# Digital Twin for Human-Robot Collaboration in Manufacturing: Review and Outlook
## 3 Review of DT in HRC
### Definitions  
Though the concept of DT dates back to the early 1990s, the definition of DT was first published in the NASA Modelling, Simulation, Information Technology, and Processing Road Map in 2010 [13, 14, 49, 50]. According to this definition, DT is \"An integrated multi-physics, multi-scale, probabilistic simulation of a vehicle or system that uses the best available physical models, sensor updates, fleet history, etc., to mirror the life of its flying twin\" [51]. The definitions widely used in the literature from 2010 to 2016 are summarized  
Figure 3: The number of papers found in Google Scholar from the period of 2016—2021. The keyword used for the search is “Digital Twin” and “Robotics” for the data represented in black and the keyword used for the data in grey is “Digital Twin” and “Human Robot Collaboration.” The trend line indicates that interest in these areas has increased significantly since 2016.  
in [13] and from 2014 to 2020 in [52]. There is a wide array of definitions for the term DT in the literature [1, 13, 53, 54, 17]. However, the concept of DT lacks clarity in a number of areas, such as the definition, interpretation and implementation. For instance, when DT is compared with system emulation, which is the concept of using an application to imitate a system behaviour, it is defined as \"A Digital Twin is the digital representation of a product or system under development representing a functionally correct, predictable and reproducible representation of the product or system at the appropriate level of fidelity to perform verification, performance analysis and system validation tasks\" [55]. In summary, a common understanding of the term DT is that it is a high fidelity multi-physics digital model capable of describing the micro and macro features and mirroring the state of and the behaviour of the physical system. DT also has the capability of performing simulation in the virtual model and mirroring the same to the physical model in real-time. Another feature of DT is the two-way real-time communication that exists with the physical system. In general, the definition of a DT tends to be based on an understanding of the concepts, the context and the system for which a particular DT is developed. DT can be defined in many ways due to the difference in understanding of DT as a model, simulation technology, or its association with IoT or CPS systems. This difference is based on the level of integration and mode of exchange of data [13, 53]. For instance, digital representations such as a digital model, a digital shadow and a digital twin are often confused with one another [1]. Figure 4 illustrates the distinction between these three in the mode of exchange of data between the physical and virtual models. There are two modes of data exchange, namely manual and automated. A change in the physical model that automatically updates the virtual model and vice versa is defined as an automated data exchange, whereas when the physical model does not update changes observed to the virtual model and vice versa, it is described as manual data exchange [1]. Although the underlying functionality of CPS and DT is to integrate the physical and the virtual world, the critical difference between them is the relative focus of each of the technologies. The core of the DT relies on the virtual model and the data to model the characteristic behaviour of the product, whereas the core of CPS involves the integration of the sensors and actuators [27].