**Preface to "Digital Twins in Industry"**

Digital twin (DT) is an emerging and fast growing technology that provides a promising way to connect and integrate physical and virtual spaces seamlessly. In brief, a DT is a digital representation of a physical object or system. It has bi-directional communication capability with the physical twin through sensors and networks. DT is an evolution and integration of the various information-communication technologies (ICT) that have proliferated in the IT scene for the last two decades. It integrates internet of things (IoT), big data, cloud and edge storage, artificial intelligence of things (AIoT), augmented reality (AR), etc., to form a comprehensive communication network for controlling, monitoring, diagnosis and health inspection of equipment and facilities, traffic and transportation systems, buildings, etc.

DT has attracted much interest and enthusiasm from the academia as well as the industry. While academia has worked on algorithms and frameworks, industry will be the final implementer, as they can see the immediate benefits offered by DT technology. This book focuses on the industrial applications of DT technology, and it provides insights for the practitioners on how DTs can be successfully planned and implemented, as well as the desirable outcomes achieved.

This book contains 11 chapters covering a broad range of applications, in-depth review and integration of DT with other technologies such as AR and Industry 4.0.

In the chapter by Warmefjord, et al., they discussed the barriers in the industry to be overcome ¨ before the use of DT for variation management and geometry assurance can be fully utilized. An extensive interview with engineers from eight different companies was conducted. They concluded that 3D models must be kept fully updated in order to maintain a robust digital thread.

The chapter by Sepasgozar advocated DT and web-based gaming technologies for online education. Not quite an industry application of DT as such, as it is more for educators. This is useful in view of COVID-19, as much of the face-to-face instruction has been virtual and online.

The chapter by Jacoby and Uslander emphasized the importance of interoperability by ¨ addressing the need to consolidate the various standards of DT and IoT. A classification scheme was created and applied to the standards to adopt serialization formats and network protocols to be used. An important issue, as this could lead to smooth and robust operations of DT and to overcome barriers of Industry 4.0.

An industrial application of DT was presented by Bambura et al. They implemented DT for engine block manufacturing processes. They constructed a DT consisting of three layers: physical, virtual and information-processing layers. Raw data were collected using programmable logic control (PLC) sensors. They concluded that even only partial results were presented, DT seems to be a prospective real-time optimization tool for the industry.

Another industrial application by Sierla et al. proposed a semi-automatic methodology for generating a DT of a brownfield plant, which is in the area of construction and urban development. Many procedures are required to construct a DT as outlined in the paper. The case study showed that only few manual edits were needed to the automatically generated simulation model.

In the chapter by Greco, a DT was used to set up models for monitoring the performance of manual work activities with near real-time feedback to support the decision-making process for improving working conditions. This is an interesting presentation of a human-centric DT for improving ergonomics and working conditions.

Autiosalo et al. presented an integrated DT for an overhead crane, providing service to machine

designers and maintainers in their daily tasks. They showed that a good-quality Application Programming Interface (API) is a significant enabler for the development of DT, and advised traditional industrial companies to start building their own API portfolios.

In another industrial application, Pang et al. developed a DT and Digital Thread framework for an "Industry 4.0"shipyard. A new framework that combines the DT and Digital Thread was proposed for better management and to ensure continuity and traceability of information. The twin/thread framework encompasses specifications that include organizational architecture layout, security, user access, databases and hardware and software requirements.

The chapter by Pareja-Corcho et al. reported the development of simulation tools for the gerotor pumps. The paper is not a direct application of the DT, but is a virtual prototype that can be considered in the context of a DT tool. Future work is necessary to further integrate the physical pump with the software tool.

Agnusdei et al. presented an interesting chapter querying if DT technology supports safety management. The study analysed existing fields of applications of DTs for supporting safety management processes and provided a comprehensive bibliometric review to identify future trends between the DT approach and safety issues.

Carvalho and da Silva reported a rarely addressed area of DT-based systems in sustainability requirements. They conducted a meta systematic literature review and concluded that DTs across the product life cycle or the DT life cycle are not sufficiently studied. In addition, they mentioned in their research that it was not possible to find a paper discussing DTs with regards to environmental sustainability.

With the myriad of academic and industrial reports on DT development, this Special Issue could only represent a small fragment of the entire DT application scenario, not to forget the highly sophisticated commercial software that has been developed in recent years that are capable of handling large-scale and complex industrial systems.

DT is a promising technology, and its impact is yet to be fully realized in time to come.

This book would not have been possible without the dedicated contributions of the authors, reviewers and the editorial team of *Applied Sciences*.

The editors wish to congratulate all the authors and thank all the sterling support from the reviewers who have helped to refine the submissions.

Finally, we would like to place on record our sincere gratitude to Ms Jennifer Li, Associated Publisher *Applied Sciences*, who mooted this book and other assistant editors who relentlessly contributed their time and effort.

> **A.Y.C. Nee, S.K. Ong** *Editors*

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