Redundancy and overactuation in cephalopod-inspired soft robot arms

Current soft robotic arms commonly follow traditional - i.e., hard - robot design conventions. Omnidirectional soft arms most commonly consist of segments that contain three parallel longitudinal actuators, or actuator groups, of which one or two are activated to induce bending. This design uses the minimum number of actuators required for omnidirectional bending, which is consistent with traditional robot design but unnecessarily constrains the soft arm design space. The cephalopods that frequently inspire soft robot arms have tens of bundles of longitudinal muscle fibers, and these bundles are distributed around the limb circumference rather than grouped. This article analyzes fluid-driven soft arm architectures with up to 12 actuators, which mimic the redundant and distributed nature of cephalopod arms and tentacles. Over-constraints (activating more than two actuators) were considered, which are possible because soft robots - unlike traditional robots - can tolerate conflicting constraints without jamming. A previously-developed generalizable model was used to simulate design performance, and a subset of the examined architectures were constructed and tested. Many-actuator soft arms achieved high strokes under equivalent load without sacrificing no-load reach and were able to execute near constant-curvature turns with a simple actuation pattern. The framework introduced here also enables cross section variations that are not possible with minimalist designs.