The Robonaut hand 16 developed by NASA, David hand 17 developed by DLR, and Shadow dexterous hand 18 developed by Shadow Robot Company can be considered as representative hands with such a mechanism. In general, their actuators are located on the forearm and connected to the joints by tendons to transmit the driving force 16, 17, 18, 19, 20, 21. The hands based on the tendon-driven mechanism are the most similar to the human driving mechanism. Therefore, it is difficult to achieve compactness and high performance without innovation in actuator technology. Further, the space between the fingers is narrow, which makes it difficult to wire the force sensor to the finger. In addition, the inertia at the finger is high owing to the weight of the motor, thus requiring complex control mechanisms. Using motors with high-end specifications or driving force transmission parts result in increased costs. However, the size and performance of the hands are highly dependent on the motor, especially the finger part. This hand is capable of human-level natural movement and tactile feedback 15. In particular, MPL v2.0, which was developed by Johns Hopkins APL, shows a high dexterity with active 22 DOF and a compact design integrating actuators and electronics. Such a structure may have high joint driving efficiency, and it is easy to arrange the joints at a desired position. The hands developed based on the motor-direct-driven mechanism are structures that intuitively position the motors with respect to the joints to drive the joints directly or using a gear or timing pulley 13, 14. Therefore, the representative core mechanisms of the dexterous robotic hand are classified into (1) motor-direct-driven, (2) tendon-driven, and (3) linkage-driven mechanisms. For performing efficient grasping motions, many effective robotic hands in a form capable of adaptive grasping or low degree of freedom (DOF) have been developed 7, 8, 9, 10, 11, 12 however, our analysis focused on multi-DOF hands with high dexterity. To implement these functions using robots, many dexterous anthropomorphic robotic hands have been developed. In particular, because most tactile corpuscles are distributed at ~1 mm intervals in the fingertips, delicate tasks are easily performed with the fingertips 6. In addition, the tactile corpuscles, which enable tactile sensation, are mostly distributed in the hand, and they help in performing delicate tasks 5.
Out of the 206 bones in the human body, 54 bones are in the hands, corresponding to a quarter of the total number of bones the muscle structure driving them is also extremely complex. In particular, the movement of the human hand involves considerably high dexterity levels, suitable for performing a wide variety of tasks requiring a strong gripping force ranging from fine object grasping to tool manipulation 3, 4.
Interpreting the extremely complex functioning of the human hand remains an unresolved challenge in the field of robotics 1, 2.
Actual manipulation tasks involving tools used in everyday life are performed with the hand mounted on a commercial robot arm. It has the following features: 15-degree-of-freedom (20 joints), a fingertip force of 34N, compact size (maximum length: 218 mm) without additional parts, low weight of 1.1 kg, and tactile sensing capabilities. Based on a linkage-driven mechanism, an integrated linkage-driven dexterous anthropomorphic robotic hand called ILDA hand, which integrates all the components required for actuation and sensing and possesses high dexterity, is developed. The actuation parts make it difficult to integrate these hands into existing robotic arms, thus limiting their applicability. However, developing integrated hands without additional actuation parts while maintaining important functions such as human-level dexterity and grasping force is challenging. Robotic hands perform several amazing functions similar to the human hands, thereby offering high flexibility in terms of the tasks performed.