# Digital Twin for Human-Robot Collaboration in Manufacturing: Review and Outlook
## 2 Potential of Digital Twin and Human-Robot Collaboration  
Through Industry 4.0 developments, the building of highly accurate virtual replicas of physical entities in manufacturing is now possible, which then enables control and visualization of factory operations [15]. The resulting DTs can be coupled with HRC operation to evaluate and ensure safe cooperation between a human operator and the robot [20]. A DT model consists of a physical environment and a virtual environment. The changes occurring in the physical environment are continuously monitored, tracked and updated in the virtual environment [21; 22]. Figure 2 illustrates an abstract representation of a digital twin model of a motor. In the scenario shown, the physical system's (motor) state is captured using sensors and communicated to the virtual model (virtual replica of the motor). The virtual model recreates the exact state and conditions, such as field currents, motor temperature, state-rpm, on/off and computes the motor's efficiency. The value of the process parameters to run the motor more efficiently can also be calculated using this virtual model. Furthermore, the optimal process parameters obtained from the virtual model can be inputted to the physical model to operate the motor more efficiently [23]. DT models operate based on multiple underlying components: CAD and CAM data, inventory data, PLM, simulations and real-time sensor information to digitally integrate processes from design to manufacturing, with a seamless transfer of data between systems [24]. DT provides flexibility in simulating and analysing various scenarios without modifying the physical setup or building physical prototypes. It can also support identification of unforeseen issues in the process design. DT can facilitate the e  
Figure 2: An abstract representation of the digital twin (DT) model of a motor. A DT model recreates real-world entities in a virtual environment with the exact characteristics of the real system. With the information from the sensors, the virtual model will reproduce the exact state and condition of the real-world system. behaviours and help overcome them at a very early stage [25]. Various companies, such as Dassault, Siemens, ANSYS, Boeing and GE incorporate DT within their business lines [26]. Some of the challenges and requirements of implementing alternative approaches to DT technologies are identified and addressed in [26]. The term DT was presented for the first time at a conference on product lifecycle management (PLM) in 2003 by [PERSON]. At this stage, DT was relatively immature due to the lack of knowledge in the field [27]. An early DT model was developed by NASA and the United States Air Force for the systems used for aerospace equipment maintenance [13]. The model was subjected to simulated flight conditions and these simulations were carried out using data from an actual space vehicle [28]. In the early stages of its development, the most widely used application of DT was related to identifying faults, performing predictive maintenance, machine health monitoring, performance and quality analysis [29]. Today, as the manufacturing industry is seeking to develop highly automated processes, DT will play an important role, particularly when it comes to collaborative robots. Collaborative robots are robots that are specifically designed to work alongside humans without the need for any safety or protective barriers that are otherwise present in an industrial robotic system/robotic cell [30]. Collaborative robots are also capable of physically interacting with a human in a safe manner. These capabilities are achieved by using sensors and safety features embodied within the robot to sense the environment, especially the presence of humans and other obstructions, and to act accordingly. Using built-in sensors, data related to the state of the robot joints, compliance control, force, and impedance control is readily accessible and can be utilized for developing a DT.