Kanban number optimisation in a supermarket warehouse feeding a mixed-model assembly system

Following just-in-time principles, a growing number of manufacturers are adopting the so-called supermarket concept. Supermarkets are decentralised storage areas scattered throughout the shopfloor that serve as an intermediate store for parts required by nearby assembly lines. From these stores, a certain number of handling operators deliver parts from the supermarket to, and collect empty bins from, assembly stations. Finally, they return to the supermarket and are refilled for their next tours. The assembly stations are typically refilled from the supermarket through the constant replacement of the consumed parts pulled by the kanban system. Considering a mixed model assembly system composed of different assembly lines, feeding problems can occur as an effect of the replenishment lead time, of the production mix variation, of the commonality between the different models assembled. The aims of this paper are (i) to highlight how the supermarket/multi-mixed assembly-line system presents specific attributes that prohibit the simple application of well-known kanban dimensioning formulations and (ii) to provide an innovative procedure to optimally set all decision variables related to such a feeding system.     

Algorithms for the design of a multi-stage adaptive kanban system

The traditional kanban system with a fixed number of cards does not work satisfactorily in an unstable environment. With the adaptive kanban-type pull control mechanism, the number of kanbans is allowed to change with respect to the inventory and backorder level. It is required to set the threshold values at which cards are added or deleted, which is part of the design. Previous studies used local search and meta-heuristic methods to design an adaptive kanban system for a single stage. In a multi-stage system the cards are circulated within the stage and their presence at designated positions signals to the neighbouring stages details concerning the inventory. In this work, a model of a multi-stage system adapted from a traditional and adaptive kanban system is developed. A genetic and simulated annealing algorithm based search is employed to set the parameters of the system. The results are compared with a traditional kanban system and signs of improvement are found. The numerical results also indicate that the use of a simulated annealing algorithm produces a better solution.     

Centralised vs. decentralised control decision in card-based control systems: comparing kanban systems and COBACABANA

Kanban systems are simple yet effective means of controlling production. Production control is decentralised or exercised locally on the shop floor, i.e. a downstream station signals to an upstream station that an item is needed. If items are always the same and known, then demands can be satisfied instantaneously from stock; but if items differ and are unknown, demands must first be propagated backwards from station to station before being satisfied. The former is defined as an inventory control problem and the latter as an order control problem. Handling the order control problem via kanban involves a decentralised card acquisition process (during which information is propagated from station to station) that is separated from the actual production process. COBACABANA (control of balance by card-based navigation), an alternative card-based solution, shares kanban’s control structure but centralises the card acquisition process. Evaluating the two systems therefore provides a unique opportunity to compare decentralised and centralised control. Using simulation, we demonstrate that it is specifically the centralised card acquisition process that allows COBACABANA to balance the workload across resources and thus to outperform kanban in an order control problem. This has major implications for research and practice.     

A control theoretical modelling for velocity tuning of the conveyor belt in a dynamic mixed-model assembly line

This paper proposes a control theoretical modelling to study dynamic behaviour of a mixed-model assembly line. First, an open-loop model is developed for the system, then examined via different conveyor’s velocity values. It is realised that the performance of the system is very sensitive to the velocity; therefore, a closed-loop (CL) model is developed taking feedback from the system. By the use of proportional-integral-derivative (PID) controller and SIMULINK, some interesting results are obtained applying CL model: regardless of the sequence of the products in the line, the total work-overload and idleness always equals to zero. Moreover, less working area within the workstation is required. Based on the statistical analysis, it is found that no significant increase in makespan is imposed by CL model. It is also shown that PID controller is robust not only to the disturbances of the velocity, also to the uncertainties in the assembly operation times. These results are supported by many numerical experiments dealing with different test problems, line configurations and sequences. Finally, using a discrete event simulation model, the proposed approach is applied into a seru production mode. Simulation results show that the feedback PID controller can deal with real-world assembly line problems, successfully.     

A simulation based approach for supply network control

We focus on the problem of strategic network design in the context of multi-site enterprises. We develop a simulation model for various supply network designs evaluation. The suggested model is generic in the sense that only input data modifications are needed to adapt the model to a given supply network structure, while the model's procedural logic stays unchanged. The model reproduces the dynamics of the totality of the supply chain activities. It takes into account control policies related to the inventory management and to the scheduling of production activities. It also carries out local optimisations by dynamically allocating the customers’ demands to the appropriate distribution site and the replenishment orders expressed by the distribution and production sites of the enterprise so as to minimise a global cost function. Simulation execution permits to compute various performance measures. These indicators inform the decision-maker about the profit of the enterprise and about its customer's service level. In this paper, we propose to detail the simulation model. Then, for a fictitious problem of supply network design based on two real life applications, we highlight the influence of the control policies on the performance of the studied enterprise.     

A dynamic approach for evaluating parameters in a numerical method

A new methodology for evaluating unknown parameters in a numerical method for solving a partial differential equation is developed. The main result is the identification of a functional form for the parameters which is derived by requiring the numerical method to yield ‘optimal’ solutions over a set of finite‐dimensional function spaces. The functional depends upon the numerical solution, the forcing function, the set of function spaces, and the definition of the optimal solution. It does not require exact or approximate analytical solutions of the continuous problem, and is derived from an extension of the variational Germano identity. This methodology is applied to the one‐dimensional, linear advection–diffusion problem to yield a non‐linear dynamic diffusivity method. It is found that this method yields results that are commensurate to the SUPG method. The same methodology is then used to evaluate the Smagorinsky eddy viscosity for the large eddy simulation of the decay of homogeneous isotropic turbulence in three dimensions. In this case the resulting method is found to be more accurate than the constant‐coefficient and the traditional dynamic versions of the Smagorinsky model. Copyright © 2004 John Wiley & Sons, Ltd.     

A multi-criteria model for evaluating design for manufacturability

The design for manufacturability (DFM) method is most effective when integrated with the new product development (NPD) process. Due to the focused nature of the associated product and process knowledge and the extensive effort required, there are many NPD situations where classical DFM techniques cannot be readily applied. In this paper, Pro-DFM a multi-criteria model for manufacturability analysis that identifies product realisation opportunities (PRO) for cost reduction is presented. The key assumption in Pro-DFM is that the NPD team has a baseline estimate of production costs, and the evaluation question is how DFM issues will affect the expected unit production cost. The Pro-DFM model analyses a new design in three different factors: part procurement and handling, product assembly fabrication processes, and inventory costs. Each of these is independently analysed using a hierarchy of multiple criteria and sub-criteria. This approach is found to be amenable to most NPD processes, and conducive to easy integration. Each evaluation sub-criterion is presented in the form of a simple query, which is associated with a set of responses that is flexibly anchored to a uni-dimensional scale. A case study example is used to demonstrate the DFM evaluation process and the derivation of the cost penalties.     

A data-driven fuzzy approach for predicting the remaining useful life in dynamic failure scenarios of a nuclear system

This paper presents a similarity-based approach for prognostics of the Remaining Useful Life (RUL) of a system, i.e. the lifetime remaining between the present and the instance when the system can no longer perform its function. Data from failure dynamic scenarios of the system are used to create a library of reference trajectory patterns to failure. Given a failure scenario developing in the system, the remaining time before failure is predicted by comparing by fuzzy similarity analysis its evolution data to the reference trajectory patterns and aggregating their times to failure in a weighted sum which accounts for their similarity to the developing pattern. The prediction on the failure time is dynamically updated as time goes by and measurements of signals representative of the system state are collected. The approach allows for the on-line estimation of the RUL. For illustration, a case study is considered regarding the estimation of RUL in failure scenarios of the Lead Bismuth Eutectic eXperimental Accelerator Driven System (LBE-XADS).     

A heuristic based on Vogel's approximation method for sequencing mixed-model assembly lines

Sequencing mixed-model assembly lines is a well researched topic in the literature. However, many methods that have been developed to solve this problem fail to cope with either the large size or the specific characteristics of real-life problems. In this paper, a heuristic is proposed that is derived from Vogel's approximation method for transportation planning. The heuristic is able to handle large and supposedly difficult problem instances. Sophisticated test scenarios considering real-life aspects were generated to evaluate the performance of the heuristic for realistic problem instances. It is shown that the proposed heuristic significantly outperforms priority rule-based methods and requires only reasonable computational effort.     

Integrated procedure of balancing and sequencing for mixed-model assembly lines: a multi-objective evolutionary approach

A mixed-model assembly line is a type of production line which is used to assemble a variety of product models with a certain level of similarity in operational characteristics. This variety causes workload variance among other problems resulting in low efficiency and line stops. To cope with these problems, a hierarchical design procedure for line balancing and model sequencing is proposed. It is structured in terms of an amelioration procedure. On the basis of our evolutionary algorithm, a genetic encoding procedure entitled priority-based multi-chromosome (PMC) is proposed. It features a multi-functional chromosome and provides efficient representation of task assignment to workstations and model sequencing. The lean production perspective recognises the U-shape assembly line system as more advanced and beneficial compared to the traditional straight line system. To assure the effectiveness of the proposed procedure, both straight and U-shape assembly lines are examined under two major performance criteria, i.e., number of workstations (or line efficiency) as static criterion and variance of workload (line and models) as dynamic criterion. The results of simulation experiments suggest that the proposed procedure is an effective management tool of a mixed-model assembly line system.     

A Petri net-based heuristic for mixed-model assembly line balancing problem of Type-E

To effectively respond to the changing market demands, a manufacturer should produce variety of products with small lots. Thus, multiple products (models) are assembled simultaneously on a same line. However, it is very challenging to balance such an assembly line. This paper conducts a study on balancing a mixed-model assembly line of Type E. To solve this problem, a coloured-timed Petri net model is developed to describe the task precedence relationship. Also, the optimisation problem is formulated as a mathematical programming model. Then, with the models, a two-stage heuristic algorithm is proposed to solve the problem. At the first stage, based on the Petri net model, a P-invariant algorithm (PA) is presented to minimise the number of workstations. At the second stage, a heuristic is proposed to further minimise the cycle time by combining the PA with a binary search algorithm (BSA). Performance of the proposed method is evaluated by an illustrative example and numerical experiments. It is shown that it works well in terms of both solution accuracy and computational efficiency for large size problems.     

A multi-decision genetic approach for workload balancing of mixed-model U-shaped assembly line systems

A mixed-model assembly line is a type of production line where a variety of product models similar in product characteristics are produced. As a consequence of introducing the just-in-time (JIT) production principle, it has been recognised that a U-shaped assembly line system offers several benefits over the traditional straight line system. This paper proposes a new evolutionary approach to deal with workload balancing problems in mixed-model U-shaped lines. The proposed method is based on the multi-decision of an amelioration structure to improve a variation of the workload. This paper considers both the traditional straight line system and the U-shaped assembly line, and is thus an unbiased examination of line efficiency. The performance criteria considered are the number of workstations (the line efficiency) and the variation of workload, simultaneously. The results of experiments enhanced the decision process during multi-model assembly line system production; thus, it is therefore suitable for the augmentation of line efficiency in workstation integration and simultaneously enhancement of the variation of the workload. A case study is examined as a validity check in collaboration with a manufacturing company.     

A rough set approach for evaluating vague customer requirement of industrial product-service system

This research mainly focuses on the customer requirements’ evaluation in the early development of industrial product-service system (IPS²). IPS² can maintain and enhance manufacturer’s competitiveness by integrating tangible product and intangible service. Accurately capturing and understanding customer requirement are critical for successfully developing the right IPS² for the right customer. However, there are few researches on requirement evaluation in the early planning phase of IPS² development. And IPS² requirement evaluation often involves much subjectivity and vagueness. To solve these problems, a systematic evaluation approach for eliciting and assessing IPS² requirement under vagueness is proposed. The proposed model of industrial customer activity cycle can efficiently support requirement elicitation from the lifecycle perspective. Besides, the integrated rough group analytic hierarchy process method can effectively deal with subjectivity and vagueness in the IPS² requirement evaluation process. To demonstrate the potential of the approach, an application in an air compressors system is also illustrated.     

A novel integer programming formulation with logic cuts for the U-shaped assembly line balancing problem

U-shaped assembly lines are regarded as an efficient configuration in Just-In-Time manufacturing. Balancing the workload in these lines is an unsolved problem that attracted significant research within the past two decades. We present a novel integer programming formulation for U-shaped line balancing problems, where cycle time, the interval between two consecutive outputs, is known and the aim is to minimize the number of workstations. To enhance the efficiency of the LP relaxation of the new formulation, we present three types of logic cuts (assignable-station-cuts, task-assignment-cuts and knapsack-cuts) that exploit the inherent logic of the problem structure. The new formulation and logic cuts are tested on an extensive set of benchmark problems to provide a comparative analysis with the existing models in the literature. The results show that our novel formulation augmented by assignable-station-cuts is significantly better than the previous formulations.     

Sequencing procedure for balancing the workloads variations in case of mixed model assembly system with multiple secondary feeder lines

In order to increase flexibility and reduce costs, assembly systems are changing from simple mixed-model assembly lines, to multi-lines systems, where some components are assembled in secondary lines, called feeder lines, connected to the main line by a ‘pull philosophy’. The production of different models in such a complex multi-lines assembly system, where the tasks to perform could be very different from model to model, impacts the production with very high workload time variations, with the consequence of lack of productivity. If operators can perform tasks on different stations on both the feeder and the main lines these time variations can generally be absorbed. But if working across the stations is not possible (closed stations), the balancing of these workload time variations becomes critical. In such contexts the consolidated approach, that allows to configure mixed-model-multi lines assembly system with a single average model representative of the whole mix, can be very limited and is unable to bring substantial results. This paper aims to introduce an innovative sequencing model for mixed-model-multi-lines system, in the case of closed stations, in which it is possible to obtain, after a first long term configuration, a short term balancing, using an appropriate sequencing, for a given production mix and characteristics of the assembly system. The proposed procedure is applied to un-paced assembly lines and validated through simulation and industrial applications.     

A structured approach to evaluating the impact of implementing a component-based system in the automotive engine manufacturing domain

Component-based (CB) technology applied to the control system of production machinery is one of the new research developments in the automotive manufacturing sector. Although it is important to evaluate the technical aspects of this new paradigm, an appreciation of the impact from the business and human aspects is equally important to the stakeholders in the industry. Current evaluation approaches do not offer a method to capture and analyse the impact of CB technology that is simple to use and produces results that are readily understood by all the interested parties. The definition of an approach for evaluating the business and human aspects of implementing a CB system in the automotive sector is discussed. An evaluation strategy has been formulated comprising (1) knowledge elicitation, (2) investigation of future implementation scenarios, (3) data representation and analysis using enterprise modelling approaches, and (4) simulation and model analysis using proprietary software toolkits.     

An integrated approach to evaluating conceptual design alternatives in a new product development environment

As many companies have demonstrated over time, a conceptual design for a new product contributes greatly to an improvement in competitiveness, because it permits a reduction of costs, an increase in quality, and, often, a shortening of the time necessary to get the product on the market. That is why the evaluation of conceptual design alternatives created in a new product development (NPD) environment has long been very vital and a critical point for the future success of companies in fast-growing markets. These alternatives can be evaluated using the analytic hierarchy process (AHP), one of the most commonly used multiple criteria decision making (MCDM) techniques. This technique is used to reduce the number of conceptual design alternatives after ranking them using the scores obtained from the process. Furthermore, another technique, simulation analysis integrated with the AHP, is also used to help decision makers (product engineers or managers) perform economic analyses of the AHP's high-score alternatives using data from the generated simulation model of a real-life manufacturing system in which the final alternative is produced. In short, the objectives of this research are: first, to use the AHP technique to evaluate conceptual design alternatives in a NPD environment; second, to use a simulation generator integrated with this technique in order to perform economic analyses for the AHP's high-score alternatives. Finally, the results of both techniques, simulation and AHP, are used in a benefit/cost (B/C) analysis to reach a final decision on the conceptual design alternatives.     

A precision gas feeding system for a PIG heavy ion source

A precision gas feeding system aiming at stable and efficient operation of a small, cold-cathode PIG heavy ion source for the K = 50 MeV AVF cyclotron is described in detail. The system can regulate the supply gas pressure with a precision of 0.6 Torr and keep a constant gas flow rate with a stability of 0.010 ± 0.003 cm³/min. The ion source operating parameters such as the arc current, the arc voltage and the gas flow rate remain fairly constant through the lifetime of the source. By adopting this system in the routine accelerations of carbon or oxygen ions, the average beam current was increased by a factor of 2–3.     

随机混合装配线的启发式平衡设计方法

把某个型号产品的数量占整个批次的比重作为它的权重,加权合并混合装配线上各个型号产品的同类型工作单元的时间.运用随机启发式方法对合并后的工作单元进行平衡排序,通过改变预设超限概率的值,获得一组排序方案.采用工作站总数、装配线效率、平滑指数和成本四个指标对这组排序方案进行综合评价,挑选出最佳的排序方案,并且用系统仿真来检验该最佳方案的实际运作效果.实证研究表明,各个工作站的平均等待时间和空闲率都有明显的下降.     

Mixed model assembly system with multiple secondary feeder lines: layout design and balancing procedure for ATO environment

The constant research for efficiency and flexibility has forced assembly systems to change from simple/single assembly lines to mixed model assembly lines, while the necessity to reduce inventory has led the transition from single to multi-line systems, where some components are assembled in secondary lines, called feeder lines, connected to the main one by a ‘pull philosophy’. A possible approach to configure such an assembly system is to balance the main line first and use the retrieved cycle time to balance each feeder line separately, which is a questionable solution, especially if operators can perform tasks on both the feeder and the main line. Moreover for its complexity the mixed model balancing problem is usually solved transforming it into a single model by creating a single ‘virtual average model’, representative of the whole production mix. The use of a virtual average model assumes that the processing times of some models are higher or lower than the cycle time, which creates overload/idle time at the stations. This approach, especially in complex multi line production systems, largely reduces the assembly line productivity and increases the buffers dimensions. This paper faces the mixed model assembly line balancing problem in the presence of multiple feeder lines, introducing an innovative integrated main-feeder lines balancing procedure in case of unpaced assembly systems. The proposed approach is compared with the classical one and validated through simulation and industrial applications.