In this paper, we discuss the development of a joint flexion mechanism for a myoelectric prosthetic hand that realizes stable power grasps and precision grasps. A number of studies on myoelectric prosthetic hands have been reported. For instance, a two-degree-of-freedom (2DOF) myoelectric prosthetic hand with high practicality has been described. It is capable of realizing the minimum required grip motion in daily living tasks. However, power grasping and precise grasping with the 2DOF myoelectric prosthetic hand are unstable because the joint angles of the fingers are fixed. To solve this problem, we proposed two joint flexion mechanisms: (i) the so-called PIP joint flexion mechanism, consisting of a wire pulling mechanism that flexes and extends the PIP joint of four fingers to realize form closure in a power grasp; (ii) an elastic joint implementing surface contact at the fingertip based on stability from the potential energy method in precision grasp. This is a passive mechanism incorporating a tension spring in the DIP joint of the thumb, index finger, and middle finger. Moreover, we developed the force and form closure (FFC) hand equipped with the PIP joint flexion mechanism and the elastic joint on the 2DOF myoelectric prosthetic hand and conducted evaluation experiments. By comparing experimental results of the FFC hand and the 2DOF myoelectric prosthetic hands, we proved that the proposed mechanisms can perform stable power grasping and precision grasping.
