# Towards Integrated Digital Twins for Industrial Products: Case Study on an Overhead Crane
## 1 Introduction  
Digital twin (DT) represents a new paradigm for the Industrial Internet of Things. DTs are linked to a real-world counterpart and leverage several technologies and other paradigms, such as simulation, artificial intelligence, and augmented reality, for optimizing the operation of the counterparts. DTs are being built at an accelerating pace in both industry and academia and the digital twin term has reached multiple expressions of recognition, such as being among the IEEE Computer Society's Top 12 Technology Trends for 2020 [1]. The roots of the digital twin concept are in mirroring physical systems as exemplified by two seminal publications: [PERSON] and [PERSON] [2] described digital twins as a set of virtual information describing a potential or actual physical manufactured product and NASA [3] described DT as an integrated ultra-realistic simulation that combines physical models and sensor data. The purpose and use cases of the concept have since been further described in further leading publications [4, 5, 6]. These concentrate on various areas of mechanical engineering, which is a trend continued by the majority of digital twinapplications as shown by multiple review articles [7; 8; 9; 10; 11; 12]. More recently, other domains, such as healthcare [13], business [14], and buildings [15], are starting to adopt digital twins as well. For a more in-depth exploration of the background of the digital twin concept, we refer to Section II in [PERSON] et al. [16]. Several earlier studies make recommendations for digital twin research. There seems to be a lot to be improved, as [PERSON] et al. [11] conclude their review by stating that current digital twin literature cannot be inherited by other researchers. Other studies point out specific issues, including both technical aspects as well as considering humans as crucial actors in twin development. [PERSON] et al. [17] list the integration of information technology and the integration of partner companies as research issues. [PERSON] et al. [10] list the cost of development and human interaction as challenges and call for a sociotechnical and collaborative approach in designing digital twins. [PERSON] et al. [18] identify three topics for future research agenda: information architectures and models, specific applications, and the role of the human. [PERSON] and [PERSON] [19] recognize the need for experts from several disciplines and the need to make building digital twins easier. They also state that there should be physical 'innovation hubs' that are accessible to experts from different fields. [PERSON] et al. [14] recognize people and their skills as an essential factor in adopting digital twins. We condense these research issues into the following research question: \"How to build integrated digital twins for industrial products?\" This study aims to answer this question through a case study on the digital twin development journey of an industrial overhead crane. Despite the high amount of manufacturing-related DT publications, only few DTs have been developed specifically for cranes. [PERSON] et al. [20] developed and validated a simulation-and-control focused digital twin for an offshore crane for lift planning purposes. [PERSON] et al. [21] experimentally verified a real-time simulation of strain on a small-scale boom crane to prove that simulation-based virtual sensors can be used for condition monitoring. [PERSON] and [PERSON] [22] developed a statistical decision-making model to efficiently schedule maintenance breaks for port gantry cranes. Additionally, an overhead crane is mentioned as a part of an ontology focused roll grinding simulation configurator [23]. The main research data of this study comes from the digital twin development journey of an overhead crane \"Ilmatar\" [24]. Most of the development was implemented as an industry-university research project called \"DigiTwin\" which is referred to as \"the project\" in this study.