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Digital twin-enabled Graduation Intelligent Manufacturing System for fixed-position assembly islands

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Highlights

  • Designed a Graduation-inspired assembly system for fixed-position layout.

  • Unified digital representations with appropriate sets of information at object, product and system level are developed.

  • IoT-enabled solutions are proposed to achieve real-time synchronization between physical space and digital space.

  • Developed cloud-based services with real-time ticket pool management mechanisms for facilitating decision-making and daily operations.

Abstract

The layout of fixed-position assembly islands is widely used in the heavy equipment industry, where the product remains at one assembly island for its entire assembly period, while required workers, equipment, and materials are moved to the island according to the assembly plan. Such layout is not only suitable for producing bulky or fragile products, but also offers considerable flexibility and competitive operational efficiency for products with medium variety and volumes. However, due to inherent complexity of the product, sophisticated assembly operations heavily rely on skilled operators, and the complexity and uncertainty are high and amplified by such massive manual interventions as well as the unique routing patterns of the fixed-position assembly process. Aiming at reducing the complexity and uncertainty, this paper introduces a digital twin-enabled Graduation Intelligent Manufacturing System (DT-GiMS) for fixed-position assembly islands. Inspired by the success of graduation ceremony, an assembly system-Graduation Manufacturing System (GMS) is proposed for fixed-position assembly islands, in which job tickets, setup tickets, operation tickets, and logistics tickets are designed to organize the production activities. Following the concept of digital twin, unified digital representations with appropriate sets of information are created at object level, product level, and system level, respectively. Through Internet of Things (IoT), smart gateway, Web 3D and industrial wearable technologies, vital information including identity, status, geometric model, and production process can be captured and mapped in physical space, and converged and synchronized with their digital representations in twin (cloud) space on a real-time basis. The overall framework of DT-GiMS is presented with physical layer, digital layer, and service layer. Real-time convergence and synchronization among them ensure that right resources are allocated and utilized to the right activities at the right time with enhanced visibility. Considering customer demand and production capacity constraints, real-time ticket pool management mechanisms are proposed to manage production activities in a near-optimal way under DT-GiMS. With the support of cloud-based services provided in service layer in DT-GiMS, managers could easily make production decisions, and onsite operators could efficiently complete their daily tasks with nearly error-free operations with enhanced visibility. A demonstrative case is carried out to verify the effectiveness of the proposed concept and approach.

Introduction

The layout of fixed-position assembly islands is widely used in the heavy equipment industry. It is normally adopted when products are too bulky or fragile, i.e., ships, aircraft, rotary printing presses and big milling machines [1, 2]. Among over 80,000 Hong Kong manufacturers in Guangdong-Hong Kong-Macao Greater Bay Area (GBA), most lift and mold manufacturers are of this type [3]. In this layout, workers move from one assembly site (often called an assembly island) to another, and required materials and equipment are moved to an assembly island, while the product normally remains at one assembly island for its entire manufacturing (assembly) period.

There are many advantages of fixed-position assembly islands comparing to typical job shop or flow shop, such as reduced damage or cost of product movement and more continuity of the assigned workforce since the product does not need to be moved frequently [4]. Such layout also offers considerable flexibility at the strategic level and competitive efficiency at the operational level for products with medium variety and volumes. However, due to inherent complexity of the product, sophisticated assembly operations heavily rely on skilled operators, and it is prone to errors and disruptions with massive human interventions. Besides, since the product remains at one fixed island, required resources including workers, equipment and materials need frequently moved to the island at different stages. These challenges increase and amplify the complexity and uncertainty in fixed-position assembly islands.

For fighting complexity and uncertainty in the manufacturing industry, many advanced manufacturing systems have been developed by the researchers and practitioners, such as enterprise resource planning (ERP) systems [5, 6] and manufacturing execution systems (MES) [7]. These advanced manufacturing systems provide effective solutions for improving collaboration and streamlining workflow for complex production processes. However, industrial practices show that they failed to reach the organization's targets and expectations because of coherent data collection requirements and high sensitivity to variability and uncertainty [8], [9], [10]. With the support of cutting-edge information technologies, Internet of Things (IoT) is considered to be an enabling technology for data acquisition. Some work has been explored the application of RFID or Auto-ID technologies in manufacturing scenarios to capture real-time data from the workshops [[11], [12]–13]. Fortunately, the power of IoT promises to reduce uncertainty through information visibility and traceability by capturing real-time production data. However, IoT alone does not guarantee that as it does not really act as effective mechanisms for information sharing, and many challenges remain unsolved in fixed-position assembly islands.

Firstly, how to design an appropriate assembly system for the unique layout of fixed-position assembly islands? Due to the specific routing pattern, the material flow and workflow in fixed-position assembly islands are quite different from that in traditional flow shop and job shop. A specific assembly system with appropriate configurations and effective production strategies for fixed-position assembly islands is crucial, which is the basis of the whole system.

Secondly, how to create unified digital representations for fixed-position assembly islands? Since the product remains at one fixed assembly island while being assembled, various manufacturing resources, including workers, equipment and materials are needed at different stages to organize and coordinate multi-echelon production activities. However, lack of unified digitization approaches, vital information, such as identification, status, geometric model, and production process cannot be captured, mapped and shared on a real-time basis.

Thirdly, how to realize convergence and synchronization between digital space and physical space in fixed-position assembly islands? Digital space is the reflection of real-life production in physical space. However, lack of convergence, it is hard to keep ultra-high fidelity and synchronization with the physical space. Besides, due to the lack of convergence and synchronization, the two spaces are isolated, and it is difficult to achieve optimization of global interaction and collaboration in fixed-position assembly islands.

Digital twin is considered as a promising approach to achieve convergence and synchronization between the physical space and its corresponding twin [14, 15]. To address the above challenges, this paper proposes a digital twin-enabled Graduation Intelligent Manufacturing System (DT-GiMS) for fixed-position assembly islands. Inspired by the success of the graduation ceremony, an assembly system-Graduation Manufacturing System (GMS) is proposed, in which three kinds of tickets are designed to organize the production activities in fixed-position assembly islands. Under GMS, a unified digitization approach is proposed to create the digital representations with appropriate sets of information at object level, product level, and system level, respectively. Following the concept of digital twin, the overall framework of DT-GiMS is presented. Besides, real-time ticket pool management mechanisms are proposed for managing production activities in a near-optimal way under DT-GiMS. Cloud-based services for managers and onsite operators are developed to facilitate their decision-making and daily operations.

The rest of this paper is organized as follows. Section 2 briefly reviews the key related research streams in assembly system design, IoT-enabled manufacturing and digital twin. Section 3 presents the assembly system-GMS for the unique layout of fixed-position assembly islands. In Section 4, DT-GiMS for fixed-position assembly islands is discussed. In Section 5, a laboratory demo is developed for demonstrating the effectiveness of DT-GiMS for fixed-position assembly islands. Section 6 summarises this paper by giving our findings, key contributions as well as future work.

Section snippets

Literature review

This section reviews the related research streams from three categories: assembly system design, IoT-enabled manufacturing and digital twin.

The configuration of the fixed-position assembly islands

As a unique production mode, the layout of fixed-position assembly islands is widely used for producing bulky or fragile products. For simplicity of discussion but without losing generality, Fig. 1 shows the typical configuration of fixed-position assembly islands. It includes two main sections. One section shows the main manufacturing resources involved in the production process, which includes workers, equipment and materials. The other section illustrates the typical assembly process at one

Digital twin-enabled Graduation Intelligent Manufacturing System (DT-GiMS) for fixed-position assembly islands

For exploring the application of digital twin in the manufacturing industry, several questions will be discussed first for better understanding its capabilities and potential benefits for intelligent manufacturing. Firstly, for digital twin, it should serve a specific purpose, for example, in this paper, digital twin services as an enabling technology to reduce complexity and uncertainty in fixed-position assembly islands by creating, establishing and utilizing information visibility and

Case study

Following the concepts and methodologies described in the preceding sections, this section will describe a demonstrative case aiming at:

  • Verifying the feasibility of DT-GiMS for fixed-position assembly islands and its technical architecture;

  • Demonstrating the effectiveness of DT-GiMS for fixed-position assembly islands, and illustrating its management and execution process.

In this case, the mini lathe machine assembly scenario is used to demonstrate the fixed-position product assembly in the lab.

Conclusion

Aim at reducing complexity and uncertainty in fixed-position assembly islands by utilizing information visibility and visualization. This paper introduces a DT-GiMS for fixed-position assembly islands. Firstly, inspired by the success of the graduation ceremony, an assembly system-GMS is developed for fixed-position assembly islands. After creating unified digital representations of manufacturing objects, the assembly process of the product and the production system, the overall framework of

Declaration of Competing Interest

We wish to draw the attention of the Editor to the following facts which may be considered as potential conflicts of interest and to significant financial contributions to this work. [OR] We wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.

We confirm that the manuscript has been read and approved by all named authors and that there are no other

Acknowledgement

Acknowledgement to Zhejiang Provincial, Hangzhou Municipal, Lin'an City Governments, Hong Kong ITF Innovation and Technology Support Program (ITP/079/16LP) and Shenzhen Science, Technology and Innovation Commission Support Program (KQJSCX20170728162555608).

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