Two-phased force and coordinates controller for a pair of 2-DOF soft fingers

We have previously shown that a hemispherical soft fingertip is in equilibrium when it is in contact with an object. The characteristics of the contact force and flexibility of a soft fingertip are important in stable grasping and manipulation. Hence, using these characteristics soft fingers can manipulate objects dexterously. We previously focused on a pair of 1-DOF fingers with soft fingertips. In this paper, we present a control scheme by which a pair of 2-DOF soft fingers can control a grasped object´s planar coordinates. First, we formulate the equations of motion of soft fingered manipulation, which has the local minimum of elastic potential energy (LMEE), defined as the characteristic of equilibrium that appear during manipulation. Next, we propose a new control scheme, which can control a grasped object´s planar coordinates. The proposed control scheme includes a positional controller of each finger joint angle and an integral controller of the object coordinates. Finally, we apply this control scheme to an experiment and a simulation, based on a parallel distributed virtual spring model to control planar coordinates of a grasped object. The agreement between our experimental and simulated results shows the validity of our control scheme. At the end of the paper, we show the possibility of controlling the grasping force by extending the theory of the proposed controller.