Mechanical Design of the Legs for OLL-E, a Fully Self-Balancing, Lower-Body Exoskeleton

• Main Author: Wilson, Bradford Asin
• Other Authors: Mechanical Engineering
• Format: Others
• Published: Virginia Tech 2019
• Subjects: Mechanical Design; Linkage Design; Gait Analysis; Humanoid Robot; Exoskeleton; Finite Element Analysis
• Online Access: http://hdl.handle.net/10919/93574

Exoskeletons show great promise in aiding people in a wide range of applications. One such application is medical rehabilitation and assistance of those with spinal cord injuries. Exoskeletons have the potential to offer several benefits over wheelchairs, including a reduction in the risk of upper-body injuries associated with extended wheelchair use. To fully mitigate this risk of injury, exoskeletons will need to be fully self-balancing, able to move and stand without crutches or other walking aid. To accomplish this, the Orthotic Lower-body Locomotion Exoskeleton (OLL-E) will actuate 12 Degrees of Freedom, six in each leg, using custom design linear series elastic actuators. The placement of these actuators relative to each joint axis, and the geometry of the linkage connecting them, were critical to ensuring each joint was capable of producing the required outputs for self-balancing locomotion. In pursuit of this goal, a general model was developed, relating the actuator's position and linkage geometry to the actual joint output over its range of motion. This model was then adapted for each joint in the legs and compared against the required outputs for humans and robots moving through a variety of gaits. This process allowed for the best placement of the actuator and linkages within the design constraints of the exoskeleton. The structure of the exoskeleton was then designed to maintain the desired geometry while meeting several other design requirements such as weight, adjustability, and range of motion. Adjustability was a key factor for ensuring the comfortable use of the exoskeleton and to minimize risk of injury by aligning the exoskeleton joint axes as close as possible to the wearer's joints. The legs of OLL-E can accommodate users between 1.60 m and 2.03 m in height while locating the exoskeleton joint axes within 2 mm of the user's joints. After detailed design was completed, analysis showed that the legs met all long-term goals of the exoskeleton project. === Master of Science === Exoskeletons show great promise in aiding people in a wide range of applications. One such application is medical rehabilitation and assistance of those with spinal cord injuries. Exoskeletons have the potential to offer several benefits over wheelchairs, including a reduction in the risk of upper-body injuries associated with extended wheelchair use. To best reduce this risk of injury, exoskeletons will need to be fully self-balancing, able to move and stand without crutches or relying on any other outside structure to stay upright. To accomplish this, the Orthotic Lower-body Locomotion Exoskeleton (OLL-E) will use a set of custom designed motors to apply power and control to 12 joints, six in each leg. Where these motors were placed, and how they connect to the joints they control, were critical to ensuring the exoskeleton was able to self-balance, walk, and climb stairs. To find the correct position, a set of equations was developed to determine how different positions changed each joints’ speed, strength, and range of motion. These equations were then put into a piece of custom software that could quickly evaluate different joint layouts and compare the capabilities against measurements from people and robots walking, climbing stairs, and standing up out of a chair. This process allowed for the best placement of the motors and joints while still keeping the exoskeleton relatively compact. The rest of the exoskeleton was then designed to connect these joints together, while meeting several other design requirements such as weight, adjustability, and range of motion. Adjustability was very important for ensuring the comfortable use of the exoskeleton and to minimize risk of injury by ensuring that the exoskeleton legs closely matched the movements of the person inside. The legs of OLL-E can accommodate users between 1.60 m and 2.03 m in increments of 7 mm. After detailed design was completed, additional analyses were performed to check the strength of the structure and ensure it met other long-term goals of the project.