Design and Modeling of a High Force Soft Actuator for Assisted Elbow Flexion

Over the past decade, soft robotics have found its way into many applications that span across different length scales. Studies have since been done in respective research fields to characterize soft robots and understand their actuation process. On the nano to micro scale, soft robots are used to study and replicate movements of living organisms in bioinspired robots. While on the macro scale, soft actuators have found their niche in assistive and rehabilitative exoskeletons due to their compliant nature, as compared to traditional rigid rehabilitative robots, making them less injury prone. Modeling the kinematics and force output of soft actuators have been the main research focuses so that actuator performance may be tuned for specific scenarios and use cases. Kinematics studies have been adequate in describing the actuation patterns of these soft actuators. However, modeling the force output of soft robotic actuators remains a challenge due to their highly compliant material characteristics and complex deformation throughout the actuation process. In addition, the maximum force output of soft robots are often several magnitudes lesser than that of rigid robots. Force modeling and profiling of soft robots are thus of paramount importance towards fine tuning the force output from soft actuators. State-of-the-art upper limb assistive exoskeletons focus on improving the mobility of fingers and wrists joints that are lesser in anatomical weight. Developing exoskeletons for heavier joints, such as elbow and shoulder, remain a technical challenge. In this letter, we describe our work in the development of a 3D printable, high force soft exoskeleton that aims to assist the user in elbow flexion. The soft actuator can provide up to 100 N of tip force. We also describe a force model that allows tuning the dimensions of the bellow based actuator to cater to different payloads should the actuator be used in other applications, which are not described in this letter.