Fig 1.
Task design and experimental setup: A. The task involved moving a two-dimensional cup with a ball sliding inside, inspired by humans carrying a cup of coffee. B. Mechanical model of a cart and pendulum. If the ball is at the bottom of the cup, the ball angle is defined as zero with positive and negative directions as shown. Hand movements that displace the cup on the horizontal line create interaction forces, Finter. C. The virtual experiment: A participant stands in front of the large screen and holds the handle of the HapticMaster robot to move the cup in the virtual environment. The inset shows the subject’s grip on the robot handle. D. Two rectangles on the screen delimited the cup movement amplitude, although they were larger than the cup allowing for some tolerance to downplay accuracy demands. Box A and Box B were 0.30 m apart, located at -0.15 m and 0.15 m, respectively.