# Digital Twin for Human-Robot Collaboration in Manufacturing: Review and Outlook
## 4 Challenges
Another issue to consider in the modelling process is the challenges associated with the validation of the models. The number of possible states the DT-HRC can take at a given time, considering the dynamic nature of the human who is collaborating with the robot is also an obstacle in using the DT-HRC for process optimization. The large number of parameters that affect these types of interactions will increase the number of possible states of the system [56]. There can be thousands of extrinsic and intrinsic parameters that can affect the HRC scenario. Finding all these parameters and simulating the same conditions in the virtual world will be tedious and computationally expensive. The modelling of a human on the shop floor to accurately replicate an exact digital version of an operator in the virtual world is extremely difficult to create. Modelling the digital twin of an operator, incorporating factors related to physical health conditions and emotional aspects, such as trembling, fatigue and emotional distress, to predict the outcome of the activities of the operator is very complex and almost unfeasible, at least, with currently available technologies. Therefore, it is highly unlikely to model the process in DT-HRC utilizing the complete model of operators in a virtual environment. The number of productive person-hours required to create a DT for simple robot applications, such as a pick and place task, is still unclear. One example is the development of DT that can mirror the exact robot in the virtual world, using a standard robot [83]. This DT provides a set of functionalities, such as plotting the robot data (current, velocity, torque, and motor temperature). However, there are no studies on how this information can be transferred to develop a DT of another robot or on how to model a more complicated process. Moreover, most articles discuss creating a high-fidelity model using DT, but are not clear on how the model should be developed. Likewise, the cost of purchasing and using commercial process simulation software with the required toolbox or plugins to create a DT is expensive. There is still a lack of knowledge and understanding about the concepts of DT, which makes it more challenging for manufacturing industry to implement DT concepts [26]. Fourthly, the integration of DT involves communication and interactions with various CPS and IoT sensors, and therefore requires reliable sensor information and faster updates of real-time data [178]. A potential challenge for implementing the DT-HRC is the problem related to real-time connectivity and synchronization. The lack of high-fidelity models in the virtual environment, along with the other challenges described previously, does not favour the possibility of establishing a closed-loop synchronization of the real and the virtual model. This increases the difficulties of capturing and processing large amounts of data. A reference model or a standard benchmark model is required to overcome these challenges [49]. An attempt to create a DT of a 3D printer showed a delay of 2-3 s to process the data and a 2-s delay in updating the digital and physical model [179]. These authors have also mentioned that their system is much simpler with limited data, and it will be much more difficult to synchronize a complex system. Another important issue to be noted from this research is the 2-3 s computational time required to process the data collected for a system with limited data [159]. This delay is not suitable for real-time control. In another example, the use of a DT to model crack propagation in a single mechanical component having machined features underlines the requirement of high computational power for DT [180]. It took four days to simulate the model when the simulation was running on a high specification personal computer. HRC systems are much more complex than the above-mentioned examples, with many assembled parts, sensors, actuators and other components in a dynamic environment.