Simultaneous Control of Passive Stiffness Characteristics and Position with Elastically Constrained Underactuated Mechanisms

Abstract

Passive compliance is a necessary feature to ensure safety of home robots and medical robots working in human daily life. Such kinds of robots are desired to adjust their stiffness characteristics according to the task at hand and the surrounding environment. In this paper, we present an approach to control both the position and passive stiffness of output links of elastically constrained underactuated link mechanisms. Specifically, we consider a planar parallel link mechanism with nine degrees-of-freedom (DoF) consisting of six active revolute pairs with rotary actuators, eighteen passive revolute pairs, and nineteen links. Six of the passive revolute pairs are elastically constrained by torsional coil springs, thus the mechanism has passive compliance. We propose an optimization procedure to control the output position and multi-directional non-linear stiffness characteristics simultaneously by determining input actuator angles. The validity of the proposed analysis and optimization procedure is also experimentally corroborated.