Evaluation of Spring Stiffness for Stable Grasp in Underactuated Fingers

The underactuated hand that combined the active actuators with passive springs has the function of maintaining a stable grasp and achieving high adaptable grasp of irregularly shaped objects by their mechanical behavior. The spring has an influence with the size of stable grasp region. If the spring becomes compliant, the adaptability of the hand will increase while the stable grasp region will decrease. On the other hand, if the spring becomes too stiff, although the system's stabilization will become better for some specific objects, the general performance of the hand will decrease. The objective of this paper is to present a metric to use the spring as a design parameter to affect the tradeoff between the ability to conform to the object and grasp stability. Firstly, both an underactuated finger and stable region would be reviewed based on the statics analysis. Secondly, the transition between various grasp types would be reviewed based on some parameters which used to design an underactuated hand. Finally, the proper spring correspond to a larger stable grasp region can be confirmed based on their relationship in a numerical way.