Toward Collaborative Robots: A Novel Hybrid Slider–Crank Mechanism with a Soft Variable-Stiffness Connecting Rod

Abstract

Robots designed for human interactions require not only precision
but also adaptability and safety during physical contact. Traditional
rigid robots perform well in structured industrial environments
but are limited in collaborative applications because of their inherent
lack of compliance. Soft robots, by contrast, offer intrinsic safety and
adaptability but struggle with accurate modeling and controllability. To
address these limitations, this paper proposes a novel design strategy
for robotic mechanisms that integrates rigid and soft components to exploit
the advantages of both principles. This strategy is demonstrated
through a hybrid slider–crank mechanism incorporating a soft, variablestiffness
connecting rod into the conventional rigid framework. The soft
link, constructed as a pneumatic pressure–responsive elastomer composite,
allows active stiffness modulation: elevated pressure increases rigidity
for accurate force transmission, whereas reduced pressure enhances
compliance for safer interaction. A theoretical model of the mechanism
is developed to describe stiffness variation, dynamic force transmission
under different pressurization levels. The design and fabrication process
of the soft link is presented, followed by indentation tests to characterize
pressure-dependent stiffness and dynamic experiments to evaluate
force transmission in the hybrid mechanism. Experimental results confirm
that pneumatic pressurization effectively tunes the stiffness of the
connecting rod, leading to controllable changes in dynamic response and
output force. This work demonstrates the feasibility of embedding soft
variable-stiffness elements into core structural components of classical
mechanisms, paving the way for collaborative robots that combine precision,
adaptability, and safety.