Development of a soft linear actuator to use in wearable assistive exosuits

Abstract

Wearable exosuits require flexible, linearly contractile, and lightweight actuators to provide sufficient force to move the respective limb. This thesis presents the concept, design, fabrication, experimental performance characterization, and numerical modeling of two types of respectively thin and low-profile vacuum-driven, soft, linearly contractile actuators. The proposed soft actuators are made of an inextensible yet flexible thin-skinned pouch supported by a collapsible skeleton that orients the collapse of the actuator in the longitudinal axis upon the evacuation of the air within the pouch. The proposed novel soft, lightweight, contractile actuators are thin (ThinVAc) and lowprofile (LPVAc). Both these actuators are lightweight (ThinVAc: 0.75 g; LPVAc: 14 g), provide high maximum blocked forces (ThinVAc: 5.2 N; LPVAc: 39 N), provide maximum stresses similar to that expected from biological muscles (ThinVAc: 184 kPa; LPVAc: 117 kPa) and have high force-to-weight ratios (ThinVAc: 477; LPVAc: 285). The ThinVAc can combine to create multifilament actuators for force scaling. Combining 15 units of 500 mm ThinVAcs generates a maximum blocked force of 54 N (Max. stress: 62 kPa), 290 times the self-weight. The LPVAc integrates a position sensor based on an inductive sensor allowing closed-loop control with minimal error at 0.25 Hz. Numerical models for the contraction and blocked force of mono- and multifilament actuators allow for predicting their behavior independent of external sensors. The proposed actuators are tested in wearable applications to check their suitability. The ThinVAc is integrated into a knee rehabilitation assist device, and the LPVAc is incorporated into a novel mono-articular sit-to-stand transition (StSt) assist exosuit, helping to reduce muscle activity by 45%. These actuators have the potential to be integrated into a wide range of assistive devices and orthoses, such as knee or ankle braces, exoskeletons, and prosthetics, to provide the necessary support for people with mobility impairments.