Design and Modeling of a Variable-Stiffness Spring Mechanism for Impedance Modulation in Physical Human–Robot Interaction

Abstract

Our goal is to investigate different approaches to modulate stiffness and apply them to human-robot interaction. Here we report on our effort employing the concept of adjustable unsupported-length cantilever leaf spring, which has been previously applied to different designs of variable stiffness actuators. By transmitting the interaction force through the elastic component directly to the supporting structure instead of the actuation unit, this type of actuator requires low power to adjust and to maintain a desired stiffness. In the design of a 1-translational degree of freedom body weight support system of a rehabilitation robot, we used a leaf spring mechanism for stiffness modulation relying only on the spring deflection in combination with a non-backdrivable actuator for adjusting the vertical equilibrium position. This paper describes our approach in determining the spring parameters to attain a desired range of stiffness with a short traveling distance of the adjuster. To model the spring stiffness under deflection, the ideal cantilever support model cannot be assumed for a conventional design of dual roller-pairs slider, especially with a soft spring. A beam deflection model considering the non-zero slopes at the contact points between the rollers and the spring is presented, along with the validation experiments using different spring thicknesses on our prototype.