# Digital Twin for Human-Robot Collaboration in Manufacturing: Review and Outlook
## 3 Review of DT in HRC
### Approaches in DT
#### 3.3.1 Simulation-Based Approach  
The simulation-based approach (SBA) combines the digital models and the data generated at each stage of the product development lifecycle to build the DT [23; 28; 49; 57]. For this, the real-world object's physical, geometrical and behavioural properties are required. Figure 5 gives an overview of the various steps in the development of DT via SBA. The SBA primarily focuses on the collection of data via multiple simulation models at various phases of the product lifecycle, such as the design, engineering, operation and service phases. SBA uses a linear approach where the information gathered at each stage is used to update the digital twin model and maintain a complete record of the product lifecycle as it evolves [58]. In SBA, the DT evolves with the real product by embedding various characteristics and properties of the product [23]. Additionally, the modules used across all four phases of the DT model in SBA need to recognise the data formats used by all other modules. Therefore, the digital thread will play a crucial role in the future of DT under SBA architecture [23; 59; 60]. Digital thread and data-driven communication architectures can provide seamless integration and transfer of data across the complete product lifecycle. Digital thread supports the process by having a unified data format with all the information required for the exchange of data between the physical and virtual models of the DT [60]. During the development of the DT using SBA, the DT model is commissioned virtually before the operation phase to evaluate the performance and to identify and solve any shortcomings of the model [61; 62; 63]. Virtual commissioning can be carried out either by using a hardware-in-the-loop simulation or by using a software-in-the-loop simulation [64]. In a hardware-in-the-loop simulation, the virtual plant is controlled using actual control hardware. In a software-in-the-loop simulation, the virtual plant is controlled by using a virtual controller. A framework that can automate the process of decision making for a simulation-based DT of a CPS is illustrated in [65]. The software-in-the-loop virtual commissioning enables the designer to validate the hardware system before it is manufactured [66]. This is particularly helpful in the case of HRC, as the system can be tested for safety before being physically implemented on the shop floor. The virtual commissioning of a collaborative robot for a bin-picking application by using software-in-the-loop simulation is explained in [67]. Experimental digital twin, as stated in [68], can be helpful in developing the DT prototype to access and verify safety in HRC scenarios. Furthermore, DT combined with VR can be utilized for safely testing the HRC scenarios [69]. A framework to integrate human-robot simulation with VR is explained in [70]. SBA can initiate multiple DT instances to validate the design, safety  
Figure 5: Overview of simulation-based DT. The data collected at each stage of the product development are used to create a digital instance of the physical object. The digital object is developed in parallel with the physical object. and process planning. For example, an event-based simulation model of a DT with a case study of an HRC scenario is discussed in [48]. This study focuses on work-load balances, trajectory optimization and collision avoidance, and shows the capabilities of DT within a HRC scenario. Hence, DT modelling can enable companies to save time and cost in the design of robotic processes [71]. An architecture to implement DT for the controlled flow of information in a smart CPS for planning is discussed in detail in [57]. Here, the foundation level is comprised of the general physical properties of the product, the second level is the interface between the machine and environment, the third level is the reaction or response system, and the final level is the planning.