Design and Validation of a Parallel Elastic Module for a Quasi-Direct Drive Exoskeleton

Compliant, powered exoskeletons have the potential to assist users in various circumstances including manual labor, rehabilitation, and elderly assistance. Parallel elastic actuators (PEAs) can potentially increase the battery life and/or assistance level of these devices, or allow the use of smaller motors while retaining comparable assistive performance. These benefits could be especially pronounced for quasi-direct drive (QDD) actuators, given the high currents required to deliver meaningful torques. Here we design and validate a spring-cam-follower PEA system to augment our existing QDD knee exoskeleton (M-BLUE) based on a novel design framework with an associated open-access Matlab application. The device employs a mixture of custom and commercial hardware to allow fast, low-cost implementation of a wide range of nonlinear parallel stiffness profiles. We use optimization to generate a stiffness profile to minimize Joule heating losses of the knee exoskeleton under a fractional assistance controller across four weighted tasks: squat lifting, level walking, and ramp ascent/descent. Benchtop dynamometer experiments characterize the empirical spring-cam stiffness curve, confirm the I2R energy savings achieved during target tasks (e.g., 40.3% reduction during squat with 25% assistance torque), and demonstrate the system's capacity for accurate open-loop torque control. An N=1 able-bodied human subject experiment serves as in situ validation of the PEA module during all four optimization tasks, with maximum Joule energy savings of 43.22 J seen during squat lifting/lowering—a 30% improvement compared to no PEA module.