## 1 Introduction and Related Works  
Collaborative robots and their implementation in the assisted assembly process is an important part of the Industry 4.0 concept. They can work in the same workspace as human workers and perform basic manipulations or simple monotonous assembly tasks. This area is open to new research, methodology development and definition of basic requirements, because real applications in production processes are currently still limited. The main advantage of using collaborative robots in the assembly process is a minimal transport delay of assembly parts between manual and automated operation. Other benefits are, for example, an integrated vision system for additional inspection of manual operation, the possibility to provide the interface for digital data collection from sensors and communication with external cloud platforms. Appropriate human-robot cooperation can significantly improve assembly time, but both must have exactly defined methods of communication between them. For example, the collaborative robot can check the success of a worker assembly operation by the integrated camera and the worker can get information about this status by mixed reality devices. The mixed reality device also can shortcut the time of staff training. Configuration principles of a collaborative robot in an assembly task were introduced in [1], a framework to implement collaborative robots in the manual assembly in [2] and a human-robot collaboration framework for improving ergonomics in [3]. The important condition for the assisted assembly process is the synchronization of augmented (AR), virtual (VR), or mixed (MR) reality devices with the digital twin for full digitalization of the used technology. A nice review of virtual, mixed, and augmented reality for immersive systems research is presented in [4]. Some other research results of the mixed assembly process between human and collaborative robots are described in [5; 6; 7]. An AR-based worker support system for human-robot collaboration using AR libraries was proposed in [8] and an anchoring support system using the AR toolkit was developed in [9]. A novel approach for end-user-oriented no-code process modeling in IoT domains using mixed reality technology is presented in [10] and a holistic analysis towards understanding consumer perceptions of virtual reality devices in the post-adoption phase in [11]. Technologies of virtual, augmented and mixed reality have an important role also in educational and training purposes as it was, for example, stated in [12; 13]. Our research in the field of a digital twin implementation into assembly processes started with the digitalization of the experimental manufacturing assembly system described in [14]. A digital twin can visualize the real status of a manufacturing system as a 3D simulation model with real-time actualization, so there is a need to have an appropriate simulation model, for example a generic simulation model of the flexible manufacturing system developed in [15]. An automatic generation of a simulation-based digital twin of an industrial process plant is described in [16]. The basic overview about the usability of a digital twin can be found in [17], the possibility of improving the efficiency of the production by the implementation of digital twins is presented in [18]. Some learning experiences after establishing digital twins are described in [19] and using a digital twin to enhance the integration of ergonomics in the workplace design in [20]. Recommendations for future research and practice in the use of digital twins in the field of collaborative robotics are given in [21]. Also, this brief overview shows that a digital twin is an important element in the implementation of an assisted assembly into the production process.