Electrohydraulic Folding Ring Actuators for Radial Contraction

Soft radially contracting actuators, inspired by biological circular muscles, enable diverse robotic functions, including manipulation, gut simulation, locomotion, and haptics. The combination of electrohydraulic actuation with radially contracting actuators holds promise for the development of high-performance radial contraction in a compact form factor. In this paper, we introduce an electrohydraulic folding ring actuator (EFRA) that utilizes folding motion to actively close its inner lumen. The EFRA is lightweight (∼25.9 g) and compact, consisting of a triangular ring with sides of ∼65 mm. Under 7 kV actuation, the EFRA achieves a high contraction ratio of 0.89 sustained over 5 s of actuation. This performance is enabled by the multiple synchronously bending segments in the folding mechanism, which enhances radial contraction compared with prior electrohydraulic radially contracting actuator designs. The EFRA also produces ∼0.96 N of inward-directed force measured at a single inner vertex under 7 kV actuation, yielding high force output relative to its low mass. Finally, we demonstrate the advantages of the high performance and compact nature of the EFRA in multiple applications, including robotic manipulation, locomotion, as well as an artificial robotic sphincter.