# Digital Twin for Human-Robot Collaboration in Manufacturing: Review and Outlook
## 3 Review of DT in HRC
### Approaches in DT
#### 3.3.2 Layer Based Approach  
The second category, the layer based approach (LBA), describes the design of a DT system generally consisting of three layers [14; 21; 53; 78]. These three general layers are the physical layer, virtual layer and connection layer/information processing layer [79; 80]. Figure 6 provides a conceptual overview of modelling the DT-HRC using LBA. The attributes of the physical systems are collected via sensors that are mapped to the virtual system with the help of the connection layer. The connection layer enables the exchange of data between the physical and virtual layers. \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Strategies and Technologies** & \
\begin{tabular}{c} **Conceptual** \\ **Frame Work** \\ \end{tabular} & **Use Case** &
\begin{tabular}{c} **Implementation/** \\ **Demonstration** \\ \end{tabular} \\ \hline Digital thread based approach & [23; 59] & & [60] \\ \hline Hardware in the loop simulation & [63] & [68; 73] & [61; 62] \\ \hline Software in the loop simulation & [68] & [74] & [66; 67; 69; 75; 76] \\ \hline AR and VR & & & [69; 70] \\ \hline Data-oriented approach & [65; 77] & & \\ \hline DT of manufacturing process & & [58; 68; 73] & [62; 69] \\ \hline Digital factory & & [66; 73] & [76] \\ \hline HRC & [63] & & [69; 70] \\ \hline Sensor technology & [58] & & [69] \\ \hline Task planning & & [73; 74] & [70] \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Summary of strategies and technologies that can potentially be used for building a simulation based DT. The three main layers in LBA are as follows [19; 21; 29; 53; 78]:  
* Physical Layer The physical layer consists of information about the state, the properties of the system and the properties of the environment, collected through various sources such as smart sensors, product development software, resource management and planning tools, networking tools and communication tools. For example, information obtained in the physical layer includes:
* Design--CAD/CAM, factory layout, drawings of all the physical parts in the production environment; Resources--inventory data, bill of materials, process plan, physical properties of the product, simulation and modelling results; IoT devices--smart sensors, PLC, SCADA, distributed control system (DCS), actuators; Network--communication protocol, machine-machine interface, feedback and control. * Virtual Layer The virtual layer consists of an information model of the physical object and a data processing module [81]. The information model represents the structure of the physical object. The data processing module collects and processes the data about the physical object for real-time reconstruction of the physical object in the virtual world. Hence, the virtual layer represents the digital environment that provides the virtual replica of the object present in the physical layer. The advantage of the virtual layer is that it can create multiple instances of the digital twin model. These multiple models along with the data acquired from the real world can be used to simulate, predict and optimize the process. Additionally, the virtual layer can interact with its physical counterpart to update the system parameters to increase the efficiency of the physical system. * Connection Layer The connection layer acts as a bridge to link the physical and the virtual layer, i.e., it provides an exchange of data between the real-world entities and the digital twin model. This layer handles data storage, processing and communication. The con  
Figure 6: Layer-based approach for modelling of DT in HRC scenario. nection layers serve to map the data from the physical layer to the virtual layer and vice versa. The changes can be updated automatically in the physical or in the virtual model. Another purpose of this connection layer is optimization by processing the collected data. The raw data obtained via the sensors are processed to extract relevant information from them.