Characterisation of Antagonistically Actuated, Stiffness-Controllable Joint-Link Units for Cobots

Abstract

Soft robotic structures may play a major role in the 4th industrial revolution. Researchers have successfully demonstrated some of the advantages of soft robotics over traditional robots made of rigid links and joints in several application areas. In some applications, robots will need to work closely together with humans in a safe manner. However, soft robots have limited ability to exert larger forces when it comes to interaction with the environment, hence, changing their stiffness on-demand over a wide range. Variable stiffness links (VSL) and joints (VSJ) have been investigated to achieve on-demand forces and, at the same time, be inherently safe in interactions with humans.This paper investigates the influence of antagonistically actuated, stiffness-controllable joint-link units (JLUs) on the performance of collaborative robots (i.e. stiffness, load capacity, repetitive accuracy) and characterizes the difference compared with rigid units. A JLU is made of a combination of a VSL, a (VSJ) and their rigid counterparts. Experimental results show that the VSL has minor differences in terms of stiffness (0.62 ∼ 0.95), output force (0.93 ∼ 0.94) and repetitive accuracy compared with the rigid link. For the VSJ, our results show a significant gap compared with the servo motor with regards to maximum stiffness (0.14 ∼ 0.21) and repetitive position accuracy (0.07 ∼ 0.25). However, similar performance on repetitive force accuracy and better performance on the maximum output force (1.54 ∼ 1.55 times) are demonstrated.