  As factories continue to evolve into collaborative spaces with multiple
robots working together with human supervisors in the loop, ensuring safety for
all actors involved becomes critical. Currently, laser-based light curtain
sensors are widely used in factories for safety monitoring. While these
conventional safety sensors meet high accuracy standards, they are difficult to
reconfigure and can only monitor a fixed user-defined region of space.
Furthermore, they are typically expensive. Instead, we leverage a controllable
depth sensor, programmable light curtains (PLC), to develop an inexpensive and
flexible real-time safety monitoring system for collaborative robot workspaces.
Our system projects virtual dynamic safety envelopes that tightly envelop the
moving robot at all times and detect any objects that intrude the envelope.
Furthermore, we develop an instrumentation algorithm that optimally places
(multiple) PLCs in a workspace to maximize the visibility coverage of robots.
Our work enables fence-less human-robot collaboration, while scaling to monitor
multiple robots with few sensors. We analyze our system in a real manufacturing
testbed with four robot arms and demonstrate its capabilities as a fast,
accurate, and inexpensive safety monitoring solution.
