Development of adjustable stiffness actuator by varying lever arm length

Abstract

For a robotic actuator to be able to work safely with humans and/or to adapt natural dynamics of locomotion, it should be able to change its stiffness. The research presented herein, in this context, discusses a new actuation concept of variable stiffness for robotic joints for the purposes of energy efficiency, safety, and enhanced speed. With the aid of two motors, the actuator developed by authors can control its angular position and link-stiffness, independently. A small motor regulates the output stiffness for intended application whereas the large motor can control the angular position of the link. Stiffness variation is accomplished using an energy-efficient way of changing the lever arm ratio using a ball screw mechanism, without inducing or removing the potential energy stored in the springs. Experiments successfully exhibit that the greater the lever arm ratio, the stiffer will be the link. Also, to observe the effect of changing the springs, a set of springs with different spring constants are tested and their results indicate a direct relation between spring and actuator stiffness.