Bioinspired actuator for assistive robot

Abstract:

The size and proportion of older persons in most countries are increasing, aging is inevitable deterioration of cellular structure, muscle strength, and biological function, leading to problems during important movements like sit-to-stand, walking, and stair climbing. To enhance the quality of life of these aged people, it is desirable to develop wearable robots at human joints to produce biologically inspired motions. Traditional exoskeletons have a rigid structure, which restricts the natural motion patterns and reduces the comfort for wearers. The proposed study is to combine a smart material shear thickening fluids (STF) into twisted string actuators (TSA) to develop a type of variable stiffness actuator to assist knee joints, achieving human skeleton muscle similar motions. The employed STF is a lightly cross-linked silicone polymer whose mechanical characters, such as storage modulus, stiffness, and viscosity increased dramatically when the strain rate increased. The proposed STF based TSA is characterized by impregnating the STF into a Kevlar rope, increase the twisting speed of string can increase the strain rate acting on STF, generating a high stiffness and a large output force for the twisted string. An experimental platform is introduced to investigate the transmission property, output axial tension and equivalent stiffness in axial direction. Contrast experiments are performed between Kevlar and STF/ Kevlar ropes under different load conditions, which include both ends fixed and load with different stiffness springs. Shear thickening performance tests including different diameters, various combinations of Kevlar tube and STF/Kevlar rope, different weaving modes, etc. are attempted. A new mathematical model of TSA is proposed taking into account the actual force on the rope and storage modulus of the STF.