Reconfigurable Assembly Systems Research Articles

A systematic strategy for the architecture design of collaborative and reconfigurable assembly lines

Current manufacturing environments have to face a competitive and evolving framework characterised by high-mix and low-volume demands. To properly deal with new requests coming from the market, Reconfigurable Assembly Systems (RASs) have to be designed. In this paper, we propose a methodology for the design of the architecture of a RAS, featuring both human and mobile robotic agents, and dealing with multi-product assembly of medium-light-weight products, originally assembled manually. In particular, a systematic methodology is proposed to define the number of workstations, specifying which set of operations must be performed in each one, and to design the system layout guaranteeing reconfigurability. To this end, the concepts of operation, datasheet of an operation, assembly tree, and macro-operation are introduced and formalised. Guidelines to automatise the required operations in a flexible way are also proposed. The method is applied to a use case where we define the proper number of workstations of a RAS to assemble several types of motorbike brakes, showing the reconfigurability of the system.

Human-robot collaborative task planning for assembly system productivity enhancement

PurposeIn the current era of Industry 4.0, the manufacturing industries are striving toward mass production with mass customization by considering human–robot collaboration. This study aims to propose the reconfiguration of assembly systems by incorporating multiple humans with robots using a human–robot task allocation (HRTA) to enhance productivity.Design/methodology/approachA human–robot task scheduling approach has been developed by considering task suitability, resource availability and resource selection through multicriteria optimization using the Linear Regression with Optimal Point and Minimum Distance Calculation algorithm. Using line-balancing techniques, the approach estimates the optimum number of resources required for assembly tasks operating by minimum idle time.FindingsThe task allocation schedule for a case study involving a punching press was solved using human–robot collaboration, and the approach incorporated the optimum number of appropriate resources to handle different types of proportion of resources.Originality/valueThis proposed work integrates the task allocation by human–robot collaboration and decrease the idle time of resource by integrating optimum number of resources.

Proposition of a software-assisted methodology to solve safety issues in reconfigurable assembly systems in a short time

ABSTRACT Reconfigurable manufacturing systems have been proposed over the last decades to deal with mass-customization problems and volatile markets. Issues regarding the physical design, control, or scheduling of these systems have been studied intensively but very few studies have investigated inherent safety problems. Yet this issue is crucial for reconfigurable assembly systems where manual and robotized workstations must cohabit. The present paper first presents the main safety issues inherent to this new type of manufacturing system. A methodological approach based on adaptable solutions (e.g. safety plans, movable barriers, and safety laser scanners) and comprising two phases (‘offline’ and ‘online’) is then proposed to address in a short time the previous issues. At each step of the ‘offline’ phase, dedicated software assists the safety manager (in charge of and responsible for safety) to design a ‘safe’ reconfigurable manufacturing system. Once this design is validated by the safety manager, the ‘online’ phase of the methodology is deployed.

Requirements and concept development for a reconfigurable assembly system with individual and interchangeable modules

Requirements and concept development for a reconfigurable assembly system with individual and interchangeable modules

A formal skill model to enable reconfigurable assembly systems

ABSTRACT As assembly systems move into the era of mass customisation, the complexity of design processes, (re)configurations and operations rises. Well-structured data are key in keeping this complexity manageable. Here to, this paper presents a multidimensional formal skill model designed to deliver generic descriptions of needs and capacities with skills as the connector between products, processes and resources. The model formalises resource structures in relation to the processes they master and products they can produce. This paper discusses the case-based evaluation in a reconfigurable assembly system and highlights the added-value of a skill-based modelling approach. The presented formal model combines concepts coming from both offline and online modelling perspectives and allows for various applications and levels of detail. The resource structures embedded in the prerequisites of a skill enable matchmaking of resources for workspace design and reconfigurations. The mapping of the model to standardised ISA-95 models couples production needs to the resources allowing for more optimal production planning, control and a structured interface between enterprise and control systems. The possibility to couple states to the assembly environment allows for optimal runtime orchestration.

Multiple-objective optimization of a reconfigurable assembly system via equipment selection and sequence planning

With the development of society and technology, the modern manufacturing system is required to handle unpredictable and rapidly changing market demands during various demand periods. This is a new challenge to the production scheduling of the production line, which requires a faster and smarter response to changes in production demand. To meet the challenge, the reconfigurable manufacturing system (RMS) has emerged as a competitive and promising manufacturing model. The reconfigurable assembly system (RAS), a critical feature of RMS, can be seamlessly incorporated into multi-variety, variable-batch, multi-functional, and fast-delivery manufacturing modes, opening up the possibility of rapid changes for production scheduling. In this paper, we first build a RAS model that can more accurately represent scheduling problems that may occur in real-world industrial operations. More specifically, we consider the assembly process of products could be divided into a number of major steps which are performed in different stations and optimize the assembly sequence of products and the equipment selection within individual assembly stations. After that, three objective functions corresponding to the workload balance, reconfiguration/assembly cost, and lead time are proposed to jointly optimize the assembly sequence and the equipment selection in individual assembly stations. Finally, a hybrid particle swarm optimization (MOHPSO) method is utilized to solve the multiple-objective optimization problem, which combines the advantages of particle swarm optimization and the hybrid method. Experimental results show the superiority of the proposed MOHPSO method over other alternatives including MOPSO and NSGA-II, achieving lower Spacing Metric (SM) and Mean Ideal Distance (MID) indexes and is suitable for solving production scheduling problems.

A framework for planning a reconfigurable assembly system for press brake production

This article presents the design of a framework for the development of a reconfigurable assembly system (RAS) to facilitate the assembly of press brakes varieties. A consideration of press brake varieties is achieved by examining the parts taxonomy of press brakes from which a framework for assembly process plan is developed that considers all the types of press brakes identified from the classification. Also, a layout for the RAS is developed by considering a prototype of the major enabling equipment of the system which is a reconfigurable assembly fixture (RAF). The development of the assembly process plan and layout of the RAS enabled the design of a reconfiguration model for operation in the RAS. The designed assembly framework will provide: rapid and physical reconfiguration of the RAF in the RAS; generation of assembly sequence for press brake varieties; and allocation of assembly operations to assembly work cells in the system. [Submitted 27 May 2020; Accepted 3 December 2020]

Multi-level approach to virtual commissioning: a reconfigurable assembly system case

Software plays a key role in modern automated systems. Testing the control software is one of the final steps in the development process and when done on the physical set up, it creates risks (e.g. project delay, system damage and operator injury). In addition, with flexible systems it is almost impossible to test every possible scenario on the real system. Using a digital model allows testing different configurations before the physical hardware is available and thus without risk for the operator and the system hardware. A digital model of the reconfigurable system can also help validate new configurations virtually while the real system still operates in its current configuration, allowing for a quick risk-less changeover. Depending on the level in the control hierarchy, the simulations can run faster than real time. This paper demonstrates the use of a digital model during the commissioning at different levels. On each level, other requirements need to be fulfilled and a different level of detail is needed in the model. Every level asks for a different set of input data, that needs to be consistent to result in a reliable emulation/simulation. The methodology is evaluated on a reconfigurable assembly system, consisting of multiple base units with modular, relocatable add-on modules. The most detailed level is the process and machine level, considering the low-level control logic, movements of the machine and robots, collisions, etc. Cell and plant level commissioning focuses on the interaction of multiple machines, transport, production schedule and assignment of tasks to the individual machines. This level typically covers a longer time span and requires less detail of the actions on the machine level. It interacts with higher level control software and a simplified version of the low-level control logic is implemented in the model itself. This paper describes the methodology and lessons learned to reuse the digital model at different levels of the control hierarchy while remaining the consistency by exchange of information within the digital model and with the real system. Further research is described towards automated virtual test case scenarios with a digital model for a reconfigurable assembly system.

Production scheduling for reconfigurable assembly systems: Mathematical modeling and algorithms

This study generalizes the scheduling problem of a reconfigurable assembly system where the configuration of the system is changeable. There are different job sets and each has a unique fittest configuration, although the job set can be still operated in a less fit configuration. Therefore, besides the classical scheduling decision, the problem decides whether to move to the configuration that most suits a job set or continue the standing configuration. The available models cannot address this specification. In this study, we first explicitly define different aspects of the problem and then develop a mathematical formulation to formally model the problem. We solve the small-sized instances with up to 6 jobs using the model and commercial software of operations research. To effectively solve large instances of the problem, we also propose two different artificial immune algorithms. In this regard, we develop an encoding scheme as well as two hypermutations and a local search that match the encoding scheme. Using numerical experiments, both the model and the performance of the proposed algorithm are being evaluated.

A flexible control system for reconfigurable assembly lines

With the rising demand for customized and frequently updated products, the current assembly systems require a higher level of reconfigurability and flexibility. Thus we proposed a scheme of reconfigurable assembly line and hierarchical control system. To increase the flexibility of control systems, a communication method and control strategy is presented in this paper. This approach will endow the reconfigurable assembly system with flexibility by encapsulating the ladder diagram program and providing an external interface for the control software to call. This approach is validated with one assembly unit assembling shuttle valve, which takes 40s during the process. The control system can not only monitor the assembling process, but also control the action of the actuator via control panel or control commands.

Solving Facility Layout Problem with safety consideration of Reconfigurable Manufacturing and Assembly Systems

Reconfigurable Manufacturing Systems (RMS) are designed to cope with mass customization problem. Reconfigurable Assembly Systems (RAS) are a variant of this RMS concept in the assembly field. One of the most crucial issues in designing the RAS is the layout problem with human safety considerations. This paper’s main motivation is to propose a methodology to determine the most efficient and safe arrangement of facilities in a workshop. The objective is twofold: (i) to minimize the Material Handling Cost and (ii) to ensure human safety by applying "safety bubble". An implementation of the approach and an illustrative case study are presented.

A model for achieving reconfiguration in a smart assembly work-cell

The reconfigurable assembly system is a type of a smart assembly system that can survive the challenges of personalised customisation of product design in the manufacturing sector. Reconfigurable assembly fixtures (RAFs) have been identified as enabling equipment in assembly work cells of reconfigurable assembly systems. RAFs can rapidly adapt to different dimensions of product sizes in the same part family. In this article, a reconfiguration model (RM) for achieving rapid positioning and immobilisation of press brake models is developed for a reconfigurable assembly fixture used for assembling press brakes. The model sets the reconfiguration limits of the RAF based on fixture workspace synthesis and physical dimensions of the press bake. It converts the physical dimensions of the press brake into travel length of locators and computes the operating parameters of the RAF. It also attempts to integrate the operation of the RAF with the assembly procedure in the system. The reconfiguration model is validated using different dimensions of press brake models. The reconfiguration model yielded results for operating parameters of the RAF, which are presented graphically and discussed extensively in order to compare the response of the model to the expected behaviour of the RAF. Comparing the responses of the RM to different press brake models shows different values for the operating parameters of the RAF which is valid. Also, the article established and supports the fact that the operations of a RAF are incomplete without a RM. The originality of the article lies in the development of customised mathematical models for the RM that is meant for the RAF and its validation using different press brake models as the product to be assembled.

The concept of “safety bubble” for reconfigurable assembly systems

In the last decade, Reconfigurable Manufacturing Systems (RMS) have been proposed to deal with mass-customization problems and volatile markets. Applied to the field of assembly, this concept gives rise to Reconfigurable Assembly Systems (RAS). The latter are composed of different basic units (e.g., conveyor units, robotized units) linked together. Issues regarding the physical design, scheduling, and control of RAS have been studied intensively but very few studies have addressed safety problems. However, this issue is of primary concern for RAS containing robotized units with frequent interventions of human operators. Indeed, traditional safety approaches fail to take into account the versatility of RAS. In this context, the paper proposes a new way of dealing with safety in RAS with the concept of “safety bubble”. The latter aims to ensure operator safety by sharing safety data between units. An implementation methodology of this safety bubble is presented and on-going developments “off-line” on a multi-agent platform and “on-line” on a real demonstrator are described.

Selection of optimal assembly strategy at an engine assembly plant using fuzzy multi-criteria decision-making tools

This paper is presenting a methodology to select an optimal assembly strategy using two fuzzy multi-criteria decision making (MCDM) tools as illustrated in a case study of an engine assembly plant. Based on an automotive engine assembly system, the existing challenges and related criteria are identified. Four potential assembly strategies compatible with existing system are developed to address the challenges. Two different fuzzy MCDM tools (VIKOR and TOPSIS) are applied for the multi-criteria problem to select the optimal assembly strategy. Four decision makers (DMs) from middle-level management are involved during the process. A comparison of results from the two MCDM tools is performed to consolidate the decision. For the present case, reconfigurable assembly system (RAS) is found to be the most preferred assembly strategy by both the MCDM tools. The ranking for rest of the alternatives are also found.

The concept of "safety bubble" to build ethical reconfigurable assembly systems

The concept of Reconfigurable Assembly System has been proposed in the last decade to deal with mass-customization problems and volatile market environment. If physical design or control issues of these systems have been studied intensively, very few works concern the inherent ethical risks and associated safety problems. However these issues are of first interest for reconfigurable robotized systems with frequent interventions of human operators. Indeed, the re-configurability features of these manufacturing systems induce new issues in safety. The manufacturing enterprises must pay attention to the possible consequence of the Reconfigurable Assembly System design on the safety of the humans and on their possible responsibility in case of an accident. The present paper proposes the concept of safety bubble aiming to insure human’s safety by cooperation among safe robotized units. After a presentation of the ethical considerations, a design methodology of such safety bubble is detailed. The on-going works in simulation and on a real demonstrator, including collaborative robots and mobile robots, are then described.

Computer Vision-enabled Human-Cyber-Physical Workstations Collaboration for Reconfigurable Assembly System

Manufacturing flexibility is strategically important to firms for improving competition in the fast-changing market. This research inspired by the assembly cells system, which significantly appears rapid response, high flexibility and efficiency on the high-mix, low-volume (HMLV) environment. However, due to the non-repetitive operations caused by various products, high dependency on workers’ skills and inevitable exceptions and disruptions in manual operation, the complexity and uncertainty within the assembly cell will cause the system disturbance propagation and amplification. This paper presents a reconfigurable assembly system consisted of multiple intelligent assembly workstations (iWorkstation), named W-RAS. Based on a proposed human-cyber-physical system (HCPS) framework, the innovated iWorkstation can assist workers with differences in abilities and characteristics. Computer vision technology is applied to monitor and analyze the operation process and status for work measurement and ergonomic assessment. This real-time information visibility and traceability can support feedback-based optimal management and enhance coordination among the iWorkstations for enhancing the adaptability and reconfigurability of W-RAS. Finally, an operational mechanism is proposed for multiple iWorkstations collaboration and a numerical example shows the benefits in shortening the lead time of various orders.

Selection of optimal assembly strategy at an engine assembly plant using fuzzy multi-criteria decision-making tools

This paper is presenting a methodology to select an optimal assembly strategy using two fuzzy multi-criteria decision making (MCDM) tools as illustrated in a case study of an engine assembly plant. Based on an automotive engine assembly system, the existing challenges and related criteria are identified. Four potential assembly strategies compatible with existing system are developed to address the challenges. Two different fuzzy MCDM tools (VIKOR and TOPSIS) are applied for the multi-criteria problem to select the optimal assembly strategy. Four decision makers (DMs) from middle-level management are involved during the process. A comparison of results from the two MCDM tools is performed to consolidate the decision. For the present case, reconfigurable assembly system (RAS) is found to be the most preferred assembly strategy by both the MCDM tools. The ranking for rest of the alternatives are also found.

Reconfiguration Control of Dynamic Reconfigurable Discrete Event Systems Based on NCESs

In order to satisfy system specifications persistently in the cases of component failure, partial communication blocks, or energy deficiency, safety-critical systems need to apply system reconfigurations at runtime. Such systems can be considered as dynamic reconfigurable discrete event systems (DRDESs) if their logic behaviors are concerned only. A reconfiguration of a DRDES includes the switch of system structures and the update of system states. Based on the assumption that all configurations of a DRDES are designed in advance and dynamic reconfigurations can be implemented only at a few predefined reconfigurable states, a novel reconfiguration control method is proposed in this paper. The aim is to implement a required dynamic reconfiguration smoothly and rapidly. A DRDES is modeled by net condition/event systems (NCESs) under the firing rule “maximal single spontaneous transition steps.” An integer linear programming-based method is developed to compute a shortest legal firing sequence (SLFS) from the state where a reconfiguration requirement arises to a reconfigurable state. Accordingly, a virtual controller for guiding the system to evolve along the obtained SLFS is proposed. The proposed method is able to implement a required reconfiguration before the maximum permissible reconfiguration delay if an SLFS exists. A simple reconfigurable assembly system is adopted to illustrate the method.

Digital twin for adaptation of robots’ behavior in flexible robotic assembly lines

Abstract Product personalization is the global trend driving the development of modern production systems. This in turn creates the need to deploy flexible and reconfigurable assembly systems. New type of factories exploiting the capabilities of multiple resources such as human operators and mobile and / or stationary robot assistants are emerging. This paper investigates the use of digital world modelling techniques in such hybrid production systems for enabling system reconfiguration through shared environment and process perception. The suggested digital world model infrastructure involves three main functionalities: a) Virtual representation of the shopfloor, combining multiple sensor data and CAD models. The digital shopfloor is rendered in the 3D environment exploiting the capabilities provided by Robot Operating System (ROS) framework, b) Semantic representation of the world through the implementation of a unified data model for representing the geometrical as well as the workload state, and c) Dynamic update of the digital twin based on real time sensor and resource data coming from the actual shopfloor. The communication and integration layer among the physical and the virtual agents is realized on top of the ROS framework. The developed infrastructure is applied to an industrial case study from the automotive industry focusing on a car’s front axle assembly. Nevertheless, it is designed to be generic enough to be able to address multiple cases.

Tri-objective optimisation of mixed model reconfigurable assembly system for modular products

ABSTRACTThe recent challenges for assembly-based manufacturing are primarily governed by dynamic market with mass customisation requirements. In this context, modular products are gaining attentions after success in automobile, electronics and many other sectors. Another contemporary research on reconfigurable assembly system (RAS) is a manifestation of reconfigurable manufacturing system (RMS) philosophy in mixed-model assembly operations to handle flexibility issues in functionality and capacity. In this paper, an integration model for these two state-of-the-art design and assembly philosophies are presented. A tri-objective formulation and module selection plans are developed for modular products in an RAS environment. Non-dominated Pareto optimal solutions considering assembly cycle time, balance delay and smoothness indices as three objectives are found to select optimal product modules during assembly process. The assumptions and features of this model are complying with a typical manufacturer from auto or electronics industries having some parts outsourced to original equipment manufacturers (OEMs). An industrial case study dealing with assembly of automobile engines is included as a base model to illustrate the application and scope of the research.