A novel variable impedance compact compliant series elastic actuator for human-friendly soft robotics applications

This paper presents the analysis of electro-mechanical design, materials, dynamics and control of a novel variable impedance compact compliant series elastic actuator (SEA) to improve human-robot interactions in human-friendly soft robotics applications. The design consists of a servomotor, a ball screw, a torsional spring connecting the servomotor and the ball screw via a pair of spur gear, and a set of translational springs connecting the ball screw nut to the output link. The translational springs have low stiffness and these are used to handle low force operations that reduce non-linear friction, output impedance, impact etc. The torsional spring is in the high speed range, has high effective stiffness and it enhances the system bandwidth for large force operations when the translational springs are fully compressed. Design, materials for construction, kinematics and working principle of the actuator are analyzed. Then, dynamics modeling and control on the physical implementation of the actuator for different conditions are analyzed. The conditions are: (i) open loop transfer function with load end fixed, (ii) closed loop transfer function with load end fixed, and (iii) output impedance with load end free for (i) low force, and (ii) high force cases. Then, the advantages of the design over its existing counterparts are discussed and its potential applications are mentioned. This novel SEA overcomes the major limitations of the existing SEAs, and can be used to develop human-friendly soft robots for various applications.