Traditional Assembly Line Research Articles

Lot streaming in workforce scheduling problem for seru production system under Shojinka philosophy

Drawing upon efficiency, flexibility, and responsiveness, this study attempts to explore the impact of lot streaming and worker assignment strategies in the workforce scheduling problem for the seru production system (SPS), which has emerged as an alternative to traditional assembly lines. The addressed problem involves decisions related to employee timetabling, lot splitting, and sublot scheduling. While scheduling problems in SPS have been extensively researched in recent years, the workforce scheduling problem has not received much attention in the literature. Moreover, to our best knowledge, it has not been investigated with the sublot division methodologies within the context of SPS so far. To bridge this gap, a generic novel optimization model is developed with the objective of minimizing average flow time (AFT) by integrating lot streaming and the Shojinka philosophy through modules within the model. Additionally, the structural properties of the problem are examined, and lower and upper bound formulations are developed based on these properties. Given the NP-hard nature of the problem, customized approaches based on the genetic algorithm (GA) are proposed for solving large-sized problems, considering the Shojinka and division methodologies. The computational results clarify that achieving Shojinka with variable sublots division methodology significantly reduces the AFT objective. Furthermore, the findings confirm that adapting variable sublots in operational scenarios results in a drastic improvement in system performance, regardless of the implemented worker assignment strategy.

Integrated Resource Optimization in a Multi-Product Separated Line Collaborative Assembly Line Balancing Problem (MPSLC-ALBP)

The traditional assembly line which produces a single-model product is not efficient for today's competitive industry and it cannot respond the customers’ expectations. This drives companies toward multi/mixed-model products. In addition, the assembly lines’ layout is of great importance for realizing such products and achieving the expected productivity improvement and capacity increase. In the current industrial context integration of Collaborative Robots (Cobots) in the assembly lines can be considered an effective way to increase the productivity while ensuring job security and flexibility . Thus, this study proposes a new Multi-Product Separated Line Collaborative Assembly Line Balancing Problem (MPSLC-ALBP) mathematical model to optimize resource assignment among lines. Besides the multi-product aspect, comparing to traditional ALB, the proposed model embraces the particular scenarios of Human and Cobot interaction as well as the diversity of workers such as experience level and its impact on task completion times. Through an illustrative case, it is shown that integrated resource optimization for all lines is more beneficial than individual resource optimization. Moreover, the effect of integrated resource optimization will be significant in the case that there are a limited number of resources available.

Modelling and numerical analysis for seru system balancing with lot splitting

ABSTRACT Lot splitting is one of the effective technologies of time-based strategy and has been widely studied in a variety of production environments. Nevertheless, the literature on its application in seru production has been highly scarce until now. Seru system is composed of simple equipment and multi-skilled workers, which can be quickly converted from the traditional assembly line to seru units. As an innovative production mode, seru production inevitably allows applying lot splitting in the real world. Therefore, a multi-objective model is studied for line–cell conversion with lot splitting, aiming at determining the trade-off among makespan, inter-seru system balancing, and intra-seru system balancing. Due to the proposed model's NP-hard nature, an improved NSGA-II was developed to solve it. Finally, extensive numerical simulations are conducted. Compared to no lot splitting, lot splitting is improved by 4.2% and 3.7% in terms of inter-seru system balancing and makespan respectively. The better efficiency and effectiveness of the improved NSGA-II are proved by comparisons with other state-of-the-art algorithms. Additionally, a sensitivity analysis is conducted to ascertain the degree of contribution of the model parameters towards the value of objectives, which provides management implications to support the decision-making of seru production for the enterprise.

Flexible assembly layouts in smart manufacturing: An impact assessment for the automotive industry

Currently, automotive manufacturers take the concept of flexible manufacturing to an unprecedented level, considering the deployment of flexible assembly layouts (also known as matrix production systems) in which automated guided vehicles transport bodyworks on individual routes between assembly stations. To this end, a methodological framework that allows the assessment of the impact of technology choice decisions between traditional assembly lines and flexible assembly layouts as well as the impact of different flexibility levers and configurations is necessary for optimal decision support. We provide such a framework based on analytical insights and a chance-constrained problem formulation. We further show how this problem formulation can be solved optimally using a tailored branch-and-price algorithm. Our results quantify the impact of different flexibility configurations in flexible assembly layouts. We show that flexibility enables a simultaneous improvement in worker utilization and work in progress, resolving a classic trade-off in manufacturing systems. Moreover, we find that worker utilization and output are up to 30% higher in flexible assembly layouts compared with line assembly layouts. Further, flexible assembly layouts prove to be especially beneficial during the ramp-up of vehicles with alternative drivetrain technologies, such as the current transition to electric vehicles.

A Heuristic-Mixed Genetic Algorithm for Type II Assembly Line Balancing with Multiple Workers in Workstations

A type II line balancing problem with multiple workers in workstations (mALBP-II) is considered given a total number of workers, group workers, and tasks into workstations so as to minimize cycle time. Different from the manufacturing environment where the traditional assembly line balancing problem (ALBP) rises, manual or semimanual manufacturing enjoys much higher flexibility allowing multiple workers to perform the same set of tasks on workpieces in the same workstation in parallel. The freedom of specifying the number of workers in workstations makes the classic NP-hard ALBP even harder to solve. A heuristic-mixed genetic algorithm (hGA) is therefore proposed to solve the problem. The algorithm minimizes cycle time as its first objective and balances workload among workstations as its second objective. A maximum-utilization heuristic and a bisection search are integrated into the decoding process of hGA so that the optimization of task assignment and worker allocation is accomplished simultaneously. Numerical results and a real-life application demonstrate the efficiency and effectiveness of hGA.

A flexible manufacturing assembly system with deep reinforcement learning

Traditional assembly line requires a significant amount of designs from engineers, especially in the case of multi-species and small-lot production. Recently, intelligent algorithms based on reinforcement learning are proposed to address this issue. However, the lower success rate and safety reasons limit their industrial applications. In this article, we proposed a systematic solution, including the automatic planning of assembly motions and the monitoring system of the production lines. In the planning stage, we built the digital twin model of the assembly line, then trained a deep reinforcement learning agent to assembly the workpieces. In the production stage, the digital twin model is used to monitor the assembly lines and predict failures. To validate the system we proposed, we conducted a peg-in-hole assembly experiment, and reached a 90% success rate for a single assembly attempt. During the whole experiment, no collision happens in the real world.

An analysis of task assignment and cycle times when robots are added to human-operated assembly lines, using mathematical programming models

Adding robots to a human-operated assembly line influences both the short- and long-term operation of the line. However, the effects of robots on assembly line capacity and on cycle time can only be studied if appropriate task assignment models are available. This paper shows how traditional assembly line balancing models can be changed in order to determine the optimal number of workstations and cycle time when robots with different technological capabilities are able to perform a predetermined set of tasks. The mathematical programming models for the following three cases are presented and analysed: i) only workers are assigned to the workstations; ii) either a worker or a robot is assigned to a workstation; iii) a robot and a worker are also assigned to specific workstations. The data of an assembly line producing power inverters is used to illustrate the proposed calculations. Both the assignment of tasks and the changes of cycle time are analysed within the AIMMS modelling environment. The computational characteristics of the proposed mathematical programming models are also examined and tested using benchmark problems. The models presented in this paper can assist operations management in making decisions relating to assembly line configuration.

EVALUATE THE RELIABILITY OF A SERU PRODUCTION SYSTEM WITH REWORKING ACTIONS

ABSTRACT This paper measures the reliability of a seru production system, which has been proven to be more flexible, efficient, and responsive than the traditional assembly lines in production practice. Seru is the Japanese pronunciation for “cell”, and it is a new type and flexible parallel low automated assembly mode originated from Japanese production practice. This is the first research that focuses on the evaluation of the reliability of seru production systems. We take into account reworking actions of some defective products, especially when these defective products have high values. In this seru production system, the capacity of each worker in the seru systems is regarded as stochastic according to the practical situation. The whole processes of each seru can be decomposed into a general process and a reworking process. In order to analyze the reliability of a seru production system, a model is constructed to describe these two process paths. A solution method is designed to obtain the seru production system reliability. Finally, some numerical examples are presented, and the reliability between seru production system and traditional assembly line is compared. We found that seru production systems are more reliable than assembly lines.

An Improved Lexicographical Whale Optimization Algorithm for the Type-II Assembly Line Balancing Problem Considering Preventive Maintenance Scenarios

In the traditional assembly line balancing, all the workstations are assumed available and hence the unavailability of any workstation brings about the stoppage of the whole line and the waste of the production capacity in the rest workstations. Considering the planning characteristic of preventive maintenance, this paper proposes a novel methodology of integrating the preventive maintenance scenarios into assembly line balancing problems to bypass the unavailable workstation. A lexicographic model is formulated to generate multiple task assignment plans that ensure primarily the high productivity under regular operation scenario and guarantee secondarily the production continuity under preventive maintenance scenarios. And, an improved whale optimization algorithm (IWOA) with three modifications is proposed to solve this problem. Specifically, a combined crossover operator enhances better combination in exploration; three best search agents promote the exploitation; partial regeneration avoids being trapped in local optima. More than five thousand experiments demonstrate that the joint of three modifications endows the IWOA significant superiority over six variants and other six well-known algorithms. Moreover, integrating preventive maintenance scenarios into the assembly line balancing problem increases the production efficiency by 1% at the cost of small production adjustment.

Investigating the performance improvement by conversion of assembly line configuration to a pure cell system in manufacturing industry

Seru production systems implement reconfiguration of traditional assembly lines to a flexible cell system that aim at reducing the required workforce while at the same time augmenting the productivity manifold. For evaluating the overall performance improvement, cell formatting and assigning workers to serus takes form of a complicated decision problem. In this paper, with the objective of reducing the total cost for training the worker, minimising the processing time and the total throughput time, mathematical insights on the solution space of a multi-objective line-cell conversion model are identified, in turn proving it to be an NP-hard model. By applying the proposed heuristic algorithm on several numerical simulations, a Pareto-optimal solution of this multi-objective model is obtained. With experimental results and comparative studies, the proposed approach proves its effectiveness that may lead to further improvement in seru production systems competitive advantage to cope with fluctuating market demands by enhancing the flexibility as well as the efficiency of the system. [Submitted: 18 June 2016; Revised 29: August 2017; Revised: 25 August 2018; Revised: 5 December 2018; Accepted: 8 January 2019]

Seru production as an alternative to a traditional assembly line

Abstract The article presents the concept of seru production and a simple simulation experiment to check the application effectiveness of the seru production concept in the assembly line of finished products. The article presents the concept of seru production created by Japanese electronics manufacturing companies in the 90s. The simulation experiment showed, better results using the seru production concept compared to a traditional assembly line. Three types of production cells were used and each option turned out to be better than a traditional assembly line.

Lean Methodology to Transform Shipbuilding Panel Assembly

The core competence of any medium-to-large sized shipyard includes the panel assembly line. Ship panels are the basic building blocks of well over 60% of the interim products of typical commercial ships. Therefore the improvement of the panel assembly process could greatly reduce the number of man-hours of all assembled panels, thereby yielding significant savings to the shipyard. Using a lean methodology to make kaizen improvements to traditional panel assembly lines will greatly reduce the costs in ship production. This means that shipyards, which are barely keeping earnings above costs, will be able to increase profits. Value stream mapping is a key way of determining how lean a production process is. The wastes in production assembly are readily identified as well as the takt time and the areas where there is push as opposed to pull. In this paper, a case study of a typical commercial shipyard, which builds a product mix of vessels is analyzed. The present state panel assembly line is mapped and then using lean tools and avantgarde technologies, such as hybrid laser arc welding, the new transformed panel assembly line is demonstrated to bring man-hour reductions of over 80%, so that a typical panel is assembled using 12 man-hours as opposed to the present 72 man-hours. 1. Introduction The core competence of virtually any medium-to-large sized shipyard includes a panel assembly line. Many times shipyard management takes for granted that the panel assembly line is just fine as is and therefore it is not necessary to make analysis to improve this core shipbuilding assembly process any further. However, since flat panels make up well over 60% of the building blocks of most commercial ships, any type of significant improvement to the panel assembly line will result in important cost savings for the shipyard.

Line-seru conversion towards reducing worker(s) without increasing makespan: models, exact and meta-heuristic solutions

Compared with the traditional assembly line, seru production can reduce worker(s) and decrease makespan. However, when the two objectives are considered simultaneously, Pareto-optimal solutions may save manpower but increase makespan. Therefore, we formulate line-seru conversion towards reducing worker(s) without increasing makespan and develop exact and meta-heuristic algorithms for the different scale instances. Firstly, we analyse the distinct features of the model. Furthermore, according to the feature of the solution space, we propose two exact algorithms to solve the small to medium-scale instances. The first exact algorithm searches the solution space from more workers to fewer workers. The second exact algorithm searches the solution space from fewer workers to more workers. The two exact algorithms search a part of solution space to obtain the optimal solution of reducing worker(s) without increasing makespan. According to the variable length of the feasible solutions, we propose a variable-length encoding heuristic algorithm for the large-scale instances. Finally, we use the extensive experiments to evaluate the performance of the proposed algorithms and to investigate some managerial insights on when and how to reduce worker(s) without increasing makespan by line-seru conversion.

Modelling and numerical analysis of seru loading problem under uncertainty

As a flexible manufacturing system, seru seisan has been proven to be more competitive in a volatile market than traditional assembly line. In this system, seru loading plays an essential role to deal with the fluctuating demands and uncertain product types. Unfortunately, in practice, seru loading decision-making is mainly based on the supervisor's experience instead of the theoretical research. In this situation, seru loading in a single period under uncertainty is considered in this paper, meanwhile, a fuzzy random bi-objective model is developed to minimise the makespan and the total tardiness penalty cost. To deal with the fuzzy random variables, these uncertain parameters are transformed into the trapezoidal fuzzy variables, which are subsequently de-fuzzified by the optimistic-pessimistic adjustment index Me. After obtaining the equivalent crisp model, a heuristic algorithm based on enumerating feasible solutions is designed. Finally, to demonstrate the practicability, a numerical example is applied, and the generated results verify the efficiency of the proposed model and algorithm for seru loading problem. [Received: 28 April 2016; Revised: 28 July 2016; Accepted: 13 September 2016]

Integration of a collaborative robot in a U-shaped production line: a real case study

In lean production environments, such as the U-shaped cells, flexibility is a priority. Therefore, any element that introduces process stiffness is negatively valued. Former studies establish that robotization of tasks in U-shaped cells presents some drawbacks. For instance: it may complicate continuous improvement, prolong changeover time, use a large space or create safety problems for the operators. However, the collaborative robots (CoBots) may change this situation, since they overcome most of the issues previously mentioned. The present study analyses a real case of de-robotization in a traditional assembly line to transform it into a manual U-shaped line. In a second step a CoBot is integrated in the cell replacing one of the workers. This study empirically compares the manufacturing process in these three scenarios. Results in real production conditions show that a U-shape cell assisted by a CoBot increases productivity and reliability while reducing the surface used. These results suggest that collaborative robotics can be integrated in U-shaped production lines and even increase the efficiency of a traditional robotized assembly line.

Complexity of line-seru conversion for different scheduling rules and two improved exact algorithms for the multi-objective optimization.

Productivity can be greatly improved by converting the traditional assembly line to a seru system, especially in the business environment with short product life cycles, uncertain product types and fluctuating production volumes. Line-seru conversion includes two decision processes, i.e., seru formation and seru load. For simplicity, however, previous studies focus on the seru formation with a given scheduling rule in seru load. We select ten scheduling rules usually used in seru load to investigate the influence of different scheduling rules on the performance of line-seru conversion. Moreover, we clarify the complexities of line-seru conversion for ten different scheduling rules from the theoretical perspective. In addition, multi-objective decisions are often used in line-seru conversion. To obtain Pareto-optimal solutions of multi-objective line-seru conversion, we develop two improved exact algorithms based on reducing time complexity and space complexity respectively. Compared with the enumeration based on non-dominated sorting to solve multi-objective problem, the two improved exact algorithms saves computation time greatly. Several numerical simulation experiments are performed to show the performance improvement brought by the two proposed exact algorithms.

A New Approach to Traditional Assembly Line Balancing Problem with Fuzzy Operation Time: Defence Industry Application

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Optimisation of stochastic assembly line for balancing under high variability

An ideal manufacturing system is characterised by deterministic task times, 100% machine availability and on-time delivery. In a real time production scenario an ideal system does not exist due to the presence of variability. Most line balancing approaches assume deterministic task times while ignoring the impact of task time variability on system performance measures. However, in manufacturing systems such as aircraft assembly, ship building and custom job shops which are characterised by high variability in task times, large number of tasks assigned to a workstation and strict task precedence relationship, traditional assembly line balancing techniques may not be effective. In addition to balancing the task times, it is also important to balance the risk of delay. In this research, a novel heuristic optimisation approach for highly variable task times is presented. Results indicate that the method increases throughput while balancing the risk of delays at each station.

Robotic U-shaped assembly line balancing using particle swarm optimization

Automation in an assembly line can be achieved using robots. In robotic U-shaped assembly line balancing (RUALB), robots are assigned to workstations to perform the assembly tasks on a U-shaped assembly line. The robots are expected to perform multiple tasks, because of their capabilities. U-shaped assembly line problems are derived from traditional assembly line problems and are relatively new. Tasks are assigned to the workstations when either all of their predecessors or all of their successors have already been assigned to workstations. The objective function considered in this article is to maximize the cycle time of the assembly line, which in turn helps to maximize the production rate of the assembly line. RUALB aims at the optimal assignment of tasks to the workstations and selection of the best fit robot to the workstations in a manner such that the cycle time is minimized. To solve this problem, a particle swarm optimization algorithm embedded with a heuristic allocation (consecutive) procedure is proposed. The consecutive heuristic is used to allocate the tasks to the workstation and to assign a best fit robot to that workstation. The proposed algorithm is evaluated using a wide variety of data sets. The results indicate that robotic U-shaped assembly lines perform better than robotic straight assembly lines in terms of cycle time.

Study on Multi-Specification Small-Batch Line Balancing and Optimization Base on the Streamline Balancing

Based on the principal concepts of streamline balance, this paper analyses the issue of the line balancing, through analyzing traditional assembly line balance method and having the bottleneck work processes to amend the parameters of the Workplace design, and then building up a multi-variety small-batch production line balancing mixed-model.