This paper proposes a design method of transmissions for underactuated mechanisms. This transmission consists of active and passive gear trains that align a series of gears over several joints. The transmission has no kinematic singular point that restricts the workspace. Thus, arbitrary joint-connected motions of the underactuated mechanisms can be designed by combining the gear trains. We investigate the kinematics and statics of underactuated mechanisms to find a transmission matrix that realizes the desired connected motions and adaptive grasps. We also develop a method to design gear trains that corresponds to the transmission matrix. We extend this design method with stepped gears and belt-gear mechanisms to increase the design flexibility. The proposed method was experimentally validated using the two- and three-jointed mechanisms.

This paper designs a new underactuated robotic finger using a special transmission mechanism. The transmission mechanism consists of a set of gears and a spring to produce connected motions and adaptive curling when the finger moves in free space and when it contacts with the environment, respectively. The connected motion is realized by kinematic constraints of the joints instead of the static equilibrium of joint elastic elements that are frequently adopted in underactuated mechanisms. The motor torque is effectively used to increase the gripping force due to the mechanism. In addition, a reduction of the transmission components also simplifies the assembly process and decreases the size and weight of the finger, which is useful in attaching it to the arm as a prosthesis.