On the Design of Underactuated Finger Mechanisms for Robotic Hands

The mechatronic design of robotic hands is a very complex task, which involves different aspects of mechanics, actuation, and control. In most of cases inspiration is taken by the human hand, which is able to grasp and manipulate objects with different sizes and shapes, but its functionality and versatility are very difficult to mimic. Human hand strength and dexterity involve a complex geometry of cantilevered joints, ligaments, and musculotendinous elements that must be analyzed as a coordinated entity. Furthermore, actuation redundancy of muscles generates forces across joints and tissues, perception ability and intricate mechanics complicate its dynamic and functional analyses. By considering these factors it is evident that the design of highly adaptable, sensor-based robotic hands is still a quite challenge objective giving in a number of cases devices that are still confined to the research laboratory. There have been a number of robotic hand implementations that can be found in literature. A selection of leading hand designs reported here is limited in scope, addressing mechanical architecture, not control or sensing schemes. Moreover, because this work is concentrated to finger synthesis and design, the thumb description is excluded, as well as two-fingered constructions, because most of them were designed to work as grippers and would not integrate in the frame of multi-finger configuration. Significant tendon operated hands are the Stanford/JPL hand and the Utah/MIT hand. In particular, the first one has three 3-DOF fingers, each of them has a double-jointed head knuckle providing 90° of pitch and jaw and another distal knuckle with a range of ±135°. The Utah/MIT dextrous hand has three fingers with 4-DOFs, each digit of this hand has a non anthropomorphic design of the head knuckle excluding circumduction. The inclusion of three fingers minimizes reliance on friction and adds redundant support to manipulations tasks. Each N-DOF finger is controlled by 2-N independent actuators and tension cables. Although these two prototypes exhibit a good overall behaviour, they suffer of limited power transmission capability. The prototype of the DLR hand possesses special designed actuators and sensors integrated in the hand’s palm and fingers. This prototype has four fingers with 3-DOFs each, a 2-DOFs base joint gives ± 45° of flexion and ±30° of abduction/adduction, and 1-DOF knuckle with 135° of flexion. The distal joint, which is passively driven, is capable of flexing 110°. A prototype of an anthropomorphic mechanical hand with pneumatic actuation has been developed at Polytechnic of Turin having 4 fingers with 1-DOF each and it is controlled through PWM modulated digital valves.