A Study of the Underactuated Mechanisms with Compliance

It is often suggested that hand prostheses and robot grippers must possess a kinematic structure that is similar to those of the natural human hand [1-4]. Such structure allows better adaptation of the gripper to the size and the shape of the grasped object and better cosmetic effect. Anthropomorphic hands with a large number of joints are highly dexterous, but their independent joint control requires a large number of actuators. While human muscles can generate very high energy per unit weight, the electrical motors used in the artificial hand design do not have high power-to-weight ratio. Therefore, the mimicking of the natural hand structure with presently available technology often leads to serious design difficulties and the designed mechanisms become large, heavy and less powerful. As a solution to the problem, many design concepts of robotic hands with fewer actuators than degree-of-freedom (DoF) have been introduced [5-12]. In such concepts, one actuator is connected with several joints and operates them simultaneously. In this research field such mechanisms are often called “underactuated” mechanisms. The idea of the underactuated gripper mechanisms is suggested by various studies of human hand motions which showed that specific human movement tasks can be performed by a small number of variables [13]. The underactuated grippers do not have the grasping dexterity of the human hand but they can replicate natural movements on a satisfactory way for a significant set of human movement tasks. Although the accuracy of the synthesized underactuated mechanisms is lower than the accuracy of the mechanisms with the same kinematic structure and independently controlled joints, the approach allows the synthesis of hands with simple structures that possess high functionality and precise motions for the named set of tasks. The underactuated gripper mechanisms were further improved by adding elastic elements or self-adaptive transmission between the output of each actuator and the corresponding robot hand joint [5-12]. Such a solution increases gripper adaptability and allows successful handling of objects with bigger variations in shape and size. Improvement of the functionalities of the underactuated robot hands have been widely studied [1416]. Ciocarlie et al. [14], introduced a quasistatic analytical tool for underactuated hands performing grasping tasks. Balasubramania et al. [15], studied the motion and force capabilities of a variety underactuated mechanisms. Prattichizzo et al. [16], explored gripper manipulability by introducing manipulability ellipsoids of underactuated hands.