Concentric Precurved Bellows: New Bending Actuators for Soft Robots

Abstract

We present a new mechanical bending actuator for soft and continuum robots based on a pair of concentric precurved bellows. Each bellows is rotated axially at its base, allowing independent control of the curvature and bending plane of the concentric bellows pair. Rotation of precurved nested tubes is a well-known principle by which needle-sized concentric-tube robots operate, but the concept has never been scaled up to large diameters due to the trade-offs of increased actuation forces, decreased range of motion, strain limits, and torsional windup. In this letter, we show that using bellows structures instead of tubes allows two important breakthroughs: (1) actuation by rotation of precurved concentric elements can be achieved at much larger scales, and (2) torsional lag (i.e. when the relative tube angle at the tip differs from that at the base) and torsional instability are virtually eliminated due to the high ratio of torsional rigidity to flexural rigidity endowed by the bellows geometry. We discuss the development of two types of 3D printed concentric precurved bellows prototypes (revolute and helical), perform model parameter identification, and experimentally verify a torsionless mechanics model which accounts for direction-dependent rigidities.