. The processing time of each machine is deterministic and may be not identical. All machines are prone to operation-dependent failures, and the times between failures and the times to repair are assumed to be exponentially distributed. Many analytical methods have been developed to evaluate the performance of such lines. In general, these methods provide fairly accurate results. However, in some real cases where the orders of magnitude of machines’ reliability parameters (mean times between failures and mean times to repair) are not at the same level, the accuracy of these existing methods may not be good enough. The purpose of this paper is to propose an improved decomposition method that performs well even in the situation above. We use generalised exponential distributions instead of exponential distributions to approximate the repair-time distributions of the fictitious machines, and a new ADDX algorithm is developed to calculate the performance parameters such as the production rate and the average buffer levels. Numerical results indicate that the improved decomposition method provides more accurate results and converges in most cases. It is feasible and valid to evaluate the performance of transfer lines with unreliable machines and finite buffers.

Reliable and Unreliable Unpaced Production Lines with Unbalanced Operation Time Means – A Comparison

Reliable and unreliable unpaced production lines are simulated with various line lengths, buffer capacities, degrees of mean processing time imbalance and patterns of imbalance. Performance measures are idle time and average buffer level. It was found that for both reliable and unreliable lines, the best configurations are a bowl arrangement and a monotone decreasing order, with the first resulting in lower idle times and the second leading to lower average buffer levels.

Performance of Reliable and Unreliable Unpaced Production Lines with Uneven Buffer Allocation

We compare the efficiency of reliable and unreliable unpaced production lines that are unbalanced in terms of their buffer storage capacities to a corresponding line with equal sized buffers. The lines were simulated with different values of line length, buffer storage size, mean buffer capacity, and configuration of buffer imbalance. The results show that, for both reliable and unreliable lines the best buffer allocation patterns in terms of generating lower idle times as compared to a balanced line are those where total available buffer capacity is allocated as evenly as possible between workstations, whereas concentrating more buffer capacity towards the end of the line gives best average buffer level results.

Unpaced production lines with three simultaneous imbalance sources

Purpose – The purpose of this paper is to investigate the performance of unpaced reliable production lines that are unbalanced in terms of their mean operation times, coefficients of variation and buffer capacities.Design/methodology/approach – Simulations were carried out for five‐ and eight‐station lines with various buffer capacities and degrees of means imbalance. Throughput, idle time and average buffer level performance indicators were generated and statistically analysed.Findings – The results show that an inverted bowl allocation of mean service times, combined with a bowl configuration for coefficients of variation and a decreasing order of buffer sizes results in higher throughput and lower idle times than a balanced line counterpart. In addition, considerable reductions in average inventory levels were consistently obtained when utilizing a configuration of progressively faster stations, coupled with a bowl‐shaped pattern for coefficients of variation and an ascending buffer size order.Research...

A decomposition method for approximate evaluation of continuous flow multi-stage lines with general Markovian machines

In this paper, a decomposition method for evaluating the performance of continuous flow lines with machines characterized by general Markovian fluid models and finite capacity buffers is proposed. This study uses the exact solution of general two-stage Markovian fluid models as a building block. Decomposition equations are provided to propagate the effect of partial and complete blocking and starvation phenomena throughout the system. A decomposition algorithm that solves the new decomposition equations is proposed. Numerical results prove the good accuracy of the developed method. In particular, a comparison with existing techniques shows that our method is generally more accurate, especially in the estimation of the average buffer levels. Moreover, additional information can be collected by the application of our approach which enables a deeper analysis of the system behavior. Finally, the generality of the approach allows for modeling and studying many different system configurations within a unique framework, also including several previously uninvestigated layouts.

Transient behaviour of unbalanced lines

The operating behavior of non-automated reliable serial production lines that are either individually or jointly imbalanced in terms of their operation time means, coefficients of variation and/or buffer sizes are investigated in this paper. Simulations are carried out on the non-steady state mode of operation with various values of line length, buffer storage size, degree of imbalance and patterns of imbalance. Idle time and average buffer level output data are generated and analyzed. The results show that for the lines simulated the transient idle time is larger than the steady-state counterpart, whereas the opposite is true for the transient average buffer level. In addition, a longer line extends the transient idle time duration and as the degree of imbalance goes up, a shorter transient period is observed.

Unpaced production lines with jointly unbalanced operation time means and buffer capacities – their behaviour and performance

The performance of unpaced production lines that are unbalanced in terms of both their operation time means and buffer storage sizes is studied in this paper. The lines were simulated under their steady-state operational mode of operation with various values of line length, buffer storage capacity, degree of imbalance, and patterns of imbalance. Output data (principally idle time and average buffer level) were analysed utilising a number of statistical tools. In terms of idle time, it was found that the best unbalanced pattern is an MT bowl configuration, coupled with a distribution of buffer capacity as evenly as possible. With respect to ABL, the best pattern turned out to be a monotone decreasing MT order, together with an ascending buffer size order.

Modelling and analysis of manufacturing systems: a review of existing models

Manufacturing system models with unreliable machines and buffers have received much attention in the past five decades. While the first studies considered only two machines and one buffer in series, the most recent ones can consider much more and present various configurations (parallel, assembly/disassembly, rework path, feed-forward path or any combination of these). By using indicators – such as the global production rate, probability of idleness and average buffer level – to evaluate the performance of such systems, it may give one the opportunity to determine the buffer sizing. This paper presents an overview of analytical manufacturing system models, starting from a two-machine-one-buffer model towards a complex manufacturing system integrating assembly/disassembly networks, rework loops and feed-forward loops.