Bond graph modelling of a tendon driven prosthetic hand finger for motion and impedance control

The human hand is a highly complex system with five co-operating manipulators and each manipulator(finger) has three links or phalanges. Hand has more than twenty degrees of freedom including abduction and adduction. Objective of any prosthetic hand would be to do as many grasps as possible with minimum weight and minimum cost. Different types of mechanisms are available like pneumatic hands, tendon driven, linkage mechanism etc. Among these tendon driven hand prosthesis has distinct advantages. All the three links of a finger i.e. proximal, middle and distal phalanges can be actuated by single actuator thus reducing the cost and weight. The underactuated tendon driven hand can be made adaptive: if a link is obstructed the motion is transferred to other links. So it can grasp wide range of objects with any shape. Grasping of objects is important in case of prosthetics. In grasping, there is interaction between the object and the fingers leading to interaction forces. These interaction forces should be controlled for an effective grasp. In this paper, tendon driven prosthetic finger is modelled using Bond Graph modelling. Interaction force control problems of a tendon driven robotic finger in performing different tasks i.e. interaction with environment is addressed using the concept of impedance control by the use of additional passive degree of freedom and with the aid of Bond Graphs (Symbols Shakti) as a simulation tool.