A Simple Free-Fold Test to Measure Bending Stiffness of Slender Soft Actuators

A reliable estimate for bending stiffness is critical to many soft robot models when predicting everything from robot-environment contact to buckling resistance. Current methods for predicting actuator bending stiffness rely on highly accurate knowledge of material characteristics, which are not trivial to obtain for composite actuators. Additionally, current models for fluidic actuators often depend on a pressure-independent bending stiffness despite pressure playing a non-negligible role in bending stiffness behavior. Methods to measure actuator stiffness often require costly instrumentation to measure or perturb the motions and forces required to measure actual bending stiffness. We introduce a simple free-fold test to empirically estimate the bending stiffness of slender soft actuators - pressurized or unpressurized and composite or homogeneous - which requires the measurement of one distance from a single image of a specific robot pose. The resulting model also shows that the change in actuator weight per unit length can be used to determine the dependence of bending stiffness on actuation pressure.