Workflow analysis of production lines with complete inspection and rework loops

We study the flows induced by different rework loops in serial manufacturing systems with inspection stations. Average values of these flows and queuing network formulas are used for performance evaluation and optimisation of production lines. An application is presented for solving jointly the problems of inventory control and inspection station allocation in a CONWIP production line.     

Controlling shop floor operations in a multi-family, multi-cell manufacturing environment through constant work-in-process

This paper discusses pertinent issues in applying CONstant Work-In-Process (CONWIP) principles to control shop floor operations in a manufacturing environment characterized by several product families processed along different routes in several production cells. The approach we take is to simultaneously answer two major questions: (1) what is the best WIP level? and (2) how to arrange the backlog list for a given system? The problem is posed as a mathematical programming model and solved via a simulated annealing heuristic. We design an experiment that captures essential elements of the systems under investigation. We then execute an extensive simulation to evaluate the effectiveness of various control schemes in a multi-cell, multi-family production environment. Specifically, we compare two variants of CONWIP control, one where containers are restricted to stay within given cells all the time and the other where containers are allowed to move through the entire system. We demonstrate the superiority of the latter in all the simulated scenarios.     

Selection of a pull production control system in multi-stage production processes

In this paper, we conduct an analytical comparison of three pull production control systems: Kanban, CONWIP and Base-stock in multi-stage production processes. First, we compare the three control systems in a multi-stage serial production process. Then, we compare them in multi-stage assembly production processes, and present guidelines that allow us to select the best system. As a result, we show which structural parameters decide the superiority of one control scheme to the others, and how they are related. A key for superiority is a configuration of parameters, such as processing times and number of cards employed in the system. We show that there is no general superiority amongst the analysed concepts. Finally, we verify the effect of variability on the system performance, and generalise the analytical results of deterministic cases by conducting numerical experiments.     

Comparison of DBR with CONWIP in an unbalanced production line with three stations

The recently developed alternatives to traditional production planning and control systems such as material requirement planning (MRP) and Kanban are the drum–buffer–rope (DBR) and CONWIP (CONstant Work In Process) systems. Each system is best described as a combination push (like an MRP)/pull (like a Kanban) logistical procedure. Materials are pulled into the shop via the appropriate logic, and once released, materials are then pushed to subsequent workcentres. The performance of the DBR and CONWIP control policies are analysed and compared in a three-stage unbalanced tandem production line. Using a continuous Markov process model, steady-state probability distributions for the systems are derived, and then the performance measures of the systems can be evaluated. To compare the two systems, an optimization model for each system is proposed. From sensitivity analyses for the optimization models, the proposed models are validated, the differences of the two systems are investigated, and it is found that DBR is better than CONWIP under the proposed performance measures.     

A mathematical programming model for production planning using CONWIP

Push-based MRP and pull-based Kanban systems are effective planning tools for a wide range of manufacturing production. Both of them, however, have certain limitations when they are implemented in different production environments. In recent years, CONWIP (CONstant Work-In-Process) has been proposed and studied to take advantage of MRP and Kanban systems for optimal work-inprocess (WIP) inventory control. In this paper, an integer nonlinear mathematical programming model was developed to determine an optimal production sequence and lot sizes in a CONWIP production line. The nonlinear programming model was linearized and solved directly for a number of illustrative example problems.     

The extended kanban control system for production coordination of assembly manufacturing systems

In assembly manufacturing systems there are points in the production process where several component parts are put together in areas called assembly cells so as to form more complex parts called subassemblies. In this paper, we present and compare two variants of the Extended Kanban Control System (EKCS) - a recently developed pull production control mechanism that combines base stock and kanban control - for the production coordination of assembly manufacturing systems. In both variants, the production of a new subassembly is authorized only when an assembly kanban is available. Assembly kanbans become available when finished subassemblies are consumed. If an assembly kanban is available, in the first variant, each component part of a subassembly is released into the assembly cell as soon as itis available (independent release). In the second variant, however, it is released only when allother component parts also become available (simultaneous release). In both variants, when a component part is released into the assembly cell, it releases its kanban, thus authorizing the production of a new component part.     

Bottleneck detection of complex manufacturing systems using a data-driven method

Bottlenecks within a production line significantly affect system productivity. Most current bottleneck detection schemes focus on the long-term bottleneck detection problem using an analytical or simulation model. Furthermore, these studies are restricted to serial tandem lines only. This research focuses on extending the newly developed data-driven method for throughput bottleneck detection from a serial line to a manufacturing system with a complex layout. Within these complex systems, two specific layouts are considered: the concurrent process and the closed-loop feedback process. The method is verified using simulation case studies. An industrial case study is examined to demonstrate the practicality of this approach and to validate the efficiency of the proposed bottleneck detection method.     

A simulation study of CONWIP assembly with multi-loop in mass production,multi-products and low volume and OKP environments

This paper studies the performance of constant work-in-process (CONWIP) assembly system with multi-loop in mass production, multi-products and low volume and one-of-a-kind production (OKP) environments using simulation. We propose five basic design patterns of CONWIP loop and develop eight control policies of CONWIP loop based on the design pattern for standard assembly system. The performance of developed loop policies is evaluated in three production environments. In particular, control policies of CONWIP loop in OKP environment provide a valuable reference for OKP shop floor controlling. A heuristic algorithm of searching work-in-process (WIP) upper bound, the deadlock phenomenon in CONWIP assembly system and suggestion are introduced specifically. The summary of CONWIP installation guidelines in the mixed assembly system can apply CONWIP theory to practise.     

A comparison of alternative kanban control mechanisms. II. Experimental results

This paper continues our study of the design of inventory control policies for serial systems. Using simulation, we compare alternate control policies where a target of mean throughput has to be met under three different objectives - minimize the maximum inventory, minimize the average inventory and minimize the variance of the output - for balanced lines and lines with one bottleneck,

The main objectives of this paper are: (1) to study several objectives not amenable to sample path analysis and (2) to show some sets of conditions and objectives under which Constant-Work-in Progress (CONWIP) is preferred over the traditional kanban mechanism, and vice-versa.     

Impact of work teams: a comparison study of assembly cells and assembly line for a variety of operating environments

This research compares assembly lines with different cellular, team working environments. Organization of an assembly system using cells requires worker flexibility where each worker is allowed to help his or her co-workers in the same cell and often to undertake a number of different tasks within the cell. We compare two different cellular systems with an unpaced, serial assembly line. The manufacturing systems are initially compared through an exploratory Markovian approach. Subsequently, the relative performance levels of the systems are examined across a variety of operating environments using simulation models. Operating environments vary based on shop size, setup times and processing time variances. The results indicate the general superiority of the cellular systems over the assembly line. However, some interesting observations are drawn for cases where the assembly line outperforms the cells. One of the cases is where workers realize substantial benefits in terms of performing their tasks in less time due to specialization on the tasks. The other scenario is where working in teams make the overall completion of the tasks more inefficient compared to working individually. Specific cross-over points are analyzed where the assembly lines start performing better than the assembly cells under these scenarios. The conclusions highlight guidelines for practicing managers on the most appropriate system under a given set of operating conditions.     

Intelligent dynamic control policies for serial production lines

Heuristic production control policies such as CONWIP, kanban, and other hybrid policies have been in use for years as better alternatives to MRP-based push control policies. It is a fact that these policies, although efficient, are far from optimal. Our goal is to develop a methodology that, for a given system, finds a dynamic control policy via intelligent agents. Such a policy while achieving the productivity (i.e., demand service rate) goal of the system will optimize a cost/reward function based on the WIP inventory. To achieve this goal we applied a simulation-based optimization technique called Reinforcement Learning (RL) on a four-station serial line. The control policy attained by the application of a RL algorithm was compared with the other existing policies on the basis of total average WIP and average cost of WIP. We also develop a heuristic control policy in light of our experience gained from a close examination of the policies obtained by the RL algorithm. This heuristic policy named Behavior-Based Control (BBC), although placed second to the RL policy, proved to be a more efficient and leaner control policy than most of the existing policies in the literature. The performance of the BBC policy was found to be comparable to the Extended Kanban Control System (EKCS), which as per our experimentation, turned out to be the best of the existing policies. The numerical results used for comparison purposes were obtained from a four-station serial line with two different (constant and Poisson) demand arrival processes.     

From loop structure to policy-making: a CONWIP design framework for hybrid flow shop control in one-of-a-kind production environment

A feasible constant work in process (CONWIP) policy can guide developer to better implement CONWIP system. The feasible policy should be selected from alternatives by evaluation. Therefore, how to generate more than one CONWIP alternative policy to evaluate is an inevitable problem in CONWIP practice. From the perspective of loop structure, we propose CONWIP design framework (CDF) which is a systematic design approach to obtain CONWIP alternative policies. The basic concepts and components for CDF are discussed in this paper. Based on CDF, we make 10 CONWIP alternative policies for hybrid flow shop in one-of-a-kind production environment, and these alternative policies are evaluated by simulation. The simulation result implies that (i) the CONWIP alternative policy with robustness has the potential to cope with more fluctuations in high-variety production environment; (ii) a better design for CONWIP policy will be able to enhance the system performance in practice; and (iii) the loop structure can serve as a parameter of CONWIP.     

Throughput analysis of production systems: recent advances and future topics

Throughput analysis is important for the design, operation and management of production systems. A substantial amount of research has been devoted to developing analytical methods to estimate the throughput of production systems with unreliable machines and finite buffers. In this paper we summarise the recent studies in this area. In addition to the performance evaluation of serial lines, approximation methods for more complex systems, such as assembly/disassembly systems, parallel lines, split and merge, closed-loop systems, etc., are discussed. Moreover, we propose future research topics from the automotive manufacturing systems perspective.     

Multiproduct Kanban-like Control Systems

Kanban Control Systems are widely used to control the release of parts in multistage manufacturing systems operating under a pull mechanism, i.e. production of new parts is triggered based on the actual demands arriving to the output of the system. Most of the work on Kanban Control Systems deals with single-product manufacturing systems, although Kanban Control Systems have also been used in multiproduct environments. The aim is to study in detail the behaviour of Kanban Control Systems in the context of multiproduct manufacturing systems. In addition to the classical Kanban Control System, we also consider two more general Kanban-Like Control Systems known as Generalized Kanban Control System and Extended Kanban Control System. When extending these pull control mechanisms to multi-product systems, we distinguish two cases, whether kanbans are dedicated to a single part type or shared among the different part types. We provide an in-depth analysis of each of the resulting control mechanism and discuss some important issues arising from the study.     

Release policies for assembly systems

We consider a production system consisting of several fabrication lines feeding an assembly machine. The machines in the fabrication lines and at assembly are assumed to have general processing time distributions. Releases to the system are governed by the kanban release mechanism. We first derive a heuristic for the throughput of this system by replacing the assembly system under kanban release by an equivalent system under CONWIP release and by making use of an approximation for the throughput of a CONWIP assembly system (Duenyas, 1992). Comparisons with simulation show that this heuristic is robust over a wide range of conditions.

We also address the issue of which release mechanism is more effective in obtaining a desired throughput level with the minimum possible work-in-process inventory level in the system. We present both analytical and simulation results to demonstrate that the CONWIP release policy seems to be a more effective release policy for assembly systems.     

A comparison of alternative kanban control mechanisms. I. Background and structural results

The design of inventory control policies for serial systems is a topic currently being explored by a number of researchers. Our goal -in two papers - is to synthesize and extend some of these efforts. We consider, simultaneously, four sources of variability in production lines - processing time variability, machine breakdowns, rework and yield loss - and show some similarities and differences in their effects on the performance of the line. In this paper we introduce an enhanced model that accommodates raw material and demand uncertainity and demonstrate that many of the sample path results obtained previously can be extended.

The main objectives of this paper are: (1) to demonstrate that Constant-Work-in-Process (CONWIP) and the traditional kanban control are just two extremes in a finite family of implementable pull controls; (2) to show that while machine breakdowns, rework and random processing times have a similar effect in terms of the optimal decisions, yield losses in the line may have to be managed differently.     

A comparative analysis of an MRP lot-for-lot system and a Kanban system for a multistage production operation

Japanese production methods and operational control techniques have been the subject of much recent attention from American production managers and the media. In this paper the Kanban aspect of the 'just-in-time' concept is analysed in order to ascertain the potential for scheduling in an ill-structured production operation and to compare its effectiveness with the traditional MRP lot-for-lot scheduling and control procedure. The analysis will be conducted using a hypothetical production operation that includes multiple workcentres, machines and product structures, and serial and assembly operations. Simulation model results are discussed and principles for selecting the more cost-effective system are outlined for the general MRP lot-for-lot versus Kanban scenarios.     

Scheduling manufacturing systems for delayed product differentiation in agile manufacturing

Production of customized products to respond to changing markets in a short time and at a low cost for agile manufacturing can be implemented with delayed product differentiation in a manufacturing system. The successful implementation of delayed product differentiation lies in efficient scheduling of the manufacturing system. Scheduling problems in implementing delayed product differentiation in a general flexible manufacturing system are defined, formulated and solved here. The manufacturing system consists of two stages: machining and assembly. At the machining stage, a single machine is used to produce standard component parts for assembly products. These parts are then assembled at the assembly stage by multiple identical assembly stations to form customized products. The products to be produced in the system are characterized by their assembly sequences represented by digraphs. The scheduling problem is to determine the sequence of products to be produced in the system so that the maximum completion time (makespan) is minimized for any given number of assembly stations at the assembly stage. Based on the representation of assembly sequence of the products, three production modes are defined: production of a single product with a simple assembly sequence ; production of a single product with a complex assembly sequence ; and production of N products . According to the three defined production modes, the associated scheduling problems are defined as G s scheduling problems, G c scheduling problems and N-product scheduling problems, respectively. Optimal and heuristic methods for solving the scheduling problems are developed. The computational experiment shows that the heuristics provide good solutions to the scheduling problems.     

One-piece flow manufacturing on U-shaped production lines: a tutorial

Now-a-days shorter product life cycles and increased demands for customization make it difficult to produce some products on traditional production lines. Often the best that can be done is to produce them in batch flow systems that have been improved through the incorporation of line flow principles. This is one-piece flow manufacturing. Traditional cells with irregular material flows are replaced by U-shaped production lines within which flow is regular and paced by a cycle time and between which flow is controlled by pull signals. This tutorial examines the research literature on one-piece flow manufacturing. It begins with the decisions rules that determine when one-piece flow is appropriate. Next the unique elements of one-piece flow (takt time, standard work, flow manufacturing on U-shaped lines, pull production, and jidoka) are reviewed. Then the mathematical models that are used to design one-piece flow systems are examined. Finally areas where more research is needed are discussed.     

Modeling arrival-to-departure sequence disorder in flow-controlled manufacturing systems

Flow control policies, such as clear the largest buffer first, have been used extensively in mediating server access between multiple job-arrival processes within manufacturing systems requiring significant set-up times between job types. Although the stability and performance characteristics of clearing policies have been studied, how these clearing policies affect the growth in arrival-to-departure sequence disorder across a machine or workcell is still an area for further research. In this paper, a closed-form model is formulated that characterises the steady-state average arrival-to-departure sequence disorder experienced by jobs processed through a stable flow controlled manufacturing system. Specifically addressed are the effects of upstream sequence disorder on the work-in-process of a downstream assembly process. A simulation study is used to validate the model’s accuracy and applicability in predicting steady-state average arrival-to-departure sequence disorder. A simulation study of an assembly process shows that by balancing upstream steady-state average sequence disorder, downstream work-in-process can be minimised while under a logical component grouping constraint.