This paper proposes a method to design low-cost robotic hands that require almost toolless assembly, and have special transmissions and joint mechanisms. The transmissions consist of active and passive gear trains manufactured by 3D printers to realize coordinated motions among joints and to transmit the driving force to underactuated joints. The joints are designed with pop-in structures to simplify the assembly of the designed robotic hands and its maintenance. The robotic hands even enabled us to exchange some components without necessitating a shutdown. By using these techniques, we manufactured two robotic hands with different passive gear trains. One of the hands had three fingers with stiff constrained gear trains and soft fingertips, and the other had five fingers with elastic gear trains. Experiments were conducted to validate the performance of the proposed robotic hands and to clarify the roles of the different gear trains.

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.