Time to Contact for Robot Safety Stop in Close Collaborative Tasks

Alberto Olivares-Alarcos, Sergi Foix and Guillem Alenyà

Institut de Robòtica i Informàtica Industrial, CSIC-UPC, C/ Llorens i Artigas 4-6, 08028 Barcelona, Spain.

Abstract - Within the next years, industrial collaborative robots will collaborate with human operators, whose safety might be compromised. To ensure a safe collaboration, robots should estimate the risk of collision with respect to the pose and motion of operators within the shared workspace. A common approach is to compute and maintain aminimum distance between humans and robots during tasks’ execution. Nevertheless, separation-based solutions do not capture the real dynamics of the human-robot interaction, and tend to be rather conservative, avoiding a close human-robot collaboration. In this chapter, we explore the concept of time-to-contact (TTC) as a softer trigger of safety stops in collaborative scenarios where humans and robots are in constant closeness. Particularly, we propose a TTC formulation and study its advantages with respect to two approaches based on the protective distance proposed by the ISO standards. We compared the three methods in some representative cases extracted from an example of collaborative task. The evaluation is firstly done in simulation, and then, in a more realistic setup with a simulated human, aiming for repeatability, and a real robot. Furthermore, we showcased our approach in a demo of a complete collaborative task. TTC allows robots to operate closer to humans and for longer times before a safety stop is issued, which benefits long-term productivity. This later stop produces shorter human-robot distances, which might affect safety. However, the increment in time that the robot moves before stopping (productivity) is greaterthan the reduction in distance (safety). Hence, our work demonstrates that TTC is a smoother, but still safe, collision risk estimator for close human-robot collaboration.


Evaluations' results and demo of the experiments done with one user who collaborated with a robot on the accomplisment of a shared task: filling a tray with tokens.


We would like to thank Sergi Martínez for his technical support, Alberto San Miguel and Antonio Andriella for the fruitful discussions, and Sergio Santos for his help with part of the video.