Instantaneous-Stiffness Plane Analysis of Underactuated Fingers

The general stiffness of an underactuated finger as seen from the actuator is a function of its internal compliant elements, such as springs, but also depends on its geometry. In this paper, a complete stiffness analysis of a general underactuated finger is presented. The objective is to shed light on important aspects to consider while designing underactuated fingers and how to take advantage of the finger’s stiffness during grasping, for instance in order to estimate information such as contact location and force magnitude. This is done using the instantaneous-stiffness plane of the finger introduced in this paper. This plane shows the relationship between the finger’s geometry and its instantaneous stiffness and how simple changes in geometrical parameters can have significant effects on the finger’s stiffness. This novel tool can be used for a wide range of underactuated finger architectures as will be shown. First, a theoretical framework including numerical simulations is presented. This is then followed by an optimization example of a finger’s geometry and a discussion.