Strain-based Pinch Force Control for Soft Object Manipulation

This paper newly proposes a force control method for soft object manipulation by multi-joint three-fingered robotic hand. In this study, the soft object is made of polyurethane resin, which enables the force control method based on strain of the grasped object to work well. First, we mention the design of anthropomorphic robotic hand, which contains eight degrees of freedom as the joint and five DC motors in total. A tendon-driven mechanism and its fine wiring enable grasping force to be 14.0N at maximum. This robotic hand has an index/middle finger and a thumb to realize stable and robust grasping motion, in which an optimal arrangement of the tendon is designed in all three fingers. In addition, this study designs a novel small sensor capable of estimating the Young’s modulus of a contacting soft object, in which we formulate an estimation algorithm based on Hertzian contact theory. The Young’s modulus measurement system has been optimized for a humanoid hand and realized with less than a 10% measurement error in verification experiments. By using the small sensor, we demonstrate that soft object manipulations can be easily achieved with a three-fingered robotic hand on the basis of inner-loop force feedback control with the strain reference of a soft object. Finally, we show secure dynamic responses by the three-fingered hand, in which strain output is able to successfully follow stepwise target signal regardless of the viscoelastic repulsive force of soft objects.