Study on switching from position to stiffness control for musculoskeletal-inspired robot arm based on direct sum decomposition

Abstract

This paper presents a method of switching from position to stiffness control for a two-degree-of-freedom robot arm driven by three pairs of antagonistic actuators by applying the position and stiffness controller with the aim of developing a robot arm that can perform tasks involving a change from free to constrained motion. The control law consists of a feedback position controller and a feedforward stiffness controller in joint space. The position controller is a traditional PD feedback. The stiffness controller is designed using a newly developed method for solving the robot redundancy and adjusts joint stiffness through the coactivation of all the actuators. The solution to the redundancy expresses the actuator force space by a direct sum of its three subspaces and gives two independent relationships between actuator forces to joint torques and to joint stiffness. The position and stiffness controller is expected to control the position and the stiffness simultaneously and seamlessly perform tasks that involve changing from free to constrained motion. However, it has been revealed that the PD feedback controller affects the joint stiffness and that the stiffness controller reduces energy efficiency during position control. Therefore, this paper attempts to solve the problem by controlling the free motion with the position controller and by controlling the stiffness after the robot reaches the desired posture with the stiffness controller. Simulation results demonstrate that the proposed method can bring the robot to the desired posture, switch the control smoothly, and generate the desired stiffness.

Keywords

• musculoskeletal-inspired robot
• biarticular actuator
• stiffness control
• position control
• direct sum decomposition