Task and workspace sharing between human and robots offers an efficient combination of individual strengths and is a promising attempt to meet the continuous increasing demand on flexibility within assembly systems using human-robot co-operation. Here, the major pre-condition is the human safety along robot-based process steps limiting the system effectiveness. Particularly the automated assembly motion implies extensive considerations due to required contacts which are difficult to handle by common safety systems. This paper presents a methodical approach for the continuous model-based supervision of the robot assembly execution in proximate workspace sharing. On this basis, a conceptual framework is derived and the requirements as well as design principles are discussed.