Joint Stiffness and Position Control of an Artificial Muscle Manipulator for Instantaneous Loads Using a Mechanical Equilibrium Model

Abstract

Robots have become an integral part of human life, and the relationship between human and robots has grown closer. Thus, it is desirable for robots to have characteristics similar to those of humans. In this context, we paid attention to an artificial muscle actuator, and studied and developed straight-fiber-type artificial muscles derived from McKibben-type muscles, which have an excellent contraction rate and force characteristics. We developed a manipulator with 6-d.o.f. using artificial muscles as actuators and considered its position control. However, artificial muscle manipulators are susceptible to load torque because they do not use any gears and are flexible. Joint stiffness must increase because accurate position control of the artificial muscle is difficult. Stiffness control must respond quickly to achieve coordination with human activity; however, conventional stiffness control by torque-based methods depends on the position control response. Therefore, position and stiffness control need to be independent of each other. In this study, we propose a new method of joint stiffness control, which adds estimated stiffness to the torque-based method. In addition, we performed experiments examining the load response to steady-state and instantaneous loads.