Effects of substituting anthropometric joints with revolute joints in humanoid robots and robotic hands: a case study

In the human body, there are many joints whose functions are very similar to revolute joints. To avoid the complexity of these joints, they are usually substituted by revolute joints in many humanoid robots. Revolute joints have purely rotational motion along their fixed axis while real joints in the human body have Instantaneous Rotational Axis (IRA) due to their configuration. Substitution of this kind of human joints with revolute (hinge) joints in robots changes the kinematics of the joints. Knowing the exact characteristics of the moving axis of rotation in human joints is a prerequisite for the kinematic study of the joint. Here, the main geometrical difference between this kind of joints in humans and their simplified (hinge-like) models in robots is described. Then, as a case study, the mechanism of the three joints of the index finger are compared with their hinge-like model using a multibody code, in order to understand when revolute joints can be substituted for anthropometric joints in hand exoskeletons and robotic hands. Furthermore, the position of Instantaneous Rotational Axis (IRA) and its distance from center of the condyle (MH) of the joint are presented. The concept and the results can be extended for other fingers and all similar joints, and can be used in humanoid robots, hand exoskeletons and robotic hands.