Comparing and Configuring Soft Tendon Designs for Variable Stiffness Actuators on a Robot Arm

A Variable Stiffness Actuator (VSA) can vary the stiffness of a robot joint. Robots which use rigid links but soft joints like VSAs are known as articulated soft robots. The articulated soft robot arm in this paper uses an agonist/antagonist VSA setup with composite tendons made out of a soft material on the inside and an ideally non-elastic material on the outside. The outer material gradually aligns with the direction of the load, and compresses the inner soft material during extension. This provides a cheap and compact tendon that can be made to exhibit suitable spring characteristics for a VSA. The focus of the work presented here is to optimize the manufacturing process of these soft tendons through methodological tuning of parameters and the usage of off-the-shelf materials. The filament, outer sleeve and pulley configurations are modeled, and tensile testing used to provide data on the effect of different design parameters on the tendon properties. Soft tendons with an outer mesh sleeve that are easy to manufacture are implemented in a proof of concept experiment on the robot arm elbow joint. The results show that variable stiffness can be achieved with the proposed design but that the available outer sleeve is too flexible resulting in only a small range of stiffness levels. Several directions for improvement are identified.