# Digital Twins: Enabling Interoperability in Smart Manufacturing Networks
## 3 Digital Twins  
A digital twin is a virtual model that replicates a physical system or process, providing real-time data and feedback on its performance. Digital twins enable manufacturers to monitor and analyse the behaviour of physical assets in real-time, enabling them to optimise performance, reduce downtime, and improve efficiency. It is a virtual representation that acts as the real-time digital counterpart of a physical thing or process. The term \"digital twin\" was coined by Dr [PERSON] in 2002 at the University of Michigan. _\"Only when we get it to where it performs to our requirements do we physically manufacture it? We then want that physical build to tie back to its digital twin through sensors so that the digital twin contains all the information that we could have by inspecting the physical build.\"_ Dr [PERSON], 2002. While digital twins are most certainly used in manufacturing today, the use cases span across many industry verticals. Over the last few years, as the world has become  
Figure 4: Interoperability of different entities in smart manufacturing. more connected, data has become more readily available, richer, and more accurate. The proliferation of IoT equipment has meant data is harvested from a wide array of networked sensors, creating a detailed map of any enterprise asset or series of assets. A digital twin creates a virtual representation of a process, system, service, product, or other physical thing using virtual or augmented reality, 3D graphic modelling, and data modelling. The physical world is mirrored in this digital duplicate. Real-time updates are used to preserve its exact duplicate status. Due to its many benefits, including the elimination of errors and cost optimisation in any system, digital twin (DT) technology is regarded as the foundation of smart manufacturing. From the literature, it is clear how the evolution of the digital twin has been deployed over the years, falling into one of three subcategories [11; 12]:  
* Digital Model: A manual data exchange between a physical object and a digital object is required, and therefore changes in the physical object are not reflected in real-time. * Digital Shadow: Data from the physical object is automatically transferred to the digital counterpart, but not the other way around. Therefore, changes in the physical object can be viewed digitally but not vice-versa. * Digital Twin: A two-way data exchange between physical and digital objects is involved. Therefore, the changes in the physical/digital objects affect each other. Digital twins can be even further categorised in their deployments of various types, according to their creation, integration level, application, and hierarchy level of the digital twin. After reviewing the literature, a summary of the relevant terminologies of a digital twin is shown below [12; 13; 14]. In manufacturing, DTs can be divided into three levels from a hierarchical viewpoint [13]. A digital twin environment (DTE) is a logical setting in which hardware, software, and occasionally both interact to imitate a whole system or a portion of a system. A virtual clone of a tangible system, process, or product is created by digital twins. By using data analysis, the replica, for instance, may forecast when a machine will break down, enabling businesses to boost production through predictive maintenance [14]. * Part/Component Twin: the smallest unit on the industry floor and is based on a geometric, functional, and operational model of the unit-level physical copy. * System level: DT is the composition of many unit-level DTs in a production floor, interconnected for wider information flow and efficient resource allocation. * System of Systems (SoS) level: Formed when multiple system-level DTs are connected, bringing different departments such as logistics, design, service, maintenance etc. together into one twinned model. This is outlined below in Figure 5[13]. Figure 5: Progression toward a system of systems for digital twins.