Twisted Rubber Variable-Stiffness Artificial Muscles

Abstract

Variable-stiffness artificial muscles are important in many applications including running and hopping robots, human-robot interaction, and active suspension systems. Previously used technologies include pneumatic muscles, layer and granular jamming, series elastic actuators, and shape memory polymers. All these are limited in terms of cost, complexity, the need for fluid power supplies, or controllability. In this article, we present a new concept for variable-stiffness artificial muscles (the twisted rubber artificial muscle, TRAM) made from twisted rubber cord that overcomes these limitations. Rubber cord is inexpensive, readily available, and inherently compliant. When an extended piece of rubber cord is twisted, the tensile force it exerts is reduced and its stiffness is altered. This behavior makes twisted rubber ideal for use as an artificial muscle, because its output force and natural stiffness are both controllable by varying twist angle. We investigate the behavior of four types of rubber cord and evaluate which type of rubber allows for the greatest reversible reduction in average stiffness (fluoroelastomer [FKM standard] rubber, 56.42% reduction) and initial stiffness (silicone rubber, 92.62%). Tensile force and stiffness can be further altered by increasing the twist angle of the artificial muscle beyond a threshold angle, which initiates nonlinear buckling behavior. This, however, can cause plastic deformation of the artificial muscle. Using a single TRAM, we show how the equilibrium position and natural frequency of a system can be simultaneously altered by controlling twist angle. We further demonstrate independent position and stiffness control of a functional robotic arm system using an antagonistic pair of TRAMs. TRAMs are ready for immediate inclusion in a wide range of robotic systems.

Design, Implementation, and Kinematics of a Twisting Robot Continuum Arm Inspired by Human Forearm Movements

“…In a comparison with the literature, the proposed arm mimics the performances of the human arm at high force. The twisting arm of [21] provides a high twisting angle but at 0.4 N, 2.5 N for the robot arm in [23], and 5 N in [24]. While the contraction PMA provides about 400 N [46] Robotics 2022, 11, x FOR PEER REVIEW 13 of 23…”

“…A robot arm by 3D printable origami has been used by [23] and it is driven by tendons. A prototype of a variable stiffness twisted rubber and it is driven by a stepper motor is presented by [24] present a variable stiffness twisted rubber and it is driven by a stepper motor.…”

Tethering of twisted-fiber artificial muscles

“…In the first section, we describe the external tethering of twisted artificial muscle fibers. 75,81,[89][90][91][92][93][94] In the second section, we discuss the self-supported, twisted artificial muscle fibers obtained via the balanced geometrical design of the twisted fibers; these methods involve the use of self-plied yarns and tendril-like architectures. 82,89,90,95,96 In the third section, we describe the self-supported, twisted artificial muscles constructed using chemical bonds or non-covalent interactions, such as polymer infiltration and coating, 76,87,98,99 solvent immersion and evaporation, 70 thermal annealing, [100][101][102][103][104][105] and cross-linking of chemical bonds.…”

Variable stiffness methods for robots: a review

“…Bioinspired soft robotic arm can achieve stable robot-environment interaction against unknown contact forces and impacts by variable stiffness mechanism [28][29][30]. Therefore, stiffness modulation is instrumental for reducing reflected inertias, improving performances and realizing safe human-robot collaboration [31][32][33][34]. These advantages of intrinsic compliance have encouraged researchers to develop a number of variable stiffness methods (VSMs) for robots [35][36][37][38][39][40].…”