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

Soft robotic structures may play a major role in the 4th industrial revolution. Researchers have successfully demonstrated the advantages of soft robotics over traditional robots made of rigid links and joints in many application areas. 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. However, a thorough characterisation of soft and rigid robotic components is still required. 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 precision) 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 similar to 0.95), output force (0.93 similar to 0.94), and repetitive precision 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 similar to 0.21) and repetitive position precision (0.07 similar to 0.25). However, similar performance on repetitive force precision and better performance on the maximum output force (1.54 similar to 1.55 times) are demonstrated.