Constant Work In Process Research Articles

Service-level performance of MRP, kanban, CONWIP and DBR due to parameter stability and environmental robustness

Decisions regarding production planning and control strategy (PPCS) choices can be classified as strategic, whereas parametrization issues are of a tactical nature. However, readjustment is often skipped either as a result of a lack of planning expertise or because it would require extended planning. For this reason, robustness, which is defined as PPCS behaviour within dynamic environments, is investigated. To achieve a greater understanding of the sensitivity on parameter changes in a production system, PPCS stability is examined. An eM-Plant based simulation model is presented that discusses the service-level performance of material requirement planning (MRP), kanban, constant work in process (CONWIP) and drum–buffer–rope (DBR) in a flow-shop with attention to the work in process (WIP). Although the service-level performance of CONWIP exceeds that of the other systems, CONWIP struggles to maintain its advantage under dynamic conditions. The paper seeks to support industrial practitioneers both in their choice of a specific PPCS and to parametrize the PPCS successfully.

Stochastic analysis and design of CONWIP controlled production systems

PurposeThis paper aims to present a model of a multi‐phase multi‐product manufacturing system considering a CONstant work‐in‐process (CONWIP) control mechanism and using continuous‐time Markov chain modelling approach.Design/methodology/approachThe model includes defining a state space then constructing the rate matrix, which contains the transition rates, followed by formulating the transition matrix. The time‐dependent probabilities that a product is in a particular state at a certain time are characterized. Performance measures related to the statistics on the waiting time and average number of work‐in‐process in the production system have been determined. Consequently, a numerical example is presented to illustrate the computations of different model aspects.FindingsThe analyses explain a foundation needed for analyzing the steady state behavior of manufacturing systems. Results have shown how production data can be easily modified for what‐if analyses by the use of Excel add‐in tool.Practical implicationsThe multi‐level model outlines a framework that provides a practical tool for production engineers seeking to enhance the performance of their production system by selecting the best order release mechanism.Originality/valueA novel aspect of the work reported in this paper is the application of Chapman‐Kolmogrov mathematics and CONWIP ordering theory, which is developed for evaluating and managing CONWIP controlled production systems.

Tracking production rate of constant work-in-process controlled job shop

A multiproduct class, multiworkstation Constant Work-In-Process (CONWIP) controlled job shop is formulated and analysed by applying the Markovian network model, in such a way that yields a feasible level of CONWIP. A new approach for analysing the job shop network model was presented and routing probability matrix and model solutions were derived and presented. Consequently the job shop ? as a single unit ? is formulated as (M/M/1: GD/∞/∞) queue and solved in such a way that matches production capacity of the shop floor with the outside customer demand under CONWIP production scheme. Numerical example is presented to illustrate the computations of different model parameters. It is concluded that the developed analytical approach, in this study, realised our objectives of achieving a good trade-off between customer demand and CONWIP level in the CONWIP controlled production environment that uses a hybrid push/pull production authorisation policy.

Response surface methodology based simulation optimization of a CONWIP controlled dual resource constrained system

We propose an optimal worker flexibility policy for a CONWIP (constant work-in-process) controlled dual resource constrained (DRC) system. A critical issue in DRC systems is the dynamic assignment of workers to the workstations. The proposed worker flexibility policy is designed to evaluate the state of the system at periodic time intervals so that the dynamic assignment of workers to the workstations can be made in an efficient manner. By integrating the response surface methodology (RSM) with simulation, this study provides an approach for optimizing worker transfer decisions. The RSM-based simulation optimization approach adopted in this study provides an effective and robust procedure to determine the optimum level of this decision variable under the given operating conditions.

Simulation study using system dynamics for a CONWIP-controlled lamp supply chain

Production inventory control is one of the most important elements of manufacturing logistics. CONstant Work In Process (CONWIP), which is a hybrid of push and pull type systems, offers an effective approach to supply chain (SC) management. The study of CONWIP is very often conducted with detailed modeling and mathematical analysis, and is not appropriate for supporting decisions from a managerial or macro perspective. In this paper, the authors present an alternative analysis of CONWIP at an aggregate level, which is appropriate to the decision-making about the SC. The model is simulated using system dynamics (SD). The implementation of this model is described using an example from the lamp manufacturing industry in mainland China. The method and its results are illustrated with a lamp manufacturing SC, which indicates that SD can be used to analyze and understand the CONWIP approach to achieve higher productivity and better performance. Moreover, SD is more suitable for simulation analyses of the SC system.

A review of production planning and control: the applicability of key concepts to the make-to-order industry

The paper reviews ‘classic approaches’ to Production Planning and Control (PPC) such as Kanban, Manufacturing Resource Planning (MRP II) and Theory of Constrains (TOC), and elaborates upon the emergence of techniques such as Workload Control (WLC), Constant Work In Process (CONWIP), Paired cell Overlapping Loops of Cards with Authorization (POLCA) and web- or e-based Supply Chain Management (SCM) solutions. A critical assessment of the approaches from the point of view of various sectors of the Make-To-Order (MTO) Industry is presented. The paper considers factors such as the importance of the customer enquiry stage, company size, degree of customization and shop floor configuration and shows them to play a large role in the applicability of planning and control concepts. The paper heightens the awareness of researchers and practitioners to the PPC options, aids managerial system selection decision-making, and highlights the importance of a clear implementation strategy. WLC emerges as the most effective Job Shop solution; whilst for other configurations there are several alternatives depending on individual company characteristics and objectives. The paper outlines key areas for future research, including the need for empirical research into the use of Workload Control in small and medium sized MTO companies.

Comparing CONWIP, synchronized CONWIP, and Kanban in complex supply chains

Three types of JIT ordering systems used for supply chain management were quality tested, the Kanban, the original constant work-in-process (CONWIP), and a synchronized CONWIP. The synchronized CONWIP system is described for its ability to handle complicated supply chains, which consist of assembly stages with different lead times. In the system, orders for each process are released while adjusting the lead time of the subsequent process, and the released and processed orders are synchronized during the assembly stages. Practical questions, that is, which system is superior and which parameter affects superiority, were investigated for the three types of systems.

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.

Benefits of Skill Chaining in Serial Production Lines with Cross-Trained Workers

To gain insight into the potential logistical benefits of worker cross-training and agile workforce policies, we study simple models of serial production systems with flexible servers operating under a constant work-in-process (CONWIP) release policy. Two important and interrelated issues are: (a) how to decide which skill(s) are strategically most desirable for workers to gain, and (b) how to coordinate these workers to respond dynamically to congestion. We address these by considering two cross-training strategies: a straightforward capacity-balancing approach, which we call cherry picking (CP), and an innovative overlapping zone strategy that we call skill chaining. Our comparison shows that skill-chaining strategies have the potential to be robust and efficient methods for implementing workforce agility in serial production lines.

Coexistence of JIT and MRP in a Manufacturing Cell

Material Requirement Planning (MRP) was the computerization of the conventional manufacturing procedure, or push system. Justin-Time (JIT) manufacturing is the pull system of manufacturing. Constant work in process is a part of MRP, and elimination of work in process is the aim of JIT. This study explores the coexistence of MRP and JIT when a manufacturing organization tries to implement JIT, with MRP becoming the checkpoint.

Job order releasing and throughput planning for multi-priority orders in wafer fabs

To meet the production target of multi-level (multiple priority rank) orders in wafer fabs, this paper uses a hierarchical framework based on a mathematical model, and without the assistance of any simulation tool, to build a production scheduling system to plan wafer lot releasing sequence and time. This system first applies capacity loading analysis to set up the batch policy for each level (rank) of orders. Next, the production cycle time of each product level is estimated with considerations of batching and loading factor. The cycle time is then used to derive system control parameters such as the most appropriate level of work in process (WIP) and the number of daily operations on the bottleneck workstation. Lastly, a Constant WIP mechanism is applied to establish a wafer release sequence table and a throughput timetable. The due date designation for each specific order can hence be confirmed. With the comparison with the result of simulation, it shows that under the designed system the performance and planning measures in the master production schedule can be drawn up quickly and accurately, and the system throughput target and due date satisfaction can be achieved. Overall, the proposed production scheduling system is both effective and practicable, and the planning results are supportive for good target planning and production activity control.

Managing multi-project environments through constant work-in-process

This paper proposes new project control mechanisms that limit the number of active projects in multi-project environments. Incoming projects first enter a backlog list. Then, they are staggered into a network of inter-related resources in a way that maintains stable load on the system. The proposed mechanisms adapt the concept of constant work-in-process (CONWIP) that was proposed earlier in the context of production management. We report on an extensive set of simulations that were conducted with several types of projects in dynamic settings. We discuss possible preference rules for the backlog list and the resource queues within the system and demonstrate the superiority of a new rule whose objective is to achieve a dynamic balance of the loads of the various resources.

A CONWIP model for FMS control

Production inventory control is one of the most important aspects of a flexible manufacturing system (FMS) design. CONstant Work In Process (CONWIP), which is a hybrid of push-and-pull type systems, offers an alternative to effective utilization of the expensive FMS equipment while still meeting customer requirements. In the selection of an FMS control method, material handling often becomes one of the capacity constraints which forms the basis of various research interests. In this paper, a structure-based model for a CONWIP-controlled FMS is proposed, and within it, the node type characteristics concept is used to describe the constraints in FMS. Furthermore, simulation is used to determine the card number based on the structure-based model. The simulation results demonstrate that the model is suitable for the design and operation of FMS. The model can be used as a manufacturing execution system of enterprise resources planning. An architecture for this integrated design based on Internet/Intranet systems is also proposed.

A Dynamic AGV Dispatcher for a C-CONWIP Controlled Job Shop

Abstract A Fuzzy Logic Controller (FLC) for dispatching Automated Guided Vehicles (AGVs) in a job shop that operates under a Capacitated Constant Work In Process (C-CONWIP) control policy is presented. The FLC utilizes the available information about the state of the job shop and suggests a real time dispatching decision. The design of the FLC is modular and the controller uses 11 rules. For an automated job shop the proposed dispatching procedure is compared with the Modified First Come First Serve (MFCFS) dispatching rule under various conditions. The simulation results show that the proposed FLC statistically outperforms the MFCFS dispatching rule.

Performance opportunity for workforce agility in collaborative and noncollaborative work systems

To gain insight into the potential logistical benefits of worker cross-training and agile workforce policies, we study simple models of flexible workers in serial production systems. The primary control issue is how to assign workers to jobs/stations over time. Under assumptions of complete worker flexibility and collaborative work, we prove that a simple expedite policy minimizes along each sample path the cycle time (delay) for each job. Therefore, the expedite policy also minimizes work in process and maximizes throughput along every sample path. We also compute the performance improvement opportunity achievable using flexible workers as opposed to the optimal static allocation of workers. This enables us to examine th e factors that make workforce agility a potentially attractive strategy. We also consider the intuitive analog of the expedite policy for the noncollaborative work environment, which we call the pick-and-run policy; however, we demonstrate by counterexample that it is not always optimal. Finally, we extend some of our insights from the demand-constrained environment to a capacity-constrained environment operating under a CONstant WIP (CONWIP) protocol

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.

Performance opportunity for workforce agility in collaborative and noncollaborative work systems

To gain insight into the potential logistical benefits of worker cross-training and agile workforce policies, we study simple models of flexible workers in serial production systems. The primary control issue is how to assign workers to jobs/stations over time. Under assumptions of complete worker flexibility and collaborative work, we prove that a simple expedite policy minimizes along each sample path the cycle time (delay) for each job. Therefore, the expedite policy also minimizes work in process and maximizes throughput along every sample path. We also compute the performance improvement opportunity achievable using flexible workers as opposed to the optimal static allocation of workers. This enables us to examine the factors that make workforce agility a potentially attractive strategy. We also consider the intuitive analog of the expedite policy for the noncollaborative work environment, which we call the pick-and-run policy; however, we demonstrate by counterexample that it is not always optimal. Finally, we extend some of our insights from the demand-constrained environment to a capacity-constrained environment operating under a CONstant WIP (CONWIP) protocol.

Performance evaluation and design of a CONWIP system with inspections

In this paper, we analyze a CONWIP (Constant Work-In-Process) system which consists of three stations in series. We assume that raw parts are always available. When a finished part is consumed by a demand, a raw part is released immediately and gets processed at each station sequentially. The processings do not always meet the requirement of quality. Therefore, at the end of the processing at each station, a part is inspected randomly to determine if it satisfies the requirement of quality. We assume that the inspection time is negligible. If a part is inspected and found to be defective because of treatment at station j, the part is fed back to station j to be treated again. We consider that the stations have an exponential or a N-stage Coxian service time distribution. We propose an analytical method to evaluate the performance of this kind of system. We consider two cases: one is the saturated case where the supply of demands is infinite and the other is the non-saturated case where external demands arrive according to a Poisson process. Then we use the analytical method to show how the optimal conwip system can be designed for a chosen design criterion. Although we present the method only for three-station systems in this paper, it can be easily extended to systems with more than three stations.

Generating a backlog list for a CONWIP production line: A simulation study

A CONWIP (CONstant Work In Process) line is a flow line in which the WIP level in the line is held constant by synchronizing releases to departures. One of the first decisions that CONWIP users must address is to generate a backlog list, which gives the sequence of orders to be introduced into the line. This study uses both two-stage (group scheduling) heuristics and single-stage heuristics to generate the backlog list, and then utilizes simulation to compare the performance of the two types of heuristics. Two-stage heuristics attempt to serially process similar orders and eliminate major set-ups required between part types. Results of the paired t-test analysis show that two-stage heuristics outperform single-stage heuristics for various measures. The superior performance of the two-stage heuristics is due to the reduction in major set-up times. The simulation results can provide guidance for schedulers in the selection of preferable heuristics for relevant CONWIP lines.

Finite Schedule Monitoring and Filtering in a Computer Integrated Manufacturing Environment

An efficient and effective method of filtering is described for reactive scheduling in an integrated manufacturing environment. The filter evaluates the significance of schedule deviations in near-real-time and gives a statistical estimate of the importance of each schedule deviation; Highly important deviations being those that prevent schedule objectives from being achieved. A typical result from STC filtering would be an indication that there is a 95% probability of failing to meet the schedule if the resource were to continue working at the same rate. The technique is well suited for bottle-neck resources but may be applied to all resources in an enterprise. The filter, based on statistical throughput control (STC) is compared to past work in DEDS and reactive scheduling. Model formulation is presented to show extensions from previous applications of STC for short-term production control in a CONWIP (Constant WIP) production system. Data requirements are detailed with reference to Business Object Documents (BODs) defined by the Open Applications Group (OAG). A summary of the CIIMPLEX framework is presented to show how the filter is applied.