  Industrial standards define safety requirements for Human-Robot Collaboration
(HRC) in industrial manufacturing. The standards particularly require real-time
monitoring and securing of the minimum protective distance between a robot and
an operator. In this work, we propose a depth-sensor based model for workspace
monitoring and an interactive Augmented Reality (AR) User Interface (UI) for
safe HRC. The AR UI is implemented on two different hardware: a
projector-mirror setup anda wearable AR gear (HoloLens). We experiment the
workspace model and UIs for a realistic diesel motor assembly task. The
AR-based interactive UIs provide 21-24% and 57-64% reduction in the task
completion and robot idle time, respectively, as compared to a baseline without
interaction and workspace sharing. However, subjective evaluations reveal that
HoloLens based AR is not yet suitable for industrial manufacturing while the
projector-mirror setup shows clear improvements in safety and work ergonomics.
