Chao Bo Yan

Raw material release rates to ensure desired production lead time in Bernoulli serial lines

Production systems are often managed to maximise their production rate ( ). In some cases, this leads to excessive work-in-process and, as a result, long production lead time ( ). This issue is of particular importance in systems with ‘unlimited’ buffer space, where may also become unlimited. The question addressed in this paper is: What should be the release rate of raw materials so that the desired is obtained, while is maximised? We answer this question for serial lines with infinite buffers and machines obeying the Bernoulli reliability model. Specifically, given a serial line, we quantify the set of attainable s (feasible set), and for each point in the feasible set provide the release rate (as a function of machine parameters) that guarantees the desired lead time, while maximising the production rate. In addition, we offer a feedback control law that enforces the desired system behaviour. The development is based on a recursive aggregation procedure that leads to analytical estimates of . The accuracy of these estimates is evaluated by simulations and is shown to be acceptable for practical applications (well within 5%). The results obtained offer small- and mid-size enterprises the possibility of managing their production lead time, when neither limited buffers nor kanban/CONWIP systems are available, by releasing raw materials on, say, an hourly basis.

Efficient Simulation Method for General Assembly Systems With Material Handling Based on Aggregated Event-Scheduling

Performance evaluation of complex manufacturing systems is challenging due to many factors such as system complexity, parameter uncertainties, problem size, just to name a few. In many cases when a system is too complex to model using mathematical formulas, simulation is used as an effective alternative to conduct system analysis. A manufacturing system is a good example of such cases where both system performance and system complexity are greatly impacted by material handling (MH) strategy, management, and operational control. In this paper, we study vehicle general assembly (GA) system with MH, and focus on developing an efficient simulation method for modeling and analysis where traditional simulation methods may suffer from computation intensity. Making use of the partial system decomposability, we introduce an aggregated event-scheduling simulation method with two-level framework. A dividing mechanism with boundary conditions is employed in top-level simulation to divide the global event list into small sizes. A timing-focuses strategy based on max-plus algebra is applied in bottom-level local simulation to further reduce local event lists. With this new method it is possible to mimic real production systems fast and accurately within a reasonable computational time frame. The effectiveness and efficiency of the new simulation method are validated through experimental results.

Production Lead Time in Serial Lines: Evaluation, Analysis, and Control

Production Lead Time \mbi (LT) is the average time a part spends in the system, being processed or waiting for processing. In systems with unlimited buffers, \mbi LT may be orders of magnitude larger than the total processing time, leading to serious economic and quality problems. At present, no systematic analytical methods for evaluation, analysis, and control of \mbi LT in systems with machines having up- and downtime characterized by continuous random variables are available. This paper is intended to develop such methods. Specifically, we address synchronous serial lines with exponential machines and derive formulas for \mbi LT as a function of machine parameters and raw material release rate. Using these formulas, we develop methods for open- and closed-loop raw material release, which result in the desired \mbi LT. For asynchronous exponential lines, we provide an upper bound on LT. For non-exponential lines (e.g., Weibull, gamma, and log-normal), we offer an empirical formula for \mbi LT as an affine function of the coefficient of variation. The results reported in this paper enable a new paradigm for production systems management, namely: manage a production system so that the desired \mbi LT is ensured, while the throughput is maximized. This paper analyzes production lead time in manufacturing systems with hardware-unlimited buffers. The main practical insights obtained are as follows. . The lead time as a function of the raw material release rate has a “knee-type” behavior. . Releasing raw material beyond the knee results in no noticeable increase of the throughput, but in unlimited increase of the lead time. . To maintain the lead time close to the knee (and, thus, maximize the throughput), a simple control law may be used: release raw material on, say, hourly or shift-type basis, if the total work-in-process is below a certain threshold (provided in the paper) and do not release otherwise.

Equilibria, Stability, and Transients in Re-Entrant Lines Under FBFS and LBFS Dispatch and Constant Release

A model of a re-entrant line, consisting of the bottleneck workcenter and time delays representing other workcenters, is considered. Its mathematical description is provided and performance metrics are introduced. The steady states of this model and their stability properties are investigated under two dispatch policies-first buffer first served (FBFS) and last buffer first served (LBFS)-and under constant release rate. The transients due to machine downtime are also analyzed. It is shown that, although LBFS may be viewed as having superior steady-state characteristics, it induces longer and more volatile transients than FBFS and, in some cases, periodic and chaotic regimes.

Formulation and a Simulation-Based Algorithm for Line-Side Buffer Assignment Problem in Systems of General Assembly Line With Material Handling

In systems of general assembly line with material handling, line-side buffers need to be carefully assigned to a limited number of material delivers (drivers) for part delivery to avoid production stoppage due to material shortage. Such a problem is referred to as line-side buffer assignment problem (LBAP). In this paper, we focus on fixed zoning version of LBAP. We formulate the problem, prove its NP-hardness, and propose an algorithm based on two structural characteristics of the LBAP problem-one being the analogousness between our problem and the parallel machine scheduling (PMS) problem and the other being the monotonicity of the system throughput in the course of assigning line-side buffers to drivers. The developed algorithm globally converges with probability one when there exist feasible assignments. The algorithm is tested on a real system, and the results show that it is effective for solving the LBAP problem.

A Unified Effective Method for Aggregating Multi-Machine Stages in Production Systems

In production lines, multi-machine stages are pervasive in factory. In order to take advantage of analytical methods developed for transfer lines and assembly lines to analyze, optimize, and design the lines with multi-machine stages, a usual way is to aggregate each of such stages into a single machine. Aggregation results of existing methods may depend on the order of aggregation, which makes the choice of the aggregation order an open problem. In addition, existing exact aggregation methods are limited to two- or three-stage production lines due to the curse of dimensionality; for the approximate methods, aggregation errors sometimes are unacceptable. Inspired by some existing methods, in this paper, we introduce the steady-state equivalence of production lines, based on which we develop a unified effective method to figure out parameters of a single machine which is equivalent to the multi-machine stage. The developed aggregation method is independent of the aggregation order. It is tested on numerous series-parallel example lines and a practical general assembly line. Extensive experiments show that under various machine reliability models with the coefficient of variation of failure and repair times less than 3, aggregation errors of the throughput are less than 5%.

Efficient simulation for serial production lines based on aggregated event-scheduling

Performance evaluation of complex manufacturing systems is very challenging. In many cases, simulation is one of the most important approaches available. For a manufacturing system with serial production lines, e.g. general assembly in automotive industry, the synchronous dependent machines have great impact on system performances, and increase the complexity in modeling and simulation. In this paper, we focus on developing an efficient simulation approach for a serial production line with synchronous dependent machines, where the traditional simulation methods may suffer from computation intensity. Making use of the partial system decomposability, we introduce an aggregated event-scheduling simulation approach for keeping the global event list in small size. Each section of the production line are simulated locally and synchronized through the global system states and events. A novel dividing mechanism based on max-plus algebra is introduced to guarantee the equivalence of the aggregated local simulation to the original problem. With this new approach it is possible to mimic real production systems fast and accurately within a reasonable computational time frame. Experiment results are presented to demonstrate the effectiveness and efficiency of our new simulation approach.

Cellular Production Lines With Asymptotically Reliable Bernoulli Machines: Lead Time Analysis and Control

Cellular lines are production systems consisting of cells comprised of machines performing similar operations. These systems are notorious for having excessively long lead time (LT) that are often an order of magnitude longer than the total processing time by all machines in the system. The main goal of this paper is to carry out an analytical investigation of this phenomenon and offer methods for its alleviation. To accomplish this, we develop a technique for performance evaluation of cellular lines with asymptotically reliable Bernoulli machines and use it for analysis and control of LT.

Permutation flow shop scheduling with delay time under time-of-use electricity tariffs

Worldwide energy crisis and global economic depression make reducing electricity costs has become a focus for many factory managers. This paper set up a Permutation Flow Shop Scheduling (PFSS) model with delay time, which means adding delay time between different jobs to make processing period more flexible. Considering the time of use (TOU) electricity tariff, the PFSS model using the TOU electricity tariffs is established. Meanwhile, the shutdown state is introduced for the proposed model to improve the performance. The proposed PFSS model is solved by genetic algorithm with constructing the new chromosome coding. The experiment demonstrates the new PFSS model can reduce electricity cost by 16.83% and realize the purpose to save electricity cost for factories.

Cellular production lines with asymptotically reliable Bernoulli machines: Performance analysis and lead time control

Cellular lines are production systems consisting of cells comprised of machines performing similar operations. These systems are notorious for having excessively long lead time (LT), often orders of magnitude longer than processing time by all machines in the system. The main goal of this paper is to provide a plausible explanation of this phenomenon and offer methods for LT reduction. To accomplish this, the paper develops a technique for performance analysis of cellular lines with asymptotically reliable Bernoulli machines and uses it to analyze and control LT. It shows, in particular, that robustness of LT is a decreasing function of the number of machines in a cell. Thus, in cells with many machines, small variations in either release rates or machine efficiencies may lead to dramatic increases of LT, if operating points are not selected appropriately. A method for selecting appropriate operating points and, thus, controlling release rates and LT, is provided.