Joint design and fabrication for multi-material soft/hybrid robots

The premises of safer interactions with surroundings and the higher adaptability to its environment make soft robotics a very interesting research field. Some robots try to achieve these feats using soft materials in their designs whereas some achieve behavioral softness through compliant use of hard materials. In this work, we present soft/hybrid robot leg designs that utilize elastomers as leg materials but hard DC motors as actuators. Two different leg designs that would convert the rotational motion of the DC motors to a foot trajectory are proposed. The different leg designs are kinematically identical; however, the hourglass design utilizes geometrical modifications to differentiate joint locations, whereas the composite design uses materials with different Young's Moduli without geometrical effects to create joints. In order to fabricate the composite design, a new method is developed involving 3D printed molds with removable joint pieces and a two-step molding process. Both of the legs are fabricated and simulations and experiments are run to compare their performances. Both mechanisms achieve a good foot trajectory, however the hourglass joint experiences higher mechanical stress during operation, which might lead to earlier failure especially under high loads. Such multi-material structures made out of elastomers can be utilized in miniature robots or mechanisms of similar size in which absolute joint locations are needed and continuum robotic limbs are not preferred.