Effect of Speed, Speed Differences, and Motion Type on Perceived Safety of Collaborative Robots

Abstract

Collaborative robotics is one of the main drivers of the fourth industrial revolution, where humans and robots work in a shared workspace. The robot movement trajectory and speed significantly affect perceived safety which plays an essential role in successful collaboration. The study aimed to measure absolute and differential sensitivity for the perception and discrimination of a collaborative robot arm movement speed and to study how robots' speed and trajectory influence the perception of pleasure, arousal, perceived unsafety, and humans' intention to collaborate with the robot. The study was performed using the Universal Robots UR5e robotic arm. Participants were assessing the perceived safety of the robot's movement for eight different TCP speeds (0.3 m/s - 1 m/s) under three conditions: (i): standing, (ii) sitting, and (iii) standing on a podium (0.3 m high) in front of the robot. For measuring the differential threshold, the participants were presented with 18 speed pairs. Each pair had one of the reference speeds (0.25 m/s, 0.5 m/s, 0.75 m/s), and the participants were asked to report if they perceived a difference in two consecutive movements. Different combinations of movement speed (slow, fast) and movement trajectory (linear, random) were evaluated. The results showed that the absolute threshold for feeling unsafe when standing in front of the robot (and the robot was operating at chest level) was higher than the one in the sitting condition and lower than the absolute threshold in the podium condition. The results also showed that the differential threshold was higher for higher speeds, meaning that participants were more sensitive to differences at lower speeds. Movement trajectories had the most substantial effect on the perceived safety parameters. Participants reported a higher sense of pleasure with a linear movement and a lower sense of arousal and unsafety than they reported with the random movement. Fast robots' movements arouse more negative feelings in participants than slow robotic movements and safe tools.