Design of A Highly Backdrivable, Powered Lower-Limb Orthosis for Improved Human Interaction
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Nearly 31 million Americans suffer from varying levels of mobility impairments, which are often caused by stroke, spine injury, arthritis, old age, and obesity. With an aging population, this number will continue to increase in the next several decades. Currently, most patients with lower-limb mobility issues receive physical training from a therapist, e.g., body weight support (BWS) training for a stroke survivor. The frequency and availability of physical training, however, are limited by finite medical resources. To address this, researchers are investigating powered lower-limb rehabilitation orthoses to relieve the repetitive and physically taxing duties of therapists, as well as to improve the efficacy of patient recovery. Currently, most lower-limb rehabilitation orthoses are stationary (due to their large size) and are quite costly, and these devices are only available in a small number of hospitals. A personal, affordable, and mobile powered lower-limb orthosis that can be used in the clinic or at home is necessary for a variety of different rehabilitation and mobility assistance purposes. These powered lower-limb orthoses provide assistance during different daily activities (e.g., walking, stair climbing, etc.) via a motorized actuation system that provides assistive or resistive torque to the joints. Due to the high torque requirements of lower-limb joints, past research has focused on increasing the torque density of powered orthoses to provide sufficient output torque with an acceptable weight. Consequently, the combination of a high speed motor and a high-ratio transmission, e.g., ball screw or harmonic drive, is common in powered lower-limb orthoses. However, the use of a high-ratio transmission results in high mechanical impedance, which restricts the user from freely moving their joints without the help from the orthosis. The significance of this research is to provide a paradigm shift to using a backdrivable actuator that allows the user to have an interaction with the orthosis, as opposed to having the user being forced to follow a pre-defined gait trajectory. An orthosis is said to be backdrivable if users can drive their joints without a high resistive torque from the orthosis. Backdrivability may not be necessary for patients who cannot contribute to their walking gait, such as patients with spinal cord injuries. In this research, we design and present several backdrivable orthoses, which can promote user participation and provide comfort during physical therapy and daily usage. Several design principles that we used in these designs could also influence the field of powered prosthetic legs, walking robotics, and electrical vehicles, etc.