Preliminary design analysis of a novel variable impedance compact compliant actuator

This paper presents a novel variable impedance compact compliant series elastic actuator (SEA) design for human-friendly robotics applications. The actuator design consists of a servomotor, a ball screw, a torsional spring connecting the motor 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 with low stiffness are used to handle the low force operation and reduce nonlinear friction, output impedance, and impact. The torsional spring, being in the high speed range, has high effective stiffness and improves the system bandwidth in large force operation when the translational springs are fully compressed. This novel design overcomes the major limitations in the existing SEA design which requires a trade-off in the selection of spring constant. The new actuator design is also more compact due to the small size translational springs. We first review the merits and the limitations of current series elastic actuators. We then explain the construction and the working principle of our new design, followed by the dynamic modeling and analysis of the actuator model. Dynamic modeling on a physical implementation of the design is presented for the three conditions: (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. We then conclude the paper by discussing the advantages of the actuator design over its existing counterparts and its potential applications.