Design of a multi-stage stiffness enhancing unit for a soft robotic finger and its robust motion control

Conventional robotics have always pursued methods to bring robots near human. Human robot interaction (HRI) has gained great momentum to realize freeing robots with various control methods to ensure their safe operations with human presence. Soft robotics, alternatively, focuses on building robots mainly made of low elastic moduli materials, which may not be capable of harming humans due to nature of the materials they are made of, nevertheless, requires stiffness augmentation to be able to transmit relatively higher forces. In this study, we designed and fabricated an underactuated soft robotic finger using fused deposition modelling type 3D printing with a thermoplastic elastomer material. Robotic finger is actuated with a tendon-cable attached to a linear servo-actuator while secondary linear servo-actuator is used to control position of a stiffness-enhancement unit made of bistable metal strips. In order to verify stiffness enhancement, we perform precise position control and estimate external disturbances. Experimental results suggest that estimated external disturbances (i.e. stiffness of soft robotic finger) change under the same position conditions (finger flexion) thanks to precise robust motion controller.