This paper investigates the posture of the manipulator that has redundant DOF similar to human upper extremity. The human arm naturally takes a posture with no wandering although it has one or two DOF redundancy. The authors consider that the posture will be determined in task-oriented, which means that human unconsciously takes a posture of his/her upper extremities, which is suitable for the task that the endpoint is about to do. This study also assumes that all joints are capable to adjust the joint stiffness so that the stiffness of the endpoint in the task space is also adjustable. Hence our study aims to establish the theoretical method to make the manipulator take a posture that provides a stiffness of the endpoint suitable for the task. The new formula for shaping the manipulator's posture to provide a desired stiffness of the endpoint is presented followed by the simulation study to verify it.

This paper deals with mechanical stiffness control of multi-DOF joint. It fundamentally mimics the skeleto-muscular system of human articulation, in which at least two muscles cooperatively handle one DOF under their antagonistic action. In the first part of the paper one describes a novel actuator called ANLES (actuator with non-linear elastic system) that mimics a skeletal muscle in the sense of having a non-linear elasticity. Next one introduces a basic formula for controlling stiffness of the joint as well as its angles using multiple ANLESes. A new mechanism for the wrist joint having three DOF is proposed on the practical feasibility base, in which only four ANLESes with one additional DC-motor are used to control the stiffness of the three DOF wrist. It follows the simulation analysis for evaluating the controllability of the stiffness. It also shows the one DOF wrist joint controlled by two ANLESes