# Digital Twin for Human-Robot Collaboration in Manufacturing: Review and Outlook
## 2 Potential of Digital Twin and Human-Robot Collaboration
The concept of collaborative robots was introduced in the automotive industry during the late 1990's to increase the safety, quality, and performance and improve the ergonomics of assembly processes [31]. Since then, the capabilities of collaborative robots have increased over the years, initially from using a single-arm collaborative robot to the use now of dual-arm collaborative robots to perform manufacturing tasks [32; 33]. The main advantage of using collaborative robots within the manufacturing sector is their ability to work together with the operator without the necessity of being located in a secure enclosure [34; 35]. The terms collaboration and cooperation represent two different aspects of HRC. Collaboration means working together to achieve a common goal, whereas cooperation means when the operator and robot work together to achieve a shared goal [36]. Furthermore, depending on the scenario, the relationship between humans and robots can be classified as coexistence, interaction, cooperation and collaboration [37]. Conventional human-robot interaction approaches often lack feedback about the environment around the robot, which can cause stress and fatigue to the operator [38]. On the other hand, the symbiotic HRC approach involves sensors that offer more information about the environment, leading to better planning of tasks, ease of programming the robot, natural form of interaction and decision support [35]. As a result, much of the research related to HRC aims to achieve improved communication and task planning [39], interaction [40; 41; 42], trust between the robot and the operator [10; 34], and safety [34; 41]. Recent developments in the area of industry 4.0 [43], artificial intelligence (AI) [40], DT [22], non-invasive operator tracking utilising sensors such as somatosensory systems [44], wearable devices and vision-based tracking [35; 45], and augmented/virtual reality (AR/VR) [42; 43] provide additional mechanisms to increase the safety and efficiency of operations and understanding the intentions of the human operator. They also allow more robust methods for humans and robots to work together in a safe, collaborative environment. Operators tend to hone their skills over a period of time, making them more adaptable to various circumstances and enabling them to make decisions in a continuous and dynamic environment. Furthermore, research in the area of bionics and robotic prosthetics can aid humans to effectively participate in HRC in manufacturing environments [46; 47]. However, robots require programming or teaching to handle unexpected situations [35]. Hence, the safety of the operator working closely with the collaborative robots should be carefully analyzed [43]. There are various challenges involved in the modelling of HRC to establish safety and trust between the human and the robot. It is necessary to continuously monitor the activities of a HRC system to make the robots adapt to the operator [10, 48]. Therefore, DT in HRC could assist in the development of more reliable and trustworthy collaborative scenarios by planning and testing multiple scenarios and configurations of the HRC system in a virtual manner using a DT model.